Optical transmission device, optical transmission system, and optical transmission method

Abstract

To provide an optical transmission device, an optical transmission system, and an optical transmission method that suppress an OSNR penalty due to return light.SOLUTION: An optical transmission device disposed at a fiber end of a single-core optical fiber and used when signal light is bidirectionally transmitted through the optical fiber, the optical transmission device comprising an optical transmitter configured to transmit first signal light in one direction belonging to the signal light, an optical receiver configured to receive second signal light in a reverse direction belonging to the signal light, and a controller configured to guide the first signal light transmitted from the optical transmitter to the optical fiber, An optical circulator configured to guide the second signal light from the optical fiber to the optical receiver; and a light source configured to output, to the optical fiber, excitation light that Raman-amplifies the second signal light to second optical power higher than first optical power of the first signal light.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical transmission device, an optical transmission system, and an optical transmission method. [Background technology]

[0002] A WDM (Wave-length-Division Multiplexing) system using both a forward-pumped distributed Raman fiber amplifier and a backward-pumped distributed Raman fiber amplifier is known (see, for example, Patent Document 1). Also known is a multi-band WDM system using multiple bands (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2007 / 0058242 [Patent Document 2] U.S. Patent No. 6,359,725 Summary of the Invention [Problem to be solved by the invention]

[0004] Optical transmission systems such as WDM systems transmit signal light using a pre-designed transmission method. Some optical transmission systems transmit signal light using a transmission method called same-wavelength, single-fiber, bidirectional transmission. Same-wavelength, single-fiber, bidirectional transmission is a transmission method that transmits signal light of the same wavelength in both directions using a single optical fiber.

[0005] However, in single-core bidirectional transmission using the same wavelength, the optical fiber is shared for both transmitting and receiving the signal light. Therefore, return light due to the transmission of the signal light may enter the optical transmission device connected to the fiber end of the optical fiber. For example, if Fresnel reflection due to the transmission of the signal light occurs at the optical connector connecting the fiber end of the optical fiber and the optical transmission device, the light due to Fresnel reflection may enter the optical transmission device as return light. Furthermore, if Rayleigh scattering due to the transmission of the signal light occurs in the fiber of the optical fiber, the light due to Rayleigh scattering may enter the optical transmission device as return light.

[0006] When such return light enters an optical transmission device when the optical transmission device receives an optical signal, the return light is superimposed on the optical signal received by the optical transmission device as noise, degrading the signal quality of the optical signal. In particular, if the amount of crosstalk, defined as the relationship between the optical signal received by the optical transmission device and the return light, is greater than a reference amount, an OSNR (Optical Signal to Noise Ratio) penalty may occur.

[0007] Therefore, in one aspect, an object is to provide an optical transmission device, an optical transmission system, and an optical transmission method that suppress the OSNR penalty caused by the returned light. [Means for solving the problem]

[0008] In one embodiment, the optical transmission device is arranged at the fiber end of a single-core optical fiber and is used to transmit signal light bidirectionally through the optical fiber, and includes an optical transmitter that transmits a first signal light in one direction belonging to the signal light, an optical receiver that receives a second signal light in the opposite direction belonging to the signal light, an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver, and a light source that outputs pump light to the optical fiber to Raman amplify the second signal light to a second optical power higher than the first optical power of the first signal light.

[0009] In one embodiment, the optical transmission device is an optical transmission device used when transmitting signal light bidirectionally through a single optical fiber, and includes an optical transmitter that transmits a first signal light in one direction belonging to the signal light, an optical receiver that receives a second signal light in the opposite direction belonging to the signal light, an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver, and an optical amplifier that amplifies the second signal light without amplifying the first signal light. [Effects of the Invention]

[0010] The OSNR penalty due to optical feedback can be suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an example of an optical transmission system according to a first embodiment. [Figure 2] 1 is an example of a block diagram of an optical transmission device according to a first embodiment. [Figure 3] FIG. 10 is a diagram illustrating a comparative example. [Figure 4] FIG. 1 is a diagram illustrating an embodiment (part 1). [Figure 5] FIG. 10 is an example of a block diagram of an optical transmission device according to a second embodiment. [Figure 6] 10 is an example of an optical transmission system according to a third embodiment. [Figure 7] FIG. 10 is an example of a block diagram of an optical transmission device according to a third embodiment. [Figure 8] FIG. 10 is an example of a block diagram of an optical repeater according to a third embodiment. [Figure 9] FIG. 10 is an example of a block diagram of an optical transmission device according to a fourth embodiment. [Figure 10] FIG. 10 is an example of a block diagram of an optical repeater according to a fourth embodiment. [Figure 11] FIG. 10 is a diagram illustrating an embodiment (part 2). [Figure 12] FIG. 10 is an example of a block diagram of an optical transmission device according to a fifth embodiment. [Figure 13]FIG. 13 is an example of a block diagram of an optical transmission device according to a sixth embodiment. [Figure 14A] FIG. 13 is an example of a block diagram of an optical repeater according to a sixth embodiment. [Figure 14B] FIG. 13 is another example of a block diagram of the optical repeater according to the sixth embodiment. [Figure 15] 10 is a flowchart illustrating an example of the operation of a control unit. [Figure 16] FIG. 13 is an example of a block diagram of an optical transmission device according to a seventh embodiment. [Figure 17] FIG. 13 is an example of a block diagram of an optical transmission device according to an eighth embodiment. [Figure 18] FIG. 10 is a diagram for explaining an embodiment (part 3). DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] (First embodiment) 1, the optical transmission system ST1 includes optical transmission devices 10 and 20. The optical transmission devices 10 and 20 are connected to each other via a single optical fiber 30. One fiber end of the optical fiber 30 is connected to the optical transmission device 10. The other fiber end of the optical fiber 30 is connected to the optical transmission device 20. The signal light 41 is an example of a first signal light, and the signal light 42 is an example of a second signal light.

[0014] When a client signal is input to the optical transmission device 10, the optical transmission device 10 converts the client signal into signal light 41 and transmits it to the optical fiber 30. As a result, the signal light 41 propagates through the optical fiber 30. The optical transmission device 20 receives the signal light 41 from the optical fiber 30. Upon receiving the signal light 41, the optical transmission device 20 converts the signal light 41 into a client signal and outputs the client signal. The client signal is an electrical digital signal such as an Ethernet (registered trademark) signal. The client signal may be a main signal or a control signal that includes only parameters for adjusting transmission characteristics, etc.

[0015] On the other hand, when a client signal is input to the optical transmission device 20, the optical transmission device 20 converts the client signal into signal light 42 and transmits it to the optical fiber 30. As a result, the signal light 42 propagates through the optical fiber 30 in the opposite direction to the signal light 41. The optical transmission device 10 receives the signal light 42 from the optical fiber 30. Upon receiving the signal light 42, the optical transmission device 10 converts the signal light 42 into a client signal and outputs it. As will be described in detail later, in order to effectively use the signal bandwidth, it is assumed that the wavelength of the signal light 42 is the same as the wavelength of the signal light 41, but the wavelengths do not have to be the same.

[0016] Here, Rayleigh scattering may occur within the optical fiber 30 due to the transmission of the signal light 41. Rayleigh scattering occurs whether or not the wavelengths of the signal lights 41 and 42 are the same. When Rayleigh scattering occurs, light due to Rayleigh scattering (hereinafter referred to as Rayleigh scattered light) 43 enters the optical transmission device 10 as returned light. The Rayleigh scattered light 43 propagates through the optical fiber 30 in a direction (e.g., an upstream direction) opposite to the direction (e.g., a downstream direction) in which the signal light 41 propagates. When the Rayleigh scattered light 43 enters the optical transmission device 10, the signal quality of the signal light 42 is degraded due to the Rayleigh scattered light 43 being superimposed on the signal light 42.

[0017] Although Rayleigh scattering also occurs due to the transmission of the signal light 42, the Rayleigh scattered light due to this Rayleigh scattering is omitted in Fig. 1. The Rayleigh scattered light based on the transmission of the signal light 42 propagates through the optical fiber 30 in a direction (e.g., a downstream direction) opposite to the direction (e.g., an upstream direction) in which the signal light 42 propagates.

[0018] Next, the optical transmission device 10 will be described in detail with reference to FIG.

[0019] The optical transmission device 10 includes a transponder 110, an optical circulator 120, and a Raman amplifier 130. The Raman amplifier 130 is an example of a light source. The transponder 110 includes an optical transmitter (represented as Tx in FIG. 2) 111 and an optical receiver (represented as Rx in FIG. 2) 112. The optical circulator 120 is connected to optical connectors 121, 122, and 123. The Raman amplifier 130 includes a primary pumping light source (represented as PUMP#1 in FIG. 2) 131, an optical filter 132, a PD (Photo Diode) 133, and a control unit (represented as CTRL in FIG. 2) 134. The primary pumping light source 131 is an example of a first light source.

[0020] An optical amplifier 11 is provided between the optical transmitter 111 and the optical connector 121. An optical amplifier 12 is provided between the optical receiver 112 and the optical connector 123. The optical amplifiers 11 and 12 include, for example, EDFAs (Erbium Doped Fiber Amplifiers). The optical amplifiers 11 and 12 may include SOAs (Semiconductor Optical Amplifiers) instead of EDFAs.

[0021] The optical transmitter 111 transmits signal light 41 toward the optical transmission device 20. The optical amplifier 11 amplifies the signal light 41. The optical circulator 120 allows the signal light 41 to pass in the direction of the optical connector 122. That is, the optical circulator 120 allows the signal light 41 to pass from the optical connector 121 toward the optical connector 122. As a result, the optical circulator 120 guides the signal light 41 transmitted from the optical transmitter 111 to the optical fiber 30. On the other hand, the optical circulator 120 blocks the signal light 41 from passing in the direction of the optical connector 123. That is, the optical circulator 120 blocks the signal light 41 from passing in the direction from the optical connector 121 toward the optical connector 123. As a result, the signal light 41 can reach the optical connector 122 but cannot reach the optical connector 123.

[0022] Here, the signal light 41 is amplified by the optical amplifier 11. Therefore, the optical power of the signal light 41 after passing through the optical circulator 120 is higher than the optical power of the signal light 41 before being amplified by the optical amplifier 11. In the optical connector 122 indirectly connected to the fiber end of the optical fiber 30, Fresnel reflection due to the transmission of the signal light 41 may occur. When Fresnel reflection occurs, light 44 due to the Fresnel reflection (hereinafter referred to as Fresnel reflected light) enters the optical circulator 120 as returned light.

[0023] Here, the optical circulator 120 allows the transmission of signal light 42, Rayleigh scattered light 43, and Fresnel reflected light 44 from the optical transmission device 20 facing the optical transmission device 10 toward the optical connector 123 via the optical fiber 30. That is, the optical circulator 120 allows the transmission of the signal light 42 and the like traveling from the optical connector 122 toward the optical connector 123. On the other hand, the optical circulator 120 blocks the transmission of the signal light 42, Rayleigh scattered light 43, and Fresnel reflected light 44 toward the optical connector 121. That is, the optical circulator 120 blocks the transmission of the signal light 42 and the like traveling from the optical connector 122 toward the optical connector 121. As a result, bidirectional transmission is performed on the optical connector 122 side, and unidirectional transmission is performed on the optical connectors 121 and 123 sides.

[0024] The optical amplifier 12 amplifies the signal light 42, the Rayleigh scattered light 43, and the Fresnel reflected light 44. The optical receiver 112 receives the signal light 42, the Rayleigh scattered light 43, and the Fresnel reflected light 44 in the opposite direction to the signal light 41 heading toward the optical transmission device 20. That is, the optical circulator 120 described above guides the signal light 42 from the optical fiber 30 to the optical receiver 112.

[0025] The primary pumping light source 131 includes a laser light source and outputs, for example, incoherent pumping light as the primary pumping light 51. The primary pumping light 51 is guided in the same direction as the signal light 41 by an optical filter 132. As a result, the primary pumping light 51 is guided to an optical connector 124 that is directly connected to the fiber end of the optical fiber 30, and propagates through the optical fiber 30. The primary pumping light 51 amplifies the signal light 41 within the optical fiber 30 due to the Raman amplification phenomenon. The Raman amplification phenomenon includes a distributed Raman method, but does not include a lumped Raman method.

[0026] Since the signal light 41 and the primary pumping light 51 propagate in the same direction through the optical fiber 30, forward pumping Raman amplification occurs. Furthermore, since the signal light 42 from the optical transmission device 20 propagates in the opposite direction to the primary pumping light 51, backward pumping Raman amplification occurs. Therefore, the primary pumping light 51 simultaneously performs forward pumping Raman amplification on the signal light 41 and backward pumping Raman amplification on the signal light 42.

[0027] The signal light 41 is co-propagated Raman amplified by the primary pump light 51, then propagates through the optical fiber 30, and is counter-propagated Raman amplified by the Raman amplifier 130 on the optical transmission device 20 side. Similarly, the signal light 42 from the optical transmission device 20 is co-propagated Raman amplified by the Raman amplifier 130 on the optical transmission device 20 side, then propagates through the optical fiber 30, and is counter-propagated Raman amplified by the primary pump light 51.

[0028] Here, a branching coupler 135 is provided between the primary pumping light source 131 and the optical filter 132. The branching coupler 135 guides a portion of the primary pumping light 51 to the PD 133. The PD 133 includes a monitor that measures the optical power of the primary pumping light 51 and electrically outputs the measurement result of the optical power to the control unit 134. The monitor includes, for example, an OCM (Optical Channel Monitor). The control unit 134 includes, for example, a processor such as an FPGA (Field Programmable Gate Array) or a CPU (Central Processing Unit), a driver circuit, and the like, and controls the operation of the primary pumping light source 131 based on the measurement result.

[0029] The optical transmission device 20 has basically the same configuration as the optical transmission device 10, and therefore a detailed description thereof will be omitted. For example, since the primary pump light 51 is emitted in the same direction as the signal light 41, the optical transmission device 10 transmits the primary pump light 51 in the same direction as the signal light 41. For the same reason, the optical transmission device 20 transmits the primary pump light 51A output from the Raman amplifier 130 of the optical transmission device 20 in the same direction as the signal light 42. The optical transmitter 111 included in the optical transmission device 20 is an example of a counter transmitter. The optical receiver 112 included in the optical transmission device 20 is an example of a counter receiver. The optical circulator 120 included in the optical transmission device 20 is an example of a counter circulator. The Raman amplifier 130 included in the optical transmission device 20 is an example of a counter light source.

[0030] Next, the effects of the present invention will be described in comparison with a comparative example with reference to Figures 3 and 4. In Figures 3 and 4, the optical power of downstream signal light 41 and upstream signal light 42, which are opposed to each other in bidirectional transmission, within the optical fiber 30 are plotted according to the distance from the optical transmission device 10.

[0031] 3, a comparative example will be described in which the optical transmission device 10 does not include the Raman amplifier 130 and the optical transmission device 20 does not include a Raman amplifier (not shown). In the comparative example, the optical power of the downstream signal light 41 decreases linearly as the transmission distance increases. That is, the optical power of the signal light 41 transmitted from the optical transmission device 10 decreases as it propagates through the optical fiber 30. By the time the optical transmission device 20, which is separated by a transmission distance D, such as several hundred kilometers, from the optical transmission device 10 receives the signal light 41, the optical power of the signal light 41 reaches a minimum.

[0032] Similarly, the optical power of the upstream signal light 42 also decreases linearly as the transmission distance increases. That is, the optical power of the signal light 42 transmitted from the optical transmission device 20 decreases as it propagates through the optical fiber 30. By the time the optical transmission device 10 receives the signal light 42, the optical power of the signal light 42 has reached a minimum.

[0033] Here, if the amount of crosstalk, defined as the relationship between the signal light 42 received by the optical transmission device 10 and the returned light, is greater than a reference amount (such as -35 dB), an OSNR penalty may occur. Given an error rate when there is no crosstalk and an error rate when there is crosstalk, the OSNR penalty represents the difference in OSNR corresponding to the error between the two cases, based on the relationship between the OSNR and the error rate when there is no crosstalk.

[0034] The amount of crosstalk is defined by, for example, the following formula (1). The relationship between the amount of crosstalk and the OSNR penalty can be determined by referring to the following document (1). In this embodiment, the sign of the amount of crosstalk defined in document (1) is inverted and defined. <Formula (1)> Crosstalk amount = -10 × log (optical power of return light / received power of signal light) <Reference (1)> Penalties from In-Band Crosstalk for Advanced Optical Modulation Formats” PJ Winzer, AH Gnauck, A Konczykowska, F Jorge, JY Dupuy 2011 37th European Conference and Exhibition on Optical Communication, 2011

[0035] In this way, if the amount of crosstalk is greater than the reference amount, there is a risk of an OSNR penalty occurring, so it is desirable that the amount of crosstalk is less than the reference amount. In this embodiment, the amount of crosstalk is defined by the optical power at the position of the optical connector 122 (see FIG. 2), for example. The amount of crosstalk may be defined at the optical connector 124 instead of the optical connector 122.

[0036] In this case, the amount of crosstalk is defined by the following equation (2): According to equation (2), if the reception power is increased relatively compared to the transmission power, the amount of crosstalk is reduced. <Formula (2)> Crosstalk amount = -10 × log((P_Tx × R) / P_Rx) where P_Tx is the transmission power, P_Rx is the reception power, and R is the return loss at the optical connector.

[0037] In the first embodiment, the optical transmission device 10 includes a Raman amplifier 130, and the optical transmission device 20 includes a Raman amplifier (not shown). Signal light 41 propagates through the optical fiber 30 in a direction from the optical transmission device 10 toward the optical transmission device 20. Signal light 42 propagates through the optical fiber 30 in a direction from the optical transmission device 20 toward the optical transmission device 10. Due to such bidirectional transmission, for example, forward pumping based on the primary pumping light 51 and backward pumping based on the primary pumping light transmitted from the optical transmission device 20 are simultaneously performed on the signal light 41. That is, the signal light 41 is amplified by the Raman amplification phenomenon. When incoherent pumping light is used as the primary pumping light 51, degradation of signal quality due to Relative Intensity Noise (RIN) is suppressed even when it acts as forward pumping.

[0038] On the other hand, although Raman amplification occurs within the optical fiber 30 up to a distance of approximately several tens of kilometers, the increase in power is only a few dB. Therefore, even if the signal light 41 is amplified by Raman amplification, the increase in power of the signal light 41 is small. Therefore, it is assumed that the optical power of Fresnel reflected light 44 resulting from the transmission of the signal light 41 will remain unchanged. When the optical amplifier 11 is set to reduce its output, the optical power of the signal light 41 decreases. This not only reduces the optical power of the Fresnel reflected light 44, but also reduces the optical power of the Rayleigh scattered light 43 resulting from the transmission of the signal light 41. In other words, the optical power of the returned light decreases.

[0039] Furthermore, near the optical transmission device 10, the optical power of the signal light 42 is amplified by backward Raman amplification due to the primary pump light 51. That is, the optical power of the signal light 42 received by the optical transmission device 10 increases. This reduces the ratio of the optical power of the returned light to the reception power of the signal light 42 received by the optical transmission device 10. As a result, the amount of crosstalk becomes smaller than the reference amount, and the occurrence of OSNR penalty is suppressed.

[0040] 4, for example, near the optical transmission device 10, unlike the case shown in FIG. 3, the optical power of the upstream signal light 42 and the optical power of the downstream signal light 41 are reversed, and the optical power of the upstream signal light 42 is relatively greater than the optical power of the downstream signal light 41. This is because, near the optical transmission device 10, the optical power of the upstream signal light 42 is amplified by backward pumping Raman amplification due to the primary pump light 51 from the optical transmission device 10. Also near the optical transmission device 20, as in the case of the optical transmission device 10, the optical power of the upstream signal light 42 transmitted by the optical transmission device 20 and the optical power of the downstream signal light 41 received by the optical transmission device 20 are reversed. That is, also near the optical transmission device 20, the optical power of the downstream signal light 41 is relatively greater than the optical power of the upstream signal light 42. This is because the optical power of the downstream signal light 41 is amplified by backward Raman amplification due to the primary pumping light 51A from the optical transmission device 20 near the optical transmission device 20. As a result, the amount of crosstalk becomes smaller than the reference amount, and the occurrence of OSNR penalty is suppressed.

[0041] (Second embodiment) Next, a second embodiment of the present invention will be described with reference to Fig. 5. Note that the same components as those in the optical transmission device 10 according to the first embodiment are basically denoted by the same reference numerals, and detailed descriptions thereof will be omitted. This also applies to the embodiments described below. The Raman amplifier 130 according to the second embodiment differs from the Raman amplifier 130 according to the first embodiment in that it further includes a secondary pumping light source (denoted as PUMP#2 in Fig. 2) 136. The secondary pumping light source 136 is an example of a second light source.

[0042] The secondary pumping light source 136 includes a laser light source and outputs coherent pumping light as secondary pumping light 52. The secondary pumping light 52 Raman amplifies the primary pumping light 51, and the wavelength band of the secondary pumping light 52 is shorter than the wavelength band of the primary pumping light 51. The secondary pumping light 52 is guided by the optical filter 132 in the same direction as the signal light 41 and the primary pumping light 51. As a result, the secondary pumping light 52 is guided to the optical connector 124 connected to the fiber end of the optical fiber 30, and propagates through the optical fiber 30. Note that the control unit 134 controls the operation of the secondary pumping light source 136 based on the measurement results obtained by the monitor included in the PD 133, just as in the case of the primary pumping light source 131.

[0043] The secondary pump light 52 amplifies the primary pump light 51 in the optical fiber 30 due to the Raman amplification phenomenon. This increases the optical power of the primary pump light 51. Therefore, not only is the signal quality degradation of the signal light 41 caused by RIN suppressed, but the increased optical power of the primary pump light 51 also amplifies the optical power of the signal light 41 more than in the case where there is no secondary pump light 52. In this second embodiment as well, the OSNR penalty caused by return light such as Rayleigh scattered light 43 and Fresnel reflected light 44 is suppressed.

[0044] (Third embodiment) Next, a third embodiment of the present invention will be described with reference to Fig. 6 to Fig. 8. As shown in Fig. 6, an optical transmission system ST2 according to the third embodiment differs from the optical transmission system ST1 according to the first embodiment (see Fig. 1) in that it further includes an optical repeater 60. Also, as shown in Fig. 7, a transponder 110 according to the third embodiment differs from the transponder 110 according to the first embodiment (see Fig. 2) in that it further includes optical transmitters 113 and 115, optical receivers 114 and 116, an optical multiplexer (represented as MUX in Fig. 7) 117, and an optical demultiplexer (represented as DEMUX in Fig. 7) 118.

[0045] First, the transponder 110 will be described. The optical transmitters 111, 113, and 115 each transmit signal light with a different center wavelength. For example, the optical transmitter 111 transmits signal light with a center wavelength λ1. The optical transmitter 113 transmits signal light with a center wavelength λ2. The optical transmitter 115 transmits signal light with a center wavelength λ3. The optical multiplexer 117 multiplexes the signal light transmitted from the optical transmitters 111, 113, and 115. After multiplexing these signal lights, the optical multiplexer 117 outputs the combined signal light as signal light 41 in the wavelength band λx. The wavelength band λx includes center wavelengths λ1, λ2, and λ3.

[0046] The optical demultiplexer 118 demultiplexes the signal light 42 in the wavelength band λx, which is the same as the wavelength band λx of the signal light 41. For example, the optical demultiplexer 118 demultiplexes the signal light 42 in the wavelength band λx into a signal light with a center wavelength λ1, a signal light with a center wavelength λ2, and a signal light with a center wavelength λ3. The optical demultiplexer 118 outputs these three signal lights to the optical receivers 112, 114, and 116. The optical receiver 112 receives the signal light with a center wavelength λ1. The optical receiver 114 receives the signal light with a center wavelength λ2. The optical receiver 116 receives the signal light with a center wavelength λ3. In this way, the third embodiment utilizes WDM technology, which multiplexes and transmits multiple signal lights with different center wavelengths.

[0047] Here, the central wavelength may be the C-band or two bands of the C-band and L-band, or may further include the S-band and O-band. Furthermore, in order to broaden the gain wavelength characteristics of the primary pumping light 51 and the secondary pumping light 52 in association with WDM of the signal, the primary pumping light source 131 and the secondary pumping light source 136 are made multi-wavelength.

[0048] Next, the optical repeater 60 will be described. As shown in Fig. 8, the optical repeater 60 includes optical connectors 125, 126 and a Raman amplifier 130. The Raman amplifier 130 included in the optical repeater 60 is an example of a separate light source. The Raman amplifier 130 included in this optical repeater 60 also has multi-wavelength primary pumping light source 131 and secondary pumping light source 136 in order to broaden the gain wavelength characteristics as signals are converted to WDM.

[0049] The optical connector 125 is connected to a fiber end of the optical fiber 30 connected to the optical transmission device 10. The optical connector 126 is connected to a fiber end of the optical fiber 30 connected to the optical transmission device 20. The optical fiber 30 connecting the optical transmission device 10 and the optical repeater 60 is an example of a first optical fiber. The optical fiber 30 connecting the optical transmission device 20 and the optical repeater 60 is an example of a second optical fiber.

[0050] The Raman amplifier 130 according to the third embodiment differs from the Raman amplifier 130 according to the second embodiment (see FIG. 5) in that it further includes an optical splitter (denoted as SPL in FIG. 8) 137 and an optical filter 138. The optical splitter 137 is provided between the optical filter 132 and the branching coupler 135. The optical filter 138 is provided between the optical filter 132 and the optical connector 126.

[0051] The optical splitter 137 splits the primary pumping light 51, and guides one of the split primary pumping lights 51 to the optical filter 132, and guides the other of the split primary pumping lights 51 to the optical filter 138. The optical splitter 137 also splits the secondary pumping light 52, and guides one of the split secondary pumping lights 52 to the optical filter 132, and guides the other of the split secondary pumping lights 52 to the optical filter 138. As a result, one of the primary pumping lights 51 and one of the secondary pumping lights 52 are guided to the optical connector 125 and transmitted from the optical repeater 60 toward the optical transmission device 10. On the other hand, the other of the primary pumping light 51 and the other of the secondary pumping lights 52 are guided to the optical connector 126 and transmitted from the optical repeater 60 toward the optical transmission device 20.

[0052] In the optical fiber 30 connected to the optical transmission device 10, one of the secondary pumping lights 52 amplifies one of the primary pumping lights 51, and one of the amplified primary pumping lights 51 amplifies the signal lights 41 and 42. In the optical fiber 30 connected to the optical transmission device 20, the other of the secondary pumping lights 52 amplifies the other of the primary pumping lights 51, and the other amplified primary pumping light 51 amplifies the signal lights 41 and 42. As described above, according to the third embodiment, the optical transmission system ST2 includes an optical repeater 60. This allows the optical transmission system ST2 to extend the transmission distance more than the optical transmission system ST1 while suppressing the OSNR penalty due to the returned light. Although the case where one optical repeater 60 is included has been described above, the same applies to a case where multiple optical repeaters 60 are included.

[0053] (Fourth embodiment) Next, a fourth embodiment of the present invention will be described with reference to Fig. 9 to Fig. 11. As shown in Fig. 9, an optical transmission device 10 according to the fourth embodiment differs from the optical transmission device 10 according to the third embodiment (see Fig. 7) in that it further includes an optical amplifier 140. Also, as shown in Fig. 10, an optical repeater 60 according to the fourth embodiment differs from the optical repeater 60 according to the third embodiment (see Fig. 8) in that it further includes optical amplifiers 140 and 150.

[0054] The optical transmission system according to the fourth embodiment has a configuration basically similar to that of the optical transmission system ST2 according to the third embodiment, and therefore a detailed description thereof will be omitted. Also, the optical transmission device 20 according to the fourth embodiment has a configuration basically similar to that of the optical transmission device 10 according to the fourth embodiment, and therefore a detailed description thereof will be omitted.

[0055] First, the optical amplifier 140 will be described with reference to Fig. 9. The optical amplifier 140 includes optical circulators 141 and 142 and an optical amplifier 143. The optical amplifier 143 includes, for example, an EDFA. The optical circulator 141 is connected to the optical connector 122, the optical circulator 142, and the optical amplifier 143, respectively. The optical circulator 142 is connected to the optical connector 127, the optical circulator 141, and the optical amplifier 143, respectively. The optical connector 127 is connected to the optical filter 132.

[0056] The optical circulator 141 allows the signal light 41 to pass in the direction of the optical circulator 142. That is, the optical circulator 141 allows the signal light 41 traveling from the optical connector 122 in the direction of the optical circulator 142 to pass. As a result, the optical circulator 141 guides the signal light 41 transmitted from the optical transmitters 111, 113, and 115 and combined to the optical fiber 30. On the other hand, the optical circulator 141 blocks the signal light 41 from passing in the direction of the optical amplifier 143. That is, the optical circulator 141 blocks the signal light 41 traveling from the optical connector 122 in the direction of the optical amplifier 143.

[0057] Furthermore, the optical circulator 141 allows the signal light 42 to pass in the direction of the optical connector 122. That is, the optical circulator 141 allows the signal light 42 traveling in the direction from the optical amplifier 143 to the optical connector 122 to pass. Therefore, the optical circulator 141 guides the signal light 42 from the optical fiber 30 in the direction of the optical receivers 112, 114, and 116. On the other hand, the optical circulator 141 blocks the signal light 42 from passing in the direction of the optical circulator 142. That is, the optical circulator 141 blocks the signal light 42 traveling in the direction from the optical amplifier 143 to the optical circulator 142.

[0058] The optical circulator 142 allows the signal light 42 to pass in the direction of the optical amplifier 143. That is, the optical circulator 142 allows the signal light 42 traveling from the optical connector 127 in the direction of the optical amplifier 143 to pass. As a result, the optical circulator 142 guides the signal light 42 from the optical fiber 30 in the direction of the optical receivers 112, 114, and 116. This causes the signal light 42 to enter the optical amplifier 143. The optical amplifier 143 amplifies the signal light 42 and guides it to the optical circulator 141. On the other hand, the optical circulator 142 blocks the signal light 42 from passing in the direction of the optical circulator 141. That is, the optical circulator 142 blocks the signal light 42 traveling from the optical connector 127 in the direction of the optical circulator 141.

[0059] Furthermore, the optical circulator 142 allows the signal light 41 to pass in the direction of the optical connector 127. That is, the optical circulator 142 allows the signal light 41 traveling from the optical circulator 141 in the direction of the optical connector 127 to pass. As a result, the optical circulator 142 guides the signal light 41, which is transmitted from the optical transmitters 111, 113, and 115 and combined, to the optical fiber 30. On the other hand, the optical circulator 142 blocks the signal light 41 from passing in the direction of the optical amplifier 143. That is, the optical circulator 142 blocks the signal light 41 traveling from the optical circulator 141 in the direction of the optical amplifier 143.

[0060] Next, the optical amplifier 150 will be described with reference to Fig. 10. As described above, the optical repeater 60 includes the optical amplifier 140, but the optical amplifier 140 included in the optical repeater 60 has basically the same configuration as the optical amplifier 140 included in the optical transmission device 10, so a detailed description will be omitted. For example, the optical circulator 141 of the optical amplifier 140 is connected to the optical connector 129, and the optical circulator 142 is connected to the optical connector 126.

[0061] The optical amplifier 150 includes optical circulators 151 and 152 and an optical amplifier 153. The optical amplifier 153 includes, for example, an EDFA. The optical circulator 151 is connected to the optical connector 125, the optical circulator 152, and the optical amplifier 153, respectively. The optical circulator 152 is connected to the optical connector 128, the optical circulator 151, and the optical amplifier 153, respectively. The optical connector 128 is connected to the optical filter 132.

[0062] The optical circulator 151 allows the signal light 41 to pass in the direction of the optical amplifier 153. That is, the optical circulator 151 allows the signal light 41 traveling from the optical connector 125 in the direction of the optical amplifier 153 to pass. This causes the signal light 41 to enter the optical amplifier 153. The optical amplifier 153 amplifies the signal light 41 and outputs it to the optical circulator 152. On the other hand, the optical circulator 151 blocks the signal light 41 from passing in the direction of the optical circulator 152. That is, the optical circulator 151 blocks the signal light 41 from passing in the direction of the optical circulator 152 from the optical connector 125.

[0063] Furthermore, the optical circulator 151 allows the transmission of the signal light 42 in the direction of the optical connector 125. That is, the optical circulator 151 allows the transmission of the signal light 42 traveling in the direction from the optical circulator 152 to the optical connector 125. On the other hand, the optical circulator 151 blocks the transmission of the signal light 42 in the direction of the optical amplifier 153. That is, the optical circulator 151 blocks the transmission of the signal light 42 traveling in the direction from the optical circulator 152 to the optical amplifier 153.

[0064] The optical circulator 152 allows the signal light 41 to pass in the direction of the optical connector 128. That is, the optical circulator 152 allows the signal light 41 traveling from the optical amplifier 153 in the direction of the optical connector 128 to pass. On the other hand, the optical circulator 152 blocks the signal light 41 from passing in the direction of the optical circulator 151. That is, the optical circulator 152 blocks the signal light 41 traveling from the optical amplifier 153 in the direction of the optical circulator 151 to pass.

[0065] Furthermore, the optical circulator 152 allows the signal light 42 to pass in the direction of the optical circulator 151. That is, the optical circulator 152 allows the signal light 42 traveling from the optical connector 128 in the direction of the optical circulator 151 to pass. On the other hand, the optical circulator 152 blocks the signal light 42 from passing in the direction of the optical amplifier 153. That is, the optical circulator 152 blocks the signal light 42 from passing in the direction of the optical amplifier 153.

[0066] Fluctuations in the optical power of the signal lights 41 and 42 according to the fourth embodiment will be described with reference to Fig. 11. Fig. 11 shows fluctuations in the optical power from the optical connector 129 of the optical repeater 60 (see Fig. 10) to an optical connector (not shown) of the optical transmission device 20 to which the fiber end of the optical fiber 30 is connected. In Fig. 11, the position of the optical connector of the optical transmission device 20 is indicated by Ekm with reference to the position of the optical connector 128.

[0067] The downstream signal light 41 transmitted from the optical repeater 60 is amplified in the optical fiber 30 based on the primary pumping light 51 and secondary pumping light 52 transmitted from the optical repeater 60 as forward pumping light. As a result, the optical power of the downstream signal light 41 gradually increases from the reference (i.e., 0 km). Thereafter, since there is no opportunity for the downstream signal light 41 to be amplified, the optical power of the downstream signal light 41 gradually decreases as the signal light 41 propagates through the optical fiber 30.

[0068] In the vicinity where the optical transmission device 20 receives the downstream signal light 41, the downstream signal light 41 is amplified in the optical fiber 30 based on the primary pumping light and secondary pumping light transmitted as backward pumping light from the optical transmission device 20. As a result, the optical power of the downstream signal light 41 gradually increases. When the optical transmission device 20 receives the signal light 41, the optical power of the signal light 41 increases rapidly due to the optical amplifier that the optical transmission device 20 has, similar to the optical transmission device 10.

[0069] The upstream signal light 42 transmitted from the optical transmission device 20 is basically the same as the signal light 41. That is, the upstream signal light 42 is amplified in the optical fiber 30 based on the primary pumping light and secondary pumping light transmitted from the optical transmission device 20 as forward pumping light. Furthermore, the upstream signal light 42 is amplified in the optical fiber 30 based on the primary pumping light 51 and secondary pumping light 52 transmitted from the optical repeater 60 as backward pumping light. When the optical repeater 60 receives the upstream signal light 42, the optical power of the upstream signal light 42 is abruptly increased by the optical amplifier 143 of the optical amplifier 140 included in the optical repeater 60.

[0070] As described above, in the fourth embodiment, the optical transmission device 10 includes the optical amplifier 140, and the optical repeater 60 includes the optical amplifiers 140 and 150. As a result, even when the fiber length of the optical fiber 30 is long and the optical power during reception is lower than the optical power during transmission with Raman amplification alone, the signal light 41 is amplified by the optical amplifier 150, and the signal light 42 is amplified by the optical amplifier 140. In other words, even when the fiber length of the optical fiber 30 is long, the signal lights 41 and 42 are amplified by both the Raman amplifier 130 and the optical amplifiers 140 and 150, thereby suppressing the OSNR penalty due to the return light.

[0071] (Fifth embodiment) Next, a fifth embodiment of the present invention will be described with reference to Fig. 12. In the fifth embodiment, a signal light 41 in the wavelength band λx and a signal light 42 in the wavelength band λy will be described as an example. That is, the fifth embodiment differs from the third embodiment in which the signal light 41, 42 in the wavelength band λx were both described as an example.

[0072] First, as in the third embodiment, optical transmitter 111 transmits signal light with a center wavelength λ1. Optical transmitter 113 transmits signal light with a center wavelength λ2. Optical transmitter 115 transmits signal light with a center wavelength λ3. Optical multiplexer 117 combines the signal lights transmitted from optical transmitters 111, 113, and 115. After combining these signal lights, optical multiplexer 117 outputs the combined signal light as signal light 41 in wavelength band λx. Wavelength band λx includes center wavelengths λ1, λ2, and λ3.

[0073] On the other hand, the optical demultiplexer 118 demultiplexes the signal light 42 in the wavelength band λy, which is different from the wavelength band λx of the signal light 41. For example, the optical demultiplexer 118 demultiplexes the signal light 42 in the wavelength band λy into a signal light with a center wavelength λ4, a signal light with a center wavelength λ5, and a signal light with a center wavelength λ6. The optical demultiplexer 118 outputs these three signal lights to the optical receivers 112, 114, and 116. The optical receiver 112 receives the signal light with a center wavelength λ4. The optical receiver 114 receives the signal light with a center wavelength λ5. The optical receiver 116 receives the signal light with a center wavelength λ6.

[0074] In this way, when the signal light 41 and the signal light 42 have different wavelength bands, the OSNR penalty caused by the Fresnel reflected light 44 is avoided. Therefore, when the signal light 41 and the signal light 42 have different wavelength bands, the OSNR penalty caused by the Rayleigh scattered light 43 is suppressed independently.

[0075] (Sixth embodiment) Next, a sixth embodiment of the present invention will be described with reference to FIGS. 13 to 15. As shown in FIG. 13, an optical transmission device 10 according to the sixth embodiment differs from the optical transmission device 10 according to the third embodiment (see FIG. 7) in that it further includes an OSC (Optical Supervisory Channel) optical first transmitter (denoted as OSC-T1 in FIG. 13) 161. As shown in FIG. 14A, an optical repeater 60 according to the sixth embodiment differs from the optical repeater 60 according to the third embodiment (see FIG. 8) in that it further includes an OSC optical first receiver (denoted as OSC-R1 in FIG. 14A) 163 and an OSC optical second transmitter 165. Although not shown, an optical transmission device 20 according to the sixth embodiment is provided with an OSC optical second receiver corresponding to the optical transmission device 10.

[0076] 13 , the OSC light first transmitter 161 is connected to an optical filter 162. The optical filter 162 is provided between the optical amplifier 11 and the optical connector 121. The OSC light first transmitter 161 transmits the OSC light 55 under the control of, for example, a network controller that manages the optical transmission system ST2. The optical filter 162 guides the OSC light 55 to the optical connector 121. Therefore, the OSC light 55 is transmitted from the optical transmission device 10 to the optical repeater 60, similar to the signal light 41.

[0077] 14A, the OSC light first receiving unit 163 is connected to the optical filter 164. The optical filter 164 is provided between the optical connector 126 and the optical connector 129. The optical filter 164 guides the OSC light 55 to the OSC light first receiving unit 163. As a result, the OSC light first receiving unit 163 receives the OSC light 55 transmitted from the optical transmission device 10.

[0078] The OSC light first receiving unit 163 and the OSC light second transmitting unit 165 communicate with each other. When the OSC light second transmitting unit 165 is notified of the reception of the OSC light 55 by the OSC light first receiving unit 163, it transmits OSC light 56, which has a wavelength different from that of the OSC light 55. The OSC light second transmitting unit 165 may transmit the OSC light 56 ​​based on the control of the network controller. The OSC light second transmitting unit 165 is connected to an optical filter 166. The optical filter 166 is provided between the optical connector 125 and the optical connector 128. The optical filter 166 guides the OSC light 56 ​​to the optical connector 128.

[0079] Therefore, the OSC light 56 ​​is transmitted from the optical repeater 60 toward the optical transmission device 20, similar to the signal light 41. Since the OSC light 56 ​​and the OSC light 55 have different wavelengths, the OSC light 56 ​​is not guided to the OSC light first receiving unit 163, but is guided to the optical connector 126. As a result, the OSC light 56 ​​is transmitted from the optical repeater 60 toward the optical transmission device 20.

[0080] The second OSC light receiving unit of the optical transmission device 20 receives the OSC light 56 ​​transmitted from the optical repeater 60 via an optical filter. Upon receiving the OSC light 56, the second OSC light receiving unit notifies the network controller described above of the communication of the OSC light 56. In response to this, the network controller instructs the control unit 134 of the optical transmission devices 10 and 20 and the optical repeater 60 to drive the primary excitation light source 131 and the secondary excitation light source 136.

[0081] As a result, each control unit 134 drives the primary excitation light source 131 and the secondary excitation light source 136. When each control unit 134 drives the primary excitation light source 131, the primary excitation light source 131 outputs the primary excitation light 51. When each control unit 134 drives the secondary excitation light source 136, the secondary excitation light source 136 outputs the secondary excitation light 52.

[0082] On the other hand, if the OSC light first receiving unit 163 does not receive the OSC light 55, it is assumed that a fiber break has occurred in the optical fiber 30 connecting the optical transmission device 10 and the optical repeater 60. Therefore, if the network controller is not notified that the OSC light 55 is connected, the network controller instructs the control unit 134 to reduce the output or stop driving the primary pumping light source 131 and the secondary pumping light source 136. This weakens the optical power of the primary pumping light 51 and the secondary pumping light 52, or stops the output of the primary pumping light 51 and the secondary pumping light 52.

[0083] This ensures the safety of workers who restore the optical fiber 30. If the OSC light second receiving unit does not receive the OSC light 56, it is assumed that a fiber break has occurred in the optical fiber 30 connecting the optical repeater 60 and the optical transmission device 20. In this case, as in the case of the OSC light 55, the network controller instructs the control unit 134 to reduce the output power of the primary excitation light source 131 and the secondary excitation light source 136 or to stop driving them.

[0084] Furthermore, when the optical transmission device 10 includes the optical amplifier 140, if the network controller is not notified of the connection of the OSC light 55, the network controller may instruct the control unit 134 to stop driving the optical amplifier 140. Specifically, the network controller may instruct the control unit 134 to stop driving the optical amplifier 143. In this case as well, the safety of workers is ensured. The same applies to the case where the optical repeater 60 includes the optical amplifiers 140 and 150.

[0085] An example of the operation of the control unit 134 will be described with reference to FIG.

[0086] When the optical transmission devices 10, 20 and the optical repeater 60 are started, first, the control units 134 of the optical transmission devices 10, 20 and the optical repeater 60 acquire span information (step S1). For example, each control unit 134 acquires span information from a network controller. The span information includes the fiber type of the optical fiber 30, the span length of the span, and the span loss. The fiber type includes, for example, SMF (Single Mode Fiber). Note that the span represents, for example, the transmission section between the optical transmission device 10 and the optical repeater 60, the transmission section between the optical repeater 60 and the optical transmission device 20, or the transmission section between the optical repeaters 60.

[0087] Upon acquiring the span information, each control unit 134 determines whether or not the optical amplifier 140 or the optical amplifier 150 is present (step S2). For example, the control unit 134 of the optical transmission device 10 determines whether or not the optical transmission device 10 includes the optical amplifier 140. Similarly, the control unit 134 of the optical repeater 60 determines whether or not the optical repeater 60 includes the optical amplifiers 140 and 150. If the optical amplifier 140 or the optical amplifier 150 is present (step S2: YES), each control unit 134 drives the optical amplifier 140 or the optical amplifier 150 (step S3). On the other hand, if the optical amplifier 140 or the optical amplifier 150 is not present (step S2: NO), each control unit 134 skips the processing of step S3.

[0088] Next, each control unit 134 transmits the OSC light (step S4). For example, the control unit 134 of the optical transmission device 10 drives the first OSC light transmission unit 161, causing the first OSC light transmission unit 161 to transmit the OSC light 55. Also, the control unit 134 of the optical repeater 60 drives the second OSC light transmission unit 165, causing the second OSC light transmission unit 165 to transmit the OSC light 56.

[0089] After transmitting the OSC light, each control unit 134 determines whether or not the OSC light has been received (step S5). For example, the control unit 134 of the optical repeater 60 determines whether or not the first OSC light receiving unit 163 has received the OSC light 55. The control unit of the optical transmission device 20 determines whether or not the second OSC light receiving unit has received the OSC light 56. If the OSC light has been received (step S5: YES), each control unit 134 causes the excitation light source to emit light (step S6).

[0090] For example, the control unit 134 of the optical transmission device 10 controls the primary excitation light source 131 and the secondary excitation light source 136 to emit light. As a result, the primary excitation light source 131 transmits the primary excitation light 51, and the secondary excitation light source 136 transmits the secondary excitation light 52. Furthermore, the control unit 134 of the optical repeater 60 controls the primary excitation light source 131 and the secondary excitation light source 136 to emit light. As a result, the primary excitation light source 131 transmits the primary excitation light 51, and the secondary excitation light source 136 transmits the secondary excitation light 52. Each control unit 134 accesses a table in which span information is associated with the set temperatures and set currents of the primary excitation light source 131 and the secondary excitation light source 136, identifies the set temperature and set current according to the span information, and controls the primary excitation light source 131 and the secondary excitation light source 136 to emit light. After the excitation light sources are emitted, each control unit 134 ends the processing.

[0091] On the other hand, if the OSC light is not received (step S5: NO), each control unit 134 stops control (step S7) and ends the process. For example, if the OSC light is not received, it is assumed that the optical fiber 30 is broken, and therefore each control unit 134 stops sending out the OSC lights 55 and 56. As described above, according to the sixth embodiment, when communication of the OSC lights 55 and 56 is confirmed, the primary pump light 51 and the secondary pump light 52 are sent out, and the OSNR penalty due to the returned light is suppressed.

[0092] Here, an example has been shown in which different wavelengths are assigned to the OSC lights 55 and 56, and a fiber break or a disconnection of a connector is monitored between the optical fiber 30 and the Raman amplifier 130. As an even simpler configuration, a fiber break or a disconnection of a connector can be monitored similarly between the optical fiber 30 and the optical transmission device 10, between the optical fiber 30 and the optical transmission device 20, or between the optical fiber 30 and the optical repeater 60. Specifically, this can be achieved by exchanging the positions of the OSC light second transmitter 165 and the optical filter 166 with the OSC light first receiver 163 and the optical filter 164, as shown in FIG. 14B .

[0093] Seventh embodiment Next, a seventh embodiment of the present invention will be described with reference to Fig. 16. The optical transmission device 10 according to the seventh embodiment differs from the optical transmission device 10 according to the third embodiment (see Fig. 7) in that it further includes an OTDR (Optical Time Domain Reflectometer) 170. The OTDR 170 inputs an optical pulse 57 into an optical fiber 30, detects reflected light 58 from the optical fiber 30, and checks the communication of the optical pulse 57 based on the reception timing of the reflected light 58.

[0094] The OTDR 170 is connected to an optical filter 171. The optical filter 171 is provided between the optical circulator 120 and the optical connector 122. The optical filter 171 guides the optical pulse 57 to the optical connector 122. This allows the optical transmission device 10 to transmit the optical pulse 57 to the optical fiber 30. Meanwhile, the optical filter 171 guides the reflected light 58 to the OTDR 170. This allows the OTDR 170 to receive the reflected light 58. In this way, the optical pulse 57 or the reflected light 58 may be used instead of the OSC lights 55 and 56 to confirm the communication of the optical pulse 57.

[0095] (Eighth embodiment) Next, an eighth embodiment of the present invention will be described with reference to Figures 17 and 18. As shown in Figure 17, the optical transmission device 10 according to the eighth embodiment differs from the optical transmission device 10 according to the fourth embodiment (see Figure 9) in that it does not include a Raman amplifier 130. The optical transmission device 20 according to the eighth embodiment has basically the same configuration as the optical transmission device 10 according to the eighth embodiment, and therefore a detailed description thereof will be omitted. As described above, even if the optical transmission device 10 does not include a Raman amplifier 130, the signal power of the signal light 42 is increased by including the optical amplifier 140. As a result, the amount of crosstalk is reduced below the reference amount, and the occurrence of an OSNR penalty is suppressed.

[0096] 18 , for example, in the vicinity of the optical transmission device 10, the optical power of the upstream signal light 42 and the optical power of the downstream signal light 41 are reversed, and the optical power of the upstream signal light 42 is relatively greater than the optical power of the downstream signal light 41. Also in the vicinity of the optical transmission device 20, as in the case of the optical transmission device 10, the optical power of the upstream signal light 42 transmitted by the optical transmission device 20 and the optical power of the downstream signal light 41 received by the optical transmission device 20 are reversed. That is, also in the vicinity of the optical transmission device 20, the optical power of the downstream signal light 41 is relatively greater than the optical power of the upstream signal light 42. As a result, the amount of crosstalk becomes smaller than the reference amount, and the occurrence of OSNR penalty is suppressed.

[0097] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

[0098] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) An optical transmission device disposed at an end of a single-core optical fiber and used for transmitting signal light bidirectionally through the optical fiber, the optical transmission device comprising: an optical transmitter that transmits a first signal light in one direction belonging to the signal light; an optical receiver that receives a second signal light in the opposite direction belonging to the signal light; an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver; and a light source that outputs pump light to the optical fiber to Raman amplify the second signal light to a second optical power higher than the first optical power of the first signal light. (Supplementary Note 2) The optical transmission device according to Supplementary Note 1, further comprising an optical amplifier that does not amplify the first signal light but amplifies the second signal light. (Supplementary Note 3) The optical transmission device according to Supplementary Note 1 or 2, characterized in that the light source includes a first light source that outputs primary pumping light that Raman-amplifies the second signal light to the optical fiber, and a second light source that outputs secondary pumping light that Raman-amplifies the primary pumping light. (Appendix 4) The optical transmission device described in Appendix 3, characterized in that the primary pumping light is incoherent pumping light in a first wavelength band, and the secondary pumping light is coherent pumping light in a second wavelength band that is shorter than the first wavelength band. (Supplementary Note 5) The optical transmission device according to Supplementary Note 1 or 2, wherein the optical transmission device is connected to another optical transmission device opposite to the optical transmission device via the optical fiber, and the other optical transmission device includes an opposite receiver that receives the first signal light, an opposite transmitter that transmits the second signal light, an opposite circulator that guides the first signal light from the optical fiber to the opposite receiver and guides the second signal light transmitted from the opposite transmitter to the optical fiber, and an opposite light source that outputs another pump light to the optical fiber, Raman-amplifying the first signal light received by the opposite receiver to a fourth optical power higher than a third optical power of the second signal light transmitted by the opposite transmitter. (Appendix 6) An optical transmission device according to appendix 1 or 2, characterized in that one or more optical repeaters are arranged between a first optical fiber belonging to the optical fiber and a second optical fiber belonging to the optical fiber, and the optical repeater includes a separate light source that outputs the excitation light to each of the first optical fiber and the second optical fiber. (Supplementary Note 7) The optical transmission device according to Supplementary Note 6, wherein the optical repeater further includes an optical amplifier that does not amplify the first signal light but amplifies the second signal light. (Supplementary Note 8) An optical transmission device used for transmitting signal light bidirectionally through a single optical fiber, the optical transmission device comprising: an optical transmitter that transmits a first signal light in one direction belonging to the signal light; an optical receiver that receives a second signal light in the opposite direction belonging to the signal light; an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver; and an optical amplifier that amplifies the second signal light without amplifying the first signal light. (Supplementary Note 9) The optical transmission device according to Supplementary Note 1 or 8, wherein the first wavelength of the first signal light and the second wavelength of the second signal light are the same. (Supplementary Note 10) The optical transmission device according to Supplementary Note 1 or 8, wherein a first wavelength of the first signal light and a second wavelength of the second signal light are different from each other. (Supplementary Note 11) An optical transmission system for transmitting signal light bidirectionally through a single optical fiber, wherein an optical transmission device arranged at an end of the optical fiber comprises: an optical transmitter for transmitting a first signal light in one direction belonging to the signal light; an optical receiver for receiving a second signal light in the opposite direction belonging to the signal light; an optical circulator for guiding the first signal light transmitted from the optical transmitter to the optical fiber and guiding the second signal light from the optical fiber to the optical receiver; and a light source for outputting pump light to the optical fiber, which Raman amplifies the second signal light to a second optical power higher than the first optical power of the first signal light. (Supplementary Note 12) An optical transmission method performed by an optical transmission device disposed at a fiber end of a single-core optical fiber and used for transmitting signal light bidirectionally through the optical fiber, the optical transmission method comprising: transmitting a first signal light in one direction belonging to the signal light from an optical transmitter; receiving a second signal light in the opposite direction belonging to the signal light by an optical receiver; guiding the first signal light transmitted from the optical transmitter to the optical fiber; guiding the second signal light from the optical fiber to the optical receiver; and outputting pump light to the optical fiber for Raman amplifying the second signal light to a second optical power higher than a first optical power of the first signal light. [Explanation of symbols]

[0099] ST1, ST2 optical transmission system 10,20 Optical transmission equipment 30 Optical Fiber 41,42 Signal light 43 Rayleigh scattered light 44 Fresnel Reflection 60 Optical repeater 110 Transponder 111,113,115 Optical transmitter 112,114,116 Optical receiver 120 Optical Circulator 130 Raman Amplifier 131 Primary excitation light source 136 Secondary excitation light source 140,150 Optical amplifier

Claims

1. An optical transmission device that is disposed at an end of a single optical fiber and is used to transmit signal light bidirectionally through the optical fiber, an optical transmitter that transmits a first signal light in one direction belonging to the signal light; an optical receiver for receiving a second optical signal in a reverse direction belonging to the optical signal; an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver; a light source that outputs pumping light to the optical fiber, Raman-amplifying the second signal light to a second optical power higher than a first optical power of the first signal light; An optical transmission device having:

2. the optical transmission device further includes an optical amplifier that amplifies the second signal light without amplifying the first signal light.

2. The optical transmission device according to claim 1.

3. the light source includes a first light source that outputs primary pumping light that Raman-amplifies the second signal light to the optical fiber, and a second light source that outputs secondary pumping light that Raman-amplifies the primary pumping light.

3. The optical transmission device according to claim 1, wherein the optical transmission device is a fiber optic cable.

4. the primary excitation light is incoherent excitation light in a first wavelength band, the secondary excitation light is coherent excitation light in a second wavelength band that is shorter than the first wavelength band; 4. The optical transmission device according to claim 3.

5. the optical transmission device is connected to another optical transmission device opposite to the optical transmission device via the optical fiber; the other optical transmission device includes an opposite receiver that receives the first signal light, an opposite transmitter that transmits the second signal light, an opposite circulator that guides the first signal light from the optical fiber to the opposite receiver and guides the second signal light transmitted from the opposite transmitter to the optical fiber, and an opposite light source that outputs another pump light to the optical fiber, Raman-amplifying the first signal light received by the opposite receiver to a fourth optical power higher than a third optical power of the second signal light transmitted by the opposite transmitter.

3. The optical transmission device according to claim 1, wherein the optical transmission device is a fiber optic cable.

6. one or more optical repeaters are disposed between a first optical fiber belonging to the optical fiber and a second optical fiber belonging to the optical fiber; the optical repeater includes separate light sources that output the excitation light to the first optical fiber and the second optical fiber, respectively; 3. The optical transmission device according to claim 1, wherein the optical transmission device is a fiber optic cable.

7. the optical repeater further includes an optical amplifier that amplifies the second signal light without amplifying the first signal light; 7. The optical transmission device according to claim 6.

8. An optical transmission device used to transmit signal light bidirectionally through a single optical fiber, an optical transmitter that transmits a first signal light in one direction belonging to the signal light; an optical receiver for receiving a second optical signal in a reverse direction belonging to the optical signal; an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver; an optical amplifier that amplifies the second signal light without amplifying the first signal light; An optical transmission device having:

9. An optical transmission system that transmits signal light bidirectionally through a single optical fiber, an optical transmission device disposed at a fiber end of the optical fiber, an optical transmitter that transmits a first signal light in one direction belonging to the signal light; an optical receiver for receiving a second optical signal in a reverse direction belonging to the optical signal; an optical circulator that guides the first signal light transmitted from the optical transmitter to the optical fiber and guides the second signal light from the optical fiber to the optical receiver; a light source that outputs pumping light to the optical fiber, Raman-amplifying the second signal light to a second optical power higher than the first optical power of the first signal light; An optical transmission system comprising:

10. An optical transmission method performed by an optical transmission device that is disposed at an end of a single optical fiber and is used to transmit signal light bidirectionally through the optical fiber, comprising: transmitting a unidirectional first signal light belonging to the signal light from an optical transmitter; receiving a second signal light in a reverse direction belonging to the signal light by an optical receiver; guiding the first signal light transmitted from the optical transmitter to the optical fiber; guiding the second signal light from the optical fiber to the optical receiver; outputting pumping light to the optical fiber, Raman-amplifying the second signal light to a second optical power higher than the first optical power of the first signal light; Optical transmission method.

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