Optical receiver and receiving method

Abstract

In order to provide an optical receiver capable of performing stable coherent detection and maintaining the quality of a received signal, the optical receiver is provided with: a local oscillation light output unit (301); a phase adjustment unit (302); a polarization control unit (303); a multiplexing unit (304); a photoelectric conversion unit (305); a demodulation unit (306); and a control unit (307). The phase adjustment unit (302) adjusts the phase of the local oscillation light. The polarization control unit (303) controls the polarization rotation of the optical signal. The multiplexing unit (304) multiplexes the local oscillation light output from the phase adjustment unit (302) with the optical signal output from the polarization control unit (303). The demodulation unit (306) performs a demodulation process based on an electrical signal obtained by conversion performed by the photoelectric conversion unit (305). Based on information about the reception state of the optical signal, the control unit (307) controls the execution of at least one of phase adjustment of the local oscillation light in the phase adjustment unit (302) and polarization rotation of the optical signal in the polarization control unit (303).

Description

Technical Field

[0001] The present invention relates to an optical communication technology based on a digital coherence method, and more particularly, to a technology for maintaining reception quality. Background Art

[0002] As an optical communication technology supporting high-speed and large-capacity transmission, a digital coherent optical communication method is used. As a digital coherent optical communication method, various modulation methods such as a polarization multiplexing method and a multi-value modulation method have been proposed. As a multi-value modulation method, binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 8-quadrature amplitude modulation (8-QAM), etc. are used.

[0003] In the digital coherent method, the received optical signal is multiplied by the output light (local oscillation light) from the local oscillator to generate a baseband signal. The baseband signal is subjected to analog / digital conversion and digital signal processing to regenerate the original transmission signal. Therefore, in order to maintain the reception quality, it is necessary to stably perform coherent detection on the optical signal. Therefore, as such a technology for stably performing coherent detection on the optical signal and maintaining the signal quality, for example, a technology such as in PTL1 is disclosed.

[0004] PTL 1 relates to an optical transmitter based on a digital coherent method. The optical transmitter in PTL 1 adjusts the wavelength and power of local oscillation light in such a manner as to increase the signal quality of a received signal, and controls the wavelength of the local oscillation light in such a manner that a wavelength difference between the optical signal and the local oscillation light does not occur. PTL 1 assumes that when such a configuration is constructed, high-precision reception performance of an optical signal can be achieved.

[0005] [Citation List]

[0006] [Patent Document]

[0007] [PTL 1] Japanese Unexamined Patent Application Publication No. 2015-170916 Summary of the invention

[0008] [Technical problem solved by the invention]

[0009] However, the technology of PTL 1 is insufficient in the following aspects. When coherent detection is performed on the receiving side, the symbol may be fixed on the in-phase (I) axis or the quadrature (Q) axis when the frequencies of the optical signal and the local oscillator light match. When in this case, the gain is automatically controlled so that the output amplitude is constant in the detection element for the optical signal, there is no input signal for the component whose fixed component on the axis is 0, so the gain can be set to be larger to increase the output amplitude. When the gain is set to be larger, the noise of the signal increases, and then the quality degradation of the signal occurs. Therefore, the technology of PTL 1 is not sufficient as a technology for maintaining the reception quality that supports stable reception processing in an optical communication system based on a digital coherent method.

[0010] In order to solve the above-mentioned problems, an object of the present invention is to provide an optical receiver capable of maintaining reception quality supporting stable reception processing.

[0011] [Solutions to technical issues]

[0012] In order to solve the above-mentioned problems, according to the optical receiver of this exemplary embodiment, the optical receiver includes: a local oscillating light output device; a phase adjusting device; a polarization controlling device; a multiplexing device; a photoelectric converting device; a demodulating device; and a controlling device. The local oscillating light output device outputs a local oscillating light of a frequency set based on the frequency of the optical signal to be input. The phase adjusting device adjusts the phase of the local oscillating light. The polarization controlling device controls the polarization rotation of the optical signal. The multiplexing device multiplexes the local oscillating light output from the phase adjusting device with the optical signal output from the polarization controlling device. The photoelectric converting device converts the optical signal multiplexed by the multiplexing device into an electrical signal. The demodulating device performs a demodulation process based on the electrical signal converted by the photoelectric converting device. The controlling device controls the execution of at least one of the adjustment of the phase of the local oscillating light in the phase adjusting device and the polarization rotation of the optical signal in the polarization controlling device based on the information about the receiving state of the optical signal.

[0013] The receiving method according to the present invention includes: outputting a local oscillator light of a frequency set based on the frequency of an optical signal to be input; and multiplexing the local oscillator light subjected to phase adjustment with the optical signal subjected to polarization rotation. The receiving method according to the present invention also includes: converting the multiplexed optical signal into an electrical signal; and performing a demodulation process based on the converted electrical signal. The receiving method according to the present invention also includes: performing at least one of adjustment of the phase of the local oscillator light and polarization rotation of the optical signal based on information about the reception state of the optical signal.

[0014] [Beneficial effects of the present invention]

[0015] According to the present invention, it is possible to perform stable coherent detection on the receiving side and maintain the quality of the received signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a diagram showing an outline of a configuration according to a first exemplary embodiment of the present invention.

[0017] Figure 2 is a diagram showing an outline of a configuration according to a second exemplary embodiment of the present invention.

[0018] Figure 3 is a diagram showing a configuration of an optical transmitter according to a second exemplary embodiment of the present invention.

[0019] Figure 4 is a diagram showing a configuration of an optical receiver according to a second exemplary embodiment of the present invention.

[0020] Figure 5 is a diagram showing an operation flow of an optical communication system according to a second exemplary embodiment of the present invention.

[0021] Figure 6 is a diagram showing an example of a measurement result of the number of errors per frequency offset according to the second exemplary embodiment of the present invention.

[0022] Figure 7 : is a diagram showing an example of a frame transmitted in another configuration example according to the second exemplary embodiment of the present invention.

[0023] Fig. 8A is a diagram showing an example of a constellation in a multi-value modulation method.

[0024] Figure 8B is a diagram showing an example of a constellation in a multi-value modulation method.

[0025] Fig. 9 is a diagram showing an example of change of a constellation in a multi-value modulation method.

[0026] Fig.10 is a diagram showing an outline of a configuration according to a third exemplary embodiment of the present invention.

[0027] Fig.11 is a diagram showing a configuration of an optical transmitter according to a third exemplary embodiment of the present invention.

[0028] Fig.12 is a diagram showing a configuration of an optical receiver according to a third exemplary embodiment of the present invention.

[0029] Fig.13 is a diagram showing an outline of a configuration according to a fourth exemplary embodiment of the present invention.

[0030] Fig.14is a diagram showing a configuration of an optical transmitter according to a fourth exemplary embodiment of the present invention.

[0031] Fig.15 is a diagram showing a configuration of an optical receiver according to a fourth exemplary embodiment of the present invention.

[0032] Fig.16 is a diagram showing an operation flow of an optical communication system according to a fourth exemplary embodiment of the present invention.

[0033] Fig.17 is a diagram showing an outline of a configuration according to a fifth exemplary embodiment of the present invention.

[0034] Fig.18 is a diagram showing a configuration of an optical transmitter according to a fifth exemplary embodiment of the present invention.

[0035] Fig.19 is a diagram showing a configuration of an optical receiver according to a fifth exemplary embodiment of the present invention.

[0036] Fig. 20 is a diagram showing an outline of a configuration according to a sixth exemplary embodiment of the present invention.

[0037] Fig.21 is a diagram showing a configuration of an optical transmitter according to a sixth exemplary embodiment of the present invention.

[0038] Fig. 22 is a diagram showing a configuration of an optical receiver according to a sixth exemplary embodiment of the present invention.

[0039] Fig.23 is a diagram showing an operation flow of an optical communication system according to a sixth exemplary embodiment of the present invention.

[0040] Fig.24 is a diagram showing an outline of a configuration according to a seventh exemplary embodiment of the present invention.

[0041] Fig.25 is a diagram showing a configuration of an optical transmitter according to a seventh exemplary embodiment of the present invention.

[0042] Fig.26 is a diagram showing a configuration of an optical receiver according to a seventh exemplary embodiment of the present invention.

[0043] Fig. 27 is a diagram showing an outline of a configuration according to an eighth exemplary embodiment of the present invention.

[0044] Fig.28 is a diagram showing a configuration of an optical transmitter according to an eighth exemplary embodiment of the present invention.

[0045] Fig.29 is a diagram showing a configuration of an optical receiver according to an eighth exemplary embodiment of the present invention.

[0046] Fig. 30A is a diagram showing an example of a constellation in a multi-value modulation method.

[0047] Fig. 30B is a diagram showing an example of a constellation in a multi-value modulation method.

[0048] Fig. 30C is a diagram showing an example of a constellation in a multi-value modulation method.

[0049] Fig.30D is a diagram showing an example of a constellation in a multi-value modulation method.

[0050] Fig.31A is a diagram showing an example of a constellation in a multi-value modulation method.

[0051] Fig.31B is a diagram showing an example of a constellation in a multi-value modulation method.

[0052] Fig.31C is a diagram showing an example of a constellation in a multi-value modulation method.

[0053] Fig.31D is a diagram showing an example of a constellation in a multi-value modulation method.

[0054] Fig.32 is a diagram showing an example of temporal changes in the number of error occurrences.

[0055] Fig.33A is a diagram showing an example of a constellation in a multi-value modulation method.

[0056] Fig.33B is a diagram showing an example of a constellation in a multi-value modulation method.

[0057] Fig.34 is a diagram showing an example of temporal changes in the number of error occurrences in an optical receiver according to an eighth example embodiment of the present invention.

[0058] Fig.35 is a diagram showing an example of another configuration of an optical receiver according to the present invention. DETAILED DESCRIPTION

[0059] (First exemplary embodiment)

[0060] A first exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Figure 1An outline of the configuration of an optical transmitter according to the present exemplary embodiment is shown. The optical transmitter according to the present exemplary embodiment includes an optical output unit 1, an optical modulation unit 2, a reception information acquisition unit 3, and a frequency adjustment unit 4. The optical output unit 1 outputs light of a frequency assigned to a local device. The optical modulation unit 2 separates the light output by the optical output unit 1 into polarized waves orthogonal to each other, modulates each polarized wave into an in-phase component and an orthogonal component, and outputs an optical signal in which the modulated component waves are polarized and synthesized. The reception information acquisition unit 3 acquires information about the reception state of the optical signal in the optical receiver as the transmission destination of the optical signal. The frequency adjustment unit 4 controls the frequency of the light output by the optical output unit 1 based on the information about the reception state, and adjusts the frequency offset which is the difference between the frequency of the local oscillator light used when the optical receiver performs coherent detection on the optical signal and the frequency of the light output by the optical output unit 1.

[0061] The optical transmitter according to the present exemplary embodiment acquires information about the reception state in the optical receiver in the reception information acquisition unit 3, and adjusts the frequency offset which is the difference between the frequency of the local oscillation light of the optical receiver and the frequency of the light output by the optical output unit 1 in the frequency adjustment unit 4. In the optical transmitter according to the present exemplary embodiment, the offset is added to the frequency of the local oscillation light and the frequency of the light output by the optical output unit 1, so that a component having an amplitude of 0 is output in the detection element of the signal of the optical receiver is not generated. Therefore, a state in which noise is generated in the signal due to an attempt to increase the gain in the optical receiver can be prevented, so that the reception quality can be maintained. As a result, when the optical transmitter according to the present exemplary embodiment is used, the quality of the received signal can be maintained by performing stable coherent detection on the receiving side.

[0062] (Second exemplary embodiment)

[0063] A second exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Figure 2 20 is a diagram showing an outline of a configuration of an optical communication system according to the present exemplary embodiment. The optical communication system according to the present exemplary embodiment includes an optical transmitter 10 and an optical receiver 20. The optical transmitter 10 and the optical receiver 20 are connected to each other via a communication path 201 and a communication path 202. The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method via the communication path 201 between the optical transmitter 10 and the optical receiver 20.

[0064] The configuration of the optical transmitter 10 is described. Figure 3The configuration of the optical transmitter 10 according to the present exemplary embodiment is shown. The optical transmitter 10 includes a client signal input unit 11, a signal processing unit 12, a signal modulation unit 13, a light source unit 14, and a frequency adjustment unit 15.

[0065] The client signal input unit 11 is an input port of a client signal transmitted via the communication path 201. The client signal input to the client signal input unit 11 is transmitted to the signal processing unit 12.

[0066] The signal processing unit 12 performs processing such as redundancy on the input client signal, and maps the processed signal in a frame used when transmitting the signal through the communication path 201 .

[0067] The signal modulation unit 13 modulates the light input from the light source unit 14 based on the signal input from the signal processing unit 12, and generates an optical signal sent to the communication path 201. The signal modulation unit 13 according to the present exemplary embodiment performs modulation based on, for example, a binary phase shift keying (BPSK) modulation method. The modulation method may be another multi-value modulation method other than BPSK, such as quadrature phase shift keying (QPSK) and 8-quadrature amplitude modulation (8-QAM). The function of the signal modulation unit 13 according to the present exemplary embodiment is equivalent to that of the optical modulation unit 2 according to the first exemplary embodiment.

[0068] The light source unit 14 outputs continuous light of a predetermined frequency to the signal modulation unit 13. The predetermined frequency is assigned based on the wavelength design of the optical communication network. The light source unit 14 uses the predetermined frequency as a set value to output light whose frequency is offset and added to the set value. The offset amount of the frequency is controlled by the frequency adjustment unit 15. The function of the light source unit 14 according to the present exemplary embodiment is equivalent to that of the optical output unit 1 according to the first exemplary embodiment.

[0069] The frequency adjustment unit 15 controls the offset of the frequency of the light source unit 14. The frequency adjustment unit 15 controls the offset of the frequency based on the error information transmitted from the optical receiver 20. The frequency adjustment unit 15 controls the offset of the frequency in a manner that reduces the bit error rate (BER) transmitted as the error information. The function of the frequency adjustment unit 15 according to the present exemplary embodiment is equivalent to the reception information acquisition unit 3 and the frequency adjustment unit 4 according to the first exemplary embodiment.

[0070] The configuration of the optical receiver 20 is described. Figure 42 shows a configuration of an optical receiver 20 according to the present exemplary embodiment. The optical receiver 20 includes a client signal output unit 21, a PBS 22, a 90-degree hybrid 23, and an optical detection unit 24. The optical receiver 20 also includes an analog-to-digital converter (ADC) 25, a digital signal processor (DSP) 26, a local oscillation light output unit 27, and an error detection unit 28.

[0071] The client signal output unit 21 is an output port that outputs the demodulated client signal.

[0072] The polarization beam splitter (PBS) 22 polarizes the input optical signal and outputs the separated signal. The PBS 22 includes a PBS 22-1 that polarizes the optical signal and a PBS 22-2 that polarizes the local oscillator light. The PBS 22-1 polarizes the optical signal input from the communication path 201, outputs the X polarization wave to the 90-degree hybrid 23-1, and transmits the Y polarization wave to the 90-degree hybrid 23-2. The PBS 22-2 polarizes the light input from the local oscillator light output unit 27, outputs the X polarization wave to the 90-degree hybrid 23-1, and transmits the Y polarization wave to the 90-degree hybrid 23-2.

[0073] 90 degree hybrid 23 multiplexes the input optical signal with the input local oscillator light through two paths with a phase difference of 90 degrees. 90 degree hybrid 23-1 multiplexes the X polarization component of the optical signal input from PBS 22-1 with the X polarization component of the local oscillator light input from PBS 22-2 through two paths with a phase difference of 90 degrees.

[0074] The 90-degree hybrid 23-1 transmits signals of an in-phase (I-phase) component and a quadrature (Q-phase) component generated by multiplexing an optical signal with a local oscillator light via a path having a phase difference of 90 degrees to the optical detection unit 24-1. The 90-degree hybrid 23-2 multiplexes the Y polarization component of the optical signal input from the PBS 22-1 with the Y polarization component of the local oscillator light input from the PBS 22-2 via two paths having a phase difference of 90 degrees. The 90-degree hybrid 23-2 transmits signals of an I-phase component and a Q-phase component generated by multiplexing an optical signal with a local oscillator light via a path having a phase difference of 90 degrees to the optical detection unit 24-2.

[0075] The optical detection unit 24 converts the input optical signal into an electrical signal and outputs the electrical signal. The optical detection unit 24 is configured using a photodiode. The optical detection unit 24-1 converts the optical signal of each of the I-phase component and the Q-phase component of the X-polarized wave input from the 90-degree hybrid 23-1 into an electrical signal, and sends the electrical signal to the ADC 25-1. The optical detection unit 24-2 converts the optical signal of each of the I-phase component and the Q-phase component of the Y-polarized wave input from the 90-degree hybrid 23-2 into an electrical signal, and sends the electrical signal to the ADC 25-2.

[0076] The ADC 25 converts the input analog signal into a digital signal. The ADC 25-1 converts the analog signal input from the optical detection unit 24-1 into a digital signal and sends the digital signal to the DSP 26. The ADC 25-2 converts the analog signal input from the optical detection unit 24-2 into a digital signal and sends the digital signal to the DSP 26.

[0077] The DSP 26 performs reception processing such as distortion correction, decoding, and error correction for the input signal, and demodulates the client signal. The DSP 26 is configured by a semiconductor device. The reception processing function of the DSP 26 can be configured using a field programmable gate array (FPGA). The reception processing function of the DSP 26 can be performed by causing a general-purpose processor such as a central processing unit (CPU) to execute a computer program. The DSP 26 sends the demodulated client signal to the client signal output unit 21.

[0078] The local oscillator light output unit 27 generates local oscillator light used when multiplexing light with the optical signal transmitted via the communication path 201 and generating an optical signal for performing coherent detection. The local oscillator light output unit 27 includes a semiconductor laser and outputs light of a frequency set based on the frequency of the optical signal transmitted via the communication path 201.

[0079] The error detection unit 28 monitors the error correction process in the DSP 26 and measures the number of errors. The error detection unit 28 according to the present exemplary embodiment calculates the BER based on the measured number of errors and transmits information about the calculated BER as error information to the optical transmitter 10 via the communication path 202. The error detection unit 28 may be integrated with the DSP 26 as a part of the DSP 26.

[0080] The communication path 201 is configured as an optical communication network using optical fibers. The communication path 201 transmits an optical signal from the optical transmitter 10 in the direction of the optical receiver 20. The communication path 202 is a communication network that transmits a control signal or the like from the optical receiver 20 to the optical transmitter. For example, the communication path 202 is provided as a control line of a device based on a communication management system.

[0081] The operation of the optical communication system according to the present exemplary embodiment is described. First, a client signal to be transmitted through the communication path 201 is input to the client signal input unit 11. As the client signal, for example, a signal based on a synchronous optical network (SONET), Ethernet (registered trademark), a fiber channel (FC), an optical transport network (OTN), etc. is used. The client signal input to the client signal input unit 11 is sent to the signal processing unit 12.

[0082] When the client signal is input, the signal processing unit 12 maps the client signal in a frame used when the client signal is transmitted through the communication path 201. When the mapping is performed, the signal processing unit 12 transmits the mapped signal to the signal modulation unit 13.

[0083] When a signal based on data of a frame subjected to mapping is input, the signal modulation unit 13 modulates the light output from the light source unit 14 based on the data of the frame input from the signal processing unit 12. The signal modulation unit 13 performs conversion from an electrical signal to an optical signal using a BPSK method. The signal modulation unit 13 transmits the optical signal generated by performing the modulation to the communication path 201.

[0084] The optical signal transmitted to the communication path 201 is transmitted through the communication path 201 and is transmitted to the optical receiver 20. The optical signal received in the optical receiver 20 is input to the PBS 22-1. When the optical signal is input, the PBS 22-1 polarization-separates the input optical signal and transmits the X-polarized optical signal to the 90-degree hybrid 23-1 and transmits the Y-polarized optical signal to the 90-degree hybrid 23-2.

[0085] When an optical signal is input from the PBS 22-1, the 90-degree hybrids 23-1 and 23-2 multiplex the optical signal input from the PBS 22-1 with the local oscillation light input from the PBS 22-2, and generate signals related to the I-phase component and the Q-phase component. The 90-degree hybrids 23-1 and 23-2 transmit the generated optical signals to the optical detection units 24-1 and 24-2.

[0086] When an optical signal is input, the optical detection unit 24-1 and the optical detection unit 24-2 convert the input optical signal into an electrical signal and send the electrical signal to the ADC 25-1 and ADC 25-2. When an electrical signal converted from an optical signal is input, the ADC 25-1 and the ADC 25-2 convert the input signal into a digital signal and send the digital signal to the DSP 26.

[0087] When a signal is input to the DSP 26, the DSP 26 performs reception processing on the input signal, demodulates the client signal, and transmits the demodulated client signal to the client signal output unit 21. The client signal output unit 21 outputs the input client signal to a communication network or a communication device.

[0088] While the reception process is performed in the DSP 26, the error detection unit 28 monitors the error correction process in the DSP 26 and measures the number of errors of the received signal. The error detection unit 28 according to the present exemplary embodiment calculates the number of errors as BER. When calculating the BER, the error detection unit 28 transmits information about the calculated BER as error information to the optical transmitter 10 via the communication path 202.

[0089] The error information received by the optical transmitter 10 via the communication path 202 is transmitted to the frequency adjustment unit 15. When receiving the error information, the frequency adjustment unit 15 adjusts the frequency offset of the light source unit 14 to reduce the value of the BER. The frequency adjustment unit 15 changes the frequency offset based on the change in the BER, and controls the frequency offset to minimize the BER. The light source unit 14 outputs light of the frequency whose offset is corrected to the signal modulation unit 13.

[0090] The operation performed when the frequency of light output by the light source unit 14 is adjusted in the optical transmitter 10 is described in more detail. Figure 5 The flow of operations performed when the frequency of light output by the light source unit 14 is adjusted is shown.

[0091] First, the frequency adjustment unit 15 sets a search range for the frequency offset, that is, searches for a frequency range in which the number of errors output by the light source unit 14 is the smallest, and changes the frequency offset (step S11). The search range for the frequency offset may be pre-stored in the frequency adjustment unit 15, or may be a set value for the search range input by a worker or the like.

[0092] When setting the search range of the frequency offset, the frequency adjustment unit 15 sets the frequency offset ofs (i.e., the deviation from the set value of the frequency of the light output from the light source unit 14) to ofs = 0 (step S12). When ofs = 0 is satisfied, the light source unit 14 outputs light of the set value (i.e., the frequency assigned to the local device).

[0093] The frequency adjustment unit 15 extracts information about the number of errors from the error information received from the optical receiver 20, and substitutes the number of errors satisfying ofs=0 into the minimum value ofs_err_best of the errors (step S13). The set value of the frequency offset ofs is substituted into ofs_best to indicate information about the frequency offset related to the data substituted into the minimum value ofs_err_best (step S14). When the number of errors satisfying ofs=0 is substituted into ofs_err_best, ofs_best=0 is satisfied.

[0094] When the number of errors in which the frequency offset is 0 is stored, the frequency adjustment unit 15 sets the set value of the frequency offset ofs to ofs=min, that is, the minimum value min of the search range of the frequency offset (step S15).

[0095] When setting the value of the frequency offset ofs, the frequency adjustment unit 15 compares the set value of the frequency offset ofs with the maximum value ofs_max of the search range of the frequency offset. When the frequency offset ofs is equal to or less than the maximum value ofs_max (No in step S16), the frequency adjustment unit 15 corrects the frequency of the light source based on the frequency offset ofs. The frequency adjustment unit 15 calculates the frequency output by the light source unit 14 as the frequency of the light source = the frequency setting value + ofs, and sets the calculated value (step S17).

[0096] When the frequency of light source unit 14 is set based on frequency offset ofs, light frequency offset from the set value is output from light source unit 14. When light frequency offset is output to communication path 201, information on the number of errors is transmitted from optical receiver 20 as a transmission destination.

[0097] When receiving information about the number of errors, the frequency adjustment unit 15 substitutes the number of errors into ofs_err (step S18), and compares the received number of errors ofs_err with ofs_err_best stored as the minimum value so far. When the number of newly received errors is smaller (yes in step S19), the frequency adjustment unit 15 updates ofs_err_best with the value of the newly received number of errors ofs_err (step S20). When updating ofs_err_best, the frequency adjustment unit 15 substitutes the value of the frequency offset ofs into ofs_best to indicate information about the frequency offset related to the minimum value ofs_err_best (step S21).

[0098] When updating the information about the frequency offset related to the minimum value ofs_err_best, the frequency adjustment unit 15 changes the frequency offset ofs to ofs=ofs+Δf (step S22), and performs the operation from step S16. The amount Δf of changing the frequency offset is set in advance. Δf can be set by dividing the search range of the frequency offset by a preset number.

[0099] When the number of newly received errors is equal to or exceeds the minimum value so far (No in step S19), the frequency adjustment unit 15 changes the frequency offset ofs to ofs=ofs+Δf, and performs the operation from step S16.

[0100] When the frequency offset ofs is greater than the maximum value ofs_max of the search range in step S16 (step S16 is yes), the frequency adjustment unit 15 sets the frequency setting of the light source unit 14 to a frequency related to the minimum value ofs_err_best. The frequency adjustment unit 15 calculates the frequency of the light source = the frequency setting value + ofs_best, and controls the frequency of the signal output by the light source unit 14 to the calculated frequency (step S23).

[0101] Figure 6 is a graph showing an example of the relationship between the frequency offset and the number of errors. Figure 6 In the example of , the frequency is shifted by an amount relative to each Δf change, thereby measuring the amount of error. Figure 6 In the example of FIG. 1 , −3Δf, at which the number of errors is the smallest, is set as the shift amount of the frequency of the light output by the light source unit 14 .

[0102] In the optical communication system according to the present example embodiment, error information is transmitted from the optical receiver 20 to the optical transmitter 10 via the communication path 202, but when bidirectional optical communication is performed, error information may be added to a frame transmitted from the optical receiver 20 to the optical transmitter 10 as a main signal. Figure 7 For example, when executing the OTN frame based on Figure 7 In data communication of the OTN frame in the optical receiver 20, error information is added to the reserved bit of the overhead, so that the error information can be transmitted from the optical receiver 20 to the optical transmitter 10. When such a configuration is constructed, communication using the communication path 202 is not required, so the configuration is simplified.

[0103] Fig. 8A is a diagram showing a constellation using a BPSK modulation method. Figure 8B is a diagram showing a constellation using the QPSK modulation method. Fig. 8A and Figure 8BIn the constellation in , the symbol of the signal is shown in a plane where the phase component identical to the carrier is designated as the I axis and the phase component orthogonal to the carrier is designated as the Q axis. In the case of the BPSK modulation method, the symbol is mapped on the I axis, so when the frequency offset between the optical signal and the local oscillation light is small, the state in FIG8 occurs and the Q phase component of the optical signal becomes 0. In this state, when the gain is automatically controlled so that the output amplitude is constant in the optical detection unit 24, a signal without a Q phase component is input to the input signal of Q-ch, so when the signal of Q-ch is amplitude, the output amplitude does not increase. Therefore, the gain is set to be large in order to increase the output amplitude of the signal of Q-ch, a noise component is added to Q-ch, and thus the signal quality deteriorates.

[0104] On the contrary, when there is a frequency offset between the source of the optical signal and the source of the local oscillator light, the constellation Fig. 9 Rotate as shown. Fig. 8A In the BPSK method shown, although only the I-axis component is included, when a frequency offset is intentionally generated, not only the I-axis component but also the Q-axis component may have a value. When the Q-axis component is included, an appropriate gain is set to avoid excessive increase in signal noise and prevent signal quality degradation.

[0105] According to the optical communication system of the present exemplary embodiment, the frequency adjustment unit 15 of the optical transmitter 10 adjusts the frequency of the light output from the light source unit 14 based on the error information detected in the error detection unit 28 of the optical receiver 20. The adjustment of the frequency is adjusted to reduce the number of errors so that an appropriate offset can be added to the frequency of the optical signal transmitted from the optical transmitter 10 and the frequency of the local oscillation light used to detect the received signal in the optical receiver 20. As a result, the optical communication system according to the present exemplary embodiment can reduce the influence of the noise generated in the received signal and maintain the reception quality.

[0106] (Third exemplary embodiment)

[0107] An optical communication system according to a third exemplary embodiment of the present invention is described. Fig.10 An outline of a configuration of an optical communication system according to the present exemplary embodiment is shown. The optical communication system according to the present exemplary embodiment includes an optical transmitter 30 and an optical receiver 40. The optical transmitter 30 and the optical receiver 40 are connected to each other via a communication path 201.

[0108] The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method through communication path 201 similarly to the second exemplary embodiment. Although the optical communication system according to the second exemplary embodiment adjusts the offset amount of the frequency of the light source of the optical transmitter, the optical communication system according to the present exemplary embodiment is characterized in adjusting the offset amount of the frequency of the local oscillation light of the optical receiver.

[0109] The configuration of the optical transmitter 30 is described. Fig.11 1 shows a configuration of an optical transmitter 30 according to the present exemplary embodiment. The optical transmitter 30 includes a client signal input unit 11, a signal processing unit 12, a signal modulation unit 13, and a light source unit 31. The configuration and function of each of the client signal input unit 11, the signal processing unit 12, and the signal modulation unit 13 according to the present exemplary embodiment are similar to those of the same name according to the second exemplary embodiment.

[0110] The light source unit 31 includes a function similar to the light source unit 14 according to the second exemplary embodiment except for the shift function of the frequency of the output light. In other words, the light source unit 31 includes a semiconductor laser and outputs continuous light of a predetermined frequency to the signal modulation unit 13. The predetermined frequency is allocated based on the wavelength design of the optical communication network.

[0111] The configuration of the optical receiver 40 is described. Fig.12 The configuration of the optical receiver 40 according to the present exemplary embodiment is shown. The optical receiver 40 includes a client signal output unit 21, a PBS 22, a 90 degree hybrid 23, an optical detection unit 24, an ADC 25, a DSP 26, a local oscillation light output unit 41, an error detection unit 42, and a frequency adjustment unit 43.

[0112] The configuration and function of each of the client signal output unit 21, the PBS 22, the 90-degree hybrid 23, the optical detection unit 24, the ADC 25, and the DSP 26 are similar to those of the parts of the same name according to the second exemplary embodiment. In other words, as the PBS 22, a PBS 22-1 that polarization-separates the optical signal input via the communication path 201 and a PBS 22-2 that polarization-separates the local oscillation light are provided. A 90-degree hybrid 23-1, an optical detection unit 24-1, and an ADC 25-1 that process an X-polarized signal are provided; and a 90-degree hybrid 23-2, an optical detection unit 24-2, and an ADC 25-2 that process a Y-polarized signal are provided.

[0113] The local oscillation light output unit 41 generates a local oscillation light of a predetermined frequency used when multiplexing light with an optical signal transmitted via the communication path 201 and generating a number of optical signals for performing coherent detection. The local oscillation light output unit 41 is configured using a semiconductor laser. The predetermined frequency is set based on the frequency of the optical signal transmitted via the communication path 201. The local oscillation light output unit 41 outputs light of a frequency whose offset is added to the predetermined frequency. The offset amount of the frequency is controlled by the frequency adjustment unit 43.

[0114] The error detection unit 42 includes a function similar to that of the error detection unit 28 according to the second exemplary embodiment. The error detection unit 42 according to the present exemplary embodiment monitors the reception processing of the signal in the DSP 26, and measures the number of errors based on the number of error corrections. The error detection unit 42 transmits error information calculated based on the error measurement result to the frequency adjustment unit 43 within the local device. The error detection unit 42 according to the present exemplary embodiment transmits the BER as the error information to the frequency adjustment unit 43. The error detection unit 42 may be integrated with the DSP 26 as a part of the DSP 26.

[0115] The frequency adjustment unit 43 controls the shift amount of the frequency of the local oscillation light output unit 41. The frequency adjustment unit 43 controls the shift amount of the frequency based on the error information transmitted from the error detection unit 42. The frequency adjustment unit 43 controls the shift amount of the frequency to reduce the BER transmitted as the error information.

[0116] The operation of the optical communication system according to the present example embodiment is described. In operations other than the adjustment of the frequency offset between the optical signal and the local oscillator light, the optical communication system according to the present example embodiment operates similarly to the optical communication system according to the second example embodiment. The optical communication system according to the present example embodiment adjusts the frequency offset between the optical signal and the local oscillator light based on the detection result of the number of errors in the optical receiver 40. In other words, in the optical communication system according to the present example embodiment, the frequency adjustment unit 43 of the optical receiver 40 changes the offset from the set value of the frequency of the local oscillator light output from the local oscillator light output unit 41, and controls the frequency of the local oscillator light based on the offset at which the number of errors is minimized.

[0117] The optical communication system according to this exemplary embodiment produces similar advantageous effects to those of the optical communication system according to the second exemplary embodiment. The optical receiver 40 side adjusts the frequency of the local oscillation light based on the number of errors, so there is no need to send the number of errors to the optical transmitter 30, whereby the configuration of the system can be further simplified.

[0118] (Fourth exemplary embodiment)

[0119] A fourth exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Fig.13 An outline of the configuration of the optical communication system according to the present exemplary embodiment is shown. The optical communication system according to the present exemplary embodiment includes an optical transmitter 50 and an optical receiver 60. The optical transmitter 50 and the optical receiver 60 are connected via a communication path 201 and a communication path 202.

[0120] The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method via a communication path 201 similarly to the second exemplary embodiment. Although the optical communication system according to the second exemplary embodiment adjusts the optical signal to minimize the number of errors, thereby adjusting the frequency offset between the optical signal and the local oscillation light. Instead of this configuration, the optical communication system according to the present exemplary embodiment is characterized in that the frequency of the optical signal is monitored and the frequency of the light output from the light source unit is adjusted so that the frequency offset between the optical signal and the local oscillation light has a set value.

[0121] The configuration of the optical transmitter 50 is described. Fig.14 The configuration of the optical transmitter 50 according to the present exemplary embodiment is shown. The optical transmitter 50 includes a client signal input unit 11, a signal processing unit 12, a signal modulation unit 13, a light source unit 14, a frequency monitoring unit 51, and a frequency adjustment unit 52.

[0122] The configuration and function of each of the client signal input unit 11 , the signal processing unit 12 , the signal modulation unit 13 , and the light source unit 14 according to the present exemplary embodiment are similar to those of the same name according to the second exemplary embodiment.

[0123] The frequency monitoring unit 51 includes a function of measuring the frequency of the output signal of the signal processing unit 12. For example, the output signal of the signal modulation unit 13 is branched through an optical coupler, and the branched signal is input to the frequency monitoring unit 51. The frequency monitoring unit 51 transmits information about the frequency of the output signal of the signal modulation unit 13 to the frequency adjustment unit 52.

[0124] The frequency adjustment unit 52 controls the offset value of the frequency of the light output by the light source unit 14 based on the frequency of the output signal transmitted from the frequency monitoring unit 51 and the frequency of the local oscillator light transmitted from the optical receiver 60 via the communication path 202. The frequency adjustment unit 52 monitors the difference (i.e., frequency offset) between the frequency of the output signal transmitted from the frequency monitoring unit 51 and the frequency of the local oscillator light transmitted from the optical receiver 60. The frequency adjustment unit 52 controls the offset amount of the frequency of the light output by the light source unit 14 based on the set value of the frequency offset set so that the frequency offset is not 0.

[0125] The configuration of the optical receiver 60 is described. Fig.15 The configuration of the optical receiver 60 according to the present exemplary embodiment is shown. The optical receiver 60 includes a client signal output unit 21, a PBS 22, a 90 degree hybrid 23, an optical detection unit 24, an ADC 25, a DSP 26, a local oscillation light output unit 27, and a frequency monitoring unit 61.

[0126] The configuration and function of each of the client signal output unit 21, PBS 22, 90-degree hybrid 23, optical detection unit 24, ADC 25, DSP 26, and local oscillation light output unit 27 are similar to those of the same-named parts according to the second exemplary embodiment. In other words, as PBS 22, PBS 22-1 that polarization-separates the optical signal input via the communication path 201 and PBS 22-2 that polarization-separates the local oscillation light are provided. A 90-degree hybrid 23-1, an optical detection unit 24-1, and an ADC 25-1 that process X-polarized waves are provided; and a 90-degree hybrid 23-2, an optical detection unit 24-2, and an ADC 25-2 that process Y-polarized waves are provided.

[0127] The frequency monitoring unit 61 includes a function of measuring the frequency of the output light of the local oscillation light output unit 27. The output light of the local oscillation light output unit 27 is branched, for example, by an optical coupler, and the branched signal is input to the frequency monitoring unit 61. The frequency monitoring unit 61 transmits information about the frequency of the output light of the local oscillation light output unit 27 to the frequency adjustment unit 52 of the optical transmitter 50 via the communication path 202.

[0128] The operation of the optical communication system according to the present exemplary embodiment is described.The optical communication system according to the present exemplary embodiment operates similarly to the optical communication system according to the second exemplary embodiment in operations other than adjustment of the frequency offset between the optical signal and the local oscillation light.

[0129] An operation of adjusting the frequency output by the light source unit 14 in the optical transmitter 50 according to the present exemplary embodiment is described. Fig.16 An operation flow used when adjusting the frequency of light output by the light source unit 14 is shown.

[0130] First, the frequency adjustment unit 52 sets a target ofs_target of the frequency shift (step S31). The target ofs_target of the frequency shift is referred to as a target of the difference between the frequency of the light output by the light source unit 14 and the frequency of the light output by the local oscillation light output unit 41. The target ofs_target of the frequency shift is pre-stored in the frequency adjustment unit 52. For the target ofs_target of the frequency shift, a set value may be input by a worker or the like.

[0131] When the target ofs_target of the frequency offset is set, the frequency adjustment unit 52 calculates the frequency offset sig_ofs of the optical signal, that is, the difference between the frequency of the optical signal actually output and the set value of the frequency of the optical signal (step S32). The frequency adjustment unit 52 calculates the frequency offset sig_ofs of the optical signal based on the monitoring result of the frequency of the optical signal sent from the frequency monitoring unit 51. The frequency adjustment unit 52 calculates the frequency offset of the optical signal as frequency offset sig_ofs=frequency monitoring value of the optical signal-frequency set value of the optical signal.

[0132] When calculating the frequency offset of the optical signal, the frequency adjustment unit 52 calculates the frequency offset lo_ofs of the local oscillator light, that is, the difference between the frequency of the local oscillator light actually output in the optical receiver 60 and the set value of the frequency of the local oscillator light (step S33). The frequency adjustment unit 52 calculates the frequency offset lo_ofs of the local oscillator light based on the monitoring result of the frequency of the local oscillator light sent from the frequency monitoring unit 61 via the communication path 202. The frequency adjustment unit 52 calculates the frequency offset of the local oscillator light as frequency offset lo_ofs=monitoring result of the frequency of the local oscillator light-the set value of the frequency of the local oscillator light.

[0133] When calculating the frequency offset of each of the optical signal and the local oscillator light, the frequency adjustment unit 52 calculates the frequency offset total_ofs between the optical signal and the local oscillator light (step S34). The frequency adjustment unit 52 calculates the frequency offset between the optical signal and the local oscillator light based on the frequency offset total_ofs=frequency offset sig_ofs of the optical signal-frequency offset lo_ofs of the local oscillator light.

[0134] When calculating the frequency difference (ie, frequency offset) between the optical signal and the local oscillation light, the frequency adjustment unit 52 confirms the positive / negative of the target ofs_target of the frequency offset and determines the coefficient SIGN used when calculating the correction amount diff of the frequency of the light output by the light source unit 14 .

[0135] When the value of the target frequency offset ofs_target is equal to or greater than 0 (Yes in step S35), the frequency adjustment unit 52 sets the coefficient SIGN to an increase value of 1 (step S36). When the value of the target frequency offset ofs_target is less than 0 (No in step S35), the frequency adjustment unit 52 sets the coefficient SIGN to a decrease value of 1 (step S39).

[0136] When determining the coefficient SIGN used in calculating the correction amount diff of the frequency of light output by the light source unit 14, the frequency adjustment unit 52 calculates the correction amount diff of the frequency shift (step S37). The frequency adjustment unit 52 calculates the correction amount diff as diff=SIGN×ofs_target−SIGN×total_ofs.

[0137] When calculating the correction amount diff of the frequency, the frequency adjustment unit 52 calculates the frequency of the light output by the light source unit 14 as the frequency setting value + SIGN × diff (step S38). When calculating the frequency of the light output by the light source unit 14, the frequency adjustment unit 52 controls the light source unit 14 to output light of the calculated frequency.

[0138] The optical communication system according to the present exemplary embodiment monitors the frequencies of the optical signal and the local oscillation light, and the frequency adjustment unit 52 controls the frequency of the light output from the light source unit 14 so that the frequency offset, which is the frequency difference between the optical light and the local oscillation light, has a set value. In this way, the frequencies of the optical signal and the local oscillation light are maintained at a set value other than 0, and a frequency offset is caused between the optical signal and the local oscillation light, so that the noise generated in the signal of Q-ch can be reduced. As a result, the optical communication system according to the present exemplary embodiment can reduce the influence of the noise generated in the received signal and maintain the reception quality.

[0139] (Fifth Exemplary Implementation)

[0140] A fifth exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Fig.17 An outline of the configuration of the optical communication system according to the present exemplary embodiment is shown. The optical communication system according to the present exemplary embodiment includes an optical transmitter 70 and an optical receiver 80. The optical transmitter 70 and the optical receiver 80 are connected via a communication path 201 and a communication path 203. The communication path 203 is a communication network that transmits a control signal or the like from the optical transmitter 70 to the optical receiver 80.

[0141] The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method via a communication path 201 similarly to the second exemplary embodiment. The optical communication system according to the present exemplary embodiment is characterized in that the frequency of the local oscillation light of the optical receiver 80 is controlled based on the measurement result of the frequencies of the optical signal and the local oscillation light so that the frequency offset between the optical signal and the local oscillation light has a set value.

[0142] The configuration of the optical transmitter 70 is described. Fig.181 shows a configuration of an optical transmitter 70 according to the present exemplary embodiment. The optical transmitter 70 includes a client signal input unit 11, a signal processing unit 12, a signal modulation unit 13, a light source unit 71, and a frequency monitoring unit 72. The configuration and function of each of the client signal input unit 11, the signal processing unit 12, and the signal modulation unit 13 according to the present exemplary embodiment are similar to those of the same name according to the second exemplary embodiment.

[0143] The light source unit 71 includes a function similar to the light source unit 14 according to the second exemplary embodiment, except for the shift function of the frequency of the output light. In other words, the light source unit 71 includes a semiconductor laser, and outputs continuous light of a predetermined frequency to the signal modulation unit 13. The predetermined frequency is allocated based on the wavelength design of the optical communication network.

[0144] The frequency monitoring unit 72 includes a function of measuring the frequency of the output signal of the signal processing unit 12. For example, the output signal of the signal processing unit 12 is branched through an optical coupler, and the branched signal is input to the frequency monitoring unit 72. The frequency monitoring unit 72 transmits information about the frequency of the output signal of the signal processing unit 12 to the frequency adjustment unit 82 of the optical receiver 80 via the communication path 203.

[0145] The configuration of the optical receiver 80 is described. Fig.19 The configuration of the optical receiver 80 according to the present exemplary embodiment is shown. The optical receiver 80 includes a client signal output unit 21, a PBS 22, a 90 degree hybrid 23, an optical detection unit 24, an ADC 25, a DSP 26, a local oscillation light output unit 27, a frequency monitoring unit 81 and a frequency adjustment unit 82.

[0146] The configuration and function of each of the client signal output unit 21, PBS 22, 90-degree hybrid 23, optical detection unit 24, ADC 25, and DSP 26 according to the present exemplary embodiment are similar to those of the same-named parts according to the second exemplary embodiment. In other words, as PBS 22, PBS 22-1 that polarization-separates the optical signal input via communication path 201 and PBS 22-2 that polarization-separates the local oscillation light are provided. A 90-degree hybrid 23-1, optical detection unit 24-1, and ADC 25-1 that process X-polarized signals are provided; and a 90-degree hybrid 23-2, optical detection unit 24-2, and ADC 25-2 that process Y-polarized signals are provided.

[0147] The frequency monitoring unit 81 includes a function of measuring the frequency of the output light of the local oscillation light output unit 27. The output light of the local oscillation light output unit 27 is branched, for example, by an optical coupler, and the branched signal is input to the frequency monitoring unit 81. The frequency monitoring unit 81 sends information about the frequency of the output light of the local oscillation light output unit 27 to the frequency adjustment unit 82 of the local device.

[0148] The frequency adjustment unit 82 controls the offset amount of the frequency of the light output by the local oscillation light output unit 27 based on the frequency of the output signal transmitted from the frequency monitoring unit 72 of the optical transmitter 70 via the communication path 203 and the frequency of the local oscillation light transmitted from the frequency monitoring unit 81 of the local device. The frequency adjustment unit 82 monitors the frequency of the optical signal transmitted from the optical transmitter 70 and the frequency of the local oscillation light, and controls the offset amount of the frequency of the local oscillation light output by the local oscillation light output unit 27 based on the set value of the frequency offset set so that the offset sum is not zero.

[0149] The operation of the optical communication system according to the present example embodiment is described. The optical communication system according to the present example embodiment operates similarly to the fourth example embodiment, except for the control of the frequency of the local oscillator light on the optical receiver side and the adjustment of the frequency offset. In the optical communication system according to the present example embodiment, the frequency adjustment unit 82 of the optical receiver 80 calculates the frequency difference based on the frequency of the optical signal transmitted from the optical transmitter 70 and the frequency of the local oscillator light measured in the local device. The frequency adjustment unit 82 adjusts the frequency of the local oscillator light based on the frequency difference between the optical signal and the local oscillator light and the set value of the frequency offset. The frequency adjustment unit 82 adjusts the frequency of the local oscillator light output from the local oscillator light output unit 27 so that the calculated frequency difference between the optical signal and the local oscillator light and the set value of the frequency offset match.

[0150] The optical communication system according to the present exemplary embodiment produces a beneficial effect similar to that of the optical communication system according to the fourth exemplary embodiment. In other words, the optical communication system according to the present exemplary embodiment monitors the frequencies of the optical signal and the local oscillation light, and the frequency adjustment unit 82 controls the frequency of the light output from the local oscillation light output unit 27 so that the frequency offset as the frequency difference between the optical signal and the local oscillation light has a set value. In this way, the frequencies of the optical signal and the local oscillation light are maintained at a set value other than 0, and a frequency offset is caused between the optical signal and the local oscillation light, thereby reducing the noise generated in the signal of Q-ch. As a result, the optical communication system according to the present exemplary embodiment can reduce the influence of the noise generated in the received signal and maintain the reception quality.

[0151] (Sixth exemplary embodiment)

[0152] A sixth exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Fig. 20 An outline of a configuration of an optical communication system according to the present exemplary embodiment is shown. The optical communication system according to the present exemplary embodiment includes an optical transmitter 90 and an optical receiver 100. The optical transmitter 90 and the optical receiver 100 are connected via a communication path 201 and a communication path 202.

[0153] The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method via communication path 201 similarly to the second exemplary embodiment. Although the optical communication systems according to the fourth and fifth exemplary embodiments measure the frequencies of the optical signal and the local oscillation light to calculate the frequency difference, the optical communication system according to the present exemplary embodiment is characterized in that signal processing of an optical receiver is monitored to obtain information on the frequency difference between the optical signal and the local oscillation light.

[0154] The configuration of the optical transmitter 90 is described. Fig.21 The configuration of the optical transmitter 90 according to the present exemplary embodiment is shown. The optical transmitter 90 includes a client signal input unit 11, a signal processing unit 12, a signal modulation unit 13, a light source unit 14, and a frequency adjustment unit 91.

[0155] The configuration and function of each of the client signal input unit 11 , the signal processing unit 12 , the signal modulation unit 13 , and the light source unit 14 according to the present exemplary embodiment are similar to those of the same name according to the second exemplary embodiment.

[0156] The frequency adjustment unit 91 controls the offset amount of the frequency of the light output by the light source unit 14 based on the offset amount between the frequency of the output signal transmitted from the frequency offset detection unit 101 of the optical receiver 100 via the communication path 202 and the frequency of the local oscillation light of the optical receiver 100. The frequency adjustment unit 91 controls the offset amount of the frequency of the light source unit 14 based on the offset amount between the frequency of the optical signal transmitted from the optical receiver 100 and the local oscillation light so that the offset sum is not 0.

[0157] The configuration of the optical receiver 100 is described. Fig. 22 The configuration of the optical receiver 100 according to the present exemplary embodiment is shown. The optical receiver 100 includes a client signal output unit 21, a PBS 22, a 90 degree hybrid 23, an optical detection unit 24, an ADC 25, a DSP 26, a local oscillation light output unit 27, and a frequency shift detection unit 101.

[0158] The configuration and function of each of the client signal output unit 21, PBS 22, 90-degree hybrid 23, optical detection unit 24, ADC 25, DSP 26, and local oscillation light output unit 27 are similar to those of the same-named parts according to the second exemplary embodiment. In other words, as PBS 22, PBS 22-1 that polarization-separates the optical signal input via communication path 201 and PBS 22-2 that polarization-separates the local oscillation light are provided. A 90-degree hybrid 23-1, optical detection unit 24-1, and ADC 25-1 that process X-polarization signals are provided; and a 90-degree hybrid 23-2, optical detection unit 24-2, and ADC 25-2 that process Y-polarization signals are provided.

[0159] The frequency offset detection unit 101 monitors the reception process in the DSP 26, and detects the difference between the frequency of the optical signal transmitted by the optical transmitter 90 and the frequency of the local oscillation light output by the local oscillation light output unit 27 as a frequency offset. The frequency offset detection unit 101 transmits information about the frequency offset indicating the difference between the detected optical signal and the frequency of the local oscillation light to the frequency adjustment unit 91 of the optical transmitter 90 via the communication path 202. The frequency offset detection unit 101 may be integrated with the DSP 26 as a part of the DSP 26.

[0160] The operation of the optical communication system according to the present exemplary embodiment is described. In operations other than adjustment of the frequency offset between the optical signal and the local oscillation light, the optical communication system according to the present exemplary embodiment operates similarly to the optical communication system according to the second exemplary embodiment. The operation of adjusting the frequency output by the light source unit 14 in the optical transmitter 90 according to the present exemplary embodiment is described. Fig.23 An operation flow used when adjusting the frequency of light output by the light source unit 14 is shown.

[0161] First, the frequency adjustment unit 91 sets a target ofs_target of the frequency shift (step S41). The target ofs_target of the frequency shift is referred to as a target of the difference between the frequency of the light output by the light source unit 14 and the frequency of the light output by the local oscillation light output unit 27. The target ofs_target of the frequency shift may be pre-stored in the frequency adjustment unit 91, or may be a set value input by a worker or the like.

[0162] When the frequency offset target ofs_target is set, the frequency adjustment unit 91 acquires data of the frequency offset total_ofs between the optical signal and the local oscillation light (step S42 ). The frequency offset total_ofs between the optical signal and the local oscillation light is received from the frequency offset detection unit 101 of the optical receiver 100 via the communication path 202 .

[0163] When receiving data of the frequency offset total_ofs between the optical signal and the local oscillation light, the frequency adjustment unit 91 confirms the positive / negative of the target ofs_target of the frequency offset, and determines a coefficient SIGN used when calculating the correction amount diff of the frequency offset.

[0164] When the value of the target frequency offset ofs_target is equal to or greater than 0 (Yes in step S43), the frequency adjustment unit 91 sets the coefficient SIGN to an increase value of 1 (step S44). When the value of the target frequency offset ofs_target is less than 0 (No in step S43), the frequency adjustment unit 91 sets the coefficient SIGN to a decrease value of 1 (step S47).

[0165] When determining the coefficient SIGN used in calculating the correction amount diff, the frequency adjustment unit 91 calculates the correction amount diff of the frequency offset (step S45). The frequency adjustment unit 91 calculates the correction amount diff as diff=SIGN×ofs_target-SIGN×total_ofs.

[0166] When calculating the correction amount diff of the frequency, the frequency adjustment unit 91 calculates the frequency of the light output by the light source unit 14 as the frequency setting value + SIGN × diff (step S46). When calculating the frequency of the light output by the light source unit 14, the frequency adjustment unit 91 controls the light source unit 14 to output light of the calculated frequency.

[0167] The optical communication system according to the present exemplary embodiment acquires the frequencies of the optical signal and the local oscillation light from the frequency shift detection unit 101, and controls the frequency of the light output from the light source unit 14 so that the frequency shift indicating the frequency difference between the optical signal and the local oscillation light has a set value. In this way, the frequencies of the optical signal and the local oscillation light are maintained at a set value other than 0 and a frequency shift is caused between the optical signal and the local oscillation light, so that the noise generated in the signal of Q-ch can be reduced. As a result, the optical communication system according to the present exemplary embodiment can reduce the influence of the noise generated in the received signal and maintain the reception quality.

[0168] (Seventh exemplary embodiment)

[0169] A seventh exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Fig.24 An outline of a configuration of an optical communication system according to the present exemplary embodiment is shown. The optical communication system according to the present exemplary embodiment includes an optical transmitter 110 and an optical receiver 120. The optical transmitter 110 and the optical receiver 120 are connected via a communication path 201.

[0170] The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method via a communication path 201 similarly to the second exemplary embodiment. In the optical communication system according to the sixth exemplary embodiment, the frequency offset detection unit 101 monitors the processing of the received signal in the DSP 26 and acquires information about the frequency difference between the optical signal and the local oscillation light, and the optical transmitter adjusts the frequency of the optical signal. The optical communication system according to the present exemplary embodiment is characterized in that the frequency offset detection unit 101 monitors the processing of the received signal in the DSP 26, adjusts the frequency of the local oscillation light, and thereby adjusts the frequency offset between the optical signal and the local oscillation light.

[0171] The configuration of the optical transmitter 110 is described. Fig.25 1 shows a configuration of an optical transmitter 110 according to the present exemplary embodiment. The optical transmitter 110 includes a client signal input unit 11, a signal processing unit 12, a signal modulation unit 13, and a light source unit 111. The configuration and function of each of the client signal input unit 11, the signal processing unit 12, and the signal modulation unit 13 according to the present exemplary embodiment are similar to those of the same name according to the second exemplary embodiment.

[0172] The light source unit 111 includes a function similar to the light source unit 14 according to the second exemplary embodiment, except for the shift function of the frequency of the output light. In other words, the light source unit 111 includes a semiconductor laser and outputs continuous light of a predetermined frequency to the signal modulation unit 13. The predetermined frequency is allocated based on the wavelength design of the optical communication network.

[0173] The configuration of the optical receiver 120 is described. Fig.26 The configuration of the optical receiver 120 according to the present exemplary embodiment is shown. The optical receiver 120 includes a client signal output unit 21, a PBS 22, a 90 degree hybrid 23, an optical detection unit 24, an ADC 25, a DSP 26, a local oscillation light output unit 121, a frequency shift detection unit 122, and a frequency adjustment unit 123.

[0174] The configuration and function of each of the client signal output unit 21, PBS 22, 90-degree hybrid 23, optical detection unit 24, ADC 25, and DSP 26 according to the present exemplary embodiment are similar to those of the same-named parts according to the second exemplary embodiment. In other words, as PBS 22, PBS 22-1 that polarization-separates the optical signal input via communication path 201 and PBS 22-2 that polarization-separates the local oscillation light are provided. A 90-degree hybrid 23-1, optical detection unit 24-1, and ADC 25-1 that process X-polarized signals are provided; and a 90-degree hybrid 23-2, optical detection unit 24-2, and ADC 25-2 that process Y-polarized signals are provided.

[0175] The local oscillation light output unit 121 generates a local oscillation light of a predetermined frequency used when causing light to interfere with an optical signal transmitted via the communication path 201 and generating an optical signal for performing coherent detection. The local oscillation light output unit 121 includes a semiconductor laser, and outputs light of a frequency set based on the frequency of the optical signal transmitted via the communication path 201. The local oscillation light output unit 121 outputs light to which a frequency offset is added by using a predetermined frequency as a center frequency. The frequency offset is controlled by the frequency adjustment unit 123.

[0176] The frequency offset detection unit 122 monitors the reception process in the DSP 26, and performs detection on the offset between the frequency of the optical signal transmitted by the optical transmitter 110 and the frequency of the local oscillation light output by the local oscillation light output unit 121. The frequency offset detection unit 122 transmits information on the offset of the frequency to the frequency adjustment unit 123 of the local device. The frequency offset detection unit 122 may be integrated with the DSP 26 as a part of the DSP 26.

[0177] The frequency adjustment unit 123 controls the frequency shift of the local oscillation light output by the local oscillation light output unit 121. The frequency adjustment unit 123 controls the frequency shift of the local oscillation light output by the local oscillation light output unit 121 based on information about the frequency shift between the optical signal sent from the frequency shift detection unit 122 and the local oscillation light.

[0178] The communication system according to the present exemplary embodiment operates similarly to the sixth exemplary embodiment except for the adjustment of the frequency offset by the control of the frequency of the local oscillator light on the optical receiver side. In the optical communication system according to the present exemplary embodiment, the frequency adjustment unit 123 of the optical receiver 120 acquires information about the frequency difference between the optical signal detected by the frequency offset detection unit 122 and the local oscillator light. The frequency adjustment unit 123 adjusts the frequency of the local oscillator light based on the set value of the frequency offset indicating the difference between the frequency of the optical signal and the frequency of the local oscillator light. The frequency adjustment unit 123 adjusts the frequency of the local oscillator light output from the local oscillator light output unit 121 so that the calculated frequency difference between the optical signal and the local oscillator light matches the set value of the frequency offset.

[0179] The optical communication system according to the present exemplary embodiment produces a beneficial effect similar to that of the optical communication system according to the sixth exemplary embodiment. In other words, the optical communication system according to the present exemplary embodiment acquires the frequencies of the optical signal and the local oscillation light from the frequency shift detection unit 122, and controls the frequency of the light output from the local oscillation light output unit 121 so that the frequency shift indicating the frequency difference between the optical signal and the local oscillation light has a set value. In this way, the frequencies of the optical signal and the local oscillation light are maintained at a set value other than 0 and a frequency shift is caused between the optical signal and the local oscillation light, thereby reducing the noise generated in the signal of Q-ch. As a result, the optical communication system according to the present exemplary embodiment can reduce the influence of the noise generated in the received signal and maintain the reception quality.

[0180] (Eighth Exemplary Implementation)

[0181] An eighth exemplary embodiment of the present invention is described in detail with reference to the accompanying drawings. Fig. 27 20 is a diagram showing an outline of a configuration of an optical communication system according to the present exemplary embodiment. The communication system according to the present exemplary embodiment includes an optical transmitter 130 and an optical receiver 140. The optical transmitter 130 and the optical receiver 140 are connected via a communication path 204. The optical communication system according to the present exemplary embodiment is a network system that performs optical communication based on a digital coherent method via the communication path 204 between the optical transmitter 130 and the optical receiver 140.

[0182] Although the frequency of the optical signal or the local oscillation light is adjusted on the optical transmitter side or the optical receiver side, the optical communication system according to the present exemplary embodiment is characterized in that the polarization state of the optical signal and the phase of the local oscillation light are adjusted in the optical receiver, thereby increasing reception quality.

[0183] The configuration of the optical transmitter 130 is described. Fig.281 is a diagram showing a configuration of an optical transmitter 130 according to the present exemplary embodiment. The optical transmitter 130 includes a light source unit 131 , a signal processing unit 132 , a signal modulation unit 133 , and a client signal input unit 134 .

[0184] The light source unit 131 outputs continuous light of a predetermined frequency to the signal modulation unit 133. The predetermined frequency is allocated based on a wavelength design of an optical communication network.

[0185] The signal processing unit 132 performs processing such as redundancy on the input client signal, and maps the processed signal in a frame used when transmitting the signal through the communication path 201 .

[0186] The signal modulation unit 133 modulates the light input from the light source unit 131 based on the signal input from the signal processing unit 132, and generates an optical signal to be sent to the communication path 204. The signal modulation unit 133 according to the present exemplary embodiment performs modulation based on, for example, a BPSK modulation method. The modulation method may be another multi-value modulation method other than BPSK (e.g., QPSK and 8-QAM).

[0187] The client signal input unit 134 is an input port of a client signal transmitted via the communication path 204. The client signal input to the client signal input unit 134 is transmitted to the signal processing unit 132.

[0188] The configuration of the optical receiver 140 is described. Fig.29 1 shows a configuration of an optical receiver 140 according to the present exemplary embodiment. The optical receiver 140 includes a control unit 141, a PBS 142, a 90-degree hybrid 143, and an optical detection unit 144. The optical receiver 140 also includes an ADC 145, a DSP 146, a local oscillation light output unit 147, a phase variable unit 148, a polarization control unit 149, and a client signal output unit 150.

[0189] The control unit 141 includes a function of adjusting the polarization rotation of the optical signal in the polarization control unit 149 and controlling the adjustment operation of the phase of the local oscillation light in the phase variable unit 148. The control unit 141 monitors the power of the optical signal in the optical detection unit 144 and the gain of the transimpedance amplifier (TIA) that amplifies the electrical signal converted from the optical signal, the power of the signal in the ADC 145, and the bit error rate (BER) in the DSP 146. The control unit 141 adds a variation to the phase of the local oscillation light and the polarization rotation of the optical signal so that the BER is less than a predetermined reference.

[0190] PBS 142 polarizes the input optical signal and outputs the separated signal. PBS 142 includes PBS 142-1 that polarizes the optical signal and PBS 142-2 that polarizes the local oscillator light. PBS 142-1 polarizes the optical signal input via communication path 204, outputs the X polarized wave to 90-degree hybrid 143-1, and transmits the Y polarized wave to 90-degree hybrid 143-2. ​​PBS 142-2 polarizes the light input via local oscillator light output unit 147, outputs the X polarized wave to 90-degree hybrid 143-1, and transmits the Y polarized wave to 90-degree hybrid 143-2.

[0191] 90 degree hybrid 143 multiplexes the input optical signal with the input local oscillator light through two paths with a phase difference of 90 degrees. 90 degree hybrid 143-1 multiplexes the X polarization component of the optical signal input from PBS 142-1 with the X polarization component of the local oscillator light input from PBS 142-2 through two paths with a phase difference of 90 degrees.

[0192] The 90-degree hybrid 143-1 transmits signals of the I-phase component and the Q-phase component generated by multiplexing the optical signal with the local oscillator light via the paths having a phase difference of 90 degrees to the optical detection unit 144-1. The 90-degree hybrid 143-2 multiplexes the Y polarization component of the optical signal input from the PBS 142-1 with the Y polarization component of the local oscillator light input from the PBS 142-2 via two paths having a phase difference of 90 degrees. The 90-degree hybrid 143-2 transmits signals of the I-phase component and the Q-phase component generated by multiplexing the optical signal with the local oscillator light via the paths having a phase difference of 90 degrees to the optical detection unit 144-2.

[0193] The optical detection unit 144 converts the input optical signal into an electrical signal and outputs the electrical signal. The optical detection unit 144 is configured using a photodiode. The optical detection unit 144 includes a TIA that amplifies the electrical signal converted from the optical signal. The detection unit 144-1 converts the optical signal of each of the I-phase component and the Q-phase component of the X-polarized wave input from the 90-degree hybrid 143-1 into an electrical signal, and sends the electrical signal to the ADC 145-1. The optical detection unit 144-2 converts the optical signal of each of the I-phase component and the Q-phase component of the Y-polarized wave input from the 90-degree hybrid 143-2 into an electrical signal, and sends the electrical signal to the ADC 145-2.

[0194] ADC 145 converts an input analog signal into a digital signal. ADC 145-1 converts an analog signal input from optical detection unit 144-1 into a digital signal and sends the digital signal to DSP 146. ADC 145-2 converts an analog signal input from optical detection unit 144-2 into a digital signal and sends the digital signal to DSP 146.

[0195] The DSP 146 performs reception processing such as distortion correction, decoding, and error correction for the input signal, and demodulates the client signal. The DSP 146 is configured by a semiconductor device. The reception processing function of the DSP 146 can be configured using an FPGA. The reception processing function of the DSP 146 can be performed by causing a general-purpose processor such as a CPU to execute a computer program. The DSP 146 transmits the demodulated client signal to the client signal output unit 150.

[0196] The local oscillation light output unit 147 multiplexes the optical signal transmitted via the communication path 204 with the local oscillation light, and generates the local oscillation light used when generating the optical signal of the frequency related to the frequency difference between the optical signal and the local oscillation light. The local oscillation light output unit 147 includes a semiconductor laser, and outputs light of a frequency set based on the frequency of the optical signal transmitted via the communication path 204.

[0197] The phase variable unit 148 includes a function of changing the phase of the local oscillation light. The phase variable unit 148 adds a change to the phase of the local oscillation light based on the control of the control unit 141.

[0198] The polarization control unit 149 includes a function of adding a change to the polarization state of the optical signal. The polarization control unit 149 adds polarization rotation to the optical signal based on the control of the control unit 141.

[0199] The communication path 204 is configured as an optical communication network using optical fibers. The communication path 204 transmits an optical signal from the optical transmitter 130 in the direction of the optical receiver 140 .

[0200] The operation of the optical communication system according to the present exemplary embodiment is described: The client signal input to the client signal input unit 134 of the optical transmitter 130 is transmitted to the signal processing unit 132 .

[0201] When the client signal is input, the signal processing unit 132 maps the client signal in a frame used when the client signal is transmitted through the communication path 204. When the mapping is performed, the signal processing unit 132 transmits the mapped signal to the signal modulation unit 133.

[0202] When a signal based on data of a frame subjected to mapping is input, the signal modulation unit 133 modulates the light output from the light source unit 131 based on the data of the frame input from the signal processing unit 132. The signal modulation unit 133 performs conversion from an electrical signal to an optical signal using, for example, a BPSK method. The signal modulation unit 133 performs modulation based on, for example, a BPSK modulation method. The signal modulation unit 133 may be configured to perform conversion from an electrical signal to an optical signal based on another multi-value modulation method other than BPSK (e.g., QPSK and 8-QAM). The signal modulation unit 133 transmits the optical signal generated by performing the modulation to the communication path 204.

[0203] The optical signal transmitted to the communication path 204 is transmitted through the communication path 204, and is transmitted to the optical receiver 140. The optical signal received in the optical receiver 140 is input to the polarization control unit 149. When the optical signal is input, the polarization control unit 149 performs polarization rotation on the optical signal based on the control of the control unit 141. The optical signal subjected to the polarization rotation is transmitted to the PBS 142-1.

[0204] When an optical signal is input, the PBS 142 - 1 polarization-splits the input optical signal, sends an X-polarized optical signal to the 90-degree hybrid 143 - 1 , and sends a Y-polarized optical signal to the 90-degree hybrid 143 - 2 .

[0205] The local oscillation light output unit 147 generates local oscillation light, and outputs the generated light to the phase variable unit 148. When the local oscillation light is input, the phase variable unit 148 adds a change to the phase of the local oscillation light based on the control of the control unit 141. The phase variable unit 148 transmits the local oscillation light to which the change is added in the phase to the PBS 142-2.

[0206] When an optical signal is input from PBS 142-1, 90-degree hybrids 143-1 and 143-2 multiplex the optical signal input from PBS 142-1 with the local oscillation light input from PBS 142-2, and generate signals related to an I-phase component and a Q-phase component. 90-degree hybrids 143-1 and 143-2 transmit the generated optical signals to optical detection units 144-1 and 144-2.

[0207] When an optical signal is input, the optical detection unit 144-1 and the optical detection unit 144-2 each convert the input optical signal into an electrical signal and send the electrical signal to the ADC 145-1 and the ADC 145-2. When an electrical signal converted from an optical signal is input, the ADC 145-1 and the ADC 145-2 convert the input signal into a digital signal and send the digital signal to the DSP 146.

[0208] When a signal is input to the DSP 146, the DSP 146 performs reception processing on the input signal, demodulates the client signal, and transmits the demodulated client signal to the client signal output unit 150. The client signal output unit 150 outputs the input client signal to a communication network or a communication device.

[0209] While the receiving process is performed in the DSP 146, the control unit 141 monitors the error correction process in the DSP 146 and measures the number of errors in the received signal. The control unit 141 according to the present exemplary embodiment calculates the number of errors as a BER. When calculating the BER, the control unit 141 confirms whether the calculated BER falls within a predetermined reference. When the calculated BER is equal to or less than the predetermined reference, the control unit 141 continues to adjust the phase of the local oscillation light and control the polarization rotation of the optical signal without changing the set value.

[0210] When the calculated BER is greater than a predetermined reference, the control unit 141 changes the amount of adjustment of the phase of the local oscillator light and the amount of polarization rotation of the optical signal. The amount of adjustment of the phase of the local oscillator light and the amount of polarization rotation of the optical signal are preset based on the amount of change in the BER. The optical receiver 140 controls the amount of adjustment of the phase of the local oscillator light and the amount of polarization rotation of the optical signal based on the BER, thereby stabilizing the reception process.

[0211] The case where the modulation method for the optical signal is multi-value modulation of four or more values ​​is described. When the modulation method for the optical signal is multi-value modulation of four or more values, the components of the symbols included in the received signal are in such a state that Fig. 30A , Fig. 30B , Fig. 30C and Fig.30D As shown, it includes an X polarization I component, an X polarization Q component, a Y polarization I component, and a Y polarization Q component. As a multi-value modulation method for four or more values, for example, using Fig. 30A and Fig. 30B Polarization multiplexing-quadrature phase shift keying (PM-QPSK) as shown and Fig. 30C and Fig.30D Polarization multiplexing-16 quadrature amplitude modulation (PM-16QAM) is shown.

[0212] The above four components each received coherently with the local oscillation light include a specific signal component. The above four components are each converted into an electrical signal by the optical detection unit 144 and the electrical signal is amplified by the TIA, after which the amplified signal is sampled as a digital signal in the ADC 145. The DSP 146 demodulates the electrical information signal through digital signal processing based on the signal sampled as a digital signal.

[0213] However, when the modulation method for the optical signal is multi-value modulation of values ​​less than four values, one component or two components among the X polarization I component, the X polarization Q component, the Y polarization I component, and the Y polarization Q component of the photoelectric conversion may be in a state substantially close to 0, depending on the polarization state of the received signal, the phase of the local oscillation light, etc. As the multi-value modulation of values ​​less than four values, for example, polarization multiplexing-binary phase shift keying (PM-BPSK) or the like is used. Fig.31A , Fig.31B , Fig.31C and Fig.31D Each shows the constellation in the polarization multiplexing-binary phase shift keying method. Fig.31A and Fig.31B In , both components have values ​​other than 0, but in Fig.31C and Fig.31D In the equation, both components are basically close to 0.

[0214] In a channel where the signal is close to 0, when amplification is performed by TIA, the electrical noise component is amplified. In a channel where the signal is close to 0, the noise component is larger than in a channel where the signal component exists. Therefore, in a channel where the signal is close to 0, the reception SNR deteriorates, and the reception BER affected by the SNR may deteriorate. Fig.32 An example of observing the time variation of the BER in a channel where the signal is close to zero is shown.

[0215] In order to reduce the variation of BER, it is effective to provide a slight change to the optical signal or local oscillator light being received so as not to be in a specific polarization state, frequency or phase condition of the local oscillator light. Fig.33A and Fig.33B Each shows a state of a demodulated symbol of polarization multiplexing-two-phase phase modulated light in which a slight change is given to the phase of the local oscillator light.

[0216] exist Fig.33A and Fig.33B In the ADC, the four signal (XI, XQ, YI, and YQ) components in the optical detection unit and ADC change at specific values. Therefore, the noise component is not excessively amplified in the TIA, so excessive degradation of the BER can be reduced. The influence caused by small changes is compensated by the frequency offset compensation function in the digital signal processing, so there is no influence on the BER. Fig.34 An example of the results obtained by observing the time variation of BER when a small change is applied is shown in FIG. Fig.32 In this way, a small change is added to the phase of the local oscillation light of the optical receiver, thereby achieving stable reception characteristics even when polarization multiplexing-two-phase phase modulated light is received.

[0217] It is applicable to intensity modulation of a non-return-to-zero (NRZ) signal, a return-to-zero (RZ) signal, etc., and a modulation method in which phase modulation and intensity modulation are combined, such as m-value quadrature amplitude modulation (m-QAM), etc.

[0218] The optical communication system according to this example embodiment adjusts the phase of the local oscillator light on the receiving side and controls the polarization rotation of the optical signal. With this configuration, when the frequency of the light source on the transmitting side changes, a shift of the signal occupied frequency band to the optical filter band of the wavelength multiplexing / demultiplexing installed on the ROADM, etc. occurs, and signal degradation due to spectrum shrinkage may occur. When the frequency of the optical signal on the transmitting side or the light source of the local oscillator light changes, it may be disadvantageous to the frequency offset compensation process in the digital signal processing. On the contrary, when a configuration such as the optical communication system according to this example embodiment is performed, signal degradation, etc. can be reduced, so that stable receiving processing can be performed.

[0219] The optical communication system according to the present exemplary embodiment adjusts the phase of the local oscillator light and adjusts the polarization rotation of the input optical signal based on the BER detected in the control unit 141 of the optical receiver 140. When the phase of the local oscillator light and the polarization state of the optical signal are adjusted so that the BER has an appropriate value, the influence of noise occurring in the received signal can be reduced and the reception quality can be maintained.

[0220] The optical receiver according to the eighth exemplary embodiment may be Fig.35 Configured in. Fig.35 The optical receiver in the embodiment includes a local oscillation light output unit 301 , a phase adjustment unit 302 , a polarization control unit 303 , a multiplexing unit 304 , a photoelectric conversion unit 305 , a demodulation unit 306 and a control unit 307 .

[0221] The local oscillation light output unit 301 outputs a local oscillation light of a frequency set based on the frequency of the input optical signal. The phase adjustment unit 302 adjusts the phase of the local oscillation light. The polarization control unit 303 controls the polarization rotation of the optical signal. The multiplexing unit 304 multiplexes the local oscillation light output from the phase adjustment unit 302 with the optical signal output from the polarization control unit 303. The photoelectric conversion unit 305 converts the optical signal multiplexed by the multiplexing unit 304 into an electrical signal. The demodulation unit 306 performs a demodulation process based on the electrical signal converted by the photoelectric conversion unit 305. The control unit 307 controls the execution of at least one of the adjustment of the phase of the local oscillation light in the phase adjustment unit 302 and the polarization rotation of the optical signal in the polarization control unit 303 based on the information about the reception state of the optical signal.

[0222] Fig.35The optical receiver in the embodiment controls the execution of at least one of the adjustment of the phase of the local oscillation light in the phase adjustment unit 302 and the polarization rotation of the optical signal in the polarization control unit 303 based on the information about the reception state of the optical signal. When such control is performed, the optical receiver according to the present exemplary embodiment can maintain the reception quality, thereby allowing stable reception processing.

[0223] In the optical communication system according to the second to eighth example embodiments, a configuration for performing unidirectional communication in which an optical signal is sent from an optical transmitter to an optical receiver is described. Instead of this configuration, bidirectional optical communication may be performed in the optical communication system according to the example embodiment. When bidirectional optical communication is performed, a frequency offset as a frequency difference between an optical signal and a local oscillator light is controlled for each direction. When bidirectional communication is performed, information such as error information, information about the frequency of light, and information about the frequency difference between an optical signal and a local oscillator light may be sent by being added to a frame sent to an opposite device.

[0224] Although the present invention has been particularly shown and described with reference to exemplary embodiments thereof, the present invention is not limited to these embodiments. It will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the spirit and scope of the present invention as defined by the claims.

[0225] This application is based upon and claims the benefit of priority from Japanese patent application No. 2018-167935, filed on September 7, 2018, the disclosure of which is incorporated herein in its entirety by reference.

[0226] [Reference mark list]

[0227] 1 Optical output unit

[0228] 2 Optical modulation unit

[0229] 3. Receiving information acquisition unit

[0230] 4 Frequency adjustment unit

[0231] 10 Optical transmitter

[0232] 11 Client signal input unit

[0233] 12 Signal Processing Unit

[0234] 13 Signal modulation unit

[0235] 14 Light source unit

[0236] 15. Frequency adjustment unit

[0237] 20 Optical receiver

[0238] 21 Client signal output unit

[0239] 22 PBS

[0240] 23 90 degree mixer

[0241] 24 Optical detection unit

[0242] 25 ADC

[0243] 26 DSP

[0244] 27 Local oscillator light output unit

[0245] 28 Error detection unit

[0246] 30 Optical transmitter

[0247] 31 Light source unit

[0248] 40 Optical receiver

[0249] 41 Local oscillator light output unit

[0250] 42 Error detection unit

[0251] 43. Frequency adjustment unit

[0252] 50 Optical transmitter

[0253] 51 Frequency Monitoring Unit

[0254] 52 Frequency adjustment unit

[0255] 60 Optical receiver

[0256] 61 Frequency Monitoring Unit

[0257] 70 Optical transmitter

[0258] 71 Light source unit

[0259] 72 Frequency Monitoring Unit

[0260] 80 Optical receiver

[0261] 81 Frequency Monitoring Unit

[0262] 82 Frequency adjustment unit

[0263] 90 Optical transmitter

[0264] 91 Frequency adjustment unit

[0265] 100 Optical receiver

[0266] 101 Frequency offset detection unit

[0267] 110 Optical transmitter

[0268] 111 Light source unit

[0269] 120 Optical receiver

[0270] 121 Local oscillator light output unit

[0271] 122 Frequency offset detection unit

[0272] 123 Frequency adjustment unit

[0273] 130 Optical transmitter

[0274] 131 Light source unit

[0275] 132 Signal Processing Unit

[0276] 133 Signal Modulation Unit

[0277] 134 Client signal input unit

[0278] 140 Optical receiver

[0279] 141 Control Unit

[0280] 142 PBS

[0281] 143 90 degree mixer

[0282] 144 Optical detection unit

[0283] 145 ADC

[0284] 146 DSP

[0285] 147 Local oscillator light output unit

[0286] 148 Phase Variable Unit

[0287] 149 Polarization Control Unit

[0288] 150 Client signal output unit

[0289] 201 Communication Path

[0290] 202 Communication Path

[0291] 203 Communication Path

[0292] 204 Communication Path

[0293] 301 Local oscillator light output unit

[0294] 302 Phase adjustment unit

[0295] 303 Polarization Control Unit

[0296] 304 Multiplexing Unit

[0297] 305 Photoelectric conversion unit

[0298] 306 Demodulation Unit

[0299] 307 Control Unit

Claims

1. An optical receiver, comprising: a local oscillation light output device for outputting a local oscillation light of a frequency set based on the frequency of an optical signal to be input, the optical signal being polarization multiplexed-two-phase phase modulated light; A phase adjustment device, used for adjusting the phase of the local oscillator light; a polarization control device for controlling the polarization rotation of the optical signal; a multiplexing device for multiplexing the local oscillator light output from the phase adjustment device and the optical signal output from the polarization control device; an optoelectronic conversion device for converting the optical signal multiplexed by the multiplexing device into an electrical signal; demodulation means for performing a demodulation process based on the electrical signal converted by the photoelectric conversion means; as well as A control device for controlling the execution of at least one of the adjustment of the phase of the local oscillator light in the phase adjustment device and the polarization rotation of the optical signal in the polarization control device in a manner that reduces the degradation of the bit error rate caused by the polarization multiplexing-two-phase phase modulated light.

2. The optical receiver according to claim 1, wherein: The control device controls the execution of at least one of the adjustment of the phase of the local oscillation light and the polarization rotation of the optical signal in a manner that optimizes a predetermined index of reception quality of the optical signal.

3. The optical receiver according to claim 2, wherein: The control means controls the execution of at least one of the adjustment of the phase of the local oscillator light and the polarization rotation of the optical signal in such a manner as to minimize the number of errors detected by the demodulation means.

4. The optical receiver according to any one of claims 1 to 3, wherein: The control device controls the execution of at least one of the adjustment of the phase of the local oscillator light and the polarization rotation of the optical signal in such a manner that the constellation of the signal is rotated during demodulation.

5. The optical receiver according to claim 2 or 3, further comprising: A monitoring device for monitoring each power of an in-phase component and an orthogonal component of polarized waves orthogonal to each other of a signal obtained by multiplexing the local oscillation light with the optical signal, wherein The control means determines a reception state based on the power.

6. A communication system comprising: An optical transmitter including means for transmitting an optical signal obtained by modulating each of an in-phase component and an orthogonal component of a polarized wave which are orthogonal to each other; as well as The optical receiver according to any one of claims 1 to 3, wherein The polarization control device of the optical receiver performs polarization rotation of the optical signal received from the optical transmitter through a communication path.

7. A receiving method, comprising: outputting a local oscillation light of a frequency set based on a frequency of an optical signal to be input, the optical signal being polarization multiplexed-two-phase phase modulated light; multiplexing the local oscillator light subjected to phase adjustment with the optical signal subjected to polarization rotation; converting the multiplexed optical signal into an electrical signal; performing a demodulation process based on the converted electrical signal; as well as At least one of the adjustment of the phase of the local oscillation light and the polarization rotation of the optical signal is performed in a manner that reduces degradation of a bit error rate due to the polarization multiplexing-binary phase modulated light.

8. The receiving method according to claim 7, further comprising: At least one of the adjustment of the phase of the local oscillator light and the polarization rotation of the optical signal is performed in a manner that optimizes a predetermined indicator of reception quality of the optical signal.

9. The receiving method according to claim 8, further comprising: At least one of the adjustment of the phase of the local oscillator light and the polarization rotation of the optical signal is performed in a manner that minimizes the number of errors detected during demodulation.

10. The receiving method according to any one of claims 7 to 9, further comprising: At least one of the adjustment of the phase of the local oscillator light and the polarization rotation of the optical signal is performed in such a way that the constellation of the signal is rotated during demodulation.

Images