Patent Publication Number: US-2022231480-A1

Title: Optical Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry of PCT Application No. PCT/JP2019/022876, filed on Jun. 10, 2019, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical device, and in particular relates to an optical device including a waveguide semiconductor laser. 
     BACKGROUND 
     Si photonics is a technique for integrating an electronic circuit constituted by Si and an optical device on the same substrate using a CMOS technique. In this technique, an optical device that emits light is important, but since Si is an indirect band gap semiconductor, the light emission efficiency is very small, and it is difficult to use Si as an optical device that emits light. 
     In general, a III-V compound semiconductor such as GaAs or InP, which are a direct band gap type and have a high light emission efficiency, is used in an optical device that emits light. In view of this, for example, as an optical device that can be applied to Si photonics, a technique has been studied in which a III-V compound semiconductor is bonded to a Si substrate and a laser structure (a III-V on Si laser) is produced using the bonded III-V semiconductor (see NPL1). For example, hydrophilic bonding or surface activation bonding, which are well-known, is used in this kind of bonding of a silicon substrate and a III-V compound semiconductor. An insulating layer such as SiO 2  is used in the bond interface of the surface activation bond or hydrophilic bond, and enables bonding of the substrate via oxygen bonding of the bond interface (NPL1). 
     In a laser composed of a III-V compound semiconductor formed on an Si substrate, the refractive index of the Si substrate is higher than the refractive index of an upper-portion cladding medium, and is about the same as the refractive index of an active layer medium. Accordingly, in order to obtain high light confinement, it is necessary to set the distance between the active layer and the Si substrate composed of the III-V compound semiconductor on the order of several micrometers, and design the waveguide mode of the laser such that the refractive index of Si is not felt. 
     Incidentally, in the above-described laser structure, since the thermal conductivity of SiO 2  is small, a problem occurs in that heat generation in the active layer is not efficiently emitted to the Si substrate. The effect of the heat generation in the active layer appears as a reduction of the light output and saturation of the modulation speed, and causes the laser characteristics to deteriorate (NPL2). 
     In order to solve the above-described problems of light confinement and heat dissipation, it has been proposed that a laser is integrated on a substrate having a lower refractive index and a higher thermal conductivity than a core. For example, since a laser structure in which SiC, which has a higher thermal conductivity and a smaller refractive index than Si or InP, is used as a substrate can improve the heat dissipation property of the laser active layer and a greater amount of current than in the conventional technique can be injected, the realization of high light output and high speed modulation is expected. 
     Incidentally, a Q switch laser, with which higher-energy laser light is obtained, and a mode-locked laser, with which a long-short pulse laser is obtained, have been attracting attention. These optical devices use graphene as an oversaturation absorption body, and an external light source such as a fiber laser is used as the light source (see NPL3). Note that graphene is formed through epitaxial growth on a 6H-SiC (0001) substrate, a 4H-SiC (0001) substrate, and a 3C-SiC (111) substrate. 
     Citation List 
     Non Patent Literature 
     NPL1—T. Fujii et al., “Epitaxial growth of InP to bury directly bonded thin active layer on SiO 2 /Si substrate for fabricating distributed feedback lasers on silicon”, IET Optoelectron, vol. 9, Iss. 4, pp. 151-157, 2015. 
     NPL2—W. Kobayashi et al., “50-Gb/s Direct Modulation of a 1.3-μm InGaAlAs-Based DFB Laser With a Ridge Waveguide Structure”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 19, no. 4, 1500908, 2013. 
     NPL3—Q. Wang et al., “Graphene on SiC as a Q-switcher for a 2 μm laser”, Optics Letters, vol. 37, no. 3, pp. 395-397, 2012. 
     SUMMARY 
     Technical Problem 
     However, an optical device in which the above-described oversaturation absorption body is used is problematic in that size reduction is not easy since an external light source such as a fiber laser is used. 
     Embodiments of the present invention were made in order to solve the above-described problems, and aims to reduce the size of an optical device in which an oversaturation absorption body is used. 
     Means for Solving the Problem 
     An optical device according to embodiments of the present invention includes: a waveguide laser that is formed above a first cladding layer and includes an active layer constituted by an InP-based compound semiconductor; a reflection portion provided on one end of the active layer, which is an emission side of the waveguide laser; a distributed Bragg reflection portion provided on another end of the active layer of the waveguide laser; an absorption layer composed of an oversaturation absorption body provided between the active layer and the distributed Bragg reflection portion; and a second cladding layer that is formed on the first cladding layer and covers the waveguide laser, in which the first cladding layer is constituted by a material having a higher thermal conductivity than InP. 
     In an exemplary configuration of the above-described optical device, the absorption layer is formed below a core forming an optical waveguide between the active layer and the distributed Bragg reflection portion. 
     In an exemplary configuration of the above-described optical device, the absorption layer is formed above a core forming an optical waveguide between the active layer and the distributed Bragg reflection portion. 
     In an exemplary configuration of the above-described optical device, the first cladding layer is constituted by SiC. 
     In an exemplary configuration of the above-described optical device, the oversaturation absorption body is graphene. 
     Effects of Embodiments of the Invention 
     As described above, according to embodiments of the present invention, an absorption layer composed of an oversaturation absorption body is arranged between an active layer and a distributed Bragg reflection portion of a waveguide laser, and these are formed on a first cladding layer constituted by a material having a higher thermal conductivity than InP, and therefore it is possible to reduce the size of an optical device in which an oversaturation absorption body is used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view showing a configuration of an optical device according to an embodiment of the present invention. 
         FIG. 1B  is a cross-sectional view showing a configuration of an optical device according to an embodiment of the present invention. 
         FIG. 1C  is a cross-sectional view showing a configuration of an optical device according to an embodiment of the present invention. 
         FIG. 1D  is a cross-sectional view showing a configuration of an optical device according to an embodiment of the present invention. 
         FIG. 2A  is a feature diagram showing a waveguide mode distribution in the cross-section shown in  FIG. 1B . 
         FIG. 2B  is a feature diagram showing a waveguide mode distribution in the cross-section shown in  FIG. 1C . 
         FIG. 2C  is a feature diagram showing a waveguide mode distribution in the cross-section shown in  FIG. 1D . 
         FIG. 3  is a cross-sectional view showing a configuration of another optical device according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, an optical device according to an embodiment of the present invention will be described with reference to  FIGS. 1A, 1B, 1C, and 1D . Note that  FIG. 1A  shows a cross-section on a plane parallel to a wave guiding direction. Also,  FIG. 1B  shows a cross-section taken along line aa′ in  FIG. 1A ,  FIG. 1C  shows a cross-section taken along line bb′ in  FIG. 1A , and  FIG. 1C  shows a cross-section taken along line cc′ in  FIG. 1A . 
     The optical device includes a first cladding layer  101 , a waveguide laser  102 , an absorption layer  103 , and a second cladding layer  104 . 
     The first cladding layer  101  is constituted by a material having a higher thermal conductivity than InP. For example, the first cladding layer  101  can be constituted by one of SiC, AIN, GaN, and diamond. These materials have lower refractive indices and larger thermal conductivities and band gaps than any material forming a later-described active layer  105 . For example, the first cladding layer  101  can be produced through lithography/etching or the like of a substrate constituted by SiC, diamond, or the like, but the method of production does not matter. The first cladding layer  101  can be constituted by 6H-SiC in which the plane orientation of a main surface is a (0001) surface, 4H-SiC in which the plane orientation of a main surface is a (0001) surface, and 3C-SiC in which the plane orientation of a main surface is a (111) surface. 
     In the first region  121 , the waveguide laser  102  includes an active layer  105  that is constituted by an InP-based compound semiconductor. Also, the waveguide laser  102  includes a reflection portion  106  that is provided on one end of the active layer  105  that is the emission side of the light (laser light), and a distributed Bragg reflection portion  107  that is arranged on another end of the active layer  105 . 
     The active layer  105  is, for example, a double quantum well structure resulting from a well layer and a barrier layer composed of InGaAlAs, InGaAs, InGaAsP, and the like, which each have a different composition. The active layer  105  can also be formed by compound semiconductors such as bulk InGaAlAs, InGaAs, and InGaAsP. For example, the width of the active layer  105  can be 0.7 μm, and the thickness of the active layer  105  can be 0.32 μm. Note that there is no limitation on the layer structure and the width. The thickness 0.32 μm of the active layer  105  is the value of the approximate upper limit at which light of the wavelength 1.31 μm, which propagates in the active layer  105 , is in a single mode with respect to the thickness direction of the active layer  105 . 
     Also, the active layer  105  is, for example, embedded in the semiconductor layer  110  composed of InP. The semiconductor layer  110  on the upper side and the lower side of the active layer  105  is constituted by non-doped InP. Also, the semiconductor layer  110  on one side surface of the active layer  105  is a p-type region  111  constituted by p-type InP, and the semiconductor layer  110  on the other side surface of the active layer  105  is an n-type region  112  constituted by n-type InP. According to this p-i-n junction, a structure through which current is injected into the active layer  105  is formed. Also, a p electrode  113  is electrically connected to the p-type region in, and an n electrode  114  is electrically connected to the n-type region  112 . The p electrode  113  can connect to the p-type region in via a p-type contact layer (not shown). Also, the n electrode  114  can connect to the n-type region  112  via an n-type contact layer (not shown). 
     The active layer  105  is formed continuously with a core  108 . The core  108  is formed spanning over a second region  122 , which is continuous with the first region  121 , and a third region  123 , which is continuous with the second region  122 . The core  108  can be constituted by, for example, InP. Also, the distributed Bragg reflection portion  107  is constituted by a diffraction grating that is formed on the third region  123  of the core  108 . The active layer  105  and the distributed Bragg reflection portion  107  are optically connected via the core  108  of the second region  122 . 
     Also, the active layer  105  is, for example, a double quantum well structure resulting from a well layer and a barrier layer composed of InGaAlAs, InGaAs, InGaAsP, and the like, which each have a different composition. The active layer  105  can also be formed by compound semiconductors such as bulk InGaAlAs, InGaAs, and InGaAsP. For example, the width of the active layer  105  can be 0.7 μm, and the thickness of the active layer  105  can be 0.32 μm. Note that there is no limitation on the layer structure and the width. The thickness 0.32 μm of the active layer  105  is the value of the approximate upper limit at which light of the wavelength 1.31 μm, which propagates in the active layer  105 , is in a single mode with respect to the thickness direction of the active layer  105 . 
     Also, the active layer  105  is, for example, embedded in the semiconductor layer  110  composed of InP. The semiconductor layer  110  on the upper side and the lower side of the active layer  105  is constituted by non-doped InP. Also, the semiconductor layer  110  on one side surface of the active layer  105  is constituted by p-type InP, and the semiconductor layer  110  on the other side surface of the active layer  105  is constituted by n-type InP. According to this p-i-n junction, a structure through which current is injected into the active layer  105  is formed. 
     Incidentally, if the operating wavelength of the waveguide laser  102  or the material used as the active layer  105  is changed, in order to achieve a single mode in the thickness direction of the active layer  105 , it is sufficient that the thickness t of the active layer  105  approximately satisfies the relationship shown in the following equation (1), where the operating wavelength is λ, the average refractive index of the active layer  105  is n core , and the refractive index of the first cladding layer  101  is n clad . 
     
       
         
           
             
               
                 
                   
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                             n 
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                             n 
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     For example, if light of a wavelength in a 1.55 μm band is to be used, the thickness t of the active layer  105  is 0.364 μm or less. 
     The absorption layer  103  is constituted by an oversaturation absorption body such as graphene. Also, the absorption layer  103  is provided between the active layer  105  and the distributed Bragg reflection portion  107 . In this example, the absorption layer  103  is formed below the core  108 , which constitutes the optical waveguide between the active layer  105  and the distributed Bragg reflection portion  107  (second region  122 ). Graphene can be epitaxially grown on 6H-SiC, in which the surface orientation of the main surface is a (0001) surface, 4H-Sic, in which the surface orientation of the main surface is a (0001) surface, and 3C-SiC, in which the surface orientation of the main surface is a (111) surface. Note that the absorption layer  103  can also have a configuration of being arranged on the core  108 , which forms the optical waveguide between the active layer  105  and the distributed Bragg reflection portion  107 . 
     Also, the second cladding layer  104  is formed on the first cladding layer  101  and covers the waveguide laser  102 . The second cladding layer  104  is constituted by, for example, SiO 2 . 
     According to the optical device in the above-described embodiment, first, deterioration of output light resulting from self-heat generation of the waveguide laser  102  can be reduced due to the high heat dissipation ability of the first cladding layer  101  constituted by SiC or the like, and therefore high light output is expected. Also, lateral-direction current injection is enabled with a PIN structure formed by the p-type region  111  and the n-type region  112  sandwiching the active layer  105 . Also, according to an embodiment, the light generated from the active layer  105  of the first region  121  is confined in the second region  122  by the distributed Bragg reflection portion  107  of the third region  123 . Then, the absorption layer  103  is arranged on the second region  122 . If the absorption layer  103  is constituted by graphene, in the optical device according to the embodiment, a Q switch or a mode-locking operation can be expected due to the oversaturation absorption property in the wavelength band 1 to 2 μm of graphene. 
     In the above-described optical device, the active layer  105 , the core  108 , and the semiconductor layer no can be formed using a well-known crystal growth technique. Also, the first cladding layer  101  can be formed using a substrate bonding technique or the like with a substrate on which the active layer  105  is formed, but there is no limitation to this production method. Also, in the embodiment, light confinement in the substrate horizontal direction is realized through a difference in the refractive indices of the active layer  105  and the semiconductor layer  110 , and the waveguide gain. However, there is no limitation to this, and any method can be used to realize light confinement resulting from a two-dimensional photonic crystal structure and the like. 
     Next, the waveguide mode distribution of the optical device according to the embodiment will be described. Note that hereinafter, it is assumed that the first cladding layer  101  is constituted by SiC, and the second cladding layer  104  is constituted by SiO2. Also, the active layer  105  has a double quantum well structure resulting from a well layer and a barrier layer, which each have different compositions and are composed of InGaAlAs. Also, the semiconductor layer  110  on one side surface of the active layer  105  is a p-type region  111  constituted by p-type InP. Also, the semiconductor layer  110  on the other side surface of the active layer  105  is the n-type region  112  constituted by n-type InP. Also, the core  108  is InP. Also, the width of the active layer  105  is 0.7 μm, the thickness is 0.33 μm, the width of the core  108  is 1.2 μm, and the thickness is 0.33 μm. 
     A waveguide mode distribution calculated based on the above-described configuration is shown in  FIGS. 2A, 2B, and 2C .  FIG. 2A  shows a waveguide mode distribution in the cross-section shown in  FIG. 1B  of the first region  121 .  FIG. 2B  shows a waveguide mode distribution in the cross-section shown in  FIG. 1C  of the second region  122 .  FIG. 2C  shows a waveguide mode distribution in the cross-section shown in  FIG. 1D  of the third region  123 . In  FIGS. 2A, 2B, and 2C , the waveguide mode distributions are shown as contour lines. 
     Since an active layer with a high refractive index or an InP core is sandwiched between SiO 2  and SiC, which has a small relative refractive index, high light confinement is realized as shown in  FIGS. 2A, 2B, and 2C . For this reason, in the second region  122 , a high photon density is achieved using an optical waveguide structure with high light confinement and a small volume. This is useful for realizing oversaturation absorption in the absorption layer  103 . 
     Incidentally, the reflection portion provided on one end of the active layer  105 , which is the emission side of the laser light, can be constituted by a distributed Bragg reflection portion  206  constituted by a diffraction grating, as shown in  FIG. 3 . For example, a core  108   a , which is similar to the core  108 , can be formed also on one end of the active layer  105 , and a diffraction grating can be formed above the core  108   a  to form the distributed Bragg reflection portion  206 . 
     As described above, according to embodiments of the present invention, an absorption layer composed of an oversaturation absorption body is arranged between an active layer and a distributed Bragg reflection portion of a waveguide laser, and these are formed on a first cladding layer constituted by a material having a higher thermal conductivity than InP, and therefore it is possible to realize a reduction in size of an optical device using an oversaturation absorption body. According to the present invention, it is possible to integrate a waveguide semiconductor laser having a high heat dissipation ability and a waveguide structure including an oversaturation absorption body. 
     Note that the present invention is not limited to the embodiment described above, and it is clear that many modifications and combinations can be implemented by a person with normal knowledge in the relevant field, within the technical idea of the present invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               101  First cladding layer 
               102  Waveguide laser 
               103  Absorption layer 
               104  Second cladding layer 
               105  Active layer 
               106  Reflection portion 
               107  Distributed Bragg reflection portion 
               108  Core 
               110  Semiconductor layer 
               111  p-type region 
               112  n-type region 
               113  p electrode 
               114  n electrode 
               121  First region 
               122  Second region 
               123  Third region.