JP3428501B2 - Semiconductor optical amplifier and method of manufacturing the same - Google Patents

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 used in an optical communication system and its manufacturing method,
In particular, the present invention relates to a semiconductor optical amplifier having a quantum well structure and having no polarization dependence in optical amplification gain, and a manufacturing method thereof.

[0002]

2. Description of the Related Art A semiconductor optical amplifier has a function of amplifying and outputting an input signal light. With this function, a loss compensation application for amplifying an attenuated input signal light in an optical fiber communication system, and It is drawing attention as an optical gate device application for turning on / off the input signal light. In such applications, it is very important that the optical amplification gain of the semiconductor optical amplifier has no polarization dependence. Here, there is no polarization dependence, even if the input signal light is TE polarized light (polarized light parallel to the active layer) or TM polarized light (polarized light perpendicular to the active layer), the same optical amplification gain is obtained. It is to amplify the input signal light.

Generally, a semiconductor optical amplifier has a structure similar to a buried structure type semiconductor laser used as a light source for an optical communication system, and a traveling wave type semiconductor optical amplifier has a polarization dependence of an optical amplification gain. In order to improve, end face anti-reflection measures such as an anti-reflection film and a window structure for suppressing laser oscillation operation when current is injected are taken.

However, the semiconductor optical amplifier has a different waveguide shape when an active layer having a width of about 1.5 μm and a thickness of about 0.1 μm is used, like the active layer used for a semiconductor laser. Since the optical confinement coefficients for the TE polarized light and the TM polarized light in the active layer are different based on the directionality, the optical amplification gain for the TE polarized light is higher than the optical amplification gain for the TM polarized light, and there is a problem of polarization dependency.

Various semiconductor optical amplifiers have been proposed in order to suppress the optical amplification gain for the TE polarized light and eliminate the polarization dependence of the optical amplification gain. As a first conventional example, there has been proposed a semiconductor optical amplifier that can realize polarization-independent operation by using a bulk active layer in which the cross-sectional shape of the active layer is a rectangular shape having substantially the same width and thickness.
However, in this semiconductor optical amplifier, it is necessary to narrow the lateral width and thickness to about 0.5 μm or less so that the active waveguide satisfies the single mode condition, and such a narrow active layer width can be realized highly uniformly. Doing so is impractical as it requires extremely sophisticated process technology.

As a second conventional example, there has been proposed a semiconductor optical amplifier in which the active layer is changed from a bulk structure to a multiple quantum well structure. By doing so, the gain coefficient with respect to the injected current is increased, and the gain of the semiconductor optical amplifier is increased. In this semiconductor optical amplifier,
By changing the valence band structure, the noise figure, which is an important parameter of the semiconductor optical amplifier, can be lowered to about 4 dB. However, in the semiconductor optical amplifier having the active layer with the multiple quantum well structure, the gain for TE polarized light is usually increased due to the effect of band splitting in the valence band as well as in the waveguide shape. The polarization dependence cannot be completely eliminated.

As a third conventional example, a semiconductor optical amplifier using extension strain in the active layer has been proposed in order to realize polarization independent optical amplification gain. There are a semiconductor optical amplifier in which extension strain is introduced into a well or a barrier, and a semiconductor optical amplifier in which compressive strain and extension strain are alternately introduced into multiple quantum wells to control gains for TE polarized light and TM polarized light. . Among these semiconductor optical amplifiers, a semiconductor optical amplifier in which tensile strain is introduced into a quantum well having a multiple quantum well structure is particularly promising.

However, in the 1.55 μm wavelength band device using the InGaAsP / InGaAsP quantum well structure widely used in optical fiber communication, there is a limit to the amount of extension strain that can be added to the quantum well, which is sufficient. You can't get the effect. For this reason, JP-A-4-2718
As a technique disclosed in No. 3, a method of applying extension strain to a barrier has been proposed, but it has not yet achieved polarization independent operation in a wide injection current range.

As a fourth conventional example, a semiconductor optical amplifier having a high gain for TE polarized light and a semiconductor optical amplifier having a high gain for TM polarized light are connected in series or in parallel so that the entire system is polarization independent. A semiconductor optical amplifier that realizes the operation has been proposed. Here, as a semiconductor optical amplifier for connecting the semiconductor optical amplifiers having different characteristics, in addition to a combination of two independent semiconductor optical amplifiers, there is also a semiconductor optical amplifier in which both are integrated in one semiconductor optical amplifier. Proposed.

Specifically, as the technique disclosed in Japanese Patent Application Laid-Open No. 10-154841, there is a region having an active layer having a lateral width larger than the thickness and a region having an active layer having a thickness larger than the lateral width. However, there has been proposed a semiconductor optical amplifier capable of performing polarization-independent operation by controlling the current to be injected into each region independently. But,
In a semiconductor optical amplifier having a structure in which a wide lateral active layer and a narrow lateral active layer are integrated in series, there is an advantage that the current injected into each active layer can be controlled, but there is a problem that the control current increases. .

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and in a semiconductor optical amplifier, particularly in a semiconductor optical amplifier having a quantum well structure, a wide injection is performed with a single injection current. An object of the present invention is to provide a semiconductor optical amplifier capable of performing polarization-independent operation over a current range and a manufacturing method thereof.

[0012]

In order to achieve the above object, a semiconductor optical amplifier according to claim 1 of the present invention comprises:
A first active layer formed on the upper surface of a stripe-shaped protrusion of a semiconductor substrate; a second active layer protruding below the first active layer and embedded in a groove provided in the protrusion of the semiconductor substrate; A third active layer projecting above the first active layer, a current blocking layer covering at least the upper portion of the semiconductor substrate and both end faces of the first active layer, and a clad contacting at least a part of the third active layer. Layer and at least a cap layer formed on the cladding layer, and antireflection at the light input portion and the light output portion which are end faces in the stripe direction of the first active layer, the second active layer and the third active layer. A semiconductor optical amplifier comprising at least a film, and means for injecting current into the first active layer, the second active layer and the third active layer, the second active layer and the third active layer The width of the above It is constituted that narrower than the width of the active layer.

By thus forming the first active layer, the second active layer and the third active layer, the collective active layer (hereinafter, referred to as
When the first active layer, the second active layer, and the third active layer are described as an integrated active layer, they are collectively referred to as a collective active layer. ) Is introduced, the optical amplification gain of the semiconductor optical amplifier is isotropic, and the polarization dependence of the optical amplification gain is improved. As a result, the semiconductor optical amplifier can perform polarization-independent operation, and, for example, has a rectangular bulk active layer having a horizontal width of the first active layer as a long side and a total thickness of each active layer as a short side. Since the cross-sectional area of the collective active layer can be made smaller than that of, the single mode condition can be maintained.

According to a second aspect of the present invention, there is provided a semiconductor optical amplifier in which a first clad layer or a first current block layer formed on a stripe-shaped protrusion of a semiconductor substrate, the first clad layer or the first current block. A first active layer formed on the upper part of the layer, a second active layer projecting on the lower part of the first active layer and embedded in a groove provided in the first cladding layer or the first current blocking layer, A third active layer projecting above the first active layer, a second cladding layer covering the upper surface of the third active layer, a current blocking layer covering at least the upper part of the semiconductor substrate and both end faces of the first active layer, At least a clad layer in contact with at least a part of the second clad layer and a cap layer formed on the clad layer are provided, and the strike of the first active layer, the second active layer and the third active layer is performed. Semiconductor optical amplifier including at least an antireflection film in the light input portion and the light output portion, which are end faces in the direction, and means for injecting current into the first active layer, the second active layer, and the third active layer. The lateral width of the second active layer and the third active layer is narrower than the lateral width of the first active layer.

By doing so, when the second active layer is buried in the first current blocking layer, both sides of the second active layer are insulated, so that the reactive current can be reduced and the TM polarized light can be reduced. The optical amplification gain can be increased. Further, the second active layer is embedded in the first cladding layer, by using a conductive type material as the material of the first cladding layer, for example, the ratio of the width to the thickness of the first active layer is large, and When the ratio of the lateral width of the first active layer to the lateral width of the second active layer is large, the injection current flows intensively in the central portion of the first active layer and hardly flows to both ends. Even if there is, a current can be injected into both ends through the first cladding layer, so that it is possible to prevent a decrease in optical amplification gain of the first active layer.

According to a third aspect of the present invention, in a semiconductor optical amplifier, a first clad layer or a first current block layer formed on a stripe-shaped projection of a semiconductor substrate, the first clad layer or the first current block is provided. A first active layer formed on the upper part of the layer, a second active layer projecting on the lower part of the first active layer and embedded in a groove provided in the first cladding layer or the first current blocking layer, A third active layer protruding above the first active layer, a second clad layer covering the upper surface of the third active layer, at least both end faces of the second clad layer and both end faces of the third active layer. A third clad layer or a second current block layer, a current block layer covering at least the upper portion of the semiconductor substrate and both end faces of the first active layer, a clad layer in contact with at least a part of the second clad layer, and on this clad layer At least a cap layer formed on the first active layer, the second active layer and the third active layer, and at least an antireflection film in the light input portion and the light output portion which are end faces in the stripe direction, and A semiconductor optical amplifier comprising means for injecting current into the first active layer, the second active layer and the third active layer,
The widths of the second active layer and the third active layer are narrower than the width of the first active layer.

By doing so, when the third active layer is buried in the second current blocking layer, both sides of the third active layer are insulated, so that the reactive current can be reduced, and the TM polarized light can be reduced. The optical amplification gain can be increased. Further, the third active layer is embedded in the third clad layer, and by using a conductive type material as the material of the third clad layer, for example, the ratio of the lateral width to the thickness of the first active layer is large, and In the case where the ratio of the width of the first active layer to the width of the third active layer is large, the injected current flows intensively in the central part of the first active layer and hardly flows in both ends. Even if there is, a current can be injected into both ends through the third cladding layer, so that it is possible to prevent a decrease in the optical amplification gain of the first active layer.

According to a fourth aspect of the present invention, in the semiconductor optical amplifier according to any one of the first to third aspects, the thickness of the second active layer and the third active layer is set to the first active layer. It is made thicker than the thickness. By doing so, the semiconductor optical amplifier can be formed so that the sectional shape of the collective active layer is, for example, a cross shape,
The fluctuation of the confinement coefficient due to the polarized light can be effectively suppressed, and more complete polarization-independent operation can be performed.

According to a fifth aspect of the present invention, in the semiconductor optical amplifier according to any one of the first to fourth aspects, the first active layer has tensile strain, and the second active layer and the third active layer are provided. The active layer has a compressive strain. By doing so, when forming the collective active layer, even when it is difficult to form the cross-sectional shape into a completely symmetrical shape, the lateral width, thickness and strain amount of each active layer are appropriately adjusted. As a result, the semiconductor optical amplifier can perform polarization independent operation. For example, tensile strain is introduced into the first active layer having high gain for TE polarized light so that the gain for TM polarized light is improved, and TE polarized light is further added to the second active layer and third active layer having high gain for TM polarized light. If compressive strain is introduced so as to improve the gain with respect to, a complete isotropic property can be introduced into the collective active layer.

According to a sixth aspect of the present invention, in the semiconductor optical amplifier according to any one of the first to fifth aspects, at least the first active layer, the second active layer, and the third active layer are at least the first active layer. One active layer has a multiple quantum well structure. As described above, by introducing the quantum well structure into at least the first active layer, the gain coefficient with respect to the injection current is increased, so that the gain of the semiconductor optical amplifier is increased.

When a multi-quantum well structure having a large number of layers is formed in the first active layer and the third active layer each having a vertically long cross-sectional shape, carriers are injected by these energy barriers due to the energy barrier. When it is feared that the layers will be non-uniform, the laterally elongated first active layer only has a multi-quantum well structure, and the second and third active layers have a bulk structure. Carriers can be uniformly injected into the entire active layer. In particular, when the first active layer and the third active layer have a bulk structure, the manufacturing process can be reduced, which is more effective in terms of manufacturing cost.

According to a seventh aspect of the present invention, in a semiconductor optical amplifier, a first active layer is formed on the upper surface of a stripe-shaped projection of a semiconductor substrate, and a third active layer is projected on the first active layer. A current blocking layer covering at least the upper part of the semiconductor substrate and both end faces of the first active layer, a clad layer in contact with at least a part of the third active layer, and a cap layer formed on the clad layer. A light input portion and a light output portion that are end faces of the first active layer and the third active layer in the stripe direction, each having at least an antireflection film, and injecting a current into the first active layer and the third active layer. In the semiconductor optical amplifier, the width of the third active layer is narrower than that of the first active layer.

By thus reducing the manufacturing process for forming the second active layer, the manufacturing cost of the semiconductor optical amplifier can be reduced. Further, by thus forming the first active layer and the third active layer, the collective active layer (hereinafter,
When the first active layer and the third active layer are described as an integrated active layer, they are collectively referred to as a collective active layer. ) Is introduced, so that the optical amplification gain of the semiconductor optical amplifier is isotropic,
The polarization dependence of the optical amplification gain is improved. As a result, the semiconductor optical amplifier can perform polarization-independent operation, and, for example, has a rectangular bulk active layer having a horizontal width of the first active layer as a long side and a total thickness of each active layer as a short side. Since the cross-sectional area of the collective active layer can be made smaller than that of, the single mode condition can be maintained.

According to a eighth aspect of the present invention, in a semiconductor optical amplifier, a first active layer formed on the upper surface of a stripe-shaped projection of a semiconductor substrate and a third active layer projected on the first active layer are provided. A second clad layer covering the upper surface of the third active layer, a third clad layer or a second current blocking layer covering at least both end surfaces of the second clad layer and both end surfaces of the third active layer, at least the semiconductor substrate Of the first active layer, a current blocking layer covering both upper surfaces of the first active layer and the first active layer, a clad layer in contact with at least a part of the second clad layer, and a cap layer formed on the clad layer. Layer and at least the antireflection film in the light output portion, which is the end face in the stripe direction of the third active layer, and injecting a current into the first active layer and the third active layer A semiconductor optical amplifier comprising means that is the width of the third active layer a structure in which narrower than the width of the first active layer.

By doing so, when the third active layer is buried in the second current blocking layer, the reactive current of the third active layer can be reduced and the optical amplification gain of TM polarized light can be increased. be able to. In addition, the third active layer is embedded in the third clad layer, and by using a conductive type material as the material of the third clad layer, for example, the injection current flows centrally in the first active layer, Even if a phenomenon such that almost no flow occurs at both ends, a current can be injected into both ends through the third cladding layer, so that a decrease in optical amplification gain of the first active layer can be prevented. .

According to a ninth aspect of the present invention, in the semiconductor optical amplifier according to the seventh or eighth aspect, the third active layer is thicker than the first active layer. By doing so, the semiconductor optical amplifier can effectively suppress the fluctuation of the confinement coefficient due to polarization, and can perform more complete polarization-independent operation.

The invention according to claim 10 is the semiconductor optical amplifier according to any one of claims 7 to 9, wherein the first active layer has tensile strain and the third active layer has compressive strain. It is configured to have. By doing so, the semiconductor optical amplifier can perform polarization independent operation by appropriately adjusting the lateral width, thickness and strain amount of each active layer when forming the collective active layer.

An eleventh aspect of the present invention is the semiconductor optical amplifier according to any one of the seventh to tenth aspects, wherein at least the first active layer is multiplexed among the first active layer and the third active layer. The structure is a quantum well structure. As described above, by introducing the quantum well structure into at least the first active layer, the gain coefficient with respect to the injection current is increased, so that the gain of the semiconductor optical amplifier is increased.

According to a twelfth aspect of the present invention, in the semiconductor optical amplifier according to any one of the first to tenth aspects, an etching stop layer is formed on an upper surface of the first active layer,
The third active layer is provided so as to project above the etching stop layer. In this way, by inserting the etching stop layer between the first active layer and the third active layer, the third active layer having a narrow width can be accurately formed on the first active layer.

In order to achieve the above-mentioned object, a method of manufacturing a semiconductor optical amplifier according to a thirteenth aspect of the present invention is a step of forming a stripe-shaped groove on the upper surface of a semiconductor substrate and forming a second active layer in this groove. And a step of forming at least a first active layer and a third active layer on the second active layer and on the semiconductor substrate on both sides of the second active layer, and a stripe-shaped first dielectric thin film on the surface of the third active layer. A step of forming
A step of etching away the third active layer in a region where the first dielectric thin film is not formed; a step of forming a current blocking layer using the first dielectric thin film as a mask; and the first dielectric thin film. And a step of forming a second dielectric thin film having a width wider than that of the groove portion and a width wider than that of the first dielectric thin film on the upper surface of the current blocking layer on both sides of the second dielectric thin film. A step of forming a mesa stripe by etching a region where is not formed until reaching the semiconductor substrate, and a step of forming the current block layer again on both sides of the mesa stripe using the second dielectric thin film as a mask, At least as a method of including.

By doing so, isotropicity is introduced into the collective active layer, and the semiconductor optical amplifier can perform polarization independent operation. Further, for example, since the cross-sectional area of the collective active layer can be made smaller than that of a rectangular bulk active layer in which the horizontal width of the first active layer is the long side and the total thickness of each active layer is the short side, the single active mode can be made smaller. The condition can be maintained. That is, the present invention is not limited to the actual semiconductor optical amplifier, but is effective as a manufacturing method thereof.

In order to achieve the above object, a method of manufacturing a semiconductor optical amplifier according to a fourteenth aspect of the present invention is the method of forming a first current blocking layer or a first cladding layer on the upper surface of a semiconductor substrate, and the first step. A step of forming a stripe-shaped groove on the upper surface of the current blocking layer or the first cladding layer; a step of forming a second active layer in this groove; and a step of forming the second active layer and the first current blocking layer on both sides thereof A step of forming at least a first active layer, a third active layer and a second clad layer on the first clad layer; a step of forming a first dielectric thin film in a stripe pattern on the surface of the second clad layer; A step of etching away the second cladding layer and the third active layer in a region where the first dielectric thin film is not formed; and forming a current blocking layer using the first dielectric thin film as a mask. And a second dielectric thin film having a width wider than that of the groove and a width wider than that of the first dielectric thin film on the upper surface of the current blocking layer on both sides of the first dielectric thin film. A step of forming a mesa stripe by etching the region where the second dielectric thin film is not formed until reaching the semiconductor substrate, and using the second dielectric thin film as a mask on both sides of the mesa stripe. The method includes at least the step of forming the current blocking layer again.

By doing so, when the second active layer is buried in the first current blocking layer, the reactive current of the second active layer can be reduced and the optical amplification gain of TM polarized light can be increased. be able to. Further, the second active layer is embedded in the first clad layer, and by using a conductive type material as the material of the first clad layer, for example, the injection current flows centrally in the first active layer, Even if a phenomenon such that almost no flow occurs at both ends, a current can be injected into both ends through the first cladding layer, so that a decrease in optical amplification gain of the first active layer can be prevented. .

In order to achieve the above-mentioned object, a method of manufacturing a semiconductor optical amplifier according to a fifteenth aspect of the present invention is a method of forming a first current blocking layer or a first cladding layer on the upper surface of a semiconductor substrate, and the first step. A step of forming a stripe-shaped groove on the upper surface of the current blocking layer or the first cladding layer; a step of forming a second active layer in this groove; and a step of forming the second active layer and the first current blocking layer on both sides thereof A step of forming at least a first active layer, a third active layer and a second clad layer on the first clad layer; a step of forming a first dielectric thin film in a stripe pattern on the surface of the second clad layer; A step of etching away the second cladding layer and the third active layer in a region where the first dielectric thin film is not formed; and a second current blocking layer using the first dielectric thin film as a mask Or a third clad layer is formed, and the first dielectric thin film and the second current block layer or the third clad layer on both sides of the first dielectric thin film have a lateral width wider than that of the groove portion, and A step of forming a second dielectric thin film having a width wider than that of the one dielectric thin film, and a step of forming a mesa stripe by etching a region where the second dielectric thin film is not formed until reaching the semiconductor substrate,
The method includes at least a step of forming current blocking layers on both sides of the mesa stripe using the second dielectric thin film as a mask.

By doing so, when the third active layer is buried in the second current blocking layer, the reactive current of the third active layer can be reduced and the optical amplification gain of TM polarized light can be increased. be able to. Further, when the third active layer is embedded in the third cladding layer, it is possible to prevent a decrease in optical amplification gain of the third active layer due to light leakage. Further, when a conductive type material is used as the material of the third cladding layer, for example, when a phenomenon occurs in which the injected current flows intensively in the central part of the first active layer and hardly flows in both ends. However, since the current can be injected into both ends through the third cladding layer, it is possible to prevent a decrease in the optical amplification gain of the first active layer.

According to a sixteenth aspect of the present invention, in the method of manufacturing a semiconductor optical amplifier according to the fourteenth aspect or the fifteenth aspect, an etching stop layer is previously formed on the upper surface of the semiconductor substrate, and the etching stop layer is formed on the upper surface. After forming the first current blocking layer or the first cladding layer,
The method includes a step of forming the groove portion by etching, and a step of automatically stopping etching on the surface of the etching stop layer in the etching step of the first current blocking layer or the first cladding layer. By doing so, in the etching process for forming the second active layer on the first current blocking layer or the first cladding layer, the etching is automatically stopped at the etching stop layer, so the etching should be performed accurately. You can

In order to achieve the above-mentioned object, a method of manufacturing a semiconductor optical amplifier according to a seventeenth aspect of the present invention comprises a step of forming at least a first active layer and a third active layer on a semiconductor substrate, and (3) forming a stripe-shaped first dielectric thin film on the surface of the three active layers, removing the third active layer in a region where the first dielectric thin film is not formed by etching, and the first dielectric Forming a current blocking layer using the thin film as a mask; and forming a second dielectric thin film having a width wider than the first dielectric thin film on the upper surface of the first dielectric thin film and the current blocking layers on both sides of the first dielectric thin film. A step of forming a mesa stripe by etching the region where the second dielectric thin film is not formed until reaching the semiconductor substrate, and the mesa strut using the second dielectric thin film as a mask. The process of the current blocking layer again formed on both sides of the flop is a method including at least.

By doing so, the isotropic property is introduced into the collective active layer, and the polarization dependence of the optical amplification gain is improved, so that the polarization independent operation can be performed and, for example, the first Since the cross-sectional area of the collective active layer can be made smaller than that of a rectangular bulk active layer in which the lateral width of the active layer is the long side and the total thickness of the active layers is the short side, the single mode condition can be maintained. Further, by reducing the manufacturing process for forming the second active layer, the manufacturing cost of the semiconductor optical amplifier can be reduced.

In order to achieve the above-mentioned object, in the method for manufacturing a semiconductor optical amplifier according to the present invention, the at least first active layer, third active layer and second cladding layer are formed on the semiconductor substrate. A step of forming a first dielectric thin film in a stripe shape on the surface of the second clad layer, and etching the second clad layer and the third active layer in a region where the first dielectric thin film is not formed. And removing the first dielectric thin film as a mask to form the second current blocking layer or the third cladding layer, the first dielectric thin film and the second current blocking layer or the third dielectric film on both sides of the first dielectric thin film. A step of forming a second dielectric thin film having a wider width than the first dielectric thin film on the upper surface of the clad layer,
A step of forming a mesa stripe by etching the region where the second dielectric thin film is not formed until reaching the semiconductor substrate, and forming a current block layer on both sides of the mesa stripe using the second dielectric thin film as a mask. The method includes at least the step of:

By doing so, when the third active layer is buried in the second current blocking layer, the reactive current of the third active layer can be reduced and the optical amplification gain of TM polarized light can be increased. be able to. In addition, the third active layer is embedded in the third clad layer, and by using a conductive type material as the material of the third clad layer, for example, the injection current flows centrally in the first active layer, Even if a phenomenon such that almost no flow occurs at both ends, a current can be injected into both ends through the third cladding layer, so that a decrease in optical amplification gain of the first active layer can be prevented. .

The invention according to claim 19 is the above-mentioned claim 13.
19. The method for manufacturing a semiconductor optical amplifier according to claim 18, wherein an etching stop layer is provided between the first active layer and the third active layer, whereby the third active layer is etched in the etching step. The method includes a step of automatically stopping etching on the surface of the etching stop layer.

By doing so, in the step of etching the third active layer, when etching is performed up to the upper surface of the first active layer while leaving a portion of the third active layer having a narrow width, the gap between the active layers is increased. When the thin etching stop layer is inserted, the etching automatically stops at the etching stop layer, so that the etching can be performed accurately. As described above, the present invention is not limited to the actual semiconductor optical amplifier, but is effective as a manufacturing method thereof.

[0043]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A semiconductor optical amplifier and a method of manufacturing the same according to embodiments of the present invention will be described below with reference to the drawings. First, a semiconductor optical amplifier according to the embodiment of the present invention will be described. <First Embodiment> FIG. 1 is a schematic enlarged sectional view of a semiconductor optical amplifier according to the first embodiment. In the figure, reference numeral 1 denotes a semiconductor optical amplifier, which is an n-type semiconductor substrate 101.
The first active layer 10 having a horizontally elongated cross-section located at the center
3. A cross-shaped assembly including a second active layer 102 having a vertically long cross-sectional shape located below the first active layer 103 and a third active layer 104 having a vertically long cross-sectional shape located above the first active layer 103. The structure has an active layer. InGaAsP is used for the first active layer 103, the second active layer 102, and the third active layer 104.

The first active layer 103 is the n-type semiconductor substrate 10
1 on the upper surface of the stripe-shaped protrusion 101a formed in
It is formed so that the lateral width of the cross-sectional shape is several times larger than the thickness. Thus, by forming the first active layer 103 to have a horizontally long cross-sectional shape, the first active layer 103 is
It has a high optical amplification gain for E-polarized light. A current blocking layer 203 is formed on both end surfaces of the first active layer 103. The current blocking layer 203 blocks the injection current and reduces the reactive current. It is possible to prevent the decrease of the optical amplification gain.

The second active layer 102 is the n-type semiconductor substrate 10
The first active layer 102 is embedded in the first protrusion 101a, and the upper surface of the second active layer 102 is in contact with the lower surface of the first active layer 103 and is electrically connected thereto. In addition, the second active layer 102
Is formed such that the thickness of the cross-sectional shape is several times or more larger than the lateral width. In this way, by forming the second active layer 102 to have a vertically long cross-sectional shape, the second active layer 102 is formed.
Has a high optical amplification gain for TM polarized light.

The third active layer 104 is formed on the upper surface of the first active layer 103 so that the thickness of the sectional shape is several times or more larger than the lateral width. Thus, by forming the third active layer 104 to have a vertically long cross-sectional shape, the third active layer 10 is formed.
4 has a high optical amplification gain for TM polarized light. The current blocking layer 2 is formed on both end surfaces of the third active layer 104.
No. 03 is formed, and the current blocking layer 203 blocks the injection current and reduces the reactive current, so that the reduction of the optical amplification gain of the third active layer 104 due to the reactive current can be prevented.

The cladding layer 106 is formed on both sides of the third active layer 104 on the current block layer 203 and the upper surface of the third active layer 104. Specifically, the p-type In made of InP is formed.
It is a P clad layer. The cap layer 107 is formed on the upper surface of the cladding layer 106.
This is a p-type InGaAs cap layer made of GaAs.

The p-side electrode 402 is formed on the upper surface of the cap layer 107, and the n-side electrode 401 is formed on the lower surface of the n-type semiconductor substrate 101. The p-side electrode 402 and the n-side electrode 401 are connected to a DC power source, and n When a current is injected into the side electrode 401, the first active layer 103, the second active layer 102, and the third active layer 104
Amplifies the input signal light by stimulated emission. Although not shown, an antireflection film is provided on the light input portion and the light output portion located on both end faces of the (001) plane orientation of the first active layer 103, the second active layer 102, and the third active layer 104. The formation prevents irregular reflection of input light and output light.

The operation of the semiconductor optical amplifier 1 in the first embodiment configured as described above will be described. Electrode 4
By injecting a current into 01 and 402, the collective active layer composed of the first active layer 103, the second active layer 102 and the third active layer 104 has a cross-shaped cross section. Similar to the active layer, the optical amplification gains for TE polarized light and TM polarized light can be made equal. That is, the semiconductor optical amplifier 1 can perform polarization-independent operation with a single injection current by introducing isotropicity into the collective active layer.

Further, since the collective active layer is formed in a cross shape by the active layers having the vertically long and horizontally long cross sections, the collective active layer has a smaller cross section than the rectangular bulk active layer having the same vertical and horizontal dimensions. The area becomes smaller, and the semiconductor optical amplifier 1
Unlike the bulk active layer, single mode conditions can be maintained without multi-mode optical amplification. Furthermore, by forming the collective active layer so that the cross-sectional shape is a cross shape, the fluctuation of the confinement coefficient due to the polarized light can be suppressed more effectively, and the semiconductor optical amplifier 1 accurately performs the polarization independent operation. be able to. The semiconductor optical amplifier 1 of the first embodiment described above corresponds to the semiconductor optical amplifier described in claim 1.

<Second Embodiment> FIG. 2 is a schematic enlarged sectional view of a semiconductor optical amplifier according to the second embodiment.
In the figure, reference numeral 2 denotes a semiconductor optical amplifier, in which an n-type semiconductor substrate 101 has a horizontally-oriented first active layer 103 located at the center and a vertically-oriented cross-sectional shape located above the first active layer 103. The structure has a cross-shaped collective active layer composed of the third active layer 104.

Here, the semiconductor optical amplifier 1 according to the first embodiment.
Compared with the above, instead of not forming the second active layer 102, the thickness of the third active layer 104 is formed to be approximately the same as the lateral width of the first active layer 103. Other structures are similar to those of the semiconductor optical amplifier 1 of the first embodiment.

By doing so, in the semiconductor optical amplifier 2, the third active layer 104 can increase the optical amplification gain of TM polarized light, and therefore the first active layer 103 and the third active layer 104 are assembled. Since the isotropic property is introduced into the active layer and the polarization dependence of the optical amplification gain is improved, the polarization independent operation can be performed. Other effects are similar to those of the semiconductor optical amplifier 1 of the first embodiment.

As described above, the semiconductor optical amplifier 2 can perform highly accurate polarization-independent operation similarly to the semiconductor optical amplifier 1 without forming the second active layer 102. Therefore, the semiconductor optical amplifier 2 includes the second active layer 10
Since the number of manufacturing processes for forming the semiconductor optical amplifier 2 can be reduced, the manufacturing cost of the semiconductor optical amplifier 2 can be reduced. In addition,
The semiconductor optical amplifier 2 according to the second embodiment described above is characterized in that
It corresponds to the semiconductor optical amplifier described in.

<Third Embodiment> Next, a semiconductor optical amplifier according to a third embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a schematic enlarged sectional view of the semiconductor optical amplifier according to the third embodiment. In the figure, reference numeral 3 denotes a semiconductor optical amplifier, which includes a first active layer 103 having a laterally elongated centrally located cross-sectional shape on the n-type semiconductor substrate 101 and a first active layer located below the first active layer 103. The second active layer 102 and the first active layer 1 which are narrower in width and thicker than 103
03 has a structure having a collective active layer composed of a third active layer 104 which is narrower in width and thicker than the first active layer 103 located on the upper side of 03.

The first active layer 103 is the n-type semiconductor substrate 10
The first current block layer 201 and the second active layer 102 formed on the upper surface of the stripe-shaped protrusion 101b of No. 1 are formed so that the lateral width of the cross-sectional shape is several times larger than the thickness. Thus, by forming the first active layer 103 to have a horizontally long sectional shape, the first active layer 103 is formed.
Has a high optical amplification gain for TE polarization. In addition,
The material of the first active layer 103 is InGaAsP (wavelength composition 1.57 μm, strain amount −0.15%, layer thickness 300 nm, layer width 1.5 μm).

Here, preferably, by introducing the quantum well structure into the first active layer 103, the gain coefficient with respect to the injection current is increased, so that the gain of the semiconductor optical amplifier is increased. Further, more preferably, by introducing a tensile strain into the first active layer 103, the optical amplification gain for TM polarized light can be improved, and complete isotropy can be introduced into the collective active layer.

The second active layer 102 is the n-type semiconductor substrate 10
1 is embedded in the first current blocking layer 201 formed on the upper surface of the first protrusion 101b, the upper surface of the second active layer 102 is in contact with the lower surface of the first active layer 103, and is electrically connected. is there. Since the second active layer 102 is formed such that its cross-sectional shape is narrower in width and thicker than the first active layer 103, it has a high optical amplification gain for TM polarized light. In addition,
The material of the second active layer 102 is InGaAsP (wavelength composition 1.57 μm, strain amount + 0.15%, layer thickness 400 nm, layer width 500 nm).

Here, preferably, by introducing the quantum well structure into the second active layer 102, the gain coefficient with respect to the injection current is increased and the internal loss is also reduced.
The semiconductor optical amplifier 3 can increase the optical amplification gain. Further, more preferably, by introducing compressive strain into the second active layer 102, the optical amplification gain for TE polarized light can be increased, and the collective active layers can introduce complete isotropicity.

The first current blocking layer 201 is formed on both sides of the second active layer 102 and made of Fe-doped high resistance InP, so that the injection current is blocked and the reactive current is reduced. It is possible to prevent a decrease in the optical amplification gain of the biactive layer 102.

The third active layer 104 is formed on the upper surface of the etching stop layer 303 formed on the upper surface of the first active layer 103 so that its cross-sectional shape is narrower in lateral width than the first active layer 103 and thicker. By forming in this way, T
It is possible to obtain a high optical amplification gain for M-polarized light, and it is possible to accurately form the third active layer 104 on the upper surface of the etching stop layer 303 formed on the upper surface of the first active layer. The material of the third active layer 104 is I
nGaAsP (wavelength composition 1.57 μm, strain amount +0.15
%, Layer thickness 400 nm, layer width 500 nm).

The second clad layer 105 is formed on the upper surface of the third active layer 104, and the third active layer 104 is formed.
A current blocking layer 203 is formed on both end surfaces of the second cladding layer 105. By thus forming the second cladding layer 105 made of p-type InP, the amplified light can be effectively confined in the third active layer 104. It is possible to prevent a decrease in optical amplification gain. Further, since the current blocking layer 203 blocks the injection current and reduces the reactive current, it is possible to prevent the decrease in the optical amplification gain of the third active layer 104 due to the reactive current.

Here, preferably, by introducing the quantum well structure into the third active layer 104, the gain coefficient with respect to the injection current is increased and the internal loss is also reduced.
The semiconductor optical amplifier 3 has a high optical amplification gain. Further, more preferably, by introducing compressive strain into the third active layer 104, the optical amplification gain for TE polarized light can be increased, and complete isotropic property can be introduced into the collective active layer. Other structures are similar to those of the semiconductor optical amplifier 1 of the first embodiment.

The operation of the semiconductor optical amplifier 3 in the third embodiment configured as described above will be described. Electrode 4
By injecting a current into 01 and 402, the second active layer 102 and the third active layer 104 can increase the optical amplification gain for TM polarized light. Here, the second active layer 102
Unlike the semiconductor optical amplifier 1 of the first embodiment, the third active layer 104 does not have a vertically long sectional shape, but the first current blocking layer 201 and the second cladding layer 105 are formed,
First active layer 103, second active layer 102 and third active layer 1
By introducing a quantum well structure and strain in 04,
The optical amplification gain can be effectively increased. Other effects are similar to those of the semiconductor optical amplifier 1 of the first embodiment.

Therefore, the semiconductor optical amplifier 3 described above is used.
Is to make the optical amplification gains for TE polarized light and TM polarized light equal even if the cross-sectional shapes of the second active layer 102 and the third active layer 104 are not formed vertically long as in the semiconductor optical amplifier 1 of the first embodiment. Therefore, the polarization independent operation can be performed.

Further, since the second active layer 102 and the third active layer 104 can be formed thinner than the semiconductor optical amplifier 1, the second active layer 102 and the third active layer 104 are formed. The time can be shortened, and the manufacturing cost of the semiconductor optical amplifier 3 can be reduced. The semiconductor optical amplifier 3 of the third embodiment described above corresponds to the semiconductor optical amplifier when the first current blocking layer is formed on the protrusion of the semiconductor substrate described in claim 2.

As a modification of the semiconductor optical amplifier 3 in the third embodiment, the semiconductor optical amplifier 3a shown in FIG. 4 is used.
Has a structure in which a first cladding layer 201a is formed instead of forming the first current blocking layer 201. Here, the material of the first clad layer 201a can be a conductive type or a high resistance material, and by doing so, the optical amplification characteristics of the second active layer 102 can be finely adjusted. Other structures and operations are similar to those of the semiconductor optical amplifier 3 of the third embodiment. In addition,
The semiconductor optical amplifier 3a corresponds to the semiconductor optical amplifier when the first cladding layer is formed on the protrusion of the semiconductor substrate described in claim 2.

<Fourth Embodiment> Next, a semiconductor optical amplifier according to a fourth embodiment of the present invention will be described with reference to the drawings. FIG. 5 shows a schematic enlarged sectional view of a semiconductor optical amplifier according to the fourth embodiment. In the figure, reference numeral 4 denotes a semiconductor optical amplifier, and like the semiconductor optical amplifier 3 of the third embodiment, a structure having a collective active layer composed of a first active layer 103, a second active layer 102 and a third active layer 104. There is.

Further, unlike the semiconductor optical amplifier 3, the semiconductor optical amplifier 4 has the third active layer 104 and the cladding layer 10.
The third clad layer 202 is formed on both end faces of the No. 5 substrate.

Here, the material of the third cladding layer 202 is
By using the conductive p-type InP, for example, the ratio of the lateral width to the thickness of the first active layer 103 is large, and
Since the ratio of the lateral width of the first active layer 103 to the lateral width of the third active layer 104 is large, the injection current is
Even when a phenomenon occurs in which the current flows intensively in the central part of the first active layer 103 and hardly flows in both ends, current can be injected into the vicinity of both ends of the first active layer 103 through the third cladding layer 202. It is possible to prevent a decrease in the optical amplification gain of the first active layer 103. Other structures and effects are similar to those of the semiconductor optical amplifier 3 of the third embodiment.

As described above, the semiconductor optical amplifier 4 described above is used.
Is formed on both end faces of the third active layer 104.
By forming, it is possible to prevent a decrease in optical amplification gain of the third active layer 104 and the first active layer 103. The semiconductor optical amplifier 4 of the fourth embodiment described above
Corresponds to the semiconductor optical amplifier in the case where both end faces of the third active layer described in claim 3 are covered with the third cladding layer.

As a modification of the semiconductor optical amplifier 4 in the fourth embodiment, the semiconductor optical amplifier 4a shown in FIG. 6 is used.
Has a structure in which the second current blocking layer 202a is formed instead of forming the third cladding layer 202. Here, the second current blocking layer 202a is the first active layer 103.
A material different from the material of the current blocking layer 203 formed on both end surfaces of the third active layer 104 can be finely adjusted. Other structures and operations are similar to those of the semiconductor optical amplifier 4 of the fourth embodiment. Incidentally, the semiconductor optical amplifier 4a is defined by claim 3.
It corresponds to the semiconductor optical amplifier in the case where both end faces of the third active layer described in 1) are covered with the second current blocking layer.

<Fifth Embodiment> Next, a semiconductor optical amplifier according to a fifth embodiment of the present invention will be described with reference to the drawings. FIG. 7 shows a schematic enlarged sectional view of a semiconductor optical amplifier according to the fifth embodiment. In the figure, reference numeral 5 denotes a semiconductor optical amplifier, in which an n-type semiconductor substrate 101 has a horizontally-oriented first active layer 103 located at the center and a vertically-oriented cross-sectional shape located above the first active layer 103. The structure has a collective active layer including the third active layer 104.

The first active layer 103 is the n-type semiconductor substrate 10
It is formed on the upper surface of the stripe-shaped projection 101b of No. 1 so that the lateral width of the cross-sectional shape is several times larger than the thickness.
As described above, by forming the first active layer 103 to have a horizontally long cross-sectional shape, the first active layer 103 has a high optical amplification gain for TE polarized light. Further, since the second active layer 102 is not formed on the lower surface of the first active layer 103, the manufacturing process of the second active layer 102 can be reduced, so that the manufacturing cost can be reduced.

The third active layer 104 is formed on the upper surface of the etching stop layer 303 formed on the upper surface of the first active layer 103 so that the cross-sectional shape is thicker than the lateral width. By thus forming the cross-sectional shape of the third active layer 104 to be vertically long, the third active layer 104 has a high optical amplification gain for TM polarized light.

The semiconductor optical amplifier 5 is similar to the semiconductor optical amplifier 4 of the fourth embodiment in that the second clad layer 105 and the third clad layer 20 are provided on the upper surface and both end surfaces of the third active layer 104.
2 is formed, and further the first active layer 103 and the third active layer 1 are formed.
By introducing a quantum well structure and strain in 04,
Since the optical amplification gain is effectively increased, the thickness of the third active layer 104 is made smaller than the lateral width of the first active layer 103. Other structures are similar to those of the semiconductor optical amplifier 4 of the fourth embodiment.

The operation of the semiconductor optical amplifier 5 in the fifth embodiment configured as described above will be described. The semiconductor optical amplifier 5 increases the optical amplification gain with respect to TM polarized light by forming the third active layer 104 thick, and the TE polarized light and the TE polarized light can be obtained without forming the second active layer 102 unlike the semiconductor optical amplifier 4. The optical amplification gains for TM polarized light can be made equal, and polarization independent operation can be performed. As described above, by not forming the second active layer 102, the manufacturing process for forming the second active layer 102 can be reduced, so that the manufacturing cost can be reduced.

Further, since the thickness of the third active layer 104 can be formed thinner than the lateral width of the first active layer 103, the formation time for forming the third active layer 104 and the material cost can be reduced. You can Other effects are similar to those of the semiconductor optical amplifier 4 of the fourth embodiment.

As described above, the semiconductor optical amplifier 5 described above is used.
Does not form the second active layer and the third active layer 10
Since the thickness of 4 can be reduced, the manufacturing cost can be further reduced. The semiconductor optical amplifier 5 of the fifth embodiment described above corresponds to the semiconductor optical amplifier in which both end faces of the third active layer described in claim 8 are covered with the third cladding layer.

As a modification of the semiconductor optical amplifier 5 in the fifth embodiment, the semiconductor optical amplifier 5a shown in FIG. 8 is used.
Has a structure in which the second current blocking layer 202a is formed instead of forming the third cladding layer 202. Here, the second current blocking layer 202a is the first active layer 103.
A material different from the material of the current blocking layer 203 formed on both end surfaces of the third active layer 104 can be finely adjusted. Other structures and operations are similar to those of the semiconductor optical amplifier 5 of the fifth embodiment. The semiconductor optical amplifier 5a is defined in claim 8.
It corresponds to the semiconductor optical amplifier in the case where both end faces of the third active layer described in 1) are covered with the second current blocking layer.

Next, a method of manufacturing the semiconductor optical amplifier according to the embodiment of the present invention will be described. (First Embodiment) As a first embodiment of the manufacturing method of the present invention, a method of manufacturing a semiconductor optical amplifier will be described with reference to the drawings. FIG. 9 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the first embodiment of the present invention.
(I) has shown the schematic expanded sectional view of the 1st process-9th process. Further, in the manufacturing method according to the first embodiment, all crystal growth is performed by a metal organic chemical vapor deposition method. In addition,
This manufacturing method corresponds to the manufacturing method of the semiconductor optical amplifier 1 shown in FIG.

First, in FIG. 9A, as a first step, a mask alignment marker (not shown) is formed on the surface of the n-type semiconductor substrate 101 having the (100) plane orientation. After that, a SiO ₂ film 403 is formed on the upper surface of the n-type semiconductor substrate 101 by the thermal CVD method.
A mask opening is formed in the [011] direction by a lithography process using a stepper. Then, the n-type semiconductor substrate 101 in the mask opening is removed by etching to form a groove 211.

In FIG. 10B, in a second step, the second active layer 102 is embedded in the groove 211. Here, the upper surface of the second active layer 102 is the SiO ₂ film 40.
Embedding is performed until the height is almost the same as the upper surface of 3.
Further, by forming the second active layer 102 so that the sectional shape of the second active layer 102 is thicker than the lateral width, the optical amplification gain for TM polarized light can be increased.

In FIG. 7C, the third step is S
After removing the iO ₂ film 403 with buffered hydrofluoric acid, the first active layer 103 and the third active layer 10 are formed on the upper surface of the second active layer 102 and the upper surfaces of the n-type semiconductor substrate 101 on both sides thereof.
4 are continuously formed. Here, by forming the third active layer 104 thick, the optical amplification gain of TM polarized light can be increased.

In the same step (d), as a fourth step, the SiO ₂ film 403 is formed again on the upper surface of the third active layer 104, and then photolithography using a stepper is performed.
The first dielectric thin film 403a is formed in a stripe shape. Here, the first dielectric thin film 403a can be formed right above the position of the second active layer 102 by performing the alignment using the alignment pattern.

In FIG. 8E, in a fifth step, the third active layer 104 in the region where the first dielectric thin film 403a is not formed is removed by etching. This etching can be performed by, for example, methane-based reactive ion etching.

In FIG. 6F, in a sixth step, the current blocking layer 203 is formed on the upper surface of the first active layer 103 so as to fill the second active layer 104 with the first dielectric thin film 403a as a mask. . This current blocking layer 203
Since the injection current is blocked and the reactive current is reduced, it is possible to prevent the reduction of the optical amplification gain of the third active layer 104 due to the reactive current.

In FIG. 9G, as a seventh step, the SiO ₂ film 403 is formed again on the upper surfaces of the current block layer 203 and the first dielectric thin film 403a. And this S
A second dielectric thin film 403b having a width wider than that of the groove 211 and a width wider than that of the first dielectric thin film 403a is formed on the iO ₂ film 403 by photolithography using a stepper. Here, by performing the alignment using the alignment pattern, the second dielectric thin film 4 is aligned so that the center line of the first dielectric thin film 403a is aligned.
03b can be formed.

In FIG. 8H, as an eighth step, a region where the second dielectric thin film 403a is not formed is etched until it reaches the n-type semiconductor substrate 101 to form a mesa stripe. This etching can be performed by, for example, methane-based reactive ion etching.

In the figure (i), as the ninth step, n
The second dielectric thin film 403b is formed on the upper surface of the semiconductor substrate 101.
Using the as a mask, the current block layers 203 are formed on both sides of the mesa stripe. Since the current blocking layer 203 blocks the injection current and reduces the reactive current, it is possible to prevent the reduction of the optical amplification gain of the first active layer 103 due to the reactive current.

As a final step, as shown in FIG. 1, after removing the second dielectric thin film 403b, the cladding layer 106 and the cap layer 107 are continuously formed on the upper surfaces of the third active layer 104 and the current block layer 203. Form. Then, after forming the p-side electrode 402 on the surface of the cap layer 107 and polishing the lower surface of the n-type semiconductor substrate 101, the n-side electrode 4 is formed on the lower surface.
01 is formed. Further, in the semiconductor optical amplifier 1 manufactured by the above manufacturing process, after cleavage, although not shown, an antireflection film is formed on both end faces of the (001) plane orientation, and after mounting on a heat sink, It is modularized by the process of.

According to the method for manufacturing the semiconductor optical amplifier in the above-described first embodiment, the isotropic property is introduced into the collective active layer of the semiconductor optical amplifier 1, and the polarization dependence of the optical amplification gain is improved, so that the polarization Independent operation is possible, and, for example, compared to a rectangular bulk active layer in which the lateral width of the first active layer is the long side and the total thickness of each active layer is the short side, the disconnection of the collective active layer is reduced. Since the area can be reduced, the single mode condition can be maintained. The manufacturing method of the semiconductor optical amplifier in this case corresponds to the manufacturing method of the semiconductor optical amplifier described in claim 13.

(Second Embodiment) Next, as a second embodiment of the manufacturing method of the present invention, a method of manufacturing a semiconductor optical amplifier will be described with reference to the drawings. FIG. 10 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the second embodiment of the present invention, in which (a) to (g) are first to seventh steps. The schematic expanded sectional view is shown. This manufacturing method corresponds to the manufacturing method of the semiconductor optical amplifier 2 shown in FIG.

First, in FIG. 10A, the first active layer 103 and the third active layer 1 are formed on the upper surface of the n-type semiconductor substrate 101.
04 are continuously formed. Here, by forming the third active layer 104 thick, the optical amplification gain of TM polarized light can be increased.

In the same figure (b), as the second step, S
After forming the iO ₂ film 403 on the upper surface of the third active layer 104, the first dielectric thin film 403a is formed in stripes by photolithography using a stepper.

In FIG. 9C, as the third step, the third active layer 104 in the region where the first dielectric thin film 403a is not formed is removed by etching. This etching can be performed by, for example, methane-based reactive ion etching.

In the same step (d), as a fourth step, the current blocking layer 203 is formed on the upper surface of the first active layer 103 so as to fill the second active layer 104 with the first dielectric thin film 403a as a mask. . This current blocking layer 203
As a result, the injection current is blocked and the reactive current is reduced, so that the reduction of the optical amplification gain of the third active layer 104 due to the reactive current can be prevented.

In the same step (e), as a fifth step, the SiO ₂ film 403 is formed again on the upper surfaces of the current block layer 203 and the first dielectric thin film 403a. And this S
A second dielectric thin film 403b having a width wider than that of the first dielectric thin film 403a is formed on the iO ₂ film 403 by photolithography using a stepper.

In FIG. 6F, as a sixth step, a region in which the second dielectric thin film 403a is not formed is etched until it reaches the n-type semiconductor substrate 101 to form a mesa stripe. This etching can be performed by, for example, methane-based reactive ion etching.

In the same figure (g), as the seventh step, n
The second dielectric thin film 403b is formed on the upper surface of the semiconductor substrate 101.
Using the as a mask, the current block layers 203 are formed on both sides of the mesa stripe. Since the current blocking layer 203 blocks the injection current and reduces the reactive current, it is possible to prevent the reduction of the optical amplification gain of the first active layer 103 due to the reactive current.

As a final step, as shown in FIG. 2, after removing the second dielectric thin film 403b, the cladding layer 106 and the cap layer 107 are continuously formed on the upper surfaces of the third active layer 104 and the current block layer 203. Form. Other manufacturing methods are the same as those in the first embodiment.

According to the method of manufacturing the semiconductor optical amplifier in the second embodiment described above, the semiconductor optical amplifier 2 can reduce the manufacturing process for forming the second active layer, and thus the manufacturing cost of the semiconductor optical amplifier 2 can be reduced. It can be reduced. Other effects are similar to those of the manufacturing method according to the first embodiment. The method of manufacturing the semiconductor optical amplifier in this case corresponds to the method of manufacturing the semiconductor optical amplifier described in claim 17.

(Third Embodiment) Next, as a third embodiment of the manufacturing method of the present invention, a method of manufacturing a semiconductor optical amplifier will be described with reference to the drawings. FIG. 11 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the third embodiment of the present invention, in which (a) to (i) are first to ninth steps. The schematic expanded sectional view is shown. This manufacturing method corresponds to the manufacturing method of the semiconductor optical amplifier 3 shown in FIG.

First, in FIG. 11A, the first current block layer 201 and the SiO ₂ film 403 are formed on the upper surface of the n-type semiconductor substrate 101 by the thermal CVD method, and then a mask is formed by a lithography process using a stepper. The opening is formed in the [011] direction. Then, by etching
The groove 211 is formed by removing the first current blocking layer 201 in the mask opening.

In the second step of FIG. 10B, the second active layer 102 is buried in the groove 211 until its upper surface is substantially level with the upper surface of the SiO ₂ film 403. By doing so, the second active layer 102 is
Since the first current blocking layer 201 blocks the injection current and reduces the reactive current, it is possible to prevent the reduction of the optical amplification gain of the second active layer 102 due to the reactive current.

In the same figure (c), as the third step, S
After removing the iO ₂ film 403 with buffered hydrofluoric acid, the first active layer 103, the etching stop layer 303, and the third active layer are formed on the upper surface of the second active layer 102 and the upper surfaces of the first current blocking layer 201 on both sides of the second active layer 102. 104 and the second cladding layer 105 are continuously formed. Here, the third active layer 10
By increasing the thickness of No. 4, the optical amplification gain of TM polarized light can be increased.

In FIG. 7D, the fourth step is S
After forming the iO ₂ film 403 on the upper surface of the third active layer 104, the first dielectric thin film 403a is formed in stripes by photolithography using a stepper.

In the same step (e), as the fifth step, the third active layer 104 and the second cladding layer 105 in the region where the first dielectric thin film 403a is not formed are removed by etching to form a mesa stripe. . Here, first, etching is performed to the vicinity of the etching stop layer 303 by methane-based reactive ion etching, and then etching is performed with a mixed solution of sulfuric acid, hydrogen peroxide solution and water. By doing so, this mixed solution can etch the third active layer 104, but cannot remove the etching stop layer, so that etching automatically stops at the etching stop layer, and accurate etching can be performed. .

In FIG. 10F, in a sixth step, the current is applied to the upper surface of the first active layer 103 so that the second clad layer 105 and the third active layer 104 are embedded with the first dielectric thin film 403a as a mask. The block layer 203 is formed.

In FIG. 9G, as the seventh step, the SiO ₂ film 403 is formed again on the upper surfaces of the current block layer 203 and the first dielectric thin film 403a. And this S
A second dielectric thin film 403b having a width wider than that of the groove portion 211 and a width wider than that of the first dielectric thin film 403a is formed on the iO ₂ film 403 by photolithography using a stepper.

In FIG. 11H, as an eighth step, a region where the second dielectric thin film 403a is not formed is etched until it reaches the n-type semiconductor substrate 101 to form a mesa stripe. This etching can be performed by, for example, methane-based reactive ion etching.

In the figure (i), as the ninth step, n
The second dielectric thin film 403b is formed on the upper surface of the semiconductor substrate 101.
Using the as a mask, the current block layers 203 are formed on both sides of the mesa stripe.

As a final step, as shown in FIG. 3, after removing the second dielectric thin film 403b, the cladding layer 106 and the cap layer 107 are continuously formed on the upper surfaces of the third active layer 104 and the current block layer 203. Form. Other manufacturing methods are the same as those in the first embodiment.

According to the method of manufacturing a semiconductor optical amplifier in the third embodiment described above, in the semiconductor optical amplifier 3, the second active layer is embedded in the first current blocking layer 201, so that
The reactive current of the second active layer can be reduced and the optical amplification gain of TM polarized light can be increased. Other effects are similar to those of the manufacturing method according to the first embodiment. The method of manufacturing the semiconductor optical amplifier in this case corresponds to the method of manufacturing the semiconductor optical amplifier in which the first current blocking layer is formed on the protrusion of the method of manufacturing the semiconductor optical amplifier described in claim 14.

(Fourth Embodiment) Next, as a fourth embodiment of the manufacturing method of the present invention, a method of manufacturing a semiconductor optical amplifier will be described with reference to the drawings. FIG. 12 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the fourth embodiment of the present invention, in which (a) to (i) are first to ninth steps. The schematic expanded sectional view is shown. This manufacturing method corresponds to the method for manufacturing the semiconductor optical amplifier 4a shown in FIG.

First, the manufacturing method from the first step shown in FIG. 12A to the fifth step shown in FIG. 12E is the same as the manufacturing method in the third embodiment.

Next, in FIG. 6F, as a sixth step, a first dielectric thin film 403 is formed on the upper surface of the first active layer 103.
The second clad layer 105 and the third active layer 1 using a as a mask
The second current block layer 202a is formed so as to embed 04. Here, the second current blocking layer 202a is the current blocking layer 20 formed on both end surfaces of the first active layer 103.
The third active layer 1 can be made of a material different from that of
The optical amplification gain of 04 can be finely adjusted.

Next, the manufacturing method from the seventh step shown in FIG. 12 (g) to the ninth step shown in FIG. 12 (i) is the same as the manufacturing method in the third embodiment.

As a final step, as shown in FIG. 6, after removing the second dielectric thin film 403b, the cladding layer 106 and the cap layer 107 are continuously formed on the upper surfaces of the third active layer 104 and the current block layer 203. Form. Other manufacturing methods are the same as those in the third embodiment.

According to the method for manufacturing a semiconductor optical amplifier in the fourth embodiment described above, in the semiconductor optical amplifier 4a, the third active layer 104 is buried in the second current blocking layer 202a, so that the third Since the decrease in the optical amplification gain of the active layer can be prevented, the optical amplification gain can be increased and the second current blocking layer 202a can be formed.
Can be made different from the material of the current blocking layer 203, and the optical amplification gain of the third active layer 104 can be finely adjusted. Other operations are similar to those of the manufacturing method according to the third embodiment. The method for manufacturing a semiconductor optical amplifier in this case is the method for manufacturing a semiconductor optical amplifier according to claim 15, wherein both end surfaces of the third active layer are covered with the second current blocking layer. Corresponding to.

Next, as a first example of the method for manufacturing the semiconductor optical amplifier in the above-described fourth embodiment, a method for manufacturing a semiconductor optical amplifier (wavelength 1.55 μm band semiconductor optical amplifier) will be described with reference to the drawings. To do. FIG. 13 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier after the final step in the method for manufacturing a semiconductor optical amplifier of the first embodiment. In the figure, 6 is a semiconductor optical amplifier, which has a structure in which etching stop layers 301 and 303 and a spacer layer 302 are formed in a semiconductor optical amplifier 4a in a modification of the fourth embodiment of the semiconductor optical amplifier shown in FIG. .

FIG. 14 is a schematic enlarged sectional view of the semiconductor optical amplifier in each manufacturing process, and (a) to (h) show schematic enlarged sectional views of the first to eighth steps. In addition, in the manufacturing method of the first example, all crystal growth was performed by the metal organic chemical vapor deposition method.

First, in FIG. 11A, as a first step, a mask alignment marker (not shown) is formed on the surface of the semiconductor substrate 101 made of n-type InP having a (100) plane orientation. did. Then, on the upper surface of the n-type semiconductor substrate 101, an etching stop layer 301 made of n-type InGaAsP (wavelength composition 1.3 μm, carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 5 nm) and a Fe-doped high-resistance InP first layer were formed. A one-current blocking layer 201 (Fe concentration 5 × 10 ¹⁶ cm ⁻³ , layer thickness 1 μm) was formed. The first current blocking layer 201 blocks the injection current and reduces the reactive current.
It is possible to prevent the optical amplification gain of 02 from decreasing.

Next, in the same figure (b), as a second step, a SiO ₂ film 403 (thickness 100 nm) is formed on the upper surface of the first current block layer 201 by a thermal CVD method, and then a stepper is used. The mask opening with a layer width of 0.7 μm and a period of 250 μm was formed by the above-mentioned lithography process.
1] direction. Here, the width of the SiO ₂ film 403 on both sides of the opening was 20 μm.

Next, by etching using a mixed solution of hydrochloric acid and phosphoric acid, the current blocking layer 20 in the mask opening is formed.
1 was removed to form a groove 211. Here, the groove 211 by etching is stopped at this surface by the etching stop layer 301, and the side surface of the groove 211 is (10
It is a plane perpendicular to the upper surface of the n-type semiconductor substrate 101 having the (0) plane orientation. Therefore, the second active layer 102 is formed in the groove 211.
The shape of the second active layer 102 can be formed with high precision in the case of embedding, and as a result, the semiconductor optical amplifier 6 can perform more complete polarization independent operation.

Next, as shown in FIG. 13C, as the third step, the second active layer 10 made of undoped InGaAsP is used.
2 (wavelength composition 1.5 μm, layer thickness 1 μm) was embedded in the groove 211. Here, the second active layer 102 is formed by embedding until its upper surface is substantially level with the upper surface of the SiO ₂ film 403.

In FIG. 10D, the fourth step is S
After removing the iO ₂ film 403 with buffered hydrofluoric acid, an n-type InP spacer layer 302 (carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 10 nm) on the entire surface, a first active layer having a multiple quantum well structure 103, etching stop layer 303 made of n-type InP (carrier concentration 1 × 10
¹⁸ cm ^-3 , layer thickness 20 nm), undoped InGaAsP
Active layer 104 (wavelength composition: 1.5 μm, layer thickness: 1 μm) and a second clad layer 105 made of p-type InP
(Carrier concentration 5 × 10 ¹⁷ cm ⁻³ , layer thickness 0.3 μm) was continuously formed.

Here, the first active layer 102 having a quantum well structure is a 10-period InGaAsP quantum well (strain amount-
0.1%, layer thickness 6 nm) and InGaAsP barrier (lattice matching, wavelength composition 1.2 μm, layer thickness 8 nm), and although not shown, light confinement with a layer thickness of 30 nm by the same composition as the barrier on both sides A layer is formed. By forming the spacer layer 302, the interface of the first active layer 103 having the quantum well structure can be made sufficiently flat.

In the same figure (e), as a fifth step, after the SiO ₂ film 403 (thickness 200 nm) is formed again on the surface of the second cladding layer 105, the lateral width of 0.7 μm is formed by photolithography using a stepper. Processed into stripes. Here, the width of 0.7 formed in the second step
The alignment was performed using the alignment pattern so that the same position as that of the second active layer 102 of μm was obtained. Then, etching is performed to a depth of about 1.2 μm by methane-based reactive ion etching, and then etching is performed with a mixed solution of sulfuric acid, hydrogen peroxide solution and water, and etching is stopped on the surface of the etching stop layer 303. At the same time, a mesa stripe having a width of 0.7 μm was formed.

In FIG. 15F, as a sixth step, the second active layer 104 is formed on the upper surface of the etching stop layer 303.
The second current blocking layer 202a (Fe concentration of 5 × 10 ¹⁶ c
m ⁻³ , layer thickness 2 μm). The second current blocking layer 202a blocks the injection current and reduces the reactive current, so that the reduction of the optical amplification gain of the third active layer 104 due to the reactive current can be prevented.

In the same figure (g), as the seventh step, after forming the SiO ₂ film 403 (thickness 200 nm) again on the surface, it was processed into a stripe shape having a lateral width of 1.5 μm by photolithography using a stepper. Here, the alignment was performed using the alignment pattern so that the center line of the SiO ₂ film having a width of 0.7 μm already formed was aligned. Then, by methane-based reactive ion etching, the semiconductor substrate 101 was etched to a depth of about 4 μm.

Next, in the same figure (h), as the eighth step, after the surface treatment with concentrated sulfuric acid, the current blocking layer 203 (made of Fe-doped high resistance InP) so as to fill the first active layer 103 ( Fe concentration 5 × 10 ¹⁶ cm ^-3 , layer thickness 4
μm) was formed.

Next, in the final step shown in FIG.
After removing the SiO ₂ film 403 on the surface, the cladding layer 10 made of p-type InP is formed on the upper surfaces of the second cladding layer 105, the second current blocking layer 202a and the current blocking layer 203.
6 (carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 4 μm) and p
A cap layer 107 (carrier concentration: 1 × 10 ¹⁹ cm ⁻³ , layer thickness: 0.3 μm) made of InGaAsP type was continuously formed.

Then, a p-side electrode 402 made of Cr or Au was formed on the surface of the cap layer 107, and after polishing the lower surface of the substrate 101, an n-side electrode 401 made of AuGeNi was formed on the lower surface. The semiconductor optical amplifier 6 manufactured by the above manufacturing process, after cleaving, forms an antireflection film (reflectance 0.001%) on both end faces of the (001) plane orientation, which is not shown, and is formed on the heat sink. After mounting on a substrate, it was modularized by a usual process.

According to the method of manufacturing a semiconductor optical amplifier in the above-described first embodiment, the semiconductor optical amplifier 6 has a lateral width of 1.
It is possible to form an active layer having a laminated structure composed of a first active layer 103 composed of a multiple quantum well of 5 μm, a second active layer 102 composed of a bulk having a width of 0.7 μm, and a third active layer. It was possible to manufacture the semiconductor optical amplifier 6 having good characteristics with almost no occurrence. Regarding the structure of the active layer for realizing the polarization independent operation,
Of course, values other than those described in the first embodiment can be used.

(Fifth Embodiment) Next, as a fifth embodiment of the manufacturing method of the present invention, a method of manufacturing a semiconductor optical amplifier will be described with reference to the drawings. FIG. 15 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step of the method for manufacturing a semiconductor optical amplifier according to the fifth embodiment of the present invention, in which (a) to (g) are first to seventh steps. The schematic expanded sectional view is shown. This manufacturing method corresponds to the manufacturing method of the semiconductor optical amplifier 5 shown in FIG.

First, in FIG. 10A, the first active layer 103, the etching stop layer 303, the third active layer 104, and the cladding layer 105 are formed on the upper surface of the n-type semiconductor substrate 101.
Are continuously formed. Here, by forming the third active layer 104 thick, the optical amplification gain of TM polarized light can be increased.

In the same figure (b), as a second step, after the SiO ₂ film 403 is formed again on the upper surface of the cladding layer 105, it is subjected to photolithography using a stepper.
The first dielectric thin film 403a is formed in a stripe shape.

In FIG. 13C, as a third step, the third active layer 104 and the cladding layer 105 in the region where the first dielectric thin film 403a is not formed are removed by etching to form a mesa stripe.

In the same step (d), as the fourth step, the third clad layer 105 and the third active layer 104 are embedded on the upper surface of the first active layer 103 by using the first dielectric thin film 403a as a mask. Form the layer 202.

In the same step (e), as a fifth step, the SiO ₂ film 403 is formed again on the upper surfaces of the current block layer 203 and the first dielectric thin film 403a. And this S
A second dielectric thin film 403b having a width wider than that of the groove 211 and a width wider than that of the first dielectric thin film 403a is formed on the iO ₂ film 403 by photolithography using a stepper.

In FIG. 16F, in a sixth step, a region where the second dielectric thin film 403a is not formed is etched until it reaches the n-type semiconductor substrate 101 to form a mesa stripe.

In the same figure (g), as the seventh step, n
The second dielectric thin film 403b is formed on the upper surface of the semiconductor substrate 101.
Using the as a mask, the current block layers 203 are formed on both sides of the mesa stripe.

As a final step, as shown in FIG. 7, after removing the second dielectric thin film 403b, the cladding layer 106 and the cap layer 107 are continuously formed on the upper surfaces of the third active layer 104 and the current block layer 203. Form. Other manufacturing methods are the same as those in the second embodiment.

According to the method for manufacturing a semiconductor optical amplifier in the fifth embodiment described above, in the semiconductor optical amplifier 5, the third active layer 104 is embedded in the third clad layer 202, and the material of the third clad layer is a conductive type. By using the material of (3), for example, even when a phenomenon occurs in which the injection current intensively flows in the central portion of the first active layer and hardly flows in both end portions, it is possible to intervene through the third cladding layer. Since current can be injected into both ends, it is possible to prevent a decrease in the optical amplification gain of the first active layer. Other operations are similar to those of the manufacturing method according to the second embodiment. The manufacturing method of the semiconductor optical amplifier in this case corresponds to the manufacturing method of the semiconductor optical amplifier described in claim 18.

As described above, the semiconductor optical amplifier and the manufacturing method thereof in each of the above-described embodiments include various modifications. For example, each current blocking layer is not limited to the above structure, but is a p-type. InP layer and n-type In
It is also possible to use a structure in which P layers are stacked. Further, it goes without saying that various configurations of the semiconductor optical amplifier, for example, a configuration in which the waveguide direction is tilted from the [011] direction by several degrees or a window structure is introduced to suppress the end facet reflection can be applied.

[0147]

As described above, according to the present invention,
In a semiconductor optical amplifier, an isotropic property is introduced by a collective active layer composed of a first active layer having a vertically long cross section, a second active layer and a third active layer having a vertically long cross section, and polarization of optical amplification gain is obtained. Since the dependence is improved, polarization independent operation can be performed with a single injection current. Further, in the semiconductor optical amplifier, the sectional area of the collective active layer can be made smaller than that of the rectangular bulk active layer, so that the single mode condition can be maintained.

Further, the semiconductor optical amplifier can perform polarization independent operation in a wide current range by introducing the quantum well structure. Further, by varying the lateral width, thickness, structure, strain amount of each active layer and the structure around each active layer, it is possible to design a wide range of semiconductor optical amplifier devices.
Further, the present invention is not limited to the actual semiconductor optical amplifier, but is effective as a manufacturing method thereof.

[Brief description of drawings]

FIG. 1 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a first embodiment.

FIG. 2 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a second embodiment.

FIG. 3 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a third embodiment.

FIG. 4 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a modification of the third embodiment.

FIG. 5 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a fourth embodiment.

FIG. 6 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a modification of the fourth embodiment.

FIG. 7 is a schematic enlarged sectional view of a semiconductor optical amplifier according to a fifth embodiment.

FIG. 8 is a schematic enlarged cross-sectional view of a semiconductor optical amplifier according to a modification of the fifth embodiment.

FIG. 9 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the first embodiment of the present invention, in which (a) to (i) show the first step to The schematic enlarged cross-sectional view of the ninth step is shown.

FIG. 10 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the second embodiment of the present invention, in which (a) to (g).
Shows a schematic enlarged cross-sectional view of the first step to the seventh step.

FIG. 11 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the third embodiment of the present invention, in which (a) to (i).
Shows schematic enlarged cross-sectional views of the first step to the ninth step.

FIG. 12 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the fourth embodiment of the present invention, which includes (a) to (i).
Shows schematic enlarged cross-sectional views of the first step to the ninth step.

FIG. 13 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier after the final step in the method for manufacturing a semiconductor optical amplifier according to the first embodiment.

FIG. 14 is a schematic enlarged cross-sectional view of a semiconductor optical amplifier in each manufacturing process, and (a) to (h) show schematic enlarged cross-sectional views of a first process to an eighth process.

FIG. 15 is a schematic enlarged cross-sectional view of the semiconductor optical amplifier in each manufacturing step in the method for manufacturing a semiconductor optical amplifier according to the fifth embodiment of the present invention, in which FIGS.
Shows a schematic enlarged cross-sectional view of the first step to the seventh step.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 semiconductor optical amplifier 2 semiconductor optical amplifier 3 semiconductor optical amplifier 3a semiconductor optical amplifier 4 semiconductor optical amplifier 4a semiconductor optical amplifier 5 semiconductor optical amplifier 5a semiconductor optical amplifier 6 semiconductor optical amplifier 101 n-type semiconductor substrate 101a protrusion 101b protrusion 102 second Active layer 103 First active layer 104 Third active layer 105 Second cladding layer 106 Cladding layer 107 Cap layer 201 First current blocking layer 201a First cladding layer 202 Third cladding layer 202a Second current blocking layer 203 Current blocking layer 211 Groove 301 Etching stop layer 302 Spacer layer 303 Etching stop layer 401 n-side electrode 402 p-side electrode 403 SiO ₂ film 403a First dielectric thin film 403b Second dielectric thin film

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (19)

(57) [Claims] 1. A first active layer formed on the upper surface of a stripe-shaped protrusion of a semiconductor substrate, protruding from a lower portion of the first active layer, and buried in a groove formed in the protrusion of the semiconductor substrate. At least one of a second active layer, a third active layer projecting above the first active layer, a current blocking layer covering at least the top of the semiconductor substrate and both end faces of the first active layer, and at least one of the third active layers And at least a cap layer formed on the clad layer and the clad layer in contact with the part,
The first active layer, the second active layer, and the third active layer are provided with at least an antireflection film in the light input portion and the light output portion that are end faces in the stripe direction, and the first active layer and the second active layer. And a semiconductor optical amplifier comprising means for injecting a current into the third active layer, wherein the widths of the second active layer and the third active layer are narrower than the width of the first active layer. Semiconductor optical amplifier. 2. A first clad layer or a first current blocking layer formed on a stripe-shaped protrusion of a semiconductor substrate,
A first active layer formed on the first clad layer or the first current blocking layer, and a groove provided on the first clad layer or the first current blocking layer so as to protrude below the first active layer. A second active layer buried in the first active layer, a third active layer protruding above the first active layer, a second clad layer covering the upper surface of the third active layer, at least the upper portion of the semiconductor substrate and the first active layer. The first active layer and the second active layer, which have at least a current blocking layer covering both end faces of the layer, a clad layer in contact with at least a part of the second clad layer, and a cap layer formed on the clad layer. And at least an antireflection film in the light input portion and the light output portion, which are the end faces of the third active layer in the stripe direction, and inject current into the first active layer, the second active layer, and the third active layer. Means A semiconductor optical amplifier are example, a semiconductor optical amplifier, characterized in that the width of said second active layer and said third active layer and narrower than the width of the first active layer. 3. A first clad layer or a first current blocking layer formed on a stripe-shaped protrusion of a semiconductor substrate,
A first active layer formed on the first clad layer or the first current blocking layer, and a groove provided on the first clad layer or the first current blocking layer so as to protrude below the first active layer. A second active layer embedded in the first active layer, a third active layer protruding above the first active layer, a second clad layer covering the upper surface of the third active layer, and at least both end faces of the second clad layer. A third clad layer or a second current blocking layer covering both end faces of the third active layer, a current blocking layer covering at least the upper part of the semiconductor substrate and both end faces of the first active layer, and at least a part of the second clad layer. A light input part and a light output which are at least a clad layer in contact with the clad layer and a cap layer formed on the clad layer, and which are end faces in the stripe direction of the first active layer, the second active layer and the third active layer. A semiconductor optical amplifier comprising at least an antireflection film, and means for injecting a current into the first active layer, the second active layer and the third active layer, A semiconductor optical amplifier, wherein the lateral width of the three active layers is narrower than the lateral width of the first active layer. 4. The semiconductor optical amplifier according to claim 1, wherein the second active layer and the third active layer are thicker than the first active layer. A semiconductor optical amplifier characterized by: 5. The semiconductor optical amplifier according to claim 1, wherein the first active layer has tensile strain, and the second active layer and the third active layer are compressed. A semiconductor optical amplifier characterized by having distortion. 6. The semiconductor optical amplifier according to claim 1, wherein at least the first active layer among the first active layer, the second active layer, and the third active layer is multiplexed. A semiconductor optical amplifier having a quantum well structure. 7. A first active layer formed on the upper surface of a stripe-shaped protrusion of a semiconductor substrate, a third active layer protruding above the first active layer, at least the upper portion of the semiconductor substrate and the first active layer. The first active layer and the third active layer have at least a current blocking layer covering both end faces of the layer, a clad layer in contact with at least a part of the third active layer, and a cap layer formed on the clad layer. Is a semiconductor optical amplifier having at least an antireflection film in the light input portion and the light output portion, which are the end faces in the stripe direction, and having means for injecting a current into the first active layer and the third active layer. The lateral width of the third active layer is narrower than the lateral width of the first active layer. 8. A first active layer formed on the upper surface of a stripe-shaped protrusion of a semiconductor substrate, a third active layer protruding above the first active layer, and a first active layer covering the upper surface of the third active layer. A second clad layer, at least a third clad layer or a second current block layer covering both end faces of the second clad layer and both end faces of the third active layer, at least an upper part of the semiconductor substrate and both end faces of the first active layer. At least a current blocking layer for covering, a clad layer in contact with at least a part of the second clad layer, and a cap layer formed on the clad layer are provided, and the first active layer and the third active layer are arranged in the stripe direction. A semiconductor optical amplifier comprising at least an antireflection film in an optical input section and an optical output section, which are end faces, and comprising means for injecting a current into the first active layer and the third active layer, Serial semiconductor optical amplifier the width of the third active layer, characterized in that narrower than the width of the first active layer. 9. The semiconductor optical amplifier according to claim 7, wherein the third active layer is thicker than the first active layer. 10. The semiconductor optical amplifier according to claim 7, wherein the first active layer has tensile strain and the third active layer has compressive strain. Characteristic semiconductor optical amplifier. 11. The semiconductor optical amplifier according to claim 7, wherein at least the first active layer of the first active layer and the third active layer has a multiple quantum well structure. A semiconductor optical amplifier characterized in that. 12. The semiconductor optical amplifier according to claim 1, wherein an etching stop layer is formed on the upper surface of the first active layer, and the third active layer is formed on the etching stop layer. A semiconductor optical amplifier characterized in that layers are provided in a protruding manner. 13. A step of forming a stripe-shaped groove on an upper surface of a semiconductor substrate, a step of forming a second active layer in the groove, and a step of forming at least a first active layer and an upper portion of the semiconductor substrate on both sides of the second active layer. A step of forming an active layer and a third active layer, a step of forming a first dielectric thin film in a stripe shape on the surface of the third active layer, and a step of forming the third dielectric layer in a region where the first dielectric thin film is not formed. A step of removing the active layer by etching; a step of forming a current block layer using the first dielectric thin film as a mask; and a width of the groove on the upper surface of the first dielectric thin film and the current block layer on both sides of the first dielectric thin film. A step of forming a second dielectric thin film having a wider width and a wider width than the first dielectric thin film, and etching a region where the second dielectric thin film is not formed until reaching the semiconductor substrate. Process and method of manufacturing a semiconductor optical amplifier which comprises at least a step of the current blocking layer again formed on both sides of the mesa stripe of the second dielectric thin film as a mask to form a mesa stripe. 14. A step of forming a first current blocking layer or a first cladding layer on the upper surface of a semiconductor substrate, and a step of forming a stripe-shaped groove on the upper surface of the first current blocking layer or the first cladding layer, A step of forming a second active layer in the groove, and at least a first active layer, a third active layer and a second clad layer on the second active layer and the first current blocking layer or the first clad layer on both sides thereof. A step of forming a first dielectric thin film on the surface of the second clad layer in a stripe shape, and the second clad layer and the third active layer in a region where the first dielectric thin film is not formed. Removing the layer by etching, forming a current blocking layer using the first dielectric thin film as a mask, the first dielectric thin film and the upper surface of the current blocking layer on both sides of the first dielectric thin film, Forming a second dielectric thin film having a width larger than that of the first dielectric thin film and a width wider than that of the first dielectric thin film, and reaching a region where the second dielectric thin film is not formed to the semiconductor substrate. A method for manufacturing a semiconductor optical amplifier, comprising at least a step of forming a mesa stripe by etching and a step of forming the current blocking layer again on both sides of the mesa stripe using the second dielectric thin film as a mask. . 15. A step of forming a first current blocking layer or a first cladding layer on the upper surface of a semiconductor substrate, and a step of forming a stripe-shaped groove on the upper surface of the first current blocking layer or the first cladding layer, A step of forming a second active layer in the groove, and at least a first active layer, a third active layer and a second clad layer on the second active layer and the first current blocking layer or the first clad layer on both sides thereof. A step of forming a first dielectric thin film on the surface of the second clad layer in a stripe shape, and the second clad layer and the third active layer in a region where the first dielectric thin film is not formed. A step of removing the layer by etching, a step of forming a second current blocking layer or a third clad layer using the first dielectric thin film as a mask, the first dielectric thin film and the second dielectric film on both sides of the first dielectric thin film. Forming on the upper surface of the flow block layer or the third cladding layer a second dielectric thin film having a width wider than that of the groove and wider than the first dielectric thin film; and the second dielectric thin film. At least including a step of forming a mesa stripe by etching a region where no film is formed to reach the semiconductor substrate, and a step of forming a current block layer on both sides of the mesa stripe by using the second dielectric thin film as a mask. And a method for manufacturing a semiconductor optical amplifier. 16. The method of manufacturing a semiconductor optical amplifier according to claim 14 or 15, wherein an etching stop layer is previously formed on the upper surface of the semiconductor substrate, and the first current block is formed on the upper surface of the etching stop layer. A step of forming the groove portion by etching after forming the layer or the first clad layer, and in the step of etching the first current blocking layer or the first clad layer, etching is automatically performed on the surface of the etching stop layer. A method for manufacturing a semiconductor optical amplifier, which comprises a step of physically stopping. 17. A step of forming at least a first active layer and a third active layer on a semiconductor substrate, a step of forming a stripe-shaped first dielectric thin film on the surface of the third active layer, and A step of etching away the third active layer in a region where the dielectric thin film is not formed; a step of forming a current blocking layer using the first dielectric thin film as a mask; and the first dielectric thin film and both sides thereof. A step of forming a second dielectric thin film having a width wider than that of the first dielectric thin film on the upper surface of the current blocking layer, and etching a region where the second dielectric thin film is not formed until reaching the semiconductor substrate. And forming a mesa stripe by using the second dielectric thin film as a mask, and forming the current blocking layer again on both sides of the mesa stripe by using the second dielectric thin film as a mask. Method for manufacturing semiconductor optical amplifier. 18. A step of forming at least a first active layer, a third active layer, and a second clad layer on a semiconductor substrate, and a step of forming a first dielectric thin film in a stripe pattern on the surface of the second clad layer. And a step of removing the second clad layer and the third active layer in a region where the first dielectric thin film is not formed by etching, a second current blocking layer or A step of forming three cladding layers, and a second dielectric thin film having a width wider than that of the first dielectric thin film on the upper surface of the first dielectric thin film and the second current blocking layer or the third cladding layer on both sides of the first dielectric thin film. A step of forming a mesa stripe by etching the region where the second dielectric thin film is not formed until reaching the semiconductor substrate, and using the second dielectric thin film as a mask A method for manufacturing a semiconductor optical amplifier, comprising at least a step of forming current blocking layers on both sides of the mesa stripe. 19. The method of manufacturing a semiconductor optical amplifier according to claim 13, wherein an etching stop layer is provided between the first active layer and the third active layer. A method of manufacturing a semiconductor optical amplifier, comprising the step of automatically stopping etching on the surface of the etching stop layer in the step of etching the third active layer.