Raman amplification system, Raman amplification method, and Raman amplification apparatus

Abstract

Ensure flatness of Raman gain in 4-band transmission. [Solution] The Raman amplification system is a Raman amplification system that amplifies multiple signal bands using a first back-excited Raman amplifier provided on the output side of a transmission line and a second forward-excited Raman amplifier provided on the input side of the transmission line, wherein the first Raman amplifier has a first power detection unit that detects the power of at least a portion of the signal band on the longest wavelength side among the multiple signal bands, and the second Raman amplifier has a second excitation light source that outputs a second excitation light of a specific wavelength that amplifies the second signal band from the shortest wavelength side among the multiple signal bands with the highest efficiency, and a second control unit that controls the power of the second excitation light source based on the results obtained from the first power detection unit.

Description

Technical Field

[0001] The present invention relates to a Raman amplification system, a Raman amplification method, and a Raman amplification device.

Background Art

[0002] A backward excitation type Raman amplifier that applies excitation light to a transmission fiber in a direction opposite to the propagation direction of the signal light is known. Also, a forward excitation type Raman amplifier that applies excitation light to a transmission fiber in the same direction as the propagation direction of the signal light is known.

[0003] Note that, as a Raman amplifier, for example, a distributed Raman amplifier using a transmission fiber as an amplification medium is known. Also, it is common that the wavelength of the excitation light in the distributed Raman amplifier is shorter than the wavelength of the signal light (for example, see Patent Documents 1 to 4).

Prior Art Documents

[0006] In transmission across these four wavelength bands (hereinafter referred to as four-band transmission), ensuring flatness of the Raman gain for the signal light is difficult with a back-excited Raman amplifier, as it is challenging to secure sufficient Raman gain for the U-band signal light. More specifically, when the wavelength of a single excitation light is close to the shorter wavelength side of the S-band signal light, the Raman gain obtained by the excitation light is maximized around 13 THz (terahertz) on the longer wavelength side of the excitation light. This Raman gain then drops sharply beyond approximately 15 THz on the longer wavelength side of the excitation light. In other words, in such cases, Raman gain is secured for the signal light from the S-band to the L-band, but not for the U-band signal light, making it difficult to ensure flatness of the Raman gain.

[0007] On the other hand, when ensuring Raman gain for signal light in four-band transmission, in a forward-excited Raman amplifier, the effect of stimulated Raman scattering on the signal light increases the Raman gain for, for example, the L-band and U-band signal light, but decreases the Raman gain for the S-band and C-band signal light. Thus, even with a forward-excited Raman amplifier, it becomes difficult to ensure flatness of the Raman gain.

[0008] Therefore, one objective is to provide a Raman amplification system, a Raman amplification method, and a Raman amplification apparatus that ensure flatness of Raman gain in 4-band transmission. [Means for solving the problem]

[0009] In one embodiment, the Raman amplification system amplifies multiple signal bands using a first back-excited Raman amplifier provided on the output side of a transmission line and a second forward-excited Raman amplifier provided on the input side of the transmission line, wherein the first Raman amplifier has a first power detection unit that detects the power of at least a portion of the signal band on the longest wavelength side among the multiple signal bands, and the second Raman amplifier has a second excitation light source that outputs a second excitation light of a specific wavelength that amplifies the second shortest wavelength signal band among the multiple signal bands with the highest efficiency, and a second control unit that controls the power of the second excitation light source based on the results obtained from the first power detection unit. [Effects of the Invention]

[0010] This ensures flatness of the Raman gain in 4-band transmission. [Brief explanation of the drawing]

[0011] [Figure 1] This is an example of an optical transmission system. [Figure 2] This is an example of a Raman amplification system. [Figure 3] This flowchart shows an example of the operation of the Raman amplification system according to the first embodiment. [Figure 4A] This is an example of a power spectrum from forward-excited Raman amplification alone. [Figure 4B] This is an example of a graph illustrating the saturation of gains. [Figure 5] This is an example of a power spectrum from back-excited Raman amplification alone. [Figure 6] This is a diagram illustrating an example of the operation according to the first embodiment. [Figure 7] This is an example of a Raman gain spectrum. [Figure 8] This is an example of a forward-excited Raman amplifier according to the second embodiment. [Figure 9] This is an example of a back-excited Raman amplifier according to the second embodiment. [Figure 10] It is a flowchart showing an example of the operation of the Raman amplification system according to the second embodiment. [Figure 11] It is an example of a power spectrum using both forward excitation Raman amplification and backward excitation Raman amplification. [Figure 12] It is a diagram for explaining an example of the operation according to the second embodiment. [Figure 13] It is another example of a power spectrum using both forward excitation Raman amplification and backward excitation Raman amplification. [Figure 14] It is an example of a graph showing the relationship between the specific center wavelength and flatness of forward excitation Raman amplification.

Mode for Carrying Out the Invention

[0012] Hereinafter, the embodiments for carrying out the present case will be described with reference to the drawings.

[0013] (First Embodiment) As shown in FIG. 1, the optical transmission system ST includes two optical transmission devices 10 and 20. The optical transmission devices 10 and 20 are, for example, ROADM (Reconfigurable Optical Add / Drop Multiplexer). The optical transmission devices 10 and 20 may be, for example, ILA (In-Line Amplifier). The optical transmission devices 10 and 20 are connected via two optical transmission paths 31 and 32. The optical transmission paths 31 and 32 include, for example, optical fibers. Note that the type of the transmission path of the optical transmission paths 31 and 32 is not particularly limited. For example, the optical transmission paths 31 and 32 may include SMF (Single Mode Fiber). Also, the optical transmission paths 31 and 32 may include DSF (Dispersion Shifted Fiber).

[0014] First, let's describe the optical transmission device 10. The optical transmission device 10 includes an optical transmitter (indicated as Tx in Figure 1) 11, an optical receiver (indicated as Rx in Figure 1) 12, a multiplexer (indicated as MUX in Figure 1) 13, and a demultiplexer (indicated as DEMUX in Figure 1) 14. The optical transmission device 10 also includes optical amplifiers 15 and 16, a forward-excited Raman amplifier (indicated as FWD Raman in Figure 1) 100, and a back-excited Raman amplifier (indicated as BWD Raman in Figure 1) 150.

[0015] The optical transmitter 11 includes an S-band transmitter 11S and a C-band transmitter 11C. It also includes an L-band transmitter 11L and a U-band transmitter 11U. The S-band transmitter 11S transmits signal light whose center wavelength belongs to the S-band. The S-band is, for example, a wavelength band from 1460 nm to 1530 nm. The C-band transmitter 11C transmits signal light whose center wavelength belongs to the C-band. The C-band is, for example, a wavelength band from 1530 nm to 1565 nm. The L-band transmitter 11L transmits signal light whose center wavelength belongs to the L-band. The L-band is, for example, a wavelength band from 1565 nm to 1625 nm. The U-band transmitter 11U transmits signal light whose center wavelength belongs to the U-band. The U-band is an example of the longest wavelength signal band in a four-band transmission, and is, for example, a wavelength band from 1625 nm to 1675 nm. Therefore, the L band is an example of the second-longest wavelength signal band, the C band is an example of the third-longest wavelength signal band, and the S band is an example of the fourth-longest wavelength signal band. In other words, the C band can be described as the second-shortest wavelength signal band.

[0016] Thus, the S-band transmitter 11S, the C-band transmitter 11C, the L-band transmitter 11L, and the U-band transmitter 11U each transmit signal light in different wavelength bands. Note that there may be multiple signal lights of different wavelengths in each wavelength band, and there may be multiple transmitters and receivers in each wavelength band simultaneously. Hereafter, the entirety of the signal light belonging to the S-band, C-band, L-band, and U-band will be denoted as WDM (Wavelength Division Multiplexing) light Ls, Lc, Ll, and Lu, respectively.

[0017] The optical receiver 12 includes an S-band receiver 12S and a C-band receiver 12C. It also includes an L-band receiver 12L and a U-band receiver 12U. The S-band receiver 12S, C-band receiver 12C, L-band receiver 12L, and U-band receiver 12U each receive signal light in different wavelength bands.

[0018] The multiplexer 13 combines WDM light Ls, Lc, Ll, and Lu of different wavelength bands to generate WDM light Lw1. The demultiplexer 14 demultiplexes the WDM light Lw2 output from the back-excited Raman amplifier 150 into WDM light Ls, Lc, Ll, and Lu of center wavelengths at regular wavelength intervals. Note that the multiplexer 13 and demultiplexer 14 include, for example, a WDM coupler.

[0019] Optical amplifiers 15s, 15c, 15l, and 15u amplify WDM light Ls, Lc, Ll, and Lu. Optical amplifiers 16s, 16c, 16l, and 16u amplify WDM light Ls, Lc, Ll, and Lu. Optical amplifiers 15s, 15c, 15l, 15u, 16s, 16c, 16l, and 16u include, for example, EDFA (Erbium Doped Fiber Amplifier). The forward-excited Raman amplifier 100 outputs excitation light Lp to the optical transmission path 31 in the same direction as the WDM light Lw1. Excitation light Lp corresponds to forward excitation light for WDM light Lw1. Stimulated Raman scattering occurs when excitation light Lp is incident on the optical transmission path 31, and the WDM light Lw1 is Raman amplified. The backward-excited Raman amplifier 150 outputs excitation light Lr to the optical transmission path 32 in the opposite direction to the WDM light Lw2. The excitation light Lr corresponds to the back excitation light for the WDM light Lw2. When the excitation light Lr is incident on the optical transmission path 32, stimulated Raman scattering occurs, and the WDM light Lw2 is Raman amplified.

[0020] Next, the optical transmission device 20 will be described. The optical transmission device 20 includes an optical transmitter 21, an optical receiver 22, a multiplexer 23, and a demultiplexer 24. The optical transmission device 20 also includes optical amplifiers 25s, 25c, 25l, 25u, 26s, 26c, 26l, 26u, a back-excited Raman amplifier 200, and a forward-excited Raman amplifier 250. The back-excited Raman amplifier 200 is an example of a first Raman amplifier. The forward-excited Raman amplifier 100 is an example of a second Raman amplifier.

[0021] Optical transmitter 21 has essentially the same configuration and function as optical transmitter 11 described above, so a detailed explanation is omitted. For example, optical transmitter 21 includes an S-band transmitter 21S and a C-band transmitter 21C. Optical transmitter 21 also includes an L-band transmitter 21L and a U-band transmitter 21U. Therefore, the S-band transmitter 21S, C-band transmitter 21C, L-band transmitter 21L, and U-band transmitter 21U each transmit signal light in different wavelength bands.

[0022] The optical receiver 22 has essentially the same configuration and function as the optical receiver 12 described above, so a detailed explanation is omitted. For example, the optical receiver 22 includes an S-band receiver 22S and a C-band receiver 22C. It also includes an L-band receiver 22L and a U-band receiver 22U. Therefore, the S-band receiver 22S, C-band receiver 22C, L-band receiver 22L, and U-band receiver 22U each receive signal light in different wavelength bands.

[0023] Optical amplifiers 25s, 25c, 25l, 25u, 26s, 26c, 26l, and 26u basically have the same functions as the optical amplifiers 15s, 15c, 15l, 15u, 16s, 16c, 16l, and 16u described above, so a detailed explanation is omitted. For example, optical amplifiers 25s, 25c, 25l, and 25u amplify WDM light Ls, Lc, Ll, and Lu. Optical amplifiers 26s, 26c, 26l, and 26u amplify WDM light Ls, Lc, Ll, and Lu. Back-excited Raman amplifier 200 and forward-excited Raman amplifier 250 basically have the same functions as the back-excited Raman amplifier 150 and forward-excited Raman amplifier 100 described above, so a detailed explanation is omitted. For example, the back-excited Raman amplifier 200 outputs the excitation light Lq to the optical transmission path 31 in the opposite direction to the WDM light Lw1. The excitation light Lq corresponds to the back excitation light for the WDM light Lw1. The forward excitation Raman amplifier 250 outputs the excitation light Lz to the optical transmission path 32 in the same direction as the WDM light Lw2. The excitation light Lz corresponds to the forward excitation light for the WDM light Lw2.

[0024] Here, the forward-excited Raman amplifier 100 is connected to the backward-excited Raman amplifier 200 via the optical transmission path 31. The forward-excited Raman amplifier 100 is located on the input side of the optical transmission path 31. The backward-excited Raman amplifier 200 is located on the output side of the optical transmission path 31. The forward-excited Raman amplifier 250 is connected to the backward-excited Raman amplifier 150 via the optical transmission path 32. The forward-excited Raman amplifier 250 is located on the input side of the optical transmission path 32. The backward-excited Raman amplifier 150 is located on the output side of the optical transmission path 32.

[0025] For example, by utilizing a forward-excited Raman amplifier 100 and a backward-excited Raman amplifier 200, a bidirectional excitation type Raman amplification system STa that amplifies multiple signal bandwidths can be realized. The Raman amplification system STa may or may not include an optical transmission line 31. Alternatively, a fiber for Raman amplification can be placed instead of the optical transmission line 31 to realize a Raman amplification system as a centrifugal Raman amplifier. By utilizing a forward-excited Raman amplifier 250 and a backward-excited Raman amplifier 150, a bidirectional excitation type Raman amplification system that amplifies multiple signal bandwidths can also be realized. Such a Raman amplification system may or may not include an optical transmission line 32. Alternatively, a fiber for Raman amplification can be placed instead of the optical transmission line 32 to realize a Raman amplification system as a centrifugal Raman amplifier.

[0026] Referring to Figure 2, the details of the forward-excited Raman amplifier 100 and the backward-excited Raman amplifier 200 provided in the Raman amplification system STa will be described.

[0027] First, the forward-excited Raman amplifier 100 will be described. The forward-excited Raman amplifier 100 includes a plurality of forward-excited light sources 101. For example, the forward-excited Raman amplifier 100 includes six forward-excited light sources 101. The forward-excited Raman amplifier 100 also includes a multiplexer 102 and a forward control unit 105. Furthermore, the forward-excited Raman amplifier 100 includes a plurality of WDM couplers 106, 107 and an OSC (Optical Supervisory Channel) communication unit 112. The plurality of WDM couplers 106, 107 are provided on the optical waveguide 103. Note that the forward control unit 105 is an example of a second control unit and a control unit.

[0028] Each of the forward excitation light sources 101 outputs unit excitation rays L1,...,L6, each with a different central wavelength. Multiple forward excitation light sources 101 that output unit excitation rays L1,...,L6, excluding unit excitation ray L4, are an example of multiple third excitation light sources. All unit excitation rays L1,...,L6 are incoherent excitation rays belonging to a wavelength band shorter than the shortest wavelength belonging to the S band. Therefore, the forward excitation light sources 101 are an example of incoherent light sources. Because the unit excitation rays L1,...,L6 are incoherent excitation rays, signal degradation caused by RIN (Relative Intensity Noise) is suppressed. The multiplexer 102 combines the unit excitation rays L1,...,L6 to generate excitation ray Lp. Excitation ray Lp is output to the optical transmission line 31 via the WDM coupler 106. As a result, in the optical transmission line 31, excitation ray Lp amplifies the WDM ray Lw1.

[0029] The OSC communication unit 112 transmits OSC light Lk. OSC light Lk is a control light that controls the operation of the forward-excited Raman amplifier 250. OSC light Lk is output to the optical transmission path 31 via the WDM coupler 107. The OSC communication unit 112 also receives OSC light Lj. OSC light Lj is a control light that controls the operation of the forward-excited Raman amplifier 100. OSC light Lj is input to the OSC communication unit 112 from the optical coupler 151 provided on the back-excited Raman amplifier 150. The OSC communication unit 112 electrically notifies the forward control unit 105 of predetermined information contained in OSC light Lj. As will be described in detail later, the predetermined information is a measured value of the optical power of the U-band WDM light Lu (hereinafter referred to as the monitor value).

[0030] The forward control unit 105 includes a forward calculation unit 105F and a forward adjustment unit 105G. The forward calculation unit 105F individually calculates adjustment amounts used to control the optical power of unit excitation rays L1,...,L6 based on predetermined information notified from the OSC communication unit 112. The forward adjustment unit 105G controls the operation of each of the multiple forward excitation light sources 101 based on the adjustment amounts calculated by the forward calculation unit 105F. Specifically, the forward adjustment unit 105G controls the optical power of unit excitation rays L1,...,L6 while keeping the excitation ratio constant or fixed, and adjusts the Raman gain for WDM light Lu.

[0031] Next, the back-excited Raman amplifier 200 will be described. The back-excited Raman amplifier 200 includes multiple back-excitement light sources 201. Multiple back-excitement light sources 201 are an example of multiple first excitation light sources. For example, the back-excited Raman amplifier 200 includes five back-excitement light sources 201. The number of back-excitement light sources 201 may be the same as or different from the number of forward-excitement light sources 101.

[0032] Furthermore, the back-excited Raman amplifier 200 includes a multiplexer 202 and a back-control unit 205. The back-excited Raman amplifier 200 also includes a plurality of optical couplers 203, 204 and a WDM coupler 206. The optical couplers 203, 204 and the WDM coupler 206 are located on the optical waveguide 215. Note that the back-control unit 205 is an example of a first control unit.

[0033] Furthermore, the back-excited Raman amplifier 200 includes a plurality of optical filters 207, 208, 209, 210 and a plurality of monitor PDs (Photo Diodes) 211, 212, 213, 214. Monitor PD 214 is an example of a first detection unit. Monitor PD 213 is an example of a second detection unit. Monitor PD 212 is an example of a third detection unit. Monitor PD 211 is an example of a fourth detection unit.

[0034] Each of the rear excitation light sources 201 outputs unit excitation light Ld,···,Lh with different central wavelengths. All of the unit excitation light Ld,···,Lh are coherent excitation light belonging to a wavelength band shorter than the shortest wavelength belonging to the S band. The multiplexer 202 combines the unit excitation light Ld,···,Lh to generate excitation light Lq. Excitation light Lq is output to the optical transmission line 31 via the WDM coupler 206. In this way, in the optical transmission line 31, excitation light Lq amplifies the WDM light Lw1.

[0035] Optical coupler 203 splits the WDM light Lw1. A portion of the WDM light Lw1 is output to the outside of the back-pumped Raman amplifier 200 via optical coupler 204. The remainder of the WDM light Lw1 is input to optical filters 207, 208, 209, and 210, respectively. Optical coupler 204 splits a portion of the WDM light Lw1 and the OSC light Lk. The OSC light Lk is output to the OSC communication unit 252 provided in the forward-pumped Raman amplifier 250. A portion of the WDM light Lw1 is output to the outside of the back-pumped Raman amplifier 200.

[0036] Optical filter 207 allows the transmission of WDM light Ls contained in the remainder of WDM light Lw1, while blocking the transmission of WDM light Lc, Ll, and Lu contained in the remainder of WDM light Lw1. Monitor PD211 detects WDM light Ls and outputs the magnitude of the detected WDM light Ls's optical power as an electrical monitor value to the rear control unit 205. Optical filter 208 allows the transmission of WDM light Lc contained in the remainder of WDM light Lw1, while blocking the transmission of WDM light Ls, Ll, and Lu contained in the remainder of WDM light Lw1. Monitor PD212 detects WDM light Lc and outputs the magnitude of the detected WDM light Lc's optical power as an electrical monitor value to the rear control unit 205.

[0037] The optical filter 209 allows the transmission of WDM light Ll contained in the remainder of WDM light Lw1, and blocks the transmission of WDM light Ls, Lc, and Lu contained in the remainder of WDM light Lw1. The monitor PD213 detects WDM light Ll and outputs the magnitude of the optical power of the detected WDM light Ll as an electrical monitor value to the rear control unit 205. The optical filter 210 allows the transmission of WDM light Lu contained in the remainder of WDM light Lw1, and blocks the transmission of WDM light Ls, Lc, and Ll contained in the remainder of WDM light Lw1. The monitor PD214 detects all of the WDM light Lu and outputs the magnitude of the optical power of the detected WDM light Lu as an electrical monitor value to the OSC communication unit 252.

[0038] As a result, the OSC communication unit 252 can generate an OSC optical signal Lj that includes the monitor value output from the monitor PD214 as predetermined information. That is, the OSC communication unit 252 can generate an OSC optical signal Lj that includes the monitor value of the optical power of the U-band WDM optical signal Luu. Once the OSC communication unit 252 generates the OSC optical signal Lj, it transmits it. The OSC optical signal Lj propagates through the optical waveguide 258 via the WDM coupler 257 and is led to the optical coupler 151 via the optical transmission path 32. The optical coupler 151 splits the WDM optical signal Lw2 and the OSC optical signal Lj and leads the OSC optical signal Lj to the OSC communication unit 112. As a result, the OSC communication unit 112 can receive the OSC optical signal Lj.

[0039] The rear control unit 205 includes a rear calculation unit 205B and a rear adjustment unit 205C. The rear calculation unit 205B individually calculates adjustment amounts used to control the optical power of unit excitation light Ld,...,Lh based on the monitor values ​​output from monitors PD211,212,213. The rear adjustment unit 205C controls the operation of each of the multiple rear excitation light sources 201 based on the adjustment amounts calculated by the rear calculation unit 205B. Specifically, the rear adjustment unit 205C controls the optical power of unit excitation light Ld,...,Lh and adjusts the Raman gain for each WDM light Ls,Lc,Ll.

[0040] Furthermore, techniques for adjusting the Raman gain for each WDM optical signal Ls, Lc, and Ll can be referenced, for example, from patent documents (e.g., Japanese Patent No. 4821037) that adjust the Raman gain by splitting the C band into multiple frequency bands. For example, based on the patent documents, the rear control unit 205 can calculate the above-mentioned adjustment amount based on the monitor values ​​output from monitors PD211, 212, and 213 and the inverse matrix of the average gain coefficient.

[0041] Referring to Figure 3, the operation of the Raman amplification system STa according to the first embodiment will be described.

[0042] First, monitor PD214 measures the optical power of the U-band WDM light (step S1). That is, monitor PD214 measures the magnitude of the optical power of the WDM light Lu. Once monitor PD214 has measured the optical power of the U-band WDM light, OSC communication unit 252 then transmits OSC light Lj (step S2). More specifically, OSC communication unit 252 transmits OSC light Lj to OSC communication unit 112, which includes the magnitude of the optical power of the WDM light Lu measured by monitor PD214 as a monitor value.

[0043] When the OSC communication unit 252 transmits OSC light Lj, the forward adjustment unit 105G then controls the forward excitation light source 101 (step S3). More specifically, the forward adjustment unit 105G adjusts the optical power of the unit excitation light L1, ... L6, while fixing the excitation ratio, based on the adjustment amounts calculated by the forward calculation unit 105F.

[0044] When the forward adjustment unit 105G controls the forward excitation light source 101, the monitors PD211, 212, and 213 then measure the optical power of WDM light other than the U band (step S4). That is, the monitors PD211, 212, and 213 measure the magnitude of the optical power of the WDM light Ls, Lc, and Ll, respectively. After the monitors PD211, 212, and 213 measure the optical power of the WDM light other than the U band, the rear adjustment unit 205C then controls the rear excitation light source 201 (step S5), and the process ends. More specifically, the rear adjustment unit 205C adjusts the optical power of the unit excitation light Ld,...,Lh based on the adjustment amounts calculated by the rear calculation unit 205B.

[0045] Thus, the Raman amplification system STa according to the first embodiment adjusts the Raman gain for WDM light Lu, and then adjusts the Raman gain for WDM light Ls, Lc, and Ll. In particular, the Raman amplification system STa according to the first embodiment controls the optical power of the excitation light Lq based on the optical powers of WDM light Ls, Lc, and Ll, and adjusts the Raman gain for WDM light Ls, Lc, and Ll. As a result, the Raman amplification system STa can ensure the flatness of the Raman gain in transmission across the four wavelength bands: S-band, C-band, L-band, and U-band.

[0046] The operation and effects of the Raman amplification system STa according to the first embodiment will be explained with reference to Figures 4A to 7.

[0047] For example, when the forward-excited Raman amplifier 100 performs Raman amplification of WDM light Lw1 on its own, the forward-excited Raman amplification is most pronounced near the input where the WDM light Lw1 is input to the optical transmission path 31. As a result, the input power, which is the optical power when the WDM light Lw1 is input to the optical transmission path 31, becomes high, making gain saturation more likely and making it difficult to secure a large gain.

[0048] Gain saturation, as shown in Figure 4B, is a phenomenon in which a constant gain can be maintained as a small-signal gain when the input power is lower than a predetermined reference power, but when the input power is higher than the reference power, the gain is gradually limited to a level lower than the small-signal gain as the input power increases. Thus, when the forward-excited Raman amplifier 100 performs Raman amplification of WDM light Lw1 on its own, gain saturation is likely to occur, making it difficult to secure a large gain.

[0049] Furthermore, when WDM light Lw1 contains WDM light Ls, Lc, Ll, and Lu, stimulated Raman scattering between the WDM light Ls, Lc, Ll, and Lu contained in WDM light Lw1 becomes significant, and the Raman gain changes across the entire wavelength band from the S band to the U band. In other words, the optical power on the shorter wavelength side, such as the S band and C band, shifts to the longer wavelength side, such as the L band and U band, resulting in insufficient gain on the shorter wavelength side.

[0050] As a result, as shown in Figure 4A, in the power spectrum of the received optical power (or incoming level), which is the optical power of WDM light Ls, Lc, Ll, and Lu received by the optical receiver 22, the received optical power on the short wavelength side is smaller than the received optical power on the long wavelength side. Thus, if the forward-excited Raman amplifier 100 performs Raman amplification of WDM light Lw1 alone without using the back-excited Raman amplifier 200, it becomes difficult to ensure the flatness of the power spectrum.

[0051] On the other hand, when the back-excited Raman amplifier 200 performs Raman amplification of the WDM light Lw1 by itself, the back-excited Raman amplification is most pronounced near the output of the WDM light Lw1 from the optical transmission path 31. In this case, since the WDM light Lw1 has already propagated through the optical transmission path 31, the input power of the WDM light Lw1 is reduced due to transmission path losses, and the occurrence of gain saturation is suppressed. In other words, when the back-excited Raman amplifier 200 performs Raman amplification of the WDM light Lw1 by itself, a large gain can be secured.

[0052] However, when the back-excited Raman amplifier 200 performs Raman amplification of WDM light Lw1 on its own, the wavelength bands in which Raman gain can be secured are limited. For example, the wavelength bands in which Raman gain can be secured are limited to three wavelength bands: the S band, the C band, and the L band. That is, there are cases in which Raman gain for the U band cannot be secured. Here, the excitation wavelengths of each of the multiple unit excitation rays used for Raman amplification are all shorter than the shortest wavelength of WDM light Ls. For this reason, for example, the Raman gain of the unit excitation ray with the longest excitation wavelength peaks at the long wavelength side, 13 THz, i.e., around the wavelength of WDM light Ll, and decreases sharply above 15 THz. That is, there is a high possibility that Raman gain for WDM light Lu cannot be secured.

[0053] As a result, as shown in Figure 5, in the power spectrum of the received optical powers, which are the respective optical powers of WDM optical Ls, Lc, Ll, and Lu received by the optical receiver 22, flatness is ensured except for the received optical power of WDM optical Lu. That is, the Raman gain does not reach WDM optical Lu, and flatness is ensured only for the optical powers of WDM optical Ls, Lc, and Ll. Thus, even if the back-excited Raman amplifier 200 performs Raman amplification of WDM optical Lw1 alone without using the forward-excited Raman amplifier 100, it becomes difficult to ensure flatness of the power spectrum in 4-band transmission.

[0054] However, in this embodiment, first, as shown in Figure 6, the forward-excited Raman amplifier 100 amplifies the optical power of WDM light Ls, Lc, and Ll by Raman amplification. Meanwhile, stimulated Raman scattering occurs as WDM light Lw1, which includes WDM light Ls, Lc, Ll, and Lu, propagates. As a result, the optical power of WDM light Ls and Lc transitions to the optical power of WDM light Ll and Lu, and the optical power of WDM light Ll and Lu increases compared to the optical power of WDM light Ls and Lc. Consequently, sufficient Raman gain in the U band is ensured.

[0055] Next, the back-excited Raman amplifier 200 amplifies the optical power of the WDM light Ls, Lc, and Ll through Raman amplification. In this case, the effect of stimulated Raman scattering is negligibly small. As a result, sufficient Raman gain is ensured only in the S, C, and L bands. Thus, by using the forward-excited Raman amplifier 100 and the back-excited Raman amplifier 200 in combination and specifying the order in which to adjust the Raman amplification gain, the Raman amplification system STa can ensure a flat Raman gain G1, as shown in Figure 7.

[0056] (Second Embodiment) The second embodiment of this invention will be described with reference to Figures 8 to 14. Components identical to those in the forward-excited Raman amplifier 100 and the backward-excited Raman amplifier 200 according to the first embodiment are basically given the same reference numerals, and their detailed descriptions are omitted.

[0057] As shown in Figure 8, in the forward-excited Raman amplifier 100 according to the second embodiment, the forward control unit 105 further includes an individual calculation unit 105H and an individual adjustment unit 105I. Thus, the forward-excited Raman amplifier 100 according to the second embodiment differs from the forward-excited Raman amplifier 100 according to the first embodiment. Also, as shown in Figure 9, the back-excited Raman amplifier 200 according to the second embodiment further includes an optical filter 216 and a monitor PD 217. The monitor PD 217 is an example of a first detection unit. That is, in the second embodiment, both monitor PD 214 and 217 correspond to the first detection unit. Thus, the back-excited Raman amplifier 200 according to the second embodiment differs from the back-excited Raman amplifier 200 according to the first embodiment.

[0058] Here, the optical filter 216 allows the transmission of a portion of the WDM light Lu contained in the remainder of the WDM light Lw1 described in the first embodiment, while blocking the transmission of the WDM light Lc, Ll, Lu contained in the remainder of the WDM light Lw1 and the remainder of the WDM light Lu. As will be described in detail later, the optical filter 216 allows the transmission of WDM light Lu from a predetermined wavelength λx to the longest wavelength λy belonging to the U band. On the other hand, the optical filter 216 blocks the transmission of WDM light Lu from the shortest wavelength λz to a predetermined wavelength λx belonging to the U band. The difference between the predetermined wavelength λx and the longest wavelength λy is about a few nanometers.

[0059] Thus, the optical filter 216 allows the transmission of a portion of the WDM light Lu contained in the remainder of the WDM light Lw1 described in the first embodiment, while blocking the transmission of the WDM light Lc, Ll, Lu contained in the remainder of the WDM light Lw1 and the remainder of the WDM light Lu. For this reason, the optical filter 210 according to the second embodiment allows the transmission of the remainder of the WDM light Lu as described above, while blocking the transmission of the WDM light Lc, Ll, Lu contained in the remainder of the WDM light Lw1 and a portion of the WDM light Lu as described above.

[0060] The monitor PD217 detects a portion of the WDM optical Lu mentioned above and outputs the magnitude of the optical power of the detected portion of the WDM optical Lu as an electrical monitor value to the OSC communication unit 252 (omitted in Figure 9). Thus, the monitor PD217 differs from the monitor PD214, which detects all of the WDM optical Lu. That is, the monitor PD214 according to the second embodiment detects all of the WDM optical Lu and outputs the magnitude of the optical power of all of the detected WDM optical Lu as an electrical monitor value to the OSC communication unit 252 (omitted in Figure 9).

[0061] As a result, the OSC communication unit 252 can generate an OSC optical signal Lj that includes each monitor value output from monitors PD214 and 217 as predetermined information. That is, the OSC communication unit 252 can generate an OSC optical signal Lj that includes the monitor values ​​of some of the optical power of the U-band WDM optical signal Lu and the monitor values ​​of all of the optical power of the U-band WDM optical signal Lu. Once the OSC communication unit 252 generates the OSC optical signal Lj, it transmits it. As a result, as shown in Figure 8, the OSC communication unit 112 can receive the OSC optical signal Lj, similar to the first embodiment.

[0062] The forward calculation unit 105F and the individual calculation unit 105H individually calculate adjustment amounts used to control the optical power of unit excitation rays L1, ..., L6 based on predetermined information notified from the OSC communication unit 112. Specifically, the forward calculation unit 105F individually calculates adjustment amounts used to control the optical power of unit excitation rays L1, ..., L6, excluding unit excitation ray L4, based on the monitored values ​​of the total optical power of the U-band WDM light Lu. The individual calculation unit 105H calculates an adjustment amount used to control the optical power of unit excitation ray L4 based on the monitored values ​​of some of the optical power of the U-band WDM light Lu. The forward excitation light source 101 that outputs unit excitation ray L4 is an example of a second excitation light source.

[0063] The forward adjustment unit 105G controls the operation of each of the corresponding forward excitation light sources 101 based on the adjustment amounts calculated by the forward calculation unit 105F. Specifically, the forward adjustment unit 105G controls the optical power of the unit excitation light sources L1, ..., L6, excluding the unit excitation light source L4, while fixing the excitation ratio, and adjusts the Raman gain for the remainder of the WDM light source Lu. The individual adjustment unit 105I controls the operation of the corresponding forward excitation light source 101 based on the adjustment amounts calculated by the individual calculation unit 105H. Specifically, the individual adjustment unit 105I controls the optical power of the unit excitation light source L4 and adjusts the Raman gain for a portion of the WDM light source Lu.

[0064] Referring to Figure 10, the operation of the Raman amplification system STa according to the second embodiment will be described.

[0065] First, monitor PD217 measures the optical power of a portion of the U-band WDM light (step S11). That is, monitor PD217 measures the magnitude of a portion of the optical power of the WDM light Lu. After monitor PD217 measures the optical power of the U-band WDM light, monitor PD214 then measures the total optical power of the U-band WDM light (step S12). That is, monitor PD214 measures the magnitude of the total optical power of the WDM light Lu.

[0066] When monitor PD214 measures the total optical power of the U-band WDM light, OSC communication unit 252 then transmits OSC light Lj (step S13). More specifically, OSC communication unit 252 transmits OSC light Lj to OSC communication unit 112 which includes the magnitude of a portion of the optical power of the WDM light Lu measured by monitor PD217 as a monitor value, and also includes the magnitude of the total optical power of the WDM light Lu measured by monitor PD214 as a monitor value.

[0067] When the OSC communication unit 252 transmits OSC light Lj, the forward adjustment unit 105G then controls the forward excitation light source 101 (step S14). More specifically, the forward adjustment unit 105G adjusts the optical power of the unit excitation light L1, ... L6 while fixing the excitation ratio, based on the adjustment amounts calculated by the forward calculation unit 105F. After the forward adjustment unit 105G controls the forward excitation light source 101, the individual adjustment unit 105I then controls a specific forward excitation light source 101 (step S15). More specifically, the forward adjustment unit 105G adjusts the optical power of the unit excitation light L4 based on the adjustment amount calculated by the forward calculation unit 105F.

[0068] When the individual adjustment unit 105I controls a specific forward excitation light source 101, the monitors PD211, 212, and 213 then measure the optical power of WDM light other than the U band (step S16). That is, the monitors PD211, 212, and 213 measure the magnitude of the optical power of WDM light Ls, Lc, and Ll, respectively. After the monitors PD211, 212, and 213 measure the optical power of WDM light other than the U band, the rear adjustment unit 205C then controls the rear excitation light source 201 (step S17) and terminates the process. More specifically, the rear adjustment unit 205C adjusts the optical power of unit excitation light Ld,...Lh based on the adjustment amounts calculated by the rear calculation unit 205B.

[0069] Thus, the Raman amplification system STa according to the second embodiment adjusts the Raman gain for a portion of the WDM light Lu and for all of the WDM light Lu individually, and then adjusts the Raman gain for the WDM light Ls, Lc, and Ll. In particular, the Raman amplification system STa according to the first embodiment controls the optical power of the excitation light Lq based on the optical power of the WDM light Ls, Lc, and Ll, and adjusts the Raman gain for the WDM light Ls, Lc, and Ll. As a result, the Raman amplification system STa can ensure the flatness of the Raman gain in transmission across the four wavelength bands: S band, C band, L band, and U band.

[0070] The operation and effects of the Raman amplification system STa according to the second embodiment will be described with reference to Figures 11 to 14.

[0071] For example, in the first embodiment described above, the unit excitation light L1,...,L6 is output at 100% excitation light power. As a result, the desired Raman gain is ensured even if the span loss is large. Consequently, as shown in Figure 11, in the power spectrum of the received optical power, which is the WDM light Ls, Lc, Ll, Lu received by the optical receiver 22, the magnitude of the received optical power is approximately the same when comparing the received optical power on the short wavelength side with the received optical power on the long wavelength side. This ensures the flatness of the received optical power.

[0072] However, depending on the distance of the optical transmission lines 31 and 32, the span loss may be small. In this case, it is sufficient to secure a Raman gain smaller than the desired Raman gain described above. That is, the unit excitation light L1,...,L6 may be adjusted to an excitation light power lower than the desired Raman gain described above (assuming an excitation light power of 100%). For example, the excitation light power of the unit excitation light L1,...,L6 may be adjusted to 55%, 60%, or 65%. However, in this case, as shown in Figure 11, in the power spectrum of the received optical power, which is the WDM light Ls, Lc, Ll, Lu received by the optical receiver 22, the magnitude of the received optical power will not be approximately the same when comparing the received optical power on the short wavelength side with the received optical power on the long wavelength side. Specifically, the received optical power on the long wavelength side will be lower than the received optical power on the short wavelength side, and the flatness of the optical power will be lost.

[0073] Therefore, as shown in Figure 12, the individual adjustment unit 105I may independently adjust the excitation light power of a specific central wavelength λ4 that is longer than the central wavelength λ1 of the unit excitation light L1 and shorter than the central wavelength λ6 of the unit excitation light L6. In other words, the individual adjustment unit 105I may adjust the excitation light power of the unit excitation light L4 independently of the fixing of the excitation ratio described above. The specific central wavelength λ4 is an example of a specific wavelength, and is a wavelength that is a unit wavelength range away from the longest wavelength λy belonging to the U band on the shorter wavelength side. The unit wavelength range is from 190 nm to 210 nm, including 200 nm. The unit wavelength range may also be from 195 nm to 205 nm.

[0074] This unit excitation light L4 with a specific center wavelength λ4 amplifies the second shortest signal band out of the four signal bands with the highest conversion efficiency. In particular, WDM light Lc with a center wavelength λm, which is approximately 100 nm longer than the specific center wavelength λ4, is Raman amplified. The optical power of the Raman amplified WDM light Lc then shifts to the longer wavelength side due to stimulated Raman scattering. As a result, WDM light Lu from a predetermined wavelength λx to the longest wavelength λy, which belongs to the U band, is amplified. As a result, as shown in Figure 13, the received optical power on the longer wavelength side increases to the same level as the received optical power on the shorter wavelength side, ensuring flatness of the received optical power.

[0075] The specific central wavelength λ4 mentioned above is the minimum central wavelength λ4, as shown in Figure 14. min From the maximum central wavelength λ4 max It is sufficient if it falls within the range. Minimum center wavelength λ4 min For example, it is 1445 nm, and the maximum central wavelength λ4 max For example, this is 1457 nm. The minimum center wavelength λ4 is the specific center wavelength λ4. min From the maximum central wavelength λ4 max If it falls within this range, the received optical power can be kept below flatness F1 (e.g., 2 dB).

[0076] Although preferred embodiments of the present invention have been described in detail above, the present invention is not limited to specific embodiments, and various modifications and changes are possible within the scope of the gist of the invention as described in the claims.

[0077] Furthermore, the following additional information is disclosed regarding the above explanation. (Note 1) A Raman amplification system that amplifies multiple signal bands using a first back-excited Raman amplifier provided on the output side of a transmission line and a second forward-excited Raman amplifier provided on the input side of the transmission line, wherein the first Raman amplifier has a first power detection unit that detects the power of at least a portion of the signal band on the longest wavelength side among the multiple signal bands, and the second Raman amplifier has a second excitation light source that outputs a second excitation light of a specific wavelength that amplifies the second signal band from the shortest wavelength side among the multiple signal bands with the highest efficiency, and a second control unit that controls the power of the second excitation light source based on the results obtained from the first power detection unit. (Note 2) The Raman amplification system according to Note 1, characterized in that the first power detection unit of the first Raman amplifier detects the power of the entire signal band on the longest wavelength side among the multiple signal bands, the second Raman amplifier further has a plurality of third excitation light sources that each output third excitation light of a plurality of different wavelengths excluding the specific wavelength, and the second control unit adjusts the power of the second excitation light source and the plurality of third excitation light sources based on the results obtained from the first power detection unit before controlling the power of the second excitation light source. (Note 3) The Raman amplification system according to Note 1, further comprising: a plurality of first excitation light sources that each output first excitation light of a plurality of different wavelengths; a second power detection unit that monitors the power of the entire second signal band from the longest wavelength side among the plurality of signal bands; a third power detection unit that monitors the power of the entire third signal band from the longest wavelength side among the plurality of signal bands; a fourth power detection unit that monitors the power of the entire fourth signal band from the longest wavelength side among the plurality of signal bands; and a first control unit that controls the plurality of first excitation light sources based on the results obtained from the second power detection unit, the third power detection unit and the fourth power detection unit after the second control unit has controlled the power of the second excitation light sources. (Note 4) The Raman amplification system according to Note 1, characterized in that the central wavelength of the second excitation light source falls within the range of 1445 nm to 1457 nm. (Note 5) The Raman amplification system according to Note 1, characterized in that the second excitation light source is an incoherent light source. (Note 6) The Raman amplification system according to Note 1, characterized in that the signal band on the longest wavelength side is the U band. (Note 7) A Raman amplification method characterized by propagating a second excitation light of a specific wavelength in the same direction as the signal light, which Raman amplifies the second signal band from the short wavelength side with the highest efficiency among multiple signal bands, and adjusting the power of the second excitation light based on the power of at least a portion of the signal band on the long wavelength side among the multiple signal bands. (Note 8) The Raman amplification method according to Note 7, characterized in that, based on the power of the entire signal band on the longest wavelength side among the multiple signal bands, the power of the second excitation light and the power of a plurality of different wavelengths of excitation light, excluding the specific wavelength, that propagate in the same direction as the signal light, are adjusted, respectively, and based on the power of the remaining signal bands, excluding the signal band on the longest wavelength side among the multiple signal bands, the power of a plurality of different wavelengths of excitation light, that propagate in the opposite direction to the signal light, is adjusted, respectively. (Note 9) A Raman amplifier provided on the input side of a transmission line for amplifying multiple signal bands, comprising: an excitation light source that outputs excitation light of a specific wavelength that amplifies the second signal band from the short wavelength side of the multiple signal bands with the highest efficiency; and a control unit that controls the power of the excitation light source based on the power of at least a portion of the signal band on the long wavelength side of the multiple signal bands. [Explanation of symbols]

[0078] ST Optical Transmission Systems STa Raman Amplification System 10,20 Optical transmission equipment 100 Forward-excited Raman amplifier 101 Forward-excited light source 105 Front control unit 200 Back-excited Raman amplifier 201 Back-excited light source 205 Rear control section 211, 212, 213, 214, 217 Monitor PD

Claims

1. In a Raman amplification system that amplifies multiple signal bandwidths using a first back-excited Raman amplifier provided on the output side of a transmission line and a second forward-excited Raman amplifier provided on the input side of the transmission line, The first Raman amplifier described above is The system includes a first power detection unit that detects the power of at least a portion of the signal band on the longest wavelength side among the multiple signal bands, The second Raman amplifier described above, A second excitation light source that outputs a second excitation light of a specific wavelength that amplifies the second signal band from the short wavelength side with the highest efficiency among the multiple signal bands, The system includes a second control unit that controls the power of the second excitation light source based on the results obtained from the first power detection unit, A Raman amplification system characterized by the following features.

2. The first power detection unit of the first Raman amplifier is The power of the entire signal band on the longest wavelength side among the multiple signal bands is detected. The second Raman amplifier described above further, The system has a plurality of third excitation light sources, each outputting a third excitation light of a plurality of different wavelengths excluding the aforementioned specific wavelength, Before controlling the power of the second excitation light source, the second control unit adjusts the power of the second excitation light source and the plurality of third excitation light sources based on the results obtained from the first power detection unit. The Raman amplification system according to feature 1.

3. The first Raman amplifier further, Multiple first excitation light sources, each emitting first excitation light of multiple different wavelengths, A second power detection unit monitors the power of the entire signal band, specifically the second longest wavelength signal band among the multiple signal bands mentioned above. A third power detection unit monitors the power of the entire signal band, specifically the third longest wavelength signal band among the multiple signal bands mentioned above. A fourth power detection unit monitors the power of the entire fourth signal band from the longest wavelength side among the multiple signal bands, After the second control unit controls the power of the second excitation light source, the system includes a second power detection unit, a third power detection unit, and a first control unit that controls each of the plurality of first excitation light sources based on the results obtained from the fourth power detection unit. The Raman amplification system according to feature 1.

4. The central wavelength of the second excitation light source is within the range of 1445 nm to 1457 nm. The Raman amplification system according to feature 1.

5. The second excitation light source is an incoherent light source. The Raman amplification system according to feature 1.

6. A second excitation light of a specific wavelength is propagated in the same direction as the signal light to achieve the highest efficiency Raman amplification of the second shortest wavelength signal band among multiple signal bands, and the power of the second excitation light is adjusted based on the power of at least a portion of the longest wavelength signal band among the multiple signal bands. A Raman amplification method characterized by the following features.

7. Based on the overall power of the signal band on the longest wavelength side among the multiple signal bands, the power of the second excitation light and the power of the third excitation light, which propagates in the same direction as the signal light and has multiple different wavelengths excluding the specific wavelength, are adjusted. Based on the power of the remaining signal bands after excluding the signal band with the longest wavelength among the aforementioned multiple signal bands, the power of multiple first excitation lights of different wavelengths that propagate in the opposite direction to the signal light is adjusted. The Raman amplification method according to feature 6.

8. A Raman amplifier device installed on the input side of a transmission line, which amplifies multiple signal bandwidths, An excitation light source that outputs excitation light of a specific wavelength that amplifies the second signal band from the short wavelength side with the highest efficiency among the multiple signal bands, A control unit controls the power of the excitation light source based on the power of at least a portion of the signal band on the longest wavelength side among the multiple signal bands, A Raman amplifier having a

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