Semiconductor laser

Abstract

It is an object of the present invention to provide a semiconductor laser having excellent high output characteristics and wavelength uniformity.SOLUTION: A semiconductor laser according to a first aspect of the present application includes a substrate, an active layer, a cladding layer including a first diffraction grating layer having a first diffraction grating structure and a second diffraction grating layer having a second diffraction grating structure, and an electrode. The active layer and the cladding layer constitute a mesa structure, and have a first reflection region and a second reflection region constituting a resonator in a direction in which the mesa structure extends. A second diffraction grating structure is formed in the first reflection region, and one of the first diffraction grating structure and the second diffraction grating structure is formed in the second reflection region. The first diffraction grating structure and the second diffraction grating structure are formed such that a normalized coupling coefficient of the first reflection region is larger than a normalized coupling coefficient of the second reflection region. The mesa structures of the first reflection region and the second reflection region have different mesa widths depending on the refractive index.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] Semiconductor lasers are widely used as light sources in optical communications. A distributed feedback semiconductor laser (DFB laser) is known as one type of semiconductor laser. DFB lasers are equipped with a diffraction grating. To improve performance, a structure in which a phase shift region is provided in the diffraction grating is also known. Stable single-wavelength operation can be achieved by forming a non-reflective film (low-reflection film) on both end faces of the semiconductor laser and arranging a π-shift region in the diffraction grating (Patent Document 1). Patent Document 1 discloses that the output from one end facet can be increased by changing the reflectivity for Bragg-reflected light before and after the phase shift region. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 61-47685 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-119434 [Patent Document 3] International Publication No. 2010 / 116460 Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable for a DFB laser to have high output power and oscillate at only a single wavelength. Patent Document 2 discloses a DFB laser in which the diffraction grating structure is removed at a fixed interval. The reflectivity at the Bragg wavelength differs between the region where the diffraction grating structure is removed and the region where it is not, allowing for increased optical output intensity from the end facet on the side where the diffraction grating structure is removed. However, the effective refractive index differs between the region where the diffraction grating structure is located and the region where it is not located, which may result in reflection or scattering of light at the boundary between them. This may degrade the high-output characteristics and the uniformity of the wavelength. Therefore, it is desirable to place a diffraction grating structure over the entire region in the resonator direction.

[0005] Patent Document 3 discloses a structure in which a diffraction grating structure is arranged over the entire cavity, and the reflectivity for the Bragg wavelength is distributed in the cavity direction depending on the number of steps in the diffraction grating layer. However, because the effective refractive index is distributed depending on the number of steps in the diffraction grating layer, the effective refractive index is not constant when viewed in the cavity direction, resulting in variations in the Bragg wavelength determined by the diffraction grating structure. As a result, wavelength uniformity deteriorates. Patent Document 3 modifies the optical waveguide structure as a means of achieving high wavelength uniformity. Also known is a semiconductor laser that forms a mesa structure in the optical waveguide structure in order to concentrate the current injected for driving in a certain area. Patent Document 3 discloses a structure in which the width (hereinafter referred to as the mesa width) in a planar view perpendicular to the direction in which the mesa structure extends is adjusted in different regions of the diffraction grating structure.

[0006] An object of the present invention is to provide a semiconductor laser that is excellent in high output power characteristics and wavelength uniformity. [Means for solving the problem]

[0007] a cladding layer including: a first diffraction grating layer disposed on the active layer, at least a portion of which has a first diffraction grating structure; and a second diffraction grating layer disposed above at least a portion of the first diffraction grating layer and spaced apart, at least a portion of which has a second diffraction grating structure; and an electrode disposed on the cladding layer, wherein the active layer and the cladding layer form a mesa structure, and the mesa structure has a first reflection region and a second reflection region that form a resonator in an extending direction of the mesa structure, the second diffraction grating structure being formed in the first reflection region, and either the first diffraction grating structure or the second diffraction grating structure being formed in the second reflection region, the first diffraction grating structure and the second diffraction grating structure being formed such that a normalized coupling coefficient of the first reflection region is larger than that of the second reflection region, and the mesa structures of the first reflection region and the second reflection region have mesa widths that differ depending on the refractive index. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a top view of a semiconductor laser according to a first embodiment. [Figure 2] 2 is a schematic cross-sectional view taken along line II-II of the semiconductor laser shown in FIG. [Figure 3] 3 is a schematic cross-sectional view taken along line III-III of the semiconductor laser shown in FIG. [Figure 4] 4 is a schematic cross-sectional view taken along line IV-IV of the semiconductor laser shown in FIG. [Figure 5] 4 is a top view of a cross section taken along line VV of the semiconductor laser shown in FIG. [Figure 6] 6 is a top view of a cross section taken along line VI-VI of the semiconductor laser shown in FIG. [Figure 7] FIG. 2 is a schematic cross-sectional view of a semiconductor laser according to a first modification of the first embodiment. [Figure 8] FIG. 2 is a top view of a semiconductor laser according to a first modification of the first embodiment. [Figure 9] FIG. 10 is a schematic cross-sectional view of a semiconductor laser according to Modification 2 of the first embodiment. [Figure 10] FIG. 10 is a top view of a semiconductor laser according to Modification 2 of the first embodiment. [Figure 11] FIG. 10 is a top view of a semiconductor laser according to a second embodiment, in which some layers are exposed. [Figure 12] 12 is a schematic cross-sectional view taken along line XII-XII of the semiconductor laser shown in FIG. 11. [Figure 13] FIG. 10 is a schematic cross-sectional view of a semiconductor laser according to a modified example of the second embodiment. [Figure 14] FIG. 10 is a top view of a semiconductor laser according to a third embodiment. [Figure 15] 15 is a schematic cross-sectional view taken along line XV-XV of the semiconductor laser shown in FIG. 14. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all drawings for explaining the embodiments, components having the same functions are designated by the same reference numerals, and repeated explanations thereof will be omitted. The drawings shown below are merely for explaining examples of the embodiments, and the size of the drawings does not necessarily correspond to the scale described in the examples. Furthermore, each example may be combined.

[0010] [First embodiment] FIG. 1 is a top view of a semiconductor laser 1 according to a first embodiment. FIG. 2 is a schematic cross-sectional view taken along line II-II in FIG. 1. FIG. 3 is a schematic cross-sectional view taken along line III-III in FIG. 1. FIG. 4 is a schematic cross-sectional view taken along line IV-IV in FIG. 1. FIGS. 5 and 6 are top views showing some exposed layers of the semiconductor laser 1. The semiconductor laser 1 has a first electrode 2 on its back surface and a second electrode 3 on its front surface. The first electrode 2 and the second electrode 3 are metal layers. When a current is injected between the first electrode 2 and the second electrode 3, light is emitted from a front end face 40 (the end face on the left in FIG. 1). A low-reflection coating 4 is formed on the front end face 40 (the end face on the left in FIG. 1) and the rear end face 50 (the end face on the right in FIG. 1). When the low-reflection coating 4 is formed, the reflectivity of the end face is preferably 1% or less.

[0011] [Semiconductor stacked structure] The semiconductor laser 1 includes a first-conductivity substrate 5 on which semiconductor layers are stacked in this order: a first-conductivity optical confinement layer 6 (SCH layer), an active layer 7, a second-conductivity optical confinement layer 8 (SCH layer), a second-conductivity cladding layer 9, and a second-conductivity contact layer 13. The direction in which the semiconductors are stacked is designated as a third direction D3. The semiconductor laser 1 is a DFB laser. The second-conductivity cladding layer 9 includes a first diffraction grating layer 11, at least a portion of which has a first diffraction grating structure 12A (described later), and a second diffraction grating layer 21, at least a portion of which has a second diffraction grating structure 22A (described later), disposed above and spaced from at least a portion of the first diffraction grating layer 11. The active layer 7 is formed of, for example, a multiple quantum well layer. The multiple quantum well layer may be an intrinsic semiconductor or an n-type semiconductor. Here, the first conductivity type is n-type and the second conductivity type is p-type, but the opposite is also possible. These semiconductor layers include a mesa structure 15. The mesa structure 15 extends in the direction in which light is extracted (first direction D1). The lower part of the mesa structure 15 is part of the substrate 5. Both sides of the mesa structure 15 are covered with a semi-insulating semiconductor burying layer 17. The burying layer 17 may be a stack of p-type and n-type semiconductor layers. The dotted line in FIG. 1 indicates the boundary between the upper part of the mesa structure 15 and the burying layer 17.

[0012] The semiconductor laser 1 has an insulating film 14 on its surface. The insulating film 14 covers the entire surface of the semiconductor laser 1 except for a portion thereof. The insulating film 14 includes an opening (through-hole) 18 in a region corresponding to the top of the mesa structure 15. The through-hole 18 connects the second electrode 3 and the second conductivity type contact layer 13, and an electrical signal is applied (current is injected) to the mesa structure 15. The through-hole 18 has a shape along the first direction D1. In addition, in a second direction D2 perpendicular to the first direction D1 and the third direction D3, the width of the through-hole 18 is wider than the width of the mesa structure 15. However, the two widths may be the same. The width of the mesa structure 15 is the width of the mesa structure formed in the second direction D2 (hereinafter referred to as the mesa width), and the mesa widths of the first region 10 and the second region 20 are referred to as the first mesa width W1 and the second mesa width W2, respectively. Details of the first region 10 and the second region 20 will be described later.

[0013] [First grating layer] The first diffraction grating layer 11 has a floating type diffraction grating structure, and is composed in cross section of a region having the same refractive index as the second conductivity type cladding layer 9 and a region having a first refractive index different from that of the second conductivity type cladding layer 9. Here, when the refractive index of the second conductivity type cladding layer 9 is defined as a second refractive index, the first diffraction grating layer 11 includes a region in which first refractive index regions 11A and second refractive index regions 11B are alternately arranged along a first direction D1.

[0014] [Second grating layer] The second diffraction grating layer 21 includes a floating-type diffraction grating structure similar to that of the first diffraction grating layer 11, and is thicker than the first diffraction grating layer 11 in the semiconductor stacking direction (third direction D3). A cladding layer 9 is disposed between the first diffraction grating layer 11 and the second diffraction grating layer 21. In a cross-sectional view, the second diffraction grating layer 21 includes a region in which third refractive index regions 21A and fourth refractive index regions 21B formed by the cladding layer 9 are alternately disposed along the first direction D1. In this embodiment, the first refractive index and the third refractive index are higher than the second refractive index and the fourth refractive index. However, the relationship between the refractive indices may be reversed. The first refractive index and the third refractive index may be the same or different.

[0015] [Mesa structure] The mesa structure 15 includes regions with different mesa widths in the first direction D1. That is, the mesa structure of the first reflective region 61 included in the first region 10 has a different mesa width from the mesa structure of the second reflective region 62 included in the second region 20. Specifically, the semiconductor laser 1 includes the first region 10 with a first mesa width W1 and the second region 20 with a second mesa width W2. In this embodiment, the second mesa width W2 is wider than the first mesa width W1. The mesa structure 15 also includes a third region 30 between the first region 10 and the second region 20, in which the mesa width gradually changes from the first mesa width W1 to the second mesa width W2. The third region 30 preferably has a tapered shape in which the mesa width gradually changes from the first region 10 to the second region 20. However, the boundary between the third region 30 and the burying layer 17 may change linearly or curvedly when viewed from above. The second electrode 3 is integrally disposed across the first region 10, the second region 20, and the third region 30. The second electrode 3 may be disposed separately in each region, but in this case, it is desirable that each of the separately disposed electrodes be connected to the same power source.

[0016] [First diffraction grating structure] FIG. 5 is a top view of the semiconductor laser 1, illustrating the position of each region included in the first diffraction grating layer 11. Specifically, as shown in FIG. 3, this is a top view of a cross section (VV cross section) of the first diffraction grating layer 11. For ease of explanation, some components are not shown. A structure (diffraction grating structure) formed in the first diffraction grating layer 11, in which the first refractive index regions 11A and the second refractive index regions 11B are alternately arranged, is referred to as a first diffraction grating structure 12A. The first diffraction grating layer 11 has the first diffraction grating structure 12A throughout the entire resonator of the semiconductor laser 1, except for a phase shift section 12C, which will be described later. The first diffraction grating structure 12A is a uniform diffraction grating structure in which the first refractive index regions 11A and the second refractive index regions 11B are alternately arranged in a first direction D1 to reflect light of a specific Bragg wavelength. The first diffraction grating structure 12A of this embodiment is arranged at the same period in the first region 10 and the second region 20. However, the first diffraction grating structure 12A arranged in the first region 10 and the first diffraction grating structure 12A arranged in the second region 20 reflect light in different phases. The phases will be described later. Here, the first diffraction grating structure 12A is configured to reflect light in the 1.3 μm band. However, it may also be configured to reflect light in other Bragg wavelengths, such as the 1.55 μm band.

[0017] [Second diffraction grating structure] FIG. 6 is a top view of the semiconductor laser 1, illustrating the position of each region included in the second diffraction grating layer 21. Specifically, as shown in FIG. 3, this is a top view of the second diffraction grating layer 21 (VI-VI cross section). For ease of explanation, some components are not shown. A structure in which the third refractive index regions 21A and the fourth refractive index regions 21B are alternately arranged in the second diffraction grating layer 21 is referred to as a second diffraction grating structure 22A. The second diffraction grating structure 22A has the same period and is arranged at the same position (first direction D1) as the first diffraction grating structure 12A. That is, the centers of the third refractive index regions 21A and the first refractive index regions 11A in the first direction D1 are aligned. Similarly, the centers of the fourth refractive index regions 21B and the second refractive index regions 11B in the first direction D1 are aligned. A structure in which only the third refractive index regions 21A or only the fourth refractive index regions 21B are arranged is referred to as a second non-diffraction grating structure 22B. In this embodiment, second non-diffraction grating structure 22B is composed only of fourth refractive index region 21B. In other words, cladding layer 9 is disposed in the region where second diffraction grating structure 22A is not formed. Second non-diffraction grating structure 22B transmits, without reflecting, light of the Bragg wavelength that would otherwise be reflected by second diffraction grating structure 22A.

[0018] [Phase shift section] The first diffraction grating layer 11 includes a phase shift portion 12C. The phase shift portion 12C has a structure in which two first refractive index regions 11A are arranged consecutively. Alternatively, the phase shift portion 12C may have a structure in which two second refractive index regions 11B are arranged consecutively. The first diffraction grating structure 12A has a phase shift of π before and after the phase shift portion 12C. In other words, the phase shift portion 12C is a λ / 4 phase shift portion. The phase shift amount here corresponds to a π shift, taking into account the optical path length. As will be described later, the first region 10 and the second region 20 have substantially the same effective refractive index, and therefore the optical path lengths of the first region 10 and the second region 20 are substantially the same. Meanwhile, the third region is a region in which the mesa width changes, and therefore includes a region with a different effective refractive index from the first region 10 and the second region 20. Therefore, the optical path length of the third region is also different from that of the first region 10 and the second region 20. In this embodiment, taking into consideration the difference in optical path length in the third region 30, fine adjustment is made so that the phases of the diffraction grating structures in the first region 10 and the second region 20 are effectively shifted by π.

[0019] [Reflection area] Here, the region from the phase shift portion 12C to the rear end facet 50 is referred to as the first reflection region 61. The region from the phase shift portion 12C to the front end facet 40 is referred to as the second reflection region 62. The region between the first reflection region 61 and the second reflection region 62 is referred to as the phase shift region 63. Here, the phase shift region 63 includes the phase shift portion 12C. The first reflection region 61 and the second reflection region 62 form a resonator with the phase shift region 63 sandwiched between them. In other words, a portion of the Bragg wavelength light reflected by the first reflection region 61 is reflected by the second reflection region 62 and returns to the first reflection region 61. Similarly, a portion of the Bragg wavelength light reflected by the second reflection region 62 is reflected by the first reflection region 61 and returns to the second reflection region 62. Furthermore, since the semiconductor laser 1 has the low-reflection coating 4 formed on the front end facet 40 and the rear end facet 50, it can achieve extremely high single-wavelength characteristics. For example, the theoretical yield of the side mode suppression ratio (SMSR) is 100%. In Fig. 2, the first reflective region 61, the second reflective region 62, and the phase shift region 63 indicate the regions where the first diffraction grating layer 11 and the second diffraction grating layer 21 are disposed. However, this is for the sake of simplicity, and in reality, the entire region including the semiconductor layers above and below the diffraction grating layer constitutes each region.

[0020] [First reflection area] The first reflection region 61 includes the first diffraction grating structure 12A of the first diffraction grating layer 11 and the second diffraction grating structure 22A of the second diffraction grating layer 21. In other words, the first reflection region 61 has a two-stage diffraction grating structure. The first reflection region 61 spans the first region 10 and part of the third region 30. Here, the first reflection region 61 in the third region 30 includes only the first diffraction grating structure 12A, and the diffraction grating structure is one stage. In other words, the end of the second diffraction grating structure 22A substantially coincides with the interface between the first region 10 and the third region 30. However, this is not limiting, and for example, the second diffraction grating structure 22A may be disposed in the third region 30. If the second diffraction grating structure 22A is disposed so as to extend over the phase shift portion 12C, a phase shift portion is also disposed in the second diffraction grating layer 21. Conversely, the end of the second diffraction grating structure 22A may be disposed closer to the rear end face 50 than the interface between the first region 10 and the third region 30. However, it is desirable that the second diffraction grating structure 22A be disposed over 80% or more of the first region 10 in the first direction D1.

[0021] [Second reflective area] The second reflective region 62 includes a first diffraction grating structure 12A in the first diffraction grating layer 11 and a second non-diffraction grating structure 22B in the second diffraction grating layer 21. In other words, the diffraction grating structure included in the second reflective region 62 is only one stage of the first diffraction grating structure 12A. The second reflective region 62 spans the second region 20 and part of the third region 30. Here, the first diffraction grating structure 12A in the second reflective region 62 is disposed over the entire surface of the second region 20 in the first direction D1, but is not limited to this. It is desirable that the first diffraction grating structure 12A be disposed over at least 80% or more of the second region 20.

[0022] Coupling coefficient Here, the coupling coefficients of the first reflection region 61 and the second reflection region 62 are denoted as κ1 and κ2, respectively. The coupling coefficient κ is determined by the structure of the semiconductor multilayer, the diffraction grating structure, etc. In this embodiment, the semiconductor multilayers included in the first reflection region 61 and the second reflection region 62 are substantially the same. Therefore, the difference between κ1 and κ2 is mainly due to the difference in the diffraction grating structure. The first reflection region 61 includes a two-stage diffraction grating structure, while the second reflection region 62 includes a one-stage diffraction grating structure, so κ1 and κ2 are different. Here, the number of stages in the diffraction grating structure of the first reflection region 61 is greater than that of the second reflection region 62, so κ1 is greater than κ2.

[0023] [Resonator length] The lengths of the first reflection region 61 and the second reflection region 62 in the first direction D1 are denoted by L1 and L2, respectively. As described above, L1 is not the length of the first region 10, which has a mesa width W1. L2 is not the length of the second region 20, which has a mesa width W2. L1 and L2 indicate the lengths of the regions in which the diffraction grating structures are arranged, as viewed from the phase shift section 12C. Here, L2 is longer than L1.

[0024] [Normalized Coupling Coefficient] In this embodiment, the normalized coupling coefficient κ1L1 of the first reflection region 61 is greater than the normalized coupling coefficient κ2L2 of the second reflection region 62. This is achieved by adjusting the thickness and composition of the diffraction grating structure to make κ1 greater than κ2. The optical output intensity from the facet with a smaller normalized coupling coefficient across the phase shift section 12C is greater. In other words, in the semiconductor laser 1, the optical output intensity from the front facet 40 is greater than the optical output intensity from the rear facet 50. If the first reflection region 61 and the second reflection region 62 have the same normalized coupling coefficient, the optical intensities output from both facets will be the same. Note that the terms "front" and "rear" are merely used for convenience, and the facet with the higher optical output is simply referred to as the "front facet." In general optical communications, higher optical intensity is preferable, and light from the front facet is used for optical communications. In this way, by arranging regions with different normalized coupling coefficients depending on the number of steps in the diffraction grating structure, the optical output intensity from one facet can be increased. Furthermore, since the diffraction grating structure (here, the first diffraction grating structure 12A) is arranged over the entire cavity, scattering of light is suppressed, contributing to higher output power.

[0025] The coupling coefficient κ1 of the first reflection region 61 is determined by the structures of both the first region 10 and the third region 30. In this embodiment, the second diffraction grating structure 22A is disposed only in the first region 10, but if the second diffraction grating structure 22A is also disposed in the third region 30, the coupling coefficient κ1 will be larger. Similarly, the second reflection region 62 includes only the first diffraction grating structure 12A, but if the second diffraction grating structure 22A is included in part of the third region 30, the coupling coefficient κ2 will be larger. Even if these are included, if κ1L1 of the first reflection region 61 is larger than κ2L2 of the second reflection region 62, the optical output intensity from the front end facet 40 can be increased.

[0026] Here, κ1L1 is preferably 60% or more (i.e., κ2L2 is 40% or less) of the normalized coupling coefficient of the entire semiconductor laser 1. Furthermore, increasing κ1L1 by lengthening L1 reduces the ratio of the second region 20, which increases the optical output, to the entire device. This is because the second region 20 is the main region of the second reflecting region 62, and its mesa width is wider than the first region 10, which is the main region of the first reflecting region 61. In other words, the mesa structures of the first reflecting region 61 and the second reflecting region 62 have different mesa widths depending on the refractive index. A wider mesa increases the total amount of light, and a longer second region 20 results in superior high-output characteristics. In this case, the high-output effect is not fully achieved, so κ1L1 is preferably 70% or more. If even higher output characteristics are required, κ1L1 is preferably 80% or more.

[0027] The cavity length of the semiconductor laser 1 is the total length in the first direction D1 of the first region 10, the second region 20, and the third region 30. More precisely, because the low-reflection coating 4 is formed on both end faces of the semiconductor laser 1, the cavity length is the length over which the diffraction grating structure is disposed. Here, the cavity length is the length over which the first diffraction grating structure 12A is disposed. To increase the optical output intensity from the front end face 40, the first reflection region 61 is preferably located on the rear side. The first reflection region 61 is preferably disposed so that its length is 40% or less of the cavity length. That is, the length of the first reflection region 61 in the first direction D1 is preferably 40% or less of the length of the entire diffraction grating layer (the region in which the first diffraction grating structure 12A is disposed), and more preferably 30% or less. However, if κ1L1 is less than 1, the oscillation threshold becomes high, which is undesirable from the viewpoint of power consumption. Therefore, κ1L1 must be set to 1 or greater, more preferably 1.5 or greater.

[0028] [Effective refractive index] To achieve high wavelength uniformity, the Bragg wavelengths of the light reflected by the first and second reflection regions 61 and 62 must be identical. The Bragg wavelength is proportional to the effective refractive index of the region through which the light propagates and the period of the diffraction grating structure. The effective refractive index depends on the semiconductor structure and mesa width of the region through which the light propagates. The first and second reflection regions 61 and 62 contain different numbers of steps in the diffraction grating structure, resulting in different semiconductor layer configurations. Therefore, even if the mesa widths of the regions containing the first and second reflection regions 61 and 62 are the same, the effective refractive indices of the two regions are different. Therefore, to achieve the same Bragg wavelengths for the first and second reflection regions 61 and 62, the periods of the diffraction grating structures must be different. However, from a manufacturing perspective, forming regions with different periods of the diffraction grating structure within a single semiconductor device is undesirable. For example, the period of a diffraction grating structure corresponding to the 1.3 μm band is approximately 200 nm, requiring extremely fine processing. Furthermore, the difference in effective refractive index due to the number of steps in the diffraction grating structure is small, and when this difference is adjusted by the diffraction grating period, the difference in period becomes very small. For example, the difference in period is 1 nm or less. Therefore, forming regions with very small differences in diffraction grating period within a single semiconductor element is undesirable from the standpoint of process precision. If the desired diffraction grating structure period cannot be obtained due to process variations, oscillation at a single wavelength cannot be obtained, which is undesirable as a semiconductor laser. Therefore, it is preferable to form the first reflection region 61 and the second reflection region 62 so that their diffraction grating periods are the same.

[0029] In this embodiment, the mesa width is changed to match the effective refractive index. The effective refractive index of the semiconductor layer in the first region 10, which occupies the main area of ​​the first reflection region 61, is greater than the effective refractive index of the semiconductor layer in the second region 20, which occupies the main area of ​​the second reflection region 62. This is mainly due to the inclusion of the third refractive index region 21A in the first region 10. Therefore, the mesa width W2 of the second region 20 is made greater than the mesa width W1 of the first region 10, so that the effective refractive indices of the first region 10 and the second region 20 are approximately equal. Here, "equal" means that the difference between the effective refractive indexes of the first reflection region 61 and the second reflection region 62 is within 0.5%. Here, W1 is, for example, 2 μm, and W2 is 2.2 μm. Compared to the period of the diffraction grating, the difference in mesa width is sufficiently large, enabling stable manufacturing. With this structure, the first reflection area 61 and the second reflection area 62 have the same diffraction grating structure period and reflect the same Bragg wavelength, resulting in oscillation at a single wavelength. Note that the period being the same here means that the period is the same within the range formed by the same manufacturing process, and is considered to be the same as long as it is within manufacturing variations (for example, variations in the etching process when forming the diffraction grating structure).

[0030] The first mesa width W1 of the first region 10 is preferably set to a width equal to or less than the width at which transverse high-order modes are not generated for light of the Bragg wavelength. In other words, W1 is preferably set to a width equal to or less than the cutoff width. For example, when the light wavelength is in the 1.3 μm band, W1 is preferably set to 2 μm or less. Furthermore, the generation conditions for transverse high-order modes may vary depending on the driving conditions. To achieve a stable suppression effect for transverse high-order modes, W1 is preferably set to 1.5 times the Bragg wavelength or less. For example, when the Bragg wavelength is 1.3 μm, W1 is preferably set to 1.95 μm or less. In this embodiment, the second mesa width W2 of the second region 20 is set to be wider than the first mesa width W1 to match the effective refractive index, as described above. The mesa width W2 of the second region is also preferably set to be equal to or less than the cutoff width to suppress the generation of transverse high-order modes, but is not limited to this. For example, when the first mesa width W1 is set near the cutoff width, the second mesa width for matching the effective refractive index is wider than the cutoff width, but this is acceptable. The wider the mesa width, the greater the total amount of light generated, thereby increasing the optical output intensity of the semiconductor laser 1. When the second mesa width W2 is equal to or greater than the cutoff width, even if a higher-order transverse mode occurs, the light in the higher-order transverse mode is not reflected by the first region 10, and therefore, no strong light in the higher-order transverse mode is output from the front end facet 40.

[0031] [Third area] The third region 30, where the mesa width varies, contributes to a decrease in wavelength uniformity. In particular, the effective refractive index of the region not including the second diffraction grating structure 22A is smaller than that of the first region 10. Furthermore, because the mesa width is smaller than W2, the effect of increasing the effective refractive index by widening the mesa width is also limited. Therefore, from the perspective of wavelength uniformity, it is preferable that the proportion of the third region 30 in the entire cavity of the semiconductor laser 1 be small. In the first reflection region 61, which spans both the first region 10 and the third region 30, if the proportion of the third region 30 is 20% or less, the effect on wavelength uniformity degradation is small. Similarly, in the second reflection region 62, which spans both the second region 20 and the third region 30, the proportion of the third region 30 is 20% or less.

[0032] [Phase shift position] Furthermore, it is desirable to arrange the phase shift portion 12C within the third region 30. Even if the phase shift portion 12C is arranged in the first region 10 or the second region 20, high output characteristics can be obtained as long as the relationship κ1L1>κ2L2 is satisfied. Let us assume a structure in which the phase shift portion 12C is arranged within the first region 10 and the second diffraction grating structure 22A is not included between the phase shift portion 12C and the front end face 40. In other words, the second diffraction grating structure 22A is arranged only in the first reflection region 61. In this case, the second reflection region 62 is arranged across part of the first region 10, the entire third region 30, and the entire second region 20. In this case, the second reflection region 62 within the first region 10 has a mesa width W1 and a single-stage diffraction grating structure. As described above, the mesa width W1 is set so that light of the desired Bragg wavelength is reflected when the diffraction grating structure is two stages. Therefore, the wavelength of light reflected by the region of the second reflection region 62 included in the first region 10 is shifted from the Bragg wavelength. As described above, the wavelength of light reflected by the region of the second reflection region 62 included in the third region 30 is shifted from the Bragg wavelength because the mesa width is smaller than W2. Therefore, when the phase shifter 12C is disposed in the first region 10, the region where the Bragg wavelength is shifted is wider than when the phase shifter 12C is disposed in the third region 30. This results in a decrease in wavelength monotony. This, for example, leads to a decrease in the side-mode suppression ratio. The same applies when the phase shifter 12C is disposed in the second region 20. The first reflection region 61 is disposed across part of the second region 20, the entire third region 30, and the entire first region 10. In part of the second region 20, the diffraction grating structure is two-stage, the effective refractive index is high, and the Bragg wavelength is shifted to the larger side. As a result, wavelength monotony is decreased. As described above, if the phase shift section 12C is disposed in the first region 10 or the second region 20, the region where the Bragg wavelength is shifted becomes larger, which is undesirable from the viewpoint of wavelength monotony. In this embodiment, by disposing the phase shift section 12C in the third region 30, the region where the Bragg wavelength is shifted can be minimized, and degradation of wavelength monotony can be suppressed. Note that, for example, if the end of the second diffraction grating structure 22A completely coincides with the end of the first region 10 and the end of the third region 30, disposing the phase shift section 12C in the first region does not expand the region where the Bragg wavelength is different.However, considering manufacturing variations, it is difficult to perfectly align the end of the diffraction grating layer structure with the end of the region where the mesa width changes, and therefore, from the viewpoint of yield, it is preferable to intentionally place the phase shift portion 12C in the third region 30.

[0033] The second electrode 3 is arranged across the first region 10, the second region 20, and the third region 30, and the change in the effective refractive index due to the amount of injected current is approximately the same, achieving high wavelength uniformity over a wide range of operating conditions.

[0034] As described above, in the semiconductor laser 1 according to this embodiment, the first diffraction grating structure 12A is disposed over the entire cavity, including the phase-shift portion 12C. Furthermore, the diffraction grating structures are disposed discretely, which prevents light from scattering and achieves high output characteristics.

[0035] [Variation 1] Fig. 7 is a cross-sectional view of the semiconductor laser 1 according to Modification 1 of the first embodiment, and corresponds to Fig. 2. Fig. 8 is a top view of the semiconductor laser 1 according to Modification 1 of the first embodiment, and corresponds to Fig. 5. The main difference from the first embodiment is the structure of the phase shift region 63.

[0036] In this modification, the first diffraction grating structures 12A of the first diffraction grating layer 11 are not arranged continuously in the first direction D1, but are not arranged in part of the first region 10, the entire third region 30, or part of the second region 20. A first non-diffraction grating structure 12B is arranged in these regions. In the first non-diffraction grating structure 12B, only either the first refractive index region 11A or the second refractive index region 11B is arranged. The first non-diffraction grating structure 12B transmits, without reflecting, light of the Bragg wavelength that is reflected by the first diffraction grating structure 12A. Here, only the second refractive index region 11B is arranged in the first non-diffraction grating structure 12B. In other words, the first diffraction grating layer 11 has a structure in which two first diffraction grating structures 12A are arranged with a cladding layer 9 between them.

[0037] In this modification, the phase shift region 63 is a region where the first diffraction grating structure 12A and the second diffraction grating structure 22A are not arranged. In other words, it is a region where the first non-diffraction grating structure 12B and the second non-diffraction grating structure 22B are arranged. The first reflection region 61 is a region between the phase shift region 63 and the rear end face 50, where at least the second diffraction grating structure 22A is arranged. In this modification, the first reflection region 61 also includes the first diffraction grating structure 12A. Therefore, the first reflection region 61 includes a two-stage diffraction grating structure. The second reflection region 62 is a region between the phase shift region 63 and the front end face 40, where the first diffraction grating structure 12A is arranged. As in the first embodiment, the first diffraction grating structures 12A arranged in the first reflection region 61 and the second reflection region 62 have the same period and are phase-shifted by π. Note that the π-shift in the phase of the diffraction grating structure here refers to a phase shift that takes into account the optical path length as described above.

[0038] In this modification, neither the first diffraction grating structure 12A nor the second diffraction grating structure 22A is disposed in the third region 30 where the mesa width changes. Therefore, reflection of light with wavelengths different from the desired Bragg wavelength, as described in the first embodiment, does not occur. In other words, a semiconductor optical device superior in terms of wavelength monotony is realized. However, if the length of the first non-diffraction grating structure 12B in the first direction D1 is greater than half the length of the first diffraction grating structure 12A disposed in the first region 10, oscillation of a wavelength different from the desired Bragg wavelength may occur. Therefore, the length of the first non-diffraction grating structure 12B is preferably equal to or less than half the length of the first diffraction grating structure 12A disposed in the first region.

[0039] In this modification, the normalized coupling coefficient κ1L1 of the first reflection region 61 is also greater than the normalized coupling coefficient κ2L2 of the second reflection region 62. Therefore, the optical output intensity output from the front end facet 40 is greater than the optical output intensity output from the rear end facet 50.

[0040] In this modification, the diffraction grating structure is not completely continuous, but is interrupted at the phase shift region 63. Therefore, compared to the first embodiment, there is a possibility that light scattering may occur. However, compared to Patent Document 1, there are fewer regions where the diffraction grating structure is discontinuous (only one), so even if light scattering does occur, the impact is small. Therefore, a semiconductor laser with excellent wavelength uniformity and high output characteristics is realized.

[0041] [Variation 2] Fig. 9 is a cross-sectional view of the semiconductor laser 1 according to Modification 2 of the first embodiment, and corresponds to Fig. 2. Fig. 10 is a top view of the semiconductor laser 1 according to Modification 2 of the first embodiment, and corresponds to Fig. 5. The main difference from the first embodiment is the structure of the phase shift region 63.

[0042] In this modification, the first diffraction grating structure 12A of the first diffraction grating layer 11 is not disposed in the third region 30 and is not continuous in the first direction D1. A first non-diffraction grating structure 12B is disposed in the region of the first diffraction grating layer 11 where the first diffraction grating structure 12A is not disposed. Here, the first non-diffraction grating structure 12B is the first refractive index region 11A. The other structures are the same as those in the modification 1 of the first embodiment.

[0043] As described above, the region of the first reflective region 61 that is located in the third region 30 reflects light of a wavelength different from the desired Bragg wavelength. However, as in the first embodiment, if the proportion of the first reflective region 61 that is occupied by the third region 30 is 20% or less, the effect is tolerable. The same applies to the second reflective region 62.

[0044] Note that Modification 1 and Modification 2 may be combined. Specifically, for example, the first reflection region 61 may have the structure of Modification 1, and the second reflection region 62 may have the structure of Modification 2. In other words, the first region 10 may include the first reflection region 61 and a portion of the phase shift region 63, and the second region 20 may include the second reflection region 62. The third region 30 may also include a portion of the second reflection region 62 and a portion of the phase shift region 63.

[0045] As described above, the phase shift region 63 may include the phase shift portion 12C as shown in the first embodiment, or may not include a diffraction grating structure. If the phase shift region 63 does not include a diffraction grating structure, the phase shift region 63 may be disposed in a part of the first region 10 or the second region 20.

[0046] [Second embodiment] Fig. 11 is a top view of a semiconductor laser 201 according to the second embodiment, and corresponds to Fig. 1. Fig. 12 is a schematic cross-sectional view taken along line XII-XII in Fig. 11, and corresponds to Fig. 2. The second embodiment differs from the first embodiment in that the semiconductor laser 201 includes a spot size conversion region 260 between the second region 20 and the front end facet 40, and in the structures of the first reflection region 61 and the phase shift region 63.

[0047] In this embodiment, the first diffraction grating layer 11 has a first non-diffraction grating structure 12B arranged from the rear end facet 50 side toward the front end facet 40 side up to the middle of the second region 20, and the first diffraction grating structure 12A arranged from the middle of the second region 20. The first non-diffraction grating structure 12B is formed from the first refractive index region 11A. The second diffraction grating layer 21 is the same as in the first embodiment.

[0048] [Mesa width] Similar to the first embodiment, the semiconductor laser 201 has different mesa widths in the first region 10 and the second region 20. That is, the mesa structure of the first reflective region 61 included in the first region 10 has a different mesa width from the mesa structure of the second reflective region 62 included in the second region 20. Specifically, the mesa structure 215 has a first mesa width W1 in the first region 10 and a second mesa width W2 in the second region 20. The second mesa width W2 is larger than the first mesa width W1. That is, the mesa structure of the first reflective region 61 has a narrower mesa width than the mesa structure of the second reflective region 62. Furthermore, the mesa width changes from the first mesa width W1 to the second mesa width W2 in the third region 30. The spot size conversion region 260 is also part of the mesa structure 215. Furthermore, in the spot size conversion region 260, the mesa width gradually becomes narrower than the second mesa width W2 toward the front end facet 40, and the narrowest point is where it contacts the front end facet 40. The mesa width of the spot size conversion region 260 closest to the front end facet 40 is set so as to obtain a desired optical output shape. This mesa width is, for example, smaller than the first mesa width W1.

[0049] [Spot size conversion area] The spot size conversion region 260 has the same semiconductor multilayer structure as the first region 10 and the second region 20, except for the diffraction grating structure. In the spot size conversion region 260, the first diffraction grating layer 11 is a first non-diffraction grating structure 12B, and the second diffraction grating layer 21 is a second non-diffraction grating structure 22B. In the spot size conversion region 260, both non-diffraction grating structures are cladding layers 9 (second refractive index region 11B, fourth refractive index region 21B). In this embodiment, a second conductivity type contact layer 13 and an insulating film 214 are disposed on the second conductivity type cladding layer 9. However, the contact layer 13 may not be included between the cladding layer 9 and the insulating film 214. Furthermore, a first diffraction grating structure 12A may be included near the connection portion of the spot size conversion region 260 on the second region 20 side. Furthermore, although a portion of the second electrode 3 extends to the spot size conversion region 260, this is not limitative, and the second electrode 3 may be disposed over the entire spot size conversion region 260.

[0050] In this embodiment, the first reflection region 61 is a region where the first non-diffraction grating structure 12B and the second diffraction grating structure 22A, both of which are formed by the first refractive index region 11A, are disposed. The second reflection region 62 is a region where the second non-diffraction grating structure 22B, which is formed by the first diffraction grating structure 12A and the fourth refractive index region 21B, is disposed. The phase shift region 63 is a region between the first reflection region 61 and the second reflection region 62. The phase shift region 63 is mainly composed of the first non-diffraction grating structure 12B, which is formed by the first refractive index region 11A. Therefore, light reflected by the first reflection region 61 is transmitted and transmitted to the second reflection region 62. Note that in the phase shift region 63, the first non-diffraction grating structure 12B may be the second refractive index region 11B (cladding layer 9).

[0051] As in the first embodiment, the period of the second diffraction grating structure 22A in the first reflection area 61 is the same as the period of the first diffraction grating structure 12A in the second reflection area 62. Furthermore, the phases of the diffraction grating structures 22A and 12A, taking into account the optical path lengths, are shifted by π. Furthermore, κ1L1 in the first reflection area 61 is greater than κ2L2 in the second reflection area 62.

[0052] In particular, the effect of diffraction broadening of the first diffraction grating layer 11 can be suppressed, and therefore the manufacturability of the diffraction grating structure can be improved.

[0053] In this embodiment, the first reflection region 61 and the second reflection region 62 each include only a single-stage diffraction grating structure. However, the thickness in the third direction D3 of the second diffraction grating structure 22A included in the first reflection region 61 is thicker than the thickness of the first diffraction grating structure 12A included in the second reflection region 62. Therefore, the coupling coefficient κ1 of the first reflection region 61 is greater than the coupling coefficient κ2 of the second reflection region 62. As described above, the first reflection region 61 does not necessarily need to include a two-stage diffraction grating structure. The difference in coupling coefficient may be generated by varying the thickness of the diffraction grating layer. The coupling coefficient may also be adjusted by varying the composition of the two diffraction grating layers. This embodiment also ensures a sufficient difference between the normalized coupling coefficients of the first reflection region 61 and the second reflection region 62 while maintaining wavelength unity, thereby increasing the optical output intensity from the front end facet 40. Furthermore, by adjusting the first mesa width W1 and the second mesa width W2 to match the effective refractive index, wavelength unity can be improved while maintaining the diffraction grating period.

[0054] [Variations] FIG. 13 is a cross-sectional view of a semiconductor laser 201 according to a modification of the second embodiment, corresponding to FIG. 2. This modification differs from the second embodiment in the configuration of the first diffraction grating layer 11. In this modification, the phase shift region 63 includes a first non-diffraction grating structure 12B, as in the second embodiment. However, this is the second refractive index region 11B (cladding layer 9). In the first reflection region 61, a concave-convex diffraction structure is formed near the surface of the first refractive index region 11A. This is an example in which this structure is formed simultaneously with the formation of the second diffraction grating structure 22A of the second diffraction grating layer 21. The second diffraction grating structure 22A is formed by etching while masking the region that will remain as the third refractive index region 21A and leaving the region that will become the fourth refractive index region 21B unmasked. While it is preferable that only the second diffraction grating layer 21 be etched, the etching may also etch the surface of the first diffraction grating layer 11. In this case, a concave-convex structure is formed near the surface of the first refractive index region 11A, as in this modification. Although the uneven structure is smaller than the first diffraction grating structure 12A, it still contributes to light reflection. In other words, it affects the coupling coefficient κ1. However, because its period is the same as that of the second diffraction grating structure 22A, it does not affect wavelength uniformity. In this modification, too, by making κ1L1 of the first reflection area 61 larger than κ2L2 of the second reflection area 62, both high output characteristics and high wavelength uniformity characteristics are achieved.

[0055] In the phase shift region 63, the first diffraction grating structure 12A is still arranged as in the first embodiment, and the phase shift portion 12C may be arranged midway therebetween.

[0056] [Third embodiment] Fig. 14 is a top view of a semiconductor laser 301 according to the third embodiment, and corresponds to Fig. 1. Fig. 15 is a schematic cross-sectional view taken along line XV-XV in Fig. 14, and corresponds to Fig. 2. The main differences between the third embodiment and the first embodiment are the diffraction grating structures included in the first reflection region 61, the second reflection region 62, and the phase shift region 63, and the mesa widths of the first region 10 and the second region 20.

[0057] In this embodiment, the first diffraction grating layer 311 includes a first refractive index region 311A ​​and a second refractive index region 311B. The second diffraction grating layer 321 includes a third refractive index region 321A and a fourth refractive index region 321B. Here, the second refractive index region 311B and the fourth refractive index region 321B form the cladding layer 9. The refractive indices of the first refractive index region 311A ​​and the third refractive index region 321A are greater than the refractive indices of the second refractive index region 311B and the fourth refractive index region 321B. However, the reverse is also possible. The refractive indices of the first refractive index region 311A ​​and the third refractive index region 321A may be the same or different. The first diffraction grating layer 311 is thicker than the second diffraction grating layer 321 in the third direction D3.

[0058] [First grating layer] The first grating layer 311 includes a first grating structure 312A in which first refractive index regions 311A ​​and second refractive index regions 311B are alternately arranged. The first grating layer 311 also includes a first non-grating structure 312B that transmits light reflected by the first grating structure 312A. Here, the first non-grating structure 312B is formed by the first refractive index region 311A, but it may also be formed by the second refractive index region 311B. The first grating structure 312A is arranged in the first region 10. The first non-grating structure 312B is arranged in the second region 20 and the third region 30. The first region 10, the second region 20, and the third region 30 will be described later.

[0059] [Second grating layer] The second diffraction grating layer 321 includes a second diffraction grating structure 322A in which third refractive index regions 321A and fourth refractive index regions 321B are alternately arranged. The second diffraction grating layer 321 also includes a second non-diffraction grating structure 322B that transmits light reflected by the second diffraction grating structure 322A. Here, the second non-diffraction grating structure 322B is formed by the fourth refractive index region 321B, but it may also be formed by the third refractive index region 321A. The second diffraction grating structure 322A is arranged in the first region 10 and the second region 20. The second non-diffraction grating structure 322B is arranged in the third region 30. Note that the second diffraction grating structure 322A may be arranged in part of the third region 30.

[0060] [Mesa structure] The mesa structure 315 includes regions with different mesa widths in the direction in which the first direction D1 extends. The semiconductor laser 301 includes a first region 10 with a first mesa width W1 and a second region 20 with a second mesa width W2. In this embodiment, the first mesa width W1 is wider than the second mesa width W2. That is, the mesa structure of the first reflection region 361 has a wider mesa width than the mesa structure of the second reflection region 62. The mesa structure 315 also includes a third region 30 between the first region 10 and the second region 20, in which the mesa width gradually changes from the first mesa width W1 to the second mesa width W2. The third region 30 preferably has a tapered shape in which the mesa width gradually changes from the first region 10 to the second region 20. However, the boundary between the third region 30 and the burying layer 17 may change linearly or curvedly when viewed from above. The second electrode 3 is integrally disposed across the first region 10, the second region 20, and the third region 30. The second electrode 3 may be disposed separately in each region, but in this case, it is desirable that each of the separately disposed electrodes be connected to the same power source.

[0061] The semiconductor laser 301 has a first reflective region 361, a second reflective region 362, and a phase shift region 363, similar to the first embodiment. The first reflective region 361 is disposed in the first region 10 and includes a first diffraction grating structure 312A of the first diffraction grating layer 311 and a second diffraction grating structure 322A of the second diffraction grating layer 321. Here, the first diffraction grating structure 312A and the second diffraction grating structure 322A are disposed so that their periods and center positions in the first direction D1 are identical, similar to the first embodiment. The second reflective region 362 is disposed in the second region 20 and includes a first non-diffraction grating structure 312B of the first diffraction grating layer 311 and a second diffraction grating structure 322A of the second diffraction grating layer 321. The phase shift region 363 includes a first non-diffraction grating structure 312B of the first diffraction grating layer 311 and a second non-diffraction grating structure 322B of the second diffraction grating layer 321. When the second diffraction grating structure 322A is disposed in the third region 363, that region is included in the first reflection region 361 or the second reflection region 362.

[0062] As in the first embodiment, the period of the second diffraction grating structure 322A in the first reflection region 361 is the same as the period of the second diffraction grating structure 322A in the second reflection region 62. Furthermore, the phases of the diffraction grating structures 322A, taking into account the optical path lengths, are shifted by π. The diffraction grating structure in the first reflection region 361 is a two-stage structure, while the diffraction grating structure in the second reflection region 362 is a single-stage structure. Therefore, the coupling coefficient κ of the first reflection region 61 is larger. Furthermore, κ1L1 of the first reflection region 361 is larger than κ2L2 of the second reflection region 362. Therefore, as in the first embodiment, the light intensity output from the front end facet 40 is larger than the light intensity output from the rear end facet 50.

[0063] In this embodiment, the first diffraction grating layer 311 in the first region 10 has a first refractive index region 311A ​​and a second refractive index region 311B. On the other hand, the first diffraction grating layer 311 in the second region 20 has only the first refractive index region 311A. Here, the refractive index of the first refractive index region 311A ​​is larger than that of the second refractive index region 311B. Therefore, when comparing only the first diffraction grating layer 311, the effective refractive index of the second region 20 is larger than that of the first region 10. Furthermore, the second diffraction grating layers 321 in the first region 10 and the second region 20 have the same second diffraction grating structure 322A. Here, when comparing only the second diffraction grating layer 321, the effective refractive index of the first region 10 is the same as that of the second region 20. The other semiconductor multilayer structures of the first region 10 and the second region 20 are identical. As a result, for the same mesa width, the effective refractive index of the first region 10 is smaller than that of the second region 20. As in the first embodiment, in order to make the periods of the diffraction grating structures of the first reflection region 361 and the second reflection region 362 the same, it is necessary to make the effective refractive indexes of the first region 10 and the second region 20 approximately the same. Therefore, in this embodiment, the first mesa width W1 of the first region 10 is wider than the second mesa width W2 of the second region 20.

[0064] As described above, the mesa width is adjusted in accordance with the diffraction grating structures included in the first reflecting region 361 and the second reflecting region 362, and high wavelength uniformity is achieved while maintaining the same period of the diffraction grating structure.

[0065] The present invention is not limited to the above-described embodiments and various modifications are possible. For example, the configurations described in the embodiments can be replaced with substantially the same configurations, configurations that achieve the same effects, or configurations that can achieve the same objectives. Furthermore, the present invention is not limited to buried semiconductor lasers, and can also be applied to ridge-type semiconductor lasers. The semiconductor laser may be a CW light source or a directly modulated laser. Furthermore, while the first to third embodiments illustrate examples in which two diffraction grating layers are included, the diffraction grating layer may be three or more layers. Even in the case of three or more layers, the normalized coupling coefficient of the first region may be greater than the normalized coupling coefficient of the second region, thereby increasing the optical output intensity from the front end facet.

[0066] The present invention provides a semiconductor laser with a mesa structure that exhibits high output power, high wavelength uniformity, and excellent manufacturability. This is achieved by arranging two diffraction grating layers, making the periods of the diffraction grating structures in the first reflection region included in the first area and the second reflection region included in the second area identical, and by differentiating the mesa widths of the first reflection region and the second reflection region, making the effective refractive indexes of the two regions approximately identical. The normalized coupling coefficient of the first reflection region is greater than the normalized coupling coefficient of the second reflection region. The phases of the diffraction grating structures, taking into account the optical path lengths of the first reflection region and the second reflection region, are shifted by π. The first reflection region has a second diffraction grating structure included in a second diffraction grating layer disposed farther from the active layer. The second reflection region has either the first diffraction grating structure or the second diffraction grating structure included in a first diffraction grating layer disposed closer to the active layer. For example, the first reflection region has a two-stage diffraction grating structure consisting of the first diffraction grating structure and the second diffraction grating structure, while the second reflection region has a single-stage diffraction grating structure consisting of only the first diffraction grating structure. In another example, the first reflective region includes a first non-diffraction grating structure that does not reflect light contained in the first diffraction grating layer and a second diffraction grating structure, and the second reflective region includes only the first diffraction grating structure. In these two examples, the second diffraction grating layer is thicker in the stacking direction than the first diffraction grating layer. The mesa width of the first region including the first reflective region is narrower than the mesa width of the second region including the second reflective region. In a different example, the first reflective region includes a two-stage diffraction grating structure consisting of a first diffraction grating structure and a second diffraction grating structure, and the second reflective region includes a first non-diffraction grating structure and a second diffraction grating structure. In this example, the second diffraction grating layer is thinner in the stacking direction than the first diffraction grating layer. The mesa width of the first region including the first reflective region is wider than the mesa width of the second region including the second reflective region. A phase shift region is also provided between the first reflective region and the second reflective region. The phase shift region may include a phase shift portion or may not include a diffraction grating structure.

[0067] [Note] (1) A semiconductor laser according to one aspect of the present disclosure includes: A substrate; an active layer disposed on the substrate; a cladding layer including: a first diffraction grating layer disposed on the active layer, at least a portion of which has a first diffraction grating structure; and a second diffraction grating layer disposed above at least a portion of the first diffraction grating layer and spaced apart, at least a portion of which has a second diffraction grating structure; an electrode disposed on the cladding layer; the active layer and the cladding layer form a mesa structure, the mesa structure has a first reflecting region and a second reflecting region that form a resonator in an extending direction of the mesa structure; the second diffraction grating structure is formed in the first reflection area, the second reflection region is formed with either the first diffraction grating structure or the second diffraction grating structure, the first diffraction grating structure and the second diffraction grating structure are formed such that a normalized coupling coefficient of the first reflection region is greater than a normalized coupling coefficient of the second reflection region; The mesa structures of the first and second reflecting regions have different mesa widths depending on the refractive indexes of the semiconductor laser. (2) The semiconductor laser according to (1), The semiconductor laser further includes the first diffraction grating structure formed in the first reflection region. (3) The semiconductor laser according to (1), the second grating layer is thicker than the first grating layer; The semiconductor laser has the first diffraction grating structure formed in the second reflection region. (4) The semiconductor laser according to (1), A semiconductor laser, wherein the period of the second diffraction grating structure included in the first reflection region is the same as the period of the first diffraction grating structure or the second diffraction grating structure included in the second reflection region. (5) 2. The semiconductor laser according to claim 1, The effective refractive index of the first reflective region and the effective refractive index of the second reflective region are substantially the same. (6) The semiconductor laser according to (1), a phase shift region between the first reflective region and the second reflective region; a phase of the second diffraction grating structure included in the first reflection region and a phase of the first diffraction grating structure or the second diffraction grating structure included in the second reflection region are shifted by π. (7) The semiconductor laser according to (6), the first diffraction grating layer includes a first refractive index region and a second refractive index region having a refractive index different from that of the first refractive index region, the first diffraction grating structure is a uniform diffraction grating structure in which the first refractive index regions and the second refractive index regions are alternately and periodically arranged, the second diffraction grating layer includes a third refractive index region and a fourth refractive index region having a refractive index different from that of the third refractive index region, the second diffraction grating structure is a uniform diffraction grating structure in which the third refractive index areas and the fourth refractive index areas are alternately and periodically arranged. (8) The semiconductor laser according to (7), the phase shift region includes a phase shift portion, The phase shift portion has a structure in which the first refractive index region or the second refractive index region is continuously arranged. (9) The semiconductor laser according to (2), the second grating layer has a second non-grating structure; the first diffraction grating structure and the second non-diffraction grating structure are formed in the second reflective area; The second non-diffraction grating structure transmits light reflected by the second diffraction grating structure. (10) The semiconductor laser according to (9), a semiconductor laser, wherein the first diffraction grating structure and the second diffraction grating structure formed in the first reflection region are arranged with the same period and the same phase; (11) The semiconductor laser according to (9), a mesa width of the mesa structure of the first reflecting region being narrower than that of the mesa structure of the second reflecting region; (12) The semiconductor laser according to (3), the first grating layer has a first non-grating structure; the second grating layer has a second non-grating structure; the first non-diffraction grating structure is formed in the first reflective region, the second non-diffraction grating structure is formed in the second reflective region, the first non-grating structure transmits light reflected by the first grating structure; The second non-diffraction grating structure transmits light reflected by the second diffraction grating structure. (13) The semiconductor laser according to (12), the first diffraction grating layer includes a first refractive index region and a second refractive index region having a refractive index different from that of the first refractive index region, the second refractive index region is formed by the cladding layer, the first non-diffraction grating structure of the first reflective region is formed by the first refractive index region, a semiconductor laser, wherein the first diffraction grating structure of the second reflection region is a uniform diffraction grating structure in which the first refractive index regions and the second refractive index regions are alternately and periodically arranged. (14) The semiconductor laser according to (12), The mesa structure of the first reflecting region has a narrower mesa width than the mesa structure of the second reflecting region. (15) The semiconductor laser according to (13), a surface layer of the first non-diffraction grating structure in the first reflection region includes a concavo-convex structure; The semiconductor laser, wherein the period of the concave-convex structure is the same as the period of the second diffraction grating structure. (16) The semiconductor laser according to (2), the first grating layer has a first non-grating structure; the first non-diffraction grating structure and the second diffraction grating structure are formed in the second reflective area; The first non-grating structure transmits light reflected by the first grating structure. (17) The semiconductor laser according to (16), the first diffraction grating layer includes a first refractive index region and a second refractive index region having a refractive index different from that of the first refractive index region, the second refractive index region is formed by the cladding layer, the first non-diffraction grating structure of the second reflective region is formed by the first refractive index region, a first diffraction grating structure in the first reflection region that is a uniform diffraction grating structure in which the first refractive index regions and the second refractive index regions are alternately and periodically arranged; (18) The semiconductor laser according to (16), The first grating layer is thicker than the second grating layer. (19) The semiconductor laser according to (16), The mesa structure of the first reflecting region has a wider mesa width than the mesa structure of the second reflecting region. (20) The semiconductor laser according to (1), A first end surface; A second end surface; a spot size conversion region between the second end face and the second reflection region; the first grating layer has a first non-grating structure; a mesa width of the mesa structure in the spot size conversion region gradually narrows from the second reflection region toward the second end facet, the first non-diffraction grating structure and the second non-diffraction grating structure are formed in the spot size conversion region, the first non-grating structure transmits light reflected by the first grating structure; The second non-diffraction grating structure is a semiconductor laser that transmits light reflected by the second diffraction grating structure. [Explanation of symbols]

[0068] 1. Semiconductor laser 2 1st electrode 3 Second electrode 4 Low reflective coating 5. Substrate 6. First conductivity type optical confinement layer 7 Active layer 8. Second conductivity type optical confinement layer 9 Second conductivity type cladding layer 11 First diffraction grating layer 11A First refractive index region 11B Second refractive index region 12A First diffraction grating structure 12B First non-diffraction grating structure 12C Phase shift section 21 Second diffraction grating layer 21A, 321A Third refractive index region 21B, 321B Fourth refractive index region 22A Second diffraction grating structure 22B Second non-diffraction grating structure 13 Second conductivity type contact layer 14 insulating film 15 Mesa structure 17 Embedding Layer 18 through holes 10 First area 20 Second area 30 Third area 40 Front end 50 Rear end face 61,361 1st reflection area 62,362 2nd reflection area 63,363 Phase Shift Region 201 Semiconductor laser 203 2nd electrode 214 insulating film 215 Mesa Structure 260 Spot size conversion area 301 Semiconductor laser 311 First diffraction grating layer 311A First refractive index region 311B Second refractive index region 312A First grating structure 312B First non-diffraction grating structure 321 Second grating layer 321A Third refractive index region 321B 4th refractive index region 322A Second diffraction grating structure 322B Second non-diffraction grating structure 315 Mesa Structure D1 1st direction D2 2nd direction D3 Third direction W1 First mesa width W2 Second mesa width L1 Length of the first diffraction grating region L2 Length of the second diffraction grating region κ1 Coupling coefficient of the first region κ2 Coupling coefficient of the second region

Claims

1. A substrate; an active layer disposed on the substrate; a cladding layer including: a first diffraction grating layer disposed on the active layer, at least a portion of which has a first diffraction grating structure; and a second diffraction grating layer disposed above at least a portion of the first diffraction grating layer and spaced apart, at least a portion of which has a second diffraction grating structure; an electrode disposed on the cladding layer; the active layer and the cladding layer form a mesa structure, the mesa structure has a first reflecting region and a second reflecting region that constitute a resonator in an extending direction of the mesa structure; the second diffraction grating structure is formed in the first reflection area, the second reflection region is formed with either the first diffraction grating structure or the second diffraction grating structure, the first diffraction grating structure and the second diffraction grating structure are formed such that a normalized coupling coefficient of the first reflection region is greater than a normalized coupling coefficient of the second reflection region; The mesa structures of the first and second reflecting regions have different mesa widths depending on the refractive indexes of the respective regions.

2. 2. The semiconductor laser according to claim 1, The semiconductor laser further includes the first diffraction grating structure formed in the first reflection region.

3. 2. The semiconductor laser according to claim 1, the second grating layer is thicker than the first grating layer; The semiconductor laser, wherein the first diffraction grating structure is formed in the second reflection region.

4. 2. The semiconductor laser according to claim 1, a period of the second diffraction grating structure included in the first reflection region and a period of the first diffraction grating structure or the second diffraction grating structure included in the second reflection region are the same.

5. 2. The semiconductor laser according to claim 1, The effective refractive index of the first reflective region and the effective refractive index of the second reflective region are substantially the same.

6. 2. The semiconductor laser according to claim 1, a phase shift region between the first and second reflecting regions; a phase of the second diffraction grating structure included in the first reflection region and a phase of the first diffraction grating structure or the second diffraction grating structure included in the second reflection region are shifted by π.

7. 7. The semiconductor laser according to claim 6, the first diffraction grating layer includes a first refractive index region and a second refractive index region having a refractive index different from that of the first refractive index region, the first diffraction grating structure is a uniform diffraction grating structure in which the first refractive index regions and the second refractive index regions are alternately and periodically arranged, the second diffraction grating layer includes a third refractive index region and a fourth refractive index region having a refractive index different from that of the third refractive index region, the second diffraction grating structure is a uniform diffraction grating structure in which the third refractive index areas and the fourth refractive index areas are alternately and periodically arranged.

8. 8. The semiconductor laser according to claim 7, the phase shift region includes a phase shift portion, The phase shift portion has a structure in which the first refractive index region or the second refractive index region is continuously arranged.

9. 3. The semiconductor laser according to claim 2, the second grating layer has a second non-grating structure; the first diffraction grating structure and the second non-diffraction grating structure are formed in the second reflective area, The second non-diffraction grating structure transmits light reflected by the second diffraction grating structure.

10. 10. The semiconductor laser according to claim 9, a semiconductor laser, wherein the first diffraction grating structure and the second diffraction grating structure formed in the first reflection region are arranged with the same period and the same phase;

11. 10. The semiconductor laser according to claim 9, a mesa width of the mesa structure of the first reflecting region being narrower than that of the mesa structure of the second reflecting region;

12. 4. The semiconductor laser according to claim 3, the first grating layer has a first non-grating structure; the second grating layer has a second non-grating structure; the first non-diffraction grating structure is formed in the first reflection area, the second non-diffraction grating structure is formed in the second reflective region, the first non-grating structure transmits light reflected by the first grating structure; The second non-diffraction grating structure transmits light reflected by the second diffraction grating structure.

13. 13. The semiconductor laser according to claim 12, the first diffraction grating layer includes a first refractive index region and a second refractive index region having a refractive index different from that of the first refractive index region, the second refractive index region is formed by the cladding layer, the first non-diffraction grating structure of the first reflection region is formed by the first refractive index region, the first diffraction grating structure of the second reflection region is a uniform diffraction grating structure in which the first refractive index regions and the second refractive index regions are alternately and periodically arranged.

14. 13. The semiconductor laser according to claim 12, a mesa structure of the first reflecting region having a narrower mesa width than the mesa structure of the second reflecting region;

15. 14. The semiconductor laser according to claim 13, a surface layer of the first non-diffraction grating structure in the first reflection region includes an uneven structure; A semiconductor laser, wherein the period of the concave-convex structure is the same as the period of the second diffraction grating structure.

16. 3. The semiconductor laser according to claim 2, the first grating layer has a first non-grating structure; the first non-diffraction grating structure and the second diffraction grating structure are formed in the second reflective area; The first non-grating structure transmits light reflected by the first grating structure.

17. 17. The semiconductor laser according to claim 16, the first diffraction grating layer includes a first refractive index region and a second refractive index region having a refractive index different from that of the first refractive index region, the second refractive index region is formed by the cladding layer, the first non-diffraction grating structure of the second reflective region is formed by the first refractive index region, the first diffraction grating structure of the first reflection region is a uniform diffraction grating structure in which the first refractive index regions and the second refractive index regions are alternately and periodically arranged.

18. 17. The semiconductor laser according to claim 16, The first grating layer is thicker than the second grating layer.

19. 17. The semiconductor laser according to claim 16, The mesa structure of the first reflecting region has a wider mesa width than the mesa structure of the second reflecting region.

20. 2. The semiconductor laser according to claim 1, A first end surface; A second end surface; a spot size conversion region between the second end face and the second reflection region; the first grating layer has a first non-grating structure; a mesa width of the mesa structure in the spot size conversion region gradually narrows from the second reflection region toward the second end face, the first non-diffraction grating structure and the second non-diffraction grating structure are formed in the spot size conversion region, the first non-grating structure transmits light reflected by the first grating structure; The second non-diffraction grating structure transmits light reflected by the second diffraction grating structure.

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