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

Abstract

The optical receiver includes: an optical coherent detection section (210) that receives modulated light subjected to optical modulation by a carrier frequency, performs coherent detection by causing the received modulated light to interfere with interference light, and outputs an analog electric signal; an analog-digital conversion section (230) that performs analog-digital conversion on the analog electric signal from the optical coherent detection section (210) to obtain a digital signal; and a reception-side digital signal processing section (240) that has a frequency shift section (241) that performs coarse compensation that shifts the center frequency of the digital signal obtained by the analog-digital conversion section (230) to a frequency that reduces the frequency difference between transmission and reception, and a frequency difference estimation compensation section (243) that performs fine compensation on the center frequency of the digital signal on which the frequency shift section (241) has performed coarse compensation of the frequency difference.

Description

Technical Field

[0001] This disclosure relates to an optical receiver and an optical receiving method for digital coherent optical communication. Background Technology

[0002] In the field of optical fiber communication, digital coherent technology is widely used in metropolitan area core networks and submarine optical cable systems.

[0003] Digital coherent optical communication has become increasingly popular since the 2010s. With its development, research has been conducted in recent years on optical communication terminals that are not only used in terrestrial fiber optic networks, but also in satellites.

[0004] For such advanced optical networks, digital coherent optical transceivers can generate multiple non-interfering subcarrier signals in the digital region with a single unit, allocate different information to each subcarrier and simultaneously accommodate multiple services, thereby achieving hardware utilization efficiency and space saving.

[0005] Non-patent document 1 illustrates a digital coherent optical communication technology capable of transmitting and receiving such subcarrier signals.

[0006] Non-patent literature: H. Sun et al. , "800G DSP ASIC Design Using ProbabilisticShaping and Digital Sub-Carrier Multiplexing, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 38, NO. 17, SEPTEMBER 1, 2020, p.p. 4744-4756

[0007] The optical receiving device shown in Non-Patent Document 1 converts the voltage signal detected and output by optical coherent detection into a digital signal via an analog-to-digital converter (ADC).

[0008] With a high sampling rate, the ADC can accurately acquire the analog received signal before analog-to-digital conversion over a wider frequency space, thereby enabling precise waveform distortion compensation in digital signal processing and improving the quality of the received signal.

[0009] However, when using FPGAs (Field Programmable Gate Arrays) with gigabit sampling rates or throughput for digital signal processing, or when the modulation speed of each subcarrier is low due to subcarrier multiplexing, the ability to compensate for waveform distortion in the digital signal processing on the receiving side is reduced if there is a frequency difference between the transmitted and received optical carriers.

[0010] Especially when the frequency difference between the transmitted and received optical carriers is large, the waveform cannot be represented correctly, resulting in a situation where waveform distortion compensation is difficult. Summary of the Invention

[0011] This disclosure was made in view of the above circumstances, and its object is to provide an optical receiver that can compensate for the frequency difference between the transmitted and received optical carriers even when a frequency difference occurs between the transmitted and received optical carriers.

[0012] The optical receiver disclosed herein comprises: an optical coherent detector that receives modulated light after optical modulation by a carrier frequency, performs coherent detection by interfering the received modulated light with interference light, and outputs an analog electrical signal; an analog-to-digital converter that performs analog-to-digital conversion on the analog electrical signal from the optical coherent detector to obtain a digital signal; and a receiving-side digital signal processing unit that includes a frequency shifting unit that performs coarse compensation to shift the center frequency of the digital signal obtained by the analog-to-digital converter towards a frequency that reduces the frequency difference between transmission and reception, and a frequency difference estimation compensation unit that performs fine compensation on the center frequency of the digital signal after the frequency difference coarse compensation is performed by the frequency shifting unit.

[0013] According to this disclosure, waveform distortion compensation can be achieved, for example, even if a frequency difference occurs between the transmitted and received optical carriers. Attached Figure Description

[0014] Figure 1 This is a block diagram showing an optical transceiver apparatus equipped with the optical receiver according to Embodiment 1.

[0015] Figure 2 This is a block diagram showing the receiving-side digital signal processing unit in the optical receiver according to Embodiment 1.

[0016] Figure 3 This is a diagram illustrating an example of the frequency relationship between the transmitted and received optical carriers in a digital coherent mode (the estimated value of the frequency difference is within an acceptable range).

[0017] Figure 4 This indicates that the optical carriers being transmitted and received are in a certain state. Figure 3The graph shows the frequency (signal spectrum) of the digital signal input to the receiving-side DSP obtained by the ADC unit under the condition of the relationship.

[0018] Figure 5 This is another example illustrating the frequency relationship between the transmitted and received optical carriers in a digital coherent mode (the estimated value of the frequency difference is outside the allowable range but within the compensable range).

[0019] Figure 6 This indicates that the optical carriers being transmitted and received are in a certain state. Figure 5 The graph shows the frequency (signal spectrum) of the digital signal input to the receiving-side DSP obtained by the ADC unit under the condition of the relationship.

[0020] Figure 7 This is a diagram showing the coarse compensation of the frequency (signal spectrum) of the digital signal input to the receiving side DSP240 in the optical receiver according to Embodiment 1.

[0021] Figure 8 It is a diagram representing the frequency difference in the IQ signal plane space between the transmit and receive signals, which is a compensable frequency difference.

[0022] Figure 9 It is a diagram representing the frequency difference in the IQ signal plane space where the frequency difference between transmission and reception is uncompensable.

[0023] Figure 10 This is a flowchart illustrating the operation of the frequency shifting unit 241 and the frequency shifting control unit 246 in the optical receiver according to Embodiment 1 on the estimated value of the frequency difference obtained by the frequency difference estimation compensation unit 243.

[0024] Figure 11 This is a diagram that briefly illustrates the shift of the estimated value of the frequency difference between transmission and reception over time in the optical receiver according to Embodiment 1.

[0025] Figure 12 This diagram schematically illustrates an example in which the estimated value of the frequency difference between transmission and reception in the optical receiver according to Embodiment 1 is within an allowable range.

[0026] Figure 13 This diagram schematically illustrates an example of coarse compensation in the optical receiver according to Embodiment 1, where the estimated value of the frequency difference between transmission and reception deviates from the allowable range towards the + side.

[0027] Figure 14 This diagram schematically illustrates an example of coarse compensation in the optical receiver according to Embodiment 1, where the estimated value of the frequency difference between transmission and reception deviates from the allowable range on one side.

[0028] Figure 15 This is a diagram showing the hardware structure of the optical receiver according to Embodiment 1.

[0029] Figure 16 This is a block diagram showing the receiving-side digital signal processing unit in the optical receiver according to Embodiment 2.

[0030] Figure 17 This is a block diagram showing the receiving-side digital signal processing unit in the optical receiver according to Embodiment 3.

[0031] Figure 18 This is a block diagram illustrating another example of the receiving-side digital signal processing unit in the optical receiver according to Embodiment 3.

[0032] Figure 19 This is a block diagram showing the receiving-side digital signal processing unit in the optical receiver according to Embodiment 4.

[0033] Figure 20 This is a diagram representing the subcarrier signals in the subcarrier multiplexing method of digital coherence on the frequency axis. Detailed Implementation

[0034] Implementation Method 1

[0035] according to Figures 1 to 15 An optical transceiver apparatus having the optical receiver described in Embodiment 1 will be described.

[0036] exist Figure 1 In the diagram, dashed arrows represent the flow of optical signals, while solid arrows represent the flow of electrical signals.

[0037] The optical transceiver apparatus equipped with the optical receiver described in Embodiment 1 is an optical communication network system that uses optical fiber as the transmission medium in optical access and optical core metropolitan area networks, and an optical communication network system that does not use optical fiber and envisions wireless space as the transmission medium for mobile bodies such as satellites in space optical communication, etc. It is a device that focuses on the transmission and reception functions of communication equipment responsible for transmitting and receiving optical signals and an optical transceiver.

[0038] In an actual system, the optical receiving device equipped with the optical receiver according to Embodiment 1 includes a control device for controlling the optical transmitter and the optical receiver together in the same housing as the optical transmitter and the optical receiver.

[0039] As an optical transceiver device having the optical receiver according to Embodiment 1, the following describes an optical transceiver device applied to an optical communication system using medium-low speed digital coherent signals of less than 100Gbps.

[0040] In addition, it is not limited to low- and medium-speed digital coherent signals of less than 100Gbps, but can also be used as a method to compensate for wide frequency differences in optical transceivers in optical communication systems that use high-speed digital coherent signals of 100Gbps or more.

[0041] An optical transceiver equipped with the optical receiver described in Embodiment 1 is used to transmit and receive optical signals, i.e., digital coherent signals, that utilize the orthogonality of phase, such as polarization-multiplexed binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or quadrature amplitude modulation (QAM).

[0042] In the following description, in an optical transceiver apparatus equipped with the optical receiver according to Embodiment 1, in a digital coherent polarization multiplexing method common in fiber optic networks, it is envisioned that an optical signal is polarized multiplexed using an X-polarized wave and a Y-polarized wave through a digital coherent method, and the X-polarized wave and the Y-polarized wave are modulated into optical signals I and Q signals with orthogonal phases, respectively. For example, the X-polarized wave is a horizontally polarized wave, and the Y-polarized wave is a vertically polarized wave.

[0043] That is, in optical transceivers corresponding to polarization-multiplexed QPSK or QAM, polarization-multiplexed digital coherent signals are used, which are divided into I signals (XI signals) in X-polarized waves, Q signals (XQ signals) in X-polarized waves, I signals (YI signals) in Y-polarized waves, and Q signals (YQ signals) in Y-polarized waves.

[0044] Furthermore, the terminals (XI output terminal, XQ output terminal, YI output terminal, and YQ output terminal) of the XI signal, XQ signal, YI signal, and YQ signal, as well as the channels (XI channel, XQ channel, YI channel, and YQ channel) of the electrical signal flow of the XI signal, XQ signal, YI signal, and YQ signal, are distinguished but summarized in the diagram, that is, shown as a single path.

[0045] In addition, it can also be applied to optical transceivers that transmit and receive digital coherent signals of a single polarized wave without polarization multiplexing, i.e., optical signals modulated into I and Q signals with orthogonal phase. In the case of digital coherent mode of single polarization wave, it can also be applied in the same way as digital coherent mode of polarization multiplexing.

[0046] Therefore, the digital coherence method for single-polarized waves can be described without distinction from the digital coherence method for polarization multiplexing. Therefore, the description of the digital coherence method for single-polarized waves is omitted in the following description.

[0047] like Figure 1 As shown, the optical transceiver includes an optical transmitter 100, an optical receiver 200, and an optical transmitter-receiver control unit (not shown).

[0048] The optical transmitter 100, the optical receiver 200, and the optical transmitter-receiver control unit are housed in the same housing.

[0049] Regarding the optical transmitter 100, since it uses commonly known equipment, it will be described simply.

[0050] The optical transmitter 100 includes a modulation signal generation unit 110, a DAC (Digital-to-Analog converter) unit 120 which is a digital-to-analog converter, an optical modulation unit 130, and a CW (Continuous Wave) optical generation unit 140.

[0051] The modulation signal generation unit 110 is a transmitting-side digital signal processing unit (transmitting digital signal processor (DSP), hereinafter referred to as transmitting-side DSP110).

[0052] The transmitting DSP110 is input with the information, i.e., data, to be sent to the destination as a digital signal. Symbol signals are inserted into the input data to generate a digital signal, i.e., a modulation signal, suitable for the transmission path for optical modulation.

[0053] A modulated signal is a time-series signal in which each symbol is represented by one or more samples.

[0054] The data to be transmitted is constructed into a frame, a signal form capable of error correction, via the transmitting DSP110.

[0055] In the case of polarization multiplexing digital coherent mode, the transmitting side DSP110 generates four modulation signals: XI signal, XQ signal, YI signal, and YQ signal.

[0056] In the case of digital coherence mode of single polarization wave, the transmitting side DSP110 generates two modulation signals, I signal and Q signal.

[0057] In optical transceivers, all modulation signals are processed in the same way, so there is no distinction between each modulation signal. The following explanation focuses on one modulation signal.

[0058] The DAC unit 120 converts the modulated signal, which is composed of digital signals, into a modulated signal composed of analog signals.

[0059] For example, when a symbol is represented as two samples in the digital region, the DAC section 120 uses a digital-to-analog converter of 2 gigabits per second for a symbol rate of 1 gigabits (GBaud).

[0060] The optical modulation unit 130 modulates 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 to generate modulated light, and outputs the generated modulated light to the transmission path.

[0061] Modulated light is a digital coherent signal, or optical signal, that carries information based on the modulation signal in CW light.

[0062] Modulated light mixing is based on information from each modulated signal output from an independent channel.

[0063] The CW light from the CW light generation unit 140 is a single-carrier signal in digital coherent mode, having a single carrier frequency f. Tx0 Continuous light.

[0064] like Figure 1 As shown, the optical receiver 200 includes: an optical coherence detector 210, an interference light (LO (local oscillator) light) generator 220, an ADC (analog-to-digital converter) unit 230 as an analog-to-digital converter, and a receiving-side digital signal processing unit 240.

[0065] The receiving-side digital signal processing unit 240 is a receiving digital signal processor (DSP), hereinafter referred to as the receiving-side DSP240.

[0066] The optical coherence detector 210 receives modulated light transmitted through the transmission path from the optical transmitter of another optical transceiver, and mixes the received modulated light with light from the interference light generator 220 having a single carrier frequency f. Rx The continuous light, also known as interferometric light, is used to perform photoelectric conversion on the optical signal obtained through interference, and an analog electrical signal based on the voltage obtained is output, thus performing optical coherent detection. The voltage signal output from the optical coherent detector 210 is either a single-ended output or a differential output.

[0067] In the case of polarization multiplexing digital coherence mode, the analog electrical signal output from the optical coherent detector 210 consists of four signals: XI, XQ, YI, and YQ, which are output from four output terminals. In the case of single-polarization wave digital coherence mode, it consists of two signals: I and Q.

[0068] However, in reality, due to polarization rotation and other factors generated during transmission in the optical fiber, the four analog electrical signals XI, XQ, YI, and YQ, which are output from independent channels on the transmitting side, are mixed in the four output terminals. After being digitally converted by the ADC unit 230, the four analog electrical signals are restored by the receiving side DSP 240 and thus demodulated.

[0069] In addition, in the case of digital coherence mode of single polarization wave, optical coherent detector 210 also performs coherent detection on the input modulated light and outputs four analog electrical signals.

[0070] 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-polarization wave signals by the receiving side DSP 240.

[0071] In addition, in the optical coherent detector 210, when the I signal and Q signal are directly obtained due to reasons such as inputting with 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 a single polarization wave signal by the receiving side DSP 240.

[0072] Each analog electrical signal is transmitted through an optical coherent detector 210, which modulates each light beam modulated by the carrier frequency and then transmits the signals to each light beam with a carrier frequency f. Rx The analog voltage signal obtained by optical interference of the interfering light is the signal before demodulation. The signal before demodulation is used to obtain the demodulated signal obtained by the receiving DSP240.

[0073] In the optical transceiver, the four signals before demodulation, namely the XI signal, XQ signal, YI signal and YQ signal in Implementation 1, are processed separately. However, since the processing is the same, the signals before demodulation are not distinguished. Hereinafter, one signal before demodulation will be described.

[0074] The carrier frequency f in the interference light from the interference light generation unit 220 Rx It is the carrier frequency f in the modulated light received by the optical coherent detector 210. Tx Roughly the same frequency.

[0075] carrier frequency f Tx and carrier frequency f Rx For example, it is approximately 193.1 THz.

[0076] The carrier frequency f in the interference light from the interference light generation unit 220 Rx The carrier frequency f in the CW light from the CW light generation unit 140 in the optical transmitter of other optical transceivers is the same as that in the CW light from the CW light generation unit 140 of other optical transceivers. Tx0 Same as or the same as the carrier frequency f Tx0There is a frequency difference between them.

[0077] Furthermore, in the following description, the explanation of the CW light from the CW light generation unit 140 in the optical transmitter of other optical transceivers becomes complicated, so only the CW light from the CW light generation unit 140 will be described.

[0078] Even in the interference light from the interference light generation unit 220, the carrier frequency f Rx The carrier frequency f in the CW light from the CW light generation unit 140 Tx0 Under the same conditions, due to the time-varying nature of the light source in the interference light generation unit 220 or the large Doppler frequency shift caused by the orbital state between satellites, the carrier frequency in the modulated light received by the optical coherence detector unit 210 differs from the carrier frequency f in the interference light from the interference light generation unit 220. Rx There are also frequency differences between them.

[0079] Furthermore, the carrier frequency f in the interference light from the interference light generation unit 220 Rx The carrier frequency f in the CW light from the CW light generation unit 140 Tx When a frequency difference exists, the carrier frequency in the modulated light received by the optical coherence detector 210 and the carrier frequency f in the interference light from the interference light generator 220 are... Rx There is a frequency difference between them.

[0080] Regardless of the frequency difference, it 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 is only described as the frequency difference between transmission and reception.

[0081] Furthermore, the frequency difference between transmission and reception is the carrier frequency f in the interference light from the interference light generation unit 220. Rx The value is the base value, which can be positive or negative, but in the following explanation, it will be explained as an absolute value except where necessary.

[0082] The ADC unit 230 is based on the accompanying modulation speed, for example, when the carrier frequency f Tx With a high modulation speed of over 25 GBaud in the QPSK mode at approximately 100 Gbps, it exhibits a high sampling frequency and sampling rate. It samples the electrical signal of the input analog region and converts it into a digital signal as a discrete signal of the digital region, thus obtaining the unmodulated signal composed of digital signals.

[0083] The sampling rate in the ADC unit 230 is higher than the sampling rate in the frequency difference estimation compensation unit 243 in the receiver-side DSP 240.

[0084] The signal before demodulation sent from the ADC unit 230 to the receiving DSP 240 is a time-series signal under high sampling rate conditions.

[0085] The digital signal from the ADC unit 230 before demodulation is the signal after processing in a wide-band digital frequency region.

[0086] The receiving-side DSP240 performs coarse compensation for the frequency difference, which shifts the signal from the ADC unit 230 before demodulation to a frequency that reduces the frequency difference Δf between transmission and reception. It then performs fine compensation for the frequency of the digital signal after the coarse compensation for the frequency difference and uses the signal before demodulation as the demodulated signal.

[0087] In Implementation 1, the receiver-side DSP240 performs digital signal processing in the digital coherence mode.

[0088] like Figure 2 As shown, the receiving-side DSP240 includes: a frequency shifting unit 241, an equalization unit 242, a frequency difference estimation compensation unit 243, a phase estimation unit 244, a symbol determination unit 245, and a frequency shifting control unit 246.

[0089] The frequency shifting unit 241 performs coarse compensation for the frequency difference, which shifts 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.

[0090] The coarse compensation in the frequency shifting unit 241 is performed by the following method: when the frequency difference estimation compensation unit 243 deviates from the allowable range of the estimated value Δf of the frequency difference between the transmitter and receiver, which allows fine compensation, the center frequency of the digital signal obtained by the ADC unit 230 is shifted so that the estimated value Δf of the frequency difference between the transmitter and receiver is within the allowable range.

[0091] As is generally 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 phase rotation of the signal points of the frequency difference on the IQ signal plane space.

[0092] The frequency shifting unit 241 receives the frequency setting signal from the frequency shifting control unit 246, obtains a sine wave for frequency shifting through the frequency setting signal, multiplies the obtained sine wave with the digital signal obtained by the ADC unit 230 in the time domain, thereby shifting the center frequency of the digital signal in a direction that is 0Hz relative to the frequency axis.

[0093] In other words, the direction is shifted toward the presumed value Δf of the frequency difference between transmission and reception becoming 0 Hz.

[0094] It can be said that the frequency shifting unit 241 has the function of receiving the frequency setting signal from the frequency shifting control unit 246 and providing the sine wave for frequency shifting to the digital signal obtained by the ADC unit 230.

[0095] The frequency shifting unit 241 adjusts the carrier frequency f in the interference light from the interference light generating unit 220 to the center frequency of the digital signal obtained by the ADC unit 230, in a manner that ensures the frequency difference estimation compensation unit 243 is within the allowable range of the fine compensation at the start of communication. Rx With the carrier frequency f in CW light Tx The frequency difference between the two frequencies, or the frequency obtained by shifting a fixed value relative to that frequency difference, is set as the initial setting value.

[0096] Preferably, the estimated value Δf of the frequency difference between transmission and reception is close to 0Hz relative to the initial setting value, and the initial setting value is set to a frequency in which the estimated value Δf is within the allowable range for the frequency difference estimation compensation unit 243 to perform fine compensation.

[0097] Furthermore, the initial setting value in the frequency shifting unit 241 is a frequency that can be appropriately represented by a digital signal at the Sub-GHz or GHz level.

[0098] In Embodiment 1, the frequency setting signal of the frequency movement control unit 246 indicates permission when the estimated value Δf is within the allowable frequency difference (-Δfallowable≤Δf≤+Δfallowable), and indicates compensation when the estimated value Δf exceeds the allowable frequency and is within the compensable frequency difference (-Δfcompensable<Δf≤+Δfcompensable).

[0099] The estimated value Δf, which is the frequency within the allowable range, is a finely compensated frequency difference that can be adaptively and precisely estimated by the frequency difference estimation compensation unit 243.

[0100] The estimated value Δf, representing the frequency difference within the compensable range, is the frequency difference that, although in a state of adaptive and precise estimated compensation, cannot be precisely compensated if it falls outside this range.

[0101] In operation, if the frequency setting signal of the frequency shift control unit 246 indicates permission, the frequency shift unit 241 will not update the shift amount setting value indicating the shift amount of frequency shift, and will provide a sine wave of frequency based on the current shift amount setting value to the digital signal obtained by the ADC unit 230.

[0102] For example, if the center value of the digital signal obtained by the ADC unit 230 is the initial setting value, then the digital signal obtained by the ADC unit 230 is set to the initial setting value as the center frequency value.

[0103] Furthermore, if the center value of the digital signal obtained by the ADC unit 230 is a value that has changed from the initial setting value due to the frequency shifting unit 241, a sine wave with a frequency based on the changed value is provided to the digital signal obtained by the ADC unit 230.

[0104] Figure 3 This refers to the carrier frequency f in the modulated light received by the optical coherence detector 210 when the estimated value Δf is within the allowable frequency difference Δf1. Tx1 The carrier frequency f in the interference light from the interference light generation unit 220 Rx , Figure 4 Indicates carrier frequency f Tx1 With carrier frequency f Rx In Figure 3 The frequency (signal spectrum) of the digital signal obtained by the ADC unit 230 and input to the receiving side DSP240 under the condition of the relationship.

[0105] Frequency shift unit 241 maintains Figure 4 The digital signal at the frequency shown.

[0106] In operation, if the frequency setting signal of the frequency shift control unit 246 indicates that compensation is possible, the shift amount setting value representing the shift amount of frequency shift is updated in stages so that the estimated value Δf of the frequency difference between transmission and reception becomes 0Hz, thereby making the center frequency of the digital signal obtained by the ADC unit 230 become 0Hz.

[0107] The center frequency of the digital signal obtained by the ADC unit 230 based on the frequency shifting unit 241 is updated slowly so that the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit 243 remains unchanged.

[0108] Figure 5 This refers to the carrier frequency f in the modulated light received by the optical coherence detector 210 when the estimated value Δf is outside the allowable range but within the compensable range. Tx2 The carrier frequency f in the interference light from the interference light generation unit 220 Rx , Figure 6 Indicates carrier frequency f Tx2 With carrier frequency f Rx In Figure 5 The frequency (signal spectrum) of the digital signal obtained by the ADC unit 230 and input to the receiving side DSP240 under the condition of the relationship.

[0109] For a frequency difference Δf2 that is outside the allowable range but within the compensable range, for example, even if the carrier frequency f in the interference light from the interference light generation unit 220 is outside the allowable range, Rx The carrier frequency f in the CW light from the CW light generation unit 140 Tx0 Under the same circumstances, it is also caused by a large shift in the Doppler frequency due to the orbital state between satellites.

[0110] like Figure 6 ( Figure 7 The digital signal obtained by the ADC unit 230 shown in (a) is processed by the equalization unit 242 and received by the frequency difference estimation compensation unit 243. Then the frequency shift control unit 246 determines that the estimated value Δf2 of the frequency difference between transmission and reception has deviated from the allowable range of fine compensation, and provides a frequency setting signal indicating that compensation is possible to the frequency shift unit 241.

[0111] like Figure 7 As shown in (b), the frequency shifting unit 241 pulls the estimated value Δf2 from the compensable range into the allowable range by using a frequency setting signal that indicates compensability, that is, it makes the estimated value Δf2 with frequency difference 0Hz (Δf 2C The coarse compensation setting for frequency shifting is updated in a manner that updates the frequency shift setting.

[0112] That is, the frequency shift unit 241 shifts the digital signal obtained by the ADC unit 230 by multiplying it in the time domain by a shift amount Δf obtained by multiplying the digital signal obtained by the ADC unit 230 by a frequency setting signal representing compensability. 2α The sine wave used for frequency shifting (apparently, Δf) 2c =Δf2 + Δf 2α This causes the center frequency of the digital signal to shift relative to the frequency axis in the direction of 0Hz.

[0113] In the update of the center frequency of the digital signal in the coarse compensation, the center frequency of the digital signal obtained by the ADC unit 230 is changed slowly in stages so that the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit 243 remains unchanged.

[0114] That is, the frequency shifting unit 241 gradually changes the frequency of the sine wave multiplied by the digital signal obtained by the ADC unit 230 in stages.

[0115] like Figure 7 As shown in (c), if the estimated value of the frequency difference after coarse compensation is Δf 2c If the frequency setting signal of the frequency shift control unit 246 indicates 0Hz, then the estimated value Δf2 of the frequency difference is 0Hz (Δf 2C The frequency shifting unit 241 stops setting the frequency shift, that is, it does not update the shift amount Δf representing the frequency shift. 2αThe movement amount setting value.

[0116] The coarse compensation performed by the frequency shifting unit 241 can be performed over a wide frequency range due to the high sampling rate in the ADC unit 230, thus accurately representing the digital signal obtained by frequency shifting the digital signal obtained by the ADC unit 230.

[0117] As a result, the frequency shift unit 241 can output a sine wave based on the frequency setting signal of the frequency shift control unit 246, and by multiplying it with the digital signal obtained by the ADC unit 230, the frequency component of the digital signal obtained by the ADC unit 230 is made close to 0Hz.

[0118] Furthermore, the frequency shifting performed by the frequency shifting unit 241 can also employ digital signal processing operations in the frequency region, thereby replacing the multiplication operations in the time domain.

[0119] The signal processing for demodulation performed by the receiver-side DSP240 in the equalization unit 242 is a general method in digital signal processing used in typical digital coherent methods, which will be briefly described.

[0120] In the following description, the digital signal obtained by the ADC unit 230 after coarse compensation by the frequency shifting unit 241 includes the digital signal after frequency shifting according to the shift amount setting value and the digital signal for stopping frequency shifting.

[0121] When the equalization unit 242 has a clock synchronization function, it compensates for the clock difference between optical transmission and optical reception of the digital signal obtained by the ADC unit 230 after coarse compensation by the frequency shifting unit 241. When it has an adaptive equalization function, it adaptively compensates for the polarization separation and frequency characteristic degradation of the X-polarized wave signal (XI signal and XQ signal) and the Y-polarized wave signal (YI signal and YQ signal).

[0122] In summary, the equalization unit 242 compensates for waveform distortion in the digital signal obtained from the ADC unit 230 after coarse compensation.

[0123] The frequency difference estimation compensation unit 243 estimates the carrier frequency f in the modulated light received by the optical coherence detector unit 210. Tx The carrier frequency f in the interference light from the interference light generation unit 220 Rx The frequency difference (the frequency difference between transmission and reception) is compensated.

[0124] The frequency difference estimation compensation unit 243 performs fine compensation on the center frequency of the digital signal for which the frequency difference estimation value Δf has been coarsely compensated by the frequency shift unit 241.

[0125] When the frequency difference Δf between transmission and reception is within the allowable range, the frequency difference estimation compensation unit 243 can perform fine compensation, which can adaptively and precisely perform estimation compensation, on the digital signal that has undergone coarse compensation.

[0126] The frequency difference estimation compensation unit 243 estimates the frequency of phase rotation of the signal points on the IQ signal plane space of the pilot symbol signal periodically inserted in the modulated light received by the optical coherent detector unit 210, thereby performing fine compensation on the frequency of the digital signal obtained by the ADC unit 230, which has been coarsely compensated by the frequency shift unit 241.

[0127] Among them, the fine compensation of the frequency of digital signals is not limited to the pilot insertion method using pilot symbol signals. It can also be performed using methods based on Fourier transform analysis or commonly known frequency estimation methods such as the power method.

[0128] The compensation for the rotation of the signal point, i.e. the compensation for the frequency difference, performed by the frequency difference estimation compensation unit 243, is carried out by performing a multiplication operation on the digital signal in the time domain.

[0129] However, in the frequency difference estimation compensation unit 243, the frequency difference changes over time, so the estimated value of the frequency difference and even the frequency of the sine wave used for compensation are updated adaptively.

[0130] The frequency difference compensation performed by the frequency difference estimation compensation unit 243 uses a general method in digital signal processing used in typical digital coherence mode.

[0131] The frequency difference estimation compensation unit 243 is conceived to be a time series signal, such as a time series signal represented by one sample per symbol or two samples per symbol, input at a sampling rate lower than that of the frequency shift unit 241.

[0132] Even at such a low sampling rate, the estimated value Δf of the frequency difference between transmission and reception is kept within the allowable range by the frequency shifting unit 241 through coarse compensation, and the frequency of the digital signal obtained by the ADC unit 230 is kept near 0Hz through coarse compensation. Therefore, even in a narrow frequency space, the frequency difference estimation compensation unit 243 can normally and accurately perform estimation compensation adaptively.

[0133] For example, such as Figure 8 As shown, when the phase rotation of the frequency difference relative to the frequency difference-free phase is within 180°, that is, when the estimated value Δf of the frequency difference between transmission and reception is compensable, the estimated compensation can be implemented normally.

[0134] Furthermore, if coarse compensation based on the frequency shift unit 241 is not performed, for example, if the center frequency of the digital signal obtained by the ADC unit 230 deviates significantly from 0Hz, and the estimated value Δf of the frequency difference between transmission and reception is outside the compensable range (not compensable), then the frequency difference estimation compensation unit 243 cannot correctly represent the waveform as an out-of-band signal. Figure 9 As shown, the phase rotation of the frequency difference relative to the frequency difference-free phase exceeds 180°, which ultimately leads to an estimation error in the frequency difference estimation compensation unit 243.

[0135] The frequency movement control unit 246 obtains an estimated value Δf of the frequency difference between transmission and reception based on the frequency difference estimation compensation unit 243, and obtains a frequency setting signal that is associated with the estimated value Δf. If the estimated value Δf is within the allowable range, it indicates that the frequency is allowed; if the estimated value Δf is outside the allowable range, it indicates that the frequency is compensable. The obtained frequency setting signal is then provided to the frequency movement unit 241.

[0136] The signal processing for demodulation performed by the receiver-side DSP240 in the phase estimation unit 244 and the symbol determination unit 245 is a general method in digital signal processing used in a typical digital coherent mode, and will be briefly described.

[0137] The phase estimation unit 244 compensates for the phase variation of the light source constituting the CW light generation unit and the light source constituting the interference light generation unit 220 in the light transmitter.

[0138] The symbol determination unit 245 determines the transmitted symbol in the demodulated signal that has undergone waveform distortion compensation.

[0139] In addition, error correction of the demodulated signal can also be performed in the stage after the symbol determination unit 245.

[0140] Next, use Figure 10 The operation of the optical receiver according to Embodiment 1, particularly the frequency shifting unit 241 and the frequency shifting control unit 246 in the receiving-side DSP 240, will be described.

[0141] When the optical transmitter of another optical transceiver device begins communication, in step ST1, an initial capture of the estimated value Δf of the frequency difference between transmission and reception is performed.

[0142] The frequency shift unit 241, which receives the frequency setting signal generated by the frequency shift control unit 246 based on the estimated value Δf initially captured by the frequency difference estimation compensation unit 243, initially sets the shift amount setting value representing the shift amount of frequency shift.

[0143] The estimated value Δf after initial setting is shown in... Figure 11 The time t0 in the middle.

[0144] exist Figure 11 In the diagram, the horizontal axis represents time, the vertical axis represents the estimated value, and the black circle represents the estimated value Δf obtained by the frequency difference estimation compensation unit 243.

[0145] The estimated value Δf is obtained on the order of symbol periods. Alternatively, the estimated value Δf can also be obtained by a moving average from multiple symbols.

[0146] To simplify the explanation, Figure 11 The estimated value Δf represented by the black circle does not represent all the estimated values ​​obtained by the frequency difference estimation compensation unit 243.

[0147] Initiate communication with the optical transmitters of other optical transceivers, such as... Figure 12 As shown, if the estimated value Δf obtained by the frequency difference estimation compensation unit 243 is within the allowable range (-Δfallowable≤Δf≤+Δfallowable:2×Δfallowable), then the frequency setting signal based on the frequency movement control unit 246 indicates that it is allowed. The frequency movement unit 241 does not update the movement amount, but provides a sine wave of the frequency based on the current movement amount setting value to the digital signal obtained by the ADC unit 230.

[0148] Therefore, the digital signal obtained by the ADC unit 230 is precisely compensated by the frequency difference estimation compensation unit 243.

[0149] 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 compensation unit 243 is within the allowable range, and then... Figure 11 As of the moment t1 shown, the estimated value Δf of the frequency difference is considered to be within the allowable range, and a frequency setting signal indicating that it is allowed is obtained. The frequency shift unit 241 does not update the shift amount, and the frequency difference estimation compensation unit 243 performs fine compensation on the digital signal obtained by the ADC unit 230, which provides a sine wave with a frequency based on the current shift amount setting value.

[0150] Step ST2 is a step of maintaining the 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.

[0151] like Figure 13 As shown, if in Figure 11 At time t1, the estimated value Δf of the frequency difference obtained by the frequency difference estimation compensation unit 243 exceeds (deviates) +Δfallowable, and the center frequency of the digital signal obtained by the ADC unit 230 deviates significantly from 0Hz. Then, the process proceeds from step ST2 to step ST3.

[0152] In step ST3, based on the frequency setting signal of the frequency movement control unit 246 indicating that compensation is possible, the frequency movement unit 241 updates the movement amount in the direction where the estimated value Δf of the frequency difference becomes 0 at a speed in which the compensation function in the frequency difference estimation compensation unit 243 is not interrupted, and proceeds to step ST4.

[0153] The frequency difference estimation compensation unit 243 performs fine compensation on the digital signal obtained by the ADC unit 230, which provides a sine wave with a frequency based on the updated shift amount.

[0154] 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 within the range of 0±β. If it is outside the range of 0±β, it returns to step ST3, the frequency shift unit 241 updates the shift amount and proceeds to step ST4.

[0155] This continues until the estimated value of the frequency difference Δf falls within the range of 0 ± β, until... Figure 11 Repeat steps ST3 and ST4 up to time t2.

[0156] β is a value that is larger than the frequency difference that can be changed by the frequency shifting unit 241 through one update of the shift amount.

[0157] Steps ST3 and ST4 are update steps whereby the frequency shifting unit 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 deviates from the allowable range.

[0158] The maintenance step of step ST2, together with the update steps of steps ST3 and ST4, constitute the steps of the frequency shifting unit 241 to perform coarse compensation for the frequency difference between the transmission and reception of the center frequency of the digital signal obtained by the ADC unit 230.

[0159] In addition, although Figure 10 It is not explicitly described in the text, but there is a step in which the frequency difference estimation compensation unit 243 performs fine compensation on the digital signal after coarse compensation of the frequency difference between transmission and reception is performed through steps ST2, ST3 and ST4.

[0160] If in Figure 11 At time t2, the estimated value of the frequency difference Δf falls within the range of 0±β. Then, proceed to step ST5, where the frequency shifting unit 241 stops updating the shift amount, maintains the shift amount, and returns to step ST2.

[0161] exist Figure 11In step ST2, from immediately after time t2 to immediately before time t3, the estimated value Δf of the frequency difference is within the allowable range. Therefore, in step ST2, the frequency shift unit 241 does not update the shift amount, and the frequency difference estimation compensation unit 243 performs fine compensation on the digital signal obtained by the ADC unit 230, which provides a sine wave of the frequency based on the current shift amount maintained after the update.

[0162] like Figure 14 As shown, if in Figure 11 At time t3, if the estimated value Δf of the frequency difference obtained by the frequency difference estimation compensation unit 243 exceeds (deviates) -Δfallowable, then proceed from step ST2 to step ST3.

[0163] In steps ST3 and ST4, as described above, the frequency shifting unit 241 updates the shift amount sequentially in the direction where the estimated value Δf of the frequency difference becomes 0 at a speed in which the compensation function in the frequency difference estimation compensation unit 243 is uninterrupted, until the estimated value Δf of the frequency difference enters the range of 0±β.

[0164] The frequency difference estimation compensation unit 243 performs fine compensation on the digital signal obtained by the ADC unit 230, which provides a sine wave with a frequency based on the updated shift amount.

[0165] If in Figure 11 At time t4, the estimated value of the frequency difference Δf falls within the range of 0±β. Then, proceed to step ST5, where the frequency shifting unit 241 stops updating the shift amount, maintains the shift amount, and returns to step ST2.

[0166] Thus, since coarse compensation is performed by the frequency shifting unit 241 and fine compensation is performed by the frequency difference estimation compensation unit 243 on the digital signal obtained by the ADC unit 230 after coarse compensation by the frequency shifting unit 241, waveform distortion compensation can be performed even if a frequency difference occurs between the transmitted and received optical carriers.

[0167] Next, use Figure 15 The hardware structure of the optical receiver 200 according to Embodiment 1 will be described.

[0168] exist Figure 15 In, with Figure 1 and Figure 2 The same reference numerals in the accompanying drawings indicate the same or equivalent parts.

[0169] 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 composed of RAM (Random Access Memory) and ROM (Read Only Memory), a communication interface 253, and an input / output interface 254.

[0170] The processor 251, memory 252, communication interface 253 and input / output interface 254 are connected to the bus 255, and exchange data and control signals with each other through the bus 255.

[0171] The processor 251 temporarily reads the program recorded in the ROM in the memory 252 into the RAM in the memory 252, and performs processing through the read program.

[0172] The ROM in memory 252 stores various data, programs for performing processing in optical receiver 200, and processing programs required for starting optical receiver 200.

[0173] The communication interface 253 is used for transmitting and receiving data and control signals with each component of the optical receiver 200 and other components of optical transceivers.

[0174] The input / output interface 254 receives and transmits control signals and modulation signals between the various components in the receiver 200 via electrical wiring.

[0175] The input / output interface 254 is, for example, an interface for supplying an injection current to the light source constituting the interference light generation unit 220 to the interference light generation unit 220.

[0176] In addition, the input / output interface 254 is, for example, an interface for outputting various control signals to the receiving-side DSP unit.

[0177] exist Figure 15 In this document, for the sake of clarity, the optical receiver control unit 250 is shown as an alternative structure for the receiving-side DSP 240. However, the optical receiver control unit 250 can also be responsible for the functional parts of the receiving-side DSP 240 that can be configured by software.

[0178] For example, the functions of the frequency shifting unit 241, equalization unit 242, frequency difference estimation compensation unit 243, phase estimation unit 244, symbol determination unit 245, and frequency shifting control unit 246 constituting the receiver-side DSP 240 can also be implemented by the processor 251 and the memory 252.

[0179] When the functions of the frequency shifting unit 241 and the frequency shifting control unit 246 are implemented by the processor 251 and the memory 252, the optical receiving program recorded in the ROM of the memory 252 and executed by the processor 251 includes the following steps: a step of performing coarse compensation for the frequency difference between transmission and reception, which includes: maintaining the shift of the center frequency of the digital signal obtained by coherent detection of an analog electrical signal obtained by optically modulating the modulated light of the carrier frequency and interfering with the interference light, if the frequency difference between transmission and reception is within an allowable range; updating the shift of the center frequency of the digital signal if the frequency difference between transmission and reception deviates from the allowable range; and performing fine compensation for the center frequency of the digital signal after performing coarse compensation for the frequency difference between transmission and reception.

[0180] The optical receiver according to Embodiment 1 includes: an optical coherent detector 210 that receives modulated light after optical modulation by a carrier frequency, performs coherent detection by interfering the received modulated light with interference light, 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 detector 210 to obtain a digital signal; and a receiving-side DSP 240 that has a frequency shifting unit 241 that performs coarse compensation to shift the center frequency of the digital signal obtained by the ADC unit 230 towards a frequency that reduces the frequency difference Δf between transmission and reception, and a frequency difference estimation compensation unit 243 that performs fine compensation on the center frequency of the digital signal after the frequency difference coarse compensation is performed by the frequency shifting unit 241. Therefore, the frequency difference estimation compensation unit 243 receives a digital signal that brings the center frequency of the digital signal obtained by the ADC unit 230 close to 0Hz, i.e., reduces the phase rotation speed. Thus, the frequency difference estimation compensation unit 243 can accurately read the phase rotation amount of the digital signal, and the frequency difference estimation compensation unit 243 can perform adaptive and precise optical frequency difference estimation compensation.

[0181] As a result, the signal quality of the demodulated signal from the receiving DSP240 can be improved, thereby expanding the compensation range of optical frequency difference in the optical receiver.

[0182] Implementation Method 2

[0183] according to Figure 16 The optical receiver involved in Embodiment 2 will be described.

[0184] The optical receiver in Embodiment 2 differs from the optical receiver in Embodiment 1 in that the receiving-side DSP240 is different, but all other aspects are the same.

[0185] Figure 16In, with Figure 1 and Figure 2 The same reference numerals in the accompanying drawings indicate the same or equivalent parts.

[0186] The equalization unit 242, frequency difference estimation compensation unit 243, phase estimation unit 244, and symbol determination unit 245 constituting the receiving-side DSP240 are the same as the equalization unit 242, frequency difference estimation compensation unit 243, phase estimation unit 244, and symbol determination unit 245 constituting the receiving-side DSP240 in the optical receiver according to Embodiment 1.

[0187] The method for setting the initial setting values ​​of the frequency shift unit 241 and the frequency shift control unit 246 at the start of communication is different from that of the frequency shift unit 241 and the frequency shift control unit 246 in the optical receiver according to Embodiment 1. In operation, the implementation of coarse compensation based on the frequency shift unit 241 and the frequency shift control unit 246, which shifts the center frequency of the digital signal obtained by the ADC unit 230 to the frequency that reduces the estimated value Δf of the frequency difference between transmission and reception, is the same.

[0188] Therefore, the initial settings at the start of communication will be explained.

[0189] The frequency shift control unit 246 obtains shift amount setting information from outside the receiving side DSP 240, which indicates the shift amount of frequency, and provides the shift amount setting information to the frequency shift unit 241.

[0190] The frequency shift control unit 246 simultaneously provides the synchronization determination value or conduction information from the clock synchronization function of the equalization unit 242 to the frequency shift unit 241.

[0191] like Figure 16 As shown, the movement amount setting information from the outside is information representing multiple discrete movement amounts, and the frequency movement control unit 246 sequentially receives information representing multiple discrete frequency movement amounts.

[0192] The frequency shifting unit 241 performs frequency shifting based on information representing multiple discrete shift amounts for the center frequency of the digital signal obtained by the ADC unit 230.

[0193] In the estimated value Δf of the frequency difference between transmission and reception obtained by sequentially implementing frequency shifts based on the synchronization determination value or conduction information in the clock synchronization function of the equalization unit 242, the frequency shift unit 241 sets the shift amount representing the frequency difference within the compensable range in the frequency difference estimation compensation unit 243, preferably the shift amount representing the optimal frequency difference, as an initial setting value.

[0194] In addition to having the same effects as the optical receiver in Embodiment 1, the optical receiver according to Embodiment 2 determines the initial setting value of the frequency shift unit 241 for the shift amount based on the shift amount setting information representing multiple discrete shift amounts. Therefore, even if the frequency difference between transmission and reception is outside the compensable range of the frequency difference estimation compensation unit 243 at the start of communication, the center frequency of the digital signal obtained by the ADC unit 230 can be pulled into the compensable range of the frequency difference in the frequency difference estimation compensation unit 243 at the initial setting, and coarse compensation and fine compensation can be reliably performed in use.

[0195] Implementation Method 3

[0196] according to Figure 17 The optical receiver involved in Embodiment 3 will be described.

[0197] The optical receiver in Embodiment 3 differs from the optical receiver in Embodiment 1 in that the receiving-side DSP240 is different, but all other aspects are the same.

[0198] Figure 17 In, with Figure 1 and Figure 2 The same reference numerals in the accompanying drawings indicate the same or equivalent parts.

[0199] The equalization unit 242, frequency difference estimation compensation unit 243, phase estimation unit 244, and symbol determination unit 245 constituting the receiving-side DSP240 are the same as the equalization unit 242, frequency difference estimation compensation unit 243, phase estimation unit 244, and symbol determination unit 245 constituting the receiving-side DSP240 in the optical receiver according to Embodiment 1.

[0200] The frequency shift control unit 246 periodically or periodically obtains shift amount setting information from outside the receiving side DSP240, which represents the shift amount of frequency, and provides a frequency setting signal based on the shift amount setting information to the frequency shift unit 241.

[0201] The frequency shift control unit 246 simultaneously provides the synchronization determination value or conduction information from the clock synchronization function of the equalization unit 242 to the frequency shift unit 241.

[0202] The motion setting information from the outside is any information that represents the Doppler motion estimated from satellite orbit information or the carrier frequency of other optical transmitters.

[0203] During initial setup and in operation, the frequency shifting unit 241 periodically or cyclically shifts the center frequency of the digital signal obtained by the ADC unit 230 based on the shift amount indicated by the frequency setting signal of the frequency shifting control unit 246.

[0204] In the optical receiver according to Embodiment 3, since the setting value of the movement amount based on the frequency movement unit 241 is determined based on any movement amount setting information that represents the Doppler movement amount estimated by satellite orbit information or the carrier frequency of other optical transmitters, coarse compensation and fine compensation can be reliably performed.

[0205] Furthermore, in the optical receiver according to Embodiment 3, although the setting value of the movement amount based on the frequency movement unit 241 is determined based on any movement amount setting information representing the Doppler movement amount estimated from the satellite orbit information or the carrier frequency of another optical transmitter, it is also possible to... Figure 18 As shown, similar to the optical receiver described in Embodiment 1, it also has the following functions: the frequency shift control unit 246 obtains an estimated value Δf of the frequency difference between transmission and reception based on the frequency difference estimation compensation unit 243, and obtains a frequency setting signal that is associated with the estimated value Δf, and indicates that if the estimated value Δf is within the allowable range, it is allowed; if the estimated value Δf is outside the allowable range, it is outside the compensable range, it is compensable. The obtained frequency setting signal is then provided to the frequency shift unit 241.

[0206] That is, the motion amount setting information is used in the initial setting value setting at the start of communication, and in use, the motion amount setting performed by the frequency motion unit 241 based on the motion amount setting information is used together with the motion amount setting performed by the frequency motion unit 241 based on the estimated value Δf of the frequency difference between transmission and reception obtained by the frequency difference estimation compensation unit 243.

[0207] In application, the motion amount setting performed by the frequency shifting unit 241 based on the estimated value Δf of the frequency difference between transmission and reception obtained by the frequency difference estimation compensation unit 243, and the motion amount setting performed by the frequency shifting unit 241 based on the motion amount setting information are implemented in the following manner.

[0208] During normal operation, similar to Embodiment 1, the frequency shifting unit 241 sets the shift amount for the center frequency of the digital signal obtained by the ADC unit 230 based on the frequency setting signal received from the frequency shifting control unit 246, which receives the estimated value Δf of the frequency difference obtained by the frequency difference estimation compensation unit 243.

[0209] When the frequency shift unit 241 receives shift amount setting information from the outside, it adds the shift amount based on the shift amount setting information from the outside to the shift amount based on the frequency setting signal based on the frequency shift control unit 246, and uses the summed shift amount as the shift amount for the center frequency of the digital signal obtained by the ADC unit 230.

[0210] The update is performed slowly in a manner that keeps the quality of the demodulated signal waveform obtained by the frequency difference estimation compensation unit 243 unchanged.

[0211] However, in application, when the estimated value Δf of the frequency difference between transmission and reception deviates from the allowable range due to the frequency shift unit 241 adding the shift amount set by the shift amount set by the shift amount set by the shift amount set by the shift amount set by the shift amount set by the shift amount set by the shift amount set by the shift unit 241, the shift amount set by the shift unit 241 is stopped, and the shift amount set by the shift unit 241 is centered on the estimated value Δf of the frequency difference between transmission and reception obtained by the frequency difference estimation compensation unit 243.

[0212] Implementation Method 4

[0213] according to Figure 19 and Figure 20 The optical receiver involved in Embodiment 4 will be described.

[0214] The optical receiver described in Embodiment 1 is a receiver that receives modulated light that modulates data information using a single carrier signal (single carrier frequency) in a conventional digital coherent manner. In contrast, the optical receiver described in Embodiment 4 receives modulated light that modulates data information using multiple different subcarrier signals (multiple different carrier frequencies) based on a subcarrier multiplexing method.

[0215] In the following explanation, to avoid unnecessary complexity, the subcarrier signal with overlapping data information will be referred to simply as the subcarrier signal.

[0216] Figure 19 In, with Figure 1 and Figure 2 The same reference numerals in the accompanying drawings indicate the same or equivalent parts.

[0217] The optical receiver involved in Implementation 4 receives subcarrier signals within the frequency band that can detect signals, and separates and demodulates the subcarriers in the digital signal processing on the receiving side.

[0218] Furthermore, in embodiment 4, the multiple different subcarrier signals in the modulated light received by the optical receiver can also be generated separately for the data signals in the independent modulated light. Therefore, it is also conceivable to receive unrelated modulated light transmitted from different locations simultaneously in the optical receiver.

[0219] The optical receiver in Embodiment 4, like the optical receiver in Embodiment 1, targets optical signals (XI signal, XQ signal, YI signal, and YQ signal) whose X-polarized and Y-polarized waves are modulated into orthogonal phase I and Q signals, respectively. It receives N signals composed of N subcarriers of different frequencies for each of the X-polarized and Y-polarized waves. Channel Optical receiver for multiplexed optical signals.

[0220] Furthermore, the following description explains the case in which, in the optical transmitter of other optical transceivers, the subcarrier signal is multiplexed in frequency in the subcarrier multiplexing section and output as modulated optical signal.

[0221] The optical coherent detector 210, interference light generator 220, and ADC unit 230 in the optical receiver according to Embodiment 4 have essentially the same structure as the optical coherent detector 210, interference light generator 220, and ADC unit 230 in the optical receiver according to Embodiment 1. They are used for the commonly known subcarrier multiplexing method, so the description is omitted.

[0222] The receiving-side DSP240 is configured to correspond with the four pre-demodulation signals obtained by optical interference from the optical coherent detector 210, namely the XI signal, XQ signal, YI signal, and YQ signal. However, similar to Embodiment 1, the pre-demodulation signals are not distinguished. Hereinafter, one pre-demodulation signal will be described.

[0223] The receiving-side DSP240 has N receiving-side DSP240-1 to 240-N corresponding to the N subcarrier signals in the received modulated light.

[0224] like Figure 20 As shown, the N subcarrier signals in the modulated light are centrally symmetrical about the center frequency of the N subcarrier signals when N is an even number.

[0225] In addition, the carrier frequency of the interference light from the interference light generation unit 220 is set to the center frequency of the N subcarrier signals.

[0226] The N receiver-side DSPs 240-1 to 240-N process each of the N subcarriers individually, and each has a frequency shifting 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 shifting control unit 246, and an adjacent signal removal unit 248.

[0227] In each of the N receiving-side DSPs 240-1 to 240-N, the signal processing for demodulation in the equalization section 242, phase estimation section 244, and symbol determination section 245 of the receiving-side DSP 240 is performed in the same manner as the signal processing for demodulation in the equalization section 242, phase estimation section 244, and symbol determination section 245 of the receiving-side DSP 240 in Embodiment 1, corresponding to the N subcarrier signals.

[0228] In addition, the adjacent signal removal unit 248 functions as a low-pass filter or band-pass filter to remove adjacent signals outside the frequency band of each of the N subcarrier signals whose shift amount is provided by the frequency shift unit 241, and performs signal separation for the subcarrier signals.

[0229] At the start of communication, the frequency shifting unit 241 in each of the N receiving-side DSPs 240-1 to 240-N sets an initial value based on the shift amount of the frequency difference between the frequency of the subcarrier signal and the center frequency of the N subcarrier signals.

[0230] As a result, in each of the N receiving-side DSPs 240-1 to 240-N, the frequency of the subcarrier signal in the digital signal obtained by the ADC unit 230 is shifted by the frequency shifting unit 241, which is initially set, in a manner that allows for fine compensation.

[0231] and Figure 18 Similarly, the initial value of the movement amount in the frequency shift unit 241 can be set by providing the frequency shift control unit 246 with the frequency difference between the frequency of the subcarrier signal and the center frequency of the N subcarrier signals, instead of providing the Doppler movement amount or the carrier frequency of the optical transmitter.

[0232] In the adjacent signal removal unit 248 of each of the N receiving side DSPs 240-1 to 240-N, the digital signal obtained by the ADC unit 230 is separated into the corresponding subcarrier signal. Therefore, the frequency difference estimation compensation unit 243 of each of the N receiving side DSPs 240-1 to 240-N is within the allowable range relative to the corresponding subcarrier signal, so fine compensation can be implemented.

[0233] After initial setting, when the frequency shifting units 241 of each of the N receiving-side DSPs 240-1 to 240-N begin communicating with the optical transmitters of other optical transceivers, if the estimated value Δf obtained by the corresponding frequency difference estimation compensation unit 243 is within the allowable range, the shift amount of the initial setting is not updated, and the sine wave of the frequency at the initial setting is provided to the digital signal obtained by the ADC unit 230.

[0234] Therefore, the digital signal of the subcarrier signal obtained by the ADC unit 230 is finely compensated by the frequency difference estimation compensation unit 243.

[0235] When the estimated value Δf obtained by the corresponding frequency difference estimation compensation unit 243 of each of the N receiving side DSPs 240-1 to 240-N deviates from the allowable range, the frequency shifting unit 241 updates the shift amount in the direction where the estimated value Δf of the frequency difference becomes 0.

[0236] That is, in each of the N receiving-side DSPs 240-1 to 240-N, coarse compensation is performed in the frequency shifting unit 241, and fine compensation is performed by the frequency difference estimation compensation unit 243, similar to the case described in Embodiment 1.

[0237] In the optical receiver according to Embodiment 4, which receives modulated light that modulates data information using multiple different subcarrier signals based on subcarrier multiplexing, the frequency shifting unit 241 in the multiple receiving-side DSPs 240-1 to 240-N corresponding to the multiple subcarrier signals updates the shift amount in a way that pulls the corresponding subcarrier signal into 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 perform adaptive and precise estimation compensation of optical frequency difference.

[0238] Furthermore, it is possible to freely combine the various embodiments, modify any constituent elements of each embodiment, or omit any constituent elements in each embodiment.

[0239] Industrial availability

[0240] The optical receivers disclosed herein can be applied to: optical receivers used in the field of digital coherent technology, which are widely used in metropolitan area core networks and submarine optical cable systems in optical fiber communication; optical receivers used in wireless access networks in the field of wireless communication; optical receivers used in optical networks containing core metropolitan areas, such as encrypted signals, imaging systems, and existing Internet protocols; and optical receivers for satellite-borne optical communication terminals in optical communication systems that do not use optical fibers, such as space or space optical communication.

[0241] It is particularly suitable for optical receivers in optical communication systems that use medium- and low-speed digital coherent signals of less than 100Gbps.

[0242] Explanation of reference numerals in the attached figures

[0243] 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 detector; 220... Interference light generation unit; 230... ADC unit; 240, 240-1 to 240-N... Receiver-side digital signal processing unit; 241... Frequency shifting unit; 242... Equalization unit; 243... Frequency difference estimation compensation unit; 244... Phase estimation unit; 245... Symbol determination unit; 246... Frequency shifting control unit; 247... Adjacent signal removal unit.

Claims

1. An optical receiver, characterized in that, have: The optical coherent detector receives modulated light that has been optically modulated by a carrier frequency, performs coherent detection by interfering the received modulated light with the interference light, and outputs an analog electrical signal. The analog-to-digital converter performs analog-to-digital conversion on the analog electrical signal from the optical coherence detector to obtain a digital signal; as well as The receiving-side digital signal processing unit includes a frequency shifting unit that performs coarse compensation to shift the center frequency of the digital signal obtained by the analog-to-digital conversion unit towards a frequency that reduces the frequency difference between transmission and reception, and a frequency difference estimation compensation unit that performs fine compensation on the center frequency of the digital signal after the frequency difference coarse compensation is performed by the frequency shifting unit.

2. The optical receiver according to claim 1, characterized in that, The modulated light has an optical signal with I and Q signals modulated into orthogonal phases. The analog-to-digital converter and the receiving-side digital signal processing unit perform signal processing on the I signal and the Q signal separately.

3. The optical receiver according to claim 1 or 2, characterized in that, The coarse compensation for the frequency difference performed by the frequency shifting unit is performed in the following manner: when the frequency difference between the transmitter and receiver deviates from the allowable range of the estimated value of the frequency difference between the transmitter and receiver that the frequency difference estimation compensation unit can allow for fine compensation, the center frequency of the digital signal obtained by the analog-to-digital converter is shifted in such a way that the estimated value of the frequency difference between the transmitter and receiver is within the allowable range.

4. The optical receiver according to claim 3, characterized in that, In the coarse compensation of frequency difference performed by the frequency shifting unit, the center frequency of the digital signal obtained by the analog-to-digital converter is shifted in stages in such a way that the quality of the waveform of the demodulated signal obtained by the frequency difference estimation compensation unit remains unchanged.

5. The optical receiver according to claim 1 or 2, characterized in that, The coarse compensation for the frequency difference performed by the frequency shifting unit is performed in the following manner: the frequency shifting unit shifts the center frequency of the digital signal obtained by the analog-to-digital converter in such a way that the estimated value of the frequency difference between transmission and reception is within an allowable range, based on shift amount setting information from outside the receiving digital signal processing unit that represents the shift amount of frequency.

6. The optical receiver according to claim 5, characterized in that, The motion setting information is any information representing the Doppler motion estimated from satellite orbit information or the carrier frequency of other optical transmitters.

7. The optical receiver according to claim 1 or 2, characterized in that, The coarse compensation for the frequency difference performed by the frequency shifting unit is performed as follows: during initial setting, the frequency shifting unit shifts the center frequency of the digital signal obtained by the analog-to-digital converter based on shift amount setting information from outside the receiving digital signal processing unit, which represents the shift amount of frequency, so that the estimated value of the frequency difference between transmission and reception is within an allowable range; in operation, when the frequency difference shifts out of the allowable range of the estimated value of the frequency difference between transmission and reception that the frequency difference estimation compensation unit can allow for fine compensation, the center frequency of the digital signal obtained by the analog-to-digital converter is shifted so that the estimated value of the frequency difference between transmission and reception is within an allowable range.

8. The optical receiver according to claim 1 or 2, characterized in that, The coarse compensation for the frequency difference performed by the frequency shifting unit is performed as follows: when the frequency difference between the transmitter and receiver deviates from the allowable range of the estimated value of the frequency difference between the transmitter and receiver that the frequency difference estimation compensation unit can allow for fine compensation, the shift amount of the frequency difference between the transmitter and receiver is reduced by adding the shift amount indicated by the shift amount setting information from outside the digital signal processing unit on the receiving side, so that the center frequency of the digital signal obtained by the analog-to-digital conversion unit is shifted. The shift amount setting information is any information that represents the Doppler shift amount estimated by satellite orbit information or the carrier frequency of other optical transmitters.

9. The optical receiver according to claim 1 or 2, characterized in that, The modulated light received by the optical coherent detector is modulated light in which each piece of data information is modulated by multiple different subcarrier signals. The receiving-side digital signal processing unit has the frequency shifting unit and the frequency difference estimation compensation unit corresponding to multiple different subcarrier signals.

10. An optical receiving method, comprising an optical receiver including an optical coherent detector, an analog-to-digital converter, and a receiving-side digital signal processing unit having a frequency shifting unit and a frequency difference estimation compensation unit. The optical receiving method is characterized in that... have: The frequency shifting unit performs a coarse compensation step for the frequency difference between transmission and reception. This coarse compensation step includes: for the shift of the center frequency of the digital signal obtained by coarse detection of the analog electrical signal obtained by optically modulating the modulated light (which is optically modulated by the carrier frequency) with interference light through coarse detection by the optical coarse detection unit, and converting the analog electrical signal into a digital signal by the analog-to-digital converter, if the frequency difference between transmission and reception is within an allowable range, maintaining the shift of the center frequency of the digital signal obtained by the analog-to-digital converter; and updating the shift of the center frequency of the digital signal obtained by the analog-to-digital converter if the frequency difference between transmission and reception deviates from the allowable range. The frequency difference estimation compensation unit performs fine compensation on the center frequency of the digital signal after coarse compensation for the frequency difference between transmission and reception.

11. An optical receiving procedure, characterized by Have the computer perform the following steps: The step of performing coarse compensation for the frequency difference between transmission and reception includes: maintaining the shift of the center frequency of the digital signal obtained by coherent detection of an analog electrical signal obtained by optically modulating the modulated light (obtained by optically modulating the carrier frequency) with the interference light (obtained by interfering with the interference light); and updating the shift of the center frequency of the digital signal if the frequency difference between transmission and reception is outside the allowable range. and The steps for fine compensation of the center frequency of the digital signal after coarse compensation for the frequency difference between transmission and reception.

12. A recording medium storing a program that causes a computer to perform the following steps: A step of coarsely compensating for the frequency difference between transmission and reception, comprising: maintaining the shift in the center frequency of a digital signal obtained by coherent detection of an analog electrical signal obtained by optically modulating a modulated light (obtained by optically modulating a carrier frequency) with an interference light, if the frequency difference between transmission and reception is within an allowable range; and updating the shift in the center frequency of the digital signal if the frequency difference between transmission and reception deviates from the allowable range; and The steps for fine compensation of the center frequency of the digital signal after coarse compensation for the frequency difference between transmission and reception.

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