Electrode structure of optical waveguide device and optical waveguide device

Abstract

To provide an electrode structure of an optical waveguide device capable of low voltage driving, and achieving flat frequency characteristics in a broad band of permeability coefficient S21.SOLUTION: There is provided an electrode structure of an optical waveguide device including a channel optical waveguide composed of a ferroelectric thin film having electrooptic effect. The electrode structure comprises: an optical modulation part including an electrode for applying voltage to the channel optical waveguide in a vertical direction to modulate an optical signal; an input part arranged on an input side of the optical modulation part and including an electrode for receiving an electrical signal from an external signal source; and an output part arranged on an output side of the optical modulation part and including an electrode for outputting an electrical signal from the optical modulation part to an external terminal resistor. Each electrode of the input part and the output part for adjusting impedance has electrode length set on the basis of wavelength of an electrical signal of maximum frequency in object band.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode structure of an optical waveguide element made of a single-crystal ferroelectric thin film having an electro-optic effect. [Background technology]

[0002] In optical waveguide devices such as optical modulators, which have coplanar waveguide electrodes for applying a voltage perpendicular to the propagation direction of light incident on the channel optical waveguide, it is preferable to narrow the electrode gap to enable low-voltage operation. However, narrowing the electrode gap reduces the electrode impedance, which can cause an impedance mismatch with the typical 50 Ω impedance of an external signal source. Impedance mismatch degrades the reflection coefficient S11 and transmission coefficient S21 of the electrical signal.

[0003] To solve this problem, it has been proposed to provide an impedance adjustment section between the external signal connection terminal and the optical modulation section (Patent Document 1).Some impedance adjustment sections have a tapered or stepped shape for the gap between the signal electrodes and ground electrodes of the input and / or output sections of the optical waveguide element (Patent Document 2).

[0004] In optical waveguide devices, LiNbO3 is widely used as a single-crystal ferroelectric material for optical waveguides. 1-x La x (Zr y Ti 1-y ) 1-x / 4 With the establishment of a method for producing a single-crystal epitaxial thin film of O3 (PLZT) (Patent Document 3), the application of PLZT ferroelectric thin films to optical waveguide elements is progressing. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-126054 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-72369 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-329959 Summary of the Invention [Problem to be solved by the invention]

[0006] However, conventional impedance adjustment methods have only the impedance adjustment function on the input side, and do not aim to obtain a wide band and flat frequency characteristic, especially for the transmission coefficient S21. An object of the present invention is to realize an electrode structure of an optical waveguide element using a ferroelectric thin film having an electro-optic effect, which can be driven at a low voltage and has a wide band and flat frequency characteristic of the transmission coefficient S21. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention provides the following configuration. [1] An aspect of the present invention is an electrode structure of an optical waveguide element including a channel optical waveguide formed from a ferroelectric thin film having an electro-optic effect, an optical modulation unit including an electrode for applying a voltage perpendicular to the channel optical waveguide to modulate an optical signal; an input section including an electrode that is disposed on the input side of the optical modulation section and to which an electrical signal is input from an external signal source; an output unit including an electrode that is disposed on the output side of the optical modulation unit and outputs an electrical signal from the optical modulation unit to an external termination resistor; The electrodes of the input section and the output section that adjust the impedance have an electrode length that is set based on the wavelength of the electrical signal with the maximum frequency in the target band. [2] In the above aspect, the impedance of each electrode in the input section and the output section is 30Ω or more at the input end and the output end, and the electrode length is 25% or more of the wavelength of the electrical signal of the maximum frequency. [3]In the above aspect, the electrodes of the input part and the output part include a tapered signal electrode and ground electrodes arranged on both sides of the signal electrode with tapered gaps provided respectively, and the angle formed by the ground electrodes on both sides is 50 degrees or less. [4]Another aspect of the present invention is an optical waveguide element having the electrode structure of the optical waveguide element according to any of the above aspects. [5]In the aspect of [4] above, it is characterized in that the optical waveguide element is a Mach-Zehnder interferometer type optical modulator. [6]In the aspect of [4] above, the ferroelectric thin film is a Pb 1-x La x (Zr y Ti 1-y ) 1-x / 4 O3 thin film. [7]In the aspect of [4] above, it is characterized in that the ferroelectric thin film is a BaTiO3 thin film. [8]In the aspect of [4] above, it is characterized in that the film thickness of the ferroelectric thin film is from 100 nm to 3000 nm. [Advantages of the Invention]

[0008] According to the present invention, in the electrode structure of an optical waveguide element using a ferroelectric thin film having an electro-optic effect, low voltage driving is possible, and broadband and flat frequency characteristics with good transmission coefficient S21 of an electrical signal can be realized. [Brief Description of the Drawings]

[0009] [Figure 1] [[ID=3�]]FIG. 1 is a schematic plan view showing a configuration example of the electrode structure of an optical waveguide element. [Figure 2] FIG. 2(a) is a schematic cross-sectional view of line A-A in FIG. 1, and (b) is a schematic cross-sectional view of line B-B in FIG. 1. [Figure 3] FIG. 3 is a graph showing the relationship between the electrode gap and impedance in the optical modulation unit 2. [Figure 4] FIG. 4 is a schematic plan view showing an example of the electrode structure of the present invention. [Figure 5] FIG. 5 shows the change in impedance with the electrode length in the input section on the x-axis for the embodiment of FIG. 4. [Figure 6] FIG. 6 is a diagram similar to FIG. 5, showing the case where the impedance at the input end of the optical modulation section is 30 Ω or more. [Figure 7] FIG. 7 is a graph showing the simulation result of the frequency characteristics of the transmission coefficient S21 of the electrical signal in the electrode structure of the present invention.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the electrode structure of the optical waveguide element according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic plan view schematically showing an example of an optical waveguide element in order to show a configuration example of the electrode structure of the optical waveguide element of the present invention. FIG. 2(a) is a schematic cross-sectional view of the A-A line in FIG. 1, and (b) is a schematic cross-sectional view of the B-B line in FIG. 1.

[0011] The optical waveguide element 5 here assumes an optical modulator. The optical waveguide element 5 includes a substrate 6, an optical waveguide layer (the layer portion is not explicitly shown in FIG. 1; refer to FIG. 2), and electrodes 10, 20, and 30. The optical waveguide layer is formed of an epitaxial ferroelectric thin film. The epitaxial ferroelectric thin film is preferably a Pb 1-x La x (Zr y Ti 1-y ) 1-x / 4 O3 (PLZT, dielectric constant 1000 - 3000) thin film, and the range of 0 < x < 0.3 and 0.2 < y < 1.0 is preferred, and particularly the range of 0.08 < x < 0.1 and 0.6 < y < 0.7 is preferred. As another example, the epitaxial ferroelectric thin film may be BaTiO3. The substrate 6 is a single crystal substrate, preferably sapphire (Al2O3, dielectric constant 9.4).

[0012] The optical waveguide layer has a pair of channel optical waveguides 7, 7 through which input light branches at a Y branch (not shown) and travels. The channel optical waveguides 7, 7 are arranged in parallel with a predetermined gap between them. The channel optical waveguides 7, 7 merge at a second Y branch (not shown) and output light is output.

[0013] The optical modulator is configured as a Mach-Zehnder interferometer. An example of optical modulation will be described. A voltage between the signal electrode 12 and the ground electrode 22, and a voltage between the signal electrode 12 and the ground electrode 32 are applied to the channel optical waveguides 7, 7, respectively. As a result, the phases of the optical signals passing through the channel optical waveguides 7, 7 are inverted and merge, and the amplitude of the output light when a voltage is applied is zero. When no voltage is applied, the amplitude of the output light is the same as that of the input light. This performs optical modulation.

[0014] In the illustrated example, the electrode structure of the present invention includes a signal electrode 10, a ground electrode 20, and a ground electrode 30. Each of the electrodes 10, 20, and 30 has an input section 1 to which an electrical signal is input, an optical modulation section 2 to perform optical modulation, and an output section 3 to which an electrical signal is output. The electrode material is preferably gold (conductivity 4.3×10 7 S / m).

[0015] The input section 1 is arranged on the input side of the optical modulation section 2, and includes electrodes 11, 21, and 31 to which an electrical signal is input from an external signal source 8. The signal electrode in the input section 1 is indicated by reference numeral 11, and the ground electrodes are indicated by reference numerals 21 and 31.

[0016] The optical modulation section 2 includes electrodes 12, 22, and 32 for modulating an optical signal by applying a voltage perpendicularly to the channel optical waveguides 7, 7. The signal electrode in the optical modulation section 2 is indicated by reference numeral 12, and the ground electrodes are indicated by reference numerals 22 and 32.

[0017] The output section 3 is arranged on the output side of the optical modulation section 30, and includes electrodes 13, 23, and 33 that output an electrical signal to the termination resistor 9. The signal electrode in the output section 3 is indicated by reference numeral 13, and the ground electrodes are indicated by reference numerals 23 and 33.

[0018] As shown in FIG. 1, the electrode length in the input section 1 and the output section 3 is indicated by the symbol Lp, and the electrode length in the optical modulation section 2 is indicated by the symbol L.

[0019] As shown in Figure 2(a), the width of the signal electrode 12 of the optical modulation unit 2 is indicated by the symbol W, the width of the ground electrodes 22 and 32 by the symbol WG, and the electrode gap between the signal electrode and the ground electrode by the symbol S. As shown in Figure 2(b), the width of the signal electrode 11 of the input unit 1 is indicated by the symbol Wp, and the electrode gap between the signal electrode and the ground electrode by the symbol Sp. The output unit 3 is arranged symmetrically to the input unit 1, and is therefore similar to the input unit 1.

[0020] As shown in Figure 2, the channel optical waveguides 7, 7 are formed in a ridge-type shape in the optical waveguide layer. The thickness of the ferroelectric thin film forming the optical waveguide layer including the channel optical waveguides 7, 7 can be set to 100 nm to 3000 nm. In addition, an insulator SiO2 (dielectric constant 3.8) is disposed between the electrodes to cover the channel optical waveguides 7, 7.

[0021] Figure 3 is a graph showing the relationship between the electrode gap and impedance in the optical modulation section 2. This is the result of calculating the impedance Z of the optical modulation section 2 when using sapphire with a dielectric constant of 9.4 as the substrate, PLZT with a dielectric constant of 1500 as the ferroelectric thin film, gold as the electrodes, and SiO2 with a dielectric constant of 3.8 as the insulator, with the signal electrode width W set to 10 μm.

[0022] The optical signal propagating through the channel optical waveguides 7, 7 is modulated by a voltage applied by the signal electrode 12 and the ground electrodes 22, 32 arranged on both sides of the channel optical waveguides 7, 7 located in the electrode gap S shown in Fig. 2. Since the modulated optical signal depends on the applied voltage (electric field), in order to achieve low-voltage driving, the narrower the electrode gap between the signal electrode 12 and the ground electrodes 22, 32, the lower the voltage driving becomes.

[0023] To drive the optical modulator under the above conditions at a low voltage, the electrode gap between the signal electrode 12 and the ground electrodes 22, 32 needs to be about 10 μm. As shown in Fig. 3, when the electrode gap S between the signal electrode 12 and the ground electrodes 22, 32 shown in Fig. 2 is in the range of 10 μm to 50 μm, the impedance of the optical modulation section 2 is 10 to 25 Ω.

[0024] On the other hand, a signal source in a general high-frequency circuit, such as signal source 8 in Figure 1, has an impedance of 50 Ω. Therefore, when an electrical signal is input from signal source 8 to optical modulation unit 2, reflection occurs due to impedance mismatch, resulting in degradation of frequency characteristics. In this way, there is a trade-off between low-voltage drive and frequency characteristics in electrical signals.

[0025] In the present invention, the input section 1 and the output section 3 are provided with a function for adjusting the impedance mismatch. Preferably, the impedance of the electrodes of the input section 1 and the output section 3 is set to 30 Ω or more at the input and output ends, respectively. More preferably, it is set to 50 Ω or less, which is the impedance of a typical external signal source. To achieve this, each electrode of the input section 1 and the output section 3 has an electrode length Lp (see FIG. 1) set based on the wavelength of the electrical signal with the highest frequency in the target band of the electrical signal. Specifically, the electrode length Lp is preferably 25% or more of the wavelength of the electrical signal with the highest frequency. In other words, the electrode length Lp is equal to or greater than one-quarter of a wavelength, or π / 2 or greater in phase. In Example 1 described below, the effective relative dielectric constant of the input section is 40, and the wavelength at 10 GHz is 4.7 mm, so the electrode length Lp is 1.2 mm or greater. At 40 GHz, the wavelength is 1.2 mm, so the electrode length Lp is 0.3 mm or greater.

[0026] The signal electrode 12 and the ground electrodes 22, 32 in the optical waveguide element are electrodes of a coplanar line. Therefore, in addition to adjusting the electrode length Lp described above, it is preferable to adjust the electrode gap S between the signal electrode 12 and the ground electrodes 22, 32 to achieve low-voltage driving, and then adjust the impedance by adjusting both the signal electrode width W and the electrode gap S.

[0027] 4 is a schematic plan view showing Example 1 of the electrode structure of the present invention. The dimensions of each part are as follows: Signal electrode width W of optical modulation section 2: 10 μm Electrode length L of optical modulation section 2: 2 mm Electrode width Wp of the input end and output end of the input section 1 and the output section 3: 100 μm Electrode gap Sp between the input end and output end of the input section 1 and the output section 3: 150 μm Electrode length Lp of input section 1 and output section 3: 1.2 mm

[0028] Although not shown in FIG. 4, the channel optical waveguide 7 in Example 1 has a ridge width of 2 μm and a ridge height of 0.3 μm, the optical waveguide layer has a thickness of 0.7 μm, and the sapphire substrate has a thickness of 500 μm.

[0029] Figure 5 shows the change in impedance for the example in Figure 4, with the electrode length Lp in the input unit 1 on the x-axis. x=0 is the input end of the optical modulation unit 2. In this case, the impedance at the input end of the optical modulation unit 2 is 30 Ω or less. The impedance is 15 Ω at the input end of the optical modulation unit 2 and 38 Ω at the input end of the input unit 1. In this way, the signal electrode width and electrode gap are adjusted so that the impedance at the input end of the input unit 1 is 30 Ω or more and 50 Ω or less. The same is true for the output unit 3, although it is not shown.

[0030] Figure 6 is a diagram similar to Figure 5, but shows the case where the impedance at the input end of the optical modulation unit 2 is 30 Ω or more. In this case, too, the signal electrode width and electrode gap are adjusted so that the impedance at the input end of the input unit 1 is 30 Ω or more and 50 Ω or less. The same applies to the output unit 3.

[0031] In this way, the impedance of the input and output terminals can be set to 30 Ω or more and 50 Ω or less by adjusting the signal electrode width and electrode gap of the input section 1 and the output section 3. It is preferable to set it as close to 50 Ω as possible to match the impedance.

[0032] 7 is a graph showing the simulation results of the frequency characteristics of the transmission coefficient S21 of an electric signal in the electrode structure of the present invention. As can be seen from the graph, a flat frequency characteristic is obtained up to about 40 GHz.

[0033] More preferably, the flat frequency characteristic can be improved by adjusting the termination resistor 9 in FIG.

[0034] As shown in the embodiment of FIG. 4, the impedance can be continuously changed by using tapered electrodes in the input section 1 and the output section 3. The tapered electrode shape includes a tapered signal electrode and a pair of ground electrodes arranged on both sides of the signal electrode with a tapered gap. The angle between the ground electrodes on both sides (see symbol α in FIG. 1) is preferably 50 degrees or less. However, the electrode shape is not limited to a tapered shape and can also be changed in a stepwise manner. In this case, the impedance changes stepwise, but as long as the impedance is within the above-mentioned allowable range, the impedance adjustment function can be achieved.

[0035] Although the present invention has been described above with reference to exemplary configurations, various modifications are possible in keeping with the spirit of the present invention, and these modifications are also encompassed by the present invention. [Explanation of symbols]

[0036] 1 Input section 2 Optical modulation section 3 Output section 4. Insulators 5 Optical waveguide elements 6 PCB 7-channel optical waveguide 8 Signal source 9 Termination Resistor 10 signal electrode 11 Signal electrode (input section) 12 Signal electrode (optical modulation section) 13 Signal electrode (output section) 20 Ground electrode 21 Ground electrode (input section) 22 Ground electrode (optical modulation section) 23 Ground electrode (output section) 30 Ground electrode 31 Ground electrode (input section) 32 Ground electrode (optical modulation section) 33 Ground electrode (output section) Lp, L electrode length Wp, W signal electrode width Sp, S Electrode gap width

Claims

1. An electrode structure of an optical waveguide element including a channel optical waveguide formed from a ferroelectric thin film having an electro-optic effect, an optical modulation unit including an electrode for applying a voltage perpendicular to the channel optical waveguide to modulate an optical signal; an input section including an electrode that is disposed on the input side of the optical modulation section and to which an electrical signal is input from an external signal source; an output unit including an electrode that is disposed on the output side of the optical modulation unit and outputs an electrical signal from the optical modulation unit to an external termination resistor; 1. An electrode structure of an optical waveguide element, wherein each electrode of the input section and the output section that adjusts impedance has an electrode length set based on the wavelength of an electrical signal with a maximum frequency in a target band.

2. 2. The electrode structure of an optical waveguide element according to claim 1, wherein the impedance of each electrode in the input section and the output section is 30 Ω or more at the input end and the output end, and the electrode length is 25% or more of the wavelength of the electrical signal of the maximum frequency.

3. 2. The electrode structure of an optical waveguide element according to claim 1, wherein the electrodes of the input and output sections include a tapered signal electrode and a ground electrode disposed on both sides of the signal electrode with a tapered gap therebetween, and the angle formed by the ground electrodes on both sides is 50 degrees or less.

4. An optical waveguide element having the electrode structure of the optical waveguide element according to any one of claims 1 to 3.

5. 5. The optical waveguide element according to claim 4, wherein the optical waveguide element is a Mach-Zehnder interferometer type optical modulator.

6. The ferroelectric thin film is Pb in the range of 0<x<0.3 and 0.2<y<1.

0. １－ｘ La ｘ (Zr ｙ Ti １－ｙ ) １－ｘ／４ O ３ 5. The optical waveguide element according to claim 4, which is a thin film.

7. The ferroelectric thin film is BaTiO ３ 5. The optical waveguide element according to claim 4, which is a thin film.

8. 5. The optical waveguide element according to claim 4, wherein the thickness of the ferroelectric thin film is 100 nm to 3000 nm.

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