US-20260128799-A1 - OPTICAL TRANSMISSION AND RECEPTION DEVICE AND OPTICAL RECEIVER

US20260128799A1US 20260128799 A1US20260128799 A1US 20260128799A1US-20260128799-A1

Abstract

An optical transmitter includes: a modulation signal generator that generates a modulation signal that is a digital signal in which a first pilot symbol signal having a first symbol period and a second pilot symbol signal having a second symbol period are inserted into data to be transmitted; and an optical modulator that generates modulated light by modulating CW light from a CW light generator, and an optical receiver includes: an optical coherent detector that receives polarization-multiplexed modulated light into which the first pilot symbol signal and the second pilot symbol signal are inserted, coherently detects the received modulated light; and a reception-side digital signal processor including a frequency difference compensator that calculates a phase difference using the first pilot symbol signal and the second pilot symbol signal adjacent to the first pilot symbol signal, and compensates a frequency of the received digital signal.

Inventors

Hayato SANO
Keisuke Matsuda

Assignees

MITSUBISHI ELECTRIC CORPORATION

Dates

Publication Date: 20260507
Application Date: 20251219

Claims (7)

1 . An optical transmission and reception device comprising: an optical transmitter; and an optical receiver, wherein the optical transmitter includes: a modulation signal generator to generate a modulation signal that is a digital signal for optical modulation in which a first pilot symbol signal having a first symbol period and a second pilot symbol signal having a second symbol period different from the first symbol period are inserted into data to be transmitted; a digital-to-analog converter to convert the modulation signal that is the digital signal generated by the modulation signal generator into a modulation signal including an analog signal; and an optical modulator to generate modulated light into which the first pilot symbol signal having the first symbol period and the second pilot symbol signal having the second symbol period are inserted by modulating CW light from a CW light generator on a basis of the modulation signal converted into the analog signal by the digital-to-analog converter, and the optical receiver includes: an optical coherent detector to receive polarization-multiplexed modulated light into which the first pilot symbol signal having the first symbol period and the second pilot symbol signal having the second symbol period different from the first symbol period are inserted, coherently detect the received modulated light, and output an analog electric signal; an analog-to-digital converter to perform analog-to digital conversion on the analog electric signal from the optical coherent detector to output the signal as a digital signal; and a reception-side digital signal processor including a frequency difference compensator to receive the digital signal from the analog-to-digital converter, calculate a phase difference using the first pilot symbol signal having the first symbol period and the second pilot symbol signal having the second symbol period different from the first symbol period and adjacent to the first pilot symbol signal, the first pilot symbol signal and the second pilot symbol signal being inserted into the received polarization-multiplexed modulated light extracted from the received digital signal, and compensate a frequency of the received digital signal.
2 . The optical transmission and reception device according to claim 1 , wherein the optical modulator generates the modulated light using a single carrier signal as an optical carrier wave, the optical coherent detector converts the modulated light using the single carrier signal as the optical carrier wave into the analog electric signal, and the frequency difference compensator in the reception-side digital signal processor compensates the frequency of the digital signal for the single carrier signal.
3 . The optical transmission and reception device according to claim 1 , wherein the optical modulator generates the modulated light frequency-multiplexed using, as an optical subcarrier, a plurality of sub-carrier signals of different frequency arrangements, the optical coherent detector converts the modulated light frequency-multiplexed using, as the optical subcarrier, the plurality of sub-carrier signals of the different frequency arrangements into the analog electric signal, and the frequency difference compensator in the reception-side digital signal processor compensates the frequency of the digital signal for each of the plurality of sub-carrier signals of the different frequency arrangements.
4 . The optical transmission and reception device according to claim 1 , wherein the modulated light generated by the optical modulator includes an optical signal in which an X-polarized wave and a Y-polarized wave are modulated into an I signal and a Q signal of a quadrature phase, respectively.
5 . The optical transmission and reception device according to claim 1 , wherein a QPSK signal is used as the first pilot symbol signal and the second pilot symbol signal.
6 . The optical transmission and reception device according to claim 1 , wherein the second symbol period of the second pilot symbol signal is an integer multiple of equal to or more than two of the first symbol period of the first pilot symbol signal.
7 . An optical receiver comprising: an optical coherent detector to receive polarization-multiplexed modulated light into which a first pilot symbol signal having a first symbol period and a second pilot symbol signal having a second symbol period different from the first symbol period are inserted, coherently detect the received modulated light, and output an analog electric signal; an analog-to-digital converter to perform analog-to digital conversion on the analog electric signal from the optical coherent detector to output the signal as a digital signal; and a reception-side digital signal processor including a frequency difference compensator to receive the digital signal from the analog-to-digital converter, calculate a phase difference using the first pilot symbol signal having the first symbol period and the second pilot symbol signal having the second symbol period different from the first symbol period and adjacent to the first pilot symbol signal, the first pilot symbol signal and the second pilot symbol signal being inserted into the received polarization-multiplexed modulated light extracted from the received digital signal, and compensate a frequency of the received digital signal.

Description

CROSS REFERENCE TO RELATED APPLICATION This application is a Continuation of PCT International Application No. PCT/JP2023/026913, filed on July 24, 2023, which is hereby expressly incorporated by reference into the present application. TECHNICAL FIELD The present disclosure relates to an optical transmission and reception device that performs digital coherent optical communication. BACKGROUND ART In the field of optical fiber communication, digital coherent technology is widely applied in optical metro core networks and submarine optical cable systems. In digital coherent optical communication, higher performance and higher functionality including improvement in a data rate are pursued. In particular, according to an optical transceiver of a digital coherent system, one optical transceiver can generate a plurality of sub-carrier signals that does not interfere on a frequency axis in a digital domain. Furthermore, in recent years, a technique of allocating different information to individual subcarriers and simultaneously accommodating various services has been disclosed. According to simultaneous accommodation in this manner, efficiency in hardware utilization and space saving can be achieved. For example, Non-Patent Literature 1 discloses a technique of digital coherent optical communication capable of transmitting and receiving sub-carrier signals. Furthermore, with regard to the digital coherent system, Patent Literature 1 discloses an optical transmission system in which, in order to perform phase compensation based on phase variation that occurs in an optical fiber transmission path between a transmission device and a reception device, a pilot symbol is inserted and output for every K symbols in a data string in the transmission device and the reception device detects a pilot symbol in a data string, estimates phase variation from a reference symbol stored in a predetermined storage device, and compensates for a residual frequency offset on the basis of the phase variation. CITATION LIST NON-PATENT LITERATURES Non-Patent Literature 1: H. Sun et al, “800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 38, NO. 17, SEPTEMBER 1, 2020, p.p.4744-4756 Patent Literature 1: WO2014/126132 SUMMARY OF INVENTION TECHNICAL PROBLEM In an optical receiver disclosed in Non-Patent Literature 1, an analog-to-digital converter (ADC) converts a voltage signal, which is detected by optical coherent detection and output, into a digital signal. In general, when a sampling speed of the analog-to-digital converter is high, an analog reception signal before analog-to-digital conversion can be accurately obtained as a digital signal, waveform distortion can be accurately compensated in digital signal processing, and signal quality after reception can be improved. However, for example, when a field programmable gate array (FPGA) having a sampling rate or a throughput of several gigabits is used for the digital signal processing, or when digital coherent signals of a low-to-medium speed are handled in an optical receiver, such as when a modulation rate of each subcarrier is low due to a sub-carrier multiplexing system, a frequency difference between transmission and reception optical carrier waves deteriorates the capability of compensation for waveform distortion in the reception-side digital signal processing. While a pilot symbol is inserted every K symbols in the optical transmission system of Patent Literature 1, a time interval between adjacent pilot symbols is long, which causes an estimation error in phase compensation when there is a difference between a frequency of the optical carrier wave of the transmission device and a frequency of the optical carrier wave of the reception device. The present disclosure has been conceived in view of the points described above, and an object is to obtain an optical transmission and reception device having a compensation function for degrading factors of waveform distortion caused by a difference in frequency between transmission and reception optical carrier waves in an optical receiver that handles digital coherent signals of a low-to-medium speed. SOLUTION TO PROBLEM An optical transmission and reception device according to the present disclosure includes an optical transmitter and an optical receiver, in which the optical transmitter includes: a modulation signal generator that generates a modulation signal that is a digital signal for optical modulation in which a first pilot symbol signal having a first symbol period and a second pilot symbol signal having a second symbol period different from the first symbol period are inserted into data to be transmitted; a digital-to-analog converter that converts the modulation signal that is the digital signal generated by the modulation signal generator into a modulation signal including an analog signal; and an optical modulator that generates modulated light into wh