Optical communication system and optical communication method

Abstract

To provide an optical communication system and an optical communication method for transmitting a wavelength multiplexed signal whose signal quality upon reception is adjusted to desired quality.SOLUTION: First optical transmitting means outputs a first optical signal having a first wavelength at first transmission intensity. Second optical transmitting means outputs a second optical signal having a second wavelength longer than the first wavelength at second transmission intensity lower than the first transmission intensity. Combining means outputs a third optical signal obtained by combining the first and second optical signals to the optical transmission line. The intensity of the first optical signal transmitted by the optical transmission line is reduced by inter-signal stimulated Raman scattering, and the second optical signal transmitted by the optical transmission line is amplified by the inter-signal stimulated Raman scattering of the first optical signal.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an optical communication system and an optical communication method.

Background Art

[0002] In the field of optical communication, a wavelength division multiplexing (WDM) method in which optical signals of a plurality of wavelengths are multiplexed is used. Generally, in order to increase the transmission capacity, it is preferable that the number of multiplexed wavelengths is large. Therefore, for example, optical signals of a plurality of wavelengths such as so-called L-band (wavelength 1565 nm to 1625 nm) optical signals and C-band (wavelength 1530 nm to 1565 nm) optical signals are multiplexed.

[0003] When a wavelength multiplexed optical signal is transmitted through an optical transmission line, transmission loss occurs. Therefore, various methods are used to compensate for the transmission loss. For example, in Patent Document 1, a method of compensating for the loss of an optical signal by coupling Raman excitation light output from a light source to an optical transmission line and Raman amplifying the optical signal has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When transmitting a wavelength-division multiplexed optical signal through an optical transmission line, due to stimulated Raman scattering, an optical signal in the C-band acts as Raman pump light, causing a phenomenon where the optical signal in the L-band is amplified (intersignal stimulated Raman scattering). Therefore, even when transmitted through the same optical transmission line, the loss of the optical signal in the C-band becomes larger than the loss of the optical signal in the L-band.

[0007] As a result, even if the optical signal in the C-band and the optical signal in the L-band are output with the same intensity at the time of transmission, at the receiving side, compared with the intensity of the optical signal in the L-band whose loss is compensated by intersignal stimulated Raman scattering, the intensity of the optical signal in the C-band decreases. This causes an imbalance between the intensity of the optical signal in the C-band and the intensity of the optical signal in the L-band, resulting in a problem of deterioration of the signal quality at the time of reception.

Means for Solving the Problems

[0008] An optical communication system according to an aspect of the present disclosure includes: a first optical transmission means that outputs a first optical signal having a first wavelength at a first transmission intensity; a second optical transmission means that outputs a second optical signal having a second wavelength longer than the first wavelength at a second transmission intensity lower than the first transmission intensity; and a multiplexing means that outputs a third optical signal obtained by multiplexing the first optical signal and the second optical signal to an optical transmission line. The intensity of the first optical signal transmitted through the optical transmission line decreases due to intersignal stimulated Raman scattering, and the second optical signal transmitted through the optical transmission line is amplified by the intersignal stimulated Raman scattering of the first optical signal.

[0009] In an optical communication method according to one aspect of the present disclosure, a first optical signal having a first wavelength is output with a first transmission intensity, and a second optical signal having a second wavelength longer than the first wavelength is output with a second transmission intensity lower than the first transmission intensity. A third optical signal obtained by multiplexing the first optical signal and the second optical signal is output to an optical transmission line. The intensity of the first optical signal transmitted through the optical transmission line decreases due to inter-signal induced Raman scattering, and the second optical signal transmitted through the optical transmission line is amplified by the first inter-signal Raman scattering.

Advantages of the Invention

[0010] According to the present disclosure, it is possible to provide an optical communication system and an optical communication method for transmitting a wavelength-division multiplexed signal adjusted so that the signal quality at reception becomes a desired quality.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each drawing, the same reference numerals are assigned to the same elements, and redundant descriptions will be omitted as necessary.

[0013] When referring to an embodiment hereinafter, it is applicable to any one of the embodiments described hereinafter, or a combination of two or more embodiments, and it also means that the application is not limited to a specific embodiment.

[0014] Embodiment 1 The optical communication system according to Embodiment 1 will be described. FIG. 1 is a block diagram schematically showing the basic configuration of the optical communication system according to an embodiment. The optical communication system 100 in FIG. 1 includes at least an optical transmitter 1 and 2 and a multiplexer 3.

[0015] The optical transmitters 1 and 2 each output an optical signal obtained by modulating light of a predetermined wavelength by a predetermined modulation method according to the input data signal. The optical transmitters 1 and 2 may be configured to have the optical signal transmission function of an optical transceiver, for example. Hereinafter, the optical transmitters 1 and 2 are also referred to as the first and second optical transmitters, respectively.

[0016] The optical transmitter 1 outputs an optical signal S1 with a wavelength λ1 to the multiplexer 3. The optical transmitter 2 outputs an optical signal S2 with a wavelength λ2 to the multiplexer 3. Here, the wavelength λ1 is shorter than the wavelength λ2. Here, the wavelength λ1 is a wavelength in the C band. The wavelength λ2 is a wavelength in the L band. Hereinafter, the wavelengths λ1 and λ2 are also referred to as the first and second wavelengths, respectively. The optical signals S1 and S2 are also referred to as the first and second optical signals, respectively.

[0017] The multiplexer 3 multiplexes the optical signal S1 with wavelength λ1 and the optical signal S2 with wavelength λ2. Then, the multiplexer 3 outputs the wavelength-division multiplexed optical signal S obtained by multiplexing the optical signal S1 and the optical signal S2 to the optical transmission line 10. The optical transmission line 10 is an optical transmission line made of a medium capable of transmitting an optical signal, such as an optical fiber. Hereinafter, the wavelength-division multiplexed optical signal S is also referred to as a third optical signal.

[0018] The configuration of the receiving side of the optical communication system 100 according to Embodiment 1 will be described. FIG. 2 is a block diagram schematically showing the basic configuration of an optical communication system according to an embodiment. The optical communication system 100 in FIG. 2 is further provided with a demultiplexer 4, and optical receivers 5 and 6 as compared with FIG. 1.

[0019] The demultiplexer 4 separates the optical signal S1 with wavelength λ1 and the optical signal S2 with wavelength λ2. The demultiplexer 4 outputs the wavelength-separated optical signal S1 to the optical receiver 5. The demultiplexer 4 outputs the wavelength-separated optical signal S2 to the optical receiver 6.

[0020] The optical receivers 5 and 6 demodulate the received optical signals and output data signals, respectively. The optical receivers 5 and 6 may be configured to have an optical signal receiving function among optical transceivers, for example. Hereinafter, the optical receivers 5 and 6 are also referred to as the first and second optical receivers, respectively.

[0021] The optical receiver 5 is configured as an optical receiver capable of receiving an optical signal with wavelength λ1. Thereby, the optical receiver 5 receives the optical signal S1 output from the demultiplexer 4. The optical receiver 6 is configured as an optical receiver capable of receiving an optical signal with wavelength λ2. Thereby, the optical receiver 6 receives the optical signal S2 output from the demultiplexer 4.

[0022] Next, the transmission of the optical signal in the optical communication system 100 will be described in more detail. In this configuration, when transmitting the wavelength-division multiplexed optical signal S through the optical transmission line 10, the optical signal S1 with wavelength λ1 multiplexed in the wavelength-division multiplexed optical signal S is shifted by a certain wavelength due to stimulated Raman scattering by the medium of the optical transmission line 10 and other optical signals with different wavelengths (λ i -λ jThe power transitions to the signal light of ( = Ω). Thus, when the second wavelength of the optical signal S2 multiplexed in the wavelength-division multiplexed optical signal S satisfies Ω (when it is the same as or approximate to the wavelength of the Raman scattered light), the optical signal S2 is amplified by inter-channel Raman scattering. As described above, in the optical transmission line 10, since the wavelength λ1 is in the C band and the wavelength λ2 is in the L band, the optical signal S2 in the L band is amplified using the optical signal S1 in the C band as the pump light, and amplification by inter-channel induced Raman scattering is performed.

[0023] Therefore, when the optical signal S2 reaches the receiving side, the amount of decrease in the received intensity is suppressed by the intensity compensation due to inter-channel induced Raman scattering in the optical transmission line 10. On the other hand, the optical signal S1 when it reaches the receiving side has a larger decrease in the received intensity compared to the optical signal S2 by the amount of power transition from the optical signal S1 to the optical signal S2 due to inter-channel induced Raman scattering. Therefore, an imbalance occurs between the received intensity of the optical signal S1 and the received intensity of the optical signal S2.

[0024] For example, when the transmission intensity of the optical signal S1 when output from the optical transmitter 1 and the transmission intensity of the optical signal S2 when output from the optical transmitter 2 are the same, and the difference in transmission loss is ignored for simplicity, the received intensity of the optical signal S1 is lower than the received intensity of the optical signal S2 by the amount of amplification of the optical signal S2 by inter-channel induced Raman scattering.

[0025] Hereinafter, the transmission intensities of the optical signals S1 and S2 are also referred to as the first and second transmission intensities, respectively. The received intensities of the optical signals S1 and S2 are also referred to as the first and second received intensities, respectively.

[0026] FIG. 3 is a diagram showing a simplified variation in intensity when the optical signal S1 in the C band and the optical signal S2 in the L band are transmitted through an optical transmission line. As described above, the optical signal S2 in the L band is amplified by inter-channel induced Raman scattering by the optical signal S1 in the C band. Therefore, as shown in FIG. 3, even when the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 are the same, the received intensity R1 of the optical signal S1 is lower than the received intensity R2 of the optical signal S2.

[0027] Therefore, in the optical communication system 100, the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 are adjusted so that the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 are equal, or so that the difference is suppressed to a negligible level. Thereby, the imbalance between the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2, which is caused by the amplification of the optical signal S2 by inter-signal induced Raman scattering in the optical transmission line 10, is corrected.

[0028] Next, regarding the significance of adjusting the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 in the optical communication system 100, an explanation will be given using a comparative example in which the relationship between the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 is simplified.

[0029] Let the transmission loss when the optical signal S1 is transmitted alone through the optical transmission line 10 be L1. At this time, the reception intensity R1 of the optical signal S1 at the optical receiver 5 is expressed by the following formula.

Equation

[0030] Let the transmission loss when the optical signal S2 is transmitted alone through the optical transmission line 10 be L2. Let the amplification amount of the optical signal S2 due to inter-signal induced Raman scattering in the optical transmission line 10 be ΔP RAM At this time, the reception intensity R2 of the optical signal S2 at the optical receiver 6 is expressed by the following formula.

Equation

[0031] From equations [1] and [2], the relationship shown in the following equation is derived.

Equation

[0032] Therefore, the transmission intensity T2 of the optical signal S2 is expressed by the following equation.

Equation

[0033] Here, to compensate for the transmission loss difference ΔL, it is assumed that the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 are adjusted by a general method. Let P1 be the transmission intensity of the optical signal S1 after transmission loss compensation. Let P2 be the transmission intensity of the optical signal S2 after transmission loss compensation. In this case, ΔL = 0 and Equation [4] is rewritten as Equation [5].

Equation

[0034] That is, in the state where the transmission loss is compensated, to make the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 equal (ΔR = 0) (from Equation [5], ΔR = P1 - P2 - 2ΔP RAM = 0 → P1 - P2 = 2ΔP RAM ), it can be understood that the transmission intensity of the optical signal S2 should be set lower than the transmission intensity of the optical signal S1 by an amount corresponding to the amplification amount 2ΔP of the optical signal S2 due to inter-signal Raman amplification. Or, it can be understood that the transmission intensity of S1 should be set higher than the transmission intensity of the optical signal S2 by an amount 2ΔP RAM by which the optical signal transitions from the optical signal 1 to the optical signal S2 due to inter-signal induced Raman scattering. RAM

[0035] ​Therefore, in this configuration, the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 are adjusted so that the transmission intensity T2 of the optical signal S2 is lower than the transmission intensity T1 of the optical signal S1. Or the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 are adjusted so that the transmission intensity T1 of the optical signal S1 is higher than the transmission intensity T2 of the optical signal S2. Thereby, the difference between the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 can be kept within an allowable range. Hereinafter, the allowable range of the difference between the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 is also referred to as a first predetermined range. More preferably, the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 can be made equal. Thereby, the imbalance between the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 can be preferably corrected.

[0036] Thereby, the difference in reception intensity between the optical signal in the C band and the optical signal in the L band can be compensated, and the reception intensity of all optical signals can be flattened. As a result, it is possible to improve the OSNR degradation on the high-frequency side, that is, the short-wavelength side, where the intensity reduction due to inter-signal induced Raman scattering is large in multi-band transmission, and the accompanying Q-value degradation.

[0037] FIG. 4 is a diagram simply showing the intensity variation when the transmission intensity of the optical signal in the L band is set lower than the transmission intensity of the optical signal in the C band by the amount of amplification due to inter-signal induced Raman scattering in the optical communication system according to one embodiment. In FIG. 4, for simplicity, it is assumed that the transmission loss L1 of the optical signal S1 and the transmission loss L2 of the optical signal S2 are the same. This is only an assumption for simplifying the figure, and does not deny the fact that the transmission losses of lights with different wavelengths may be different.

[0038] According to the optical communication system 100, as shown in FIG. 4, by previously making the transmission intensity T2 of the optical signal S2 in the L band lower than the transmission intensity T1 of the optical signal S1 in the C band by the power transition amount of the induced Raman scattering, or making the transmission intensity T1 of the optical signal S1 in the C band higher than the transmission intensity T2 of the optical signal S2 in the L band, the imbalance between the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 can be corrected.

[0039] Also, when the transmission intensity of the optical signal is increased and the total power in the fiber becomes too high, it is known that non-linear effects such as self-phase modulation (SPM) become significant in the optical signal at reception. Therefore, if the transmission intensity T1 of the optical signal S1 is increased too much to correct the imbalance between the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2, the generalized signal-to-noise ratio (hereinafter, GSNR: Generalized Signal to Noise Ratio) and the optical signal-to-noise ratio (hereinafter, OSNR: Optical Signal to Noise Ratio) of the optical signal S1 tend to deteriorate easily.

[0040] Therefore, it is desirable to keep the transmission intensity T1 of the optical signal S1 as low as possible. In contrast, in the optical communication system 100, instead of increasing the transmission intensity T1 of the optical signal S1, the transmission intensity T2 of the optical signal S2 can be decreased by an amount compensated by Raman amplification, relative to the transmission intensity T1 of the optical signal S1. As a result, the transmission intensity T1 of the optical signal S1 can be maintained at a low level, and it is also possible to prevent the deterioration of the GSNR and OSNR of the optical signal S1 at reception.

[0041] Furthermore, when transmitting an optical signal through an optical transmission line, for example, as described in Patent Document 2, it is known that a so-called tilt phenomenon occurs in which the reception intensity of the optical signal varies according to the wavelength. The tilt phenomenon appears as a wavelength characteristic in which the reception intensity within one band becomes lower on the high-frequency side, that is, the short-wavelength side, due to the occurrence of induced Raman scattering within the band. Therefore, also in the optical communication system 100, in order to compensate for the tilt phenomenon, one or both of the transmission intensities T1 and T2 may be appropriately adjusted according to the wavelengths of the optical signals S1 and S2.

[0042] In this configuration, the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 may be preset, for example, by a user of the optical communication system 100.

[0043] Alternatively, a control unit may be provided in the optical communication system 100, and by giving commands from the control unit to the optical transmitters 1 and 2, the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2 may be set.

[0044] FIG. 5 is a diagram schematically showing a configuration of a modified example of the optical communication system according to an embodiment. The optical communication system 110 in FIG. 5 has a configuration in which a control unit 7 is further provided in the optical communication system 100. The control unit 7 outputs control signals CON1 and CON2 to the optical transmitters 1 and 2, respectively, based on a command INS given from, for example, a user of the optical communication system 110. The optical transmitters 1 and 2 may set the transmission intensity T1 of the optical signal S1 and the transmission intensity T2 of the optical signal S2, respectively, according to the control signals CON1 and CON2.

[0045] Embodiment 2 In the present embodiment, an optical communication system that Raman-amplifies a wavelength-division multiplexed optical signal S by coupling pump light for Raman amplification to the optical transmission line 10 will be described. FIG. 6 is a diagram schematically showing a configuration of the optical communication system according to an embodiment. The optical communication system 200 in FIG. 6 has a configuration in which a pump light source 11 and 12 and optical couplers 13 and 14 are further provided in the optical communication system 100.

[0046] The pump light source 11 outputs pump light PL1 to the optical transmission line 10. In this example, an optical coupler 13 is provided in the optical transmission line 10. The optical coupler 13 is configured as, for example, an optical coupler. The pump light source 11 outputs the pump light PL1 to the optical coupler 13. The optical coupler 13 couples the pump light PL1 to the optical transmission line 10 so that the pump light PL1 propagates in the transmission direction of the wavelength-division multiplexed optical signal S. Thereby, the wavelength-division multiplexed optical signal S is amplified by forward Raman amplification by the pump light PL1. Hereinafter, the pump light source 11 is also referred to as a first pump light source. The pump light PL1 is also referred to as a first pump light. The optical coupler 13 is also referred to as a first optical coupler.

[0047] The pump light sources 11 and 12 may be configured as any laser light source such as a laser module that outputs pump light.

[0048] The pump light source 12 outputs pump light PL2 to the optical transmission path 10. In this example, an optical coupler 14 is provided in the optical transmission path 10. The optical coupler 14 is configured as, for example, an optical coupler. The pump light source 12 outputs the pump light PL2 to the optical coupler 14. The optical coupler 14 couples the pump light PL2 to the optical transmission path 10 so that the pump light PL2 propagates in a direction opposite to the transmission direction of the wavelength-division multiplexed optical signal S. Thereby, the wavelength-division multiplexed optical signal S is amplified by backward Raman amplification by the pump light PL2. Hereinafter, the pump light source 12 is also referred to as a second pump light source. The pump light PL2 is also referred to as second pump light. The optical coupler 14 is also referred to as a second optical coupler.

[0049] In the optical communication system 200, by coupling the pump lights PL1 and PL2 to the optical transmission path, the wavelength-division multiplexed optical signal can be Raman amplified. Therefore, the transmission intensities T1 of the optical signal S1 and T2 of the optical signal S2, which are necessary to make the reception intensities R1 of the optical signal S1 and R2 of the optical signal S2 into desired values, can be reduced. Thereby, non-linear effects such as SPM, which occur in accordance with an increase in the transmission intensity of the optical signal, can be further suppressed.

[0050] Also, in the optical communication system 200, the wavelengths λ3 of the pump lights PL1 and PL2 may be set so that the gain of the Raman amplification of the optical signal S1 in the C band becomes larger than the gain of the Raman amplification of the optical signal S2 in the L band. Hereinafter, the wavelength λ3 is also referred to as a third wavelength. Note that the wavelength of the pump light PL1 and the wavelength of the pump light PL2 may be the same or different.

[0051] Thereby, Raman amplification of the wavelength-division multiplexed signal S by the pump lights PL1 and PL2 can be performed so that an imbalance does not occur in the reception intensities between the optical signals S1 and S2.

[0052] Embodiment 3 In an optical communication system, it is required to maintain the optical signal received by an optical receiver at a certain quality. Therefore, for example, the GSNR of the optical signal received by the optical receiver is used as an index of the signal quality. For example, Non-Patent Document 1 describes the calculation simulation of GSNR. Here, a process of converting the OSNR received by the optical receiver into GSNR is performed.

[0053] On the other hand, in this configuration, an optical communication system that monitors the quality of the optical signal received by the optical receiver based on the OSNR will be described. FIG. 7 is a diagram schematically showing the configuration of an optical communication system according to an embodiment. The optical communication system 300 in FIG. 7 has a configuration in which a control unit 8 is further provided in the optical communication system 100. The control unit 8 may be configured to manage the optical communication system 300 such as a network management system (NMS).

[0054] In the optical communication system 300, the optical receiver 5 measures the OSNR of the optical signal S1. The optical receiver 5 outputs a measurement signal M1 indicating the measured OSNR of the optical signal S1 to the control unit 8. The optical receiver 6 measures the OSNR of the optical signal S2. The optical receiver 6 outputs a measurement signal M2 indicating the measured OSNR of the optical signal S2 to the control unit 8.

[0055] Also, as shown in FIG. 8, the optical communication system 300 may include an optical coupler 15 and an optical channel monitor (OCM) 16. The optical coupler 15 is inserted at a position on the receiving side of the optical transmission line 10, for example, in front of the demultiplexer 4. The optical coupler 15 branches a part of the wavelength-division multiplexed optical signal S transmitted through the optical transmission line 10 and outputs it to the OCM 16. The OCM 16 measures the intensity of the wavelength-division multiplexed optical signal S transmitted through the optical transmission line 10 based on a part of the input wavelength-division multiplexed optical signal S. Then, the OCM 16 outputs a measurement signal M3 indicating the measurement result to the control unit 8. Note that the optical coupler 15 is merely an example, and any optical branching means that can branch the wavelength-division multiplexed optical signal S in the same manner may be used. The OCM 16 is merely an example, and any measurement means that measures the intensity of the wavelength-division multiplexed optical signal S may be used.

[0056] Based on the measurement signals M1 and M2, or the measurement signal M3, the control unit 8 calculates the average value of the optical signal. Using this average value or a value instructed in advance from an external system as a target value, it determines whether the OSNR of the optical signal S1 is within an allowable range. Also, the control unit 8 determines whether the OSNR of the optical signal S2 is within an allowable range based on the measurement signal M2. Hereinafter, the allowable range of the OSNR of the optical signal S1 is also referred to as the second predetermined range. The allowable range of the OSNR of the optical signal S2 is also referred to as the second predetermined range.

[0057] According to the determination result, the control unit 8 outputs control signals CON1 and CON2 to the multiplexer 3 respectively. The multiplexer 3 adjusts the transmission intensity of the optical signal S1 and the transmission intensity of the optical signal S2 output from the multiplexer 3 according to the control signals CON1 and CON2 respectively. The multiplexer 3 is assumed to be a WSS or the like, but the device type is not limited as long as it has the same function.

[0058] Next, the intensity adjustment operation of the optical signals S1 and S2 output from the multiplexer 3 in the optical communication system 300 will be described. FIG. 8 is a flowchart of the intensity adjustment operation of the optical signal output from the multiplexer in the optical communication system according to an embodiment.

[0059] Step ST1 In a state where the wavelength-division multiplexed optical signal S is being transmitted, the intensity on the receiving side of the wavelength-division multiplexed optical signal S is measured. First, the first measurement method will be described. The optical receiver 3 measures the reception intensity R1 of the optical signal S1. The optical receiver 3 outputs the information indicating the measured reception intensity R1 included in the measurement signal M1. The optical receiver 4 measures the reception intensity R2 of the optical signal S2. The optical receiver 4 outputs the information indicating the measured reception intensity R2 included in the measurement signal M2.

[0060] Next, the second measurement method will be described. The OCM16 measures the intensity of the optical wavelength-division multiplexed optical signal S on the receiving side based on the optical wavelength-division multiplexed optical signal S branched by the optical coupler 15. Then, the OCM16 outputs a measurement signal M3 indicating the measurement result to the control unit 3. The measurement signal M3 may include signals indicating the intensities of the optical signals S1 and S2 on the receiving side. Also, the measurement signal M3 may be a signal from which the control unit 8 can obtain the intensities of the optical signals S1 and S2 on the receiving side by analysis.

[0061] Step ST2 Based on the measurement signals M1 and M2, or the measurement signal M3, the control unit 8 calculates the average value R of the reception intensity R1 and the reception intensity R2. AVE to calculate.

[0062] Step ST3 The control unit 8 calculates the difference R between the calculated average value R AVE and the target value R TRG and the difference R D to calculate.

[0063] Step ST4 The control unit 8 determines whether the difference R D is smaller than the reference value R D_REF . If the difference R D is smaller than the reference value D_REF , the process ends.

[0064] Step ST5 If the difference R D is larger than the reference value R D_REF , to adjust the output intensities of the optical signals S1 and S2 in the multiplexer 3 so that the difference R D becomes smaller, the control unit 8 instructs the output intensities of the adjusted optical signals S1 and S2 by the control signals CON1 and CON2.

[0065] Step ST6 The multiplexer 3 sets the output intensity of the instructed optical signal S1 and the output intensity of the optical signal S2. Then, the process returns to Step ST1.

[0066] As described above, by the adjustment operation of the multiplexer 3, the differential R D can be converged to a range smaller than the reference value R D_REF . As a result, the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 can be set to suitable values. Consequently, the OSNR of the optical signals S1 and S2 can be maintained at sufficient values.

[0067] Therefore, according to the optical communication system 300, while monitoring whether the OSNR is within a predetermined range, the reception intensity R1 of the optical signal S1 and the reception intensity R2 of the optical signal S2 can be set to suitable values.

[0068] Other Embodiments Although the present disclosure has been described with reference to the embodiments above, the present disclosure is not limited to the above-described embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. And each embodiment can be combined with other embodiments as appropriate.

[0069] In the above-described embodiment, the wavelength λ1 is a wavelength in the C band and the wavelength λ2 is a wavelength in the L band, but this is merely an example. As long as the wavelength λ1 is shorter than the wavelength λ2, the wavelengths λ1 and λ2 can be any wavelengths.

[0070] In the embodiment, the configuration for performing forward Raman amplification and backward Raman amplification on the wavelength-division multiplexed optical signal S has been described, but this is merely an example. For the wavelength-division multiplexed optical signal S, both forward Raman amplification and backward Raman amplification may be performed, or only one of them may be performed.

[0071] Also in the optical communication system according to Embodiment 3, similar to the optical communication system according to Embodiment 2, by providing the pump light sources 11 and 12 and the optical couplers 13 and 14, one or both of forward Raman amplification and backward Raman amplification may be performed on the wavelength-division multiplexed optical signal S.

[0072] The figures are merely illustrative for explaining one or more embodiments. Each figure may be associated with not only one specific embodiment but also one or more other embodiments. As can be understood by those skilled in the art, various features or steps described with reference to any one figure can be combined with features or steps shown in one or more other figures to create, for example, embodiments that are not explicitly illustrated or described. Not all of the features or steps shown in any one figure for explaining exemplary embodiments are necessarily essential, and some features or steps may be omitted. The order of the steps described in any figure may be changed as appropriate.

[0073] Some or all of the above embodiments may be described as follows, but are not limited thereto.

[0074] (Appendix 1) An optical communication system comprising: a first optical transmission means for outputting a first optical signal having a first wavelength at a first transmission intensity; a second optical transmission means for outputting a second optical signal having a second wavelength longer than the first wavelength at a second transmission intensity lower than the first transmission intensity; and a multiplexing means for outputting a third optical signal obtained by multiplexing the first optical signal and the second optical signal to an optical transmission line, wherein the intensity of the first optical signal transmitted through the optical transmission line is decreased by inter-signal induced Raman scattering, and the second optical signal transmitted through the optical transmission line is amplified by amplification due to inter-signal induced Raman scattering of the first optical signal.

[0075] (Appendix 2) The optical communication system according to Appendix 1, wherein the second transmission intensity is determined according to the intensity compensation amount in amplification due to inter-signal induced Raman scattering of the first optical signal.

[0076] (Appendix 3) Demultiplexing means for demultiplexing the third optical signal transmitted through the optical transmission path into the first optical signal and the second optical signal by wavelength; first optical receiving means for receiving the first optical signal demultiplexed by the demultiplexing means; and second optical receiving means for receiving the second optical signal demultiplexed by the demultiplexing means. The first and second transmission intensities are determined such that the difference between the first reception intensity of the first optical signal received by the first optical receiving means and the second reception intensity of the second optical signal received by the second optical receiving means is within a first predetermined range. The optical communication system according to Appendix 1 or 2.

[0077] (Appendix 4) The first and second transmission intensities are determined so as to compensate for a tilt phenomenon in which the first reception intensity varies according to the wavelength of the first optical signal and a tilt phenomenon in which the second reception intensity varies according to the wavelength of the second optical signal. The optical communication system according to Appendix 3.

[0078] (Appendix 5) Further comprising a first pump light source for outputting a first pump light, and first optical coupling means for coupling the first pump light to the optical transmission path so that the first pump light is transmitted through the optical transmission path in the transmission direction of the third optical signal. The third optical signal is amplified by forward Raman amplification by the first pump light. The optical communication system according to Appendix 1 or 2.

[0079] (Appendix 6) The wavelength of the first pump light is set to a wavelength at which the gain of the forward Raman amplification of the first optical signal by the first pump light is greater than the gain of the forward Raman amplification of the second optical signal by the first pump light. The optical communication system according to Appendix 5.

[0080] (Appendix 7) Further comprising a second pump light source for outputting a second pump light, and second optical coupling means for coupling the second pump light to the optical transmission path so that the second pump light is transmitted through the optical transmission path in a direction opposite to the transmission direction of the third optical signal. The third optical signal is amplified by backward Raman amplification by the second pump light. The optical communication system according to Appendix 1 or 2.

[0081] (Appendix 8) The wavelength of the second pump light is set to a wavelength at which the gain of the backward Raman amplification of the first optical signal by the second pump light is greater than the gain of the backward Raman amplification of the second optical signal by the second pump light. The optical communication system according to Appendix 7.

[0082] (Appendix 9) Further provided is control means for controlling the output intensities of the first and second optical signals in the multiplexing means such that the difference between the average value of the first reception intensity and the second reception intensity and a predetermined target value is smaller than a predetermined reference value. The optical communication system according to Appendix 3.

[0083] (Appendix 10) Further provided is control means for setting the first transmission intensity for the first optical transmission means and setting the second transmission intensity for the second optical transmission means in response to a given command. The optical communication system according to Appendix 1 or 2.

[0084] (Appendix 11) An optical communication method in which a first optical signal having a first wavelength is output at a first transmission intensity, a second optical signal having a second wavelength longer than the first wavelength is output at a second transmission intensity lower than the first transmission intensity, a third optical signal obtained by multiplexing the first optical signal and the second optical signal is output to an optical transmission line, the intensity of the first optical signal transmitted through the optical transmission line is decreased by Raman scattering, and the second optical signal transmitted through the optical transmission line is amplified by forward Raman amplification using the Raman scattered light of the first optical signal as excitation light.

Explanation of Reference Numerals

[0085] 1, 2 Optical transmitters 3 Multiplexer 4 Demultiplexer 5, 6 Optical receivers 7, 8 Control units 10 Optical transmission line 11, 12 Pump light sources 13, 14 Optical couplers 15 Optical coupler 16 OCM 100, 110, 200, 300 optical communication systems CON1, CON2 control signals INS instruction M1, M2 measurement signals PL1, PL2 pump lights S wavelength multiplexed optical signal S1, S2 optical signals

Claims

1. A first optical transmission means for outputting a first optical signal having a first wavelength at a first transmission intensity; A second optical transmission means for outputting a second optical signal having a second wavelength longer than the first wavelength at a second transmission intensity lower than the first transmission intensity; A multiplexing means for outputting a third optical signal obtained by multiplexing the first optical signal and the second optical signal to an optical transmission line, comprising: The intensity of the first optical signal transmitted through the optical transmission line is reduced by inter-signal induced Raman scattering; The second optical signal transmitted through the optical transmission line is amplified by the inter-signal induced Raman scattering of the first optical signal; An optical communication system.

2. The second transmission intensity is determined according to the intensity compensation amount in the amplification by the inter-signal induced Raman scattering of the first optical signal; The optical communication system according to Claim 1.

3. A demultiplexing means for wavelength-separating the third optical signal transmitted through the optical transmission line into the first optical signal and the second optical signal; A first optical receiving means for receiving the first optical signal wavelength-separated by the demultiplexing means; A second optical receiving means for receiving the second optical signal wavelength-separated by the demultiplexing means, comprising: The first and second transmission intensities are determined such that the difference between the first reception intensity of the first optical signal received by the first optical receiving means and the second reception intensity of the second optical signal received by the second optical receiving means is within a first predetermined range; The optical communication system according to Claim 1 or 2.

4. The first and second transmission intensities are determined so as to compensate for a tilt phenomenon in which the first reception intensity varies according to the wavelength of the first optical signal and a tilt phenomenon in which the second reception intensity varies according to the wavelength of the second optical signal; The optical communication system according to Claim 3.

5. A first pump light source for outputting a first pump light; A first optical coupling means for coupling the first pump light to the optical transmission line so that the first pump light is transmitted through the optical transmission line in the transmission direction of the third optical signal; The third optical signal is amplified by forward Raman amplification by the first pump light; The optical communication system according to Claim 1 or 2.

6. The wavelength of the first pump light is set to a wavelength at which the gain of the forward Raman amplification of the first optical signal by the first pump light is greater than the gain of the forward Raman amplification of the second optical signal by the first pump light; The optical communication system according to claim 5.

7. A second pump light source that outputs a second pump light; Second optical coupling means for coupling the second pump light to the optical transmission line so that the second pump light is transmitted in a direction opposite to the transmission direction of the third optical signal through the optical transmission line; and The third optical signal is amplified by backward Raman amplification by the second pump light. The optical communication system according to claim 1 or 2.

8. The wavelength of the second pump light is set to a wavelength at which the gain of backward Raman amplification of the first optical signal by the second pump light is greater than the gain of backward Raman amplification of the second optical signal by the second pump light. The optical communication system according to claim 7.

9. Further comprising control means for controlling the output intensities of the first and second optical signals at the multiplexing means so that the difference between the average value of the first received intensity and the second received intensity and a predetermined target value is smaller than a predetermined reference value. The optical communication system according to claim 3.

10. Output a first optical signal having a first wavelength at a first transmission intensity; Output a second optical signal having a second wavelength longer than the first wavelength at a second transmission intensity lower than the first transmission intensity; Output a third optical signal obtained by multiplexing the first optical signal and the second optical signal to an optical transmission line; The intensity of the first optical signal transmitted through the optical transmission line decreases due to inter-signal induced Raman scattering; The second optical signal transmitted through the optical transmission line is amplified by inter-signal induced Raman scattering of the first optical signal. Optical communication method.

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