Semiconductor laser

Abstract

A semiconductor laser (10) according to the present invention comprises: a substrate (101); a waveguide (102); over cladding (103) that covers the waveguide; an active layer (111) that is disposed in part of the waveguide; a heater (14) that is disposed near the side of the waveguide at a prescribed distance from the active layer; and a groove (15) that is disposed between the active layer and the heater and in a region which includes the substrate. Furthermore, the groove is disposed near the side of the waveguide, between the active layer and the heater, and from the surface of the over cladding to inside of the substrate.　As a result, the present invention makes it possible to provide a semiconductor laser that can operate in a wide frequency band.

Description

semiconductor laser

[0001] The present invention relates to a semiconductor laser with excellent frequency characteristics.

[0002] With the development of a wide variety of Internet services, such as cloud computing, smartphones, and IoT, Internet traffic at data centers is increasing year by year. Similarly, the amount of power consumed by Internet traffic is also increasing. Therefore, there is a need for faster and more energy-efficient optical devices used in data centers.

[0003] Modulation using a directly modulated semiconductor laser is based on a simple method of encoding data "0" and "1" into optical output by modulating the amount of current injection, and is used as an optical transmitter for short distances due to its low power consumption and low cost.

[0004] However, the modulation bandwidth of a typical directly modulated laser is limited to about 30 GHz. The modulation bandwidth of a semiconductor laser is mainly determined by the damping constant, RC time constant, and relaxation oscillation frequency. Among these, the factor that limits the bandwidth is the relaxation oscillation frequency, which is about 20 GHz.

[0005] In order to increase the relaxation oscillation frequency, it is important to (1) prevent gain degradation at the operating point by increasing the heat dissipation of the semiconductor active layer to reduce the self-heating effect when current is injected, or (2) increase the optical confinement factor in the semiconductor active layer.

[0006] A membrane laser on a SiC substrate has been reported as a structure that simultaneously satisfies these requirements (Non-Patent Document 1). SiC substrates have a refractive index of approximately 2.6 and a thermal conductivity of approximately 490 W / m / K, enabling high optical confinement and high heat dissipation. These features enable a relaxation oscillation frequency of 42 GHz.

[0007] To further increase the speed, it has been reported that the bandwidth can be expanded by extrinsic optical feedback from, for example, a DBR waveguide or a passive optical waveguide (Non-Patent Documents 1 to 3). One of the effects that leads to the bandwidth expansion is photon-photon resonance (PPR), which is an effect in which the laser oscillation mode and one of the resonator modes formed by optical feedback interact with each other, improving the frequency response at the inter-mode frequency. In a membrane laser on SiC, a modulation bandwidth of 108 GHz has been reported due to the improvement in frequency response at 95 GHz caused by the PPR effect.

[0008] To obtain a flat frequency response over a wide band, it is necessary to appropriately control the PPR frequency according to the relaxation oscillation frequency, which decreases due to gain degradation as the temperature of the active layer increases.

[0009] One of the factors that determines the PPR frequency is the phase of the feedback light (Non-Patent Documents 2 and 3). Temperature control of the feedback region, such as the DBR or passive waveguide, is effective for phase control. For example, a heater is integrated for temperature control.

[0010] When heaters are integrated, it is necessary to heat the heated structure (for example, the feedback region) with high efficiency and to suppress temperature rise caused by the heater in regions other than the heated structure (for example, the active layer).

[0011] As a method for suppressing this temperature rise, in other words, a method for improving thermal isolation, for example, in a Mach-Zehnder interferometer used as an optical switch, a thin-film heater is placed on the waveguide core via the cladding to adjust the temperature of one arm. To efficiently heat the heater and reduce its power consumption, adiabatic grooves are formed in the cladding region on both sides of the heater (Non-Patent Document 4).

[0012] S. Yamaoka, N. -P. Diamantopoulos, H. Nishi, R. Nakao, T. Fujii, K. Takeda, T. Hiraki, T. Tsurugaya, S. Kanazawa, H. Tanobe, T. Kakitsuka, T. Tsuchizawa, F. Koyama, and S. Matsuo, “Directly modulated membrane lasers with 108 GHz bandwidth on a high-thermal-conductivity silicon carbide 35, no. 3, pp. 397-403, Feb. 2017.Y. Matsui, R. Schatz, Di Che, F. Khan, M. Kwakernaak, and T. Sudo, “Low-chirp isolator-free 65-GHz-bandwidth directly modulated lasers,” Nature Photon., vol. 15, no. 1, pp. 59-63, Jan. 2021.S. Sohma, T. Goh, H. Okazaki, M. Okuno, and A. Sugita:“Low switching power silica-based super high delta thermo-optic switch with heat insulating grooves,”Electronics. Letters, Vol. 38, No. 3, pp. 127-128, 2002.

[0013] As described above, in the prior art, in order to improve the heat insulation of the heater, a heat insulating groove is formed only in the cladding region. ２This can be easily achieved by selective etching between the substrate and the semiconductor. However, this structure has the problem that heat is dissipated through the substrate, which does not provide sufficient thermal insulation. This problem is particularly pronounced when a heat-dissipating substrate is used.

[0014] Furthermore, when a heater is disposed above a heated portion (for example, a waveguide core), if the heater is made of a metal material that is subject to high stress, the internal stress can cause the metal film to peel off.

[0015] Furthermore, in a structure using multilayer wiring, in order to suppress the height of the stack, it is necessary to integrate the heater so that its height is approximately the same as that of the active layer or the passive waveguide region.

[0016] In order to solve the above-mentioned problems, the semiconductor laser according to the present invention includes a substrate, a waveguide, an overclad covering the waveguide, an active layer disposed in a part of the waveguide, a heater disposed near a side of the waveguide at a predetermined distance from the active layer, and a groove disposed between the active layer and the heater in a region including the substrate.

[0017] According to the present invention, it is possible to provide a semiconductor laser that can operate over a wide frequency band.

[0018] FIG. 1A is a schematic top view showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 1B is a schematic IB-IB' cross-sectional view showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 1C is a schematic IC-IC' cross-sectional view showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 1D is a schematic ID-ID' cross-sectional view showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 1E is a schematic IE-IE' cross-sectional view showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 2 is a diagram for explaining the effects of a semiconductor laser according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the effects of a semiconductor laser according to the first embodiment of the present invention. FIG. 4A is a schematic top view showing the configuration of a semiconductor laser according to a second embodiment of the present invention. FIG. 4B is a schematic IVB-IVB' cross-sectional view showing the configuration of a semiconductor laser according to the second embodiment of the present invention.

[0019] First Embodiment A semiconductor laser 10 according to a first embodiment of the present invention will be described with reference to Figures 1 to 3. In the figures, the x direction indicates the "length" direction, the y direction indicates the "width" direction, and the z direction indicates the "height" direction, "thickness" direction, or vertical direction.

[0020] <Configuration of Semiconductor Laser> As shown in FIGS. 1A to 1E , a semiconductor laser 10 according to this embodiment includes an active region 11 having an active layer 111, a front-stage waveguide region 12 having a front-stage waveguide connected to one end of the active layer 111, a rear-stage waveguide region 13 having a rear-stage waveguide connected to the other end of the active layer 111, a heater 14 disposed near the front-stage waveguide and the rear-stage waveguide, and a groove 15 disposed between the heater 14 and the active region 11.

[0021] As shown in FIG. 1B, the semiconductor laser 10 includes, in order, a substrate 101, a waveguide 102 made of a semiconductor, an overclad 103 covering the waveguide 102, and an active layer 111 in a part of the waveguide 102.

[0022] As shown in FIG. 1C , the semiconductor laser 10 has a lateral p-i-n diode formed in the active region 11. The semiconductor laser 10 includes a layer structure consisting of an i (intrinsic) type semiconductor layer (lower) 112, an active layer 111, and an i-type semiconductor layer (upper) 113, a p-type semiconductor layer 114 disposed on one sidewall of the layer structure, and an n-type semiconductor layer 115 disposed on the other sidewall of the layer structure. Electrodes 116 and 117 are provided on the surfaces of the p-type semiconductor layer 114 and the n-type semiconductor layer 115, respectively. This allows the semiconductor laser 10 to operate by injecting a current laterally into the active layer 111. When a signal current is injected into the active layer 111, the semiconductor laser 10 outputs an optically modulated signal in response to the signal.

[0023] The overclad 103 is made of SiO ２ The overclad 103 may be formed by, for example, surface activated bonding of the support substrate and the InP substrate, or by other methods.

[0024] As an example of the layer structure, the active layer 111 is an InGaAsP-based multiple quantum well structure for the 1.31 μm wavelength band, with six quantum well layers. The active layer 111 is 150 nm thick. The i-type semiconductor layers 112 and 113 are each 50 nm thick and made of undoped InP.

[0025] The p-type semiconductor layer 114 is, for example, Zn-doped (1×10 １８ cm －３ The n-type semiconductor layer 115 is, for example, Si-doped (2×10 １８ cm －３ ) n-type InP.

[0026] The active layer 111 has a length of 0.7 μm and a thickness of 0.32 μm. The thickness of 0.32 μm of the active layer 111 is approximately the upper limit value at which light with a wavelength of 1.31 μm propagating through the active layer 111 becomes single mode in the thickness direction of the active layer 111.

[0027] The waveguide 102 is made of undoped InP and has a thickness of 350 μm. The waveguide 102 includes i-type semiconductor layers 112 and 113 in a part thereof.

[0028] The dimensions of the layer structure and the waveguide are not limited to these, and other dimensions may be used.

[0029] When changing the operating wavelength or the material used for the active layer 111, in order to obtain a single mode in the thickness direction of the active layer 111, the operating wavelength is set to λ and the average refractive index of the active layer 111 is set to n ｃｏｒｅ , the refractive index of the second cladding layer is n ｃｌａｄ In this case, the thickness t of the active layer 111 should approximately satisfy the relationship of formula (1).

[0030]

[0031] For example, when light having a wavelength in the 1.55 μm band is used, the thickness t of the active layer (core layer) 111 is 0.364 μm or less.

[0032] In the semiconductor laser 10, as shown in FIGS. 1B and 1E, a heater 14 is disposed in the overclad 103 at a position away from the active layer 111 and adjacent to the side of the waveguide 102.

[0033] The distance between the heater 14 and the active layer 111 depends on the thermal conductivity of the substrate 101. For example, if the substrate 101 is made of SiO ２ In this case, the distance between the heater 14 and the active layer 111 is about 1 to 10 μm, preferably about several μm. Also, when the substrate 101 is SiC, the distance between the heater 14 and the active layer 111 is about 10 to 100 μm, preferably about several tens of μm.

[0034] The distance between the heater 14 and the waveguide 102 is, for example, about 1 to 10 μm, and preferably about 3 μm.

[0035] It is desirable that the height of the heater 14 is approximately equal to the height of the waveguide 102. In other words, it is desirable that the position of the surface of the heater 14 is approximately equal to the position of the surface of the waveguide 102 in the vertical direction. This allows for integration with the heater and the waveguide having approximately the same height when using multilayer wiring for the semiconductor laser, thereby reducing the height of the stacked structure of the multilayer wiring.

[0036] 1B and 1D, in the semiconductor laser 10, a groove 15 is formed in the overclad 103 between the active region 11 and the heater 14. The groove 15 is formed from the overclad 103 through the waveguide 102 to the inside of the substrate 101.

[0037] The groove 15 is disposed on the side of the waveguide 102 at a distance approximately equal to the distance between the waveguide 102 and the heater 14. The position of the groove 15 is not limited thereto, and it need only be disposed so as to include a heat conduction path between the active region 11 and the heater 14.

[0038] The length of the groove 15 is, for example, about 1 to 10 μm, preferably about several μm. The width of the groove 15 is equal to or greater than the width of the heater 14, preferably the longer the width. The depth of the groove 15 is, for example, about 200 to 300 μm.

[0039] The grooves 15 are filled with a gas such as air, or may be filled with a material with low thermal conductivity.

[0040] In the semiconductor laser 10, the heater 14 heats and controls the temperature of the front-stage waveguide and rear-stage waveguide, thereby controlling the PPR frequency, increasing the relaxation oscillation frequency, and widening the modulation bandwidth of the semiconductor laser.

[0041] In the semiconductor laser 10, the groove 15 improves thermal isolation between the active layer 111 and the heater 14, thereby suppressing a temperature rise in the active layer 111. Furthermore, the improved thermal insulation in the vicinity of the heater 14 reduces the power consumption required to achieve a desired temperature rise.

[0042] Therefore, the positions of the heater 14 and the groove 15 are set at a distance that allows heat conduction from the heater 14 to the waveguide 102 to the extent that the temperature of the waveguide 102 can be changed, and at a distance that allows heat conduction from the heater 14 to the active layer 111 via the groove 15 to be suppressed.

[0043] The support substrate 101 is made of Si or SiO ２ The support substrate 101 is made of SiC, GaN, AlN, or Al. ２ O ３ Alternatively, a heat-dissipating substrate such as diamond may be used. The substrate is not limited to these, and may be made of other materials.

[0044] <Effects> The effects of the semiconductor laser 10 according to this embodiment will be described with reference to FIGS.

[0045] In the semiconductor laser 10 according to this embodiment, the dependency of the temperature rise in the heater 14 and the active layer 111 when heated by the heater 14 on the depth of the groove 15 in the substrate (SiC) 101 was calculated using the finite element method.

[0046] The structure used in the calculation is shown in Figure 2. The heater 14 is made of platinum and has a resistance of 1.14 Ω. The active layer 111 is made of InP, and the overclad 103 is made of SiO ２ The substrate 101 was made of SiC. The heater 14 and the active layer 111 were arranged at the same height, with a gap of 30 μm between the heater 14 and the active layer 111, and a groove (length: 10 μm) 15 was arranged in the middle. The groove 15 was filled with air.

[0047] In this structure, the temperature rise in the heater 14 and active layer 111 when 700 mW of power is applied to the heater 14 is calculated by changing the depth of the groove 15 from 0 to 50 μm. At this time, the platinum 14 is heated, but the InP 111 is not heated. Other setting values ​​are shown below.

[0048] Thickness of the first overcladding 103_1 (T ＯＣ１ Thickness of the second overclad 103_2 (T ＯＣ２ ): 5 μm Platinum 14 length (L Ｐｒ ): 3 μm Thickness of platinum 14 (T Ｐｒ ): 0.36 μm Length of InP111 (L ＩｎＰ ): 0.5 μm Thickness of InP111 (T ＩｎＰ ): 0.36 μm ・SiO ２ The thickness of 101_2 (T ＳｉＯ２ ): 40 nm Thickness of SiC101 (T ＳｉＣ ): 300 μm

[0049] 3 shows the calculation results of the dependency of the temperature rise in the heater 14 and the active layer 111 on the depth of the groove 15 in the substrate (SiC). In the figure, the black circles and dotted line indicate the temperature rise in the heater 14, and the white circles and solid line indicate the temperature rise in the active layer (InP) 111.

[0050] As the depth of the grooves 15 in the substrate (SiC) 101 increases, the amount of temperature rise in the heater 14 increases. On the other hand, the amount of temperature rise in the active layer (InP) 111 decreases. For example, when a 50 μm groove 15 is formed in SiC, the amount of temperature rise in the heater 14 increases by 28% and the amount of temperature rise in the active layer (InP) 111 decreases by 23% compared to when no groove is formed (when the groove depth is zero).

[0051] In this way, by forming the groove 15 between the heater 14 and the active layer 111, the power consumption of the heater 14 required to obtain a desired temperature rise can be reduced, and thermal isolation can be improved.

[0052] In the semiconductor laser according to this embodiment, the grooves improve thermal isolation between the active layer and the heater, thereby suppressing temperature rise in the active layer. Furthermore, improved thermal insulation in the vicinity of the heater reduces power consumption required to achieve a desired temperature rise. This allows the semiconductor laser to operate over a wide frequency band.

[0053] In the present embodiment, an example has been shown in which the groove 15_2 is provided near the waveguide in addition to the groove 15 arranged between the heater 14 and the active layer 111, but this is not limiting. A configuration may also be provided in which only the groove 15 is arranged between the heater 14 and the active layer 111.

[0054] The number of heaters is not limited to two, but may be one, or three or more. The number of grooves is not limited to two or four, but may be one, or three or more.

[0055] Second Embodiment A semiconductor laser according to a second embodiment of the present invention will be described with reference to FIGS. 4A and 4B.

[0056] <Configuration of Semiconductor Laser> As shown in FIGS. 4A and 4B, the semiconductor laser 20 according to this embodiment includes a substrate 101, an active layer 111, a front-stage waveguide, a rear-stage waveguide, an overclad 103, a heater 14, and a groove structure 25.

[0057] The substrate 101, the active layer 111, the waveguide 102 (including the front-stage waveguide and the rear-stage waveguide), the overclad 103, and the heater 14 have the same configuration as in the first embodiment.

[0058] The groove structure 25 is composed of a plurality of periodically arranged grooves, and is arranged in the substrate 101 below the waveguides in the front and rear waveguide regions and the overclad 103 on the sides of the waveguides. Each groove is filled with a gas such as air, or may be filled with a material with low thermal conductivity.

[0059] As a result, similar to the first embodiment, the grooves improve thermal isolation between the active layer 111 and the heater 14, thereby suppressing a temperature rise in the active layer 111. Furthermore, the thermal insulation in the vicinity of the heater 14 is improved, thereby reducing the power consumption required to achieve a desired temperature rise. This allows the semiconductor laser to operate over a wide frequency band.

[0060] Furthermore, the groove structure 25 can be made to function as a diffraction grating. For example, by setting the period of the groove structure 25 according to the wavelength of light guided through the waveguide, the front-stage waveguide and the rear-stage waveguide can be made to function as DBR waveguides.

[0061] In the embodiment of the present invention, an example has been shown in which an InGaAsP-based multiple quantum well structure for the 1.31 μm wavelength band is used for the active layer of the semiconductor laser, but this is not limiting. Other wavelength bands such as the 1.55 μm wavelength band may also be used, and other materials such as AlGaAs, GaAs, and GaN may also be used.

[0062] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration and manufacturing method of the semiconductor laser are shown, but the present invention is not limited to these examples. Anything that can exhibit the functions and effects of the semiconductor laser can be used.

[0063] It should be noted that the present invention is not limited to the above-described embodiments, 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.

[0064] A part or all of the above-described embodiment or an example thereof can be described as, but is not limited to, the following supplementary notes.

[0065] (Supplementary Note 1) A semiconductor laser comprising: a substrate; a waveguide; an overclad covering the waveguide; an active layer disposed in a portion of the waveguide; a heater disposed near a side of the waveguide at a predetermined distance from the active layer; and a groove disposed between the active layer and the heater in a region including the substrate.

[0066] (Supplementary Note 2) A semiconductor laser according to Supplementary Note 1, wherein the groove is positioned near the side of the waveguide between the active layer and the heater, from the surface of the overclad to within the substrate.

[0067] (Supplementary Note 3) The semiconductor laser according to Supplementary Note 1 or Supplementary Note 2, further comprising: a p-type semiconductor layer disposed on one side wall of the active layer; an n-type semiconductor layer disposed on the other side wall of the active layer; and electrodes disposed on the surfaces of the p-type semiconductor layer and the n-type semiconductor layer.

[0068] (Supplementary Note 4) The semiconductor laser according to any one of Supplementary Note 1 to Supplementary Note 3, wherein the height of the heater and the height of the waveguide are approximately equal.

[0069] (Appendix 5) The semiconductor laser according to appendix 1, appendix 3, or appendix 4, wherein the grooves are periodically arranged in the substrate below the heater.

[0070] The present invention relates to a semiconductor laser and can be applied to optical communication systems and optical transmitters.

[0071] REFERENCE SIGNS LIST 10 Semiconductor laser 101 Substrate 102 Waveguide 103 Overclad 111 Active layer 14 Heater 15 Groove

Claims

1. A semiconductor laser comprising: a substrate; a waveguide; an overclad covering the waveguide; an active layer disposed in a portion of the waveguide; a heater disposed a predetermined distance from the active layer and adjacent to a side of the waveguide; and a groove disposed between the active layer and the heater in a region including the substrate.

2. A semiconductor laser as claimed in claim 1, wherein said groove is disposed adjacent to a side of said waveguide between said active layer and said heater, from a surface of said overclad into said substrate.

3. The semiconductor laser according to claim 1 or claim 2, further comprising: a p-type semiconductor layer disposed on one side wall of the active layer; an n-type semiconductor layer disposed on the other side wall of the active layer; and electrodes disposed on the respective surfaces of the p-type semiconductor layer and the n-type semiconductor layer.

4. The semiconductor laser according to claim 1 or 2, wherein the height of said heater and the height of said waveguide are approximately equal.

5. The semiconductor laser of claim 1, wherein said grooves are periodically disposed within said substrate beneath said heater.

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