Optical devices and optical communication devices

Abstract

To provide an optical device and the like that improve modulation efficiency and coupling efficiency while suppressing crosstalk.SOLUTION: An optical device has a thin-film LN (Lithium Niobate) layer, a first optical waveguide, and a second optical waveguide. The thin-film LN layer is an X-cut or Y-cut LN layer. The first optical waveguide is an optical waveguide formed in the thin-film LN layer along a direction substantially orthogonal to a Z-direction of a crystal axis of the thin-film LN layer. The second optical waveguide is a lead-around optical waveguide connected to the first optical waveguide. At least part of a core of the first optical waveguide is made thicker than a core of the second optical waveguide.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an optical device and an optical communication apparatus. [Background technology]

[0002] For example, an optical device such as an optical modulator has a signal electrode arranged on a surface optical waveguide. When a voltage is applied to the signal electrode, an electric field perpendicular to the surface of the optical modulator is generated in the optical waveguide. This electric field changes the refractive index of the optical waveguide, changing the phase of the light propagating through the optical waveguide and enabling light modulation. That is, the optical waveguide of the optical modulator constitutes, for example, a Mach-Zehnder interferometer, and the phase difference of the light between multiple parallel optical waveguides can output, for example, an XY polarization multiplexed IQ signal.

[0003] Fig. 18 is a schematic plan view showing an example of the configuration of optical modulator 100, and Fig. 19 is a schematic cross-sectional view showing an example of the cross-sectional portion taken along the line E-E shown in Fig. 18. Optical modulator 100 shown in Fig. 18 has an input section 101, a branching section 102, two optical waveguides 103, an electrode section 104, a multiplexing section 105, and an output section 106. The length direction of optical modulator 100 is defined as the Y direction, the width direction as the Z direction, and the thickness direction as the X direction.

[0004] The input unit 101 inputs light from an optical fiber. The branching unit 102 branches the light from the input unit 101. The two optical waveguides 103 are arranged in parallel and modulate each of the lights 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 has a pair of ground electrodes 104B and a signal electrode 104A that is arranged so as to be sandwiched between the pair of ground electrodes 104B. The signal electrode 104A is an electrode that applies an electric signal to the optical waveguide 103. The ground electrode 104B is an electrode that grounds the electric signal applied to the optical waveguide 103. Each optical waveguide 103 is a Mach-Zehnder (MZ) interaction unit that modulates light by changing the optical refractive index in response to the electric signal from the signal electrode 104A, thereby changing the phase of the light. Then, each optical waveguide 103 outputs the modulated light to the multiplexing section 105. The multiplexing section 105 multiplexes the modulated light from each optical waveguide 103. The output section 106 outputs the modulated light from the multiplexing section 105 to an optical fiber.

[0005] 19 includes a substrate 111, an intermediate layer 112 laminated on the substrate 111, and a thin-film LN substrate 113 made of LN (LiNbO3) material laminated on the intermediate layer 112. The optical modulator 100 further includes two optical waveguides 103 formed in the thin-film LN substrate 113, a buffer layer 114 formed on the thin-film LN substrate 113, and an electrode portion 104 formed on the buffer layer 114.

[0006] The substrate 111 is made of a material such as Si or LN. The intermediate layer 112 is made of a material with a lower optical refractive index than LN, such as a SiO2 layer. The thin-film LN substrate 113 is a thin-film substrate that has strong optical confinement and is advantageous for miniaturization. The crystal axis of the thin-film LN substrate 113 is in the Z direction.

[0007] The optical waveguide 103 is formed of a thin-film LN substrate 113, and is therefore excellent in terms of, for example, insertion loss and transmission characteristics. The thin-film LN substrate 113 is an X-cut substrate, and its structural symmetry enables chirp-free operation, making it suitable for long-distance transmission. Each optical waveguide 103 is disposed between a ground electrode 104B and a signal electrode 104A. The signal electrode 104A is an electrode made of a metal material such as gold or copper. The ground electrode 104B is an electrode made of a metal material such as aluminum. The buffer layer 114 is, for example, a layer of SiO2, and is provided to prevent light propagating through the optical waveguide 103 from being absorbed by the electrode portion 104.

[0008] The crystal orientation of the thin-film LN substrate 113 is the width direction (Z direction) perpendicular to the signal propagation direction (Y direction). The optical refractive index of each optical waveguide 103 changes depending on the electric field in the electric field direction from the signal electrode 104A to the ground electrode 104B.

[0009] Fig. 20 is an explanatory diagram showing an example of the TE mode and the TM mode. In the optical waveguide 103, two modes exist depending on the direction of the main component of the electric field of the optical field. As shown in Fig. 20, there is the TE mode, in which the main component of the electric field is in the planar direction, and the TM mode, in which the main component of the electric field is in the vertical direction. Note that the arrows in Fig. 20 indicate the direction of the main component of the electric field, and the range enclosed by the dotted lines in Fig. 20 is the light distribution range.

[0010] 20A, the optical modulator 100 on the X-cut thin-film LN substrate 113 has the Z direction aligned with the planar direction of the optical waveguide 103, and therefore the signal light modulated by the electric field applied from the electrode section 104 is in the TE mode, the main component of which is the electric field in the planar direction. 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 for the operation of the optical modulator 100.

[0011] Furthermore, the modulation efficiency of the optical modulator 100 is greatly affected by the length of the interaction section of each optical waveguide 103 to which the electric field is applied, i.e., the optical length, and therefore, in order to achieve miniaturization while maintaining modulation efficiency, a structure in which the interaction section is lengthened and folded back is required.

[0012] Fig. 21 is a schematic plan view showing an example of the configuration of an optical modulator 100A with a folded structure, and Fig. 22 is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line FF and line GG shown in Fig. 21. Note that the same components as those of the optical modulator 100 shown in Figs. 18 and 19 are given the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The optical waveguide of the optical modulator 100A shown in Fig. 21 has an outgoing optical waveguide 103A (103), a folded optical waveguide 108, and a returning optical waveguide 103B (103).

[0013] The outgoing optical waveguide 103A and the returning optical waveguide 103B are optical waveguides formed on the thin-film LN substrate 113 along the Y direction, in which the propagation direction (propagation angle) is 0 degrees. The cross section taken along the line FF shown in FIG. 22(A) corresponds to the outgoing optical waveguide 103A. The core thickness of the outgoing optical waveguide 103A is Ha10. The core thicknesses of the outgoing optical waveguide 103A and the returning optical waveguide 103B are the same.

[0014] The folded optical waveguide 108 is an optical waveguide formed on the thin-film LN substrate 113 along the Z direction, where the propagation direction (propagation angle) is other than 0 degrees, for example, 90 degrees. The cross section taken along line GG shown in FIG. 22(B) corresponds to the folded optical waveguide 108. The core thickness of the folded optical waveguide 108 is Hb10. The core thickness Ha10 of the outgoing optical waveguide 103A and the returning optical waveguide 103B is the same as the core thickness Hb10 of the folded optical waveguide 108.

[0015] 23A is an explanatory diagram showing an example of the dependence of the effective refractive index on the propagation angle when the core thickness of the optical waveguide 103 is 1 μm. When the core thickness of the optical waveguide 103 is 1 μm, in the X-cut thin-film LN substrate 113, the refractive index in the planar direction changes depending on the propagation direction of the optical waveguide 103, so the effective refractive index of the TE mode also changes. In addition, the refractive index in the vertical direction is constant regardless of the propagation direction of the optical waveguide 103, so the effective refractive index of the TM mode hardly changes even when the propagation direction of the optical waveguide 103 changes. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-264487 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-102891 [Patent Document 3] International Publication No. 2018 / 031916 Summary of the Invention [Problem to be solved by the invention]

[0017] In the optical modulator 100A, when the propagation angle of the optical waveguide 103 is 0 degrees in the Y direction and 90 degrees in the Z direction, as shown in Fig. 23A, the effective refractive index of the TE mode increases as the propagation angle of the optical waveguide 103 approaches 90 degrees. Therefore, in the optical modulator 100A, when the core of the optical waveguide 103 is thickened, a propagation angle occurs at which the effective refractive index of the TE mode and the effective refractive index of the TM mode match. As a result, the TE mode of the signal light is converted to the TM mode of unwanted light, and the unwanted TM mode causes crosstalk with respect to the TE mode.

[0018] Therefore, if the core thickness of the optical waveguide 103 is reduced, crosstalk for the TM mode can be suppressed. Fig. 23B is an explanatory diagram showing an example of the dependence of the propagation angle on the effective refractive index when the core thickness of the optical waveguide 103 is set to 0.4 µm. When the core thickness of the optical waveguide 103 is set to, for example, 0.4 µm, no propagation angle occurs at which the effective refractive index of the TE mode and the effective refractive index of the TM mode match, so crosstalk for the TE mode can be suppressed.

[0019] However, in the optical modulator 100A, if the core thickness of the optical waveguide 103 is reduced to suppress crosstalk, the light confinement in the thin-film LN substrate 113 is weakened. FIG. 24 is an explanatory diagram showing an example of how the modulation efficiency changes depending on the core thickness of the optical waveguide 103. In the optical modulator 100A, if the core thickness of the optical waveguide 103 is reduced, the light confinement in the thin-film LN substrate 113 is weakened, resulting in a deterioration in modulation efficiency. FIG. 25 is an explanatory diagram showing an example of how the coupling efficiency changes depending on the core thickness of the optical waveguide 103. In the optical modulator 100A, if the core thickness of the optical waveguide 103 is reduced, the light confinement in the thin-film LN substrate 113 is weakened, resulting in a reduction in the optical mode field due to the reduced core thickness, resulting in a deterioration in the coupling efficiency with the optical fiber. Therefore, in the optical modulator 100A, there is a trade-off between suppressing crosstalk for the TE mode and improving modulation efficiency and coupling efficiency.

[0020] The disclosed technology has been made in view of the above points, and aims to provide an optical device or the like that improves modulation efficiency and coupling efficiency while suppressing crosstalk. [Means for solving the problem]

[0021] In one embodiment, the optical device disclosed herein comprises a thin-film LN (lithium niobate) layer, a first optical waveguide, and a second optical waveguide. The thin-film LN layer is an X-cut or Y-cut LN layer. The first optical waveguide is an optical waveguide formed in the thin-film LN layer along a direction substantially perpendicular to the Z direction of the crystal axis of the thin-film LN layer. The second optical waveguide is a routing optical waveguide connected to the first optical waveguide. At least a portion of the core of the first optical waveguide is thicker than the core of the second optical waveguide. [Effects of the Invention]

[0022] According to one aspect of the optical device etc. disclosed in the present application, crosstalk is suppressed while modulation efficiency and coupling efficiency are improved. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a block diagram illustrating an example of the configuration of an optical communication device according to a first embodiment. [Figure 2] FIG. 2 is a schematic plan view showing an example of the configuration of the optical modulator according to the first embodiment. [Figure 3] FIG. 3 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA and line BB shown in FIG. [Figure 4] FIG. 4 is an explanatory diagram showing an example of a joining portion. [Figure 5] FIG. 5 is a schematic plan view showing an example of the configuration of the optical modulator according to the second embodiment. [Figure 6] FIG. 6 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA and line BB shown in FIG. [Figure 7] FIG. 7 is a schematic cross-sectional view showing a modification of the cross-sectional portion taken along line AA and the cross-sectional portion taken along line BB shown in FIG. [Figure 8] FIG. 8 is a schematic plan view showing an example of the configuration of the optical modulator according to the third embodiment. [Figure 9A] FIG. 9A is an explanatory diagram showing an example of a joint portion. [Figure 9B] FIG. 9B is an explanatory diagram showing an example of a joint portion. [Figure 10] FIG. 10 is a schematic plan view showing an example of the configuration of the optical modulator according to the fourth embodiment. [Figure 11] FIG. 11 is a schematic plan view showing an example of the configuration of the optical modulator according to the fifth embodiment. [Figure 12] FIG. 12 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line CC shown in FIG. [Figure 13] FIG. 13 is a schematic plan view showing an example of the configuration of the optical modulator according to the sixth embodiment. [Figure 14] FIG. 14 is a schematic plan view showing an example of the configuration of the optical modulator according to the seventh embodiment. [Figure 15] FIG. 15 is a schematic plan view showing an example of the configuration of the optical modulator according to the eighth embodiment. [Figure 16] FIG. 16 is a schematic plan view showing an example of the configuration of the optical modulator according to the ninth embodiment. [Figure 17] FIG. 17 is a schematic cross-sectional view showing an example of the cross-sectional portions taken along the lines AA, BB, and DD shown in FIG. [Figure 18] FIG. 18 is a schematic plan view showing an example of the configuration of an optical modulator. [Figure 19] FIG. 19 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line EE shown in FIG. [Figure 20] FIG. 20 is an explanatory diagram showing an example of the TE mode and the TM mode. [Figure 21] FIG. 21 is a schematic plan view showing an example of the configuration of an optical modulator with a folded structure. [Figure 22] FIG. 22 is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line FF and line GG shown in FIG. [Figure 23A] FIG. 23A is an explanatory diagram showing an example of the dependence of the effective refractive index on the propagation angle when the core thickness of the optical waveguide is 1 μm. [Figure 23B] FIG. 23B is an explanatory diagram showing an example of the dependence of the effective refractive index on the propagation angle when the core thickness of the optical waveguide is set to 0.4 μm. [Figure 24] FIG. 24 is an explanatory diagram showing an example of the change in modulation efficiency depending on the thickness of the core of the optical waveguide. [Figure 25] FIG. 25 is an explanatory diagram showing an example of the change in coupling efficiency depending on the thickness of the core of the optical waveguide. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, embodiments of the optical device and the like disclosed in the present application will be described in detail with reference to the drawings. However, the present invention is not limited to these embodiments. [Example]

[0025] FIG. 1 is a block diagram showing an example of the configuration of an optical communication device 1 according to a first embodiment. The optical communication device 1 shown in FIG. 1 is connected to an optical fiber 2A(2) on the output side and an optical fiber 2B(2) on the input side. The optical communication device 1 includes a DSP (Digital Signal Processor) 3, a light source 4, an optical modulator 5, and an optical receiver 6. The DSP 3 is an electrical component that performs digital signal processing. For example, the DSP 3 performs processing such as encoding of transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the optical modulator 5. The DSP 3 also obtains an electrical signal including reception data from the optical receiver 6 and performs processing such as decoding of the obtained electrical signal to obtain the reception data.

[0026] The light source 4 is, for example, a laser diode or the like, which generates light of a predetermined wavelength and supplies it to the optical modulator 5 and the optical receiver 6 through the optical fiber 4A. The optical modulator 5 is an optical device that modulates the light supplied from the light source 4 with an electrical signal output from the DSP 3 and outputs the obtained optical transmission signal to the optical fiber 2A. The optical modulator 5 is, for example, an optical device such as an LN (Lithium Niobate) optical modulator that includes an LN optical waveguide and a signal electrode with a coplanar waveguide (CPW) structure.

[0027] The optical receiver 6 receives an optical signal from the optical fiber 2B and demodulates the received optical signal using light supplied from the light source 4. The optical receiver 6 then converts the demodulated received optical signal into an electrical signal and outputs the converted electrical signal to the DSP 3.

[0028] Fig. 2 is a schematic plan view showing an example of the configuration of the optical modulator 5 of Example 1. The optical modulator 5 shown in Fig. 2 has an input section 11, a branching section 12, two first optical waveguides 13A (13) on the outgoing path side, an electrode section 14, a multiplexing section 15, a second optical waveguide 16, a first optical waveguide 13B (13) on the returning path side, and an output section 17. The length direction (propagation direction) of the optical modulator 5 is defined as the Y direction, the width direction (planar direction) as the Z direction, and the thickness direction as the X direction.

[0029] The input unit 11 inputs light from an optical fiber 4A. The light from the optical fiber 4A is light from the light source 4. The branching unit 12 branches the light from the input unit 11. The two first optical waveguides 13A on the outgoing path side are arranged in parallel and are optical waveguides of a Mach-Zehnder (MZ) interaction part that modulate each light branched by the branching unit 12. The two first optical waveguides 13A on the outgoing path side are, for example, ridge-type waveguides having a core and slabs on both sides of the core that are thinner than the core. The two first optical waveguides 13A on the outgoing path side are optical waveguides formed in the thin-film LN layer 33 along a direction substantially perpendicular to the Z direction of the crystal axis of the thin-film LN layer 33 described later, for example, along the Y direction.

[0030] The electrode unit 14 is an electrode that applies an electric signal to the two first optical waveguides 13A on the outgoing path. The two first optical waveguides 13A on the outgoing path change their optical refractive index in response to the electric signal from the electrode unit 14 to modulate light, and output each modulated light to the multiplexing unit 15. The electrode unit 14 has a signal electrode 14A and a pair of ground electrodes 14B. The signal electrode 14A is an electrode that applies an electric signal to the first optical waveguide 13A on the outgoing path. The ground electrode 14B is an electrode that grounds the electric signal applied to the first optical waveguide 13A on the outgoing path. The two first optical waveguides 13A on the outgoing path change their optical refractive index in response to the electric signal from the signal electrode 14A to change the phase of the light, modulating the light, and outputting the modulated light to the multiplexing unit 15. The multiplexing unit 15 multiplexes each modulated light modulated by the two first optical waveguides 13A on the outgoing path. The optical modulator 5 has a modulation section 20 formed by the two first optical waveguides 13A on the outgoing path side and the electrode section 14.

[0031] The second optical waveguide 16 is a folded optical waveguide that transmits the modulated light from the multiplexing section 15. The second optical waveguide 16 is an optical waveguide that includes at least one of a straight waveguide formed in the thin-film LN layer 33 along a direction other than a direction substantially perpendicular to the Z direction, and a curved waveguide in which the propagation direction changes smoothly.

[0032] The first optical waveguide 13B on the return path is an optical waveguide that passes the modulated light from the second optical waveguide 16. The first optical waveguide 13B on the return path is an optical waveguide that is formed in the thin-film LN layer 33 along a direction that is approximately perpendicular 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 2A, and outputs the modulated light from the first optical waveguide 13B on the return path to the optical fiber 2A.

[0033] Fig. 3 is a schematic cross-sectional view showing an example of the cross-sectional portions taken along line AA and line BB shown in Fig. 2. The optical modulator 5 shown in Fig. 3 has a substrate 31, an intermediate layer 32 laminated on the substrate 31, a thin-film LN layer 33 laminated on the intermediate layer 32, and two first optical waveguides 13 formed by the thin-film LN layer 33. Furthermore, the optical modulator 5 has a buffer layer 34 laminated on the thin-film LN layer 33, and an electrode portion 14 formed on the buffer layer 34.

[0034] The substrate 31 is made of a material such as SiO2 (silicon dioxide), TiO2 (titanium dioxide), Si, or LN. The intermediate layer 32 is made of a material with a lower optical refractive index than LN, such as a layer of SiO2 or TiO2. The thin-film LN layer 33 is a substrate using a thin film of LN crystal, and is formed with a first optical waveguide 13 that has a convex shape that protrudes upward at a predetermined location. Because it is made of an LN material, it has strong light confinement properties, which is advantageous for miniaturization.

[0035] The thin-film LN layer 33 is an X-cut substrate. The thin-film LN layer 33 is a substrate made of LN (LiNbO3) material. The two first optical waveguides 13A on the outgoing path side are formed of the thin-film LN layer 33, and since the material is LN, it is excellent in terms of, for example, insertion loss and transmission characteristics. The optical modulator 5 is capable of chirp-free operation due to its structural symmetry, and is suitable for long-distance transmission.

[0036] The buffer layer 34 is, for example, a layer of SiO2, and is provided to prevent light propagating through the first optical waveguide 13 from being absorbed by the electrode portion 14. A 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. The width of the convex protrusions formed on the thin-film LN layer 33 to become the first optical waveguide 13 is, for example, about 1 to 8 μm. The thin-film LN layer 33 and the first optical waveguide 13 are covered with the buffer layer 34.

[0037] The signal electrode 14A is made of a metal material such as gold or copper and has a width of 2 to 10 μm and a thickness of 1 to 20 μm. The ground electrode 14B is made of a metal material such as aluminum and has a thickness of 1 μm or more. When a drive voltage corresponding to an electrical signal output from the DSP 3 is transmitted by the signal electrode 14A, an electric field is generated in a direction from the signal electrode 14A to the ground electrode 14B, and this electric field is applied to the first optical waveguide 13. As a result, the refractive index of the first optical waveguide 13 changes in response to the application of the electric field to the first optical waveguide 13, making it possible to modulate the light propagating through the first optical waveguide 13.

[0038] The two first optical waveguides 13A on the outgoing path side are linear optical waveguides formed in the chip of the optical modulator 5, extending along the Y direction at approximately 0 degrees, assuming that the Z direction of the crystal axis of the thin-film LN layer 33 is 90 degrees and the Y direction is 0 degrees. The core thickness of the two first optical waveguides 13A on the outgoing path side is Ha, as shown in FIG. 3A. Note that the term "approximately 0 degrees" includes not only 0 degrees but also a range of manufacturing error of the first optical waveguide 13 when manufacturing an optical waveguide aiming for 0 degrees, for example, within ±20 degrees. The two first optical waveguides 13A on the outgoing path side are formed in the thin-film LN layer 33 along a direction approximately perpendicular to the Z direction of the crystal axis of the thin-film LN layer 33. Therefore, the thicknesses are thicker than the core thickness Hb of the second optical waveguide 16, thereby improving modulation efficiency and coupling efficiency. The thickness of the cores of the first optical waveguide 13B on the return path, the input portion 11, and the output portion 17 is Ha in the X direction as shown in FIG. 3A, similar to the cores of the two first optical waveguides 13A on the outward path.

[0039] The core of 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, i.e., a direction other than the Y direction at approximately 0 degrees, and has a core thickness of Hb, as shown in FIG. 3B. The direction not orthogonal to the Z direction refers to, for example, a direction greater than the actual angle deviated from the Y direction and less than 90 degrees from the Z direction. Because the second optical waveguide 16 is formed in the thin-film LN layer 33 along a direction other than a direction substantially orthogonal to the Z direction, its thickness is thinner than the thickness Ha of the first optical waveguide 13, thereby suppressing unwanted crosstalk of the TM mode relative to the TE mode. The cores of the branching section 12 and the multiplexing section 15 also have the same thickness Hb as the core of the second optical waveguide 16. The second optical waveguide 16, the branching section 12, and the multiplexing section 15 are second optical waveguides routed to connect to the first optical waveguide 13.

[0040] 4 is an explanatory diagram showing an example of a joint portion. At the joint portion joining the core of the first optical waveguide 13 and the core of the multiplexing section 15, the heights of the core of the first optical waveguide 13 and the core of the multiplexing section 15 are different. The core of the multiplexing section 15 has the same thickness as the core of the second optical waveguide 16. In other words, the cores of the two first optical waveguides 13A on the outgoing path, the first optical waveguide 13B on the incoming path, the input section 11, and the output section 17 are thicker than the core of the second optical waveguide 16.

[0041] The optical modulator 5 of the first embodiment includes an X-cut thin-film LN layer 33, a first optical waveguide 13 formed in the thin-film LN layer 33 along a direction substantially perpendicular to the Z direction of the crystal axis of the thin-film LN layer 33, and a second optical waveguide 16 routed to connect to the first optical waveguide 13. At least a portion of the core of the first optical waveguide 13 is thicker than the core of the second optical waveguide 16. Because the first optical waveguide 13 is formed along a direction substantially perpendicular to the Z direction of the crystal axis, e.g., the Y direction, its core is thicker than the core of the second optical waveguide 16, thereby improving modulation efficiency and coupling efficiency. Furthermore, because the second optical waveguide 16 is thinner than the core of the first optical waveguide 13, unwanted crosstalk of the TM mode relative to the TE mode can be suppressed. In other words, modulation efficiency and coupling efficiency can be improved while suppressing crosstalk.

[0042] The second optical waveguide 16 includes at least one of a straight waveguide and a curved waveguide formed in the thin-film LN layer 33 along a direction other than a direction substantially perpendicular to the Z direction. As a result, the second optical waveguide 16 is thinner than the core of the first optical waveguide 13, and therefore, unnecessary crosstalk of the TM mode with respect to the TE mode can be suppressed.

[0043] For ease of explanation, an X-cut thin-film LN layer 33 is illustrated, but the optical modulator 5 may also use a Y-cut thin-film LN layer. The optical modulator 5 includes a Y-cut thin-film LN layer 33, a first optical waveguide 13 formed in the thin-film LN layer 33 along a direction substantially perpendicular to the Z direction of the crystal axis of the thin-film LN layer, and a second optical waveguide 16 routed to connect to the first optical waveguide 13. At least a portion of the core of the first optical waveguide 13 is thicker than the core of the second optical waveguide 16. As a result, since the first optical waveguide 13 is formed along a direction substantially perpendicular to the Z direction of the crystal axis, for example, the Y direction, its core is thicker than the core of the second optical waveguide 16, resulting in improved modulation efficiency and coupling efficiency. Furthermore, the second optical waveguide 16 is thinner than the core of the first optical waveguide 13, which can suppress unwanted crosstalk of the TM mode with respect to the TE mode. In other words, the modulation efficiency and coupling efficiency can be improved while suppressing crosstalk.

[0044] The joint portion between the first optical waveguide 13 and the second optical waveguide 16 may include an optical waveguide having a part of the core formed along the approximately 0 degree direction with a thickness Hb.

[0045] The thickness of the core in the first optical waveguide 13 does not have to be Ha over the entire length, and the thickness of the core in part may be Ha, and can be changed as appropriate. The thickness of the core in the second optical waveguide 16 does not have to be Hb over the entire length, and the thickness of the core in part may be Hb.

[0046] Although the second optical waveguide 16, the branching section 12, and the multiplexing section 15 are exemplified as the optical waveguides to be routed and connected to the first optical waveguide 13, the present invention is not limited to these and can be changed as appropriate.

[0047] In Example 1, the first optical waveguide 13 and the second optical waveguide 16 are configured as ridge-type waveguides, but the present invention is not limited to this, and an embodiment thereof will be described below as Example 2. [Example]

[0048] FIG. 5 is a schematic plan view showing an example of the configuration of an optical modulator 5A of Example 2. Note that the same components as those of the optical modulator 5 of Example 1 are assigned the same reference numerals, and explanations of the overlapping configurations and operations will be omitted. FIG. 6 is a schematic cross-sectional view showing an example of the cross-sectional portions along line AA and line BB shown in FIG. 5. The cross-sectional portion along line AA of the optical modulator 5A is the portion of the first optical waveguide 13A on the outgoing path side. The first optical waveguide 13A on the outgoing path side is a ridge-type waveguide. The cross-sectional portion along line BB is the portion of the second optical waveguide 16. The second optical waveguide 16A (16) is a channel waveguide. In a channel waveguide, the core of the optical waveguide has a rectangular cross-sectional shape as shown in FIG. 6(B).

[0049] The cores of the two first optical waveguides 13A on the outgoing path, the input section 11, the first optical waveguide 13B on the returning path, and the output section 17 are ridge-type waveguides with a core thickness of Ha. The cores of the second optical waveguide 16A, the branching section 12, and the multiplexing section 15 are channel waveguides with a core thickness of Hb. In other words, the cores of the two first optical waveguides 13A on the outgoing path, the first optical waveguide 13B on the returning path, the input section 11, and the output section 17 are thicker than the core of the second optical waveguide 16A.

[0050] In the optical modulator 5A of the second embodiment, the outgoing first optical waveguide 13A and the returning first optical waveguide 13B are ridge-type waveguides, and the second optical waveguide 16A is a channel waveguide. Furthermore, in the optical modulator 5A, the core thicknesses of the outgoing first optical waveguide 13A and the returning first optical waveguide 13B are thicker than the core thickness of the second optical waveguide 16A. As a result, the first optical waveguide 13 is formed along a direction substantially perpendicular to the Z direction of the crystal axis, e.g., the Y direction, and its core is thicker than the core of the second optical waveguide 16A, thereby improving modulation efficiency and coupling efficiency. Furthermore, the second optical waveguide 16A is thinner than the core of the first optical waveguide 13, thereby suppressing unnecessary crosstalk of the TM mode relative to the TE mode. In other words, modulation efficiency and coupling efficiency can be improved while suppressing crosstalk.

[0051] For convenience of explanation, the first optical waveguide 13 is a ridge-type waveguide and the second optical waveguide 16A is a channel waveguide. However, the first optical waveguide 13 may be a channel waveguide and the second optical waveguide 16A may be a ridge-type waveguide, and other appropriate modifications may be made.

[0052] Figure 7 is a schematic cross-sectional view showing a modified example of the cross-sectional portions taken along line AA and line BB shown in Figure 5. The first optical waveguide 13 in the cross-sectional portion taken along line AA is a ridge-type waveguide, and the cross-sectional shape of the core is not rectangular, but may be a core having sidewall surfaces with a predetermined inclination angle, as shown in Figure 7(A). The inclination angle of the sidewall surfaces of the core is, for example, about 80 degrees.

[0053] The second optical waveguide 16A at the cross section of line BB is a channel waveguide, and the cross section of the core may not be rectangular but may have sidewalls with a predetermined inclination angle as shown in Fig. 7(B). The inclination angle of the sidewalls of the core is, for example, about 80 degrees.

[0054] In the second embodiment, the core of the second optical waveguide 16A is configured as a channel waveguide, but the core of the second optical waveguide 16A may be configured as a channel waveguide or a ridge waveguide, and the core of the first optical waveguide 13 may be configured as a channel waveguide or a ridge waveguide.

[0055] In Example 1, at a joining portion between the core of the first optical waveguide 13 and the core of the second optical waveguide 16 (combining portion 15), a step X1 occurs as shown in FIG. 4 because the thicknesses of the core of the first optical waveguide 13 and the core of the second optical waveguide 16 (combining portion 15) are different. At the joining portion, the step X1 occurs between the core of the first optical waveguide 13 and the core of the second optical waveguide 16 (combining portion 15), and therefore the coupling efficiency deteriorates. Therefore, in order to address this situation, an embodiment thereof will be described below as Example 3. [Example]

[0056] Fig. 8 is a schematic plan view showing an example of the configuration of an optical modulator 5B according to Example 3. Note that the same components as those of the optical modulator 5 according to Example 1 are denoted by the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The optical modulator 5B shown in Fig. 8 differs from the optical modulator 5 shown in Fig. 2 in that, for example, the optical modulator 5B has a junction 18D (18) that joins the first optical waveguide 13B and the second optical waveguide 16 on the return path side.

[0057] The optical modulator 5B has a junction 18A joining the input section 11 and the branching section 12, a junction 18B joining the branching section 12 and the first optical waveguide 13A on one outgoing path, and a junction 18B joining the branching section 12 and the first optical waveguide 13A on the other outgoing path. The optical modulator 5B further has a junction 18C joining the first optical waveguide 13A on one outgoing path and the multiplexing section 15, and a junction 18C joining the first optical waveguide 13A on the other outgoing path and the multiplexing section 15. The optical modulator 5B further has a junction 18D joining the second optical waveguide 16 and the first optical waveguide 13B on the return path.

[0058] 9A is an explanatory diagram showing an example of a joint 18. The core of joint 18C (18) joining the outgoing first optical waveguide 13A and the multiplexing section 15 has a tapered structure that gradually becomes thinner from the core of the outgoing first optical waveguide 13A toward the core of the multiplexing section 15. The core of joint 18A joining the input section 11 and the branching section 12 has a tapered structure that gradually becomes thinner from the core of the input section 11 toward the core of the branching section 12. The core of joint 18B joining the branching section 12 and the outgoing first optical waveguide 13A has a tapered structure that gradually becomes thinner from the core of the branching section 12 toward the core of the outgoing first optical waveguide 13A.

[0059] The core of the joint 18D that joins the second optical waveguide 16 and the first optical waveguide 13B on the return path side has a tapered structure that gradually becomes thicker from the core of the second optical waveguide 16 toward the core of the first optical waveguide 13B on the return path side.

[0060] In the optical modulator 5 of the third embodiment, the core of the joint 18 that joins the first optical waveguide 13 and the second optical waveguide 16 has a tapered shape that gradually becomes thinner from the core of the first optical waveguide 13 toward the core of the second optical waveguide 16. As a result, the step X1 in the core of the joint 18 is minimized, thereby improving the coupling efficiency.

[0061] The tapered joint 18, in which the height of the core gradually changes, may be formed along a direction at approximately 90 degrees to the Z direction, and can be changed as appropriate.

[0062] The structure of the joint 18 joining the first optical waveguide 13 and the second optical waveguide 16 (combining section 15) has been illustrated as a tapered structure in which the core of the first optical waveguide 13 gradually becomes thinner toward the core of the second optical waveguide 16. However, the tapered structure of the core of the joint 18 is not limited to this, and it does not have to be continuous, as shown in FIG. 9B . FIG. 9B is an explanatory diagram showing an example of a joint 18E. The joint 18E shown in FIG. 9B can improve coupling efficiency compared to the case where the first optical waveguide 13 and the second optical waveguide 16 are directly connected, even when a step X2 of, for example, 0.1 μm occurs between the joint 18E and the first optical waveguide 13. [Example]

[0063] FIG. 10 is a schematic plan view showing an example of the configuration of an optical modulator 5C according to a fourth embodiment. The same components as those of the optical modulator 5 according to the first embodiment are designated by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The optical modulator 5C shown in FIG. 10 includes an input section 11, a branching section 12, two first optical waveguides 13A (13) on the outgoing path, an electrode section 14, and two second optical waveguides 16B (16) on the return path. Furthermore, the optical modulator 5C includes two first optical waveguides 13C (13) on the return path, a multiplexing section 15A, and an output section 17.

[0064] The input unit 11 inputs light from the optical fiber 4A. The branching unit 12 branches the light from the input unit 11. The two first optical waveguides 13A on the outgoing path are arranged in parallel and are optical waveguides in the interaction part of the MZ that modulate each light branched by the branching unit 12. The electrode unit 14 applies an electric signal to the two first optical waveguides 13A on the outgoing path. The two first optical waveguides 13A on the outgoing path modulate the light by changing the optical refractive index in response to the electric signal from the electrode unit 14.

[0065] The two second optical waveguides 16B are connected to the two first optical waveguides 13A on the outgoing path and are return optical waveguides through which light modulated by the two first optical waveguides 13A on the outgoing path passes. The two first optical waveguides 13C on the return path are optical waveguides connected to the two second optical waveguides 16B. The multiplexing unit 15A multiplexes the modulated light passing through the two first optical waveguides 13C on the return path. The output unit 17 outputs the modulated light multiplexed by the multiplexing unit 15A to the optical fiber 2A.

[0066] The core thickness of the two first optical waveguides 13A on the outgoing path, the two first optical waveguides 13C on the return path, the input section 11, and the output section 17 is Ha. The core thickness of the second optical waveguide 16B, the branching section 12, and the multiplexing section 15A is Hb. In other words, the cores of the two first optical waveguides 13A on the outgoing path, the first optical waveguide 13B on the return path, the input section 11, and the output section 17 are thicker than the core of the second optical waveguide 16B.

[0067] The waveguide length of the interaction section is the optical length obtained by adding the waveguide lengths of the first optical waveguide 13A and the second optical waveguide 16B on the outgoing path and the first optical waveguide 13C on the returning path. As a result, the optical modulator 5C of the fourth embodiment has a longer waveguide length in the interaction section than the optical modulator 5 of the first embodiment, thereby increasing modulation efficiency.

[0068] In the optical modulator 5C of the fourth embodiment, the thicknesses of the cores of the two first optical waveguides 13A on the outgoing path, the two first optical waveguides 13C on the returning path, the input section 11, and the output section 17 are made thicker than the core of the second optical waveguide 16C. As a result, it is possible to improve the modulation efficiency and coupling efficiency while suppressing crosstalk.

[0069] The optical modulator 5C of Example 4 illustrates a case where an electrical signal is applied from the electrode portion 14 to the two first optical waveguides 13A on the outgoing path side. However, in addition to the two first optical waveguides 13A on the outgoing path side, an interaction portion may also be disposed in the two first optical waveguides 13C on the return path side, and this embodiment will be described below as Example 5. [Example]

[0070] FIG. 11 is a plan view schematic diagram showing an example of the configuration of an optical modulator 5D according to a fifth embodiment. The same components as those of the optical modulator 5C according to the fourth embodiment are denoted by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The optical modulator 5D shown in FIG. 11 includes an input section 11, a branching section 12, two first optical waveguides 13A on the outgoing path, and two second optical waveguides 16B. Furthermore, the optical modulator 5D includes two first optical waveguides 13C on the return path, a first electrode section 141 (14), a second electrode section 142 (14), a multiplexing section 15A, and an output section 17.

[0071] The input unit 11 inputs light from the optical fiber 4A. The branching unit 12 branches the light from the input unit 11. The two first optical waveguides 13A on the outgoing path are arranged in parallel and are optical waveguides in the interaction part of the MZ that modulate each light branched by the branching unit 12. The two second optical waveguides 16B are folded optical waveguides that connect to the two first optical waveguides 13A on the outgoing path. The two first optical waveguides 13C on the return path are arranged in parallel and are optical waveguides in the interaction part of the MZ that connect to the two second optical waveguides 16B.

[0072] The first optical waveguide 13A on one outgoing path side is connected to the second optical waveguide 16B on one side, and the second optical waveguide 16B is connected to the first optical waveguide 13C on one returning path side. The first optical waveguide 13A on the other outgoing path side is connected to the second optical waveguide 16B on the other side, and the second optical waveguide 16B on the other returning path side is connected to the first optical waveguide 13C on the other returning path side.

[0073] The first electrode unit 141 has a first electrode 141A arranged between the two first optical waveguides 13A on the outward path side, and a second electrode 141B and a third electrode 141C arranged outside the two first optical waveguides 13C on the return path side. The second electrode unit 142 has a fourth electrode 142A arranged between the two first optical waveguides 13C on the return path side, and a fifth electrode 142B and a sixth electrode 142C arranged outside the two first optical waveguides 13A on the outward path side. The second electrode unit 142 has a higher potential than the first electrode unit 141.

[0074] Fig. 12 is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line CC shown in Fig. 11. The first optical waveguide 13A on one outgoing path applies an electric field in the -Z direction from the sixth electrode 142C of the second electrode portion 142 to the first electrode 141A of the first electrode portion 141. The first optical waveguide 14C on one returning path applies an electric field in the -Z direction from the fourth electrode 142A of the second electrode portion 142 to the third electrode 141C of the first electrode portion 141. As a result, since the direction of the electric field applied to the first optical waveguide 13A on one outgoing path and the first optical waveguide 13C on one returning path is the same as the -Z direction, a phase change occurs in the same direction, thereby improving modulation efficiency.

[0075] The first optical waveguide 13A on the other outgoing path applies an electric field in the +Z direction from the fifth electrode 142B of the second electrode portion 142 to the first electrode 141A of the first electrode portion 141. The first optical waveguide 13C on the other returning path applies an electric field in the +Z direction from the fourth electrode 142A of the second electrode portion 142 to the second electrode 141B of the first electrode portion 141. As a result, since the direction of the electric field applied to the first optical waveguide 13A on the other outgoing path and the first optical waveguide 13C on the other returning path is the same as the +Z direction, a phase change occurs in the same direction, and modulation efficiency is improved.

[0076] The multiplexing section 15A multiplexes the modulated light passing through the two first optical waveguides 13C on the return path side. The output section 17 outputs the modulated light multiplexed by the multiplexing section 15A to the optical fiber 4A.

[0077] The core thickness of the two first optical waveguides 13A on the outgoing path, the two first optical waveguides 13C on the return path, the input section 11, and the output section 17 is Ha. The core thickness of the second optical waveguide 16B, the branching section 12, and the multiplexing section 15A is Hb. In other words, the cores of the two first optical waveguides 13A on the outgoing path, the first optical waveguide 13C on the return path, the input section 11, and the output section 17 are thicker than the core of the second optical waveguide 16B.

[0078] In the optical modulator 5D of the fifth embodiment, the thicknesses of the cores of the two first optical waveguides 13A on the outgoing path, the two first optical waveguides 13C on the returning path, the input section 11, and the output section 17 are made thicker than the core of the second optical waveguide 16B. As a result, it is possible to improve the modulation efficiency and coupling efficiency while suppressing crosstalk.

[0079] In the optical modulator 5D, an electric field is applied to the two first optical waveguides 13A on the outgoing path before and after the return and the two first optical waveguides 13C on the returning path, thereby improving modulation efficiency and achieving miniaturization.

[0080] In the optical modulator 5C of Example 4, an electrode portion 14 is disposed in the first optical waveguide 13A on the outgoing path to form an interaction portion. However, in addition to the first optical waveguide 13A on the outgoing path, another electrode portion 14 may be disposed in the first optical waveguide 13C on the return path to provide another interaction portion, and such an embodiment will be described below as Example 6. [Example]

[0081] 13 is a plan view schematic diagram showing an example of the configuration of an optical modulator 5E of Example 6. Note that the same components as those of the optical modulator 5C of Example 4 are denoted by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The optical modulator 5E shown in FIG. 13 has an input section 11, a branching section 12, two first optical waveguides 13A on the outgoing path side, a third electrode section 143, a second optical waveguide 16B, two first optical waveguides 13C on the incoming path side, a fourth electrode section 144, a multiplexing section 15A, and an output section 17.

[0082] The input unit 11 inputs light from the optical fiber 4A. The branching unit 12 branches the light from the input unit 11. The two first optical waveguides 13A on the outgoing path side are arranged in parallel and are first optical waveguides 13 of the MZ interaction unit that modulate each light branched by the branching unit 12. The third electrode unit 143 has a signal electrode 143A that applies a first electrical signal to the two first optical waveguides 13A on the outgoing path side, and a ground electrode 143B that grounds the first electrical signal from the two first optical waveguides 13A on the outgoing path side. The two first optical waveguides 13A on the outgoing path side and the third electrode unit 143 constitute a first modulation unit 20A (20). The first modulation section 20A is, for example, a DC (Direct Current) modulation section that applies a bias voltage as a first electrical signal from the third electrode section 143 to the two first optical waveguides 13A on the outgoing path side.

[0083] The second optical waveguide 16B is arranged in parallel, connected to the two first optical waveguides 13A on the outgoing path, and is a folded optical waveguide through which light modulated by the two first optical waveguides 13A on the outgoing path passes.

[0084] The two first optical waveguides 13C on the return path side are arranged in parallel and are first optical waveguides 13 connected to the two second optical waveguides 16B. The fourth electrode unit 144 has a signal electrode 144A that applies a second electrical signal to the two first optical waveguides 13C on the return path side and a ground electrode 114B that grounds the second electrical signal from the two first optical waveguides 13C on the return path side. The two first optical waveguides 13C on the return path side and the fourth electrode unit 144 constitute a second modulation unit 20B (20). The second modulation unit 20B is, for example, an RF (Radio Frequency) modulation unit in which the fourth electrode unit 144 applies a high-frequency signal as the second electrical signal to the two first optical waveguides 13C on the return path side.

[0085] The multiplexing unit 15A is connected to the two first optical waveguides 13C on the return path side, and multiplexes the modulated light passing through the two first optical waveguides 13C on the return path side. The output unit 17 outputs the modulated light multiplexed by the multiplexing unit 15A to the optical fiber 2A.

[0086] The core thickness of the two first optical waveguides 13A on the outgoing path, the two first optical waveguides 13C on the return path, the input section 11, and the output section 17 is Ha. The core thickness of the second optical waveguide 16B, the branching section 12, and the multiplexing section 15A is Hb. In other words, the cores of the two first optical waveguides 13A on the outgoing path, the first optical waveguide 13C on the return path, the input section 11, and the output section 17 are thicker than the core of the second optical waveguide 16B.

[0087] In the optical modulator 5E of the sixth embodiment, the thicknesses of the cores of the two first optical waveguides 13A on the outgoing path, the two first optical waveguides 13C on the returning path, the input section 11, and the output section 17 are made thicker than the core of the second optical waveguide 16B. As a result, even when the first modulation section 20A is disposed in the stage before the return and the second modulation section 20B is disposed in the stage after the return, it is possible to improve the modulation efficiency and coupling efficiency while suppressing crosstalk.

[0088] For ease of explanation, the first modulation unit 20A is a DC modulation unit and the second modulation unit 20B is an RF modulation unit, but this is not limited to this. For example, the second modulation unit 20B may be a DC modulation unit, and changes can be made as appropriate. [Example]

[0089] FIG. 14 is a plan view schematic diagram showing an example of the configuration of an optical modulator 5F according to a seventh embodiment. The optical modulator 5F shown in FIG. 14 is an IQ modulator. The optical modulator 5F includes an input section 11, a first branching section 12A, a pair of second branching sections 12B, two first optical waveguides 13A1 and 13A2 on the forward path side of the previous stage, a fifth electrode section 145, and second optical waveguides 16A1 and 16A2 on the backward path side of the previous stage. The optical modulator 5F also includes two first optical waveguides 13C1 and 13C2 on the backward path side of the previous stage, a sixth electrode section 146, a pair of first multiplexing sections 15B, and two second optical waveguides 16B1 on the middle path side. Furthermore, the optical modulator 5F has two first optical waveguides 13D on the outward side of the subsequent stage, a seventh electrode portion 147, a second optical waveguide 16C on the subsequent stage, an optical waveguide 13E on the return side of the subsequent stage, and an output portion 17.

[0090] The input unit 11 receives light from the optical fiber 4A. The first branching unit 12A branches the light from the input unit 11. Each second branching unit 12B outputs the light branched by the first branching unit 12A to two first optical waveguides 13A1 and 13A2 on the outgoing path, which modulate the light.

[0091] The two first optical waveguides 13A1 and 13A2 on the forward path side of each front stage are arranged in parallel and are optical waveguides in the interaction section of the MZ that modulate each light branched at the second branch section 12B. The fifth electrode section 145 has a first signal electrode 145A1 and a second signal electrode 145A2, and a first ground electrode 145C1, a second ground electrode 145C2, and a third ground electrode 145C3. The first signal electrode 145A1 applies a first electrical signal to the two first optical waveguides 13A1 on the forward path side of one front stage. The first ground electrode 145B1 grounds the first electrical signal from one of the two first optical waveguides 13A1 on the forward path side. The second ground electrode 145B2 grounds the first electrical signal from the other of the two first optical waveguides 13A1 on the forward path side of the preceding stage. The first optical waveguide 13A1 on the forward path side of the preceding stage and the fifth electrode unit 145 form the first RF modulation unit 20A1.

[0092] The second signal electrode 145A2 applies a second electrical signal to the two first optical waveguides 13A2 on the forward path side of the other preceding stage. The third ground electrode 145B3 grounds the second electrical signal from the first optical waveguide 13A2 on the forward path side of the other preceding stage, among the two first optical waveguides 13A2 on the forward path side of the other preceding stage. The second ground electrode 145B2 grounds the second electrical signal from one of the two first optical waveguides 13A2 on the forward path side of the other preceding stage, among the two first optical waveguides 13A2 on the forward path side of the other preceding stage. The first optical waveguide 13A2 on the forward path side of the other preceding stage and the fifth electrode unit 145 form a second RF modulation unit 20A2.

[0093] The second optical waveguides 16A in the front stage are arranged in parallel and include two second optical waveguides 16A1 in one front stage and two second optical waveguides 16A2 in the other front stage. The two second optical waveguides 16A1 in one front stage are connected to the two first optical waveguides 13A1 on the outward path side of one of the front stages and are folded optical waveguides through which light modulated by the two first optical waveguides 13A1 on the outward path side of one of the front stages passes. The two second optical waveguides 16A2 in the other front stage are connected to the two first optical waveguides 13A2 on the outward path side of the other front stage and are folded optical waveguides through which light modulated by the two first optical waveguides 13A2 on the outward path side of the other front stage passes.

[0094] The two first optical waveguides 13C1 on the return path side of one of the preceding stages are arranged in parallel and are first optical waveguides that connect to the two second optical waveguides 16A1 on one of the preceding stages. The two first optical waveguides 13C2 on the return path side of the other preceding stage are arranged in parallel and are first optical waveguides that connect to the two second optical waveguides 16A2 on the other preceding stage.

[0095] The sixth electrode unit 146 has a first signal electrode 146A1 and a second signal electrode 146A2, and a first ground electrode 146B1, a second ground electrode 146B2, and a third ground electrode 146B3. The first signal electrode 146A1 of the sixth electrode unit 146 applies a third electrical signal to two first optical waveguides 13C1 on the return path side of one of the preceding stages. The first ground electrode 146B1 of the sixth electrode unit 146 grounds the third electrical signal from one of the two first optical waveguides 13C1 on the return path side of one of the preceding stages. The second ground electrode 146B2 of the sixth electrode unit 146 grounds the third electrical signal from the first optical waveguide 13C1 on the return path side of the other of the two first optical waveguides 13C1 on the return path side of the one preceding stage. The first optical waveguide 13C1 on the return path side of the one preceding stage and the sixth electrode unit 146 form a first DC modulation unit 20B1(20).

[0096] The second signal electrode 146A2 of the sixth electrode unit 146 applies a fourth electrical signal to the two first optical waveguides 13C2 on the return path side of the other preceding stage. The third ground electrode 146B3 of the sixth electrode unit 146 grounds the fourth electrical signal from the first optical waveguide 13C2 on the return path side of the other preceding stage, among the two first optical waveguides 13C2 on the return path side of the other preceding stage. The second ground electrode 146B2 of the sixth electrode unit 146 grounds the fourth electrical signal from the first optical waveguide 13C2 on the return path side of one preceding stage, among the two first optical waveguides 13C2 on the return path side of the other preceding stage. The first optical waveguide 13C2 on the return path side of the other preceding stage and the sixth electrode unit 146 form a second DC modulation unit 20B2 (20).

[0097] One first multiplexing section 15B1 is connected to two first optical waveguides 13C1 on the return path side of one preceding stage and multiplexes modulated light passing through the two first optical waveguides 13C1 on the return path side of one preceding stage. The other first multiplexing section 15B2 is connected to two first optical waveguides 13C2 on the return path side of the other preceding stage and multiplexes modulated light passing through the two first optical waveguides 13C2 on the return path side of the other preceding stage.

[0098] The middle second optical waveguides 16B are arranged in parallel and include one middle second optical waveguide 16B1 and the other middle second optical waveguide 16B2. One middle second optical waveguide 16B1 is connected to one first multiplexing section 15B1 and is a folded optical waveguide through which light from one first multiplexing section 15B1 passes. The other middle second optical waveguide 16B2 is connected to the other first multiplexing section 15B2 and is a folded optical waveguide through which light from the other first multiplexing section 15B2 passes.

[0099] The two first optical waveguides 13D on the outward path side of the rear stage are first optical waveguides connected to the second optical waveguides 16B1 and 16B2 on the middle stage. The seventh electrode unit 147 has a signal electrode 147A that applies a fifth electrical signal to the two first optical waveguides 13D on the outward path side of the rear stage, and a pair of ground electrodes 147B that ground the fifth electrical signal from the two first optical waveguides 13D on the outward path side of the rear stage. The two first optical waveguides 13D on the outward path side of the rear stage and the seventh electrode unit 147 form a parent DC modulation unit 20C.

[0100] The second multiplexing section 15C is connected to the two first optical waveguides 13D on the outward path side of the subsequent stage and multiplexes the light modulated by the two first optical waveguides 13D on the outward path side of the subsequent stage. The second optical waveguide 16C on the subsequent stage is connected to the second multiplexing section 15C and is a return optical waveguide through which light from the second multiplexing section 15C passes. The single first optical waveguide 13E on the return path side of the subsequent stage is a first optical waveguide that connects to the second optical waveguide 16C on the subsequent stage. The output section 17 is connected to the first optical waveguide 13E on the return path side of the subsequent stage and outputs light from the first optical waveguide 13E on the return path side of the subsequent stage to the optical fiber 2A.

[0101] That is, the optical modulator 5F has a first RF modulation section 20A1 and a second RF modulation section 20A2 on the first optical waveguide 13A on the forward path side of the previous stage, and a first DC modulation section 20B1 and a second DC modulation section 20B2 on the first optical waveguide 13C on the backward path side of the previous stage. Furthermore, the optical modulator 5F has a parent DC modulation section 20C on the first optical waveguide 13D on the forward path side of the subsequent stage. As a result, the optical modulator 5F constitutes an IQ modulator.

[0102] The core thickness of the two first optical waveguides 13A1 and 13A2 on the forward path side of the front stage, the two first optical waveguides 13C1 and 13C2 on the return path side of the front stage, the two first optical waveguides 13D on the forward path side of the rear stage, the first optical waveguide 13E on the return path side of the rear stage, the input section 11, and the output section 17 is Ha. In contrast, the core thickness of the second optical waveguides 16A1 and 16A2 on the front stage, the second optical waveguides 16B1 and 16B2 on the middle stage, the second optical waveguide 16C on the rear stage, the first branching section 12A, the second branching section 12B, the first multiplexing section 15B, and the second multiplexing section 15C is Hb. That is, the cores of the two first optical waveguides 13A1 and 13A2 on the forward path side of the front stage and the two first optical waveguides 13C1 and 13C2 on the return path side of the front stage are made thicker than the core of the second optical waveguide 16. Furthermore, the cores of the two first optical waveguides 13D on the forward path side of the rear stage, the first optical waveguide 13E on the return path side of the rear stage, the input section 11, and the output section 17 are made thicker than the core of the second optical waveguide 16.

[0103] In the optical modulator 5F of the seventh embodiment, the core thicknesses of the two first optical waveguides 13A1 and 13A2 on the forward path side of the front stage, the two first optical waveguides 13C1 and 13C2 on the return path side of the front stage, and the two first optical waveguides 13D on the forward path side of the rear stage are made thicker than the core of the second optical waveguide 16. Furthermore, in the optical modulator 5F, the core thicknesses of the first optical waveguide 13E on the return path side of the rear stage, the input section 11, and the output section 17 are made thicker than the core of the second optical waveguide 16. As a result, even when an IQ modulator is configured, it is possible to improve modulation efficiency and coupling efficiency while suppressing crosstalk. [Example]

[0104] 15 is a schematic plan view showing an example of the configuration of an optical modulator 5G of Example 8. Note that the same components as those of the optical modulator 5 of Example 1 are given the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The optical modulator 5G of Example 8 is characterized in that the core in the first coupler 50A (50) in the branching unit 12 and the core in the second coupler 50B (50) in the multiplexing unit 15 are thinner than the thickness of the core of the first optical waveguide 13 and wider than the waveguide width of the core of the second optical waveguide 16.

[0105] The first coupler 50A and the second coupler 50B are, for example, MMI (Multimode Interference) couplers. MMI couplers have the advantage of small wavelength dependency. The MMI coupler uses a multimode optical waveguide as its core, which has a waveguide width wider than that of the second optical waveguide 16. The core of the MMI coupler is made thinner than the core thickness of the two first optical waveguides 13A on the outgoing path side and the first optical waveguide 13B on the incoming path side.

[0106] In the optical modulator 5G of the eighth embodiment, the thickness of the core in the first coupler 50A and the core in the second coupler 50B is thinner than the thickness of the core in the outgoing first optical waveguide 13A and the incoming first optical waveguide 13B, and is thicker than the waveguide width of the core in the second optical waveguide 16. As a result, even when an MMI coupler is used, it is possible to improve the modulation efficiency and coupling efficiency while suppressing crosstalk. [Example]

[0107] FIG. 16 is a schematic plan view showing an example of the configuration of an optical modulator 5H of Example 9. Note that the same components as those of the optical modulator 5 of Example 1 are assigned the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. FIG. 17 is a schematic cross-sectional view showing examples of the cross-sectional portions along lines AA, BB, and DD shown in FIG. 16. The cross-sectional portion along line AA corresponds to the portions of the two first optical waveguides 13A on the outgoing path. The cross-sectional portion along line BB corresponds to the portions of the branching section 12, the multiplexing section 15, and the second optical waveguide 16. The cross-sectional portion along line DD corresponds to the portions of the input section 11A, the first optical waveguide 13F on the incoming path, and the output section 17A.

[0108] The input section 11A, which is the chip end face of the optical modulator 5H, is externally connected to the optical fiber 4A via a lens or the like. The output section 17A, which is the chip end face, is externally connected to the optical fiber 2A via a lens or the like. The chip end face of the optical modulator 5H tends to have better coupling efficiency with the optical fibers 4A and 2A as the optical field distribution becomes larger. Therefore, because the optical field distribution depends on the cross-sectional area of ​​the core, the width and thickness of the core of the input section 11A and the output section 17A are increased to widen the optical field distribution.

[0109] The core thickness of the two first optical waveguides 13A on the outgoing path is Ha. The core thickness of the branching section 12, the multiplexing section 15, and the second optical waveguide 16 is Hb. The core thickness of the input section 11A, the first optical waveguide 13F on the return path, and the output section 17A is Ha2. The core thicknesses satisfy the relationship Ha2>Ha>Hb.

[0110] In the optical modulator 5H of Example 9, the cores of the input section 11A and the output section 17A are thicker than the core of the first optical waveguide 13A on the outgoing path side, which results in improved coupling efficiency between the input section 11A and the optical fiber 4A and between the output section 17A and the optical fiber 2A.

[0111] In the optical modulator 5H, the core of the first optical waveguide 13F on the return path is also thicker than the core of the first optical waveguide 13A on the outgoing path. As a result, the output section 17A and the first optical waveguide 13F on the return path are joined with the same core thickness and width, thereby improving the coupling efficiency. [Explanation of symbols]

[0112] 1 Optical communication equipment 3 DSP 4 light source 5,5A,5B,5C,5D,5E,5F,5G,5H optical modulator 11 Input section 12 Branch 13 First optical waveguide 13A: First optical waveguide on the outgoing path 13B First optical waveguide on the return path side 13C First optical waveguide on the return path side 14 Electrode part 15 Multiplexing section 16 Second optical waveguide 17 Output section 33 Thin film LN layer

Claims

1. an X-cut or Y-cut thin-film LN (Lithium Niobate) layer; a first optical waveguide formed in the thin film LN layer along a direction substantially perpendicular to the Z direction of the crystal axis of the thin film LN layer; a second optical waveguide that is connected to the first optical waveguide; At least a portion of the core of the second optical waveguide is An optical device, comprising: a first optical waveguide having a thickness smaller than that of a core thereof, the first optical waveguide being thinner than the core of the first optical waveguide so as to suppress crosstalk of a TM mode with respect to a TE mode.

2. The second optical waveguide is 2. The optical device according to claim 1, further comprising a curved waveguide formed in the thin-film LN layer along a direction other than a direction substantially perpendicular to the Z direction, the core of the curved waveguide being thinner than the core of the first optical waveguide so as to suppress crosstalk of the TM mode with respect to the TE mode.

3. At least a portion of the first optical waveguide is A ridge-type waveguide, At least a portion of the second optical waveguide is 3. The optical device according to claim 1, which is a channel waveguide.

4. A core of a joint portion that joins the first optical waveguide and the second optical waveguide is 4. The optical device according to claim 1, wherein the first optical waveguide has a tapered shape that is gradually thinner from a core of the first optical waveguide toward a core of the second optical waveguide.

5. An input unit for inputting light; A branching unit that branches the light from the input unit; two first optical waveguides on an outgoing path side, the first optical waveguide being branched at the branching portion; an electrode for applying an electric signal to the two first optical waveguides on the outgoing path side; a multiplexing section that multiplexes the light modulated in the two first optical waveguides on the outgoing path side in response to the electrical signal; the second optical waveguide through which the modulated light from the multiplexing section passes; a return-path first optical waveguide, which is the first optical waveguide and through which modulated light from the second optical waveguide passes; an output section that outputs modulated light from the first optical waveguide on the return path side, At least a portion of the core of the second optical waveguide is The optical device according to any one of claims 1 to 4, characterized in that the first optical waveguide is thinner than the cores of the two first optical waveguides on the outgoing path side and the first optical waveguide on the return path side so as to suppress crosstalk of the TM mode with respect to the TE mode.

6. The input and output cores include The second optical waveguide is thicker than the core of the second optical waveguide. The thickness of the core of the branching section and the multiplexing section is 6. The optical device according to claim 5, wherein the core is the same as that of the second optical waveguide.

7. A core in a first coupler in the branching section and a core in a second coupler in the multiplexing section are 6. The optical device according to claim 5, wherein the first optical waveguide has a thickness smaller than a core thickness, and the second optical waveguide has a width larger than a core width.

8. The input and output cores include 8. The optical device according to claim 5, wherein the core of the first optical waveguide is thicker than the cores of the two first optical waveguides on the outgoing path side.

9. An input unit for inputting light; A branching unit that branches the light from the input unit; two first optical waveguides on the outgoing path side, which are the first optical waveguides branched at the branching portion; an electrode for applying an electric signal to the two first optical waveguides on the outgoing path side; the second optical waveguide connected to the two first optical waveguides on the outgoing path side and through which light modulated by the two first optical waveguides on the outgoing path side in response to the electrical signal passes; two first optical waveguides on the return path side, which are the first optical waveguides and are connected to the second optical waveguide; a multiplexing section that multiplexes modulated light passing through the two first optical waveguides on the return path side; an output section that outputs the modulated light multiplexed in the multiplexing section, At least a portion of the core of the second optical waveguide is The optical device according to any one of claims 1 to 4, characterized in that the thickness of the first optical waveguide is made thinner than the cores of the two first optical waveguides on the outgoing path side and the two first optical waveguides on the return path side so as to suppress crosstalk of the TM mode with respect to the TE mode.

10. An input unit for inputting light; A branching unit that branches the light from the input unit; two first optical waveguides on the outgoing path side, which are the first optical waveguides branched at the branching portion; the second optical waveguide connected to the two first optical waveguides on the outgoing path side; two first optical waveguides on the return path side, which are the first optical waveguides and are connected to the second optical waveguide; an electrode for applying voltage to the two first optical waveguides on the outgoing path side and the two first optical waveguides on the return path side; a multiplexing section that multiplexes modulated light passing through the two first optical waveguides on the return path side; an output section that outputs the modulated light multiplexed in the multiplexing section, At least a portion of the core of the second optical waveguide is The optical device according to any one of claims 1 to 4, characterized in that the thickness of the first optical waveguide is made thinner than the cores of the two first optical waveguides on the outgoing path side and the two first optical waveguides on the return path side so as to suppress crosstalk of the TM mode with respect to the TE mode.

11. An input unit for inputting light; A branching unit that branches the light from the input unit; two first optical waveguides on the outgoing path side, which are the first optical waveguides branched at the branching portion; a first electrode for applying a first electric signal to the two first optical waveguides on the outgoing path side; the second optical waveguide connected to the two first optical waveguides on the outgoing path side and through which light modulated by the two first optical waveguides on the outgoing path side in response to the first electrical signal passes; two first optical waveguides on the return path side, which are the first optical waveguides and are connected to the second optical waveguide; a second electrode for applying a second electric signal to the two first optical waveguides on the return path side; a multiplexing section that is connected to the two first optical waveguides on the return path side and multiplexes modulated light passing through the two first optical waveguides on the return path side in response to the second electrical signal; an output section that outputs the modulated light multiplexed in the multiplexing section, At least a portion of the core of the second optical waveguide is The optical device according to any one of claims 1 to 4, characterized in that the thickness of the first optical waveguide is made thinner than the cores of the two first optical waveguides on the outgoing path side and the two first optical waveguides on the return path side so as to suppress crosstalk of the TM mode with respect to the TE mode.

12. An input unit for inputting light; a first branching unit that branches light from the input unit; a second branching unit that branches the light branched by the first branching unit; two first optical waveguides on the forward path side of the first optical waveguide branched at the second branching portion; a first electrode for applying a first electrical signal to the two first optical waveguides on the forward path side of the front stage; a first optical waveguide which is connected to the two first optical waveguides on the forward path side of the first stage and through which light modulated by the two first optical waveguides on the forward path side of the first stage in response to the first electrical signal passes; two first optical waveguides on the return path side of the first stage, which are the first optical waveguides and are connected to the second optical waveguide of the first stage; a second electrode for applying a second electrical signal to the two first optical waveguides on the return path side of the previous stage; a first multiplexing unit that is connected to the two first optical waveguides on the return path side of the previous stage and multiplexes modulated light passing through the light modulated by the two first optical waveguides on the return path side of the previous stage in response to the second electrical signal; a middle second optical waveguide that is connected to the first multiplexing section and through which light from the first multiplexing section passes; two first optical waveguides on the outward path side of a rear stage, which are the first optical waveguides and are connected to the second optical waveguides in the middle stage; a third electrode for applying a third electric signal to the two first optical waveguides on the outgoing path side of the rear stage; a second multiplexing unit that is connected to the two first optical waveguides on the outward path side of the rear stage and multiplexes modulated light passing through the two first optical waveguides on the outward path side of the rear stage in response to the third electrical signal; a second optical waveguide at a rear stage, which is connected to the second multiplexing section and through which light from the second multiplexing section passes, the second optical waveguide being the second optical waveguide; a first optical waveguide on a return path side of a rear stage, which is the first optical waveguide and is connected to the second optical waveguide of the rear stage; an output section connected to the first optical waveguide on the return path side of the rear stage and outputting light from the first optical waveguide on the return path side of the rear stage, At least a part of the cores of the front-stage second optical waveguide, the middle-stage second optical waveguide, and the rear-stage second optical waveguide The optical device according to any one of claims 1 to 4, characterized in that the thickness is made thinner than the cores of the two first optical waveguides on the forward path side of the front stage, the two first optical waveguides on the return path side of the front stage, the two first optical waveguides on the forward path side of the rear stage, and the first optical waveguide on the return path side of the rear stage, so as to suppress crosstalk of the TM mode with respect to the TE mode.

13. A processor that performs signal processing on the electrical signal; A light source that generates light; an optical device that modulates light generated from the light source using an electrical signal output from the processor; The optical device comprises: an X-cut or Y-cut thin-film LN (Lithium Niobate) layer; a first optical waveguide formed in the thin film LN layer along a direction substantially perpendicular to the Z direction of the crystal axis of the thin film LN layer; a second optical waveguide that is connected to the first optical waveguide; At least a portion of the core of the second optical waveguide is 13. An optical communication device, comprising: a first optical waveguide having a thickness smaller than that of a core thereof so as to suppress crosstalk of a TM mode relative to a TE mode.

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