Semiconductor laser

Abstract

To provide a semiconductor laser that achieve both of high optical output characteristics and reduction in occurrence of transverse higher modes.SOLUTION: An semiconductor laser includes a substrate, an active layer formed on the substrate, a cladding layer formed on the active layer, and a diffraction grating layer including a first refractive index region and a second refractive index region. A part of the cladding layer forms a mesa structure, and the mesa structure includes a first region 10 having a first width and a second region 20 having a second width larger than the first width. The first region includes a diffraction grating region in which the first refractive index region and the second refractive index region that reflect light of the Bragg wavelength are alternately arranged with the same period. The second region includes a diffraction grating region in which the first refractive index region and the second refractive index region that reflect light of the Bragg wavelength are alternately arranged, and a non-diffraction grating region that transmits light of the Bragg wavelength. The diffraction grating region of the first region and the diffraction grating region of the second region form a resonator, and the normalized coupling coefficient of the first region is larger than the normalized coupling coefficient of the second region.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] Semiconductor lasers are widely used as light sources for 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. A structure in which a phase shift section is provided on the diffraction grating to improve characteristics is also known. Stable single-wavelength operation can be obtained by forming a non-reflective film (low-reflection film) on both end faces of the semiconductor laser and arranging a λ / 4 shift section on the diffraction grating (Patent Document 1). Patent Document 1 discloses that the output from one end facet can be increased by changing the reflectance of Bragg-reflected light before and after the phase shift section.

[0003] In addition, a structure in which the diffraction grating is removed at a certain interval in order to adjust the coupling coefficient κ is known. (Patent Documents 2, 3, 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 61-47685 [Patent Document 2] Patent Publication No. 2004-128372 [Patent Document 3] Patent Publication No. 2004-259924 [Patent Document 4] Patent Publication No. 2011-119434 Summary of the Invention [Problem to be solved by the invention]

[0005] Semiconductor lasers with a mesa structure are known for the purpose of concentrating the current injected for driving in one area. The area into which the current is injected can be limited by narrowing the width in the direction perpendicular to the direction in which the mesa structure extends (hereinafter referred to as the mesa width). Increasing the mesa width is effective for increasing the optical output. When the mesa width is wide, the width of the active layer that is effectively driven becomes wider, and high optical output can be obtained.

[0006] The mesa width also plays a role in determining the mode of light propagating through the waveguide. When the mesa width is narrow, only the fundamental transverse mode propagates, but as the mesa width becomes wider, higher-order transverse modes also propagate. Optical output that includes higher-order transverse modes is undesirable as a light source for optical communications. As described above, when the mesa width is widened to increase the optical output, the occurrence of higher-order transverse modes becomes a problem. The occurrence of higher-order transverse modes leads to a decrease in the continuity of the current and optical output characteristics, the occurrence of so-called kinks, and is undesirable as a light source for communications.

[0007] An object of the present invention is to provide a semiconductor laser that achieves both high optical output characteristics and reduced generation of higher-order transverse modes. [Means for solving the problem]

[0008] a first diffraction grating region for reflecting light of a Bragg wavelength and a second diffraction grating region for alternately arranging the first refractive index region and the second refractive index region at the same period; and a non-diffraction grating region for transmitting light of the Bragg wavelength. The first diffraction grating region and the second diffraction grating region constitute a resonator. A semiconductor laser according to a first aspect of the present application comprises: a substrate; an active layer formed in a mesa structure on the substrate; a cladding layer formed on the active layer in the mesa structure; a diffraction grating layer formed in the cladding layer and including a plurality of first refractive index regions and a plurality of second refractive index regions; and a low-reflection end face coating film provided on both end faces in an extension direction of the mesa structure. The mesa structure comprises a first region having a first width in a plan view and a second region having a second width wider than the first width. The first region comprises a first diffraction grating region in which the first refractive index regions and the second refractive index regions are alternately arranged with the same period, for reflecting light of a Bragg wavelength. The second region comprises a second diffraction grating region in which the first refractive index regions and the second refractive index regions are alternately arranged with the same period, and a non-diffraction grating region for transmitting light of the Bragg wavelength. 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. [Brief description of the drawings]

[0009] [Figure 1] 1 is a top view of a semiconductor laser according to a first embodiment. [Diagram 2] 2 is a schematic cross-sectional view taken along line II-II of the semiconductor laser shown in FIG. [Diagram 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. [Diagram 5] 1 is a top view of a semiconductor laser according to a first embodiment. [Figure 6] FIG. 4 is a top view of a semiconductor laser according to a modified example of the first embodiment. [Figure 7] FIG. 4 is a top view of a semiconductor laser according to a second embodiment. [Figure 8] 8 is a schematic cross-sectional view taken along line VIII-VIII of the semiconductor laser shown in FIG. 7. [Figure 9] FIG. 11 is a top view of a semiconductor laser according to a third embodiment. [Figure 10] FIG. 11 is a top view of a semiconductor laser according to a fourth embodiment. [Figure 11] FIG. 11 is a schematic cross-sectional view of a semiconductor laser according to a fourth embodiment. [Figure 12] FIG. 13 is a top view of a semiconductor laser according to a modified example of the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In all the drawings for explaining the embodiment, the same reference numerals are used for components having the same functions, and the repeated explanations are omitted. Note that the drawings shown below are merely for explaining the embodiment, and the size of the drawings does not necessarily correspond to the scale described in the embodiment.

[0011] [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. FIG. 5 is a top view of the semiconductor laser 1, and is a diagram for explaining the position of each region included inside. The semiconductor laser 1 has 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 a front end face 40 (the end face on the left side in FIG. 1). A low-reflection end face coating film 4 is formed on both end faces in the direction in which a mesa structure described later extends, that is, on the front end face 40 and the rear end face 50 (the end face on the right side in FIG. 1). The reflectance of the low-reflection end face coating film 4 is 1% or less.

[0012] In the semiconductor laser 1, semiconductor layers are laminated on a substrate 5 of a first conductivity type in the order of a first conductivity type light confinement layer (SCH layer) 6, an active layer 7, a second conductivity type light confinement layer 8 (SCH layer), a second conductivity type cladding layer 9, and a second conductivity type contact layer 13. A diffraction grating layer 11 is formed in the second conductivity type cladding layer 9. The semiconductor laser 1 is a DFB laser. 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 they may be reversed. The semiconductor layer including the above layers has a mesa structure 15. For example, the active layer 7 is formed in a mesa structure on the substrate 5, and the cladding layer 9 is formed on the active layer 7 in the mesa structure 15. The mesa structure 15 extends in a direction (first direction D1) in which light is extracted. The bottom of mesa structure 15 is part of substrate 5. Both sides of mesa structure 15 are covered with semi-insulating semiconductor burying layer 17. Buried 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 top of mesa structure 15 and burying layer 17.

[0013] The semiconductor laser 1 has an insulating film 14 on its surface. The insulating film 14 covers the 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 upper portion of the mesa structure 15. The second electrode 3 and the second conductive type contact layer 13 are connected via the through hole 18, and an electric signal is applied (current is injected) to the semiconductor layer having the mesa structure 15. Here, the through hole 18 is provided along the first direction D1. In addition, in the second direction D2 perpendicular to the first direction D1, the width of the through hole 18 is wider than the width of the mesa structure 15. However, the two may have the same width.

[0014] The diffraction grating layer 11 includes a plurality of first refractive index regions and a plurality of second refractive index regions. Specifically, the diffraction grating layer 11 is of a floating type, and is composed of a region of a first refractive index different from the refractive index of the second conductive type cladding layer 9 in a cross-sectional view, and a region of a second refractive index in which the second conductive type cladding 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 are alternately disposed. Here, the first refractive index is higher than the second refractive index. However, the relationship of the refractive indexes may be reversed.

[0015] The mesa structure 15 has regions with different widths in the second direction D2 perpendicular to the first direction D1 in the direction in which the first direction D1 extends. Hereinafter, the width of the mesa structure 15 in the second direction D2 is referred to as the mesa width. The semiconductor laser 1 includes a first region 10 with a mesa width W1 and a second region 20 with a mesa width W2. Here, W2 is wider than W1. That is, the mesa structure includes a first region with a first width in a plan view and a second region with a second width wider than the first width. 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 W1 to W2. 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 individually in each region, but in this case, it is preferable that each electrode disposed individually is connected to the same power source.

[0016] FIG. 5 is a top view of the semiconductor laser 1, and is a diagram for explaining the position of each region included in the diffraction grating layer 11. For the sake of explanation, some elements are not shown. In the diffraction grating layer 11, a region in which the first refractive index region 11A and the second refractive index region 11B are alternately arranged at the same period is called a diffraction grating region 12A. On the other hand, in the diffraction grating layer 11, a region in which only the first refractive index region 11A or only the second refractive index region 11B is arranged is called a non-diffraction grating region 12B. In this embodiment, the non-diffraction grating region 12B is composed of the second refractive index region 11B. In the diffraction grating layer 11 in the first region 10, one diffraction grating region 12A is arranged. Note that the diffraction grating region 12A in this embodiment has a uniform diffraction grating structure in which the first refractive index region 11A and the second refractive index region 11B are alternately arranged at the same period in the first direction D1. That is, the first region 10 has a diffraction grating region in which the first refractive index region 11A and the second refractive index region 11B for reflecting light of the Bragg wavelength are alternately arranged at the same period. The Bragg wavelength of this diffraction grating corresponds to the 1.3 μm band. It is also acceptable to use other Bragg wavelengths such as the 1.55 μm band. The first region 10 has a non-diffraction grating region 12B closer to the connection point with the third region 30. It is also acceptable for the first region 10 not to include the non-diffraction grating region 12B, and for the diffraction grating region 12A to be arranged in the entire region of the first region 10.

[0017] In the second region 20, the diffraction grating regions 12A and the non-diffraction grating regions 12B are alternately arranged. Here, the diffraction grating regions 12A and the non-diffraction grating regions 12B are arranged so as to reflect the light of the Bragg wavelength transmitted from the first region 10 into the second region 20. In order to strongly reflect the light of a specific wavelength (Bragg wavelength), it is necessary to adjust the period (diffraction grating period) of the first refractive index region 11A and the second refractive index region 11B. Furthermore, the relationship between the diffraction grating period and the Bragg wavelength depends on the effective refractive index. In this embodiment, the second region 20 has a larger mesa width than the first region 10, so that the effective refractive index is larger. However, at the same time, there are many regions (non-diffraction grating regions 12B) without a diffraction grating structure when viewed as a whole of the second region 20, so that the effective refractive index is smaller by that amount. The average effective refractive index of the whole second region 20 is determined by both the increase in the effective refractive index due to the wider mesa width and the decrease in the effective refractive index due to the absence of a diffraction grating structure. The diffraction grating region 12A and the non-diffraction grating region 12B arranged in the second region 20 are determined in consideration of the difference in the average effective refractive index between the first region 10 and the second region 20. Here, the diffraction grating period of the diffraction grating region 12A in the second region 20 may be different from or the same as the diffraction grating period of the diffraction grating region 12A in the first region 10. When the diffraction grating period is the same, the length in the first direction D1 of each of the multiple diffraction grating regions 12A and the multiple non-diffraction grating regions 12B and the interval between each of them may be adjusted so that the average diffraction grating period of the entire second region 20 reflects light of a desired Bragg wavelength. Hereinafter, when particularly distinguishing between them, 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. 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 from each other. For example, under conditions that result in reflection at a desired Bragg wavelength, the lengths of each are preferably different to suppress higher order post-diffraction scattering for the Bragg wavelength.

[0018] In order to obtain high wavelength uniformity, it is desirable to configure a resonator with the first region 10 and the second region 20. In order to configure a resonator with the first region 10 and the second region 20, the phases of the diffraction grating regions need to be shifted by π. Here, the phase of the diffraction grating region is the phase of the diffraction grating structure. For example, when the effective refractive indexes of the first region 10 and the second region 20 are the same, the phase of the second diffraction grating region (diffraction grating region 12A of the second region 20) may simply be shifted by π with respect to the phase of the first diffraction grating region (diffraction grating region 12A of the first region 10). However, in this embodiment, the effective refractive indexes 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 shifting the phase of the first diffraction grating region by π. In this embodiment, the phase of the first diffraction grating region and the phase of the second diffraction grating region are not shifted by π, but the resonator is configured with the first region 10 and the second region 20. Furthermore, in this embodiment, the low-reflection end face coating film 4 is formed on both end faces, which provides very high wavelength uniformity. The second electrode 3 is disposed across the first region 10 and the second region 20, and the change in the effective refractive index due to the amount of injected current is substantially the same, achieving high wavelength uniformity over a wide range of operating conditions.

[0019] In this embodiment, the normalized coupling coefficient κ1L1 of the first region 10 is larger than the normalized coupling coefficient κ2L2 of the second region 20. Here, the coupling coefficient κ1 of the first region 10 and the coupling coefficient κ2 of the second region 20 may not be the same due to differences in mesa widths, etc. However, the difference is small, and the difference in the normalized coupling coefficients of 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 region. Here, the length L1 of the first region 10 is the length of the first diffraction grating region. That is, the length L1 of the first region 10 is not the length of the region where the mesa width is W1, but the length of the region in which the diffraction grating region 12A is arranged. The length L2 of the second region 20 is the total length of the multiple second diffraction grating regions. Since the non-diffraction grating region 12B does not reflect light of the Bragg wavelength, the coupling coefficient κ can be considered to be virtually 0. Therefore, the length L2 contributing to the normalized coupling coefficient κL is the total length of the region in which the multiple 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, the optical output intensity output from the front end face 40 and the rear end face 50 are the same. In the present invention, the total length of the region in which the multiple second diffraction grating regions are arranged is made shorter than the length of the region in which the first diffraction grating region is arranged, thereby increasing the intensity of the light output from the front end face 40. Note that the terms front and rear are merely names for convenience, and the end face with the larger optical output is simply called the front end face. In general optical communication, a larger optical intensity is preferable, and the light from the front end face is used for communication. The total length of the multiple second diffraction grating regions in the first direction D1 is shorter than the total length of the multiple 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.

[0020] The third region 30 does not include the diffraction grating region 12A. However, the third region 30 may include a diffraction grating structure similar to that of the diffraction grating region 12A arranged in the first region 10 near the connection with the first region 10. Similarly, the third region 30 may include a diffraction grating structure similar to that of the diffraction grating region 12A arranged in the second region 20 near the connection with the second region 20. When the diffraction grating region 12A is arranged over the entire third region 30, it is desirable to adjust the widths of the first refractive index region 11A and the second refractive index region 11B and the distance between them in accordance with the change in the effective refractive index accompanying the change in the mesa width.

[0021] The mesa width W1 of the first region 10 is set to a width that does not generate high-order transverse modes for light of the Bragg wavelength. In other words, W1 is set to a width that is equal to or smaller than the cutoff width. For example, W1 is 2 μm or less. The conditions for generating high-order transverse modes may change depending on the driving conditions. In order to obtain a stable high-order transverse mode suppression effect, it is preferable that W1 is 1.5 times the Bragg wavelength or less. For example, if the Bragg wavelength is 1.3 μm, it is preferable that W1 is 1.95 μm or less.

[0022] The mesa width W2 of the second region 20 is wider than W1. The mesa width W2 is preferably 1.2 times or more, more preferably 1.5 times or more, of W1. W2 may be equal to or larger than the cutoff width. In other words, the second region 20 may have a width that allows the transverse higher-order mode of light of the Bragg wavelength to be guided. For example, W2 is 3 μm or more. The wider the mesa width, the greater the total amount of light generated, and therefore the optical output intensity of the semiconductor laser 1 can be increased. It is desirable that the mesa width of the third region 30 gradually widens from the first region 10 toward the second region 20 so that the first region 10 and the second region 20 are smoothly connected. That is, the mesa structure of the third region 30 preferably has a tapered shape inclined with respect to the first direction D1 when viewed from above, but is not limited thereto. For example, the boundary between the mesa structure of the third region 30 and the buried layer 17 may change linearly or curved when viewed from above.

[0023] In the semiconductor laser 1 according to the present invention, even if the mesa width W2 of the second region 20 is equal to or greater than the cutoff width, the light output from the end face does not include a transverse high-order mode, and has excellent optical output characteristics in which kinks caused by the transverse high-order mode do not occur. This is because the mesa width W1 of the first region 10 is equal to or less than the cutoff width. The laser light resonating inside the semiconductor laser 1 is determined by the entire structure including the first region 10, the second region 20, and the third region 30. Even if light in a transverse high-order mode occurs in the second region 20, the light cannot be guided in the first region 10 and is therefore not reflected by the first region 10. Therefore, the light cannot exist as a strong intensity in the semiconductor laser 1. On the other hand, light in the transverse fundamental mode that can be guided in the first region 10 can also be guided in the second region 20. Therefore, the light in the transverse fundamental mode is reflected between the first region 10 and the second region 20 and constructively interacts with each other, becoming a very strong light (so-called laser light) and being output from the end face. Here, in order to fully obtain this effect, it is necessary to sufficiently suppress the generation of light in the higher order transverse mode in the first region 10. To achieve this, the normalized coupling coefficient κ1L1 of the first region 10 needs to be larger than the normalized coupling coefficient κ2L2 of the second region 20. For example, it is preferable that κ1L1 is 60% or more (i.e., κ2L2 is 40% or less) of the normalized coupling coefficient of the entire semiconductor laser 1. If κ1L1 is increased by further lengthening L1, the ratio of the second region 20 that increases the optical output to the entire element will decrease. In this case, the effect of increasing the output will not be fully obtained, so κ1L1 is preferably 80% or less. If higher output characteristics are required, κ1L1 is preferably 70% or less.

[0024] Here, 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 as a whole. In this embodiment, it is also an object to make the optical output intensity output from the front end face 40 greater 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 located on the rear side. For example, it is preferable that the first diffraction grating region is disposed in a region from the rear end of the cavity (here, the rear end face 50) to 40% or less of the cavity length. That is, it is preferable that the length of the first diffraction grating region in the direction in which the mesa structure extends is 40% or less of the entire length of the diffraction grating layer. More preferably, it is 30% or less. In other words, it is preferable that the end Gt of the first diffraction grating region on the third region 30 side is disposed at a position 40% or less or 30% or less from the rear side of the cavity length. It is preferable that the position of Gt is small, but if it is too small, κ1L1 becomes too small. For example, if κ1L1 is smaller than 1, the oscillation threshold becomes large, which is undesirable from the viewpoint of power consumption. Therefore, it is necessary to set κ1L1 to 1 or more, and more preferably to 1.5 or more.

[0025] With the above configuration, the semiconductor laser 1 has a high wavelength uniformity because it has the first region 10 and the second region 20 in which the diffraction grating phase is shifted by π in consideration of the optical path length, and the low-reflection end face coating film 4 is formed on both end faces. Furthermore, since the normalized coupling coefficient κ1L1 of the first region 10 is larger than the normalized coupling coefficient κ2L2 of the second region 20, the optical output intensity from the front end face 40 can be increased. Furthermore, since the mesa width W2 of the second region 20 is wider than the mesa width W1 of the first region 10, the semiconductor laser 1 according to this embodiment has high output characteristics compared to a structure in which the mesa widths of the second region 20 and the first region 10 are the same. And, since the mesa width W1 of the first region 10 is equal to or less than the cutoff width, it is possible to realize excellent optical output characteristics in which no transverse high-order mode occurs, that is, no kink occurs.

[0026] [Variations] FIG. 6 is a top view of the semiconductor laser 1 according to a modification of the first embodiment. FIG. 6 is a view corresponding to FIG. 5. The second region 20 of the semiconductor laser 1 according to the modification includes one diffraction grating region 12A and a non-diffraction grating region 12B. In this modification, the length that determines the normalized coupling coefficient of the second region 20 is the length of one diffraction grating region 12A in the first direction D1. As in the first embodiment, the normalized coupling coefficient κ2L2 of the second region 20 is smaller than the normalized coupling coefficient κ1L1 of the first region 10. As described above, it is not necessary to arrange a plurality of diffraction grating regions 12A and non-diffraction grating regions 12B in the second region 20. However, it is preferable that the length from the tip Gt of the first diffraction grating region to the tip Gt2 of the second diffraction grating region on the first region 10 side is half or less of the length of the first diffraction grating region. The region between Gt and Gt2 is non-grating region 12B, and oscillation of a wavelength different from the desired Bragg wavelength may occur if the length of this non-grating region 12B is greater than half the length of first region 10. When multiple non-grating regions 12B are included as in the first embodiment, it is sufficient that the lengths of all non-grating regions 12B including third region 30 are each less than half the length of the first grating region, and the total length of the multiple non-grating regions 12B may be longer than the first grating region.

[0027] [Second embodiment] Fig. 7 is a top view of a semiconductor laser 201 according to the second embodiment. Fig. 8 is a schematic cross-sectional view taken along line VIII-VIII in Fig. 7. The second embodiment differs from the first embodiment in that the semiconductor laser 201 includes a spot size conversion portion 60 between the second region 20 and the front end face 40.

[0028] The spot size conversion section 60 has a semiconductor layer formed on the substrate 5 in the following order: a first conductive type SCH layer 6, an active layer 7, a second conductive type SCH layer 8, and a second conductive type cladding layer 9. An insulating film 14 is disposed on the second conductive type cladding layer 9. A contact layer 13 may be disposed between the cladding layer 9 and the insulating film 14. The spot size conversion section 60 does not include a diffraction grating region 12A. The second conductive type cladding layer 9, that is, the second refractive index region 11B, is disposed in a region at the same height as the diffraction grating layer 11. The spot size conversion section 60 is also a part of the mesa structure 215. The mesa width at the boundary between the spot size conversion section 60 and the second region 20 is W2, and the mesa width of the spot size conversion section 60 gradually becomes narrower than W2 toward the front end face 40. The narrowest point is the point where the spot size conversion section 60 contacts the front end face 40. The mesa width of the spot size conversion section 60 on the front end face 40 side is set so as to obtain a desired optical output shape. The mesa width is, for example, smaller than W1. Although a part of the second electrode 203 reaches the spot size conversion section 60, this is not limiting, and the second electrode 203 may be disposed over the entire spot size conversion section 60. The contact layer 13 and the second electrode 203 may be disposed in the spot size conversion section 60.

[0029] In this embodiment, the structures of the first region 10 and the second region 20, particularly the structure of the diffraction grating layer 11, are the same as those in the first embodiment, and high output, high wavelength uniformity, and kink-free characteristics are achieved.

[0030] [Third embodiment] Fig. 9 is a top view of a semiconductor laser 301 according to the third embodiment. Except for the shape of the through-hole 318, the structure is the same as that of the semiconductor laser 1 of the first embodiment. For the sake of explanation, in Fig. 9, the position of the mesa structure 315 is indicated by a dashed line, and the position of the through-hole 318 is indicated by a two-dot chain line. The second electrode 3 is not shown.

[0031] Through holes 318 are openings provided in insulating film 314. In first region 310, through holes 318 are not disposed on a portion of the upper surface of mesa structure 315. In other words, through holes 318 are disposed discretely in first region 310. That is, in first region 310, insulating film 314 is disposed on a portion of the upper surface of mesa structure 315. In second region 320 and third region 330, through holes 318 are provided throughout. Note that second electrode 3 covers entire through holes 318.

[0032] As described in the first embodiment, the normalized coupling coefficient κ1L1 of the first region 310 is larger than the normalized coupling coefficient κ2L2 of the second region 320. The boundary between the first direction D1 of the first region 310 and the third region 330 is disposed on the rear side (right side in FIG. 9) of the center of the semiconductor laser 301. Therefore, the photon density of the first region 310 is smaller than the photon density of the second region 320. Since the consumption of carriers by stimulated emission increases as the photon density increases, when the injection current densities of the first region 310 and the second region 320 are equal, the carrier density of the first region 310 is larger than the carrier density of the second region 320. An increase in carrier density is a factor that deteriorates the characteristics of the semiconductor laser, for example, the relative noise intensity characteristics. The carrier density depends on the amount of injected current. Therefore, in this embodiment, the increase in carrier density is reduced by limiting the region into which the current is injected in the first region 310. In order to limit the region into which current is injected, the through holes 318 in the first region 310 are not disposed on the entire upper surface of the mesa structure 315, but are disposed on a portion of the upper surface. In other words, the upper surface of the mesa structure 315 in the first region 310 has openings (through holes 318) and non-openings (insulating film 314) alternately disposed along the first direction D1. Since current is not injected into the region where the non-openings (insulating film 314) are disposed, the carrier density in the entire first region 310 can be reduced.

[0033] The ratio of the openings to the entire first region 310 is preferably 60% or less. More preferably, the through holes 318 are arranged in the first region 310 in an area that is 20% to 50% of the entire first region 310 in plan view.

[0034] [Fourth embodiment] FIG. 10 is a top view of a semiconductor laser 401 according to the fourth embodiment. FIG. 10 corresponds to FIG. 5 of the first embodiment, and for the purpose of explanation, the positions of the regions included in the diffraction grating layer 511 are shown. FIG. 11 corresponds to FIG. 2 of the first embodiment, and is a schematic cross-sectional view in the direction in which the mesa structure 415 extends. The difference from the first embodiment is that the first region 410 includes a phase shift portion 470. As shown in FIG. 11, the phase shift portion 470 is a λ / 4 phase shift portion having a structure in which two first refractive index regions 11A are continuously arranged (i.e., twice the length of the other first refractive index regions 11A). In other words, the phase of the diffraction grating structure is shifted by π before and after the phase shift portion 470. The phase shift portion 470 may also have a structure in which two second refractive index regions 11B are continuously arranged. Here, the region of the diffraction grating structure located in the first region 410 on the second region 420 side of the phase shift portion 470 is referred to as the phase shift diffraction grating region 12C. The first region 410 includes one diffraction grating region 12A having a uniform diffraction grating structure, the phase shift portion 470, and the phase shift diffraction grating region 12C. Note that, although the non-diffraction grating region 12B is included between the phase shift diffraction grating region 12C and the third region 430 in FIG. 11, this non-diffraction grating region 12B may be omitted. It can be said that the diffraction grating region 12A and the phase shift diffraction grating region 12C of the first region 410 form a resonator structure with the phase shift portion 470 sandwiched therebetween.

[0035] The phase of the diffraction grating region 12A in the second region 420 is the same as that of the phase shift diffraction grating region 12C. However, this is finely adjusted taking into account the difference in optical path length according to the difference in effective refractive index caused by the difference in width of the mesa structure 415. In other words, the diffraction grating region 12A in the first region 410 and the second region 420 form a resonator, as in the first embodiment.

[0036] In this embodiment, the length L1 of the diffraction grating structure that determines the normalized coupling coefficient κ1L1 of the first region 410 is the length of the first diffraction grating region (the diffraction grating region 12A of the first region 410). Here, the diffraction grating region 12A is between the rear end face 50 and the phase shift section 470. On the other hand, the length L2 of the diffraction grating structure that determines the normalized coupling coefficient κ2L2 of the second region 420 is the total length of the multiple second diffraction grating regions (the diffraction grating regions 12A included in the second region 420). In this embodiment, κ1L1 is also larger than κ2L2. Moreover, the width W1 of the mesa structure 415 of the first region 410 is narrower than the width W2 of the mesa structure 415 of the second region 420. The normalized coupling coefficient of the phase shift diffraction grating region 12C also affects the intensity of the light output from the front end face 40. For this reason, the sum of the normalized coupling coefficient κ2L2 of the phase shift diffraction grating region 12C and the normalized coupling coefficient κ2L2 of the second region 420 needs to be smaller than the normalized coupling coefficient κ1L1 of the first region 410. For example, the length of the phase shift diffraction grating region 12C in the first direction D1 is desirably ⅕ or less of the length of the diffraction grating region 12A of the first region 410.

[0037] [Variations] FIG. 12 is a top view of a semiconductor laser 501 according to a modification of the fifth embodiment. In this modification, the phase shift section 470 is disposed in the third region 430. The phase shift section 470 does not necessarily have to be disposed in the first region 410 as in this modification. In order to increase the light intensity output from the front end face 40, the normalized coupling coefficient on the rear side (the right side in FIG. 12) of the phase shift section 470 must be larger than the normalized coupling coefficient on the front side. The normalized coupling coefficient on the rear side is determined by the length of the diffraction grating region disposed on the rear end face 50 side from the phase shift section 470. Specifically, it is determined by the sum of the length L1 (here, the same as the length of the first region) of the first diffraction grating region (the diffraction grating region 12A of the first region 410) and the length L3 of the region on the first region 410 side from the phase shift section 470 among the diffraction grating regions disposed in the third region 430. On the other hand, the length of the diffraction grating region that determines the normalized coupling coefficient on the front side is the total length of the phase shift diffraction grating region 12C and the multiple diffraction grating regions 12A in the second region 420. However, the length L3 of the diffraction grating region arranged in the third region 430 is shorter than the diffraction grating regions arranged in the first region 410 and the second region 420. For example, L3 is 1 / 2 or less compared to the length (L1) of the first diffraction grating region. Therefore, in this embodiment as well, if the normalized coupling coefficient κ1L1 of the first region 410 is made larger than the normalized coupling coefficient κ2L2 of the second region 420, the optical output intensity from the front end face 40 can be increased.

[0038] As described above, the present invention can achieve high output characteristics and high single wavelength characteristics by forming a resonator with the first region having a narrow mesa width and the second region having a wide mesa width. Furthermore, by making the mesa width W1 of the first region equal to or less than the cutoff width, it is possible to obtain high output characteristics while suppressing the generation of high-order transverse modes. Here, the first region and the second region forming a resonator indicate that the diffraction grating phase taking into account the optical path length is in a π-shifted relationship. For example, in the first embodiment, the region in which the phase is shifted by π is the region between the tip Gt of the first diffraction grating region and the tip of the second diffraction grating region closest to the first region 10. The light transmitted through the first region 10 is reflected by the second region 20 toward the first region 10. Similarly, the light transmitted through the second region 20 is reflected by the first region 10 to the second region 20. In this way, the boundary of reflection becomes a phase shift portion. And it can be said that a resonator is formed in order to reflect each other. As shown in the fourth embodiment, the diffraction grating layer 11 may be provided with a phase shift section 470 in which the first refractive index area 11A or the second refractive index area 11B is continuously arranged.

[0039] The present invention is not limited to the above-mentioned embodiment, and various modifications are possible. For example, the configurations described in the embodiments can be replaced with substantially the same configurations, configurations that provide the same operational effects, or configurations that can achieve the same purpose. The present invention is not limited to buried semiconductor lasers, and can be applied to ridge-type semiconductor lasers. The semiconductor laser may be a CW light source or a directly modulated laser. In the first to fifth embodiments, examples are given in which the mesa structure has two widths, W1 and W2, but the mesa structure may have three or more widths. Specifically, for example, in the first embodiment, a fourth region having a mesa structure having a width W3 wider than W2 and a fifth region provided between the fourth region and the second region may be provided on the front end face 40 (the end face on the left side in FIG. 1) side of the second region 20. Here, the fifth region has a mesa width that gradually changes from W2 to W3, similar to the third region. Even with such a configuration, it is possible to improve the high optical output characteristics while reducing the occurrence of higher-order transverse modes.

[0040] The present invention achieves high output characteristics, high wavelength uniformity, and reduced transverse mode kink in a semiconductor laser having a mesa structure. In an embodiment of the present invention, the mesa structure is composed of a first region having a width W1 and a second region having a width W2 wider than W1, and a resonator is formed by the first diffraction grating region of the first region and the second diffraction grating region of the second region, and the normalized coupling coefficient of the first region is greater than the normalized coupling coefficient of the second region. The phase of the diffraction grating taking into account the optical path lengths of the first region and the second region is shifted by π. 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 of the Bragg wavelength are alternately arranged with the same period. The second region includes a second diffraction grating region in which the first refractive index region and the second refractive index region are alternately arranged with the same period, and a non-diffraction grating region that transmits light of the Bragg wavelength. The non-diffraction grating region is composed of only the first refractive index region or the second refractive index region. The second region includes a plurality of diffraction grating regions and non-diffraction grating regions. However, it is also acceptable to have one of each. The multiple diffraction grating regions and the multiple non-diffraction grating regions may have different lengths. The width W1 of the mesa structure in the first region may be equal to or smaller than the cutoff width for light of the Bragg wavelength. W1 may be equal to or smaller than 1.5 times the Bragg wavelength. The width W2 of the mesa structure in the second region may be equal to or larger than 1.2 times W1, more preferably equal to or larger than 1.5 times W1. The width W2 of the mesa structure in the second region may be equal to or larger than the cutoff width for light of the Bragg wavelength. The normalized coupling coefficient of the first region may be equal to or larger than 60% of the normalized coupling coefficient of the entire semiconductor laser. A third region in which the width of the mesa structure changes from W1 to W2 may be disposed between the first region and the second region. A spot size conversion section may be provided in front of the second region. The diffraction grating layer region may be located above or below the active layer. The semiconductor laser may be a buried type in which the mesa structure is buried with a semiconductor, or a ridge type in which the mesa structure is not buried with a semiconductor layer. The characteristics can be improved by reducing the carrier density in the first region. In order to reduce the carrier density, the opening ratio of the through holes in the first region is 60% or less, and preferably 20% to 50%. The semiconductor laser of the present invention oscillates at a wavelength in the 1.3 μm band or 1.55 μm band. However, other wavelength bands may be used. [Explanation of symbols]

[0041] 1. Semiconductor laser 2 1st electrode 3 Second electrode 4. Low-reflection end face coating film 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 Diffraction grating layer 11A First refractive index region 11B Second refractive index region 12A Diffraction Grating Area 12B Non-diffraction grating area 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 60 Spot size conversion section 201 Semiconductor Laser 203 2nd electrode 215 Mesa Structure 220 1st area 230 Second area 301 Semiconductor Laser 314 Insulating film 315 Mesa Structure 318 through hole 310 1st area 320 Second area 330 Third area 401 Semiconductor Laser 415 Mesa Structure 410 1st area 420 Second area 430 Third area 12C Phase shift grating area 470 Phase shift section D1 1st direction D2 2nd direction W1 Mesa width W2 Mesa width

Claims

1. A substrate; an active layer formed in a mesa structure on the substrate; a cladding layer formed on the active layer within the mesa structure; a grating layer formed in the cladding layer, the grating layer including a plurality of first refractive index regions and a plurality of second refractive index regions; a low-reflection end face coating film provided on both end faces in the extending direction of the mesa structure; Equipped with the mesa structure includes, in a plan view, a first region having a first width and a second region having a second width greater than the first width; the first region includes a first diffraction grating region in which the first refractive index region and the second refractive index region for reflecting light of a Bragg wavelength are alternately arranged, the second region includes a second diffraction grating region in which the first refractive index region and the second refractive index region are alternately arranged, and a non-diffraction grating region that transmits light of the Bragg wavelength, the first diffraction grating region and the second diffraction grating region form a resonator, A semiconductor laser, wherein a normalized coupling coefficient of the first region is greater than a normalized coupling coefficient of the second region.

2. 2. The semiconductor laser according to claim 1 , The first width is equal to or smaller than a cutoff width for light of the Bragg wavelength.

3. 3. The semiconductor laser according to claim 2, The second width is equal to or greater than a cutoff width for light of the Bragg wavelength.

4. 4. The semiconductor laser according to claim 2, The second width is 1.2 times or more the first width.

5. 2. The semiconductor laser according to claim 1 , The first width is less than or equal to 1.5 times the Bragg wavelength.

6. 2. The semiconductor laser according to claim 1 , A semiconductor laser, wherein a normalized coupling coefficient of the first region is 60% or more of a normalized coupling coefficient of the entire semiconductor laser.

7. 2. The semiconductor laser according to claim 1 , A semiconductor laser, wherein a normalized coupling coefficient of the first region is 80% or less of a normalized coupling coefficient of the entire semiconductor laser.

8. 2. The semiconductor laser according to claim 1 , a length of the first diffraction grating region in a direction in which the mesa structure extends is 40% or less of an entire length of the diffraction grating layer.

9. 2. The semiconductor laser according to claim 1 , The semiconductor laser includes a plurality of the second diffraction grating regions and a plurality of the non-diffraction grating regions.

10. 10. The semiconductor laser according to claim 9, a length of each of the second diffraction grating regions and the non-diffraction grating regions in a direction in which the mesa structure extends is different from one another.

11. 2. The semiconductor laser according to claim 1 , A semiconductor laser, wherein the number of the second diffraction grating region and the number of the non-diffraction grating region are each one.

12. 2. The semiconductor laser according to claim 1 , the mesa structure further comprises a third region between the first region and the second region; A mesa width in the third region changes from the first width to the second width.

13. 2. The semiconductor laser according to claim 1 , A spot size conversion unit is further provided, the spot size conversion portion is in contact with the second region, the spot size conversion portion does not include the diffraction grating region, a width of the mesa structure of the spot size conversion portion gradually becomes smaller than the second width.

14. 2. The semiconductor laser according to claim 1 , The first region includes a non-grating region on the second region side.

15. 2. The semiconductor laser according to claim 1 , an electrode for applying a voltage to the active layer; The electrode is integrally disposed across the first region and the second region.

16. 2. The semiconductor laser according to claim 1 , the first region includes a λ / 4 phase shift portion at an end of the first diffraction grating region on the second region side.

17. 2. The semiconductor laser according to claim 1 , A semiconductor laser comprising a through hole for injecting a current into the active layer.

18. 18. The semiconductor laser according to claim 17, The through hole is disposed across the first region and the second region.

19. 18. The semiconductor laser according to claim 17, The through holes are discretely arranged in the first region.

20. 18. The semiconductor laser according to claim 17, The through holes are disposed in an area of ​​the first region that occupies 20% to 50% of the entire first region in a plan view.

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