Optical transmission system and wavelength monitoring method

Abstract

To monitor the passage characteristics of a wavelength selective switch through which an optical signal passes during optical signal transmission in an optical communication system.SOLUTION: A WSS 11 wavelength-separates an optical signal L11, outputs optical signals of some wavelengths to a first path, and outputs the optical signals of the remaining wavelengths to a second path. A spectrum analyzer 13 observes the spectrum of a first optical signal of a first wavelength included in the optical signal L11 and the spectrum of a second optical signal of the first wavelength output from the WSS 11, and outputs first observation results. A WSS 22 outputs an optical signal L12 that is obtained by wavelength-multiplexing the optical signal input from a third path with the optical signal of the remaining wavelength input from the second path. A spectrum analyzer 14 observes the spectrum of a third optical signal of a second wavelength included in the optical signal input to the WSS 22 from the second path, and the spectrum of a fourth optical signal of the second wavelength output from the WSS 12 and outputs a second observation result.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present disclosure relates to a node device, a wavelength monitoring mechanism, and a wavelength monitoring method. [Background Art]

[0002] In optical communication systems, to increase transmission capacity, a wavelength division multiplexing (WDM) transmission method, in which optical signals of different wavelengths are multiplexed for transmission, is employed. In the WDM transmission method, in order to transmit an optical signal of a desired wavelength to a destination, transmission control is performed on a wavelength-by-wavelength basis, such as adding (ADD) an optical signal of a desired wavelength to a wavelength-multiplexed optical signal, or dropping (DROP) an optical signal of a desired wavelength from a wavelength-multiplexed optical signal.

[0003] A node device incorporated in an optical communication system may have a function of performing add / drop (ADD / DROP) of optical signals by performing wavelength filtering on an input wavelength-multiplexed optical signal. As a result, the spectrum of an optical signal is affected by the pass characteristics of the filter (Patent Documents 1 to 4). [Prior Art Literature] [Patent Literature]

[0004] [Patent Document 1] International Publication No. WO2017 / 163993 [Patent Document 2] Japanese Patent Laid-Open No. 2014-143614 [Patent Document 3] Japanese Patent Laid-Open No. 2011-254309 [Patent Document 4] Japanese Patent Laid-Open No. 2009-229784 [Summary of the Invention] [Problem to be Solved by the Invention]

[0005] In WDM systems, narrowing of the wavelength bandwidth used is required to improve channel capacity efficiency, and there are two factors that narrow the wavelength bandwidth: the miniaturization of the Wavelength Selective Switch (WSS) degrades the passband characteristics. As a result, the transmission characteristics deteriorate.

[0006] Therefore, in WDM systems, it is necessary to establish a method for monitoring the pass characteristics of wavelength-selective switches through which optical signals pass during the transmission process.

[0007] This disclosure is made in view of the above circumstances and aims to monitor the pass characteristics of wavelength-selective switches through which optical signals pass in optical signal transmission in optical communication systems. [Means for solving the problem]

[0008] A node device according to one aspect of the present disclosure includes: a first wavelength selector switch that wavelength-separates an incoming first wavelength-division multiplexed optical signal, outputs some wavelengths of the optical signal to a first path, and outputs the remaining wavelengths of the optical signal to a second path; a first spectrum analyzer that observes the spectrum of a first optical signal of a first wavelength included in the first wavelength-division multiplexed optical signal and the spectrum of a second optical signal of the first wavelength output from the first wavelength selector switch, and outputs a first observation result; a second wavelength selector switch that outputs a second wavelength-division multiplexed optical signal obtained by wavelength-division multiplexing the remaining wavelengths of the optical signal input from the second path with an optical signal input from a third path; and a second spectrum analyzer that observes the spectrum of a third optical signal of a second wavelength included in the second wavelength-division multiplexed optical signal input to the second wavelength selector switch from the second path, and the spectrum of a fourth optical signal of the second wavelength output from the second wavelength selector switch, and outputs a second observation result.

[0009] A wavelength monitoring mechanism according to one aspect of the present disclosure includes: a first wavelength selection switch that separates an input wavelength-division multiplexed optical signal by wavelength, outputs an optical signal of some wavelengths to a first path, and outputs an optical signal of the remaining wavelengths to a second path; and a spectrum analyzer that observes the spectrum of a first optical signal of a first wavelength included in the wavelength-division multiplexed optical signal and the spectrum of a second optical signal of the first wavelength output from the wavelength selection switch, and outputs the observation results.

[0010] A wavelength monitoring mechanism according to one aspect of the present disclosure includes a wavelength selector switch that outputs a wavelength-multiplexed optical signal obtained by wavelength-multiplexing an optical signal input from a third path with an optical signal input from a second path, and a spectrum analyzer that observes the spectrum of a third optical signal of a second wavelength included in the optical signal input to the wavelength selector switch from the second path, and the spectrum of a fourth optical signal of the second wavelength output from the wavelength selector switch, and outputs the observation results.

[0011] A wavelength monitoring method in one aspect of the present disclosure involves a first wavelength selection switch that separates the wavelength of an input first wavelength-division multiplexed optical signal, outputs the optical signal of some wavelengths to a first path, and outputs the optical signal of the remaining wavelengths to a second path, observing the spectrum of a first optical signal of a first wavelength included in the first wavelength-division multiplexed optical signal and the spectrum of a second optical signal of the first wavelength output from the first wavelength selection switch, and outputting a first observation result. A second wavelength selection switch then outputs a second wavelength-division multiplexed optical signal obtained by wavelength-division multiplexing the optical signal input from a third path to the remaining wavelength optical signal input from the second path, observing the spectrum of a third optical signal of a second wavelength included in the second wavelength-division multiplexed optical signal input from the second path to the second wavelength selection switch, and outputting a second observation result. [Effects of the Invention]

[0012] According to the present disclosure, in transmission of an optical signal in an optical communication system, it is possible to monitor the pass characteristics of a wavelength selective switch through which the optical signal passes. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] [Figure 1] FIG. 1 is a diagram showing an example network configuration of a WDM transmission system according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing a configuration example of a WDM transmission system having another network topology. [Figure 3] FIG. 3 is a diagram schematically showing a configuration of a node device according to the first embodiment. [Figure 4] FIG. 4 is a diagram schematically showing a configuration of a wavelength cross connect (WXC) functional unit according to the first embodiment. [Figure 5] FIG. 5 is a diagram showing an example of a wavelength separation function of a wavelength selective switch (WSS) according to the first embodiment. [Figure 6] FIG. 6 is a diagram showing an example of a wavelength multiplexing function of a WSS according to an embodiment. [Figure 7] FIG. 7 is a diagram schematically showing a configuration of a WXC functional unit according to the first embodiment. [Figure 8] FIG. 8 is a diagram schematically showing an add-drop mechanism including two WSSs and a wavelength monitoring mechanism including the same. [Figure 9] FIG. 9 is a diagram schematically showing an add-drop mechanism including two WSSs and a wavelength monitoring mechanism including the same. [Figure 10] FIG. 10 is a diagram showing an outline of spectrum analysis in a spectrum analyzer according to the first embodiment. [Figure 11] FIG. 11 is a diagram showing an outline of adjustment of pass characteristics of a WSS. [Figure 12] FIG. 12 is a diagram schematically showing a main part of a WDM transmission system according to a second embodiment. [Figure 13] FIG. 13 is a diagram showing an output spectrum of a preceding-stage WSS, an output spectrum of a subsequent-stage WSS, and a corrected spectrum. DETAILED DESCRIPTION OF EMBODIMENTS

[0014] Embodiments of the present invention will be described below with reference to the drawings. In each drawing, the same elements are denoted by the same reference numerals, and repeated description is omitted as necessary.

[0015] Embodiment 1 A network configuration example of a WDM (Wavelength Division Multiplexing) transmission system according to Embodiment 1 will be described. FIG. 1 shows an example of a network configuration of a WDM transmission system 100 according to Embodiment 1. The WDM transmission system 100 is configured as an optical communication system that performs WDM transmission. The WDM transmission system 100 includes five node devices (hereinafter, NE: Node Equipment) 10A to 10E and a network management system (hereinafter, NMS: Network Management System) 110. Hereinafter, when individual NEs 10A to 10E are not specified, they are simply referred to as NE 10. NE 10A to NE 10E are connected by optical fibers F to form a ring topology. Although five NEs 10A to 10E are shown in FIG. 1, the number of NEs 10 is not limited to five and may be any number. In addition, although FIG. 1 shows an example in which the number of routes of NE 10 is two, the number of routes of NE 10 is not limited to two.

[0016] Although FIG. 1 shows a ring network as the WDM transmission system, the network topology is not limited to a ring type. FIG. 2 shows a configuration example of a WDM transmission system 100 having another network topology. As shown in FIG. 2, NE 10A to NE 10E may be connected by optical fibers F to form a linear topology.

[0017] Next, the NE10 according to this embodiment will be described. Figure 3 schematically shows the configuration of the NE10 according to Embodiment 1. Note that, as with Figures 1 and 2, Figure 3 shows an example configuration when the number of paths for the NE10 is two. The NE10 has at least an add-drop function for the transmitted optical signal, and can be configured, for example, as an optical add-drop multiplexer (OADM) device or a wavelength cross-connect node device.

[0018] The NE10 comprises Wavelength Cross-Connect (WXC) function units 1 and 2, an optical switch circuit 3, an NE control unit 4, and transponder function units TP_1 to TP_N. N is an integer greater than or equal to 2. When individual transponder function units TP_1 to TP_N are not specified, they are simply referred to as transponder function unit TP.

[0019] The WXC function unit 1 separates the wavelength of the optical signal L11 (also called the first wavelength-division multiplexed signal), which is a wavelength-division multiplexed signal input from the optical fiber F11, outputs the optical signal of the selectively separated wavelength to the optical switch circuit 3 (DROP), and passes the optical signal of the remaining wavelength to the WXC function unit 2 (THRU). The WXC function unit 1 also wavelength-divisions (ADD) the optical signal input from the optical switch circuit 3 with the optical signal input from the WXC function unit 2, and outputs the multiplexed optical signal L12 to the optical fiber F12.

[0020] The WXC function unit 2 separates the wavelength of the optical signal L21 (also called the second wavelength-division multiplexed signal), which is a wavelength-division multiplexed signal input from the optical fiber F21, outputs the optical signal of the selectively separated wavelength to the optical switch circuit 3 (DROP), and passes the optical signal of the remaining wavelength to the WXC function unit 1 (THRU). The WXC function unit 2 also wavelength-divisions (ADD) the optical signal input from the optical switch circuit 3 with the optical signal input from the WXC function unit 1, and outputs the multiplexed optical signal L22 to the optical fiber F22.

[0021] The optical switch circuit 3 combines the optical signals input from transponder function units TP_1 to TP_N as needed and outputs them to one or both of WXC function units 1 and 2. As a result, the optical signals output from the optical switch circuit 3 are added by WXC function units 1 and 2. The optical switch circuit 3 also outputs the signals dropped by WXC function unit 1 and WXC function unit 2 to transponder function units TP_1 to TP_N as needed.

[0022] The NE control unit 4 is configured to control the operation of the WXC function units 1 and 2 in response to commands from the NMS 110. For example, when the NE control unit 4 specifies an optical path (optical signal route), route specification information for cross-connection to specify the optical signal route is transmitted from the NMS 110 to each of the NE units 10A to 10E. In each of the NE units 10A to 10E, the NE control unit 4 receives the route specification information INF from the NMS 110 and passes the received route specification information INF to the WXC function units 1 and 2.

[0023] Next, the WXC function unit 1 will be described. Figure 4 schematically shows the configuration of the WXC function unit 1 according to Embodiment 1. The WXC function unit 1 includes wavelength selective switches (hereinafter referred to as WSS: Wavelength Selective Switch) 11 and 12, spectrum analyzers 13 and 14, and a WSS control unit 15.

[0024] The configurations of WSS11 and 12 will be described below. Figure 5 shows an example of the wavelength separation function of WSS11 according to Embodiment 1. As shown in Figure 5, WSS11 (also referred to as the first wavelength selector switch) has the function of wavelength-separating the wavelength-multiplexed optical signal IN passing through WSS11 into optical signals OUT 1 to OUT N of each wavelength. Figure 6 shows an example of the wavelength-multiplexing function of WSS12 according to Embodiment. As shown in Figure 6, WSS12 has the function of wavelength-multiplexing the optical signals IN 1 to IN N of each wavelength passing through WSS12 and outputting the wavelength-multiplexed optical signal OUT.

[0025] The spectrum analyzer 13 (also referred to as the first spectrum analyzer) is configured to compare and analyze the spectrum of the optical signal input to the WSS 11 and the spectrum of the optical signal output from the WSS. Specifically, the spectrum analyzer 13 is configured to compare the spectrum of the optical signal L11 input from the optical fiber F11 and the optical signal output to the WXC function unit 2 (THRU path) and the optical signal output to the optical switch circuit 3. DROP The spectrum of the path is compared and analyzed.

[0026] The spectrum analyzer 14 (also referred to as the second spectrum analyzer) is configured to compare and analyze the spectrum of the optical signal input to the WSS 12 and the spectrum of the optical signal output from the WSS. The spectrum analyzer 14 compares and analyzes the spectra of the optical signal input from the WXC function unit 2 (THRU path) and the optical signal input from the optical switch circuit 3 (ADD path) and the spectrum of the optical signal L12 output to the optical fiber F12.

[0027] The WSS control unit 15 (also referred to as the first wavelength selection switch control unit) controls the operation of WSS 11 and 12 in response to commands from the NW control unit 4. For example, based on the route designation information INF received from the NE control unit 4, the WSS control unit 15 can appropriately switch the wavelength of the optical signal dropped by WSS 11 and the wavelength of the optical signal added by WSS 12 to establish a desired optical signal route.

[0028] Next, the WXC function unit 2 will be described. Figure 7 schematically shows the configuration of the WXC function unit 2 according to Embodiment 1. As shown in Figure 7, the WXC function unit 2 has WSS21 and 22, spectrum analyzers 23 and 24, and a WSS control unit 25 (also referred to as the second wavelength selective switch control unit). In this embodiment, the WXC function unit 2 has the same configuration and functions as the WXC function unit 1. Therefore, the WSS21 and 22, spectrum analyzers 23 and 24, and WSS control unit 25 of the WXC function unit 2 correspond to WSS11 and 12, spectrum analyzers 13 and 14, and WSS control unit 15, respectively. In addition, the WSS21 receives the optical signal L21 from the optical fiber F21, and the WSS22 outputs the optical signal L22 from the optical fiber F22. Other redundant explanations are omitted for simplification.

[0029] Based on the configurations of the WXC functional units 1 and 2 described above, the optical signal transmission path in NE10 will now be explained. As mentioned above, NE10 has two paths, which means that two single-path add-drop functions are implemented, each having the function of adding an optical signal from one path and dropping an optical signal to the other path, with two WSSs arranged in series. Therefore, the basic single-path add-drop mechanism and the wavelength monitoring mechanism including it will be explained here.

[0030] Figure 8 schematically shows the add-drop mechanism consisting of WSS11 and 22 and the wavelength monitoring mechanism including it. WSS11 of the WXC function unit 1 separates the optical signal input from the optical fiber F11 by wavelength and outputs the optical signal of the selected wavelength to the optical switch circuit 3 (drop; the drop path is also called the first path). WSS11 also outputs the optical signal of wavelengths other than the selected wavelength directly to WSS22 of the WXC function unit 2 (through; the through path is also called the first path). 2 (Also referred to as the route.)

[0031] The WSS22 of the WXC function unit 2 wavelength-multiplexes (adds) the optical signal input from the optical switch circuit 3 (the input path is also referred to as the fourth path) to the optical signal output from the WSS11 of the WXC function unit 1, i.e., the optical signal that has been passed through the WSS11 (the input path is also referred to as the third path), and outputs it to the optical fiber F22.

[0032] Figure 9 schematically shows the add-drop mechanism consisting of WSS21 and 12, and the wavelength monitoring mechanism including it. WSS21 of the WXC function unit 2 separates the optical signal input from the optical fiber F21 by wavelength and outputs the optical signal of the selected wavelength to the optical switch circuit 3 (drop; the drop path is also referred to as the first path). WSS21 also outputs the optical signals of wavelengths other than the selected wavelength directly to WSS12 of the WXC function unit 1 (through; the through path is also referred to as the second path).

[0033] The WSS12 of the WXC function unit 1 wavelength-multiplexes (adds) the optical signal input from the optical switch circuit 3 (the input path is also referred to as the fourth path) to the optical signal output from the WSS21 of the WXC function unit 2, i.e., the optical signal that has been passed through the WSS21 (the input path is also referred to as the third path), and outputs it to the optical fiber F12.

[0034] Next, the comparison and analysis of spectra in each spectrum analyzer will be explained. The spectrum analyzer compares the optical signal of each input wavelength with the spectrum of the same wavelength (first wavelength, second wavelength) and the spectrum in between. Figure 10 shows an overview of the spectrum analysis in the spectrum analyzer according to Embodiment 1. In the figure, when spectra are shown, the horizontal axis represents frequency f and the vertical axis represents power P. The spectrum analyzer can monitor the pass characteristics of the corresponding WSS by acquiring the difference between the optical signal OUT of the optical signal of the same wavelength and the optical signal IN of each input wavelength.

[0035] Specifically, the spectrum analyzer 13 compares and analyzes the spectrum of a specific wavelength (first wavelength) optical signal (first optical signal) among the optical signals input to WSS11 and the spectrum of a specific wavelength (first wavelength) optical signal (second optical signal) among the optical signals output from WSS11. The spectrum analyzer 23 compares and analyzes the spectrum of a specific wavelength (first wavelength) optical signal (first optical signal) among the optical signals input to WSS21 and the spectrum of a specific wavelength (first wavelength) optical signal (second optical signal) among the optical signals output from WSS21.

[0036] Spectrum analyzer 14 compares and analyzes the spectrum of a specific wavelength (second wavelength) optical signal (third optical signal) among the optical signals input to WSS12 and the spectrum of a specific wavelength (second wavelength) optical signal (fourth optical signal) among the optical signals output from WSS12. Spectrum analyzer 24 compares and analyzes the spectrum of a specific wavelength (second wavelength) optical signal (third optical signal) among the optical signals input to WSS22 and the spectrum of a specific wavelength (second wavelength) optical signal (fourth optical signal) among the optical signals output from WSS22.

[0037] This configuration makes it possible to adjust the pass characteristics of each WSS to a desired state based on the monitoring results of the pass characteristics of each WSS.

[0038] The spectrum analyzer may output the monitoring results of the pass characteristics of each WSS to the NMS110. In this case, the NMS110 may, in light of the functions required of the NE10, the source of the monitoring results within the WDM transmission system 100, determine, for example, a target value for the pass characteristics of each WSS and issue a command to the target NE10. In this case, the NE control unit 4 of the NE10 gives a command to the NE control unit 4 of the WXC function unit that is the target of control, to perform the necessary control of the WSS. Based on this, the WSS control units 15 and 25 can adjust the pass characteristics (filter characteristics) of the target WSS according to the command.

[0039] Figure 11 shows an overview of the adjustment of the WSS pass-through characteristics. Here, as an example, we assume that the pass-through characteristics of WSS11 obtained by observation (dashed line in Figure 11) are shifted to the higher frequency side relative to the target pass-through characteristics of WSS11 (solid line in Figure 11). In this case, the WSS control unit 15 should control WSS11 so that the pass-through characteristics of WSS11 (dashed line) match the target (solid line). At this time, the WSS control unit 15 can appropriately adjust the center frequency and width of the pass-through characteristics of WSS11.

[0040] Furthermore, the target values ​​for the pass characteristics (filter characteristics) of each WSS may be predetermined and stored in the WSS control units 15 and 25. In this case, the WSS control units 15 and 25 may adjust the pass characteristics (filter characteristics) of the WSS as appropriate, based on the monitoring results of the pass characteristics (filter characteristics) of the controllable WSS, so that they reach the target values.

[0041] Embodiment 2 Embodiment 2 describes an example in which a spectrum analyzer performs spectral analysis after correcting the spectrum. In Embodiment 2, although the operation in the WDM transmission system is different, the configuration of the WDM transmission system and the WXC function unit is the same as in Embodiment 1, so redundant explanations will be omitted.

[0042] Figure 12 schematically shows the main components of the WDM transmission system according to Embodiment 2. In this example, when focusing on a specific wavelength (channel) λ of the transmitted optical signal, the spectrum observed by the spectrum analyzer attached to the preceding WSS is compared with the spectrum observed by the spectrum analyzer attached to the succeeding WSS. Then, by correcting the frequency observation error obtained from the observation results, the transmission characteristics of the WSS are evaluated with high accuracy. Here, it is assumed that the preceding WSS is WSS11 of the WXC function unit 1, and the succeeding WSS is WSS22 of the WXC function unit 2.

[0043] Figure 13 shows the output spectrum of the preceding WSS11, the output spectrum of the succeeding WSS22, and the corrected spectrum. Generally, there are discrepancies in frequency detection accuracy between different spectrum analyzers. For example, as shown in the uncorrected spectrum in Figure 13, the output spectrum of the succeeding WSS22 observed by spectrum analyzer 24 has a center frequency that is shifted to the positive side compared to the output spectrum of the preceding WSS11 observed by spectrum analyzer 13.

[0044] As shown in Figure 13, comparing observation results from different spectral analyzers can be achieved, for example, by transmitting the observation results from each spectral analyzer to the NMS110, which then performs the comparison. The NMS110 can then detect the frequency shift and command the spectral analyzer 13 of the WXC function unit 1 and the spectral analyzer 24 of the WXC function unit 2 to correct the shift. This makes it possible to match the center frequencies of the spectra observed by both analyzers.

[0045] Various methods can be used to correct the center frequency of the spectrum observed in the first stage to match the center frequency of the spectrum observed in the second stage. Here, one example is described. For example, one or both spectra of spectrum analyzers 13 and 24 may be corrected so as to maximize the portion where the spectra in a predetermined range R centered on the peak of each spectrum, as shown in Figure 13, coincide. In this case, since the correction can be performed based on the spectrum near the peak that is not affected by filtering in WSS, high-precision correction can be achieved.

[0046] The above described spectral correction between spectra observed by two spectral analyzers. However, even when the WSS is set up in multiple stages, it is possible to monitor the transmission characteristics of the optical signal throughout the entire system with high precision by sequentially performing spectral correction between spectra observed by two spectral analyzers.

[0047] Furthermore, the two spectra to be compared are not limited to two spectral analyzers within the same NE, but may be two spectral analyzers located in different NEs. That is, as shown in Figures 1 and 2, if the WDM transmission system includes multiple NEs, the spectrum from a spectral analyzer monitoring a WSS in one NE connected by an optical transmission path may be compared with the spectrum from a spectral analyzer monitoring a WSS in another NE, and appropriate corrections may be made.

[0048] With this configuration, as described above, the WSS pass-through characteristics can be adjusted as needed even after the optical communication system has started operation. Therefore, the WSS pass-through characteristics that need to be considered during system design can be relaxed, enabling a more flexible system design.

[0049] Other embodiments It should be noted that the present invention is not limited to the embodiments described above, and can be modified as appropriate without departing from the spirit of the invention. For example, although an example in which the NMS110 commands a correction has been described, the spectral observation results may be aggregated in either the WXC function unit 1 or the WXC function unit 2. In this case, the WSS control unit of the WXC function unit where the observation results are aggregated may detect the amount of frequency shift and command the spectrum analyzer to correct the amount of shift.

[0050] Furthermore, the spectrum analyzer in the WXC function unit, which aggregates the observation results, may simultaneously detect and correct the frequency shift. Additionally, if other processing means besides the spectrum analyzer and WSS control unit are provided within the WXC function unit, these processing means may detect the frequency shift and instruct the spectrum analyzer to correct the shift. [Explanation of Symbols]

[0051] 1, 2 WXC function section 3. Optical switch circuit 4 NE Control Unit 10 NE (Node Device) 11, 12, 21, 22 WSS 13, 14, 23, 24 Spectrum analyzers 15, 25 WSS Control Unit 100 Transmission Systems 110 NMS F, F11, F12, F21, F22 Optical Fiber L11, L12, L21, L22 optical signal TP_1~TP_N Transponder Function Unit

Claims

1. A first wavelength selection switch that separates the wavelengths of an incoming first wavelength-division multiplexed optical signal, outputs some wavelengths of the optical signal to a first path, and outputs the remaining wavelengths of the optical signal to at least a second path, A first spectrum analyzer that outputs a first observation result based on the spectrum of a first optical signal included in the first wavelength-division multiplexed optical signal and the spectrum of a second optical signal output from the first wavelength-selective switch, A second wavelength selector switch that outputs a second wavelength-division multiplexed optical signal that includes at least a portion of the remaining wavelength optical signal input from the second path, A second spectrum analyzer that outputs a second observation result based on the spectrum of a third optical signal included in the second wavelength-division multiplexed optical signal input to the second wavelength-selective switch, and the spectrum of a fourth optical signal output from the second wavelength-selective switch, The system includes a detection means for detecting the error between the center frequency of the spectrum of the second optical signal and the center frequency of the spectrum of the fourth optical signal, based on the first and second observation results. Optical transmission system.

2. The first spectrum analyzer obtains the transmission characteristics of the optical signal at the first wavelength selector switch by subtracting the spectrum of the first optical signal from the spectrum of the second optical signal based on the first observation result. The second spectrum analyzer obtains the transmission characteristics of the optical signal at the second wavelength selector switch by subtracting the spectrum of the third optical signal from the spectrum of the fourth optical signal based on the second observation result. The optical transmission system according to claim 1.

3. A first wavelength selector switch control unit that controls the first wavelength selector switch, The system further comprises a second wavelength selective switch control unit for controlling the second wavelength selective switch, The first wavelength selective switch control unit adjusts the optical signal transmission characteristics of the first wavelength selective switch based on the acquired optical signal transmission characteristics of the first wavelength selective switch. The second wavelength selective switch control unit adjusts the optical signal transmission characteristics of the second wavelength selective switch based on the acquired optical signal transmission characteristics of the second wavelength selective switch. The optical transmission system according to claim 2.

4. Based on the detected error, the detection means commands the first and second wavelength-selective switch control units to match the center frequency of the spectrum of the second optical signal and the center frequency of the spectrum of the fourth optical signal. The first and second wavelength-selective switch control units control the first and second wavelength-selective switches, respectively, in accordance with the command, so that the center frequency of the spectrum of the second optical signal matches the center frequency of the spectrum of the fourth optical signal. The optical transmission system according to claim 3.

5. One or both of the first and second wavelength selective switch control units function as the detection means. The optical transmission system according to claim 4.

6. The first wavelength selection switch separates the wavelength of the input first wavelength-division multiplexed optical signal, outputs some wavelengths of the optical signal to the first path, and outputs the remaining wavelengths of the optical signal to at least the second path. The first spectrum analyzer outputs a first observation result based on the spectrum of the first optical signal included in the first wavelength-division multiplexed optical signal and the spectrum of the second optical signal output from the first wavelength-selective switch. A second wavelength selection switch outputs a second wavelength-division multiplexed optical signal that includes at least a portion of the remaining wavelengths of the input optical signal. The second spectrum analyzer outputs a second observation result based on the spectrum of the third optical signal included in the second wavelength-division multiplexed optical signal input to the second wavelength-selective switch, and the spectrum of the fourth optical signal output from the second wavelength-selective switch. The detection means detects the error between the center frequency of the spectrum of the second optical signal and the center frequency of the spectrum of the fourth optical signal based on the first and second observation results. Wavelength monitoring method.

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