JP2004191539A - Waveguide type optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make speeding up and expanding of a working temperature range be compatible with each other in relation to a waveguide type optical device. <P>SOLUTION: The waveguide type optical device is provided with a waveguide substrate 2 formed of a material exhibiting an electrooptic effect and having a core part 4 to waveguide light and electrodes 22, 24, 26 disposed on the waveguide substrate to apply an electric field to the core part. The waveguide type optical device comprises the electrodes which contain a first part having a thermal expansion coefficient larger than that of the waveguide substrate and a second part having a thermal expansion coefficient smaller than that of the waveguide substrate. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a waveguide type optical device such as a waveguide type optical modulator.
[0002]
[Prior art]
Conventionally, a relatively high refractive index core is formed on a waveguide substrate exhibiting an electro-optic effect such as LiNbO ₃ and LiTaO _2, and electrodes for applying an electric field are formed on the core on the waveguide substrate. Waveguide optical devices are known. A waveguide-type phase modulator can be obtained by forming a linear core portion by patterning technology, and a waveguide-type intensity modulator can be obtained by modifying the shape of the core portion so as to function as a Mach-Zehnder interferometer.
[0003]
In the Mach-Zehnder type intensity modulator, the core section includes an input waveguide to which light of a constant intensity is input, first and second parallel waveguides formed so as to branch off from the tip of the input waveguide, And an output waveguide where the first and second parallel waveguides merge. First and second electrodes are formed on the first and second parallel waveguides, respectively.
[0004]
By applying a voltage of a predetermined magnitude between the first and second electrodes, electric fields in opposite directions are applied to the first and second parallel waveguides, and the first and second parallel waveguides are applied. One of the parallel waveguides has a higher refractive index and the other has a lower refractive index. As a result, a phase difference occurs in the light guided to the first and second parallel waveguides, and when the two lights having the phase difference merge in the output waveguide, the light having the intensity corresponding to the phase difference is generated. Output light will be obtained. For example, by setting the phase difference to be zero or π in accordance with the binary value of the binary input signal, efficient intensity modulation can be performed.
[0005]
When the above-described Mach-Zehnder modulator is applied to high-speed modulation in which a modulation signal is in a microwave region, the first and second electrodes are configured to have a traveling wave type. That is, one of the first and second electrodes is a ground electrode, the other is a signal electrode, one end of which is terminated with a resistor, and a modulation signal as a microwave is input from the other end.
[0006]
Since the effective refractive index of the microwave can be adjusted by changing the cross-sectional shape of the electrode and the like, broadband optical response characteristics can be obtained by matching the speeds of the light and the microwave.
[0007]
As a related prior art, there is an optical waveguide device disclosed in Japanese Patent Application Laid-Open No. 2001-133641.
[0008]
[Problems to be solved by the invention]
When trying to optimize the cross-sectional shape of a traveling-wave-type electrode in order to match the speed of light and microwaves, the thickness of the electrode may be required to be 10 μm or more. In this case, the stress that occurs when there is a temperature change due to the difference in the linear thermal expansion coefficient between the electrode and the waveguide substrate increases, causing problems such as an unstable operating point and the separation of the electrode from the waveguide substrate. Occurs.
[0009]
As described above, in the case of the related art, there is a problem that it is difficult to achieve both a high modulation speed and a wide operating temperature range.
[0010]
Therefore, an object of the present invention is to provide a waveguide-type optical device that can achieve both a high modulation speed and a wide operating temperature range. Other objects of the present invention will become clear from the following description.
[0011]
[Means for Solving the Problems]
According to the present invention, there is provided a waveguide substrate formed of a material exhibiting an electro-optical effect and having a core portion for guiding light, and an electrode provided on the waveguide substrate for applying an electric field to the core portion. A waveguide optical device is provided. This waveguide-type optical device has a first portion in which an electrode has a larger thermal expansion coefficient than the waveguide substrate and a second portion in which an electrode has a smaller thermal expansion coefficient than the waveguide substrate. Is characterized in that
[0012]
Generally, it is necessary to provide an electrode having a coefficient of thermal expansion that exactly matches the coefficient of thermal expansion of a waveguide substrate exhibiting an electro-optic effect suitable for providing a waveguide-type optical device and providing an electrode. It is difficult to select an appropriate conductor.
[0013]
According to the configuration of the present invention, an electrode is formed from the first portion having a larger coefficient of thermal expansion than the waveguide substrate and the second portion having a smaller coefficient of thermal expansion than the waveguide substrate. With this configuration, the apparent thermal expansion coefficient of the electrode can be made to match the thermal expansion coefficient of the waveguide substrate, and a waveguide type that can achieve both a higher modulation speed and a wider operating temperature range. An optical device can be provided.
[0014]
For example, the waveguide substrate is made of LiNbO ₃ , the material of the second part is gold, and the material of the first part is selected from the group consisting of silver, aluminum and copper.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0016]
FIG. 1 is a plan view showing a first embodiment of a waveguide type optical device according to the present invention, and FIG. 2 is a sectional view taken along line II-II. Here, a Mach-Zehnder optical modulator is shown as an example of a waveguide optical device.
[0017]
As shown in FIG. 1, this optical modulator is formed on a waveguide substrate 2 made of a material exhibiting an electro-optic effect such as LiNbO ₃ and LiTaO _2, and on the surface of the waveguide substrate 2 or very close to the surface. And an electrode (22, 24, and 26) formed on the waveguide substrate 2 for applying an electric field to the core portion 4.
[0018]
The core unit 4 includes an input waveguide 8 for transmitting the light supplied to the optical input port 6 and a parallel conductor for connecting the input waveguide 8 at the Y branch 10 and transmitting the light bisected by the Y branch 10. Waveguides 12 and 14 and output waveguide 18 connected to parallel waveguides 12 and 14 by Y branch 16 and propagating light from parallel waveguides 12 and 14 joined by Y branch 16 are included. The output end of the output waveguide 18 becomes the output port 20 of this optical modulator.
[0019]
A signal electrode 22 is provided on the parallel waveguide 12, and a ground electrode 24 is provided on the parallel waveguide 14. Another ground electrode 26 is provided on the side of the signal electrode 22 opposite to the ground electrode 24. A microwave as a modulation signal is supplied to one end of the signal electrode 12 from an input terminal 28, and the other end of the signal electrode 12 is terminated by a resistor 30.
[0020]
As shown in FIG. 2, the core portion 4 (parallel waveguides 12 and 14 in the illustrated range) is formed so that a portion near the surface of the waveguide substrate 2 has a partially high refractive index. For example, the core portion 4 can be obtained by forming a high refractive index portion by Ti (titanium) diffusion or proton exchange on the surface of the waveguide substrate 2 made of z-cut LiNbO ₃ .
[0021]
On the waveguide substrate 2, a dielectric layer (buffer layer) 32 made of SiO ₂ or the like is formed. The reason why the dielectric layer 32 is provided is that, in the absence of this, the light propagating through the core portion 4 is absorbed by the metal electrode to increase the loss, and the thickness of the dielectric layer 32 is, for example, 0.2 to 1. 0.0 μm.
[0022]
On the dielectric layer 32, metal layers 34, 36 and 38 made of, for example, Ti are provided at positions where the ground electrode 26, the signal electrode 22 and the ground electrode 24 are to be formed, in order to improve the adhesion to the dielectric layer 32. Are formed respectively. Then, the ground electrode 26, the signal electrode 22, and the ground electrode 24 having a multilayer structure are formed on the metal layers 34, 36, and 38 by vapor deposition or plating, respectively. As a material of the metal layers 34, 36 and 38, Cr can be used.
[0023]
In this embodiment, each of the ground electrode 26, the signal electrode 22, and the ground electrode 24 is configured by stacking an Ag (silver) layer and an Au (gold) layer in this order from the waveguide substrate 2 side. The total thickness of the Ag layer and the Au layer is set to, for example, several tens of μm in order to achieve velocity matching between light and microwave in the traveling wave type, and the thickness ratio is set according to the coefficient of thermal expansion. Specifically, it is as follows.
[0024]
The thermal expansion coefficient of LiNbO ₃ as the material of the waveguide substrate 2 is 15.4 × 10 ⁻⁶ ° C.- ¹ . It is difficult to find a metal material that has the same coefficient of thermal expansion and that is suitable for the electrode. Therefore, in this embodiment, Au layer and larger thermal expansion coefficient than the thermal expansion coefficient of the LiNbO ₃ having a smaller thermal expansion coefficient than (14.1 × ^{10 -6} ℃ ^-1) thermal expansion coefficient of the LiNbO ₃ (19 2 × 10 ⁻⁶ ° C. ⁻¹ ), the apparent thermal expansion coefficient of the entire electrode is made to match the thermal expansion coefficient of the waveguide substrate 2.
[0025]
As a result, even if the thickness of the electrode is set to a desired value suitable for a traveling wave type of several tens μm or more, there is no possibility that a trouble due to a difference in the coefficient of thermal expansion between the electrode and the waveguide substrate occurs, and the modulation speed is increased. Thus, it is possible to provide a waveguide type optical device capable of achieving both the realization and the expansion of the operating temperature range.
[0026]
In addition, as a metal having a thermal expansion coefficient larger than that of LiNbO ₃ , in addition to Ag, Al (23.2 × 10 ⁻⁶ ° C. ⁻¹ ) or Cu (16.8 × 10 ⁻⁶ ° C. ⁻¹⁾ ).
[0027]
In this embodiment, placing the Ag layer under the Au layer minimizes the exposed area of the Ag layer, which has relatively low electrochemical stability, and increases the chemical stability of the electrode. That's why.
[0028]
FIG. 3 is a sectional view showing a second embodiment of the waveguide type optical device according to the present invention, and the sectional position is the same as that of FIG. In the first embodiment, since each of the signal electrode 22 and the ground electrodes 24 and 26 has a substantially two-layer structure, there is a possibility that each electrode may be curved when there is a temperature change due to the bimetal effect. is there. Therefore, in this embodiment, the signal electrode 22 and the ground electrodes 24 and 26 are each formed in substantially three layers to prevent this.
[0029]
That is, in this embodiment, the Ag layer, the Au layer, and the Ag layer are laminated in this order on the metal layers 34, 36, and 38, respectively, to prevent the electrode from being curved due to the difference in the thermal expansion coefficient between Ag and Au. are doing.
[0030]
In this embodiment, each of the signal electrode 22 and the ground electrodes 24 and 26 has substantially three layers to prevent the electrode from being curved. However, a structure having four or more layers may be employed. Alternatively, various materials may be selected and laminated.
[0031]
FIG. 4 is a sectional view showing a third embodiment of the waveguide type optical device according to the present invention, and the sectional position is the same as that of FIG.
[0032]
In this embodiment, each of the signal electrode 22 and the ground electrodes 24 and 26 are connected to the ground electrode 24 and 26 in order to reduce the exposed area of the Ag layer having a relatively low electrochemical stability to increase the chemical stability of the electrodes. It is composed of one Ag layer and an Au layer formed so as to surround them. Such a multilayered electrode can be obtained, for example, by patterning a plurality of times.
[0033]
As described above, according to the present embodiment, the exposed portion of the electrode becomes a chemically stable Au layer, so that it is possible to prevent the electrode from deteriorating with time and to ensure long-term reliability of the waveguide type optical device. Will be possible.
[0034]
FIG. 5 is a plan view showing a fourth embodiment of the waveguide type optical device according to the present invention. In this embodiment, a linear core portion 4 'is provided, and an optical phase modulator is provided.
[0035]
The signal electrode 22 is provided on the core part 4 ′, and the ground electrodes 24 and 26 are arranged together with the signal electrode 22 so as to effectively apply an electric field to the core part 4 ′. It is the same as the first embodiment in that it is configured as a traveling wave type so that the velocity modulation between the microwave modulation signal and the propagation light of the core unit 4 ′ is performed.
[0036]
When the electric field is applied to the core portion 4 'by the signal electrode 22 and the ground electrodes 24 and 26, the refractive index of the core portion 4' is changed. Accordingly, the phase modulation is performed on the light propagating through the core portion 4 '. You. Also in this phase modulation, the point that the cross-sectional shape of the traveling-wave-type electrode affects the high-speed response is the same as in the previous embodiments, and therefore, the electrode structure of a specific configuration according to the present invention, for example, By adopting the electrode cross-sectional shapes shown in FIGS. 2 to 4, it becomes possible to provide a waveguide type optical device that can achieve both a high modulation speed and an extended use temperature range.
[0037]
Incidentally, as a guide of the thickness of the electrode in the case of carrying out the present invention, it is a high speed that the thickness of the electrode is made larger than the frequency (bit rate) of the applied modulation signal, for example, the skin thickness of the microwave at 10 GHz. This is effective in ensuring operability.
[0038]
【The invention's effect】
As described above, according to the present invention, there is an effect that it is possible to provide a waveguide-type optical device that can achieve both a higher modulation speed and a wider use temperature range. The effects obtained by the specific embodiment of the present invention are as described above, and the description thereof will be omitted.
[Brief description of the drawings]
FIG. 1 is a plan view showing a first embodiment of a waveguide type optical device according to the present invention.
FIG. 2 is a sectional view taken along line II-II of the waveguide type optical device shown in FIG.
FIG. 3 is a sectional view showing a second embodiment of the waveguide type optical device according to the present invention.
FIG. 4 is a sectional view showing a third embodiment of the waveguide type optical device according to the present invention.
FIG. 5 is a plan view showing a fourth embodiment of the waveguide type optical device according to the present invention.
[Explanation of symbols]
2 waveguide substrates 4, 4 'core 6 input port 8 input waveguide 10, 16 Y branch 12, 14 parallel waveguide 18 output waveguide 20 output waveguide 22 signal electrodes 24, 26 ground electrode

Claims (5)

A waveguide substrate having a core portion formed of a material exhibiting an electro-optic effect and guiding light,
An electrode provided on the waveguide substrate to apply an electric field to the core portion,
The electrode includes a first portion having a larger coefficient of thermal expansion than the waveguide substrate and a second portion having a smaller coefficient of thermal expansion than the waveguide substrate. Waveguide type optical device. 2. The waveguide type optical device according to claim 1, wherein said first and second portions are laminated on said waveguide substrate. 2. The waveguide type optical device according to claim 1, wherein one of said first and second portions surrounds the other. 2. The waveguide type optical device according to claim 1, further comprising a dielectric layer provided between said waveguide substrate and said electrode. Said waveguide substrate is made of LiNbO _3, the material of the second part is a gold claim 1 material of the first portion, characterized in that it is selected from the group consisting of silver, aluminum and copper 2. The waveguide type optical device according to 1.

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