JPH11238931A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide with a simple constitution an optical amplifier which suppresses the leakage of inner oscillation light. SOLUTION: In an optical amplifier having rare-earth added light fibers 1a and 1b which are excited by excitation light and amplify light and a feedback means feeding back the light of a prescribed wavelength band in amplified light to the rare-earth added light fibers 1a and 1b, a filter 9 for attenuating the light of the wavelength band is inserted into the rare-earth added light fibers 1a and 1b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier for stabilizing a gain by causing oscillation in an amplifier using a rare-earth-doped optical fiber. The present invention relates to an optical amplifier capable of suppressing the above.

[0002]

2. Description of the Related Art An optical fiber amplifier using a rare earth-doped optical fiber has excellent characteristics such as high gain, high output and low noise, so that the performance of an optical communication system has been greatly improved. The principle of the optical fiber amplifier is that the excitation light having a wavelength corresponding to the excitation level of the added rare earth is incident on the rare earth doped optical fiber, and the signal light is amplified by the stimulated emission phenomenon caused by the inverted distribution of the energy level of the rare earth ions. It is to do.

In particular, in an optical fiber amplifier (hereinafter referred to as EDFA) using an optical fiber doped with erbium (Er), the amplification wavelength band coincides with the lowest loss wavelength band (1.55 μm band) of a silica-based optical fiber. In addition, since it is easy to obtain high efficiency and high gain and low noise amplification characteristics, it has been widely used.

[0004] When an optical fiber amplifier such as an EDFA is used, a wavelength-multiplexed optical signal can be amplified collectively, so that a large-capacity and flexible transmission system by wavelength division 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. Have been. However, in this method, in addition to the complicated structure, it is difficult to stabilize the gain with high accuracy over a wide input range, and there is a problem in a transient response characteristic to a time variation of the input signal light power. .

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. 3 shows an example. As shown, this optical amplifier has the same basic parts as a normal optical fiber amplifier. The basic parts are a rare earth-doped optical fiber 1 that is excited by the pump light to amplify the signal light, a pump light source 2 that supplies the pump light, an optical multiplexer 3 that multiplexes the signal light and the pump light, and a rare earth Additive optical fiber 1
And an output-side optical isolator 4b for extracting signal light. In addition to this configuration, the optical amplifier has optical fiber grating filters 5a and 5b connected to both ends of the rare-earth-doped optical fiber 1 to cause oscillation.

The optical fiber grating filters 5a,
Reference numeral 5b shows a diffraction grating formed in an optical fiber containing germanium at a high concentration by irradiating a high-output short-wavelength laser beam by a holographic method. According to such a filter, it is possible to reflect only light having 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 where the connection is made. Thus, the optical fiber grating filter 5
Reference numerals a and 5b selectively reflect light in a predetermined wavelength band out of the light amplified in the rare-earth-doped optical fiber 1, and respectively convert the light from the rare-earth-doped optical fiber 1 into the rare-earth-doped optical fiber 1. It is arranged to reflect inside. Therefore, for light in this wavelength band, a feedback loop is formed by the rare-earth-doped optical fiber 1.

In the example shown in FIG. 3, oscillation occurs at the wavelength selected by the optical fiber grating filters 5a and 5b, and the gain of the amplifier is fixed. The magnitude of the gain of the amplifying section is determined by the reflectance of each of the optical fiber grating filters 5a and 5b because the gain of the feedback loop including the amplifying section and the reflecting section is determined to be 1.

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 a transient response characteristic to a time change of the input signal light power is also stable. doing. Moreover, complicated electric control is not required. FIG. 5 shows the difference in the dependence of the gain on the input signal light power depending on the presence or absence of optical feedback. In the absence of optical feedback, the gain varies greatly depending on the input signal light power, but by performing optical feedback control (oscillation), the input signal light power range that is stabilized at a constant gain is expanded. You. In addition, when there is no control, the wavelength dependence of the gain changes according to the input signal light power. However, by performing control using a feedback loop, the wavelength dependence of the gain is maintained in a range where the gain is stabilized. Is also stabilized.

[0010]

However, the method using optical feedback has a problem that the internal oscillation light is output to the outside together with the signal light. In a signal light power region in which the gain is stabilized by internal oscillation, the oscillating light has a higher optical power than the signal light, so that such light is problematic to be emitted to the outside.

As a method of suppressing the leakage of the internal oscillation light, it is conceivable to increase the reflectance of the rear (optical signal light output side) optical fiber grating filter 5b in the configuration of FIG. At this time, in order to set the gain to a certain appropriate value, it is necessary to reduce the reflectance of the optical fiber grating filter 5a in front (signal light input side). When the reflectivity is set asymmetrically in this way, the longitudinal optical power distribution of the internal oscillation light in the rare-earth-doped optical fiber 1 shows a maximum distribution near the front optical fiber grating filter 5a. If there is a strong oscillation light power, the population inversion density of rare earth ions is reduced, and the population inversion density on the signal light input side is reduced by lowering the noise characteristics of the optical fiber amplifier (noise figure NF;
e figure increase).

An object of the present invention is to solve the above-mentioned problems and to provide an optical amplifier having a simple configuration and capable of suppressing leakage of internal oscillation light.

[0013]

In order to achieve the above object, the present invention provides a rare-earth-doped optical fiber which is excited by pumping light and amplifies the light; An optical amplifier having feedback means for feeding back to the rare-earth-doped optical fiber, wherein a filter for attenuating light in the wavelength band is inserted into the rare-earth-doped optical fiber.

The feedback means may comprise an optical fiber grating filter arranged at both ends of the rare-earth-doped optical fiber to reflect light in the wavelength band into the rare-earth-doped optical fiber.

The feedback means includes an optical coupler for extracting amplified light at one end of the rare-earth-doped optical fiber,
The other end may be provided with an optical coupler for taking in the returned light, and a band-pass filter for transmitting light in the above-mentioned wavelength band may be provided between the optical couplers.

The feedback means is provided with an optical circulator for extracting amplified light at one end of the rare-earth-doped optical fiber, and an optical circulator for receiving feedback light at the other end. A band pass filter that transmits band light may be provided.

The filter for attenuating the light in the wavelength band is
It may have a wavelength loss characteristic that cancels out the wavelength dependence of the amplification gain by the rare-earth-doped optical fiber.

[0018]

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

As shown in FIG. 1, the optical amplifier of the present invention is obtained by adding a filter to the optical amplifier of FIG.
The description of the overlapping part is omitted. The added optical band reject filter 9 is inserted into the rare earth-doped optical fiber, and connects the rare earth-doped optical fiber to the rare earth-doped optical fiber 1.
a and 1b. The optical band reject filter 9 includes optical fiber grating filters 5a and 5a.
It has wavelength characteristics such that the loss is large at the oscillation wavelength determined by b and the loss is small in the signal light wavelength band and the pump light wavelength band.

According to such a configuration, the reflectivities of the optical fiber grating filters 5a and 5b are both increased, and the signal light gain (gain of the amplifier) is set by the loss amount at the oscillation wavelength of the optical band reject filter 9. It becomes possible. Therefore, the oscillation light power distribution on the signal light input side can be reduced, and a decrease in noise characteristics due to a reduction in the population inversion density of rare earth ions can be prevented. In addition, since the reflectance of the optical fiber grating filter 5b on the rear side (signal light output side) can be made as large as possible, leakage of the internal oscillation light to the outside can be reduced.

Next, another embodiment of the present invention will be described.

First, FIG. 4 shows an optical amplifier having a feedback loop different from that of FIG. 3 as a prior art. This optical amplifier is
A rare earth-doped optical fiber 1 that is excited by pump light to amplify signal light, an excitation light source 2 that supplies pump light, an optical multiplexer 3 that multiplexes signal light and pump light, and a signal light that is transmitted to the rare earth-doped optical fiber 1 An input-side optical circulator 6a for taking in and taking out oscillating light, an output-side optical circulator 6b for taking out signal light from the rare-earth-doped optical fiber 1 and taking in oscillating light, an optical bandpass filter 7 having a transmission characteristic in an oscillating light band, And an optical attenuator 8.

Each of the optical circulators 6a and 6b has three terminals A, B and C.
From terminal C to terminal C. Therefore, the signal light input reaches the terminal B from the terminal A of the input-side optical circulator 6a, is multiplexed with the pump light by the optical multiplexer 3, and is taken into the rare-earth-doped optical fiber 1. The signal light amplified by the rare-earth-doped optical fiber 1 reaches the terminal C from the terminal B of the output-side optical circulator 6b and is extracted as a signal light output.

On the other hand, the terminal C of the input-side optical circulator 6a and the terminal A of the output-side optical circulator 6b are connected via an optical band-pass filter 7 and an optical attenuator 8. Thereby, the terminal B of the input side optical circulator 6a
To the terminal C of the output-side optical circulator 6b through the optical attenuator 8 and the optical bandpass filter 7, from the terminal A to the terminal B, and again through the rare-earth-doped optical fiber 1, which is an optical amplifier, to the input-side optical circulator 6a. A feedback loop returning to the terminal B is formed. Therefore, the optical bandpass filter 7
Oscillation occurs at the wavelength selected by the above, and the gain of the optical amplifying section composed of the rare-earth-doped optical fiber 1 is fixed at a constant value. The fixed gain can be adjusted by the optical attenuator 8.

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

However, the oscillation light in the rare-earth-doped optical fiber 1 has such a distribution that the optical power is maximized on the signal light input side, so that the noise characteristics deteriorate for the above-mentioned reason. Therefore, the optical amplifier of FIG. 2 is an example in which the present invention is applied to the prior art of FIG. The optical amplifier of FIG.
And an optical filter is added to the optical amplifier described above, and the description of the overlapping part will be omitted. The optical band reject filter 9 is inserted into the rare-earth-doped optical fiber, and divides the rare-earth-doped optical fiber into rare-earth-doped optical fibers 1a and 1b. The optical band reject filter 9 has such wavelength characteristics that the loss is large at the oscillation wavelength determined by the optical bandpass filter 7 and the loss is small in the signal light wavelength band and the pump light wavelength band.

According to such a configuration, it is possible to reduce the optical power distribution on the signal light input side, and it is possible to prevent noise characteristic deterioration due to reduction of the population inversion density. Further, the advantage that the internal oscillation light does not leak to the outside is utilized as it is.

In this embodiment, the optical circulator is used to take out the oscillating light from the amplifying unit to the feedback loop and take in the oscillating light from the feedback loop to the amplifying unit. However, an optical coupler is used instead of the optical circulator. be able to.

1 and 2, the filter 9 inserted into the rare-earth-doped optical fiber 1 is provided with a loss characteristic that cancels out the wavelength dependence of the gain of the amplifying section, thereby achieving wavelength dependence. , A broadband optical fiber amplifier having a small size can be realized.

[0030]

The present invention exhibits the following excellent effects.

(1) Since an optical filter for attenuating oscillating light is inserted in the middle of the amplifying section, leakage of the oscillating light inside the amplifying section to the outside is suppressed, and deterioration of noise characteristics is reduced.

(2) The wavelength characteristics can be flattened by providing the optical filter with a loss characteristic for compensating the wavelength dependence of the gain of the amplifier.

(3) Since the number of parts is small and the configuration is simple, miniaturization and cost reduction are possible.

[Brief description of the drawings]

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

FIG. 2 is a configuration diagram of an optical amplifier according to another embodiment 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 characteristic diagram showing the input signal light power dependence of the gain of the optical amplifier.

[Explanation of symbols]

 1a, 1b Rare-earth-doped optical fiber 2 Pump light source 3 Optical multiplexer 5a, 5b Optical fiber grating filter 6a, 6b Optical circulator 7 Optical bandpass filter 9 Optical band reject filter

Claims (5)

[Claims] 1. An optical amplifier comprising: a rare-earth-doped optical fiber that is excited by pumping light to amplify light; and feedback means that feeds light of a predetermined wavelength band out of the amplified light to the rare-earth-doped optical fiber. An optical amplifier, wherein a filter for attenuating light in the wavelength band is inserted into the rare earth-doped optical fiber. 2. The return means comprises an optical fiber grating filter disposed at both ends of the rare-earth-doped optical fiber for reflecting light in the wavelength band into the rare-earth-doped optical fiber. 2. The optical amplifier according to 1. 3. The feedback means includes: an optical coupler for extracting amplified light at one end of the rare-earth-doped optical fiber; and an optical coupler for capturing feedback light at the other end.
2. The optical amplifier according to claim 1, wherein a band-pass filter that transmits light in the wavelength band is provided between the optical couplers. 4. An optical circulator for taking out amplified light is disposed at one end of the rare-earth-doped optical fiber, and an optical circulator is provided at the other end for taking in the returned light. 2. The optical amplifier according to claim 1, further comprising a band-pass filter that transmits light in the wavelength band. 5. The filter according to claim 1, wherein the filter for attenuating the light in the wavelength band has a wavelength loss characteristic for canceling a wavelength dependency of an amplification gain by the rare-earth-doped optical fiber. Optical amplifier.