Semiconductor laser

Abstract

To provide a semiconductor laser having excellent characteristics.SOLUTION: A semiconductor laser comprises: a first conductivity type semiconductor layer; an active layer formed on the first conductivity type semiconductor layer; a second conductivity type semiconductor layer formed on the active layer; a first electrode electrically connected to the first conductivity type semiconductor layer; a second electrode electrically connected to the second conductivity type semiconductor layer; a diffraction grating layer; an insulating film disposed at a portion between the second electrode and the second conductivity type semiconductor layer; and a mesa structure. The semiconductor laser includes a first region and a second region in a direction in which the mesa structure extends. The diffraction grating layer in the first region includes a first diffraction grating region. The diffraction grating layer in the second region includes a second diffraction grating region. The first diffraction grating region and the second diffraction grating region constitute a resonator. A normalized coupling coefficient in the first region is greater than a normalized coupling coefficient in the second region. A current per unit area supplied from the second electrode to the mesa structure is smaller in the first region than that in the second region.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a semiconductor laser.

Background Art

[0002] Semiconductor lasers are widely used as light sources for optical communication. One type of semiconductor laser is a distributed feedback semiconductor laser (DFB laser). The DFB laser has a diffraction grating. In order to improve the characteristics, a structure having a phase shift portion in the diffraction grating is known. By forming antireflection films (low reflection films) on both end faces of the semiconductor laser and arranging a λ / 4 shift portion in the diffraction grating, stable single wavelength operation can be obtained. Patent Documents 1, 2, and 3 disclose that by arranging two regions having different reflectivities for the light of Bragg reflection in the resonator direction, the output from one end face can be increased.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] Patent Documents 1, 2, and 3 each disclose a structure in which regions with high and low reflectivities with respect to the Bragg wavelength are combined. Also, the electrodes into which the driving current is injected are arranged across two regions with different reflectivities. Therefore, it is presumed that the injection current densities in the two regions with different reflectivities are substantially the same. The region with high reflectivity has a lower photon density compared to the region with low reflectivity, so the carrier density increases and the refractive index decreases. Also, in the region with high reflectivity, the gain increases. The decrease in the refractive index causes deterioration of the side mode suppression ratio (SMSR) and a decrease in yield. The increase in gain reduces the light output extraction efficiency and the relative noise intensity characteristics.

[0005] An object of the present invention is to provide a semiconductor laser with excellent characteristics.

Means for Solving the Problems

[0006] The semiconductor laser includes a first conductivity type semiconductor layer, an active layer formed on the first conductivity type semiconductor layer, a second conductivity type semiconductor layer formed on the active layer, a first electrode electrically connected to the first conductivity type semiconductor layer, a second electrode electrically connected to the second conductivity type semiconductor layer, a diffraction grating layer disposed on the first conductivity type semiconductor layer side or the second conductivity type semiconductor layer side as viewed from the active layer, an insulating film disposed in a part between the second electrode and the second conductivity type semiconductor layer, and a mesa structure including at least one of the active layer or the second conductivity type semiconductor layer. The mesa structure has a first region and a second region in the extending direction. The diffraction grating layer of the first region includes a first diffraction grating region, and the diffraction grating layer of the second region includes a second diffraction grating region. The first diffraction grating region and the second diffraction grating region constitute a resonator. The normalized coupling coefficient of the first region is larger than that of the second region, and the current per unit area supplied from the second electrode to the mesa structure is smaller in the first region than in the second region.

Brief Description of the Drawings

[0007]

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Embodiments for Carrying Out the Invention

[0008] Hereinafter, embodiments of the present invention will be specifically and detailedly described with reference to the drawings. Members denoted by the same reference numerals throughout the drawings have the same or equivalent functions, and repeated descriptions thereof will be omitted. Note that the sizes of the figures do not necessarily match the magnification.

[0009] [First Embodiment] FIG. 1 is a top view of a semiconductor laser 1 according to the first embodiment. FIG. 2 represents a schematic cross-sectional view taken along line II-II of FIG. 1. FIG. 3 represents a schematic cross-sectional view taken along line III-III of FIG. 1. FIG. 4 is a top view of the semiconductor laser 1 and is a diagram for explaining the positions of the respective regions included therein. The semiconductor laser 1 includes a first electrode 2 on the back surface and a second electrode 3 on the front surface. The first electrode 2 and the second electrode 3 are metal layers. By injecting a current between the first electrode 2 and the second electrode 3, light is emitted from the front end face 40 (the left end face in FIGS. 1 and 2). Here, the first electrode 2 is an electrode electrically connected to a first conductivity type semiconductor layer described later. The second electrode 3 is an electrode electrically connected to a second conductivity type semiconductor layer described later. Low-reflection end face coating films 4 are formed on both end faces in the extending direction of the mesa structure (hereinafter, the first direction D1), that is, the front end face 40 (the left end face in FIGS. 1 and 2) and the rear end face 50 (the right end face in FIGS. 1 and 2). The reflectivity of the low-reflection end face coating film 4 is 1% or less.

[0010] The semiconductor laser 1 has a semiconductor multilayer structure laminated in the order of a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. In the first embodiment, semiconductor layers are laminated in the order of a core layer 7, a second conductivity type clad layer 9, and a second conductivity type contact layer 13 on a first conductivity type substrate 5. Here, the first conductivity type semiconductor layer corresponds to the substrate 5. Note that another first conductivity type semiconductor layer may be disposed on the substrate 5. The core layer 7 is a semiconductor layer including at least the active layer. For example, the core layer 7 is a semiconductor layer laminated in the order of a first conductivity type optical confinement layer, an active layer composed of an i-type multiple quantum well layer, and a second conductivity type optical confinement layer in this order from the substrate 5 side toward the second conductivity type clad layer 9. Note that the core layer 7 may include layers other than these semiconductor layers. Also, the active layer may be an n-type semiconductor layer. In the first embodiment, the clad layer 9 and the contact layer 13 constitute the second conductivity type semiconductor layer. A diffraction grating layer 11 is formed in the second conductivity type clad layer 9. The semiconductor laser 1 is a DFB laser. Here, the first conductivity type is n-type and the second conductivity type is p-type, but the reverse may also be possible. As shown in FIG. 3, the semiconductor multilayer including the above-described layers has a mesa structure 15. Here, a part of the substrate 5, the core layer 7, the clad layer 9, and the contact layer 13 are included in the mesa structure 15. The mesa structure 15 extends in the light extraction direction. Both sides of the mesa structure 15 are covered with a semi-insulating semiconductor embedding layer 17. Note that the embedding layer 17 may be a laminate of p-type and n-type semiconductor layers. The dotted line in FIG. 1 indicates the position of the boundary between the mesa structure 15 and the embedding layer 17. In a direction (hereinafter referred to as the second direction D2) perpendicular to the first direction D1 in plan view, the width of the mesa structure 15 is constant.

[0011] The semiconductor laser 1 includes an insulating film 14. The insulating film 14 covers the surface of the semiconductor laser 1 except for a part. The insulating film 14 is, for example, silicon oxide, silicon nitride, or resin. The insulating film 14 includes one or a plurality of opening regions where the second electrode 3 and the second conductivity type semiconductor layer are in contact with each other in a part above the mesa structure 15. The second electrode 3 and the second conductivity type contact layer 13 are physically and electrically connected through the opening region, and an electrical signal (current is injected) is applied to the active layer through the second conductivity type semiconductor layer. Since the opening region is a hole for electrically connecting the second electrode 3 and the second conductivity type contact layer 13, hereinafter, the opening region is also referred to as a through hole 18. In the first embodiment, the through holes 18 are arranged side by side in the first direction D1. Also, the width of the through hole 18 (the length in the second direction D2) is wider than the width of the mesa structure 15. However, even if they have the same width, or even if the width of the through hole 18 is narrower than the mesa width, it does not matter. Details of the through hole 18 will be described later.

[0012] The diffraction grating layer 11 includes a plurality of first refractive index regions 11A and a plurality of second refractive index regions 11B. Specifically, the diffraction grating layer 11 is of a floating type and, in a cross-sectional view, is composed of a region having a first refractive index different from that of the second conductivity type clad layer 9 and a region having a second refractive index where the second conductivity type clad layer 9 is disposed. That is, the diffraction grating layer 11 has a structure in which the first refractive index regions 11A and the second refractive index regions 11B of the same length are alternately arranged. Here, the first refractive index is higher than the second refractive index. However, the relationship between the refractive indices may be reversed.

[0013] Here, in the diffraction grating layer 11, a region in which the first refractive index regions 11A and the second refractive index regions 11B are alternately arranged with the same length is referred to as a diffraction grating region 12A. On the other hand, in the diffraction grating layer 11, a region in which only the first refractive index regions 11A or only the second refractive index regions 11B are arranged is referred to as a non-diffraction grating region 12B. In the first embodiment, the non-diffraction grating region 12B is composed of the second refractive index regions 11B. Note that the diffraction grating region 12A of the first embodiment has a uniform diffraction grating structure in which the first refractive index regions 11A and the second refractive index regions 11B are alternately arranged with the same length in the first direction D1. The Bragg wavelength of this diffraction grating structure corresponds to the 1.3 μm band. Note that other Bragg wavelengths such as the 1.55 μm band may also be used.

[0014] As shown in FIG. 2, the semiconductor laser 1 is composed of two regions. On the rear end face 50 side, a first region 10 including one diffraction grating region 12A is arranged. On the front end face 40 side, a second region 20 in which a plurality of diffraction grating regions 12A and a plurality of non-diffraction grating regions 12B are alternately arranged is arranged. Here, the diffraction grating region 12A and the non-diffraction grating region 12B are arranged so as to reflect the light of the Bragg wavelength transmitted from the first region 10 in the second region 20. In order to strongly reflect the light of a specific wavelength (Bragg wavelength), it is necessary to adjust the lengths (diffraction grating periods) of the first refractive index region 11A and the second refractive index region 11B. Also, the relationship between the diffraction grating period and the Bragg wavelength depends on the effective refractive index. In the first embodiment, the first region 10 has a larger effective refractive index than the second region 20. This is because, when looking at the entire second region 20, there are many regions without a diffraction grating structure (regions of the non-diffraction grating region 12B), and the effective refractive index is reduced accordingly. Considering the difference in the average effective refractive index of each of the first region 10 and the second region 20, the diffraction grating region 12A and the non-diffraction grating region 12B arranged in the second region 20 are determined. Here, the diffraction grating period of the diffraction grating region 12A arranged in the second region 20 may be different from or the same as the diffraction grating period of the diffraction grating region 12A arranged in the first region 10. In the case of the same diffraction grating period, the lengths and the intervals in the first direction D1 of each of the plurality of diffraction grating regions 12A and the plurality of non-diffraction grating regions 12B are adjusted so that the average diffraction grating period of the entire second region 20 reflects the light of the desired Bragg wavelength. Hereinafter, when particularly distinguishing, the diffraction grating region 12A included in the first region 10 is referred to as the first diffraction grating region, and the diffraction grating region 12A included in the second region 20 is referred to as the second diffraction grating region. Note that the lengths in the first direction D1 of each of the diffraction grating region 12A and the non-diffraction grating region 12B in the second region 20 may be different. For example, under the condition of reflecting at the desired Bragg wavelength, it is preferable that the lengths are different so as to suppress the higher-order diffraction backscattering with respect to the Bragg wavelength.

[0015] In order to obtain high wavelength monochromaticity, it is desirable to form a resonator in the first region 10 and the second region 20. In order to form a resonator in the first region 10 and the second region 20, the phases of the diffraction grating regions of each other need to be π-shifted. Here, the phase of the diffraction grating region is the phase of the diffraction grating structure. For example, when the effective refractive indices of the first region 10 and the second region 20 are the same, simply with respect to the phase of the first diffraction grating region (diffraction grating region 12A of the first region 10), the phase of the second diffraction grating region (diffraction grating region 12A of the second region 20) may be arranged with a π-shift. However, in the first embodiment, the effective refractive indices of the first region 10 and the second region 20 are different. Therefore, the phase of the second diffraction grating region needs to be different from the phase obtained by π-shifting the phase of the first diffraction grating region. In the first embodiment, although the phases of the first diffraction grating region and the second diffraction grating region are not π-shifted, a resonator is formed in the first region 10 and the second region 20. Furthermore, in the first embodiment, since the low-reflection end-face coating films 4 are formed on both end faces, it has very high wavelength monochromaticity.

[0016] In the first embodiment, the normalized coupling coefficient κ1L1 of the first region 10 is larger than the normalized coupling coefficient κ2L2 of the second region 20. The coupling coefficient κ is determined by a semiconductor multilayer or a diffraction grating structure. Here, the coupling coefficient κ1 of the first region 10 and the coupling coefficient κ2 of the second region 20 are not the same due to, for example, a difference in the effective refractive index. However, the difference is small, and the factor of the difference in the normalized coupling coefficients between the first region 10 and the second region 20 is dominated by the length in the first direction D1 of the diffraction grating region 12A included in each. Here, the length L1 of the first region 10 is the length in the first direction D1 of the first diffraction grating region. The length L2 of the second region 20 is the total length of the plurality of second diffraction grating regions. That is, L2 is the total length in the first direction D1 of the plurality of diffraction grating regions 12A included in the second region 20. Since the non-diffraction grating region 12B does not reflect light at the Bragg wavelength, the coupling coefficient κ of the non-diffraction grating region 12B can be regarded as being substantially 0. Therefore, the length L2 contributing to the normalized coupling coefficient κL of the second region 20 is the total length of the region where the plurality of second diffraction grating regions are arranged. Since κ1L1 of the first region 10 is larger than κ2L2 of the second region 20, the optical output intensity output from the front end face 40 is larger than the optical output intensity output from the rear end face 50. When the normalized coupling coefficients of the first region 10 and the second region 20 are the same value, the optical output intensities output from the front end face 40 and the rear end face 50 are the same. In the present disclosure, by making the total length of the region where the plurality of second diffraction grating regions are arranged shorter than the length of the region where the first diffraction grating region is arranged, the intensity of the light output from the front end face 40 is increased. Here, the front and rear are merely for convenience of naming, and the end face with the larger optical output is simply called the front end face. In general optical communication, a larger light intensity is preferable, and the light from the front end face is used for communication. Also, the total length in the first direction D1 of the plurality of second diffraction grating regions is shorter than the total length of the plurality of non-diffraction gratings 12B. With this structure, the effect of increasing the optical output from the front by reducing the coupling coefficient κ2 of the second region 20 can be enhanced. Here, it is preferable that κ1L1 is 60% or more (that is, κ2L2 is 40% or less) with respect to the normalized coupling coefficient of the entire semiconductor laser 1.

[0017] Here, the resonator length of the semiconductor laser 1 is the total length in the first direction D1 of the first region 10 and the second region 20. In the first embodiment, the resonator length corresponds approximately to the length between the front end face 40 and the rear end face 50. The first embodiment also aims to make the optical output intensity output from the front end face 40 larger than the optical output intensity output from the rear end face 50. For this purpose, it is preferable that the first region 10 (more precisely, the first diffraction grating region) is on the rear side. For example, the first diffraction grating region is preferably arranged in a region from the rear end of the resonator (here, the rear end face 50) to 40% or less of the resonator length. That is, it is preferable that the length in the direction in which the mesa structure of the first diffraction grating region extends is 40% or less of the total length of the diffraction grating layer. More preferably, it is 30% or less. However, when κ1L1 becomes smaller than 1, the oscillation threshold increases, which is not preferable from the viewpoint of power consumption. Therefore, it is preferable to set κ1L1 so that κ1L1 is 1 or more, more preferably 1.5 or more.

[0018] FIG. 4 is a top view of the semiconductor laser 1 and is a diagram for explaining the position of the opening region (through hole 18). The second electrode 3 is not shown for the sake of explanation. Also, it is shown as a view through the diffraction grating layer 11. The position of the through hole 18 is indicated by a two-dot chain line.

[0019] The through-hole 18 is an opening provided in the insulating film 14. In the first region 10, the through-hole 18 is disposed only on a part of the upper surface of the mesa structure 15. In other words, in the first region 10, the through-holes 18 are discretely arranged. Also, in the first region 10, a high-resistance body 30 is disposed in a region other than the opening region in the region directly above the mesa structure 15 (hereinafter also referred to as a non-opening region). That is, in the first region 10, the high-resistance body 30 is disposed on a part of the upper surface of the mesa structure 15. Note that high resistance means that the electrical resistance is higher compared to the contact layer 13. In the first embodiment, the high-resistance body 30 is a part of the insulating film 14, but the high-resistance body 30 and the region of the insulating film 14 other than the high-resistance body 30 may be made of different materials. Examples of other materials will be described later. The high-resistance body 30 has a higher electrical resistance than the contact layer 13 of the second conductivity type. In the opening region above the mesa structure 15 where the insulating film 14 is not disposed, the second electrode 3 and the contact layer 13 of the second conductivity type are in electrical and physical contact. The through-hole 18 is square in plan view, but is not limited thereto. For example, it may be a square with rounded corners.

[0020] [Effect] As described above, the normalized coupling coefficient κ1L1 of the first region 10 is larger than the normalized coupling coefficient κ2L2 of the second region 20. Therefore, the photon density in the first region 10 is smaller than the photon density in the second region 20. Since the consumption of carriers by stimulated emission increases as the photon density increases, when the injection current densities of the first region 10 and the second region 20 are equal, the carrier density in the first region 10 becomes larger than the carrier density in the second region 20. In the first embodiment, since the second electrode 3 is disposed across both the first region 10 and the second region 20, the injection current densities of both are equal. As the carrier density increases, the refractive index of the first region 10 decreases and the gain increases. The decrease in the refractive index causes deterioration of the SMSR characteristics, and the increase in the gain causes a decrease in the light output efficiency from the front end face 40 and a decrease in the relative noise intensity characteristics. Therefore, in the present disclosure, the current per unit area supplied from the second electrode 3 to the mesa structure 15 is smaller in the first region 10 than in the second region 20. In the first embodiment, in the first region 10, by restricting the region where current is injected, the current per unit area becomes smaller in the first region 10 than in the second region 20. By reducing the injection current density in the first region 10 compared to the second region 20, an increase in the carrier density in the first region 10 is suppressed. In order to restrict the region where current is injected, the through holes 18 in the first region 10 are arranged only in a part, not in the entire region directly above the mesa structure 15. A high resistor 30 is arranged in the region where the through holes 18 are not arranged. In other words, the upper surface of the mesa structure 15 in the first region 10 has an opening region (through hole 18) and a non-opening region (high resistor 30) alternately arranged along the first direction D1. Since no current is injected into the non-opening region, the carrier density of the entire first region 10 can be reduced.

[0021] In the first region 10, the ratio of the area occupied by the opening region (through-hole 18) directly above the mesa structure 15 is preferably 60% or less. Hereinafter, the ratio of the area occupied by the opening region directly above the mesa structure 15 is referred to as the opening ratio. More preferably, the opening ratio is desirably 20% or more and 50%. Here, the opening ratio can be equivalently read as the ratio of the total area of the upper surface of the mesa structure 15 in the first region 10 to the area of the region (current-carrying region) where the second electrode 3 is in contact with the second conductivity type semiconductor layer (contact layer 13). In FIG. 2, the second region 20 includes a region where the high-resistance body 30 is disposed near the connection portion with the first region 10. However, this region is sufficiently small compared to the opening region of the entire second region 20 and has little influence on the carrier density. In the first embodiment, by making the opening ratio of the first region 10 smaller than the opening ratio of the second region 20, the influence of the difference in carrier density due to the difference in the normalized coupling coefficient κL can be reduced.

[0022] Note that in the first embodiment, the plurality of through-holes 18 and the plurality of high-resistance bodies 30 provided in the first region 10 are not limited to having the same length in the first direction D1. They may have different lengths. However, it is desirable that the plurality of through-holes 18 be discretely arranged so that the injection current in the first region 10 is uniform. Preferably, the minimum length in the first direction D1 between the plurality of through-holes 18 is desirably 1 / 5 or less of the length of the first region 10.

[0023] Also, in the first embodiment, the first electrode 2 and the second electrode 3 are integrally formed across both the first region 10 and the second region 20, but are not limited thereto. Electrodes may be provided individually as long as current is injected so that the same injection current density is obtained for the electrodes in the first region 10 and the second region 20.

[0024] The semiconductor laser 1 according to the first embodiment includes a first region 10 having a large normalized coupling coefficient κ1L1 and a second region 20 having a smaller normalized coupling coefficient κ2L2 than that of the first region 10, which form a resonator, achieving both high wavelength single-mode property and high output power characteristics from the front end face 40. Further, by making the aperture ratio of the through-hole 18 in the first region 10 smaller than that of the through-hole 18 in the second region, a semiconductor laser excellent in SMSR characteristics and relative noise intensity characteristics is realized.

[0025] [Modification Example 1] FIG. 5 is a top view of the semiconductor laser 1 according to Modification Example 1 of the first embodiment, and is a diagram for explaining the form of the through-hole 18. The difference from the first embodiment is only the shape of the through-hole 18 and the high-resistance body 30. In this Modification Example 1, the through-hole 18 has a parallelogram shape in plan view. In other words, the sides forming the through-hole 18 and the high-resistance body 30 are arranged inclined with respect to the first direction D1 in which light resonates. Here, all the inclination angles are constant, but it is not limited thereto and different inclination angles may be used.

[0026] [Modification Example 2] FIG. 6 is a top view of the semiconductor laser 1 according to Modification Example 2 of the first embodiment, and is a diagram for explaining the form of the through-hole 18. The difference from the first embodiment is only the shape of the through-hole 18 and the high-resistance body 30. Each through-hole 18 has substantially the same width in the second direction D2 of the mesa structure 15. The through-hole 18 in the first region 10 is hexagonal. Thus, the shape of the through-hole 18 is not limited to a quadrilateral or a hexagon, and may be a polygon, or a circle or an ellipse. Also, combinations thereof may be used. As long as the aperture ratio of the first region 10 is smaller than that of the second region 20, the effects of the present disclosure can be obtained.

[0027] [Second Embodiment] FIG. 7 shows a schematic cross-sectional view of the semiconductor laser 201 according to the second embodiment along the first direction D1, corresponding to the schematic cross-sectional view taken along the line II-II in FIG. 1. FIG. 8 is a top view of the semiconductor laser 201, and is a diagram for explaining the form of the opening region (through-hole 218).

[0028] The semiconductor multilayer structure of the semiconductor laser 201 is almost the same as that of the semiconductor laser 1 in the first embodiment, but the number of diffraction grating layers and the diffraction grating structure are different. In the second embodiment, the first region 210 includes two layers, namely, the first diffraction grating layer 211A and the second diffraction grating layer 211B, from the side closer to the core layer 7. A cladding layer 9 is disposed between the first diffraction grating layer 211A and the second diffraction grating layer 211B. On the other hand, the second region 220 includes only the first diffraction grating layer 211A. Further, a λ / 4 phase shift portion 219 is disposed between the first region 210 and the second region 220. Note that the λ / 4 phase shift portion 219 may also be disposed in the second diffraction grating layer 211B. The first region 210 has a uniform diffraction grating structure in which the lengths of the first refractive index region 11A and the second refractive index region 11B in the first direction D1 are the same and are alternately arranged, except for the phase shift portion 219. In the first region 210, the lengths and phases of the first refractive index region 11A and the second refractive index region 11B included in the first diffraction grating layer 211A and the second diffraction grating layer 211B are the same. The second region 220 has a uniform diffraction grating structure in which the lengths of the first refractive index region 11A and the second refractive index region 11B in the first direction D1 are the same and are alternately arranged, except for the phase shift portion 219. The first region 210 has a first diffraction grating region composed of two diffraction grating layers (the first diffraction grating layer 211A and the second diffraction grating layer 211B). The second region 220 has a second diffraction grating region composed of one diffraction grating layer (the first diffraction grating layer 211A). The first diffraction grating region and the second diffraction grating region constitute a resonator with the phase shift portion 219 interposed therebetween.

[0029] Since the first region 210 includes the second diffraction grating layer 211B, the coupling coefficient κ is larger than that of the second region 220. Also, in the second embodiment, in order to make the light intensity output from the front end face 40 larger than the light intensity output from the rear end face 50, the normalized coupling coefficient κ1L1 of the first region 210 is larger than the normalized coupling coefficient κ2L2 of the second region 220. Here, L1 is the length of the entire first region 210 in the first direction D1. L2 is the length of the entire second region 220. Also, L1 is less than or equal to half of the entire length of the semiconductor laser 201. That is, in the first embodiment, the difference in the normalized coupling coefficient κL between the first region 10 and the second region 20 was mainly due to the difference in the total length of the diffraction grating region 12A, but in the second embodiment, the main factors are the difference in the number of layers of the diffraction grating layer and the difference in the length of each region. Note that if the number of layers of the diffraction grating layer in the first region 210 is more than that in the second region 220, the number of layers of the diffraction grating layer is not limited to two layers. For example, the number of layers of the diffraction grating layer in the first region 210 may be three layers and the number of layers of the diffraction grating layer in the second region 220 may be two layers.

[0030] Furthermore, the second embodiment is different from the first embodiment in the structure of the high-resistance body 230. In the first embodiment, the high-resistance body 230 disposed on the upper surface of the mesa structure 15 in the first region 210 is continuous in the second direction D2. On the other hand, in the second embodiment, the opening region has a wide portion and a narrow portion in a direction (second direction D2) perpendicular to the direction (first direction D1) in which the mesa structure 15 extends in plan view. High-resistance bodies 230 are disposed on both sides of the narrow portion of the opening region. According to this structure, although the current injected from the second electrode 3 in the opening region circulates and current is also injected below the high-resistance body 230, the density of the current injected below the high-resistance body 230 is smaller than the density of the current injected below the opening region (through hole 218). On the other hand, in the second region 220, the high-resistance body 230 is not disposed except at the connection portion with the first region 210, and the opening region (through hole 218) is disposed in most regions. In other words, in the first region 210, in the second direction D2, regions where the entire upper surface of the mesa structure 15 is open (wide portions) and regions where a part is open (narrow portions) are alternately arranged. On the other hand, in the second region 220, the opening region (through hole 218) is disposed in the entire region directly above the mesa structure 15. Therefore, the aperture ratio of the through hole 218 in the first region 210 is smaller than that in the second region 220. As a result, the injection current density in the first region 210 becomes smaller than that in the second region 220, and the same effect as in the first embodiment is obtained.

[0031] Needless to say, the same effect can be obtained by applying the structures of the through hole 18 and the high-resistance body 30 shown in the first embodiment and its modified examples to the semiconductor laser 201. Similarly, the structure of the high-resistance body 230 shown in the second embodiment may be combined with the first embodiment.

[0032] [Third Embodiment] FIG. 9 shows a schematic cross-sectional view of the semiconductor laser 301 according to the third embodiment in the direction along the first direction D1, corresponding to the schematic cross-sectional view taken along line II-II in FIG. 1. FIG. 10 is a top view of the semiconductor laser 301 and is a diagram for explaining the form of the opening region (through hole 318).

[0033] The semiconductor multilayer structure of the semiconductor laser 301 is substantially the same as that of the semiconductor laser 1 of the first embodiment, but the diffraction grating structure is different. In the third embodiment, a diffraction grating structure is formed by forming irregularities on the surface of the diffraction grating layer 311 disposed on the core layer 7. The clad layer 9 is disposed in the concave region. Here, the convex region corresponds to the first refractive index region 11A, and the concave region corresponds to the second refractive index region 11B. The height of each convex (depth of each concave) of the diffraction grating structure in the first region 310 is larger than that of the diffraction grating structure in the second region 320. Here, the height / depth is defined in the stacking direction of the semiconductor layers. Also, a λ / 4 phase shift portion 319 is disposed between the first region 310 and the second region 320. The first region 310 has a uniform diffraction grating structure in which the first refractive index region 11A and the second refractive index region 11B having the same length in the first direction are alternately arranged. The second region 320 has a uniform diffraction grating structure in which the first refractive index region 11A and the second refractive index region 11B having the same length in the first direction are alternately arranged. The first region 310 has a first diffraction grating region with a deep diffraction grating structure. The second region 320 has a second diffraction grating region with a shallower diffraction grating structure than the first region 210. The first diffraction grating region and the second diffraction grating region constitute a resonator with the phase shift portion 319 interposed therebetween.

[0034] Since the first region 310 has a deep diffraction grating structure, the coupling coefficient κ is larger than that of the second region 320. Also, in the third embodiment, in order to make the light intensity output from the front end face 40 larger than the light intensity output from the rear end face 50, the normalized coupling coefficient κ1L1 of the first region 310 is larger than the normalized coupling coefficient κ2L2 of the second region 320. Here, L1 is the length of the entire first region 310 in the first direction D1. Also, L2 is the length of the entire second region 320. Also, L1 is less than or equal to half of the total length of the semiconductor laser 301. That is, in the first embodiment, the difference in the normalized coupling coefficient κL between the first region 10 and the second region 20 was mainly due to the difference in the total length of the diffraction grating region 12A, but in the third embodiment, the main factors are the difference in the depth of the diffraction grating structure and the difference in the length of each region.

[0035] Furthermore, in the third embodiment, the material of the high-resistance body 330 is different from that of the first embodiment. In the first embodiment, the high-resistance body 30 was the insulating film 14, but in the third embodiment, it is formed of an insulating material different from the insulating film 314. For example, if the insulating film 14 is silicon oxide, the high-resistance body 330 is silicon nitride or resin. Note that the material of the high-resistance body 30 is not limited to this, and a high-resistance body formed of another material may be disposed as long as its resistivity is at least greater than that of the contact layer 13.

[0036] The insulating film 314 is disposed except for a part of the upper surface of the mesa structure 15. In the second region 320, the second electrode 3 is in contact with the contact layer 13 in the entire region directly above the mesa structure 15. That is, the aperture ratio of the second region 320 is 100%. On the other hand, in the first region 310, the high-resistance body 330 is discretely disposed between the second electrode 3 and the contact layer 13. In other words, the aperture regions (through holes 318) where the second electrode 3 and the contact layer 13 are in contact are discretely disposed. Therefore, the aperture ratio of the first region 310 is smaller than that of the second region 320. The aperture ratio of the first region 310 is preferably, for example, 20% or more and 50% or less as shown in the first embodiment. Also in the third embodiment, the injection current density in the first region 310 is smaller than the injection current density in the second region 320, and the same effect as in the first embodiment can be obtained.

[0037] [Fourth Embodiment] FIG. 11 is a top view of a semiconductor laser 401 according to the fourth embodiment. FIG. 12 shows a schematic cross-sectional view taken along line XII-XII of FIG. 11. FIG. 13 is a top view of the semiconductor laser 401 for explaining the positions of the respective regions included therein. The main differences from the semiconductor laser 1 according to the first embodiment are the difference in the aperture region (through hole 418) and the shape of the high-resistance body 430 in the first region 410, and the fact that window structures 460 are disposed between the first region 410 and the rear end face 50 and between the second region 420 and the front end face 40. The other semiconductor multilayer structures are the same as those in the first embodiment.

[0038] The window structure 460 is composed of a semiconductor material having a refractive index smaller than that of the active layer included in the core layer 7. For example, the window structure 460 is made of the same material as the embedded layer 17. The window structure 460 has the effect of reducing the light returning to the core layer 7 and further improving the SMSR characteristics, such as the yield. Also, the window structure 460 is not included in the mesa structure 415. In the first direction D1, the end face of the mesa structure 415 is in contact with the window structure 460. The window structure 460 is arranged from one end to the other end of the semiconductor laser 401 in the second direction D2.

[0039] In the first region 410, the opening region (through hole 418) is substantially square in plan view. In the second direction D1, the through hole 418 is provided so that the center of the through hole 418 is displaced from the center of the mesa structure 415. That is, only a part of the through hole 418 overlaps with the mesa structure 415. A high resistor 430 (here, the insulating film 414) is arranged in a region where the through hole 418 and the mesa structure 415 do not overlap. Thus, the through hole 418 and the high resistor 430 do not have to be arranged alternately in a discrete manner. In the second region 420, except in the vicinity of the connection portion with the window structure 460, no resistor 430 is arranged directly above the mesa structure 415. Therefore, the aperture ratio of the second region 420 is larger than that of the first region 410.

[0040] The insulating film 414 is arranged on the upper part of the window structure 460. The insulating film 414 is also arranged in a part of the first region 310 and a part of the second region 420 and functions as the high resistor 430. Note that in the first region 410, the through holes 418 may be arranged discretely as in the first embodiment. The second electrode 3 is also arranged on a part of the upper surface of the window structure 460. Also in the fourth embodiment, the above-described effects can be obtained.

[0041] [Fifth Embodiment] FIG. 14 is a top view of a semiconductor laser 501 according to a fifth embodiment. FIG. 15 shows a schematic cross-sectional view taken along line XV-XV of FIG. 14. FIG. 16 shows a schematic cross-sectional view taken along line XVI-XVI of FIG. 14. FIG. 17 shows a schematic cross-sectional view taken along line XVII-XVII of FIG. 14. FIG. 18 is a top view of the semiconductor laser 501 and is a diagram for explaining the positions of the respective regions included therein. The semiconductor laser 501 has a PBH (Planar Buried Hetero-Structure) structure in which a second-conductivity-type clad layer 509 and a second-conductivity-type contact layer 513 are widely disposed on a mesa structure 515.

[0042] The semiconductor laser 501 has a semiconductor multilayer structure laminated in the order of a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. In the fifth embodiment, semiconductor layers are laminated in the order of a first conductivity type buffer layer 516, a core layer 507, a second conductivity type clad layer 509, and a second conductivity type contact layer 513 on a first conductivity type substrate 505. Here, the first conductivity type semiconductor layer corresponds to the substrate 505 and the buffer layer 516. The core layer 507 is a semiconductor layer including at least an active layer. For example, the core layer 507 is a semiconductor layer laminated in the order of a first conductivity type optical confinement layer, an active layer composed of an i-type multiple quantum well layer, and a second conductivity type optical confinement layer from the substrate 5 side. Note that the core layer 507 may include layers other than these semiconductor layers. Also, the active layer may be an n-type semiconductor layer. In the fifth embodiment, the clad layer 509 and the contact layer 513 constitute the second conductivity type semiconductor layer. A diffraction grating layer 511 is formed in the buffer layer 516. The semiconductor laser 501 is a DFB laser. Here, the first conductivity type is n-type and the second conductivity type is p-type, but the reverse may also be true. Also, as shown in FIG. 16, a part of the buffer layer 516 and the core layer 507 form a mesa structure 515. The mesa structure 515 extends in the first direction D1. Both sides of the mesa structure 515 are covered with a semi-insulating semiconductor embedding layer 517. Note that the embedding layer 517 may be a laminate of p-type and n-type semiconductor layers. The clad layer 509 and the contact layer 513 are disposed on the mesa structure 515 and the embedding layer 517. The dotted line in FIG. 14 indicates the position of the boundary between the mesa structure 515 and the embedding layer 517. In the second direction D2 perpendicular to the first direction D1 in plan view, the width of the mesa structure 515 is constant.

[0043] The folded grating layer 511 has the same structure as the folded grating layer 11 of the first embodiment, except that it is disposed below the core layer 507 (on the substrate 505 side). That is, the first region 510 includes a first diffraction grating region that is a uniform diffraction grating region 12A. The second region 520 includes a plurality of diffraction grating regions 12A (second diffraction grating regions) and a plurality of non-diffraction grating regions 12B. Resonators are formed by the first diffraction grating region of the first region and the second diffraction grating region of the second region. As described above, the phase of the diffraction grating structure considering the optical path lengths of the first region and the second region is π-shifted.

[0044] FIG. 18 is a top view of the semiconductor laser 501 and is a diagram for explaining the position of the through hole 518. The second electrode 3 is not shown for the sake of explanation. Also, it is shown passing through the diffraction grating layer 511. The position of the opening region (through hole 518) is indicated by a two-dot chain line. In the second region 520, the through hole 518 is disposed above the mesa structure 515. As shown in FIG. 16, the through hole 518 overlaps the entire mesa structure 515 in the second direction D2. On the other hand, in the first region 510, the through hole 518 is not disposed directly above the mesa structure 515 but is disposed in a region shifted from the mesa structure 515 (see FIG. 17). In other words, in the first region 510, the opening region is disposed shifted in plan view from the region directly above the mesa structure 515, and the high resistor 530 is disposed so as to overlap the region directly above the mesa structure 515 in plan view. Since the PBH structure has a wide second conductivity type semiconductor layer (here, the cladding layer 509 and the contact layer 513) disposed on the mesa structure 515, even if the mesa structure 515 and the through hole 518 are shifted in the second direction D2, the current injected from the second electrode 3 flows into the mesa structure 515. However, since the distance between the through hole 518 and the upper surface of the mesa structure 515 is longer in the first region 510 than in the second region 520, the effective current injection density is smaller in the first region 510.

[0045] In the present disclosure, the ratio of the area occupied by the opening region in the region directly above the mesa structure is defined as the opening ratio. That is, the opening ratio is not defined by the area ratio where the mesa structure and the electrode are in contact. In the fifth embodiment, since the mesa structure 515 and the opening region do not overlap in plan view, the opening ratio of the first region 510 is 0%. That is, the high-resistance body 530 is disposed over the entire region directly above the mesa structure 515. On the other hand, the opening ratio of the second region 520 is approximately 100%. Thus, if the opening ratio of the first region having a large normalized coupling coefficient is made smaller than the opening ratio of the second region, the effects of the present disclosure can be obtained even if the opening region does not necessarily overlap with the mesa structure.

[0046] In the first region 510, the through hole 518 may partially overlap with the region directly above the mesa structure 515 as shown in FIG. 13.

[0047] [Sixth Embodiment] FIG. 19 is a top view of a semiconductor laser 601 according to the sixth embodiment. FIG. 20 represents a schematic cross-sectional view taken along line XX-XX of FIG. 19. FIG. 21 is a top view of the semiconductor laser 601 and is a diagram for explaining the positions of the respective regions included therein. The semiconductor laser 601 is a ridge-type semiconductor laser in which the active layer is not included in the mesa structure. The stacked structure of the semiconductor in the sixth embodiment is the same as that in the first embodiment. The difference is that the core layer 7 including the active layer is not included in the mesa structure 615. In the sixth embodiment, the mesa structure 615 includes a cladding layer 9, a diffraction grating layer 11, and a contact layer 13.

[0048] Similar to the first embodiment, the normalized coupling coefficient of the first region 610 is larger than that of the second region 620. Also, as shown in FIG. 21, the opening ratio of the first region 610 is smaller than the opening ratio of the second region 620. The above-described effects can also be obtained in the sixth embodiment.

[0049] In the above-described embodiments and modified examples, the above-described effects can be obtained even when they are combined. For example, the semiconductor laser may be any of an embedded type, a PBH type, and a ridge type. As the structure for making the normalized coupling coefficients of the first region and the second region different, any of the arrangement, number of layers, depth, etc. of the diffraction grating structure may be applied. As for the shape and position of the opening region and the high-resistance body in the first region, any of the plurality of examples described above may be applied. However, the example shown in FIG. 18 is applicable only to the PBH structure. Conversely, similar effects can be obtained even when other examples are applied to the PBH structure. The diffraction grating layer may be disposed above or below the active layer in the stacking direction of the semiconductor layers.

[0050] The present disclosure improves high output characteristics, high wavelength single-mode characteristics, SMSR characteristics, and relative noise intensity characteristics in a semiconductor laser having a mesa structure. Embodiments of the present disclosure are composed of a first region having a large normalized coupling coefficient and a second region having a normalized coupling coefficient smaller than that of the first region, and this is realized by the aperture ratio of the first region being smaller than the aperture ratio of the second region. By having a small aperture ratio, the injection current density in the first region is made smaller than the injection current density in the second region. A resonator is formed by a first diffraction grating region in the first region and a second diffraction grating region in the second region. The phase of the diffraction grating structure taking into account the optical path lengths of the first region and the second region with respect to each other is π-shifted. The first region includes a first diffraction grating region in which a first refractive index region and a second refractive index region for reflecting light having a Bragg wavelength are alternately arranged with the same length. The second region includes a second diffraction grating region in which a first refractive index region and a second refractive index region are alternately arranged with the same length, and a non-diffraction grating region that transmits light having a Bragg wavelength. The non-diffraction grating region is composed of only a first refractive index region or a second refractive index region. The second region includes a plurality of diffraction grating regions and non-diffraction grating regions. The structure for making the normalized coupling coefficient of the first region larger than the normalized coupling coefficient of the second region is not limited to the above. For example, it may be realized by making the number of layers of the diffraction grating layer in the first region larger than the number of layers of the diffraction grating layer in the second region. Also, it may be realized by making the depth of the diffraction grating structure in the first region deeper than the depth of the diffraction grating structure in the second region. The aperture ratio of the first region is preferably 60% or less, more preferably 20% or more and 50% or less. A high resistor is disposed in a non-aperture region where no through hole (aperture) is formed. In the first region, the aperture region and the high resistor may be alternately arranged in the direction in which light travels (the direction in which the mesa structure extends). The aperture region of the first region may overlap only a part directly above the mesa structure. Here, the high resistor may be an insulator such as silicon oxide, silicon nitride, and resin. Also, the high resistor does not have to completely cover the region directly above the mesa structure. By providing a portion where the width of the aperture region in the second direction is small, the aperture area may be reduced. The semiconductor laser may have low-reflection end face coating films formed on the front end face side and the rear end face side. Also, it may have window structures on both end faces. The semiconductor laser may be an embedded type, a PBH type, or a ridge type.The semiconductor laser of the present disclosure oscillates at a wavelength in the 1.3 μm band or the 1.55 μm band. However, other wavelength bands may also be acceptable. The diffraction grating layer may be disposed either above or below the active layer.

Explanation of Reference Numerals

[0051] 1 Semiconductor laser 2 First electrode 3 Second electrode 4 Low-reflection end-face coating film 5 Substrate 7 Core layer 9 Clad layer 10 First region 11 Diffraction grating layer 11A First refractive index region 11B Second refractive index region 12A Diffraction grating region 12B Non-diffraction grating region 13 Contact layer 14 Insulating film 15 Mesa structure 17 Embedded layer 18 Through hole 20 Second region 30 High resistor 40 Front end face 50 Rear end face 201 Semiconductor laser 210 First region 211A First diffraction grating layer 211B Second diffraction grating layer 214 Insulating film 218 Through hole 219 Phase shift portion 220 Second region 230 High resistor 301 Semiconductor laser 310 First region 311 Diffraction grating layer 314 Insulating film 318 Through hole 319 Phase shift portion 320 Second region 330 High resistor 401 Semiconductor laser 410 First region 411 Diffraction grating layer 414 Insulating film 415 Mesa structure 418 Through hole 420 Second region 430 High resistor 460 Window structure 501 Semiconductor laser 505 Substrate 507 Core layer 509 Clad layer 510 First region 511 Diffraction grating layer 513 Contact layer 514 Insulating film 515 Mesa structure 516 Buffer layer 517 Buried layer 518 Through hole 520 Second region 530 High resistor 601 Semiconductor laser 610 First region 614 Insulating film 615 Mesa structure 618 Through hole 620 Second region 630 High resistor

Claims

1. A first conductivity type semiconductor layer; an active layer formed on the first conductive type semiconductor layer; a second conductive type semiconductor layer formed on the active layer; a first electrode electrically connected to the first conductive type semiconductor layer; A second electrode electrically connected to the second conductive type semiconductor layer; a diffraction grating layer disposed on the first conductive type semiconductor layer side or the second conductive type semiconductor layer side as viewed from the active layer; an insulating film disposed in a portion between the second electrode and the second conductive type semiconductor layer; a mesa structure including at least one of the active layer and the second conductive type semiconductor layer; Equipped with The mesa structure has a first region and a second region in an extending direction thereof, the diffraction grating layer in the first region includes a first diffraction grating region, the diffraction grating layer in the second region includes a second diffraction grating region, the first diffraction grating region and the second diffraction grating region form a resonator, a normalized coupling coefficient of the first region is greater than a normalized coupling coefficient of the second region; a current per unit area supplied from the second electrode to the mesa structure is smaller in the first region than in the second region.

2. 2. The semiconductor laser according to claim 1 , the insulating film has one or a plurality of opening regions in which the second electrode and the second conductive type semiconductor layer are in contact with each other in a portion above the mesa structure; a ratio of the opening region to the region immediately above the mesa structure is smaller in the first region than in the second region.

3. 3. The semiconductor laser according to claim 2, a high-resistance resistor is disposed in a non-aperture region other than the aperture region in the region directly above the mesa structure in the first region, the semiconductor laser.

4. 4. The semiconductor laser according to claim 3, The apertured regions and the non-apertured regions are arranged alternately in a direction in which the mesa structure extends.

5. 4. The semiconductor laser according to claim 3, The aperture region of the semiconductor laser includes any one of a polygonal shape, a circular shape, and an elliptical shape in a plan view.

6. 4. The semiconductor laser according to claim 3, The opening region is a parallelogram in a plan view, A semiconductor laser, wherein the sides of the opening region are inclined in a direction in which the mesa structure extends.

7. 4. The semiconductor laser according to claim 3, The opening region has a wide portion and a narrow portion in a direction perpendicular to a direction in which the mesa structure extends in a plan view.

8. 4. The semiconductor laser according to claim 3, The high-resistance resistor is formed of the insulating film.

9. 4. The semiconductor laser according to claim 3, The semiconductor laser, wherein the high-resistance resistor is any one of silicon oxide, silicon nitride, and resin.

10. 3. The semiconductor laser according to claim 2, a ratio of the opening region to the area directly above the mesa structure in the first region is 60% or less.

11. 3. The semiconductor laser according to claim 2, a ratio of the opening region to a region immediately above the mesa structure in the first region is equal to or greater than 20% and equal to or less than 50%.

12. 2. The semiconductor laser according to claim 1 , the diffraction grating layer has a first refractive index region and a second refractive index region; the first diffraction grating region of the first region has a diffraction grating region in which the first refractive index region and the second refractive index region are alternately arranged, the second region has a non-diffraction grating region in which only the first refractive index region or only the second refractive index region is arranged, The second region comprises a plurality of the diffraction grating regions and a plurality of the non-diffraction grating regions.

13. 2. The semiconductor laser according to claim 1 , the diffraction grating layer is composed of a plurality of diffraction grating layers, A semiconductor laser, wherein the number of layers of the diffraction grating layer in the first region is greater than the number of layers of the diffraction grating layer in the second region.

14. 2. The semiconductor laser according to claim 1 , the diffraction grating layer has two regions with different diffraction grating depths; A semiconductor laser, wherein a depth of the diffraction grating in the first region is greater than a depth of the diffraction grating in the second region.

15. 2. The semiconductor laser according to claim 1 , A low-reflection end face coating film is formed on an end face in the extending direction of the mesa structure.

16. 2. The semiconductor laser according to claim 1 , a window structure having a refractive index smaller than that of the active layer is disposed between the mesa structure and an end face in an extending direction of the mesa structure.

17. 2. The semiconductor laser according to claim 1 , the mesa structure includes the active layer; The semiconductor laser further comprises a semiconductor burying layer on a side surface of the mesa structure.

18. 18. The semiconductor laser according to claim 17, The second conductive type semiconductor layer is disposed on the mesa structure and the semiconductor buried layer.

19. 20. The semiconductor laser according to claim 18, In the first region, the opening region is disposed so as to be shifted from a region directly above the mesa structure in a plan view, In the first region, the high resistor is disposed so as to overlap with a region directly above the mesa structure in a plan view; In the second region, the opening region is disposed directly above the mesa structure.

20. 2. The semiconductor laser according to claim 1 , The second electrode is disposed across both the first region and the second region.

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