Semiconductor laser

Abstract

A semiconductor laser includes: a multiple quantum well layer in a mesa structure; a buried layer including a semi-insulating semiconductor, the buried layer being in contact with each of the two sides of the mesa structure; a first cladding layer having a first conductivity type, the first cladding layer having a lower refractive index than the multiple quantum well layer; a high refractive index layer configured to not absorb light oscillated in the multiple quantum well layer, the high refractive index layer having a higher refractive index than the first cladding layer; a diffraction grating layer at least partially constituting a diffraction grating capable of diffracting light oscillated in the multiple quantum well layer, the diffraction grating layer not contacting the high refractive index layer; a substrate having the first conductivity type; and a second cladding layer having a second conductivity type, the second cladding layer being over the multiple quantum well layer.

Description

[0001] Divisional Application Instructions

[0002] This application is a divisional application of the invention patent application with the application date of May 9, 2022, the Chinese national application number 202210498507.9, and the invention name “Semiconductor Laser”.

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims priority from Japanese Patent Application JP 2021-079577 filed on May 10, 2021, and Japanese Patent Application JP 2021-110929 filed on July 2, 2021, the contents of which are expressly incorporated herein by reference. Technical Field

[0005] The present disclosure relates to a semiconductor laser. Background Art

[0006] Semiconductor lasers with buried heterostructures (BH) offer excellent power characteristics and reliability. Due to the large optical confinement coefficient of the mesa structure, BH can make the aspect ratio of the output light close to 1. In addition, by widening the mesa width, the current density of the multiple quantum well (MQW) can be reduced, thereby achieving higher power output and higher reliability.

[0007] When the optical confinement coefficient of the mesa structure is large, the area of ​​the near field pattern (NFP) is small, so the diffusion angle of the far field pattern (FFP) becomes larger, resulting in a decrease in the optical coupling tolerance. In addition, when the mesa width exceeds the high-order lateral mode cutoff width, lateral high-order modes appear, resulting in a decrease in optical coupling efficiency, thereby reducing power output.

[0008] In some cases, an n-InP buffer layer and an n-InGaAsP guide layer are provided below the mesa structure. The n-InGaAsP guide layer is provided to fill the diffraction grating. The diffraction grating is a main element having a very large influence on the optical properties, specifically, it determines κ which is a coupling coefficient of light. Therefore, in addition to the necessity of considering κ, there is less flexibility in designing the n-InGaAsP guide layer. In addition, since the n-InP buffer layer is as thin as 30 nm, the n-InGaAsP guide layer below it may increase the optical confinement coefficient to the mesa structure, so that the cutoff width may be narrowed. Summary of the invention

[0009] The present disclosure improves the output characteristics and reliability of semiconductor lasers.

[0010] In some embodiments, a semiconductor laser includes: a multi-quantum well layer included in a mesa structure; a buried layer including a semi-insulating semiconductor, the buried layer contacting each of the two sides of the mesa structure; a first cladding layer having a first conductivity type, the first cladding layer being below the mesa structure and the buried layer, the first cladding layer having a lower refractive index than the multi-quantum well layer; a high refractive index layer, the high refractive index layer being configured not to absorb light oscillating in the multi-quantum well layer, the high refractive index layer being below the mesa structure and the buried layer and below the first cladding layer, the high refractive index layer having a higher refractive index than the first cladding layer; a diffraction grating layer at least partially constituting a diffraction grating capable of diffracting light oscillating in the multi-quantum well layer, the diffraction grating layer not contacting the high refractive index layer; a substrate having the first conductivity type, the substrate being below the high refractive index layer; and a second cladding layer having a second conductivity type opposite to the first conductivity type, the second cladding layer being above the multi-quantum well layer.

[0011] The high refractive index layer having a high refractive index expands the distribution of light. Therefore, the high-order transverse mode cutoff width becomes larger, the area of ​​the NFP becomes larger, and the diffusion angle of the FFP becomes smaller, thereby improving the output characteristics and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a plan view of a semiconductor laser according to a first exemplary embodiment.

[0013] Figure 2 yes Figure 1 II-II cross-sectional view of the semiconductor laser shown.

[0014] Figure 3 yes Figure 1 III-III cross-sectional view of the semiconductor laser shown.

[0015] Figure 4 is a cross-sectional view of a laser oscillator section of a semiconductor laser according to a comparative example.

[0016] Figure 5A This is a diagram showing the comparison results of the relationship between the cutoff width and the first cladding layer.

[0017] Figure 5B is a graph comparing the relationship of the vertical FFP to the first cladding layer.

[0018] Figure 5C It is a diagram showing the comparison results of the relationship between the horizontal FFP and the first cladding layer.

[0019] Figure 5D This is a diagram showing the comparison results of the relationship between the aspect ratio and the first cladding layer.

[0020] Fig. 6A This is a diagram showing the comparison results of the relationship between the cutoff width and the high refractive index layer.

[0021] Figure 6B is a graph showing the comparison results of the relationship between the vertical FFP and the high refractive index layer.

[0022] Figure 6C This is a diagram showing the comparison results of the relationship between the horizontal FFP and the high refractive index layer.

[0023] Fig.6D This is a diagram showing the comparison results of the relationship between the aspect ratio and the high refractive index layer.

[0024] Figure 7 is a cross-sectional view of a semiconductor laser according to modified Configuration 1.

[0025] Figure 8 is a cross-sectional view of a semiconductor laser according to Modified Configuration 2.

[0026] Fig. 9 is a plan view of a semiconductor laser according to a second exemplary embodiment.

[0027] Fig.10 yes Fig. 9 XX cross-sectional view of the semiconductor laser shown. DETAILED DESCRIPTION

[0028] Some embodiments are described in detail below with reference to the accompanying drawings. In the accompanying drawings, the same components are represented by the same reference numerals and have the same or equivalent functions, and their repeated descriptions are omitted. The dimensions of the drawings are not necessarily consistent with the magnification.

[0029] Silicon photonics, which has attracted attention in recent years, inherently requires high-power semiconductor optical devices, especially high-power semiconductor lasers. Ridge-type or buried heterostructure (BH) is known as one of the structures of semiconductor lasers. Semiconductor lasers having BH are called BH-type semiconductor lasers.

[0030] In a ridge-type semiconductor laser, multiple quantum wells (MQWs) are widely distributed on a substrate, and a semiconductor layer (mainly a cladding layer) thereon is provided with a ridge portion. The side surface of the ridge portion is covered by an insulating film or a semiconductor layer. In a BH-type semiconductor laser, a semiconductor layer including MQWs has a mesa structure (striped shape), and a buried layer including a semi-insulating semiconductor is adjacent to both sides of the mesa structure.

[0031] Comparing the ridge type semiconductor laser and the BH type semiconductor laser, both sides of the MQW of the BH type semiconductor laser are covered with a buried layer, which has high thermal conductivity and provides excellent heat dissipation.

[0032] In a BH type semiconductor laser, generally, the MQW is surrounded by substantially the same semiconductor material (e.g., InP) on the top, bottom, left, and right sides, and the surface irregularities are smaller than those of a ridge type semiconductor laser, whereby the stress applied to the MQW is smaller. Therefore, from the viewpoint of reliability due to stress, the BH type semiconductor laser is superior to the ridge type semiconductor laser.

[0033] The shape of the output light affects the optical coupling characteristics with external optical components. If the optical coupling efficiency is poor, the output light intensity of the semiconductor laser cannot be fully utilized even if the output light intensity of the semiconductor laser is large. For example, when the semiconductor laser is optically coupled to a lens, from the viewpoint of the optical coupling efficiency and tolerance when adjusting the optical axis, the diffusion angle of the far field pattern (FFP) of the output light of the semiconductor laser should be small and the aspect ratio should be close to 1. BH-type semiconductor lasers generally have an aspect ratio of output light closer to 1 than ridge-type semiconductor lasers, which is superior in terms of optical coupling characteristics.

[0034] For the above reasons, a semiconductor laser having BH satisfies high power characteristics and provides high reliability. In addition, a ridge type semiconductor laser and a BH type semiconductor laser can be optimized to provide a semiconductor laser that can withstand sufficient practical use.

[0035] Figure 1 is a plan view of a semiconductor laser according to a first exemplary embodiment. Figure 2 yes Figure 1 II-II cross-sectional view of the semiconductor laser shown. Figure 3 yes Figure 1 The semiconductor laser is shown as a III-III cross-sectional view. The semiconductor laser is a BH type semiconductor laser, which may be a continuous oscillation laser or a directly modulated laser.

[0036] The semiconductor laser may include a mesa structure 10. The mesa structure 10 may include a laser oscillator section 12 capable of laser oscillating light having a predetermined wavelength. The laser oscillator section 12 may be configured to output continuous light. The oscillation wavelength of the laser oscillator section 12 may be a 1.3 μm band, or may be another wavelength band, such as a 1.55 μm band. The laser oscillator section 12 may be a distributed feedback (DFB) laser. The mesa structure 10 may have a width (mesa width) of 1.7 μm at the laser oscillator section 12.

[0037] The mesa structure 10 may include a spot size converter section 14. The spot size converter section 14 may be adjacent to the laser oscillator section 12. The width (mesa width) of the spot size converter section 14 perpendicular to the light emission direction gradually decreases. As the mesa width decreases, the optical confinement coefficient of the mesa structure 10 decreases. The spot size converter section 14 may be configured to adjust the area of ​​the NFP and adjust the diffusion angle of the FFP by adjusting the emission area of ​​the laser beam.

[0038] The laser oscillator section 12 and the spot size converter section 14 may include a common pin structure. "A common pin structure" in the present disclosure means a pin structure composed of the same material and formed by the same process. Since they have a common pin structure, the gain band remains static from the laser oscillator section 12 to the spot size converter section 14. The gain band may be a wavelength range of a gain spectrum. The mesa structure 10 may be composed of some layers of the pin structure.

[0039] The semiconductor laser may include a multi-quantum well layer 16. The multi-quantum well layer 16 may be included in the mesa structure 10. The multi-quantum well layer 16 may have a multiple structure, in which some quantum well layers and some barrier layers may be alternately stacked to have a total layer thickness of 100 nm. The multi-quantum well layer 16 may be composed of InGaAsP or may be composed of InGaAlAs.

[0040] The semiconductor laser may include a buried layer 18. The buried layer 18 may be in contact with each of the two sides of the mesa structure 10. Each of the two sides of the mesa structure 10 may be covered by the buried layer 18. The buried layer 18 may be made of a semi-insulating semiconductor. The buried layer 18 may include semi-insulating InP (e.g., Fe-InP).

[0041] The semiconductor laser may include a first cladding layer 20. The first cladding layer 20 may be below the mesa structure 10 and the buried layer 18. A portion of the first cladding layer 20 (e.g., a protrusion on the top surface) may be included in the mesa structure 10. Alternatively, the first cladding layer 20 may not be included in the mesa structure 10. The first cladding layer 20 may include InP. For example, the thermal conductivity of InP may be higher than that of InGaAsP, so that the first cladding layer 20 may be made of InP to increase heat dissipation from the multi-quantum well layer 16. The first cladding layer 20 may have a first conductivity type (e.g., n-type). The refractive index of the first cladding layer 20 may be lower than that of the multi-quantum well layer 16. The first cladding layer 20 may have a thickness of 500 nm or more (e.g., 1000 nm) and 1500 nm or less.

[0042] The semiconductor laser may include a second cladding layer 22. The second cladding layer 22 may be above the multi-quantum well layer 16. The second cladding layer 22 may have a second conductivity type (e.g., p-type) opposite to the first conductivity type. The second cladding layer 22 may be included in the mesa structure 10. The second cladding layer 22 may include InP and may have a thickness of 2000 nm.

[0043] The semiconductor laser may include a diffraction grating layer 24. The diffraction grating layer 24 may include InGaAsP and may have a thickness of 10 nm. The diffraction grating layer 24 may be inside the second cladding layer 22. The diffraction grating layer 24 may be included in the mesa structure 10. The diffraction grating layer 24 at least partially constitutes a diffraction grating capable of diffracting light oscillating in the multi-quantum well layer 16. For example, the diffraction grating layer 24 corresponding to the composition of the multi-quantum well layer 16 may be configured to allow the laser oscillator section 12 to emit light in the 1.3 μm band.

[0044] The diffraction grating layer 24 at the spot size converter section 14 constitutes a grating configured not to diffract light oscillating in the multi-quantum well layer 16. For example, the diffraction grating layer 24 at the spot size converter section 14 may include a grating having a spacing in which neither refraction nor reflection of the 1.3 μm band occurs. That is, the diffraction grating layer 24 may include a grating having a spacing configured not to effectively function as a diffraction grating. Alternatively, the diffraction grating layer 24 may not include a grating at the spot size converter section 14. The spot size converter section 14 and the laser oscillator section 12 may have structural differences only in terms of the mesa width and the diffraction grating.

[0045] The semiconductor laser may include a first individually confined heterostructure (SCH) layer 26. The first SCH layer 26 may be between the multi-quantum well layer 16 and the first cladding layer 20. The first SCH layer 26 may be included in the mesa structure 10. The first SCH layer 26 may have a first conductivity type. The first SCH layer 26 may include InGaAsP and may have a thickness of 50 nm. Alternatively, the first SCH layer 26 may include InGaAlAs.

[0046] The semiconductor laser may include a second SCH layer 28. The second SCH layer 28 may be between the multi-quantum well layer 16 and the second cladding layer 22. The second SCH layer 28 may be included in the mesa structure 10. The second SCH layer 28 may have a second conductivity type. The second SCH layer 28 may include InGaAsP and may have a thickness of 50 nm. Alternatively, the second SCH layer 28 may include InGaAlAs. The second SCH layer 28 may be identical to the first SCH layer 26 in component wavelength, but they may be different.

[0047] The semiconductor laser may include a high refractive index layer 30. The high refractive index layer 30 may be below the mesa structure 10 and the buried layer 18. The high refractive index layer 30 may not be included in the mesa structure 10. The diffraction grating layer 24 may not be in contact with the high refractive index layer 30. The diffraction grating layer 24 may be provided separately from the high refractive index layer 30, so the design flexibility is high. The high refractive index layer 30 may be wider than the mesa structure 10 and may overlap at least a portion of the buried layer 18, but may not overlap the entire buried layer 18.

[0048] The high refractive index layer 30 may be below the first cladding layer 20. The high refractive index layer 30 may have a higher refractive index than the first cladding layer 20. The high refractive index layer 30 may have a lower refractive index than the multi-quantum well layer 16. The high refractive index layer 30 may have a component wavelength that is set not to absorb light oscillated by the semiconductor laser. Here, not absorbing light means that the synthetic wavelength of the high refractive index layer 30 may be shorter than the center of the oscillation wavelength of the semiconductor laser, and does not necessarily mean that the wavelength included in the spectrum of the semiconductor laser is not absorbed at all. The high refractive index layer 30 may have a thickness of 50nm or more. The high refractive index layer 30 may have a thickness of 100nm or less. The high refractive index layer 30 may include at least one of InGaAsP, InGaAs or InGaAlAs. The high refractive index layer 30 may have a first conductivity type. The high refractive index layer 30 having a high refractive index may expand the distribution of the light L. However, since the first cladding layer 20 has a sufficient thickness, the expansion of the distribution of the light L cannot lead to an increase in the optical confinement coefficient of the multi-quantum well layer 16.

[0049] The semiconductor laser may include a substrate 32. The substrate 32 may be below the high refractive index layer 30. The substrate 32 may have a first conductivity type. The substrate 32 may include InP.

[0050] The semiconductor laser may include a low-reflection coating film 34 on the laser emitting surface. The semiconductor laser may include a high-reflection coating film 36 on the surface opposite to the laser emitting surface. On the upper surface of the buried layer 18, except near the mesa structure 10, an insulating film 38 may be provided.

[0051] The semiconductor laser may include a back electrode 40 on the back side. The semiconductor laser may include a top electrode 42 on the second cladding layer 22. A contact layer not shown may be interposed between the second cladding layer 22 and the top electrode 42. The back electrode 40 and the top electrode 42 may be used to inject current into the semiconductor laser from an external power source (not shown).

[0052] The back electrode 40 and the top electrode 42 may be provided not only at the laser oscillator section 12 but also at the spot size converter section 14. This may be used to drive the laser oscillator section 12 and also to use the spot size converter section 14 as an amplifier. Alternatively, the top electrode 42 may not be on the spot size converter section 14. Alternatively, a top electrode not shown that is separated from the top electrode 42 of the laser oscillator section unit 12 may be provided on the spot size converter section 14. In addition, a window structure not shown that includes, for example, Fe-InP may be provided at the front end of the spot size converter section 14.

[0053] Figure 4 : is a cross-sectional view of a laser oscillator section of a semiconductor laser according to a comparative example. This laser oscillator section is different from the laser oscillator section 12 in the first exemplary embodiment in that it has neither the first cladding layer 20 nor the high refractive index layer 30. They are the same in other features such as material and component wavelength.

[0054] In this comparative example, the light L' oscillating in the laser oscillator section penetrates from the first SCH layer 26, the multi-quantum well layer 16, and the second SCH layer 28 into the second cladding layer 22, the buried layer 18, and the substrate 32 having a low refractive index. Therefore, the light L' is guided around the first SCH layer 26, the multi-quantum well layer 16, and the second SCH layer 28. The regions (buried layer 18, substrate 32) on both sides of the region in and below the mesa structure 10 and where the light L' is strongly concentrated are made of a material having a low refractive index (e.g., InP). Note that the distribution of the light L and the light L' is only schematically shown in order to explain the difference between the first exemplary embodiment and the comparative example, and is not necessarily consistent with the NFP.

[0055] Comparison was made to find the cutoff width, vertical FFP, horizontal FFP, and aspect ratio (vertical / horizontal) in the first exemplary embodiment and the comparative example. The mesa width was 1.7 μm. The aspect ratio was vertical FFP / horizontal FFP. The results are shown in Table 1.

[0056] Table 1 Results

[0057]

[0058] As is clear from the results, the effects of widening the cutoff width and reducing the FFP are obtained in the first example embodiment.

[0059] In the first exemplary embodiment, the high refractive index layer 30 (e.g., InGaAsP) has a higher refractive index than the first cladding layer 20 (e.g., InP) below the mesa structure 10. Therefore, the overall distribution of the light L is widened in the downward direction by the high refractive index layer 30 compared to the comparative example. In addition, since the high refractive index layer 30 is extended below the buried layer 18, the distribution of the light L is also widened in the horizontal direction.

[0060] In the first embodiment, the refractive index of the regions on both sides of the region where the light L is strongly concentrated is larger than that of the comparative example. As a result, the refractive index difference between the region where the light L is strongly concentrated and the regions on both sides thereof becomes smaller. When the refractive index difference decreases, the high-order lateral mode cutoff width (hereinafter referred to as the cutoff width) becomes wider. If the cutoff width becomes wider, the mesa width can be widened without generating a high-order mode in the horizontal direction, so that the current density of the multi-quantum well layer 16 can be reduced. This can improve reliability and achieve higher power output because more current is injected.

[0061] The dependence on the thickness of the first cladding layer 20 was examined. Specifically, a comparison was made on how the characteristics would change when the thickness of the first cladding layer 20 was changed. The mesa width was 1.7 μm, and the thickness of the high refractive index layer 30 was 50 nm.

[0062] Figure 5A is a graph of the comparison results of the relationship between the cutoff width and the first cladding layer 20. The dotted line represents the value of the cutoff width in the comparative example shown in Table 1. When the thickness of the first cladding layer 20 is less than 500nm, the cutoff width is narrower than the cutoff width in the comparative example. This is because the effect of gathering light in the mesa structure 10 is strongly exhibited due to the proximity of the high refractive index layer 30 to the mesa structure 10. Therefore, the thickness of the first cladding layer 20 can be 500nm or more. In addition, when the thickness exceeds 1000nm, the cutoff width becomes substantially constant, so the characteristics can be stabilized by setting the thickness to 1000nm or more.

[0063] Figure 5B is a graph of the comparison results of the relationship between the vertical FFP and the first cladding layer 20. The dotted line represents the value of the vertical FFP in the comparative example shown in Table 1. When the thickness of the first cladding layer 20 is less than 500nm, the vertical FFP is greater than the vertical FFP in the comparative example. Therefore, the thickness of the first cladding layer 20 may be 500nm or more.

[0064] Figure 5Cis a graph of comparison results of the relationship between the horizontal FFP and the first cladding layer 20. The dotted line represents the value of the horizontal FFP in the comparative example shown in Table 1. When the thickness of the first cladding layer 20 is less than 500 nm, the horizontal FFP is greater than the horizontal FFP in the comparative example. Therefore, the thickness of the first cladding layer 20 may be 500 nm or more.

[0065] Figure 5D It is a graph of the comparison results of the relationship between the aspect ratio and the first cladding layer 20. The dotted line represents the value of the aspect ratio in the comparative example shown in Table 1. The aspect ratio is vertical FFP / horizontal FFP. When the thickness of the first cladding layer 20 can be about 1000nm, the aspect ratio can be minimized, and when the thickness of the first cladding layer 20 exceeds 1500nm, there may be a tendency for the aspect ratio to increase. Since the required aspect ratio depends on the lens used, the thickness of the first cladding layer 20 can be set accordingly.

[0066] Next, the dependence on the thickness of the high refractive index layer 30 was examined. In detail, a comparison was made on how the characteristics would change when the thickness of the high refractive index layer 30 was changed. The mesa width was 1.7 μm, and the thickness of the first cladding layer 20 was 1000 nm.

[0067] Fig. 6A 30 is a graph showing the comparison results of the cut-off width and the relationship between the high refractive index layer 30. The dotted line represents the value of the cut-off width in the comparative example shown in Table 1. When the thickness of the high refractive index layer 30 is 30nm or less, it is found that the cut-off width is almost the same as the cut-off width in the comparative example, with almost no influence. If the thickness of the high refractive index layer 30 is 50nm or more, the cut-off width increases.

[0068] Figure 6B is a graph of the comparison results of the relationship between the vertical FFP and the high refractive index layer 30. The dotted line represents the value of the vertical FFP in the comparative example shown in Table 1. As the high refractive index layer 30 becomes thicker, the value of the vertical FFP becomes smaller. However, it is found that when the thickness of the high refractive index layer 30 exceeds 150nm, the value of the vertical FFP tends to increase rapidly.

[0069] Figure 6C TABLE 1 is a graph showing the comparison results of the horizontal FFP in relation to the high refractive index layer 30. The dotted line indicates the value of the horizontal FFP in the comparative example shown in Table 1. As the high refractive index layer 30 becomes thicker, the value of the FFP becomes smaller. The horizontal FFP tends to decrease monotonically.

[0070] Fig.6D 1 is a graph showing the comparison results of the aspect ratio and the relationship between the high refractive index layer 30. The dotted line represents the value of the aspect ratio in the comparative example shown in Table 1. The aspect ratio is vertical FFP / horizontal FFP. When the high refractive index layer 30 exceeds 100 nm or thereafter, the aspect ratio tends to increase significantly.

[0071] From the above, from the viewpoint of cut-off width, the thickness of the high refractive index layer 30 may be greater than 50 nm, no matter how large. On the other hand, if FFP is taken into consideration, especially the aspect ratio, the high refractive index layer 30 may be 100 nm or less.

[0072] The numerical values ​​in this example embodiment may be merely examples. The thickness of the first cladding layer 20 and the thickness of the high refractive index layer 30 may be appropriately selected according to the desired characteristics. Figure 5A As can be clearly seen in FIG. 1 , in order to obtain good effects, the first cladding layer 20 may be 500 nm or more. If it is thinner than 500 nm, there is a concern that the optical confinement coefficient of the multi-quantum well layer 16 increases. The thickness of the high refractive index layer 30 may be 50 nm or more.

[0073] Since the high refractive index layer 30 may have a uniform width, the spot size converter section 14 (whose width may be narrower than the laser oscillator section 12) shapes the overall distribution of the light L close to the high refractive index layer 30. Since the high refractive index layer 30 may not be a part of the mesa structure 10 and may be widely extended, the light L is also extended horizontally. As a result, the vertical FFP and the horizontal FFP may be increased. For example, when the mesa width at the tip of the spot size converter section 14 is 0.9 μm, the vertical FFP is 22.3°, the horizontal FFP is 16.1°, and the aspect ratio is 1.38. Compared with the light distribution of the laser oscillator section 12 shown in Table 1, the FFP may be reduced without changing the aspect ratio. As a result, the tolerance of the optical coupling with the lens or the waveguide may be improved.

[0074] The high refractive index layer 30 having a high refractive index expands the vertical and horizontal distribution of the light L. Therefore, the area of ​​the NFP becomes larger and the diffusion angle of the FFP becomes smaller, resulting in improved reliability such as greater tolerance of optical coupling with optical components such as lenses.

[0075] The high refractive index layer 30 having a high refractive index increases the high-order lateral mode cutoff width. Therefore, the mesa width can be increased. When the mesa width is wider, since the current density of the mesa structure 10 becomes lower, more current can be injected, thereby achieving high power output.

[0076] In the case where the high refractive index layer 30 has a high refractive index, the distribution of the light L expands downward (toward the substrate 32). Therefore, although depending on the material, a higher power output is possible. For example, the amount of light absorption by the lower n-type semiconductor layer such as the first cladding layer 20 or the high refractive index layer 30 is less than that of the upper p-type semiconductor layer such as the second cladding layer 22. This reduces the optical confinement coefficient of the upper semiconductor layer, minimizes the internal loss due to light absorption, and enhances power output.

[0077] To fully achieve the effects of the first exemplary embodiment, the high refractive index layer 30 may have a component wavelength that does not absorb light oscillating in the multi-quantum well layer 16, may have a higher refractive index than the first cladding layer 20, and may include InGaAsP, InGaAlAs, or InGaAs.

[0078] The first cladding layer 20 may include a material (e.g., InP) having a refractive index smaller than that of any one of the high refractive index layer 30 and the first SCH layer 26. If the first cladding layer 20 is made of a material having a refractive index greater than that of InP, the distribution of the light L may be closer to the multi-quantum well layer 16. The above effects may be achieved by adjusting the thickness and composition, but the design flexibility is reduced. In addition, from the viewpoint of higher power output, the heat generated in the multi-quantum well layer 16 may be dissipated to other regions.

[0079] The first conductivity type in the above example embodiment may be an n-type, and the second conductivity type may be a p-type, but vice versa. If the first conductivity type is a p-type, the high refractive index layer 30 may be a p-type, so the internal loss may increase and a higher power output may not be fully achieved, but the cutoff width may be widened, so a higher output may be achieved by injecting more current. In addition, the effect of reducing the diffusion angle of the FFP may be fully achieved.

[0080] Figure 7 2 is a cross-sectional view of a semiconductor laser according to a modified configuration 1. The first cladding layers 20A, 20B and the high refractive index layers 30A, 30B may be alternately stacked. The high refractive index layer 30A, the first cladding layer 20A, the high refractive index layer 30B and the first cladding layer 20B may be provided on a substrate 32. The first cladding layer 20B may have a protruding portion serving as a lower end portion of the mesa structure 10.

[0081] The thickness of the first cladding layers 20A, 20B may be different, being thicker when closer to the multi-quantum well layer 16. The first cladding layer 20A may include n-InP and may have a thickness of 750 nm. The first cladding layer 20B may include n-InP and may have a thickness of 1000 nm.

[0082] The high refractive index layers 30A, 30B may be composed of two layers: an upper layer close to the multi-quantum well layer 16 and a lower layer away from the multi-quantum well layer 16. The upper layer may be thinner than the lower layer. The high refractive index layer 30A may be a lower layer, may include n-InGaAsP, and may have a thickness of 75nm. The high refractive index layer 30B may be an upper layer, may include n-InGaAsP, and may have a thickness of 50nm.

[0083] When the structure of the modified configuration 1 is adopted, the cutoff width becomes 2.07 μm, which is wider than the cutoff width of 1.74 μm in the laser oscillator section 12 in the first exemplary embodiment. In addition, when the mesa width is 1.7 μm, the vertical FFP becomes 22.4°, and the horizontal FFP becomes 15.9°, both of which are smaller than those in the laser oscillator section 12 in the first exemplary embodiment. In addition, the aspect ratio can be 1.41.

[0084] In the modified configuration, when the spot size converter section 14 is applied and the mesa width is 0.9 μm, the vertical FFP becomes 18.1°, and the horizontal FFP becomes 12.3°. The aspect ratio becomes 1.47, but it is not as large as an absolute value, and the benefit of higher power output by increasing the cutoff width is large, thereby achieving higher power output overall.

[0085] Figure 8 2 is a cross-sectional view of a semiconductor laser according to a modified configuration 2. The first cladding layers 20C, 20D, 20E, 20F and the high refractive index layers 30C, 30D, 30E, 30F may be alternately stacked. The high refractive index layer 30C, the first cladding layer 20C, the high refractive index layer 30D, the first cladding layer 20D, the high refractive index layer 30E, the first cladding layer 20E, the high refractive index layer 30F and the first cladding layer 20F may be disposed on a substrate 32. The first cladding layer 20F may have a protruding portion serving as a lower end portion of the mesa structure 10.

[0086] The thicknesses of the first cladding layers 20C, 20D, 20E, 20F may be different, being thicker as they are closer to the multi-quantum well layer 16. The first cladding layers 20C, 20D, 20E, 20F may include n-InP, and may have thicknesses of 600nm, 800nm, 1000nm, and 1200nm, respectively, from the lowermost layer.

[0087] The high refractive index layers 30C, 30D, 30E, 30F may include an uppermost layer closest to the multi-quantum well layer 16, a lowermost layer farthest from the multi-quantum well layer 16, and at least one intermediate layer between the uppermost layer and the lowermost layer. The at least one intermediate layer may be thinner than any one of the uppermost layer and the lowermost layer. The high refractive index layers 30C, 30D, 30E, 30F may include n-InGaAsP. In thickness, each of the high refractive index layers 30C, 30F as the lowermost layer and the uppermost layer may be 70nm, and each of the high refractive index layers 30D, 30E as the intermediate layer may be 50nm.

[0088] When the structure of this modified configuration is adopted, the cutoff width becomes 2.23 μm, which is wider than the laser oscillator section 12 in the first exemplary embodiment and the laser oscillator section 12 in the modified configuration 1. In addition, when the mesa width is 1.7 μm, the vertical FFP becomes 21.9°, and the horizontal FFP becomes 14.5°, both of which are smaller than the FFP at the laser oscillator section 12 in the first exemplary embodiment. The aspect ratio increases to 1.51.

[0089] In this modified configuration, when the spot size converter section 14 is applied and the mesa width is 0.9 μm, the vertical FFP becomes 14.7°, and the horizontal FFP becomes 10.9°. Furthermore, the aspect ratio becomes 1.34, which is equal to the laser oscillator section 12 in the first exemplary embodiment.

[0090] As described above, the high refractive index layers 30C, 30D, 30E, 30F and the first cladding layers 20C, 20D, 20E, 20F may be combined to achieve an increase in cutoff width and a decrease in FFP, and also to adjust the aspect ratio by using the spot size converter section 14 .

[0091] Fig. 9 is a plan view of a semiconductor laser according to a second exemplary embodiment. Fig.10 yes Fig. 9 XX cross-sectional view of the semiconductor laser shown. The semiconductor laser may be a directly modulated semiconductor laser, and may be a DFB laser in which light oscillates in a 1.3 μm band or a 1.55 μm band. The semiconductor laser may include a planar buried heterostructure (PBH).

[0092] The mesa structure 210 may include a first SCH layer 226, a multi-quantum well layer 216, and a second SCH layer 228. The first SCH layer 226 may have a first conductivity type, and the second SCH layer 228 may have a second conductivity type. In the PBH, the mesa structure 210 may be lower than the BH because it does not include the second cladding layer 222.

[0093] The buried layer 218 may be disposed on each of the two sides of the mesa structure 210. Figure 3 Compared to the BH shown, the mesa structure 210 can be lower, and thus the buried layer 218 can be thin, resulting in improved manufacturability. The mesa structure 210 does not have a spot size converter section.

[0094] The second cladding layer 222 may be on the multi-quantum well layer 216 and the buried layer 218. Alternatively, the second cladding layer 222 may be on the mesa structure 210 and the buried layer 218. The second cladding layer 222 may have a second conductivity type (e.g., p-type) and may include an InP layer. The contact layer 244 may be disposed on the second cladding layer 222. The contact layer 244 may have a second conductivity type and may be made of InGaAs. The PBH may be a structure in which the second cladding layer 222 and the contact layer 244 may be widely disposed on the mesa structure 210.

[0095] The third cladding layer 246 may be between the high refractive index layer 230 and the substrate 232. The high refractive index layer 230 and the first cladding layer 220 may be stacked on the third cladding layer 246. The third cladding layer 246 may have a first conductivity type (eg, n-type), and may include InP.

[0096] The diffraction grating layer 224 may be between the third cladding layer 246 and the substrate 232. The third cladding layer 246 may be disposed on the diffraction grating layer 224. The diffraction grating layer 224 may be disposed on the substrate 232. The diffraction grating layer 224 may be closer to the substrate 232 than the high refractive index layer 230, but may be used as a diffraction grating by adjusting the thickness of the third cladding layer 246, whereby the semiconductor laser may be driven as a DFB laser.

[0097] The semiconductor laser may include a pair of grooves 248. The pair of grooves 248 along the direction in which the mesa structure 210 extends may be formed to extend to both end faces (the low-reflective coating 234 may be formed at one end face, and the high-reflective coating 236 may be formed at the other end face). The pair of grooves 248 may be formed to have a depth from the surface of the uppermost layer (e.g., the contact layer 244) of the semiconductor laminate to the substrate 232. The pair of grooves 248 may have a depth reaching the substrate 232 through the buried layer 218 and the first cladding layer 220.

[0098] The insulating film 238 may be disposed on the surface of the uppermost layer of the semiconductor laminate except for the upper region of the mesa structure 210, and may be disposed on the inner surface of the pair of grooves 248. The surface electrode 242 may include a mesa top electrode 250 extending along the mesa structure 210, a pad electrode 252 disposed on only one side adjacent to the mesa structure 210, and an extraction electrode 254 connected therebetween. The extraction electrode 254 may contact the inner surface of one of the pair of grooves 248.

[0099] The mesa structure 210 may be between the pair of grooves 248. The mesa structure 210 may be located away from the center of the semiconductor laser in a direction away from the pad electrode 252. Fig.10In the embodiment, light can be guided around the mesa structure 210 to be horizontally diffused. The paired grooves 248 can be disposed in a region that has substantially no effect on the value of the FFP. That is, light that can be substantially coupled to the external lens remains in the region between the paired grooves 248.

[0100] In the PBH, the contact layer 244 may be widely provided, and thus the parasitic capacitance may be larger than that of the BH. However, the paired grooves 248 may reduce the area of ​​the contact layer 244 directly conducting to the second cladding layer 222, thereby making it possible to reduce the parasitic capacitance.

[0101] The pair of grooves 248 may separate the high refractive index layer 230 and the first cladding layer 220. Therefore, although light is not diffused in a region beyond the pair of grooves 248, similar to the first exemplary embodiment, a high-order transverse mode cutoff width may be wider, thereby achieving high power output of single-mode transmission.

[0102] The high refractive index layer 230 may not be included in the mesa structure 210 that may include the multi-quantum well layer 216 , may be wider than the mesa structure 210 , and may continuously overlap the entire mesa structure 210 and at least a portion of the buried layer 218 (between the pair of trenches 248 ).

[0103] In the first embodiment, the semiconductor laser includes: a multi-quantum well layer 16, which is included in the mesa structure 10; a buried layer 18, which is made of a semi-insulating semiconductor, the buried layer 18 is in contact with each of the two sides of the mesa structure 10; a first cladding layer 20, which has a first conductivity type, below the mesa structure 10 and the buried layer 18, the first cladding layer 20 has a lower refractive index than the multi-quantum well layer 16; a high refractive index layer 30, which is configured not to absorb light oscillating in the multi-quantum well layer 16, the high refractive index layer 30 is formed on the mesa structure 10; Below the surface structure 10 and the buried layer 18, below the first cladding layer 20, a high refractive index layer 30 has a higher refractive index than the first cladding layer 20; a diffraction grating layer 24, which at least partially constitutes a diffraction grating capable of diffracting light oscillating in the multi-quantum well layer 16, and the diffraction grating layer 24 does not contact the high refractive index layer 30; a substrate 32, which has a first conductivity type, below the high refractive index layer 30; and a second cladding layer 22, which has a second conductivity type opposite to the first conductivity type, above the multi-quantum well layer 16.

[0104] In a second embodiment alone or in combination with the first embodiment, the semiconductor laser further includes: a first SCH layer 26 having a first conductivity type, between the multi-quantum well layer 16 and the first cladding layer 20, the first SCH layer 26 being included in the mesa structure 10; and a second SCH layer 28 having a second conductivity type, between the multi-quantum well layer 16 and the second cladding layer 22, the second SCH layer 28 being included in the mesa structure 10.

[0105] In a third embodiment alone or in combination with one or more of the first and second implementations, the first cladding layer 20 has a thickness of 500 nm or more.

[0106] In a fourth embodiment alone or in combination with one or more of the first to third embodiments, the first cladding layer 20 has a thickness of 1500 nm or less.

[0107] In the fifth embodiment alone or in combination with one or more of the first to fourth embodiments, the high refractive index layer 30 has a thickness of 50 nm or more.

[0108] In the sixth embodiment alone or in combination with one or more of the first to fifth embodiments, the high refractive index layer 30 has a thickness of 100 nm or less.

[0109] In the seventh embodiment, alone or in combination with one or more of the first to sixth embodiments, the first cladding layer 20 is one of a plurality of first cladding layers 20A-20F, the high refractive index layer 30 is one of a plurality of high refractive index layers 30A-30F, and the plurality of first cladding layers 20A-20F and the plurality of high refractive index layers 30A-30F are alternately stacked.

[0110] In the eighth embodiment alone or in combination with one or more of the first to seventh embodiments, the plurality of first cladding layers 20A- 20F have different thicknesses, being thicker as they are closer to the multi-quantum well layer 16 .

[0111] In a ninth embodiment alone or in combination with one or more of the first to eighth embodiments, the plurality of high refractive index layers 30A-30F include two layers: an upper layer closer to the multi-quantum well layer 16 and a lower layer farther from the multi-quantum well layer 16, and the upper layer is thinner than the lower layer.

[0112] In a tenth embodiment alone or in combination with one or more of the first to ninth embodiments, the plurality of high refractive index layers 30A-30F include an uppermost layer closest to the multi-quantum well layer 16, a lowermost layer farthest from the multi-quantum well layer 16, and at least one intermediate layer between the uppermost layer and the lowermost layer, and the at least one intermediate layer is thinner than either the uppermost layer or the lowermost layer.

[0113] In an eleventh embodiment alone or in combination with one or more of the first to tenth embodiments, the mesa structure 10 includes a spot size converter section 14 whose width perpendicular to the light emission direction gradually decreases, and the diffraction grating layer 24 constitutes a grating at the spot size converter section 14 configured not to diffract light oscillating in the multi-quantum well layer 16.

[0114] In the twelfth embodiment alone or in combination with one or more of the first to eleventh embodiments, the second cladding layer 22 is included in the mesa structure 10 .

[0115] In the thirteenth embodiment, alone or in combination with one or more of the first to twelfth embodiments, the diffraction grating layer 24 is within the second cladding layer 22 .

[0116] In the fourteenth embodiment alone or in combination with one or more of the first to thirteenth embodiments, the second cladding layer 222 is on the multiple quantum well layer 216 and the buried layer 218 .

[0117] In a fifteenth embodiment alone or in combination with one or more of the first to fourteenth embodiments, the semiconductor laser further comprises a third cladding layer 246 between the high refractive index layer 230 and the substrate 232, wherein the diffraction grating layer 224 is between the third cladding layer 246 and the substrate 232.

[0118] In the sixteenth embodiment alone or in combination with one or more of the first to fifteenth embodiments, the mesa structure 210 is between the pair of grooves 248 having a depth through the buried layer 218 and the first cladding layer 220 to the substrate 232 .

[0119] In a seventeenth embodiment alone or in combination with one or more of the first to sixteenth embodiments, the first conductivity type is n-type, and the second conductivity type is p-type.

[0120] In the eighteenth embodiment alone or in combination with one or more of the first to seventeenth embodiments, the high refractive index layer 30 includes at least one of InGaAsP, InGaAs, or InGaAlAs.

[0121] In a nineteenth embodiment, alone or in combination with one or more of the first to eighteenth embodiments, at least one of the substrate 32 , the buried layer 18 , the first cladding layer 20 , or the second cladding layer 22 includes InP.

[0122] In the twentieth embodiment alone or in combination with one or more of the first to nineteenth embodiments, the high refractive index layer 30 has a lower refractive index than the multiple quantum well layer 16 .

[0123] The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Modifications and variations may be made based on the foregoing disclosure, or may be obtained from the practice of the embodiments. In addition, any embodiments described herein may be combined, unless the foregoing disclosure explicitly provides reasons why one or more embodiments may not be combined.

[0124] Even if specific combinations of features are described in the claims and / or disclosed in the specification, these combinations are not intended to limit the disclosure of the various embodiments. In fact, many of these features can be combined in ways that are not specifically described in the claims and / or disclosed in the specification. Although each dependent claim listed below may be directly subordinate to only one claim, the disclosure of the various embodiments includes the combination of each dependent claim with each other claim in the claim set. As used herein, a phrase referring to "at least one" in a list of items refers to any combination of these items, including single members. As an example, "at least one of a, b, or c" is intended to cover a, b, c, ab, ac, bc, and abc, as well as any combination with multiple identical items.

[0125] Unless clearly described, the elements, actions or instructions used herein should not be interpreted as key or necessary. In addition, as used herein, the articles "one" and "an" are intended to include one or more projects, and can be used interchangeably with "one or more". In addition, as used herein, the article "the" is intended to include one or more projects quoted in conjunction with the article "the", and can be used interchangeably with "one or more". In addition, as used herein, the term "set" is intended to include one or more projects (for example, a combination of related projects, unrelated projects or related and unrelated projects), and can be used interchangeably with "one or more". In the case of only expecting one project, the phrase "only one" or similar language is used. In addition, as used herein, the term "has", "have", "having" etc. are intended to be open terms. In addition, unless otherwise clearly stated, the phrase "based on" is intended to represent "at least partially based on". In addition, as used herein, the term "or" is intended to be inclusive when used in series, and can be used interchangeably with "and / or", unless otherwise clearly stated (for example, if used in combination with "any one" or "only one"). In addition, for ease of description, spatially relative terms, such as "below," "lower," "above," "upper," etc., may be used herein to describe the relationship of one element or feature to another element or features as shown in the accompanying drawings. Spatially relative terms are intended to encompass different orientations of the device, apparatus, and / or element in use or operation in addition to the orientation depicted in the accompanying drawings. The device may be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Claims

1. A semiconductor laser comprising: substrate; a multi-quantum well layer included in the mesa structure and located above the substrate; a buried layer in contact with the first side and the second side of the mesa structure; a first cladding layer, having a first conductivity type and located below the buried layer; a second cladding layer, having a second conductivity type different from the first conductivity type, and located above the mesa structure; A high refractive index layer, located below the first cladding layer; a third cladding layer, located between the high refractive index layer and the substrate; as well as a diffraction grating layer, located between the third cladding layer and the substrate; wherein the first cladding layer has a lower refractive index than the multi-quantum well layer, and The high refractive index layer has a higher refractive index than the first cladding layer. 2 . The semiconductor laser according to claim 1 , wherein the second cladding layer is located on the buried layer. 3 . The semiconductor laser according to claim 2 , wherein the mesa structure is located between a pair of grooves extending through the buried layer and the first cladding layer to the substrate. 4 . The semiconductor laser according to claim 1 , wherein the third cladding layer has a lower refractive index than the multiple quantum well layer. 5 . The semiconductor laser according to claim 1 , wherein a portion of the first cladding layer is included in the mesa structure. 6 . The semiconductor laser according to claim 1 , wherein the high refractive index layer has a lower refractive index than the multiple quantum well layer.

7. The semiconductor laser according to claim 1, wherein the high refractive index layer comprises at least one of the following: InGaAsP, InGaAs or InGaAlAs. 8 . The semiconductor laser according to claim 1 , wherein at least one of the substrate, the buried layer, the first cladding layer, the second cladding layer, or the third cladding layer comprises InP. 9 . The semiconductor laser according to claim 1 , wherein the high refractive index layer is configured to have a component wavelength that is shorter than a center of an oscillation wavelength of the semiconductor laser. 10 . The semiconductor laser according to claim 1 , wherein the diffraction grating layer is configured to diffract light oscillating in the multiple quantum well layer.

11. The semiconductor laser according to claim 1, further comprising: A first separated confinement heterostructure layer, having the first conductivity type, is located between the multi-quantum well layer and the first cladding layer, and the first separated confinement heterostructure layer is included in the mesa structure; as well as A second separated confinement heterostructure layer, having the second conductivity type, is located between the multi-quantum well layer and the second cladding layer, and the second separated confinement heterostructure layer is included in the mesa structure. 12 . The semiconductor laser according to claim 1 , wherein the first cladding layer has a thickness of 500 nm or more. 13 . The semiconductor laser according to claim 12 , wherein the thickness of the first cladding layer is 1500 nm or less.

14. The semiconductor laser according to claim 1, wherein the high refractive index layer has a thickness of 50 nm or more. 15 . The semiconductor laser according to claim 14 , wherein the high refractive index layer has a thickness of 100 nm or less.

16. The semiconductor laser according to claim 1, wherein: The first coating layer is one of a plurality of first coating layers, The high refractive index layer is one high refractive index layer among a plurality of high refractive index layers, and The plurality of first cladding layers and the plurality of high refractive index layers are alternately stacked.

17. The semiconductor laser according to claim 16, wherein the plurality of first cladding layers have different thicknesses, wherein a fourth cladding layer among the plurality of first cladding layers is closer to the multi-quantum well layer than the first cladding layer, and The fourth cladding layer is thicker than the first cladding layer.

18. The semiconductor laser according to claim 16, wherein the plurality of high refractive index layers include an upper layer and a lower layer, wherein the upper layer is closer to the multi-quantum well layer than the lower layer, and The upper layer is thinner than the lower layer.

19. The semiconductor laser according to claim 16, wherein the plurality of high refractive index layers include an uppermost layer, a lowermost layer, and at least one intermediate layer, the at least one intermediate layer being located between the uppermost layer and the lowermost layer, wherein the uppermost layer is closer to the multi-quantum well layer than the lowermost layer, and The at least one intermediate layer is thinner than the uppermost layer or the lowermost layer.

20. A semiconductor laser comprising: substrate; a multi-quantum well layer included in the mesa structure and located above the substrate; a buried layer in contact with the first side and the second side of the mesa structure; a first cladding layer, having a first conductivity type and located below the buried layer; a second cladding layer, having a second conductivity type different from the first conductivity type, and located above the mesa structure; A high refractive index layer, located below the first cladding layer; as well as Diffraction grating layer; wherein the first cladding layer has a lower refractive index than the multi-quantum well layer, wherein the high refractive index layer has a higher refractive index than the first cladding layer, wherein the mesa structure comprises a spot size converter section, the width of the spot size converter section perpendicular to the light emission direction gradually decreases, and The portion of the diffraction grating layer located at the other section of the mesa structure is configured to diffract light oscillating in the multiple quantum well layer.

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