JPH0895098A - Variable gain light amplifier and optical communication system - Google Patents

Abstract

PURPOSE: To provide a variable gain light amplifier which can change gain without causing deterioration of characteristic and deal with various systems. CONSTITUTION: A light amplifier in which the polarized wave of input light or output light is controlled so as to make variable gain possible is constituted by the use of a polarized wave element such as polarizer 3. The variable gain light amplifier is provided with a light amplifying part 1 giving gain to an input light from the outside, and a polarized wave control means 7 controlling the polarized state of a light. By controlling an input light 5 prescribed in a specified polarized wave state by the polarizer 3 using the polarized wave control means 7, effective amplification factor of the whole light amplifier is changed without changing the gain of the light amplifying part 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifying device capable of changing an effective amplification factor and an optical communication system using the same.

[0002]

2. Description of the Related Art Conventionally, an optical amplifier that amplifies an optical signal as light has been an important technique in optical fiber communication. The gain performance of the optical amplifier is limited by the configuration of the amplifier and is mainly determined by the saturated output, but the gain actually used is smaller than the saturated output, and control is performed to keep the gain constant. . Therefore, for example, in the case of an optical fiber amplifier, the intensity of pumping light can be changed, and in the case of a semiconductor optical amplifier, the bias current given to the amplifier can be changed.

However, it is desired that the gain be arbitrarily changed depending on the application of the amplifier. In that case, the simplest is to control the gain control unit.
However, the above-mentioned adjustment may change not only the magnitude of the gain itself but also various characteristics during amplification. For example, in a semiconductor optical amplifier, lowering the bias current leads to lowering of gain, lowering of saturated output, and deterioration of noise characteristics, which is not preferable.

Therefore, the next conceivable method is to use an attenuator such as an ND filter in the front and rear stages while keeping the gain of the amplifier set to the maximum. In this case, various characteristics of amplification do not change, which is very useful in designing a communication system. However, when using the ND filter, attention must be paid to the influence of the return light due to the reflection from the ND filter. Special attention should be paid to absorption filters. The presence of the returning light makes the communication system unstable and causes a reduction in the error rate.

Also, one of the reasons for wanting to control the gain
For one thing, the gain of each signal may be small, but there is a desire to amplify several signals at once. It is also possible that only a specific signal is amplified and other signals need not be forcibly amplified. Also,
Since an optical amplifier has a saturated output, it is impossible to amplify a number of signals unnecessarily, but when the number of signal lights is large, each gain is suppressed individually, and conversely when there is only one kind of signal light. It is also conceivable to give a large gain and make the most effective use of one optical amplifier.

Gain control for these applications can be cumbersome with conventional optical amplifiers.

[0007]

As described above, the conventional optical amplifier has a drawback that it is difficult to construct a variable gain optical amplifier without degrading the characteristics.
In addition, there is a drawback that it cannot support a wide variety of systems.

Therefore, it is an object of the present invention to provide a variable gain optical amplifier and an optical communication system using the same, which overcome these drawbacks.

[0009]

According to the present invention, an optical amplifying device capable of variable gain is constructed by using a polarization element such as a polarizer by controlling the polarization of input or output light.

More specifically, the variable gain optical amplifying apparatus of the present invention comprises an optical amplifying section for giving a gain to the input light from the outside and a polarization control means for controlling the polarization state of the light, and has a specific polarization. It is characterized in that the effective amplification factor is changed by controlling the input light defined in the state using the polarization control means.

Specifically, the polarization control means has a rotation means for the polarization maintaining fiber, and the combination of the rotation means and the polarizer attenuates the light. Further, the polarization control means has a rotation means for the polarizer, and attenuates light defined in a specific polarization state. Further, the polarization control means has a wave plate rotation means, and a combination of this and a polarizer attenuates light. Further, the optical amplifier is a semiconductor optical amplifier having a gain polarization dependency, and the gain is changed by changing the polarization component ratio of the input light to the semiconductor optical amplifier by the polarization control means.

Further, the variable gain optical amplifying apparatus of the present invention is provided with an optical amplifying section for giving a gain to the input light from the outside and at least one of a front stage and a rear stage of the optical amplifying section to change the polarization state of the light. Effectively by providing a polarization element (polarizer or the like) for controlling the light intensity in accordance with the polarization element and polarization control means for controlling the relative relationship of the polarization state of the light input to the polarization element to the polarization element. It is characterized by changing the amplification factor.

Further, the variable gain optical amplifying apparatus of the present invention comprises a semiconductor optical amplifier having a polarization dependency of gain and a polarization control means for changing the polarization component ratio of the input light to the semiconductor optical amplifier. , Which is characterized by changing the gain.

Further, the optical communication system of the present invention is characterized in that there is one input signal, and the optical amplification device gives an arbitrary gain to the input signal. Also,
It is characterized in that the optical amplifying device collectively gives a gain to a plurality of input signals. Further, it is characterized in that it has means for demultiplexing the input signals with respect to a plurality of input signals, and gives different gains to the demultiplexed signals depending on the optical amplifying device. Further, it is characterized in that the optical amplification device and the light detection device are provided, and the signal of the light detection device is fed back to the polarization control device in the optical amplification device to prevent gain saturation of the optical amplification device. .

[0015]

[First Embodiment] FIG. 1 shows a first embodiment of the present invention.
In FIG. 1, 1 is a semiconductor optical amplifier, 2 is a lens, 3 is a polarizer, 4 is a polarization maintaining fiber, 5 and 6 are input light and output light, 7 is a polarization controller, and 8 is a semiconductor optical amplifier 1. It is a driver that supplies current.

The operation principle of the variable gain optical amplifier in this embodiment will be described. The input light 5 is transmitted through the polarization maintaining fiber 4 according to a certain linearly polarized light. Thereafter, the polarization direction and power of this linearly polarized light are controlled in relation to the transmission axis of the polarizer 3 adjusted by the polarization controller 7, and the lens 2 inputs it to the optical amplifier 1 from one end face of the semiconductor optical amplifier 1. To be done. The semiconductor optical amplifier 1 is a so-called traveling wave type optical amplifier in which both end surfaces of a semiconductor laser structure are coated with antireflection coating, and is set to a constant bias current. The input light is amplified while being guided through the semiconductor optical amplifier 1, and is output from the other end surface. The output light 6 is again coupled to the polarization maintaining fiber 4 by the lens 2 and transmitted. The polarization direction of the output light 6 is the direction of the transmission axis of the polarizer 3. The semiconductor optical amplifier 1 may or may not have polarization dependence.

At this time, as described above, the light is input to the semiconductor optical amplifier 1 and is controlled by the controller 7 using the polarizer 3 in the preceding stage to adjust the amount of light input to the semiconductor optical amplifier 1. That is, the polarization direction of the polarization maintaining fiber 4 is
The maximum output is obtained when the transmission direction of the polarizer 3 is matched, and the minimum output is obtained when the transmission direction of the polarizer 3 is matched, and the effective gain from the fiber 4 to the fiber 4 is changed.

Various types of polarizers 3 can be considered.
However, in an absorption-type polarizer, the amount of transmitted light itself is generally attenuated, and like the ND filter, the influence of return light can be considered. The prism-type polarizer is effective because it can avoid the influence of returning light. However, it is necessary to devise the design so that the rotation of the prism type polarizer does not change the axial direction of the light and the efficiency of optical coupling to the semiconductor optical amplifier does not change.

Further, in the present embodiment, the polarizer 3 is placed in the front stage of the semiconductor optical amplifier 1, but it is of course possible to place it in the rear stage to control the output light. However, considering the saturated output of the optical amplifier 1, it is advantageous to arrange the optical amplifier 1 in the preceding stage so as to adjust the amount of light input to the optical amplifier 1 first. In the semiconductor optical amplifier 1, the fact that the bias current is not controlled due to the variable gain helps prevent the characteristic deterioration which is the effect of the present invention. However, in order to compensate for the gain change of the optical amplifier 1 caused by the change of the ambient temperature, the control of the bias current by the automatic gain control may be necessary in some cases.

In this embodiment, the polarization state of the input light is all linearly polarized. This is because the use of linearly polarized waves can realize the variable gain most efficiently. When the input light is elliptically polarized light, the variable range is narrowed although the same effect can be obtained. In addition, completely circular polarization has no effect. Therefore, it is a condition that the polarization state of the input light is defined so as to exclude circular polarization.

On the other hand, in a general transmission line, a single mode fiber is often used, and in this case, the polarization state is considered to fluctuate with time. Therefore, in order to define the polarization state of the input light and keep it in the same state, in this case, a polarization controller using a polarization coil or the like (in front of the polarizer 3) is connected to the optical amplifier. Will be required separately. Of course, a transmission system using a polarization-maintaining fiber can be dealt with by adopting a configuration in which the polarization state does not change during connection. However, caution is required when using a modulation method that uses the polarization state as a part of the signal.

In this embodiment, the optical amplifier is used as a repeater, but the optical amplifier may be used as a preamplifier placed in front of the receiver. In that case, the components after the semiconductor optical amplifier 1 are different. In addition, although the present configuration has shown the possibility of reducing the influence of return light due to reflection, if it is necessary to completely suppress reflection, it is also necessary to consider applying antireflection coating to the components.

[0023]

[Second Embodiment] FIG. 2 shows a second embodiment. In this embodiment, the polarization maintaining fiber 4 on the input side is rotated instead of the rotation of the polarizer 3 in the first embodiment.
In this case, the output fiber 4 is also the input fiber 4
Is rotated by the controller 17 in accordance with the rotation of the. The operating principle of the variable gain optical amplifier of this embodiment is substantially the same as that of the first embodiment.

[0024]

Third Embodiment FIG. 3 shows a third embodiment. In this embodiment, a wave plate 28 is added to the structure of the first embodiment, and the polarizer 3 and the polarization maintaining fiber 4 are fixed, and the wave plate 28 is rotated by the controller 27. In this case, the output side wave plate 28 is also rotated by the controller 27 in accordance with the rotation of the input side wave plate 28. As the wave plate 28, various types such as a λ / 2 plate, a λ / 4 plate, and a combination thereof can be considered. Further, other than the wave plate, another polarization element such as a Faraday rotation element may be used.

Although all of the above are composed of individual components, an integrated polarization element integrated with the semiconductor optical amplifier 1 on the same substrate may be used. Further, the lens 2 is an optical fiber 4
It is a component for coupling the light from the semiconductor optical amplifier 1 to the semiconductor optical amplifier 1, but any component other than the lens may be used as long as it can be coupled while maintaining the polarization state. The type of lens may be another component such as a Selfoc lens. The operating principle of the variable gain optical amplifier of the present embodiment is substantially the first.
Same as the embodiment.

[0026]

[Fourth Embodiment] FIG. 4 shows a fourth embodiment of the present invention.
The present embodiment is an example in which the second embodiment shown in FIG. 2 is further devised. That is, it is possible to eliminate the polarizer by using an optical amplifier having a gain polarization dependency. In the case of the semiconductor optical amplifier 31, the polarization characteristic of the gain mainly depends on the active layer thickness, and the end face reflectance and the optical confinement coefficient have polarization dependency due to the difference in the light distribution during propagation between TE light and TM light. As a result, the gain of TE light is normally about 6 dB higher than that of TM light. Therefore, if the controller 37 causes the polarization of the polarization-maintaining fiber 4 to match the TE mode of the semiconductor optical amplifier 31, the maximum gain can be obtained, and conversely, the minimum gain can be obtained. .

FIG. 5 shows a graph of the gain characteristic in this embodiment. The variable range of the gain in this embodiment depends on the polarization characteristic of the gain of the optical amplifier 31, and the adjustment range is between the maximum gain and the minimum gain of the optical amplifier. Therefore, its performance is determined by the optical amplifier 31 used. As mentioned above, in a typical semiconductor optical amplifier,
It is possible to adjust the dB. Further, in the semiconductor optical amplifier using the quantum well structure in the active layer, the gain difference between TE light and TM light becomes large due to the polarization characteristics of the optical gain itself.
A variable range of about 0 dB is possible. In order to further increase the variable range, it is possible to use a so-called strained superlattice structure having a semiconductor layer in which in-plane compression or in-plane tensile stress is applied to the active layer.

Further, the present configuration in which the polarizer is eliminated by utilizing the polarization dependency of the gain of the optical amplifier can be applied to the above-mentioned third embodiment and modified. That is, the polarizer 3 can be eliminated in FIG.

The configuration of the present embodiment can eliminate the polarizer, but this has the effect of reducing the influence of the returning light as compared with the first to third embodiments. That is, in the first to third embodiments, it can be considered that the polarization is controlled to attenuate the signal light in order to effectively change the gain, and the reflection is less likely to occur than the attenuation by the ND filter. There is. But these are not perfect. On the other hand, in this embodiment, no attenuation of signal light is used. Therefore, the influence of reflection can be reduced. Also,
In this configuration, polarization control needs to be performed before being input to the semiconductor optical amplifier 31. The effect of the variable gain cannot be expected if the polarization control means is provided after the amplifier 31.

[0030]

[Fifth Embodiment] FIG. 6 shows an embodiment of an optical communication system according to the present invention. (A) is a case where a single signal is amplified. In this case, the required gain can be changed arbitrarily. (B) is a case where the wavelength multiplexed signals are collectively amplified.
If the gain is adjusted according to the number of input signals, all the signals can be amplified so that the signal light is not deteriorated. For example, the saturation output of the semiconductor optical amplifier 1 is plus 3 dBm.
Then, if the input light level is -15 dBm, 1 wave is 1
It can be amplified up to a gain of 8 dB, and up to a gain of 12 dB with four waves.

(C) is another embodiment for collectively amplifying the wavelength division multiplexed signals. The present embodiment is a case where different gains are to be given depending on the type of signal (wavelength in this case). Therefore, the configuration is rather complicated. A semiconductor optical amplifier 31 having polarization dependency is used. First, input light 5
Are separated by wavelength by the demultiplexer-multiplexer 9. Then, λ1 is adjusted to TE light for the semiconductor optical amplifier 31 by the wave plate 28 so as to have maximum gain, while λ2 is adjusted to TM light so as to have minimum gain. Both are combined again by using the mirror 10 and the polarization beam splitter 11,
It is input to the semiconductor optical amplifier 31. The output side has the same configuration as the input side. This system is effective for systems that assign wavelengths by function. For example, it is effective to use λ1 for a long-distance transmission signal, λ2 for a short-distance transmission control signal, and increase the amplification degree only for λ1.

Further, FIG. 7 shows another embodiment of the optical communication system. This system has a configuration in which a communication control function is considered in the system of FIG. 6, and (a) and (b) of FIG.
Both systems can be compatible. That is, the amplification factor is automatically controlled according to the state of the signal light. For example, the light output from the optical amplifier 1 is branched using the branching device 14 and the monitor light is extracted to the photodetector 12. Then, the power is detected, and the communication controller 13 controls the polarizer 3 so that the gain saturation of the semiconductor optical amplifier 1 does not always occur. As a result, a large gain is obtained when the signal light is one wave, and the gain of each signal light is suppressed when the signal light is plural.

By constructing such a system, it is possible to maximize the effective use of the optical amplifier and prevent the deterioration of the transmission signal while keeping the number of signal lights and the communication form flexible. .

As shown in the above embodiments, the optical amplifying device according to the present invention not only enables the variable gain of a single signal, but also collectively amplifies wavelength multiplexed signals and further changes the number of signal lights. For example, there is an advantage that it can flexibly support various systems.

[0035]

As described above, according to the present invention,
An optical amplification device capable of variable gain can be configured by using the polarization element. In addition, it is possible to make the configuration less susceptible to the influence of the returning light, and to stabilize the communication system. Further, this configuration has an effect that it can flexibly cope with various systems such as batch amplification of wavelength-multiplexed signals.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing a third embodiment of the present invention.

FIG. 4 is a diagram showing a fourth embodiment of the present invention.

FIG. 5 is a graph showing gain characteristics in the fourth embodiment.

FIG. 6 is a diagram showing various system embodiments of the present invention.

FIG. 7 is a diagram showing another system embodiment of the present invention.

[Explanation of symbols]

 1, 31 Semiconductor Optical Amplifier 2 Lens 3 Polarizer 4 Polarization Preserving Fiber 7, 17, 27, 37 Controller 8 Driver 9 Demultiplexer / Multiplexer 10 Mirror 11 Polarizing Beam Splitter 12 Photodetector 13 Communication Controller 14 Divider 28 Wave Plate

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[Procedure amendment]

[Submission date] April 27, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

FIG.

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[Figure 5]

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Claims (12)

[Claims] 1. An optical amplifier unit for giving a gain to an input light from the outside, and a polarization control means for controlling a polarization state of the light,
A variable gain optical amplifying device characterized in that an effective amplification factor is changed by controlling input light defined in a specific polarization state by using the polarization control means. 2. The variable gain optical amplifier according to claim 1, wherein the polarization control means has a rotation means of a polarization maintaining fiber, and the light is attenuated by a combination of the rotation means and a polarizer. apparatus. 3. The variable gain optical amplifier according to claim 1, wherein the polarization control means has a rotation means for a polarizer, and attenuates light defined in a specific polarization state. . 4. The variable gain optical amplifier according to claim 1, wherein the polarization control means has a wave plate rotation means, and attenuates the light when combined with a polarizer. 5. The semiconductor optical amplifier in which the optical amplification section has a gain polarization dependency, and the gain is obtained by changing the polarization component ratio of the input light to the semiconductor optical amplifier by the polarization control means. The variable gain optical amplifier according to claim 1, wherein the variable gain optical amplifier is changed. 6. An optical amplifying section for giving a gain to an input light from the outside, and a polarization element arranged in at least one of a front stage and a rear stage of the optical amplifying section for controlling a light intensity according to a polarization state of light. And a polarization control means for controlling the relative relationship of the polarization state of the light input to the polarization element with respect to the polarization element, thereby changing the effective amplification factor. Amplification device. 7. A semiconductor optical amplifier having a polarization dependence of gain, and a polarization control means for changing a polarization component ratio of input light to the semiconductor optical amplifier, thereby changing the gain. Variable gain optical amplifier. 8. An input signal is present and the input signal is present in any one of claims 1 to 6.
Alternatively, an optical communication system characterized by giving an arbitrary gain to the input signal by the optical amplifying device described in 7. 9. An optical communication system, wherein a gain is collectively applied to a plurality of input signals by the optical amplifying device according to claim 1, 6 or 7. 10. A plurality of input signals are provided with means for demultiplexing the input signals, and the demultiplexed signals are provided with different gains according to the optical amplifying device according to claim 1, 6 or 7. An optical communication system characterized by the above. 11. The optical communication system according to claim 8, wherein the light input to the optical amplification device and the light output from the optical amplification device have the same polarization state. 12. The optical amplifying device according to claim 1, 6 or 7, and the photodetecting means, wherein the signal of the photodetecting means is fed back to the polarization control means in the optical amplifying device to obtain the optical amplifying device. The optical communication system is characterized by preventing the gain saturation of.