JPH07202813A - Optical transmission system - Google Patents

Abstract

PURPOSE:To input an optical signal with a prescribed intensity to an light receiving device or the like by controlling the intensity of the optical signal automatically in response to a loss of an optical transmission line. CONSTITUTION:A signal light outputted from an optical signal amplifier section 19 of an optical amplifier 5 is reflected in an optical partial reflector 6 to be returned to an optical transmission line 3 and a reflected returned light is separated from the optical transmission line 3 by an optical circulator 22. A gain control circuit 24 changes the gain of an optical signal amplifier section 19 based on the reflected returned light and an optical output signal to control the luminous intensity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system for inputting an optical output signal output from an optical transmitter to an optical receiver via an optical transmission line, and particularly to a gain of an amplifier according to a loss of the optical transmission line. Alternatively, it relates to an optical transmission system that automatically adjusts the transmission output.

[0002]

2. Description of the Related Art In a general optical transmission system, as shown in FIG. 7, an optical output signal output from an optical transmitter 101 is passed through an optical transmission line 102 using an optical fiber to an optical receiver 103.
I am trying to receive it. Here, the optical transmitter 101 includes a light emitting element 104 and a light emitting element driving circuit 105,
According to the signal input to the transmission signal input terminal 106, the light emitting element 104 is driven to output an optical signal, and the optical receiver 103
The light receiving element 107 and the receiving circuit 108 convert the input optical signal into an electric signal, amplify or identify the electric signal, and output it as a reception signal from the reception signal output terminal 109.

An optical transmission system using a general optical amplifier is the same as the optical transmission system as described above in FIG.
As shown in FIG. 5, an optical amplifier 111 is provided in the optical transmission line 102 from the optical transmitter 101 to the optical receiver 103. The optical amplifier 111 includes an optical signal amplifier 112, an optical isolator 113, an optical filter 114, an optical branching device 115, an output signal light monitor light receiving element 116, a reference voltage source 117 and a gain control circuit 118. A part of the optical output signal is branched, the intensity thereof is detected by the light receiving element 116, and is compared with the reference voltage of the reference voltage source 117. In accordance with the comparison result, the gain control circuit 118 causes the optical signal amplifying section 112 to operate.
I'm trying to control the gain.

[0004]

However, the above-mentioned FIG.
In the conventional general optical transmission system shown in (1), if the optical output signal transmitted to the optical fiber that is the optical transmission line is larger than the maximum received power of the optical receiver, the optical receiver becomes an excessive input. , The receiving circuit is saturated, or the photocurrent generated in the light receiving element exceeds the rated value and reception becomes impossible. In this case, an optical attenuator must be used to attenuate the light intensity so that an optical signal can be received, or the optical output of the optical transmitter must be reduced to a receivable level.

However, the optical attenuator used to prevent an excessive input of the optical receiver needs to individually adjust the amount of optical attenuation according to the loss of the optical transmission line. Since there is an increase in loss due to connection, bending, and twisting due to the laying of the optical fiber, it is only possible to accurately determine the loss of the optical transmission line after actually installing the optical fiber. Adjustment will be performed after the optical transmission line is installed. As described above, in order to adjust the light attenuation amount, individual adjustment and on-site adjustment must be performed, and each adjustment is troublesome and troublesome, and the cost becomes expensive.

Further, the fact that the optical transmitter outputs a signal intensity more than necessary is a loss itself, and the life of the light emitting element is shortened. Furthermore, the method of preventing the excessive input of the optical receiver by adjusting the optical output of the optical transmitter is the same as the case of using the optical attenuator in that the loss of the optical transmission line is individually obtained.

Next, in the optical transmission system using the conventional optical amplifier shown in FIG. 8, the gain is controlled so that the optical output level becomes constant, but the optical transmission between the optical amplifier and the optical receiver is controlled. When the loss of the transmission line fluctuates, it is necessary to adjust the output level of the optical amplifier according to the loss of the optical transmission line by the optical fiber, and it takes a lot of time and effort even if the optical attenuator is used. It takes time and effort to adjust. Further, in order to cope with the transmission loss due to the optical fiber in the dynamic range of the optical receiver, it is necessary to use a high-performance optical receiver having a large dynamic range, which increases the cost.

[0008]

In order to solve the above-mentioned problems, the present invention provides an optical transmission system in which an optical signal amplification means for amplifying an optical output signal is provided in an optical transmission line, which is output from the optical signal amplification means. Optical returning means for reflecting the signal light reflected back to the optical transmission path, optical separating means for separating the reflected return light returned to the optical transmission path by the optical returning means from the optical transmission path, and separating by the optical separating means Gain control means for controlling the light intensity by changing the gain of the optical signal amplification means based on the reflected light returned and the optical output signal.

Further, according to another invention of the present application, an optical returning means for reflecting the optical output signal of the optical transmitter inputted to the optical receiver and returning it to the optical transmission line, and the optical returning means for returning the optical output signal to the optical transmission line. A light separating means for separating the reflected light returned from the optical transmission line, and a light intensity by changing the optical transmission output of the optical transmitter based on the reflected return light separated by the light separating means and the optical output signal. And an optical transmission output control means for controlling the.

[0010]

According to the present invention, the signal light output from the optical signal amplifying means is reflected and returned to the optical transmission line, the reflected return light returned to the optical transmission line is separated from the optical transmission line, and this separated light is separated. Since the light intensity is controlled by changing the gain of the optical signal amplifying means based on the reflected return light and the optical output signal, the gain of the optical signal amplifying means is automatically adjusted according to the loss of the optical transmission line, An optical output signal having a constant intensity can be transmitted to the optical receiver in response to a change in transmission line loss.

According to another aspect of the invention, the optical output signal of the optical transmitter input to the optical receiver is reflected and returned to the optical transmission line, and the reflected return light returned to the optical transmission line is separated from the optical transmission line. Then, the transmission intensity of the optical transmitter is controlled by changing the transmission output of the optical transmitter based on the separated reflected return light and the optical output signal, so that the optical power of the optical transmitter is automatically adjusted according to the loss of the optical transmission line. The output can be adjusted so that an optical output signal of constant intensity can be transmitted to the optical receiver in response to changes in the optical transmission line loss.

[0012]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Here, FIG. 1 is a block diagram of a first embodiment of an optical transmission system according to the present invention, FIG. 2 is a block diagram of a second embodiment of the optical transmission system, and FIG. 3 is a block diagram of an application example of the same transmission system. FIG. 4 is a block diagram of a first embodiment of another optical transmission system according to the present invention, FIG. 5 is a block diagram of a second embodiment of the optical transmission system, and FIG. 6 is a block diagram of an application example of the same transmission system. Is.

The optical transmission system shown in FIG. 1 includes an optical transmitter 1.
The optical transmission lines 2 and 3 formed of optical fibers for transmitting the optical output signal from the optical transmitter 1, the optical receiver 4 for receiving the optical output signal, and the optical transmission lines 2 and 3 are interposed. Optical amplifier 5 and a part of the signal light from the optical amplifier 5 which is input to the optical receiver 4 and which is interposed in the optical transmission line 3 is reflected to reflect the optical transmission line 3
The optical partial reflector 6 is composed of a half mirror which is a light returning means.

The optical transmitter 1 comprises a light emitting element 11 and a light emitting element drive circuit 12, and drives the light emitting element 11 in response to a signal input to a transmission signal input terminal 13 to output an optical signal. The optical receiver 4 comprises a light receiving element 15 and a receiving circuit 16, which converts an input optical signal into an electric signal, amplifies or discriminates it, and outputs it as a reception signal from a reception signal output terminal 17.

The optical amplifier 5 includes an optical signal amplifier 19 for amplifying the optical output signal from the optical transmitter 1, an optical branching device 20 for branching the optical output signal, and an optical output signal for branching by the optical branching device 20. A light receiving element 21 for receiving light, an optical circulator 22 for separating the reflected return light returned to the optical transmission path 3 by the optical partial reflector 6 from the optical transmission path 3, and a reflected return light separated by the optical circulator 22 for the light receiving element. 23, and a gain control circuit 24 that controls the gain of the optical signal amplification unit 19 by comparing the optical output signal received and detected by the light receiving element 21 with the reflected return light received and detected by the light receiving element 23.

In this transmission system, the optical transmitter 1
The optical output signal output from the optical transmission line 2 is transmitted through the optical transmission line 2, input into the optical amplifier 5, amplified by the optical signal amplification section 19, and then transmitted through the optical branching device 20 and the optical circulator 22 through the optical transmission line 3. The reflected light is reflected by the optical partial reflector 6 such as a half mirror and is returned to the optical transmission line 3 by reflecting a part of the signal light transmitted through the optical transmission line 3 to the optical receiver 4. Then, the light is input from the optical transmission line 3 to the optical amplifier 5, separated from the optical transmission line 3 by the optical circulator 22, and input to the light receiving element 23.

Then, the gain control circuit 24 of the optical signal amplifying section 19 detects the level of the optical output signal (amplified signal light) received by the light receiving element 21 after being branched by the optical branching device 20 and the light receiving element 23. The level of the reflected return light received is compared, and the gain of the optical signal amplifier 19 is controlled according to the comparison result.

If the reflectance of the optical partial reflector 6 is constant, the reflected return light becomes small when the loss of the optical transmission line 3 is large, and the reflected return light becomes small when the loss of the optical transmission line 3 is small. Therefore, the optical output of the optical amplifier 5 can be controlled according to the loss of the optical transmission line 3 by changing the gain of the optical signal amplifier 19 according to the intensity of the reflected return light. Therefore, the transmission loss of the optical transmission line 3 can be known by comparing the level of the reflected return light with the level of the optical output signal. Therefore, by controlling the gain of the optical signal amplifier 19 according to this transmission loss. , And can be controlled so that the level of the signal light input to the optical receiver 4 becomes constant.

In this way, the reflected return light of the output signal of the optical amplifier 5 is detected and the gain is controlled, so that it is automatically input to the optical receiver 4 in accordance with the transmission loss of the optical transmission line 3. The intensity of the optical signal can be kept constant, the receiving power range of the optical receiver 4 can be reduced, and the optical receiver can be made only in consideration of the receiving sensitivity, which significantly improves the optical receiver. It becomes possible to increase the sensitivity, and it becomes possible to deal with a large fluctuation of the optical transmission line loss without adjustment.

In this embodiment, an optical partial reflector consisting of a half mirror is used as the light returning means, but instead of this, Fresnel reflection of the optical fiber or the end face of the optical connector may be used.

Next, in the optical transmission system shown in FIG. 2, instead of the optical partial reflector 6 of the above embodiment, a part of the signal light intensity transmitted by the optical transmission line 3 is branched as a light returning means. Optical branching device 26, and an optical total reflector 27 that totally reflects the signal light branched by the optical branching device 26 and returns it to the optical transmission line 3.
Further, the optical amplifier 25 is provided with an optical isolator 28 and an optical branching device 29 in place of the optical circulator 22 of the above embodiment.

As a result, similarly to the above embodiment, the reflected return light is received and detected by the optical amplifier 25, and the intensity of the signal light input to the optical receiver 4 is kept constant regardless of the transmission loss of the optical transmission line 3. Can be kept at Here, the optical isolator 28
Prevents the operation of the optical signal amplification unit 19 from becoming unstable due to the reflected return light being input to the optical signal amplification unit 19 from the optical branching unit 29.

In each of the above embodiments, the signal light from the optical amplifier is input to the optical receiver. However, a multistage optical amplifier is provided so that the signal light of the optical amplifier in the previous stage is input to the optical amplifier in the next stage. A multi-stage optical repeater can also be configured by inputting. Therefore, the gain of the optical amplifier can be made equal to the loss of the optical transmission line, and it is not necessary to increase the optical output more than necessary.
It is possible to suppress power consumption due to optical signal amplification. Further, since the input signal strength of the optical amplifier can be set to the strength at which the noise figure of the optical amplifier is the best, it is possible to perform relay amplification with lower noise.

In the transmission system shown in FIG. 2, the reflected return light can be modulated by providing an optical switch between the optical branching device 26 and the total optical reflector 27. For example, when the optical switch is ON, a part of the optical output signal of the optical amplifier 25 is reflected by the total optical reflector 27 and becomes return light.
When the optical switch is OFF, no light is input to the total light reflector 27, and no return light is generated. That is, by turning the optical switch on and off, it is possible to perform intensity modulation depending on the presence or absence of reflected return light. In addition, combine an optical modulator,
By using an optical modulator instead of the optical switch, AM modulation and FM modulation are also possible. Then, the light receiving element 23 of the optical amplifier 25 receives the modulated signal of the reflected return light, so that the signal from the optical receiver 4 side can be transmitted to the optical amplifier 25 side. That is, from the optical amplifier 25 side to the optical receiver 4
In addition to the signal transmission to the optical receiver 25, the signal from the optical receiver 4 side can be transmitted to the optical amplifier 25 side without using the light emitting element or the light source. Further, an optical reflector is similarly provided on the signal input side of the optical transmitter 25 of the optical amplifier 25,
Based on the reflected return optical signal transmitted from the optical receiver 4 side,
By modulating the reflectance, relay transmission of the reflected return optical signal becomes possible.

Next, the application example shown in FIG. 3 is applied to a network using an optical branching device, in which an optical branching device 30 is provided in the optical transmission line 3 to branch to a large number of optical receivers 4. , And is also branched into the optical total reflector 27 to return the reflected return light to the optical transmission line 3 and input it to the optical amplifier 5. Thereby, a constant optical signal input can be given to each optical receiver 4 connected to the optical branching device 30. Therefore, even when the number of branches significantly changes and the branch loss fluctuates, the optical amplifier 5 automatically changes the gain and controls the optical signal intensity input to the optical receiver to stabilize the reception state. be able to.

As described above, in the optical transmission system using the optical amplifier, since the intensity of the signal light input to the optical receiver and the optical amplifier in the next stage can be automatically controlled, the loss of the optical transmission line is lost. An optical attenuator for adjusting the fluctuation is unnecessary.

Next, the optical transmission system using no optical amplifier shown in FIG. 4 has an optical transmitter 1 of the transmission system and an optical transmission line 2 including an optical fiber for transmitting an optical output signal from the optical transmitter 1. And an optical receiver 4 for receiving the optical output signal, and a part of the optical output signal from the optical transmitter 1 which is interposed in the optical transmission path 2 and is input to the optical receiver 4 to reflect the optical output path. Light partial reflector 6 consisting of a half mirror which is a light returning means for returning to 2.
And an optical output control section 36 for controlling the optical output of the optical transmitter 1 based on the reflected return light returned to the optical transmission line 2 by the optical partial reflector 6.

The optical output control unit 36 includes an optical circulator 37 for separating the reflected return light from the optical transmission line 2, a light receiving element 38 for receiving the reflected return light separated by the optical circulator 37, and an optical transmitter 1. A drive current detection circuit 39 that detects a drive current flowing through the light emitting element 11, and a drive current that flows through the light emitting element 12 is controlled based on the current flowing through the light receiving element 38 and the current detected by the drive current detecting circuit 39 to generate an optical output. And a light output control circuit 40 for controlling.

In this transmission system, the optical transmitter 1
The optical output signal output from the optical transmission line 2 is transmitted through the optical transmission line 2 and input to the optical receiver 4. However, a part of the optical output signal transmitted through the optical transmission line 2 is reflected by the optical partial reflector 6. Optical transmission line 2
The reflected light is reflected by the optical transmission line 2 and is input to the optical output control unit 36. The optical circulator 37 separates the optical transmission line 2 from the optical transmission line 2 and inputs it to the light receiving element 38.

Then, the light output control circuit 40 of the light output control section 36, based on the current flowing through the light receiving element 38 and the current detected by the driving current detecting circuit 39, drives the current flowing through the light emitting element 11 of the optical transmitter 1. To control the optical output of the optical transmitter 1.

Here, assuming that the reflectance of the optical partial reflector 6 is constant, the reflected return light becomes small when the loss of the optical transmission line 2 is large and becomes small when the loss of the optical transmission line 2 is small, as described above. Since the reflected return light becomes large, it is possible to control the optical output signal according to the loss of the optical transmission line 2 by changing the optical output of the optical transmitter 1 according to the intensity of the reflected return light.

That is, the optical output of the optical transmitter 1 is set to Pt [dB
m], the loss of the optical transmission line (optical fiber) 2 is d [dB],
Assuming that the reflectance of the partial light reflector 6 is α, the optical signal intensity Pr [dBm] input to the optical receiver 4 and the reflected return light Pi [d] input to the light receiving element 38 serving as a reflected return light detector.
Bm] is represented by Formula 1 and Formula 2.

Pr = Pt−d + ₁₀ log ₁₀ (1-α) ... Equation 1

Pi = Pt−2d + ₁₀ log ₁₀ α ... Equation 2

In this equation 2, the light output Pt and the reflected return light P
If i is given, the loss d of the optical transmission line 2 can be obtained. By substituting d into Equation 1, the optical signal intensity Pr input to the optical receiver 4 is obtained. Therefore, the optical output Pt
Is controlled according to the loss d of the optical transmission line 2,
The optical signal intensity Pr input to the optical receiver 4 can be kept constant.

The light output Pt can be obtained by detecting the current flowing through the light emitting element 11, and the reflected return light Pi can be obtained by detecting the current flowing through the light receiving element 38. The monitor signals of Pt and the reflected return light Pi are input to the optical output control circuit 40, the loss d of the optical transmission line 2 and the optical signal intensity Pr input to the optical receiver 4 are obtained, and Pr is kept constant. The light output Pt is controlled by controlling the current flowing through the light emitting element 11.

In this way, the reflected return light of the optical output signal input to the optical receiver 4 is detected and the optical output of the optical transmitter 1 is controlled to match the transmission loss of the optical transmission line 2. The intensity of the optical signal input to the optical receiver 4 can be automatically kept constant, the reception power range of the optical receiver 4 can be reduced, and the optical receiver can be considered only in the reception sensitivity. Therefore, the sensitivity of the optical receiver can be significantly increased, and it is possible to cope with a large fluctuation of the optical transmission line loss without adjustment.

Further, in this embodiment, since the optical circulator 37 is used for separating the optical output signal of the optical transmitter 1 and the reflected return light from the optical partial reflector 6, the light emitting element 11 is provided with a laser diode (LD). Even when (1) is used, the reflected return light does not return to the LD, so that the LD can be operated stably, which is particularly effective for a high-speed, wide-band optical transmission system.

In this embodiment, an optical partial reflector consisting of a half mirror is used as the light returning means, but instead of the above, instead of the above, the optical fiber and the Fresnel reflection of the end face of the optical connector are replaced. It can also be used. Although the optical output Pt of the optical transmitter 1 is monitored by detecting the current flowing through the light emitting element 11, it may be monitored using a light receiving element for optical output monitoring.

Next, in the optical transmission system shown in FIG. 5, instead of the optical partial reflector 6 of the above-mentioned embodiment, a part of the signal light intensity transmitted by the optical transmission line 2 is branched as the light returning means. Optical branching device 41, and an optical total reflector 42 that totally reflects the signal light branched by the optical branching device 41 and returns it to the optical transmission line 2.
Further, instead of the optical circulator 37 of the optical output control unit 36, an optical isolator 43 and an optical branching device 44 are provided.

Incidentally, non-reflective terminators 45 and 46 are attached to the vacant ports of the optical branchers 41 and 44. The non-reflective terminators 45 and 46 are optical waveguides having a structure that diffuses or absorbs the light distributed to the terminals, for example, a rough waveguide on the side surface of the core, a waveguide in which bubbles are mixed, or a waveguide with large radiation loss. It can be configured with a waveguide or the like. In addition, antireflection measures such as a non-reflective coating film may be attached or an end face may be cut obliquely with respect to the optical waveguide.

As a result, similarly to the above embodiment, the optical output control section 36 receives and detects the reflected return light, and the intensity of the signal light input to the optical receiver 4 irrespective of the transmission loss of the optical transmission line 2. Can be kept constant. Here, the optical isolator 4
Reference numeral 3 prevents the operation of the optical transmitter 1 from becoming unstable due to the reflected return light being input to the optical transmitter 1 from the optical branching device 44.

Next, the application example shown in FIG. 6 is applied to a network using an optical branching device, in which a plurality of optical branching devices 48 are provided in the optical transmission line 2 to output the optical output from the optical transmitter 1. The signal is split into a number of optical receivers 4 and the optical total reflector 4
It is also branched to 2 and the reflected return light is returned to the optical transmission line 2 and input to the optical output control unit 36. As a result, the optical splitter 48
It is possible to always provide a constant optical signal input to each optical receiver 4 connected to. Therefore, even when the number of branches changes significantly and the branch loss fluctuates, the optical output controller 36
Automatically changes the optical output of the optical transmitter 1, and the optical receiver 4
It is possible to stabilize the reception state by controlling the intensity of the optical signal input to the.

[0044]

As described above, according to the present invention,
In an optical transmission system using the optical signal amplification means, the signal light output from the optical signal amplification means is reflected and returned to the optical transmission path, and the reflected return light returned to the optical transmission path is separated from the optical transmission path. The light intensity is controlled by changing the gain of the optical signal amplifying means based on the separated reflected return light and the optical output signal. Therefore, the optical signal amplifying means is automatically adjusted according to the loss of the optical transmission line. By adjusting the gain of the optical transmission line, it is possible to transmit an optical output signal of constant intensity to the optical receiver according to the change of the optical transmission line loss, and the optical attenuator that adjusts the loss variation of the optical transmission line becomes unnecessary. At the same time, the receiving range of the optical receiver can be narrowed, and it is only necessary to design and manufacture in consideration of sensitivity, and it is possible to apply a light receiving element and receiving circuit with high sensitivity characteristics, and to improve sensitivity. As a result, the optical transmission line loss is further increased. It is possible to cope with.

Further, in an optical transmission system using no optical signal amplification means, the optical output signal input to the optical receiver is reflected and returned to the optical transmission line, and the reflected return light returned to the optical transmission line is transmitted optically. Since it is separated from the optical path and the optical output is controlled by changing the transmission output of the optical transmitter based on the separated reflected return light and optical output signal, it is automatically adjusted according to the loss of the optical transmission path. The optical output of the optical transmitter can be adjusted to transmit the optical output signal of constant intensity to the optical receiver according to the change of the optical transmission line loss, and the optical intensity input to the optical receiver can be kept constant. It is possible to reduce the value to less than the value, the optical attenuator becomes unnecessary, the receiving range of the optical receiver can be narrowed, and it suffices to design and manufacture considering only the sensitivity. , A receiver circuit can be applied to increase the sensitivity. As a result, it is possible to further correspond to the increase in the optical transmission line loss.

Further, since the light receiving element for receiving and detecting the reflected return light, the gain control circuit and the light output control circuit do not require high-speed response characteristics, they can be realized by an easy method, and the optical coupling system can be realized. Including, it can be realized at low cost. Also,
The light output control circuit can be made into a monolithic IC together with the drive circuit of the light emitting element, and the optical transmitter / receiver is small and lightweight.
High integration becomes possible.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of an optical transmission system according to the present invention.

FIG. 2 is a block diagram of a second embodiment of the optical transmission system.

FIG. 3 is a block diagram of an application example of the transmission system.

FIG. 4 is a block diagram of a first embodiment of another optical transmission system according to the present invention.

FIG. 5 is a block diagram of a second embodiment of the optical transmission system.

FIG. 6 is a block diagram of an application example of the transmission system.

FIG. 7 is a block diagram of a conventional optical transmission system.

FIG. 8 is a block diagram of another conventional optical transmission system.

[Explanation of symbols]

1 ... Optical transmitter, 2, 3 ... Transmission line, 4 ... Optical receiver, 5, 2
5 ... Optical amplifier, 6 ... Optical partial reflector, 11 ... Light emitting element, 1
2 ... Light emitting element drive circuit, 19 ... Optical signal amplification section, 22 ... Optical circulator, 23 ... Reflection return light detection light receiving element, 2
4 ... Gain control circuit, 26, 29 ... Optical branching device, 27 ... Optical total reflector, 28 ... Optical isolator, 36 ... Optical output control section,
37 ... Optical circulator, 38 ... Reflection return light detection light receiving element, 39 ... Light emitting element drive current detection circuit, 40 ... Optical output control circuit, 41, 44 ... Optical branching device, 43 ... Optical isolator.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G02F 1/35 501 H04B 10/17 10/16

Claims (12)

[Claims] 1. An optical transmission system in which an optical signal amplifying means for amplifying an optical output signal outputted from an optical transmitter is interposed in an optical transmission line, wherein the signal light outputted from the optical signal amplifying means is reflected. Light returning means for returning to the optical transmission path; light separating means for separating the reflected return light returned to the optical transmission path by the light returning means from the optical transmission path; and reflected return light separated by the light separating means An optical transmission system, comprising: a gain control unit that controls a light intensity by changing a gain of the optical signal amplification unit based on an optical output signal. 2. The optical transmission system according to claim 1, wherein the light returning means comprises an optical partial reflector that reflects a part of the optical output signal. 3. The optical transmission system according to claim 1, wherein the optical returning means returns the optical output signal to the optical transmission line by Fresnel reflection of the end face of the optical fiber or the optical connector. 4. The optical transmission according to claim 1, wherein the light returning means comprises an optical branching device and an optical total reflector or an optical partial reflector provided at one end of the optical branching device. system. 5. The optical transmission system according to claim 1, wherein the light splitting means comprises an optical circulator. 6. The optical transmission system according to claim 1, wherein the light splitting means comprises an optical branching device. 7. An optical transmission system for inputting an optical output signal output from an optical transmitter to an optical receiver via an optical transmission line, wherein the optical output signal input to the optical receiver is reflected to generate an optical signal. Light returning means for returning to the transmission path, light separating means for separating the reflected return light returned to the optical transmission path by the light returning means from the optical transmission path, reflected return light separated by the light separating means and the light An optical transmission system comprising: an optical transmission output control means for controlling an optical intensity by changing an optical transmission output of the optical transmitter based on an output signal. 8. The optical transmission system according to claim 7, wherein the light returning means comprises an optical partial reflector that reflects a part of the optical output signal. 9. The optical transmission system according to claim 7, wherein the optical returning means returns the optical output signal to the optical transmission line by Fresnel reflection of the end face of the optical fiber or the optical connector. 10. The optical transmission according to claim 7, wherein the light returning means comprises an optical branching device and a total light reflector or an optical partial reflector provided at one end of the optical branching device. system. 11. The optical transmission system according to claim 7, wherein the light separating unit is an optical circulator. 12. The optical transmission system according to claim 7, wherein the light splitting means comprises an optical branching device.