Optical devices and optical communication devices

Abstract

To provide an optical device and the like capable of suppressing the occurrence of DC drift.SOLUTION: An optical device includes: a rib waveguide of a thin-film LN (Lithium Niobate) crystal; a buffer layer laminated on the rib waveguide; and an electrode for applying voltage to the rib waveguide. The buffer layer has: a thick-film portion that is laminated on a rib of the rib waveguide; and thin-film portions that are laminated on slabs of the rib waveguide situated on both sides of the rib, and made thinner than the thick-film portion.SELECTED DRAWING: Figure 3

Description

[Technical field]

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

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

[0003] In the optical modulator, for example, a four-channel Mach-Zehnder modulator is integrated. Each Mach-Zehnder interferometer has an RF modulation section and a DC modulation section. A high-frequency signal having a band of, for example, several tens of GHz is input to the electrode of the RF modulation section to perform high-speed modulation. A bias voltage is applied to the electrode of the DC modulation section, and the bias voltage is adjusted so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal.

[0004] The optical waveguide of the optical modulator constitutes, for example, a Mach-Zehnder interferometer, and outputs, for example, an XY polarization multiplexed IQ signal due to the optical phase difference between multiple optical waveguides arranged in parallel. Then, two channels of output from the four channels are combined to generate two IQ signals, one of which is subjected to polarization rotation and polarization multiplexed by a polarization beam combiner before being output.

[0005] On the other hand, there is a diffused optical waveguide, which is formed at a position where it does not overlap with the signal electrode by diffusing a metal such as titanium from the substrate surface. An electrode is placed on a buffer layer that covers the diffused optical waveguide. The composition and thickness of the buffer layer are determined so that the resistance value is low in order to suppress DC drift (change over time of the emitted light caused by the applied bias voltage). However, this diffused optical waveguide has a low light confinement, so the application efficiency of the electric field is poor, and therefore the driving voltage is high. Therefore, there is a thin film optical waveguide, in which an optical waveguide using a thin film of LN (Lithium Niobate: LiNbO3) crystal is formed at a position where it does not overlap with the signal electrode. The thin film optical waveguide can confine light more strongly than the diffused optical waveguide in which a metal is diffused, improve the application efficiency of the electric field, and reduce the driving voltage.

[0006] The optical modulator has an RF modulation section and a DC modulation section. Fig. 7 is a schematic cross-sectional view showing an example of a DC modulation section 200 of an optical modulator. The DC modulation section 200 shown in Fig. 7 has a support substrate 201 such as Si (silicon) and an intermediate layer 202 laminated on the support substrate 201. Furthermore, the DC modulation section 200 has a thin-film LN substrate 203 laminated on the intermediate layer 202, and a buffer layer 204 of SiO2 laminated on the thin-film LN substrate 203.

[0007] The thin-film LN substrate 203 is a thin-film optical waveguide 207 having a convex shape protruding upward. The thin-film optical waveguide 207 is a rib-type waveguide having a rib 207A and a slab 207B formed on both sides of the rib 207A. The rib 207A and the slab 207B are covered with a buffer layer 204, and a signal electrode 205 and a pair of ground electrodes 206 having a coplanar (CPW) structure are arranged on the surface of the buffer layer 204. That is, the signal electrode 205 and a pair of ground electrodes 206 sandwiching the signal electrode 205 are arranged on the buffer layer 204. The buffer layer 204 can prevent the light propagating through the thin-film optical waveguide 207 from being absorbed by the signal electrode 205 and the ground electrodes 206.

[0008] A convex thin-film optical waveguide 207 is formed on the thin-film LN substrate 203 located between the signal electrode 205 and the ground electrode 206. Furthermore, the buffer layer 204 located between the signal electrode 205 and the ground electrode 206 also has a step portion 204A that covers the entire convex thin-film optical waveguide 207.

[0009] With such a thin film optical waveguide 207, a bias voltage is applied to the signal electrode 205 to generate an electric field, and the refractive index of the thin film optical waveguide 207 is changed, thereby modulating the light propagating through the thin film optical waveguide 207. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 5-158001 [Patent Document 2] US Patent Application Publication No. 2002 / 0106141 Summary of the Invention [Problem to be solved by the invention]

[0011] In the thin film optical waveguide 207 of the optical modulator, the buffer layer 204 serves as a cladding, and therefore the mode field of the light propagating through the thin film optical waveguide 207 penetrates into the buffer layer 204. Therefore, in order to prevent the light of the thin film optical waveguide 207 from being absorbed by the signal electrode 205 and the ground electrode 206, it is necessary to make the buffer layer 204 thick.

[0012] However, even if the thickness of the buffer layer 204 is increased, the resistance value of the buffer layer 204 becomes higher than that of LN crystal when the temperature rises. Therefore, even if a bias voltage is applied to the signal electrode 205, the bias voltage applied to the buffer layer 204 gradually increases with time, while a DC drift occurs in which the bias voltage applied to the rib 207A of the thin film optical waveguide 207 relatively decreases. As a result, the electric field gradually decreases, making it impossible to normally control the optical modulator. Therefore, it is necessary to suppress the occurrence of the DC drift.

[0013] The disclosed technology has been made in view of the above points, and has an object to provide an optical device etc. that can suppress the occurrence of DC drift. [Means for solving the problem]

[0014] In one embodiment, the optical device disclosed in the present application includes a rib waveguide made of a thin-film LN (lithium niobate) crystal, a buffer layer laminated on the rib waveguide, and an electrode for applying a voltage to the rib waveguide. The buffer layer includes a thick film portion laminated on the rib of the rib waveguide, and thin film portions on both sides of the rib that are laminated on a slab of the rib waveguide and are thinner than the thick film portions. Effect of the Invention

[0015] According to one aspect of the optical device etc. disclosed in the present application, the occurrence of DC drift can be suppressed. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a block diagram illustrating an example of a configuration of an optical communication device according to a first embodiment. [Diagram 2] FIG. 2 is a schematic plan view illustrating an example of the configuration of the optical modulator according to the first embodiment. [Diagram 3] FIG. 3 is a schematic cross-sectional view illustrating an example of a first DC modulation unit of the optical modulator according to the first embodiment. [Figure 4]FIG. 4 is a schematic cross-sectional view illustrating an example of a first DC modulation unit of an optical modulator according to a second embodiment. [Diagram 5] FIG. 5 is a schematic cross-sectional view illustrating an example of a first DC modulation unit of an optical modulator according to a third embodiment. [Figure 6] FIG. 6 is a schematic cross-sectional view illustrating an example of a first DC modulation unit of an optical modulator according to a fourth embodiment. [Figure 7] FIG. 7 is a schematic cross-sectional view showing an example of a DC modulation section of an optical modulator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] 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 the embodiments. EXAMPLES

[0018] FIG. 1 is a block diagram showing an example of the configuration of an optical communication device 1 according to a first 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 executes digital signal processing. For example, the DSP 3 executes 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 acquires an electrical signal including reception data from the optical receiver 6, and executes processing such as decoding the acquired electrical signal to obtain reception data.

[0019] 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 obtained optical transmission signal to the optical fiber 2A. The optical modulator 5 is, for example, an optical device such as an LN optical modulator including an LN (Lithium Niobate: LiNbO3) optical waveguide and a modulation section. The LN optical waveguide is formed of an LN crystal substrate. The optical modulator 5 generates an optical transmission signal by modulating the light supplied from the light source 4 with an electrical signal input to the modulation section when the light propagates through the LN optical waveguide.

[0020] The optical receiver 6 receives an optical signal from the optical fiber 2B and demodulates the received optical signal using the light supplied from the light source 4. Then, the optical receiver 6 converts the demodulated received optical signal into an electrical signal and outputs the converted electrical signal to the DSP 3.

[0021] FIG. 2 is a schematic plan view showing an example of the configuration of the optical modulator 5 of the first embodiment. The optical modulator 5 shown in FIG. 2 has an optical fiber 4A from a light source 4 connected to the input side, and an optical fiber 2A for sending a transmission signal connected to the output side. The optical modulator 5 has a first optical input unit 11, an RF (Radio Frequency) modulation unit 12, a DC (Direct Current) modulation unit 13, and a first optical output unit 14. The first optical input unit 11 has a first optical waveguide 11A and a first waveguide joint unit 11B. The first optical waveguide 11A has one optical waveguide connected to the optical fiber 4A, two optical waveguides branching from the one optical waveguide, four optical waveguides branching from each of the two optical waveguides, and eight optical waveguides branching from each of the four optical waveguides. The first waveguide junction 11B joins the eight optical waveguides in the first optical waveguide 11A and the eight LN optical waveguides in the LN optical waveguide .

[0022] The RF modulation section 12 has an LN optical waveguide 21, an electrode section 22, and an RF terminator 23. When light supplied from the first optical waveguide 11A propagates through the LN optical waveguide 21, the RF modulation section 12 modulates the light by an electric field applied from a signal electrode 22A of the electrode section 22. The LN optical waveguide 21 is, for example, a rib-type optical waveguide formed using a thin-film LN substrate 53, and has eight parallel LN optical waveguides that repeatedly branch from the input side. The light propagated through the LN optical waveguide 21 and modulated is output to a first DC modulation section 32 in the DC modulation section 13. The thin-film LN substrate 53 is an X-cut substrate that has a high refractive index when a DC voltage is applied in the direction of the X-axis of the crystal.

[0023] The signal electrode 22A in the electrode section 22 is provided at a position not overlapping with the LN optical waveguide 21, and applies an electric field to the LN optical waveguide 21 in response to an electrical signal output from the DSP 3. The terminal end of the signal electrode 22A in the electrode section 22 is connected to an RF terminator 23. The RF terminator 23 is connected to the terminal end of the signal electrode 22A, and prevents unnecessary reflection of the signal transmitted by the signal electrode 22A.

[0024] The DC modulation unit 13 has an LN optical waveguide 31 joined to the LN optical waveguide 21 of the RF modulation unit 12, a first DC modulation unit 32, and a second DC modulation unit 33. The first DC modulation unit 32 is four child side MZs (Mach-Zehnder). The second DC modulation unit 33 is two parent side MZs. The first DC modulation unit 32 has an LN optical waveguide 31 and an electrode unit 22.

[0025] The LN optical waveguide 31 has eight LN optical waveguides and four LN optical waveguides that merge with two of the eight LN optical waveguides. The eight LN optical waveguides 31 have a first DC modulation unit 32 arranged for every two LN optical waveguides. The first DC modulation unit 32 applies a bias voltage to the signal electrode 22A on the LN optical waveguide 31, and adjusts the bias voltage so that the ON / OFF of the electrical signal corresponds to the ON / OFF of the optical signal, thereby outputting an I signal of an in-phase axis component or a Q signal of an orthogonal axis component. The four LN optical waveguides in the LN optical waveguide 31 have a second DC modulation unit 33 arranged for every two LN optical waveguides. The second DC modulation unit 33 applies a bias voltage to the signal electrode 22A on the LN optical waveguide 31, adjusts 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.

[0026] The first optical output unit 14 has a second waveguide junction 41, a second optical waveguide 42, a PR (Polarization Rotator) 43, and a PBC (Polarization Beam Combiner) 44. The second waveguide junction 41 joins the LN optical waveguide 31 in the DC modulation unit 13 and the second optical waveguide 42. The second optical waveguide 42 has four optical waveguides connected to the second waveguide junction 41 and two optical waveguides that merge with two of the four optical waveguides.

[0027] The PR43 rotates the I signal or Q signal input from one of the second DC modulation units 33 by 90 degrees to obtain a vertically polarized optical signal after 90-degree rotation. Then, the PR43 inputs the vertically polarized optical signal to the PBC44. The PBC44 multiplexes the vertically polarized optical signal from the PR43 and the horizontally polarized optical signal input from the other second DC modulation unit 33, and outputs a polarization multiplexed signal.

[0028] 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 first DC modulation section 32 of the optical modulator 5 of the first embodiment. Since the second DC modulation section 33 has the same configuration as the first DC modulation section 32, the same components are given the same reference numerals, and the description of the overlapping configuration and operation will be omitted. The first DC modulation section 32 shown in FIG. 3 has a support substrate 51 and an intermediate layer 52 laminated on the support substrate 51. Furthermore, the first DC modulation section 32 has a thin-film LN substrate 53 made of thin-film LN crystal laminated on the intermediate layer 52, a buffer layer 54 laminated on the thin-film LN substrate 53, and an electrode section 22. The electrode section 22 has a signal electrode 22A and a pair of ground electrodes 22B.

[0029] The support substrate 51 is, for example, a substrate made of SiO2 (silicon dioxide), TiO2 (titanium dioxide), Si, LN, etc. The intermediate layer 52 is, for example, a layer made of a transparent material having a refractive index lower than that of LN, such as SiO2 or TiO2. Similarly, the buffer layer 54 is, for example, a layer made of a transparent material having a refractive index lower than that of LN, such as SiO2 or TiO2.

[0030] The thin-film LN substrate 53 is a thin-film optical waveguide 60 in the form of a protrusion protruding upward. The thin-film optical waveguide 60 is the LN optical waveguide 21 of the first DC modulation section 32. The thin-film optical waveguide 60 is a rib-type optical waveguide having a rib 60A and slabs 60B on both sides of the rib 60A. The rib 60A has an upper surface 60A1 and a side wall surface 60A2 of the rib 60A. The thin-film optical waveguide 60 is covered with a buffer layer 54. The buffer layer 54 is provided to prevent the light propagating through the thin-film optical waveguide 60 from being absorbed by the electrode section 22.

[0031] The buffer layer 54 has a thick film portion 54A covering the upper surface 60A1 of the rib 60A of the thin film optical waveguide 60, and a thin film portion 54B covering the slab 60B of the thin film optical waveguide 60. The thickness X2 of the thin film portion 54B is thinner than the thickness X1 of the thick film portion 54A. The signal electrode 22A and the pair of ground electrodes 22B are disposed on the thin film portion 54B.

[0032] The thin film optical waveguide 60 located between the signal electrode 22A and the ground electrode 22B is a rib 60A in the thin film optical waveguide 60. Furthermore, the buffer layer 54 covering the rib 60A in the thin film optical waveguide 60 located between the signal electrode 22A and the ground electrode 22B is a thick film portion 54A. The thin film optical waveguide 60 in which the signal electrode 22A and the ground electrode 22B are located is a slab 60B in the thin film optical waveguide 60. Furthermore, the buffer layer 54 covering the slab 60B in the thin film optical waveguide 60 in which the signal electrode 22A and the ground electrode 22B are located is a thin film portion 54B. The step portion 54D covering the side wall surface 60A2 of the rib 60A is spaced apart from the ground electrode 22B or the signal electrode 22A.

[0033] A thin-film optical waveguide 60 of a thin-film LN substrate 53 having a thickness of 0.5 to 3 μm is sandwiched between the intermediate layer 52 and the buffer layer 54. The width of the rib 60A which becomes the thin-film optical waveguide 60 is, for example, about 1 to 8 μm.

[0034] The signal electrode 22A is an electrode made of a metal material such as gold or copper, and has a width of 2 to 10 μm and a thickness of 1 to 20 μm. The ground electrode 22B is an electrode made of a metal material such as gold or copper, and has a thickness of 1 μm or more. When a bias voltage corresponding to an electric signal output from the DSP 3 is applied to the signal electrode 22A, an electric field is generated in a direction from the signal electrode 22A to the ground electrode 22B, and this electric field is applied to the thin film optical waveguide 60. As a result, the refractive index of the thin film optical waveguide 60 changes in response to the application of the electric field to the thin film optical waveguide 60, and it becomes possible to modulate the light propagating through the thin film optical waveguide 60.

[0035] In the first DC modulation section 32, the buffer layer 54 covering the rib 60A of the thin-film optical waveguide 60 is made into a thick film section 54A, and the buffer layer 54 at the portion where the electrode section 22 is laminated is made into a thin film section 54B. As a result, in the thick film section 54A, the thickness of the buffer layer 54 is increased, so that the scattering loss of light that is guided through the rib 60A and absorbed by the electrode section 22 can be suppressed. Furthermore, in the thin film section 50B, the thickness of the buffer layer 54 is reduced, so that the occurrence of DC drift due to changes over time can be suppressed. In addition, by reducing the thickness of the buffer layer 54 between the electrode section 22 and the thin-film optical waveguide 60 of LN crystal, the influence of the resistance value of the buffer layer 54 when the temperature rises is reduced. As a result, the occurrence of DC drift when the temperature rises can be suppressed.

[0036] Therefore, the applicant verified the 50% arrival time of the DC drift of the first DC modulation section 32 under the following conditions. The conditions of the first DC modulation section 32 were that the thick film portion 54A of the buffer layer 54 had a thickness of 1.2 μm, the thin film portion 54B had a thickness of 0.2 μm, and the environmental temperature was 120° C. The conditions of the conventional DC modulation section 200 were that the buffer layer 204 (204A, 204B) had a thickness of 1.2 μm, and the environmental temperature was 120° C. As a result, the first DC modulation section 32 can extend the 50% arrival time of the DC drift four times longer than the conventional DC modulation section 200.

[0037] In the first DC modulation section 32 of the first embodiment, the buffer layer 54 covering the rib 60A of the thin-film optical waveguide 60 is a thick film section 54A, and the buffer layer 54 at the lamination portion where the electrode section 22 is laminated is a thin film section 54B. As a result, in the thick film section 54A, the thickness of the buffer layer 54 is increased, so that it is possible to suppress the scattering loss of light, such as the light guided in the rib 60A being absorbed by the electrode section 22. In the thin film section 54B, the influence of the resistance value of the buffer layer 54 is reduced, so that it is possible to suppress the occurrence of DC drift when the temperature rises due to aging. Furthermore, by suppressing the occurrence of DC drift, it is possible to extend the life of the first DC modulation section 32.

[0038] In the second DC modulation section 33, the buffer layer 54 covering the rib 60A of the thin-film optical waveguide 60 is a thick film section 54A, and the buffer layer 54 at the lamination section where the electrode section 22 is laminated is a thin film section 54B. As a result, in the thick film section 54A, the thickness of the buffer layer 54 is increased, so that it is possible to suppress the scattering loss of light that passes through the rib 60A and is absorbed by the electrode section 22. Furthermore, in the thin film section 54B, the influence of the resistance value of the buffer layer 54 is reduced, so that it is possible to suppress the occurrence of DC drift when the temperature rises due to aging. Furthermore, by suppressing the occurrence of DC drift, it is possible to extend the life of the second DC modulation section 33.

[0039] In the first DC modulation section 32 of Example 1, the buffer layer 54 covering the upper surface 60A1 of the rib 60A is a thick film portion 54A, but the buffer layer 54 between the electrode portion 22 and the side wall surface 60A2 of the rib 60A is thin. As a result, it is possible that the buffer layer 54 between the electrode portion 22 and the side wall surface 60A2 of the rib 60A is thin, and thus light guided through the rib 60A may be absorbed by the electrode portion 22, increasing the scattering loss of light. Therefore, it is preferable to thicken the buffer layer 54 between the electrode portion 22 and the side wall surface 60A2 of the rib 60A, and an embodiment of this will be described below as Example 2. EXAMPLES

[0040] Fig. 4 is a schematic cross-sectional view showing an example of the first DC modulation section 32A of the optical modulator 5 of the second embodiment. The same components as those of the optical modulator 5 of the first embodiment are given the same reference numerals, and the description of the overlapping components and operations will be omitted. The first DC modulation section 32A shown in Fig. 4 differs from the first DC modulation section 32 shown in Fig. 3 in that the width L2 of the thick film portion 54A is wider than the width L1 of the rib 60A.

[0041] The thick film portion 54A of the buffer layer 54 has a first thick film portion 54A1 that covers the upper surface 60A1 of the rib 60A of the thin film optical waveguide 60, and a second thick film portion 54A2 that covers the side wall surface 60A2 of the rib 60A and covers a part of the slab 60B on the rib 60A side. The second thick film portion 54A2 is structured so that the thickness of the second thick film portion 54A2 is thicker than the thickness of the thin film portion 54B. Furthermore, the width L2 of the thick film portion 54A is wider than the width L1 of the rib 60A. For convenience of explanation, if the thickness of the second thick film portion 54A2 is the same as the thickness of the first thick film portion 54A1, the manufacturing process of the thick film portion 54A can be simplified.

[0042] In the first DC modulation section 32A of the second embodiment, the width L2 of the thick film portion 54A is made wider than the width L1 of the rib 60A, and the second thick film portion 54A2 of the thick film portion 54A covers the side wall surface 60A2 of the rib 60A. As a result, the second thick film portion 54A2 separates the electrode portion 22 from the side wall surface 60A2 of the rib 60A, thereby suppressing the scattering loss of light guided in the rib 60A. Furthermore, in the thin film portion 54B, the thickness of the buffer layer 54 is reduced, so that the occurrence of DC drift due to changes over time can be suppressed.

[0043] In the second DC modulation section 33A, the width L2 of the thick film portion 54A is made wider than the width L1 of the rib 60A, and the second thick film portion 54A2 of the thick film portion 54A covers the side wall surface 60A2 of the rib 60A. As a result, the second thick film portion 54A2 separates the electrode portion 22 from the side wall surface 60A2 of the rib 60A, thereby suppressing the scattering loss of light guided in the rib 60A. Furthermore, in the thin film portion 54B, the thickness of the buffer layer 54 is reduced, so that the occurrence of DC drift due to changes over time can be suppressed.

[0044] In the first DC modulation section 32A of Example 2, the width of the thin film portion 54B is the same as the width of the laminated region of the electrode portion 22 on the thin film portion 54B. Therefore, for example, if there is a misalignment of the mask when laminating the electrode portion 22 on the thin film region 54B in the manufacturing process, a part of the electrode portion 22 may be laminated on the second thick film portion 54A2. Therefore, an embodiment of the optical modulator 5 that deals with such a situation will be described as Example 3. EXAMPLES

[0045] FIG. 5 is a schematic cross-sectional view showing an example of the first DC modulation section 32B of the optical modulator 5 of the third embodiment. The same components as those of the optical modulator 5 of the second embodiment are given the same reference numerals, and the description of the overlapping components and operations is omitted. The first DC modulation section 32B shown in FIG. 5 is different from the first DC modulation section 32A shown in FIG. 4 in that the width of the thin film section 54B is made wider so that one end of the electrode section 22, i.e., the tip section X5, is spaced apart from the second thick film section 54A2. In other words, by providing a gap X8 between the tip section X5 of the electrode section 22 and the tip section X6 of the second thick film section 54A2, the tip section of the thin film section 54B is located closer to the rib 60A than the tip section X5 of the electrode section 22.

[0046] The width of the thin film portion 54B is increased so that the tip portion X5 of the electrode portion 22 is spaced from the second thick film portion 54A2. As a result, even if a mask misalignment occurs when the electrode portion 22 is laminated on the thin film region 54B, it is possible to prevent a part of the electrode portion 22 from being laminated on the second thick film portion 54A2.

[0047] The width of the thin film portion 54B is increased so that the tip portion X5 of the ground electrode 22B is spaced from the second thick film portion 54A2. As a result, even if a mask misalignment occurs when laminating the ground electrode 22B on the thin film region 54B, it is possible to prevent a part of the ground electrode 22B from being laminated on the second thick film portion 54A2.

[0048] The width of the thin film portion 54B is increased so that the tip portion X5 of the signal electrode 22A is spaced from the second thick film portion 54A2. As a result, even if a mask misalignment occurs when laminating the signal electrode 22A on the thin film region 54B, it is possible to prevent a part of the signal electrode 22A from being laminated on the second thick film portion 54A2.

[0049] In the first DC modulation section 32B of the third embodiment, the width of the thin film portion 54B is increased so that one end of the electrode portion 22 is spaced from the second thick film portion 54A2. As a result, even if a mask misalignment occurs when the electrode portion 22 is laminated on the thin film region 54B, it is possible to prevent a part of the electrode portion 22 from being laminated on the second thick film portion 54A2. Furthermore, in the thin film portion 54B, the thickness of the buffer layer 54 is reduced, so that the occurrence of DC drift due to changes over time can be suppressed.

[0050] In the second DC modulation section 33B, the width of the thin film portion 54B is increased so that one end of the electrode portion 22 is separated from the second thick film portion 54A2. As a result, even if a mask misalignment occurs when the electrode portion 22 is laminated on the thin film region 54B, it is possible to prevent a part of the electrode portion 22 from being laminated on the second thick film portion 54A2. Furthermore, in the thin film portion 54B, the thickness of the buffer layer 54 is reduced, so that the occurrence of DC drift due to changes over time can be suppressed.

[0051] In the first DC modulation section 32B of Example 3, the width of the thin film portion 54B is widened so that one end of the electrode portion 22 is separated from the second thick film portion 54A2. The electrode portion 22 needs a certain width to ensure adhesion with the buffer layer 54. However, if the width of the thin film portion 54B that thins the buffer layer 54 according to the width of the electrode portion 22 is made too wide, a lot of etching waste is generated in plasma etching, etc., making the etching work of the buffer layer 54 difficult. Therefore, an embodiment that can deal with such a situation will be described below as Example 4. EXAMPLES

[0052] Fig. 6 is a schematic cross-sectional view showing an example of the first DC modulation section 32 of the optical modulator 5 of the fourth embodiment. The same components as those of the optical modulator 5 of the third embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The first DC modulation section 32C shown in Fig. 6 differs from the first DC modulation section 32B shown in Fig. 5 in that a part 22C of the electrode section 22 is laminated on a third thick film section 54C on the opposite side to the rib 60A, in the buffer layer 54 other than the thin film section 54B.

[0053] Furthermore, if the width of the laminated portion where the electrode portion 22 contacts the thin-film portion 54B is made too narrow, the effect of suppressing the occurrence of DC drift cannot be obtained because the resistance value of the buffer layer 54 becomes large. Therefore, the width L4 of the laminated portion where the electrode portion 22 contacts the thin-film portion 54B is made wider than the thickness L3 of the thin-film portion 54B.

[0054] The first DC modulation section 32C of the fourth embodiment allows a part 22C of the electrode section 22 to be laminated on the third thick film section 54C in addition to the thin film section 54B. Since the width of the thin film section 54B does not need to be increased as compared with the thin film section 54B of the third embodiment, the etching operation of the buffer layer 54 can be simplified. Furthermore, the width L4 of the laminated portion where the electrode section 22 contacts the thin film section 54B is made wider than the thickness L3 of the thin film section 54B. As a result, the resistance value of the buffer layer 54 can be reduced, and the effect of suppressing DC drift can be ensured.

[0055] The second DC modulation section 33C allows a part 22C of the electrode section 22 to be laminated on the third thick film section 54C in addition to the thin film section 54B. Since the width of the thin film section 54B does not need to be increased as compared to the thin film section 54B of the third embodiment, the etching operation of the buffer layer 54 can be simplified. Furthermore, the width L4 of the laminated portion where the electrode section 22 contacts the thin film section 54B is made wider as compared to the thickness L3 of the thin film section 54B. As a result, the resistance value of the buffer layer 54 can be reduced, and the effect of suppressing DC drift can be ensured.

[0056] The following supplementary notes are further disclosed regarding the above-described embodiments including the present example.

[0057] (Note 1) A rib waveguide made of thin-film LN (Lithium Niobate) crystal, a buffer layer laminated on the rib waveguide; an electrode for applying a voltage to the rib waveguide; The buffer layer is a thick film portion laminated on a rib of the rib waveguide; thin film portions laminated on the slab of the rib waveguide on both sides of the rib and having a thickness smaller than that of the thick film portions; An optical device comprising:

[0058] (Appendix 2) The thick film portion is 2. The optical device according to claim 1, characterized in that the width of the thick film portion is wider than the width of the rib.

[0059] (Appendix 3) The thick film portion is a first thick film portion covering the rib; The optical device described in Appendix 2 is characterized in that it has a second thick film portion that covers the side wall surface of the rib and also covers a part of the slab on the rib side, and the thickness of the second thick film portion is made thicker than the thickness of the thin film portion.

[0060] (Appendix 4) The optical device according to appendix 3, wherein the width of the thin film portion is increased so that one end of the electrode is spaced apart from the second thick film portion.

[0061] (Appendix 5) A part of the electrode is 5. The optical device of claim 4, wherein the buffer layer other than the thin film portion is laminated in a third thick film portion different from the first thick film portion and the second thick film portion.

[0062] (Appendix 6) The width of the laminated portion where the electrode is in contact with the thin film portion is 6. The optical device according to claim 5, wherein the width of the thin film portion is wider than the thickness of the thin film portion.

[0063] (Appendix 7) The electrode is 7. The optical device according to claim 1, wherein the electrode is a direct current (DC) electrode.

[0064] (Appendix 8) The electrode is 7. The optical device according to any one of claims 1 to 6, which is an RF (Radio Frequency) electrode.

[0065] (Additional Note 9) The buffer layer is 7. The optical device according to any one of claims 1 to 6, characterized in that the optical device is made of a material whose resistance value becomes higher as the temperature increases compared to the resistance value of the thin film LN crystal.

[0066] (Appendix 10) A processor that performs signal processing on an electrical signal; A light source that generates light; an optical device that modulates light generated from the light source using an electrical signal output from the processor; The optical device comprises: A rib waveguide made of thin-film LN (Lithium Niobate) crystal, a buffer layer laminated on the rib waveguide; an electrode for applying a voltage to the rib waveguide; The buffer layer is a thick film portion laminated on a rib of the rib waveguide; thin film portions laminated on the slab of the rib waveguide on both sides of the rib and having a thickness smaller than that of the thick film portions; An optical communication device comprising: [Explanation of symbols]

[0067] 1 Optical communication equipment 3. DSP 4 light source 5 Optical Modulator 22 Electrode section 22A signal electrode 22B Ground electrode 32, 32A, 32B, 32C First DC modulation section 33, 33A, 33B, 33C Second DC modulation section 54 Buffer Layer 54A Thick film area 54A1 First thick film section 54A2 Second thick film section 54B Thin film part 54C Third thick film section 60 Thin-film optical waveguide 60A Rib 60A2 Side wall 60B Slab

Claims

1. A rib waveguide made of a thin-film LN (Lithium Niobate) crystal; a buffer layer laminated on the rib waveguide; a signal electrode adjacent to the rib waveguide and configured to input an electrical signal for applying a voltage to the rib waveguide; a ground electrode adjacent to the rib waveguide and for outputting the electrical signal from the rib waveguide; The buffer layer is a thick film portion laminated on a rib of the rib waveguide; thin film portions laminated on the slab of the rib waveguide on both sides of the rib and having a thickness smaller than that of the thick film portions; The thick film portion is a first thick film portion covering the rib; a second thick film portion covering a side wall surface of the rib and a portion of the slab on the rib side; The second thick film portion is An optical device characterized in having a first step portion on a side wall surface of the rib that is spaced apart from a side wall surface on the signal electrode side adjacent to the rib, and a second step portion on the side wall surface of the rib that is spaced apart from a side wall surface on the ground electrode side adjacent to the rib.

2. The thick film portion is 2. The optical device according to claim 1, wherein the width of the thick film portion is wider than the width of the rib.

3. The thick film portion is a first thick film portion covering the rib; 3. The optical device according to claim 2, further comprising a second thick film portion covering the side wall surface of the rib and covering a portion of the slab on the rib side, the second thick film portion being thicker than the thin film portion.

4. 4. The optical device according to claim 3, wherein the width of the thin film portion is increased so that one ends of the signal electrode and the ground electrode are spaced apart from the second thick film portion.

5. The signal electrode and a portion of the ground electrode are 5. The optical device according to claim 4, wherein the buffer layer other than the thin film portion is laminated in a third thick film portion different from the first thick film portion and the second thick film portion.

6. The width of the laminated portion where the signal electrode and the ground electrode are in contact with the thin film portion is 6. The optical device according to claim 5, wherein the width of the thin film portion is larger than the thickness of the thin film portion.

7. A processor that performs signal processing on the electrical signal; A light source that generates light; an optical device that modulates light generated from the light source using an electrical signal output from the processor; The optical device comprises: A rib waveguide made of a thin-film LN (Lithium Niobate) crystal; a buffer layer laminated on the rib waveguide; a signal electrode adjacent to the rib waveguide and configured to input an electrical signal for applying a voltage to the rib waveguide; a ground electrode adjacent to the rib waveguide and for outputting the electrical signal from the rib waveguide; The buffer layer is a thick film portion laminated on a rib of the rib waveguide; thin film portions laminated on the slab of the rib waveguide on both sides of the rib and having a thickness smaller than that of the thick film portions; The thick film portion is a first thick film portion covering the rib; a second thick film portion covering a side wall surface of the rib and a portion of the slab on the rib side; The second thick film portion is a first step portion spaced apart from a side wall surface of the rib adjacent to the signal electrode side, and a second step portion spaced apart from a side wall surface of the rib adjacent to the ground electrode side, An optical communication device comprising:

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