JPH11168258A - Optical amplifier - Google Patents

Abstract

(57) Abstract: An object of the present invention is to provide an optical amplifier capable of outputting an optical signal in which signal waveform distortion in a semiconductor optical amplifier has been compensated. A semiconductor optical amplifier, a saturable absorber to which output light of the semiconductor optical amplifier is input, and a saturable absorber.
Light output control means 3 for controlling the output light from the light source.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier used in an optical communication system.

[0002]

2. Description of the Related Art In a conventional optical amplifying device of an optical communication system, a semiconductor optical amplifier for amplifying and outputting the light intensity of input signal light is used. The semiconductor optical amplifier has a feature that it is small and can be integrated with other optical circuits.

[0003]

However, in such a conventional optical amplifying device, when a signal light having a high light intensity is usually input to a semiconductor optical amplifier, the carrier density in the amplifier is reduced due to stimulated emission. As a result, the amplification gain decreases. If the signal light is intensity-modulated, the carrier density is low when the light intensity is on and the carrier density is high when the light intensity is off, but the carrier density does not respond instantaneously to a change in the light intensity. It changes with a time constant.

For example, a case where a single-pulse signal light is input to a semiconductor optical amplifier will be described with reference to FIG. The head of the signal light that is input when the carrier density of the semiconductor optical amplifier is high receives a high gain, and the carrier density decreases toward a predetermined steady-state value when the light intensity is on. That is, the output optical pulse waveform of the semiconductor optical amplifier has a different shape from the rectangular input optical pulse waveform.
Due to such a mechanism, there is a problem that signal waveform distortion occurs when the intensity-modulated signal light is input to the semiconductor optical amplifier at a high light intensity.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to provide an optical amplifier capable of outputting an optical signal in which signal waveform distortion in a semiconductor optical amplifier has been compensated.

[0006]

In order to achieve this object, the present invention provides a semiconductor optical amplifier, a saturable absorber to which the output light of the semiconductor optical amplifier is input, and a saturable absorber from the saturable absorber. Light output control means for controlling output light.

The saturable absorber and the semiconductor optical amplifier have the same structure, and the optical output control means receives a current injected into the active layer of the saturable absorber, and the medium acts as a loss medium. Means for controlling and setting the value to be performed.

Further, the saturable absorber and the semiconductor optical amplifier have the same structure, and the light output control means supplies an injection current to the saturable absorber with the above-mentioned saturable absorber at a stage preceding the saturable absorber. There is provided means for controlling according to the input light intensity to the semiconductor optical amplifier.

The saturable absorber and the semiconductor optical amplifier have the same structure. The semiconductor optical amplifier includes means for controlling according to a voltage between terminals of the active layer of the semiconductor optical amplifier.

The semiconductor optical amplifier and the saturable absorber are integrated on the same substrate.

[0011]

FIG. 1 is a diagram showing a first embodiment of an optical amplifying device according to the present invention. As shown in the figure, a semiconductor saturable absorber 2 and an optical output control means 3 are provided at the subsequent stage of the semiconductor optical amplifier 1.

The semiconductor saturable absorber 2 has the same element structure as the semiconductor optical amplifier 1. However, the amount of current flowing into the semiconductor active layer is different. In the case of the semiconductor optical amplifier 1, a high current is injected. As a result, a carrier inversion distribution is formed, and a gain is given to the input light. On the other hand, zero or a small amount of current is injected into the semiconductor saturable absorber 2. In this case, many carriers exist in the lower level, and the input light is absorbed. (Strictly speaking, it is a lower-level carrier hole, and the term “lower-level carrier” is strange, but from the viewpoint of easy association with ordinary laser theory and easy explanation, When the light is absorbed, the carriers are excited to the upper level. When the input light intensity is low, the number of carriers to be excited is small, but when the input light intensity is high, the number of excited carriers increases, and the upper input light intensity is higher than the lower input light intensity. Carrier density is higher. Incidentally, the absorptance of light depends on the carrier distribution state. When the carrier density of the lower level is large, the absorptance is large, and when the carrier density of the upper level is large, the absorptivity is small. Therefore, when the input light intensity is small, the absorption rate is large, and when the input light intensity is large, the absorption rate is small. Thus, the current injected into the active layer of the semiconductor saturable absorber 2 is controlled by the optical output control means 3 to increase the absorptivity of the medium of the semiconductor saturable absorber 2 and act as a loss medium. Means for setting the current value is provided.

FIG. 2 shows the results of experiments on input intensity versus transmittance actually performed by the inventor. As shown in the figure, it can be seen that as the input light intensity increases, the absorptance decreases, that is, the transmittance increases. In the limit where the input light intensity is increased, the carrier density of the upper level becomes equal to the carrier density of the lower level, and the absorptivity is zero, that is, the medium is transparent to the input light.

The above is the static characteristics of the semiconductor saturable absorber 2. When the input light intensity changes with time, the carrier density cannot respond instantaneously, but changes to a steady value with a predetermined time constant. For example, the output for a single pulse input is as shown in FIG. Since the carrier is not sufficiently excited at the leading part of the pulse, the absorption is large, that is, the transmittance is small. However, the carrier is excited with time and shifts to a steady value of large transmittance. Here, the time constant of the change is generally determined by the carrier lifetime.

By utilizing the time response characteristic of the semiconductor saturable absorber 2, it is possible to compensate for the signal waveform distortion generated in the semiconductor optical amplifier 1. As shown in FIG. 8, the waveform distortion in the semiconductor optical amplifier 1 is such that the output level is large at the beginning of the pulse and the output level is reduced to a low level. On the other hand, the pulse response of the semiconductor saturable absorber 2 is reduced from a low level to a high level steady value, and has characteristics opposite to those of the semiconductor optical amplifier 1. Furthermore, the relaxation rates of both are determined by the carrier lifetime and are almost equal. Therefore, as shown in FIG.
Is provided at the subsequent stage of the semiconductor optical amplifier 1 and the optical output control means 3 is provided, so that the signal light with less waveform distortion can be obtained as shown in FIG.

Since the manner in which the absorptance of the semiconductor saturable absorber 2 changes depends on the intensity of the input light, it is necessary to obtain input / output characteristics for compensating waveform distortion generated in the semiconductor optical amplifier 1. Means for adjusting the light intensity may be inserted between the semiconductor optical amplifier 1 and the semiconductor saturable absorber 2.

The degree of waveform distortion in the semiconductor optical amplifier 1 depends on the input light intensity. Therefore, the dynamic characteristic of the semiconductor saturable absorber 2 that compensates for this has an optimum value depending on the intensity of the input light to the semiconductor optical amplifier 1. The dynamic characteristics of the semiconductor saturable absorber 2 can be controlled by the current injected into the active layer. Therefore, if the injection current to the semiconductor saturable absorber 2 is controlled according to the intensity of the input light to the semiconductor optical amplifier 1, the waveform distortion can be always compensated in an optimum state.

FIG. 5 is a diagram showing a second embodiment of the optical amplifying device according to the present invention. As shown in the figure, the basic configuration and the operation principle are the same as those in FIG. 1, but a part of the input light to the preceding semiconductor optical amplifier 1 is split by the optical splitting mirror 3c and the intensity is detected by light. And a control circuit 3a for controlling an injection current to the semiconductor saturable absorber 2 according to the detected light intensity detected by the detector 3b. The control circuit 3a, the photodetector 3b, the light splitting mirror 3c Constitute the light output control means. This makes it possible to always compensate for the waveform distortion in an optimum state.

The reason why the waveform distortion in the semiconductor optical amplifier 1 depends on the input light intensity is that the state of gain saturation differs depending on the input light intensity. In other words, it can be rephrased that the waveform distortion depends on the gain saturation state. Therefore, if the gain saturation state is detected and the injection current to the semiconductor saturable absorber 2 is controlled accordingly, the optimum compensation state can always be realized.

FIG. 6 is a view showing a third embodiment of the optical amplifying device according to the present invention. As shown in the figure, the basic configuration and the operation principle are the same as those in FIG. 1, except that an inter-terminal voltage monitor 3e for detecting the inter-terminal voltage of the active layer of the preceding semiconductor optical amplifier 1 is provided. It has a control circuit 3d for controlling the injection current to the semiconductor saturable absorber 2 according to the value, and the control circuit 3d and the inter-terminal voltage monitor 3e constitute an optical output control means. The gain saturation of the semiconductor optical amplifier 1 is caused by a decrease in the upper-level carrier density in the active layer. The degree of decrease in the carrier density can be monitored by the voltage between terminals of the active layer. Therefore, the waveform compensation operation by the semiconductor saturable absorber 2 can be optimized by the configuration of FIG.

In the above embodiment, the semiconductor optical amplifier 1 and the semiconductor saturable absorber 2 have been described as separate elements.
Both can be configured as the same element. FIG.
FIG. The semiconductor optical amplifier 1 and the semiconductor saturable absorber 2 are integrated on one substrate 5, and two current applying electrodes 4a and 4b are provided on each of them, and currents are independently injected. If a high current is injected into one current application electrode 4a and a low current is injected into the other current application electrode 4b, the former can be an optical amplification region and the latter can be a saturable absorption region. Can be realized on the same substrate.

Also in the second and third embodiments, it is possible to realize the embodiment on the same substrate by using the hybrid integration technology of mounting a semiconductor element on a glass or plastic waveguide. It is.

By using the various embodiments described above, it is possible to compensate for the signal waveform distortion of the semiconductor optical amplifier.

[0024]

As described above, the optical amplifying device according to the present invention can output an optical signal in which the signal waveform distortion in the semiconductor optical amplifier has been compensated, and constitute a good optical communication system. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of an optical amplifying device according to the present invention.

FIG. 2 is an experimental result of input intensity versus transmittance of a semiconductor saturable absorber.

FIG. 3 is a pulse output waveform of a semiconductor saturable absorber.

FIG. 4 is a pulse output waveform in a configuration in which a semiconductor saturable absorber is provided at a stage subsequent to a semiconductor optical amplifier.

FIG. 5 is a diagram showing a second embodiment of the optical amplifying device according to the present invention.

FIG. 6 is a diagram showing a third embodiment of the optical amplifying device according to the present invention.

FIG. 7 is an example in which a semiconductor optical amplifier and a semiconductor saturable absorber are formed on the same substrate.

FIG. 8 is a pulse output waveform in a conventional semiconductor optical amplifier.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor optical amplifier 2 Semiconductor saturable absorber 3 Optical output control means 3a Control circuit 3b Photodetector 3c Optical branching mirror 3d Control circuit 3e Voltage monitor between terminals 4a Current application electrode 4b Current application electrode 5 Substrate

Claims (5)

[Claims] 1. A semiconductor optical amplifier comprising: a semiconductor optical amplifier; a saturable absorber to which output light of the semiconductor optical amplifier is input; and optical output control means for controlling output light from the saturable absorber. Characteristic optical amplifier. 2. The saturable absorber has the same structure as that of the semiconductor optical amplifier. A current injected into an active layer of the saturable absorber is supplied to the optical output control means by using a medium as a loss medium. 2. The optical amplifying device according to claim 1, further comprising means for controlling and setting the value to act on. 3. The saturable absorber has the same structure as that of the semiconductor optical amplifier, and the optical output control means supplies an injection current to the saturable absorber before the saturable absorber. 2. The optical amplifying device according to claim 1, further comprising means for controlling according to the intensity of the input light to the semiconductor optical amplifier. 4. The saturable absorber has the same structure as the semiconductor optical amplifier, and the light output control means supplies an injection current to the saturable absorber before the saturable absorber. 2. The optical amplifying device according to claim 1, further comprising means for controlling according to a voltage between terminals of an active layer of the semiconductor optical amplifier. 5. The optical amplifying device according to claim 1, wherein said semiconductor optical amplifier and said saturable absorber are integrated on the same substrate.