JP2000312041A - Optical amplifying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To assure planarity in gain characteristics related to a wavelength, even under temperature change. SOLUTION: An optical signal detecting means 8 detects, for example, the weakest level of optical signal among optical signal for each channel amplified by an optical fiber 110, an automatic level control circuit 10 so generates a control voltage that the level of optical signal detected by the optical signal detecting means 8 which comes close to a prescribed value, which is supplied to a light-emitting means 14, and the light-emitting means 14 supplies the exciting light of a level corresponding to the control voltage to the optical fiber 110 through an optical coupler 12. Here for an erbium-doped optical fiber 110, a gain drops as the higher the temperature rises, the lower the gain drops, and when larger the amount of drop is, the longer the wavelength side. Although the gain rises, when the exciting light becomes intensified, a gain rises more on the longer wavelength side. Under such a control, the change in gain characteristics under temperature change and that under change of level of exciting light cancel each other, thus flattening the gain characteristics of the optical fiber 110.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifying device for amplifying an optical wavelength division multiplexed signal using an optical fiber.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing an example of a conventional optical amplifier. The conventional optical amplifying device 102 shown in FIG.
Includes first and second optical amplification units 104 and 106,
The optical signal amplified by the first optical amplifier 104 is applied to the optical attenuator 1
08, the signal is supplied to the second optical amplifier 106 and further amplified in the second optical amplifier 106, for example, forming an optical regenerative repeater of a wavelength division multiplexing transmission system.

[0003] First and second optical amplifiers 104 and 106
Has basically the same configuration, and the optical fiber 110
And peripheral elements for gain adjustment. The optical fiber 110 is an erbium-doped optical fiber and amplifies an optical signal with a gain according to the intensity of the pump light. In the first optical amplifier 104, an optical coupler 112 is inserted at the input of the optical fiber 110, while in the second optical amplifier 106, the optical coupler 112 is inserted at the output of the optical fiber 110. . And a light emitting means 1 using a laser light emitting diode.
Reference numeral 14 (LD) generates excitation light, and this excitation light is supplied to each optical coupler 112. These light emitting means 1
The optical coupler 14 and the optical coupler 112 constitute an amplification control means for controlling the amplification of the optical signal in each optical fiber 110.

[0004] In the first optical amplifying section 104, the optical coupler 11
Reference numeral 2 denotes an optical signal that is supplied from an input port 116 of the optical fiber 110 and propagates through the optical fiber 110, while passing the optical signal in the direction of the optical fiber body 118,
Is reflected and propagated in the direction of the optical fiber body 118. On the other hand, in the second optical amplifying unit 106, the optical coupler 112 allows the optical signal from the optical fiber main body 118 to pass through to the output port 120, and the light emitting unit 1
The excitation light from 14 is reflected and propagated in the direction of optical fiber body 118.

The first and second optical amplifiers 104,
At 106, optical splitters 122 and 124 are inserted into the input portion and the output portion of the optical fiber 110, respectively, and a part of the optical signal propagating through the optical fiber 110 is
The light is branched at 22 and 124, and the light detectors 126 and
128. The photodetectors 126 and 128 detect the optical signals from the splitters 122 and 124, convert them into electric signals, and output the electric signals to the AGC circuit 130 (AGC). The AGC circuit 130 supplies a control voltage to the light emitting unit 114 such that the ratio of the magnitude of the electric signal from each of the photodetectors 126 and 128 becomes a predetermined substantially constant value.

Therefore, the first and second optical amplifiers 1
At 04 and 106, the optical signals input to the optical fiber 110 by the photodetectors 126 and 128 and the optical fiber 1
The AGC circuit 130 supplies a control voltage to the light emitting means 114 so that the ratio of the intensity of these optical signals becomes a preset substantially constant value. To adjust the emission intensity. Thus, the gains of the first and second optical amplifiers 104 and 106 are always set to a substantially constant value.

Specifically, for example, when the gain in the optical fiber 110 is small and the optical signal on the output side of the optical fiber 110 is not sufficiently large with respect to the optical signal on the input side, the AGC circuit 130 By outputting a high control voltage, the light emission intensity of the light emitting means 114 is increased.
As a result, stronger excitation light is supplied to the optical fiber 110, and the gain of the optical fiber 110 increases.

Conversely, when the gain in the optical fiber 110 is large and the optical signal on the output side of the optical fiber 110 is too large relative to the optical signal on the input side, the AGC circuit 130 outputs a low control voltage. Thus, the light emission intensity of the light emitting means 114 is reduced. As a result, weaker pump light is supplied to the optical fiber 110, and the gain of the optical fiber 110 is reduced.

As shown in FIG. 6, the first optical amplifier 10
The optical signal amplified in step 4 is input from the optical splitter 124 to the optical splitter 124 of the second optical amplifier 106 via the optical attenuator 108 (ATT), and is input to the optical fiber 110 of the second optical amplifier 106. Supplied. And the optical attenuator 1
08 is controlled by the automatic level control circuit 132 (ALC).

The optical attenuator 108 and the automatic level control circuit 132 control the intensity of the optical signal at the output of the second optical amplifying unit 106 so as to have a substantially constant value. belongs to. That is,
At the output of the second optical amplifier 106, for example, when the optical signal is strong and the electric signal output from the photodetector 128 is large, the automatic level control circuit 132 sets the optical attenuator 108
Optical attenuator 1 so that the optical signal attenuation at
08 is controlled. On the other hand, when the optical signal is weak at the output of the second optical amplifier 106 and the electric signal output from the photodetector 128 is small, the automatic level control circuit 132 determines that the amount of attenuation of the optical signal in the optical attenuator 108 is small. The optical attenuator 108 is controlled to be smaller. As a result, output port 1
20 always outputs an optical signal of almost constant intensity.

[0011]

By the way, in such an optical amplifying apparatus 102 used in a wavelength division multiplexing transmission system, if the difference in the intensity between the optical signals of each wavelength is large, especially when performing multistage relay, S / N is significantly deteriorated on the wavelength side where the signal strength is weak, and transmission errors are likely to occur. Therefore, it is desirable that the gain characteristic be as flat as possible within the wavelength band used.

However, in the 1580 nm band or the like, the temperature dependence of the optical amplifier is large, and the flatness of the gain characteristic may be significantly deteriorated by a change in temperature, and the improvement is an important issue. The present invention has been made to solve such a problem, and an object of the present invention is to provide an optical amplifying device in which the flatness of gain characteristics with respect to wavelength does not deteriorate even when the temperature changes.

[0013]

According to the present invention, there is provided an optical amplifier comprising an optical fiber for amplifying an optical signal and amplifying an optical wavelength multiplex signal comprising optical signals of a plurality of channels having different wavelengths. An apparatus, comprising: an amplification control unit configured to supply an excitation light having an intensity corresponding to a control voltage to the optical fiber to control an amplification degree of the optical signal in the optical fiber; and Optical signal detection means for detecting the intensity of the strongest or the weakest optical signal among the optical signals of the channels,
An automatic level control circuit for generating the control voltage to supply the intensity of the optical signal detected by the optical signal detection means to a predetermined value and supplying the control voltage to the amplification degree control means. And

In the optical amplifying apparatus according to the present invention, the optical signal detecting means detects the intensity of the strongest or the weakest optical signal of each channel amplified by the optical fiber, and performs an automatic level control circuit. Generates a control voltage so that the intensity of the optical signal detected by the optical signal detection means approaches a predetermined value, and supplies the control voltage to the amplification degree control means. For example, in an optical fiber to which erbium is added, the gain (amplification degree) decreases as the temperature increases, but at this time, the gain decreases more toward the longer wavelength side. When the pumping light is strengthened, the gain increases. At this time, the gain increases greatly on the longer wavelength side.

Therefore, in the optical amplifying device of the present invention, when the temperature around the optical fiber rises, for example, and the temperature of the optical fiber rises, the gain of the optical fiber decreases. The intensity decreases, and the automatic level control circuit generates a control voltage to increase the optical signal and supplies the control voltage to the amplification degree control means. As a result, the amplification degree control means supplies stronger pumping light to the optical signal. As a result, the gain of the optical fiber is increased as a whole, and the gain is further increased particularly on the long wavelength side. Therefore, the change in the gain characteristic due to the temperature change and the change in the gain characteristic due to the change in the intensity of the pump light cancel each other, and the gain characteristic of the optical fiber becomes flat. That is, in the optical amplifying device of the present invention, the flatness of the gain characteristic does not deteriorate even when the temperature changes.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of the optical amplifier according to the present invention. In the figure, the same elements as those in FIG. 6 are denoted by the same reference numerals, and a description thereof will be omitted here. Optical amplification device 2 shown in FIG.
6 differs from the optical amplifying device 102 shown in FIG. 6 in the configuration of the circuit related to the second optical amplifying unit 4 and the optical attenuator 108. The second optical amplifying unit 4 includes an amplification degree control unit 6, an optical signal detection unit 8, and an automatic level control circuit (ALC) 10 on the output side of the optical fiber 110. The amplification degree control means 6 supplies the excitation light having the intensity according to the control voltage to the optical fiber 110 to control the amplification degree of the optical signal in the optical fiber 110. More specifically, the amplification degree control means 6
Is inserted into the output section of the optical fiber 110 and the optical fiber 1
An optical coupler 12 that transmits light propagating through the optical fiber 10 and introduces pump light supplied from the outside into the optical fiber 110 and propagates the optical fiber 110 toward the optical fiber main body 118; And a light emitting means 14 for generating the above light and supplying the light to the optical coupler 12 as the excitation light, and the light emitting means 14 is constituted by a laser light emitting diode in the present embodiment.

The optical signal detecting means 8 detects the intensity of the strongest or weakest optical signal of each channel amplified by the optical fiber 110. The optical signal detection means 8 is specifically configured to include an optical splitter 16, a wavelength separation light detector 18, and a signal detection means (DET) 20. The optical splitter 16 is an optical fiber 1
A part of the optical signal from the main body 10 is separated into a wavelength separation photodetector 18.
The wavelength-separated optical detector 18 detects the intensity of the optical signal for each optical signal having a different wavelength in the branched optical signal, and outputs an electric signal indicating the detection result. The signal detecting means 20 detects an electric signal corresponding to the strongest or the weakest optical signal among the electric signals output from the wavelength separation photodetector 18 and detects the electric signal corresponding to the weakest optical signal.
Supply 0.

In this embodiment, the wavelength separation photodetector 18 includes a filter means (AWG) 22 composed of a plurality of optical filters for extracting optical signals of each wavelength, and each optical filter of the filter means 22. A photodetector (P) comprising a plurality of photodetectors for converting the extracted optical signals into electric signals, respectively.
D) The array 24 is included. The automatic level control circuit 10 generates a control voltage so that the intensity of the optical signal detected by the optical signal detection means 8 approaches a predetermined value, and supplies the control voltage to the amplification degree control means 6.

The optical amplifying device 2 of the present embodiment is
Optical attenuator 108 inserted at the input of optical fiber 110
The optical attenuator 108 and the optical fiber body 11
8, a photodetector 28 for detecting the intensity of the optical signal split by the optical splitter 26 and outputting an electric signal indicating the detection result, and a photodetector 28. And outputs an electric signal to stabilize the intensity of the optical signal supplied to the optical fiber main body based on the output signal of the optical attenuator 1.
08 for controlling the automatic level control circuit 08.

Next, the optical amplifying device 2 thus configured
Will be described. FIGS. 2A to 2F are graphs showing gain characteristics and the like in the second optical amplifier 4. FIG. In the figure, the horizontal axis represents the wavelength, and the vertical axis represents (A).
And (C) and (F) represent relative values of gain,
(D) and (E) show the relative values of the output levels.

FIG. 2A shows the gain characteristic of the second optical amplifying unit 4 at room temperature. By setting the emission intensity of the light emitting means 14 appropriately, the gain becomes almost flat over the entire wavelength band used. Gain characteristics are obtained. Therefore, the output level of the optical signal of each wavelength is almost constant as shown in FIG.

On the other hand, when the temperature of the optical fiber 110 rises together with the temperature around the optical fiber 110,
When the present invention is not implemented, the gain decreases over the entire used wavelength band, and as shown in FIG. 2B, the gain decreases greatly toward the longer wavelength side, and the flatness of the gain characteristic is reduced. Will be lost. Therefore, the output level of the optical signal of each wavelength also decreases as a whole, and as shown in FIG.

As described above, in the optical fiber 110, as the temperature increases, the gain decreases as the wavelength becomes longer. However, the gain characteristic of the optical fiber 110 also changes depending on the intensity of the pump light. As shown in FIG. 2C, the gain increases over the entire used wavelength band, and the gain increases greatly as the wavelength becomes longer. In the figure, the solid line shows the gain characteristics when the pumping light is intensified, and the dotted line shows the gain characteristics when the temperature of the pumping light is increased with normal intensity.

In the present embodiment, the optical signal detecting means 8 detects the intensity of the strongest or the weakest optical signal among the optical signals of the respective channels having different wavelengths amplified by the optical fiber 110. Then, the automatic level control circuit 10 generates a control voltage so that the intensity of the optical signal detected by the optical signal detection means 8 approaches a predetermined value, and supplies the control voltage to the amplification degree control means 6.

Therefore, in the optical amplifying device 2 of the present embodiment, for example, when the temperature around the optical fiber 110 rises and the temperature of the optical fiber 110 rises, the optical fiber 1
Since the gain of the optical signal 10 decreases, the intensity of the optical signal detected by the optical signal detecting means 8 decreases, and the automatic level control circuit 10 generates a control voltage to increase the optical signal and supplies the control voltage to the amplification degree controlling means 6. I do. As a result, the amplification degree control means 6 supplies stronger excitation light to the optical signal, and as a result, the optical fiber 11
As the gain of 0 increases as a whole, the gain increases more particularly on the long wavelength side.

Therefore, the change in the gain characteristic due to the temperature change (FIG. 2B) and the change in the gain characteristic due to the change in the intensity of the pump light (FIG. 2C) cancel each other out. As shown in FIG. 2 (F), the gain characteristic of the optical fiber 110 becomes flat. That is, the optical amplifying device 2 of the present embodiment
Then, even if the temperature changes, the flatness of the gain characteristic in the second amplifier 4 does not deteriorate. The change in the gain characteristic of the optical fiber as shown in FIG. 2 is described in “IEICE Technical Report: OQE92” (P.21) and “IEICE Technical Report: OQCS92” (P.58) issued by the Institute of Electronics, Information and Communication Engineers. ).

The optical attenuator 108 and the automatic level control circuit 30 operate to keep the intensity of the optical signal supplied to the optical fiber 110 of the second optical amplifier 4 almost constant. That is, the optical signal split by the optical splitter 26 is the photodetector 2
8, the automatic level control circuit 30
The amount of attenuation of the optical attenuator 108 is controlled based on the magnitude of the electric signal. For example, when the optical signal supplied to the optical fiber 110 is strong, a large electric signal is input to the automatic level control circuit 30, and as a result, the automatic level control circuit 30 sets the attenuation in the optical signal attenuator to a large value. . Conversely, when the optical signal supplied to the optical fiber 110 is weak, a small electric signal is input to the automatic level control circuit 30, and as a result, the automatic level control circuit 30 sets the attenuation in the optical signal attenuator to a small value. .

In the above-described embodiment, the gain characteristic is flattened only in the second optical amplifying unit 4. However, the first optical amplifying unit 104 has the same configuration as the second optical amplifying unit 4. As a configuration, it is of course possible to prevent the flattening of the gain characteristic from being deteriorated due to the temperature change and to further improve the performance of the entire optical amplifying device 2. Further, in the second optical amplifier 4, when the light emitting means 14 is controlled by the automatic level control circuit 10, the compensation is more strongly performed so as to cancel the deterioration of the flattening of the gain characteristic in the first optical amplifier 104. It is also effective to do so. In the present embodiment, the optical amplifying device 2 includes the first and second optical amplifying units 104 and 4, but only the second optical amplifying unit 4 depends on the required performance level. It is also possible to adopt a configuration using. Further, in the present embodiment, the optical coupler 12 is connected to the second
Although the optical fiber 110 is inserted into the output section of the optical fiber 110 of the amplifying section, it may be provided at the input section of the optical fiber 110 to supply the light from the light emitting means 14 to the optical fiber body 118.

Next, a second embodiment of the present invention will be described. FIG. 3 is a block diagram showing an optical amplifying device according to the second embodiment. In the figure, the same elements as those in FIG. 1 are denoted by the same reference numerals, and a description thereof will be omitted here. The optical amplifying device 32 shown in FIG. 3 differs from the optical amplifying device 2 in the configuration of a wavelength separation photodetector. That is, in the optical amplifying device 32 according to the second embodiment, the wavelength separation photodetector 34 scans the wavelength band of the optical signal and sequentially passes the optical signals of each wavelength to extract the optical filter (AOTF). ) 36 and a photodetector 38 that converts the optical signal extracted by the optical filter 36 into an electric signal.

The wavelength separation photodetector 34 functions in the same manner as the wavelength separation photodetector 18 of the first embodiment. For each of the optical signals having different wavelengths in the optical signals split by the optical splitter 16. It detects the intensity of the optical signal and outputs an electric signal indicating the detection result. From this electric signal, the signal detecting means 20 detects an electric signal corresponding to the strongest or the weakest optical signal and detects the electric signal corresponding to the weakest light signal.
To supply. Accordingly, the same effects as those of the optical amplifier 2 according to the first embodiment can be obtained in the optical amplifier 32 according to the second embodiment.

Next, a third embodiment will be described. FIG. 4 is a block diagram showing an optical amplifying device according to the third embodiment. In the figure, the same elements as those in FIG. 1 are denoted by the same reference numerals, and a description thereof will be omitted here. The optical amplifier 40 shown in FIG.
The difference from the first embodiment is that a channel number detection circuit 42 and a reference voltage generation circuit 44 are added in connection with the automatic level control circuit 30. The channel number detection circuit 42
When a plurality of electric signals output from the photodetector array 24 are input and the magnitude of each electric signal is larger than a reference value, it is determined that the corresponding channel is used,
The total number of used channels is counted, and a signal representing the counting result is output to the reference voltage generation circuit 44.

The reference voltage generation circuit 44 generates a reference voltage which increases as the number of channels counted by the channel number detection circuit 42 increases, and supplies the reference voltage to the automatic level control circuit 30. Then, in the automatic level control circuit 30, the optical attenuator 1 based on the electric signal from the photodetector 28 as described above.
When setting the attenuation of 08, the attenuation of the optical attenuator 108 is set to a smaller value as the reference voltage supplied from the reference voltage generating circuit 44 is higher (that is, as the number of channels is larger).

Therefore, in the present embodiment, for example, even when the number of channels increases and the electric signal from the photodetector 28 increases, the amount of attenuation in the optical attenuator 108 is set to be larger than necessary. Without being set to an appropriate level, the optical signal for each channel is
Regardless of the number of channels, the intensity is supplied to the subsequent optical fiber 110 with almost constant strength.

The amplification degree control means 6, optical signal detection means 8, automatic level control circuit 10, etc., which are directly related to the present invention, operate independently of the channel number detection circuit 42, the reference voltage generation circuit 44, etc. In the optical amplifier 40 according to the third embodiment, the same effects as those of the optical amplifier 2 can be obtained.

Next, a fourth embodiment will be described. FIG. 5 is a block diagram showing an optical amplifying device according to the fourth embodiment. In the figure, the same elements as those in FIG. 3 are denoted by the same reference numerals, and a description thereof will be omitted here. The optical amplifier 46 shown in FIG.
2 is different from the automatic level control circuit 30,
The point is that a channel number detection circuit 42 and a reference voltage generation circuit 44 are added. Channel number detection circuit 42
Receives the electric signal output from the photodetector 38, and when the magnitude of the electric signal for each channel (that is, for each wavelength) is larger than the reference value, it is determined that the corresponding channel is used. Thus, the total number of used channels is counted, and a signal representing the counting result is output to the reference voltage generating circuit 44. Therefore, this optical amplifier 4
6, the same effects as those of the optical amplifier 40 can be obtained.

[0036]

As described above, in the optical amplifying apparatus of the present invention, the optical signal detecting means determines the intensity of the strongest or the weakest optical signal of each channel amplified by the optical fiber. Upon detection, the automatic level control circuit generates a control voltage to bring the intensity of the optical signal detected by the optical signal detection means closer to a predetermined value, and supplies the control voltage to the amplification degree control means. For example, in an optical fiber to which erbium is added, the gain (amplification degree) decreases as the temperature increases, but at this time, the gain decreases more toward the longer wavelength side. When the pumping light is strengthened, the gain increases. At this time, the gain increases greatly on the longer wavelength side.

Therefore, in the optical amplifying device of the present invention, when the temperature around the optical fiber rises, for example, and the temperature of the optical fiber rises, the gain of the optical fiber decreases. The intensity decreases, and the automatic level control circuit generates a control voltage to increase the optical signal and supplies the control voltage to the amplification degree control means. As a result, the amplification degree control means supplies stronger pumping light to the optical signal. As a result, the gain of the optical fiber is increased as a whole, and the gain is further increased particularly on the long wavelength side. Therefore, the change in the gain characteristic due to the temperature change and the change in the gain characteristic due to the change in the intensity of the pump light cancel each other, and the gain characteristic of the optical fiber becomes flat. That is, in the optical amplifying device of the present invention, the flatness of the gain characteristic does not deteriorate even when the temperature changes.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of an optical amplifier according to the present invention.

FIGS. 2A to 2F are graphs showing gain characteristics and the like in a second optical amplifier.

FIG. 3 is a block diagram illustrating an optical amplifying device according to a second embodiment.

FIG. 4 is a block diagram illustrating an optical amplifying device according to a third embodiment.

FIG. 5 is a block diagram showing an optical amplifying device according to a fourth embodiment.

FIG. 6 is a block diagram illustrating an example of a conventional optical amplifier.

[Explanation of symbols]

2 ... amplifying device, 4 ... second optical amplifying unit, 6 ... amplifying degree control means, 8 ... optical signal detecting means, 10 ... automatic level control circuit, 12 ... optical coupler, 14 ... ... Light emitting means, 1
6 optical splitter, 18 wavelength separation photodetector, 20
Signal detecting means, 22 Filter means, 24 Array, 26 Optical splitter, 28 Photodetector, 30 Automatic level control circuit, 32 Optical amplifying device, 34 Wavelength separated light Detector 36, optical filter 38, photodetector 40, optical amplifying device 42, channel number detecting circuit 44, reference voltage generating circuit 46, optical amplifying device 46
102 ... optical amplifying device, 104 ... first optical amplifying unit, 1
06 second optical amplifier 108 optical attenuator 110
.., Optical fiber, 112, optical coupler, 114, light emitting means, 116, input port, 118, optical fiber body, 120, output port, 122, optical splitter, 12
4 ... optical splitter, 126 ... photodetector, 128 ... photodetector, 130 ... AGC circuit, 132 ... automatic level control circuit.

Claims (8)

[Claims] 1. An optical amplifying device comprising an optical fiber for amplifying an optical signal and amplifying an optical wavelength-division multiplex signal comprising optical signals of a plurality of channels having different wavelengths, wherein the pumping light has an intensity corresponding to a control voltage. Amplification degree control means for controlling the amplification degree of the optical signal in the optical fiber by supplying the optical fiber to the optical fiber, The strongest or the weakest of the optical signals of each channel amplified by the optical fiber Optical signal detecting means for detecting the intensity of the optical signal; and the amplification control means for generating the control voltage so as to bring the intensity of the optical signal detected by the optical signal detecting means closer to a predetermined value. And an automatic level control circuit for supplying to the optical amplifier. 2. The amplification degree control means according to claim 1, wherein said amplification degree control means is inserted into an input portion or an output portion of said optical fiber, passes light propagating through said optical fiber, and introduces pump light supplied from outside into said optical fiber. An optical coupler that propagates the light toward the optical fiber body, and a light emitting unit that generates light having an intensity corresponding to the magnitude of the control voltage and supplies the light as the excitation light to the optical coupler. The optical amplifying device according to claim 1, wherein 3. The optical signal detecting means detects an intensity of the optical signal for each of the optical signals having different wavelengths through an optical splitter inserted into an output part of the optical fiber and the optical splitter. A wavelength separation photodetector that outputs an electric signal indicating a detection result, and among the electric signals output by the wavelength separation photodetector, the strongest or the weakest electric signal indicating the intensity of the light signal is detected. 2. The optical amplifying device according to claim 1, further comprising signal detection means for supplying the signal to said automatic level control circuit. 4. A plurality of optical filters for respectively extracting the optical signals of each wavelength, and a plurality of optical detectors for converting the optical signals extracted by each optical filter into electric signals, respectively. 4. The optical amplifying device according to claim 3, comprising: 5. A second optical filter for scanning the wavelength division multiplexed signal within the wavelength band of the optical wavelength division multiplexed signal and sequentially passing and extracting the optical signals of each wavelength, and the second optical filter. 4. The optical amplifying device according to claim 3, further comprising a second photodetector that converts the extracted optical signal into an electric signal by an optical filter. 6. The optical amplifier according to claim 1, wherein the optical fiber is an erbium-doped optical fiber. 7. The optical amplifying device according to claim 2, wherein said light emitting means comprises a laser light emitting diode. 8. An optical attenuator inserted into an input section of the optical fiber, an optical splitter inserted between the optical attenuator and the optical fiber main body, and an optical signal split by the optical splitter. A third photodetector for detecting the intensity of the optical signal and outputting an electric signal representing the detection result, and the intensity of the optical signal supplied to the optical fiber body based on the output signal of the third photodetector. 2. The optical amplifying device according to claim 1, further comprising a second automatic level control circuit that outputs an electric signal to control the optical attenuator so as to stabilize the optical attenuator.