Optical device, optical transmitter, and optical transceiver

Abstract

To provide an optical device and the like, composed of a first chip including a first electrode and a second chip including a second electrode.SOLUTION: An optical device comprises a first chip including a first waveguide and a first electrode, and a second chip mounted on the first chip and including a second waveguide and a second electrode. The second waveguide has an electro-optic effect higher than that of the first waveguide and has a return structure that places an end of the second waveguide at an end surface of the second chip. The second waveguide is optically coupled to the first waveguide at the end surface. The second electrode has a return structure that places an end of the second electrode at the end surface, and the second electrode and the first electrode are electrically connected to each other in an area where the end has been placed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A conventional optical modulator has, for example, 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, thereby changing the phase of the light. The optical waveguide constitutes a Mach-Zehnder interferometer, and the optical output changes depending on the difference in the phase of the light between the optical waveguides.

[0003] The optical modulator is, for example, a Mach-Zehnder modulator. FIG. 34 is a plan view schematic diagram showing an example of a conventional optical modulator 200. The optical modulator 200 is configured, for example, with a thin-film LN (Lithium Niobate: LiNbO3) chip. The optical modulator 200 has an input waveguide 221, a branching unit 222, an RF (Radio Frequency) modulation unit 223, a DC (Direct Current) modulation unit 224, a multiplexing unit 225, and an output waveguide 226. The input waveguide 221 is, for example, an LN waveguide 215, which inputs signal light. The branching unit 222 branches the signal light from the input waveguide 221 into two optical waveguides 223A in the RF modulation unit 223. The RF modulation unit 223 is a modulation unit that performs high-speed modulation of the signal light guided through the two optical waveguides 223A.

[0004] The RF modulation unit 223 has two optical waveguides 223A arranged in parallel and a plurality of RF electrodes 223B arranged in parallel to the two optical waveguides 223A. The two optical waveguides 223A are LN waveguides 215. The RF electrode 223B has two ground electrodes 214B arranged in parallel on the outside of the two optical waveguides 223A, and one signal electrode 214A arranged between the two optical waveguides 223A and in parallel to the optical waveguide 223A. When a high-frequency signal having a band of, for example, several tens of GHz is input to the signal electrode 214A, the RF modulation unit 223 can quickly modulate the signal light propagating through the optical waveguide 223A in accordance with the high-frequency signal.

[0005] Fig. 35 is a cross-sectional schematic diagram showing an example of the cross section taken along line AA in Fig. 34. The cross-sectional portion taken along line AA is an RF modulation section 223 of the optical modulator 200. The RF modulation section 223 shown in Fig. 35 includes a support substrate 211 made of Si or the like, an intermediate layer 212 formed on the support substrate 211, a thin-film LN substrate 213 formed on the intermediate layer 212, and an electrode 214 formed on the thin-film LN substrate 213. The thin-film LN substrate 213 includes a thin-film LN waveguide 215 having a convex shape that protrudes upward. The LN waveguide 215 is a rib-type waveguide having a rib and slabs formed on both sides of the rib. The electrode 214 includes a signal electrode 214A and a pair of ground electrodes 214B. Then, on the surface of the slab, there are arranged electrodes 214 having a coplanar (CPW: Coplanar Waveguide) structure, that is, a signal electrode 214A and a pair of ground electrodes 214B sandwiching the signal electrode 214A. The electrode 214 of the RF modulation section 223 is an RF electrode 223B.

[0006] The DC modulation unit 224 is a modulation unit that connects two optical waveguides 223A in the RF modulation unit 223 and two optical waveguides 224A in the DC modulation unit 224, and modulates the signal light that is guided through the two optical waveguides 224A in the DC modulation unit 224. The multiplexing unit 225 multiplexes the signal light from the two optical waveguides 224A in the DC modulation unit 224. The output waveguide 226 is an LN waveguide 215 that outputs the multiplexed signal light from the multiplexing unit 225.

[0007] The DC modulation unit 224 includes two parallel optical waveguides 224A and a plurality of DC electrodes 224B arranged parallel to the two optical waveguides 224A. The two optical waveguides 224A are LN waveguides 215. The DC electrode 224B includes two ground electrodes 214B arranged parallel to the outer sides of the two optical waveguides 224A and one signal electrode 214A arranged parallel to the optical waveguide 224A between the two optical waveguides 224A. When a bias voltage is applied to the signal electrode 214A, the DC modulation unit 224 adjusts the bias at which the signal light guided through the optical waveguide 224A is turned on / off in response to the ON / OFF state of the bias voltage. The DC modulation unit 224 also includes a support substrate 211, an intermediate layer 212, a thin-film LN substrate 213, and an electrode 214. The electrode 214 of the DC modulation section 224 is a DC electrode 224B. That is, both the RF modulation section 223 and the DC modulation section 224 are made of thin-film LN chips. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2007 / 058366 [Patent Document 2] US Patent Application Publication No. 2017 / 0285437 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-221874 Summary of the Invention [Problem to be solved by the invention]

[0009] When a bias voltage is continuously applied to the DC electrode 224B of the optical modulator 200, the output light intensity changes over time due to DC drift (changes in the emitted light over time caused by the applied bias voltage). Therefore, to compensate for this change, it is necessary to increase the bias voltage. However, when the bias voltage reaches the maximum voltage of the control power supply, it becomes uncontrollable.

[0010] Therefore, the DC modulation unit 224 can solve the problem of DC drift by using a heater electrode instead of the DC electrode 224B. When a current flows through the heater electrode provided on the optical waveguide 224A via a buffer layer, the temperature of the optical waveguide 224A increases due to the heat generated by the heater electrode. As a result, the refractive index of the optical waveguide 224A changes due to the thermo-optic effect, and the phase of the signal light propagating through the optical waveguide 224A can be adjusted.

[0011] However, the DC modulation section using the heater electrode is composed of a SiPh (silicon photonics) chip, and the RF modulation section using the RF electrode is composed of a thin-film LN chip. Therefore, the optical modulator must be constructed by mounting a thin-film LN chip on a SiPh chip.

[0012] The disclosed technology has been developed in consideration of these points, and aims to provide an optical device, etc., consisting of a first chip having a first electrode and a second chip having a second electrode. [Means for solving the problem]

[0013] In one aspect, the optical device disclosed herein comprises a first chip including a first waveguide and a first electrode, and a second chip mounted on the first chip and including a second waveguide and a second electrode. The second waveguide has a stronger electro-optic effect than the first waveguide, and an end of the second waveguide is gathered on an end face of the second chip by a folded structure. The second waveguide is optically coupled to the first waveguide at the end face. The second electrode has a folded structure such that an end of the second electrode is gathered on the end face, and the first electrode and the second electrode are electrically connected in the gathered region. [Effects of the Invention]

[0014] According to one aspect of the optical device etc. disclosed in the present application, it is possible to provide an optical device etc. that is configured with a first chip having a first electrode and a second chip having a second electrode. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic plan view showing an example of an optical modulator according to a first embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 3] FIG. 3 is a cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 4] FIG. 4 is a schematic plan view illustrating an example of an optical modulator according to the second embodiment. [Figure 5] FIG. 5 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 6] FIG. 6 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 7] FIG. 7 is a schematic plan view showing an example of an optical modulator according to a third embodiment. [Figure 8] FIG. 8 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 9] FIG. 9 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 10] FIG. 10 is a schematic plan view showing an example of an optical modulator according to a fourth embodiment. [Figure 11] FIG. 11 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 12] FIG. 12 is a cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 13] FIG. 13 is a schematic plan view showing an example of an optical modulator according to a fifth embodiment. [Figure 14] FIG. 14 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 15] FIG. 15 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 16] FIG. 16 is a schematic plan view showing an example of an optical modulator according to a sixth embodiment. [Figure 17] FIG. 17 is a cross-sectional view showing an example of a cross section taken along line AA 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 the optical modulator according to the seventh embodiment. [Figure 20] FIG. 20 is a cross-sectional view showing an example of the cross section taken along the line AA shown in FIG. [Figure 21] FIG. 21 is a cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 22] FIG. 22 is a schematic plan view showing an example of the optical modulator according to the eighth embodiment. [Figure 23] FIG. 23 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 24] FIG. 24 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 25] FIG. 25 is a schematic plan view showing an example of the optical modulator according to the ninth embodiment. [Figure 26] FIG. 26 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. [Figure 27] FIG. 27 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 28] FIG. 28 is an explanatory diagram illustrating an example of an optical transceiver according to this embodiment. [Figure 29] FIG. 29 is a schematic plan view showing an example of an optical modulator of a comparative example. [Figure 30] FIG. 30 is a cross-sectional view showing an example of the cross section taken along line AA shown in FIG. [Figure 31] FIG. 31 is a cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 32] FIG. 32 is a cross-sectional view showing an example of a cross section taken along line CC shown in FIG. [Figure 33] FIG. 33 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in FIG. [Figure 34] FIG. 34 is a schematic plan view showing an example of a conventional optical modulator. [Figure 35]FIG. 35 is a cross-sectional view showing an example of a cross section taken along line AA shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] <Comparative Example> Fig. 29 is a schematic plan view showing an example of a comparative optical modulator 100. The optical modulator 100 shown in Fig. 29 has a SiPh chip 101 on which a DC modulation section 130 having a heater electrode 135 is mounted, and a thin-film LN chip 102 on which an RF modulation section 140 having an RF electrode 142 is mounted.

[0017] The SiPh chip 101 has a Si substrate 111, an oxide layer 112 on the Si substrate 111, and an opening 113 that opens a part of the oxide layer 112. The opening 113 is a portion where the thin-film LN chip 102 is mounted. The oxide layer 112 has a first oxide layer 112A and a second oxide layer 112B.

[0018] The first oxide layer 112A has an input-side Si waveguide 121, a branching portion 122, two input-side Si waveguides 123, an input-side electrode 124, and an input-side SiN waveguide 125. The input-side Si waveguide 121 is a Si waveguide to which signal light is input. The branching portion 122 is connected to the input-side Si waveguide 121 and the two input-side Si waveguides 123, and branches and outputs the signal light input from the input-side Si waveguide 121 to the two input-side Si waveguides 123. The two input-side Si waveguides 123 are connected to the branching portion 122 and the input-side SiN waveguide 125, and output the signal light branched and output from the branching portion 122 to the input ends of two LN waveguides 141 in the thin-film LN chip 103. The input-side electrode 124 is, for example, an Al electrode that is electrically connected to the input-side RF electrode 142 in the thin-film LN chip 103. The input-side SiN waveguide 125 is a waveguide that connects the two input-side Si waveguides 123 to the input ends of the two LN waveguides 141 in the thin-film LN chip 103.

[0019] The second oxide layer 112B has two Si waveguides 131 on the output side in the DC modulation section 130, a multiplexing section 132, an output Si waveguide 133, a heater 134 in the DC modulation section 130, and a heater electrode 135. The second oxide layer 112B has an output electrode 136 and an output SiN waveguide 137. The output SiN waveguide 137 is connected to the two output Si waveguides 131 and also to the output ends of two LN waveguides 141 in the thin-film LN chip 103, and outputs signal light from the output ends of the two LN waveguides 141 to the multiplexing section 132. The two output-side Si waveguides 131 are connected to the output-side SiN waveguide 137 and also to the multiplexing section 132, and output the signal light from the output-side SiN waveguide 137 to the multiplexing section 132. The multiplexing section 132 is connected to the two output-side Si waveguides 131 and also to the output-side Si waveguide 133, and multiplexes the signal light from the two output-side Si waveguides 131 and outputs the multiplexed signal light to the output-side Si waveguide 133. The output-side Si waveguide 133 is a waveguide that outputs the multiplexed signal light from the multiplexing section 132.

[0020] The heater 134 of the DC modulation unit 130 is disposed on the two Si waveguides 131 on the output side. The heater electrode 135 is, for example, an Al electrode, and is electrically connected to the heater 134. The DC modulation unit 130 heats the Si waveguide 131 on the output side by heating the heater 134 in response to the current from the heater electrode 135, thereby modulating the signal light guided through the Si waveguide 131 on the output side. The electrode 136 on the output side is, for example, an Al electrode, and is electrically connected to the RF electrode 142 of the RF modulation unit 140 in the thin-film LN chip 103.

[0021] The thin-film LN chip 103 has two LN waveguides 141 and an RF electrode 142 arranged parallel to the two LN waveguides 141. The two LN waveguides 141 and the RF electrode 142 together form an RF modulation section 140. The RF electrode 142 is, for example, an Au electrode. The RF electrode 142 has one signal electrode 142A and two ground electrodes 142B, and is electrically connected to the input electrode 124 and the output electrode 136 of the SiPh chip 101.

[0022] Fig. 30 is a cross-sectional schematic diagram showing an example of the cross section taken along line AA shown in Fig. 29. The cross-sectional portion taken along line AA is a bonding portion on the second oxide layer 112B side where the SiPh chip 101 and the thin-film LN chip 102 are bonded. The thin-film LN chip 102 has a support substrate 151 made of Si or the like, an intermediate layer 152 made of SiO2 or the like formed on the support substrate 151, and a thin-film LN substrate 153 formed on the intermediate layer 152. The thin-film LN substrate 153 has a convex LN waveguide 141 that protrudes from the surface opposite the intermediate layer 152.

[0023] The LN waveguide 141 is a rib-type waveguide having a rib and slabs formed on both sides of the rib. An RF electrode 142 having a coplanar (CPW) structure, i.e., a signal electrode 142A and a pair of ground electrodes 142B sandwiching the signal electrode 142A, is arranged on the surface of the slab.

[0024] The SiPh chip 101 has a Si substrate 111 and a second oxide layer 112B. The second oxide layer 112B has a SiN waveguide 137 that connects to two Si waveguides 131 on the output side. The SiN waveguide 137 is optically connected to an LN waveguide 141 of the thin-film LN chip 102.

[0025] The output electrode 136 of the SiPh chip 101 is electrically connected to the RF electrode 142 of the thin-film LN chip 102 via the bump 155 of the connection pad 154. That is, the signal electrode of the output electrode 136 is electrically connected to the signal electrode 142A of the RF electrode 142 via the bump 155 of the connection pad 154. The ground electrode of the output electrode 136 is electrically connected to the ground electrode 142B of the RF electrode 142 via the bump 155 of the connection pad 154.

[0026] Fig. 31 is a cross-sectional schematic diagram showing an example of a cross section taken along line BB shown in Fig. 29. The cross-sectional portion taken along line BB is the portion of RF modulation section 140 of thin-film LN chip 102. RF modulation section 140 of thin-film LN chip 102 is mounted in opening 113 on Si substrate 111 of SiPh chip 101, with RF electrode 142 and Si substrate 111 spaced apart from each other.

[0027] Fig. 32 is a cross-sectional schematic diagram showing an example of the cross section taken along line CC shown in Fig. 29. The cross-sectional portion of line CC electrically connects, via bumps 155, RF electrode 142 arranged on thin-film LN substrate 153 of thin-film LN chip 102 to input electrode 124 and output electrode 136 arranged on Si substrate 111 in opening 113 of SiPh chip 101.

[0028] The input electrode 124 arranged on the Si substrate 111 on the first oxide layer 112A side is electrically connected to the input RF electrode 142 arranged on the thin-film LN substrate 153 of the thin-film LN chip 102 via a bump 155. The output electrode 136 arranged on the Si substrate 111 on the second oxide layer 112B side is electrically connected to the output RF electrode 142 arranged on the thin-film LN substrate 153 of the thin-film LN chip 102 via a bump 155.

[0029] Fig. 33 is a cross-sectional schematic diagram showing an example of the cross section of the DD line shown in Fig. 29. The cross-sectional portion of the DD line is, for example, a portion where the connection portion between the input-side Si waveguide 123, the LN waveguide 141, and the output-side Si waveguide 131 is cut.

[0030] The first oxide layer 112A of the SiPh chip 101 has two input-side SiN waveguides 125 that are optically connected to the two input-side Si waveguides 123 and optically connected to the input ends of the two LN waveguides 141 of the thin-film LN chip 102, and an input-side electrode 124.

[0031] The second oxide layer 112B of the SiPh chip 101 has two output-side SiN waveguides 137 that are optically connected to the two output-side Si waveguides 131 and are optically connected to the output ends of the two LN waveguides 141 of the thin-film LN chip 102. The second oxide layer 112B further has a heater 134 and a heater electrode 135 that are arranged on the two output-side Si waveguides 131.

[0032] In the comparative example, a thin-film LN chip 102 having an RF electrode 142 is flip-chip bonded face-down to an opening 113 formed by etching a portion of an oxide layer 112 of a SiPh chip 101. The RF electrode 142 of the thin-film LN chip 102 is electrically connected to an electrode 124 (136) of the SiPh chip 101 via a bump 155. The LN waveguide 141 of the thin-film LN chip 102 couples the input-side Si waveguide 123 and the output-side Si waveguide 131 via a SiN waveguide 125 (137) provided in the SiPh chip 101.

[0033] However, in optical modulator 100 of the comparative example, the portion connected to input-side Si waveguide 123 and the portion connected to output-side Si waveguide 131 are located at both ends of thin-film LN chip 102. As a result, when flip-chip bonding thin-film LN chip 102 to opening 113 of SiPh chip 101, optical axis adjustment between the SiN waveguide and the LN waveguide is required at both ends of the chip, making the mounting work difficult.

[0034] Furthermore, thin-film LN chip 102 has a length of about 10 mm in the longitudinal direction of LN waveguide 141. As a result, when thin-film LN chip 102 is fixed to SiPh chip 101 at both ends, stress occurs during temperature changes due to the difference in thermal expansion between thin-film LN chip 102 and SiPh chip 101, resulting in unstable optical output. In other words, even when optical modulator 100 is configured with thin-film LN chip 102 including RF modulation section 140 and SiPh chip 101 including DC modulation section 130, there is a need for an optical modulator that avoids the situation in which stress occurs during temperature changes and unstable optical output occurs.

[0035] Therefore, an embodiment of an optical modulator that can deal with such a situation will be described in detail with reference to the drawings, but the present invention is not limited to this embodiment. [Example]

[0036] Fig. 1 is a plan view showing an example of an optical modulator 1 according to a first embodiment, Fig. 2 is a cross-sectional view showing an example of a cross section taken along line AA in Fig. 1, and Fig. 3 is a cross-sectional view showing an example of a cross section taken along line BB in Fig. 1. The optical modulator 1 shown in Fig. 1 is, for example, a Mach-Zehnder modulator. The optical modulator 1 has a SiPh chip 2 on which a DC (Direct Current) modulation unit 30 having a heater electrode is mounted, and a thin-film LN (Lithium Niobate: LiNbO3) chip 3 on which an RF (Radio Frequency) modulation unit 40 having an RF electrode is mounted.

[0037] The SiPh chip 2 has a Si substrate 11, an oxide layer 12 made of, for example, SiO2 or the like formed on the Si substrate 11, and an opening 13 formed by opening a portion of the oxide layer 12. The oxide layer 12 has a first oxide layer 12A and a second oxide layer 12B through the opening 13. The first chip is, for example, the SiPh chip 2.

[0038] The first oxide layer 12A has an input-side Si waveguide 21, a branching section 22, two input-side Si waveguides 23, two output-side Si waveguides 24, a multiplexing section 25, and an output-side Si waveguide 26. The input-side Si waveguide 21 is a waveguide to which signal light is input. The branching section 22 is connected to the input-side Si waveguide 21 and also to the two input-side Si waveguides 23, and branches and outputs the signal light input from the input-side Si waveguide 21 to the two input-side Si waveguides 23. The two input-side Si waveguides 23 are connected to the branching section 22 and also to a SiN waveguide 32, which will be described later. The two Si waveguides 23 on the input side are connected to the input ends of two LN waveguides 41 on the input side within the thin-film LN chip 3, which are connected to the SiN waveguide 32, and the signal light branched and output from the branching section 22 is output to the two LN waveguides 41 on the input side within the thin-film LN chip 3.

[0039] The two output-side Si waveguides 24 are connected to the SiN waveguide 32 which is connected to the output ends of the two output-side LN waveguides 42 in the thin-film LN chip 3, and are also connected to the multiplexing unit 25, so that the signal light from the two output-side LN waveguides 42 is output to the multiplexing unit 25. The multiplexing unit 25 is connected to the two output-side Si waveguides 24 and also to the output-side Si waveguide 26, so that the signal light from the two output-side Si waveguides 24 is multiplexed and the multiplexed signal light is output to the output-side Si waveguide 26. The output-side Si waveguide 26 is a waveguide that outputs the multiplexed signal light from the multiplexing unit 25. The first waveguide is, for example, the input-side Si waveguide 23 and the output-side Si waveguide 24.

[0040] The DC modulation unit 30 has two Si waveguides 24 on the output side, a heater 29 arranged on the two Si waveguides 24 on the output side, and a heater electrode 31 electrically connected to the heater 29. In the DC modulation unit 30, the heater 29 is heated in accordance with the current to the heater electrode 31, thereby heating the Si waveguide 24 on the output side and modulating the signal light guided through the Si waveguide 24 on the output side. The first electrode is, for example, the heater electrode 31.

[0041] The thin-film LN chip 3 includes a support substrate 51 made of Si or the like, an intermediate layer 52 made of SiO2 or the like formed on the support substrate 51, and a thin-film LN substrate 53 formed on the intermediate layer 52. The thin-film LN substrate 53 includes LN waveguides 41, 42, and 43 that protrude from the surface opposite the intermediate layer 52. The LN waveguides 41, 42, and 43 are rib-type waveguides having ribs and slabs formed on both sides of the ribs. RF electrodes 44, 45, and 46 with a coplanar (CPW) structure are arranged on the surfaces of the slabs. The RF electrodes 44, 45, and 46 include a signal electrode and a pair of ground electrodes sandwiching the signal electrode. The second chip has a higher electro-optic effect than the first waveguide, and is, for example, the thin-film LN chip 3.

[0042] The thin-film LN chip 3 has LN waveguides 41, 42, and 43 with a folded structure, and RF electrodes 44, 45, and 46 with a folded structure. In the thin-film LN chip 3, both ends of the LN waveguides 41 and 42 are gathered at a first end face 3A of the thin-film LN chip 3, and both ends of the RF electrodes 44 and 45 are gathered at the first end face 3A of the thin-film LN chip 3.

[0043] The LN waveguide includes an outgoing LN waveguide 41, a returning LN waveguide 42, and a U-shaped folded LN waveguide 43 connecting the outgoing LN waveguide 41 and the returning LN waveguide 42. The input end of the outgoing LN waveguide 41 and the output end of the returning LN waveguide 42 are integrated at a first end face 3A of the thin-film LN chip 3. The first end face 3A is the end face of the thin-film LN chip 3 on the first oxide layer 12A side of the SiPh chip 2. The second waveguide is, for example, an LN waveguide such as the outgoing LN waveguide 41 and the returning LN waveguide 42. The second outgoing waveguide is, for example, the outgoing LN waveguide 41. The second waveguide on the return path side is, for example, the LN waveguide 42 on the return path side.

[0044] The RF electrodes include an outgoing RF electrode 44, a return RF electrode 45, and a U-shaped folded RF electrode 46 that electrically connects the outgoing RF electrode 44 and the return RF electrode 45. The input end of the outgoing RF electrode 44 and the output end of the return RF electrode 45 are gathered on the first end face 3A of the thin-film LN chip 3. The second electrode is, for example, an RF electrode including the outgoing RF electrode 44 and the return RF electrode 45. The second outgoing electrode is, for example, the outgoing RF electrode 44. The second return electrode is, for example, the return RF electrode 45.

[0045] The outgoing RF electrode 44 has a signal electrode 44A and a pair of ground electrodes 44B arranged parallel to the signal electrode 44A and sandwiching the signal electrode 44A. The returning RF electrode 45 has a signal electrode 45A and a pair of ground electrodes 45B arranged parallel to the signal electrode 45A and sandwiching the signal electrode 45A. The folded RF electrode 46 has a signal electrode 46A and a pair of ground electrodes 46B arranged parallel to the signal electrode 46A and sandwiching the signal electrode 46A.

[0046] Because the thin-film LN chip 3 uses an X-cut LN crystal, when the LN waveguide and RF electrode have a folded structure, the polarization of the modulation is reversed between the outgoing LN waveguide 41 and the return-side LN waveguide 42 due to the folding of the RF electrode. As a result, modulation is canceled before and after the folding of the folded LN waveguide 43. Therefore, the thin-film LN chip 3 is configured so that the electric field of the electrical signal is applied to the LN waveguide only when the signal light travels leftward as shown in FIG. 1. For example, the return-side LN waveguide 42 is only affected by the electric field of the return-side RF electrode 45. In other words, the return-side RF electrode 45 is configured so that the electric field from the return-side RF electrode 45 is applied to the return-side LN waveguide 42.

[0047] Although the outgoing-side LN waveguide 41 is disposed in parallel with the outgoing-side RF electrode 44, all of the outgoing-side RF electrodes 44 are disposed on the inner periphery side of the outgoing-side LN waveguide 41. As a result, it is possible to reduce the influence of the electric field applied from the outgoing-side RF electrode 44 to the outgoing-side LN waveguide 41. In other words, the outgoing-side RF electrode 44 is disposed in such a manner that the electric field from the outgoing-side RF electrode 44 is not applied to the outgoing-side LN waveguide 41.

[0048] Although the folded LN waveguide 43 is disposed near the folded RF electrode 46, the folded RF electrode 46 is entirely disposed on the inner circumferential side of the folded LN waveguide 43. As a result, the influence of the electric field applied from the folded RF electrode 46 to the folded LN waveguide 43 can be reduced.

[0049] The return-side LN waveguide 42 is disposed in parallel with the return-side RF electrode 45, and a signal electrode 45A is disposed between the return-side LN waveguides 42. Furthermore, in the return-side LN waveguide 42, one ground electrode 45B is disposed on the inner peripheral side of the return-side LN waveguide 42, and the other ground electrode 45B is disposed on the inner peripheral side of the return-side LN waveguide 42. In other words, the return-side LN waveguide 42 is configured to modulate the guided signal light in response to the electric field from the return-side RF electrode 45.

[0050] The RF modulation unit 40 has two return-side LN waveguides 42 arranged in parallel, and a return-side RF electrode 45 arranged in parallel to the two return-side LN waveguides 42. The return-side RF electrode 45 has two ground electrodes 45B arranged in parallel on the outside of the two return-side LN waveguides 42, and one signal electrode 45A arranged between the two return-side LN waveguides 42 and in parallel to the return-side LN waveguide 42. When a high-frequency signal having a band of, for example, several tens of GHz is input to the signal electrode 45A, the RF modulation unit 40 performs high-speed modulation of the signal light guided through the two return-side LN waveguides 42 in accordance with the high-frequency signal.

[0051] The two Si waveguides 23 on the input side of the first oxide layer 12A are connected to two LN waveguides 41 on the outgoing side in the thin-film LN chip 3 via two SiN waveguides 32 on the input side. The two Si waveguides 24 on the output side of the first oxide layer 12A are connected to two LN waveguides 42 on the returning side in the thin-film LN chip 3 via the SiN waveguides 32 on the output side. In other words, the LN waveguides in the thin-film LN chip 3 have a folded structure, and optical coupling points connecting the two Si waveguides 23 on the input side to the input ends of the two outgoing LN waveguides 41 and optical coupling points connecting the two Si waveguides 24 on the output side to the output ends of the two returning LN waveguides 42 are located on the thin-film LN chip 3. As a result, the work required for adjusting the optical axis, such as the tilt of the element, when flip-chip bonding the thin-film LN chip 3 to the opening 13 of the SiPh chip 2 can be simplified.

[0052] The input-side electrode 27 of the first oxide layer 12A is, for example, an Al electrode, and is electrically connected to the outgoing RF electrode 44 in the thin-film LN chip 3 via the metal fusion bump 55 of the connection pad 54. The output-side electrode 28 of the first oxide layer 12A is, for example, an Al electrode, and is electrically connected to the return-side RF electrode 45 in the thin-film LN chip 3 via the bump 55 of the connection pad 54. In other words, the RF electrode in the thin-film LN chip 3 has a folded structure, and the junction electrically connecting the input-side electrode 27 to the outgoing RF electrode 44 and the junction electrically connecting the output-side electrode 28 to the return-side RF electrode 45 are concentrated on the first end face 3A of the thin-film LN chip 3.

[0053] In the optical modulator 1 of the first embodiment, the optical junctions connecting the input-side Si waveguide 23 and the outgoing-side LN waveguide 41 and the output-side Si waveguide 24 and the incoming-side LN waveguide 42 are concentrated on the first end face 3A of the thin-film LN chip 3. As a result, the optical axis adjustment between the SiN waveguide and the LN waveguide when flip-chip bonding the thin-film LN chip 3 to the opening 13 of the SiPh chip 2 can be concentrated on the first end face 3A, simplifying the mounting work.

[0054] Furthermore, in optical modulator 1, the junction connecting input-side electrode 27 and outgoing-side RF electrode 44 and the junction connecting output-side electrode 28 and incoming-side RF electrode 45 are concentrated on first end face 3A of thin-film LN chip 3. As a result, the junction on the first end face 3A side is fixed on one side of thin-film LN chip 3, which reduces the effect of stress during temperature changes due to differences in the thermal expansion coefficients of thin-film LN chip 3 and SiPh chip 2, thereby improving operational stability.

[0055] In the thin-film LN chip 3, the signal light guided through the return-side LN waveguide 42 of the folded LN waveguide and RF electrode undergoes high-speed modulation in response to the electric field of the return-side RF electrode 45. As a result, even when an LN waveguide made of X-cut LN crystal is used, it is possible to avoid a situation in which modulation is canceled before and after folding.

[0056] For ease of explanation, the RF modulation unit 40 has been exemplified as performing high-speed modulation of the signal light guided through the return-side LN waveguide 42 in accordance with the electric field of the return-side RF electrode 45. However, instead of the return-side RF electrode 45, a structure may be used in which an outward-side RF electrode 44 is disposed so that an electric field is applied to the outward-side LN waveguide 41. In this case, the RF modulation unit 40 can perform high-speed modulation of the signal light guided through the outward-side LN waveguide 41 in accordance with the electric field of the outward-side RF electrode 44.

[0057] In the optical modulator 1 of Example 1, the Si waveguide in the SiPh chip 2 and the LN waveguide in the thin-film LN chip 3 are optically coupled via the SiN waveguide 32. However, optical power is transferred between the SiN waveguide 32 and the LN waveguide by adiabatic conversion. Although adiabatic conversion has high tolerance for misalignment between the chips relative to the light propagation direction, misalignment between the chips in the direction perpendicular to the light propagation direction, for example, deviation of the spacing between the SiN waveguide 32 and the LN waveguide from the design value, may degrade the conversion efficiency. Therefore, an embodiment of the optical modulator 1A that can deal with such a situation will be described below as Example 2. [Example]

[0058] Fig. 4 is a plan view schematic diagram showing an example of an optical modulator 1A of the second embodiment, Fig. 5 is a cross-sectional view schematic diagram showing an example of a cross section taken along line AA shown in Fig. 4, and Fig. 6 is a cross-sectional view schematic diagram showing an example of a cross section taken along line BB shown in Fig. 4. Note that the same components as those in the optical modulator 1 of the first embodiment are given the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical modulator 1 of the first embodiment differs from the optical modulator 1A of the second embodiment in that, instead of the SiN waveguide 32, the Si waveguide of the SiPh chip 2A and the LN waveguide of the thin-film LN chip 3 are optically coupled by butt coupling X.

[0059] The two Si waveguides 23 on the input side of the SiPh chip 2A are optically coupled to the two LN waveguides 41 on the outgoing side of the thin-film LN chip 3 by butt coupling X. Furthermore, the two Si waveguides 24 on the output side of the SiPh chip 2A are optically coupled to the two LN waveguides 42 on the incoming side of the thin-film LN chip 3 by butt coupling X.

[0060] In the optical modulator 1A of the second embodiment, the Si waveguide of the SiPh chip 2 and the LN waveguide of the thin-film LN chip 3 are optically coupled by butt coupling X. As a result, in the optical modulator 1A, it is possible to improve the tolerance not only for misalignment between the chips in the light propagation direction, but also for misalignment between the chips in the direction perpendicular to the light propagation direction.

[0061] In the optical modulator 1A of Example 2, a portion of the oxide layer 12 of the SiPh chip 2A is etched to form an opening 13, and the thin-film LN chip 3 is mounted on the opening 13. However, because the surface of the Si substrate 11 is exposed at the opening 13 of the SiPh chip 2A, an electrical signal from an RF electrode disposed on the thin-film LN substrate 53 of the thin-film LN chip 3 is absorbed by the Si substrate 11, resulting in significant attenuation of the electrical signal. Therefore, an embodiment of the optical modulator 1B that addresses this situation will be described below as Example 3. [Example]

[0062] Fig. 7 is a schematic plan view showing an example of an optical modulator 1B of the third embodiment, Fig. 8 is a schematic cross-sectional view showing an example of a cross section taken along line AA shown in Fig. 7, and Fig. 9 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in Fig. 7. Note that the same components as those in the optical modulator 1A of the second embodiment are given the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical modulator 1A of the second embodiment differs from the optical modulator 1B of the third embodiment in that a recess 13B is formed by digging down the surface of the Si substrate 11 exposed from an opening 13 in the SiPh chip 2B on which the thin-film LN chip 3 is mounted.

[0063] The recess 13B formed in the Si substrate 11 is structured to be flush with the end face of the first oxide layer 12A, which includes the output end of the Si waveguide 23 on the input side of the SiPh chip 2B and the input end of the Si waveguide 24 on the output side.

[0064] In optical modulator 1B of Example 3, recess 13B is formed on the surface of Si substrate 11 exposed from opening 13 of SiPh chip 2A. As a result, the amount of electrical signal flowing through the RF electrode arranged on thin-film LN substrate 53 of thin-film LN chip 3 mounted in opening 13 that is absorbed by Si substrate 11 can be reduced, thereby suppressing attenuation of the electrical signal flowing through the RF electrode. This also makes it possible to suppress deterioration of RF characteristics.

[0065] In optical modulator 1B, in order to suppress an increase in coupling loss due to chip misalignment at the location where the Si waveguide of SiPh chip 2B and the LN waveguide of thin-film LN chip 3 are optically coupled by butt coupling X, the optical mode field is increased by weakening the confinement of the optical waveguide. However, at the optical coupling location, a large amount of light is radiated to Si substrate 11, increasing light loss. Therefore, an embodiment of optical modulator 1C that can deal with such a situation will be described below as Example 4. [Example]

[0066] Fig. 10 is a plan view schematic diagram showing an example of an optical modulator 1C of Example 4, Fig. 11 is a cross-sectional view schematic diagram showing an example of a cross section taken along line AA in Fig. 10, and Fig. 12 is a cross-sectional view schematic diagram showing an example of a cross section taken along line BB in Fig. 10. Note that the same components as those in the optical modulator 1B of Example 3 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical modulator 1B differs from the optical modulator 1C of Example 4 in that, in addition to the recess 13B formed in the surface of the Si substrate 11, a recess 13C is formed by digging out the surface of the Si substrate 11 below a portion where the Si waveguide and the LN waveguide of the first oxide layer 12A are optically coupled.

[0067] The optical coupling portion for butt-coupling between the input-side Si waveguide 23 and the outgoing-side LN waveguide 41, i.e., the surface of the Si substrate 11 near the output end of the input-side Si waveguide 23 in the first oxide layer 12A, is excavated to form a recess 13C in the opening 13.

[0068] The optical coupling portion for butt-coupling between the output-side Si waveguide 24 and the return-side LN waveguide 42, i.e., the surface of the Si substrate 11 near the input end of the output-side Si waveguide 24 in the first oxide layer 12A, is excavated to form a recess 13C in the opening 13.

[0069] In the optical modulator 1C of the fourth embodiment, the surface of the Si substrate 11 below the portion of the first oxide layer 12A where the Si waveguide and the LN waveguide are optically coupled is excavated to form a recess 13C in addition to the recess 13B within the opening 13. As a result, radiation of light to the Si substrate 11 at the optically coupled portion can be suppressed, thereby suppressing light loss at the optically coupled portion.

[0070] In the optical modulator 1C of Example 4, the Si waveguide of the SiPh chip 2C and the LN waveguide of the thin-film LN chip 3 are optically coupled by butt coupling X. However, with butt coupling X, if air gets between the Si waveguide and the LN waveguide, reflection and loss of the signal light guided at the optical coupling point increases. Therefore, an embodiment of the optical modulator 1D that can deal with such a situation will be described below as Example 5. [Example]

[0071] Fig. 13 is a plan view showing an example of an optical modulator 1D of Example 5, Fig. 14 is a cross-sectional view showing an example of a cross section taken along line AA in Fig. 13, and Fig. 15 is a cross-sectional view showing an example of a cross section taken along line BB in Fig. 13. Note that the same components as those in the optical modulator 1C of Example 4 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical modulator 1C of Example 4 differs from the optical modulator 1D of Example 5 in that the first oxide layer 12A of the SiPh chip 2C and the first end face 3A of the thin-film LN chip 3 are bonded together with an optical adhesive 61.

[0072] In the optical modulator 1D of the fifth embodiment, the first oxide layer 12A of the SiPh chip 2C and the first end face 3A of the thin-film LN chip 3 are bonded together with an optical adhesive 61. As a result, it is possible to prevent air from entering the optical coupling portion where the Si waveguide and the LN waveguide are optically coupled by butt coupling X, thereby suppressing radiation and loss of light guided through the optical coupling portion.

[0073] However, in optical modulator 1D, because adhesive 61 is used to bond SiPh chip 2C and thin-film LN chip 3 at first end facet 3A, i.e., one end face of the chip, it is possible that optical modulator 1D may be affected by impacts or vibrations on SiPh chip 2C and thin-film LN chip 3. Therefore, an embodiment of optical modulator 1E that can deal with such a situation will be described below as Example 6. [Example]

[0074] Fig. 16 is a schematic plan view showing an example of an optical modulator 1E of Example 6, Fig. 17 is a schematic cross-sectional view showing an example of a cross section taken along line AA in Fig. 16, and Fig. 18 is a schematic cross-sectional view showing an example of a cross section taken along line BB in Fig. 17. Note that the same components as those in the optical modulator 1D of Example 5 are given the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical modulator 1D of Example 5 differs from the optical modulator 1E of Example 6 in that the second oxide layer 12B and the second end face 3B of the thin-film LN chip 3 are bonded with adhesive 62, in addition to the first oxide layer 12A and the first end face 3A of the thin-film LN chip 3.

[0075] The optical adhesive 62 used to bond the second oxide layer 12B and the second end face 3B of the thin-film LN chip 3 is softer than the adhesive 61 used to bond the first oxide layer 12A and the first end face 3A of the thin-film LN chip 3. The adhesive 62 has a hardness that is lower after curing than the hardness of the adhesive near the bump bonding portion.

[0076] In the optical modulator 1E of Example 6, the first oxide layer 12A and the first end face 3A of the thin-film LN chip 3 are bonded together with an optical adhesive 61, and the second oxide layer 12B and the second end face 3B of the thin-film LN chip 3 are bonded together with an adhesive 62. Furthermore, a softer material is used for the adhesive 62 than for the adhesive 61. As a result, by bonding the SiPh chip 2C and the thin-film LN chip 3 on both sides, the effects of shock and vibration on the SiPh chip 2C and the thin-film LN chip 3 can be suppressed. Moreover, because the adhesive 62 used on one side is made of a soft material, the stress applied to the thin-film LN chip 3 can be suppressed, thereby stabilizing the operation of the optical modulator 1E.

[0077] Furthermore, if the stress applied to the thin-film LN chip 3 can be reduced, the same adhesive may be used for the optical adhesive 61 that bonds the first oxide layer 12A and the first end face 3A and the adhesive 62 that bonds the second oxide layer 12B and the second end face 3B.

[0078] In the optical modulator 1E of the sixth embodiment, the SiPh chip 2 is bonded to the first end face 3A of the thin-film LN chip 3 and the second end face 3B opposite to the first end face 3A. However, this is not limited to the second end face 3B, and instead of the second end face 3B, the SiPh chip 2 may be bonded to an end face of the thin-film LN chip 3 that is perpendicular to the first end face 3A, and this can be modified as appropriate.

[0079] Furthermore, in the optical modulator 1E of Example 6, three connection pads 54 are illustrated as connecting the input electrode 27 and the outgoing RF electrode 44, and three connection pads 54 are illustrated as electrically connecting the output electrode 28 and the incoming RF electrode 45. However, the number of connection pads 54 may be reduced, and such an embodiment will be described below as Example 7. [Example]

[0080] Fig. 19 is a schematic plan view showing an example of an optical modulator 1F of the seventh embodiment, Fig. 20 is a schematic cross-sectional view showing an example of a cross section taken along line AA shown in Fig. 19, and Fig. 21 is a schematic cross-sectional view showing an example of a cross section taken along line BB shown in Fig. 19. Note that the same components as those in the optical modulator 1E of the sixth embodiment are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. The optical modulator 1E of the sixth embodiment differs from the optical modulator 1F of the seventh embodiment in that the ground electrode 27B1 of the input-side electrode 27 and the ground electrode 28B1 of the output-side electrode 28 are connected to a shared pad 54A instead of the connection pad 54.

[0081] The shared pad 54A electrically connects the ground electrode 27B1 of the input-side electrode 27 to the ground electrode 44B1 of the outgoing-side RF electrode 44. Furthermore, the shared pad 54A electrically connects the ground electrode 28B1 of the output-side electrode 28 to the ground electrode 45B1 of the returning-side RF electrode 45. The ground electrode 46B1 of the folded RF electrode 46 electrically connects the ground electrode 44B1 of the outgoing-side RF electrode 44 to the ground electrode 45B1 of the returning-side RF electrode 45.

[0082] The optical modulator 1F has a connection pad 54 connecting the ground electrode 27B of the input-side electrode 27 to the ground electrode 44B of the outgoing-side RF electrode 44, and a connection pad 54 connecting the signal electrode 27A of the input-side electrode 27 to the signal electrode 44A of the outgoing-side RF electrode 44. The optical modulator 1F also has a connection pad 54 connecting the ground electrode 28B of the output-side electrode 28 to the ground electrode 45B of the return-side RF electrode 45, and a connection pad 54 connecting the signal electrode 28A of the output-side electrode 28 to the signal electrode 45A of the return-side RF electrode 45. The optical modulator 1F also has a shared pad 54A connecting the ground electrode 27B1 of the input-side electrode 27 to the ground electrode 44B1 of the outgoing-side RF electrode 44 and connecting the ground electrode 28B1 of the output-side electrode 28 to the ground electrode 45B1 of the return-side RF electrode 45.

[0083] In the optical modulator 1F of the seventh embodiment, the ground electrode 27B1 of the input electrode 27 and the ground electrode 28B1 of the output electrode 28 are connected to the shared pad 54A. As a result, the number of connection pads electrically connecting the SiPh chip 2C and the thin-film LN chip 3 is set to five, which reduces the stress on the bump bonding points and contributes to a reduction in the chip size.

[0084] In the optical modulator 1F of Example 7, the two Si waveguides and two LN waveguides constituting the Mach-Zehnder interferometer are folded back, which causes the waveguide lengths of the inner and outer waveguides to differ, affecting the temperature characteristics and wavelength characteristics of the optical modulator. Therefore, an embodiment that addresses this situation will be described below as Example 8. [Example]

[0085] FIG. 22 is a schematic plan view showing an example of an optical modulator 1G of Example 8, FIG. 23 is a schematic cross-sectional view showing an example of a cross section taken along line AA in FIG. 22, and FIG. 24 is a schematic cross-sectional view showing an example of a cross section taken along line BB in FIG. 22. The same components as those in the optical modulator 1F of Example 7 are denoted by the same reference numerals, and redundant descriptions of the components and operations will be omitted. The optical modulator 1F of Example 7 differs from the optical modulator 1G of Example 8 in that, of the two outgoing LN waveguides 41, the inner outgoing LN waveguide 41 is replaced with a curved waveguide 41A in which the waveguide length of the inner outgoing LN waveguide 41 is adjusted. The curved waveguide 41A has a folded structure in which the waveguide length of the inner LN waveguide is the same as the waveguide length of the outer LN waveguide.

[0086] The inner LN waveguide constituting the Mach-Zehnder interferometer is an LN waveguide having an inner outgoing LN waveguide 41, an inner returning LN waveguide 42, and an inner folded LN waveguide 43. The outer LN waveguide constituting the Mach-Zehnder interferometer is an LN waveguide having an outer outgoing LN waveguide 41, an outer returning LN waveguide 42, and an outer folded LN waveguide 43.

[0087] The inner outgoing LN waveguide 41 was replaced with a curved waveguide 41A so that the waveguide lengths of the inner and outer LN waveguides of the thin-film LN chip 3 were the same. In other words, the curved waveguide 41A is such that the waveguide length of one of the two outgoing LN waveguides 41 is longer or shorter than the waveguide length of the other outgoing LN waveguide 41. As a result, the waveguide length of the inner LN waveguide and the waveguide length of the outer LN waveguide are the same.

[0088] In the optical modulator 1G of Example 8, the inner outgoing LN waveguide 41 in the thin-film LN chip 3 is replaced with a curved waveguide 41A so that the waveguide lengths of the inner LN waveguide and the outer LN waveguide are the same. As a result, the waveguide lengths of the inner LN waveguide and the outer LN waveguide that constitute the Mach-Zehnder interferometer are the same, which can suppress the influence on the temperature characteristics and wavelength characteristics of the optical modulator 1G.

[0089] In the optical modulator 1G of Example 8, the case where the waveguide length of the outgoing LN waveguide 41 is adjusted has been exemplified, but the waveguide length of the folded LN waveguide 43 may be adjusted instead of the outgoing LN waveguide 41, and changes can be made as appropriate.

[0090] In addition, the optical modulator 1G has been described as an example in which the waveguide length of the outgoing LN waveguide 41 is adjusted so that the waveguide lengths of the inner LN waveguide and the outer LN waveguide are the same. However, the present invention is not limited to this, and an embodiment thereof will be described below as Example 9. [Example]

[0091] Fig. 25 is a schematic plan view showing an example of an optical modulator 1H according to a ninth embodiment, Fig. 26 is a schematic cross-sectional view showing an example of a cross section taken along line AA in Fig. 25, and Fig. 27 is a schematic cross-sectional view showing an example of a cross section taken along line BB in Fig. 25. The same components as those in the optical modulator 1E according to the sixth embodiment are denoted by the same reference numerals, and redundant descriptions of the components and operations will be omitted. The optical modulator 1E according to the sixth embodiment differs from the optical modulator 1H according to the ninth embodiment in that the inner Si waveguide 23 on the input side of the two Si waveguides 23 is replaced with a bent waveguide 23A in which the waveguide length of the inner Si waveguide 23 on the input side is adjusted. The bent waveguide 23A has a folded structure in which the inner and outer waveguides have the same waveguide length.

[0092] The inner waveguide constituting the Mach-Zehnder interferometer is a waveguide having an inner input-side Si waveguide 23, an inner outgoing LN waveguide 41, an inner return-side LN waveguide 42, an inner folded LN waveguide 43, and an inner output-side Si waveguide 24. The outer waveguide constituting the Mach-Zehnder interferometer is a waveguide having an outer input-side Si waveguide 23, an outer outgoing LN waveguide 41, an outer return-side LN waveguide 42, an outer folded LN waveguide 43, and an outer output-side Si waveguide 24.

[0093] The inner input-side Si waveguide 23 was replaced with a curved waveguide 23A so that the waveguide lengths of the inner and outer waveguides were the same. The curved waveguide 23A is formed by making the waveguide length of one of the two input-side Si waveguides 23 longer or shorter than that of the other input-side Si waveguide 23. As a result, the waveguide lengths of the inner and outer waveguides are the same. The Si waveguide of the SiPh chip 2C allows for the formation of a curved waveguide with a smaller bending radius than the LN waveguide of the thin-film LN chip 3.

[0094] In the optical modulator 1H of the ninth embodiment, the inner input-side Si waveguide 23 is replaced with a curved waveguide 23A so that the waveguide lengths of the inner and outer waveguides are the same. As a result, the waveguide lengths of the inner and outer waveguides constituting the Mach-Zehnder interferometer are the same, which can suppress the influence on the temperature characteristics and wavelength characteristics of the optical modulator 1H.

[0095] In the optical modulator 1G of Example 9, the waveguide length of the Si waveguide 23 on the inner input side is adjusted, but the waveguide length of the Si waveguide 24 on the inner output side may also be adjusted, and can be changed as appropriate.

[0096] Although the optical modulator 1 has been shown as an example of the optical device of the present invention, the present invention is not limited to the optical modulator 1 and can be applied to various optical devices such as optical receivers.

[0097] FIG. 28 is an explanatory diagram illustrating an example of an optical transceiver 70 according to this embodiment. The optical transceiver 70 shown in FIG. 28 is connected to an output optical fiber and an input optical fiber. The optical transceiver 70 includes a light source 71, a DSP (Digital Signal Processor) 72, and an optical transmitter / receiver 73. The optical transmitter / receiver 73 includes an optical transmitter 73A and an optical receiver 73B. The DSP 72 is an electrical component that performs digital signal processing. For example, the DSP 72 performs processing such as encoding transmission data, generates an electrical signal containing the transmission data, and outputs the generated electrical signal to the optical transmitter 73A. The DSP 72 also acquires an electrical signal containing reception data from the optical receiver 73B and performs processing such as decoding the acquired electrical signal to obtain the reception data.

[0098] The light source 71 is, for example, an LD equipped with a laser diode or the like, which generates light of a predetermined wavelength and supplies it to the optical transmitter 73A and the optical receiver 73B. The optical transmitter 73A is an optical device such as an optical modulator that modulates the light supplied from the light source 71 using an electrical signal output from the DSP 72 and outputs the modulated signal light to an optical fiber. The optical transmitter 73A incorporates an optical device such as an optical modulator constituted by, for example, a SiPh chip 2 and a thin-film LN chip 3. The optical transmitter 73A generates signal light by modulating the light supplied from the light source 71 using an electrical signal as the light is guided through the waveguide.

[0099] The optical receiver 73 B receives the received light from the optical fiber, converts the received light into an electrical signal using the light supplied from the light source 71 , and outputs the converted electrical signal to the DSP 72 .

[0100] Although the optical transceiver 70 has been exemplified as having an optical transmitter 73A and an optical receiver 73B built in, the present invention can also be applied to an optical transmitter that has only an optical transmitter 73A that has an optical device built in. It can also be applied to an optical receiver that has only an optical receiver 73B that has an optical device built in. Furthermore, the present invention is not limited to the optical transceiver 70, and the optical device can also be applied to the optical transmitter / receiver 73.

[0101] In this embodiment, a thin film LN chip 3 is used as an example, but the present invention is not limited to this and may be, for example, TF-Barium Titanate, or may be changed as appropriate. Materials with an 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]

[0102] 1 Optical modulator 2 SiPh chips 3 Thin-film LN chip 3A First end face 23 Si waveguide on the input side 24 Output Si waveguide 31 heater electrode 41 LN waveguide on the outgoing side 42 Return LN waveguide 43 Folded LN waveguide 44 RF electrode on the outgoing path 45 Return RF electrode 46 Folded RF electrode 61 Adhesive 62 Adhesive 70 Optical Transceiver 71 Light source 72 DSP 73 Optical Transmitter / Receiver 73A Optical Transmitter 73B Optical Receiver

Claims

1. a first chip including a first waveguide and a first electrode; a second chip mounted on the first chip and including a second waveguide and a second electrode, The second waveguide comprises: the second waveguide has a higher electro-optic effect than the first waveguide, and an end of the second waveguide is gathered on an end face of the second chip by a folded structure, and is optically coupled to the first waveguide at the end face; The second electrode is An optical device characterized in that an end of the second electrode is concentrated on the end face by a folded structure, and the first electrode and the second electrode are electrically connected in the concentrated area.

2. The second waveguide of the folded structure is a second waveguide on an outgoing path side and a second waveguide on a returning path side connected to the second waveguide on the outgoing path side, The second electrode of the folded structure is A second electrode on the outgoing path and a second electrode on the return path, The second electrode on the return path side is The second electrode is disposed to apply an electric field from the return path side second electrode to the return path side second waveguide, The second electrode on the outgoing path is 2. The optical device according to claim 1, wherein the second electrode on the outgoing path is disposed so that an electric field from the second electrode on the outgoing path is not applied to the second waveguide on the outgoing path.

3. The first chip includes:

2. The optical device according to claim 1, wherein the second chip is bonded to the end surface of the second chip with an adhesive.

4. The first chip includes:

4. The optical device according to claim 3, wherein the second chip is bonded to an end face other than the end face with an adhesive that is softer than the adhesive.

5. The second waveguide of the folded structure is a second waveguide on an outgoing path side and a second waveguide on a returning path side connected to the second waveguide on the outgoing path side, The second electrode of the folded structure is A second electrode on the outgoing path and a second electrode on the return path, The second waveguide on the outgoing path side is It is composed of two outgoing waveguides, The second waveguide on the return path side is The waveguide is configured with two return waveguides connected to the two outgoing waveguides, 2. The optical device according to claim 1, wherein the length of one of said two outgoing waveguides is set to be the same as the length of the other outgoing waveguide.

6. The first waveguide is A first waveguide on an input side and a first waveguide on an output side, The first waveguide on the input side is It consists of two input waveguides, The first waveguide on the output side is It consists of two output waveguides, The second waveguide of the folded structure is a second waveguide on an outgoing path side and a second waveguide on a returning path side connected to the second waveguide on the outgoing path side, The second waveguide on the outgoing path side is It is composed of two outgoing waveguides, The second waveguide on the return path side is The waveguide is configured with two return waveguides connected to the two outgoing waveguides, 2. The optical device according to claim 1, wherein the waveguide length of one of the two input side waveguides is set to be the same as the waveguide length of the other input side waveguide, or the waveguide length of one of the two output side waveguides is set to be the same as the waveguide length of the other output side waveguide.

7. A branching unit that branches the input light; Two first waveguides on an input side connected to the branching portion; two second waveguides on an outgoing path side connected to the two first waveguides on the input side; two second waveguides on a return path side connected to the two second waveguides on the outward path side; a second electrode for applying a second electrical signal to the two second optical waveguides on the return path side; two first waveguides on an output side connected to the two second waveguides on the return path side; a first electrode for applying a first electrical signal to the two first optical waveguides on the output side; a multiplexing section that is connected to the two first optical waveguides on the output side, multiplexes modulated light passing through the light modulated by the two first optical waveguides on the output side in response to the first electrical signal, and outputs the multiplexed light to an output section, The first chip includes: The optical waveguide is configured by a silicon photonics chip, and has the branching section, two first waveguides on the input side, two first waveguides on the output side, the first electrode, and the multiplexing section; The second chip includes:

2. The optical device according to claim 1, comprising a thin-film LN chip, the optical device comprising: two second waveguides on the outgoing path side; two second waveguides on the return path side; and the second electrode.

8. The second waveguide comprises:

2. The optical device according to claim 1, wherein when the electro-optic crystal layer of the second chip is an X-cut thin-film LN chip, the optical waveguide transmits light in the Y-axis direction of the crystal axis of the thin-film LN chip.

9. 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 first chip including a first waveguide and a first electrode; a second chip mounted on the first chip and including a second waveguide and a second electrode, The second waveguide comprises: the second waveguide has a higher electro-optic effect than the first waveguide, and an end of the second waveguide is gathered on an end face of the second chip by a folded structure, and is optically coupled to the first waveguide at the end face; The second electrode is An optical transmitter characterized in that an end of the second electrode is concentrated on the end face by a folded structure, and the first electrode and the second electrode are electrically connected in the concentrated area.

10. A processor that performs signal processing on the electrical signal; A light source that generates light; an optical device that modulates light generated from the light source using an electrical signal output from the processor; an optical receiver that converts received light into an electrical signal using the light generated by the light source, The optical device comprises: a first chip including a first waveguide and a first electrode; a second chip mounted on the first chip and including a second waveguide and a second electrode, The second waveguide comprises: the second waveguide has a higher electro-optic effect than the first waveguide, and an end of the second waveguide is gathered on an end face of the second chip by a folded structure, and is optically coupled to the first waveguide at the end face; The second electrode is An optical transceiver characterized in that an end of the second electrode is concentrated on the end face by a folded structure, and the first electrode and the second electrode are electrically connected in the concentrated area.

Images