Optical receiver and optical receiving method

Abstract

To provide an optical receiver and an optical receiving method that suppress degradation of the transmission characteristics of optical signals.SOLUTION: An optical receiver includes a dispersion compensation circuit that compensates for chromatic dispersion in an optical transmission path for an electrical signal corresponding to an optical signal received via the optical transmission path, an adaptive equalization circuit that adaptively compensates for residual chromatic dispersion that remains in the electrical signal after compensation by the dispersion compensation circuit due to insufficient compensation in the dispersion compensation circuit, and a monitor circuit that monitors a dispersion slope of the residual chromatic dispersion on the basis of a tap coefficient of the adaptive equalization circuit, and the dispersion compensation circuit compensates for the chromatic dispersion on the basis of a monitored value of the dispersion slope.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] This invention relates to an optical receiver and an optical receiving method.

Background Art

[0002] In a wavelength division multiplexing optical transmission system of 40 Gbit / s or more, a system that performs dispersion compensation collectively for an accommodation wavelength band is known. In recent years, it has also become possible to compensate for wavelength dispersion accumulated in an optical receiver by digital signal processing technology. In a wavelength division multiplexing optical transmission system, the transmission distance and transmission capacity are greatly limited by the wavelength dispersion slope (hereinafter simply referred to as the dispersion slope), which is the higher-order dispersion of the wavelength dispersion of an optical fiber. Therefore, it is important to accurately grasp the dispersion value and dispersion slope of an optical transmission line and perform dispersion compensation including the dispersion slope (see, for example, Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Various types of optical transmission lines may be mixed between an optical transmitter and an optical receiver included in a wavelength division multiplexing optical transmission system. For example, optical fibers such as ELEAF (Enhanced Large Effective Area Fiber) and SMF (Single-Mode Fiber) are mixed as optical transmission lines between the optical transmitter and the optical receiver. In addition, optical transmission lines of various lengths are also mixed between the optical transmitter and the optical receiver.

[0005] The type and length of the optical transmission path described above are not always known and may be partially unknown. When the type and length of the optical transmission path are unknown, it is difficult for the optical receiver to accurately determine the dispersion value and dispersion slope of the optical transmission path. As a result, it becomes difficult for the optical receiver to perform high-precision dispersion compensation, including the dispersion slope, which leads to a problem of degraded optical signal transmission characteristics.

[0006] Therefore, one objective is to provide an optical receiver and optical receiving method that suppress the degradation of optical signal transmission characteristics. [Means for solving the problem]

[0007] In one embodiment, the optical receiver includes a dispersion compensation circuit that compensates for the chromatic dispersion of the optical transmission path with respect to an electrical signal corresponding to an optical signal received via the optical transmission path, an adaptive equalization circuit that adaptively compensates for residual chromatic dispersion remaining due to insufficient compensation by the dispersion compensation circuit with respect to the electrical signal after compensation by the dispersion compensation circuit, and based on the tap coefficient of the adaptive equalization circuit, The aforementioned residual wavelength dispersion and, The dispersion slope of the residual wavelength dispersion and The circuit comprises a monitoring circuit that monitors the dispersion compensation circuit, The monitoring value of the residual wavelength dispersion and, Monitor value of the aforementioned distributed slope and Based on this, the wavelength dispersion is compensated. The monitoring circuit calculates the residual wavelength dispersion and the dispersion slope by fitting the relationship between the monitored value of the residual wavelength dispersion and the difference from the center wavelength of the optical signal to a linear function. do. [Effects of the Invention]

[0008] This can suppress the degradation of the transmission characteristics of optical signals. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 shows an example of a wavelength division multiplexing optical transmission system. [Figure 2] Figure 2 shows an example of a block diagram of an optical receiver. [Figure 3] Figure 3 is an example of a block diagram of a DSP (Digital Signal Processor). [Figure 4] Figure 4 is a graph illustrating an example of transmission performance degradation caused by distributed slope. [Figure 5] Figs. 5(a) to (c) are graphs showing monitor values for each transmission distance of residual wavelength dispersion. [Figure 6] Fig. 6 is a diagram for explaining the range of a signal band. [Figure 7] Fig. 7 is a diagram for explaining the determination of the range of a signal band. [Figure 8] Fig. 8(a) is a graph showing an example of a monitor value of residual wavelength dispersion. Fig. 8(b) is a graph showing another example of a monitor value of residual wavelength dispersion. [Figure 9] Fig. 9 is a flowchart showing an example of the processing of a monitor circuit. [Figure 10] Fig. 10(a) is a graph showing another example of a monitor value of residual wavelength dispersion at a high bit rate. Fig. 10(b) is a graph showing another example of a monitor value of residual wavelength dispersion at a low bit rate. [Figure 11] Fig. 11(a) is a diagram for explaining an example of splitting an optical signal when subcarrier modulation is applied. Fig. 11(b) is a diagram showing an example of calculating a dispersion slope when subcarrier modulation is applied.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments for carrying out the present case will be described with reference to the drawings.

[0011] As shown in Fig. 1, the wavelength division multiplexing optical transmission system ST includes an optical transmitter 100 and an optical receiver 200. The optical receiver 200 receives the optical signal O(t) transmitted from the optical transmitter 100. The optical signal O(t) is a signal output from the optical transmitter 100 and can be represented as a function of time t. In Fig. 1, a wavelength division multiplexing optical transmission system ST including one optical transmitter 100 and one optical receiver 200 is shown, but there are also cases where different wavelengths are multiplexed for optical transmission. In this case, the wavelength division multiplexing optical transmission system ST includes a plurality of optical transmitters and a plurality of optical receivers, demultiplexes the optical signal O(t) into optical signals for each wavelength, and multiplexes the optical signals for each wavelength to generate the optical signal O(t). The optical transmitter 100 and the optical receiver 200 are connected by various types of optical fibers 51, 52, 53, 54 and a plurality of optical amplifiers 55, 56, 57, 58. The optical fibers 51, 52, 53, 54 are an example of an optical transmission path. For example, the optical fiber 51 is an SMF. The optical fiber 52 is an ELEAF. Both of the optical fibers 53, 54 are unspecified optical fibers whose types are not clear.

[0012] Also, optical fibers 51, 52, 53, 54 of various lengths are used for connecting the optical transmitter 100 and the optical receiver 200. For example, the length L1 of the optical fiber 53 is 80 km (kilometer). The length L2 of the optical fiber 54 is 30 km. On the other hand, neither the length L3 of the optical fiber 51 nor the length L4 of the optical fiber 52 is known. Thus, in the wavelength division multiplexing optical transmission system ST, there may be cases where the types of the optical fibers 53, 54 are not known, and there may also be cases where the lengths L3 of the optical fiber 51 and the length L4 of the optical fiber 52 are not known.

[0013] Next, referring to Fig. 2, the details of the optical receiver 200 will be described.

[0014] The optical receiver 200 includes a DSP 210, an ADC (Analogue Digital Converter) 220, an ICR (Integrated Coherent Receiver) 230, an ITLA (Integrable Tunable Laser Assembly) 240, and a control unit 250.

[0015] The ICR230 receives an optical signal O(t) transmitted from the optical transmitter 100. The ICR230 includes a polarization beam splitter and an optical-to-electrical converter. The ICR230 separates the optical signal O(t) into H-polarization and V-polarization components, mixes them with the local light emitted from the ITLA240 to extract information corresponding to the optical signal O(t), and then converts it into an electrical signal (specifically, an electric field signal) E(t) and outputs it to the ADC220. The electrical signal E(t) converted from the optical signal O(t) can be expressed as a function of time t.

[0016] The ADC220 converts the electrical signal E(t) from analog to digital and outputs it to the DSP210. As will be described in detail later, the DSP210 performs various digital signal processing on the electrical signal E(t), such as compensating for the chromatic dispersion of optical fibers 51, 52, 53, and 54, and outputs the result. The control unit 250 includes a processor and memory and controls the operation of the DSP210, ICR230, and ITLA240.

[0017] Refer to Figure 3 for a detailed explanation of the DSP210.

[0018] The DSP210 includes a dispersion compensation circuit 211, an adaptive equalization circuit 212, a monitor circuit 213, and a frequency offset compensation circuit 214. The DSP210 also includes a carrier phase estimation circuit 215, an FEC (Forward Error Correction) decoding circuit 216, and a deframer circuit 217. Alternatively, instead of the DSP210 having the monitor circuit 213, the control unit 250 described above may have the monitor circuit 213. In this case, the control unit 250 should perform processing according to the flowchart described later.

[0019] The dispersion compensation circuit 211 fixedly and collectively compensates for the accumulated chromatic dispersion of the optical fibers 51, 52, 53, and 54 in relation to the electrical signal E(t) input from the ADC 220. For example, the dispersion compensation circuit 211 performs an FFT (Fast Fourier Transform) on the input electrical signal E(t) to convert it into a frequency domain signal. Next, the dispersion compensation circuit 211 fixedly compensates for chromatic dispersion by multiplying the frequency domain signal by the inverse transfer function H(fn) input from the monitor circuit 213 as a dispersion compensation coefficient. Then, the dispersion compensation circuit 211 performs an IFFT (Inverse FFT) on the compensated frequency domain signal to compensate for waveform distortion due to chromatic dispersion of the received electrical signal E(t), which is expressed as a function of time t, and outputs it to the adaptive equalization circuit 212.

[0020] The inverse transfer function H(fn) can be expressed by the following equation (1). The second-order wavelength dispersion is the term related to the dispersion slope. <Formula (1)> H(fn) = exp[j*{1 / 2*(2π*fn)] 2 *C*1 Next wavelength dispersion / (2π*Fc 2 ) + 1 / 6 * (2π * fn) 3 *C*(C / Fc*2nd order wavelength dispersion + 2*1st order wavelength dispersion) / ((2π) 2 *Fc 3 )}] C: Speed ​​of light Fc: Signal frequency First-order chromatic dispersion: The reciprocal of the cumulative chromatic dispersion of optical fibers 51, 52, 53, and 54 (ps / nm). Second-order chromatic dispersion: The reciprocal of the cumulative second-order chromatic dispersion of optical fibers 51, 52, 53, and 54 (ps / nm) 2 ) fn = n * sampling rate / N_FFT (where n is an integer, from 0 to N_FFT / 2, and from -(N_FFT / 2)+1 to -1) j: Imaginary unit

[0021] The adaptive equalization circuit 212 adaptively compensates for waveform distortion in accordance with the distortion mainly caused by polarization fluctuations and polarization mode dispersion in the optical fibers 51, 52, 53, and 54. The adaptive equalization circuit 212 may also include a digital filter circuit, such as an FIR (Finite Impulse Response) filter.

[0022] Furthermore, the adaptive equalization circuit 212 adaptively compensates for residual chromatic dispersion remaining due to insufficient compensation by the dispersion compensation circuit 211, in addition to the two complex time series of electrical signals E(t) after chromatic dispersion compensation by the dispersion compensation circuit 211. The dispersion compensation circuit 211 performs dispersion compensation based on information about the amount of chromatic dispersion of the transmission line and its monitoring, but if this information is uncertain, dispersion compensation may not be fully performed. For this reason, the adaptive equalization circuit 212 adaptively compensates for residual chromatic dispersion in the compensated electrical signal E(t).

[0023] The adaptive equalization circuit 212 compensates for distortion and residual wavelength dispersion caused by polarization fluctuations and polarization mode dispersion, and outputs the compensated electrical signal E(t) to the frequency offset compensation circuit 214. The frequency offset compensation circuit 214 compensates for the frequency shift (offset) between the optical signal O(t) and the local emission based on the compensated electrical signal E(t), and outputs it to the carrier phase estimation circuit 215. The carrier phase estimation circuit 215 estimates the correct carrier phase from the compensated electrical signal E(t) and restores the carrier phase. The carrier phase estimation circuit 215 restores the transmitted signal from the estimated carrier phase and outputs the restored electrical signal E(t) to the FEC decoding circuit 216.

[0024] The FEC decoding circuit 216 errors-corrects and decodes the electrical signal E(t) based on an error correction code added to the optical signal O(t) by digital signal processing in the optical transmitter 100, for example. The deframer circuit 217 performs deframer processing on the electrical signal E(t). Deframer processing is the process of demapping the client signal that is mapped to the frame of the electrical signal E(t). The client signal may be an Ethernet® frame signal, or an SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical NETwork) frame signal.

[0025] Here, the monitor circuit 213 estimates and monitors the residual chromatic dispersion based on the tap coefficients Hxx, Hxy, Hyx, Hyy of the adaptive equalization circuit 212, and calculates the residual chromatic dispersion and the dispersion slope, which is the wavelength derivative of the residual chromatic dispersion, based on the monitored value of the residual chromatic dispersion. More specifically, the residual dispersion monitor 213A of the monitor circuit 213 estimates and monitors the residual chromatic dispersion, and calculates and determines the residual chromatic dispersion based on the monitored value of the residual chromatic dispersion. Then, the dispersion slope monitor 213B of the monitor circuit 213 calculates and determines the dispersion slope based on the monitored value of the residual chromatic dispersion. Once the residual chromatic dispersion and dispersion slope are determined, the monitor circuit 213 takes the residual chromatic dispersion as the first-order chromatic dispersion and the dispersion slope as the second-order chromatic dispersion, and calculates the inverse transfer function H(fn) based on the above formula (1).

[0026] Furthermore, the estimation of residual wavelength dispersion using the residual dispersion monitor 213A is disclosed in the following literature, for example. On the other hand, the calculation of dispersion slope using the dispersion slope monitor 213B is not disclosed in the following literature and is not publicly known. (1)Md. Saifuddin Faruk,et al, “Multi-Impairments Monitoring from the Equalizer in a Digital Coherent Optical Receiver”, ECOC 2010, Th.10.A.1, 19-23 September, 2010, Torino, Italy (2) Gabriella Bosco, et al, “Joint DGD, PDL and Chromatic Dispersion Estimation in Ultra-Long-Haul WDM Transmission Experiments with Coherent Receivers”, ECOC 2010, Th.10.A.2, 19-23 September, 2010, Torino, Italy

[0027] The monitor circuit 213 calculates the inverse transfer function H(fn) and inputs it to the dispersion compensation circuit 211 as the dispersion compensation coefficient. This allows the dispersion compensation circuit 211 to compensate for chromatic dispersion by considering not only first-order chromatic dispersion such as residual chromatic dispersion, but also higher-order (specifically second-order) chromatic dispersion such as dispersion slope.

[0028] Now, referring to Figure 4, we will explain the degradation of the transmission characteristics of the optical signal O(t) caused by the dispersion slope.

[0029] It is known that the chromatic dispersion of optical fibers 51, 52, 53, and 54 is frequency-dependent, and this frequency dependence can be represented by the dispersion slope. When the optical receiver 200 receives an optical signal O(t) with a low baud rate such as 32 Gbaud or 64 Gbaud, the signal bandwidth range of the optical signal O(t) is narrow, so the chromatic dispersion is considered to be constant within the signal bandwidth range. Therefore, the effect of the dispersion slope on the degradation of the transmission characteristics of the optical signal O(t) is assumed to be small and can be ignored.

[0030] On the other hand, when the optical receiver 200 receives an optical signal O(t) with a high baud rate exceeding 100 Gbaud, the signal bandwidth range of the optical signal O(t) is wide, so the chromatic dispersion within the signal bandwidth range is not constant and is expected to vary. For this reason, it is assumed that the effect of the dispersion slope on the degradation of the transmission characteristics of the optical signal O(t) is significant. For example, as shown in Figure 4, when the optical receiver 200 receives a 130 Gbaud optical signal O(t) modulated with the 16QAM optical modulation scheme, a large difference in the transmission characteristics of the optical signal O(t) appears between the optical fiber 51 and the optical fiber 52.

[0031] To explain in more detail, graph G1, which represents the transmission characteristics of optical fiber 51 (SMF), shows that the SNR (Signal to Noise Ratio) penalty increases gradually with increasing transmission distance. In other words, graph G1 has a gentle slope. Thus, with optical fiber 51, even if the transmission distance increases, the effect of the dispersion slope, which is the wavelength derivative of chromatic dispersion, is small, so the SNR penalty is small and the deterioration of transmission characteristics is expected to be small.

[0032] On the other hand, graph G2, which represents the transmission characteristics of optical fiber 52 (ELEAF), shows that the SNR penalty increases sharply in the ultra-long distance region as the transmission distance increases. In other words, graph G2 has a steep slope. Thus, with optical fiber 52, as the transmission distance increases, the effect of the dispersion slope is greater, so the SNR penalty is expected to be large, and the deterioration of transmission characteristics will be significant. For example, when the transmission distance is around 6000 km, with optical fiber 51, the transmission characteristics will decrease by about 1.5 dB due to the dispersion slope, but with optical fiber 52, the transmission characteristics will decrease by more than 3.0 dB due to the dispersion slope.

[0033] If the type and length of optical fibers 51 and 52 are known, the dispersion slope can be calculated accurately based on the fiber specifications, and as a result, compensation for the dispersion slope can be prepared and implemented in advance. However, if optical fibers 53 and 54, whose type and length are unknown, are included in the connection between the optical transmitter 100 and the optical receiver 200, the dispersion slope is also uncertain and cannot be calculated accurately. Therefore, as a result, it is difficult to prepare and implement compensation for the dispersion slope in advance.

[0034] In this embodiment, when the connection between the optical transmitter 100 and the optical receiver 200 includes, for example, optical fibers 53 and 54 of unknown type, or optical fibers 51 and 52 of unknown length, dispersion compensation is realized that takes into account the case where both the accumulated residual wavelength dispersion and dispersion slope are undefined.

[0035] Referring to Figure 5, the relationship between residual chromatic dispersion and dispersion slope monitored by the monitor circuit 213 for each transmission distance will be explained. Figures 5(a) to (c) show the relationship between the difference from the center wavelength of the optical signal O(t) and the residual chromatic dispersion when the optical transmitter 100 and the optical receiver 200 are connected by an optical fiber 52, and the first-order chromatic dispersion is compensated by the dispersion compensation circuit 211.

[0036] First, as shown in Figure 5(a), when the transmission distance is 0 km, the variation in residual wavelength dispersion is small and constant around 0 ps / nm for the difference from the center wavelength between -0.3 nm (nanometers) and 0.3 nm. In other words, when the transmission distance is 0 km, the gradient of the dispersion slope DS1 is almost zero, indicating that the frequency dependence of residual wavelength dispersion is absent or minimal.

[0037] Next, as shown in Figure 5(b), when the transmission distance is 3200 km, there is variation in residual wavelength dispersion between -0.3 nm and 0.3 nm from the center wavelength. Specifically, it is around -100 ps / nm near -0.3 nm and around 100 ps / nm near 0.3 nm. Thus, when the transmission distance is 3200 km, the dispersion slope DS2 slopes upward from the dispersion slope DS1, and a larger gradient is generated in the dispersion slope DS2 than in the dispersion slope DS1. In other words, it can be seen that the frequency dependence of residual wavelength dispersion increases as the transmission distance increases.

[0038] Furthermore, as shown in Figure 5(c), when the transmission distance is 4800 km, there is variation in residual wavelength dispersion between -0.3 nm and 0.3 nm from the center wavelength. Specifically, it is around -100 ps / nm near -0.3 nm, but around 190 ps / nm near 0.3 nm. Thus, when the transmission distance is 4800 km, the dispersion slope DS3 slopes even more steeply to the right than the dispersion slope DS2, resulting in a larger gradient in the dispersion slope DS3 than in the dispersion slope DS2. In other words, it can be seen that the frequency dependence of residual wavelength dispersion increases further as the transmission distance increases.

[0039] Next, the estimation method for the distributed slope according to this embodiment will be described with reference to Figures 6 and 7.

[0040] As described above, residual chromatic dispersion is estimated and monitored by the residual dispersion monitor 213A. Therefore, as shown in Figure 6, the dispersion slope monitor 213B estimates the dispersion slope DS2 using the line graph G3 representing the monitored value of residual chromatic dispersion. Here, outside the signal bandwidth SB limit L of the optical signal O(t), the residual dispersion monitor 213A performs out-of-band noise suppression, which reduces the accuracy of the monitored value. Even inside the signal bandwidth SB limit L, the variation in residual chromatic dispersion increases due to the effects of monitoring accuracy and frequency resolution.

[0041] The dispersion slope monitor 213B estimates the dispersion slope DS2 by approximating the line graph G3 of the residual wavelength dispersion monitor values, which show greater variability inside the signal bandwidth limit L of the signal bandwidth, with a linear function. The approximation of the line graph G3 with a linear function can be done using methods such as the least squares method. For example, if the linear function is expressed as y = ax + b, the coefficient a can be associated with the dispersion slope and the coefficient b can be associated with the residual wavelength dispersion. In this way, the dispersion slope monitor 213B calculates and determines the coefficient a corresponding to the dispersion slope and the coefficient b corresponding to the residual wavelength dispersion.

[0042] Furthermore, the distributed slope monitor 213B can determine the range of the signal bandwidth SB that approximates the line graph G3 as a linear function, for example, based on the monitored value of the transmission path bandwidth, which is the bandwidth of the transceiver or transmission path. Specifically, the distributed slope monitor 213B calculates the monitored value M(f) of the transmission path bandwidth based on the following formula (2). <Formula (2)> M(f)=Hxx(f)*Hyy(f)-Hyx(f)*Hxy(f) Hxx(f), Hyy(f), Hyx(f), and Hxy(f) represent the inverse transfer functions of the tap coefficients Hxx, Hyy, Hyx, and Hxy, respectively.

[0043] Once the monitor value M(f) is calculated, the distributed slope monitor 213B calculates the inverse characteristic P(f) of the transmission path bandwidth based on the following formula (3). <Formula (3)> P(f) = 10 * log(abs(M(f)))

[0044] When the inverse characteristic P(f) of the transmission line bandwidth is calculated, the dispersion slope monitor 213B determines the range of the signal band SB as the range in which P(f) is constant, as shown in Figure 7. This is because this range often corresponds to a linear change in the monitored value of residual wavelength dispersion. Alternatively, the dispersion slope monitor 213B may also determine the range of the signal band SB as the interval between the inflection points where P(f) is convex upwards. This is because it is closer to the actual range of the signal band SB. In this way, the dispersion slope monitor 213B determines the range of the signal band SB, approximates the monitored value of residual wavelength dispersion within that range to a linear function, and calculates and determines the residual wavelength dispersion and dispersion slope based on that linear function.

[0045] Next, the effect of first-order residual wavelength dispersion will be explained with reference to Figures 8 and 9.

[0046] First, as shown in Figure 8(a), compensation by the dispersion compensation circuit 211 can result in a very small coefficient b, such as coefficient b1 (e.g., 20 ps / nm), at a difference of 0 nm from the center wavelength. In other words, sufficient compensation is performed by the dispersion compensation circuit 211, resulting in almost no first-order residual wavelength dispersion, sometimes approaching 0 ps / nm. In this state, where the first-order residual wavelength dispersion is close to 0 ps / nm, the dispersion slope monitor 213B can accurately calculate the dispersion slope DS2.

[0047] However, as shown in Figure 8(b), compensation by the dispersion compensation circuit 211 may result in a large coefficient b (e.g., 100 ps / nm) at which the difference from the center wavelength is 0 nm, where the coefficient b corresponding to the first-order residual wavelength dispersion is larger than the coefficient b1. In other words, if the dispersion compensation circuit 211 does not adequately compensate for the first-order residual wavelength dispersion, the first-order residual wavelength dispersion may increase and deviate significantly from 0 ps / nm. In this state, where the residual wavelength dispersion is far from 0 ps / nm, the dispersion slope monitor 213B may not be able to accurately calculate the dispersion slope DS2.

[0048] Therefore, as shown in Figure 9, in the monitor circuit 213, first, the residual dispersion monitor 213A independently feeds back the monitor value of the first-order residual wavelength dispersion to the dispersion compensation circuit 211 (step S1). Then, the residual dispersion monitor 213A determines whether the monitor value of the first-order residual wavelength dispersion of the compensated electrical signal E(t) restored by the dispersion compensation circuit 211 is below a threshold for determining low residual wavelength dispersion (step S2). For example, the threshold can be 20 ps / nm as described above. If the monitor value of the first-order residual wavelength dispersion is not below the threshold (step S2: NO), the residual dispersion monitor 213A executes the process of step S1 again.

[0049] In this way, by repeating the processes of step S1 and step S2 until the monitoring value of the first-order residual wavelength dispersion falls below the threshold, the first-order residual wavelength dispersion converges below the threshold, and the decrease in the accuracy of the dispersion slope calculation can be suppressed.

[0050] When the monitoring value of the primary residual wavelength dispersion falls below a threshold (Step S2: YES), the residual dispersion monitor 213A and the dispersion slope monitor 213B feed back the monitoring values ​​of the primary residual wavelength dispersion and dispersion slope, respectively, to the dispersion compensation circuit 211 (Step S3). Then, the residual dispersion monitor 213A determines whether the monitoring value of the primary residual wavelength dispersion of the electrical signal E(t) after wavelength dispersion compensation of the transmission line by the dispersion compensation circuit 211 is below the aforementioned threshold, and whether the monitoring value of the dispersion slope is below another threshold (Step S4). This other threshold is a threshold for determining the smallness of the dispersion slope, for example, 5 ps / nm 2 or 10 ps / nm 2 You can adopt such methods.

[0051] If the monitoring value of the primary residual wavelength dispersion is not below a threshold, or if the monitoring value of the dispersion slope is not below another threshold (step S4: NO), the residual dispersion monitor 213A and the dispersion slope monitor 213B each execute the process in step S3 again. Since the processes in steps S3 and S4 are repeated after it has been determined that the monitoring value of the primary residual wavelength dispersion has become below a threshold through the processes in steps S1 and S2, the dispersion slope monitor 213B can calculate the dispersion slope with high accuracy. When the monitoring value of the primary residual wavelength dispersion becomes below a threshold, and the monitoring value of the dispersion slope becomes below another threshold (step S4: YES), the residual dispersion monitor 213A and the dispersion slope monitor 213B each terminate their processes.

[0052] Next, the effects of low baud rates will be explained with reference to Figures 10 and 11.

[0053] In the embodiments described above, as shown in Figure 10(a), the monitor value of residual chromatic dispersion was explained for high baud rates such as 128 Gbaud. On the other hand, as shown in Figure 10(b), for low baud rates such as 32 Gbaud, the signal bandwidth range of residual chromatic dispersion narrows, so the slope of the dispersion slope becomes smaller, and the variation in the monitor value of residual chromatic dispersion with respect to the slope of the dispersion slope becomes larger. This may reduce the accuracy of calculating the dispersion slope.

[0054] Such low baud rates occur when subcarrier modulation is applied to the transmission of an optical signal O(t). For example, as shown in Figure 11(a), when subcarrier modulation is applied to the transmission of a single-carrier 128 Gbaud optical signal O(t), the optical transmitter 100 divides this optical signal O(t) into four parts and transmits four 32 Gbaud optical signals O(t) using the FDM (Frequency Division Multiplex) method. Thus, when subcarrier modulation is applied, the accuracy of calculating the dispersion slope may decrease.

[0055] In such cases, the dispersion slope monitor 213B calculates the dispersion slope based on the monitored values ​​of the first-order residual wavelength dispersion for each subcarrier. For example, as shown in Figure 11(b), the dispersion slope monitor 213B calculates the dispersion slope DS4 from the monitored values ​​of the first-order residual wavelength dispersion of subcarriers SC1, SC2, SC3, and SC4. The least squares method may be used to calculate the dispersion slope. The dispersion slope monitor 213B may also input such a dispersion slope DS4 to the dispersion compensation circuit 211.

[0056] As described above, according to this embodiment, the optical receiver 200 includes a dispersion compensation circuit 211, an adaptive equalization circuit 212, and a monitor circuit 213. The dispersion compensation circuit 211 compensates for the chromatic dispersion of optical fibers 51, 52, 53, and 54 in relation to the electrical signal E(t) corresponding to the optical signal O(t) received via the optical fibers 51, 52, 53, and 54. The adaptive equalization circuit 212 adaptively compensates for the residual chromatic dispersion remaining due to insufficient compensation by the dispersion compensation circuit 211 in relation to the electrical signal E(t) after compensation by the dispersion compensation circuit 211. The monitor circuit 213 monitors the dispersion slope of the residual chromatic dispersion based on the tap coefficient of the adaptive equalization circuit 212. The dispersion compensation circuit 211 then compensates for the chromatic dispersion based on at least the monitored value of the dispersion slope. This makes it possible to suppress a decrease in the transmission characteristics of the optical signal O(t).

[0057] Although preferred embodiments of the present invention have been described in detail above, the present invention is not limited to specific embodiments, and various modifications and changes are possible within the scope of the gist of the invention as described in the claims. [Explanation of Symbols]

[0058] 100 Optical Transmitters 200 Optical Receivers 210 DSP 211 Dispersion compensation circuit 212 Adaptive Equalization Circuit 213 Monitor Circuit 213A Residual Dispersion Monitor 213B Distributed Slope Monitor

Claims

1. A dispersion compensation circuit that compensates for the wavelength dispersion of the optical transmission path with respect to an electrical signal corresponding to an optical signal received via the optical transmission path, An adaptive equalization circuit is provided to adaptively compensate for residual wavelength dispersion remaining due to insufficient compensation by the dispersion compensation circuit in the electrical signal after compensation by the dispersion compensation circuit, The system includes a monitoring circuit that monitors the residual wavelength dispersion and the dispersion slope of the residual wavelength dispersion based on the tap coefficient of the adaptive equalization circuit, The dispersion compensation circuit compensates for the chromatic dispersion based on the monitored value of the residual chromatic dispersion and the monitored value of the dispersion slope. The monitoring circuit calculates the residual wavelength dispersion and the dispersion slope by fitting the relationship between the monitored value of the residual wavelength dispersion and the difference from the center wavelength of the optical signal to a linear function. An optical receiver characterized by the following features.

2. The monitoring circuit determines the range in which the relationship fits the linear function based on the signal bandwidth of the optical transmission path. The optical receiver according to feature 1.

3. The monitoring circuit calculates the monitoring value of the dispersion slope after the monitoring value of the residual wavelength dispersion has been compensated to be below a threshold. The optical receiver according to feature 1.

4. The monitoring circuit calculates the monitoring value of the dispersion slope based on the monitoring value of the residual wavelength dispersion for each subcarrier when subcarrier modulation is applied to the transmission of the optical signal. The optical receiver according to feature 1.

5. For an electrical signal corresponding to an optical signal received via an optical transmission path, the chromatic dispersion of the optical transmission path is compensated. The residual wavelength dispersion remaining due to the insufficient compensation of the wavelength dispersion in the compensated electrical signal is adaptively compensated. Based on the tap coefficients of an adaptive equalization circuit that adaptively compensates for the residual wavelength dispersion, the residual wavelength dispersion and the dispersion slope of the residual wavelength dispersion are monitored. Based on the monitored value of the residual wavelength dispersion and the monitored value of the dispersion slope, the wavelength dispersion is compensated. The residual wavelength dispersion and the dispersion slope are calculated by fitting the relationship between the monitored value of the residual wavelength dispersion and the difference from the center wavelength of the optical signal to a linear function. A method for receiving light characterized by the following features.

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