JP5310533B2 - Optical semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve stable single transverse mode operation while suppressing saturation of optical output, for sufficient characteristics. <P>SOLUTION: The optical semiconductor device includes an active region 1 and a first distribution reflecting mirror region 2 which is provided on one end side of the active region 1 and contains a first waveguide 10 mounted with a first diffraction grating 6. The first diffraction grating 6 is tilted to the opposite side of the active region 1 relative to the central portion in width direction whose coupling factor is identical in width direction of the first waveguide 10, with the both side portions in width direction being parallel in width direction of the first waveguide 10. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an optical semiconductor device such as a semiconductor laser.

Along with the explosive increase in Internet demand, efforts to increase the speed and capacity in optical communication / transmission have become active.
In particular, an uncooled semiconductor laser capable of high-speed direct modulation is demanded for an ultrahigh-speed optical fiber transmission system or datacom.
A DFB (Distributed Feed-Back) laser is expected as a semiconductor laser capable of high-speed direct modulation in this uncooled state.

Basically, in the semiconductor laser, if the volume of the active layer is made as small as possible, the value of the relaxation oscillation frequency increases, and the bit rate that can be directly modulated increases.
In fact, there is a DFB laser having a cavity length as short as 100 μm that enables 40 Gb / s modulation at room temperature.
However, in such a DFB laser, as shown in FIG. 22, a non-reflective coating (antireflection film) is provided on the front end face, and a high reflection coat (high reflection film; reflectivity of about 90%) is provided on the rear end face. A diffraction grating having no phase shift is provided along the active layer.

  For this reason, the yield of the element capable of obtaining single longitudinal mode oscillation strongly depends on the phase of the diffraction grating at the rear end face. And since the period of the diffraction grating is as fine as about 200 nm and it is almost impossible to precisely control the position of the end face when cleaved into the element, the phase at the rear end face must be random. . Therefore, it is not possible to increase the yield of elements that can obtain good single longitudinal mode oscillation.

  In addition, there is a distributed reflector (DR) laser that improves the yield of an element capable of obtaining single longitudinal mode oscillation and can perform highly efficient laser operation. In such a DR laser, the reflecting mirror on the rear end face is not a highly reflective film, but has a diffraction grating having the same period as the diffraction grating of the active region, and has a passive region functioning as a passive reflector, and the active region and the passive mirror. A phase shift is provided between the regions.

  Also, in the DR laser, a λ / 4 phase shift is provided in the center of the active region, a passive region having a diffraction grating is provided as a reflecting mirror on the rear end surface, and depending on the difference in composition between the active region and the passive region In some cases, the pitch of the diffraction grating is changed between the active region and the passive region. As a result, the optical pitch of the diffraction grating is made equal in the active region and the passive region, and the Bragg reflection wavelength in the passive region is made equal to the wavelength of the laser light generated in the active region, so that the laser light can be reflected efficiently. I have to.

Japanese Patent Publication No. 7-70785 JP 2002-353559 A

K. Nakahara et al., “High Extinction Ratio Operation at 40-Gb / s Direct Modulation in 1.3-μm InGaAlAs-MQW RWG DFB Lasers”, OFC / NFOEC 2006, lecture number OWC5

By the way, a DR laser having a structure as shown in FIG. 23 having a buried waveguide structure and having an active region shorter than 100 μm in order to improve high-speed response characteristics is manufactured. It was found that the light output was saturated during high temperature operation and sufficient characteristics could not be obtained.
In a DR laser having a buried waveguide structure, the width of the mesa structure, that is, the width of the active layer as the waveguide core layer (waveguide width) is about 1.3 μm, for example, and is narrow. It was found that the Joule heat generated as a result was one of the causes.

For this reason, if the waveguide width is widened, the device resistance decreases, so that the problem that the optical output is saturated at the time of the above-described high-temperature operation and sufficient characteristics cannot be obtained can be solved.
However, when the waveguide width is increased to, for example, 3 μm, as shown in FIGS. 24A to 24C, it becomes possible to oscillate a transverse higher-order mode in the waveguide, and a stable single transverse mode operation is obtained. Disappear.

  Therefore, it is desired to realize a stable single transverse mode operation while suppressing saturation of the light output and obtaining sufficient characteristics.

  Therefore, the optical semiconductor device includes an active region and a first distributed reflector region that is provided on one end side of the active region and has a first waveguide loaded with the first diffraction grating. The coupling coefficient is the same in the width direction of the first waveguide, and both side portions in the width direction are inclined to the opposite side of the active region with respect to the center portion in the width direction parallel to the width direction of the first waveguide. And

  Therefore, according to the present optical semiconductor device, there is an advantage that stable single transverse mode operation can be realized while suppressing saturation of the optical output and obtaining sufficient characteristics.

FIG. 3 is a schematic plan view showing a configuration of a diffraction grating provided in the optical semiconductor device according to the first embodiment. It is a typical sectional view showing the composition of the optical semiconductor device concerning a 1st embodiment. (A)-(C) are the figures for demonstrating the effect | action by the optical semiconductor device concerning 1st Embodiment. It is a figure which shows the relationship between the curvature radius of the diffraction grating for reflection provided in the distributed reflector area | region of the optical semiconductor device concerning 1st Embodiment, and the coupling coefficient reduction rate of a transverse mode. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 1st Embodiment. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 1st Embodiment. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 1st Embodiment. (A), (B) is a typical perspective view for demonstrating the manufacturing method of the optical semiconductor device concerning 1st Embodiment. It is a figure for demonstrating the effect by the optical semiconductor device concerning 1st Embodiment. FIG. 6 is a schematic plan view showing the configuration of a modification of the diffraction grating provided in the optical semiconductor device according to the first embodiment. It is a typical top view which shows the structure of the diffraction grating with which the optical semiconductor device concerning 2nd Embodiment is equipped. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 2nd Embodiment. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 2nd Embodiment. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 2nd Embodiment. (A), (B) is a typical perspective view for demonstrating the manufacturing method of the optical semiconductor device concerning 2nd Embodiment. It is a typical top view showing composition of a modification of a diffraction grating with which an optical semiconductor device concerning a 2nd embodiment is provided. It is a typical top view which shows the structure of the diffraction grating with which the optical semiconductor device concerning 3rd Embodiment is equipped. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 3rd Embodiment. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 3rd Embodiment. (A)-(C) are typical perspective views for demonstrating the manufacturing method of the optical semiconductor device concerning 3rd Embodiment. (A), (B) is a typical perspective view for demonstrating the manufacturing method of the optical semiconductor device concerning 3rd Embodiment. It is typical sectional drawing which shows the structure of the conventional semiconductor laser. It is typical sectional drawing which shows the structure of the semiconductor laser produced in the creation process of this invention. (A)-(C) are the figures for demonstrating the subject of this invention.

Hereinafter, an optical semiconductor device according to an embodiment of the present invention will be described with reference to the drawings.
[First Embodiment]
First, the optical semiconductor device according to the first embodiment will be described with reference to FIGS.
The optical semiconductor device according to this embodiment is, for example, a semiconductor laser as a light source for an optical fiber transmission system, and is a distributed reflection laser (DR laser) having a distributed reflection laser structure. The distributed reflection laser is also referred to as a distributed reflector integrated distributed feedback semiconductor laser.

  As shown in FIG. 2, the present distributed reflection type laser has an active region 1 functioning as a distributed feedback (DFB) laser region in which a laser is oscillated when current is injected, and is emitted from the active region 1 without being injected with current. And a distributed reflector region 2 that reflects the laser beam back to the active region 1. The active region 1 is also referred to as a current injection region or a distributed feedback active region. The distributed reflector region 2 is also referred to as a distributed Bragg reflection region, a passive region, or a current non-injection region.

In this distributed reflection type laser, since current injection is performed only in the active region 1, a current injection electrode (p-side electrode) 115 is provided only in the active region 1. An n-side electrode 116 is provided on the back side of the n-type doped InP substrate 101. In this distributed reflection type laser, antireflection coatings (antireflection films; antireflection films) 117 and 118 are applied to both end faces.
Here, the active region 1 includes an active layer 105 that generates a gain by current injection, a diffraction grating 3 and a phase shift 4 that determine an oscillation wavelength, a p-type InP cladding layer 110, and a p-type GaInAs contact layer 111. Here, the phase shift 4 is a λ / 4 phase shift, and the phase shift amount is a value in the vicinity of λ / 4. That is, the active region 1 includes an active waveguide 9 loaded with a diffraction grating (third diffraction grating) 3 having a phase shift 4.

In the present embodiment, the active region 1 includes a diffraction grating layer 5 including a phase shift 4 and an n-type doped GaInAsP layer 103, an undoped AlGaInAs / AlGaInAs quantum well active layer 105 as a waveguide core layer, and Is provided.
Here, the diffraction grating layer 5 is formed by embedding the diffraction grating 3 including the phase shift 4 formed on the surface of the n-type doped InP substrate 101 with the n-type doped GaInAsP layer 103. The n-type doped GaInAsP layer 103 has a composition wavelength of 1.20 μm and a thickness of 120 nm.

  The undoped AlGaInAs / AlGaInAs quantum well active layer 105 includes an undoped AlGaInAs well layer and an undoped AlGaInAs barrier layer. Here, the undoped AlGaInAs well layer has a thickness of 6 nm and a compressive strain of 1.2%. The undoped AlGaInAs barrier layer has a composition wavelength of 1.05 μm and a thickness of 10 nm. The number of undoped AlGaInAs / AlGaInAs quantum well active layers 105 is 15 and the emission wavelength (oscillation wavelength) is 1310 nm.

  Here, the length of the active region 1 is about 125 μm. Thereby, a high-speed response characteristic can be improved. For example, a semiconductor laser (direct modulation laser) capable of direct modulation of 25 Gb / s or more is realized. Note that the period of the diffraction grating 3 in the active region 1 is constant. Further, in the active region 1, the coupling coefficient (duty ratio and depth) and phase (excluding the portion of the phase shift 4) of the diffraction grating 3 are constant. In the active region 1, the equivalent refractive index of the waveguide (active layer 6) is constant along the resonator direction. Further, a λ / 4 phase shift 4 having a phase shifted by π radians (corresponding to λ / 4 shift) is provided at a position 10 μm rear end face side from the center of the active region 1.

  The distributed reflector region 2 includes a light guide layer (passive waveguide core layer) 108 that does not generate gain, a reflection diffraction grating 6, and a p-type InP cladding layer 110. That is, the distributed reflector region (first distributed reflector region) 2 includes a passive waveguide (first waveguide) 10 loaded with a reflection diffraction grating (first diffraction grating) 6. The light guide layer 108 has a composition of 1.15 μm so as not to cause absorption loss.

In the present embodiment, the distributed reflector region 2 includes a reflection diffraction grating layer 7 including a reflection diffraction grating 6 and an n-type doped GaInAsP layer 103, and an undoped AlGaInAs light guide layer 108 as a waveguide core layer. .
Here, the reflective diffraction grating layer 7 is formed by embedding the reflective diffraction grating 6 formed on the surface of the n-type doped InP substrate 101 with the n-type doped GaInAsP layer 103. The n-type doped GaInAsP layer 103 has a composition wavelength of about 1.20 μm and a thickness of about 120 nm, for example. The undoped AlGaInAs light guide layer 108 has, for example, a composition wavelength of about 1.15 μm and a thickness of about 250 nm.

  Here, as shown in FIG. 2, the distributed reflector region 2 is provided so as to be continuous with the rear end face side of the active region 1 [right side in FIG. 2; one end side]. In the present embodiment, a distributed reflector region 2 having a length of, for example, about 75 μm is provided continuously to the active region 1. That is, the reflective diffraction grating 6 is patterned continuously to the active region 1 over a length of, for example, about 75 μm. Further, the period of the reflection diffraction grating 6 in the distributed reflector region 2 is constant. In the distributed reflector region 2, the coupling coefficient (duty ratio and depth) and phase of the reflecting diffraction grating 6 are constant. In the distributed reflector region 2, the equivalent refractive index of the waveguide (light guide layer) is constant along the resonator direction.

  Further, the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 have the same phase at the central portion in the waveguide width direction. In addition, the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 have the same period. Furthermore, the coupling coefficient of the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 are the same. That is, the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 have the same depth. Here, the depths of the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 are both 100 nm. Further, the diffraction grating 3 in the active region 1 and the reflecting diffraction grating 6 in the distributed reflector region 2 have the same duty ratio (the ratio of the portion left by etching with respect to the period of the diffraction grating; here, about 50%). ing.

This distributed reflection type laser has a buried waveguide structure [see FIG. 8A]. However, the width of the mesa structure 8, that is, the waveguide width is increased to about 3.0 μm, for example. ing. Thereby, element resistance can be reduced and saturation of optical output can be suppressed. In particular, saturation of the light output during high temperature operation can be suppressed, and sufficient modulation characteristics can be obtained.
By the way, in the present embodiment, the diffraction grating 3 in the active region 1 is parallel to the width direction of the active waveguide 9, that is, the width direction of the mesa structure 8 (vertical direction in FIG. 1), as shown in FIG. It is a linear diffraction grating. On the other hand, the reflecting diffraction grating 6 in the distributed reflector region 2 has a curved shape bent along the width direction of the active waveguide 9, that is, the width direction of the mesa structure 8 (vertical direction in FIG. 1). It is a diffraction grating. That is, the reflection diffraction grating 6 in the distributed reflector region 2 is curved so as to be convex toward the active region 1 side. The diffraction grating 3 in the active region 1 is also referred to as a linear diffraction grating. The reflection diffraction grating 6 in the distributed reflector region 2 is also referred to as a curved diffraction grating or a curved diffraction grating.

As a result, the coupling coefficient of the transverse higher-order mode can be reduced. As a result, the oscillation threshold value increases, and the oscillation of the lateral higher order mode can be suppressed.
That is, when the reflecting diffraction grating 6 in the distributed reflector region 2 is a curved diffraction grating, as shown in FIGS. 3A to 3C, the waveguide mode component is more than the central portion in the waveguide width direction. The intensity components of both side portions (outer portions) are radiated out of the waveguide by the curved diffraction grating.

As shown in FIGS. 3A to 3C, the lateral high-order mode has more components in the outer portion than the central portion in the waveguide width direction compared to the transverse fundamental mode, and therefore, the active by the curved diffraction grating. The amount of feedback to region 1 is reduced. As a result, oscillation in the lateral higher order mode is suppressed.
Here, rather than lowering the coupling coefficient at both sides in the width direction of the reflecting diffraction grating 6 in the distributed reflector region 2 to suppress oscillation in the lateral higher order mode, the lateral higher order mode is changed by using a curved diffraction grating. It radiates out of the waveguide to suppress oscillation in the lateral higher order mode. For this reason, the reflection diffraction grating 6 in the distributed reflector region 2 has the same coupling coefficient in the waveguide width direction. That is, the diffraction grating 6 for reflection in the distributed reflector region 2 does not have a small duty ratio at both sides in the width direction. Therefore, many transverse higher-order modes are reflected and radiated out of the waveguide at both sides in the width direction of the reflecting diffraction grating 6. As a result, the coupling coefficient of the transverse fundamental mode does not decrease so much, and the coupling coefficient of the transverse higher-order mode greatly decreases.

As described above, the waveguide width is widened, the element resistance is reduced, the saturation of the optical output is suppressed, and the single transverse mode operation can be realized by using the curved diffraction grating. Thereby, it is possible to prevent the occurrence of mode competition and the deterioration of the characteristics (modulation characteristics).
Specifically, as described above, the length of the active region 1 is set to 125 μm in order to improve the high-speed response characteristic. Further, in order to reduce the element resistance and suppress the saturation of the optical output, the width of the mesa structure 8, that is, the waveguide width is set to 3.0 μm. In order to suppress the oscillation of the lateral higher-order mode, an arc-shaped diffraction grating (curved diffraction grating) having an arc shape with a radius of curvature of 10 μm is used as the reflection diffraction grating 6 in the distributed reflector region 2.

The length of the active region 1, the waveguide width, and the radius of curvature of the reflection diffraction grating 6 in the distributed reflector region 2 are not limited to these values.
For example, in order to realize a direct modulation laser of about 25 Gb / s or more, the length of the active region 1 may be about 125 μm or less. In order to realize a direct modulation laser of about 40 Gb / s or more, the length of the active region 1 may be about 100 μm or less.

  In this case, by using a curved diffraction grating as the reflection diffraction grating 6 in the distributed reflector region 2, the waveguide width is 1.5 times the general waveguide width of 1.3 μm to 1.6 μm. The width can be doubled. That is, the waveguide width can be set to 1.95 μm to 2.6 μm, or 2.4 μm to 3.2 μm, that is, 1.95 μm to 3.2 μm. Here, the waveguide width is set to about 3.2 μm, even if it is wider than this, the current path injected into the active layer is almost the same as that of the ridge waveguide type laser, and the current confinement effect. Is saturated.

  As described above, when the waveguide width is about 3.0 μm, the radius of curvature of the curved diffraction grating as the reflection diffraction grating 6 in the distributed reflector region 2 is preferably in the range of about 5 μm to about 13 μm. . This is due to the following reason. Here, the preferable range of the radius of curvature is defined on the assumption that the waveguide width is about 3.0 μm, but this is not restrictive, and the waveguide width is in the above range. Even if it exists, it is preferable to make a curvature radius into this range.

Here, FIG. 4 shows the relationship between the radius of curvature and the transverse mode coupling coefficient reduction rate when a curved diffraction grating is used as the reflection diffraction grating 6 in the distributed reflector region 2. In FIG. 4, solid lines A, B, and C indicate the radius of curvature and the transverse mode in the transverse basic mode (0th order), the transverse higher order mode (primary), and the transverse higher order mode (secondary), respectively. The relationship with the coupling coefficient reduction rate is shown.
First, the larger the radius of curvature, the closer to a straight line. For this reason, as shown in FIG. 4, the coupling coefficient of the transverse mode when the linear diffraction grating is used, that is, the larger the radius of curvature of the curved diffraction grating is, as shown in FIG. Nearly 100%.

  For example, if the radius of curvature is reduced from 100 μm, where the coupling coefficient of all transverse modes of the 0th, 1st and 2nd is almost 100%, all of the transverse modes of 0th, 1st and 2nd are obtained. The coupling coefficient decreases. In this case, the degree of reduction of the coupling coefficient in the transverse mode becomes more prominent as the order of the transverse mode increases. That is, the greater the order of the transverse mode, the greater the change in the coupling coefficient reduction rate of the transverse mode with respect to the change in the radius of curvature. For this reason, by using a curved diffraction grating, the coupling coefficient of the transverse higher-order mode can be reduced without significantly reducing the coupling coefficient of the transverse fundamental mode, thereby realizing a single transverse mode operation. Is possible.

  Specifically, it is preferable that the coupling coefficient of the transverse higher-order mode when the curved diffraction grating is used is about 50% or less of the coupling coefficient of the transverse mode when the linear diffraction grating is used. Here, when the curved diffraction grating is used, the coupling coefficient of all the transverse higher-order modes is about 50% when the radius of curvature is about 13 μm. For this reason, the radius of curvature of the curved diffraction grating as the reflection diffraction grating 6 in the distributed reflector region 2 is preferably about 13 μm or less.

  In addition, it is preferable that the coupling coefficient of the transverse fundamental mode when the curved diffraction grating is used is not less than about 50% of the coupling coefficient of the transverse mode when the linear diffraction grating is used. Here, the coupling coefficient of the transverse fundamental mode when the curved diffraction grating is used is about 50% when the radius of curvature is about 5 μm. Therefore, the radius of curvature of the curved diffraction grating as the reflection diffraction grating 6 in the distributed reflector region 2 is preferably about 5 μm or more.

Next, a manufacturing method of a distributed reflection type laser (optical semiconductor device) according to a specific configuration example of this embodiment will be described with reference to FIGS.
First, as shown in FIG. 5A, on a surface of an n-type doped InP substrate 101, a mask 102 made of an electron beam resist (ZEP 520 manufactured by Nippon Zeon Co., Ltd.) and having a diffraction grating pattern is formed by, for example, an electron beam exposure method. Form. The diffraction grating pattern includes a diffraction grating pattern for forming the diffraction grating 3 (including the phase shift 4) in the active region 1 and a reflection diffraction pattern for forming the reflection diffraction grating 6 in the distributed reflector region 2. And a diffraction grating pattern.

Next, by using this mask 102, for example, reactive ion etching (RIE; Reactive Ion Etching) using an ethane / hydrogen mixed gas, as shown in FIG. The diffraction grating pattern is transferred by removing the portion. Here, the etching is stopped in the middle of the n-type InP substrate 101.
As a result, the diffraction grating 3 (including the phase shift 4) in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 are collectively formed. That is, λ whose phase is shifted by π radians (corresponding to λ / 4 shift) to the position on the rear end face side of 10 μm from the center of the active region 1 over the entire length of the region to be the active region 1 of each element (here 125 μm). A diffraction grating 3 having a / 4 phase shift 4 is formed. Further, the reflection diffraction grating 6 is formed over the entire length (here, 75 μm) of the region that becomes the distributed reflector region 2 continuously to the diffraction grating 3 of the active region 1.

  In the present embodiment, the diffraction grating 3 in the active region 1 is a linear diffraction grating. On the other hand, the reflecting diffraction grating 6 in the distributed reflector region 2 is a curved diffraction grating. That is, the reflection diffraction grating 6 in the distributed reflector region 2 is a curved diffraction grating that is curved so as to be convex toward the active region 1 side. Here, the reflection diffraction grating 6 in the distributed reflector region 2 is an arc-shaped diffraction grating having an arc shape with a radius of curvature of 10 μm.

Further, the period of the diffraction grating 3 in the active region 1 is constant. Further, in the active region 1, the coupling coefficient and phase of the diffraction grating 3 are constant. In the active region 1, the equivalent refractive index of the waveguide is constant along the resonator direction.
Furthermore, the period of the reflection diffraction grating 6 in the distributed reflector region 2 is constant. Further, in the distributed reflector region 2, the coupling coefficient and the phase of the reflecting diffraction grating 6 are constant. In the distributed reflector region 2, the equivalent refractive index of the waveguide is constant along the resonator direction.

  The depths of the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 are both 100 nm and are the same. In addition, the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 have the same duty ratio (here, 50%). That is, the coupling coefficient of the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 are the same. Further, the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 have the same phase at the central portion in the waveguide width direction. In addition, the diffraction grating 3 in the active region 1 and the reflection diffraction grating 6 in the distributed reflector region 2 have the same period.

  Next, the mask 102 is removed from the surface using a normal resist stripping process. Thereafter, as shown in FIG. 5C, on the surface of the n-type InP substrate 101 on which the diffraction grating pattern is formed, for example, using metal organic chemical vapor deposition (MOVPE) method, An n-type doped GaInAsP layer (guide layer) 103 is grown. Here, the n-type doped GaInAsP layer 103 has a composition wavelength of 1.15 μm and a thickness of 120 nm. Thereby, the groove (diffraction grating pattern) formed by stopping the etching in the middle of the n-type InP substrate 101 is filled with the n-type doped GaInAsP layer 103.

  Next, as shown in FIG. 5C, the n-type doped InP layer 104, the undoped AlGaInAs / AlGaInAs quantum well active layer 105, and the p-type doped InP cladding layer 106 are formed on the n-type doped GaInAsP layer 103 by, for example, MOVPE. Grow sequentially. Here, the thickness of the n-type doped InP layer 104 is 20 nm. The thickness of the p-type doped InP cladding layer 106 is 250 nm.

  Here, the quantum well active layer 105 is configured using an AlGaInAs-based compound semiconductor material. That is, the quantum well active layer 105 includes an undoped AlGaInAs well layer and an undoped AlGaInAs barrier layer. Here, the undoped AlGaInAs well layer has a thickness of 6 nm and a compressive strain of 1.2%. The undoped AlGaInAs barrier layer has a composition wavelength of 1.05 μm and a thickness of 10 nm. The number of stacked quantum well active layers 105 is 15, and the emission wavelength (oscillation wavelength) is 1310 nm.

An undoped AlGaInAs-SCH (Separate Confinement Heterostructure) layer may be provided above and below the quantum well active layer 105 so as to sandwich the quantum well active layer 105. Here, the undoped AlGaInAs-SCH layer (light guide layer) has a composition wavelength of 1.05 μm and a thickness of 20 nm.
Next, as shown in FIG. 6A, the active region 1 is formed on the surface of the p-type doped InP cladding layer 106 by using a normal chemical vapor deposition (CVD) method and a photolithography technique. A striped SiO ₂ mask (etching mask) 107 is formed so as to cover it. Here, the thickness of the SiO ₂ mask 107 is 400 nm.

6B, from the surface of the p-type doped InP cladding layer 106 to the surface of the n-type doped InP layer 104 using the mask 107, that is, the p-type doped InP cladding layer 106 and The quantum well active layer 105 is removed by, for example, etching.
Thereafter, as shown in FIG. 6C, an undoped AlGaInAs light guide layer 108 and an undoped InP layer 109 are sequentially grown on the n-type doped InP layer 104 by, eg, MOVPE. Here, the undoped AlGaInAs light guide layer 108 has a composition wavelength of 1.15 μm and a thickness of 250 nm. The undoped InP layer 109 has a thickness of 250 nm. At this time, these layers 108 and 109 do not grow on the SiO ₂ mask 107 by selective growth, but grow only on the n-type doped InP layer 104 exposed on the surface.

Thereafter, after removing the SiO ₂ mask 107, as shown in FIG. 7A, the MOVPE method is used again, for example, on the entire surface of the semiconductor crystal wafer, for example, the p-type InP clad layer 110, followed by the p-type GaInAs contact. Layer 111 is stacked. Here, the p-type InP clad layer 110 is doped with Zn and has a thickness of 2.0 μm. The p-type GaInAs contact layer 111 is doped with Zn and has a thickness of 300 nm.

Then, as shown in FIG. 7B, a SiO ₂ mask 112 is formed on the surface of the p-type GaInAs contact layer 111 using, for example, a normal CVD method and a photolithography technique. Here, the SiO ₂ mask 112 is a striped etching mask having a thickness of 400 nm and a width of 3.0 μm.
Thereafter, as shown in FIG. 7C, the semiconductor multilayer structure formed as described above is etched to a depth at which the n-type InP substrate 101 is dug, for example, by about 0.7 μm, using, for example, a dry etching method. Then, it is processed into a striped mesa structure (mesa stripe) 8. Here, the width of the mesa structure 8, that is, the waveguide width is 3.0 μm.

Next, as shown in FIG. 8A, current confinement layers 113 made of, for example, Fe-doped semi-insulating InP are grown on both sides of the mesa structure 8 by using, for example, the MOVPE method. Thereby, a semi-insulating buried heterostructure (SI-BH structure; Semi-Insulating Buried Heterostructure) is formed.
Next, after removing the etching mask 112 with, for example, hydrofluoric acid, the p-type GaInAs contact layer 111 other than the active region 1 is removed using a normal photolithography technique and etching.

Thereafter, as shown in FIG. 8B, a SiO ₂ passivation film 114 is formed. Then, the SiO ₂ passivation film 114 is removed using a normal photolithography technique and etching so that a window opens only in a portion of the active region 1 above the p-type GaInAs contact layer 111. Thereafter, the p-side electrode 115 and the n-side electrode 116 are formed. FIG. 2 is a cross-sectional view taken along the line AA ′ in FIG.

Then, as shown in FIG. 8B, non-reflective coatings 117 and 118 are formed on both end faces of the element, thereby completing the element.
Therefore, according to the optical semiconductor device (semiconductor laser) according to the present embodiment, stable single transverse mode operation can be realized while suppressing saturation of optical output and obtaining sufficient characteristics (modulation characteristics). There is an advantage.

  In particular, in the above-described embodiment, the length of the active region 1 is set to 125 μm in order to improve high-speed response characteristics. Further, in order to reduce the element resistance and suppress the saturation of the optical output, the width of the mesa structure 8, that is, the waveguide width is set to 3.0 μm. In order to suppress the oscillation of the transverse higher-order mode, the reflection diffraction grating 6 in the distributed reflector region 2 is a curved diffraction grating having a curvature radius of 10 μm.

Here, FIG. 9 is a diagram showing the relationship between the transverse mode coupling coefficient and the oscillation threshold gain, and the reflecting diffraction grating 6 in the distributed reflector region 2 is a curved diffraction grating as in the above-described embodiment. The effect of the above is shown in comparison with the case of using a linear diffraction grating.
In FIG. 9, numbers 0, 1 and 2 in circles or squares indicate the order of the transverse mode. That is, in FIG. 9, the numbers 0, 1, and 2 in circles or squares indicate the coupling in each case of the horizontal basic mode (0th order), the horizontal higher order mode (primary), and the horizontal higher order mode (secondary). The relationship between the coefficient and the oscillation threshold gain is shown.

  As shown in FIG. 9, the coupling coefficient of the transverse fundamental mode (0th order) in the case of the curved diffraction grating is 80% as compared with the case of the linear diffraction grating. On the other hand, the coupling coefficient of the transverse higher-order mode (first order) and the coupling coefficient of the transverse higher-order mode (second order) in the case of the curved diffraction grating are 40% and 30%, respectively, compared to the case of the linear diffraction grating. %. As a result, the oscillation threshold gain of the transverse higher-order mode (first order and second order) in the case of the curved diffraction grating is at least twice the oscillation threshold gain of the transverse fundamental mode. As a result, in the device manufactured as in the above-described embodiment, oscillation in the lateral high-order mode was stopped, and a stable single transverse mode operation was obtained.

  In the above-described embodiment, a curved diffraction grating is used as the reflection diffraction grating 6 in the distributed reflector region 2. However, the present invention is not limited to this, and a convex shape is formed toward the active region 1 side. A diffraction grating may be used. That is, the reflection diffraction grating 6 in the distributed reflector region 2 has the same coupling coefficient in the width direction of the passive waveguide 10, and both side portions in the width direction are in the center portion in the width direction parallel to the width direction of the passive waveguide 10. In contrast, it may be inclined to the opposite side of the active region 1.

  However, it is preferable to use a curved diffraction grating such as an arc-shaped diffraction grating, an elliptical diffraction grating, or a second-order curved diffraction grating in terms of wide design tolerance. When such a curved diffraction grating is used, even if the diffraction grating is displaced in the width direction of the waveguide when the diffraction grating is formed, the characteristics can be prevented from being affected. In other words, when a curved diffraction grating is used, even if the diffraction grating is shifted in the width direction of the waveguide, the coupling coefficient of the transverse fundamental mode does not decrease so much, while the coupling coefficient of the transverse higher-order mode is large. Therefore, desired characteristics can be obtained. Note that there is a degree of freedom in design as to what function is adopted as a function that defines the shape of the diffraction grating (for example, a function indicating a change in curvature). Therefore, the “curved diffraction grating” includes not only a curved diffraction grating but also a diffraction grating having a shape approximate to a curved shape.

In the above-described embodiment, the diffraction grating 3 in the active region 1 includes the phase shift 4, but is not limited to this. For example, as shown in FIG. 10, the diffraction grating 3 in the active region 1. May be configured not to include a phase shift.
Further, in the above-described embodiment, the distributed reflector region 2 is provided so as to be continuous with the rear end surface side of the active region 1. However, the present invention is not limited to this, and the active region has a rear end surface side and a front end surface side. A distributed reflector region may be provided so as to be continuous. In this case, the reflection diffraction grating in the distributed reflector region connected to the front end face side of the active region may be a linear diffraction grating or a curved diffraction grating.

In the above-described embodiment, the diffraction grating 3 in the active region 1 is a linear diffraction grating. However, the diffraction grating 3 is not limited to this. For example, a curved diffraction grating is used as in a third embodiment described later. Also good.
[Second Embodiment]
Next, an optical semiconductor device according to a second embodiment will be described with reference to FIGS.

The optical semiconductor device (DR laser) according to the present embodiment differs from the above-described first embodiment in the following five points.
That is, the first different point is that the distributed reflector regions 2A and 2B are provided so as to be continuous with the rear end surface side and the front end surface side of the active region 1. The second difference is that no phase shift is provided. A third difference is that the length of the active region 1 is 100 μm. A fourth different point is that the radius of curvature of the curved diffraction grating as the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B is set to 8 μm. The fifth different point is that the oscillation wavelength band is 1.55 μm band.

  In the present embodiment, as shown in FIG. 11, the distributed reflector regions 2 </ b> A and 2 </ b> B are provided so as to continue to the front end surface side and the rear end surface side of the active region 1. In this embodiment, a distributed reflector region 2B (second distributed reflector region) having a length of 25 μm is provided on the front end face side continuously with the active region 1, and the rear end face side continuously with the active region 1 is provided. A distributed reflector region 2A (first distributed reflector region) having a length of 75 μm is provided. That is, the reflection diffraction grating 6B (second diffraction grating) is patterned on the front end face side over the length of 25 μm continuously to the active region 1, and the reflection diffraction grating 6A over the length of 75 μm on the rear end face side. The (first diffraction grating) is patterned.

As described above, the distributed reflector region 2B provided on the front end face side of the active region 1 is shorter in the cavity direction than the distributed reflector region 2A provided on the rear end face side of the active region 1. Yes. This is because the laser beam is output from the front end face side of the element, and thus the reflectance in the distributed reflector region 2B on the front end face side is lowered.
The distributed reflector regions 2A and 2B on the rear end face side and the front end face side have the same structure.

In the present embodiment, the active region 1 has no phase shift. That is, the active region 1 includes an active waveguide 9 loaded with a diffraction grating (third diffraction grating) 3 having no phase shift.
In the present embodiment, the length of the active region 1 is about 100 μm. Thereby, a high-speed response characteristic can be improved. For example, a semiconductor laser capable of direct modulation of 40 Gb / s or more can be realized.

  In addition, the distributed reflection laser has an embedded waveguide structure [see FIG. 15A] as in the case of the first embodiment, but the width of the mesa structure 8, that is, the guiding wavelength. The waveguide width is increased to, for example, about 3.0 μm. Thereby, element resistance can be reduced and saturation of optical output can be suppressed. In particular, saturation of the light output during high temperature operation can be suppressed, and sufficient modulation characteristics can be obtained.

  In this embodiment, the diffraction grating 3 in the active region 1 is parallel to the width direction of the active waveguide 9, that is, the width direction of the mesa structure 8 (vertical direction in FIG. 11), as shown in FIG. 11. It is a linear diffraction grating. On the other hand, the reflection diffraction grating 6A in the distributed reflector region 2A is along the width direction of the passive waveguide 10 (first waveguide), that is, the width direction of the mesa structure 8 (vertical direction in FIG. 11). It is a curved curved diffraction grating. The reflection diffraction grating 6B in the distributed reflector region 2B is bent along the width direction of the passive waveguide 10 (second waveguide), that is, the width direction of the mesa structure 8 (vertical direction in FIG. 11). The diffraction grating has a curved shape. That is, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved so as to be convex toward the active region 1 side. The diffraction grating 3 in the active region 1 is also referred to as a linear diffraction grating. The reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are also referred to as curved diffraction gratings or curved diffraction gratings.

As a result, the coupling coefficient of the transverse higher-order mode can be reduced. As a result, the oscillation threshold value increases, and the oscillation of the lateral higher order mode can be suppressed.
As described above, the waveguide width is widened, the element resistance is reduced, the saturation of the optical output is suppressed, and the single transverse mode operation can be realized by using the curved diffraction grating.
Specifically, as described above, the length of the active region 1 is set to 100 μm in order to improve the high-speed response characteristic. Further, in order to reduce the element resistance and suppress the saturation of the optical output, the width of the mesa structure 8, that is, the waveguide width is set to 3.0 μm. In order to suppress the oscillation of the transverse higher-order mode, an arc-shaped diffraction grating (curved diffraction grating) having an arc shape with a radius of curvature of 8 μm is used as the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B. Used.

In this embodiment, the oscillation wavelength band of the distributed reflection type laser is set to the 1.55 μm band.
Therefore, the diffraction grating layer 5 has a composition wavelength of 1.25 μm and a thickness of 120 nm of the n-type doped GaInAsP layer 203 embedding the diffraction grating 3 formed on the surface of the n-type doped InP substrate 201.
The undoped AlGaInAs well layer constituting the undoped AlGaInAs / AlGaInAs quantum well active layer 205 has a thickness of 5.1 nm and a compressive strain of 1.2%, and the undoped AlGaInAs barrier layer has a composition wavelength of 1.20 μm and a thickness of 10 nm. . The number of undoped AlGaInAs / AlGaInAs quantum well active layers 205 is 15 and the emission wavelength (oscillation wavelength) is 1550 nm.

When an undoped AlGaInAs-SCH layer is provided above and below the quantum well active layer 205 to sandwich the quantum well active layer 205, the composition wavelength of the undoped AlGaInAs-SCH layer is 1.15 μm and the thickness is 20 nm. good.
The undoped AlGaInAs light guide layer 208 has a composition wavelength of 1.35 μm and a thickness of 230 nm.

Since other details are the same as those of the first embodiment and its modification, the description thereof is omitted here.
Next, a manufacturing method of a distributed reflection type laser (optical semiconductor device) according to a specific configuration example of the present embodiment will be described with reference to FIGS.
First, as shown in FIG. 12A, a mask 202 made of an electron beam resist (ZEP 520 manufactured by Nippon Zeon Co., Ltd.) and having a diffraction grating pattern is formed on the surface of an n-type doped InP substrate 201 by, for example, an electron beam exposure method. Form. The diffraction grating pattern includes a diffraction grating pattern for forming the diffraction grating 3 in the active region 1 and reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B on the front end surface side and the rear end surface side. And a diffraction grating pattern for reflection.

Next, using this mask 202, a part of the surface of the n-type InP substrate 201 is removed by reactive ion etching (RIE) using, for example, an ethane / hydrogen mixed gas, as shown in FIG. To transfer the diffraction grating pattern. Here, the etching is stopped in the middle of the n-type InP substrate 201.
As a result, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are collectively formed. That is, the diffraction grating 3 is formed over the entire length (here, 100 μm) of the active region 1 of each element. Further, a reflection diffraction grating 6B is formed over the entire length (here, 25 μm) of the distributed reflector region 2B on the front end face side in succession to the diffraction grating 3 of the active region 1. Further, the diffraction grating 6A for reflection is formed over the entire length (here, 75 μm) of the distributed reflector region 2A on the rear end face side continuously to the diffraction grating 3 of the active region 1.

  In the present embodiment, the diffraction grating 3 in the active region 1 is a linear diffraction grating. On the other hand, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved diffraction gratings. That is, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved diffraction gratings that are curved so as to be convex toward the active region 1 side. Here, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are arc-shaped diffraction gratings having an arc shape with a radius of curvature of 8 μm.

Further, the period of the diffraction grating 3 formed in the active region 1 is constant. Further, in the active region 1, the coupling coefficient and phase of the diffraction grating 3 are constant. In the active region 1, the equivalent refractive index of the waveguide is constant along the resonator direction.
Further, the period of the reflection diffraction grating 6A formed in the distributed reflector region 2A on the rear end face side is constant. Further, in the distributed reflector region 2A, the coupling coefficient and phase of the reflection diffraction grating 6A are constant. In the distributed reflector region 2A, the equivalent refractive index of the waveguide is constant along the resonator direction.

The period of the reflection diffraction grating 6B formed in the distributed reflector region 2B on the front end face side is constant. Further, the coupling coefficient and the phase of the reflection diffraction grating 6B are constant in the distributed reflector region 2B. In the distributed reflector region 2B, the equivalent refractive index of the waveguide is constant along the resonator direction.
Further, the depths of the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are 100 nm, which are the same. Further, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same duty ratio (here, 50%). That is, the coupling coefficients of the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are the same. Further, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same phase at the center in the waveguide width direction. Further, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same period.

  Next, the mask 202 is removed from the surface using a normal resist stripping process. Thereafter, as shown in FIG. 12C, an n-type doped GaInAsP layer (guide layer) 203 is grown on the surface of the n-type InP substrate 201 on which the diffraction grating pattern is formed, using, for example, the MOVPE method. . Here, the n-type doped GaInAsP layer 203 has a composition wavelength of 1.25 μm and a thickness of 120 nm. Thereby, the groove (diffraction grating pattern) formed by stopping the etching in the middle of the n-type InP substrate 201 is filled with the n-type doped GaInAsP layer 203.

  Next, as shown in FIG. 12C, an n-type doped InP layer 204, an undoped AlGaInAs / AlGaInAs quantum well active layer 205, and a p-type doped InP cladding layer 206 are formed on the n-type doped GaInAsP layer 203 by, for example, MOVPE. Grow sequentially. Here, the thickness of the n-type doped InP layer 204 is 20 nm. The p-type doped InP cladding layer 206 has a thickness of 250 nm.

  Here, the quantum well active layer 205 is configured using an AlGaInAs-based compound semiconductor material. That is, the quantum well active layer 205 includes an undoped AlGaInAs well layer and an undoped AlGaInAs barrier layer. Here, the undoped AlGaInAs well layer has a thickness of 5.1 nm and a compressive strain of 1.2%. The undoped AlGaInAs barrier layer has a composition wavelength of 1.20 μm and a thickness of 10 nm. The number of stacked quantum well active layers 205 is 15, and the emission wavelength is 1550 nm.

An undoped AlGaInAs-SCH layer (light guide layer) may be provided above and below the quantum well active layer 205 so as to sandwich the quantum well active layer 205. Here, the undoped AlGaInAs-SCH layer has a composition wavelength of 1.15 μm and a thickness of 20 nm.
Next, as shown in FIG. 13A, a striped SiO ₂ mask is formed on the surface of the p-type doped InP clad layer 206 so as to cover the active region 1 by using a normal CVD method and a photolithography technique. An (etching mask) 207 is formed. Here, the thickness of the SiO ₂ mask 207 is 400 nm.

Then, as shown in FIG. 13B, from the surface of the p-type doped InP cladding layer 206 to the surface of the n-type doped InP layer 204 using the mask 207, that is, the p-type doped InP cladding layer 206 and The quantum well active layer 205 is removed by etching, for example.
Thereafter, as shown in FIG. 13C, an undoped AlGaInAs light guide layer 208 and an undoped InP layer 209 are sequentially grown on the n-type doped InP layer 204 by, for example, the MOVPE method. Here, the undoped AlGaInAs light guide layer 208 has a composition wavelength of 1.35 μm and a thickness of 230 nm. The undoped InP layer 209 has a thickness of 250 nm. At this time, these layers 208 and 209 do not grow on the SiO ₂ mask 207 by selective growth, but grow only on the n-type doped InP layer 204 exposed on the surface.

Thereafter, after the SiO ₂ mask 207 is peeled off, as shown in FIG. 14A, for example, a p-type InP cladding layer 210 is formed on the entire surface of the semiconductor crystal wafer by using the MOVPE method, followed by a p-type GaInAs contact. The layer 211 is stacked. Here, the p-type InP clad layer 210 is doped with Zn and has a thickness of 2.0 μm. The p-type GaInAs contact layer 211 is doped with Zn and has a thickness of 300 nm.

Then, as shown in FIG. 14B, a SiO ₂ mask 212 is formed on the surface of the p-type GaInAs contact layer 211 using, for example, a normal CVD method and a photolithography technique. Here, the SiO ₂ mask 212 is a striped etching mask having a thickness of 400 nm and a width of 3.0 μm.
Thereafter, as shown in FIG. 14C, the semiconductor stacked structure formed as described above is etched to a depth at which the n-type InP substrate 201 is dug, for example, about 0.7 μm, using, for example, a dry etching method. Then, it is processed into a striped mesa structure (mesa stripe) 8.

Next, as shown in FIG. 15A, current confinement layers 213 made of, for example, Fe-doped semi-insulating InP are grown on both sides of the mesa structure 8 by using, for example, the MOVPE method. Thereby, a semi-insulating buried heterostructure (SI-BH structure) is formed.
Next, after removing the etching mask 212 with, for example, hydrofluoric acid, the p-type GaInAs contact layer 211 other than the active region 1 is removed using a normal photolithography technique and etching.

Thereafter, as shown in FIG. 15B, a SiO ₂ passivation film 214 is formed. Then, the SiO ₂ passivation film is removed using a normal photolithography technique and etching so that a window is opened only in a portion above the p-type GaInAs contact layer 211 in the active region 1. Thereafter, the p-side electrode 215 and the n-side electrode 216 are formed.
Then, as shown in FIG. 15B, non-reflective coatings 217 and 218 are formed on both end faces of the element, thereby completing the element.

Since other details are the same as those of the first embodiment and its modification, the description thereof is omitted here.
Therefore, according to the optical semiconductor device (semiconductor laser) according to the present embodiment, stable single transverse mode operation can be realized while suppressing saturation of optical output and obtaining sufficient characteristics (modulation characteristics). There is an advantage.

  In particular, in the above-described embodiment, the length of the active region 1 is set to 100 μm in order to improve high-speed response characteristics. Further, in order to reduce the element resistance and suppress the saturation of the optical output, the width of the mesa structure 8, that is, the waveguide width is set to 3.0 μm. In order to suppress the oscillation of the transverse higher-order mode, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved diffraction gratings having a curvature radius of 8 μm.

  When such a curved diffraction grating is used, the coupling coefficient of the transverse fundamental mode (0th order) is 70% compared to the case of the linear diffraction grating. On the other hand, the coupling coefficient of the transverse higher-order mode (first order) and the coupling coefficient of the transverse higher-order mode (second order) when the curved diffraction grating is used are 20% as compared with the case of the linear diffraction grating. 25%. As a result, the oscillation threshold gain of the transverse higher-order mode (first order and second order) when the curved diffraction grating is used is at least twice the oscillation threshold gain of the transverse fundamental mode. As a result, in the device manufactured as in the above-described embodiment, oscillation in the lateral high-order mode was stopped, and a stable single transverse mode operation was obtained.

  In the above-described embodiment, the curved diffraction gratings are used as the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B. However, the present invention is not limited to this, and is directed toward the active region 1 side. A convex diffraction grating may be used. That is, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same coupling coefficient in the width direction of the passive waveguide, and both widthwise center portions in the width direction are parallel to the width direction of the passive waveguide. What is necessary is just to incline to the opposite side of an active region with respect to a part.

  However, it is preferable to use a curved diffraction grating such as an arc-shaped diffraction grating, an elliptical diffraction grating, or a second-order curved diffraction grating in terms of wide design tolerance. When such a curved diffraction grating is used, even if the diffraction grating is displaced in the width direction of the waveguide when the diffraction grating is formed, the characteristics can be prevented from being affected. In other words, when a curved diffraction grating is used, even if the diffraction grating is shifted in the width direction of the waveguide, the coupling coefficient of the transverse fundamental mode does not decrease so much, while the coupling coefficient of the transverse higher-order mode is large. Therefore, desired characteristics can be obtained. Note that there is a degree of freedom in design as to what function is adopted as a function that defines the shape of the diffraction grating (for example, a function indicating a change in curvature). Therefore, the “curved diffraction grating” includes not only a curved diffraction grating but also a diffraction grating having a shape approximate to a curved shape.

In the above-described embodiment, the diffraction grating 3 in the active region 1 does not include the phase shift 4. However, the present invention is not limited to this, and the diffraction grating 3 in the active region 1 includes the phase shift. You may comprise as.
In the above-described embodiment, the distributed reflector regions 2A and 2B are provided so as to be continuous with the rear end surface side and the front end surface side of the active region 1. However, the present invention is not limited to this, and the rear end surface of the active region 1 is not limited thereto. You may provide a distributed reflector area | region so that it may continue only to the side.

  In the above-described embodiment, the reflection diffraction gratings 6A and 6B of the distributed reflector regions 2A and 2B connected to the rear end surface side and the front end surface side of the active region 1 are both curved diffraction gratings. It is not limited. For example, in the configuration of the above-described embodiment, either one of the reflecting reflector regions connected to the rear end surface side and the front end surface side of the active region may be a linear diffraction grating. Usually, the distributed reflector region continuous to the rear end surface side is made longer than the distributed reflector region continuous to the front end surface side. Therefore, as shown in FIG. 16, the reflecting diffraction grating region 6 </ b> A of the distributed reflector region 2 </ b> A connected to the rear end surface side of the active region 1 is a curved diffraction grating, and the distributed reflector region connected to the front end surface side of the active region 1. The 2B reflecting diffraction grating 6B is preferably a linear diffraction grating.

In the above-described embodiment, the diffraction grating 3 in the active region 1 is a linear diffraction grating. However, the diffraction grating 3 is not limited to this. For example, a curved diffraction grating is used as in a third embodiment described later. Also good.
[Third Embodiment]
Next, an optical semiconductor device according to a third embodiment will be described with reference to FIGS.

The optical semiconductor device (DR laser) according to the present embodiment differs from the above-described first embodiment in the following three points.
That is, the first different point is that the distributed reflector regions 2A and 2B are provided so as to be continuous with the rear end surface side and the front end surface side of the active region 1. The second difference is that a phase shift 4 is provided in the center of the active region 1. A third difference is that the length of the active region 1 is 100 μm. A fourth difference is that the diffraction grating 3 in the active region 1 is a curved diffraction grating. The fifth difference is that the radius of curvature of the curved diffraction grating as the diffraction grating 3 in the active region 1 and the radius of curvature of the curved diffraction grating as the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are as follows. The point is 8 μm. The sixth different point is that the oscillation wavelength band is 1.55 μm band.

  In the present embodiment, as shown in FIG. 17, the distributed reflector regions 2 </ b> A and 2 </ b> B are provided so as to continue to the front end surface side and the rear end surface side of the active region 1. In this embodiment, a distributed reflector region 2B (second distributed reflector region) having a length of 25 μm is provided on the front end face side continuously with the active region 1, and the rear end face side continuously with the active region 1 is provided. A distributed reflector region 2A (first distributed reflector region) having a length of 75 μm is provided. That is, the reflection diffraction grating 6B (second diffraction grating) is patterned on the front end face side over the length of 25 μm continuously to the active region 1, and the reflection diffraction grating 6A over the length of 75 μm on the rear end face side. The (first diffraction grating) is patterned.

As described above, the distributed reflector region 2B provided on the front end face side of the active region 1 is shorter in the cavity direction than the distributed reflector region 2A provided on the rear end face side of the active region 1. Yes. This is because the laser beam is output from the front end face side of the element, and thus the reflectance in the distributed reflector region 2B on the front end face side is lowered.
The distributed reflector regions 2A and 2B on the rear end face side and the front end face side have the same structure.

In the present embodiment, the active region 1 includes a diffraction grating 3 and a phase shift 4 that determine an oscillation wavelength. Here, as the phase shift 4, a λ / 4 phase shift in which the phase is shifted by π radians (corresponding to λ / 4 shift) is provided in the center of the active region 1. That is, the active region 1 includes an active waveguide 9 loaded with a diffraction grating (third diffraction grating) 3 having a phase shift 4.
In the present embodiment, the length of the active region 1 is about 100 μm. Thereby, a high-speed response characteristic can be improved. For example, a semiconductor laser capable of direct modulation of 40 Gb / s or more can be realized.

  Further, the distributed reflection type laser has an embedded waveguide structure [see FIG. 21A] as in the case of the first embodiment described above, but the width of the mesa structure 8, that is, the guiding structure. The waveguide width is increased to, for example, about 3.0 μm. Thereby, element resistance can be reduced and saturation of optical output can be suppressed. In particular, saturation of the light output during high temperature operation can be suppressed, and sufficient modulation characteristics can be obtained.

  In the present embodiment, the diffraction grating 3 in the active region 1 is arranged along the width direction of the active waveguide 9, that is, the width direction of the mesa structure 8 (vertical direction in FIG. 17), as shown in FIG. It is a curved curved diffraction grating. That is, the diffraction grating 3 of the active region 1 is curved so as to be convex toward the center position of the active region 1. The diffraction grating 3 in the active region 1 is also referred to as a curved diffraction grating or a curved diffraction grating.

  In the present embodiment, the reflection diffraction grating 6A in the distributed reflector region 2A has a width direction of the passive waveguide 10 (first waveguide), that is, a width direction of the mesa structure 8 (vertical direction in FIG. 17). Is a curved diffraction grating bent along the line. Further, the reflection diffraction grating 6B in the distributed reflector region 2B is bent along the width direction of the passive waveguide 10 (second waveguide), that is, the width direction of the mesa structure 8 (vertical direction in FIG. 17). The diffraction grating has a curved shape. That is, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved so as to have a convex shape toward the active region 1, that is, toward the center position of the active region 1. The reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are also referred to as curved diffraction gratings or curved diffraction gratings.

As a result, the coupling coefficient of the transverse higher-order mode can be reduced. As a result, the oscillation threshold value increases, and the oscillation of the lateral higher order mode can be suppressed.
As described above, the waveguide width is widened, the element resistance is reduced, the saturation of the optical output is suppressed, and the single transverse mode operation can be realized by using the curved diffraction grating.
Specifically, as described above, the length of the active region 1 is set to 100 μm in order to improve the high-speed response characteristic. Further, in order to reduce the element resistance and suppress the saturation of the optical output, the width of the mesa structure 8, that is, the waveguide width is set to 3.0 μm. In order to suppress the oscillation of the transverse higher-order mode, as the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B, an arc shape having an arc shape with a curvature radius of 8 μm. A diffraction grating (curved diffraction grating) is used.

In this embodiment, the oscillation wavelength band of the distributed reflection type laser is set to the 1.55 μm band.
Therefore, the diffraction grating layer 5 has a composition wavelength of 1.25 μm and a thickness of 120 nm of the n-type doped GaInAsP layer 303 that embeds the diffraction grating 3 formed on the surface of the n-type doped InP substrate 301.
The undoped AlGaInAs well layer constituting the undoped AlGaInAs / AlGaInAs quantum well active layer 305 has a thickness of 5.1 nm and a compressive strain of 1.2%, and the undoped AlGaInAs barrier layer has a composition wavelength of 1.20 μm and a thickness of 10 nm. . The number of undoped AlGaInAs / AlGaInAs quantum well active layers 305 is 15 and the emission wavelength (oscillation wavelength) is 1550 nm.

When an undoped AlGaInAs-SCH layer is provided above and below the quantum well active layer 305 so as to sandwich the quantum well active layer 305, the composition wavelength of the undoped AlGaInAs-SCH layer is 1.15 μm and the thickness is 20 nm. good.
The undoped AlGaInAs light guide layer 308 has a composition wavelength of 1.35 μm and a thickness of 230 nm.

Since other details are the same as those of the first embodiment and its modification, the description thereof is omitted here.
Next, a manufacturing method of a distributed reflection type laser (optical semiconductor device) according to a specific configuration example of the present embodiment will be described with reference to FIGS.
First, as shown in FIG. 18A, on a surface of an n-type doped InP substrate 301, a mask 302 made of an electron beam resist (ZEP 520 manufactured by Nippon Zeon Co., Ltd.) and having a diffraction grating pattern is formed by, for example, an electron beam exposure method. Form. The diffraction grating pattern includes a diffraction grating pattern for forming the diffraction grating 3 in the active region 1 and reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B on the front end surface side and the rear end surface side. And a diffraction grating pattern for reflection.

Next, using this mask 302, a part of the surface of the n-type InP substrate 301 is removed by reactive ion etching (RIE) using an ethane / hydrogen mixed gas, for example, as shown in FIG. To transfer the diffraction grating pattern. Here, the etching is stopped in the middle of the n-type InP substrate 301.
As a result, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are collectively formed. That is, the diffraction grating 3 is formed over the entire length (here, 100 μm) of the active region 1 of each element. In addition, a reflection diffraction grating 6B is formed over the entire length (here, 25 μm) of the region that becomes the distributed reflector region 2B on the front end face side, continuously from the diffraction grating 3 of the active region 1. Further, the diffraction grating 6A for reflection is formed over the entire length (here, 75 μm) of the distributed reflector region 2A on the rear end face side in succession to the diffraction grating 3 of the active region 1.

  In the present embodiment, the diffraction grating 3 in the active region 1 is a curved diffraction grating. The reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are also curved diffraction gratings. That is, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved diffraction gratings curved so as to be convex toward the center position of the active region 1. It has become. Here, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are arc-shaped diffraction gratings having an arc shape with a curvature radius of 8 μm. A phase shift (λ / 4 phase shift in this case) is provided at the center of the active region 1.

Further, the period of the diffraction grating 3 formed in the active region 1 is constant. Further, in the active region 1, the coupling coefficient and phase of the diffraction grating 3 are constant. In the active region 1, the equivalent refractive index of the waveguide is constant along the resonator direction.
Further, the period of the reflection diffraction grating 6A formed in the distributed reflector region 2A on the rear end face side is constant. Further, in the distributed reflector region 2A, the coupling coefficient and phase of the reflection diffraction grating 6A are constant. In the distributed reflector region 2A, the equivalent refractive index of the waveguide is constant along the resonator direction.

The period of the reflection diffraction grating 6B formed in the distributed reflector region 2B on the front end face side is constant. Further, the coupling coefficient and the phase of the reflection diffraction grating 6B are constant in the distributed reflector region 2B. In the distributed reflector region 2B, the equivalent refractive index of the waveguide is constant along the resonator direction.
Further, the depths of the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are 100 nm, which are the same. Further, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same duty ratio (here, 50%). That is, the coupling coefficients of the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are the same. Further, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same phase at the center in the waveguide width direction.

  Next, the mask 302 is removed from the surface using a normal resist stripping process. Thereafter, as shown in FIG. 18C, an n-type doped GaInAsP layer (guide layer) 303 is grown on the surface of the n-type InP substrate 301 on which the diffraction grating pattern is formed by using, for example, the MOVPE method. . Here, the n-type doped GaInAsP layer 303 has a composition wavelength of 1.25 μm and a thickness of 120 nm. As a result, a groove (diffraction grating pattern) formed by stopping etching in the middle of the n-type InP substrate 301 is filled with the n-type doped GaInAsP layer 303.

  Next, as shown in FIG. 18C, an n-type doped InP layer 304, an undoped AlGaInAs / AlGaInAs quantum well active layer 305, and a p-type doped InP cladding layer 306 are formed on the n-type doped GaInAsP layer 303 by, for example, MOVPE. Grow sequentially. Here, the thickness of the n-type doped InP layer 304 is 20 nm. The p-type doped InP cladding layer 306 has a thickness of 250 nm.

  Here, the quantum well active layer 305 is configured using an AlGaInAs-based compound semiconductor material. That is, the quantum well active layer 305 includes an undoped AlGaInAs well layer and an undoped AlGaInAs barrier layer. Here, the undoped AlGaInAs well layer has a thickness of 5.1 nm and a compressive strain of 1.2%. The undoped AlGaInAs barrier layer has a composition wavelength of 1.20 μm and a thickness of 10 nm. The number of stacked quantum well active layers 305 is 15, and the emission wavelength is 1550 nm.

An undoped AlGaInAs-SCH layer (light guide layer) may be provided above and below the quantum well active layer 305 so as to sandwich the quantum well active layer 305. Here, the undoped AlGaInAs-SCH layer has a composition wavelength of 1.15 μm and a thickness of 20 nm.
Next, a striped SiO ₂ mask is formed on the surface of the p-type doped InP cladding layer 306 so as to cover the active region 1 by using a normal CVD method and a photolithography technique as shown in FIG. (Etching mask) 307 is formed. Here, the thickness of the SiO ₂ mask 307 is 400 nm.

Then, as shown in FIG. 19B, from the surface of the p-type doped InP cladding layer 306 to the surface of the n-type doped InP layer 304 using the mask 307, that is, the p-type doped InP cladding layer 306 and The quantum well active layer 305 is removed by, for example, etching.
Thereafter, as shown in FIG. 19C, an undoped AlGaInAs light guide layer 308 and an undoped InP layer 309 are sequentially grown on the n-type doped InP layer 304 by, eg, MOVPE. Here, the undoped AlGaInAs light guide layer 308 has a composition wavelength of 1.35 μm and a thickness of 230 nm. The undoped InP layer 309 has a thickness of 250 nm. At this time, these layers 308 and 309 do not grow on the SiO ₂ mask 307 by selective growth, but grow only on the n-type doped InP layer 304 exposed on the surface.

Thereafter, after the SiO ₂ mask 307 is peeled off, as shown in FIG. 20A, the MOVPE method is used again, for example, on the entire surface of the semiconductor crystal wafer, for example, a p-type InP clad layer 310, followed by a p-type GaInAs contact. The layer 311 is stacked. Here, the p-type InP cladding layer 310 is doped with Zn and has a thickness of 2.0 μm. The p-type GaInAs contact layer 311 is doped with Zn and has a thickness of 300 nm.

Then, as shown in FIG. 20B, a SiO ₂ mask 312 is formed on the surface of the p-type GaInAs contact layer 311 using, for example, a normal CVD method and a photolithography technique. Here, the SiO ₂ mask 312 is a striped etching mask having a thickness of 400 nm and a width of 3.0 μm.
After that, as shown in FIG. 20C, the semiconductor stacked structure formed as described above is etched to a depth at which the n-type InP substrate 301 is dug by about 0.7 μm, for example, using a dry etching method. Then, it is processed into a striped mesa structure (mesa stripe) 8.

Next, as shown in FIG. 21A, current confinement layers 313 made of, for example, Fe-doped semi-insulating InP are grown on both sides of the mesa structure 8 by using, for example, the MOVPE method. Thereby, a semi-insulating buried heterostructure (SI-BH structure) is formed.
Next, after removing the etching mask 312 with, for example, hydrofluoric acid, the p-type GaInAs contact layer 311 other than the active region 1 is removed using a normal photolithography technique and etching.

Thereafter, as shown in FIG. 21B, a SiO ₂ passivation film 314 is formed. Then, the SiO ₂ passivation film is removed using a normal photolithography technique and etching so that a window opens only in a portion above the p-type GaInAs contact layer 311 in the active region 1. Thereafter, a p-side electrode 315 and an n-side electrode 316 are formed.
Then, as shown in FIG. 21B, non-reflective coatings 317 and 318 are formed on both end faces of the element, thereby completing the element.

Since other details are the same as those of the first embodiment and its modification, the description thereof is omitted here.
Therefore, according to the optical semiconductor device (semiconductor laser) according to the present embodiment, stable single transverse mode operation can be realized while suppressing saturation of optical output and obtaining sufficient characteristics (modulation characteristics). There is an advantage.

  In particular, in the above-described embodiment, the length of the active region 1 is set to 100 μm in order to improve high-speed response characteristics. Further, in order to reduce the element resistance and suppress the saturation of the optical output, the width of the mesa structure 8, that is, the waveguide width is set to 3.0 μm. In order to suppress the oscillation of the transverse higher-order mode, the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B are curved diffraction gratings having a curvature radius of 8 μm. .

  When such a curved diffraction grating is used, the coupling coefficient of the transverse fundamental mode (0th order) is 65% as compared with the case of the linear diffraction grating. On the other hand, the coupling coefficient of the transverse higher-order mode (primary) and the coupling coefficient of the transverse higher-order mode (secondary) when the curved diffraction grating is used are 10% as compared with the case of the linear diffraction grating. 15%. As a result, the oscillation threshold gain of the transverse higher-order mode (first order and second order) when the curved diffraction grating is used is at least twice the oscillation threshold gain of the transverse fundamental mode. As a result, in the device manufactured as in the above-described embodiment, oscillation in the lateral high-order mode was stopped, and a stable single transverse mode operation was obtained.

In the above-described embodiment, curved diffraction gratings are used as the diffraction grating 3 in the active region 1 and the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B. However, the present invention is not limited to this. A diffraction grating having a convex shape toward the active region 1 may be used.
That is, the reflection diffraction gratings 6A and 6B in the distributed reflector regions 2A and 2B have the same coupling coefficient in the width direction of the passive waveguide 10, and widths on both sides in the width direction are parallel to the width direction of the passive waveguide 10. What is necessary is just to incline to the opposite side of the active region 1 with respect to the direction center part.

  Further, the diffraction grating 3 from the center position of the active region 1 to the rear end face side has the same coupling coefficient in the width direction of the active waveguide 9, and the width direction both side portions are parallel to the width direction of the active waveguide 9. What is necessary is just to incline to the distributed reflector area | region 2A side of the rear end surface side with respect to a center part. The diffraction grating 3 on the rear end face side from the center position of the active region 1 is referred to as a fourth diffraction grating. Further, the distributed reflector region 2A on the rear end face side is also referred to as a first distributed reflector region.

  Further, the diffraction grating 3 on the front end face side from the center position of the active region 1 has the same coupling coefficient in the width direction of the active waveguide 9, and both sides in the width direction are parallel to the width direction of the active waveguide 9. What is necessary is just to incline to the opposite side of 2 A of distributed reflector area | regions of the rear end surface side with respect to a center part. Note that the diffraction grating 3 on the front end face side from the center position of the active region 1 is also referred to as a fifth diffraction grating. Further, the distributed reflector region 2A on the rear end face side is also referred to as a first distributed reflector region.

  However, it is preferable to use a curved diffraction grating such as an arc-shaped diffraction grating, an elliptical diffraction grating, or a second-order curved diffraction grating in terms of wide design tolerance. When such a curved diffraction grating is used, even if the diffraction grating is displaced in the width direction of the waveguide when the diffraction grating is formed, the characteristics can be prevented from being affected. In other words, when a curved diffraction grating is used, even if the diffraction grating is shifted in the width direction of the waveguide, the coupling coefficient of the transverse fundamental mode does not decrease so much, while the coupling coefficient of the transverse higher-order mode is large. Therefore, desired characteristics can be obtained. Note that there is a degree of freedom in design as to what function is adopted as a function that defines the shape of the diffraction grating (for example, a function indicating a change in curvature). Therefore, the “curved diffraction grating” includes not only a curved diffraction grating but also a diffraction grating having a shape approximate to a curved shape.

In the above-described embodiment, the diffraction grating 3 in the active region 1 includes the phase shift 4. However, the present invention is not limited to this, and the diffraction grating 3 in the active region 1 does not include the phase shift. You may comprise as.
In the above-described embodiment, the distributed reflector regions 2A and 2B are provided so as to be continuous with the rear end surface side and the front end surface side of the active region 1. However, the present invention is not limited to this, and the rear end surface of the active region 1 is not limited thereto. You may provide a distributed reflector area | region so that it may continue only to the side.

In the above-described embodiment, the reflection diffraction grating 6B of the distributed reflector region 2B connected to the front end face side of the active region 1 is a curved diffraction grating. However, the present invention is not limited to this. A linear diffraction grating may be used as in the modification of the embodiment.
In the above-described embodiment, the diffraction grating 3 of the active region 1 is a curved diffraction grating. However, the present invention is not limited to this. For example, as in the first and second embodiments described above, linear diffraction is performed. It may be a lattice. In this case, either one of the diffraction gratings 3 on the front end face side and the rear end face side from the center position of the active region 1 may be a linear diffraction grating. The diffraction grating 3 located on the rear end face side from the center position of the active region 1 is a curved diffraction grating (fourth diffraction grating), and the diffraction grating 3 located on the front end face side from the center position of the active area 1 is a linear diffraction grating. (Third diffraction grating) is preferable.
[Others]
In each of the above-described embodiments and modifications, the case where the present invention is applied to a DR laser in which the diffraction grating 3 is also provided in the active region 1, that is, a DR laser having a function of a distributed feedback laser is taken as an example. The explanation is given by way of example, but the present invention is not limited to this. For example, the present invention can also be applied to a distributed reflection (DBR) laser in which no diffraction grating is provided in the active region. In this case, in the configuration of the first embodiment described above, a highly reflective film is provided on the cleavage surface on the front end face side.

Further, in each of the above-described embodiments and modifications, the case where the active region and the distributed reflection region have the same coupling coefficient value of the diffraction grating has been described as an example. However, the present invention is not limited to this. Depending on the design of the element, it may be different.
In each of the above-described embodiments and modifications, the case where the duty ratio (Yamatani ratio) of the diffraction grating is set to 50% has been described as an example. However, the present invention is not limited to this, and element design is performed. Depending on the case, the duty ratio may be changed.

  In the first embodiment described above, the oscillation wavelength band is set to 1.31 μm band, and in the second and third embodiments, the oscillation wavelength band is set to 1.55 μm band. It is not a thing. For example, in the first embodiment, the oscillation wavelength band may be 1.55 μm, and in the second and third embodiments, the oscillation wavelength band may be 1.31 μm.

  In each of the above-described embodiments and modifications, a semiconductor laser (DR laser) including a quantum well active layer using an AlGaInAs-based compound semiconductor material on an n-type InP substrate is taken as an example in order to realize a high-speed modulation operation. The explanation is given by way of example, but the present invention is not limited to this. For example, the quantum well active layer may be formed using a GaInAsP-based compound semiconductor material in each of the embodiments and modifications. For example, the quantum well active layer may be formed using other compound semiconductor materials such as a GaInNAs-based compound semiconductor material. In these cases as well, the lateral higher-order mode suppression effect can be obtained as in the case of the above-described embodiments and modifications thereof.

  In the above-described embodiments and modifications, the substrate having n-type conductivity is used. However, the present invention is not limited to this. For example, a substrate having p-type conductivity may be used. In this case, the conductivity of each layer formed on the substrate is reversed. A semi-insulating substrate may be used. Further, for example, it may be produced by a method of bonding onto a silicon substrate. In these cases, the same effects as those of the above-described embodiments and modifications thereof can be obtained.

Alternatively, for example, a GaAs substrate may be used, and each layer may be formed using a semiconductor material capable of crystal growth (for example, epitaxial growth) on the GaAs substrate. Also in this case, the same effects as those of the above-described embodiments and modifications thereof can be obtained.
In each of the above-described embodiments and modifications, the quantum well active layer is used. However, the present invention is not limited to this. For example, other active layer structures such as a bulk active layer using a bulk semiconductor material or a quantum dot active layer may be employed. In this case as well, the lateral higher-order mode suppression effect can be obtained as in the case of the above-described embodiments and modifications thereof.

Further, in each of the above-described embodiments and modifications, a case where a current confinement structure using a semi-insulating embedded structure (SI-BH structure) is used for high-speed modulation operation is described as an example. . However, the present invention is not limited to these, and a current confinement structure using another buried structure such as a current confinement structure using a pnpn thyristor structure can also be adopted.
Further, in each of the above-described embodiments and the modifications thereof, the surface diffraction grating structure formed on the surface of the substrate is described as an example. However, the invention is not limited to these, and an embedded diffraction grating structure formed by embedding a periodically divided semiconductor layer with another semiconductor layer can also be used.

  Further, in each of the above-described embodiments and modifications thereof, an example in which the diffraction grating is loaded on the lower side of the waveguide core layer (active layer or light guide layer) (substrate side with respect to the waveguide core layer) is an example. It is listed and explained. However, the present invention is not limited to these, and for example, it may be loaded on the upper side of the waveguide core layer (the side opposite to the substrate with respect to the waveguide core layer). Similar effects can be obtained.

Further, in the above-described first and third embodiments and the modifications thereof, the phase shift is provided at the position on the rear end face side of 10 μm from the center of the active region 1 or at the center position of the active region 1. The position of is not limited to this. In other words, the phase shift can be arranged at other positions within the design range within the active region.
Further, in each of the above-described embodiments and modifications thereof, the case where only one phase shift is provided has been described as an example. However, the present invention is not limited to this, and for example, a structure having a plurality of phase shifts. There may be. Further, the amount of one or a plurality of phase shifts can be arbitrarily set.

Further, the present invention is not limited to the configurations described in the above-described embodiments and modifications, and various modifications can be made without departing from the spirit of the present invention.
Hereinafter, additional notes will be disclosed regarding the above-described embodiments and modifications thereof.
(Appendix 1)
An active region;
A first distributed reflector region provided on one end side of the active region and having a first waveguide loaded with a first diffraction grating;
The first diffraction grating has the same coupling coefficient in the width direction of the first waveguide, and both side portions in the width direction of the active region with respect to the center portion in the width direction parallel to the width direction of the first waveguide. An optical semiconductor device which is inclined to the opposite side.

(Appendix 2)
The optical semiconductor device according to appendix 1, wherein the first diffraction grating is a curved diffraction grating.
(Appendix 3)
The optical semiconductor device according to appendix 1 or 2, further comprising a second distributed reflector region provided on the other end side of the active region and having a second waveguide loaded with a second diffraction grating.

(Appendix 4)
The second diffraction grating has the same coupling coefficient in the width direction of the second waveguide, and both side portions in the width direction of the active region with respect to the center portion in the width direction parallel to the width direction of the second waveguide. 4. The optical semiconductor device according to appendix 3, wherein the optical semiconductor device is inclined to the opposite side.
(Appendix 5)
The optical semiconductor device according to appendix 4, wherein the second diffraction grating is a curved diffraction grating.

(Appendix 6)
4. The optical semiconductor device according to appendix 3, wherein the second diffraction grating is a linear diffraction grating parallel to the width direction of the second waveguide.
(Appendix 7)
The optical semiconductor device according to any one of appendices 1 to 6, wherein the active region includes an active waveguide loaded with a third diffraction grating.

(Appendix 8)
8. The optical semiconductor device according to appendix 7, wherein the third diffraction grating is a linear diffraction grating parallel to the width direction of the active waveguide.
(Appendix 9)
The active region comprises an active waveguide loaded with a third diffraction grating and a fourth diffraction grating,
The third diffraction grating is a linear diffraction grating parallel to the width direction of the active waveguide,
The fourth diffraction grating has the same coupling coefficient in the width direction of the active waveguide, and the first distributed reflector with respect to the center portion in the width direction whose both side portions are parallel to the width direction of the active waveguide. The optical semiconductor device according to any one of appendices 1 to 6, wherein the optical semiconductor device is inclined toward the region.

(Appendix 10)
The active region comprises an active waveguide loaded with a fourth diffraction grating and a fifth diffraction grating,
The fourth diffraction grating has the same coupling coefficient in the width direction of the active waveguide, and the first distributed reflector with respect to the center portion in the width direction whose both side portions are parallel to the width direction of the active waveguide. Tilted to the area side,
The fifth diffraction grating has the same coupling coefficient in the width direction of the active waveguide, and the first distributed reflectors with respect to the center portion in the width direction whose both side portions are parallel to the width direction of the active waveguide. The optical semiconductor device according to any one of appendices 1 to 6, wherein the optical semiconductor device is inclined to the opposite side of the region.

DESCRIPTION OF SYMBOLS 1 Active area | region 2 Distributed reflector area | region 2A Distributed reflector area | region of the rear end surface side 2B Distributed reflector area | region of the front end surface side 3 Diffraction grating 4 Phase shift 5 Diffraction grating layer 6, 6A, 6B Reflection diffraction grating 7 Reflection diffraction grating Layer 8 mesa structure 9 active waveguide 10 passive waveguide 101, 201, 301 n-type doped InP substrate 102, 202, 302 mask 103, 203, 303 n-type doped GaInAsP layer 104, 204, 304 n-type doped InP layer 105, 205,305 Undoped AlGaInAs / AlGaInAs quantum well active layer 106,206,306 p-type doped InP cladding layer 107,207,307 SiO ₂ mask 108,208,308 Undoped AlGaInAs light guide layer 109,209,309 Undoped InP layer 110, 210, 310 p-type InP cladding layer 111, 211, 311 p-type GaInAs contact layer 112, 212, 312 SiO ₂ mask 113, 213, 313 Current confinement layer 114, 214, 314 SiO ₂ passivation film 115, 215, 315 Current injection electrode (P-side electrode)
116, 216, 316 n-side electrode 117, 217, 317, 118, 218, 318 Non-reflective coating

Claims (5)

An active region;
A first distributed reflector region provided on one end side of the active region and having a first waveguide loaded with a first diffraction grating;
The first diffraction grating has the same coupling coefficient in the width direction of the first waveguide, and both side portions in the width direction of the active region with respect to the center portion in the width direction parallel to the width direction of the first waveguide. An optical semiconductor device which is inclined to the opposite side.   The optical semiconductor device according to claim 1, wherein the first diffraction grating is a curved diffraction grating.   3. The optical semiconductor device according to claim 1, further comprising a second distributed reflector region provided on the other end side of the active region and having a second waveguide loaded with a second diffraction grating.   The second diffraction grating has the same coupling coefficient in the width direction of the second waveguide, and both side portions in the width direction of the active region with respect to the center portion in the width direction parallel to the width direction of the second waveguide. The optical semiconductor device according to claim 3, wherein the optical semiconductor device is inclined to the opposite side.   The optical semiconductor device according to claim 1, wherein the active region includes an active waveguide loaded with a third diffraction grating.

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