WO2024257152A1 - Semiconductor laser - Google Patents

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

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

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 electricity consumed by Internet traffic is also increasing. This has created a need for faster, less power-consuming optical devices used in data centers.

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.

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.

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.

As a structure that simultaneously satisfies all these conditions, a membrane laser on a SiC substrate has been reported (Non-Patent Document 1). The SiC substrate has a refractive index of about 2.6 and a thermal conductivity of about 490 W/m/K, which allows for high optical confinement and high heat dissipation. As a result, a relaxation oscillation frequency of 42 GHz can be achieved.

In order 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 1-3). One of the effects that results in the expansion of the bandwidth is photon-photon resonance (PPR), which is an effect in which the laser oscillation mode and one of the resonator modes formed by the optical feedback interact with each other to improve the frequency response at the inter-mode frequency of these modes. In a membrane laser on SiC, a modulation bandwidth of 108 GHz has been reported due to an improvement in frequency response at 95 GHz caused by the PPR effect.

To obtain a flat frequency response over a wide bandwidth, it is necessary to appropriately control the PPR frequency according to the relaxation oscillation frequency. In particular, the relaxation oscillation frequency decreases due to gain degradation as the temperature of the active layer increases.

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

When heaters are integrated, it is necessary to heat the heated structure (e.g., the feedback region) with high efficiency and suppress the temperature rise caused by the heater in areas other than the heated structure (e.g., the active layer).

As a method for suppressing this temperature rise, in other words, 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. In order to efficiently heat with this heater and reduce the power consumption of the heater, thermal insulating grooves are formed in the cladding region on both sides of the heater (Non-Patent Document 4).

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 substrate,” Nature Photonics volume 15, pages28-35(2021). Y. Matsui et al., “55 GHz bandwidth distributed reflector laser,” J. Lightw. Technol., vol. 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.

As described above, in the conventional technology, in order to improve the heat insulation of the heater, a heat insulating groove is formed only in the cladding region. This heat insulating groove can be easily formed by selective etching between _SiO2 or the like and the semiconductor. However, in this configuration, the heat is dissipated through the path via the substrate, so there is a problem that sufficient heat insulation cannot be obtained. This problem is particularly noticeable when a heat dissipating substrate is used.

Furthermore, when the heater is placed above the heated part (e.g., the waveguide core), there is a problem that the internal stress of the heater, which is made of a metal material that is subject to high stress, can cause the metal film to peel off.

In addition, in structures that use multilayer wiring, in order to reduce 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 passive waveguide region.

In order to solve the problems described above, the semiconductor laser according to the present invention comprises a substrate, a waveguide, an overcladding covering the waveguide, an active layer disposed in a portion of the waveguide, a heater disposed near the 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.

The present invention provides a semiconductor laser that can operate over a wide frequency band.

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 cross-sectional view taken along line IB-IB' showing the configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 1C is a schematic cross-sectional view taken along the line IC-IC' showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 1D is a schematic cross-sectional view taken along line ID-ID' showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 1E is a schematic cross-sectional view taken along line IE-IE' showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a diagram for explaining the effect of the semiconductor laser according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the effect of the 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 the second embodiment of the present invention. FIG. 4B is a schematic cross-sectional view taken along line IVB-IVB' showing the configuration of a semiconductor laser according to a second embodiment of the present invention.

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, the "thickness" direction, or the vertical direction.

<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 in the vicinity of the front-stage waveguide and the rear-stage waveguide, and a groove 15 disposed between the heater 14 and the active region 11.

As shown in FIG. 1B, the semiconductor laser 10 comprises, in order, a substrate 101, a waveguide 102 made of a semiconductor, and an overclad 103 that covers the waveguide 102, and an active layer 111 in a portion of the waveguide 102.

As shown in FIG. 1C, the semiconductor laser 10 has a lateral p-i-n diode formed in the active region 11. It has a layer structure consisting of an i (intrinsic) type semiconductor layer (lower part) 112, an active layer 111, and an i-type semiconductor layer (upper part) 113, a p-type semiconductor layer 114 disposed on one side wall of the layer structure, and an n-type semiconductor layer 115 disposed on the other side wall of the layer structure. Electrodes 116, 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 outputs an optical modulation signal in response to the signal.

The overclad 103 is a low refractive index medium such as SiO _2. The overclad 103 may be formed, for example, by surface activated bonding of the support substrate and the InP substrate, or may be formed by other methods.

As an example of a layer structure, the active layer 111 is an InGaAsP-based multiple quantum well structure in 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.

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

The length of the active layer 111 is 0.7 μm and the thickness is 0.32 μm. The thickness of the active layer 111, 0.32 μm, is approximately the upper limit value at which the 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.

The waveguide 102 is made of undoped InP and has a layer thickness of 350 μm. A part of the waveguide 102 includes i-type semiconductor layers 112 and 113.

The dimensions of the layer structure and the waveguide are not limited to these. Other dimensions are also acceptable.

When changing the operating wavelength or the material used for the active layer 111, in order for the active layer 111 to have a single mode in the thickness direction, the thickness t of the active layer 111 only needs to roughly satisfy the relationship in equation (1), where λ is the operating wavelength, n _core is the average refractive index of the active layer 111, and n _clad is the refractive index of the second clad layer.

For example, when using light with a wavelength in the 1.55 μm band, the thickness t of the active layer (core layer) 111 is 0.364 μm or less.

In the semiconductor laser 10, as shown in Figures 1B and 1E, the heater 14 is disposed near the side of the waveguide 102 in the overclad 103, away from the active layer 111.

The distance between the heater 14 and the active layer 111 depends on the thermal conductivity of the substrate 101. For example, when the substrate 101 is made of _SiO2 , the distance between the heater 14 and the active layer 111 is about 1 to 10 μm, preferably about several μm. When the substrate 101 is made of SiC, the distance between the heater 14 and the active layer 111 is about 10 to 100 μm, preferably about several tens of μm.

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

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 the heater and the waveguide to be integrated with approximately the same height when multilayer wiring is used in the semiconductor laser, thereby reducing the height of the stacked structure of the multilayer wiring.

In the semiconductor laser 10, as shown in Figures 1B and 1D, 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.

The groove 15 is disposed to 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.

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

The groove 15 is filled with a gas, such as air, or may be filled with a material with low thermal conductivity.

In the semiconductor laser 10, the heater 14 heats the front-stage waveguide and rear-stage waveguide to control the temperature. This controls the PPR frequency, increases the relaxation oscillation frequency, and expands the modulation bandwidth of the semiconductor laser.

In the semiconductor laser 10, the groove 15 improves thermal isolation between the active layer 111 and the heater 14, making it possible to suppress a rise in temperature of the active layer 111. In addition, the insulation properties in the vicinity of the heater 14 are improved, making it possible to reduce the power consumption required to achieve a desired temperature rise.

Therefore, the positions of the heater 14 and the groove 15 are set at a distance that allows thermal 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 thermal conduction from the heater 14 to the active layer 111 via the groove 15 to be suppressed.

The support substrate 101 is made of Si or SiO ₂ /Si. The support substrate 101 may be made of a heat dissipation substrate such as SiC, GaN, AlN, Al ₂ O ₃ , diamond, etc. The support substrate 101 is not limited to these, and may be made of other materials.

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

In the semiconductor laser 10 according to this embodiment, the dependence 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.

The structure used in the calculation is shown in Fig. 2. The heater 14 was made of platinum and had a resistance of 1.14Ω. The active layer 111 was made of InP, the overcladding 103 was made of _SiO2 , and the substrate 101 was made of SiC. The heater 14 and the active layer 111 were placed at the same height, the distance between the heater 14 and the active layer 111 was set to 30 μm, and a groove (length: 10 μm) 15 was placed in the middle. The groove 15 was assumed to be filled with air.

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 was 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.

Thickness (T _OC1 ) of the first overclad 103_1: 3 μm
Thickness (T _OC2 ) of the second overclad 103_2: 5 μm
Length of platinum 14 (L _Pr ): 3 μm
Platinum 14 thickness (T _Pr ): 0.36 μm
Length of InP 111 (L _InP ): 0.5 μm
Thickness of InP111 (T _InP ): 0.36 μm
Thickness of _SiO2 101_2 ( _TSiO2 ): 40 nm
・Thickness of SiC 101 (T _SiC ): 300 μm

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

As the depth of the groove 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).

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

In the semiconductor laser according to this embodiment, the grooves improve thermal isolation between the active layer and the heater, making it possible to suppress temperature rise in the active layer. In addition, the insulation properties in the vicinity of the heater are improved, making it possible to reduce the power consumption required to achieve a desired temperature rise. This allows the semiconductor laser to operate over a wide frequency band.

In this embodiment, an example is shown in which a 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 limited to the example. A configuration may also be used in which only a groove 15 is provided between the heater 14 and the active layer 111.

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

Second Embodiment
A semiconductor laser according to a second embodiment of the present invention will be described with reference to FIGS.

<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.

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

The groove structure 25 is composed of a number of periodically arranged grooves, and is arranged in the substrate 101 under the waveguides in the front-stage waveguide region and the rear-stage waveguide region and the overclad 103 on the sides of the waveguides. Each groove is filled with a gas, such as air. Alternatively, the grooves may be filled with a material with low thermal conductivity.

As a result, as in the first embodiment, the grooves improve thermal isolation between the active layer 111 and the heater 14, making it possible to suppress a rise in temperature of the active layer 111. In addition, the insulation properties in the vicinity of the heater 14 are improved, making it possible to reduce the power consumption required to achieve a desired temperature rise. This allows the semiconductor laser to operate over a wide frequency band.

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

In the embodiment of the present invention, an example is shown in which an InGaAsP-based multiple quantum well structure in the 1.31 μm wavelength band is used for the active layer of the semiconductor laser, but this is not limited to this. 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.

In the embodiment 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. Anything that can exert the functions and effects of the semiconductor laser can be used.

The present invention is not limited to the above-described embodiment, and it is clear that many modifications and combinations can be implemented by those with ordinary skill in the art within the technical concept of the present invention.

The above-described embodiment or an example thereof, in whole or in part, may be described as, but is not limited to, the following notes.

(Note 1) A semiconductor laser comprising a substrate, a waveguide, an overcladding covering the waveguide, an active layer disposed in a portion of the waveguide, a heater disposed near the 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.

(Appendix 2) The semiconductor laser described in Appendix 1, in which the groove is disposed near the side of the waveguide between the active layer and the heater, from the surface of the overclad into the substrate.

(Appendix 3) The semiconductor laser according to appendix 1 or 2, further comprising a p-type semiconductor layer disposed on one sidewall of the active layer, an n-type semiconductor layer disposed on the other sidewall of the active layer, and electrodes disposed on the surfaces of the p-type semiconductor layer and the n-type semiconductor layer.

(Appendix 4) A semiconductor laser according to any one of appendices 1 to 3, in which the height of the heater and the height of the waveguide are approximately equal.

(Appendix 5) A semiconductor laser as described in appendix 1, appendix 3, or appendix 4, in which the grooves are periodically arranged below the heater within the substrate.

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

10 Semiconductor laser 101 Substrate 102 Waveguide 103 Overcladding 111 Active layer 14 Heater 15 Groove

Claims (5)

A substrate;
A waveguide;
an overclad covering the waveguide;
an active layer disposed in a portion of the waveguide;
a heater disposed adjacent to a side of the waveguide at a predetermined distance from the active layer;
a groove disposed between the active layer and the heater in a region including the substrate. The semiconductor laser of claim 1, wherein the groove is disposed near the side of the waveguide between the active layer and the heater, from the surface of the overclad into the substrate. a p-type semiconductor layer disposed on one side wall of the active layer;
an n-type semiconductor layer disposed on the other sidewall of the active layer;
3. The semiconductor laser according to claim 1, further comprising: electrodes disposed on a surface of each of the p-type semiconductor layer and the n-type semiconductor layer. The semiconductor laser according to claim 1 or 2, wherein the height of the heater and the height of the waveguide are approximately equal. The semiconductor laser of claim 1, wherein the grooves are periodically arranged in the substrate below the heater.

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