semiconductor laser

Abstract

To improve output characteristics and reliability.SOLUTION: A semiconductor laser includes: a multiple quantum well layer 16 included in a mesa structure 10; a buried layer 18 made of a semi-insulating semiconductor in contact with both sides of the mesa structure 10; a first clad layer 20 of a first conductivity type that is below the mesa structure 10 and the buried layer 18 and being lower in refractive index than the multiple quantum well layer 16; a high refractive index layer 30 that is under the first clad layer 20 below the mesa structure 10 and the buried layer 18, has a higher refractive index than the first clad layer 20, and absorbs no light oscillating in the multiple quantum well layer 16; a diffraction grating layer 24 that is not in contact with the high refractive index layer 30 and at least partially constitutes a diffraction grating capable of diffracting light oscillated in the multiple quantum well layer 16; a substrate 32 of a first conduction type under the high refraction index layer 30; and a second clad layer 22 of a second conduction type opposite to the first conduction type on the multiple quantum well layer 16.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] Semiconductor lasers with a buried heterostructure (BH structure) have excellent output characteristics and reliability. The BH structure has a large optical confinement factor in the mesa structure, making it easy to bring the aspect ratio of the output light close to 1. In addition, by widening the mesa width, the current density of the multi-quantum well (MQW) can be reduced, further increasing output power and reliability. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-212664 Summary of the Invention [Problem to be solved by the invention]

[0004] If the optical confinement factor of the mesa structure is large, the area of ​​the NFP (Near Field Pattern) becomes small, which increases the divergence angle of the FFP (Far Field Pattern), leading to a decrease in optical coupling tolerance. Also, if the mesa width exceeds the cutoff width of higher-order transverse modes, higher-order transverse modes appear, which leads to a decrease in optical coupling efficiency and a decrease in output power.

[0005] Patent Document 1 discloses that an n-InP buffer layer 4 and an n-InGaAsP guide layer 3 are disposed below the mesa structure. The n-InGaAsP guide layer 3 is disposed to fill the diffraction grating. The diffraction grating is a factor that has a significant impact on optical characteristics, specifically, it is the main factor that determines the optical coupling coefficient κ. Therefore, due to the need to balance this with κ, the design freedom of the n-InGaAsP guide layer 3 is limited. Furthermore, because the n-InP buffer layer 4 is thin (30 nm), it is presumed that the underlying n-InGaAsP guide layer 3 increases the optical confinement coefficient of the mesa structure, thereby narrowing the cutoff width.

[0006] The present invention aims to improve output characteristics and reliability. [Means for solving the problem]

[0007] The semiconductor laser includes: a multiple quantum well layer included in a mesa structure; burying layers made of a semi-insulating semiconductor and in contact with both sides of the mesa structure; a first cladding layer of a first conductivity type located below the mesa structure and the burying layer and having a refractive index lower than that of the multiple quantum well layer; a high refractive index layer located below the mesa structure and the burying layer and below the first cladding layer, having a refractive index higher than that of the first cladding layer and not absorbing light oscillated in the multiple quantum well layer; a diffraction grating layer not in contact with the high refractive index layer and at least partially constituting a diffraction grating capable of diffracting the light oscillated in the multiple quantum well layer; a substrate of the first conductivity type located below the high refractive index layer; and a second cladding layer of a second conductivity type opposite to the first conductivity type located above the multiple quantum well layer.

[0008] The high refractive index layer expands the light distribution, which increases the cutoff width of the higher-order transverse modes, increases the area of ​​the NFP, and reduces the divergence angle of the FFP, improving the output characteristics and reliability. [Brief explanation of the drawings]

[0009] [Figure 1]1 is a plan view of a semiconductor laser according to a first embodiment. [Figure 2] 2 is a cross-sectional view taken along line II-II of the semiconductor laser shown in FIG. [Figure 3] 3 is a cross-sectional view taken along line III-III of the semiconductor laser shown in FIG. [Figure 4] FIG. 10 is a cross-sectional view of a laser oscillation portion of a semiconductor laser according to a comparative example. [Figure 5A] FIG. 10 is a diagram showing the simulation results of the cutoff width for the first cladding layer. [Figure 5B] FIG. 10 is a diagram showing simulation results of the longitudinal FFP for the first cladding layer. [Figure 5C] FIG. 10 is a diagram showing simulation results of lateral FFP for the first cladding layer. [Figure 5D] FIG. 10 is a diagram showing the simulation results of the aspect ratio for the first cladding layer. [Figure 6A] FIG. 10 is a diagram showing the simulation results of the cutoff width for a high refractive index layer. [Figure 6B] FIG. 10 is a diagram showing simulation results of the vertical FFP for a high refractive index layer. [Figure 6C] FIG. 10 shows simulation results of transverse FFP for a high refractive index layer. [Figure 6D] FIG. 10 is a diagram showing the results of a simulation of the aspect ratio for a high refractive index layer. [Figure 7] FIG. 10 is a cross-sectional view of a semiconductor laser according to Modification 1. [Figure 8] FIG. 10 is a cross-sectional view of a semiconductor laser according to Modification 2. [Figure 9] FIG. 4 is a plan view of a semiconductor laser according to a second embodiment. [Figure 10] 10 is a cross-sectional view of the semiconductor laser shown in FIG. 9 taken along line XX. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Elements with the same reference numerals in all the drawings have the same or equivalent functions, and their repeated explanation will be omitted. Note that the size of the figures does not necessarily correspond to the magnification.

[0011] Silicon photonics, which has been attracting attention in recent years, requires high-power semiconductor optical devices due to its characteristics, and in particular high-power semiconductor lasers. Known semiconductor laser structures are the ridge type and the buried heterostructure (BH structure). A semiconductor laser with a BH structure is called a BH type semiconductor laser.

[0012] In a ridge-type semiconductor laser, a multi-quantum well (MQW) is widely arranged on a substrate, and the semiconductor layer (mainly the cladding layer) on top of that forms a ridge portion. The sides of the ridge portion are covered with an insulating film and a semiconductor layer. In a BH-type semiconductor laser, the semiconductor layer including the MQW has a mesa structure (stripe shape), and buried layers made of semi-insulating semiconductors are adjacent to both sides of the mesa structure.

[0013] When comparing ridge-type semiconductor lasers with BH-type semiconductor lasers, the BH-type semiconductor laser, in which both sides of the MQW are covered with buried layers with high thermal conductivity, has superior heat dissipation properties.

[0014] In BH-type semiconductor lasers, the MQWs are generally surrounded by the same semiconductor material (e.g., InP) from above, below, left, and right, and the surface has fewer irregularities compared to ridge-type semiconductor lasers, so the stress on the MQWs is small. Therefore, from the perspective of reliability due to stress, BH-type semiconductor lasers are superior to ridge-type semiconductor lasers.

[0015] 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 a semiconductor laser cannot be fully utilized, even if it is high. For example, when optically coupling a semiconductor laser to a lens, from the perspective of optical coupling efficiency and tolerance during optical axis adjustment, it is preferable that the spread angle of the FFP (Far Field Pattern) of the output light from the semiconductor laser is small, and that the aspect ratio is close to 1. BH-type semiconductor lasers generally have an aspect ratio of output light closer to 1 than ridge-type semiconductor lasers, and are therefore superior in terms of optical coupling characteristics.

[0016] For the reasons mentioned above, semiconductor lasers with the BH structure are superior as semiconductor lasers that satisfy high output characteristics, high reliability, etc. Note that the above is merely a comparison, and by optimizing both ridge-type semiconductor lasers and BH-type semiconductor lasers, it is possible to provide semiconductor lasers that are fully suitable for practical use.

[0017] [First embodiment] Fig. 1 is a plan view of a semiconductor laser according to a first embodiment. Fig. 2 is a cross-sectional view of the semiconductor laser shown in Fig. 1 taken along line II-II. Fig. 3 is a cross-sectional view of the semiconductor laser shown in Fig. 1 taken along line III-III. The semiconductor laser is a BH type semiconductor laser, and is a continuous wave laser or a directly modulated laser.

[0018] [Mesa structure] The semiconductor laser has a mesa structure 10. The mesa structure 10 has a laser oscillation section 12 that oscillates light of a predetermined wavelength. The laser oscillation section 12 is configured to output continuous light. The oscillation wavelength of the laser oscillation section 12 is in the 1.3 μm band, but it may be other wavelengths such as the 1.55 μm band. The laser oscillation section 12 is a DFB (Distributed Feedback) laser. The mesa structure 10 has a width (mesa width) of 1.7 μm at the laser oscillation section 12.

[0019] The mesa structure 10 includes a spot size conversion portion 14. The spot size conversion portion 14 is adjacent to the laser oscillation portion 12. The spot size conversion portion 14 has a width (mesa width) that is perpendicular to the light output direction, which gradually decreases. As the mesa width becomes narrower, the optical confinement factor of the mesa structure 10 decreases. The spot size conversion portion 14 adjusts the emission area of ​​the laser light, thereby adjusting the area of ​​the NFP and the divergence angle of the FFP.

[0020] The laser oscillation section 12 and the spot size conversion section 14 have one common pin structure. In this application, "one common pin structure" means a pin structure made of the same material and formed by the same process. Because they have one common pin structure, the gain band does not vary from the laser oscillation section 12 to the spot size conversion section 14. The gain band is the wavelength range of the gain spectrum. The mesa structure 10 is made up of multiple layers of pin structures.

[0021] [Multiple quantum well layer] The semiconductor laser has a multiple quantum well layer 16. The multiple quantum well layer 16 is included in the mesa structure 10. The multiple quantum well layer 16 has a multilayer structure in which multiple quantum well layers and multiple barrier layers are alternately sandwiched, and has a total layer thickness of 100 nm. The multiple quantum well layer 16 is made of InGaAsP, but may also be made of InGaAlAs.

[0022] [Embedding Layer] The semiconductor laser has a burying layer 18. The burying layer 18 contacts each of both sides of the mesa structure 10. Each of the two side surfaces of the mesa structure 10 is covered with the burying layer 18. The burying layer 18 is made of a semi-insulating semiconductor. The burying layer 18 contains semi-insulating InP (e.g., Fe—InP).

[0023] [First cladding layer] The semiconductor laser has a first cladding layer 20. The first cladding layer 20 is located below the mesa structure 10 and the burying layer 18. A portion of the first cladding layer 20 (e.g., a protruding portion on the upper surface) is 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 is made of InP. Since InP has a higher thermal conductivity than InGaAsP and the like, it is preferable to make the first cladding layer 20 of InP to improve heat dissipation from the multiple quantum well layer 16. The first cladding layer 20 is of a first conductivity type (e.g., n-type). The first cladding layer 20 has a lower refractive index than the multiple quantum well layer 16. The thickness of the first cladding layer 20 is 500 nm or more (e.g., 1000 nm) and 1500 nm or less.

[0024] [Second cladding layer] The semiconductor laser has a second cladding layer 22. The second cladding layer 22 is on the multiple quantum well layer 16. The second cladding layer 22 is of a second conductivity type (e.g., p-type) opposite to the first conductivity type. The second cladding layer 22 is included in the mesa structure 10. The second cladding layer 22 is made of InP and has a thickness of 2000 nm.

[0025] [Grating layer] The semiconductor laser has a diffraction grating layer 24. The diffraction grating layer 24 is made of InGaAsP and has a thickness of 10 nm. The diffraction grating layer 24 is located inside the second cladding layer 22. The diffraction grating layer 24 is included in the mesa structure 10. The diffraction grating layer 24 at least partially constitutes a diffraction grating that can diffract light oscillated in the multiple quantum well layer 16. For example, the diffraction grating layer 24 is formed in accordance with the composition of the multiple quantum well layer 16 so that the laser oscillation portion 12 emits light in the 1.3 μm band.

[0026] In the spot size converter 14, the diffraction grating layer 24 forms a grating that does not diffract the light oscillated in the multiple quantum well layer 16. For example, in the spot size converter 14, the diffraction grating layer 24 has a grating formed at a spacing that does not cause refraction or reflection in the 1.3 μm band. In other words, the diffraction grating layer 24 has a grating formed at a spacing that does not effectively function as a diffraction grating. Alternatively, the diffraction grating layer 24 may not have a grating formed in the spot size converter 14. The structural differences between the spot size converter 14 and the laser oscillator 12 are only the mesa width and the diffraction grating.

[0027] [First optical confinement layer] The semiconductor laser has a first optical confinement (SCH: Separate Confinement Heterostructure) layer 26. The first optical confinement layer 26 is located between the multiple quantum well layer 16 and the first cladding layer 20. The first optical confinement layer 26 is included in the mesa structure 10. The first optical confinement layer 26 is of a first conductivity type. The first optical confinement layer 26 is made of InGaAsP and has a thickness of 50 nm. Alternatively, the first optical confinement layer 26 may be made of InGaAlAs.

[0028] [Second optical confinement layer] The semiconductor laser has a second optical confinement layer 28. The second optical confinement layer 28 is located between the multiple quantum well layer 16 and the second cladding layer 22. The second optical confinement layer 28 is included in the mesa structure 10. The second optical confinement layer 28 is of the second conductivity type. The second optical confinement layer 28 is made of InGaAsP and has a thickness of 50 nm. Alternatively, the second optical confinement layer 28 may be made of InGaAlAs. The second optical confinement layer 28 may be the same as or different from the first optical confinement layer 26 in terms of composition wavelength.

[0029] [High refractive index layer] The semiconductor laser has a high-refractive index layer 30. The high-refractive index layer 30 is below the mesa structure 10 and the burying layer 18. The high-refractive index layer 30 is not included in the mesa structure 10. The diffraction grating layer 24 is not in contact with the high-refractive index layer 30. Since the diffraction grating layer 24 is provided separately from the high-refractive index layer 30, there is a high degree of freedom in design. The high-refractive index layer 30 only needs to be wider than the mesa structure 10 and overlap at least a portion of the burying layer 18, but does not have to overlap the entire burying layer 18.

[0030] The high-refractive index layer 30 is located below the first cladding layer 20. The high-refractive index layer 30 has a higher refractive index than the first cladding layer 20. The high-refractive index layer 30 has a lower refractive index than the multiple quantum well layer 16. The high-refractive index layer 30 has a composition wavelength set so as not to absorb the light emitted by the semiconductor laser. Here, "not absorbing light" means that the composition wavelength of the high-refractive index layer 30 is shorter than the center of the oscillation wavelength of the semiconductor laser, but does not mean that it does not absorb all wavelengths included in the optical spectrum of the semiconductor laser. The high-refractive index layer 30 has a thickness of 50 nm or more. The high-refractive index layer 30 has a thickness of 100 nm or less. The high-refractive index layer 30 contains any of InGaAsP, InGaAs, and InGaAlAs. The high-refractive index layer 30 is of the first conductivity type. The presence of the high-refractive index layer 30 expands the distribution of light L. However, the presence of the first cladding layer 20 with a sufficient thickness prevents the expansion of the distribution of light L from increasing the optical confinement factor of the multiple quantum well layer 16.

[0031] [substrate] The semiconductor laser has a substrate 32. The substrate 32 is located below the high refractive index layer 30. The substrate 32 is of a first conductivity type. The substrate 32 is made of InP.

[0032] [others] The semiconductor laser has a low-reflection coating film 34 on its laser emission surface. The semiconductor laser has a high-reflection coating film 36 on the surface opposite to the laser emission surface. An insulating film 38 is disposed on the top surface of the burying layer 18 except for the vicinity of the mesa structure 10.

[0033] The semiconductor laser has a back electrode 40 on the back side. The semiconductor laser has a front electrode 42 on the second cladding layer 22. A contact layer (not shown) is interposed between the second cladding layer 22 and the front electrode 42. The back electrode 40 and the front electrode 42 are used to inject current from an external power supply (not shown) into the semiconductor laser.

[0034] The back electrode 40 and the front electrode 42 are provided not only on the laser oscillator unit 12 but also on the spot size converter 14. This is because the spot size converter 14 is used as an amplifier in addition to driving the laser oscillator unit 12. Alternatively, the front electrode 42 does not have to be located on the spot size converter 14. Alternatively, a front electrode (not shown) separate from the front electrode 42 of the laser oscillator unit 12 may be located on the spot size converter 14. Furthermore, a window structure (not shown) made of, for example, Fe-InP may be provided at the tip of the spot size converter 14.

[0035] [Comparative Example] 4 is a cross-sectional view of the laser oscillation section of a semiconductor laser according to the comparative example. The laser oscillation section differs from the laser oscillation section 12 of the first embodiment in that it does not have the first cladding layer 20 and the high refractive index layer 30. Other characteristics (materials, composition wavelength, etc.) are the same between the two.

[0036] In the comparative example, light L' oscillated in the laser oscillation portion seeps out from the first optical confinement layer 26, the multiple quantum well layer 16, and the second optical confinement layer 28 into the second cladding layer 22, the burying layer 18, and the substrate 32, which have a low refractive index. Therefore, the light L' is guided mainly through the first optical confinement layer 26, the multiple quantum well layer 16, and the second optical confinement layer 28. The mesa structure 10 and the regions immediately below it on both sides of the region where the light L' is strongly concentrated (the burying layer 18 and the substrate 32) are made of a material with a low refractive index (e.g., InP). Note that the distributions of light L and light L' are merely shown schematically to explain the difference between the first embodiment and the comparative example, and do not necessarily coincide with the NFP.

[0037] For the first embodiment and the comparative example, simulations were performed on the cutoff width, vertical FFP, horizontal FFP, and aspect ratio (vertical / horizontal) of the laser oscillation section 12. The mesa width was set to 1.7 μm. The aspect ratio is vertical FFP / horizontal FFP. The results are shown in Table 1.

[0038] [Table 1]

[0039] As is clear from these results, the effects of widening the cutoff width and reducing FFP were observed.

[0040] In the first embodiment, the high-refractive-index layer 30 (e.g., InGaAsP) below the mesa structure 10 has a higher refractive index than the first cladding layer 20 (e.g., InP). Therefore, compared to the comparative example, the high-refractive-index layer 30 causes the distribution of light L to spread generally downward. Moreover, because the high-refractive-index layer 30 spreads below the burying layer 18, the distribution of light L also spreads laterally.

[0041] In the first embodiment, the refractive index of the regions on both sides of the region where light L is strongly concentrated is greater than that of the comparative example. As a result, the difference in refractive index between the region where light L is strongly concentrated and the regions on both sides of it is smaller. As the refractive index difference becomes smaller, the cutoff width of the higher-order transverse mode (hereinafter referred to as cutoff width) becomes wider. If the cutoff width is wider, it becomes possible to widen the mesa width without generating higher-order modes in the transverse direction, and it becomes possible to reduce the current density of the multiple quantum well layer 16. As a result, reliability is improved, and a larger current can be injected, thereby achieving higher output.

[0042] [Simulation of laser oscillation section 12] The dependence of the characteristics on the thickness of the first cladding layer 20 was investigated. Specifically, a simulation was performed to examine how the characteristics change when the thickness of the first cladding layer 20 is changed. The mesa width was set to 1.7 μm, and the thickness of the high refractive index layer 30 was set to 50 nm.

[0043] FIG. 5A shows the results of a simulation of the cutoff width for the first cladding layer 20. The dashed line indicates the cutoff width value for the comparative example shown in Table 1. When the thickness of the first cladding layer 20 is less than 500 nm, the cutoff width is narrower than that of the comparative example. This is presumably because the high-refractive-index layer 30 is close to the mesa structure 10, which has a strong effect of focusing light into the mesa structure 10. Therefore, it is clear that a thickness of 500 nm or more is preferable for the first cladding layer 20. Furthermore, when the thickness exceeds 1000 nm, the cutoff width becomes roughly constant, and by setting it to 1000 nm or more, stable characteristics can be achieved.

[0044] 5B is a diagram showing the simulation results of the longitudinal FFP for the first cladding layer 20. The dashed line indicates the longitudinal FFP value of the comparative example shown in Table 1. When the thickness of the first cladding layer 20 is less than 500 nm, the longitudinal FFP is larger than that of the comparative example. Therefore, the thickness of the first cladding layer 20 is preferably 500 nm or more.

[0045] 5C is a diagram showing the simulation results of the lateral FFP for the first cladding layer 20. The dashed line indicates the lateral FFP value of the comparative example shown in Table 1. When the thickness of the first cladding layer 20 is less than 500 nm, the lateral FFP is larger than that of the comparative example. Therefore, the thickness of the first cladding layer 20 is preferably 500 nm or more.

[0046] FIG. 5D shows the simulation results of the aspect ratio for the first cladding layer 20. The dashed line indicates the aspect ratio values ​​for the comparative examples shown in Table 1. The aspect ratio is vertical FFP / horizontal FFP. The aspect ratio was smallest when the thickness of the first cladding layer 20 was around 1000 nm, and tended to increase when the thickness exceeded 1500 nm. The required aspect ratio differs depending on the lens used, so the thickness of the first cladding layer 20 can be set appropriately.

[0047] Next, we investigated the dependency on the thickness of the high refractive index layer 30. Specifically, we performed a simulation to see how the characteristics change when the thickness of the high refractive index layer 30 is changed. The mesa width was set to 1.7 μm, and the thickness of the first cladding layer 20 was set to 1000 nm.

[0048] 6A is a diagram showing the results of a simulation of the cutoff width for the high-refractive-index layer 30. The dashed line indicates the cutoff width value for the comparative example shown in Table 1. When the thickness of the high-refractive-index layer 30 is 30 nm or less, the cutoff width is almost the same as that of the comparative example, and the effect is small. When the thickness of the high-refractive-index layer 30 is 50 nm or more, the cutoff width increases.

[0049] 6B is a diagram showing the simulation results of the longitudinal FFP for the high refractive index layer 30. The dashed line indicates the longitudinal FFP value of the comparative example shown in Table 1. As the high refractive index layer 30 becomes thicker, the longitudinal FFP value decreases. However, when the thickness of the high refractive index layer 30 exceeds 150 nm, the longitudinal FFP value tends to increase sharply.

[0050] 6C is a diagram showing the simulation results of the lateral FFP for the high refractive index layer 30. The dashed line indicates the lateral FFP value of the comparative example shown in Table 1. As the high refractive index layer 30 becomes thicker, the FFP value becomes smaller. The lateral FFP showed a monotonically decreasing trend.

[0051] 6D is a diagram showing the results of a simulation of the aspect ratio of the high refractive index layer 30. The dashed line indicates the aspect ratio values ​​of the comparative examples shown in Table 1. The aspect ratio is vertical FFP / horizontal FFP. The aspect ratio tends to increase significantly once the high refractive index layer 30 exceeds 100 nm.

[0052] From the viewpoint of the cutoff width, the thickness of the high refractive index layer 30 may be larger as long as it is 50 nm or more. On the other hand, if FFP, particularly the aspect ratio, is also taken into consideration, the thickness of the high refractive index layer 30 is preferably 100 nm or less.

[0053] Note that the numerical values ​​in this embodiment are merely examples. The thickness of the first cladding layer 20 and the high refractive index layer 30 may be selected appropriately depending on the required characteristics. However, as is clear from FIG. 5A, to obtain a sufficient effect, the first cladding layer 20 is preferably 500 nm or more. If it is thinner than this, there is a concern that the optical confinement coefficient of the multiple quantum well layer 16 may become too large. Furthermore, the thickness of the high refractive index layer 30 is preferably 50 nm or more.

[0054] [Simulation of spot size conversion section 14] Because the width of the high-refractive-index layer 30 remains constant, the light L distribution in the spot-size conversion region 14, which is narrower than the laser oscillation region 12, is concentrated in the high-refractive-index layer 30 as a whole. Because the high-refractive-index layer 30 is not part of the mesa structure 10 and is widely distributed, the light L also spreads laterally. As a result, the FFP can be increased in both the vertical and horizontal directions. For example, if the mesa width at the tip of the spot-size conversion region 14 is 0.9 μm, the FFP in the vertical direction is 22.3°, the FFP in the horizontal direction is 16.1°, and the aspect ratio is 1.38. Compared to the light distribution in the laser oscillation region 12 shown in Table 1, the FFP can be reduced without significantly changing the aspect ratio. As a result, the tolerance of optical coupling with a lens or waveguide can be improved.

[0055] [effect] The presence of the high refractive index layer 30 expands the distribution of light L in both the vertical and horizontal directions, thereby increasing the area of ​​the NFP and reducing the divergence angle of the FFP, thereby improving reliability by increasing the tolerance of optical coupling with optical components such as lenses.

[0056] The presence of the high refractive index layer 30 increases the cutoff width of the higher-order transverse mode. This allows the mesa width to be widened. A wider mesa width reduces the current density in the mesa structure 10, allowing more current to be injected, thereby achieving higher output power.

[0057] The presence of the high refractive index layer 30 expands the distribution of light L downward (toward the substrate 32). This can increase output power depending on the material. For example, the n-type semiconductor layers located below, such as the first cladding layer 20 and the high refractive index layer 30, absorb less light than the p-type semiconductor layers located above, such as the second cladding layer 22. This reduces the optical confinement coefficient of the upper semiconductor layers, reducing internal loss due to light absorption and increasing output intensity.

[0058] To fully obtain the effects of the first embodiment, it is preferable that the high refractive index layer 30 has a composition wavelength that does not absorb the light oscillated by the multiple quantum well layer 16 and has a refractive index higher than that of the first cladding layer 20. Specifically, it is preferable that the high refractive index layer 30 be made of InGaAsP, InGaAlAs, or InGaAs.

[0059] The first cladding layer 20 is made of a material (specifically, InP) with a refractive index lower than that of the high-refractive-index layer 30 and the first optical confinement layer 26. If the first cladding layer 20 were made of a material with a refractive index higher than that of InP, the distribution of light L may be shifted more toward the multiple quantum well layer 16. While the above-mentioned effects may be achieved by adjusting the thickness and composition, this reduces the degree of freedom in design. Furthermore, from the perspective of increasing output, it is preferable to dissipate as much heat generated in the multiple quantum well layer 16 as possible to other regions.

[0060] In the above-described embodiment, the first conductivity type is n-type and the second conductivity type is p-type, but the reverse is also possible. If the first conductivity type is p-type, the high refractive index layer 30 is p-type, which increases internal loss and may prevent sufficient high output. However, since the cutoff width can be widened, high output can be achieved by injecting more current. Furthermore, the effect of reducing the FFP divergence angle can be sufficiently obtained.

[0061] [Variation 1] 7 is a cross-sectional view of a semiconductor laser according to Modification 1. A plurality of first cladding layers 20A, 20B and a plurality of high-refractive-index layers 30A, 30B are 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 are disposed on a substrate 32. A convex portion that becomes the lower end of the mesa structure 10 is formed on the first cladding layer 20B.

[0062] The first cladding layers 20A, 20B have different thicknesses, with the thickness increasing toward the multiple quantum well layer 16. The first cladding layer 20A is made of n-InP and has a thickness of 750 nm. The first cladding layer 20B is made of n-InP and has a thickness of 1000 nm.

[0063] The multiple high-refractive-index layers 30A, 30B consist of two layers: an upper layer close to the multiple quantum well layer 16 and a lower layer far from the multiple quantum well layer 16. The upper layer is thinner than the lower layer. The lower high-refractive-index layer 30A is made of n-InGaAsP and has a thickness of 75 nm. The upper high-refractive-index layer 30B is made of n-InGaAsP and has a thickness of 50 nm.

[0064] When the structure of this modification is adopted, the cutoff width becomes 2.07 μm, which is significantly wider than 1.74 μm of the laser oscillation section 12 of the first embodiment. Furthermore, at a mesa width of 1.7 μm, the vertical FFP becomes 22.4° and the horizontal FFP becomes 15.9°, which are smaller than those of the laser oscillation section 12 of the first embodiment. However, the aspect ratio is slightly degraded to 1.41.

[0065] In this modification, when the spot size conversion portion 14 is applied and the mesa width is set to 0.9 μm, the vertical FFP is reduced to 18.1° and the horizontal FFP to 12.3°. Although the aspect ratio deteriorates to 1.47, the absolute value is not that large, and the benefit of higher output due to the increased cutoff width is significant, so overall higher output can be achieved.

[0066] [Variation 2] 8 is a cross-sectional view of a semiconductor laser according to Modification 2. Multiple first cladding layers 20C, 20D, 20E, and 20F and multiple high-refractive-index layers 30C, 30D, 30E, and 30F are alternately stacked. High-refractive-index layer 30C, first cladding layer 20C, high-refractive-index layer 30D, first cladding layer 20D, high-refractive-index layer 30E, first cladding layer 20E, high-refractive-index layer 30F, and first cladding layer 20F are arranged on substrate 32. A convex portion that becomes the lower end of mesa structure 10 is formed on first cladding layer 20F.

[0067] The first cladding layers 20C, 20D, 20E, and 20F have different thicknesses, with the layers being thicker closer to the multiple quantum well layer 16. The first cladding layers 20C, 20D, 20E, and 20F are all made of n-InP, and have thicknesses of 600 nm, 800 nm, 1000 nm, and 1200 nm, respectively, from the bottom layer.

[0068] The multiple high-refractive-index layers 30C, 30D, 30E, and 30F include a top layer closest to the multiple quantum well layer 16, a bottom layer farthest from the multiple quantum well layer 16, and at least one intermediate layer between the top and bottom layers. The at least one intermediate layer is thinner than the top and bottom layers. The multiple high-refractive-index layers 30C, 30D, 30E, and 30F are all made of n-InGaAsP. The bottom and top high-refractive-index layers 30C and 30F are 70 nm thick, and the intermediate high-refractive-index layers 30D and 30E are 50 nm thick.

[0069] When the structure of this modification is adopted, the cutoff width becomes 2.23 μm, which is significantly wider than the laser oscillation section 12 of the first embodiment and the laser oscillation section 12 of modification 1. Furthermore, at a mesa width of 1.7 μm, the vertical FFP becomes 21.9° and the horizontal FFP becomes 14.5°, which are smaller than the laser oscillation section 12 of the first embodiment. The aspect ratio becomes larger at 1.51.

[0070] In this modification, when the spot size conversion portion 14 is applied and the mesa width is set to 0.9 μm, the vertical FFP can be further reduced to 14.7° and the horizontal FFP to 10.9°. Furthermore, the aspect ratio is 1.34, which is equivalent to that of the laser oscillation portion 12 of the first embodiment.

[0071] In this way, by combining multiple high refractive index layers 30C, 30D, 30E, and 30F with multiple first cladding layers 20C, 20D, 20E, and 20F, it is possible to increase the cutoff width and reduce the FFP, while also adjusting the aspect ratio by using the spot size conversion section 14.

[0072] [Second embodiment] Fig. 9 is a plan view of a semiconductor laser according to a second embodiment. Fig. 10 is a cross-sectional view of the semiconductor laser shown in Fig. 9 taken along line XX. The semiconductor laser is a directly modulated semiconductor laser, and is a DFB laser that oscillates in the 1.3 μm band or the 1.55 μm band. The semiconductor laser has a PBH (Planer BH) structure.

[0073] The mesa structure 210 includes a first optical confinement layer 226, a multiple quantum well layer 216, and a second optical confinement layer 228. The first optical confinement layer 226 is of a first conductivity type, and the second optical confinement layer 228 is of a second conductivity type. In the PBH structure, the mesa structure 210 does not include the second cladding layer 222, and therefore is lower than the BH structure.

[0074] A burying layer 218 is disposed on each side of the mesa structure 210. In the PBH structure, the mesa structure 210 is lower than in the BH structure shown in Fig. 3, and therefore the burying layer 218 is thinner, which improves manufacturability. The mesa structure 210 does not have a spot size conversion portion.

[0075] The second cladding layer 222 is located on the multiple quantum well layer 216 and the buried layer 218. Alternatively, the second cladding layer 222 is located on the mesa structure 210 and the buried layer 218. The second cladding layer 222 is of a second conductivity type (e.g., p-type) and is made of an InP layer. A contact layer 244 is located on the second cladding layer 222. The contact layer 244 is of the second conductivity type and is made of InGaAs. The PBH structure is a structure in which the second cladding layer 222 and the contact layer 244 are located widely on the mesa structure 210.

[0076] A third cladding layer 246 is located between the high refractive index layer 230 and the substrate 232. The high refractive index layer 230 and the first cladding layer 220 are stacked on the third cladding layer 246. The third cladding layer 246 is of a first conductivity type (e.g., n-type) and is made of InP.

[0077] The diffraction grating layer 224 is located between the third cladding layer 246 and the substrate 232. The third cladding layer 246 is disposed on the diffraction grating layer 224. The diffraction grating layer 224 is disposed on the substrate 232. The diffraction grating layer 224 is closer to the substrate 232 than the high refractive index layer 230, but by adjusting the thickness of the third cladding layer 246, it functions as a diffraction grating, and the semiconductor laser is driven as a DFB laser.

[0078] The semiconductor laser has a pair of grooves 248. The pair of grooves 248 are formed along the direction in which the mesa structure 210 extends, so as to reach two end faces (the end face on which the low-reflection coating film 234 is formed and the end face on which the high-reflection coating film 236 is formed). The pair of grooves 248 are formed to a depth that reaches 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 have a depth that reaches the substrate 232 through the buried layer 218 and the first cladding layer 220.

[0079] An insulating film 238 is disposed on the surface of the uppermost layer of the semiconductor laminate except for the region above the mesa structure 210, and is also disposed on the inner surfaces of the pair of grooves 248. The surface electrode 242 includes an on-mesa electrode 250 extending along the mesa structure 210, a pad electrode 252 disposed only on one side of the mesa structure 210, and an extraction electrode 254 connecting these. The extraction electrode 254 is in contact with the inner surface of one of the pair of grooves 248.

[0080] The mesa structure 210 is located between a pair of grooves 248. The mesa structure 210 is located at a position offset from the center of the semiconductor laser in a direction away from the pad electrode 252. Light is guided, spreading laterally in the lateral direction of FIG. 10, with the mesa structure 210 as the center. The pair of grooves 248 are located in an area that does not substantially affect the FFP value. In other words, light that is substantially coupled to an external lens remains within the area sandwiched between the pair of grooves 248.

[0081] In the PBH structure, the contact layer 244 is widely arranged, which results in a larger parasitic capacitance than in the BH structure. However, by arranging the pair of grooves 248, the area of ​​the contact layer 244 that is directly connected to the second cladding layer 222 can be reduced, thereby reducing the parasitic capacitance.

[0082] The pair of grooves 248 separates the high refractive index layer 230 and the first cladding layer 220. As a result, light does not spread beyond the pair of grooves 248, but as in the first embodiment, the cutoff width of the higher-order transverse mode can be widened, enabling high-output single-mode transmission.

[0083] The high refractive index layer 230 is not included in the mesa structure 210 that includes the multiple quantum well layer 216, is wider than the mesa structure 210, and continuously overlaps the entire mesa structure 210 and at least a portion of the buried layer 218 (the portion between the pair of grooves 248).

[0084] [Outline of the embodiment] (1) A semiconductor laser comprising: a multiple quantum well layer 16 included in a mesa structure 10; burying layers 18 made of a semi-insulating semiconductor and in contact with both sides of the mesa structure 10; a first cladding layer 20 of a first conductivity type located below the mesa structure 10 and the burying layer 18 and having a refractive index lower than that of the multiple quantum well layer 16; a high refractive index layer 30 located below the mesa structure 10 and the burying layer 18 and below the first cladding layer 20, having a refractive index higher than that of the first cladding layer 20 and not absorbing light oscillated in the multiple quantum well layer 16; a diffraction grating layer 24 not in contact with the high refractive index layer 30 and at least partially constituting a diffraction grating capable of diffracting light oscillated in the multiple quantum well layer 16; a substrate 32 of the first conductivity type located below the high refractive index layer 30; and a second cladding layer 22 of a second conductivity type opposite to the first conductivity type located above the multiple quantum well layer 16.

[0085] (2) The semiconductor laser according to (1), further comprising: a first optical confinement layer 26 of a first conductivity type, which is located between the multiple quantum well layer 16 and the first cladding layer 20 and is included in the mesa structure 10; and a second optical confinement layer 28 of a second conductivity type, which is located between the multiple quantum well layer 16 and the second cladding layer 22 and is included in the mesa structure 10.

[0086] (3) A semiconductor laser according to (1) or (2), wherein the thickness of the first cladding layer 20 is 500 nm or more.

[0087] (4) The semiconductor laser according to (3), wherein the thickness of the first cladding layer 20 is 1500 nm or less.

[0088] (5) The semiconductor laser according to any one of (1) to (4), wherein the high refractive index layer 30 has a thickness of 50 nm or more.

[0089] (6) The semiconductor laser according to (5), wherein the thickness of the high refractive index layer 30 is 100 nm or less.

[0090] (7) A semiconductor laser according to any one of (1) to (6), wherein the first cladding layer is each of the plurality of first cladding layers 20A to 20F, the high refractive index layer is each of the plurality of high refractive index layers 30A to 30F, and the plurality of first cladding layers 20A to 20F and the plurality of high refractive index layers 30A to 30F are alternately stacked.

[0091] (8) The semiconductor laser according to (7), wherein the first cladding layers 20A to 20F have different thicknesses, and the closer they are to the multiple quantum well layer 16, the thicker they are.

[0092] (9) A semiconductor laser according to (8), wherein the plurality of high refractive index layers 30A-30B are composed of two layers, an upper layer close to the multiple quantum well layer 16 and a lower layer far from the multiple quantum well layer 16, and the upper layer is thinner than the lower layer.

[0093] (10) A semiconductor laser according to (8), wherein the plurality of high refractive index layers 30C to 30F include a top layer closest to the multiple quantum well layer 16, a bottom layer farthest from the multiple quantum well layer 16, and at least one intermediate layer between the top layer and the bottom layer, and the at least one intermediate layer is thinner than the top layer and the bottom layer.

[0094] (11) A semiconductor laser according to any one of (1) to (10), wherein the mesa structure 10 includes a spot size conversion section 14 whose width in a direction perpendicular to the light output direction gradually decreases, and the diffraction grating layer 24 forms a grating in the spot size conversion section 14 that does not diffract light oscillated in the multiple quantum well layer 16.

[0095] (12) The semiconductor laser according to any one of (1) to (11), wherein the second cladding layer 22 is included in the mesa structure 10.

[0096] (13) The semiconductor laser according to (12), wherein the diffraction grating layer 24 is located inside the second cladding layer 22.

[0097] (14) The semiconductor laser according to any one of (1) to (11), wherein the second cladding layer 222 is located on the multiple quantum well layer 216 and the burying layer 218.

[0098] (15) A semiconductor laser according to (14), further comprising a third cladding layer 246 between the high refractive index layer 230 and the substrate 232, and the diffraction grating layer 224 is located between the third cladding layer 246 and the substrate 232.

[0099] (16) A semiconductor laser according to (14) or (15), wherein the mesa structure 210 is located between a pair of grooves 248 having a depth that extends through the buried layer 218 and the first cladding layer 220 to the substrate 232.

[0100] (17) A semiconductor laser according to any one of (1) to (16), wherein the first conductivity type is n-type and the second conductivity type is p-type.

[0101] (18) The semiconductor laser according to any one of (1) to (17), wherein the high refractive index layer 30 contains any one of InGaAsP, InGaAs, and InGaAlAs.

[0102] (19) A semiconductor laser according to any one of (1) to (18), wherein at least one of the substrate 32, the burying layer 18, the first cladding layer 20, and the second cladding layer 22 contains InP.

[0103] (20) The semiconductor laser according to any one of (1) to (19), wherein the high refractive index layer 30 has a refractive index lower than that of the multiple quantum well layer 16.

[0104] The present invention is not limited to the above-described embodiments and various modifications are possible. For example, the configurations described in the embodiments can be replaced with substantially the same configurations, configurations that achieve the same effects, or configurations that can achieve the same objectives. [Explanation of symbols]

[0105] 10 mesa structure, 12 laser oscillation section, 14 spot size conversion section, 16 multiple quantum well layer, 18 burying layer, 20 first cladding layer, 20A first cladding layer, 20B first cladding layer, 20C first cladding layer, 20D first cladding layer, 20E first cladding layer, 20F first cladding layer, 22 second cladding layer, 24 diffraction grating layer, 26 first optical confinement layer, 28 second optical confinement layer, 30 high refractive index layer, 30A high refractive index layer, 30B high refractive index layer, 30C high refractive index layer, 30D high refractive index layer, 30E high refractive index layer, 30F high refractive index layer, 32 substrate, 34 low reflection coating film, 36 high reflection coating film, 38 insulating film, 40 back electrode, 42 front electrode, 210 mesa structure, 216 multiple quantum well layer, 218 burying layer, 220 First cladding layer, 222 second cladding layer, 224 diffraction grating layer, 226 first optical confinement layer, 228 second optical confinement layer, 230 high refractive index layer, 232 substrate, 234 low reflection coating film, 236 high reflection coating film, 238 insulating film, 242 surface electrode, 244 contact layer, 246 third cladding layer, 248 groove, 250 mesa electrode, 252 pad electrode, 254 extraction electrode.

Claims

1. a multiple quantum well layer included in a mesa structure; a buried layer made of a semi-insulating semiconductor and contacting both sides of the mesa structure; a first cladding layer of a first conductivity type located below the mesa structure and the buried layer and having a refractive index lower than that of the multiple quantum well layer; a high refractive index layer located below the mesa structure and the burying layer and below the first cladding layer, the high refractive index layer having a refractive index higher than that of the first cladding layer and not absorbing light oscillated in the multiple quantum well layer; a diffraction grating layer that is not in contact with the high refractive index layer and at least partially constitutes a diffraction grating that can diffract the light oscillated in the multiple quantum well layer; a substrate of the first conductivity type underlying the high refractive index layer; a second cladding layer of a second conductivity type opposite to the first conductivity type overlying the multiple quantum well layer; and the thickness of the first cladding layer is 500 nm or more and 1500 nm or less; the first cladding layer is each of a plurality of first cladding layers; the high refractive index layer is each of a plurality of high refractive index layers, the plurality of first cladding layers and the plurality of high refractive index layers are alternately stacked, the plurality of high refractive index layers each include an upper layer close to the multiple quantum well layer and a lower layer far from the multiple quantum well layer; The upper layer is thinner than the lower layer.

2. 2. The semiconductor laser according to claim 1, a first optical confinement layer of the first conductivity type that is located between the multiple quantum well layer and the first cladding layer and is included in the mesa structure; a second optical confinement layer of the second conductivity type, which is located between the multiple quantum well layer and the second cladding layer and is included in the mesa structure; The semiconductor laser further comprises:

3. 3. The semiconductor laser according to claim 1, The thickness of the high refractive index layer is 50 nm or more.

4. 4. The semiconductor laser according to claim 3, The thickness of the high refractive index layer is 100 nm or less.

5. 5. The semiconductor laser according to claim 1, The plurality of first cladding layers have different thicknesses, and the thickness increases as the cladding layers approach the multiple quantum well layer.

6. 6. The semiconductor laser according to claim 5, the plurality of high refractive index layers include an uppermost layer closest to the multiple quantum well layer, a lowermost layer farthest from the multiple quantum well layer, and at least one intermediate layer between the uppermost layer and the lowermost layer; The at least one intermediate layer is thinner than the top layer and the bottom layer.

7. 7. The semiconductor laser according to claim 1, the mesa structure includes a spot size conversion portion whose width in a direction perpendicular to the light output direction gradually decreases, The diffraction grating layer in the spot size conversion portion constitutes a grating that does not diffract the light oscillated in the multiple quantum well layer.

8. 8. The semiconductor laser according to claim 1, The second cladding layer is included in the mesa structure of the semiconductor laser.

9. 9. The semiconductor laser according to claim 8, The diffraction grating layer is located inside the second cladding layer of the semiconductor laser.

10. 8. The semiconductor laser according to claim 1, The second cladding layer is located above the multiple quantum well layer and the burying layer.

11. 11. The semiconductor laser according to claim 10, a third clad layer between the high refractive index layer and the substrate; The grating layer is located between the third cladding layer and the substrate.

12. 12. The semiconductor laser according to claim 10 or 11, The mesa structure is located between a pair of grooves having a depth that passes through the buried layer and the first cladding layer and reaches the substrate.

13. 13. The semiconductor laser according to claim 1, The first conductivity type is n-type, and the second conductivity type is p-type.

14. 14. The semiconductor laser according to claim 1, The high refractive index layer contains any one of InGaAsP, InGaAs, and InGaAlAs.

15. 15. The semiconductor laser according to claim 1, At least one of the substrate, the burying layer, the first cladding layer, and the second cladding layer contains InP.

16. 16. The semiconductor laser according to claim 1, The high refractive index layer has a refractive index lower than that of the multiple quantum well layer.

17. a multiple quantum well layer included in a mesa structure; a buried layer made of a semi-insulating semiconductor and contacting both sides of the mesa structure; a first cladding layer of a first conductivity type located below the mesa structure and the buried layer and having a refractive index lower than that of the multiple quantum well layer; a high refractive index layer located below the mesa structure and the burying layer and below the first cladding layer, the high refractive index layer having a refractive index higher than that of the first cladding layer and not absorbing light oscillated in the multiple quantum well layer; a diffraction grating layer that is not in contact with the high refractive index layer and at least partially constitutes a diffraction grating that can diffract the light oscillated in the multiple quantum well layer; a substrate of the first conductivity type underlying the high refractive index layer; a second cladding layer of a second conductivity type opposite to the first conductivity type overlying the multiple quantum well layer; and the first cladding layer is each of a plurality of first cladding layers; the high refractive index layer is each of a plurality of high refractive index layers, the plurality of first cladding layers and the plurality of high refractive index layers are alternately stacked, The plurality of first cladding layers have different thicknesses, and the thickness increases as the cladding layers approach the multiple quantum well layer.

Images