JP2000077771A - Semiconductor optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain laser oscillators from starting in wavelength of signal light by a method wherein the laser is equipped with a variable optical attenuating mechanism that is capable of controlling laser oscillation light in resonator loss, while keeping signal light constant in effective transmission loss. SOLUTION: A laser resonator is composed of two reflecting mirrors, where at least a wavelength selective reflecting mirror 4 is made to serve as one of the two reflecting mirrors, and a variable light attenuator 3 is provided as a variable optical attenuating mechanism in the laser resonator. As mentioned above, a laser resonator is composed of two reflecting mirrors, where at least the wavelength selective reflecting mirror 4 is made to serve as one of the two reflecting mirrors, whereby a feedback mechanism functions only for a laser oscillation wavelength, so that laser oscillation can be restrained from starting in wavelength of signal light 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical amplifier, and more particularly to a gain-clamped semiconductor having a wide wavelength band characterized by a mechanism for adjusting an allowable incident power used in a wavelength division multiplexing (WDM) communication system. The present invention relates to an optical amplifier.

[0002]

2. Description of the Related Art In recent years, introduction of a wavelength division multiplexing communication system in which a plurality of signal lights having different wavelengths are multiplexed and transmitted simultaneously through one optical fiber has been studied in response to a dramatic increase in communication demand. A wide wavelength band is required for an optical amplifier used for a wavelength division multiplexing communication system.

As an optical amplifying device having such a wide wavelength band, a semiconductor optical amplifying device is known, and since this semiconductor optical amplifying device is small in size and can operate with low power consumption, it is used for a wavelength division multiplexing communication system. Is promising as an optical amplifying device.

In general, in an optical amplifying device, carriers are consumed in the process of amplifying a signal light. Therefore, when the power of the signal light increases, the carrier decrease due to stimulated emission becomes remarkable, and as a result, the gain decreases. The phenomenon of saturation occurs.

In the above-described semiconductor optical amplifier, since the carrier lifetime is as short as several hundred picoseconds, the gain saturation can follow the fluctuation of the power of the signal light due to the modulation, so that the gain modulation may occur and the signal light may be distorted. In particular, in a situation where a large number of signal lights are simultaneously amplified in the semiconductor optical amplifier,
Crosstalk may occur due to mutual gain modulation between signal lights.

In order to suppress such gain modulation, it is necessary to add a mechanism for maintaining a constant carrier density to the semiconductor optical amplifying device regardless of a change in the power of the signal light. This is a method in which an oscillation mechanism is introduced into a semiconductor optical amplifier, and a phenomenon in which a carrier density is locked to an oscillation threshold carrier density is used.

In this case, as long as the power of the signal light is smaller than the power of the laser oscillation light and the laser oscillation continues, the gain of the semiconductor optical amplifier is clamped at a constant value. In this gain clamp type semiconductor optical amplifier,
In order to form a traveling wave optical amplifier without resonating with the signal light, the wavelength of the laser oscillation light is set apart from the wavelength of the signal light, and a high reflectance is obtained only for the wavelength of the laser oscillation light. It is widely used to use a reflecting mirror having a certain wavelength selectivity.

For example, a diffraction grating is used as an external reflecting mirror having wavelength selectivity (if necessary, JCS
imon et al, Electronics Le
ters, vol. 30, pp. 49-50,199
4) or using a fiber grating (fiber grating) (if necessary, L. Lablo).
nde et al, ECOC'94, processed
ings, pp. 715-718, 1994).

As an example of using an internal reflecting mirror having wavelength selectivity, a monolithic DFB (distributed feedback type) is used.
Using a laser (if necessary, P. Doussie
reet al, International Sem
Icon Laser Conferen
ce, proceedings, pp. 185-18
6, 1994) or using a DBR (distributed Bragg reflection type) laser (if necessary, LF Ti
emijer et al, IEEE Photon
ics Technology Letters, p
p. 284-286, 1995).

[0010] Further, as another method of constructing the gain-clamped semiconductor optical amplifying device, in a region other than the semiconductor optical amplifying medium, two pairs of laser oscillation light and signal light are spatially separated. Type 2 optical splitter, ie, 2 ×
A semiconductor optical amplifying medium is provided on both arms of a Mach-Zehnder interferometer composed of a type 2 optical multiplexer / demultiplexer, and one of an input port of the input optical multiplexer / demultiplexer and an output optical multiplexer / demultiplexer located at a cross position thereof. It has also been proposed to arrange a reflecting mirror made of a multilayer dielectric film at an output port to form a laser resonator to provide a gain-clamped semiconductor optical amplifier (if necessary, Ch Holtsmann et a).
1, ECOC '96, proceedings, pp.
199-202, 1996).

[0011]

In the gain-clamped semiconductor optical amplifier as described above, the resonator loss is an important parameter that determines the gain and the allowable incident power of the signal light. It will be described with reference to FIG. FIG. 18A is a diagram showing the dependence of the gain-optical output characteristic on the resonator loss in the semiconductor optical amplifying device.
2 (= ₁₀ log ₁₀ 2 ≒ 3.01 ≒ 3 dB) is also shown. The amplifier ₁ is a non-gain-clamp type ordinary semiconductor optical amplifying device. As the optical output P increases, the gain G gradually decreases and the optical output P
_{Although s1} is the saturation light output power, in the gain-clamped semiconductor optical amplifier, the gain is clamped to a constant value until laser oscillation stops.

Therefore, a laser having a large cavity loss_Two
In the case of
Great value G_TwoAt the injection current
Since the power of the laser oscillation light becomes smaller, the signal light
, The allowable incident power becomes lower. On the other hand, small resonator loss
Laser_ThreeIn the case of, the oscillation threshold carrier density is small.
Therefore, the gain is small G_Three(<G_Two) Clamped in
However, the power of the laser oscillation light for a certain injection current is large.
The incident power of signal light increases,
For example, looking at the saturation light output power, the laser_Two→ Laser
_ThreeThe gain is G _Two→ G_Three, The saturation light
Output power is P_S2→ P_S3And therefore the resonator
Since it can be properly amplified by reducing the loss,
Can increase the allowable input power of the signal light
Now, this situation will be described with reference to FIGS.
I do.

FIGS. 18 (b) and 18 (c) FIGS. 18 (b) and 18 (c) are diagrams showing light output-current characteristics (LI characteristics) of the lasers ₂ and ₃ , respectively. For a relatively large laser ₂ , the threshold current I
_th-2 becomes larger than the threshold current I _{th-3 of the} laser ₃ ,
As a result, the optical output when a certain current flows is larger for the laser ₃ having a relatively small cavity loss,
The allowable input power of signal light that can be properly amplified can be increased.

In the case of using such a gain-clamped semiconductor optical amplifying device whose allowable input power has resonator loss dependence in various applications, the gain-clamped semiconductor optical amplifying device depends on the application. However, in order to optimize the characteristics, it is desirable that the above-described resonator loss can be continuously and arbitrarily controlled.

However, in the conventional monolithic semiconductor optical amplifier, there is a problem that the resonator loss cannot be made variable depending on the application since the resonator loss is fixed.

On the other hand, in an external resonator type semiconductor optical amplifying device, the resonator loss can be changed by replacing the diffraction grating or the fiber diffraction grating constituting the external resonator, but these are continuously changed. It is difficult to optimize, and when modularized, there is a problem that the resonator loss is fixed as in the case of the monolithic type.

Accordingly, it is an object of the present invention to continuously control the resonator loss of a gain-clamped semiconductor optical amplifier and optimize the characteristics of the gain-clamped semiconductor optical amplifier according to the application.

It is another object of the present invention to provide a gain-clamped semiconductor optical amplifier having a simple structure, having excellent characteristics of separating signal light and laser light and having high saturation light input / output characteristics. I do.

[0019]

FIG. 1 is an explanatory view of the principle configuration of the present invention. Referring to FIG. 1, means for solving the problems in the present invention will be described. FIG. 1 is a conceptual configuration diagram of a semiconductor optical amplifier of the present invention. FIG. 1 (1) The present invention provides a gain-clamped semiconductor optical amplifier 1 composed of a semiconductor optical amplifier 2 and a laser resonator including the semiconductor optical amplifier 2 therein. The present invention is characterized by having a variable optical attenuation mechanism capable of controlling the amount of resonator loss with respect to laser oscillation light while maintaining the transmission loss at a constant value.

As described above, in the laser resonator, a variable optical attenuation mechanism capable of controlling the resonator loss amount with respect to the laser oscillation light while maintaining the effective transmission loss amount with respect to the signal light 7 at a constant value, that is, By providing the variable optical attenuator 3, the resonator loss can be continuously and arbitrarily controlled,
Thus, the characteristics of the gain-clamped semiconductor optical amplifier 1 can be optimized according to the application. In the specification of the present application, "semiconductor optical amplifier" and "variable optical attenuator" mean a single "semiconductor optical amplifier" and "variable optical attenuator", and electrodes and the like are provided on a predetermined semiconductor region. It also means a semiconductor light amplifying region or a variable light attenuating region provided with a light amplifying function or a light attenuating function.

(2) According to the present invention, in the above (1), at least one of the laser resonators is constituted by a reflecting mirror 4 having wavelength selectivity, and a variable optical attenuation mechanism is provided in the laser resonator. The variable optical attenuator 3 is provided.

Thus, a feedback mechanism is formed only for the laser oscillation wavelength by configuring the laser resonator with the reflecting mirror 4 having at least one of which has wavelength selectivity. Oscillation can be suppressed.

(3) In the present invention, in the above (2), a portion through which the signal light 7 does not pass is provided in the laser resonator, and the variable optical attenuator 3 is arranged in a portion through which the signal light 7 does not pass. It is characterized by the following.

As described above, a portion through which the signal light 7 does not pass is provided in the laser resonator, and the variable optical attenuator 3 is disposed in a portion through which the signal light 7 does not pass. Since there is no need to consider the attenuation by the variable optical attenuator 3, the degree of freedom of the variable optical attenuator 3 that can be used can be increased.

(4) According to the present invention, in the above (3), the laser resonator has an external resonator structure consisting of a reflecting mirror 4 having wavelength selectivity and a reflecting mirror 5 having no wavelength selectivity. The beam splitter 6 is inserted between the reflecting mirror 4 having the laser beam and the semiconductor optical amplifying device 1, and the signal light 7 is made incident on the laser resonator from the direction perpendicular to the laser resonator via the beam splitter 6 to provide wavelength-selective reflection. It is characterized in that a portion where the signal light 7 does not pass between the mirror 4 and the beam splitter 6 is provided.

By adopting such a configuration, the signal light 7 whose intensity is reduced by half by the beam splitter 6 is incident on the semiconductor optical amplifier 2, so that even when the signal light 7 is large, gain saturation hardly occurs. The upper limit of the allowable incident power of the signal light 7 can be doubled.

(5) According to the present invention, in the above (3), the laser resonator has an external resonator structure consisting of a reflecting mirror 4 having wavelength selectivity and a reflecting mirror 5 having no wavelength selectivity. The beam splitter 6 is inserted between the reflecting mirror 4 having the laser beam and the semiconductor optical amplifier 1, and the signal light 7 is emitted from the direction perpendicular to the laser resonator through the beam splitter 6 to provide wavelength-selective reflection. It is characterized in that a portion where the signal light 7 does not pass between the mirror 4 and the beam splitter 6 is provided.

By adopting such a configuration, the signal light 7 is not lost on the entrance side and the semiconductor optical amplifier 2 has no loss.
Therefore, even when the signal light 7 is weak, an appropriate amplified signal can be obtained, and the lower limit of the allowable incident power can be halved.

(6) According to the present invention, in the above (3), the laser resonator has an external resonator structure comprising a pair of wavelength-selective reflecting mirrors 4, and the semiconductor optical amplifier 2 and both wavelength-selective mirrors are provided. A beam splitter 6 is inserted between the reflecting mirrors 4 and the signal mirror 7 through the beam splitter 6, and the signal light 7 is emitted and emitted from a direction perpendicular to the laser resonator. The two regions between the beam splitter 6 and the beam splitter 6 are portions through which the signal light 7 does not pass.

By adopting such a configuration, although the loss of the signal light 7 is increased, it is not necessary to use the input / output end face of the optical fiber as a part of the reflecting mirror. Return light of the signal light 7 due to undesired reflection can be prevented.

(7) According to the present invention, in the above (2), a portion where the signal light 7 does not pass is not provided in the laser resonator, and the variable optical attenuator 3 is used for optically attenuating the laser oscillation wavelength. The variable optical attenuator 3 is provided, which can change the amount but does not attenuate the signal light 7 effectively.

By not providing a portion through which the signal light 7 does not pass in the laser resonator, the loss by the beam splitter 6 is eliminated, so that the signal light 7 can be used efficiently and the number of parts can be reduced. Although the signal light 7 can be reduced, since the signal light 7 passes through the variable optical attenuator 3, it is necessary to consider the attenuation characteristics of the variable optical attenuator 3 so that the signal light 7 is not effectively attenuated. In the present application, "the signal light 7 is not effectively attenuated" means that no attenuation is intended, and even if there is an inevitable attenuation, it is not regarded as being attenuated. .

(8) According to the present invention, in the above (7), the laser oscillation wavelength is set to be shorter than the wavelength of the signal light 7 by the wavelength-selective reflecting mirror 4, and the variable optical attenuator 3 is used. An electroabsorption optical modulator having an absorption edge wavelength on the shorter wavelength side than the laser oscillation wavelength is used.

As described above, the signal light 7 is not effectively attenuated,
In addition, as the variable optical attenuator 3 for selectively attenuating only the laser oscillation light, an electroabsorption optical modulator having an absorption edge wavelength shorter than the laser oscillation wavelength is preferable.

(9) The present invention is characterized in that, in the above (8), the semiconductor optical amplifier 2 and the variable optical attenuator 3 comprising an electro-absorption type optical modulator are monolithically integrated.

An electro-absorption type optical modulator uses a semiconductor
Since it can be formed in an in-structure, it is suitable for monolithic integration. By monolithically integrating, the alignment between the semiconductor optical amplifier 2 and the variable optical attenuator 3 becomes unnecessary, and a lens becomes unnecessary. Therefore, the number of parts can be reduced, and the entire device can be downsized.

(10) Further, according to the present invention, in any one of the above (7) to (9), the reflecting mirror 4 having wavelength selectivity can be used.
Is characterized by using a fiber grating.

As described above, when a portion through which the signal light 7 does not pass is not provided in the laser resonator, a wavelength grating can be used as the reflecting mirror 4 having wavelength selectivity, that is, a fiber diffraction grating can be used. It is possible to simplify a laser resonator structure having a characteristic.

(11) According to the present invention, in the above (9), a distributed Bragg reflector is used as the wavelength-selective reflecting mirror 4, and the distributed Gragg reflector is combined with the semiconductor optical amplifier 2 and the electroabsorption type optical modulator. It is characterized in that it is monolithically integrated with the variable optical attenuator 3 composed of a device.

As described above, when a distributed Bragg reflector (DBR) is used as the wavelength-selective reflecting mirror 4, this distributed Gragg reflector can be changed by the semiconductor optical amplifier 2 and the electroabsorption type optical modulator. By monolithically integrating the optical attenuator 3, the main components of the device can be formed only by the semiconductor manufacturing process.

(12) Further, according to the present invention, in the above (1), the semiconductor optical amplifier 2 is provided on two arms of a Mach-Zehnder interferometer composed of two 2 × 2 type optical couplers and a laser resonator. Is constituted by one of the input ports of the optical multiplexer / demultiplexer on the input side and a reflecting mirror arranged at the output port of the optical multiplexer / demultiplexer on the output side which is located at a cross position with respect to the input port. The attenuator is provided between the optical coupler and the reflecting mirror in the laser resonator.

As described above, by using the Mach-Zehnder interferometer composed of two 2 × 2 optical couplers, signal light can be amplified without loss of signal light due to the use of a beam splitter or the like. Is simplified, and the laser oscillation light and the signal light are spatially separated in regions other than the semiconductor optical amplifier. Therefore, the signal light can be transmitted to the laser resonator without giving the laser resonator wavelength selectivity. No resonance occurs.

(13) The present invention is characterized in that, in the above (12), a directional coupler is used as the 2 × 2 type optical coupler / branch.

As described above, the Mach-Zehnder interferometer can be miniaturized by using the directional coupler as the 2 × 2 type optical multiplexer / demultiplexer.

(14) The present invention is characterized in that, in the above (12), a multi-mode interferometer is used as the 2 × 2 optical multiplexer / demultiplexer.

As described above, by using the multi-mode coupler as the 2 × 2 type optical multiplexer / demultiplexer, the manufacturing tolerance can be improved as compared with the case where the directional coupler is used as the 2 × 2 type optical multiplexer / demultiplexer.

(15) The present invention is characterized in that, in the above (12) to (14), at least one of the reflecting mirrors is constituted by a reflecting mirror having wavelength selectivity.

As described above, by forming at least one of the reflecting mirrors constituting the laser resonator with a reflecting mirror having wavelength selectivity, the laser oscillation wavelength is stabilized, and the wavelength of the signal light and the laser oscillation wavelength are reduced. Can be maintained stably, whereby the nonlinear effect due to the undesired interaction between the signal light and the laser light, for example, the phase conjugate wave due to the four-wave mixing does not occur, and the signal is amplified. Only the output signal light can be output from the output port.

(16) The present invention is characterized in that, in the above (12) to (15), an electroabsorption type optical modulator is used as the variable optical attenuator.

As described above, when the Mach-Zehnder interferometer is used, since the signal light does not pass through the variable optical attenuator, it is not always necessary to use an electro-absorption optical modulator as the variable optical attenuator. An electro-absorption type optical modulator may be used. In particular, when monolithically integrated, it is preferable to use an electro-absorption type optical modulator which can be formed by a pin structure.

(17) Further, according to the present invention, in the above (12) to (16), the semiconductor optical amplifier, two 2 × 2 optical couplers, the variable optical attenuator 3 and the reflecting mirror are monolithically integrated. It is characterized by having

Thus, the semiconductor optical amplifier 2 and the two 2 ×
By monolithically integrating the type 2 optical multiplexer / demultiplexer, the variable optical attenuator, and the reflector, the overall configuration is reduced, and the lens system between the 2 × 2 type optical multiplexer / demultiplexer and the variable optical attenuator. Becomes unnecessary.

(18) Further, according to the present invention, in a gain-clamped semiconductor optical amplifier 1 comprising a semiconductor optical amplifier 2 and a laser resonator including the semiconductor optical amplifier 2, the semiconductor optical amplifier 2 An optical waveguide connected to one input / output port of the Sagnac type optical interferometer, and a reflector is arranged on the optical waveguide connected to one input / output port of the Sagnac type optical interferometer to form a laser resonator. Are input / output optical waveguides for the signal light 7.

As described above, when a Sagnac-type optical interferometer is used, only one semiconductor optical amplifier 2 is required. Therefore, a symmetrical gain-clamped semiconductor optical amplifier using a pair of semiconductor optical amplifiers 2 is used. Since the requirement regarding the symmetrical operation required for the semiconductor optical amplifier 2 is not required as compared with the device 1, the possibility that the laser oscillation light and the signal light 7 are mixed is further reduced,
A stable optical amplification operation becomes possible.

(19) The present invention is characterized in that, in the above (18), a reflecting mirror having wavelength selectivity is used as the reflecting mirror.

When a Sagnac type optical interferometer is used,
Since the laser oscillation light and the signal light 7 are spatially separated,
Even if the laser resonator does not have wavelength selectivity, resonance of the signal light 7 does not occur in the laser resonator, but the laser oscillation wavelength is stabilized to stabilize the difference between the wavelength of the signal light 7 and the laser oscillation wavelength. In order to maintain it, as a reflector,
It is desirable to use a reflecting mirror having wavelength selectivity, so that non-linear effects due to undesired interaction between the signal light 7 and the laser light, such as a phase conjugate wave due to four-wave mixing, do not occur. Only the amplified signal light 7 can be output from the other input / output port connected to the input / output optical waveguide.

(20) Further, in the present invention, in the above (18) or (19), in the Sagnac type optical interferometer, the optical waveguide section and the optical coupler section other than the semiconductor optical amplifier 2 are constituted by optical fibers. Features.

As described above, by configuring the optical waveguide and optical coupler other than the semiconductor optical amplifier 2 in the Sagnac type optical interferometer with the optical fiber, the gain clamp type semiconductor is not required without special fine processing technology. The optical amplifying device 1 can be easily configured.

(21) In the present invention according to (18) or (19), in the Sagnac-type optical interferometer, the optical waveguide and optical coupler other than the semiconductor optical amplifier 2 are formed by a planar dielectric optical circuit. It is characterized by comprising.

As described above, in the Sagnac-type optical interferometer, the optical waveguide section and the optical coupler section other than the semiconductor optical amplifier 2 are constituted by the planar dielectric optical circuit (PLC), so that an optical fiber is not used. The gain-clamped semiconductor optical amplifier 1 can be downsized.

(22) In the present invention according to (18) or (19), the optical waveguide, the optical coupler, the arm, and the semiconductor optical amplifier 2 constituting the Sagnac-type optical interferometer are monolithically formed of a semiconductor. It is characterized by being integrated.

As described above, the optical waveguide, the optical coupler, the arm, and the semiconductor optical amplifier 2 constituting the Sagnac-type optical interferometer are monolithically integrated with a semiconductor, so that the gain-clamped semiconductor optical amplifier 1 Can be further miniaturized, and the hybrid mounting of the semiconductor optical amplifier 2 becomes unnecessary, so that the problems associated with the accuracy such as the optical axis alignment between the semiconductor optical amplifier 2 and the arm can be solved.

(23) The present invention is characterized in that, in the above (22), the optical waveguide, the optical coupler and the arm constituting the Sagnac type optical interferometer are also semiconductor optical amplification regions.

As described above, the optical waveguide, the optical coupler, and the arm constituting the Sagnac-type optical interferometer are also semiconductor optical amplification regions, so that the laser oscillation light and the laser light in the optical waveguide, the optical coupler, and the arm can be obtained. Signal light 7
Can be prevented, and the manufacturing process is greatly simplified.

(24) The present invention is characterized in that, in any one of the above (18) to (22), the variable optical attenuator 3 is arranged in the optical waveguide constituting the laser resonator.

As described above, even when the Sagnac type optical interferometer is used, the loss of the resonator can be continuously and arbitrarily controlled by disposing the variable optical attenuator 3 in the optical waveguide constituting the laser resonator. Accordingly, the characteristics of the gain-clamped semiconductor optical amplifier 1 can be optimized according to the application.

[0067]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, first to sixth embodiments of the present invention will be described with reference to FIGS.
First, a first embodiment of the present invention will be described with reference to FIG. FIG. 2 is a conceptual configuration diagram of the gain-clamped semiconductor optical amplifier according to the first embodiment of the present invention, in which an optical fiber 12 for inputting an optical input 11 which is a signal light, an optical fiber Lens 1 that converts light input 11 from 12 into a parallel light flux
3. Transmit half of the converted light input 11 and the remaining 1
/ 2, a lens 15 for converging the reflected optical input 11, a semiconductor optical amplifier 16 for amplifying the converged optical input 11, and an amplified and expanded light in the semiconductor optical amplifier 16. A pair of lenses 17 and 1 that converge the light input 11 and guide it to the optical fiber 19
8 and the optical fiber 19 constitute an optical amplification system for signal light, and the amplified signal light is emitted from the optical fiber 19 as an optical output 20.

The gain-clamped semiconductor optical amplifying device includes a variable optical attenuator 21 and a diffraction grating 22, and a semiconductor optical device is formed by a cleavage end face formed by cleaving the incident side of the diffraction grating 22 and the optical fiber 19. A laser resonator having wavelength selectivity with respect to the amplifier 16 is formed, and a variable optical attenuator 21 is disposed at a portion where the signal light does not pass between the beam splitter 14 and the diffraction grating 22. Control.

The “portion where the signal light does not pass” in this case means a region where the optical input 11 effectively used as the signal light does not pass, and the optical input 11 is a cleaved end face of the optical fiber 19. The reflected light passes through the variable optical attenuator 21 and reaches the diffraction grating 22.
2 has wavelength selectivity, the reflected light input 11 does not go back to the semiconductor optical amplifier side as signal light again, so that the light input 11 effectively used as such signal light is used.
Are defined as "portions through which signal light does not pass".

In this case, the semiconductor optical amplifier 16
An anti-reflection coating (AR coating) made of a multilayer dielectric film is used so that reflection does not occur at the input / output end face, and the cross-sectional shape perpendicular to the optical axis of the active layer is square or Strain MQW with tensile strain
It is desirable to use a (multiple quantum well) structure so that the gain does not have polarization dependence.

The semiconductor optical amplifier 16 has an oscillation wavelength of 1.5 μm band (1.5 μm in consideration of the material and structure of the active layer) from the viewpoint of attenuation characteristics in an optical fiber used for long-distance optical communication. (Approximately 1.6 μm), and the inclination of the diffraction grating 22 with respect to the optical axis is controlled so as to form a laser resonator only for a certain set oscillation wavelength in the 1.5 μm band.

As the variable optical attenuator 21, various commercially available variable optical attenuators are used. Since the optical input 11 effectively used as signal light does not pass through the variable optical attenuator 21, the variable optical attenuator 21 is used. As an attenuation characteristic of the attenuator 21, a variable optical attenuator having any configuration may be used as long as it can attenuate light having a laser oscillation wavelength, for example, a wavelength of 1.51 μm. It is arbitrarily controlled.

In the first embodiment, before amplification by the semiconductor amplifier 16, the beam splitter 14
The signal light is attenuated by half by the semiconductor amplifier 16.
, The effective saturation input power of the system as viewed from the optical fiber 12 increases, and the upper limit of the allowable input power of the optical input 11 as signal light at the stage of entering the optical fiber 12 can be doubled. it can. Therefore, this configuration is suitable when the power of the signal light is large.

Since the external resonator constituted by the diffraction grating 22 and the cleavage end face of the optical fiber 19 has a wavelength dependence depending on the tilt angle of the diffraction grating 22, the active layer of the semiconductor optical amplifier 16 has Acts as a laser resonator only at the wavelength specified by the material and structure, and does not generate laser oscillation at the wavelength of the signal light, so the carrier density is locked to the oscillation threshold carrier density determined by the resonator loss. So that the gain can be clamped to a predetermined value.

Further, in the first embodiment,
Since the variable optical attenuator 21 is provided in the external resonator, the resonator loss can be continuously controlled to an arbitrary value by the attenuation amount of the variable optical attenuator 21. Can be set to optimal values according to various applications.

Next, a second embodiment of the present invention will be described with reference to FIG. 3. In the second embodiment, the input side and the output side of the first embodiment are described. The other configuration is exactly the same as that of the first embodiment. FIG. 3 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a second embodiment of the present invention, in which an optical input 11 serving as signal light parallel to the optical axis of a semiconductor optical amplifier 16 is connected. Optical fiber 19 for input, light input 1 from optical fiber 19
A pair of lenses 18 and 17 for converging 1; a semiconductor optical amplifier 16 for amplifying the converged optical input 11; and a lens for converting the optical input 11 amplified and spread out in the semiconductor optical amplifier 16 to a parallel light flux. 15, a beam splitter 14 that transmits one half of the converted light input 11 and vertically reflects the remaining half, and the reflected light input 11
The optical fiber 12 and the lens 13 that converges the optical signal 12 and the optical fiber 12 constitute an amplification system for signal light. The amplified signal light is emitted from the optical fiber 12 as an optical output 20.

This gain-clamped semiconductor optical amplifying device also includes a variable optical attenuator 21 and a diffraction grating 22, and the semiconductor optical device is formed by a cleavage end face formed by cleaving the emission side of the diffraction grating 22 and the optical fiber 19. A laser resonator having wavelength selectivity with respect to the amplifier 16 is formed, and a variable optical attenuator 21 is disposed at a portion where the signal light does not pass between the beam splitter 14 and the diffraction grating 22. Control.

In this case, half of the amplified optical input 11 also passes through the region between the beam splitter 14 and the diffraction grating 22, but since the diffraction grating 22 has wavelength selectivity, this amplification is performed. Since the 1/2 optical input 11 thus reflected is not effectively used again as a signal light, the region between the beam splitter 14 and the diffraction grating 22 is also referred to as "a" in the second embodiment. Part where signal light does not pass "
It is defined.

In the second embodiment, the signal light is amplified by the semiconductor amplifier 16 and then attenuated by the beam splitter 14 to 1/2.
Cannot increase the allowable incident power of
Since the signal is amplified before being attenuated to 2, it can be properly amplified even when the optical input 11 as the signal light is weak.

Also, in the second embodiment,
Since the variable optical attenuator 21 is provided in the external resonator, the resonator loss can be continuously controlled to an arbitrary value by the attenuation of the variable optical attenuator 21, and the characteristics of the semiconductor optical amplifier can be improved. The optimum value can be set according to various uses.

Also in this case, the variable optical attenuator 21
Since the signal light is arranged between the beam splitter 14 and the diffraction grating 22 at a portion where the signal light does not pass, the degree of freedom of the attenuation characteristic of the variable optical attenuator 21 that can be used can be increased.

Next, a third embodiment of the present invention will be described with reference to FIG. 4. In the third embodiment, the optical input 11 enters through the beam splitter 14, and The optical output 20 is extracted through a beam splitter 23, and the other configuration is exactly the same as that of the first embodiment. FIG. 4 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a third embodiment of the present invention, in which an optical fiber 12 for inputting an optical input 11 as a signal light, an optical fiber Lens 1 that converts light input 11 from 12 into a parallel light flux
3. Transmit half of the converted light input 11 and the remaining 1
/ 2, a lens 15 for converging the reflected optical input 11, a semiconductor optical amplifier 16 for amplifying the converged optical input 11, and an amplified and expanded light in the semiconductor optical amplifier 16. A lens 17 for converting the light input 11 into a parallel light beam, the converted light input 11
, A pair of lenses 24 that converge the reflected light input 11 and guide it to the optical fiber 19, and an optical fiber 19.
Thus, a signal light amplification system is configured, and the amplified signal light is emitted from the optical fiber 19 as an optical output 20.

In the third embodiment, a pair of diffraction gratings 22 and 25 and a variable optical attenuator 21 are provided, and the pair of diffraction gratings 22 and 25 constitute an external resonator having wavelength selectivity. In addition, the variable optical attenuator 21 is disposed in a portion where the signal light does not pass between the beam splitter 23 and the diffraction grating 22, and the cavity loss of the laser cavity is arbitrarily and continuously controlled according to the application. Can be.

Further, half of the signal light before and after amplification is lost in the beam splitters 14 and 23, so that the loss is large. However, since the optical fiber 12 or the optical fiber 19 is not used as a component of the external resonator, it is not used. Since it is not necessary to set the input end face or the output end face of the optical fiber 12 or the optical fiber 19 serving as the input / output end as the cleavage end face, and therefore, it can be made a non-reflection surface or a low reflection surface.
The reflection loss at the input / output end is reduced.

Also, in the third embodiment,
Since the variable optical attenuator 21 is arranged in a portion between the beam splitter 23 and the diffraction grating 22 where the signal light does not pass, the degree of freedom of the attenuation characteristic of the variable optical attenuator 21 that can be used can be increased. . The variable optical attenuator 21
May be arranged in a portion where the signal light does not pass between the beam splitter 14 and the diffraction grating 25 on the opposite side.

Next, a fourth embodiment of the present invention will be described with reference to FIG. 5. In the fourth embodiment, signal light is input and output without using a beam splitter. Therefore, there is no portion in the laser resonator through which the signal light does not pass. FIG. 5 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifying device according to a fourth embodiment of the present invention. A fiber grating provided with a diffraction grating for forming a resonator,
That is, the fiber diffraction grating 26, the pair of lenses 13 and 15 for focusing the light input 11 from the fiber diffraction grating 26, the focused light input 11 is transmitted without attenuating, and the laser oscillation light is arbitrarily attenuated. A variable optical attenuator 21 and a pair of lenses 1 for focusing the optical input 11 transmitted through the variable optical attenuator 21
8, 17; a semiconductor optical amplifier 16 for amplifying the focused optical input 11; and an optical resonator 11 which is amplified and expanded and output in the semiconductor optical amplifier 16 to form an external resonator on the incident end side. A pair of lenses 27 and 28 for guiding to a fiber diffraction grating 29 provided with a diffraction grating for
The amplified signal light constituted by the fiber diffraction grating 29 is emitted from the fiber diffraction grating 29 as an optical output 20.

In this case, the external resonator is formed between a pair of fiber diffraction gratings 26 and 29 which are provided with a refractive index distribution on the incident end face side or the output end face side to form a diffraction grating. Is not used, the structure is simplified. However, since the wavelength dependence of the external resonator is determined by the period of the diffraction gratings provided in the fiber diffraction gratings 26 and 29, the fiber diffraction gratings 26 and 29 are not exchanged. As long as it is fixed. However, there is usually no problem because the laser oscillation wavelength of the semiconductor optical amplifier 16 is determined at the time of element design. It should be noted that one of the fiber gratings 26 and 29 is replaced with a normal optical fiber having an end face cleaved,
A combination of a fiber diffraction grating and a normal optical fiber may be used.

In this case, since the variable optical attenuator 21 is provided in a portion where the signal light passes, the variable optical attenuator 2
It is necessary to accurately set the attenuation characteristic of the variable optical attenuator 21 so that the signal light is not attenuated in 1. For example,
When an electro-absorption optical modulator having a pin structure is used as such a variable optical attenuator 21 and the absorption edge wavelength of the electro-absorption optical modulator is shorter than the laser oscillation wavelength and an electric field is applied. It is necessary to accurately set the absorption edge wavelength to move to the longer wavelength side so that the laser oscillation light is absorbed, and to make the wavelength of the signal light longer than the laser oscillation wavelength so as not to be absorbed even by the application of an electric field. It must be set on the wavelength side.

The configuration of the semiconductor optical amplifier 16 used in the fourth embodiment is exactly the same as that of the first embodiment, and a multi-layer dielectric is used to prevent reflection at the end face. The one having a non-reflective coating made of a film is used, and the active layer has a square cross section perpendicular to the optical axis or a strain MQW having a tensile strain.
It is desirable to use a (multiple quantum well) structure so that the gain does not have polarization dependence. Further, from the viewpoint of attenuation characteristics in an optical fiber used for long-distance optical communication, the material of the active layer It is desirable to set the gain wavelength to be in the 1.5 μm band by considering the structure and the structure.

As described above, since the beam splitter is not used in the fourth embodiment, light loss caused by the beam splitter, that is, transmitted light loss can be eliminated, and a normal diffraction grating can be used. The structure of the apparatus is simplified and miniaturized, and the input / output end faces of the fiber diffraction gratings 26 and 29 do not constitute external resonators. Therefore, the end faces need not be cleaved end faces. Thus, the reflection loss of the signal light and the undesired return light can be prevented by providing the lens function as a non-reflective or low-reflection end face.

Next, a fifth embodiment of the present invention will be described with reference to FIG. 6. In the fifth embodiment, a semiconductor optical amplifier and a variable optical attenuator are monolithically integrated. The other configuration is exactly the same as that of the fourth embodiment. FIG. 6 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifying device according to a fifth embodiment of the present invention. A fiber grating 26 provided with a diffraction grating for forming a resonator, a pair of lenses 13 and 15 for focusing the optical input 11 from the fiber grating 26, and transmitting the focused optical input 11 without attenuation, In addition, a variable optical attenuator that arbitrarily attenuates laser oscillation light, that is, a variable optical attenuation region 33, and a semiconductor optical amplifier that amplifies the optical input 11, that is, a semiconductor optical amplification region 32 are monolithically formed on a semiconductor substrate 31. An integrated monolithic optical semiconductor device 30 and a diffraction grating provided with a diffraction grating for converging the optical input 11 amplified and expanded and emitted in the semiconductor optical amplification region 32 to form an external resonator on the incident end side. A pair of lenses 17 leading to the fiber diffraction grating 29,
18, and a fiber diffraction grating 29,
The amplified signal light is output from the fiber diffraction grating 29 to the optical output 2.
It is emitted as 0. Also in this case, one of the fiber diffraction gratings 26 and 29 may be replaced with a normal optical fiber whose end face is cleaved, and a combination of the fiber diffraction grating and a normal optical fiber may be used.

In this case, the monolithic optical semiconductor device 30
Is, for example, on a semiconductor substrate 31 made of n-type InP,
InGaAsP /
An InP double heterojunction structure is formed, and one region is a semiconductor light amplification region 32 and the other region is a variable light attenuation region 33 composed of a pin structure electroabsorption optical modulator.

For example, in order to set the period of the optical fiber diffraction gratings 26 and 29 so that the laser oscillation wavelength in the semiconductor optical amplification region 32 becomes 1.51 μm, for example, and to eliminate the polarization dependence of the gain. The active layer has a square cross section perpendicular to the optical axis or a strained MQW (multiple quantum well) structure having tensile strain.

On the other hand, in order to prevent the attenuation of the signal light in the variable light attenuation region 33 and the attenuation of the laser oscillation light when no electric field is applied, for example, InG
The composition of the aAsP light absorption layer may be set to 1.48 μm, and the variable light attenuation region 33 may be formed by regrowth in a portion where a part of the semiconductor light amplification region 32 is removed.
The attenuation characteristic of the variable optical attenuation region can be accurately determined at the element design stage. In this case as well, an anti-reflection coating made of a multilayer dielectric film is applied so that reflection does not occur on the input / output end face of the monolithic optical semiconductor device 30.

As described above, since the beam splitter is not used in the fifth embodiment as well, light loss caused by the beam splitter can be eliminated, and the semiconductor optical amplifier and the variable optical attenuator are monolithically integrated. The variable optical attenuator can be downsized,
Since a pair of lenses provided between the semiconductor optical amplifier and the variable optical attenuator becomes unnecessary, the overall configuration of the device can be simplified and downsized.

Also in this case, the fiber diffraction grating 2
Since the input / output end faces 6 and 29 do not constitute an external resonator, they need not be cleavage end faces. Therefore, by making the input / output end faces non-reflective or low-reflective, reflection loss of signal light and undesired reflection are reduced. Return light can be prevented.

Next, a sixth embodiment of the present invention will be described with reference to FIG. 7. In the sixth embodiment, a laser resonator is used for a semiconductor optical amplifier and a variable optical attenuator. The constituent mirrors are monolithically integrated as a DBR (distributed Bragg reflector) region. FIG. 7 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a sixth embodiment of the present invention. A pair of lenses 13 and 15 for converging the light input 11, a variable light attenuating region 33 that transmits the condensed light input 11 without attenuating and arbitrarily attenuates the laser oscillation light, and amplifies the light input 11. Semiconductor light amplification region 32 and semiconductor substrate 3
5, a monolithic optical semiconductor device 3 having a pair of DBR regions 36 and 37 for forming a laser resonator on its input / output end face side.
4. Amplified in the semiconductor optical amplification region 32 and D
A pair of lenses 17, 18 for converging the light input 11 emitted from the BR region 37 and guiding the light input 11 to an optical fiber 19 provided with a diffraction grating for forming an external resonator on the incident end side;
The amplified signal light is constituted by the optical fiber 19 and emitted from the optical fiber 19 as an optical output 20.

In this case, the monolithic optical semiconductor device 34
Is, for example, on a semiconductor substrate 31 made of n-type InP,
InGaAsP /
An InP-based double heterojunction structure is formed, one region is a semiconductor optical amplification region 32, the other region is a variable light attenuation region 33 composed of a pin structure electroabsorption optical modulator, and a periodic The diffraction grating is formed by providing the concave and convex portions to form the DBR regions 36 and 37.

The laser oscillation wavelength in the semiconductor optical amplification region 32 is set to 1.51 μm, for example, so that the DBR
While setting the period of the diffraction grating in the regions 36 and 37,
The active layer has a square cross section perpendicular to the optical axis or a strained MQW (multiple quantum well) structure having tensile strain so that the gain does not have polarization dependence. In order to prevent the signal light from attenuating at the time of and the laser oscillation light when no electric field is applied, the composition of the InGaAsP light absorbing layer constituting the variable light attenuating region 33 is set to 1.48 μm. It is desirable to set.

On the other hand, the DBR regions 36, 3 are controlled so that the laser oscillation light is not absorbed in the DBR regions 36, 37.
7, the composition of the optical waveguide core layer is set to a shorter wavelength side than the laser oscillation wavelength, for example, so as to have a composition of 1.30 μm;
Periodic unevenness may be formed near the optical waveguide core layer.

As described above, since the beam splitter is not used in the sixth embodiment as well, light loss due to the beam splitter can be eliminated, and the semiconductor optical amplifier 16, the variable optical attenuator 21, and the DBR Regions 36 and 3
Since the device 7 is monolithically integrated, the overall configuration of the device including the laser resonator structure can be simplified and reduced in size.

In this case, since the laser resonator is constituted by the pair of DBR regions 36 and 37, there is no need to form an external resonator using a fiber diffraction grating or the like. Normal optical fibers 12 and 19 may be used as the fibers, and the input and output end faces of the optical fibers 12 and 19 are made non-reflective or low-reflective to prevent signal signal reflection loss and unwanted return light. be able to.

Next, gain-clamped semiconductor optical amplifiers using the Mach-Zehnder interferometers according to the seventh to eleventh embodiments of the present invention will be described with reference to FIGS. FIG. 8 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a seventh embodiment of the present invention. A Mach-Zehnder interferometer which is a basic component of the gain-clamped semiconductor optical amplifier is shown. 40 uses two directional couplers 46 and 47 formed on a semiconductor substrate 45 as two 2 × 2 (2 to 2) type optical couplers, and uses two directional couplers 46 and 47.
Semiconductor optical amplifiers 48 and 49
The optical input 41 as signal light input from the converging optical fiber 42 having a lens at the tip to one input port of the directional coupler 46 at the front stage is provided by the directional coupler 46. The light is branched into halves at the coupling section, amplified at the semiconductor optical amplifiers 48 and 49, and then coupled at the coupling section of the directional coupler 47 at the subsequent stage, where the signal light is input to the input port and the cross position. Output port 44 through a focusing optical fiber 43 from the output port of
Is output as

On the other hand, the laser resonator has a high reflection film 50 made of a multilayer dielectric film provided on the cleavage end face of the other output port of the directional coupler 47 at the subsequent stage, and an end face outside the variable optical attenuator 52. Reflection film 53 composed of a multilayer dielectric film provided on the substrate
And a lens pair 51 in the laser cavity.
The variable optical attenuator 52 is inserted through the
Thus, the resonator loss can be arbitrarily and continuously controlled.

In this case, the Mach-Zehnder interferometer 40
Is formed on a semiconductor substrate 45 made of n-type InP, for example,
The active layer has a strained MQW structure having a wavelength composition of 1.55 μm.
A stacked structure for forming the semiconductor optical amplifiers 48 and 49 is formed by stacking nGaAsP / InP-based double heterojunction structures, and after removing a region other than the region for forming the semiconductor optical amplifiers 48 and 49, the removing portion is formed. Then, a double heterojunction structure having an optical waveguide core layer having a wavelength composition of, for example, 1.30 μm is laminated and mesa-etched into a shape as shown in FIG. Devices 46 and 47 and semiconductor optical amplifier 4 interposed therebetween
8, 49 are formed.

In the seventh embodiment, since the laser oscillation light and the signal light are spatially separated in a region other than the amplification region, it is not necessary to make the laser resonator have wavelength dependency. The resonator structure can be simplified because the reflection mirror can be constituted by the cleavage plane itself or the multilayer dielectric film having a high reflectance provided on the cleavage plane, and the oscillation whose carrier density is determined by the resonator loss It can be locked to a threshold carrier density, which allows the gain to be clamped to a predetermined value.

Since the variable optical attenuator 52 is provided in a portion through which the signal light does not pass, the variable optical attenuator 52 can be used because only the attenuation characteristic with respect to the laser oscillation light needs to be considered. The degree of freedom of the optical attenuator 52 is increased, and various types of commercially available optical attenuators can be used.

Further, in the seventh embodiment,
Since no beam splitter is used, light loss due to the beam splitter can be eliminated, and the configuration of the entire apparatus is simplified. Although the converging optical fibers 42 and 43 are used as the input and output optical fibers, the present invention is not limited to the converging optical fibers, and a pair of lenses and a pair of lenses are usually used as in the sixth embodiment. May be used.

Next, an eighth embodiment of the present invention will be described with reference to FIG. 9. In this eighth embodiment, a 3 dB multimode interferometer is used as a 2 × 2 optical multiplexer / demultiplexer. The other configuration is exactly the same as that of the seventh embodiment. FIG. 9 is a conceptual diagram of a gain-clamped semiconductor optical amplifier according to an eighth embodiment of the present invention. The Mach-Zehnder interferometer as a basic component of the gain-clamped semiconductor optical amplifier is shown in FIG. The reference numeral 60 designates two 3 dB multimode interferometers 66 and 67 formed on a semiconductor substrate 65 as two 2 ×
A semiconductor optical amplifier 68, 69 is provided on both of a pair of arms between two multi-mode interferometers 66, 67, and a converging optical fiber 62 having a lens at the tip. The optical input 61 as the signal light input from the input port to one input port of the multi-mode interferometer 66 in the preceding stage is split into halves at the interfering part of the multi-mode interferometer 66 and the semiconductor optical amplifiers 68 and 69 After being amplified, the signal light is coupled in the interference section of the multi-mode interferometer 67 at the subsequent stage, and is output as an optical output 64 from the input port to which the signal light is input and the output port at the cross position via the focusing optical fiber 63.

In this case, the laser resonator also includes a high-reflection film 70 composed of a multilayer dielectric film provided on the cleavage end face of the other output port of the multi-mode interferometer 67 at the subsequent stage, and a portion outside the variable optical attenuator 72. A variable optical attenuator 72 is inserted into the laser resonator via a pair of lenses 71, and a variable optical attenuator 72 is used to form an arbitrary resonator loss. Can be controlled continuously.

The Mach-Zehnder interferometer 60 in this case
Is formed on a semiconductor substrate 65 made of n-type InP, for example,
The active layer has a strained MQW structure having a wavelength composition of 1.55 μm.
An nGaAsP / InP double heterojunction structure is stacked to form a stacked structure for forming the semiconductor optical amplifiers 68 and 69, and after removing a region other than the region for forming the semiconductor optical amplifiers 68 and 69, the removing portion is formed. By stacking a double heterojunction structure having an optical waveguide core layer having a wavelength composition of, for example, 1.30 μm, and performing mesa etching into a shape as shown in FIG. In this embodiment, devices 66 and 67 and semiconductor optical amplifiers 68 and 69 sandwiched therebetween are formed.

In the eighth embodiment, 2 × 2
3 dB multi-mode interferometer 66 as an optical coupler
Since 68 is used, the manufacturing tolerance of the 2 × 2 type optical coupler / branch is larger than that in the case of using the directional couplers 46 and 47 as the 2 × 2 type optical coupler / branch as in the seventh embodiment. That is, the tolerance of the dimensional accuracy can be increased, whereby the semiconductor optical amplifier can be manufactured with a high yield.

Also, in the eighth embodiment,
Since the laser oscillation light and the signal light are spatially separated in regions other than the amplification region, there is no need to provide the laser resonator with wavelength dependence, and therefore the normal cleavage plane itself or the high reflection provided on the cleavage plane Since the reflecting mirror can be composed of a multilayer dielectric film having a high efficiency, the resonator structure is simplified, and further, since a beam splitter is not used, light loss due to the beam splitter can be eliminated. The overall configuration can be further simplified. In this case as well, the converging optical fibers 42 and 43 are used as the input / output optical fibers. However, the present invention is not limited to the converging optical fibers. It is also possible to use a combination of this lens and a normal optical fiber.

Since the variable optical attenuator 72 is provided in a portion through which the signal light does not pass, the variable optical attenuator 72 can be used because only the attenuation characteristic with respect to the laser oscillation light needs to be considered. Various commercially available optical attenuators in which the degree of freedom of the optical attenuator 72 is increased can be used.

Next, a ninth embodiment of the present invention will be described with reference to FIG. 10. In the ninth embodiment, a laser resonator is provided with wavelength selectivity using a fiber diffraction grating. The other configuration is exactly the same as that of the eighth embodiment. FIG. 10 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a ninth embodiment of the present invention. A Mach-Zehnder interferometer which is a basic component of the gain-clamped semiconductor optical amplifier is shown. 60 uses two 3 dB multi-mode interferometers 66 and 67 formed on a semiconductor substrate 65 as two 2 × 2 type optical multiplexer / demultiplexers, just like the eighth embodiment. Semiconductor optical amplifiers 68 and 69 are provided on both of a pair of arms between the multi-mode interferometers 66 and 67. The optical input 61 as the signal light input to the input port of
An input port into which the signal light is input after being split in half at the interference unit of the multi-mode interferometer 66 and amplified by the semiconductor optical amplifiers 68 and 69 and then combined at the interference unit of the multi-mode interferometer 67 at the subsequent stage. And an optical output 6 from the output port at the cross position via the focusing optical fiber 63.
4 is output.

On the other hand, the laser resonator in this case includes a high-reflection film 70 made of a multilayer dielectric film provided on the end face of the other output port of the multi-mode interferometer 67 at the subsequent stage, And a variable optical attenuator 72 is inserted into the resonator via a lens pair 71. The variable optical attenuator 72 allows the resonator loss to be arbitrarily and continuously reduced. Can be controlled.

In the ninth embodiment, since the laser resonator has wavelength selectivity by the fiber diffraction grating 74, the laser oscillation wavelength is stabilized, and undesired nonlinear interaction with the signal light is performed. Does not occur, and stable signal light amplification becomes possible. For example, when the laser oscillation wavelength is unstable and the laser oscillation wavelength transitions to a longer wavelength side than the expected wavelength, the wavelength difference λ ₁ −λ _{2 from} the signal light is reduced, and a phase conjugate wave is generated by four-wave mixing. Since the light is output from the output port together with the signal light as the optical output 64, it is necessary to provide a filter or the like for separating and removing the phase conjugate wave in order to stably perform the optical communication. In the embodiment described above, such a problem does not occur.

Also, in the ninth embodiment,
Since the 3 dB multi-mode interferometers 66 and 67 are used as the 2 × 2 optical multiplexer / demultiplexer, the directional couplers 46 and 67 are used as the 2 × 2 optical multiplexer / demultiplexer as in the seventh embodiment.
The manufacturing tolerance of the 2 × 2 type optical multiplexer / demultiplexer can be increased as compared with the case where 47 is used, whereby the semiconductor optical amplifier can be manufactured with a high yield. The other functions, effects, characteristics, and the like are the same as those in the above-described eighth embodiment.

Next, a tenth embodiment of the present invention will be described with reference to FIG. 11. In the tenth embodiment, the wavelength selection is performed on the laser resonator by using a pair of fiber diffraction gratings. And a variable optical attenuator is monolithically integrated into the Mach-Zehnder interferometer 60 as an electro-absorption optical modulator, and the other configuration is exactly the same as that of the ninth embodiment. It is. FIG. 11 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a tenth embodiment of the present invention. A Mach-Zehnder interferometer which is a basic component of the gain-clamped semiconductor optical amplifier is shown. Reference numeral 60 denotes a case where two 3 × 2 multimode interferometers 66 and 67 formed on a semiconductor substrate 65 are used as two 2 × 2 type optical multiplexer / demultiplexers, similarly to the eighth embodiment. Semiconductor optical amplifiers 68 and 69 are provided on both of a pair of arms between the mode interferometers 66 and 67, and one end of the multi-mode interferometer 66 in the preceding stage is provided from the converging optical fiber 62 provided with a lens at the tip. The optical input 61 as the signal light input to the input port is split into １ ／ at the interference section of the multi-mode interferometer 66 and amplified by the semiconductor optical amplifiers 68 and 69. Are combined at interferometer of MMI coupler 67 of stage, the light output 64 from the output port of the input port and the cross position where the signal light is input through a focused optical fiber 63
Is output as

On the other hand, the laser resonator in this case is composed of an optical fiber diffraction grating 76 provided on the end face side of the other output port of the subsequent multi-mode interferometer 67 and the other input port of the preceding multi-mode interferometer 66. In the tenth embodiment, a variable optical attenuator is used as an electro-absorption optical modulator 75, and a multi-mode interferometer of a preceding stage is provided. 6
6 is provided monolithically and integrally on the other input port side.

In this case, the Mach-Zehnder interferometer 60
Is formed on a semiconductor substrate 65 made of n-type InP, for example,
The active layer has a strained MQW structure having a wavelength composition of 1.55 μm.
An nGaAsP / InP double heterojunction structure is stacked to form a stacked structure for forming the semiconductor optical amplifiers 68 and 69, and after removing a region other than the region for forming the semiconductor optical amplifiers 68 and 69, the removing portion is formed. Then, a double heterojunction structure having an optical waveguide core layer having a wavelength composition of, for example, 1.30 μm is laminated, and then, a part of the laminated structure in the vicinity of the region where the multi-mode interferometer 66 in the previous stage is formed is removed After that, a pin structure for forming an electroabsorption optical modulator composed of an InGaAsP layer having a wavelength composition of 1.48 μm as an absorption layer is laminated, and mesa etching is performed in a shape as shown in FIG. A multi-mode interferometer 66, 67, semiconductor optical amplifiers 68, 69 interposed therebetween, and an electro-absorption optical modulator 75 are formed. May optionally be continuously controlled cavity loss by the electric field absorption type optical modulator 75.

As described above, the size of the variable optical attenuator can be reduced by using the integrated electro-absorption optical modulator 75 as the variable optical attenuator. Since the lens pair 71 with the multimode interferometer 66 is not required, the size of the semiconductor optical amplifier can be further reduced.

In the tenth embodiment, since the laser resonator is constituted by a pair of fiber diffraction gratings 74 and 76, the laser oscillation wavelength is more stable, and the unwanted nonlinearity with the signal light is improved. No interaction occurs, and stable amplification of signal light becomes possible. Other effects, characteristics, and the like are the same as those of the eighth embodiment.

In the tenth embodiment, an electroabsorption optical modulator 75 that can be easily integrated is used as a variable optical attenuator. In this case, a Mach-Zehnder interferometer 60 is used. Therefore, the signal light and the laser oscillation light are spatially separated in a region other than the amplification region. Therefore, no special light absorption characteristics are required for the electroabsorption optical modulator 75. For example,
A current injection type semiconductor light absorbing element or the like may be used.

Next, an eleventh embodiment of the present invention will be described with reference to FIG. 12. In the eleventh embodiment, a DBR which is a reflecting mirror having wavelength selectivity as a laser resonator is used. The region is monolithically integrated with the input / output port side of the multi-mode interferometer, and the other configuration is exactly the same as that of the tenth embodiment. FIG. 12 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to an eleventh embodiment of the present invention. A Mach-Zehnder interferometer which is a basic component of the gain-clamped semiconductor optical amplifier is shown. Reference numeral 60 denotes a case where two 3 × 2 multimode interferometers 66 and 67 formed on a semiconductor substrate 65 are used as two 2 × 2 type optical multiplexer / demultiplexers, similarly to the eighth embodiment. Semiconductor optical amplifiers 68 and 69 are provided on both of a pair of arms between the mode interferometers 66 and 67, and one end of the multi-mode interferometer 66 in the preceding stage is provided from the converging optical fiber 62 provided with a lens at the tip. The optical input 61 as the signal light input to the input port is split into １ ／ at the interference section of the multi-mode interferometer 66 and amplified by the semiconductor optical amplifiers 68 and 69. Are combined at the interference portion of the MMI coupler 67 of stage, the light output 64 from the output port of the input port and the cross position where the signal light is input through a focused optical fiber 63
Is output as

On the other hand, the laser resonator in this case includes a DBR region 77 provided on the other output port side of the subsequent multi-mode interferometer 67 and a multi-mode interferometer 66 of the preceding stage.
In the eleventh embodiment, the variable optical attenuator 79 includes an electro-absorption optical modulator as a variable optical attenuator in the preceding multi-mode interferometer. It is provided monolithically and integrally on the other input port side 66.

The Mach-Zehnder interferometer 60 in this case
Also, on a semiconductor substrate 65 made of n-type InP, for example,
The active layer has a strained MQW structure having a wavelength composition of 1.55 μm.
An nGaAsP / InP double heterojunction structure is stacked to form a stacked structure for forming the semiconductor optical amplifiers 68 and 69, and after removing a region other than the region for forming the semiconductor optical amplifiers 68 and 69, the removing portion is formed. A double heterojunction structure having an optical waveguide core layer having a wavelength composition of, for example, 1.30 μm is laminated on the shorter wavelength side than the laser oscillation wavelength, and then the vicinity of the region where the multi-mode interferometer 66 of the preceding stage is formed After removing a part of the laminated structure, a pin structure for forming an electroabsorption optical modulator composed of an InGaAsP layer having a wavelength composition of 1.48 μm as an absorption layer is laminated, and mesa-etched into a shape as shown in the figure. By doing so, two 2 × 2 multimode interferometers 66, 6
7 and semiconductor optical amplifiers 68, 69, variable optical attenuators 79, and DBR regions 77, 78 sandwiched therebetween, and the variable optical attenuator 79 can continuously and arbitrarily reduce the resonator loss. Can be controlled. DBR
After growing the optical waveguide core layer, or before growing the optical waveguide core layer, the regions 77 and 78 form periodic irregularities near the optical waveguide core layer to form a diffraction grating. ,
It is formed by laminating a predetermined layer structure.

As described above, the laser resonator is connected to the DBR region 7.
7 and 78, it is formed as an internal resonator.
Since the device configuration is simplified, assembly of the device becomes easy,
Further, since the variable optical attenuator 79 is also integrated, the variable optical attenuator 79 can be reduced in size, whereby the semiconductor optical amplifier can be further reduced in size.

In the eleventh embodiment, since the laser resonator is constituted by a pair of DBR regions 77 and 78, the wavelength selectivity of the laser resonator is set at the element design stage and the manufacturing stage. Therefore, attention on wavelength selectivity, such as when using an external resonator, is reduced.
Other functions, effects, characteristics, and the like are the same as those in the tenth embodiment.

In the eleventh embodiment, an electro-absorption optical modulator which can be easily integrated is used as the variable optical attenuator 79. In this case, too, the variable optical attenuator 7 is used.
Since no special light absorption characteristics are required for 9, a variable optical attenuator having another configuration, for example, a current injection type semiconductor light absorption element or the like may be used.

Next, a gain-clamped semiconductor optical amplifying device using a Sagnac interferometer according to twelfth to sixteenth embodiments of the present invention will be described with reference to FIGS. FIG. 13 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a twelfth embodiment of the present invention. A Sagnac interferometer 80 constituting the gain-clamped semiconductor optical amplifier includes: An input / output optical waveguide 82 for signal light composed of optical fibers, a directional coupler 83 also composed of optical fibers, arms 84 and 86, and a semiconductor optical amplifier 85 arranged in the middle of the arms 84 and 86. It is composed of a laser side optical waveguide 88 composed of an optical fiber and constituting a laser resonator, and a high reflection film 89 composed of a multilayer dielectric film.

In this case, the laser resonator is made of the high reflection film 8
9, laser-side optical waveguide 88, directional coupler, arm 8
The laser oscillation light generated by the semiconductor optical amplifier 85 is reflected by the high-reflection film 89 and returns to the arm side.
At this time, the clockwise laser oscillation light and the counterclockwise laser oscillation light are divided into two equal parts in the directional coupler 83 constituting the optical coupler. The phase of the surrounding laser oscillation light is shifted by π / 2. On the other hand, when the clockwise laser oscillation light returns from the arm 84 to the cross-side laser waveguide 88 in the directional coupling 83, the phase also changes. Since it is shifted by π / 2, in the laser-side optical waveguide 88, the left-handed laser oscillation light and the right-handed laser oscillation light have the same phase, and do not disappear due to interference.

On the other hand, of the clockwise laser oscillation light, the input / output side optical waveguide 8 from the arm 84 via the directional coupler 83.
Although there is no change in the phase of the component going to 2, the component of the counterclockwise laser oscillation light going from the arm 86 to the cross-side input / output optical waveguide 82 via the directional coupler 83 is:
Since the directional coupler 83 is crossed twice, its phase is (π
/ 2) × 2 = π, so the input / output side optical waveguide 82
In this case, the clockwise laser oscillation light and the counterclockwise laser oscillation light have opposite phases and interfere with each other and disappear, so that no laser oscillation light is output from the input / output side optical waveguide 82.

The same applies to the signal light. The optical input 81 input to the input / output side optical waveguide 82 is amplified by the semiconductor optical amplifier 85, propagates through the annular arms 84 and 86, and then enters again. The amplified light output 87 is output from the output side optical waveguide 82 and does not propagate through the laser side optical waveguide 88.

Therefore, the Sagnac type optical interferometer 8
In the case of 0, the laser oscillation light and the signal light are completely spatially separated from each other. Therefore, it is not necessary to make the laser resonator have wavelength dependency, and the cleavage plane itself or the cleavage plane of an ordinary optical fiber is not required. Since the reflecting mirror can be constituted by a high-reflectance multilayer dielectric film or the like provided in the above, the resonator structure is simplified.

In the twelfth embodiment, since the Sagnac-type optical interferometer 80 is used, only one semiconductor optical amplifier 85 is required, and the seventh to eleventh embodiments using a pair of semiconductor optical amplifiers are used. Compared with the embodiment, the requirement for the symmetric operation required for the semiconductor optical amplifier 85 can be eliminated, and therefore, the laser oscillation light and the signal light can be spatially separated more stably.

Further, in the twelfth embodiment, the parts other than the optical amplifier 85 of the Sagnac type optical interferometer 80 are constituted by using optical fibers, so that a special fine processing technique is required. Thus, it is possible to configure a gain-clamped semiconductor optical amplifying device capable of stable operation without any problem.

Next, a thirteenth embodiment of the present invention will be described with reference to FIG. 14. In the thirteenth embodiment, the high reflection film 8 of the twelfth embodiment is used.
9 is replaced by a diffraction grating 9 at the end of the laser-side optical waveguide 88.
0, that is, the laser-side optical waveguide 88 is formed of a fiber diffraction grating, and the other configuration is completely the same as that of the twelfth embodiment. . FIG. 14 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a thirteenth embodiment of the present invention. A Sagnac interferometer 80 constituting the gain-clamped semiconductor optical amplifier includes: An input / output optical waveguide 82 for signal light composed of optical fibers, a directional coupler 83 also composed of optical fibers, arms 84 and 86, and a semiconductor optical amplifier 85 arranged in the middle of the arms 84 and 86. It is composed of a laser-side optical waveguide 88 composed of an optical fiber and constituting a laser resonator, and a diffraction grating 90 formed at one end of the laser-side optical waveguide 88.

The operating principle and main features in this case are as follows.
The same as the twelfth embodiment, except that the thirteenth embodiment
In the embodiment, the diffraction grating 90 is formed at one end of the laser-side optical waveguide 88 to form a reflecting mirror having wavelength selectivity, so that the oscillation wavelength of the laser oscillation light is stabilized,
It is possible to stably maintain the difference between the wavelength of the signal light and the wavelength of the laser oscillation light, thereby preventing the occurrence of a non-linear effect or the like due to an undesirable interaction between the signal light and the laser oscillation light. Can be.

Next, a fourteenth embodiment of the present invention will be described with reference to FIG. 15. In the fourteenth embodiment, the portions other than the semiconductor optical amplifier are made of planar light using a dielectric. It is constituted by a circuit (PLC). FIG. 15 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifier according to a fourteenth embodiment of the present invention. A Sagnac interferometer 100 constituting the gain-clamped semiconductor optical amplifier includes: An input / output optical waveguide 103 for signal light provided on a PLC substrate 102 such as a quartz substrate, a directional coupler 104, arms 105 and 107, and arms 105 and 10.
7, a semiconductor optical amplifier 106 provided in a hybrid manner, and a laser-side optical waveguide 10 constituting a laser resonator
9 and a diffraction grating 110 formed at one end of the laser-side optical waveguide 109.

The Sagnac interferometer 100 in this case is
For example, a lower cladding layer made of a SiO ₂ film and a GeO ₂ film are formed on a PLC substrate 102 such as a quartz substrate by a CVD method.
After depositing a core layer made of a SiO ₂ film having a high refractive index by doping, the input / output side optical waveguide 103, the directional coupler 104, the arms 105 and 107, the laser side optical waveguide are formed by reactive ion etching. The core layer is etched into an “Ω” -shaped pattern constituting 109.

Next, the SiO ₂ is again formed by the CVD method.
After depositing an upper clad layer made of a film, a diffraction grating 1 is formed on one end of the laser-side optical waveguide 109 by using an interference exposure method.
After the formation of the semiconductor optical amplifier 10, a recess reaching the PLC substrate 102 for hybrid mounting the semiconductor optical amplifier 106 is formed in the center of the optical waveguide of the annular portion constituting the arms 105 and 107. Amplifier 106
Are mounted in a hybrid state with the optical axes aligned with the arms 105 and 107. In this case, as the semiconductor optical amplifier 106, for example, an InGaAsP / InP-based semiconductor optical amplifier in which the active layer has a strained MQW structure having a wavelength composition of 1.55 μm may be used.

In the fourteenth embodiment, the portions other than the semiconductor optical amplifier of the Sagnac interferometer 100 are formed by a planar optical circuit (PLC) using a dielectric. Twelfth and thirteenth
The device configuration can be greatly simplified and reduced in size as compared with the embodiment. The operation principle and main features are the same as those of the above-described thirteenth embodiment.
01 is an optical output 1 after being amplified by the semiconductor optical amplifier 106.
08 is output from the input / output side optical waveguide 103.

In the fourteenth embodiment, the material configuration and the manufacturing method of the PLC are not limited to the above-described material configuration and the manufacturing method. For example, the method of forming the cladding layer and the core layer is not limited to the above. It is not limited to the normal atmospheric pressure CVD method, but may be a plasma CVD method or a low pressure CV method.
The method D may be used, and further, the flame deposition method may be used.

The dopant for doping SiO ₂ to form the core layer is not limited to GeO ₂ , but may be TiO ₂ or P ₂ O ₅ . it is intended the core layer may be formed using another material system, such as SiO _x N _y H _z.

Next, a fifteenth embodiment of the present invention will be described with reference to FIG. 16. In the fifteenth embodiment, the entire Sagnac type optical interferometer is monolithically formed by using a semiconductor. It was done. FIG. 16 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifying device according to a fifteenth embodiment of the present invention. A Sagnac interferometer 120 that constitutes the gain-clamped semiconductor optical amplifying device includes: An input / output optical waveguide 123 for a signal light provided on a semiconductor substrate 122, a directional coupler 124, arms 125 and 127, a semiconductor optical amplifier 126 provided between arms 125 and 127, and a laser constituting a laser resonator. Side optical waveguide 129 and laser side optical waveguide 1
It is constituted by a diffraction grating 130 formed at one end of 29.

The Sagnac interferometer 120 in this case is
For example, to form a semiconductor optical amplifier 126 by stacking, for example, an InGaAsP / InP-based double heterojunction structure in which an active layer has a strained MQW structure with a 1.55 μm wavelength composition on a semiconductor substrate 122 made of n-type InP. After removing the region other than the region for forming the semiconductor optical amplifier 126, the removed portion is provided with an optical waveguide core layer having a wavelength composition shorter than the laser oscillation wavelength, for example, 1.30 μm. The input / output-side optical waveguide 123, the directional coupler 124, the arms 125 and 127, and the laser-side optical waveguide 129 were formed by laminating the provided double heterojunction structures and then performing mesa etching in an “Ω” shape. Things. The diffraction grating 130, that is, DB
After growing the optical waveguide core layer or before growing the optical waveguide core layer, the R region forms a periodic grating near the optical waveguide core layer to form a diffraction grating. Are formed by laminating the predetermined layer structures.

As described above, in the fifteenth embodiment of the present invention, the overall configuration of the Sagnac interferometer 120 is formed monolithically using semiconductors, so that the overall configuration can be further reduced in size. In addition, since the hybrid mounting as in the fourteenth embodiment is not required, the optical axis alignment at the time of mounting becomes unnecessary, and the deterioration of the optical characteristics depending on the accuracy of the optical axis alignment occurs. Nothing. The operation principle and main features are described in
The optical input 121 is amplified by a semiconductor optical amplifier 126 and then output from an input / output side optical waveguide 123 as an optical output 128.

Next, a sixteenth embodiment of the present invention will be described with reference to FIG. 17. In the sixteenth embodiment, the Sagnac type optical interferometer of the fifteenth embodiment is described. In addition, an electro-absorption optical modulator is monolithically incorporated as a variable optical attenuator. FIG. 17 is a conceptual configuration diagram of a gain-clamped semiconductor optical amplifying device according to a sixteenth embodiment of the present invention. The Sagnac interferometer 120 constituting the gain-clamped semiconductor optical amplifying device has: An input / output optical waveguide 123 for a signal light provided on a semiconductor substrate 122, a directional coupler 124, arms 125 and 127, a semiconductor optical amplifier 126 provided between arms 125 and 127, and a laser constituting a laser resonator. Side optical waveguide 129, an electro-absorption type optical modulator 131 provided at an intermediate portion of the laser side optical waveguide 129, and
Diffraction grating 1 provided at one end of laser-side optical waveguide 129
30.

The Sagnac interferometer 120 in this case is also
For example, to form a semiconductor optical amplifier 126 by stacking, for example, an InGaAsP / InP-based double heterojunction structure in which an active layer has a strained MQW structure with a 1.55 μm wavelength composition on a semiconductor substrate 122 made of n-type InP. After removing the region other than the region for forming the semiconductor optical amplifier 126, the removed portion is provided with an optical waveguide core layer having a wavelength composition shorter than the laser oscillation wavelength, for example, 1.30 μm. After laminating the double heterojunction structure provided, and then removing a part of the laminated structure in the middle of the region constituting the laser-side optical waveguide 129, an electric field composed of an InGaAsP layer having a wavelength composition of 1.48 μm as an absorption layer A pin structure for forming the absorption-type optical modulator 131 is laminated, and then the mesa-etched “Ω” shape is used to form the input / output light guide. Road 123, directional coupler 12
4, arms 125 and 127, laser-side optical waveguide 129,
And an electro-absorption type optical modulator 131 is formed. The diffraction grating 130, that is, the DBR region is formed by forming periodic irregularities near the optical waveguide core layer after the optical waveguide core layer is grown or before the optical waveguide core layer is grown. A lattice is formed, and thereafter, the above-mentioned predetermined layer structure is laminated and formed.

As described above, in the sixteenth embodiment of the present invention, when the entire configuration of the Sagnac interferometer 120 is formed monolithically using a semiconductor, the electroabsorption type optical modulator serving as a variable optical attenuator is used. Since the modulator 131 is also monolithically incorporated, the resonator loss can be continuously controlled to an arbitrary value by the attenuation of the electro-absorption optical modulator 131 as in the first to eleventh embodiments. Thus, the characteristics of the semiconductor optical amplifier can be set to optimal values according to various uses. The operation principle and main features are the same as those in the fifteenth embodiment. The optical input 121 is amplified by the semiconductor optical amplifier 126 and then output from the input / output optical waveguide 123 as an optical output 128. You.

In the sixteenth embodiment, the electro-absorption optical modulator 131 which can be easily integrated is used as the variable optical attenuator. Since absorption characteristics are not required, a variable optical attenuator having another configuration, for example, a current injection type semiconductor light absorbing element or the like may be used.

The embodiments of the present invention have been described above. However, the present invention is not limited to the configuration described in each embodiment, and various modifications are possible. For example, the active layer of the semiconductor optical amplifier has a 1.55 μm wavelength composition so that attenuation in an optical long-distance communication fiber is small, but other compositions such as a 1.3 μm wavelength composition may be used. Is the attenuation characteristic such as the absorption edge wavelength in the variable optical attenuator according to the wavelength composition of the active layer, or 2 × 2
Optical splitter, Sagnac optical interferometer, or DBR
What is necessary is just to change the composition of the core layer for optical waveguides which comprises an area | region etc.

Further, the present invention is not limited to the InGaAsP / InP system when the semiconductor optical amplifier, the variable optical attenuator, the optical waveguide, the optical coupler, or the like is formed of a semiconductor, but is not limited to the GaAs / AlGaAs. It may be formed using another compound semiconductor such as a system.

Further, the points of the ninth to eleventh embodiments different from the eighth embodiment, that is, the use of a wavelength-selective reflecting mirror such as a fiber diffraction grating,
Monolithically integrating the variable optical attenuator, and D
The point that the BR region is monolithically integrated is described in the seventh aspect.
The present embodiment is also applied to the embodiment as it is.

In the seventh to eleventh embodiments, the variable optical attenuator is provided on the input port side of the input 2 × 2 optical multiplexer / demultiplexer, but the output 2 × 2 optical multiplexer / demultiplexer is provided. A variable optical attenuator may be provided on the output port side of the optical coupler.

In the description of the seventh to eleventh embodiments, in order to prevent undesired absorption of laser oscillation light, the core layer for optical waveguide of the DBR region or the 2 × 2 optical coupler is used. Is set on the shorter wavelength side than the wavelength composition of the active layer of the semiconductor optical amplifier. However, the structure may be the same as that of the semiconductor optical amplifier. In this case, the DBR is set so as to cancel the absorption loss. What is necessary is just to forward-bias an area | region or a 2x2 type optical coupler / branch.

In the description of the twelfth to fourteenth embodiments, it is assumed that the gain-clamped semiconductor optical amplifying device using the Sagnac-type optical interferometer is a novel one. Although no variable optical attenuator is provided, a variable optical attenuator may be provided in the middle of the laser-side optical waveguide as in the above-described sixteenth embodiment, whereby the resonator loss can be arbitrarily set. , The characteristics of the semiconductor optical amplifier can be set to optimal values according to various applications.

As the variable optical attenuator, various commercially available variable optical attenuators are used. Since the optical input effectively used as signal light does not pass through the variable optical attenuator, the variable optical attenuator is used. As the attenuation characteristic, a variable optical attenuator having any configuration may be used as long as it can attenuate light having a laser oscillation wavelength, for example, a wavelength of 1.51 μm. , The amount of attenuation can be arbitrarily controlled.

Also, in the description of the fifteenth and sixteenth embodiments, the input / output side optical waveguide, the directional coupler, the arm, the laser side light guide are also provided in order to prevent undesired absorption of laser oscillation light. Although the wavelength composition of the waveguide and the core layer for optical waveguide in the DBR region is set to be shorter than the wavelength composition of the active layer of the semiconductor optical amplifier, it may have the same structure as the semiconductor optical amplifier. Have the same configuration, the manufacturing process can be greatly simplified.
In this case, it is necessary to forward bias each region so as to cancel the absorption loss.

In the description of the thirteenth to sixteenth embodiments, the diffraction grating is provided in the laser-side optical waveguide to provide wavelength selectivity. Similarly, only a high reflection film may be provided on the end face of the laser-side optical waveguide, or a cleavage plane of a dielectric or a semiconductor may be used.

[0162]

According to the present invention, the gain and the allowable incident power of the signal light can be continuously adjusted, and the gain-clamped semiconductor light whose characteristics can be optimized for each application requirement by one structure. An amplifier can be realized, which greatly contributes to practical use of a wavelength division multiplexing optical communication system.

By constructing a gain-clamped semiconductor optical amplifier using a Sagnac-type optical interferometer,
Since the symmetric operation characteristic of the semiconductor optical amplifier is not required, the laser oscillation light and the signal light can be surely spatially separated from each other, and the overall configuration of the device can be further reduced. This greatly contributes to the practical use of the wavelength division multiplexing optical communication system.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is a conceptual configuration diagram of the first embodiment of the present invention.

FIG. 3 is a conceptual configuration diagram of a second embodiment of the present invention.

FIG. 4 is a conceptual configuration diagram of a third embodiment of the present invention.

FIG. 5 is a conceptual configuration diagram of a fourth embodiment of the present invention.

FIG. 6 is a conceptual configuration diagram according to a fifth embodiment of the present invention.

FIG. 7 is a conceptual configuration diagram according to a sixth embodiment of the present invention.

FIG. 8 is a conceptual configuration diagram according to a seventh embodiment of the present invention.

FIG. 9 is a conceptual configuration diagram of an eighth embodiment of the present invention.

FIG. 10 is a conceptual configuration diagram according to a ninth embodiment of the present invention.

FIG. 11 is a conceptual configuration diagram according to a tenth embodiment of the present invention.

FIG. 12 is a conceptual configuration diagram according to an eleventh embodiment of the present invention.

FIG. 13 is a conceptual configuration diagram according to a twelfth embodiment of the present invention.

FIG. 14 is a conceptual configuration diagram according to a thirteenth embodiment of the present invention.

FIG. 15 is a conceptual configuration diagram according to a fourteenth embodiment of the present invention.

FIG. 16 is a conceptual configuration diagram according to a fifteenth embodiment of the present invention.

FIG. 17 is a conceptual configuration diagram according to a sixteenth embodiment of the present invention.

FIG. 18 is an explanatory diagram of the dependence of the gain-optical output characteristics of the semiconductor optical amplifier on the resonator loss.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor optical amplifier 2 Semiconductor optical amplifier 3 Variable optical attenuator 4 Reflector with wavelength selectivity 5 Reflector without wavelength selectivity 6 Beam splitter 7 Signal light 11 Optical input 12 Optical fiber 13 Lens 14 Beam splitter 15 Lens 16 Semiconductor optical amplifier 17 lens 18 lens 19 optical fiber 20 optical output 21 variable optical attenuator 22 diffraction grating 23 beam splitter 24 lens 25 diffraction grating 26 fiber diffraction grating 27 lens 28 lens 29 fiber diffraction grating 30 monolithic optical semiconductor device 31 semiconductor substrate 32 Semiconductor optical amplification area 33 Variable optical attenuation area 34 Monolithic optical semiconductor device 35 Semiconductor substrate 36 DBR area 37 DBR area 40 Mach-Zehnder interferometer 41 Optical input 42 Focusing optical fiber 43 Focusing optical fiber 44 Optical output 45 Semiconductor substrate 46 Directional coupler 47 Directional coupler 48 Semiconductor optical amplifier 49 Semiconductor optical amplifier 50 High reflection film 51 Lens pair 52 Variable optical attenuator 53 High reflection film 60 Mach-Zehnder interferometer 61 Optical input 62 Focusing optical fiber 63 Focusing Optical fiber 64 Optical output 65 Semiconductor substrate 66 Multimode interferometer 67 Multimode interferometer 68 Semiconductor optical amplifier 69 Semiconductor optical amplifier 70 High reflection film 71 Lens pair 72 Variable optical attenuator 73 High reflection film 74 Fiber diffraction grating 75 Electroabsorption type Optical modulator 76 Fiber diffraction grating 77 DBR region 78 DBR region 79 Variable optical attenuator 80 Sagnac type optical interferometer 81 Optical input 82 Input / output side optical waveguide 83 Directional coupler 84 Arm 85 Semiconductor optical amplifier 86 Arm 87 Optical output 88 Laser side optical waveguide 89 High reflection film 0 Diffraction grating 100 Sagnac type optical interferometer 101 Optical input 102 PLC substrate 103 Input / output side optical waveguide 104 Directional coupler 105 Arm 106 Semiconductor optical amplifier 107 Arm 108 Optical output 109 Laser side optical waveguide 110 Diffraction grating 120 Sagnac type optical interference Total 121 Optical input 122 Semiconductor substrate 123 Input / output side optical waveguide 124 Directional coupler 125 Arm 126 Semiconductor optical amplifier 127 Arm 128 Optical output 129 Laser side optical waveguide 130 Diffraction grating 131 Electroabsorption optical modulator

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H079 AA02 AA13 BA01 CA04 DA16 DA22 EA03 EA05 EA07 HA04 HA11 KA01 KA06 KA11 5F073 AA63 AA65 AA74 AA83 AB21 AB25 AB27 AB28 BA03 CA04 CA12 EA12 EA15

Claims (24)

[Claims] In a gain-clamped semiconductor optical amplifier comprising a semiconductor optical amplifier and a laser resonator including the semiconductor optical amplifier therein, an effective transmission loss for signal light is maintained at a constant value. A semiconductor optical amplifying device having a variable optical attenuation mechanism capable of controlling the amount of resonator loss with respect to laser oscillation light. 2. The laser resonator according to claim 1, wherein at least one of the laser resonators is constituted by a reflecting mirror having wavelength selectivity, and a variable optical attenuator is provided in the laser resonator as the variable optical attenuation mechanism. Item 2. The semiconductor optical amplifier according to Item 1. 3. A semiconductor optical amplifier according to claim 2, wherein a portion through which signal light does not pass is provided in said laser resonator, and said variable optical attenuator is arranged in a portion through which said signal light does not pass. . 4. An external resonator structure comprising a laser mirror having a wavelength selectivity and a mirror having no wavelength selectivity, wherein the laser resonator is provided between the wavelength-selective reflector and the semiconductor optical amplifier. A beam splitter is inserted into the beam splitter, and the signal light is made incident on the laser resonator through the beam splitter in a direction perpendicular to the laser resonator, and the signal light does not pass between the wavelength-selective reflecting mirror and the beam splitter. 4. The semiconductor optical amplifying device according to claim 3, wherein: 5. An external resonator structure comprising a laser mirror having a wavelength selectivity and a mirror having no wavelength selectivity, wherein the laser resonator is provided between the reflector having the wavelength selectivity and the semiconductor optical amplifier. A beam splitter is inserted into the beam splitter, and the signal light is emitted from the direction perpendicular to the laser resonator via the beam splitter, and the signal light does not pass between the wavelength-selective reflecting mirror and the beam splitter. 4. The semiconductor optical amplifying device according to claim 3, wherein: 6. An external resonator structure comprising a pair of wavelength-selective reflecting mirrors, wherein a beam splitter is provided between the semiconductor optical amplifier and the two wavelength-selective reflecting mirrors. At the same time, the signal light enters and exits from the direction perpendicular to the laser resonator via the beam splitter, and the signal light passes through two regions between the pair of wavelength-selective reflecting mirrors and the beam splitter. 4. The semiconductor optical amplifying device according to claim 3, wherein said portion does not pass through. 7. The variable attenuator can change the amount of light attenuation with respect to a laser oscillation wavelength without providing a portion through which signal light does not pass in the laser resonator, but the signal light is effectively attenuated. 3. The semiconductor optical amplifier according to claim 2, wherein a variable optical attenuator is provided. 8. A laser mirror for setting the laser oscillation wavelength to be shorter than the wavelength of the signal light by the wavelength-selective reflector, and the variable optical attenuator having an absorption end wavelength shorter than the laser oscillation wavelength. 8. The semiconductor optical amplifying device according to claim 7, wherein a certain electro-absorption type optical modulator is used. 9. The semiconductor optical amplifying device according to claim 8, wherein said semiconductor optical amplifier and said variable optical attenuator comprising said electro-absorption type optical modulator are monolithically integrated. 10. The reflecting mirror having wavelength selectivity,
10. The semiconductor optical amplifying device according to claim 7, wherein a fiber grating is used. 11. A distributed Bragg reflector is used as said wavelength-selective reflector, and said distributed Gragg reflector is monolithically integrated with a variable optical attenuator comprising said semiconductor optical amplifier and said electroabsorption optical modulator. 10. The semiconductor optical amplifying device according to claim 9, wherein: 12. The semiconductor optical amplifier is provided on two arms of a Mach-Zehnder interferometer comprising two 2 × 2 optical couplers, and the laser resonator is provided at one of input ports of the optical coupler on the input side. And a reflecting mirror arranged at an output port of the optical coupler on the output side at a cross position with respect to the input port, wherein a variable optical attenuator as the variable optical attenuating mechanism is provided in the laser resonator. 2. The semiconductor optical amplifying device according to claim 1, wherein said semiconductor optical amplifying device is provided between an optical coupler and said reflecting mirror. 13. The semiconductor optical amplifier according to claim 12, wherein a directional coupler is used as said 2 × 2 optical coupler. 14. The semiconductor optical amplifying device according to claim 12, wherein a multi-mode interferometer is used as said 2 × 2 optical multiplexer / demultiplexer. 15. At least one of said reflecting mirrors,
15. The semiconductor optical amplifying device according to claim 12, wherein the semiconductor optical amplifying device is constituted by a reflecting mirror having wavelength selectivity. 16. An electro-absorption type optical modulator as the variable optical attenuator.
The semiconductor optical amplifier according to any one of the above. 17. The semiconductor optical amplifier, wherein the two 2 ×
2. A monolithically integrated type 2 optical multiplexer / demultiplexer, said variable optical attenuator, and said reflecting mirror.
17. The semiconductor optical amplifier according to any one of 2 to 16. 18. A gain-clamped semiconductor optical amplifier comprising a semiconductor optical amplifier and a laser resonator including the semiconductor optical amplifier therein, wherein the semiconductor optical amplifier is provided on an arm of a Sagnac optical interferometer. A reflector is arranged on an optical waveguide connected to one input / output port of the Sagnac type optical interferometer to form a laser resonator, and an optical waveguide connected to the other input / output port is an input / output optical waveguide for signal light. A semiconductor optical amplifying device characterized in that: 19. The semiconductor optical amplifier according to claim 18, wherein a reflecting mirror having wavelength selectivity is used as said reflecting mirror. 20. The Sagnac-type optical interferometer,
20. The semiconductor optical amplifying device according to claim 18, wherein an optical waveguide portion and an optical coupler portion other than the semiconductor optical amplifier are constituted by optical fibers. 21. The Sagnac-type optical interferometer,
20. The semiconductor optical amplifying device according to claim 18, wherein the optical waveguide section and the optical coupler section other than the semiconductor optical amplifier are configured by a planar dielectric optical circuit. 22. The semiconductor device according to claim 18, wherein the optical waveguide, the optical coupler, the arm, and the semiconductor optical amplifier constituting the Sagnac-type optical interferometer are monolithically integrated with a semiconductor. Semiconductor optical amplifier. 23. The semiconductor optical amplifier according to claim 22, wherein the optical waveguide, the optical coupler, and the arm constituting the Sagnac-type optical interferometer are also semiconductor optical amplification regions. 24. The semiconductor optical amplifying device according to claim 18, wherein a variable optical attenuator is disposed in the optical waveguide on the side forming the laser resonator.