Patent Publication Number: US-2023141163-A1

Title: Optical device and optical communication apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2021-182515, filed on Nov. 9, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical device and an optical communication apparatus. 
     BACKGROUND 
     For example, in an optical device such as an optical modulator, a signal electrode is disposed over an optical waveguide on a surface thereof. When a voltage is applied to the signal electrode, an electric field is generated in the optical waveguide in a direction perpendicular to the surface of the optical modulator. The electric field changes a refractive index of the optical waveguide, resulting in a change in phase of light propagating through the optical waveguide, making it possible to modulate the light. That is, the optical waveguide of the optical modulator constitutes, for example, a Mach-Zehnder interferometer, and it is possible to output an IQ signal that is, for example, XY-polarization multiplexed due to a difference in phase of light between a plurality of optical waveguides arranged in parallel with each other. 
       FIG.  7    is a schematic plan view illustrating an example of a configuration of an optical modulator  100 , and  FIG.  8    is a schematic cross-sectional view illustrating an example of a cross-sectional portion taken along line E-E illustrated in  FIG.  7   . The optical modulator  100  illustrated in  FIG.  7    includes an input unit  101 , a branching unit  102 , two optical waveguides  103 , an electrode unit  104 , a multiplexing unit  105 , and an output unit  106 . Note that a length direction of the optical modulator  100  is defined as a Y direction, a width direction of the optical modulator  100  is defined as a Z direction, and a thickness direction of the optical modulator  100  is defined as an X direction. 
     The input unit  101  receives light from an optical fiber. The branching unit  102  branches the light from the input unit  101 . The two optical waveguides  103  are optical waveguides that are arranged in parallel with each other to each modulate the light branched by the branching unit  102 . The electrode unit  104  is an electrode that applies an electric signal to the two optical waveguides  103 . The electrode unit  104  includes a pair of ground electrodes  104 B and a signal electrode  104 A disposed to be sandwiched between the pair of ground electrodes  104 B. The signal electrode  104 A is an electrode that applies an electric signal to the optical waveguides  103 . The ground electrodes  104 B are electrodes that ground the electric signal applied to the optical waveguides  103 , respectively. The optical waveguides  103  constitute a Mach-Zehnder (MZ) interaction unit that modulates light by changing a light refractive index according to the electric signal from the signal electrode  104 A so as to change a phase of the light. Then, each of the optical waveguides  103  outputs the modulated light to the multiplexing unit  105 . The multiplexing unit  105  multiplexes the modulated light from the optical waveguides  103 . The output unit  106  outputs the modulated light from the multiplexing unit  105  to the optical fiber. 
     The optical modulator  100  illustrated in  FIG.  8    includes a substrate  111 , an intermediate layer  112  laminated on the substrate  111 , and a thin film LN layer  113  laminated on the intermediate layer  112  and made of an LN material (LiNbO 3 ). Further, the optical modulator  100  includes two optical waveguides  103  formed in the thin film LN layer  113 , a buffer layer  114  formed on the thin film LN layer  113 , and an electrode unit  104  formed on the buffer layer  114 . 
     For example, the substrate  111  is a substrate made of Si, LN, or the like. The intermediate layer  112  is a layer having a lower light refractive index than LN, for example, SiO 2 . The thin film LN layer  113  is a thin film substrate that is advantageous in size reduction because it is strong in terms of confinement of light. Note that the crystal axis of the thin film LN layer  113  is the Z direction. 
     Since the optical waveguides  103  are formed in the thin film LN layer  113 , they are superior, for example, in terms of insertion loss and transmission characteristics. Since the thin film LN layer  113  is an X-cut substrate, structural symmetry is suitable for long-distance transmission while enabling chirp-free operation. The optical waveguides  103  are disposed between the ground electrodes  104 B and the signal electrode  104 A, respectively. The signal electrode  104 A is, for example, an electrode of a metal material such as gold or copper. The ground electrode  104 B is, for example, an electrode of a metal material such as gold or copper. The buffer layer  114  is, for example, a SiO 2  layer provided to prevent light propagating on the optical waveguides  103  from being absorbed into the electrode unit  104 . 
     The crystal direction in which the electro-optical effect of the thin film LN layer  113  is most strongly exhibited is the width direction (Z direction) orthogonal to the signal progress direction (Y direction). Each of the optical waveguides  103  has a light refractive index that varies depending on an electric field in an electric field direction from the signal electrode  104 A to the ground electrode  104 B. 
       FIGS.  9 A and  9 B  are explanatory diagrams illustrating examples of a TE mode and a TM mode, respectively. In the optical waveguide  103 , there are two modes depending on a direction of a main component of an electric field of an optical field. As illustrated in  FIGS.  9 A and  9 B , there are a TE mode in which the main component of the electric field is in a planar direction and a TM mode in which the main component of the electric field is in a vertical direction. An arrow in  FIGS.  9 A and  9 B  indicates a direction of the main component of the electric field, and a range of a dotted line in  FIGS.  9 A and  9 B  indicates a light distribution range. 
     The optical modulator  100  having an X-cut thin film LN layer  113  is in the TE mode in which signal light modulated by the electric field applied from the electrode unit  104  uses an electric field in the planar direction as a main component, because the Z direction is located as the planar direction of the optical waveguide  103  as illustrated in  FIG.  9 A . Therefore, the TM mode in which the main component of the electric field is in the vertical direction can be said to be an unnecessary mode in the operation of the optical modulator  100 . 
     In addition, in order to facilitate size reduction while maintaining modulating efficiency of the optical modulator  100  by increasing a length of an interaction unit constituted by the optical waveguides  103  to which the electric field is applied, that is, an influence of an optical length, it has been a demand for a structure in which the interaction unit is long such that the interaction unit is folded. 
       FIG.  10    is a schematic plan view illustrating an example of a configuration of an optical modulator  100 A having a folded structure, and  FIGS.  11 A and  11 B  are schematic cross-sectional views illustrating examples of cross-sectional portions taken along lines F-F and G-G illustrated in  FIG.  10   , respectively. Note that the same components as those of the optical modulator  100  illustrated in  FIGS.  7  and  8    will be denoted by the same reference signs, and the description of the overlapping components and operations will be omitted. The optical waveguides of the optical modulator  100 A illustrated in  FIG.  10    include an optical waveguide  103 A ( 103 ) on a forward path side, a folded optical waveguide  108 , and an optical waveguide  103 B ( 103 ) on a backward path side. 
     The optical waveguide  103 A on the forward path side and the optical waveguide  103 B on the backward path side are optical waveguides formed in the thin film LN layer  113  along the Y direction in which the propagation direction (propagation angle) is 0 degrees. The cross-sectional portion taken along line F-F illustrated in  FIG.  11 A  corresponds to the optical waveguide  103 A on the forward path side. Note that the cores of the optical waveguide  103 A on the forward path side and the optical waveguide  103 B on the backward path side have the same thickness. The cores of the optical waveguide  103 A on the forward path side and the optical waveguide  103 B on the backward path side have, for example, a refractive index of 2.21 in the X and Y directions and have a refractive index of 2.14 in the Z direction. 
     The folded optical waveguide  108  is an optical waveguide formed in the thin film LN layer  113  along the Z direction in which the propagation direction (propagation angle) is other than 0 degrees, for example, 90 degrees. The cross-sectional portion taken along line G-G illustrated in  FIG.  11 B  corresponds to the folded optical waveguide  108 . The core of the folded optical waveguide  108  has the same thickness as the cores of the optical waveguide  103 A on the forward path side and the optical waveguide  103 B on the backward path side. The core of the folded optical waveguide  108  has, for example, a refractive index of 2.21 in the X and Y directions and has a refractive index of 2.14 in the Z direction. That is, the cores of the folded optical waveguide  108 , the optical waveguide  103 A on the forward path side, and the optical waveguide  103 B on the backward path side have the same refractive index for each direction. 
       FIG.  12 A  is an explanatory diagram illustrating an example of dependence of a propagation angle on an effective refractive index in a case where the core of the optical waveguide  103  has a thickness of 1 μm. In a case where the core of the optical waveguide  103  has a thickness of 1 μm, the X-cut thin film LN layer  113 , a refractive index in the planar direction varies depending on the propagation direction of the optical waveguide  103 , resulting in a change in effective refractive index in the TE mode. In addition, a refractive index in the vertical direction is constant regardless of the propagation direction of the optical waveguide  103 , and thus, an effective refractive index in the TM mode hardly changes even when the propagation direction of the optical waveguide  103  changes.
     Patent Literature 1: U.S. patent Ser. No. 07/970,241   Patent Literature 2: Japanese Laid-open Patent Publication No. 2017-156454   

     In the optical modulator  100 A, in a case where the Y direction is set to 0 degrees and the Z direction is set to 90 degrees for the propagation angle of the optical waveguide  103 , the effective refractive index in the TE mode is higher as the propagation angle of the optical waveguide  103  is closer to 90 degrees as illustrated in  FIG.  12 A . Therefore, in the optical modulator  100 A, when the core of the optical waveguide  103  is thick, there is a propagation angle at which the effective refractive index in the TE mode and the effective refractive index in the TM mode coincide with each other. As a result, the TE mode for signal light transitions to the TM mode for unnecessary light, and the crosstalk of the unnecessary TM mode occurs with respect to the TE mode. 
     Therefore, the crosstalk of the TM mode can be suppressed by reduce the thickness of the core of the optical waveguide  103 .  FIG.  12 B  is an explanatory diagram illustrating an example of dependence of a propagation angle on an effective refractive index in a case where the core of the optical waveguide  103  has a thickness of 0.4 μm. In a case where the thickness of the core of the optical waveguide  103  is, for example, 0.4 μm, there is no propagation angle at which the effective refractive index in the TE mode and the effective refractive index in the TM mode coincide with each other, and thus, the crosstalk with respect to the TE mode can be suppressed. 
     However, when the core of the optical waveguide  103  is thin in order to suppress crosstalk, the optical modulator  100 A is weak in terms of confinement of light in the thin film LN layer  113 .  FIG.  13    is an explanatory diagram illustrating an example of a change in modulating efficiency depending on the thickness of the core of the optical waveguide  103 . When the core of the optical waveguide  103  is thin, the optical modulator  100 A is weak in terms of confinement of light in the thin film LN layer  113 , resulting in a deterioration in modulating efficiency.  FIG.  14    is an explanatory diagram illustrating an example of a change in coupling efficiency depending on the thickness of the core of the optical waveguide  103 . When the core of the optical waveguide  103  is thin, the optical modulator  100 A is weak in terms of confinement of light in the thin film LN layer  113 . The decrease in thickness of the core causes a reduction in optical mode field, resulting in a deterioration in coupling efficiency with respect to the optical fiber. Therefore, in the optical modulator  100 A, there is a trade-off relationship between the suppression of the crosstalk with respect to the TE mode and the improvements in modulating efficiency and coupling efficiency. 
     SUMMARY 
     According to an aspect of an embodiment, an optical device includes an electro-optic crystal layer, a first optical waveguide formed in the electro-optic crystal layer, an electrode that applies an electric signal to the first optical waveguide, and a second optical waveguide in an amorphous state formed in the electro-optic crystal layer and connected to the first optical waveguide. 
     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 block diagram illustrating an example of a configuration of an optical communication apparatus according to a first embodiment; 
         FIG.  2    is a schematic plan view illustrating an example of a configuration of an optical modulator according to the first embodiment; 
         FIGS.  3 A and  3 B  are schematic cross-sectional views illustrating examples of cross-sectional portions taken along lines A-A and B-B illustrated in  FIG.  2   , respectively; 
         FIG.  4    is a schematic plan view illustrating an example of a configuration of an optical modulator according to a second embodiment; 
         FIG.  5    is a schematic plan view illustrating an example of a configuration of an optical modulator according to a third embodiment; 
         FIG.  6    is a schematic plan view illustrating an example of a configuration of an optical modulator according to a fourth embodiment; 
         FIG.  7    is a schematic plan view illustrating an example of a configuration of an optical modulator; 
         FIG.  8    is a schematic cross-sectional view illustrating an example of a cross-sectional portion taken along line E-E illustrated in  FIG.  7   ; 
         FIGS.  9 A and  9 B  are explanatory diagrams illustrating examples of a TE mode and a TM mode, respectively; 
         FIG.  10    is a schematic plan view illustrating an example of a configuration of an optical modulator having a folded structure; 
         FIGS.  11 A and  11 B  are schematic cross-sectional views illustrating examples of cross-sectional portions taken along lines F-F and G-G illustrated in  FIG.  10   , respectively; 
         FIG.  12 A  is an explanatory diagram illustrating an example of dependence of a propagation angle on an effective refractive index in a case where a core of an optical waveguide has a thickness of 1 μm; 
         FIG.  12 B  is an explanatory diagram illustrating an example of dependence of a propagation angle on an effective refractive index in a case where a core of an optical waveguide has a thickness of 0.4 μm; 
         FIG.  13    is an explanatory diagram illustrating an example of a change in modulating efficiency depending on the thickness of the core of the optical waveguide; and 
         FIG.  14    is an explanatory diagram illustrating an example of a change in coupling efficiency depending on the thickness of the core of the optical waveguide. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. Note that the present invention is not limited by these embodiments. 
     [a] First Embodiment 
       FIG.  1    is a block diagram illustrating an example of a configuration of an optical communication apparatus  1  according to a first embodiment. The optical communication apparatus  1  illustrated in  FIG.  1    is connected to an optical fiber  2 A ( 2 ) on an output side and an optical fiber  2 B ( 2 ) on an input side. The optical communication apparatus  1  includes a digital signal processor (DSP)  3 , a light source  4 , an optical modulator  5 , and an optical receiver  6 . The DSP  3  is an electric component that executes digital signal processing. For example, the DSP  3  executes processing including encoding of transmission data, generates an electric signal including the transmission data, and outputs the generated electric signal to the optical modulator  5 . In addition, the DSP  3  acquires an electric signal including reception data from the optical receiver  6 , and executes processing including decoding of the acquired electric signal to obtain the reception data. 
     The light source  4  is, for example, a laser diode or the like, and generates light having a predetermined wavelength and supplies the light to the optical modulator  5  and the optical receiver  6  through an optical fiber  4 A. The optical modulator  5  is an optical device that outputs an optical transmission signal obtained by modulating light supplied from the light source  4  according to an electric signal output from the DSP  3  to the optical fiber  2 A. For example, the optical modulator  5  is an optical device such as an LN optical modulator including an optical waveguide of lithium niobate (LN) and a signal electrode having a coplanar waveguide (CPW) structure. 
     The optical receiver  6  receives an optical signal from the optical fiber  2 B and demodulates the received optical signal using light supplied from the light source  4 . Then, the optical receiver  6  converts the demodulated received optical signal into an electric signal, and outputs the converted electric signal to the DSP  3 . 
       FIG.  2    is a schematic plan view illustrating an example of a configuration of the optical modulator  5  according to the first embodiment. The optical modulator  5  illustrated in  FIG.  2    includes an input unit  11 , a branching unit  12 , two first optical waveguides  13 A ( 13 ) on a forward path side, an electrode unit  14 , a multiplexing unit  15 , a second optical waveguide  16 , a first optical waveguide  13 B ( 13 ) on a backward path side, and an output unit  17 . Note that a length direction (propagation direction) of the optical modulator  5  is defined as a Y direction, a width direction (planar direction) of the optical modulator  5  is defined as a Z direction, and a thickness direction of the optical modulator  5  is defined as an X direction. 
     The input unit  11  receives light from the optical fiber  4 A. Note that the light from the optical fiber  4 A is light from the light source  4 . The branching unit  12  branches the light from the input unit  11 . The two first optical waveguides  13 A on the forward path side are optical waveguides for forming a Mach-Zehnder (MZ) interaction unit that are arranged in parallel with each other to each modulate the light branched by the branching unit  12 . For example, the two first optical waveguides  13 A on the forward path side are ridge waveguides, each including a core and slabs disposed on both sides of the core, the slabs being thinner than the core. The two first optical waveguides  13 A on the forward path side are optical waveguides formed on a thin film LN layer  33 , which will be described later, along a direction substantially orthogonal to the Z direction of the crystal axis of the thin film LN layer  33 , for example, the Y direction. 
     The electrode unit  14  is an electrode that applies an electric signal to the two first optical waveguides  13 A on the forward path side. Each of the two first optical waveguides  13 A on the forward path side modulates the light by changing a light refractive index according to the electric signal from the electrode unit  14 , and outputs the modulated light to the multiplexing unit  15 . The electrode unit  14  includes a signal electrode  14 A and a pair of ground electrodes  14 B. The signal electrode  14 A is an electrode that applies an electric signal to the first optical waveguides  13 A on the forward path side. The ground electrodes  14 B are electrodes that ground the electric signal applied to the first optical waveguides  13 A on the forward path side, respectively. The two first optical waveguides  13 A on the forward path side modulate the light by changing the light refractive index according to the electric signal from the signal electrode  14 A to change a phase of the light, and output the modulated light to the multiplexing unit  15 . The multiplexing unit  15  multiplexes the light modulated by the two first optical waveguides  13 A on the forward path side. In the optical modulator  5 , the two first optical waveguides  13 A on the forward path side and the electrode unit  14  form a modulation unit  20 . 
     The second optical waveguide  16  is a folded optical waveguide that allows the modulated light from the multiplexing unit  15  to pass therethrough. The second optical waveguide  16  is an optical waveguide including at least one of a straight waveguide formed in the thin film LN layer  33  along a direction other than the Y direction substantially orthogonal to the Z direction or a curved waveguide whose propagation direction smoothly changes. 
     The first optical waveguide  13 B on the backward path side is an optical waveguide that allows the modulated light from the second optical waveguide  16  to pass therethrough. The first optical waveguide  13 B on the backward path side is an optical waveguide formed in the thin film LN layer  33  along the Y direction substantially orthogonal to the Z direction of the crystal axis of the thin film LN layer  33 . The output unit  17  is connected to the optical fiber  2 A, and outputs the modulated light from the first optical waveguide  13 B on the backward path side to the optical fiber  2 A. 
       FIGS.  3 A and  3 B  are schematic cross-sectional views illustrating examples of cross-sectional portions taken along lines A-A and B-B illustrated in  FIG.  2   , respectively. A portion including the first optical waveguides  13  illustrated in  FIG.  3 A  includes a substrate  31 , an intermediate layer  32  laminated on the substrate  31 , a thin film LN layer  33  laminated on the intermediate layer  32 , the two first optical waveguides  13  formed in the thin film LN layer  33 , and a buffer layer  34  on the thin film LN layer  33 . Also, a portion including the second optical waveguide  16  illustrated in  FIG.  3 B  includes a substrate  31 , an intermediate layer  32  laminated on the substrate  31 , a thin film LN layer  33  laminated on the intermediate layer  32 , the second optical waveguide  16  formed in the thin film LN layer  33 , and a buffer layer  34  on the thin film LN layer  33 . 
     For example, the substrate  31  is a substrate formed of SiO 2  (silicon dioxide), TiO 2  (titanium dioxide), Si, LN, or the like. The intermediate layer  32  is a layer formed of a material having a lower light refractive index than LN, for example, SiO 2  or TiO 2 . Note that the intermediate layer  32  has a certain thickness to form an optically sufficient distance between the thin film LN layer  33  and the substrate  31 , for example, so that the optical signal propagating through the first optical waveguides  13  and the second optical waveguide  16  formed in the thin film LN layer  33  is not radiated to the substrate  31 . For example, the thickness is about 2 μm to 6 μm. The thin film LN layer  33  is a substrate using a thin film of an LN crystal which is an electro-optic crystal, with the first optical waveguides  13  formed thereon to each have a convex shape protruding upward at a predetermined position. The LN material is advantageous in size reduction because it is strong in terms of confinement of light. Moreover, the LN material is an anisotropic material whose refractive index changes when an electric field is applied thereto, for example, having a Pockels coefficient of about 30 pm/V. 
     The thin film LN layer  33  is an X-cut substrate. The thin film LN layer  33  is a substrate of an electro-optic crystal, for example, a LN(LiNbO 3 ) material. The two first optical waveguides  13 A on the forward path side have superior insertion loss and transmission characteristics because they are formed in the thin film LN layer  33  and their material is LN. The optical modulator  5  is structurally symmetrical, which is suitable for long-distance transmission while enabling a chirp-free operation. 
     The buffer layer  34  is, for example, an SiO 2  layer provided to prevent light propagating through the first optical waveguides  13  from being absorbed into the electrode unit  14 . The thin film LN layer  33  having a thickness of 0.5 to 3 μm is sandwiched between the intermediate layer  32  and the buffer layer  34 . Protrusions formed in the thin film LN layer  33  as the first optical waveguides  13  having a convex shape have a width of, for example, about 1 to 8 μm. The thin film LN layer  33  and the first optical waveguides  13  are covered by the buffer layer  34 . 
     The signal electrode  14 A is, for example, an electrode made of a metal material such as gold or copper and having a width of 2 to 10 μm and a thickness of 1 to 20 μm. The ground electrode  14 B is, for example, an electrode made of a metal material such as gold or copper and having a thickness of 1 μm or more. When a driving voltage corresponding to the electric signal output from the DSP  3  is transmitted by the signal electrode  14 A, an electric field is generated in a direction from the signal electrode  14 A toward the ground electrode  14 B, and this electric field is applied to the first optical waveguide  13 . As a result, it is possible to change a refractive index of the first optical waveguide  13  according to the electric field applied to the first optical waveguide  13 , thereby modulating the light propagating through the first optical waveguide  13 . 
     When the crystal axis of the thin film LN layer  33  is set to be 90 degrees in the Z direction and 0 degrees in the Y direction, the two first optical waveguides  13 A on the forward path side are straight optical waveguides formed in the Y direction of substantially 0 degrees, among optical waveguides formed in a chip of the optical modulator  5 . Note that the substantially 0 degrees includes not only 0 degrees but also an error range in which the first optical waveguide  13  can be manufactured, for example, when the optical waveguide is formed aiming at 0 degrees, for example, within ±20 degrees. The two first optical waveguides  13 A on the forward path side have cores set to a non-amorphous state because the cores are formed in the thin film LN layer  33  along the Y direction substantially orthogonal to the Z direction of the crystal axis of the thin film LN layer  33 . Similarly to the two first optical waveguides  13 A on the forward path side, the first optical waveguide  13 B on the backward path side, the input unit  11 , and the output unit  17  also have cores set to a non-amorphous state. Since the cores of the first optical waveguides  13 A on the forward path side and the first optical waveguide  13 B on the backward path side are in the non-amorphous state, the cores are set, for example, to have a refractive index of about 2.21 in the X and Y directions and have a refractive index of about 2.14 in the Z direction. 
     The second optical waveguide  16  is an optical waveguide formed in the thin film LN layer  33  along a direction not orthogonal to the Z direction, that is, a direction other than the Y direction of substantially 0 degrees, and has a core in an amorphous state. Note that the direction not orthogonal to the Z direction is, for example, in a range of larger than an angle actually deviated from the Y direction and equal to or smaller than 90 degrees in the Z direction. Since the cores of the second optical waveguide  16 , the branching unit  12 , and the multiplexing unit  15  are in the amorphous state, the cores are set, for example, to have a refractive index of about 2.175 in the X, Y, and Z directions. Since the cores of the second optical waveguides  16  are in the amorphous state, there is no propagation angle at which an effective refractive index in a TE mode and an effective refractive index in a TM mode intersect. As a result, it is possible to suppress crosstalk of the unnecessary TM mode with respect to the TE mode. Note that, similarly to the second optical waveguide  16 , the branching unit  12  and the multiplexing unit  15  also have cores set to an amorphous state. Note that the second optical waveguide  16 , the branching unit  12 , and the multiplexing unit  15  are defined as a routed second optical waveguide connected to the first optical waveguides  13 . 
     Since the cores of the first optical waveguides  13 A on the forward path side, the first optical waveguide  13 B on the backward path side, the input unit  11 , and the output unit  17  are in the non-amorphous state, the cores are set, for example, to have a refractive index of about 2.21 in the X and Y directions and have a refractive index of about 2.14 in the Z direction. On the other hand, since the cores of the second optical waveguide  16 , the branching unit  12 , and the multiplexing unit  15  are in the amorphous state, the cores are set, for example, to have a refractive index of about 2.175 in the X, Y, and Z directions. 
     As an example of a process of forming the amorphous state and the non-amorphous state, argon (Ar + ) ions are implanted from a surface of the thin film LN layer  33  while masking cores to be set to the non-amorphous state. By implanting the Ar +  ions, the LN crystal orientation of cores of the thin film LN layer  33  is collapsed to remove the anisotropy of the light refractive index, thereby bring the cores into an amorphous state. Also, the masked cores remain in the non-amorphous state. As a result, the cores in the amorphous state have no angle at which an effective refractive index of signal light in the TE mode and an effective refractive index of unnecessary light in the TM mode coincide with each other, making it possible to suppress crosstalk. 
     In the optical modulator  5  according to the first embodiment, the cores of the first optical waveguides  13 A on the forward path side, the first optical waveguide  13 B on the backward path side, the input unit  11 , and the output unit  17  are in a non-amorphous state, and the cores of the second optical waveguide  16 , the branching unit  12 , and the multiplexing unit  15  are in an amorphous state. The cores in the amorphous state have no propagation angle at which an effective refractive index in the TE mode and an effective refractive index in the TM mode intersect. As a result, it is possible to suppress crosstalk of the unnecessary TM mode with respect to the TE mode. Furthermore, the crosstalk of the unnecessary TM mode with respect to the TE mode can be suppressed even though the cores have a large thickness, thereby improving modulating efficiency and coupling efficiency. That is, it is possible to improve modulating efficiency and coupling efficiency while suppressing crosstalk. 
     The second optical waveguide  16  is a straight waveguide or a curved waveguide formed in the thin film LN layer  33  along a direction other than the Y direction of the crystal axis of the X-cut thin film LN layer  33  in which light propagates. As a result, since the core of the second optical waveguide  16  is in the amorphous state, it is possible to suppress crosstalk of the unnecessary TM mode with respect to the TE mode. 
     Note that, although the X-cut thin film LN layer  33  has been exemplified for convenience of description, the optical modulator  5  may use a Y-cut thin film LN layer. The optical modulator  5  includes a Y-cut thin film LN layer  33 , first optical waveguides  13  formed in the thin film LN layer  33  along a direction substantially orthogonal to the Z direction of the crystal axis of the thin film LN layer, and a routed second optical waveguide  16  connected to the first optical waveguide  13 . The second optical waveguide  16  is a straight waveguide or a curved waveguide formed in the thin film LN layer  33  along a direction other than the X direction of the crystal axis of the thin film LN layer  33  in which light propagates. In addition, the cores of the first optical waveguides  13 A on the forward path side, the first optical waveguide  13 B on the backward path side, the input unit  11 , and the output unit  17  are in a non-amorphous state, and the cores of the second optical waveguide  16 , the branching unit  12 , and the multiplexing unit  15  are in an amorphous state. The cores in the amorphous state have no propagation angle at which an effective refractive index in the TE mode and an effective refractive index in the TM mode intersect. As a result, it is possible to suppress crosstalk of the unnecessary TM mode with respect to the TE mode. Furthermore, the crosstalk of the unnecessary TM mode with respect to the TE mode can be suppressed even though the cores have a large thickness, thereby improving modulating efficiency and coupling efficiency. 
     In addition, although the thin film LN layer  33  has been exemplified, the material of the electro-optic crystal is not limited to LN, and any type of anisotropic electro-optic crystal may be used. For example, the material of the electro-optic crystal may be a perovskite type oxide such as lead zirconate titanate (PZT), lanthanum-doped lead zirconate titanate (PLZT), or barium titanate (BaTiO 3 ), and can be appropriately modified. Since PZT has a Pockels coefficient of about 110 pm/V, PLZT has a Pockels coefficient of about 700 pm/V, and BaTiO 3  has a Pockels coefficient of about 1850 pm/V, the material of the electro-optic crystal applied to the present invention has a Pockels coefficient in the range of 10 to 2000 pm/V. 
     In addition, although it has been exemplified that the first optical waveguides  13  and the second optical waveguide  16  are ridge waveguides, but the first optical waveguides  13  and the second optical waveguide  16  are not limited to ridge waveguides, and for example, channel waveguides can be used. 
     Although the second optical waveguide  16 , the branching unit  12 , and the multiplexing unit  15  have been exemplified as a routed optical waveguide connected to the first optical waveguides  13 , the routed optical waveguide connected to the first optical waveguides  13  is not limited thereto, and can be appropriately modified. In addition, it has been exemplified that the cores of the second optical waveguide  16 , other than the first optical waveguides  13  to which the electric signal from the electrode unit  14  is applied, are in the amorphous state. However, among the cores of the second optical waveguide  16 , for example, a core whose light propagation direction is parallel to the first optical waveguides  13  may be in a non-amorphous state, and the states of the cores can be appropriately modified. 
     Note that it has been exemplified for the optical modulator  5  according to the first embodiment that the electric signal from the electrode unit  14  is applied to the two first optical waveguides  13 A on the forward path side. However, in addition to the two first optical waveguides  13 A on the forward path side, two first optical waveguides  13 C may be arranged to form an interaction unit on the backward path side as well, and this will be described below as a second embodiment. 
     [b] Second Embodiment 
       FIG.  4    is a schematic plan view illustrating an example of a configuration of an optical modulator  5 A according to the second embodiment. Note that the same components as those of the optical modulator  5  according to the first embodiment will be denoted by the same reference signs, and the description of the overlapping components and operations will be omitted. The optical modulator  5 A illustrated in  FIG.  4    includes an input unit  11 , a branching unit  12 , two first optical waveguides  13 A on a forward path side, and two folded second optical waveguides  16 B. Further, the optical modulator  5 A includes two first optical waveguides  13 C on a backward path side, a first electrode unit  141  ( 14 ), a second electrode unit  142  ( 14 ), a multiplexing unit  15 A, and an output unit  17 . 
     The input unit  11  receives light from the optical fiber  4 A. The branching unit  12  branches the light from the input unit  11 . The two first optical waveguides  13 A on the forward path side are optical waveguides for forming an MZ interaction unit that are arranged in parallel with each other to each modulate the light branched by the branching unit  12 . The two second optical waveguides  16 B are folded optical waveguides connected to the two first optical waveguides  13 A on the forward path side. The two first optical waveguides  13 C on the backward path side are optical waveguides for forming an MZ interaction unit that are arranged in parallel with each other and connected to the two second optical waveguides  16 B, respectively. 
     One of the first optical waveguides  13 A on the forward path side is connected to one of the second optical waveguides  16 B, and one of the second optical waveguides  16 B is connected to one of the first optical waveguides  13 C on the backward path side. The other one of the first optical waveguides  13 A on the forward path side is connected to the other one of the second optical waveguides  16 B, and the other one of the second optical waveguides  16 B is connected to the other one of the first optical waveguides  13 C on the backward path side. 
     The first electrode unit  141  includes a first electrode  141 A disposed between the two first optical waveguides  13 A on the forward path side, and a second electrode  141 B and a third electrode  141 C disposed outside the two first optical waveguides  13 C on the backward path side, respectively. The second electrode unit  142  includes a fourth electrode  142 A disposed between the two first optical waveguides  13 C on the backward path side, and a fifth electrode  142 B and a sixth electrode  142 C disposed outside the two first optical waveguides  13 A on the forward path side, respectively. The second electrode unit  142  has a higher potential than the first electrode unit  141 . 
     One of the first optical waveguides  13 A on the forward path side applies an electric field in the −Z direction from the sixth electrode  142 C of the second electrode unit  142  to the first electrode  141 A of the first electrode unit  141 . One of first optical waveguides  14 C on the backward path side applies an electric field in the −Z direction from the fourth electrode  142 A of the second electrode unit  142  to the third electrode  141 C of the first electrode unit  141 . As a result, since the direction of the electric field applied to one of the first optical waveguides  13 A on the forward path side and the direction of the electric field applied to one of the first optical waveguide  13 C on the backward path side are the same as the −Z direction, a phase change occurs in the same direction, resulting in an improvement in modulating efficiency. 
     The other one of the first optical waveguides  13 A on the forward path side applies an electric field in the +Z direction from the fifth electrode  142 B of the second electrode unit  142  to the first electrode  141 A of the first electrode unit  141 . The other one of the first optical waveguides  13 C on the backward path side applies an electric field in the +Z direction from the fourth electrode  142 A of the second electrode unit  142  to the second electrode  141 B of the first electrode unit  141 . As a result, since the direction of the electric field applied to the other one of the first optical waveguides  13 A on the forward path side and the direction of the electric field applied to the other one of the first optical waveguide  13 C on the backward path side are the same as the +Z direction, a phase change occurs in the same direction, resulting in an improvement in modulating efficiency. 
     The multiplexing unit  15 A multiplexes the modulated light passing through the two first optical waveguides  13 C on the backward path side. The output unit  17  outputs the modulated light multiplexed by the multiplexing unit  15 A to the optical fiber  4 A. 
     The two first optical waveguides  13 A on the forward path side, the two first optical waveguides  13 C on the backward path side, the input unit  11 , and the output unit  17  have cores set to a non-amorphous state. Since the cores of the first optical waveguides  13 A on the forward path side, the first optical waveguides  13 C on the backward path side, the input unit  11 , and the output unit  17  are in the non-amorphous state, the cores are set, for example, to have a refractive index of about 2.21 in the X and Y directions and have a refractive index of about 2.14 in the Z direction. The second optical waveguides  16 B, the branching unit  12 , and the multiplexing unit  15 A have cores set to an amorphous state. Since the cores of the second optical waveguides  16 , the branching unit  12 , and the multiplexing unit  15 A are in the amorphous state, the cores are set, for example, to have a refractive index of about 2.175 in the X, Y, and Z directions. 
     As an example of a process of forming the amorphous state and the non-amorphous state, Ar +  ions are implanted from a surface of the thin film LN layer  33  while masking cores to be set to the non-amorphous state. By implanting the Ar +  ions, the LN crystal orientation of cores of the thin film LN layer  33  is collapsed to remove the anisotropy of the light refractive index, thereby bring the cores into an amorphous state. Also, the masked cores remain in the non-amorphous state. As a result, the cores in the amorphous state have no angle at which an effective refractive index of signal light in the TE mode and an effective refractive index of unnecessary light in the TM mode coincide with each other, making it possible to suppress crosstalk. 
     In the optical modulator  5 A according to the second embodiment, the cores of the two first optical waveguides  13 A on the forward path side, the two first optical waveguides  13 C on the backward path side, the input unit  11 , and the output unit  17  are in a non-amorphous state, and the cores of the second optical waveguides  16 B, the branching unit  12 , and the multiplexing unit  15 A are in an amorphous state. The cores in the amorphous state have no propagation angle at which an effective refractive index in the TE mode and an effective refractive index in the TM mode intersect. As a result, it is possible to suppress crosstalk of the unnecessary TM mode with respect to the TE mode. Furthermore, the crosstalk of the unnecessary TM mode with respect to the TE mode can be suppressed even though the cores have a large thickness, thereby improving modulating efficiency and coupling efficiency. 
     In the optical modulator  5 A, by applying an electric field to the two first optical waveguides  13 A on the forward path side disposed before the folded optical waveguides and the two first optical waveguides  13 C on the backward path side disposed after the folded optical waveguides, it is possible to facilitate improvements in modulating efficiency and size reduction. 
     Note that it has been exemplified for the optical modulator  5  according to the first embodiment that the electrode unit  14  is disposed around the first optical waveguides  13 A on the forward path side to form an interaction unit. However, another electrode unit  14  may be disposed around the first optical waveguides  13 C on the backward path side to form another interaction unit, in addition to the interaction unit around the first optical waveguides  13 A on the forward path side, and this will be described below as a third embodiment. 
     [c] Third Embodiment 
       FIG.  5    is a schematic plan view illustrating an example of a configuration of an optical modulator  5 B according to the third embodiment. Note that the same components as those of the optical modulator  5  according to the first embodiment will be denoted by the same reference signs, and the description of the overlapping components and operations will be omitted. The optical modulator  5 B illustrated in  FIG.  5    includes an input unit  11 , a branching unit  12 , two first optical waveguides  13 A on a forward path side, a third electrode unit  143 , second optical waveguides  16 B, two first optical waveguides  13 C on a backward path side, a fourth electrode unit  144 , a multiplexing unit  15 A, and an output unit  17 . 
     The input unit  11  receives light from the optical fiber  4 A. The branching unit  12  branches the light from the input unit  11 . The two first optical waveguides  13 A on the forward path side are first optical waveguides  13  for forming an MZ interaction unit that are arranged in parallel with each other to each modulate the light branched by the branching unit  12 . The third electrode unit  143  includes a signal electrode  143 A that applies a first electric signal to the two first optical waveguides  13 A on the forward path side, and ground electrodes  143 B that ground the first electric signal from the two first optical waveguides  13 A on the forward path side, respectively. Note that the two first optical waveguides  13 A on the forward path side and the third electrode unit  143  constitute a first modulation unit  20 A ( 20 ). For example, the first modulation unit  20 A is a direct current (DC) modulation unit in which a bias voltage is applied as the first electric signal from the third electrode unit  143  to the two first optical waveguides  13 A on the forward path side. 
     The second optical waveguides  16 B are folded optical waveguides that are arranged in parallel with each other and connected to the two first optical waveguides  13 A on the forward path side, respectively, to allow the light modulated by the two first optical waveguides  13 A on the forward path side to pass therethrough. 
     The two first optical waveguides  13 C on the backward path side are first optical waveguides  13  that are arranged in parallel with each other and connected to the two second optical waveguides  16 B, respectively. The fourth electrode unit  144  includes a signal electrode  144 A that applies a second electric signal to the two first optical waveguides  13 C on the backward path side, and ground electrodes  144 B that ground the second electric signal from the two first optical waveguides  13 C on the backward path side, respectively. Note that the two first optical waveguides  13 C on the backward path side and the fourth electrode unit  144  constitute a second modulation unit  20 B ( 20 ). For example, the second modulation unit  20 B is a radio frequency (RF) modulation unit in which the fourth electrode unit  144  applies a high-frequency signal as the second electric signal to the two first optical waveguides  13 C on the backward path side. 
     The multiplexing unit  15 A is connected to the two first optical waveguides  13 C on the backward path side to multiplex the light passing therethrough after being modulated by the two first optical waveguides  13 C on the backward path side. The output unit  17  outputs the modulated light multiplexed by the multiplexing unit  15 A to the optical fiber  2 A. 
     The two first optical waveguides  13 A on the forward path side, the two first optical waveguides  13 C on the backward path side, the input unit  11 , and the output unit  17  have cores set to a non-amorphous state. Since the cores of the first optical waveguides  13 A on the forward path side, the first optical waveguides  13 C on the backward path side, the input unit  11 , and the output unit  17  are in the non-amorphous state, the cores are set, for example, to have a refractive index of about 2.21 in the X and Y directions and have a refractive index of about 2.14 in the Z direction. The second optical waveguides  16 B, the branching unit  12 , and the multiplexing unit  15 A have cores set to an amorphous state. Since the cores of the second optical waveguides  16 , the branching unit  12 , and the multiplexing unit  15 A are in the amorphous state, the cores are set, for example, to have a refractive index of about 2.175 in the X, Y, and Z directions. 
     As an example of a process of forming the amorphous state and the non-amorphous state, Ar +  ions are implanted from a surface of the thin film LN layer  33  while masking cores to be set to the non-amorphous state. By implanting the Ar +  ions, the LN crystal orientation of cores of the thin film LN layer  33  is collapsed to remove the anisotropy of the light refractive index, thereby bring the cores into an amorphous state. Also, the masked cores remain in the non-amorphous state. As a result, the cores in the amorphous state have no angle at which an effective refractive index of signal light in the TE mode and an effective refractive index of unnecessary light in the TM mode coincide with each other, making it possible to suppress crosstalk. 
     In the optical modulator  5 B according to the third embodiment, the cores of the two first optical waveguides  13 A on the forward path side, the two first optical waveguides  13 C on the backward path side, the input unit  11 , and the output unit  17  are in a non-amorphous state, and the cores of the second optical waveguides  16 B, the branching unit  12 , and the multiplexing unit  15 A are in an amorphous state. Since the cores of the second optical waveguides  16  are in the amorphous state, there is no propagation angle at which an effective refractive index in a TE mode and an effective refractive index in a TM mode intersect. As a result, even in a case where the first modulation unit  20 A is disposed before the folded optical waveguides and the second modulation unit  20 B is disposed after the folded optical waveguides, it is possible to suppress crosstalk of the unnecessary TM mode with respect to the TE mode. Furthermore, the crosstalk of the unnecessary TM mode with respect to the TE mode can be suppressed even though the cores have a large thickness, thereby improving modulating efficiency and coupling efficiency. 
     Note that, although it has been exemplified for convenience of description that the first modulation unit  20 A is a DC modulation unit and the second modulation unit  20 B is an RF modulation unit, but the first modulation unit  20 A and the second modulation unit  20 B are not limited thereto. For example, the second modulation unit  20 B may be a DC modulation unit, and the first modulation unit  20 A and the second modulation unit  20 B can be appropriately modified. 
     [d] Fourth Embodiment 
       FIG.  6    is a schematic plan view illustrating an example of a configuration of an optical modulator  5 C according to a fourth embodiment. The optical modulator  5 C illustrated in  FIG.  6    is an IQ modulator. The optical modulator  5 C includes an input unit  11 , a first branching unit  12 A, a pair of second branching units  12 B, two first optical waveguides  13 A 1  and two first optical waveguides  13 A 2  on a forward path side at a leading stage, a fifth electrode unit  145 , and second optical waveguides  16 A 1  and  16 A 2  at the leading stage. The optical modulator  5 C includes two first optical waveguides  13 C 1  and two first optical waveguides  13 C 2  on a backward path side at the leading stage, a sixth electrode unit  146 , a pair of first multiplexing units  15 B, and two second optical waveguides  16 B 1  and  16 B 2  at a middle stage. Further, the optical modulator  5 C includes two first optical waveguides  13 D on a forward path side at a trailing stage, a seventh electrode unit  147 , a second optical waveguide  16 C at the trailing stage, an first optical waveguide  13 E on a backward path side at the trailing stage, and an output unit  17 . 
     The input unit  11  receives light from the optical fiber  4 A. The first branching unit  12 A branches the light from the input unit  11 . The second branching units  12 B output the light branched by the first branching unit  12 A to the two first optical waveguides  13 A 1  and the two first optical waveguides  13 A 2  on the forward path side, respectively, to modulate the light. 
     The two first optical waveguides  13 A 1  and the two first optical waveguides  13 A 2  on the forward path side at the leading stage are optical waveguides for forming MZ interaction units that are arranged in parallel with each other to each modulate the light branched by the second branching unit  12 B. The fifth electrode unit  145  includes a first signal electrode  145 A 1 , a second signal electrode  145 A 2 , a first ground electrode  145 B 1 , a second ground electrode  145 B 2 , and a third ground electrode  145 B 3 . The first signal electrode  145 A 1  applies a first electric signal to one pair of two first optical waveguides  13 A 1  on the forward path side at the leading stage. The first ground electrode  145 B 1  grounds the first electric signal from one first optical waveguide  13 A 1  of the two first optical waveguides  13 A 1  on the forward path side. The second ground electrode  145 B 2  grounds the first electric signal from the other first optical waveguide  13 A 1  of the two first optical waveguides  13 A 1  on the forward path side at the leading stage. The one pair of first optical waveguides  13 A 1  on the forward path side at the leading stage and the fifth electrode unit  145  constitute a first RF modulation unit  20 A 1 . 
     The second signal electrode  145 A 2  applies a second electric signal to the other pair of two first optical waveguides  13 A 2  on the forward path side at the leading stage. The third ground electrode  145 B 3  grounds the second electric signal from the other first optical waveguide  13 A 2  of the other pair of two first optical waveguides  13 A 2  on the forward path side at the leading stage. The second ground electrode  145 B 2  grounds the second electric signal from one first optical waveguide  13 A 2  of the other pair of two first optical waveguides  13 A 2  on the forward path side at the leading stage. The other pair of first optical waveguides  13 A 2  on the forward path side at the leading stage and the fifth electrode unit  145  constitute a second RF modulation unit  20 A 2 . 
     A second optical waveguides  16 A at the leading stage are arranged in parallel with each other, and include one pair of two second optical waveguides  16 A 1  at the leading stage and the other pair of two second optical waveguides  16 A 2  at the leading stage. The one pair of two second optical waveguides  16 A 1  at the leading stage are folded optical waveguides connected to the one pair of two first optical waveguides  13 A 1  on the forward path side to allow the light modulated by the one pair of two first optical waveguides  13 A 1  on the forward path side at the leading stage to pass therethrough. The other pair of two second optical waveguides  16 A 2  at the leading stage are folded optical waveguides connected to the other pair of two first optical waveguides  13 A 2  on the forward path side at the leading stage to allow the light modulated by the other pair of two first optical waveguides  13 A 2  on the forward path side at the leading stage to pass therethrough. 
     One pair of two first optical waveguides  13 C 1  on the backward path side at the leading stage are first optical waveguides arranged in parallel with each other, and connected to the one pair of two second optical waveguides  16 A 1  at the leading stage. The other pair of two first optical waveguides  13 C 2  on the backward path side at the leading stage are first optical waveguides arranged in parallel with each other, and connected to the other pair of two second optical waveguides  16 A 2  at the leading stage. 
     The sixth electrode unit  146  includes a first signal electrode  146 A 1 , a second signal electrode  146 A 2 , a first ground electrode  146 B 1 , a second ground electrode  146 B 2 , and a third ground electrode  146 B 3 . The first signal electrode  146 A 1  of the sixth electrode unit  146  applies a third electric signal to the one pair of two first optical waveguides  13 C 1  on the backward path side at the leading stage. The first ground electrode  146 B 1  of the sixth electrode unit  146  grounds the third electric signal from one first optical waveguide  13 C 1  of the one pair of two first optical waveguides  13 C 1  on the backward path side at the leading stage. The second ground electrode  146 B 2  of the sixth electrode unit  146  grounds the third electric signal from the other first optical waveguide  13 C 1  of the one pair of two first optical waveguides  13 C 1  on the backward path side at the leading stage. The one pair of first optical waveguides  13 C 1  on the backward path side at the leading stage and the sixth electrode unit  146  constitute a first DC modulation unit  20 B 1  ( 20 ). 
     The second signal electrode  146 A 2  of the sixth electrode unit  146  applies a fourth electric signal to the other pair of two first optical waveguides  13 C 2  on the backward path side at the leading stage. The third ground electrode  146 B 3  of the sixth electrode unit  146  grounds the fourth electric signal from the other first optical waveguide  13 C 2  of the other pair of two first optical waveguides  13 C 2  on the backward path side at the leading stage. The second ground electrode  146 B 2  of the sixth electrode unit  146  grounds the fourth electric signal from one first optical waveguide  13 C 2  of the other pair of two first optical waveguides  13 C 2  on the backward path side at the leading stage. The other pair of first optical waveguides  13 C 2  on the backward path side at the leading stage and the sixth electrode unit  146  constitute a second DC modulation unit  20 B 2  ( 20 ). 
     One first multiplexing unit  15 B 1  is connected to the one pair of two first optical waveguides  13 C 1  on the backward path side at the leading stage to multiplex the light passing therethrough after being modulated by the one pair of two first optical waveguides  13 C 1  on the backward path side at the leading stage. The other first multiplexing unit  15 B 2  is connected to the other pair of two first optical waveguides  13 C 2  on the backward path side at the leading stage to multiplex the light passing therethrough after being modulated by the other pair of two first optical waveguides  13 C 2  on the backward path side at the leading stage. 
     The second optical waveguides  16 B at the middle stage are arranged in parallel with each other, and include one second optical waveguide  16 B 1  at the middle stage and the other second optical waveguide  16 B 2  at the middle stage. The one second optical waveguide  16 B 1  at the middle stage is a folded optical waveguide connected to the one first multiplexing unit  15 B 1  to allow the light from the one first multiplexing unit  15 B 1  to pass therethrough. The other second optical waveguide  16 B 2  at the middle stage is a folded optical waveguide connected to the other first multiplexing unit  15 B 2  to allow the light from the other first multiplexing unit  15 B 2  to pass therethrough. 
     The two first optical waveguides  13 D on the forward path side at the trailing stage are first optical waveguides connected to the second optical waveguides  16 B 1  and  16 B 2  at the middle stage. The seventh electrode unit  147  includes a signal electrode  147 A that applies a fifth electric signal to the two first optical waveguides  13 D on the forward path side at the trailing stage, and a pair of ground electrodes  147 B that grounds the fifth electric signal from the two first optical waveguides  13 D on the forward path side at the trailing stage, respectively. The two first optical waveguides  13 D on the forward path side at the trailing stage and the seventh electrode unit  147  constitute a parent DC modulation unit  20 C. 
     A second multiplexing unit  15 C is connected to the two first optical waveguides  13 D on the forward path side at the trailing stage to multiplex the light modulated by the two first optical waveguides  13 D on the forward path side at the trailing stage. The second optical waveguide  16 C at the trailing stage is a folded optical waveguide connected to the second multiplexing unit  15 C to allow the light from the second multiplexing unit  15 C to pass therethrough. One first optical waveguide  13 E on the backward path side at the trailing stage is a first optical waveguide connected to the second optical waveguide  16 C at the trailing stage. The output unit  17  is connected to the first optical waveguide  13 E on the backward path side at the trailing stage to output the light from the first optical waveguide  13 E on the backward path side at the trailing stage to the optical fiber  2 A. 
     That is, the optical modulator  5 C includes a first RF modulation unit  20 A 1  and a second RF modulation unit  20 A 2  over the first optical waveguides  13 A on the forward path side at the leading stage, and a first DC modulation unit  20 B 1  and a second DC modulation unit  20 B 2  over the first optical waveguides  13 C on the backward path side at the leading stage. Further, the optical modulator  5 C includes a parent DC modulation unit  20 C over the first optical waveguides  13 D on the forward path side at the trailing stage. As a result, the optical modulator  5 C constitutes an IQ modulator. 
     The two first optical waveguides  13 A 1  and the two first optical waveguides  13 A 2  on the forward path side at the leading stage, the two first optical waveguides  13 C 1  and the two first optical waveguides  13 C 2  on the backward path side at the leading stage, the two first optical waveguides  13 D on the forward path side at the trailing stage, the first optical waveguide  13 E on the backward path side at the trailing stage, the input unit  11 , and the output unit  17  have cores set to a non-amorphous state. Since the cores of the two first optical waveguides  13 A 1  and the two first optical waveguides  13 A 2  on the forward path side at the leading stage, the two first optical waveguides  13 C 1  and the two first optical waveguides  13 C 2  on the backward path side at the leading stage, the two first optical waveguides  13 D on the forward path side at the trailing stage, the first optical waveguide  13 E on the backward path side at the trailing stage, the input unit  11 , and the output unit  17  are in the non-amorphous state, the cores are set, for example, to have a refractive index of about 2.21 in the X and Y directions and have a refractive index of about 2.14 in the Z direction. 
     On the other hand, the second optical waveguides  16 A 1  and  16 A 2  at the leading stage, the second optical waveguides  16 B 1  and  16 B 2  at the middle stage, the second optical waveguide  16 C at the trailing stage, the first branching unit  12 A, the second branching units  12 B, the first multiplexing units  15 B, and the second multiplexing unit  15 C have cores set to an amorphous state. Since the cores of the second optical waveguides  16 A 1  and  16 A 2  at the leading stage, the second optical waveguides  16 B 1  and  16 B 2  at the middle stage, the second optical waveguide  16 C at the trailing stage, the first branching unit  12 A, the second branching units  12 B, the first multiplexing units  15 B, and the second multiplexing unit  15 C are in the amorphous state, the cores are set, for example, to have a refractive index of about 2.175 in the X, Y, and Z directions. 
     As an example of a process of forming the amorphous state and the non-amorphous state, Ar +  ions are implanted from a surface of the thin film LN layer  33  while masking cores to be set to the non-amorphous state. By implanting the Ar +  ions, the LN crystal orientation of cores of the thin film LN layer  33  is collapsed to remove the anisotropy of the light refractive index, thereby bring the cores into an amorphous state. Also, the masked cores remain in the non-amorphous state. As a result, the cores in the amorphous state have no angle at which an effective refractive index of signal light in the TE mode and an effective refractive index of unnecessary light in the TM mode coincide with each other, making it possible to suppress crosstalk. 
     In the optical modulator  5 C according to the fourth embodiment, the cores of the two first optical waveguides  13 A 1  and the two first optical waveguides  13 A 2  on the forward path side at the leading stage, the two first optical waveguides  13 C 1  and the two first optical waveguides  13 C 2  on the backward path side at the leading stage, the two first optical waveguides  13 D on the forward path side at the trailing stage are in a non-amorphous state. In addition, in the optical modulator  5 C, the cores of the first optical waveguide  13 E on the backward path side at the trailing stage, the input unit  11 , and the output unit  17  are in an amorphous state. The cores in the amorphous state have no propagation angle at which an effective refractive index in the TE mode and an effective refractive index in the TM mode intersect. As a result, even when the optical modulator is formed of an IQ modulator, it is possible to facilitate improvements in modulating efficiency and coupling efficiency while suppressing crosstalk of the unnecessary TM mode with respect to the TE mode. 
     According to one aspect of an optical device or the like disclosed in the present application, improvements in modulating efficiency and coupling efficiency are facilitated while crosstalk is suppressed. 
     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 embodiments of the present invention have 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.