Optical devices and optical communication devices

Abstract

To provide an optical device and an optical communication apparatus that can be miniaturized and driven at a low voltage by improving modulation efficiency.SOLUTION: An optical device includes: a silicon substrate; a waveguide laminated on the silicon substrate and formed of a thin film of a perovskite-type oxide having a greater electrooptical effect than lithium niobate; and a clad layer that covers the waveguide. The optical device further includes: a ground electrode of a ground potential and a signal electrode that is arranged at a position facing the ground electrode and applies a drive voltage to the waveguide.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical device and an optical communication apparatus. [Background technology]

[0002] Generally, optical devices such as optical modulators include an optical modulator chip with an optical waveguide formed on its surface. A signal electrode is disposed on the optical waveguide of the optical modulator chip. When a voltage is applied to the signal electrode, an electric field perpendicular to the surface of the optical modulator chip is generated within the optical waveguide. This electric field changes the refractive index of the optical waveguide, thereby changing the phase of the light propagating through the optical waveguide and modulating the light. In other words, the optical waveguide of the optical modulator chip constitutes, for example, a Mach-Zehnder interferometer, and can output, for example, an XY polarization multiplexed IQ signal based on the phase difference of light between multiple parallel optical waveguides.

[0003] When the optical modulator chip performs high-speed modulation, a high-speed signal having a band of, for example, several tens of GHz is input to a signal electrode arranged along the optical waveguide.

[0004] Furthermore, the optical waveguide may be formed at a position where it overlaps with the signal electrode, for example, as a thin-film optical waveguide using a thin film of LN (Lithium Niobate: LiNbO3) crystal. Thin-film optical waveguides can confine light more strongly than diffused optical waveguides that use diffused metal, improving the efficiency of applying an electric field and reducing the driving voltage.

[0005] 20 is a plan view schematically illustrating an example of the configuration of an optical modulator 100 (LN modulator). The optical modulator 100 shown in FIG. 20 has an optical fiber from a light source connected to its input side and an optical fiber for transmitting a transmission signal connected to its output side. The optical modulator 100 has an optical input section 110, an RF modulation section 120, and an optical output section 130. The optical input section 110 has a first Si waveguide 111 and a first LN-Si waveguide junction section 112. The first Si waveguide 111 has one Si waveguide connected to the optical fiber on the input side, two Si waveguides branching from the one Si waveguide, four Si waveguides branching from each of two Si waveguides, and eight Si waveguides branching from each of four Si waveguides. The first LN-Si waveguide junction 112 joins the eight Si waveguides in the first Si waveguide 111 and the eight LN waveguides in the LN waveguide 121 in the RF modulation section 120 .

[0006] The RF modulation unit 120 has an LN waveguide 121, a signal electrode 122, and an RF terminator 123. When light supplied from the first Si waveguide 111 propagates through the LN waveguide 121, the RF modulation unit 120 modulates the light by an electric field applied from the signal electrode 122. The LN waveguide 121 is an optical waveguide formed using, for example, a thin-film LN substrate 154, and has eight parallel LN waveguides that are joined to each of the first LN-Si waveguide junctions 112 in the optical input unit 110. The modulated light propagating through the LN waveguide 121 is output to the optical output unit 130.

[0007] The signal electrode 122 is an electrode with a CPW (Coplanar Waveguide) structure provided at a position overlapping the LN waveguide 121, and applies an electric field to the LN waveguide 121 in response to an electrical signal of several tens of GHz output from the DSP. The terminal end of the signal electrode 122 is connected to an RF terminator 123. The RF terminator 123 is connected to the terminal end of the signal electrode 122 and prevents unnecessary reflection of the signal transmitted by the signal electrode 122.

[0008] The optical output unit 130 has a second LN-Si waveguide junction 131, a second Si waveguide 132, eight child MZs (Mach-Zehnder) 133, and four parent MZs 134. The optical output unit 130 further has a PR (Polarization Rotator) 135 and a PBC (Polarization Beam Combiner) 136. The second LN-Si waveguide junction 131 joins the eight LN waveguides 121 in the RF modulation unit 120 with the eight second Si waveguides 132. The second Si waveguides 132 have eight Si waveguides connected to the second LN-Si waveguide junction 131 and four Si waveguides that merge with two of the eight Si waveguides. Furthermore, the second Si waveguide 132 has two Si waveguides that merge with two of the four Si waveguides, and one Si waveguide that merges with the two Si waveguides and is connected to an optical fiber on the output side.

[0009] A daughter MZ 133 is provided for each of the eight Si waveguides in the second Si waveguide 132. Each set of daughter MZs 133 applies a bias voltage to the DC electrodes on the Si waveguide, adjusting the bias voltage so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal, and outputs an I signal or a Q signal. A parent MZ 134 is provided for each of the four Si waveguides in the second Si waveguide 132. Each set of parent MZs 134 applies a bias voltage to the DC electrodes on the Si waveguide, adjusting the bias voltage so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal, and outputs an I signal or a Q signal.

[0010] The PR 135 rotates the I or Q signal input from one of the parent MZs 134 by 90 degrees to obtain a vertically polarized optical signal after the 90-degree rotation. The PR 135 then inputs the vertically polarized optical signal to the PBC 136. The PBC 136 multiplexes the vertically polarized optical signal from the PR 135 with the horizontally polarized optical signal input from the other parent MZ 134, and outputs a polarization multiplexed signal.

[0011] 21 is a schematic cross-sectional view showing an example of the cross section of the optical modulator 100 shown in FIG. 20 taken along line GG. The cross section of line GG shown in FIG. 20 is a first LN-Si waveguide junction 112. The first LN-Si waveguide junction 112 shown in FIG. 21 includes a Si substrate 151, a box layer 152 made of SiO (silicon dioxide) stacked on the Si substrate 151, and a first clad layer 153 made of SiO stacked on the box layer 152. The first LN-Si waveguide junction 112 further includes a thin-film LN substrate 154 stacked on the first clad layer 153, and a second clad layer 155 made of SiO stacked on the thin-film LN substrate 154. A first Si waveguide 111 is formed in the center of the first clad layer 153. An LN waveguide 121 protruding upward is formed in the center of the thin-film LN substrate 154. By vertically approaching the first Si waveguide 111 and the LN waveguide 121, the first Si waveguide 111 and the LN waveguide 121 are directionally coupled.

[0012] 22 is a schematic cross-sectional view showing an example of the HH line cross section of the optical modulator 100 shown in FIG. 20. The HH line cross section shown in FIG. 20 is a schematic cross section of the RF modulation section 120. The RF modulation section 120 shown in FIG. 22 has a Si substrate 151, a SiO2 box layer 152 stacked on the Si substrate 151, and a first clad layer 153 stacked on the box layer 152. The RF modulation section 120 further has a thin-film LN substrate 154 stacked on the first clad layer 153, and a SiO2 second clad layer 155 stacked on the thin-film LN substrate 154. An LN waveguide 121 protruding upward is formed in the center of the thin-film LN substrate 154. A signal electrode 122 with a CPW structure is arranged on the surface of the second clad layer 155. That is, the signal electrode 122 is disposed at a position overlapping the LN waveguide 121, and a pair of ground electrodes 122A sandwiching the signal electrode 122 are disposed on the second cladding layer 155.

[0013] In such an LN waveguide 121, a high-frequency signal is applied to the signal electrode 122 to generate an electric field, which changes the refractive index of the LN waveguide 121, thereby modulating the light propagating through the LN waveguide 121. Furthermore, since the thin-film LN substrate 154 and the LN waveguide 121 are stacked on the first cladding layer 153, light can be tightly confined in the LN waveguide 121, and the driving voltage applied to the signal electrode 122 can be reduced. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] U.S. Patent No. 5,189,713 [Patent Document 2] International Publication No. 2015 / 087988 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-195239 [Patent Document 4] U.S. Patent No. 7,095,920 [Non-patent literature]

[0015] [Non-Patent Document 1] Mingbo He et al., “High-performance hybrid silicon and lithium Niobate Mach-Zehender modulators for 100 Gbit / s and beyond” NATURE PHOTONICS vol.13, may 2019, 359-364 Summary of the Invention [Problem to be solved by the invention]

[0016] However, even for the optical modulator 100 that uses an LN single crystal, further miniaturization and lower drive voltage are required.

[0017] The disclosed technology has been made in view of the above points, and aims to provide an optical device or the like that is miniaturized and requires a low driving voltage by improving modulation efficiency. [Means for solving the problem]

[0018] In one aspect, the optical device disclosed herein includes a silicon substrate and a waveguide formed of a thin film of a perovskite oxide having a larger electro-optic effect than lithium niobate, stacked on the silicon substrate. The optical device further includes a cladding layer covering the waveguide, a ground electrode at ground potential, and a signal electrode positioned opposite the ground electrode for applying a drive voltage to the waveguide. [Effects of the Invention]

[0019] According to one aspect of the optical device etc. disclosed in the present application, the modulation efficiency is improved, thereby enabling miniaturization and a lower driving voltage. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a block diagram showing an example of the configuration of an optical communication device according to this embodiment. [Figure 2] FIG. 2 is a schematic plan view showing an example of the configuration of the optical modulator (PZT modulator) according to the first embodiment. [Figure 3] FIG. 3 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in FIG. 2 taken along line AA. [Figure 4] FIG. 4 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in FIG. 2 taken along the line BB. [Figure 5] FIG. 5 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in FIG. 2 taken along line CC. [Figure 6] FIG. 6 is a schematic cross-sectional view showing an example of an optical modulator taken along line DD (Si optical integrated circuit wafer). [Figure 7] FIG. 7 is a schematic cross-sectional view showing an example of a thin-film PZT substrate wafer. [Figure 8A]FIG. 8A is an explanatory diagram showing an example of a manufacturing process for the RF modulation section. [Figure 8B] FIG. 8B is an explanatory diagram showing an example of a manufacturing process (adhesion layer) of the RF modulation section. [Figure 8C] FIG. 8C is an explanatory diagram showing an example of a manufacturing process of the RF modulation section (adjusting the dimension between the bottom surface of the PZT layer and the top surface of the Si waveguide). [Figure 8D] FIG. 8D is an explanatory diagram showing an example of a manufacturing process (polishing process) of the RF modulation section. [Figure 8E] FIG. 8E is an explanatory diagram showing an example of a manufacturing process (forming a thin film PZT substrate) for an RF modulation section. [Figure 9] FIG. 9 is an explanatory diagram showing another example of the manufacturing process of the RF modulation section. [Figure 10] FIG. 10 is a schematic cross-sectional view showing a modification of the cross section of the optical modulator shown in FIG. 2 taken along line BB. [Figure 11A] 11A is a schematic cross-sectional view showing a modification of the cross section of the optical modulator shown in FIG. 2 taken along line BB. [Figure 11B] FIG. 11B is a schematic cross-sectional view showing an example of a thin-film BTO substrate wafer. [Figure 12] FIG. 12 is a schematic plan view showing an example of the configuration of the optical modulator (PLZT modulator) according to the second embodiment. [Figure 13] FIG. 13 is a schematic cross-sectional view showing an example of the EE line cross section of the optical modulator shown in FIG. [Figure 14] FIG. 14 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in FIG. 12 taken along the line FF. [Figure 15] FIG. 15 is a schematic cross-sectional view showing an example of a thin-film PLZT substrate wafer. [Figure 16A] FIG. 16A is an explanatory diagram showing an example of a manufacturing process for an RF modulation section. [Figure 16B] FIG. 16B is an explanatory diagram showing an example of a manufacturing process (adhesion layer) of the RF modulation section. [Figure 16C] FIG. 16C is an explanatory diagram showing an example of a manufacturing process of the RF modulation section (adjusting the dimension between the bottom surface of the PLZT layer and the top surface of the Si waveguide). [Figure 16D]FIG. 16D is an explanatory diagram showing an example of a manufacturing process (polishing process) of the RF modulation section. [Figure 16E] FIG. 16E is an explanatory diagram showing an example of a manufacturing process (forming a thin film PLZT substrate) for an RF modulation section. [Figure 17] FIG. 17 is an explanatory diagram showing an example of the results of a comparison between an LN waveguide and a PLZT waveguide in terms of the driving voltage relative to the electrode length. [Figure 18] FIG. 18 is an explanatory diagram showing another example of the manufacturing process of the RF modulation section. [Figure 19] FIG. 19 is a schematic cross-sectional view showing a modified example of the optical modulator taken along the EE line. [Figure 20] FIG. 20 is a schematic plan view showing an example of the configuration of an optical modulator (LN modulator). [Figure 21] FIG. 21 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in FIG. 20 taken along line GG. [Figure 22] FIG. 22 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in FIG. 20 taken along line HH. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the optical device and the like disclosed in the present application will be described in detail with reference to the drawings. Note that the present invention is not limited to these embodiments. Furthermore, for the sake of convenience, the dimensions of various parts are shown as examples, but the dimensions are not limited to these and can be changed as appropriate. [Example]

[0022] FIG. 1 is a block diagram showing an example of the configuration of an optical communication device 1 according to this embodiment. The optical communication device 1 shown in FIG. 1 is connected to an optical fiber 2A(2) on the output side and an optical fiber 2B(2) on the input side. The optical communication device 1 includes a DSP (Digital Signal Processor) 3, a light source 4, an optical modulator 5, and an optical receiver 6. The DSP 3 is an electrical component that performs digital signal processing. For example, the DSP 3 performs processing such as encoding transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the optical modulator 5. The DSP 3 also obtains an electrical signal including reception data from the optical receiver 6 and performs processing such as decoding of the obtained electrical signal to obtain the reception data.

[0023] The light source 4 includes, for example, a laser diode, and generates light of a predetermined wavelength and supplies it to the optical modulator 5 and the optical receiver 6. The optical modulator 5 is an optical device that modulates the light supplied from the light source 4 with an electrical signal output from the DSP 3 and outputs the resulting optical transmission signal to the optical fiber 2A. The optical modulator 5 includes, for example, a PZT (lead zirconate titanate) waveguide 31 and a signal electrode 32 with a microstripline (MSL) structure. When the light supplied from the light source 4 propagates through the PZT waveguide 31, the optical modulator 5 modulates the light with an electrical signal input to the signal electrode 32, thereby generating an optical transmission signal.

[0024] The optical receiver 6 receives an optical signal from the optical fiber 2B and demodulates the received optical signal using light supplied from the light source 4. The optical receiver 6 then converts the demodulated received optical signal into an electrical signal and outputs the converted electrical signal to the DSP 3.

[0025] FIG. 2 is a plan view schematic diagram showing an example of the configuration of an optical modulator (PZT modulator) 5 according to the first embodiment. The optical modulator 5 shown in FIG. 2 is, for example, a PZT modulator, having an optical fiber 4A from a light source 4 connected to its input side and an optical fiber 2A for transmitting a transmission signal connected to its output side. The optical modulator 5 has a first optical input unit 11, an RF modulation unit 12, and a first optical output unit 13. The first optical input unit 11 has a first Si waveguide 21 and a first PZT-Si waveguide junction 22. The first Si waveguide 21 has one Si waveguide connected to the optical fiber 4A, two Si waveguides branching from the single Si waveguide, four Si waveguides branching from each of the two Si waveguides, and eight Si waveguides branching from each of the four Si waveguides. The first PZT-Si waveguide junction 22 joins the eight Si waveguides in the first Si waveguide 21 and the eight PZT waveguides in the PZT waveguide 31 .

[0026] The RF modulation unit 12 has a PZT waveguide 31, a signal electrode 32, and an RF terminator 33. When light supplied from the first Si waveguide 21 propagates through the PZT waveguide 31, the RF modulation unit 12 modulates the light by an electric field applied from the signal electrode 32. The PZT waveguide 31 is an optical waveguide formed using, for example, a thin-film PZT substrate 55, and has eight parallel PZT waveguides that branch repeatedly from the input side. The modulated light propagating through the PZT waveguide 31 is output to the first optical output unit 13. PZT is an inorganic material, such as a perovskite oxide, that has a larger electro-optic effect, such as a higher optical refractive index, than LN.

[0027] The signal electrode 32 is a transmission line with an MSL structure provided at a position overlapping the PZT waveguide 31, and applies an electric field to the PZT waveguide 31 in response to the electrical signal output from the DSP 3. The terminal end of the signal electrode 32 is connected to an RF terminator 33. The RF terminator 33 is connected to the terminal end of the signal electrode 32 and prevents unnecessary reflection of the signal transmitted by the signal electrode 32.

[0028] The thin-film PZT substrate 55 is a PZT single crystal, and the crystal direction in which the electro-optic coefficient of the thin-film PZT substrate is high is perpendicular to the Si substrate 51 (X direction). Therefore, the optical modulator 5 has a ground electrode 53 between the Si substrate 51 and the signal electrode 32, and has a structure in which the direction of the electric field is perpendicular to the Si substrate 51 (X direction).

[0029] The first optical output unit 13 includes a second PZT-Si waveguide junction 41, a second Si waveguide 42, eight daughter MZs 43, four parent MZs 44, a PR 45, and a PBC 46. The second PZT-Si waveguide junction 41 connects the PZT waveguide 31 in the RF modulation unit 12 to the second Si waveguide 42. The second Si waveguide 42 includes eight Si waveguides connected to the second PZT-Si waveguide junction 41 and four Si waveguides that merge with two of the eight Si waveguides. Furthermore, the second Si waveguide 42 includes two Si waveguides that merge with two of the four Si waveguides and one Si waveguide that merges with the two Si waveguides. A daughter MZ (Mach-Zehnder) 43 is provided for each of the eight Si waveguides in the second Si waveguide 42. Each set of daughter MZs 43 applies a bias voltage to DC electrodes on the Si waveguide, adjusting the bias voltage so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal, and outputs an I signal, which is an in-phase axis component, or a Q signal, which is an orthogonal axis component. A parent MZ 44 is provided for each of the four Si waveguides in the second Si waveguide 42. Each set of parent MZs 44 applies a bias voltage to DC electrodes on the Si waveguide, adjusting the bias voltage so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal, and outputs an I signal or a Q signal.

[0030] The PR 45 rotates the I or Q signal input from one of the parent MZs 44 by 90 degrees to obtain a vertically polarized optical signal after the 90-degree rotation. The PR 45 then inputs the vertically polarized optical signal to the PBC 46. The PBC 46 multiplexes the vertically polarized optical signal from the PR 45 with the horizontally polarized optical signal input from the other parent MZ 44, and outputs a polarization multiplexed signal.

[0031] Next, the configuration of the optical modulator 5 of the first embodiment will be specifically described. Fig. 3 is a schematic cross-sectional view showing an example of the cross section of the optical modulator 5 shown in Fig. 2 taken along line AA, and Fig. 4 is a schematic cross-sectional view showing an example of the cross section of the optical modulator shown in Fig. 2 taken along line BB. The cross sections of line AA shown in Fig. 3 and line BB shown in Fig. 4 are the RF modulation section 12. The RF modulation section 12 has a Si substrate 51, a box layer 52 made of SiO2 (silicon dioxide) or the like stacked on the Si substrate 51, and a ground electrode 53 having an MSL structure stacked on the box layer 52. The RF modulation section 12 has a first Si waveguide 21 formed on the box layer 52 and a first clad layer 54 stacked on the ground electrode 53. Furthermore, the RF modulation section 12 has a thin-film PZT substrate 55 laminated on the first clad layer 54, a second clad layer 56 laminated on the thin-film PZT substrate 55, and a signal electrode 32 with an MSL structure laminated on the second clad layer 56.

[0032] The Si substrate 51 is, for example, a Si substrate with a thickness of approximately several hundred μm. The box layer 52 is, for example, a substrate made of SiO2 or TiO2 (titanium dioxide). The ground electrode 53 is, for example, an electrode made of metal such as aluminum with a thickness of 0.1 μm or more and at ground potential. The ground electrode 53 can reduce the influence of the electric field from the signal electrode 32 to the Si substrate 51, thereby reducing high-frequency loss. The first cladding layer 54 is, for example, made of SiO2 or TiO2, and is, for example, a layer with a thickness of 0.3 to 0.5 nm. Similarly, the second cladding layer 56 is, for example, made of SiO2 or TiO2, and is, for example, a layer with a thickness of 0.2 to 3 μm. The second cladding layer 56 can suppress light loss caused by the signal electrode 32 arranged on the thin-film PZT substrate 55.

[0033] A thin-film PZT substrate 55 having a thickness of, for example, 0.5 to 3 μm is sandwiched between the first cladding layer 54 and the second cladding layer 56, and a PZT waveguide 31 is formed at the center of the thin-film PZT substrate 55, protruding upward. The width of the protrusion that becomes the PZT waveguide 31 is, for example, approximately 1 to 8 μm. The thin-film PZT substrate 55 and the PZT waveguide 31 are covered with a second cladding layer 56, and a signal electrode 32 is disposed on the surface of the second cladding layer 56. In other words, the signal electrode 32 faces the ground electrode 53 across the PZT waveguide 31, forming a transmission path with an MSL structure.

[0034] The ground electrode 53 of the MSL structure is preferably formed by a Si wafer process. Furthermore, it is preferable to select a material taking into consideration the adhesion between the ground electrode 53 and the first cladding layer 54. Furthermore, it is preferable that the signal electrode 32 is made of a material different from that of the ground electrode 53, which has low high-frequency loss.

[0035] The signal electrode 32 is made of a metal material such as gold or copper and has a width of, for example, 2 to 10 μm and a thickness of, for example, 1 to 20 μm. The ground electrode 53 is made of a metal material such as aluminum and has a thickness of 0.1 μm or more. When a high-frequency signal corresponding to the electrical signal output from the DSP 3 is transmitted through the signal electrode 32, an electric field is generated in a direction from the signal electrode 32 to the ground electrode 53, and this electric field is applied to the PZT waveguide 31. As a result, the refractive index of the PZT waveguide 31 changes in response to the application of the electric field to the PZT waveguide 31, making it possible to modulate the light propagating through the PZT waveguide 31. The thin-film PZT substrate 55 forming the PZT waveguide 31 is a PZT single crystal, and therefore the crystal direction (crystal orientation) is perpendicular to the Si substrate 51 (X direction), which is the same as the electric field direction.

[0036] The optical modulator 5 has a Si optical integrated circuit wafer 500 and a thin-film PZT substrate wafer 550. The Si optical integrated circuit wafer 500 is a wafer for forming the first optical input section 11, RF modulation section 12, and first optical output section 13 in the optical modulator 5. The thin-film PZT substrate wafer 550 is a wafer for forming the thin-film PZT substrate 55 of PZT single crystal, which forms the RF modulation section 12.

[0037] 5 is a schematic cross-sectional view showing an example of a CC cross section of the optical modulator 5 shown in FIG. 5. The CC cross section shown in FIG. 5 is a first PZT-Si waveguide junction 22. The first PZT-Si waveguide junction 22 has a Si substrate 51, a box layer 52 stacked on the Si substrate 51, a first Si waveguide 21 stacked on the box layer 52, and a first clad layer 54 covering the first Si waveguide 21. The first PZT-Si waveguide junction 22 further has a thin-film PZT substrate 55 having a PZT waveguide 31 stacked on the first clad layer 54, and a second clad layer 56 stacked on the thin-film PZT substrate 55.

[0038] FIG. 6 is a schematic cross-sectional view showing an example of a cross section (Si optical integrated circuit wafer) 500 of the optical modulator 5 taken along line DD. The Si optical integrated circuit wafer 500 shown in FIG. 6 is an SOI (Silicon On Insulator) wafer. The Si optical integrated circuit wafer 500 has a Si substrate 51, a box layer 52 stacked on the Si substrate 51, a first Si waveguide 21 stacked on the box layer 52, and a first clad layer 54 stacked on the first Si waveguide 21. The first clad layer 54 is a dielectric material with a low refractive index, such as an SiO2 film. The surface of the first clad layer 54 is planarized by chemical mechanical polishing (CMP) to remove unevenness. The dielectric material covering the entire wafer surface may be, for example, a low refractive index resin.

[0039] FIG. 7 is a schematic cross-sectional view showing an example of a thin-film PZT substrate wafer 550. The thin-film PZT substrate wafer 550 shown in FIG. 7 includes a Si substrate 551, a ZrO film 552, a Pt layer 553, an SRO film 554, and a PZT layer 555. The ZrO film 552 is, for example, a film portion of 10 nm or more and a protrusion portion of 3 to 8 nm, and is a film formed by epitaxial growth on a Si single crystal as a first layer containing ZrO. The ZrO film 552 has, for example, a film portion of 10 nm or more and a protrusion portion of 3 to 8 nm, and contains up to 8% crystal defects, thereby providing elasticity. The Pt layer 553 is, for example, a layer formed by epitaxial growth on the ZrO film 552 as a second layer containing Pt and having a thickness of 20 nm or more. The SRO film 554 is, for example, a film formed by epitaxial growth on the Pt layer 553 as a third layer containing SRO (strontium oxide) and having a thickness of 20 nm or more. The PZT layer 555 is a layer formed by epitaxial growth on the SRO film 554. The fourth layer includes a thin film having piezoelectric and electro-optical effects, such as PZT, having a thickness of approximately 2 μm. The first, second, third, and fourth layers are epitaxially grown in sequence to form a multilayer thin-film PZT substrate wafer 550 including the PZT layer 555 of single crystal PZT (see International Publication WO 2020 / 179210). The epitaxial growth can be performed by vacuum deposition for the ZrO film 552, or by physical vapor deposition (PVD) such as sputtering for the Pt layer 553, SRO film 554, and PZT layer 555. Furthermore, the Si substrate 551 may be heated to a temperature of 450 to 600°C to promote epitaxial growth.

[0040] Here, the ZrO2 crystal in the first layer is tetragonal, but contains up to 8% crystal defects. It is believed that the presence of crystal defects causes the defect vacancies and adjacent atoms to have elasticity in the direction that reduces lattice distortion. This elasticity of the ZrO2 crystal can be utilized to make the crystal structure variable. Furthermore, the formation of protruding ZrO2 crystals is thought to be due to the fact that, when the source material concentration is supersaturated during the film formation process, the crystal grows anisotropically, with one axis of the crystal growing along a certain edge, forming a pyramidal structure.

[0041] 8A to 8E are explanatory diagrams showing an example of the manufacturing process of the RF modulation section 12 of the optical modulator 5. Fig. 8A is an explanatory diagram showing an example of the manufacturing process of the RF modulation section 12, and Fig. 8B is an explanatory diagram showing an example of the manufacturing process (adhesion layer) of the RF modulation section 12. A PZT layer 555 of the thin-film PZT substrate wafer 550 shown in Figs. 8A and 8B is wafer-bonded onto the first cladding layer 54 in the Si optical integrated circuit wafer 500 via an adhesion layer 556.

[0042] FIG. 8C is an explanatory diagram showing an example of a manufacturing process of the RF modulation section 12 (adjusting the dimension between the bottom surface of the PZT layer 555 and the top surface of the first Si waveguide 21). For ease of explanation, it is assumed that the thickness of the first Si waveguide 21 is, for example, 220 nm and the thickness of the PZT layer 555 is, for example, 1 μm. A thin-film PZT substrate wafer 550 is wafer-bonded onto the Si optical integrated circuit wafer 500 shown in FIG. 8C. The thicknesses of the adhesion layer 556 and the first cladding layer 54 are then adjusted so that the dimension between the bottom surface of the PZT layer 555 and the top surface of the first Si waveguide 21 is, for example, in the range of 100 to 300 nm. As a result, light can be coupled from the first Si waveguide 21 to the PZT waveguide 31.

[0043] 8D is an explanatory diagram showing an example of a manufacturing process (polishing process) for the RF modulation unit 12. After bonding the PZT layer 555 of the thin-film PZT substrate wafer 550 onto the Si optical integrated circuit wafer 500, the Si substrate 551, ZrO film 552, Pt layer 553, and SRO film 554 are removed using a polishing process, leaving the PZT layer 555 in the thin-film PZT substrate wafer 550.

[0044] 8E is an explanatory diagram showing an example of a manufacturing process (thin-film PZT substrate formation) for the RF modulation section 12. Furthermore, as shown in FIG. 8E, after polishing, a PZT layer 555 of the thin-film PZT substrate wafer 550 is polished, and then a convex PZT waveguide 31 and a PZT slab 558 are formed on the surface of the PZT layer 555 using photolithography. As a result, the thin-film PZT substrate 55 is formed on the first cladding layer 54. The PZT slab 558 of the thin-film PZT substrate 55 can strengthen the adhesive strength between the first cladding layer 54 on the Si optical integrated circuit wafer 500 and the thin-film PZT substrate 55.

[0045] Then, a second cladding layer 56 is formed on the thin-film PZT substrate 55 (see FIG. 4). Furthermore, a signal electrode 32 having an MSL structure is formed on the second cladding layer 56. As a result, the RF modulation section 12 shown in FIG. 4 is completed.

[0046] The optical modulator 5 of the first embodiment includes a Si substrate 51, a ground electrode 53 at ground potential stacked on the Si substrate 51, and a PZT waveguide 31 formed by a thin-film PZT substrate 55 stacked on the ground electrode 53. The optical modulator 5 further includes a signal electrode 32 that is positioned facing the ground electrode 53 and the Si substrate 51 in the vertical direction across the PZT waveguide 31 and applies a high-frequency signal to the PZT waveguide 31. As a result, the use of a PZT waveguide 31, which has a greater electro-optic effect than an LN waveguide, improves modulation efficiency, thereby enabling miniaturization and a lower driving voltage. The optical modulator 5 uses a thin-film PZT substrate 55 made of PZT single crystal, which has a greater electro-optic effect than LN, resulting in an electro-optic coefficient that is three times greater than that of an LN single crystal.

[0047] The optical modulator 5 includes a first cladding layer 54 laminated between a ground electrode 53 and a thin-film PZT substrate 55, and a second cladding layer 56 laminated on the thin-film PZT substrate 55 and covering the PZT waveguide 31. The signal electrode 32 is positioned on the surface of the second cladding layer 56, overlapping the PZT waveguide 31. The signal electrode 32 generates an electric field in the PZT waveguide 31 in a direction perpendicular to the Si substrate 51 (the vertical direction (X direction) in Figure 4). The crystal orientation of the PZT waveguide 31 is also perpendicular to the Si substrate 51 (X direction). In other words, the crystal orientation of the PZT waveguide 31 is the same as the electric field direction, improving the efficiency of electric field application and reducing the driving voltage, thereby significantly improving modulation efficiency. Furthermore, the use of PZT increases modulation efficiency (voltage × electrode length). As a result, lower voltages and smaller size are possible. Moreover, since higher modulation efficiency can be obtained compared to LN even when the electrode length is shortened, the optical modulator 5 can be made smaller in size by the amount of the shortened electrode length.

[0048] The optical modulator 5 has a first cladding layer 54 that covers the first Si waveguide 21 and a second cladding layer 36 that covers the thin-film PZT substrate 55. The thickness of the first cladding layer 54 between the top surface of the first Si waveguide 21 and the bottom surface of the thin-film PZT substrate 55 is set to within 100 nm to 300 nm, thereby optically coupling the first Si waveguide 21 and the PZT waveguide 31. As a result, the first Si waveguide 21 and the PZT waveguide 31 can be optically coupled.

[0049] The first cladding layer 54 is made of a dielectric material or resin with a low refractive index, so that the first cladding layer 54 and the thin-film PZT substrate 55 can be bonded together.

[0050] The optical modulator 5 has a dielectric 57 made of an SiO2 film, which is a dielectric material formed on the back surface of the Si substrate 51. As a result, the influence of warping due to thermal history during the manufacturing process of the optical modulator 5 can be suppressed.

[0051] In the first embodiment, the RF modulation section 12 of the optical modulator 5 is exemplified by bonding a PZT layer 555 onto an optical integrated circuit wafer 500 via an adhesive layer 556, as shown in FIG. 8D . However, if the warpage of the optical integrated circuit wafer 500 increases due to thermal history or the like during the manufacturing process of the optical modulator 5, it is possible that the optical integrated circuit wafer 500 will not be able to be adsorbed to a wafer stage in an exposure apparatus or the like used to form a pattern on the optical modulator 5. To address this issue, as shown in FIG. 9 , a dielectric film 57 such as an SiO film is formed on the back surface of the Si substrate 51 of the Si optical integrated circuit wafer 500, thereby suppressing warpage of the Si optical integrated circuit wafer 500. As a result, it is possible to avoid a situation in which the optical integrated circuit wafer 500 will not be able to be adsorbed to a wafer stage in an exposure apparatus or the like used to form a pattern on the optical modulator 5.

[0052] Also, the RF modulation section 12 has been exemplified as having the PZT layer 555 of the thin-film PZT substrate wafer 550 bonded onto the first cladding layer 54 of the optical integrated circuit wafer 500 via the adhesive layer 556A. However, even if the adhesive layer 556A is not used, a bonding portion may be formed between the first cladding layer 54 and the PZT layer 555, and this can be modified as appropriate.

[0053] FIG. 10 is a schematic cross-sectional view showing a modified example of the optical modulator 5 shown in FIG. 2 taken along the line BB. In the RF modulation unit 12 shown in FIG. 10, an opening 561 is formed by etching a portion of a second cladding layer 56 covering both sides of a PZT waveguide 31 of a thin-film PZT substrate 55. Furthermore, in the RF modulation unit 12, a signal electrode 32 is disposed on the second cladding layer 56 on the PZT waveguide 31. By forming an opening in the portion of the second cladding layer 56 covering both sides of the PZT waveguide 31, the influence of the second cladding layer 56 is reduced, and a vertical electric field is applied to the PZT waveguide 31 from the signal electrode 32 to the ground electrode 53. As a result, modulation efficiency can be improved.

[0054] 11A is a schematic cross-sectional view showing a modified example of the cross section along line BB of the optical modulator 5 shown in FIG. 11A. In the RF modulation section 12 shown in FIG. 11A, an opening 562 is formed by etching the second cladding layer 56, the thin-film PZT substrate 55, and the first cladding layer 54 to expose a portion of the surface of the ground electrode 53 covered with the first cladding layer 54. Then, a metal film made of the same material as the ground electrode 53 is formed in the opening 562 to form an exposed ground electrode 531. As a result, the ground electrode 53 and the exposed ground electrode 531 are electrically connected, and thus, by exposing the ground electrode 531 on the second cladding layer 56, they can be easily grounded.

[0055] For the sake of convenience, the optical modulator 5 of Example 1 illustrates a case in which the first Si waveguide 21 and the PZT waveguide 31 are directionally coupled, but the first Si waveguide 21 and the PZT waveguide may be butt-coupled, and other modifications can be made as appropriate.

[0056] The first clad layer 54 is provided between the thin-film PZT substrate 55 and the ground electrode 53, and it is necessary to thicken the first clad layer 54 in order to laminate the ground electrode 53. Therefore, the distance between the PZT waveguide 31 and the first Si waveguide 21 increases by the amount of the thickened first clad layer 54, and the coupling length between the PZT waveguide 31 and the first Si waveguide 21 increases accordingly. To address this situation, the PZT waveguide 31 and the first Si waveguide 21 may be optically coupled by a Si-PZT waveguide.

[0057] Although the optical modulator 5 in Example 1 is a PZT modulator, a BTO modulator using BiTiO (barium titanate, hereinafter referred to as BTO) instead of PZT may also be used. FIG. 11B is a schematic cross-sectional view showing an example of a thin-film BTO substrate wafer 550B. The thin-film BTO substrate wafer 550B shown in FIG. 11B includes a Si substrate 551, a ZrO film 552, a Pt layer 553, an SRO film 554, and a BTO film 555B. As mentioned above, tetragonal ZrO crystals contain crystal defects, and the defect vacancies and adjacent atoms have elasticity, allowing the crystal structure to be variable. When the raw material concentration is supersaturated during the film formation process, the crystal grows anisotropically along a certain axis and a certain edge, forming a pyramidal structure. Therefore, epitaxial growth is possible even with BTO.

[0058] Furthermore, since BTO can undergo a phase transition near 0°C to 5°C within the operating temperature range of the modulator (e.g., -5°C to 75°C), in order to stabilize its characteristics within the operating temperature range, BTO can be doped with Sr, Zr, La, KF, etc. to lower its Curie temperature, thereby lowering the phase transition temperature near 0°C to 5°C to below -5°C, or a Peltier element can be used to control the temperature to, for example, 25°C to 45°C. The Peltier element is mounted on the surface of the Si substrate 51 shown in FIG. 11B opposite to the layered surface. By controlling the temperature of the Peltier element to a constant value, the characteristics can be stabilized within the operating temperature range of the modulator.

[0059] The BTO film 555B of the thin-film BTO substrate wafer 550 is then wafer-bonded to the first cladding layer 54 of the Si optical integrated circuit wafer 500 via an adhesive layer 556. As a result, a thin-film BTO substrate is formed on the first cladding layer 54 instead of the thin-film PZT substrate 55 through the manufacturing processes shown in FIGS. 8A to 8E. The process for forming the thin-film BTO substrate differs from that for forming the thin-film BTO substrate wafer 550, in that a BTO film is used instead of a PZT film. Since the other steps are essentially the same as those for forming the thin-film PZT substrate 55, detailed description thereof will be omitted. A second cladding layer 56 is then formed on the thin-film BTO substrate (see FIG. 4). Furthermore, a signal electrode 32 with an MSL structure is formed on the second cladding layer 56. As a result, the RF modulation section of the BTO modulator is completed. The BTO waveguide formed on the thin-film BTO substrate has a shape similar to that of the PZT waveguide 31 formed on the thin-film PZT substrate 55 shown in FIG. 8E.

[0060] Furthermore, although a PZT modulator was exemplified as the optical modulator 5 in Example 1, a PLZT modulator employing PLZT (lanthanum-doped lead zirconate titanate) instead of PZT may also be used, and such an embodiment will be described below as Example 2. Note that the same components as in Example 1 are given the same reference numerals, and descriptions of the overlapping components and operations will be omitted. [Example]

[0061] 12 is a plan view schematically illustrating an example of the configuration of an optical modulator (PLZT modulator) 5A according to a second embodiment. The optical modulator 5A illustrated in FIG. 12 is a PLZT modulator. The optical modulator 5A includes a first optical input unit 11, an RF modulation unit 12A, and a first optical output unit 13. The first optical input unit 11 includes a first Si waveguide 21 and a first PLZT-Si waveguide junction 22A. The first Si waveguide 21 includes one Si waveguide connected to the optical fiber 4A, two Si waveguides branching from the one Si waveguide, four Si waveguides branching from each of the two Si waveguides, and eight Si waveguides branching from each of the four Si waveguides. The first PLZT-Si waveguide junction 22A joins the eight Si waveguides in the first Si waveguide 21 and the eight PLZT waveguides in the PLZT waveguide 31A.

[0062] The RF modulation unit 12A includes a PLZT waveguide 31A, a signal electrode 32A, and an RF terminator 33. When light supplied from the first Si waveguide 21 propagates through the PLZT waveguide 31A, the RF modulation unit 12A modulates the light by an electric field applied from the signal electrode 32A. The PLZT waveguide 31A is an optical waveguide formed using, for example, a thin-film PLZT substrate 55A, and has eight parallel PLZT waveguides that branch repeatedly from the input side. The modulated light propagating through the PLZT waveguide 31A is output to the first optical output unit 13. PLZT is an inorganic material, such as a perovskite oxide, that has a larger electro-optic effect, such as a higher optical refractive index, than LN.

[0063] The signal electrode 32A and the ground electrode 53A have a coplanar waveguide (CPW) structure. The signal electrode 32A and a pair of ground electrodes 53A sandwiching the signal electrode 32A are arranged above the PLZT waveguide 31A. The signal electrode 32A applies an electric field to the PLZT waveguide 31A in response to an electrical signal output from the DSP 3. The end of the signal electrode 32A is connected to the RF terminator 33.

[0064] The optical modulator 5A has a signal electrode 32A with a CPW structure and a pair of ground electrodes 53A above a PLZT waveguide 31A, and the direction of the electric field is the width direction (left-right direction (Z direction) in FIG. 13) with respect to the Si substrate 51. The thin-film PLZT substrate 55A is a PLZT single crystal, and the crystal direction of the thin-film PLZT substrate 55A is the width direction (Z direction) with respect to the Si substrate 51, which is the same as the direction of the electric field.

[0065] The first optical output unit 13 has a second PLZT-Si waveguide junction 41A, a second Si waveguide 42, eight daughter MZs 43, four parent MZs 44, a PR 45, and a PBC 46. The second PLZT-Si waveguide junction 41A joins the PLZT waveguide 31A in the RF modulation unit 12A to the second Si waveguide 42. The second Si waveguide 42 has eight Si waveguides connected to the second PLZT-Si waveguide junction 41A and four Si waveguides that merge with two of the eight Si waveguides.

[0066] Next, the configuration of the optical modulator 5A of the second embodiment will be described in detail. Fig. 13 is a schematic cross-sectional view showing an example of the EE line cross-section of the optical modulator 5A shown in Fig. 12. The EE line cross-section shown in Fig. 13 is the RF modulation section 12A. The RF modulation section 12A has a Si substrate 51, a SiO2 box layer 52 stacked on the Si substrate 51, and a first clad layer 54 stacked on the box layer 52. The RF modulation section 12A has a thin-film PLZT substrate 55A stacked on the first clad layer 54, a second clad layer 56 stacked on the thin-film PLZT substrate 55A, and a signal electrode 32A and a pair of ground electrodes 53A with a CPW structure stacked on the surface of the second clad layer 56A.

[0067] The Si substrate 51 is, for example, a Si substrate with a thickness of approximately several hundred μm. The box layer 52 is, for example, a substrate made of SiO2 or TiO2. The ground electrode 53A is, for example, a ground potential electrode made of a metal such as copper with a thickness of 1 μm or more. The first clad layer 54 is, for example, a layer made of SiO2 or TiO2 with a high refractive index and with a thickness of, for example, 0.3 to 0.5 μm. Similarly, the second clad layer 56 is, for example, a layer made of SiO2 or TiO2 and with a thickness of, for example, 0.2 to 3 μm. The second clad layer 56 can, for example, suppress light loss caused by the signal electrode 32A arranged on the thin-film PLZT substrate 55A.

[0068] A thin-film PLZT substrate 55A having a thickness of, for example, 0.5 to 3 μm is sandwiched between the first cladding layer 54 and the second cladding layer 56, and a PLZT waveguide 31A is formed at the center of the thin-film PLZT substrate 55A, protruding upward. The width of the protrusion that becomes the PLZT waveguide 31A is, for example, approximately 1 to 8 μm. The thin-film PLZT substrate 55A and the PLZT waveguide 31A are covered with a second cladding layer 56, and a signal electrode 32A and a ground electrode 53A are disposed on the surface of the second cladding layer 56. In other words, the signal electrode 32A and a pair of ground electrodes 53A are disposed on the PLZT waveguide 31A, forming a transmission path with a CPW structure.

[0069] The signal electrode 32A is made of a metal material such as gold or copper and has a width of, for example, 2 to 10 μm and a thickness of, for example, 1 to 20 μm. The ground electrode 53A is made of a metal material such as gold or copper and has a thickness of 1 μm or more. When a high-frequency signal corresponding to the electrical signal output from the DSP 3 is transmitted through the signal electrode 32A, an electric field is generated in a direction from the signal electrode 32A to the ground electrode 53A, and this electric field is applied to the PLZT waveguide 31A. As a result, the refractive index of the PLZT waveguide 31A changes in response to the application of the electric field to the PLZT waveguide 31A, making it possible to modulate the light propagating through the PLZT waveguide 31A. Furthermore, the thin-film PLZT substrate 55A on which the PLZT waveguide 31A is formed is a PLZT single crystal, and therefore the crystal orientation is also the width direction (Z direction). In other words, the crystal orientation of the PLZT waveguide 31A is the same as the electric field direction, improving the efficiency of applying the electric field and reducing the driving voltage.Furthermore, the modulation efficiency can be significantly improved.

[0070] The optical modulator 5A has a Si optical integrated circuit wafer 500 and a thin-film PLZT substrate wafer 550A. The thin-film PLZT substrate wafer 550A is a wafer for forming a thin-film PLZT substrate 55A of PLZT single crystal, which forms the RF modulation section 12A.

[0071] Fig. 14 is a schematic cross-sectional view showing an example of the FF line cross section of the optical modulator 5A shown in Fig. 12. The portion of the FF line cross section shown in Fig. 14 is a first PLZT-Si waveguide junction 22A. The first PLZT-Si waveguide junction 22A has a Si substrate 51, a Box layer 52 stacked on the Si substrate 51, a first Si waveguide 21 stacked on the Box layer 52, and a first clad layer 54 covering the first Si waveguide 21. Furthermore, the first PLZT-Si waveguide junction 22A has a thin-film PLZT substrate 55A having a PLZT waveguide 31A stacked on the first clad layer 54, and a second clad layer 56 stacked on the thin-film PLZT substrate 55A.

[0072] Fig. 15 is a schematic cross-sectional view showing an example of a thin-film PLZT substrate wafer 550A. The thin-film PLZT substrate wafer 550A shown in Fig. 15 has a sapphire substrate 551A and a PLZT layer 552A. The sapphire substrate 551A is a material with a lattice constant close to that of PLZT.

[0073] An organometallic compound, which is a reaction product of Pb, La, Zr, Ti, and an organic compound, is applied onto the sapphire substrate 551A without being hydrolyzed by spin coating or the like (application step).The sapphire substrate 551A that has undergone the application step is thermally decomposed in an oxygen-containing atmosphere at a temperature of, for example, 200 to 400°C, at which crystallization does not occur, at a temperature rise rate of, for example, 1 to 100°C / sec, to form an amorphous thin film with a thickness of, for example, 200 nm or less (thermal decomposition step).

[0074] Next, the sapphire substrate 551A that has undergone the thermal decomposition process is heated to a crystal growth temperature of, for example, 600 to 800°C in a dry, oxygen-containing atmosphere, and heated for, for example, 10 seconds to 12 hours. Then, a PLZT single crystal thin film is grown on the sapphire substrate 551A by solid-phase epitaxial growth (crystallization process). After the crystal growth, the temperature is maintained at, for example, 100 to 600°C, and the film is cooled at a rate of, for example, 0.01 to 100°C / second. By repeating the above coating process to crystallization process multiple times, a PLZT layer 552A having a desired thickness of, for example, about 2 μm can be obtained.

[0075] The first layer of PLZT film, which is initially formed on the sapphire (Al2O3) substrate 551A, is Pb(1-x)La(x)(Zr(y)Ti(1-y))(1-x / 4)O3, and the PLZT film with a composition of 0 < x < 0.30 and 0 < y < 0.20 is made to have a film thickness of, for example, 1 to 40 nm so that the crystal grains are separated in an island shape. As a result, it does not become a pyrochlore layer, and epitaxial growth of a perovskite single phase can be easily performed. The second layer and subsequent layers on the first layer are made of PLZT with a composition of Pb(1-x)La(x)(Zr(y)Ti(1-y))(1-x / 4)O3, 0 < x < 0.20, and 0.20 < y < 1.0 to cause epitaxial growth of a perovskite single phase. As a result, even when the PLZT film of the first layer is separated in an island shape, it is possible to fill the gap without creating voids between the islands, and the surface of the PLZT film of the second layer and subsequent layers becomes smooth, suppressing scattering. Incidentally, the refractive indices of the PLZT film of the first layer and the PLZT film of the second layer and subsequent layers are adjusted by appropriately adjusting the amounts of x and y to make the refractive index difference, for example, 0.01 or less, in order to prevent scattering at the interface.

[0076] Figs. 16A to 16E are explanatory diagrams showing an example of the manufacturing process of the RF modulation section 12A of the optical modulator 5A. Fig. 16A is an explanatory diagram showing an example of the manufacturing process of the RF modulation section, and Fig. 16B is an explanatory diagram showing an example of the manufacturing process (adhesion layer) of the RF modulation section. In Figs. 16A and 16B, an adhesion layer 556A is laminated on the surface of the first cladding layer 54 of the Si optical integration circuit wafer 500, and the PLZT layer 552A of the thin film PLZT substrate wafer 550A is wafer-bonded on the adhesion layer 556A.

[0077] 16C is an explanatory diagram showing an example of a manufacturing process of the RF modulation section 12A (adjusting the dimension between the bottom surface of the PLZT layer 552A and the top surface of the first Si waveguide 21). For convenience of explanation, it is assumed that the thickness of the first Si waveguide 21 is, for example, 220 nm and the thickness of the PLZT layer 552A is, for example, 1 μm. The thickness between the top surface of the first Si waveguide 21 and the bottom surface of the PLZT layer 552A is, for example, 100 nm to 300 nm. Since the thickness between the top surface of the first Si waveguide 21 and the bottom surface of the PLZT layer 552A is, for example, 100 nm to 300 nm, it is possible to couple light from the first Si waveguide 21 to the PLZT waveguide 31A.

[0078] 16D is an explanatory diagram showing an example of a manufacturing process (polishing process) for the RF modulation unit 12A. After bonding the thin-film PLZT substrate wafer 550A onto the first cladding layer 54 of the Si optical integrated circuit wafer 500 via an adhesion layer 556A, the sapphire substrate 551A is removed using a polishing process, leaving behind the PLZT layer 552A in the thin-film PLZT substrate wafer 550A.

[0079] 16E is an explanatory diagram showing an example of a manufacturing process (thin-film PLZT substrate formation) for the RF modulation section 12A. After polishing, a PLZT layer 552A constituting the thin-film PLZT substrate 55A is polished to form a convex PLZT waveguide 31A and a PLZT slab 558A on the surface of the PLZT layer 552A using photolithography. The thin-film PLZT substrate 55A is then formed on the first cladding layer 54 of the Si integrated optical circuit wafer 500. The PLZT slab 558A of the thin-film PLZT substrate 55A can strengthen the adhesive strength between the first cladding layer 54 on the Si integrated optical circuit wafer 500 and the thin-film PLZT substrate 55A.

[0080] Then, a second clad layer 56 is laminated on the thin-film PLZT substrate 55A and the PLZT waveguide 31A. The RF modulation section 12A is formed by laminating the second clad layer 56 on the thin-film PLZT substrate 55A, and the signal electrode 32A and a pair of ground electrodes 53A with a CPW structure laminated on the second clad layer 56. As a result, the RF modulation section 12A is completed.

[0081] FIG. 17 is an explanatory diagram showing an example of the results of a comparison between the LN waveguide 121 and the PLZT waveguide 31A in terms of the drive voltage versus electrode length. In FIG. 17, the vertical axis represents the drive voltage, and the horizontal axis represents the electrode length of the signal electrode. The electrode length is the length of the signal electrode. The LN waveguide 121 is the waveguide of the RF modulation unit 120 shown in FIG. 20. The PLZT waveguide 31A is the waveguide of the RF modulation unit 12A shown in FIG. 12. To ensure the same refractive index, a drive voltage of "1" is required when the electrode length of the signal electrode 32A of the PLZT waveguide 31A is 21 mm, whereas a drive voltage of "3" is required when the electrode length of the signal electrode 122 of the LN waveguide 121 is 21 mm. Therefore, the PLZT waveguide 31A has approximately three times the electro-optic effect of the LN waveguide 121.

[0082] The optical modulator 5A of the second embodiment includes a Si substrate 51 and a PLZT waveguide 31A formed by a thin-film PLZT substrate 55A laminated on the Si substrate 51. The optical modulator 5A further includes a second cladding layer 56 laminated on the PLZT waveguide 31A, and a signal electrode 32A and a ground electrode 53A with a CWP structure formed on the second cladding layer 56. As a result, the optical modulator 5A uses a PLZT waveguide 31A with a greater electro-optic effect than an LN waveguide, thereby improving modulation efficiency and enabling miniaturization and a lower driving voltage. The optical modulator 5A uses a thin-film PLZT substrate 55A made of PLZT single crystal, which has a greater electro-optic effect than LN, and the PLZT single crystal has an electro-optic coefficient three times greater than that of an LN single crystal.

[0083] The optical modulator 5A includes a thin-film PLZT substrate 55A, a second cladding layer 56 laminated on the thin-film PLZT substrate 55A and covering the PLZT waveguide 31A, and a signal electrode 32A and a ground electrode 53A with a CWP structure formed on the second cladding layer 56. The signal electrode 32A generates an electric field in the width direction (Z direction) of the Si substrate 51 within the PLZT waveguide 31A. The crystal orientation of the PLZT waveguide 31A is the width direction (Z direction). In other words, the crystal orientation of the PLZT waveguide 31A is the same as the electric field direction, improving the electric field application efficiency and reducing the driving voltage, thereby significantly improving modulation efficiency. Furthermore, the use of PLZT increases modulation efficiency (voltage × electrode length). As a result, lower voltages and smaller sizes are possible. Moreover, because higher modulation efficiency can be achieved compared to LN even with a shorter electrode length, the optical modulator 5A can be made more compact by shortening the electrode length.

[0084] The optical modulator 5A has a first cladding layer 54 that covers the first Si waveguide 21 and a second cladding layer 56 that covers the thin-film PLZT substrate 55A. The thickness of the first cladding layer 54 between the top surface of the first Si waveguide 21 and the bottom surface of the thin-film PLZT substrate 55A is set to within 100 nm to 300 nm, thereby optically coupling the first Si waveguide 21 and the PLZT waveguide 31A. As a result, the first Si waveguide 21 and the PLZT waveguide 31A can be optically coupled.

[0085] The first cladding layer 54 is made of a dielectric material or resin with a low refractive index, and as a result, the first cladding layer 54 and the thin-film PLZT substrate 55A can be bonded together.

[0086] The optical modulator 5A has a dielectric 57A such as an SiO2 film, which is a dielectric, formed on the back surface of the Si substrate 51. As a result, the influence of warping due to thermal history during the manufacturing process of the optical modulator 5A can be suppressed.

[0087] In the RF modulation section 12A of the optical modulator 5A of the second embodiment, a PLZT layer 552A is bonded to an optical integrated circuit wafer 500 via an adhesive layer 556A, as shown in FIG. 16D. However, if the warpage of the optical integrated circuit wafer 500 increases due to thermal history or the like during the manufacturing process of the optical modulator 5A, the optical integrated circuit wafer 500 may become unable to be adsorbed to a wafer stage in an exposure apparatus or the like used for pattern formation of the optical modulator 5A. To address this issue, as shown in FIG. 18, a dielectric film 57A such as an SiO film is formed on the back surface of the Si substrate 51 of the Si optical integrated circuit wafer 500, thereby suppressing warpage of the Si optical integrated circuit wafer 500. As a result, it is possible to avoid a situation in which the optical integrated circuit wafer 500 may become unable to be adsorbed to a wafer stage in an exposure apparatus or the like used for pattern formation of the optical modulator 5A.

[0088] Also, the RF modulation section 12A has been exemplified as having the PLZT layer 552A of the thin-film PLZT substrate wafer 550A bonded via an adhesive layer 556A onto the first cladding layer 54 of the optical integrated circuit wafer 500. However, even if the adhesive layer 556A is not used, a bonding portion may be formed between the first cladding layer 54 and the PLZT layer 552A, and this can be modified as appropriate.

[0089] The RF modulation section 12A shown in Figure 13 illustrates an example of a CPW electrode structure in which a signal electrode 32A is disposed between a pair of ground electrodes 53A on the first cladding layer 54, but the electrode structure is not limited to that shown in Figure 13 and can be modified as appropriate.

[0090] FIG. 19 is a schematic cross-sectional view showing a modified example of the optical modulator 5A taken along the EE line. The RF modulation unit 12A shown in FIG. 19 is formed by etching portions of the second cladding layer 56 on both sides of the PLZT waveguide 31A of the thin-film PLZT substrate 55A. Then, openings 563A and 564A are formed in the second cladding layer 56, exposing portions of the surface of the PLZT slab 558A of the thin-film PLZT substrate 55A. The ground electrode 53B is formed in the opening 563A so as to protrude beyond the surface of the second cladding layer 56. Furthermore, the signal electrode 32B is formed in the opening 564A so as to protrude beyond the surface of the second cladding layer 56. The signal electrode 32B and the ground electrode 53B are disposed on the side surfaces of the PLZT waveguide 31A. As a result, since the signal electrode 32B and the ground electrode 53B are arranged on both sides of the PLZT waveguide 31A, an electric field is applied in the width direction (left-right direction (Z direction) in the figure), which allows for further improvement in modulation efficiency.

[0091] For the sake of convenience, the optical modulator 5A of the second embodiment illustrates a case in which the first Si waveguide 21 and the PLZT waveguide 31A are directionally coupled. However, the first Si waveguide 21 and the PLZT waveguide 31A may be butt-coupled, and other modifications can be made as appropriate.

[0092] 17 illustrates the relationship between the electrode length and the driving voltage in which the PLZT waveguide 31A has an electro-optic effect three times or more times that of the LN waveguide 121. However, it goes without saying that the PZT waveguide 31 also has the same effect as the PLZT waveguide 31A, and has an electro-optic effect three times or more times that of the LN waveguide 121.

[0093] Although the thin film substrate (50, 55A) is a single crystal perovskite oxide substrate having a larger electro-optic effect than lithium niobate, it may be a polycrystalline perovskite oxide substrate having a larger electro-optic effect than lithium niobate, and may be modified as appropriate. The thin film substrate may also be a thin film without a slab, and may be modified as appropriate.

[0094] The following additional notes are provided regarding the above-described embodiments including the present example.

[0095] (Appendix 1) A silicon substrate, a waveguide formed of a thin film of a perovskite oxide having a larger electro-optic effect than lithium niobate, laminated on the silicon substrate; a clad layer covering the waveguide; a ground electrode at ground potential; a signal electrode disposed opposite the ground electrode and configured to apply a drive voltage to the waveguide; An optical device comprising:

[0096] (Supplementary Note 2) The ground electrode having a ground potential stacked on the silicon substrate; a PZT waveguide formed of thin film PZT (lead zirconate titanate), which is a thin film of the perovskite oxide, stacked on the ground electrode; the signal electrode is disposed on the clad layer at a position facing the ground electrode across the PZT waveguide, and applies the drive voltage to the PZT waveguide; The crystal orientation of the PZT waveguide is The optical device according to claim 1, wherein the electric field direction is the same as that from the signal electrode to the ground electrode.

[0097] (Supplementary Note 3) A silicon waveguide formed on the silicon substrate, 3. The optical device according to claim 2, wherein the silicon waveguide and the PZT waveguide are optically coupled.

[0098] (Appendix 4) A silicon waveguide formed on the silicon substrate; a first clad layer covering the silicon waveguide; a second clad layer that is the clad layer covering the thin film PZT that forms the PZT waveguide; The optical device according to claim 2 or 3, characterized in that the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film PZT is set within a range of 100 nm to 300 nm, and the silicon waveguide and the PZT waveguide are optically coupled.

[0099] (Appendix 5) The optical device according to appendix 4, wherein openings are formed in the second clad layer on both sides of the PZT waveguide, and the signal electrode is disposed on the second clad layer above the PZT waveguide.

[0100] (Appendix 6) An optical device as described in Appendix 4, characterized in that an opening is formed in the second cladding layer and the thin film PZT to expose a portion of the ground electrode, and another ground electrode electrically connected to the ground electrode is disposed in the opening.

[0101] (Supplementary Note 7) An optical input unit that inputs signal light into the PZT waveguide; a modulation unit that applies a drive voltage to the PZT waveguide to change the refractive index of the PZT waveguide and modulates signal light passing through the PZT waveguide; an optical output unit that outputs modulated signal light from the modulation unit, The PZT waveguide is disposed in the modulation section, The silicon waveguide is 5. The optical device according to claim 3, wherein the optical device is disposed at the optical input section and the optical output section.

[0102] (Appendix 8) The ground electrode having a ground potential stacked on the silicon substrate; a BTO waveguide formed of a thin film BTO (barium titanate) that is a thin film of the perovskite oxide laminated on the ground electrode; the signal electrode is disposed on the cladding layer at a position facing the ground electrode across the BTO waveguide, and applies the drive voltage to the BTO waveguide; The crystal orientation of the BTO waveguide is: The optical device according to claim 1, wherein the electric field direction is the same as that from the signal electrode to the ground electrode.

[0103] (Supplementary Note 9) A silicon waveguide formed on the silicon substrate; a first clad layer covering the silicon waveguide; a second cladding layer that is the cladding layer covering the thin film BTO that forms the BTO waveguide; The optical device described in Appendix 8, characterized in that the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film BTO is set to within 100 nm to 300 nm, and the silicon waveguide and the BTO waveguide are optically coupled.

[0104] (Appendix 10) The optical device according to appendix 9, wherein openings are formed in the second cladding layer on both sides of the BTO waveguide, and the signal electrode is disposed on the second cladding layer above the BTO waveguide.

[0105] (Appendix 11) An optical device according to appendix 9, characterized in that an opening is formed in the second cladding layer and the thin film BTO to expose a portion of the ground electrode, and another ground electrode electrically connected to the ground electrode is disposed in the opening.

[0106] (Supplementary Note 12) An optical input unit for inputting signal light into the BTO waveguide; a modulation unit that applies a drive voltage to the BTO waveguide to change the refractive index of the BTO waveguide and modulates the signal light passing through the BTO waveguide; an optical output unit that outputs modulated signal light from the modulation unit, The BTO waveguide is disposed in the modulation section, The silicon waveguide is 10. The optical device according to claim 9, wherein the optical device is disposed at the optical input portion and the optical output portion.

[0107] (Appendix 13) The optical device according to any one of Appendices 8 to 12, wherein a Peltier element is mounted on the surface of the silicon substrate opposite to the layered surface.

[0108] (Appendix 14) A PLZT waveguide, which is the waveguide formed of a thin film PLZT (lanthanum-doped lead zirconate titanate) which is a thin film of the perovskite oxide laminated on the silicon substrate; a cladding layer covering the PLZT waveguide; a signal electrode disposed on the cladding layer and configured to apply the driving voltage to the PLZT waveguide; a pair of ground electrodes disposed on the clad layer so as to sandwich the signal electrode therebetween; The crystal orientation of the PLZT waveguide is The optical device according to claim 1, wherein the electric field direction is the same as that from the signal electrode to the ground electrode.

[0109] (Supplementary Note 15) A silicon waveguide formed on the silicon substrate, 15. The optical device according to claim 14, wherein the silicon waveguide and the PLZT waveguide are optically coupled.

[0110] (Appendix 16) A silicon waveguide formed on the silicon substrate; a first clad layer covering the silicon waveguide; a second cladding layer that is the cladding layer covering the thin film PLZT that forms the PLZT waveguide; The optical device according to claim 14 or 15, characterized in that the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film PLZT is set within 100 nm to 300 nm, and the silicon waveguide and the PLZT waveguide are optically coupled.

[0111] (Appendix 17) The signal electrode and the ground electrode are 17. The optical device of claim 16, wherein the PLZT waveguide is disposed in an opening formed in the second cladding layer at a side portion of the PLZT waveguide.

[0112] (Supplementary Note 18) An optical input section for inputting signal light into the PLZT waveguide; a modulation section that applies a drive voltage to the PLZT waveguide to change the refractive index of the PLZT waveguide and modulates signal light passing through the PLZT waveguide; an optical output unit that outputs modulated signal light from the modulation unit, The PLZT waveguide is disposed in the modulation section, The silicon waveguide is 17. The optical device according to claim 15, wherein the optical device is disposed at the optical input section and the optical output section.

[0113] (Supplementary Note 19) 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 the electrical signal output from the processor; The optical device is a waveguide formed of a thin film of a perovskite oxide having a larger electro-optic effect than lithium niobate, laminated on the silicon substrate; a clad layer covering the waveguide; a ground electrode at ground potential; a signal electrode disposed opposite the ground electrode and configured to apply a drive voltage to the waveguide; An optical communication device comprising: [Explanation of symbols]

[0114] 1 Optical communication equipment 3 DSP 4 light source 5 Optical Modulator 5A Optical Modulator 21 First Si waveguide 31 PZT waveguide 31A PLZT waveguide 32 signal electrode 32A signal electrode 51 Si substrate 53 Ground electrode 53A ground electrode 54 First cladding layer 55 Thin film PZT substrate 55A thin film PLZT substrate 56 Second cladding layer 563A opening 564A opening

Claims

1. A silicon substrate; a silicon waveguide formed on the silicon substrate; a first clad layer covering the silicon waveguide; a waveguide formed of a thin film of a perovskite oxide having a larger electro-optic effect than lithium niobate, the bottom surface of the thin film being bonded to the surface of the first clad layer, the waveguide having a convex slab at the center; a second clad layer covering the waveguide; a ground electrode at ground potential; a signal electrode disposed opposite the ground electrode and configured to apply a drive voltage to the waveguide; The crystal direction of the waveguide formed of the single crystal of the perovskite oxide is The electric field direction is the same as the electric field direction from the signal electrode to the ground electrode, and The thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film is set within 100 nm to 300 nm, and the silicon waveguide and the thin film are optically coupled. An optical device characterized by:

2. the ground electrode at ground potential stacked on the silicon substrate; a PZT waveguide formed of a thin film of the perovskite oxide, PZT (lead zirconate titanate), laminated on the ground electrode; the signal electrode is disposed on the second clad layer at a position facing the ground electrode across the PZT waveguide, and applies the drive voltage to the PZT waveguide; The crystal orientation of the PZT waveguide is: The electric field direction is the same as the electric field direction from the signal electrode to the ground electrode, and 2. The optical device according to claim 1, wherein the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film PZT is set within 100 nm to 300 nm, and the silicon waveguide and the PZT waveguide are optically coupled.

3. 3. The optical device according to claim 2, wherein openings are formed in the second cladding layer on both sides of the PZT waveguide, and the signal electrode is disposed on the second cladding layer above the PZT waveguide.

4. 3. The optical device according to claim 2, wherein an opening is formed in the second cladding layer and the thin film PZT to expose a portion of the ground electrode, and another ground electrode electrically connected to the ground electrode is disposed in the opening.

5. an optical input section for inputting signal light into the PZT waveguide; a modulation unit that applies a drive voltage to the PZT waveguide to change the refractive index of the PZT waveguide and modulates signal light passing through the PZT waveguide; an optical output unit that outputs modulated signal light from the modulation unit, The PZT waveguide is disposed in the modulation section, The silicon waveguide is The optical device according to claim 2 , wherein the optical input section and the optical output section are arranged.

6. the ground electrode at ground potential stacked on the silicon substrate; a BTO (barium titanate) waveguide, which is the waveguide formed of a thin film BTO (barium titanate) that is a thin film of the perovskite oxide laminated on the ground electrode; the signal electrode is disposed on the second clad layer at a position facing the ground electrode across the BTO waveguide, and applies the drive voltage to the BTO waveguide; The crystal orientation of the BTO waveguide is: The electric field direction is the same as the electric field direction from the signal electrode to the ground electrode, and 2. The optical device according to claim 1, wherein the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film BTO is set within a range of 100 nm to 300 nm, and the silicon waveguide and the BTO waveguide are optically coupled.

7. 7. The optical device according to claim 6, wherein openings are formed in the second cladding layer on both sides of the BTO waveguide, and the signal electrode is disposed on the second cladding layer above the BTO waveguide.

8. 7. The optical device according to claim 6, wherein an opening is formed in the second cladding layer and the thin film BTO to expose a portion of the ground electrode, and another ground electrode electrically connected to the ground electrode is disposed in the opening.

9. an optical input section for inputting signal light into the BTO waveguide; a modulation unit that applies a drive voltage to the BTO waveguide to change the refractive index of the BTO waveguide and modulates signal light passing through the BTO waveguide; an optical output unit that outputs modulated signal light from the modulation unit, The BTO waveguide is disposed in the modulation section, The silicon waveguide is The optical device according to claim 6 , wherein the optical input section and the optical output section are disposed on the optical input section and the optical output section.

10. 10. The optical device according to claim 6, wherein a Peltier element is mounted on a surface of the silicon substrate opposite to the layered surface.

11. a PLZT waveguide formed of a thin film PLZT (lanthanum-doped lead zirconate titanate) which is a thin film of the perovskite oxide laminated on the silicon substrate; the second clad layer covering the PLZT waveguide; a signal electrode disposed on the second clad layer and configured to apply the driving voltage to the PLZT waveguide; a pair of ground electrodes disposed on the second clad layer so as to sandwich the signal electrode therebetween; The crystal orientation of the PLZT waveguide is The electric field direction is the same as the electric field direction from the signal electrode to the ground electrode, and 2. The optical device according to claim 1, wherein the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film PLZT is set within 100 nm to 300 nm, and the silicon waveguide and the PLZT waveguide are optically coupled.

12. The signal electrode and the ground electrode are 12. The optical device according to claim 11, wherein the PLZT waveguide is disposed in an opening formed in the second cladding layer at a side portion of the PLZT waveguide.

13. an optical input section for inputting signal light into the PLZT waveguide; a modulation unit that applies a drive voltage to the PLZT waveguide to change the refractive index of the PLZT waveguide and modulates signal light passing through the PLZT waveguide; an optical output unit that outputs modulated signal light from the modulation unit, The PLZT waveguide is disposed in the modulation section, The silicon waveguide is The optical device of claim 11 , wherein the optical input and output are disposed at the optical input and output.

14. 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 the electrical signal output from the processor; The optical device is A silicon substrate; a silicon waveguide formed on the silicon substrate; a first clad layer covering the silicon waveguide; a waveguide formed of a thin film of a perovskite oxide having a larger electro-optic effect than lithium niobate, the bottom surface of the thin film being bonded to the surface of the first clad layer, the waveguide having a convex slab at the center; a second clad layer covering the waveguide; a ground electrode at ground potential; a signal electrode disposed opposite the ground electrode and configured to apply a drive voltage to the waveguide; The crystal direction of the waveguide formed of the single crystal of the perovskite oxide is The electric field direction is the same as the electric field direction from the signal electrode to the ground electrode, and An optical communication device, characterized in that the thickness of the first clad layer between the top surface of the silicon waveguide and the bottom surface of the thin film is set to within 100 nm to 300 nm, and the silicon waveguide and the waveguide are optically coupled.

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