Patent Publication Number: US-2023161184-A1

Title: Optical device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2021-188354, filed on Nov. 19, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to an optical device. 
     BACKGROUND 
     A silicon photonics component is able to strongly confine light in a minute area due to a large refractive index difference between a core and a clad, and therefore is effective to achieve downsizing and high integration of various silicon optical elements, such as an optical modulator, a light receiving element, a phase control element, or a polarization multiplexer/demultiplexer. However, a general silicon optical modulator is, for example, a carrier control type with doped PN junction, and therefore, there is a problem in further expanding a modulation bandwidth. 
     To cope with this, for example, an optical modulator using an electro-optic crystal, such as lithium niobate (LiNbO 3 : LN), with an electro-optic effect is able to expand the modulation bandwidth and prevent an absorption loss, so that it is possible to realize a high-performance optical modulator. However, it is difficult to integrate a silicon optical element, such as a light receiving element, a phase control element, or a polarization multiplexer/demultiplexer, other than the optical modulator into an electro-optic crystal. 
     Therefore, in recent years, a hybrid optical device, in which a silicon photonics component and a crystal with the electro-optic effect are combined, is attracting attention. In the hybrid optical device, an optical device that realizes high integration of the silicon photonics component and high modulation characteristics of the crystal with the electro-optic effect is demanded. 
     In a conventional optical device, an electro-optic crystal layer with the electro-optic effect is laminated on a buffer layer in a silicon photonics component that is formed in advance, an optical waveguide made of an electro-optic crystal is formed on the electro-optic crystal layer, and a cladding layer is laminated on the electro-optic crystal layer. Further, an electrode is arranged on the cladding layer, so that the optical modulator with the electro-optic crystal can be formed. 
     Patent Literature 1: U.S. Unexamined Patent Application Publication No. 2020/0150467 
     Patent Literature 2; Japanese Laid-open Patent Publication No. 2011-102891 
     In the conventional optical device that is laminated on the electro-optic crystal layer on the silicon photonics component, a first optical waveguide is formed on an intermediate layer in the silicon photonics component, and a second optical waveguide is formed on the electro-optic crystal layer. However, if a distance between the intermediate layer on which the first optical waveguide is formed and the electro-optic crystal layer on which the second optical waveguide is formed is excessively increased, it becomes difficult to optically couple the first optical waveguide and the second optical waveguide, so that an optical loss occurs due to deterioration of optical coupling characteristics. 
     SUMMARY 
     According to an aspect of an embodiment, an optical device includes a substrate, a first cladding layer that is laminated on one surface of the substrate, and a first optical waveguide that is formed in the first cladding layer at a side opposite to the substrate in the first cladding layer. The optical device further includes an electro-optic crystal layer that is laminated on a surface of the first cladding layer at a side opposite to the substrate, a second optical waveguide that is formed of the electro-optic crystal layer on a surface of the electro-optic crystal layer at a side opposite to the first cladding layer, and a second cladding layer that is laminated on a surface of the electro-optic crystal layer at a side opposite to the first cladding layer. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic plan view illustrating an example of a configuration of an optical device according to a present embodiment; 
         FIG.  2    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line A-A in  FIG.  1   ; 
         FIG.  3    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line B-B in  FIG.  1   ; 
         FIG.  4    is a schematic cross-sectional view illustrating an example of a cross-sectional part (Mach-Zehnder interferometer) cut along a line C-C in  FIG.  1   ; 
         FIG.  5    is a schematic cross-sectional view illustrating an example of a cross-sectional part (phase control element) cut along a line D-D in  FIG.  1   ; 
         FIG.  6    is a schematic cross-sectional view illustrating an example of a cross-sectional part (light receiving element) cut along a line E-E in  FIG.  1   ; 
         FIG.  7    is a flowchart illustrating an example of a process of manufacturing the optical device; 
         FIG.  8 A  is a schematic cross-sectional view illustrating an example of a configuration of a silicon photonics component; 
         FIG.  8 B  is a schematic cross-sectional view illustrating an example of a attaching process; 
         FIG.  9 A  is a schematic cross-sectional view illustrating an example of a substrate removal process; 
         FIG.  9 B  is a schematic cross-sectional view illustrating an example of a thickness adjustment process and an electro-optic crystal layer formation process; 
         FIG.  10 A  is a schematic cross-sectional view illustrating an example of a second optical waveguide formation process; 
         FIG.  10 B  is a schematic cross-sectional view illustrating an example of a second cladding layer formation process, an electrode formation process, and a via formation process; 
         FIG.  11 A  is a schematic plan view illustrating an example of a configuration of an optical modulator; 
         FIG.  11 B  is a schematic plan view illustrating an example of a configuration of an IQ optical modulator; 
         FIG.  11 C  is a schematic plan view illustrating an example of a configuration of a DP-IQ optical modulator; 
         FIG.  11 D  is a schematic plan view illustrating an example of a configuration of an optical communication device; 
         FIG.  12    is a schematic plan view illustrating an example of a configuration of an optical device according to a comparative example; 
         FIG.  13    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line A 1 -A 1  in  FIG.  12   ; 
         FIG.  14    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line B 1 -B 1  in  FIG.  12   ; 
         FIG.  15    is a schematic cross-sectional view illustrating an example of a cross-sectional part (Mach-Zehnder interferometer) cut along a line C 1 -C 1  in  FIG.  12   ; 
         FIG.  16    is a schematic cross-sectional view illustrating an example of a cross-sectional part (phase control element) cut along a line D 1 -D 1  in  FIG.  12   ; and 
         FIG.  17    is a schematic cross-sectional view illustrating an example of a cross-sectional part (light receiving element) cut along a line E 1 -E 1  in  FIG.  12   . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     [a] COMPARATIVE EXAMPLE 
       FIG.  12    is a schematic plan view illustrating an example of a configuration of an optical device  100  according to a comparative example. The optical device  100  illustrated in  FIG.  12    includes an input unit  111 , a first optical waveguide  102 , a branching unit  112 , two optical modulators  103 , two phase control elements  104 , a multiplexing unit  113 , an output unit  114 , and a light receiving element  105 . 
     The input unit  111  inputs signal light from a light source (not illustrated) to the first optical waveguide  102 . The first optical waveguide  102  is, for example, a silicon optical waveguide through which the signal light coming from the input unit  111  passes. 
     The optical modulators  103  are, for example, LN modulators. The optical modulators  103  are, for example, Mach-Zehnder modulators that include the branching unit  112 , two Mach-Zehnder interferometers  103 A, and the multiplexing unit  113 , and optically modulate optically-split signal light that comes from the first optical waveguide  102  in accordance with an electrical signal. The branching unit  112  optically splits the signal light coming from the first optical waveguide  102  into light for the two first optical waveguides  102 , and outputs the optically-split signal light to each of the Mach-Zehnder interferometers  103 A. Each of the Mach-Zehnder interferometers  103 A includes, for example, a second optical waveguide  132 A that is made of an electro-optic crystal, such as LN, and an electrode  134 . The electrode  134  includes a signal electrode  134 A and a ground electrode  134 B. Each of the Mach-Zehnder interferometers  103 A generates an electric field from the signal electrode  134 A to the ground electrode  134 B in accordance with an electrical signal applied to the signal electrode  134 A, changes an optical refractive index of the second optical waveguide  132 A in accordance with the electrical field, and adjusts a phase of light that passes through the second optical waveguide  132 A in accordance with the change of the optical refractive index. Each of the Mach-Zehnder interferometers  103 A outputs the light for which the phase has been adjusted to each of the phase control elements  104 . The multiplexing unit  113  multiplexes the signal light that comes from each of the phase control elements  104  and that is subjected to phase shift, and outputs the multiplexed signal light to the output unit  114  via the first optical waveguide  102 . 
     Each of the phase control elements  104  is a silicon component that shifts a phase of the signal light that has been subjected to optical modulation by the optical modulators  103 . The phase control elements  104  output the signal light that has been subjected to phase shift to the multiplexing unit  113  via the first optical waveguides  102 . The multiplexing unit  113  multiplexes the signal light that comes from each of the phase control elements  104  and that is subjected to phase shift, and outputs the multiplexed signal light to the output unit  114  via the first optical waveguide  102 . The output unit  114  is connected to an optical fiber (not illustrated) and outputs the multiplexed signal light that comes from the first optical waveguides  102 . Further, the light receiving element  105  is a silicon component that converts a part of the signal light, which is an output of the multiplexing unit  113 , to an electrical signal. 
     Meanwhile, for example, a silicon photonics component  120  is provided in which the input unit  111 , the first optical waveguide  102 , the branching unit  112 , the two phase control elements  104 , the multiplexing unit  113 , the output unit  114 , and the light receiving element  105  in the optical device  100  are integrated. Meanwhile, the silicon photonics component  120  is a component that is formed in advance. 
       FIG.  13    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line A 1 -A 1  in  FIG.  12   . The A 1 -A 1  cross-sectional part as illustrated in  FIG.  13    includes the silicon photonics component  120 , an electro-optic crystal layer  132 , and a second cladding layer  133 . 
     The silicon photonics component  120  includes a first substrate  121 , a first cladding layer  124  that is laminated on the first substrate  121 , and the first optical waveguide  102  that is formed in the first cladding layer  124 . The first substrate  121  has resistivity of smaller than 1000 Ωcm, for example. The first cladding layer  124  includes an intermediate layer  122  that is laminated on the first substrate  121 , the first optical waveguide  102  that is formed in the intermediate layer  122 , and a buffer layer  123  that is laminated on the intermediate layer  122 . The electro-optic crystal layer  132  is a layer that is laminated on the buffer layer  123  in the silicon photonics component  120  and that is made of LN or the like with an electro-optic effect, for example. The second cladding layer  133  is a layer that is laminated on the electro-optic crystal layer  132  and made of Si 02  or the like, for example. 
       FIG.  14    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line B 1 -B 1  in  FIG.  12   . The B 1 -B 1  cross-sectional part as illustrated in  FIG.  14    includes the first substrate  121 , the intermediate layer  122 , the first optical waveguide  102 , the buffer layer  123 , the electro-optic crystal layer  132 , the second optical waveguide  132 A that is formed on the electro-optic crystal layer  132 , and the second cladding layer  133 . The second optical waveguide  132 A is an LN optical waveguide that is formed of the electro-optic crystal layer  132 . The first optical waveguide  102  and the second optical waveguide  132 A are optically coupled. 
       FIG.  15    is a schematic cross-sectional view illustrating an example of a cross-sectional part (the Mach-Zehnder interferometer  103 A) cut along a line C 1 -C 1  in  FIG.  12   . The C 1 -C 1  cross-sectional part as illustrated in  FIG.  15    is a cross-sectional part of the Mach-Zehnder interferometer  103 A in the optical modulators  103 . The C 1 -C 1  cross-sectional part includes the first substrate  121 , the intermediate layer  122 , the buffer layer  123 , the electro-optic crystal layer  132 , the second optical waveguide  132 A, the second cladding layer  133 , and the electrode  134  that is formed on the second cladding layer  133 . The electrode  134  includes the signal electrode  134 A and the ground electrode  134 B. Each of the Mach-Zehnder interferometers  103 A changes the optical refractive index of the second optical waveguide  132 A in accordance with an electric field generated from the signal electrode  134 A to the ground electrode  134 B depending on an electrical signal applied to the signal electrode  134 A, and optically modulates signal light that passes through the second optical waveguide  132 A in accordance with the change of the optical refractive index. 
       FIG.  16    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line D 1 -D 1  (phase control elements  104 ) in  FIG.  12   . The D 1 -D 1  cross-sectional part as illustrated in  FIG.  16    includes the first substrate  121 , the intermediate layer  122 , the first optical waveguide  102 , the buffer layer  123 , the phase control element  104  that is formed in the vicinity of the first optical waveguide  102  in the buffer layer  123 , the electro-optic crystal layer  132 , and the second cladding layer  133 . A via  135  that exposes a metal wire  104 A 2  in the phase control element  104  is formed in the second cladding layer  133 , the electro-optic crystal layer  132 , and the buffer layer  123 . 
     The phase control element  104  includes a thermo-optical heater  104 A 1  that is an electrical resistance made of TiN or the like and arranged above the first optical waveguide  102 , and the metal wire  104 A 2  that is electrically connected to the thermo-optical heater  104 A 1  and supplies an electric current to the thermo-optical heater  104 A 1 . The thermo-optical heater  104 A 1  generates heat by causing an electric current to flow from the metal wire  104 A 2  to the thermo-optical heater  104 A 1 . The phase control element  104  shifts a phase of light that passes through the inside of the first optical waveguide  102 , due to a change of a silicon refractive index inside the first optical waveguide  102  by the heat of the thermo-optical heater  104 A 1 . 
       FIG.  17    is a schematic cross-sectional view illustrating an example of a cross-sectional part (the light receiving element  105 ) cut along a line E 1 -E 1  in  FIG.  12   . The E 1 -E 1  cross-sectional part as illustrated in  FIG.  17    includes the first substrate  121 , the intermediate layer  12 , the first optical waveguide  102 , the buffer layer  123 , the light receiving element  105  that is formed on the first optical waveguide  102  in the buffer layer  123 , the electro-optic crystal layer  132 , and the second cladding layer  133 . The via  135  that exposes a metal wire  105 A 2  in the light receiving element  105  is formed in the second cladding layer  133 , the electro-optic crystal layer  132 , and the buffer layer  123 . 
     The light receiving element  105  includes a photoelectric conversion element  105 A 1  that is made of Ge or the like and arranged on the first optical waveguide  102 , and the metal wire  105 A 2  that is connected to the photoelectric conversion element  105 A 1  and outputs an electrical signal from the photoelectric conversion element  105 A 1 . The light receiving element  105  converts signal light that passes through the first optical waveguide  102  to an electrical signal via the photoelectric conversion element  105 A 1 , and outputs the electrical signal to a monitor (not illustrated) via the metal wire  105 A 2 . 
     In the optical device  100  of the comparative example, the electro-optic crystal layer  132  is laminated on the buffer layer  123  in the silicon photonics component  120  that is formed in advance, the second optical waveguide  132 A is formed on the electro-optic crystal layer  132 , and the second cladding layer  133  is laminated on the electro-optic crystal layer  132 . Further, the electrode  134  is arranged on the the second cladding layer  133 , so that the optical modulators  103  with the electro-optic crystal can be formed. 
     In the optical device  100  that is laminated on the electro-optic crystal layer  132  on the silicon photonics component  120 , the first optical waveguide  102  is formed on the intermediate layer  122  in the silicon photonics component  120 , and the second optical waveguide  132 A is formed on the electro-optic crystal layer  132 . However, for example, when focus is given to the light receiving element  105  and the phase control elements  104 , and if a distance Ll between the intermediate layer  122  on which the first optical waveguide  102  is formed and the electro-optic crystal layer  132  on which the second optical waveguide  132 A is formed is excessively increased, it becomes difficult to optically couple the first optical waveguide  102  and the second optical waveguide  132 A, so that an optical loss occurs due to deterioration of optical coupling characteristics. 
     Furthermore, in the optical modulators  103 , the resistivity of the first substrate  121  in the silicon photonics component  120  is smaller than 1000 Ωcm, so that a modulation bandwidth of the optical modulators  103  is deteriorated. 
     Therefore, there is a need to provide an optical device according to a present embodiment, in which it is possible to stabilize optical coupling performance by reducing a distance between an electro-optic crystal layer on which a second optical waveguide is formed and a first optical waveguide that is formed on an intermediate layer, and it is possible to prevent deterioration of the modulation bandwidth of an optical modulator. 
     An embodiment of the optical device or the like disclosed in the present application will be described in detail below with reference to the drawings. The present invention is not limited by the embodiment below. 
     [b] EMBODIMENT 
       FIG.  1    is a schematic plan view illustrating an example of an optical device  1  according to the present embodiment. The optical device  1  illustrated in  FIG.  1    includes an input unit  11 , a first optical waveguide  2 , a branching unit  12 , two optical modulators  3 , two phase control elements  4 , a multiplexing unit  13 , an output unit  14 , and a light receiving element  5 . 
     The input unit  11  inputs signal light from a light source (not illustrated) to the first optical waveguide  2 . The first optical waveguide  2  is, for example, a silicon optical waveguide through which the signal light coming from the input unit  11  passes. 
     The optical modulators  3  are, for example, LN optical modulators made of a crystal, such as LN, with an electro-optic effect. The optical modulators  3  are, for example, Mach-Zehnder modulators that include the branching unit  12 , two Mach-Zehnder interferometers  3 A, and the multiplexing unit  13 , and optically modulate optically-split signal light that comes from the first optical waveguide  2  in accordance with an electrical signal. The branching unit  12  optically splits the signal light coming from the first optical waveguide  2  into the two first optical waveguides  2 , and outputs the optically-split signal light to each of the Mach-Zehnder interferometers  3 A. Each of the Mach-Zehnder interferometers  3 A includes, for example, a second optical waveguide  32 A and an electrode  34 . The second optical waveguide  32 A is, for example, an LN optical waveguide. The electrode  34  includes a signal electrode  34 A and a ground electrode  34 B. Each of the Mach-Zehnder interferometers  3 A generates an electric field from the signal electrode  34 A to the ground electrode  34 B in accordance with an electrical signal applied to the signal electrode  34 A, changes an optical refractive index of the second optical waveguide  32 A in accordance with the electrical field, and adjusts a phase of light that passes through the second optical waveguide  32 A in accordance with the change of the optical refractive index. Each of the Mach-Zehnder interferometers  3 A outputs the light for which the phase has been adjusted to each of the phase control elements  4 . The multiplexing unit  13  multiplexes the signal light that comes from each of the phase control elements  4  and that is subjected to phase shift, and outputs the multiplexed signal light to the output unit  14  via the first optical waveguide  2 . 
     Each of the phase control elements  4  is a silicon component that shifts a phase of the signal light that has been subjected to optical modulation by the optical modulators  3 . The phase control elements  4  output the signal light that has been subjected to phase shift to the multiplexing unit  13  via the first optical waveguides  2 . The output unit  14  is connected to an optical fiber (not illustrated) and outputs the multiplexed signal light that comes from the first optical waveguides  2 . Further, the light receiving element  5  is a silicon component that electrically converts a part of the signal light that is an output of the multiplexing unit  13 . 
     Meanwhile, for example, a silicon photonics component  20  is provided in which the input unit  11 , the first optical waveguide  2 , the branching unit  12 , the two phase control elements  4 , the multiplexing unit  13 , the output unit  14 , and the light receiving element  5  in the optical device  1  are integrated. Meanwhile, the silicon photonics component  20  is a component that is formed in advance. 
       FIG.  2    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line A 1 -A 1  in  FIG.  1   . The A-A cross-sectional part as illustrated in  FIG.  2    includes a second substrate  31 , a silicon photonics component  20 A, an electro-optic crystal layer  32 , and a second cladding layer  33 . The silicon photonics component  20 A is the silicon photonics component  20  from which a first substrate  21  is removed. 
     The second substrate  31  is a substrate that has resistivity of, for example, equal to or larger than 1000 Ωcm. A thickness of the second substrate  31  is, for example, 1000 micrometers (μm). Further, the second substrate  31  is a substrate that is made of, for example, silicon, LN, or quartz. The silicon photonics component  20  includes the first substrate  21 , a first cladding layer  24  that is laminated on the first substrate  21 , and the first optical waveguide  2  that is formed in the first cladding layer  24 . The first cladding layer  24  is made of, for example, SiO 2 . Further, the first optical waveguide  2  is, for example, a silicon waveguide. The first optical waveguide  2  is, for example, a rib waveguide. The first optical waveguide  2  is an optical waveguide that is formed in the first cladding layer  24  at a side opposite to the second substrate  31  in the first cladding layer  24 . 
     The first cladding layer  24  includes an intermediate layer  22  that is laminated on the first substrate  21 , the first optical waveguide  2  that is formed on the intermediate layer  22 , and a buffer layer  23  that is laminated on the intermediate layer  22 . The buffer layer  23  is a layer that is laminated on one surface of the second substrate  31 . The intermediate layer  22  is a layer that is laminated on one surface of the buffer layer  23  at a side opposite to the second substrate  31 . 
     The second substrate  31  is in a state in which the buffer layer  23  in the silicon photonics component  20 A is attached. In other words, the A-A cross-sectional part illustrated in  FIG.  2    includes the second substrate  31 , the buffer layer  23  that is laminated on the second substrate  31 , the intermediate layer  22  which is laminated on the buffer layer  23  and in which the first optical waveguide  2  is formed, the electro-optic crystal layer  32  that is laminated on the intermediate layer  22 , and the second cladding layer  33  that is laminated on the electro-optic crystal layer  32 . The second cladding layer  33  is made of, for example, SiO 2 . The second cladding layer  33  is a layer that is laminated on one surface of the electro-optic crystal layer  32  at a side opposite to the first cladding layer  24 . 
     The first optical waveguide  2  is formed on the intermediate layer  22  at a side of the second substrate  31 . The electro-optic crystal layer  32  is, for example, an X-cut LN layer. LN is an anisotropic material whose refractive index changes by application of an electric field and which has the Pockels coefficient of about 30 pm/V, for example. The electro-optic crystal layer  32  is a layer that is laminated on the first cladding layer  24  at a side opposite to the second substrate  31 . The second optical waveguide  32 A is an optical waveguide that is formed of the electro-optic crystal layer  32  on a surface of the electro-optic crystal layer  32  at a side opposite to the first cladding layer  24 . Meanwhile, the first optical waveguide  2  and the second optical waveguide  32 A have trapezoidal shapes such that respective long sides face each other across the intermediate layer  22 . 
     The intermediate layer  22  is a layer made of SiO 2  with a lower optical refractive index than LN, for example. A thickness of the intermediate layer  22  between the first optical waveguide  2  and the second optical waveguide  32 A is, for example, about 2 μm to 6 μm. The buffer layer  23  is a layer that is made of SiO 2  and that is arranged to prevent light propagating through the first optical waveguide  2  from being absorbed by the electrode  34 . A thickness of the electro-optic crystal layer  32  is, for example, about 0.5 μm to 3 μm. 
       FIG.  3    is a schematic cross-sectional view illustrating an example of a cross-sectional part cut along a line B-B in  FIG.  1   . The B-B cross-sectional part as illustrated in  FIG.  3    is a couplig part  2 A connecting the first optical waveguide  2  with the second optical waveguide  32 A. The B-B cross-sectional part includes the second substrate  31 , the buffer layer  23 , the first optical waveguide  2 , the intermediate layer  22 , the electro-optic crystal layer  32 , the second optical waveguide  32 A that is formed on the electro-optic crystal layer  32 , and the second cladding layer  33 . The first optical waveguide  2  and the second optical waveguide  32 A are optically coupled. The first optical waveguide  2  and the second optical waveguide  32 A are located close to each other in a vertical direction, and the width of the first optical waveguide  2  is adiabatically reduced to weaken optical confinement such that the first optical waveguide  2  is gradually coupled with the second optical waveguide  32 A, for example. Meanwhile, the thickness of the intermediate layer  22  between the first optical waveguide  2  formed on the intermediate layer  22  and the second optical waveguide  32 A formed on the electro-optic crystal layer  32 , that is, a distance L between the first optical waveguide  2  and the second optical waveguide  32 A, is reduced. 
       FIG.  4    is a schematic cross-sectional view illustrating an example of a cross-sectional part (the Mach-Zehnder interferometers  3 A) cut along a line C-C in  FIG.  1   . The C-C cross-sectional part as illustrated in  FIG.  4    is a cross-sectional part of the Mach-Zehnder interferometers  3 A in the optical modulators  3 . The C-C cross-sectional part includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the electro-optic crystal layer  32 , the second optical waveguide  32 A, the second cladding layer  33 , and the electrode  34  formed on the second cladding layer  33 . 
     The electrode  34  includes the signal electrode  34 A and the ground electrode  34 B. The signal electrode  34 A is an electrode that is made of a metal material, such as gold or copper, has a width of 2 to 10 μm, and has a thickness of 1 to 20 μm, for example. The ground electrode  34 B is an electrode that is made of a metal material, such as gold or copper, and has a thickness of equal to or larger than 1 μm, for example. 
     If the electro-optic crystal layer  32  is an X-cut LN, a refractive index is changed by application of an electric field in a horizontal direction, and an electric field is applied from the signal electrode  34 A to the ground electrode  34 B in a left-right direction of the second optical waveguide  32 A. Each of the Mach-Zehnder interferometers  3 A changes the optical refractive index of the second optical waveguide  32 A in accordance with the electric field that is applied from the signal electrode  34 A to the ground electrode  34 B by application of an electrical signal to the signal electrode  34 A, and optically modulates signal light that passes through the second optical waveguide  32 A in accordance with the change of the optical refractive index. 
       FIG.  5    is a schematic cross-sectional view illustrating an example of a cross-sectional part (the phase control element  4 ) cut along a line D-D in  FIG.  1   . The D-D cross-sectional part as illustrated in  FIG.  5    includes the second substrate  31 , the buffer layer  23 , the first optical waveguide  2 , the intermediate layer  22 , the phase control element  4  is are formed in the vicinity of the first optical waveguide  2  in the buffer layer  23 , the electro-optic crystal layer  32 , and the second cladding layer  33 . A via  35  that exposes a metal wire  4 A 2  in the phase control element  4  is formed in the second cladding layer  33 , the electro-optic crystal layer  32 , the intermediate layer  22 , and the buffer layer  23 . 
     The phase control element  4  includes a thermo-optical heater  4 A 1  that is an electrical resistance made of TiN or the like and arranged at a position in the vicinity of the first optical waveguide  2 , and the metal wire  4 A 2  that is electrically connected to the thermo-optical heater  4 A 1  and supplies an electric current to the thermo-optical heater  4 A 1 . The thermo-optical heater  4 A 1  generates heat by causing an electric current to flow from the metal wire  4 A 2  to the thermo-optical heater  4 A 1 . The phase control element  4  shifts a phase of light that passes through the inside of the first optical waveguide  2 , due to a change of a silicon refractive index inside the first optical waveguide  2  by the heat of the thermo-optical heater  4 A 1 . 
       FIG.  6    is a schematic cross-sectional view illustrating an example of a cross-sectional part (the light receiving element  5 ) cut along a line E-E in  FIG.  1   . The E-E cross-sectional part as illustrated in  FIG.  6    includes the second substrate  31 , the buffer layer  23 , the first optical waveguide  2 , the intermediate layer  22 , the light receiving element  5  that is formed in the vicinity of the first optical waveguide  2  in the buffer layer  23 , the electro-optic crystal layer  32 , and the second cladding layer  33 . The via  35  that exposes a metal wire  5 A 2  in the light receiving element  5  is formed in the second cladding layer  33 , the electro-optic crystal layer  32 , the intermediate layer  22 , and the buffer layer  23 . 
     The light receiving element  5  includes a photoelectric conversion element  5 A 1  that is made of Ge or the like and arranged at a position in the vicinity of the first optical waveguide  2 , and the metal wire  5 A 2  that is connected to the photoelectric conversion element  5 A 1  and outputs an electrical signal from the photoelectric conversion element  5 A 1 . The light receiving element  5  converts signal light that passes through the first optical waveguide  2  to an electrical signal via the photoelectric conversion element  5 A 1 , and outputs the electrical signal to a monitor (not illustrated) via the metal wire  5 A 2 . 
       FIG.  7    is a flowchart illustrating an example of a process of manufacturing the optical device  1 . As a manufacturing process, a preparation process of preparing the silicon photonics component  20  that is formed in advance is performed (Step S 11 ). An attaching process of inverting the silicon photonics component  20  and attaching the second substrate  31  to a surface of the buffer layer  23  in the silicon photonics component  20  is performed (Step S 12 ). Meanwhile, the second substrate  31  is a substrate with high resistivity of equal to or larger than 1000 Ωcm, which is higher than the first substrate  21 . 
     After attaching the second substrate  31 , a removal process of removing the first substrate  21  in the silicon photonics component  20  is performed (Step S 13 ). A thickness adjustment process of adjusting the thickness of the intermediate layer  22  in the silicon photonics component  20 A from which the first substrate  21  is removed is performed (Step S 14 ). Meanwhile, by adjusting the thickness of the intermediate layer  22 , it is possible to reduce the distance L between the second optical waveguide  32 A of the electro-optic crystal layer  32  and the first optical waveguide  2  and realize highly efficient optical coupling at the time of completion. 
     An electro-optic crystal layer formation process of laminating the electro-optic crystal layer  32  on the intermediate layer  22  in the silicon photonics component  20 A, in which the thickness of the intermediate layer  22  has been adjusted, is performed (Step S 15 ). A second optical waveguide formation process of forming the second optical waveguide  32 A on the electro-optic crystal layer  32  is performed (Step S 16 ). 
     A second cladding layer formation process of forming the second cladding layer  33  on the electro-optic crystal layer  32 , on which the second optical waveguide  32 A has been formed, is performed (Step S 17 ). Further, an electrode formation process of arranging the electrode  34  on the second cladding layer  33  is performed (Step S 18 ). Furthermore, after the electrode  34  is arranged, a via formation process of forming the via  35  in the second cladding layer  33 , the electro-optic crystal layer  32 , and the buffer layer  23  is performed so as to expose the metal wire  4 A 2  of the phase control element  4  and the metal wire  5 A 2  of the light receiving element  5  in the buffer layer  23  (Step S 19 ). As a result, the optical device  1  as illustrated in  FIG.  10 B  is formed. 
     In the optical device  1  as illustrated in  FIG.  10 B , the second substrate  31  with higher resistivity than the first substrate  21  is adopted, so that it is possible to avoid a situation in which the modulation bandwidth of the optical modulators  3  is deteriorated, for example. Furthermore, in the optical device  1 , the thickness of the intermediate layer  22  is adjusted and the distance L between the second optical waveguide  32 A in the electro-optic crystal layer  32  and the first optical waveguide  2  is reduced, so that it is possible to improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A. 
       FIG.  8 A  is a schematic cross-sectional view illustrating an example of a configuration of the silicon photonics component  20 . The silicon photonics component  20  includes the first substrate  21 , the intermediate layer  22 , the plurality of first optical waveguides  2 , and the buffer layer  23 . The phase control element  4  and the light receiving element  5  are incorporated in the arbitrary first optical waveguides  2  in the buffer layer  23 . The silicon photonics component  20  is a component that is formed in advance. 
       FIG.  8 B  is a schematic cross-sectional view illustrating an example of the attaching process. After the silicon photonics component  20  illustrated in  FIG.  8 A  is inverted upside down, the second substrate  31  is attached onto the buffer layer  23  of the silicon photonics component  20 . 
       FIG.  9 A  is a schematic cross-sectional view illustrating an example of the substrate removal process. By removing the first substrate  21  in the silicon photonics component  20  to which the second substrate  31  is attached, the silicon photonics component  20 A as illustrated in  FIG.  9 A  is formed. 
       FIG.  9 B  is a schematic cross-sectional view illustrating an example of the thickness adjustment process and the electro-optic crystal layer formation process. The thickness of the intermediate layer  22  in the silicon photonics component  20 A in which the first substrate  21  has been removed is adjusted. By adjusting the thickness of the intermediate layer  22 , the distance between the first optical waveguide  2  and the second optical waveguide  32 A is adjusted. Further, after the thickness of the intermediate layer  22  is adjusted, the electro-optic crystal layer  32  is laminated on the intermediate layer  22  of the silicon photonics component  20 A. 
       FIG.  10 A  is a schematic cross-sectional view illustrating an example of the second optical waveguide formation process. After the electro-optic crystal layer  32  is laminated on the intermediate layer  22  of the silicon photonics component  20 A, the second optical waveguide  32 A is formed on the electro-optic crystal layer  32 . 
       FIG.  10 B  is a schematic cross-sectional view illustrating an example of the second cladding layer formation process, the electrode formation process, and the via formation process. After the second optical waveguide  32 A is formed, the second cladding layer  33  is laminated on the electro-optic crystal layer  32 . Further, after the second cladding layer  33  is laminated, the signal electrode  34 A and the ground electrode  34 B are arranged on the second cladding layer  33  on which the second optical waveguide  32 A is arranged in the Mach-Zehnder interferometer  3 A. Furthermore, the vias  35  are formed in the second cladding layer  33 , the electro-optic crystal layer  32 , the intermediate layer  22 , and the buffer layer  23  such that the metal wire  4 A 2  of the phase control element  4  and the metal wire  5 A 2  of the light receiving element  5  in the silicon photonics component  20 A are exposed from the second cladding layer  33 , so that the optical device  1  is formed. 
     In the optical device  1  according to the present embodiment, the second substrate  31  with higher resistivity than the first substrate  21  is adopted, so that it is possible to avoid a situation in which the modulation bandwidth of the optical modulators  3  is deteriorated, for example. Furthermore, in the optical device  1 , the thickness of the intermediate layer  22  is adjusted and the distance L between the electro-optic crystal layer  32  and the first optical waveguide  2  is reduced, so that it is possible to improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A. 
     The optical device  1  includes the first optical waveguide  2  that is formed in the first cladding layer  24  at a side opposite to the second substrate  31  in the first cladding layer  24 , the electro-optic crystal layer  32  that is laminated on a surface of the first cladding layer  24  at a side opposite to the second substrate  31 , and the second optical waveguide  32 A that is formed of the electro-optic crystal layer  32  on a surface of the electro-optic crystal layer  32  at a side opposite to the first cladding layer  24 . As a result, the distance L between the electro-optic crystal layer  32  and the first optical waveguide  2  is reduced, so that it is possible to improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A. Furthermore, it is possible to realize an optical device that can reduce a loss and that is suitable for mass production while simultaneously using high integration of silicon photonics and high modulation characteristics of a crystal with the electro-optic effect. 
     In the optical device  1 , even if the silicon photonics component  20  in which, for example, the phase control elements  4  and the light receiving element  5  are incorporated is used, it is possible to integrate the LN optical modulators  3  in the silicon photonics component  20 , so that it is possible to provide the optical device  1  that implements different functions depending on silicon elements to be integrated. 
     The first optical waveguide  2  is formed on a surface of the intermediate layer  22  that comes into contact with the buffer layer  23 , and the electro-optic crystal layer  32  is laminated on a surface of the intermediate layer  22  at a side opposite to the buffer layer  23 . Therefore, the distance L between the electro-optic crystal layer  32  and the first optical waveguide  2  is reduced, so that it is possible to improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A. 
     At least one silicon component, e.g., the silicon components  4  and  5 , are further arranged in the first cladding layer  24  between the first optical waveguide  2  and the second substrate  31 . As a result, it is possible to maintain high integration. 
     Each of the optical modulators  3  includes the electrode  34  that is arranged on a surface of the second cladding layer  33  at a side opposite to the electro-optic crystal layer  32  and that applies an electrical signal to the second optical waveguide  32 A, and the second substrate  31  is a substrate with resistivity of equal to or larger than 1000 Ωcm. Thus, the second substrate  31  with higher resistivity than the first substrate  21  is adopted, so that it is possible to avoid a situation in which the modulation bandwidth of the optical modulators  3  is deteriorated, for example. 
     The signal electrode  34 A and the ground electrode  34 B are arranged on a surface of the second cladding layer  33  at a side opposite to the electro-optic crystal layer  32  so as to apply an electric signal in the horizontal direction in the second optical waveguide  32 A when the electro-optic crystal layer  32  is X-cut. Therefore, the technology is applicable to the optical modulator  3  that includes the X-cut electro-optic crystal layer  32 . 
     Meanwhile, for convenience of explanation, the X-cut LN electro-optic crystal layer  32  is described as an example, but the technology is applicable to the optical device  1  that uses the Z-cut LN electro-optic crystal layer  32 . The optical device  1  includes the Z-cut LN electro-optic crystal layer  32 , and the second optical waveguide  32 A that is formed on the electro-optic crystal layer  32  along an X direction or a Y direction of a crystal axis of the electro-optic crystal layer  32 . In the case of the electro-optic crystal layer  32  whose refractive index is changed by application of an electric field in the vertical (Z) direction, the electro-optic crystal layer  32  arranges the signal electrode  34 A just above the second optical waveguide  32 A to apply the electric field in the vertical direction of the second optical waveguide  32 A. In this case, the ground electrode  34 B may be embedded in the second cladding layer  33 . Further, the buffer layer  23  may be formed between the second optical waveguide  32 A and the signal electrode  34 A. For example, by forming the buffer layer  23  using Si 02 , it is possible to reduce an electrode absorption loss of a propagating optical signal even if the signal electrode  34 A is located just above the second optical waveguide  32 A. 
     Furthermore, while the electro-optic crystal layer  32  is described as one example, a material of the electro-optic crystal is not limited to LN, and any kind of electro-optic crystal is applicable. For example, perovskite oxides, such as lead zirconate titanate (PZT), lanthanum-doped lead zirconate-lead titanate (PLZT), or barium titanate (BaTiO 3 ), may be applicable, and an appropriate change is acceptable. Meanwhile, the Pockels coefficient of PZT is about 110 pm/V, the Pockels coefficient of PLZT is about 700 pm/V, and the Pockels coefficient of BaTiO 3  is about 1850 pm/V, and therefore, the Pockels coefficient of an electro-optic crystal applied to the present invention is a material with the coefficient in a range of 10 to 2000 pm/V. 
     Further, the example has been described in which the first optical waveguide  2  and the second optical waveguide  32 A are ridge waveguides, but embodiments are not limited to the ridge waveguide, but the technology is applicable to, for example, a channel waveguide. 
       FIG.  11 A  is a schematic plan view illustrating an example of an optical device  1 A in which the optical modulator  3  is incorporated. Meanwhile, the same components as those of the optical device  1  illustrated in  FIG.  1    are denoted by the same reference symbols, and explanation of the same components and operation will be omitted. The optical device  1 A illustrated in  FIG.  11 A  includes the optical modulator  3  and the two phase control elements  4 . The optical modulator  3  includes the branching unit  12 , the two Mach-Zehnder interferometers  3 A, and the multiplexing unit  13 . For example, a portion of each of the Mach-Zehnder interferometers  3 A includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the electro-optic crystal layer  32 , the second optical waveguide  32 A, the second cladding layer  33 , and the electrode  34 . A portion of each of the phase control elements  4  includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the phase control element  4 , the electro-optic crystal layer  32 , and the second cladding layer  33 . Therefore, in the optical device  1 A in which the optical modulator  3  is incorporated as illustrated in  FIG.  11 A , it is possible to maintain high integration of silicon photonics, improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A, and prevent deterioration of the modulation bandwidth. 
       FIG.  11 B  is a schematic plan view illustrating an example of an optical device  1 B in which IQ optical modulators  3 B are incorporated. The same components as those of the optical device  1 A illustrated in  FIG.  11 A  are denoted by the same reference symbols, and explanation of the same configuration and operation will be omitted. The IQ optical modulators  3 B in the optical device  1 B illustrated in  FIG.  11 B  include a first branching unit  12 A, an optical modulator  3 B 1  for inphase (I) component, an optical modulator  3 B 2  for quadrature (Q) component, two first phase control elements  4 A, and a first multiplexing unit  13 A. The optical modulator  3 B 1  for I component performs phase modulation on an optical signal of I component. The optical modulator  3 B 2  for Q component performs phase modulation on an optical signal of Q component. The optical modulator  3 B 1  for I component includes the branching unit  12 , two Mach-Zehnder interferometers  3 A 1  ( 3 A 2 ), and the multiplexing unit  13 . The optical modulator  3 B 2  for Q component includes the branching unit  12 , the two Mach-Zehnder interferometers  3 A 1  ( 3 A 2 ), and the multiplexing unit  13 . 
     The first branching unit  12 A optically splits signal light coming from the first optical waveguide  2  and outputs the optically-split signal light to each of the IQ optical modulators  3 B. The optical modulator  3 B 1  for I component outputs the signal light, which is subjected to phase modulation on I component and which comes from the multiplexing unit  13  in the optical modulator  3 B 1 , to the first phase control element  4 A. The first phase control element  4 A shifts a phase of the signal light that has been subjected to the phase modulation on I component, and outputs the signal light of I component subjected to the phase shift to the first multiplexing unit  13 A. 
     Further, the optical modulator  3 B 2  for Q component outputs the signal light, which is subjected to phase modulation on Q component and which comes from the multiplexing unit  13  in the optical modulator  3 B 2 , to the first phase control element  4 A. The first phase control element  4 A shifts a phase of the signal light that has been subjected to the phase modulation on Q component, and outputs the signal light of Q component subjected to the phase shift to the first multiplexing unit  13 A. The first multiplexing unit  13 A multiplexes the signal light of I component and the signal light of Q component, and outputs the multiplexed signal light of IQ component to the output unit  14 . 
     For example, a portion of the Mach-Zehnder interferometer  3 A 1  ( 3 A 2 ) includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the electro-optic crystal layer  32 , the second optical waveguide  32 A, the second cladding layer  33 , and the electrode  34 . A portion of the phase control element  4  (the first phase control element  4 A) includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the phase control element  4  (the first phase control element  4 A), the electro-optic crystal layer  32 , and the second cladding layer  33 . Therefore, in the optical device  1 B in which the IQ optical modulators  3 B are incorporated as illustrated in  FIG.  11 B , it is possible to maintain high integration of silicon photonics, improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A, and prevent deterioration of the modulation bandwidth. 
       FIG.  11 C  is a schematic plan view illustrating an example of a configuration of DP-IQ optical modulators  3 C. The same components as those of the optical device  1 B illustrated in  FIG.  11 B  are denoted by the same reference symbols, and explanation of the same configuration and operation will be omitted. The DP-IQ optical modulators  3 C in an optical device  1 C illustrated in  FIG.  11 C  include a second branching unit  12 B, IQ optical modulator  3 C 1  for X-polarization component, an IQ optical modulator  3 C 2  for Y-polarization component, a polarization rotator (PR)  15 , and a polarization beam combiner (PBC)  16 . 
     The second branching unit  12 B optically splits signal light coming from the first optical waveguide  2  and outputs the optically-split signal light to each of the IQ optical modulators  3 C 1  and  3 C 2 . The IQ optical modulator  3 C 1  for X-polarization component includes the optical modulator  3 B 1  for I component of X-polarization component, and the optical modulator  3 B 2  for Q component of X-polarization component. 
     The first multiplexing unit  13 A in the IQ optical modulator  3 C 1  for X-polarization component multiplexes the signal light of I component of X-polarization component coming from the multiplexing unit  13  in the optical modulator  3 B 1  for I component and the signal light of Q component of X-polarization component coming from the multiplexing unit  13  in the optical modulator  3 B 2  for Q component, and outputs the signal light of IQ component of X-polarization component to the PBC  16 . 
     The first multiplexing unit  13 A in the IQ optical modulator  3 C 2  for Y polarization multiplexes the signal light of I component of Y-polarization component coming from the multiplexing unit  13  in the optical modulator  3 B 2  for I component and the signal light of Q component of Y-polarization component coming from the multiplexing unit  13  in the optical modulator  3 B 2  for Q component, and outputs the signal light of IQ component of the Y-polarization component to the PR  15 . The PR  15  performs polarization rotation on the signal light of IQ component of Y-polarization component, and outputs the signal light of IQ component of Y-polarization component subjected to the polarization rotation to the PBC  16 . The PBC  16  multiplexes the signal light of IQ component of X-polarization component and the signal light of IQ component of Y-polarization component subjected to the polarization rotation, and outputs the multiplexed signal light of XY-polarization component to the output unit. 
     For example, a portion of the Mach-Zehnder interferometer  3 A 1  ( 3 A 2 ) in the DP-IQ optical modulator  3 C includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the electro-optic crystal layer  32 , the second optical waveguide  32 A, the second cladding layer  33 , and the electrode  34 . A portion of the phase control element  4  (the first phase control element  4 A) includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the phase control element  4  (the first phase control element  4 A), the electro-optic crystal layer  32 , and the second cladding layer  33 . Further, a portion of each of the PR  15  and the PBC  16  includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the PR  15 , the PBC  16 , the electro-optic crystal layer  32 , and the second cladding layer  33 . Therefore, in the optical device  1 C in which the DP-IQ optical modulators  3 C are incorporated as illustrated in  FIG.  11 C , it is possible to maintain high integration of silicon photonics, improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A, and prevent deterioration of the modulation bandwidth. 
       FIG.  11 D  is a schematic plan view illustrating an example of a configuration of an optical communication device  1 D. Meanwhile, the same components as those of the DP-IQ optical modulators  3 C illustrated in  FIG.  11 C  are denoted by the same reference symbols, and explanation of the same configuration and operation will be omitted. The optical communication device  1 D illustrated in  FIG.  11 D  includes DP-IQ optical modulators  3 C, a third branching unit  12 C, a light-receiving input unit  41 , a polarization beam splitter (PBS)  42 , a PR  43 , a first optical hybrid circuit  44 A ( 44 ), a second optical hybrid circuit  44 B ( 44 ), four light receiving elements  5 A, and four light receiving elements  5 B. 
     The third branching unit  12 C optically splits local light from a light source (not illustrated) via the input unit  11 , and outputs the optically-split light to the DP-IQ optical modulators  3 C and each of the hybrid circuits  44 . The light-receiving input unit  41  receives input of reception light from an optical fiber (not illustrated). The PBS  42  splits the light coming from the light-receiving input unit  41  into X-polarization reception light and Y-polarization reception light, outputs the X-polarization reception light to the first optical hybrid circuit  44 A, and outputs the Y-polarization reception light to the PR  43 . The PR  43  performs  90 -degree polarization rotation on the Y-polarization reception light, and outputs the Y-polarization reception light subjected to the polarization rotation to the second optical hybrid circuit  44 B. 
     The first optical hybrid circuit  44 A causes locally-emitted light to interfere with X-polarization component of the reception light, and acquires an optical signal of I component and an optical signal of Q component. The first optical hybrid circuit  44 A outputs the optical signal of I component in X-polarization component to the light receiving element  5 A. The first optical hybrid circuit  44 A outputs the optical signal of Q component in X-polarization component to the light receiving element  5 A. 
     The second optical hybrid circuit  44 B causes locally-emitted light to interfere with Y-polarization component of the reception light, and acquires an optical signal of I component and an optical signal of Q component. The second optical hybrid circuit  44 B outputs the optical signal of I component in Y-polarization component to the light receiving element  5 B. The second optical hybrid circuit  44 B outputs the optical signal of Q component in Y-polarization component to the light receiving element  5 B. 
     The light receiving element  5 A performs electrical conversion on the optical signal of I component of X-polarization component coming from the first optical hybrid circuit  44 A, and outputs the electrical signal of I component subjected to the electrical conversion. Further, the light receiving element  5 A performs electrical conversion on the optical signal of Q component of X-polarization component coming from the first optical hybrid circuit  44 A, and outputs the electrical signal of Q component subjected to the electrical conversion. The light receiving element  5 B performs electrical conversion on the optical signal of I component of Y-polarization component coming from the second optical hybrid circuit  44 B, and outputs the electrical signal of I component subjected to the electrical conversion. The light receiving element  5 B performs electrical conversion on the optical signal of Q component of Y-polarization component coming from the second optical hybrid circuit  44 B, and outputs the electrical signal of Q component subjected to the electrical conversion. 
     For example, a portion of the Mach-Zehnder interferometer  3 A 1  ( 3 A 2 ) in the DP-IQ optical modulators  3 C includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the electro-optic crystal layer  32 , the second optical waveguide  32 A, the second cladding layer  33 , and the electrode  34 . A portion of the phase control element  4  (the first phase control element  4 A) includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the phase control element  4  (the first phase control element  4 A), the electro-optic crystal layer  32 , and the second cladding layer  33 . Further, a portion of each of the PR  15  and the PBC  16  includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the PR  15 , the PBC  16 , the electro-optic crystal layer  32 , and the second cladding layer  33 . 
     A portion of the light receiving element  5  includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the light receiving element  5 , the electro-optic crystal layer  32 , and the second cladding layer  33 . Further, a portion of each of the PR  43  and the PBS  42  includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the PR  43 , the PBS  42 , the electro-optic crystal layer  32 , and the second cladding layer  33 . Similarly, a portion of the hybrid circuit  44  includes the second substrate  31 , the buffer layer  23 , the intermediate layer  22 , the first optical waveguide  2 , the hybrid circuit  44 , the electro-optic crystal layer  32 , and the second cladding layer  33 . Therefore, in the optical communication device  1 D as illustrated in  FIG.  11 D , it is possible to maintain high integration of silicon photonics, improve optical coupling efficiency between the first optical waveguide  2  and the second optical waveguide  32 A, and prevent deterioration of the modulation bandwidth. 
     According to one embodiment of the optical device or the like disclosed in the present application, it is possible to improve coupling efficiency while ensuring high integration of a silicon photonics component. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.