Optical devices and optical communication devices

Abstract

The present disclosure relates to an optical device and an optical communication apparatus. The optical device includes: an optical waveguide; a buffer layer stacked on the optical waveguide; and an opening formed in the buffer layer at least above a portion located near a side surface of the optical waveguide. The optical device also includes: an electrode stacked in 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.

Description

Technical Field

[0001] The embodiments discussed herein relate to optical devices and optical communication apparatuses. Background Art

[0002] A conventional optical modulator includes an optical waveguide disposed on a substrate and a modulator disposed near the optical waveguide. The modulator includes 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, so the refractive index of the optical waveguide changes and the phase of light changes. The optical waveguide configures a Mach-Zehnder interferometer, and the optical phase difference between the waveguides changes the light output.

[0003] In the optical modulator, for example, a four-channel Mach-Zehnder modulator is integrated. Each Mach-Zehnder interferometer includes an RF modulator and a DC modulator. For example, a high-frequency signal with a bandwidth of tens of gigahertz (GHz) is input to the electrode of the RF modulator to perform rapid modulation. A bias voltage is applied to the electrode of the DC modulator, 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] For example, the optical waveguide of the optical modulator configures a Mach-Zehnder interferometer and outputs an IQ signal obtained by performing XY polarization multiplexing according to the phase difference of light between a plurality of optical waveguides arranged in parallel. The outputs of the four channels are combined into two IQ signals in a manner of two channels each, and polarization rotation is performed on one of the two IQ signals, and the polarization beam combiner performs polarization multiplexing and generates an output.

[0005] On the other hand, the optical waveguide includes, for example, a diffused optical waveguide formed at a position not overlapping with the signal electrode by diffusing a metal such as titanium from the surface of a substrate. However, the diffused optical waveguide has a small optical confinement, so the efficiency of applying an electric field is poor, and thus the driving voltage increases. Therefore, there is a film optical waveguide in which an optical waveguide using a lithium niobate (LN) crystal film is formed at a position not overlapping with the signal electrode. The film optical waveguide makes it possible to enhance optical confinement more than the diffused optical waveguide in which the metal is diffused, improve the efficiency of applying an electric field, and reduce the driving voltage.

[0006] Optical modulators include RF modulators and DC modulators. Fig. 9 is a schematic cross-sectional diagram illustrating an example of an RF modulator 200 that illustrates an optical modulator. Fig. 9 The RF modulator 200 shown includes a support substrate 201 of silicon (Si) or the like and an intermediate layer 202 stacked on the support substrate 201. The RF modulator 200 also includes a film LN substrate 203 stacked on the intermediate layer 202 and a SiO2 buffer layer 204 stacked on the film LN substrate 203.

[0007] A protruding film optical waveguide 207 protruding upward is formed on the film LN substrate 203. The film LN substrate 203 and the film optical waveguide 207 are covered with a buffer layer 204, and a signal electrode 205 and a pair of ground electrodes 206 forming a coplanar (coplanar waveguide (CPW)) structure are arranged on the surface of the buffer layer 204. In other words, the signal electrode 205 and the pair of ground electrodes 206 between which the signal electrode 205 is located are arranged on the buffer layer 204. Note that the buffer layer 204 makes it possible to prevent light propagating through the film optical waveguide 207 from being absorbed into the signal electrode 205 and the ground electrode 206.

[0008] The protruding film optical waveguide 207 is formed on the film LN substrate 203 between the signal electrode 205 and the ground electrode 206. The protruding film optical waveguide 207 includes a side wall surface 207A and a flat surface 207B. In addition, the buffer layer 204 located between the signal electrode 205 and the ground electrode 206 has a step portion 204A covering the entire protruding film optical waveguide 207.

[0009] According to the above-described film optical waveguide 207, an electric field is generated by applying a high-frequency signal to the signal electrode 205 and the refractive index of the film optical waveguide 207 is changed, so that light propagating through the film optical waveguide 207 can be modulated. Note that although the DC modulator is different from the RF modulator 200 in that the DC modulator applies a bias voltage to the signal electrode 205 instead of a high-frequency signal, the DC modulator has substantially the same structure as the RF modulator 200, and a description of the configuration and operation of the DC modulator will be omitted.

[0010] Patent Document 1: Japanese Unexamined Patent Publication No. 2013-37243

[0011] In the film optical waveguide 207 of the optical modulator, since the buffer layer 204 serves as a cladding, the mode field of light propagating through the film optical waveguide 207 enters the buffer layer 204. By increasing the thickness of the buffer layer 204, it is possible to prevent the light in the film optical waveguide 207 from being absorbed into the signal electrode 205 and the ground electrode 206. However, increasing the thickness of the buffer layer 204 reduces the electric field applied to the film optical waveguide 207, which therefore reduces the efficiency of applying the electric field.

[0012] Therefore, an object of an aspect of an embodiment of the present invention is to provide an optical device that enables improvement in the efficiency of applying an electric field. Summary of the invention

[0013] According to one aspect of the embodiment, an optical device includes an optical waveguide, a buffer layer, an opening, an electrode, and a silicon layer. The buffer layer is stacked on the optical waveguide. The opening is formed in the buffer layer at least above a portion located near a side surface of the optical waveguide. The electrode is stacked in the opening and is configured to apply a signal to the optical waveguide. The silicon layer is stacked on the buffer layer except for the opening. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a block diagram illustrating an example of a configuration of an optical communication device of a first embodiment;

[0015] Figure 2 is a plan view schematically illustrating an example of a configuration of the optical modulator of the first embodiment;

[0016] Figure 3 is a schematic cross-sectional view illustrating an example of an RF modulator of the optical modulator of the first embodiment;

[0017] Figure 4 is a schematic cross-sectional view illustrating an example of an RF modulator of the optical modulator of the second embodiment;

[0018] Figure 5 is a schematic cross-sectional view illustrating an example of an RF modulator of the optical modulator of the third embodiment;

[0019] Figure 6 is a schematic cross-sectional view illustrating an example of an RF modulator of the optical modulator of the fourth embodiment;

[0020] Figure 7 is a schematic cross-sectional view illustrating an example of an RF modulator of an optical modulator of a fifth embodiment;

[0021] Figure 8 is a schematic cross-sectional view illustrating an example of an RF modulator of an optical modulator of a comparative example; and

[0022] Fig. 9 is a schematic cross-sectional view of an example of an RF modulator illustrating an optical modulator. DETAILED DESCRIPTION

[0023] exist Fig. 9 In the RF modulator 200 of the optical modulator shown, increasing the thickness of the buffer layer 204 increases the distance between the film optical waveguide 207 and the signal electrode 205 (ground electrode 206), so that the electric field applied from the signal electrode 205 to the film optical waveguide 207 is smaller. As a result, the efficiency of applying the electric field is reduced. Therefore, the RF modulator 100 of the optical modulator of the comparative example has been designed, which can improve the efficiency of applying the electric field even when the thickness of the buffer layer 204 is increased.

[0024] Comparative Example

[0025] Figure 8 is a schematic cross-sectional view illustrating an example of an RF modulator 100 of an optical modulator of a comparative example. Figure 8 The RF modulator 100 shown includes a support substrate 101 and an intermediate layer 102 stacked on the support substrate 101. The RF modulator 100 also includes a film LN substrate 103 stacked on the intermediate layer 102, a buffer layer 104 stacked on the film LN substrate 103, a silicon layer 105 stacked on the buffer layer 104, and an electrode portion 106. The electrode portion 106 includes a signal electrode 106A and a pair of ground electrodes 106B.

[0026] A protruding film optical waveguide 160 protruding upward is formed on the film LN substrate 103. The film LN substrate 103 and the film optical waveguide 160 are covered with a buffer layer 104. The buffer layer 104 is provided so as to prevent light propagating through the film optical waveguide 160 from being absorbed into the electrode portion 106. A first opening 103A is partially formed in a portion (plate) of the film LN substrate 103 near the film optical waveguide 160. A second opening 104A communicating with the first opening 103A is formed in a portion of the buffer layer 104 located above the first opening 103A, the first opening 103A being formed in a portion (plate) of the film LN substrate 103. Further, a third opening 102A communicating with the first opening 103A is formed in a portion of the intermediate layer 102 located below the first opening 103A. The first opening 103A, the second opening 104A, and the third opening 102A form an opening 170.

[0027] In addition, the silicon layer 105 is provided in order to prevent localization of charges due to temperature change and stabilize temperature characteristics. The silicon layer 105 covers the buffer layer 104 and the opening 170. The electrode portion 106 is stacked on the silicon layer 105 located in the opening 170.

[0028] The protruding film optical waveguide 160 is formed on the film LN substrate 103 between the signal electrode 106A and the ground electrode 106B. In addition, a step portion 104B covering the entire protruding film optical waveguide 160 exists in the buffer layer 104 between the signal electrode 106A and the ground electrode 106B.

[0029] The film LN substrate 103 is interposed between the intermediate layer 102 and the buffer layer 104, and a protruding film optical waveguide 160 protruding upward is formed on the film LN substrate 103. The film LN substrate 103 and the 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 a portion of the silicon layer 105 overlapping with the opening 170.

[0030] The signal electrode 106A is, for example, an electrode made of a metal material such as gold or copper. The ground electrode 106B is, for example, an electrode made of a metal material such as gold or copper. In the RF modulator 100, a high-frequency signal corresponding to the electric signal is transmitted by the signal electrode 106A, and thus an electric field in a direction from the signal electrode 106A to the ground electrode 106B is generated, and the electric field is applied to the film optical waveguide 160. As a result, the refractive index of the film optical waveguide 160 changes according to the electric field applied to the film optical waveguide 160, which makes it possible to modulate the light propagating through the film optical waveguide 160.

[0031] The ground electrode 106B and the signal electrode 106A are arranged in the opening 170, and therefore the distance between the ground electrode 106B and the film optical waveguide 160 and the distance between the signal electrode 106A and the film optical waveguide 160 are small. As a result, even when the thickness of the buffer layer 104 increases, the efficiency of applying the electric field can be improved.

[0032] However, the silicon layer 105 is close to the film LN substrate 103 on both sides of the film light waveguide 160. The silicon layer 105 has a high light refractive index, and therefore, when the silicon layer 105 is close to the film LN substrate 103, light in the film light waveguide 160 is extracted and light loss increases.

[0033] Preferred embodiments of the present invention will be explained with reference to the accompanying drawings. The embodiments do not limit the present disclosure.

[0034] [a] First Embodiment

[0035] Figure 1 is a block diagram illustrating an example configuration of the optical communication device 1 of the first embodiment. Figure 1 The optical communication device 1 shown 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 digital signal processor (DSP) 3, a light source 4, an optical modulator 5, and an optical receiver 6. The DSP 3 is an electronic component that performs digital signal processing. For example, the DSP 3 performs processing such as encoding transmission data, generates an electrical signal containing the transmission data, and outputs the generated electrical signal to the optical modulator 5. The DSP 3 acquires the electrical signal containing the received data from the optical receiver 6, and performs processing such as decoding the acquired electrical signal, thereby obtaining the received data.

[0036] The light source 4 includes, for example, a laser diode, etc., generates light of a given wavelength, and supplies the generated light 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 according to the electrical signal output from the DSP 3 and outputs the resulting optical transmission signal to the optical fiber 2A. For example, the optical modulator 5 is an optical device such as a lithium niobate (LN) optical modulator including an LN optical waveguide and a modulator. The LN optical waveguide is formed by a substrate of an LN crystal. When the light supplied from the light source 4 propagates through the LN optical waveguide, the optical modulator 5 modulates the light according to the electrical signal input to the modulator, thereby generating an optical transmission signal.

[0037] 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. The optical receiver 6 converts the demodulated received optical signal into an electrical signal, and outputs the converted electrical signal to the DSP 3.

[0038] Figure 2 1 is a schematic plan view illustrating an example of the configuration of the optical modulator 5 according to the first embodiment. Figure 2 In the optical modulator 5 shown, the optical fiber 4A from the light source 4 is connected to the input side, and the optical fiber 2A for outputting the transmission signal is connected to the output side. The optical modulator 5 includes a first optical input unit 11, a radio frequency (RF) modulator 12, a direct current (DC) modulator 13, and a first optical output unit 14. The first optical input unit 11 includes a first optical waveguide 11A and a first waveguide junction 11B. The first optical waveguide 11A includes a single optical waveguide connected to the optical fiber 4A, two optical waveguides branched therefrom, four optical waveguides branched therefrom, and eight optical waveguides branched therefrom by the four optical waveguides. The first waveguide junction 11B joins eight optical waveguides in the first optical waveguide 11A and eight LN optical waveguides in the LN optical waveguide 21.

[0039] The RF modulator 12 includes an LN optical waveguide 21, an electrode portion 22, and an RF terminator 23. When light supplied from the first optical waveguide 11A propagates through the LN optical waveguide 21, the RF modulator 12 modulates the light using an electric field applied from a signal electrode 22A of the electrode portion 22. The LN optical waveguide 21 is, for example, an optical waveguide formed using a film LN substrate 53, which repeatedly branches from the input side, and thus includes eight parallel LN optical waveguides. Light propagating through the LN optical waveguide 21 and modulated is output to the first DC modulator 32 in the DC modulator 13. The film LN substrate 53 is an X-cut substrate whose refractive index increases when a DC voltage is applied in the X-axis direction of the crystal.

[0040] The signal electrode 22A in the electrode portion 22 is disposed at a position not overlapping the LN optical waveguide 21, and applies an electric field to the LN optical waveguide 21 according to the electric signal output from the DSP 3. The terminal of the signal electrode 22A in the electrode portion 22 is connected to the RF terminator 23. The RF terminator 23 is connected to the terminal of the signal electrode 22A, and prevents unnecessary reflection of a signal transmitted by the signal electrode 22A.

[0041] The DC modulator 13 includes an LN optical waveguide 31 joined to the LN optical waveguide 21 of the RF modulator 12, a first DC modulator 32, and a second DC modulator 33. The first DC modulator 32 includes four child Mach-Zehnder (MZ) modulators. The second DC modulator 33 includes two father Mach-Zehnder (MZ) modulators. The first DC modulator 32 includes the LN optical waveguide 31 and the electrode portion 22.

[0042] The LN optical waveguide 31 includes eight LN optical waveguides and four LN optical waveguides that join two of the eight LN optical waveguides. In the eight LN optical waveguides 31, the first DC modulator 32 is arranged at every two LN optical waveguides. The first DC modulator 32 applies a bias voltage to the signal electrode 22A above the LN optical waveguide 31, thereby adjusting the bias voltage so that the on / off of the electric signal corresponds to the on / off of the optical signal, and outputs an I signal of an in-phase component or a Q signal of an orthogonal component. In the four LN optical waveguides in the LN optical waveguide 31, the second DC modulator 33 is arranged at every two LN optical waveguides. The second DC modulator 33 applies a bias voltage to the signal electrode 22A above the LN optical waveguide 31, thereby adjusting the bias voltage so that the on / off of the electric signal corresponds to the on / off of the optical signal, and outputs an I signal or a Q signal.

[0043] The first light output unit 14 includes a second waveguide junction 41, a second optical waveguide 42, a polarization rotator (PR) 43, and a polarization beam combiner (PBC) 44. The second waveguide junction 41 joins the LN optical waveguide 31 in the DC modulator 13 and the second optical waveguide 42. The second optical waveguide 42 includes four optical waveguides connected to the second waveguide junction 41 and two optical waveguides joining two optical waveguides among the four optical waveguides.

[0044] The PR 43 rotates the I signal or Q signal input from one of the second DC modulators 33 by 90 degrees and obtains a vertically polarized optical signal after the 90-degree rotation. The PR 43 inputs the vertically polarized optical signal to the PBC 44. The PBC 44 multiplexes the vertically polarized optical signal from the PR 43 and the horizontally polarized optical signal from the other second DC modulator 33 and outputs a polarization multiplexed signal.

[0045] The configuration of the optical modulator 5 of the first embodiment will be described in detail below. Figure 3 is a schematic cross-sectional view illustrating an example of the RF modulator 12 of the optical modulator 5 of the first embodiment. Figure 3 The RF modulator 12 shown includes a support substrate 51 and an intermediate layer 52 stacked on the support substrate 51. The RF modulator 12 also includes: a film LN substrate 53, which is a film substrate and is stacked on the intermediate layer 52; a buffer layer 54, which is stacked on the film LN substrate 53; a silicon layer 55, which is stacked on the buffer layer 54; and an electrode portion 22. The electrode portion 22 includes a signal electrode 22A and a pair of ground electrodes 22B.

[0046] A protruding film optical waveguide 60 protruding upward is formed on the film LN substrate 53. The film optical waveguide 60 is the LN optical waveguide 21 of the RF modulator 12. The film LN substrate 53 and the film optical waveguide 60 are covered with a buffer layer 54. The buffer layer 54 is provided so as to prevent light propagating through the film optical waveguide 60 from being absorbed into the electrode portion 22. A first opening 53A is formed near the side surface of the film optical waveguide 60, for example, in a portion (plate) of the film LN substrate 53 located on both sides. A second opening 54A communicating with the first opening 53A is formed in a portion of the buffer layer 54 located above the first opening 53A, the first opening 53A being formed in a portion (plate) of the film LN substrate 53. In addition, a third opening 52A communicating with the first opening 53A is formed in a portion of the intermediate layer 52 located below the first opening 53A. The first opening 53A, the second opening 54A, and the third opening 52A form an opening 70. The electrode portion 22 is stacked in the opening 70.

[0047] The silicon layer 55 is provided in order to prevent the localization of the charge due to the temperature change and stabilize the temperature characteristics. The silicon layer 55 is stacked in the portion of the buffer layer 54 except the opening 70. The silicon side opening 55A is formed in the portion of the silicon layer 55 overlapping with the second opening 54A of the buffer layer 54. Note that the silicon side opening 55A is formed by etching the silicon layer 55 when patterning the second opening 54A of the buffer layer 54.

[0048] The protruding film optical waveguide 60 is formed on the film LN substrate 53 located between the signal electrode 22A and the ground electrode 22B. The film optical waveguide 60 is a protruding rib-shaped optical waveguide provided at a given position on the film LN substrate 53. The protruding film optical waveguide 60 includes a side wall surface 60B and a flat surface 60A. In addition, the buffer layer 54 located between the signal electrode 22A and the ground electrode 22B also has a step portion 54B covering the entire protruding film optical waveguide 60. The step portion 54B covering the side wall surface 60B of the film optical waveguide 60 separates the ground electrode 22B and the signal electrode 22A.

[0049] The film LN substrate 53 having a thickness of 0.5 μm to 3 μm is inserted between the intermediate layer 52 and the buffer layer 54, and a protruding film optical waveguide 60 protruding upward is formed on the film LN substrate 53. The width of the protruding portion as the film optical waveguide 60 is, for example, about 1 μm to 8 μm. The film LN substrate 53 and the 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. Note that the silicon layer 55 and the signal electrode 22A or the ground electrode 22B are not in contact in the silicon side opening 55A.

[0050] Preferably, the material of the signal electrode 22A is different from that of the ground electrode 22B and has a small high-frequency loss.

[0051] The signal electrode 22A is, for example, an electrode made of a metal material such as gold or copper, and has a width of 2 μm to 10 μm and a thickness of 1 μm to 20 μm. The ground electrode 22B is, for example, an electrode made of a metal material such as gold or copper, and has a thickness of 1 μm or more. A high-frequency signal corresponding to the electric signal output from the DSP 3 is transmitted by the signal electrode 22A, thereby generating an electric field in a direction from the signal electrode 22A to the ground electrode 22B, and the electric field is applied to the film optical waveguide 60. As a result, the refractive index of the film optical waveguide 60 changes according to the electric field applied to the film optical waveguide 60, which makes it possible to modulate the light propagating through the film optical waveguide 60.

[0052] In the RF modulator 12 of the first embodiment, the ground electrode 22B and the signal electrode 22A are arranged in the opening 70. In the RF modulator 12, since the distance between the ground electrode 22B and the film optical waveguide 60 and the distance between the signal electrode 22A and the film optical waveguide 60 are set shorter, the electric field applied to the film optical waveguide 60 is stronger. As a result, even when the thickness of the buffer layer 54 is increased, the efficiency of applying the electric field is improved, making it possible to reduce the driving voltage.

[0053] Furthermore, in the RF modulator 12, compared to the RF modulator 100 of the comparative example, since the silicon layer 55 is not in the opening 70, the silicon layer 55 does not come close to the film optical waveguide 60. As a result, light loss due to the silicon layer 55 (where the silicon layer 55 extracts light in the film optical waveguide 60 and the electrode portion 22 absorbs light) can be suppressed.

[0054] Furthermore, the buffer layer 54 except for the second opening 54A is covered with the silicon layer 55 , which makes it possible to prevent the localization of charges due to temperature changes and stabilize the temperature characteristics.

[0055] Figure 3The RF modulator 12 is illustrated, and the first DC modulator 32 of the DC modulator 13 has substantially the same configuration as the RF modulator 12. The first DC modulator 32 is different from the RF modulator 12 in that a driving voltage is applied to the signal electrode 22A instead of a high-frequency signal. Therefore, redundant descriptions of the configuration and operation will be omitted. Note that the film optical waveguide 60 corresponds to the LN optical waveguide 31 of the first DC modulator 32.

[0056] In the first DC modulator 32, the ground electrode 22B and the signal electrode 22A are arranged in the opening 70. In the first DC modulator 32, since the distance between the ground electrode 22B and the film optical waveguide 60 and the distance between the signal electrode 22A and the film optical waveguide 60 are set shorter, the electric field to be applied to the film optical waveguide 60 is stronger. As a result, even when the thickness of the buffer layer 54 is increased, the driving voltage can be reduced by improving the efficiency of applying the electric field.

[0057] Furthermore, in the first DC modulator 32, compared to the RF modulator 100 of the comparative example, since the silicon layer 55 is not in the opening 70, the silicon layer 55 does not come close to the film optical waveguide 60. As a result, light loss due to the silicon layer 55 (where the silicon layer 55 extracts light in the film optical waveguide 60 and the electrode portion 22 absorbs the light) can be suppressed.

[0058] Similarly, the second DC modulator 33 of the DC modulator 13 has substantially the same configuration as the RF modulator 12. The second DC modulator 33 is different from the RF modulator 12 in that a driving voltage is applied to the signal electrode 22A instead of a high-frequency signal. Therefore, descriptions of redundant configurations and operations will be omitted. Note that the film optical waveguide 60 corresponds to the LN optical waveguide 31 of the second DC modulator 33.

[0059] In the second DC modulator 33, the ground electrode 22B and the signal electrode 22A are arranged in the opening 70. In the second DC modulator 33, since the distance between the ground electrode 22B and the film optical waveguide 60 and the distance between the signal electrode 22A and the film optical waveguide 60 are set shorter, the electric field applied to the film optical waveguide 60 is stronger. As a result, even when the thickness of the buffer layer 54 is increased, the driving voltage can be reduced by improving the efficiency of applying the electric field.

[0060] Furthermore, in the second DC modulator 33, compared with the RF modulator 100 of the comparative example, since the silicon layer 55 is not in the opening 70, the silicon layer 55 does not come close to the film optical waveguide 60. As a result, light loss due to the silicon layer 55 (where the silicon layer 55 extracts light in the film optical waveguide 60 and the electrode portion 22 absorbs the light) can be suppressed.

[0061] Note that the case where the opening is constituted by the first opening 53A, the second opening 54A, and the third opening 52A is exemplified. Alternatively, the opening 70 may be constituted by only the second opening 54A of the buffer layer 54 without the third opening 52A and the first opening 53A of the film LN substrate 53. Reducing the distance between the electrode portion 22 and the film optical waveguide 60 in the opening 70 makes it possible to reduce the driving voltage by improving the efficiency of applying the electric field even when the thickness of the buffer layer 54 increases. The opening 70 may be constituted by the first opening 53A and the second opening 54A without the third opening 52A, and may be changed appropriately.

[0062] In the RF modulator 12 of the first embodiment, 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 preferred that the charges appearing on the surface of the buffer layer 54 are transferred to the electrode portion 22 via the silicon layer 55, and its implementation will be described below as a second embodiment.

[0063] [b] Second Embodiment

[0064] Figure 4 1 is a schematic cross-sectional view illustrating an example of the RF modulator 12 of the optical modulator 5 of the second embodiment. Note that the same components as those of the optical modulator 5 of the first embodiment will be denoted by the same reference numerals as those of the first embodiment, and thus descriptions of redundant components and operations will be omitted. Figure 4 The RF modulator 12 shown is Figure 3 The RF modulator 12 shown is different in that it is configured so that the signal electrode 22A1 or the ground electrode 22B1 exposed from the silicon side opening 55A protrudes from the silicon layer 55, and electrical contact is formed between the signal electrode 22A1 and the silicon layer 55, and between the ground electrode 22B1 and the silicon layer 55. For example, in the RF modulator 12, the electrode portion 22 is formed in the opening 70 with the positions of the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode portion 22 being kept aligned with each other.

[0065] The RF modulator 12 of the second embodiment is configured so that electrical contact is formed between the signal electrode 22A1 and the silicon layer 55 and between the ground electrode 22B1 and the silicon layer 55, and thus the charges appearing on the surface of the buffer layer 54 are transferred to the electrode portion 22 via the silicon layer 55. As a result, localization of charges due to temperature changes is avoided, which makes it possible to stabilize temperature characteristics.

[0066] In the first DC modulator 32, electrical contacts are formed between the signal electrode 22A1 and the silicon layer 55 and between the ground electrode 22B1 and the silicon layer 55, and thus charges appearing on the surface of the buffer layer 54 are transferred to the electrode portion 22 through the silicon layer 55. As a result, localization of charges due to temperature changes is avoided, which makes it possible to stabilize temperature characteristics.

[0067] In the second DC modulator 33, electrical contacts are formed between the signal electrode 22A1 and the silicon layer 55 and between the ground electrode 22B1 and the silicon layer 55, and thus charges appearing on the surface of the buffer layer 54 are transferred to the electrode portion 22 through the silicon layer 55. As a result, localization of charges due to temperature changes is avoided, which makes it possible to stabilize temperature characteristics.

[0068] In the RF modulator 12 of the second embodiment, for example, in the case where a positional difference occurs between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode portion 22 when forming the electrode portion 22, there may be no electrical contact between the signal electrode 22A and the silicon layer 55 or between the ground electrode 22B and the silicon layer 55. An embodiment of the optical modulator 5 for dealing with such a situation will be described as a third embodiment.

[0069] [c] Third Embodiment

[0070] Figure 5 1 is a schematic cross-sectional view illustrating an example of the RF modulator 12 of the optical modulator of the third embodiment. Note that the same components as those of the optical modulator 5 of the second embodiment will be denoted by the same reference numerals as those of the second embodiment, and thus descriptions of redundant components and operations will be omitted. Figure 5 The RF modulator 12 shown is Figure 4 The RF modulator 12 shown is different in that it is configured so that the electrode portion 22 exposed from the silicon side opening 55A has a width L1 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. In addition, 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.

[0071] In the RF modulator 12 of the third embodiment, the width L1 of the signal electrode 22A2 or the ground electrode 22B2 is set to be wider than the opening width L2 of the silicon side opening 55A. As a result, even when a positional difference occurs between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode portion 22, electrical contact between the signal electrode 22A2 and the silicon layer 55 and between the ground electrode 22B2 and the silicon layer 55 can be achieved.

[0072] In the first DC modulator 32, the width L1 of the signal electrode 22A2 or the ground electrode 22B2 is set to be wider than the opening width L2 of the silicon side opening 55A. As a result, even when a positional difference occurs between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode portion 22, electrical contact between the signal electrode 22A2 and the silicon layer 55 and between the ground electrode 22B2 and the silicon layer 55 can be achieved.

[0073] In the second DC modulator 33, the width L1 of the signal electrode 22A2 or the ground electrode 22B2 is set to be wider than the opening width L2 of the silicon side opening 55A. As a result, even when a positional difference occurs between the pattern of the second opening 54A of the buffer layer 54 and the pattern of the electrode portion 22, electrical contact between the signal electrode 22A2 and the silicon layer 55 and between the ground electrode 22B2 and the silicon layer 55 can be achieved.

[0074] For the RF modulator 12 of the third embodiment, the case where the silicon layer 55 is etched and the silicon side opening 55A is formed by patterning the second opening 54A of the buffer layer 54 in order to remove the silicon layer 55 except the surface of the buffer layer 54 is illustrated. However, etching is not limited to this, and its embodiment will be described below as a fourth embodiment.

[0075] [d] Fourth Embodiment

[0076] Figure 6 1 is a schematic cross-sectional view illustrating an example of the RF modulator 12 of the optical modulator 5 of the fourth embodiment. Note that the same components as those of the optical modulator of the second embodiment will be denoted by the same reference numerals as those of the second embodiment, and thus descriptions of redundant components and operations will be omitted. Figure 6 The RF modulator 12 shown is Figure 4 The RF modulator 12 shown is different in that the silicon-side opening 55A of the silicon layer 55 has an opening width L3 wider than the opening width L4 of the second opening 54A of the buffer layer 54. Patterning in which the silicon-side opening 55A is etched and patterning in which the second opening 54A is etched are performed separately. The patterning makes it possible to absorb the difference between the pattern obtained by etching the silicon-side opening 55A and the pattern obtained by etching the second opening 54A.

[0077] Since the opening width L3 of the silicon-side opening 55A of the silicon layer 55 is set 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 set 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.

[0078] In the RF modulator 12 of the fourth embodiment, the opening width L3 of the silicon-side opening 55A of the silicon layer 55 is set to be wider than the opening width L4 of the second opening 54A of the buffer layer 54. As a result, a reduction in yield due to a positional difference between a pattern obtained by etching the silicon-side opening 55A of the silicon layer 55 and a pattern obtained by etching the second opening 54A of the buffer layer 54 can be suppressed.

[0079] In the first DC modulator 32, the opening width L3 of the silicon-side opening 55A of the silicon layer 55 is set to be wider than the opening width L4 of the second opening 54A of the buffer layer 54. As a result, a reduction in yield due to a positional difference between a pattern obtained by etching the silicon-side opening 55A of the silicon layer 55 and a pattern obtained by etching the second opening 54A of the buffer layer 54 can be suppressed.

[0080] In the second DC modulator 33, the opening width L3 of the silicon-side opening 55A of the silicon layer 55 is set to be wider than the opening width L4 of the second opening 54A of the buffer layer 54. As a result, a reduction in yield due to a positional difference between a pattern obtained by etching the silicon-side opening 55A of the silicon layer 55 and a pattern obtained by etching the second opening 54A of the buffer layer 54 can be suppressed.

[0081] Note that in the RF modulator 12 of the fourth embodiment, the silicon layer 55 on the buffer layer 54 extracts light in the film optical waveguide 60 and is therefore responsible for increased absorption of light entering the electrode portion 22. An embodiment for dealing with this situation will be described below as a fifth embodiment.

[0082] [e] Fifth Embodiment

[0083] Figure 7 1 is a schematic cross-sectional view illustrating an example of the RF modulator 12 of the optical modulator 5 of the fifth embodiment. Note that the same components as those of the optical modulator 5 of the fourth embodiment will be denoted by the same reference numerals as those of the fourth embodiment, and thus descriptions of redundant components and operations will be omitted. Figure 7 The RF modulator 12 shown is Figure 4 The RF modulator 12 shown is different in that the buffer layer 54 has a thickness L5 that is greater than a thickness L6 of the film light waveguide 60 , according to the size of the mode field of light propagating through the film light waveguide 60 .

[0084] In the RF modulator 12 of the fifth embodiment, the thickness L5 of the buffer layer 54 is set to be larger than the thickness L6 of the film optical waveguide 60, and thus the amount of light in the film optical waveguide 60 absorbed by the silicon layer 55 on the buffer layer 54 is reduced, which makes it possible to suppress light absorption into the electrode portion 22.

[0085] In the first DC modulator 32 , the thickness L5 of the buffer layer 54 is set larger than the thickness L6 of the film optical waveguide 60 , and thus the amount of light in the film optical waveguide 60 extracted by the silicon layer 55 on the buffer layer 54 is reduced, which makes it possible to suppress light absorption into the electrode portion 22 .

[0086] In the second DC modulator 33 , the thickness L5 of the buffer layer 54 is set larger than the thickness L6 of the film optical waveguide 60 , and thus the amount of light in the film optical waveguide 60 extracted by the silicon layer 55 on the buffer layer 54 is reduced, which makes it possible to suppress light absorption into the electrode portion 22 .

[0087] Note that, for convenience of description, a rib waveguide is exemplified as the film optical waveguide 60; however, a channel waveguide may be used, and changes may be made as appropriate.

[0088] According to a mode of an optical device disclosed herein, etc., the efficiency of applying an electric field can be improved.

Claims

1. An optical device, comprising: Optical waveguides; a buffer layer, the buffer layer being stacked on the optical waveguide; an opening formed in the buffer layer at least over a portion located near a side surface of the optical waveguide; an electrode stacked in the opening and configured to apply a signal to the optical waveguide; as well as A silicon layer is stacked on the buffer layer except for the opening.

2. The optical device according to claim 1, wherein: The optical waveguide is disposed in a given position on the film substrate, The buffer layer is stacked on the film substrate, and The opening is formed in at least a portion of the film substrate located near the side surface of the optical waveguide and in the buffer layer located above the portion.

3. The optical device according to claim 1, further comprising a silicon side opening formed in a portion of the silicon layer overlapping with the opening, in, The optical device is configured such that the electrode stacked in the opening is exposed from the silicon-side opening, and an electrical contact is formed between the electrode and the silicon layer.

4. The optical device according to claim 3, wherein: The electrode exposed from the silicon-side opening is configured such that a width of the electrode is wider than an opening width of the silicon-side opening.

5. The optical device according to claim 3, wherein: The silicon-side opening is configured such that an opening width of the silicon-side opening is wider than an opening width of the opening.

6. The optical device according to claim 1, wherein: The optical waveguide is configured such that a thickness of the buffer layer stacked on the optical waveguide is greater than a thickness of the optical waveguide.

7. The optical device according to claim 1, wherein: The optical waveguide is an optical waveguide of a lithium niobate LN crystal film.

8. The optical device according to claim 7, wherein: The optical waveguide is formed using an X-cut substrate of the film lithium niobate LN crystal.

9. The optical device according to claim 1, wherein: The electrodes are radio frequency, RF, electrodes.

10. The optical device according to claim 1, wherein: The electrodes are direct current (DC) electrodes.

11. An optical communication device, comprising: a processor configured to perform signal processing on the electrical signal; a light source configured to generate light; as well as an optical device configured to modulate light generated from the light source using an electrical signal output from the processor, Wherein, the optical device comprises: Optical waveguides; a buffer layer, the buffer layer being stacked on the optical waveguide; an opening formed in the buffer layer at least over a portion located near a side surface of the optical waveguide; an electrode stacked in the opening and configured to apply a signal to the optical waveguide; and A silicon layer is stacked on the buffer layer except for the opening.

Images