JPH1022556A - Light gain control type of optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To oppress even the light surge by very short instantaneous break by equipping this light amplifier with a gain control light source, which outputs a gain control light whose center wavelength is in the gain band of a rare earth doped optical fiber, and a light composing means which inputs the gain control light into the rare earth doped optical fiber. SOLUTION: An input light signal and an excitation light outputted from an excitation light source 1 are composed into united waveform by a light composing means 2, and it is inputted into a rare earth doped optical fiber 3. The output light of the rare earth doped optical fiber 3 is outputted through a light wave uniting means 4. Moreover, the gain control light outputted from a gain control light source 5 is inputted from the output side of the rare earth doped optical fiber 3 through the light wave uniting means 4. The gain control light outputted from the gain control light source 5 is always inputted into the rare earth doped optical fiber 3. The center wavelength of the gain control light set within the gain band of the rare earth doped optical fiber 3, and even if instantaneous break of about one millisecond occurs in the input light signal, an optical fiber amplifier operates with the gain saturation range, and does not bring about light surge.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical gain control type optical amplifier having a function of suppressing an optical surge generated in an optical transmission system using an optical fiber amplifier.

[0002]

2. Description of the Related Art FIG. 11 shows a configuration example of an optical fiber amplifier. In the figure, 1 is an excitation light source, 12 is an optical multiplexer, 1
3 is an erbium-doped optical fiber, and 20-1 and 20-2 are optical isolators. It is known that when an optical fiber doped with a rare earth element is excited by laser light, an optical signal of a specific wavelength is amplified. Erbium-doped optical fiber 13
In this case, when pumping with laser light having a wavelength of 1.48 μm or 0.98 μm, 1.55 μm, which is the signal light wavelength of the optical transmission system, can be amplified. Such an optical fiber amplifier has high output, low noise, and stable characteristics, is easily connected to an optical fiber, and has small connection loss, and is therefore used as an optical amplifier in an optical transmission system.

Incidentally, there is a phenomenon called optical surge in an optical fiber amplifier. Hereinafter, the generation principle of the optical surge will be described with reference to FIGS. FIG.
Shows the principle of optical surge generation due to the saturation characteristics of the optical fiber amplifier. (a) shows the input / output characteristics of the optical fiber amplifier, which has a linear gain in a region where the optical input level is small, and a saturation gain when the optical input level exceeds a predetermined value. Here, as shown in the timing charts of (b) to (d), when optical power that causes saturation to the optical fiber amplifier in the non-input state is input, the optical fiber amplifier initially operates with a linear gain. As a result, a transient phenomenon called an optical surge appears.

FIG. 13 shows the principle of generating an optical surge by controlling the output of an optical fiber amplifier to be constant. In an optical transmission system, the lengths of optical fibers are different, and the loss values of individual optical fibers are not constant. In the optical fiber amplifier in this case, FIG.
As shown in FIG. 3 (a), the optimum gain is automatically adjusted from the light input / output level, and the constant output control for obtaining a constant output is performed.
This is to detect the optical input / output level by the input level detection circuit 31 and the output level detection circuit 32 and to control the excitation light of the optical fiber amplifier 33 accordingly. Here, as shown in the timing charts of FIGS. 13 (b) to 13 (d), when the input is lost, the gain becomes maximum, and when recovering from the non-input state, the control of the optical fiber amplifier is not in time, and the optical surge is reduced. appear.

In an optical transmission system in which optical fiber amplifiers are connected in multiple stages, when an optical surge occurs according to the principle shown in FIGS. 12 and 13, an optical surge grows in each stage of the optical fiber amplifier, and a very large optical surge occurs. Can occur. FIG. 14 shows the causes of the light surge. In the optical transmission system, (a) instantaneous interruption caused by maintenance personnel attaching and detaching optical connectors, (b) instantaneous interruption caused by switching optical fibers by optical switches, and (c) zero sign continued A situation similar to the instantaneous interruption occurs. Such an instantaneous interruption causes an optical surge according to the above principle. (C)
Can be avoided by coding techniques such as scrambling, but (a) and (b) require some countermeasures.

A conventional optical surge suppression technique in an optical transmission system in which optical fiber amplifiers are connected in multiple stages is described in a paper (Yoneyama et al., 1993 IEICE Spring Conference, B-94).
There is a method described in 1). In this method, FIG.
As shown in the figure, the optical surge grows every time it passes through the optical fiber amplifier. However, the optical surge is slowed down as the rise time of the optical signal in the optical signal transmission unit is delayed, and the optical surge is suppressed when the optical surge is delayed up to about 4 milliseconds. It shows that you can do it.

Further, as another conventional technique, there is a method of suppressing the optical surge by controlling the intensity of the pumping light in response to the intensity change of the input optical signal. FIG. 16 shows a configuration of an optical fiber amplifier having a pump light source control function. In the figure, 1 is an excitation light source, 12 is an optical multiplexer, 13 is an erbium-doped optical fiber, 20-1 and 20-2 are optical isolators, 16 is an optical coupler, 17 is a photodiode, 8 is a threshold value judgment circuit,
0 is an excitation light source control circuit. The input optical signal is input to the photodiode 17 via the optical coupler 16, and the input level is monitored. The threshold value judging circuit 8 monitors the monitor output, and when the intensity of the input optical signal falls below the threshold value as shown in FIG. 17 (a), the pumping light source control circuit 10 turns on the pumping light source as shown in FIG. 17 (b). Decrease or stop the output of 1. Further, when the intensity of the input optical signal is restored, the output of the pump light source 1 is restored. As described above, by reducing the intensity of the pump light with the decrease in the intensity of the input optical signal, the phenomenon that large energy is accumulated is limited. As a result, even if the intensity of the input optical signal suddenly returns to the threshold value or more, it is possible to suppress the occurrence of the optical surge.

[0008]

In an optical fiber amplifier, for example, when a high-speed instantaneous interruption occurs due to switching of an optical switch, the occurrence of an optical surge may not be suppressed. Erbium-doped optical fiber is approximately 1
Even if the output of the pump light source is completely stopped in response to an instantaneous interruption of milliseconds or less, the potential gain does not suddenly become zero, and an optical surge occurs when the input is restored.

The cause will be described with reference to a three-level model shown in FIG. Excitation light source is excited from the ground level N1 to excited level N3, a transition to the upper laser level N2 by spontaneous emission lifetime tau _32, then transition to the ground level N1. When a non-input state occurs due to an instantaneous interruption of light or the like, energy in the laser upper level N2 is accumulated. Here, the energy accumulated in the laser upper level N2 is consumed by the time of the spontaneous emission lifetime τ ₂₁ by reducing or setting the excitation light intensity to 0, but the energy cannot be consumed at a high speed above τ _21. . The spontaneous emission lifetime τ ₂₁ of erbium used in an actual optical fiber amplifier is about 13 milliseconds,
Energy cannot be sufficiently consumed for an instantaneous interruption of about 1 millisecond, causing an optical surge.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical gain control type optical amplifier capable of suppressing an optical surge caused by a short interruption of about 1 millisecond irrespective of the presence or absence of constant output control for an input optical signal. I do.

[0011]

According to a first aspect of the present invention, there is provided an optical gain control type optical amplifier comprising:
A gain control light source for outputting gain control light having a center wavelength within the gain band of the rare earth-doped optical fiber, and optical multiplexing means for inputting the gain control light to the rare earth-doped optical fiber. Thus, since the gain control light is always input into the rare-earth-doped optical fiber, the energy state in the fiber is maintained in a gain-saturated state irrespective of the instantaneous interruption of the input optical signal. That is, even if an instantaneous interruption of about 1 millisecond occurs in the input optical signal,
The energy in the laser upper level N2 is not stored, and the occurrence of the optical surge can be prevented.

An optical gain control type optical amplifier according to claim 2 is
In the configuration of Item 1, the intensity of the input optical signal is monitored, and
Gain control light is emitted from the gain control light source only when the
The force is controlled. That is, the intensity of the input optical signal is
When the value falls below the value, the gain control light is
By entering the fiber, the energy in the fiber
The state can be returned to the gain saturation state. to this
From the spontaneous emission lifetime τ _{twenty one}Store energy faster
Is forcibly emitted, so that the input optical signal
Optical surge can be prevented even if a momentary interruption occurs.
You.

An optical gain control type optical amplifier according to a third aspect of the present invention monitors the intensity of an input optical signal in the configuration of the first aspect, and performs control to reduce or stop the output of the pumping light source when the intensity is below a threshold value. That is, the gain control light is always input into the rare earth-doped optical fiber, and when the intensity of the input optical signal falls below the threshold, the accumulation of energy by the pump light is limited. Thus, even if an instantaneous interruption of about 1 millisecond occurs in the input optical signal, the release of the stored energy by the gain control light can be accelerated, and the occurrence of an optical surge can be prevented.

According to a fourth aspect of the present invention, in the optical gain control type optical amplifier according to the second aspect, the intensity of the input optical signal is monitored, and when the intensity of the input optical signal is equal to or less than the threshold value, the gain control light is output.
Control is performed to reduce or stop the output of the excitation light source. As a result, it is possible to maintain the gain saturation state by the gain control light while limiting the accumulation of energy by the pump light, thereby preventing the occurrence of an optical surge even if an instantaneous interruption of about 1 millisecond occurs in the input optical signal. can do.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a first embodiment of an optical gain control type optical amplifier according to the present invention. In the figure,
The input optical signal and the pump light output from the pump light source 1 are multiplexed by the optical multiplexing means 2 and input to the rare-earth-doped optical fiber 3. The output light of the rare earth-doped optical fiber 3 is output via the optical multiplexing means 4. On the other hand, the gain control light output from the gain control light source 5 is input from the output side of the rare earth doped optical fiber 3 via the optical multiplexing means 4.

In this embodiment, the gain control light output from the gain control light source 5 is always input into the rare earth doped optical fiber 3. The center wavelength of the gain control light is set within the gain band of the rare-earth-doped optical fiber 3, and the optical fiber amplifier operates in the gain saturation region even if an instantaneous interruption of about 1 millisecond occurs in the input optical signal. No surge occurs. In addition,
By inputting the gain control light from the output side of the rare-earth-doped optical fiber 3, the propagation direction is opposite to that of the main signal light.
It does not affect the transmission system connected to the output side of the optical amplifier.

FIG. 2 shows a specific configuration example of the first embodiment. In the figure, the center wavelength of the input optical signal is 1.552 μm
And 11 is a 1.48 μm pump laser corresponding to the pump light source 1, 12 is an optical multiplexer corresponding to the optical multiplexing means 2, 13 is an erbium-doped optical fiber corresponding to the rare-earth-doped optical fiber 3, and 14 is an optical multiplexing means 4 The optical coupler 15 is a 1.55 μm DFB laser corresponding to the gain control light source 5. Optical isolators 20-1 and 20-2 are inserted between the optical multiplexer 12 and the erbium-doped optical fiber 13, and between the optical coupler 14 and the output terminal.

In an experiment for confirming the optical surge suppression effect of this configuration, an optical surge of about 11 dBm was generated for an input optical signal having an intensity of −5.0 dBm when no gain control light was input. As a result, it was confirmed that no light surge occurred. The DFB laser is a Fabry-Perot laser, a mode-locked laser, a DBR laser,
LEDs, SLDs, and other light sources may be used. Also,
A 0.98 μm pump laser may be used as the pump light source 1.

(Second Embodiment ... Claim 1) FIG. 3 shows a second embodiment of the optical gain control type optical amplifier of the present invention.
The feature of the present embodiment lies in that the first embodiment has a backward pumping configuration, whereas the first embodiment has a forward pumping configuration.
That is, the excitation light output from the excitation light source 1 is input to the rare earth-doped optical fiber 3 via the optical multiplexing means 2 provided on the output side of the rare earth-doped optical fiber 3. Other configurations are the same as those of the first embodiment.

Third Embodiment FIG. 4 shows a third embodiment of the optical gain control type optical amplifier according to the present invention.
This embodiment is characterized in that the first embodiment has a forward excitation configuration,
The second embodiment is different from the backward pumping configuration in that a bidirectional pumping configuration is used. That is, the excitation light output from the excitation light source 1-1 is input to the rare earth-doped optical fiber 3 via the optical multiplexing means 2-1 provided on the input side of the rare earth-doped optical fiber 3. The pumping light output from the pumping light source 1-2 is input to the rare-earth-doped optical fiber 3 via the optical multiplexing means 2-2 provided on the output side of the rare-earth-doped optical fiber 3. Other configurations are the same as those of the first embodiment.

(Fourth Embodiment ... Claim 1) FIG. 5 shows a fourth embodiment of the optical gain control type optical amplifier of the present invention.
The feature of this embodiment lies in that the gain control light output from the gain control light source 5 is input from the input side of the rare earth doped optical fiber 3 in the first embodiment. Other configurations are the same as those of the first embodiment. Note that, also in the present embodiment, a backward excitation configuration or a bidirectional excitation configuration can be adopted.

FIG. 6 shows a fifth embodiment of the optical gain control type optical amplifier according to the present invention.
The feature of this embodiment lies in that, in the fourth embodiment, an optical filter 21 for blocking the gain control light and passing an input optical signal (main signal light) is provided on the output side of the rare-earth-doped optical fiber 3. Other configurations are the same as in the fourth embodiment.

(Sixth Embodiment ... Claim 2) FIG. 7 shows a sixth embodiment of the optical gain control type optical amplifier according to the present invention.
In the figure, an input optical signal is input to an optical multiplexing unit 2 via an optical demultiplexing unit 6. The optical multiplexing means 2 includes an optical demultiplexing means 6
And the pumping light output from the pumping light source 1 are multiplexed and input to the rare-earth-doped optical fiber 3. The output light of the rare earth-doped optical fiber 3 is output via the optical multiplexing means 4. Note that, also in the present embodiment, a backward excitation configuration or a bidirectional excitation configuration can be adopted.

The input optical signal partially branched by the optical demultiplexing means 6 is input to a photodetector 7 and converted into an electric signal. This electrical signal indicates a voltage proportional to the intensity of the input optical signal. The threshold determination circuit 8 compares the electric signal voltage output from the photodetector 7 with the threshold voltage, and notifies the gain control light source control circuit 9 of the result. Here, when the intensity of the input optical signal is equal to or higher than the threshold value and the electric signal voltage is higher than the threshold voltage, the rare-earth-doped optical fiber 3 pumped by the pumping light source 1 is in a gain saturated state, and the gain control light source control circuit 9 stops the operation of the gain control light source 5. On the other hand, when the intensity of the input optical signal is equal to or lower than the threshold value and the electric signal voltage is lower than the threshold voltage, the gain control light source control circuit 9 operates the gain control light source 5 to output the gain control light. The gain control light output from the gain control light source 5 is input from the output side of the rare earth doped optical fiber 3 via the optical multiplexing means 4. The gain control light may be input from the input side of the rare earth doped optical fiber 3.

The gain control light is output within a few hundred μsec after the threshold determination circuit 8 detects that the intensity of the input optical signal has dropped below the threshold, and is input into the rare earth-doped optical fiber 3. When the intensity of the input optical signal returns to the threshold or more, the gain control light source control circuit 9 decreases or stops the output of the gain control light source 5. The center wavelength of the gain control light is set within the gain band of the rare-earth-doped optical fiber 3, and the optical fiber amplifier instantaneously returns to the gain saturation region even if an instantaneous interruption occurs in the input optical signal. Thus, no optical surge occurs even if an instantaneous interruption of about 1 millisecond occurs.

FIG. 8 shows a specific configuration example of the sixth embodiment. In the figure, the center wavelength of the input optical signal is 1.552 μm
And 11 is a 1.48 μm pump laser corresponding to the pump light source 1, 12 is an optical multiplexer corresponding to the optical multiplexing means 2, 13 is an erbium-doped optical fiber corresponding to the rare-earth-doped optical fiber 3, and 14 is an optical multiplexing means 4 An optical coupler 15 is a 1.55 μm DFB laser corresponding to the gain control light source 5,
Denotes an optical coupler having a branching ratio of 1: 9 corresponding to the optical demultiplexing means 6;
Reference numeral 7 denotes a photodiode corresponding to the photodetector 7. An optical isolator 20 is provided between the optical multiplexer 12 and the erbium-doped optical fiber 13 and between the optical coupler 14 and the output terminal.
-1 and 20-2 are inserted. The DFB laser is
Fabry-Perot lasers, mode-locked lasers, DBR lasers, LEDs, SLDs, and other light sources may be used.
Further, a 0.98 μm pump laser may be used as the pump light source 1.

FIG. 9 shows an optical gain control type optical amplifier according to a seventh embodiment of the present invention.
In the figure, an input optical signal is input to an optical multiplexing unit 2 via an optical demultiplexing unit 6. The optical multiplexing means 2 includes an optical demultiplexing means 6
And the pumping light output from the pumping light source 1 are multiplexed and input to the rare-earth-doped optical fiber 3. The output light of the rare earth-doped optical fiber 3 is output via the optical multiplexing means 4. On the other hand, the gain control light output from the gain control light source 5 is transmitted through the optical multiplexing means 4 to the rare earth doped optical fiber 3.
Is input from the output side. Note that, also in the present embodiment, a backward excitation configuration or a bidirectional excitation configuration can be adopted. Further, a configuration may be adopted in which the gain control light is input from the input side of the rare-earth-doped optical fiber 3.

The input optical signal partially branched by the optical demultiplexing means 6 is input to a photodetector 7 and converted into an electric signal. This electrical signal indicates a voltage proportional to the intensity of the input optical signal. The threshold determination circuit 8 compares the electric signal voltage output from the photodetector 7 with the threshold voltage, and notifies the pump light source control circuit 10 of the result. Here, when the intensity of the input optical signal is equal to or less than the threshold and the electric signal voltage is lower than the threshold voltage,
The excitation light source control circuit 10 stops or reduces the output of the excitation light source 1. When the intensity of the input optical signal returns above the threshold, the output of the pump light source 1 is restored.

In the present embodiment, the gain control light output from the gain control light source 5 is always input into the rare-earth-doped optical fiber 3. The center wavelength of the gain control light is set within the gain band of the rare-earth-doped optical fiber 3, and the optical fiber amplifier operates in the gain saturation region even if an instantaneous interruption occurs in the input optical signal. Further, when the intensity of the input optical signal falls below the threshold, the pumping light output from the pumping light source 1 is reduced or stopped. Thus, no optical surge occurs even if an instantaneous interruption of about 1 millisecond occurs. Note that the present embodiment can be configured using the elements of each unit in the sixth embodiment shown in FIG.

(Eighth Embodiment: Claim 4) FIG.
An eighth embodiment of the optical gain control type optical amplifier of the present invention is shown. In the figure, an input optical signal is input to an optical multiplexing unit 2 via an optical demultiplexing unit 6. The optical multiplexing means 2 multiplexes the input optical signal that has passed through the optical demultiplexing means 6 and the excitation light output from the excitation light source 1 and inputs the multiplexed optical signal to the rare-earth-doped optical fiber 3.
The output light of the rare earth-doped optical fiber 3 is output via the optical multiplexing means 4. Note that, also in the present embodiment, a backward excitation configuration or a bidirectional excitation configuration can be adopted.

The input optical signal partially branched by the optical demultiplexing means 6 is input to a photodetector 7 and converted into an electric signal. This electrical signal indicates a voltage proportional to the intensity of the input optical signal. The threshold determination circuit 8 compares the electric signal voltage output from the photodetector 7 with the threshold voltage, and notifies the gain control light source control circuit 9 and the excitation light source control circuit 10 of the result.
Here, when the intensity of the input optical signal is equal to or higher than the threshold value and the electric signal voltage is higher than the threshold voltage, the rare-earth-doped optical fiber 3 pumped by the pumping light source 1 is in a gain saturated state, and the gain control light source control circuit 9 stops the operation of the gain control light source 5.

On the other hand, when the intensity of the input optical signal is lower than the threshold and the electric signal voltage is lower than the threshold voltage, the gain control light source control circuit 9 operates the gain control light source 5 to output the gain control light. The gain control light output from the gain control light source 5 is input from the output side of the rare earth doped optical fiber 3 via the optical multiplexing means 4. Note that a configuration may be adopted in which the gain control light is input from the input side of the rare-earth-doped optical fiber 3.

Further, when the intensity of the input optical signal is lower than the threshold value and the electric signal voltage is lower than the threshold voltage, the pumping light source control circuit 10 stops or reduces the output of the pumping light source 1. When the intensity of the input optical signal returns to or above the threshold, the output of the pump light source 1 is restored, and the output of the gain control light source 5 is stopped. In the present embodiment, when the intensity of the input optical signal falls below the threshold, the gain control light is input from the gain control light source 5 into the rare earth doped optical fiber 3 and the pump light output from the pump light source 1 decreases. Or stop. The center wavelength of the gain control light is set within the gain band of the rare-earth-doped optical fiber 3, and the optical fiber amplifier instantaneously returns to the gain saturation region even if an instantaneous interruption occurs in the input optical signal. This allows
Even if an instantaneous interruption of about 1 millisecond occurs, no optical surge occurs. Note that the present embodiment can be configured using the elements of each unit in the sixth embodiment shown in FIG.

(Other Embodiments) In each of the embodiments described above, the wavelength of the gain control light is set as follows. (1) Set within 10 nm from the center wavelength of the input optical signal.
For example, if the center wavelength of the input optical signal is 1.552 μm, a 1.552 μm DFB laser is used as the gain control light source 5.

(2) Set at a distance of 10 nm or more from the center wavelength of the input optical signal. For example, if the center wavelength of the input optical signal is
If it is 1.552 μm, the gain control light source 5 is 1.565 μm
A DFB laser is used. Thus, the input optical signal and the gain control light can be separated from each other, and the instantaneous interruption of the input optical signal can be reliably detected. (3) ± 5 from the gain peak wavelength of rare earth doped optical fiber 3
Set within nm. For example, when an erbium-doped optical fiber is used as the rare-earth-doped optical fiber 3,
Since the gain peak wavelength is 1.53 nm, a 1.53 μm DFB laser is used as the gain control light source 5. This allows efficient gain control of the rare-earth-doped optical fiber 3.

[0036]

As described above, the optical gain control type optical amplifier according to the present invention changes the energy state in the fiber by inputting the gain control light within the gain band of the optical fiber amplifier into the optical fiber amplifier. The gain can be maintained (claim 1) or restored (claim 2). As a result, even if an instantaneous interruption of about 1 millisecond occurs in the input optical signal, no energy is accumulated in the upper laser level (claim 1), or even if it is accumulated, it is forcibly released (claim 2). ), The occurrence of light surge can be prevented.

Further, by simultaneously lowering or stopping the output of the pump light source in accordance with the decrease in the intensity of the input optical signal, it is possible to further limit the accumulation of energy due to the instantaneous interruption and to enhance the effect of suppressing the occurrence of optical surge. Yes (Claim 3,
4).

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical gain control type optical amplifier according to the present invention.

FIG. 2 is a diagram showing a specific configuration example of the first embodiment.

FIG. 3 is a block diagram showing a second embodiment of the optical gain control type optical amplifier according to the present invention.

FIG. 4 is a block diagram showing a third embodiment of the optical gain control type optical amplifier according to the present invention.

FIG. 5 is a block diagram showing a fourth embodiment of the optical gain control type optical amplifier according to the present invention.

FIG. 6 is a block diagram showing a fifth embodiment of the optical gain control type optical amplifier according to the present invention.

FIG. 7 is a block diagram showing a sixth embodiment of the optical gain control type optical amplifier of the present invention.

FIG. 8 is a diagram showing a specific configuration example of a sixth embodiment.

FIG. 9 is a block diagram showing a seventh embodiment of the optical gain control type optical amplifier according to the present invention.

FIG. 10 is a block diagram showing an optical gain control type optical amplifier according to an eighth embodiment of the present invention.

FIG. 11 is a diagram showing a configuration example of an optical fiber amplifier.

FIG. 12 is a diagram illustrating a principle of generation of an optical surge due to a saturation characteristic of an optical fiber amplifier.

FIG. 13 is a diagram showing the principle of generating an optical surge by controlling the output of the optical fiber amplifier to be constant.

FIG. 14 is a diagram showing factors that cause an optical surge.

FIG. 15 is a diagram showing a conventional example of suppressing an optical surge in an optical transmission system in which optical fiber amplifiers are connected in multiple stages.

FIG. 16 is a diagram showing a configuration of an optical fiber amplifier having a pump light source control function.

FIG. 17 is a diagram showing an example of controlling the intensity of excitation light.

FIG. 18 is a diagram showing energy levels of a three-level system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Excitation light source 2, 4 Optical multiplexing means 3 Rare earth-doped optical fiber 5 Gain control light source 6 Optical demultiplexing means 7 Photodetector 8 Threshold judgment circuit 9 Gain control light source control circuit 10 Excitation light source control circuit 11 1.48 μm pump laser 12 Optical multiplexing Device 13 Erbium-doped optical fiber 14, 16 Optical coupler 15 1.55 μm DFB laser 17 Photodiode 20 Optical isolator 21 Optical filter

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/02

Claims (4)

[Claims] A rare earth-doped optical fiber for amplifying an input optical signal; a pump light source for outputting a pump light for causing population inversion in the rare earth-doped optical fiber; a multiplexing of the input optical signal and the pump light; First optical multiplexing means for inputting to the rare earth-doped optical fiber, a gain control light source for outputting a gain control light having a center wavelength within a gain band of the rare earth-doped optical fiber, and An optical gain control type optical amplifier, comprising: a second optical multiplexing means for inputting to the additional optical fiber. 2. The optical gain control type optical amplifier according to claim 1, wherein: an optical demultiplexing unit that splits a part of the input optical signal; and a light that detects the intensity of the optical signal split by the optical splitting unit. A detector, a threshold determination circuit for comparing the optical signal intensity detected by the photodetector with a predetermined threshold, and a gain when the optical signal intensity is equal to or less than a threshold based on a determination result of the threshold determination circuit. An optical gain control type optical amplifier, comprising: a gain control light source control circuit for performing control for outputting control light. 3. The optical gain control type optical amplifier according to claim 1, wherein: an optical demultiplexing unit that splits a part of the input optical signal; and a light that detects the intensity of the optical signal split by the optical splitting unit. A detector, a threshold value determination circuit that compares the optical signal intensity detected by the photodetector with a predetermined threshold value, based on a determination result of the threshold value determination circuit, when the optical signal intensity is equal to or less than the threshold value, and is excited. An optical gain control type optical amplifier, comprising: an excitation light source control circuit for performing control to reduce or stop the output of the light source. 4. The optical gain control type optical amplifier according to claim 2, wherein based on a determination result of a threshold value determination circuit, a control to reduce or stop an output of the pump light source when the optical signal intensity is equal to or less than a threshold value. An optical gain control type optical amplifier comprising a pump light source control circuit for performing the control.