JPH08102718A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE: To provide an optical fiber amplifier which can always operate in a stable way and also can surely control an optical signal at a prescribed level. CONSTITUTION: An excitation LD drive circuit 8 fixes the light emission intensity of an excitation LD 3 at its highest level, and an excitation LD drive circuit 9 controls the light emission intensity of an excitation LD 4 so that the output signal light of a 2nd-stage amplifier 2 is set at its control target level when the signal level of a 1st-stage amplifier 1 cannot be amplified up to its control target level even though the light emission intensity of the LD 3 is set at its prescribed highest level. Meanwhile the circuit 9 fixes the light emission intensity of the LD 4 at the lowest level higher than the level where the operation of the LD 4 becomes unstable, and the circuit 8 controls the light emission intensity of the LD 3 so that the output signal light of the amplifier 2 is set at its target level as long as the signal light of the amplifier 1 can intensity of the LD 3 is set at a level lower than its highest level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier for amplifying signal light by using an amplification optical fiber.

[0002]

2. Description of the Related Art In recent years, optical fiber amplifiers capable of directly amplifying an optical signal without converting it into an electric signal have been widely used.

In an optical fiber amplifier, an exciting optical fiber is made incident on the amplifying optical fiber such as quartz glass doped with a rare earth element such as Er (erbium), together with the signal light, so that the inside of the amplifying optical fiber is excited by the exciting light. The signal light is amplified by stimulated emission generated in 1.

By the way, such an optical fiber amplifier is used as an optical preamplifier in order to increase the sensitivity of an optical receiver. However, one amplification optical fiber may not be able to obtain a dynamic range required as an optical preamplifier, and therefore two amplification optical fibers are cascaded in two stages.

In the optical preamplifier, since it is necessary to keep the input level to the optical receiver constant, automatic gain control (AGC) is applied to keep the output level constant. The optical fiber amplifier can change the gain by changing the intensity of the pumping light, and when the two amplification optical fibers are cascaded in two stages, each of the two amplification optical fibers is changed. To control the emission intensity of each of the laser diodes (excitation light emission sources) provided corresponding to. However, a decrease in gain in the amplification optical fiber at the first stage causes an increase in noise figure.

Therefore, in the conventional optical fiber amplifier, if the total gain above a certain level is required, the pumping light to be given to the first-stage amplification optical fiber is fixed to the maximum,
The intensity of the pumping light given to the second-stage amplification optical fiber is changed. If the optimum total gain cannot be obtained simply by changing the intensity of the pumping light given to the second-stage amplification optical fiber, the pumping light given to the second-stage amplification optical fiber is stopped to The intensity of the excitation light given to the optical fiber is changed. That is, when the level of the input optical signal is small (when the total gain is large), the gain of the first stage is fixed to the maximum and the gain of the second stage is changed to control the total gain. When the level of the input optical signal is high (the total gain is small), the gain of the second stage is fixed to "0" and the gain of the first stage is changed to control the total gain.

However, as shown in FIG. 9, the laser diode has a threshold value I having a drive current (LD current) to be supplied.
Below th, the operation becomes unstable and the IL characteristic (current-optical output characteristic) becomes non-linear, but in the above-mentioned optical fiber amplifier, the gain of the second-stage amplification optical fiber is up to "0". Since it needs to be lowered, a region where the IL characteristic becomes non-linear is used. For this reason, there is a risk that the control does not work sufficiently and unstable operation such as oscillation occurs.

[0008]

As described above, in the conventional optical fiber amplifier, when the two-stage cascade connection configuration is adopted, the gain in the second-stage amplification optical fiber is set to "0". The drive current may be reduced to "0" in order to stop the operation of the light source that supplies the pumping light to the second-stage amplification optical fiber, and the drive current is equal to or less than the threshold value, that is, the current-light of the light source. The region where the output characteristic is non-linear is used, and there is a problem that unstable operation such as oscillation occurs due to insufficient control.

The present invention has been made in consideration of such circumstances, and the object thereof is to always stably operate and reliably control an optical signal to a predetermined level. It is to provide an optical fiber amplifier that can be used.

[0010]

To achieve the above object, the first invention is a first-stage amplification optical fiber and a second-stage amplification optical fiber which are cascade-connected in two stages, and the first-stage amplification optical fiber. A first light emitting means such as a pump laser diode for emitting a first pump light for pumping the amplification optical fiber;
For example, a second light emitting means such as a pumping laser diode that emits second pumping light for pumping the second-stage amplification optical fiber and the intensity of output signal light of the second-stage amplification optical fiber are detected, for example. A detection means such as a photodiode, and the emission intensity of the first light emission means is increased or decreased below a predetermined second level so that the intensity detected by this detection means is set to a predetermined first level (control target level), Further, when the second pumping light is at a predetermined third level set to a level above which the operation of the second light emitting means becomes unstable, the first pumping light at the second level or lower is used for the second amplification. When the intensity of the output signal light of the optical fiber falls below the first level, the emission intensity of the first light emitting means is set to the second level.
A first control unit such as an excitation LD drive circuit for controlling the level to a constant level, and a light emission intensity of the second light emission unit at the third level or higher so that the intensity detected by the detection unit is set to the first level. If the emission intensity of the first light emitting means is increased or decreased and the second level is constant and the second pump light is equal to or higher than the third level, the intensity of the output signal light of the second amplification optical fiber is the first level. If it exceeds the above, the second light emitting means is provided with a second control means such as an excitation LD drive circuit for controlling the light emission intensity of the second light emitting means to the third level constant.

A second aspect of the present invention is that the two stages are connected in cascade.
First-stage amplification optical fiber, second-stage amplification optical fiber, and first for exciting the first-stage amplification optical fiber
First light emitting means for emitting pumping light, second light emitting means for emitting second pumping light for pumping the second-stage amplification optical fiber, and input signal light to the first-stage amplification optical fiber When the intensity of is below a predetermined fourth level, the first
The light emission intensity of the second light emitting means is set so that the intensity of the output signal light of the second-stage amplification optical fiber is set to the predetermined first level after the light emission intensity of the light emitting means is kept constant at the predetermined second level. A first control unit that increases and decreases at a predetermined third level or higher that is set to a level at which the operation of the second light emitting unit becomes unstable and a strength of input signal light to the first-stage amplification optical fiber is predetermined. Is higher than the fifth level, the light emission intensity of the second light emitting means is kept constant at the third level, and
And a second control unit for increasing or decreasing the light emission intensity of the first light emitting unit at the second level or less so that the intensity of the output signal light of the second-stage amplification optical fiber is at the first level.

[0012]

By taking these measures, since the input signal light is at a low level, the intensity of the signal light in the first amplification optical fiber is reduced to the first level in the first pumping light having the second level or lower. When it cannot be amplified, the first
The light emission intensity of the light emitting means is kept constant at the second level.
The gain of the amplification optical fiber is fixed to the maximum, and the emission intensity of the second light emitting means is increased or decreased at a predetermined third level or higher set to a level at which the operation of the second light emitting means becomes unstable. Then, the gain in the second amplification optical fiber is changed, and the intensity of the output signal light of the second-stage amplification optical fiber detected by the detection means is controlled to a predetermined first level. Also, since the input signal light is at a high level,
When the emission intensity of the light emitting means is constant at the second level and the intensity of the output signal light from the second amplification optical fiber exceeds the first level when the second pumping light is at the third level or higher, The light emission intensity of the second light emitting means is fixed to the third level, and the gain in the second amplification optical fiber is fixed to the minimum level within a range in which the operation of the second amplification optical fiber is not unstable. The emission intensity of the first light emitting means is increased or decreased below the second level to change the gain in the first amplification optical fiber, and the output signal light of the second amplification optical fiber detected by the detection means is detected. The intensity is controlled to a predetermined first level.

[0013]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the structure of an optical fiber amplifier according to this embodiment. In the figure, 1 is a first-stage amplifier, 2 is a second-stage amplifier, and the first-stage amplifier 1, the second-stage amplifier 2 are cascade-connected in two stages in this order.

Reference numerals 3 and 4 denote pump laser diodes (hereinafter referred to as pump LDs). The pump LD 3 emits pump light for the first-stage amplifier 1 and the pump LD 4 emits pump light for the second-stage amplifier 2. To do.

Reference numeral 5 denotes an optical coupler, which branches monitor signal light from the output signal light of the second stage amplifier 2 and supplies it to a photodiode (PD) 6. The photodiode 6 converts the monitoring signal light supplied from the optical coupler 5 into an electric signal and supplies the electric signal to the error amplifier 7. The error amplifier 7 compares the level of the electric signal given from the photodiode 6 with a preset output set value, and outputs a signal of a level corresponding to the difference.

Reference numerals 8 and 9 denote pump LD drive circuits, respectively. The pump LD drive circuit 8 controls the emission intensity of the pump LD 3, that is, the gain of the first-stage amplifier 1, and the pump LD drive circuit 9 controls the emission intensity of the pump LD 4, that is, 2 The gain of the stage amplifier 2 is controlled respectively.

FIG. 2 shows a first stage amplifier 1 and a second stage amplifier 2.
It is a figure which shows the detailed structure of. As shown in this figure, the first-stage amplifier 1 and the second-stage amplifier 2 include an isolator 21,
It includes a WDM coupler 22, an amplification optical fiber 23, an isolator 24, and an optical bandpass filter 25.

The amplification optical fiber 23 is a quartz optical fiber doped with a rare earth element such as Er (erbium). Signal light (wavelength: 1.55 μm) taken in via the isolator 21 is supplied to the amplification optical fiber 23.
m band) and the pumping light (wavelength: 1.48 μm band) emitted from the pumping LDs 3, 4 are combined and input into the WDM coupler 22. The isolator 4 suppresses the reflection of light.

The light output from the amplification optical fiber 23 is input to the optical bandpass filter 25 via the isolator 24. The optical bandpass filter 25 transmits only the signal light component (wavelength: 1.55 μm band) and removes the excitation light component (wavelength: 1.48 μm band).

FIG. 3 is a diagram showing a detailed structure of the excitation LD drive circuit 8. As shown in this figure, the excitation LD drive circuit 8
Is composed of a buffer amplifier 31, a rectifying diode 32, a buffer amplifier 33, an LD current control amplifier 34, a power transistor 35, a current detection resistor 36, and an automatic power control circuit (APC circuit) 37.

In the excitation LD drive circuit 8, the output signal of the error amplifier 7 is supplied to the buffer amplifier 31 and the rectifying diode 3.
2 and the buffer amplifier 33, and input to the non-inverting input terminal of the LD current control amplifier 34 which is a differential amplifier. The inverting input terminal of the LD current control amplifier 34 is connected to the connection point between the power transistor 35 and the current detection resistor 36, which are connected in series to the pump LD 3, respectively, and the connection between the power transistor 34 and the current detection resistor 36 is performed. The voltage generated at the point (the voltage of the level corresponding to the LD current flowing through the excitation LD 3) is input. And LD current control amplifier 3
Reference numeral 4 outputs an LD current control signal having a voltage level corresponding to the difference between the voltage level of the signal given from the buffer amplifier 33 and the voltage level corresponding to the LD current.

The power transistor 35 increases or decreases the LD current flowing through the exciting LD 3 according to the voltage level of the LD current control signal supplied from the LD current control amplifier 34.

The APC circuit 37 further includes a photodiode 37a and a current-voltage conversion circuit (IV conversion circuit) 37.
b and an APC error detection amplifier 37c.

The photodiode 37a receives the excitation light emitted from the excitation LD 3 and generates an electric signal having a current level corresponding to its intensity.

The IV conversion circuit 37b converts the electric signal generated by the photodiode 37a into a voltage level signal corresponding to the current level thereof.

The APC error detection amplifier 37c is composed of a differential amplifier, and the signal output from the IV conversion circuit 37b is input to the inverting input terminal and the predetermined reference voltage Vref is input to the non-inverting input terminal. Then, the APC error detection amplifier 37c outputs a signal of a voltage level corresponding to the difference between both inputs. The signal output from the APC error detection amplifier 37c is added to the signal supplied from the buffer amplifier 31 to the LD current control amplifier 34 via the rectifying diode 32 and input to the LD current control amplifier 34.

FIG. 4 is a diagram showing a detailed structure of the excitation LD drive circuit 9. As shown in this figure, the excitation LD drive circuit 9
Is composed of a buffer amplifier 41, an LD current control amplifier 42, a level shift diode 43, a power transistor 44, and a current detection resistor 45.

In the excitation LD drive circuit 9, the output signal of the error amplifier 7 is input to the non-inverting input terminal of the LD current control amplifier 42 which is a differential amplifier via the buffer amplifier 41. The inverting input terminal of the LD current control amplifier 42 is connected to the connection point between the power transistor 44 and the current detection resistor 45, which are connected in series to the excitation LD 4, and the connection between the power transistor 44 and the current detection resistor 45. The voltage generated at the point (the voltage of the level corresponding to the LD current flowing through the excitation LD 4) is input. Then, the LD current control amplifier 42 outputs an LD current control signal having a voltage level corresponding to the difference between the voltage level of the signal given from the buffer amplifier 41 and the voltage level corresponding to the LD current.

The level shift diode 43 is inserted between the output terminal of the LD current control amplifier 42 and the base of the power transistor 44. The level shift diode 43 has the same characteristic as that of the power transistor 44 between B and E, and sets the minimum value of the LD current flowing through the excitation LD 4 as a predetermined threshold value.

The power transistor 44 is pumped to an L level according to the voltage level of the LD current control signal supplied from the LD current control amplifier 42 via the level shift diode 43.
The LD current flowing through D4 is increased or decreased.

Next, the operation of the optical fiber amplifier configured as described above will be described.

First, the signal light input from the input terminal is amplified by the first stage amplifier 1 as follows.

That is, first, the pumping light emitted from the pumping LD 3 passes through the WDM coupler 22 and the amplification optical fiber 23.
When the incident light enters, the rare earth element ions doped in the amplification optical fiber 23 are excited by the excitation light, and stimulated emission occurs. When the signal light enters the amplification optical fiber 23 in this state, the signal light is amplified by the stimulated emission. Then, the amplified signal light is input to the optical bandpass filter 25 via the isolator 24. Then, in the optical bandpass filter 25, the pumping light component is removed, only the signal light component is extracted, and this signal light is output.

The signal light thus amplified by the first-stage amplifier 1 is next input to the second-stage amplifier 2 and further amplified. The amplification of the signal light in the second stage amplifier 2 is
It is performed in the same manner as in the case of the first stage amplifier 1. The signal light output from the second stage amplifier 2 is output from the output terminal.

At this time, a part of the signal light output from the second-stage amplifier 2 is branched by the optical coupler 5 as a monitor signal light and given to the photodiode 6, which is converted into an electric signal by the photodiode 6. It That is, the photodiode 6 generates an electric signal having a signal level corresponding to the level of the signal light output from the second stage amplifier 2. Then, in the error amplifier 7, the difference between the level of the electric signal given from the photodiode 6 and the preset output setting value is obtained, and the signal of the level corresponding to the difference is obtained. Since the output set value is set to a value corresponding to the control target level of the signal light output from the second stage amplifier 2, the signal output from the error amplifier 7 is 2
The signal has a level corresponding to the error between the actual level of the signal light output from the stage amplifier 2 and the control target level.

Therefore, the pumping LD drive circuits 8 and 9 use the pumping LD 3 to match the actual level of the signal light output from the second stage amplifier 2 with the control target level based on the signal output from the error amplifier 7. , 4, that is, the gains of the first-stage amplifier 1 and the second-stage amplifier 2 are controlled as follows.

That is, first, when the level of the input signal light is low and the pump light of the second-stage amplifier 2 is minimum, the first-stage amplifier 1
When it is not possible to amplify the signal light level to the control target level by only using the ALD circuit 37 in the pump LD drive circuit 8, the emission intensity of the pump LD 3 is changed to the reference voltage Vre.
The intensity is controlled to correspond to f. The reference voltage Vref is set to a value corresponding to the intensity of the pumping light that can obtain the maximum gain within the range in which the first-stage amplifier 1 can stably operate, and the gain of the first-stage amplifier 1 is controlled to a maximum value. To be done.

At this time, since the signal light output from the first-stage amplifier 1 is lower than the control target level, the gain that can compensate the difference between the level of the signal light output from the first-stage amplifier 1 and the control target level is the second stage. Excitation LD as obtained with amplifier 2
The drive circuit 9 controls the emission intensity of the excitation LD 4.
This is achieved by controlling the emission intensity of the pump LD 4 so that the error amplifier 7 has a level indicating an error “0”.

If the level of the signal light output from the second stage amplifier 2 is controlled to the control target level in this way,
Since the output of the error amplifier 7 does not become "+", the pump LD
In the drive circuit 8, only the output signal of the APC circuit 37 is input to the buffer amplifier 33 due to the action of the rectifying diode 32. Therefore, as described above, the gain of the first-stage amplifier 1 is constantly controlled to the maximum value.

FIG. 5 is a diagram showing the setting values of the gains of the first-stage amplifier 1 and the second-stage amplifier 2 with respect to the level of the input signal light, and the range indicated by A in this figure corresponds to the above operating state.

When the level of the input signal light is large and the level of the signal light has already exceeded the control target level in a state where the gain of the first-stage amplifier 1 is constantly controlled to a maximum value, the pumping LD drive circuit 9 , To reduce the emission intensity of the pumping LD 4 and reduce the gain of the second stage amplifier 2. However, the base potential of the power transistor 44 does not become "0" due to the action of the level shift diode 43, and is fixed to a predetermined value slightly larger than "0". As a result, as shown in FIG. 5, the gain of the second-stage amplifier 2 is also fixed to a predetermined value slightly larger than "0".

As described above, even if the gain of the second-stage amplifier 2 is reduced to the minimum value, the level of the signal light output from the second-stage amplifier 2 is still higher than the control target level. Therefore, the error is detected in the error amplifier 7, and the excitation L
In the D drive circuit 8, the output of the buffer amplifier 31 becomes “+”. Then, in the buffer amplifier 33,
The output of the buffer amplifier 31 is added to the output of the APC circuit 37 and given. That is, the input level of the buffer amplifier 33 rises, and the input level of the non-inverting input terminal of the LD current control amplifier 34 rises as shown in FIG. In response to this, the LD current control amplifier 34 works to reduce the emission intensity of the pumping LD 3 to reduce the gain of the first stage amplifier 1 and controls the level of the signal light output from the second stage amplifier 2 to the control target level. . The range indicated by B in FIG. 5 corresponds to this operating state.

Thus, according to the present embodiment, when a large gain is required, the gain of the first-stage amplifier 1 is set to a predetermined maximum value and the gain of the second-stage amplifier 2 is changed to set the level of the output signal light. To control target level. If a small gain is required, the gain of the second-stage amplifier 2 is set to a predetermined minimum value and then the gain of the first-stage amplifier 1 is changed to control the output signal light level to the control target level. Therefore, the total gain can be changed over a wide range, and a sufficient dynamic range can be obtained.

Moreover, even when the total gain is reduced, the gain of the second-stage amplifier 2 is not "0" but "0".
Since it is fixed to a predetermined value that is slightly larger than this, the LD current is always supplied to the exciting LD 4. Therefore the excitation L
Since the D4 is not used in a region where the operation becomes unstable and always operates stably, the second-stage amplifier 2 can also always operate stably. As a result, it is possible to reliably control the level of the signal light to the control target level over the entire dynamic range.

The present invention is not limited to the above embodiment. For example, instead of grounding the inverting input terminal of the buffer amplifier 31 in the excitation LD drive circuit 8 as in the above embodiment, a predetermined voltage V + is applied to control the first stage amplifier 1 as shown in FIG. It may have an offset. In this way, the amplifier gain control at the time of high input is deviated, but it is possible to prevent the malfunction of the first-stage amplifier 1 against the noise at the normal time.

In the above embodiment, the level of the output signal light is detected and the gains of the first stage amplifier 1 and the second stage amplifier 2 are controlled so that the level coincides with the control target level. It is also possible to detect the level of the input signal light and control the respective gains of the first-stage amplifier 1 and the second-stage amplifier 2 based on the relationship shown in FIG.

In addition, various modifications can be made without departing from the spirit of the present invention.

[0049]

According to the present invention, the second excitation light is the second excitation light.
When the first pumping light having a second level or lower is set to a predetermined third level which is set to a level above which the operation of the light emitting device becomes unstable, the intensity of the signal light in the first amplification optical fiber is set to the first level. When you can not amplify up to
The light emission intensity of the first light emitting means is fixed to the second level and the gain in the first amplification optical fiber is fixed to the maximum, and then the light emission intensity of the second light emitting means is increased or decreased to the third level or higher to increase the second intensity. The gain in the amplification optical fiber is changed to control the intensity of the output signal light of the second-stage amplification optical fiber detected by the detection means to a predetermined first level.

When the emission intensity of the first light emitting means is constant at the second level and the second pumping light is at the third level or higher, the intensity of the output signal light from the second amplifying optical fiber is at the first level. If it exceeds the second,
The light emission intensity of the light emitting means is fixed to the third level, and the gain of the second amplification optical fiber is fixed to a minimum value within a range where the operation of the second amplification optical fiber does not become unstable. Increase or decrease the intensity below the second level to reach the first
The gain in the amplification optical fiber is changed, and the intensity of the output signal light of the second-stage amplification optical fiber detected by the detection means is controlled to the first level.

Thus, the optical fiber amplifier can be always operated stably and the optical signal can be surely controlled to a predetermined level.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical fiber amplifier according to an embodiment of the present invention.

FIG. 2 is a diagram showing a detailed configuration of a first stage amplifier 1 and a second stage amplifier 2 in FIG.

3 is a diagram showing a detailed configuration of an excitation LD drive circuit 8 in FIG.

FIG. 4 is a diagram showing a detailed configuration of an excitation LD drive circuit 9 in FIG.

FIG. 5 is a diagram showing setting values of respective gains of the first-stage amplifier 1 and the second-stage amplifier 2 with respect to the level of input signal light.

FIG. 6 is a diagram showing changes in the input level of the non-inverting input terminal of the LD current control amplifier 34 with respect to the output of the error amplifier 7.

7 is a diagram showing a modified configuration example of the excitation LD drive circuit 8 in FIG.

8 is a diagram showing changes in the input level of the non-inverting input terminal of the LD current control amplifier 34 with respect to the output of the error amplifier 7 when the pumping LD drive circuit 8 has the configuration shown in FIG.

FIG. 9 is a diagram showing operating characteristics of a laser diode.

[Explanation of symbols]

 1 ... First stage amplifier 2 ... Second stage amplifier 3,4 ... Excitation laser diode (excitation LD) 5 ... Optical coupler 6 ... Photodiode 7 ... Error amplifier 8,9 ... Excitation LD drive circuit 23 ... Amplification optical fiber

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Claims (2)

[Claims] 1. A first-stage amplification optical fiber and a second-stage amplification optical fiber, which are cascade-connected in two stages, and a first pumping light for emitting a first pumping light for exciting the first-stage amplification optical fiber. A light emitting means; a second light emitting means for emitting a second pumping light for exciting the second-stage amplification optical fiber; and a detection means for detecting the intensity of the output signal light of the second-stage amplification optical fiber. , The emission intensity of the first light emitting means is set to a predetermined second level so that the intensity detected by the detecting means is set to a predetermined first level.
When the second excitation light is increased or decreased below a level and the second excitation light is a predetermined third level set above a level at which the operation of the second light emitting means becomes unstable, the first level below the second level
When the intensity of the output signal light from the second amplification optical fiber with the pumping light falls below the first level, the first
First control means for controlling the light emission intensity of the light emitting means to the second level constant, and the light emission intensity of the second light emitting means for the third level so that the intensity detected by the detection means is the first level. When the light emission intensity of the first light emitting means is constant and the second level is constant and the second pumping light is equal to or higher than the third level, the intensity of the output signal light of the second amplification optical fiber is An optical fiber amplifier, comprising: second control means for controlling the light emission intensity of the second light emitting means to be constant at the third level when the level exceeds one level. 2. A first-stage amplification optical fiber and a second-stage amplification optical fiber, which are cascade-connected in two stages, and a first pumping light for emitting the first-stage amplification optical fiber for exciting the first-stage amplification optical fiber. A light emitting means, a second light emitting means for emitting a second pumping light for pumping the second-stage amplification optical fiber; and a fourth intensity-inputting light having a predetermined intensity to the first-stage amplification optical fiber. When it is below the level, the light emission intensity of the first light emitting means is made constant at a predetermined second level,
The intensity of the output signal light of the optical fiber for the stage amplification is set to the first
First control means for increasing or decreasing the light emission intensity of the second light emitting means to a level above a predetermined third level which is set to a level above which the operation of the second light emitting means becomes unstable, and the first stage When the intensity of the input signal light to the amplification optical fiber is higher than a predetermined fifth level, the emission intensity of the second light emitting means is kept constant at the third level, and then the second amplification optical fiber An optical fiber amplifier, comprising: second control means for increasing or decreasing the light emission intensity of the first light emitting means below the second level so that the intensity of the output signal light becomes the first level.