Optical transmission device, optical transmission method, and optical transmission system

Abstract

To provide an optical transmission device, an optical transmission method, and an optical transmission system for suppressing degradation of transmission performance due to band extension.SOLUTION: An optical transmission device includes: a receiver receiving, from an optical transmission line, a wavelength-multiplexed signal obtained by multiplexing first to third optical signals after power of each of the first to third optical signals is adjusted, the first to third optical signals having wavelengths belonging to first to third wavelength bands, respectively, the second wavelength band being longer than the first wavelength band, the third wavelength band being shorter than the first wavelength band; an amplifier amplifying the wavelength-multiplexed signal based on excitation light output in the optical transmission line, the excitation light propagating in a direction opposite a propagation direction of the wavelength-multiplexed signal; and a controller controlling a gain of the amplifier based on the quality of the third optical signal calculated by optical power of the third optical signal before being transmitted to the optical transmission line and optical power of the third optical signal included in the wavelength-multiplexed signal received by the receiver.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] WDM (Wavelength Division Multiplexing) transmission is sometimes performed using multiple wavelength bands such as the C band (Conventional Band) and the L band (Long Band) (see, for example, Patent Document 1). The C band tends to be defined as the band from 1529.55 nm (nanometers) to 1563.86 nm. The L band tends to be defined as the band from 1567.95 nm to 1604.02 nm. Thus, the L band is a longer wavelength band than the C band.

[0003] When WDM transmission is performed, the optical energy on the short wavelength side excites the light on the long wavelength side due to the SRS (Stimulated Raman Scattering) phenomenon. As a result, a tilt occurs on the receiving side, where the optical power of the signal light on the short wavelength side is small and the optical power of the signal light on the long wavelength side is large. When a tilt occurs in the optical power, a tilt also occurs in the OSNR (Optical Signal to Noise Ratio), where the OSNR on the short wavelength side is small and the OSNR on the long wavelength side is large. The occurrence of such a tilt induces a deterioration in the transmission performance of WDM transmission. For example, when a tilt occurs in the OSNR, it becomes difficult to increase the transmission capacity and extend the possible transmission distance.

[0004] In order to suppress the occurrence of OSNR tilt, a control called pre-emphasis is sometimes implemented. Pre-emphasis is a control that adjusts the optical power of the short wavelength band on the transmitting side in advance, for example. Pre-emphasis suppresses the tilt of SNR in each wavelength band on the receiving side. In other words, the SNR in each wavelength band on the receiving side becomes flat. In this way, the transmission performance of WDM transmission is improved by implementing pre-emphasis. Note that such optical pre-emphasis is sometimes called OSNR pre-emphasis (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2003-188831 A [Patent Document 2] US Patent Application Publication No. 2002 / 0191903 Summary of the Invention [Problem to be solved by the invention]

[0006] Meanwhile, new WDM transmissions are being considered that further improve transmission performance by using not only the above-mentioned C and L bands but also the S band (Short band). The S band tends to be defined as the band from 1489.70 nm to 1522.56 nm. In other words, the S band is a shorter wavelength band than the C band. In this new WDM transmission with the band extended to the S band, the wider wavelength band may increase the tilt of the received power caused by SRS compared to the WDM transmissions using the C and L bands.

[0007] Although the above-mentioned existing pre-emphasis is expected to eliminate the tilt of the S-band receiving power caused by SRS, there is a possibility that the transmission performance of WDM transmission will not be improved sufficiently. In other words, in new WDM transmission with the band extended to the S-band, even if pre-emphasis is implemented, tilt will remain in the Generalized SNR (GSNR), which is the quality of the optical signal that takes into account the nonlinear noise generated in the optical fiber transmission line, and the transmission performance may not be improved. This is a factor that makes it difficult to extend the band to the S-band.

[0008] Therefore, in one aspect, an object of the present invention is to provide an optical transmission device, an optical transmission method, and an optical transmission system that suppress a decrease in transmission performance that accompanies band expansion. [Means for solving the problem]

[0009] In one embodiment, an optical transmission device includes a receiving unit that receives wavelength-multiplexed signal light from an optical transmission line in which a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, after adjusting the optical power of each of the first signal light, the second signal light, and the third signal light, and an amplifying unit that amplifies the wavelength-multiplexed signal light based on output of pump light propagating in a direction opposite to the propagation direction of the wavelength-multiplexed signal light to the optical transmission line, and a control unit that controls a gain of the amplifying unit based on a quality of the third signal light calculated based on the optical power of the third signal light before being transmitted to the optical transmission line and the optical power of the third signal light included in the wavelength-multiplexed signal light received by the receiving unit. Effect of the Invention

[0010] It is possible to suppress the degradation of transmission performance that accompanies band expansion. [Brief description of the drawings]

[0011] [Figure 1] 1 is an example of an optical network. [Diagram 2] 1 is an example of an optical transmission system. [Diagram 3] 3 is an example of the functional configuration of a transmission side control unit and a reception side control unit. [Figure 4] 4 is a flowchart showing an example of the operation of the optical transmission system in the first embodiment. [Diagram 5] (a) is an example of optical power before implementing pre-emphasis control in a transmitting node, (b) is an example of optical power when pre-emphasis control is not implemented in a receiving node, and (c) is an example of SNR when pre-emphasis control is not implemented. [Figure 6] (a) is an example of optical power after performing pre-emphasis control at a transmitting node, (b) is an example of optical power at a receiving node after performing pre-emphasis control, and (c) is an example of SNR after performing pre-emphasis control. [Figure 7] 1A is an example of optical power at a transmitting node in a comparative example, FIG. 1B is an example of optical power at a receiving node in a comparative example, and FIG. 1C is an example of SNR in a comparative example. [Figure 8] 1 is an example of gain characteristics of backward Raman amplification. [Figure 9] 1A is an example of optical power at a transmitting node in the first embodiment, FIG. 1B is an example of optical power at a receiving node in the first embodiment, and FIG. 1C is an example of SNR in the first embodiment. [Figure 10] FIG. 4 is a diagram illustrating an example of an effect of the first embodiment. [Figure 11] 10 is a flowchart partially illustrating an example of the operation of the optical transmission system in the second embodiment. [Figure 12] 13A is an example of optical power at a transmitting node in the second embodiment, FIG. 13B is an example of optical power at a receiving node in the second embodiment, and FIG. 13C is an example of SNR in the second embodiment. [Figure 13] FIG. 11 is a diagram illustrating an example of an effect of the second embodiment. [Figure 14] 13 is a flowchart partially illustrating an example of an operation of an optical transmission system according to another embodiment. [Figure 15] 13 is an example of a functional configuration of a backward Raman control unit in the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0013] (First embodiment) As shown in Fig. 1, the optical network NW includes a C-band transponder (denoted as TRPN in Fig. 1) 11, an L-band transponder 12, and an S-band transponder 13. The optical network NW also includes an optical transmission device 100 as a transmitting node and an optical transmission device 200 as a receiving node. The optical transmission device 100 is an example of a first optical transmission device. The optical transmission device 200 is an example of a second optical transmission device.

[0014] The optical transmission devices 100 and 200 may be, for example, a ROADM (Reconfigurable Optical Add / Drop Multiplexer) or an ILA (In-Line Amplifier Equipment). The C-band transponder 11, the L-band transponder 12, the S-band transponder 13, and the optical transmission device 100 are installed, for example, in a first station 10 of a telecommunications company. The first station 10 includes, for example, a terminal station. The optical transmission device 200 is installed, for example, in a second station 20 located several tens of kilometers away from the first station 10. The second station 20 includes, for example, a relay station.

[0015] The optical transmission devices 100 and 200 are connected to each other by an optical transmission line 300. The optical transmission line 300 may be an optical fiber such as SMF (Single Mode Fiber), or an optical fiber other than SMF such as DSF (Dispersion Shifted Fiber). The optical transmission devices 100 and 200 can realize an optical transmission system ST.

[0016] The C-band transponder 11 includes a transmitter (represented as Tx-C in FIG. 1) 11C that transmits a C-band signal light L1. The C-band is an example of a first wavelength band. The signal light L1 includes a plurality of wavelengths (e.g., 48 wavelengths) that belong to the C-band. In this embodiment, the wavelength band from 1531.12 nm to 1563.97 nm is used as the C-band.

[0017] The L-band transponder 12 includes a transmitter (denoted as Tx-L in FIG. 1) 12L that transmits an L-band signal light L2. The L-band is an example of a second wavelength band. The signal light L2 includes a plurality of wavelengths (e.g., 48 wavelengths) that belong to the L-band. In this embodiment, the wavelength band from 1566.31 nm to 1600.71 nm is used as the L-band. Thus, the L-band is a longer wavelength band than the C-band.

[0018] The S-band transponder 13 includes a transmitter (denoted as Tx-S in FIG. 1) 13S that transmits an S-band signal light L3. The S-band is an example of a third wavelength band. The signal light L3 includes a plurality of wavelengths (e.g., 48 wavelengths) that belong to the S-band. In this embodiment, the wavelength band from 1497.47 nm to 1528.87 nm is used as the S-band. Thus, the S-band is a shorter wavelength band than the C-band.

[0019] The optical transmission device 100 receives the signal lights L1, L2, and L3 transmitted from the transmitters 11C, 12L, and 13S, respectively. When the optical transmission device 100 receives the signal lights L1, L2, and L3, it controls and adjusts the optical power of each of the signal lights L1, L2, and L3. As will be described in detail later, the optical transmission device 100 performs pre-emphasis control on the signal lights L1, L2, and L3. After controlling and adjusting the optical power of each of the signal lights L1, L2, and L3, the optical transmission device 100 multiplexes the signal lights L1, L2, and L3, and outputs the wavelength-multiplexed signal light obtained by multiplexing the signal lights L1, L2, and L3 as WDM light Lw to the optical transmission line 300. As a result, the WDM light Lw propagates through the optical transmission line 300 toward the optical transmission device 200.

[0020] The optical transmission device 200, as a receiving node, receives the WDM light Lw from the optical transmission line 300. When the optical transmission device 200 receives the WDM light Lw, it amplifies the WDM light Lw and outputs it downstream of the optical transmission device 200. Note that, as will be described in detail later, the optical transmission device 200 outputs pump light Lp, which propagates in the opposite direction to the propagation direction of the WDM light Lw, to the optical transmission line 300. The pump light Lp amplifies the WDM light Lw by the Raman amplification phenomenon. In this way, the WDM light Lw is optically amplified in the optical transmission line 300.

[0021] The optical transmission devices 100 and 200 included in the optical transmission system ST will be described in detail with reference to FIG.

[0022] First, the optical transmission device 100 will be described. The optical transmission device 100 includes a first optical power adjustment unit 110, a second optical power adjustment unit 120, a transmitting-side multiplexer 130, and a transmitting-side control unit 140. The optical transmission device 100 also includes a plurality of ASE (Amplified Spontaneous Emission) light sources 151, 152, and 153, a plurality of branching couplers 161, 162, and 163, and a plurality of OCM (Optical channel monitors) 171, 172, and 173. Although details will be described later, the ASE light sources 151, 152, and 153 do not necessarily have to be included in the optical transmission device 100.

[0023] The first optical power adjustment unit 110 includes a WSS (Wavelength Selective Switch) 111 corresponding to the C band, a WSS 112 corresponding to the L band, and a WSS 113 corresponding to the S band. Although omitted in Fig. 2, the WSS 111 is optically connected to the ASE light source 151 and a fiber from the other path. The WSS 112 is optically connected to the ASE light source 152 and a fiber from the other path. The WSS 113 is optically connected to the ASE light source 153 and a fiber from the other path.

[0024] The WSS 111 splits the ASE light emitted from the ASE light source 151 into wavelengths and generates dummy signal light for each wavelength. The dummy signal light is a pseudo signal light. The WSS 111 controls the wavelengths of the signal light so that the dummy wavelengths of the ASE light source are transmitted, so that the number of wavelengths is always the maximum. This makes it possible to avoid the influence of changes in the number of wavelengths, such as SRS tilt, that cause the level of each channel to fluctuate.

[0025] The WSS 111 adjusts the transmission power of the signal light L1 based on the first optical power control by the transmitting side controller 140. For example, when the first optical power adjuster 110 detects the first optical power control, it controls the WSS 111 to increase the transmission power of the short wavelength side of the signal light L1 more than the transmission power of the long wavelength side. If the transmission power of the signal light L1 is flat, an intentional tilt occurs in the transmission power of the signal light L1 due to the first optical power control. In this way, when the transmission power of the signal light L1 is adjusted, the tilt of the reception power of the signal light L1 is improved in the optical transmission device 200 compared to when the transmission power is not adjusted.

[0026] Since WSSs 112 and 113 are basically the same as WSS 111, detailed description thereof will be omitted.

[0027] The second optical power adjustment unit 120 includes optical amplifiers 121 and 123 corresponding to the C band, and a VOA (Variable Optical Attenuator) 122 corresponding to the C band. The optical amplifiers 121 and 123 include, for example, an EDFA (Erbium Doped Fiber Amplifier). The optical amplifier 121 amplifies the signal light L1 output from the WSS 111. The VOA 122 attenuates the signal light L1 output from the optical amplifier 121. The optical amplifier 123 amplifies the signal light L1 output from the VOA 122.

[0028] The second optical power adjustment unit 120 adjusts the optical power of each of the signal lights L1, L2, and L3 based on the second optical power control by the transmission side control unit 140. For example, when the second optical power adjustment unit 120 detects the second optical power control, it controls the optical amplifiers 121 and 123 and the VOA 122 to increase the transmission power of the short wavelength side of the signal light L1 more than the transmission power of the long wavelength side. If the transmission power of the signal light L1 is flat, an intentional tilt occurs in the transmission power of the signal light L1 due to the second optical power control. In this way, when the transmission power of the signal light L1 is adjusted, the tilt of the reception power of the signal light L1 is improved in the optical transmission device 200 compared to when the transmission power is not adjusted.

[0029] The second optical power adjustment unit 120 includes optical amplifiers 124 and 126 corresponding to the L band and a VOA 125 corresponding to the L band. The second optical power adjustment unit 120 also includes optical amplifiers 127 and 129 corresponding to the S band and a VOA 128 corresponding to the S band. Therefore, when the second optical power adjustment unit 120 adjusts the transmission power of each of the signal lights L2 and L3 in the same manner as in the case of the signal light L1, the tilt of each of the reception powers of the signal lights L2 and L3 is improved in the optical transmission device 200. In this embodiment, the second optical power adjustment unit 120 performs tilt control and average level control of the transmission power of the signal light before being sent to the optical transmission line 300, but the second optical power adjustment unit 120 may perform only tilt control and provide a VOA for each band at the output end of the second optical power adjustment unit 120 to control the average level of the transmission power of each wavelength band.

[0030] In this way, even if the pre-emphasis control is performed solely on the signal lights L1, L2, and L3, the tilt of the reception power of the signal lights L1, L2, and L3 appearing from the shortest wavelength side of the S band to the longest wavelength side of the L band is generally improved. When the tilt of the reception power is improved, the tilt of the GSNR is also improved due to the improvement of the tilt, and the deterioration of the transmission performance is suppressed. However, as will be described in detail later, even if the pre-emphasis control is performed solely, the suppression of the deterioration of the transmission performance is insufficient, so in this embodiment, further suppression of the deterioration of the transmission performance is described.

[0031] The transmitting-side multiplexer 130 multiplexes the signal light L1 output from the optical amplifier 123, the signal light L2 output from the optical amplifier 126, and the signal light L3 output from the optical amplifier 129 to generate the WDM light Lw. After generating the WDM light Lw, the transmitting-side multiplexer 130 outputs the WDM light Lw to the optical transmission line 300.

[0032] The transmitting side control unit 140 is electrically connected to the OCMs 171, 172, and 173. The OCM 171 measures the optical power of the signal light L1 branched by the branching coupler 161. Here, the branching coupler 161 is provided between the second optical power adjustment unit 120 and the transmitting side multiplexer 130. Therefore, the OCM 171 measures the optical power of the signal light L1 before it is multiplexed by the transmitting side multiplexer 130. The OCM 171 notifies the transmitting side control unit 140 of the optical power of the signal light L1. Note that the OCMs 172 and 173 and the branching couplers 162 and 163 are basically the same as the OCM 171 and the branching coupler 161, so detailed description thereof will be omitted.

[0033] The transmitting side control unit 140 calculates a nonlinear SNR (to be described later) based on the transmission power of each of the signal lights L1, L2, and L3 notified from each of the OCMs 171, 172, and 173. The nonlinear SNR is calculated for a section up to the branch couplers 271, 272, and 273 after the wavelength multiplexed signal light multiplexed by the transmitting side multiplexer 130 is output to the optical transmission line 300 as the WDM light Lw, demultiplexed by the receiving side demultiplexer 210, and amplified by the optical amplifiers 261, 262, and 263. The transmitting side control unit 140 also calculates the GSNR as the signal quality of the signal lights L1, L2, and L3 based on the nonlinear SNR and the linear SNR output from the optical transmission device 200. After calculating the GSNR, the transmitting side control unit 140 performs a first optical power control and a second optical power control based on the GSNR. The first optical power control and the second optical power control perform tilt control of the signal light before it is sent to the optical transmission line 300 and control of the average level of the transmission power of each wavelength band. In addition, the transmitting-side control unit 140 calculates the tilt amount of the GSNR of the S band and notifies the receiving-side control unit 240 of it.

[0034] The linear SNR is notified from the optical transmission device 200 using, for example, an optical supervisory channel (OSC) light. The linear SNR may be notified to the transmission side control unit 140 via a network controller that manages the optical network NW.

[0035] Next, the optical transmission device 200 will be described. The optical transmission device 200 includes a demultiplexer 210, an optical power adjustment unit 220, a receiving side multiplexer 230, a receiving side control unit 240, and a backward Raman amplifier 250. The optical transmission device 200 also includes a WDM coupler 260, a plurality of optical amplifiers 261, 262, and 263, a plurality of branching couplers 271, 272, and 273, and a plurality of OCMs 281, 282, and 283. The demultiplexer 210 is an example of a receiving unit and a demultiplexing unit. The receiving side control unit 240 is an example of a control unit. The backward Raman amplifier 250 is an example of an amplifier.

[0036] The demultiplexer 210 receives the WDM light Lw from the optical transmission line 300. Upon receiving the WDM light Lw, the demultiplexer 210 demultiplexes the WDM light Lw into signal light L1, L2, and L3. The signal light L1 is guided to an optical amplifier 261. The signal light L2 is guided to an optical amplifier 262. The signal light L3 is guided to an optical amplifier 263. The optical amplifier 261 amplifies the signal light L1. The optical amplifier 262 amplifies the signal light L2. The optical amplifier 263 amplifies the signal light L3.

[0037] The optical power adjuster 220 includes optical amplifiers 221 and 223 corresponding to the C band, and a VOA 222 corresponding to the C band. The optical amplifiers 221 and 223 include, for example, EDFAs. The optical amplifier 221 amplifies the signal light L1 output from the optical amplifier 261. The VOA 222 attenuates the signal light L1 output from the optical amplifier 221. The optical amplifier 223 amplifies the signal light L1 output from the VOA 222.

[0038] The optical power adjuster 220 adjusts the optical power of each of the signal lights L1, L2, and L3 based on a control (not shown) output from the receiving-side controller 240. For example, when the optical power adjuster 220 detects the control, it controls the optical amplifiers 221 and 223 and the VOA 222 to adjust the optical power of the signal light L1.

[0039] The optical power adjuster 220 includes optical amplifiers 224 and 226 corresponding to the L band, and a VOA 225 corresponding to the L band. The optical power adjuster 220 also includes optical amplifiers 227 and 229 corresponding to the S band, and a VOA 228 corresponding to the S band. Therefore, the optical power adjuster 220 can adjust the optical power of each of the signal light L2 and L3 in the same way as in the case of the signal light L1.

[0040] The receiving side multiplexer 230 multiplexes the signal light L1 output from the optical amplifier 223, the signal light L2 output from the optical amplifier 226, and the signal light L3 output from the optical amplifier 229 to generate WDM light, and outputs the WDM light downstream of the optical transmission device 200.

[0041] The receiving side control unit 240 is electrically connected to the OCMs 281, 282, and 283. The OCM 281 measures the optical power of the signal light L1 branched by the branching coupler 271. The branching coupler 271 is provided between the optical amplifier 261 and the optical power adjusting unit 220. Therefore, the OCM 281 measures the optical power of the signal light L1 after it has been amplified by the optical amplifier 261. The OCM 281 notifies the receiving side control unit 240 of the optical power of the signal light L1. Note that the OCMs 282 and 283 and the branching couplers 272 and 273 are basically the same as the OCM 281 and the branching coupler 271, and therefore detailed description thereof will be omitted.

[0042] The receiving side control unit 240 calculates a linear SNR, which will be described later, based on the reception power of each of the signal lights L1, L2, and L3 notified from each of the OCMs 281, 282, and 283. The linear SNR is calculated for a section up to the branch couplers 271, 272, and 273 after the wavelength-multiplexed signal light multiplexed by the transmitting side multiplexer 130 is output to the optical transmission line 300 as the WDM light Lw, demultiplexed by the receiving side demultiplexer 210, and amplified by the optical amplifiers 261, 262, and 263. The receiving side control unit 240 also controls the gain of the backward Raman amplifier 250 based on the tilt amount of the S-band GSNR notified from the optical transmission device 100. Specifically, the receiving side control unit 240 adjusts the power of each pump wavelength of the pump light Lp output by the LD (Laser Diode) 251 of the backward Raman amplifier 250 to realize a predetermined gain characteristic related to the backward Raman amplification.

[0043] The pump light Lp is input to the optical transmission line 300 via the WDM coupler 260. As a result, the pump light Lp propagates through the optical transmission line 300 in the opposite direction to the propagation direction of the WDM light Lw. As a result, in the optical transmission line 300, the WDM light Lw is Raman amplified by the pump light Lp. That is, the backward Raman amplifier 250 Raman amplifies the WDM light Lw. The tilt amount of the S-band GSNR is notified from the optical transmission device 100 by, for example, OSC light. This tilt amount may be notified to the receiving side control unit 240 via the above-mentioned network controller.

[0044] 3, the transmitting side control unit 140 and the receiving side control unit 240 will be described in detail. The transmitting side control unit 140 includes a non-linear SNR calculation unit 141, a GSNR calculation unit 142, and a power control unit 143. The receiving side control unit 240 includes a linear SNR calculation unit 241 and a backward Raman control unit 242.

[0045] The transmitting side control unit 140 can be realized by a hardware circuit such as a field programmable gate array (FPGA) and a memory. The transmitting side control unit 140 may be realized by a hardware circuit such as an application specified integrated circuit (ASIC), a digital signal processor (DSP), or a central processing unit (CPU) instead of an FPGA. The hardware configuration of the receiving side control unit 240 is basically the same as that of the transmitting side control unit 140, so a detailed description will be omitted.

[0046] The nonlinear SNR calculation unit 141 of the transmission side control unit 140 calculates the nonlinear SNR as SNR_NL for each channel (band) based on the optical powers of the signal lights L1, L2, and L3 notified from the OCMs 171, 172, and 173 and the following formula (1).

number

[0047] Here, the numerator P_CH(T) represents the optical power of the wavelength channel to be measured in the optical transmission device 100 as the transmitting node. That is, P_CH(T) represents the optical power of the C-band signal light L1 measured by the OCM 171, for example. P_CH(T) may represent the optical power of the L-band signal light L2 measured by the OCM 172, or may represent the optical power of the S-band signal light L3 measured by the OCM 173. The numerator B_CH represents the bandwidth of the wavelength channel. Therefore, P_CH(T) / B_CH corresponds to the optical power per unit bandwidth of the wavelength channel to be measured.

[0048] The denominator G_NLI represents the optical power of nonlinear noise per unit bandwidth. When P_NLI, which represents the optical power of nonlinear noise, is expressed by the following formula (2), G_NLI is expressed by the following formula (3).

number

number

[0049] Here, η in formula (2) represents a known proportionality coefficient for calculating the nonlinear SNR. Therefore, P_NLI is proportional to, for example, the cube of the optical power of the signal light L1 input to the transmitting-side multiplexer 130. ηd in formula (3) represents a known proportionality coefficient determined by the fiber type of the optical transmission line 300, etc. Since B_CH is known, when the optical powers of the signal lights L1, L2, and L3 are notified to the nonlinear SNR calculation unit 141, the nonlinear SNR calculation unit 141 can calculate the nonlinear SNR.

[0050] On the other hand, the linear SNR calculation unit 241 of the reception side control unit 240 calculates the linear SNR as SNR_L for each channel (band) based on the optical power of the signal lights L1, L2, and L3 notified from the OCMs 281, 282, and 283 and the following formula (4).

number

[0051] Here, P_CH represents the optical power of the wavelength channel to be measured in the optical transmission device 200 as the receiving node. That is, P_CH represents the optical power of the C-band signal light L1 measured by the OCM 281, for example. P_CH may represent the optical power of the L-band signal light L2 measured by the OCM 282, or the optical power of the S-band signal light L3 measured by the OCM 283. P_ASE represents the optical power of the ASE noise. In this way, the linear SNR is represented by the ratio of the optical power of the signal light to the optical power of the ASE noise.

[0052] The GSNR calculation unit 142 of the transmission side control unit 140 calculates the GSNR based on the non-linear SNR calculated by the non-linear SNR calculation unit 141, the linear SNR calculated by the linear SNR calculation unit 241, and the following formula (5).

number

[0053] After calculating the GSNR, the GSNR calculation unit 142 notifies the GSNR to the power control unit 143. Furthermore, after calculating the GSNR, the GSNR calculation unit 142 calculates the amount of tilt of the GSNR in the S band and notifies the backward Raman control unit 242 of the optical transmission device 200.

[0054] For the GSNR calculated using the formulas (1) to (5), reference can be made to, for example, the following documents 1 and 2. In particular, the P_NLI is calculated using the GN (Gaussian noise) model described in document 1. In the first embodiment and the second embodiment described later, the nonlinear SNR is calculated using the closed-form EGN (Enhanced Gaussian noise) model described in document 2, and the GSNR is calculated using the calculation result. Reference 1: P. Poggiolini, Analytical Modeling of Non-Linear Propagation in Coherent Systems, in Proc. OFC 2013, Anaheim, CA, USA, Mar. 2013. Reference 2: M. Zefreh et al., “A Closed-Form Nonlinearity Model for Forward-Raman-Amplified WDM Optical Links,” in Proc. Opt. Fiber Commun. Conf. (OFC), Paper M5C.1, 2021.

[0055] The power control unit 143 of the transmission side control unit 140 performs a first optical power control for the first optical power adjustment unit 110 and a second optical power control for the second optical power adjustment unit 120 based on the GSNR notified from the GSNR calculation unit 142. For example, when the GSNR is not flat, the power control unit 143 performs a pre-emphasis control to suppress tilt of the reception power generated in the optical transmission device 200 and achieve flatness of the GSNR. The power control unit 143 can improve the GSNR by performing the pre-emphasis control based on the frequency characteristics of the input optical power for each channel described in the following literature 3. Document 3: Pierluigi Poggiolini et al. “Closed Form Expressions of the Nonlinear Interference for UWB Systems,” ECOC 2022, paper Tu1D.1.

[0056] The backward Raman control unit 242 of the receiving side control unit 240 controls the gain of the backward Raman amplifier unit 250 based on the tilt amount of the GSNR of the S band notified from the GSNR calculation unit 142. For example, when a GSNR tilt occurs such that the minimum GSNR on the short wavelength side of the S band is smaller than the GSNR on the long wavelength side of the S band, the backward Raman control unit 242 performs tilt control to compensate for this GSNR tilt. In this case, the backward Raman control unit 242 adjusts the wavelength and pump light power of the pump light Lp output from the backward Raman amplifier unit 250 so that the improvement of the minimum GSNR on the short wavelength side of the S band is greater than the improvement of the GSNR on the long wavelength side of the S band. This improves the tilt of the GSNR in the S band.

[0057] The operation of the optical transmission system ST will be described with reference to Fig. 4 to Fig. 9. More specifically, the operation performed by the transmitting side control unit 140 and the receiving side control unit 240 in cooperation with each other will be described.

[0058] First, as shown in FIG. 4, the power control unit 143 performs average level control of the transmission power of each wavelength band (step S0), and then repeats pre-emphasis control until the GSNRs of both the C band and the L band become flat (step S1, step S2: NO). The power control unit 143 can determine whether the GSNR is flat or not, for example, based on the threshold deviation of the GSNR. For example, as shown in FIG. 5(a), the optical powers of the S band, the C band, and the L band are all 1.32 dBm / ch, so that the signal lights L1, L2, and L3 whose transmission powers are constant are input to the second optical power adjustment unit 120. In this case, after the average level control of the transmission power of each wavelength band, the pre-emphasis control is repeated, so that a tilt occurs in which the short wavelength side of the transmission power of the S band is increased more than the long wavelength side, as shown in FIG. 6(a). A tilt also occurs in the C band and the L band. For example, the transmission power of the shortest wavelength of the S band is controlled to 3.2 dBm / ch. The second optical power adjuster 120 inputs the signal lights L1, L2, and L3 in such a tilted state to the transmitting-side multiplexer .

[0059] In addition, when such a tilt occurs, if the OCMs 281, 282, and 283 included in the optical transmission device 200 serving as a receiving node measure the optical power of the signal lights L1, L2, and L3, the tilt of the received power is suppressed as shown in Fig. 6(b). When the linear SNR calculation unit 241 calculates the linear SNR based on the optical power measured by the OCMs 281, 282, and 283, a linear SNR with high flatness is obtained for the C band and the L band as shown in Fig. 6(c).

[0060] On the other hand, when the non-linear SNR calculation unit 141 calculates the non-linear SNR based on the optical power measured by OCMs 171, 172, and 173, as shown in Fig. 6(c), for example, the non-linear SNR on the short-wavelength side of the S band deteriorates compared to the non-linear SNRs of the C band and the L band, and a non-linear SNR with a large tilt appears across the S band, C band, and L band. This is because, as shown in equations (1), (2), and (3), due to pre-emphasis control, the transmission power on the short-wavelength side of the S band increases, and the non-linear noise in the S band increases. For example, compared with Fig. 5(c) under the condition of constant transmission power, due to pre-emphasis control, in Fig. 6(c), the tilt of the non-linear SNR is large. When the GSNR calculation unit 142 calculates the GSNR based on such linear SNR and non-linear SNR according to equation (5), the minimum GSNR on the short-wavelength side of the S band shown in Fig. 6(c) (denoted as Ry in Fig. 6(c)) is improved by about 3.1 dB compared to the minimum GSNR on the short-wavelength side of the S band shown in Fig. 5(c) (denoted as Rx in Fig. 6(c)). That is, due to pre-emphasis control, the minimum GSNR on the short-wavelength side of the S band is improved.

[0061] Note that when pre-emphasis control is not implemented, as shown in Fig. 5(b), in WDM transmission using the C band, L band, and S band, due to SRS, a very large tilt occurs in the received power of the received signals in the S band, C band, and L band. In this case, as shown in Fig. 5(c), while a non-linear SNR with high flatness appears, a linear SNR with low flatness and deterioration also appears. Therefore, the minimum GSNR on the short-wavelength side of the S band (denoted as Rx (<Ry) in Fig. 5(c)) deteriorates significantly.

[0062] Returning to Fig. 4, when pre-emphasis control is repeated and the GSNRs of both the C band and the L band become flat (step S2: YES), the power control unit 143 repeats the pre-emphasis control until the minimum GSNR of the S band becomes maximum (steps S3, S4: NO). For example, when the pre-emphasis control is repeated, as shown in Fig. 7(a), the transmission power of the shortest wavelength in the S band is controlled to 4.2 dBm / ch.

[0063] As a result, as shown in Fig. 7(c), the tilt of the linear SNR in the S band is suppressed compared to the case shown in Fig. 6(c). As a result, as shown in Fig. 7(c), the minimum GSNR in the S band is improved while maintaining the flatness of the GSNR in the C band and the GSNR in the L band. For example, the minimum GSNR "Ry" of 26.3 dB (see Fig. 6(c)) is improved to the minimum GSNR "R0" of 26.5 dB (see Fig. 7(c)).

[0064] However, as shown in Fig. 7(c), a tilt of about 1.8 dB still remains in the GSNR of the S band due to degradation of the nonlinear SNR. Therefore, as shown in Fig. 4, when the minimum GSNR of the S band becomes maximum (step S4: YES), the GSNR calculation unit 142 calculates the GSNR tilt amount of the S band (step S5). Specifically, the GSNR calculation unit 142 calculates the difference between the maximum GSNR and the minimum GSNR of the S band, and calculates the calculated difference as the tilt amount indicating the magnitude of the tilt of the S band.

[0065] After calculating the tilt amount, the GSNR calculation unit 142 notifies the tilt amount to the backward Raman control unit 242. The backward Raman control unit 242 adjusts the wavelength of the pump light Lp and the pump light power based on the tilt amount of the GSNR in the S band notified from the GSNR calculation unit 142 (step S6), and determines whether the minimum GSNR in the S band is equal to or greater than a threshold value (step S7). This threshold value can be the GSNR "Rc" (for example, 28.6 dB or 29.2 dB) in WDM transmission using only the C band.

[0066] If the minimum GSNR is not equal to or greater than the threshold (step S7: NO), the GSNR calculation unit 142 and the backward Raman control unit 242 repeat the processes of steps S6 and S7. If the minimum GSNR in the S band is equal to or greater than the threshold (step S7: YES), the transmission-side control unit 140 and the reception-side control unit 240 each end the process.

[0067] Here, the above-mentioned step S6 will be described in detail. The backward Raman control unit 242 adjusts the wavelength and pumping light power of the pumping light Lp, and controls the gain of the backward Raman amplifier 250 so as to compensate for the tilt related to the GSNR of the S band notified by the GSNR calculation unit 142. For example, the backward Raman control unit 242 adjusts the pumping light Lp having a wavelength of 1395 nm so as to be input to the optical transmission line 300 with a pumping light power of 0.08 W (watts).

[0068] By such adjustment, as shown in Fig. 8, the backward Raman amplifier 250 can realize a gain characteristic in which the gain is higher on the shorter wavelength side of the S band. Since it is sufficient that the gain is higher on the shorter wavelength side of the S band, the gain on the short wavelength side of the S band only needs to be higher than the gain on the long wavelength side of the S band. Therefore, the gain on the short wavelength side of the S band may be higher than the gain on the short wavelength side or long wavelength side of the C band, or may be higher than the gain on the short wavelength side or long wavelength side of the L band. In this way, the backward Raman control unit 242 controls the gain of the backward Raman amplifier 250.

[0069] As a result, as shown in FIG. 9(a), when Raman amplification is performed by pump light Lp in a state where a tilt occurs in the transmission power as in the case shown in FIG. 7(a), the reception power of each of the S band, C band, and L band increases overall as shown in FIG. 9(b). For example, the reception power of the shortest wavelength in the S band increases by about 5 dBm compared to the reception power of the shortest wavelength in the S band described with reference to FIG. 7(b). As a result, as shown in FIG. 9(c), the minimum GSNR of the S band is improved while maintaining the flatness of the GSNR of the C band and the GSNR of the L band. For example, in the entire S band, C band, and L band, the minimum GSNR "R0" of 26.5 dB (see FIG. 7(c)) is improved to the minimum GSNR "R1" of 28.6 dB (see FIG. 9(c)).

[0070] The effects of the first embodiment will be described in comparison with the comparative example with reference to Fig. 10. The frequency band "f0" in the first embodiment (for example, 12.6 THz (Terahertz)) is common to the comparative example. This frequency band "f0" corresponds to the wavelength band from the shortest wavelength in the S band to the longest wavelength in the L band in the first embodiment.

[0071] In such a frequency band "f0", the minimum GSNR "R0" was calculated in the comparative example. On the other hand, in the first embodiment, the minimum GSNR "R1" was calculated, which was larger than the minimum GSNR "R0". Thus, the first embodiment improves the GSNR compared to the comparative example. Furthermore, when the C band ratio of the comparative example is compared to the C band ratio of the first embodiment, the GSNR "R1" of the first embodiment is closer to the GSNR "Rc" in WDM transmission using only the C band than the comparative example. That is, according to the first embodiment, it is possible to obtain transmission performance closer to WDM transmission using only the C band than the comparative example. The C band ratio of the comparative example represents the difference between the minimum GSNR "R0" and the GSNR "Rc", and the C band ratio of the first embodiment represents the difference between the minimum GSNR "R1" and the GSNR "Rc".

[0072] Regarding the flatness of the GSNR, in the comparative example, the GSNR tilt remains at least in the S band, and the improvement in flatness is insufficient. However, in the first embodiment, the tilt in the S band is improved, and the deviation D1 in the overall GSNR of the S band, C band, and L band is suppressed to about 1.6 dB. If the threshold deviation is set to, for example, 2.0 dB, the flatness is ensured to a certain degree. In this way, the GSNR is improved in the first embodiment compared to the comparative example. The reason for this is as follows.

[0073] That is, since the counter-propagating Raman amplification has low nonlinearity and noise, it is assumed that there is no or very little effect on SNR_NL defined as the nonlinear SNR in the above formula (1). In other words, even if the counter-propagating Raman amplification is used, the decrease in the nonlinear SNR is suppressed. On the other hand, the gain of the counter-propagating Raman amplification improves SNR_NL defined as the linear SNR in the above formula (4). Therefore, when the GSNR is calculated based on the above formula (5), the GSNR can be improved compared to the case where the counter-propagating Raman amplification is not used.

[0074] Thus, according to the first embodiment, the minimum GSNR in the S band is improved and the GSNR tilt is suppressed while the flatness of the GSNR in the C band and the L band is ensured. As a result, it is possible to suppress the deterioration of the transmission performance even in WDM transmission with the band expanded to the S band. In addition, by improving the flatness of the GSNR to a certain extent by using pre-emphasis and then applying backward pumping Raman amplification, the power of the pump light can be reduced, thereby achieving low power consumption.

[0075] (Second Example) Next, a second embodiment of the present invention will be described with reference to Fig. 11 to Fig. 13. In the second embodiment, pre-emphasis control for fine adjustment is further repeated. This improves the GSNR of all of the S band, C band, and L band, and improves the flatness of all of the S band, C band, and L band. Note that the flow chart shown in Fig. 11 omits parts common to the flow chart shown in Fig. 4, and therefore detailed description thereof will be omitted.

[0076] 11, if the minimum GSNR in the S band is equal to or greater than the threshold in the process of step S7 described above, the power control unit 143 performs pre-emphasis control (step S11). Then, the GSNR calculation unit 142 determines whether the minimum GSNR is equal to or greater than the threshold and whether the GSNR is flat across the S band, C band, and L band (step S12). The GSNR calculation unit 142 executes the process of step S12 based on the calculated tilt amount of the overall GSNR across the S band, C band, and L band.

[0077] If the minimum GSNR is not equal to or greater than the threshold, or if the GSNR is not flat (step S12: NO), the GSNR calculation unit 142 repeats the processes of steps S11 and S12. If the minimum GSNR is equal to or greater than the threshold and the GSNR is flat (step S12: YES), the transmission-side control unit 140 and the reception-side control unit 240 each end the process.

[0078] As a result, as shown in Fig. 12(a), a tilt of the transmission power different from that shown in Fig. 9(a) occurs. When Raman amplification is performed by pump light Lp in a state where such a tilt occurs, the GSNR tilt of the S band, C band, and L band is reduced compared to the case shown in Fig. 9(c), as shown in Fig. 12(c). For example, by the processing of steps S11 and S12, the GSNR tilt of the S band, C band, and L band is suppressed as shown in Fig. 12(c).

[0079] As a result, the GSNR of the S band, the GSNR of the C band, and the GSNR of the L band are generally flat, as shown in Fig. 12(c). For example, in the S band, C band, and L band as a whole, the minimum GSNR "R1" (see Fig. 9(c)) of 28.6 dB is improved to the minimum GSNR "R2" (see Fig. 12(c)) of 29.2 dB.

[0080] The effects of the second embodiment will be described in comparison with the first embodiment with reference to Fig. 13. The frequency band "f0" in the second embodiment is common to the first embodiment. In such a frequency band "f0", a minimum GSNR "R1" was calculated in the first embodiment. On the other hand, a minimum GSNR "R2" larger than the minimum GSNR "R1" was calculated in the second embodiment.

[0081] Thus, according to the second embodiment, the GSNR is improved compared to the first embodiment. Furthermore, when comparing the C band ratio of the first embodiment with the C band ratio of the second embodiment, the GSNR "R2" of the second embodiment is the same or almost the same as the GSNR "Rc" in WDM transmission using only the C band. In other words, according to the second embodiment, it is possible to obtain the same or almost the same transmission performance as in WDM transmission using only the C band, compared to the first embodiment. The C band ratio of the second embodiment represents the difference between the minimum GSNR "R2" and the GSNR "Rc".

[0082] Regarding the flatness of the GSNR, in the second embodiment, the deviation D2 is suppressed to about 0.7 dB in the overall GSNR for the S, C, and L bands, and the flatness is further improved compared to the first embodiment. Thus, in the second embodiment, the GSNR is further improved compared to the first embodiment by fine-tuning the pre-emphasis control. As a result, the degradation of the transmission performance is further suppressed in the second embodiment compared to the first embodiment.

[0083] (Other Examples) Another embodiment will be described with reference to Fig. 14. As shown in Fig. 14, the backward Raman control unit 242 may generate a target Raman gain profile between the processing of step S5 and the processing of step S6 (step S21). The target Raman gain profile is an example of gain information. An optical fiber may contain optical loss (so-called water peak) caused by hydroxide ions. If the pump light Lp is affected by this optical loss, the gain may vary and the backward Raman amplification may become insufficient. Since the relationship between the type of optical fiber and the optical loss varies, the gain of the backward Raman amplification may also vary.

[0084] Therefore, before the process of step S6 is executed, the backward Raman control unit 242 generates various target Raman gain profiles according to the type of optical fiber, based on the tilt amount in the S band notified by the GSNR calculation unit 142. Then, the backward Raman control unit 242 executes the process of step S6 based on the calculated target Raman gain profile. This suppresses the influence of the water peak and prevents the deterioration of the transmission performance.

[0085] (Third Example) A third embodiment will be described with reference to FIG. 15. In this embodiment, the backward Raman control unit 242 has a target Raman gain calculation unit 243 and a pump laser control unit 244. In FIG. 4 and FIG. 14, it has been described that the GSNR tilt amount is notified to the backward Raman control unit 242 from the GSNR calculation unit 142. In this embodiment, the target Raman gain calculation unit 243 acquires a nonlinear SNR from the GSNR calculation unit 142 (or the nonlinear SNR calculation unit 141). In addition, in this embodiment, the target Raman gain calculation unit 243 acquires a linear SNR from the linear SNR calculation unit 241 (or the GSNR calculation unit 142). The target Raman gain calculation unit 243 calculates a target linear SNR based on the acquired nonlinear SNR, the target GSNR, and Equation (5). Furthermore, the target Raman gain calculation unit 243 generates a target Raman gain profile from the difference between the target linear SNR and the acquired linear SNR. Since the nonlinearity and noise of the backward pumping Raman amplification are low, it is assumed that the effect on the nonlinear SNR of the received signal before and after the Raman amplification is zero or very small. The target GSNR may be stored in memory beforehand, or may be input and set later.

[0086] Then, the pump laser control unit 244 controls the backward Raman amplifier 250 based on the target Raman gain profile notified from the target Raman gain calculation unit 243, and adjusts the pump light. This allows the backward Raman amplification to be controlled based on the GSNR, which is the quality of the optical signal taking into account the nonlinear noise generated in the optical transmission line 300, and suppresses degradation of the transmission performance. Furthermore, the effect of the water peak is also suppressed, further suppressing degradation of the transmission performance.

[0087] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims.

[0088] For example, in the above embodiment, GSNR is used as an example, but GOSNR (Generalized OSNR) may be used instead of GSNR. Even when GOSNR is used, the same results as when GSNR is used can be obtained.

[0089] In the above embodiment, WDM transmission using S band, C band, and L band is used as an example, but WDM transmission using C band, L band, and U band (Ultralong Band) may be used. It is expected that the same results can be obtained with WDM transmission using C band, L band, and U band.

[0090] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) An optical transmission device comprising: a receiving unit that receives wavelength-multiplexed signal light obtained by adjusting the optical powers of a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, from an optical transmission line, the first signal light, the second signal light, and the third signal light, and then amplifying the wavelength-multiplexed signal light based on output of pump light propagating in a direction opposite to a propagation direction of the wavelength-multiplexed signal light to the optical transmission line; and a control unit that controls a gain of the amplifying unit based on a quality of the third signal light calculated based on the optical power of the third signal light before being transmitted to the optical transmission line and the optical power of the third signal light included in the wavelength-multiplexed signal light received by the receiving unit. (Supplementary Note 2) The optical transmission device described in Supplementary Note 1, characterized in that the control unit makes the gain on the short wavelength side of the third wavelength band higher than at least one of the gain on the long wavelength side of the first wavelength band, the gain on the long wavelength side of the second wavelength band, and the gain on the long wavelength side of the third wavelength band. (Supplementary Note 3) The optical transmission device described in Supplementary Note 2, characterized in that the control unit makes the gain on the short wavelength side of the third wavelength band higher than at least one of the gain on the long wavelength side of the first wavelength band, the gain on the long wavelength side of the second wavelength band, and the gain on the long wavelength side of the third wavelength band by adjusting the wavelength and power of the pumping light. (Supplementary Note 4) The optical transmission device according to Supplementary Note 1, further comprising a demultiplexing unit which demultiplexes the wavelength-multiplexed signal light into the first signal light, the second signal light, and the third signal light, and the optical powers of the first signal light and the second signal light are adjusted before the amplifier unit amplifies the wavelength-multiplexed signal light so that qualities of the first signal light and the second signal light calculated based on the optical powers of the first signal light and the second signal light before being sent out to the optical transmission line and the optical powers of the first signal light and the second signal light demultiplexed by the demultiplexing unit are uniform, and after the qualities of the first signal light and the second signal light are uniform, the optical powers of the first signal light, the second signal light, and the third signal light demultiplexed by the demultiplexing unit are further adjusted so that qualities of the first signal light, the second signal light, and the third signal light demultiplexed by the demultiplexing unit are uniform. (Supplementary Note 5) The optical transmission device described in Supplementary Note 1, characterized in that the control unit generates information on the gain of the amplification unit based on a tilt amount of the quality of the third signal light, and controls the gain of the amplification unit based on the information. (Supplementary Note 6) The optical transmission device described in Supplementary Note 1, characterized in that the control unit generates information on the gain of the amplification unit based on a target linear SNR calculated based on a target value of the quality of the third signal light and a nonlinear SNR calculated based on the optical power of the third signal light before it is transmitted to the optical transmission path, and a linear SNR calculated based on the optical power of the third signal light included in the wavelength-multiplexed signal light received by the receiving unit, and controls the gain of the amplification unit based on the information. (Supplementary Note 7) An optical transmission method comprising: receiving, from an optical transmission line, wavelength-multiplexed signal light obtained by multiplexing a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, after adjusting the optical powers of the first signal light, the second signal light, and the third signal light; amplifying the wavelength-multiplexed signal light based on output, to the optical transmission line, of pump light propagating in a direction opposite to a propagation direction of the wavelength-multiplexed signal light; and controlling an amplification gain based on a quality of the third signal light calculated based on the optical power of the third signal light before being transmitted to the optical transmission line and the optical power of the third signal light included in the received wavelength-multiplexed signal light. (Supplementary Note 8) An optical transmission system comprising: a first optical transmission device that adjusts the optical power of a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, and then transmits to an optical transmission line a wavelength multiplexed signal light obtained by multiplexing the first signal light, the second signal light, and the third signal light; and a second optical transmission device that receives the wavelength multiplexed signal light from the optical transmission line, amplifies the wavelength multiplexed signal light based on output of pump light propagating in a direction opposite to a propagation direction of the wavelength multiplexed signal light to the optical transmission line, and controls an amplification gain based on the optical power of the third signal light before being transmitted to the optical transmission line and a quality of the third signal light calculated based on the optical power of the third signal light included in the wavelength multiplexed signal light received via the optical transmission line. [Explanation of symbols]

[0091] NW Optical Network ST Optical Transmission System 100,200 Optical transmission equipment 110 First optical power adjustment unit 120 Second optical power adjustment unit 140 Transmission side control section 141 Nonlinear SNR calculation unit 142 GSNR calculation unit 143 Power control section 240 Receiving side control section 241 Linear SNR calculation unit 242 Rear Raman control section 243 Target Raman Gain Calculation Unit 244 Excitation laser control unit 250 Backward Raman amplifier 300 Optical Transmission Line

Claims

1. a receiving unit that receives, from an optical transmission line, a wavelength-multiplexed signal light obtained by multiplexing a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, after adjusting the optical power of each of the first signal light, the second signal light, and the third signal light; an amplifier that amplifies the wavelength-multiplexed signal light based on output of pump light propagating in a direction opposite to a propagation direction of the wavelength-multiplexed signal light to the optical transmission line; a control unit that controls a gain of the amplifying unit based on a quality of the third signal light calculated based on an optical power of the third signal light before being transmitted to the optical transmission line and an optical power of the third signal light included in the wavelength multiplexed signal light received by the receiving unit; An optical transmission device comprising:

2. the control unit sets a gain on the short wavelength side of the third wavelength band to be higher than at least one of a gain on the long wavelength side of the first wavelength band, a gain on the long wavelength side of the second wavelength band, and a gain on the long wavelength side of the third wavelength band.

2. The optical transmission device according to claim 1.

3. the control unit adjusts a wavelength and a power of the pumping light to make a gain on the short wavelength side of the third wavelength band higher than at least one of a gain on the long wavelength side of the first wavelength band, a gain on the long wavelength side of the second wavelength band, and a gain on the long wavelength side of the third wavelength band.

3. The optical transmission device according to claim 2.

4. a demultiplexing unit that demultiplexes the wavelength multiplexed signal light into the first signal light, the second signal light, and the third signal light, Before the amplifier unit amplifies the wavelength multiplexed signal light, the optical powers of the first signal light and the second signal light are adjusted so that the qualities of the first signal light and the second signal light calculated based on the optical powers of the first signal light and the second signal light before being sent to the optical transmission line and the optical powers of the first signal light and the second signal light separated by the demultiplexer unit are uniform, and after the qualities of the first signal light and the second signal light are uniform, the optical powers of the first signal light, the second signal light, and the third signal light separated by the demultiplexer unit are further adjusted so that the qualities of the first signal light, the second signal light, and the third signal light are uniform.

2. The optical transmission device according to claim 1.

5. the control unit generates information on a gain of the amplifier unit based on an amount of tilt of the quality of the third signal light, and controls the gain of the amplifier unit based on the information.

2. The optical transmission device according to claim 1.

6. the control unit generates information on a gain of the amplifier unit based on a target linear SNR calculated based on a nonlinear SNR calculated based on a target value of the quality of the third signal light and the optical power of the third signal light before being transmitted to the optical transmission line, and a linear SNR calculated based on the optical power of the third signal light included in the wavelength multiplexed signal light received by the receiving unit, and controls the gain of the amplifier unit based on the information.

2. The optical transmission device according to claim 1.

7. receiving, from an optical transmission line, a wavelength-multiplexed signal light obtained by multiplexing a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, after adjusting the optical power of each of the first signal light, the second signal light, and the third signal light; amplifying the wavelength-multiplexed signal light based on output of pump light propagating in a direction opposite to a propagation direction of the wavelength-multiplexed signal light to the optical transmission line; controlling an amplification gain based on a quality of the third signal light calculated based on an optical power of the third signal light before being transmitted to the optical transmission line and an optical power of the third signal light included in the received wavelength multiplexed signal light; Optical transmission method.

8. a first optical transmission device that adjusts the optical power of a first signal light having a wavelength belonging to a first wavelength band, a second signal light having a wavelength belonging to a second wavelength band longer than the first wavelength band, and a third signal light having a wavelength belonging to a third wavelength band shorter than the first wavelength band, and then transmits a wavelength-multiplexed signal light obtained by multiplexing the first signal light, the second signal light, and the third signal light to an optical transmission line; a second optical transmission device that receives the wavelength multiplexed signal light from an optical transmission line, amplifies the wavelength multiplexed signal light based on output of pump light propagating in a direction opposite to a propagation direction of the wavelength multiplexed signal light to the optical transmission line, and controls an amplification gain based on a quality of the third signal light calculated based on an optical power of the third signal light before being transmitted to the optical transmission line and an optical power of the third signal light included in the wavelength multiplexed signal light received via the optical transmission line; An optical transmission system comprising:

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