Optical waveguide element, optical modulation device using the same, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element in which light absorption is suppressed even when an electrode interval is narrowed.　This optical waveguide element has a substrate (1) in which an optical waveguide (10) is formed, and a control electrode (3) that is arranged close to the optical waveguide on the substrate, wherein the optical waveguide element is characterized in that the optical waveguide (10) is a convex optical waveguide, and the shape of a side surface of the control electrode (3) that faces the optical waveguide is constituted from an inclined surface (32) having a prescribed angle from the substrate, and a curved surface (33) that connects to the inclined surface and forms a curved recess.

Description

Technical Field

[0001] The present invention relates to an optical waveguide device, an optical modulation device using the same, and an optical transmission device, and particularly to an optical waveguide device having a substrate on which an optical waveguide is formed and a control electrode disposed on the substrate in proximity to the optical waveguide, an optical modulation device using the same, and an optical transmission device.

Background Art

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide devices such as optical modulators using a substrate on which an optical waveguide is formed are widely used. In a general optical waveguide device, an optical waveguide is formed on a substrate having an electro-optic effect such as lithium niobate (LN), and a control electrode for applying an electric field to the optical waveguide is formed on the substrate.

[0003] As shown in FIG. 1, in Patent Document 1, the control electrode 3 is configured in a two-stage structure, and the first-stage electrode 30 is formed thinly, so that even if the electrode interval is narrow, it can be accurately manufactured and the driving voltage can be reduced. Moreover, by forming the second-stage electrode 31 thick in an inverted trapezoid shape, it is possible to prevent deterioration of high-frequency characteristics. Note that reference numeral 1 denotes a substrate, reference numeral 2 denotes an optical waveguide, and reference numeral 4 denotes a buffer layer.

[0004] As shown in FIG. 2, in Patent Document 2, it is disclosed that the side surface of the control electrode 3 has an inclination and the cross section is formed in a trapezoidal shape. With this configuration, an increase in the driving voltage can be suppressed, high-frequency characteristics can be improved, and furthermore, the manufacturing cost can be suppressed.

[0005] In recent years, high-bandwidth-coherent driver modulators (HB-CDMs) have attracted attention. The optical waveguides formed on the substrate are typically convex optical waveguides (e.g., rib-type waveguides, ridge-type waveguides, slot-type waveguides) with a width and height of about 1 μm and composed of convex sections extending in a band shape. Such fine convex waveguides have strong light confinement, making it possible to bend the optical waveguide with a small curvature, and allowing for the compact formation of optical waveguide elements.

[0006] However, the spacing between control electrodes, such as the distance between the signal electrode and the ground electrode, or the distance between the DC bias electrodes, has decreased from the conventional tens of micrometers to just a few micrometers, resulting in extremely narrow electrode spacing. This makes it easier for the electrodes to absorb light waves propagating through the optical waveguide, leading to a problem of increased light wave propagation loss (optical absorption loss).

[0007] Furthermore, if an electrode structure like that in Patent Document 1 is adopted, adjacent electrodes are too close together in the upper (second) stage, making such a structure difficult to implement. Also, if an inclined surface like that in Patent Document 2 is provided, the convex optical waveguide and the inclined surface of the electrode are in close proximity, making it difficult to effectively suppress light absorption by the electrode. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] JP1997-185025A (JP1997-185025A) [Patent Document 2] Japanese Patent Publication No. 2011-215294 (JP2011-215294A) [Overview of the project] [Problems that the invention aims to solve]

[0009] The problem that this invention aims to solve is to provide an optical waveguide element that solves the above-mentioned problems and suppresses light absorption even when the electrode spacing is narrowed. Furthermore, it aims to provide an optical modulation device and an optical transmission device using this optical waveguide element. [Means for solving the problem]

[0010] To solve the above problems, the optical waveguide element, optical modulation device using the same, and optical transmission device of the present invention have the following technical features. (1) An optical waveguide element having a substrate on which an optical waveguide is formed and a control electrode disposed on the substrate in close proximity to the optical waveguide, wherein the optical waveguide is a convex optical waveguide, and the shape of the side surface of the control electrode facing the optical waveguide is composed of an inclined surface having a predetermined angle with respect to the substrate and a curved surface that follows the inclined surface and forms a curved recess.

[0011] (2) The optical waveguide element described in (1) above is characterized in that the position where the inclined surface changes to a curved surface is located at a position lower than the height of the optical waveguide.

[0012] (3) The optical waveguide element described in (1) above is characterized in that the thickness of the electrode is 1 μm or less.

[0013] (4) The optical waveguide element described in any of (1) to (3) above is housed in a housing and is an optical modulation device characterized by comprising an optical fiber that inputs or outputs an optical wave to the optical waveguide.

[0014] (5) In the optical modulation device described in (4) above, the control electrode is a modulation electrode for modulating the light wave propagating through the optical waveguide, and the device is characterized in that it has an electronic circuit inside the housing for amplifying the modulation signal input to the modulation electrode.

[0015] An optical transmission device comprising the optical modulation device according to (5) above, a light source for inputting light waves to the optical modulation device, and an electronic circuit for outputting a modulation signal to the optical modulation device.

Advantages of the Invention

[0016] In an optical waveguide element having a substrate on which an optical waveguide is formed and a control electrode disposed on the substrate in proximity to the optical waveguide, the optical waveguide is a convex optical waveguide, and the shape of the side surface of the control electrode facing the optical waveguide is composed of an inclined surface having a predetermined angle with the substrate and a curved surface that follows the inclined surface and forms a curved depression. Therefore, the distance between the convex optical waveguide and the control electrode, particularly the distance between the two at the upper part of the control electrode, can be increased, so that light absorption by the control electrode can be suppressed. Furthermore, by using an optical waveguide element having such excellent characteristics, it is possible to provide an optical modulation device and an optical transmission device that exhibit similar effects.

Brief Description of the Drawings

[0017] [Figure 1] A cross-sectional view showing an example of a conventional optical waveguide element. [Figure 2] A cross-sectional view showing another example of a conventional optical waveguide element. [Figure 3] A cross-sectional view showing an example of the optical waveguide element of the present invention. [Figure 4] A diagram showing an example in the case where the entire side surface of the control electrode is formed by a curve forming a curved depression. [Figure 5] A diagram for explaining the shape of the control electrode used in the optical waveguide element of the present invention. [Figure 6] A diagram for explaining the manufacturing method of the optical waveguide element shown in FIG. 3. [Figure 7] A diagram for explaining the state before lift-off in the manufacturing method of FIG. 6. [Figure 8] A diagram for explaining the change in optical absorption loss due to the shape of the side surface of the control electrode. [Figure 9]This is a diagram showing an example of an optical waveguide device used in HB-CDM. [Figure 10] FIG. 9 shows a cross-sectional view (a) of the modulation electrode RF and a cross-sectional view (b) of the DC bias electrode. [Figure 11] This is a diagram showing an example of an optical transmission device of the present invention.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, the optical waveguide device of the present invention will be described in detail using preferred examples. FIG. 3 shows a cross-sectional view showing an example of the optical waveguide device of the present invention. The optical waveguide device of the present invention is an optical waveguide device having a substrate 1 on which an optical waveguide 10 is formed and a control electrode 3 disposed on the substrate in proximity to the optical waveguide. The optical waveguide 10 is a convex optical waveguide, and the shape of the side surface of the control electrode 3 facing the optical waveguide is composed of an inclined surface 32 at a predetermined angle from the substrate and a curved surface 33 that follows the inclined surface and forms a curved depression. [[ID=二十]]

[0019] [[ID=二十一]] [[ID=二十二]]As the substrate 1 used in the optical waveguide device of the present invention, a substrate having an electro-optic effect can be used. Specifically, substrates such as lithium niobate (LN), lithium tantalate (LT), PLZT (lead lanthanum zirconate titanate), and substrates doped with MgO or the like in these substrate materials can be used. Further, it is also possible to form a film using a vapor phase growth method such as sputtering, evaporation, or CVD method for these materials. Furthermore, a semiconductor substrate or the like can also be used. [[ID=二十三]] [[ID=二十四]]

[0020] [[ID=二十五]] As a method for forming the optical waveguide 10, it is possible to use a convex optical waveguide in which the portion of the substrate corresponding to the optical waveguide is made convex by etching the substrate 1 other than the optical waveguide or forming grooves on both sides of the optical waveguide. It is also possible to use a slot-type waveguide in which the portion other than the optical waveguide is completely removed by etching or other methods. Furthermore, it is possible to increase the refractive index by diffusing Ti or other materials onto the substrate surface by thermal diffusion or proton exchange methods in accordance with the convex optical waveguide. In terms of size, the convex optical waveguide is a fine convex optical waveguide with a width and height of about 1 μm in order to enhance light confinement.

[0021] The thickness of the substrate (thin plate) 1 on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to match the speed of the microwave and optical waves of the modulated signal. The height of the convex optical waveguide is set to 4 μm or less, more preferably 3 μm or less, and even more preferably 1 μm or less or 0.4 μm or less.

[0022] To increase the mechanical strength of the substrate 1 on which the optical waveguide is formed, a reinforcing substrate (not shown) is bonded to the underside of the substrate 1. The substrate 1 and the reinforcing substrate are bonded together either directly or via an adhesive layer such as resin. In the case of direct bonding, an intermediate layer such as a metal oxide or metal may be included. The reinforcing substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but is not limited to this. Furthermore, the reinforcing substrate is preferably made of a material with a similar coefficient of thermal expansion to the substrate 1, such as a substrate containing an oxide layer of quartz or glass. In addition, it is also possible to use the same LN substrate as the substrate 1, or a composite substrate in which a silicon oxide layer is formed on a silicon substrate abbreviated as SOI or LNOI, or a composite substrate in which a silicon oxide layer is formed on an LN substrate.

[0023] A control electrode 3 is formed on the substrate 1 in close proximity to the optical waveguide 10. The control electrode includes a modulation electrode that applies a modulation signal to the optical waveguide and a DC bias electrode that applies a DC bias voltage. The control electrode is formed by first creating a base electrode using a sputtering method, a vapor deposition method, etc., and then forming a thick electrode using a plating method. In this invention, the shape of the side surface of the control electrode 3 facing the optical waveguide 10 is composed of an inclined surface 32 having a predetermined angle with respect to the substrate, and a curved surface 33 that follows the inclined surface 32 and forms a curved recess.

[0024] One way to retract the side surface of the control electrode from the optical waveguide is to form a curved surface 34 that forms a curved recess across the entire side surface of the control electrode, as shown in Figure 4. However, with such an electrode shape, the electrode close to the optical waveguide (the electrode at the bottom of the curved surface 34) is thin, making it difficult to effectively apply an electric field to the optical waveguide. Moreover, the electrode at the bottom of the curved surface 34 is extremely thin, leading to the problem of it easily peeling off from the substrate 1.

[0025] Therefore, as shown in Figure 5, the lower part of the control electrode 3 is an inclined surface 32 having a predetermined angle θ with respect to the substrate 1. Considering the efficiency of applying the electric field (Vπ) and the adhesion of the electrode (prevention of electrode peeling), the height Ay of the inclined surface 32 in Figure 5 is 10 nm or more, more preferably 100 nm or more. Also, the angle θ is set to 30 degrees ≤ θ ≤ 90 degrees. Considering the adhesion between the electrode and the substrate, an angle θ of 30 degrees or more is preferable. Furthermore, if the electrode is formed with an angle θ exceeding 90 degrees, the electrode approaches the optical waveguide and light absorption by the electrode occurs, so an angle θ of 90 degrees or less is preferable.

[0026] As shown in Figure 3, the height h at the point where the inclined surface 32 changes to a curved surface 33 is located lower than the height H of the optical waveguide 10. For the curved surface 33 of the present invention to function effectively, if the inclined surface 32 extends higher than the height H of the optical waveguide, the light waves spreading from the optical waveguide 10, particularly the light waves on the upper side of the optical mode diameter, are absorbed by the inclined surface. The curved surface 33 of the present invention increases the distance between the optical waveguide 10 and the control electrode, suppressing the absorption of light waves.

[0027] Considering the optical bandwidth (high frequency characteristics), it is preferable that the sum of Ay (height of the inclined surface) and By (height of the curved surface) in Figure 5 be 200 nm or more. Naturally, as shown in Figure 3, the top of the curved surface 33 will be higher than the height H of the optical waveguide.

[0028] The thickness (Ay+By) of the control electrode 3 is preferably 1000 nm or less, considering the manufacturing process such as lift-off described later, as this is the range in which it can be stably formed.

[0029] The length (Ax + Bx) from the position of the control electrode 3 adjacent to the optical waveguide 10 to the top of the curved surface 33 is preferably set to 100 nm or more, more preferably 200 nm or more, in order to reduce losses due to electrode absorption. Here, Ax is the length in the lateral direction (left-right direction in the drawing) of the inclined surface, and Bx is the length in the lateral direction of the curved surface.

[0030] In this invention, the "curved surface that forms a curved depression" means, as shown in Figure 5, that the surface of the curved surface 33 is recessed toward the electrode side than the dashed line D connecting the two ends of the curved surface. Furthermore, the position of the curved surface 33 is set back toward the electrode side than the dashed line C, which is the extension of the inclined surface 32.

[0031] Figure 6 illustrates the process of forming the control electrode. Figure 6(a) shows the state in which a resist film 5 is applied to a substrate 1 on which an optical waveguide 10 is formed. The resist film is exposed to light using UV, laser, electron beam, etc., and removed by development, and processed into the shape shown in Figure 6(b). In particular, it is preferable that the resist film 5 is undercut near the contact area between the resist film 5 and the substrate 1 (see dotted line frame E).

[0032] In Figure 6(c), the material that will become the electrode 3 is deposited on the surface of the substrate 1 containing the resist film 5 using methods such as vapor deposition, plating, sputtering, or CVD. Next, the electrode structure shown in Figure 6(d) is formed by lifting off the resist film 5. Figure 6 shows a magnified view of the area around the convex optical waveguide 10.

[0033] Figure 7 is a magnified view of the lower part of the resist film 5 before lift-off. When the resist film 5 is lift-off, an inclined surface 32 is formed on the undercut surface 51 of the resist film, and a depression (curved surface 33) is formed between the resist film 5 and the corner of the electrode 3 along the dotted line.

[0034] To evaluate the effect of the side shape of the control electrode of the present invention, simulations of the optical absorption loss (Loss, dB) were performed for side shapes that were rectangular only, trapezoidal (inclined only) as shown in Figure 2, and a combination of inclined and curved surfaces as in the present invention. The graph in Figure 8 shows the results of evaluating the optical absorption loss by changing the gap between the control electrodes shown in Figure 3 from 3.5 μm to 5.0 μm. The wavelength of the light wave used was 1570 nm. The graph at the top of Figure 8 represents the case of a rectangular electrode, the graph in the middle represents the case of a trapezoidal electrode, and the graph at the bottom represents the shape of the present invention.

[0035] It can be understood that when the electrode spacing GAP is 5 μm or more, the effect of changes in the shape of the electrode sides is almost negligible. However, as the electrode spacing GAP narrows, especially when it is 4 μm or less, the effect of the shape of the electrode sides becomes significant, and it can be understood that the electrode shape of the present invention effectively suppresses light absorption loss.

[0036] Figure 9 is a plan view of substrate 1, showing an example of an optical waveguide element used in HB-CDM and the like. The optical waveguide 10 consists of two nested optical waveguides, each incorporating multiple Mach-Zehnder type optical waveguides in a nested manner, arranged in parallel. In such a complex optical waveguide, the width and height of the optical waveguide are extremely narrow. Similarly, the thickness of the electrodes is also thin. Therefore, in the region where the modulation electrode is formed (code RF) and the region where the DC bias electrode is formed (code DC), it is possible to employ a configuration using resin materials or inorganic dielectric materials, also known as "permanent resist" (PR), as shown in Figure 10. Examples of resin materials include polyamide resins, melamine resins, phenolic resins, amino resins, and epoxy resins. Examples of inorganic dielectric materials include SiO2, Al2O3, MgF, La2O3, ZnO, MgO, CaF2, and Y2O3.

[0037] Figure 10(a) is an example of a cross-sectional view showing a modulation electrode for region RF. Below the signal electrode 3S and ground electrode 3G, the shape of the control electrode of the present invention described above is adopted, and a permanent resist PR is arranged to cover a portion of these control electrodes and the optical waveguide 10. The permanent resist suppresses scattering due to the roughness of the optical waveguide surface and prevents the electrodes from peeling off from the substrate. Furthermore, by arranging the electrodes (3S, 3G) to cover a portion of the permanent resist PR, it is possible not only to improve the high-frequency characteristics of the modulation electrode but also to suppress the peeling of the permanent resist.

[0038] In Figure 10(b), the electrodes are not positioned to cover the permanent resist PR because there is no need to bring the upper part of the DC bias electrode (3D) into close proximity. The effect of the permanent resist on the optical waveguide 10 and the area below the electrodes is as described above.

[0039] Next, we will describe examples of applying the optical waveguide element of the present invention to optical modulation devices and optical transmitting devices. In the following description, we will use an example of HB-CDM, but the present invention is not limited to this and can also be applied to optical phase modulators, optical modulators with polarization combining functions, optical waveguide elements integrating more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide elements made of other materials such as silicon, and devices for sensor applications.

[0040] As shown in Figure 11, the optical waveguide element has an optical waveguide 10 formed on an optical waveguide substrate 1 and control electrodes (not shown), such as modulation electrodes, that modulate the light waves propagating through the optical waveguide 10, and is housed in a housing CA. Furthermore, by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide, an optical modulation device MD can be constructed. In Figure 11, the optical fiber F is optically coupled to the optical waveguide 10 in the optical waveguide element using an optical block equipped with an optical lens, a lens barrel, a polarization multiplexer 6, etc. However, it is not limited to this, and the optical fiber may be introduced into the housing through a through-hole that penetrates the side wall of the housing and the optical fiber may be directly joined to an optical component or substrate, or an optical fiber with a lens function at the end of the optical fiber may be optically coupled to the optical waveguide in the optical waveguide element. In addition, to ensure stable joining with the optical fiber and optical block, it is also possible to place reinforcing members (not shown) on top of each other along the end face of the optical waveguide substrate 1.

[0041] An optical transmitter (OTA) can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So to the optical modulation device MD to perform modulation operations on the optical modulation device MD. In order to obtain the modulation signal S to be applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. For this reason, in Figure 11, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and the digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.

[0042] The input light L1 to the optical modulation device MD may be supplied from outside the optical transmitter OTA, but as shown in Figure 11, a semiconductor laser (LD) can also be used as the light source. The output light L2 modulated by the optical modulation device MD is output externally via an optical fiber F. [Industrial applicability]

[0043] As described above, the present invention makes it possible to provide an optical waveguide element that suppresses light absorption even when the electrode spacing is narrowed. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using this optical waveguide element. [Explanation of symbols]

[0044] 1. Substrate (thin plate, film) for forming an optical waveguide 3. Control electrodes 10 Optical waveguide 32 Slope 33 Curved surface F Optical Fiber LD light source CA cabinet MD Optical Modulation Device DRV driver circuit DSP (Digital Signal Processor) OTA Optical Transmitter

Claims

1. An optical waveguide element having a substrate on which an optical waveguide is formed and a control electrode arranged on the substrate in close proximity to the optical waveguide, The optical waveguide is a convex optical waveguide, An optical waveguide element characterized in that the shape of the side surface of the control electrode facing the optical waveguide is composed of an inclined surface having a predetermined angle with respect to the substrate and a curved surface that follows the inclined surface and forms a curved recess.

2. An optical waveguide element according to claim 1, characterized in that the position where the inclined surface changes to a curved surface is located at a position lower than the height of the optical waveguide.

3. An optical waveguide element according to claim 1, characterized in that the thickness of the electrode is 1 μm or less.

4. The optical waveguide element according to any one of claims 1 to 3 is housed in a housing, An optical modulation device characterized by comprising an optical fiber that inputs or outputs an optical wave to the optical waveguide.

5. In the optical modulation device according to claim 4, The control electrode is a modulation electrode for modulating the light wave propagating through the optical waveguide. An optical modulation device characterized by having an electronic circuit inside the housing that amplifies the modulation signal input to the modulation electrode.

6. The optical modulation device according to claim 5, A light source that inputs light waves to the optical modulation device, An optical transmitting device characterized by having an electronic circuit that outputs a modulation signal to the optical modulation device.

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