JPH10154840A - Light amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light amplifier which can have a constant gain always independently of input signal light power and have a wavelength dependency of gain, and which can have a simple structure while securing a good amplifier efficiency. SOLUTION: The amplifier has a first optical circulator 9a having forward characteristics from a terminal A to a terminal B and from the terminal B to a terminal C, and a second optical circulator 9b having forward characteristics from a terminal D to a terminal E and from the terminal E to a terminal F. An optical fiber 1 additive of a rare-earth element is connected between the terminal B of the first light circulator 8a and the terminal E of the second light circulator 9b, exciting light is inputted to the optical fiber 1 from an exciting light source 2 through an optical multiplexer/demultiplexer 3, and an optical attenuator 10 or the attenuator 10 and optical band pass filter 6 are connected between the terminal C of the first circulator 9a and the terminal D of the second circulator 9b. Further, signal light is inputted from the terminal S of the first circulator 9a and an amplified light signal is outputted from the terminal F of the second circulator 9b.

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.

[0002]

2. Description of the Related Art A conventional optical amplifier will be described with reference to FIG. The input signal light Pi enters the rare earth-doped optical fiber 1 via the optical isolator 4a and the optical multiplexer 3.
On the other hand, the excitation light emitted from the excitation light source 2 is
And enters the rare-earth-doped optical fiber 1 through. The excitation light has a wavelength corresponding to the excitation level of the added rare earth ion, and amplifies the input signal light Pi by a stimulated emission phenomenon caused by a population inversion of the energy level of the rare earth ion formed by the excitation light. The amplified optical signal is output as the output signal light Po via the optical isolator 4b. Here, the optical isolators 4a and 4b provided at the input / output terminals
Prevents the signal light and the spontaneous emission light from returning to the inside of the amplifier due to the reflection from the outside and falling into an unstable state such as oscillation.

[0003] In addition, although the method of pumping from the front (signal light incident side) of the rare-earth-doped optical fiber has been described here, the same function can be realized by pumping from the rear (signal light output side).

[0004]

A practical example of a rare earth-doped optical fiber amplifier is an optical fiber amplifier (hereinafter abbreviated as EDFA) doped with erbium (Er). This is because the amplification wavelength band coincides with the lowest loss wavelength band (1.55 μm band) of the silica-based optical fiber, and high efficiency, high gain and low noise amplification characteristics can be easily obtained. It is rapidly spreading in transmission systems.

When an optical fiber amplifier such as an EDFA is used, a wavelength multiplexed optical signal can be amplified at a time, so that a large-capacity and flexible transmission system by wavelength multiplexing can be economically constructed. Becomes However, in such a wavelength division multiplexing transmission system, if the number of multiplexed channels dynamically fluctuates,
The gain per wave will also vary.

In order to avoid this, a method has been devised in which the signal light intensity input to the optical fiber amplifier and the amplified output signal light intensity are monitored and the pump light intensity is controlled so that the ratio becomes constant. ing. However, this method not only complicates the configuration, but also changes the wavelength dependence of the gain by changing the pumping light intensity, making it difficult to ensure uniform gain for each channel having a different wavelength.

[0007] As another method for solving the above problem, a method has been devised in which oscillation is generated in an amplifier, thereby stabilizing the gain. FIG. 4 shows an example. The basic configuration of the amplifying section including the rare earth-doped optical fiber 1, the pumping light source 2, the optical multiplexer 3, and the optical isolators 4a and 4b is the same as that of the conventional example shown in FIG. A part of the output light amplified by the amplifying unit is branched by the optical branching unit 5b, passes through the optical bandpass filter 6, and is again input to the amplifying unit via the optical branching unit 5a. By configuring such a feedback loop, the device operates as a ring laser, and its oscillation frequency is determined in the pass wavelength band of the optical bandpass filter 6. In addition, since the gain of the feedback loop is 1, the gain of the optical amplifier is determined to a value that compensates for the loss of the optical splitters 5a and 5b and the optical bandpass filter 6. Therefore, as long as oscillation occurs, the optical amplifier is fixed at a constant gain regardless of the magnitude of the input signal light power. The optical band reject filter 7 connected to the amplifier output terminal prevents the output of the internally oscillated optical signal. Another conventional example using the same principle is shown in FIG. In this example, optical fiber grating filters 8a and 8b are connected to both ends of the rare-earth-doped optical fiber 1. Optical fiber grating filters 8a, 8
b is an optical fiber containing germanium at a high concentration,
A diffraction grating is formed in an optical fiber by irradiating a high-output short-wavelength laser beam by a holographic method. According to such a filter, it is possible to reflect only an optical signal of a desired wavelength and bandwidth by changing the period of the diffraction grating, the number of gratings, and the like, and since the filter is a fiber type, it is configured with conventional optical components. There is an advantage that the loss due to connection or the like is smaller than in the case. Oscillation occurs at the wavelength selected by the optical fiber grating filters 8a and 8b, and the gain of the amplifying unit is fixed as in the example of FIG. The magnitude of this gain is determined by the reflectance of each of the optical fiber grating filters 8a and 8b. The optical band reject filter 7 connected to the amplifier output terminal prevents the output of the internally oscillated optical signal.

According to such a method using internal oscillation, a constant gain can be always maintained irrespective of a change in input signal light power, and fluctuations such as wavelength dependence of the gain are small. Moreover, complicated electrical control is not required.

However, in this method, it is necessary to avoid the internal oscillation wavelength in the signal wavelength band, and excessive loss is caused by connecting an optical branching device or a band reject filter for forming a feedback loop, and an amplifier is generated. There is a problem that the efficiency of the device is deteriorated.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide a constant gain regardless of the input signal light power.
It is another object of the present invention to provide a novel optical amplifier which has wavelength dependency of gain and can be realized with a simple configuration without deteriorating the efficiency of the amplifier.

[0011]

According to a first aspect of the present invention, there is provided a first optical circulator having a forward characteristic from a terminal A to a terminal B and a terminal B to a terminal C; A second optical circulator having forward characteristics from D to terminal E and from terminal E to terminal F, and a rare-earth-doped light between the terminal B of the first optical circulator and the terminal E of the second optical circulator. A fiber is connected, excitation light is input from the excitation light source to the rare earth-doped optical fiber via an optical multiplexer / demultiplexer, and light is transmitted between the terminal C of the first optical circulator and the terminal D of the second optical circulator. Connected to an attenuator or optical attenuator via an optical bandpass filter,
An optical amplifier is characterized in that signal light is input from a terminal A of the first optical circulator and an amplified optical signal is output from a terminal F of the second optical circulator.

According to a second aspect of the present invention, there is provided an optical circulator having forward characteristics from terminal A to terminal B, terminal B to terminal C, terminal C to terminal D, and terminal D to terminal E. A rare earth-doped optical fiber is connected therebetween, excitation light is input from the excitation light source to the rare earth-doped optical fiber via an optical multiplexer / demultiplexer, and an optical reflector or wavelength is connected to a terminal C of the optical circulator via an optical attenuator. A selective optical reflector is connected, a signal light is input from a terminal A of the optical circulator, and an amplified optical signal is output from a terminal E of the optical circulator.

The invention according to claim 3 is that the wavelength-selective light reflector is an optical fiber grating filter.

According to a fourth aspect of the present invention, an equalizing optical filter for compensating for the wavelength dependence of the amplification gain is inserted at one end or in the middle of the rare-earth-doped optical fiber.

According to the configuration of the present invention, an internal oscillation light is prevented from being mixed with the signal light by forming an oscillation loop in a direction opposite to a path for amplifying the signal light by using the optical circulator. There is no need for a branching device or the like for forming a loop.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIG. An optical circulator 9a having forward characteristics from terminal A to terminal B and from B to C, and an optical circulator 9 having forward characteristics from terminal D to terminal E and from terminal E to terminal F.
Use b. The input signal light Pi reaches the terminal B from the terminal A of the optical circulator 9a.
Is multiplexed with the excitation light of
The signal is output from the terminal E of the optical circulator 9b to the terminal F.

On the other hand, the terminal C of the optical circulator 9a and the terminal D of the optical circulator 9b are connected via the optical band-pass filter 6 and the optical attenuator 10. As a result, from the terminal B of the optical circulator 9a to C, through the optical attenuator 10 and the optical bandpass filter 6, from the terminal D of the optical circulator 9b to the terminal E, and again through the optical amplifier, to the terminal B of the optical circulator 9a. , A feedback loop is formed.

Accordingly, oscillation occurs at the wavelength selected by the optical band-pass filter 6, and the gain of the optical amplifier comprising the rare-earth-doped optical fiber 1 is fixed at a constant value. The fixed gain can be adjusted by the optical attenuator 10.

Since the feedback loop is in the opposite direction to the signal light path, the internal oscillation light is not output outside the amplifier. Therefore, any wavelength including the signal light wavelength band can be selected as the oscillation wavelength of the internal oscillation light.

FIG. 6 shows the dependence of the gain on the input signal light power when the present invention is used. When the control by the feedback loop is not performed, the gain greatly varies depending on the input signal light power. However, by performing the control (oscillation) by the feedback loop, the input signal power range stabilized at a constant gain. Is enlarged. In addition, when there is no control, the wavelength dependence of the gain changes according to the input signal light power.However, by performing the control by the feedback loop, the wavelength dependence of the gain is reduced in a range where the gain is stabilized. Be stabilized.

Although the two optical circulators 9a and 9b are used in the embodiment of FIG. 1, the configuration can be further simplified by using a multi-port (five-port) optical circulator 11.
An example is shown in FIG. The operation principle is the same as that of the embodiment of FIG. A five-port optical circulator 11 having forward characteristics from terminal A to terminal B, terminal B to terminal C, terminal C to terminal D, and terminal D to terminal E is used. The input signal light Pi reaches the terminal B from the terminal A of the optical circulator 11, is multiplexed with the pumping light of the pumping light source 2 by the optical multiplexer 3, is incident on the rare-earth-doped optical fiber 1, is amplified, and then is output from the optical circulator 11. Output from terminal D to terminal E.

On the other hand, one end of the optical fiber grating filter 8 is connected to the terminal C of the optical circulator 11 via the optical attenuator 10, and the other end of the optical fiber grating filter 8 is matched by an optical non-reflection terminator 12. .
Accordingly, the light reaches the terminal C from the terminal B of the optical circulator 11, passes through the optical attenuator 10, is reflected by the optical fiber grating filter 8, passes through the optical attenuator 10 again, and reaches the terminal D from the terminal C of the optical circulator 11, and Then, a feedback loop is formed which returns to the terminal B of the optical circulator 11 again. Oscillation occurs due to this feedback loop,
The gain of the optical amplifier is fixed at a fixed value. The fixed gain can be adjusted by the optical attenuator 10. The oscillation wavelength is determined by the reflection wavelength of the optical fiber grating filter 8.

The advantages of the embodiment shown in FIG. 2 over the embodiment shown in FIG. 1 are that only one optical circulator is required and that the optical fiber grating filter 8 is graded, so that the size and cost can be reduced.

As another embodiment, in order to further improve the wavelength dependency of the gain, an equalizing filter for compensating the wavelength dependency of the gain may be inserted before, after or in the middle of the rare-earth-doped optical fiber 1. As described above, according to the present invention, the wavelength dependence of the gain is stabilized regardless of the input signal light power in the range where the gain is stabilized. An optical fiber amplifier having a flat gain wavelength characteristic can be realized. This is suitable for a wavelength division multiplex communication system.

[0025]

According to the present invention, the following excellent effects can be obtained.

(1) By providing a feedback loop inside the optical amplifier and oscillating, the gain of the amplifying section is stabilized, and an optical fiber amplifier having a constant gain regardless of the fluctuation of the input signal light power can be realized. .

(2) Since the feedback loop is in the opposite direction to the signal light path, the internal oscillation light does not leak to the outside.
Therefore, the oscillation light does not mix with the signal light.

(3) In the region where the gain is stabilized, the wavelength dependence of the gain is also stabilized. Therefore, by incorporating the equalizing filter, an optical fiber amplifier having a flat gain wavelength characteristic over a wide operating range can be realized.

(4) The above characteristics can be obtained with a simple configuration.

(5) Since the number of parts is small and the configuration is simple,
It is possible to reduce the size and cost.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical amplifier using two optical circulators of the present invention.

FIG. 2 is a configuration diagram showing an embodiment of an optical amplifier using one optical circulator of the present invention.

FIG. 3 is a configuration diagram of a conventional optical amplifier.

FIG. 4 is a configuration diagram of a conventional optical amplifier.

FIG. 5 is a configuration diagram of a conventional optical amplifier.

FIG. 6 is a characteristic diagram showing the input signal light power dependency of the gain of the optical amplifier of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 rare earth doped optical fiber 2 pumping light source 3 optical multiplexer / demultiplexer 4a, 4b optical isolator 5a, 5b optical splitter 6 optical bandpass filter 7 optical band reject filter 8, 8a, 8b optical fiber grating filter 9a, 9b optical circulator 10 light Attenuator 11 Optical circulator 12 Optical non-reflective terminator

Claims (4)

[Claims] A first optical circulator having a forward characteristic from a terminal A to a terminal B and from a terminal B to a terminal C;
And a second optical circulator having a forward characteristic from the terminal E to the terminal F, and a rare earth-doped optical fiber between the terminal B of the first optical circulator and the terminal E of the second optical circulator. And an excitation light is input from the excitation light source to the rare-earth-doped optical fiber via an optical multiplexer / demultiplexer, and optical attenuation occurs between the terminal C of the first optical circulator and the terminal D of the second optical circulator. And an optical attenuator via an optical band-pass filter, input signal light from a terminal A of the first optical circulator, and output an amplified optical signal from a terminal F of the second optical circulator. An optical amplifier, characterized in that: 2. A rare earth-doped optical fiber between terminals B and D of an optical circulator having forward characteristics from terminal A to terminal B, terminal B to terminal C, terminal C to terminal D, and terminal D to terminal E. And an excitation light is input from the excitation light source to the rare-earth-doped optical fiber via an optical multiplexer / demultiplexer, and an optical reflector or a wavelength-selective optical reflector is connected to a terminal C of the optical circulator via an optical attenuator. An optical amplifier connected to the optical circulator for inputting a signal light from a terminal A and outputting an amplified optical signal from a terminal E of the optical circulator. 3. The optical amplifier according to claim 2, wherein said wavelength-selective optical reflector is an optical fiber grating filter. 4. An optical amplifier according to claim 1, wherein an equalizing optical filter for compensating for the wavelength dependence of the amplification gain is inserted at one end or in the middle of said rare-earth-doped optical fiber.