Optical device, optical transmitter, and optical transceiver

Abstract

The invention relates to an optical device, an optical transmitter and an optical transceiver. An optical device has 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 higher electro-optic effect than the electro-optic effect of the first waveguide, with a return structure that places an end of the second waveguide at an end face of the second chip. The second waveguide is optically coupled to the first waveguide at the end face. The second electrode has a return structure that places an end portion of the second electrode at the end face, and the second electrode and the first electrode are electrically connected to each other in a region where the end portion of the second electrode has been placed.

Description

Technical Field

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

[0002] For example, a conventional optical modulator has an optical waveguide disposed on a substrate, and a signal electrode and a ground electrode disposed near the optical waveguide. In response to application of a voltage to the signal electrode, an electric field is generated in the optical waveguide, the refractive index of the optical waveguide changes due to the electric field in the optical waveguide, and the phase of light also changes. The optical waveguide forms a Mach-Zehnder interferometer, and the light output changes due to the difference between the light phases in the optical waveguide.

[0003] The light modulator is, for example, a Mach-Zehnder modulator. Fig.34 2 is a plan schematic diagram illustrating an example of a conventional optical modulator 200. For example, the optical modulator 200 includes a thin film lithium niobate (LiNbO3:LN) chip. The optical modulator 200 has an input waveguide 221, a beam splitter 222, a radio frequency (RF) modulation unit 223, a direct current (DC) modulation unit 224, a multiplexer 225, and an output waveguide 226. For example, the input waveguide 221 is an LN waveguide 215 of an input signal light. The beam splitter 222 splits 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 in which the signal light guided through the two optical waveguides 223A is subjected to high-speed modulation.

[0004] The RF modulation unit 223 has two optical waveguides 223A arranged in parallel with each other and a plurality of RF electrodes 223B arranged in parallel with the two optical waveguides 223A. The two optical waveguides 223A are LN waveguides 215. The RF electrode 223B includes two ground electrodes 214B arranged in parallel with each other outside the two optical waveguides 223A and one signal electrode 214A arranged between the two optical waveguides 223A and in parallel with the optical waveguide 223A. In the case where a high-frequency signal having a frequency band of several tens of GHz has been input to the signal electrode 214A, the RF modulation unit 223 enables the signal light guided through the optical waveguide 223A to be subjected to high-speed modulation according to the high-frequency signal.

[0005] Fig.35 yes Fig.34 2 is a cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. The cross section on line AA is a cross section of the RF modulation unit 223 in the optical modulator 200 . Fig.35The RF modulation unit 223 shown in FIG. 2 has: a support substrate 211 of, for example, Si; 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 has a thin film LN waveguide 215 protruding upward. The LN waveguides 215 are each a rib-shaped waveguide having a rib and a plate formed on both sides of the rib. The electrode 214 has a signal electrode 214A and a pair of ground electrodes 214B. The surface on the plate has an electrode 214 having a coplanar waveguide (CPW) structure arranged thereon, that is, a signal electrode 214A and a pair of ground electrodes 214B, wherein the signal electrode 214A is located between the pair of ground electrodes 214B. The electrode 214 in the RF modulation unit 223 is an RF electrode 223B.

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

[0007] The DC modulation unit 224 has two optical waveguides 224A arranged in parallel with each other and a plurality of DC electrodes 224B arranged in parallel with the two optical waveguides 224A. The two optical waveguides 224A are LN waveguides 215. The DC electrode 224B includes: two ground electrodes 214B arranged in parallel with each other outside the two optical waveguides 224A; and one signal electrode 214A arranged between the two optical waveguides 224A and in parallel with the optical waveguide 224A. In the case where a bias voltage has been applied to the signal electrode 214A, the DC modulation unit 224 adjusts the bias according to the on state or off state of the bias voltage to turn on and off the signal light guided through the optical waveguide 224A. The DC modulation unit 224 also has a supporting substrate 211, an intermediate layer 212, a thin film LN substrate 213, and an electrode 214. The electrode 214 in the DC modulation unit 224 is a DC electrode 224B. That is, both the RF modulation unit 223 and the DC modulation unit 224 are formed of thin-film LN chips.

[0008] Patent Document 1: International Publication No. WO 2007 / 058366

[0009] Patent Document 2: U.S. Patent Application Publication No. 2017 / 0285437

[0010] Patent Document 3: Japanese Patent Application Publication No. 2005-221874

[0011] If the bias voltage is continuously applied to the DC electrode 224B, the intensity of the light output by the DC modulation unit 224 of the optical modulator 200 changes over time due to DC drift (the change in the emitted light over time, which is caused by the applied bias voltage). The bias voltage needs to be increased to compensate for this change. However, when the bias voltage reaches the maximum voltage of the control power supply, the control fails.

[0012] This problem due to DC drift can be solved by using a heater electrode instead of the DC electrode 224B in the DC modulation unit 224. When current passes through the heater electrode provided on the optical waveguide 224A via the buffer layer, the heat generated by the heater electrode increases the temperature of the optical waveguide 224A. As a result, the refractive index of the optical waveguide 224A changes due to the thermo-optical effect, thereby being able to adjust the phase of the signal light guided through the optical waveguide 224A.

[0013] However, the DC modulation unit using the heater electrode will be formed by a silicon photonics (SiPh) chip, and the RF modulation unit using the RF electrode will be formed by a thin film LN chip. Therefore, it is necessary to mount the thin film LN chip on the SiPh chip to form an optical modulator.

[0014] Therefore, an aspect of an embodiment of the present invention aims to provide, for example, an optical device including a first chip and a second chip, the first chip including a first electrode, the second chip including a second electrode. Summary of the invention

[0015] According to one aspect of the embodiment, an optical device includes: 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-optical effect higher than that of the first waveguide, has a return structure for placing an end of the second waveguide at an end face of the second chip, and is optically coupled to the first waveguide at the end face. The second electrode has a return structure for placing an end of the second electrode at the end face. The second electrode and the first electrode are electrically connected to each other in a region where the end of the second electrode has been placed. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic plan view illustrating an example of the light modulator according to the first embodiment;

[0017] Figure 2 It is an example Figure 1 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0018] Figure 3 It is an example Figure 1 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0019] Figure 4 is a plan view schematically illustrating an example of an optical modulator according to a second embodiment;

[0020] Figure 5 It is an example Figure 4 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0021] Figure 6 It is an example Figure 4 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0022] Figure 7 is a plan view schematically illustrating an example of an optical modulator according to a third embodiment;

[0023] Figure 8 It is an example Figure 7 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0024] Fig. 9 It is an example Figure 7 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0025] Fig.10 is a schematic plan view illustrating an example of an optical modulator according to a fourth embodiment;

[0026] Fig.11 It is an example Fig.10 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0027] Fig.12 It is an example Fig.10 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0028] Fig.13 is a plan view schematically illustrating an example of an optical modulator according to a fifth embodiment;

[0029] Fig.14 It is an example Fig.13 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0030] Fig.15 It is an example Fig.13 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0031] Fig.16is a plan view schematically illustrating an example of an optical modulator according to a sixth embodiment;

[0032] Fig.17 It is an example Fig.16 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0033] Fig.18 It is an example Fig.16 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0034] Fig.19 is a plan view schematically illustrating an example of an optical modulator according to a seventh embodiment;

[0035] Fig. 20 It is an example Fig.19 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0036] Fig.21 It is an example Fig.19 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0037] Fig. 22 is a plan view schematically illustrating an example of an optical modulator according to an eighth embodiment;

[0038] Fig.23 It is an example Fig. 22 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0039] Fig.24 It is an example Fig. 22 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0040] Fig.25 is a plan view schematically illustrating an example of an optical modulator according to a ninth embodiment;

[0041] Fig.26 It is an example Fig.25 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0042] Fig. 27 It is an example Fig.25 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0043] Fig.28 is a diagram illustrating an example of an optical transceiver according to an embodiment;

[0044] Fig.29 is a plan view schematically illustrating an example of an optical modulator according to a comparative example;

[0045] Fig.30 It is an example Fig.29 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG.

[0046] Fig.31 It is an example Fig.29 A cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG.

[0047] Fig.32 It is an example Fig.29 A cross-sectional schematic diagram of an example of a cross section on line CC shown in FIG.

[0048] Fig.33 It is an example Fig.29 A cross-sectional schematic diagram of an example of a cross section on line DD shown in FIG.

[0049] Fig.34 is a plan view schematically illustrating an example of a conventional light modulator; and

[0050] Fig.35 It is an example Fig.34 Schematic cross-sectional view of an example of a cross section on line AA shown in FIG. DETAILED DESCRIPTION

[0051] (a) Comparative Example

[0052] Fig.29 is a plan schematic diagram illustrating an example of an optical modulator 100 according to a comparative example. Fig.29 The optical modulator 100 shown has: a SiPh chip 101 on which a DC modulation unit 130 including a heater electrode 135 is mounted; and a thin-film LN chip 102 on which an RF modulation unit 140 including an RF electrode 142 is mounted.

[0053] The SiPh chip 101 has a Si substrate 111, an oxide layer 112 on the Si substrate 111, and an opening portion 113 which is an opening provided in a portion of the oxide layer 112. The opening portion 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.

[0054] The first oxide layer 112A has a Si waveguide 121 near the input end, a beam splitter 122, two Si waveguides 123, an electrode 124 near the input end, and a SiN waveguide 125 near the input end. The Si waveguide 121 near the input end is a Si waveguide for inputting signal light. The beam splitter 122 is connected to the Si waveguide 121 near the input end, and is also connected to the two Si waveguides 123 near the input end, and splits the signal light input from the Si waveguide 121 near the input end and outputs it to the two Si waveguides 123 near the input end. The two Si waveguides 123 near the input end are connected to the beam splitter 122, and are also connected to the SiN waveguide 125 near the input end, and the signal light split and output from the beam splitter 122 is output to the input terminals of the two LN waveguides 141 in the thin film LN chip 103. The electrode 124 near the input end is, for example, an Al electrode electrically connected to the RF electrode 142 near the input end in the thin film LN chip 103. The SiN waveguide 125 near the input end is a waveguide connected between the input terminals of the two Si waveguides 123 near the input end and the two LN waveguides 141 in the thin film LN chip 103.

[0055] The second oxide layer 112B has two Si waveguides 131 near the output end and in the DC modulation unit 130, a multiplexer 132, a Si waveguide 133 near the output end, a heater 134 in the DC modulation unit 130, and a heater electrode 135. The second oxide layer 112B has an electrode 136 near the output end and a SiN waveguide 137 near the output end. The SiN waveguide 137 near the output end is connected between the two Si waveguides 131 near the output end and the output terminals of the two LN waveguides 141 in the thin film LN chip 103, and outputs the signal light from the output terminals of the two LN waveguides 141 to the multiplexer 132. The two Si waveguides 131 near the output end are connected to the SiN waveguide 137 near the output end, are also connected to the multiplexer 132, and output the signal light from the SiN waveguide 137 near the output end to the multiplexer 132. The multiplexer 132 is connected to the two Si waveguides 131 near the output end and also connected to the Si waveguide 133 near the output end, multiplexes the signal lights from the two Si waveguides 131 near the output end, and outputs the multiplexed signal lights to the Si waveguide 133 near the output end. The Si waveguide 133 near the output end is a waveguide that outputs the multiplexed signal lights from the multiplexer 132.

[0056] The heater 134 in the DC modulation unit 130 is arranged on the two Si waveguides 131 near the output end. The heater electrode 135 is, for example, an Al electrode electrically connected to the heater 134. In the DC modulation unit 130, the signal light guided through the Si waveguide 131 near the output end is modulated by heating by the heater 134 according to the current from the heater electrode 135. The electrode 136 near the output end is, for example, an Al electrode electrically connected to the RF electrode 142 of the RF modulation unit 140 in the thin film LN chip 103.

[0057] The thin film LN chip 103 has two LN waveguides 141 and an RF electrode 142 arranged in parallel with the two LN waveguides 141, and the two LN waveguides 141 and the RF electrode 142 form an RF modulation unit 140. The RF electrode 142 is, for example, an Au electrode. The RF electrode 142 includes a signal electrode 142A and two ground electrodes 142B, and is electrically connected to the electrode 124 and the electrode 136 in the SiPh chip 101, the electrode 124 is close to the input end, and the electrode 136 is close to the output end.

[0058] Fig.30 It is an example Fig.29 1 is a cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. The cross section on line AA is a portion where the SiPh chip 101 and the thin film LN chip 102 are bonded to each other, which portion is close to the second oxide layer 112B. The thin film LN chip 102 has: a support substrate 151 of, for example, Si; an intermediate layer 152 of, for example, SiO2 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 LN waveguides 141, each of which has a protruding shape and protrudes from a surface of the thin film LN substrate 153 that is not the other surface facing the intermediate layer 152.

[0059] The LN waveguide 141 is a rib-shaped waveguide each having a rib and a plate formed on both sides of the rib. The plate has an RF electrode 142 having a coplanar waveguide (CPW) structure arranged on the surface of the plate, that is, a signal electrode 142A and a pair of ground electrodes 142B, wherein the signal electrode 142A is located between the pair of ground electrodes 142B.

[0060] 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 connected to two Si waveguides 131 near the output end. The SiN waveguide 137 is optically connected to an LN waveguide 141 of the thin-film LN chip 102.

[0061] The electrode 136 near the output end 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 in the electrode 136 near the output end is electrically connected to the signal electrode 142A in the RF electrode 142 via one of the bumps 155 of the connection pad 154. The ground electrode of the electrode 136 near the output end is electrically connected to the ground electrode 142B in the RF electrode 142 via the other bump 155 of the connection pad 154.

[0062] Fig.31 It is an example Fig.29 1 is a cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG. The cross section on line BB is a cross section of the RF modulation unit 140 of the thin film LN chip 102. In a state where the RF modulation unit 140 of the thin film LN chip 102 has been mounted in the opening portion 113 on the Si substrate 111 of the SiPh chip 101, the RF electrode 142 and the Si substrate 111 are separated from each other.

[0063] Fig.32 It is an example Fig.29 Schematic cross-sectional view of an example of a cross section on line CC shown in FIG. At the cross section on line CC, the RF electrode 142 arranged on the thin film LN substrate 153 of the thin film LN chip 102 is electrically connected via the bump 155 to the electrode 124 near the input end and the electrode 136 near the output end on the Si substrate 111 already arranged in the opening portion 113 of the SiPh chip 101.

[0064] The electrode 124 disposed on the Si substrate 111 near the first oxide layer 112A and near the input end is electrically connected to the RF electrode 142 disposed on the thin film LN substrate 153 of the thin film LN chip 102 and near the input end via the bump 155. The electrode 136 disposed on the Si substrate 111 near the second oxide layer 112B and near the output end is electrically connected to the RF electrode 142 disposed on the thin film LN substrate 153 of the thin film LN chip 102 and near the output end via the bump 155.

[0065] Fig.33 It is an example Fig.29 2 is a cross-sectional schematic diagram of an example of a cross section on line DD shown in . For example, the cross section on line DD is located at a position where the Si waveguide 123 near the input end, the LN waveguide 141, and the Si waveguide 131 near the output end are connected to each other.

[0066] The first oxide layer 112A of the SiPh chip 101 has: an electrode 124 near the input end; and two SiN waveguides 125 near the input end, the two SiN waveguides 125 are optically connected to the two Si waveguides 123 near the input end, and optically connected to the input terminals of the two LN waveguides 141 of the thin film LN chip 102.

[0067] The second oxide layer 112B of the SiPh chip 101 has two SiN waveguides 137 near the output end, which are optically connected to the two Si waveguides 131 near the output end, and optically connected to the output terminals of the two LN waveguides 141 of the thin film LN chip 102. The second oxide layer 112B also has a heater 134 arranged on the two Si waveguides 131 near the output end, and a heater electrode 135.

[0068] In this comparative example, a thin film LN chip 102 having an RF electrode 142 is flip-chip bonded face-down by etching to an opening portion 113 dug out in 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 connects a Si waveguide 123 near an input end and a Si waveguide 131 near an output end to each other via a SiN waveguide 125 (137) provided in the SiPh chip 101.

[0069] However, in the optical modulator 100 according to the comparative example, the connection with the Si waveguide 123 near the input end and the connection with the Si waveguide 131 near the output end are respectively at both ends of the thin film LN chip 102. As a result, when the thin film LN chip 102 is flip-chip bonded to the opening portion 113 of the SiPh chip 101, it is necessary to adjust the optical axis between the SiN waveguide and the LN waveguide at both ends of the chip, making the mounting work difficult.

[0070] In addition, the thin film LN chip 102 has a length of about 10 mm in the longitudinal direction of the LN waveguide 141. As a result, in the case where the thin film LN chip 102 has been fixed to the SiPh chip 101 at both ends of the thin film LN chip 102, stress is generated upon temperature change according to the difference between the thermal expansions of the thin film LN chip 102 and the SiPh chip 101, thereby destabilizing the optical output. That is, even in the case where the optical modulator 100 is formed of the thin film LN chip 102 including the RF modulation unit 140 and the SiPh chip 101 including the DC modulation unit 130, an optical modulator capable of preventing destabilization of the optical output due to stress generated upon temperature change is required.

[0071] Preferred embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the embodiments.

[0072] (b) First Embodiment

[0073] Figure 1 is a schematic plan view illustrating an example of the optical modulator 1 according to the first embodiment, Figure 2 It is an example Figure 1 A cross-sectional schematic diagram of an example of a cross section on line AA is shown in FIG. 1 , and Figure 3 It is an example Figure 1 , which is a cross-sectional schematic diagram of an example of a cross section on line BB shown in FIG. Figure 1 The optical modulator 1 shown is a Mach-Zehnder modulator and has a SiPh chip 2 on which a direct current (DC) modulation unit 30 including a heater electrode is mounted, and a thin-film lithium niobate (LiNbO3:LN) chip 3 on which an RF modulation unit 40 including a radio frequency (RF) electrode is mounted.

[0074] The SiPh chip 2 includes: a Si substrate 11; an oxide layer 12 such as SiO2 formed on the Si substrate 11; and an opening portion 13 which is an opening provided in a portion of the oxide layer 12. The oxide layer 12 includes a first oxide layer 12A and a second oxide layer 12B via the opening portion 13. The first chip is, for example, the SiPh chip 2.

[0075] The first oxide layer 12A has a Si waveguide 21 near the input end, a beam splitter 22, two Si waveguides 23 near the input end, two Si waveguides 24 near the output end, a multiplexer 25, and a Si waveguide 26 near the output end. The Si waveguide 21 near the input end is a waveguide for input signal light. The beam splitter 22 is connected to the Si waveguide 21 near the input end, connected to the two Si waveguides 23 near the input end, and splits the signal light input from the Si waveguide 21 near the input end and outputs it to the two Si waveguides 23 near the input end. The two Si waveguides 23 near the input end are connected to the beam splitter 22, and connected to the SiN waveguide 32 described later. The two Si waveguides 23 near the input end are connected to the input terminals of the two LN waveguides 41 near the input end in the thin film LN chip 3, the input terminals are connected to the SiN waveguide 32, and the two Si waveguides 23 near the input end output the signal light split and output from the beam splitter 22 to the two LN waveguides 41 near the input end in the thin film LN chip 3.

[0076] The two Si waveguides 24 near the output end are connected to the SiN waveguide 32 connected to the output terminals of the two LN waveguides 42 near the output end in the thin film LN chip 3, connected to the multiplexer 25, and output the signal light from the two LN waveguides 42 near the output end to the multiplexer 25. The multiplexer 25 is connected to the two Si waveguides 24 near the output end, multiplexes the signal light from the two Si waveguides 24 near the output end, and outputs the multiplexed signal light to the Si waveguide 26 near the output end. The Si waveguide 26 near the output end is a waveguide that outputs the signal light that has been multiplexed and originated from the multiplexer 25. For example, the first waveguide is the Si waveguide 23 near the input end and the Si waveguide 24 near the output end.

[0077] The DC modulation unit 30 has two Si waveguides 24 near the output end, a heater 29 arranged on the two Si waveguides 24 near the output end, and a heater electrode 31 electrically connected to the heater 29. In the DC modulation unit 30, the signal light guided through the Si waveguide 24 near the output end is modulated in the following manner: the heater 29 is heated according to the current to the heater electrode 31; thereby heating the Si waveguide 24 near the output end. For example, the first electrode is the heater electrode 31.

[0078] The thin film LN chip 3 has: a support substrate 51 of, for example, Si; an intermediate layer 52 of, for example, SiO2 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 has LN waveguides 41 and 42 and an LN waveguide 43 (each of which has a protruding shape) protruding and located on a surface of the thin film LN substrate 53, which is not the other surface facing the intermediate layer 52. The LN waveguides 41, 42, and 43 are rib-shaped waveguides each having a rib and a plate formed on both sides of the rib. RF electrodes 44, 45, and 46 having a coplanar waveguide (CPW) structure are arranged on the surface of the plate. The RF electrodes 44, 45, and 46 include a signal electrode and a pair of ground electrodes, wherein the signal electrode is located between the pair of ground electrodes. For example, the second chip is a thin film LN chip 3 having an electro-optical effect higher than that of the first waveguide.

[0079] The thin film LN chip 3 has: LN waveguides 41, 42 and 43 having a return structure and RF electrodes 44, 45 and 46 having a return structure. In the thin film LN chip 3, the ends of both the LN waveguides 41 and 42 are located on the first end surface 3A of the thin film LN chip 3, and the ends of both the RF electrodes 44 and 45 are also located on the first end surface 3A of the thin film LN chip 3.

[0080] The LN waveguide includes an LN waveguide 41 on the forward path, an LN waveguide 42 on the backward path, and an LN waveguide 43 connected between the LN waveguide 41 on the forward path and the LN waveguide 42 on the backward path, and the LN waveguide 43 is U-shaped and returns. For the LN waveguide, the input terminal of the LN waveguide 41 on the forward path and the output terminal of the LN waveguide 42 on the backward path are both located on the first end face 3A of the thin film LN chip 3. The first end face 3A is an end face of the thin film LN chip 3, which is close to the first oxide layer 12A of the SiPh chip 2. For example, the second waveguide is an LN waveguide such as the LN waveguide 41 on the forward path and the LN waveguide 42 on the backward path. For example, the second waveguide on the forward path is the LN waveguide 41 on the forward path. For example, the second waveguide on the backward path is the LN waveguide 42 on the backward path.

[0081] The RF electrode includes an RF electrode 44 on the forward path, an RF electrode 45 on the backward path, and an RF electrode 46 electrically connected between the RF electrode 44 on the forward path and the RF electrode 45 on the backward path, and the RF electrode 46 is U-shaped and returns. For the RF electrode, both the input terminal of the RF electrode 44 on the forward path and the output terminal of the RF electrode 45 on the backward path are located on the first end surface 3A of the thin film LN chip 3. For example, the second electrode is an RF electrode including the RF electrode 44 on the forward path and the RF electrode 45 on the backward path. For example, the second electrode on the forward path is the RF electrode 44 on the forward path. For example, the second electrode on the backward path is the RF electrode 45 on the backward path.

[0082] The RF electrode 44 on the forward path includes a signal electrode 44A and a pair of ground electrodes 44B arranged in parallel with the signal electrode 44A, wherein the signal electrode 44A is located between the pair of ground electrodes 44B. The RF electrode 45 on the backward path includes a signal electrode 45A and a pair of ground electrodes 45B arranged in parallel with the signal electrode 45A, wherein the signal electrode 45A is located between the pair of ground electrodes 45B. The returning RF electrode 46 includes a signal electrode 46A and a pair of ground electrodes 46B arranged in parallel with the signal electrode 46A, wherein the signal electrode 46A is located between the pair of ground electrodes 46B.

[0083] In the case where the LN waveguide and the RF electrode have a return structure, since an X-cut LN crystal is used for the thin-film LN chip 3, the return of the RF electrode causes the modulation polarity to be reversed from the LN waveguide 41 on the forward path to the LN waveguide 42 on the backward path. As a result, after the return LN waveguide 43 returns, the modulation before the return LN waveguide 43 returns is cancelled. Figure 1In order to determine the electric field of the electric signal applied to the LN waveguide in the case of traveling leftward in the backward direction, for example, the thin-film LN chip 3 is configured so that only the LN waveguide 42 on the backward path is affected by the electric field from the RF electrode 45 on the backward path. That is, the RF electrode 45 on the backward path is arranged to apply the electric field from the RF electrode 45 on the backward path to the LN waveguide 42 on the backward path.

[0084] The LN waveguide 41 on the forward path is arranged in parallel with the RF electrode 44 on the forward path, but all of the RF electrodes 44 on the forward path are arranged on the inner peripheral side of the LN waveguide 41 on the forward path. As a result, the influence of the electric field applied from the RF electrode 44 on the forward path to the LN waveguide 41 on the forward path can be reduced. That is, the RF electrode 44 on the forward path is arranged so as not to apply the electric field from the RF electrode 44 on the forward path to the LN waveguide 41 on the forward path.

[0085] The return LN waveguide 43 is arranged close to the return RF electrode 46, but all the return RF electrodes 46 are arranged on the inner peripheral side of the return LN waveguide 43. As a result, the influence of the electric field applied from the return RF electrode 46 to the return LN waveguide 43 can be reduced.

[0086] The LN waveguides 42 on the backward path are arranged in parallel with the RF electrodes 45 on the backward path, and the signal electrode 45A is arranged between the LN waveguides 42 on the backward path. In addition, one ground electrode 45B is arranged for the inner LN waveguide 42 among the LN waveguides 42 on the backward path, and another ground electrode 45B is arranged for the outer LN waveguide 42 among the LN waveguides 42 on the backward path. That is, the LN waveguide 42 on the backward path is configured to modulate the signal light guided therethrough according to the electric field from the RF electrode 45 on the backward path.

[0087] The RF modulation unit 40 has: two LN waveguides 42 on the backward path arranged in parallel with each other; and an RF electrode 45 on the backward path arranged in parallel with the two LN waveguides 42 on the backward path. The RF electrode 45 on the backward path includes: two ground electrodes 45B arranged outside the two LN waveguides 42 on the backward path; and one signal electrode 45A arranged in parallel with the LN waveguides 42 on the backward path and located between the two LN waveguides 42 on the backward path. In the case where a high-frequency signal having a bandwidth of several tens of GHz has been input to the signal electrode 45A, for example, the signal light guided through the two LN waveguides 42 on the backward path is subjected to high-speed modulation in the RF modulation unit 40 according to the high-frequency signal.

[0088] The two Si waveguides 23 near the input end in the first oxide layer 12A are connected to the two LN waveguides 41 on the forward path in the thin film LN chip 3 via the two SiN waveguides 32 near the input end. The two Si waveguides 24 near the output end in the first oxide layer 12A are connected to the two LN waveguides 42 on the backward path in the thin film LN chip 3 via the SiN waveguides 32 near the output end. That is, the LN waveguide in the thin film LN chip 3 has a return structure in which the optical connection of the input terminal of the two Si waveguides 23 near the input end connected to the LN waveguide 41 on the forward path and the optical connection of the output terminal of the two Si waveguides 24 near the output end connected to the LN waveguide 42 on the backward path are both located on the first end face 3A of the thin film LN chip 3. As a result, it is possible to simplify the work required to adjust the optical axis for the tilt of the element when, for example, the thin film LN chip 3 is flip-chip bonded to the opening portion 13 of the SiPh chip 2.

[0089] For example, the electrode 27 near the input end in the first oxide layer 12A is an Al electrode electrically connected to the RF electrode 44 on the forward path in the thin film LN chip 3 via the metal melting bump 55 of the connection pad 54. For example, the electrode 28 near the output end in the first oxide layer 12A is an Al electrode electrically connected to the RF electrode 45 on the backward path in the thin film LN chip 3 via the bump 55 of the connection pad 54. That is, the RF electrode in the thin film LN chip 3 has a return structure, in which the joint where the electrode 27 near the input end is electrically connected to the RF electrode on the forward path and the joint where the electrode 28 near the output end is electrically connected to the RF electrode 45 on the backward path are both located on the first end surface 3A of the thin film LN chip 3.

[0090] The optical modulator 1 according to the first embodiment has an optical joint in which the Si waveguide 23 near the input end is connected to the LN waveguide 42 on the forward path, and an optical joint in which the Si waveguide 24 near the output end is connected to the LN waveguide 42 on the backward path, which are located on the first end face 3A of the thin film LN chip 3. As a result, adjustment of the optical axis between the SiN waveguide and the LN waveguide when flip-chip bonding the thin film LN chip 3 to the opening portion 13 of the SiPh chip 2 can be performed entirely on the first end face 3A, so that the mounting work can be simplified.

[0091] Furthermore, the optical modulator 1 has a joint where the electrode 27 near the input end is connected to the RF electrode 44 on the forward path and a joint where the electrode 28 near the output end is connected to the RF electrode 45 on the backward path, which are located on the first end surface 3A of the thin film LN chip 3. As a result, the joint at the first end surface 3A is fixed at one end of the thin film LN chip 3, thereby reducing the influence of stress at the time of temperature change caused by the difference between the thermal expansion coefficients of the thin film LN chip 3 and the SiPh chip 2, thereby being able to improve stability at the time of operation.

[0092] In the thin film LN chip 3, the signal light guided through the LN waveguide 42 on the backward path among the LN waveguide and the RF electrode having the return structure is subjected to high-speed modulation according to the electric field from the RF electrode 45 on the backward path. As a result, even in the case of the LN waveguide using the X-cut LN crystal, it is possible to prevent the modulation from being canceled at the return.

[0093] For convenience of description, in the case described above as an example, the signal light guided through the LN waveguide 42 on the backward path is subjected to high-speed modulation in the RF modulation unit 40 according to the electric field from the RF electrode 45 on the backward path. However, the RF electrode 44 on the forward path instead of the RF electrode 45 on the backward path may be arranged so that the electric field is applied to the LN waveguide 41 on the forward path. In this case, the signal light guided through the LN waveguide 41 on the forward path can be subjected to high-speed modulation in the RF modulation unit 40 according to the electric field from the RF electrode 44 on the forward path.

[0094] In the case of describing the optical modulator 1 according to the first embodiment as an example, the Si waveguide in the SiPh chip 2 and the LN waveguide in the thin film LN chip 3 are optically connected to each other through the SiN waveguide 32. However, the conversion of optical power is caused by the thermal insulation conversion between the SiN waveguide 32 and the LN waveguide. The thermal insulation conversion has a high tolerance for the misalignment between the chips in the light propagation direction, but the conversion efficiency may be degraded due to the misalignment between the chips in the direction perpendicular to the light propagation direction (for example, the difference in the design value of the interval between the SiN waveguide 32 and the LN waveguide). An embodiment related to the optical modulator 1A capable of solving such degradation will be described below as a second embodiment.

[0095] (c) Second Embodiment

[0096] Figure 4 is a schematic plan view illustrating an example of an optical modulator 1A according to the second embodiment, Figure 5 It is an example Figure 4 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Figure 6 It is an example Figure 4. By assigning the same reference numerals to the same components as those of the optical modulator 1 according to the first embodiment, descriptions of the same components and operations thereof will be omitted. The optical modulator 1A according to the second embodiment is different from the optical modulator 1 according to the first embodiment in that, in the optical modulator 1A, a butt connection X optically connects the Si waveguide of the SiPh chip 2A and the LN waveguide of the thin film LN chip 3 to each other instead of the SiN waveguide 32.

[0097] The two Si waveguides 23 near the input end in the SiPh chip 2A are X-ray connected to the two LN waveguides 41 on the forward path of the thin film LN chip 3 via butt-joint X-ray. In addition, the two Si waveguides 24 near the output end of the SiPh chip 2A are X-ray connected to the two LN waveguides 42 on the backward path of the thin film LN chip 3 via butt-joint X-ray.

[0098] In the optical modulator 1A according to the second embodiment, the Si waveguide in the SiPh chip 2 and the LN waveguide in the thin-film LN chip 3 are optically coupled to each other by butt coupling X. As a result, in the optical modulator 1A, it is possible to improve the tolerance for misalignment between chips in a direction perpendicular to the light propagation direction, not to mention the tolerance for misalignment between chips in the light propagation direction.

[0099] In the case of describing the optical modulator 1A according to the second embodiment as an example, a part of the oxide layer 12 of the SiPh chip 2A is etched to form the opening portion 13, and the thin film LN chip 3 is mounted in the opening portion 13. However, since the surface of the Si substrate 11 is exposed at the opening portion 13 of the SiPh chip 2A, the electric signal from the RF electrode arranged on the thin film LN substrate 53 of the thin film LN chip 3 is absorbed by the Si substrate 11, thereby increasing the attenuation of the electric signal. Therefore, an embodiment related to the optical modulator 1B capable of solving such attenuation will be described below as a third embodiment.

[0100] (d) Third Embodiment

[0101] Figure 7 is a plan view schematically illustrating an example of an optical modulator 1B according to the third embodiment, Figure 8 It is an example Figure 7 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig. 9 It is an example Figure 7. A cross-sectional schematic diagram of an example of a cross section on line BB shown in . By assigning the same reference numerals to the same components as those of the optical modulator 1A according to the second embodiment, description of the same components and their operations will be omitted. The optical modulator 1B according to the third embodiment is different from the optical modulator 1A according to the second embodiment in that a recessed portion 13B dug out from and opened on the surface of the Si substrate 11 has been formed, the surface being exposed from the opening portion 13 of the SiPh chip 2B, the opening portion 13 being a position where the thin film LN chip 3 is mounted.

[0102] The end face of the recessed portion 13B formed in the Si substrate 11 is constructed to be coplanar with the end face of the first oxide layer 12A, and the end face of the first oxide layer 12A is where the output terminal of the Si waveguide 23 near the input end in the SiPh chip 2B and the input terminal of the Si waveguide 24 near the output end in the SiPh chip 2B are located.

[0103] The optical modulator 1B according to the third embodiment has the recessed portion 13B formed on the surface of the Si substrate 11, which is exposed from the opening portion 13 of the SiPh chip 2A. As a result, absorption of the electric signal by the Si substrate can be reduced, thereby reducing attenuation of the electric signal flowing to the RF electrode arranged in the thin film LN substrate 53 of the thin film LN chip 3 mounted in the opening portion 13. Therefore, degradation of RF characteristics can be reduced.

[0104] In the optical modulator 1B, in order to minimize the increase in connection loss due to chip misalignment at the joint where the Si waveguide in the SiPh chip 2B and the LN waveguide in the thin film LN chip 3 are optically connected to each other by butt connection X, the constraint in the optical waveguide is weakened to increase the optical mode field. However, at the optical joint, the radiation of light to the Si substrate 11 is large, and the optical loss increases. Therefore, an embodiment related to the optical modulator 1C capable of solving this increase in optical loss will be described below as a fourth embodiment.

[0105] (e) Fourth Embodiment

[0106] Fig.10 is a schematic plan view illustrating an example of an optical modulator 1C according to the fourth embodiment, Fig.11 It is an example Fig.10 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig.12 Illustrated Fig.100 is a cross-sectional schematic diagram of an example of a cross section on line BB shown in . By assigning the same reference numerals to the same components as those of the optical modulator 1B according to the third embodiment, description of the same components and their operations will be omitted. The optical modulator 1C according to the fourth embodiment is different from the optical modulator 1B in that, in addition to the recessed portion 13B formed on the surface of the Si substrate 11, a recessed portion 13C is formed in the optical modulator 1C, which is additionally dug out from a portion of the surface of the Si substrate 11, which portion is located below the optical joint between the Si waveguide and the LN waveguide of the first oxide layer 12A.

[0107] A recessed portion 13C has been formed in the opening portion 13 by additionally digging from a portion of the surface of the Si substrate 11, which portion is close to the optical joint where the Si waveguide 23 near the input end and the LN waveguide 41 on the forward path are connected to each other by butt connection, that is, close to the output terminal of the Si waveguide 23 near the input end in the first oxide layer 12A.

[0108] A recessed portion 13C has been formed in the opening portion 13 by additionally digging from a portion of the surface of the Si substrate 11, which portion is close to the optical joint where the Si waveguide 24 near the output end and the LN waveguide 42 on the backward path are connected to each other by butt connection, that is, close to the input terminal of the Si waveguide 24 near the output end in the first oxide layer 12A.

[0109] In the optical modulator 1C according to the fourth embodiment, in addition to the recessed portion 13B, a recessed portion 13C has been formed in the opening portion 13 by additionally digging from a portion of the surface of the Si substrate 11, the portion being located below the optical joint between the Si waveguide and the LN waveguide in the first oxide layer 12A. As a result, radiation of light to the Si substrate 11 at the optical joint can be reduced, so that light loss at the optical joint can be reduced.

[0110] In the case of describing the optical modulator 1C according to the fourth embodiment as an example, the Si waveguide in the SiPh chip 2C and the LN waveguide in the thin film LN chip 3 are optically connected to each other by the butt joint X. However, if air enters between the Si waveguide and the LN waveguide at the butt joint X, the reflection and loss of the guided signal light increase at the optical joint. Therefore, an embodiment related to the optical modulator 1D capable of solving such reflection and loss increase will be described below as a fifth embodiment.

[0111] (f) Fifth Embodiment

[0112] Fig.13 is a schematic plan view illustrating an example of an optical modulator 1D according to a fifth embodiment, Fig.14It is an example Fig.13 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig.15 It is an example Fig.13 . By assigning the same reference numerals to the same components as those of the optical modulator 1C of the fourth embodiment, descriptions of the same components and their operations will be omitted. The optical modulator 1D according to the fifth embodiment is different from the optical modulator 1C according to the fourth embodiment in that the first oxide layer 12A in the SiPh chip 2C and the first end face 3A of the thin film LN chip 3 have been bonded to each other by the optical adhesive 61 in the optical modulator 1D.

[0113] In the optical modulator 1D according to the fifth embodiment, the first oxide layer 12A in the SiPh chip 2C and the first end face 3A of the thin film LN chip 3 have been bonded to each other by the optical adhesive 61. Therefore, air can be prevented from entering the optical connector in which the Si waveguide and the LN waveguide are optically coupled to each other by the butt coupling X, so that radiation and loss of light guided through the optical connector can be reduced.

[0114] However, since the SiPh chip 2C and the thin film LN chip 3 are bonded to each other at the first end surface 3A (i.e., at one of the end surfaces of the chips) by the adhesive 61, the optical modulator 1D may be affected by shock or vibration on the SiPh chip 2C and the thin film LN chip 3. Therefore, an embodiment related to the optical modulator 1E capable of coping with such influence will be described below as a sixth embodiment.

[0115] (g) Sixth Embodiment

[0116] Fig.16 is a plan view schematically illustrating an example of an optical modulator 1E according to a sixth embodiment, Fig.17 It is an example Fig.16 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig.18 It is an example Fig.17 . A cross-sectional schematic diagram of an example of a cross section on line BB shown in . By assigning the same reference numerals to the same components as those of the optical modulator 1D according to the fifth embodiment, description of the same components and their operations will be omitted. The optical modulator 1E according to the sixth embodiment is different from the optical modulator 1D according to the fifth embodiment in that: in the optical modulator 1E, in addition to the bonding between the first oxide layer 12A and the first end face 3A of the thin film LN chip 3, the second oxide layer 12B and the second end face 3B of the thin film LN chip 3 have also been bonded to each other by an adhesive 62.

[0117] As the optical adhesive 62 for bonding between the second oxide layer 12B and the second end surface 3B of the thin film LN chip 3, an adhesive softer than the adhesive 61 for bonding between the first oxide layer 12A and the first end surface 3A of the thin film LN chip 3 is used. The hardness of the adhesive 62 after curing is lower than the hardness of the adhesive near the bump joint.

[0118] In the optical modulator 1E according to the sixth embodiment, the first oxide layer 12A and the first end face 3A of the thin film LN chip 3 have been bonded to each other by the optical adhesive 61, and the second oxide layer 12B and the second end face 3B of the thin film LN chip 3 have been bonded to each other by the adhesive 62. In addition, a material softer than the adhesive 61 is used as the adhesive 62. As a result, the bonding of the SiPh chip 2C and the thin film LN chip 3 at both ends can reduce the influence of shock or vibration on the SiPh chip 2C and the thin film LN chip 3. More importantly, since a softer material is used as the adhesive 62 used at one end, the stress on the thin film LN chip 3 can be reduced, so that the operation of the optical modulator 1E can be stabilized.

[0119] If the stress on the thin film LN chip 3 can be reduced thereby, the same adhesive can be used as the optical adhesive 61 for bonding between the first oxide layer 12A and the first end face 3A and the adhesive 62 for bonding between the second oxide layer 12B and the second end face 3B.

[0120] In the case of describing the optical modulator 1E according to the sixth embodiment as an example, the SiPh chip 2 is bonded to the first end face 3A and the second end face 3B opposite to the first end face 3A of the thin film LN chip 3. However, this example can be appropriately modified without being limited to the second end face 3B, and an end face orthogonal to the first end face 3A of the thin film LN chip 3 can be bonded to the SiPh chip 2 instead of the second end face 3B.

[0121] Furthermore, in the example described with the optical modulator 1E according to the sixth embodiment, three connection pads 54 are connected to the electrode 27 near the input end and the RF electrode 44 on the forward path, and three connection pads 54 electrically connect the electrode 28 near the output end and the RF electrode 45 on the backward path to each other. However, the number of connection pads 54 may be reduced, and such an embodiment will be described hereinafter as the seventh embodiment.

[0122] (h) Seventh Embodiment

[0123] Fig.19 is a plan view schematically illustrating an example of an optical modulator 1F according to a seventh embodiment, Fig. 20 It is an example Fig.19 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig.21 It is an example Fig.19 1 is a cross-sectional schematic diagram of an example of a cross section on line BB shown in . The same reference numerals are assigned to the same components as those of the optical modulator 1E according to the sixth embodiment, and description of the same components and operations thereof will be omitted. The optical modulator 1F according to the seventh embodiment is different from the optical modulator 1E according to the sixth embodiment in that the ground electrode 27B1 in the electrode 27 near the input end and the ground electrode 28B1 in the electrode 28 near the output end have been connected to the common pad 54A instead of the connection pad 54.

[0124] The common pad 54A is electrically connected between the ground electrode 27B1 in the electrode 27 near the input end and the ground electrode 44B1 in the RF electrode 44 on the forward path. The common pad 54A is also electrically connected between the ground electrode 28B1 in the electrode 28 near the output end and the ground electrode 45B1 in the RF electrode 45 on the backward path. The ground electrode 46B1 in the return RF electrode 46 is electrically connected between the ground electrode 44B1 in the RF electrode 44 on the forward path and the ground electrode 45B1 in the RF electrode 45 on the backward path.

[0125] The optical modulator 1F has a connection pad 54 that connects the ground electrode 27B among the electrodes 27 near the input end and the ground electrode 44B among the RF electrodes 44 on the forward path, and a connection pad 54 that connects the signal electrode 27A among the electrodes 27 near the input end and the signal electrode 44A among the RF electrodes 44 on the forward path. The optical modulator 1F also has a connection pad 54 that connects the ground electrode 28B among the electrodes 28 near the output end and the ground electrode 45B among the RF electrodes 45 on the backward path, and that connects the signal electrode 28A among the electrodes 28 near the output end and the signal electrode 45A among the RF electrodes 45 on the backward path. The optical modulator 1F also has a common pad 54A, which connects the ground electrode 27B1 in the electrode 27 near the input end and the ground electrode 44B1 in the RF electrode 44 on the forward path, and connects the ground electrode 28B1 in the electrode 28 near the output end and the ground electrode 45B1 in the RF electrode 45 on the backward path.

[0126] In the optical modulator 1F according to the seventh embodiment, the ground electrode 27B1 of the electrode 27 near the input end and the ground electrode 28B1 of the electrode 28 near the output end have been connected to the common pad 54A. As a result, the number of connection pads for electrical connection between the SiPh chip 2C and the thin film LN chip 3 can be reduced to five, the stress on the bump joint can be reduced, and the chip can be reduced in size.

[0127] Since two Si waveguides and two LN waveguides included in the Mach-Zehnder interferometer are folded back in the optical modulator 1F according to the seventh embodiment, the inner waveguide and the outer waveguide have different waveguide lengths, thereby affecting the temperature characteristics and wavelength characteristics of the optical modulator 1F. Therefore, an embodiment that solves this influence will be described below as an eighth embodiment.

[0128] (i) Eighth Embodiment

[0129] Fig. 22 is a plan view schematically illustrating an example of an optical modulator 1G according to an eighth embodiment, Fig.23 It is an example Fig. 22 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig.24 It is an example Fig. 22 1 is a cross-sectional schematic diagram of an example of a cross section on line BB shown in . By assigning the same reference numerals to the same components as those of the optical modulator 1F according to the seventh embodiment, description of the same components and operations thereof will be omitted. The optical modulator 1G according to the eighth embodiment is different from the optical modulator 1F according to the seventh embodiment in that, in the optical modulator 1G, the inner LN waveguide 41 on the forward path of the two LN waveguides 41 on the forward path is replaced by a curved waveguide 41A having a waveguide length adjusted according to the waveguide length of the inner LN waveguide 41 on the forward path. The curved waveguide 41A makes the waveguide length of the inner LN waveguide having a return structure the same as the waveguide length of the outer LN waveguide having a return structure.

[0130] The inner LN waveguide included in the Mach-Zehnder interferometer is an LN waveguide including an inner LN waveguide 41 on a forward path, an inner LN waveguide 42 on a backward path, and an inner return LN waveguide 43. The outer LN waveguide included in the Mach-Zehnder interferometer is an LN waveguide including an outer LN waveguide 41 on a forward path, an outer LN waveguide 42 on a backward path, and an outer return LN waveguide 43.

[0131] The inner LN waveguide 41 on the forward path has been replaced by the curved waveguide 41A so that the inner LN waveguide and the outer LN waveguide in the thin film LN chip 3 have the same waveguide length. That is, the curved waveguide 41A has made the waveguide length of one of the two LN waveguides 41 on the forward path longer or shorter than the waveguide length of the other of the two LN waveguides 41 on the forward path. As a result, the waveguide length of the inner LN waveguide and the waveguide length of the outer LN waveguide have become the same.

[0132] In the optical modulator 1G according to the eighth embodiment, the inner LN waveguide 41 on the forward path in the thin film LN chip 3 has been replaced by the bent waveguide 41A so that the inner LN waveguide and the outer LN waveguide have the same waveguide length. As a result, the waveguide length of the inner LN waveguide included in the Mach-Zehnder interferometer and the waveguide length of the outer LN waveguide included in the Mach-Zehnder interferometer become the same, and the influence on the temperature characteristics and wavelength characteristics of the optical modulator 1G can be reduced.

[0133] In the case described as an example with respect to the optical modulator 1G according to the eighth embodiment, the waveguide length of the LN waveguide 41 on the forward path is adjusted, but this example can be modified appropriately, and the waveguide length of the return LN waveguide 43 may be adjusted instead of the waveguide length of the LN waveguide 41 on the forward path.

[0134] In addition, in the case of describing the optical modulator 1G as an example, the waveguide length of the LN waveguide 41 on the forward path is adjusted so that the waveguide length of the inner LN waveguide and the waveguide length of the outer LN waveguide are the same. However, an embodiment not limited to this example will be described below as a ninth embodiment.

[0135] (j) Ninth Embodiment

[0136] Fig.25 is a plan view schematically illustrating an example of an optical modulator 1H according to a ninth embodiment, Fig.26 Illustrated Fig.25 A cross-sectional schematic diagram of an example of a cross section on line AA shown in FIG. 1 , and Fig. 27 Illustrated Fig.25 A cross-sectional schematic diagram of an example of a cross section on line BB shown in . By assigning the same figure numbers to the same components as those of the optical modulator 1E according to the sixth embodiment, the description of the same components and their operations will be omitted. The optical modulator 1H according to the ninth embodiment is different from the optical modulator 1E according to the sixth embodiment in that: in the optical modulator 1H, the inner Si waveguide 23 near the input end of the two Si waveguides 23 near the input end has been replaced by a bent waveguide 23A, and the bent waveguide 23A has a waveguide length adjusted according to the waveguide length of the inner Si waveguide 23 near the input end. The bent waveguide 23A makes the inner waveguide and the outer waveguide of the return structure have the same waveguide length.

[0137] The inner waveguide included in the Mach-Zehnder interferometer is a waveguide including the inner Si waveguide 23 near the input end, the inner LN waveguide 41 on the forward path, the inner LN waveguide 42 on the backward path, the inner return LN waveguide 43, and the inner Si waveguide 24 near the output end. The outer waveguide included in the Mach-Zehnder interferometer is a waveguide including the outer Si waveguide 23 near the input end, the outer LN waveguide 41 on the forward path, the outer LN waveguide 42 on the backward path, the outer return LN waveguide 43, and the outer Si waveguide 24 near the output end.

[0138] The inner Si waveguide 23 near the input end has been replaced by a curved waveguide 23A so that the inner waveguide and the outer waveguide have the same waveguide length. The curved waveguide 23A has made one of the two Si waveguides 23 near the input end have a longer or shorter waveguide length than the other of the two Si waveguides 23 near the input end. As a result, the waveguide length of the inner LN waveguide and the waveguide length of the outer LN waveguide have become the same. The Si waveguide of the SiPh chip 2C can be formed as a curved waveguide with a smaller bending radius than the LN waveguide of the thin film LN chip 3.

[0139] In the optical modulator 1H according to the ninth embodiment, the inner Si waveguide 23 near the input end has been replaced by the bent waveguide 23A so that the inner waveguide and the outer waveguide have the same waveguide length. Therefore, the waveguide length of the inner waveguide included in the Mach-Zehnder interferometer and the waveguide length of the outer waveguide included in the Mach-Zehnder interferometer become the same, and the influence on the temperature characteristics and wavelength characteristics of the optical modulator 1H can be reduced.

[0140] While the optical modulator 1G according to the ninth embodiment is described as an example, the waveguide length of the inner Si waveguide 23 near the input end is adjusted, this example can be modified appropriately and the waveguide length of the inner Si waveguide 24 near the output end can be adjusted instead.

[0141] The optical modulator 1 has been described as an example of the optical device according to the present invention, but the optical device is not limited to the optical modulator 1 and may be any of various optical devices including an optical receiver.

[0142] Fig.28 is a schematic diagram illustrating an example of an optical transceiver 70 according to an embodiment. Fig.28The optical transceiver 70 shown in FIG. 1 is connected to an optical fiber near the output end and an optical fiber near the input end. The optical transceiver 70 has a light source 71, a digital signal processor (DSP) 72, and an optical transmitter-receiver 73. The optical transmitter-receiver 73 has an optical transmitter 73A and an optical receiver 73B. The DSP 72 is an electronic component that performs digital signal processing. For example, the DSP 72 performs processing such as encoding the transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the optical transmitter 73A. In addition, the DSP 72 obtains the electrical signal including the received data from the optical receiver 73B, performs processing such as decoding the obtained electrical signal, and obtains the received data.

[0143] For example, the light source 71 is an LD including a laser diode, generates light having a predetermined wavelength, and supplies the light 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 the electrical signal output from the DSP 72 and outputs the modulated signal light to the optical fiber. For example, the optical transmitter 73A has a built-in optical device such as an optical modulator including a SiPh chip 2 and a thin film LN chip 3. When the light supplied from the light source 71 is guided through a waveguide in the optical transmitter 73A, the signal light is generated by modulating the light by means of the electrical signal.

[0144] The optical receiver 73B receives reception light from the optical fiber, converts the reception light into an electric signal by using the light supplied from the light source 71 , and outputs the converted electric signal to the DSP 72 .

[0145] In the case described above as an example, the optical transceiver 70 has a built-in optical transmitter 73A and an optical receiver 73B, but the present invention is applicable to an optical transmitter having only a built-in optical transmitter 73A, which has a built-in optical device. In addition, the present invention is also applicable to an optical receiver having only a built-in optical receiver 73B, which has a built-in optical device. The optical device according to the present invention is also applicable to the optical transmitter-receiver 73, not limited to the optical transceiver 70.

[0146] With respect to the embodiments, the thin film LN chip 3 has been described as an example, but is not limited to this example, the example can be appropriately modified, and for example, TF-barium titanate can be used. Examples of materials having an electro-optical effect include BaTiO3 (TF-BTO), PbLaZrTiO3 (TF-PLZT), and PbZrTiO3 (TF-PZT), and the material can be appropriately modified.

[0147] According to an embodiment related to an optical device disclosed in the present application, for example, there can be provided an optical device including: a first chip including a first electrode; and a second chip including a second electrode.

Claims

1. An optical device, comprising: a first chip, the first chip comprising a first waveguide and a first electrode; as well as a second chip mounted on the first chip and comprising a second waveguide and a second electrode, in, The second waveguide has an electro-optic effect higher than that of the first waveguide, has a return structure in which an end of the second waveguide is placed at an end surface of the second chip, and is optically coupled to the first waveguide at the end surface, and The second electrode has a return structure in which an end portion of the second electrode is placed at the end surface, and the second electrode and the first electrode are electrically connected to each other in a region in which the end portion of the second electrode has been placed.

2. The optical device according to claim 1, wherein: The second waveguide having the return structure has a second waveguide on a forward path and a second waveguide on a backward path connected to the second waveguide on the forward path, The second electrode having the return structure has a second electrode on a forward path and a second electrode on a backward path, The second electrode on the backward path is arranged to apply an electric field from the second electrode on the backward path to the second waveguide on the backward path, and The second electrode on the forward path is arranged not to apply an electric field from the second electrode on the forward path to the second waveguide on the forward path.

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

4. The optical device according to claim 3, wherein: The first chip is bonded to the other end surface of the second chip using another adhesive softer than the adhesive, the other end surface being an end surface other than the end surface.

5. The optical device according to claim 1, wherein: The second waveguide having the return structure has a second waveguide on a forward path and a second waveguide on a backward path connected to the second waveguide on the forward path, The second electrode having the return structure has a second electrode on a forward path and a second electrode on a backward path, the second waveguide on the forward path comprises two waveguides on the forward path, The second waveguide on the backward path includes two waveguides on the backward path, the two waveguides on the backward path are connected to the two waveguides on the forward path, and The waveguide length of one of the two waveguides on the forward path is made the same as the waveguide length of the other of the two waveguides on the forward path.

6. The optical device according to claim 1, wherein: The first waveguide has a first waveguide close to the input end and a first waveguide close to the output end, The first waveguide close to the input end includes two waveguides close to the input end, The first waveguide close to the output end includes two waveguides close to the output end, The second waveguide having the return structure has a second waveguide on a forward path and a second waveguide on a backward path connected to the second waveguide on the forward path, the second waveguide on the forward path comprises two waveguides on the forward path, The second waveguide on the backward path includes two waveguides on the backward path, the two waveguides on the backward path are connected to the two waveguides on the forward path, and One of the two waveguides near the input end has the same waveguide length as the other of the two waveguides near the input end, or one of the two waveguides near the output end has the same waveguide length as the other of the two waveguides near the output end.

7. The optical device according to claim 1, wherein: The first waveguide has two first waveguides close to the input end and two first waveguides close to the output end, The second waveguide has two second waveguides on a forward path and two second waveguides on a backward path, and The optical device comprises: A spectrometer that splits light input to the spectrometer; The two first waveguides close to the input end are connected to the optical splitter; two second waveguides on the forward path, the two second waveguides on the forward path connected to the two first waveguides close to the input end; two second waveguides on the backward path, the two second waveguides on the backward path connected to the two second waveguides on the forward path; a second electrode that applies a second electrical signal to two second waveguides on the backward path; The two first waveguides close to the output end are connected to the two second waveguides on the backward path; a first electrode that applies a first electrical signal to the two first waveguides near the output end; and a multiplexer connected to the two first waveguides near the output end, multiplexing the light modulated by the two first waveguides near the output end according to the first electrical signal, and outputting the multiplexed light to the output unit, in, The first chip is a silicon photonic chip and has the optical splitter, the two first waveguides close to the input end, the two first waveguides close to the output end, the first electrode and the multiplexer, and The second chip is a thin-film lithium niobate (LN) chip and has two second waveguides on the forward path, two second waveguides on the backward path, and the second electrode.

8. The optical device according to claim 1, wherein: When the electro-optical crystal layer of the second chip is an X-cut thin-film lithium niobate LN chip, the second waveguide is an optical waveguide in which light propagates in the Y-axis direction along the crystal axis of the X-cut thin-film lithium niobate LN chip.

9. An optical transmitter, comprising: a light source that emits light; as well as an optical device that modulates light from the light source according to an electrical signal, in, The optical device comprises: a first chip, the first chip comprising a first waveguide and a first electrode; and a second chip mounted on the first chip and comprising a second waveguide and a second electrode, The second waveguide has an electro-optic effect higher than that of the first waveguide, has a return structure in which an end of the second waveguide is placed at an end surface of the second chip, and is optically coupled to the first waveguide at the end surface, and The second electrode has a return structure in which an end portion of the second electrode is placed at the end surface, and the second electrode and the first electrode are electrically connected to each other in a region in which the end portion of the second electrode has been placed.

10. An optical transceiver, comprising: a processor that performs signal processing of the electrical signal; a light source that generates light; an optical device that modulates light generated by the light source by using an electrical signal output from the processor; as well as a light receiver that converts received light into an electrical signal by using the light generated by the light source, in, The optical device comprises: a first chip, the first chip comprising a first waveguide and a first electrode; and a second chip mounted on the first chip and comprising a second waveguide and a second electrode, The second waveguide has an electro-optic effect higher than that of the first waveguide, has a return structure in which an end of the second waveguide is placed at an end surface of the second chip, and is optically coupled to the first waveguide at the end surface, and The second electrode has a return structure in which an end portion of the second electrode is placed at the end surface, and the second electrode and the first electrode are electrically connected to each other in a region in which the end portion of the second electrode has been placed.

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