JPH0333823A - Electrode structure for waveguide type optical device - Google Patents

Abstract

PURPOSE:To obtain the effect of discharging the charge of a desired low- resistance film over entire part of a substrate without increasing the operating voltage of the optical device by constructing electrode films in such a manner that the films overlap on the low-resistance film in the outer peripheral part of electrode patterns and the other parts come into direct contact with a buffer layer. CONSTITUTION:After the low-resistance film 4 consisting of Si is formed, metallic films are so formed as to constitute the electrode patterns and the electrodes 3 are provided. The waveguide width of optical waveguides 2 is 7mum and the width of the electrodes 3 is confined to 11mum in the upper part of the optical waveguides in order to impress electric fields to the optical waveguides 2. The low-resistance film 4 consisting of the Si is so formed as to extend respectively 2mum inner than the patterns of the electrodes 3 on both sides. The low-resistance film 4 and the electrodes 3 are, therefore, overlapped respectively 2mum on both sides so that the electrodes 3 are eventually in direct contact with the buffer layer 5 of SiO2 by 7mum. The effect of discharging the charge of the desired low-resistance film over the entire part of the substrate is obtd. without increasing the operating voltage of the optical device in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrode structure of a waveguide type optical device, and particularly to the structure of an electrode formed on an optical waveguide substrate.

[Conventional technology] Waveguide type optical devices with switching and modulation functions,
There are waveguide type optical devices using substrates having an electro-optic effect such as L i Mb Oi and L L T a O1. This optical device uses Ti diffusion, proton exchange, etc. to form a part on the substrate surface with a slightly higher refractive index than the substrate, and this is used as an optical waveguide, and an electrode is provided above or near the optical waveguide. By applying a voltage to this, an electric field is generated in the optical waveguide, and the refractive index of the optical waveguide is changed by the electro-optic effect of the substrate, thereby performing switching and modulation.

Here, when a ferroelectric material such as LiNbO5 or LiTaO3 is used as an optical waveguide substrate, when a temperature change occurs around the device, a potential difference is generated within the substrate due to the pyroelectric effect of the substrate itself, and the optical waveguide formed on the surface of the substrate An unnecessary electric field is generated near the wave path and the electrodes provided above or near the wave path, making the characteristics unstable.

In order to eliminate such factors that cause the characteristics of the substrate to become unstable due to temperature changes, as a conventional technology 2, for example, as shown in Figure 2, Electronic Information and Communication Technology Report 0QE86-44 rS
Z-CU by L coating also improves the temperature characteristics of LiNbO3 waveguide devices by coating the entire substrate 1 with a low resistance film 4 from above the electrode 3 formed on the surface of the optical waveguide substrate 1, or As shown in Figure 3, the 1988 Institute of Electronics, Information and Communication Engineers Spring National Conference C-489rTi
: L f N b 03 Improving Temperature Characteristics of Matsuhatsu Enda Modulator", a method was used in which the entire surface of the optical waveguide substrate 1 was coated with a resistive film 4 and the electrodes 3 were formed thereon.

All of these require a low resistance film having a desired resistance value on the optical waveguide substrate (if this resistance value is too low, current will flow when a voltage is applied between the electrodes, making it impossible to generate an electric field in the optical waveguide. If it is too high, the effect of discharging the charge will be lost.

Usually, a film is used in which the resistance value between the electrode terminals is approximately 1 to IOMΩ. ) is used to discharge local charges on the substrate surface, such as near the electrodes, to the entire surface.

[Problem to be Solved by the Invention J] However, in the conventional electrode structure of the waveguide type optical device described above, the method of coating the entire optical waveguide substrate with a low resistance film on the electrode as shown in FIG.

The thickness of the electrode film is usually 2000 to 4000 Å or more,
Particularly in devices where electrode capacitance reduction is important, such as high-speed modulators, the thickness may be 1 μm or more, and if a low-resistance film is coated over such a relatively thick electrode film, a large step difference will occur between the areas with and without electrodes. As a result, the contact between the electrode and the low resistance film is insufficient, and the charges near the electrode cannot be sufficiently discharged. To prevent this, if the low-resistance film is made thicker, the sheet resistance of the low-resistance film itself decreases, making it impossible to obtain the desired resistance value, as well as causing distortion between the electrodes and the optical waveguide substrate. This has the drawback of causing stress and deteriorating the characteristics of the device itself.

On the other hand, in the method shown in FIG. 3, in which the entire optical waveguide substrate is coated with a low resistance value, and electrodes are formed on it.

Since a low resistance film layer is added between the optical waveguide substrate and the electrode,
When the same voltage is applied, the electric field generated in the optical waveguide becomes weaker than in the above method. Therefore, in order to generate the electric field necessary for switching modulation in the optical waveguide, a higher voltage is required, which has the disadvantage of causing a voltage increase.

[Means for Solving the Problems] Namely, the electrode structure of the waveguide type optical device of the present invention is an optical waveguide substrate having an optical waveguide formed on the surface thereof.

Alternatively, a buffer layer is further formed on the top surface of the optical waveguide substrate, and a low-resistance film is coated on the entire portion of the substrate where no electrodes are present, and on a portion slightly inside the outer periphery of the electrode pattern where the electrodes are formed. The low resistance film is on the bottom only in the area where the low resistance film is coated on the outer periphery of the electrode pattern.
The electrode films are stacked on top of each other, and the other parts are formed directly on the optical waveguide substrate or on the buffer layer formed on the top surface of the optical waveguide substrate.

[Embodiments] Next, two embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of an optical waveguide coupling portion of a directional coupling type 2×2 optical switch according to an embodiment of the present invention.

The optical waveguide 2 consists of a Ti film with a thickness of 600 nm and a Ti film patterned into a waveguide pattern on a LiNbO3 optical waveguide substrate 1.
It was manufactured by thermal diffusion in a wet atmosphere at 50° C. for 8 hours. The surface of the optical waveguide substrate 1 is coated with a 5fO2 buffer layer (5cm thick, 25OA) in order to prevent absorption loss of light by the electrodes 3. This SiO
□ Further on the upper surface of the buff 7 layer 5, a Si low resistance film 4 is coated on a portion where there is no electrode pattern so as to enter slightly inside the electrode pattern where the electrode 3 is formed.

Here, the film thickness of the Si low-resistance film 4 is assumed to be 1000, and in order to stabilize the resistance value of the Si low-resistance film 4 after film formation,
Annealing was performed for 1 hour in a constant temperature bath at 150°C. After coating the Si low resistance film 4, a metal film (Cr-
An electrode 3 was formed by depositing a film of 2,000 An. Here, the waveguide width of the optical waveguide 2 was 7 μm, and in order to apply an electric field to the optical waveguide 2, the width of the electrode 3 was set to 11 μm at the top of the optical waveguide. Moreover, the Si low resistance film 4 is
The film was coated in an amount of 2μ on each side of the pattern. As a result, the Si low resistance film and the electrode 3 overlap by 2μII on each side.

The electrode 3 is in direct contact with the 7 layers of 5in2 buffer by an amount of 7μ. In addition, in this embodiment, the length of the coupling part of the switch is 16 mm, and the resistance value between the electrode terminals is 3.
It was MΩ.

Figure 4 shows a SiO
2. From above the buffer layer 5, apply the low resistance film 4 to the optical waveguide substrate 1.
A conventional 2 x 2 optical switch in which the electrode is formed on the low resistance film 4, which is entirely coated.The temperature characteristics of loss for each boat of the 2 x 2 optical switch having the electrode structure of the present invention are shown.

As can be seen from FIG. 4, in the conventional optical switch, a low resistance film 4 is coated over the entire electrode 3.

In particular, the amount of variation in loss due to temperature changes in the voltage OFF state is extremely large. It can be seen that in the method of forming the electrode 3 from above the low resistance film 4, both the conventional first switch and the optical switch having the electrode structure of the present invention are sufficiently stable in the temperature range of 5 to 45°C.

Next, we measured the switch operating voltage of an optical switch with the above two electrode configurations that have stable temperature characteristics, and found that in a conventional optical switch in which the entire optical waveguide substrate 1 is coated with the low resistance film 4, with respect to 7M incident light, .

The optical switch having the electrode structure of the present invention was able to operate at a voltage of 4.5 V, approximately 40% lower than that of conventional switches.

[Effects of the Invention] As explained above, the present invention provides a low resistance film only on the non-electrode portion of the optical waveguide substrate of a waveguide type optical device and slightly inside the outer periphery of the electrode pattern. By creating a structure in which the outer periphery of the pattern overlaps with the low-resistance film and the other parts are in direct contact with the optical waveguide substrate or buffer layer, the desired low-resistance film can be formed without increasing the operating voltage of the optical device. This has the effect of discharging the electric charge to the entire substrate, which has the effect of realizing stable characteristics of the optical device.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the electrode structure of a waveguide-type optical device of the present invention, and FIGS. 2 and 3 are cross-sectional views of the electrode structure of a conventional waveguide-type optical device. Also. Figure 4 shows a waveguide type optical switch (a) with a conventional electrode structure.
), (b) and the temperature characteristics of the insertion loss of the waveguide optical switch (C) having the electrode structure of the present invention are shown. 1... Optical waveguide substrate, 2... Optical waveguide, 3... Electrode. 4...Low resistance film, 5...Buffer layer.

Claims (1)

[Claims] 1. An optical waveguide substrate with an optical waveguide formed on the surface of the substrate;
In a waveguide type optical device having an electrode and a low resistance film provided on the surface of the optical waveguide substrate, after coating a low resistance film except for the optical waveguide portion where the electrode is formed, the low resistance film is coated. An electrode structure for a waveguide type optical device, characterized in that an electrode film pattern is formed with a width slightly wider than the width of an uncoated part. 2. The electrode structure of a waveguide type optical device according to claim 1, characterized in that an insulating layer is provided between the optical waveguide substrate and the electrode.