Semiconductor laser

Abstract

A semiconductor laser having a distribution of coupling coefficients is provided. [Solution] The semiconductor laser comprises a substrate and a semiconductor multilayer disposed on the substrate, the semiconductor multilayer including an active layer, a diffraction grating layer and a flat light confinement adjustment layer, the semiconductor multilayer comprising a first region and a second region in a first direction in which the diffraction grating layer extends, the light confinement adjustment layer including a high refractive index region and a low refractive index region with a refractive index lower than that of the high refractive index region, the high refractive index region being located in either the first or second region and the low refractive index region being located in the other of the first or second region such that the first coupling coefficient of the first region is greater than the second coupling coefficient of the second region.

Description

Technical Field

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

Background Art

[0002] Semiconductor lasers are widely used as light sources for optical communication. As one type of semiconductor laser, a distributed feedback semiconductor laser (DFB laser) is known. The DFB laser has a diffraction grating. Further, for improving characteristics, a structure in which a phase shift portion is provided in the diffraction grating is known. By forming antireflection films (low reflection films) on both end faces of the semiconductor laser and arranging a λ / 4 shift portion in the diffraction grating, stable single wavelength operation can be obtained. In Patent Documents 1, 2, and 3, a structure is disclosed in which the coupling coefficient κ indicating the strength of the interaction between the diffraction grating structure and light has a distribution in the direction in which the resonator is formed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, a distribution is provided in the coupling coefficient by changing the height of the diffraction grating (grating) in the optical axis direction. In Patent Documents 2 and 3, although the height of the diffraction grating is constant in the optical axis direction, a distribution is formed by changing the thickness or composition of the layer filling therebetween. However, in order to realize a desired distribution of the coupling coefficient by these methods, precise film thickness control is required.

[0005] The present invention aims to provide a semiconductor laser having a distribution of coupling coefficients. [Means for solving the problem]

[0006] The semiconductor laser comprises a substrate and a semiconductor multilayer disposed on the substrate, the semiconductor multilayer including an active layer, a diffraction grating layer and a flat light confinement adjustment layer, the semiconductor multilayer comprising a first region and a second region in a first direction in which the diffraction grating layer extends, the light confinement adjustment layer including a high refractive index region and a low refractive index region with a refractive index lower than that of the high refractive index region, the high refractive index region being located in either the first or second region and the low refractive index region being located in the other of the first or second region such that the first coupling coefficient of the first region is greater than the second coupling coefficient of the second region. [Brief explanation of the drawing]

[0007] [Figure 1] This is a top view of a semiconductor laser according to the first embodiment. [Figure 2] Figure 1 is a schematic cross-sectional view of a semiconductor laser along the II-II line. [Figure 3] Figure 1 is a schematic cross-sectional view of a semiconductor laser along the line III-III. [Figure 4] Figure 1 is a schematic cross-sectional view along the IV-IV line of a semiconductor laser. [Figure 5] This is a calculation result showing the relationship between the thickness of the light confinement adjustment layer and the coupling coefficient. [Figure 6] This is a schematic cross-sectional view of a semiconductor laser according to a second embodiment. [Figure 7] This is a schematic cross-sectional view of a semiconductor laser according to a second embodiment. [Figure 8] This is a schematic cross-sectional view of a semiconductor laser according to a second embodiment. [Figure 9] This is a schematic cross-sectional view of a semiconductor laser according to the third embodiment. [Figure 10] This is a schematic cross-sectional view of a semiconductor laser according to the fourth embodiment. [Figure 11] This is a schematic cross-sectional view of a semiconductor laser according to the fifth embodiment. [Figure 12] This is a top view of a semiconductor laser according to the sixth embodiment. [Figure 13] Figure 12 shows a schematic cross-sectional view of a semiconductor laser along the XIII-XIII line. [Figure 14] Figure 12 shows a schematic cross-sectional view of a semiconductor laser along the XIV-XIV line. [Figure 15] Figure 12 shows a schematic cross-sectional view of a semiconductor laser along the XV-XV line. [Modes for carrying out the invention]

[0008] Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the drawings. Components denoted by the same reference numerals in all figures have the same or equivalent function, and repeated explanations will be omitted. Note that the size of the figures does not necessarily correspond to the magnification.

[0009] [First Embodiment] Figure 1 is a top view of the semiconductor laser 1 according to the first embodiment. Figure 2 shows a schematic cross-sectional view of the semiconductor laser 1 shown in Figure 1 along the line II-II. Figure 3 shows a schematic cross-sectional view of the semiconductor laser 1 shown in Figure 1 along the line III-III. Figure 4 shows a schematic cross-sectional view of the semiconductor laser 1 shown in Figure 1 along the line IV-IV. The semiconductor laser 1 is an end-face emission type laser, and is either a continuous-wave laser or a direct-modulation type laser.

[0010] [Mesa structure] The semiconductor laser 1 has a substrate 32. The substrate 32 is a semiconductor substrate of the first conductivity type. For example, the substrate 32 is an n-type InP substrate. The semiconductor laser 1 has a mesa structure 10 on the substrate 32. The mesa structure 10 includes a semiconductor multilayer 5. The bottom layer of the mesa structure 10 may or may not include a part of the substrate 32. The semiconductor multilayer 5 is defined by layers disposed above the substrate 32. Here, the substrate 32 also functions as a clad layer of the first conductivity type. Here, the direction in which the mesa structure 10 extends is defined as the first direction D1, and the direction perpendicular to the first direction D1 in plan view is defined as the second direction D2. In the second direction D2, the width of the mesa structure 10 is the same in the region where the diffraction grating layer 24 described later is disposed. Note that the width of the mesa structure 10 does not necessarily have to be constant in the second direction D2. For example, in the phase shift portion 25 described later, a structure that shifts the phase by changing the width of the mesa structure 10 may be employed.

[0011] [Semiconductor Multilayer] The semiconductor multilayer 5 has a first optical confinement layer 26, an active layer 16, a second optical confinement layer 28, a spacer layer 18, a diffraction grating layer 24, a first clad layer 29, an optical confinement adjustment layer 30, and a second clad layer 22. The first optical confinement layer 26 is of the first conductivity type. The active layer 16 may be a multiple quantum well layer in which a plurality of quantum well layers and a plurality of barrier layers are alternately stacked, or may have another structure. The second optical confinement layer 28 is of the second conductivity type opposite to the first conductivity type. Here, the second optical confinement layer 28 is p-type. The first optical confinement layer 26, the active layer 16, and the second optical confinement layer 28 may be made of, for example, InGaAsP or InGaAlAs. Note that the first optical confinement layer 26 and / or the second optical confinement layer 28 may be omitted. The spacer layer 18 is a layer disposed between the active layer 16 and the diffraction grating layer 24. The spacer layer 18 is made of, for example, InP and is of the second conductivity type. The first clad layer 29 and the second clad layer 22 are made of, for example, InP and are of the second conductivity type. Details of the diffraction grating layer 24 and the optical confinement adjustment layer 30 will be described later. Note that the material of each layer is merely an example. Here, the direction in which each layer is stacked, in other words, the normal direction of the substrate 32, is hereinafter defined as the third direction D3.

[0012] [Embedded layer] The semiconductor laser 1 has an embedded layer 44. The embedded layer 44 is disposed adjacent to each of both sides of the mesa structure 10. The embedded layer 44 is formed of a semi-insulating semiconductor. For example, the embedded layer 44 is composed of semi-insulating InP (such as Fe-InP). Note that the embedded layer 44 may be formed of a stacked structure of an n-type semiconductor layer and a p-type semiconductor layer.

[0013] [End face] The semiconductor laser 1 has a first end face 46 and a second end face 48 on the opposite side of the first end face 46. A low-reflection coating film (not shown) is formed on the first end face 46 and the second end face 48. The reflectance of the low-reflection coating film is 1% or less. The mesa structure 10 extends in a direction (first direction D1) connecting the first end face 46 and the second end face 48. Note that the mesa structure 10 may not reach the first end face 46 and / or the second end face 48. For example, a window structure may be included between the mesa structure 10 and the first end face 46 and / or the second end face 48.

[0014] [Electrode] The semiconductor laser 1 has a back surface electrode 40 on a side of the substrate 32 where the mesa structure 10 is not formed. The semiconductor laser 1 also has a surface electrode 42 on the upper surface of the mesa structure 10 and a part of the upper surface of the embedded layer 44. Note that a contact layer of a second conductivity type may be disposed between the surface electrode 42 and the second cladding layer 22. The surface electrode 42 is disposed across both a first region 12 and a second region 14 described later, and injects the same current into the two regions. The back surface electrode 40 and the surface electrode 42 are used to inject a current supplied from an external power source (not shown) into the semiconductor laser 1. Note that the surface electrode 42 may be separately disposed on each of the upper surface of the first region 12 and the upper surface of the second region 14. At this time, the same current may be injected into the first region 12 and the second region 14, or different currents may be injected.

[0015] [Insulating film] The semiconductor laser 1 has an insulating film 38 on the upper surface of the embedded layer 44, except near the upper surface of the mesa structure 10. The insulating film 38 is, for example, silicon oxide or silicon nitride.

[0016] [Diffraction grating layer] The diffraction grating layer 24 has a diffraction grating structure in which two regions with different refractive indices are alternately arranged in the first direction D1. Here, the diffraction grating layer 24 can be considered as a floating-type diffraction grating structure placed within the spacer layer 18 and the first cladding layer 29. The diffraction grating layer 24 has a phase shift portion 25. For example, the phase shift portion 25 is a λ / 4 phase shift portion. The diffraction grating layer 24 is formed to reflect light generated in the active layer 16 so that the semiconductor laser 1 oscillates in the 1.3 μm band. Note that the wavelength band is not limited to the 1.3 μm band, but may be the 1.55 μm band or other wavelength bands. Here, the diffraction grating layer 24 is extended in the first direction D1 between the first end face 46 and the second end face 48. The thickness of the diffraction grating layer 24 in the third direction D3 is the same throughout the entire region. Here, the same thickness indicates that it is the same within the range of manufacturing variations, and that the thickness is not intentionally varied. As will be described later, the effective refractive index in the optical axis direction is not constant due to the difference in refractive index between the high refractive index region 51 and the low refractive index region 52 contained in the optical confinement adjustment layer 30. Therefore, the diffraction grating period is not constant across the entire region, and the diffraction grating period is finely adjusted according to the effective refractive index to achieve the desired Bragg wavelength.

[0017] [Light confinement adjustment layer] The light confinement adjustment layer 30 is sandwiched between the first cladding layer 29 and the second cladding layer 22. The light confinement adjustment layer 30 has a high refractive index region 51 and a low refractive index region 52. The high refractive index region 51 and the low refractive index region 52 are arranged adjacent to each other in the direction of the first direction D1. In the first direction D1, the high refractive index region 51 is located on the first end face 46 side, and the low refractive index region 52 is located on the second end face 48 side. The refractive index of the high refractive index region 51 is greater than the refractive index of the low refractive index region 52. The high refractive index region 51 may be composed of, for example, InGaAsP or InGaAlAs. The low refractive index region 52 is composed of InP in this case. In Figures 2 and 4, the interface between the low refractive index region 52 and the first cladding layer 29 and the second cladding layer 22 is shown by a line, but in reality, the interface may not be clearly defined because the two cladding layers are made of the same material. The optical confinement adjustment layer 30 in the high refractive index region 51 and the optical confinement adjustment layer 30 in the low refractive index region 52 are both flat films. When the optical confinement adjustment layer 30 in the low refractive index region 52 is made of the same material as the first cladding layer 29 and the second cladding layer 22, the upper surface of the optical confinement adjustment layer 30 in the low refractive index region 52 can be considered to be on the same plane (i.e., at the same position in the third direction D3) as the upper surface of the optical confinement adjustment layer 30 in the high refractive index region 51. However, the low refractive index region 52 may be made of a different material from the first cladding layer 29 or the second cladding layer 22. The refractive index of the high refractive index region 51 and the low refractive index region 52 is lower than that of the active layer 16. The composition wavelength is set so that the optical confinement adjustment layer 30 does not absorb light at the Bragg wavelength. The refractive index of the high refractive index region 51 is higher than that of the second cladding layer 22. Also, the high refractive index region 51 is thicker than the active layer 16 in the third direction D3. The optical confinement adjustment layer 30 is of the second conductivity type.

[0018] Here, the semiconductor multilayer can be divided into a first region 12 that includes a high refractive index region 51 and a second region 14 that includes a low refractive index region 52. In this embodiment, the phase shift portion 25 is included in the second region 14.

[0019] [Coupling coefficient] The coupling coefficient κ, which indicates the strength of the interaction between the diffraction grating structure of the semiconductor laser 1 and light, has a distribution in the optical axis direction (first direction D1). Let κ1 be the coupling coefficient of the first region 12 and κ2 be the coupling coefficient of the second region 14. In this embodiment, κ1 is greater than κ2. The coupling coefficient κ is determined by the thickness of the diffraction grating layer 24 in the third direction D3 and the refractive index of the diffraction grating layer 24. The diffraction grating layer 24 has the same thickness in the first region 12 and the second region 14. Furthermore, the first region 12 and the second region 14 have the same semiconductor multilayer structure except for the structure of the optical confinement adjustment layer 30 and the presence or absence of the phase shift portion 25. The phase shift portion 25 is only a small part of the diffraction grating layer 24, and its effect on the effective refractive index is practically negligible. Therefore, the difference in the effective refractive index between the first region 12 and the second region 14 is mainly due to the structure of the optical confinement adjustment layer 30, that is, the difference in refractive index between the high refractive index region 51 and the low refractive index region 52.

[0020] The high refractive index region 51 has a higher refractive index than the low refractive index region 52. Therefore, the distribution of propagating light differs between the first region 12 and the second region 14. In this embodiment, the thickness of the high refractive index layer 51 in the third direction D3 is set such that the distribution of light propagating in the first region 12 is shifted towards the light confinement adjustment layer 30 compared to the second region 14. Therefore, the light confinement rate in the diffraction grating layer 24 is greater in the first region 12 than in the second region 14. As a result, the coupling coefficient κ1 in the first region 12 is greater than the coupling coefficient κ2 in the second region 14.

[0021] By configuring the optical confinement adjustment layer 30 to include a high refractive index region 51 and a low refractive index region 52, a distribution can be made in the coupling coefficient κ in the optical axis direction. Here, L1 is the length of the first region 12 in the first direction D1, and L2 is the length of the second region 14. For example, if the normalized coupling coefficient κ2L2 of the second region 14 is made smaller than the normalized coupling coefficient κ1L1 of the first region 12, the light intensity output from the second end face 48 can be made greater than the light intensity output from the first end face 46. By utilizing this feature, a high-power laser can be realized. Furthermore, since a low-reflection coating film is formed on the first end face 46 and the second end face 48, and a λ / 4 phase shift portion is provided, wavelength uniformity is also excellent. Generally, semiconductor lasers used in optical communication use only the output light from one end face. Therefore, it is desirable to increase the light intensity output from one end face (here, the second end face 48). To do this, it is necessary to increase the difference between κ1 and κ2. To achieve this, it is desirable that the thickness of the third direction D3 of the light confinement adjustment layer 30 in the high refractive index region 51 be greater than or equal to the thickness of the active layer 16. Hereinafter, the thickness of the third direction D3 of the light confinement adjustment layer 30 in the high refractive index region 51 will be referred to as the thickness of the high refractive index region 51. The thickness of the third direction D3 of the light confinement adjustment layer 30 in the low refractive index region 52 will be referred to as the thickness of the low refractive index region 52.

[0022] Furthermore, in this embodiment, the thickness of the diffraction grating layer 24 in the third direction D3 is constant, and is the same thickness in the first region 12 and the second region 14. Therefore, compared to the method of adjusting the coupling coefficient by varying the thickness of the diffraction grating layer, as in Patent Document 1, it is easier to manufacture. Similarly, as in Patent Documents 2 and 3, the composition of the semiconductor layer within the diffraction grating layer does not differ in the optical axis direction, so it can be manufactured using a simple manufacturing method. Moreover, when a distribution of coupling coefficients is created by adjusting the thickness and composition of the diffraction grating layer, it is difficult to achieve a large difference in coupling coefficients from the viewpoint of manufacturability. For example, the difference in coupling coefficients can be increased by significantly changing the thickness of the diffraction grating layer in the first region 12 and the second region 14, but it is difficult to form a diffraction grating layer with a step, and this step may adversely affect the next manufacturing process. On the other hand, in this embodiment, the coupling coefficient is adjusted by the configuration of the optical confinement adjustment layer 30, which is a layer separate from the diffraction grating layer 24, so the desired coupling coefficient can be obtained independently of the design of the diffraction grating layer.

[0023] Figure 5 shows the calculation results illustrating the relationship between the thickness of the high refractive index region 51 and the coupling coefficient. The horizontal axis represents the ratio of the thickness Dadj of the high refractive index region 51 to the thickness Dact of the active layer 16 (Dadj / Dact). The vertical axis represents the ratio of the coupling coefficient κ1 of the first region 12 to the coupling coefficient κ2 of the second region 14 (κ1 / κ2). A value of 0 on the horizontal axis indicates that the high refractive index region 51 is not present, in other words, that the refractive indices of the high refractive index region 51 and the low refractive index region 52 are the same. In this case, κ1 and κ2 are the same value, so the vertical axis is 1. As κ1 / κ2 increases, the optical output intensity output from the second end face 48 increases, and the optical output intensity output from the first end face 46 decreases. Generally, when the thickness Dact of the active layer 16 is thick, the optical confinement rate of the active layer 16 increases. Accordingly, the optical confinement rate of the diffraction grating layer 24 decreases. The optical confinement rate of the diffraction grating layer 24 cannot be increased unless the high refractive index region 51 of the optical confinement adjustment layer 30 is sufficiently thick. In other words, Dadj needs to be set appropriately according to Dact. This is shown on the horizontal axis of Figure 5.

[0024] Our diligent research has shown that a κ1 / κ2 ratio of 1.33 or higher is sufficient for optical output intensity for optical communication light sources. To accommodate this, Dadj / Dact needs to be set between 1 and 6.7. In other words, it is desirable that the high refractive index region 51 has a thickness greater than or equal to the active layer thickness Dact. Furthermore, to meet the recent demands for high-power semiconductor lasers, a κ1 / κ2 ratio of 1.5 or higher is desirable. In this case, Dadj / Dact will be between 1.6 and 6.2. Moreover, with the increase in optical communication volume, even higher-power semiconductor lasers are required to cope with the rise in ambient temperature (increase in driving temperature) due to high-density mounting of optical components and to enable low-power operation. To meet this, a κ1 / κ2 ratio of 1.65 or higher is desirable, and Dadj / Dact is desirable between 2.5 and 5.25. Figure 5 shows the calculation results for the case where the low refractive index region 52 is InP.

[0025] [Second Embodiment] Figure 6 shows a schematic cross-sectional view of the semiconductor laser 201 according to the second embodiment, along the optical axis, and corresponds to the schematic cross-sectional view along line II-II in Figure 1. Figure 7 corresponds to the schematic cross-sectional view along line III-III in Figure 1. Figure 8 corresponds to the schematic cross-sectional view along line IV-IV in Figure 1.

[0026] The semiconductor multilayer 205 of the semiconductor laser 201 is stacked in the following order from the substrate 32 side: optical confinement adjustment layer 230, first cladding layer 229, diffraction grating layer 224, spacer layer 218, first optical confinement layer 26, active layer 16, second optical confinement layer 28, and second cladding layer 22. Similar to the first embodiment, the optical confinement adjustment layer 230 has a high refractive index region 251 and a low refractive index region 252. Similar to the first embodiment, the high refractive index region 251 has a higher refractive index than the low refractive index region 252. The optical confinement adjustment layer 230 is of the first conductivity type. The diffraction grating layer 224 and the diffraction grating structure are the same as in the first embodiment, except that their positions in the third direction D3 are different.

[0027] In this embodiment as well, the region where the high refractive index region 251 is located is designated as the first region 212, and the region where the low refractive index region 252 is located is designated as the second region 214. Similar to the first embodiment, in the first region 212, the distribution of light propagated by the high refractive index region 251 is shifted towards the high refractive index region 251 side, so the coupling coefficient κ1 of the first region 212 becomes larger than the coupling coefficient κ2 of the second region 214.

[0028] As described above, even in a structure where the diffraction grating layer 224 is placed between the active layer 16 and the substrate 32, a distribution of coupling coefficients in the optical axis direction can be formed by placing the optical confinement adjustment layer 230 between the diffraction grating layer 224 and the substrate 32.

[0029] [Third Embodiment] Figure 9 shows a schematic cross-sectional view of the semiconductor laser 301 according to the third embodiment, along the optical axis, and corresponds to the schematic cross-sectional view along line II-II in Figure 1.

[0030] The semiconductor multilayer 305 of the semiconductor laser 301 is stacked in the following order from the substrate 32 side: optical confinement adjustment layer 330, first cladding layer 329, first optical confinement layer 26, active layer 16, second optical confinement layer 28, spacer layer 18, diffraction grating layer 24, and second cladding layer 22. The optical confinement adjustment layer 330 has a high refractive index region 351 and a low refractive index region 352, similar to the first embodiment. Similar to the first embodiment, the high refractive index region 351 has a higher refractive index than the low refractive index region 352. The optical confinement adjustment layer 330 is of the first conductivity type.

[0031] In this embodiment, the region where the low refractive index region 352 is located is designated as the first region 312, and the region where the high refractive index region 351 is located is designated as the second region 314. In the second region 314, because the high refractive index region 351 is located, the distribution of propagating light is pulled towards the high refractive index region 351. Therefore, the optical confinement rate of the diffraction grating layer 24 in the second region 314 is lower than that of the first region 312, and κ2 becomes smaller than κ1. In the first and second embodiments, the high refractive index regions 51 and 251 were arranged to increase the optical confinement rate of the diffraction grating layer, but in the third embodiment, they are arranged to decrease the optical confinement rate of the diffraction grating layer. In any case, the coupling coefficient κ of the first region 312 can be made larger than that of the second region 314.

[0032] As described above, the first region is not defined by the region where the high refractive index region of the optical confinement adjustment layer is located, but rather by the region in which the coupling coefficient becomes larger due to the optical confinement adjustment layer composed of two refractive index regions.

[0033] [Fourth Embodiment] Figure 10 shows a schematic cross-sectional view of the semiconductor laser 401 according to the fourth embodiment, along the optical axis, and corresponds to the schematic cross-sectional view along line II-II in Figure 1.

[0034] The semiconductor multilayer 405 of the semiconductor laser 401 is stacked in the following order from the substrate 32 side: diffraction grating layer 424, spacer layer 418, first optical confinement layer 26, active layer 16, second optical confinement layer 28, first cladding layer 429, optical confinement adjustment layer 430, and second cladding layer 22. Similar to the first embodiment, the optical confinement adjustment layer 430 has a high refractive index region 451 and a low refractive index region 452. Similar to the first embodiment, the high refractive index region 451 has a higher refractive index than the low refractive index region 452. The optical confinement adjustment layer 430 is of the second conductivity type.

[0035] In this embodiment, the region where the low refractive index region 452 is located is designated as the first region 412, and the region where the high refractive index region 451 is located is designated as the second region 414. Similar to the third embodiment, in the second region 414, the optical confinement rate in the diffraction grating layer is lower than that in the first region 412 due to the high refractive index region 451. Therefore, as in the other embodiments, it becomes possible to form a coupling coefficient distribution in the first direction D1.

[0036] As described above, the semiconductor laser of the present invention is characterized by comprising an active layer and a diffraction grating layer (diffraction grating structure), and having an optical confinement adjustment layer with a high refractive index region and a low refractive index region positioned above or below these two structures in the stacking direction (third direction D3). By adjusting the optical confinement rate of the diffraction grating layer with the high refractive index region, the coupling coefficient can be made different from that of the region where the low refractive index region is located. The stacking order of the active layer and the diffraction grating layer is not important. These two layers and the spacer layer positioned between them have the same structure in the direction of the optical axis. The coupling coefficient of the semiconductor laser is mainly determined by the active layer, the diffraction grating layer and the spacer layer. This base coupling coefficient can be adjusted by positioning the optical confinement adjustment layer, and a coupling coefficient distribution can be formed in the direction of the optical axis. The coupling coefficient distribution can produce effects such as increasing the optical output intensity from one end face.

[0037] When a region with a large coupling coefficient is designated as the first region and a region with a smaller coupling coefficient than the first region is designated as the second region, if a high refractive index region is placed in the first region, a low refractive index region is placed in the second region. This is because the coupling coefficient is increased by increasing the optical confinement rate of the diffraction grating layer with the high refractive index region. Conversely, if a low refractive index region is placed in the first region and a high refractive index region is placed in the second region, the coupling coefficient of the second region is reduced by decreasing the optical confinement rate of the diffraction grating layer with the high refractive index region of the second region. As described above, the high refractive index region is placed in either the first or second region, and the low refractive index region is placed in the other of the first or second region, such that the first coupling coefficient of the first region is greater than the second coupling coefficient of the second region.

[0038] In the first and fourth embodiments, the optical confinement adjustment layer is positioned above the diffraction grating layer and the active layer in the stacking direction of the third direction D3. The semiconductor multilayer is grown in multiple layers sequentially from the substrate 32 towards the top of Figures 2 and 10. The optical confinement adjustment layer consists of a high-refractive-index region and a low-refractive-index region with different refractive indices. When these two regions are formed with the same thickness, there is a risk of a step difference occurring between them. In particular, as mentioned above, in order to create a large difference in coupling coefficients between the first and second regions, it is preferable for the optical confinement adjustment layer to be thicker, and if the layer thickness is thick, there is a risk of this step difference becoming larger. When the diffraction grating layer and the active layer are placed on top of the region where the step difference occurs, these layers may also develop steps. Step differences in the diffraction grating layer and the active layer are factors that degrade the optical properties. However, in the first and fourth embodiments, since the optical confinement adjustment layer is formed on top of the active layer and the diffraction grating layer, the step difference in the optical confinement adjustment layer does not affect the active layer and the diffraction grating layer. Therefore, a semiconductor laser with better manufacturability can be realized.

[0039] In the second and third embodiments, the diffraction grating layer and the active layer are arranged on top of the light confinement adjustment layer, which may cause an impact due to the step height of the light confinement adjustment layer. However, because the first cladding layer is arranged on top of the light confinement adjustment layer, the step height of the light confinement adjustment layer is reduced, thereby reducing the impact on the diffraction grating layer and the active layer.

[0040] [Fifth Embodiment] Figure 11 shows a schematic cross-sectional view of the semiconductor laser 501 according to the fifth embodiment, along the optical axis, and corresponds to the schematic cross-sectional view along line II-II in Figure 1.

[0041] The difference between semiconductor laser 501 and semiconductor laser 1 of the first embodiment lies in the difference in the diffraction grating structure. In this embodiment, the semiconductor multilayer is stacked in the following order from the substrate 32 side: first optical confinement layer 26, active layer 16, second optical confinement layer 528, first cladding layer 29, optical confinement adjustment layer 30, and second cladding layer 22. A diffraction grating structure is formed on the surface side of the second optical confinement layer 528. In other words, the second optical confinement layer 528 has the function of optical confinement as well as the function of a diffraction grating layer 524. The diffraction grating structure consists of a region where the surface of the second optical confinement layer 528 (diffraction grating layer 524) is convex, and the first cladding layer 29 positioned between the two convex regions. The refractive index of the second optical confinement layer 528 is higher than that of the first cladding layer 29. The diffraction grating structure also has a phase shift portion 525. As shown in this embodiment, when a diffraction grating structure is formed on the second optical confinement layer 528, a spacer layer is not required.

[0042] In this embodiment as well, by providing a high refractive index region 51, the coupling coefficient κ1 of the first region 12 becomes larger than the coupling coefficient κ2 of the second region 14. In the first embodiment, the diffraction grating structure may be arranged with irregularities on the surface of the diffraction grating layer as shown in this embodiment, rather than being of the floating type.

[0043] [Sixth Embodiment] Figure 12 is a top view of the semiconductor laser 601 according to the sixth embodiment. Figure 13 shows a schematic cross-sectional view of the semiconductor laser 601 shown in Figure 12 along the line XIII-XIII. Figure 14 shows a schematic cross-sectional view of the semiconductor laser 601 shown in Figure 12 along the line XIV-XIV. Figure 15 shows a schematic cross-sectional view of the semiconductor laser 601 shown in Figure 12 along the line XV-XV.

[0044] The difference between the semiconductor laser 601 according to the sixth embodiment and the semiconductor laser 1 according to the first embodiment is that, in addition to the first region 12 and the second region 14, the third region 615 is adjacent in that order along the first direction D1. The semiconductor multilayer 5 of the third region 615 does not have a diffraction grating layer 24 and is identical to the second region 14 in the first embodiment except that the first cladding layer 29 and the spacer layer 18 are in contact. The semiconductor multilayer 5 of the first region 12 and the second region 14 have the same structure as in the first embodiment. The surface electrode 42 and the back electrode 40 are arranged across the first region 12, the second region 14, and the third region 615, and the same current is injected into the three regions.

[0045] The width of the mesa structure 610 in the third region 615 (mesa width) gradually decreases toward the second end face 48. As the mesa width narrows, the light confinement rate of the mesa structure 610 decreases, and the Near Field Pattern (NFP) in the third direction D3 expands. By adjusting the shape of the NFP, the third region 615 functions as a spot size conversion unit that can adjust the spread angle of the Far Field Pattern (FFP) in the vertical direction. In some cases, the desired FFP cannot be obtained as a result of placing the light confinement adjustment layer 30 to adjust the coupling coefficient. By placing the third region 615 here, the FFP can be adjusted. Furthermore, since the third region 615 includes the active layer 16 and has a structure to which current is injected, it also has the effect of amplifying light.

[0046] Furthermore, the mesa width of the third region 615 may gradually increase toward the second end face 48. The mesa width can be adjusted according to the desired FFP specifications. In addition, the mesa width may be constant. When the mesa width is constant, the third region 615 functions as a semiconductor optical amplifier that purely amplifies light. The embodiment shown here is merely an example, and of course, a third region 615 may be added between the first region 12 and the first end face 46.

[0047] The present invention is not limited to the embodiments described above, and various modifications are possible. For example, the number of regions with different refractive indices included in the light confinement adjustment layer is not limited to two, but may be three or more.

[0048] The present invention relates to a semiconductor laser having a distribution of coupling coefficients in the optical axis direction. Embodiments of the present invention have an active layer, a diffraction grating layer, and an optical confinement adjustment layer including a high refractive index region and a low refractive index region, and this is achieved by adjusting the optical confinement rate of the diffraction grating layer with respect to the high refractive index region. By increasing the optical confinement rate of the diffraction grating layer with respect to the high refractive index region, a region with a large coupling coefficient may be formed, or by decreasing the optical confinement rate of the diffraction grating layer with respect to the high refractive index region, a region with a small coupling coefficient may be formed. The diffraction grating layer has a common structure in a first region with a large coupling coefficient and a second region with a small coupling coefficient, and the coupling coefficient is adjusted by the optical confinement adjustment layer. The optical confinement adjustment layer may be placed on top of the diffraction grating layer or between the substrate and the diffraction grating layer. The thickness of the high refractive index region is preferably greater than or equal to the thickness of the active layer. To obtain high output characteristics, the ratio of the thickness of the high refractive index region to the thickness of the active layer is preferably 1.6 or more and 6.2 or less, more preferably 2.5 or more and 5.25 or less. The semiconductor laser may further include a window structure, a spot size conversion unit, and an optical amplifier. [Explanation of Symbols]

[0049] 1, 201, 301, 401, 501, 601 Semiconductor Lasers 5, 205, 305, 405, 505 Semiconductor Multilayer 10,610 Mesa structure 12, 212, 312, 412 1st area 14, 214, 314, 414 2nd area 16 Active layer 18, 218, 418 spacer layers 22 Second Cladding Layer 24, 224, 424, 524 diffraction grating layers 25, 525 Phase shift section 26. First light confinement layer 28,528 Second light confinement layer 29, 229, 329, 429 First cladding layer 30, 230, 330, 430 Light confinement adjustment layer 32 circuit boards 38 Insulating film 40 Backside electrode 42 Surface electrode 44. Embedding layer 46 1st end face 48 Second end face 51, 251, 351, 451 High refractive index regions 52, 252, 352, 452 Low refractive index region 615 Third area D1 1st direction D2 2nd direction D3 Third direction

Claims

1. circuit board and The substrate comprises a semiconductor multilayer arranged on the substrate, The semiconductor multilayer includes an active layer, a diffraction grating layer, and a flat light confinement adjustment layer. The semiconductor multilayer comprises a first region and a second region in the first direction in which the diffraction grating layer extends. The light confinement adjustment layer includes a high refractive index region and a low refractive index region with a refractive index lower than that of the high refractive index region. A semiconductor laser in which the high refractive index region is located in either the first or second region, and the low refractive index region is located in the other of the first or second region, such that the first coupling coefficient of the first region is greater than the second coupling coefficient of the second region.

2. A semiconductor laser according to claim 1, The aforementioned light confinement adjustment layer is positioned to avoid the space between the active layer and the diffraction grating layer, in a semiconductor laser.

3. A semiconductor laser according to claim 1, The semiconductor multilayer is stacked on the substrate in the order of the active layer, the diffraction grating layer, and the light confinement adjustment layer. The first region includes the high refractive index region, The second region is a semiconductor laser including the low refractive index region.

4. A semiconductor laser according to claim 1, The semiconductor multilayer is stacked on the substrate in the following order: the light confinement adjustment layer, the diffraction grating layer, and the active layer. The first region includes the high refractive index region, The second region is a semiconductor laser including the low refractive index region.

5. A semiconductor laser according to claim 1, The semiconductor multilayer is stacked on the substrate in the order of the light confinement adjustment layer, the active layer, and the diffraction grating layer. The first region includes the low refractive index region, The second region is a semiconductor laser including the high refractive index region.

6. A semiconductor laser according to claim 1, The semiconductor multilayer is stacked on the substrate in the order of the diffraction grating layer, the active layer, and the photoconfinement adjustment layer. The first region includes the low refractive index region, The second region is a semiconductor laser including the high refractive index region.

7. A semiconductor laser according to claim 1, The diffraction grating layer includes a phase-shift portion, and the semiconductor laser is provided.

8. A semiconductor laser according to claim 7, The phase shift portion is a semiconductor laser included in the second region.

9. A semiconductor laser according to claim 1, The semiconductor multilayer further comprises electrodes arranged on the aforementioned semiconductor multilayer, The semiconductor multilayer includes a cladding layer between the active layer and the electrode, in a semiconductor laser.

10. A semiconductor laser according to claim 9, A semiconductor laser in which the refractive index of the high refractive index region is higher than that of the cladding layer.

11. A semiconductor laser according to claim 9, A semiconductor laser wherein the refractive index of the low refractive index region is the same as that of the cladding layer.

12. A semiconductor laser according to claim 9, A semiconductor laser in which the refractive index of the low refractive index region is different from that of the cladding layer.

13. A semiconductor laser according to claim 1, The aforementioned optical confinement adjustment layer is a semiconductor laser positioned at a distance from the diffraction grating layer.

14. A semiconductor laser according to claim 1, A semiconductor laser in which the normalized coupling coefficient of the first region is greater than the normalized coupling coefficient of the second region.

15. A semiconductor laser according to claim 1, A semiconductor laser in which, in the stacking direction of the semiconductor multilayer, the thickness of the high refractive index region is greater than or equal to the thickness of the active layer.

16. A semiconductor laser according to claim 15, A semiconductor laser in which, in the stacking direction of the semiconductor multilayer, the thickness of the high refractive index region is 1 to 6.7 times the thickness of the active layer.

17. A semiconductor laser according to claim 15, A semiconductor laser in which, in the stacking direction of the semiconductor multilayer, the thickness of the high refractive index region is 2.5 times or more and 5.25 times or less the thickness of the active layer.

18. A semiconductor laser according to claim 1, The third region further includes a third region that is in contact with the first region or the second region in the first direction, The third region is a semiconductor laser that does not include a diffraction grating layer.

19. A semiconductor laser according to claim 18, The semiconductor multilayer has a mesa structure, A semiconductor laser in which the width of the semiconductor multilayer in a direction perpendicular to the direction of extension of the mesa structure in a plan view gradually decreases or increases toward the end face in the extension direction in the third region, relative to the widths of the first and second regions.

20. A semiconductor laser according to claim 18, The third region includes the active layer, A semiconductor laser further comprising electrodes arranged across the first region, the second region, and the third region.

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