Optical device, optical transmitter, optical receiver, and optical transceiver

Abstract

To provide an optical device and the like that can reduce the loss of light due to a heater electrode.SOLUTION: An optical device includes a waveguide which light is guided through and has an electro-optic effect and a thermo-optic effect, and a heater electrode which is disposed on one side surface of the waveguide and heats the waveguide. The optical device includes a parallel electrode disposed on the other side surface of the waveguide, is electrically connected to the heater electrode, and has higher resistance than the heater electrode.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

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

[0003] The optical modulator is, for example, a Mach-Zehnder modulator. FIG. 21 is a schematic plan view showing an example of a conventional optical modulator 200. The optical modulator 200 is, for example, configured with a thin-film LN (Lithium Niobate: LiNbO3) chip. The optical modulator 200 has one first input waveguide 201, one first branching section 202, two second input waveguides 203, two second branching sections 204, four third input waveguides 205, and four third branching sections 206. The optical modulator 200 has eight first waveguides 207, four RF (Radio Frequency) modulation sections 208, four first DC (Direct Current) modulation sections 209, and four first multiplexing sections 210. The optical modulator 200 has four second waveguides 211, two second DC modulation sections 212, a second multiplexing section 213, and two first output waveguides 214. The optical modulator 200 has one PR (Polarization Rotator) 215, one PBC (Polarization Beam Combiner) 216, and one second output waveguide 217.

[0004] The first input waveguide 201 is, for example, an LN waveguide that guides the signal light. The first branching unit 202 branches the signal light from the first input waveguide 201 into two second input waveguides 203. The second input waveguide 203 is, for example, an LN waveguide that guides the signal light from the first branching unit 202. The second branching unit 204 branches the signal light from the second input waveguide 203 into two third input waveguides 205. The third branching unit 206 branches the signal light from the third input waveguide 205 into two first waveguides 207A in the RF modulation unit 208.

[0005] The RF modulation unit 208 is a modulation unit that performs high-speed modulation of the signal light guided through the two first waveguides 207A. The RF modulation unit 208 has two first waveguides 207A (207) arranged in parallel, and a plurality of RF electrodes 221 arranged in parallel to the two first waveguides 207A. The two first waveguides 207A are LN waveguides. The RF electrode 221 has two ground electrodes 221B arranged in parallel on the outside of the two first waveguides 207A, and one signal electrode 221A arranged in parallel to the first waveguide 207A between the two first waveguides 207A. When a high-frequency signal having a band of, for example, several tens of GHz is input to the signal electrode 221A, the RF modulation unit 208 can perform high-speed modulation of the signal light guided through the first waveguide 207A in response to the high-frequency signal.

[0006] The first DC modulation unit 209 has two first waveguides 207B (207) arranged in parallel and a plurality of heater electrodes 222 arranged in parallel on the two first waveguides 207B. The two first waveguides 207B are, for example, LN waveguides. The first DC modulation unit 209 is a modulation unit that connects the two first waveguides 207A in the RF modulation unit 208 and the two first waveguides 207B in the first DC modulation unit 209 and modulates the signal light guided through the two first waveguides 207B in the first DC modulation unit 209. When a current flows through the heater electrode 222, the first DC modulation unit 209 heats the first waveguide 207B with heat generated by the heater electrode 222. As a result, the refractive index of first waveguide 207B changes due to the thermo-optic effect, and the phase of the signal light guided through first waveguide 207B can be adjusted. First multiplexing section 210 multiplexes the signal light from the two first waveguides 207B in first DC modulation section 209, and outputs the multiplexed signal light to second waveguide 211.

[0007] The second DC modulation section 212 has two second waveguides 211 arranged in parallel and a heater electrode 223 arranged on the two second waveguides 211. The two second waveguides 211 are, for example, LN waveguides. The second DC modulation section 212 is a modulation section that connects two first multiplexing sections 210 and two second waveguides 211 in the second DC modulation section 212 and modulates the signal light guided through the two second waveguides 211 in the second DC modulation section 212. When a current flows through the heater electrode 223, the second DC modulation section 212 heats the second waveguide 211 with heat generated by the heater electrode 223. As a result, the refractive index of the second waveguide 211 changes due to the thermo-optic effect, and the phase of the signal light guided through the second waveguide 211 can be adjusted. The second DC modulation unit 212 modulates the signal light guided through the second waveguide 211, and outputs the modulated signal light to the second multiplexing unit 213. The second multiplexing unit 213 multiplexes the modulated signal light from the two second waveguides 211 in the second DC modulation unit 212, and outputs the multiplexed signal light to the first output waveguide 214.

[0008] One second multiplexing section 213 multiplexes signal light from two second waveguides 211 in one second DC modulation section 212, and outputs the multiplexed signal light to the PR 215 via one first output waveguide 214. The other second multiplexing section 213 multiplexes signal light from two second waveguides 211 in the other second DC modulation section 212, and outputs the multiplexed signal light to the PBC 216 via the other first output waveguide 214.

[0009] The PR 215 rotates the polarization of the signal light from one of the second multiplexing units 213, and outputs the signal light after the polarization rotation to the PBC 216. The PBC 216 performs polarization multiplexing on the signal light after the polarization rotation from the PR 215 and the signal light from the other second multiplexing unit 213, and outputs the polarization multiplexed signal light to the second output waveguide 217.

[0010] FIG. 22 is a schematic cross-sectional view showing an example of the cross section of the line AA shown in FIG. 21. The cross-sectional portion of the line AA is an RF modulation section 208 of an optical modulator. The RF modulation section 208 shown in FIG. 22 has a support substrate 231 such as Si, a first buffer layer 232 formed on the support substrate 231, a thin-film LN substrate 233 formed on the first buffer layer 232, and a second buffer layer 234 formed on the thin-film LN substrate 233. The RF modulation section 208 has an RF electrode 221 formed on the second buffer layer 234. The thin-film LN substrate 233 has a first waveguide 207A which is a thin-film rib-type waveguide having a convex shape protruding upward. The RF electrode 221 has a signal electrode 221A and a pair of ground electrodes 221B. Then, on the second buffer layer 234 on the slab, an RF electrode 221 having a coplanar waveguide (CPW) structure, that is, a signal electrode 221A and a pair of ground electrodes 221B sandwiching the signal electrode 221A, are arranged.

[0011] FIG. 23 is a schematic cross-sectional view showing an example of a cross section along the line BB shown in FIG. 21. The cross-sectional portion along the line BB is the first DC modulation section 209 of the optical modulator 200. The first DC modulation section 209 shown in FIG. 23 has a support substrate 231, a first buffer layer 232 formed on the support substrate 231, and a thin-film LN substrate 233 formed on the first buffer layer 232. The first DC modulation section 209 has a second buffer layer 234 formed on the thin-film LN substrate 233, and a heater electrode 222 formed on the second buffer layer 234. The thin-film LN substrate has a first waveguide 207B which is a thin-film rib-type waveguide having a convex shape protruding upward. A heater electrode 222 is disposed on the second buffer layer 234 on the rib of the first waveguide 207B. The second DC modulation section 212 also has approximately the same cross-sectional structure as the first DC modulation section 209 , and a heater electrode 222 is disposed on the second buffer layer 234 on the second waveguide 211 .

[0012] Therefore, the first DC modulation section 209 can solve the problem of DC drift by using a heater electrode 222 instead of a DC electrode. This is not limited to the first DC modulation section 209, but the second DC modulation section 212 can also obtain a similar effect by using a heater electrode 223 instead of a DC electrode. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2013-003442 A [Patent Document 2] International Publication No. 2022 / 091980 [Patent Document 3] U.S. Pat. No. 10,394,059 [Patent Document 4] Special Publication No. 2002-541516 [Patent Document 5] JP 2003-021815 A Summary of the Invention [Problem to be solved by the invention]

[0014] However, in the first DC modulation section 209, the heater electrode 222 is located directly above the first waveguide 207A, so that the first waveguide 207A and the heater electrode 222 are close to each other, and a part of the light propagating through the first waveguide 207A is absorbed by the heater electrode 222, causing a loss of light. In the second DC modulation section 212, the second waveguide 211 and the heater electrode 223 are also close to each other, so that a part of the light propagating through the second waveguide 211 is absorbed by the heater electrode 223, causing a loss of light.

[0015] One object of the present invention is to provide an optical device or the like that can reduce the loss of light due to a heater electrode. [Means for solving the problem]

[0016] In one embodiment, the optical device disclosed in the present application includes a waveguide having an electro-optic effect and a thermo-optic effect through which light is guided, and a heater electrode disposed on one side of the waveguide for heating the waveguide. The optical device includes a parallel electrode disposed on the other side of the waveguide, electrically connected to the heater electrode, and having a higher resistance than the heater electrode. Effect of the Invention

[0017] According to one aspect of the optical device etc. disclosed in the present application, the loss of light due to the heater electrode can be reduced. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic plan view showing an example of an optical modulator according to this embodiment. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Diagram 3] FIG. 3 is a schematic plan view illustrating an example of a first DC modulation unit according to the first embodiment. [Figure 4] FIG. 4 is an explanatory diagram showing an example of the relationship between the position and the potential of the heater electrode. [Diagram 5]FIG. 5 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 6] FIG. 6 is a schematic cross-sectional view showing an example of a cross section taken along line CC shown in FIG. [Figure 7] FIG. 7 is a schematic plan view illustrating an example of a first DC modulation unit according to the second embodiment. [Figure 8] FIG. 8 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 9] FIG. 9 is a schematic cross-sectional view showing an example of a cross section taken along line CC shown in FIG. [Figure 10] FIG. 10 is a schematic plan view illustrating an example of a first DC modulation unit according to the third embodiment. [Figure 11] FIG. 11 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 12] FIG. 12 is a schematic cross-sectional view showing an example of a cross section taken along line CC shown in FIG. [Figure 13] FIG. 13 is a schematic plan view illustrating an example of a first DC modulation unit according to the fourth embodiment. [Figure 14] FIG. 14 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 15] FIG. 15 is an explanatory diagram illustrating an example of an optical transceiver according to this embodiment. [Figure 16] FIG. 16 is a schematic plan view showing an example of an optical modulator of a comparative example. [Figure 17] FIG. 17 is a schematic plan view showing an example of a first DC modulation section of Comparative Example 1. As shown in FIG. [Figure 18] FIG. 18 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 19] FIG. 19 is a schematic plan view showing an example of a first DC modulation section of Comparative Example 2. As shown in FIG. [Figure 20] FIG. 20 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 21] FIG. 21 is a schematic plan view showing an example of a conventional optical modulator. [Figure 22]FIG. 22 is a schematic cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 23] FIG. 23 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] <Comparative Example 1> Fig. 16 is a schematic plan view showing an example of an optical modulator 100 of a comparative example. The optical modulator 100 shown in Fig. 16 is configured, for example, with a thin-film LN (Lithium Niobate: LiNbO3) chip. The optical modulator 100 has one first input waveguide 101, one first branching section 102, two second input waveguides 103, two second branching sections 104, four third input waveguides 105, and four third branching sections 106. The optical modulator 100 has eight first waveguides 107, four RF (Radio Frequency) modulation sections 108, four first DC (Direct Current) modulation sections 109, and four first multiplexing sections 110. The optical modulator 100 has four second waveguides 111, two second DC modulation sections 112, a second multiplexing section 113, and two first output waveguides 114. The optical modulator 100 has one PR (Polarization Rotator) 115, one PBC (Polarization Beam Combiner) 116, and one second output waveguide 117.

[0020] The first input waveguide 101 is, for example, an LN waveguide that guides the signal light. The first branching unit 102 branches the signal light from the first input waveguide 101 into two second input waveguides 103. The second input waveguide 103 is, for example, an LN waveguide that guides the signal light from the first branching unit 102. The second branching unit 104 branches the signal light from the second input waveguide 103 into two third input waveguides 105. The third branching unit 106 branches the signal light from the third input waveguide 105 into two first waveguides 107A (107) in the RF modulation unit 108.

[0021] The RF modulation unit 108 is a modulation unit that performs high-speed modulation of the signal light guided through the two first waveguides 107A. The RF modulation unit 108 has two first waveguides 107A arranged in parallel and a plurality of RF electrodes 121 arranged in parallel to the two first waveguides 107A. The two first waveguides 107A are, for example, LN waveguides. The RF electrode 121 has two ground electrodes 121B arranged in parallel on the outside of the two first waveguides 107A, and one signal electrode 121A arranged in parallel to the first waveguide 107A between the two first waveguides 107A. When a high-frequency signal having a band of, for example, several tens of GHz is input to the signal electrode 121A, the RF modulation unit 108 can perform high-speed modulation of the signal light guided through the first waveguide 107A in response to the high-frequency signal.

[0022] The first DC modulation unit 109 has two first waveguides 107B (107) arranged in parallel and a plurality of heater electrodes 122 arranged in parallel on both sides of the two first waveguides 107B. The two first waveguides 107B are, for example, LN waveguides. The first DC modulation unit 109 is a modulation unit that connects the two first waveguides 107A in the RF modulation unit 108 and the two first waveguides 107B in the first DC modulation unit 109 and modulates the signal light guided through the two first waveguides 107B in the first DC modulation unit 109. When a current flows through the heater electrode 122, the first DC modulation unit 109 heats the first waveguide 107B with heat generated by the heater electrode 122. As a result, the refractive index of first waveguide 107B changes due to the thermo-optic effect, and the phase of the signal light guided through first waveguide 107B can be adjusted. First multiplexing section 110 multiplexes the signal light from the two first waveguides 107B in first DC modulation section 109, and outputs the multiplexed signal light to second waveguide 111.

[0023] The second DC modulation unit 112 has two second waveguides 111 arranged in parallel and heater electrodes 123 arranged on both sides of the two second waveguides 111. The two second waveguides 111 are, for example, LN waveguides. The second DC modulation unit 112 is a modulation unit that connects two first multiplexing units 110 and two second waveguides 111 in the second DC modulation unit 112 and modulates the signal light guided through the two second waveguides 111 in the second DC modulation unit 112. When a current is applied to the heater electrode 123, the second DC modulation unit 112 heats the second waveguide 111 with heat generated by the heater electrode 123. As a result, the refractive index of the second waveguide 111 changes due to the thermo-optic effect, so that the phase of the signal light guided through the second waveguide 111 can be adjusted. The second DC modulation unit 112 modulates the signal light guided through the second waveguide 111, and outputs the modulated signal light to the second multiplexing unit 113. The second multiplexing unit 113 multiplexes the modulated signal light from the two second waveguides 111 in the second DC modulation unit 112, and outputs the multiplexed signal light to the first output waveguide 114.

[0024] One second multiplexing section 113 multiplexes the signal light from the two second waveguides 111 in one second DC modulation section 112, and outputs the multiplexed signal light to the PR 115 via one first output waveguide 114. The other second multiplexing section 113 multiplexes the signal light from the two second waveguides 111 in the other second DC modulation section 112, and outputs the multiplexed signal light to the PBC 116 via the other first output waveguide 114.

[0025] The PR 115 rotates the polarization of the signal light from one of the second multiplexing units 113, and outputs the signal light after the polarization rotation to the PBC 116. The PBC 116 performs polarization multiplexing on the signal light after the polarization rotation from the PR 115 and the signal light from the other second multiplexing unit 113, and outputs the polarization multiplexed signal light to the second output waveguide 117.

[0026] FIG. 17 is a schematic plan view showing an example of the first DC modulation section 109 of Comparative Example 1. The first DC modulation section 109 shown in FIG. 17 has two first waveguides 107B, two heater electrodes 122 arranged in parallel on both sides of the first waveguide 107B, and two electrode pads 124 electrically connected to both ends of the heater electrodes 122. The electrode pads 124 are made of an electrode material such as Au. The electrode pads 124 have a first electrode pad 124A for inputting a current and a second electrode pad 124B which is a ground electrode. The heater electrode 122 electrically connects the input end 122A of the heater electrode 122 to the first electrode pad 124A through a via 125, and also electrically connects the output end 122B of the heater electrode 122 to the second electrode pad 124B through the via 125. In the heater electrode 122, a current flows from the first electrode pad 124A to the second electrode pad 124B.

[0027] FIG. 18 is a schematic cross-sectional view showing an example of the cross section of the line BB shown in FIG. 17. The cross-sectional portion of the line BB is the first DC modulation section 109 of the optical modulator 100. The first DC modulation section 109 shown in FIG. 18 has a support substrate 131 such as Si, a first buffer layer 132 formed on the support substrate 131, and a thin-film LN substrate 133 formed on the first buffer layer 132. The first DC modulation section 109 has a heater electrode 122 arranged parallel to the first waveguide 107B so as to sandwich both side surfaces of the first waveguide 107B on the thin-film LN substrate 133, and a second buffer layer 134 formed on the thin-film LN substrate 133 and the heater electrode 122. The thin-film LN substrate 133 has the first waveguide 107B, which is a thin-film rib-type waveguide having a convex shape protruding upward. The second DC modulation section 112 also has almost the same cross-sectional structure as the first DC modulation section 109, and has heater electrodes 123 arranged parallel to the second waveguide 111 so as to sandwich both side surfaces of the second waveguide 111.

[0028] The RF modulation section 108 has a support substrate 131, a first buffer layer 132 formed on the support substrate 131, and a thin-film LN substrate 133 formed on the first buffer layer 132. The RF modulation section 108 has a second buffer layer 134 formed on the thin-film LN substrate 133, and an RF electrode 121 formed on the second buffer layer 134. The thin-film LN substrate 133 has a first waveguide 107A that is a thin-film rib-type waveguide having a convex shape that protrudes upward. The RF electrode 121 has a signal electrode 121A and a pair of ground electrodes 121B. And, on the second buffer layer 134 on the slab, an RF electrode 121 having a coplanar (CPW) structure, that is, a signal electrode 121A and a pair of ground electrodes 121B sandwiching the signal electrode 121A, are arranged.

[0029] In first DC modulation section 109 of comparative example 1, heater electrodes 122 are arranged on both side surfaces of first waveguide 107B, so that it is possible to reduce the amount of light that propagates through first waveguide 107B and is absorbed by heater electrodes 122, thereby suppressing light loss. In second DC modulation section 112, heater electrodes 123 are arranged on both side surfaces of second waveguide 111, so that it is possible to reduce the amount of light that propagates through second waveguide 111 and is absorbed by heater electrodes 123, thereby suppressing light loss.

[0030] However, in the first DC modulation section 109 of Comparative Example 1, heater electrodes 122 are arranged on both side surfaces of the first waveguide 107B, which reduces the heating efficiency of the first waveguide 107B and increases power consumption. In the second DC modulation section 112, heater electrodes 123 are arranged on both side surfaces of the second waveguide 111, which reduces the heating efficiency of the second waveguide 111 and increases power consumption.

[0031] Moreover, the first DC modulation section 109 uses the heater electrode 122, but since it uses the thin-film LN substrate 133, which has a smaller thermo-optic coefficient than a Si substrate, it is necessary to make the waveguide length of the first waveguide 107B sufficiently long. The second DC modulation section 112 also uses the thin-film LN substrate 133, which has a smaller thermo-optic coefficient, so it is necessary to make the waveguide length of the second waveguide 111 sufficiently long.

[0032] <Comparative Example 2> Therefore, a DC modulation section for improving the heating efficiency of the waveguide will be described below as Comparative Example 2. FIG. 19 is a schematic plan view showing an example of a first DC modulation section 109A of Comparative Example 2. The first DC modulation section 109A shown in FIG. 19 has two first waveguides 107B1, a heater electrode 122 arranged in parallel on both side surfaces of a part of the first waveguide 107B1, and two electrode pads 124 electrically connected to both ends of the heater electrode 122. The electrode pad 124 has a first electrode pad 124A for injecting a current and a second electrode pad 124B which is a ground electrode. The heater electrode 122 electrically connects an input end 122A of the heater electrode 122 to the first electrode pad 124A through a via 125, and also electrically connects an output end 122B of the heater electrode 122 to the second electrode pad 124B through a via 125. In the heater electrode 122, a current flows from the first electrode pad 124A to the second electrode pad 124B.

[0033] The first waveguide 107B1 in the first DC modulation unit 109A has a first outgoing waveguide 141, a first folded waveguide 142, a return waveguide 143, a second folded waveguide 144, and a second outgoing waveguide 145. The first outgoing waveguide 141 is, for example, a first waveguide, the input end of which is connected to the output end of the first waveguide 107A in the RF modulation unit 108 and the output end of which is connected to the input end of the first folded waveguide 142. The first folded waveguide 142 has an input end connected to the output end of the first outgoing waveguide 141 and an output end connected to the input end of the return waveguide 143. The return waveguide 143 is, for example, a third waveguide, the input end of which is connected to the output end of the first folded waveguide 142 and the output end of which is connected to the input end of the second folded waveguide 144. The second folded waveguide 144 is, for example, a third waveguide, the input end of which is connected to the output end of the return waveguide 143 and the output end of which is connected to the input end of the second outgoing waveguide 145. The second outgoing waveguide 145 is, for example, a second waveguide, the input end of which is connected to the output end of the second folded waveguide 144 and the output end of which is connected to the first multiplexing section 110. The first outgoing waveguide 141, the return waveguide 143, and the second outgoing waveguide 145 are arranged in parallel.

[0034] The two heater electrodes 122 are arranged in parallel to the first outgoing waveguide 141 and the second outgoing waveguide 145 so as to sandwich both side surfaces of the return waveguide 143. One heater electrode 122 is arranged in parallel to one side surface of the first outgoing waveguide 141. The other heater electrode 122 is arranged in parallel to the other side surface of the second outgoing waveguide 145. The two heater electrodes 122 are electrically connected to the first electrode pad 124A through the via 125, and are also electrically connected to the second electrode pad 124B through the via 125.

[0035] FIG. 20 is a schematic cross-sectional view showing an example of the cross section along the line BB shown in FIG. 19. The cross-sectional portion along the line BB shown in FIG. 20 is a cross-sectional portion of the first DC modulation section 109A. The first DC modulation section 109A shown in FIG. 20 has a support substrate 131, a first buffer layer 132 formed on the support substrate 131, and a thin-film LN substrate 133 formed on the first buffer layer 132. The first DC modulation section 109A has a heater electrode 122 arranged parallel to the return waveguide 143 so as to sandwich both side surfaces of the return waveguide 143 in the first waveguide 107B1 on the thin-film LN substrate 133. The first DC modulation section 109A has a second buffer layer 134 formed on the thin-film LN substrate 133 and the heater electrode 122. One of the heater electrodes 122 is arranged parallel to one side surface of the first outward waveguide 141. The other heater electrode 122 is disposed in parallel to the other side surface of the second outgoing waveguide 145 .

[0036] The second DC modulation section 112A has a cross-sectional structure almost identical to that of the first DC modulation section 109A. The second waveguide 111 in the second DC modulation section 112A also has a first outward waveguide 141, a first folded waveguide 142, a return waveguide 143, a second folded waveguide 144, and a second outward waveguide 145. The second DC modulation section 112A has heater electrodes arranged in parallel to the return waveguide 143 of the second waveguide 111 so as to sandwich both side surfaces of the return waveguide 143. One heater electrode 123 is arranged in parallel to one side surface of the first outward waveguide 141. The other heater electrode 123 is arranged in parallel to the other side surface of the second outward waveguide 145.

[0037] In the first DC modulation section 109A of Comparative Example 2, the first waveguide 107B1 is folded back to increase the waveguide length of the first waveguide 107B1. As a result, the first DC modulation section 109A can ensure a stable thermo-optic effect even when the thin-film LN substrate 133 is used, compared to the first DC modulation section 109 of Comparative Example 1. Moreover, in the first DC modulation section 109A, the heater electrodes 122 are arranged on both side surfaces of the return waveguide 143, and on the side surfaces of the first outgoing waveguide 141 and the second outgoing waveguide 145. As a result, the first DC modulation section 109A can reduce power consumption by improving the heating efficiency of the first waveguide 107B1. In the second DC modulation section 112A of Comparative Example 2, the second waveguide 111 is folded back to increase the waveguide length of the second waveguide 111. As a result, compared to the second DC modulation section 112A of Comparative Example 1, the first DC modulation section 109A can reduce power consumption by improving the heating efficiency of the second waveguide 111 while ensuring a stable thermo-optical effect.

[0038] However, in the first DC modulation section 109A of the second comparative example, the heater electrode 122 is disposed on one side of the first outgoing waveguide 141 and one side of the second outgoing waveguide 145. As a result, an electric field is applied from the heater electrode 122 to the first outgoing waveguide 141 and the second outgoing waveguide 145, causing a DC drift. Since the first waveguide 107B1 has an electro-optic effect, the optical phase changes due to the DC drift, making the DC control unstable and preventing stable phase control by the heater electrode 122. DC drift also occurs in the first outgoing waveguide 141 and the second outgoing waveguide 145 of the second DC modulation section 112A, preventing stable phase control.

[0039] Therefore, an embodiment of an optical modulator having a DC modulation section capable of realizing stable phase control by a heater electrode will be described as an example. EXAMPLES

[0040] FIG. 1 is a schematic plan view showing an example of an optical modulator 1 according to the present embodiment. The optical modulator 1 shown in FIG. 1 is, for example, configured with a thin-film LN (Lithium Niobate: LiNbO3) chip. The optical modulator 1 has one first input waveguide 11, one first branching section 12, two second input waveguides 13, two second branching sections 14, four third input waveguides 15, and four third branching sections 16. The optical modulator 1 has eight first waveguides 17, four RF (Radio Frequency) modulation sections 18, four first DC (Direct Current) modulation sections 19, and four first multiplexing sections 20. The optical modulator 1 has four second waveguides 21, two second DC modulation sections 22, a second multiplexing section 23, and two first output waveguides 24. The optical modulator has one PR (Polarization Rotator) 25, one PBC (Polarization Beam Combiner) 26, and one second output waveguide 27.

[0041] The first input waveguide 11 is, for example, an LN waveguide that guides the signal light. The first branching unit 12 branches the signal light from the first input waveguide 11 into two second input waveguides 13. The second input waveguide 13 is, for example, an LN waveguide that guides the signal light from the first branching unit 12. The second branching unit 14 branches the signal light from the second input waveguide 13 into two third input waveguides 15. The third branching unit 16 branches the signal light from the third input waveguide 15 into two first waveguides 17A (17) in the RF modulation unit 18.

[0042] The RF modulation unit 18 is a modulation unit that performs high-speed modulation of the signal light guided through the two first waveguides 17A. The RF modulation unit 18 has two first waveguides 17A arranged in parallel and a plurality of RF electrodes 31 arranged in parallel to the two first waveguides 17A. The two first waveguides 17A are, for example, LN waveguides. The RF electrode 31 has two ground electrodes 31B arranged in parallel on the outside of the two first waveguides 17A, and one signal electrode 31A arranged in parallel to the first waveguide 17A between the two first waveguides 17A. When a high-frequency signal having a band of, for example, several tens of GHz is input to the signal electrode 31A, the RF modulation unit 18 can perform high-speed modulation of the signal light guided through the first waveguide 17A in response to the high-frequency signal.

[0043] The first DC modulation unit 19 has two first waveguides 17B, two heater electrodes 32 arranged in parallel to parts of the two first waveguides 17B, and two parallel electrodes 35 arranged in parallel to the first waveguides 17B. The two first waveguides 17B are, for example, LN waveguides having electro-optical and thermo-optical effects. The first DC modulation unit 19 is a modulation unit that connects the two first waveguides 17A in the RF modulation unit 18 and the two first waveguides 17B in the first DC modulation unit 19, and modulates the signal light guided through the two first waveguides 17B in the first DC modulation unit 19. When a current is applied to the heater electrode 32, the first DC modulation unit 19 heats the first waveguide 17B with the heat generated by the heater electrode 32. As a result, the refractive index of the first waveguide 17B changes due to the thermo-optic effect, and the phase of the signal light guided through the first waveguide 17B can be adjusted. The first multiplexing unit 20 multiplexes the signal light from the two first waveguides 17B in the first DC modulation unit 19, and outputs the multiplexed signal light to the second waveguide 21.

[0044] The second DC modulation section 22 has two second waveguides 21, a heater electrode 32 arranged on the two second waveguides 21, and two parallel electrodes 35 arranged in parallel to a part of the second waveguide 21. The two second waveguides 21 are, for example, LN waveguides having an electro-optic effect and a thermo-optic effect. The second DC modulation section 22 is a modulation section that connects two first multiplexing sections 20 and two second waveguides 21 in the second DC modulation section 22 and modulates the signal light guided through the two second waveguides 21 in the second DC modulation section 22. When a current flows through the heater electrode 32, the second DC modulation section 22 heats the second waveguide 21 with heat generated by the heater electrode 32. As a result, the refractive index of the second waveguide 21 changes due to the thermo-optic effect, so that it is possible to adjust the phase of the signal light guided through the second waveguide 21. The second DC modulation unit 22 modulates the signal light guided through the second waveguide 21, and outputs the modulated signal light to the second multiplexing unit 23. The second multiplexing unit 23 multiplexes the modulated signal light from the two second waveguides 21 in the second DC modulation unit 22, and outputs the multiplexed signal light to the first output waveguide 24.

[0045] One of the second multiplexing sections 23 multiplexes the signal light from the two second waveguides 21 in one of the second DC modulation sections 22, and outputs the multiplexed signal light to the PR 25 via one of the first output waveguides 24. The other of the second multiplexing sections 23 multiplexes the signal light from the two second waveguides 21 in the other of the second DC modulation sections 22, and outputs the multiplexed signal light to the PBC 26 via the other of the first output waveguides 24.

[0046] The PR 25 rotates the polarization of the signal light from one of the second multiplexing units 23, and outputs the signal light after the polarization rotation to the PBC 26. The PBC 26 polarization multiplexes the signal light after the polarization rotation from the PR 25 and the signal light from the other second multiplexing unit 23, and outputs the polarization multiplexed signal light to the second output waveguide 27.

[0047] FIG. 2 is a schematic cross-sectional view showing an example of the cross section taken along the line AA shown in FIG. 1. The RF modulation section 18 shown in FIG. 2 has a support substrate 41 such as Si, a first buffer layer 42 formed on the support substrate 41, and a thin-film LN substrate 43 formed on the first buffer layer 42. Furthermore, the RF modulation section 18 has a second buffer layer 44 formed on the thin-film LN substrate 43, and an RF electrode 31 formed on the second buffer layer 44. The thin-film LN substrate 43 has a thin-film LN waveguide having a convex shape that protrudes upward. The LN waveguide is a first waveguide 17A (17) of a rib-type waveguide having a rib and a slab formed on both sides of the rib. The RF electrode 31 has a signal electrode 31A and a pair of ground electrodes 31B. Then, on the second buffer layer 44 on the slab, an RF electrode 31 having a coplanar waveguide (CPW) structure, that is, a signal electrode 31A and a pair of ground electrodes 31B sandwiching the signal electrode 31A, are arranged.

[0048] FIG. 3 is a schematic plan view showing an example of the first DC modulation section 19 of the first embodiment. The first DC modulation section 19 shown in FIG. 3 has two first waveguides 17B (17), two heater electrodes 32, two parallel electrodes 35, and two electrode pads 36. The heater electrode 32 is made of an electrode material including, for example, Ti, TiN, or the like. The heater electrode 32 has a first heater electrode 32A and a second heater electrode 32B. The electrode pad 36 is made of an electrode material including, for example, Au, or the like. The electrode pad 36 has a first electrode pad 36A connected to the heater electrode 32 and injecting a current into the heater electrode 32, and a second electrode pad 36B connecting the heater electrode 32 to the ground.

[0049] The heater electrode 32 electrically connects an input end 321 of the heater electrode 32 to the first electrode pad 36A through a via 37, and also electrically connects an output end 322 of the heater electrode 32 to the second electrode pad 36B through the via 37. The heater electrode 32 passes a current from the first electrode pad 36A to the second electrode pad 36B. The input end 321 of the heater electrode 32 is an electrode part near a part connected to the first electrode pad 36A that injects a current into the heater electrode 32, and the output end 322 of the heater electrode 32 is an electrode part near a part connected to the second electrode pad 36B that passes a current from the heater electrode 32.

[0050] The parallel electrode 35 is made of an electrode material having a higher resistance than the heater electrode 32, for example, an electrode material containing Si or doped Si. In other words, while a current flows through the heater electrode 32, the parallel electrode 35 is configured such that a current does not easily flow through it and a voltage is applied thereto. The parallel electrode 35 electrically connects the input end of the parallel electrode 35 to the input end 321 of the heater electrode 32, and also electrically connects the output end of the parallel electrode 35 to the output end 322 of the heater electrode 32.

[0051] The first waveguide 17B in the first DC modulation unit 19 has a first outgoing waveguide 51, a first folded waveguide 52, a return waveguide 53, a second folded waveguide 54, and a second outgoing waveguide 55. The first outgoing waveguide 51 has an input end connected to an output end of the first waveguide 17A in the RF modulation unit 18, and an output end connected to an input end of the first folded waveguide 52. The first folded waveguide 52 has an input end connected to an output end of the first outgoing waveguide 51, and an output end connected to an input end of the return waveguide 53. The return waveguide 53 has an input end connected to an output end of the first folded waveguide 52, and an output end connected to an input end of the second folded waveguide 54. The second folded waveguide 54 has an input end connected to the output end of the return waveguide 53, and an output end connected to an input end of the second outgoing waveguide 55. The second outgoing waveguide 55 has an input end connected to the output end of the second folded waveguide 54, and an output end connected to the first multiplexing section 20. The first outgoing waveguide 51, the return waveguide 53, and the second outgoing waveguide 55 are arranged in parallel.

[0052] The two heater electrodes 32 are arranged parallel to the return waveguide 53 so as to sandwich both side surfaces of the return waveguide 53. The first heater electrode 32A is arranged parallel to one side surface of the first outward waveguide 51. The second heater electrode 32B is arranged parallel to the other side surface of the second outward waveguide 55.

[0053] The two parallel electrodes 35 are disposed on the other side of the first outgoing waveguide 51, i.e., the side opposite to the side on which the first heater electrode 32A is disposed, and on one side of the second outgoing waveguide 55, i.e., the side opposite to the side on which the second heater electrode 32B is disposed. The first parallel electrode 35A is disposed in parallel to the other side of the first outgoing waveguide 51, i.e., the side opposite to the side on which the first heater electrode 32A is disposed. The second parallel electrode 35B is disposed in parallel to one side of the second outgoing waveguide 55, i.e., the side opposite to the side on which the second heater electrode 32B is disposed.

[0054] That is, the first outgoing waveguide 51 has both side surfaces sandwiched between the first heater electrode 32A and the first parallel electrode 35A, and is disposed in parallel to the first heater electrode 32A and the first parallel electrode 35A. The second outgoing waveguide 55 has both side surfaces sandwiched between the second heater electrode 32B and the second parallel electrode 35B, and is disposed in parallel to the second heater electrode 32B and the second parallel electrode 35B.

[0055] FIG. 4 is an explanatory diagram showing an example of the relationship between the position and the potential of the heater electrode 32. The potential at each position X of the first heater electrode 32A between the first electrode pad 36A and the second electrode pad 36B is the same between the first parallel electrode 35A. The position X is a position between the input end 321 and the output end 322 of the first heater electrode 32A. The potential at each position X of the second heater electrode 32B between the first electrode pad 36A and the second electrode pad 36B is the same between the second parallel electrode 35B. The position X is a position between the input end 321 and the output end 322 of the second heater electrode 32B.

[0056] In the first outgoing waveguide 51, a current flows through the first heater electrode 32A on one side, but a current does not easily flow through the first parallel electrode 35A on the other side, and the first heater electrode 32A and the first parallel electrode 35A are at the same potential. As a result, the electric field from the first heater electrode 32A to the first outgoing waveguide 51 is suppressed, and the occurrence of DC drift can be suppressed.

[0057] In the second outgoing waveguide 55, a current also flows through the second heater electrode 32B on one side, but a current does not easily flow through the second parallel electrode 35B on the other side, and the second heater electrode 32B and the second parallel electrode 35B are at the same potential. As a result, the electric field from the second heater electrode 32B to the second outgoing waveguide 55 is suppressed, and the occurrence of DC drift can be suppressed.

[0058] FIG. 5 is a schematic cross-sectional view showing an example of the cross section along the line BB shown in FIG. 5. The cross-sectional portion along the line BB shown in FIG. 5 is a cross-sectional portion of the first DC modulation section 19. The first DC modulation section 19 has a support substrate 41, a first buffer layer 42 formed on the support substrate 41, and a thin-film LN substrate 43 formed on the first buffer layer 42. The first DC modulation section 19 has two heater electrodes 32 arranged in parallel to the return waveguide 53 so as to sandwich both side surfaces of the return waveguide 53 in the first waveguide 17B on the thin-film LN substrate 43. The first DC modulation section 19 has a first parallel electrode 35A arranged in parallel to the first outward waveguide 51 on the thin-film LN substrate 43, and a second parallel electrode 35B arranged in parallel to the second outward waveguide 55 on the thin-film LN substrate 43. The first DC modulation section 19 has a thin-film LN substrate 43, a heater electrode 32, and a second buffer layer 44 formed on the first parallel electrode 35A and the second parallel electrode 35B.

[0059] Fig. 6 is a schematic cross-sectional view showing an example of the cross section of the CC line shown in Fig. 4. The first DC modulation section 19 shown in Fig. 6 includes a support substrate 41, a first buffer layer 42, a thin-film LN substrate 43, a second buffer layer 44, a first outgoing waveguide 51, a return waveguide 53, and a second outgoing waveguide 55. The second buffer layer 44 in the first DC modulation section 19 further includes two heater electrodes 32 arranged in parallel to the return waveguide 53 so as to sandwich both side surfaces of the return waveguide 53, and a via 37 electrically connected to the two heater electrodes 32. The second buffer layer 44 further includes a first electrode pad 36A electrically connected to the via 37.

[0060] The first DC modulation section 19 has two heater electrodes 32 arranged in parallel on both sides of the return waveguide 53, and a first parallel electrode 35A arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The first DC modulation section 19 has a second parallel electrode 35B arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A and the second parallel electrode 35B prevent an electric field from being applied to the first waveguide 17B. Since no electric field is applied to the first waveguide 17B, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0061] It is also considered that the current flowing through the first parallel electrode 35A and the second parallel electrode 35B increases the power consumption of the heater electrode 32. However, the electrode material of the first parallel electrode 35A and the second parallel electrode 35B has a higher resistance than the electrode material of the heater electrode 32. As a result, it is possible to suppress an increase in the drive power of the heater electrode 32 while suppressing the power consumption of the parallel electrode 35.

[0062] The second DC modulation section 22 has a cross-sectional structure almost identical to that of the first DC modulation section 19. The second waveguide 21 in the second DC modulation section 22 also has a first outward waveguide 51, a first folded waveguide 52, a return waveguide 53, a second folded waveguide 54, and a second outward waveguide 55. The second DC modulation section 22 has a heater electrode 32 arranged parallel to the return waveguide 53 so as to sandwich both side surfaces of the return waveguide 53 in the second waveguide 21. The first parallel electrode 35A in the second DC modulation section 22 is arranged parallel to the first outward waveguide 51 and the first heater electrode 32A so as to sandwich the first outward waveguide 51 between the first parallel electrode 35A and the first heater electrode 32A. The second parallel electrode 35B in the second DC modulation section 22 is arranged parallel to the second outgoing waveguide 55 and the second heater electrode 32B so as to sandwich the second outgoing waveguide 55 between the second parallel electrode 35B and the second heater electrode 32B.

[0063] The second DC modulation section 22 has two heater electrodes 32 arranged in parallel on both side surfaces of the return waveguide 53, and a first parallel electrode 35A arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The second DC modulation section 22 has a second parallel electrode 35B arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A and the second parallel electrode 35B prevent an electric field from being applied to the second waveguide 21. Since no electric field is applied to the second waveguide 21, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0064] The DC modulation section of the first embodiment has two heater electrodes 32 arranged in parallel on both side surfaces of the return waveguide 53, and a first parallel electrode 35A arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The DC modulation section has a second parallel electrode 35B arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A and the second parallel electrode 35B prevent an electric field from being applied to the first waveguide 17B. Since no electric field is applied to the first waveguide 17B, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0065] In the optical modulator 1 of the first embodiment, the first parallel electrode 35A is disposed on the side of the first outgoing waveguide 51, and the second parallel electrode 35B is disposed on the side of the second outgoing waveguide 55. However, the same effect can be obtained by disposing a heater electrode on one side of a single optical waveguide and a parallel electrode on the other side of the single optical waveguide. For example, the heater electrode 32 may be disposed only on one side of the first outgoing waveguide 51, and the parallel electrode 35 may be disposed only on the other side of the first outgoing waveguide 51. As a result, even if a voltage is applied to the heater electrode 32, the parallel electrode 35 prevents an electric field from being applied to the first outgoing waveguide 51. Since no electric field is applied to the first outgoing waveguide 51, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized. Furthermore, although the first outgoing waveguide 51 has been exemplified as a single optical waveguide, the waveguide does not have to have a folded structure and can be modified as appropriate.

[0066] In the DC modulation section of Example 1, an example is shown in which the input end of the parallel electrode 35 is electrically connected to the input end 321 of the heater electrode 32, and the output end of the parallel electrode 35 is electrically connected to the output end 322 of the heater electrode 32. The input end 321 is near the input end of the heater electrode 32, and the output end 322 is near the output end of the heater electrode 32. However, the present invention is not limited to this, and an embodiment thereof will be described below as Example 2. The same components as those of the first DC modulation section 19 of Example 1 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. EXAMPLES

[0067] Fig. 7 is a schematic plan view showing an example of the first DC modulation section 19 of the second embodiment, Fig. 8 is a schematic cross-sectional view showing an example of the cross section along the line BB shown in Fig. 7, and Fig. 9 is a schematic cross-sectional view showing an example of the cross section along the line CC shown in Fig. 7. The first DC modulation section 19 of the first embodiment and the first DC modulation section 19A of the second embodiment differ in that the input end of the parallel electrode 35 and the input end 321 of the heater electrode 32 are electrically connected by a first electrode pad 36A1. Another difference is that the output end of the parallel electrode 35 and the output end 322 of the heater electrode 32 are electrically connected by a second electrode pad 36B1.

[0068] The parallel electrode 35 has a first parallel electrode 35A1 and a second parallel electrode 35B1. The first electrode pad 36A1 is electrically connected to the input end 321 of the heater electrode 32 through the first via 37A, and is electrically connected to the input end of the parallel electrode 35 through the second via 37B. The second electrode pad 36B1 is electrically connected to the output end 322 of the heater electrode 32 through the first via 37A, and is electrically connected to the output end of the parallel electrode 35 through the second via 37B.

[0069] The first DC modulation section 19A has two heater electrodes 32 arranged in parallel on both sides of the return waveguide 53, and a first parallel electrode 35A1 arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The first DC modulation section 19A has a second parallel electrode 35B1 arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A1 and the second parallel electrode 35B1 prevent an electric field from being applied to the first waveguide 17B. Since no electric field is applied to the first waveguide 17B, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0070] The second DC modulation section 22 has two heater electrodes 32 arranged in parallel on both sides of the return waveguide 53, and a first parallel electrode 35A1 arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The second DC modulation section 22 has a second parallel electrode 35B1 arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A1 and the second parallel electrode 35B1 prevent an electric field from being applied to the second waveguide 21. Since no electric field is applied to the second waveguide 21, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0071] In the DC modulation section of Example 1, the parallel electrode 35 is made of a material different from that of the heater electrode 32, but the present invention is not limited to this, and an embodiment of the same will be described below as Example 3. The same components as those of the first DC modulation section 19 of Example 1 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. EXAMPLES

[0072] Fig. 10 is a plan view schematic diagram showing an example of a first DC modulation section 19B of Example 3, Fig. 11 is a cross-sectional view schematic diagram showing an example of a cross section taken along line BB shown in Fig. 10, and Fig. 12 is a cross-sectional view schematic diagram showing an example of a cross section taken along line CC shown in Fig. 10. The first DC modulation section 19 of Example 1 differs from the first DC modulation section 19B of Example 3 in that although the same material as that of the heater electrode 32 is used for the parallel electrode 35, the width of the parallel electrode 35 is narrower than that of the heater electrode 32, thereby making the parallel electrode 35 highly resistant.

[0073] The parallel electrode 35 has a first parallel electrode 35A2 and a second parallel electrode 35B2. The electrode width of the first parallel electrode 35A2 is narrower than the electrode width of the heater electrode 32, thereby providing high resistance. The electrode width of the second parallel electrode 35B2 is narrower than the electrode width of the heater electrode 32, thereby providing high resistance.

[0074] The first DC modulation section 19B has two heater electrodes 32 arranged in parallel on both sides of the return waveguide 53, and a first parallel electrode 35A2 arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The first DC modulation section 19B has a second parallel electrode 35B2 arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A2 and the second parallel electrode 35B2 prevent an electric field from being applied to the first waveguide 17B. Since no electric field is applied to the first waveguide 17B, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0075] The second DC modulation section 22 has two heater electrodes 32 arranged in parallel on both side surfaces of the return waveguide 53, and a first parallel electrode 35A2 arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The second DC modulation section 22 has a second parallel electrode 35B2 arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A2 and the second parallel electrode 35B2 prevent an electric field from being applied to the second waveguide 21. Since no electric field is applied to the second waveguide 21, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0076] In the optical modulator 1 of the third embodiment, the parallel electrodes 35 are made of the same material as the heater electrodes 32, and therefore the manufacturing process can be simplified.

[0077] Incidentally, there is no limitation to the electrode width of the parallel electrodes 35, and the thickness of the parallel electrodes 35 may be made thinner than the thickness of the heater electrodes 32, and can be changed as appropriate.

[0078] In addition, in the DC modulation section 19 of Example 1, the parallel electrode 35 is made of a material different from that of the heater electrode 32, but the present invention is not limited to this, and an embodiment thereof will be described below as Example 3. Note that the same components as those of the first DC modulation section 19 of Example 1 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. EXAMPLES

[0079] Fig. 13 is a schematic plan view showing an example of a first DC modulation section of Example 4, and Fig. 14 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in Fig. 13. The first DC modulation section 19 of Example 1 differs from the first DC modulation section 19C of Example 4 in that the parallel electrode 35 is made of the same material as the heater electrode 32 and the electrode length of the parallel electrode 35 is made longer than the electrode length of the heater electrode 32, thereby making the parallel electrode 35 highly resistant.

[0080] The parallel electrode 35 has a first parallel electrode 35A3 and a second parallel electrode 35B3. The first parallel electrode 35A3 is arranged to meander within a range in which the potential of the first heater electrode 32A adjacent to the first outgoing waveguide 51 is the same. The second parallel electrode 35B3 is arranged to meander within a range in which the potential of the second heater electrode 32B adjacent to the second outgoing waveguide 55 is the same.

[0081] The first DC modulation section 19C has two heater electrodes 32 arranged in parallel on both sides of the return waveguide 53, and a first parallel electrode 35A3 arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The first DC modulation section 19C has a second parallel electrode 35B3 arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A3 and the second parallel electrode 35B3 prevent an electric field from being applied to the first waveguide 17B. Since no electric field is applied to the first waveguide 17B, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0082] The second DC modulation section 22 has two heater electrodes 32 arranged in parallel on both side surfaces of the return waveguide 53, and a first parallel electrode 35A3 arranged in parallel on the outside of the first outgoing waveguide 51 arranged in parallel to the return waveguide 53. The second DC modulation section 22 has a second parallel electrode 35B3 arranged in parallel on the outside of the second outgoing waveguide 55 arranged in parallel to the return waveguide 53. As a result, even when a voltage is applied to the heater electrode 32, the first parallel electrode 35A3 and the second parallel electrode 35B3 prevent an electric field from being applied to the second waveguide 21. Since no electric field is applied to the second waveguide 21, no DC drift occurs, and stable phase control by the heater electrode 32 can be realized.

[0083] In this embodiment, a DC modulation section in the optical modulator 1 using the heater electrode 32 is exemplified, but the present invention is not limited to this and can also be applied to optical devices such as phase shifters and VOAs that use the heater electrode 32.

[0084] FIG. 15 is an explanatory diagram showing an example of an optical transceiver 80 according to the present embodiment. The optical transceiver 80 shown in FIG. 15 is connected to an output optical fiber and an input optical fiber. The optical transceiver 80 includes a light source 81, a DSP (Digital Signal Processor) 82, and an optical transceiver 83. The optical transceiver 83 includes an optical transmitter 84 and an optical receiver 85. The DSP 82 is an electrical component that performs digital signal processing. For example, the DSP 82 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 84. The DSP 82 also obtains an electrical signal including reception data from the optical receiver 85, and obtains reception data by performing processing such as decoding the obtained electrical signal.

[0085] The light source 81 includes, for example, a laser diode, and generates light of a predetermined wavelength and supplies it to the optical transmitter 84 and the optical receiver 85. The optical transmitter 84 is an optical device such as an optical modulator that modulates the light supplied from the light source 81 with an electrical signal output from the DSP 82 and outputs the modulated signal light to an optical fiber. The optical transmitter 84 includes, for example, an optical device such as an optical modulator including a first DC modulation unit 19 and a second DC modulation unit 22. The optical transmitter 84 generates signal light by modulating the light supplied from the light source 81 with an electrical signal when the light is guided through the waveguide.

[0086] The optical receiver 85 receives the incoming light from the optical fiber, converts the incoming light into an electrical signal using the light supplied from the light source 81, and outputs the converted electrical signal to the DSP .

[0087] In the optical transceiver 80, an example is shown in which an optical transmitter 84 and an optical receiver 85 are built in, but the present invention can also be applied to an optical transmitter that only has an optical transmitter 84 that has an optical device built in. The present invention can also be applied to an optical receiver that only has an optical receiver 85 that has an optical device built in. The present invention is not limited to the optical transceiver 80, and the optical device can also be applied to the optical transmitter / receiver 83.

[0088] In this embodiment, a thin film LN chip is exemplified, but it is not limited to this, and may be, for example, TF-Barium Titanate, and may be changed as appropriate. The material for the electro-optic effect may be, for example, TF-BTO (BaTiO3), TF-PLZT (PbLaZrTiO3), or TF-PZT (PbZrTiO3), and may be changed as appropriate. [Explanation of symbols]

[0089] 17B First Waveguide 19 First DC modulation section 22 Second DC modulation section 32 Heater electrode 32A First Heater Electrode 32B Second heater electrode 35 parallel electrodes 35A First Parallel Electrode 35B Second parallel electrode 51 First outgoing waveguide 52 First folded waveguide 53 Return Waveguide 54 Second folded waveguide 55 Second outgoing waveguide

Claims

1. A waveguide having an electro-optic effect and a thermo-optic effect in which light is guided; a heater electrode disposed on one side of the waveguide and configured to heat the waveguide; a parallel electrode disposed on the other side of the waveguide, electrically connected to the heater electrode, and having a higher resistance than the heater electrode; An optical device comprising:

2. The parallel electrodes include 2. The optical device according to claim 1, wherein an input end of the parallel electrode is electrically connected to an input end of the heater electrode, and an output end of the parallel electrode is electrically connected to an output end of the heater electrode.

3. The parallel electrodes include The optical device of claim 1 , comprising a material different from the heater electrode.

4. The parallel electrodes include 2. The optical device according to claim 1, wherein the heater electrode is made of the same material as the heater electrode and has a structure having a width narrower than that of the heater electrode.

5. The parallel electrodes include 2. The optical device according to claim 1, wherein the heater electrode is made of the same material as the heater electrode and has a meandering structure that makes the electrode length longer than that of the heater electrode.

6. The waveguide is A first waveguide; and a second waveguide disposed in parallel with the first waveguide; a third waveguide disposed in parallel between one side surface of the first waveguide and one side surface of the second waveguide; The heater electrode is a first heater electrode disposed on one side of the third waveguide; a second heater electrode disposed on the other side of the third waveguide; The parallel electrodes include a first parallel electrode disposed on the other side of the first waveguide and electrically connected to the first heater electrode; a second parallel electrode disposed on the other side of the second waveguide and electrically connected to the second heater electrode; 2. The optical device according to claim 1, further comprising:

7. The waveguide is a first folded waveguide connecting between an output end of the first waveguide and an input end of the third waveguide; a second folded waveguide connecting between an output end of the third waveguide and an input end of the second waveguide; 7. The optical device according to claim 6, further comprising:

8. A light source that emits light; an optical device that modulates the light from the light source in response to an electrical signal, The optical device comprises: A waveguide having an electro-optic effect and a thermo-optic effect in which light is guided; a heater electrode disposed on one side of the waveguide and configured to heat the waveguide; a parallel electrode that is disposed on the other side of the waveguide, is electrically connected to the heater electrode, and has a higher resistance than the heater electrode; 1. An optical transmitter comprising:

9. A light source that emits light; and an optical device that converts received light into an electrical signal using light generated from the light source, The optical device comprises: A waveguide having an electro-optic effect and a thermo-optic effect in which light is guided; a heater electrode disposed on one side of the waveguide and configured to heat the waveguide; a parallel electrode that is disposed on the other side of the waveguide, is electrically connected to the heater electrode, and has a higher resistance than the heater electrode; 1. An optical receiver comprising:

10. 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 transceiver includes: A waveguide having an electro-optic effect and a thermo-optic effect in which light is guided; a heater electrode disposed on one side of the waveguide and configured to heat the waveguide; an optical device disposed on the other side of the waveguide, electrically connected to the heater electrode, and having a parallel electrode having a higher resistance than the heater electrode;

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