JP2015220322A - Semiconductor optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor optical device capable of solving such problems that it is important to secure sufficient optical output level for applying the lateral implantation laser to an access system network or a metro network, however, in a conventional lateral implantation laser technique, the maximum optical output level of the lateral implantation laser is still limited to 10 mW or less and thus further enhancement of an optical output is required, and that the lateral implantation laser configured to implant current in the lateral direction of an active layer, causes non-uniformity in current distribution in the active layer, thereby producing no effect of enhancing the optical output by increasing the width of the active layer.SOLUTION: A semiconductor optical device is so configured as to form an active layer and an embedded waveguide structure on a semiconductor board, subject embedded layers at both sides of the active layer to impurity doping, and laterally inject a current into the active layer in a lateral direction (width direction). The width of the active layer is minimized as much as possible in a range where an oscillation operation becomes a single waveguide mode and a waveguide mode is present, and the value of the equivalent refractive index of the waveguide is set to about the refractive index of the board.

Description

  The present invention relates to a structure of an element used for a light source for an optical transmitter. More specifically, the present invention relates to a semiconductor element applicable to a semiconductor laser for a light source, a modulator, and the like.

  Due to the explosive increase in the amount of network traffic accompanying the spread of the Internet, optical fiber transmission continues to increase in speed and capacity. Semiconductor lasers have continued to develop as basic light source devices that support optical fiber communications. Direct modulation lasers that generate intensity-modulated signals by current intensity modulation have a simple laser configuration and low power consumption, so they are currently used as low-cost optical transmitters used in access networks and the like. ing.

  In a conventional semiconductor laser, a lower cladding layer, an active layer, and an upper cladding layer are formed on a semiconductor substrate, impurity doping is performed on the cladding layers above and below the active layer, and the longitudinal direction (direction perpendicular to the substrate surface) ) Had a structure for injecting current. In contrast, Namizaki devised a so-called lateral injection laser that injects a current in the active layer in a horizontal direction (a direction parallel to the substrate surface) (Non-patent Document 1). The lateral injection laser performs impurity doping on the cladding layer next to the active layer and injects a current in the lateral direction (width direction) of the active layer. In the lateral injection laser, the cross section of the active layer is generally formed by a flat structure that is long in a direction parallel to the substrate. When an active layer having the same structure is used for a laser, the parasitic capacitance of the element is lower in the lateral injection structure than in the vertical injection structure. Therefore, the laser having the lateral injection structure is more suitable for high-speed modulation operation because it responds more quickly to signals. In addition, the lateral injection laser can form an electrode for current injection on the surface of the laser, and is therefore an excellent feature that it is suitable for integration of electronic devices and modularization.

  Specifically, Namizaki et al. Realized a lateral injection semiconductor laser configured on a GaAs substrate, and Kawamura et al. Also realized a lateral injection laser configured on an InP substrate (Non-Patent Document 2). Hereinafter, first, a more specific configuration of the lateral injection laser will be described.

  FIG. 7 is a diagram showing the structure of a waveguide for a lateral injection laser according to the prior art. A waveguide structure 100 is shown in which a waveguide used for a resonator portion of a laser is cut by a plane perpendicular to the light reciprocation direction. In the waveguide structure 100, a light-carrier separation confinement (SCH) layer 103 and an active layer 104 are formed on a semi-insulating InP substrate 102, and a light-carrier is further formed on the active layer 104. A separate confinement layer 105 is formed. Further, an InP layer 106 is formed on the SCH layer 105. The sides of the active layer 104 are embedded with InP on both the left and right sides. In FIG. 7, the buried layer 107 on the left side of the active layer 504 is subjected to n-type doping for current injection, and the buried layer 108 on the right side of the active layer 104 is subjected to p-type doping. On the buried layer 107 and the buried layer 108, InGaAs contact layers 109a and 109b for current injection are formed, respectively. Furthermore, electrodes 110a and 110b for current injection are formed on the contact layers 109a and 109b, respectively.

  In the study by Kawamura et al., The thickness of the InP clad layer 106 above the active layer 104 is 1.5 μm, which is the same structure as the vertical injection laser of the prior art. In recent years, Shindo et al. Realized a thin-film lateral injection laser composed of a thin active layer having a thickness of less than 400 nm and a thin InP layer on an InP substrate (Non-patent Document 3). As already described, in the case of the lateral injection laser structure, there is an advantage that the parasitic capacitance of the element can be suppressed by reducing the thickness of the cladding layers above and below the active layer. Shindo et al. Realized a wide modulation band up to 5 GHz by adopting this thin film structure in a lateral injection laser.

H. Namizaki et al., Journal of Applied Physics, vol. 45, pp. 2785-2786 (1974) Y. Kawamura et al., Electronics Letters, vol. 29, pp. 102-104 (1993) T. Shindo et al., Optics Express, vol. 19, pp. 1884-1891 (2011)

  Although it is a transverse injection laser that can realize a wide modulation band, it is important to secure a sufficient light output level in order to apply the transverse injection laser to an access network or a metro network. However, in the conventional lateral injection laser, as shown in Non-Patent Document 3, the maximum light output level is still 10 mW or less, and it is desired to further increase the light output.

  In the conventional vertical current injection laser, a structure in which the active layer width is 1 μm or more and the volume of the active layer is increased is a general method for increasing the optical output. However, since the lateral injection laser has a structure in which current is injected in the lateral direction (width direction) of the active layer, nonuniformity occurs in the current distribution in the active layer, and the output can be increased by increasing the active layer width. It was known that the effect of could not be obtained. Therefore, in the lateral injection laser, it is required to avoid the problem of non-uniformity of current distribution in the active layer and to obtain a higher light output.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a lateral injection laser capable of realizing a higher light output level.

In order to achieve the above object, the present invention provides a semiconductor substrate, an active layer formed on the semiconductor substrate, an upper cladding layer, and both sides of the active layer. A waveguide structure having a buried layer formed thereon, p-type impurity doping is performed on one of the buried layers, and n-type impurity doping is performed on the other of the buried layers. In the semiconductor optical device provided with a reflector for enabling laser oscillation by current injection, wherein a structure for performing current injection in the lateral direction is formed in the active layer between the other buried layers. The width in the horizontal direction is a value within a range in which the oscillation mode becomes a single waveguide mode and the waveguide mode can exist when the semiconductor element oscillates, and the waveguide having the waveguide structure Equivalent refractive index of Value is a semiconductor optical device characterized in that it is substantially identical to the refractive index of the substrate.
According to a second aspect of the present invention, in the semiconductor optical device according to the first aspect, the thickness of the upper clad layer is thinner than 1 μm.

  According to a third aspect of the present invention, in the semiconductor optical device of the first or second aspect, the reflector is a surface diffraction grating formed above the active layer.

  According to a fourth aspect of the present invention, in the semiconductor optical device according to any one of the first to third aspects, the equivalent refractive index is in the range of 100% to 100.3% of the refractive index of the substrate. .

  The invention according to claim 5 is the semiconductor optical device according to any one of claims 1 to 4, wherein the semiconductor substrate is a semi-insulating InP substrate, and the upper clad layer and the buried layer are made of InP, The active layer is composed of InGaAsP or InGaAlAs.

  A sixth aspect of the present invention is a distributed feedback semiconductor laser having a waveguide structure in the semiconductor optical device according to any one of the first to fifth aspects.

  As described above, the semiconductor optical device of the present invention can provide a lateral injection laser that can realize a higher light output level.

FIG. 1 is a diagram showing a waveguide and an overall structure of a lateral injection laser according to the present invention. FIG. 2 is a diagram showing the active layer width dependence of the equivalent refractive index in the waveguide structure of the lateral injection laser of the present invention. FIG. 3 is a conceptual diagram comparing the electric field distribution between the lateral injection laser of the prior art and the lateral injection laser of the present invention in which the active layer width is made as narrow as possible. FIG. 4 is a diagram showing the active layer width dependence of optical confinement in the p-clad region in the lateral injection laser of the present invention. FIG. 5 is a diagram showing the dependence of the propagation loss generated in the p-clad region on the active layer width. FIG. 6 is a diagram showing a current-output curve of the lateral injection laser of the present invention. FIG. 7 is a diagram showing the structure of a waveguide for a lateral injection laser according to the prior art. FIG. 8 is a diagram showing the result of simulating the electron and hole distribution in the cross section of the active layer of the lateral injection laser. FIG. 9 is a diagram showing the distribution of stimulated emission intensity in the cross section of the active layer in the same cases as in FIG. FIG. 10 is a diagram showing a simplified structure of another waveguide in which the semiconductor optical device of the present invention is applied to a lateral injection laser. FIG. 11 is a diagram showing the optical layer dependence of the optical confinement and the ratio of the effective refractive index to the refractive index of the substrate when the thickness of the upper cladding InP layer is changed for an element having a core layer height of 250 nm. is there. FIG. 12 is a diagram showing the optical layer dependence of the optical confinement and the ratio of the effective refractive index to the refractive index of the substrate when the thickness of the upper cladding InP layer is changed for an element having a core layer height of 350 nm. is there. FIG. 13 is a diagram showing the optical confinement and the active layer width dependence of the ratio of the effective refractive index and the refractive index of the substrate when the thickness of the upper cladding InP layer is changed for an element having a core layer height of 450 nm. is there.

  The inventors have reviewed the above-mentioned problems in the active layer of the lateral injection laser once again and have found a configuration of a semiconductor optical device that increases the light output. In the semiconductor optical device of the present invention, an active layer and a buried waveguide structure are formed on a semiconductor substrate, impurity doping is performed on the buried layers on both sides of the active layer, and current is applied to the active layer in the lateral direction (width direction). In the structure in which injection is performed, the active layer width becomes a single waveguide mode and is narrowed within the range in which the waveguide mode exists, and the value of the equivalent refractive index of the waveguide is approximately the same as the equivalent refractive index of the substrate. Is set to This structure was applied to a semiconductor laser, and a distributed feedback semiconductor laser was constructed by forming a diffraction grating on the active layer. The present invention can also be applied to an electroabsorption modulator by applying a reverse bias to the active layer. Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1A is a diagram showing the structure of a waveguide in which the semiconductor optical device of the present invention is applied to a lateral injection laser. FIG. 1A shows a waveguide structure 1 in which a waveguide used for a laser resonator is cut by a plane perpendicular to the reciprocating direction of light in the resonator. In the waveguide structure 1, an InGaAsP optical carrier separation and confinement (SCH) layer 3 having a band gap wavelength of 1.2 μm and an InGaAsP active layer 4 having an emission wavelength of 1.55 μm are sequentially formed on a semi-insulating InP substrate 2. The active layer 4 is formed of a quantum well composed of 14 layers having a well layer thickness of 6 nm and a barrier layer thickness of 9 nm. On the active layer 4, an InGaAsP SCH layer 5 having a band gap wavelength of 1.2 μm is formed. Further, an InP layer 6 having a thickness of 100 nm is formed on the SCH layer 5 above the active layer 4.

On the surface of the InP layer 6, a diffraction grating having a Bragg wavelength of 1.55 μm made of InP and air is formed by etching InP. This diffraction grating functions as a reflector that enables laser oscillation by current injection. Both sides of the active layer 4 are filled with InP with different types of doping. That is, for current injection, a Si n-type doping layer 7 having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ is disposed on the left side of the active layer 4 in FIG. 1, and a doping concentration of 1 × 10 6 is disposed on the right side of the active layer 4. ^{A 18} cm ⁻³ Zn p-type doping layer 8 is formed. On the buried layers 7 and 8, InGaAs contact layers 9a and 9b for current injection are formed, respectively, and electrodes 10a and 10b for current injection are formed on the contact layer regions 9a and 9b, respectively.

  FIG. 1B is a bird's-eye view of a lateral injection laser to which the structure of the semiconductor optical device of the present invention is applied, viewed from obliquely above. The lateral injection laser of the present invention is a distributed feedback type in which a surface diffraction grating 11 having a repetitive structure is formed on a top InP layer 6 and between electrodes 10a and 10b, thereby forming a resonator. It is a laser. Anti-reflective coating is applied to the laser output end faces at both ends of the resonator. In producing the lateral injection laser of the present invention, metal organic vapor phase epitaxy (MOVPE) is used for crystal growth, and a general semiconductor laser such as wet etching or dry etching is used for producing a laser waveguide structure and a diffraction grating. The method can be used. The buried doping layers 7 and 8 for current injection on the left and right sides of the active layer 4 can be formed by embedding and regrowing n-type doped InP and p-type doped InP, respectively. Further, after the active layer 4 is formed, intrinsic InP may be buried and regrown, and then a dopant may be formed by a technique such as ion implantation or thermal diffusion.

  As already described, in the vertical injection laser of the prior art, high output of light output is realized by widening the waveguide (active layer) width. The inventors focused attention on the behavior of the oscillator in a configuration in which the waveguide width is narrowed in order to avoid nonuniform carrier distribution in the lateral injection laser. Then, the inventors have found a configuration that can increase the optical output by adopting a configuration opposite to the expansion of the waveguide width, which is considered effective for the longitudinal laser. In the lateral injection laser of the present invention, the width of the active layer is made narrow so that the oscillation operation in the resonator becomes a single waveguide mode and the waveguide mode can exist, and the value of the equivalent refractive index of the waveguide is obtained. Is set to be approximately the refractive index of the substrate. Specifically, in the case of the structure corresponding to FIG. 1, the waveguide width is 0.5 μm. As will be described later, since stimulated emission occurs in the entire region of the active layer, the maximum width of the active layer is about 1 μm. The operation of the lateral injection laser having a narrow waveguide width according to the present invention will be described below.

FIG. 8 is a diagram showing the result of simulating the electron and hole distribution in the cross section of the active layer of the lateral injection laser. In the cross section of the active layer, the carrier density (cm ⁻³ ) is compared for each case where the active layer width in the direction perpendicular to the light traveling direction is 2 μm and 0.5 μm. 8A shows the case of an element having an active layer width of 2 μm, and FIG. 8B shows the case of an element having an active layer width of 0.5 μm. The buried layers on both sides of the active layer are doped with n doping and p doping concentrations of 1 × 10 ¹⁸ cm ⁻³ , respectively. As the active layer, a quantum well in which a well layer thickness of 6 nm and a barrier layer thickness of 9 nm are formed is calculated as an example. It should be noted that each layer in the quantum well in the cross-sectional view of FIG.

  Referring to FIG. 8A, it can be seen that electrons and holes are localized in a region on the p-type buried layer side in the active layer region indicated by the arrow. As already described, due to the localization of carriers, the internal quantum efficiency in a laser with a wide waveguide width as in the case where the width of the active layer is 2 μm is reduced, and the light emission efficiency is greatly reduced. As shown in FIG. 8B, when the generation layer width is as narrow as 0.5 μm, the carrier localization is not remarkable.

  FIG. 9 is a diagram showing a distribution of stimulated emission in the cross section of the active layer in each case of the same waveguide width as that in FIG. As shown in FIG. 9B, in an element having an active layer width of 0.5 μm, stimulated emission occurs in almost the entire region of the active layer width. On the other hand, in the device having an active layer width of 2 μm shown in FIG. 9A, the stimulated emission occurs only in a half (1 μm) region on the p-type buried layer side in the active layer indicated by the arrow. It has not occurred. Since the inventors have a slower response to modulation as the carrier travels longer, reducing the width of the active layer is rather desirable from the standpoint of realizing a fast response of a directly modulated laser and a wide modulation band. I thought.

  FIG. 2 is a diagram showing the active layer width dependence of the equivalent refractive index in the waveguide structure of the lateral injection laser of the present invention. The horizontal axis represents the active layer width (μm), and the vertical axis represents the equivalent refractive index. In order for the laser to operate in a single mode, it is necessary to control the active layer width so that a higher-order mode does not appear. Referring to FIG. 2, in order to prevent the primary mode from appearing, in the configuration of the lateral injection laser according to the present invention, it is necessary to first set the active layer width to 1.6 μm or less. Further, in order to cause stimulated emission in the entire region in the active layer, the maximum width of the active layer is approximately 1 μm as a guide in consideration of the lateral width in which stimulated emission occurs in FIG. It becomes.

  Preferably, the width of the active layer in the lateral injection laser according to the present invention is such that the equivalent refractive index of the waveguide in the fundamental mode is substantially the same as the refractive index of 3.17 of the InP substrate 2, and the zero-order mode can exist. Let the value be the minimum waveguide width. Here, the equivalent refractive index is a refractive index related to a phase change of propagating light in the waveguide mode. In the case of the configuration of the lateral injection laser shown in FIG. 1, 0.5 μm corresponds to this minimum waveguide width value. Hereinafter, effects obtained by the lateral injection laser of the present invention using a structure having the minimum waveguide width will be described.

The external differential quantum efficiency η _d of the semiconductor laser of the Fabry-Perot laser is expressed as follows using the internal differential quantum efficiency η _i , the propagation loss α _{i of} the active layer, the reflectance R of the laser end face, and the active layer length L. Is done.

The external differential quantum efficiency η _d is defined as the ratio of photons extracted with respect to the number of incident carriers and is a well-known parameter. According to equation (1), the internal differential quantum efficiency η _i of the active layer can be increased, and the propagation loss α _i of the active layer can be further decreased to increase the laser efficiency η _d . As described above, in the lateral injection laser, the internal differential quantum efficiency is higher as the width of the active layer is narrower. On the other hand, considering the propagation loss of the active layer, that is, the waveguide loss, the main factor is considered to be an absorption loss generated in the p-clad region formed beside the active layer. The absorption loss of the p-type InP layer 8 is about 20 cm ⁻¹ per doping concentration of 1 × 10 ¹⁸ cm ⁻³ . In the vertical injection laser of the prior art, as the width of the active layer becomes narrower, the electric field distribution of the waveguide mode spreads toward the p-clad region (p-type InP layer 8), which is a buried layer, and the optical loss in the cladding region increases. It was common.

  However, in the structure of the lateral injection laser of the present invention, the clad layer InP layer 6 above the active layer is as thin as 100 nm, so that the light confinement is more strongly affected by the air on the upper surface of the device. As a result, the electric field distribution spreads asymmetrically in the substrate direction with respect to the active layer, so that loss in the p-clad region can be suppressed.

  In the vertical injection structure, it is necessary to dispose an electrode on the upper cladding layer. For this reason, in order to suppress loss, it is necessary to increase the thickness of the upper clad layer so that no electric field exists above the clad, and as described above, a thickness of about 1.5 μm is necessary. On the other hand, the transverse injection laser according to the present invention is characterized in that there is little concern about propagation loss above the active layer, the upper clad layer is 1 μm or less, and the electric field distribution asymmetry can be used positively. Next, a more specific difference in waveguide loss between the configuration of the present invention and the configuration of the prior art will be described by comparing electric field distributions.

  FIG. 3 is a conceptual diagram comparing the electric field distribution between the lateral injection laser of the prior art and the lateral injection laser of the present invention in which the active layer width is made as narrow as possible. 3A shows the electric field distribution in the case of a conventional lateral injection laser with a wide active layer width, and FIG. 3B shows the electric field in the case of a lateral injection laser with the active layer width made as narrow as possible according to the present invention. Distribution is shown. As shown in (a), when the waveguide width is relatively wide, the electric field spread 31 spreads in an approximately elliptical shape with the active layer 4 as the center. At this time, a waveguide loss occurs in the overlapping portion 33 between the electric field spread 31 and the p-type InP layer 8.

  On the other hand, as shown in FIG. 3B, when the waveguide width is narrower, the electric field spread 32 is approximately a circle centered on the position shifted to the InP substrate 2 side below the active layer 4. Spread in shape. Also at this time, a waveguide loss occurs in the overlapping portion 34 between the electric field spreading region 32 and the p-type InP layer 8.

  The light confinement in the p-type cladding region, which is the buried region, is expressed by the ratio of the light energy in the p-type cladding region to the light energy in the entire region. In the structure of the lateral injection laser of the present invention, as shown in (b), since the electric field distribution spreads more widely in the direction of the InP substrate 2, the area of the entire electric field spread is the area of the electric field overlapping portion in the p-type cladding region. The ratio to is small compared to the prior art structure. This is because the area ratio between the electric field spreading region 31 in FIG. 3A and the overlapping region 33 in the p-type cladding region is the same as the overlapping region in the electric field spreading region 32 and the p-type cladding region in FIG. Compared with the area ratio of 34, it will be easy to understand. Due to the difference in the electric field distribution described above, in the lateral injection laser of the present invention, optical confinement in the p-cladding region, which is a buried layer, is reduced, and propagation loss can be suppressed.

  Accordingly, the present invention provides a waveguide structure including a semiconductor substrate, an active layer formed on the semiconductor substrate, an upper cladding layer, and buried layers on both sides of the active layer, and the buried layer P-type impurity doping is applied to one of the first and n-type impurity dopings to the other of the buried layer, and a current flows laterally to the active layer between the one buried layer and the other buried layer. In a semiconductor optical device having a structure for performing injection and having a reflector capable of laser oscillation by current injection, the width of the active layer in the lateral direction is determined when the semiconductor device oscillates. In addition, the oscillation mode is a single waveguide mode, and is a value in a range where the waveguide mode can exist, and the value of the equivalent refractive index of the waveguide of the waveguide structure is substantially the same as the refractive index of the substrate. There is It is implemented as a semiconductor optical device characterized.

  FIG. 4 is a diagram showing the active layer width dependence of optical confinement in the p-clad region in the lateral injection laser of the present invention. The horizontal axis represents the width of the active layer, and the vertical axis represents optical confinement in the p-type cladding region. The optical confinement is maximum when the waveguide width is around 0.6 μm. The light confinement in the p-type region means light leakage to the outside of the active layer. The smaller the light confinement in the p-type buried region 8, that is, the smaller the leak into the p-type buried region 8, the more light Loss is reduced.

FIG. 5 is a diagram showing the dependence of the propagation loss generated in the p-clad region on the active layer width. Active layer width and p-clad when the doping concentration of the p-clad region 8 which is a buried layer is changed to three types of 1 × 10 ¹⁸ cm ⁻³ , 2 × 10 ¹⁸ cm ⁻³ and 3 × 10 ¹⁸ cm ⁻³ The propagation loss occurring in the region is shown. As shown on the right side of the peak of each curve, there is a region where propagation loss decreases due to the strong confinement effect of the core (active layer) as the active layer width increases, while the active layer width is narrowed as shown on the left side of the peak. Even in this case, there is a region where the optical confinement in the p-clad region is small and the propagation loss is reduced. According to FIG. 5, the effect of suppressing the propagation loss of the element by setting the waveguide width to the minimum waveguide width value at which the mode can exist (in the case of this embodiment, 0.5 μm). Has been revealed.

  FIG. 6 is a diagram showing a current-output curve of the lateral injection laser of the present invention in comparison with the prior art. The configuration of a conventional lateral injection laser having an active layer width of 2 μm and the configuration of a lateral injection laser of the present invention having an active layer width of 0.5 μm were compared. In the configuration in which the active layer of the present invention is narrowed, the optical confinement of the active layer is lowered, so the oscillation threshold current is slightly higher than in the configuration of the prior art, but at the same current value, the optical output level is The lateral injection laser is larger. From FIG. 6 as well, the lateral injection laser having a narrow active layer (waveguide) width according to the present invention has an increased external quantum efficiency due to a reduction in propagation loss and an increase in internal quantum efficiency, thereby increasing the optical output. It is clear that it is very useful for.

  In the above embodiment, the InGaAsP material is used for the active layer, but it goes without saying that other optical semiconductor material systems such as InAlGaAs material can be applied. When other optical semiconductor materials are used, the width of the minimum active layer in the above-described embodiment is naturally different from 0.5 μm. The minimum active layer width also varies depending on the substrate material and doping parameters. However, the equivalent refractive index of the fundamental mode is almost the same as the refractive index of the substrate material, and there is no change in setting the minimum waveguide width value at which the zero-order mode can exist. Next, in the lateral injection laser having the structure of the narrow active layer according to the present invention, another configuration for explaining the effect of using a different core material and having a narrow waveguide width more generally and the preferable waveguide width range is used. An example will be described.

  FIG. 10 is a diagram showing another waveguide structure in which the semiconductor optical device of the present invention is applied to a lateral injection laser. In order to explain the effect of the waveguide width, the structure of the waveguide structure 51 is simplified by including the lower and upper SCH layers in the core layer as compared with the structure of FIG. That is, the core layer 54 is composed of an active layer made of InGaAsP or InGaAlAs material having an emission wavelength of 1.55 μm and upper and lower SCH layers. The buried InP layers 57 and 58, the contact layers 59a and 59b, and the electrodes 60a and 60b are the same as those in FIG.

  In FIG. 10, the active layer and the SCH layer included in the core layer 54 are not distinguished from each other, and are simply shown as the core layer 54 including the active layer and the SCH layer. The active layer in the core layer 54 may be a bulk layer or a quantum well active layer. The examination of the effect of the active layer width described below is based on the thickness (height) of the entire core layer 54 including the active layer and the SCH layer. At this time, in the case of the thickness (height) of the specific core layer 54, the optimum active layer (core layer) width was examined for each different thickness of the upper InP cladding layer 53.

FIG. 11 shows the optical confinement and effective refractive index of the p-cladding layer and the refractive index of the substrate when the thickness of the upper clad InP layer is 50, 150, and 250 nm for an element having a core layer height of 250 nm. It is a figure which shows the active layer width dependence of a ratio. The upper graph of FIG. 11 shows the active layer width dependence of the optical confinement of the p-clad layer, and the lower graph shows the ratio n _eq / n _InP (percentage display) of the effective refractive index n _eq and the refractive index n _InP of the substrate. ) Showed an active layer width dependency. The effective active layer width of the present invention is indicated by arrows a, b and c. When the thickness of the upper InP layer 53 is 150 nm (H_InP 150), if the width of the active layer (core layer) is narrower than about 0.7 μm, optical confinement starts to decrease, and absorption loss can be suppressed. The case where the effective refractive index n _eq is the same as the refractive index n _{InP of the} substrate is the minimum width in which the propagation mode exists, and in this case, the active layer width of 0.6 μm is the minimum width. Therefore, it can be seen that the effect of the structure of the narrow active layer according to the present invention is effective in the range of the width of 0.6 μm to 0.7 μm (arrow a).

  Similarly, when the thickness of the upper InP layer is 100 nm (H_InP 100), the range from 0.75 μm to 0.82 μm (arrow b) is effective. Further, when the thickness of the upper InP layer is 50 nm (H_InP 50), it is effective in the range of 1 μm to 1.1 μm (arrow c) from the viewpoint of loss, but the minimum width considering the uniformity of the current distribution 1 μm is preferable. The ratio between the effective refractive index and the refractive index of the substrate is in the range of approximately 100% to 100.1% regardless of the thickness of the upper InP layer 53.

  FIG. 12 shows the optical confinement and effective refractive index of the p-clad layer and the refractive index of the substrate when the thickness of the upper clad InP layer is 50, 150, and 250 nm for an element having a core layer height of 350 nm. It is a figure which shows the active layer width dependence of a ratio. When the thickness of the upper InP layer 53 is 150 nm (H_InP 150), it can be seen that the effect of the structure of the narrow active layer of the present invention is effective at the waveguide width indicated by the arrow a. Similarly, when the thickness of the upper InP layer is 100 nm (H_InP 100), the arrow b indicates the case where the thickness of the upper InP layer is 50 nm (H_InP 50). In the case of an element having a core layer height of 350 nm, it is effective when the ratio of the effective refractive index to the refractive index of the substrate is in the range of approximately 100% to 100.3%.

  FIG. 13 shows the optical confinement and effective refractive index of the p-clad layer and the refractive index of the substrate when the thickness of the upper clad InP layer is 50, 150, and 250 nm for an element having a core layer height of 450 nm. It is a figure which shows the active layer width dependence of a ratio. In the case of an element having a core layer height of 450 nm, it is effective when the ratio of the effective refractive index to the refractive index of the substrate is in the range of approximately 100% to 100.3%. Therefore, in the present invention, the equivalent refractive index of the core layer is preferably in the range of 100% to 100.3% of the refractive index of the substrate.

In each of the above-described embodiments, the example of the distributed feedback semiconductor laser that realizes the oscillation operation by the diffraction grating formed above the active layer has been described. However, other forms such as a Fabry-Perot laser and a distributed Bragg reflection laser are described. The present invention can also be applied to other oscillators. Regarding the shape of the diffraction grating, the upper InP was etched to obtain an InP and air layer, but a protective film such as SiN or SiO ₂ was formed on the upper part to form a diffraction grating made of InP and a protective film. It may be formed. The same effect can be obtained by forming a diffraction grating of SiN or SiO ₂ on InP.

  In the above-described embodiment, the present invention is applied to a semiconductor laser, but since an effect of reducing optical loss in the waveguide structure can be obtained, even if an electroabsorption modulator is realized by applying a reverse bias to the active layer, It is clear that the same characteristics can be improved in the electro-absorption characteristics.

  As described above in detail, the semiconductor optical device of the present invention can provide a lateral injection laser capable of realizing a higher light output level.

  The present invention is generally applicable to communication systems. In particular, it can be used for an optical transmitter of an optical communication system.

1, 51, 100 Waveguide structure 2, 52, 102 Substrate 3, 103 Lower SCH layer 4, 104 Active layer 5, 105 Upper SCH layer 6, 53, 106 InP layer 7, 8, 57, 58, 107, 108 Embedding InP layer
9a, 9b, 59a, 59b, 109a, 109b Contact layer 10a, 10b, 60a, 60b, 110a, 110b Electrode 11 Diffraction grating 31, 32 Electric field distribution 54 Core layer

Claims (6)

A waveguide structure comprising a semiconductor substrate, an active layer formed on the semiconductor substrate, an upper cladding layer, and buried layers on both sides of the active layer is formed,
One of the buried layers is doped with p-type impurities, the other of the buried layers is doped with n-type impurities, and the active layer is laterally disposed between the one buried layer and the other buried layer. In a semiconductor optical device having a reflector that enables laser oscillation by current injection, in which a structure for current injection in the direction is formed,
The lateral width of the active layer is a value within a range in which the oscillation mode becomes a single waveguide mode and the waveguide mode can exist when the semiconductor element oscillates, and the waveguide structure The value of the equivalent refractive index of the waveguide is substantially the same as the refractive index of the substrate.   2. The semiconductor optical device according to claim 1, wherein a thickness of the upper clad layer is thinner than 1 [mu] m.   The semiconductor optical device according to claim 1, wherein the reflector is a surface diffraction grating formed above the active layer.   4. The semiconductor optical device according to claim 1, wherein the equivalent refractive index is in the range of 100% to 100.3% of the refractive index of the substrate.   5. The semiconductor substrate according to claim 1, wherein the semiconductor substrate is a semi-insulating InP substrate, the upper cladding layer and the buried layer are made of InP, and the active layer is made of InGaAsP or InGaAlAs. A semiconductor optical device according to claim 1.   6. A distributed feedback semiconductor laser having a waveguide structure in the semiconductor optical device according to claim 1.

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