Optical transmission system and control method

Abstract

An optical transmission system according to the present disclosure is provided with: a first optical sending means for outputting, at a first sending power, a first optical signal to be sent to a first optical reception means; a second optical sending means for outputting, at a second sending power, a second optical signal to be sent to a second optical reception means; a multiplexing means for multiplexing the first optical signal and the second optical signal and outputting the multiplexed signal to an optical transmission path; and a setting means for setting the first sending power and the second sending power. In the optical transmission path, the first optical signal is amplified by using a first gain, and in the optical transmission path, the second optical signal is amplified by using a second gain different from the first gain. The setting means sets the first sending power in accordance with the first gain and a first target reception power associated with the first optical reception means, and sets the second sending power in accordance with the second gain and a second target reception power associated with the second optical reception means.

Description

Optical Transmission System and Control Method

[0001] The present disclosure relates to an optical transmission system and a control method.

[0002] In recent years, in the field of optical transmission systems, multiband transmission that uses a plurality of wavelength bands has been put into practical use in order to cope with broadband demands. Furthermore, the optical power of an optical signal transmitted via a single optical transmission path is regulated from the perspective of safety, so it is necessary to effectively utilize the limited optical power.

[0003] For this reason, in recent years, techniques for controlling the transmission power of optical signals in optical transmission systems that perform multiband transmission have been proposed. For example, Patent Document 1 discloses a technique for setting the transmission power of an optical signal in consideration of the influence of SRS (Stimulated Raman Scattering) tilt.

[0004] Here, SRS tilt will be described. FIG. 1 is a diagram for explaining an example of SRS tilt. In FIG. 1, it is assumed that an optical signal is transmitted from a transmitter (Tx) and received by a receiver (Rx) via an optical transmission path. Furthermore, the left diagram shows the relationship between the transmission power of the optical signal transmitted from the transmitter (Tx) and wavelength, and the right diagram shows the relationship between the reception power of the optical signal received by the receiver (Rx) and wavelength.

[0005] As shown in FIG. 1, when an optical signal is transmitted through an optical transmission path, a gain gradient occurs in the optical transmission path due to the influence of SRS. As a result, the optical power of the optical signal transitions from the short wavelength side to the long wavelength side. This physical phenomenon is SRS tilt.

[0006] The technique disclosed in Patent Document 1 detects the reception power of an optical signal for each wavelength band, and sets the transmission power of the optical signal such that the tilt of the optical signal for each wavelength band is reduced, and the reception power difference between optical signals in each wavelength band after tilt compensation is also reduced.

[0007] Japanese Unexamined Patent Publication No. 2019-186735

[0008] As described above, the technology disclosed in Patent Document 1 sets the transmission power of the optical signal so that the difference in received power between optical signals in each wavelength band is small. As a result, there is a problem that optical signals of a particular wavelength may have excess received power, and if the transmission power of the optical signal is set to result in such received power, the overall average transmission power of the optical signal will increase.

[0009] Therefore, in view of the above-mentioned problems, the purpose of this disclosure is to provide an optical transmission system and control method that can reduce the overall average transmission power of an optical signal.

[0010] An optical transmission system according to one embodiment includes: a first optical transmitting means that outputs a first optical signal transmitted to a first optical receiving means with a first transmission power; a second optical transmitting means that outputs a second optical signal transmitted to a second optical receiving means with a second transmission power; a multiplexing means that combines the first optical signal and the second optical signal and outputs them to an optical transmission path; and a setting means for setting the first transmission power and the second transmission power, wherein in the optical transmission path, the first optical signal is amplified with a first gain; in the optical transmission path, the second optical signal is amplified with a second gain different from the first gain; and the setting means sets the first transmission power according to a first target receiving power and a first gain associated with the first optical receiving means; and sets the second transmission power according to a second target receiving power and a second gain associated with the second optical receiving means.

[0011] A control method according to one embodiment is a control method performed by an optical transmission system comprising: a first optical transmitting means that outputs a first optical signal transmitted to a first optical receiving means with a first transmission power; a second optical transmitting means that outputs a second optical signal transmitted to a second optical receiving means with a second transmission power; and a multiplexing means that combines the first optical signal and the second optical signal and outputs them to an optical transmission line, wherein in the optical transmission line, the first optical signal is amplified with a first gain, and in the optical transmission line, the second optical signal is amplified with a second gain different from the first gain, and the control method is performed by an optical transmission system comprising: setting the first transmission power according to a first target receiving power and a first gain associated with the first optical receiving means; and setting the second transmission power according to a second target receiving power and a second gain associated with the second optical receiving means.

[0012] According to the above-described embodiment, the effect is obtained that an optical transmission system and control method can be provided that can reduce the overall average transmission power of the optical signal.

[0013] This is a diagram illustrating an example of SRS tilt. This is a diagram illustrating an example of the configuration of an optical transmission system according to this disclosure. This is a diagram illustrating an example of comparing the transmission power of an optical signal set according to this disclosure with the transmission power of an optical signal set according to related technology. This is a diagram illustrating an example of comparing the transmission power of an optical signal set according to this disclosure with the transmission power of an optical signal set according to related technology. This is a diagram illustrating an example of comparing the transmission power of an optical signal set according to this disclosure with the transmission power of an optical signal set according to related technology. This is a diagram illustrating an example of the configuration of an optical transmission system according to this disclosure. This is a flowchart illustrating an example of the operation flow of the setting unit according to this disclosure. This is a diagram illustrating an example of the hardware configuration of a computer that implements the setting unit according to this disclosure.

[0014] Embodiments of this disclosure will be described below with reference to the drawings. Note that the following description and drawings have been omitted and simplified as appropriate for clarity of explanation. Furthermore, in the following drawings, the same elements are denoted by the same reference numerals, and redundant explanations have been omitted where necessary.

[0015] <Embodiment 1> First, the configuration of the optical transmission system 1 according to the present disclosure will be described. In the following, the optical transmission system 1 will be assumed to perform multiband transmission using two wavelength bands, the C band (1530-1565 nm) and the L band (1565-1625 nm), but it is not limited to this. The optical transmission system 1 may also use other bands (for example, the S band (1460-1530 nm)), or it may perform multiband transmission using three or more wavelength bands.

[0016] Figure 2 shows an example configuration of the optical transmission system 1 according to the present disclosure. As shown in Figure 2, the optical transmission system 1 includes a transmitter (Tx) 10, a receiver (Rx) 20, and a setting unit 30. The transmitter (Tx) 10 and the receiver (Rx) 20 are connected to each other via an optical transmission path OL.

[0017] The transmitter (Tx) 10 includes TPNDs (Transponders) 11C, 11L, WSSs (Wavelength Selective Switches) 12C, 12L, AMPs (Amplifiers) 13C, 13L, and a multiplexer 14. In the transmitter (Tx) 10, TPND 11C, WSS 12C, and AMP 13C are components for the C band, while TPND 11L, WSS 12L, and AMP 13L are components for the L band.

[0018] TPND11C outputs an optical signal. WSS12C selects and outputs the C-band optical signal from the optical signal output by TPND11C. AMP13C amplifies the C-band optical signal output by WSS12C and outputs it.

[0019] TPND11L outputs an optical signal. WSS12L selects and outputs the L-band optical signal from the optical signal output by TPND11L. AMP13L amplifies the L-band optical signal output by WSS12L and outputs it.

[0020] The multiplexer 14 combines the C-band optical signal output from AMP 13C and the L-band optical signal output from AMP 13L, and transmits the combined optical signal to the receiver (Rx) 20 via the optical transmission path OL.

[0021] The receiver (Rx) 20 includes TPND 21C, 21L, WSS 22C, 22L, AMP 23C, 23L, and a demultiplexer 24. In the receiver (Rx) 20, TPND 21C, WSS 22C, and AMP 23C are components for the C band, and TPND 21L, WSS 22L, and AMP 23L are components for the L band.

[0022] The demultiplexer 24 receives an optical signal from the transmitter (Tx) 10 via the optical transmission path OL, and demultiplexes the received optical signal before outputting it.

[0023] AMP23C amplifies and outputs one of the optical signals output from the demultiplexer 24. WSS22C selects and outputs the C-band optical signal from the optical signals output from AMP23C. TPND21C receives the C-band optical signal output from WSS22C.

[0024] AMP23L amplifies and outputs the other optical signal output from the demultiplexer 24. WSS22L selects and outputs the L-band optical signal from the optical signal output from AMP23L. TPND21L receives the L-band optical signal output from WSS22L.

[0025] The setting unit 30 sets the transmission power of the optical signal transmitted from the transmitter (Tx) 10. More specifically, the setting unit 30 sets the transmission power of the C-band optical signal output from AMP 13C and the transmission power of the L-band optical signal output from AMP 13L. Although the setting unit 30 is located outside the transmitter (Tx) 10, it is not limited to this and may also be located inside the transmitter (Tx) 10.

[0026] The operation of the setting unit 30 will be described in detail below with reference to Figure 3. Figure 3 is a diagram illustrating an example of comparing the transmission power of the optical signal set by this disclosure with the transmission power of the optical signal set by related technologies.

[0027] In Figure 3, the related technology is assumed to be the technology disclosed in Patent Document 1. Also in Figure 3, the linear graph in the upper figure shows the relationship between the wavelength of the received power set for the optical signal received by the receiver (Rx) 20. The band graph in the upper figure shows the relationship between the wavelength of the target received power, which is the received power required by TPND 21C and 21L, respectively, on the receiver (Rx) 20 side. The linear graph in the lower figure shows the relationship between the wavelength of the transmitted power set for the optical signal transmitted from the transmitter (Tx) 10 (the same applies to Figures 4 to 6 thereafter).

[0028] As shown in Figure 3, the L-band has a longer wavelength and a higher target received power compared to the C-band. Also, the L-band has a higher gain in the optical transmission path OL due to the influence of SRS compared to the C-band.

[0029] Furthermore, margins are set for both the L-band and C-band to account for the gain due to the SRS effect, and it is necessary to secure a receiving power that is equal to or greater than the target receiving power.

[0030] In this related technology, the transmission power of the optical signal is set so that the difference in received power between optical signals in each wavelength band is small. In Figure 3, the target received power for the L band is larger than that for the C band. In addition, for both the L band and the C band, it is necessary to secure a received power that is equal to or greater than the target received power plus a margin.

[0031] Therefore, in related technologies, the C-band receiving power is matched to the L-band receiving power. The L-band receiving power is the target L-band receiving power plus the L-band margin.

[0032] However, as a result, in the related technology, the C-band reception power is the C-band target reception power plus an excess margin. Therefore, if the transmission power of the optical signal transmitted from the transmitter (Tx) 10 is set to achieve such reception power, the transmission power of the C-band optical signal increases in particular, which increases the overall average transmission power of the optical signal.

[0033] Therefore, the setting unit 30 sets the transmission power of the optical signal transmitted from the transmitter (Tx) 10 in the following manner in order to reduce the overall average transmission power of the optical signal transmitted from the transmitter (Tx) 10.

[0034] First, the setting unit 30 sets the slope of the received power of the optical signal received by the receiver (Rx) 20 so that, for each wavelength in the C band, a received power equal to the target received power of the C band plus the margin of the C band is secured, and for each wavelength in the L band, a received power equal to the target received power of the L band plus the margin of the L band is secured.

[0035] In this case, for example, the setting unit 30 may set the slope of the received power as follows. The setting unit 30 determines a value of received power on the short wavelength side within the C band by adding the margin of the C band to the target received power of the C band. The setting unit 30 also determines a value of received power on the short wavelength side within the L band by adding the margin of the L band to the target received power of the L band. Then, the setting unit 30 draws a straight line connecting the value of received power determined on the short wavelength side within the C band and the value of received power determined on the short wavelength side within the L band, and sets the slope of that straight line as the slope of the received power. However, this setting method is just one example and is not limited to this.

[0036] Subsequently, the setting unit 30 sets the slope of the transmission power of the optical signal transmitted from the transmitter (Tx) 10 so that the slope of the received power of the optical signal received by the receiver (Rx) 20 becomes the slope set above.

[0037] In detail, the setting unit 30 sets the transmission power of the C-band optical signal output from AMP 13C and the transmission power of the L-band optical signal output from AMP 13L so that the transmission power of the optical signal transmitted from transmitter (Tx) 10 follows a set slope.

[0038] Here, the overall average transmission power of the optical signal when the transmission power of the optical signal is set by the setting unit 30 was compared with the overall average transmission power of the optical signal when the transmission power of the optical signal is set by the related technology. As a result, it was confirmed that the overall average transmission power of the optical signal set by the setting unit 30 is reduced compared to the related technology.

[0039] Furthermore, in Figure 3, the slope of the optical signal transmission power set by the setting unit 30 is in the same direction as the slope of the gain generated in the optical transmission path OL due to the influence of SRS. Therefore, it is possible to simplify the design of the transmitter (Tx) 10.

[0040] The following describes in detail variations of the operation of the setting unit 30 with reference to Figures 4 to 6. In Figure 3, the setting unit 30 sets the slope of the received power over the entire wavelength band, including the C band and L band, but it is not limited to this. As shown in Figures 4 to 6, the setting unit 30 may set the slope of the received power for each of the C band and L band. In this case, the setting unit 30 sets the slope of the transmitted power for each of the C band and L band.

[0041] Furthermore, in Figures 3 and 4, the setting unit 30 sets the slope of the received power to be in the same direction as the slope of the gain generated in the optical transmission path OL due to the influence of SRS, but it is not limited to this. As shown in Figure 5, the setting unit 30 may also set the slope of the received power to be in the opposite direction to the slope of the gain generated in the optical transmission path OL due to the influence of SRS. In this case, by increasing the transmission power of the short-wavelength optical signal on the transmitter (Tx) 10 side, it becomes possible to increase the received power of the short-wavelength optical signal on the receiver (Rx) 20 side.

[0042] Furthermore, in FIGS. 3 to 5, the setting unit 30 tilts the reception power, but the present invention is not limited to this. As shown in FIG. 6, the setting unit 30 may flatten the reception power without tilting the reception power for each of the C-band and the L-band.

[0043] It has been confirmed that in any of FIGS. 4 to 6, the overall average transmission power of the optical signal set by the setting unit 30 is reduced compared with that in the related art.

[0044] As described above, according to the first embodiment, the slope of the reception power of the optical signal received by the receiver (Rx) 20 is set in accordance with the target reception power of the optical signal at the receiver (Rx) 20 side and the gain slope generated in the optical transmission line OL due to the influence of SRS, and the transmission power of the optical signal transmitted from the transmitter (Tx) 10 is set in accordance with the set slope of the reception power.

[0045] This makes it possible to reduce the overall average transmission power of the optical signal transmitted from the transmitter (Tx) 10. In addition, since the optical power limited by a single optical transmission line OL can be effectively utilized, it is possible to increase the number of channels that can be transmitted through the optical transmission line OL, and as a result, it is possible to contribute to an increase in transmission capacity.

[0046] <Embodiment 2> The second embodiment corresponds to an embodiment obtained by generalizing the above-described first embodiment. FIG. 7 is a diagram illustrating a configuration example of an optical transmission system 2 according to the present disclosure.

[0047] As shown in FIG. 7, the optical transmission system 2 includes a first optical transmission unit 41, a second optical transmission unit 42, a multiplexer 43, and a setting unit 44. The first optical transmission unit 41 corresponds to TPND21C, WSS22C, and AMP23C, the second optical transmission unit 42 corresponds to TPND21L, WSS22L, and AMP23L, the multiplexer 43 corresponds to the multiplexer 14, and the setting unit 44 corresponds to the setting unit 30.

[0048] The first optical transmission unit 41 outputs a first optical signal to be transmitted to a first optical reception unit at a first transmission power. The second optical transmission unit 42 outputs a second optical signal to be transmitted to a second optical reception unit at a second transmission power. The multiplexer 43 multiplexes the first optical signal and the second optical signal, and outputs the multiplexed signal to the optical transmission line OL. The setting unit 44 sets the first transmission power and the second transmission power.

[0049] In the optical transmission line OL, the first optical signal is amplified with a first gain, and in the optical transmission line OL, the second optical signal is amplified with a second gain different from the first gain. The setting unit 44 sets the first transmission power according to a first target reception power and the first gain associated with the first optical reception unit, and sets the second transmission power according to a second target reception power and the second gain associated with the second optical reception unit.

[0050] FIG. 8 is a flowchart describing an example of an operation flow of the setting unit 44 according to the present disclosure. As shown in FIG. 8, the setting unit 44 sets the first transmission power according to the first target reception power and the first gain associated with the first optical reception unit (step S11). Further, the setting unit 44 sets the second transmission power according to the second target reception power and the second gain associated with the second optical reception unit (step S12). Note that the processes of steps S11 and S12 may be performed in reverse order, or may be performed in parallel.

[0051] According to the second embodiment, the first and second transmission powers of the first and second optical signals respectively are set in consideration of the first and second target reception powers and the first and second gains in the optical transmission line OL, so it is possible to reduce the overall average transmission power of the optical signals.

[0052] The setting unit 44 may set a first received power of the first optical signal received by the first optical receiving unit according to a first target received power and a first gain, and set a first transmitted power according to the first received power. The setting unit 44 may also set a second received power of the second optical signal received by the second optical receiving means according to a second target received power and a second gain, and set a second transmitted power according to the second received power.

[0053] Furthermore, the setting unit 44 may set the first receiving power according to the first target receiving power and the first margin corresponding to the first gain. Also, the setting unit 44 may set the second receiving power according to the second target receiving power and the second margin corresponding to the second gain.

[0054] Furthermore, the second gain may be higher than the first gain. Also, the second target received power may be higher than the first target received power. Also, the wavelength of the second optical signal may be longer than the wavelength of the first optical signal. Furthermore, the first and second gains may be generated in response to stimulated Raman scattering in the optical transmission line OL.

[0055] <Hardware configuration of the setting unit related to this disclosure> Figure 9 is a diagram showing an example of the hardware configuration of a computer 90 that implements the setting units 30 and 44 related to this disclosure.

[0056] As shown in Figure 9, the computer 90 includes a processor 91, memory 92, storage 93, input / output interface (I / F) 94, and communication interface (Communication I / F) 95. The processor 91, memory 92, storage 93, input / output interface 94, and communication interface 95 are connected to each other by a data transmission path for sending and receiving data.

[0057] The processor 91 is a processing unit such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit). The memory 92 is a memory such as RAM (Random Access Memory) or ROM (Read Only Memory). The storage 93 is a storage device such as an HDD (Hard Disk Drive), SSD (Solid State Drive), or memory card. The storage 93 may also be a memory such as RAM or ROM.

[0058] A program is stored in the storage 93. This program includes a set of instructions (or software code) that, when loaded into a computer, causes the computer 90 to perform one or more functions in the setting units 30 and 44 according to this disclosure. The components of the setting units 30 and 44 according to this disclosure may be realized by the processor 91 loading and executing the program stored in the storage 93. The storage function of the setting units 30 and 44 according to this disclosure may also be realized by memory 92 or storage 93.

[0059] Furthermore, the programs described above may be stored on non-temporary computer-readable media or tangible storage media. Examples, but not limited to, include RAM, ROM, flash memory, SSD, or other memory technologies; CD (Compact Disc)-ROM, DVD (Digital Versatile Disc), Blu-ray® disc, or other optical disc storage; magnetic cassettes, magnetic tapes, magnetic disk storage, or other magnetic storage devices. The programs may also be transmitted over temporary computer-readable media or communication media. Examples, but not limited to, include temporary computer-readable media or communication media, including electrical, optical, acoustic, or other forms of transmitted signals.

[0060] The input / output interface 94 is connected to a display device 941, an input device 942, a sound output device 943, and the like. The display device 941 is a device that displays a screen corresponding to the drawing data processed by the processor 91, such as an LCD (Liquid Crystal Display), a CRT (Cathode Ray Tube) display, or a monitor. The input device 942 is a device that receives operator input, such as a keyboard, mouse, and touch sensor. The display device 941 and the input device 942 may be integrated and implemented as a touch panel. The sound output device 943 is a device that outputs sound corresponding to the acoustic data processed by the processor 91, such as a speaker.

[0061] The communication interface 95 transmits and receives data to and from an external device. For example, the communication interface 95 communicates with an external device via a wired communication path or a wireless communication path.

[0062] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0063] Furthermore, each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated not only with one specific embodiment but also with one or more other embodiments. As those skilled in the art will understand, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps described in any of the drawings may be changed as appropriate.

[0064] Furthermore, some or all of the embodiments described above may also be described as follows, but are not limited to the following. (Note 1) An optical transmission system comprising: a first optical transmitting means that outputs a first optical signal to be transmitted to a first optical receiving means with a first transmitting power; a second optical transmitting means that outputs a second optical signal to be transmitted to a second optical receiving means with a second transmitting power; a multiplexing means that combines the first optical signal and the second optical signal and outputs them to an optical transmission path; and a setting means for setting the first transmitting power and the second transmitting power, wherein in the optical transmission path, the first optical signal is amplified with a first gain; in the optical transmission path, the second optical signal is amplified with a second gain different from the first gain; the setting means sets the first transmitting power according to a first target receiving power and a first gain associated with the first optical receiving means; and sets the second transmitting power according to a second target receiving power and a second gain associated with the second optical receiving means. (Note 2) The optical transmission system according to Note 1, wherein the setting means sets a first received power of the first optical signal received by the first optical receiving means according to the first target received power and the first gain, sets a first transmitted power according to the first received power, sets a second received power of the second optical signal received by the second optical receiving means according to the second target received power and the second gain, and sets a second transmitted power according to the second received power. (Note 3) The optical transmission system according to Note 2, wherein the setting means sets a first received power according to the first target received power and a first margin according to the first gain, and sets a second received power according to the second target received power and a second margin according to the second gain. (Note 4) The optical transmission system according to Note 1, wherein the second gain is higher than the first gain, and the second target received power is higher than the first target received power.(Note 5) The optical transmission system according to Note 1, wherein the wavelength of the second optical signal is longer than the wavelength of the first optical signal, and the first and second gains are generated in accordance with stimulated Raman scattering in the optical transmission path. (Note 6) A control method performed by an optical transmission system comprising: a first optical transmitting means that outputs a first optical signal to be transmitted to a first optical receiving means with a first transmission power; a second optical transmitting means that outputs a second optical signal to be transmitted to a second optical receiving means with a second transmission power; and a multiplexing means that combines the first optical signal and the second optical signal and outputs them to an optical transmission line, wherein in the optical transmission line, the first optical signal is amplified with a first gain, and in the optical transmission line, the second optical signal is amplified with a second gain different from the first gain, the control method comprising: setting the first transmission power according to a first target receiving power and a first gain associated with the first optical receiving means; and setting the second transmission power according to a second target receiving power and a second gain associated with the second optical receiving means.

[0065] Furthermore, some or all of the elements (e.g., configuration and function) described in Appendices 2 to 5 that are subordinate to Appendice 1 may also be subordinate to Appendice 6 in the same manner as those described in Appendices 2 to 5. Some or all of the elements described in any appendice may be applied to various hardware, software, recording means, systems, and methods for recording software.

[0066] 1,2 Optical transmission system 10 Transmitter (Tx) 11C,11L TPND 12C,12L WSS 13C,13L AMP 14 Multiplexer 20 Receiver (Rx) 21C,21L TPND 22C,22L WSS 23C,23L AMP 24 Demultiplexer 30 Setting unit 41 First optical transmission unit 42 Second optical transmission unit 43 Multiplexer 44 Setting unit OL Optical transmission line 90 Computer 91 Processor 92 Memory 93 Storage 94 Input / output interface 941 Display device 942 Input device 943 Sound output device 95 Communication interface

Claims

1. An optical transmission system comprising: a first optical transmitting means that outputs a first optical signal to be transmitted to a first optical receiving means with a first transmission power; a second optical transmitting means that outputs a second optical signal to be transmitted to a second optical receiving means with a second transmission power; a multiplexing means that combines the first optical signal and the second optical signal and outputs them to an optical transmission path; and a setting means for setting the first transmission power and the second transmission power, wherein in the optical transmission path, the first optical signal is amplified with a first gain; in the optical transmission path, the second optical signal is amplified with a second gain different from the first gain; and the setting means sets the first transmission power according to a first target receiving power and a first gain associated with the first optical receiving means; and sets the second transmission power according to a second target receiving power and a second gain associated with the second optical receiving means.

2. The optical transmission system according to claim 1, wherein the setting means sets a first received power of the first optical signal received by the first optical receiving means according to the first target received power and the first gain, sets a first transmitted power according to the first received power, sets a second received power of the second optical signal received by the second optical receiving means according to the second target received power and the second gain, and sets a second transmitted power according to the second received power.

3. The optical transmission system according to claim 2, wherein the setting means sets the first receiving power according to the first target receiving power and the first margin corresponding to the first gain, and sets the second receiving power according to the second target receiving power and the second margin corresponding to the second gain.

4. The optical transmission system according to claim 1, wherein the second gain is higher than the first gain, and the second target received power is higher than the first target received power.

5. The optical transmission system according to claim 1, wherein the wavelength of the second optical signal is longer than the wavelength of the first optical signal, and the first and second gains are generated in accordance with stimulated Raman scattering in the optical transmission path.

6. A control method performed by an optical transmission system comprising: a first optical transmitting means that outputs a first optical signal to be transmitted to a first optical receiving means with a first transmission power; a second optical transmitting means that outputs a second optical signal to be transmitted to a second optical receiving means with a second transmission power; and a multiplexing means that combines the first optical signal and the second optical signal and outputs them to an optical transmission line, wherein in the optical transmission line, the first optical signal is amplified with a first gain, and in the optical transmission line, the second optical signal is amplified with a second gain different from the first gain, the control method comprising: setting the first transmission power according to a first target receiving power and a first gain associated with the first optical receiving means; and setting the second transmission power according to a second target receiving power and a second gain associated with the second optical receiving means.

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