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WO2002001773A2 - Multiplexeur-demultiplexeur canal pour communications optiques - Google Patents

Multiplexeur-demultiplexeur canal pour communications optiques Download PDF

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Publication number
WO2002001773A2
WO2002001773A2 PCT/US2001/020321 US0120321W WO0201773A2 WO 2002001773 A2 WO2002001773 A2 WO 2002001773A2 US 0120321 W US0120321 W US 0120321W WO 0201773 A2 WO0201773 A2 WO 0201773A2
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WO
WIPO (PCT)
Prior art keywords
birefringent element
birefringent
interleaver
light
element assembly
Prior art date
Application number
PCT/US2001/020321
Other languages
English (en)
Other versions
WO2002001773A3 (fr
Inventor
Bin Zhao
Original Assignee
Cirvine Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Cirvine Corporation filed Critical Cirvine Corporation
Priority claimed from US09/891,794 external-priority patent/US6628449B2/en
Priority claimed from US09/892,224 external-priority patent/US6563641B2/en
Priority claimed from US09/891,795 external-priority patent/US20010055158A1/en
Publication of WO2002001773A2 publication Critical patent/WO2002001773A2/fr
Publication of WO2002001773A3 publication Critical patent/WO2002001773A3/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29302—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means based on birefringence or polarisation, e.g. wavelength dependent birefringence, polarisation interferometers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
    • G02B6/29386—Interleaving or deinterleaving, i.e. separating or mixing subsets of optical signals, e.g. combining even and odd channels into a single optical signal
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
    • G02B6/2713—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations
    • G02B6/272—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations comprising polarisation means for beam splitting and combining
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
    • G02B6/2766—Manipulating the plane of polarisation from one input polarisation to another output polarisation, e.g. polarisation rotators, linear to circular polarisation converters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
    • G02B6/2773—Polarisation splitting or combining

Definitions

  • the present invention relates generally to optical devices and relates more particularly to a dispersion mitigating comb filter and interleaver for optical communications and the like.
  • the present invention relates generally to optical devices and relates more particularly to an interleaver for optical communications and the like.
  • the present invention relates generally to optical devices and relates more particularly to an interleaver for optical communications and the like.
  • wavelength-division multiplexing WDM
  • dense wavelength-division multiplexing DWDM
  • a plurality of different wavelengths of light preferably infrared light
  • Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels.
  • the plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system.
  • wavelength-division multiplexing and dense wavelength-division multiplexing technologies a much greater amount of information can be transmitted than is possible utilizing a single wavelength optical communication system.
  • the individual channels of a wavelength-division multiplexed or dense wavelength-division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof.
  • This separation or demultiplexing process can be performed or assisted by a comb filter or an interleaver.
  • a similar device facilitates multiplexing of the individual channels by a transmitter.
  • crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel.
  • crosstalk interferes with the detection and/or demodulation process.
  • the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal.
  • optical interleaver which multiplexes and demultiplexes optical channels with respect to the physical media, i.e., optical fiber, offers a potential upgrade path, so as to facilitate scalability in both channel spacing and number of channel counts in a manner which enhances the performance of optical communication networks.
  • an interleaver can combine two streams of optical signals, wherein one stream contains odd channels and the other stream contains even channels, into a single, more densely spaced optical signal stream.
  • an interleaver can separate a dense signal stream into two, wider spaced streams, wherein one stream contains the odd channels and the other stream contains the even channels.
  • the interleaver offers scalability which allows contemporary communication technologies that perform well at wider channel spacing to address narrower, more bandwidth efficient, channel spacings.
  • interleavers suitable for multiplexing and demultiplexing optical signals. These include birefringent filters, thin-film dielectric devices, planar waveguides, and fiber-based devices. All of these contemporary interleaving technologies suffer from substantial limitations with respect to channel spacing, dispersion, insertion loss, channel isolation, temperature stability, cost, reliability and flexibility. For example, most commercially available interleavers provide only 100 GHz and 50 GHz channel spacings. Reduction of channel spacing to 25 GHz, 12.5 GHz and beyond appears to be difficult and challenging. Since it is generally impractical and undesirably expensive to provide precise control during manufacturing, the actual wavelength of communication channels and the center wavelength of filters generally tend to mismatch with each other.
  • the passband be wide enough so as to include a selected signal, even when both the carrier wavelength of the selected signal and the center wavelength of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time.
  • dispersion is the non-linear phase response of an optical device or system wherein light of different wavelengths is spread or dispersed, such that the phase relationship among the different wavelengths varies undesirably as the light passes through the device or system.
  • Such dispersion undesirably distorts optical signals, such as those used in optical communication systems.
  • the nonlinear phase response or dispersion of WDM and DWDM devices is responsible for signal distortion which results in undesired limitations on channel capability. That is, such dispersion undesirably limits the useable bandwidth of a channel, such as that of a fiber optic communication system. Such undesirable limitation of the bandwidth of a channel in a fiber optic communication system inherently reduces the bit rate of data transmitted thereby.
  • Contemporary interleavers have dispersion versus wavelength curves which have zero dispersion value at a particular wavelength, such as at nominal channel center wavelength.
  • the dispersion versus wavelength curve of such contemporary interleavers departs drastically from this zero dispersion value as the wavelength moves away from the nominal channel center wavelength.
  • small deviations in channel center wavelength can result in undesirably large dispersion values being realized. Since, as discussed in detail above, it is extremely difficult, if not impossible, to maintain the actual channel wavelength precisely at its nominal value, such channel center wavelengths do vary, thereby resulting in undesirably large dispersion values.
  • wavelength-division multiplexing WDM
  • dense wavelength-division multiplexing DWDM
  • a plurality of different wavelengths of light preferably infrared light
  • Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels.
  • the plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system.
  • wavelength-division multiplexing and dense wavelength-division multiplexing technologies a much greater amount of information can be transmitted than is possible utilizing a single wavelength optical communication system.
  • the individual channels of a wavelength-division multiplexed or dense wavelength- division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed or assisted by an interleaver.
  • a similar device facilitates multiplexing of the individual channels by a transmitter.
  • crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel.
  • crosstalk interferes with the detection and/or demodulation process.
  • the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection/demodulation of the signal.
  • optical interleaver which multiplexes and demultiplexes optical channels with respect to the physical media, i.e., optical fiber, offers a potential upgrade path, so as to facilitate scalability in both channel spacing and number of channel counts in a manner which enhances the performance of optical communication networks.
  • an interleaver can combine two streams of optical signals, wherein one stream contains odd channels and the other stream contains even channels, into a single, more densely spaced optical signal stream.
  • an interleaver can separate a dense signal stream into two, wider spaced streams, wherein one stream contains the odd channels and the other stream contains the even channels.
  • the interleaver offers scalability which allows contemporary communication technologies that perform well at wider channel spacing to address narrower, more bandwidth efficient, channel spacings.
  • interleavers suitable for multiplexing and demultiplexing optical signals. These include birefringent filters, thin-film dielectric devices, planar waveguides, and fiber-based devices. All of these contemporary interleaving technologies suffer from substantial limitations with respect to channel spacing, dispersion, insertion loss, channel isolation, temperature stability, cost, reliability and flexibility. For example, most commercially available interleavers provide only 100 GHz and 50 GHz channel spacings. Reduction of channel spacing to 25 GHz, 12.5 GHz and beyond appears to be difficult and challenging. Since it is generally impractical and undesirably expensive to provide precise control during manufacturing, the actual wavelength of communication channels and the center wavelength of filters generally tend to mismatch with each other.
  • the passband be wide enough so as to include a selected signal, even when both the carrier wavelength of the selected signal and the center wavelength of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time.
  • dispersion is the non-linear phase response of an optical device or system wherein light of different wavelengths is spread or dispersed, such that the phase relationship among the different wavelengths varies undesirably as the light passes through the device or system.
  • Such dispersion undesirably distorts optical signals, such as those used in optical communication systems.
  • the nonlinear phase response or dispersion of WDM and DWDM devices is responsible for signal distortion which results in undesired limitations on channel capability. That is, such dispersion undesirably limits the useable bandwidth of a channel, such as that of a fiber optic communication system. Such undesirable limitation of the bandwidth of a channel in a fiber optic communication system inherently reduces the bit rate of data transmitted thereby.
  • Contemporary interleavers have dispersion versus wavelength curves which have zero dispersion value at a particular wavelength, such as at nominal channel center wavelength.
  • the dispersion versus wavelength curve of such contemporary interleavers departs drastically from this zero dispersion value as the wavelength moves away from the nominal channel center wavelength.
  • small deviations in channel center wavelength can result in undesirably large dispersion values being realized.
  • wavelength-division multiplexing WDM
  • dense wavelength- division multiplexing DWDM
  • a plurality of different wavelengths of light are transmitted via a single medium such as an optical fiber.
  • Each wavelength corresponds to a separate channel and carries information generally independently with respect to the other channels.
  • the plurality of wavelengths (and consequently the corresponding plurality of channels) are transmitted simultaneously without interference with one another, so as to substantially enhance the transmission bandwidth of the communication system.
  • a much greater amount of information can be transmitted than is possible utilizing a single wavelength optical communication system.
  • the individual channels of a wavelength-division multiplexed or dense wavelength-division multiplexed signal must be selected or separated from one another at a receiver in order to facilitate detection and demodulation thereof. This separation or demultiplexing process can be performed or assisted by an interleaver.
  • a similar device facilitates multiplexing of the individual channels by a transmitter.
  • optical interleaver which multiplexes and demultiplexes optical channels with respect to the physical media, i.e., optical fiber, offers a potential upgrade path, so as to facilitate scalability in both channel spacing and number of channel counts in a manner which enhances the performance of optical communication networks.
  • an interleaver can combine two streams of optical signals, wherein one stream contains odd channels and the other stream contains even channels, into a single, more densely spaced optical signal stream.
  • an interleaver can separate a dense signal stream into two, wider spaced streams, wherein one stream contains the odd channels and the other stream contains the even channels.
  • the interleaver offers scalability which allows contemporary communication technologies that perform well at wider channel spacing to address narrower, more bandwidth efficient, channel spacings.
  • crosstalk occurs when channels overlap, i.e., remain substantially unseparated, such that some portion of one or more non-selected channels remains in combination with a selected channel.
  • crosstalk interferes with the detection and/or demodulation process.
  • the effects of crosstalk must be compensated for by undesirably increasing channel spacing and/or reducing the communication speed, so as to facilitate reliable detection demodulation of the signal.
  • the actual wavelength of communication channels and the center wavelength of filters generally tend to mismatch with each other. Precise control of manufacturing processes is difficult because it involves the use of more stringent tolerances which inherently require more accurate manufacturing equipment and more time consuming procedures.
  • the actual wavelength of the communication channel and the center wavelength of the filter also tend to drift over time due to inevitable material and device degradation over time and also due to changes in the optical characteristics of optical components due to temperature changes. Therefore, it is important that the passband be wide enough so as to include a selected signal, even when both the carrier wavelength of the selected signal and the center wavelength of the passband are not precisely matched or aligned during manufacturing and have drifted substantially over time.
  • dispersion is the non-linear phase response of an optical device or system wherein light of different wavelengths is spread or dispersed, such that the phase relationship among the different wavelengths varies undesirably as the light passes through the device or system.
  • Such dispersion undesirably distorts optical signals, such as those used in optical communication systems.
  • Contemporary interleavers have dispersion versus wavelength curves which have zero dispersion value at a particular wavelength, such as at nominal channel center wavelength.
  • the dispersion versus wavelength curve of such contemporary interleavers departs drastically from this zero dispersion value as the wavelength moves away from the nominal channel center wavelength.
  • small deviations in channel center wavelength can result in undesirably large dispersion values being realized.
  • interleavers suitable for multiplexing and demultiplexing optical signals. These include birefringent filters, thin-film dielectric devices, planar waveguides, and fiber-based devices. All of these contemporary interleaving technologies suffer from substantial limitations with respect to channel spacing, dispersion, insertion loss, channel isolation, temperature stability, cost, reliability and flexibility.
  • Birefringent crystals are commonly used in birefringent filters for separating multiplexed optical channels in DWDM communication systems. Birefringent crystals are materials in which the phase velocity of an optical beam propagating therein depends upon the polarization direction of the optical beam.
  • birefringent crystals suffer from inherent limitations which seriously degrade their performance, limit their application and reduce their desirability.
  • Contemporary crystal birefringent devices suffer from limitations imposed by the crystal's physical, mechanical and optical properties, as well as by problems associated with temperature instability. Further, such contemporary crystal birefringent devices have comparatively small and fixed birefringent values.
  • the present invention comprises a comb filter or an interleaver comprising a first birefringent element assembly which comprises at least one birefringent element and a second birefringent element assembly which comprises at least one other birefringent element.
  • the first birefringent element assembly and the second birefringent element assembly are configured so as to cooperate with one another in a manner which mitigates dispersion of the interleaver.
  • the present invention comprises an interleaver comprising a birefringent element assembly which comprises at least one birefringent element.
  • the interleaver further comprises a reflector configured so as to direct light from the birefringent element assembly back into and through the birefringent element assembly, such that the light traverses the birefringent element assembly in two different and opposite directions. Directing light from the birefringent element assembly back through the birefringent element assembly such that the light traverses the birefringent element assembly in both directions substantially mitigates crosstalk and dispersion of the interleaver.
  • the present invention comprises an interleaver comprising a birefringent element assembly and a reflector configured so as to direct light from the birefringent element assembly back through the birefringent element assembly.
  • the birefringent element assembly comprises at least one spacial birefringent element.
  • Such spatial birefringent element utilizes a difference in optical path length caused by a difference in physical path lengths or a difference in refraction indices along different paths, rather than utilizing birefringent crystals.
  • interleavers having narrower channel spacings may be constructed.
  • narrower interleaver channel spacing facilitates enhanced bandwidth utilization and an desirably increased number of channel counts.
  • Fig. 1 is a schematic top view of a contemporary birefringent filter or interleaver having a single birefringent element assembly
  • Fig. 2 is a schematic illustration of exemplary birefringent element orientations for the first stage (first interleaver element) and the second stage (second interleaver element) of an interleaver constructed according to one embodiment of the present invention, wherein the polarization direction of the first and second components entering the second stage are aligned with a polarization direction of a second one of the two components output from the first stage;
  • Fig. 3 is a schematic illustration of exemplary birefringent element orientations for the first stage (first interleaver element) and the second stage (second interleaver element) of an interleaver constructed according to an alternative configuration of the present invention, wherein the polarization direction of the first and second components entering the second stage are aligned with a polarization direction of a first one of the two components output from the first stage;
  • Fig. 4 is a schematic top view of an interleaver according to the present invention, wherein the first stage (first interleaver element) and the second stage (second interleaver element) thereof are disposed generally along a common axis and wherein the interleaver can operate, for example, according to either Fig. 2 or Fig. 3, depending upon the half- wave waveplate orientations thereof;
  • Fig. 5 is a series of schematic diagrams showing the optical beam states, crystal orientations and half-wave waveplate orientations at different locations for the interleaver of Fig. 4, wherein the polarizations of both components input to the second stage are aligned with the polarization of a second one of the two components exiting the first stage, as shown in Fig. 2;
  • Fig. 6 is a series of schematic diagrams showing the optical beam states, crystal orientations and half-waveplate orientations at different locations for the interleaver of
  • Fig. 7 is a chart showing dispersion versus wavelength for the interleaver (two stage interleaver having three elements in each stage thereof) of Fig. 4, after light has passed through only the first stage thereof, the first stage having birefringent element orientations of 45°, -21° and 7° and having phase delays of T, 2r and 2T;
  • Fig. 8 is a chart showing phase versus wavelength for the interleaver (two stage interleaver having three elements in each stage thereof) of Fig. 4, after light has passed through only the first stage thereof, having birefringent element orientations of 45°, -21° and 7° and having phase delays of T, 2r and 2T;
  • Fig. 9 is a chart showing transmission versus wavelength for the interleaver (two stage interleaver having three elements in each stage thereof) of Fig. 4, after light has passed through only the first stage thereof, having birefringent element orientations of 45°, -21° and 7° and having phase delays of r, 2r and 2T;
  • Fig. 10 is a chart showing dispersion versus wavelength for the interleaver (two stage interleaver having three elements in each stage thereof) of Fig. 4, after light has passed through both stages thereof, having first stage birefringent element orientations of 45°, -21° and 7° and having phase delays of r, 2r and 217;
  • Fig. 11 is a chart showing phase versus wavelength for the interleaver (two stage interleaver having three elements in each stage thereof) of Fig. 4, after light has passed through both stages thereof, having first stage birefringent element orientations of 45°, -21° and 7° and having phase delays of T, 2T and 21";
  • Fig. 12 is a chart showing transmission versus wavelength for the interleaver (two stage interleaver having three elements in each stage thereof) of Fig. 4, after light has passed through both stages thereof, having first stage birefringent element orientations of 45°, -21° and 7° and having phase delays of r, 2r and 2r;
  • Fig. 13 is a chart showing dispersion versus wavelength for an interleaver (two stage interleaver having two elements in each stage thereof), after light has passed only through the first stage thereof, having birefringent element orientations of 45° and -15° and having phase delays of T and 21";
  • Fig. 14 is a chart showing phase versus wavelength for the interleaver (two stage interleaver having two elements in each stage thereof), after light has passed only through the first stage thereof, having birefringent element orientations of 45° and -15° and having phase delays of T and 2r;
  • Fig. 15 is a chart showing transmission versus wavelength for the interleaver (two stage interleaver having two elements in each stage thereof), after light has passed only through the first stage thereof, having birefringent element orientations of 45° and -15° and having phase delays of T and 217;
  • Fig. 16 is a chart showing dispersion versus wavelength for the interleaver (two stage interleaver having two elements in each stage thereof), after light has passed through both stages thereof, having first stage birefringent element orientations of 45° and -15° and having phase delays of T and 217;
  • Fig. 17 is a chart showing phase versus wavelength for the interleaver (two stage interleaver having two elements in each stage thereof), after light has passed through both stages thereof, having first stage birefringent element orientations of 45° and -15° and having phase delays of T and 217;
  • Fig. 18 is a chart showing transmission versus wavelength for the interleaver (two stage interleaver having two elements in each stage thereof), after light has passed only through both stages thereof, having first stage birefringent element orientations of 45°, and -15° and having phase delays of T and 2r; and
  • Fig. 19 is a schematic top view of an interleaver according to the present invention, wherein the first stage (first interleaver element) and second stage (second interleaver element) thereof are not disposed generally along a common axis.
  • Figure 1 is a schematic diagram of a contemporary birefringent filter or interleaver.
  • Figure 2 is a schematic diagram of a fold interleaver configuration according to the present invention, showing birefringent element angular orientations (with respect to a polarization direction of an input light) for incident light and for returning light;
  • Figure 3 is a chart showing dispersion vs. wavelength for a three-element 50 GHz non-fold interleaver having birefringent element angular orientations of 45°, -21°, and 7° and having phase delays of T , IT and 2T , for each of the three birefringent elements, respectively;
  • Figure 4 is a chart showing phase distortion vs. wavelength for a three-element
  • Figure 5 is a chart showing transmission vs. wavelength for a three-element 50 GHz non-fold interleaver having birefringent element angular orientations of 45°, -21°, and 7° and having phase delays of T , IT and IT , for each of the three birefringent elements, respectively;
  • Figure 6 a chart showing dispersion vs. wavelength for a three-element 50 GHz fold interleaver having birefringent element angular orientations of 45°, -21°, and 7° and having phase delays of , 1 T and 2 T , for each of the three birefringent elements, respectively;
  • Figure 7 is a chart showing phase distortion vs. wavelength for a three-element 50 GHz fold interleaver having birefringent element angular orientations of 45°, -21°, and 7° and having phase delays of T , 2T and IT , for each of the three birefringent element, respectively;
  • Figure 8 is a chart showing transmission vs. wavelength for a three-element 50 GHz fold interleaver having birefringent element angular orientations of 45°, -21°, and 7° and having phase delays of T , IT and IT , for each of the three birefringent element, respectively;
  • Figure 9 is a schematic diagram of a three-element fold interleaver according to one embodiment of the present invention wherein a prism is used to cause light to be transmitted back through the birefringent elements;
  • Figure 10 is a series of frames showing the optical beam states, the crystal orientations and the half-wave waveplate orientations at different locations in an exemplary fold interleaver of Figure 9;
  • Figure 11 is a schematic diagram of a two-element fold interleaver according to another embodiment of the present invention wherein a prism is used to cause light to be transmitted back through the birefringent elements;
  • Figure 12 is a series of frames showing the optical beam states, the crystal orientations and the half-wave waveplate orientations at different locations in an exemplary fold interleaver of Figure 11;
  • Figure 13 a chart showing dispersion vs. wavelength for a two-element 50 GHz non- fold interleaver having birefringent element angular orientations of 45° and -15° and having phase delays of T and IT for each of the two birefringent elements, respectively;
  • Figure 14 is a chart showing phase distortion vs. wavelength for a two-element
  • Figure 15 is a chart showing transmission vs. wavelength for a two-element 50 GHz non-fold interleaver having birefringent element angular orientations of 45° and -15° and having phase delays of T and 2 T for each of the two birefringent elements, respectively;
  • Figure 16 a chart showing dispersion vs. wavelength for a two-element 50 GHz fold interleaver having birefringent element angular orientations of 45° and -15° and having phase delays of T and IT for each of the two birefringent elements, respectively;
  • Figure 17 is a chart showing phase distortion vs. wavelength for a two-element 50 GHz fold interleaver having birefringent element angular orientations of 45° and -15° and having phase delays of T and 2 T for each of the two birefringent elements, respectively;
  • Figure 18 is a chart showing transmission vs. wavelength for a two-element 50 GHz fold interleaver having birefringent element angular orientations of 45° and -15° and having phase delays of T and 2T for each of the two birefringent elements, respectively;
  • Figure 19 a chart showing dispersion vs. wavelength for a one-element 50 GHz non- fold interleaver having a birefringent element angular orientation of 45°;
  • Figure 20 is a chart showing phase distortion vs. wavelength for a one-element 50 GHz non-fold interleaver having a birefringent element angular orientation of 45°;
  • Figure 21 is a chart showing transmission vs. wavelength for a one-element 50 GHz non-fold interleaver having a birefringent element angular orientation of 45°;
  • Figure 22 a chart showing dispersion vs. wavelength for a one-element 50 GHz fold interleaver having a birefringent element angular orientation of 45°;
  • Figure 23 is a chart showing phase distortion vs. wavelength for a one-element 50 GHz fold interleaver having a birefringent element angular orientation of 45°
  • Figure 24 is a chart showing transmission vs. wavelength for a one-element 50 GHz fold interleaver having a birefringent element angular orientation of 45°;
  • Figure 25 is a schematic diagram of a three-element fold interleaver according to another embodiment of the present invention, wherein a mirror is used to cause light to be transmitted back through the birefringent elements;
  • Figure 26 is a series of frames showing the optical beam states, the crystal orientations and the quarter-wave and half-wave waveplate orientations at different locations in an exemplary fold interleaver of Figure 25;
  • Figure 27 is a schematic diagram of a two-element fold interleaver according to another embodiment of the present invention, wherein a mirror is used to cause light to be transmitted back through the birefringent elements;
  • Figure 28 is a series of frames showing the optical beams states, the crystal orientations and the quarter-wave and half-wave waveplate orientations at different locations in an exemplary fold interleaver of Figure 27.
  • Figure 1 is a top view schematic diagram of a two-element fold interleaver constructed according to the present invention.
  • Figures 2a and 2b are schematic diagrams showing the optical beams states and the quarter-wave and half-wave waveplate orientations at different locations for an exemplary two-element fold interleaver of Figure 1 which has equivalent birefringent element orientation angles of 45° and -15° and birefringent phase delays of T and 2T , respectively, for the two spatial birefringent elements.
  • Figure 3 is a dispersion vs. wavelength chart for an exemplary 50 GHz fold interleaver having equivalent birefringent element orientations of 45° and -15° and having phase delays of T and 2 T and constructed as shown in Figure 1 ;
  • Figure 4 is a phase vs. wavelength chart for an exemplary 50 GHz fold interleaver having equivalent birefringent element orientations of 45° and -15° and having phase delays of T and 2 T and constructed as shown in Figure 1 ;
  • Figure 5 is a transmission vs. wavelength chart for an exemplary 50 GHz fold interleaver having equivalent birefringent element orientations of 45° and -15° and having phase delays of T and IT and constructed as shown in Figure 1;
  • Figure 6 is a dispersion vs. wavelength chart for a non-fold interleaver having birefringent element orientations of 45° and -15° and having phase delays of T and 2T
  • Figure 7 is a phase vs. wavelength chart for a non-fold interleaver having birefringent element orientations of 45° and -15° and having phase delays of T and 2T ;
  • Figure 8 is a transmission vs. wavelength chart for a non-fold interleaver having birefringent element orientations of 45° and -15° and having phase delays of T and 2T ;
  • Figure 9 is a top view schematic diagram of a three-element fold interleaver constructed according to the present invention.
  • Figures 10a and 10b are schematic diagrams showing the optical beams states and the quarter-wave and half-wave waveplate orientations at different locations for an exemplary the three-element birefringent fold interleaver of Figure 9 which has equivalent birefringent element orientation angles of 45°, -21° and 7° and birefringent phase delays of T , IT and 2T , respectively, for the three spatial birefringent elements;
  • Figure 11 is a dispersion vs. wavelength chart for an exemplary 50 GHz fold interleaver having equivalent birefringent element orientations of 45°, 21° and 7° and having phase delays of T , 2 T and 2 T and constructed as shown in Figure 9;
  • Figure 12 is a phase vs. wavelength chart for an exemplary 50 GHz fold interleaver having equivalent birefringent element orientations of 45°, 21° and 7° and having phase delays of T , 2 T and 2 T and constructed as shown in Figure 9;
  • Figure 13 is a transmission vs. wavelength chart for an exemplary 50 GHz fold interleaver having equivalent birefringent element orientations of 45°, 21° and 7° and having phase delays of T , 2 T and 2 T and constructed as shown in Figure 9;
  • Figure 14 is a dispersion vs. wavelength chart for a 50 GHz non-fold interleaver having birefringent element orientations of 45°, 21° and 7° and having phase delays of T , 2T and 2T ;
  • Figure 15 is a phase vs. wavelength chart for a 50 GHz non-fold interleaver having birefringent element orientations of 45°, 21° and 7° and having phase delays of T , 2T and 2T ;
  • Figure 16 is a transmission vs. wavelength chart for a 50 GHz non-fold interleaver having birefringent element orientations of 45°, 21° and 7° and having phase delays of T , 2T and 2r ;
  • Figure 17 is a top view schematic diagram of an alternative configuration of a two-element fold interleaver according to the present invention.
  • Figure 18 is a top view schematic diagram of an alternative configuration of a three-element fold interleaver according to the present invention
  • Figure 19 is a top view schematic diagram of a configuration of a one-element fold interleaver according to the present invention.
  • Two different reference systems are used in this patent application for the determination of angular orientations.
  • One reference system is used for the determination of the angular orientations of birefringent elements, such as birefringent crystals, with respect to the polarization direction of input light.
  • Another reference system is used for the determination of the angular orientations of birefringent elements and the angular orientations of waveplates with respect to a moving (x, y, z) coordinate system.
  • two separate reference systems are utilized for the birefringent element angular orientations.
  • the angular orientation is typically the fast axis of the birefringent element with respect to the polarization direction of incoming light just prior to the incoming light reaching the birefringent element. Determination of the angular orientation is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the oncoming light and is negative if the rotation is counter-clockwise with respect to the polarization direction of the oncoming light.
  • each of the elements of the filter is measured by their fast axes with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of the filter. If there are more than one birefringent filters in a sequence, then the angular orientations are determined separately for each birefringent filter (the angular orientations are measured with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of each different filter). Thus, each birefringent filter has its own independent reference for the determination of the angular orientations of the birefringent elements thereof.
  • the angular orientation of birefringent elements and angular orientations of waveplates are also measured by the fast axes of birefringent elements and the optic axes of waveplates with respect to the +x axis at that location.
  • the +x axis is part of the moving coordinate system. This coordinate system travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate system rotates with the +y axis thereof so as to provide a new coordinate system.
  • an interleaver is an optical device which typically includes at least one birefringent filter.
  • a birefringent filter is one example of a comb filter.
  • the present invention comprises an interleaver which comprises a first birefringent element assembly (which is a first interleaver element and defines a first stage) which has at least one birefringent element and a second birefringent element assembly (which is a second interleaver element and defines a second stage) which has at least one other birefringent element.
  • the first birefringent element assembly and the second birefringent element assembly are configured so as to cooperate with one another in a manner which mitigates dispersion of the interleaver.
  • the first birefringent element assembly and the second birefringent element assembly are configured so as to have dispersion versus wavelength curves which are flipped with respect to one another for both even set and odd set of channels.
  • each point on the dispersion versus wavelength curve of one of the first and second birefringent element assemblies has a value which is approximately equal in value and opposite in sign with respect to the corresponding point on the curve for the other birefringent element assembly.
  • the dispersion versus wavelength curve for the first birefringent element assembly is added to the dispersion versus wavelength curve for the second birefringent element assembly, the net or total dispersion for the two birefringent element assemblies is approximately zero for all wavelengths.
  • dispersion introduced by one birefringent element assembly substantially cancels dispersion introduced by the other birefringent element assembly.
  • cancellation of the dispersion of one birefringent element assembly by another birefringent element assembly is achieved by providing birefringent element_orientations and phase delays in one birefringent element assembly that are related to the angular orientations and phase delays of birefringent elements in the other birefringent element assembly, such that both birefringent element assemblies cooperate with one another in a manner which desirably mitigates dispersion.
  • the first birefringent assembly may be configured so as to comprise birefringent elements which have angular orientations with respect to the polarization direction of incoming light at the input of the first birefringent assembly of ⁇ , wherein i indicates the position of the birefringent element counting in the direction in which light propagates
  • the second birefringent element assembly may comprise birefringent elements which have angular orientations with respect to the polarization direction of incoming light at the input of the second birefringent assembly of either 90° + ⁇ ,- or 90° - ⁇ it wherein i again indicates the position of the birefringent element counting in the direction in which light travels.
  • the first and second birefringent element assemblies comprise birefringent crystals.
  • various other birefringent devices are likewise suitable. For example, spatial birefringent devices are likewise suitable. For example, spatial birefringent devices are likewise suitable. For example, spatial birefringent devices are
  • Each birefringent element assembly may comprise one, two, three .or more birefringent elements, as desired.
  • additional birefringent elements generally facilitates the construction of a birefringent filter or interleaver having more enhanced transmission characteristics (such as a flatter and wider passband and/or a deeper and wider stopband).
  • the interleaver comprises an input polarization beam displacer configured to provide light to the first birefringent element assembly (which itself at least partially defines an interleaver), an intermediate polarization beam displacer configured to receive light from the first birefringent element assembly and to provide light to the second birefringent element assembly (which itself at least partially defines an interleaver) and an output polarization beam displacer configured to receive light from the second birefringent element assembly.
  • the first birefringent element assembly provides two orthogonal light components which are aligned with respect to one another prior to being input to the second birefringent element assembly.
  • the low dispersion interleaver of the present invention comprises two tandem or sequential interleaver elements.
  • This exemplary embodiment of the present invention preferably further comprises a first input half-wave waveplate assembly disposed intermediate the input polarization beam displacer and the first birefringent element assembly; a second input half-wave waveplate assembly disposed between intermediate polarization beam displacer and the second birefringent element assembly; and an output half-wave waveplate assembly disposed intermediate the second birefringent element assembly and the output polarization beam displacer.
  • a first input half-wave waveplate assembly disposed intermediate the input polarization beam displacer and the first birefringent element assembly
  • a second input half-wave waveplate assembly disposed between intermediate polarization beam displacer and the second birefringent element assembly
  • an output half-wave waveplate assembly disposed intermediate the second birefringent element assembly and the output polarization beam displacer.
  • birefringent element orientations of the first birefringent element assembly there are two sets of suitable birefringent element angular orientations for the second birefringent element assembly.
  • birefringent element orientations of ⁇ i, ⁇ 2 , and ⁇ y with respect to the polarization direction of incoming light at the input of the first birefringent element assembly both the angular orientations of 90° - ⁇ ⁇ , 90° - ⁇ 2 , 90° - ⁇ (these angles are referred to the incoming light polarization direction at the input of the second birefringent element assembly, e.g., Pi', P ', at location 10 as shown in Figs.
  • the first and third birefringent elements of the birefringent element assembly may be swapped with one another. Swapping the first and third birefringent elements of the first and/or second birefringent element assemblies provides the same transmission and dispersion characteristics as in an interleaver element_wherein the first and third birefringent elements are not swapped. Swapping of the first and third birefringent elements of the first birefringent element assembly and/or the second birefringent element assembly may be performed so as to facilitate manufacturability.
  • orientation angles for the first birefringent element assembly is 45°, -21° and 7° with respect to the incoming light polarization direction (at the first-stage input) for the birefringent elements of the first birefringent element assembly.
  • Birefringent elements having angular orientations of either 45°, -69° and 83° or 135°, 69° and 97° may then be utilized for the birefringent elements of the second birefringent element assembly, all with respect to the light polarization direction at the second stage input.
  • the phase delays for both the first and second birefringent element assemblies are T, 2T and 2r.
  • the first and third birefringent elements of the first and second birefringent element assemblies may be swapped, if desired.
  • the first birefringent element assembly and the second birefringent element assembly are disposed generally along the same axis with respect to one another.
  • an in-line or linear interleaver is formed.
  • the first and second birefringent element assemblies are not disposed generally along the same axis with respect to one another.
  • the first and second birefringent element assemblies may be disposed side-by-side with respect to one another, orthogonally with respect to one another, or in any other desired orientation with respect to one another.
  • a reflector such as a prism or a plurality of mirrors, is preferably used to deflect light from the first birefringent element assembly to the second birefringent element assembly.
  • a Sole birefringent filter can be utilized in the construction of an interleaver in which several birefringent elements, typically birefringent crystals, are located between two polarizing devices, such as an input polarizer and an output polarizer.
  • a typical layout of such a Sole birefringent filter utilizes birefringent crystals of lengths L, 2L, and 2L, which correspond to the relative phase delays T, 2T, IT provided thereby.
  • Angular orientations for the three birefringent crystals of a Sole birefringent filter are 45°, -15° and 10°.
  • the low dispersion interleaver of the present invention may utilize any desired angular orientations of the birefringent elements thereof.
  • such a contemporary Sole birefringent filter typically comprises an input polarizer 11, an output polarizer 12 and a birefringent element assembly 13 disposed intermediate the input polarizer 11 and the output polarizer 12.
  • the birefringent element assembly comprises a first birefringent crystal 15, second birefringent crystal 16 and third birefringent crystal 17.
  • birefringent crystal orientations e.g., 45°, -15° and 10°
  • a birefringent filter or interleaver can be designed having enhanced transmission characteristics.
  • Such enhanced transmission characteristics may be provided by constructing the birefringent filter or interleaver so as to have birefringent elements disposed at desired angular orientations and having desired phase delays.
  • All contemporary birefringent filters and interleavers introduce a certain, undesirable amount of dispersion into a communication system. As discussed above, such dispersion undesirably inherently limits effective communication bandwidth or communication capacity. Communication capacity is limited by such dispersion by, for example, inhibiting further reductions in interleaver channel spacing.
  • birefringent element orientations of ⁇ i, ⁇ 2 , and ⁇ for the first, second, and third birefringent elements, respectively, the same transmission performance can be obtained at birefringent element orientations of 90° - ⁇ i, 90° - ⁇ 2 , and 90° - ⁇ 3 , as well as at 90° + ⁇ ls 90° + ⁇ 2 , and 90° + ⁇ 3 , respectively.
  • the dispersion curves are flipped about the zero-dispersion axis for the two latter recited sets of orientation angles, when compared to the orientation angles of ⁇ i, ⁇ 2 , and ⁇ 3 .
  • interleaver elements it is also possible to utilize three separate interleaver elements to mitigate dispersion, by utilizing the first interleaver element to interleave channels into two sets thereof (odd and even channels), using a second interleaver element to compensate for the dispersion of the odd channels, and using a third interleaver element to compensate for the dispersion of the even channels.
  • the second and third interleaver elements are utilized, then the polarization directions of the even and odd channels do not have to be parallel at the input of the second and the third interleaver elements.
  • this configuration requires extra interleaver element.
  • the present invention accomplishes such mitigation of dispersion using only two interleaver elements.
  • FIG. 2 one configuration of first and second stages or interleaver elements which achieves zero or nearly zero dispersion is shown schematically.
  • P is the polarization direction for the input optical beam.
  • the fast axes of the birefringent elements are represented by f ⁇ , f 2 , and f 3 , respectively, in the first stage or interleaver element.
  • the fast axes of the birefringent elements are represented by fi', f 2 ' and f 3 ', respectively, in the second stage or interleaver element (birefringent element assembly).
  • the orientations of the birefringent elements in the first stage are represented by a set of angles, i.e., ⁇ ⁇ , ⁇ 2 , and ⁇ 3, which are all with respect to the input polarization direction P when looking at the incoming light.
  • two sets of interleaved signals having orthogonal polarizations with respect to one another are provided.
  • the odd channels are polarized along the direction Pi and the even channels are polarized along the direction P 2 , as shown.
  • the polarization directions thereof are positioned so as to be in the same direction, in order to compensate the dispersion of both even and odd channels simultaneously by the second stage or interleaver element.
  • the polarization direction of Pi is rotated by 90° to Pi', which is parallel to P 2 ' (which is the same as P 2 ).
  • the birefringent elements in the second stage are oriented so as to have birefringent element orientations of 90° - ⁇ 1 , 90° - ⁇ 2 , and 90° - ⁇ 3 with respect to the input polarization direction Pi' and P 2 '.
  • the two light components output from the first stage birefringent element assembly are made to be parallel with respect to one another prior to entering the second stage birefringent element assembly. This is done so that both components are acted upon in the same fashion by the second stage. In this manner, the second stage introduces dispersion into both the first and second components which substantially cancels dispersion introduced thereto by the first stage. If dispersion mitigation is not important, then the first and second components need not be so aligned.
  • the transmission characteristics include a generally flat passband and a comparatively deep and wide stopband. That is, for the stopband, the -30 dB bandwidth is comparatively wider than for a corresponding single stage interleaver (such as that shown in Figure 9) and the crosstalk is almost -80 dB.
  • the linear low dispersion interleaver of the present invention comprises an input polarization beam displacer 10 which provides light to first input half- wave waveplates 11. After the light is transmitted through half- wave waveplates 11 , the light is transmitted through a first stage or first birefringent element assembly, which at least partially defines a first birefringent filter or interleaver element.
  • the first birefringent element assembly comprises a first birefringent element 12, a second birefringent element 13 and a third birefringent element 14. After being transmitted through the first birefringent element assembly, light is transmitted through an intermediate polarization beam displacer 16.
  • Second input half- wave waveplates 17 and a second birefringent element assembly which at least partially defines a second birefringent filter or interleaver element.
  • the second birefringent element assembly is comprised of a first birefringent element 18, a second birefringent element 19 and a third birefringent element 20.
  • Light from the second birefringent element assembly is transmitted through output half- wave waveplates 21 to the output polarization beam displacer 22.
  • a right-hand coordinate system of axes is used to characterize the optical beam propagation in the system at various locations with a convention that the light is propagating in the +z direction and the +y direction is out of the plane of the paper in
  • Figure 4 are shown for an instance wherein Figure 4 is configured as shown in Figure 3
  • each of the four boxes corresponds to a physical beam position at various locations within the low dispersion interleaver of Figure 4.
  • the polarization beam displacers of Figure 4 shift the optical beams between these beam positions according to the orientation of the polarization beam displacers and the optical beam polarizations.
  • the first stage birefringent element orientations of 45°, -21° and 7°, and the phase delays of T , 2T and IT are utilized to illustrate how an exemplary linear interleaver may be constructed.
  • phase delays are otherwise suitable.
  • an input optical beam has two linearly polarized components: 1
  • the optic axis of the half-wave waveplate for component 1 is oriented at 45° with respect to the +x axis at that location and the optic axis of the half- wave waveplate for component 2 is oriented at 0° with respect to the +x axis at that location.
  • the first birefringent element is oriented with its fast axis at -45° with respect to the +x axis at that location.
  • the orientations for the birefringent elements 2 and 3 are 21° and - 7° with respect to the +x axis at locations 5 and 6, respectively.
  • the vertically polarized (y direction) components correspond to one set of the interleaved channels (e.g., the even channels) and the horizontally polarized (x-direction) components correspond to another set of interleaved channels (e.g., the odd channels).
  • the vertically polarized beams move to the bottom beam positions as shown at location 8.
  • the polarization direction for all components is changed to or remain at the y direction at location 10.
  • the orientation of optic axes of the half- wave waveplates with respect to +x axis are shown in frame 9 in Fig. 5, as 45°, 45°, 90°, 90°, respectively. It is worthwhile to note that the polarization directions are all vertical at location 10. Thus, all of the beams are polarized in the same direction at this point.
  • the la' and the 2a' components are the corresponding odd channels and la" and 2a" are the crosstalk noise from the even channels (typcially very small).
  • the lb" and the 2b" components are the corresponding even channels and lb', 2b' are the crosstalk noise from the odd channel (typically very small).
  • the two output beams la' and 2a' as well as lb" and 2b" are combined at location 17, which corresponds to the two series of interleaved channels with zero or nearly zero dispersion, respectively.
  • the orientation of optic axes of the half-wave waveplates with respective to +x axis are shown in frame 15 in Fig.
  • Thick arrows (such as those of frames 14, 16, and 17) are used to indicate the desired (non-crosstalk) signal and thin arrows are used to indicate crosstalk when thick arrows are present.
  • the polarization mode dispersion (PMD) is minimized.
  • the birefringent element assemblies of the present invention described above comprise three birefringent elements
  • any desired number of birefringent elements may alternatively be utilized.
  • the use of four, five, or more birefringent elements tends to provide enhanced transmission characteristics with respect to the use of three birefringent elements.
  • the low dispersion interleaver of the present invention may be constructed so as to have fewer than three birefringent elements in each interleaver element thereof, without adversely affecting the dispersion provided thereby.
  • the curves of Figures 13-18 are for a 50 GHz interleaver having birefringent element orientations for the first stage thereof of 45° and -15° with respect to input polarization direction at entry of the first stage and having phase delays for the first stage thereof of T and 2 r .
  • the half-wave waveplates orientations at location 2, 9, and 15 can be chosen as shown in Fig. 5 so that the output of the first stage can be aligned so as to be perpendicular to the input thereto as shown in Fig. 2.
  • the half- wave waveplates orientations at locations 2, 9, and 15 can be chosen as shown in Fig. 6 so that the output of the first stage can be aligned so as to be parallel to the input thereto as shown in Fig. 3.
  • FIG. 19 an exemplary low dispersion interleaver wherein the first birefringent element assembly and the second birefringent element assembly thereof are not collinear with respect to one another is shown.
  • a prism 15 deflects light from the first birefringent element assembly to the second birefringent element assembly.
  • various other devices such as mirrors, may be similarly utilized to deflect light from the first birefringent element assembly to the second birefringent element assembly.
  • the first birefringent element assembly and the second birefringent element assembly may be at any desired angle with respect to one another and need not be either collinear (as shown in Figure 4) or parallel (as shown in Figure 19).
  • birefringent devices other than birefringent crystals, may alternatively be utilized.
  • birefringent devices other than birefringent crystals
  • the angular orientations of the birefringent devices can be converted to relative angles between the optical beam polarization direction and the equivalent fast axes of such birefringent devices.
  • birefringent device which does not utilize birefringent crystals is a device wherein an incoming composite optical beam is separated into two generally orthogonally polarized optical beams and each of the two generally orthogonally polarized beams travel over different optical path lengths prior to being recombined, so as to obtain a birefringent effect.
  • a 50 GHz interleaver is utilized as an example.
  • Those skilled in the art will appreciate that the use of a 50 GHz interleaver as an example by way of illustration only, and not by way of limitation.
  • phase delays of T , 2T and 2T may alternatively be utilized.
  • interleavers described herein are suitable for demultiplexing optical signals. Those skilled in the art will appreciate similar structures may be utilized to multiplex optical signals.
  • the waveplates which are utilized in the present invention can optionally be omitted in some instances by rotating subsequent components appropriately.
  • various devices and/or materials may alternatively be utilized to orient the polarization direction of light beams.
  • devices and/or materials which are responsive to applied voltages, currents, magnetic fields and/or electrical fields may be used to orient the polarization direction of light beams.
  • the use of waveplates herein is by way of example only, and not by way of limitations.
  • the stages of the low dispersion interleaver of the present invention need not comprise substantially identical devices, but rather may comprise any two devices having generally flipped or opposite dispersion curves with respect to one another.
  • the first stage may comprise birefringent crystals while the second stage comprises spatial birefringent devices.
  • birefringent elements having various different angular orientations and/or phase delays may be utilized.
  • birefringent elements having various different angular orientations and/or phase delays may be utilized.
  • Two different reference systems are used in this patent application for the determination of angular orientations.
  • One reference system is used for the determination of the angular orientations of birefringent elements, such as birefringent crystals, with respect to the polarization direction of input light.
  • Another reference system is used for the determination of the angular orientations of birefringent elements and the angular orientations of waveplates with respect to a moving (x, y, z) coordinate system.
  • two separate reference systems are utilized for the birefringent element angular orientations.
  • the angular orientation is typically the fast axis of the birefringent element with respect to the polarization direction of incoming light just prior to the incoming light reaching the birefringent element. Determination of the angular orientation is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the oncoming light and is negative if the rotation is counterclockwise with respect to the polarization direction of the oncoming light.
  • each of the elements of the filter is measured by their fast axes with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of the filter. If there are more than one birefringent filters in a sequence, then the angular orientations are determined separately for each birefringent filter (the angular orientations are measured with respect to the polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of each different filter). Thus, each birefringent filter has its own independent reference for the determination of the angular orientations of the birefringent elements thereof.
  • the angular orientation of birefringent elements and angular orientations of waveplates are also measured by the fast axes of birefringent elements and the optic axes of waveplates with respect to the + ⁇ axis.
  • the +x axis is part of the moving coordinate system. This coordinate system travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate system rotates with the +y axis thereof so as to provide a new coordinate system.
  • Determination of the angular orientations in (x, y, z) coordinate system is made by observing oncoming light with the convention that the angle is positive if the rotation of the corresponding optical axis is counter-clockwise with respect to +x axis and is negative if the rotation is clockwise with respect to the +x axis (which is consistent with the conventional use of (x, y, z) coordinate system, but which is contrary to the sign convention for determining the angular orientations of birefringent elements with respect to the input polarization direction, as discussed above).
  • an interleaver is an optical device which typically includes at least one birefringent filter.
  • a birefringent filter is one example of a comb filter.
  • the present invention comprises an interleaver which comprises a birefringent element assembly and a reflector configured so as to direct light which was emitted from the birefringent element assembly back into and through the birefringent element assembly.
  • the birefringent element assembly comprises at least one birefringent element, such as a birefringent crystal. Directing light from the birefringent element assembly back into and through the birefringent element assembly causes the light to be transmitted through the birefringent element assembly twice, one in each of two different opposite directions. By transmitting the light through the birefringent element assembly in both directions, crosstalk can be substantially mitigated. Further, dispersion can be substantially mitigated or eliminated.
  • the reflector preferably comprises a single prism. However, those skilled in the art will appreciate that the reflector may alternatively comprise more than one prism and/or one or more mirrors or etalons.
  • the birefringent element assembly may contain any desired number of birefringent elements, such as birefringent crystals.
  • the birefringent element assembly may contain one, two, three, four, five or more birefringent elements.
  • additional birefringent elements tend to enhance the transmission vs. wavelength curve of the birefringent filter or interleaver defined by the birefringent elements, so as to tend to provide a flatter and wider passband and/or so as to provide a deeper and wider stopband.
  • the birefringent element assembly comprises birefringent crystals.
  • the birefringent element assembly is disposed intermediate (in an optical sense) an input polarization beam displacer and an intermediate polarization beam displacer.
  • the interleaver comprises an input polarization beam displacer from which light is transmitted to the birefringent element assembly; a first input half-wave waveplate assembly configured to receive light from the input polarization beam displacer and change the light polarization directions; an intermediate polarization beam displacer configured to transmit light from the birefringent element assembly before the light is transmitted back through the birefringent element assembly; a second input half-wave waveplate assembly configured to control the light polarization direction before the light is transmitted back through the birefringent element assembly; an output half-wave waveplate assembly configured to control the light polarization direction after the light is transmitted back through the birefringent element assembly; and an output polarization beam displacer to which light is transmitted after the light has been transmitted back through the birefringent element assembly.
  • the present invention comprises a method for interleaving a light beam, such as for use in multiplexing and de-multiplexing dense wavelength-division multiplex (DWDM) communication signals.
  • the method comprises transmitting a beam of light through a birefringent element assembly in a first direction and then transmitting the beam of light through the same birefringent element assembly in a second direction, which is generally opposite to the first direction. Transmitting the beam of light through the birefringent element assembly in both the first and second directions mitigates crosstalk. Further, dispersion can be mitigated in interleavers having more than a single birefringent element or birefringent crystal.
  • a Sole birefringent filter can be utilized to construct an interleaver.
  • birefringent element assembly 13 is disposed intermediate two polarizing devices, such as input polarizer 11 and output polarizer 12.
  • the birefringent element assembly may comprise a plurality of birefringent crystals such as first birefringent crystal 15, second birefringent crystal 16 and third birefringent crystal 17.
  • the birefringent crystals have lengths of Li, L 2 and L 3 .
  • such birefringent element assemblies commonly comprise three birefringent crystals, wherein the first birefringent crystal 15 has an angular orientation of 45°, the second birefringent crystal 16 has an angular orientation of -15° and third birefringent crystal 17 has an angular orientation of 10°, all with respect to a polarization direction of the input polarizer 11.
  • the second birefringent crystal 16 and the third birefringent crystal 17 provide phase delays (2T) which are approximately twice the phase delay (T) provided by the first birefringent crystal 15.
  • birefringent element orientations of 45°, -15° and 10° do provide generally satisfactory transmission characteristics to a certain degree, i.e., provide a generally flat passband, enhanced transmission characteristics can be provided via the use of other sets of angles and phase delays for the birefringent elements.
  • orientations angles of 45°, -21° and 7° and phase delays T , 1T , 1T for the first birefringent element, second birefringent element and third birefringent element, respectively, may alternatively be utilized to provide enhanced passband and stopband characteristics, so as to mitigate undesirable crosstalk between channels in a dense wavelength-division multiplexing (DWDM) optical communication system or the like.
  • the crosstalk can be further reduced by letting optical signals pass through another birefringent filter. But this leads to higher cost due to the doubling in device numbers.
  • birefringent filters wherein light passes therethrough only once and in a single direction always introduce a finite, undesirably high, amount of dispersion.
  • an interleaver utilizing a birefringent filter is constructed in a manner which substantially mitigates crosstalk without additional birefringent elements. Further, dispersion can be substantially mitigated and eliminated without additional birefringent elements. This is accomplished by configuring the present invention such that light travels through the same birefringent filter twice or more times, in two generally opposite directions.
  • the present invention facilitates the construction of an interleaver which makes possible substantially reduced channel spacing, so as to desirably increase the effective bandwidth of an optical medium and thereby enhance the potential for channel count increases.
  • a birefringent filter or interleaver can be formed, such that the dispersion vs. wavelength curve thereof is approximately zero for all wavelengths and thus such that the birefringent filter or interleaver itself contributes very little or no dispersion. Therefore, the interleaver of the present invention may be utilized to mitigate total dispersion within an optical system by minimizing its own introduction of undesirable dispersion.
  • dispersion curves are flipped about the zero dispersion axis for the sets of angles of 90° - ⁇ 1, 90° - ⁇ and 90° - ⁇ 3, as well as 90° + ⁇ 1, 90° + ⁇ 2 and 90° + ⁇ 3 , when taken with respect to the orientations of ⁇ l5 ⁇ 2 and ⁇ 3 .
  • the dispersion curve of a birefringent filter having birefringent element orientations of ⁇ l5 ⁇ 2 and ⁇ 3 will be opposite the dispersion curve of either a birefringent filter having birefringent element orientations of 90° - ⁇ 1, 90° - ⁇ 2 and 90° - ⁇ 3 or a birefringent filter having birefringent element orientations of 90° + ⁇ 1, 90° + ⁇ 2 and 90° + ⁇ 3 .
  • an optical beam is transmitted through two interleavers sequentially, wherein the two interleavers have been designed such that they have flipped dispersion curves with respect to one another (such as by having the first interleaver utilize birefringent element orientations of ⁇ ⁇ , ⁇ 2 and ⁇ 3 and having the second interleaver utilize birefringent element orientations of 90° - ⁇ ⁇ , 90° - ⁇ 2 and 90° - ⁇ 3 or by having the second interleaver utilize birefringent element orientations of 90° + ⁇ 1 , 90° + ⁇ 2 and 90° + ⁇ 3 ), then the dispersion of the two interleavers cancels and the total dispersion of the two interleavers is zero or approximately zero.
  • this configuration requires at least two separate interleavers to achieve zero or approximately zero dispersion for both odd and even channels.
  • dispersion is substantially mitigated by transmitting a beam of light (incident light as labeled in Figure 2) through a birefringent element assembly, such as a birefringent element assembly (such as item 13 of Fig.
  • first birefringent crystal is of Figure 1
  • second birefringent element such as second birefringent crystal 16 of Figure 1
  • third birefringent element such as third birefringent crystal 17 of Figure 1
  • third birefringent crystal 17 of Figure 1 has a fast axis f 3 thereof oriented at an angle of ⁇ 3 with respect to the polarization direction of the input light P.
  • two separate sets of interleaved signals (channels) having polarizations which are orthogonal to one another are obtained.
  • the odd channels may be polarized along the Pi direction and the even channels polarized along the P 2 direction.
  • the incident light is reflected, such as by a mirror or prism, and then travels back through the same set of birefringent elements in the reverse direction (and is labeled returning light in Figure 2).
  • Pi and P 2 are aligned so as to be parallel with respet to one another (such as by rotating Pi) ,so as to form Pi' and P 2 ', which are parallel to one another and are also aligned so as both to be perpendicular with respect to P.
  • the birefringent element angles are ⁇ 1 , ⁇ 2 , ⁇ 3
  • the birefringent element angles are 90° - ⁇ 3 , 90° - ⁇ 2 , and 90° - ⁇ 1 , in the order in which light encounters the birefringent elements and with respect to the polarization direction Pi' and P 2 ' (which are parallel to one another).
  • Pi' and P 2 ' are pe ⁇ endicular to P.
  • Such a zero dispersion interleaver may be constructed by folding the light path (as shown generally in Figure 2 and as shown more particularly in Figure 9 and 11), such that incident light traveling through the birefringent filter in a forward direction is reflected back through the filter in a reverse direction.
  • Pi Before being transmitted back the birefringent element assembly, Pi is made to be parallel with respect to P 2 so as to form Pi' and P 2 '. However, if dispersion mitigation is not required, then it is not necessary to make Pi parallel with respect to P 2 , so as to form Pi' and P 2 '. However, it is necessary that Pi' and P 2 ' each be either parallel or perpendicular with respect to P (the polarization direction of light input to the birefringent element assembly as shown in Figure 2) for crosstalk mitigation. Referring now to Figures 3-5, the dispersion vs. wavelength, phase vs. wavelength and transmission vs.
  • wavelength for a 50 GHz three-element non-fold interleaver having birefringent element orientations of 45°, -21° and 7° and having phase delays of T , 2T , 2T respectively, are shown for incident light traveling through the interleaver (and not being reflected back therethrough).
  • the dispersion has non-zero values at various different wavelengths.
  • this non-fold interleaver introduces undesirable dispersion.
  • the dispersion vs. wavelength, phase vs. wavelength, and transmission vs. wavelength curves for a 50 GHz three-element fold interleaver are shown. This may be accomplished by using the birefringent elements that produced the dispersion vs. wavelength curve of Figure 3 and by aligning Pi ' so as to be parallel with respect to P 2 ' and by aligning both Pi' and P ' so as to be pe ⁇ endicular with P and then by reflecting light back through the birefringent elements after the light has already passed therethrough once. The light is transmitted back through the birefringent elements in a direction opposite to the direction in which it was first transmitted. Thus, as shown in Figure 6, reflecting the incident light back through the birefringent filter such that returning light is transmitted through the filter in the reverse direction results in approximately zero dispersion for all wavelengths. Similarly, zero phase distortion is provided at all wavelengths, as shown in Figure 7.
  • the depth of the stopband is substantially enhanced, thereby further enhancing filter performance.
  • the stopband of the fold interleaver shown in Figure 8 has wider -30 dB points than the stopband of the non-fold interleaver shown in Figure 5 and also has a crosstalk of almost -80 dB.
  • birefringent filter having birefringent element orientations of 90° - ⁇ 3 , 90° - ⁇ 2 and 90° - ⁇ ⁇ is equivalent to having the beam pass through a birefringent filter having orientations of 90° - ⁇ ls 90° - ⁇ 2 and 90° - ⁇ 3 .
  • a birefringent filter having orientations of 90° - ⁇ 1 , 90° - ⁇ 2 and 90° - ⁇ 3 provides flipped dispersion with respect to a birefringent filter having birefringent element orientations of ⁇ 1 , ⁇ 2 and ⁇ 3 .
  • both the odd channels (those channels polarized along the Pi" direction) and the even channels (those channels polarized along the P 2 " direction) have approximately zero dispersion. That is, such a fold interleaver does not introduce substantial dispersion into an optical signal transmitted therethrough.
  • the first and third birefringent elements may be swapped with one another. Referring now to Figure 9, a three-element fold interleaver is constructed so as to reflect incident light traveling in a first direction therethrough back into the interleaver (or more particularity, back into the birefringent elements of the interleaver) such that returning light travels in a reverse direction therethrough.
  • Half-wave waveplates 32 align Pi ' parallel with respect to P 2 ' and Pi' and P 2 ' pe ⁇ endicular with respect to P prior to Pi' and P 2 ' being transmitted back through the birefringent element assembly 20 in the reverse direction.
  • a right-hand coordinate system of axes is used to characterize the optical beam propagation in the system at various locations with a convention that the light is always propagating in the +z direction and the +y direction is always out of the plane of the paper.
  • the fold interleaver comprises a birefringent element assembly 20 disposed intermediate (in an optical sense) an input polarization beam displacer 21 and an intermediate polarization beam displacer 28.
  • the birefringent element assembly 20 comprises a first birefringent element, such as first birefringent crystal 23; a second birefringent element, such as second birefringent crystal 24; and a third birefringent element, such as third birefringent crystal 25.
  • a reflector such as one or more mirrors, or such as prism 27 (more than one prism may be utilized, if desired) deflects incident light which has passed through the birefringent element assembly 20 back into and through the birefringent element assembly 20, such that the returning light travels through the birefringent element assembly 20 in a reverse direction.
  • a reflector such as one or more mirrors, or such as prism 27 (more than one prism may be utilized, if desired) deflects incident light which has passed through the birefringent element assembly 20 back into and through the birefringent element assembly 20, such that the returning light travels through the birefringent element assembly 20 in a reverse direction.
  • prism 27 more than one prism may be utilized, if desired
  • intermediate polarization beam displacer 28 receives incident light from the birefringent element assembly 20 and transmits the incident light to the prism 27. But, it can be placed between the birefringent element assembly 20 and the prism 27 on the path of returning light.
  • a first input half- wave waveplate assembly 29 comprises two half- wave waveplates disposed intermediate the input polarization beam displacer 21 and the birefringent element assembly 20.
  • the first one of the two half- wave waveplates 29 preferably has an optical axis thereof oriented at an angle of approximately 0° with respect to the +x axis at that location.
  • a second half-wave waveplate of the two half-wave waveplates 29 preferably has an optical axis thereof oriented at an angle of approximately 45° with respect to the +x axis at that location.
  • an output half- wave waveplate assembly 30 comprises four half- wave waveplates disposed intermediate the birefringent element assembly 20 and the output polarization beam displacer 22.
  • a first one of the four half-wave waveplates 30 preferably has an optic axis thereof oriented at an angle of approximately 0° with respect to the +x axis at that location
  • a second one of the half- wave waveplates 30 preferably has an optic axis thereof oriented at an angle of approximately 45° with respect to the +x axis at that location
  • a third one of the half-wave waveplates 30 preferably has an optic axis thereof oriented at an angle of approximately 45° with respect to the + ⁇ axis at that location
  • a fourth one of the half-wave waveplates 30 preferably has an optic axis thereof oriented at an angle of approximately 90° with respect to the +x axis at that location.
  • a second input half- wave waveplate assembly 32 comprises four half- wave waveplates disposed intermediate the prism 27 and the birefringent element assembly 20.
  • the first one of the half-wave waveplates 32 preferably has an optic axis thereof oriented at an angle of 45° with respect to the +x axis at that location
  • a second one of the half-wave waveplates 32 preferably has an optic axis thereof oriented at an angle of approximately 45° with respect to the +x axis at that location
  • a third one of the half-wave waveplates 32 preferably has an optic axis thereof oriented at an angle of approximately 90° with respect to the +x axis at that location
  • a fourth one of the half-wave waveplates 32 preferably has an optic axis thereof oriented at an angle of approximately 90° with respect to the +x axis at that location.
  • the positions and orientations of all the half-wave waveplates are provided by Figure 10.
  • each frame of Figure 10 has an underlined number associated therewith which corresponds to a location in the interleaver of Figure 9 having the same underlined number.
  • Each of the four boxes of a frame corresponds to a physical beam position at the various locations.
  • the polarization beam displacers 21, 22 and 28 shift the optical beams between these beam positions according to the orientation of the polarization beam displacer 21 , 22 and 28 and the polarization of the optical beam transmitted therethrough.
  • polarization beam displacers 21 and 22 may be either separate polarization beam displacers or may be the same polarization beam displacer, since they are located at approximately at the same position and have the same optical properties.
  • utilizing a common polarization beam displacer rather than two separate polarization beam displacers may be desirable so as to reduce materials and assembly costs.
  • angles of 45°, -21° and 7° for the birefringent element orientations and phase delays of T , IT , IT are utilized.
  • angles of 45°, -21° and 7° for the birefringent element orientations and phase delays of T , IT , IT are utilized.
  • phase delays may be utilized.
  • dispersion cancellation will be achieved via the folded configuration of the interleaver (wherein light from the birefringent element assembly is transmitted back therethrough) and by properly aligning the polarization directions of the odd channels and the even channels before transmitting them back through the birefringent element assembly regardless of the angular orientations of the birefringent elements.
  • an input composite optical beam has two linearly polarized components 1 (along the y direction) and 2 (along the x direction) at the top right beam position.
  • component 2 shifts to the top-left beam position and component 1 remains at the top-right beam position.
  • the arrows shown on the polarization beam displacers indicate the beam shift direction for the beam displacers of Figure 9.
  • the linearly polarized components 1 and 2 become polarized along the +x axis at location 3.
  • the optical axis of the half-wave waveplate for component 1 is oriented at 45° with respect to the +x axis and the optical axis of the half- wave waveplate for component 2 is oriented at 0° with respect to the +x axis.
  • the first birefringent crystal is oriented with its fast axis at -45° with respect to the +x axis.
  • the crystal orientations for crystals 2 and 3 are shown at locations 5 and 6, respectively.
  • the vertically polarized (y direction) components correspond to one set of the interleaved channels, e.g., the even channels
  • the horizontally polarized (x direction) components correspond to another set of interleaved channels, e.g., the odd channels.
  • the polarization direction for all of the components have been changed to or remain at the y direction at location JO.
  • the crystal orientations do not change, since the same birefringent crystals are used.
  • the + ⁇ axis changes its direction with respect to the crystals, because the coordinate system travels along with the optical beams.
  • the crystal orientation angles, with respect to the + ⁇ axis are changed conventionally.
  • the polarization directions are all vertical at location 10.
  • the polarization directions are horizontal at location 3.
  • the la' and 2a' components are the corresponding odd channels and la" and 2a" components are the crosstalk noise from the even channels (typically very small).
  • the lb" and the 2b" components are the corresponding even channels and lb' and 2b' components are the crosstalk noise from the odd channels (typically very small).
  • Thick arrows (such as those of frames 14, 16 and 17) are used to indicate the desired (non-crosstalk) signal and thin arrows are used to indicate crosstalk when thick arrows are present.
  • the two output beams la' and 2a' as well as lb" and 2b" are combined at location 17.
  • the two output beams (la', 2a' and lb", 2b") are the two series of interleaved channels with zero dispersion. Since the same crystals are used in the fold interleaver for both the incident light path and the returning light path, zero dispersion is achieved at a low cost, and in a small device. Further, using the same set of birefringent crystals in both instances mitigates alignment requirements.
  • the present invention may optionally be configured so as to cause light to travel through the birefringent element assembly 3, 4, 5, 6 or any other desired number of times, so as to further enhance the transmission characteristics of the interleaver.
  • the present invention may be configured such that light passes there through an odd number of times. It is worthwhile to note that the folded configuration of the interleaver of the present invention automatically matches elements between successive stages of birefringent filtering for effective mitigation of crosstalk and/or dispersion.
  • each pass through the birefringent assembly in a direction opposite to the previous pass therethrough apparently occurs through a birefringent element assembly which is perfectly matched to the birefringent element assembly which the light previously pass through since the light passes through the same birefringent element assembly in both instances.
  • the polarization mode dispersion (PMD) is minimized.
  • the exemplary fold interleaver shown in Figure 9 is a three-element_interleaver utilizing a birefringent element assembly 20 which comprises three separate birefringent elements or crystals.
  • interleavers having more than three birefringent elements may alternatively be utilized so as to provide enhanced passband and stopband characteristics.
  • the more elements utilized in a birefringent filter or interleaver the better the passband and stopband characteristics thereof.
  • the interleaver may be simplified by providing only one or two birefringent elements. Although the transmission characteristics will tend to suffer when fewer birefringent elements are utilized, there may be instances wherein adequate transmission characteristics are provided at the advantageously lower cost associated with fewer stages.
  • one or two birefringent crystals may alternatively be utilized, rather than three birefringent crystals as shown in Figure 9.
  • FIG 11 a schematic top view of an exemplary fold interleaver having two birefringent elements is shown. That is, the birefringent element assembly 20 thereof comprises only a first birefringent element or crystal 23 and a second birefringent element or crystal 24. However, light is reflected through the two birefringent crystals in both directions, as it is in the fold interleaver of Figure 9.
  • the optical beam states, the crystal orientations, and the half-wave waveplate orientations at various locations in an exemplary two-element fold interleaver of Figure 11 are shown with birefringent element orientations of 45° and -15° and phase delays of T and IT , respectively.
  • the dispersion vs. wavelength, phase distortion vs. wavelength and transmission vs. wavelength curves are shown, respectively, for a 50 GHz two-element non-fold interleaver with birefringent element orientations of 45° and -15° and phase delays of T and T , respectively.
  • Figures 16-18 show the dispersion vs. wavelength, phase distortion vs. wavelength and transmission vs. wavelength curves for a 50 GHz two-element fold interleaver having birefringent element orientations of 45° and -15° and phase delays for T and IT , respectively.
  • FIG. 13 A comparison of Figure 13 and Figure 16 clearly shows the advantages of a fold interleaver with respect to a non-fold interleaver.
  • the dispersion shown in Figure 13 is substantial, while the dispersion shown in Figure 16 is approximately zero.
  • a low or zero dispersion interleaver comprising only two birefringent elements can be provided according to the present invention.
  • a dispersion vs. wavelength curve, a phase distortion vs. wavelength curve and a transmission vs. wavelength curve are provided for a non-fold interleaver having only one birefringent element or crystal.
  • the birefringent element has an angular orientation of 45°. It is worthwhile to note that the dispersion of a single element non-fold birefringent filter or interleaver is approximately zero. That is, all single element birefringent filters or interleavers generally have approximately zero dispersion, whether they utilize a fold configuration or not. Thus, providing a fold single element interleaver generally does not enhance dispersion mitigation therefor.
  • a dispersion vs. wavelength curve, phase distortion vs. wavelength curve and the transmission vs. wavelength curve for a single element 50 GHz fold interleaver are provided.
  • the single birefringent element again has an angular orientation of 45°.
  • the folded configuration does cause the light beam to travel through the birefringent element twice, thereby, at least in some instances, providing enhanced transmission characteristics.
  • wavelength curve of Figure 21 for the non-folded interleaver shows that the stopbands of the folded interleaver tend to be wider and deeper than the stopbands for the non-folded interleaver.
  • a fold interleaver utilizing a mirror rather than a prism is shown.
  • Light input to the interleaver enters a first beam displacer 51, which is preferably both an input and output beam displacer.
  • Light from the input/output polarization beam displacer 51 passes through the birefringent element assembly 60, which comprises three birefringent elements or crystals. The light then passes through an intermediate beam displacer 52. Light from the intermediate beam displacer 52 is reflected back through the interleaver such that the light passes through the birefringent element assembly 60 in the opposite direction.
  • separate beam displacers may be utilized (such as one beam displacer for input light and two separate beam displacers for output light).
  • the upper beam of light After exiting the input beam displacer 51, the upper beam of light passes through glass plate 54 which compensates for a difference in paths lengths between the upper beam and the lower beam.
  • the glass plate 54 without the glass plate 54 the upper beam has a shorter optical path length since the lower beam was displaced by the input polarization beam displacer 51 and therefore traveled further therein. By compensating for this difference in optical path length, the glass plate 54 mitigates undesirable polarization mode dispersion (PMD) within the interleaver.
  • the thickness of the glass plate 54 is determined by the optical properties and physical dimensions of the polarization beam displacers.
  • a fold interleaver having a mirror rather than a prism may alternatively comprise only two birefringent elements or crystals, if desired.
  • two birefringent elements rather than three, tends to degrade the transmission characteristics of the interleaver, in some instances sufficient performance may be provided.
  • the optical beam states, the crystal orientations, and waveplate orientations at different locations for the fold interleaver of Figure 27 are shown.
  • the invention described herein comprises an interleaver having birefringent crystals in the birefringent element assembly thereof, it is also possible to form an interleaver utilizing birefringent effects formed other than with birefringent crystals.
  • a birefringent effect utilizing polarization beam splitters and/or polarization beam displacers to separate an incoming optical beam into two orthogonally polarized optical beams and then causing the two orthogonally polarized beams to be transmitted along two paths, wherein each path has a different optical path length.
  • the above-mentioned angles associated with the birefringent crystals can be converted into equivalent relative angles between the optical beam polarization direction and the equivalent fast axes of the birefringent elements.
  • the phase delays for the first, second and third birefringent elements will preferably be the same as when birefringent crystals are utilized.
  • a fold birefringent filter or interleaver wherein reflecting light from a birefringent element assembly thereof back into and through the birefringent element assembly enhances transmission characteristic thereof and/or mitigates dispersion introduced thereby.
  • phase delays of r , 2T and T may alternatively be utilized.
  • interleavers described herein are suitable for demultiplexing optical signals. Those skilled in the art will appreciate similar structures may be utilized to multiplex optical signals.
  • the waveplates which are utilized in the present invention can optionally be omitted in some instances by rotating subsequent components appropriately. Further, various devices and/or materials may alternatively be utilized to orient the nnlariVatinn direction of lip t heam ⁇ For examnle Hevir ⁇ j anH/nr
  • Such configuration of an interleaver of the present invention may be desirable when the cost is an important factor.
  • Enhanced transmission characteristics without necessarily improving dispersion may be provided by omitting the waveplate which aligns the component light beams parallel to one another prior to the component light beams being transmitted back through the birefringent element assembly.
  • a waveplate or the like is provided so as to align the polarization directions of the two component light beams parallel to one another and both pe ⁇ endicular with respect to the polarization direction of light initially input to the birefringent element assembly.
  • the description contained herein is directed primarily to the configuration of an interleaver as a demultiplexer. However, as those skilled in the art will appreciate, the present invention may be used in both demultiplexers and multiplexers. The difference between demultiplexers and multiplexers is small and the configuration of the present invention as either desired device is well within the ability of one of the ordinary skill in the art.
  • Two different reference systems are used in this patent application for the determination of angular orientations.
  • One reference system is used for the determination of the equivalent angular orientations of spatial birefringent elements, with respect to an equivalent polarization direction of input light.
  • Another reference system is used for the determination of the angular orientations of waveplates with respect to a moving (x, y, z) coordinate system.
  • the angular orientation is typically the fast axis of the birefringent element with respect to the polarization direction of incoming light just prior to the incoming light reaching the birefringent element. Determination of the angular orientation is made by observing oncoming light with the convention that the angle is positive if the rotation of the fast axis is clockwise with respect to the polarization direction of the incoming light and is negative if the rotation is counter-clockwise with respect to the polarization direction of the oncoming light.
  • the equivalent angular orientations of each of the elements of the filter are measured by their fast axes with respect to an equivalent polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of the filter. If there are more than one birefringent filters in a sequence, then the equivalent angular orientations are determined separately for each birefringent filter (the equivalent angular orientations are measured with respect to an corresponding equivalent polarization direction of incoming light just prior to the incoming light reaching the first birefringent element of each different filter).
  • each birefringent filter has its own independent reference for the determination of the angular orientations of the birefringent elements thereof.
  • Each spatial birefringent element has its own equivalent polarization direction of incoming light just prior to the incoming light reaching the first birefringent element.
  • the angular orientations of waveplates are measured by the optic axes of waveplates with respect to the +x axis.
  • the +x axis is part of the moving coordinate system. This coordinate system travels with the light, such that the light is always traveling in the +z direction and such that the +y axis is always up as shown in the drawings. Thus, when the light changes direction, the coordinate system rotates with the +y axis thereof so as to provide a new coordinate system.
  • Determination of the angular orientations in (x, y, z) coordinate system is made by observing oncoming light with the convention that the angle is positive if the rotation of the corresponding optical axis is counter-clockwise with respect to +x axis and is negative if the rotation is clockwise with respect to the +x axis (which is consistent with the conventional use of (x, y, z) coordinate system, but which is contrary to the sign convention for determining the angular orientations of birefringent elements with respect to the input polarization direction, as discussed above).
  • the present invention comprises an interleaver which comprises a birefringent element assembly.
  • the birefringent element assembly comprises at least one spatial birefringent element.
  • a reflector is configured so as to direct light which is emitted from the birefringent element assembly back into and through the birefringent element assembly, such that the light travels through the birefringent element assembly in two different, generally opposite directions.
  • the birefringent element assembly provides two output components of the light input thereto. One output component corresponds to the interleaved odd channels and the other corresponds to the interleaved even channels.
  • the reflector is configured to direct the two components back through the birefringent element assembly. By transmitting the light through the birefringent element assembly in both directions, crosstalk can be substantially mitigated. Further, dispersion can be substantially mitigated or eliminated.
  • the reflector preferably comprises a single prism. However, those skilled in the art will appreciate that the reflector may alternatively comprise more than one prism and/or one or more mirrors or etalons.
  • the birefringent element assembly may contain any desired number of spatial birefringent elements.
  • the birefringent element assembly may contain one, two, three, four, five or more spatial birefringent elements.
  • additional birefringent elements tend to enhance the transmission vs. wavelength curve of the birefringent filter or interleaver defined by the birefringent elements, so as to tend to provide a flatter and wider passband and/or so as to provide a deeper and wider stopband.
  • the birefringent element assembly is disposed intermediate (in an optical sense) an input polarization beam displacer and an intermediate polarization beam displacer.
  • the birefringent element assembly comprises at least one spatial birefringent element.
  • the spatial birefringent element physically separate two orthoganally polarized optical beams and provides differences in physical path lengths and/or refraction indices for the two optical beams so as to provide a birefringent effect. In this manner, the use of birefringent crystals and disadvantages commonly associated therewith are eliminated.
  • the interleaver comprises an input polarization beam displacer from which light is transmitted to the birefringent element assembly; a first input half-wave waveplate assembly configured to receive light from the input polarization beam displacer and control the light polarization directions; an intermediate polarization beam displacer configured to transmit light from the birefringent element assembly before the light is transmitted back through the birefringent element assembly; a second input half- wave waveplate assembly configured to control the light polarization directions before the light is transmitted back through the birefringent element assembly; an output half-wave waveplate assembly configured to control the light polarization directions after the light is transmitted back through the birefringent element assembly; and an output polarization beam displacer to which light is transmitted after the light has been transmitted back through the birefringent element assembly.
  • the spatial birefringent element preferably comprises a polarization beam splitter (which separates an optical beam into two orthoganally polarized optical components); a first mirror; a second mirror; first quarter-wave waveplate(s) having an optic axis thereof oriented at an angle of approximately 45° with respect to the +x axis at that location, the first quarter-wave waveplate(s) being disposed intermediate the polarization beam splitter and the first mirror; second quarter-wave waveplate(s) having an optic axis thereof oriented at an angle of approximately 45° with respect to the +x axis at that location, the second quarter-wave waveplate(s) being disposed intermediate the polarization beam splitter and the second mirror.
  • a polarization beam splitter which separates an optical beam into two orthoganally polarized optical components
  • a birefringent effect is obtained by defining a first and a second light paths at each birefringent element, wherein light input into the birefringent element is split into two composite beams, each of the two composite beams travels along separate paths.
  • the two paths have different optical path lengths, such that when the two beams recombine a birefringent effect is achieved.
  • the splitting of light into two components and the recombining of the two components are achieved utilizing a polarization beam splitter.
  • polarization beam displacers are likewise suitable.
  • Reflectors such as mirrors, or prisms
  • each path will be from a polarization beam splitter to a mirror or prism and back to the polarization beam splitter.
  • Different optical path lengths between the two paths may be obtained by defining the two paths so as to have different physical path lengths or by inserting a material having a different refraction index into one of the two paths, so as to cause the two paths to have different optical path lengths.
  • a material having a different refraction index into one of the two paths
  • Half-wave waveplates are used to control the light polarization direction before light enters a polarization beam splitter, so as to define a desired angle between input light polarization direction and the fast axis of the spatial birefringent element, which further defines an equivalent angle for birefringent element orientation.
  • the fast axis is usually along x-axis or y-axis, which is determined by the configuration of spatial birefringent element using a polarization beam splitter.
  • the equivalent angle is the angle which would be utilized in a birefringent filter having birefringent crystals in order to obtain the same effect.
  • the equivalent angle of a special birefringent element according to the present invention is the angle between the fast axis of a birefringent crystal and the polarization direction of light input thereto which would be required in order to obtain the same optical effect that the spatial birefringent device of the present invention provides.
  • one or more half- waveplates are typically disposed between two adjacent polarization beam splitters, so as to control the light polarization direction before light entering each subsequent polarization beam splitter in order to define the equivalent angle.
  • the half-wave waveplates which light passes through prior to entering the polarization beam splitter of the present invention define the transmission characteristics (e.g., cross-talk) of the birefringent element assembly.
  • a half-wave waveplate is used to define the equivalent orientation angle for each birefringent element of the present invention. It is worthwhile to note that the equivalent orientation angle is controlled by manipulating the polarization direction of light input to the polarization beam splitter of each birefringent element. At the beam split point of the polarization beam splitter, the polarization direction of light which travels along the shorter of the two paths is the fast axis of the spatial birefringent element.
  • the polarization directions of light traveling along the short path and the long path are manipulated so as to cause_that light to be either transmitted or reflected again by the polarization beam splitter, such that the light from the two paths recombines and is transmitted in the desired direction (such as to the next birefringent element). Therefore, the polarization direction of light input to each birefringent element must be manipulated so as to obtain the desired equivalent angle. Manipulation of the polarization of light input to a birefringent element is accomplished by rotating the polarization direction of light input to a birefringent element by the desired amount utilizing a half- wave waveplate.
  • the present invention thus comprises a method for interleaving, wherein the method comprises transmitting light through a birefringent element assembly in a first direction and then transmitting the light through the same birefringent element assembly in a second direction.
  • the birefringent element assembly comprises at least one spatial birefringent element and the spatial birefringent element causes a first beam of light to travel along a first path and causes a second beam of light to travel along a second path.
  • the first and second beams of light are preferably generally orthogonal with respect to one another.
  • the first and second paths have different optical path lengths with respect to one another.
  • the different optical paths length may be formed by either providing different physical path lengths or by providing materials having different refraction indices along the first and second paths.
  • Transmitting the light through the same birefringent assembly in a second direction preferably comprises transmitting the light through the same birefringent assembly along generally the same path along with the light was transmitted in the first direction.
  • the second direction is preferably opposite the first direction. More particularly, the second direction is preferably parallel to the first direction and may be offset, i.e., laterally translated, with respect to the first direction.
  • light traveling in the first direction will pass through some of the same components as light traveling in the second direction, light traveling in the first direction may also typically pass through unique components which light traveling in the second direction does not pass through and vice versa.
  • light traveling in one direction may preferably pass through different quarter-wave waveplates and half-wave waveplates from light which travels in opposite direction.
  • Transmitting the light through the birefringent element assembly in both the first and the second directions mitigates crosstalk. Further, dispersion can be mitigated in interleavers having more than one spatial birefringent element.
  • a birefringent filter or interleaver is constructed by utilizing the birefringent effect which results from differences in optical path lengths, either in free space, e.g., air, or in materials having desired indices of refraction.
  • free space e.g., air
  • materials having desired indices of refraction e.g., materials having desired indices of refraction.
  • the need for birefringent crystals is eliminated.
  • the device construction is simplified and cost are minimized when birefringent crystals are eliminated.
  • various limitations associated with the use of birefringent crystals do not present which are inherent to the optical, physical, mechanical, and thermal properties of the birefringent crystals.
  • birefringent crystals provide a fixed birefringent value and are therefore not variable or tunable.
  • the use of optical path length differences to obtain a birefringent affect facilitates easy tunability of birefringent values by simply varying the length of one or both of the paths and/or varying an index of refraction along one or both of the paths.
  • optical signal interleaving can be achieved utilizing a Sole birefringent filter, in which at least one, typically, a plurality, of birefringent elements are located intermediate two polarizing devices, such as an input polarizer and an output polarizer.
  • a typical Sole birefringent filter comprises three birefringent crystals having orientation angles of 45°, -15° and 10° and birefringent phase delays of T , 2T and 2T , respectively.
  • the use of a birefringent filter having such crystal orientation angles and phase delays provides a generally acceptably flat passband.
  • other sets of orientation angles (or equivalent orientation angles when spatial birefringent devices are utilized) and phase delays can provide transmission characteristics which are enhanced with respect to those of contemporary practice.
  • one such set of orientation angles which provides enhanced transmission characteristics is 45°, -21° and 7° for birefringent filters having first, second and third birefringent elements of phase delays of T , 2T and 2T , respectively.
  • the transmission characteristics of such a device include a flatter passband and a deeper and/or wider stopband, so as to substantially mitigate undesirable crosstalk.
  • birefringent filters wherein light passes therethrough only once and in a single direction
  • the dispersion introduced by such contemporary birefringent filters is sufficient to significantly degrade optical signal quality. Because of this degradation in optical signal quality, further advances in channel spacing reduction are difficult, if not impossible.
  • an interleaver utilizing a birefringent filter is constructed in a manner which substantially mitigates crosstalk without additional birefringent elements. Further, dispersion can be substantially mitigated and eliminated without additional birefringent elements. This is accomplished by configuring the present invention such that light travels through the same birefringent filter twice or more times, in two generally opposite directions. Therefore, the present invention facilitates the construction of an interleaver which makes possible substantially reduced channel spacing, so as to desirably increase the effective bandwidth of an optical medium and thereby enhance the potential for channel count increases.
  • a birefringent filter or interleaver can be formed, such that the dispersion vs. wavelength curve thereof is approximately zero for all wavelengths and thus such that the birefringent filter or interleaver itself contributes very little or no dispersion. Therefore, the interleaver of the present invention may be utilized to mitigate total dispersion within an optical system by minimizing its own introduction of undesirable dispersion.
  • birefringent filter In a birefringent filter, if ⁇ ⁇ , ⁇ 2 , and ⁇ 3 are the orientation angles for the first, second and third birefringent elements, then the same transmission performance is obtained for birefringent element orientations of 90° - ⁇ 1 , 90° - ⁇ 2 and 90° - ⁇ 3, as well as for birefringent element orientations of 90° + ⁇ 1 , 90° + ⁇ 2 and 90° + ⁇ 3 , respectively.
  • dispersion curves are flipped about the zero dispersion axis for the sets of angles of 90° - ⁇ i, 90° - ⁇ 2 and 90° - ⁇ 3 , as well as 90° + ⁇ i, 90° + ⁇ 2 and 90° + ⁇ 3 , when taken with respect to the orientations of ⁇ ls ⁇ 2 and ⁇ 3 .
  • the dispersion curve of a birefringent filter having birefringent element orientations of ⁇ 1 , ⁇ 2 and ⁇ 3 will be opposite to the dispersion curve of either a birefringent filter having birefringent element orientations of 90° - ⁇ ⁇ , 90° - ⁇ 2 and 90° - ⁇ 3 or a birefringent filter having birefringent element orientations of 90° + ⁇ 1, 90° + ⁇ 2 and 90° + ⁇ 3 .
  • an optical beam is transmitted through two interleavers sequentially, wherein the two interleavers have been designed such that they have flipped dispersion curves with respect to one another (such as by having the first interleaver utilize birefringent element orientations of ⁇ 1 , ⁇ 2 and ⁇ 3 and having the second interleaver utilize birefringent element orientations of 90° - ⁇ 1 , 90° - ⁇ 2 and 90° - ⁇ 3 (or by having the second interleaver utilize birefringent element orientations of 90° + ⁇ ⁇ , 90° + ⁇ 2 and 90° + ⁇ 3 ), then the dispersion of the two interleavers cancels and the total dispersion of the two interleavers is zero or approximately zero.
  • this configuration typically requires at least two separate interleavers to achieve zero or approximately zero dispersion for both odd and even channels.
  • Dispersion can be substantially mitigated by transmitting an optical beam through a birefringent element assembly, such as a birefringent element assembly comprising three different birefringent elements, wherein the first element has a fast axis oriented at an angle of ⁇ 1 , a second birefringent element has a fast axis thereof oriented at an angle of ⁇ 2 , and a third birefringent element has a fast axis thereof oriented at an angle of ⁇ 3 , all with respect to the polarization direction of the input.
  • a birefringent element assembly such as a birefringent element assembly comprising three different birefringent elements, wherein the first element has a fast axis oriented at an angle of ⁇ 1 , a second birefringent element has a fast axis thereof oriented at an angle of ⁇ 2 , and a third birefringent element has a fast axis thereof oriented at an angle of ⁇ 3 , all
  • the incident light is reflected, such as by a mirror or prism, and then travels back through the same set of birefringent elements in the reverse direction.
  • the polarization directions of the odd channels and the even channels are aligned in parallel and so as to be pe ⁇ endicular to the input polarization direction of the incident light. This results in that the angular orientation of the birefringent elements are 90°- ⁇ 3 , 90°- ⁇ 2 , 90°- ⁇ l5 respectively, with respect to the input polarization direction of the returning light.
  • the birefringent element angles are ⁇ i, ⁇ 2 , ⁇ 3
  • the birefringent element angles are 90° - ⁇ 3, 90° - ⁇ 2 , and 90° - ⁇ i, in the order in which light encounters the birefringent elements.
  • an interleaver which provides zero or approximately zero dispersion and which does not require the use of two separate birefringent filters, as discussed above.
  • Such a zero dispersion interleaver may be constructed by folding the light path, such that incident light traveling through the birefringent filter in a forward direction is reflected back through the filter in a reverse direction.
  • the fold interleavers of the present invention provide low cross-talk and/or zero or very low dispersion by directing light which passes through a birefringent element assembly thereof back through the same birefringent element assembly in a direction opposite to the direction in which the light was first transmitted through the birefringent element assembly.
  • dispersion introduced into the light during its first pass through the birefringent element assembly is compensated for or cancelled during its second pass through the birefringent element assembly. That is, when light passes through the birefringent element assembly in the first direction, a first dispersion vs.
  • wavelength curve results and when light passes through the birefringent element assembly in a second direction, generally opposite to the first direction, a second dispersion vs. wavelength curve results which is flipped or generally opposite to the first dispersion vs. wavelength curve, thus, result in a net dispersion resulting from both passes through the birefringent element assembly of zero or approximately zero dispersion. Since light travels through the birefringent element assembly twice (once in a first or forward direction and again in the second or reverse direction) the transmission characteristics of the interleaver are enhanced with respect to the transmission characteristics of light which passes through such an interleaver only once (such as in the forward direction only). Such enhanced transmission characteristics improve cross-talk.
  • light may be transmitted through the birefringent element of the assembly of the present invention any desired number of times, so as to provide the desired transmission characteristics.
  • transmitting light through the birefringent element assembly of the present invention an even number of times results in zero or nearly zero dispersion, since the dispersion introduced during transmission through the birefringent element assembly in one direction is substantially canceled by dispersion introduced during transmission through the birefringent element assembly in the opposite direction.
  • the dispersion characteristics of the interleaver are not important, then light may be transmitted through the birefringent element assembly an odd number of times.
  • a right-hand coordinate system of axes is used to characterized the optical beam propagation in the system at various locations with a convention that the coordinate system is traveling with light and the light is always propagating in the +z direction and the +y direction is always out of the paper, as shown in Figure 1.
  • each of the four boxes correspond to a physical beam position at various locations.
  • the polarization beam displacers 10, 11 and 18 shift the optical beams to these various beam positions according to the orientation of polarization beam displacer and the optical beam polarization.
  • the optic axis orientation angles of the quarter- wave and half- wave waveplates shown in Figs. 2a and 2b are referred to the +x axis at the corresponding locations.
  • the birefringent effect derived by each spatial birefringent element of the birefringent element assembly 12 is determined by the distance difference between the polarization beam splitter and the mirrors thereof.
  • the polarization beam splitter 19a, the quarter- wave waveplate 23 a, the mirror 14a, the quarter- wave waveplate 22a, the mirror 15a and the half- wave waveplates 30 define a portion of the first birefringent element of the birefringent element assembly 12.
  • An input polarization beam displacer 10 provide light to half- wave waveplates 30 from which the light is transmitted into polarization beam splitter 19a.
  • the input polarization beam displacer 10 separates light input to the interleaver into two optical beams having known polarization directions, such that the polarization directions of the two optical beams can be controlled (such as by a half-wave waveplate) to define the desired equivalent birefringent element orientation angles.
  • the input beam displacer 10 may be eliminated (and the two composite beams resulting therefrom will be reduced to a single beam).
  • Polarization beam splitter 19a separates an optical beam into two components.
  • the first component having polarization direction along x-axis is transmitted straight there through to quarter- wave waveplate 23 a and mirror 14a.
  • Mirror 14a reflects the light back through quarter- wave waveplate 23 a and into polarization beam splitter 19a.
  • the second component of the light having a polarization generally orthogonal to the first component (along y-axis) is deflected by polarization beam splitter 19a through quarter- wave waveplate 22a and is reflect by mirror 15a back through polarization beam splitter 19a.
  • the polarization direction of the first component is changed by 90° by the combination of the mirror and the quarter- wave waveplate 23 a, (having an optical axis thereof oriented at 45° with respect to the +x axis), so that the first component is reflected by the polarization beam splitter 19a to location 10 when the first component is transmitted back to the polarization beam splitter 19a.
  • the polarization direction of the second component is changed by 90° by the cooperation of the mirror and the quarter- wave waveplate 22a (having an optical axis thereof oriented at 45° with respect to the +x axis), so that it is transmitted through the polarization beam splitter 19a to location 10 when it is transmitted back to the polarization beam splitter 19a.
  • the first and second components are together at location 10.
  • Light from the polarization beam splitter 19a is transmitted to a second birefringent element of the birefringent element assembly 12 which comprises half- wave waveplates 33 a, a polarization beam splitter 19b, quarter- wave waveplate 23b, mirror 14b, quarter- wave waveplate 21b and mirror 15b, all of which operate in a manner analogous to the corresponding components of the first birefringent element.
  • the birefringent element assembly comprises two elements, as shown in Figure 1.
  • the quarter-wave waveplates 21a, 22a, 23a, 24a, 21b, 22b, 23b and 24b orient light returning from the mirrors so that the light is either transmitted through or reflected by the corresponding polarization beam splitter and the two components recombine.
  • quarter- wave waveplate 22a orients the polarization direction of light from mirror 15a such that that component of the light is transmitted through the polarization beam splitter 19a
  • quarter- wave waveplate 23 a orients the polarization direction of light from mirror 14a such that light from mirror 14a is reflected by the polarization beam splitter 19a to location 10.
  • the polarization beam splitters may comprise either single polarization beam splitters as shown, or may alternatively comprise multiple polarization beam splitters. For example, separate polarization beam splitters may be utilized at each point where light is separated and recombined, thereby replacing each polarization beam splitter shown in Figure 1 or Figure 9 with four separate polarization beam splitters.
  • each polarization beam splitter shown in Figure 1 and Figure 9 may be replaced with two polarization beam splitters, wherein one polarization beam splitter splits and recombines light traveling in the forward direction through the birefringent element assembly and the other polarization beam splitter separates and combines the light traveling in the opposite direction (back through the birefringent element assembly).
  • distance Li and distance L 2 are different with respect to one another, so as to provide the desired phase delay and the consequent birefringent effect.
  • distances L 3 and L of the second birefringent element are different, again so as to provide the desired phase delay and the consequent birefringent effect for the second birefringent element.
  • Half- wave waveplates 30 and 33a are used to manipulate the input light polarization directions for desired equivalent birefringent element orientation angles ⁇ and ⁇ 2 , respectively. After exiting the birefringent element assembly 12, light from the polarization beam splitter 19b is transmitted through half- wave waveplate 34 to prism 13.
  • the input light provided to the interleaver of Figure 1 passes through two interleavers wherein the first interleaver introduces dispersion and the second interleaver (which comprises the same physical components as the first interleaver) introduces substantially the opposite dispersion, such that the dispersion of the first interleaver and the dispersion of the second interleaver substantially cancel one another.
  • the input beam displacer 10 receives a composite (light of unknown polarization direction) beam and separates the composite beam into two beams of known polarization directions.
  • the half- wave waveplates 30 orient the polarization directions of the two composite beams such that the two composite beams have the same polarization direction and such that the polarization direction provides the desired equivalent angle (the angle which provides birefringent filter element performance similar to that of a corresponding birefringent crystal).
  • the polarization beam splitter in cooperation with associated mirrors and associated quarter-waveplates provide two separate paths, wherein each path has a different optical path length with respect to the other path.
  • the polarization beam splitter splits each of the two beams provided by the polarization beam displacer 10 into two orthogonally polarized components, respectively. Each component travels along one of the two paths (having different optical path lengths) so as to provide a birefringent effect when the two components are recombined. This process is repeated as necessary and additional birefringent elements (comprised of additional polarization beam splitters, additional quarter-wave waveplates and additional mirrors) so as to provide the desired birefringent filtering effect.
  • the equivalent angle of each birefringent element is determined by the half-wave waveplate through which light is transmitted prior to entering the polarization beam splitter.
  • Half- wave waveplates 35 aligns the odd and even channels in parallel in frame 24 of Figure 2b and performs a function analogous to that of half- wave waveplates 30. If only cross-talk mitigation is required, it is not necessary to make the polarization directions of the odd channels and the even channels in parallel. However, it is necessary that they each be either parallel or pe ⁇ endicular with respect to the equivalent input polarization direction of incident light at this location.
  • Prism 13 deflects light through polarization beam displacer 18 and back into the birefringent element assembly 12 where the light passes through half- wave waveplates 35, polarization beam splitter 19b, quarter- wave waveplate 24b, quarter- wave waveplate 22b, half-wave waveplate 32a, quarter-wave waveplate 24a, and quarter-wave waveplate 21a, while being reflected by mirrors 14a, 14b, 15a and 15b in a manner analogous to the manner in which light is transmitted through birefringent element assembly 12 in the first direction.
  • an equivalent interleaver is constructed by making the first birefringent element have the second equivalent angle and the second phase delay and making the second birefringent element have the first equivalent angle and the first phase delay.
  • the beam components 1' and 2' (odd channels) as well as the beam components 3' and 4' (even channels) correspond to the two series of interleaved channels.
  • the half-wave waveplate at location 23 changes the optical beam polarization directions in such a way that they align the polarization directions of the odd and the even channels along the same direction.
  • the dispersion caused by optical beams propagating from location 22 to location 43 cancels the dispersion caused by optical beams propagating from location 0 to 22.
  • the half-wave waveplates at various locations in the apparatus are controlled to ensure that the optical beams are polarized along the appropriate direction required to obtain the desired passband and stopband characteristics.
  • the two output beams 1" and 2" (even channels) and 3" and 4" (odd channels) are the two series interleaved channels having zero or approximately zero dispersion.
  • Figure 3 the dispersion provided by the two-element fold interleaver of Figs. 1, 2a and 2b is shown for one of the interleaved channels.
  • Figure 4 shows the phase vs. wavelength
  • Figure 5 shows the transmission vs. wavelength for the two-element fold interleaver of Figure 1, where the equivalent birefringent orientation angles are 45°, -15° and phase delays are T , 2T , respectively.
  • FIG. 6 the dispersion for a two-element non-fold interleaver having birefringent element orientations of 45° and -15° and having phase delays of T and 2 r is shown. It is clear that the dispersion of the non-fold interleaver shown in Figure 6 is substantially greater than that of the corresponding fold interleaver of Figure 3.
  • Figure 7 shows the phase vs. wavelength and
  • Figure 8 shows the transmission vs. wavelength for the two-element non-fold interleaver.
  • Figure 9 a top schematic view of a three-element fold interleaver is shown.
  • three birefringent elements can provide a flatter and wider passband and a deeper and wider stopband as compared to the two-element fold interleaver of Figure 1.
  • Structure and operation of the three-element fold birefringent filter is generally analogous to that of the two-element fold birefringent filter.
  • FIGS. 10a and 10b the optical beam states and the quarter- wave and half-wave waveplate orientations at various locations for an exemplary three- element fold interleaver of Figure 9 are shown, where the equivalent birefringent element orientations are 45°, -21°, 7° and phase delays are T , 2T , 2T , respectively, for the three birefringent elements.
  • the optic axis orientation angles of the quarter-wave and half- wave waveplates shown in Figs. 10a and 10b are referred to the + ⁇ axis at the corresponding locations.
  • the channel spacing is determined by the phase delay of the first element (T ⁇ ).
  • the half- wave waveplates at various locations in the apparatus are controlled to ensure that the optical beams are polarized along the appropriate direction so that the desired passband and stopband characteristics are obtained.
  • the two output beams 1" and 2" (odd channels) and 3" and 4" (even channels) are the two series of interleaved channels of having zero or nearly zero dispersion.
  • the equivalent angle and phase delay associated with the first birefringent element may be swapped with the equivalent angle and phase delay associated with the third birefringent element. That is, for a first birefringent element having a first equivalent angle and a first phase delay and a third birefringent element having a third equivalent angle and a third phase delay, then equivalent performance is obtained when the first birefringent element has the third equivalent angle and the third phase delay and the third birefringent element has the first equivalent angle and the first phase delay.
  • the dispersion vs. wavelength for the three-element fold interleaver of Figs. 9, 10a and 10b for one of the interleaved channels is shown.
  • the dispersion is zero or approximately zero for all wavelengths.
  • Figure 12 shows the phase vs. wavelength and
  • Figure 13 shows the transmission vs. wavelength for the exemplary three-element fold interleaver of Figures 9, 10a and 10b.
  • the dispersion vs. wavelength for a non-fold interleaver having equivalent birefringent element orientation of 45°, -21° and 7° and having phase delays of T , 2T and 2T is shown. It is clear that the dispersion for the non-fold three-element interleaver shown in Figure 14 is substantially greater than the dispersion for the three-element fold interleaver shown in Figure 11.
  • the transmission characteristic of the fold interleavers of Fig. 5 (two-element) and Fig. 13 (three-element) are superior to those of the non-fold interleavers of Fig. 8 (two element) and Fig. 16 (three-element). More particularly, the stopband, the -30dB bandwidth is substantially wider for the fold interleaver than for the non-fold interleaver. Additionally, crosstalk of almost -80dB is obtained for the three- element fold interleaver, which is substantially better than that for the three-element non- fold interleaver.
  • an interleaver may be formed so as to have four elements, five elements, or more elements, as desired.
  • a one-element fold interleaver may be useful in some applications. Although dispersion in a one-element non-fold interleaver is zero, the use of a one-element fold interleaver provides enhanced stopband characteristics. More particularly, a wider stopband can be obtained with a one-element fold interleaver than can be obtained with a corresponding one-element non-fold interleaver.
  • the light beams can comprise a plurality or array of separate light beams or channels. Thus, a plurality of such channels can be processed simultaneously by a fold interleaver constructed according to the present invention.
  • the fold interleavers of the present invention overcome many of the limitations associated with the optical, physical, mechanical and thermal properties of the birefringent crystal. For example, since a spatial distance determines the amount of birefringence obtained in any element of the birefringent element assembly, variable or tuned birefringence may be obtained by making at least one mirror of a element movable or by facilitating the introduction of different materials, having different indices of refraction, into at least one of the two optical paths of a spatial birefringent element. Thus, tunable fold interleaver can be constructed.
  • the fold interleaver of the present invention provides a low cost and small device size. It is worthwhile to note that the folded configuration of the interleaver of the present invention provides automatic device match between successive stages of birefringent filtering for effective mitigation of crosstalk and/or dispersion. That is, each pass through the birefringent assembly in a direction opposite to the previous pass therethrough apparently occurs through a birefringent element assembly which is matched to the birefringent element assembly which the light previously pass through since the light passes through the same birefringent element assembly in both instances.
  • orientations for the waveplates described herein are given and specific values for the distance between the polarization beam splitter and the mirrors are given, those skilled in the art will appreciate the various other waveplate orientations and distance between polarization beams splitter and mirror can likewise be used. Further, the use of a 50 GHz interleaver by way of example only and not by way of limitation. Those skilled in the art will appreciate that various other channels spacing, particularly smaller channel spacings, may likewise be utilized.
  • One important aspect of this invention is the ability to control the difference in optical path length between the first and second paths in the spatial birefringent element, so that the birefringence value provided by this difference in optical path length does not vary undesirably during operation of the invention, such as due to temperature changes.
  • the birefringence values of a device determine the operational characteristics, i.e., transmission, dispersion, phase distortion, thereof. Therefore, it is very important that the optical path length differences (and consequently the birefringence values) remain substantially fixed during operation of the devices.
  • Portions of the first and second paths, other than the portions which contribute the optical path length differences, are less critical since these other portions do not determined birefringence values.
  • portions of the first and second paths, other than the portions which contribute to the optical path length differences tend to vary (changes in physical length and/or changes in an index of refraction thereof) in response to environment (e.g., temperature) changes by approximately the same amount, due to structural similarity and symmetry of the first and second paths, and thus do not generally tend to change the optical path length difference. Therefore, it is that portion of the first and second paths which directly provides the difference in optical path length that must be most carefully controlled.
  • the difference in optical path length between the first and second paths in a spatial birefringent element may optionally be controlled by inserting a material having desired optical, thermal and/or mechanical properties into at least the longer of the two paths, so as to substantially fix the optical path length which defines the difference between the first and second paths.
  • a material having desired optical, thermal and/or mechanical properties into at least the longer of the two paths, so as to substantially fix the optical path length which defines the difference between the first and second paths.
  • those portions of the first and second paths which do not contribute to the optical path length difference comprise air, vacuum or any other material.
  • these portions of the first and second paths are inherently equal in physical lengths to one another (since they do not contribute to the optical path length difference).
  • birefringence is obtained by optical path length differences, which may occur in free space, e.g., air or vacuum.
  • a material of desired optical, thermal, and/or mechanical properties and having a desired index of refraction may be inserted along desired portion of the light paths of the present invention.
  • such a material may be utilized to shorten any desired path lengths and/or to provide a difference in optical path lengths to achieve a birefringent effect.
  • both paths can have the same physical dimensions, and birefringence may be obtained by inserting material having desired optical properties, e.g., an index of refraction greater than one, so as to cause the two paths to have different optical paths lengths.
  • the optical path lengths may be made so as to be variable, thus providing adjustability of the birefringence value and a tunable interleaver.
  • the interleaver of the present invention is simple in construction and low in cost.
  • the present invention overcomes many of the limitations associated with contemporary birefringent crystal interleavers, such as those limitations associated with the optical, physical, mechanical and thermal properties of birefringent crystals. It is important to appreciate that, as mentioned above, the phase delay necessary for providing a birefringent effect may be obtained by inserting a material having desired optical, thermal, and/or mechanical properties into at least a portion of either the first or second path in a spatial birefringent element.
  • phase delays of T , 2T and 2T may alternatively be utilized.
  • interleavers described herein are suitable for demultiplexing optical signals. Those skilled in the art will appreciate similar structures may be utilized to multiplex optical signals.
  • the waveplates which are utilized in the present invention can be replaced by other devices.
  • Various devices and/or materials may alternatively be utilized to orient the polarization direction of light beams.
  • devices and/or materials which are responsive to applied voltages, currents, magnetic fields and/or electrical fields may be used to orient the polarization direction of light beams.
  • the use of waveplates herein is by way of example only, and not by way of limitations.
  • waveplates either half-wave waveplates or quarter-wave waveplates having identical orientations are dispose next to one another, then a common waveplate may be substituted therefor.
  • gasket is defined to include any bracket, mount, optical bench, host, enclosure or any other structure which is used to maintain components of the present invention in desired positions relative to one another.
  • gasket is comprised of an ultra low expansion (ULE) material, fused silica or any other material having a very low thermal expansion coefficient.
  • UEE ultra low expansion

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Abstract

La présente invention concerne un filtre en peigne ou imbricateur à faible niveau de dispersion, comprenant un premier ensemble d'éléments biréfringents comportant au moins un élément biréfringent, et un second ensemble d'éléments biréfringents comportant au moins un autre élément biréfringent. De par leur configuration, les deux ensembles d'éléments biréfringents coopèrent entre eux de façon à atténuer la dispersion de l'imbricateur. Pour réaliser une dispersion presque ou totalement nulle simultanément pour les canaux pairs et impairs, on oriente les axes de polarisation des canaux pairs et impairs de façon qu'ils soient parallèles entre eux avant l'entrée dans le second ensemble d'éléments biréfringents.
PCT/US2001/020321 2000-06-23 2001-06-25 Multiplexeur-demultiplexeur canal pour communications optiques WO2002001773A2 (fr)

Applications Claiming Priority (12)

Application Number Priority Date Filing Date Title
US21337000P 2000-06-23 2000-06-23
US21337100P 2000-06-23 2000-06-23
US60/213,370 2000-06-23
US60/213,371 2000-06-23
US24461400P 2000-11-01 2000-11-01
US60/244,614 2000-11-01
US09/891,794 US6628449B2 (en) 2000-11-01 2001-06-25 Tandem comb filter
US09/892,224 2001-06-25
US09/892,224 US6563641B2 (en) 2000-06-23 2001-06-25 Fold interleaver
US09/891,795 2001-06-25
US09/891,795 US20010055158A1 (en) 2000-06-23 2001-06-25 Apparatus for channel interleaving in communications
US09/891,794 2001-06-25

Publications (2)

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WO2002001773A2 true WO2002001773A2 (fr) 2002-01-03
WO2002001773A3 WO2002001773A3 (fr) 2002-05-23

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WO2017048899A1 (fr) * 2015-09-15 2017-03-23 The Arizona Board Of Regents On Behalf Of The University Of Arizona Techniques de détection fondées sur un analyseur de polarisation pour une émission optique de bande latérale unique, auto-cohérente et multiplexée par polarisation

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US5414540A (en) * 1993-06-01 1995-05-09 Bell Communications Research, Inc. Frequency-selective optical switch employing a frequency dispersive element, polarization dispersive element and polarization modulating elements
US5771120A (en) * 1995-12-26 1998-06-23 Lucent Technologies Inc. Optical apparatus with combined polarization functions
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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2017048899A1 (fr) * 2015-09-15 2017-03-23 The Arizona Board Of Regents On Behalf Of The University Of Arizona Techniques de détection fondées sur un analyseur de polarisation pour une émission optique de bande latérale unique, auto-cohérente et multiplexée par polarisation

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