Optical transmitter-receiver and optical receiver

Abstract

An optical transmitter (100) is provided with: a modulation signal generation unit (110) that generates a modulation signal that is a digital signal in which a first pilot symbol signal comprising a first symbol period and a second pilot symbol signal comprising a second symbol period are inserted into the data to be transmitted; and an optical modulation unit (130) that modulates CW light from a CW light generation unit (140) on the basis of the modulation signal converted to an analog signal by a digital / analog conversion unit (120) to generate modulated light into which the first pilot symbol signal and the second pilot symbol signal are inserted. An optical receiver (200) is provided with: a coherent detection unit (210) that receives polarization-multiplexed modulated light into which the first pilot symbol signal comprising the first symbol period and the second pilot symbol signal comprising the second symbol period are inserted, and outputs an analog electrical signal by coherent detection of the received modulated light; and a reception-side digital signal processing unit (240) having a frequency difference compensation unit (247) that calculates a phase difference using the first pilot symbol signal inserted into the received polarization-multiplexed modulated light and the second pilot symbol signal adjacent to the first pilot symbol signal having been extracted from the digital signal from an analog / digital converter unit (230), and compensates for the frequency of the received digital signal.

Description

Optical transmitting / receiving device and optical receiver

[0001] The present disclosure relates to an optical transceiver that performs digital coherent optical communication.

[0002] In the field of optical fiber communications, digital coherent technology is widely applied in metro / core networks and submarine optical cable systems, and efforts are being made to improve performance and functionality in digital coherent optical communications, including improving data rates.

[0003] In particular, digital coherent optical transceivers are capable of generating multiple subcarrier signals in the digital domain that do not interfere with each other on the frequency axis within a single optical transceiver. Recently, a method has been proposed in which different information is assigned to each subcarrier, allowing for simultaneous accommodation of a variety of services. This simultaneous accommodation makes it possible to achieve efficient hardware utilization and space savings. Non-Patent Document 1, for example, describes a digital coherent optical communication technology capable of transmitting and receiving subcarrier signals.

[0004] In addition, in a digital coherent system, in order to perform phase compensation according to phase fluctuations that occur in an optical fiber transmission path between a transmitting device and a receiving device, a transmitting device inserts a pilot symbol into a data string every K symbols and outputs the data string, and a receiving device detects the pilot symbol in the data string, estimates the phase fluctuations from a reference symbol stored in a predetermined storage device, and compensates for the residual frequency offset based on the phase fluctuations.This optical transmission system is disclosed in Patent Document 1.

[0005] H. Sun et al, "800G DSP ASIC Design Using Probabilistic Shaping and Digital Sub-Carrier Multiplexing", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 38, NO. 17, SEPTEMBER 1, 2020, pp4744-4756WO2014 / 126132

[0006] The optical receiver disclosed in Non-Patent Document 1 detects signals using optical coherent detection, and converts the output voltage signal into a digital signal using an analog-to-digital converter (ADC). Generally, if the sampling rate of the analog-to-digital converter is high, the analog received signal before analog-to-digital conversion can be accurately acquired as a digital signal, and waveform distortion compensation in digital signal processing can be accurately performed, improving the quality of the received signal.

[0007] However, when an FPGA (Field Programmable Gate Array) with a sampling rate or throughput of several gigabits is used for digital signal processing, or when a low- to medium-speed digital coherent signal is handled in an optical receiver, such as when the modulation speed of each subcarrier is low due to a subcarrier multiplexing method, if there is a frequency difference between the transmitted and received optical carrier waves, the ability to compensate for waveform distortion in the digital signal processing on the receiving side will be reduced.

[0008] Although the optical transmission system described in Patent Document 1 inserts a pilot symbol every K symbols, if the time interval between adjacent pilot symbols is long and there is a difference between the frequency of the optical carrier wave of the transmitting device and the frequency of the optical carrier wave of the receiving device, an estimation error will occur in phase compensation.

[0009] The present disclosure has been made in consideration of the above points, and aims to provide an optical transceiver device that has a compensation function for factors that cause waveform distortion degradation due to frequency differences between transmitted and received optical carrier waves in an optical receiver that handles low- to medium-speed digital coherent signals.

[0010] An optical transmission / reception apparatus according to the present disclosure includes an optical transmitter and an optical receiver, wherein the optical transmitter includes a modulation signal generation unit that generates a modulation signal, which 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 conversion unit that converts the modulation signal, which is a digital signal generated by the modulation signal generation unit, into a modulation signal consisting of an analog signal, and an optical modulation unit that modulates CW light from a CW light generation unit based on the modulation signal converted into an analog signal by the digital-to-analog conversion unit to generate modulated light in which the first pilot symbol signal having the first symbol period and the second pilot symbol signal having the second symbol period are inserted, and the optical receiver includes: and a receiving-side digital signal processing unit having a frequency difference compensation unit that receives the digital signal from the analog-to-digital conversion unit, and calculates a phase difference using the first pilot symbol signal having the first symbol period and inserted into the received polarization-multiplexed modulated light, and a second pilot symbol signal having a second symbol period different from the first symbol period, which are extracted from the received digital signal, and a frequency difference compensation unit that compensates for the frequency of the received digital signal.

[0011] According to the present disclosure, in an optical receiver that handles low- to medium-speed digital coherent signals, it is possible to suppress a decrease in the ability to compensate for waveform distortion in digital signal processing by an analog-to-digital converter.

[0012] FIG. 1 is a block diagram showing a configuration of an optical transmission / reception device according to a first embodiment; FIG. 2 is a block diagram showing a configuration of a transmission-side digital signal processing unit in the optical transmission / reception device according to the first embodiment; FIG. 3 is a block diagram showing a configuration of another transmission-side digital signal processing unit in the optical transmission / reception device according to the first embodiment; FIG. 4 is a block diagram showing a configuration of a reception-side digital signal processing unit in the optical transmission / reception device according to the first embodiment; Ａ and the second pilot symbol signal P Ｂ 1 is a diagram illustrating an example of a symbol signal sequence into which a first pilot symbol signal P is inserted in a digital coherent system in an optical transmission / reception device according to a first embodiment. Ａ and the second pilot symbol signal P Ｂ 1 is a diagram illustrating another example of a symbol signal sequence into which one pilot symbol signal P is inserted in the digital coherent system in a comparative example. FIG. 2 is a diagram illustrating, on a frequency axis, a single carrier signal in a digital coherent system used in the optical transmission / reception device according to the first embodiment. FIG. 3 is a diagram illustrating, in an IQ signal space, the effect of compensation for a difference in optical frequency in the optical transmission / reception device according to the first embodiment. FIG. 4 is a diagram illustrating, on a frequency axis, a subcarrier signal of a subcarrier multiplexing system in a digital coherent system used in the optical transmission / reception device according to the second embodiment. FIG. 5 is a diagram illustrating, on a frequency axis, a subcarrier signal of a subcarrier multiplexing system in a digital coherent system used in the optical transmission / reception device according to the second embodiment.

[0013] First Embodiment An optical transmission / reception device according to a first embodiment will be described with reference to Figures 1 to 12. In the figures, dashed arrows indicate the flow of optical signals, and solid arrows indicate the flow of electrical signals. The optical transmission / reception device according to the first embodiment is a device that focuses on the transmission and reception functions of communication equipment and optical transceivers that control the transmission and reception of optical signals in optical communication network systems that use optical fiber as the transmission medium in optical access and optical core / metro networks, as well as optical communication network systems that do not use optical fiber and use radio space as the transmission medium, such as those intended for space and free-space optical communications.

[0014] The optical transceiver according to the first embodiment is applied to an optical transceiver that transmits and receives digital coherent signals, which are optical signals that utilize phase orthogonality such as polarization-multiplexed binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or quadrature amplitude modulation (QAM) of a digital coherent system. Note that the optical transceiver may also be applied to an optical transceiver that transmits and receives single-polarized digital coherent signals that do not perform polarization multiplexing.

[0015] In the following description, in the optical transmission / reception device according to the first embodiment, an optical signal is assumed to be polarization-multiplexed by a digital coherent method using X polarization and Y polarization, and the X polarization and Y polarization are modulated into orthogonal phase I and Q signals. For example, the X polarization is horizontally polarized, and the Y polarization is vertically polarized. A digital coherent signal is assumed to be polarization-multiplexed into an I signal (XI signal) in the X polarization, a Q signal (XQ signal) in the X polarization, an I signal (YI signal) in the Y polarization, and a Q signal (YQ signal) in the Y polarization.

[0016] Although the terminals for the XI signal, XQ signal, YI signal, and YQ signal (XI output terminal, XQ output terminal, YI output terminal, and YQ output terminal), as well as the lanes through which the electrical signals for the XI signal, XQ signal, YI signal, and YQ signal flow (XI lane, XQ lane, YI lane, and YQ lane), are distinguished, they are shown together in the figure.

[0017] As shown in FIG. 1 , the optical transmission / reception device according to the first embodiment includes an optical transmitter 100, an optical receiver 200, and an optical transmitter / receiver control unit 300. The optical transmitter 100, the optical receiver 200, and the optical transmitter / receiver control unit 300 are housed in the same housing. The optical transmitter 100 generates polarization-multiplexed modulated light 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, and outputs the generated modulated light to an optical receiver of another optical transmission / reception device via a transmission path such as an optical fiber or wireless space. In the first embodiment, the modulated light output from the optical transmitter 100 is an optical signal obtained by modulating continuous wave (CW) light having one carrier frequency with a modulation signal, which is an electrical signal.

[0018] The optical receiver 200 receives 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 have been inserted, and which has propagated through a transmission path from an optical transmitter of another optical transmission / reception device, and acquires data from the received modulated light. The data acquired from the modulated light received by the optical receiver 200 is data demodulated using an analog electrical signal obtained by interfering the modulated light, which is an optical signal, with interference light having one carrier frequency.

[0019] The optical transmitter 100 includes a modulation signal generation unit 110, a digital-to-analog converter (DAC) unit 120, an optical modulation unit 130, and a CW light generation unit 140. The modulation signal generation unit 110 is a transmission-side digital signal processing unit (transmission digital signal processor (DSP)), hereinafter referred to as the transmission-side DSP 110.

[0020] The transmitting-side DSP 110 receives data, which is information to be sent to a destination, as a digital signal, and generates a modulated signal, which 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 the input data, i.e., the data to be transmitted. The transmitting-side DSP 110 constructs the data to be transmitted as a frame, which is a signal format that allows error correction.

[0021] The transmitter DSP 110 generates four modulated signals, XI, XQ, YI, and YQ signals. Each of the four modulated signals is a symbol signal sequence with two types of pilot symbol signals inserted as shown in Fig. 5 or a symbol signal sequence with two types of pilot symbol signals inserted as shown in Fig. 6.

[0022] 5 and 6, the horizontal axis represents time, S represents a data symbol signal, and P Ａ (P Ａ１ , P Ａ２ , ... P Ａ５ . . ) denotes the first pilot symbol signal, P Ｂ (P Ｂ１ , P ＢＡ２ , ...) indicates a second pilot symbol signal. The data symbol signal S is a symbol signal (complex signal) obtained by converting a bit signal of data to be transmitted for IQ modulation. In other words, the data symbol signal S is a digital coherent signal. The first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂare digital coherent signals each having one bit, that is, a symbol length of 1.

[0023] The symbol signal sequence shown in FIG. 5 is not frame-by-frame, but the first pilot symbol signal P is generated at a first symbol interval, i.e., a first symbol period, relative to the data symbol signal S. Ａ is inserted, and the second pilot symbol signal P is generated not in frame units but at a second symbol interval different from the first symbol interval, i.e., at a second symbol period different from the first symbol period, for the data symbol signal S. Ｂ is a symbol signal sequence with

[0024] Second pilot symbol signal P Ｂ The second symbol interval of the first pilot symbol signal P Ａ In other words, the second pilot symbol signal P Ｂ The second symbol period of the first pilot symbol signal P Ａ The symbol period is an integer multiple of two or more, and in this example, four times the first symbol period.

[0025] That is, the second pilot symbol signal P Ｂ is the first pilot symbol signal P Ａ The second pilot symbol signal P is inserted at a frequency of 1 / 4 of the data symbol signal S. Ｂ is the first pilot symbol signal P every second symbol period. Ａ That is, the second pilot symbol signal P Ｂ is the first pilot symbol signal P every second symbol period. Ａ The first pilot symbol signal and the adjacent second pilot symbol signal are arranged at a short time interval, that is, at one symbol interval in the first embodiment.

[0026] The symbol signal sequence shown in FIG. 6 is a first pilot symbol signal P at a first symbol interval, i.e., a first symbol period, determined at a fixed position in frame units.Ａ is inserted, and a second pilot symbol signal P is generated at a second symbol interval different from the first symbol interval, that is, a second symbol period different from the first symbol period, which is determined at a fixed position in frame units. Ｂ is a symbol signal sequence with

[0027] Second pilot symbol signal P Ｂ The second symbol interval of the first pilot symbol signal P Ａ In other words, the second pilot symbol signal P Ｂ The second symbol period of the first pilot symbol signal P Ａ The symbol period is an integer multiple of two or more, and in this example, four times the first symbol period.

[0028] That is, the second pilot symbol signal P Ｂ is the first pilot symbol signal P Ａ The second pilot symbol signal P is inserted at a frequency of 1 / 4 of the data symbol signal S. Ｂ is the first pilot symbol signal P every second symbol period. Ａ That is, the second pilot symbol signal P Ｂ is the first pilot symbol signal P every second symbol period. Ａ The first pilot symbol signal and the adjacent second pilot symbol signal are arranged at a short time interval, that is, at one symbol interval in the first embodiment.

[0029] 2, the transmitting-side DSP 110 includes digital circuits having the signal processing functions of a frame generation unit 111, a mapping unit 112, a spectrum shaping unit 113, a skew adjustment unit (deskewing unit) 114, a pilot generation unit 115, and a pilot insertion unit 116. The frame generation unit 111 is a framer unit that constructs data to be transmitted in a signal frame format, which is a signal format that allows error correction.

[0030] The mapping unit 112 converts a data sequence based on bit signals in the digital signal from the frame generation unit 111 into a symbol signal (complex signal) for IQ modulation. That is, the mapping unit 112 converts a data sequence based on data to be transmitted into a modulated signal, which is a digital signal for optical modulation. Generally, as the multi-level degree of IQ modulation increases, the number of bits allocated to the data symbol signal S increases, and the data rate improves.

[0031] The mapping unit 112 has four output terminals for the XI signal, XQ signal, YI signal, and YQ signal corresponding to the polarization-multiplexed BPSK signal or QPSK signal obtained by the mapping unit 112. Note that the mapping unit 112 also has four output terminals when the polarization-multiplexed signal obtained by the mapping unit 112 is a QAM signal. In short, the mapping unit 112 has four output terminals for the digital coherent signal.

[0032] The pilot inserter 116 generates a first pilot symbol signal P using the digital coherent signal generated by the pilot generator 115. Ａ is inserted into the data string for each first symbol period before mapping section 112 processes the data string made up of bit signals in the digital signal from frame generation section 111 .

[0033] Furthermore, the pilot inserter 116 generates a second pilot symbol signal P using the digital coherent signal generated by the pilot generator 115. Ｂ is inserted into the data sequence for each second symbol period before the mapping unit 112 processes the data sequence of the bit signals in the digital signal from the frame generation unit 111. In this case, the mapping unit 112 inserts the first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ The data string into which the symbol signal sequence shown in FIG. 5 is inserted is converted into a modulated signal by the symbol signal sequence shown in FIG.

[0034] As shown in FIG. 3, the pilot inserter 116 generates a first pilot symbol signal P using the digital coherent signal generated by the pilot generator 115. Ａ and the second pilot symbol signal P Ｂ When the mapping unit 112 converts the data string of bit signals in the digital signal from the frame generation unit 111 into a modulated signal, the first symbol period and the second symbol period may be inserted on a symbol-by-symbol basis, respectively, to obtain a modulated signal based on the symbol signal sequence shown in FIG. 5 or the symbol signal sequence shown in FIG. 6 .

[0035] The spectrum shaping unit 113 shapes the frequency characteristics of the modulated signal obtained by the mapping unit 112 in accordance with the frequency characteristics of the optical transmitting and receiving device and the frequency characteristics in the transmission path. The skew adjustment unit 114 compensates for skew (delay difference) occurring between signals caused by the optical transmitter 100 and generated by the XI lane, XQ lane, YI lane, and YQ lane through which the electrical signals of the XI signal, XQ signal, YI signal, and YQ signal, respectively, that are modulated signals from the spectrum shaping unit 113, flow.

[0036] As described above, the transmitting side DSP 110 converts the data to be transmitted into a signal frame format using a digital coherent signal using the frame generation unit 111, converts it into a modulated signal using the mapping unit 112, and generates the first pilot symbol signal P using a digital coherent signal using the pilot insertion unit 116. Ａ and the second pilot symbol signal P Ｂ The modulated signal is then subjected to frequency characteristic shaping by spectrum shaping section 113, and skew compensation by skew adjustment section 114 to generate a modulated signal that is a digital signal for optical modulation 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.

[0037] The DAC unit 120 converts the first pilot symbol signal P Ａ and the second pilot symbol signal P ＢThe optical modulator 130 modulates the CW light from the CW light generator 140 based on the modulation signal converted into the analog signal by the DAC 120 to generate 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, and outputs the generated modulated light to a transmission path.

[0038] The modulated light is an optical signal that is a digital coherent signal in which information from a modulated signal is carried on CW light. The modulated light is a mixture of information from the XI signal, XQ signal, YI signal, and YQ signal output from independent lanes. The CW light from the CW light generating unit 140 is a single carrier signal in a digital coherent system, as shown on the frequency axis in Figure 7.

[0039] 1, the optical receiver 200 includes an optical coherent detection unit 210, an interference light generation unit 220, an ADC (Analog-to-digital converter) unit 230, which is an analog-to-digital converter, and a receiving-side digital signal processing unit 240. The receiving-side digital signal processing unit 240 is a receiving digital signal processor (DSP), and will be referred to as the receiving-side DSP 240 hereinafter.

[0040] The optical coherent detection unit 210 receives polarization-multiplexed modulated light transmitted over a transmission path from an optical transmitter of another optical transmission / reception device, causes the received modulated light to interfere with interference light, which is continuous wave (CW) light having a single carrier frequency fc, from the interference light generation unit 220, and performs optical coherent detection to output an analog electrical signal based on the voltage obtained by photoelectric conversion of the optical signal obtained by the interference. The voltage signal output from the optical coherent detection unit 210 is a single-ended output or a differential output.

[0041] In the first embodiment, the analog electrical signals output from the optical coherent detection unit 210 are four signals, namely, an XI signal, an XQ signal, a YI signal, and a YQ signal, which are output from four output terminals, respectively. The XI signal, the XQ signal, the YI signal, and the YQ signal are pre-demodulation signals, which are analog voltage signals for obtaining a demodulated signal obtained by optically interfering the polarization-multiplexed modulated light and the interference light having a single carrier frequency fc using the optical coherent detection unit 210.

[0042] The ADC unit 230 is a collection of ADCs corresponding to the XI lane of the XI signal, the XQ lane of the XQ signal, the YI lane of the YI signal, and the YQ lane of the YQ signal. Each ADC in the ADC unit 230 samples the analog electrical signal input to the corresponding lane based on a sampling frequency, converts it into a digital signal, which is a discrete signal in the digital domain, and obtains a pre-demodulation signal consisting of a digital signal. The XI signal, XQ signal, YI signal, and YQ signal converted into digital signals by the ADC unit 230 are each a symbol signal sequence into which two types of pilot symbol signals are inserted, as shown in FIG. 5 or 6 .

[0043] The receiving-side DSP 240 performs digital signal processing and demodulation on the XI signal, XQ signal, YI signal, and YQ signal that have been converted into digital signals by the ADC unit 230. Note that the receiving-side DSP 240 may combine the XI signal and XQ signal for the X polarization, which have been converted into digital signals by the ADC unit 230, to generate a complex signal in the digital domain for the X polarization, and may combine the YI signal and YQ signal for the Y polarization to generate a complex signal in the digital domain for the Y polarization.

[0044] The transmitting-side DSP 110 in the optical transmitter 100 generates a modulated signal, while the receiving-side DSP 240 in the optical receiver 200 demodulates the modulated signal to obtain a demodulated signal. The receiving-side DSP 240 compensates for the input digital signal, such as chromatic dispersion and nonlinear optical effects that the modulated light has experienced in the transmission path, and polarization mode dispersion, as well as compensation for a frequency offset that occurs due to a difference in light source frequency of the CW light between the optical transmitter and the optical receiver.

[0045] The receiving-side DSP 240 performs digital signal processing on each of the XI signal, the XQ signal, the YI signal, and the YQ signal. However, to simplify the explanation, the flow of the digital signal processing is shown as a flow of one electrical signal in Fig. 4. Also, Fig. 4 mainly shows the first pilot symbol signal P inserted into the polarization-multiplexed modulated light transmitted from the optical transmitter via the transmission line, which is a feature of the first embodiment. Ａ and the second pilot symbol signal P Ｂ This shows the optical demodulation function for detecting the error, but does not show error correction etc.

[0046] As shown in FIG. 4, the receiving side DSP 240 includes a skew adjustment unit 241, an amplitude adjustment unit 242, a dispersion compensation unit 243, a clock synchronization unit 244, a band compensation unit 245, an adaptive equalization unit 246, a frequency difference compensation unit 247, a phase estimation unit 248, a demapping unit 249, a frame synchronization unit 250, and a first pilot symbol signal P Ａ and a first pilot reading unit 251 that reads a second pilot symbol signal P Ｂ and a pilot removal unit 253.

[0047] The XI signal, XQ signal, YI signal, and YQ signal before being input to the demapping unit 249 are pre-demodulation signals that undergo signal processing prior to demodulation, and the data signal output from the demapping unit 249 is a demodulated signal that has undergone signal processing for demodulation. Also, in Figure 4, the signal input from the ADC unit 230 to the receiving side DSP 240 is a pre-demodulation signal that will be demodulated, but is simply shown as a demodulated signal.

[0048] The skew adjustment unit 241 compensates for skews originating from the optical transmitter and optical receiver devices of the lane for each of the XI signal, XQ signal, YI signal, and YQ signal. The amplitude adjustment unit 242 adjusts amplitude differences originating from the optical transmitter and optical receiver devices of the lane for each of the XI signal, XQ signal, YI signal, and YQ signal, as necessary. The dispersion compensation unit 243 compensates for signal degradation due to chromatic dispersion occurring in the transmission path. The clock synchronization unit 244 compensates for clock differences between the optical transmitter and optical receiver. This function of compensating for clock differences may be performed on analog signals before they are converted to digital by the ADC unit 230.

[0049] The band compensation unit 245 compensates for deterioration of frequency characteristics under fixed conditions. The adaptive equalization unit 246 adaptively compensates for deterioration of polarization separation and frequency characteristics of the X polarization signal (XI signal and XQ signal) and the Y polarization signal (YI signal and YQ signal). In order to determine compensation conditions for equalization processing, the adaptive equalization unit 246 reads the first pilot symbol signal P extracted by the first pilot reading unit 251 from the symbol signal sequence into which two types of pilot symbol signals have been inserted. Ａ When quadrature phase modulation such as 16QAM is used for the data symbol signal S, the first pilot symbol signal P Ａ Since a QPSK signal with constant IQ amplitude is used, the adaptive equalizer 246 can determine the compensation conditions for the equalization process.

[0050] The skew adjustment unit 241, amplitude adjustment unit 242, dispersion compensation unit 243, clock synchronization unit 244, bandwidth compensation unit 245, and adaptive equalization unit 246 are configured using units that are commonly used in digital coherent systems. That is, the signal processing performed prior to demodulation by the receiving DSP 240 is a common technique in digital signal processing that is commonly used in digital coherent systems.

[0051] The frequency difference compensator 247 compensates for the frequency difference between the CW light from the CW light generator 140 in the optical transmitter 100 and the interference light from the interference light generator 220. The frequency difference compensator 247 reads the first pilot symbol signal P extracted by the first pilot reader 251 from the symbol signal sequence into which two types of pilot symbol signals have been inserted. Ａ The first pilot symbol signal P in the received symbol signal sequence Ａ When quadrature phase modulation such as 16QAM is used for the data symbol signal S, the position of the first pilot symbol signal P Ａ Since a QPSK signal with constant IQ amplitude is used for the first pilot symbol signal P Ａ The position can be detected.

[0052] The frequency difference compensator 247 receives the XI signal, XQ signal, YI signal, and YQ signal converted into digital signals by the ADC unit 230, and extracts the first pilot symbol signal P extracted by the first pilot reader 251 from the received digital signals. Ａ and the second pilot symbol signal P extracted by the second pilot reading unit 252. Ｂ ' and the adjacent first pilot symbol signal P inserted into the symbol signal sequence by the transmitting side DSP 110 in the optical transmitter 100. Ａ and the second pilot symbol signal P Ｂ The second pilot symbol signal P Ｂ The second pilot symbol signal P Ｂ The phase difference of the received digital signal is calculated and the frequency of the received digital signal is compensated.

[0053] The frequency difference compensator 247 reads the first pilot symbol signal P extracted by the first pilot reader 251 from the symbol signal sequence into which two types of pilot symbol signals have been inserted. Ａ and a second pilot symbol signal P extracted by the second pilot reading unit 252 from the symbol signal sequence into which two types of pilot symbol signals are inserted. Ｂ', and the first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ The second pilot symbol signal P Ｂ The frequency difference between the frequency of the CW light from the CW light generating unit 140 and the frequency of the interference light from the interference light generating unit 220 in the optical transmitter 100 is calculated, and the frequency difference is compensated for, thereby performing frequency compensation for the signal of the symbol signal sequence in the optical receiver 200.

[0054] The frequency difference compensator 247 compensates for the adjacent first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ ', that is, the first pilot symbol signal P extracted by the first pilot reading unit 251. Ａ and the extracted first pilot symbol signal P Ａ The second pilot symbol signal P extracted by the second pilot reading unit 252 adjacent to Ｂ ' and the first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ In other words, the relative position of the adjacent first pilot symbol signal P inserted into the signal of the symbol signal sequence by the transmitting side DSP 110 in the optical transmitter 100. Ａ and the second pilot symbol signal P Ｂ The first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ The second pilot symbol signal P Ｂ Estimate the frequency difference at

[0055] The estimated second pilot symbol signal P Ｂ ', the frequency difference between adjacent first pilot symbol signals P Ａ and the second pilot symbol signal P ＢThe phase difference between the adjacent first pilot symbol signal P' is analyzed, and the frequency of the received symbol signal sequence is compensated for using the calculated compensation amount. Ａ and the second pilot symbol signal P Ｂ are inserted at short time intervals (symbol intervals), the phase rotation relative to the time interval can be reduced. As a result, the amount of phase rotation can be accurately read, and the frequency difference compensator 247 can accurately compensate for the difference in optical frequency.

[0056] This point will be explained using Fig. 8. Fig. 8 shows an IQ signal space, with the horizontal axis representing the Q axis and the vertical axis representing the I axis. Ａ is the first pilot symbol signal P Ａ signal point at P Ｂ is the second pilot symbol signal P when there is no difference in frequency Ｂ signal point at P Ｂ ' is the second pilot symbol signal P when there is a difference in frequency Ｂ The signal point at

[0057] In FIG. 8, signal point P Ａ is the first pilot symbol signal P extracted by the first pilot reading unit 251 Ａ The signal point P is the observation signal point indicated by P and serves as the reference point for analyzing the phase difference. Ｂ is the signal point P Ａ , that is, the adjacent first pilot symbol signal P inserted into the symbol signal sequence by the transmitting side DSP 110 in the optical transmitter 100. Ａ and the second pilot symbol signal P Ｂ are signal points obtained from the relative positions in the IQ signal space of

[0058] Signal point P Ｂ ' is the second pilot symbol signal P extracted by the second pilot reading unit 252. Ｂ The signal point P Ａ and signal point P Ｂ(No difference) is the signal point P in the IQ signal space due to the passage of one symbol period in the symbol signal sequence by the transmitting side DSP 110. Ａ The relative positional relationship is shown with respect to signal point P. Ａ and signal point P Ｂ ' (with difference) is the signal point P Ａ Signal point P based on Ｂ The relative positional relationship in ´ is shown.

[0059] Signal point P Ａ and signal point P Ｂ ´ (with difference) and signal point P Ａ and signal point P Ｂ (no difference) indicates the relative positional relationship in the IQ signal space. Ａ Signal point P based on Ｂ The phase difference between the signal point P Ｂ It is possible to observe whether the signal point P Ａ Signal point P based on Ｂ and signal point P Ｂ The phase difference between the two points is calculated by extracting the phase term from the complex number value output by multiplication using the complex conjugate when the two points are represented by complex numbers on the complex plane.

[0060] That is, the first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ Assuming that there is no difference in frequency between the modulated light 100 from the optical transmitter and the modulated light received by the optical receiver 200 with respect to the relative position of the original IQ signal space, the first pilot symbol signal P extracted by the first pilot reading unit 251 is Ａ The signal point P Ａ The second pilot symbol signal P Ｂ The signal point P Ｂ An example of the relative positions in the IQ signal space is shown in FIG.

[0061] Also, the first pilot symbol signal P extracted by the first pilot reading unit 251 Ａ The signal point PＡ The second pilot symbol signal P extracted by the second pilot reading unit 252 based on Ｂ The signal point P Ｂ An example of the relative position of the second pilot symbol signal P′ in the IQ signal space is shown in FIG. Ｂ The signal point P Ｂ ' is the second pilot symbol signal P Ｂ is the first pilot symbol signal P Ａ In the first embodiment, the phase rotation relative to the time interval can be reduced because the symbols are arranged adjacent to each other, that is, at a short time interval, and as a result, the amount of phase rotation can be read accurately.

[0062] In this way, the frequency difference compensator 247 calculates the signal point P Ｂ Signal point P at Ｂ The first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ The frequency difference (amount of phase rotation per unit time) is calculated from the phase shift during the sampling time between the optical transmitter 100 and the optical receiver 200. The frequency difference compensator 247 generates a sine wave (digital data) of unit amplitude that imparts a phase in the opposite direction during the sampling time so as to cancel out the calculated frequency difference, and multiplies the generated sine wave by the symbol signal sequence signal. As a result, the frequency difference compensator 247 compensates for the optical frequency difference between the optical transmitter 100 and the optical receiver 200, i.e., the frequency difference between the frequency of the CW light from the CW light generating unit 140 and the frequency of the interference light from the interference light generating unit 220 in the optical transmitter 100.

[0063] That is, the first pilot symbol signal P extracted by the first pilot reading unit 251 Ａ and the extracted first pilot symbol signal P Ａ The second pilot symbol signal P extracted by the second pilot reading unit 252 adjacent to Ｂ and the adjacent first pilot symbol signal P inserted into the symbol signal sequence by the transmitting side DSP 110 in the optical transmitter.Ａ and the second pilot symbol signal P Ｂ The first pilot symbol signal P extracted by the first pilot reading unit 251 from the relative position of the IQ signal space Ａ The extracted first pilot symbol signal P Ａ The second pilot symbol signal P extracted by the second pilot reading unit 252 adjacent to Ｂ The extracted first pilot symbol signal P Ａ The second pilot symbol signal P inserted into the symbol signal sequence by the transmitting DSP 110 adjacent to Ｂ The frequency difference can be analyzed.

[0064] As a result, by using the frequency difference obtained as a result of the above analysis, the frequency difference compensator 247 can compensate for the frequency of the symbol signal sequence in the optical receiver 200. Ａ and the second pilot symbol signal P Ｂ The frequency compensation by the above method is as explained above, and a method of performing frequency compensation using two pilot symbol signals that are generally known between the transmitting side and the receiving side can be used.

[0065] The frequency compensation for the symbol signal sequence in the optical receiver 200 is performed by the first pilot symbol signal P Ａ and the extracted first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ , that is, two pilot symbol signals with a short time interval of the symbol interval are used, so the first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ Since the phase rotation of the first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ The accurate phase rotation amount in

[0066] For example, the second pilot symbol signal PＢ 9, if an optical receiver performs frequency compensation on modulated light based on a symbol signal sequence in which pilot symbol signals P are inserted into data symbol signals S at symbol intervals rather than frame units as shown in FIG. 10, or a symbol signal sequence in which pilot symbol signals P are inserted at predetermined symbol intervals at predetermined positions in frame units as shown in FIG. 11, problems may occur as shown in FIG. 9.

[0067] FIG. 9 shows an IQ signal space, with the horizontal axis representing the Q axis and the vertical axis representing the I axis. １ is the signal point of the first pilot symbol signal P1 for frequency compensation, P ２ is the second pilot symbol signal P when there is no frequency difference ２ signal point at P ２ ' is the second pilot symbol signal P when there is a difference in frequency ２ The signal point at

[0068] In FIG. 9, signal point P １ is the observed signal point indicated by the first pilot symbol signal P1 extracted by the pilot reading unit in the optical receiver, and serves as a reference point for analyzing the phase difference. ２ is the signal point P １ In other words, there is no difference in frequency between the first pilot symbol signal P1 and the second pilot symbol signal P2 inserted into the symbol signal sequence in the optical transmitter. ２ The signal point P is obtained from the relative position of the IQ signal space. ２ ' is the second pilot symbol signal P extracted by the pilot reading unit ２ is the observed signal point indicated by

[0069] As can be seen from FIG. 9, the time when the first pilot symbol signal P1 is extracted is the time when the second pilot symbol signal P ２is extracted is long, and there is a difference between the frequency of the optical carrier wave from the optical transmitter and the frequency of the optical carrier wave of the interference light at the optical receiver 200, the time interval from the first pilot symbol signal P1 to the second pilot symbol signal P2 will be long over a long period of time. ２ Signal point P at ２ That is, a large phase rotation occurs in the first pilot symbol signal P １ The second pilot symbol signal P ２ The phase rotation is large, and the phase rotation occurs in the IQ signal space, causing the phase to rotate cyclically, resulting in an error in estimating the amount of phase rotation.

[0070] In the optical transmission / reception device according to the first embodiment, the first pilot symbol signal P Ａ and the extracted first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ Since the first pilot symbol signal P Ａ The second pilot symbol signal P Ｂ The phase rotation is small, there is no estimation error in the amount of phase rotation, and an accurate amount of phase rotation can be obtained.

[0071] In short, the optical transmission / reception device according to the first embodiment generates first pilot symbol signals P Ａ and the second pilot symbol signal P Ｂ is inserted into the signal of the symbol signal sequence, and the second pilot symbol signal P Ｂ The first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ The frequency difference between the frequency of the optical carrier wave in the optical transmitter 100 and the frequency of the interference light in the optical receiver 200 is compensated for.

[0072] Therefore, the optical transmission / reception device according to the first embodiment transmits the first pilot symbol signal P at short time intervals. Ａ and the second pilot symbol signal P Ｂ The first pilot symbol signal PＡ and the second pilot symbol signal P Ｂ Signal point P Ａ , P Ｂ , P Ｂ The amount of phase rotation calculated from the information, and therefore the amount of compensation for waveform distortion caused by the calculated amount of phase rotation (frequency difference), can be accurately calculated.

[0073] This improves the signal quality of the received signal in the optical receiver 200, and in turn expands the compensation range for the frequency difference between the optical carrier in the optical transmitter 100 and the frequency of the interference light in the optical receiver 200. Therefore, the optical transmission / reception device according to the first embodiment can compensate for degradation factors caused by waveform distortion resulting from the frequency difference between the optical carrier in the optical transmitter 100 and the frequency of the interference light in the optical receiver 200, for low- to medium-speed digital coherent signals.

[0074] In estimating the frequency difference of the modulated light, the adjacent first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ and the adjacent first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ The compensation amount may be calculated after averaging the relative positions of the two sensors. By performing averaging, the speed response can be adjusted as desired.

[0075] The phase estimator 248 compensates for phase fluctuations in the light sources constituting the CW light generator 140 and the interference light generator 220. The phase estimator 248 is configured using a method used in a typical digital coherent system. That is, the signal processing for demodulation in the receiver DSP 240 other than the compensation for the frequency difference by the frequency difference compensator 247 is a general method in digital signal processing used in a typical digital coherent system.

[0076] The demapping unit 249 receives the first pilot symbol signal P Ａ and the second pilot symbol signal P ＢAfter the distortion is removed, the symbol signal with the waveform distortion compensated is converted into a bit signal. A frame synchronization unit 250 synchronizes the bit signal sequence with the frame. The digital signal of the bit signal sequence synchronized with the frame is then subjected to error correction.

[0077] Next, the hardware configuration of the optical transmitter / receiver according to the first embodiment will be described with reference to Fig. 12. In Fig. 12, the same reference numerals as those in Figs. 1 to 4 indicate the same or corresponding parts. The optical transmitter / receiver control unit 300 includes a processor 31 such as a CPU (Central Processing Unit) or a system LSI (Large Scale Integration), a memory 32 configured from RAM (Random Access Memory) and ROM (Read Only Memory), a communication interface 33, and an input / output interface 34. The processor 31, the memory 32, the communication interface 33, and the input / output interface 34 are connected to a bus 35, and data, control signals, and the like are exchanged between them via the bus 35.

[0078] The processor 31 temporarily loads a program recorded in the ROM in the memory 32 into the RAM in the memory 32 and executes processing according to the loaded program. The ROM in the memory 32 stores various data, programs for executing processing in the optical transmitter 100 and the optical receiver 200, and processing programs required to start the optical transmitter / receiver control unit 300. The communication interface 33 is used to send and receive data and control signals to and from each component in the optical transmitter 100, each component in the optical receiver 200, and each component in other optical transmitter / receiver devices.

[0079] The input / output interface 34 transmits and receives control signals and modulation signals between the components of the optical transmitter 100 and the components of the optical receiver 200 via electrical wiring. The input / output interface 34 is, for example, an interface for supplying the interference light generating unit 220 with an injection current to a light source constituting the interference light generating unit 220 for generating light. The input / output interface 34 is also, for example, an interface for outputting various control signals to the transmitting-side DSP unit 110 and the receiving-side DSP unit.

[0080] As described above, in the optical transmission / reception device according to the first embodiment, the optical transmitter 100 adds the first pilot symbol signal P having the first symbol period to the data to be transmitted. Ａ and a second pilot symbol signal P having a second symbol period different from the first symbol period. Ｂ a modulation signal generator 110 for generating a modulation signal into which a first pilot symbol P having a first symbol period is inserted, and a digital / analog converter 120 for converting the modulation signal into an analog signal. Ａ and a second pilot symbol P consisting of a second symbol period. Ｂ and an optical modulator 130 for generating modulated light into which a first pilot symbol signal P having a first symbol period is inserted. Ａ and a second pilot symbol signal P having a second symbol period different from the first symbol period. Ｂ and an optical coherent detection unit 210 that coherently detects the polarization-multiplexed modulated light into which the first pilot symbol signal P is inserted and outputs an analog electrical signal. The analog / digital conversion unit 230 converts the analog electrical signal into a digital signal, and the first pilot symbol signal P is extracted from the digital signal. Ａ and the first pilot symbol signal P Ａ and the second pilot symbol signal P ＢThe optical receiver 200 is provided with a receiving-side digital signal processing unit 240 having a frequency difference compensation unit 247 that calculates a phase difference using the above and compensates for the frequency of the received digital signal, so that the difference in frequency of the modulated light received by the optical receiver 200 relative to the frequency of the modulated light from the optical transmitter 100 can be compensated for.

[0081] As a result, it is possible to compensate for the cause of waveform distortion degradation due to the difference in signal frequency between transmission and reception, improve the signal quality of the received signal at the optical receiver 200, and ultimately expand the compensation range for the difference in optical frequency between the optical transmitter 100 and the optical receiver 200. This is particularly effective for the optical receiver 200 that handles low- to medium-speed digital coherent signals.

[0082] Second Embodiment An optical transceiver according to a second embodiment will be described with reference to Fig. 13 to Fig. 15. The optical transceiver according to the first embodiment uses a single carrier signal in a digital coherent system shown on the frequency axis in Fig. 7 as an optical carrier wave. In contrast, the optical transceiver according to the second embodiment is an optical transceiver using a subcarrier multiplexing system, using, as optical subcarriers, each of N subcarrier signals with different frequency allocations in a digital coherent system shown on the frequency axis in Fig. 15.

[0083] The basic idea of ​​the optical transmission / reception device according to the second embodiment is to apply the optical transmission / reception device according to the first embodiment to subcarrier signals as N optical subcarriers. Therefore, the following description will focus on the differences from the optical transmission / reception device according to the first embodiment. As an example, the second embodiment assumes an optical transmission / reception device that transmits and receives an optical signal multiplexed with N channels of subcarrier signals SC#1 to SC#N of N different frequencies in X polarization and N channels of subcarrier signals SC#1 to SC#N of N different frequencies in Y polarization, as shown in FIG.

[0084] In the optical receiver 200, the optical coherent detection unit 210 collectively receives modulated light that has been polarization-multiplexed onto each of N subcarrier signals with different frequency allocations within a band in which signals can be detected, and the optical coherent detection unit 210 performs optical coherent detection on the modulated light that has been collectively received and outputs it as an analog electrical signal. The collectively processed analog electrical signal is converted into a digital signal by the ADC unit 230 and input to the receiving-side DSP 240. The receiving-side DSP 240 separates the signal into subcarrier signals SC#1 to SC#N and demodulates each channel of the subcarrier signals SC#1 to SC#N. The optical transmission / reception apparatus according to the second embodiment includes an optical transmitter 100, an optical receiver 200, and an optical transmitter / receiver control unit 300, as shown in FIG. 1, similar to the optical transmission / reception apparatus according to the first embodiment.

[0085] The optical transmitter 100 includes a transmission-side DSP 110A, a DAC unit 120, an optical modulation unit 130, and a CW light generation unit 140. As shown in FIG. １ ~11 Ｎ The subcarrier multiplexing unit 117 and the skew adjustment unit (deskew unit) 114 are provided.

[0086] N subcarrier generation units 11 １ ~11 Ｎ Each of the digital modulation units receives as input, as digital signals, different data signals #1 to #N, which are the information to be sent to the destination, and generates a modulated signal, which 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 the data #1 to #N to be transmitted.

[0087] N subcarrier generation units 11 １ ~11 Ｎ Although the data #1 to #N to be transmitted and the subcarrier signals SC#1 to SC#N as carrier waves are different, they basically have the same configuration. １ ~11 Ｎ Each of them is a frame generator 111 １ ~111 Ｎand mapping unit 112 １ ~112 Ｎ and the spectrum shaping unit 113 １ ~113 Ｎ and the pilot generation unit 115 １ ~115 Ｎ and pilot insertion section 116 １ ~116 Ｎ The optical transmitter 110 has a digital circuit having the above signal processing functions, and has basically the same configuration as the frame generation unit 111, mapping unit 112, spectrum shaping unit 113, pilot generation unit 115, and pilot insertion unit 116 in the transmission-side DSP 110 of the optical transmitter 110 in the optical transmission and reception device according to the first embodiment.

[0088] In short, N subcarrier generation units 11 １ ~11 Ｎ Each of them is a frame generator 111 １ ~111 Ｎ The data to be transmitted is converted into a signal frame format using a digital coherent signal by a mapping unit 112, and converted into XI signals, XQ signals, YI signals, and YQ signals corresponding to QPSK signals that are polarization-multiplexed on the frequency axis using different subcarrier signals SC#1 to SC#N by a pilot insertion unit 116. １ ~116 Ｎ The first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ The N subcarrier generation units 11 １ ~11 Ｎ Each has four output terminals.

[0089] The subcarrier multiplexing unit 117 generates N subcarriers. １ ~11 ＮThe skew adjustment unit 114 compensates for skew (delay difference) occurring between signals due to the optical transmitter 100 and caused by the XI lane, XQ lane, YI lane, and YQ lane through which the electrical signals of the XI signal, XQ signal, YI signal, and YQ signal, which are the modulated signals multiplexed on the frequency axis from the spectrum shaping unit 113, flow.

[0090] The modulated signals multiplexed on different frequency axes thus generated are converted into analog signals by the DAC unit 120. The optical modulator 130 modulates the CW light from the CW light generator 140 based on the modulated signals converted into analog signals by the DAC unit 120 to generate polarization-multiplexed modulated light in which a plurality of different subcarrier signals into which a first pilot symbol having a first symbol period and a second pilot symbol having a second symbol period different from the first symbol period are inserted are frequency-multiplexed, and the optical modulator 130 outputs the generated modulated light to a transmission path.

[0091] The optical receiver 200 includes an optical coherent detection unit 210, an interference light generation unit 220, an ADC unit 230, and a receiving-side DSP 240A. The optical coherent detection unit 210 detects a first pilot symbol signal P Ａ and a second pilot symbol signal P having a second symbol period different from the first symbol period. Ｂ The optical coherent detection unit 210 receives modulation light that has been frequency-multiplexed and polarization-multiplexed using a plurality of different subcarrier signals with the subcarrier signals inserted therein, causes the received modulation light to interfere with interference light that is CW light having a single carrier frequency fc from the interference light generating unit 220, and performs photoelectric conversion of the optical signal obtained by the interference to obtain an analog electrical signal based on the voltage obtained.

[0092] The ADC unit 230 samples the analog electrical signal from the optical coherent detection unit 210 based on a sampling frequency, converts it into a digital signal, which is a discrete signal in the digital domain, and obtains a pre-demodulation signal consisting of a digital signal for obtaining a demodulated signal. The receiving side DSP 240A includes a skew adjustment unit (deskewing unit) 241, a subcarrier separation unit 254, and N subcarrier detection units 24１ ~24 Ｎ The skew adjustment unit 241 compensates for the skews caused by the optical transmitter and receiver devices of the lane for the XI signal, XQ signal, YI signal, and YQ signal, respectively.

[0093] The subcarrier separator 254 extracts the first pilot symbol signal P Ａ and the second pilot symbol signal P Ｂ The signal before demodulation, which is a digital signal that is frequency-multiplexed by a plurality of different subcarrier signals into which N subcarrier signals SC#1 to SC#N are inserted and then subjected to signal separation by corresponding the signal to N subcarrier signals SC#1 to SC#N on the frequency axis. １ ~24 Ｎ Each of these performs compensation for the N digital signals separated by the subcarrier separation unit 254, such as compensation for chromatic dispersion and nonlinear optical effects that the modulated light has experienced in the transmission path, and compensation for polarization mode dispersion, as well as compensation for frequency offsets that arise due to differences in the light source frequencies of the CW light between the optical transmitter and the optical receiver.

[0094] N subcarrier detection units 24 １ ~24 Ｎ Each of them is an amplitude adjustment unit 242 １ ~242 Ｎ and the dispersion compensator 243 １ ~243 Ｎ and the clock synchronization unit 244 １ ~244 Ｎ and the bandwidth compensation unit 245 １ ~245 Ｎ and the adaptive equalizer 246 １ ~246 Ｎ and the frequency difference compensator 247 １ ~247 Ｎ and the phase estimation unit 248 １ ~248 Ｎ and the demapping unit 249 １ ~249 Ｎ and the frame synchronization unit 250 １ ~250 Ｎ and the first pilot symbol signal P Ａ A first pilot reading unit 251 that reads １ ~251 Ｎand the second pilot symbol signal P Ｂ A second pilot reader 252 reads １ ~252 Ｎ and pilot removal unit 253 １ ~253 Ｎ and has basically the same configuration as the amplitude adjustment unit 242, dispersion compensation unit 243, clock synchronization unit 244, bandwidth compensation unit 245, adaptive equalization unit 246, frequency difference compensation unit 247, phase estimation unit 248, demapping unit 249, frame synchronization unit 250, first pilot reading unit 251, second pilot reading unit 252, and pilot removal unit 253 in the receiving-side DSP 240 of the optical receiver 200 in the optical transmission / reception device according to the first embodiment.

[0095] In short, N subcarrier detectors 24 １ ~24 Ｎ Each of the N subcarrier detectors 24 performs digital processing similar to that performed by the receiving DSP 240 in the optical transmitting and receiving device according to the first embodiment. １ ~24 Ｎ The frequency difference compensation unit 247 in each １ ~247 Ｎ compensates for the frequency difference between the CW light from the CW light generating unit 140 and the interference light from the interference light generating unit 220 in the optical transmitter 100 for each of the plurality of different subcarrier signals SC#1 to SC#N. １ ~24 Ｎ Thus, N data signals are output from the receiving DSP 240A.

[0096] The hardware configuration of the optical transceiver of embodiment 2 is similar to the hardware configuration of the optical transceiver of embodiment 1, in that the receiver control unit 30 is composed of a processor 31 such as a CPU or system LSI, a memory 32 composed of RAM, ROM, etc., a communication interface 33, and an input / output interface 34.

[0097] As described above, the optical transmission / reception device of embodiment 2 is an optical transmission / reception device in which the optical transmitter 100 outputs modulated light that is frequency-multiplexed using subcarrier signals SC#1 to SC#N as N optical subcarriers, and the optical receiver 200 collectively receives polarization-multiplexed modulated light in which the N subcarrier signals SC#1 to SC#N are frequency-multiplexed on the frequency axis, and can compensate for the difference in frequency of the optical carrier of the interference light in the optical receiver 200 relative to the frequency of the optical carrier from the optical transmitter 100 for each subcarrier signal SC#1 to SC#N, as in the optical transmission / reception device of embodiment 1.

[0098] It should be noted that the embodiments may be freely combined, or any of the components in each embodiment may be modified, or any of the components in each embodiment may be omitted.

[0099] The optical transceiver according to the present disclosure is not limited to communication systems that use optical fiber as a transmission medium in optical access and optical core / metro networks, but is also applicable to optical communication systems that do not use optical fiber, such as space or free-space optical communications.

[0100] 100 Optical transmitter, 110, 110A Modulation signal generation unit (transmission side DSP), 11 １ ~11 Ｎ Subcarrier generation unit, 111, 111 １ ~111 Ｎ Frame generation unit (framer unit), 112, 112 １ ~112 Ｎ Mapping unit, 113, 113 １ ~113 Ｎ Spectral shaping unit, 114 Skew adjustment unit (deskew unit), 115, 115 １ ~115 Ｎ Pilot generation unit, 116, 116 １ ~116 Ｎ Pilot insertion unit, 117 subcarrier multiplexing unit, 120 digital / analog conversion (DAC) unit, 130 optical modulation unit, 140 CW light generation unit, 200 optical receiver, 210 optical coherent detection unit, 220 interference light generation unit, 230 ADC unit, 240, 240A receiving side digital signal processing unit (receiving side DSP), 24 １ ~24 ＮSubcarrier detection unit, 241 Skew adjustment unit, 242, 242 １ ~242 Ｎ Amplitude adjustment section, 243, 243 １ ~243 Ｎ Dispersion compensation section, 244, 244 １ ~244 Ｎ Clock synchronization unit, 245, 245 １ ~245 Ｎ Bandwidth compensation unit, 246, 246 １ ~246 Ｎ Adaptive equalization unit, 247, 247 １ ~247 Ｎ Frequency difference compensation unit, 248, 248 １ ~248 Ｎ Phase estimation unit, 249, 249 １ ~249 Ｎ Demapping unit, 250, 250 １ ~250 Ｎ Frame synchronization unit, 251, 251 １ ~251 Ｎ First pilot reading unit, 252, 252 １ ~252 Ｎ Second pilot reading unit, 253, 253 １ ~253 Ｎ Pilot removal section.

Claims

1. An optical transmitter and an optical receiver, the optical transmitter comprising: a modulation signal generation unit that generates a modulation signal, which 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 / analog conversion unit that converts the modulation signal, which is a digital signal generated by the modulation signal generation unit, into a modulation signal consisting of an analog signal; and an optical modulation unit that modulates CW light from a CW light generation unit based on the modulation signal converted into an analog signal by the digital / analog conversion unit to generate modulated light in which the first pilot symbol signal having the first symbol period and the second pilot symbol signal having the second symbol period are inserted; the optical receiver comprising: an optical coherent detection unit that receives polarization multiplexed modulated light in 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 detects the received modulated light, and outputs an analog electrical signal; an analog / digital conversion unit that performs analog / digital conversion on an analog electrical signal from the optical coherent detection unit and outputs a digital signal; and a receiving-side digital signal processing unit having a frequency difference compensation unit that receives the digital signal from the analog / digital conversion unit and compensates for a frequency of the received digital signal by calculating a phase difference using a first pilot symbol signal having the first symbol period and a 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 being extracted from the received digital signal and inserted into the received polarization multiplexed modulated light.

2. The optical transmission / reception device according to claim 1, wherein the optical modulation unit generates modulated light using a single carrier signal as an optical carrier, the optical coherent detection unit converts the modulated light using the single carrier signal as the optical carrier into an analog electrical signal, and the frequency difference compensation unit in the receiving digital signal processing unit compensates for the frequency of the digital signal with respect to the single carrier signal.

3. The optical transmission / reception device according to claim 1, wherein the optical modulation unit generates modulated light which is frequency-multiplexed using subcarrier signals of a plurality of different frequency allocations as optical subcarriers, the optical coherent detection unit converts the modulated light which is frequency-multiplexed using subcarrier signals of a plurality of different frequency allocations as optical subcarriers into an analog electrical signal, and a frequency difference compensation unit in the receiving digital signal processing unit compensates the frequency of the digital signal for each of the subcarrier signals of the plurality of different frequency allocations.

4. An optical transmitting / receiving device according to any one of claims 1 to 3, wherein the modulated light generated by the optical modulation section is an optical signal in which the X-polarized wave and the Y-polarized wave are modulated into an I signal and a Q signal in quadrature phase, respectively.

5. An optical transmission / reception device according to any one of claims 1 to 3, wherein the first pilot symbol signal and the second pilot symbol signal are QPSK signals.

6. An optical transmission / reception device according to any one of claims 1 to 3, wherein the second symbol period of the second pilot symbol signal is an integer multiple of two or more of the first symbol period of the first pilot symbol signal.

7. An optical receiver comprising: an optical coherent detection unit that receives 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, and coherently detects the received modulated light and outputs an analog electrical signal; an analog / digital conversion unit that performs analog / digital conversion on the analog electrical signal from the optical coherent detection unit and outputs a digital signal; and a receiving-side digital signal processing unit that receives the digital signal from the analog / digital conversion unit and calculates 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 that are extracted from the received digital signal and inserted into the received polarization multiplexed modulated light, thereby compensating for a frequency of the received digital signal.

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