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WO2007103410A2 - Récepteurs multivoie à modulation par déplacement de phase et codage différentiel - Google Patents

Récepteurs multivoie à modulation par déplacement de phase et codage différentiel Download PDF

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Publication number
WO2007103410A2
WO2007103410A2 PCT/US2007/005778 US2007005778W WO2007103410A2 WO 2007103410 A2 WO2007103410 A2 WO 2007103410A2 US 2007005778 W US2007005778 W US 2007005778W WO 2007103410 A2 WO2007103410 A2 WO 2007103410A2
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Prior art keywords
optical signal
channels
optical
interferometer
dpsk
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PCT/US2007/005778
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English (en)
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WO2007103410A3 (fr
Inventor
David O. Caplan
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Massachusetts Institute Of Technology
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Publication of WO2007103410A2 publication Critical patent/WO2007103410A2/fr
Publication of WO2007103410A3 publication Critical patent/WO2007103410A3/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/66—Non-coherent receivers, e.g. using direct detection
    • H04B10/67—Optical arrangements in the receiver
    • H04B10/676—Optical arrangements in the receiver for all-optical demodulation of the input optical signal
    • H04B10/677—Optical arrangements in the receiver for all-optical demodulation of the input optical signal for differentially modulated signal, e.g. DPSK signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/66—Non-coherent receivers, e.g. using direct detection

Definitions

  • DPSK Differential Phase Shift Keying
  • OOK On-Off-Keying
  • WDM Wavelength Division Multiplexed
  • DPSK conveys information by encoding a relative phase difference between two optical bits. Differential encoding simplifies the receiver by eliminating the need for a stable absolute phase reference that is required for coherent homodyne Phase Shift Keyed (PSK) modulation. Instead of mixing the received signal with a local oscillator phase reference to determine its phase, DPSK mixes the incoming signal with a delayed version of itself. Consequently the DPSK receiver is often referred to as a "self-homodyne" receiver.
  • PSK Phase Shift Keyed
  • DPSK is often used to refer to differentially encoded binary PSK (DBPSK) in which the binary information preferably corresponds to either a 0 or 180 degree ( ⁇ ) relative phase difference, which can be demodulated using a single delay-line interferometer.
  • DBPSK differentially encoded binary PSK
  • ⁇ 180 degree
  • DQPSK Differential Quadrature Phase Shift Keying
  • DBPSK Differential Quadrature Phase Shift Keying
  • DQPSK conveys 2-bits of information per symbol by differentially encoding one of four relative phases, preferably 0, 90, 180 or 270 degrees [0, ⁇ /2, ⁇ , 3 ⁇ /2], which can essentially be demodulated using two DBPSK receivers consisting of interferometers shifted by + ⁇ /4 and - ⁇ /4 so that they are orthogonal ( ⁇ /2 out of phase).
  • DQPSK requires half the bandwidth to convey the same information, (see, e.g., R. A. Griffin, R. I. Johnstone, R. G. Walker, J. Hall, S. D. Wadsworth, K. Berry, A. C. Carter, and M.
  • optical DPSK optical DPSK
  • the interferometer is the most technically challenging and the least mature. Control hardware is necessary to ensure stable operation, which requires that the arms of the interferometer be stable to small fractions of a wavelength.
  • carefully designed thermo-mechanical packaging is necessary in addition to stabilization electronics, adding to size, weight, power, and cost.
  • An optical, multi-channel, Differential Phase Shift Keying (DPSK) receiver, and corresponding method, employing the principles of the present invention demodulates multiple Wavelength Division Multiplexed (WDM) channels using a single interferometer for DBDSPK and two interferometers for DQPSK.
  • a DPSK receiver using a single interferometer for WDM demultiplexing is achieved by constraining the received wavelength spacing ( ⁇ v Ch ) and leveraging the interferometer's periodic transfer function to perform demodulation on all channels ( ⁇ 's). This distributes the expense of the interferometer over all channels, allowing for deployment of cost-effective, scalable, wideband, WDM DPSK systems.
  • SWAP size, weight, and power
  • an optical receiver or corresponding method for demodulating optical signal(s) having DPSK channels includes a delay line interferometer that (i) demodulates optical signal(s) having DPSK channels to optical signal(s) having channels modulated in intensity, and (ii) outputs the demodulated optical signal(s) onto at least one main output optical path.
  • the receiver also includes channel selectors, such as optical filters or Wavelength Division de-Multiplexers (WDMs), in the main output optical path(s) directing channels onto tributary optical paths.
  • the channels may be predefined wavelengths or wavelength ranges (e.g, 1557.0nm +/- 2.0nm).
  • the receiver may include optoelectronic converters in the tributary optical paths that convert the demodulated optical signal into respective, corresponding, electrical signals.
  • the delay line interferometer may be a one-bit delay line interferometer, a multiple bit delay line interferometer, or selectably adjustable delay line interferometer to interfere optical signal pulses offset by selectable numbers.
  • interferometer may also include an electronically tunable phase shifter for stabilization and/or for receiving channels of different wavelengths.
  • the interferometer may be adapted to demodulate optical signals having wavelength spacing between carrier wavelengths defining the channels with an integer multiple of a channel rate. For example, for wavelengths on a 100 GHz International Telecommunications Union (ITU) grid (i.e., 100 GHz channel separation), standard Synchronous Optical Network (SONET) rates of 2.5 Gbps and 10 Gbps, for instance, factor evenly into the 100 GHz spacing. Therefore, these rates are compatible with this multi-wavelength DPSK receiver design. In another example, 40 Gbps channel rates requires 200 GHz channel spacing, which is also compatible with the ITU grid and the multi-wavelength DPSK receiver.
  • ITU International Telecommunications Union
  • SONET Synchronous Optical Network
  • the interferometer may also be adapted to demodulate optical signals having wavelength spacing between carrier wavelengths defining the channels evenly divisible by an odd number of half channel rates of the optical signal. For example, 40 Gbps goes into 100 GHz channel spacing exactly 2 1 A times. The received
  • SNR Signal-to-Noise Ratio
  • detection electronics perform the post processing by correcting polarity of electrical signals of received optical channels as necessary, which may correspond to predefined optical channels.
  • optical elements perform the post processing by correcting polarity of the phase demodulated optical signal.
  • the optical receiver may also include a feedback processor that generates signals transmitted to transmitter(s) of the optical signal(s) to cause the transmitter(s) to tune carrier wavelengths defining the channels. Tuning the carrier wavelengths allows for independent channel adjustment and alignment to the interferometer for improved demodulation performance. With this feedback, independent transmitter wavelengths can be precisely aligned to the interferometer, despite potential drift in the transmitter wavelengths relative to the interferometer. Such feedback, for example, could be used to compensate for laser aging or for Doppler shifts.
  • the optical receiver includes a low noise optical amplifier that receives the optical signal(s) and outputs the received, amplified, optical signal(s) to the interferometer, where "optical signal(s)" in this case refers to the collection of one or more WDM channels being received.
  • the low noise optical amplifier may be an Erbium Doped Fiber Amplifier (EDFA) or other optical amplifier known in the art.
  • EDFA Erbium Doped Fiber Amplifier
  • the interferometer or orthogonally phased interferometers may receive a pilot signal or plurality of pilot signals that serve(s) as reference wavelengths that the interferometer(s) can lock-on to for stabilization and/or alignment to incoming optical signal(s).
  • the pilot signal(s) may be locally resident or distributed, may be tunable, and may be selected to be outside the standard communications band so as not to impact the available communications spectrum.
  • the pilot tone to which the interferometer locks-on can be a dominant incoming channel that may be selectable by the network. With feedback, the interferometer and all other channels including locally resident references can be spaced at wavelengths relative to the dominant signal. In the event the dominant channel drops out, locally resident pilot tone(s) or other incoming channels, which have been aligned relative to the dominant channel can take over as the reference, allowing for continued wavelength alignment and control.
  • the use of multiple pilot tones can be used to improve interferometer control and provide a means of measuring interferometer parameters, such as Free Spectral Range (FSR).
  • FSR Free Spectral Range
  • the receiver may be used in various applications, such as an optical regenerator.
  • the receiver may also be used in various optical networking environments, such as Free Space Optic (FSO) and fiber optic environments.
  • FSO Free Space Optic
  • the networks may be point-to-point networks, mesh networks, ring networks, broadcast networks, multi-access networks, and so forth.
  • Figs. IA, IB, and 1C are a schematic diagrams of an optical, multi-channel, Differential Phase Shift Keying (DPSK) receiver according to the principles of the present invention
  • Fig. 2A is a schematic diagram of an example optical delay line interferometer that may be used in the receiver of Figs. 1 A or 1 B;
  • Fig. 2B is a plot of sensitivity as a function of normalized carrier frequency offset (e.g., center wavelength) for the interferometer of Fig. 2A and receiver of Figs. IA or IB;
  • normalized carrier frequency offset e.g., center wavelength
  • Fig. 2C is a schematic diagram of example optical delay line interferometers that may be used in an orthogonal DPSK receiver such as the quadrature or DQPSK receiver of Fig. 1C;
  • Fig. 3A is a spectral diagram indicating channel wavelength spacing as an integer multiple of a channel rate that optimizes use of one embodiment of the receiver of Figs. IA or IB;
  • Fig. 3B is a spectral diagram indicating channel wavelength spacing as an integer multiple of a half channel rate that optimizes use of an alternative embodiment of the receiver of Figs. IA or IB;
  • Fig. 3 C is a spectral diagram indicating targeted wavelength spacing on the ITU grid that optimizes use of an alternative embodiment of the receiver of Figs. IA or IB, illustrating the use of a pilot tone to lock the interferometer to a known reference and wavelength measurement to generate corrective feedback to transmitters of a DPSK optical signal;
  • Fig. 4 is a schematic diagram of network nodes employing the receiver of Figs. IA or IB;
  • Fig. 5 A is an illustration of a Free Space Optic (FSO) application in which the receiver of Figs. IA or IB is deployed in mobile platforms such as satellites or aircraft; and
  • FSO Free Space Optic
  • Fig. 5B is a network diagram of an optical communications network application in which the receiver of Figs. IA or IB is deployed in network nodes.
  • Fig. IA and Fig. I B are schematic diagrams of example embodiments of an optical, multi-channel, binary DPSK receiver 100 according to the principles of the present invention.
  • the receiver 100 receives optical signal(s) 105 from optical transmitter(s) 90.
  • the optical signal(s) 105 may have many DPSK channels defined by distinct wavelengths, ⁇ i, ⁇ 2, ... , ⁇ n .
  • the optical signal(s) 105 are received by an optical amplifier 110, such as a low noise Erbium Doped Fiber Amplifier (EDFA) 110, that may be separate from the receiver 100 or integrated into the receiver 100.
  • EDFA Erbium Doped Fiber Amplifier
  • the EDFA 1 10 outputs amplified optical signal(s) 105 to a first optical splitter 115, which directs a portion of the amplified optical signal(s) 105 to a binary DPSK demodulator 199, including an interferometer 120, such as a 1-bit delay line interferometer 120, which receives the amplified signals via a primary input line 1 18.
  • the delay line interferometer 120 is sometimes referred to as a "delay and multiply demodulator.”
  • the remaining portion of the amplified optical signal(s) 105 is directed to a wavemeter 170.
  • the receiver 100 may leverage the periodic transfer functions (cos or sin ) from either arm of the interferometer 120 to perform a delay line (e.g., one bit) demodulation on all channels ( ⁇ 's) with a single interferometer. Such leveraging is preferably performed by defining the wavelength spacing ( ⁇ v Ch ) to be an integer multiple of a channel rate (R).
  • the interferometer 120 demodulates the optical signal(s) 105 by interfering the received optical signal pulses from each channel with an offset version of itself, where the offset is equal to the interferometer delay.
  • the delay is usually a one bit delay (i.e., interfering adjacent signal pulses), but more generally can be an integer multiple of the one bit delay duration (i.e., interfering non-adjacent pulses).
  • the interference converts the optical signal in each channel from being differentially phase modulated to being intensity modulated.
  • An example of an interferometer that may be used in the receiver 100 is described below in reference to Fig. 2A.
  • the interferometer 120 outputs a processed form of the optical signal(s) 105 onto first and second main optical paths 132-a and 132-b, respectively (collectively, optical paths 132).
  • the demodulated optical signal(s) on the two main optical paths 132 are complementary; if the optical signal pulses in each channel constructively interfere (i.e., they are in-phase), the interferometer 120 directs the phase demodulated optical signals onto one main optical path (132-a), and if the optical signal pulses destructively interfere (i.e., they are out-of-phase), the interferometer 120 directs the phase demodulated optical signal onto the other main optical path (132-b).
  • the optical paths 132 traverse a channel selector network 130 shown as 1 x (n+1) Wavelength Division de-Multiplexers (WDMs) 131-a, 131-b (collectively 131) (e.g., prisms, diffraction gratings, or Arrayed Waveguide Gratings (AWGs) in Fig. IA, and alternatively defined by a series of cascaded fiber Bragg grating (FBG) WDM filter pairs 133-la/133-lb, 133-2a/133-2b, . . . , 133-na/133-nb (collectively, WDM filters 133) in Fig. IB.
  • the WDMs 131 (Fig. IA) or filter pairs 133 (Fig. IB) is/are coupled to respective tributary optical paths 140-la/140-lb, 140-2a/140-2b, . . . , 140-na/140-nb (collectively, tributary optical paths 140).
  • the tributary paths for each channel are sent to post processing elements 160, which may be all optical, or optoelectronic, which includes elements such as communications electronics (not shown), balanced detection hardware 150, and detection electronics 155, as illustrated in Fig. IB.
  • the communications electronics pass network traffic to network communications systems (Figs. 4, 5A, and 5B).
  • the detection electronics 155 may include peak RF power detection (for signal-to- interferometer alignment), clock recovery, Forward Error Correction (FEC) decoding hardware, and so forth.
  • FEC Forward Error Correction
  • 160-n may send channel performance information or metrics, such as Bit Error Rate (BER) and peak RF power, to a control processor 125 via bus lines 165-1, 165-2, ..., 165-n (collectively 165).
  • Other inputs to the control processor 125 may include measurements by a wavemeter 170 of a pilot tone 108 and incoming optical signal(s) wavelength information, which can be used along with the BER and peak RF power, among other information or metrics, to determine feedback signal(s) 185 and 188.
  • the feedback signals 185 and 188 are communicated from the control processor 125 to the pilot tone generator 107 and optical transmitter(s) 90 via feedback paths 190 and 195, respectively.
  • the feedback signals 185 and 188 are generated to control the pilot tone 108 and incoming received signal wavelengths, respectively.
  • an optical pilot tone generator 107 generates an optical pilot or reference tone 108 at wavelength ⁇ p that may be injected into the interferometer 120 via a secondary input line 119 at an output of a second optical splitter 1 16. A portion of the pilot tone 108 may also be directed via the optical splitter 1 16 to the wavemeter 170 for wavelength measurement. Pilot tone outputs from the interferometer 120 are directed through the channel selector network 130. The a-side and b-side WDM pilot tone outputs on tributary optical paths 140p-a and 140p-b, respectively (collectively 14Op), are directed to a control processor 125, which may measure a contrast between the pilot tone outputs on tributary optical paths 14Op. The measured contrast (D), which is the normalized difference between the pilot tone outputs on the tributary optical paths 140 ⁇ :
  • pilot tone wavelength ⁇ p (or equivalently center frequency) and the interferometer bias (e.g., interferometer phase), which is discussed in further detail below in reference to Figs. 2A and 2B.
  • the pilot tone contrast ratio is a function of the interferometer bias. Therefore, sending pilot tone outputs on the tributary optical paths 14Op to the control processor 125 enables the pilot contrast to be determined, which can provide feedback parameters that may be used to measure and the control interferometer 120, as discussed above. Referring to Fig.
  • each of the tributary optical paths 140 undergo balanced detection by balanced detectors 150, the output of which is electrically coupled to detection electronics 155-1, 155-2, .... 155-n (collectively 155).
  • Balanced detection can be accomplished in many ways, for example, using balanced detector pairs that output a difference photocurrent directly or using discrete photodetectors that output their respective photocurrents to electronic elements, such as a differential amplifier, which subsequently performs the differencing.
  • ITU International Telecommunications Union
  • TDM Time Division Multiplexed
  • SONET Synchronous Optical Network
  • 40 Gbps channel rates requires 200 GHz channel spacing, which is also compatible with the interferometer 120 in the optical receiver 100 and the ITU grid.
  • the channel spacing is evenly divisible by an odd number of half channel rates, for example, 40 Gbps goes into 100 GHz channel spacing exactly 2 1 A times, the received Signal-to-Noise Ratio (SNR) remains intact, but the received data is inverted on every other channel (see also Fig. 3) — a condition that can be anticipated or detected and corrected via a polarity corrector 162-1 (e.g., a conditional inverter) in the post processing electronics 160 or optically inverted prior to detection by suitable optical element(s) 162-2, 162-3.
  • a polarity corrector 162-1 e.g., a conditional inverter
  • An example of such an optical element is a controllable delay line interferometer in which the relative optical phase between the two arms can be switched by approximately a half- wavelength of the carrier frequency, or odd multiples thereof.
  • the interferometer is a controllable delay line interferometer in which the relative optical phase between the two arms can be switched by approximately a half- wavelength of the carrier frequency,
  • a multi-wavelength DPSK receiver can receive all channels without any penalty whenever the channel spacing is evenly divisible by the half channel rate (R/2), shown in the equation below.
  • the multichannel binary DPSK demodulators of Fig IA and IB can be extended to accommodate multichannel orthogonal DPSK signals such as quadrature DPSK or DQPSK signals using two binary DPSK demodulators 199-a and 199-b, with interferometers 120-a with a phase offset of ⁇ /4 and 120-b with a phase offset of - ⁇ /4. Further detail regarding the interferometer phase offset is given in Fig. 2C.
  • Fig. 2A is a schematic diagram of a representative delay line interferometer 120 composed of a first 50% optical splitter 210 that accepts primary and secondary inputs 118, 1 19 and splits them equally between two internal paths or "arms" 205, 206 with relative time delay ⁇ .
  • the optical signals in the two paths 205, 206 are recombined with a second 50% optical splitter 220.
  • the interferometer 120 (i) may be constructed as a Mach-Zehnder (as shown) or Michelson interferometer, (ii) may be waveguide, free-space, or fiber based, and/or (iii) may include Faraday rotator elements to achieve polarization independence.
  • the transfer functions of the two interferometer output arms 132a, 132b are periodic and complementary, following a cos 2 ( ⁇ f/FSR+ ⁇ ) and sin 2 ( ⁇ f/FSR+ ⁇ ) dependence, respectively.
  • the biasing term ⁇ is a measure of the relative optical phase between the internal interferometer arms and is typically controlled to maximize the signal interference in one or both of the output arms 132a, 132b.
  • the term ⁇ f is the frequency deviation from optimum alignment of the incoming signal to the interferometer, which is periodic.
  • FSR is the interferometer Free Spectral Range, which is the interferometer spectral period, the inverse of the interferometer time delay ( ⁇ ).
  • the contrast determines the impact on the signal-to-noise ratio (SNR) of the interferometer 120.
  • SNR signal-to-noise ratio
  • the signal loss is represented as the solid line with diamonds; the solid line represents the interferometer cos 2 () transfer function; the dashed line represents the interferometer sin 2 () transfer function.
  • a performance penalty is incurred with the multi-channel DPSK receiver 100 whenever the condition of Equation 2 presented above is not met since it is not possible for all channels to align to the interferometer simultaneously. This can occur, for example, when commonly used 7%-overhead G.709 compliant Forward Error Correction (FEC) coding is used with standard SONET rates, which brings a 10 Gbps SONET data rate to a 10.7 Gbps coded channel rate. These rates do not factor evenly into the 100 GHz ITU grid spacing, so it is impossible for all of the ITU grid-based WDM channels to align with the periodicity of the interferometer 120.
  • FEC Forward Error Correction
  • Fig. 2C is a schematic diagram of a representative orthogonal DPSK demodulator such as a quadrature DPSK or DQPSK demodulator 222 composed a splitter 129 followed by two delay line interferometers.
  • a first interferometer 120-a has a ⁇ /4 phase shift and a second 120-b has a - ⁇ /4 phase shift relative to the first so that the two are orthogonal.
  • Interferometer 120-a demodulates a first series of DBPSK signals and 120-b demodulates a second series of DBPSK signals that are orthogonal to the first.
  • Fig. 3 A is a graphical representation of a channel spacing evenly divisible by an integer multiple of a channel rate, superimposed on the interferometer transfer function for one output arm, (e.g., cos 2 ).
  • Xi, X 2 , and X n are spaced by integer multiples of the channel rate.
  • Fig. 3B is a graphical representation of a channel spacing evenly divisible by the half channel rate, superimposed on the interferometer transfer function for both output arms, (i.e., cos 2 and sin 2 ).
  • ⁇ i and X n are at an odd half channel rate spacing multiple (requiring polarity correction)
  • X 2 is spaced by both integer and half rate multiples of the channel rate.
  • FIG. 3C illustrates the use of the pilot tone to lock the interferometer to a known reference.
  • Fig. 3C also illustrates the generation of feedback signals that can be used to tune transmitted channels ⁇ , ⁇ 2 , and X n to the desired wavelengths, in this case aligned to the interferometer on ITU grid channels 57, 58, and 59.
  • a 40 GHz FSR interferometer is used to receive, for example, 40 Gbps DPSK data on the 100 GHz ITU grid.
  • ⁇ i and X n are at an odd half channel rate spacing multiple being located on an inverted fringe (requiring polarity correction), and ⁇ 2 is spaced by both integer and half rate multiples of the channel rate.
  • the target contrast is zero, which places an inverted fringe precisely on ITU channel #59 (195,900 GHz) where ⁇ is expected.
  • a positive fringe is aligned to ITU channel #58 (195,800 GHz) where ⁇ 2 is expected, and another inverted fringe is aligned to ITU channel #57 (195,700 GHz) where ⁇ 3 is expected.
  • the phase of the interferometer 120 may be iteratively updated until the phase error reaches an acceptably small level and converges in a stable manner to a unique phase, at which point, the interferometer 120 is "locked" to the target contrast D and a corresponding phase, ⁇ .
  • the incoming channel wavelengths are measured by wavemeter 170 to be 195,902.2, 195,800.0, and 195,698.2 GHz for ⁇ i, ⁇ 2 , and ⁇ ,, respectively.
  • the control processor 125 calculates the wavelength error for each channel, which corresponds to wavelength error ⁇ 's of 4.2 GHz, 0.0 GHz, and —1.8 GHz for ⁇ i, ⁇ 2 , and X n. .
  • the control processor 125 sends accurate wavelength error correction information 188 back to the transmitters 90 to achieve rapid convergence to optimum link performance. Note that error information may be useful for calibrating local wavelengths and measurement hardware, such as the local pilot tone generator 107 or wavemeter 170.
  • the control processor 125 may shift the pilot tone 108 by 4.5 GHz to compensate for the error common to all the transmitters.
  • the half channel rate and channel spacing can be forced to factor evenly (upholding Equation 2) by either: (i) adjusting the channel spacing to be a multiple of the channel rate, abandoning, for example, a standard such as the ITU grid if necessary, or (ii) adjusting the half channel rate to be an even factor of the channel spacing, abandoning, for example, SONET or G.709 standards, if necessary. Note that applying strong Rate Vi codes to SONET data rates yields channel rates consistent with the multi-wavelength DPSK receiver and standard ITU grid channel spacing.
  • misalignment cannot be avoided, e.g., if conforming with existing standards is a priority, performance penalties can be constrained to an acceptable level by operating in a regime where the incoming channel wavelength and the interferometer alignment are close enough so that the misalignment penalty or conformance concerns become negligible.
  • a 10.7 Gbps channel rate can be received by a 10.7 GHz FSR interferometer, which can accept optical center frequencies every 5.35 GHz (assuming the ability to anticipate or detect and correct inverted data). While most of the interferometer fringes do not align exactly to the 100 GHz ITU grid, none of the fringes are more than 5.35/2 or ⁇ 2.7 GHz from the ITU grid, with the average deviation of ⁇ 1.4 GHz. Note that if a smaller deviation is required, the deviation can be reduced by a factor of n by using Non-Adjacent (NA)-DPSK with an n NA-pulse separation, which corresponds to an interferometer FSR narrowed by a factor of n.
  • NA Non-Adjacent
  • transmitted wavelengths can be tuned to align precisely with the interferometer 120, providing penalty-free performance while maintaining a sub-GHz average deviation from the ITU grid.
  • harmonically related channel rates e.g., 2.5 Gbps, 10 Gbps, and 40 Gbps
  • a single interferometer 120 with delay equal to an integer multiple of the lowest channel rate bit period may also be used to demodulate multi-rate optical signals (e.g., n-NA-DPSK channels with varying n).
  • a pair of orthogonally phased delay line interferometers can be used to demodulate multi-rate orthogonal DPSK signals such as-DQPSK optical signals.
  • one or two interferometers may be used to demodulate multiple orthogonal DBDPSK or DQPSK WDM channels having multiple rates simultaneously.
  • a single interferometer with a 400 psec delay can be used to demodulate multiple optical DPSK signals with ITU grid compliant 100 GHz spaced carrier wavelengths carrying DBPSK data at 2.5 Gbps, 10 Gbps, and/or 40 Gbps rates.
  • the pair With a second orthogonally phased interferometer, the pair could demodulate 100 GHz spaced carrier wavelengths carrying DQPSK data at 5 Gbps, 20 Gbps, and/or 80 Gbps rates.
  • FIG. 4 is a schematic diagram of a two node network 400 that includes two network nodes 405-1 and 405-2 (collectively, network nodes 405). Each of these network nodes 405 includes transmitter(s) 90 and a receiver 100, as described in reference to Figs. IA and IB.
  • the network nodes 405 communicate via optical paths 410, which may be free space or fiber optic optical paths.
  • the transmitter(s) 90 transmits WDM DPSK signal(s) 105 via the optical path 410 to a receiver 100.
  • Proper wavelength spacing is achieved in this embodiment optionally through use of a feedback signal 188, which is fed back by the receiver 100 to the transmitter 90.
  • the feedback signal 188 may be determined through use of the wavemeter 170, which measures the wavelength of each channel (e.g., 1555.000 nm vs. 1554.800 nm).
  • a reference or pilot tone 108 may be temporarily or continuously introduced into the interferometer 120.
  • the wavelength of the reference or pilot tone 108 may be known in advance or measured with the wavemeter 170.
  • the reference or pilot tone 108 can be used for stabilization or to lock the interferometer 120 to a preselected comb of wavelength(s) at which incoming channel(s) are expected to be.
  • the position of the entire comb of interferometer channels i.e., location of periodic interferometer peaks and troughs
  • This information may be relayed back to the transmitter(s) as target wavelength information or may be combined with measurement of incoming wavelengths to generate an error signal indicating the error between the received and targeted wavelength for each channel.
  • the error or a representation of the error is fed back to the transmitter(s) 90 of the optical signal(s) for correction of the given channel's wavelength.
  • the error signal(s) may be used to align the interferometer 120 to the incoming channels, thereby minimizing the aggregate error.
  • this capability may be particularly beneficial if all incoming wavelengths deviate from the optimum by the same amount, a condition that can occur whenever the incoming wavelengths are (properly) spaced at an integer multiple of the interferometer FSR. In this case, it may be preferable to tune the interferometer 120 to the incoming signals rather than tuning all the incoming signals to the interferometer 120.
  • other information available to the receiver may be used to determine the contents of the feedback signal.
  • information includes (i) a bit error rate (BER) or other metrics associated with detection of the optical signal(s) 105, such as FEC based error rate estimates or Doppler shifit(s) that can be measured, for example, via the wavemeter 170
  • the transmitter(s) 90 of the optical signal(s) 105 may "step around" the wavelength of the channel(s) until a minimum bit error rate, for example, is found.
  • a minimum bit error rate for example, is found.
  • Other min/max search techniques known in the art may also be employed.
  • the feedback signals 185, 188 may be implemented in many different forms.
  • the feedback signals 185, 188 may be analog, digital, or packetized.
  • the feedback signals may be communicated via electrical, Radio Frequency (RF), or optical paths using applicable communications techniques and protocols.
  • RF Radio Frequency
  • the feedback signals 185, 188 may be handled via a network-level maintenance and control channel, traffic channel, or other suitable communications channel.
  • the feedback signals 185, 188 may be a command signal or informational signal, depending on the processing capability associated with the transmitter(s) 90.
  • the feedback signals 185, 188 may be included in overhead or payload portions of communications packets.
  • the feedback signals 185, 188 may be transmitted over the optical communications path 410, optical maintenance path (not shown), electrical communications path (not shown), electrical maintenance path (not shown), or other path(s) configured for transporting the feedback signals 185, 188. Since optical transmitter wavelength(s) are generally stable (i.e., have a slow drift rate), correcting the transmitted wavelength(s) can be done relatively infrequently (i.e., every few minutes, hours, days, weeks, months, or years) depending on the transmitter 90 and possibly environmental conditions at the transmitter(s) 90, receiver 100, or optical paths therebetween and be sent from the control processor 125 on a periodic, aperiodic, event driven, or request driven basis.
  • Fig. 5A is an example communications network 500 in which the receiver 100 may be employed.
  • the communications network 500 is a Free Space Optic (FSO) network.
  • the optical signal 105 is transmitted by the transmitters 90 between mobile platforms, such as communications satellites 501-1 and 501-2, and received by the DPSK receiver(s) 100 in a manner as described above.
  • the optical receiver 100 can also be employed by other platforms, such as aircraft, and relatively stable links, such as building-to-building and ship-to-shore FSO links.
  • Fig. 5B is another example of a network in which the DPSK receiver 100 may be employed.
  • the optical network 515 includes optical nodes 405-1, 405-2, 405-3, and 405-4 (collectively, nodes 405).
  • the optical communications network 515 is configured as a Unidirectional Path Switched Ring (UPSR) or Bi-directional Line Switched Ring (BLSR) that may be optically coupled to other rings or optical networks having other configurations.
  • UPSR Unidirectional Path Switched Ring
  • BLSR Bi-directional Line Switched Ring
  • the interferometers 120 of Figs. 1 and 2 can be more than a 1- bit delay line interferometer. It may be a multiple bit (multi-bit) delay line interferometer in one embodiment or a dynamically selectable bit delay line interferometer in another embodiment.
  • the pilot tone reference wavelength of Fig. IA rather than being local, can be selected as one of the incoming channels.
  • the interferometer 120 and optical channels 105 can align to wavelengths relative to the selected channel through feedback from the control processor 125.
  • the main optical paths 132 (Figs. IA and IB) may also be a single main optical path instead of the two main optical paths.
  • the single main optical path may carry a signal when the interference is constructive and no signal when the interference is destructive, or vice-versa. Because the two main optical path embodiment provides 3 dB more sensitivity than the single main optical path embodiment, the two main optical path embodiment is generally preferable.
  • the single main path embodiment may be easier to implement since it does not require both a-side and b-side elements (such as WDMs 131), balanced detection, or amplitude and time alignment of received photocurrents for each channel.
  • the main optical paths 132 may also include more than two main optical paths. Such an embodiment may be used in cases where the interferometer interferes more than two optical signal pulses in the DPSK channels. For example, future noise reduction or security schemes may facilitate or warrant higher order DPSK demodulation, in which case, three, four, or more main optical paths 132 may be employed.
  • Figs. IA and IB instead of having a tributary optical path for each wavelength, multiple wavelengths may be directed onto the same tributary optical paths and separated onto yet other tributary optical paths to respective optical receivers for processing.
  • the receiver 100 may be used in other network configurations, such as mesh networks, Bi-directional Line Switched Ring (BLSR) networks, broadcast networks, or multi-access networks.
  • BLSR Bi-directional Line Switched Ring

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne un récepteur optique multivoie à modulation par déplacement de phase différentielle (MDPD) qui utilise au moins un interféromètre pour démoduler de multiples voies multiplexées par répartition en longueur d'onde (WDM). Ce récepteur, qui répartit la charge du ou des interféromètres sur toutes les voies d'un signal optique, permet de déployer des systèmes MDPD WDM à large bande extensibles et rentables. Par exemple, pour une liaison WDM à 80 voies, le récepteur utilise un seul interféromètre au lieu de quatre-vingts interféromètres et d'un matériel informatique de stabilisation associé, ce qui réduit considérablement la taille, le poids, la puissance et le coût du système. L'architecture du récepteur, compatible avec les technologies d'interféromètre existantes, permet de renforcer les efforts de développement et de qualification antérieurs. L'invention peut donc être introduite rapidement dans des réseaux de communication optiques existants, y compris des réseaux optiques terrestres et spatiaux.
PCT/US2007/005778 2006-03-08 2007-03-08 Récepteurs multivoie à modulation par déplacement de phase et codage différentiel WO2007103410A2 (fr)

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WO2009103041A1 (fr) * 2008-02-14 2009-08-20 Infinera Corporation Émetteur à haute capacité réalisé sur un circuit photonique intégré
WO2013015859A3 (fr) * 2011-05-02 2013-04-11 Massachusetts Institute Of Technology Récepteur optique apte à être configuré pour recevoir une diversité de formats de modulation
US9264147B2 (en) 2010-03-24 2016-02-16 Massachusetts Institute Of Technology Method and apparatus for phase shift keyed optical communications

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2009103041A1 (fr) * 2008-02-14 2009-08-20 Infinera Corporation Émetteur à haute capacité réalisé sur un circuit photonique intégré
US8233510B2 (en) 2008-02-14 2012-07-31 Infinera Corporation Dual output laser source
US8238759B2 (en) 2008-02-14 2012-08-07 Infinera Corporation High capacity transmitter implemented on a photonic integrated circuit
US9264147B2 (en) 2010-03-24 2016-02-16 Massachusetts Institute Of Technology Method and apparatus for phase shift keyed optical communications
US9647765B2 (en) 2010-03-24 2017-05-09 Massachusetts Institute Of Technology Method and apparatus for transmitting phase shift keyed optical signals
WO2013015859A3 (fr) * 2011-05-02 2013-04-11 Massachusetts Institute Of Technology Récepteur optique apte à être configuré pour recevoir une diversité de formats de modulation
US10009115B2 (en) 2011-05-02 2018-06-26 Massachusetts Institute Of Technology Optical receiver configurable to accommodate a variety of modulation formats

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