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

Abstract

The invention relates to an optical device, an optical transmitter, an optical receiver, and an optical transceiver. An optical device includes: a waveguide in which light is guided and which has an electro-optical effect and a thermo-optical effect; and a heater electrode disposed on one of the side surfaces of the waveguide. The optical device includes a parallel electrode disposed on the other one of the side surfaces of the waveguide, electrically connected to the heater electrode, and having a higher resistance than the heater electrode.

Description

Technical Field

[0001] The embodiments discussed herein relate to optical devices, optical transmitters, optical receivers, and optical transceivers. Background Art

[0002] A conventional optical modulator includes an optical waveguide disposed on, for example, a substrate, and further includes a signal electrode and a ground electrode disposed near the optical waveguide. When a voltage is applied to the signal electrode, an electric field is generated inside the optical waveguide, and the refractive index of the optical waveguide changes due to the electric field existing inside the optical waveguide, so that the phase of light changes. The optical waveguide constitutes a Mach-Zehnder interferometer, and the output level of light changes due to the phase difference of each light traveling through the optical waveguide.

[0003] For example, the optical modulator is a Mach-Zehnder modulator. Fig.21 1 is a schematic plan view illustrating an example of a conventionally used optical modulator 200. The optical modulator 200 is composed of, for example, a thin film lithium niobate (LN: LiNbO3) chip. The optical modulator 200 includes a first input waveguide 201, a first branch portion 202, two second input waveguides 203, two second branch portions 204, four third input waveguides 205, and four third branch portions 206. The optical modulator 200 includes eight first waveguides 207, four radio frequency (RF) modulation units 208, four first direct current (DC) modulation units 209, and four first multiplexing portions 210. The optical modulator 200 includes four second waveguides 211, two second DC modulation units 212, two second multiplexing portions 213, and two first output waveguides 214. The optical modulator 200 includes a single polarization rotator (PR) 215, a single polarization beam combiner (PBC) 216, and a single second output waveguide 217.

[0004] For example, the first input waveguide 201 is an LN waveguide that guides signal light. The first branch portion 202 branches the signal light received from the first input waveguide 201 into two second input waveguides 203. For example, each second input waveguide 203 is an LN waveguide that guides the signal light received from the first branch portion 202. Each second branch portion 204 branches the signal light received from the associated second input waveguide 203 into two third input waveguides 205. Each third branch portion 206 branches the signal light received from the associated third input waveguide 205 into two first waveguides 207A included in the RF modulation unit 208.

[0005] The RF modulation unit 208 is a modulation unit that performs high-speed modulation on the signal light passing through each of the two first waveguides 207A. The RF modulation unit 208 includes two first waveguides 207A (207) arranged in parallel and a plurality of RF electrodes 221 arranged in parallel with the two first waveguides 207A. The two first waveguides 207A are LN waveguides. The RF electrode 221 includes: two ground electrodes 221B, each of which is arranged in parallel with the outer side of an associated one of the two first waveguides 207A; and a single signal electrode 221A, which is arranged in parallel with the first waveguide 207A at a position between the two first waveguides 207A. In the case where a high-frequency signal having a frequency band of, for example, several tens of gigahertz (GHz) is input to the signal electrode 221A, the RF modulation unit 208 can perform high-speed modulation on the signal light passing through the first waveguide 207A according to the high-frequency signal.

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

[0007] The second DC modulation unit 212 includes two second waveguides 211 arranged in parallel and heater electrodes 223 arranged on the two second waveguides 211. The two second waveguides 211 are, for example, LN waveguides. The second DC modulation unit 212 is a modulation unit that connects the two first multiplexing parts 210 and the two second waveguides 211 included in the second DC modulation unit 212 and modulates the signal light passing through each of the two second waveguides 211 included in the second DC modulation unit 212. When current flows in each heater electrode 223, the second DC modulation unit 212 heats the second waveguide 211 by the heat generated in the heater electrode 223. As a result, the refractive index of the second waveguide 211 changes due to the thermo-optical effect, so that the phase of the signal light passing through the second waveguide 211 can be adjusted. The second DC modulation unit 212 modulates the signal light passing through the second waveguide 211 and outputs the modulated signal light to the second multiplexing part 213. Each second multiplexing section 213 multiplexes the modulated signal lights received from the associated two second waveguides 211 included in the second DC modulation unit 212 , and outputs the multiplexed signal lights to the first output waveguide 214 .

[0008] One second multiplexing section 213 multiplexes the respective signal lights received from the two second waveguides 211 included in one second DC modulation unit 212, and outputs the multiplexed signal lights to the PR 215 via an associated one of the first output waveguides 214. Another second multiplexing section 213 multiplexes the respective signal lights received from the two second waveguides 211 included in another second DC modulation unit 212, and outputs the multiplexed signal lights to the PBC 216 via another first output waveguide 214.

[0009] The PR 215 performs polarization rotation on the signal light received from one second multiplexing section 213, and outputs the polarization-rotated signal light to the PBC 216. The PBC 216 performs polarization division multiplexing on both the signal light that has been polarization-rotated and received from the PR 215 and the signal light that has been received from the other second multiplexing section 213, and outputs the polarization-division multiplexed signal light to the second output waveguide 217.

[0010] Fig. 22 is along Fig.21 1 is a cross-sectional schematic diagram of an example of a cross-sectional view taken along line AA shown in FIG. The cross-sectional portion taken along line AA corresponds to the RF modulation unit 208 included in the optical modulator. Fig. 22The RF modulation unit 208 shown in the figure includes a support substrate 231 made of Si or the like, 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 unit 208 includes an RF electrode 221 formed on the second buffer layer 234. The thin film LN substrate 233 includes a first waveguide 207A, which is a rib-type waveguide made of a thin film having a convex shape protruding upward. The RF electrode 221 includes a signal electrode 221A and a pair of ground electrodes 221B. In addition, on the second buffer layer 234 provided on the board, an RF electrode 221 (i.e., a signal electrode 221A) having a coplanar waveguide (CPW) structure and a pair of ground electrodes 221B sandwiching the signal electrode 221A are arranged.

[0011] Fig.23 is along Fig.21 1 is a cross-sectional schematic diagram of one example of a cross-sectional view taken along line BB shown in FIG. The cross-sectional portion taken along line BB is the first DC modulation unit 209 included in the optical modulator 200 . Fig.23 The first DC modulation unit 209 shown in the figure includes 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 unit 209 includes 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 includes a first waveguide 207B, which is a rib-type waveguide made of a thin film having a convex shape protruding upward. In addition, the heater electrode 222 is arranged on the second buffer layer 234 provided on the rib of the first waveguide 207B. In addition, the second DC modulation unit 212 also has a substantially same cross-sectional structure as the first DC modulation unit 209, and the heater electrode 222 is arranged on the second buffer layer 234 provided on the second waveguide 211.

[0012] Therefore, the first DC modulation unit 209 can solve the problem of DC drift by using the heater electrode 222 instead of the DC electrode. This is not limited to the first DC modulation unit 209, but in the second DC modulation unit 212, the same effect can be obtained by using the heater electrode 223 instead of the DC electrode.

[0013] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2013-003442

[0014] Patent Document 2: International Publication No. WO 2022 / 091980

[0015] Patent Document 3: U.S. Patent No. 10394059

[0016] Patent Document 4: Japanese National Publication No. 2002-541516 of International Patent Application

[0017] Patent Document 5: Japanese Patent Application Laid-Open No. 2003-021815

[0018] However, in the first DC modulation unit 209, the heater electrode 222 is disposed just above the first waveguide 207A, so that the first waveguide 207A is located near the heater electrode 222, and some of the light propagating through the first waveguide 207A is absorbed into the heater electrode 222, thereby causing light loss. In the second DC modulation unit 212, the second waveguide 211 is also located near the heater electrode 223, so that some of the light propagating through the second waveguide 211 is absorbed into the heater electrode 223, thereby causing light loss.

[0019] Therefore, an object of one aspect of an embodiment of the present invention is to provide an optical device or the like capable of reducing light loss caused by a heater electrode. Summary of the invention

[0020] According to one aspect of the embodiment, an optical device includes a waveguide, a heater electrode and a parallel electrode. Light is guided in the waveguide and the waveguide has an electro-optical effect and a thermo-optical effect. The heater electrode is arranged on one of the side surfaces of the waveguide and heats the waveguide. The parallel electrode is arranged on the other side surface of the side surface of the waveguide. The parallel electrode is electrically connected to the heater electrode and has a high resistance compared to the heater electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic plan view illustrating one example of an optical modulator according to the present embodiment;

[0022] Figure 2 is along Figure 1 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line AA shown in FIG.

[0023] Figure 3 is a schematic plan view illustrating one example of a first DC modulation unit according to the first embodiment;

[0024] Figure 4 is an explanatory diagram illustrating an example of the relationship between the position of the heater electrode and the potential;

[0025] Figure 5 is along Figure 3 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG.

[0026] Figure 6 is along Figure 3 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line CC shown in FIG.

[0027] Figure 7 is a schematic plan view illustrating one example of a first DC modulation unit according to a second embodiment;

[0028] Figure 8 is along Figure 7 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG.

[0029] Fig. 9 is along Figure 7 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line CC shown in FIG.

[0030] Fig.10 is a schematic plan view illustrating one example of a first DC modulation unit according to a third embodiment;

[0031] Fig.11 is along Fig.10 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG.

[0032] Fig.12 is along Fig.10 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line CC shown in FIG.

[0033] Fig.13 is a schematic plan view illustrating an example of a first DC modulation unit according to a fourth embodiment;

[0034] Fig.14 is along Fig.13 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG.

[0035] Fig.15 is an explanatory diagram illustrating one example of an optical transceiver according to the present embodiment;

[0036] Fig.16 is a schematic plan view illustrating one example of a light modulator according to a comparative example;

[0037] Fig.17 is a schematic plan view illustrating an example of a first DC modulation unit according to Comparative Example 1;

[0038] Fig.18 is along Fig.17 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG.

[0039] Fig.19is a schematic plan view illustrating an example of a first DC modulation unit according to Comparative Example 2;

[0040] Fig. 20 is along Fig.19 A cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG.

[0041] Fig.21 is a schematic plan view illustrating an example of a conventionally used light modulator;

[0042] Fig. 22 is along Fig.21 A cross-sectional schematic diagram of one example of a cross-sectional view taken along line AA shown in FIG. 1 ; and

[0043] Fig.23 is along Fig.21 0 is a cross-sectional schematic diagram of one example of a cross-sectional view taken along line BB shown in FIG. DETAILED DESCRIPTION

[0044] Comparative Example 1

[0045] Fig.16 is a schematic plan view illustrating one example of an optical modulator 100 according to a comparative example. Fig.16 The optical modulator 100 shown in the figure is composed of, for example, a thin film lithium niobate (LN: LiNbO3) chip. The optical modulator 100 includes a single first input waveguide 101, a single first branch portion 102, two second input waveguides 103, two second branch portions 104, four third input waveguides 105 and four third branch portions 106. The optical modulator 100 includes eight first waveguides 107, four radio frequency (RF) modulation units 108, four first direct current (DC) modulation units 109 and four first multiplexing portions 110. The optical modulator 100 includes four second waveguides 111, two second DC modulation units 112, two second multiplexing portions 113 and two first output waveguides 114. The optical modulator 100 includes a single polarization rotator (PR) 115, a single polarization beam combiner (PBC) 116 and a single second output waveguide 117.

[0046] The first input waveguide 101 is, for example, an LN waveguide that guides signal light. The first branch portion 102 branches the signal light received from the first input waveguide 101 into two second input waveguides 103. Each second input waveguide 103 is, for example, an LN waveguide that guides the signal light received from the first branch portion 102. Each second branch portion 104 branches the signal light received from the associated second input waveguide 103 into two third input waveguides 105. Each third branch portion 106 branches the signal light received from the associated third input waveguide 105 into two first waveguides 107A (107) included in the RF modulation unit 108.

[0047] The RF modulation unit 108 is a modulation unit that performs high-speed modulation on the signal light passing through each of the two first waveguides 107A. The RF modulation unit 108 includes two first waveguides 107A arranged in parallel and a plurality of RF electrodes 121 arranged in parallel on the two first waveguides 107A. The two first waveguides 107A are, for example, LN waveguides. The RF electrode 121 includes: two ground electrodes 121B arranged in parallel outside an associated one of the two first waveguides 107A; and a single signal electrode 121A arranged in parallel between the two first waveguides 107A so as to be parallel to the first waveguide 107A. In the case where a high-frequency signal having a frequency band of, for example, several tens of gigahertz (GHz) is input to the signal electrode 121A, the RF modulation unit 108 can perform high-speed modulation on the signal light passing through the first waveguide 107A according to the high-frequency signal.

[0048] The first DC modulation unit 109 includes two first waveguides 107B (107) arranged in parallel and a plurality of heater electrodes 122 arranged in parallel on both side surfaces 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 included in the RF modulation unit 108 and the two first waveguides 107B included in the first DC modulation unit 109, and modulates the signal light passing through the two first waveguides 107B included in the first DC modulation unit 109. When current flows in each heater electrode 122, the first DC modulation unit 109 heats the first waveguide 107B by the heat generated in the heater electrode 122. As a result, the refractive index of the first waveguide 107B changes due to the thermo-optical effect, so that the phase of the signal light passing through the first waveguide 107B can be adjusted. The first multiplexing section 110 multiplexes the respective signal lights received from the two first waveguides 107B included in the first DC modulation unit 109 , and outputs the multiplexed signal lights to the second waveguide 111 .

[0049] The second DC modulation unit 112 includes two second waveguides 111 arranged in parallel and heater electrodes 123 arranged on both side surfaces 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 the two second waveguides 111 included in the two first multiplexing parts 110 and the second DC modulation unit 112 and modulates the signal light passing through each of the two second waveguides 111 included in the second DC modulation unit 112. When current flows in each heater electrode 123, the second DC modulation unit 112 heats the second waveguide 111 by the heat generated in the heater electrode 123. As a result, the refractive index of the second waveguide 111 changes due to the thermo-optical effect, so that the phase of the signal light passing through the second waveguide 111 can be adjusted. The second DC modulation unit 112 modulates the signal light passing through the second waveguide 111 and outputs the modulated signal light to each second multiplexing part 113. Each second multiplexing section 113 multiplexes modulated signal lights received from associated two second waveguides 111 included in the second DC modulation unit 112 , and outputs the multiplexed signal lights to the first output waveguide 114 .

[0050] One of the second multiplexing sections 113 multiplexes the signal lights received from the two second waveguides 111 included in one second DC modulation unit 112, and outputs the multiplexed signal light to the PR 115 via an associated one of the first output waveguides 114. The other second multiplexing section 113 multiplexes the signal lights received from the two second waveguides 111 included in the other second DC modulation unit 112, and outputs the multiplexed signal light to the PBC 116 via the other first output waveguide 114.

[0051] The PR 115 performs polarization rotation on the signal light received from one second multiplexing section 113, and outputs the polarization-rotated signal light to the PBC 116. The PBC 116 performs polarization division multiplexing on both the signal light that has been polarization-rotated and received from the PR 115 and the signal light that has been received from the other second multiplexing section 113, and outputs the polarization-multiplexed signal light to the second output waveguide 117.

[0052] Fig.17 is a schematic plan view illustrating one example of the first DC modulation unit 109 according to Comparative Example 1. Fig.17The first DC modulation unit 109 shown includes 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 composed of an electrode material made of, for example, Au. The electrode pads 124 include a first electrode pad 124A for inputting current and a second electrode pad 124B as a ground electrode. Each heater electrode 122 is electrically connected via a through hole 125 to a portion between an input end 122A of the heater electrode 122 and the first electrode pad 124A, and is electrically connected via a through hole 125 to a portion between an output end 122B of the heater electrode 122 and the second electrode pad 124B. Then, in the heater electrode 122, the current received from the first electrode pad 124A flows in the second electrode pad 124B.

[0053] Fig.18 is along Fig.17 1 is a cross-sectional schematic diagram of one example of a cross-sectional view taken along line BB shown in FIG. The cross-sectional portion taken along line BB is the first DC modulation unit 109 included in the optical modulator 100 . Fig.18 The first DC modulation unit 109 shown in the figure includes a support substrate 131 made of Si or the like, 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 unit 109 includes: a heater electrode 122 arranged in parallel with the first waveguide 107B to sandwich both side surfaces of each first waveguide 107B provided 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 includes the first waveguides 107B, each of which is a rib-type waveguide made of a thin film having a convex shape protruding upward. In addition, the second DC modulation unit 112 also has substantially the same cross-sectional structure as the first DC modulation unit 109, and includes a heater electrode 123 arranged in parallel with the second waveguide 111 to sandwich both side surfaces of each second waveguide 111.

[0054] The RF modulation unit 108 includes 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 unit 108 includes 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 includes a first waveguide 107A, which is a rib-type waveguide made of a thin film having a convex shape protruding upward. The RF electrode 121 includes a signal electrode 121A and a pair of ground electrodes 121B. In addition, on the second buffer layer 134 provided on the board, an RF electrode 121 (i.e., a signal electrode 121A) having a coplanar waveguide (CPW) structure and a pair of ground electrodes 121B sandwiching the signal electrode 121A are arranged.

[0055] In the first DC modulation unit 109 according to Comparative Example 1, the heater electrodes 122 are arranged on both side surfaces of the first waveguide 107B, so that light loss can be suppressed by reducing the amount of some light that propagates through the first waveguide 107B and is absorbed into the heater electrodes 122. In the second DC modulation unit 112, the heater electrodes 123 are also arranged on both side surfaces of the second waveguide 111, so that light loss can be suppressed by reducing the amount of some light that propagates through the second waveguide 111 and is absorbed into the heater electrodes 123.

[0056] However, in the first DC modulation unit 109 according to Comparative Example 1, the heater electrodes 122 are arranged on both side surfaces of the first waveguide 107B, so that the heating efficiency of the first waveguide 107B is reduced and the power consumption is increased. In the second DC modulation unit 112, the heater electrodes 123 are also arranged on both side surfaces of the second waveguide 111, so that the heating efficiency of the second waveguide 111 is reduced and the power consumption is increased.

[0057] Furthermore, the heater electrode 122 is used in the first DC modulation unit 109, and the thin film LN substrate 133 having a smaller thermo-optic coefficient than a substrate made of Si is used, and the waveguide length of the first waveguide 107B needs to be made sufficiently long. In the second DC modulation unit 112, the thin film LN substrate 133 having a smaller thermo-optic coefficient is also used, and the waveguide length of the second waveguide 111 needs to be made sufficiently long.

[0058] Comparative Example 2

[0059] Therefore, a DC modulation unit that improves the heating efficiency of the waveguide will be described as Comparative Example 2. Fig.19 is a schematic plan view illustrating one example of a first DC modulation unit 109A according to Comparative Example 2. Fig.19The first DC modulation unit 109A shown includes two first waveguides 107B1, heater electrodes 122 arranged in parallel on a portion of both side surfaces of the first waveguide 107B1, and two electrode pads 124 electrically connected to both ends of each heater electrode 122. The electrode pads 124 include a first electrode pad 124A for injecting current and a second electrode pad 124B as a ground electrode. The heater electrode 122 electrically connects a portion between an input end 122A of the heater electrode 122 and the first electrode pad 124A via a through hole 125, and electrically connects a portion between an output end 122B of the heater electrode 122 and the second electrode pad 124B via the through hole 125. Then, in the heater electrode 122, the current received from the first electrode pad 124A flows in the second electrode pad 124B.

[0060] The first waveguide 107B1 included in the first DC modulation unit 109A includes a first inbound side waveguide 141, a first folded waveguide 142, an outbound side waveguide 143, a second folded waveguide 144, and a second inbound side waveguide 145. For example, the first inbound side waveguide 141 is a first waveguide having a structure in which an input end of the first inbound side waveguide 141 is connected to an output end of the first waveguide 107A included in the RF modulation unit 108, and an output end of the first inbound side waveguide 141 is connected to an input end of the first folded waveguide 142. The first folded waveguide 142 has a structure in which an input end of the first folded waveguide 142 is connected to an output end of the first inbound side waveguide 141, and an output end of the first folded waveguide 142 is connected to an input end of the outbound side waveguide 143. For example, the outward waveguide 143 is a third waveguide having a structure in which an input end of the outward waveguide 143 is connected to an output end of the first folded waveguide 142, and an output end of the outward waveguide 143 is connected to an input end of the second folded waveguide 144. The second folded waveguide 144 has a structure in which an input end of the second folded waveguide 144 is connected to an output end of the outward waveguide 143, and an output end of the second folded waveguide 144 is connected to an input end of the second inward waveguide 145. For example, the second inward waveguide 145 is a second waveguide having a structure in which an input end of the second inward waveguide 145 is connected to an output end of the second folded waveguide 144, and an output end of the second inward waveguide 145 is connected to the first multiplexing section 110. Furthermore, it is assumed that the first inward waveguide 141, the outward waveguide 143, and the second inward waveguide 145 are arranged in parallel.

[0061] The two heater electrodes 122 are arranged parallel to both the first inward waveguide 141 and the second inward waveguide 145 so as to sandwich the two side surfaces of the outward waveguide 143. In addition, one heater electrode 122 is correspondingly arranged parallel to one side surface of the first inward waveguide 141. In addition, the other heater electrode 122 is correspondingly arranged parallel to the other side surface of the second inward waveguide 145. The two heater electrodes 122 are electrically connected to the first electrode pad 124A via the through hole 125, and are electrically connected to the second electrode pad 124B via the through hole 125.

[0062] Fig. 20 is along Fig.19 0 is a cross-sectional schematic diagram of one example of a cross-sectional view taken along line BB shown in FIG. Fig. 20 The cross-sectional portion taken along line BB shown in corresponds to the cross-sectional portion of the first DC modulation unit 109A. Fig. 20 The first DC modulation unit 109A shown in FIG. 1 includes 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 unit 109A includes a heater electrode 122 arranged in parallel to the outward waveguide 143 so as to sandwich both side surfaces of the outward waveguide 143 included in the first waveguide 107B1 provided on the thin film LN substrate 133. The first DC modulation unit 109A includes a second buffer layer 134 formed on the thin film LN substrate 133 and the heater electrode 122. Therefore, one heater electrode 122 is arranged in parallel to one side surface of the first inward waveguide 141. In addition, another heater electrode 122 is arranged in parallel to the other side surface of the second inward waveguide 145.

[0063] The second DC modulation unit 112A also has a cross-sectional structure substantially the same as that of the first DC modulation unit 109A. The second waveguide 111 included in the second DC modulation unit 112A also has a first inward waveguide 141, a first folded waveguide 142, an outward waveguide 143, a second folded waveguide 144, and a second inward waveguide 145. The second DC modulation unit 112A includes heater electrodes, which are arranged in parallel to the outward waveguide 143 to sandwich the two side surfaces of the outward waveguide 143 included in the second waveguide 111. Therefore, one heater electrode 123 is arranged in parallel to one side surface of the first inward waveguide 141. In addition, another heater electrode 123 is arranged in parallel to the other side surface of the second inward waveguide 145, respectively.

[0064] The first DC modulation unit 109A according to the comparative example 2 is constructed so that the first waveguide 107B1 has a folded structure and the waveguide length of the first waveguide 107B1 is made longer. Thus, compared with the case of the first DC modulation unit 109 according to the comparative example 1, the first DC modulation unit 109A can ensure a stable thermo-optical effect even when the thin film LN substrate 133 is used. In addition, the first DC modulation unit 109A is constructed so that the heater electrodes 122 are arranged on both side surfaces of the outward waveguide 143 and the side surfaces of the first inward waveguide 141 and the second inward waveguide 145. Thus, the first DC modulation unit 109A can reduce power consumption by improving the heating efficiency of the first waveguide 107B1. The second DC modulation unit 112A according to the comparative example 2 is also constructed so that the second waveguide 111 has a folded structure, and further, the waveguide length of the second waveguide 111 is made longer. Thus, compared with the case of the second DC modulation unit 112A according to Comparative Example 1, the first DC modulation unit 109A can reduce power consumption by improving the heating efficiency of the second waveguide 111 while ensuring a stable thermo-optical effect.

[0065] However, in the first DC modulation unit 109A according to the comparative example 2, the heater electrode 122 is arranged on one side surface of the first inward waveguide 141 and one side surface of the second inward waveguide 145. As a result, the electric field received from the heater electrode 122 is applied to both the first inward waveguide 141 and the second inward waveguide 145, and thus a DC drift occurs. In the first waveguide 107B1, due to the electro-optical effect, the optical phase changes due to the DC drift, so that stable phase control cannot be achieved by the heater electrode 122 due to the unstable DC control. In the first inward waveguide 141 and the second inward waveguide 145 included in the second DC modulation unit 112A, stable phase control cannot be achieved due to the occurrence of DC drift.

[0066] Therefore, an embodiment of an optical modulator including a DC modulation unit capable of performing stable phase control by using a heater electrode will be described as an embodiment.

[0067] (a) First Embodiment

[0068] Figure 1 is a schematic plan view illustrating one example of the optical modulator 1 according to the present embodiment. Figure 1The optical modulator 1 shown is, for example, composed of a thin film lithium niobate (LN: LiNbO3) chip. The optical modulator 1 includes a single first input waveguide 11, a single first branch portion 12, two second input waveguides 13, two second branch portions 14, four third input waveguides 15 and four third branch portions 16. The optical modulator 1 includes eight first waveguides 17, four radio frequency (RF) modulation units 18, four first direct current (DC) modulation units 19 and four first multiplexing portions 20. The optical modulator 1 includes four second waveguides 21, two second DC modulation units 22, two second multiplexing portions 23 and two first output waveguides 24. The optical modulator includes a single polarization rotator (PR) 25, a single polarization beam combiner (PBC) 26 and a single second output waveguide 27.

[0069] The first input waveguide 11 is, for example, an LN waveguide that guides signal light. The first branch portion 12 branches the signal light received from the first input waveguide 11 into two second input waveguides 13. Each second input waveguide 13 is, for example, an LN waveguide that guides the signal light received from the first branch portion 12. Each second branch portion 14 branches the signal light received from the associated second input waveguide 13 into two third input waveguides 15. Each third branch portion 16 branches the signal light received from the associated third input waveguide 15 into two first waveguides 17A (17) included in the associated RF modulation unit 18.

[0070] The RF modulation unit 18 is a modulation unit that performs high-speed modulation on the signal light passing through each of the two first waveguides 17A. The RF modulation unit 18 includes two first waveguides 17A arranged in parallel, and a plurality of RF electrodes 31 arranged in parallel with the two first waveguides 17A. The two first waveguides 17A are, for example, LN waveguides. The RF electrode 31 includes: two ground electrodes 31B, each of which is arranged in parallel with the outer side of an associated one of the first waveguides 17A; and a single signal electrode 31A, which is arranged in parallel with the first waveguide 17A at a position between the two first waveguides 17A. In the case where a high-frequency signal having a frequency band of, for example, several tens of gigahertz (GHz) is input to the signal electrode 31A, the RF modulation unit 18 can perform high-speed modulation on the signal light passing through the first waveguide 17A according to the high-frequency signal.

[0071] The first DC modulation unit 19 includes two first waveguides 17B, two heater electrodes 32 arranged in parallel with a part of the two first waveguides 17B, and two parallel electrodes 35 arranged in parallel with the first waveguides 17B. The two first waveguides 17B are, for example, LN waveguides having an electro-optic effect and a thermo-optic effect. The first DC modulation unit 19 is a modulation unit that connects the two first waveguides 17A included in the RF modulation unit 18 and the two first waveguides 17B included in the first DC modulation unit 19 and modulates the signal light passing through the two first waveguides 17B included in the first DC modulation unit 19. When current flows in each heater electrode 32, the first DC modulation unit 19 heats the first waveguide 17B by the heat generated in the heater electrode 32. As a result, the refractive index of the first waveguide 17B changes due to the thermo-optic effect, so that the phase of the signal light passing through the first waveguide 17B can be adjusted. The first multiplexing section 20 multiplexes the respective signal lights received from the two first waveguides 17B included in the first DC modulation unit 19 , and outputs the multiplexed signal lights to the second waveguide 21 .

[0072] The second DC modulation unit 22 includes two second waveguides 21, heater electrodes 32 arranged on the two second waveguides 21, and two parallel electrodes 35 arranged in parallel with a portion of the second waveguide 21. The two second waveguides 21 are, for example, LN waveguides having an electro-optical effect and a thermo-optical effect. The second DC modulation unit 22 is a modulation unit that connects the two second waveguides 21 included in the two first multiplexing parts 20 and the second DC modulation unit 22 and modulates the signal light passing through each of the two second waveguides 21 included in the second DC modulation unit 22. When current flows in each heater electrode 32, the second DC modulation unit 22 heats the second waveguide 21 by the heat generated in the heater electrode 32. As a result, the refractive index of the second waveguide 21 changes due to the thermo-optical effect, so that the phase of the signal light passing through the second waveguide 21 can be adjusted. The second DC modulation unit 22 modulates the signal light passing through the second waveguide 21, and outputs the modulated signal light to the second multiplexing part 23. Each second multiplexing section 23 multiplexes the modulated signal lights received from the associated two second waveguides 21 included in the second DC modulation unit 22 , and outputs the multiplexed signal lights to the first output waveguide 24 .

[0073] One of the second multiplexing parts 23 multiplexes the signal lights received from the two second waveguides 21 included in one second DC modulation unit 22, and outputs the multiplexed signal lights to the PR 25 via an associated first output waveguide 24. The other second multiplexing part 23 multiplexes the signal lights received from the two second waveguides 21 included in the other second DC modulation unit 22, and outputs the multiplexed signal lights to the PBC 26 via another first output waveguide 24.

[0074] The PR 25 performs polarization rotation on the signal light received from one second multiplexing section 23, and outputs the polarization-rotated signal light to the PBC 26. The PBC 26 performs polarization division multiplexing on both the signal light which has been polarization-rotated and received from the PR 25 and the signal light received from the other second multiplexing section 23, and outputs the polarization-division multiplexed signal light to the second output waveguide 27.

[0075] Figure 2 is along Figure 1 A cross-sectional schematic diagram of one example of a cross-sectional view taken along line AA shown in FIG. Figure 2 The RF modulation unit 18 shown in FIG. 1 includes a support substrate 41 made of Si or the like, 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. In addition, the RF modulation unit 18 includes 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 includes an LN waveguide made of a thin film having a convex shape protruding upward. The LN waveguide is a first waveguide 17A (17) having a rib-type waveguide including a rib and a plate formed on both sides of the rib. The RF electrode 31 includes a signal electrode 31A and a pair of ground electrodes 31B. In addition, on the second buffer layer 44 provided on the plate, an RF electrode 31 (i.e., a signal electrode 31A) having a coplanar waveguide (CPW) structure and a pair of ground electrodes 31B sandwiching the signal electrode 31A are arranged.

[0076] Figure 3 is a schematic plan view illustrating one example of the first DC modulation unit 19 according to the first embodiment. Figure 3The first DC modulation unit 19 shown includes two first waveguides 17B (17), two heater electrodes 32, two parallel electrodes 35 and two electrode pads 36. The heater electrodes 32 are composed of an electrode material including, for example, Ti, TiN, etc. The heater electrodes 32 include a first heater electrode 32A and a second heater electrode 32B. Each electrode pad 36 is composed of an electrode material made of, for example, Au, etc. The electrode pads 36 are connected to the corresponding heater electrodes 32, and include a first electrode pad 36A into which current is injected into the heater electrode 32, and a second electrode pad 36B connecting the heater electrode 32 and the ground.

[0077] The heater electrode 32 is electrically connected to a portion between an input terminal 321 of the heater electrode 32 and the first electrode pad 36A via a through hole 37, and is electrically connected to a portion between an output terminal 322 of the heater electrode 32 and the second electrode pad 36B via a through hole 37. Then, the current received from the first electrode pad 36A flows into the second electrode pad 36B through the heater electrode 32. The input terminal 321 of the heater electrode 32 corresponds to an electrode region located near a region connected to the first electrode pad 36A, in which current is injected into the heater electrode 32, and the output terminal 322 of the heater electrode 32 is an electrode region located near a region connected to the second electrode pad 36B, through which current from the heater electrode 32 flows.

[0078] The parallel electrode 35 is composed of an electrode material having a high resistance compared with the heater electrode 32 (that is, an electrode material including, for example, Si, doped Si, etc.). In other words, the heater electrode 32 is configured to have a structure in which a current flows through the heater electrode 32 but does not flow through the parallel electrode 35 easily, so a voltage is applied to the heater electrode 32. The parallel electrode 35 electrically connects a portion between an input end of the parallel electrode 35 and an input end 321 of the heater electrode 32, and electrically connects a portion between an output end of the parallel electrode 35 and an output end 322 of the heater electrode 32.

[0079] The first waveguide 17B included in the first DC modulation unit 19 includes: a first inward waveguide 51, a first folded waveguide 52, an outward waveguide 53, a second folded waveguide 54, and a second inward waveguide 55. The first inward waveguide 51 has a structure in which an input end of the first inward waveguide 51 is connected to an output end of the first waveguide 17A included in the RF modulation unit 18, and an output end of the first inward waveguide 51 is connected to an input end of the first folded waveguide 52. The first folded waveguide 52 has a structure in which an input end of the first folded waveguide 52 is connected to an output end of the first inward waveguide 51, and an output end of the first folded waveguide 52 is connected to an input end of the outward waveguide 53. The outward waveguide 53 has a structure in which an input end of the outward waveguide 53 is connected to an output end of the first folded waveguide 52, and an output end of the outward waveguide 53 is connected to an input end of the second folded waveguide 54. The second folded waveguide 54 has a structure in which an input end of the second folded waveguide 54 is connected to an output end of the outward waveguide 53, and an output end of the second folded waveguide 54 is connected to an input end of the second inward waveguide 55. The second inward waveguide 55 has a structure in which an input end of the second inward waveguide 55 is connected to an output end of the second folded waveguide 54, and an output end of the second inward waveguide 55 is connected to the first multiplexing section 20. Furthermore, it is assumed that the first inward waveguide 51, the outward waveguide 53, and the second inward waveguide 55 are arranged in parallel.

[0080] The two heater electrodes 32 are arranged in parallel to the outward waveguide 53 so as to sandwich both side surfaces of the outward waveguide 53. Therefore, the first heater electrode 32A is arranged in parallel with one of the side surfaces of the first inward waveguide 51. Furthermore, the second heater electrode 32B is arranged in parallel with the other of the side surfaces of the second inward waveguide 55, respectively.

[0081] The two parallel electrodes 35 are arranged on the other of the side surfaces of the first inward waveguide 51, that is, on the side surface opposite to the side surface on which the first heater electrode 32A is arranged, and are arranged on one side surface of the second inward waveguide 55, that is, on the side surface opposite to the side surface on which the second heater electrode 32B is arranged. Therefore, the first parallel electrode 35A is arranged on the other of the side surfaces of the first inward waveguide 51, that is, so arranged in parallel to the side surface opposite to the side surface on which the first heater electrode 32A is provided. Therefore, the second parallel electrode 35B is arranged on one of the side surfaces of the second inward waveguide 55, that is, so arranged in parallel to the side surface opposite to the side surface on which the second heater electrode 32B is provided.

[0082] In other words, the first inward waveguide 51 is disposed in parallel with both the first heater electrode 32A and the first parallel electrode 35A, so that the two side surfaces of the first inward waveguide 51 are sandwiched between the first heater electrode 32A and the first parallel electrode 35A. The second inward waveguide 55 is also disposed in parallel with the second heater electrode 32B and the second parallel electrode 35B, so that the two side surfaces of the second inward waveguide 55 are sandwiched between the second heater electrode 32B and the second parallel electrode 35B.

[0083] Figure 4 36A and 36B. The potential at each position X of the first heater electrode 32A provided between the first electrode pad 36A and the second electrode pad 36B becomes the same as the potential of the first parallel electrode 35A. The position X is a position between the input terminal 321 and the output terminal 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 becomes the same as the potential of the second parallel electrode 35B. The position X is a position between the input terminal 321 and the output terminal 322 of the second heater electrode 32B.

[0084] In the first inward waveguide 51, current flows into the first heater electrode 32A located on one side surface, but current does not easily flow through the first parallel electrode 35A located on the other side surface, and the potential at the first heater electrode 32A and the potential at the first parallel electrode 35A are the same. As a result, by suppressing the electric field from the first heater electrode 32A to the first inward waveguide 51, the occurrence of DC drift can be suppressed.

[0085] In the second inward waveguide 55, current also flows in the second heater electrode 32B located on one side surface, but current does not easily flow through the second parallel electrode 35B located on the other side surface, and the potential at the second heater electrode 32B is the same as the potential at the second parallel electrode 35B. As a result, by suppressing the electric field from the second heater electrode 32B to the second inward waveguide 55, the occurrence of DC drift can be suppressed.

[0086] Figure 5 is along Figure 3 0 is a cross-sectional schematic diagram of one example of a cross-sectional view taken along line BB shown in FIG. Figure 5The cross-sectional portion taken along BB shown in FIG. 1 is a cross-sectional portion of the first DC modulation unit 19. The first DC modulation unit 19 includes 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 unit 19 includes two heater electrodes 32 arranged in parallel with the outward waveguide 53 so as to sandwich both side surfaces of the outward waveguide 53 included in the first waveguide 17B provided on the thin film LN substrate 43. The first DC modulation unit 19 includes a first parallel electrode 35A arranged in parallel with the first inward waveguide 51 provided on the thin film LN substrate 43, and a second parallel electrode 35B arranged in parallel with the second inward waveguide 55 provided on the thin film LN substrate 43. The first DC modulation unit 19 includes the thin film LN substrate 43, the heater electrode 32, the first parallel electrode 35A, and the second buffer layer 44 formed on the second parallel electrode 35B.

[0087] Figure 6 is along Figure 3 In addition to the support substrate 41, the first buffer layer 42, the thin film LN substrate 43 and the second buffer layer 44, Figure 6 The first DC modulation unit 19 shown in FIG. 1 further includes a first inward waveguide 51, an outward waveguide 53, and a second inward waveguide 55. In addition, in the second buffer layer 44 included in the first DC modulation unit 19, two heater electrodes 32 arranged in parallel with the outward waveguide 53 to sandwich both side surfaces of the outward waveguide 53, and a through hole 37 electrically connecting the two heater electrodes 32 are included. In addition, in the second buffer layer 44, a first electrode pad 36A electrically connected to the through hole 37 is included.

[0088] The first DC modulation unit 19 includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The first DC modulation unit 19 includes a second parallel electrode 35B arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the first waveguide 17B accordingly due to the first parallel electrode 35A and the second parallel electrode 35B. Since no electric field is applied to the first waveguide 17B, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0089] Furthermore, it is conceivable that, since the current flows in the first parallel electrode 35A and the second parallel electrode 35B, the power consumed by the heater electrode 32 increases. However, the electrode material of each of the first parallel electrode 35A and the second parallel electrode 35B is configured to have a high resistance compared to the electrode material of the heater electrode 32. As a result, it is possible to suppress the increase in the power for driving the heater electrode 32 while suppressing the power consumed by the parallel electrode 35.

[0090] The second DC modulation unit 22 also has a cross-sectional structure substantially the same as that of the first DC modulation unit 19. The second waveguide 21 included in the second DC modulation unit 22 also includes a first inward waveguide 51, a first folded waveguide 52, an outward waveguide 53, a second folded waveguide 54, and a second inward waveguide 55. The second DC modulation unit 22 includes a heater electrode 32, which is arranged in parallel with the outward waveguide 53 to sandwich both side surfaces of the outward waveguide 53 included in the second waveguide 21. The first parallel electrode 35A included in the second DC modulation unit 22 is arranged in parallel with both the first inward waveguide 51 and the first heater electrode 32A to sandwich the first inward waveguide 51 together with the first heater electrode 32A. The second parallel electrode 35B included in the second DC modulation unit 22 is arranged in parallel with the second inward waveguide 55 and the second heater electrode 32B to sandwich the second inward waveguide 55 together with the second heater electrode 32B.

[0091] The second DC modulation unit 22 includes two heater electrodes 32 parallel to both side surfaces of the outward waveguide 53, and a first parallel electrode 35A arranged in parallel to the outer side of the first inward waveguide 51 arranged in parallel to the outward waveguide 53. The second DC modulation unit 22 includes a second parallel electrode 35B arranged in parallel to the outer side of the second inward waveguide 55 arranged in parallel to the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the second waveguide 21 accordingly due to the first parallel electrode 35A and the second parallel electrode 35B. Since the electric field is not applied to the second waveguide 21, a DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0092] The DC modulation unit according to the first embodiment includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A parallel to the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The DC modulation unit includes a second parallel electrode 35B parallel to the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the first waveguide 17B accordingly due to the first parallel electrode 35A and the second parallel electrode 35B. Since the electric field is not applied to the first waveguide 17B, a DC drift does not occur, so that a stable phase control can be achieved due to the heater electrode 32.

[0093] Furthermore, in the optical modulator 1 according to the first embodiment, the case where the first parallel electrode 35A is arranged on the side surface of the first inward waveguide 51 and the second parallel electrode 35B is arranged on the side surface of the second inward waveguide 55 has been described as an example. However, even if the heater electrode is arranged on one of the side surfaces of the single optical waveguide and the parallel electrode is arranged on the other of the side surfaces of the single optical waveguide, the same effect can be obtained. For example, the heater electrode 32 may be arranged only on one of the side surfaces of the first inward waveguide 51, and the parallel electrode 35 may be arranged only on the other of the side surfaces of the first inward waveguide 51. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the first inward waveguide 51 accordingly due to the parallel electrode 35. Since the electric field is not applied to the first inward waveguide 51, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32. Furthermore, as a single optical waveguide, the first inward waveguide 51 has been exemplified, but the waveguide does not necessarily have a folded structure, and appropriate modifications are possible.

[0094] In addition, in the DC modulation unit according to the first embodiment, the case where the portion between the input terminal of the parallel electrode 35 and the input terminal 321 of the heater electrode 32 is electrically connected, and the portion between the output terminal of the parallel electrode 35 and the output terminal 322 of the heater electrode 32 is electrically connected has been described as an example. The input terminal 321 is located near the input terminal of the heater electrode 32, and the output terminal 322 is located near the output terminal of the heater electrode 32. However, the embodiment is not limited to this, and the embodiment thereof will be described below as a second embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the first DC modulation unit 19 according to the first embodiment, repeated descriptions of their configurations and operations will be omitted.

[0095] (b) Second Embodiment

[0096] Figure 7 is a schematic plan view illustrating one example of a first DC modulation unit 19A according to the second embodiment, Figure 8 is along Figure 7 BB is a cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG. Fig. 9 is along Figure 7 Schematic cross-sectional view of an example of a cross-sectional view taken along the line CC shown in . The first DC modulation unit 19A according to the second embodiment is different from the first DC modulation unit 19 according to the first embodiment in that the input end of the parallel electrode 35 and the input end 321 of the heater electrode 32 are electrically connected at the first electrode pad 36A1. In addition, the first DC modulation unit 19A is also different from the first DC modulation unit 19 in that the output end of the parallel electrode 35 and the output end 322 of the heater electrode 32 are electrically connected at the second electrode pad 36B1.

[0097] The parallel electrodes 35 include 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 via the first through hole 37A, and is electrically connected to the input end of the parallel electrode 35 via the second through hole 37B. The second electrode pad 36B1 is electrically connected to the output end 322 of the heater electrode 32 via the first through hole 37A, and is electrically connected to the output end of the parallel electrode 35 via the second through hole 37B.

[0098] The first DC modulation unit 19A includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A1 arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The first DC modulation unit 19A includes a second parallel electrode 35B1 arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the first waveguide 17B accordingly due to the first parallel electrode 35A1 and the second parallel electrode 35B1. Since the electric field is not applied to the first waveguide 17B, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0099] The second DC modulation unit 22 includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A1 arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The second DC modulation unit 22 includes a second parallel electrode 35B1 arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the second waveguide 21 accordingly due to the first parallel electrode 35A1 and the second parallel electrode 35B1. Since the electric field is not applied to the second waveguide 21, a DC drift does not occur, so that a stable phase control can be achieved due to the heater electrode 32.

[0100] In addition, in the first DC modulation unit 19 according to the first embodiment, the case where the parallel electrode 35 is composed of a material different from that of the heater electrode 32 has been described as an example, but the embodiment is not limited thereto, and an embodiment thereof will be described below as a third embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the first DC modulation unit 19 according to the first embodiment, repeated description of their configuration and operation will be omitted.

[0101] (c) Third Embodiment

[0102] Fig.10 is a schematic plan view illustrating one example of a first DC modulation unit 19B according to the third embodiment, Fig.11 is along Fig.10 BB is a cross-sectional schematic diagram of an example of a cross-sectional view taken along line BB shown in FIG. Fig.12 is along Fig.10 The first DC modulation unit 19B according to the third embodiment is different from the first DC modulation unit 19 according to the first embodiment in that the same material as that used for the heater electrode 32 is used for the parallel electrode 35, but since the width of the parallel electrode 35 is made narrower than that of the heater electrode 32, the parallel electrode 35 is configured to have a high resistance.

[0103] The parallel electrodes 35 include a first parallel electrode 35A2 and a second parallel electrode 35B2. The electrode width of the first parallel electrode 35A2 is made narrower than the electrode width of the heater electrode 32, so that the first parallel electrode 35A2 has a high resistance accordingly. The electrode width of the second parallel electrode 35B2 is made narrower than the electrode width of the heater electrode 32, so that the second parallel electrode 35B2 has a high resistance accordingly.

[0104] The first DC modulation unit 19B includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A2 arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The first DC modulation unit 19B includes a second parallel electrode 35B2 arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the first waveguide 17B accordingly due to the first parallel electrode 35A2 and the second parallel electrode 35B2. Since the electric field is not applied to the first waveguide 17B, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0105] The second DC modulation unit 22 includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A2 arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The second DC modulation unit 22 includes a second parallel electrode 35B2 arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the second waveguide 21 due to the first parallel electrode 35A2 and the second parallel electrode 35B2. Since the electric field is not applied to the second waveguide 21, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0106] In the optical modulator 1 according to the third embodiment, the parallel electrode 35 is configured of the same material as that of the heater electrode 32 , which makes it possible to simplify the manufacturing process.

[0107] Furthermore, this embodiment is not limited to the electrode width of the parallel electrode 35 , but the thickness of the parallel electrode 35 may be made thinner than the thickness of the heater electrode 32 , and appropriate modifications are also possible.

[0108] In addition, in the first DC modulation unit 19 according to the first embodiment, the case in which the parallel electrode 35 is configured to have a material different from that of the heater electrode 32 has been described as an example, but the embodiment is not limited thereto, and an embodiment thereof will be described below as a fourth embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the first DC modulation unit 19 according to the first embodiment, repeated descriptions of their configurations and operations will be omitted.

[0109] (d) Fourth Embodiment

[0110] Fig.13 is a schematic plan view illustrating an example of a first DC modulation unit according to a fourth embodiment, and Fig.14 is along Fig.13 The first DC modulation unit 19C according to the fourth embodiment is different from the first DC modulation unit 19 according to the first embodiment in that the parallel electrode 35 is constructed of the same material as the material used for 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, so that the parallel electrode 35 has a high resistance.

[0111] The parallel electrodes 35 include a first parallel electrode 35A3 and a second parallel electrode 35B3. The first parallel electrode 35A3 is arranged in a meandering manner within the same potential range as the first heater electrode 32A adjacent to the first inward waveguide 51. The second parallel electrode 35B3 is arranged in a meandering manner within the same potential range as the second heater electrode 32B adjacent to the second inward waveguide 55.

[0112] The first DC modulation unit 19C includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A3 arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The first DC modulation unit 19C includes a second parallel electrode 35B3 arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the first waveguide 17B accordingly due to the first parallel electrode 35A3 and the second parallel electrode 35B3. Since the electric field is not applied to the first waveguide 17B, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0113] The second DC modulation unit 22 includes two heater electrodes 32 arranged in parallel with both side surfaces of the outward waveguide 53, and a first parallel electrode 35A3 arranged in parallel with the outer side of the first inward waveguide 51 arranged in parallel with the outward waveguide 53. The second DC modulation unit 22 includes a second parallel electrode 35B3 arranged in parallel with the outer side of the second inward waveguide 55 arranged in parallel with the outward waveguide 53. As a result, even if a voltage is applied to the heater electrode 32, an electric field is not applied to the second waveguide 21 accordingly due to the first parallel electrode 35A3 and the second parallel electrode 35B3. Since the electric field is not applied to the second waveguide 21, DC drift does not occur, so that stable phase control can be achieved due to the heater electrode 32.

[0114] Furthermore, in the present embodiment, the DC modulation unit included in the optical modulator 1 and using the heater electrode 32 has been described as an example, but the embodiment is not limited thereto and the embodiment can be applied to a phase shifter using the heater electrode 32, an optical device such as a VOA.

[0115] Fig.15 is an explanatory diagram illustrating one example of the optical transceiver 80 according to the present embodiment. Fig.15 The optical transceiver 80 shown is connected to an optical fiber provided on the output side and an optical fiber provided on the input side. The optical transceiver 80 includes a light source 81, a digital signal processor (DSP) 82, and an optical transmitter / receiver 83. The optical transmitter / receiver 83 includes an optical transmitter 84 and an optical receiver 85. The DSP 82 is an electronic component that performs digital signal processing. The DSP 82 performs processing such as encoding transmission data, generates an electrical signal including transmission data, and outputs the generated electrical signal to the optical transmitter 84. In addition, the DSP 82 acquires an electrical signal including reception data from the optical receiver 85, and obtains reception data by performing processing for decoding the acquired electrical signal.

[0116] The light source 81 includes, for example, a laser diode, etc., generates light of a predetermined wavelength, and supplies the generated light to the optical transmitter 84 and the optical receiver 85. The optical transmitter 84 is an optical device such as an optical modulator, which modulates the light supplied from the light source 81 by using the electrical signal output from the DSP 82, and outputs the modulated signal light to the optical fiber. The optical transmitter 84 includes therein an optical device such as an optical modulator as a built-in element, and the optical device includes, for example, the first DC modulation unit 19, the second DC modulation unit 22, etc. When the light supplied from the light source 81 is guided through the waveguide, the optical transmitter 84 generates signal light by modulating the light by using the electrical signal.

[0117] The optical receiver 85 receives reception light from the optical fiber, converts the reception light into an electric signal by using the light supplied from the light source 81 , and outputs the converted electric signal to the DSP 82 .

[0118] An example has been described in which the optical transceiver 80 includes the optical transmitter 84 and the optical receiver 85 as built-in components, but this example can be applied to an optical transmitter including only the optical transmitter 84 as a built-in component, and the optical transmitter 84 includes an optical device as a built-in component. In addition, the embodiment can also be applied to an optical receiver including only the optical receiver 85 as a built-in component, and the optical receiver 85 includes an optical device as a built-in component. In addition, the embodiment is not limited to the optical transceiver 80, and the optical device can also be applied to the optical transmitter / receiver 83.

[0119] In this embodiment, a thin film LN chip has been described as an example, but the embodiment is not limited thereto. For example, TF-barium titanate may also be used, and appropriate modifications are also possible. For example, the material for the electro-optical effect may be TF-BTO (BaTiO3), TF-PLZT (PbLaZrTiO3), or TF-PZT (PbZrTiO3), and appropriate modifications are also possible.

[0120] According to one aspect of an embodiment of an optical device and the like disclosed in the present application, light loss caused by a heater electrode can be reduced.

Claims

1. An optical device, comprising: a waveguide in which light is guided and which has an electro-optic effect and a thermo-optic effect; a heater electrode disposed on one of the side surfaces of the waveguide and heating the waveguide; as well as A parallel electrode is arranged on the other of the side surfaces of the waveguide, is electrically connected to the heater electrode, and has a high resistance compared with the heater electrode.

2. The optical device according to claim 1, wherein: The parallel electrode electrically connects a portion between an input end of the parallel electrode and an input end of the heater electrode, and electrically connects a portion between an output end of the parallel electrode and an output end of the heater electrode.

3. The optical device according to claim 1, wherein: The parallel electrodes include a material different from a material used for the heater electrodes.

4. The optical device according to claim 1, wherein: The parallel electrode is composed of the same material as that used for the heater electrode, and has a structure in which an electrode width of the parallel electrode is made narrower than an electrode width of the heater electrode.

5. The optical device according to claim 1, wherein: The parallel electrode is composed of the same material as that used for the heater electrode, and has a meandering structure in which an electrode length of the parallel electrode is made longer than an electrode length of the heater electrode.

6. The optical device according to claim 1, wherein The waveguide comprises: First waveguide, a second waveguide arranged parallel to the first waveguide, and a third waveguide arranged in parallel with the first waveguide and the second waveguide at a position between one of the side surfaces of the first waveguide and one of the side surfaces of the second waveguide, The heater electrode comprises: a first heater electrode arranged on one of the side surfaces of the third waveguide, and a second heater electrode disposed on the other of the side surfaces of the third waveguide, and The parallel electrodes include: a first parallel electrode arranged on the other of the side surfaces of the first waveguide and electrically connected to the first heater electrode, and A second parallel electrode is arranged on the other of the side surfaces of the second waveguide and is electrically connected to the second heater electrode.

7. The optical device according to claim 6, wherein: The waveguide comprises: a first folded waveguide connecting a portion between an output end of the first waveguide and an input end of the third waveguide, and A second folded waveguide connects a portion between an output end of the third waveguide and an input end of the second waveguide.

8. An optical transmitter, comprising: a light source that emits light; as well as An optical device that modulates light received from the light source according to an electrical signal, wherein The optical device comprises: a waveguide in which light is guided and which has an electro-optic effect and a thermo-optic effect; a heater electrode disposed on one of the side surfaces of the waveguide and heating the waveguide; and A parallel electrode is arranged on the other of the side surfaces of the waveguide, is electrically connected to the heater electrode, and has a high resistance compared with the heater electrode.

9. An optical receiver, comprising: a light source that emits light; as well as An optical device that converts received light into an electrical signal by using the light emitted from the light source, wherein, The optical device comprises: a waveguide in which light is guided and which has an electro-optic effect and a thermo-optic effect; a heater electrode disposed on one of the side surfaces of the waveguide and heating the waveguide; and A parallel electrode is arranged on the other of the side surfaces of the waveguide, is electrically connected to the heater electrode, and has a high resistance compared with the heater electrode.

10. An optical transceiver, comprising: a processor that performs signal processing on the electrical signal; a light source that emits light; an optical transmitter that modulates light emitted from the light source by using the electrical signal output from the processor; as well as A light receiver converts received light into an electrical signal by using the light emitted from the light source, wherein: The optical transceiver comprises: a waveguide in which light is guided and which has an electro-optic effect and a thermo-optic effect; a heater electrode disposed on one of the side surfaces of the waveguide and heating the waveguide; and A parallel electrode is arranged on the other of the side surfaces of the waveguide, is electrically connected to the heater electrode, and has a high resistance compared with the heater electrode.

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