Raman amplifier, wavelength division multiplexing optical transmission system, and method of controlling excitation light of raman amplifier

Abstract

To appropriately control a gain of Raman amplification in each of a case where light is transmitted in all wavelength bands and a case where light is transmitted only in a part of wavelength bands among predetermined wavelength bands.SOLUTION: When the signal unit does not receive the optical signal of the first band among the plurality of bands, the control unit 14 controls the excitation light source 111 to output the excitation light having a power smaller than that when the optical signal of the first band is received or to stop the output of the excitation light. The excitation light source 111 is controlled so as to output the excitation light of power smaller than the power of the excitation light outputted when the optical signal of the first band is received, and the excitation light source 112 is controlled so as to output the excitation light of power larger than the power of the excitation light outputted when the optical signal of the first band is received.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a Raman amplifier that amplifies an optical signal in wavelength-division multiplexed optical transmission, a wavelength-division multiplexed optical transmission system, and a method for controlling pump light in a Raman amplifier. [Background technology]

[0002] Raman amplification is known as a method for amplifying an optical signal during transmission over long distances. For example, techniques related to Raman amplification have been proposed (see, for example, Patent Document 1). Patent Document 1 discloses that broadband Raman amplification is performed in a wavelength division multiplexing optical transmission system. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7077794 Summary of the Invention [Problem to be solved by the invention]

[0004] In wavelength-multiplexed optical transmission, the number of wavelengths and wavelength allocation of signal light may change during operation. The Raman amplifier described in Patent Document 1 measures a first optical power including a first signal light in a first wavelength band and ASS light (noise light), subtracts the first ASS optical power in the first wavelength band and the ASS optical power in a second wavelength band calculated based on a model formula from the measured first optical power to obtain the power of the first signal light, and adjusts the power of the first pump light or the second pump light based on the difference between the power of the first signal light and the power of a first target signal light. However, it is unclear how to control the power of the first pump light or the second pump light, and adjusting the power of the first pump light or the second pump light is not easy. It is necessary to take measures such as changing the target value of the total signal light power in response to changes in the number of wavelengths and the wavelength allocation.

[0005] An object of the present disclosure is to appropriately control the gain of Raman amplification in both the case where light is transmitted in all wavelength bands of a predetermined wavelength band and the case where light is transmitted in only a portion of the wavelength bands. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, there is provided a Raman amplifier, a wavelength-multiplexed optical transmission system, and a method for controlling pumping light in a Raman amplifier. The Raman amplifier is capable of receiving optical signals including wavelengths in multiple bands, and includes a signal processing unit for receiving the optical signals and a control unit. When the signal processing unit is not receiving an optical signal in a first band among the multiple bands, the control unit controls a pumping light source that outputs pumping light for the first band and also controls a pumping light source that outputs pumping light for other bands. Alternatively, when the signal processing unit is receiving an optical signal in the first band among the multiple bands, the control unit controls a pumping light source for the first band and also controls a pumping light source that outputs pumping light for the other bands. [Effects of the Invention]

[0007] According to the present disclosure, Raman amplification can be performed appropriately both when transmitting light in all wavelength bands of a predetermined wavelength band and when transmitting light in only a portion of the wavelength bands. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating an example of a wavelength multiplexing optical transmission system according to a first embodiment. [Figure 2] 1 is a block diagram showing the functions of a Raman amplifier according to a first embodiment. [Figure 3] 3 is a block diagram showing the functions of a control unit according to the first embodiment. FIG. [Figure 4] 10 is a diagram illustrating a specific example in which a C-band pumping light source and an L-band pumping light source according to the first embodiment output pumping light. FIG. [Figure 5] 10 is a diagram illustrating a comparative example in which a pumping light source for C band outputs pumping light. FIG. [Figure 6]10 is a diagram illustrating a specific example in which a C-band pumping light source according to the first embodiment outputs pumping light. FIG. [Figure 7] FIG. 10 is a diagram illustrating a comparative example in which an L-band pumping light source outputs pumping light. [Figure 8] 10 is a diagram illustrating a specific example in which an L-band pumping light source according to the first embodiment outputs pumping light. FIG. [Figure 9] FIG. 10 is a block diagram showing the functions of a Raman amplifier according to a third embodiment. [Figure 10] FIG. 10 is a block diagram showing the functions of a Raman amplifier according to a fourth embodiment. [Figure 11] FIG. 10 is a block diagram showing the functions of a Raman amplifier according to a fifth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments will be described with reference to the drawings. The following embodiments are merely examples, and various modifications are possible within the scope of the present disclosure.

[0010] Embodiment 1 1 is a diagram illustrating an example of a wavelength-multiplexed optical transmission system according to a first embodiment. The wavelength-multiplexed optical transmission system includes Raman amplifiers 100a and 100b, a wavelength-multiplexed optical transmitter 200, and a wavelength-multiplexed optical receiver 300. The Raman amplifiers 100a and 100b, the wavelength-multiplexed optical transmitter 200, and the wavelength-multiplexed optical receiver 300 communicate with each other via an optical fiber transmission line. For example, an optical signal (also referred to as a wavelength-multiplexed optical signal, for example) transmitted by the wavelength-multiplexed optical transmitter 200 is received by the wavelength-multiplexed optical receiver 300 via the Raman amplifiers 100a and 100b and the optical fiber transmission line.

[0011] Furthermore, a plurality of Raman amplifiers 100 may be present between the Raman amplifier 100a and the Raman amplifier 100b. That is, the optical signal may pass through a plurality of Raman amplifiers 100 and optical fiber transmission lines in the dotted line portion of FIG. 1 . Here, the Raman amplifiers 100a and 100b are collectively referred to as the Raman amplifier 100. Thus, as shown in FIG. 1 , the wavelength multiplexing optical transmission system of the first embodiment includes a wavelength multiplexing optical transmitter 200 capable of transmitting optical signals including wavelengths in a plurality of bands, an optical fiber transmission line having one end connected to the wavelength multiplexing optical transmitter 200 and through which the optical signal transmitted from the wavelength multiplexing optical transmitter 200 propagates, and a wavelength multiplexing optical receiver 300 connected to the other end of the optical fiber transmission line and receiving the optical signal propagated through the optical fiber transmission line. Furthermore, as also shown in FIG. 1, the wavelength multiplexing optical transmission system of the first embodiment is provided with a plurality of Raman amplifiers 100 (Raman amplifier 100a, Raman amplifier 100b) on the optical fiber transmission line.

[0012] Next, the functions of the Raman amplifier 100 will be described. FIG. 2 is a block diagram showing the functions of a Raman amplifier according to the first embodiment. The Raman amplifier 100 is a device capable of receiving an optical signal including wavelengths in a first band and wavelengths in a second band shorter than the wavelengths in the first band (i.e., an optical signal including wavelengths in multiple bands, which is a wavelength-multiplexed optical signal). In the following description, the first band is referred to as the L band, and the second band is referred to as the C band. Note that the first band and the second band do not necessarily correspond to the names of specific bands in the optical communication wavelength band. For example, in the description of the first embodiment, band A may be referred to as a short wavelength, band C as a long wavelength, and band B as a wavelength between bands A and C, and band B may be referred to as the first band, as will be described later. In this case, a band (second band) shorter in wavelength than the first band is band A, and a band longer in wavelength than the first band is band C.

[0013] The Raman amplifier 100 includes a pump light multiplexer 101, a demultiplexer 102, a splitter 103, a splitter 104, a photodetector 105, a photodetector 106, an optical amplifier 107, an optical amplifier 108, a multiplexer 109, a control unit 110, a C-band pump light source 111, an L-band pump light source 112, and a multiplexer 113. The optical amplifiers 107 and 108 do not necessarily need to be included in the Raman amplifier 100. The photodetectors 105 and 106 may be referred to as a signal unit, or the signal unit may include the photodetectors 105 and 106 as well as circuits from the photodetectors 105 and 106 to the downstream Raman amplifier 100 (Raman amplifier 100a, Raman amplifier 100b). The signal unit receives an optical signal containing wavelengths in multiple bands. Therefore, the wavelength multiplexing optical transmission system of the first embodiment can be said to include Raman amplifiers 100 (Raman amplifiers 100a and 100b) and an optical fiber transmission line, as shown in Fig. 2. Therefore, the signal section can be said to receive at least a part of the optical signal propagating through the optical fiber transmission line.

[0014] The pumping light source 111 for the C band includes pumping light source 111a and pumping light source 111b. The number of pumping light sources for the C band may be one, or may be three or more. The pumping light source 112 for the L band includes pumping light source 112a and pumping light source 112b. The number of pumping light sources for the L band may be one, or may be three or more.

[0015] Here, the control unit 110 is also referred to as a first control unit. The C-band pumping light source 111 is also referred to as a first pumping light source. The L-band pumping light source 112 is also referred to as a second pumping light source.

[0016] The pumping light multiplexer 101 passes the optical signal input to the Raman amplifier 100. The optical signal is then input to the demultiplexer 102. As will be described later, the pumping light multiplexer 101 outputs pumping light in the direction of arrow 10. The arrow 10 points to one end of the optical fiber transmission line, that is, the direction of the wavelength multiplexing optical transmitter 200 on the optical fiber transmission line. Of course, as described above in the wavelength multiplexing optical transmission system of the first embodiment, one or more other Raman amplifiers 100 may be present in the direction toward the wavelength multiplexing optical transmitter 200.

[0017] Demultiplexer 102 demultiplexes the input optical signal into an optical signal in the C band (1530-1560 nm) and an optical signal in the L band (1570-1600 nm). The optical signal in the C band is input to splitter 103. The optical signal in the L band is input to splitter 104.

[0018] The splitter 103 splits the C-band optical signal. As a result of the splitting, a portion of the C-band optical signal is input to the optical receiver 105. The remaining optical signal (specifically, most of the optical signal) is input to the optical amplifier 107.

[0019] The splitter 104 splits the L-band optical signal. As a result of the splitting, a portion of the L-band optical signal is input to the optical receiver 106. The remaining optical signal (specifically, the majority of the optical signal) is input to the optical amplifier 108.

[0020] The optical receiver 105 outputs to the control unit 110 an electrical signal corresponding to the optical power of the input optical signal. The optical receiver 106 outputs to the control unit 110 an electrical signal corresponding to the optical power of the input optical signal.

[0021] That is, a part of the optical power is split by splitters 103 and 103, and input to optical receivers 105 and 106, which output electrical signals according to the total signal optical power of each wavelength band. Most of the power of the signal light travels straight through splitters 103 and 103, and is sent to optical amplifiers 107 and 108.

[0022] The optical amplifier 107 is typically an erbium-doped optical fiber amplifier (EDFA), and is not a Raman amplifier that amplifies an optical signal in an optical fiber transmission line. Alternatively, other optical amplifiers such as a semiconductor optical amplifier may be used. The optical amplifier 107 is not essential and may be omitted. The optical amplifier 107 amplifies the input optical signal. The amplified optical signal is input to the multiplexer 109. Like the optical amplifier 107, the optical amplifier 108 is typically an EDFA, and is not a Raman amplifier that amplifies an optical signal in an optical fiber transmission line. Alternatively, other optical amplifiers, such as a semiconductor optical amplifier, may be used. The optical amplifier 108 is not essential and may be omitted. The optical amplifier 108 amplifies the input optical signal. The amplified optical signal is input to the multiplexer 109.

[0023] The multiplexer 109 multiplexes the optical signal output from the optical amplifier 107 and the optical signal output from the optical amplifier 108. The multiplexed optical signal is then output to the outside of the Raman amplifier 100, and all the signal lights are again sent to the optical fiber transmission line via a single optical fiber.

[0024] Next, the function of the control unit 110 will be described with reference to FIG. 3 is a block diagram showing the functions of the control unit of embodiment 1. The control unit 110 has a determination unit 110a, a determination unit 110b, a storage unit 110c, a calculation and setting unit 110d, and an excitation light control unit 110e and an excitation light control unit 110f. Note that the storage unit 110c may be located outside the control unit 110.

[0025] In the following description, it is assumed that the C-band pumping light source 111 and the L-band pumping light source 112 output pumping light.

[0026] Here, the determination of the presence or absence of signal light in each wavelength band will be described. The optical receiver 105 and the determination unit 110a constitute an optical power monitor for the C band. The determination unit 110a monitors the optical power of the C-band optical signal demultiplexed by the demultiplexer 102 based on the electrical signal output by the optical receiver 105, and determines whether the Raman amplifier 100 has received the C-band optical signal based on the monitored optical power. For example, the determination unit 110a determines whether the Raman amplifier 100 has received the C-band optical signal based on the magnitude of the optical power indicated by the electrical signal. If the optical power is equal to or greater than a threshold, the determination unit 110a determines that the Raman amplifier 100 has received the C-band optical signal. The determination result is input to the pump light control unit 110e and the pump light control unit 110f. In other words, the optical receiver 105 and the determination unit 110a do not measure the input power to obtain numerical values ​​such as mW or dBm of the propagating signal light, but only determine the presence or absence of signal light in each wavelength band.

[0027] Furthermore, the optical receiver 106 and the determination unit 110b constitute an optical power monitor for the L band, and the determination unit 110b monitors the optical power of the L-band optical signal demultiplexed by the demultiplexer 102 based on the electrical signal output by the optical receiver 106, and determines whether the Raman amplifier 100 has received an L-band optical signal based on the monitored optical power. For example, the determination unit 110b determines whether the Raman amplifier 100 has received an L-band optical signal based on the magnitude of the optical power indicated by the electrical signal. If the optical power is equal to or greater than a threshold, the determination unit 110b determines that the Raman amplifier 100 has received an L-band optical signal. The determination result is input to the pump light control unit 110e and the pump light control unit 110f. In other words, like the optical receiver 105 and the determination unit 110a, the optical receiver 106 and the determination unit 110b do not measure the input power to obtain numerical values ​​such as mW and dBm of the signal light being propagated, but only determine the presence or absence of signal light in each wavelength band.

[0028] The storage unit 110c stores a table. The table indicates the relationship between the power of the pump light output from the Raman amplifier 100 and the Raman amplification gain that an optical signal passing through an optical fiber transmission line (an optical signal input to the Raman amplifier 100) obtains due to the power of the pump light. The relationship is, for example, a relationship obtained in advance by calculation or experiment and stored as a table. Alternatively, an approximation formula that approximates the relationship may be used as long as it can obtain the power of the pump light for a predetermined Raman amplification gain.

[0029] When the calculation setting unit 110d receives information indicating the magnitude of the Raman amplification gain obtained by the optical signal input to the Raman amplifier 100, the calculation setting unit 110d uses the table or the like to calculate a target value of the pumping light power required to obtain the desired Raman amplification gain. The calculation setting unit 110d sets the target values ​​of the pumping light power in the pumping light control units 110e and 110f. Note that the information indicating the magnitude of the Raman amplification gain is, for example, information transmitted by a monitoring and control device used by a user.

[0030] The pump light control unit 110e can control the C-band pump light sources 111 (more specifically, the pump light sources 111a and 111b) so that pump light is output. More specifically, the pump light control unit 110e can control the C-band pump light sources 111 based on a target value of the pump light power.

[0031] The pump light control unit 110f can control the L-band pump light sources 112 (more specifically, the pump light sources 112a and 112b) so that pump light is output. More specifically, the pump light control unit 110f can control the L-band pump light sources 112 based on a target value of the pump light power.

[0032] When the determination result of the determination unit 110a indicates that a C-band optical signal has been received and the determination result of the determination unit 110b indicates that an L-band optical signal has been received, the pump light controller 110e and the pump light controller 110f control the C-band pump light source 111 and the L-band pump light source 112 so that pump light of a target power value is output. Here, the controller 110 stores in advance in the storage unit 110c pump light source control information indicating the relationship between each band (C-band and L-band, C-band only, L-band only) of the received optical signal, the target value of the pump light power, and the control values ​​of the C-band pump light source 111 and the L-band pump light source 112 so that the output pump light power reaches the target value. The pump light controller 110e and the pump light controller 110f control the C-band pump light source 111 and the L-band pump light source 112 based on the pump light source control information stored in the storage unit 110c. This pump light source control information may be stored in a storage unit other than the storage unit 110c.

[0033] When the determination result of the determination unit 110a indicates that a C-band optical signal has not been received and the determination result of the determination unit 110b indicates that an L-band optical signal has been received, the pump light controller 110e controls the C-band pump light source 111 to stop outputting pump light or to output predetermined pump light with a power lower than that when a C-band optical signal has been received.When the determination result of the determination unit 110a indicates that a C-band optical signal has not been received and the determination result of the determination unit 110b indicates that an L-band optical signal has been received, the pump light controller 110f controls the L-band pump light source 112 to output pump light with a power higher than that of the pump light output by the L-band pump light source 112 when a C-band optical signal and an L-band optical signal have been received.

[0034] The pumping light source control information stored in the storage unit 110c is set with a control value for the C-band pumping light source 111 so that, when an L-band optical signal is received but no C-band optical signal is received, the output of pumping light for the C-band optical signal is stopped, or the power of the pumping light output from the C-band pumping light source 111 is reduced compared to when the pumping light power is the same target value as when a C-band optical signal and an L-band optical signal are received. Also, a control value for the L-band pumping light source 112 is set so that the power of the pumping light output from the L-band pumping light source 112 is increased compared to when the C-band optical signal and an L-band optical signal are received and the pumping light power is the same target value.

[0035] When the determination result of the determination unit 110a indicates that an optical signal in the C band has been received and the determination result of the determination unit 110b indicates that an optical signal in the L band has not been received, the pump light control unit 110f controls the L-band pump light source 112 to stop outputting pump light or to output predetermined pump light with lower power than when an L-band optical signal has been received.When the determination result of the determination unit 110a indicates that an optical signal in the C band has been received and the determination result of the determination unit 110b indicates that an optical signal in the L band has not been received, the pump light control unit 110e controls the C-band pump light source 111 to output pump light with lower power than the pump light power output by the C-band pump light source 111 when an optical signal in the C band and an optical signal in the L band have been received.

[0036] The pumping light source control information stored in the storage unit 110c has set thereto a control value for the L-band pumping light source 112 such that, when a C-band optical signal is received but an L-band optical signal is not received, the output of pumping light for the L-band optical signal is stopped, or the power of the pumping light output from the L-band pumping light source 112 is reduced compared to when the pumping light power has the same target value as when a C-band optical signal and an L-band optical signal are received. Also, a control value for the C-band pumping light source 111 is set thereto such that the power of the pumping light output from the C-band pumping light source 111 is reduced compared to when the C-band optical signal and an L-band optical signal are received and the pumping light power has the same target value.

[0037] Returning to FIG. 2, the C-band pumping light source 111 and the L-band pumping light source 112 will be described. The pump light source 111 for the C band (that is, the pump light source 111a and the pump light source 111b) outputs pump light for the C band under the control of the pump light control unit 110e. The pump light source 112 for the L band (that is, the pump light source 112a and the pump light source 112b) outputs pump light for the L band under the control of the pump light control unit 110f. The C-band pumping light source 111 and the L-band pumping light source 112 may be built into the Raman amplifier 100, or may be connected so as to be attachable later.

[0038] The multiplexer 113 multiplexes the pumping light output from the C-band pumping light source 111 and the pumping light output from the L-band pumping light source 112. The multiplexed pumping light is output via the pumping light multiplexer 101 in the opposite direction to the optical signal input to the Raman amplifier 100. In other words, the multiplexed pumping light is output in the direction indicated by arrow 10. In this way, backward pumping is performed. The optical signal input to the Raman amplifier 100 obtains a gain based on the pumping light.

[0039] Furthermore, when only the pump light output from the C-band pump light source 111 is input to the multiplexer 113, no multiplexing is performed and the pump light is output in the direction indicated by the arrow 10. Furthermore, when only the pump light output from the L-band pump light source 112 is input to the multiplexer 113, no multiplexing is performed and the pump light is output in the direction indicated by the arrow 10.

[0040] The pumping light sources 111a, 111b, L-band pumping light source 112a, and 112b, which are the pumping light sources that receive control signals from the control unit 110, output pumping light of their respective wavelengths as necessary in accordance with the control signals, and the pumping light of different wavelengths is multiplexed by a multiplexer 113 and guided in the direction of arrow 10 into an optical fiber that transmits signal light from the Raman amplifier 100. The control signals are signals sent from the control unit 110 shown in Fig. 2 to the pumping light sources 111a, 111b, L-band pumping light source 112a, and pumping light source 112b via lines indicated by four arrows.

[0041] 2 shows a backward pumping configuration, in which the Raman pump light propagates in the opposite direction to the propagation direction of the signal light in the optical fiber transmission line, and the signal light obtains gain in the optical fiber transmission line through Raman amplification according to the pump light power, and the power of the signal light incident on the Raman amplifier 100 increases accordingly. In the wavelength-multiplexed optical transmission system and the method for controlling pump light for a Raman amplifier according to the first embodiment, the pumping method is also backward pumping. Note that in the method for controlling pump light for a Raman amplifier according to the first embodiment, backward pumping can be said to be one type of processing step in the method for controlling pump light, and the pumping method is not limited to backward pumping. The processing steps in the method for controlling pump light for a Raman amplifier according to the first embodiment correspond to operations according to the configuration of the Raman amplifier according to the first embodiment (mainly those with reference numerals).

[0042] Next, the control of the pump light control units 110e and 110f will be described using a specific example, that is, the control of pump light power. Fig. 4 is a diagram showing a specific example in which a pumping light source for a C band and a pumping light source for an L band according to the first embodiment output pumping light. The vertical axis of the graph in Fig. 4 represents optical intensity. The horizontal axis of the graph in Fig. 4 represents wavelength. Fig. 4 shows an optical signal in the C band (1530-1560 nm) and an optical signal in the L band (1570-1600 nm) input to the Raman amplifier 100.

[0043] Pumping light of 1430 nm and pumping light of 1450 nm are used as pumping light for Raman amplifying optical signals in the C band, and pumping light of 1470 nm and pumping light of 1490 nm are used as pumping light for Raman amplifying optical signals in the L band.

[0044] Raman amplification in silica optical fiber has the characteristic of amplifying optical signals positioned at a frequency approximately 13.2 THz lower than the pump light, which corresponds to a wavelength of approximately 100 to 110 nm. When the power of the pump light is transferred to the optical signal, the power of the optical signal increases, while at the same time the power of the pump light decreases.

[0045] Furthermore, the gain band of Raman amplification is broad, so that in Raman amplification, not only the signal light with a wavelength obtained by adding approximately 100 to 110 nm to the wavelength of the pump light, but also optical signals with wavelengths around that wavelength are amplified to some extent.

[0046] For example, in Raman amplification, a power transition may also occur from a 1430 nm pump light to a 1490 nm pump light, and a power transition may occur from a 1530 nm optical signal to a 1600 nm optical signal.

[0047] The amount of power transition from wavelength to wavelength depends on the broadening of the gain band of the Raman amplifier. Power transitions occur depending on wavelength or frequency differences.

[0048] Figure 4 shows that wavelength-multiplexed optical signals are amplified to equal power, with the shorter the wavelength of the pump light, the higher the power, and the longer the wavelength of the pump light, the lower the power. The reason why the pump power for each wavelength is different is as follows: as the signal propagates through the optical fiber, the power transitions from short wavelengths to long wavelengths. Therefore, by setting the power of the pump light for short wavelengths high, approximately equal gain is given to all wavelengths. By setting the power of the pump light as shown in Figure 4, the power of the C-band optical signal and the power of the L-band optical signal become approximately equal.

[0049] Next, a case where only the C-band pumping light source 111 outputs pumping light will be described. Fig. 5 is a diagram showing a comparative example in which a C-band pumping light source outputs pumping light. The vertical axis of the graph in Fig. 5 represents light intensity, and the horizontal axis of the graph in Fig. 5 represents wavelength. FIG. 5 shows a case where the L-band signal light is lost from the state shown in FIG. 4. That is, it shows a case where only the C-band signal light is propagating through the optical fiber transmission line. In this case, if the pump light source for the C-band outputs pump light and the pump light source for the L-band does not output pump light, there is no power transition from the pump light for the C-band to the pump light for the L-band. Therefore, if the pump light source for the C-band outputs pump light at the same power as when the pump light source for the C-band and the pump light source for the L-band are outputting pump light, the attenuation of the pump light for the C-band is reduced, and the power of the pump light for the C-band becomes excessive. Furthermore, there is no power transition from the optical signal for the C-band to the optical signal for the L-band. Therefore, the attenuation of the optical signal for the C-band is reduced. Because the attenuation of the optical signal for the C-band is reduced, the state of the optical signals for each wavelength in the C-band becomes greater than the predetermined power, as shown in FIG. 5. In this way, when there is no L-band optical signal, if the C-band pumping light source outputs pumping light with the same power as when the C-band pumping light source and the L-band pumping light source are outputting pumping light, proper Raman amplification will not occur.

[0050] Therefore, when the C-band pumping light source 111 outputs pumping light and the L-band pumping light source 112 does not output pumping light, the Raman amplifier 100 reduces the power of the C-band pumping light more than when both the C-band pumping light source 111 and the L-band pumping light source 112 output pumping light. A specific example will be described below.

[0051] Fig. 6 is a diagram showing a specific example in which the C-band pumping light source according to the first embodiment outputs pumping light and the L-band pumping light source does not output pumping light. The vertical axis of the graph in Fig. 6 represents light intensity. The horizontal axis of the graph in Fig. 6 represents wavelength. 6, when the C-band pumping light source 111 outputs pumping light and the L-band pumping light source 112 does not output pumping light, the Raman amplifier 100 reduces the power of the C-band pumping light compared to when both the C-band pumping light source 111 and the L-band pumping light source 112 output pumping light. This allows the Raman amplifier 100 to prevent the state of the optical signal of each wavelength in the C-band from exceeding a predetermined power.

[0052] Here, the table stored in the storage unit 110c pre-stores information indicating the power when the C-band pump light source 111 outputs pump light and the L-band pump light source 112 does not output pump light. The calculation and setting unit 110d uses the table or the like to calculate a target value to be set in the pump light control unit 110e. Then, the calculation and setting unit 110d sets the target value in the pump light control unit 110e. In this way, when a C-band optical signal is received but an L-band optical signal is not received, the pump light control unit 110e can control the C-band pump light source 111 so that the pump light output has a power lower than the power of the pump light that the C-band pump light source 111 outputs when both the C-band optical signal and the L-band optical signal are received.

[0053] In addition, when an optical signal for the C band is received but an optical signal for the L band is not received, the pumping light source 111 for the C band may output pumping light, and the pumping light source 112 for the L band may reduce the output of the pumping light. In this case, the pumping light source 111 for the C band is controlled so that the pumping light output has a power lower than the power of the pumping light output by the pumping light source 111 for the C band when an optical signal for the C band and an optical signal for the L band are received.

[0054] Next, a case where only the L-band pumping light source outputs pumping light will be described. Fig. 7 is a diagram showing a comparative example in which the L-band pumping light source 112 outputs pumping light. The vertical axis of the graph in Fig. 7 represents light intensity, and the horizontal axis of the graph in Fig. 7 represents wavelength. FIG. 7 illustrates a case where the C-band signal light is lost from the state illustrated in FIG. 4 . That is, it illustrates a case where only the L-band signal light is propagating through the optical fiber transmission line. In this case, if the pump light source for the L-band outputs pump light and the pump light source for the C-band does not output pump light, no power transition occurs from the pump light for the C-band to the pump light for the L-band. Therefore, if the pump light source for the L-band outputs pump light at the same power as when the pump light source for the C-band and the pump light source for the L-band are outputting pump light, the power of the pump light for the L-band will be somewhat insufficient. Furthermore, no power transition occurs from the optical signal for the C-band to the optical signal for the L-band. Therefore, the gain of the optical signal for the L-band is reduced. Because the gain of the optical signal for the L-band is reduced, the state of the optical signals for each wavelength in the L-band becomes smaller than the predetermined power, as illustrated in FIG. 7. Thus, if the pump light source for the C-band outputs pump light at the same power as when the pump light source for the C-band and the pump light source for the L-band are outputting pump light when there is no C-band optical signal, proper Raman amplification will not be achieved.

[0055] Therefore, when the L-band pumping light source 112 outputs pumping light and the C-band pumping light source 111 does not output pumping light, the Raman amplifier 100 increases the power of the L-band pumping light more than when both the C-band pumping light source 111 and the L-band pumping light source 112 output pumping light. A specific example will be described below.

[0056] Fig. 8 is a diagram showing a specific example in which the L-band pumping light source of the first embodiment outputs pumping light and the C-band pumping light source does not output pumping light. The vertical axis of the graph in Fig. 8 represents light intensity. The horizontal axis of the graph in Fig. 8 represents wavelength. When the L-band pumping light source 112 outputs pumping light and the C-band pumping light source 111 does not output pumping light, the Raman amplifier 100 increases the power of the L-band pumping light compared to when both the C-band pumping light source 111 and the L-band pumping light source 112 output pumping light, as shown in Fig. 8. This allows the Raman amplifier 100 to prevent the state of the optical signal of each wavelength in the L-band from becoming lower than a predetermined power.

[0057] Here, the table stored in the storage unit 110c pre-stores information indicating the power when the L-band pump light source 112 outputs pump light and the C-band pump light source 111 does not output pump light. The calculation and setting unit 110d uses the table to calculate a target value to be set in the pump light control unit 110f. Then, the calculation and setting unit 110d sets the target value in the pump light control unit 110f. In this way, when an L-band optical signal is received but a C-band optical signal is not received, the pump light control unit 110f can control the L-band pump light source 112 so that the pump light output has a power greater than the power of the pump light that the L-band pump light source 112 outputs when both the C-band optical signal and the L-band optical signal are received.

[0058] In addition, when an L-band optical signal is received but a C-band optical signal is not received, the L-band pumping light source 112 may output pumping light and the C-band pumping light source 111 may reduce the output of the pumping light. In this case, the L-band pumping light source 112 is controlled so that the pumping light output has a power greater than the power of the pumping light output by the L-band pumping light source 112 when a C-band optical signal and an L-band optical signal are received.

[0059] According to the first embodiment, when no optical signal in the C band or L band is present, the Raman amplifier 100 adjusts the power of the pump light of the present optical signal so that it is greater or smaller than when optical signals in both the C band and the L band are present, thereby enabling the Raman amplifier 100 to achieve appropriate Raman amplification.

[0060] Here, the operation of the Raman amplifier 100 according to the first embodiment will be described as a method for controlling pumping light of the Raman amplifier according to the first embodiment. That is, this is a method for controlling pumping light of a Raman amplifier that can receive, among optical signals including wavelengths of a plurality of bands, at least an optical signal of a first band (C band) and an optical signal of a second band (L band) having a longer wavelength than the optical signal of the first band. As described above, the processing steps in the method for controlling pumping light of the Raman amplifier according to the first embodiment correspond to the operation of the configuration of the Raman amplifier according to the first embodiment, but in the description of the method for controlling pumping light of the Raman amplifier according to the first embodiment, the names of the corresponding configurations of the Raman amplifier according to the first embodiment may be omitted.

[0061] First, as a method for controlling pumping light in the Raman amplifier of the first embodiment, a case where a C-band optical signal is received and an L-band optical signal is no longer received will be described. At this time, the power of the C-band pumping light, which is the pumping light for the C-band optical signal, is lowered compared to when the C-band optical signal and the L-band optical signal are received, and the power of the L-band pumping light, which is the pumping light for the L-band optical signal, is lowered, or the output of the L-band pumping light is stopped. Similarly, a case where an L-band optical signal is received and a C-band optical signal is no longer received will be described. At this time, the power of the L-band pumping light is increased compared to when the C-band optical signal and the L-band optical signal are received, and the power of the C-band pumping light is lowered, or the output of the C-band pumping light is stopped.

[0062] Furthermore, as an operation opposite to the pump light control method of the Raman amplifier of the first embodiment, a case where a C-band optical signal is received and then an L-band optical signal is received will be described. In this case, the power of the pump light for the C-band is increased compared to when an L-band optical signal is not received, and the pump light for the L-band is output. Similarly, a case where an L-band optical signal is received and then an C-band optical signal is received will be described. In this case, the power of the pump light for the L-band is decreased compared to when an C-band optical signal is not received, and the pump light for the C-band is output. These operations correspond to the operations of the Raman amplifier 100 of the second embodiment, which will be described later, and can therefore also be said to be a pump light control method of the Raman amplifier of the second embodiment. A detailed description of the operation of this Raman amplifier 100 will be given in the second embodiment.

[0063] Although the Raman amplifier 100 has been described so far as amplifying optical signals in two bands, the C band and the L band, the number of wavelength bands of optical signals amplified by the Raman amplifier 100 of the first embodiment may be three. In this case, the control unit 110 has a pumping light source, a determination unit, and a pumping light control unit for each of bands A, B, and C. The number of pumping light sources for each of bands A, B, and C may be one, or three or more. For example, assuming that band A is a short wavelength, band C is a long wavelength, and band B is a wavelength between bands A and C, the power of the pumping light from the pumping light source for each band is appropriately adjusted when receiving optical signals from bands A, B, and C, and the optical signals from any one of bands A, B, or C are no longer received.

[0064] When band A, which has a shorter wavelength, is no longer received, the power of the optical signals in bands B and C, which have longer wavelengths than band A, becomes insufficient, as in Fig. 7. In this case, as in Fig. 8, the pumping light sources for bands B and C, which have longer wavelengths than band A, are controlled so that the power of the pumping light output by the pumping light sources is increased. In this case, it is also possible to control either band B or band C, for example, only the pumping light source for band C, which has lower power.

[0065] Next, when band C, which has a longer wavelength, is no longer received, the power of the optical signals in bands A and B, which have shorter wavelengths than band C, becomes excessive, as in Fig. 5. In this case, as in Fig. 6, the pumping light sources for bands A and B, which have shorter wavelengths than band C, are controlled so that the power of the pumping light output by the pumping light sources is reduced. In this case, it is also possible to control either band A or band B, for example, only the pumping light source for band A, which has the greater power.

[0066] Furthermore, when band B, which has a wavelength between band A and band C, is no longer received, the power of the optical signal of band C, which has a longer wavelength than band B, will be insufficient, so the pumping light source of band C is controlled to increase the power of the pumping light output by the pumping light source of band C. Also, the power of the optical signal of band A, which has a shorter wavelength than band B, will be excessive, so the pumping light source of band A is controlled to decrease the power of the pumping light output by the pumping light source of band A.

[0067] In addition, we will explain the case where band A is a short wavelength, band C is a long wavelength, and band B is a wavelength between bands A and C, and the power of the pumping light from the pumping light source for each band is appropriately adjusted when receiving optical signals from bands A, B, and C, and optical signals from any two consecutive bands A, B, or C are no longer received. When bands A and B are no longer received, the power of the optical signal in band C, which has a longer wavelength than bands A and B, becomes insufficient, so the pumping light source for band C is controlled so that the power of the pumping light output by the pumping light source for band C is increased.

[0068] Next, when bands B and C are no longer received, the power of the optical signal in band A, which has a shorter wavelength than bands B and C, becomes excessive, so the pumping light source for band A is controlled so that the power of the pumping light output by the pumping light source for band A is reduced.

[0069] Furthermore, the number of wavelength bands of the optical signal amplified by the Raman amplifier 100 of the first embodiment may be four or more. In this case, the control unit 110 has a pumping light source, a determination unit, and a pumping light control unit for each of bands A, B, C, and D. The number of pumping light sources for each of bands A, B, C, and D may be one, or three or more.

[0070] For example, assuming that the wavelengths are bands A, B, C, D, and E in ascending order, the power of the pump light from the pump light source for each band when receiving optical signals from bands A, B, C, D, and E is appropriately adjusted, if an optical signal from one of bands A, B, C, D, or E is no longer received, as in the case of three wavelength bands, the power of the optical signals in bands longer than the band that is no longer received will be insufficient, so some or all of the pump light sources in bands longer than the band that is no longer received will be controlled so that the power of the pump light output from the pump light source for the band longer than the band that is no longer received is increased. Also, the power of the optical signals in bands shorter than the band that is no longer received will be excessive, so some or all of the pump light sources in bands shorter than the band that is no longer received will be controlled so that the power of the pump light output from the pump light source for the band shorter than the band that is no longer received is reduced.

[0071] Furthermore, in a state where the power of the pump light of the pump light source for each band when receiving optical signals of bands A, B, C, D, and E is appropriately adjusted, if optical signals of multiple consecutive bands A, B, C, D, or E are no longer received, as in the case of three wavelength bands, the power of the optical signals of bands longer than the multiple consecutive bands that are no longer received will be insufficient, so some or all of the pump light sources of bands longer in wavelength than the multiple consecutive bands that are no longer received will be controlled so that the power of the pump light output by the pump light sources of bands longer in wavelength than the multiple consecutive bands that are no longer received will be increased. Also, since the power of optical signals of bands shorter in wavelength than the multiple consecutive bands that are no longer received will be excessive, some or all of the pump light sources of bands shorter in wavelength than the multiple consecutive bands that are no longer received will be controlled so that the power of the pump light output by the pump light sources of bands shorter in wavelength than the multiple consecutive bands that are no longer received will be reduced.

[0072] Furthermore, in a state where the power of the pump light of the pump light source for each band when receiving optical signals of bands A, B, C, D, and E is appropriately adjusted, if optical signals of multiple non-consecutive bands A, B, C, D, or E are no longer received, for example, bands B and D are no longer received, the pump light source for band E is controlled so that the power of the pump light output by the pump light source for band E, which has a longer wavelength than band D, which has a longer wavelength among bands B and D that are no longer received, is increased. Also, the pump light source for band A is controlled so that the power of the pump light output by the pump light source for band A, which has a shorter wavelength than band B, which has a shorter wavelength among bands B and D that are no longer received, is decreased. At this time, the pump light source for band C may be controlled so that the power of the pump light output by the pump light source for band C is increased or decreased, taking into account the influence of bands B and D that are no longer received.

[0073] It has been explained that the number of wavelength bands of the optical signal amplified by the Raman amplifier 100 of the first embodiment may be three or more, and the same applies to the method of controlling the pumping light of the Raman amplifier of the first embodiment. When the number of wavelength bands is three, a method of controlling the pumping light of a Raman amplifier that can receive an optical signal including wavelengths of multiple bands (band A being a short wavelength, band C being a long wavelength, and band B being a wavelength between band A and band C) will be explained as an example.

[0074] As a method for controlling pumping light in the Raman amplifier of the first embodiment, a case will be described in which an optical signal in a band (band A) having a shorter wavelength than the optical signal in a first band (band B) and an optical signal in a band (band C) having a longer wavelength than the optical signal in the first band are received, and the optical signal in band B is no longer received. In this case, the power of the pumping light for band B, which is the pumping light for the optical signal in band B, is reduced compared to when the optical signal in band B, the optical signal in band A, and the optical signal in band C are received, or the output of the pumping light for band B is stopped, and the power of the pumping light for band A is reduced and the power of the pumping light for band B is increased compared to when the optical signal in band A, the optical signal in band B, and the optical signal in band C are received.

[0075] As an operation opposite to the pump light control method of the Raman amplifier of the first embodiment, a case where an optical signal of band A and an optical signal of band C are received and an optical signal of band B is also received will be described. In this case, pump light of the optical signal of band B is output, and the power of the pump light for band A is increased and the power of the pump light for band C is decreased compared to when no optical signal of band B is received. This corresponds to the operation of the Raman amplifier 100 of the second embodiment, which will be described later, and can therefore also be said to be a pump light control method of the Raman amplifier of the second embodiment. A detailed description of the operation of this Raman amplifier 100 will be given in the second embodiment.

[0076] Next, an example of a method for controlling pumping light in a Raman amplifier capable of receiving an optical signal containing wavelengths in a plurality of bands when the number of wavelength bands is three or more will be described.

[0077] As a method for controlling pumping light in the Raman amplifier of the first embodiment, a case will be described in which, among optical signals containing wavelengths in a plurality of bands, an optical signal in the band with the shortest wavelength (band A) is no longer received and optical signals in the remaining bands on the longer wavelength side are received. In this case, the power of pumping light for at least one band among the remaining long-wavelength bands, which is pumping light for optical signals in the remaining long-wavelength bands, is increased compared to when all optical signals containing wavelengths in a plurality of bands are received, and the power of pumping light for the shortest wavelength band, which is pumping light for optical signals in the band with the shortest wavelength (band A), is decreased, or the output of pumping light for the shortest wavelength band is stopped.

[0078] Next, as a method for controlling pumping light in the Raman amplifier of the first embodiment, a case will be described in which, among optical signals containing wavelengths in multiple bands, an optical signal in the longest wavelength band (corresponding to band C when the number of wavelength bands is three, or band D when the number of wavelength bands is four) is no longer received, and optical signals in the remaining shorter wavelength bands are received. In this case, the power of the pumping light for at least one band among the pumping light for the remaining shorter wavelength bands, which is the pumping light for the optical signals in the remaining shorter wavelength bands, is reduced compared to when all optical signals containing wavelengths in multiple bands are received, and the power of the pumping light for the longest wavelength band is reduced, or the output of the pumping light for the longest wavelength band is stopped.

[0079] The following describes the reverse operation of the pump light control method of the Raman amplifier of the first embodiment in the previous two examples. That is, the case where, among optical signals containing wavelengths in multiple bands, optical signals in the remaining longer wavelength bands other than the optical signal in the shortest wavelength band (band A) are received, and then an optical signal in the shortest wavelength band (band A) is also received. In this case, the power of the pump light for at least one band among the remaining longer wavelength bands is reduced compared to when no optical signal in the shortest wavelength band (band A) is received, and the pump light for the shortest wavelength band is output.

[0080] Next, we will explain what happens when, among optical signals containing wavelengths in multiple bands, optical signals in the remaining short wavelength bands other than the optical signal in the longest wavelength band (corresponding to band C when there are three wavelength bands, and band D when there are four wavelength bands) are received, and then an optical signal in the longest wavelength band is also received. Compared to when an optical signal in the longest wavelength band is not received, the power of the pump light for at least one band among the remaining short wavelength bands is increased, and the pump light for the longest wavelength band is output.

[0081] These correspond to the operation of the Raman amplifier 100 according to the second embodiment, which will be described later, and can therefore also be considered as a method for controlling pump light in the Raman amplifier according to the second embodiment. The operation of the Raman amplifier 100 will be described in detail in the second embodiment.

[0082] Next, an example of a method for controlling pumping light in a Raman amplifier capable of receiving an optical signal containing wavelengths in a plurality of bands when the number of wavelength bands is three or more will be described.

[0083] As a method for controlling pumping light in the Raman amplifier of the first embodiment, a case will be described in which, among optical signals containing wavelengths in a plurality of bands, an optical signal in the band with the shortest wavelength (band A) and an optical signal in the band (band B) adjacent to the shortest wavelength band on the long wavelength side relative to the optical signal in the shortest wavelength band are no longer received, and optical signals in the remaining long wavelength bands are received. In this case, compared to when all optical signals containing wavelengths in a plurality of bands are received, the power of pumping light for at least one band among the remaining long wavelength bands, which is pumping light for optical signals in the remaining long wavelength bands, is increased, the power of pumping light for pumping light for optical signals in the band (band B) adjacent to the long wavelength side, which is pumping light for optical signals in the band adjacent to the long wavelength side, is decreased, or the output of pumping light for the band adjacent to the long wavelength side is stopped, and the power of pumping light for the shortest wavelength band, which is pumping light for optical signals in the shortest wavelength band (band A), is decreased, and the output of pumping light for the shortest wavelength band is stopped.

[0084] Next, as a method for controlling pumping light in the Raman amplifier of the first embodiment, a case will be described in which, among optical signals including wavelengths in a plurality of bands, an optical signal in the longest wavelength band (corresponding to band C when the number of wavelength bands is three, or band D when the number of wavelength bands is four) and an optical signal in a band adjacent to the longest wavelength band on the short wavelength side (corresponding to band B when the number of wavelength bands is three, or band C when the number of wavelength bands is four) are no longer received, and optical signals in the remaining short wavelength bands are received. In this case, the power of the pumping light for at least one band among the remaining short wavelength bands, which is the pumping light for the optical signals in the remaining short wavelength bands, is reduced compared to when all optical signals including wavelengths in a plurality of bands are received, and the output of the pumping light for the band adjacent to the short wavelength side, which is the pumping light for the optical signal in the band adjacent to the short wavelength side, and the output of the pumping light for the longest wavelength band, which is the pumping light for the optical signal in the longest wavelength band, are stopped or the power of the pumping light is reduced. This also includes a case where the output of either the pump light for the band adjacent to the short wavelength side or the pump light for the band with the longest wavelength is stopped and the power of the other is reduced.

[0085] Furthermore, as a method for controlling pumping light in the Raman amplifier of the first embodiment, a case will be described in which, among optical signals including wavelengths in multiple bands, an optical signal in the shortest wavelength band (band A) and an optical signal in the longest wavelength band (corresponding to band C when the number of wavelength bands is three, or band D when the number of wavelength bands is four) are no longer received, and optical signals in the remaining bands are received. In this case, the power of the pumping light for the longest wavelength band, which is the pumping light for the optical signal including wavelengths in the longest wavelength band, is reduced or the output of the pumping light for the longest wavelength band is stopped, and the power of the pumping light for the shortest wavelength band, which is the pumping light for the optical signal including wavelengths in the shortest wavelength band, is reduced or the output of the pumping light for the shortest wavelength band is stopped. The power of the pumping light for the remaining bands, which is the pumping light for the optical signals in the remaining bands, is output at a value obtained by offsetting the amount of power transition from the pumping light for the shortest wavelength band or a value reduced compared to when all optical signals including wavelengths in multiple bands are received, and the amount of power transition from the pumping light for the shortest wavelength band and the amount of power transition to the pumping light for the longest wavelength band are no longer necessary. The offset value here refers to the power originally output by the pump light for the remaining bands, that is, the power originally required even before the optical signals of the shortest wavelength band and the longest wavelength band were received, taking into account course.

[0086] The following describes the reverse operation of the pump light control method of the Raman amplifier of the first embodiment in the three previous examples. That is, the case where optical signals in the remaining long-wavelength bands other than the optical signal in the shortest wavelength band (band A) and the optical signal in the band adjacent to it on the long-wavelength side (band B) are received, and further optical signals in the shortest wavelength band and the band adjacent to it on the long-wavelength side are received. In this case, the power of the pump light for at least one band among the remaining long-wavelength bands is reduced compared to when the optical signal in the shortest wavelength band and the optical signal in the band adjacent to it on the long-wavelength side are not received, and the pump light for the shortest wavelength band and the pump light for the band adjacent to it on the long-wavelength side are output.

[0087] Next, we will explain what happens when optical signals in the remaining short-wavelength bands other than the optical signal in the longest wavelength band (corresponding to band C when there are three wavelength bands, and band D when there are four wavelength bands) and the optical signal in the band adjacent to it on the short-wavelength side (corresponding to band B when there are three wavelength bands, and band C when there are four wavelength bands) are received, and also when an optical signal in the longest wavelength band and an optical signal in the band adjacent to it on the short-wavelength side are received. In this case, the power of the pump light for at least one band among the remaining short-wavelength bands is increased compared to when an optical signal in the longest wavelength band and an optical signal in the band adjacent to it on the short-wavelength side are not received, and the pump light for the longest wavelength band and the pump light for the band adjacent to it on the short-wavelength side are output.

[0088] Furthermore, when optical signals of the remaining bands other than the optical signal of the shortest wavelength band (band A) and the optical signal of the longest wavelength band (corresponding to band C when there are three wavelength bands, and corresponding to band D when there are four wavelength bands) are received and further the optical signal of the shortest wavelength band and the optical signal of the longest wavelength band are received, pump light for the shortest wavelength band and pump light for the longest wavelength band are output, and the power of the pump light for the remaining bands is output at a value obtained by offsetting the amount of power transition from the pump light for the shortest wavelength band and the amount of power transition required to the pump light for the longest wavelength band. The offset value here naturally takes into account the power originally output by the pump light for the remaining bands, that is, the power originally required before the optical signal of the shortest wavelength band and the optical signal of the longest wavelength band were received.

[0089] These correspond to the operation of the Raman amplifier 100 according to the second embodiment, which will be described later, and can therefore also be considered as a method for controlling pump light in the Raman amplifier according to the second embodiment. The operation of the Raman amplifier 100 will be described in detail in the second embodiment.

[0090] Embodiment 2 Next, a second embodiment will be described. In the second embodiment, differences from the first embodiment will be mainly described. Furthermore, in the second embodiment, description of matters common to the first embodiment will be omitted. Of course, a similar relationship to that between the Raman amplifier of the first embodiment and the wavelength-division multiplexed optical transmission system and the method for controlling pumping light of the Raman amplifier of the first embodiment also holds in the second embodiment, so the description of the Raman amplifier of the second embodiment will be mainly focused on, and the wavelength-division multiplexed optical transmission system and the method for controlling pumping light of the Raman amplifier of the second embodiment will be briefly described. In particular, for convenience of explanation, a part of the method for controlling pumping light of the Raman amplifier of the second embodiment will be described first when describing the method for controlling pumping light of the Raman amplifier of the first embodiment.

[0091] Initially, there may be a case where the L-band pumping light source 112 is not included in the Raman amplifier 100, and only a C-band optical signal is input to the Raman amplifier 100. If the L-band pumping light source 112 is subsequently added to the Raman amplifier 100 and an L-band optical signal also begins to be input to the Raman amplifier 100, the Raman amplifier 100 executes the same process as in the first embodiment, but in reverse order. More specifically, in the first embodiment, the process is changed from the state in which the C-band pumping light source and the L-band pumping light source are outputting pumping light shown in FIG. 4 to the state in which the C-band pumping light source only outputs pumping light shown in FIG. 6. However, although the process is the same, the second embodiment executes the reverse process of changing from the state shown in FIG. 6 to the state shown in FIG. 4.

[0092] In the second embodiment, the case where only a C-band optical signal is initially input to the Raman amplifier 100 can be said to be the case where the C-band pumping light source outputs pumping light as shown in FIG. 6. When an L-band optical signal is input to the Raman amplifier 100 in this state, if the power of the C-band pumping light remains unchanged, it is obvious that the power of the C-band pumping light is not excessive, as shown in the comparative example in FIG. 5 where the C-band pumping light source outputs pumping light, and therefore appropriate Raman amplification of the C-band optical signal and the L-band optical signal as shown in FIG. 4 is not achieved. Therefore, when an L-band optical signal is additionally present, if the L-band pumping light source outputs pumping light and further outputs pumping light with a power greater than the power when only the C-band pumping light source outputs pumping light, appropriate Raman amplification of the C-band optical signal and the L-band optical signal is achieved. Note that the state where the power of the C-band pumping light is excessive here means an excessive state when only the C-band optical signal is Raman amplified.

[0093] In this way, in the Raman amplifier 100 of the second embodiment, as in the Raman amplifier 100 of the first embodiment, it is possible to transmit the signal light in the C band and the L band all at predetermined power levels as shown in Fig. 4, that is, with a good bit error rate, without changing the power of the signal light in the C band. Note that the instruction value (target value) of the pump light power when the signal light in the L band is added may also be stored in advance in a table or the like stored in the storage unit 110c, so that the pump light power for the C band can be appropriately increased when the signal light in the L band is added.

[0094] Furthermore, there may be a case where the C-band pumping light source 111 is not initially included in the Raman amplifier 100, and only an L-band optical signal is input to the Raman amplifier 100. If the C-band pumping light source 111 is subsequently added to the Raman amplifier 100 and an C-band optical signal also begins to be input to the Raman amplifier 100, the Raman amplifier 100 executes the same process as in the first embodiment, but in reverse order. More specifically, in the first embodiment, the process is changed from the state in which the C-band pumping light source and the L-band pumping light source are outputting pumping light, as shown in FIG. 4, to the state in which the L-band pumping light source is outputting only pumping light, as shown in FIG. 8. However, although the process is the same, the second embodiment executes the reverse process of changing from the state shown in FIG. 8 to the state shown in FIG. 4.

[0095] In the second embodiment, the case where only an L-band optical signal is initially input to the Raman amplifier 100 can be said to be the case where the L-band pumping light source outputs pumping light as shown in FIG. 8. When a C-band optical signal is input to the Raman amplifier 100 in this state, if the power of the L-band pumping light remains unchanged, it is obvious that the power of the L-band pumping light is not insufficient as shown in the comparative example in FIG. 7 where the L-band pumping light source outputs pumping light, and therefore appropriate Raman amplification of the C-band optical signal and the L-band optical signal as shown in FIG. 4 is not achieved. Therefore, when an additional C-band optical signal is present, if the C-band pumping light source outputs pumping light and the L-band pumping light source outputs pumping light with a power lower than that when only the L-band pumping light source outputs pumping light, appropriate Raman amplification of the C-band optical signal and the L-band optical signal is achieved. Note that the state where the power of the L-band pumping light is insufficient here means a state where the power is insufficient when only the L-band optical signal is Raman amplified.

[0096] As with the Raman amplifier 100 of the first embodiment, the Raman amplifier 100 of the second embodiment can transmit the C-band and L-band signal lights at predetermined power levels, as shown in Fig. 4, without changing the power of the L-band signal light, i.e., with a good bit error rate. Note that the instruction value (target value) of the pumping light power when the C-band signal light is added may also be stored in advance in a table or the like stored in the storage unit 110c, so that the L-band pumping light power can be appropriately reduced when the C-band signal light is added.

[0097] Even if the Raman amplifier 100 of the second embodiment does not initially have a pumping light source for C band or L band in the system, when C band signal light or L band signal light is later added to the signal light transmitted through the optical fiber transmission line, the Raman amplifier 100 can add a pumping light source for C band or L band and Raman amplify the added signal light. In other words, the system can be updated later. The same applies to the wavelength multiplexing optical transmission system and the method for controlling pumping light for a Raman amplifier of the second embodiment. Note that system update does not only refer to system update of the wavelength multiplexing optical transmission system of the second embodiment, but also refers to updating the configuration (system) of the Raman amplifier 100 of the second embodiment. Furthermore, system update in the method for controlling pumping light for a Raman amplifier of the second embodiment means changing or adding a processing step in the method for controlling pumping light.

[0098] Furthermore, according to the second embodiment, the Raman amplifier 100 achieves the same effects as those of the first embodiment. Although the two bands of optical signals amplified by the Raman amplifier 100 have been described as being the C band and the L band, the number of wavelength bands of optical signals amplified by the Raman amplifier 100 in the second embodiment may of course be three or more.

[0099] Embodiment 3 Next, a third embodiment will be described. In the third embodiment, differences from the first and second embodiments will be mainly described. Furthermore, in the third embodiment, description of matters common to the first and second embodiments will be omitted. Of course, the same relationship as that between the Raman amplifiers of the first and second embodiments and the wavelength-multiplexed optical transmission systems and methods for controlling pumping light for Raman amplifiers of the first and second embodiments also holds true in the third embodiment, so the description of the Raman amplifier of the third embodiment will be mainly focused on, and the wavelength-multiplexed optical transmission system and method for controlling pumping light for Raman amplifiers of the third embodiment will be briefly described.

[0100] In the first and second embodiments, the case of backward pumping has been described. In the third embodiment, the case of forward pumping will be described. Specifically, in the first and second embodiments, a backward pumping configuration in which pump light propagates only in the opposite direction to the signal light through the optical fiber transmission line has been described as an example, but the Raman amplifier 100 can also be applied to a forward pumping configuration in which pump light propagates in the same direction as the signal light through the optical fiber transmission line.

[0101] FIG. 9 is a block diagram showing the function of a Raman amplifier according to the third embodiment. As described above, the third embodiment is a case of forward pumping. Therefore, the arrangement of the third embodiment is different from that of the first embodiment. Although the arrangement is different, the function is the same. For example, as shown in FIG. 4, when a C-band optical signal and an L-band optical signal are received, the control unit 110 controls the pumping light source 111 for the C band and the pumping light source 112 for the L band so that pump light is output.

[0102] Also in the third embodiment, similarly to the first embodiment, for example, as shown in Fig. 6, when only a C-band optical signal is received, the control unit 110 controls the C-band pumping light source 111 so as to output pumping light with a power smaller than the power of the pumping light output by the C-band pumping light source 111 when a C-band optical signal and an L-band optical signal are received. As shown in Fig. 8, when only an L-band optical signal is received, the control unit 110 controls the L-band pumping light source 112 so as to output pumping light with a power larger than the power of the pumping light output by the L-band pumping light source 112 when a C-band optical signal and an L-band optical signal are received.

[0103] Forward pumping can also be applied to the wavelength division multiplexing optical transmission system and the method for controlling pumping light in a Raman amplifier according to embodiment 3. In the method for controlling pumping light in a Raman amplifier according to embodiment 3, forward pumping can be considered to be one of the processing steps of the method for controlling pumping light.

[0104] Also in the third embodiment, as in the second embodiment, for example, when an L-band optical signal is additionally received from a state in which only C-band optical signals are being received, the control unit 110 controls the L-band pumping light source 112 to output pumping light, and controls the C-band pumping light source 111 so that the pumping light has a power greater than the power of the pumping light output by the C-band pumping light source 111 when only the C-band optical signal is being received. This results in the state shown in FIG. 4. Also, when an L-band optical signal is additionally received from a state in which only L-band optical signals are being received, the control unit 110 controls the C-band pumping light source 111 to output pumping light, and controls the L-band pumping light source 112 so that the pumping light has a power less than the power of the pumping light output by the L-band pumping light source 112 when only the L-band optical signal is being received. This results in the state shown in FIG. 4.

[0105] Furthermore, when the C-band pumping light source 111 and the L-band pumping light source 112 output pumping light, the multiplexer 113 multiplexes the pumping light output by the C-band pumping light source 111 and the pumping light output by the L-band pumping light source 112. The multiplexed pumping light is output via the pumping light multiplexer 101 in the same direction as the optical signal input to the Raman amplifier 100. That is, the multiplexed pumping light is output in the direction indicated by the arrow 11. In this manner, forward pumping is performed. The optical signal input to the Raman amplifier 100 obtains gain based on the pumping light. The arrow 11 points to the other end of the optical fiber transmission line, i.e., the direction of the wavelength-division multiplexing optical receiver 300 on the optical fiber transmission line. Of course, as described above in the first embodiment, one or more other Raman amplifiers 100 may be present in the direction toward the wavelength-division multiplexing optical receiver 300.

[0106] Furthermore, when only the pumping light output from the C-band pumping light source 111 is input to the multiplexer 113, no multiplexing is performed, and the pumping light output from the C-band pumping light source 111 is output in the direction indicated by the arrow 11. Furthermore, when only the pumping light output from the L-band pumping light source 112 is input to the multiplexer 113, no multiplexing is performed, and the pumping light output from the L-band pumping light source 112 is output in the direction indicated by the arrow 11.

[0107] The forward-pumping configuration of the Raman amplifier 100 shown in Fig. 9 operates in the same manner as the backward-pumping configuration of the Raman amplifier 100 shown in Fig. 2, and in a system performing wavelength-division multiplexed optical transmission using Raman amplification in multiple wavelength bands, even when signal light in some wavelength bands is lost or added, the configuration suppresses changes in the power of the signal light, thereby suppressing deterioration in the bit error rate due to an increase or decrease in the number of transmitted signal wavelength bands and maintaining high transmission quality. As described above, according to the third embodiment, the Raman amplifier 100 performs forward pumping, thereby achieving the same effect as the backward pumping in the Raman amplifiers of the first and second embodiments. Although the Raman amplifier 100 has been described as amplifying optical signals in two bands, the C band and the L band, the number of wavelength bands of optical signals amplified by the Raman amplifier 100 in the third embodiment may, of course, be three or more.

[0108] Embodiment 4 Next, a fourth embodiment will be described. In the fourth embodiment, differences from the first to third embodiments will be mainly described. Furthermore, in the fourth embodiment, description of matters common to the first to third embodiments will be omitted. Of course, the same relationship as that between the Raman amplifiers of the first to third embodiments and the wavelength-multiplexed optical transmission systems and methods for controlling pumping light of the Raman amplifiers of the first to third embodiments also holds true in the fourth embodiment, so the description will be focused on the Raman amplifier of the fourth embodiment, and the wavelength-multiplexed optical transmission system and method for controlling pumping light of the Raman amplifier of the fourth embodiment will be briefly described.

[0109] In the fourth embodiment, a case where bidirectional pumping is performed will be described. Specifically, in the first and second embodiments, a backward pumping configuration in which pump light propagates only in the opposite direction to the signal light through the optical fiber transmission line was illustrated, and in the third embodiment, a forward pumping configuration in which pump light propagates only in the same direction as the signal light through the optical fiber transmission line was illustrated. However, the Raman amplifier 100 can also be applied to a bidirectional pumping configuration in which pump light propagates both in the same direction as the signal light through the optical fiber transmission line and in the opposite direction. As shown in FIG. 10, the arrow 10 illustrated in the backward pumping configuration also points toward one end of the optical fiber transmission line, i.e., toward the wavelength-multiplexed optical transmitter 200 on the optical fiber transmission line. As shown in FIG. 10, the arrow 11 illustrated in the forward pumping configuration also points toward the other end of the optical fiber transmission line, i.e., toward the wavelength-multiplexed optical receiver 300 on the optical fiber transmission line.

[0110] 10 is a block diagram showing the functions of the Raman amplifier according to the embodiment 4. The Raman amplifier 100 further includes a control unit 114, a C-band pumping light source 115, an L-band pumping light source 116, a multiplexer 117, and a signal unit 118. Here, the control unit 114 is also referred to as a second control unit. The C-band pumping light source 115 is also referred to as a third pumping light source. The L-band pumping light source 116 is also referred to as a fourth pumping light source. The control unit 110 of the fourth embodiment has a determination unit, a storage unit, a calculation and setting unit, and a pumping light control unit, similar to the control unit 110 of the first embodiment. The control unit 114 has a determination unit, a storage unit, a calculation and setting unit, and a pumping light control unit, similar to the control unit 110 of the third embodiment. In other words, the control unit 110 of the fourth embodiment controls backward pumping, similar to the control unit 110 of the first and second embodiments, and the control unit 114 of the fourth embodiment controls forward pumping, similar to the control unit 110 of the second and third embodiments.

[0111] The functions of the control unit 114, the C-band pumping light source 115, the L-band pumping light source 116, the multiplexer 117, and the signal unit 118 are the same as the functions of the control unit 110, the C-band pumping light source 111, the L-band pumping light source 112, the multiplexer 113, and the pumping light multiplexer 101 in the third embodiment. For example, as shown in Fig. 4, when an optical signal in a C band and an optical signal in an L band are received, the control unit 110 and the control unit 114 control the C-band pumping light source 111 and the L-band pumping light source 112, and the C-band pumping light source 115 and the L-band pumping light source 116 so that pump light is output.

[0112] Bidirectional pumping can also be applied to the wavelength division multiplexing optical transmission system and the method for controlling pumping light in a Raman amplifier according to embodiment 4. In the method for controlling pumping light in a Raman amplifier according to embodiment 4, bidirectional pumping can be considered to be one of the processing steps of the method for controlling pumping light.

[0113] Also in the fourth embodiment, similarly to the first and third embodiments, for example, as shown in Fig. 6, when only a C-band optical signal is received, the control unit 110 and the control unit 114 control the C-band pumping light source 111 and the C-band pumping light source 115 so that the pumping light has a power smaller than the power of the pumping light that the C-band pumping light source 111 and the C-band pumping light source 115 output when the C-band optical signal and the L-band optical signal are received. As shown in Fig. 8, when only an L-band optical signal is received, the control unit 110 and the control unit 114 control the L-band pumping light source 112 and the L-band pumping light source 116 so that the pumping light has a power larger than the power of the pumping light that the L-band pumping light source 112 and the L-band pumping light source 116 output when the C-band optical signal and the L-band optical signal are received.

[0114] Also in the fourth embodiment, similarly to the second and third embodiments, for example, when an L-band optical signal is additionally received from a state in which only a C-band optical signal is being received, the control unit 110 and the control unit 114 control the L-band pumping light source 112 and the L-band pumping light source 116 to output pumping light, and control the C-band pumping light source 111 and the C-band pumping light source 115 so that the pumping light output has a power greater than the power of the pumping light output by the C-band pumping light source 111 and the C-band pumping light source 115 when only a C-band optical signal is received. In this way, the state shown in FIG. 4 is achieved. Furthermore, when an optical signal in the C band is additionally received from a state in which only an optical signal in the L band is being received, the control unit 110 and the control unit 114 control the pump light source 111 for the C band and the pump light source 115 for the C band to output pump light, and control the pump light source 112 for the L band and the pump light source 116 for the L band so that pump light having a power smaller than the power of the pump light output by the pump light source 112 for the L band and the pump light source 116 when only an optical signal in the L band is received. In this way, the state shown in FIG. 4 is achieved.

[0115] In this way, the functions of the control unit 114 and the like are the same as the functions of the control unit 110 and the like in the first to third embodiments. Therefore, a detailed description of the functions of the control unit 114 and the like will be omitted.

[0116] The pumping light output from the C-band pumping light source 111 and the C-band pumping light source 115, or the L-band pumping light source 112 and the L-band pumping light source 116, provides bidirectional pumping.

[0117] The bidirectional pumping configuration of the Raman amplifier 100 shown in FIG. 10 operates in the same manner as the backward pumping configuration of the Raman amplifier 100 shown in FIG. 2 and the forward pumping configuration of the Raman amplifier 100 shown in FIG. 9 . In a system performing wavelength-division multiplexed optical transmission using Raman amplification in multiple wavelength bands, the configuration suppresses changes in the power of the signal light even when signal light in some wavelength bands is lost or added. This suppresses deterioration in the bit error rate due to an increase or decrease in the number of transmitted signal wavelength bands, and makes it possible to maintain high transmission quality. As described above, according to the fourth embodiment, the Raman amplifier 100 performs bidirectional pumping, thereby achieving the same effects as the backward pumping or forward pumping in the Raman amplifiers of the first to third embodiments. While the Raman amplifier 100 has been described as amplifying optical signals in two bands, the C band and the L band, the number of wavelength bands of optical signals amplified by the Raman amplifier 100 in the third embodiment may, of course, be three or more.

[0118] Embodiment 5. Next, a fifth embodiment will be described. In the fifth embodiment, differences from the first to fourth embodiments will be mainly described. Furthermore, in the fifth embodiment, a description of the commonalities with the first to fourth embodiments will be omitted. Of course, the same relationships as those between the Raman amplifiers of the first to fourth embodiments and the wavelength-multiplexed optical transmission systems and the methods for controlling pumping light of the Raman amplifiers of the first to fourth embodiments also hold true in the fifth embodiment, so the description will be focused on the Raman amplifier of the fifth embodiment, and the wavelength-multiplexed optical transmission system and the method for controlling pumping light of the Raman amplifier of the fifth embodiment will be briefly described.

[0119] 11 is a block diagram showing the functions of a Raman amplifier 100 according to the fifth embodiment. The Raman amplifier 100 further includes an optical filter 119. The optical filter 119 removes light having a wavelength shorter than the wavelength of an optical signal in the C-band, which is the shortest wavelength band. The inclusion of the optical filter 119 makes it possible to more accurately determine the presence or absence of signal light in the C-band. Fig. 11 shows a case where an optical filter 119 is present between the splitter 103 and the optical receiver 105 of the Raman amplifier 100 of Fig. 2. The optical filter 119 may also be present between the splitter 103 and the optical receiver 105 of the Raman amplifier 100 of Figs. 9 and 10.

[0120] Here, if the optical filter 119 is not present, a portion of the pump light may be input to the optical receiver 105. This may result in an error in detecting the C-band optical signal. This is because, of the two paths branched by the demultiplexer 102, the wavelength component of the pump light with a shorter wavelength is branched to the C-band path, and the pump light component may be mixed in with the optical receiver 105 that detects the power of the C-band signal light, which may result in a detection error (misdetection error). Furthermore, there may be cases where the reflected light of the pump light is strong or where forward pump light is input to the optical receiver 105. Even in these cases, an error in detecting the C-band optical signal may occur. Therefore, the optical filter 119 is disposed between the branching filter 103 and the optical receiver 105. This prevents an error in detecting the C-band optical signal. Therefore, by including the optical filter 119, the Raman amplifier 100 can improve the detection accuracy of the C-band optical signal.

[0121] As described above, the Raman amplifier 100 of the fifth embodiment is provided with the optical filter 119, and the optical receiver 105 receives only the C-band signal light and accurately converts its power into an electrical signal for output. This suppresses changes in the power of the signal light even when the wavelength band in which the signal light is transmitted changes, thereby suppressing deterioration in the bit error rate that accompanies an increase or decrease in the transmitted signal wavelength band and making it possible to maintain high transmission quality. This also applies to the wavelength division multiplexing optical transmission system and the method for controlling pumping light for a Raman amplifier of the fifth embodiment. Note that in the method for controlling pumping light for a Raman amplifier of the fifth embodiment, the filtering process by the optical filter 119 may be incorporated as a processing step in the preceding processing step.

[0122] The following also applies to the first to fifth embodiments. Generally, optical amplifiers have a lifespan during which excited ions remain in the excited state of the amplification medium. EDFAs, in particular, have a long lifespan. Therefore, when an optical signal changes from a state in which it is not received to a state in which it is received, excess energy accumulated in the optical amplifier is released all at once, causing a transient optical surge at the output of the optical amplifier, which may damage optical components in the wavelength-division multiplexing optical transmission system. By stopping an optical amplifier to which no optical signal is input, the accumulation of excess energy can be avoided, thereby preventing an optical surge from occurring when the optical signal is re-input. Therefore, the Raman amplifier 100 stops the optical amplifier 107 when a C-band optical signal is not received, and stops the optical amplifier 108 when an L-band optical signal is not received. Whether a C-band optical signal has been received may be determined using the monitoring results of a C-band optical power monitor configured by the optical receiver 105 and the determining unit 110a, or may use the monitoring results of another similar optical power monitor (not shown). Similarly, the determination of whether an L-band optical signal has been received may be made using the monitoring results of an L-band optical power monitor configured by the optical receiver 106 and the judgment unit 110b, or may be made using the monitoring results of another similar optical power monitor not shown.

[0123] Although the Raman amplifier 100 has been described as amplifying optical signals in two bands, the C band and the L band, the number of wavelength bands of optical signals amplified by the Raman amplifier 100 in the fifth embodiment may of course be three or more.

[0124] In the first to fifth embodiments, the Raman amplifier, the wavelength division multiplexing optical transmission system, and the method for controlling pump light for a Raman amplifier have been described using wavelengths in the C-band and the L-band, which are wavelength bands used in optical communications and are optical communication wavelength bands other than those designated by convenient names such as band A and band B. Wavelengths other than the C-band and L-band wavelengths may also be used. For example, wavelengths in the O-band and the S-band may also be used. The present invention may also be applied to bands that are not classified according to the conventional divisions such as the C-band and the L-band. For example, the first band may be 1100 to 1280 nm, and the second band may be 1370 to 1470 nm.

[0125] The features of the above-described embodiments can be combined with each other as appropriate. That is, the configurations or processing steps (operations of the configurations) of the Raman amplifier, wavelength-division multiplexed optical transmission system, and Raman amplifier pumping light control method of the embodiments can be combined with each other as appropriate.

[0126] Generally, in a wavelength-division multiplexing optical transmission system, the number of wavelengths and wavelength allocation of signal light are not constant, and the system is operated in various states, and the number of wavelengths and wavelength allocation often change during operation. The Raman amplifier 100 according to the first to fifth embodiments can provide appropriate Raman amplification even if the number of wavelengths and wavelength allocation change during operation in a wavelength-division multiplexing optical transmission system. Furthermore, in order to control the signal light power of each wavelength to be constant, the Raman pump light power corresponding to the desired gain required for Raman amplification is calculated or experimentally determined in advance and prepared, and the required Raman pump light power is output based on this to obtain the desired gain. As a result, the Raman amplifier 100, wavelength-division multiplexing optical transmission system, and Raman amplifier pump light control method according to the first to fifth embodiments do not require control to keep the signal light power of each wavelength constant. Therefore, the Raman amplifier 100, wavelength division multiplexing optical transmission system, and method for controlling pumping light of a Raman amplifier according to the first to fifth embodiments do not suffer from the problem that the power of each signal light cannot be accurately controlled to a desired value depending on the conditions of the band being used, which is a characteristic of Raman amplification in that the Raman amplification gain obtained with the same Raman pumping light power differs when the signal wavelength band to be used is wide and transmission is performed using a plurality of wavelength bands, when signal light is transmitted over all wavelength bands, and when signal light is transmitted over only some wavelength bands.

[0127] The Raman amplifier 100, wavelength-division multiplexing optical transmission system, and pumping light control method for a Raman amplifier according to the first to fifth embodiments can suppress fluctuations in the power of signal light by changing the power of pumping light when all or some of the wavelength bands are used in a wavelength-division multiplexing optical transmission system that uses a plurality of wavelength bands. The Raman amplifier 100, wavelength-division multiplexing optical transmission system, and pumping light control method for a Raman amplifier according to the first to fifth embodiments can maintain a constant gain for the signal light by increasing the power of pumping light in wavelength bands longer than unused wavelength bands and decreasing the power of pumping light in wavelength bands shorter than unused wavelength bands, thereby enabling the signal light to be transmitted under optimal power conditions.

[0128] As described above, the Raman amplifier 100, the wavelength-multiplexed optical transmission system, and the method for controlling pumping light for a Raman amplifier according to the first to fifth embodiments can prevent a loss of signal light in some wavelength bands and a change in the power of signal light in the remaining wavelength bands due to the characteristics of Raman amplification in a system that performs wavelength-multiplexed optical transmission using Raman amplification in multiple wavelength bands. In a system that performs wavelength-multiplexed optical transmission, if the power of each wavelength of the wavelength-multiplexed signal light is excessive, it can cause a deterioration (increase) in the bit error rate due to nonlinear effects, and if it is too low, it can cause a deterioration (increase) in the bit error rate due to an insufficient optical SNR (signal-to-noise ratio). To avoid this and continue signal transmission with a low error rate and high transmission quality, it is necessary to maintain the signal light at a predetermined power. The Raman amplifier 100, the wavelength-multiplexed optical transmission system, and the method for controlling pumping light for a Raman amplifier according to the first to fifth embodiments can suppress the deterioration of the bit error rate that occurs when the number of transmitted signal wavelength bands increases or decreases, thereby maintaining high transmission quality. [Explanation of symbols]

[0129] 10 Arrow, 11 Arrow, 100 Raman amplifier, 100a, 100b Raman amplifier, 101 Pumping light multiplexer, 102 Demultiplexer, 103 Branching device, 104 Branching device, 105 Optical receiver, 106 Optical receiver, 107 Optical amplifier, 108 Optical amplifier, 109 Multiplexer, 110 Control unit, 110a, 110b Determination unit, 110c Memory unit, 110d Calculation setting unit, 110e, 110f Pumping light control unit, 111 C band pumping light source, 111a, 111b Pumping light source, 112 L band pumping light source, 112a, 112b Pumping light source, 113 Multiplexer, 114 Control unit, 115 C band pumping light source, 116 L-band pumping light source, 117 multiplexer, 118 signaling device, 119 optical filter, 200 wavelength-multiplexed optical transmitter, 300 wavelength-multiplexed optical receiver.

Claims

1. A Raman amplifier capable of receiving an optical signal including wavelengths in two bands, a signal unit that receives the optical signal; A control unit; and The control unit When the signal unit does not receive an optical signal in a first band of the two bands, a pumping light source that outputs pumping light for the first band is controlled so as to output pumping light with a power lower than that when an optical signal of the first band is being received, or so as to stop outputting pumping light; controlling a pumping light source that outputs pumping light for a wavelength band shorter than the first band so that the pumping light has a power smaller than the power of the pumping light output when an optical signal of the first band is received; the signal unit does not receive an optical signal in a band having a shorter wavelength than the first band out of the two bands, a pumping light source that outputs pumping light for the short wavelength band is controlled so that pumping light with a power lower than that when an optical signal in the short wavelength band is received is output, or so that output of pumping light is stopped; controlling a pumping light source that outputs pumping light for the first band so that the pumping light has a power greater than the power of the pumping light output when an optical signal in the short wavelength band is being received; Raman amplifier.

2. A Raman amplifier capable of receiving an optical signal including wavelengths in two bands, a signal unit that receives the optical signal; A control unit; and The control unit When the signal unit receives an optical signal in a first band of the two bands, controlling a pumping light source that outputs pumping light for the first band so that the pumping light is output; controlling a pumping light source that outputs pumping light for a wavelength band shorter than the first band so that the pumping light has a power greater than the power of the pumping light output when no optical signal of the first band is received; When the signal unit receives an optical signal in a band having a shorter wavelength than the first band out of the two bands, controlling a pumping light source that outputs pumping light for the short wavelength band so as to output pumping light; controlling a pumping light source that outputs pumping light for the first band so that the pumping light has a power smaller than the power of the pumping light output when no optical signal in the short wavelength band is received; Raman amplifier.

3. A Raman amplifier capable of receiving optical signals including wavelengths in multiple bands, a signal unit that receives the optical signal; A control unit; and The control unit When the signal unit does not receive an optical signal in a first band among the plurality of bands, a pumping light source that outputs pumping light for the first band is controlled so as to output pumping light with a power lower than that when an optical signal of the first band is being received, or so as to stop outputting pumping light; a pumping light source that outputs pumping light for a wavelength band shorter than the first band, so that the pumping light has a power smaller than the power of the pumping light output when an optical signal of the first band is received; a pumping light source that outputs pumping light for a wavelength band longer than the first band, so that the pumping light has a power greater than the power of the pumping light output when an optical signal of the first band is received; Control, Raman amplifier.

4. A Raman amplifier capable of receiving optical signals including wavelengths in multiple bands, a signal unit that receives the optical signal; A control unit; and The control unit When the signal unit receives an optical signal of a first band among the plurality of bands, controlling a pumping light source for the first band to output pumping light for the first band; a pumping light source that outputs pumping light for a wavelength band longer than the first band, so that the pumping light has a power smaller than the power of the pumping light output when no optical signal of the first band is received; The pumping light source outputs pumping light for a wavelength band shorter than the first band so that the pumping light has a power greater than the power of the pumping light output when no optical signal of the first band is received, Control, Raman amplifier.

5. The excitation light source is further provided. A Raman amplifier according to any one of claims 1 to 4.

6. a demultiplexing unit that demultiplexes the optical signal output from the signal unit into optical signals of a plurality of bands; an optical power monitor that monitors the optical power of each of the optical signals in the bands demultiplexed by the demultiplexing unit, the optical power monitor determines the presence or absence of an optical signal in each of the bands based on the monitoring result of the optical power; A Raman amplifier according to any one of claims 1 to 4.

7. an optical filter is further provided between the optical power monitor that monitors the optical power of an optical signal in a band having the shortest wavelength among the plurality of bands and the demultiplexing unit; the optical filter removes light having a wavelength shorter than the wavelength of the optical signal having the shortest wavelength band from the light input to the optical power monitor.

7. The Raman amplifier of claim 6.

8. an optical amplifier for amplifying the optical signal received by a signal receiving unit; The optical amplifier is provided for each band of the optical signal, Turn off the optical amplifiers for bands that are not being received. A Raman amplifier according to any one of claims 1 to 4.

9. a storage unit that stores in advance information indicating a relationship between the power of the pump light that can obtain a desired gain of Raman amplification, the bandwidth of the received optical signal, and the control value of each of the pump light sources, the control unit controls the excitation light source based on the information stored in the storage unit. A Raman amplifier according to any one of claims 1 to 4.

10. The plurality of wavelength bands are a C band including 1550 nm and an L band including 1580 nm, A Raman amplifier according to any one of claims 1 to 4.

11. a Raman amplifier according to any one of claims 1 to 4 and an optical fiber transmission line, wherein the signal section receives at least a part of an optical signal propagating through the optical fiber transmission line; Wavelength multiplexing optical transmission system.

12. a wavelength division multiplexing optical transmitter capable of transmitting an optical signal including wavelengths in a plurality of bands; an optical fiber transmission line connected at one end to the wavelength division multiplexing optical transmitter and through which the optical signal transmitted from the wavelength division multiplexing optical transmitter propagates; and a wavelength division multiplexing optical receiver connected at the other end to the optical fiber transmission line and for receiving the optical signal propagated through the optical fiber transmission line, a plurality of Raman amplifiers according to any one of claims 1 to 4 provided on the optical fiber transmission line; Wavelength multiplexing optical transmission system.

13. 1. A method for controlling pumping light of a Raman amplifier capable of receiving optical signals including wavelengths in multiple bands, comprising: When the optical signal having the shortest wavelength among the optical signals including wavelengths of the plurality of bands is no longer received and the optical signal having the remaining longer wavelength band is received, increasing the power of at least one of the pump lights for the remaining long wavelength bands, which is the pump light for the optical signals in the remaining long wavelength bands, compared to when all of the optical signals including the wavelengths in the plurality of bands are received; reducing the power of the pumping light for the shortest wavelength band, which is the pumping light for the optical signal having the shortest wavelength band, or stopping the output of the pumping light for the shortest wavelength band; When the optical signal having the longest wavelength among the optical signals including wavelengths of the plurality of bands is no longer received and the optical signal having the remaining shorter wavelength band is received, lowering the power of at least one of the pump lights for the remaining short wavelength bands, which is the pump light for the optical signals in the remaining short wavelength bands, compared to when all of the optical signals including the wavelengths in the plurality of bands are received; reducing the power of the pumping light for the longest wavelength band, which is the pumping light for the optical signal having the longest wavelength band, or stopping the output of the pumping light for the longest wavelength band; When the optical signal including wavelengths in the plurality of bands is received, the optical signal in the remaining long wavelength band other than the optical signal in the shortest wavelength band, and the optical signal in the shortest wavelength band is further received, reducing the power of at least one of the pump lights for the remaining longer wavelength bands compared to when the optical signal of the shortest wavelength band is not being received; outputting pump light for the shortest wavelength band; When the optical signal including wavelengths in the plurality of bands is received, the optical signal in the remaining short wavelength band other than the optical signal in the longest wavelength band and the optical signal in the longest wavelength band is further received, increasing the power of at least one of the pump lights for the remaining short wavelength bands compared to when the optical signal of the longest wavelength band is not being received; outputting the excitation light for the longest wavelength band; A method for controlling pump light in a Raman amplifier.

14. 1. A method for controlling pumping light of a Raman amplifier capable of receiving optical signals including wavelengths in multiple bands, comprising: When the optical signal having the shortest wavelength band and the optical signal having the band adjacent to the shortest wavelength band on the longer wavelength side of the optical signal having the wavelengths of the plurality of bands are no longer received, and the optical signal having the remaining longer wavelength band is being received, increasing the power of at least one of the pump lights for the remaining long wavelength bands, which is the pump light for the optical signals in the remaining long wavelength bands, compared to when all of the optical signals including the wavelengths in the plurality of bands are received; reducing the power of the pumping light for the band adjacent to the long wavelength side, which is the pumping light for the optical signal in the band adjacent to the long wavelength side, or stopping the output of the pumping light for the band adjacent to the long wavelength side; reducing the power of the pumping light for the shortest wavelength band, which is the pumping light for the optical signal in the shortest wavelength band, and stopping the output of the pumping light for the shortest wavelength band; When the optical signal having the longest wavelength band and the optical signal having the band adjacent to the longest wavelength band on the shorter wavelength side of the optical signal having the longest wavelength band are no longer received among the optical signals including wavelengths of the plurality of bands, and the optical signal having the remaining shorter wavelength band is being received, lowering the power of at least one of the pump lights for the remaining short wavelength bands, which is the pump light for the optical signals in the remaining short wavelength bands, compared to when all of the optical signals including the wavelengths in the plurality of bands are received; Stopping the output of the pumping light for the band adjacent to the short wavelength side, which is the pumping light for the optical signal in the band adjacent to the short wavelength side, and the output of the pumping light for the longest wavelength band, which is the pumping light for the optical signal in the longest wavelength band, or reducing the power of the pumping light; When the optical signal having the shortest wavelength band and the optical signal having the longest wavelength band are no longer received among the optical signals having wavelengths in the plurality of bands, and the optical signal having the remaining bands is being received, reducing the power of the pumping light for the longest wavelength band, which is the pumping light for the optical signal having the longest wavelength band, or stopping the output of the pumping light for the longest wavelength band; reducing the power of the pumping light for the shortest wavelength band, which is the pumping light for the optical signal having the shortest wavelength band, or stopping the output of the pumping light for the shortest wavelength band; The power of the pump light for the remaining band, which is the pump light for the optical signal of the remaining band, is an amount of power transition that is no longer received from the pump light for the shortest wavelength band, or an amount of power transition that is received from the pump light for the shortest wavelength band that is reduced compared to when all optical signals including wavelengths of the plurality of bands are received; and the amount of power that is no longer required to be shifted to the pump light for the longest wavelength band; Output the offset value, When the optical signal in the remaining long wavelength band other than the optical signal in the shortest wavelength band and the optical signal in the band adjacent to the long wavelength side is received, and the optical signal in the shortest wavelength band and the optical signal in the band adjacent to the long wavelength side are further received, lowering the power of at least one of the pump lights for the remaining long wavelength bands compared to when the optical signal of the shortest wavelength band and the optical signal of the band adjacent to the long wavelength side are not received; outputting pumping light for the shortest wavelength band and pumping light for the band adjacent to the long wavelength side; When an optical signal in the remaining short wavelength band other than the optical signal in the longest wavelength band and the optical signal in the band adjacent to the short wavelength side is received, and the optical signal in the longest wavelength band and the optical signal in the band adjacent to the short wavelength side are further received, increasing the power of at least one of the pump lights for the remaining short wavelength bands compared to when the optical signal of the longest wavelength band and the optical signal of the band adjacent to the short wavelength side are not received; outputting pumping light for the longest wavelength band and pumping light for the band adjacent to the short wavelength side; When the optical signals of the remaining bands other than the optical signal of the shortest wavelength band and the optical signal of the longest wavelength band are received, and the optical signal of the shortest wavelength band and the optical signal of the longest wavelength band are further received, outputting pumping light for the shortest wavelength band and pumping light for the longest wavelength band; The power of the pump light for the remaining bands is output at a value obtained by offsetting the amount of power transition from the pump light for the band having the shortest wavelength and the amount of power transition required to the pump light for the band having the longest wavelength. A method for controlling pump light in a Raman amplifier.

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