Optical transmission system and equalization method

Abstract

An optical transmission system comprising an optical transmitter that transmits an optical signal, an equalizer that equalizes the optical signal transmitted from the optical transmitter in the optical domain, an optical receiver that receives the optical signal equalized by the equalizer and acquires quality information relating to the quality of the received optical signal, and a filter generation unit that generates a filter for use in equalization processing and sets the filter to the equalization unit on the basis of the quality information and spectrum information pertaining to the optical signal equalized by the equalization unit.　

Description

[Technical Field]

[0001] The present invention relates to an optical transmission system and an equalization method. [Background technology]

[0002] In digital coherent optical transmission systems, efforts are being made to increase communication capacity by using higher-order modulation and higher baud rates. However, signals with higher-order modulation and higher baud rates are susceptible to waveform distortion and noise originating from the transmitter / receiver and transmission path. Therefore, signal quality is improved by digitally compensating for transmitter imperfections. Digital equalization provides high compensation accuracy, but increases the peak-to-average power ratio (PAPR) of the processed signal, resulting in a high error rate. Therefore, optical equalization (OEQ) has been attracting attention as a technology for compensating devices while suppressing the increase in PAPR (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] M. Nakamura, T. Kobayashi, F. Hamaoka, and Y. Miyamoto, “High Information Rate of 128-GBaud 1.8-Tb / s and 64-GBaud 1.03-Tb / s Signal Generation and Detection Using Frequency-Domain 8×2 MIMO Equalization”, OFC 2022 Optica Publishing Group 2022, Summary of the Invention [Problem to be solved by the invention]

[0004] However, while OEQ can suppress the increase in PAPR, increasing the amount of equalization leads to a greater decrease in output power due to equalization. Because of this trade-off between the amount of equalization and output power, the relationship between the amount of equalization and signal quality was unclear. Therefore, it was not possible to optimize the filter shape used in OEQ to maximize signal quality, resulting in insufficient compensation.

[0005] In view of the above circumstances, an object of the present invention is to provide a technique that can improve compensation accuracy compared to conventional techniques. [Means for solving the problem]

[0006] One aspect of the present invention is an optical transmission system comprising: an optical transmitter that transmits an optical signal; an equalization unit that equalizes the optical signal transmitted from the optical transmitter in the optical domain; an optical receiver that receives the optical signal equalized by the equalization unit and acquires quality information regarding the quality of the received optical signal; and a filter generation unit that generates a filter to be used for equalization processing based on spectral information of the optical signal equalized by the equalization unit and the quality information, and sets the filter in the equalization unit.

[0007] One aspect of the present invention is an equalization method in which an optical transmitter transmits an optical signal, an equalization unit equalizes the transmitted optical signal in the optical domain, an optical receiver receives the equalized optical signal and acquires quality information regarding the quality of the received optical signal, and a filter generation unit generates a filter to be used in the equalization process based on spectral information of the equalized optical signal and the quality information, and sets the filter in the equalization unit. [Effects of the Invention]

[0008] According to the present invention, it is possible to improve the compensation accuracy more than before. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating an example of the configuration of an optical transmission system according to a first embodiment. [Figure 2] FIG. 4 is a diagram illustrating a method for determining a frequency width to be compensated in the first embodiment. [Figure 3] FIG. 3 is a sequence diagram showing a processing flow of the optical transmission system according to the first embodiment. [Figure 4] FIG. 3 is a sequence diagram showing a processing flow of the optical transmission system according to the first embodiment. [Figure 5] FIG. 4 is a diagram for explaining the effects of the first embodiment. [Figure 6] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmission system according to a second embodiment. [Figure 7] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmission system according to a third embodiment. [Figure 8] FIG. 11 is a sequence diagram showing a processing flow of the optical transmission system according to the third embodiment. [Figure 9] FIG. 11 is a sequence diagram showing a processing flow of the optical transmission system according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) 1 is a diagram showing an example of the configuration of an optical transmission system 100 according to the first embodiment. The optical transmission system 100 includes an optical transmitter 10, an equalization unit 11, an optical receiver 12, an OSA 13, and a filter calculation unit 14. The optical transmitter 10 and the optical receiver 12 are connected via an optical transmission path 20. The equalization unit 11 and the filter calculation unit 14 are configured as an equalizer.

[0011] The optical transmitter 10 transmits an optical signal. For example, the optical transmitter 10 transmits a polarization multiplexed signal. Note that the optical transmitter 10 may transmit an optical signal that has been wavelength-multiplexed or spatially multiplexed in addition to polarization multiplexing.

[0012] The equalizer 11 is provided on the optical transmission path 20 between the optical transmitter 10 and the optical receiver 12. The equalizer 11 equalizes the optical signal transmitted from the optical transmitter 10 in the optical domain. The equalizer 11 performs compensation using, for example, OEQ.

[0013] The optical receiver 12 receives the optical signal that has been equalized by the equalizer 11. The optical receiver 12 acquires quality information of the received optical signal. The quality information of the optical signal is information about the signal quality of the optical signal, such as a Bit Error Rate (BER), a Q factor, a Signal-to-Noise Ratio (SNR), the number of Forward Error Correction (FEC) corrections, and an Adaptive Equalizer (AEQ) error signal. During operation, the optical receiver 12 acquires the quality information through a General Communication Channel (GCC), a Digital Signal Processor (DSP) Com-ch, a control plane control channel, a Network Operation System (NW-OpS) control channel, and the like. This information may be set at the time of shipment from the factory. The optical receiver 12 feeds back the acquired quality information to the filter calculator 14.

[0014] The OSA 13 acquires the optical signal that has been equalized by the equalizer 11, and acquires the signal spectrum of the acquired equalized optical signal. The OSA 13 is an optical spectrum analyzer. The OSA 13 outputs information on the acquired signal spectrum (hereinafter referred to as "spectral information") to the filter calculator 14. The OSA 13 is one aspect of a spectrum information acquisition unit.

[0015] The filter calculation unit 14 calculates a filter to be set in the equalization unit 11 based on the quality information output from the optical receiver 12 and the spectrum information output from the OSA 13. The filter calculation unit 14 sets the calculated filter in the equalization unit 11. The filter calculation unit 14 may be configured as an equalization processing unit in combination with the equalization unit 11, or may be provided in the optical receiver 12. The function of the filter calculation unit 14 may be realized by one or more processors.

[0016] Fig. 2 is a diagram showing how to calculate the frequency width to be compensated in the first embodiment. As shown in the left diagram of Fig. 2, the filter calculation unit 14 calculates the frequency width to be compensated based on the ratio of the frequency band to the baud rate determined when the modulation method is set for the optical transmission system 100. The right diagram of Fig. 2 shows the signal spectrum compensated by a filter generated based on the calculated frequency width.

[0017] 3 and 4 are sequence diagrams showing the flow of processing in the optical transmission system 100 in the first embodiment. An initial value is set in the equalizer 11 at the start of processing (step S101). Here, as the initial value, no compensation by a filter (compensated frequency width α=0%) is set in the equalizer 11. Thereafter, the optical transmitter 10 transmits an optical signal (step S102). The optical signal transmitted by the optical transmitter 10 is input to the equalizer 11 via an optical transmission path.

[0018] The equalizer 11 performs equalization processing on the input optical signal (step S103). Since no compensation by a filter (frequency width to be compensated α=0%) is set as the initial value, the optical signal input to the equalizer 11 is output without compensation (step S104). The optical signal output from the equalizer 11 is branched in the optical transmission line 20. The branched optical signal is received by the optical receiver 12 and the OSA 13.

[0019] The OSA 13 measures the signal spectrum of the received optical signal (step S105). The OSA 13 outputs spectrum information obtained by the measurement to the filter calculation unit 14 (step S106). As a result, the spectrum information is input to the filter calculation unit 14. The optical receiver 12 measures the signal quality of the received optical signal (step S107). The optical receiver 12 outputs the signal quality information obtained by the measurement to the filter calculation unit 14 as quality information (step S108). As a result, the quality information is input to the filter calculation unit 14.

[0020] The filter calculation unit 14 generates a filter based on the input spectrum information and quality information (step S109). Specifically, the filter calculation unit 14 generates an inverse characteristic H(f) of the frequency width to be compensated (for example, α=10%) based on the amplitude characteristic H(f) of the input signal spectrum. -β A filter is generated using (f) (0≦β≦1). For example, when β=1, the signal spectrum after compensation approaches flatness. The filter calculation unit 14 sets the generated filter in the equalization unit 11 (step S110). The filter calculation unit 14 associates the obtained quality information with the values ​​of the frequency width α and β to be compensated, which are parameters of the filter shape when the quality information was obtained, and stores them.

[0021] Here, β represents a parameter that determines the degree of equalization within the set frequency band (α). As mentioned above, when β is 1, the optical power distribution within the frequency band α approaches flatness (see the right diagram in Figure 2), resulting in a completely equalized state. On the other hand, when β<1, the optical power distribution within the frequency band α becomes less flat. In other words, when β<1, the state is no longer completely equalized. Since equalization using OEQ involves power loss, β is defined as a parameter that alleviates this flatness when β<1, providing a mechanism for fine-tuning the power loss.

[0022] The optical transmitter 10 transmits an optical signal again (step S111). The optical signal transmitted by the optical transmitter 10 is input to the equalizer 11 via the optical transmission path. The equalizer 11 performs equalization processing on the input optical signal (step S112). At this point, a filter is set in the equalizer 11 (compensation frequency width α=10%), so the equalizer 11 performs equalization processing on the optical signal input to the equalizer 11. This compensates the optical signal. The equalizer 11 outputs the optical signal after equalization processing to the optical transmission path 20 (step S113). The optical signal output from the equalizer 11 is branched in the optical transmission path 20. The branched optical signal is received by the optical receiver 12 and the OSA 13.

[0023] The OSA 13 measures the signal spectrum of the received optical signal (step S114). The OSA 13 outputs spectrum information obtained by the measurement to the filter calculation unit 14 (step S115). As a result, the spectrum information is input to the filter calculation unit 14. The optical receiver 12 measures the signal quality of the received optical signal (step S116). The optical receiver 12 outputs the signal quality information obtained by the measurement to the filter calculation unit 14 as quality information (step S117). As a result, the quality information is input to the filter calculation unit 14.

[0024] The filter calculation unit 14 generates a filter based on the input spectrum information and quality information (step S118). Specifically, the filter calculation unit 14 generates a filter by increasing the frequency bandwidth α and coefficient β to be compensated based on the amplitude characteristic H(f) of the input signal spectrum. The filter calculation unit 14 sets the regenerated filter in the equalization unit 11 (step S119).

[0025] Thereafter, the processes from step S111 to step S119 are repeatedly executed until the optimum frequency range α and coefficient β are reached (step S120). The filter calculation unit 14 determines the optimum frequency range α and coefficient β based on one of the following methods. The following methods differ in whether the frequency range α or the coefficient β is determined first. The first and second methods described below are merely examples, and any other method may be used as long as it is possible to determine the optimum frequency range α and coefficient β.

[0026] (First method) The filter calculation unit 14 first fixes the coefficient β (for example, β=1) and determines the compensated frequency width α that provides the best signal quality. Next, the filter calculation unit 14 fixes the determined frequency width α and changes the coefficient β to determine the coefficient β that provides the best signal quality.

[0027] (Second method) The filter calculation unit 14 first fixes the frequency width α (for example, α=100%) and determines the coefficient β that provides the best signal quality. Next, the filter calculation unit 14 fixes the coefficient β and changes the frequency width α to determine the frequency width α that provides the best signal quality.

[0028] In the first and second methods described above, the criterion for determining the optimum frequency width α may be any one of the following (Decision Criterion 1) to (Decision Criterion 3). Note that the coefficient β may be determined in a similar manner. The following (Decision Criterion 1), (Decision Criterion 2), and (Decision Criterion 3) are merely examples, and other criteria may be set as the criteria for determining the optimum frequency width α and coefficient β. (Decision criterion 1): A signal quality value to be satisfied is determined in advance, and the frequency width α when a value close to that value (the difference from the satisfied signal quality value is less than a threshold) is obtained is determined to be the best frequency width α. (Decision criterion 2): Candidate values ​​to be set as the frequency width α are determined in advance, the signal quality is measured for each of them, and the frequency width α that results in the maximum signal quality is determined to be the best frequency width α. (Criterion 3): The initial value, maximum value, and minimum value of α are determined, and the amount of change in frequency width α is increased (or decreased) from there to find the best signal quality, and the frequency width α at that point is determined to be the best frequency width α. Note that if a highly accurate signal is required, the amount of change in frequency width α is set to a small value and fine adjustments are made. If the calculation load needs to be reduced, the amount of change in frequency width α is set to a large value and rough adjustments are made to find the optimal α. The flow of the process for determining the optimum frequency width α and coefficient β will be described below.

[0029] (When determining the best frequency band α and coefficient β in the first method (criterion 1)) The filter calculation unit 14 generates a filter by fixing the coefficient β and changing (e.g., increasing) the compensation frequency width α. The filter calculation unit 14 associates and stores quality information obtained by setting the generated filter in the equalization unit 11 with the values ​​of the compensation frequency widths α and β, which are parameters of the filter shape when the quality information was obtained. The filter calculation unit 14 compares the value indicated by the obtained quality information with a predetermined signal quality value and determines whether the difference between the value indicated by the quality information and the predetermined signal quality value is less than a threshold.

[0030] If the difference between the value indicated by the quality information and the predetermined signal quality value is less than the threshold, the filter calculation unit 14 determines the frequency width α when the quality information was obtained as the best frequency width α. On the other hand, if the difference between the value indicated by the quality information and the predetermined signal quality value is equal to or greater than the threshold, the filter calculation unit 14 changes (e.g., increases) the frequency width α to be compensated and generates a new filter. By repeating this process, the filter calculation unit 14 determines the frequency width α at which the difference between the value indicated by the quality information and the predetermined signal quality value is less than the threshold.

[0031] When determining the optimal coefficient β, the filter calculation unit 14 fixes the determined frequency range α (the optimal compensation frequency range α) and changes (for example, increases) the coefficient β to generate a filter. The subsequent processing is the same as the flow for determining the optimal compensation frequency range α.

[0032] (When determining the best frequency band α and coefficient β in the first method (criterion 2)) The filter calculation unit 14 generates a filter by fixing the coefficient β and changing (e.g., increasing) the frequency bandwidth α to be compensated. At this time, the filter calculation unit 14 selects one candidate value from candidate values ​​(e.g., 70%, 78%, 86%, etc.) to be set as the frequency bandwidth α. The filter calculation unit 14 associates and stores quality information obtained by setting the generated filter in the equalization unit 11 with the values ​​of the frequency bandwidth α and β to be compensated, which are parameters of the filter shape when the quality information was obtained. Next, the filter calculation unit 14 selects an unselected candidate value from the candidate values ​​of the frequency bandwidth α and fixes the coefficient β to generate a filter. The filter calculation unit 14 associates and stores the quality information obtained by setting the generated filter in the equalization unit 11 with the values ​​of the frequency bandwidth α and β to be compensated, which are parameters of the filter shape when the quality information was obtained.

[0033] By performing such processing, the filter calculation unit 14 acquires quality information for each candidate value of the frequency width α. The filter calculation unit 14 selects quality information with the best quality from the acquired quality information for each candidate value of the frequency width α. The filter calculation unit 14 determines the frequency width α associated with the selected quality information with the best quality as the best frequency width α.

[0034] When determining the optimal coefficient β, the filter calculation unit 14 fixes the determined frequency range α (optimal compensated frequency range α) and changes (e.g., increases) the coefficient β to generate a filter. At this time, the filter calculation unit 14 selects one candidate value from candidate values ​​(e.g., 0.1, 0.5, 1, etc.) to be set as the coefficient β. The subsequent processing is the same as the flow for determining the optimal compensated frequency range α.

[0035] (When determining the best frequency band α and coefficient β in the first method (criterion 3)) The initial value, maximum value, minimum value, and variation value of the frequency width α to be compensated and the initial value, maximum value, minimum value, and variation value of the coefficient β are determined in advance, and this information is saved in the filter calculation unit 14. The filter calculation unit 14 fixes the coefficient β and generates a filter with the initial value of the frequency width α. The filter calculation unit 14 associates and stores quality information obtained by setting the generated filter in the equalization unit 11 with the values ​​of the frequency width α to be compensated (for example, the initial value) and β, which are parameters of the filter shape when the quality information was obtained.

[0036] Next, the filter calculation unit 14 changes (e.g., increases or decreases) the frequency bandwidth α from its initial value by the change amount, fixes the coefficient β, and generates a filter. The filter calculation unit 14 determines the frequency bandwidth α so that the value of the changed frequency bandwidth α does not fall below a minimum value or exceed a maximum value. The filter calculation unit 14 associates and stores quality information obtained by setting the generated filter in the equalization unit 11 with the values ​​of the frequency bandwidths α and β to be compensated, which are parameters of the filter shape when the quality information was obtained.

[0037] The filter calculation unit 14 performs this process a predetermined number of times or on all frequency widths α after the change based on the amount of change, thereby acquiring quality information for each of the multiple frequency widths α. The filter calculation unit 14 selects the quality information with the best quality from the acquired quality information for each of the multiple frequency widths α. The filter calculation unit 14 determines the frequency width α associated with the selected quality information with the best quality as the best frequency width α.

[0038] When determining the optimal coefficient β, the filter calculation unit 14 fixes the determined frequency width α (optimal compensated frequency width α) and generates a filter with the initial value of the coefficient β. The subsequent processing is the same as the flow for determining the optimal compensated frequency width α.

[0039] When the optimum frequency width α and coefficient β are reached, the filter calculation unit 14 sets the filter obtained by the optimum frequency width α and coefficient β in the equalization unit 11 (step S121).

[0040] Fig. 5 is a diagram for explaining the effects of the first embodiment. The results shown in Fig. 5 were obtained under the following conditions. -Experiment conducted with 128GBd, 64QAM signal The filter setting range was changed to no filter setting (0%), 90GHz (70%), 100GHz (78%), 110GHz (86%), 120GHz (94%), 130GHz (101%), 140GHz (109%), and 150GHz (117%).

[0041] As a result, we were able to confirm an improvement in OSNR (Optical SNR) tolerance of up to 5.4 dB compared to when no filter was set, as shown in the left diagram of Figure 5. The compensated frequency width α when an OSNR improvement of up to 5.4 dB was observed was 94% (120 GHz), as shown in the right diagram of Figure 5.

[0042] The optical transmission system 100 configured as described above enables improved compensation accuracy compared to conventional systems. Specifically, the optical transmission system 100 includes an optical transmitter 10 that transmits an optical signal, an equalizer 11 that equalizes the optical signal transmitted from the optical transmitter 10 in the optical domain, an optical receiver 12 that receives the optical signal equalized by the equalizer 11 and acquires quality information related to the quality of the received optical signal, and a filter generator 14 that generates a filter to be used for equalization processing based on the spectral information and quality information of the optical signal equalized by the equalizer 11 and sets the filter in the equalizer 11. This makes it possible to determine the filter shape of the equalizer 11 that maximizes the quality of the received signal according to the characteristics of the optical amplifier, modulation method, transmission distance, and transmission capacity, and apply the filter to the equalizer 11. This makes it possible to improve compensation accuracy compared to conventional systems, thereby achieving high-quality optical transmission.

[0043] (Second embodiment) In the second embodiment, a configuration in which the optical transmission system 100 does not include an OPA will be described. 6 is a diagram showing an example of the configuration of an optical transmission system 100a according to the second embodiment. The optical transmission system 100a includes an optical transmitter 10, an equalization unit 11, an optical receiver 12a, and a filter calculation unit 14a. The optical transmitter 10 and the optical receiver 12a are connected via an optical transmission path 20. The optical transmission system 100a differs in configuration from the optical transmission system 100 in that it does not include an OSA 13. Other configurations of the optical transmission system 100a are similar to those of the optical transmission system 100. The differences from the optical transmission system 100 will be described below.

[0044] The optical receiver 12a receives an optical signal that has been equalized by the equalizer 11. The optical receiver 12a acquires quality information of the received optical signal. Furthermore, the optical receiver 12a acquires the signal spectrum of the received optical signal. The optical receiver 12a feeds back the acquired quality information and spectrum information to the filter calculator 14a.

[0045] The filter calculation unit 14a calculates a filter to be set in the equalization unit 11 based on the quality information and spectrum information output from the optical receiver 12. The filter calculation unit 14a sets the calculated filter in the equalization unit 11.

[0046] The specific processing in the optical transmission system 100a differs from that in the optical transmission system 100 in that the signal spectrum is acquired by the optical receiver 12a, and the optical receiver 12a outputs quality information and spectrum information to the filter calculation unit 14a.

[0047] The optical transmission system 100a configured as above does not include the OSA 13, and therefore can achieve the same effects as the optical transmission system 100 with a cheaper configuration.

[0048] (Third embodiment) In the third embodiment, a configuration will be described in which equalization processing is performed in both the electrical and optical domains, whereas in the first and second embodiments, equalization processing is performed only in the optical domain. 7 is a diagram showing an example of the configuration of an optical transmission system 100b according to the third embodiment. The optical transmission system 100b includes an optical transmitter 10b, an equalization unit 11, an optical receiver 12, an OSA 13, and a filter calculation unit 14b. The optical transmitter 10b and the optical receiver 12 are connected via an optical transmission path 20. The optical transmission system 100b differs in configuration from the optical transmission system 100 in that the optical transmitter 10b also performs equalization in the electrical domain. Other configurations of the optical transmission system 100b are similar to those of the optical transmission system 100. The differences from the optical transmission system 100 will be described below.

[0049] The optical transmitter 10b transmits an optical signal. For example, the optical transmitter 10b transmits a polarization multiplexed signal. The optical transmitter 10b may transmit an optical signal that has been subjected to wavelength multiplexing or spatial multiplexing in addition to polarization multiplexing. Furthermore, the optical transmitter 10b performs signal equalization processing in the electrical domain based on the filter set by the filter calculation unit 14b.

[0050] The filter calculation unit 14b calculates a filter to be set in the optical transmitter 10b and the equalization unit 11 based on the quality information output from the optical receiver 12 and the spectrum information output from the OSA 13. The filter calculation unit 14b sets the calculated filter in the optical transmitter 10b and the equalization unit 11.

[0051] 8 and 9 are sequence diagrams showing the flow of processing in the optical transmission system 100b according to the third embodiment. An initial value is set in the equalizer 11 at the start of processing (step S201). Here, as the initial value, no compensation by a filter (compensated frequency width α=0%) is set in the equalizer 11. At the start of processing, an initial value for performing equalization processing in the electrical domain is set in the optical transmitter 10 (step S202). Here, as the initial value, no compensation by a filter is set in the optical transmitter 10b.

[0052] Thereafter, the optical transmitter 10 transmits the optical signal without performing equalization processing in the electrical domain (step S203). The optical signal transmitted by the optical transmitter 10 is input to the equalizer 11 via the optical transmission path.

[0053] The equalizer 11 performs equalization processing on the input optical signal (step S204). Since no compensation by a filter (compensated frequency width α=0%) is set as the initial value, the optical signal input to the equalizer 11 is output without compensation (step S205). The optical signal output from the equalizer 11 is branched in the optical transmission line 20. The branched optical signal is received by the optical receiver 12 and the OSA 13.

[0054] The OSA 13 measures the signal spectrum of the received optical signal (step S206). The OSA 13 outputs spectrum information obtained by the measurement to the filter calculation unit 14b (step S207). As a result, the spectrum information is input to the filter calculation unit 14b. The optical receiver 12 measures the signal quality of the received optical signal (step S208). The optical receiver 12 outputs the signal quality information obtained by the measurement to the filter calculation unit 14b as quality information (step S209). As a result, the quality information is input to the filter calculation unit 14b.

[0055] The filter calculation unit 14b generates a filter based on the input spectrum information and quality information (step S210). Specifically, the filter calculation unit 14b calculates an inverse characteristic H(f) of a frequency width to be compensated (for example, α=10%) based on the amplitude characteristic H(f) of the input signal spectrum. -β (f) (0≦β) is used to generate a filter. For example, when β=1, the signal spectrum after compensation approaches flatness. From this, the filter calculation unit 14b calculates the coefficient β e is the electrical domain equalization, and the coefficient β o is an index that determines the inverse characteristics in optical domain equalization, and β = β e +β o (H -β (f)=H -βe (f) × H-βo For example, the filter calculation unit 14b determines the filter β e =0.1β, β o =β-β e =0.9β The filter calculation unit 14b sets the generated filter in the optical transmitter 10b and the equalization unit 11 (step S211).

[0056] The optical transmitter 10b generates an optical signal after performing equalization processing on the electrical signal using the filter set by the filter calculation unit 14b. The optical transmitter 10b transmits the generated optical signal (step S212). The optical signal transmitted by the optical transmitter 10b is input to the equalization unit 11 via the optical transmission path. The equalization unit 11 performs equalization processing on the input optical signal (step S213). At this point, a filter is set in the equalization unit 11 (compensation frequency width α=10%), so the equalization unit 11 performs equalization processing on the optical signal input to the equalization unit 11. This compensates the optical signal. The equalization unit 11 outputs the optical signal after equalization processing to the optical transmission path 20 (step S214). The optical signal output from the equalization unit 11 is branched in the optical transmission path 20. The branched optical signal is received by the optical receiver 12 and the OSA 13.

[0057] The OSA 13 measures the signal spectrum of the received optical signal (step S215). The OSA 13 outputs spectrum information obtained by the measurement to the filter calculation unit 14b (step S216). As a result, the spectrum information is input to the filter calculation unit 14b. The optical receiver 12 measures the signal quality of the received optical signal (step S217). The optical receiver 12 outputs the signal quality information obtained by the measurement to the filter calculation unit 14b as quality information (step S218). As a result, the quality information is input to the filter calculation unit 14b.

[0058] The filter calculation unit 14b generates a filter based on the input spectrum information and quality information (step S219). Specifically, the filter calculation unit 14b calculates a frequency width α to be compensated, a coefficient β, and a coefficient β based on the amplitude characteristic H(f) of the input signal spectrum.e The filter calculation unit 14b generates a filter by increasing the coefficient β e Instead of β o The filter calculation unit 14b sets the regenerated filter in the optical transmitter 10b and the equalization unit 11 (step S220).

[0059] Then, the best frequency band α, coefficient β and coefficient β e =β(β o When increasing β o The processes from step S212 to step S220 are repeated until the optimum frequency range α and coefficient β are reached (step S221). The method for determining the optimum frequency range α and coefficient β is the same as in the first embodiment. The optimum frequency range α, coefficient β, and coefficient β are determined in the same manner as in the first embodiment. e = β, the filter calculation unit 14b calculates the optimum frequency width α, coefficient β, and coefficient β e The filter obtained by β=β is set in the optical transmitter 10b and the equalizer 11 (step S222).

[0060] The optical transmission system 100b configured as described above performs equalization not only in the optical domain but also in the electrical domain, which allows for finer adjustments in digital equalization compared to optical domain equalization, thereby achieving higher signal quality.

[0061] (Modification of the third embodiment) The optical transmission system 100b does not need to include the OSA 13. In this configuration, the optical receiver 12 acquires the signal spectrum of the optical signal in addition to the quality information of the optical signal, as in the optical receiver 12a in the second embodiment. The optical receiver 12 feeds back the acquired quality information and spectrum information to the filter calculation unit 14b.

[0062] The optical transmitters 10 and 10b, some functional units of the optical receivers 12 and 12a, or the filter calculation units 14, 14a and 14b in the above-described embodiments may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into and executed by a computer system. Note that the term "computer system" here includes hardware such as an OS and peripheral devices.

[0063] Furthermore, "computer-readable recording media" refers to portable media such as flexible disks, optical magnetic disks, ROMs (Read Only Memory), and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, "computer-readable recording media" may also include devices that dynamically store programs for a short period of time, such as communication lines used when transmitting programs over networks like the Internet or over communication lines like telephone lines, or devices that store programs for a fixed period of time, such as volatile memory within a computer system that serves as a server or client in such cases. Furthermore, the program may be one that implements some of the aforementioned functions, or one that can realize the aforementioned functions in combination with a program already stored in the computer system, or one that can be implemented using a programmable logic device such as an FPGA (Field-Programmable Gate Array).

[0064] Although an embodiment of the present invention has been described in detail above 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]

[0065] The present invention can be applied to equalization techniques in digital coherent optical transmission. [Explanation of symbols]

[0066] 10, 10b... optical transmitter, 11... equalization unit, 12, 12a... optical receiver, 13... OSA, 14, 14a, 14b... filter calculation unit, 20... optical transmission path, 100, 100a, 100b... optical transmission system

Claims

1. an optical transmitter for transmitting an optical signal; an equalization unit that equalizes the optical signal transmitted from the optical transmitter in an optical domain; an optical receiver that receives the optical signal equalized by the equalizer and acquires quality information regarding the quality of the received optical signal; a filter generator that generates a filter to be used in equalization processing based on spectrum information of the optical signal equalized by the equalizer and the quality information, and sets the filter in the equalizer; An optical transmission system comprising:

2. a spectral information acquisition unit that acquires spectral information of the optical signal equalized by the equalization unit, the filter generation unit generates a filter to be used for equalization processing based on the spectral information of the optical signal acquired by the spectral information acquisition unit and the quality information, and sets the filter in the equalization unit.

2. The optical transmission system according to claim 1.

3. the optical receiver further acquires spectral information of the optical signal equalized by the equalization unit; the filter generation unit generates a filter to be used for equalization processing based on the spectrum information of the optical signal acquired by the optical receiver and the quality information, and sets the filter in the equalization unit.

2. The optical transmission system according to claim 1.

4. the filter generation unit generates the filter including a compensation frequency width and coefficients that provide the best signal quality, and sets the filter in the equalization unit.

4. The optical transmission system according to claim 1.

5. the filter generation unit generates the filter including coefficients for performing equalization processing in the electrical domain in the optical transmitter and coefficients for performing equalization processing in the optical domain in the equalization unit, and sets the filter in the optical transmitter and the equalization unit.

4. The optical transmission system according to claim 1.

6. an optical transmitter transmitting an optical signal; an equalization unit equalizing the transmitted optical signal in the optical domain; an optical receiver receives the equalized optical signal and obtains quality information regarding the quality of the received optical signal; an equalization method, wherein a filter generation unit generates a filter to be used for equalization processing based on spectrum information of the equalized optical signal and the quality information, and sets the filter in the equalization unit;

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