Optical transmission system, network controller, and optical transmission method

Abstract

In an optical transmission system according to one aspect of the present invention, one or more optical transmitters communicate with one or more optical receivers via an optical transmission path. The optical transmission path comprises a plurality of relay nodes that include optical amplifiers for amplifying optical signals. The one or more optical transmitters transmit optical signals at a plurality of channel frequencies. The one or more optical receivers infer fluctuations in the intensity of optical signals on the optical transmission path for the plurality of channel frequencies, respectively. A network controller acquires inference information items indicating inference results of the fluctuations from the one or more optical receivers, and acquires gain spectra of the optical amplifiers on the optical transmission path on the basis of the acquired plurality of the inference information items.

Description

[Technical Field]

[0001] The present invention relates to an optical transmission system, a network controller, and an optical transmission method. [Background technology]

[0002] In conventional optical transmission systems, relay nodes equipped with optical amplifiers (hereinafter also referred to as "optical amplifiers") such as EDFAs (Erbiull-doped Fiber Amplifiers) and forward / backward Raman amplifiers are inserted along the transmission path to compensate for optical signal loss in the transmission path (optical fiber) (see, for example, Non-Patent Document 1). Such optical amplifiers have characteristics such as gain (amplification amount) and tilt (gain frequency characteristics, gain spectrum), and these characteristics are often measured using dedicated equipment such as OSAs (Optical Spectrum Analyzers) (see, for example, Non-Patent Document 2). In relation to the above, Non-Patent Document 3 describes a method for visualizing optical power at each distance across an entire multi-span link using signal waveforms acquired by a coherent receiver located on the receiver side. Once optical amplifiers are incorporated into a transmission system and put into operation, they are monitored by optical channel monitors (OCMs) or other devices that function as OSAs at relay nodes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 124415 [Non-patent literature]

[0004] [Non-Patent Document 1] T. Sasai, et al., “Simultaneous Detection of Anomaly Points and Fiber types in Multi-span Transmission Links Only by Receiver-side Digital Signal Processing”, OFC2020, Paper Th1F.1. [Non-patent document 2] T. Sasai, et al., “Physics-oriented learning of nonlinear Schrodinger equation: optical fiber loss and dispersion profile identification”, arXiv:2104.05890. [Non-patent document 3] T. Tanimura, et al., “Fiber-Longitudinal Anomaly Position Identification Over Multi-Span Transmission Link Out of Receiver-end Signals”, J. Lightw. Technol., 38(9), 2020. Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional monitoring methods, it is necessary to branch off the signal from the transmission line for measurement, which results in a loss of signal power, which can lead to a deterioration in signal quality.

[0006] In view of the above circumstances, an object of the present invention is to provide a technique that can monitor relay nodes while suppressing degradation of signal quality in an optical transmission system. [Means for solving the problem]

[0007] One aspect of the present invention is an optical transmission system in which one or more optical transmitters and one or more optical receivers communicate via an optical transmission path, the optical transmission path having a plurality of relay nodes each equipped with an optical amplifier for amplifying an optical signal, the one or more optical transmitters transmitting optical signals at a plurality of channel frequencies, the one or more optical receivers estimating fluctuations in the intensity of the optical signal on the optical transmission path for each of the plurality of channel frequencies, acquiring estimation information indicating the estimated results of the fluctuations from the one or more optical receivers, and the optical transmission system having a network controller acquiring the gain spectrum of the optical amplifier on the optical transmission path based on the acquired plurality of pieces of estimation information.

[0008] One aspect of the present invention is a network controller for an optical transmission system in which one or more optical transmitters and one or more optical receivers communicate via an optical transmission path, the optical transmission path having a plurality of relay nodes each equipped with an optical amplifier for amplifying an optical signal, the one or more optical transmitters transmitting optical signals at a plurality of channel frequencies, the one or more optical receivers estimating fluctuations in the intensity of the optical signal on the optical transmission path for each of the plurality of channel frequencies, the network controller acquiring estimation information indicating the estimated results of the fluctuations from the one or more optical receivers, and acquiring the gain spectrum of the optical amplifier on the optical transmission path based on the acquired plurality of pieces of estimation information.

[0009] One aspect of the present invention is an optical transmission method in which, in an optical transmission system in which one or more optical transmitters and one or more optical receivers communicate via an optical transmission path, the optical transmission path amplifies and relays optical signals using optical amplifiers at multiple relay nodes, the one or more optical transmitters transmit optical signals at multiple channel frequencies, the one or more optical receivers estimate fluctuations in the intensity of the optical signals on the optical transmission path for each of the multiple channel frequencies, and a network controller acquires estimation information indicating the estimated results of the fluctuations from the one or more optical receivers and acquires the gain spectrum of the optical amplifiers on the optical transmission path based on the acquired multiple pieces of estimation information. [Effects of the Invention]

[0010] According to the present invention, it is possible to monitor relay nodes while suppressing degradation of signal quality in an optical transmission system. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating a system configuration of an optical transmission system according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of the functional configuration of a digital signal processing unit. [Figure 3] FIG. 2 is a schematic block diagram illustrating the functional configuration of a nonlinear optical compensation unit. [Figure 4] 10 is a flowchart showing the flow of a transmission characteristics estimation process performed by an optical receiver. [Figure 5] FIG. 10 is a diagram illustrating a method for realizing loss distribution estimation. [Figure 6] FIG. 10 is a diagram illustrating a method for realizing variance distribution estimation. [Figure 7] FIG. 10 is a diagram showing a specific example of a signal power profile. [Figure 8] FIG. 10 is a diagram showing the signal power profiles of the optical receivers arranged in order of frequency channels. [Figure 9] 10 is a plot of signal power estimates extracted from each optical receiver's signal power profile for a target node versus the optical receiver's channel frequency. [Figure 10] FIG. 4 is a diagram illustrating an example of an effect achieved by the optical transmission system according to the first embodiment. [Figure 11] FIG. 4 is a diagram illustrating an example of an effect achieved by the optical transmission system according to the first embodiment. [Figure 12] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmission system according to a second embodiment. [Figure 13] FIG. 10 is a diagram illustrating a modified example of the optical transmission system according to the embodiment. [Figure 14] FIG. 10 is a diagram illustrating a modified example of a digital signal processing unit. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] (First embodiment) 1 is a diagram showing an example of the configuration of an optical transmission system 100A in the first embodiment. The optical transmission system 100A includes a plurality of optical transmitters 1A, a plurality of optical receivers 2A, an optical transmission line 3, a multiplexer 4, a demultiplexer 5, and a network controller 6A. The plurality of optical transmitters 1A and the plurality of optical receivers 2A are communicatively connected via the optical transmission line 3. The optical transmission line 3 is formed of an optical fiber F. A relay node 31A including an optical amplifier (hereinafter also referred to as an "optical amplifier") that amplifies an optical signal that is attenuated during propagation is inserted midway along the optical transmission line 3.

[0014] Specifically, the optical transmission system 100A communicates using wavelength division multiplexing (WDM) using multiple optical transmitters 1A and multiple optical receivers 2A. Each of the multiple optical transmitters 1A encodes transmission information provided from an external information source to generate an electrical signal, converts the generated electrical signal into an optical signal, and transmits it to the optical receiver 2A via an optical transmission path 3. The multiple optical transmitters 1A each generate optical signals of different channel frequencies. A multiplexer 4 multiplexes the optical signals output by the multiple optical transmitters 1A and sends the multiplexed optical signals to the optical transmission path 3. A demultiplexer 5 demultiplexes the optical signal propagated through the optical transmission path 3 into optical signals for each channel frequency and outputs each optical signal to the optical receiver 2A according to the channel frequency.

[0015] Each of the optical receivers 2A includes a coherent receiver 21 and a digital signal processor 22. The coherent receiver 21 separates a baseband optical signal into two optical signals with orthogonal polarization planes. These optical signals and local light from a local light source (not shown) are input to a 90° hybrid circuit (not shown), which generates a total of four output lights: one set of output lights caused by in-phase and anti-phase interference between the two lights, and another set of output lights caused by orthogonal (90°) and anti-orthogonal (-90°) interference. These output lights are each converted into analog signals by a photodiode (not shown). The coherent receiver 21 converts these analog signals into digital signals.

[0016] When an optical signal propagates through the optical transmission line 3, the signal waveform is distorted due to a nonlinear optical effect in which the phase of the signal rotates in proportion to the optical power of the signal. The digital signal processing unit 22 receives the digital signal output by the coherent receiver 21 as a received signal and performs nonlinear optical compensation on the received signal.

[0017] Furthermore, the digital signal processing unit 22 estimates the transmission characteristics of the optical transmission line 3 based on the received optical signal and notifies the network controller 6A. Specifically, the digital signal processing unit 22 generates a signal power profile that indicates the intensity distribution of the optical signal in the propagation direction as information indicating the transmission characteristics of the optical transmission line 3. The multiple digital signal processing units 22 each supply the generated signal power profile to the network controller 6A.

[0018] The network controller 6A collects signal power profiles from multiple optical receivers 2A and, based on the collected signal power profiles, acquires gain spectra of amplifiers that amplify propagating optical signals at multiple observation points on the optical transmission line 3. Here, optical amplifiers include not only centralized amplifiers such as EDFAs but also distributed amplifiers such as Raman amplifiers. The gain spectrum is information that represents the frequency characteristics of the gain of an optical amplifier. Since the network controller 6A can acquire gain spectra at multiple observation points on the optical transmission line 3, the optical transmission system 100A can monitor the status of relay nodes 31A arranged on the optical transmission line 3.

[0019] 2 is a diagram showing an example of the functional configuration of the digital signal processing unit 22. The digital signal processing unit 22 includes a nonlinear optical compensation unit 23, an adaptive equalization unit 24, a frequency offset compensation unit 25, a carrier phase noise compensation unit 26, a coefficient update unit 27, and a transmission characteristics estimation unit 28.

[0020] 3 is a schematic block diagram showing the functional configuration of the nonlinear optical compensation unit 23. The nonlinear optical compensation unit 23 includes a plurality of linear compensation units 231-1 to 231-N and a plurality of nonlinear compensation units 232-1 to 232-N. One linear compensation unit 231 and one nonlinear compensation unit 232 form one set that performs linear compensation and nonlinear compensation, and the nonlinear optical compensation unit 23 includes N sets in order to perform N steps of processing using these sets.

[0021] The linear compensation unit 231-1 includes a Fourier transform unit 233-1, a chromatic dispersion compensation unit 234-1, and an inverse Fourier transform unit 235-1. The Fourier transform unit 233-1 performs an FFT on the received signal in the time domain to convert the received signal in the time domain into a received signal in the frequency domain.

[0022] The chromatic dispersion compensator 234-1 compensates for the received signal in the frequency domain by a predetermined value (for example, exp^(-jβ k ω 2)) to perform chromatic dispersion compensation. The symbol "^" means that the value after "^" is a superscript of exp. For example, exp^(-jβ k ω 2 ), then (-jβ k ω 2 ) is a superscript of exp. The same applies to the "^" in the following explanation. At the start of processing, the chromatic dispersion compensation unit 234-1 uses the dispersion coefficient β k The wavelength dispersion compensation is performed using the coefficient update unit 27, and the dispersion coefficient β k Each time is updated, the updated dispersion coefficient β k The wavelength dispersion compensation is performed using

[0023] The inverse Fourier transform unit 235-1 performs IFFT on the signal output from the chromatic dispersion compensator 234-1 to convert the chromatic dispersion compensated received signal into a received signal in the time domain. The nonlinear optical compensation unit 23 applies a predetermined value (for example, exp^(-jφ)) to the signal sequence output from the inverse Fourier transform unit 235. k Specifically, the nonlinear optical compensation unit 23 compensates for the nonlinear optical effect by multiplying the phase rotation amount φ set as an initial value at the start of processing. k The coefficient update unit 27 compensates for the nonlinear optical effect using the phase rotation amount φ k Each time is updated, the phase rotation amount φ k is used to compensate for the nonlinear optical effect.

[0024] The linear compensation unit 231-N performs the same processing as the linear compensation unit 231-1, and the nonlinear compensation unit 232-N performs the same processing as the nonlinear compensation unit 232-1.

[0025] Returning to Fig. 2, the description of the optical receiver 2A will continue. The adaptive equalizer 24 is a functional unit that compensates for distortion that occurs in the waveform of the optical signal in the optical transmission line 3. In other words, the adaptive equalizer 24 is a functional unit that corrects code errors that occur in the optical signal due to inter-symbol interference (inter-symbol interference) in the optical transmission line 3. The adaptive equalizer 24 performs adaptive equalization processing using an FIR filter (finite impulse response filter) in accordance with set tap coefficients.

[0026] The frequency offset compensator 25 performs a process of compensating for the frequency offset on the four digital signals that have been subjected to the adaptive equalization process.

[0027] The carrier phase noise compensator 26 performs a process of compensating for the phase offsets on the four digital signals whose frequency offsets have been compensated for.

[0028] The coefficient update unit 27 updates all coefficients used in the nonlinear optical compensation unit 23 (for example, the dispersion coefficient β k , phase rotation amount φ k In the first embodiment, for example, the coefficient updating unit 27 updates all the coefficients (for example, the dispersion coefficient β k , phase rotation amount φ k The coefficient update unit 27 updates the coefficients in each functional unit of the nonlinear optical compensation unit 23. In the first embodiment, the training signal to be compared with the output signal is a transmission signal converted into an electrical signal.

[0029] The transmission characteristic estimator 28 estimates the transmission characteristic of the optical transmission line 3. For example, the transmission characteristic estimator 28 estimates the optimized phase rotation amount φ k Furthermore, for example, the transmission characteristic estimator 28 estimates the loss distribution using the optimized dispersion coefficient β k The transmission characteristic estimating unit 28 supplies information indicating the estimated loss distribution and dispersion distribution to the network controller 6A as a signal power profile.

[0030] FIG. 4 is a flowchart showing the flow of the transmission characteristic estimation process by the optical receiver 2A in the first embodiment. First, the initial setting before starting the process of FIG. 4 will be described. Compensation coefficients used by functional units other than the nonlinear optical compensation unit 23 provided in the digital signal processing unit 22, such as the adaptive equalization unit 24, frequency offset compensation unit 25, and carrier phase noise compensation unit 26, are estimated in advance, and the estimated compensation coefficients are set. Note that the estimation method may be a conventional technique. Next, all coefficients (for example, the dispersion coefficient β k , phase rotation amount φ k The initial values of all the coefficients used in the nonlinear optical compensation unit 23 are arbitrary. For example, the initial value may be set to 0, or may be set to a known amount of phase rotation φ such as input power. k (Dispersion coefficient β k ) may use that value. The above is the initial setting.

[0031] Once the initial setting is complete, the coherent receiver 21 of the optical receiver 2A receives the optical signal transmitted from the optical transmitter 1A (step S101). The coherent receiver 21 converts the received optical signal into a digital signal and outputs it to the digital signal processing unit 22. The nonlinear optical compensation unit 23 performs nonlinear optical compensation on each digital signal output from the coherent receiver 21 (step S102). Specifically, the Fourier transform unit 233-1 converts the time-domain received signal into a frequency-domain received signal by performing an FFT on the time-domain received signal. The Fourier transform unit 233-1 outputs the frequency-domain received signal to the chromatic dispersion compensation unit 234-1.

[0032] The chromatic dispersion compensation unit 234-1 performs chromatic dispersion compensation on the received signal in the frequency domain. At this time, the chromatic dispersion compensation unit 234-1 compensates for chromatic dispersion using a dispersion coefficient β kThe chromatic dispersion compensator 234-1 outputs the chromatic dispersion compensated received signal to the inverse Fourier transformer 235-1. The inverse Fourier transformer 235-1 converts the chromatic dispersion compensated received signal into a time domain received signal by performing IFFT on the signal output from the chromatic dispersion compensator 234-1. The inverse Fourier transformer 235-1 outputs the time domain received signal to the nonlinear compensator 232-1. The nonlinear compensator 232-1 compensates for the nonlinear optical effect on the time domain received signal. At this time, the nonlinear compensator 232-1 compensates for the phase rotation amount φ set as an initial value. k The nonlinear compensation unit 232-1 outputs the received signal, in which the nonlinear optical effect has been compensated, to the Fourier transform unit 233 at the subsequent stage.

[0033] After the processing of step 102 has been executed for N steps, the adaptive equalization unit 24 performs adaptive equalization processing to compensate for distortion occurring in the waveform of the output signal output from the nonlinear optical compensation unit 23 (step S103). Note that the method of adaptive equalization processing is the same as conventional, and therefore description thereof will be omitted. The adaptive equalization unit 24 outputs the signal after adaptive equalization processing to the frequency offset compensation unit 25.

[0034] The frequency offset compensator 25 performs frequency offset compensation processing to compensate for the frequency offset of the signal output from the adaptive equalizer 24 (step S104). Note that the method of frequency offset compensation processing is the same as conventional, so a description thereof will be omitted. The frequency offset compensator 25 outputs the signal after frequency offset compensation processing to the carrier phase noise compensator 26. The carrier phase noise compensator 26 performs carrier phase compensation processing to compensate for the phase offset of the digital signal whose frequency offset has been compensated (step S105). Note that the method of carrier phase compensation processing is the same as that of the conventional method, and therefore a description thereof will be omitted. The carrier phase noise compensator 26 outputs the signal after carrier phase compensation processing to the coefficient updater 27.

[0035] The coefficient update unit 27 compares the output signal output from the carrier phase noise compensation unit 26 with a training signal acquired in advance, and creates a predetermined evaluation function (step S106). Any evaluation function may be used as the evaluation function. For example, the residual sum of squares shown in the following formula (1) or an equation obtained by adding a regularization term to the residual sum of squares shown in the following formula (2) may be used as the evaluation function.

[0036]

number

[0037]

number

[0038] In equation (1), J represents the evaluation function, and x i represents the i-th (i is an integer equal to or greater than 1) sample of the received signal, and t i represents the correct signal for the i-th sample. In addition, the two terms on the right side of equation (2) are regularization terms. φ k represents the amount of nonlinear phase rotation at the kth step. By adding a regularization term, the estimation accuracy of the loss (estimated) distribution can be improved. Note that the regularization term can be any function as long as it can improve the estimation accuracy of the loss distribution.

[0039] Next, the coefficient update unit 27 updates all coefficients (for example, the dispersion coefficient β k , phase rotation amount φ k The optimization algorithm updates the distribution coefficient β k and the phase rotation amount φ k Alternatively, a method of optimizing each point, or an existing method in the field of machine learning such as backpropagation or steepest descent may be used.

[0040] Dispersion coefficient β k and the phase rotation amount φ kWhen the method of optimizing each point is used, the coefficient update unit 27 performs the following processes (1) to (3). (1) Optimize φ1 so that the evaluation function is minimized (other φ k is fixed) (2) Similarly, from φ2 to φ N Optimize each point (3) Optimization is performed again from φ2, and then from φ1 to φ N Repeat until all coefficients of converge In addition, the optimized φ k The order of the phase rotation φ k The above processes (1) to (3) are also executed.

[0041] When the steepest descent method is used, the coefficient update unit 27 updates all coefficients (for example, the dispersion coefficient β k and the phase rotation amount φ k By using the steepest descent method, the coefficient update unit 27 updates all the dispersion coefficients β k and the phase rotation amount φ k Since the updates can be performed simultaneously, the estimation time can be shortened. Furthermore, there is a possibility of improving the estimation accuracy.

[0042]

number

[0043] In equation (3), μ represents the step size. As shown in equation (3), the β of the evaluation function J k and φ k This requires differentiation by . There are various ways to calculate this differentiation, and any method can be used. For example, this differentiation can be calculated using backpropagation (see, for example, Reference 1), which is often used in the field of machine learning, or numerical differentiation. (Reference 1: R.P. Lippmann, “An introduction to computing with neural nets,” IEEE ASSP Mag., 4(2)1987.)

[0044] The coefficient update unit 27 sets the updated coefficients in the nonlinear optical compensation unit 23. Thereafter, the optical receiver 2A uses the newly set coefficients to repeatedly execute the processes of steps S102 to S107 until the coefficients converge (step S108).

[0045] The transmission characteristic estimation unit 28 estimates all the optimized coefficients (for example, the dispersion coefficient β k , phase rotation amount φ k For example, the transmission characteristic estimation unit 28 obtains the optimized total coefficients (for example, the dispersion coefficient β k , phase rotation amount φ k The transmission characteristic estimation unit 28 may acquire all the optimized coefficients (for example, the dispersion coefficient β k , phase rotation amount φ k Specifically, the transmission characteristic estimator 28 estimates the transmission characteristics by using all the optimized phase rotation amounts φ k (φ1 to φ N ) as shown in FIG. 5 to estimate the loss distribution. In addition, the transmission characteristic estimator 28 estimates the loss distribution by plotting all the optimized dispersion coefficients β k (β1 to β N ) is plotted as shown in Fig. 6 to estimate the dispersion distribution. In Figs. 5 and 6, the horizontal axis Z represents distance. The transmission characteristic estimation unit 28 supplies a signal power profile indicating the estimation results (loss distribution and dispersion distribution) to the network controller 6A.

[0046] The above-described method for generating a signal power profile is merely an example, and is not limited to the above-described method. The signal power profile may be generated by other similar methods as long as they are generated by digital signal processing (see, for example, Patent Document 1, Non-Patent Documents 1, 2, and 3, etc.).

[0047] FIG. 7 shows a specific example of a signal power profile. In FIG. 7, the horizontal axis represents the transmission distance from the optical transmitter 1A, and the vertical axis represents the estimated value of NLPR (Non-Linear Phase Rotation). The optical transmission line 3 at the top of the graph shows an example of the arrangement of a relay node 31A (optical amplifier) and an attenuator 32 in the optical transmission line 3, corresponding to the transmission distance on the horizontal axis. Here, the attenuator 32 is intentionally placed midway along the transmission line to see whether intentional attenuation of the optical signal can be detected. Here, the optical transmission line 3 is assumed to be a 70 km x 4 span transmission line using standard single-mode fiber (SSMF). The solid line in the graph represents the estimated value of NLPR at different attenuation levels, and the dashed line represents the reference signal power profile (normal) obtained by OTDR.

[0048] 7, it can be seen that the signal power profile generated by the above-described digital signal processing makes it possible to accurately detect the amplification of the optical signal by the optical amplifier and the attenuation of the optical signal by the optical fiber and the attenuator 32. More specifically, it can be seen that the start of the peak corresponds to the position of the optical amplifier 31A. The network controller 6A collects the signal power profiles thus obtained from multiple optical receivers 2A with different channel frequencies, and estimates the gain spectrum of the optical transmission line 3 based on the collected multiple signal power profiles.

[0049] More specifically, the network controller 6A estimates the position of each relay node 31A (optical amplifier) from the signal power profile of each optical receiver 2A, and plots the estimated value of optical signal strength (i.e., gain) at the position of each relay node 31A in the frequency direction.

[0050] 8 is a diagram showing the signal power profiles of the optical receivers 2A arranged in order of frequency channels. For example, when a relay node 31A (optical amplifier) is the target, the network controller 6A extracts an estimated value set V1 corresponding to the position of the target relay node 31A (hereinafter referred to as the "target node") from each signal power profile.

[0051] FIG. 9 shows a group of estimated signal power values extracted from the signal power profile of each optical receiver 2A for the target node 31A, as a function of the channel frequencies (f1, f2, ..., f k ,…f K ) is plotted against the signal power and channel frequency. Hereinafter, the information indicating the relationship between the signal power and the channel frequency obtained in this manner will be referred to as the "gain spectrum." The network controller 6A extracts and plots a group of estimates for all the relay nodes 31A arranged in the optical transmission line 3 as target nodes, thereby obtaining the gain spectrum for all the relay nodes 31A arranged in the optical transmission line 3.

[0052] 10 and 11 are diagrams illustrating an example of the effect of the optical transmission system 100A according to the embodiment. Specifically, Fig. 10 and Fig. 11 show the results of an experiment in which a signal power profile and a gain spectrum were acquired using the method of the present embodiment for the first and second spans of the optical transmission system 100A, in which the optical transmission line 3 has a distance of 50 km per span. In this experiment, Raman amplifiers that perform distributed Raman amplification by backward pumping were used for the relay nodes 31A-1 and 31A-2.

[0053] In this experiment, for comparison, relay nodes 31A-1 and 31A-2 were set to different gain spectra for the output gain of the pump light. Specifically, relay node 31A-1 was set to output pump light with a wavelength of 1455 nm, and relay node 31A-2 was set to output pump light with a wavelength of 1440 nm, which is shorter than that of relay node 31A-1. As a result, relay nodes 31A-1 and 31A-2 exhibit different gain spectra. Figure 10 shows a graph of the signal power profile for each channel frequency observed under these conditions.

[0054] Fig. 11 shows the results of estimating the gain spectrum at, for example, the 40 km point and the 90 km point based on the signal power profile estimated as in Fig. 10. In Fig. 11, the curve represents the gain spectrum of the Raman amplifier measured for reference by the OSA, and the dots represent the gain spectrum estimated by the transmission characteristics estimator 28. It can be seen from the estimation results in Fig. 11 that the gain spectrum estimation results by the network controller 6A roughly match the actual measurement values by the OSA.

[0055] The optical transmission system 100A configured as described above includes a plurality of optical transmitters 1A that transmit optical signals at different channel frequencies, a plurality of optical receivers 2A that generate signal power profiles for each channel frequency by digital signal processing of the optical signals received from the optical transmitters 1A, and a network controller 6A that collects signal power profiles of optical signals with different channel frequencies from the plurality of optical receivers 2A and obtains the gain spectrum of the optical amplifier of each relay node 31A based on the collected signal power profiles.

[0056] According to this configuration, the optical transmission system 100A of the embodiment does not need to branch an optical signal from the optical transmission line 3 for measurement, and therefore can monitor relay nodes while suppressing degradation of signal quality. Furthermore, according to this configuration, there is no need for measuring instruments such as an OSA or an OCM for branching an optical signal from the optical transmission line 3 for measurement, and therefore relay nodes can be monitored while suppressing cost increases. Furthermore, according to this configuration, relay nodes can be monitored by digital signal processing of optical signals, and therefore relay nodes can be automatically monitored from remote locations. Furthermore, according to this configuration, individual characteristics of multiple relay nodes 31A (optical amplifiers) in the optical transmission system 100A can be monitored collectively. Furthermore, according to this configuration, digital signal processing of optical signals that are arbitrarily transmitted and received can be used to monitor the characteristics of the relay nodes 31A even during operation.

[0057] (Second embodiment) 12 is a diagram showing a configuration example of an optical transmission system 100B according to the second embodiment. The optical transmission system 100B differs from the optical transmission system 100A according to the first embodiment in that it includes a network controller 6B instead of the network controller 6A and a relay node 31B instead of the relay node 31A. The other configuration of the optical transmission system 100B is the same as that of the optical transmission system 100A. In FIG. 12, functional units similar to those in the first embodiment are denoted by the same reference numerals as in FIG. 1, and description thereof will be omitted.

[0058] The network controller 6B has the same function as the network controller 6A in acquiring the gain spectrum at the position of each relay node 31B, and also has the function of performing feedback control on the relay node 31B at each corresponding position based on each acquired gain spectrum. Here, the network controller 6 is assumed to be communicably connected to each relay node 31B via a control communication network (not shown).

[0059] The relay node 31B includes an optical amplifier 311 and an optical filter 312. The optical amplifier 311 is similar to the optical amplifier included in the relay node 31A. The optical filter 312 receives optical signals and outputs optical signals that satisfy predetermined conditions among the input optical signals.

[0060] Specifically, the network controller 6B performs feedback control to compensate for the gain tilt of each relay node 31B while monitoring the gain spectrum corresponding to each position of the relay node 31B. For example, the network controller 6 controls the pump wavelength or pump power of the optical amplifier 311 for each relay node 31B so that the gain spectrum has a desired shape. For example, the network controller 6 may be configured to control the pump wavelength or pump power of each optical amplifier 311 so that the gain spectrum obtained as an estimated result approaches a preset gain spectrum.

[0061] Furthermore, if the optical filter 312 has a wavelength selection function such as a WSS (Wavelength Selective Switch), the network controller 6 may be configured to control the optical filter 312 so that the output light becomes a frequency-flat WDM signal by applying an inverse function of the obtained gain spectrum to the output gain of the optical filter 312.

[0062] In the optical transmission system 100B configured as above, the network controller 6B controls each relay node 31B by feedback control based on the gain spectrum of the optical amplifier acquired for each relay node 31B. With this configuration, the optical transmission system 100B of the second embodiment can monitor the relay nodes while suppressing degradation of signal quality, and can adjust the output of each relay node 31B based on the observed gain spectrum.

[0063] <Modifications common to the first and second embodiments> Below, we will explain modified examples common to the first and second embodiments. Below, unless there is a particular need to distinguish between them, the alphabetic characters of the reference symbols will be omitted. For example, unless there is a particular need to distinguish between the optical transmission system 100A of the first embodiment and the optical transmission system 100B of the second embodiment, they will be collectively referred to as the "optical transmission system 100."

[0064] [First Modification] 13 is a diagram illustrating a first modified example of the optical transmission system 100 of the embodiment. While the optical transmission system 100 of the above embodiment is configured such that a plurality of optical transmitters 1 transmit optical signals at different channel frequencies, the optical transmission system 100C of the first modified example is different from the optical transmission system 100 of the embodiment in that it includes one optical transmitter 1C, one optical receiver 2C, and a network controller 6C, and the optical transmitter 1C transmits optical signals while sweeping its own channel frequency. The network controller 6C in the first modified example is the same as the network controller 6A of the embodiment.

[0065] In this case, in the optical receiver 2C, the digital signal processing unit 22 acquires a signal power profile for each of the multiple channel frequencies by sweeping the frequency and sequentially generating a signal power profile based on the sequentially received optical signals with different channel frequencies. The optical receiver 2C supplies the generated signal power profile for each of the multiple channel frequencies to the network controller 6C. The network controller 6C acquires the gain spectrum of each relay node 31 based on the signal power profile for each of the multiple channel frequencies supplied from the optical receiver 2C in a manner similar to that of the network controller 6 in the embodiment.

[0066] With this configuration, it is not necessary to prepare as many pairs of optical transmitters and optical receivers as there are channel frequencies, so the gain spectrum estimation method of the embodiment can be applied to small-scale optical transmission systems.

[0067] In addition, if there are multiple pairs of optical transmitters 1C and optical receivers 2C, the gain spectrum may be estimated using any one of the pairs, or the gain spectrum may be estimated using multiple pairs by dividing the frequency band to be swept.

[0068] [Second Modification] In the first modification, the gain of the optical amplifier is estimated using only one channel. In this case, since there are no other channels, the gain of the relay node 31 (optical amplifier) may be different from that during operation when wavelength multiplexing is performed. To avoid this, the optical transmitter 1C and the optical receiver 2C may be configured to estimate the gain spectrum while preparing and transmitting dummy light or another signal at another wavelength using another optical transmitter.

[0069] [Third Modification] In the above embodiment, the optical receiver 2 generates a signal power profile and the network controller 6 acquires a gain spectrum based on the signal power profile. However, the function of generating the signal power profile may be implemented in the network controller 6. Specifically, the digital signal processing unit 22 of the optical receiver 2 may be provided in the network controller 6. In this case, the optical receiver 2 may be configured to supply the optical signal received by the coherent receiver 21 to the network controller 6.

[0070] [Fourth Variation] Fig. 14 is a diagram showing a modified example of the digital signal processing unit 22. The digital signal processing unit 22 may be configured as a digital signal processing unit 22a shown in Fig. 14. The digital signal processing unit 22a includes a chromatic dispersion compensator 29, an adaptive equalizer 24, a frequency offset compensator 25, a carrier phase noise compensator 26, a chromatic dispersion adding unit 30, a nonlinear optical compensator 23, a coefficient updating unit 27, and a transmission characteristics estimator 28. The digital signal processing unit 22a further includes the chromatic dispersion compensator 29 and the chromatic dispersion adding unit 30 in addition to the configuration of the digital signal processing unit 22.

[0071] The functions of the nonlinear optical compensation unit 23, adaptive equalization unit 24, frequency offset compensation unit 25, carrier phase noise compensation unit 26, coefficient update unit 27, and transmission characteristic estimation unit 28 are the same as those in FIG. 2, so a description thereof will be omitted. The chromatic dispersion compensation unit 29 compensates for distortion due to chromatic dispersion occurring in the optical transmission line 3. Specifically, the chromatic dispersion compensation unit 29 compensates for distortion due to chromatic dispersion occurring in the frequency domain with respect to the received signal. k ω 2 )) to the received signal in the frequency domain. The chromatic dispersion adding unit 30 is a functional unit that adds the chromatic dispersion compensated for by the chromatic dispersion compensating unit 29 back to the signal. Specifically, the chromatic dispersion adding unit 30 adds the reciprocal of the value multiplied by the chromatic dispersion compensating unit 29 to the received signal in the frequency domain (for example, exp^(+jβ k ω 2 )) to add chromatic dispersion.

[0072] [Other variations] The coefficient update unit 27 may compare the output signal with the training signal at another location. Specifically, the optical receiver 2 may compare the output signal with the training signal between the adaptive equalization unit 24 and the frequency offset compensation unit 25, between the frequency offset compensation unit 25 and the carrier phase noise compensation unit 26, or between the carrier phase noise compensation unit 26 and the coefficient update unit 27. Note that, in the embodiment, when the output signal is compared with the training signal between the carrier phase noise compensation unit 26 and the coefficient update unit 27, the digital signal processing unit 22 needs to additionally include a demodulation unit (not shown).

[0073] The transmission characteristics estimation unit 28 may estimate the transmission characteristics at another location. Specifically, the transmission characteristics estimation unit 28 may estimate the transmission characteristics after the processing by the adaptive equalization unit 24, after the processing by the frequency offset compensation unit 25, or after the processing by the carrier phase noise compensation unit 26.

[0074] The transmission characteristics may be estimated by online processing within a chip constituting the digital signal processing unit 22 of the optical receiver 2, or by offline processing by extracting the received signal from the optical receiver 2. When offline processing is performed, all of the functional units included in the digital signal processing unit 22 are provided in an external device such as an external computer. The external device acquires the signal immediately before it enters the nonlinear optical compensation unit 23, and performs processing similar to that described in each of the above embodiments.

[0075] In the embodiment, the comparison between the received signal and the training signal is performed after adaptive equalization processing, compensation for frequency offset, and compensation for phase offset, but the comparison between the received signal and the training signal may also be performed between the nonlinear optical compensation unit 23 and the adaptive equalization unit 24 (see, for example, Patent Document 1).

[0076] The optical transmitter 1, optical receiver 2, relay node 31, and network controller 6 may be partially or entirely implemented by a computer. In this case, a program for implementing these functions may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. The term "computer system" as used herein includes hardware such as an operating system (OS) and peripheral devices. The term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into a computer system. The term "computer-readable recording medium" may also include media that dynamically store programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or telephone lines, or media that store programs for a fixed period of time, such as volatile memory within a computer system serving as a server or client. The program may be designed to implement some of the functions described above, or may be capable of implementing the functions in combination with a program already stored in the computer system. The program may also be implemented using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0077] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Industrial Applicability]

[0078] The present invention is applicable to an optical transmission system that communicates by wavelength division multiplexing (WDM). [Explanation of symbols]

[0079] 100, 100A, 100B, 100C...optical transmission system, 1, 1A, 1C...optical transmitter, 2, 2A, 2C...optical receiver, 21...coherent receiver, 22...digital signal processing unit, 23...nonlinear optical compensation unit, 231...linear compensation unit, 232...nonlinear compensation unit, 233...Fourier transform unit, 234...chromatic dispersion compensation unit, 235...inverse Fourier transform unit, 24...adaptive equalization unit, 25...frequency offset compensation unit, 26...carrier phase noise compensation unit, 27...coefficient update unit, 28...transmission characteristic estimation unit, 3...optical transmission path, 31, 31A, 31B...relay node, 311...optical amplifier (optical amplifier), 312...optical filter, 32...attenuator, 4...multiplexer (MUX), 5...demultiplexer (DEMUX), 6, 6A, 6B, 6C...network controller

Claims

1. An optical transmission system in which a plurality of optical transmitters corresponding to a plurality of channel frequencies and a plurality of optical receivers corresponding to the plurality of optical transmitters communicate with each other via an optical transmission path, the optical transmission line includes a plurality of relay nodes each including an optical amplifier for amplifying an optical signal; each of the plurality of optical transmitters transmits an optical signal at a corresponding channel frequency among the plurality of channel frequencies; each of the plurality of optical receivers receives an optical signal of a corresponding channel frequency among the plurality of channel frequencies, and estimates fluctuations in intensity of the optical signal in a transmission direction on the optical transmission path; a network controller that acquires a plurality of pieces of estimated information indicating estimated results of the fluctuation from the plurality of optical receivers, and acquires a gain spectrum of an optical amplifier at an arbitrary position on the optical transmission line based on the acquired plurality of pieces of estimated information; An optical transmission system comprising:

2. each of the plurality of optical transmitters transmits an optical signal at a channel frequency different from each other; each of the plurality of optical receivers receives an optical signal transmitted by an optical transmitter associated with the optical receiver, and estimates fluctuations in intensity of the received optical signal on the optical transmission path; 2. The optical transmission system according to claim 1.

3. each of the plurality of optical transmitters transmits an optical signal while sweeping a frequency band different from that of the other optical transmitters for its own channel frequency; each of the plurality of optical receivers receives an optical signal transmitted by an optical transmitter associated with the optical receiver, and estimates a fluctuation in intensity of the received optical signal on the optical transmission path for each channel frequency swept in the frequency band; 2. The optical transmission system according to claim 1.

4. the network controller acquires the gain spectrum for a position corresponding to a position of the relay node on the optical transmission line based on the plurality of pieces of estimated information acquired from the plurality of optical receivers.

4. The optical transmission system according to claim 1.

5. the network controller compensates for the gain tilt of each relay node by feedback control based on the acquired gain spectrum; 5. An optical transmission system according to claim 1.

6. The network controller adjusts the pump wavelength or pump power of the optical amplifier of the relay node by the feedback control so that the gain spectrum of the relay node has a predetermined shape, or equalizes the intensity of the optical signal of each channel frequency by applying an inverse function of the gain spectrum to the wavelength selection function of the relay node.

6. The optical transmission system according to claim 5.

7. A network controller of an optical transmission system in which a plurality of optical transmitters corresponding to a plurality of channel frequencies and a plurality of optical receivers corresponding to the plurality of optical transmitters communicate with each other via an optical transmission path, comprising: the optical transmission line includes a plurality of relay nodes each including an optical amplifier for amplifying an optical signal; each of the plurality of optical transmitters transmits an optical signal at a corresponding channel frequency among the plurality of channel frequencies; each of the plurality of optical receivers receives an optical signal of a corresponding channel frequency among the plurality of channel frequencies, and estimates fluctuations in intensity of the optical signal in a transmission direction on the optical transmission path; The network controller acquiring a plurality of pieces of estimated information indicating estimated results of the fluctuation from the plurality of optical receivers, and acquiring a gain spectrum of an optical amplifier at an arbitrary position on the optical transmission line based on the acquired plurality of pieces of estimated information; Network controller.

8. In an optical transmission system in which a plurality of optical transmitters corresponding to a plurality of channel frequencies and a plurality of optical receivers corresponding to the plurality of optical transmitters communicate with each other via an optical transmission path, the optical transmission line amplifies and relays the optical signal by optical amplifiers at a plurality of relay nodes; each of the plurality of optical transmitters transmits an optical signal at a corresponding channel frequency among the plurality of channel frequencies; each of the plurality of optical receivers receives an optical signal of a corresponding channel frequency among the plurality of channel frequencies, and estimates fluctuations in intensity of the optical signal in a transmission direction on the optical transmission path; a network controller acquires a plurality of pieces of estimated information indicating estimated results of the fluctuation from the plurality of optical receivers, and acquires a gain spectrum of an optical amplifier at an arbitrary position on the optical transmission line based on the acquired plurality of pieces of estimated information; Optical transmission method.

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