Optical transmission device, optical transmission system, and control method for a Raman amplifier

Abstract

To provide an optical transmission device that autonomously recovers from an OSC communication breakdown at the startup of bidirectional Raman excitation.SOLUTION: An optical transmission device includes an optical transmitting / receiving unit that transmits and receives signal light and monitoring light, a forward Raman amplifier provided at the transmitting end of the optical transmitting / receiving unit, a backward Raman amplifier provided at the receiving end of the optical transmitting / receiving unit, and a control unit that controls the backward Raman amplifier and the forward Raman amplifier. The control unit turns off the power of the forward Raman amplifier or the backward Raman amplifier according to a monitoring signal received from an optical transmission line when the forward Raman amplifier or the backward Raman amplifier is started up, and releases the off state of the forward Raman amplifier or the backward Raman amplifier after a predetermined period of time.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present disclosure relates to an optical transmission device, an optical transmission system, and a method for controlling a Raman amplifier.

Background Art

[0002] In a wavelength division multiplexing (WDM) optical transmission system, by compensating for the degradation of the optical signal to noise ratio (OSNR) due to the optical loss of the transmission path optical fiber, it is possible to cope with the expansion of the transmission distance and transmission capacity. One of the techniques for compensating for the degradation of OSNR is Raman amplification that utilizes the stimulated Raman scattering effect generated in an optical fiber. So far, the backward Raman excitation method, which supplies pump light in the opposite direction to the propagation of signal light from the downstream side of the transmission path where the signal light power decreases, has been mainly used. In next-generation WDM optical transmission, Raman amplification using a bidirectional excitation method that combines forward Raman excitation, which supplies pump light in the same direction as the propagation direction of signal light, and backward Raman excitation is expected to become the main technology (see, for example, Patent Document 1). Further high-speed and long-distance operation of the optical transmission path is expected with bidirectional excitation Raman amplification.

[0003] When the signal light is amplified by Raman amplification, the spontaneous Raman scattered light is also amplified at the same time. The amplified spontaneous Raman scattered light (ASS) becomes noise and reaches the downstream optical node together with the signal light. Since the signal light and the ASS noise cannot be distinguished, the pump-to-noise ratio is measured in advance at startup, and the amount of generated ASS noise when a certain excitation light source is turned on is recognized. Based on the obtained ASS noise and Raman gain, noise correction is correctly performed to control the signal level. Along with the measurement of the noise and gain of the Raman amplifier, the transmission and reception of instructions and information necessary for the measurement, and the transfer of the measurement results between the upstream optical node and the downstream optical node are performed using an optical supervisory channel (OSC).

Prior Art Documents

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-29231 [Patent Document 2] Japanese Patent Publication No. 2010-11384 [Patent Document 3] Japanese Patent Publication No. 2004-274265 [Overview of the project] [Problems that the invention aims to solve]

[0005] When measuring the noise and gain of a Raman amplifier, the excitation light of the Raman amplifier being measured and the Raman amplifier facing it across the optical transmission path are turned off. For example, when measuring the optical amplification characteristics of an upstream forward Raman amplifier, the downstream backward Raman amplifier is turned off. Until now, OSC communication between optical nodes was established even if either the backward or forward Raman excitation light was turned off or powered down. However, the demand for extending the optical transmission distance without relays is becoming stricter, and in some cases, the optical loss of the optical fiber in the transmission path between optical nodes can increase to about 50 dB. In this case, a new problem arises in which OSC communication is interrupted if the excitation light of the upstream or downstream Raman amplifier is turned off or powered down during the startup of the Raman amplifier. Generally, hardware for Ethernet communication using OSC-SFP (Small Form Factor Pluggable) is equipped with a Far-End Fault Indication (FEFI) function to maintain the normality of communication. This FEFI function causes both paths to go down if communication is lost in one direction. If OSC communication is lost in either path due to the upstream or downstream Raman excitation light being turned off or powered down, OSC communication will be lost in both directions. This problem becomes more pronounced as the optical transmission distance increases without an inline amplifier.

[0006] One objective of this disclosure is to provide an optical transmission device that autonomously recovers from OSC communication interruptions during the startup of bidirectional Raman excitation. [Means for solving the problem]

[0007] In one embodiment, the optical transmission device is An optical transceiver unit that transmits and receives signal light and monitoring light, A forward Raman amplifier is provided at the transmitting end of the optical transceiver, A back Raman amplifier is provided at the receiving end of the optical transceiver, The rear Raman amplifier and the control unit that controls the front Raman amplifier, It has, The control unit, upon startup of the forward Raman amplifier or the backward Raman amplifier, turns off the power of the forward Raman amplifier or the backward Raman amplifier according to a monitoring signal received from the optical transmission path, and after a predetermined time has elapsed, releases the off state of the forward Raman amplifier or the backward Raman amplifier. [Effects of the Invention]

[0008] This enables the realization of an optical transmission device that autonomously recovers from OSC communication interruptions during the startup of a Raman amplifier. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows new problems that arise as the transmission distance between optical nodes is extended. [Figure 2] This is a schematic diagram of an optical transmission system using an optical transmission device of an embodiment. [Figure 3] Figure 2 is a functional block diagram of the control unit implemented in the processor. [Figure 4] This figure shows the effect of bidirectional excitation Raman amplification. [Figure 5] This is a flowchart for starting up a bidirectional excitation Raman amplifier. [Figure 6] This is a schematic diagram of the noise measurement sequence for a back-Raman amplifier. [Figure 7]It is a flowchart of the operation in FIG. 6. [Figure 8] It is a schematic diagram of a Raman gain measurement sequence of a backward Raman amplifier. [Figure 9] It is a flowchart of the operation in FIG. 8. [Figure 10] It is a schematic diagram of a noise measurement sequence of a forward Raman amplifier. [Figure 11] It is a flowchart of the operation in FIG. 10. [Figure 12] It is a schematic diagram of a Raman gain measurement sequence of a forward Raman amplifier. [Figure 13] It is a flowchart of the operation in FIG. 12.

Mode for Carrying Out the Invention

[0010] Before describing the configuration and control method of the embodiment, referring to FIG. 1, new technical problems caused by the extension of the transmission distance between optical nodes will be described. Optical nodes A and B are interconnected by optical transmission lines 2 and 3. In the path from optical node A through optical transmission line 2 to optical node B, optical node A is the upstream node and optical node B is the downstream node. In the path from optical node B through optical transmission line 3 to optical node A, optical node B is the upstream node and optical node A is the downstream node. Focusing on the transmission from optical node A to B, consider a scenario where the noise of the forward Raman amplifier (denoted as "FwdRaman" in the figure) 213A of optical node A is measured at optical node B.

[0011] When starting up the forward Raman amplifier 213A, in order to know the pump-to-noise ratio of this forward Raman amplifier 213A, the noise of the forward Raman amplifier 213A is measured at optical node B. During the measurement, optical node A shuts down its own transmission amplifier (denoted as "AMPtx" in the figure) and instructs optical node B to shut down the backward Raman amplifier (denoted as "BwdRaman" in the figure) 215B. This shutdown instruction is sent to optical node B by the OSC signal. Optical node B shuts down the backward Raman amplifier 215B based on the OSC signal.

[0012] When the requirements for extending the lengths of the optical transmission paths 2 and 3 are not very strict, even if the rear Raman amplifier 215B or the front Raman amplifier 213A is shut down or powered down, the power of the OSC signal is maintained above the threshold value and is received at the optical node B. However, as the lengths of the optical transmission paths 2 and 3 are extended and the optical loss of the optical fiber increases, the power level of the OSC signal drops below the threshold value from the shutdown of the rear Raman amplifier 215B, and the OSC signal becomes interrupted. When the optical transmission path 2 goes into link-down, the optical transmission path 3 from the optical node B to the optical node A also goes into link-down. After measuring and recording the noise of the front Raman amplifier 213A, the optical node B cannot transfer the measurement result to the optical node A.

[0013] The optical node A is planned to set the pump power of the front Raman amplifier 213A and adjust the pump power based on the measured noise volume, but it cannot obtain the noise volume from the optical node B. Also, an instruction cannot be sent to the optical node B to release the shutdown of the rear Raman amplifier 215B. Even when the rear Raman amplifier is starting up, the OSC communication becomes interrupted due to the shutdown or power-down of the front Raman amplifier. Such a problem of OSC communication interruption accompanying the extension of the optical transmission path length has not been recognized so far, and a configuration for dealing with OSC communication interruption during the startup of the Raman amplifier has not been adopted.

[0014] In the embodiment, the optical node autonomously recovers from the state of OSC communication interruption during the startup of the Raman amplifier and realizes the startup of bidirectional Raman pumping. The following forms are examples for embodying the technical idea of the present disclosure and do not limit the disclosed content. The sizes, positional relationships, etc. of the components shown in each drawing may be exaggerated for easy understanding of the invention. The same name or reference numeral may be given to the same component or function, and redundant explanations may be omitted.

[0015] <Configuration of Optical Transmission Device and Optical Transmission System> Figure 2 is a schematic diagram of an optical transmission system 1 using optical transmission devices 10A and 10B of the embodiment. Optical transmission devices 10A and 10B function as optical nodes constituting the optical transmission system 1. Optical transmission devices 10A and 10B have the same configuration and are interconnected via optical transmission paths 2 and 3. In the path from optical transmission device 10A to optical transmission device 10B via optical transmission path 2, optical transmission device 10A is the upstream device and optical transmission device 10B is the downstream device. In the path from optical transmission device 10B to optical transmission device 10A via optical transmission path 3, optical transmission device 10B is the upstream device and optical transmission device 10A is the downstream device.

[0016] Since optical transmission devices 10A and 10B have the same configuration, the configuration of the optical transmission device 10A will be described using it as an example. Optical transmission device 10A includes an optical transceiver unit 11A that transmits and receives signal light and monitoring light, a forward Raman amplifier 13A provided at the transmitting end of the optical transceiver unit 11A, a backward Raman amplifier 15A provided at the receiving end of the optical transceiver unit 11A, and a control device 20A. The control device 20A is connected to the forward Raman amplifier 13A, the backward Raman amplifier 15A, and the optical transceiver unit 11A, and controls the overall operation of the optical transmission device 10A.

[0017] The control device 20A includes a processor 201, a memory 202, and a timer 203. The timer 203 may be implemented by a function of the processor 201. The processor 201 implements a control unit that controls the startup of the forward Raman amplifier 13A and the backward Raman amplifier 15A. The functional configuration of the control unit and the control operation during Raman amplification startup will be described later with reference to Figure 3 and subsequent figures.

[0018] The optical transceiver unit 11A has a transmitting amplifier 111, an optical splitter 112, an optical monitor (indicated as "PD" in the figure) 113, an optical coupler 14, and an OSC transmitter 115 as its transmitting side configuration. Multiple optical transceivers that handle signals of different wavelengths are connected to the optical transceiver unit 11A, and WDM signal light, which is multiplexed with optical signals of multiple wavelengths, is incident on the transmitting amplifier 111. The transmitting amplifier 111 is, for example, an erbium-doped fiber amplifier (EDFA), which amplifies the WDM signal light over a wide bandwidth. A portion of the WDM signal light amplified by the transmitting amplifier 111 is branched by the optical splitter 112 and input to the optical monitor 113. The optical monitor 113 is a photodetector such as a photodiode (PD) and measures the input power of the WDM signal light to the optical transmission path 2.

[0019] The OSC transmitter 115 generates an OSC signal containing monitoring information. The monitoring information is generated by intensity-modulating light of a different wavelength than the WDM signal light. The wavelength of the monitoring light is set adjacent to the WDM signal band, for example, on the shorter wavelength side of the WDM signal band. The monitoring light output from the OSC transmitter 115 is input to the same optical fiber that transmits the WDM signal via the optical coupler 114 and output to the optical transmission path 2.

[0020] The forward Raman amplifier 13A includes an excitation light source 131 and an optical coupler 132. By amplifying the WDM signal light output to the optical transmission path 2 with the forward Raman amplifier 13A, the input level of the optical amplifier 120 is increased, and the degradation of OSNR due to ASE generated in the optical amplifier 120 is mitigated.

[0021] The excitation light source 131 has multiple light source elements (e.g., i laser elements, i being an integer greater than or equal to 2) of different wavelengths to amplify the signal light across the bandwidth of the WDM signal. The excitation light from each light source element generates a Raman gain on the lower frequency side (longer wavelength side) by a Stokes shift amount corresponding to the excitation wavelength. The sum of the gains of the individual light source elements becomes the gain of the forward Raman amplifier 13A. For example, to amplify a WDM signal light in the range of 1520 to 1625 nm, the excitation light source 131 outputs i excitation lights with a wide linewidth wavelength range in the range of 1420 to 1500 nm. The excitation lights of different wavelengths output from each light source element may be combined, for example, by a multiplexer, and then combined into the WDM signal light and monitoring light by an optical coupler 132. The excitation light, along with the signal light and monitoring light, is incident on the optical transmission path 2, and the WDM signal light and monitoring light are amplified by stimulated Raman scattering occurring in the optical fiber.

[0022] At the receiving end of the optical transmission device 10A, WDM signal light and monitoring light are received from the optical transmission device 10B via the optical transmission path 3. The WDM signal light from the optical transmission device 10B is amplified by the transmitting amplifier 111 and the forward Raman amplifier 13B of the optical transmission device 10B. The backward Raman amplifier 15A, located at the receiving end of the optical transmission device 10A, injects excitation light into the optical transmission path 3 in the opposite direction to the propagation direction of the signal light and monitoring light. As a result, the signal light and monitoring light are Raman amplified before they enter the receiving end of the optical transceiver unit 11A, suppressing the degradation of the OSNR. Compared to forward Raman excitation, backward Raman excitation is less prone to gain saturation, and the signal light and monitoring light can be amplified at the output end of the optical transmission path 3, flattening the optical loss in the propagation direction.

[0023] The back-Raman amplifier 15A includes an excitation light source 151 and an optical filter 152. The excitation light source 151 has multiple light source elements (e.g., j laser elements, where j is an integer greater than or equal to 2) of different wavelengths to amplify the signal light (and monitoring light) over the bandwidth of the WDM signal propagating through the optical transmission path 3. The excitation light source 151 includes, for example, j laser elements with oscillation wavelengths in the range of 1400 to 1500 nm. The number i of light source elements in the excitation light source 131 and the number j of light source elements in the excitation light source 151 may be the same or different. The light output from each light source element of the excitation light source 151 may be combined in a multiplexer and incident on the optical filter 152. The optical filter 152 reflects the excitation light in the direction of the optical transmission path 3. The excitation light is incident on the optical transmission path 2 in the opposite direction to the propagation direction, causing Raman amplification in the optical fiber.

[0024] The signal light and monitoring light, amplified by back-Raman excitation, pass through the optical filter 152 of the back-Raman amplifier 15A and are incident on the receiving side of the optical transceiver unit 11A. The optical transceiver unit 11A has, as its receiving side configuration, an optical filter 116, an OSC receiver 117, an optical splitter 118, an optical monitor (indicated as "PD" in the figure) 119, and a receiving amplifier 120. The OSC receiver 117 on the receiving side and the OSC transmitter 115 on the transmitting side form the OSC processing unit 110A. The monitoring light extracted by the optical filter 116 is supplied to the OSC receiver 117, demodulated, and monitoring information is extracted. A portion of the signal light that has passed through the optical filter 116 is split by the optical splitter 118 and monitored by the optical monitor 119 along with the noise contained in the signal light. The signal light that has passed through the optical splitter 118 is amplified by the receiving amplifier 120.

[0025] The receiving amplifier 120 is, for example, an EDFA, which generates ASE, but the WDM signal light is amplified by a back-Raman amplifier 15A, which has lower noise compared to ASE, before it enters the receiving amplifier 120, thus suppressing the degradation of OSNR. The receiving side of the optical transceiver 11A is connected to the receiving circuits of multiple optical transceivers that handle signals of different wavelengths, and the WDM signal light amplified by the receiving amplifier 120 is separated into optical signals of each wavelength and supplied to the corresponding optical transceivers.

[0026] The optical transmitting / receiving unit 11B, forward Raman amplifier 13B, backward Raman amplifier 15B, and control device 20B of the optical transmission device 10B have the same configuration as the optical transmitting / receiving unit 11A, forward Raman amplifier 13A, backward Raman amplifier 15A, and control device 20A described above, and perform the same operations in the reverse direction.

[0027] <Functions of the control unit> Figure 3 is a functional block diagram of the control unit 200 implemented by the processor 201 of the control device 20A. The control unit 200 includes a forward Raman control unit 205, a backward Raman control unit 206, a transmit amplifier control unit 207, a receive amplifier control unit 208, and a timer control unit 209. The forward Raman control unit 205 sets the excitation power of the excitation light source 131 of the forward Raman amplifier 13A and controls the on / off operation and excitation ratio of each light source element. In particular, it controls the excitation power and excitation ratio of the forward Raman amplifier 13A based on power information from the upstream (e.g., optical transmission device 10A) and power information from the downstream (e.g., optical transmission device 10B) so that a flat Raman gain spectrum is obtained across the WDM band on the downstream side.

[0028] The back-Raman control unit 206 sets the excitation power of the excitation light source 151 of the back-Raman amplifier 15A, and controls the on / off operation and excitation ratio of each light source element. In particular, based on power information from the upstream (e.g., optical transmission device 10B) and power information from the downstream (e.g., optical transmission device 10A), it controls the excitation power and excitation ratio of the back-Raman amplifier 15A so that a flat Raman gain spectrum is obtained across the WDM band on the downstream side.

[0029] The transmitting amplifier control unit 207 sets the gain of the transmitting amplifier 111 and controls the timing for shutting down the transmitting amplifier 111. The receiving amplifier control unit 208 sets the gain of the receiving amplifier 120 and controls the timing for its shutdown. The timer control unit 209 activates the timer 203 when the forward Raman amplifier 13A is turned off or powered down. After a certain period of time, when the timer 203 expires, the timer control unit 209 notifies the forward Raman control unit 205 that time is up. Upon receiving the time-up information, the forward Raman control unit 205 turns on or powers up the forward Raman amplifier 13A. The timer control unit 209 also activates the timer 203 when the rear Raman amplifier 15A is turned off or powered down, and when the timer 203 expires, it notifies the rear Raman control unit 206 of time-up information. Upon receiving the time-up information, the rear Raman control unit 206 turns on or powers up the rear Raman amplifier 15A.

[0030] Figure 4 shows the effect of bidirectional excitation Raman amplification. The horizontal arrows represent the propagation direction, and the vertical axis represents the signal light power. The power of the signal light incident at the input end of the propagation path is amplified by forward Raman excitation. Subsequently, as the signal light travels along the transmission path, the power of the signal light decreases due to optical losses in the transmission path. Just before reaching the output end of the transmission path, the power of the signal light increases again due to backward Raman excitation. This flattens the loss characteristics of the transmission optical fiber with respect to the propagation direction, improving the OSNR and extending the transmission distance.

[0031] When Raman amplification is not performed, the power of the signal light decreases as it propagates along the transmission path. In the case of back-Raman amplification only, the signal light power recovers to some extent near the output terminal, so the OSNR can be improved when the transmission distance is not very long. However, when the transmission path becomes long, the optical loss increases, and the effect of improving the OSNR becomes insufficient. In this embodiment, bidirectional Raman excitation is employed, and the OSC communication interruption caused by shutdown during Raman amplifier startup is autonomously recovered.

[0032] <Startup process for a bidirectional excitation Raman amplifier> Figure 5 is a flowchart of the startup of a bidirectional excitation Raman amplifier. First, startup conditions are determined according to the span loss size and network configuration (S11). For example, the startup conditions are determined by whether OSC communication can be established even if the excitation light of the Raman amplifier is off or at low power. Here, we assume that the transmission path length has been extended and OSC communication is not possible unless both the forward Raman amplifier 13A and the backward Raman amplifier 15B (or the forward Raman amplifier 13B and the backward Raman amplifier 15A) facing each other across the transmission path are turned on.

[0033] With optical transmission device 10A as the upstream node and optical transmission device 10B as the downstream node, the ASS noise and Raman gain of the forward Raman amplifier 13A and the backward Raman amplifier 15B are measured. The ASS noise of the backward Raman amplifier 15B is measured by optical transmission device 10B (S12). At this time, the transmitting amplifier 111 of optical transmission device 10A and the forward Raman amplifier 13A are turned off. At the timing when the forward Raman amplifier 13A is turned off, the timer 203 of optical transmission device 10A is activated, and after a certain period of time has elapsed, the forward Raman excitation light, which had been turned off, is turned on due to the timeout. Each time the ASS noise of the multiple light source elements included in the excitation light source 151 of the backward Raman amplifier 15B is measured sequentially, the forward Raman amplifier 13A is turned off and then recovered from the off state repeatedly. The time set in the timer 203 for the recovery of the forward Raman amplifier 13A is longer than the time required to measure the noise of each light source element of the excitation light source 151 of the opposing rear Raman amplifier 15B.

[0034] The Raman gain of the back-Raman amplifier 15B is measured by the optical transmission device 10B (S13). The Raman gain is obtained by subtracting the ASS noise of the back-Raman amplifier 15B from the increase in signal optical power due to back-Raman excitation. At this time, the front-Raman amplifier 13A of the optical transmission device 10A is turned off. When the front-Raman excitation is turned off, the timer 203 of the optical transmission device 10A is activated, and after a certain period of time, the front-Raman excitation is turned on due to the timeout. Each time the Raman gain of the multiple light source elements included in the excitation light source 151 of the back-Raman amplifier 15B is measured sequentially, the front-Raman amplifier 13A is turned off and then recovered from the off state repeatedly. The time set in the timer 203 for the recovery of the front-Raman amplifier 13A is longer than the time required to measure the gain of each light source element of the excitation light source 151 of the opposing back-Raman amplifier 15B.

[0035] Once the measurement of the ASS noise and Raman gain of the back-Raman amplifier 15B is complete, the noise generated by Raman amplification in the transmission path due to the excitation light from the excitation light source 151 is corrected based on the acquired noise information. In addition, the set values ​​of the excitation light power of the j light source elements of the excitation light source 151 are adjusted. This completes the startup of back-Raman excitation.

[0036] The ASS noise of the forward Raman amplifier 13A is measured by the optical transmission device 10B (S14). At this time, the transmitting amplifier 111 of the optical transmission device 10A and the backward Raman amplifier 15B of the optical transmission device 10B are turned off. When the backward Raman excitation is turned off, the timer 203 of the optical transmission device 10B is activated, and after a certain period of time, the backward Raman excitation is turned on due to the timeout. Each time the ASS noise of the multiple light source elements included in the excitation light source 131 of the forward Raman amplifier 13A is measured sequentially, the backward Raman amplifier 15B is turned off and then recovered from the off state repeatedly. The time set in the timer 203 for the recovery of the backward Raman amplifier 15B is longer than the time required to measure the noise of each light source element of the excitation light source 131 of the opposing forward Raman amplifier 13A.

[0037] The Raman gain of the forward Raman amplifier 13A is measured by the optical transmission device 10B (S15). At this time, the backward Raman amplifier 15B of the optical transmission device 10B is turned off. When the backward Raman excitation is turned off, the timer 203 of the optical transmission device 10B is activated, and after a certain period of time, the backward Raman excitation is turned on due to the timeout. Each time the Raman gain due to the excitation light of the multiple light source elements included in the excitation light source 131 of the forward Raman amplifier 13A is measured sequentially, the backward Raman amplifier 15B is turned off and then recovered from the off state repeatedly. The time set in the timer 203 for the recovery of the backward Raman amplifier 15B is longer than the time required to measure the gain of each light source element of the excitation light source 131 of the opposing forward Raman amplifier 13A.

[0038] Once the measurement of the ASS noise and Raman gain of the forward Raman amplifier 13A is complete, the set values ​​of the excitation light power of the i light source elements of the excitation light source 131 are corrected based on the acquired noise information, and the startup of forward Raman excitation is completed.

[0039] Once the forward and backward Raman excitations have started up, both the forward Raman amplifier 13A and the backward Raman amplifier 15B are turned on to start up the transmitting amplifier 111 of the optical transmission device 10A and the receiving amplifier 120 of the optical transmission device 10B (S16). This completes the startup process and the system enters a signal communication state.

[0040] In the bidirectional Raman startup of this embodiment, when measuring the noise and gain of the forward Raman amplifier 13A and the backward Raman amplifier 15B, each time either Raman amplifier is turned off, it autonomously recovers from the off state after a predetermined time has elapsed. The time during which OSC communication is interrupted is kept to a minimum, and the transmission and reception of information necessary for Raman startup are carried out smoothly via OSC.

[0041] <Noise measurement of back-Raman excitation> Figure 6 is a schematic diagram of the noise measurement sequence for a back-Raman amplifier, and Figure 7 is a flowchart of the operation in Figure 6. The ASS noise of back-Raman excitation in optical transmission line 2 is measured with optical transmission device 10A as the upstream node and optical transmission device 10B as the downstream node. Measuring the ASS noise of back-Raman excitation in optical transmission line 3 is the same process as measuring the noise of back-Raman excitation in optical transmission line 2, just with the operation reversed. In Figure 6, the configurations of optical transmission devices 10A and 10B are simplified, but as shown in Figure 2, the forward Raman amplifier 13A and the back-Raman amplifier 15A are connected to the control device 20A by control lines. Similarly, the forward Raman amplifier 13B and the back-Raman amplifier 15B are connected to the control device 20B by control lines.

[0042] The optical transmission device 10B, which measures the noise of the back-Ramin amplifier 15B, instructs the optical transmission device 10A to shut down the transmitting amplifier 111 (S31). Specifically, the control device 20B of the optical transmission device 10B instructs the OSC processing unit 110B to generate and transmit an OSC monitoring signal including the shutdown instruction. The OSC transmitting unit 115 generates an OSC monitoring signal including the shutdown instruction and transmits the monitoring signal over the optical transmission path 3. The OSC processing unit 110A of the optical transmission device 10A demodulates the monitoring signal including the shutdown instruction and supplies the demodulated information to the control device 20A. The control device 20A shuts down the transmitting amplifier 111 in accordance with the shutdown instruction (S21). This process corresponds to operation (1) in Figure 6.

[0043] The optical transmission device 10B instructs the optical transmission device 10A to shut down the forward Raman amplifier 13A (S32). This shutdown instruction is also transmitted via OSC. The shutdown instruction for the transmitting amplifier 111 (S31) and the shutdown instruction for the forward Raman amplifier 13A (S32) may be given simultaneously. The control device 20A of the optical transmission device 10A shuts down the forward Raman amplifier 13A and starts the timer according to the shutdown instruction (S22). This process corresponds to operation (2) in Figure 6. When the forward Raman amplifier 13A is shut down, OSC communication between the optical transmission devices 10A and 10B is lost.

[0044] The control device 20B of the optical transmission device 10B sets the power of the excitation light for each light source element of the excitation light source 151 of the back-Raman amplifier 15B (S33) and measures the noise light of the back-Raman excitation (S34). The noise light of the back-Raman excitation may be detected by a photodetector provided inside the back-Raman amplifier 15B, or by using the optical monitor 119 of the optical transceiver unit 11B. Steps S33 and S34 are repeated until the measurement of the noise light for all j light source elements is completed (YES in S35) to create a back-Raman noise profile. This process corresponds to operations (3), (4), and (5) in Figure 6.

[0045] The control device 20A of the optical transmission device 10A waits for the time set in the timer to elapse (S23), and when the time is up, it turns on the forward Raman amplifier 13A (S26). This process corresponds to operation (6) in Figure 6. With the forward Raman amplifier 13A turned on, OSC communication between the optical transmission devices 10A and 10B is restored. The optical transmission devices 10A and 10B confirm the restoration of OSC in their respective OSC processing units 110A and 110B (S25 and S26). The restoration of OSC is confirmed by checking the power level of the monitoring light or the validity of the monitoring packets. This process corresponds to operation (7) in Figure 6.

[0046] In this way, the time during which the forward Raman amplifier 13A is off during noise measurement of back Raman excitation is minimized, OSC communication is quickly restored, and the startup operation of bidirectional Raman excitation is performed smoothly.

[0047] <Gain measurement of back-Raman excitation> Figure 8 is a schematic diagram of the gain measurement sequence for the back-Raman amplifier, and Figure 9 is a flowchart of the operation shown in Figure 8. The Raman gain of the back-Raman excitation in the optical transmission path 2 is measured with optical transmission device 10A as the upstream node and optical transmission device 10B as the downstream node. In Figure 8, the configurations of optical transmission devices 10A and 10B are simplified, but the forward Raman amplifier 13A and the back-Raman amplifier 15A are connected to the control device 20A by control lines, and the forward Raman amplifier 13B and the back-Raman amplifier 15B are connected to the control device 20B by control lines.

[0048] The optical transmission device 10B, which measures the gain of the back-Ramin amplifier 15B, instructs the optical transmission device 10A to release the shutdown of the transmitting amplifier 111 (S51). This shutdown release instruction is sent via OSC. The OSC processing unit 110A of the optical transmission device 10A demodulates the monitoring signal including the shutdown release instruction and supplies the demodulated result to the control device 20A. The control device 20A releases the shutdown of the transmitting amplifier 111 in accordance with the shutdown release instruction (S21). This process corresponds to operation (11) in Figure 8.

[0049] The optical transmission device 10A transfers the transmission power information of the WDM signal light to the optical transmission device 10B (S42). This process corresponds to operation (12) in Figure 8. The control device 20B of the optical transmission device 10B records the received transmission power information in the memory 202 (S52). The control device 20B sends a shutdown instruction for the forward Raman amplifier 13A to the optical transmission device 10A via the OSC processing unit 110B (S32). The control device 20A of the optical transmission device 10A shuts down the forward Raman amplifier 13A and starts the timer according to the shutdown instruction (S43). This process corresponds to operation (13) in Figure 8.

[0050] The control device 20B of the optical transmission device 10B sets the excitation optical power of each light source element of the excitation light source 151 of the back-Raman amplifier 15B (S54) and measures the gain of back-Raman excitation (S55). Steps S54 and S55 are repeated until the gain measurement is completed for all light source elements (YES in S56) to create a back-Raman gain profile. These processes correspond to operations (14), (15), and (16) in Figure 8.

[0051] Meanwhile, the control device 20A of the optical transmission device 10A waits for the time set by the timer (S44), and turns on the forward Raman amplifier 13A when the time is up (S45). This process corresponds to operation (17) in Figure 8. With the shutdown of the forward Raman amplifier 13A released, OSC communication between the optical transmission devices 10A and 10B is restored. The optical transmission devices 10A and 10B confirm the restoration of OSC (S46 and S57). This process corresponds to operation (18) in Figure 8.

[0052] In this way, the time during which the forward Raman amplifier is turned off during gain measurement of back Raman excitation is minimized, OSC communication is quickly restored, and the startup operation of bidirectional Raman excitation is performed smoothly.

[0053] <Noise measurement of forward-Raman excitation> Figure 10 is a schematic diagram of the noise measurement sequence for a forward Raman amplifier, and Figure 11 is a flowchart of the operation shown in Figure 10. The ASS noise of forward Raman excitation in optical transmission path 2 is measured with optical transmission device 10A as the upstream node and optical transmission device 10B as the downstream node. Measuring the ASS noise of forward Raman excitation in optical transmission path 3 is the same process as measuring the noise of forward Raman excitation in optical transmission path 2, but the operation is reversed. In Figure 10, the configurations of optical transmission devices 10A and 10B are simplified, but the forward Raman amplifier 13A and the backward Raman amplifier 15A are connected to the control device 20A by control lines, and the forward Raman amplifier 13B and the backward Raman amplifier 15B are connected to the control device 20B by control lines.

[0054] The control device 20A of the optical transmission device 10A shuts down the transmitting amplifier 111 (S61) and sends a shutdown instruction for the backward Raman amplifier 15B to the optical transmission device 10B via the OSC processing unit 110A (S62). The shutdown instruction for the backward Raman amplifier 15B may also include an instruction to start measuring noise caused by forward Raman excitation. These processes correspond to operations (31) and (32a) in Figure 10. Simultaneously with or around the transmission of the shutdown instruction, the excitation optical power of each light source element of the excitation light source 131 of the forward Raman amplifier 13A is set (S63).

[0055] When the control device 20B of the optical transmission device 10B receives a shutdown instruction from the OSC processing unit 110B, it shuts down the back-Raman amplifier 15B and starts the timer (S71). This process corresponds to the operation (32b) in Figure 10. At this stage, back-Raman excitation is turned off, and OSC communication is lost.

[0056] The optical transmission device 10B records the noise power caused by forward Raman excitation (S72) and waits for the time to expire (S73). This process corresponds to operation (33) in Figure 10. The noise caused by forward Raman excitation may also be measured by the optical monitor 119 of the optical transceiver unit 11B. When the predetermined time has elapsed and the time has expired, the control device 20B releases the shutdown of the rear Raman amplifier 15B (S74). This process corresponds to operation (34) in Figure 10. As a result, OSC communication between the optical transmission devices 10A and 10B is restored.

[0057] The OSC processing unit 110A of optical transmission device 10A and the OSC processing unit 110B of optical transmission device 10B each confirm the recovery of the OSC (S64 and S75). This process corresponds to operation (35) in Figure 10. Upon OSC recovery, optical transmission device 10B transfers noise information caused by forward Raman excitation to optical transmission device 10A (S76). This process corresponds to operation (36) in Figure 10.

[0058] When the control device 20A of the optical transmission device 10A acquires noise information caused by forward Raman excitation (S65), it corrects the noise of each light source element of the excitation light source 131 and adjusts the excitation optical power. S63 and S65 are repeated until noise correction and power adjustment for all light source elements are completed (YES in S66). These processes correspond to operations (37) and (38) in Figure 10. In this way, the time during which the backward Raman amplifier 15B is in the off state during noise measurement of forward Raman excitation is minimized, OSC communication is quickly restored, and the startup operation of bidirectional Raman excitation is performed without interruption.

[0059] <Gain measurement of forward Raman excitation> Figure 12 is a schematic diagram of the gain measurement sequence for the forward Raman amplifier, and Figure 13 is a flowchart of the operation shown in Figure 12. The Raman gain of forward Raman excitation in the optical transmission path 2 is measured with optical transmission device 10A as the upstream node and optical transmission device 10B as the downstream node. In Figure 12, the configurations of optical transmission devices 10A and 10B are simplified, but the forward Raman amplifier 13A and the backward Raman amplifier 15A are connected to the control device 20A by control lines, and the forward Raman amplifier 13B and the backward Raman amplifier 15B are connected to the control device 20B by control lines.

[0060] The optical transmission device 10A releases the shutdown of the transmitting amplifier 111 (S81), records the transmission power of the WDM signal light, and transmits the WDM signal (S82). These processes correspond to operations (41) and (42) in Figure 12. The control device 20A of the optical transmission device 10A sends a shutdown instruction for the backward Raman amplifier 15B to the optical transmission device 10B via the OSC processing unit 110A (S83). This process corresponds to operation (43a) in Figure 12. Simultaneously with or around the transmission of the shutdown instruction, the power of each light source element of the excitation light source 131 of the forward Raman amplifier 13A is set (S84).

[0061] The control device 20B of the optical transmission device 10B shuts down the rear Raman amplifier 15B and starts the timer in accordance with the shutdown instruction (S91). This process corresponds to operation (43b) in Figure 12. At this point, OSC communication between optical transmission devices 10A and 10B is lost. The optical transmission device 10B records the received power of the previously received WDM signal light (S92) and waits for the time set in the timer (S93). Upon timeout, the control device 20B releases the shutdown of the rear Raman amplifier 15B (S94). These processes correspond to operations (44) and (45) in Figure 12.

[0062] The OSC communication is restored when the shutdown of the rear Raman amplifier 15B is released. OSC restoration is confirmed between optical transmission devices 10A and 10B (S85 and S95). This process corresponds to operation (46) in Figure 12. Optical transmission device 10B transfers the measured signal reception power to optical transmission device 10A (S96). This process corresponds to operation (47) in Figure 12. Optical transmission device 10A calculates the gain of each light source element of the excitation light source 131 based on the received power at optical transmission device 10B (S86) and adjusts the excitation light power (S84). S84 and S86 are repeated until the gain calculation and excitation light power adjustment are completed for all light source elements (YES in S87). These processes correspond to operations (48) and (49) in Figure 12.

[0063] In this way, the time during which the rear Raman amplifier 15B is off during gain measurement of forward Raman excitation is minimized, OSC communication is quickly restored, and the startup operation of bidirectional Raman excitation is performed smoothly.

[0064] Although the disclosure has been described above based on a specific configuration and method, the disclosure is not limited to the above-described configuration and operation examples. The configuration and method of the disclosure are also applicable when the power of either the back-Raman excitation or the forward-Raman excitation decreases, causing OSC communication to be lost. In this application example, the configuration may be such that when the power level of the OSC monitoring light falls below a certain level, the power is automatically increased after a predetermined time has elapsed. The measurement of the amplification characteristics of the Raman amplifier is not limited to ASS noise and Raman gain, but other optical amplification characteristics based on stimulated Raman scattering may also be measured. Instead of providing a timer in the control device, the timer function may be implemented by the processor's functions, or a timer may be provided inside the Raman amplifier. In any configuration, it is possible to release the shutdown or power-down state of either of the two Raman amplifiers facing each other across the optical transmission path. Even if the optical transmission path length is extended, the system can autonomously recover from the OSC communication interruption state during the startup of bidirectional Raman excitation and smoothly transmit and receive the information necessary for startup. [Explanation of Symbols]

[0065] 1. Optical transmission system 2, 3 Optical transmission paths 10A, 10B Optical Transmission Equipment 11A, 11B Optical Transceiver Unit 13A, 13B Forward Raman Amplifier 15A, 15B Backward Raman Amplifier 110A, 110B OSC Processing Unit 111 Transmitter Amplifier 115 OSC Transmitter 117 OSC Receiver 120 Receiving Amplifier 131, 151 Excitation light source 20A, 20B control devices 201 Processor 202 memory 203 Timer

Claims

1. An optical transceiver unit that transmits and receives signal light and monitoring light, A forward Raman amplifier is provided at the transmitting end of the optical transceiver, A back Raman amplifier is provided at the receiving end of the optical transceiver, The rear Raman amplifier and the control unit that controls the front Raman amplifier, It has, The control unit, when one of the forward Raman amplifiers or the rear Raman amplifiers is started up, turns off the power of the other forward Raman amplifier or the rear Raman amplifier in accordance with a monitoring signal received from the optical transmission path, and after a predetermined time has elapsed, releases the off state of the other forward Raman amplifier or the rear Raman amplifier. Optical transmission device.

2. The predetermined time is longer than the time required to measure the optical amplification characteristics of the forward Raman amplifier and the backward Raman amplifier facing the other of the two Raman amplifiers across the optical transmission path. The optical transmission device according to claim 1.

3. When the control unit receives a shutdown instruction for the forward Raman amplifier from the second optical transmission device located across the optical transmission path, it shuts down the forward Raman amplifier and starts a timer, and when the timer expires, it releases the shutdown of the forward Raman amplifier. The optical transmission device according to claim 1.

4. When the control unit receives a shutdown instruction for the back-Raman amplifier from the second optical transmission device located across the optical transmission path, it shuts down the back-Raman amplifier, starts a timer, and records the noise power or received signal power caused by the forward-Raman excitation of the second optical transmission device. The optical transmission device according to claim 1.

5. Upon the expiration of the timer, the control unit releases the shutdown of the rear Raman amplifier and notifies the second optical transmission device of the noise power or the received signal power. The optical transmission device according to claim 4.

6. A first optical transmission device having a first forward Raman amplifier and a first backward Raman amplifier, A second optical transmission device having a second forward Raman amplifier and a second backward Raman amplifier, and connected to the first optical transmission device by an optical transmission path, Includes, When initiating bidirectional Raman excitation between the first optical transmission device and the second optical transmission device, one of the first and second optical transmission devices receives a monitoring signal from the other via the optical transmission path, turns off the Raman amplifier specified by the monitoring signal, and autonomously turns on the Raman amplifier after a predetermined time has elapsed. Optical transmission system.

7. The predetermined time is longer than the time required to measure the optical amplification characteristics of the opposing Raman amplifier, which is located across the optical transmission path from the Raman amplifier. The optical transmission system according to claim 6.

8. When measuring the optical amplification characteristics of the second backward Raman amplifier, the first optical transmission device receives a shutdown instruction for the first forward Raman amplifier from the second optical transmission device, shuts down the first forward Raman amplifier and starts a timer, and releases the shutdown of the first forward Raman amplifier when the timer expires. The optical transmission system according to claim 6.

9. When measuring the optical amplification characteristics of the first forward Raman amplifier, the second optical transmission device receives a shutdown instruction for the second backward Raman amplifier from the first optical transmission device, shuts down the second backward Raman amplifier, starts a timer, and records the noise power or received signal power caused by the first forward Raman amplifier. The optical transmission system according to claim 6.

10. Upon the expiration of the timer, the second optical transmission device releases the shutdown of the second rear Raman amplifier and notifies the first optical transmission device of the noise power or the received signal power. The optical transmission system according to claim 9.

11. In an optical transmission device having a forward Raman amplifier and a backward Raman amplifier, When one of the forward Raman amplifier and the backward Raman amplifier is started up, the power of the other forward Raman amplifier and the backward Raman amplifier is turned off according to a monitoring signal received from the optical transmission path. After a predetermined time has elapsed, the optical transmission device autonomously releases the off state of the other of the forward Raman amplifier and the backward Raman amplifier. Control method for Raman amplifiers.

12. The predetermined time is longer than the time required to measure the optical amplification characteristics of the forward Raman amplifier and the backward Raman amplifier facing the other of the two Raman amplifiers across the optical transmission path. A method for controlling a Raman amplifier according to claim 11.

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