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

Abstract

The optical receiver 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 analog / digital conversion unit (230) that performs analog / digital conversion of the analog electrical signal from the optical coherent detection unit (210) to obtain a digital signal, and a receiving-side digital signal processing unit (240) having a frequency shift unit (241) that performs coarse compensation to shift the center frequency of the digital signal obtained by the analog / digital conversion unit (230) to a frequency that reduces the frequency difference between transmission and reception, and a frequency difference estimation compensation unit (243) that finely compensates the center frequency of the digital signal after the coarse compensation for the frequency difference has been performed by the frequency shift unit (241).

Description

[Technical field]

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

[0002] In the field of optical fiber communications, digital coherent technology has been widely applied in metro core networks and submarine optical cable systems. Digital coherent optical communications has become increasingly widespread since the 2010s, and with this progress, in recent years, there have been considerations of applying it to optical communication terminals mounted on satellites 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 by assigning different information to each subcarrier and accommodating a variety of services simultaneously, it is possible to achieve more efficient hardware utilization and save space. Non-Patent Document 1 discloses a digital coherent optical communication technology capable of transmitting and receiving such subcarrier signals. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] 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 Summary of the Invention [Problem to be solved by the invention]

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

[0006] However, when an optical receiver handles low- to medium-speed digital coherent signals, for example when a field programmable gate array (FPGA) with a sampling rate or throughput of several gigabits is used for digital signal processing, or when the modulation speed 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 receiving digital signal processing is reduced. In particular, when the difference in frequency between the transmitted and received optical carrier waves is large, the waveform cannot be expressed correctly, which may result in making it difficult to compensate for waveform distortion.

[0007] The present disclosure has been made in consideration of the above-mentioned points, and aims to provide an optical receiver that is capable of compensating 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. [Means for solving the problem]

[0008] The optical receiver according to the present disclosure includes an optical coherent detection unit 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 analog / digital conversion unit that analog-to-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 having a frequency shift unit that performs coarse compensation to shift the center frequency of the digital signal obtained by the analog / digital conversion unit to a frequency that reduces a frequency difference between transmission and reception, and a frequency difference estimation compensation unit that finely compensates the center frequency of the digital signal after the frequency difference has been coarsely compensated for by the frequency shift unit. The coarse compensation of the frequency difference by the frequency shifter is performed by gradually shifting the center frequency of the digital signal obtained by the analog / digital converter so that when the estimated value of the frequency difference between the transmission and reception falls within the allowable range in which the frequency difference estimator / compensator can tolerate fine compensation, the estimated value of the frequency difference between the transmission and reception falls within the allowable range and the quality of the waveform of the demodulated signal obtained by the frequency difference estimator / compensator does not change. . Effect of the Invention

[0009] According to the present disclosure, even if a difference in frequency occurs between the transmitted and received optical carrier waves, it is possible to compensate for waveform distortion. [Brief description of the drawings]

[0010] [Figure 1] 1 is a block diagram showing an optical transmitting / receiving device including an optical receiver according to a first embodiment. [Diagram 2] 3 is a block diagram showing a receiving-side digital signal processing unit in the optical receiver according to the first embodiment. FIG. [Diagram 3] 1 is a diagram showing an example of a relationship between the frequencies of transmitted and received optical carriers in a digital coherent system (the estimated value of the frequency difference is within a permissible range). [Figure 4] 4 is a diagram showing the frequency (signal spectrum) of a digital signal obtained by an ADC unit and input to a receiving DSP when the transmitting and receiving optical carrier waves have the relationship shown in FIG. [Diagram 5] 13 is a diagram showing another example of the relationship between the frequencies of the transmitting and receiving optical carriers in the digital coherent system (the estimated value of the frequency difference is outside the allowable range and within the compensable range). FIG. [Figure 6]6 is a diagram showing the frequency (signal spectrum) of a digital signal obtained by an ADC unit and input to a receiving DSP when the transmitting and receiving optical carrier waves have the relationship shown in FIG. 5. FIG. [Figure 7] FIG. 11 is a diagram illustrating coarse compensation of the frequency (signal spectrum) of a digital signal input to a reception-side DSP 240 in the optical receiver according to the first embodiment. [Figure 8] FIG. 13 is a diagram showing frequency differences on the IQ signal plane space where the frequency difference between transmission and reception can be compensated. [Figure 9] FIG. 13 is a diagram showing frequency differences on an IQ signal plane space where the frequency difference between transmission and reception cannot be compensated for. [Figure 10] 13 is a flowchart showing operations of a frequency shifter 241 and a frequency shift controller 246 with respect to an estimated value of a frequency difference obtained by a frequency difference estimating and compensating unit 243 in the optical receiver according to the first embodiment. [Figure 11] 10 is a diagram for explaining a schematic transition of an estimated value of a frequency difference between transmission and reception over time in the optical receiver according to the first embodiment. FIG. [Figure 12] 10 is a diagram illustrating an example in which an estimated value of a frequency difference between transmission and reception is within a permissible range in the optical receiver according to the first embodiment. FIG. [Figure 13] FIG. 10 is a diagram illustrating an example of rough compensation when an estimated value of a frequency difference between transmission and reception deviates from a tolerable range to the positive side in the optical receiver according to the first embodiment. [Figure 14] FIG. 10 is a diagram illustrating an example of rough compensation when an estimated value of a frequency difference between transmission and reception deviates from an allowable range on the negative side in the optical receiver according to the first embodiment. [Figure 15] 1 is a diagram illustrating a hardware configuration of an optical receiver according to a first embodiment. [Figure 16] FIG. 11 is a block diagram showing a receiving-side digital signal processing unit in an optical receiver according to a second embodiment. [Figure 17] FIG. 11 is a block diagram showing a receiving-side digital signal processing unit in an optical receiver according to a third embodiment. [Figure 18] 13 is a block diagram showing another example of a receiving-side digital signal processing unit in the optical receiver according to the third embodiment. FIG. [Figure 19] FIG. 11 is a block diagram showing a receiving-side digital signal processing unit in an optical receiver according to a fourth embodiment. [Figure 20] 1 is a diagram showing subcarrier signals in a subcarrier multiplexing method in a digital coherent system on the frequency axis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Embodiment 1 An optical transmitting / receiving device including an optical receiver according to a first embodiment will be described with reference to FIGS. In FIG. 1, dashed arrows indicate the flow of optical signals, and solid arrows indicate the flow of electrical signals. The optical transmitting / receiving device equipped with the optical receiver of embodiment 1 is a device that focuses on the transmission and receiving 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 objects 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 and an optical receiver in the same housing as an optical transmitter and a control device for controlling the optical transmitter. As an optical transmission / reception device including the optical receiver according to the first embodiment, a case where the optical transmission / reception device is applied to an optical communication system using low-medium speed digital coherent signals of less than 100 Gbps will be described below. However, the present invention is not limited to low- to medium-speed digital coherent signals of less than 100 Gbps, but can also be applied as a method for compensating for wide frequency differences in optical transceivers 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 that is common for optical fiber networks, an optical signal is assumed to be polarization multiplexed by the digital coherent system using X polarization and Y polarization, and the X polarization and the Y polarization are respectively modulated into an I signal and a Q signal with orthogonal phase in the optical signal. For example, the X polarization is a horizontal polarization, and the Y polarization is a vertical polarization. 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, respectively (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, that is, as a single path.

[0016] The present invention may also be applied to optical transceivers that transmit and receive single-polarized digital coherent signals that do not perform polarization multiplexing, i.e., optical signals modulated into orthogonal phase 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 method can be described without distinction from the polarization multiplexed digital coherent method, the description of the single-polarized digital coherent method will be omitted in the following description.

[0017] As shown in FIG. 1, the optical transmitter and receiver includes an optical transmitter 100, an optical receiver 200, and an optical transmitter and 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. The optical transmitter 100 will be described briefly since it is a commonly known device.

[0018] The optical transmitter 100 includes a modulating signal generating section 110, a digital-to-analog converter (DAC) section 120 which is a digital / analog converter, an optical modulating section 130, and a continuous wave (CW) light generating section 140. The modulated signal generating unit 110 is a transmitting-side digital signal processing unit (transmitting digital signal processor (DSP: digital signal processor), hereinafter referred to as the transmitting-side DSP 110).

[0019] The transmitting side DSP 110 receives data, which is information to be sent to a 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 data to be transmitted is constructed by the transmitting DSP 110 into a frame, which is a signal format capable of error correction.

[0020] In the case of a polarization multiplexed digital coherent system, the transmitting side 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 transmitting-side DSP 110 generates two modulated signals, an I signal and a Q signal. In the optical transmitting / receiving device, each modulated signal is processed in the same manner, so no distinction is made between the modulated signals, and the following description will be given for 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 expressed by two samples in the digital domain, the DAC unit 120 uses a digital / analog converter of 2 gigasamples / second for a symbol rate of 1 gigabaud.

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

[0023] As shown in FIG. 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 receiving side DSP 240 hereinafter.

[0024] The optical coherent detection unit 210 receives modulated light transmitted from an optical transmitter of another optical transmission / reception device via a transmission line, and converts the received modulated light into a single carrier frequency f Rx 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 constant frequency, and outputting an analog electrical signal based on the voltage obtained by photoelectric conversion. The voltage signal output from the optical coherent detection unit 210 is a single-ended output or a differential output.

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

[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 section 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 converted into digital signals by the ADC unit 230, and then demodulated by the receiving DSP 240 into a single polarized signal. In addition, when the I signal and Q signal are 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 converted to digital form by the ADC unit 230 and then demodulated into a single polarized signal 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 Rx 1 is a signal before demodulation, which is an analog voltage signal obtained by optically interfering interference light having the above-mentioned characteristic by the optical coherent detection unit 210. The signal before demodulation is a signal for obtaining a demodulated signal obtained by the receiving side DSP 240. In the optical transmission / reception device, each pre-demodulation signal, in the first embodiment, four signals, XI signal, XQ signal, YI signal, and YQ signal, are processed, but since the processing is similar, no distinction will be made between the pre-demodulation signals, and the following description will be given for one pre-demodulation signal.

[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 It is almost the same frequency. Carrier frequency f Tx and carrier frequency f Rx is, for example, approximately 193.1 THz. The carrier frequency f in the interference light from the interference light generating unit 220 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 carrier frequency f Tx0 There is a frequency difference between In the following description, the CW light from the CW light generating section 140 in the optical transmitter of the other optical transmitting / receiving device will be simply described 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 above is the same, the carrier frequency 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 differ due to aging of the light source of the interference light generation unit 220 or a large Doppler shift caused by 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. Tx When 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 described simply as the frequency difference between transmission and reception. The frequency difference between transmission and reception is the carrier frequency f Rx These are values ​​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 modulation rate, e.g., a carrier frequency f Tx In the case of a QPSK system, when the modulation speed is about 100 Gbps, 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, which is a discrete signal in the digital domain, to obtain 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 compensation unit 243 in the receiving DSP 240 . The signal before demodulation sent from the ADC unit 230 to the receiving side DSP 240 is a time-series signal under high sampling rate conditions. The digital signal before demodulation from the ADC unit 230 is a signal that has been processed in the wideband digital frequency domain.

[0035] The receiving side DSP 240 performs coarse compensation for the frequency difference by shifting the pre-demodulation signal from the ADC unit 230 to a frequency that reduces the frequency difference Δf between transmission and reception, and finely compensates the frequency of the digital signal after the coarse frequency difference compensation has been performed, thereby converting the pre-demodulation signal into a demodulated signal. In the first embodiment, the receiving side DSP 240 performs digital signal processing in a digital coherent system.

[0036] As shown in FIG. 2, the receiving side DSP 240 includes a frequency shift unit 241 and It has 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 rough compensation for the frequency difference by shifting 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.

[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 tolerate fine compensation.

[0038] As is commonly known, the estimated value Δf of the frequency difference between transmission and reception is estimated in the frequency difference estimation compensation unit 243 by the frequency of the phase rotation of the signal points 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 from 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 estimate of the frequency difference between transmission and reception, Δf, is shifted toward 0 Hz.

[0039] It can be said that the frequency shift section 241 has a function of receiving a frequency setting signal from the frequency shift control section 246 and providing a sine wave for frequency shifting to the digital signal obtained by the ADC section 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 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 obtained by shifting this frequency difference by a fixed value 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 (Allowable) in which the estimated value Δf can be performed by the frequency difference estimation compensation unit 243 to perform fine compensation. 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 the first embodiment, 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 estimated value Δf is compensable when the frequency difference exceeds the allowable 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 a frequency difference that can be adaptively and precisely estimated and compensated for, but if it deviates from this range, precise compensation is not possible.

[0044] During operation, when the frequency setting signal from the frequency shift control unit 246 indicates permission, 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 the initial setting value, the digital signal obtained by the ADC unit 230 has the initial setting value as the center frequency value. In addition, when 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 having a frequency based on the changed value is applied to the digital signal obtained by the ADC unit 230.

[0046] FIG. 3 shows the carrier frequency f in the modulated light received by the optical coherent detection unit 210 when the estimated value Δf is a frequency difference Δf1 within the allowable range. Tx1 and the carrier frequency f Rx 4 shows the carrier frequency f Tx1 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 in FIG. The frequency shifter 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 adjusting the center frequency of the digital signal obtained by the ADC unit 230 to 0 Hz. The frequency shifter 241 updates the center frequency of the digital signal obtained by the ADC unit 230 gradually so that the quality of the waveform of the demodulated signal obtained by the frequency difference estimator / compensator 243 does not change.

[0048] FIG. 5 shows the carrier frequency f in the modulated light received by the optical coherent detection unit 210 when the estimated value Δf is outside the allowable range and the frequency difference Δf2 is within the compensable range. Tx2 and the carrier frequency f Rx 6 shows the carrier frequency f Tx2 and carrier frequency f Rx 6 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 in FIG. 5 is satisfied.

[0049] The frequency difference Δf2 that is outside the allowable range and within the compensable range is, for example, the carrier frequency f Rx and the carrier frequency f Tx0 This occurs because even if the orbital conditions between satellites are the same, a large Doppler shift occurs.

[0050] When the digital signal obtained by the ADC unit 230 shown in FIG. 6 (FIG. 7(a)) is processed by the equalization unit 242 and received by the frequency difference estimation compensation unit 243, the frequency shift control unit 246 determines that the estimated value Δf2 of the frequency difference between transmission and reception is outside the allowable range for fine compensation, and provides a frequency setting signal to the frequency shift unit 241 indicating that compensation is possible.

[0051] As shown in FIG. 7B, the frequency shifter 241 shifts the estimated value Δf2 from the compensable range to the allowable range by a frequency setting signal indicating that compensation is possible. In other words, the frequency shifter 241 shifts the frequency shift so that the estimated value Δf2 of the frequency difference is 0 Hz (Δf 2C ) to perform coarse compensation. That is, the frequency shift unit 241 calculates the shift amount Δf obtained by the frequency setting signal indicating that compensation is possible for the digital signal obtained by the ADC unit 230. 2α Multiplying a sine wave in the time domain for a frequency shift of Δf 2c =Δf2+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 changes the center frequency of the digital signal obtained by the ADC unit 230 gradually and stepwise so that the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit 243 does not change. That is, the frequency shift unit 241 changes the frequency of the sine wave by which the digital signal obtained by the ADC unit 230 is multiplied gradually in stages.

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

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

[0055] The signal processing for demodulation by the receiving side DSP 240 in the equalizer 242 is a general method in digital signal processing used in a normal digital coherent system, and will be explained briefly below. In the following description, the digital signal obtained by the ADC unit 230 and roughly compensated 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 ADC unit 230 that has been roughly compensated by 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 Y polarization signal (YI signal and YQ signal). In short, the equalization section 242 performs compensation for waveform distortion on the roughly compensated digital signal obtained by the ADC section 230 .

[0057] The frequency difference estimation compensation unit 243 estimates the carrier frequency f Tx and the carrier frequency f Rx This compensates for the frequency difference (frequency difference between transmission and reception). The frequency difference estimating and compensating unit 243 finely compensates the center frequency of the digital signal after the frequency shifting unit 241 has roughly compensated for the frequency difference estimate Δf. If the frequency difference Δf between transmission and reception is within a permissible range, the frequency difference estimating and compensating unit 243 can perform fine compensation that can adaptively and precisely estimate and compensate the digital signal that has been roughly compensated.

[0058] The frequency difference estimation compensation unit 243 estimates the frequency difference of the modulated light received by the optical coherent detection unit 210. The frequency is estimated based on the phase rotation of the signal point of the periodically inserted pilot symbol signal expressed in the IQ signal plane space, and the frequency of the digital signal obtained by the ADC unit 230 that has been roughly compensated by the frequency shift unit 241 is finely compensated. It should be noted that the precise compensation of the frequency of a digital signal is not limited to compensation using a pilot insertion method using a pilot symbol signal, but may be compensation using a commonly known frequency estimation method such as a method based on Fourier transform analysis or the power method.

[0059] The frequency difference estimation and compensation unit 243 compensates for the rotation of the signal points, that is, compensates for the frequency difference, by multiplying the digital signal in the time domain. However, since the frequency difference varies with the elapsed time, the frequency difference estimation and compensation unit 243 adaptively updates the estimated value of the frequency difference and therefore the frequency of the sine wave for compensation. The frequency difference estimation / compensation unit 243 compensates for the frequency difference using a general method in digital signal processing used in a typical digital coherent system.

[0060] It is assumed that the frequency difference estimation compensation unit 243 receives as input a signal with a lower sampling rate than that of the frequency shift unit 241, such as a time series signal expressed with one sample per symbol or a time series signal expressed with two samples per symbol. Even with such a low sampling rate, the estimated value Δf of the frequency difference between transmission and reception is roughly compensated by the frequency shifter 241 and falls within an allowable range, and the frequency of the digital signal obtained by the ADC unit 230 is near 0 Hz due to the rough compensation, so that the frequency difference estimation and compensation unit 243 can normally perform adaptive and precise estimation and compensation even in a narrow frequency space. For example, as shown in FIG. 8, if the phase rotation when there is a frequency difference is within 180° of when there is no frequency difference, that is, if the estimated value Δf of the frequency difference between transmission and reception can be compensated for, then estimation and compensation can be performed normally.

[0061] Note that if rough compensation is not performed by frequency shifter 241 and, for example, the center frequency of the digital signal obtained by ADC unit 230 deviates significantly from 0 Hz and the estimated value Δf of the frequency difference between transmission and reception deviates from the compensable range (is not compensable), then frequency difference estimation compensation unit 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 of the case with a frequency difference exceeds 180° compared to the case without a frequency difference, which ultimately causes an estimation error in frequency difference estimation compensation unit 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 for demodulation in the receiving side DSP 240 in the phase estimation unit 244 and the symbol decision unit 245 is a general method in digital signal processing used in a normal digital coherent system, and will be described briefly below. The phase estimation section 244 compensates for phase fluctuations of the light source constituting the CW light generating section in the optical transmitter and the light source constituting the interference light generating section 220 . The symbol decision unit 245 decides the transmission symbol in the demodulated signal that has been compensated for waveform distortion. In addition, after the symbol decision unit 245, error correction of the demodulated signal may be performed.

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

[0065] The estimated value Δf after being initialized is shown at time t0 in FIG. 11, the horizontal axis indicates time, the vertical axis indicates the estimated value, and the black circle indicates the estimated value Δf obtained by the frequency difference estimation compensation unit 243. The estimated value Δf is obtained on the order of a symbol period. Alternatively, the estimated value Δf may be calculated by a moving average from multiple symbols. In order to simplify the explanation, the estimated values ​​Δf indicated by black circles in FIG. 11 do not represent all the estimated values ​​Δf obtained by the frequency difference estimation and compensation unit 243.

[0066] When communication is started with an optical transmitter of another optical transceiver device, and the estimated value Δf obtained by the frequency difference estimation compensation unit 243 is within the allowable range (−Δfallowable≦Δf≦+Δfallowable: 2×Δfallowable) as shown in FIG. 12, the frequency setting signal from the frequency shift control unit 246 indicates allowance, and the frequency shift unit 241 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 without updating the shift amount. Therefore, the digital signal obtained by the ADC unit 230 is subjected to precise compensation by the frequency difference estimation compensation 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 estimating and compensating unit 243 is within the allowable range, and determines that the estimated value Δf of the frequency difference is within the allowable range until immediately before time t1 shown in FIG. 11, so obtains a frequency setting signal indicating allowance. Then, the frequency shift unit 241 does not update the shift amount, and 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 current shift amount setting value has been applied. Step ST2 is a step for maintaining the amount of shift in 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] At time t1 shown in FIG. 11, as shown in FIG. 13, when the estimated value Δf of the frequency difference obtained by the frequency difference estimation compensation 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 in which the frequency difference estimate value Δf becomes 0 at a speed that does not interrupt the compensation function of the frequency difference estimation and compensation unit 243, and then proceeds to step ST4.

[0069] The frequency difference estimation and compensation unit 243 performs precision compensation on the digital signal obtained by the ADC unit 230 to which a sine wave having 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 estimation compensation 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 shift unit 241 updates the shift amount and proceeds to step ST4.

[0070] Steps ST3 and ST4 are repeated until the estimated value Δf of the frequency difference falls within the range of 0±β, up to time t2 shown in FIG. β is a value larger than the changeable frequency difference that the frequency shifter 241 can obtain by updating the shift amount once. Steps ST3 and ST4 are update steps in which the frequency shifter 241 updates the amount of shift in 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 shift unit 241 performs coarse compensation of the frequency difference between the transmission and reception with respect to the center frequency of the digital signal obtained by the ADC unit 230. Also, although not explicitly shown in FIG. 10, there is a step in which the frequency difference estimation and compensation unit 243 performs fine compensation on the digital signals on which the frequency difference between the transmitting and receiving signals has been roughly compensated for in steps ST2, ST3, and ST4.

[0072] When the estimated value Δf of the frequency difference falls within the range of 0±β at time t2 shown in FIG. 11, 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. In FIG. 11, since the estimated value Δf of the frequency difference is within the allowable range immediately after time t2 until just before time t3, in step ST2, the frequency shifter 241 does not update the shift amount, and the frequency difference estimation and compensation unit 243 precisely compensates the digital signal obtained by the ADC unit 230 that has been given a sine wave of a frequency based on the current shift amount maintained after the update.

[0073] As shown in FIG. 14, at time t3 shown in FIG. 11, when the estimated value Δf of the frequency difference obtained by the frequency difference estimating and 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 value Δf at a speed that does not interrupt the compensation function in the frequency difference estimation and compensation unit 243 until the frequency difference estimate value Δf falls within the range of 0±β. The frequency difference estimation and compensation unit 243 performs precision compensation on the digital signal obtained by the ADC unit 230 to which a sine wave having a frequency based on the updated shift amount has been applied.

[0074] When the estimated value Δf of the frequency difference falls within the range of 0±β at time t4 shown in FIG. 11, 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. In this way, coarse compensation is performed by the frequency shift unit 241, and the frequency difference estimation compensation unit 243 performs fine compensation on the digital signal obtained by the ADC unit 230 after the coarse compensation has been performed by the frequency shift unit 241. Therefore, waveform distortion compensation can be performed even if a frequency difference occurs between the transmitted and received optical carrier waves.

[0075] Next, the hardware configuration of the optical receiver 200 according to the first embodiment will be described with reference to FIG. In FIG. 15, the same reference numerals as those in FIG. 1 and FIG. 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 with a RAM (Random Access Memory) and a ROM (Read Only Memory), a communication interface 253, and an input / output interface 254. The processor 251, memory 252, communication interface 253, and 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 of the memory 252 into the RAM of the memory 252, and executes processing according to the loaded program. The ROM in the memory 252 stores various data, programs for executing processes in the optical receiver 200, processing programs required for starting up the optical receiver 200, and the like. The communication interface 253 is used for transmitting and receiving data and control signals between each component in the optical receiver 200 and each component in other optical transmitting and receiving devices.

[0077] The input / output interface 254 transmits and receives control signals and modulation signals between each component in the optical receiver 200 via electrical wiring. The input / output interface 254 is, for example, 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 an interface for outputting various control signals to, for example, the receiving DSP unit.

[0078] In FIG. 15, for ease of understanding, the optical receiver control unit 250 is shown as a separate component from the receiving DSP 240. However, the functional parts of the receiving DSP 240 that can be configured by software may be handled by the optical receiver control unit 250. For example, the functions of the frequency shifter 241 , the equalizer 242 , the frequency difference estimator 243 , the phase estimator 244 , the symbol decider 245 and the frequency shift controller 246 that constitute the receiver 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 a ROM in the memory 252 and executed by the processor 251 includes a procedure for performing coarse compensation for the frequency difference between transmission and reception, the procedure having a procedure for updating the amount of shift in the center frequency of the digital signal obtained by coherently detecting an analog electrical signal by interfering with an interference light, which is optically modulated at a carrier frequency, and performing coarse compensation for the frequency difference between transmission and reception, and a procedure for fine compensation for the center frequency of the digital signal obtained by coarsely compensating for the frequency difference between transmission and reception. The procedure for updating the shift amount of the center frequency of the digital signal is to shift it stepwise so that the quality of the waveform of the demodulated signal does not change.

[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 / digital conversion of the analog electrical signal from the optical coherent detection unit 210 to obtain a digital signal, and a receiving-side DSP 240 having 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 difference estimation compensation unit 243 that finely compensates the center frequency of the digital signal obtained by the frequency shift unit 241 after the coarse compensation for the frequency difference has been performed. Therefore, the frequency difference estimation compensation unit 243 receives a digital signal with the center frequency of the digital signal obtained by the ADC unit 230 close to 0 Hz, that is, a digital signal with a reduced phase rotation speed, and therefore the frequency difference estimation compensation unit 243 can accurately read the amount of phase rotation of the digital signal, and the frequency difference estimation compensation unit 243 can adaptively and precisely estimate and compensate the optical frequency difference. As a result, the signal quality of the demodulated signal from the receiving DSP 240 can be improved, and the compensation range for the optical frequency difference in the optical receiver can be expanded.

[0081] Embodiment 2 The optical receiver according to the second embodiment will be described with reference to FIG. The optical receiver according to the second embodiment is different from the optical receiver according to the first embodiment in that it has a receiving side DSP 240, but is otherwise the same. In FIG. 16, the same reference numerals as those in FIG. 1 and FIG. 2 indicate the same or corresponding parts. The equalization unit 242, the frequency difference estimation compensation unit 243, the phase estimation unit 244, and the symbol decision unit 245 constituting the receiving side DSP 240 are the same as the equalization unit 242, the frequency difference estimation compensation unit 243, the phase estimation unit 244, and the symbol decision unit 245 constituting the receiving side DSP 240 in the optical receiver of embodiment 1.

[0082] The frequency shifter 241 and the frequency shift control unit 246 differ from those of the frequency shifter 241 and the frequency shift control unit 246 in the optical receiver according to the first embodiment in the method of setting the initial setting value at the start of communication, but they are the same in the implementation of coarse compensation during operation, in which the frequency shifter 241 and the frequency shift control unit 246 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. Therefore, the setting of the initial settings at the start of communication will be described.

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

[0084] The frequency shifter 241 sequentially performs frequency shifts on the center frequency of the digital signal obtained by the ADC unit 230 by shift amounts based on information indicating a plurality of discrete shift amounts. The frequency shift unit 241 sets an initial setting value to a shift amount indicating a frequency difference within a compensation range in the frequency difference estimation and compensation unit 243, preferably 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 judgment value in the clock synchronization function of the equalization unit 242 or the continuity information.

[0085] The optical receiver of the second embodiment has the same effects as the optical receiver of 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] Embodiment 3 The optical receiver according to the third embodiment will be described with reference to FIG. The optical receiver according to the third embodiment is different from the optical receiver according to the first embodiment in that it has a receiving side DSP 240, but is otherwise the same. In FIG. 17, the same reference numerals as those in FIG. 1 and FIG. 2 indicate the same or corresponding parts. The equalization unit 242, the frequency difference estimation compensation unit 243, the phase estimation unit 244, and the symbol decision unit 245 constituting the receiving side DSP 240 are the same as the equalization unit 242, the frequency difference estimation compensation unit 243, the phase estimation unit 244, and the symbol decision unit 245 constituting the receiving side DSP 240 in the optical receiver of embodiment 1.

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

[0088] At the time of 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 an amount indicated by a 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 any other shift amount setting information indicating the carrier frequency of another optical transmitter, and therefore can perform coarse compensation and fine compensation accurately.

[0090] In the optical receiver according to the third embodiment, the setting value of the shift amount by the frequency shift unit 241 is determined based on the Doppler shift amount estimated by satellite orbit information or on any one of shift amount setting information indicating the carrier frequency of another optical transmitter. However, as shown in FIG. 18, like the optical receiver according to the first embodiment, the frequency shift control unit 246 may have a function of obtaining an estimate Δf of the frequency difference between transmission and reception by the frequency difference estimation and compensation unit 243, obtaining a frequency setting signal to which the estimate Δf is linked, indicating acceptance when the estimate Δf is within an allowable range, and indicating compensation when the estimate 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 shift unit 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 setting of the shift amount by the frequency shifter 241 based on the shift amount setting information and the 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 estimation compensation unit 243 are used in combination.

[0092] The 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 estimation and compensation unit 243 during operation and the setting of the shift amount by the frequency shifter 241 based on the shift amount setting information are performed in the following manner. During normal operation during operation, as in embodiment 1, the frequency shift unit 241 sets the amount of shift 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 which receives the estimated value Δf of the frequency difference obtained by the frequency difference estimation compensation unit 243.

[0093] When the frequency shifter 241 receives information on the amount of shift setting from the outside, the frequency shifter 241 The shift amount based on the external shift amount setting information is added to the shift amount based on the frequency setting signal, and the added shift amount is set as the shift amount for the center frequency of the digital signal obtained by the ADC unit 230. The updating is performed gradually so that the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit 243 does not change.

[0094] However, during operation, when the frequency shifter 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 shifter 241 stops setting the shift amount by adding the shift amount indicated by the shift amount setting information and focuses on setting 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 estimation compensation unit 243.

[0095] Embodiment 4 An optical receiver according to the fourth embodiment will be described with reference to FIGS. The optical receiver of embodiment 4 receives modulated light in which data information is modulated by multiple different subcarrier signals (multiple different carrier frequencies) using a subcarrier multiplexing system, whereas the optical receiver of embodiment 1 receives modulated light in which data information is modulated by a single carrier signal (single carrier frequency) in a conventional digital coherent system. In the following description, the subcarrier signal on which data information is superimposed will be simply referred to as the subcarrier signal in order to avoid complicating the description. In FIG. 19, the same reference numerals as those in FIG. 1 and FIG. 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 by digital signal processing on the receiving side. In the fourth embodiment, the multiple different subcarrier signals in the modulated light received by the optical receiver may be generated individually for the data signals in the independent modulated light. Therefore, it is also assumed that the optical receiver receives uncorrelated modulated light sent from different points all at once.

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

[0098] The optical coherent detection unit 210, the interference light generating unit 220, and the ADC unit 230 in the optical receiver of embodiment 4 basically have the same configuration as the optical coherent detection unit 210, the interference light generating unit 220, and the ADC unit 230 in the optical receiver of embodiment 1, and are used in a commonly known subcarrier multiplexing method, so description thereof will be omitted.

[0099] The receiving side DSP 240 is provided to correspond to four pre-demodulation signals, namely, an XI signal, an XQ signal, a YI signal, and a 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 be given for 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] As shown in FIG. 20, the N subcarrier signals in the modulated light are center-symmetric on the frequency axis with respect to the central frequency for the N subcarrier signals when N is an even number. Moreover, the carrier frequency of the interference light from the interference light generating section 220 is set to the center frequency for the N subcarrier signals.

[0101] The N receiving side DSPs 240-1 to 240-N each perform processing individually corresponding to the N subcarriers, 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 decision 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 in the receiving side DSP 240 in the equalization unit 242, the phase estimation unit 244, and the symbol decision unit 245 is performed in the same manner as the signal processing for demodulation in the receiving side DSP 240 in the equalization unit 242, the phase estimation unit 244, and the symbol decision unit 245 in embodiment 1, corresponding to each of the N subcarrier signals. In addition, the adjacent signal remover 248 functions as a low-pass filter or band-pass filter that removes adjacent signals outside the band of each subcarrier signal for the N subcarrier signals that have been given a shift amount by the frequency shifter 241, and performs signal separation for the subcarrier signals.

[0103] At the start of communication, frequency shift section 241 in each of N receiving side DSPs 240-1 to 240-N sets an initial value as a shift amount based on the frequency difference of the subcarrier signals with respect to the center frequency for N subcarrier signals. As a result, in each of the N receiving side DSPs 240-1 to 240-N, the initially set frequency shift section 241 shifts the frequency of the subcarrier signal in the digital signal obtained by the ADC section 230 so that it falls 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 frequency of the subcarrier signals with respect to 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, so that the frequency difference estimation compensation unit 243 in each of the N receiving side DSPs 240-1 to 240-N is within the allowable 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 starts communication with an optical transmitter of another optical transmitting / receiving device after the initial setting, if the estimated value Δf obtained by the corresponding frequency difference estimation compensation unit 243 is within the allowable range, the frequency shift unit 241 provides a sine wave of the initially set frequency to the digital signal obtained by the ADC unit 230 without updating the initially set shift amount. Therefore, the digital signal of the subcarrier signal obtained by the ADC unit 230 is subjected to precise compensation by the frequency difference estimation compensation unit 243 .

[0107] When the estimated value Δf obtained by the corresponding frequency difference estimation and compensation unit 243 in each of the N receiving side DSPs 240-1 to 240-N deviates from the allowable range, the frequency shift unit 241 updates the shift amount in the direction in which the frequency difference estimated value Δf becomes zero. That is, in each of the N receiving side DSPs 240-1 to 240-N, similarly to the first embodiment, the frequency shifter 241 performs coarse compensation, and the frequency difference estimator 243 performs fine compensation.

[0108] The optical receiver of embodiment 4 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 the allowable range. Therefore, the phase rotation amount of the corresponding subcarrier signal can be accurately read in each receiving side DSP 240-1 to 240-N, and the frequency difference estimation compensation unit 243 can 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. [Industrial Applicability]

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

[0111] 100 optical transmitter, 110 modulated signal generating unit, 120 DAC unit, 130 optical modulation unit, 140 CW light generating unit, 200 optical receiver, 210 optical coherent detection unit, 220 interference light generating 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 decision unit, 246 frequency shift control unit, 247 adjacent signal removal unit.

Claims

1. an optical coherent detection unit that receives modulated light that has been optically modulated by a carrier frequency, causes the received modulated light to interfere with an interference light, performs 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 reception-side digital signal processing unit having a frequency shift unit that performs coarse compensation for shifting a center frequency of the digital signal obtained by the analog / digital conversion unit to a frequency that reduces a frequency difference between transmission and reception, and a frequency difference estimation and compensation unit that performs fine compensation for the center frequency of the digital signal that has been subjected to coarse compensation for the frequency difference by the frequency shift unit; An optical receiver in which the coarse compensation of the frequency difference by the frequency shift unit is performed by gradually shifting the center frequency of the digital signal obtained by the analog / digital conversion unit so that, when the estimated value of the frequency difference between transmission and reception deviates from the allowable range for which the frequency difference estimation compensation unit can tolerate fine compensation, the estimated value of the frequency difference between transmission and reception falls within the allowable range and the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit does not change.

2. the modulated light has an optical signal modulated to quadrature phase I and Q signals; The analog / digital conversion unit and the receiving side digital signal processing unit process the I signal and the Q signal separately.

2. The optical receiver according to claim 1.

3. 3. The optical receiver according to claim 1, wherein the frequency shifter performs coarse compensation of the frequency difference based on shift amount setting information from outside the receiving digital signal processor, the shift amount indicating the frequency shift amount.

4. an optical coherent detection unit that receives modulated light that has been optically modulated by a carrier frequency, causes the received modulated light to interfere with an interference light, performs 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; an optical receiver comprising: a frequency shift unit that performs coarse compensation for shifting a center frequency of a digital signal obtained by the analog / digital conversion unit to a frequency that reduces a frequency difference between a transmission frequency and a reception frequency; and a reception-side digital signal processing unit having a frequency difference estimation compensation unit that performs fine compensation for a center frequency of the digital signal obtained by the frequency shift unit that has been subjected to coarse compensation for a frequency difference, The frequency shift unit roughly compensates for the frequency difference 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 falls within a tolerable range, based on shift amount setting information, which is either a Doppler shift amount estimated from orbit information of a satellite equipped with another optical transceiver device not including the optical receiver, or information indicating the carrier frequency of an optical transmitter of another optical transceiver device not including the optical receiver, from outside the receiving-side digital signal processing unit indicating the amount of frequency shift.

5. an optical coherent detection unit that receives modulated light that has been optically modulated by a carrier frequency, causes the received modulated light to interfere with an interference light, performs 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 reception-side digital signal processing unit having a frequency shift unit that performs coarse compensation for shifting a center frequency of the digital signal obtained by the analog / digital conversion unit to a frequency that reduces a frequency difference between transmission and reception, and a frequency difference estimation and compensation unit that performs fine compensation for the center frequency of the digital signal that has been subjected to coarse compensation for the frequency difference by the frequency shift unit; The optical receiver performs coarse compensation of the frequency difference by the frequency shift unit during initial setting by shifting the center frequency of the digital signal obtained by the analog-to-digital conversion unit so that the estimated value of the frequency difference between transmission and reception falls within an acceptable range based on shift amount setting information from outside the receiving digital signal processing unit indicating the amount of frequency shift, and during operation, when the estimated value of the frequency difference between transmission and reception deviates from the acceptable range that allows fine compensation, by shifting the center frequency of the digital signal obtained by the analog-to-digital conversion unit so that the estimated value of the frequency difference between transmission and reception falls within the acceptable range.

6. an optical coherent detection unit that receives modulated light that has been optically modulated by a carrier frequency, causes the received modulated light to interfere with an interference light, performs 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; an optical receiver comprising: a frequency shift unit that performs coarse compensation for shifting a center frequency of a digital signal obtained by the analog / digital conversion unit to a frequency that reduces a frequency difference between a transmission frequency and a reception frequency; and a reception-side digital signal processing unit having a frequency difference estimation compensation unit that performs fine compensation for a center frequency of the digital signal obtained by the frequency shift unit that has been subjected to coarse compensation for a frequency difference, The coarse compensation of the frequency difference by the frequency shift unit is performed by adding, to a shift amount for reducing the frequency difference between transmission and reception when the estimated value of the frequency difference between transmission and reception deviates from an allowable range for which the frequency difference estimation compensation unit can allow fine compensation, a Doppler shift amount estimated from orbit information of a satellite equipped with another optical transmission / reception device not including the optical receiver from outside the receiving-side digital signal processing unit, or a shift amount indicated by shift amount setting information, which is information indicating the carrier frequency of an optical transmitter of another optical transmission / reception device not including the optical receiver, and shifting the center frequency of the digital signal obtained by the analog / digital conversion unit.

7. the modulated light has an optical signal modulated to quadrature phase I and Q signals; The analog / digital conversion unit and the receiving side digital signal processing unit process the I signal and the Q signal separately.

7. The optical receiver according to claim 4, wherein the first and second optical fibers are arranged in a first direction.

8. The modulated light received by the optical coherent detection unit is modulated light in which data information is modulated by a plurality of different subcarrier 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 subcarrier signals; 3. The optical receiver according to claim 1 or 2.

9. An optical receiving method in an optical receiver including an optical coherent detection unit, an analog / digital conversion unit, and a receiving-side digital signal processing unit having a frequency shift unit and a frequency difference estimation compensation unit, comprising: a step of performing rough compensation for the frequency difference between transmission and reception, the step including a step of the frequency shift unit maintaining an amount of shift in the center frequency of the digital signal obtained by the analog / digital conversion unit converting the analog electrical signal obtained by the optical coherent detection unit into digital form by coherently detecting the interference light caused by the optical coherent detection unit optically modulating the modulated light with a carrier frequency, so that the frequency difference between transmission and reception is within an allowable range, and a step of updating, when the frequency difference between transmission and reception deviates from the allowable range, the amount of shift in the center frequency of the digital signal obtained by the analog / digital conversion unit so as to shift stepwise so as not to change the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit; A step in which the frequency difference estimation and compensation unit finely compensates a center frequency of the digital signal on which coarse compensation of the frequency difference between the transmission and reception has been performed; An optical receiving method comprising:

10. a step of roughly compensating for the frequency difference between the transmitter and the receiver, the step including a step of maintaining the amount of shift in the center frequency of a digital signal obtained by coherently detecting an analog electrical signal by interfering with an interference light, the modulated light being optically modulated with a carrier frequency, while keeping the amount of shift in the center frequency of the digital signal within an allowable range between the transmitter and the receiver, and a step of updating the amount of shift in the center frequency of the digital signal so as to shift it stepwise so as not to change the quality of the waveform of the demodulated signal when the frequency difference between the transmitter and the receiver deviates from the allowable range; A step of finely compensating the center frequency of the digital signal after the coarse compensation of the frequency difference between the transmission and reception has been performed; An optical receiving program that causes a computer to execute the above.

11. a step of roughly compensating for the frequency difference between the transmitter and the receiver, the step including a step of maintaining the amount of shift in the center frequency of a digital signal obtained by coherently detecting an analog electrical signal by interfering with an interference light, the modulated light being optically modulated with a carrier frequency, while keeping the amount of shift in the center frequency of the digital signal within an allowable range between the transmitter and the receiver, and a step of updating the amount of shift in the center frequency of the digital signal so as to shift it stepwise so as not to change the quality of the waveform of the demodulated signal when the frequency difference between the transmitter and the receiver deviates from the allowable range; A step of finely compensating the center frequency of the digital signal after the coarse compensation of the frequency difference between the transmission and reception has been performed; A recording medium storing a program for causing a computer to execute the above.