Optical receiver, optical reception method, optical reception program, and recording medium

Abstract

This optical receiver comprises: an optical coherent detection unit (210) that receives modulated light optically modulated at a carrier frequency, causes the received modulated light to interfere with interference light, performs coherent detection, and outputs an analog electrical signal; an analog / digital conversion unit (230) that analog / digital-converts the analog electrical signal from the optical coherent detection unit (210) to obtain a digital signal; and a reception-side digital signal processing unit (240) that has a frequency shift unit (241) that performs rough compensation for shifting the center frequency of the digital signal obtained by the analog / digital conversion unit (230) to a frequency at which the frequency difference between transmission and reception is reduced, and a frequency difference estimation compensation unit (243) that precisely compensates the center frequency of the digital signal on which the rough compensation of the frequency difference has been performed by the frequency shift unit (241).

Description

Optical receiver, optical receiving method, optical receiving program, and recording medium

[0001] The present disclosure relates to an optical receiver and an optical receiving method used in digital coherent optical communications.

[0002] In the field of optical fiber communications, digital coherent technology is widely applied in metro / core networks and submarine optical cable systems. Digital coherent optical communications has been gaining popularity since the 2010s, and with this progress, its application to satellite-mounted optical communication terminals has been considered in addition to terrestrial optical fiber networks.

[0003] In response to these advances in optical networks, a single digital coherent optical transceiver can generate multiple subcarrier signals in the digital domain that do not interfere with each other on the frequency axis, and can simultaneously accommodate a variety of services by assigning different information to each subcarrier, thereby achieving efficient hardware utilization and space savings. Non-Patent Document 1 discloses digital coherent optical communication technology capable of transmitting and receiving such subcarrier signals.

[0004] H. Sun et al, “800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 38, NO. 17, SEPTEMBER 1, 2020, pp4744-4756

[0005] The optical receiving device disclosed in Non-Patent Document 1 detects signals using optical coherent detection and converts the output voltage signal into a digital signal using an analog-to-digital converter (ADC). If the sampling rate of the ADC is high, the analog received signal before analog-to-digital conversion can be accurately acquired over a wider frequency space, which in turn allows for precise compensation for waveform distortion in digital signal processing and improves the quality of the received signal.

[0006] However, when an FPGA (Field Programmable Gate Array) with a sampling rate or throughput of several gigabits is used for digital signal processing, or when a low- to medium-speed digital coherent signal is handled in an optical receiver, such as when the modulation rate of each subcarrier is low due to a subcarrier multiplexing method, if there is a frequency difference between the transmitted and received optical carrier waves, the ability to compensate for waveform distortion in the digital signal processing on the receiving side will be reduced. In particular, if the frequency difference between the transmitted and received optical carrier waves is large, the waveform will not be able to be accurately represented, which may result in difficulty in compensating for waveform distortion.

[0007] The present disclosure has been made in consideration of the above points, and aims to provide an optical receiver that can compensate for the difference in frequency between the transmitted and received optical carrier waves, even if a difference in frequency occurs between the transmitted and received optical carrier waves.

[0008] The optical receiver according to the present disclosure includes an optical coherent detection unit that receives modulated light optically modulated with a carrier frequency, causes the received modulated light to interfere with interference light, performs coherent detection, and outputs an analog electrical signal, an analog / digital conversion unit that analog / digital converts the analog electrical signal from the optical coherent detection unit to obtain a digital signal, and a receiving-side digital signal processing unit that has a frequency shift unit that performs coarse compensation by shifting the center frequency of the digital signal obtained by the analog / digital conversion unit to a frequency that reduces the frequency difference between transmission and reception, and a frequency difference estimation compensation unit that finely compensates the center frequency of the digital signal whose frequency difference has been coarsely compensated for by the frequency shift unit.

[0009] According to the present disclosure, even if a difference in frequency occurs between the transmitted and received optical carrier waves, waveform distortion can be compensated for.

[0010] 1 is a block diagram showing an optical transmission / reception device including an optical receiver according to a first embodiment. FIG. 2 is a block diagram showing a receiving-side digital signal processing unit in the optical receiver according to the first embodiment. FIG. 3 is a diagram showing an example of the frequency relationship between transmission and reception optical carriers in a digital coherent system (the estimated value of the frequency difference is within a tolerable range). FIG. 4 is a diagram showing the frequency (signal spectrum) of a digital signal obtained by an ADC unit input to a receiving-side DSP when the transmission and reception optical carriers have the relationship of FIG. 3. FIG. 5 is a diagram showing another example of the frequency relationship between transmission and reception optical carriers in a digital coherent system (the estimated value of the frequency difference is outside the tolerable range and within a compensable range). FIG. 6 is a diagram showing the frequency (signal spectrum) of a digital signal obtained by an ADC unit input to a receiving-side DSP when the transmission and reception optical carriers have the relationship of FIG. 5. FIG. 7 is a diagram showing coarse compensation of the frequency (signal spectrum) of a digital signal input to a receiving-side DSP 240 in an optical receiver according to the first embodiment. FIG. 8 is a diagram showing a frequency difference in an IQ signal plane space where the frequency difference between transmission and reception can be compensated. FIG. 9 is a diagram showing a frequency difference in an IQ signal plane space where the frequency difference between transmission and reception cannot be compensated. FIG. 1 is a flowchart showing the operation of the frequency shift unit 241 and the frequency shift control unit 246 with respect to the estimated value of the frequency difference obtained by the frequency difference estimating / compensating unit 243 in the optical receiver according to the first embodiment. FIG. 2 is a diagram schematically explaining the transition of the estimated value of the frequency difference between transmission and reception over time in the optical receiver according to the first embodiment. FIG. 3 is a diagram schematically showing an example in which the estimated value of the frequency difference between transmission and reception is within the allowable range in the optical receiver according to the first embodiment. FIG. 4 is a diagram schematically showing an example for explaining coarse compensation when the estimated value of the frequency difference between transmission and reception deviates from the allowable range on the positive side in the optical receiver according to the first embodiment. FIG. 5 is a diagram showing an example for explaining coarse compensation when the estimated value of the frequency difference between transmission and reception deviates from the allowable range on the negative side in the optical receiver according to the first embodiment. FIG. 6 is a diagram showing the hardware configuration of the optical receiver according to the first embodiment. FIG. 7 is a block diagram showing a receiving-side digital signal processing unit in the optical receiver according to the second embodiment. FIG. 8 is a block diagram showing a receiving-side digital signal processing unit in the optical receiver according to the third embodiment.Fig. 10 is a block diagram showing another example of a receiving-side digital signal processing unit in an optical receiver according to embodiment 3. Fig. 11 is a block diagram showing a receiving-side digital signal processing unit in an optical receiver according to embodiment 4. Fig. 12 is a diagram showing subcarrier signals in a subcarrier multiplexing method in a digital coherent method on a frequency axis.

[0011] First Embodiment An optical transmission / reception device equipped with an optical receiver according to a first embodiment will be described with reference to Fig. 1 to Fig. 15. In Fig. 1, dashed arrows indicate the flow of optical signals, and solid arrows indicate the flow of electrical signals. The optical transmission / reception device equipped with the optical receiver according to the first embodiment is a device that focuses on the transmission and reception functions of communication equipment and optical transceivers that control the transmission and reception of optical signals in optical communication network systems that use optical fiber as the transmission medium in optical access and optical core / metro networks, as well as optical communication network systems that do not use optical fiber and use wireless space as the transmission medium intended for mobile devices such as satellites, such as space and free-space optical communications.

[0012] In an actual system, an optical receiving device including the optical receiver according to the first embodiment includes an optical transmitter, an optical receiver, and a control device for controlling the optical transmitter in the same housing. The following describes an example of an optical transmitting / receiving device including the optical receiver according to the first embodiment, which is applied to an optical transmitting / receiving device in an optical communication system using low- to medium-speed digital coherent signals of less than 100 Gbps. However, the present invention is not limited to low- to medium-speed digital coherent signals of less than 100 Gbps, and can also be applied as a method for compensating for wide frequency differences in optical transmitting / receiving devices in optical communication systems using high-speed digital coherent signals of 100 Gbps or more.

[0013] An optical transmission / reception device including an optical receiver according to the first embodiment is applied to an optical transmission / reception device that transmits and receives digital coherent signals, which are optical signals that utilize phase orthogonality, such as polarization-multiplexed binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or quadrature amplitude modulation (QAM) of a digital coherent system.

[0014] In the following description, in an optical transmission / reception device equipped with an optical receiver according to the first embodiment, in a polarization-multiplexed digital coherent system commonly used for optical fiber networks, an optical signal is assumed to be polarization-multiplexed by the digital coherent system using X and Y polarizations, and the X and Y polarizations are modulated into orthogonal phase I and Q signals. For example, the X polarization is horizontally polarized, and the Y polarization is vertically polarized. That is, in an optical transmission / reception device compatible with polarization-multiplexed QPSK or QAM, a polarization-multiplexed digital coherent signal is used that is distinguished into an I signal (XI signal) in the X polarization, a Q signal (XQ signal) in the X polarization, an I signal (YI signal) in the Y polarization, and a Q signal (YQ signal) in the Y polarization.

[0015] Note that the terminals for the XI signal, XQ signal, YI signal, and YQ signal (XI output terminal, XQ output terminal, YI output terminal, and YQ output terminal), as well as the lanes through which the electrical signals for the XI signal, XQ signal, YI signal, and YQ signal flow (XI lane, XQ lane, YI lane, and YQ lane), are distinct, but are shown together in the figure, i.e., as one path.

[0016] The present invention may also be applied to optical transceivers that transmit and receive single-polarized digital coherent signals that do not undergo polarization multiplexing, i.e., optical signals modulated into quadrature I and Q signals, and can be applied to single-polarized digital coherent systems in the same way as polarization multiplexed digital coherent systems. Therefore, since the single-polarized digital coherent system can be described without distinction from the polarization multiplexed digital coherent system, the following description will omit the description of the single-polarized digital coherent system.

[0017] 1, the optical transmitting and receiving device includes an optical transmitter 100, an optical receiver 200, and an optical transmitter / receiver control unit (not shown). The optical transmitter 100, the optical receiver 200, and the optical transmitter / receiver control unit are housed in the same housing. As a commonly known device is used for the optical transmitter 100, a brief description will be given.

[0018] The optical transmitter 100 includes a modulation signal generator 110, a digital-to-analog converter (DAC) 120, an optical modulator 130, and a continuous wave (CW) light generator 140. The modulation signal generator 110 is a transmission-side digital signal processor (DSP), hereinafter referred to as the transmission-side DSP 110.

[0019] The transmitter DSP 110 receives data, which is the information to be sent to the destination, as a digital signal, inserts a symbol signal into the input data, and generates a modulated signal, which is a digital signal for optical modulation suitable for the transmission path. The modulated signal is a time-series signal represented by one or more samples per symbol. The transmitter DSP 110 constructs the data to be transmitted as a frame, which is a signal format that allows error correction.

[0020] In the case of a polarization-multiplexed digital coherent system, the transmitter DSP 110 generates four modulated signals: an XI signal, an XQ signal, a YI signal, and a YQ signal. In the case of a single-polarized digital coherent system, the transmitter DSP 110 generates two modulated signals: an I signal and a Q signal. Since each modulated signal is processed in the same way in the optical transmitter / receiver, no distinction is made between the modulated signals, and the following description will focus on one modulated signal.

[0021] The DAC unit 120 converts the modulated signal made up of a digital signal into a modulated signal made up of an analog signal. For example, when one symbol is represented by two samples in the digital domain, the DAC unit 120 uses a digital-to-analog converter with a 2 gigasamples / second rate for a symbol rate of 1 gigabaud.

[0022] The optical modulation unit 130 generates modulated light by modulating the CW light from the CW light generation unit 140 based on the modulation signal converted into an analog signal by the DAC unit 120, and outputs the generated modulated light to the transmission path. The modulated light is an optical signal that is a digital coherent signal in which information from the modulation signal is carried on the CW light. The modulated light is a mixture of information from each modulation signal output from an independent lane. The CW light from the CW light generation unit 140 is a single carrier signal in a digital coherent system, and has a single carrier frequency f Ｔｘ０ is a continuous light with

[0023] 1, the optical receiver 200 includes an optical coherent detection unit 210, an interference light (LO (local oscillator) light) generation unit 220, an ADC (Analog-to-digital converter) unit 230 which is an analog-to-digital converter, and a receiving-side digital signal processing unit 240. The receiving-side digital signal processing unit 240 is a receiving digital signal processor (DSP), and will be referred to as the receiving-side DSP 240 hereinafter.

[0024] The optical coherent detection unit 210 receives modulated light transmitted through a transmission path from an optical transmitter of another optical transmission / reception device, and converts the received modulated light into a single carrier frequency fＲｘ The optical coherent detection unit 210 performs optical coherent detection by causing the optical signal obtained by the interference to interfere with an interference light, which is continuous light having a frequency of 100 kHz, and then photoelectrically converting the optical signal obtained by the interference to obtain an analog electrical signal based on the voltage. The voltage signal output from the optical coherent detection unit 210 is a single-ended output or a differential output.

[0025] In the case of a polarization-multiplexed digital coherent system, the analog electrical signals output from the optical coherent detection unit 210 are four signals, namely, an XI signal, an XQ signal, a YI signal, and a YQ signal, output from four output terminals, and in the case of a single-polarization digital coherent system, they are two signals, namely, an I signal and a Q signal.

[0026] However, in reality, due to factors such as polarization rotation that occurs during transmission via optical fiber, the four analog electrical signals of the XI signal, XQ signal, YI signal, and YQ signal output from independent lanes on the transmitting side are mixed at four output terminals, and the four analog electrical signals are converted to digital form by the ADC unit 230 and then demodulated by the receiving side DSP 240 by restoring them to their original form.

[0027] Also, in the case of a single-polarized digital coherent system, the optical coherent detection unit 210 coherently detects the input modulated light and outputs it as four analog electrical signals. The analog electrical signals of the I and Q signals among the four analog signals are digitally converted by the ADC unit 230 and then demodulated into single-polarized signals by the receiving-side DSP 240. Furthermore, in the case where the I and Q signals can be directly obtained in the optical coherent detection unit 210 by being input in the same polarization state as the modulated light, the two analog electrical signals are digitally converted by the ADC unit 230 and then demodulated into single-polarized signals by the receiving-side DSP 240.

[0028] Each analog electrical signal is modulated by a carrier frequency and the modulated light is a carrier frequency f ＲｘThe signals before demodulation are analog voltage signals obtained by optically interfering interference light having the above characteristics by the optical coherent detection unit 210. The signals before demodulation are signals for obtaining demodulated signals obtained by the receiving-side DSP 240. In the optical transmitting and receiving device, each of the signals before demodulation, i.e., four signals, XI signal, XQ signal, YI signal, and YQ signal in the first embodiment, are processed individually, but since the processing is the same, no distinction is made between the signals before demodulation, and the following description will be given for one signal before demodulation.

[0029] The carrier frequency f in the interference light from the interference light generating unit 220 Ｒｘ is the carrier frequency f in the modulated light received by the optical coherent detection unit 210 Ｔｘ The carrier frequency f is approximately the same as Ｔｘ and carrier frequency f Ｒｘ is, for example, approximately 193.1 THz. The carrier frequency f Ｒｘ is the carrier frequency f of the CW light from the CW light generating unit 140 in the optical transmitter of the other optical transceiver device. Ｔｘ０ or the same as the carrier frequency f Ｔｘ０ In the following description, the CW light from the CW light generating section 140 in the optical transmitter of the other optical transmitting and receiving device will be simply referred to as the CW light from the CW light generating section 140 to avoid complicating the explanation.

[0030] The carrier frequency f in the interference light from the interference light generating unit 220 Ｒｘ is the carrier frequency f in the CW light from the CW light generating unit 140. Ｔｘ０ Even if the carrier frequency f in the modulated light received by the optical coherent detection unit 210 and the carrier frequency f in the interference light from the interference light generation unit 220 are the same, the carrier frequency f in the modulated light received by the optical coherent detection unit 210 and the carrier frequency f in the interference light from the interference light generation unit 220 may be different due to aging of the light source of the interference light generation unit 220 or a large Doppler shift occurring due to the orbital state between the satellites. Ｒｘ There is a frequency difference between

[0031] In addition, the carrier frequency f Ｒｘ is the carrier frequency f in the CW light from the CW light generating unit 140. ＴｘWhen there is a frequency difference between the carrier frequency in the modulated light received by the optical coherent detection unit 210 and the carrier frequency f Ｒｘ There is a frequency difference between

[0032] In either case, the frequency difference is referred to as the frequency difference between the frequency of the CW light from the CW light generating unit 140 and the frequency of the interference light from the interference light generating unit 220, and will be simply described as the frequency difference between transmission and reception. Also, the frequency difference between transmission and reception is the carrier frequency f Ｒｘ The values ​​are based on the standard and may vary in plus or minus, but in the following explanation they are explained as absolute values ​​unless necessary.

[0033] The ADC unit 230 converts the input analog electrical signal into a modulated signal at a modulation rate, for example, a carrier frequency f Ｔｘ When the modulation speed is approximately 100 Gbps, in the case of the QPSK method, sampling is performed based on a sampling frequency that indicates a high sampling rate associated with a high modulation speed of 25 GBaud or more, and the signal is converted into a digital signal that is a discrete signal in the digital domain, thereby obtaining a pre-demodulation signal consisting of a digital signal.

[0034] The sampling rate in the ADC unit 230 is higher than the sampling rate in the frequency difference estimation and compensation unit 243 in the receiving DSP 240. The pre-demodulation signal sent from the ADC unit 230 to the receiving DSP 240 is a time-series signal under high sampling rate conditions. The pre-demodulation digital signal from the ADC unit 230 is a signal processed in the wideband digital frequency domain.

[0035] The receiving-side DSP 240 performs coarse frequency compensation for the digital signal before demodulation from the ADC unit 230 to a frequency that reduces the frequency difference Δf between the transmission and reception, and then performs fine frequency compensation for the digital signal after the coarse frequency difference compensation, thereby converting the digital signal before demodulation into a demodulated signal. In the first embodiment, the receiving-side DSP 240 performs digital signal processing using a digital coherent system.

[0036] 2, the receiving side DSP 240 includes a frequency shifter 241, an equalizer 242, a frequency difference estimator / compensator 243, a phase estimator 244, a symbol determiner 245, and a frequency shift controller 246. The frequency shifter 241 performs coarse compensation for the frequency difference by shifting the digital signal obtained by the ADC 230 to a frequency that reduces the estimated value Δf of the frequency difference between the transmitting and receiving signals.

[0037] The coarse compensation in the frequency shifter 241 is performed by shifting the center frequency of the digital signal obtained by the ADC unit 230 so that the estimated value Δf of the frequency difference between transmission and reception falls within the allowable range when the estimated value Δf of the frequency difference between transmission and reception deviates from the allowable range for which the frequency difference estimation and compensation unit 243 can perform fine compensation.

[0038] As is generally known, the estimated value Δf of the frequency difference between transmission and reception is estimated in the frequency difference estimation and compensation unit 243 using the frequency of the phase rotation of the signal point based on the frequency difference in the IQ signal plane space. The frequency shift unit 241 receives a frequency setting signal from the frequency shift control unit 246, obtains a sine wave for frequency shifting using the frequency setting signal, and multiplies the obtained sine wave by the digital signal obtained by the ADC unit 230 in the time domain, thereby shifting the center frequency of the digital signal toward 0 Hz on the frequency axis. In other words, the estimated value Δf of the frequency difference between transmission and reception is shifted toward 0 Hz.

[0039] It can be said that the frequency shifter 241 has a function of receiving a frequency setting signal from the frequency shift controller 246 and providing a sine wave for frequency shifting to the digital signal obtained by the ADC unit 230 .

[0040] At the start of communication, the frequency shifter 241 shifts the center frequency of the digital signal obtained by the ADC unit 230 to the carrier frequency f in the interference light from the interference light generator 220 so that the center frequency is within the tolerance range of the frequency difference estimator / compensator 243. Ｒｘ and the carrier frequency f in CW light Ｔｘ The frequency difference between these frequencies or a frequency shifted by a fixed value from the frequency difference is set as the initial setting value.

[0041] It is preferable that the estimated value Δf of the frequency difference between transmission and reception for the initial setting value is close to 0 Hz, and the initial setting value is set to a frequency within an allowable range where the estimated value Δf can be precisely compensated by the frequency difference estimation and compensation unit 243. The initial setting value in the frequency shifter 241 is a frequency that can be appropriately represented by a sub-GHz or GHz-class digital signal.

[0042] In embodiment 1, the frequency setting signal from the frequency shift control unit 246 indicates that the estimated value Δf is acceptable when the frequency difference is within the allowable range (-Δfallowable≦Δf≦+Δfallowable), and indicates that the frequency difference is compensable when the estimated value Δf exceeds the allowable frequency range and is within the compensable range (-Δfcompensable<Δf≦+Δfcompensable).

[0043] The estimated value Δf, which is a frequency within the allowable range, is a frequency difference for fine compensation that can be adaptively and precisely estimated and compensated for by the frequency difference estimating and compensating unit 243. The estimated value Δf, which is a frequency difference within the compensable range, is in a fine compensation possible state that can be adaptively and precisely estimated and compensated for, but if it deviates from this range, it is a frequency difference that cannot be finely compensated for.

[0044] During operation, when the frequency setting signal from the frequency shift control unit 246 indicates that the frequency shift is acceptable, the frequency shift unit 241 does not update the shift amount setting value indicating the amount of frequency shift, and provides a sine wave of a frequency based on the current shift amount setting value to the digital signal obtained by the ADC unit 230.

[0045] For example, if the center value of the digital signal obtained by the ADC unit 230 is an initial setting value, the digital signal obtained by the ADC unit 230 has the initial setting value as the center frequency value. Also, if the center value of the digital signal obtained by the ADC unit 230 is changed from the initial setting value by the frequency shift unit 241, a sine wave with a frequency based on the changed value is applied to the digital signal obtained by the ADC unit 230.

[0046] FIG. 3 shows the frequency difference Δf within the allowable range. １When the carrier frequency f Ｔｘ１ and the carrier frequency f in the interference light from the interference light generating unit 220 Ｒｘ 4 shows the carrier frequency f Ｔｘ１ and carrier frequency f Ｒｘ 4 shows the frequency (signal spectrum) of the digital signal obtained by the ADC unit 230 and input to the receiving DSP 240 when the relationship shown in FIG. 3 is satisfied. The frequency shift unit 241 maintains the digital signal at the frequency shown in FIG.

[0047] During operation, when the frequency setting signal from the frequency shift control unit 246 indicates that compensation is possible, the frequency shift unit 241 gradually updates the shift amount setting value indicating the amount of frequency shift in a stepwise manner so that the estimated value Δf of the frequency difference between transmission and reception becomes 0 Hz, thereby causing the center frequency of the digital signal obtained by the ADC unit 230 to become 0 Hz. The frequency shift unit 241 gradually updates the center frequency of the digital signal obtained by the ADC unit 230 so that the quality of the waveform of the demodulated signal obtained by the frequency difference estimation and compensation unit 243 does not change.

[0048] FIG. 5 shows the frequency difference Δf within the compensable range when the estimated value Δf is outside the allowable range. ２ When the carrier frequency f Ｔｘ２ and the carrier frequency f in the interference light from the interference light generating unit 220 Ｒｘ 6 shows the carrier frequency f Ｔｘ２ and carrier frequency f Ｒｘ 5 shows the frequency (signal spectrum) of the digital signal obtained by the ADC unit 230 and input to the receiving DSP 240 when the relationship shown in FIG.

[0049] A frequency difference Δf that is outside the allowable range but within the compensable range ２ is, for example, the carrier frequency f Ｒｘ and the carrier frequency f in the CW light from the CW light generating unit 140 Ｔｘ０ Even if the distances are the same, this occurs due to a large Doppler shift occurring due to the orbital conditions between satellites.

[0050] The digital signal obtained by the ADC unit 230 shown in FIG. 6 ((a) of FIG. 7) is processed by the equalizer 242 and received by the frequency difference estimation and compensation unit 243. Then, the frequency shift control unit 246 calculates an estimated value Δf ２ is outside the allowable range for fine compensation, and a frequency setting signal indicating that compensation is possible is provided to the frequency shifter 241 .

[0051] As shown in FIG. 7B, the frequency shifter 241 adjusts the estimated value Δf ２ is brought from the compensable range to the allowable range, that is, the frequency shift setting is adjusted to the estimated value Δf ２ is 0 Hz (Δf ２Ｃ That is, the frequency shifter 241 performs coarse compensation by updating the digital signal obtained by the ADC unit 230 to a shift amount Δf obtained by a frequency setting signal indicating that compensation is possible. ２α Multiplying the sine wave for the frequency shift of Δf in the time domain (apparently, ２ｃ = Δf ２ +Δf ２α ) shifts the center frequency of the digital signal toward 0 Hz on the frequency axis.

[0052] The update of the center frequency of the digital signal in this coarse compensation gradually changes the center frequency of the digital signal obtained by the ADC unit 230 in a stepwise manner so as not to change the quality of the waveform of the demodulated signal obtained by the frequency difference estimation and compensation unit 243. In other words, the frequency shifter 241 gradually changes the frequency of the sine wave by which the digital signal obtained by the ADC unit 230 is multiplied in a stepwise manner.

[0053] As shown in FIG. 7(c), the estimated value of the roughly compensated frequency difference Δf ２ｃ indicates 0 Hz, the frequency setting signal from the frequency shift control section 246 is ２ is 0 Hz (Δf ２Ｃ ), and the frequency shifter 241 stops setting the frequency shift, that is, the frequency shift amount Δf ２α The shift amount setting value indicating this is not updated.

[0054] The coarse compensation by the frequency shifter 241 can be performed over a wide frequency range due to the high sampling rate in the ADC unit 230, and therefore, can accurately represent a digital signal obtained by frequency-shifting the digital signal obtained by the ADC unit 230. As a result, the frequency shifter 241 outputs a sine wave based on the frequency setting signal from the frequency shift control unit 246, and multiplies the digital signal obtained by the ADC unit 230 by this sine wave, thereby making it possible to bring the frequency component of the digital signal obtained by the ADC unit 230 closer to 0 Hz. Note that the frequency shift by the frequency shifter 241 may be a digital signal processing operation in the frequency domain instead of multiplication in the time domain.

[0055] The signal processing for demodulation by the receiving DSP 240 in the equalizer 242 is a common technique in digital signal processing used in a typical digital coherent system, and will be explained briefly below. In the following explanation, the digital signal obtained by the ADC 230 after rough compensation by the frequency shifter 241 includes a digital signal whose frequency has been shifted by the shift amount setting value and a digital signal whose frequency shift setting has been stopped.

[0056] If the equalization unit 242 has a clock synchronization function, it compensates for the clock difference between optical transmission and optical reception for the digital signal obtained by the ADC unit 230 that has been roughly compensated by the frequency shift unit 241, and if it has an adaptive equalization function, it adaptively compensates for the polarization separation and deterioration of the frequency characteristics of the X polarization signal (XI signal and XQ signal) and the Y polarization signal (YI signal and YQ signal). In short, the equalization unit 242 compensates for waveform distortion in the digital signal obtained by the ADC unit 230 that has been roughly compensated.

[0057] The frequency difference estimation compensation unit 243 estimates the carrier frequency f Ｔｘ and the carrier frequency f in the interference light from the interference light generating unit 220 ＲｘThe frequency difference estimating and compensating unit 243 finely compensates the center frequency of the digital signal for which the frequency difference estimate Δf has been roughly compensated for by the frequency shifting unit 241. If the frequency difference Δf between the transmission and reception is within an allowable range, the frequency difference estimating and compensating unit 243 can perform fine compensation, which can adaptively and precisely estimate and compensate the digital signal for which coarse compensation has been performed.

[0058] The frequency difference estimation / compensation unit 243 estimates the frequency based on the phase rotation of the signal point expressed in the IQ signal plane space of the pilot symbol signal periodically inserted into the modulated light received by the optical coherent detection unit 210, and finely compensates the frequency of the digital signal obtained by the ADC unit 230 after the rough compensation by the frequency shift unit 241. Note that the fine compensation of the frequency of the digital signal is not limited to compensation using a pilot insertion method using a pilot symbol signal, and compensation may also be performed using a commonly known frequency estimation method such as a method based on Fourier transform analysis or the power method.

[0059] The frequency difference estimating / compensating unit 243 compensates for the rotation of the signal points, i.e., compensates for the frequency difference, by multiplying the digital signal in the time domain. However, since the frequency difference fluctuates over time, the frequency difference estimating / compensating unit 243 adaptively updates the estimated value of the frequency difference and, therefore, the frequency of the sine wave used for compensation. The frequency difference estimating / compensating unit 243 compensates for the frequency difference using a general method in digital signal processing used in ordinary digital coherent systems.

[0060] It is assumed that the frequency difference estimating / compensating unit 243 receives a signal with a lower sampling rate than the frequency shifting unit 241, such as a time-series signal represented by one sample per symbol or two samples per symbol. Even with such a low sampling rate, the frequency difference estimating / compensating unit 243 can correctly perform adaptive and precise estimation / compensation even in a narrow frequency space because the estimated value Δf of the frequency difference between transmission and reception is roughly compensated by the frequency shifting unit 241 and falls within an allowable range, and the frequency of the digital signal obtained by the ADC unit 230 is roughly compensated to be near 0 Hz. For example, as shown in FIG. 8 , if the phase rotation of the case with a frequency difference is within 180° of the case without a frequency difference, that is, if the estimated value Δf of the frequency difference between transmission and reception can be compensated, estimation / compensation can be performed correctly.

[0061] Note that if rough compensation is not performed by the frequency shifter 241 and, for example, the center frequency of the digital signal obtained by the ADC unit 230 deviates significantly from 0 Hz, causing the estimated value Δf of the frequency difference between transmission and reception to deviate from the compensable range (to be uncompensable), the frequency difference estimator / compensator 243 will not be able to correctly represent the waveform as an out-of-band signal, and as shown in FIG. 9 , the phase rotation with a frequency difference will exceed 180° compared to the case without a frequency difference, which will ultimately result in an estimation error in the frequency difference estimator / compensator 243.

[0062] The frequency shift control unit 246 obtains an estimate Δf of the frequency difference between transmission and reception by the frequency difference estimation compensation unit 243, obtains a frequency setting signal to which the estimate Δf is linked, indicating acceptance if the estimate Δf is within an allowable range, and indicating compensation if the estimate Δf deviates from the allowable range, that is, exceeds the allowable range and is within a compensable range, and provides the obtained frequency setting signal to the frequency shift unit 241.

[0063] The signal processing performed by the phase estimator 244 and symbol determiner 245 for demodulation by the receiver DSP 240 is a common technique in digital signal processing used in typical digital coherent systems, and will be briefly described below. The phase estimator 244 compensates for phase fluctuations in the light source constituting the CW light generator in the optical transmitter and the light source constituting the interference light generator 220. The symbol determiner 245 determines the transmission symbol in the demodulated signal that has been compensated for waveform distortion. Note that error correction of the demodulated signal may be performed downstream of the symbol determiner 245.

[0064] Next, the optical receiver according to the first embodiment, particularly the operation of the frequency shifter 241 and the frequency shift controller 246 in the receiver DSP 240, will be described with reference to FIG. 10. When starting communication with an optical transmitter of another optical transmitter / receiver, initial acquisition of an estimated value Δf of the frequency difference between transmission and reception is performed in step ST1. The frequency shifter 241 receives a frequency setting signal from the frequency shift controller 246 based on the estimated value Δf initially acquired by the frequency difference estimator / compensator 243, and initializes a shift amount setting value indicating the amount of frequency shift.

[0065] The estimated value Δf after the initial setting is set to the time t ０ 11 , the horizontal axis represents time, the vertical axis represents estimated values, and the black circles represent estimated values ​​Δf obtained by the frequency difference estimating / compensating unit 243. The estimated values ​​Δf are obtained on the order of a symbol period. Alternatively, the estimated values ​​Δf may be calculated by a moving average of multiple symbols. For the sake of simplicity, the estimated values ​​Δf indicated by the black circles in FIG. 11 do not represent all of the estimated values ​​Δf obtained by the frequency difference estimating / compensating unit 243.

[0066] 12 , when communication with an optical transmitter of another optical transceiver is started and the estimated value Δf obtained by the frequency difference estimating / compensating unit 243 is within the allowable range (−Δfallowable≦Δf≦+Δfallowable: 2×Δfallowable), the frequency setting signal from the frequency shift control unit 246 indicates allowance, and the frequency shift unit 241 does not update the shift amount but provides a sine wave of a frequency based on the current shift amount setting value to the digital signal obtained by the ADC unit 230. Therefore, the digital signal obtained by the ADC unit 230 is precisely compensated by the frequency difference estimating / compensating unit 243.

[0067] In step ST2, the frequency shift control unit 246 determines whether the estimated value Δf of the frequency difference obtained by the frequency difference estimation and compensation unit 243 is within the allowable range, and determines whether the estimated value Δf of the frequency difference obtained by the frequency difference estimation and compensation unit 243 is within the allowable range. １ Assuming that the estimated value Δf of the frequency difference was within the allowable range up until just before, a frequency setting signal indicating allowance is obtained, the frequency shifter 241 does not update the shift amount, and the frequency difference estimator / compensator 243 precisely compensates the digital signal obtained by the ADC unit 230 to which a sine wave of a frequency based on the current shift amount setting value has been applied. Step ST2 is a step for maintaining the shift amount of the center frequency of the digital signal obtained by the ADC unit 230 when the estimated value Δf of the frequency difference is within the allowable range.

[0068] Time t shown in FIG. １ 13 , when the frequency difference estimate Δf obtained by the frequency difference estimating and compensating unit 243 exceeds (deviates from) +Δfallowable and the center frequency of the digital signal obtained by the ADC unit 230 deviates significantly from 0 Hz, the process proceeds from step ST2 to step ST3. In step ST3, the frequency setting signal from the frequency shift control unit 246 indicates that compensation is possible, and the frequency shift unit 241 updates the shift amount in the direction of bringing the frequency difference estimate Δf to 0 at a speed that does not interrupt the compensation function of the frequency difference estimating and compensating unit 243, and then the process proceeds to step ST4.

[0069] The frequency difference estimating and compensating unit 243 precisely compensates the digital signal obtained by the ADC unit 230 to which a sine wave of a frequency based on the updated shift amount has been applied. In step ST4, the frequency shift control unit 246 determines whether the estimated value Δf of the frequency difference obtained by the frequency difference estimating and compensating unit 243 is 0 or in the range of 0±β. If it is outside the range of 0±β, the process returns to step ST3, and the frequency shifter 241 updates the shift amount and proceeds to step ST4.

[0070] The estimated value of the frequency difference Δf is kept within the range of 0±β until the time t ２ Steps ST3 and ST4 are repeated until β is reached. β is a value greater than the changeable frequency difference that can be obtained by the frequency shifter 241 in one update of the shift amount. Steps ST3 and ST4 are update steps in which the frequency shifter 241 updates the shift amount of the center frequency of the digital signal obtained by the ADC unit 230 when the estimated value Δf of the frequency difference falls outside the allowable range.

[0071] The maintenance step of step ST2 and the update steps of steps ST3 and ST4 constitute a step in which the frequency shifter 241 roughly compensates for the frequency difference between the transmission and reception frequencies relative to the center frequency of the digital signal obtained by the ADC unit 230. In addition, although not explicitly shown in Fig. 10, there is a step in which the frequency difference estimator / compensator 243 finely compensates the digital signal for which the frequency difference between the transmission and reception frequencies has been roughly compensated for by steps ST2, ST3, and ST4.

[0072] Time t shown in FIG. ２ When the estimated value Δf of the frequency difference falls within the range of 0±β at time t, the process proceeds to step ST5, where the frequency shifter 241 stops updating the shift amount, maintains the shift amount, and returns to step ST2. ２ Immediately after time t ３ Since the estimated value Δf of the frequency difference was within the allowable range until just before, in step ST2, the frequency shift unit 241 does not update the shift amount, and the frequency difference estimation compensation unit 243 performs precision compensation on the digital signal obtained by the ADC unit 230, which has been given a sine wave of a frequency based on the current shift amount maintained after the update.

[0073] Time t shown in FIG. ３ 14 , when the frequency difference estimate Δf obtained by the frequency difference estimating / compensating unit 243 exceeds (deviates from) −Δfallowable, the process proceeds from step ST2 to step ST3. In steps ST3 and ST4, as described above, the frequency shifter 241 sequentially updates the shift amount in the direction toward 0 of the frequency difference estimate Δf at a rate that does not interrupt the compensation function of the frequency difference estimating / compensating unit 243, until the frequency difference estimate Δf falls within the range of 0±β. The frequency difference estimating / compensating unit 243 performs precision compensation on the digital signal obtained by the ADC unit 230 to which a sine wave of a frequency based on the updated shift amount has been applied.

[0074] Time t shown in FIG. ４ When the estimated value Δf of the frequency difference falls within the range of 0±β in step ST5, the frequency shifter 241 stops updating the shift amount, maintains the shift amount, and returns to step ST2. In this way, the frequency shifter 241 performs coarse compensation, and the frequency difference estimating and compensating unit 243 performs fine compensation on the digital signal obtained by the ADC unit 230 after the coarse compensation by the frequency shifter 241. Therefore, even if a frequency difference occurs between the transmitted and received optical carrier waves, waveform distortion compensation can be performed.

[0075] Next, the hardware configuration of the optical receiver 200 according to the first embodiment will be described with reference to Fig. 15. In Fig. 15, the same reference numerals as those in Figs. 1 and 2 indicate the same or corresponding parts. The optical receiver control unit 250 includes a processor 251 such as a CPU (Central Processing Unit) or a system LSI (Large Scale Integration), a memory 252 configured from a RAM (Random Access Memory) and a ROM (Read Only Memory), a communication interface 253, and an input / output interface 254. The processor 251, the memory 252, the communication interface 253, and the input / output interface 254 are connected to a bus 255, and data, control signals, and the like are exchanged between them via the bus 255.

[0076] The processor 251 temporarily loads a program recorded in the ROM in the memory 252 into the RAM in the memory 252 and executes processing according to the loaded program. The ROM in the memory 252 stores various data, programs for executing processing in the optical receiver 200, and processing programs required for starting up the optical receiver 200. The communication interface 253 is used for transmitting and receiving data and control signals to and from each component of the optical receiver 200 and each component of other optical transmitting and receiving devices.

[0077] The input / output interface 254 transmits and receives control signals and modulation signals via electrical wiring between the components of the optical receiver 200. For example, the input / output interface 254 is an interface for supplying the interference light generating unit 220 with an injection current to a light source constituting the interference light generating unit 220 for generating light. The input / output interface 254 is also an interface for outputting various control signals to the receiving-side DSP unit, for example.

[0078] 15, for ease of explanation, the optical receiver control unit 250 is shown as a separate configuration from the receiving DSP 240, but functional parts that can be configured by software in the receiving DSP 240 may be handled by the optical receiver control unit 250. For example, the functions of the frequency shift unit 241, equalization unit 242, frequency difference estimation compensation unit 243, phase estimation unit 244, symbol determination unit 245, and frequency shift control unit 246 that configure the receiving DSP 240 may be realized by a processor 251 and a memory 252.

[0079] When the functions of the frequency shift unit 241 and the frequency shift control unit 246 are realized by the processor 251 and the memory 252, an optical receiving program recorded in the ROM in the memory 252 and executed by the processor 251 includes the following steps: a procedure for roughly compensating for the frequency difference between transmission and reception, the procedure for maintaining the amount of shift of the center frequency of a digital signal obtained by coherently detecting an analog electrical signal by interfering with interference light, which is optically modulated at a carrier frequency, so that the frequency difference between transmission and reception is within an allowable range; and a procedure for updating the amount of shift of the center frequency of the digital signal when the frequency difference between transmission and reception deviates from the allowable range; and a procedure for finely compensating the center frequency of the digital signal after roughly compensating for the frequency difference between transmission and reception.

[0080] The optical receiver according to the first embodiment includes an optical coherent detection unit 210 that receives modulated light optically modulated by a carrier frequency, causes the received modulated light to interfere with interference light, performs coherent detection, and outputs an analog electrical signal, an ADC unit 230 that performs analog-to-digital conversion on the analog electrical signal from the optical coherent detection unit 210 to obtain a digital signal, a frequency shift unit 241 that performs coarse compensation to shift the center frequency of the digital signal obtained by the ADC unit 230 to a frequency that reduces the frequency difference Δf between transmission and reception, and a frequency Since the receiver DSP 240 includes a frequency difference estimating / compensating unit 243 that finely compensates the center frequency of the digital signal that has undergone coarse frequency difference compensation by the shift unit 241, the frequency difference estimating / compensating unit 243 receives a digital signal obtained by the ADC unit 230 with the center frequency approaching 0 Hz, i.e., a digital signal with a reduced phase rotation rate, the frequency difference estimating / compensating unit 243 can accurately read the amount of phase rotation of the digital signal, enabling the frequency difference estimating / compensating unit 243 to adaptively and precisely estimate and compensate for the optical frequency difference. As a result, the signal quality of the demodulated signal from the receiver DSP 240 can be improved, and ultimately the compensation range for the optical frequency difference in the optical receiver can be expanded.

[0081] Embodiment 2 An optical receiver according to embodiment 2 will be described with reference to FIG. 16. The optical receiver according to embodiment 2 is the same as the optical receiver according to embodiment 1 except for the receiving DSP 240. In FIG. 16, the same reference numerals as those in FIGS. 1 and 2 indicate the same or corresponding parts. The equalizer 242, frequency difference estimator / compensator 243, phase estimator 244, and symbol determiner 245 that constitute the receiving DSP 240 are the same as the equalizer 242, frequency difference estimator / compensator 243, phase estimator 244, and symbol determiner 245 that constitute the receiving DSP 240 in the optical receiver according to embodiment 1.

[0082] The frequency shifter 241 and the frequency shift controller 246 differ from the frequency shifter 241 and the frequency shift controller 246 in the optical receiver according to embodiment 1 in the method of setting the initial setting values ​​at the start of communication, but the implementation of coarse compensation during operation by the frequency shifter 241 and the frequency shift controller 246 to shift the center frequency of the digital signal obtained by the ADC unit 230 to a frequency that reduces the estimated value Δf of the frequency difference between transmission and reception is the same. Therefore, the setting of the initial setting values ​​at the start of communication will be described.

[0083] The frequency shift control unit 246 receives shift amount setting information indicating the frequency shift amount from outside the receiving DSP 240, and provides the shift amount setting information to the frequency shift unit 241. At the same time, the frequency shift control unit 246 provides a synchronization determination value or continuity information in the clock synchronization function of the equalization unit 242 to the frequency shift unit 241. The shift amount setting information from outside is information indicating a plurality of discrete shift amounts as shown in FIG. 16 , and the frequency shift control unit 246 sequentially receives information indicating the plurality of discrete frequency shift amounts.

[0084] The frequency shifter 241 sequentially performs frequency shifts based on information indicating a plurality of discrete shift amounts relative to the center frequency of the digital signal obtained by the ADC unit 230. The frequency shifter 241 sets as an initial setting a frequency difference within a compensable range in the frequency difference estimation / compensation unit 243, preferably a shift amount indicating an optimal frequency difference, from among the estimated values ​​Δf of the frequency difference between transmission and reception obtained by sequentially performing frequency shifts based on the synchronization determination value or continuity information in the clock synchronization function of the equalizer 242.

[0085] The optical receiver according to the second embodiment has the same effects as the optical receiver according to the first embodiment. In addition, the initial setting value of the shift amount by the frequency shift unit 241 is determined based on shift amount setting information indicating a plurality of discrete shift amounts. Therefore, even if the frequency difference between transmission and reception at the start of communication is outside the range that can be compensated for by the frequency difference estimation and compensation unit 243, the center frequency of the digital signal obtained by the ADC unit 230 can be pulled into a frequency difference that is within the range that can be compensated for by the frequency difference estimation and compensation unit 243 at the time of initial setting, and coarse compensation and fine compensation can be performed accurately during operation.

[0086] Third Embodiment An optical receiver according to the third embodiment will be described with reference to FIG. 17 . The optical receiver according to the third embodiment is the same as the optical receiver according to the first embodiment except for the receiving DSP 240. In FIG. 17 , the same reference numerals as those in FIGS. 1 and 2 indicate the same or corresponding parts. The equalizer 242, frequency difference estimator / compensator 243, phase estimator 244, and symbol determiner 245 that constitute the receiving DSP 240 are the same as the equalizer 242, frequency difference estimator / compensator 243, phase estimator 244, and symbol determiner 245 that constitute the receiving DSP 240 in the optical receiver according to the first embodiment.

[0087] The frequency shift control unit 246 periodically or periodically obtains shift amount setting information indicating the frequency shift amount from outside the receiving DSP 240, and provides a frequency setting signal based on the shift amount setting information to the frequency shift unit 241. At the same time, the frequency shift control unit 246 provides a synchronization determination value or continuity information in the clock synchronization function of the equalization unit 242 to the frequency shift unit 241. The external shift amount setting information is either information indicating the Doppler shift amount estimated from satellite orbit information or the carrier frequency of another optical transmitter.

[0088] During initial setup and during operation, the frequency shift unit 241 periodically or cyclically performs a frequency shift on the center frequency of the digital signal obtained by the ADC unit 230 by the amount of shift indicated by the frequency setting signal from the frequency shift control unit 246.

[0089] The optical receiver according to the third embodiment determines the setting value of the shift amount by the frequency shifter 241 based on either the Doppler shift amount estimated from satellite orbit information or shift amount setting information indicating the carrier frequency of another optical transmitter, and therefore can perform coarse compensation and fine compensation accurately.

[0090] The optical receiver according to the third embodiment determines the setting value of the shift amount by the frequency shifter 241 based on either the Doppler shift amount estimated from satellite orbit information or shift amount setting information indicating the carrier frequency of another optical transmitter. However, as shown in FIG. 18 , similar to the optical receiver according to the first embodiment, the frequency shift controller 246 may also have a function of obtaining an estimated value Δf of the frequency difference between transmission and reception by the frequency difference estimator / compensator 243, obtaining a frequency setting signal to which the estimated value Δf is linked, indicating acceptance when the estimated value Δf is within an allowable range, and indicating compensation when the estimated value Δf deviates from the allowable range, that is, exceeds the allowable range and is within a compensable range, and providing the obtained frequency setting signal to the frequency shifter 241.

[0091] That is, the shift amount setting information is used to set the initial setting value at the start of communication, and during operation, the frequency shift unit 241 sets the shift amount based on the shift amount setting information and the frequency shift unit 241 sets the shift amount based on the estimated value Δf of the frequency difference between transmission and reception by the frequency difference estimation compensation unit 243.

[0092] The combination of setting of the shift amount by the frequency shifter 241 based on the estimated value Δf of the frequency difference between transmission and reception by the frequency difference estimating and compensating unit 243 during operation and setting of the shift amount by the frequency shifter 241 based on the shift amount setting information is performed as follows: During normal operation during operation, similarly to the first embodiment, the frequency shifter 241 sets the shift amount for the center frequency of the digital signal obtained by the ADC unit 230 based on a frequency setting signal from the frequency shift control unit 246 that has received the estimated value Δf of the frequency difference obtained by the frequency difference estimating and compensating unit 243.

[0093] When the frequency shifter 241 receives the shift amount setting information from the outside, it adds the shift amount based on the external shift amount setting information to the shift amount based on the frequency setting signal from the frequency shift controller 246, and sets the added shift amount as the shift amount for the center frequency of the digital signal obtained by the ADC unit 230. The update is performed gradually so as not to change the quality of the waveform of the demodulated signal obtained by the frequency difference estimator / compensator 243.

[0094] However, during operation, when the frequency shift unit 241 sets the shift amount by adding the shift amount indicated by the shift amount setting information and the estimated value Δf of the frequency difference between transmission and reception deviates from the allowable range, the frequency shift unit 241 stops setting the shift amount by adding the shift amount indicated by the shift amount setting information, and instead focuses on setting the shift amount by the frequency shift unit 241 using the estimated value Δf of the frequency difference between transmission and reception by the frequency difference estimation compensation unit 243.

[0095] Fourth Embodiment An optical receiver according to a fourth embodiment will be described with reference to Figures 19 and 20. While the optical receiver according to the first embodiment receives modulated light in which data information is modulated by a single carrier signal (single carrier frequency) in a typical digital coherent system, the optical receiver according to the fourth embodiment receives modulated light in which data information is modulated by multiple different subcarrier signals (multiple different carrier frequencies) in a subcarrier multiplexing system. In the following description, the subcarrier signal on which data information is superimposed will be simply referred to as a subcarrier signal to avoid complicating the explanation. In Figure 19, the same reference numerals as those in Figures 1 and 2 indicate the same or corresponding parts.

[0096] The optical receiver according to the fourth embodiment collectively receives subcarrier signals within a band in which signals can be detected, and separates and demodulates the subcarriers using digital signal processing on the receiving side. Note that in the fourth embodiment, multiple different subcarrier signals in modulated light received by the optical receiver may each be generated individually for a data signal in an independent modulated light. Therefore, it is also possible that the optical receiver collectively receives uncorrelated modulated light sent from different points.

[0097] The optical receiver according to the fourth embodiment is an optical receiver that, like the optical receiver according to the first embodiment, targets optical signals (XI signal, XQ signal, YI signal, and YQ signal) in which X polarization and Y polarization are modulated into I signal and Q signal of quadrature phase, respectively, and further receives an optical signal in which N channels are multiplexed by N subcarriers of different frequencies for each of the X polarization and the Y polarization. Note that in the following explanation, it is assumed that in an optical transmitter of another optical transmission / reception device, subcarrier signals are multiplexed on the frequency scale in a subcarrier multiplexing section and output as modulated light.

[0098] The optical coherent detection unit 210, the interference light generation unit 220, and the ADC unit 230 in the optical receiver according to the fourth embodiment have basically the same configuration as the optical coherent detection unit 210, the interference light generation unit 220, and the ADC unit 230 in the optical receiver according to the first embodiment, and are used in a commonly known subcarrier multiplexing method, so a description thereof will be omitted.

[0099] The receiving DSP 240 is provided corresponding to four pre-demodulation signals, i.e., XI signal, XQ signal, YI signal, and YQ signal, which are analog voltage signals obtained by optical interference using the optical coherent detection unit 210. However, as in the first embodiment, no distinction is made between the pre-demodulation signals, and the following description will focus on one pre-demodulation signal. The receiving DSP 240 has N receiving DSPs 240-1 to 240-N corresponding to the N subcarrier signals in the received modulated light.

[0100] 20, the N subcarrier signals in the modulated light are centrosymmetric on the frequency axis with respect to the center frequency of the N subcarrier signals when N is an even number. Furthermore, the carrier frequency of the interference light from the interference light generating unit 220 is set to the center frequency of the N subcarrier signals.

[0101] The N receiving side DSPs 240-1 to 240-N each process N subcarriers individually, and each has a frequency shift unit 241, an equalization unit 242, a frequency difference estimation compensation unit 243, a phase estimation unit 244, a symbol determination unit 245, a frequency shift control unit 246, and an adjacent signal removal unit 248.

[0102] In each of the N receiving-side DSPs 240-1 to 240-N, the signal processing for demodulation by the receiving-side DSP 240 in the equalizer 242, the phase estimator 244, and the symbol determiner 245 is performed in the same manner as the signal processing for demodulation by the receiving-side DSP 240 in the equalizer 242, the phase estimator 244, and the symbol determiner 245 in embodiment 1, for each of the N subcarrier signals. In addition, the adjacent signal remover 248 functions as a low-pass filter or a band-pass filter that removes adjacent signals outside the band of each subcarrier signal from the N subcarrier signals given a shift amount by the frequency shifter 241, and performs signal separation for the subcarrier signals.

[0103] At the start of communication, the frequency shift unit 241 in each of the N receiving-side DSPs 240-1 to 240-N sets an initial value for the shift amount based on the frequency difference between the center frequency of the N subcarrier signals and the frequencies of the subcarrier signals. As a result, in each of the N receiving-side DSPs 240-1 to 240-N, the initially set frequency shift unit 241 shifts the frequency of the subcarrier signals in the digital signal obtained by the ADC unit 230 so that the frequency is within a range that can be precisely compensated.

[0104] The initial value of the shift amount in the frequency shifter 241 may be set by providing the frequency difference of the subcarrier signal frequencies from the center frequency for N subcarrier signals to the frequency shift controller 246, instead of the Doppler shift amount or the carrier frequency of the optical transmitter, as shown in FIG. 18 .

[0105] In the adjacent signal removal unit 248 in each of the N receiving side DSPs 240-1 to 240-N, the digital signal obtained by the ADC unit 230 is separated into corresponding subcarrier signals, and therefore the frequency difference estimation compensation unit 243 in each of the N receiving side DSPs 240-1 to 240-N is within the tolerance range for the corresponding subcarrier signals, making it possible to perform precise compensation.

[0106] After the frequency shift unit 241 in each of the N receiving-side DSPs 240-1 to 240-N is initially set, when communication with an optical transmitter of another optical transmitting / receiving device is started, if the estimated value Δf obtained by the corresponding frequency difference estimating / compensating unit 243 is within the allowable range, the frequency shift unit 241 does not update the initially set shift amount, but provides a sine wave of the initially set frequency to the digital signal obtained by the ADC unit 230. Therefore, the digital signal of the subcarrier signal obtained by the ADC unit 230 is precisely compensated by the frequency difference estimating / compensating unit 243.

[0107] When the estimated value Δf obtained by the corresponding frequency difference estimating / compensating unit 243 deviates from the allowable range, the frequency shifting unit 241 in each of the N receiving side DSPs 240-1 to 240-N updates the shift amount in a direction such that the estimated value Δf of the frequency difference becomes 0. That is, in each of the N receiving side DSPs 240-1 to 240-N, as described in the first embodiment, the frequency shifting unit 241 performs coarse compensation and the frequency difference estimating / compensating unit 243 performs fine compensation.

[0108] The optical receiver according to the fourth embodiment is an optical receiver that receives modulated light in which data information is modulated by a plurality of different subcarrier signals using a subcarrier multiplexing method, and frequency shift units 241 in a plurality of receiving-side DSPs 240-1 to 240-N corresponding to each of the plurality of subcarrier signals update the shift amount so that the corresponding subcarrier signal falls within an allowable range. This allows each receiving-side DSP 240-1 to 240-N to accurately read the amount of phase rotation of the corresponding subcarrier signal, enabling the frequency difference estimation and compensation unit 243 to adaptively and precisely estimate and compensate the optical frequency difference.

[0109] It should be noted that the embodiments may be freely combined, or any of the components in each embodiment may be modified, or any of the components in each embodiment may be omitted.

[0110] The optical receiver according to the present disclosure can be applied to optical receivers used in the field of digital coherent technology, which is widely applied in metro / core networks and submarine optical cable systems in optical fiber communications, optical receivers used in wireless access networks in the field of wireless communications, optical receivers used in optical networks including core / metro networks used for encrypted signals, video systems, existing Internet protocols, etc., and optical receivers for satellite-mounted optical communication terminals in optical communication systems that do not use optical fiber, such as space or free-space optical communications. In particular, the optical receiver is suitable for optical receivers in optical communication systems that use low- to medium-speed digital coherent signals of less than 100 Gbps.

[0111] 100 Optical transmitter, 110 Modulation signal generation unit, 120 DAC unit, 130 Optical modulation unit, 140 CW light generation unit, 200 Optical receiver, 210 Optical coherent detection unit, 220 Interference light generation unit, 230 ADC unit, 240, 240-1 to 240-N Receiving side digital signal processing unit, 241 Frequency shift unit, 242 Equalization unit, 243 Frequency difference estimation compensation unit, 244 Phase estimation unit, 245 Symbol determination unit, 246 Frequency shift control unit, 247 Adjacent signal removal unit.

Claims

1. An optical receiver comprising: an optical coherent detection unit that receives modulated light optically modulated by a carrier wave frequency, interferes the received modulated light with interfering light to perform coherent detection, and outputs an analog electrical signal; an analog / digital conversion unit that performs analog / digital conversion on the analog electrical signal from the optical coherent detection unit to obtain a digital signal; a frequency shift unit that performs coarse compensation for shifting the center frequency of the digital signal obtained by the analog / digital conversion unit to a frequency that reduces the frequency difference between transmission and reception; and a frequency difference estimation compensation unit that performs fine compensation for the center frequency of the digital signal for which coarse compensation of the frequency difference has been performed by the frequency shift unit.

2. The optical receiver according to claim 1, wherein the modulated light has an optical signal modulated into an I signal and a Q signal of quadrature phase, and the analog / digital conversion unit and the receiving-side digital signal processing unit perform signal processing on the I signal and the Q signal individually.

3. The optical receiver according to claim 1 or 2, wherein the coarse compensation of the frequency difference by the frequency shift unit is performed by shifting the center frequency of the digital signal obtained by the analog / digital conversion unit so that the estimated value of the frequency difference between transmission and reception is within the allowable range when the estimated value of the frequency difference between transmission and reception deviates from the allowable range within which the frequency difference estimation compensation unit can perform fine compensation.

4. The optical receiver according to claim 3, wherein the coarse compensation of the frequency difference by the frequency shift unit is performed by stepwise shifting the center frequency of the digital signal obtained by the analog / digital conversion unit so that the waveform quality of the demodulated signal obtained by the frequency difference estimation compensation unit does not change.

5. The optical receiver according to claim 1 or 2, wherein the coarse compensation of the frequency difference by the frequency shift unit is performed by shifting the center frequency of the digital signal obtained by the analog / digital conversion unit so that the estimated value of the frequency difference between transmission and reception is within the allowable range based on shift amount setting information from outside the receiving-side digital signal processing unit indicating the amount of frequency shift by the frequency shift unit.

6. The optical receiver according to claim 5, wherein the shift amount setting information is either information indicating the Doppler shift amount estimated from satellite orbit information or the carrier frequency of another optical transmitter.

7. The rough compensation of the frequency difference by the frequency shift unit is performed as follows: at the initial setting, the frequency shift unit shifts the center frequency of the digital signal obtained by the analog / digital conversion unit based on the shift amount setting information from outside the receiving-side digital signal processing unit indicating the amount of frequency shift, so that the estimated value of the frequency difference between transmission and reception falls within the allowable range. During operation, when the estimated value of the frequency difference between transmission and reception deviates from the allowable range of the estimated value of the frequency difference between transmission and reception that the frequency difference estimation compensation unit can tolerate for fine compensation, the center frequency of the digital signal obtained by the analog / digital conversion unit is shifted so that the estimated value of the frequency difference between transmission and reception falls within the allowable range. The optical receiver according to claim 1 or claim 2.

8. The rough compensation of the frequency difference by the frequency shift unit is performed as follows: when the estimated value of the frequency difference between transmission and reception deviates from the allowable range of the estimated value of the frequency difference between transmission and reception that the frequency difference estimation compensation unit can tolerate for fine compensation, the shift amount for reducing the frequency difference between transmission and reception is added to the shift amount indicated by the shift amount setting information, which is either the Doppler shift amount estimated from the satellite orbit information from outside the receiving-side digital signal processing unit or the shift amount indicated by any information indicating the carrier frequency of another optical transmitter, and the center frequency of the digital signal obtained by the analog / digital conversion unit is shifted. The optical receiver according to claim 1 or claim 2.

9. The modulated light received by the optical coherent detection unit is modulated light in which data information is modulated by a plurality of different sub-carrier signals. The receiving-side digital signal processing unit has the frequency shift unit and the frequency difference estimation compensation unit corresponding to a plurality of different sub-carrier signals. The optical receiver according to claim 1 or claim 2.

10. An optical reception method in an optical receiver including a light coherent detection unit, an analog / digital conversion unit, a frequency shift unit, and a reception-side digital signal processing unit having a frequency difference estimation and compensation unit, wherein the frequency shift unit maintains the shift amount of the center frequency of a digital signal obtained by analog / digital conversion by the analog / digital conversion unit of an analog electrical signal obtained by coherently detecting, by interference with an interference light, modulated light optically modulated by a carrier frequency, such that the frequency difference between transmission and reception is within an allowable range, and updates the shift amount of the center frequency of the digital signal obtained by the analog / digital conversion unit when the frequency difference between transmission and reception deviates from the allowable range, thereby performing rough compensation of the frequency difference between transmission and reception; and the frequency difference estimation and compensation unit performs fine compensation of the center frequency of the digital signal on which rough compensation of the frequency difference between transmission and reception has been performed.

11. A program for causing a computer to execute a procedure for performing rough compensation of the frequency difference between transmission and reception, the procedure including maintaining the shift amount of the center frequency of a digital signal obtained by analog / digital conversion of an analog electrical signal obtained by coherently detecting, by interference with an interference light, modulated light optically modulated by a carrier frequency, such that the frequency difference between transmission and reception is within an allowable range, and updating the shift amount of the center frequency of the digital signal when the frequency difference between transmission and reception deviates from the allowable range, and a procedure for performing fine compensation of the center frequency of the digital signal on which rough compensation of the frequency difference between transmission and reception has been performed.

12. A recording medium storing a program for causing a computer to execute a procedure for performing rough compensation of the frequency difference between transmission and reception, the procedure including maintaining the shift amount of the center frequency of a digital signal obtained by analog / digital conversion of an analog electrical signal obtained by coherently detecting, by interference with an interference light, modulated light optically modulated by a carrier frequency, such that the frequency difference between transmission and reception is within an allowable range, and updating the shift amount of the center frequency of the digital signal when the frequency difference between transmission and reception deviates from the allowable range, and a procedure for performing fine compensation of the center frequency of the digital signal on which rough compensation of the frequency difference between transmission and reception has been performed.

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