Optical transmission system and signal distortion compensation method

Abstract

In this optical transmission system: an optical transmitter comprises an operating mode control unit that sets any of a first operating mode for generating signals with a low symbol rate and a second operating mode for generating signals with a high symbol rate, and a transmission signal generating unit for generating transmission signals corresponding to the operating mode; an optical receiver comprises an optical coherent receiver, a digital signal processing unit, and a reception data demodulating unit; the optical transmitter or the optical receiver comprises a nonlinear signal distortion compensation unit that multiplies the transmission signals by a value that represents an inverse characteristic of a gain coefficient of nonlinear signal distortion produced in the transmission signals by a semiconductor optical amplifier, a nonlinear signal distortion estimating unit that estimates the nonlinear signal distortion, and a physical parameter estimating unit that estimates a physical parameter of the semiconductor optical amplifier; and the optical receiver switches the output destination so that the transmission signals are outputted to the nonlinear signal distortion estimating unit when the optical transmitter is operating in the first operating mode, and switches the output destination so that the transmission signals are outputted to the reception data demodulating unit when the optical transmitter is operating in the second operating mode.　

Description

[Technical Field]

[0001] The present invention relates to an optical transmission system and a signal distortion compensation method. [Background technology]

[0002] In order to reduce the size and power consumption of optical transmission systems, development of optical transceivers that use semiconductor optical amplifiers (SOAs) is underway. In such optical transceivers, nonlinear distortion occurs in signals when the signals are amplified by the SOA. Therefore, a technology has been proposed that uses digital signal processing to compensate for distortion caused by the SOA (see, for example, Patent Document 1).

[0003] Increasing the symbol rate of a signal can reduce nonlinear distortion caused by a semiconductor optical amplifier. However, as described in Non-Patent Document 1, an ultra-high symbol rate signal of 168 GBaud incurs a signal-to-noise ratio (SNR) penalty of approximately 1 dB. To achieve high-quality communications, high-precision compensation technology is required for signal processing, even for high symbol rate signals. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] F. Hamaoka, et al., “Experimental Investigation of Influence of SOA-Induced Nonlinear Distortion on High-Symbol-Rate 168-GBaud Signal for Achieving Ultra-Broadband Optical Frontend”, in Proceedings Optical Fiber Communication Conference (OFC), San Diego, CA, USA, 2022, M3H.2. [Patent documents]

[0005] [Patent Document 1] Patent No. 6633470 Summary of the Invention [Problem to be solved by the invention]

[0006] The technology described in Patent Document 1 calculates a gain coefficient for distortion caused by a semiconductor optical amplifier based on physical parameters of the semiconductor optical amplifier estimated using a received signal, and compensates for nonlinear signal distortion by multiplying the signal by the inverse characteristic of the calculated gain coefficient. However, when the nonlinear distortion caused by the semiconductor optical amplifier is small, such as in high symbol rate signals, the estimation accuracy of the physical parameters of the semiconductor optical amplifier deteriorates. This causes a problem of degraded compensation accuracy for nonlinear distortion caused by the semiconductor optical amplifier.

[0007] In view of the above circumstances, an object of the present invention is to provide a technique capable of improving the accuracy of compensation for nonlinear distortion caused by a semiconductor optical amplifier. [Means for solving the problem]

[0008] One aspect of the present invention is an optical transmission system including an optical transmitter and an optical receiver, wherein the optical transmitter includes an operation mode control unit that sets an operation mode to either a first operation mode for generating a signal with a low symbol rate or a second operation mode for generating a signal with a high symbol rate, and a signal processing unit that outputs a signal corresponding to the operation mode set by the operation mode control unit. Electric and a transmission signal generating unit that generates a transmission signal, and the optical receiver comprises an optical coherent receiver that acquires an optical signal based on the transmission signal and generates a reception signal based on the optical signal, a digital signal processing unit that performs digital signal processing on the reception signal, and a reception data demodulation unit that demodulates binary information of a transmission data series based on the reception signal that has been subjected to the digital signal processing, and the optical transmitter or the optical receiver demodulates the transmission signal by a semiconductor optical amplifier. Optical signal based on a nonlinear signal distortion compensation unit that calculates a gain coefficient of nonlinear signal distortion occurring in a received signal based on an estimated value of a physical parameter of the semiconductor optical amplifier and multiplies the transmission signal by a value representing an inverse characteristic of the gain coefficient; a nonlinear signal distortion estimation unit that acquires the transmission signal and estimates the nonlinear signal distortion based on a comparison result of the transmission signal and the received signal; and a physical parameter estimation unit that estimates the physical parameter of the semiconductor optical amplifier based on the nonlinear signal distortion estimated by the nonlinear signal distortion estimation unit, wherein when the optical transmitter is operating in a first operation mode, the optical receiver switches an output destination so that the received signal that has been digitally processed is output to the nonlinear signal distortion estimation unit, and when the optical transmitter is operating in a second operation mode, the optical receiver switches an output destination so that the received signal that has been digitally processed is output to the received data demodulation unit, When the optical transmitter is operating in a first operation mode, the optical transmitter transmits an optical signal to the optical receiver in a state in which the nonlinear signal distortion compensator does not compensate for nonlinear signal distortion caused in an optical signal based on a transmission signal by the semiconductor optical amplifier, the nonlinear signal distortion estimator estimates the nonlinear signal distortion, and the physical parameter estimator estimates physical parameters of the semiconductor optical amplifier based on the estimated nonlinear signal distortion. After that, the optical transmitter operates in a second operation mode, and the nonlinear signal distortion compensator calculates a gain coefficient of the nonlinear signal distortion based on the physical parameters of the semiconductor optical amplifier estimated by the physical parameter estimator, and compensates for the nonlinear signal distortion using a gain coefficient having an inverse characteristic of the calculated gain coefficient. It is an optical transmission system.

[0009] One aspect of the present invention is a signal distortion compensation method performed by an optical transmission system including an optical transmitter and an optical receiver, wherein the optical transmitter sets an operation mode to one of a first operation mode for generating a signal with a low symbol rate and a second operation mode for generating a signal with a high symbol rate, and the optical transmitter performs a signal distortion compensation process according to the set operation mode. Electric a transmission signal is generated, the optical receiver acquires an optical signal based on the transmission signal and generates a reception signal based on the optical signal, the optical receiver performs digital signal processing on the reception signal, the optical receiver demodulates binary information of a transmission data series based on the reception signal that has been subjected to the digital signal processing, and the optical transmitter or the optical receiver acquiring the transmission signal, estimating nonlinear signal distortion based on a comparison result between the transmission signal and the reception signal, and estimating physical parameters of the semiconductor optical amplifier based on the estimated nonlinear signal distortion; The transmitted signal is Optical signal based on a gain coefficient of nonlinear signal distortion occurring in the semiconductor optical amplifier based on estimated values ​​of physical parameters of the semiconductor optical amplifier, and multiplying the transmission signal by a value representing an inverse characteristic of the gain coefficient; ,beforeWhen the optical transmitter is operating in a first operation mode, the optical receiver switches an output destination so that the received signal that has been subjected to the digital signal processing is output to a nonlinear signal distortion estimation unit that estimates the nonlinear signal distortion, and when the optical transmitter is operating in a second operation mode, switches an output destination so that the received signal that has been subjected to the digital signal processing is output to a received data demodulation unit that demodulates binary information of the transmission data series. When the optical transmitter is operating in a first operation mode, the optical transmitter transmits an optical signal to the optical receiver without compensating for nonlinear signal distortion caused by the semiconductor optical amplifier in an optical signal based on a transmission signal, the nonlinear signal distortion estimator estimates the nonlinear signal distortion, the optical transmitter or the optical receiver estimates a physical parameter of the semiconductor optical amplifier based on the estimated nonlinear signal distortion, and then the optical transmitter operates in a second operation mode, and the optical transmitter or the optical receiver calculates a gain coefficient of the nonlinear signal distortion based on the estimated physical parameter of the semiconductor optical amplifier, and compensates for the nonlinear signal distortion using a gain coefficient having an inverse characteristic of the calculated gain coefficient. This is a signal distortion compensation method. [Effects of the Invention]

[0010] According to the present invention, it is possible to improve the accuracy of compensation for nonlinear distortion caused by a semiconductor optical amplifier. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram illustrating a configuration example of an optical transceiver according to the first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of the configuration of a transmission signal generating unit in the first embodiment. [Figure 3] FIG. 3 is a sequence diagram showing a processing flow of the optical transmission system in the first embodiment. [Figure 4] FIG. 3 is a sequence diagram showing a processing flow of the optical transmission system in the first embodiment. [Figure 5] FIG. 10 is a diagram illustrating an example of the configuration of an optical transceiver according to a second embodiment. [Figure 6] FIG. 10 is a diagram illustrating an example of the configuration of an optical transceiver according to a third embodiment. [Figure 7] FIG. 10 is a diagram illustrating a configuration example of an optical transceiver according to a fourth embodiment. [Figure 8] FIG. 10 is a diagram showing the relationship between the injection current and the physical parameters and the linewidth enhancement factor α. [Figure 9] FIG. 10 is a diagram for explaining the effect of the fourth embodiment. [Figure 10] FIG. 10 is a diagram showing a constellation when the injection current of the semiconductor optical amplifier is 350 mA for a high symbol rate signal of 168 GBaud. [Figure 11] FIG. 13 is a diagram illustrating an example of the configuration of an optical transceiver according to a fifth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] (First embodiment) FIG. 1 is a diagram illustrating an example of the configuration of an optical transceiver 1a according to the first embodiment. The optical transceiver 1a (optical transmission system) is a communication device that uses optical signals. The optical transceiver 1a includes an optical transmitter 2a, a transmission path 3, an optical receiver 4a, and a control channel 5. The optical transmitter 2a is a communication device that transmits optical signals. The optical transmitter 2a operates in multiple operation modes. For example, the optical transmitter 2a switches between a first operation mode and a second operation mode. The first operation mode is an operation mode used when estimating physical parameters of a semiconductor optical amplifier. When operating in the first operation mode, the optical transmitter 2a generates a low symbol rate signal. The second operation mode is an operation mode used after estimating the physical parameters of the semiconductor optical amplifier. When operating in the second operation mode, the optical transmitter 2a generates a high symbol rate signal. In the following description, an example will be described in which the low symbol rate signal is a 42 GBaud signal and the high symbol rate signal is a 168 GBaud signal. The low symbol rate signal and the high symbol rate signal are not limited to the above examples. The optical receiver 4a may include a semiconductor optical amplifier 22. The transmission line 3 may include a semiconductor optical amplifier 22.

[0014] The optical transmitter 2a includes an operation mode control unit 10, a transmission signal generation unit 20a, an optical modulator 21, and a semiconductor optical amplifier 22. The optical transmitter 2a includes the semiconductor optical amplifier 22 either internally or externally.

[0015] The operation mode control unit 10 controls the operation mode of the optical transmitter 2a. Specifically, the operation mode control unit 10 sets the optical transmitter 2a to operate in either a first operation mode or a second operation mode.

[0016] The transmission signal generation unit 20a generates a modulated signal sequence (XI, XQ, YI, YQ) based on a transmission data sequence that is binary information. The transmission signal generation unit 20a generates modulated signal sequences (XI, XQ, YI, YQ) with different symbol rates according to the operation mode set by the operation mode control unit 10. For example, when the operation mode set by the operation mode control unit 10 is the first operation mode, the transmission signal generation unit 20a generates a modulated signal sequence (XI, XQ, YI, YQ) signal with a low symbol rate (hereinafter referred to as a "low symbol rate signal"). For example, when the operation mode set by the operation mode control unit 10 is the second operation mode, the transmission signal generation unit 20a generates a modulated signal sequence (XI, XQ, YI, YQ) signal with a high symbol rate (hereinafter referred to as a "high symbol rate signal").

[0017] The optical modulator 21 includes a driver amplifier, a laser module, and a polarization multiplexed Mach-Zehnder type vector modulator (polarization multiplexed IQ modulator). The driver amplifier amplifies the amplitude of the transmission signal for each lane. The optical modulator 21 may include a direct modulation light source instead of a laser module. The optical modulator 21 may also be an intensity modulator. The optical modulator 21 modulates the modulation signal sequence. The optical modulator 21 outputs a polarization multiplexed optical modulated signal to the semiconductor optical amplifier 22. Note that the optical modulator 21 may also output a single-polarized optical modulated signal to the semiconductor optical amplifier 22.

[0018] The semiconductor optical amplifier 22 receives the modulated optical signal (optically modulated signal) from the optical modulator 21. The semiconductor optical amplifier 22 amplifies the intensity of the optically modulated signal. In the semiconductor optical amplifier 22, gain fluctuations occur depending on the intensity of the optically modulated signal due to carrier recombination within the semiconductor. This causes nonlinear signal distortion due to the pattern effect in the optical signal, degrading the signal characteristics. The semiconductor optical amplifier 22 outputs the optically modulated signal with nonlinear signal distortion to the optical receiver 4a via the transmission path 3.

[0019] As shown in Fig. 2, the transmission signal generation unit 20a includes a bit mapping unit 200, a waveform shaping unit 201, a nonlinear signal distortion compensation unit 202, and a DAC 203 (digital-to-analog converter). Fig. 2 is a diagram showing an example of the configuration of the transmission signal generation unit 20a in the first embodiment.

[0020] The bit mapping unit 200 acquires a transmission data sequence. The bit mapping unit 200 assigns transmission bits of the transmission data sequence to symbol points of a modulation method according to the operation mode. For example, the bit mapping unit 200 uses QPSK (Quadrature Phase Shift Keying) or 16QAM (Quadrature Amplitude Modulation) as the modulation method. Note that the symbol points may be symbol points of a modulation format other than QPSK or 16QAM.

[0021] The waveform shaping unit 201 performs filtering processing on the transmission signal using a raised cosine filter, a root raised cosine filter, or the like. The waveform shaping unit 201 may perform pre-signal compensation such as pre-chromatic dispersion compensation or pre-nonlinear optical effect compensation. The waveform shaping unit 201 may also perform compensation for the frequency characteristics of the electrical circuit or the optical modulator 21.

[0022] The nonlinear signal distortion compensator 202 acquires the transmission signal from the waveform shaping unit 201. In the first embodiment, the nonlinear signal distortion compensator 202 acquires estimated values ​​of physical parameters of the semiconductor optical amplifier 22 from the optical receiver 4a. The nonlinear signal distortion compensator 202 estimates nonlinear signal distortion caused by the semiconductor optical amplifier 22 in the unknown transmission signal based on the estimated values ​​of the physical parameters of the semiconductor optical amplifier 22, and compensates for the nonlinear signal distortion.

[0023] The nonlinear signal distortion compensator 202 calculates a gain coefficient h(t) of nonlinear signal distortion caused in the transmission signal by the semiconductor optical amplifier 22, based on estimated values ​​of physical parameters of the semiconductor optical amplifier 22. The nonlinear signal distortion compensator 202 calculates a value (exp((-h(t)(1+jα)) / 2)) representing the inverse characteristic of the gain coefficient h(t) of nonlinear signal distortion, using a gain coefficient -h(t) having the inverse characteristic of the gain coefficient h(t) of nonlinear signal distortion. The nonlinear signal distortion compensator 202 multiplies the transmission signal by the value (exp((-h(t)(1+jα)) / 2)) representing the inverse characteristic of the gain coefficient h(t) of nonlinear signal distortion. In this way, the nonlinear signal distortion compensator 202 can pre-equalize the nonlinear signal distortion caused in the transmission signal by the semiconductor optical amplifier 22.

[0024] When the optical transmitter 2a operates in the first operation mode, the nonlinear signal distortion compensator 202 sets the gain coefficient h(t) to 0. When the optical transmitter 2a operates in the second operation mode, the nonlinear signal distortion compensator 202 calculates the gain coefficient h(t) based on estimated values ​​of the physical parameters of the semiconductor optical amplifier 22.

[0025] The relationship between the optical signal acquired by the semiconductor optical amplifier 22 and the optical signal output by the semiconductor optical amplifier 22 is expressed by a physical model shown in equation (1).

[0026]

number

[0027] Here, E shown in equation (1) I(t) represents the complex amplitude of the optical signal (transmission signal from the transmission signal generator 20a) acquired by the semiconductor optical amplifier 22. o (t) represents the complex amplitude of the optical signal transmitted by the semiconductor optical amplifier 22 (the signal received by the optical receiver 4a). h(t) in equation (1) represents the gain coefficient. α in equation (1) represents the linewidth enhancement coefficient. j in equation (1) represents the imaginary unit. exp(h(t)(1+jα) / 2) in equation (1) represents the nonlinear signal distortion caused in the transmission signal by the semiconductor optical amplifier 22.

[0028] In the physical model shown in equation (1), the gain coefficient h(t) is expressed by the differential equation shown in equation (2).

[0029]

number

[0030] Here, τ shown in equation (2) c indicates the carrier lifetime. h0 in equation (2) indicates the non-saturation gain. P in equation (2) sat indicates the saturated output power. These, together with the linewidth enhancement factor α shown in equation (1), are physical parameters of the semiconductor optical amplifier 22. In the physical model shown in equation (1), if these physical parameters are determined, the behavior of the nonlinear signal distortion caused in the transmission signal by the semiconductor optical amplifier 22 can be expressed.

[0031] Equation (2) shows that the time change of the gain coefficient h(t) depends on the power of the optical signal acquired by the semiconductor optical amplifier 22. Therefore, if the physical parameters of the semiconductor optical amplifier 22 are known, the gain coefficient h(t) that depends on the power of the optical signal can be found from equation (2).

[0032] The nonlinear signal distortion compensator 202 can obtain the gain coefficient h(t) as a numerical solution from equation (2) by, for example, a time evolution solution using the Euler method or the Nth-order (N is a positive integer) Runge-Kutta method. When obtaining the gain coefficient h(t) from equation (2), the nonlinear signal distortion compensator 202 may use an analytical solution if one exists.

[0033] The nonlinear signal distortion compensation unit 202 calculates a value (exp(-h(t)(1+jα) / 2)) representing the inverse characteristic of the gain coefficient h(t) of the nonlinear signal distortion using a gain coefficient (-h(t)) having the inverse characteristic of the gain coefficient h(t).

[0034] The nonlinear signal distortion compensator 202 calculates a value (exp(-h(t)(1+jα) / 2)) representing the inverse characteristic of the gain coefficient h(t) of the nonlinear signal distortion as the transmission signal E I (t) to compensate for the nonlinear signal distortion that occurs in the transmission signal in the semiconductor optical amplifier 22. In this way, the nonlinear signal distortion compensator 202 can pre-equalize the nonlinear signal distortion that occurs in the transmission signal in the semiconductor optical amplifier 22.

[0035] The nonlinear signal distortion compensator 202 may obtain the gain coefficient h(t) from equation (2) by a solution other than the time evolution solution. When the optical transmitter 2a operates in the first operation mode, the transmission signal may be input to the DAC 203, bypassing the nonlinear signal distortion compensator 202. In other words, when the optical transmitter 2a operates in the first operation mode, the transmission signal may be output from the waveform shaping unit 201 directly to the DAC 203 without passing through the nonlinear signal distortion compensator 202.

[0036] The DAC 203 converts the digital signal into an analog signal and outputs the analog transmission signal to the optical modulator 21.

[0037] Returning to Figure 1, we continue the explanation. The transmission path 3 includes an optical fiber 30 and an optical amplifier 31. The transmission path 3 may further include a path switch for switching the path of the optical signal. The optical fiber 30 transmits an optical signal (e.g., an optically modulated signal). The optical amplifier 31 amplifies the intensity of the optical signal.

[0038] The optical receiver 4a is a communication device that receives an optical signal. The optical receiver 4a includes an optical coherent receiver 40, a digital signal processing unit 41, a switching unit 42, a received data demodulation unit 43, a nonlinear signal distortion estimation unit 44, and a physical parameter estimation unit 45. The optical receiver 4a further includes a laser module.

[0039] The optical coherent receiver 40 converts the optical signal transmitted through the transmission line 3 into a received signal (baseband signal), which is an analog signal, using local light emitted by a laser module.

[0040] The digital signal processing unit 41 converts the received signal, which is an analog signal, into a digital signal. The digital signal processing unit 41 uses digital signal processing to compensate for waveform degradation caused by chromatic dispersion, polarization fluctuation, and nonlinear optical effects that occur in the optical signal on the transmission path 3. The digital signal processing unit 41 uses digital signal processing to compensate for the frequency error between the laser of the optical transmitter 2a and the laser of the optical receiver 4a, and the phase noise caused by the linewidth of the lasers of the optical transmitter 2a and the optical receiver 4a.

[0041] The switching unit 42 switches the output path of the received signal output from the digital signal processing unit 41 depending on the operation mode of the optical transmitter 2a. The switching unit 42 is, for example, a switch. When the optical transmitter 2a is operating in the first operation mode, the switching unit 42 switches the path to connect the digital signal processing unit 41 and the nonlinear signal distortion estimation unit 44. As a result, the switching unit 42 outputs the received signal output from the digital signal processing unit 41 to the nonlinear signal distortion estimation unit 44. In this way, when the optical transmitter 2a is operating in the first operation mode, the switching unit 42 outputs the received signal to the nonlinear signal distortion estimation unit 44 in order to estimate physical parameters of the semiconductor optical amplifier 22.

[0042] When the optical transmitter 2a is operating in the second operation mode, the switching unit 42 switches the path to connect the digital signal processing unit 41 and the received data demodulation unit 43. As a result, the switching unit 42 outputs the received signal output from the digital signal processing unit 41 to the received data demodulation unit 43. The operation mode of the optical transmitter 2a may be notified from the optical transmitter 2a via a control channel 5, which may be a communication channel, NE-OpS (Network Element-Operations Systems), NW-OpS (Network-Operations System), or the like, or may be notified to the optical receiver 4a using an external device. The switching unit 42 may be incorporated in the digital signal processing unit 41.

[0043] The received data demodulation unit 43 acquires the received signal, for which waveform deterioration has been compensated, from the switching unit 42. The received data demodulation unit 43 demodulates the binary information of the transmission data sequence based on the received signal, which is a digital signal.

[0044] The nonlinear signal distortion estimator 44 acquires the received signal, which is a digital signal based on the transmission signal of the optical transmitter 2a, from the switcher 42. The nonlinear signal distortion estimator 44 acquires the transmission signal of the optical transmitter 2a as a reference signal from the optical transmitter 2a. For example, the nonlinear signal distortion estimator 44 acquires the transmission signal of the optical transmitter 2a as a reference signal from the optical transmitter 2a via the control channel 5. For example, the nonlinear signal distortion estimator 44 acquires a known signal that is part of the transmission data sequence as a reference signal from the optical transmitter 2a. For example, the nonlinear signal distortion estimator 44 acquires a series of symbol values ​​of the received signal as a reference signal from the optical transmitter 2a.

[0045] The nonlinear signal distortion estimation unit 44 estimates the measured optical signal E o (t) is the optical signal E I Based on the result of division by (t) and equation (1), the value (exp(h(t)(1+jα) / 2)) representing the nonlinear signal distortion caused in the transmission signal by the semiconductor optical amplifier 22 and the linewidth enhancement factor α are calculated as shown in equation (3).

[0046]

number

[0047] The nonlinear signal distortion estimation unit 44 may calculate the average value of the same symbol by using the reference signal as a repetitive signal. This allows the nonlinear signal distortion estimation unit 44 to reduce signal distortion caused by white noise. The nonlinear signal distortion estimation unit 44 can improve the accuracy of estimating nonlinear signal distortion.

[0048] When the nonlinear signal distortion due to the semiconductor optical amplifier 22 is large, the nonlinear signal distortion estimation unit 44 may not be able to accurately estimate the nonlinear signal distortion due to degradation of signal quality. The nonlinear signal distortion estimation unit 44 may repeat the estimation by feeding back a value that has been estimated once. This allows the nonlinear signal distortion estimation unit 44 to compensate for the transfer function with higher accuracy even when the nonlinear signal distortion due to the semiconductor optical amplifier 22 is large.

[0049] The measured physical parameters of the semiconductor optical amplifier 22 may differ from the design values ​​of the physical parameters of the semiconductor optical amplifier 22 due to individual differences caused by manufacturing errors of the semiconductor optical amplifier 22, etc. Therefore, the physical parameter estimator 45 estimates the physical parameters of the semiconductor optical amplifier 22 by digital signal processing. This allows the nonlinear signal distortion compensator 202 of the optical transmitter 2a to absorb the individual differences of the semiconductor optical amplifier 22 of the optical transmitter 2a and compensate for the nonlinear signal distortion. The nonlinear signal distortion compensator 202 can compensate for the nonlinear signal distortion even for a semiconductor optical amplifier 22 having unknown physical parameters. The physical parameter estimator 45 estimates the physical parameters of the semiconductor optical amplifier 22 based on the result of the nonlinear signal distortion estimated by the nonlinear signal distortion estimator 44 based on the reference signal.

[0050] The physical parameter estimation unit 45 estimates the physical parameters of the semiconductor optical amplifier 22 based on the gain coefficient h(t) calculated using equation (3) and equation (2). The physical parameters of the semiconductor optical amplifier 22 include, for example, the carrier lifetime τc , unsaturated gain h0, saturated output P sat Specifically, the physical parameter estimation unit 45 calculates the carrier lifetime τ from the discrete equation of equation (2) using the least squares method so as to minimize the sum of squares of the residuals shown in equation (4). c , unsaturated gain h0, saturated output P sat Ask for.

[0051]

number

[0052] There is no particular limitation to the method by which the physical parameter estimation unit 45 estimates the physical parameters of the semiconductor optical amplifier 22. For example, the physical parameter estimation unit 45 may estimate the physical parameters of the semiconductor optical amplifier 22 by fitting using the least squares method, calculation using simultaneous equations, or the like.

[0053] The physical parameter estimation unit 45 feeds back the physical parameters of the semiconductor optical amplifier 22 to the nonlinear signal distortion compensation unit 202 via the control channel 5 .

[0054] If the optical transmitter 2a and the optical receiver 4a are directly connected by a dedicated line, the physical parameter estimator 45 may estimate the physical parameters of the semiconductor optical amplifier 22 based on the optical signal transmitted through the dedicated line instead of the transmission path 3. After estimating the physical parameters of the semiconductor optical amplifier 22, the physical parameter estimator 45 may transmit to the optical transmitter 2a and the received data demodulator 43 a signal obtained by removing the reference signal from the transmission signal to which the reference signal was added. If it is necessary to re-estimate the physical parameters after this, the physical parameter estimator 45 may add the reference signal back to the transmission signal. If the current injected into the semiconductor optical amplifier 22 does not change or the intensity of the optical signal acquired by the semiconductor optical amplifier 22 does not change, the physical parameter estimator 45 can continuously use the estimated results of the physical parameters of the semiconductor optical amplifier 22. In these cases, the physical parameter estimator 45 may temporarily suspend the estimation of the physical parameters of the semiconductor optical amplifier 22 or may continuously and periodically calculate the physical parameters of the semiconductor optical amplifier 22. The physical parameter estimation unit 45 recalculates the physical parameters of the semiconductor optical amplifier 22 when the current injected into the semiconductor optical amplifier 22 does not change or when the intensity of the optical signal acquired by the semiconductor optical amplifier 22 changes.

[0055] 3 and 4 are sequence diagrams showing the flow of processing in the optical transmission system in the first embodiment. In response to an external instruction, the operation mode control unit 10 sets the operation mode of the optical transmitter 2a to the first operation mode (step S101). Note that, in response to the operation mode being set by the operation mode control unit 10, the optical transmitter 2a may notify the optical receiver 4a of information about the operation mode. Since the operation mode set by the operation mode control unit 10 is the first operation mode, the transmission signal generation unit 20a generates a low symbol rate signal (step S102). Note that, since the optical transmitter 2a is operating in the first operation mode, the nonlinear signal distortion compensation unit 202 performs processing with the gain coefficient h(t) of nonlinear signal distortion set to 0. The transmission signal generation unit 20a outputs the generated low symbol rate signal to the optical modulator 21.

[0056] The optical modulator 21 generates an optical modulated signal by modulating the low symbol rate signal output from the transmission signal generator 20a (step S103). The optical modulator 21 outputs the generated optical modulated signal to the semiconductor optical amplifier 22. The semiconductor optical amplifier 22 amplifies the intensity of the optical modulated signal output from the optical modulator 21 (step S104). The semiconductor optical amplifier 22 transmits the amplified optical modulated signal to the optical receiver 4a via the transmission path 3 (step S105).

[0057] The optical coherent receiver 40 of the optical receiver 4a converts the optically modulated signal transmitted over the transmission path 3 into an analog received signal (baseband signal) using local light emitted by the laser module (step S106). The optical coherent receiver 40 outputs the converted received signal to the digital signal processing unit 41. The digital signal processing unit 41 converts the received signal output from the optical coherent receiver 40 into a digital received signal (step S107). The digital signal processing unit 41 outputs the digital received signal to the switching unit 42.

[0058] The switching unit 42 is notified that the optical transmitter 2a is operating in the first operation mode. The switching unit 42 then outputs the received signal output from the digital signal processing unit 41 to the nonlinear signal distortion estimation unit 44 (step S108). The nonlinear signal distortion estimation unit 44 estimates nonlinear signal distortion based on the received signal output from the switching unit 42 (step S109). The nonlinear signal distortion estimation unit 44 outputs the result of estimating the nonlinear signal distortion to the physical parameter estimation unit 45. The physical parameter estimation unit 45 estimates physical parameters of the semiconductor optical amplifier 22 based on the result of estimating the nonlinear signal distortion based on the reference signal by the nonlinear signal distortion estimation unit 44 (step S110). The physical parameter estimation unit 45 notifies the optical transmitter 2a of the estimated physical parameters of the semiconductor optical amplifier 22 via the control channel 5 (step S111).

[0059] In response to an external instruction, the operation mode control unit 10 sets the operation mode of the optical transmitter 2a to the second operation mode (step S112). In response to the operation mode being set by the operation mode control unit 10, the optical transmitter 2a may notify the optical receiver 4a of information about the operation mode. Since the operation mode set by the operation mode control unit 10 is the second operation mode, the transmission signal generation unit 20a generates a high symbol rate signal (step S113). Since the optical transmitter 2a is operating in the second operation mode, the nonlinear signal distortion compensation unit 202 estimates nonlinear signal distortion based on the estimated values ​​of the physical parameters of the semiconductor optical amplifier 22 notified by the optical receiver 4a, and compensates for the nonlinear signal distortion. The transmission signal generation unit 20a outputs the generated high symbol rate signal to the optical modulator 21.

[0060] The optical modulator 21 generates an optical modulated signal by modulating the high symbol rate signal output from the transmission signal generator 20a (step S114). The optical modulator 21 outputs the generated optical modulated signal to the semiconductor optical amplifier 22. The semiconductor optical amplifier 22 amplifies the intensity of the optical modulated signal output from the optical modulator 21 (step S115). The semiconductor optical amplifier 22 transmits the amplified optical modulated signal to the optical receiver 4a via the transmission path 3 (step S116).

[0061] The optical coherent receiver 40 of the optical receiver 4a converts the optical modulated signal transmitted over the transmission path 3 into an analog received signal (baseband signal) using local light emitted by the laser module (step S117). The optical coherent receiver 40 outputs the converted received signal to the digital signal processing unit 41. The digital signal processing unit 41 converts the received signal output from the optical coherent receiver 40 into a digital received signal (step S118). The digital signal processing unit 41 outputs the digital received signal to the switching unit 42.

[0062] The switching unit 42 is notified that the optical transmitter 2a is operating in the second operation mode. Therefore, the switching unit 42 outputs the reception signal output from the digital signal processing unit 41 to the reception data demodulation unit 43 (step S119). The reception data demodulation unit 43 demodulates the binary information of the transmission data sequence based on the reception signal output from the switching unit 42 (step S120).

[0063] The optical transceiver 1a of the first embodiment configured as described above can improve the accuracy of compensation for nonlinear distortion caused by the semiconductor optical amplifier 22. Specifically, in the optical transceiver 1a, when the optical transmitter 2a operates in the first operation mode, it transmits an optical signal based on a low symbol rate signal to the optical receiver 4a. The optical receiver 4a estimates the nonlinear signal distortion of the semiconductor optical amplifier 22 caused when the optical signal is amplified by the semiconductor optical amplifier 22, based on the optical signal based on the low symbol rate signal. The optical receiver 4a then estimates physical parameters of the semiconductor optical amplifier 22 based on the estimated nonlinear signal distortion. While the nonlinear distortion caused by the semiconductor optical amplifier 22 decreases as the symbol rate of the signal increases, the nonlinear distortion caused by the semiconductor optical amplifier 22 is greater for low symbol rate signals than for high symbol rate signals. As a result, the optical receiver 4a can accurately estimate the physical parameters of the semiconductor optical amplifier 22. In the optical transmitter 2 a, by using the physical parameters of the semiconductor optical amplifier 22 estimated using the low symbol rate signal to accurately calculate the gain coefficient of the nonlinear signal distortion when the high symbol rate signal is amplified by the semiconductor optical amplifier 22, it becomes possible to improve the accuracy of compensation for the nonlinear distortion caused by the semiconductor optical amplifier 22 for the high symbol rate signal.

[0064] (Second embodiment) The second embodiment differs from the first embodiment in that the optical transmitter includes a physical parameter estimation unit 45. The second embodiment will be described mainly focusing on the differences from the first embodiment.

[0065] 5 is a diagram illustrating an example of the configuration of an optical transceiver 1b according to the second embodiment. The optical transceiver 1b includes an optical transmitter 2b, a transmission path 3, an optical receiver 4b, and a control channel 5. The optical transmitter 2b includes an operation mode control unit 10, a transmission signal generation unit 20a, an optical modulator 21, a semiconductor optical amplifier 22, and a physical parameter estimation unit 45. The optical transmitter 2b includes the semiconductor optical amplifier 22 either internally or externally. The optical receiver 4b includes an optical coherent receiver 40, a digital signal processing unit 41, a switching unit 42, a received data demodulation unit 43, and a nonlinear signal distortion estimation unit 44. The optical receiver 4b may include the semiconductor optical amplifier 22. The transmission path 3 may include the semiconductor optical amplifier 22.

[0066] The nonlinear signal distortion estimator 44 transmits the gain coefficient h(t) and the linewidth enhancement coefficient α, which are the results of estimating the nonlinear signal distortion, to a physical parameter estimator 45 included in the optical transmitter 2b via the control channel 5.

[0067] The physical parameter estimator 45 estimates the physical parameters of the semiconductor optical amplifier 22 based on the result of the nonlinear signal distortion estimator 44 estimating the nonlinear signal distortion based on the reference signal.

[0068] According to the optical transceiver 1b of the first embodiment configured as above, it is possible to obtain the same effects as those of the first embodiment.

[0069] (Third embodiment) The third embodiment differs from the second embodiment in that the optical transmitter includes a nonlinear signal distortion estimation unit 44 and a physical parameter estimation unit 45. The third embodiment will be described mainly focusing on the differences from the second embodiment.

[0070] 6 is a diagram illustrating an example of the configuration of an optical transceiver 1c according to the third embodiment. The optical transceiver 1c includes an optical transmitter 2c, a transmission path 3, an optical receiver 4c, and a control channel 5. The optical transmitter 2c includes an operation mode control unit 10, a transmission signal generation unit 20c, an optical modulator 21, a semiconductor optical amplifier 22, a nonlinear signal distortion estimation unit 44, and a physical parameter estimation unit 45. The optical transmitter 2c includes the semiconductor optical amplifier 22 either internally or externally. The optical receiver 4c includes an optical coherent receiver 40, a digital signal processing unit 41, a switching unit 42, and a received data demodulation unit 43. The optical receiver 4c may include the semiconductor optical amplifier 22. The transmission path 3 may include the semiconductor optical amplifier 22.

[0071] When the optical transmitter 2c operates in the first operation mode, the switching unit 42 transmits the received signal output from the digital signal processing unit 41 to a nonlinear signal distortion estimating unit 44 included in the optical transmitter 2c via the control channel 5. The nonlinear signal distortion estimating unit 44 outputs the gain coefficient h(t) and the linewidth enhancement coefficient α, which are the results of estimating the nonlinear signal distortion, to a physical parameter estimating unit 45. The physical parameter estimating unit 45 estimates the physical parameters of the semiconductor optical amplifier 22 based on the results of the nonlinear signal distortion estimating unit 44 estimating the nonlinear signal distortion based on the reference signal.

[0072] According to the optical transceiver 1c of the third embodiment configured as above, it is possible to obtain the same effects as those of the first embodiment.

[0073] (Fourth embodiment) The fourth embodiment differs from the first embodiment in that the optical receiver includes a nonlinear signal distortion compensation unit 202 in the subsequent stage of the coherent optical receiver 40. The fourth embodiment will be described mainly focusing on the differences from the first embodiment.

[0074] 7 is a diagram showing an example of the configuration of an optical transceiver 1d according to the fourth embodiment. The optical transceiver 1d includes an optical transmitter 2d, a transmission path 3, an optical receiver 4d, and a control channel 5. The optical transmitter 2d includes an operation mode control unit 10, a transmission signal generation unit 20d, an optical modulator 21, and a semiconductor optical amplifier 22. The optical transmitter 2d includes the semiconductor optical amplifier 22 either internally or externally. The transmission signal generation unit 20d includes a bit mapping unit 200, a waveform shaping unit 201, and a DAC 203.

[0075] The optical receiver 4d includes an optical coherent receiver 40, a digital signal processing unit 41, a switching unit 42, a received data demodulation unit 43, a nonlinear signal distortion estimation unit 44, a physical parameter estimation unit 45, and a nonlinear signal distortion compensation unit 202. The optical receiver 4d includes the nonlinear signal distortion compensation unit 202 in a stage subsequent to the optical coherent receiver 40. The optical receiver 4d includes the digital signal processing unit 41 in a stage subsequent to the nonlinear signal distortion compensation unit 202. The optical receiver 4d may include a semiconductor optical amplifier 22. The transmission path 3 may include the semiconductor optical amplifier 22.

[0076] The switching unit 42 switches the output path of the received signal output from the digital signal processing unit 41 depending on the operation mode of the optical transmitter 2d. The switching unit 42 is, for example, a switch. When the optical transmitter 2d is operating in the first operation mode, the switching unit 42 switches the path to connect the digital signal processing unit 41 and the nonlinear signal distortion estimation unit 44. As a result, the switching unit 42 outputs the received signal output from the digital signal processing unit 41 to the nonlinear signal distortion estimation unit 44. In this way, when the optical transmitter 2d is operating in the first operation mode, the switching unit 42 outputs the received signal to the nonlinear signal distortion estimation unit 44 in order to estimate physical parameters of the semiconductor optical amplifier 22.

[0077] When the optical transmitter 2d is operating in the second operation mode, the switching unit 42 switches the path to connect the digital signal processing unit 41 and the received data demodulation unit 43. As a result, the switching unit 42 outputs the received signal output from the digital signal processing unit 41 to the received data demodulation unit 43. The operation mode of the optical transmitter 2d may be notified from the optical transmitter 2d via the control channel 5, or may be notified to the optical receiver 4d using an external device. The switching unit 42 may be incorporated in the digital signal processing unit 41.

[0078] The nonlinear signal distortion compensator 202 obtains the received signal, which is an analog signal, from the optical coherent receiver 40. The nonlinear signal distortion compensator 202 obtains the physical parameters of the semiconductor optical amplifier 22 from the physical parameter estimator 45.

[0079] The nonlinear signal distortion compensator 202 outputs an analog signal corresponding to a value (exp(-h(t)(1+jα) / 2)) representing the inverse characteristic of the gain coefficient h(t) of the nonlinear signal distortion to the received signal E o By multiplying (t) by (t), the nonlinear signal distortion occurring in the transmission signal in the semiconductor optical amplifier 22 is compensated for. In this way, the nonlinear signal distortion compensator 202 can equalize the nonlinear signal distortion occurring in the transmission signal by the semiconductor optical amplifier 22.

[0080] When the optical transmitter 2d operates in the first operation mode, the nonlinear signal distortion compensator 202 sets the gain coefficient h(t) to 0. When the optical transmitter 2d operates in the second operation mode, the nonlinear signal distortion compensator 202 calculates the gain coefficient h(t) based on estimated values ​​of physical parameters of the semiconductor optical amplifier 22. The operation mode of the optical transmitter 2d may be notified from the optical transmitter 2d via the control channel 5, or may be notified to the optical receiver 4d using an external device. When the operation mode of the optical transmitter 2d is not notified from the optical transmitter 2d via the control channel 5, the optical transceiver 1d does not need to include the control channel 5.

[0081] When the nonlinear signal distortion compensator 202 determines the gain coefficient h(t) as a numerical solution from equation (2), the gain coefficient h(t) is determined based on the following equations (5) to (9). This also applies to the first to third embodiments and the fifth embodiment described later.

[0082]

number

[0083]

number

[0084]

number

[0085]

number

[0086]

number

[0087] In addition, when the gain coefficient h(t) is calculated on the optical receiver 4d side (when the nonlinear signal distortion compensator 202 is provided in the optical receiver 4d) as in this embodiment, the nonlinear signal distortion compensator 202 calculates the gain coefficient h(t) of the transmission signal E I Instead of the received signal E o Calculate the gain coefficient h(t) using

[0088]

number

[0089] Next, the effects of the present invention will be described. Fig. 8 is a diagram showing the relationship between the injection current, the physical parameter, and the linewidth enhancement factor α. Here, the injection current represents the current injected into the semiconductor optical amplifier 22. Fig. 8(a) shows the relationship between the injection current and the linewidth enhancement factor α, and Fig. 8(b) shows the relationship between the injection current and the physical parameter, the carrier lifetime τ c 8(c) shows the relationship between the injection current and the non-saturation gain h0, which is a physical parameter, and FIG. 8(d) shows the relationship between the injection current and the saturated output P sat This shows the relationship between

[0090] 9 is a diagram illustrating the effects of the present invention. As shown in FIG. 9, for a high symbol rate signal of 168 Gbaud, an SNR penalty of 0.96 dB occurs without distortion compensation in the semiconductor optical amplifier 22. However, distortion compensation in the semiconductor optical amplifier 22 using physical parameters estimated with a low symbol rate signal of 42 GBaud can reduce the SNR penalty to 0.29 dB or less. It can be seen that distortion compensation in the semiconductor optical amplifier 22 using physical parameters estimated with a high symbol rate signal of 168 GBaud does not improve the SNR penalty. In each embodiment, the high symbol rate signal is, for example, a signal exceeding 100 GBaud, and the low symbol rate signal is, for example, a signal of 100 GBaud or less.

[0091] Fig. 10 shows the constellation for a high symbol rate signal of 168 GBaud when the injection current of the semiconductor optical amplifier 22 is 350 mA. Fig. 10(a) shows the constellation when the semiconductor optical amplifier 22 is not compensated for distortion, and Fig. 10(b) shows the constellation when the semiconductor optical amplifier 22 is compensated for distortion using physical parameters estimated with a 42 GBaud signal. As is clear from Fig. 10, the constellation when compensation is performed using physical parameters estimated with a 42 GBaud signal has more compensated for nonlinear distortion than the constellation when no compensation is performed.

[0092] The optical transceiver 1d of the fourth embodiment configured as described above can improve the accuracy of compensation for nonlinear distortion caused by the semiconductor optical amplifier 22. Specifically, in the optical transceiver 1d, when the optical transmitter 2d operates in the first operation mode, the optical transceiver 1d transmits an optical signal based on a low symbol-rate signal to the optical receiver 4d. The optical receiver 4d estimates the nonlinear signal distortion of the semiconductor optical amplifier 22 that occurs when the semiconductor optical amplifier 22 amplifies the optical signal based on the low symbol-rate signal. The optical receiver 4d then estimates physical parameters of the semiconductor optical amplifier 22 based on the estimated nonlinear signal distortion. The optical receiver 4d then uses the physical parameters of the semiconductor optical amplifier 22 estimated using the low symbol-rate signal to accurately calculate the gain coefficient of the nonlinear signal distortion that occurs when the semiconductor optical amplifier 22 amplifies a high symbol-rate signal. This makes it possible to improve the accuracy of compensation for nonlinear distortion of the high symbol-rate signal that occurs in the semiconductor optical amplifier 22.

[0093] (Fifth embodiment) The fifth embodiment differs from the fourth embodiment in that the optical receiver includes a nonlinear signal distortion compensation unit 202 after the digital signal processing unit 41. The fifth embodiment will be described mainly focusing on the differences from the fourth embodiment.

[0094] 11 is a diagram showing an example of the configuration of an optical transceiver 1e according to the fifth embodiment. The optical transceiver 1e includes an optical transmitter 2e, a transmission path 3, an optical receiver 4e, and a control channel 5. The optical transmitter 2e includes an operation mode control unit 10, a transmission signal generation unit 20d, an optical modulator 21, and a semiconductor optical amplifier 22. The optical transmitter 2e includes the semiconductor optical amplifier 22 either internally or externally. The transmission signal generation unit 20e includes a bit mapping unit 200, a waveform shaping unit 201, and a DAC 203.

[0095] The optical receiver 4e includes an optical coherent receiver 40, a digital signal processing unit 41, a switching unit 42, a received data demodulation unit 43, a nonlinear signal distortion estimation unit 44, a physical parameter estimation unit 45, and a nonlinear signal distortion compensation unit 202. The optical receiver 4e includes the switching unit 42 and the nonlinear signal distortion compensation unit 202 after the digital signal processing unit 41. The optical receiver 4e includes the received data demodulation unit 43 after the nonlinear signal distortion compensation unit 202. The optical receiver 4e may include a semiconductor optical amplifier 22. The transmission path 3 may include the semiconductor optical amplifier 22.

[0096] The switching unit 42 switches the output path of the received signal output from the digital signal processing unit 41 depending on the operation mode of the optical transmitter 2e. The switching unit 42 is, for example, a switch. When the optical transmitter 2e is operating in the first operation mode, the switching unit 42 switches the path to connect the digital signal processing unit 41 and the nonlinear signal distortion estimation unit 44. As a result, the switching unit 42 outputs the received signal output from the digital signal processing unit 41 to the nonlinear signal distortion estimation unit 44. In this way, when the optical transmitter 2e is operating in the first operation mode, the switching unit 42 outputs the received signal to the nonlinear signal distortion estimation unit 44 in order to estimate physical parameters of the semiconductor optical amplifier 22.

[0097] When the optical transmitter 2e is operating in the second operation mode, the switching unit 42 switches the path to connect the digital signal processing unit 41 and the nonlinear signal distortion compensator 202. As a result, the switching unit 42 outputs the received signal output from the digital signal processing unit 41 to the nonlinear signal distortion compensator 202. The operation mode of the optical transmitter 2e may be notified from the optical transmitter 2e via the control channel 5, or may be notified to the optical receiver 4e using an external device. The switching unit 42 may be incorporated in the digital signal processing unit 41.

[0098] The nonlinear signal distortion compensator 202 acquires the received signal, which is a digital signal, from the switch 42. The nonlinear signal distortion compensator 202 acquires the physical parameters of the semiconductor optical amplifier 22 from the physical parameter estimator 45.

[0099] The nonlinear signal distortion compensator 202 applies a value (exp(-h(t)(1+jα) / 2)) representing the inverse characteristic of the gain coefficient h(t) of the nonlinear signal distortion to the received signal E o By multiplying (t) by (t), the nonlinear signal distortion occurring in the transmission signal in the semiconductor optical amplifier 22 is compensated for. In this way, the nonlinear signal distortion compensator 202 can equalize the nonlinear signal distortion occurring in the transmission signal by the semiconductor optical amplifier 22.

[0100] According to the optical transceiver 1e of the fifth embodiment configured as above, it is possible to obtain the same effects as those of the fourth embodiment.

[0101] At least a portion of the optical transmitter, optical receiver, and optical transceiver in the above-described embodiments may be implemented by a computer. In this case, a program for implementing the functions may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. Note that the term "computer system" herein includes hardware such as an OS and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into a computer system. Furthermore, the term "computer-readable recording medium" may also include devices that dynamically store programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or telephone lines, or devices that store programs for a fixed period of time, such as volatile memory within a computer system serving as a server or client. The program may be for implementing some of the above-described functions, or may be capable of implementing the above-described functions in combination with a program already stored in the computer system, or may be implemented using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0102] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Industrial Applicability]

[0103] The present invention is applicable to communication equipment in an optical transmission system. [Explanation of symbols]

[0104] 1a to 1e...Optical transceiver, 2a...Optical transmitter, 3...Transmission path, 4a to 4e...Optical receiver, 5...Control channel, 20a to 20e...Transmission signal generation unit, 21...Optical modulator, 22...Semiconductor optical amplifier, 30...Optical fiber, 31...Optical amplifier, 40...Optical coherent receiver, 41...Digital signal processing unit, 42...Switching unit, 43...Received data demodulation unit, 44...Nonlinear signal distortion estimation unit, 45...Physical parameter estimation unit, 200...Bit mapping unit, 201...Waveform shaping unit, 202...Nonlinear signal distortion compensation unit, 203...DAC

Claims

1. An optical transmission system comprising an optical transmitter and an optical receiver, The optical transmitter comprises: an operation mode control unit that sets either a first operation mode in which a signal having a low symbol rate is generated or a second operation mode in which a signal having a high symbol rate is generated; a transmission signal generating unit that generates an electrical transmission signal according to the operation mode set by the operation mode control unit, The optical receiver includes: an optical coherent receiver that receives an optical signal based on the transmission signal and generates a received signal based on the optical signal; a digital signal processing unit that performs digital signal processing on the received signal; a received data demodulation unit that demodulates binary information of a transmission data sequence based on the received signal that has been digitally processed, The optical transmitter or the optical receiver comprises: a nonlinear signal distortion compensator that calculates a gain coefficient of nonlinear signal distortion caused in an optical signal based on a transmission signal by a semiconductor optical amplifier based on estimated values ​​of physical parameters of the semiconductor optical amplifier, and multiplies the transmission signal by a value that represents an inverse characteristic of the gain coefficient; a nonlinear signal distortion estimation unit that acquires the transmission signal and estimates the nonlinear signal distortion based on a comparison result between the transmission signal and the received signal; a physical parameter estimator configured to estimate physical parameters of the semiconductor optical amplifier based on the nonlinear signal distortion estimated by the nonlinear signal distortion estimator, the optical receiver switches an output destination so that the received signal that has been subjected to the digital signal processing is output to the nonlinear signal distortion estimation unit when the optical transmitter is operating in a first operation mode, and switches an output destination so that the received signal that has been subjected to the digital signal processing is output to the received data demodulation unit when the optical transmitter is operating in a second operation mode; When the optical transmitter is operating in a first operation mode, the optical transmitter transmits an optical signal to the optical receiver in a state in which the nonlinear signal distortion compensator does not compensate for nonlinear signal distortion caused in an optical signal based on a transmission signal by the semiconductor optical amplifier, the nonlinear signal distortion estimator estimates the nonlinear signal distortion, and the physical parameter estimator estimates physical parameters of the semiconductor optical amplifier based on the estimated nonlinear signal distortion. Thereafter, the optical transmitter is operated in a second operation mode, and the nonlinear signal distortion compensator calculates a gain coefficient of the nonlinear signal distortion based on the physical parameters of the semiconductor optical amplifier estimated by the physical parameter estimator, and compensates for the nonlinear signal distortion using a gain coefficient having an inverse characteristic of the calculated gain coefficient. Optical transmission system.

2. the optical transmitter includes the nonlinear signal distortion compensator, The optical receiver includes: the nonlinear signal distortion estimation unit; the physical parameter estimation unit, 2. The optical transmission system according to claim 1.

3. The optical transmitter comprises: the nonlinear signal distortion compensation unit; the physical parameter estimation unit, The optical receiver includes: The nonlinear signal distortion estimation unit is provided.

2. The optical transmission system according to claim 1.

4. The optical transmitter comprises: the nonlinear signal distortion compensation unit; the nonlinear signal distortion estimation unit; the physical parameter estimation unit, 2. The optical transmission system according to claim 1.

5. The optical receiver includes: the nonlinear signal distortion compensation unit; the nonlinear signal distortion estimation unit; the physical parameter estimation unit, 2. The optical transmission system according to claim 1.

6. A signal distortion compensation method performed by an optical transmission system including an optical transmitter and an optical receiver, comprising: the optical transmitter is configured to set either a first operation mode for generating a low symbol rate signal or a second operation mode for generating a high symbol rate signal; the optical transmitter generates an electrical transmission signal according to a set operation mode; the optical receiver acquires an optical signal based on the transmission signal and generates a received signal based on the optical signal; the optical receiver performs digital signal processing on the received signal; the optical receiver demodulates binary information of the transmission data sequence based on the received signal that has been subjected to the digital signal processing; The optical transmitter or the optical receiver is acquiring the transmitted signal and estimating nonlinear signal distortion based on a comparison of the transmitted signal and the received signal; estimating physical parameters of the semiconductor optical amplifier based on the estimated nonlinear signal distortion; calculating a gain coefficient of nonlinear signal distortion caused by the semiconductor optical amplifier in an optical signal based on a transmission signal based on estimated values ​​of physical parameters of the semiconductor optical amplifier; multiplying the transmission signal by a value representing an inverse characteristic of the gain coefficient; the optical receiver switches an output destination when the optical transmitter is operating in a first operation mode so that the received signal that has been subjected to the digital signal processing is output to a nonlinear signal distortion estimation unit that estimates the nonlinear signal distortion, and when the optical transmitter is operating in a second operation mode, switches an output destination so that the received signal that has been subjected to the digital signal processing is output to a received data demodulation unit that demodulates binary information of the transmission data series; When the optical transmitter is operating in a first operation mode, the optical transmitter transmits an optical signal to the optical receiver without compensating for nonlinear signal distortion caused by the semiconductor optical amplifier in an optical signal based on the transmission signal, the nonlinear signal distortion estimator estimates the nonlinear signal distortion, the optical transmitter or the optical receiver estimates physical parameters of the semiconductor optical amplifier based on the estimated nonlinear signal distortion, and then the optical transmitter operates in a second operation mode, and the optical transmitter or the optical receiver calculates a gain coefficient of the nonlinear signal distortion based on the estimated physical parameters of the semiconductor optical amplifier, and compensates for the nonlinear signal distortion using a gain coefficient having an inverse characteristic of the calculated gain coefficient. Signal distortion compensation method.

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