Semiconductor laser

Abstract

A first distributed Bragg reflector region (132a) and a second distributed Bragg reflector region (132b) are formed continuously with an active region (131), with the active region (131) interposed therebetween in a waveguide direction. The first distributed Bragg reflector region (132a) is provided with a first diffraction grating (121a), and the second distributed Bragg reflector region (132b) is provided with a second diffraction grating (121b). The first diffraction grating (121a) is composed of recesses formed in the first distributed Bragg reflector region (132a) and protrusions adjacent to the recesses. Similarly, the second diffraction grating (121b) is composed of recesses formed in the second distributed Bragg reflector region (132b) and protrusions adjacent to the recesses. In the first diffraction grating (121a) and the second diffraction grating (121b), the depth of the recesses becomes shallower toward the active region (131).

Description

semiconductor laser

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

[0002] In order to suppress the effects of dispersion during propagation, light sources for optical communications are required to oscillate in a single longitudinal mode. To oscillate a semiconductor laser in a single longitudinal mode, a diffraction grating is generally used as a resonator, as in DFB lasers, DR lasers, and DBR lasers (Non-Patent Documents 1, 2, and 3).

[0003] Attempts are being made to introduce optical communications using these semiconductor lasers not only into metro networks but also into optical interconnects for short-distance communications such as between chips. One of the challenges in this study is that no light source suitable for inter-chip optical interconnects has been reported to date, and short-active-layer length lasers, which enable low power consumption, are expected to be a candidate.

[0004] In a configuration with a short active layer length, the loss due to light emission (mirror loss) increases, and unless the reflectivity of the resonator is high, the oscillation conditions will not be met. The reflectivity of a diffraction grating is determined by the product of the coupling coefficient and the length, and the larger this value, the higher the reflectivity. For this reason, diffraction gratings with a large coupling coefficient are used in short active layer length lasers (Non-Patent Documents 2 and 3).

[0005] K. Utaka et al., "l / 4-Shifted InGaAsP / InP DFB Lasers" IEEE Journal of Quantum Electrons, Vol. QE-22, No. 7, pp. 1042-1051, 1986.E. Kanno, K. Takeda, T. Fujii, K. Hasebe, H. Nishi, T. Yamamoto, T. Kakitsuka and S. Matsuo, "Twin-mirror membrane distributed-reflector lasers using 20-mm-long active region on Si substrates" Opt. Express, vol. 26, no. 2, pp. 1268-1277, 2018.K. Takeda, E. Kanno, T. Fujii, K. Hasebe, H. Nishi, T. Yamamoto, T. Kakitsuka and S. Matsuo, "Continuous-wave Operation of Ultra-short Cavity Distributed Bragg Reflector Lasers on Si Substrates", Proc. CSW, paper ThD1-2, Toyama, Japan, 2016.

[0006] Increasing the coupling coefficient of a diffraction grating requires increasing the change in refractive index. By increasing the depth of the concave portions of the periodic concaves and convexes that make up the diffraction grating, the change in refractive index can be increased, resulting in a high reflectance. However, this increases the difference in equivalent refractive index between the active region and the waveguide region, which increases optical loss due to scattering at the connection between the active region and the waveguide region.

[0007] The present invention has been made to solve the above problems, and has as its object to suppress scattering at the connection portion between the waveguide region and the active region and to increase the refractive index change of the diffraction grating.

[0008] A semiconductor laser according to the present invention comprises: an active region including an active layer made of a compound semiconductor formed on a substrate; and a first distributed Bragg reflector region including a first diffraction grating and a second distributed Bragg reflector region including a second diffraction grating, which are formed contiguously with the active region on either side of the active region in the waveguiding direction, the first diffraction grating consisting of recesses formed in the first distributed Bragg reflector region and protrusions adjacent to the recesses, and the second diffraction grating consisting of recesses formed in the second distributed Bragg reflector region and protrusions adjacent to the recesses, and the first diffraction grating and the second diffraction grating have at least one of a configuration in which the depth of the recesses becomes shallower as they approach the active region and a configuration in which the pitch becomes larger as they approach the active region.

[0009] As described above, according to the present invention, the first diffraction grating formed in the first distributed Bragg reflector region and the second diffraction grating in the second distributed Bragg reflector region are configured so that the depth of the recesses becomes shallower as they approach the active region, or the pitch becomes larger as they approach the active region, thereby suppressing scattering at the connection between the waveguide region and the active region and making it possible to increase the change in refractive index of the diffraction grating.

[0010] FIG. 1A is a cross-sectional view showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 1B is a cross-sectional view showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 1C is a cross-sectional view showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 2 is a plan view showing the configuration of a diffraction grating used in a calculation of the effect of reducing optical loss by gradually changing the depth of recesses of periodic asperities constituting the diffraction grating from the end. FIG. 3A is a characteristics diagram showing the calculation results of the effect of reducing optical loss by gradually changing the depth of recesses of periodic asperities constituting the diffraction grating from the end. FIG. 3B is a characteristics diagram showing the calculation results of the effect of reducing optical loss by gradually changing the depth of recesses of periodic asperities constituting the diffraction grating from the end. FIG. 3C is a characteristics diagram showing the calculation results of the effect of reducing optical loss by gradually changing the depth of recesses of periodic asperities constituting the diffraction grating from the end. FIG. 4 is a distribution diagram showing the light distribution when light is incident on a distributed Bragg reflector region on which a diffraction grating is formed. FIG. 5 is a characteristics diagram showing the effective length of a diffraction grating. FIG. 6 is a cross-sectional view showing the configuration of a semiconductor laser according to a second embodiment of the present invention. FIG. 7 is a plan view showing the configuration of a diffraction grating used in a calculation of the effect of reducing optical loss by gradually changing the pitch of the periodic asperities constituting the diffraction grating from the end. FIG. 8A is a characteristics diagram showing the calculation results of the effect of reducing optical loss by gradually changing the pitch of the periodic asperities constituting the diffraction grating from the end. FIG. 8B is a characteristics diagram showing the calculation results of the effect of reducing optical loss by gradually changing the pitch of the periodic asperities constituting the diffraction grating from the end. FIG. 8C is a characteristics diagram showing the calculation results of the effect of reducing optical loss by gradually changing the pitch of the periodic asperities constituting the diffraction grating from the end. FIG. 9 is a distribution diagram showing the light distribution when light is incident on a distributed Bragg reflector region in which a diffraction grating is formed.

[0011] A semiconductor laser according to an embodiment of the present invention will now be described.

[0012] First Embodiment First, a semiconductor laser according to a first embodiment of the present invention will be described with reference to Figures 1A, 1B, and 1C. Figure 1A shows a cross section of a plane parallel to the waveguide direction. Figure 1B shows a cross section taken along line bb' in Figure 1A. Figure 1C shows a cross section taken along line cc' in Figure 1A.

[0013] This semiconductor laser includes an active region 131, and a first distributed Bragg reflector region 132a and a second distributed Bragg reflector region 132b disposed contiguous to the active region 131. The active region 131, the first distributed Bragg reflector region 132a, and the second distributed Bragg reflector region 132b are formed on the same substrate 101 with a lower cladding layer 102 made of an insulating material such as SiO2 interposed therebetween. The substrate 101 may be made of, for example, Si. The lower cladding layer 102 may have a thickness of, for example, 2 μm.

[0014] First distributed Bragg reflector region 132a and second distributed Bragg reflector region 132b are formed contiguously with active region 131, sandwiching active region 131 in the waveguiding direction. First distributed Bragg reflector region 132a includes first diffraction grating 121a, and second distributed Bragg reflector region 132b includes second diffraction grating 121b. This semiconductor laser is a so-called DBR laser.

[0015] Active region 131 has active layer 103 made of a compound semiconductor. Active layer 103 may have a quantum well structure with a thickness of 150 nm, in which well layers and barrier layers made of InGaAsP are alternately stacked. First distributed Bragg reflector region 132a is formed contiguously with active layer 103 and includes first core layer 113a made of a compound semiconductor. Second distributed Bragg reflector region 132b is formed contiguously with active layer 103 and includes second core layer 113b made of a compound semiconductor.

[0016] First diffraction grating 121a is composed of recesses formed in first distributed Bragg reflector region 132a and protrusions adjacent to the recesses. Similarly, second diffraction grating 121b is composed of recesses formed in second distributed Bragg reflector region 132b and protrusions adjacent to the recesses.

[0017] 1B , the active region 131 includes an n-type semiconductor layer 105 and a p-type semiconductor layer 106 formed in contact with the active layer 103. The n-type semiconductor layer 105 and the p-type semiconductor layer 106 can be made of a compound semiconductor such as InP. In this example, the n-type semiconductor layer 105 and the p-type semiconductor layer 106 are disposed in the planar direction of the substrate 101, sandwiching the active layer 103 therebetween, and are formed in contact with the side surfaces of the active layer 103.

[0018] The semiconductor laser according to the first embodiment also includes an n-type electrode 107 electrically connected to the n-type semiconductor layer 105, and a p-type electrode 108 electrically connected to the p-type semiconductor layer 106. In the semiconductor laser according to the first embodiment, a current is injected into the active layer 103 of the active region 131 in a direction parallel to the plane of the substrate 101 (lateral direction). It is also possible to form the n-type electrode 107 on the n-type semiconductor layer 105 via an n-type contact layer in which n-type impurities are introduced at a higher concentration. Similarly, it is also possible to form the p-type electrode 108 on the p-type semiconductor layer 106 via a p-type contact layer in which p-type impurities are introduced at a higher concentration.

[0019] Furthermore, the active layer 103 is sandwiched between semiconductor layers 104a and 104b in the vertical direction when viewed from the substrate 101. The semiconductor layers 104a and 104b can be made of a compound semiconductor such as undoped InP. The stacked structure of the semiconductor layers 104a, the active layer 103, and the semiconductor layers 104b is sandwiched between an n-type semiconductor layer 105 and a p-type semiconductor layer 106. The p-type semiconductor layer 106 and the n-type semiconductor layer 105 are formed to sandwich the active layer 103 in a direction parallel to the plane of the substrate 101.

[0020] In this example, an active layer 103 is formed on and in contact with a semiconductor layer 104a, and a semiconductor layer 104b is formed on and in contact with the active layer 103. An n-type semiconductor layer 105 and a p-type semiconductor layer 106 are formed on and in contact with the side portions of the stacked structure of the semiconductor layer 104a, the active layer 103, and the semiconductor layer 104b.

[0021] On the other hand, as shown in Figure 1C, in the first distributed Bragg reflector region 132a, the n-type semiconductor layer 105 and the p-type semiconductor layer 106 are not formed, and the side surface of the first core layer 113a in the waveguiding direction is open. The first core layer 113a in the first distributed Bragg reflector region 132a is formed in a stripe shape extending from the back to the front of the page in Figure 1C. The second core layer 113b is similar to the first core layer 113a described above. Note that the n-type electrode 107 and the p-type electrode 108 are not formed in the first distributed Bragg reflector region 132a and the second distributed Bragg reflector region 132b.

[0022] Although not shown, a first upper cladding layer made of InP or the like may be provided on the first core layer 113a. Similarly, although not shown, a second upper cladding layer made of InP or the like may be provided on the second core layer 113b.

[0023] In the first embodiment, first diffraction grating 121a and second diffraction grating 121b have recesses that are shallower toward active region 131. For example, in a configuration in which laser light is emitted from first distributed Bragg reflector region 132a, first diffraction grating 121a has recesses that are shallower toward emission end 141. In this case, second diffraction grating 121b can have recesses that are uniform (the same) toward end 142, but second diffraction grating 121b can also have recesses that are shallower toward end 142.

[0024] According to the first embodiment, the depth of the recesses in first diffraction grating 121a and second diffraction grating 121b becomes shallower as they approach active region 131, thereby reducing the refractive index difference at first connection 151 between active region 131 and first distributed Bragg reflector region 132a. Similarly, the refractive index difference at second connection 152 between active region 131 and second distributed Bragg reflector region 132b can be reduced. Furthermore, the depth of the recesses in first diffraction grating 121a becomes shallower as they approach output end 141, thereby reducing the refractive index difference at the connection (output end 141) between the emitted laser and the optical waveguide to which it is optically coupled. As a result, the coupling coefficient (refractive index change) of the diffraction grating can be increased while suppressing scattering at the connection.

[0025] Next, we present the results of calculations on the effect of reducing optical loss by gradually changing the depth of the concave portions of the periodic concaves and convexes that make up the diffraction grating, using the finite-difference time-domain (FDTD) method.

[0026] The calculations were performed on the structure shown in Figure 2, in which the depth was changed in stages. Here, a diffraction grating (concave and convex) with a pitch of 300 nm and a duty ratio of 50% was used for 50 periods. For comparison, calculations were also performed on the same structure as above, but with the depth kept constant at 100 nm. The calculation results are shown in Figures 3A, 3B, and 3C. Figure 3A shows the transmission and reflection spectrum when light is incident on a distributed Bragg reflector region (waveguide) in which a diffraction grating of the comparative configuration is formed. Figure 3B shows the transmission and reflection spectrum when light is incident on a distributed Bragg reflector region (waveguide) in which a diffraction grating of the configuration shown in Figure 2 is formed.

[0027] As shown in Figure 3A, when the recess depth of the diffraction grating is constant, the reflectance R (dashed line) within the stop band decreases due to scattering loss. In contrast, as shown in Figure 3B, when the depth is changed stepwise, the reflectance R approaches 1.

[0028] If there is no waveguide loss at the connection between the active region and the distributed Bragg reflector region, the sum of the transmittance T (solid line) and the reflectance R is 1. Therefore, the magnitude of loss can be estimated from the value of the sum of the transmittance T and the reflectance R. As shown in Figure 3C, when the depth is changed stepwise (solid line), the sum of the transmittance T and the reflectance R approaches 1, indicating that the loss is reduced. Note that the dashed line in Figure 3C shows the results of a comparison.

[0029] Next, Figure 4 shows the propagation of light as seen from above when light is incident on a distributed Bragg reflector region (waveguide) on which a diffraction grating is formed. As shown in Figure 4(a), when the depth is constant, light is scattered near the connection point of the diffraction grating (x = 0). In contrast, as shown in Figure 4(b), when the recess depth of the diffraction grating is changed in stages, scattering is suppressed.

[0030] While reducing optical loss is effective, there is a concern that decreasing the depth of the diffraction grating recesses reduces the coupling coefficient and increases the effective length. As the effective length increases and more light penetrates the distributed Bragg reflector region, the optical confinement in the active layer decreases, degrading the laser's performance. Figure 5 shows the effective length of the diffraction grating. The vertical axis represents the absolute value of the Poynting vector of light propagating through the distributed Bragg reflector region. Within the stop band, light that penetrates the interior of the diffraction grating (x = 0 or greater) is reflected, resulting in a decrease in light intensity. When the depth of the diffraction grating recesses is gradually changed (dotted line), the penetration depth until the light intensity is reduced by half is 1.7 μm. However, this increase is limited to just under 1 μm compared to when the depth is constant (solid line), and the impact on the laser's performance is minimal.

[0031] Second Embodiment Next, a semiconductor laser according to a second embodiment of the present invention will be described with reference to Fig. 6. This semiconductor laser includes an active region 131, and a first distributed Bragg reflector region 132a and a second distributed Bragg reflector region 132b arranged contiguous to active region 131. Active region 131, first distributed Bragg reflector region 132a and second distributed Bragg reflector region 132b are formed on the same substrate 101 with lower cladding layer 102 interposed therebetween.

[0032] The first distributed Bragg reflector region 132a and the second distributed Bragg reflector region 132b are formed contiguously with the active region 131, sandwiching the active region 131 in the waveguiding direction. The first distributed Bragg reflector region 132a includes a first diffraction grating 221a, and the second distributed Bragg reflector region 132b includes a second diffraction grating 221b.

[0033] Active region 131 has active layer 103 made of a compound semiconductor. First distributed Bragg reflector region 132a includes first core layer 113a made of a compound semiconductor and formed contiguous with active layer 103. Second distributed Bragg reflector region 132b includes second core layer 113b made of a compound semiconductor and formed contiguous with active layer 103.

[0034] First diffraction grating 221a is composed of recesses formed in first distributed Bragg reflector region 132a and protrusions adjacent to the recesses. Similarly, second diffraction grating 221b is composed of recesses formed in second distributed Bragg reflector region 132b and protrusions adjacent to the recesses.

[0035] 1B , active region 131 includes n-type semiconductor layer 105 and p-type semiconductor layer 106 formed in contact with active layer 103. It also includes n-type electrode 107 electrically connected to n-type semiconductor layer 105 and p-type electrode 108 electrically connected to p-type semiconductor layer 106. Active layer 103 is sandwiched between semiconductor layers 104a and 104b made of a compound semiconductor such as undoped InP in the vertical direction when viewed from substrate 101.

[0036] On the other hand, in first distributed Bragg reflector region 132 a, n-type semiconductor layer 105 and p-type semiconductor layer 106 are not formed, and the side surface of first core layer 113 a in the waveguide direction is open. Furthermore, in first distributed Bragg reflector region 132 a and second distributed Bragg reflector region 132 b, n-type electrode 107 and p-type electrode 108 are not formed.

[0037] The above-described configuration is the same as that of the first embodiment except for first diffraction grating 221a and second diffraction grating 221b. In the second embodiment, the pitch (the distance between adjacent convex portions or adjacent concave portions) of first diffraction grating 221a and second diffraction grating 221b increases toward active region 131. For example, in a configuration in which laser light is emitted from first distributed Bragg reflector region 132a, the pitch of first diffraction grating 221a increases toward output end 141. In this case, the pitch of second diffraction grating 221b can be made uniform (the same) toward end 142, but second diffraction grating 221b can also be configured so that the pitch increases toward end 142.

[0038] According to the second embodiment, the pitch of first diffraction grating 221a and second diffraction grating 221b increases as they approach active region 131, thereby reducing the refractive index difference at first connection 151 between active region 131 and first distributed Bragg reflector region 132a. Similarly, the refractive index difference at second connection 152 between active region 131 and second distributed Bragg reflector region 132b can be reduced. Furthermore, the pitch of first diffraction grating 121a increases as they approach output end 141, thereby reducing the refractive index difference at the connection (output end 141) between the emitted laser and the optical waveguide to which it is optically coupled. As a result, the coupling coefficient (refractive index change) of the diffraction grating can be increased while suppressing scattering at the connection.

[0039] Next, we present the results of calculations on the effect of reducing optical loss by gradually changing the period of the periodic concaves and convexes that make up the diffraction grating, starting from the edge. The calculations were performed using the finite-difference time-domain method (FDTD).

[0040] The calculations were performed on the structure shown in Figure 7, in which the pitch was changed in stages. Here, the recess depth was 100 nm, and the diffraction grating had a duty ratio of 50% and 50 periods. For comparison, calculations were also performed on the same configuration as above, but with the pitch kept constant at 300 nm. The calculation results are shown in Figures 8A, 8B, and 8C. Figure 8A shows the transmission and reflection spectrum when light is incident on a distributed Bragg reflector region (waveguide) in which a diffraction grating of the comparative configuration is formed. Figure 8B shows the transmission and reflection spectrum when light is incident on a distributed Bragg reflector region (waveguide) in which a diffraction grating of the configuration shown in Figure 7 is formed.

[0041] As shown in Figure 8A, when the pitch of the diffraction grating is constant, the reflectance R (dashed line) within the stop band decreases due to scattering loss. In contrast, as shown in Figure 8B, when the pitch is changed in steps, the reflectance R approaches 1.

[0042] If there is no waveguide loss at the connection between the active region and the distributed Bragg reflector region, the sum of the transmittance T (solid line) and the reflectance R is 1. Therefore, the magnitude of loss can be estimated from the value of the sum of the transmittance T and the reflectance R. As shown in FIG. 8C, when the pitch is changed in stages (solid line), the sum of the transmittance T and the reflectance R approaches 1, indicating that the loss is reduced. Note that the dashed line in FIG. 8C shows the results of a comparison.

[0043] Next, Figure 4 shows the propagation of light as seen from above when light is incident on a distributed Bragg reflector region (waveguide) on which a diffraction grating is formed. As shown in Figure 4(a), when the pitch is constant, light is scattered near the connection point (x = 0) of the diffraction grating. In contrast, as shown in Figure 4(b), when the pitch of the diffraction grating is changed in stages, scattering is suppressed.

[0044] As described above, according to the present invention, the first diffraction grating formed in the first distributed Bragg reflector region and the second diffraction grating formed in the second distributed Bragg reflector region are configured with at least one of a configuration in which the depth of the recesses becomes shallower as they approach the active region and a configuration in which the pitch becomes larger as they approach the active region, thereby suppressing scattering at the connection between the waveguide region and the active region and increasing the refractive index change of the diffraction grating. Note that the first diffraction grating and the second diffraction grating can be configured with a configuration in which the depth of the recesses becomes shallower as they approach the active region and a configuration in which the pitch becomes larger as they approach the active region. This configuration also suppresses scattering at the connection between the waveguide region and the active region and increases the refractive index change of the diffraction grating.

[0045] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0046] 101...substrate, 102...lower cladding layer, 103...active layer, 104a...semiconductor layer, 104b...semiconductor layer, 105...n-type semiconductor layer, 106...p-type semiconductor layer, 107...n-type electrode, 108...p-type electrode, 113a...first core layer, 113b...second core layer, 121a...first diffraction grating, 121b...second diffraction grating, 131...active region, 132a...first distributed Bragg reflector region, 132b...second distributed Bragg reflector region, 141...output end, 142...end, 151...first connection portion, 152...second connection portion.

Claims

1. An active region including an active layer made of a compound semiconductor formed on a substrate, a first distributed Bragg reflector region including a first diffraction grating and a second distributed Bragg reflector region including a second diffraction grating, which are continuously formed on the active region with the active region sandwiched in the waveguide direction, wherein the first diffraction grating is composed of a concave portion formed in the first distributed Bragg reflector region and a convex portion adjacent to the concave portion, the second diffraction grating is composed of a concave portion formed in the second distributed Bragg reflector region and a convex portion adjacent to the concave portion, and the first diffraction grating and the second diffraction grating are configured to have at least one of a configuration in which the depth of the concave portion becomes shallower as approaching the active region and a configuration in which the pitch becomes larger as approaching the active region. A semiconductor laser.

2. The semiconductor laser according to claim 1, wherein laser light is emitted from the side of the first distributed Bragg reflector region, and the first diffraction grating is configured to have at least one of a configuration in which the pitch becomes larger as approaching the emission end of the laser light and a configuration in which the depth of the concave portion becomes shallower as approaching the active region. A semiconductor laser.

3. The semiconductor laser according to claim 1 or 2, wherein the first distributed Bragg reflector region includes a first core layer made of a compound semiconductor continuously formed on the active layer, and the second distributed Bragg reflector region includes a second core layer made of a compound semiconductor continuously formed on the active layer. A semiconductor laser.

4. The semiconductor laser according to claim 3, further including a first upper cladding layer formed on the first core layer and a second upper cladding layer formed on the second core layer. A semiconductor laser.

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