USE OF PHASE SHIFT KEYED COHERENT OPTICAL SIGNALING IN A WAVELENGTH DIVISION MULTIPLEXED OPTICAL FIBER LINKS

Abstract: A wavelength division multiplexed (23) fiber optic link (40) including a phase shift keyed coherent transmitter (20) and receiver (30) coupled together by a way of periodic optically amplified (42, 44) fiber link. The transmitter has a plurality of phase shift keyed modulators (22) that each receive data for transmission over the fiber link. A separate laser (21) is coupled to each of the modulators (22) whose respective output beams are modulated with the data for transmission. Outputs of the modulators are input to a wavelength division multiplexer (23) that multiplexes the phase shift keyed modulated outputs of the phase shift keyed modulators for transmission over the fiber link. The receiver (30) includes an optical power splitter (31) or wavelength division multiplexed demultiplexer (32) that produces a plurality of data channels. A coherent optical demodulator (33) demodulates the plurality of data channels to produce data for each data channel. A corresponding optical signaling method is also disclosed.

Description: USE OF PHASE SHIFT KEYED COHERENT OPTICAL SIGNALING IN WAVELENGTH DIVISION MULTIPLEXED OPTICAL FIBER LINKS BACKGROUND The present invention relates generally to wavelength division multiplexed optical fiber links, and more particularly, to the use of phase shift keyed coherent optical signaling to mitigate the effects of fiber refractive index nonlinearity in such links. The effects of refractive index nonlinearities are three-fold, and are commonly known as self-phase modulation, cross-phase modulation and four wave mixing. The motivation for the mitigation of these phenomena is that they lead to two limits on fiber optic link performance. The first is a maximum limit on transmitted power, which leads to a maximum distance between optical amplifiers. The second is a minimum limit on the frequency spacing between wavelength division multiplexed optical channels, which limits the total number of optical channels within the optical amplifier bandwidth and hence the total aggregate data capacity on a fiber. These are important factors in the overall cost of transmitting information on fiber optic cables greater than 300 kilometers in length. Attempts to mitigate these three phenomena in prior art on-off keyed wavelength division multiplexed optical fiber links involved replacing existing inexpensive chromatically dispersive fiber with that of lessened chromatic dispersion. However, replacement of hundreds of kilometers of fiber cable is expensive. Moreover, although lessened chromatic dispersion minimizes self- and cross-phase modulation, it magnifies the four wave mixing between wavelength division multiplexed channels. To summarize, with prior art on-off keyed signaling, mitigation has primarily consisted of reducing the dispersion of the fiber until the optimum trade off between self- and cross-phase modulation versus four wave mixing is reached. Accordingly, the present invention provides for the use of phase shift keyed coherent optical signaling, in combination with inexpensive unmodified chromatic dispersion fiber (dispersion coefficient approximately 17 ps/(nm x km)), to permit simultaneous reduction of self- and cross-phase modulation and four wave mixing. This in turn permits higher transmitted power and thus increased fiber length between optical amplifiers, and also allows tighter spacing between wavelength division multiplexed channels. This reduces the cost of the fiber link and increases the aggregate data transmission capacity per fiber. SUMMARY OF THE INVENTION To accomplish the above and other objectives, the present invention uses a combination of phase shift keyed coherent optical signaling to mitigate self- and cross- phase modulation, and dispersive fiber to mitigate four wave mixing. How phase shift keying mitigates self- and cross-phase modulation is now explained. Self- and cross-phase modulation and four wave mixing all arise from the inherently nonlinear refractive index of the glass optical fiber medium. In other words, the origin of all three of these phenomena is that the refractive index of glass optical fiber varies instantaneously according to the local intensity of the laser light. On-off keying, which encodes the data through maximal variation of the intensity of the light, by its very nature maximizes this nonlinear action. The nonlinear action distorts on-off keyed optical pulse symbols in the following way. The nonlinear action in an optical pulse consists of the pulse itself locally changing the refractive index of the fiber medium in which it is traveling. The peak of the pulse has a different local refractive index than the flanks. The more intense the pulse, the larger the refractive index distortion. Self-phase modulation is defined as the resultant distortion of the optical phase within the pulse caused by its own optical energy. Cross-phase modulation is defined as the distortion of the optical phase within a pulse caused by a coincident pulse from another wavelength division multiplexed channel. Note that the total local refractive index variation in a fiber is due to the instantaneous total optical energy of all the coincident pulses from all channels. The more closely spaced the optical channels, the more severe the cross phase modulation. The distortion of the optical phase within the pulse due to the nonlinear action broadens the spectral content of the pulse, but does not in and of itself change the temporal pulse shape. Temporal pulse shape change is brought about by the chromatic dispersion of the optical fiber acting more rapidly on the broadened pulse spectrum. Once a pulse has been nonlinearly distorted by this combination of nonlinearity and chromatic dispersion, simple linear chromatic dispersion compensation will not restore it. This accelerated, uncorrectable chromatic dispersion distortion due to pulse spectral broadening places fundamental limits on the distance and optical channel density on prior art on-off keyed fiber links. This is why prior art reduction of the chromatic dispersion of fiber is helpful to reduce self- and cross-phase modulation action in such links. In the limit of zero chromatic dispersion fiber there is no change in pulse shape, and hence in zero dispersion on-off keyed links there are no deleterious consequences of self- and cross-phase modulation. However, as mentioned in the background discussion, use of reduced chromatic dispersion fiber gives rise to four wave mixing, and so a compromise value of dispersion must be chosen. Four wave mixing is classical mixing of the different optical channels due to the nonlinear fiber medium, and will occur regardless of modulation format, even with unmodulated optical carriers. In an equally spaced wavelength division multiplexed system, these mixing products will fall on top of adjacent channels. As mentioned, this effect places Umits on launched optical power and optical channel spacing. Chromatic dispersion suppresses four wave mixing. In contrast to on-off-keying, phase-shift keying presents a locally constant, uninterrupted optical intensity envelope to the fiber. The nonhnear action therefore simply yields an altered but locally constant refractive index, which ideally results in no self- and cross-phase modulation. Rather, the nonhnear action yields a simple constant optical phase shift that causes no harm to a phase-shift keyed format. Therefore, the dispersion of the fiber need not be reduced, and four wave mixing remains suppressed. This results in higher limits on optical channel power, and thus improved overall link length. It also results in reduced allowable wavelength division multiplexed channel spacing. In fact, in new links, it may be advantageous to install increased chromatic dispersion fiber (> 17 ps/(nm x km)) to suppress four wave mixing further. This would allow even higher optical power and narrower optical channel spacing. Note that many formats of electrical information may be transmitted by means of phase shift modulation, for instance analog, binary phase shift keying, quaternary phase shift keying, and even higher orders of phase shift keying. All have a constant optical intensity envelope, and thus benefit from this invention. Of particular interest is quaternary phase shift keying (QPSK) which, as is well known, doubles the spectral efficiency of data transmission vs. binary signaling with no optical power penalty. Also, the narrower optical spectrum of QPSK is less susceptible to chromatic dispersion than binary signaling. An original coherent QPSK optical demodulator is described herein. Note also that, if fiber nonlinearities are discounted, coherent binary phase shift keying and quaternary phase shift keying have been shown to have theoretically a 3.5 dB smaller optical power requirement than incoherent on-off-keying for typical fiber link bit error rates of 1 error per billion bits. Therefore, relative to prior art incoherent on-off keying, the binary and quaternary phase shift keyed formats allow even longer fiber lengths between optical amplifiers than only mitigation of fiber nonlinearity suggests. The phase shift keyed wavelength division multiplexed fiber optic link is now described. It comprises a phase shift keyed coherent transmitter and receiver coupled together by way of a periodically optically amplified fiber cable. The phase shift keyed coherent transmitter comprises a plurality of phase shift keyed optical modulators that each receive a separate data waveform to be transmitted over the periodically optically amplified fiber cable. A separate laser, each at a unique wavelength, is coupled into each of the plurality of phase shift keyed modulators whose respective optical outputs are each modulated with one of the separate data waveforms. Outputs of each of the phase shift keyed modulators are input to a wavelength division multiplexer which combines the phase shift keyed modulated optical outputs of the phase shift keyed modulators for transmission over the optically amplified fiber cable. An exemplary periodically amplified fiber cable comprises the following. It is similar in nature to that typically used with on-off keying. Depending on the total length of the link, each period of fiber consists of a 30 to 150 kilometer length of dispersive fiber. At the end of each fiber length period, there is a low noise optical preamplifier, a fiber chromatic dispersion compensator and a booster optical amplifier. The amplifier gains are such that the total link period has unity gain. The total fiber link may include many such periods totaling thousands of kilometers. The positioning of the chromatic dispersion compensator is now discussed. It is well known that positioning the dispersion compensation module between the optical preamplifier and booster amplifier is the best configuration for minimizing optical noise. For this reason, the dispersion compensator in prior art on-off keyed links is positioned in this way. Fortunately, this position is also ideal for the phase-shift keyed signaling method under consideration. This is because, although the phase shift keyed signal launched into the fiber link has a constant envelope, if the fiber is chromatically dispersive, the envelope will not remain constant. This may be understood by the following qualitative description of how the optical envelope of the phase-shift keyed signal changes as it propagates down the periodically amplified fiber link. At start, the signal will be very close to the ideal constant envelope of the phase shift keyed format. As the signal propagates down a fiber segment, the fiber chromatic dispersion will result in increasing differential phase distortion within the phase shift keyed optical signal spectrum. In terms of the optical intensity envelope, this manifests itself as an evolving intensity modulation. In other words, chromatically dispersive fiber is a phase modulation to amplitude modulation converter. This evolving intensity modulation is, like on-off keyed signals, subject to distortion by the fiber nonlinearity. However, as the signal travels it is also subject to inherent optical fiber loss, so as the phase-to-amplitude modulation conversion evolves, the signal is simultaneously attenuated. Depending on launch power levels, symbol rate and fiber dispersion magnitude, this loss can be rapid enough such that the absolute power fluctuation in the envelope will be insignificant as far as the fiber nonhnearity is concerned. Note that the QPSK modulation format is much less susceptible to this phase-to-amplitude modulation conversion and its attendant nonlinear effects than is binary signaling. This is because the QPSK optical spectrum is half as wide, and chromatic dispersion causes only half as much differential spectrum phase distortion. Given therefore that the nonlinearity may be neglected, at the end of a link period the chromatic dispersion compensator restores the original optical spectrum phase and thus the original constant optical envelope. Importantly, it does so immediately before the booster optical amplifier increases the optical signal power back into the nonlinear region. To summarize, the chromatic dispersion compensator in existing prior art on- off keyed links is positioned to minimize optical noise. However, coincidentally this position is also such that the ideal phase shift keyed constant optical amplitude envelope is restored in the peak power regions immediately after amplification where fiber nonlinearity is most significant. Subsequently, natural fiber attenuation reduces the signal rapidly enough such that the evolving phase to amplitude modulation conversion due to chromatic dispersion does not result in significant nonlinear action. In phase modulated links, the nonlinear action on the result of phase to amplitude modulation conversion imposes an upper limit on the amount of fiber dispersion that may be introduced to suppress four wave mixing. Higher dispersion results in faster phase to amplitude modulation conversion, thus larger absolute optical power envelope variation, thus more nonlinear action. Full optimization of the phase shift keyed format involves a trade between launched power, symbol rate and dispersion value. Note that on-off keying results in maximum relative optical power variation at the peak power region after the booster optical amplifier and hence maximizes harmful fiber nonlinear action, while phase shift keying does the opposite. The last periodic length of the fiber cable is coupled into the front end of the coherent receiver. The front end of the receiver comprises either an optical power splitter or a wavelength division demultiplexing optical filter. The splitter front end type would be used if access to all channels by any single demodulator is desired, or if optical channel spacing is closer than can be resolved by available demultiplexing filters. The wavelength division demultiplexing optical filter would be used in case conservation of individual optical channel power is the dominant consideration. In either case, the front end of the receiver splits the received signal into a plurality of signals. Each of these is then coupled to a coherent optical demodulator. The electrical data signal output of the demodulator is coupled to an electronic filter to remove undesired spectral elements from adjacent wavelength division multiplexed channels. Finally, the electronic output of the filter is coupled into an equalizer to compensate for various channel nonidealities. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages of the present invention may be more read- ily understood with reference to the following detailed description taken in conjunction with the accompanying drawing, wherein like reference numerals designate like structural elements, and in which: Fig. 1 illustrates an exemplary wavelength division multiplexed fiber optic link in accordance with the principles of the present invention; Fig. 2 illustrates an original conception for a four phase coherent optical costas loop demodulator, which would be used to demodulate the particularly beneficial Quaternary Phase Shift Keyed (QPSK) data format; and Fig. 3 illustrates an exemplary method in accordance with the principles of the present invention. DETAILED DESCRIPTION Referring to the drawing figures, Fig. 1 illustrates an exemplary wavelength division multiplexed fiber optic link 10 in accordance with the principles of the present invention. The wavelength division multiplexed fiber optic link comprises a phase shift keyed (PSK) coherent transmitter 20 and receiver 30 coupled together by way of a periodically optically amplified fiber link 40. The phase shift keyed coherent transmitter 20 comprises a plurality of phase shift keyed optical modulators 22 that each receive a separate electronic data waveform for transmission over the optically amplified fiber link 40. Each of a plurality of lasers LI, L2, ... LN producing respective optical wavelengths Wl, W2, ... WN is uniquely coupled to one of a plurality of optical phase shift keyed modulators 22 whose respective output beams are each modulated with a data waveform. Outputs of each of the phase shift keyed modulators 22 are input to a wavelength division multiplexer 23 that combines the phase shift keyed modulated outputs of the phase shift keyed modulators 22 for transmission over the optically amplified fiber link 40. The fiber link 40 is periodic in nature. Each period consists of a length of nondispersion shifted optical fiber 41 coupled to a two-stage erbium doped optical fiber amplifier (EDFA). The two stage optical amplifier comprises a low noise optical preamplifier (EDFA) 42 and a high power booster amplifier (EDFA) 44. Disposed between the first and second EDFAs 42, 44 is a chromatic dispersion compensation module (DCM) 43. An optical link may include many such periods totaling thousands of kilometers. Although the chromatic dispersion of the fiber 41 causes waveform amplitude variation to evolve as the signal travels down the length of fiber 41, the phase shift keyed ideal constant envelope is restored by the chromatic dispersion compensation module immediately before the second stage optical amplifier 44 increases the optical power into the nonlinear region of operation. Thus, the signal has a constant optical amplitude envelope in peak power regions immediately after amplification where nonlinearity is most significant. The nonlinearity thus yields only a harmless uniform optical phase shift. The amplified output of the booster fiber amplifier 44 of the final link period is coupled into the receiver 30. There are two types of receivers 30, differing only in their respective front ends. The front end of the receiver 30 may comprise an optical power splitter 31 whose input is coupled to the optically amplified fiber link 40. This splitter 31 splits optical power equally into each of a plurality of coherent optical demodulators 33. In this way each demodulator 33 has simultaneous access to all the data channels, and any demodulator 33 may be tuned to demodulate any channel. Additionally, the spacing of the channels is not limited by the resolution of available optical filters. Alternately, the front end of receiver 30 may comprise a wavelength division multiplexed (WDM) demultiplexing optical filter 32, whose input is coupled to the optically ampUfied fiber hnk 40. This filter 32 splits the incoming optical signal into a pluraUty of individual optical channels. This sphtting of the optical signal conserves the optical power of each channel. The optical outputs of either type receiver front end 31, 32 are coupled to the coherent optical demodulators 33. The structure of the demodulators 33 is dependent on the format of the phase shift keyed signal. An embodiment of an exemplary four phase (QPSK) demodulator 33 is illustrated in Fig. 2. The electrical signal output of the demodulators 33 is a reproduction of the transmitted data. Both self- and cross-phase modulation are due to the nonlinearity with optical power of the propagation constant of the fiber 41. Prior art on-off keyed pulses maximize distortion due to this nonlinearity. This limits the maximum launched optical power, which Umits the length of the link. The coherent phase shift keyed modulation format used in the present invention transmits a constant optical power envelope which is minimally distorted, even in common high dispersion characteristic fiber 41. The high dispersion characteristic fiber 41 also suppresses four wave mixing. This allows higher optical power to be transmitted, and hence allows a longer length fiber optic link 40. Additionally, suppression of four wave mixing allows for narrower wavelength division multiplexed channel spacing which results in more total capacity per fiber link 40. A better understanding of the present invention may be had from the following discussion. The present invention uses a combination of phase shift keyed coherent optical signaling to mitigate self- and cross-phase modulation, and dispersive fiber 41 to mitigate four wave mixing. How phase shift keying mitigates self- and cross-phase mixing will now be explained in detail. Self- and cross-phase modulation and four wave mixing all arise from the inherently nonhnear refractive index of the glass optical fiber medium. Thus, the origin of all three of these phenomena is that the refractive index of glass optical fiber 41 varies instantaneously according to the local intensity of the laser light. On-off keying, which encodes the data through maximal variation of the intensity of the light, by its very nature maximizes this nonlinear action. The nonlinear action distorts on-off keyed optical pulse symbols in the following way. The nonlinear action in an optical pulse includes the laser pulse that locally changes the refractive index of the fiber medium in which it is traveUng. The peak of the pulse has a different local refractive index than its flanks. The more intense the pulse, the larger the refractive index distortion. Self -phase modulation is defined as the resultant distortion of the optical phase within the pulse caused by its own optical energy. Cross-phase modulation is defined as the distortion of the optical phase within a pulse caused by a coincident pulse from another wavelength division multiplexed channel. The total local refractive index variation in a fiber 41 is due to the instantaneous total optical energy of all the coincident pulses from all channels. The more closely spaced the optical channels, the more severe the cross phase modulation. The distortion of the optical phase within the pulse due to the nonlinear action broadens the spectral content of the pulse, but does not in and of itself change the temporal pulse shape. Temporal pulse shape change is brought about by the chromatic dispersion of the optical fiber 41 acting more rapidly on the broadened pulse spectrum. Once a pulse has been nonlinearly distorted by this combination of nonlinearity and chromatic dispersion, simple linear chromatic dispersion compensation will not restore it. This accelerated, uncorrectable chromatic dispersion distortion due to pulse spectral broadening places fundamental limits on the distance and optical channel density on prior art on-off keyed fiber links. This is why a reduction of the chromatic dispersion of a fiber 41 is helpful to reduce self- and cross-phase modulation action in such links. In the limit of zero chromatic dispersion fiber there is no change in pulse shape, and hence in zero dispersion on-off keyed links there are no deleterious consequences of self- and cross-phase modulation. However, as mentioned in the background discussion, use of reduced chromatic dispersion fiber gives rise to four wave mixing, and so a compromise value of dispersion must be chosen. Four wave mixing is classical mixing of the different optical channels due to the nonlinear fiber medium, and occurs regardless of modulation format, even with unmodulated optical carriers. In an equally spaced wavelength division multiplexed system, these mixing products fall on top of adjacent channels. As mentioned above, this effect places limits on launched optical power and optical channel spacing. Chromatic dispersion suppresses four wave mixing. In contrast to on-off-keying, phase-shift keying presents a locally constant, uninterrupted optical intensity envelope to the fiber. The nonlinear action therefore yields an altered but locaUy constant refractive index, which ideally results in no self- and cross-phase modulation. Rather, the nonlinear action yields a simple constant optical phase shift that causes no harm to a phase-shift keyed format. Therefore, the dispersion of the fiber 41 need not be reduced, and four wave mixing remains suppressed. This results in higher limits on optical channel power, and thus improved overall tink length. It also results in reduced aUowable wavelength division multiplexed channel spacing. In fact, in new links, it may be advantageous to install increased chromatic dispersion fiber (> 17 ps/(nm x km)) to suppress four wave mixing further. This would aUow even higher optical power and narrower optical channel spacing. Many formats of electrical information may be transmitted by means of phase shift modulation, such as analog, binary phase shift keying, quaternary phase shift keying, and higher orders of phase shift keying, for example. All have a constant optical intensity envelope, and thus benefit from the present invention. Of particular interest is quaternary phase shift keying (QPSK) which, as is well known, doubles the spectral efficiency of data transmission versus binary signaling with no optical power penalty. Also, the narrower optical spectrum of QPSK is less susceptible to chromatic dispersion than binary signaling. An original coherent QPSK optical demodulator is described herein. If fiber nonlinearities are discounted, coherent binary phase shift keying and quaternary phase shift keying have been theoretically shown to have a 3.5 dB smaller optical power requirement than incoherent on-off-keying for typical fiber link bit error rates of 1 error per billion bits. Therefore, relative to prior art incoherent on-off keying, the binary and quaternary phase shift keyed formats allow even longer fiber lengths between optical amplifiers 42, 44 than only mitigation of fiber nonlinearity suggests. The phase shift keyed wavelength division multiplexed fiber optic link 40 will now described in more detail. The fiber optic link 40 comprises a phase shift keyed coherent transmitter 20 and receiver 30 coupled together by way of a periodically optically amplified fiber cable. The phase shift keyed coherent transmitter 20 comprises a plurality of phase shift keyed optical modulators 22 that each receive a separate data waveform to be transmitted over the periodically optically amplified fiber link 40. A separate laser 21, each at a unique wavelength, is coupled into each of the plurality of phase shift keyed modulators 22 whose respective optical outputs are each modulated with one of the separate data waveforms. Outputs of each of the phase shift keyed modulators 22 are input to a wavelength division multiplexer 23 which combines the phase shift keyed modulated optical outputs of the phase shift keyed modulators 22 for transmission over the optically amplified fiber link 40. An exemplary periodically amplified fiber link 40 comprises the following. It is similar in nature to that typically used with on-off keying. Depending on the total length of the link 40, each period of fiber 41 is a 30 to 150 kilometer length of dispersive fiber 41. At the end of each fiber length period, there is a low noise optical pre-amplifier 42, a fiber chromatic dispersion compensator 43 and a booster optical amplifier 44. The gains of the respective amplifiers 42, 44 are such that the total link period has unity gain. The total fiber link 40 may include many such periods totaling thousands of kilometers. The positioning of the chromatic dispersion compensator 43 will now be discussed. Positioning the dispersion compensation module 22 between the optical preamptifier 42 and booster amplifier 44 is the best configuration for minimizing optical noise. For this reason, the dispersion compensator in prior art on-off keyed links is positioned in this way. Fortunately, this position is also ideal for the phase-shift keyed signahng method provided by the present invention. This is because, although the phase shift keyed signal launched into the fiber link 40 has a constant envelope, if the fiber 41 is chromatically dispersive, the envelope wiU not remain constant. This may be understood by the following qualitative description of how the optical envelope of the phase-shift keyed signal changes as it propagates down the periodically amplified fiber link 40. At start, the signal will be very close to the ideal constant envelope of the phase shift keyed format. As the signal propagates down a fiber segment, the fiber chromatic dispersion produces increasing differential phase distortion within the phase shift keyed optical signal spectrum. In terms of the optical intensity envelope, this manifests itself as an evolving intensity modulation. In other words, chromatically dispersive fiber is a phase modulation to amplitude modulation converter. This evolving intensity modulation is, like on-off keyed signals, subject to distortion by the fiber nonlinearity. However, as the signal travels, it is also subject to inherent optical fiber loss, so as the phase-to-amplitude modulation conversion evolves, the signal is simultaneously attenuated. Depending on launch power levels, symbol rate and fiber dispersion magnitude, this loss can be rapid enough such that the absolute power fluctuation in the envelope will be insignificant as far as the fiber nonUnearity is concerned. Note that the QPSK modulation format is much less susceptible to this phase-to-amplitude modulation conversion and its attendant nonlinear effects than is binary signaling. This is because the QPSK optical spectrum is half as wide, and chromatic dispersion causes only half as much differential spectrum phase distortion. Given that the nonUnearity may be neglected, at the end of a link period, the chromatic dispersion compensation module 43 restores the original optical spectrum phase and thus the original constant optical envelope. Importantly, it does so immediately before the booster optical amplifier 44 increases the optical signal power back into the nonlinear region. Thus, the chromatic dispersion compensator in prior art on-off keyed links is positioned to minimize optical noise. However, coincidentally this position is also such that the ideal phase shift keyed constant optical amplitude envelope is restored in the peak power regions immediately after amplification where fiber nonlinearity is most significant. Subsequently, natural fiber attenuation reduces the signal rapidly enough such that the evolving phase to amplitude modulation conversion due to chromatic dispersion does not result in significant nonhnear action. In phase modulated links, the nonlinear action on the result of phase to amplitude modulation conversion imposes an upper limit on the amount of fiber dispersion that may be introduced to suppress four wave mixing. Higher dispersion results in faster phase to amplitude modulation conversion, thus larger absolute optical power envelope variation, thus more nonlinear action. Full optimization of the phase shift keyed format involves a trade between launched power, symbol rate and dispersion value. On-off keying results in maximum relative optical power variation at the peak power region after the booster optical ampUfier 44 and hence maximizes harmful fiber nonlinear action, while phase shift keying does the opposite. The last periodic length of fiber 45 is coupled into the front end of the coherent receiver 30. The front end of the receiver 30 comprises either an optical power splitter 31 or a wavelength division demultiplexing optical filter 32. The splitter front end type is used if access to all channels by any single demodulator 33 is desired, or if optical channel spacing is closer than can be resolved by available demultiplexing filters. The wavelength division demultiplexing optical filter 32 is used in case conservation of individual optical channel power is the dominant consideration. In either case, the front end of the receiver 30 splits the received signal into a plurality of signals. Each of these is then coupled to a coherent optical demodulator 33. The electrical data signal output of each demodulator 33 is coupled to an electronic filter 34 to remove undesired spectral elements from adjacent wavelength division multiplexed channels. Finally, the electronic output of each filter 34 is coupled into an equalizer 35 to compensate for various channel nonidealities. Fig. 2 illustrates an exemplary embodiment of a four phase coherent optical costas loop demodulator 50. In Fig. 1, this optical demodulator 50 fulfills the function of each coherent optical demodulator 33 used in the fiber optic link 10 shown in Fig. 1. The QPSK modulated optical input (Iopt) and the local oscillator output (LO) of a high power, narrow linewidth semiconductor distributed feedback (DFB) local laser 52 are input into a 90 degree optical hybrid 51. There are two outputs from the 90 degree optical hybrid 51. Each output includes equal parts of the inputs, but there is a 90 degree relative optical phase difference introduced in one output. Each output of the 90 degree optical hybrid 51 is input into one of two photodiodes 53 which act as optical mixers to produce in-phase and quadrature data signals I, Q. The in-phase and quadrature data signals are low pass filtered 54 to remove undesired remnants of adjacent wavelength division multiplexed optical channels to produce I and Q data. Copies of the data are electrically mixed in mixers 55, and a sum 56 of the mixing products is produced. This sum corresponds to the phase error of the LO to the input signal optical carrier. This error signal is low pass filtered 57 and used to adjust the laser current 58 and hence the frequency of the local laser 52 to maintain phase lock. The temperature of the laser is controlled 59 and may be varied to tune the distributed feedback laser 52 to a particular input optical wavelength. Fig. 3 iUustrates an exemplary optical signaling method 70 in accordance with the principles of the present invention. The exemplary optical signaling method 70 comprises the following steps. Data in a plurality of data channels that are to be transmitted over an optically amplified fiber link are phase shift key modulated 71, such as by using an individual plurality of lasers, for example. The respective phase shift keyed modulated signals are then wavelength division multiplexed 72. The wavelength division multiplexed signals are transmitted 76 over an optical fiber 73, amplified 74, processed 75 to compensate for chromatic dispersion, and amplified 76 a second time. The previous four steps (73-76) are repeated 77 a predetermined number of times to achieve a desired total optical fiber link length. The signals transmitted over the fiber link are separated 78 into a pluraUty of data channels. The signals in each of the data channels are coherently optically demodulated 79 to produce data for the respective channels. Thus, systems and methods that use phase shift keyed coherent optical signaling to mitigate the effects of fiber refractive index nonUnearity in wavelength division multiplexed optical fiber links have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.

Claims: CLAIMS What is claimed is: 1. A wavelength division multiplexed fiber optic hnk comprising: a phase shift keyed coherent transmitter; a receiver; and a periodic optically amplified fiber link having a relatively large fiber chromatic dispersion coupled between the transmitter and the receiver. 2. The fiber optic tink recited in Claim 1 wherein the fiber chromatic dispersion is greater than or equal to a nominal unshifted value of approximately 16 ps/(nm x km) to resist four wave mixing. 3. The fiber optic link recited in Claim 1 wherein the periodic optically amplified fiber link comprises periodic chromatic dispersion compensation and periodic optical amplification to provide optical signals with a substantially constant optical power envelope at peak optical power points to resist self -phase modulation and cross-phase modulation. 4. The fiber optic hnk recited in Claim 2 wherein the periodic optically ampUfied fiber link comprises periodic chromatic dispersion compensation and periodic optical amplification to provide optical signals with a substantially constant optical power envelope at peak optical power points to resist self-phase modulation and cross-phase modulation. 5. The fiber optic link recited in Claim 1 wherein the phase shift keyed coherent transmitter comprises: a plurality of phase shift keyed optical modulators that each receive a separate data stream for transmission; and a wavelength division multiplexer for wavelength division multiplexing the phase shift keyed modulated optical data signals generated by the phase shift keyed modulators. 6. The fiber optic hnk recited in claim 5 wherein the pluraUty of phase shift keyed optical modulators modulate in quadrature phase shift keyed format in order to reduce optical bandwidth and thereby enable tighter wavelength division multiplexed optical channel spacing and reduced chromatic dispersion/phase-to-amplitude modulation conversion/nonlinear action. 7. The fiber optic link recited in Claim 5 wherein the phase shift keyed coherent transmitter further comprises a plurality of local oscillator lasers, each at a unique frequency, respectively coupled to each of the plurality of phase shift keyed modulators whose respective output beams are modulated according to the data. 8. The fiber optic Unk recited in Claim 1 wherein the receiver comprises: an optical power splitter for splitting optical power equally into a plurality of identical spectra each containing all the transmitted wavelength division multiplexed optical signals; and a plurality of coherent optical demodulators for coherently optically demodulating the plurality of optical signals, and that each produce data according to the optical signal to which its local oscillator laser is tuned. 9. The fiber optic link recited in Claim 1 wherein the receiver comprises: a wavelength division multiplexed demultiplexer for splitting the multiplexed phase shift ke}'ed modulated data into a plurality of individual optical signals; and a plurality of coherent optical demodulators for coherently optically demodulating the data in the plurality of individual optical signals to produce data for the respective channels. 10. The fiber optic link recited in Claim 1 wherein the periodic optically amplified fiber link comprises: a plurality of periodic lengths of non-dispersion shifted optical fiber; a plurality of first stage Erbium-doped fiber amplifiers respectively coupled to the plurality of periodic lengths of non-dispersion shifted optical fiber; a plurality of chromatic dispersion compensation modules which restore net accumulated chromatic dispersion to neghgible levels, respectively coupled to the plurality of first stage Erbium-doped fiber amplifiers; and a plurality of second stage Erbium-doped fiber amplifiers respectively coupled to the plurality of chromatic dispersion compensation modules; a final length of optical fiber coupled between a final second stage fiber amplifier and the receiver. 11. The fiber optic link recited in Claim 1 wherein each of the plurality of coherent optical demodulators comprises a four phase coherent optical costas loop demodulator. 12. The fiber optic link recited in Claim 11 wherein each of four phase coherent optical costas loop demodulator comprises a local oscillator laser for providing a local oscillator output signal; a 90 degree optical hybrid for combining the local oscillator output signal and QPSK modulated optical signal into first and second output signals that include equal parts of the respective inputs, such that the second output yields a 90 degree relative optical phase difference between the combined equal parts of the respective inputs compared to the phase difference between the combined equal parts of the respective inputs of the first output; first and second optical mixers for producing in-phase and quadrature data signals; first and second low pass filters for filtering the in-phase and quadrature data signals to remove undesired signals from adjacent wavelength division multiplexed optical channels to produce I and Q data; first and second mixers for mixing the I and Q data; a summing device for generating a sum of the mixing products generated by the mixers that comprises a phase error signal corresponds to the phase error between the local osciUator output signal and the optical input signal, which phase error signal is coupled to the local oscillator laser to maintain phase lock. 13. The fiber optic link recited in Claim 12 further comprising: a low pass filter coupled to the local oscillator laser for filtering the phase error signal which is used to adjust the laser current and hence the frequency of the local osciUator laser to maintain phase lock; and a temperature control coupled to the local oscillator laser to tune it to a particular input optical wavelength. 14. The fiber optic link 10 recited in Claim 12 wherein the local oscillator laser comprises a narrow linewidth semiconductor distributed feedback laser. 15. An optical signaling method comprising the steps of: phase shift key modulating a plurality of optical laser carriers according to a plurality of respective data streams that are to be transmitted over an optically amplified fiber link; wavelength division multiplexing the respective phase shift keyed modulated optical signals; transmitting the wavelength division multiplexed signals over an optical fiber; amplifying the wavelength division multiplexed signals; processing the wavelength division multiplexed signals to compensate for chromatic dispersion; ampUfying compensated wavelength division multiplexed signals; repeating the previous four steps a predetermined number of times to achieve a desired total length of the fiber link; separating the signals transmitted over the fiber link into a plurality of data channels; and coherently opticaUy demodulating the signals in each data channel to produce data for the respective channel.

Applicants: LOCKHEED CORP, LOCKHEED MARTIN CORPORATION
Application year: 2001
Main classification: H04B10
IPC codes: H04B10