JPH049012A - Method and circuit for stabilizing superposed signal of optical amplifier - Google Patents

Abstract

PURPOSE:To stabilize the degree of modulation of modulated light superposed upon the output light of the optical amplifier by superposing a continuous signal of 2nd frequency and a modulated signal one over the other, extracting the continuous signal at the output terminal of the optical amplifier, and feeding the extracted signal back for the modulation of the optical amplifier as a reference signal. CONSTITUTION:A superposing means 21 superposes the continuous signal of 2nd frequency f2 and the modulated signal one over the other. Consequently, the continuous signal of 2nd frequency f2 is superposed on the output light of the optical amplifier 20 together with a modulated signal superposed upon a carrier signal of 1st frequency f1. The continuous signal of frequency f2 which is thus superposed is extracted by an extracting means 22 at the output terminal of the optical amplifier 20 and fed as the reference signal back for the modulation of the optical amplifier 20 by a stabilizing means 23. Consequently, the modulation degree of the modulated light superposed upon the output light of the optical amplifier 20 can easily be stabilized without limiting the band of this loop.

Description

[Detailed Description of the Invention] [Summary] It relates to an optical communication system using an optical amplifier that can directly amplify an optical signal. Regarding a method and circuit for stabilizing the modulated light superimposed on the output light when transmitting the modulated light superimposed on the output light, the present invention relates to a method and circuit for stabilizing the modulated light superimposed on the output light of an optical amplifier without limiting the loop band. With the aim of easily stabilizing the modulation degree of the optical amplifier, the modulated light superimposed on the output light of the optical amplifier based on the modulated signal applied to the carrier signal with the first frequency collects information from the optical amplifier and its surroundings. In an optical communication system for transmitting a continuous signal having a second frequency, a continuous signal having a second frequency is superimposed with the modulated signal, the continuous signal is extracted at the output end of the optical amplifier, and the extracted signal is used as a reference signal to transmit the modulated signal. It is configured to stabilize the modulation degree of the optical amplifier by feeding back to the modulation of the optical amplifier, or it is superimposed on the output light of the optical amplifier based on the angle modulation signal applied to the continuous wave carrier signal having the frequency. In an optical communication system that transmits information from the optical amplifier and its surroundings using modulated light, a part of the carrier signal is extracted at the output end of the optical amplifier,
The extracted signal is fed back to the modulation of the optical amplifier as a reference signal, thereby stabilizing the modulation degree of the optical amplifier.

[Industrial application field]

The present invention relates to an optical communication system using an optical amplifier that can directly amplify an optical signal, and more specifically, the present invention relates to a modulated optical system in which various information from the optical amplifier and its surroundings is superimposed on the output light of the optical amplifier. The present invention relates to a method and a circuit for stabilizing the superimposed modulated light when transmitting it.

Optical amplifiers are capable of directly amplifying input light waves, so they are classified into power amplifiers (booster amplifiers) to increase transmission power, preamplifiers (briamps) to improve reception sensitivity, and optical direct amplifiers. It is expected to be applied to future optical communication systems, such as being able to be used as repeaters.

[Traditional techniques]

As an example of an optical communication system using an optical amplifier, -
In general, the configuration of the optical amplification repeater transmission system 1 in the 7th stage is as follows.
As shown in the figure.

This consists of k optical amplifying repeaters 43.4...
・,4. II) The loss of the signal light transmitted from the transmitter 1 in the transmission fiber 3 is compensated for by the optical amplifiers in the optical amplification repeaters 41 to 4k, and the receiver This is a method of optical transmission up to 2. This method is
Basically, regardless of the signal modulation/demodulation method, signal speed, etc.
They are the same.

Optical amplifiers used in each optical amplification repeater in such an optical amplification medium-phase transmission system 1 include a Raman amplifier,
There are semiconductor optical amplifiers, -fiber type optical amplifiers, etc. As an example of an optical amplification repeater, FIG. 8 shows a configuration using a semiconductor optical amplifier, and FIG. 9 shows a configuration using a fiber type optical amplifier.

In FIG. 8, an optical signal transmitted from a transmitter 1 is input to a semiconductor optical amplifier 5 in an optical amplification repeater 4, where it is amplified and then sent to a receiver 2 via a transmission fiber 3. sent to the side. In FIG. 9, transmitter 1
The optical signal transmitted from the side is combined with the pumping light output from the pumping light source 7 by the multiplexer 6 at the entrance of the optical amplification repeater 4, and is input to the fiber-type optical amplifier 8, where it is combined with the pumping light output from the pumping light source 7. After undergoing amplification4. Receive a2 via 7 via transmission fiber 3
sent to the side.

The following shows the basic configuration of an optical amplification repeater, but in an actual optical amplification and transmission system, it is necessary to monitor and control whether a distant optical amplification repeater is operating stably or not. If an abnormality occurs in system I, it is necessary to know where the abnormality is occurring, so a function is required to respond with a monitor signal from each designated repeater. There are various ways to create such a response signal, but the most common method is the 10th one.
As shown in the figure.

In this method, the gain of the optical amplifier 9, such as the semiconductor optical amplifier 5 or the fiber-type optical amplifier 8 shown in FIGS. This is a method of applying modulation to the output light of the optical amplifier 9.

In the following, the method for creating this response signal will be specifically described.

First, a method of creating a response signal when the semiconductor optical amplifier 5 is used as the optical amplification repeater 4 is shown in FIG. In this case, by modulating the bias current of the semiconductor optical amplifier 5 to the data signal containing information of the optical amplification repeater 4, the gain of the semiconductor optical amplifier 5 is modulated. Apply modulation to the output light.

On the other hand, when a fiber type optical amplifier 8 is used as the optical amplification repeater 4, a method of modulating the pumping light as shown in FIG. 12 is effective. That is, the pumping light source 7 is modulated by the modulation signal created based on the data signal containing the information of the optical amplification repeater 4, and thereby the fiber-type optical amplifier 8
modulates the output light. Generally, a semiconductor laser is used as the excitation light source 7, and its modulation can be realized by applying modulation to the bias current of the semiconductor laser.

The most common method for such modulation is the 13th
As shown in the figure, this is a method of creating a response signal by applying AM modulation based on a data signal.

By the way, in order for the created response signal to be transmitted and stably received at the station side, the response signal created by the optical amplification repeater must first be stabilized. Therefore,
As shown in FIG. 14, the optical amplification repeater 4 requires an automatic gain control (AGC) function to stabilize the response signal.

That is, basically, by extracting a part of the output light of the optical amplifier 9 and applying feedback to the AM modulation signal based on this, the modulation degree of the optical amplifier 9 (i.e., AM modulation with respect to the total power of the output light) (ratio of amplitudes by)
control to keep it constant.

FIG. 15 shows a specific response signal stabilizing circuit 10. This stabilizing circuit 10 is a circuit applied to a method of creating a response signal using AM modulation, in which A, SK modulation is performed using a digital signal as a data signal.

In the figure, the modulator 15 first modulates the frequency f+
ASK modulation is applied to a signal obtained by mixing a digital data signal with a carrier signal.

In this case, an ASK signal as shown in FIG. 15 is superimposed on the amplified light of the optical amplifier 9. In the stabilizing circuit 10,
A part of the amplified light is extracted at the output side of the repeater, photoelectrically converted by a photodetector 11, and then an AsK signal is extracted by a band pass filter (BPF) 12 with a center frequency f1 and detected by a detector 13. Then, using the signal obtained by this detection as a reference signal, the AGC amplifier 14 applies feedback to the modulation signal (ASK signal) to the optical amplifier 9, and controls the degree of modulation to be kept constant.

[Problems to be Solved by the Invention] The problem with the conventional stabilizing circuit 10 shown in FIG. This is because a time lag occurs between the input modulation signal and the reference signal. When a deviation like this occurs, there is no reference signal (
Only the modulated signal is sent to the AGC amplifier 14 (signal “0” state).
As a result, the moment the pulse of the modulation signal is input to the AGC amplifier 14, a signal is generated that tries to rapidly increase the gain.
This will cause an error.

If you want to prevent this kind of error from occurring,
The reference signal input to the GC amplifier 14 is continuous wave or DC.
However, if the output of the detector 13 is made to be a DC signal, for example, a problem arises in that the loop band is significantly limited.

An object of the present invention is to easily stabilize the modulation degree of modulated light superimposed on the output light of an optical amplifier without limiting the loop band.

[Means to solve the problem]

The basic structure of the present invention is shown in FIGS. 1 and 2.

As shown in these figures, the stabilization method and stabilization circuit of the present invention provide a carrier signal having a first frequency f1,
A modulated signal (angle modulated signal in FIG. 2) is created by mixing data signals containing information from the optical amplifier 20 and its surroundings, and this modulated signal causes the optical amplifier 20 to
It is applied to an optical communication system that transmits the above information by superimposing modulated light on the output light of. Of course, the present invention is not limited to the optical amplification repeater as described above.

In FIG. 1, first, the superimposing means 21 superimposes a continuous signal having a second frequency f2 together with the above modulated signal. As a result, the output light of the optical amplifier 20 includes the first
With the modulation signal applied to the carrier signal of frequency f,
A continuous signal of a second frequency f2 is superimposed. The continuous signal of frequency f2 superimposed in this way is transferred to the optical amplifier 20.
At the output end of , extraction means 22 extracts. and,
Using this extracted signal as a reference signal, the stabilizing means 23
By feeding back to the modulation of the optical amplifier 20, the modulation degree of the optical amplifier 20 is stabilized.

Here, as the optical amplifier 20, a semiconductor optical amplifier, a fiber type optical amplifier, etc. can be used, and by modulating the bias current and pumping light, it is possible to modulate the output of the optical amplifier. .

Further, as a modulation method applied to the optical amplifier 20, AM modulation (including ASK modulation), FM modulation (including FSK modulation), FM modulation (including PSK modulation), etc. can be adopted.

In FIG. 2, first, a part of the modulated light superimposed on the output light of the optical amplifier 20 by the angle modulation signal applied to the continuous wave carrier signal having the frequency f1 is extracted by the extraction means 24 at the output end of the optical amplifier 20. Extract. The extracted signal is used as a reference signal and fed back to the modulation of the optical amplifier 20 by the stabilizing means 25, thereby stabilizing the degree of modulation of the optical amplifier 20.

Here, as the optical amplifier 20, a semiconductor optical amplifier, a fiber type optical amplifier, etc. can be used as in the case of FIG. It is possible to modulate the output of the amplifier.

3. The modulation method applied to the optical amplifier 20 is F
M modulation (including FSX modulation) and PM modulation (including PSK modulation) can be employed.

Furthermore, when applying angular modulation to the optical amplifier 20, in addition to modulating the signal superimposed on the output light (transmission signal light) of the optical amplifier 20, the output light itself is also affected. The modulation L7 may be used to directly apply modulation.

[For production]

In the case of FIG. 1, since the signal extracted by the extraction means 22 is a continuous wave of frequency f2, the stabilization means 23
The reference signal that is returned to the zero modulation also becomes a continuous wave (or can easily be converted to DC).

Since the base signal is a continuous wave (or DC signal), there is no problem with the time gaps that occur between the modulation signal and the reference signal as in the past, and the optical amplifier 20
It becomes possible to easily stabilize the degree of modulation. Moreover, 1. In this case, since it is only necessary to extract the continuous wave from the output light of the optical amplifier 20, there are no problems such as limiting the loop band.

In addition, in the case of FIG. 2, by applying angular modulation to the optical amplifier 20, the modulation signal superimposed on the output light of the optical amplifier 20 changes in amplitude like an ASK modulation signal. The signal does not reach the maximum signal, but becomes a signal with a constant amplitude. Therefore, such a variation with constant amplitude, the base 711: signal -4) obtained by extracting the iJQ signal, has a constant amplitude,
It can be easily converted into DC. Therefore, for the same reason as in the case of FIG. 1, it is possible to easily stabilize the modulation degree of the optical amplifier 20 without limiting the loop band.

[Examples] Examples of the present invention will be described above with reference to the drawings.

FIG. 3 shows a stabilizing circuit 30 according to a first embodiment of the present invention.
FIG. 2 is a configuration diagram of an optical amplifying repeater incorporating This stabilization circuit 30 uses a digital signal as a data signal to
This circuit is applied to a device in which a response signal is created by K modulation.

In the figure, first, the modulator 15 modulates the ASK modulation (3P) with a signal obtained by mixing a digital data signal with a carrier signal of frequency [1]. The data signal includes information from the optical amplifier 9 and its surroundings.

Further, the adder 36 adds a continuous signal with a frequency f2 different from the frequency r, and this continuous signal and A
The optical amplifier 9 is modulated by the combined signal with the SK modulation signal.

As a result, the amplified light of the optical amplifier 9 has an ASK applied to the carrier signal of the frequency f, as shown in FIG.
The modulated wave and the continuous wave of frequency "2" are superimposed together.

In the stabilizing circuit 30, the - part of the amplified light shown in I- is extracted on the output side of the repeater, photoelectrically converted in the photoreceiver 31, and only the continuous wave is extracted in the bandpass filter (BPF) 32 with a center frequency f2. It is extracted and smoothed using a smoother 33. Then, using this smoothed signal as a reference signal, the AGC amplifier 34.
35 gives feedback to the modulation signal (ASK43 and continuous signal) to the optical amplifier 9, and changes the degree of modulation to -.
Control oil to keep it constant.

According to this embodiment, since it is a continuous wave of frequency f2 that is extracted via the bandpass filter 32, the output (reference signal) of the smoother 33 can be easily adjusted to 11 Can be converted into C. In this way, the reference signal is a continuous wave (
or a DC signal), the time lag that occurs between the modulation signal and the reference signal (see Fig. 15) as in the conventional case does not pose any problem, and the modulation degree of the optical amplifier 9 can be easily adjusted. It can be stabilized.

In addition, in this case, the continuous wave of frequency "2" is used from the viewpoint of not increasing the number of parts in the repeater as much as possible.
It is desirable to create it by dividing or multiplying the - part of the output of the oscillator with frequency f1.

Furthermore, when using the fiber type optical amplifier 8 as shown in FIG. 12 as the optical amplifier 9, as shown in FIG. 4, by applying amplitude modulation to the pump light output from the pump light source 7, It is possible to modulate the output light of the fiber type optical amplifier 8. Here, excitation light source 7
When a semiconductor laser is used, amplitude modulation can be applied to the bias current to suppress the amplitude modulation of the excitation light.

Further, as the optical amplifier 9, a semiconductor optical amplifier 5 as shown in FIG. 11 may be used. In this case, the output light of the semiconductor optical amplifier 5 can be modulated by modulating its bias current. is possible.

Next, FIG. 5 is a block diagram of an optical amplifying repeater incorporating a stabilizing circuit 40 according to a second embodiment of the present invention. This embodiment uses FSK modulation or PSK modulation instead of ASK modulation.
This method uses modulation to create a response signal, and is applied to such optical amplification repeaters.

In the figure, first, a modulator 47 converts a carrier signal of frequency fl into FSX based on a digital data signal.
Or apply PSK modulation. Furthermore, an adder 46 adds a continuous signal having a frequency f2 different from the frequency f, and modulates the optical amplifier 9 with a composite signal of this continuous signal and an FSK or PSK modulated signal. As a result, the FSK or PSK modulated wave applied to the carrier signal of frequency f and the continuous wave of frequency f2 are superimposed on the amplified light of optical amplifier 9.

The operation of the stabilizing circuit 40 is similar to that of the stabilizing circuit 3 shown in FIG.
Same as 0. That is, a part of the amplified light is extracted at the output side of the repeater, photoelectrically converted by the optical receiver 41, only the continuous wave is extracted by the bandpass filter (BPF) 42 with center frequency f2, and then the continuous wave is extracted by the smoother 43. smooth. Using this smoothed signal as a reference signal, the AGC amplifier 44.45 outputs a modulation signal (FSK) to the optical amplifier 9.
Or apply feedback to PSK signal and continuous signal),
The degree of modulation is controlled to be kept constant.

In this embodiment, as in the first embodiment, the output (reference signal) of the smoother 43 can be easily converted to DC without limiting the loop band, so that the optical amplifier 9 can be modulated. The temperature can be easily stabilized.

Furthermore, by using FSK or PSK modulation, it can be expected that the reception sensitivity of the transmitted response signal at the station side will be better than in the case of ASK modulation. For example, when performing heterodyne detection, an improvement of 10 dB or more is expected compared to when directly detecting an ASK signal, and ideally, sensitivity can be expected to improve by 3 dB in the order of ASK, FSK, and PSK. .

Next, FIG. 6(a) is a block diagram of an optical amplification repeater incorporating a stabilizing circuit 50 according to a third embodiment of the present invention. This embodiment, like the second embodiment, uses FSK modulation or P
This is applied to an optical amplification repeater that generates a response signal using SK modulation.

In the figure, a modulator 47 applies FSK or P to a carrier signal of frequency [based on a digital data signal].
SK modulation is applied, and the optical amplifier is modulated by this modulation signal. Here, a fiber-type optical amplifier 8 is used as an optical amplifier, and by applying FSK or PSK modulation to the amplitude modulation applied to the pumping light source 7, the superimposed signal of the output light of the fiber-type optical amplifier 8 is subjected to FSK or PSK modulation. P.S.K.
I'm trying to modulate it. When a semiconductor laser is used as the excitation light source 7, the excitation light is subjected to FSK or PSK modulation by applying FSK or PSK modulation to the amplitude modulation applied to the bias current. As a result, the amplified light of the fiber type optical amplifier 8 has a frequency f+
The FSK or PSK modulated wave applied to the gear signal is superimposed.

In the stabilization circuit 50, a part of the amplified light is extracted on the output side of the repeater, photoelectrically converted in the photoreceiver 51, and then only the FSK or PSK modulated wave is extracted in the carrier extraction circuit 52. This carrier extraction circuit 52 uses FSK as a modulation method.
The difference is between the case of using modulation and the case of using PSK modulation, examples of which are shown in FIGS. 6(b) and 6(c), respectively.

In other words, in the case of FSX modulation, the center frequency is f1+Δf
As shown in FIG. 6(b), the two bandpass filters 52a and 52b with the center same wave numbers f11Δf and r, -Δf are passed from the point where the two outputs f and -Δf (No. 3 is added and 2g
52 (7), the FSK modulated wave is extracted. On the other hand, in the case of PSK modulation, as shown in FIG. After doubling the frequency, it passes through a bandpass filter 52f with a center frequency of 2ft.
1, and then each time the frequency is halved by a frequency divider 52g, a PSK modulated wave is extracted. Next, the FSK or PSK modulated wave thus obtained is smoothed by a smoother 53.

Then, using this smoothed signal as a reference signal,
The AGC amplifier 54 sends a modulation signal (F
SK or PSK signal),
The degree of modulation is controlled to be kept constant.

In the case of this embodiment, since the FSK or PSK modulated wave with a constant amplitude is extracted via the carrier extraction circuit 52, the output of the smoother 53 (basic "signal") limits the band of the loop. Therefore, similarly to each of the embodiments described above, the modulation degree of the optical amplifier can be easily stabilized.

Generally, the amplitude of the amplified light of an optical amplifier can be modulated if the speed of the modulated signal is approximately the relaxation time of the optical amplifier, but in the case of a fiber-type optical amplifier, for example, In a fiber-type optical amplifier doped with (Er""), this relaxation time τ is approximately 0.
Since it is 1 to 1 ms, the modulation signal is approximately 1 to 10k.
The speed is limited to about Hz (kb/S). On the other hand, in the case of a semiconductor optical amplifier, the relaxation time τ is about μS, so
Modulation with a signal on the order of MHz (Mb/s) is possible.

In addition, when a semiconductor optical amplifier is used as an optical amplifier,
By modulating the bias current, it is also possible to operate the semiconductor optical amplifier as a phase modulator. In this case, it is possible to transmit the response signal by directly applying phase modulation to the optically amplified transmission signal itself using the response transmission signal.

Also, the number of carrier signals used to create a modulation signal (response signal) does not necessarily have to be one; if there is multiple data, it is also possible to use multiple carrier signals, each with a different frequency. It is. In this case 4
J. It is possible to construct a configuration in which carrier signals of different frequencies are modulated for each piece of data, and these are added together to form one modulated signal, which is then modulated to an optical amplifier.

Furthermore, although the embodiment described above shows an IJ that is incorporated in an optical amplification repeater, the present invention is not limited to this, and the present invention is not limited to this, but can be applied to the use of the optical amplifier in any optical communication system using an optical amplifier. It can be similarly applied to the places where

In addition, in order to demodulate the modulated signal (response signal) superimposed on the output light of the optical amplifier at the station side, in addition to the direct detection method, it is possible to employ an optical heterodyne detection method, an optical homodyne detection method, or the like.

〔Effect of the invention〕

According to the present invention, it is possible to easily stabilize the degree of modulation of the modulated light superimposed on the output light of the optical amplifier, and in doing so, there is no problem such as the loop band being limited.

Therefore, especially when the present invention is applied to an optical communication system using an optical amplifier as an optical amplifying repeater, it can be widely used for monitoring and controlling optical amplifying repeaters. Further, by transmitting the response signal using a coherent method, it becomes possible to transmit the response signal with higher sensitivity.

[Brief explanation of drawings]

1 and 2 are the principle configuration diagrams of the present invention, FIG. 3 is the configuration diagram of an optical amplification repeater incorporating a stabilizing circuit according to the first embodiment of the present invention, and FIG. 4 is the diagram 3. Fig. 5 is a block diagram of an optical amplification repeater incorporating a stabilizing circuit according to a second embodiment of the present invention; FIG. 6(a) is a block diagram of an optical amplifier repeater incorporating a stabilizing circuit according to the third embodiment of the present invention, and FIG. 6(b) and (C) are respectively FS
A configuration diagram of an example of a carrier extraction circuit in the case of K modulation and PSK modulation, Figure 7 is a configuration diagram of a general optical amplification repeater transmission system, and Figure 8 is a configuration diagram of an optical amplification repeater using a semiconductor optical amplifier. , Fig. 9 is a block diagram of an optical amplification repeater using a fiber-type optical amplifier, Fig. 10 is a diagram showing a method for creating a response signal in an optical amplification repeater, and Fig. 11 is an optical amplification repeater using a semiconductor optical amplifier. 12 is a diagram showing a method for creating a response signal in an optical amplifier repeater using a fiber-type optical amplifier, and FIG. 13 is a diagram showing a method for creating a response signal in an optical amplifier repeater using AM modulation. 14 is a diagram showing a configuration for stabilizing a response signal in an optical amplification repeater that uses AM modulation to create a response signal, and FIG. 15 is a diagram showing a conventional response signal stabilization circuit. FIG. 2 is a configuration diagram of an incorporated optical amplification repeater. DESCRIPTION OF SYMBOLS 1... Transmitter, 2... Receiver, 3... Transmission fiber, 4... Optical amplification repeater, 5... Semiconductor optical amplifier, 6... Multiplexer, 7... Pumping light source, 8... Fiber type optical amplifier, 9... Optical amplifier, 15... Modulator, 20... Optical amplifier, 21... Superimposing means, 22... Extracting means, 23... - Stabilizing means, 24... Extracting means, 25... Stabilizing means, 30... Stabilizing circuit, 31... Light receiver, 32... Bandpass filter, 33... Smoother, 34 .35--- AGC777', 36... Adder, 40... Stabilization circuit, 41... Light receiver, 42... Bandpass filter, 43... Smoother, 44.45... AGC amplifier, 46... Adder, 47... Modulator, 50... Stabilization circuit, 51... Light receiver, 52... Carrier extraction circuit, 53... Smoother, 54...・AGC amplifier.

Claims (1)

[Claims] 1) Modulated light superimposed on the output light of the optical amplifier (20) based on a modulated signal applied to a carrier signal having a first frequency (f_1) causes damage to the optical amplifier (20) and its surroundings. In an optical communication system for transmitting information from
An optical amplifier characterized in that the degree of modulation of the optical amplifier (20) is stabilized by extracting the signal at the output end of the optical amplifier and feeding back the extracted signal as a reference signal to the modulation of the optical amplifier (20). superimposed signal stabilization method. 2) A semiconductor optical amplifier is used as the optical amplifier (20), and the output light of the optical amplifier is modulated by modulating its bias current.
A method for stabilizing a superimposed signal in an optical amplifier as described above. 3) The optical amplifier superimposition according to claim 1, characterized in that a fiber type optical amplifier is used as the optical amplifier (20), and the output light of the optical amplifier is modulated by applying amplitude modulation to the pumping light. Signal stabilization method. 4) Information from the optical amplifier (20) and its surroundings is transmitted by modulated light superimposed on the output light of the optical amplifier (20) based on the angle modulated signal applied to the continuous wave carrier signal having the frequency (f_1). In the optical communication system, a part of the carrier signal is extracted at the output end of the optical amplifier (20), and the extracted signal is fed back to the modulation of the optical amplifier (20) as a reference signal. A method for stabilizing a superimposed signal of an optical amplifier, the method comprising stabilizing the modulation degree of an optical amplifier (20). 5) A semiconductor optical amplifier is used as the optical amplifier (20), and frequency modulation or phase modulation is applied to the superimposed signal of the output light of the optical amplifier by modulating its bias current. A method for stabilizing superimposed signals in optical amplifiers. 6) By using a fiber type optical amplifier as the optical amplifier (20) and applying frequency modulation or phase modulation to the amplitude modulation applied to the pumping light, frequency modulation or phase modulation is applied to the superimposed signal of the output light of the optical amplifier. 5. The method for stabilizing a superimposed signal in an optical amplifier according to claim 4, further comprising: 7) Transmit information from the optical amplifier (20) and its surroundings using modulated light superimposed on the output light of the optical amplifier (20) based on the modulated signal applied to the carrier signal having the first frequency (f_1). In the optical communication system, a superimposing means (21) for superimposing a continuous signal having a second frequency (f_2) together with the modulated signal, and an extraction means for extracting the continuous signal at the output end of the optical amplifier (20). (2
2), and the optical amplifier (20) using the extracted signal as a reference signal.
), the optical amplifier (20)
A superimposed signal stabilizing circuit for an optical amplifier, comprising: stabilizing means (23) for stabilizing the modulation degree of the optical amplifier. 8) Information from the optical amplifier (20) and its surroundings is transmitted by modulated light superimposed on the output light of the optical amplifier (20) based on the angle modulation signal applied to the continuous wave carrier signal having the frequency (f_1). An optical communication system comprising: extracting means (24) for extracting a part of the carrier signal at an output end of the optical amplifier (20); and using the extracted signal as a reference signal, the optical amplifier (20)
), the optical amplifier (20)
A superimposed signal stabilizing circuit for an optical amplifier, comprising: stabilizing means (25) for stabilizing the modulation degree of the optical amplifier.