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WO2002025368A2 - Appareil de conversion de longueurs d'ondes optique - Google Patents

Appareil de conversion de longueurs d'ondes optique Download PDF

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Publication number
WO2002025368A2
WO2002025368A2 PCT/US2001/023849 US0123849W WO0225368A2 WO 2002025368 A2 WO2002025368 A2 WO 2002025368A2 US 0123849 W US0123849 W US 0123849W WO 0225368 A2 WO0225368 A2 WO 0225368A2
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WIPO (PCT)
Prior art keywords
wavelength
signal
local oscillator
quasi
tunable
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PCT/US2001/023849
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English (en)
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WO2002025368A3 (fr
WO2002025368A9 (fr
Inventor
Gayle R. Link
Calvin J. Martin
Zelda Gills
Derek Robertson
Marcus W. Shute
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Luxcore Networks, Inc.
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Application filed by Luxcore Networks, Inc. filed Critical Luxcore Networks, Inc.
Priority to AU2001280885A priority Critical patent/AU2001280885A1/en
Publication of WO2002025368A2 publication Critical patent/WO2002025368A2/fr
Publication of WO2002025368A9 publication Critical patent/WO2002025368A9/fr
Publication of WO2002025368A3 publication Critical patent/WO2002025368A3/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
    • G02B6/29304—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 operating by diffraction, e.g. grating
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29317—Light guides of the optical fibre type
    • G02B6/29319—With a cascade of diffractive elements or of diffraction operations
    • G02B6/2932—With a cascade of diffractive elements or of diffraction operations comprising a directional router, e.g. directional coupler, circulator
    • 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
    • G02B6/29346—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 operating by wave or beam interference
    • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
    • G02B6/29368—Light guide comprising the filter, e.g. filter deposited on a fibre end
    • 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
    • G02B6/29395—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 configurable, e.g. tunable or reconfigurable
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2/00—Demodulating light; Transferring the modulation of modulated light; Frequency-changing of light
    • G02F2/004—Transferring the modulation of modulated light, i.e. transferring the information from one optical carrier of a first wavelength to a second optical carrier of a second wavelength, e.g. all-optical wavelength converter
    • 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
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12133—Functions
    • G02B2006/1215—Splitter
    • 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
    • G02B6/29346—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 operating by wave or beam interference
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G02B6/29359—Cavity formed by light guide ends, e.g. fibre Fabry Pérot [FFP]
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/37—Non-linear optics for second-harmonic generation
    • G02F1/377—Non-linear optics for second-harmonic generation in an optical waveguide structure
    • G02F1/3775—Non-linear optics for second-harmonic generation in an optical waveguide structure with a periodic structure, e.g. domain inversion, for quasi-phase-matching [QPM]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0201—Add-and-drop multiplexing
    • H04J14/0202—Arrangements therefor
    • H04J14/0213—Groups of channels or wave bands arrangements

Definitions

  • the present invention relates to optical signal processing and, more particularly, to wavelength-conversion, demultiplexing, switching, and broadcasting in optical networks.
  • silica optical fiber has now been used in telecommunications for approximately three decades.
  • the advantages include low signal attenuation, immunity to electromagnetic interference (EMI), low crosstalk, fast propagation speed, physical flexibility, small size, and low weight — all at a reasonable cost.
  • EMI electromagnetic interference
  • light modulated with a data signal is coupled to a fiber at a source node, transmitted by a fiber to a destination node, possibly through several intermediate nodes, received at the destination node, demodulated and converted into an electrical data signal.
  • Light in the present context includes infrared light; in fact, two of the more commonly used bands are centered around 1550 nanometers and 1310 nanometers, both lying in the near infrared region of the electromagnetic spectrum.
  • WDM wavelength division multiplexed
  • NWDM narrowband wavelength division multiplexed
  • DWDM dense wavelength division multiplexed
  • Wavelength division multiplexed systems also provide much needed flexibility in selecting protocol and network topology. Both topology and protocol selection are severely restricted in telecommunication systems where data of multiple channels are embedded in the same stream.
  • An example of such transmission scheme is the synchronous optical network/synchronous digital hierarchy (SONET/SDH), a three-layer transport network architecture.
  • SONET/SDH synchronous optical network/synchronous digital hierarchy
  • individual data flows e.g., tributaries, are mapped into payloads and transported across the network's spans in envelopes, in a synchronous time division multiplexed (TDM) manner.
  • TDM synchronous time division multiplexed
  • each multiplexed wavelength channel can be independent from formats and rates of other channels propagating in the same fiber, because each multiplexed wavelength channel is independent from other channels.
  • one fiber can carry ⁇ , ⁇ 2 , and ⁇ 3 wavelength channels, where ⁇ is a 2.5 Gbit/s SONET OC-48 channel, ⁇ 2 is a 10 Gbit/s SONET OC-192 channel, and ⁇ 3 is a proprietary format channel.
  • each of the three wavelength channels can be optically routed or switched. In other words, each wavelength channel can be switched independently.
  • Independent switching avoids the need for opto-electronic (O-E) conversion of the aggregate data carried by the fiber, electronic processing of the data, and subsequent electro-optic (E-O) conversion for further transmission.
  • the conversions and electronic processing typically require arrays of photodetectors and transponders.
  • Photodetectors optically detect signals, and translate them into electronic signals that can be demultiplexed and switched electronically.
  • Transponders can then be employed to receive the separate wavelength channels and translate them to different wavelengths for subsequent multiplexing and transmission through appropriate fibers.
  • the use of photodetector and transponder arrays is expensive. Even more important is that photodetectors and transponders are usually wavelength-specific components, requiring a priori knowledge of the wavelengths. Switching flexibility is therefore lost. And redundancy, often needed for reliability expected from modern providers of telecommunication services, becomes a rather costly one-to-one redundancy, instead of the more affordable N-to-M redundancy with N ⁇ M.
  • optical switches e.g., optical cross-connects (OXCs)
  • OXCs optical cross-connects
  • the ability to separate channels based on wavelength is thus important to various elements in WDM networks, including demultiplexers and optical cross connects. (These and similar network elements that perform channel separation and re-direction will be referred to as switches) To take better advantage of the routing flexibility afforded by WDM, it is often important to configure the network's switches dynamically, essentially in real time, and without manual intervention. Note also that demultiplexing and switching are usually performed on received channels, i.e., channels that have already traveled some distance and, consequently, have been attenuated. It is, therefore, preferable to separate the channels with little additional attenuation.
  • a diffraction grating is a periodic arrangement of elements that reflect or transmit electromagnetic radiation. The elements are spaced apart by a distance comparable to the wavelength of the radiation. For example, a pattern of transparent slits in an opaque screen, or a pattern of reflecting grooves on a substrate can be used to build a diffraction grating.
  • coherent radiation incident on a grating diffracts the resulting multiple fields interfere constructively or destructively in different locations, creating a wavelength-dependent pattern of peaks and nulls.
  • Signals carried by different wavelengths can be spatially separated in a diffraction grating because the peaks of the different wavelengths occur in different locations.
  • a diffraction grating of a high diffraction order i.e., having many elements
  • Diffraction gratings for wavelength demultiplexing can be expensive to manufacture and difficult to tune, even within a narrow range. Additionally, the various ports may not be independently tunable. Therefore, a diffraction grating- based switch will generally perform only static routing, with specific wavelength channels being transmitted to specific physical outputs. Diffraction gratings are also somewhat lossy.
  • An interleaver is essentially a l-by-2 comb filter with periodically spaced band-pass and band-reject sections.
  • the general principle underlying interleaver technology is an interferometric overlap of two beams. The overlap causes periodic interference, allowing alternating wavelength channels to pass from an input of the device to one output, and allowing complementary alternating channels to pass from the input to a second output. Controlling the fringe pattern sets the desired channels of the device.
  • interleavers One problem with interleavers is that a single interleaver can separate wavelengths channels into only two sets of channels, with a relatively wide channel spacing. Thus, multiple interleaver stages, or other additional components, are required to separate more than two individual channels. This increases, cost, size, and signal attenuation. Like diffraction gratings discussed above, interleavers may be difficult to tune or reconfigure, requiring a priori knowledge of wavelength-port assignments. A major benefit of the WDM scheme — dynamic routing — may therefore be lost. Yet another channel separation method is described in U.S. Patent No.
  • a wavelength- selective switch according to the '992 patent uses multimode interference (MMI) waveguides, Bragg gratings, Mach-Zehnder waveguides, and controllable phase shifters. Briefly, the switch works as follows. A multi-wavelength signal introduced into an MMI waveguide produces a number of images equal to the number of wavelengths sought to be separated. Each image is then passed through a different series of Bragg gratings and controllable phase shifters. Each Bragg grating reflects a specific wavelength, allowing all other wavelengths to pass through.
  • MMI multimode interference
  • each reflected signal depends on the phase shifters through which the reflected signal has passed on its way to and from the Bragg grating.
  • the reflected signals return to the MMI waveguide, they exit out of different ports of the MMI waveguide, depending on their phases.
  • the different wavelengths pass through different phase shifters, incurring different phase shifts, they exit out of different ports.
  • the switch of the '992 patent is inherently lossy because of the optical power division (1/N) of the MMI waveguide.
  • the signals are subjected to the attenuation of several phase shifters and Bragg gratings.
  • the switch scales rather poorly.
  • the switch uses N x (N-l) Bragg gratings and N 2 phase shifters.
  • N x (N-l) Bragg gratings and N 2 phase shifters are stacked in the switch.
  • this second-order dependence of the numbers of gratings and phase shifters on the number of channels quickly become unwieldy as the number of channels increases.
  • the power losses in the switch increase with N because of the power division and also because of the increased number of components that the signals pass through.
  • a wavelength-selective switch is a fiber-based switch with impressed tunable gratings.
  • the switch is architecturally similar to the switch described in the '809 patent, using multiple cascaded 1 x N and N x 1 couplers and isolators or circulators. Scalability is therefore also a problem with this switch.
  • output ports and input ports of this switch are connected by cascading couplers, the switch requires elaborate active real time control of filters to block transmission of unwanted wavelengths.
  • Other problems with this scheme include wavelength contention, back reflectance, and optical power losses.
  • a data stream may be associated with a single wavelength as the data stream traverses an optical network. This is known as wavelength path routing.
  • the problem with wavelength path routing is that some routing flexibility is lost because of the additional constraint imposed on the routing algorithms by the requirement that the wavelength remain constant across all spans traversed by each data stream from the stream's point of origin to its destination point.
  • a data stream needs to travel from node A to node D, and that a route connecting nodes A and D via nodes B and C exists.
  • only wavelength K] is available between nodes A and B, and between nodes C and D; and that ⁇ is not available between nodes B and C.
  • the A-B-C-D route then cannot be chosen if only wavelength path routing techniques are used.
  • the data stream may also be routed without a permanent association with a single wavelength. Instead, the wavelength carrying the data stream may be reassigned at wavelength converting switches as the data stream travels across the spans of the network from one node to another. This is the virtual wavelength path routing technique.
  • the data stream between nodes A and D can first cross the span from node A to node B on the available wavelength ⁇ . Next, the data stream can be demultiplexed and converted into ⁇ 2 , a wavelength available on the span between nodes B and C. After the data stream is multiplexed with other data streams, it will travel across the B-C span.
  • the process is repeated at node C, where the data stream can be demultiplexed for conversion back to ⁇ , the wavelength that is available between nodes C and D.
  • the previously unavailable route A-B-C-D is made available because of the use of virtual path routing and wavelength conversion techniques.
  • the above example illustrates additional routing flexibility afforded by the virtual wavelength path routing technique: The data stream can now travel across multiple spans where no single wavelength can carry the data stream from its origination point to its termination point. But the example also illustrates the need to demultiplex a wavelength channel and to convert the channel to another wavelength.
  • wavelength conversion in a WDM system usually necessitates conversion of only a single wavelength channel, rather than conversion of the aggregate signal of a fiber, the other disadvantages associated with the use of opto-electronic and electro-optic conversions remain.
  • a data stream that needs to be wavelength converted has likely already traversed at least one span of the network and will traverse at least one more span. Thus, attenuation during the conversion process is a concern, as it was in the demultiplexer case discussed above.
  • the data stream's pulses traveling through the network are subject to chromatic dispersion caused by the variation of the group propagation velocity as a function of the wavelength. Dispersion causes each pulse to stretch out in time because some of the pulse's constituent wavelengths travel faster than other constituent wavelengths. As a result, the pulse becomes less distinct with distance and may eventually overlap with its adjacent pulses, preventing detection. This phenomenon is called inter-symbol interference.
  • Pulse distortion can also result from nonlinear effects in the fiber.
  • One such nonlinear effect is the variation of the fiber's refractive index (permittivity and or permeability) with the amplitude of the optical signal.
  • the variation in the index phase-modulates the optical signal, distorting the spectrum of the signal and causing frequency "chirping.” This is the so-called Kerr effect.
  • the Kerr effect also may stretch out pulses, aggravating the effect of chromatic dispersion.
  • Pulse shaping can be achieved, for example, at mediate nodes by serially performing opto-electronic and electro-optic conversions. This, of course, involves the disadvantages of the use of photodetector and transponder arrays.
  • MSSI mid-span spectral inversion
  • the spectral distribution of the inverted signal with respect to the signal's center wavelength is a mirror image of the non-inverted signal: Higher frequencies become lower frequencies, and vice versa.
  • the dispersion that follows the mid-point where spectral inversion is performed affects the signal in a manner exactly opposite to the manner in which the signal was affected by the dispersion that preceded the mid-point.
  • the spectral inverter (a phase conjugator) needs to be located approximately equidistant to the endpoints of the span. (Hence the "mid-span” in "mid-span spectral inversion.' 1 )
  • This requirement of a mid-point device is a disadvantage.
  • coupling the signal into and out of the spectral inverter may cause additional signal attenuation.
  • a third way to compensate for dispersion is by means of a special, highly dispersive fiber with a dispersion characteristic opposite to that of the main transmission fiber.
  • a signal received at a node after traversing a span is transmitted through a relatively short length of the special fiber to compensate for the dispersion incurred in traversing the span.
  • the special fiber tends to have high attenuation per unit of length, and the distance required for compensation, though relatively short in comparison to the span, may still be substantial.
  • Another scheme for dispersion compensation involves the use of multiple Bragg fiber gratings.
  • the gratings are disposed in a dispersion compensator to reflect different wavelengths at different locations along the length of the compensator. For example, the shorter wavelengths entering the compensator may be reflected sooner than the longer wavelength, forcing the longer wavelength to accumulate additional delay before exiting the compensator. This arrangement would compensate for the fiber dispersion where the shorter wavelengths have been subjected to longer delays.
  • the order of the gratings can be reversed to compensate for fiber dispersion where the longer wavelengths have been subjected to longer delays.
  • An example of a compensator using the Bragg grating scheme may be found in U.S. Patent No. 5,982,963 to Feng et al. It has been noted that Bragg grating-based dispersion compensator requires an additional component, such as a circulator, to separate the input signal from the compressed output signal.
  • U.S. Patent No. 5,815,307 to Arbore et al. discloses a method for second harmonic generation with concurrent dispersion compensation.
  • the method uses a quasi-phasematching grating with a gradually varying period of the grating. Because the period of the structure varies along the propagation dimension, different wavelengths are transformed into their second harmonic counterparts at different locations along the structure.
  • a signal having a particular wavelength ⁇ o when input into the device will travel part of the distance at ⁇ 0 wavelength, experiencing refractive index n 0 ; after undergoing wavelength conversion, the signal will travel the remainder of the distance as ⁇ 0 /2, experiencing refractive index of n 0 ', which may differ substantially from n 0 .
  • the methods for dispersion compensation described above need to be applied periodically, so that the accumulated dispersion does not exceed the dispersion compensation physically available from the compensating device.
  • a signal received at a node must be broadcast to various destinations over different wavelength channels. If the number of the distinct wavelength channels is large, individual wavelength conversions may become expensive. It would be advantageous to convert the received channel to a different wavelength at the same time and in one device.
  • the present invention is directed to a wavelength converter including an optical waveguiding structure with a signal input for receiving an optical input signal, a local oscillator input for receiving local oscillator signal or signals, and one or more signal outputs; the wavelength converter further includes a quasi-phasematching structure for mixing the input signal and the local oscillator signal or signals to produce one or more output signals; the wavelength converter also includes one or more wavelength filters for filtering each of the output signals before the signal is output. Multiple outputs of the wavelength converter may be realized, for example, as an optical power splitter, or as a staggered combination of one or more circulators and filters.
  • the wavelength converter may additionally include one or more of the following: an amplifier, such as active fiber and optical pump combination, for amplifying the output signals; local oscillator sources for generating the local oscillator signals; monitors for obtaining data relating to the input signal and to the output signals; and a computer for receiving the signal data and tuning the filters, the optical pump, the local oscillator sources, and the quasi-phasematching structure.
  • an amplifier such as active fiber and optical pump combination
  • local oscillator sources for generating the local oscillator signals
  • monitors for obtaining data relating to the input signal and to the output signals
  • a computer for receiving the signal data and tuning the filters, the optical pump, the local oscillator sources, and the quasi-phasematching structure.
  • the quasi-phasematching structure may have a variable dimensional period to provide dispersion compensation to the input signal.
  • Figure 1 is a schematic diagram of an embodiment of a wavelength converter
  • Figure 2 is a schematic diagram of a wavelength converter with a local oscillator source
  • Figure 3 is a schematic diagram of a computer-controlled wavelength converter with a local oscillator source
  • Figure 4 is a schematic diagram of a wavelength converter with a plurality of output ports
  • Figure 5 is a schematic diagram of a representative output port of a wavelength converter
  • Figure 6 is a schematic diagram of an amplifying wavelength converter with a plurality of output ports
  • Figure 7 is a schematic diagram of an amplifying wavelength converter with an optical pump and a plurality of output ports
  • Figure 8 is a schematic diagram of a wavelength converter with a plurality of output ports and a plurality of local oscillator sources
  • Figure 9 is a schematic diagram of a wavelength converter with a plurality of local oscillator input ports and a circulator-based output section.
  • FIG. 1 illustrates an embodiment of a wavelength converter 100.
  • the wavelength converter has a waveguiding medium 110, a nonlinear optical quasi- phasematching structure 120, a signal input port 130, a local oscillator port 140, an output port 150, and a wavelength filter 160.
  • the wavelength converter 100 optically mixes a signal input through the signal input port 130 with a local oscillator signal input through the local oscillator port 140.
  • the product of the mixing process is a wavelength-converted signal, modulated similarly to the input signal, and filtered by the wavelength filter 160 to suppress undesirable mixing products and other noise.
  • the filtered signal is output tlirough the output port 150.
  • Optical signals can be shifted in wavelength through processing of one or more signals in a nonlinear medium.
  • the process is analogous to radio frequency or microwave harmonic generation and mixing, which allows generation of second and higher harmonics of a single signal, and of various mixing products of two or more signals.
  • an optical signal of wavelength ⁇ s (corresponding to angular frequency ⁇ s ) can be mixed with a locally generated signal K LO (corresponding to ⁇ w) in a nonlinear waveguide to produce, for example, sum and difference products at frequencies ( ⁇ ur ⁇ s) and ( ⁇ L ⁇ + ⁇ L ⁇ ), respectively.
  • K LO local oscillator
  • K LO - S refers to the free-space wavelength corresponding to frequency of ⁇ l£>- ⁇ s-
  • n s and n Sh g are the refractive indices experienced by the signal and the signal's second harmonic, respectively. Because phasematchmg is needed to preserve momentum necessary for coupling the signal energy into the second harmonic, the energy will be coupled into the second harmonic through the first coherence length of the propagation medium. From that point and through the second coherence length, the energy will be coupled back into the fundamental signal. The coupling and decoupling cycles are then repeated through successive coherent lengths of the propagation medium.
  • the maximum or close to maximum coupling occurs at one coherence length. If, however, the nonlinearity of the medium is changed (e.g., inverted) with respect to the signals every coherence length, decoupling can be minimized and coupling of energy into the second harmonic may continue through multiple coherence lengths, increasing the efficiency of the wavelength conversion process.
  • the basic formulas for computing period K and coherence length l c remain essentially the same, with the appropriate refractive indices being substituted in the formula for computing l c .
  • L O may be approximately one half of ⁇ s ⁇ (i.e., ⁇ i-o may be approximately twice ⁇ sl ), and therefore ⁇ sl and ⁇ s2 may lie relatively close together, for example, in one optical band.
  • several optical bands have been defined, e.g., the L-band between 1570 and 1625 nm, and the C-band between 1525 and 1570 nm, by ⁇ s ⁇ and ⁇ s2 lying in "one optical band" we simply mean that the distance between the two wavelengths is no more than approximately 15% of their average value.
  • Quasi-phasematching is known to those skilled in the art.
  • the literature includes Martin M. Fejer et al., Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, 28 IEEE J. QUANTUM ELEC. 2631-54 (1992); and U.S. Patent No. 5,815,307 to Arbore et al, which patent is hereby incorporated by reference as if fully set forth herein.
  • Ferroelectric crystals with a periodically poled pattern of domains can be used for constructing the waveguiding section of an optical mixer.
  • one or more of the following materials can host the mixer medium: lithium niobate (LinbO 3 ), potassium titanyl phosphate (KTiOPO 4 or KTP), potassium titanyl arsenide (KTA), rubidium titanyl arsenide (RTA), rubidium titanyl phosphate (RTP), cesium titanyl arsenide (CTA), and lithium tantalate (LiTaO ).
  • the potential dopants include, also by way of example, Scandium (Sc), Gallium (Ga), Cromium (Cr), and Rubidium (Rb).
  • the quasi-phasematching region of the wavelength converter can be manufactured by rotationally twinning crystals, stacking of alternately oriented thin plates, growing periodic domain structures in ferroelectrics, and periodically axmihilating the nonlinear coefficient in the waveguiding medium.
  • Periodic domains can also be produced by electric fields applied to a ferroelectric waveguiding medium, e.g., by applying voltages to electrodes on the surface of the medium, thereby creating a periodically poled device. Because a minimal depth penetration may suffice to develop the grating, lithographic techniques may be used to depose periodically spaced electrodes on the surface of a ferroelectric waveguide.
  • the electric fields can be coupled to the ferroelectric waveguide by other means as well, e.g., electrodes epoxied to the waveguide or otherwise positioned proximate the waveguide.
  • the grating may also be made adjustable, that is, having a dimensional period that varies as a function of some external stimulus, e.g., a voltage signal. The variation of the dimensional period would vary the wavelength to which the grating is tuned. Such tunability can be achieved through, for example, changing the spacing between the electrodes, or stretching the grating by mechanical, electrostatic, piezoelectric, or thermo-mechanical actuators.
  • gratings are generally periodic structures, the period need not be the same throughout the grating. Instead, the period may vary (gradually or discretely) as a function of the propagation dimension (dimension "x" in Figure 1). Because the period of the structure varies along the propagation dimension, different wavelengths can be converted at different locations of the grating. Thus, the effective passband of the grating would be increased, or several discrete passbands would be created.
  • the grating does not provide immediate pulse shaping inside the wavelength converter, it nevertheless may help in reducing the effects of chromatic dispersion in the fiber.
  • This phenomenon is a byproduct of the mixing function performed by the grating: Mixing not only shifts the signal in wavelength (i.e., frequency), but also results in spectral inversion of the signal. For example, if a signal on wavelength ⁇ is received from fiber link A and then is wavelength-converted by mixing to ⁇ before it is transmitted on fiber link B, an effect akin to that resulting from mid-span spectral inversion may result.
  • the filter 160 may be a band-pass filter, i.e., a line filter that transmits wavelengths within a specific range, rejecting some or all other wavelengths.
  • the filter may be a Fabry-Perot resonator (an etalon), i.e., an optical resonator formed by mirrors.
  • the filter 160 can be a tunable filter.
  • Fabry-Perot resonators can be tuned, for example, with low voltage piezoelectric actuators varying the gap between a resonator's mirrors by positioning one or more of the mirrors.
  • a Fabry-Perot filter can also be tuned by inserting a liquid crystal layer between the opposed mirrors of the filter, and then applying an electric field across the layer. The electric field changes the refractive index of the liquid crystal material, thus changing the resonant frequency of the cavity.
  • Tunable Fabry-Perot liquid crystal filters are described in, for example, U.S. Patents with numbers 5,068,749 and 5,111,321, both to Patel, and U.S. Patent No. 6,154,591 to Kershaw.
  • Another type of optical filter is a tunable acousto-optical filter.
  • Acousto- optical filters operate based on elasto-optical effect, which is the phenomenon of physical stresses in a material causing changes in the material's refractive index.
  • radio frequency waves are often used to generate surface acoustic waves in appropriate electro-optic medium, such as LiNbO 3 crystal.
  • appropriate electro-optic medium such as LiNbO 3 crystal.
  • the compressions and rarefications of the surface acoustic waves create a temporary diffraction grating within the crystal.
  • the temporary diffraction grating works like its more permanent counterpart discussed above; that is, it spatially separates ' the various wavelengths of a diffracted optical signal.
  • the temporary grating is tuned by controlling the radio frequency emitter.
  • United States Patent No. 6,157,025 to Katagiri teaches a disc-shaped transparent substrate with an optical filter layer deposited on the substrate.
  • the filter layer is such that the center wavelength of the band-pass region varies with the angular dimension of the filter. Rotating the disc in relation to a light beam incident upon it exposes different angular portions of the disc to the beam, thereby changing the center wavelength of the filter. Therefore, different wavelengths can be selected by rotating the disc.
  • a tunable filter can be realized in an arrangement that allows physical movement of a filter element in some dimension in relation to an optical path of a beam of light being filtered. If the center wavelength of the band-pass region of the filter element is a variable of the dimension, the filter can be tuned by controlling an actuator that moves the filter element in the dimension of interest.
  • the actuator may include a servomechanism, a position encoder, and a controller.
  • the servomechanism moves the filter element, whose position the encoder senses and transmits to the controller.
  • the controller receives the position data from the encoder and directs the servomechanism to position the filter element in accordance with an input signal. See U.S. Patent No. 6,111,997 to Jeong for examples of such tunable filters.
  • Starodubov teaches an optical fiber including a core covered by a cladding.
  • a grating within the core couples light at some resonant wavelength either into the cladding or into a coating surrounding the fiber adjacent to the grating.
  • the resonant wavelength depends on the refractive index of the coating.
  • the coating is made of a material whose refractive index is a function of an externally controllable stimulus, such as an electric or a magnetic field.
  • a tunable optical filter somewhat similar to that taught by Starodubov is disclosed in U.S. Re-Examined Patent No. RE. 36,710 to Baets et al.
  • Baets's filter is also based on a tunable optical grating embedded in a multi-waveguide structure.
  • Another type of a tunable optical filter uses an optical splitter to divide a beam into several components.
  • the several components are transmitted through different phase shifters, and then combined.
  • the combined components interfere constructively or destructively, depending on their relative phases, which, of course, depend on the specific wavelengths carried by the beam. Controlling the phase shifters tunes such interferometric filter to different wavelengths.
  • Still another type of optical filter uses a dielectric multi-layered filter element.
  • Varying the optical lengths of the layers varies the passband of the filter.
  • a simple method of varying the optical lengths of the layers is to change the angle of incidence of a beam upon the filter element. This can be done by, for example, rotating the filter element. See U.S. Patent No. 5,481,402 to Cheng et al. for a polarization- independent tunable filter based on this principle.
  • FIG 2 illustrates an embodiment of a wavelength converter 200 that, in addition to the elements shown in Figure 1, includes a local oscillator source 210.
  • the local oscillator 210 may be a laser. It may also be a tunable source, allowing a user to configure the wavelength converter 200 to receive different input wavelengths, or, in conjunction with the tunable (or broadband) wavelength filter 160 and quasi- phasematching structure 120, to produce different output wavelengths.
  • a wavelength converter 300 is similar to the wavelength converter 200 of Figure 2, and further includes a computer 370, an input signal monitor 380, and an output signal monitor 390.
  • the computer 370 communicates with a tunable source 395, a tunable quasi-phasematching structure 320, and a tunable wavelength filter 360, adjusting these tunable elements. Adjustments may be based on operator inputs received from port 375, based on data received from the monitor 380, and on data received from the monitor 390.
  • the data may include signal power and/or wavelength data. For example, automated routines can be prescribed and triggered by the detection of predetermined incoming wavelengths.
  • the computer 370 may be a general purpose computer, e.g., a Wintel machine, a microcontroller, a semi-custom application specific integrated circuit, or a custom data processing device.
  • a wavelength converter in accordance with the present invention may have more than one output.
  • a three-output wavelength converter 400 is illustrated in Figure 4.
  • the output signal is divided among outputs 450a, 450b, and 450c.
  • Each of the outputs has its corresponding wavelength-selective element 460a, 460b, and 460c.
  • Signal division is the function performed by an optical power splitter section 494 of a waveguiding medium 410.
  • the power splitter section 494 is disposed to the right of line 492.
  • the multi-output version may also include monitors for the input and/or output signals, and a computer responsive to operator inputs and to the data provided by the monitors.
  • a wavelength-selective element may include a shutter for selectively blocking the output from the port associated with the shutter.
  • Figure 5 illustrates a representative output port 500 with a filter 510 and a shutter 520 disposed within the port. Note that the absolute and relative locations of the filter and the shutter are not critical to the operation of the mvention. Thus, the shutter may precede or follow the filter, and each of these elements may be disposed in various locations of the output port 500. This statement is equally true with respect to filter position in embodiments that do not use a shutter.
  • the shutter 520 is essentially an ON/OFF switch: It either allows incident light to pass through, or blocks the light.
  • the shutter 520 can be an electro-absorptive modulator using shutter effect to block or transmit the light selectively.
  • An electro-absorptive modulator becomes either transparent or absorptive (opaque) in response to a voltage applied across it.
  • a lithium niobate modulator can be used as a shutter.
  • Lithium niobate modulators often used to modulate laser transmitters in 2.5 Gbit/s and even faster WDM systems, have response time of the order of 100 picoseconds, generally fast enough for dynamic wavelength routing.
  • a gallium arsenide indium phosphate modulator can also be used as a shutter.
  • the shutter can be a micro-electromechanical system (MEMS).
  • MEMS-based shutter is described in U.S. Patent No. 6,173,105 to Aksyuk, et al.
  • MEMS rotary structures that may be used in building a shutter are described in U.S. Patent No. 6,137,206 to Hill.
  • a shutter can also be a digital mirror device (DMD), a micro- mechanical spatial light modulator array of small mirrors (or a single mirror) supported above silicon addressing circuitry by small hinges attached to a support post. Each mirror can be made to rotate about its axis by, for example, electrostatic, electromagnetic, piezoelectric, or thermo-mechanical actuation.
  • the wavelength converter may include an active fiber portion for amplifying the signal.
  • Figure 6 illustrates a wavelength converter 600, which is similar to the wavelength converter 400 illustrated in Figure 4, but also includes an active fiber portion 610 that amplifies the signal or signals output by the wavelength converter.
  • the active fiber portion 610 can compensate for some or all of the losses in the wavelength converter; it can also overcompensate for the losses, providing a net amplification effect in the wavelength converter.
  • Typical active fiber is fiber doped with rare earth element ions.
  • the doped fiber becomes fluorescent, meaning that it can absorb excitation energy at one wavelength and emit the absorbed energy at a different wavelength.
  • active fiber is excited or "pumped” by a source of light (an “optical pump"), e.g., a diode laser, elevating the energy states of the fiber's constituent particles. The particles then emit light when triggered by the propagating signal at the signal's wavelength, thus amplifying the signal.
  • a source of light an “optical pump”
  • Fluorescent dopants often used in active fiber of non-coherent optical systems operating in the 1310 nm and 1550 nm bands are erbium and praseodymium.
  • the wavelength converter 600 may provide amplification with a relatively small penalty to the noise figure of the device if the output filters are sufficiently narrowband. This benefit results because the noise contribution of the active fiber portion 610, as well as the unwanted mixing products generated in the quasi-phasematching structure 420, will be filtered out by the output filters.
  • the output filters may be too broad to provide a meaningful reduction of the noise figure, but the filters may still be useful in discriminating among the various signals at the output of the quasi- phasematching structure.
  • Active fiber uses an optical pump to provide energy needed for signal amplification.
  • the optical pump can be part of the wavelength converter, or separate therefrom.
  • the former arrangement is illustrated in Figure 7, in which wavelength converter 700 includes an optical pump 710 coupled to the active fiber portion 610 in order to inject excitation light into the active fiber.
  • the optical pump 710 may be a laser diode.
  • the wavelength converter may convert more than a single wavelength at the same time.
  • the wavelength converter may receive several input signals, and convert each of them into a different wavelength.
  • the wavelength converter may filter and output each of the converted signals through a different output port, thus providing a demultiplexing function in addition to the wavelength converting function.
  • the wavelength converter may also simultaneously convert one input signal into several different wavelengths. If the quasi-phasematching structure has a sufficiently broad spectral conversion capability, the multiple conversions may be achieved by providing two or more local oscillator input wavelengths.
  • a single local oscillator source capable of producing several wavelengths at the same time can be coupled to the local oscillator port 440.
  • several local oscillator sources may be supplied in order to provide different local oscillator wavelengths.
  • a wavelength converter 800 is similar to the wavelength converter 400 of Figure 4.
  • the wavelength converter 800 includes three local oscillator ports 840a, 840b, and 840c, each coupled to one of local oscillators 845a, 845b, and 845c.
  • Each of the wavelength-selective elements 860a, 860b, and 860c is tuned to one of the wavelength generated by mixing, in quasi-phasematching structure 820, the signal input through port 830 with one of the local oscillator wavelengths supplied by the three local oscillators 840a, 840b, and 840c.
  • the input signal is effectively broadcast to three different outputs, i.e., destinations, each at a distinct wavelength determined by the wavelength of the input signal, by one of the local oscillator wavelengths, and by the wavelength-selective element associated with the particular output.
  • the quasi-phasematching structure and the wavelength-selective elements may be tunable, and the converter 800 may include the local oscillators and an active fiber portion. So far we have described the output section of a multi-output wavelength converter as an optical power splitter. Other means may be used for this purpose. For example, signal division following mixing can also be accomplished by using staggered filters and circulators.
  • the output section of wavelength converter 900 includes circulators 970, 980 and 990, and filters 960-1 through 960-6.
  • the filters 960-3, 960-4, and 960-5 which may be, for example, Bragg gratings, transmit through some wavelengths, while reflecting back other wavelengths to be collected by circulators.
  • the circulators may be, for example, three-port devices as illustrated in Figure 9. An optical signal input at a first port of a three-port circulator is transmitted to the second port; an optical signal input at the second port is transmitted to the third port. But the optical signals will not be transmitted in the reverse direction: For example, a signal input at the third port will not be transmitted to the second port.
  • input port 930 The functions of input port 930, local oscillator ports 940a, 940b and 940c, quasi-phasematching structure 920 are the same as the functions of the corresponding components in the wavelength converter 800 of Figure 8.
  • the inventive wavelength converter and some of its features in considerable detail for illustration purposes only. Neither the specific embodiments of the invention as a whole nor those of its features limit the general principles underlying the invention.
  • the invention is not limited to specific regions of the light spectrum mentioned in this document, or to use in WDM optical transmission systems.
  • the specific quasi-phasematching structures, shutters, filters, power splitters, and active fiber fillers described may be used in some embodiments, but not in others, without departure from the spirit and scope of the invention as set forth. Different geometries of the wavelength converter and of the active fiber filler also fall within the intended scope of the invention.
  • the use of active fiber and shutters on the outputs is optional.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Nonlinear Science (AREA)
  • Optical Communication System (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

L'invention concerne un convertisseur de longueurs d'ondes entièrement optique utilisant une structure de quasi-adaptation de phase non linéaire pour mélanger un signal d'entrée avec un signal d'oscillateur local, et filtrant ensuite le signal de sortie à longueur d'onde décalée résultant. Le filtre qui exécute la fonction de filtrage, la structure de quasi-adaptation de phase, et la source d'oscillateur local, peuvent être accordables et commandés par ordinateur. Plusieurs sorties, chacune équipée de son propre filtre et, facultativement, un obturateur, peuvent être prévus. Plus d'une source d'oscillateur local peut être utilisée pour créer plusieurs signaux à longueur d'onde décalée, et les filtres peuvent être accordés sur les longueurs d'ondes des différents signaux de sortie, créant ainsi une fonction de diffusion. Une fibre active peut être utilisée dans le convertisseur de longueurs d'ondes afin d'amplifier le niveau de puissance des signaux avant leur sortie. La structure de quasi-adaptation de phase peut avoir une période dimensionnelle variable pour que le convertisseur de longueurs d'ondes ait une capacité de compensation de dispersion.
PCT/US2001/023849 2000-09-22 2001-07-30 Appareil de conversion de longueurs d'ondes optique WO2002025368A2 (fr)

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AU2001280885A AU2001280885A1 (en) 2000-09-22 2001-07-30 Optical wavelength-converting apparatus

Applications Claiming Priority (6)

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US23457100P 2000-09-22 2000-09-22
US60/234,571 2000-09-22
US24536700P 2000-11-02 2000-11-02
US60/245,367 2000-11-02
US81092101A 2001-03-16 2001-03-16
US09/810,921 2001-03-16

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US5825517A (en) * 1995-12-06 1998-10-20 Tellium, Inc. Parametric wavelength interchanging cross-connect
EP0828178B1 (fr) * 1996-09-05 2003-10-29 Oki Electric Industry Co., Ltd. Dispositif de conversion de longeur d'onde à rendement amélioré, réglage simple et insensibilité à la polarisation

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