Optical communication device, optical amplifier, and optical amplification method

Abstract

The present invention provides optical communication equipment and the like that improves nonlinear SNR by wavelength division amplification of signal light. [Solution] The optical communication device includes a demultiplexer, a first amplifier, a second amplifier, and a combiner. The demultiplexer demultiplexers the input signal light into a first signal light and a second signal light. The first signal light is amplified by the first amplifier and then amplified by the second amplifier. The second signal light is amplified by the second amplifier and then amplified by the first amplifier. The combiner combines the amplified first signal light and the amplified second signal light.

Description

[Technical Field]

[0001] The present disclosure relates to an optical communication device, an optical amplifier, and an optical amplification method. [Background Art]

[0002] Multi-band optical transmission using a plurality of wavelength bands is effective for increasing the transmission capacity of an optical communication network. In recent years, optical communication devices that use ion-doped fiber amplifiers, semiconductor optical amplifiers, or Raman amplifiers for optical amplification of signal light in, for example, the C-band, L-band, and S-band in a multi-band Wavelength Division Multiplexing (WDM) system have been studied. [Prior Art Documents] [Patent Documents]

[0003] [Patent Document 1] U.S. Pat. No. 6,882,466 [Patent Document 2] Japanese Unexamined Patent Publication No. 9-83270 [Summary of the Invention] [Problem to be Solved by the Invention]

[0004] However, in a conventional optical communication device, for example, when a Raman amplifier that optically amplifies signal light of a plurality of wavelength bands is used, Cross Phase Modulation (XPM) occurs due to wavelength division amplification of the signal light. As a result, the nonlinear Signal to Noise Ratio (SNR) of the signal light deteriorates due to XPM.

[0005] In one aspect, an object of the present invention is to provide an optical communication device or the like that improves nonlinear SNR achieved by wavelength division amplification of signal light. [Means for Solving the Problem]

[0006] One embodiment of an optical communication device includes a demultiplexer, a first amplifier, a second amplifier, and a combiner. The demultiplexer demultiplexes an input signal light into a first signal light and a second signal light. The first signal light is amplified by the first amplifier and then amplified by the second amplifier. The second signal light is amplified by the second amplifier and then amplified by the first amplifier. The combiner combines the amplified first signal light and the amplified second signal light. [Effects of the Invention]

[0007] One aspect of this approach is to improve the nonlinear signal-to-noise ratio (SNR) through wavelength division amplification of the signal light. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is an explanatory diagram showing an example of an optical amplifier in Example 1. [Figure 2] Figure 2 is an explanatory diagram showing an example of the simulation results for the optical amplifier in Example 1 and the optical amplifier in Comparative Example 1. [Figure 3] Figure 3 is an explanatory diagram showing an example of the optical amplifier in Example 2. [Figure 4] Figure 4 is an explanatory diagram showing an example of the optical amplifier in Example 3. [Figure 5] Figure 5 is an explanatory diagram showing an example of the optical amplifier in Example 4. [Figure 6] Figure 6 is an explanatory diagram showing an example of the optical amplifier in Example 5. [Figure 7] Figure 7 is an explanatory diagram showing an example of an optical amplifier in Comparative Example 1. [Figure 8] Figure 8 is an explanatory diagram showing an example of an optical amplifier in Comparative Example 2. [Modes for carrying out the invention]

[0009] For example, in the development of bandwidth extension nodes that handle S-band, C-band, and L-band signal light, the application of condensed Raman amplifiers to amplify S-band signal light is being considered. However, due to the limitations of the amount of gain obtainable with a single amplification fiber, condensed Raman amplifiers require a multi-stage configuration using multiple amplification fibers, for example, a two-stage configuration using two amplification fibers. For example, if the gain is rapidly increased to amplify the signal light with a single amplification fiber, it can lead to a degradation of the signal light quality. Therefore, the optical amplifier in Comparative Example 1, which uses two condensed Raman amplifiers to amplify the optical power of an S-band WDM signal in a two-stage configuration, can be considered.

[0010] <Comparative Example 1> Figure 7 is an explanatory diagram showing an example of the optical amplifier 100 of Comparative Example 1. The optical amplifier 100 shown in Figure 7 has an input section 101, an output section 102, a first amplification section 110A, and a second amplification section 110B. The input section 101 is the input section of the optical amplifier 100 that receives, for example, S-band signal light in a WDM (Wavelength Division Multiplexing) signal. The S-band signal light can include, for example, short-wavelength S-band signal light and long-wavelength S-band signal light. The output section 102 is the output section of the optical amplifier 100 that outputs the S-band signal light after optical amplification.

[0011] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 100 will be amplified stepwise, for example, in the order of P0 → P1 → P2. The gain amounts of the first amplifier 110A and the second amplifier 110B will be assumed to be approximately the same. Optical power P0 is the optical power of the signal light before optical amplification, which is the input stage of the first amplifier 110A. Optical power P1 is the optical power of the signal light after the first optical amplification, which is the output stage of the first amplifier 110A and the input stage of the second amplifier 110B. Optical power P2 is the optical power of the signal light after the second optical amplification, which is the output stage of the second amplifier 110B.

[0012] The first amplification unit 110A optically amplifies the S-band short-wavelength signal light and the S-band long-wavelength signal light input from the input unit 101, and inputs the optically amplified S-band short-wavelength signal light and the S-band long-wavelength signal light to the second amplification unit 110B. The first amplification unit 110A includes a first amplification fiber 103A, a first pump light source 105A, and a pump WDM filter 104A located downstream of the first amplification fiber 103A. The first amplification fiber 103A is, for example, a Raman amplification fiber that optically amplifies the S-band short-wavelength signal light and the S-band long-wavelength signal light input from the input unit 101 in response to the pump light. The first pump light source 105A is a light source that emits pump light to excite the first amplification fiber 103A. The WDM filter 104A for the pump is placed between the first amplification fiber 103A and the second amplification fiber 103B, and is a filter that inputs the pump light from the first pump light source 105A to the first amplification fiber 103A.

[0013] In other words, the first amplification fiber 103A optically amplifies the optical power P0 of the short-wavelength signal light and the long-wavelength signal light input from the input unit 101 into optical power P1 of the short-wavelength signal light and the long-wavelength signal light, respectively.

[0014] The second amplification unit 110B optically amplifies the S-band short-wavelength signal light and the S-band long-wavelength signal light input from the first amplification unit 110A, and inputs the optically amplified S-band short-wavelength signal light and the S-band long-wavelength signal light to the output unit 102. The second amplification unit 110B includes a second amplification fiber 103B, a second pump light source 105B, and a pump WDM filter 104B located downstream of the second amplification fiber 103B. The second amplification fiber 103B is, for example, a Raman amplification fiber that optically amplifies the S-band short-wavelength signal light and the S-band long-wavelength signal light input from the first amplification fiber 103A in response to the pump light. The second pump light source 105B is a light source that emits pump light to excite the second amplification fiber 103B. The WDM filter 104B for the pump is positioned between the second amplification fiber 103B and the output unit 102, and is a filter that inputs the pump light from the second pump light source 105B to the second amplification fiber 103B.

[0015] That is, the second amplification fiber 103B optically amplifies the optical power P1 of the short-wavelength band signal light and the long-wavelength band signal light input from the first amplification fiber 103A to optical power P2 of the short-wavelength band signal light and the long-wavelength band signal light, respectively, and outputs the amplified optical power to the output unit 102.

[0016] In the second amplification fiber 103B at the output stage, since the optical power of the short-wavelength band signal light is P2 and the optical power of the long-wavelength band signal light is P2, the total power increases to P2+P2=P4.

[0017] In the optical amplifier 100 of Comparative Example 1, since the total power in the second amplification fiber 103B at the output stage increases, XPM increases, and the increase in XPM degrades the nonlinear SNR. Accordingly, an optical amplifier 100A of Comparative Example 2, which can suppress degradation of nonlinear SNR of the optical amplifier 100 of Comparative Example 1, is also conceivable.

[0018] <Comparative Example 2> FIG. 8 is an explanatory diagram showing an example of the optical amplifier 100A of Comparative Example 2. The optical amplifier 100A shown in FIG. 8 includes an input unit 101A, an output unit 102A, a demultiplexing unit 111, and a multiplexing unit 112. Further, the optical amplifier 100A includes a first amplification unit 110A1, a second amplification unit 110B1, a third amplification unit 110A2, and a fourth amplification unit 110B2. The input unit 101A is, for example, an input unit of the optical amplifier 100A that receives S-band signal light in a WDM (Wavelength Division Multiplexing) signal. Note that the S-band signal light includes, for example, S-band short-wavelength band signal light and S-band long-wavelength band signal light. The output unit 102A is an output unit of the optical amplifier 100A that outputs the optically amplified S-band signal light.

[0019] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 100A will be amplified stepwise, for example, in the order of P0 → P1 → P2. The gain amounts of the first amplifier 110A1, the second amplifier 110B1, the third amplifier 110A2, and the fourth amplifier 110B2 will be assumed to be approximately the same. Optical power P0 is the optical power of the signal light before optical amplification, optical power P1 is the optical power of the signal light after the first optical amplification, and optical power P2 is the optical power of the signal light after the second optical amplification.

[0020] The demultiplexer 111 demultiplexes the S-band signal light from the input unit 101A into short-wavelength signal light and long-wavelength signal light. The demultiplexer 111 outputs the demultiplexed short-wavelength signal light to the first amplifier 110A1 and outputs the demultiplexed long-wavelength signal light to the third amplifier 110A2.

[0021] The wave combining unit 112 combines the short-wavelength signal light amplified by the second amplification unit 110B1 with the long-wavelength signal light amplified by the fourth amplification unit 110B2. The wave combining unit 112 outputs the combined long-wavelength signal light and short-wavelength signal light to the output unit 102A.

[0022] The first amplification unit 110A1 optically amplifies the S-band short-wavelength signal light input from the demultiplexer 111 and inputs the optically amplified short-wavelength signal light to the second amplification unit 110B1. The first amplification unit 110A1 includes a first amplification fiber 103A1, a first pump light source 105A1, and a pump WDM filter 104A1 located downstream of the first amplification fiber 103A1. The first amplification fiber 103A1 is, for example, a Raman amplification fiber that optically amplifies the short-wavelength signal light input from the demultiplexer 111 in response to the pump light. The first pump light source 105A1 is a light source that emits pump light to excite the first amplification fiber 103A1. The WDM filter 104A1 for the pump is placed between the first amplification fiber 103A1 and the second amplification fiber 103B1, and is a filter that inputs the pump light from the first pump light source 105A1 to the first amplification fiber 103A1.

[0023] In other words, the first amplification fiber 103A1 optically amplifies the optical power P0 of the short-wavelength signal light input from the demultiplexer 111 to the optical power P1 of the short-wavelength signal light.

[0024] The second amplification unit 110B1 optically amplifies the S-band short-wavelength signal light input from the first amplification unit 110A1 and inputs the optically amplified short-wavelength signal light to the multiplexer 112. The second amplification unit 110B1 includes a second amplification fiber 103B1, a second pump light source 105B1, and a pump WDM filter 104B1 located downstream of the second amplification fiber 103B1. The second amplification fiber 103B1 is, for example, a Raman amplification fiber that optically amplifies the S-band short-wavelength signal light input from the first amplification fiber 103A1 in response to the pump light. The second pump light source 105B1 is a light source that emits pump light to excite the second amplification fiber 103B1. The WDM filter 104B1 for the pump is positioned between the second amplification fiber 103B1 and the multiplexer 112, and is a filter that inputs the pump light from the second pump light source 105B1 to the second amplification fiber 103B1.

[0025] In other words, the second amplification fiber 103B1 optically amplifies the optical power P1 of the short-wavelength signal light input from the first amplification fiber 103A1 to the optical power P2 of the short-wavelength signal light and outputs it to the multiplexer 112.

[0026] The third amplification unit 110A2 optically amplifies the S-band long-wavelength signal light input from the demultiplexer 111 and inputs the optically amplified long-wavelength signal light to the fourth amplification unit 110B2. The third amplification unit 110A2 includes a first amplification fiber 103A2, a first pump light source 105A2, and a pump WDM filter 104A2 located downstream of the first amplification fiber 103A2. The first amplification fiber 103A2 is, for example, a Raman amplification fiber that optically amplifies the S-band long-wavelength signal light input from the demultiplexer 111 in response to the pump light. The first pump light source 105A2 is a light source that emits pump light to excite the first amplification fiber 103A2. The WDM filter 104A2 for the pump is placed between the first amplification fiber 103A2 and the second amplification fiber 103B2, and is a filter that inputs the pump light from the first pump light source 105A2 to the first amplification fiber 103A2.

[0027] In other words, the first amplification fiber 103A2 optically amplifies the optical power P0 of the long-wavelength signal light input from the demultiplexer 111 to the optical power P1 of the long-wavelength signal light.

[0028] The fourth amplification unit 110B2 optically amplifies the S-band long-wavelength signal light input from the third amplification unit 110A2 and inputs the optically amplified long-wavelength signal light to the multiplexer 112. The fourth amplification unit 110B2 includes a second amplification fiber 103B2, a second pump light source 105B2, and a pump WDM filter 104B2 located downstream of the second amplification fiber 103B2. The second amplification fiber 103B2 is, for example, a Raman amplification fiber that optically amplifies the S-band long-wavelength signal light input from the first amplification fiber 103A2 in response to the pump light. The second pump light source 105B2 is a light source that emits pump light to excite the second amplification fiber 103B2. The WDM filter 104B2 for the pump is positioned between the second amplification fiber 103B2 and the multiplexer 112, and is a filter that inputs the pump light from the second pump light source 105B2 to the second amplification fiber 103B2.

[0029] In other words, the second amplification fiber 103B2 optically amplifies the optical power P1 of the long-wavelength signal light input from the first amplification fiber 103A2 to the optical power P2 of the long-wavelength signal light and outputs it to the multiplexer 112.

[0030] In the first amplifier 110A1, the second amplifier 110B1, the third amplifier 110A2, and the fourth amplifier 110B2, the generation of XPM is suppressed. The combined wave unit 112 combines the short-wavelength signal light of optical power P2 from the second amplifier 110B1 and the long-wavelength signal light of optical power P2 from the fourth amplifier 110B2 and outputs it to the output unit 102A.

[0031] In the optical amplifier 100A of Comparative Example 2, short-wavelength signal light is optically amplified via the first amplification section 110A1 and the second amplification section 110B1, while long-wavelength signal light is optically amplified via the third amplification section 110A2 and the fourth amplification section 110B2. As a result, by optically amplified short-wavelength and long-wavelength signal light individually, the generation of XPM can be suppressed and the degradation of the nonlinear SNR can be suppressed.

[0032] However, in the optical amplifier 100A of Comparative Example 2, two amplification units are required for each wavelength band, resulting in a large number of components. Therefore, there is a need for an optical amplifier that can improve the nonlinear SNR by suppressing XPM while keeping the number of components down.

[0033] Therefore, in order to address this situation, an embodiment that can improve the nonlinear SNR by suppressing XPM will be described below. The embodiments of the optical communication device, etc., of the present invention will be described below with reference to the drawings. Note that the disclosed technology is not limited by this embodiment. Furthermore, the embodiments shown below may be combined as appropriate, as long as they do not cause inconsistencies.

[0034] <Example 1> Figure 1 is an explanatory diagram showing an example of the optical amplifier 1 of Embodiment 1. The optical amplifier 1 is built into an optical communication device such as an optical add-drop multiplexer (OADM) node that selectively adds or separates signals of desired wavelengths included in a predetermined wavelength band. The optical amplifier 1 shown in Figure 1 has an input section 2, an output section 3, a demultiplexer section 4, a multiplexer section 5, a first amplification section 6A, and a second amplification section 6B. The input section 2 is, for example, the input section of the optical amplifier 1 that optically amplifies S-band signal light. The S-band signal light includes, for example, S-band short-wavelength signal light and S-band long-wavelength signal light. The output section 3 is the output section of the optical amplifier 1 that optically amplifies S-band signal light.

[0035] The demultiplexer 4 demultiplexes the S-band signal light from the input 2 into short-wavelength signal light and long-wavelength signal light. The demultiplexer 4 outputs the demultiplexed short-wavelength signal light to the first WDM filter 11A in the first amplifier 6A, and outputs the demultiplexed long-wavelength signal light to the third WDM filter 11C in the second amplifier 6B.

[0036] The combined wave unit 5 combines the long-wavelength signal light separated by the second WDM filter 11B in the first amplification unit 6A with the short-wavelength signal light separated by the fourth WDM filter 11D in the second amplification unit 6B. The combined wave unit 5 outputs the combined long-wavelength signal light and short-wavelength signal light to the output unit 3.

[0037] The first amplification unit 6A optically amplifies the input S-band short-wavelength signal light and the long-wavelength signal light. The first amplification unit 6A includes a first WDM filter 11A, a first amplification fiber 10A, a first pump light source 13A, a pump WDM filter 12A located downstream of the first amplification fiber 10A, and a second WDM filter 11B.

[0038] The second amplification unit 6B optically amplifies the input S-band short-wavelength signal light and the long-wavelength signal light. The second amplification unit 6B includes a third WDM filter 11C, a second amplification fiber 10B, a second pump light source 13B, a pump WDM filter 12B located downstream of the second amplification fiber 10B, and a fourth WDM filter 11D.

[0039] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 1 will be amplified stepwise, for example, in the order of P0 → P1 → P2. The gain amounts of the first amplifier 6A and the second amplifier 6B will be assumed to be approximately the same. In Figure 1, the signal light in the short wavelength band will be represented by a solid line, and the signal light in the long wavelength band will be represented by a dotted line. Optical power P0 is the optical power of the signal light before the first optical amplification. Optical power P1 is the optical power of the signal light after the first optical amplification by the first amplifier 6A or the second amplifier 6B. Optical power P2 is the optical power of the signal light after the second optical amplification by the first amplifier 6A and the second amplifier 6B.

[0040] The first WDM filter 11A is, for example, the first combiner, which combines the short-wavelength signal light separated by the demultiplexer 4 and the long-wavelength signal light amplified by the second amplification fiber 10B, which was separated by the fourth WDM filter 11D. The optical power of the short-wavelength signal light input to the first WDM filter 11A is P0, and the optical power of the long-wavelength signal light is P1. The first WDM filter 11A combines the combined short-wavelength signal light and the long-wavelength signal light and outputs the combined signal light to the first amplification fiber 10A.

[0041] The first amplification fiber 10A is, for example, a Raman amplification fiber that optically amplifies the short-wavelength and long-wavelength signal light from the first WDM filter 11A in response to the pump light. The first amplification fiber 10A outputs the optically amplified short-wavelength and long-wavelength signal light to the second WDM filter 11B. The optical power of the optically amplified short-wavelength signal light output from the first amplification fiber 10A is P1, and the optical power of the optically amplified long-wavelength signal light is P2. The first pump light source 13A is a light source that emits pump light to excite the first amplification fiber 10A. The pump WDM filter 12A is placed between the first amplification fiber 10A and the second WDM filter 11B and is a WDM filter that inputs the pump light from the first pump light source 13A to the first amplification fiber 10A.

[0042] In the first amplification fiber 10A, the short-wavelength signal light undergoes the first optical amplification, so the optical power of the short-wavelength signal light is P1, and the long-wavelength signal light undergoes the second optical amplification, so the optical power of the long-wavelength signal light is P2. In other words, the total power of the first amplification fiber 10A is smaller than that of the second amplification fiber 103B in Comparative Example 1, where the optical power of both the short-wavelength and long-wavelength signal light is P2, with P1 + P2 = P3. As a result, XPM can be suppressed in the first amplification fiber 10A.

[0043] The second WDM filter 11B is, for example, the first demultiplexer, which separates the short-wavelength and long-wavelength signal light, which have been optically amplified by the first amplification fiber 10A, into short-wavelength and long-wavelength signal light. The second WDM filter 11B outputs the separated long-wavelength signal light to the multiplexer 5 and outputs the separated short-wavelength signal light to the third WDM filter 11C. The optical power of the short-wavelength signal light after demultiplexing by the second WDM filter 11B is P1, and the optical power of the long-wavelength signal light is P2.

[0044] The third WDM filter 11C is, for example, a second multiplexer that combines the long-wavelength signal light separated in the demultiplexer 4 with the short-wavelength signal light amplified by the first amplification fiber 10A, which was separated in the second WDM filter 11B. The third WDM filter 11C combines the short-wavelength signal light and the long-wavelength signal light and outputs the combined signal light to the second amplification fiber 10B. The optical power of the short-wavelength signal light input to the third WDM filter 11C is P1, and the optical power of the long-wavelength signal light is P0.

[0045] The second amplification fiber 10B is, for example, a Raman amplification fiber that optically amplifies the short-wavelength and long-wavelength signal light from the third WDM filter 11C in response to the pump light. The second amplification fiber 10B outputs the optically amplified short-wavelength and long-wavelength signal light to the fourth WDM filter 11D. The optical power of the optically amplified short-wavelength signal light output from the second amplification fiber 10B is P2, and the optical power of the optically amplified long-wavelength signal light is P1. The second pump light source 13B is a light source that emits pump light to excite the second amplification fiber 10B. The pump WDM filter 12B is placed between the second amplification fiber 10B and the fourth WDM filter 11D and is a WDM filter that inputs the pump light from the second pump light source 13B to the second amplification fiber 10B.

[0046] In the second amplification fiber 10B, the output is P2 because the short-wavelength signal light undergoes the second optical amplification, and P1 because the long-wavelength signal light undergoes the first optical amplification. In other words, the total power of the second amplification fiber 10B is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P2 + P1 = P3. As a result, XPM can be suppressed in the second amplification fiber 10B.

[0047] The fourth WDM filter 11D is, for example, a second demultiplexer that separates the short-wavelength and long-wavelength signal light, which have been optically amplified by the second amplification fiber 10B, into short-wavelength and long-wavelength signal light. The fourth WDM filter 11D outputs the separated short-wavelength signal light to the multiplexer 5 and outputs the separated long-wavelength signal light to the first WDM filter 11A. The optical power of the short-wavelength signal light and the optical power of the long-wavelength signal light after demultiplexing by the fourth WDM filter 11D are P2 and P1, respectively.

[0048] The combined wave unit 5 combines the long-wavelength signal light from the second WDM filter 11B with the short-wavelength signal light from the fourth WDM filter 11D, and outputs the combined signal light to the output unit 3. The optical power of the short-wavelength signal light input to the combined wave unit 5 is P2, and the optical power of the long-wavelength signal light is also P2. As a result, the optical amplifier 1 optically amplifies the short-wavelength and long-wavelength signal light to optical power P2 and outputs it.

[0049] Figure 2 is an explanatory diagram showing an example of the simulation results of optical amplifier 1 of Example 1 and optical amplifier 100 of Comparative Example 1. The wavelength of the signal light used in the calculation conditions of the simulation was 32 wavelengths within the range of 1489.7 to 1524.9 nm in the S band, and was divided into 16 waves from the short wavelength band and 16 waves from the long wavelength band. The signal used in the calculation conditions was 83.67 GBd-16QAM, and the others were 83.67 GHz dummy light. For the first and second amplification fibers, for example, a 1 km DCF was used, and the pump light used for the first and second amplification fibers was 1396 nm and 1424 nm pump light. Furthermore, the amplifier input / output conditions of optical amplifier 1 were set to an input of 7 dBm at the input section, an output of 24 dBm at the output section, and a gain of 17 dB, and the input / output power was the total value of 32 waves.

[0050] For the comparison of effects, the average of the calculated nonlinear SNR at 1497.5 nm, which is the center of the 16 wavelengths in the short-wavelength band, and the average of the calculated value at 1515.6 nm, which is the center of the 16 wavelengths in the long-wavelength band were used. In the simulation, the nonlinear SNR related to the output signal of output section 3 of optical amplifier 1 and the nonlinear SNR related to the output signal of output section 102 of optical amplifier 100 were calculated.

[0051] As a result, the pump optical power, as an evaluation item, is 1822mW for optical amplifier 100 in Comparative Example 1 and 1840mW for optical amplifier 1 in Example 1, as shown in Figure 2, which is a 0.06% increase compared to Comparative Example 1. The increase in pump optical power is minor. Note that the pump optical power is the sum of the optical power of the two LDs in the first pump light source in the first amplification unit and the optical power of the two LDs in the second pump light source in the second amplification unit.

[0052] Furthermore, as shown in Figure 2, the Noise Figure (NF) as an evaluation item was 3.58 dB for optical amplifier 100 in Comparative Example 1 and 3.89 dB for optical amplifier 1 in Example 1, which is an increase of 0.04 dB compared to Comparative Example 1. The increase in NF is minor.

[0053] As shown in Figure 2, the nonlinear SNR, which is an evaluation item, is 27.7 dB for optical amplifier 100 in Comparative Example 1 and 31.4 dB for optical amplifier 1 in Example 1, which is an increase of 3.7 dB compared to Comparative Example 1.

[0054] In other words, in the optical amplifier 100 of Comparative Example 1, the total power of the second amplification fiber 103B increases, resulting in a larger XPM and thus a deterioration in the nonlinear SNR. In contrast, in the optical amplifier 1 of Example 1, the total power of the first amplification fiber 10A and the second amplification fiber 10B is reduced, resulting in a larger XPM and thus an improvement in the nonlinear SNR. As a result, the optical amplifier 1 of Example 1 shows a significant improvement in the nonlinear SNR of 3.7 dB compared to the optical amplifier 100 of Comparative Example 1.

[0055] In the optical amplifier 1 of Example 1, the S-band signal light is separated into short-wavelength and long-wavelength signal light. The short-wavelength signal light is optically amplified in the first amplification fiber 10A and then optically amplified in the second amplification fiber 10B. Furthermore, the optical amplifier 1 optically amplified the long-wavelength signal light in the second amplification fiber 10B and then optically amplified in the first amplification fiber 10A. The optical amplifier 1 then outputs a combined short-wavelength signal light and a long-wavelength signal light. The first amplification fiber 10A has a small total power due to the optical power P2 of the long-wavelength signal light and the optical power P1 of the short-wavelength signal light, thus suppressing XPM. Furthermore, the second amplification fiber 10B has a small total power due to the optical power P1 of the long-wavelength signal light and the optical power P2 of the short-wavelength signal light, thus suppressing XPM. As a result, it is possible to improve the nonlinear SNR by wavelength division amplification of the signal light while suppressing the number of components.

[0056] In the optical amplifier 1 of Example 1, the input S-band signal light was shown as an example of being split into short-wavelength and long-wavelength signal light. However, it is not limited to S-band signal light, and for example, the signal light may be split into short-wavelength and long-wavelength bands in each band such as the L-band, C-band, and U-band, and can be changed as appropriate.

[0057] Furthermore, in the optical amplifier 1 of Example 1, the case in which the input signal light is split into short-wavelength signal light and long-wavelength signal light was illustrated. However, the input signal light may also be split into even-numbered channel (even ch) signal light and odd-numbered channel (odd ch) signal light, and such an embodiment will be described below as Example 2.

[0058] <Example 2> Figure 3 is an explanatory diagram showing an example of the optical amplifier 1A of Embodiment 2. The optical amplifier 1A shown in Figure 3 has an input section 2A, an output section 3A, a demultiplexer 4A, a multiplexer 5A, a first amplification section 6A1, and a second amplification section 6B1. The input section 2A is, for example, the input section of the optical amplifier 1A that optically amplifies signal light. The signal light may include, for example, signal light for odd-numbered channels and signal light for even-numbered channels. The output section 3A is the output section of the optical amplifier 1A that optically amplifies the signal light.

[0059] The demultiplexer 4A splits the signal light from the input 2A into odd-numbered channel signal light and even-numbered channel signal light. The demultiplexer 4A outputs the split odd-numbered channel signal light to the first interleaver 21A in the first amplifier 6A1, and outputs the split even-numbered channel signal light to the third interleaver 21C in the second amplifier 6B1.

[0060] The combined wave unit 5A combines the even-channel signal light separated by the second interleaver 21B in the first amplifier unit 6A1 with the odd-channel signal light separated by the fourth interleaver 21D in the second amplifier unit 6B1. The combined wave unit 5A outputs the combined even-channel signal light and odd-channel signal light to the output unit 3A.

[0061] The first amplifier 6A1 optically amplifies the input odd-channel signal light and even-channel signal light. The first amplifier 6A1 includes a first interleaver 21A, a first amplification fiber 10A1, a first pump light source 13A, a pump WDM filter 12A located downstream of the first amplification fiber 10A1, and a second interleaver 21B.

[0062] The second amplifier 6B1 optically amplifies the input odd-channel signal light and even-channel signal light. The second amplifier 6B1 includes a third interleaver 21C, a second amplification fiber 10B1, a second pump light source 13B, a pump WDM filter 12B located downstream of the second amplification fiber 10B1, and a fourth interleaver 21D.

[0063] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 1A will be amplified stepwise, for example, in the order of P0 → P1 → P2. The gain amounts of the first amplifier 6A1 and the second amplifier 6B1 will be assumed to be approximately the same. In Figure 3, the signal light of odd-numbered channels will be represented by solid lines, and the signal light of even-numbered channels will be represented by dotted lines. Optical power P0 is the optical power of the signal light before optical amplification. Optical power P1 is the optical power of the signal light after the first optical amplification by the first amplifier 6A1 or the second amplifier 6B1. Optical power P2 is the optical power of the signal light after the second optical amplification by the first amplifier 6A1 and the second amplifier 6B1.

[0064] The first interleaver 21A is an interleaver that combines the odd-channel signal light separated by the demultiplexer 4A with the even-channel signal light amplified by the second amplification fiber 10B1, which was separated by the fourth interleaver 21D. The optical power of the odd-channel signal light input to the first interleaver 21A is P0, and the optical power of the even-channel signal light is P1. The first interleaver 21A combines the combined odd-channel signal light and the even-channel signal light and outputs the combined signal light to the first amplification fiber 10A1.

[0065] The first amplification fiber 10A1 is, for example, a Raman amplification fiber that optically amplifies the odd-channel signal light and even-channel signal light from the first interleaver 21A in response to the pump light. The first amplification fiber 10A1 outputs the optically amplified odd-channel signal light and even-channel signal light to the second interleaver 21B. The optical power of the optically amplified odd-channel signal light output from the first amplification fiber 10A1 is P1, and the optical power of the optically amplified even-channel signal light is P2. The first pump light source 13A is a light source that emits pump light to excite the first amplification fiber 10A1. The pump WDM filter 12A is placed between the first amplification fiber 10A1 and the second interleaver 21B and is a WDM filter that inputs the pump light from the first pump light source 13A to the first amplification fiber 10A1.

[0066] In the first amplification fiber 10A1, the signal light of the odd-numbered channels undergoes the first optical amplification, so the optical power of the signal light of the odd-numbered channels is P1, and the signal light of the even-numbered channels undergoes the second optical amplification, so the optical power of the signal light of the even-numbered channels is P2. In other words, the total power of the first amplification fiber 10A1 is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P1 + P2 = P3. As a result, XPM can be suppressed in the first amplification fiber 10A1.

[0067] The second interleaver 21B is, for example, an interleaver that separates the odd-channel signal light and even-channel signal light, which have been optically amplified by the first amplification fiber 10A1, into odd-channel signal light and even-channel signal light. The second interleaver 21B outputs the separated even-channel signal light to the multiplexer 5A and outputs the separated odd-channel signal light to the third interleaver 21C. The optical power of the odd-channel signal light after separation by the second interleaver 21B is P1, and the optical power of the even-channel signal light is P2.

[0068] The third interleaver 21C is an interleaver that combines the even-channel signal light separated by the demultiplexer 4A with the odd-channel signal light amplified by the first amplification fiber 10A1, which was separated by the second interleaver 21B. The third interleaver 21C combines the combined odd-channel signal light and the even-channel signal light and outputs the combined signal light to the second amplification fiber 10B1. The optical power of the odd-channel signal light input to the third interleaver 21C is P1, and the optical power of the even-channel signal light is P0.

[0069] The second amplification fiber 10B1 is, for example, a Raman amplification fiber that optically amplifies the odd-channel and even-channel signal light from the third interleaver 21C in response to the pump light. The second amplification fiber 10B1 outputs the optically amplified odd-channel and even-channel signal light to the fourth interleaver 21D. The optical power of the optically amplified odd-channel signal light output from the second amplification fiber 10B1 is P2, and the optical power of the optically amplified even-channel signal light is P1. The second pump light source 13B is a light source that emits pump light to excite the second amplification fiber 10B1. The pump WDM filter 12B is placed between the second amplification fiber 10B1 and the fourth interleaver 21D and is a WDM filter that inputs the pump light from the second pump light source 13B to the second amplification fiber 10B1.

[0070] In the second amplification fiber 10B1, the odd-numbered channel signal light undergoes a second optical amplification, resulting in an optical power of P2 for the odd-numbered channel signal light, while the even-numbered channel signal light undergoes a first optical amplification, resulting in an optical power of P1 for the even-numbered channel signal light. In other words, the total power of the second amplification fiber 10B1 is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P2 + P1 = P3. As a result, XPM can be suppressed in the second amplification fiber 10B1.

[0071] The fourth interleaver 21D is, for example, an interleaver that separates the odd-channel signal light and the even-channel signal light, which have been optically amplified by the second amplification fiber 10B1, into odd-channel signal light and even-channel signal light. The fourth interleaver 21D outputs the separated odd-channel signal light to the multiplexer 5A and outputs the separated even-channel signal light to the first interleaver 21A. The optical power of the odd-channel signal light after separation by the fourth interleaver 21D is P2, and the optical power of the even-channel signal light is P1.

[0072] The combined wave unit 5A combines the even-numbered channel signal light from the second interleaver 21B with the odd-numbered channel signal light from the fourth interleaver 21D, and outputs the combined signal light to the output unit 3A. The optical power of the odd-numbered channel signal light input to the combined wave unit 5A is P2, and the optical power of the even-numbered channel signal light is also P2. As a result, the optical amplifier 1A optically amplifies the odd-numbered channel signal light and the even-numbered channel signal light to optical power P2 and outputs them.

[0073] In the optical amplifier 1A of Example 2, the signal light is separated into odd-channel and even-channel signal light. The odd-channel signal light is optically amplified in the first amplification fiber 10A1 and then optically amplified in the second amplification fiber 10B1. Furthermore, the optical amplifier 1A optically amplified the even-channel signal light in the second amplification fiber 10B1 and then optically amplified in the first amplification fiber 10A1. The optical amplifier 1A then outputs a combined signal of the odd-channel signal light after two optical amplifications and the even-channel signal light after two optical amplifications. The first amplification fiber 10A1 has a small total power, consisting of the optical power P2 of the even-channel signal light and the optical power P1 of the odd-channel signal light, thus suppressing XPM. Furthermore, the second amplification fiber 10B1 has a small total power, consisting of the optical power P1 of the even-channel signal light and the optical power P2 of the odd-channel signal light, thus suppressing XPM. As a result, it is possible to improve the nonlinear SNR by wavelength division amplification of the signal light while suppressing the number of components.

[0074] Furthermore, in the optical amplifier 1 of Example 1, the case in which the input signal light is split into short-wavelength signal light and long-wavelength signal light was illustrated. However, the input signal light may also be split into X-polarized signal light and Y-polarized signal light, and such an embodiment will be described below as Example 3.

[0075] <Example 3> Figure 4 is an explanatory diagram showing an example of the optical amplifier 1B of Embodiment 3. The optical amplifier 1B shown in Figure 1 has an input section 2B, an output section 3B, a demultiplexer 4B, a multiplexer 5B, a first amplification section 6A2, and a second amplification section 6B2. The input section 2B is, for example, the input section of the optical amplifier 1B that optically amplifies signal light. The signal light can be, for example, X-polarized signal light and Y-polarized signal light. The output section 3B is the output section of the optical amplifier 1B that optically amplifies the signal light.

[0076] The demultiplexing unit 4B is, for example, a Polarizing Beam Splitter (PBS) that splits the signal light from the input unit 2B into an X-polarized signal light and a Y-polarized signal light. The demultiplexing unit 4B outputs the split X-polarized signal light to the first Polarization Beam Combiner (PBC) 32A in the first amplifier unit 6A2, and outputs the split Y-polarized signal light to the second PBC 32B in the second amplifier unit 6B2.

[0077] The combined wave unit 5B is, for example, a PBC, which combines the Y-polarized signal light separated by the first PBS 31A in the first amplification unit 6A2 and the X-polarized signal light separated by the second PBS 31B in the second amplification unit 6B2. The combined wave unit 5B outputs the combined Y-polarized signal light and X-polarized signal light to the output unit 3B.

[0078] The first amplification unit 6A2 optically amplifies the input X-polarized signal light and Y-polarized signal light. The first amplification unit 6A2 includes a first PBC 32A, a first amplification fiber 10A2, a first pump light source 13A, a pump WDM filter 12A located downstream of the first amplification fiber 10A2, and a first PBS 31A.

[0079] The second amplification unit 6B2 optically amplifies the input X-polarized signal light and Y-polarized signal light. The second amplification unit 6B2 includes a second PBC 32B, a second amplification fiber 10B2, a second pump light source 13B, a pump WDM filter 12B located downstream of the second amplification fiber 10B2, and a second PBS 31B.

[0080] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 1B will be amplified stepwise, for example, in the order of P0 → P1 → P2. The gain amounts of the first amplifier 6A2 and the second amplifier 6B2 will be assumed to be approximately the same. In Figure 1, the X-polarized signal light will be represented by a solid line, and the Y-polarized signal light will be represented by a dotted line. Optical power P0 is the optical power of the signal light before the first optical amplification. Optical power P1 is the optical power of the signal light after the first optical amplification by the first amplifier 6A2 or the second amplifier 6B2. Optical power P2 is the optical power of the signal light after the second optical amplification by the first amplifier 6A2 and the second amplifier 6B2.

[0081] The first PBC32A is a PBC (Pulse Block Converter) that combines the X-polarized signal light separated in the demultiplexer 4B and the Y-polarized signal light amplified by the second amplification fiber 10B2, which is separated in the second PBS31B. The first PBC32A combines the combined X-polarized signal light and the Y-polarized signal light and outputs the combined signal light to the first amplification fiber 10A2. The optical power of the X-polarized signal light input to the first PBC32A is P0, and the optical power of the Y-polarized signal light is P1.

[0082] The first amplification fiber 10A2 is, for example, a Raman amplification fiber that optically amplifies the X-polarized signal light and the Y-polarized signal light from the first PBC 32A in response to the pump light. The first amplification fiber 10A2 outputs the optically amplified X-polarized signal light and the Y-polarized signal light to the first PBS 31A. The optical power of the optically amplified X-polarized signal light output from the first amplification fiber 10A is P1, and the optical power of the optically amplified Y-polarized signal light is P2. The first pump light source 13A is a light source that emits pump light to excite the first amplification fiber 10A2. The pump WDM filter 12A is placed between the first amplification fiber 10A2 and the first PBS 31A and is input to the first amplification fiber 10A2, which receives the pump light from the first pump light source 13A.

[0083] In the first amplification fiber 10A2, the X-polarized signal light undergoes the first optical amplification, so its optical power is P1, and the Y-polarized signal light undergoes the second optical amplification, so its optical power is P2. In other words, the total power of the first amplification fiber 10A2 is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P1 + P2 = P3. As a result, XPM can be suppressed in the first amplification fiber 10A2.

[0084] The first PBS31A is a PBS that splits the X-polarized signal light and Y-polarized signal light, which have been optically amplified by the first amplification fiber 10A2, into X-polarized signal light and Y-polarized signal light. The first PBS31A outputs the split Y-polarized signal light to the multiplexer 5B and outputs the split X-polarized signal light to the second PBC32B. The optical power of the X-polarized signal light and the optical power of the Y-polarized signal light after splitting by the first PBS31A are P1 and P2, respectively.

[0085] The second PBC32B is a PBC (Particle Block Converter) that combines the Y-polarized signal light separated in the decoupler 4B with the X-polarized signal light amplified by the first amplification fiber 10A2, which was separated in the first PBS31A. The second PBC32B combines the combined X-polarized signal light and the Y-polarized signal light and outputs the combined signal light to the second amplification fiber 10B2. The optical power of the X-polarized signal light input to the second PBC32B is P1, and the optical power of the Y-polarized signal light is P0.

[0086] The second amplification fiber 10B2 is, for example, a Raman amplification fiber that optically amplifies the X-polarized signal light and Y-polarized signal light from the second PBC 32B in response to the pump light. The second amplification fiber 10B2 outputs the optically amplified X-polarized signal light and Y-polarized signal light to the second PBS 31B. The optical power of the optically amplified X-polarized signal light output from the second amplification fiber 10B2 is P2, and the optical power of the Y-polarized long-wavelength signal light is P1. The second pump light source 13B is a light source that emits pump light to excite the second amplification fiber 10B2. The pump WDM filter 12B is placed between the second amplification fiber 10B2 and the second PBS 31B and is an input WDM filter to the second amplification fiber 10B2, which receives the pump light from the second pump light source 13B.

[0087] In the second amplification fiber 10B2, the X-polarized signal light undergoes a second optical amplification, resulting in an optical power of P2 for the X-polarized signal light, while the Y-polarized signal light undergoes a first optical amplification, resulting in an optical power of P1 for the Y-polarized signal light. In other words, the total power of the second amplification fiber 10B2 is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P2 + P1 = P3. As a result, XPM can be suppressed in the second amplification fiber 10B2.

[0088] The second PBS31B is a PBS that splits the X-polarized and Y-polarized signal light, which has been optically amplified by the second amplification fiber 10B2, into X-polarized and Y-polarized signal light. The second PBS31B outputs the split X-polarized signal light to the multiplexer 5B and outputs the split Y-polarized signal light to the first PBC32A. The optical power of the X-polarized signal light and the optical power of the Y-polarized signal light after splitting by the second PBS31B are P2 and P1, respectively.

[0089] The combined wave unit 5B combines the Y-polarized signal light from the first PBS 31A with the X-polarized signal light from the second PBS 31B, and outputs the combined signal light to the output unit 3B. The optical power of the X-polarized signal light input to the combined wave unit 5B is P2, and the optical power of the Y-polarized signal light is also P2. As a result, the optical amplifier 1B optically amplifies the X-polarized signal light and the Y-polarized signal light to optical power P2 and outputs them.

[0090] In the optical amplifier 1B of Example 3, the signal light is split into X-polarized and Y-polarized signal light. The X-polarized signal light is optically amplified in the first amplification fiber 10A2 and then optically amplified in the second amplification fiber 10B2. Furthermore, in the optical amplifier 1B, the Y-polarized signal light is optically amplified in the second amplification fiber 10B2 and then optically amplified in the first amplification fiber 10A2. The optical amplifier 1B then outputs a combined signal of the X-polarized signal light after two optical amplifications and the Y-polarized signal light after two optical amplifications. In the first amplification fiber 10A2, the total power is reduced as the optical power P2 of the Y-polarized signal light and the optical power P1 of the X-polarized signal light, thus suppressing XPM. Furthermore, in the second amplification fiber 10B2, the total power is reduced as the optical power P1 of the Y-polarized signal light and the optical power P2 of the X-polarized signal light, thus suppressing XPM. As a result, it is possible to improve the nonlinear SNR by wavelength division amplification of the signal light while suppressing the number of components.

[0091] In the optical amplifier 1 of Example 1, the case in which unidirectional amplification fibers are used as the first amplification fiber 10A and the second amplification fiber 10B was illustrated. However, bidirectional amplification fibers may also be used as the first amplification fiber 10A and the second amplification fiber 10B, and such an embodiment will be described below as Example 4.

[0092] <Example 4> Figure 5 is an explanatory diagram showing an example of the optical amplifier 1C of Embodiment 4. The optical amplifier 1C shown in Figure 5 has an input section 2C, an output section 3C, a demultiplexer 4C, a multiplexer 5C, a first amplification section 6A3, and a second amplification section 6B3. The input section 2C is, for example, the input section of the optical amplifier 1C that optically amplifies S-band signal light. The S-band signal light can be, for example, S-band short-wavelength signal light and S-band long-wavelength signal light. The output section 3C is the output section of the optical amplifier 1C that optically amplifies S-band signal light.

[0093] The demultiplexer 4C demultiplexes the S-band signal light from the input 2C into short-wavelength signal light and long-wavelength signal light. The demultiplexer 4C outputs the demultiplexed short-wavelength signal light to the first WDM filter 41A in the first amplifier 6A3, and outputs the demultiplexed long-wavelength signal light to the second WDM filter 41B in the second amplifier 6B3.

[0094] The wave-combining unit 5C combines the long-wavelength signal light separated by the first WDM filter 41A with the short-wavelength signal light separated by the second WDM filter 41B, and outputs the combined long-wavelength and short-wavelength signal light to the output unit 3C.

[0095] The first amplification unit 6A3 is a bidirectional amplification unit that optically amplifies short-wavelength signal light input from the forward direction and optically amplifies long-wavelength signal light input from the reverse direction in response to the pump light. The first amplification unit 6A3 includes a first WDM filter 41A, a first amplification fiber 10A3, a first pump light source 13A, and a pump WDM filter 12A located downstream of the first amplification fiber 10A3.

[0096] The second amplification unit 6B3 is a bidirectional amplification unit that optically amplifies short-wavelength signal light input from the forward direction and optically amplifies long-wavelength signal light input from the reverse direction in response to the pump light. The second amplification unit 6B3 includes a second WDM filter 41B, a second amplification fiber 10B3, a second pump light source 13B, and a pump WDM filter 12B located downstream of the second amplification fiber 10B3.

[0097] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 1C will be amplified stepwise, for example, in the order of P0 → P1 → P2. The gain amounts of the first amplifier 6A3 and the second amplifier 6B3 will be assumed to be approximately the same. In Figure 1, the signal light in the short wavelength band will be represented by a solid line, and the signal light in the long wavelength band will be represented by a dotted line. Optical power P0 is the optical power of the signal light before the first optical amplification. Optical power P1 is the optical power of the signal light after the first optical amplification by the first amplifier 6A3 or the second amplifier 6B3. Optical power P2 is the optical power of the signal light after the second optical amplification by the first amplifier 6A3 and the second amplifier 6B3.

[0098] The first WDM filter 41A outputs the short-wavelength signal light separated by the demultiplexer 4C to the first amplification fiber 10A3, and outputs the long-wavelength signal light amplified by the first amplification fiber 10A3 to the multiplexer 5C, for example, the first multiplexer. The optical power of the short-wavelength signal light input to the first WDM filter 41A is P0, and the optical power of the long-wavelength signal light is P2.

[0099] The first amplification fiber 10A3 optically amplifies the short-wavelength signal light from the first WDM filter 41A and outputs the amplified short-wavelength signal light to the second amplification fiber 10B3. The optical power of the amplified short-wavelength signal light output from the first amplification fiber 10A3 is P1. The first amplification fiber 10A3 optically amplifies the long-wavelength signal light from the second amplification fiber 10B3 and outputs the amplified long-wavelength signal light to the first WDM filter 41A. The optical power of the amplified long-wavelength signal light output from the first amplification fiber 10A3 is P2. The first amplification fiber 10A3 is, for example, a bidirectional Raman amplification fiber. The first pump light source 13A is a light source that emits pump light to excite the first amplification fiber 10A3. The WDM filter 12A for the pump is placed between the first amplification fiber 10A3 and the second amplification fiber 10B3, and is a WDM filter that inputs the pump light from the first pump light source 13A to the first amplification fiber 10A3.

[0100] In the first amplification fiber 10A3, the short-wavelength signal light undergoes the first optical amplification, so its optical power is P1, and the long-wavelength signal light undergoes the second optical amplification, so its optical power is P2. In other words, the total power of the first amplification fiber 10A3 is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P1 + P2 = P3. As a result, XPM can be suppressed in the first amplification fiber 10A3.

[0101] The second WDM filter 41B is, for example, the first demultiplexer, which outputs the long-wavelength signal light demultiplexed by the demultiplexer 4C to the second amplification fiber 10B3, and outputs the short-wavelength signal light amplified by the second amplification fiber 10B3 to the combiner 5C. The optical power of the short-wavelength signal light input to the second WDM filter 41B is P2, and the optical power of the long-wavelength signal light is P0.

[0102] The second amplification fiber 10B3 optically amplifies the long-wavelength signal light from the second WDM filter 41B and outputs the amplified long-wavelength signal light to the first amplification fiber 10A3. The optical power of the amplified long-wavelength signal light output from the second amplification fiber 10B3 is P1. The second amplification fiber 10B3 optically amplifies the short-wavelength signal light from the first amplification fiber 10A3 and outputs the amplified short-wavelength signal light to the second WDM filter 41B. The optical power of the amplified short-wavelength signal light output from the second amplification fiber 10B3 is P2. The second amplification fiber 10B3 is, for example, a bidirectional Raman amplification fiber. The second pump light source 13B is a light source that emits pump light to excite the second amplification fiber 10B3. The WDM filter 12B for the pump is positioned between the second amplification fiber 10B3 and the second WDM filter 41B, and is a WDM filter that inputs to the second amplification fiber 10B3, which receives the pump light from the second pump light source 13B.

[0103] In the second amplification fiber 10B3, the output consists of a short-wavelength signal light which undergoes a second optical amplification, resulting in an optical power of P2 for the short-wavelength signal light and a long-wavelength signal light which undergoes a first optical amplification, resulting in an optical power of P1 for the long-wavelength signal light. In other words, the total power of the second amplification fiber 10B3 is smaller than that of the second amplification fiber 103B in Comparative Example 1, with P2 + P1 = P3. As a result, XPM can be suppressed in the second amplification fiber 10B3.

[0104] The combined wave unit 5C combines the long-wavelength signal light from the first WDM filter 41A with the short-wavelength signal light from the second WDM filter 41B, and outputs the combined signal light to the output unit 3C. The optical power of the short-wavelength signal light input to the combined wave unit 5C is P2, and the optical power of the long-wavelength signal light is also P2. As a result, the optical amplifier 1C optically amplifies both the short-wavelength and long-wavelength signal light to optical power P2 and outputs it.

[0105] In the optical amplifier 1C of Example 4, the S-band signal light is split into short-wavelength and long-wavelength signal light. The short-wavelength signal light is input to a bidirectional first amplification fiber 10A3, and the long-wavelength signal light is input to a bidirectional second amplification fiber 10B3. In the optical amplifier 1C, the short-wavelength signal light is optically amplified in the first amplification fiber 10A3 and then optically amplified in the second amplification fiber 10B3. Furthermore, in the optical amplifier 1C, the long-wavelength signal light is optically amplified in the second amplification fiber 10B3 and then optically amplified in the first amplification fiber 10A3. The optical amplifier 1C then outputs a combined short-wavelength signal light and a long-wavelength signal light that have been optically amplified twice. Since the total power of the first amplification fiber 10A3 is reduced as the optical power P2 of the long-wavelength signal light and the optical power P1 of the short-wavelength signal light, XPM can be suppressed. Furthermore, the second amplification fiber 10B3 has a reduced total power as the optical power P1 of the long-wavelength signal light and the optical power P2 of the short-wavelength signal light, thus suppressing XPM. As a result, it is possible to improve the nonlinear SNR by wavelength division amplification of the signal light while reducing the number of components.

[0106] In Example 1, the optical amplifier 1 was illustrated as an example of splitting the signal light into short-wavelength signal light and long-wavelength signal light. However, the signal light may be split into, for example, four wavelength bands, and such an embodiment will be described below as Example 5.

[0107] <Example 5> Figure 6 is an explanatory diagram showing an example of the optical amplifier 1D of Embodiment 5. The optical amplifier 1D shown in Figure 6 has an input section 2D, an output section 3D, a first demultiplexer 4D1, a second demultiplexer 4D2, a third demultiplexer 4D3, a first multiplexer 5D1, a second multiplexer 5D2, and a third multiplexer 5D3. The optical amplifier 1D also has a first amplification section 6A4, a second amplification section 6B4, a third amplification section 6C4, and a fourth amplification section 6D4. The input section 2D is, for example, the input section of the optical amplifier 1D that optically amplifies signal light. The signal light may include, for example, signal light in a first wavelength band, signal light in a second wavelength band, signal light in a third wavelength band, and signal light in a fourth wavelength band. Output unit 3D is the output unit of optical amplifier 1D, which optically amplifies the signal light.

[0108] The first demultiplexer 4D1 demultiplexes the signal light from the input unit 2D into signal light of a first wavelength band and signal light of a second wavelength band, and signal light of a third wavelength band and signal light of a fourth wavelength band. The first demultiplexer 4D1 outputs the demultiplexed signal light of the first and second wavelength bands to the second demultiplexer 4D2, and outputs the demultiplexed signal light of the third and fourth wavelength bands to the third demultiplexer 4D3.

[0109] The second wave combiner 5D2 combines the signal light of the first wavelength band with the signal light of the second wavelength band and outputs the combined signal light of the first and second wavelength bands to the first wave combiner 5D1. The third wave combiner 5D3 combines the signal light of the third wavelength band with the signal light of the fourth wavelength band and outputs the combined signal light of the third and fourth wavelength bands to the first wave combiner 5D1. The first wave combiner 5D1 combines the signal light of the first and second wavelength bands from the second wave combiner 5D2 with the signal light of the third and fourth wavelength bands from the third wave combiner 5D3 and outputs the combined signal light to the output unit 3D.

[0110] The first amplification unit 6A4 optically amplifies the input signal light in the first to fourth wavelength bands and outputs the signal light in the second wavelength band to the second multiplexer 5D2. The first amplification unit 6A4 includes a first WDM filter 51A, a second WDM filter 51B, a first amplification fiber 10A4, a third WDM filter 51C, and a fourth WDM filter 51D. For the sake of explanation, components such as the pump light source that inputs pump light to the first amplification fiber 10A4 are not shown in the diagram.

[0111] The second amplification unit 6B4 optically amplifies the input signal light in the first to fourth wavelength bands and outputs the signal light in the third wavelength band to the third multiplexer 5D3. The second amplification unit 6B4 includes a fifth WDM filter 51E, a sixth WDM filter 51F, a second amplification fiber 10B4, a seventh WDM filter 51G, and an eighth WDM filter 51H.

[0112] The third amplifier 6C4 optically amplifies the input signal light in the first to fourth wavelength bands and outputs the signal light in the fourth wavelength band to the third multiplexer 5D3. The third amplifier 6C4 includes a ninth WDM filter 51I, a tenth WDM filter 51J, a third amplification fiber 10C4, an eleventh WDM filter 51K, and a twelfth WDM filter 51L.

[0113] The fourth amplifier 6D4 optically amplifies the input signal light in the first to fourth wavelength bands and outputs the signal light in the first wavelength band to the second multiplexer 5D2. The fourth amplifier 6D4 includes a 13th WDM filter 51M, a 14th WDM filter 51N, a fourth amplification fiber 10D4, a 15th WDM filter 51O, and a 16th WDM filter 51P.

[0114] For the sake of explanation, the optical power of the signal light passing through the optical amplifier 1D will be amplified stepwise, for example, in the order of P0 → P1 → P2 → P3 → P4. The gain amounts of the first amplifier 6A4, the second amplifier 6B4, the third amplifier 6C4, and the fourth amplifier 6D4 will be assumed to be approximately the same. Optical power P0 is the optical power of the signal light before the first optical amplification in the first amplifier 6A4, the second amplifier 6B, the third amplifier 6C4, or the fourth amplifier 6D4. Optical power P1 is the optical power of the signal light after the first optical amplification in the first amplifier 6A4, the second amplifier 6B4, the third amplifier 6C3, or the fourth amplifier 6D4. Optical power P2 is the optical power of the signal light after the second optical amplification, optical power P3 is the optical power of the signal light after the third optical amplification, and optical power P4 is the optical power of the signal light after the fourth optical amplification.

[0115] The first WDM filter 51A in the first amplification unit 6A4 combines the signal light of the first wavelength band, which has been separated by the second demultiplexer 4D2, with the signal light of the second wavelength band, which has been optically amplified by the fourth amplification fiber 10D4, which has been separated by the 16th WDM filter 51P. The first WDM filter 51A combines the combined signal light of the first wavelength band and the signal light of the second wavelength band, and outputs the combined signal light to the second WDM filter 51B. The optical power of the signal light of the first wavelength band input to the first WDM filter 51A is P0, and the optical power of the signal light of the second wavelength band is P3.

[0116] The second WDM filter 51B combines the signal light in the first and second wavelength bands from the first WDM filter 51A with the signal light in the third and fourth wavelength bands, which has been optically amplified by the fourth amplification fiber 10D4 that has been decoupled by the 15th WDM filter 51O. The second WDM filter 51B combines the signal light in the first and second wavelength bands with the signal light in the third and fourth wavelength bands and outputs the combined signal light to the first amplification fiber 10A4. The optical power of the signal light in the first wavelength band input to the second WDM filter 51B is P0, the optical power of the signal light in the second wavelength band is P3, the optical power of the signal light in the third wavelength band is P2, and the optical power of the signal light in the fourth wavelength band is P1.

[0117] The first amplification fiber 10A4 is, for example, a Raman amplification fiber that optically amplifies the signal light in the first to fourth wavelength bands from the second WDM filter 51B and outputs the optically amplified signal light in the first to fourth wavelength bands to the third WDM filter 51C. The optical power of the signal light in the first wavelength band output from the first amplification fiber 10A4 is P1, the optical power of the signal light in the second wavelength band is P4, the optical power of the signal light in the third wavelength band is P3, and the optical power of the signal light in the fourth wavelength band is P2.

[0118] In the first amplification fiber 10A4, the output is as follows: the optical power of the signal light in the first wavelength band is P1, the optical power of the signal light in the second wavelength band is P4, the optical power of the signal light in the third wavelength band is P3, and the optical power of the signal light in the fourth wavelength band is P2. In other words, the total power of the first amplification fiber 10A4 is smaller than when all wavelength bands are P4, as P1 + P2 + P3 + P4 = P10. As a result, XPM can be suppressed in the first amplification fiber 10A4.

[0119] The third WDM filter 51C separates the signal light in the first to fourth wavelength bands, which has been amplified by the first amplification fiber 10A4, into signal light in the first and second wavelength bands and signal light in the third and fourth wavelength bands. The third WDM filter 51C outputs the separated signal light in the first and second wavelength bands to the fourth WDM filter 51D, and outputs the separated signal light in the third and fourth wavelength bands to the sixth WDM filter 51F in the second amplification unit 6B4. The optical power of the signal light in the first wavelength band input to the third WDM filter 51C is P1, the optical power of the signal light in the second wavelength band is P4, the optical power of the signal light in the third wavelength band is P3, and the optical power of the signal light in the fourth wavelength band is P2.

[0120] The fourth WDM filter 51D separates the signal light in the first wavelength band and the signal light in the second wavelength band from the signal light in the first wavelength band and the signal light in the second wavelength band from the signal light in the third WDM filter 51C. The fourth WDM filter 51D outputs the separated signal light in the first wavelength band to the fifth WDM filter 51E and outputs the separated signal light in the second wavelength band to the second multiplexer 5D2. The optical power of the signal light in the first wavelength band input to the fourth WDM filter 51D is P1, and the optical power of the signal light in the second wavelength band is P4.

[0121] The fifth WDM filter 51E in the second amplification unit 6B4 combines the signal light of the second wavelength band, which is separated by the second demultiplexer 4D2, with the signal light of the first wavelength band, which is optically amplified by the first amplification fiber 10A4, which is separated by the fourth WDM filter 51D. The fifth WDM filter 51E combines the combined signal light of the first wavelength band and the signal light of the second wavelength band, and outputs the combined signal light to the sixth WDM filter 51F. The optical power of the signal light of the second wavelength band input to the fifth WDM filter 51E is P0, and the optical power of the signal light of the first wavelength band is P1.

[0122] The sixth WDM filter 51F combines the signal light in the first and second wavelength bands from the fifth WDM filter 51E with the signal light in the third and fourth wavelength bands, which is optically amplified by the first amplification fiber 10A that has been decoupled by the third WDM filter 51C. The sixth WDM filter 51F combines the signal light in the first and second wavelength bands with the signal light in the third and fourth wavelength bands and outputs the combined signal light to the second amplification fiber 10B4. The optical power of the signal light in the second wavelength band input to the sixth WDM filter 51F is P0, the optical power of the signal light in the first wavelength band is P1, the optical power of the signal light in the third wavelength band is P3, and the optical power of the signal light in the fourth wavelength band is P2.

[0123] The second amplification fiber 10B4 is, for example, a Raman amplification fiber that optically amplifies the signal light in the first to fourth wavelength bands from the sixth WDM filter 51F and outputs the optically amplified signal light in the first to fourth wavelength bands to the seventh WDM filter 51G. The optical power of the signal light in the first wavelength band output from the second amplification fiber 10B4 is P2, the optical power of the signal light in the second wavelength band is P1, the optical power of the signal light in the third wavelength band is P4, and the optical power of the signal light in the fourth wavelength band is P3.

[0124] In the second amplification fiber 10B4, the output is as follows: the optical power of the signal light in the first wavelength band is P2, the optical power of the signal light in the second wavelength band is P1, the optical power of the signal light in the third wavelength band is P4, and the optical power of the signal light in the fourth wavelength band is P3. In other words, the total power of the second amplification fiber 10B4 is smaller than when all wavelength bands are P4, calculated as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the second amplification fiber 10B4.

[0125] The seventh WDM filter 51G separates the signal light in the first to fourth wavelength bands, which has been amplified by the second amplification fiber 10B4, into signal light in the first and second wavelength bands and signal light in the third and fourth wavelength bands. The seventh WDM filter 51G outputs the separated signal light in the first and second wavelength bands to the tenth WDM filter 51J in the third amplification unit 6C4, and outputs the separated signal light in the third and fourth wavelength bands to the eighth WDM filter 51H. The optical power of the signal light in the first wavelength band input to the seventh WDM filter 51G is P2, the optical power of the signal light in the second wavelength band is P1, the optical power of the signal light in the third wavelength band is P4, and the optical power of the signal light in the fourth wavelength band is P3.

[0126] The eighth WDM filter 51H separates the signal light in the third and fourth wavelength bands from the seventh WDM filter 51G into the signal light in the third wavelength band and the signal light in the fourth wavelength band. The eighth WDM filter 51H outputs the separated signal light in the fourth wavelength band to the ninth WDM filter 51I, and outputs the separated signal light in the third wavelength band to the third multiplexer 5D3. The optical power of the signal light in the third wavelength band input to the eighth WDM filter 51H is P4, and the optical power of the signal light in the fourth wavelength band is P3.

[0127] The ninth WDM filter 51I within the third amplifier 6C4 combines the signal light of the third wavelength band, which is separated by the third demultiplexer 4D3, with the signal light of the fourth wavelength band, which is optically amplified by the second amplification fiber 10B4, which is separated by the eighth WDM filter 51H. The ninth WDM filter 51I combines the combined signal light of the third wavelength band and the signal light of the fourth wavelength band, and outputs the combined signal light to the tenth WDM filter 51J. The optical power of the signal light of the third wavelength band input to the ninth WDM filter 51I is P0, and the optical power of the signal light of the fourth wavelength band is P3.

[0128] The 10th WDM filter 51J combines the signal light in the third and fourth wavelength bands from the 9th WDM filter 51I with the signal light in the first and second wavelength bands, which has been optically amplified by the second amplification fiber 10B4, which has been decoupled by the 7th WDM filter 51G. The 10th WDM filter 51J combines the signal light in the first and second wavelength bands with the signal light in the third and fourth wavelength bands and outputs the combined signal light to the third amplification fiber 10C4. The optical power of the signal light in the third wavelength band input to the 10th WDM filter 51J is P0, the optical power of the signal light in the fourth wavelength band is P3, the optical power of the signal light in the first wavelength band is P2, and the optical power of the signal light in the second wavelength band is P1.

[0129] The third amplification fiber 10C4 is, for example, a Raman amplification fiber that optically amplifies the signal light in the first to fourth wavelength bands from the tenth WDM filter 51J and outputs the optically amplified signal light in the first to fourth wavelength bands to the eleventh WDM filter 51K. The optical power of the signal light in the first wavelength band output from the third amplification fiber 10C4 is P3, the optical power of the signal light in the second wavelength band is P2, the optical power of the signal light in the third wavelength band is P1, and the optical power of the signal light in the fourth wavelength band is P4.

[0130] In the third amplification fiber 10C4, the output is as follows: the optical power of the signal light in the first wavelength band is P3, the optical power of the signal light in the second wavelength band is P2, the optical power of the signal light in the third wavelength band is P1, and the optical power of the signal light in the fourth wavelength band is P4. In other words, the total power of the third amplification fiber 10C4 is smaller than when all wavelength bands are P4, calculated as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the third amplification fiber 10C4.

[0131] The 11th WDM filter 51K decomposes the signal light in the first to fourth wavelength bands, which has been amplified by the third amplification fiber 10C4, into signal light in the first and second wavelength bands and signal light in the third and fourth wavelength bands. The 11th WDM filter 51K outputs the decomposed signal light in the first and second wavelength bands to the 14th WDM filter 51N in the fourth amplification unit 6D4, and outputs the decomposed signal light in the third and fourth wavelength bands to the 12th WDM filter 51L. The optical power of the signal light in the first wavelength band input to the 11th WDM filter 51K is P3, the optical power of the signal light in the second wavelength band is P2, the optical power of the signal light in the third wavelength band is P1, and the optical power of the signal light in the fourth wavelength band is P4.

[0132] The 12th WDM filter 51L decouples the signal light in the third and fourth wavelength bands from the 11th WDM filter 51K into the signal light in the third wavelength band and the signal light in the fourth wavelength band. The 12th WDM filter 51L outputs the decoupled signal light in the third wavelength band to the 13th WDM filter 51M and outputs the decoupled signal light in the fourth wavelength band to the 3rd multiplexer 5D3. The optical power of the signal light in the third wavelength band input to the 12th WDM filter 51L is P1, and the optical power of the signal light in the fourth wavelength band is P4.

[0133] The 13th WDM filter 51M in the 4th amplification unit 6D4 combines the signal light of the 4th wavelength band, which is separated by the 3rd demultiplexer 4D3, with the signal light of the 4th wavelength band, which is optically amplified by the 3rd amplification fiber 10C, which is separated by the 12th WDM filter 51L. The 13th WDM filter 51M combines the combined signal light of the 3rd wavelength band and the signal light of the 4th wavelength band, and outputs the combined signal light to the 14th WDM filter 51N. The optical power of the signal light of the 3rd wavelength band input to the 13th WDM filter 51M is P1, and the optical power of the signal light of the 4th wavelength band is P0.

[0134] The 14th WDM filter 51N combines the signal light in the third and fourth wavelength bands from the 13th WDM filter 51M with the signal light in the first and second wavelength bands, which is optically amplified by the third amplification fiber 10C4 that was decoupled by the 11th WDM filter 51K. The 14th WDM filter 51N combines the signal light in the first and second wavelength bands with the signal light in the third and fourth wavelength bands and outputs the combined signal light to the fourth amplification fiber 10D4. The optical power of the signal light in the third wavelength band input to the 14th WDM filter 51N is P1, the optical power of the signal light in the fourth wavelength band is P0, the optical power of the signal light in the first wavelength band is P3, and the optical power of the signal light in the second wavelength band is P2.

[0135] The fourth amplification fiber 10D4 is, for example, a Raman amplification fiber that optically amplifies the signal light in the first to fourth wavelength bands from the 14th WDM filter 51N and outputs the optically amplified signal light in the first to fourth wavelength bands to the 15th WDM filter 51O. The optical power of the signal light in the first wavelength band output from the fourth amplification fiber 10D4 is P4, the optical power of the signal light in the second wavelength band is P3, the optical power of the signal light in the third wavelength band is P2, and the optical power of the signal light in the fourth wavelength band is P1.

[0136] In the fourth amplification fiber 10D4, the output is as follows: the optical power of the signal light in the first wavelength band is P4, the optical power of the signal light in the second wavelength band is P3, the optical power of the signal light in the third wavelength band is P2, and the optical power of the signal light in the fourth wavelength band is P1. In other words, the total power of the fourth amplification fiber 10D4 is smaller than when all wavelength bands are P4, calculated as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the fourth amplification fiber 10D4.

[0137] The 15th WDM filter 51O decomposes the signal light in the first to fourth wavelength bands, which has been amplified by the fourth amplification fiber 10D4, into signal light in the first and second wavelength bands and signal light in the third and fourth wavelength bands. The 15th WDM filter 51O outputs the decomposed signal light in the third and fourth wavelength bands to the second WDM filter 51B in the first amplification unit 6A4, and outputs the decomposed signal light in the first and second wavelength bands to the 16th WDM filter 51P. The optical power of the signal light in the first wavelength band input to the 15th WDM filter 51O is P4, the optical power of the signal light in the second wavelength band is P3, the optical power of the signal light in the third wavelength band is P2, and the optical power of the signal light in the fourth wavelength band is P1.

[0138] The 16th WDM filter 51P separates the signal light in the first wavelength band and the signal light in the second wavelength band from the 15th WDM filter 51O into a signal light in the first wavelength band and a signal light in the second wavelength band. The 16th WDM filter 51P outputs the separated signal light in the second wavelength band to the first WDM filter 51A, and outputs the separated signal light in the first wavelength band to the second multiplexer 5D2. The optical power of the signal light in the first wavelength band input to the 16th WDM filter 51P is P4, and the optical power of the signal light in the second wavelength band is P3.

[0139] The third multiplexer 5D3 combines the signal light in the third wavelength band from the eighth WDM filter 51H and the signal light in the fourth wavelength band from the twelfth WDM filter 51L, and outputs the combined signal light in the third and fourth wavelength bands to the first multiplexer 5D1. The optical power of the signal light in the third wavelength band output from the third multiplexer 5D3 is P4, and the optical power of the signal light in the fourth wavelength band is also P4.

[0140] The second multiplexer 5D2 combines the signal light of the second wavelength band from the fourth WDM filter 51D and the signal light of the first wavelength band from the sixteenth WDM filter 51P, and outputs the combined signal light of the second and first wavelength bands to the first multiplexer 5D1. The optical power of the signal light of the first wavelength band output from the second multiplexer 5D2 is P4, and the optical power of the signal light of the first wavelength band is P4.

[0141] The first wave combiner 5D1 combines the signal light in the first and second wavelength bands from the second wave combiner 5D2 with the signal light in the third and fourth wavelength bands from the third wave combiner 5D3, and outputs the combined signal light to the output unit 3D. The optical power of the signal light in the first wavelength band output from the first wave combiner 5D1 is P4, the optical power of the signal light in the second wavelength band is P4, the optical power of the signal light in the third wavelength band is P4, and the optical power of the signal light in the fourth wavelength band is P4.

[0142] In the optical amplifier 1D of Example 5, the signal light is split into signal light in the first, second, third, and fourth wavelength bands. The optical amplifier 1D optically amplifies the signal light in the first wavelength band in the order of the first amplification fiber 10A4, the second amplification fiber 10B4, the third amplification fiber 10C4, and the fourth amplification fiber 10D4. In other words, the total power of the fourth amplification fiber 10D4 is reduced as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the fourth amplification fiber 10D4.

[0143] The optical amplifier 1D optically amplifies the signal light in the second wavelength band in the following order: second amplification fiber 10B4, third amplification fiber 10C4, fourth amplification fiber 10D4, and first amplification fiber 10A4. The total power of the first amplification fiber 10A4 is reduced as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the first amplification fiber 10A4.

[0144] The optical amplifier 1D optically amplifies the signal light in the third wavelength band in the following order: third amplification fiber 10C4, fourth amplification fiber 10D4, first amplification fiber 10A4, and second amplification fiber 10B4. The total power of the second amplification fiber 10B4 is reduced as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the second amplification fiber 10B4.

[0145] The optical amplifier 1D optically amplifies the signal light in the fourth wavelength band in the following order: fourth amplification fiber 10D4, first amplification fiber 10A4, second amplification fiber 10B4, and third amplification fiber 10C4. The total power of the third amplification fiber 10C4 is reduced as P1+P2+P3+P4=P10. As a result, XPM can be suppressed in the third amplification fiber 10C4.

[0146] The optical amplifier 1D then combines the signal light with optical power P4 in the first wavelength band, the signal light with optical power P4 in the second wavelength band, the signal light with optical power P4 in the third wavelength band, and the signal light with optical power P4 in the fourth wavelength band and outputs it to the output unit 3D. As a result, even when the signal light is wavelength-divided and amplified into the first to fourth wavelength bands, the nonlinear SNR can be improved.

[0147] In Example 5, the optical amplifier 1D is shown as an example where the wavelength band is divided into four parts, but it is not limited to four divisions; it may be divided into three or more parts, and can be changed as appropriate. Furthermore, it may be interleaved into three or more channel bands, and can be changed as appropriate.

[0148] The example given for the demultiplexer includes a first demultiplexer 4D1, a second demultiplexer 4D2, and a third demultiplexer 4D3, but it is not limited to this. For example, it may be composed of a single demultiplexer that demultiplexes the signal light into signal light in the first wavelength band, the second wavelength band, the third wavelength band, and the fourth wavelength band, and this can be changed as appropriate. Similarly, the example given for the combiner includes a first combiner 5D1, a second combiner 5D2, and a third combiner 5D3, but it is not limited to this. For example, it may be composed of a single combiner that combines signal light with optical power P4 in the first wavelength band, signal light with optical power P4 in the second wavelength band, signal light with optical power P4 in the third wavelength band, and signal light with optical power P4 in the fourth wavelength band, and this can be changed as appropriate.

[0149] Furthermore, for the sake of explanation, a Raman amplification fiber was used as an example of the amplification fiber, but it is not limited to this and can be changed as appropriate. For example, other types of amplification fibers such as DCF (Dispersion Compensating Fiber), HNLF (Highly Non-Linear Fiber), DSF (Dispersion Shifted Fiber), or SMF (Single Mode Fiber) may also be used.

[0150] Furthermore, while amplification fibers such as Raman amplification fibers were given as examples of optical amplifiers, the method is not limited to amplification fibers. For example, semiconductor optical amplifiers (SOAs) may also be used, and can be changed as appropriate.

[0151] For example, the case where the optical amplifier 1 is built into an optical communication device such as an OADM was illustrated, but the optical amplifier 1 may also be built into an optical transmitter or an optical receiver, and can be changed as appropriate.

[0152] In optical amplifier 1, the S-band signal light within the WDM signal is used as an example, but other wavelength bands such as the C-band, L-band, or U-band may also be used and can be changed as appropriate. Also, a WDM filter is used as an example for the pump's WDM filter, but a circulator or the like may also be used and can be changed as appropriate.

[0153] Although the example of a Raman amplification fiber was given for Raman amplification of an optical signal using a back-excitation method, the excitation method can also be forward-excitation or bidirectional-excitation, and can be changed as appropriate.

[0154] Furthermore, the components of each part shown in the diagram do not necessarily have to be physically configured as depicted. In other words, the specific forms of distribution and integration of each part are not limited to those shown in the diagram, and all or part of them can be functionally or physically distributed and integrated in any unit according to various loads, usage conditions, etc. [Explanation of Symbols]

[0155] 1. Optical amplifier 4 Demultiplexer 5. Wave section 6A First Amplifier 6B Second Amplifier 10A First amplification fiber 10B Second amplification fiber 11A First WDM filter 11B Second WDM filter 11C Third WDM filter 11D 4th WDM filter

Claims

1. It has a wave splitter, a first amplification unit, a second amplification unit, and a wave combiner. The aforementioned wave splitter is, The input signal light is split into a first signal light and a second signal light. The first signal light is, After being amplified in the first amplification unit, it is amplified in the second amplification unit. The second signal light is, After being amplified in the second amplification unit, it is amplified in the first amplification unit. The aforementioned wave-combining section is The amplified first signal light and the amplified second signal light are combined. An optical communication device characterized by the following features.

2. At the output of the first amplifier, the power of the second signal light becomes greater than the power of the first signal light. At the output of the second amplification unit, the power of the first signal light is greater than the power of the second signal light. The optical communication device according to feature 1.

3. The aforementioned wave splitter is, The optical communication device according to claim 1, characterized in that the signal light is dewaverated into a first signal light in a first wavelength band and a second signal light in a second wavelength band.

4. The aforementioned wave splitter is, The optical communication device according to claim 1, characterized in that the signal light is split into the first signal light of an odd channel and the second signal light of an even channel.

5. The aforementioned wave splitter is, The optical communication device according to claim 1, characterized in that the signal light is split into the first signal light with X polarization and the second signal light with Y polarization.

6. The first amplification unit is, A first optical amplification unit that amplifies the first signal light and the second signal light, A first multiplexer is positioned in front of the first optical amplification unit, It comprises a first demultiplexer positioned after the first optical amplification unit, The second amplification section described above is A second optical amplification unit that amplifies the first signal light and the second signal light, A second multiplexer is positioned before the second optical amplification unit, It comprises a second demultiplexer positioned after the second optical amplifier, The first wave combining section is, The first signal light separated by the aforementioned demultiplexer and the second signal light amplified by the second optical amplifier, which is separated by the second demultiplexer, are combined and output to the first optical amplifier. The first wave splitter is, Of the first signal light and the second signal light amplified in the first optical amplification unit, the first signal light is separated into the second multiplexer, and the second signal light is separated into the multiplexer. The second wave-combining section is, The second signal light separated by the aforementioned demultiplexer and the first signal light amplified by the first optical amplifier, which was separated by the first demultiplexer, are combined and output to the second optical amplifier. The second wave splitter described above is, The optical communication device according to claim 1 or 2, characterized in that, of the first signal light and the second signal light amplified by the second optical amplification unit, the second signal light is separated into the first multiplexer, and the first signal light is also separated into the multiplexer.

7. The first amplification unit is, A first optical amplifier that amplifies the first signal light input from the first direction and the second signal light input from the second direction, It comprises a first demultiplexer positioned before the first optical amplifier, The second amplification section described above is A second optical amplifier that amplifies the first signal light input from the first direction and the second signal light input from the second direction, It comprises a second demultiplexer positioned after the second optical amplifier, The first wave splitter is, The first signal light separated by the dewave-decompressor is input to the first optical amplifier, and the second signal light from the first optical amplifier is output to the multiplexer. The second wave splitter described above is, The optical communication apparatus according to claim 1 or 2, characterized in that the second signal light separated by the dewave splitter is input to the second optical amplification unit, and the first signal light from the second optical amplification unit is output to the multiplexing unit.

8. The optical communication device is It further comprises a third amplification section and a fourth amplification section, The aforementioned wave splitter is, The signal light is divided into the first signal light, the second signal light, the third signal light, and the fourth signal light. The first signal light and the fourth signal light are amplified from the first amplifier to the fourth amplifier, respectively. The aforementioned wave-combining section is The first signal light, the second signal light, the third signal light, and the fourth signal light are combined. The optical communication device according to feature 1.

9. It has a wave splitter, a first amplification unit, a second amplification unit, and a wave combiner. The aforementioned wave splitter is, The signal light is split into a first signal light and a second signal light. The first signal light is, After being amplified in the first amplification unit, it is amplified in the second amplification unit. The second signal light is, After being amplified in the second amplification unit, it is amplified in the first amplification unit. The aforementioned wave-combining section is The amplified first signal light and the amplified second signal light are combined. A light amplifier characterized by the following features.

10. The optical amplifier, The signal light is split into a first signal light and a second signal light. The decoupled first signal light is amplified by the first amplification unit. The first signal light amplified in the first amplification unit is amplified in the second amplification unit. The decoupled second signal light is amplified by the second amplification unit. The second signal light amplified by the second amplification unit is amplified by the first amplification unit. The first signal light amplified by the second amplification unit and the second signal light amplified by the first amplification unit are combined. A method for amplifying light characterized by the following features.

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