Optical device, optical transmitter, and optical transceiver

Abstract

To provide a compact optical device and the like.SOLUTION: An optical device comprises a first chip and a second chip that includes a material having a high electro-optic effect. The first chip includes a first waveguide that propagates signal light from a first port, a first branched waveguide that propagates the signal light from the first waveguide, a turned-back parallel waveguide that connects with the first branched waveguide, and a first parallel waveguide that propagates the signal light from the turned-back parallel waveguide. The first chip includes a second waveguide that propagates signal light to a second port, a second branched waveguide that propagates the signal light to the second waveguide, and a phase adjustment portion that adjusts the phase of the signal light propagating in the first branched waveguide according to a direct current signal. The second chip includes a second parallel waveguide that couples the first parallel waveguide and the second branched waveguide, and a phase modulation portion that modulates the phase of the signal light propagating in the second parallel waveguide according to a high-frequency signal.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical device, an optical transmitter, and an optical transceiver. [Background technology]

[0002] A conventional optical modulator has, for example, a waveguide provided on a substrate, and a signal electrode and a ground electrode arranged near the waveguide. When a voltage is applied to the signal electrode, an electric field is generated in the waveguide, and the refractive index of the waveguide changes due to the electric field in the waveguide, and the phase of the light changes. The waveguide constitutes a Mach-Zehnder interferometer, and the optical output changes due to the difference in the phase of the light between the waveguides.

[0003] The optical modulator is, for example, a Mach-Zehnder modulator. FIG. 6 is a schematic plan view showing an example of a conventional optical modulator 200. The optical modulator 200 is, for example, a thin-film LN (Lithium Niobate: LiNbO 3 The optical modulator 200 includes a polarization beam combiner (PBC) 204 and an optical fiber array 205. The polarization beam combiner 204 and an optical fiber array 205 are also included in the optical modulator 200. The polarization beam combiner 204 and an optical fiber array 205 are also included in the optical modulator 200. The polarization beam combiner 204 and an optical fiber array 205 are also included in the optical modulator 200. The polarization beam combiner 204 and an optical fiber array 205 are also included in the optical modulator 200.

[0004] The thin-film LN chip 201 has a first port 201A, a first waveguide 211, a folded waveguide 212, one first branching section 213, and two first branching waveguides 214. The thin-film LN chip 201 has two second branching sections 215, four second branching waveguides 216, and four third branching sections 217. The thin-film LN chip 201 has eight third branching waveguides 218, four RF (Radio Frequency) modulation sections 230, four first DC (Direct Current) modulation sections 240, and four first multiplexing sections 219. The thin-film LN chip 201 has four fourth branching waveguides 220, two second DC modulation sections 250, a second multiplexing section 221, two output waveguides 222, and a second port 201B.

[0005] The first port 201A is a port disposed at one end of the thin-film LN chip 201, and is connected to the MLA 202 as well as to the first waveguide 211. The first waveguide 211 is, for example, an LN waveguide that propagates the signal light from the first port 201A. The input end of the first waveguide 211 is exposed on one end face of the thin-film LN chip 201. The first waveguide 211 outputs the signal light from the input end to the folded waveguide 212. The folded waveguide 212 is, for example, an LN waveguide that propagates the signal light from the first waveguide 211.

[0006] The first branching section 213 branches the signal light from the folded waveguide 212 into two first branching waveguides 214. The first branching waveguide 214 is, for example, an LN waveguide that propagates the signal light from the first branching section 213. The second branching section 215 branches the signal light from the first branching waveguide 214 into two second branching waveguides 216. The second branching waveguide 216 is, for example, an LN waveguide that propagates the signal light from the second branching section 215. The third branching section 217 branches the signal light from the second branching waveguide 216 into two third branching waveguides 218 in the RF modulation section 230.

[0007] The RF modulation unit 230 is a phase modulation unit that modulates the signal light propagating through the two third branch waveguides 218 at high speed. The RF modulation unit 230 has two third branch waveguides 218 arranged in parallel, a plurality of RF electrodes 231 arranged in parallel to the two third branch waveguides 218, and an RF driver 232 that inputs a high-frequency signal to the RF electrode 231. The RF modulation unit 230 further has an RF termination 233 that terminates the high-frequency signal of the RF electrode 231, and an electrode line 234 that electrically connects between the RF electrode 231 and the RF driver 232. The two third branch waveguides 218 are LN waveguides. When a high-frequency signal having a band of, for example, several tens of GHz is input from the RF driver 232 to the RF electrode 231, the RF modulation unit 230 can modulate the signal light propagating through the third branch waveguide 218 at high speed in response to the high-frequency signal.

[0008] The first DC modulation unit 240 has two third branch waveguides 218 arranged in parallel and a plurality of first DC electrodes 241 arranged in parallel on the two third branch waveguides 218. The two third branch waveguides 218 are, for example, LN waveguides. The first DC modulation unit 240 is a phase adjustment unit that connects the two third branch waveguides 218 in the RF modulation unit 230 and the two third branch waveguides 218 in the first DC modulation unit 240 and modulates the signal light propagating through the two third branch waveguides 218 in the first DC modulation unit 240. When a bias voltage is applied to the first DC electrode 241, the first DC modulation unit 240 adjusts the bias that turns on / off the signal light propagating through the third branch waveguide 218 depending on the ON / OFF of the bias voltage. As a result, the bias at which the signal light propagating through the third branch waveguide 218 is turned ON / OFF is adjusted, thereby adjusting the phase of the signal light propagating through the third branch waveguide 218. The first multiplexing section 219 multiplexes the signal light from the two third branch waveguides 218 in the first DC modulation section 240, and outputs the multiplexed signal light to the fourth branch waveguide 220.

[0009] The second DC modulation section 250 has two fourth branch waveguides 220 arranged in parallel and a second DC electrode 251 arranged on the two fourth branch waveguides 220. The two fourth branch waveguides 220 are, for example, LN waveguides. The second DC modulation section 250 is a phase adjustment section that connects two first multiplexing sections 219 and the two fourth branch waveguides 220 in the second DC modulation section 250 and modulates the signal light propagating through the two fourth branch waveguides 220 in the second DC modulation section 250. When a bias voltage is applied to the second DC electrode 251, the second DC modulation section 250 adjusts the bias at which the signal light propagating through the fourth branch waveguide 220 is turned ON / OFF depending on the ON / OFF of the bias voltage. As a result, the bias at which the signal light propagating through the fourth branch waveguide 220 is turned ON / OFF is adjusted, so that it is possible to adjust the phase of the signal light propagating through the fourth branch waveguide 220. The second DC modulation unit 250 modulates the signal light propagating through the fourth branch waveguide 220, and outputs the modulated signal light to the second multiplexing unit 221. The second multiplexing unit 221 multiplexes the modulated signal light from the two fourth branch waveguides 220 in the second DC modulation unit 250, and outputs the multiplexed signal light to the output waveguide 222.

[0010] One second multiplexing section 221 multiplexes signal light from two fourth branching waveguides 220 in one second DC modulation section 250, and outputs the multiplexed signal light to one output waveguide 222. The other second multiplexing section 221 multiplexes signal light from two fourth branching waveguides 220 in the other second DC modulation section 250, and outputs the multiplexed signal light to the other output waveguide 222. The second port 201B is disposed at one end of the thin-film LN chip 201, and has a port connected to one output waveguide 222 and a port connected to the other output waveguide 222.

[0011] The output waveguide 222 is, for example, an LN waveguide that propagates the signal light from the second multiplexing section 221. The second port 201B connected to the output end of the output waveguide 222 is exposed on one end surface of the thin-film LN chip 201. The MLA 202 is an optical component that optically connects one output waveguide 222 and the PR 203, and optically connects the other output waveguide 222 and the PBC 204. The MLA 202 optically connects the first waveguide 211 and the optical fiber 205A on the input side in the optical fiber array 205. The MLA 202 focuses the signal light from the optical fiber 205A on the input side in the optical fiber array 205 into the first waveguide 211. The MLA 202 focuses the signal light from one output waveguide 222 onto the PR 203 , and also focuses the signal light from the other output waveguide 222 onto the PBC 204 .

[0012] The PR 203 rotates the polarization of the signal light from one output waveguide 222 via the MLA 202, and outputs the signal light after the polarization rotation to the PBC 204. The PBC 204 performs polarization multiplexing on the signal light after the polarization rotation from the PR 203 and the signal light from the other output waveguide 222 via the MLA 202, and outputs the polarization multiplexed signal light to the optical fiber 205B on the output side of the optical fiber array 205.

[0013] In the optical modulator 200, a high frequency signal output from an RF driver 232 is input to an RF electrode 231 via an electrode wire 234, and the high frequency signal is propagated in the same direction as the signal light propagating through the third branch waveguide 218, thereby modulating the signal light. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] US Patent Application Publication No. 2023 / 0152660 [Patent Document 2] US Patent Application Publication No. 2021 / 0373412 [Patent Document 3] International Publication No. 2015 / 012213 [Patent Document 4] JP 2012-163876 A Summary of the Invention [Problem to be solved by the invention]

[0015] In the optical modulator 200, the size of the first DC modulation section 240 and the second DC modulation section 250 increases, which results in a large size of the thin-film LN chip 201. In addition, the MLA 202, the PR 203, and the PBC 204 are individual components, which increases the package size of the thin-film LN chip 201.

[0016] Therefore, it is desirable to reduce the chip size of the optical modulator by integrating the first DC modulation section 240, the second DC modulation section 250, the PR 203 and the PBC 204 on a SiPh (silicon photonics) chip and mounting a thin-film LN chip on the SiPh chip.

[0017] In one aspect, an object is to provide a small optical device or the like. [Means for solving the problem]

[0018] In one embodiment, the optical device disclosed in the present application includes a first chip having a first port and a second port, and a second chip disposed on the first chip and having a material with a higher electro-optic effect than the first chip. The first chip includes a first waveguide connected to the first port and propagating a signal light from the first port, and a first branch waveguide of a branching structure connected to the first waveguide and propagating the signal light from the first waveguide. The first chip includes a folded parallel waveguide connected to the first branch waveguide and having a folded structure, and a first parallel waveguide connected to the folded parallel waveguide and propagating the signal light from the folded parallel waveguide. The first chip has a second waveguide connected to the second port and propagating the signal light to the second port, and a second branch waveguide of a branching structure connected to the second waveguide and propagating the signal light to the second waveguide. The first chip has a phase adjustment unit disposed in the first branch waveguide and adjusting the phase of the signal light propagating in the first branch waveguide in response to a DC electric signal. The second chip has a second parallel waveguide coupled to the first parallel waveguide at a first end face and coupled to the second branch waveguide at a second end face different from the first end face. The second parallel waveguide propagates the signal light from the first parallel waveguide to the second branch waveguide. The second chip has a phase modulation unit disposed in the second parallel waveguide and modulating the phase of the signal light propagating in the second parallel waveguide in response to a high frequency signal. Effect of the Invention

[0019] According to one aspect, an optical device or the like having a reduced chip size can be provided. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic plan view showing an example of an optical modulator according to a first embodiment. [Diagram 2] FIG. 2 is a schematic plan view illustrating an example of an optical modulator according to a second embodiment. [Diagram 3]FIG. 3 is a schematic plan view illustrating an example of an optical modulator according to a third embodiment. [Figure 4] FIG. 4 is an explanatory diagram illustrating an example of an optical transceiver according to the present embodiment. [Diagram 5] FIG. 5 is a schematic plan view showing an example of an optical modulator of a comparative example. [Figure 6] FIG. 6 is a schematic plan view showing an example of a conventional optical modulator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] <Comparative Example 1> 5 is a schematic plan view showing an example of an optical modulator 100 of a comparative example. The optical modulator 100 shown in FIG. 5 has a SiPh chip 101 on which a first DC modulation section 140 having a first heater electrode 141 and a second DC modulation section 150 having a second heater electrode 151 are mounted. The optical modulator 100 includes a thin-film LN (Lithium Niobate: LiNbO 3 ) chip 102 and an optical fiber array 103 including an input optical fiber 103A and an output optical fiber 103B.

[0022] The SiPh chip 101 has a Si substrate, an opening 101A that opens a part of the Si substrate, a first port 101B arranged on one end face, and a second port 101C arranged on the other end face. The opening 101A is a structure in which a part of the Si substrate is dug down by etching. The opening 101A is a portion where the thin film LN chip 102 is mounted.

[0023] The SiPh chip 101 has one first waveguide 111, a folded waveguide 112, one first branching portion 113, two first branching waveguides 114, two second branching portions 115, and four second branching waveguides 116. The SiPh chip 101 has four third branching portions 117, and four third branching waveguides 118A.

[0024] The SiPh chip 101 has eight fourth branching waveguides 118B, four first DC (Direct Current) modulation sections 140, and four first multiplexing sections 119. The SiPh chip 101 has four fifth branching waveguides 120, two second DC modulation sections 150, a second multiplexing section 121, and two first output waveguides 122. The SiPh chip 101 has one PR (Polarization Rotator) 124, one PBC (Polarization Beam Combiner) 125, and one second output waveguide 123.

[0025] The thin-film LN chip 102 has eight parallel waveguides 131 and four RF (Radio Frequency) modulation units 130. The first waveguide 111 in the SiPh chip 101 is, for example, a Si waveguide that propagates the signal light from the first port 101B. The input end of the first waveguide 111 is coupled to an input-side optical fiber 103A in the optical fiber array 103. The first waveguide 111 outputs the signal light from the first port 101B to the return waveguide 112. The return waveguide 112 is, for example, a Si waveguide that propagates the signal light from the first waveguide 111.

[0026] The first branching section 113 branches the signal light from the folded waveguide 112 into two first branching waveguides 114. The first branching waveguide 114 is, for example, a Si waveguide that propagates the signal light from the first branching section 113. The second branching section 115 branches the signal light from the first branching waveguide 114 into two second branching waveguides 116. The second branching waveguide 116 is, for example, a Si waveguide that propagates the signal light from the second branching section 115. The third branching section 117 branches the signal light from the second branching waveguide 116 into two third branching waveguides 118A.

[0027] The RF modulation unit 130 is a phase modulation unit that performs high-speed modulation of the signal light propagating through the parallel waveguide 131 connected to the third branch waveguide 118A. The RF modulation unit 130 has two parallel waveguides 131 arranged in parallel, a plurality of RF electrodes 132 arranged in parallel to the two parallel waveguides 131, and an RF driver 133 that inputs a high-frequency signal to the RF electrode 132. The RF modulation unit 130 further has an RF termination 134 that terminates the high-frequency signal of the RF electrode 132. The two parallel waveguides 131 are LN waveguides. When a high-frequency signal having a band of, for example, several tens of GHz is input from the RF driver 133 to the RF electrode 132, the RF modulation unit 130 can perform high-speed modulation of the signal light propagating through the parallel waveguide 131 in response to the high-frequency signal. The SiPh chip 101 has an electrode wire 126 that electrically connects the RF electrode 132 in the thin-film LN chip 102 and the RF driver 133. The electrode wire 126 is made of, for example, Al.

[0028] The first DC modulation unit 140 has two fourth branch waveguides 118B arranged in parallel, a plurality of first heater electrodes 141 arranged in parallel on the two fourth branch waveguides 118B, and an electrode wire 142 electrically connected to the first heater electrode 141. The two fourth branch waveguides 118B are, for example, Si waveguides. The first DC modulation unit 140 is a phase adjustment unit that connects the two parallel waveguides 131 in the RF modulation unit 130 and the two fourth branch waveguides 118B in the first DC modulation unit 140 and modulates the signal light propagating through the two fourth branch waveguides 118B. When a current flows through the first heater electrode 141, the first DC modulation unit 140 heats the fourth branch waveguide 118B with heat generated by the first heater electrode 141. As a result, the refractive index of the fourth branch waveguide 118B changes due to the thermo-optic effect, and the phase of the signal light propagating through the fourth branch waveguide 118B can be adjusted. The first multiplexing section 119 multiplexes the signal light from the two fourth branch waveguides 118B in the first DC modulation section 140, and outputs the multiplexed signal light to the fifth branch waveguide 120.

[0029] The second DC modulation section 150 has two fifth branch waveguides 120 arranged in parallel, a second heater electrode 151 arranged on the two fifth branch waveguides 120, and an electrode line 152 electrically connected to the second heater electrode 151. The two fifth branch waveguides 120 are, for example, Si waveguides. The second DC modulation section 150 is a phase adjustment section that connects two first multiplexing sections 119 and the two fifth branch waveguides 120 in the second DC modulation section 150 and modulates the signal light propagating through the two fifth branch waveguides 120 in the second DC modulation section 150. When a current flows through the second heater electrode 151, the second DC modulation section 150 heats the fifth branch waveguide 120 with heat generated by the second heater electrode 151. As a result, the refractive index of the fifth branch waveguide 120 changes due to the thermo-optic effect, and it is possible to adjust the phase of the signal light propagating through the fifth branch waveguide 120. The second DC modulation unit 150 modulates the signal light propagating through the fifth branch waveguide 120, and outputs the modulated signal light to the second multiplexing unit 121. The second multiplexing unit 121 multiplexes the modulated signal light from the two fifth branch waveguides 120 in the second DC modulation unit 150, and outputs the multiplexed signal light to the first output waveguide 122.

[0030] One second multiplexing section 121 multiplexes the signal light from the two fifth branching waveguides 120 in one second DC modulation section 150, and outputs the multiplexed signal light to one first output waveguide 122. The other second multiplexing section 121 multiplexes the signal light from the two fifth branching waveguides 120 in the other second DC modulation section 150, and outputs the multiplexed signal light to the other first output waveguide 122. The first output waveguide 122 is, for example, a Si waveguide that propagates the signal light from the second multiplexing section 121.

[0031] The PR 124 rotates the polarization of the signal light from the second multiplexing unit 121 on one side via the first output waveguide 122 on one side, and outputs the signal light after the polarization rotation to the PBC 125. The PBC 125 performs polarization multiplexing of the signal light after the polarization rotation from the PR 124 and the signal light from the other second multiplexing unit 121 via the other first output waveguide 122, and outputs the signal light after the polarization multiplexing to the optical fiber 103B on the output side of the optical fiber array 103.

[0032] In the optical modulator 100, the first DC modulation section 140, the second DC modulation section 150, the PR 124, and the PBC 125 are integrated on the SiPh chip 101, and the thin-film LN chip 102 is mounted on the SiPh chip 101, thereby making it possible to reduce the chip size of the optical modulator 100. Moreover, since it is not necessary to adjust the optical axes of the input optical fiber 103A and the output optical fiber 103B, the PR 145, and the PBC 146 in the optical fiber array 103, respectively, it is possible to reduce the mounting cost.

[0033] However, in the optical modulator 100, the RF electrode 132 of the thin-film LN chip 102 is connected to the RF driver 133 via the electrode wire 126 on the SiPh chip 101. As a result, since the electrode wire 126 uses Al, the propagation loss of the high-frequency signal increases, degrading the modulation bandwidth. Moreover, the loss per unit length of the waveguide of the SiPh chip 101 is large, resulting in a large loss of light.

[0034] Therefore, an embodiment for dealing with such a situation will be described below as Example 1. EXAMPLES

[0035] 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 has a SiPh chip 2 which is a first chip on which a first DC modulation section 40 having a first heater electrode 41 and a second DC modulation section 50 having a second heater electrode 51 are mounted. The optical modulator 1 has a thin-film LN (Lithium Niobate: LiNbO 3 The thin-film LN chip 3 has a material with a higher electro-optic effect than the SiPh chip 2, and an optical fiber array 4 including an input optical fiber 4A and an output optical fiber 4B.

[0036] The SiPh chip 2 has a Si substrate, an opening 2A that opens a part of the Si substrate, a first port 2B arranged on one end face, and a second port 2C arranged on the other end face. The opening 2A is a structure in which a part of the Si substrate is dug down by etching. The opening 2A is the portion where the thin film LN chip 3 is mounted.

[0037] The SiPh chip 2 has one first waveguide 11, one first branching section 12, two first pre-stage branching waveguides 13, two second branching sections 14, and four second pre-stage branching waveguides 15. The first pre-stage branching waveguide 13 and the second pre-stage branching waveguide 15 form a pre-stage branching waveguide 60. The SiPh chip 101 has four third branching sections 16, eight first branching waveguides 17A, a folded parallel waveguide 18, and eight first parallel waveguides 17B. The first branching waveguide 17A is formed as a post-stage branching waveguide.

[0038] The SiPh chip 2 has eight second branch waveguides 17C, four first multiplexing sections 19, four third branch waveguides 20, two second multiplexing sections 21, and two first output waveguides 22. The SiPh chip 2 has one PR (Polarization Rotator) 24, one PBC (Polarization Beam Combiner) 25, and one second output waveguide 23.

[0039] The SiPh chip 2 has four first DC (Direct Current) modulation units 40 and two second DC modulation units 50. The thin-film LN chip 3 has eight second parallel waveguides 31 and four RF (Radio Frequency) modulation units 30.

[0040] The first waveguide 11 in the SiPh chip 2 is, for example, a Si waveguide that propagates the signal light from the first port 2B. The input end of the first waveguide 11 is coupled to the input side optical fiber 4A in the optical fiber array 4. The first waveguide 11 outputs the signal light from the first port 2B to the first branching section 12.

[0041] The first branching section 12 branches the signal light from the first waveguide 11 into two first pre-stage branching waveguides 13. The first pre-stage branching waveguide 13 is, for example, a Si waveguide that propagates the signal light from the first branching section 12. The first pre-stage branching waveguide 13 outputs the signal light from the first branching section 12 to the second branching section 14. The second branching section 14 branches the signal light from the first pre-stage branching waveguide 13 into two second pre-stage branching waveguides 15. The second pre-stage branching waveguide 15 is, for example, a Si waveguide that propagates the signal light from the second branching section 14. The second pre-stage branching waveguide 15 outputs the signal light from the second branching section 14 to the third branching section 16. The third branching section 16 branches the signal light from the second front-stage branching waveguide 15 into two first branching waveguides 17A.

[0042] The first branching waveguide 17A is, for example, a Si waveguide that propagates the signal light from the third branching section 16. The first branching waveguide 17A outputs the signal light from the third branching section 16 to the folded parallel waveguide 18. The folded parallel waveguide 18 is, for example, a Si waveguide that propagates the signal light from the first branching waveguide 17A. The folded parallel waveguide 18 outputs the signal light from the first branching waveguide 17A to the first parallel waveguide 17B. The first parallel waveguide 17B is, for example, a Si waveguide that propagates the signal light from the first branching waveguide 17A. The first parallel waveguide 17B is coupled to the second parallel waveguide 31 in the thin-film LN chip 3.

[0043] The second DC modulation unit 50 has two second pre-branching waveguides 15 arranged in parallel, a second heater electrode 51 arranged on the two second pre-branching waveguides 15, and an electrode line 52 electrically connected to the second heater electrode 51. The two second pre-branching waveguides 15 are, for example, Si waveguides. The second DC modulation unit 50 is a phase adjustment unit that modulates the signal light propagating through the two second pre-branching waveguides 15. When a current flows through the second heater electrode 51, the second DC modulation unit 50 heats the second pre-branching waveguide 15 with heat generated by the second heater electrode 51. As a result, the refractive index of the second pre-branching waveguide 15 changes due to the thermo-optic effect, so that the phase of the signal light propagating through the second pre-branching waveguide 15 can be adjusted. The second DC modulation section 50 modulates the signal light propagating through the second front-stage branching waveguide 15 , and outputs the modulated signal light to the third branching section 16 .

[0044] The first DC modulation unit 40 has two first branch waveguides 17A arranged in parallel, a plurality of first heater electrodes 41 arranged in parallel on the two first branch waveguides 17A, and an electrode wire 42 electrically connected to the first heater electrodes 41. The first DC modulation unit 40 is a phase adjustment unit that modulates the signal light propagating through the two first branch waveguides 17A. When a current flows through the first heater electrode 41, the first DC modulation unit 40 heats the first branch waveguide 17A with the heat generated by the first heater electrode 41. As a result, the refractive index of the first branch waveguide 17A changes due to the thermo-optic effect, and the phase of the signal light propagating through the first branch waveguide 17A can be adjusted. The first DC modulation section 40 modulates the signal light propagating through the first branching waveguide 17A, and outputs the modulated signal light to the first parallel waveguide 17B.

[0045] The RF modulation unit 30 is a phase modulation unit that performs high-speed modulation of the signal light propagating through the second parallel waveguide 31 connected to the first parallel waveguide 17B. The RF modulation unit 30 has two second parallel waveguides 31 arranged in parallel, a plurality of RF electrodes 32 arranged in parallel to the two second parallel waveguides 31, and an RF driver 33 that is a driver circuit that inputs a high-frequency signal to the RF electrode 32. Furthermore, the RF modulation unit 30 has an RF termination 34 that terminates the high-frequency signal of the RF electrode 32. When a high-frequency signal having a band of, for example, several tens of GHz is input from the RF driver 33 to the RF electrode 32, the RF modulation unit 30 can perform high-speed modulation of the signal light propagating through the second parallel waveguide 31 in response to the high-frequency signal. The SiPh chip 2 has an electrode line 26 that electrically connects the RF electrode 32 in the thin-film LN chip 3 to the RF driver 33. The RF modulation section 30 performs high-speed modulation on the signal light propagating through the second parallel waveguide 31, and outputs the high-speed modulated signal light to the second branching waveguide 17C.

[0046] The optical modulator 1 is configured with eight branch waveguides Wg1 to Wg8 that constitute a Mach-Zehnder interferometer. Each branch waveguide is configured by connecting the waveguide of the first branch waveguide 17A, the waveguide of the folded parallel waveguide 18, the waveguide of the first parallel waveguide 17B, the waveguide of the second parallel waveguide 31, and the waveguide of the second branch waveguide 17C. The Mach-Zehnder interferometer has six branch waveguides Wg2 to Wg7 arranged in parallel between the branch waveguide Wg1 on the inner circumference side of the folded parallel waveguide 18 and the branch waveguide Wg8 on the outer circumference side of the folded parallel waveguide 18.

[0047] The second branching waveguide 17C is coupled to the second parallel waveguide 31 in the RF modulation unit 30, and outputs the signal light modulated by the RF modulation unit 30 to the first multiplexing unit 19. The first multiplexing unit 19 multiplexes the signal lights from the second branching waveguides 17C, and outputs the multiplexed signal light to the third branching waveguide 20. The second multiplexing unit 21 multiplexes the signal lights from the respective third branching waveguides 20, and outputs the multiplexed signal light to the first output waveguide 22.

[0048] One of the second multiplexing sections 21 multiplexes the signal light from the two third branching waveguides 20, and outputs the multiplexed signal light to one of the first output waveguides 22. The other of the second multiplexing sections 21 multiplexes the signal light from the other two third branching waveguides 20, and outputs the multiplexed signal light to the other first output waveguide 22.

[0049] The PR 24 rotates the polarization of the signal light from one of the second multiplexing sections 21 via one of the first output waveguides 22, and outputs the signal light after the polarization rotation to the PBC 25. The PBC 25 performs polarization multiplexing of the signal light after the polarization rotation from the PR 24 and the signal light from the other of the second multiplexing sections 21 via the other of the first output waveguides 22, and outputs the signal light after the polarization multiplexing to the optical fiber 4B on the output side of the optical fiber array 4.

[0050] In the optical modulator 1 of the first embodiment, the first DC modulation section 40 and the second DC modulation section 50 are arranged before the input stage of the folded parallel waveguide 18, and the RF modulation section 30 is arranged after the output stage of the folded parallel waveguide 18, so that the chip size of the entire optical modulator 1 can be reduced.

[0051] In the optical modulator 1, the RF electrode 32 of the thin-film LN chip 3 is connected to the RF driver 33 via the electrode wire 26 on the SiPh chip 2, but the electrode wire 26 on the SiPh chip 2 is shorter than that in Fig. 5. As a result, the propagation loss of the high-frequency signal is reduced, and the speed of the high-frequency signal and the signal light are matched, thereby suppressing deterioration of the modulation bandwidth. Moreover, although the loss per unit length of the waveguide of the SiPh chip 2 increases, the length of the waveguide of the SiPh chip 2 can be shortened in the longitudinal direction of the SiPh chip 2, so that the loss of light can be reduced.

[0052] In the optical modulator 1, since the eight branch waveguides Wg1 to Wg8 constituting the Mach-Zehnder interferometer have different waveguide lengths, a phase difference between the branch waveguides may occur when the temperature changes, causing the output of the signal light to become unstable. Therefore, an embodiment for dealing with such a situation will be described below as Example 2. EXAMPLES

[0053] 2 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 according to the first embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The optical modulator 1A according to the second embodiment differs from the optical modulator 1 according to the first embodiment in that the optical modulator 1A according to the second embodiment includes a front-stage branching waveguide 60A that is disposed in a direction substantially perpendicular to the first waveguide 11 and connects between the first waveguide 11 and the first branching waveguide 17A1.

[0054] The first waveguide 11 and the first branching waveguide 17A1 are arranged in parallel with the second parallel waveguide 31. The pre-stage branching waveguide 60A has two first pre-stage branching waveguides 13A connected to the first waveguide 11, and two second pre-stage branching waveguides 15A connected to the first pre-stage branching waveguide 13A. The pre-stage branching waveguide 60A is a waveguide that is connected to the first waveguide 11 and changes the traveling direction of the first waveguide 11. The two first branching waveguides 17A1 connected to the second pre-stage branching waveguide 15A are rear-stage branching waveguides that connect between the pre-stage branching waveguide 60A and the folded parallel waveguide 18 and return the traveling direction to the original traveling direction.

[0055] The first branch waveguide 17A1 has a fourth branch waveguide 17A11 connected to the second front-stage branch waveguide 15A, and a curved parallel waveguide 17A12 having a curved structure connected to the fourth branch waveguide 17A11. The first branch waveguide 17A1 has a parallel waveguide 17A13 connecting between the curved parallel waveguide 17A12 and the folded parallel waveguide 18.

[0056] The eight branch waveguides Wg1 to Wg8 constituting the Mach-Zehnder interferometer of the optical modulator 1A are waveguides between the second branching section 14 and the first multiplexing section 19. Each branch waveguide is formed by connecting the waveguide of the first branch waveguide 17A1, the waveguide of the folded parallel waveguide 18, the waveguide of the first parallel waveguide 17B, the waveguide of the second parallel waveguide 31, and the waveguide of the second branch waveguide 17C. In the Mach-Zehnder interferometer, six branch waveguides Wg2 to Wg7 are arranged in parallel between the branch waveguide Wg1 on the inner circumference side of the folded parallel waveguide 18 and the branch waveguide Wg8 on the outer circumference side of the folded parallel waveguide 18.

[0057] The optical modulator 1A has a first pitch P1 indicating the pitch between the inner fourth branch waveguide 17A11 (Wg1) and the outer fourth branch waveguide 17A11 (Wg8) among the multiple parallel fourth branch waveguides 17A11. The optical modulator 1A has a second pitch P2 indicating the pitch between the inner folded parallel waveguide 18 (Wg1) and the outer folded parallel waveguide 18 (Wg8) among the multiple parallel folded parallel waveguides 18. Furthermore, the optical modulator 1A has a third pitch P3 indicating the pitch between the inner first parallel waveguide 17B (Wg1) and the outer first parallel waveguide 17B (Wg8) among the multiple parallel first parallel waveguides 17B.

[0058] When P1 = P2 = P3 = 0, there is no waveguide length difference among the branched waveguides Wg1 to Wg8. That is, the difference in waveguide length between the inner peripheral side branched waveguide Wg1 and the outer peripheral side branched waveguide Wg8 is 0. Also, when P1 = P2 = 0 and P3 > 0, the difference in waveguide length between the inner peripheral side branched waveguide Wg1 and the outer peripheral side branched waveguide Wg8 is -P3. That is, the waveguide length of the branched waveguide Wg8 becomes P3 longer than that of the branched waveguide Wg1. Further, when P1 = P3 = 0 and P2 > 0, the difference in waveguide length between the inner peripheral side branched waveguide Wg1 and the outer peripheral side branched waveguide Wg8 is -2×P2. That is, the waveguide length of the branched waveguide Wg8 becomes 2×P2 longer than that of the branched waveguide Wg1. Also, when P2 = P3 = 0 and P1 > 0, the difference in waveguide length between the inner peripheral side branched waveguide Wg1 and the outer peripheral side branched waveguide Wg8 is P1. That is, the waveguide length of the branched waveguide Wg1 becomes P1 longer than that of the branched waveguide Wg8. Therefore, by setting P1 = 2×P2 + P3, the lengths of the waveguide of the branched waveguide Wg1 and the branched waveguide Wg8 can be made equal. For the other branched waveguides Wg2 to Wg7, the waveguide lengths can be made equal by establishing the same relationship. Also, the condition for shortening the folded parallel waveguide 18 and shortening the electrode line 26 on the SiPh chip 2 is P3 > P2. To make the waveguide lengths of each of the branched waveguides Wg1 to Wg8 equal, P1 > P3 is required.

[0059] Then, the optical modulator 1A arranges the fourth branched waveguide 17A11, the folded parallel waveguide 18, and the first parallel waveguide 17B so that the magnitude relationship of P1 > P3 > P2 is satisfied. As a result, since the waveguide lengths of the eight branched waveguides constituting the Mach-Zehnder interferometer are the same, even when the temperature changes, there is no optical phase difference among the waveguides in the branched waveguide, and the output of the signal light can be stabilized.

[0060] The second DC modulation unit 50A is arranged for each waveguide in the second pre-stage branched waveguide 15A, has a second heater electrode 51A to which a DC electrical signal is applied to the second DC modulation unit 50A, and each second heater electrode 51A is arranged in parallel in a direction substantially orthogonal to the first waveguide 11.

[0061] In the optical modulator 1A of the second embodiment, the fourth branch waveguide 17A11, the folded parallel waveguide 18, and the first parallel waveguide 17B are arranged so that the magnitude relationship of P1>P3>P2 is satisfied. As a result, the eight branch waveguides Wg1 to Wg8 constituting the Mach-Zehnder interferometer have the same waveguide length, so that even if the temperature changes, there is no optical phase difference between the waveguides in the branch waveguides, and the output of the signal light can be stabilized.

[0062] In the optical modulator 1A of the second embodiment, the fourth branch waveguide 17A11, the folded parallel waveguide 18, and the first parallel waveguide 17B are arranged so that the magnitude relationship of P1>P3>P2 is satisfied. However, the present invention is not limited to this. As described above, the optical modulator 1A may arrange the fourth branch waveguide 17A11, the folded parallel waveguide 18, and the first parallel waveguide 17B so that the relationship of (P1=P2×2+P3) is satisfied. Even in this case, the eight branch waveguides constituting the Mach-Zehnder interferometer have the same waveguide length, so that there is no optical phase difference between the waveguides in the branch waveguide even when the temperature changes, and the output of the signal light can be stabilized.

[0063] In the optical modulator 1A of the second embodiment, the second pre-branching waveguide 15A between the second branching section 14 and the third branching section 16 is disposed in a direction substantially perpendicular to the first waveguide 11, and the second heater electrode 51A in the second DC modulation section 50A is disposed in the second pre-branching waveguide 15A. However, by disposing the second heater electrode 51A in the second pre-branching waveguide 15A, the chip size of the optical modulator 1A in the direction perpendicular to the first waveguide 11 increases. Therefore, an embodiment capable of dealing with such a situation will be described below as the third embodiment. EXAMPLES

[0064] 3 is a schematic plan view showing an example of an optical modulator 1B according to a third embodiment. The same components as those of the optical modulator 1A according to the second embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The optical modulator 1B according to the third embodiment differs from the optical modulator 1A according to the second embodiment in that the second pre-branching waveguide 15B is arranged in a direction substantially parallel to the first waveguide 11, rather than being arranged in a direction perpendicular to the first waveguide 11. The optical modulator 1B has a pre-branching waveguide 60B between the first waveguide 11 and the first branching waveguide 17A1.

[0065] The pre-stage branching waveguide 60B has two first pre-stage branching waveguides 13B connected to the first waveguide 11, and two second pre-stage branching waveguides 15B connected to the first pre-stage branching waveguide 13B. The pre-stage branching waveguide 60B is a waveguide that is connected to the first waveguide 11 and changes the traveling direction of the first waveguide 11. The two first branching waveguides 17A1 connected to the second pre-stage branching waveguide 15B are rear-stage branching waveguides that connect between the pre-stage branching waveguide 60B and the folded parallel waveguide 18 and return the traveling direction to the original traveling direction.

[0066] The first branch waveguide 17A1 has a fourth branch waveguide 17A11 connected to the second front-stage branch waveguide 15B, and a curved parallel waveguide 17A12 having a curved structure connected to the fourth branch waveguide 17A11. The first branch waveguide 17A1 has a parallel waveguide 17A13 connecting between the curved parallel waveguide 17A12 and the folded parallel waveguide 18.

[0067] The second DC modulation section 50B is arranged for each waveguide in the second front-stage branching waveguide 15B, and has a second heater electrode 51B that applies a DC electrical signal to the second DC modulation section 50B, and each second heater electrode 51B is arranged in parallel in a direction approximately parallel to the first waveguide 11.

[0068] In the optical modulator 1B of the third embodiment, the second heater electrodes 51B are arranged for each waveguide in the second front-stage branching waveguide 15B, and the second heater electrodes 51B of the second DC modulation section 50B are arranged in parallel in a direction substantially parallel to the first waveguide 11. As a result, the chip size of the optical modulator 1B in the direction perpendicular to the first waveguide 11 can be reduced.

[0069] FIG. 4 is an explanatory diagram showing an example of an optical transceiver 70 according to the present embodiment. The optical transceiver 70 shown in FIG. 4 is connected to an output optical fiber and an input optical fiber. The optical transceiver 70 includes a light source 71, a DSP (Digital Signal Processor) 72, and an optical transceiver 73. The optical transceiver 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 including the transmission data, and outputs the generated electrical signal to the optical transmitter 73A. The DSP 72 also obtains an electrical signal including reception data from the optical receiver 73B, and obtains reception data by performing processing such as decoding the obtained electrical signal.

[0070] The light source 71 includes, for example, a laser diode, and generates light of a predetermined wavelength and supplies it to the optical transmitter 73A and the optical receiver 73B. The optical transmitter 73A includes an optical modulator element 73A1 that modulates the light supplied from the light source 71 by an electrical signal output from the DSP 72 and outputs the modulated signal light to an optical fiber.

[0071] The optical modulator element 73A1 includes a first chip having a first port and a second port, and a second chip disposed on the first chip and having a material with a higher electro-optic effect than the first chip. The first chip includes a first waveguide connected to the first port and propagating a signal light from the first port, and a first branch waveguide having a branch structure connected to the first waveguide and propagating the signal light from the first waveguide. The first chip includes a folded parallel waveguide connected to the first branch waveguide and having a folded structure, and a first parallel waveguide connected to the folded parallel waveguide and propagating the signal light from the folded parallel waveguide. The first chip has a second waveguide connected to the second port and propagating the signal light to the second port, and a second branch waveguide of a branch structure connected to the second waveguide and propagating the signal light to the second waveguide. The first chip has a phase adjustment unit disposed in the first branch waveguide and adjusting the phase of the signal light propagating through the first branch waveguide in response to a DC electric signal. The second chip has a second parallel waveguide coupled to the first parallel waveguide at a first end face and coupled to the second branch waveguide at a second end face different from the first end face. The second parallel waveguide propagates the signal light from the first parallel waveguide to the second branch waveguide. The second chip has a phase modulation unit disposed in the second parallel waveguide and modulating the phase of the signal light propagating through the second parallel waveguide in response to a high frequency signal.

[0072] The optical transmitter 73A generates signal light by modulating the light supplied from the light source 71 with an electrical signal as the light propagates through the waveguide. The optical receiver 73B has an optical receiver element 73B1 that receives incoming light from an optical fiber, converts the incoming light into an electrical signal using the light supplied from the light source 71, and outputs the converted electrical signal to the DSP 72.

[0073] In the optical transceiver 70, the optical transmitter 73A and the optical receiver 73B are built in, but the present invention can also be applied to an optical transmitter that only has the optical transmitter 73A with an optical device built in. In addition, the present invention is not limited to the optical transceiver 70, and the optical device can also be applied to the optical transmitter / receiver 73.

[0074] For ease of explanation, the optical modulator 1 of the embodiment is illustrated as being arranged along the path of the first port 2B → first waveguide 11 → second DC modulation unit 50 → first DC modulation unit 40 → folded parallel waveguide 18 → RF modulation unit 30 of the thin-film LN chip 3 → PBC 25 → second port 2C. However, this is not limited to this, and the arrangement may be along the path of the first port 2B → first waveguide 11 → RF modulation unit 30 of the thin-film LN chip 3 → folded parallel waveguide 18 → first DC modulation unit 40 → second DC modulation unit 50 → PBC 25 → second port 2C.

[0075] In addition, the phase adjustment unit is configured with the first DC modulation unit 40 and the second DC modulation unit 50, but is not limited to this and may be any one of the first DC modulation unit 40 and the second DC modulation unit 50, and can be modified as appropriate. Furthermore, the first DC modulation unit 40 and the second DC modulation unit 50 use heater electrodes, but may use electrodes that apply a bias voltage, and can be modified as appropriate.

[0076] In this embodiment, a thin film LN chip is exemplified, but the present invention is not limited to this, and may be, for example, TF-Barium Titanate, and may be changed as appropriate. As a material for the electro-optic effect, for example, TF-BTO (BaTiO 3 ), TF-PLZT(PbLaZrTiO 3 ), TF-PZT(PbZrTiO 3 ) may be used and can be changed as appropriate.

[0077] In this embodiment, the material of the electrode wire is not limited to Al, Au, Cu, etc., and can be changed as appropriate. [Explanation of symbols]

[0078] 1 Optical Modulator 2. SiPh chip 3 Thin film LN chip 11 First Waveguide 17A First branch waveguide 17B First parallel waveguide 17C Second branch waveguide 18 Folded parallel waveguide 23 Second output waveguide 26 Electrode wire 30 RF Modulation Section 31 Second parallel waveguide 32 RF electrode 33 RF Driver 40 First DC modulation section 41 First heater electrode 50 Second DC Modulation Section 51 Second heater electrode

Claims

1. a first chip having a first port and a second port; a second chip disposed on the first chip and having a material with a higher electro-optic effect than the first chip, The first chip includes: a first waveguide connected to the first port and propagating a signal light from the first port; a first branching waveguide having a branching structure, the first branching waveguide being connected to the first waveguide and propagating a signal light from the first waveguide; a folded parallel waveguide connected to the first branch waveguide and having a folded structure; a first parallel waveguide connected to the folded parallel waveguide and propagating a signal light from the folded parallel waveguide; a second waveguide connected to the second port and configured to propagate the signal light to the second port; a second branching waveguide having a branching structure, the second branching waveguide being connected to the second waveguide and propagating the signal light to the second waveguide; a phase adjustment unit disposed in the first branching waveguide and configured to adjust the phase of the signal light propagating through the first branching waveguide in response to a DC electric signal; having The second chip includes: a second parallel waveguide that is coupled to the first parallel waveguide at a first end face and is coupled to the second branch waveguide at a second end face different from the first end face, and that propagates signal light from the first parallel waveguide to the second branch waveguide; a phase modulation unit disposed in the second parallel waveguide and phase-modulating the signal light propagating through the second parallel waveguide in response to a high-frequency signal; An optical device comprising:

2. The first chip includes:

2. The optical device according to claim 1, further comprising an electrode line arranged in parallel with the first parallel waveguide, electrically connecting between an electrode in the phase modulation section and a driver circuit.

3. The first branch waveguide has a front-stage branching waveguide connected to the first waveguide and changing a propagation direction of the first waveguide; a rear branching waveguide that connects between the front branching waveguide and the folded parallel waveguide and returns the propagation direction to the original propagation direction; 2. The optical device according to claim 1, further comprising:

4. The front-stage branching waveguide is a plurality of first pre-branching waveguides connected to the first waveguide; a second pre-branching waveguide connected to the first pre-branching waveguide, The rear branching waveguide is a fourth branching waveguide connected to each of the waveguides in the second front-stage branching waveguide; a parallel waveguide connecting the fourth branch waveguide and the folded parallel waveguide; a first pitch indicating a pitch interval between the fourth branch waveguides at both ends among the plurality of fourth branch waveguides running in parallel; a second pitch indicating a pitch interval between the folded parallel waveguides at both ends of the plurality of folded parallel waveguides running in parallel; a third pitch indicating a pitch interval between the first parallel waveguides at both ends among the plurality of first parallel waveguides running in parallel; 4. The optical device according to claim 3, wherein the first pitch, the second pitch, and the third pitch satisfy the relationship: (first pitch>third pitch>second pitch).

5. The front-stage branching waveguide is a plurality of first pre-branching waveguides connected to the first waveguide; a second pre-branching waveguide connected to the first pre-branching waveguide, The rear branching waveguide is a fourth branching waveguide connected to each of the waveguides in the second front-stage branching waveguide; a parallel waveguide connecting the fourth branch waveguide and the folded parallel waveguide; a first pitch indicating a pitch interval between the fourth branch waveguides at both ends among the plurality of fourth branch waveguides running in parallel; a second pitch indicating a pitch interval between the folded parallel waveguides at both ends of the plurality of folded parallel waveguides running in parallel; a third pitch indicating a pitch interval between the first parallel waveguides at both ends among the plurality of first parallel waveguides running in parallel; 4. The optical device according to claim 3, wherein the first pitch, the second pitch, and the third pitch satisfy a relationship of (first pitch=second pitch×2+third pitch).

6. The phase adjustment unit is 6. The optical device according to claim 4, further comprising an electrode arranged for each waveguide in the second front-stage branching waveguide, for applying a DC electrical signal to the phase adjustment section, the electrodes being arranged in parallel in a direction substantially perpendicular to the first waveguide.

7. The second pre-branching waveguide is disposed substantially parallel to the first waveguide; The phase adjustment unit is 6. The optical device according to claim 4, further comprising an electrode arranged for each waveguide in the second front-stage branching waveguide, for applying a DC electrical signal to the phase adjustment section, the electrodes being arranged in parallel in a direction substantially parallel to the first waveguide.

8. A light source that emits light; an optical modulator element that modulates the light from the light source in response to an electrical signal, The optical modulator element includes: a first chip having a first port and a second port; a second chip disposed on the first chip and having a material with a higher electro-optic effect than the first chip; The first chip includes: a first waveguide connected to the first port and propagating a signal light from the first port; a first branching waveguide having a branching structure, the first branching waveguide being connected to the first waveguide and propagating a signal light from the first waveguide; a folded parallel waveguide connected to the first branch waveguide and having a folded structure; a first parallel waveguide connected to the folded parallel waveguide and propagating a signal light from the folded parallel waveguide; a second waveguide connected to the second port and configured to propagate the signal light to the second port; a second branching waveguide having a branching structure, the second branching waveguide being connected to the second waveguide and propagating the signal light to the second waveguide; a phase adjustment unit disposed in the first branching waveguide and configured to adjust the phase of the signal light propagating through the first branching waveguide in response to a DC electric signal; having The second chip includes: a second parallel waveguide that is coupled to the first parallel waveguide at a first end face and is coupled to the second branch waveguide at a second end face different from the first end face, and that propagates signal light from the first parallel waveguide to the second branch waveguide; a phase modulation unit disposed in the second parallel waveguide and phase-modulating the signal light propagating through the second parallel waveguide in response to a high-frequency signal; 1. An optical transmitter comprising:

9. A processor that performs signal processing on the electrical signal; A light source that generates light; an optical transmitter that modulates light generated from the light source using an electrical signal output from the processor; an optical receiver that converts received light into an electrical signal using the light generated by the light source, The optical transmitter includes: a first chip having a first port and a second port; a second chip disposed on the first chip and having a material with a higher electro-optic effect than the first chip; The first chip includes: a first waveguide connected to the first port and propagating a signal light from the first port; a first branching waveguide having a branching structure, the first branching waveguide being connected to the first waveguide and propagating a signal light from the first waveguide; a folded parallel waveguide connected to the first branch waveguide and having a folded structure; a first parallel waveguide connected to the folded parallel waveguide and propagating a signal light from the folded parallel waveguide; a second waveguide connected to the second port and configured to propagate the signal light to the second port; a second branching waveguide having a branching structure, the second branching waveguide being connected to the second waveguide and propagating the signal light to the second waveguide; a phase adjustment unit disposed in the first branching waveguide and configured to adjust the phase of the signal light propagating through the first branching waveguide in response to a DC electric signal; having The second chip includes: a second parallel waveguide that is coupled to the first parallel waveguide at a first end face and is coupled to the second branch waveguide at a second end face different from the first end face, and that propagates signal light from the first parallel waveguide to the second branch waveguide; a phase modulation unit disposed in the second parallel waveguide and phase-modulating the signal light propagating through the second parallel waveguide in response to a high-frequency signal; 1. An optical transceiver comprising:

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