Raman amplification device, Raman amplification method, and Raman amplification system

Abstract

To provide a Raman amplification device, a Raman amplification method and a Raman amplification system that effectively control a gain for signal light.SOLUTION: A Raman amplification device includes: a first light source which outputs primary excitation light, propagated in the same direction as the propagation direction of signal light, to an optical transmission line for Raman amplification; a second light source which amplifies the primary excitation light and also outputs secondary excitation light, propagated in the same direction as the propagation direction, to the optical transmission line; and a control part which adjusts the power of the secondary excitation light to control a gain for the signal light.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] A technique for Raman amplifying signal light using pump light is known (see, for example, Patent Document 1). Also known is counter-propagating Raman amplification, in which pump light is incident on a transmission line for Raman amplification (specifically, an optical fiber) so that it propagates in the opposite direction to the propagation direction of the signal light. Furthermore, co-propagating Raman amplification, in which pump light is incident on a transmission line for Raman amplification so that it propagates in the same direction as the propagation direction of the signal light, is also known. In addition, bi-directional Raman amplification, in which co-propagating Raman amplification and counter-propagating Raman amplification are used simultaneously, is also known (see, for example, Patent Document 2).

[0003] In addition, a technique is known in which, in co-propagating Raman amplification, primary pumping light capable of optically amplifying the wavelength band of signal light is optically amplified by secondary pumping light, and the optically amplified primary pumping light is used to optically amplify the signal light. In this technique, the primary pumping light and secondary pumping light propagate in the same direction as the signal light in the transmission line (see, for example, Patent Document 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-309250 [Patent Document 2] Japanese Patent Publication No. 2020-129143 [Patent Document 3] Patent Publication No. 2021-063857 Summary of the Invention [Problem to be solved by the invention]

[0005] In Raman amplification, the transmission line itself is used as the amplification medium, so the target gain must be changed depending on the type of transmission line. Changing the target gain causes gain tilt. When tilt occurs, the quality of the signal light deteriorates. For example, the S / N ratio of signal light in wavelength bands with low gain decreases compared to wavelength bands with high gain. In this way, the quality of signal light can vary depending on the wavelength band. Deterioration in the quality of signal light can lead to a deterioration in the transmission performance (e.g., transmission distance) of the transmission equipment. For this reason, tilt control is performed in Raman amplification to suppress tilt.

[0006] Furthermore, in Raman amplification, the power of the pump light introduced into the transmission line, which is the amplification medium, decreases due to connection loss (so-called lamp loss) around the pump light transmission line output section. When the power of the pump light decreases, the gain for the signal light decreases, resulting in a smaller output of the signal light. For this reason, gain control is performed in Raman amplification to suppress the decrease in gain. In this way, both tilt control and gain control are required in Raman amplification.

[0007] Unlike counter-propagating Raman amplification, co-propagating Raman amplification involves the phenomenon whereby the signal light and pump light propagate together over a long distance in a transmission line, causing the pump light noise to gradually transfer to the signal light as noise. This phenomenon is called RIN (Relative Intensity Noise) transfer.

[0008] As described above, co-pumping Raman amplification using primary and secondary pumping light optically amplifies the primary pumping light with the secondary pumping light, and then optically amplifies the signal light using the optically amplified primary pumping light. However, when the primary pumping light is optically amplified with the secondary pumping light, noise from the secondary pumping light is transferred to the primary pumping light. Furthermore, because the signal light is optically amplified using the primary pumping light, noise transferred from the secondary pumping light to the primary pumping light is further transferred to the signal light. When noise is transferred to the signal light, transmission errors increase and transmission performance deteriorates. Therefore, co-pumping Raman amplification using primary and secondary pumping light requires effective control of the gain for the signal light.

[0009] Therefore, in one aspect, an object is to provide a Raman amplification device, a Raman amplification method, and a Raman amplification system that effectively control the gain of signal light. [Means for solving the problem]

[0010] In one embodiment, the Raman amplifier is configured to transmit a signal light beam propagating in the same direction as the signal light. , for Raman amplification Primary excitation light The light a first light source that outputs the primary pumping light to a transmission line; and a second light source that amplifies the primary pumping light and outputs secondary pumping light that propagates in the same direction as the propagation direction to the optical transmission line; The power of the primary excitation light is maximized at a first ratio, and then increased at a second ratio. and a control unit that controls the gain of the signal light by adjusting the power of the secondary pump light. [Effects of the Invention]

[0011] The gain for the signal light can be effectively controlled. [Brief explanation of the drawings]

[0012] [Figure 1] Figure 1 shows an example of a transmission system. [Figure 2] FIG. 2 shows an example of a Raman amplification system according to an embodiment. [Figure 3] FIG. 3 shows an example of a Raman amplification system according to a comparative example. [Figure 4] FIG. 4 is a flowchart showing an example of the operation of the forward control unit. [Figure 5] FIG. 5 is a flowchart showing an example of the initial state transition process. [Figure 6] Fig. 6(a) is a diagram illustrating an example of an initial state according to an embodiment, Fig. 6(b) is a diagram illustrating an example of tilt control according to an embodiment, and Fig. 6(c) is a diagram illustrating an example of gain control according to an embodiment. [Figure 7] 7(a) and 7(b) are flowcharts showing an example of tilt control processing and gain control processing, respectively. [Figure 8]Fig. 8(a) is a diagram illustrating an example of an initial state according to a comparative example, Fig. 8(b) is a diagram illustrating an example of tilt control according to a comparative example, and Fig. 8(c) is a diagram illustrating an example of gain control according to a comparative example. [Figure 9] 9(a) is an example of a graph showing the relationship between lamp loss and the pump light power required to compensate for the lamp loss, and FIG. 9(b) is an example of a graph showing the noise characteristics of i-pump and c-pump. [Figure 10] Fig. 10(a) is a diagram illustrating an example of amplification of signal light, and Fig. 10(b) is a diagram illustrating an example of the variable range of the power of secondary pump light. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] As shown in FIG. 1, the transmission system ST includes two transmission devices 10 and 20. The transmission devices 10 and 20 are connected via two optical transmission paths 31 and 32. The optical transmission paths 31 and 32 are an example of a transmission path. The optical transmission paths 31 and 32 include, for example, optical fibers. The type of transmission path for 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) or DSF (Dispersion Shifted Fiber). The optical transmission paths 31 and 32 may or may not be included in the transmission system ST.

[0015] The transmission device 10 includes multiple transceivers 11 and 12, a multiplexer 13, a demultiplexer 14, optical amplifiers 15 and 16, a co-pumped Raman amplifier 100, and a counter-pumped Raman amplifier 150. The co-pumped Raman amplifier 100 is an example of a Raman amplifier device (more specifically, a first Raman amplifier device). The transmission device 20 includes multiple transceivers 21 and 22, a multiplexer 23, a demultiplexer 24, optical amplifiers 25 and 26, a counter-pumped Raman amplifier 200, and a co-pumped Raman amplifier 250. The counter-pumped Raman amplifier 200 is an example of a second Raman amplifier device. The co-pumped Raman amplifier 100 is connected to the counter-pumped Raman amplifier 200 via an optical transmission line 31. The co-pumped Raman amplifier 250 is connected to the counter-pumped Raman amplifier 150 via an optical transmission line 32.

[0016] For example, a Raman amplification system STa can be constructed using a forward-pumping Raman amplifier 100 and a backward-pumping Raman amplifier 200. The Raman amplification system STa may or may not include an optical transmission line 31. The Raman amplification system STa can also be constructed using a forward-pumping Raman amplifier 250 and a backward-pumping Raman amplifier 150. Such a Raman amplification system STa may or may not include an optical transmission line 32.

[0017] The multiple transceivers 11 each transmit multiple wavelength light beams with different wavelengths. The multiple transceivers 12 each receive multiple wavelength light beams with different wavelengths. The multiplexer 13 multiplexes the multiple wavelength light beams with different wavelengths to generate WDM (Wavelength Division Multiplexing) signal light (hereinafter simply referred to as signal light). The demultiplexer 14 demultiplexes wavelength light beams with center wavelengths spaced at regular wavelength intervals from the signal light beams. The multiplexer 13 and demultiplexer 14 include, for example, optical couplers.

[0018] Both optical amplifiers 15 and 16 amplify signal light. Optical amplifiers 15 and 16 include, for example, an EDFA (Erbium Doped Fiber Amplifier). Optical amplifier 15 is sometimes called, for example, a post-amplifier. Optical amplifier 16 is sometimes called, for example, a pre-amplifier. Co-propagating Raman amplifier 100 outputs pump light to optical transmission line 31 in the same direction as the signal light. When the pump light enters optical transmission line 31, stimulated Raman scattering occurs, and the signal light is Raman amplified. Co-propagating Raman amplifier 150 outputs pump light to optical transmission line 32 in the opposite direction to the signal light. When the pump light enters optical transmission line 32, stimulated Raman scattering occurs, and the signal light is Raman amplified.

[0019] The multiple transceivers 21 and 22 have basically the same functions as the multiple transceivers 11 and 12 described above, and therefore detailed descriptions thereof will be omitted. Similarly, the multiplexer 23 and the demultiplexer 24 have basically the same functions as the multiplexer 13 and the demultiplexer 14 described above, and therefore detailed descriptions thereof will be omitted. The optical amplifiers 25 and 26 have basically the same functions as the optical amplifiers 15 and 16 described above, and therefore detailed descriptions thereof will be omitted. The counter-pumping Raman amplifier 200 and the co-pumping Raman amplifier 250 have basically the same functions as the counter-pumping Raman amplifier 150 and the co-pumping Raman amplifier 100 described above, and therefore detailed descriptions thereof will be omitted.

[0020] With reference to FIG. 2, the forward pumping Raman amplifier 100 and the backward pumping Raman amplifier 200 included in the Raman amplification system STa will be described in detail.

[0021] First, a description will be given of co-pumping Raman amplifier 100. Co-pumping Raman amplifier 100 includes a plurality of i-pumps (abbreviated as ip in FIG. 2) 101 and a plurality of c-pumps (abbreviated as cp in FIG. 2) 102. i-pump 101 is an example of a first light source. c-pump 102 is an example of a second light source. Examples of i-pump 101 and c-pump 102 are given in the following references.

[0022] ·References Patent No. 6774753 (JP 2016-212370 A) “Co-Propagating Dual-Order Distributed Raman Amplifier Utilizing Incoherent Pumping”, Masahito Morimoto et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.29, NO.7, APRIL 1, 2017

[0023] The forward-pumping Raman amplifier 100 also includes a first forward driver (abbreviated as Drv in FIG. 2) 103, a second forward driver 104, and a forward control unit 105. The forward control unit 105 is an example of a control unit. The forward control unit 105 includes a tilt control unit 105T and a gain control unit 105G. The forward-pumping Raman amplifier 100 also includes a plurality of optical filters 106, 107, 108, 109, and 110, and an optical isolator (denoted as ISO in FIG. 2) 111. The forward-pumping Raman amplifier 100 also includes an OSC transmitter 112, a C-mPD 113, and an L-mPD 114.

[0024] Each of the i-pumps 101 outputs a primary pump light Li having a different wavelength. The primary pump light Li is incoherent pump light belonging to a first wavelength band (for example, the 1450 nm (nanometer) band). Therefore, each of the i-pumps 101 may be referred to as an incoherent light source.

[0025] Each of the c-pumps 102 outputs secondary pumping light Lc having a different wavelength. The secondary pumping light Lc is coherent pumping light belonging to a second wavelength band (for example, the 1350 nm band). Therefore, each of the c-pumps 102 may be referred to as a coherent light source. In this way, the second wavelength band is different from the first wavelength band and is narrower than the first wavelength band. Note that the multiple c-pumps 102 may include at least one of an FBG-LD (Fiber Bragg Grating - Laser Diode), a DFB (Distributed Feed-Back)-LD, a DBR (Distributed Bragg Reflector)-LD, and an FP (Fabry-Perot)-LD.

[0026] The primary pumping light Li amplifies the signal light Ls in a third wavelength band that is different from both the first and second wavelength bands. The third wavelength band is, for example, the C band, which is the 1550 nm band, or the L band, which is the 1600 nm band. The secondary pumping light Lc amplifies the primary pumping light Li. The primary pumping light Li and the secondary pumping light Lc propagate in the same direction as the propagation direction of the signal light Ls.

[0027] As will be described in detail later, the primary pump light Li has noise characteristics in which it contains a relatively small RIN compared to the secondary pump light Lc. RIN is a parameter that represents the temporal fluctuation (intensity noise) of the intensity of a laser beam, and is a value obtained by dividing the fluctuation (noise) of the optical intensity per unit frequency by the average optical power. When the signal light Ls, the primary pump light Li, and the secondary pump light Lc propagate together through the optical transmission line 31 over a long distance, the RINs of the primary pump light Li and the secondary pump light Lc gradually transfer to the signal light Ls as noise. In other words, RIN transfer occurs. When RIN transfer occurs, transmission errors due to RIN increase, thereby degrading the transmission performance of the transmission device 10. Specifically, the transmission distance of the transmission device 10 becomes shorter. In this embodiment, the gain for the signal light Ls is effectively controlled to suppress the RIN transferred to the signal light Ls, thereby preventing degradation of transmission performance.

[0028] The first forward driver 103 is a drive circuit that controls the drive of the i-pump 101. The first forward driver 103 controls the power of the primary pumping light Li output by the i-pump 101 based on a first control signal output from the tilt control unit 105T. The second forward driver 104 is a drive circuit that controls the drive of the c-pump 102. The second forward driver 104 controls the power of the secondary pumping light Lc output by the c-pump 102 based on a second control signal output from the gain control unit 105G.

[0029] The forward control unit 105 controls the operations of the first forward driver 103 and the second forward driver 104. In particular, the tilt control unit 105T controls the operation of the first forward driver 103 by outputting a first control signal. The tilt control unit 105T controls the operation of the first forward driver 103 to adjust the inter-wavelength power ratio of the primary pumping light Li output by the i-pump 101. Furthermore, the gain control unit 105G controls the operation of the second forward driver 104 by outputting a second control signal. The gain control unit 105G controls the operation of the second forward driver 104 to adjust the average power of the secondary pumping light Lc output by the c-pump 102. The forward control unit 105 includes hardware circuits such as a memory and a CPU (Central Processing Unit). The forward control unit 105 may be a hardware circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array) instead of a CPU.

[0030] The optical filter 106 receives the primary pumping light Li output from each of the i-pumps 101. The optical filter 106 guides the received primary pumping light Li to an optical filter 108. The optical filter 107 receives the secondary pumping light Lc output from each of the c-pumps 102. The optical filter 107 guides the received secondary pumping light Lc to the optical filter 108. The optical filter 108 receives the primary pumping light Li output from the optical filter 106 and the secondary pumping light Lc output from the optical filter 107. The optical filter 108 guides the received primary pumping light Li and secondary pumping light Lc to an optical filter 109 via an optical isolator 111.

[0031] The optical filter 109 guides the primary pumping light Li and secondary pumping light Lc input from the optical filter 108 to the optical transmission line 31. When the primary pumping light Li and secondary pumping light Lc are guided to the optical transmission line 31, a lamp loss occurs in the optical transmission line 31. The gain control unit 105G can identify the lamp loss by, for example, calculating the ratio between the power of the secondary pumping light Lc immediately before it is input to the optical filter 109 and the power immediately after it is output from the optical filter 109. Note that the optical filter 109 removes the signal light Ls input from the optical filter 110 from the optical transmission line 31 without inputting it to the optical filter 108. The optical filter 109 guides the signal light Ls, together with the primary pumping light Li and secondary pumping light Lc, to the counter-pumping Raman amplifier 200 via the optical transmission line 31.

[0032] The OSC communication device 112 includes an SFP (Small Form-Factor Pluggable) transceiver. The OSC communication device 112 transmits requests and information from the forward control unit 105 to the OSC communication device 215 of the backward-pumped Raman amplifier 200 using OSC (Optical Supervisory Channel) light Lx. The OSC light Lx transmitted by the OSC communication device 112 is input to the optical filter 110. The optical filter 110 guides the input OSC light Lx to the optical filter 109. The optical filter 110 removes the signal light Ls from the optical transmission path 31 without inputting it to the OSC communication device 112.

[0033] Furthermore, the OSC communication device 112 receives requests and information from a forward control unit (not shown) included in the forward-pumped Raman amplifier 250, using the OSC light Ly. The OSC light Ly is output from an OSC communication device (not shown) included in the forward-pumped Raman amplifier 250. The OSC light Ly is input to the OSC communication device 112 from an optical filter 155 included in the backward-pumped Raman amplifier 150. In this embodiment, the optical filter 155 is provided in the backward-pumped Raman amplifier 150, but the optical filter 155 may also be provided in the optical amplifier 16 (see FIG. 1). Similarly, in this embodiment, the optical filters 156 and 157 are provided in the backward-pumped Raman amplifier 150, but the optical filters 156 and 157 may also be provided in the optical amplifier 16 (see FIG. 1). The optical filter 158 included in the counter-pumped Raman amplifier 150 is connected to an OSC communication device included in the counter-pumped Raman amplifier 150, but due to space limitations, this OSC communication device is omitted from Fig. 2 and Fig. 3, which will be described later. The optical filter 158 guides the OSC light Ly to the OSC communication device of the counter-pumped Raman amplifier 150.

[0034] The C-mPD 113 includes a PD (Photo Diode) that monitors (measures) the power of the C-band signal light Lt. The C-mPD 113 detects the power of the C-band signal light Lt output from the forward-pumped Raman amplifier 250 and input to the backward-pumped Raman amplifier 150. The C-band signal light Lt is input to the C-mPD 113 from the optical filter 156. The tilt control unit 105T of the forward control unit 105 obtains the power of the C-band signal light Lt from the output signal of the C-mPD 113.

[0035] The L-mPD 114 includes a PD that monitors the power of the L-band signal light Lt. The L-mPD 114 detects the power of the L-band signal light Lt output from the forward-pumped Raman amplifier 250 and input to the backward-pumped Raman amplifier 150. The L-band signal light Lt is input to the L-mPD 114 from the optical filter 157. The tilt control unit 105T of the forward control unit 105 obtains the power of the L-band signal light Lt from the output signal of the L-mPD 114.

[0036] The tilt control unit 105T performs tilt control to suppress gain tilt occurring between the CL bands of the signal light Ls, based on the power of the C-band signal light Lt, the power of the L-band signal light Lt, information included in the OSC light Ly received by the OSC communication device 112, and the like. Gain tilt refers to a change in the wavelength characteristic of gain when the gain given to the signal light Ls changes. For example, the tilt control unit 105T performs tilt control to suppress tilt by adjusting the ratio of inter-wavelength power of the primary pump light Li. To adjust the ratio of inter-wavelength power of the primary pump light Li, the tilt control unit 105T performs tilt control on the first forward driver 103. Details of tilt control will be described later.

[0037] The gain control section 105G performs gain control to suppress a decrease in gain based on a lamp loss in the optical transmission line 31. For example, the gain control section 105G performs tilt control to suppress the average gain by adjusting the average power of the secondary excitation light Lc. To adjust the average power of the secondary excitation light Lc, the gain control section 105G performs gain control on the second forward driver 104. Details of the gain control will be described later.

[0038] Next, the backward-pumped Raman amplifier 200 will be described. The backward-pumped Raman amplifier 200 includes a plurality of FBG-LDs (abbreviated as LDs in FIG. 2) 201. The FBG-LDs 201 are an example of a third light source. The backward-pumped Raman amplifier 200 also includes a backward driver 203 and a backward control unit 205. The backward control unit 205 includes a tilt control unit 205T and a gain control unit 205G. The backward-pumped Raman amplifier 200 also includes a plurality of optical filters 206, 207, 208, 209, 210, and 212, and an optical isolator 211. The backward-pumped Raman amplifier 200 also includes an OSC communication device 215, a C-mPD 213, an L-mPD 214, and optical filters 255, 256, 257, and 258. The backward control unit 205 basically has the same hardware configuration as the forward control unit 105. Furthermore, optical filters 255, 256, 257, and 258 correspond to the optical filters 155, 156, 157, and 158 described above. Therefore, for example, optical filter 255 guides OSC light Lx to an OSC communication device (not shown) included in the co-pumped Raman amplifier 250. For example, optical filter 258 guides OSC light Lx to OSC communication device 215 in the co-pumped Raman amplifier 200. Note that the OSC communication device in the co-pumped Raman amplifier 250 is omitted due to space limitations.

[0039] Each of the FBG-LDs 201 outputs primary pump light Lp having a different wavelength. The primary pump light Lp is coherent pump light that belongs to the same first wavelength band as the first wavelength band described above. Because the primary pump light Lp is coherent pump light, it differs from the incoherent primary pump light Li. In this way, the primary pump light Lp is a separate pump light that is different from the primary pump light Li. The primary pump light Lp amplifies the signal light Ls. The primary pump light Lp propagates in the opposite direction to the propagation direction of the signal light Ls.

[0040] The rear driver 203 is a drive circuit that controls the driving of the FBG-LD 201. The rear driver 203 controls the power of the primary excitation light Lp output by the FBG-LD 201 based on a third control signal output from the tilt control unit 205T. The rear driver 203 also controls the power of the primary excitation light Lp output by the FBG-LD 201 based on a fourth control signal output from the gain control unit 205G.

[0041] The rear control unit 205 controls the operation of the rear driver 203. In particular, the tilt control unit 205T controls the operation of the rear driver 203 by outputting a third control signal. Furthermore, the gain control unit 205G controls the operation of the rear driver 203 by outputting a fourth control signal. The tilt control unit 205T and the gain control unit 205G control the operation of the rear driver 203 to adjust the power of the primary excitation light Lp output by the FBG-LD 201.

[0042] The optical filter 206 receives the primary excitation light Lp output from some of the multiple FBG-LDs 201. The optical filter 207 receives the primary excitation light Lp output from the remaining multiple FBG-LDs 201. Both the optical filters 206 and 207 guide the input primary excitation light Lp to the optical filter 208. The optical filter 208 receives the primary excitation light Lp output from the optical filters 206 and 207. The optical filter 208 guides the input primary excitation light Lp to the optical filter 209 via the optical isolator 211.

[0043] The optical filter 209 guides the primary pumping light Lp input from the optical filter 208 to the optical transmission line 31. When the primary pumping light Lp is guided to the optical transmission line 31, a lamp loss occurs. The gain control unit 205G can identify the lamp loss in the optical transmission line 31 by, for example, calculating the ratio between the power of the primary pumping light Lp immediately before it is input to the optical filter 209 and the power immediately after it is output from the optical filter 209. Note that the optical filter 209 removes the signal light Ls input from the forward-pumping Raman amplifier 100 to the backward-pumping Raman amplifier 200 from the optical transmission line 31 without allowing it to be input to the optical filter 208. The optical filter 209 guides the signal light Ls to the optical filter 210 via the optical transmission line 31.

[0044] The C-mPD 213 includes a PD that monitors the power of the C-band signal light Ls. The C-mPD 213 detects the power of the C-band signal light Ls output from the forward-pumped Raman amplifier 100 and input to the backward-pumped Raman amplifier 200. The C-band signal light Ls is input to the C-mPD 213 from the optical filter 210. The tilt control unit 205T of the rear control unit 205 obtains the power of the C-band signal light Ls from the output signal of the C-mPD 213.

[0045] The L-mPD 214 includes a PD that monitors the power of the L-band signal light Ls. The L-mPD 214 detects the power of the L-band signal light Ls output from the forward-pumped Raman amplifier 100 and input to the backward-pumped Raman amplifier 200. The L-band signal light Ls is input to the L-mPD 214 from the optical filter 212. The tilt control unit 205T of the rear control unit 205 obtains the power of the L-band signal light Ls from the output signal of the L-mPD 214.

[0046] The tilt control unit 205T performs tilt control to suppress gain tilt occurring between the CL bands of the signal light Ls, based on the power of the C-band signal light Ls, the power of the L-band signal light Ls, information included in the OSC light Lx received by the OSC communication device 215, etc. For example, the tilt control unit 205T performs tilt control to suppress tilt by adjusting the ratio between the wavelength powers of the primary pump light Lp. For this reason, the tilt control unit 205T performs tilt control on the rear driver 203.

[0047] The gain control unit 205G performs gain control to suppress a decrease in gain based on a lamp loss in the optical transmission line 31. For example, the gain control unit 205G performs gain control to suppress the average gain by adjusting the average power of the primary excitation light Lp. Therefore, the gain control unit 205G performs gain control on the rear driver 203, similar to the tilt control unit 205T.

[0048] A Raman amplification system STb according to a comparative example will be described in comparison with a Raman amplification system STa according to the embodiment with reference to Fig. 3. In Fig. 3, the same components as those in the Raman amplification system STa described with reference to Fig. 2 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0049] As shown in Fig. 3, in the Raman amplification system STb according to the comparative example, as in the embodiment, the tilt control unit 105T controls the operation of the first forward driver 103 by outputting a first control signal. The tilt control unit 105T controls the operation of the first forward driver 103 to adjust the inter-wavelength power ratio of the primary pumping light Li output by the i-pump 101. On the other hand, in the Raman amplification system STb according to the comparative example, unlike the embodiment, the gain control unit 105G controls the operation of the first forward driver 103 by outputting a second control signal. The gain control unit 105G controls the operation of the first forward driver 103 to adjust the average power of the primary pumping light Li output by the i-pump 101.

[0050] As described above, the control target of tilt control unit 105T is the same in the embodiment and the comparative example, but the control target of gain control unit 105G is different. That is, gain control unit 105G according to the embodiment controls the operation of second front driver 104 by outputting a second control signal. Gain control unit 105G according to the comparative example controls the operation of first front driver 103 by outputting a second control signal. Differences in action and effect due to the difference in the control target will be described later.

[0051] Next, the operation of the front control unit 105 will be described with reference to FIGS.

[0052] First, as shown in FIG. 4, the forward control unit 105 executes an initial state transition process (step S10). The initial state transition process is a process for increasing the power of the primary pump light Li and the power of the secondary pump light Lc from minimum power to the power of the initial state. Specifically, when the forward control unit 105 starts executing the initial state transition process, as shown in FIG. 5, the forward control unit 105 sets the fiber input power, a predetermined ratio, and a target gain in the forward-pumping Raman amplifier 100 (step S11). The fiber input power is the power of the signal light Ls output from the transmission device 10 to the optical transmission line 31 and input to the optical transmission line 31. The predetermined ratio is a ratio for increasing the power of the secondary pump light Lc.

[0053] The fiber input power and the target gain are set based on the transmission line type of the optical transmission lines 31 and 32. For example, the forward control unit 105 has a table in which the correspondence between the transmission line type and the fiber input power is recorded in advance, and when the transmission line type is specified, the forward control unit 105 refers to this table and sets the fiber input power according to the specified transmission line type. The forward control unit 105 also has a table in which the correspondence between the transmission line type and the target gain is recorded in advance, and when the transmission line type is specified, the forward control unit 105 refers to this table and sets the target gain according to the specified transmission line type. A predetermined ratio specified in advance is set for the power of the secondary excitation light Lc.

[0054] When the process of step S11 is completed, the forward control unit 105 turns on the primary excitation light Li and the secondary excitation light Lc at minimum power (step S12). That is, the forward control unit 105 controls the first forward driver 103 to make the i-pump 101 output the primary excitation light Li at minimum power. In addition, the forward control unit 105 controls the second forward driver 104 to make the c-pump 102 output the secondary excitation light Lc at minimum power.

[0055] When the process of step S12 is completed, the forward control unit 105 calculates the pumping ratio of the primary pumping light Li (step S13). For example, the forward control unit 105 calculates the pumping ratio of the primary pumping light Li based on the target gain and the total input power obtained by multiplying the fiber input power by the number of bands (or the number of wavelengths) of the signal light Ls.

[0056] When the processing of step S13 is completed, the forward control unit 105 maximizes the power of the primary excitation light Li using the excitation ratio (step S14). Since the primary excitation light Li is output at minimum power, maximizing the excitation ratio causes the primary excitation light Li to be output at a power corresponding to the excitation ratio (for example, several times). When the processing of step S14 is completed, the forward control unit 105 increases the power of the secondary excitation light Lc by a set predetermined ratio (step S15). When the processing of step S15 is completed, the forward control unit 105 ends the initial state transition processing.

[0057] By completing the initial state transition process, the power characteristics in the initial state of the primary excitation light Li and the power characteristics in the initial state of the secondary excitation light Lc are specified, as shown in Fig. 6(a). Note that Fig. 6(a) shows, as an example, the primary excitation light Li with three mutually different wavelengths and the secondary excitation light Lc with five mutually different wavelengths, but the number of wavelengths is not particularly limited. The number of wavelengths of the primary excitation light Li and the number of wavelengths of the secondary excitation light Lc may be different or the same.

[0058] Furthermore, since the target gain is set in the process of step S11, the gain is changed, and a gain tilt occurs between the C-band and the L-band, as shown in Fig. 6(a). Note that Fig. 6(a) shows, as an example, signal light Ls in the C-band having several tens of wavelengths and signal light Ls in the L-band having several tens of wavelengths, but the number of wavelengths is not particularly limited, and the number of wavelengths in the C-band and the L-band may be different or the same. Since a gain tilt occurs between the C-band and the L-band in this way, the forward control unit 105 starts loop processing (step S20), as shown in Fig. 4, and first, the tilt control unit 105T executes tilt control processing (step S30).

[0059] 7(a), the tilt control unit 105T first calculates the gain difference between the C-band and the L-band (step S31). For example, the tilt control unit 105T calculates a first gain of the signal light Lt of each wavelength belonging to the C-band due to Raman amplification in the optical transmission line 32, based on information indicating the power of the C-band signal light Lt and the output power of the C-band signal light Lt contained in the OSC light Ly. Similarly, the tilt control unit 105T calculates a second gain of the L-band signal light Lt due to Raman amplification in the optical transmission line 32, based on information indicating the power of the L-band signal light Lt and the output power of the L-band signal light Lt contained in the OSC light Ly. After calculating the first gain and the second gain, the tilt control unit 105T calculates the gain difference between the C-band and the L-band based on the difference between the first gain and the second gain. Therefore, in the case of the gain characteristics shown in FIG. 6(a), the gain difference, which is the difference between the gain in the L-band, which is a wavelength band with high gain, and the gain in the C-band, which is a wavelength band with low gain, is calculated.

[0060] After calculating the gain difference, the tilt control unit 105T adjusts the pumping power ratio of the primary pumping light Li based on the gain difference (step S32). Here, the tilt control unit 105T is provided with a pumping ratio table that stores ratio information representing the pumping power ratio of the primary pumping light Li to obtain a specified gain characteristic with respect to the gain due to Raman amplification in the optical transmission line 32. Therefore, the tilt control unit 105T adjusts the pumping power ratio of the primary pumping light Li based on the gain difference and the ratio information in the pumping ratio table, and determines whether the gain difference is within the allowable range. When the gain difference is within the allowable range, the tilt control unit 105T ends the tilt control process.

[0061] For example, as shown in FIG. 6(b), when the ratio between the wavelengths of the power of the primary pump light Li is adjusted based on the gain difference and the ratio information, the gain in the C band increases and the gain in the L band decreases. As a result, the gain characteristics become approximately uniform or approximately flat. In such a case, the tilt control unit 105T determines that the gain difference is within the allowable range and terminates the tilt control process.

[0062] When the tilt control unit 105T finishes the tilt control process, the gain control unit 105G executes the gain control process (step S40), as shown in FIG. 4. Specifically, as shown in FIG. 7(b), the gain control unit 105G first calculates the average gain of the entire C-band and L-band (step S41). For example, the gain control unit 105G calculates the average gain by summing the first gain of the signal light Lt in the C-band and the second gain of the signal light Lt in the L-band and dividing the sum by the number of bands in the C-band and L-band. The gain control unit 105G may also calculate the average gain by summing the first gain of the signal light Lt for each wavelength belonging to the C-band and the second gain of the signal light Lt for each wavelength belonging to the L-band and dividing the sum by the total number of wavelengths in the C-band and L-band.

[0063] After calculating the average gain, the gain control unit 105G adjusts the average power of the secondary excitation light Lc (step S42). For example, the gain control unit 105G identifies the lamp loss by calculating the ratio between the power of the secondary excitation light Lc immediately before input to the optical filter 109 and the power immediately after output from the optical filter 109. Then, the gain control unit 105G adjusts the average power of the secondary excitation light Lc up or down based on the identified lamp loss until the target gain is reached.

[0064] As shown in Figure 6(c), when the average power of the secondary pumping light Lc is adjusted downward, the gain in the C band and the gain in the L band decrease. As the average power of the secondary pumping light Lc decreases, the amount of noise in the RIN of the secondary pumping light Lc decreases. This makes it possible to reduce the amount of noise in the RIN of the secondary pumping light Lc that is transferred to the signal light Ls via the primary pumping light Li. Once the average power of the secondary pumping light Lc is adjusted, the gain control unit 105G ends the gain control process.

[0065] 4, when the gain control process is completed, the forward control unit 105 ends the loop process (step S50). For example, if the forward control unit 105 finds that both the gain difference and the average gain are within the allowable ranges, the forward control unit 105 exits the loop process, fixes the power of the primary excitation light Li and the secondary excitation light Lc (step S60), and ends the process. On the other hand, if the forward control unit 105 finds that at least one of the gain difference and the average gain is not within the allowable ranges, the forward control unit 105 returns to the process of step S20, and the tilt control unit 105T and the gain control unit 105G repeat the same process.

[0066] Next, tilt control and gain control according to a comparative example will be described with reference to Fig. 8 and Fig. 9. Note that the initial state shown in Fig. 8(a) is the same as the initial state shown in Fig. 6(a), and therefore a detailed description thereof will be omitted. Also, the tilt control shown in Fig. 8(b) is the same as the tilt control shown in Fig. 6(b), and therefore a detailed description thereof will be omitted.

[0067] As shown in FIG. 8(c), the gain control section 105G according to the comparative example performs gain control processing on the primary pumping light Li. In this way, even when the gain control section 105G according to the comparative example performs gain control processing on the primary pumping light Li, the average gain can be controlled in the same way as in the embodiment. However, since the gain control section 105G according to the comparative example does not perform gain control processing on the secondary pumping light Lc, the power of the secondary pumping light Lc remains fixed from the initial state. Therefore, the amount of noise in the RIN of the secondary pumping light Lc remains unchanged. Therefore, compared to the embodiment, it is not possible to reduce the amount of noise in the RIN of the secondary pumping light Lc that is transferred to the signal light Ls via the primary pumping light Li. In other words, the amount of noise in the RIN of the secondary pumping light Lc that is transferred to the signal light Ls increases relatively compared to the embodiment.

[0068] 9(a), when the lamp loss is large, it is necessary to increase the power of the pump light for Raman amplification to compensate for the lamp loss. When increasing the power of the pump light for Raman amplification, it is assumed that at least one of the power of the primary pump light Li and the power of the secondary pump light Lc will be increased.

[0069] As shown in FIG. 9(b), even if the power of the primary pumping light Li output by the i-pump 101 increases, the amount of RIN noise of the primary pumping light Li is not proportional to the increase in power but remains approximately constant. That is, the amount of RIN noise of the primary pumping light is approximately constant regardless of whether the power of the primary pumping light increases or decreases. In contrast, when the power of the secondary pumping light Lc output by the c-pump 102 is increased, the amount of RIN noise increases in proportion to the increase in power. That is, the amount of RIN noise of the secondary pumping light changes according to whether the power of the secondary pumping light increases or decreases. In particular, in many cases, the amount of RIN noise of the secondary pumping light Lc is greater than that of the primary pumping light Li.

[0070] For this reason, when realizing the same average gain, it is not desirable to adjust the power of the primary pumping light Li alone without adjusting the power of the secondary pumping light Lc using both tilt control and gain control, as explained in the comparative example. When realizing the same average gain, it is desirable to use tilt control to adjust the power of the primary pumping light Li and gain control to adjust the power of the secondary pumping light Lc, as explained in the embodiment. By adjusting the power of the secondary pumping light Lc, which contains a larger amount of RIN noise than the primary pumping light Li, it is possible to reduce the amount of RIN noise transferred to the signal light Ls, compared to the comparative example. As a result, the embodiment can achieve improved transmission performance, such as an increase in transmission distance.

[0071] Here, for example, tilt control may be performed by adjusting the wavelength power ratio of the secondary pump light Lc. However, as shown in FIG. 10(a), the secondary pump light Lc amplifies the signal light Ls via the primary pump light Li, making it difficult to change the gain ratio for the signal light Ls. As a result, the variable range of the gain wavelength ratio becomes narrow. In this way, it is expected that the gain for the signal light Ls will not change much even if the wavelength power ratio of the secondary pump light Lc is adjusted.

[0072] Furthermore, as shown on the left side of Fig. 10(b), when the power ratio between wavelengths of the secondary pump light Lc is adjusted, the power variable range of the secondary pump light Lc is narrowed by the amount of the power difference because there is a power difference between the wavelengths of the secondary pump light Lc. Therefore, as shown on the right side of Fig. 10(b), there is an advantage in that the power variable range of the secondary pump light Lc can be widened by changing the average power without adjusting the power ratio between wavelengths of the secondary pump light Lc. In this way, it is highly desirable to use tilt control to adjust the power ratio between wavelengths of the primary pump light Li and gain control to adjust the average power of the secondary pump light Lc.

[0073] As described above, the forward-propagating Raman amplifier 100 according to this embodiment includes an i-pump 101, a c-pump 102, and a forward control unit 105. The i-pump 101 outputs primary pumping light Li, which propagates in the same direction as the signal light Ls, to the optical transmission line 31 for Raman amplification. The c-pump 102 amplifies the primary pumping light Li and outputs secondary pumping light Lc, which propagates in the same direction as the signal light Ls, to the optical transmission line 31. The forward control unit 105 controls the gain for the signal light Ls by adjusting the average power of the secondary pumping light Lc. This configuration makes it possible to effectively control the gain for the signal light Ls, thereby improving transmission performance.

[0074] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention as set forth in the claims. For example, in the above-described embodiment, tilt control is performed before gain control, but tilt control may be performed after gain control.

[0075] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) A Raman amplification device including: a first light source that outputs primary pumping light propagating in the same direction as the propagation direction of signal light to an optical transmission line for Raman amplification; a second light source that amplifies the primary pumping light and outputs secondary pumping light propagating in the same direction as the propagation direction of signal light to the optical transmission line; and a control unit that controls the gain of the signal light by adjusting the power of the secondary pumping light. (Appendix 2) The Raman amplification device according to appendix 1, characterized in that the first light source outputs incoherent light as the primary pumping light, and the second light source outputs pumping light different from the incoherent light as the secondary pumping light. (Supplementary Note 3) The Raman amplifier according to Supplementary Note 1 or 2, wherein the control unit controls the gain tilt by adjusting a power ratio of the primary pump light. (Appendix 4) The Raman amplification device according to appendix 1 or 2, characterized in that the control unit controls the gain tilt by adjusting the power ratio of the primary pumping light, and then controls the gain relative to the signal light by adjusting the power of the secondary pumping light. (Supplementary Note 5) The Raman amplification device according to Supplementary Note 1 or 2, characterized in that the control unit controls the gain tilt by adjusting a power ratio of the primary pumping light, and then fixes the adjusted power of the primary pumping light, and controls the gain relative to the signal light by adjusting the power of the secondary pumping light after fixing the power of the primary pumping light. (Appendix 6) The Raman amplification device according to appendix 1 or 2, characterized in that the second light source includes at least one of an FBG (Fiber Bragg Grating) laser, a DFB (Distributed Feedback) laser, a DBR (Distributed Bragg Reflector) laser, and a Fabry-Perot laser. (Appendix 7) The Raman amplifier according to appendix 1 or 2, characterized in that the control unit controls the gain of the signal light by adjusting the average power of the secondary pump light up or down based on a lamp loss in the optical transmission path. (Appendix 8) The Raman amplifier according to appendix 1 or 2, wherein the secondary pumping light contains a larger noise than the primary pumping light. (Appendix 9) The Raman amplification device according to appendix 1 or 2, characterized in that a first noise of the primary pumping light is constant regardless of an increase or decrease in the power of the primary pumping light, a second noise of the secondary pumping light changes according to an increase or decrease in the power of the secondary pumping light, and the second noise is smaller than the first noise. (Supplementary Note 10) A Raman amplification method comprising: outputting primary pumping light propagating in the same direction as the propagation direction of signal light; amplifying the primary pumping light; and outputting secondary pumping light propagating in the same direction as the propagation direction of the signal light to an optical transmission line for Raman amplification; and adjusting the power of the secondary pumping light to control the gain of the signal light. (Supplementary Note 11) A Raman amplification system comprising: a first Raman amplification device including: a first light source that outputs primary pumping light propagating in the same direction as the propagation direction of signal light to an optical transmission line for Raman amplification; a second light source that amplifies the primary pumping light and outputs secondary pumping light propagating in the same direction as the propagation direction to the optical transmission line; and a forward control unit that controls a gain for the signal light by adjusting the power of the secondary pumping light; and a second Raman amplification device including: a third light source that outputs another primary pumping light propagating in the opposite direction to the propagation direction to the optical transmission line; and a backward control unit that controls the gain for the signal light by adjusting the power of the another primary pumping light. [Explanation of symbols]

[0076] ST Transmission System STa, STb Raman amplification system 100 Coaxial Raman Amplifier 101 i-pump 102 c-pump 105 Front control section 105T Tilt control unit 105G Gain Control Unit 200 Counter-pumped Raman amplifier 201 FBG-LD 205 Rear control section 205T Tilt control unit 205G Gain control section

Claims

1. a first light source that outputs primary pump light for Raman amplification to an optical transmission line, the primary pump light propagating in the same direction as the propagation direction of the signal light; a second light source that amplifies the primary pumping light and outputs secondary pumping light that propagates in the same direction as the propagation direction to the optical transmission line; a control unit that controls a gain for the signal light by maximizing the power of the primary pump light at a first ratio and then adjusting the power of the secondary pump light at a second ratio; A Raman amplifier comprising:

2. the first light source outputs incoherent light as the primary excitation light, the second light source outputs excitation light different from the incoherent light as the secondary excitation light; 2. The Raman amplifier according to claim 1,

3. the control unit controls the gain tilt by adjusting the power ratio of the primary pump light.

3. The Raman amplifier according to claim 1, wherein the first and second amplifiers are arranged in series.

4. A first light source that outputs primary pump light propagating in the same direction as the propagation direction of the signal light to an optical transmission line for Raman amplification; a second light source that amplifies the primary pumping light and outputs secondary pumping light that propagates in the same direction as the propagation direction to the optical transmission line; a control unit that controls a gain for the signal light by adjusting the power of the secondary pump light, the control unit controls the gain tilt by adjusting the power ratio of the primary pump light, and then controls the gain relative to the signal light by adjusting the power of the secondary pump light. A Raman amplifier comprising:

5. A first light source that outputs primary pump light propagating in the same direction as the propagation direction of the signal light to an optical transmission line for Raman amplification; a second light source that amplifies the primary pumping light and outputs secondary pumping light that propagates in the same direction as the propagation direction to the optical transmission line; a control unit that controls a gain for the signal light by adjusting the power of the secondary pump light, the control unit adjusts a power ratio of the primary pumping light to control the gain tilt, and then fixes the adjusted power of the primary pumping light, and controls the gain relative to the signal light by adjusting the power of the secondary pumping light after fixing the power of the primary pumping light. A Raman amplifier comprising:

6. The first ratio represents an excitation ratio, and the second ratio represents a predetermined set ratio.

3. The Raman amplifier according to claim 1, wherein the first and second amplifiers are arranged in series.

7. A first light source that outputs primary pump light propagating in the same direction as the propagation direction of the signal light to an optical transmission line for Raman amplification; a second light source that amplifies the primary pumping light and outputs secondary pumping light that propagates in the same direction as the propagation direction to the optical transmission line; a control unit that controls a gain for the signal light by adjusting the power of the secondary pump light, the control unit controls the gain of the signal light by adjusting the average power of the secondary pump light up or down based on a lamp loss in the optical transmission line. A Raman amplifier comprising:

8. outputting, to an optical transmission line, primary pumping light for Raman amplification, which propagates in the same direction as the propagation direction of the signal light, and secondary pumping light, which amplifies the primary pumping light and propagates in the same direction as the propagation direction of the signal light; a power of the primary pumping light is maximized at a first ratio, and then the power of the secondary pumping light is adjusted at a second ratio, thereby controlling the gain relative to the signal light; Raman amplification method.

9. a first Raman amplifier including: a first light source that outputs primary pumping light for Raman amplification to an optical transmission line, the primary pumping light propagating in the same direction as the propagation direction of signal light; a second light source that amplifies the primary pumping light and outputs secondary pumping light that propagates in the same direction as the propagation direction of signal light to the optical transmission line; and a forward control unit that maximizes the power of the primary pumping light at a first ratio and then adjusts the power of the secondary pumping light at a second ratio, thereby controlling a gain for the signal light; a second Raman amplifier including: a third light source that outputs another primary pump light propagating in a direction opposite to the propagation direction to the optical transmission line; and a rear control unit that controls the gain of the signal light by adjusting the power of the another primary pump light; A Raman amplification system comprising:

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