JPS6370827A - Production of optical control device - Google Patents

Abstract

PURPOSE:To obtain stable operation even when a DC voltage is impressed to a device by diffusing metal ions to a substrate to decrease the refractive index under electrodes, thereby decreasing the quantity of transfer of the energy of the light wave propagating in optical waveguides to the substrate surface. CONSTITUTION:The optical waveguide pattern of a Ti film 11 which is a 1st metal ion is formed on a lithium niobate crystal 1. The Ti film is thermally diffused to form a Ti diffused region 12 to 3-10mum depth. A thin MgO film 13 contg. Mg which is the 2nd metal ion is formed thereon. The film 13 is diffused down to about 1-3mum depth to form the optical waveguide 14 consisting of the diffused region of the 1st and 2nd metal ions. The pattern of the electrodes 5 is then formed to part above the optical waveguide 14. The electrodes 5 are finally thermally annealed, by which part of the electrode atoms thereof are diffused to the substrate 1 surface to form electrode metal diffused parts 15. The unstableness and deterioration with lapse of time occurring in a buffer film are thereby eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a waveguide type optical control device in which modulation of light waves, optical path switching, etc. are controlled using an optical waveguide provided in a substrate.

[Conventional technology]

As the practical use of optical communication systems progresses, advanced systems with higher capacity and multiple functions are required, and new functions such as generation of faster optical signals and switching and switching of optical transmission lines are required. is necessary. In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers or light emitting diodes, but with direct modulation, high-speed modulation of several GHz or more is difficult due to effects such as relaxation oscillation, and the wavelength The cohesin 1-optical transmission method has drawbacks such as difficulty in application due to fluctuations.

One way to solve this problem is to use an external optical modulator. In particular, a waveguide type optical modulator constructed from an optical waveguide formed in a substrate has the advantage of being small, highly efficient, and fast.

On the other hand, optical switches are used as means for switching optical transmission lines and providing network switching functions. Optical switches currently in practical use mechanically move prisms, mirrors, fibers, etc., and have the disadvantages of slow speed, insufficient reliability, large size, and unsuitability for matrix formation.

A waveguide type optical switch using an optical waveguide is being developed as a means to solve this problem, and has features such as high speed, integration of multiple elements, and high reliability. In particular, those using ferroelectric materials such as lithium niobate (LiNb03) crystals have features such as low light absorption, low loss, and large electro-optic effects, resulting in high efficiency. Various types of light control elements such as directional coupling type optical modulators, switches, and total reflection type optical switches have been reported. When applying such waveguide-type optical control elements to actual optical communication systems, basic performance such as low loss and high speed, as well as operational stability and long-term reliability are essential for practical use. .

[Problem that the invention seeks to solve]

However, with conventional waveguide type optical control devices, stability and
Regarding reliability, sufficient characteristics were not obtained.

FIGS. 3(a) and 3(b) show a plan view and a side view of a directional coupling type optical switch as an example of a conventional optical control device. This optical switch is made of lithium niobate (LiNb0.
) Strip-shaped optical waveguides 2 and 3 are formed on a crystal substrate 1 by diffusing titanium so that the refractive index is larger than that of the substrate. The directional coupler 4 is configured by making the two adjacent to each other by several micrometers. Optical waveguide 2 constituting this directional coupler 4
A control electrode 5 is formed on the electrode 3 with a buffer film 6 interposed therebetween.

The incident light 7 that has entered the optical waveguide 2 is transmitted to the directional coupler 4
As the light propagates through the portion, the light energy gradually transfers to the adjacent optical waveguide 3, and after passing through the directional coupler 4, approximately 10'O% of the energy is transferred to the optical waveguide 3 and becomes the output light 8.

On the other hand, when a voltage is applied to the control electrode 5, the refractive index of the optical waveguide under the electrode 5 changes due to the electro-optic effect, causing a phase velocity mismatch between the guided modes propagating in the optical waveguides 2 and 3. occurs, and the bonding state between the two changes. When the voltage applied to this electrode 5 is increased, the amount of optical energy transferred from the optical waveguide 2 to the optical waveguide 3 is decreased, and at a certain voltage value VS, the incident light 7 has 100% of the optical energy after passing through the directional coupler 4.
% returns to the optical waveguide 2. That is,
Depending on whether or not a voltage is applied to the control electrode 5, the incident light 7 becomes the output light 9 from the optical waveguide 2 or the output light 8 from the optical waveguide 3.

Waveguide-type optical control devices using lithium niobate crystals have a substrate cut perpendicular to the Z-axis, which provides the greatest electro-optic effect and can be expected to operate at low voltage, and which facilitates low-loss coupling with fibers. That is, the combination of the Z plate and the polarization mode in the direction perpendicular to the substrate, that is, the 7M mode, is most often used. In such a combination, when an electrode is installed on the optical waveguide, a buffer film 6 as shown in FIGS. 3(a> and 3(b)) is conventionally installed in order to prevent light absorption by the electrode.

This buffer film 6 has a refractive index lower than that of lithium niobate and is an insulating material in order to prevent light energy in the waveguide from seeping into the electrode 5 and to effectively apply an electric field to the lithium niobate crystal. A transparent material with no light absorption, such as silicon dioxide (5i02) IIi oxide 7
Lumi (Ae z03)pA, etc. are used. When such a buffer film 6 is provided, it is well known that the quality of its insulation greatly affects device stability.

In other words, if the insulation of the buffer film 6 is insufficient and the specific resistance is small, when a DC voltage is applied, charges will drift in the buffer film 6, and over time, charges will accumulate at the interfaces between the buffer film, electrodes, and crystals. However, the voltage applied to the crystal becomes smaller, and the switching characteristics change.

In the case of optical switches, this change in operating characteristics over time when the C voltage is applied causes the generation of tarostoke and fluctuations in the switch voltage, and in the case of optical modulators, it causes drift in the bias voltage value, etc. This is a major obstacle in applying waveguide optical control devices to actual systems.

In order to increase the resistance of this buffer film, attempts have been made to improve the coating method and improve quality by oxygen annealing after coating, but sufficient stability and long-term reliability have not been achieved.

An object of the present invention is to provide a method for manufacturing a light control device that eliminates these conventional drawbacks and provides stable operation even when a DC voltage is applied.

[Means for solving problems]

The method for manufacturing a light control device of the present invention includes diffusing into a dielectric crystal substrate a first metal ion that increases the refractive index of the crystal substrate, and then a second metal ion that further decreases the refractive index of the crystal substrate. A first step of diffusing metal ions to a depth shallower than the diffusion depth of the first metal ions to form an optical waveguide, forming an electrode on the top of the optical waveguide in direct contact with the crystal substrate, and then thermal annealing. and a second step of diffusing some of the atoms of the electrode into the substrate through processing.

[Effect]

The major difference between the structure of the present invention and the conventional structure is that a buffer film is not used. In the present invention, after forming an optical waveguide by diffusing a first metal ion that increases the refractive index of a substrate that is normally used, a second metal ion that further decreases the refractive index of the substrate is added to the upper part where an electrode is installed. By diffusing the metal ions, the refractive index under the electrode is lowered, reducing the amount of energy of the light wave propagating in the optical waveguide seeping out to the substrate surface, thereby preventing light absorption by the electrode in the M mode. In the present invention, metal atoms, which are the electrode material, are diffused into the substrate by thermal annealing to prevent the formation of barriers such as Schottky barriers at the interface between the electrode and the crystal, and to prevent the operation caused by the accumulation of charges on these barriers. As described above, according to the present invention, by annealing the electrode without using a buffer film, the instability and deterioration over time caused by the conventional buffer film can be avoided. Can be removed.

〔Example〕

FIGS. 1(a) to 1(e) are cross-sectional views schematically showing an embodiment of the method for manufacturing a light control device according to the present invention in the order of steps, and show a cross section of a part of the light control device in each manufacturing step. As shown in FIG. 1 (a), this embodiment includes the first step of forming an optical waveguide pattern of a Ti film 11, which is a first metal ion, on a lithium niobate crystal 1, and
The film 11 is thermally diffused at 900 to 1100°C for several hours to a depth of 3.
The second step of forming a Ti diffusion region 12 of ~10 μm (
FIG. 1(b)) and a third step of forming an MgO thin film 13 containing J, which is a second metal ion, on at least a portion of this Ti diffusion region 12 (FIG. 1(c)). and Kono M
The gO thin film 13 is heated at about 600 to 10oO℃ for 1 to several hours.
A fourth step (FIG. 1(d)) of forming an optical waveguide 14 consisting of a diffusion region of the first and second metal ions by diffusion to a depth of about 1 to 3 μm, and A fifth step of forming a pattern of the electrode 5 made of a material such as Ti, Cr, ^u, f't, Si, etc. at least in part (
By thermally annealing this electrode 5 at 200 to 600°C, some of the electrode atoms are transferred to the substrate 1 (Fig. 1(e)).
A sixth step (FIG. 1(f)) of forming an electrode metal diffusion portion 15 by diffusing the metal on the surface of the electrode metal is sequentially performed.

FIGS. 2(a) and 2(b) show a plan view and a side view of a directional coupling type optical switch, which is an example of a light control device manufactured by the manufacturing method shown in FIG. 1. 900% of titanium, which is the first metal ion, is placed on the niobium lithium crystal substrate 1.
A guide waveguide 2.3 with a depth of about 3 to 10 μm formed by thermal diffusion at about 1100”C for several hours is installed, and both optical waveguides are close to each other within a few μm in the center of the substrate, and the directionality is A portion 1o of the surface of the substrate 1 on which the directional coupler 4 is formed has a depth of 1 to 2.
Magnesium, which is the second metal ion, is diffused to a region of about μm, and the refractive index of this diffused portion 10 is lower than that of a portion where magnesium is not diffused.

This diffusion of magnesium is accomplished by first forming a guide waveguide 2.3, then coating the substrate surface with magnesium oxide using a method such as a sprinkling method, and then thermally diffusing it at 800 to 1000°C. When magnesium is used as the second metal ion, the diffusion rate of magnesium is sufficiently higher than that of titanium, and the diffusion depth of magnesium is sufficiently smaller than that of titanium. Therefore, when magnesium is diffused, the optical waveguide that has already been formed by diffusing titanium is hardly affected. It should be noted that the decrease in the refractive index of a lithium niobate crystal substrate when magnesium is diffused is reported in the journal ``Journal of Applied Physics (J, ^ppl.

The basic switch operation of the directional coupling type optical switch of this embodiment is the same as that of the conventional example shown in FIG. 3, and the incident light 7 is The output light becomes 8 or 9 depending on the presence or absence of a voltage applied to 5. However, in this embodiment, the buffer film 6 is not provided, and instead, the optical waveguide 2.3 under the control electrode 5 is
There is a portion 10 on the surface of which has a reduced refractive index due to the diffusion of magnesium. Therefore, in this embodiment as well, similarly to the case where the buffer film 6 is provided, the TM mode light energy propagating through the optical waveguide is prevented from seeping into the substrate surface by the region 10 in which magnesium is diffused, and the control electrode 5 absorbs light. is excluded. Furthermore, in this example, since a buffer film is not used, DC
There are no unstable phenomena when voltage is applied. Furthermore, thermal annealing of the electrodes prevents the formation of a barrier such as a Schottky barrier at the interface between the electrodes and the crystal.

Note that the present invention is not limited to the substrate material and metal ions used in this example, and any crystal having an electro-optic effect, such as LiTaO3 crystal, can be used as the substrate material. As the metal ion 1, ions having the effect of increasing the refractive index, such as copper and niobium, can be used. Furthermore, the control electrode material is Ti-^U.

Metal multilayer films such as Cr-Au and semiconductor materials can also be used.

〔Effect of the invention〕

As explained above, the method for manufacturing an optical control device of the present invention is characterized in that, since a buffer film is not used, stable operation can be obtained compared to conventional optical control devices.

[Brief explanation of the drawing]

FIGS. 1(a) to (f) are cross-sectional views showing the manufacturing method of a light control device according to the present invention in the order of steps, and FIGS. 2(a) to (b)
) are a front view and a side view of an example of a light control device using the manufacturing method of the present invention, and FIGS. 3(a) and 3(b) are a plan view and a side view of an example of a conventional light control device.1 ...Lithium niobate crystal substrate, 2.3.14...
・Optical waveguide, 5... control electrode, 6... buffer film,
7. Incident light, 8.9... Outgoing light, 10... Diffusion part of second metal ion (Mg), 11... Ti film, 12...,?噌-21I5ii [茅 2 times 茅 3 fig.

Claims (1)

[Claims] Diffusion of a first metal ion into the dielectric crystal substrate that increases the refractive index of the crystal substrate, and then diffusion of the first metal ion with a second metal ion that further decreases the refractive index of the crystal substrate. A first step is to form an optical waveguide by diffusion shallower than the depth, and an electrode is formed on the top of the optical waveguide in direct contact with the crystal substrate, and then a part of the atoms of the electrode is removed by thermal annealing. A method for manufacturing a light control device, comprising: a second step of diffusing into a substrate.