JP2000077758A - Light amplifier and light transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light amplifier that can be freely changed in direction of amplification. SOLUTION: A light amplifying fiber 24,a wavelength multiplexer 26, and an optical circulator 28 are connected in series between signal light input/output terminals 20 and 22. Exciting light is inputted into the wavelength multiplexer 26 from an exciting light source 30. Exciting light emitted from the exciting light source 30 is directed to the light amplifying fiber 24 by the wavelength multiplexer 26. The optical circulator 28 is equipped with three ports A, B, and C and can be reversed in the direction of rotation by reversing an applied magnetic field. That is, either the one direction of rotation where the optical circular 28 outputs light inputted through the port A through the port B and outputs light inputted through the port B through the port C or the other direction of rotation where th optical circulator 28 outputs light inputted through the port B through the port A and outputs light inputted through the port A through the port C can be selected from outside. The port A of the optical circulator 28 is connected to the wavelength8 multiplexer 26, the port B is connected to the input/output terminal 22, and the port C is connected to a non-reflection terminal end 32.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical amplifier and an optical transmission system using the same.

[0002]

2. Description of the Related Art In an optical amplification repeater transmission system using an optical fiber amplifier, as the gain of the optical amplifier increases,
The effect of return light due to reflection on an optical connector, an optical component, an optical fiber, or the like connected to the optical amplifier becomes remarkable, and the optical amplifier oscillates or the optical amplification characteristics become unstable. In order to prevent this, conventionally, an optical isolator for suppressing the reflected return light is inserted at least on one side of the optical amplification fiber.

FIG. 9 shows a schematic block diagram of a conventional example. The signal light to be amplified enters the optical amplification fiber 10 and is optically amplified while propagating there. The amplified signal light output from the optical amplifying fiber 10 passes through the wavelength multiplexing element 12, further passes through the optical isolator 14, and
The data is transmitted to the optical fiber transmission line. The excitation light source 16 generates excitation light for exciting the optical amplification fiber 10. Excitation light source 1
The pumping light output from 6 is multiplexed on the signal light transmission path by the wavelength multiplexing element 12 and is incident on the optical amplification fiber 10 in a direction opposite to that of the signal light. As a means for making the pump light enter the optical amplification fiber, there are a light coming from the front and a light coming from both, in addition to the light coming from the rear.

A conventional optical amplifier can transmit signal light only in one determined direction. As a result, in the conventional optical transmission system, a pair of optical fiber lines composed of two optical fibers is prepared, and optical amplifiers having a unidirectional amplification characteristic are respectively arranged in the opposite directions, Two-way transmission is realized. That is, in the conventional example, one optical fiber transmission line cannot be used bidirectionally.

[0005]

At both ends of the optical fiber transmission system, a transmitting terminal station and a receiving terminal station are connected to face each other. In the conventional system, the transmission capacity is designed to be the same for both uplink and downlink. However, depending on the traffic demand, a situation in which a signal is transmitted only in one direction (for example, upstream) may be assumed. In this case, the traffic in the opposite direction, that is, downstream traffic transmission becomes unnecessary, and the downstream line is not used while having a predetermined transmission capacity. If the down line can be used easily for up,
The transmission system can be operated efficiently.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical amplifier having a novel structure for preventing return of reflected light.

Another object of the present invention is to provide an optical amplifier whose amplification direction can be changed and an optical transmission system using the same.

[0008]

According to the present invention, an optical circulator that guides light traveling in a direction opposite to the signal light transmission direction to a non-reflection terminal is disposed on one side of an optical amplification medium for signal light. Thereby, the return of the reflected light can be efficiently prevented.
The amplification direction can be changed by changing the rotation direction of the optical circulator. Similar optical circulators may be arranged on both sides of the optical amplification medium.

In an optical transmission system using an optical amplifier whose amplification direction can be changed as a repeater, the transmission direction of signal light can be changed freely by remotely controlling the amplification direction of the optical amplifier. As a result, a symmetric optical transmission system can be constructed for the time being, and the transmission capacity can be easily made asymmetric afterwards according to the actual traffic.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of the present invention. An optical amplification fiber 24, a wavelength multiplexing element 26, and an optical circulator 28 are serially connected between the signal light input / output terminals 20 and 22. Excitation light from the excitation light source 30 is input to the wavelength multiplexing element 26. The excitation light from the excitation light source 30 is directed to the optical amplification fiber 24 by the wavelength multiplexing element 26 and excites the optical amplification fiber 24 while passing through the optical amplification fiber 24.

The optical circulator 28 has three ports A, B, and C, and can reverse the direction of rotation by reversing the applied magnetic field. That is, the optical circulator 28 outputs the input light of the port A from the port B, outputs the input light of the port B from the port C, and outputs the input light of the port B from the port A.
And the rotation direction in which the input light is output from the port C can be externally selected. The port A of the optical circulator 28 is connected to the wavelength multiplexing element 26, the port B is connected to the input / output terminal 22, and the port C is connected to the reflectionless termination 32.

In FIG. 1, a signal light to be amplified is input to an input / output terminal 20, amplified by an optical amplification fiber 24, and input to a port A of an optical circulator 28 via a wavelength multiplexing element 26. In FIG. 1, since the optical circulator 28 outputs the input light of the port A from the port B, the amplified signal light is output from the input / output terminal 22 to the outside.

The light reflected from the input / output terminal 22 and the light reflected from the input / output terminal 22 are input to the port B of the optical circulator 28. Since the optical circulator 28 outputs the input light of the port B from the port C, the light reflected at the input / output terminal 22 and the light reflected from the input and output terminals are input to the non-reflection terminal 32 and disappear there. That is, reflected light or return light can be prevented from entering the optical amplification fiber 24, and the optical circulator 28
However, it functions in exactly the same way as the conventional optical isolator 14.

When the signal light to be amplified is input to the input / output terminal 22 and the amplified signal light is to be output from the input / output terminal 20, the rotation direction of the optical circulator 28 is reversed. FIG. 2 shows the flow directions of the signal light and the reflected light at this time.
The signal light is input from the input / output terminal 22 to the port B of the optical circulator 28. In the state shown in FIG. 2, the optical circulator 28 outputs the input light of the port B from the port A, so that the signal light eventually passes through the optical circulator 28 and the wavelength multiplexing element 26 and enters the optical amplification fiber 24. I do. Excitation light output from the excitation light source 30 is incident on the optical amplification fiber 24 by the wavelength multiplexing element 26 and excites the optical amplification fiber 24. The optical amplification fiber 24 optically amplifies the signal light incident from the wavelength multiplexing element 26, and
Output to 0.

Light input to the input / output terminal 20 (for example, light reflected from the end) enters an optical amplification fiber 24, where it is amplified, passes through a wavelength division multiplexing element 26, and passes through a port of an optical circulator 28. Input to A. In FIG. 2, the optical circulator 28 outputs the input light of the port A from the port C. Therefore, the light input to the input / output terminal 20 is
Although the light is amplified, it is incident on the non-reflection terminal 32 and is absorbed there.

As described above, in the configuration shown in FIG. 2, while the optical circulator 28 has the function of an optical isolator,
By changing the rotation direction of the optical circulator 28, the light amplification direction can be changed.

In FIG. 1 (and FIG. 2), the optical circulator 28 is disposed on the right side of the optical amplification fiber 26. However, it is apparent that the optical circulator 28 may be disposed on the left side of the optical amplification fiber 26. FIG. 3 is a schematic block diagram of a second embodiment of the present invention in which an optical circulator is arranged on one side of an optical amplification fiber and a wavelength multiplexing element is arranged on the other side.

In FIG. 3, between the optical input / output terminals 40 and 42,
The optical circulator 44, the optical amplification fiber 46, and the wavelength multiplexing element 48 are serially connected. Excitation light from an excitation light source 50 is input to the wavelength multiplexing element 48. Excitation light source 50
Is pumped by the wavelength multiplexing element 48 to the optical amplification fiber 46, and excites the optical amplification fiber 46 while passing through the optical amplification fiber 46.

The optical circulator 44, like the optical circulator 28, has three ports A, B, and C, and can reverse the direction of rotation by reversing the applied magnetic field. The port A of the optical circulator 44 is connected to the input / output terminal 40, the port B is connected to the optical amplification fiber 46, and the port C is connected to the non-reflection terminal 52.

The signal light to be amplified is externally supplied to the input / output terminal 4
The operation when the signal is input to 0 and output from the input / output terminal 42 to the outside after amplification will be described.

The pump light output from the pump light source 50 is introduced into the optical amplification fiber 46 by the wavelength multiplexing element 48,
The optical amplification fiber 46 is excited. The signal light from the input / output terminal 40 enters the port A of the optical circulator 44,
The signal is output from the port B and input to the optical amplification fiber 46. The signal light is amplified while propagating through the optical amplification fiber 46, passes through the wavelength multiplexing element 48, and is output from the input / output terminal 42 to the outside.

On the other hand, the reflected light of the signal light generated in the input / output terminal 42 and the optical transmission line connected to the input / output terminal 42
8, the light enters the optical amplification fiber 46, where it is amplified. The amplified reflected light enters the port B of the optical circulator 44, is output from the port C, and is absorbed by the non-reflection terminal 52. Thus, the reflected light is not output from the input / output terminal 40 to the outside.

When the signal light input to the input / output terminal 42 is amplified and output from the input / output terminal 40 to the outside, the optical circulator 44 may be rotated from the port B to the port A and from the port A to the port C. . The reflected light input from the input / output terminal 40 to the port A of the optical circulator 44 enters the non-reflective terminal 52 from the port C and is absorbed.

In the embodiment shown in FIGS. 1 and 3, the optical circulators 28 and 44 whose rotation directions can be freely changed are arranged on one side of the optical amplification fibers 24 and 44, however, the optical circulators 28 and 44 have a specific amplification direction (for example, FIG. 1). In this case, the optical amplification fibers 24, 44
The reflected light generated between the optical circulators 28 and 44 is amplified by the optical amplification fibers 24 and 44 and returns to the upstream side of the signal light. This can be solved by disposing optical circulators whose rotation directions can be freely changed on both sides of the optical amplification fiber. FIG. 4 is a schematic block diagram of the embodiment.

The configuration of FIG. 4 will be described. Optical input / output terminal 6
Between 0 and 62, an optical circulator 64, an optical amplification fiber 66, a wavelength multiplexing element 68, and an optical circulator 70 are serially connected. The wavelength multiplexing element 68 has an excitation light source 72
The excitation light from is input. The excitation light from the excitation light source 72 is directed to the optical amplification fiber 66 by the wavelength multiplexing element 68 and excites the optical amplification fiber 66 while passing through the optical amplification fiber 66.

The optical circulators 64 and 70 have three ports A, B and C, like the optical circulators 28 and 44.
And the direction of rotation can be reversed by reversing the applied magnetic field. The port A of the optical circulator 64 is connected to the input / output terminal 60, the port B is connected to the optical amplification fiber 66, and the port C is connected to the non-reflection terminal 74. The port A of the optical circulator 70 is
8, port B is connected to the input / output terminal 62, and port C is connected to the reflectionless termination 76.

Regardless of which direction the signal light propagates, the pump light output from the pump light source 72 is directed to the optical amplification fiber 66 by the wavelength multiplexing element 68, and the light amplification fiber 66 , While exciting the optical amplification fiber 66.

The flow of signal light and reflected light when the signal light to be amplified is input to the input / output terminal 60 and the amplified signal light is output from the input / output terminal 62 to the outside will be described. in this case,
The signal light input from the outside to the input / output terminal 60 is input to the port A of the optical circulator 64, output from the port B, and input to the optical amplification fiber 66. The signal light is amplified while propagating through the optical amplification fiber 66, and
8 and input to the port A of the optical circulator 70. Since the optical circulator 70 outputs the input light of the port A from the port B, the amplified signal light is output from the input / output terminal 62 to the outside.

On the other hand, reflected light generated on the downstream side of the signal light from the port B of the optical circulator 70 (for example, reflected light generated on the input / output terminal 62 or downstream of the signal light therefrom).
Is input to the port B of the optical circulator 70 and output from the port C, and is absorbed by the non-reflection terminal 76. The reflected light generated at the connection between the port B of the optical circulator 64 and the port A of the optical circulator 70 is input to the port B of the optical circulator 64, is output from the port C, and has a non-reflection terminal. Absorbed by 74.

Conversely, when the signal light to be amplified is input to the input / output terminal 62 and the amplified signal light is output from the input / output terminal 60 to the outside, the optical circulators 64 and 70 are connected to the port B From port to port A. Similarly, the signal light is amplified by the optical amplification fiber 66 and output from the input / output terminal 60 to the outside. Reflected light generated on the downstream side of the signal light from the port A of the optical circulator 64 (for example, reflected light generated on the input / output terminal 60 or the downstream side of the signal light therefrom) enters the port A of the optical circulator 64, Since the signal is output from the port C, it is absorbed by the non-reflection terminal 74. The reflected light generated at the connection between the port B of the optical circulator 64 and the port A of the optical circulator 70 is reflected by the optical circulator 7.
0 is input to port A, output from its port C, and absorbed by the non-reflective termination 76.

Thus, in the embodiment shown in FIG.
In either case, the optical circulator 64,
The reflected light generated between the optical fibers 70 and amplified by the optical amplification fiber 66 can be absorbed and eliminated.

FIG. 5 is a schematic block diagram showing an embodiment of an optical transmission system using the embodiment shown in FIG. Two optical fiber transmission lines 1 between the terminal station 110 and the terminal station 112
14, 116 are connected. Optical fiber transmission line 11
Optical amplifiers 118 and 120 having the same configuration as that of the embodiment shown in FIG. The optical fiber transmission line 114 is further provided on the optical fiber transmission line 114.
1 that splits a part of the signal light in both directions propagating through the splitter 4
22 are provided. The photodiode 124 converts the signal light, which is demultiplexed by the demultiplexer 122, from the terminal station 110 to the terminal station 112 into an electric signal, and
From the terminal station 112 to the terminal station 11 demultiplexed by the demultiplexer 122.
The signal light going to 0 is converted into an electric signal. The outputs of the photodiodes 124 and 126 are applied to a control circuit 128 that controls the amplification direction of the optical amplifier 120.

In the initial state, the optical amplifier 118
The signal light traveling from the terminal station 10 to the terminal station 112 is amplified, and the optical amplifier 120 amplifies the signal light traveling from the terminal station 112 to the terminal station 110. The control circuit 128 includes the photodiode 12
The amplification direction of the optical amplifier 120 is controlled in the same manner as in the embodiment shown in FIG. 1 in accordance with the outputs of 4,126, that is, the control signals from the terminal station 110 or 112.

The terminal station 110 selectively supplies the optical transmitter 130, the optical receiver 132, and the output of the optical transmitter 130 or the input of the optical receiver 132 to the optical fiber transmission line 114. An optical switch is connected to the optical switch. The terminal station 110 also has an optical fiber transmission line 116.
, An optical transmitter 136, an optical receiver 138, and an optical switch 140 for selectively connecting the output of the optical transmitter 136 or the input of the optical receiver 138 to the optical fiber transmission line 116.
Is provided.

Similarly, the terminal station 112 selectively transmits an optical transmitter 142, an optical receiver 144, and an output of the optical transmitter 142 or an input of the optical receiver 144 to the optical fiber transmission line 114. An optical switch 146 connected to the fiber transmission line 114 is provided. The terminal station 112 also transmits an optical transmitter 148, an optical receiver 150,
And an optical switch 152 for selectively connecting the output of the optical transmitter 148 or the input of the optical receiver 150 to the optical fiber transmission line 116.

The control signal for the control circuit 128 is applied to the optical transmitter 130 at the terminal station 110 and to the optical transmitter 142 at the terminal station 112.

Although the configuration shown in FIG. 5 is versatile, in practice, one of the optical fiber transmission lines 114 and 116 has a fixed signal light transmission direction. For example, when the optical fiber transmission line 114 is fixed for signal transmission from the terminal station 110 to the terminal station 112, the optical receiver 132 and the optical switch 134 of the terminal station 110, and the optical transmitter 142 and the optical switch of the terminal station 112 The switch 146 becomes unnecessary, and the demultiplexer 122 also only demultiplexes the signal light from the terminal station 110 to the terminal station 112, and the photodiode 126 becomes unnecessary.

FIG. 5 shows the initial state. That is, the optical switch 134 connects the output of the optical transmitter 130 to the optical fiber transmission line 114 and
Connects the optical fiber transmission line 114 to the input of the optical receiver 144. Thereby, the signal light propagates from the terminal station 110 to the terminal station 112 on the optical fiber transmission line 114. An optical switch 152 connects the output of the optical transmitter 148 to the optical fiber transmission line 116, and an optical switch 140 connects the optical fiber transmission line 116 to the input of the optical receiver 138. Thus, the signal light is transmitted on the optical fiber transmission line 116 to the terminal station 11.
2 to the terminal station 110.

In this embodiment, the optical fiber transmission line 114,
It is possible to change from a state in which each 116 is used in a different direction to a state in which it is used in the same direction by remote control, for example, a state shown in FIG.

For this purpose, first, a control signal light for inverting the amplification direction of the optical amplifier 120 is transmitted from the optical transmitter 130 onto the optical fiber transmission line 114. The control signal light propagates through the optical fiber transmission line 114, is amplified by the optical amplifier 18, is partially demultiplexed by the demultiplexer 122, and is input to the photodiode 124. The control signal converted into an electric signal by the photodiode 124 is input to the control circuit 128. In this way, a control signal for controlling the amplification direction of the optical amplifier 120 is transmitted from the terminal station 110 to the control circuit 128. The control circuit 128 reverses the rotation direction of the optical circulator of the optical amplifier 120 according to the control signal,
The amplification direction of the optical amplifier 120 is changed from the terminal station 110 to the terminal station 11.
Switch to the direction toward 2. At the same time, in the terminal station 110, the switch 140
6 is connected to the optical fiber transmission line 116,
In 2, switch 140 connects optical fiber transmission line 116 to the input of optical receiver 150. As a result, the state transits to the state shown in FIG. 6, and both the optical fiber transmission lines 114 and 116 are used for transmitting signal light from the terminal station 110 to the terminal station 112.

In FIG. 5, the optical fiber transmission lines 114, 11
Although only one optical amplifier 118, 120 is disposed on each of the optical amplifiers 11, in practice, a plurality of similar optical amplifiers 11 and 120 are arranged.
It is clear that 8,120 are arranged. In such a case, the duplexer 122, the photodiodes 124 and 126, and the control circuit 128 may be accommodated for each optical amplification repeater accommodating each optical amplifier 120.

In the embodiment shown in FIG. 5, the control circuit 128 controls only the amplification direction of the optical amplifier 120.
The amplification direction of the optical amplifier 118 may also be controlled. In this case, both the optical fiber transmission lines 114 and 116 are connected to the terminal station 11 or 11 by remote control from the terminal station 110 or 112.
2 to a state in which signal light is transmitted to the terminal station 110.

By using this embodiment, for example,
It is possible to freely respond to fluctuations in traffic between stations. For example, as shown in FIG.
It is assumed that two optical fiber pairs 164 and 166 are laid between them. The optical amplifier of the present embodiment is arranged in the optical fibers 164a and 164b that constitute one of the optical fiber pairs 164. A conventional optical amplifier is arranged in the two optical fibers 166a and 166b constituting the optical fiber pair 166.

Initially, assuming that the traffic volume between the terminal stations 160 and 162 is substantially equal, as shown in FIG. 7, the optical fibers 164a and 166a are used for signal light traveling from the terminal station 160 to the terminal station 162. Optical fibers 164b, 16
6b is used for signal light traveling from the terminal station 162 to the terminal station 160.

As a result, the amount of traffic from the terminal station 162 to the terminal station 160 increases due to a change in the situation.
It is assumed that the traffic amount from the terminal to the terminal station 162 relatively decreases. In this case, the optical fiber 164a can be used to transmit signal light from the terminal station 162 to the terminal station 160 by reversing the amplification direction of the optical amplifier provided in the optical fiber 164a. As a result, the propagation direction of the signal light is as shown in FIG.
Allocate 3 lines to the signal light going to 0,
An asymmetric operation of allocating one line to the signal light traveling from the terminal station 160 to the terminal station 162 can be selected. The reverse is also possible by setting the amplification direction of the optical amplifiers provided in the optical fibers 164a and 164b to the direction from the terminal station 160 to the terminal station 162.

In conventional optical amplification relay transmission, especially in long-distance optical amplification relay transmission, it was impossible to change the direction of the transmission direction. As described above, although the transmission capacity cannot be secured, according to the present embodiment, the capacity in the opposite direction can be used. That is, the present invention is particularly effective when the traffic flow is asymmetric.

[0048]

As can be easily understood from the above description, according to the present invention, since the amplification direction can be freely selected or changed, the signal transmission direction of the optical transmission line can be freely selected after the start of operation. Therefore, it becomes possible to freely construct an optical transmission system in which traffic is asymmetric, and to flexibly cope with fluctuations in traffic volume.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a first embodiment of the optical amplifier of the present invention.

FIG. 2 is a diagram showing flows of signal light and reflected light when the amplification direction is changed in the embodiment shown in FIG.

FIG. 3 is a schematic block diagram of a second embodiment of the optical amplifier of the present invention.

FIG. 4 is a schematic block diagram of a third embodiment of the optical amplifier of the present invention.

FIG. 5 is a schematic block diagram of an embodiment of an optical transmission system using the optical amplifier shown in FIG. 1;

FIG. 6 is a schematic block diagram showing a state in which the signal transmission direction of the optical fiber transmission line 16 is changed.

FIG. 7 is a schematic diagram of an optical transmission system using two optical fiber pairs.

FIG. 8 shows the transmission capacity of the optical transmission system shown in FIG.
It is a schematic diagram in the case of dividing into 1.

FIG. 9 is a schematic block diagram of a conventional optical amplifier.

[Explanation of symbols]

 10: Optical amplification fiber 12: Wavelength multiplexing device 14: Optical isolator 16: Pump light source 20, 22: Signal light input / output terminal 24: Optical amplification fiber 26: Wavelength multiplexing device 28: Optical circulator 30: Pump light source 32: Non-reflection termination 40, 42: Optical input / output terminal 44: Optical circulator 46: Optical amplification fiber 48: Wavelength multiplexing device 50: Excitation light source 52: Non-reflection termination 60, 62: Optical input / output terminal 64: Optical circulator 66: Optical amplification fiber 68: Wavelength multiplexing device 70: Optical circulator 72: Pump light source 74, 76: Non-reflection terminal 110, 112: Terminal station 114, 116: Optical fiber transmission line 118, 120: Optical amplifier 122: Duplexer 124, 126: Photodiode 128 : Control circuit 130: Optical transmitter 132: Optical receiver 134: Optical switch 136: Optical Transmitter 138: Optical receiver 140: Optical switch 142: Optical transmitter 144: Optical receiver 146: Optical switch 148: Optical transmitter 150: Optical receiver 152: Optical switch 160, 162: Terminal station 164, 166: Optical Fiber pair 164a, 164b, 166a, 166b: optical fiber

Claims (6)

[Claims] 1. An optical amplification medium for transmitting signal light, an excitation source for exciting the optical amplification medium, and light that is disposed on one of the optical amplification media and travels in a direction opposite to a transmission direction of the signal light. An optical amplifier comprising a first optical circulator for guiding to a non-reflection end, wherein a rotation direction of the first optical circulator is changeable. And a second optical circulator disposed on the other side of the optical amplifying medium for guiding light traveling in a direction opposite to a transmission direction of the signal light to a non-reflection terminal. 2. The optical amplifier according to claim 1, wherein the rotation direction of the optical circulator can be changed. 3. An optical fiber transmission line including one or more optical amplifiers capable of remotely controlling the amplification direction, and an optical transmission connected to one end of the optical fiber transmission line and selectively connected to the optical fiber transmission line. And a second terminal having an optical transmitter and an optical receiver connected to the other end of the optical fiber transmission line and selectively connected to the optical fiber transmission line. An optical transmission system comprising a terminal station. 4. A first optical fiber transmission line including one or more optical amplifiers capable of remotely controlling the amplification direction, a second optical fiber transmission line, and the first and second optical fiber transmission lines. Connect to one end,
A first terminal having an optical transmitter and an optical receiver selectively connected to the first optical fiber transmission line, and connected to the other ends of the first and second optical fiber transmission lines;
A second terminal station comprising an optical transmitter and an optical receiver selectively connected to the first optical fiber transmission line, and detecting a control signal light flowing through the second optical fiber transmission line, Control means for controlling the amplification direction of the one or more optical amplifiers on the one optical fiber transmission line. 5. The optical transmission system according to claim 4, wherein said control means is provided for each of said one or more optical amplifiers. 6. The demultiplexing means for demultiplexing the control signal light flowing through the second optical fiber transmission line, and the photoelectric conversion means for converting the light demultiplexed by the demultiplexing means into an electric signal. The optical transmission system according to claim 4, further comprising: a control unit configured to control an amplification direction of the optical amplifier in accordance with an output of the photoelectric conversion unit.