Optical device, optical transmitter, and optical transceiver

Abstract

To provide an optical device and the like for improving modulation efficiency.SOLUTION: An optical device comprises: a substrate; an optical waveguide provided on the substrate; and a first chip and a second chip, each of which is mounted on the substrate, includes material having an electro-optical effect higher than that of the substrate, and includes a crystal axis in which the strongest electro-optical effect is exerted. The optical waveguide includes: a first optical waveguide of a folded structure, which connects an input side first waveguide and an output side first waveguide; and a second optical waveguide of the folded structure, which connects an input side second waveguide and an output side second waveguide. The first chip includes a first electrode, which is arranged in the vicinity of the input side first waveguide, and which applies an electric field in the same direction as an orientation of the crystal axis of the first chip to the input side first waveguide. The second chip includes a second electrode, which is arranged in the vicinity of the output side first waveguide, and which applies an electric field in the same direction as an orientation of the crystal axis of the second chip to the output side first waveguide.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical device, an optical transmitter, and an optical transceiver. [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. The optical waveguide of the optical modulator then 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. 10 is a plan view showing an example of a conventional optical modulator 100, and FIG. 11 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA in FIG. 10. The optical modulator 100 shown in FIGS. 10 and 11 includes a substrate 101, a lower clad 102 laminated on the substrate 101, and an electro-optic crystal layer 103 having an electro-optic effect, such as an LN (LiNbO) material, laminated on the lower clad 102. The optical modulator 100 further includes a pair of waveguides 104 formed by the electro-optic crystal layer 103 and a pair of ground electrodes 105 formed on the electro-optic crystal layer 103. The optical modulator 100 further includes a signal electrode 106 formed on the electro-optic crystal layer 103 and sandwiched between the pair of ground electrodes 105. The pair of ground electrodes 105 and the signal electrode 106 form a coplanar structure.

[0004] Substrate 101 is made of a material such as Si (Silicon) or LN. Lower cladding 102 is a layer of, for example, SiO2, which has a lower optical refractive index than LN. Electro-optic crystal layer 103 is a thin-film substrate that has strong optical confinement and is advantageous for miniaturization.

[0005] The optical waveguide 104 is formed of an electro-optic crystal layer 103, and therefore has excellent insertion loss and transmission characteristics. The electro-optic crystal layer 103 is an X-cut substrate, and its structural symmetry enables chirp-free operation, making it suitable for long-distance transmission. The optical waveguide 104 includes one waveguide 104A, another waveguide 104B, a first coupler 104C, and a second coupler 104D. The first coupler 104C is connected to the input waveguide and splits and outputs the signal light from the input waveguide to one waveguide 104A and the other waveguide 104B. The second coupler 104D is connected to the output waveguide and splits and outputs the signal light from one waveguide 104A and the other waveguide 104B.

[0006] The ground electrode 105 has one ground electrode 105A and the other ground electrode 105B. The one waveguide 104A is disposed between the one ground electrode 105A and the signal electrode 106. Furthermore, the other waveguide 104B is disposed between the other ground electrode 105B and the signal electrode 106.

[0007] The direction Z11 of the crystal axis of electro-optic crystal layer 103 is the width direction (Z direction) perpendicular to the light propagation direction (Y direction). The optical refractive index of one waveguide 104A changes in response to the electric field in the electric field direction a11 from signal electrode 106 to one ground electrode 105A. Furthermore, the optical refractive index of the other waveguide 104B changes in response to the electric field in the electric field direction b11 from signal electrode 106 to the other ground electrode 105B.

[0008] The modulation efficiency of the optical modulator 100 is greatly affected by the length of the interaction section, such as the waveguide 104A on one side and the waveguide 104B on the other side, to which the electric field is applied. Therefore, in order to achieve miniaturization while maintaining the modulation efficiency, a structure in which the interaction section is folded back is required.

[0009] Fig. 12 is a schematic plan view showing an example of the configuration of a conventional optical modulator 100A, and Fig. 13 is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line AA shown in Fig. 12. The optical modulator 100A shown in Fig. 12 has an interaction section 110A on the outgoing path side, an interaction section 110B on the returning path side, and a folded waveguide 130 that optically connects a pair of waveguides 114 in the interaction section 110A on the outgoing path side to a pair of waveguides 114 in the interaction section 110B on the returning path side.

[0010] The optical modulator 100A has a substrate 111, a lower clad 112 laminated on the substrate 111, and an electro-optic crystal layer 113 having an electro-optic effect, such as an LN material, laminated on the lower clad 112. The optical modulator 100A further has a pair of waveguides 114 formed by the electro-optic crystal layer 113. The optical modulator 100A has a pair of ground electrodes 115 formed on the electro-optic crystal layer 113, and a signal electrode 116 formed on the electro-optic crystal layer 113 and arranged so as to be sandwiched between the pair of ground electrodes 115.

[0011] Substrate 111 is made of a material such as Si (Silicon) or LN. Lower cladding 112 is a layer of, for example, SiO2, which has a lower optical refractive index than LN. Electro-optic crystal layer 113 is a thin-film substrate that has strong optical confinement and is advantageous for miniaturization.

[0012] The optical waveguide 114 is formed of an electro-optic crystal layer 113, and therefore has excellent insertion loss and transmission characteristics. Because the electro-optic crystal layer 113 is an X-cut substrate, chirp-free operation is possible due to its structural symmetry, making it suitable for long-distance transmission. The optical waveguide 114 includes an outgoing waveguide 114A1, an outgoing waveguide 114B1, an incoming waveguide 114A2, and an incoming waveguide 114B2. The optical waveguide 114 further includes a first coupler 114C and a second coupler 114D. The first coupler 114C is connected to the input waveguide and splits and outputs the signal light from the input waveguide to the outgoing waveguide 114A1 and the outgoing waveguide 114B1. The second coupler 114D is a coupler that is connected to the output waveguide and multiplexes the signal light from the eleventh waveguide 114A2 on the return path side and the twelfth waveguide 114B2 on the return path side, and outputs the multiplexed signal light.

[0013] The outgoing side interaction section 110A has an outgoing side eleventh waveguide 114A1, an outgoing side twelfth waveguide 114B1, an outgoing side ground electrode 115A, an outgoing side signal electrode 116A, and a common ground electrode 115C. The outgoing side ground electrode 115A is a ground electrode arranged in parallel with the outgoing side eleventh waveguide 114A1. The common ground electrode 115C is a ground electrode arranged in parallel with the outgoing side twelfth waveguide 114B1. The outgoing side signal electrode 116A is a signal electrode arranged in parallel between the outgoing side eleventh waveguide 114A1 and the outgoing side twelfth waveguide 114B1.

[0014] The homeward interaction section 110B has a homeward eleventh waveguide 114A2, a homeward twelfth waveguide 114B2, a homeward ground electrode 115B, and a homeward signal electrode 116B. The homeward ground electrode 115B is a ground electrode arranged in parallel with the homeward eleventh waveguide 114A2. The common ground electrode 115C is a ground electrode arranged in parallel with the homeward twelfth waveguide 114B2. The homeward signal electrode 116B is a signal electrode arranged in parallel between the homeward eleventh waveguide 114A2 and the homeward twelfth waveguide 114B2.

[0015] One waveguide 114 has an eleventh waveguide 114A1 on the outgoing side and an eleventh waveguide 114A2 on the return side, and the eleventh waveguide 114A1 on the outgoing side and the eleventh waveguide 114A2 on the return side are optically connected by one folded waveguide 130A. The other waveguide 114 has a twelfth waveguide 114B1 on the outgoing side and a twelfth waveguide 114B2 on the return side, and the twelfth waveguide 114B1 on the outgoing side and the twelfth waveguide 114B2 on the return side are optically connected by the other folded waveguide 130B.

[0016] The direction Z21 of the crystal axis of electro-optic crystal layer 113 is the width direction (Z direction) perpendicular to the light propagation direction (Y direction). The optical refractive index of outgoing eleventh waveguide 114A1 changes in response to the electric field in electric field direction a21 from outgoing signal electrode 116A to outgoing ground electrode 115A. Furthermore, the optical refractive index of outgoing twelfth waveguide 114B1 changes in response to the electric field in electric field direction b21 from outgoing signal electrode 116A to common ground electrode 115C.

[0017] The optical refractive index of the eleventh waveguide 114A2 on the return path changes in response to the electric field in the electric field direction a22 from the signal electrode 116B on the return path to the ground electrode 115B on the return path. Furthermore, the optical refractive index of the twelfth waveguide 114B2 on the return path changes in response to the electric field in the electric field direction b22 from the signal electrode 116B on the return path to the common ground electrode 115C. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] US Patent Application Publication No. 2023 / 0107837 [Patent Document 2] Japanese Patent Publication No. 2022-73196 Summary of the Invention [Problem to be solved by the invention]

[0019] However, in the optical modulator 100A with a folded structure, the electric field direction a21 of the eleventh waveguide 114A1 on the outgoing side is the same as the crystal direction (Z1 direction) of the LN crystal, while the electric field direction a22 of the eleventh waveguide 114A2 on the homeward side is different from the crystal direction (Z1 direction) of the LN crystal. In other words, the electric field direction a21 of the eleventh waveguide 114A1 on the outgoing side is opposite to the electric field direction a22 of the eleventh waveguide 114A2 on the homeward side. Therefore, the electric field of the electric field direction a21 of the eleventh waveguide 114A1 on the outgoing side is canceled out by the electric field of the eleventh waveguide 114A2 on the homeward side in the electric field direction a22, resulting in a decrease in modulation efficiency.

[0020] Similarly, the electric field direction b21 of the twelfth waveguide 114B1 on the outbound side is the same as the crystal direction (Z1 direction) of the LN crystal, while the electric field direction b22 of the twelfth waveguide 114B2 on the inbound side is different from the crystal direction (Z1 direction) of the LN crystal. In other words, the electric field direction b21 of the twelfth waveguide 114B1 on the outbound side is opposite to the electric field direction b22 of the twelfth waveguide 114B2 on the inbound side. Therefore, the electric field in the electric field direction b21 of the twelfth waveguide 114B1 on the outbound side is canceled out by the electric field in the electric field direction b22 of the twelfth waveguide 114B2 on the inbound side, resulting in a decrease in modulation efficiency.

[0021] In the optical modulator 100A, which is an X-cut LN modulator with a single signal electrode and single-ended drive, the crystal axis is reversed with respect to the direction of light propagation (Y direction) between the forward and return paths. As a result, a phase change occurs in the opposite direction, and the phase change in the forward path is canceled out by the phase change in the return path, reducing modulation efficiency.

[0022] Another possible method is to switch the left and right positions of the optical waveguides relative to the direction of travel between the outbound and return paths, but a crossing waveguide or a reflective structure using an external mirror to switch the optical waveguides would cause reflection and attenuation of the optical signal.

[0023] Another possible method is to swap the left-right positions of the signal electrodes and ground electrodes relative to the direction of travel between the outbound and return paths, but this would require a significant change in the design of the signal electrodes between the outbound and return paths, and could lead to signal reflections and mismatches between the electrical and optical speeds.

[0024] The disclosed technology has been made in view of the above points, and aims to provide an optical device or the like that can improve modulation efficiency. [Means for solving the problem]

[0025] In one embodiment, the optical device disclosed herein includes a substrate, an optical waveguide provided on the substrate, and first and second chips mounted on the substrate, each chip containing a material with a stronger electro-optic effect than the substrate and having a crystal axis along which the electro-optic effect is most strongly expressed. The optical waveguide includes a first coupler, an input-side first waveguide and an input-side second waveguide, each connected to the first coupler, a second coupler, and an output-side first waveguide and an output-side second waveguide, each connected to the second coupler. The optical waveguide further includes a first bent waveguide connecting the input-side first waveguide and the output-side first waveguide, and a second bent waveguide connecting the input-side second waveguide and the output-side second waveguide. The first chip is arranged near the first input waveguide and has a first electrode that applies an electric field to the first input waveguide in the same direction as the crystal axis of the first chip, and the second chip is arranged near the first output waveguide and has a second electrode that applies an electric field to the first output waveguide in the same direction as the crystal axis of the second chip. [Effects of the Invention]

[0026] According to one aspect of the optical device disclosed in the present application, modulation efficiency can be improved. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a schematic plan view showing an example of an optical modulator according to a first embodiment. [Figure 2A]FIG. 2A is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA shown in FIG. [Figure 2B] FIG. 2B is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line BB shown in FIG. [Figure 3A] FIG. 3A is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line CC shown in FIG. [Figure 3B] 3B is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line DD shown in FIG. [Figure 4] FIG. 4 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line EE shown in FIG. [Figure 5] FIG. 5 is a schematic plan view illustrating an example of an optical modulator according to the second embodiment. [Figure 6A] FIG. 6A is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA shown in FIG. [Figure 6B] FIG. 6B is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line BB shown in FIG. [Figure 6C] FIG. 6C is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line CC shown in FIG. [Figure 7A] FIG. 7A is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line DD shown in FIG. [Figure 7B] FIG. 7B is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line EE shown in FIG. [Figure 8] FIG. 8 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line FF shown in FIG. [Figure 9] FIG. 9 is an explanatory diagram illustrating an example of an optical transceiver according to this embodiment. [Figure 10] FIG. 10 is a schematic plan view showing an example of a conventional optical modulator. [Figure 11] FIG. 11 is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line AA shown in FIG. [Figure 12] FIG. 12 is a schematic plan view showing an example of a conventional optical modulator. [Figure 13] FIG. 13 is a schematic cross-sectional view showing an example of a cross-sectional portion taken along line AA shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0028] 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]

[0029] Fig. 1 is a schematic plan view showing an example of an optical modulator 1 according to a first embodiment. The optical modulator 1 shown in Fig. 1 includes a chip 2, and a first chip 3A and a second chip 3B mounted on the chip 2. The chip 2 is, for example, a silicon photonics (SiPh) chip, and includes an outgoing path section 2A, a returning path section 2B, and a turning section 4 that optically connects the outgoing path section 2A and the returning path section 2B via the first chip 3A and the second chip 3B.

[0030] The chip 2 has a substrate 11 made of, for example, Si, an optical waveguide 14 provided on the substrate 11, a pair of ground electrodes 15, and a signal electrode 16 sandwiched between the pair of ground electrodes 15. The substrate 11 is exemplified by a Si substrate, but may be made of at least one material selected from the group consisting of SiO2 (silicon dioxide), TiO2 (titanium dioxide), QTZ, and sapphire, and may be modified as appropriate. The pair of ground electrodes 15 and signal electrode 16 constitute a coplanar GSG electrode made of, for example, aluminum (Al), or the like.

[0031] The outgoing section 2A includes a first outgoing waveguide 14A1, a second outgoing waveguide 14B1, a pair of first ground electrodes 15A, and a first signal electrode 16A. The first outgoing waveguide 14A1 and the second outgoing waveguide 14B1 are, for example, Si waveguides. The first outgoing waveguide 14A1 and the second outgoing waveguide 14B1 are not limited to Si waveguides and may be made of materials such as SiN and LN, and may be modified as appropriate. The first ground electrode 15A and the first signal electrode 16A are, for example, Au electrodes. The first ground electrode 15A and the first signal electrode 16A are not limited to Au electrodes and may be made of materials such as Al and Cu, and may be modified as appropriate.

[0032] The first ground electrode 15A has one first ground electrode 15A1 and the other first ground electrode 15A2. The first ground electrode 15A1 is a ground electrode arranged in parallel to the outgoing first waveguide 14A1. The first ground electrode 15A2 is a ground electrode arranged in parallel to the outgoing second waveguide 14B1. The first signal electrode 16A is a signal electrode arranged in parallel between the outgoing first waveguide 14A1 and the outgoing second waveguide 14B1.

[0033] The return path section 2B includes a return-side first waveguide 14A2, a return-side second waveguide 14B2, a pair of second ground electrodes 15B, and a second signal electrode 16B. The return-side first waveguide 14A2 and the return-side second waveguide 14B2 are optical waveguides made of, for example, Si. Note that the return-side first waveguide 14A2 and the return-side second waveguide 14B2 are not limited to Si waveguides and may be made of materials such as SiN and LN, and may be modified as appropriate. The second ground electrode 15B and the second signal electrode 16B are, for example, Au electrodes. Note that the second ground electrode 15B and the second signal electrode 16B are not limited to Au electrodes and may be made of, for example, Al and Cu, and may be modified as appropriate.

[0034] The second ground electrode 15B has one second ground electrode 15B1 and the other second ground electrode 15B2. The second ground electrode 15B1 is a ground electrode arranged in parallel to the first waveguide 14A2 on the return path side. The second ground electrode 15B2 is a ground electrode arranged in parallel to the second waveguide 14B2 on the return path side. The second signal electrode 16B is a signal electrode arranged in parallel between the first waveguide 14A2 on the return path side and the second waveguide 14B2 on the return path side.

[0035] The optical waveguide 14 includes a first coupler 14C and an outgoing first waveguide 14A1 and an outgoing second waveguide 14B1, each connected to the first coupler 14C. The optical waveguide 14 also includes a second coupler 14D and a return-side first waveguide 14A2 and a return-side second waveguide 14B2, each connected to the second coupler 14D. The first coupler 14C is connected to an input waveguide and splits and outputs the signal light from the input waveguide to the outgoing first waveguide 14A1 and the outgoing second waveguide 14B1. The second coupler 14D is a coupler that is connected to the output waveguide and multiplexes the signal light from the first waveguide 14A2 on the return path side and the second waveguide 14B2 on the return path side, and outputs the multiplexed signal light.

[0036] The folded section 4 has a folded waveguide 51 and a folded electrode 52. The folded waveguide 51 has a first folded waveguide 51A that optically connects the first waveguide 14A1 on the outgoing side and the first waveguide 14A2 on the return side, and a second folded waveguide 51B that optically connects the second waveguide 14B1 on the outgoing side and the second waveguide 14B2 on the return side. The first folded waveguide 51A is a waveguide in which the traveling direction of light guided through the first waveguide 14A1 on the outgoing side changes by 180 degrees from the traveling direction of light guided through the first waveguide 14A2 on the return side. The second folded waveguide 51B is a waveguide in which the traveling direction of light guided through the second waveguide 14B1 on the outgoing path changes by 180 degrees from the traveling direction of light guided through the second waveguide 14B2 on the return path. For convenience of explanation, the folded section 4 has been illustrated as having a folded structure in which the traveling direction of light changes by 180 degrees, but any structure in which the traveling direction of light changes by at least 90 degrees or more may be used, and this can be changed as appropriate.

[0037] The folded electrode 52 includes a first folded ground electrode 52A1, a second folded ground electrode 52B1, and a folded signal electrode 53. The first folded ground electrode 52A1 is a ground electrode arranged in parallel with the first folded waveguide 51A. The second folded ground electrode 52B1 is a ground electrode arranged in parallel with the second folded waveguide 51B. The folded signal electrode 53 is a signal electrode arranged in parallel between the first folded waveguide 51A and the second folded waveguide 51B.

[0038] The first chip 3A and the second chip 3B are mounted on the substrate 11 and are made of a material having a higher electro-optic effect than the substrate 11, such as LN (LiNbO), and have a crystal axis that maximizes the electro-optic effect. The material having the electro-optic effect may be, for example, a material with a γ33 of 25 pm / V or greater, and can be changed as appropriate. The first chip 3A and the second chip 3B are, for example, X-cut thin-film LN. The first chip 3A and the second chip 3B are mounted on the substrate 11 using a thin-film transfer technique, such as microtransfer printing (μ-TP) technology. When μ-TP technology is used, the width of the first chip 3A and the second chip 3B in the substrate plane direction and in the direction perpendicular to the electrode lines is, for example, 300 μm or less.

[0039] The first chip 3A is mounted on the substrate 11 so that the direction Z1 of the crystal axis of the first chip 3A is perpendicular to the direction of propagation of light guided through the outgoing first waveguide 14A1 and the outgoing second waveguide 14B1. The first chip 3A is disposed near the outgoing fifth waveguide 34A1 of the outgoing first waveguide 14A1 and has a first electrode that applies an electric field to the outgoing fifth waveguide 34A1 in the same direction as the direction Z1 of the crystal axis of the first chip 3A. The first chip 3A is disposed near the outgoing sixth waveguide 34B1 of the outgoing second waveguide 14B1 and has a first electrode that applies an electric field to the outgoing sixth waveguide 34B1 in the opposite direction to the direction Z1 of the crystal axis of the first chip 3A.

[0040] The second chip 3B is mounted on the substrate 11 so that the direction Z2 of the crystal axis of the second chip 3B is perpendicular to the direction of propagation of light guided through the first waveguide 14A2 on the return path and the second waveguide 14B2 on the return path. The second chip 3B is disposed near the fifth waveguide 34A2 on the return path of the first waveguide 14A2 on the return path and has a second electrode that applies an electric field to the fifth waveguide 34A2 on the return path in the same direction as the direction Z2 of the crystal axis of the second chip 3B. The second chip 3B is disposed near the sixth waveguide 34B2 on the return path of the second waveguide 14B2 on the return path and has a second electrode that applies an electric field to the sixth waveguide 34B2 on the return path in the opposite direction to the direction Z2 of the crystal axis of the second chip 3B. That is, the first chip 3A and the second chip 3B are mounted on the substrate 11 so as not to share the Z-axis direction.

[0041] The first waveguide 14A1 on the outgoing path has a third waveguide 24A1 on the outgoing path and a fifth waveguide 34A1 on the outgoing path, which are provided on the substrate 11 of the chip 2. The first waveguide 14A2 on the return path has a third waveguide 24A2 on the return path and a fifth waveguide 34A2 on the return path, which are provided on the substrate 11 of the chip 2.

[0042] The first chip 3A and the second chip 3B are mounted on the substrate 11 of the chip 2 so that the direction Z1 of the crystal axis of the first chip 3A differs by 180 degrees from the direction Z2 of the crystal axis of the second chip 3B. In addition, the first chip 3A and the second chip 3B are mounted on the substrate 11 so that the direction a1 of the electric field applied from the first electrode to the fifth waveguide 34A1 on the outgoing path side differs by 180 degrees from the direction a2 of the electric field applied from the second electrode to the fifth waveguide 34A2 on the return path side.

[0043] The first electrode in the first chip 3A is a GSG coplanar electrode having a first signal electrode 36A, one first ground electrode 35A1, and the other first ground electrode 35A2. The first signal electrode 36A is arranged in parallel between the outgoing fifth waveguide 34A1 and the outgoing sixth waveguide 34B1. The first ground electrode 35A1 is arranged in parallel with the outgoing fifth waveguide 34A1. The first ground electrode 35A2 is arranged in parallel with the outgoing sixth waveguide 34B1.

[0044] The second electrode in the second chip 3B is an electrode with a GSG coplanar structure including a second signal electrode 36B, one second ground electrode 35B1, and the other second ground electrode 35B2. The second signal electrode 36B is arranged in parallel between the fifth waveguide 34A2 on the return path and the sixth waveguide 34B2 on the return path. The second ground electrode 35B1 is arranged in parallel with the fifth waveguide 34A2 on the return path. The second ground electrode 35B2 is arranged in parallel with the sixth waveguide 34B2 on the return path.

[0045] 2A is a schematic cross-sectional view showing an example of the cross section taken along line AA in FIG. 1. The cross section taken along line AA is a cross-sectional view of the outgoing third waveguide 24A1 and the outgoing fourth waveguide 24B1 in the chip 2. The optical modulator 1 shown in FIG. 2A includes a substrate 11, a lower clad 12 stacked on the substrate 11, and the outgoing third waveguide 24A1 and the outgoing fourth waveguide 24B1 stacked on the lower clad 12. The optical modulator 1 further includes an upper clad 13 formed on the lower clad 12, the outgoing third waveguide 24A1, and the outgoing fourth waveguide 24B1, and a first electrode formed on the upper clad 13. The lower clad 12 and the upper clad 13 are formed of, for example, SiO2.

[0046] The first electrode in the chip 2 is a coplanar electrode having a first signal electrode 16A, one first ground electrode 15A1, and the other first ground electrode 15A2. The first signal electrode 16A is arranged in parallel between the outgoing third waveguide 24A1 and the outgoing fourth waveguide 24B1. The first ground electrode 15A1 is arranged in parallel to the outgoing third waveguide 24A1. The first ground electrode 15A2 is arranged in parallel to the outgoing fourth waveguide 24B1.

[0047] The first electrode has a connection portion 41A3 that electrically connects the first signal electrode 16A in the chip 2 and the first signal electrode 36A in the first chip 3A. The first electrode has a connection portion 41A1 that electrically connects the first ground electrode 15A1 in the chip 2 and the first ground electrode 35A1 in the first chip 3A. The first electrode has a connection portion 41A2 that electrically connects the first ground electrode 15A2 in the chip 2 and the first ground electrode 35A2 in the first chip 3A.

[0048] 2A, the description focuses on the portion of the first electrode of the chip 2, but the same applies to the portion of the second electrode of the chip 2. The optical modulator 1 has a third waveguide 24A2 on the return path and a fourth waveguide 24B2 on the return path stacked on the lower clad 12. The optical modulator 1 further has an upper clad 13 formed on the lower clad 12, the third waveguide 24A2 on the return path, and the fourth waveguide 24B2 on the return path, and a second electrode formed on the upper clad 13.

[0049] The second electrode in the chip 2 is a coplanar electrode having a second signal electrode 16B, one second ground electrode 15B1, and the other second ground electrode 15B2. The second signal electrode 16B is arranged in parallel between the third waveguide 24A2 on the return path and the fourth waveguide 24B2 on the return path. The second ground electrode 15B1 is arranged in parallel with the third waveguide 24A2 on the return path. The second ground electrode 15B2 is arranged in parallel with the fourth waveguide 24B2 on the return path.

[0050] The second electrode has a connection portion 41B3 that electrically connects the second signal electrode 16B in the chip 2 and the second signal electrode 36B in the second chip 3B. The second electrode has a connection portion 41B1 that electrically connects the second ground electrode 15B1 in the chip 2 and the second ground electrode 35B1 in the second chip 3B. The second electrode has a connection portion 41B2 that electrically connects the second ground electrode 15B2 in the chip 2 and the second ground electrode 35B2 in the second chip 3B.

[0051] 2B is a schematic cross-sectional view showing an example of the cross-sectional portion along line BB shown in FIG. 1. The cross-sectional portion along line BB is a cross-sectional portion of the outgoing-side fifth waveguide 34A1 and the outgoing-side sixth waveguide 34B1 on which the first chip 3A is mounted. The optical modulator 1 shown in FIG. 2B has a substrate 11, a lower clad 12 stacked on the substrate 11, an upper clad 13A stacked on the lower clad 12, and the first chip 3A mounted on the upper clad 13A. The upper clad 13A is a clad layer formed by etching the upper clad 13 shown in FIG. 2A or the like.

[0052] The first chip 3A is disposed in the opening 13A3 of the upper cladding 13A, and is disposed near the outgoing fifth waveguide 34A1 and the outgoing sixth waveguide 34B1 disposed on the lower cladding 12. The first chip 3A further has an electro-optic crystal layer 32A mounted on the upper cladding 13A, and first electrodes including a first signal electrode 36A, a first ground electrode 35A1, and a first ground electrode 35A2 formed on the electro-optic crystal layer 32A.

[0053] 2B, the first chip 3A has been focused on, but the same applies to the second chip 3B. The optical modulator 1 has the second chip 3B mounted on the upper cladding 13A.

[0054] The second chip 3B is disposed in the opening 13A3 of the upper cladding 13A, and is disposed near the fifth waveguide 34A2 on the return path side and the sixth waveguide 34B2 on the return path side disposed on the lower cladding 12. The second chip 3B further includes an electro-optic crystal layer 32B mounted on the upper cladding 13A, and second electrodes including a second signal electrode 36B and a second ground electrode 35B1 and a second ground electrode 35B2 formed on the electro-optic crystal layer 32B.

[0055] FIG. 3A is a schematic cross-sectional view showing an example of the cross-sectional area along line CC shown in FIG. 1. The cross-sectional area along line CC is a cross-sectional area of ​​the optical modulator 1 electrically connecting the first ground electrode 15A1 in the chip 2 and the first ground electrode 35A1 in the first chip 3A. The optical modulator 1 shown in FIG. 3A includes an upper clad 13 and an upper clad 13A stacked on the lower clad 12, and a first ground electrode 15A1 disposed on the upper clad 13. The optical modulator 1 also includes an electro-optic crystal layer 32A of the first chip 3A mounted on the upper clad 13A, and a first ground electrode 35A1 disposed on the electro-optic crystal layer 32A. The optical modulator 1 electrically connects the first ground electrode 15A1 disposed on the upper clad 13 in the chip 2 to the first ground electrode 35A1 disposed on the electro-optic crystal layer 32A in the first chip 3A via a connection portion 41A1.

[0056] 3A, the description focuses on the first chip 3A, but the second chip 3B is similar. The optical modulator 1 has an upper clad 13 and an upper clad 13A stacked on the lower clad 12, and a second ground electrode 15B1 arranged on the upper clad 13. The optical modulator 1 has an electro-optic crystal layer 32B of the second chip 3B mounted on the upper clad 13A, and a second ground electrode 35B1 arranged on the electro-optic crystal layer 32B. The optical modulator 1 electrically connects the second ground electrode 15B1 arranged on the upper clad 13 in the chip 2 to the second ground electrode 35B1 arranged on the electro-optic crystal layer 32B of the second chip 3B via a connection portion 41B1.

[0057] FIG. 3B is a schematic cross-sectional view showing an example of the cross-sectional portion of the DD line shown in FIG. 1. The cross-sectional portion of the DD line is a cross-sectional portion of the optical modulator 1 optically connecting the outgoing fourth waveguide 24B1 in the chip 2 and the outgoing sixth waveguide 34B1 on which the first chip 3A is mounted. The optical modulator 1 shown in FIG. 3B has the outgoing fourth waveguide 24B1 and the outgoing sixth waveguide 34B1 arranged on the lower cladding 12, and an upper cladding 13 covering the outgoing fourth waveguide 24B1. Furthermore, the optical modulator 1 has an electro-optic crystal layer 32A arranged on the outgoing sixth waveguide 34B1.

[0058] 3B, the description focuses on the first chip 3A, but the same applies to the second chip 3B. The optical modulator 1 has an outgoing-side fourth waveguide 24B2 and an outgoing-side sixth waveguide 34B2 arranged on the lower cladding 12, an upper cladding 13 covering the outgoing-side fourth waveguide 24B2, and an electro-optic crystal layer 32B arranged on the outgoing-side sixth waveguide 34B2.

[0059] 4 is a schematic cross-sectional view showing an example of the EE-line cross section shown in FIG. 1. The crystal axis direction Z1 of the electro-optic crystal layer 32A of the first chip 3A is the width direction (Z direction) perpendicular to the light propagation direction (Y direction). The optical refractive index of the fifth waveguide 34A1 on the outgoing path on which the first chip 3A is mounted changes depending on the electric field in the electric field direction a1 from the first signal electrode 36A to the first ground electrode 35A1. Furthermore, the optical refractive index of the sixth waveguide 34B1 on the outgoing path on which the first chip 3A is mounted changes depending on the electric field in the electric field direction b1 from the first signal electrode 36A to the first ground electrode 35A2.

[0060] The crystal axis direction Z2 of the electro-optic crystal layer 32B of the second chip 3B is the width direction (Z direction) perpendicular to the light propagation direction (Y direction). The crystal axis direction Z1 of the electro-optic crystal layer 32A of the first chip 3A and the crystal axis direction Z2 of the electro-optic crystal layer 32B of the second chip 3B are 180 degrees different from each other. The optical refractive index of the fifth waveguide 34A2 on the return path side on which the second chip 3B is mounted changes depending on the electric field in the electric field direction a2 from the second signal electrode 36B to the second ground electrode 35B1. Furthermore, the optical refractive index of the sixth waveguide 34B2 on the return path side on which the second chip 3B is mounted changes depending on the electric field in the electric field direction b2 from the second signal electrode 36B to the second ground electrode 35B2.

[0061] The electric field direction a1 of the fifth waveguide 34A1 on the outbound side on which the first chip 3A is mounted is the same as the crystal direction (Z1 direction) of the electro-optic crystal layer 32A of the first chip 3A. The electric field direction a2 of the fifth waveguide 34A2 on the return side on which the second chip 3B is mounted is the same as the crystal direction (Z2 direction) of the electro-optic crystal layer 32B of the second chip 3B. Therefore, the electric field in the electric field direction a1 of the fifth waveguide 34A1 on the outbound side and the electric field in the electric field direction a2 of the fifth waveguide 34A2 on the return side are the same direction. In other words, the electric field in the electric field direction a1 of the fifth waveguide 34A1 on the outbound side and the electric field in the electric field direction a2 of the fifth waveguide 34A2 on the return side are not canceled out, resulting in high modulation efficiency.

[0062] The electric field direction b1 of the sixth waveguide 34B1 on the outbound side on which the first chip 3A is mounted is opposite to the crystal direction (Z1 direction) of the electro-optic crystal layer 32A of the first chip 3A. The electric field direction b2 of the sixth waveguide 34B2 on the return side on which the second chip 3B is mounted is opposite to the crystal direction (Z2 direction) of the electro-optic crystal layer 32B of the second chip 3B. Therefore, the electric field in the electric field direction b1 of the sixth waveguide 34B1 on the outbound side and the electric field in the electric field direction b2 of the sixth waveguide 34B2 on the return side are in the same direction. In other words, the electric field in the electric field direction b1 of the sixth waveguide 34B1 on the outbound side and the electric field in the electric field direction b2 of the sixth waveguide 34B2 on the return side are not canceled out, resulting in high modulation efficiency.

[0063] The first chip 3A of the first embodiment is disposed near the fifth waveguide 34A1 on the outgoing path and has a first electrode that applies an electric field to the fifth waveguide 34A1 on the outgoing path in the same direction as the crystal axis of the first chip 3A. The second chip 3B is disposed near the fifth waveguide 34A2 on the return path and has a second electrode that applies an electric field to the fifth waveguide 34A2 on the return path in the same direction as the crystal axis of the second chip 3B. As a result, the electric field in the electric field direction a1 of the fifth waveguide 34A1 on the outgoing path and the electric field in the electric field direction a2 of the fifth waveguide 34A2 on the return path are not canceled out, resulting in high modulation efficiency.

[0064] The first chip 3A is mounted on the substrate 11 so that the crystal axis direction Z1 of the first chip 3A is perpendicular to the propagation direction of light guided through the outgoing fifth waveguide 34A1 and the outgoing sixth waveguide 34B1. The second chip 3B is mounted on the substrate 11 so that the crystal axis direction Z2 of the second chip 3B is perpendicular to the propagation direction of light guided through the return fifth waveguide 34A2 and the return sixth waveguide 34B2. As a result, the electric field in the electric field direction a1 of the outgoing fifth waveguide 34A1 and the electric field in the electric field direction a2 of the return fifth waveguide 34A2 are not canceled out, thereby increasing modulation efficiency. Furthermore, the electric field in the electric field direction b1 of the outgoing sixth waveguide 34B1 and the electric field in the electric field direction b2 of the return sixth waveguide 34B2 are not canceled out, thereby increasing modulation efficiency. That is, the first chip 3A and the second chip 3B are arranged so that the axis where the electro-optic effect is most pronounced is oriented in a direction substantially perpendicular to the direction of propagation of the high frequency signal, and further so that phase modulation is not canceled between the first chip 3A and the second chip 3B. Furthermore, the X-cut thin film LN optical modulator can be folded back within the optical modulation element without crossing the signal line or optical waveguide.

[0065] The first electrode is disposed near the sixth waveguide 34B1 on the outgoing side and applies an electric field to the sixth waveguide 34B1 on the outgoing side in a direction opposite to the direction of the crystal axis of the first chip 3A. The second electrode is disposed near the sixth waveguide 34B2 on the return side and applies an electric field to the sixth waveguide 34B2 on the return side in a direction opposite to the direction of the crystal axis of the second chip 3B. As a result, the electric field in the electric field direction b1 of the sixth waveguide 34B1 on the outgoing side and the electric field in the electric field direction b2 of the sixth waveguide 34B2 on the return side are not canceled out, thereby increasing modulation efficiency.

[0066] The first chip 3A and the second chip 3B are mounted on the substrate 11 so that the crystal axis direction Z1 of the first chip 3A is 180 degrees different from the crystal axis direction Z2 of the second chip 3B. As a result, the electric field in the electric field direction a1 of the fifth waveguide 34A1 on the outgoing path and the electric field in the electric field direction a2 of the fifth waveguide 34A2 on the return path are not canceled out, thereby increasing modulation efficiency. Furthermore, the electric field in the electric field direction b1 of the sixth waveguide 34B1 on the outgoing path and the electric field in the electric field direction b2 of the sixth waveguide 34B2 on the return path are not canceled out, thereby increasing modulation efficiency.

[0067] The first chip 3A and the second chip 3B are mounted on the substrate 11 so that the direction a1 of the electric field applied from the first electrode to the fifth waveguide 34A1 on the outward path is 180 degrees different from the direction a2 of the electric field applied from the second electrode to the fifth waveguide 34A1 on the return path. As a result, the electric field in the electric field direction a1 of the fifth waveguide 34A1 on the outward path and the electric field in the electric field direction a2 of the fifth waveguide 34A2 on the return path are not canceled out, thereby increasing modulation efficiency.

[0068] The first folded waveguide 51A is a folded waveguide in which the propagation direction of light guided through the fifth waveguide 34A1 on the outgoing side changes by 180 degrees from the propagation direction of light guided through the fifth waveguide 34A2 on the return side. The second folded waveguide 51B is a folded waveguide in which the propagation direction of light guided through the sixth waveguide 34B1 on the outgoing side changes by 180 degrees from the propagation direction of light guided through the sixth waveguide 34B2 on the return side. As a result, the optical modulator 1 can be made smaller.

[0069] The outgoing first waveguide 14A1 optically couples the outgoing third waveguide 24A1 provided on the substrate 11 with the outgoing fifth waveguide 34A1 on which the first chip 3A is mounted. The returning first waveguide 14A2 optically couples the returning third waveguide 24A2 provided on the substrate 11 with the returning fifth waveguide 34A2 on which the second chip 3B is mounted. The thin-film LN first chip 3A and second chip 3B can be mounted on the Si substrate 11.

[0070] Although the optical waveguide 14 provided on the substrate 11 of the chip 2 of the optical modulator 1 in Example 1 has been illustrated as having a channel structure, it is not limited to a channel structure and can be modified as appropriate. Therefore, an embodiment in which the optical waveguide 14 has a rib structure will be described below as Example 2. [Example]

[0071] 5 is a schematic plan view showing an example of an optical modulator 1A according to the second embodiment. The same components as those in the optical modulator 1 of the first embodiment are denoted by the same reference numerals, and explanations of the overlapping components and operations will be omitted. The optical modulator 1 of the first embodiment differs from the optical modulator 1A of the second embodiment in that the optical waveguides 14 in the first chip 3A1 and the second chip 3B1 have a rib structure.

[0072] The first chip 3A1 and the second chip 3B1 are mounted on the substrate 11 and are made of a material having a higher electro-optic effect than the substrate 11, such as LN (LiNbO), and have a crystal axis that maximizes the electro-optic effect. The material having the electro-optic effect may be, for example, a material with a γ33 of 25 pm / V or more, and can be changed as appropriate. The first chip 3A1 and the second chip 3B1 are, for example, X-cut thin-film LN. The first chip 3A1 and the second chip 3B1 are mounted on the substrate 11 using a thin-film transfer technique, such as microtransfer printing (μ-TP) technology. In other words, the first chip 3A1 and the second chip 3B1 are mounted on the substrate 11 so that they do not share the Z-axis direction.

[0073] The first chip 3A1 in the optical modulator 1A shown in Figure 5 has a seventh waveguide 37A1 on the outgoing side of the rib structure and an eighth waveguide 37B1 on the outgoing side, and the second chip 3B1 has a seventh waveguide 37A2 on the outgoing side of the rib structure and an eighth waveguide 37B2 on the outgoing side.

[0074] The outgoing-side first waveguide 14A1 includes an outgoing-side third waveguide 24A1, an outgoing-side first tapered waveguide 34A11, an outgoing-side seventh waveguide 37A1, and an outgoing-side first reverse tapered waveguide 34A12. The outgoing-side third waveguide 24A1 is a waveguide that optically connects the first coupler 14C and the outgoing-side first tapered waveguide 34A11. The outgoing-side third waveguide 24A1 is a waveguide made of, for example, Si or the like. Note that the outgoing-side third waveguide 24A1 is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0075] The first tapered waveguide 34A11 on the outgoing path is optically connected to the third waveguide 24A1 on the outgoing path, and is a tapered waveguide whose width gradually narrows toward the seventh waveguide 37A1 on the outgoing path. The first tapered waveguide 34A11 on the outgoing path is a waveguide made of, for example, Si. Note that the first tapered waveguide 34A11 on the outgoing path is not limited to a Si waveguide, and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0076] The first inverse tapered waveguide 34A12 on the outgoing path side is optically connected to the first folded waveguide 51A and is a tapered waveguide whose waveguide width gradually increases toward the first folded waveguide 51A. The first inverse tapered waveguide 34A12 on the outgoing path side is a waveguide made of, for example, Si. Note that the first inverse tapered waveguide 34A12 on the outgoing path side is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0077] The outgoing-side seventh waveguide 37A1 is a rib-structured waveguide in which the outgoing-side first tapered waveguide 34A11 is disposed below the input portion of the outgoing-side seventh waveguide 37A1, and optically connected to the outgoing-side first tapered waveguide 34A11 by indirect transition. The outgoing-side seventh waveguide 37A1 is, for example, an LN waveguide. The outgoing-side seventh waveguide 37A1 is in which the outgoing-side first reverse tapered waveguide 34A12 is disposed below the output portion of the outgoing-side seventh waveguide 37A1, and optically connected to the outgoing-side first reverse tapered waveguide 34A12 by indirect transition.

[0078] The outgoing-side second waveguide 14B1 includes an outgoing-side fourth waveguide 24B1, an outgoing-side second tapered waveguide 34B11, an outgoing-side eighth waveguide 37B1, and an outgoing-side second reverse tapered waveguide 34B12. The outgoing-side fourth waveguide 24B1 is a waveguide that optically connects the first coupler 14C and the outgoing-side second tapered waveguide 34B11. The outgoing-side fourth waveguide 24B1 is a waveguide made of, for example, Si. Note that the outgoing-side fourth waveguide 24B1 is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0079] The outgoing-side second tapered waveguide 34B11 is optically connected to the outgoing-side fourth waveguide 24B1 and is a tapered waveguide whose width gradually narrows toward the outgoing-side eighth waveguide 37B1. The outgoing-side second tapered waveguide 34B11 is a waveguide made of, for example, Si. Note that the outgoing-side second tapered waveguide 34B11 is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0080] The second inverse tapered waveguide 34B12 on the outgoing path side is optically connected to the second folded waveguide 51B and is a tapered waveguide whose waveguide width gradually increases toward the second folded waveguide 51B. The second inverse tapered waveguide 34B12 on the outgoing path side is a waveguide made of, for example, Si. Note that the second inverse tapered waveguide 34B12 on the outgoing path side is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0081] The outgoing-side eighth waveguide 37B1 is a rib-structured waveguide in which the outgoing-side second tapered waveguide 34B11 is disposed below the input portion of the outgoing-side eighth waveguide 37B1, and optically connected to the outgoing-side second tapered waveguide 34B11 by indirect transition. The outgoing-side eighth waveguide 37B1 is, for example, an LN waveguide. The outgoing-side eighth waveguide 37B1 is in which the outgoing-side second reverse tapered waveguide 34B12 is disposed below the output portion of the outgoing-side eighth waveguide 37B1, and optically connected to the outgoing-side second reverse tapered waveguide 34B12 by indirect transition.

[0082] The first waveguide 14A2 on the return path includes a first reverse tapered waveguide 34A13 on the return path, a seventh waveguide 37A2 on the return path, a first tapered waveguide 34A14 on the return path, and a third waveguide 24A2 on the return path. The first reverse tapered waveguide 34A13 on the return path is optically connected to the first folded waveguide 51A and is a tapered waveguide whose waveguide width gradually narrows toward the seventh waveguide 37A2 on the return path. The first reverse tapered waveguide 34A13 on the return path is a waveguide made of, for example, Si. Note that the first reverse tapered waveguide 34A13 on the return path is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0083] The first tapered waveguide 34A14 on the return path is optically connected to the third waveguide 24A2 on the return path, and is a tapered waveguide whose width gradually increases toward the seventh waveguide 37A2 on the return path. The first tapered waveguide 34A14 on the return path is a waveguide made of, for example, Si. Note that the first tapered waveguide 34A14 on the return path is not limited to a Si waveguide, and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0084] The third waveguide 24A2 on the return path is a waveguide that optically connects the second coupler 14D and the first tapered waveguide 34A14 on the return path. The third waveguide 24A2 on the return path is a waveguide made of, for example, Si. Note that the third waveguide 24A2 on the return path is not limited to a Si waveguide, and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0085] The seventh waveguide 37A2 on the return side is a waveguide with a rib structure in which the first reverse tapered waveguide 34A13 on the return side is disposed below the input portion of the seventh waveguide 37A2 on the return side, and optically connected to the first reverse tapered waveguide 34A13 on the return side by indirect transition. The seventh waveguide 37A2 on the return side is, for example, an LN waveguide. The first tapered waveguide 34A14 on the return side is disposed below the output portion of the seventh waveguide 37A2 on the return side, and optically connected to the first tapered waveguide 34A14 on the return side by indirect transition.

[0086] The second waveguide 14B2 on the return path includes a second reverse tapered waveguide 34B13 on the return path, an eighth waveguide 37B2 on the return path, a second tapered waveguide 34B14 on the return path, and a fourth waveguide 24B2 on the return path. The second reverse tapered waveguide 34B13 on the return path is optically connected to the second folded waveguide 51B and is a tapered waveguide whose waveguide width gradually narrows toward the eighth waveguide 37B2 on the return path. The second reverse tapered waveguide 34B13 on the return path is a waveguide made of, for example, Si. Note that the second reverse tapered waveguide 34B13 on the return path is not limited to a Si waveguide and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0087] The second tapered waveguide 34B14 on the return path is optically connected to the fourth waveguide 24B2 on the return path, and is a tapered waveguide whose width gradually increases toward the eighth waveguide 37B2 on the return path. The second tapered waveguide 34B14 on the return path is a waveguide made of, for example, Si. Note that the second tapered waveguide 34B14 on the return path is not limited to a Si waveguide, and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0088] The fourth waveguide 24B2 on the return path is a waveguide that optically connects the second coupler 14D and the second tapered waveguide 34B14 on the return path. The fourth waveguide 24B2 on the return path is a waveguide made of, for example, Si. Note that the fourth waveguide 24B2 on the return path is not limited to a Si waveguide, and may be made of, for example, SiN, LN, or other materials, and can be changed as appropriate.

[0089] The eighth waveguide 37B2 on the return side is a rib-structured waveguide in which the second reverse tapered waveguide 34B13 on the return side is disposed below the input portion of the eighth waveguide 37B2 on the return side, and optically connected to the second reverse tapered waveguide 34B13 on the return side by indirect transition. The eighth waveguide 37B2 on the return side is, for example, an LN waveguide. The second tapered waveguide 34B14 on the return side is disposed below the output portion of the eighth waveguide 37B2 on the return side, and optically connected to the second tapered waveguide 34B14 on the return side by indirect transition.

[0090] FIG. 6A is a schematic cross-sectional view showing an example of the cross section taken along line AA in FIG. 5. The cross section taken along line AA is a cross-sectional view of the outgoing third waveguide 24A1 and the outgoing fourth waveguide 24B1 in the chip 2. The optical modulator 1A shown in FIG. 6A includes a substrate 11, a lower clad 12 stacked on the substrate 11, and the outgoing third waveguide 24A1 and the outgoing fourth waveguide 24B1 stacked on the lower clad 12. The optical modulator 1A further includes an upper clad 13 formed to cover the outgoing third waveguide 24A1 and the outgoing fourth waveguide 24B1. The optical modulator 1A includes first electrodes formed on the upper clad 13, including a first signal electrode 16A, a first ground electrode 15A1, and a first ground electrode 15A2.

[0091] The first electrode has a connection portion 41A3 that electrically connects the first signal electrode 16A in the chip 2 and the first signal electrode 36A in the first chip 3A1. The first electrode has a connection portion 41A1 that electrically connects the first ground electrode 15A1 in the chip 2 and the first ground electrode 35A1 in the first chip 3A1. The first electrode has a connection portion 41A2 that electrically connects the first ground electrode 15A2 in the chip 2 and the first ground electrode 35A2 in the first chip 3A1.

[0092] 6A, the description focuses on the first electrode in the chip 2, but the same applies to the second electrode in the chip 2. The optical modulator 1A has a third waveguide 24A2 on the return path side, a fourth waveguide 24B2 on the return path side, and an upper clad 13 formed to cover the third waveguide 24A2 on the return path side and the fourth waveguide 24B2 on the return path side. The optical modulator 1A has second electrodes formed on the upper clad 13, including a second signal electrode 16B, a second ground electrode 15B1, and a second ground electrode 15B2.

[0093] The second electrode has a connection portion 41B3 that electrically connects second signal electrode 16B in chip 2 and second signal electrode 36B in second chip 3B1. The second electrode has a connection portion 41B1 that electrically connects second ground electrode 15B1 in chip 2 and second ground electrode 35B1 in second chip 3B1. The second electrode has a connection portion 41B2 that electrically connects second ground electrode 15B2 in chip 2 and second ground electrode 35B2 in second chip 3B1.

[0094] 6B is a schematic cross-sectional view showing an example of the cross-sectional portion along line BB shown in FIG. 5. The cross-sectional portion along line BB is a cross-sectional portion of the seventh waveguide 37A1 on the outgoing path side and the eighth waveguide 37B1 on the outgoing path side in the first chip 3A1. The optical modulator 1A shown in FIG. 6B has a substrate 11, a lower clad 12 stacked on the substrate 11, an upper clad 13A1 stacked on the lower clad 12, and a first chip 3A1 mounted on the upper clad 13A1. The upper clad 13A1 is a clad layer formed by etching the upper clad 13 shown in FIG. 6A or the like.

[0095] The first chip 3A1 is disposed in the opening 13A3 of the upper cladding 13A1 and has an outgoing-side first tapered waveguide 34A11 and an outgoing-side second tapered waveguide 34B11 disposed on the lower cladding 12. The first chip 3A1 also has an electro-optic crystal layer 32A1 mounted on the upper cladding 13A1, and an outgoing-side seventh waveguide 37A1 and an outgoing-side eighth waveguide 37B1 formed by the electro-optic crystal layer 32A1. The first chip 3A1 has first electrodes formed on the electro-optic crystal layer 32A1, including a first signal electrode 16A, a first ground electrode 15A1, and a first ground electrode 15A2.

[0096] The seventh waveguide 37A1 on the outgoing side is formed of a rib-type waveguide and is optically connected to the first tapered waveguide 34A11 on the outgoing side by indirect transition. The eighth waveguide 37B1 on the outgoing side is formed of a rib-type waveguide and is optically connected to the second tapered waveguide 34B11 on the outgoing side by indirect transition.

[0097] The first electrode includes a first signal electrode 36A arranged in parallel between the outgoing first tapered waveguide 34A11 and the outgoing second tapered waveguide 34B11, and between the outgoing seventh waveguide 37A1 and the outgoing eighth waveguide 37B1. The first electrode includes a first ground electrode 35A1 arranged in parallel with the outgoing first tapered waveguide 34A11 and the outgoing seventh waveguide 37A1. The first electrode includes a first ground electrode 35A2 arranged in parallel with the outgoing second tapered waveguide 34B11 and the outgoing eighth waveguide 37B1.

[0098] 6B, the first chip 3A1 has been described, but the same is true for the second chip 3B1. The optical modulator 1A has a second chip 3B1 mounted on the upper clad 13A1. The second chip 3B1 is disposed in the opening 13A3 of the upper clad 13A1, and has a first tapered waveguide 34A14 on the return path and a second tapered waveguide 34B14 on the return path that are disposed on the lower clad 12. The second chip 3B1 has an electro-optic crystal layer 32B1 mounted on the upper clad 13A1, and a seventh waveguide 37A2 on the return path and an eighth waveguide 37B2 on the return path that are formed by the electro-optic crystal layer 32B1. The second chip 3B1 has second electrodes formed on the electro-optic crystal layer 32B1, including a second signal electrode 16B, a second ground electrode 15B1 and a second ground electrode 15B2.

[0099] The seventh waveguide 37A2 on the return side is formed of a rib-type waveguide and is optically connected to the first tapered waveguide 34A14 on the return side by indirect transition. The eighth waveguide 37B2 on the return side is formed of a rib-type waveguide and is optically connected to the second tapered waveguide 34B14 on the return side by indirect transition.

[0100] The second electrode includes a second signal electrode 36B arranged in parallel between the first tapered waveguide 34A14 on the homeward side and the second tapered waveguide 34B14 on the homeward side, and between the seventh waveguide 37A2 on the homeward side and the eighth waveguide 37B2 on the homeward side. The second electrode includes a second ground electrode 35B1 arranged in parallel with the first tapered waveguide 34A14 on the homeward side and the seventh waveguide 37A2 on the homeward side. The second electrode includes a second ground electrode 35B2 arranged in parallel with the second tapered waveguide 34B14 on the homeward side and the eighth waveguide 37B2 on the homeward side.

[0101] Fig. 6C is a schematic cross-sectional view showing an example of the cross-sectional portion taken along line CC shown in Fig. 5. The cross-sectional portion is a cross-sectional portion of the seventh waveguide 37A1 on the outgoing path side and the eighth waveguide 37B1 on the outgoing path side in the first chip 3A1. The optical modulator 1A shown in Fig. 6C has a substrate 11, a lower clad 12 stacked on the substrate 11, an upper clad 13A1 stacked on the lower clad 12, and a first chip 3A1 mounted on the upper clad 13A1.

[0102] The first chip 3A1 has an electro-optic crystal layer 32A1 mounted on the upper clad 13A1, and a seventh waveguide 37A1 and an eighth waveguide 37B1 on the outgoing path formed by the electro-optic crystal layer 32A1. The first chip 3A1 has first electrodes formed on the electro-optic crystal layer 32A1, including a first signal electrode 36A, a first ground electrode 35A1, and a first ground electrode 35A2.

[0103] While FIG. 6C focuses on the first chip 3A1, the second chip 3B1 is similar. The optical modulator 1A includes a second chip 3B1 mounted on the upper clad 13A1. The second chip 3B1 includes an electro-optic crystal layer 32B1 mounted on the upper clad 13A1, and a seventh waveguide 37A2 on the return path and an eighth waveguide 37B2 on the return path, both of which are formed by the electro-optic crystal layer 32B1. The second chip 3B1 includes second electrodes formed on the electro-optic crystal layer 32B1, including a second signal electrode 36B, a second ground electrode 35B1, and a second ground electrode 35B2.

[0104] 7A is a schematic cross-sectional view showing an example of the cross-sectional portion of the optical modulator 1A taken along the line DD shown in FIG. 5. The cross-sectional portion of the line DD is a cross-sectional portion of the optical modulator 1A electrically connecting the first ground electrode 15A1 in the chip 2 and the first ground electrode 35A1 in the first chip 3A1. The optical modulator 1A shown in FIG. 7A has an upper clad 13 and an upper clad 13A1 stacked on the lower clad 12, and a first ground electrode 15A1 disposed on the upper clad 13. The optical modulator 1A has an electro-optic crystal layer 32A1 of the first chip 3A1 mounted on the upper clad 13A1, and a first ground electrode 35A1 disposed on the electro-optic crystal layer 32A1. The optical modulator 1A electrically connects a first ground electrode 15A1 arranged on the upper clad 13 in the chip 2 and a first ground electrode 35A1 arranged on the electro-optic crystal layer 32A1 in the first chip 3A1 via a connection portion 41A1.

[0105] 7A, the first chip 3A1 is described in detail, but the second chip 3B1 is similar. The optical modulator 1A includes an upper clad 13 and an upper clad 13A1 stacked on a lower clad 12, and a second ground electrode 15B1 disposed on the upper clad 13. The optical modulator 1A includes an electro-optic crystal layer 32B1 of a second chip 3B1 mounted on the upper clad 13A1, and a second ground electrode 35B1 disposed on the electro-optic crystal layer 32B1. The optical modulator 1A electrically connects the second ground electrode 15B1 disposed on the upper clad 13 in the chip 2 to the second ground electrode 35B1 disposed on the electro-optic crystal layer 32B1 of the second chip 3B1 via a connection portion 41B1.

[0106] FIG. 7B is a schematic cross-sectional view showing an example of the E-E line cross section shown in FIG. 5. The E-E line cross section is a cross-sectional view of an optical modulator 1A optically connecting the outgoing fourth waveguide 24B1 and the outgoing second tapered waveguide 34B11 in the chip 2 to the outgoing eighth waveguide 37B1 in the first chip 3A1. The optical modulator 1A shown in FIG. 7B has the outgoing fourth waveguide 24B1 and the outgoing second tapered waveguide 34B11 arranged on the lower cladding 12, and an upper cladding 13 covering the outgoing fourth waveguide 24B1. The optical modulator 1A has an electro-optic crystal layer 32A1 arranged on the outgoing eighth waveguide 37B1.

[0107] 7B, the first chip 3A1 has been described, but the second chip 3B1 is similar. The optical modulator 1A has a fourth waveguide 24B2 on the return path side and a second tapered waveguide 34B14 on the return path side disposed on the lower cladding 12, and an upper cladding 13 that covers the fourth waveguide 24B2 on the return path side. The optical modulator 1A has an electro-optic crystal layer 32B1 disposed on an eighth waveguide 37B2 on the return path side.

[0108] 8 is a schematic cross-sectional view showing an example of the cross section taken along the line FF shown in FIG. 5. The crystal axis direction Z1 of the electro-optic crystal layer 32A1 of the first chip 3A1 is the width direction (Z direction) perpendicular to the light propagation direction (Y direction). The seventh waveguide 37A1 on the outgoing path side in the first chip 3A1 changes its optical refractive index in response to the electric field in the electric field direction a1 from the first signal electrode 36A to the first ground electrode 35A1. Furthermore, the eighth waveguide 37B1 on the outgoing path side in the first chip 3A1 changes its optical refractive index in response to the electric field in the electric field direction b1 from the first signal electrode 36A to the first ground electrode 35A2.

[0109] The crystal axis direction Z2 of the electro-optic crystal layer 32B1 of the second chip 3B1 is the width direction (Z direction) perpendicular to the light propagation direction (Y direction). The crystal axis direction Z1 of the electro-optic crystal layer 32A1 of the first chip 3A1 and the crystal axis direction Z2 of the electro-optic crystal layer 32B1 of the second chip 3B1 are 180 degrees different from each other. The optical refractive index of the seventh waveguide 37A2 on the return path side in the second chip 3B1 changes depending on the electric field in the electric field direction a2 from the second signal electrode 36B to the second ground electrode 35B1. Furthermore, the optical refractive index of the eighth waveguide 37B2 on the return path side in the second chip 3B1 changes depending on the electric field in the electric field direction b2 from the second signal electrode 36B to the second ground electrode 35B2.

[0110] The electric field direction a1 of the seventh waveguide 37A1 on the outgoing path in the first chip 3A1 is the same as the crystal direction (Z1 direction) of the electro-optic crystal layer 32A1. The electric field direction a2 of the seventh waveguide 37A2 on the return path in the second chip 3B1 is the same as the crystal direction (Z2 direction) of the electro-optic crystal layer 32B1. Therefore, the electric field in the electric field direction a1 of the seventh waveguide 37A1 on the outgoing path and the electric field in the electric field direction a2 of the seventh waveguide 37A2 on the return path are the same direction. In other words, the electric field in the electric field direction a1 of the seventh waveguide 37A1 on the outgoing path and the electric field in the electric field direction a2 of the seventh waveguide 37A2 on the return path are not canceled out, resulting in high modulation efficiency.

[0111] The electric field direction b1 of the eighth waveguide 37B1 on the outward path side in the first chip 3A1 is opposite to the crystal direction (Z1 direction) of the electro-optic crystal layer 32A1. The electric field direction b2 of the eighth waveguide 37B2 on the return path side in the second chip 3B1 is opposite to the crystal direction (Z2 direction) of the electro-optic crystal layer 32B1. Therefore, the electric field in the electric field direction b1 of the eighth waveguide 37B1 on the outward path side is the same as the electric field in the electric field direction b2 of the eighth waveguide 37B2 on the outward path side. In other words, the electric field in the electric field direction b1 of the eighth waveguide 37B1 on the outward path side and the electric field in the electric field direction b2 of the eighth waveguide 37B2 on the return path side are not canceled out, resulting in high modulation efficiency.

[0112] In the second embodiment, the first chip 3A1 is disposed near the seventh waveguide 37A1 on the outbound side and has a first electrode that applies an electric field to the seventh waveguide 37A1 on the outbound side in the same direction as the crystal axis of the first chip 3A1. The second chip 3B1 is disposed near the seventh waveguide 37A2 on the inbound side and has a second electrode that applies an electric field to the seventh waveguide 37A2 on the inbound side in the same direction as the crystal axis of the second chip 3B1. As a result, the electric field in the electric field direction a1 of the seventh waveguide 37A1 on the outbound side and the electric field in the electric field direction a2 of the seventh waveguide 37A2 on the inbound side are not canceled out, resulting in high modulation efficiency.

[0113] The first chip 3A1 is mounted on the substrate 11 so that the crystal axis direction Z1 of the first chip 3A1 is perpendicular to the propagation direction of light guided through the outgoing seventh waveguide 37A1 and the outgoing eighth waveguide 37B1. The second chip 3B1 is mounted on the substrate 11 so that the crystal axis direction Z2 of the second chip 3B1 is perpendicular to the propagation direction of light guided through the return seventh waveguide 37A2 and the return eighth waveguide 37B2. As a result, the electric field in the electric field direction a1 of the outgoing seventh waveguide 37A1 and the electric field in the electric field direction a2 of the return seventh waveguide 37A2 are not canceled out, thereby increasing modulation efficiency. Furthermore, the electric field of the eighth waveguide 37B1 on the outgoing path in the electric field direction b1 and the electric field of the eighth waveguide 37B2 on the return path in the electric field direction b2 are not canceled out, resulting in high modulation efficiency. In other words, the first chip 3A1 and the second chip 3B1 are arranged so that the axis along which the electro-optic effect is most pronounced is oriented substantially perpendicular to the direction of propagation of the high-frequency signal, and further so that phase modulation is not canceled out between the first chip 3A1 and the second chip 3B1. Furthermore, the X-cut thin-film LN optical modulator can be folded within the optical modulation element without crossing the signal line or optical waveguide.

[0114] The first electrode is disposed near the eighth waveguide 37B1 on the outgoing side and applies an electric field to the eighth waveguide 37B1 on the outgoing side in a direction opposite to the direction of the crystal axis of the first chip 3A1. The second electrode is disposed near the eighth waveguide 37B2 on the return side and applies an electric field to the eighth waveguide 37B2 on the return side in a direction opposite to the direction of the crystal axis of the second chip 3B1. As a result, the electric field in the electric field direction b1 of the eighth waveguide 37B1 on the outgoing side and the electric field in the electric field direction b2 of the eighth waveguide 37B2 on the return side are not canceled out, thereby increasing modulation efficiency.

[0115] The first chip 3A1 and the second chip 3B1 are mounted on the substrate 11 so that the crystal axis direction Z1 of the first chip 3A1 is 180 degrees different from the crystal axis direction Z2 of the second chip 3B1. As a result, the electric field in the electric field direction a1 of the seventh waveguide 37A1 on the outgoing path and the electric field in the electric field direction a2 of the seventh waveguide 37A2 on the return path are not canceled out, thereby increasing modulation efficiency. Furthermore, the electric field in the electric field direction b1 of the eighth waveguide 37B1 on the outgoing path and the electric field in the electric field direction b2 of the eighth waveguide 37B2 on the return path are not canceled out, thereby increasing modulation efficiency.

[0116] The first chip 3A1 and the second chip 3B1 are mounted on the substrate 11 so that the direction a1 of the electric field applied from the first electrode to the seventh waveguide 37A1 on the outgoing path is 180 degrees different from the direction a2 of the electric field applied from the second electrode to the seventh waveguide 37A1 on the return path. As a result, the electric field in the electric field direction a1 of the seventh waveguide 37A1 on the outgoing path and the electric field in the electric field direction a2 of the seventh waveguide 37A2 on the return path are not canceled out, thereby increasing modulation efficiency.

[0117] The seventh waveguides 37A1 and 37A2 and the eighth waveguides 37B1 and 37B2 are waveguides with a rib structure of an electro-optic crystal layer. As a result, compared to the optical modulator 1 of Example 1, light is confined only within the electro-optic crystal, resulting in higher modulation efficiency. In the optical modulator 1 of Example 1, the waveguide mode also extends to the waveguide on the substrate 11 side.

[0118] In the optical modulator 1 (1A) of this embodiment, the description has focused on one MZM structure, but it can also be applied to an IQ modulator by having two MZM structures, and it can also be applied to a DP-IQ modulator by having four MZM structures.

[0119] FIG. 9 is an explanatory diagram illustrating an example of an optical transceiver 70 according to this embodiment. The optical transceiver 70 shown in FIG. 9 is connected to an output optical fiber and an input optical fiber. The optical transceiver 70 includes a DSP (Digital Signal Processor) 72 and an optical transmitter / receiver 73. The optical transmitter / receiver 73 includes an optical transmitter 73A and an optical receiver 73B. The DSP 72 is an electrical component that performs digital signal processing. For example, the DSP 72 performs processing such as encoding transmission data, generates an electrical signal containing the transmission data, and outputs the generated electrical signal to the optical transmitter 73A. The DSP 72 also acquires an electrical signal containing reception data from the optical receiver 73B and performs processing such as decoding the acquired electrical signal to obtain the reception data.

[0120] The optical transmitter 73A has an optical modulator element 73A1 that modulates supplied light with an electrical signal output from the DSP 72, and outputs the transmission light modulated by the electrical signal to an optical fiber. The optical modulator element 73A1 has a built-in optical device that guides the light to be output to the optical fiber.

[0121] The optical receiver 73B has an optical receiver element 73B1 that receives an optical signal from an optical fiber and demodulates the received light using the supplied light, converts the demodulated received light into an electrical signal, and outputs the converted electrical signal to the DSP 72.

[0122] The optical device in the optical transceiver 70 includes a substrate, an optical waveguide provided on the substrate, and first and second chips mounted on the substrate, each containing a material with a stronger electro-optic effect than the substrate and having a crystal axis along which the electro-optic effect is most pronounced. The optical waveguide includes a first coupler and an input-side first waveguide and an input-side second waveguide, each connected to the first coupler. The optical waveguide includes a second coupler and an output-side first waveguide and an output-side second waveguide, each connected to the second coupler. The optical waveguide includes a first bent waveguide connecting the input-side first waveguide and the output-side first waveguide, and a second bent waveguide connecting the input-side second waveguide and the output-side second waveguide. The first chip is mounted on the substrate so that the direction of the crystal axis of the first chip is perpendicular to the propagation direction of light guided through the first waveguide on the input side and the second waveguide on the input side.

[0123] The first chip is disposed near the input-side first waveguide and has a first electrode that applies an electric field to the input-side first waveguide in the same direction as the crystal axis of the first chip. The second chip is mounted on the substrate so that the crystal axis of the second chip is perpendicular to the propagation direction of light guided through the output-side first waveguide and the output-side second waveguide. The second chip is disposed near the output-side first waveguide and has a second electrode that applies an electric field to the output-side first waveguide in the same direction as the crystal axis of the second chip. As a result, the optical device can achieve improved modulation efficiency.

[0124] For convenience of explanation, the optical transceiver 70 has been illustrated as having an optical transmitter 73A and an optical receiver 73B built in, but the optical transceiver 70 may have either the optical transmitter 73A or the optical receiver 73B built in. For example, an optical device may be applied to the optical transceiver 70 having the optical receiver 73B built in, and modifications can be made as appropriate.

[0125] Furthermore, the components of each unit shown in the figure do not necessarily have to be physically configured as shown in the figure. In other words, the specific form of distribution and integration of each unit is not limited to that shown in the figure, and all or part of them can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc.

[0126] Furthermore, the various processing functions performed by each device may be executed in whole or in part on a CPU (Central Processing Unit) (or a microcomputer such as an MPU (Micro Processing Unit) or MCU (Micro Controller Unit)). Needless to say, the various processing functions may be executed in whole or in part on a program analyzed and executed by a CPU (or a microcomputer such as an MPU or MCU), or on hardware using wired logic.

[0127] The following additional notes are provided regarding the above-described embodiments.

[0128] (Appendix 1) A substrate, an optical waveguide provided on the substrate; a first chip and a second chip mounted on the substrate, containing a material having a stronger electro-optic effect than the substrate, and having a crystal axis in which the electro-optic effect is most strongly exhibited; The optical waveguide is a first coupler; a first input waveguide and a second input waveguide, each connected to the first coupler; a second coupler; and a first output waveguide and a second output waveguide, each connected to the second coupler; a first bent waveguide connecting the first waveguide on the input side and the first waveguide on the output side, and a second bent waveguide connecting the second waveguide on the input side and the second waveguide on the output side, The first chip comprises: a first electrode disposed near the first waveguide on the input side and applying an electric field in the same direction as the crystal axis direction of the first chip to the first waveguide on the input side; The second chip comprises: An optical device characterized by having a second electrode arranged near the first waveguide on the output side and applying an electric field to the first waveguide on the output side in the same direction as the crystal axis direction of the second chip.

[0129] (Supplementary Note 2) The first chip is The first chip is mounted on the substrate so that the crystal axis of the first chip is perpendicular to the propagation direction of light guided through the first input waveguide and the second input waveguide, The second chip comprises: The optical device described in Appendix 1, characterized in that the second chip is mounted on the substrate so that the orientation of the crystal axis of the second chip is perpendicular to the propagation direction of light guided through the output side first waveguide and the output side second waveguide.

[0130] (Supplementary Note 3) The first electrode is The first chip is disposed near the second waveguide on the input side, and applies an electric field to the second waveguide on the input side in a direction opposite to the direction of the crystal axis of the first chip, The second electrode is An optical device as described in Appendix 1, characterized in that it is arranged near the second waveguide on the output side and applies an electric field to the second waveguide on the output side in a direction opposite to the direction of the crystal axis of the second chip.

[0131] (Appendix 4) An optical device described in Appendix 1 or 2, characterized in that the first chip and the second chip are mounted on the substrate so that the orientation of the crystal axis of the first chip is 180 degrees different from the orientation of the crystal axis of the second chip.

[0132] (Appendix 5) The optical device described in Appendix 1 or 2, characterized in that the first chip and the second chip are mounted on the substrate so that the direction of the electric field applied from the first electrode to the first waveguide on the input side is 180 degrees different from the direction of the electric field applied from the second electrode to the first waveguide on the output side.

[0133] (Supplementary Note 6) The first curved waveguide and the second curved waveguide are 3. The optical device according to claim 1, wherein the optical device is a folded waveguide in which the propagation direction of light guided through the first input waveguide and the second input waveguide changes by 180 degrees from the propagation direction of light guided through the first output waveguide and the second output waveguide.

[0134] (Supplementary Note 7) The first waveguide on the input side is a third waveguide on the input side provided on the substrate and a fourth waveguide on the input side provided on the first chip are optically coupled to each other; The first waveguide on the output side is The optical device described in Appendix 1, characterized in that it is configured by optically coupling a fifth waveguide on the output side provided on the substrate with a sixth waveguide on the output side provided on the second chip.

[0135] (Supplementary Note 8) The fourth waveguide and the sixth waveguide are 8. The optical device according to claim 7, wherein the waveguide has a rib structure.

[0136] (Supplementary Note 9) The first electrode is The electrode has a coplanar structure including a first signal electrode arranged in parallel between the first input waveguide and the second input waveguide, one first ground electrode arranged in parallel with the first input waveguide, and the other first ground electrode arranged in parallel with the second input waveguide, The second electrode is 2. The optical device according to claim 1, wherein the electrodes have a coplanar structure and include: a second signal electrode arranged in parallel between the first waveguide on the output side and the second waveguide on the output side; one second ground electrode arranged in parallel to the first waveguide on the output side; and the other second ground electrode arranged in parallel to the second waveguide on the output side.

[0137] (Appendix 10) The optical device according to appendix 1, wherein the material having the electro-optic effect includes a material with γ33 of 25 pm / V or more.

[0138] (Appendix 11) The optical device according to appendix 1, wherein the material of the substrate includes at least one material selected from the group consisting of Si, SiO2, QTZ, and sapphire.

[0139] (Appendix 12) The optical device according to appendix 1, wherein the material of the optical waveguide includes at least one of Si, SiN, and LN.

[0140] (Appendix 13) The material of the first electrode is 2. The optical device according to claim 1, comprising at least one material selected from the group consisting of Au, Al, and Cu.

[0141] (Appendix 14) The optical device according to appendix 1, wherein the width of the first chip is 300 μm or less.

[0142] (Supplementary Note 15) An optical transmitter having an optical modulator element that modulates guided light in response to an electrical signal, The optical modulator element comprises: A substrate; an optical waveguide provided on the substrate; a first chip and a second chip mounted on the substrate, containing a material having a stronger electro-optic effect than the substrate, and having a crystal axis in which the electro-optic effect is most strongly exhibited; The optical waveguide is a first coupler; a first input waveguide and a second input waveguide, each connected to the first coupler; a second coupler; and a first output waveguide and a second output waveguide, each connected to the second coupler; a first bent waveguide connecting the first waveguide on the input side and the first waveguide on the output side, and a second bent waveguide connecting the second waveguide on the input side and the second waveguide on the output side, The first chip comprises: a first electrode disposed near the first waveguide on the input side and applying an electric field in the same direction as a crystal axis of the first chip to the first waveguide on the input side; The second chip comprises: An optical transmitter characterized by having a second electrode arranged near the first waveguide on the output side and applying an electric field to the first waveguide on the output side in the same direction as the crystal axis direction of the second chip.

[0143] (Supplementary Note 16) An optical modulator element that modulates guided light in response to an electrical signal; an optical receiver element that converts received signal light into an electrical signal; a signal processing unit that generates an electrical signal to be output to the optical modulator element and processes the electrical signal obtained from the optical receiver element, The optical modulator element comprises: A substrate; an optical waveguide provided on the substrate; a first chip and a second chip mounted on the substrate, containing a material having a stronger electro-optic effect than the substrate, and having a crystal axis in which the electro-optic effect is most strongly exhibited; The optical waveguide is a first coupler; a first input waveguide and a second input waveguide, each connected to the first coupler; a second coupler; and a first output waveguide and a second output waveguide, each connected to the second coupler; a first bent waveguide connecting the first waveguide on the input side and the first waveguide on the output side, and a second bent waveguide connecting the second waveguide on the input side and the second waveguide on the output side, The first chip comprises: a first electrode disposed near the first waveguide on the input side and applying an electric field in the same direction as a crystal axis of the first chip to the first waveguide on the input side; The second chip comprises: An optical transceiver characterized by having a second electrode arranged near the first waveguide on the output side and applying an electric field to the first waveguide on the output side in the same direction as the crystal axis direction of the second chip. [Explanation of symbols]

[0144] 1 Optical modulator 2 chips 3A First Chip 3B Second Chip 11 Circuit Board 14 Optical waveguide 14A1 First waveguide on the outgoing path 14A2 First waveguide on the return path side 14B1 Second waveguide on the outgoing path 14B2 Second waveguide on the return path 14C First Coupler 14D Second Coupler 34A1 Fifth waveguide on the outgoing side 34A2 Fifth waveguide on the return path 51A First folded waveguide 51B Second folded waveguide 35A1 First ground electrode 35A2 First ground electrode 36A First signal electrode 35B1 Second ground electrode 35B2 Second ground electrode 36B Second signal electrode

Claims

1. A substrate; an optical waveguide provided on the substrate; a first chip and a second chip mounted on the substrate, each chip containing a material having a stronger electro-optic effect than the substrate, and each chip having a crystal axis along which the electro-optic effect is most strongly expressed; The optical waveguide is a first coupler; a first input waveguide and a second input waveguide, each connected to the first coupler; a second coupler; and a first output waveguide and a second output waveguide, each connected to the second coupler; a first bent waveguide connecting the first waveguide on the input side and the first waveguide on the output side, and a second bent waveguide connecting the second waveguide on the input side and the second waveguide on the output side, The first chip includes: a first electrode disposed near the first waveguide on the input side and applying an electric field in the same direction as the crystal axis direction of the first chip to the first waveguide on the input side; The second chip includes: An optical device characterized by having a second electrode arranged near the first waveguide on the output side and applying an electric field to the first waveguide on the output side in the same direction as the crystal axis direction of the second chip.

2. The first chip includes: The first chip is mounted on the substrate so that the orientation of the crystal axis of the first chip is perpendicular to the propagation direction of light guided through the first input waveguide and the second input waveguide, The second chip includes:

2. The optical device according to claim 1, wherein the second chip is mounted on the substrate so that the orientation of the crystal axis of the second chip is perpendicular to the propagation direction of light guided through the first waveguide on the output side and the second waveguide on the output side.

3. The first electrode is The first chip is disposed near the second waveguide on the input side, and applies an electric field to the second waveguide on the input side in a direction opposite to the direction of the crystal axis of the first chip. The second electrode is 2. The optical device according to claim 1, wherein the second chip is disposed near the second waveguide on the output side, and applies an electric field to the second waveguide on the output side in a direction opposite to the direction of the crystal axis of the second chip.

4. 3. The optical device according to claim 1, wherein the first chip and the second chip are mounted on the substrate so that the orientation of the crystal axis of the first chip is 180 degrees different from the orientation of the crystal axis of the second chip.

5. 3. The optical device according to claim 1, wherein the first chip and the second chip are mounted on the substrate such that the direction of the electric field applied from the first electrode to the first waveguide on the input side is 180 degrees different from the direction of the electric field applied from the second electrode to the first waveguide on the output side.

6. The first bent waveguide and the second bent waveguide are 3. The optical device according to claim 1, wherein the optical device is a folded waveguide in which the propagation direction of light guided through the first input waveguide and the second input waveguide changes by 180 degrees from the propagation direction of light guided through the first output waveguide and the second output waveguide.

7. The first waveguide on the input side is a third waveguide on an input side provided on the substrate and a fourth waveguide on an input side provided on the first chip are optically coupled to each other; The first waveguide on the output side is 2. The optical device according to claim 1, wherein the optical device is configured by optically coupling a fifth waveguide on the output side provided on the substrate with a sixth waveguide on the output side provided on the second chip.

8. The fourth waveguide and the sixth waveguide are 8. The optical device according to claim 7, wherein the waveguide has a rib structure.

9. The first electrode is an electrode having a coplanar structure including a first signal electrode arranged in parallel between the first input waveguide and the second input waveguide, one first ground electrode arranged in parallel with the first input waveguide, and the other first ground electrode arranged in parallel with the second input waveguide; The second electrode is 2. The optical device according to claim 1, wherein the electrodes have a coplanar structure including: a second signal electrode arranged in parallel between the first waveguide on the output side and the second waveguide on the output side; one second ground electrode arranged in parallel to the first waveguide on the output side; and another second ground electrode arranged in parallel to the second waveguide on the output side.

10. An optical transmitter including an optical modulator element that modulates guided light in response to an electrical signal, The optical modulator element comprises: A substrate; an optical waveguide provided on the substrate; a first chip and a second chip mounted on the substrate, each chip containing a material having a stronger electro-optic effect than the substrate, and each chip having a crystal axis along which the electro-optic effect is most strongly expressed; The optical waveguide is a first coupler; a first input waveguide and a second input waveguide, each connected to the first coupler; a second coupler; and a first output waveguide and a second output waveguide, each connected to the second coupler; a first bent waveguide connecting the first waveguide on the input side and the first waveguide on the output side, and a second bent waveguide connecting the second waveguide on the input side and the second waveguide on the output side, The first chip includes: a first electrode disposed near the first waveguide on the input side and applying an electric field in the same direction as a crystal axis of the first chip to the first waveguide on the input side; The second chip includes: an optical transmitter having a second electrode disposed near the first waveguide on the output side and applying an electric field to the first waveguide on the output side in the same direction as the crystal axis direction of the second chip.

11. an optical modulator element that modulates guided light in response to an electrical signal; an optical receiver element that converts received signal light into an electrical signal; a signal processing unit that generates an electrical signal to be output to the optical modulator element and processes the electrical signal obtained from the optical receiver element, The optical modulator element comprises: A substrate; an optical waveguide provided on the substrate; a first chip and a second chip mounted on the substrate, each chip containing a material having a stronger electro-optic effect than the substrate, and each chip having a crystal axis along which the electro-optic effect is most strongly expressed; The optical waveguide is a first coupler; a first input waveguide and a second input waveguide, each connected to the first coupler; a second coupler; and a first output waveguide and a second output waveguide, each connected to the second coupler; a first bent waveguide connecting the first waveguide on the input side and the first waveguide on the output side, and a second bent waveguide connecting the second waveguide on the input side and the second waveguide on the output side, The first chip includes: a first electrode disposed near the first waveguide on the input side and applying an electric field in the same direction as a crystal axis of the first chip to the first waveguide on the input side; The second chip includes: An optical transceiver characterized by having a second electrode arranged near the first waveguide on the output side and applying an electric field to the first waveguide on the output side in the same direction as the crystal axis direction of the second chip.

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