Optical devices and optical communication devices

Abstract

To provide an optical device for improving electric field application efficiency.SOLUTION: An optical device provided herein comprises an optical waveguide, a buffer layer laminated on the optical waveguide, and openings formed through the buffer layer at least at sites near a side face of the optical waveguide. The optical device further comprises electrodes stacked on the openings to apply a signal to the optical waveguide, and a silicon layer laminated on the buffer layer excluding the openings.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, and when a voltage is applied to the signal electrode, an electric field is generated in the optical waveguide, which changes the refractive index of the optical waveguide and the phase of the light. The optical waveguide constitutes a Mach-Zehnder interferometer, and the optical output changes depending on the difference in the phase of the light between the optical waveguides.

[0003] 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. However, this diffused optical waveguide has low light confinement, so the efficiency of applying an electric field is poor, and the driving voltage is therefore high. Therefore, there is a thin film optical waveguide, in which an optical waveguide using a thin film of LN (lithium niobate) 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 a diffused optical waveguide in which a metal is diffused, improve the efficiency of applying an electric field, and reduce the driving voltage.

[0006] The optical modulator has an RF modulation section and a DC modulation section. Fig. 9 is a schematic cross-sectional view showing an example of an RF modulation section 200 of an optical modulator. The RF modulation section 200 shown in Fig. 9 has a support substrate 201 such as Si (silicon) and an intermediate layer 202 laminated on the support substrate 201. Furthermore, the RF 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] A thin-film optical waveguide 207 having a convex shape protruding upward is formed on the thin-film LN substrate 203. The thin-film LN substrate 203 and the thin-film optical waveguide 207 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 disposed 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 disposed on the buffer layer 204. The buffer layer 204 can prevent 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. The convex thin-film optical waveguide 207 has a side wall surface 207A and a flat surface 207B. 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] According to such a thin film optical waveguide 207, it is possible to modulate the light propagating through the thin film optical waveguide 207 by applying a high frequency signal to the signal electrode 205 to generate an electric field and changing the refractive index of the thin film optical waveguide 207. The DC modulation section also has almost the same structure as the RF modulation section 200, although it differs in that a bias voltage is applied to the signal electrode 205 instead of a high frequency signal, and therefore a description of the configuration and operation thereof will be omitted. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2013-37243 A 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 increase the thickness of the buffer layer 204. However, when the thickness of the buffer layer 204 is increased, the electric field applied to the thin film optical waveguide 207 becomes smaller, and the efficiency of applying the electric field decreases.

[0012] The disclosed technology has been made in consideration of the above points, and aims to provide an optical device or the like that improves the efficiency of applying an electric field. [Means for solving the problem]

[0013] In one embodiment, the optical device disclosed in the present application includes an optical waveguide, a buffer layer stacked on the optical waveguide, and an opening formed in the buffer layer at least on a portion near a side surface of the optical waveguide. The optical device further includes an electrode stacked on the opening and configured to apply a signal to the optical waveguide, and a silicon layer stacked on the buffer layer except for the opening. Effect of the Invention

[0014] According to one aspect of the optical device etc. disclosed in the present application, it is possible to improve the efficiency of application of an electric field. [Brief description of the drawings]

[0015] [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 showing an example of an RF modulation section of the optical modulator according to the first embodiment. [Figure 4] FIG. 4 is a schematic cross-sectional view showing an example of an RF modulation section of an optical modulator according to a second embodiment. [Diagram 5] FIG. 5 is a schematic cross-sectional view showing an example of an RF modulation section of an optical modulator according to a third embodiment. [Figure 6] FIG. 6 is a schematic cross-sectional view showing an example of an RF modulation section of an optical modulator according to a fourth embodiment. [Figure 7] FIG. 7 is a schematic cross-sectional view showing an example of an RF modulation section of an optical modulator according to a fifth embodiment. [Figure 8] FIG. 8 is a schematic cross-sectional view showing an example of an RF modulation section of an optical modulator of a comparative example. [Figure 9] FIG. 9 is a schematic cross-sectional view showing an example of an RF modulation section of an optical modulator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In the RF modulation section 200 of the optical modulator shown in FIG. 9, when the thickness of the buffer layer 204 is increased, the distance between the thin film optical waveguide 207 and the signal electrode 205 (ground electrode 206) increases, and the electric field applied from the signal electrode 205 to the thin film optical waveguide 207 becomes smaller. As a result, the efficiency of the electric field application decreases. Therefore, an RF modulation section 100 of the optical modulator of a comparative example has been devised that can improve the efficiency of the electric field application even when the thickness of the buffer layer 204 is increased. Comparative Example

[0017] Fig. 8 is a schematic cross-sectional view showing an example of an RF modulation section 100 of an optical modulator of a comparative example. The RF modulation section 100 shown in Fig. 8 has a support substrate 101 and an intermediate layer 102 laminated on the support substrate 101. The RF modulation section 100 further has a thin-film LN substrate 103 laminated on the intermediate layer 102, a buffer layer 104 laminated on the thin-film LN substrate 103, a silicon layer 105 laminated on the buffer layer 104, and an electrode section 106. The electrode section 106 has a signal electrode 106A and a pair of ground electrodes 106B.

[0018] The thin-film LN substrate 103 is formed with a thin-film optical waveguide 160 that is a protruding stripe protruding upward. The thin-film LN substrate 103 and the thin-film optical waveguide 160 are covered with a buffer layer 104. The buffer layer 104 is provided to prevent light propagating through the thin-film optical waveguide 160 from being absorbed by the electrode portion 106. A first opening 103A is formed in a part (slab) of the thin-film LN substrate 103 near the thin-film optical waveguide 160. A second opening 104A communicating with the first opening 103A is formed in a part of the buffer layer 104 on the first opening 103A formed in the part (slab) of the thin-film LN substrate 103. Furthermore, a third opening 102A communicating with the first opening 103A is formed in a part of the intermediate layer 102 below the first opening 103A. The first opening 103A, the second opening 104A, and the third opening 102A form an opening 170.

[0019] Furthermore, the silicon layer 105 is provided to prevent localization of electric charges generated by temperature changes and to stabilize temperature characteristics. The silicon layer 105 covers the buffer layer 104 and the opening 170. An electrode portion 106 is laminated on the silicon layer 105 in the opening 170.

[0020] A convex thin-film optical waveguide 160 is formed on the thin-film LN substrate 103 located between the signal electrode 106A and the ground electrode 106B. Furthermore, a step portion 104B that covers the entire convex thin-film optical waveguide 160 is also formed on the buffer layer 104 located between the signal electrode 106A and the ground electrode 106B.

[0021] The thin-film LN substrate 103 is sandwiched between the intermediate layer 102 and the buffer layer 104, and a convex thin-film optical waveguide 160 that protrudes upward is formed on the thin-film LN substrate 103. The thin-film LN substrate 103 and the thin-film optical waveguide 160 are covered with the buffer layer 104 and the silicon layer 105, and the signal electrode 106A and the ground electrode 106B are exposed from the portion of the silicon layer 105 that overlaps with the opening 170.

[0022] The signal electrode 106A is an electrode made of a metal material such as gold or copper. The ground electrode 106B is an electrode made of a metal material such as gold or copper. In the RF modulation section 100, a high frequency signal corresponding to an electric signal is transmitted by the signal electrode 106A, whereby an electric field is generated in a direction from the signal electrode 106A to the ground electrode 106B, and this electric field is applied to the thin film optical waveguide 160. As a result, the refractive index of the thin film optical waveguide 160 changes in response to the application of the electric field to the thin film optical waveguide 160, and it becomes possible to modulate the light propagating through the thin film optical waveguide 160.

[0023] Since the ground electrode 106B and the signal electrode 106A are disposed inside the opening 170, the distance between the ground electrode 106B and the thin film optical waveguide 160 and the distance between the signal electrode 106A and the thin film optical waveguide 160 are shortened. As a result, even if the thickness of the buffer layer 54 is increased, the efficiency of application of the electric field can be improved.

[0024] However, the silicon layer 105 is close to the thin-film LN substrate 103 on both sides of the thin-film optical waveguide 160. Since the silicon layer 105 has a high optical refractive index, bringing it close to the thin-film LN substrate 103 attracts light in the thin-film optical waveguide 160, increasing optical loss.

[0025] Therefore, an optical modulator that deals with such a situation will be described below as Example 1. 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. EXAMPLES

[0026] 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.

[0027] 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) 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.

[0028] 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.

[0029] 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 .

[0030] The RF modulation unit 12 has an LN optical waveguide 21, an electrode unit 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 unit 12 modulates the light by an electric field applied from a signal electrode 22A of the electrode unit 22. The LN optical waveguide 21 is an optical waveguide formed, for example, using a thin-film LN substrate 53, and has eight parallel LN optical waveguides that are repeatedly branched from the input side. The light propagated through the LN optical waveguide 21 and modulated is output to a first DC modulation unit 32 in the DC modulation unit 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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 RF modulation section 12 of the optical modulator 5 of the first embodiment. The RF modulation section 12 shown in Fig. 3 has a support substrate 51 and an intermediate layer 52 laminated on the support substrate 51. Furthermore, the RF modulation section 12 has a thin-film LN substrate 53, which is a thin-film substrate, laminated on the intermediate layer 52, a buffer layer 54 laminated on the thin-film LN substrate 53, a silicon layer 55 laminated on the buffer layer 54, and an electrode section 22. The electrode section 22 has a signal electrode 22A and a pair of ground electrodes 22B.

[0037] The thin-film LN substrate 53 is formed with a thin-film optical waveguide 60 having a ridge shape projecting upward. The thin-film optical waveguide 60 is the LN optical waveguide 21 of the RF modulation section 12. The thin-film LN substrate 53 and the thin-film optical waveguide 60 are 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. A first opening 53A is formed near the side of the thin-film optical waveguide 60, for example, in the portions (slabs) of the thin-film LN substrate 53 on both sides. A second opening 54A communicating with the first opening 53A is formed in the portion of the buffer layer 54 on the first opening 53A formed in the portion (slab) of the thin-film LN substrate 53. Furthermore, a third opening 52A communicating with the first opening 53A is formed in the portion of the intermediate layer 52 below the first opening 53A. The first opening 53A, the second opening 54A and the third opening 52A form an opening 70. In the opening 70, the electrode portion 22 is laminated.

[0038] The silicon layer 55 is provided to prevent localization of charges generated by temperature changes and stabilize temperature characteristics. The silicon layer 55 is laminated on the buffer layer 54 at a portion other than the opening 70. A silicon-side opening 55A is formed at a portion of the silicon layer 55 overlapping with the second opening 54A of the buffer layer 54. The silicon layer 55 is etched by patterning the second opening 54A of the buffer layer 54 to form the silicon-side opening 55A.

[0039] A convex thin-film optical waveguide 60 is formed on the thin-film LN substrate 53 located between the signal electrode 22A and the ground electrode 22B. The thin-film optical waveguide 60 is a protruding rib-type optical waveguide provided at a predetermined position on the thin-film LN substrate 53. The convex thin-film optical waveguide 60 has a side wall surface 60A and a flat surface 60B. Furthermore, the buffer layer 54 located between the signal electrode 22A and the ground electrode 22B also has a step portion 54B that covers the entire convex thin-film optical waveguide 60. The step portion 54B that covers the side wall surface 60A of the thin-film optical waveguide 60 separates the ground electrode 22B from the signal electrode 22A.

[0040] Between the intermediate layer 52 and the buffer layer 54, a thin-film LN substrate 53 having a thickness of 0.5 to 3 μm is sandwiched, and a thin-film optical waveguide 60 having a convex shape protruding upward is formed on the thin-film LN substrate 53. The width of the protrusion that becomes the thin-film optical waveguide 60 is, for example, about 1 to 8 μm. The thin-film LN substrate 53 and the thin-film optical waveguide 60 are covered with the buffer layer 54 and the silicon layer 55, and the signal electrode 22A and the ground electrode 22B in the opening 70 are exposed from the silicon-side opening 55A of the silicon layer 55. It is assumed that the silicon layer 55 does not contact the signal electrode 22A or the ground electrode 22B in the silicon-side opening 55A.

[0041] Moreover, it is desirable that the signal electrode 22A is made of a material that has small high frequency loss and is different from the material of the ground electrode 22B.

[0042] 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 high-frequency signal corresponding to an electric signal output from the DSP 3 is transmitted by 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 an 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.

[0043] In the RF modulation section 12 of the first embodiment, the ground electrode 22B and the signal electrode 22A are disposed within the opening 70. In the RF modulation section 12, the distance between the ground electrode 22B and the thin film optical waveguide 60 and the distance between the signal electrode 22A and the thin film optical waveguide 60 are shortened, so that the electric field applied to the thin film optical waveguide 60 is strengthened. As a result, even if the thickness of the buffer layer 54 is increased, the driving voltage can be reduced by improving the efficiency of application of the electric field.

[0044] Moreover, in the RF modulation section 12, unlike the RF modulation section 100 of the comparative example, the silicon layer 55 is not present in the opening 70, and therefore the silicon layer 55 does not come close to the thin-film optical waveguide 60. As a result, it is possible to suppress optical loss caused by the silicon layer 55, such as the silicon layer 55 attracting light in the thin-film optical waveguide 60 and absorbing it in the electrode section 22.

[0045] Moreover, since the buffer layer 54 other than the second opening 54A is covered with the silicon layer 55, localization of charges generated due to temperature changes can be prevented, and the temperature characteristics can be stabilized.

[0046] 3 shows the RF modulation section 12 as an example, the first DC modulation section 32 of the DC modulation section 13 has almost the same configuration as the RF modulation section 12. The RF modulation section 12 and the first DC modulation section 32 differ in that a drive voltage is applied to the signal electrode 22A instead of a high-frequency signal. Therefore, a description of the overlapping configuration and operation will be omitted. The thin film optical waveguide 60 corresponds to the LN optical waveguide 31 of the first DC modulation section 32.

[0047] In the first DC modulation section 32, the ground electrode 22B and the signal electrode 22A are disposed within the opening 70. In the first DC modulation section 32, the distance between the ground electrode 22B and the thin film optical waveguide 60 and the distance between the signal electrode 22A and the thin film optical waveguide 60 are shortened, so that the electric field applied to the thin film optical waveguide 60 is strengthened. As a result, even if the thickness of the buffer layer 54 is increased, the driving voltage can be reduced by improving the efficiency of application of the electric field.

[0048] Moreover, in the first DC modulation section 32, unlike the RF modulation section 100 of the comparative example, there is no silicon layer 55 in the opening 70, and therefore the silicon layer 55 does not come close to the thin-film optical waveguide 60. As a result, it is possible to suppress optical loss caused by the silicon layer 55, such as the silicon layer 55 attracting light in the thin-film optical waveguide 60 and absorbing it in the electrode section 22.

[0049] Similarly, the second DC modulation section 33 of the DC modulation section 13 has almost the same configuration as the RF modulation section 12. The difference between the RF modulation section 12 and the second DC modulation section 33 is that a drive voltage is applied to the signal electrode 22A instead of a high-frequency signal. Therefore, a description of the overlapping configuration and operation will be omitted. The thin film optical waveguide 60 corresponds to the LN optical waveguide 31 of the second DC modulation section 33.

[0050] In the second DC modulation section 33, the ground electrode 22B and the signal electrode 22A are disposed within the opening 70. In the second DC modulation section 33, the distance between the ground electrode 22B and the thin film optical waveguide 60 and the distance between the signal electrode 22A and the thin film optical waveguide 60 are shortened, so that the electric field applied to the thin film optical waveguide 60 is strengthened. As a result, even if the thickness of the buffer layer 54 is increased, the driving voltage can be reduced by improving the efficiency of application of the electric field.

[0051] Moreover, in the second DC modulation section 33, unlike the RF modulation section 100 of the comparative example, the silicon layer 55 is not present in the opening 70, and therefore the silicon layer 55 does not come close to the thin-film optical waveguide 60. As a result, it is possible to suppress optical loss caused by the silicon layer 55, such as the silicon layer 55 attracting light in the thin-film optical waveguide 60 and absorbing it in the electrode section 22.

[0052] In the above, the opening 70 is formed of the first opening 53A, the second opening 54A, and the third opening 52A. However, the first opening 53A and the third opening 52A of the thin-film LN substrate 53 may be eliminated, and the opening 70 may be formed only of the second opening 54A of the buffer layer 54. By shortening the distance between the electrode portion 22 and the thin-film optical waveguide 60 in the opening 70, the efficiency of application of the electric field can be improved, and the driving voltage can be reduced, even if the thickness of the buffer layer 54 is increased. In addition, the opening 70 may be formed of the first opening 53A and the second opening 54A without the third opening 52A, and can be modified as appropriate.

[0053] In the RF modulation section 12 of Example 1, there is no electrical contact between the silicon layer 55 and the signal electrode 22A, and between the silicon layer 55 and the ground electrode 22B. However, it is preferable to transmit the charge generated on the surface of the buffer layer 54 to the electrode section 22 via the silicon layer 55, and this embodiment is described below as Example 2. EXAMPLES

[0054] FIG. 4 is a schematic cross-sectional view showing an example of the RF modulation section 12 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 is omitted. The RF modulation section 12 shown in FIG. 4 differs from the RF modulation section 12 shown in FIG. 3 in that the signal electrode 22A1 or the ground electrode 22B1 exposed from the silicon side opening 55A protrudes from the silicon layer 55, and the signal electrode 22A1 and the silicon layer 55 and the ground electrode 22B1 and the silicon layer 55 are electrically in contact with each other. In the RF modulation section 12, for example, the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode section 22 are aligned to form the electrode section 22 in the opening 70.

[0055] The RF modulation section 12 of the second embodiment has a structure in which electrical contact is established between the signal electrode 22A1 and the silicon layer 55 and between the ground electrode 22B1 and the silicon layer 55, so that the charge generated on the surface of the buffer layer 54 is transmitted to the electrode section 22 via the silicon layer 55. As a result, localization of the charge due to temperature change is avoided, and the temperature characteristics can be stabilized.

[0056] The first DC modulation section 32 electrically contacts between the signal electrode 22A1 and the silicon layer 55 and between the ground electrode 22B1 and the silicon layer 55, and therefore transmits the charge generated on the surface of the buffer layer 54 to the electrode section 22 via the silicon layer 55. As a result, localization of the charge due to temperature change is avoided, and the temperature characteristics can be stabilized.

[0057] The second DC modulation section 33 electrically contacts between the signal electrode 22A1 and the silicon layer 55 and between the ground electrode 22B1 and the silicon layer 55, and therefore transmits the charge generated on the surface of the buffer layer 54 to the electrode section 22 via the silicon layer 55. As a result, localization of the charge due to temperature change is avoided, and the temperature characteristics can be stabilized.

[0058] In the RF modulation section 12 of Example 2, for example, when forming the electrode section 22, if a positional misalignment occurs between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode section 22, it may not be possible to electrically contact between the signal electrode 22A and the silicon layer 55 or between the ground electrode 22B and the silicon layer 55. Therefore, an embodiment of the optical modulator 5 that deals with such a situation will be described as Example 3. EXAMPLES

[0059] FIG. 5 is a schematic cross-sectional view showing an example of the RF modulation section 12 of the optical modulator 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 RF modulation section 12 shown in FIG. 5 is different from the RF modulation section 12 shown in FIG. 4 in that the electrode section 22 exposed from the silicon side opening 55A has a structure in which the width L1 of the electrode section 22 is wider than the opening width L2 of the silicon side opening 55A. The signal electrode 22A2 exposed from the silicon side opening 55A has a width L1 wider than the opening width L2 of the silicon side opening 55A. Furthermore, the ground electrode 22B2 exposed from the silicon side opening 55A has a width L1 wider than the opening width L2 of the silicon side opening 55A.

[0060] In the RF modulation section 12 of the third embodiment, the width L1 of the signal electrode 22A2 or the ground electrode 22B2 is made wider than the opening width L2 of the silicon side opening 55A. As a result, even if there is a positional misalignment between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode section 22, electrical contact can be made between the signal electrode 22A2 and the silicon layer 55, and between the ground electrode 22B2 and the silicon layer 55.

[0061] In the first DC modulation section 32, the width L1 of the signal electrode 22A2 or the ground electrode 22B2 is made wider than the opening width L2 of the silicon side opening 55A. As a result, even if there is a positional misalignment between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode section 22, electrical contact can be established between the signal electrode 22A2 and the silicon layer 55, and between the ground electrode 22B2 and the silicon layer 55.

[0062] In the second DC modulation section 33, the width L1 of the signal electrode 22A2 or the ground electrode 22B2 is made wider than the opening width L2 of the silicon side opening 55A. As a result, even if there is a positional misalignment between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode section 22, electrical contact can be established between the signal electrode 22A2 and the silicon layer 55, and between the ground electrode 22B2 and the silicon layer 55.

[0063] In the RF modulation section 12 of Example 3, when removing the silicon layer 55 other than the surface of the buffer layer 54, the silicon layer 55 is etched by patterning the second opening 54A of the buffer layer 54 to form the silicon side opening 55A. However, the present invention is not limited to this, and an embodiment thereof will be described below as Example 4. EXAMPLES

[0064] FIG. 6 is a schematic cross-sectional view showing an example of the RF modulation section 12 of the optical modulator 5 of the fourth embodiment. The same components as those of the optical modulator of the second embodiment are given the same reference numerals, and the description of the overlapping components and operations is omitted. The RF modulation section 12 shown in FIG. 6 differs from the RF modulation section 12 shown in FIG. 4 in that the opening width L3 of the silicon side opening 55A of the silicon layer 55 is wider than the opening width L4 of the second opening 54A of the buffer layer 54. The patterning for etching the silicon side opening 55A and the patterning for etching the second opening 54A are performed separately. At this time, the deviation between the etching pattern of the silicon side opening 55A and the etching pattern of the second opening 54A can be absorbed.

[0065] Since the opening width L3 of the silicon side opening 55A of the silicon layer 55 is made wider than the opening width L4 of the second opening 54A of the buffer layer 54, the signal electrode 22A3 is formed in the opening 70. Furthermore, since the opening width L3 of the silicon side opening 55A of the silicon layer 55 is made wider than the opening width L4 of the second opening 54A of the buffer layer 54, the ground electrode 22B3 is formed in the opening 70.

[0066] In the RF modulation section 12 of the fourth embodiment, the opening width L3 of the silicon side opening 55A of the silicon layer 55 is made wider than the opening width L4 of the second opening 54A of the buffer layer 54. As a result, it is possible to suppress a decrease in yield caused by misalignment between the etching pattern of the silicon side opening 55A of the silicon layer 55 and the etching pattern of the second opening 54A of the buffer layer 54.

[0067] In the first DC modulation section 32, the opening width L3 of the silicon-side opening 55A of the silicon layer 55 is made wider than the opening width L4 of the second opening 54A of the buffer layer 54. As a result, it is possible to suppress a decrease in yield caused by misalignment between the etching pattern of the silicon-side opening 55A of the silicon layer 55 and the etching pattern of the second opening 54A of the buffer layer 54.

[0068] In the second DC modulation section 33, the opening width L3 of the silicon-side opening 55A of the silicon layer 55 is made wider than the opening width L4 of the second opening 54A of the buffer layer 54. As a result, it is possible to suppress a decrease in yield caused by misalignment between the etching pattern of the silicon-side opening 55A of the silicon layer 55 and the etching pattern of the second opening 54A of the buffer layer 54.

[0069] In the RF modulation section 12 of Example 4, the silicon layer 55 on the buffer layer 54 attracts the light of the thin film optical waveguide 60, which causes an increase in the absorption of light in the electrode section 22. In order to deal with such a situation, an embodiment of the present invention will be described below as Example 5. EXAMPLES

[0070] Fig. 7 is a schematic cross-sectional view showing an example of the RF modulation section 12 of the optical modulator 5 of the embodiment 5. The same components as those of the optical modulator 5 of the embodiment 4 are given the same reference numerals, and the description of the overlapping components and operations will be omitted. The RF modulation section 12 shown in Fig. 7 differs from the RF modulation section 12 shown in Fig. 4 in that the thickness L5 of the buffer layer 54 is made thicker than the thickness L6 of the thin film optical waveguide 60 in accordance with the size of the mode field of the light propagating through the thin film optical waveguide 60.

[0071] In the RF modulation section 12 of the fifth embodiment, the thickness L5 of the buffer layer 54 is made thicker than the thickness L6 of the thin-film optical waveguide 60. Therefore, the silicon layer 55 on the buffer layer 54 reduces the amount of light that is attracted by the thin-film optical waveguide 60, thereby suppressing the absorption of light in the electrode section 22.

[0072] In the first DC modulation section 32, the thickness L5 of the buffer layer 54 is made thicker than the thickness L6 of the thin-film optical waveguide 60, so that the silicon layer 55 on the buffer layer 54 reduces the amount of light that is attracted to the thin-film optical waveguide 60, thereby suppressing light absorption in the electrode section 22.

[0073] In the second DC modulation section 33, the thickness L5 of the buffer layer 54 is made thicker than the thickness L6 of the thin-film optical waveguide 60, so that the silicon layer 55 on the buffer layer 54 reduces the amount of light that is attracted to the thin-film optical waveguide 60, thereby suppressing light absorption in the electrode section 22.

[0074] For convenience of explanation, a rib-type waveguide is exemplified as the thin film optical waveguide 60, but a channel waveguide may also be used, and appropriate modifications can be made.

[0075] The following supplementary notes are further disclosed regarding the above-described embodiments including the present example.

[0076] (Appendix 1) An optical waveguide; a buffer layer laminated on the optical waveguide; an opening formed in the buffer layer at least on a portion near a side surface of the optical waveguide; an electrode that is laminated in the opening and applies a signal to the optical waveguide; a silicon layer laminated on the buffer layer except for the opening; An optical device comprising: (Additional Note 2) The optical waveguide is provided at a predetermined location on the thin film substrate, The buffer layer is laminated on the thin film substrate, The opening is 2. The optical device according to claim 1, characterized in that the optical waveguide is formed at least in a portion of the thin film substrate near a side surface thereof and in the buffer layer above the portion. (Additional Note 3) A silicon-side opening is formed in a portion of the silicon layer that overlaps the opening, 3. The optical device according to claim 1 or 2, characterized in that the electrode laminated in the opening is exposed from the silicon side opening, and electrical contact is established between the electrode and the silicon layer. (Additional Note 4) The electrode exposed from the silicon side opening is 4. The optical device according to claim 3, wherein the width of the electrode is wider than the width of the silicon side opening. (Additional Note 5) The silicon side opening is 4. The optical device according to claim 3, wherein the opening has a width wider at the silicon side than the opening. (Additional Note 6) The optical waveguide is 6. The optical device according to any one of claims 1 to 5, wherein the optical waveguide has a thickness greater than a thickness of the buffer layer laminated on the optical waveguide. (Supplementary Note 7) The optical waveguide is 7. The optical device according to any one of claims 1 to 6, which is an optical waveguide made of a thin-film LN (Lithium Niobate) crystal. (Additional Note 8) The optical waveguide is 8. The optical device according to claim 7, which is formed on an X-cut substrate of the thin-film LN crystal. (Appendix 9) The electrode is 8. The optical device according to claim 1, which is an RF (Radio Frequency) electrode. (Appendix 10) The electrode is 8. The optical device according to claim 1, wherein the electrode is a direct current (DC) electrode. (Appendix 11) 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: An optical waveguide; a buffer layer laminated on the optical waveguide; an opening formed in the buffer layer at least on a portion near a side surface of the optical waveguide; an electrode that is laminated in the opening and applies a signal to the optical waveguide; a silicon layer laminated on the buffer layer except for the opening; An optical communication device comprising: [Explanation of symbols]

[0077] 1 Optical communication devices 3. DSP 4 light source 5 Optical Modulator 12 RF Modulation Section 22 Electrode section 22A signal electrode 22B Ground electrode 32 First DC modulation section 33 Second DC Modulation Section 53 Thin film LN substrate 54 Buffer Layer 60 Thin-film optical waveguide

Claims

1. A thin film substrate; an optical waveguide formed by the thin film substrate; a buffer layer laminated on the optical waveguide; an opening formed in at least a portion of the thin film substrate near a side surface of the optical waveguide and in the buffer layer above the portion; an electrode that is laminated in the opening and applies a signal to the optical waveguide; a silicon layer laminated on the buffer layer except for the opening; An optical device comprising:

2. The buffer layer The optical device according to claim 1 , which is laminated on the thin film substrate.

3. a silicon-side opening formed in a portion of the silicon layer overlapping the opening, 3. The optical device according to claim 1, wherein the electrode laminated in the opening is exposed from the silicon side opening, and an electrical contact is established between the electrode and the silicon layer.

4. The electrode exposed from the silicon side opening is 4. The optical device according to claim 3, wherein the width of the electrode is wider than the width of the opening on the silicon side.

5. The silicon side opening is 4. The optical device according to claim 3, wherein the opening has a width wider at the silicon side than the opening.

6. The optical waveguide is 6. The optical device according to claim 1, wherein the buffer layer laminated on the optical waveguide has a thickness greater than a thickness of the optical waveguide.

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 thin film substrate; an optical waveguide formed by the thin film substrate; a buffer layer laminated on the optical waveguide; an opening formed in at least a portion of the thin film substrate near a side surface of the optical waveguide and in the buffer layer above the portion; an electrode that is laminated in the opening and applies a signal to the optical waveguide; a silicon layer laminated on the buffer layer except for the opening; An optical communication device comprising:

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