Optical receiver and signal processing method

Abstract

An optical receiver comprising: an acquisition unit that acquires a signal indicating each of an in-phase component and a quadrature component of each polarized wave of received light; and an error detection unit which uses the signals to detect, as an angle error, a deviation in orthogonality that is between the in-phase component and the quadrature component and occurs in an optical hybrid circuit for performing coherent detection of the received light on the basis of the signals and which uses the angle error to update a setting value that is used for compensation processing for the deviation in orthogonality in at least one of the optical hybrid circuit and an equalizer.

Description

Technical Field

[0001] The present invention relates to an optical receiver and a signal processing method.

Background Art

[0002] Digital coherent optical communication can achieve high-capacity and long-distance optical transmission by compensating for signal distortion generated in an optical transmitter, an optical fiber transmission line, and an optical receiver through digital signal processing. Recently, in order to further increase the capacity of optical transmission, an improvement in the accuracy of imperfect compensation in optical transceivers has been demanded. For example, the optical transmission characteristic compensation system described in Patent Document 1 estimates the imperfection of an optical transceiver using a known reference signal and performs imperfect compensation based on the estimation result.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Conventional technologies have a problem in that the IQ orthogonality of the receiver's optical hybrid circuit can change due to factors such as temperature changes and wavelength changes. To solve this problem, compensation technologies that compensate for the effects of 90-degree error and IQ gain imbalance are being investigated. However, such compensation technologies have the problem of complicating the configuration of digital signal processing because they require the addition of a new equalizer.

[0006] In view of the above circumstances, the present invention aims to provide a technology that can improve the estimation accuracy of IQ orthogonality error while suppressing the complexity of the digital signal processing configuration, thereby realizing higher quality optical communication. [Means for solving the problem]

[0007] One aspect of the present invention is an optical receiver comprising: an acquisition unit that acquires signals indicating the in-phase component and the orthogonal component for each polarization of the received light, respectively; and an error detection unit that, based on the signals, detects as an angular error the deviation in orthogonality between the in-phase component and the orthogonal component that occurs in an optical hybrid circuit that performs coherent detection of the received light, and updates a set value used for compensating for the deviation in orthogonality in at least one of the optical hybrid circuit and the equalizer based on the angular error.

[0008] Furthermore, one aspect of the present invention is a signal processing method by computer for an optical receiver, comprising: an acquisition step of acquiring electrical signals representing the in-phase component and the orthogonal component for each polarization of the received light; a detection step of detecting, based on the electrical signals, the deviation in orthogonality between the in-phase component and the orthogonal component occurring in an optical hybrid circuit that performs coherent detection of the received light as an angular error; and an update step of updating a set value used for compensating for the deviation in orthogonality in at least one of the optical hybrid circuit and the equalizer based on the angular error. [Effects of the Invention]

[0009] This invention makes it possible to improve the estimation accuracy of IQ orthogonality error while suppressing the complexity of the digital signal processing configuration, thereby realizing higher quality optical communication. [Brief explanation of the drawing]

[0010] [Figure 1] This is a block diagram showing the overall configuration of the optical receiver 1 in the first embodiment of the present invention. [Figure 2] This is a block diagram showing the configuration of the 90-degree error detection unit 24 in the first embodiment of the present invention. [Figure 3] This is a flowchart showing the operation of the optical receiver 1 in the first embodiment of the present invention. [Figure 4] This figure shows the simulation results for estimating the 90-degree error amount of the IQ orthogonal deviation using the configuration of the 90-degree error detection unit 24 in the first embodiment of the present invention. [Figure 5] This is a block diagram showing the configuration of the 90-degree error detection unit 24a in a second embodiment of the present invention. [Figure 6] This is a flowchart showing the operation of the optical receiver 1 in a second embodiment of the present invention. [Modes for carrying out the invention]

[0011] The optical receiver and signal processing method of the embodiment will be described below with reference to the drawings.

[0012] This invention relates to a technique for compensating for deviations in IQ orthogonality in an optical receiver of an optical transmission system for digital coherent optical communication. Digital coherent optical communication is a communication method that utilizes the wave properties of light. Coherent means having coherence, and in the field of communication, it means that frequency or phase modulation can be used. Compared to the conventional intensity modulation (IM)-direct detection (DD) method, which detects changes in the intensity of signal light with a photodiode, digital coherent optical communication has higher reception sensitivity and is a fundamental technology for wavelength division multiplexing communication that enables high-capacity information transmission of terabits per second.

[0013] For example, a quadrature phase shift keying (QPSK) optical transmission system, one of the modulation schemes, can transmit twice as much information as the IM-DD system by using the phase information of the light. Furthermore, by utilizing the property that two orthogonal light waves do not intersect, and by loading different information onto the X-polarization and Y-polarization respectively, it is possible to transmit twice as much information as the above QPSK optical transmission system. This type of modulation scheme is called dual polarization (DP)-QPSK. A DP-QPSK optical transmission system can transmit four times as much information in the same signal bandwidth compared to a conventional IM-DD optical transmission system.

[0014] The optical transmitter in the DP-QPSK optical transmission system converts the digital signals "0" and "1" into in-phase (I) and quadrature (Q) components of X-polarization (vertical polarization) and Y-polarization (horizontal polarization), respectively. The optical transmitter uses the electrical signals XI, XQ, YI, and YQ to drive a Mach-Zehnder modulator, and then performs polarization synthesis to generate a phase-modulated and polarization-multiplexed optical signal.

[0015] On the one hand, the optical receiver of an optical transmission system using the DP-QPSK method separates the polarization multiplexed signal light with phase modulation. Then, the optical receiver detects the I components and Q components of the X polarization and Y polarization respectively by interfering the signal light with the laser light (local oscillation light) sent from the local oscillation laser mounted on the receiving unit. Since the signal is detected by interfering the signal light with the local oscillation light, this is called coherent detection.

[0016] The optical receiver converts the detected I components and Q components of the X polarization and Y polarization into electrical signals by a light receiving element. Then, the optical receiver converts these I components and Q components into digital sampling data by an analog-to-digital converter (ADC: Analog to Digital Converter) having a high-speed sampling rate. Then, the optical receiver performs advanced signal equalization by digital signal processing using a DSP (Digital Signal Processor) on this digital sampling data. Thereby, the optical receiver can correct signal distortion caused by wavelength dispersion and polarization dispersion peculiar to the optical fiber.

[0017] The optical receiver 1 in the embodiment of the present invention described below is characterized in that it includes a 90-degree error detection unit that calculates the 90-degree error amount of the IQ orthogonal deviation of the optical hybrid circuit as angle information (angle error), and updates at least one receiver compensation value (initial setting value) of the optical hybrid circuit and the equalizer by summing the calculated value and the initially set receiver compensation value.

[0018] By having such a configuration, the optical receiver 1 in the embodiment of the present invention can estimate the 90-degree error amount of the IQ orthogonal deviation of the optical hybrid circuit without using a known reference signal, and feedback the estimated value to the optical hybrid circuit or update the compensation value used in the digital signal processing unit (equalizer) of the conventional configuration according to the estimated value.

[0019] As a result, the optical receiver 1 in the embodiment of the present invention can compensate for the IQ orthogonality error of the optical hybrid circuit with higher precision, thereby reducing signal quality degradation and enabling higher quality optical communication. Furthermore, in an optical transmission system having the optical receiver 1 in the embodiment of the present invention, there is no need to use a known reference signal, making it possible to compensate for the IQ orthogonality error even during operation of the optical transmission system.

[0020] <First Embodiment> The configuration of the optical receiver and signal processing method in the first embodiment will be described below.

[0021] [Overall configuration of the optical receiver] The overall configuration of the optical receiver 1 in the first embodiment will be described below. Figure 1 is a block diagram showing the overall configuration of the optical receiver 1 in the first embodiment of the present invention. As shown in Figure 1, the optical receiver 1 is configured to include an optical coherent detection unit 10 and a digital signal processing unit 20.

[0022] The optical coherent detection unit 10 is composed of an optical hybrid circuit 11 and four sets of balanced photodetectors (BPDs) and transimpedance amplifiers (TIAs) 12 (hereinafter referred to as "BPD+TIA12") (BPD+TIA12-1 to BPD+TIA12-4).

[0023] The digital signal processing unit 20 is composed of four analog-to-digital converters 21 (hereinafter referred to as "ADC21") (ADC21-1 to ADC21-4), two equalizers 22 (equalizer 22-1 and equalizer 22-2), an adaptive equalization unit 23, and two 90-degree error detection units 24 (90-degree error detection unit (X side) 24-1 and 90-degree error detection unit (Y side) 24-2).

[0024] The optical hybrid circuit 11 receives signal light transmitted from an opposing optical transmitter (not shown) and transmitted through an optical transmission path (not shown). The optical hybrid circuit 11 also receives local light emitted from a local oscillator laser (not shown) provided by the optical receiver 1. The optical hybrid circuit 11 performs coherent detection by interfering the received signal light (hereinafter also referred to as "received light") with the local light emitted. Through coherent detection, the optical hybrid circuit 11 converts the received light into four components: X-polarization I component, X-polarization Q component, Y-polarization I component, and Y-polarization Q component.

[0025] The optical hybrid circuit 11 outputs the four generated components to the corresponding four BPD+TIA12s. Specifically, the optical hybrid circuit 11 outputs the X-polarized I component to BPD+TIA12-1, the X-polarized Q component to BPD+TIA12-2, the Y-polarized I component to BPD+TIA12-3, and the Y-polarized Q component to BPD+TIA12-4.

[0026] The BPD+TIA12 converts the optical signal input from the optical hybrid circuit 11 into an electrical signal and amplifies the electrical signal. Each of the four BPD+TIA12s outputs its electrical signal to the corresponding ADC21. Specifically, BPD+TIA12-1 outputs its electrical signal to ADC21-1, BPD+TIA12-2 outputs its electrical signal to ADC21-2, BPD+TIA12-3 outputs its electrical signal to ADC21-3, and BPD+TIA12-4 outputs its electrical signal to ADC21-4.

[0027] Each of the four ADCs 21 converts the electrical signal input from the BPD+TIA 12 into a digital signal, and outputs the converted digital signal to the corresponding equalizer 22 and the corresponding 90-degree error detection unit 24, respectively.

[0028] Specifically, ADC21-1 converts the X-polarized I component electrical signal input from BPA+TIA12-1 into a digital signal, and outputs the converted X-polarized I component digital signal to equalizer 22-1 and 90-degree error detection unit (X side) 24-1, respectively. ADC21-2 converts the X-polarized Q component electrical signal input from BPA+TIA12-2 into a digital signal, and outputs the converted X-polarized Q component digital signal to equalizer 22-1 and 90-degree error detection unit (X side) 24-1, respectively. ADC21-3 converts the Y-polarized I component electrical signal input from BPA+TIA12-3 into a digital signal, and outputs the converted Y-polarized I component digital signal to equalizer 22-2 and 90-degree error detection unit (Y side) 24-2, respectively. Furthermore, the ADC21-4 converts the Y-polarized Q-component electrical signal input from the BPA+TIA12-4 into a digital signal, and outputs the converted Y-polarized Q-component digital signal to the equalizer 22-2 and the 90-degree error detection unit (Y side) 24-2, respectively.

[0029] The equalizer 22 compensates for signal distortion due to imperfections in the optical coherent detection unit 10 and signal distortion due to wavelength dispersion occurring in the optical transmission path (optical fiber) for each polarization of the digital signals input from the two corresponding ADCs 21. These imperfections include, for example, IQ gain imbalance, IQ skew, and 90-degree error. The equalizer 22 outputs the compensated digital signals for each polarization to the adaptive equalizer 23.

[0030] Specifically, equalizer 22-1 compensates for X-polarization signal distortion due to imperfections in the optical coherent detection unit 10 and X-polarization signal distortion due to wavelength dispersion occurring in the optical transmission path, based on the X-polarization I component digital signal input from ADC 21-1 and the X-polarization Q component digital signal input from ADC 21-2. Then, equalizer 22-1 outputs the compensated X-polarization digital signal to adaptive equalization unit 23. Equalizer 22-2 compensates for Y-polarization signal distortion due to imperfections in the optical coherent detection unit 10 and Y-polarization signal distortion due to wavelength dispersion occurring in the optical transmission path, based on the Y-polarization I component digital signal input from ADC 21-3 and the Y-polarization Q component digital signal input from ADC 21-4. The equalizer 22-2 then outputs the compensated Y-polarized digital signal to the adaptive equalizer 23.

[0031] The adaptive equalization unit 23 compensates for rapidly dynamically fluctuating channel responses (e.g., polarization rotation) in the digital signals for each polarization input from the equalizer 22. Specifically, the adaptive equalization unit 23 compensates for rapidly dynamically fluctuating channel responses of the digital signals based on the X-polarized digital signal input from equalizer 22-1 and the Y-polarized digital signal input from equalizer 22-2. The adaptive equalization unit 23 outputs the digital signal with the corrected channel response to, for example, an external device.

[0032] The configuration of the 90-degree error detection unit 24 will be explained in detail below with reference to Figure 2.

[0033] [Configuration of the 90-degree error detection unit] The configuration of the 90-degree error detection unit 24 will be described below. The 90-degree error detection unit 24 calculates the 90-degree error amount of the IQ orthogonal deviation as angular information, and updates the 90-degree error amount of the IQ orthogonal deviation set in the optical hybrid circuit 11 and equalizer 22 by adding the calculated value to the 90-degree error amount of the receiver compensation value of the IQ orthogonal deviation (initialized in the optical hybrid circuit 11 and equalizer 22).

[0034] Figure 2 is a block diagram showing the configuration of the 90-degree error detection unit 24 in the first embodiment of the present invention. As shown in Figure 2, the 90-degree error detection unit 24 includes an imperfection compensation unit 241, a Widely Linear compensation coefficient calculation unit 242 (hereinafter referred to as "WL compensation coefficient calculation unit 242"), and a 90-degree error calculation unit 243.

[0035] Note that the configuration of the 90-degree error detection unit (X side) 24-1 and the configuration of the 90-degree error detection unit (Y side) 24-2 are the same.

[0036] The imperfection compensation unit 241 acquires the digital signals of the I and Q components of the corresponding polarizations from the two corresponding ADCs 21. The imperfection compensation unit 241 also acquires the receiver compensation value of the corresponding polarization from the corresponding equalizer 22.

[0037] Specifically, the imperfection compensation unit 241 of the 90-degree error detection unit (X side) 24-1 acquires the digital signal of the I component of the X polarization from ADC 21-1 and the digital signal of the Q component of the X polarization from ADC 21-2. The imperfection compensation unit 241 of the 90-degree error detection unit (X side) 24-1 also acquires the receiver compensation value of the X polarization from equalizer 22-1. Similarly, the imperfection compensation unit 241 of the 90-degree error detection unit (Y side) 24-2 acquires the digital signal of the I component of the Y polarization from ADC 21-3 and the digital signal of the Q component of the Y polarization from ADC 21-4. The imperfection compensation unit 241 of the 90-degree error detection unit (Y side) 24-2 also acquires the receiver compensation value of the Y polarization from equalizer 22-2.

[0038] The receiver compensation values ​​include the IQ gain imbalance value, the IQ skew value, and the initial setting value for the 90-degree error amount of the IQ orthogonal shift.

[0039] The imperfection compensation unit 241 performs imperfection compensation on the digital signals of the I and Q components for each polarization using the acquired receiver compensation values ​​for each polarization. Specifically, the imperfection compensation unit 241 of the 90-degree error detection unit (X side) 24-1 performs imperfection compensation on the digital signals of the I and Q components of the X polarization using the acquired receiver compensation values ​​for the X polarization. Similarly, the imperfection compensation unit 241 of the 90-degree error detection unit (Y side) 24-2 performs imperfection compensation on the digital signals of the I and Q components of the Y polarization using the acquired receiver compensation values ​​for the Y polarization.

[0040] The imperfection compensation unit 241 outputs digital signals of the I component and Q component for each polarization, after imperfection compensation has been performed. For the output digital signal of the Q component, the imaginary unit multiplication unit j advances the phase of the Q component (orthogonal component) by 90 degrees on the complex plane. Subsequently, the addition unit combines the output of the imaginary unit multiplication unit j with the digital signal of the I component to generate an X-polarized (vertical polarization) or Y-polarized (horizontal polarization) received signal with the I component (in-phase component) as the real component and the Q component (orthogonal component) as the imaginary component, and this received signal is input to the WL compensation coefficient calculation unit 242.

[0041] Specifically, the imperfection compensation unit 241 of the 90-degree error detection unit (X side) 24-1 outputs digital signals of the I component and Q component of the X-polarization, respectively, after imperfection compensation has been performed. Then, a received signal of X-polarization (vertical polarization) is generated with the I component as the real number component and the Q component as the imaginary number component, and this received signal is input to the WL compensation coefficient calculation unit 242. Similarly, the imperfection compensation unit 241 of the 90-degree error detection unit (Y side) 24-2 outputs digital signals of the I component and Q component of the Y-polarization, respectively, after imperfection compensation has been performed. Then, a received signal of Y-polarization (horizontal polarization) is generated with the I component as the real number component and the Q component as the imaginary number component, and this received signal is input to the WL compensation coefficient calculation unit 242.

[0042] The WL compensation coefficient calculation unit 242 calculates the WL (wide-area linear) compensation coefficient W input from the summing unit. The WL compensation coefficient W is a coefficient that compensates for the deviation in the IQ symmetry of the received signal. The WL compensation coefficient W can be determined by the method described in Non-Patent Literature 1. Specifically, the WL compensation coefficient W can be determined using the following equation (1), which corresponds to equation (24) in Non-Patent Literature 1.

[0043] W = -E[(.) 2 ] / 2E[|.| 2 ] ···(1)

[0044] Here, |.| represents the absolute value, and E[.] represents the expected value.

[0045] The WL compensation coefficient calculation unit 242 outputs the calculated value of the WL compensation coefficient W to the 90-degree error calculation unit 243.

[0046] The 90-degree error calculation unit 243 extracts only the 90-degree error amount of the IQ orthogonal deviation from the value of the WL compensation coefficient W input from the WL compensation coefficient calculation unit 242. The 90-degree error calculation unit 243 calculates the 90-degree error amount of the IQ orthogonal deviation as angular information. Here, the 90-degree error θ is calculated using the following equation (2).

[0047] θ=arg∠[(1+w) / (1-w)] ···(2)

[0048] Here, arg∠[.] represents the argument of a complex number.

[0049] The 90-degree error calculation unit 243 updates the 90-degree error amount of IQ orthogonal deviation set in the optical hybrid circuit 11 and equalizer 22 by adding the calculated value to the 90-degree error amount of IQ orthogonal deviation of the receiver compensation value (which is initially set in the optical hybrid circuit 11 and equalizer 22).

[0050] The above update of the 90-degree error amount of IQ orthogonal shift is performed for each polarization. Specifically, the 90-degree error calculation unit 243 of the 90-degree error detection unit (X side) 24-1 extracts only the 90-degree error amount of the IQ orthogonal shift for the X polarization from the WL compensation coefficient W of the X polarization input from the WL compensation coefficient calculation unit 242. The 90-degree error calculation unit 243 calculates the 90-degree error amount of the IQ orthogonal shift for the X polarization as angular information. The 90-degree error calculation unit 243 updates the 90-degree error amount of the IQ orthogonal shift for the X polarization set in the optical hybrid circuit 11 and equalizer 22-1 by adding the calculated value to the 90-degree error amount of the IQ orthogonal shift of the receiver compensation value for the X polarization (initialized in the optical hybrid circuit 11 and equalizer 22-1). Similarly, the 90-degree error calculation unit 243 of the 90-degree error detection unit (Y side) 24-2 extracts only the 90-degree error amount of the Y-polarization IQ orthogonal shift from the WL compensation coefficient W of the Y-polarization input from the WL compensation coefficient calculation unit 242. The 90-degree error calculation unit 243 calculates the 90-degree error amount of the Y-polarization IQ orthogonal shift as angular information. The 90-degree error calculation unit 243 updates the 90-degree error amount of the Y-polarization IQ orthogonal shift set in the optical hybrid circuit 11 and equalizer 22-2 by adding the calculated value to the 90-degree error amount of the Y-polarization receiver compensation value IQ orthogonal shift (initialized in the optical hybrid circuit 11 and equalizer 22-2).

[0051] Furthermore, the method described in Non-Patent Document 1 is configured to apply the WL compensation coefficient W to the received signal. In other words, the incomplete compensation method described in Non-Patent Document 1 is configured to compensate for the effects of the 90-degree error amount of IQ orthogonal shift and IQ gain imbalance on the received signal. Therefore, the incomplete compensation method described in Non-Patent Document 1 does not have a configuration to calculate the 90-degree error amount of IQ orthogonal shift as angular information (angular error).

[0052] In contrast, the optical receiver 1 in the first embodiment differs from the incomplete compensation method described in Non-Patent Literature 1 in that it does not directly apply the WL compensation coefficient W to the received signal. As described above, the 90-degree error calculation unit 243 of the optical receiver 1 in the first embodiment extracts only the 90-degree error amount of IQ orthogonal shift from the WL compensation coefficient W. The 90-degree error calculation unit 243 calculates the 90-degree error amount of IQ orthogonal shift as angular information (angular error). The 90-degree error calculation unit 243 then updates the 90-degree error amount of IQ orthogonal shift of the initially set receiver compensation value by summing the calculated value with the 90-degree error amount of IQ orthogonal shift of the receiver compensation value initially set in at least one of the optical hybrid circuit 11 and the equalizer 22.

[0053] By having this configuration, the optical receiver 1 in the first embodiment can compensate not only for the 90-degree error of the IQ orthogonal shift in the digital signal processing unit 20 (equalizer 22), but also for the 90-degree error of the IQ orthogonal shift in the optical hybrid circuit 11. In particular, when the 90-degree error of the IQ orthogonal shift in the optical hybrid circuit 11 is large, the effectiveness of the compensation in the digital signal processing unit 20 decreases. Therefore, even in such cases, the optical receiver 1 in the first embodiment can suppress the decrease in the effectiveness of the compensation in the digital signal processing unit 20, and can realize higher quality optical communication.

[0054] [Operation of the optical receiver] The following describes an example of the operation of the optical receiver 1 in relation to updating the receiver compensation value. Figure 3 is a flowchart showing the operation of the optical receiver 1 in the first embodiment of the present invention.

[0055] The optical hybrid circuit 11 receives signal light transmitted from an opposing optical transmitter (not shown) and transmitted through an optical transmission path (not shown). The optical hybrid circuit 11 also receives local light emitted from a local oscillator laser (not shown) provided by the optical receiver 1. The optical hybrid circuit 11 performs coherent detection by interfering the received signal light (received light) with the local light emitted (step S001). Through coherent detection, the optical hybrid circuit 11 converts the received light into four components: X-polarization I component, X-polarization Q component, Y-polarization I component, and Y-polarization Q component. The optical hybrid circuit 11 outputs the four generated components to the corresponding four BPD+TIA 12.

[0056] The BPD+TIA12 converts the optical signal input from the optical hybrid circuit 11 into an electrical signal and amplifies it (step S002). Each of the four BPD+TIA12 outputs an electrical signal to the corresponding ADC21.

[0057] Each of the four ADCs 21 converts the electrical signal input from the BPD+TIA 12 into a digital signal, and outputs the converted digital signal to the corresponding equalizer 22 and the corresponding 90-degree error detection unit 24, respectively (step S003).

[0058] The imperfection compensation unit 241 of the 90-degree error detection unit 24 acquires the digital signals of the I component and Q component of the corresponding polarization from the two corresponding ADCs 21. The imperfection compensation unit 241 also acquires the receiver compensation value of the corresponding polarization from the corresponding equalizer 22. The receiver compensation value includes the initial setting value of the 90-degree error amount of the IQ orthogonal shift. The imperfection compensation unit 241 performs imperfection compensation on the digital signals of the I component and Q component for each polarization using the acquired receiver compensation value for each polarization (step S004).

[0059] The imperfection compensation unit 241 outputs digital signals of the I component and Q component for each polarization, after imperfection compensation has been performed. For the output digital signal of the Q component, the imaginary unit multiplication unit j advances the phase of the Q component by 90 degrees on the complex plane. Subsequently, the output of the imaginary unit multiplication unit j is combined with the digital signal of the I component by the addition unit to generate X-polarized and Y-polarized received signals in which the I component is the real component and the Q component is the imaginary component, and these received signals are input to the WL compensation coefficient calculation unit 242.

[0060] The WL compensation coefficient calculation unit 242 calculates the WL (wide-area linear) compensation coefficient W input from the addition unit (step S005). The WL compensation coefficient calculation unit 242 outputs the calculated WL compensation coefficient W to the 90-degree error calculation unit 243.

[0061] The 90-degree error calculation unit 243 extracts only the 90-degree error amount of the IQ orthogonal deviation from the WL compensation coefficient W input from the WL compensation coefficient calculation unit 242 (step S006). The 90-degree error calculation unit 243 calculates the 90-degree error amount of the IQ orthogonal deviation as angular information.

[0062] The 90-degree error calculation unit 243 updates the 90-degree error amount of the IQ orthogonal deviation of the receiver compensation value set in the optical hybrid circuit 11 and equalizer 22 by adding the calculated value to the 90-degree error amount of the IQ orthogonal deviation of the receiver compensation value (initialized in the optical hybrid circuit 11 and equalizer 22) (step S007).

[0063] The above describes the process for updating the receiver compensation value of the optical receiver 1 as shown in the flowchart of Figure 3. The operation will end.

[0064] [Simulation Results] The following describes the results of a simulation conducted based on the configuration of the 90-degree error detection unit 24 mentioned above, regarding the estimation of the 90-degree error amount of the IQ orthogonal deviation.

[0065] In this simulation, the 90-degree error amount of the IQ orthogonal shift was estimated using a 16QAM (Quadrature Amplitude Modulation) signal and the 90-degree error detection unit 24 shown in Figure 2. In this simulation, a load was applied to the 90-degree angle between the polarizations of the optical hybrid circuit 11 while varying the value. Then, in this simulation, the estimated value output from the 90-degree error detection unit 24 was observed for each load value, and the estimation accuracy was confirmed by comparing the load value with the estimated value.

[0066] Figure 4 shows the results of a simulation for estimating the 90-degree error amount of the IQ orthogonal shift, based on the configuration of the 90-degree error detection unit 24 in the first embodiment of the present invention. In the graph shown in Figure 4, the horizontal axis represents the load value [unit: deg] given to the 90-degree angle between the polarizations of the optical hybrid circuit 11, and the vertical axis represents the estimated value [unit: deg] output by the 90-degree error detection unit 24.

[0067] As shown in Figure 4, the load value and the estimated value were in close agreement, confirming that the configuration of the 90-degree error detection unit 24 in the first embodiment of the present invention makes it possible to estimate the 90-degree error amount of the IQ orthogonal deviation.

[0068] <Second Embodiment> The configuration of the optical receiver and signal processing method in the second embodiment will be described below.

[0069] The overall configuration of the optical receiver 1 in the second embodiment is the same as that of the optical receiver 1 in the first embodiment described above with reference to the overall configuration diagram in Figure 1, so the explanation will be omitted.

[0070] [Configuration of the 90-degree error detection unit] The configuration of the 90-degree error detection unit 24a of the optical receiver 1 in the second embodiment will be described below. Figure 5 is a block diagram showing the configuration of the 90-degree error detection unit 24a in the second embodiment of the present invention.

[0071] The configuration of the 90-degree error detection unit 24a in the second embodiment shown in Figure 5 differs from the configuration of the 90-degree error detection unit 24 in the first embodiment shown in Figure 2, in that it further includes a repeatability determination unit 244. The configurations of the imperfection compensation unit 241, the WL compensation coefficient calculation unit 242, and the 90-degree error calculation unit 243 are the same as those included in the 90-degree error detection unit 24 in the first embodiment, and therefore the same reference numerals are used, and their descriptions are omitted.

[0072] The 90-degree error calculation unit 243 calculates the 90-degree error amount of the IQ orthogonal deviation as angular information and outputs the calculated value to the repeat determination unit 244. The repeat determination unit 244 determines whether or not to repeatedly execute the calculation process of the 90-degree error amount of the IQ orthogonal deviation, which is performed by the imperfection compensation unit 241, the WL compensation coefficient calculation unit 242, and the 90-degree error calculation unit 243.

[0073] If the repeat determination unit 244 determines that the calculation process for the 90-degree error amount of the IQ orthogonal shift should be repeated, it outputs the value of the 90-degree error amount of the IQ orthogonal shift input from the 90-degree error calculation unit 243 to the imperfection compensation unit 241. When the imperfection compensation unit 241 receives the value of the 90-degree error amount of the IQ orthogonal shift from the 90-degree error calculation unit 243, it performs imperfection compensation on the digital signals of the I component and Q component using the receiver compensation value which includes the input value of the 90-degree error amount of the IQ orthogonal shift. Thereafter, the calculation process for the 90-degree error amount of the IQ orthogonal shift is repeated.

[0074] On the other hand, if the repeatability determination unit 244 determines that the calculation process for the 90-degree error amount of the IQ orthogonal shift should not be repeated, it sets the value of the 90-degree error amount of the IQ orthogonal shift input from the 90-degree error calculation unit 243 as the final updated value. The 90-degree error calculation unit 243 updates the 90-degree error amount of the IQ orthogonal shift set in the optical hybrid circuit 11 and the equalizer 22 by adding the final updated value of the 90-degree error amount of the IQ orthogonal shift with the receiver compensation value of the 90-degree error amount of the IQ orthogonal shift (initialized in the optical hybrid circuit 11 and the equalizer 22).

[0075] The repetition determination unit 244 determines, for example, that the calculation process for the 90-degree error amount of the IQ orthogonal deviation should not be repeated if the value of the 90-degree error amount of the IQ orthogonal deviation input from the 90-degree error calculation unit 243 has converged. For example, the repetition determination unit 244 determines that the value of the 90-degree error amount of the IQ orthogonal deviation has converged if the difference between the acquired value of the 90-degree error amount of the IQ orthogonal deviation and the previously acquired value of the 90-degree error amount of the IQ orthogonal deviation is less than or equal to a predetermined threshold.

[0076] By having such a configuration, the optical receiver 1 in the second embodiment can avoid updating the 90-degree error amount of the IQ orthogonal shift set in the optical hybrid circuit 11 and the equalizer 22 when, for example, the 90-degree error of the IQ orthogonal shift changes suddenly and significantly.

[0077] [Operation of the optical receiver] The following describes an example of the operation of the optical receiver 1 in relation to updating the receiver compensation value. Figure 6 is a flowchart showing the operation of the optical receiver 1 in the second embodiment of the present invention.

[0078] Note that the operation of the optical receiver 1 in the second embodiment from steps S101 to S103 of the flowchart shown in Figure 6 is the same as the operation of the optical receiver 1 in the first embodiment from steps S001 to S003 of the flowchart shown in Figure 3, so the explanation is omitted.

[0079] The imperfection compensation unit 241 of the 90-degree error detection unit 24 acquires the digital signals of the I component and Q component of the corresponding polarization from the two corresponding ADCs 21. The imperfection compensation unit 241 also acquires the receiver compensation value of the corresponding polarization from the corresponding equalizer 22. The receiver compensation value includes the initial setting value of the 90-degree error amount of the IQ orthogonal shift. The imperfection compensation unit 241 performs imperfection compensation on the digital signals of the I component and Q component for each polarization using the acquired receiver compensation value for each polarization (step S104).

[0080] The imperfection compensation unit 241 outputs digital signals of the I component and Q component for each polarization, after imperfection compensation has been performed. For the output digital signal of the Q component, the imaginary unit multiplication unit j advances the phase of the Q component by 90 degrees on the complex plane. Subsequently, the output of the imaginary unit multiplication unit j is combined with the digital signal of the I component by the addition unit to generate X-polarized and Y-polarized received signals in which the I component is the real component and the Q component is the imaginary component, and these received signals are input to the WL compensation coefficient calculation unit 242.

[0081] The WL compensation coefficient calculation unit 242 calculates the WL (wide-area linear) compensation coefficient W input from the addition unit (step S105). The WL compensation coefficient calculation unit 242 outputs the calculated WL compensation coefficient W to the 90-degree error calculation unit 243.

[0082] The 90-degree error calculation unit 243 extracts only the 90-degree error amount of the IQ orthogonal deviation from the WL compensation coefficient W input from the WL compensation coefficient calculation unit 242 (step S106). The 90-degree error calculation unit 243 calculates the 90-degree error amount of the IQ orthogonal deviation as angle information. The 90-degree error calculation unit 243 calculates the 90-degree error amount of the IQ orthogonal deviation as angle information and outputs the calculated value to the repeat determination unit 244.

[0083] The repeat determination unit 244 determines whether or not to repeatedly execute the process of calculating the 90-degree error amount of the IQ orthogonal deviation (step S107).

[0084] If the repeat determination unit 244 determines that the calculation process for the 90-degree error amount of the IQ orthogonal shift should be repeated (step S107, YES), it outputs the value of the 90-degree error amount of the IQ orthogonal shift input from the 90-degree error calculation unit 243 to the imperfection compensation unit 241. When the imperfection compensation unit 241 receives the value of the 90-degree error amount of the IQ orthogonal shift from the 90-degree error calculation unit 243, it performs imperfection compensation on the digital signals of the I component and Q component using the receiver compensation value which includes the input value of the 90-degree error amount of the IQ orthogonal shift (step S104). Thereafter, the calculation process for the 90-degree error amount of the IQ orthogonal shift is repeated (steps S105 to S106).

[0085] On the other hand, if the repeatability determination unit 244 determines that the calculation process for the 90-degree error amount of the IQ orthogonal shift should not be repeated (step S107, NO), it sets the value of the 90-degree error amount of the IQ orthogonal shift input from the 90-degree error calculation unit 243 as the final updated value. The 90-degree error calculation unit 243 updates the 90-degree error amount of the IQ orthogonal shift set in the optical hybrid circuit 11 and the equalizer 22 by adding the final updated value of the 90-degree error amount of the IQ orthogonal shift with the 90-degree error amount of the receiver compensation value (initialized in the optical hybrid circuit 11 and the equalizer 22) (step S108).

[0086] The above describes the process for updating the receiver compensation value of the optical receiver 1 as shown in the flowchart of Figure 6. The operation will end.

[0087] As described above, the optical receiver 1 in each embodiment of the present invention is characterized by comprising a 90-degree error detection unit that calculates the 90-degree error amount of the IQ orthogonal deviation of the optical hybrid circuit as angular information (angular error), and updates the receiver compensation value (initial setting value) of at least one of the optical hybrid circuit and the equalizer by summing the calculated value with the initially set receiver compensation value.

[0088] By having such a configuration, the optical receiver 1 in each embodiment of the present invention can estimate the 90-degree error amount of the IQ orthogonal shift of the optical hybrid circuit without using a known reference signal, and can feed the estimated value back to the optical hybrid circuit or update the compensation value used in the conventional digital signal processing unit (equalizer) based on the estimated value.

[0089] As a result, the optical receiver 1 in the embodiment of the present invention can compensate for the IQ orthogonality error of the optical hybrid circuit with higher precision, thereby reducing signal quality degradation and enabling higher quality optical communication. Furthermore, in an optical transmission system having the optical receiver 1 in the embodiment of the present invention, there is no need to use a known reference signal, making it possible to compensate for the IQ orthogonality error even during operation of the optical transmission system.

[0090] Furthermore, by having such a configuration, the optical receiver 1 in each embodiment of the present invention does not require the addition of a new equalizer, unlike conventional techniques that perform compensation processing using a known reference signal. Therefore, it is possible to improve the estimation accuracy of the IQ orthogonality error while suppressing the complexity of the digital signal processing configuration.

[0091] According to the embodiment described above, the optical receiver comprises an acquisition unit and an error detection unit. For example, the optical receiver is the optical receiver 1 in the embodiment, the acquisition unit is the digital signal processing unit 20 in the embodiment, and the error detection unit is the 90-degree error detection unit 24 (90-degree error detection unit (X side) 24-1 and 90-degree error detection unit (Y side) 24-2) and the 90-degree error detection unit 24a (90-degree error detection unit (X side) 24a-1 and 90-degree error detection unit (Y side) 24a-2) in the embodiment.

[0092] The acquisition unit described above acquires signals representing the in-phase and orthogonal components for each polarization of the received light. For example, the received light is the signal light in the embodiment, the polarization is the X-polarization and Y-polarization in the embodiment, and the signal is the electrical signal for each polarization and each IQ component in the embodiment.

[0093] The error detection unit, based on the above signal, detects the deviation in orthogonality between the common-phase component and the orthogonal component occurring in the optical hybrid circuit that performs coherent detection of the received light as an angular error, and updates the set value used for compensating for the deviation in orthogonality in at least one of the optical hybrid circuit and the equalizer based on the angular error. For example, the optical hybrid circuit is the optical hybrid circuit 11 in the embodiment, the angular error is the output value of the 90-degree error calculation unit 243 in the embodiment, the equalizer is the equalizer 22 (equalizer 22-1 and equalizer 22-1) in the embodiment, and the set value is the receiver compensation value in the embodiment.

[0094] In the optical receiver described above, the error detection unit may perform an iterative process to repeatedly calculate the orthogonality deviation using the detected angular error, and update the set value with the angular error value converged by the iterative process. For example, the iterative process is performed by the iterative determination unit 244 in the embodiment.

[0095] In the above-described optical receiver, the error detection unit may calculate a coefficient to compensate for the symmetry difference between the in-phase component and the orthogonal component based on the above-described signal, and detect the orthogonality difference extracted from this coefficient as an angular error. For example, the coefficient is the WL compensation coefficient W in the embodiment.

[0096] Furthermore, in the optical receiver described above, the error detection unit may calculate the angle error based on the following formula.

[0097] θ = arg∠[(1+w) / (1-w)]

[0098] Here, W represents the coefficient mentioned above, θ represents the angular error mentioned above, and arg∠[.] represents the argument of the complex number.

[0099] In the optical receiver described above, the error detection unit may calculate the coefficient based on a set value used for compensation processing set in the equalizer. For example, the set value is the receiver compensation value in the embodiment.

[0100] Furthermore, in the optical receiver described above, the error detection unit may calculate the coefficient based on the following formula. W = -E[(.) 2 ] / 2E[|.| 2 ] Here, W represents the coefficient mentioned above, and |.| is A vertically polarized or horizontally polarized received signal in which the in-phase component is the real component and the orthogonal component is the imaginary component. It represents the absolute value, and E[.] is The received signal This represents the expected value. Note that the above calculation formula corresponds to formula (24) in Non-Patent Document 1.

[0101] A part of the optical receiver 1 in the above-described embodiment may be implemented using a computer. In that case, the program for implementing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be loaded into a computer system and executed. Here, "computer system" includes hardware such as the OS and peripheral devices. Furthermore, "computer-readable recording medium" refers to portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, and storage devices such as hard disks built into a computer system.

[0102] Furthermore, "computer-readable recording media" may include those that dynamically hold programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or communication lines such as telephone lines, as well as those that hold programs for a fixed period of time, such as volatile memory inside computer systems that act as servers or clients in such cases. In addition, the above-mentioned program may be for the purpose of realizing a part of the aforementioned functions, or it may be a program that can realize the aforementioned functions in combination with a program already recorded in the computer system, or it may be implemented using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0103] Although embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention. [Explanation of symbols]

[0104] 1…Optical receiver, 10…Optical coherent detection unit, 11…Optical hybrid circuit, 12…Balanced photodetector and transimpedance amplifier (BPD+TIA), 20…Digital signal processing unit, 21…Analog-to-digital converter (ADC), 22…Equalizer, 23…Adaptive equalization unit, 24…90-degree error detection unit, 241…Imperfection compensation unit, 242…WL compensation coefficient calculation unit, 243…90-degree error calculation unit, 244…Determination unit

Claims

1. An acquisition unit that acquires signals representing the in-phase and orthogonal components for each polarization of the received light, An error detection unit calculates a 90-degree error amount as an angular error, which represents the deviation in orthogonality between the common-phase component and the orthogonal component that occurs in the optical hybrid circuit that performs coherent detection of the received light based on the signal, and adds the calculated 90-degree error amount to the receiver compensation value, which is the 90-degree error amount initially set in the optical hybrid circuit and the equalizer, thereby updating the setting value used for compensating for the deviation in orthogonality in at least one of the optical hybrid circuit and the equalizer. Equipped with, The error detection unit calculates a coefficient to compensate for the symmetry difference between the in-phase component and the orthogonal component based on the signal, using the following formula, and detects the orthogonality difference as the angular error by extracting it from the coefficient. Optical receiver. W=-E[(.) 2 ] / 2E[|. | 2] Here, W represents the coefficient, |.| represents the absolute value of the vertically or horizontally polarized received signal with the in-phase component as the real component and the orthogonal component as the imaginary component, and E[.] represents the expected value of the received signal.

2. The error detection unit performs an iterative process to repeatedly calculate the orthogonality deviation using the detected angular error, and updates the set value with the angular error value converged by the iterative process. The optical receiver according to claim 1.

3. An acquisition unit that acquires signals representing the in-phase and orthogonal components for each polarization of the received light, An error detection unit detects, based on the signal, the deviation in orthogonality between the in-phase component and the orthogonal component occurring in the optical hybrid circuit that performs coherent detection of the received light as an angular error, and updates a set value used for compensating for the deviation in orthogonality in at least one of the optical hybrid circuit and the equalizer based on the angular error. Equipped with, The error detection unit calculates a coefficient to compensate for the symmetry difference between the in-phase component and the orthogonal component based on the signal, detects the orthogonality difference as the angular error by extracting it from the coefficient, and calculates the angular error based on the following formula. Optical receiver. θ=arg∠[(1+W) / (1-W)] Here, W represents the coefficient, θ represents the angular error, and arg∠[.] represents the argument of the complex number.

4. The error detection unit calculates the coefficient based on the set value used in the compensation process set in the equalizer. The optical receiver according to claim 1.

5. A computer-based signal processing method for an optical receiver, An acquisition step in which electrical signals representing the in-phase and orthogonal components for each polarization of the received light are acquired, A calculation step of calculating a 90-degree error amount as an angular error, which represents the deviation in orthogonality between the in-phase component and the orthogonal component that occurs in the optical hybrid circuit that performs coherent detection of the received light based on the electrical signal, An update step to update the setting value used for compensating for the orthogonality deviation in at least one of the optical hybrid circuit and the equalizer by summing the calculated 90-degree error amount with the receiver compensation value, which is the 90-degree error amount initially set in the optical hybrid circuit and the equalizer; It has, In the calculation step, a coefficient to compensate for the symmetry difference between the in-phase component and the orthogonal component is calculated based on the electrical signal using the following formula, and the orthogonality difference is detected as the angular error by extracting it from the coefficient. Signal processing method. W=-E[(.) 2 ] / 2E[|. | 2] Here, W represents the coefficient, |.| represents the absolute value of the vertically or horizontally polarized received signal with the in-phase component as the real component and the orthogonal component as the imaginary component, and E[.] represents the expected value of the received signal.

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