JPS62258419A - Optical control device - Google Patents

Abstract

PURPOSE:To perform stable operation even when a DC voltage is impressed by diffusing metal ions in an optical waveguide under an electrode and decreasing its refractive index when the electrode is installed on the optical waveguide. CONSTITUTION:Optical waveguides 12 and 13 of about 3-10mum in depth are formed on a lithium niobate crystal substrate 1 by the heat diffusion of Ti and both optical waveguides 12 and 13 come closer to each other at the center part of the substrate 1 to form a directional coupler 14. In this case, Mg, however, is diffused at part 10 of the surface of the substrate 1 where the coupler 14 is formed up to an area of about 1-2mum in depth and the part 10 is lower in refractive index than the remaining part where no Mg is diffused. Consequently, the leak quantity of the energy of a light wave propagated in the optical waveguides to the surface of the substrate is reduced to prevent the light from being absorbed by a control electrode 5 for a TM mode. Thus, no buffer film is used, so unstableness due to a buffer film and deterioration with time are eliminated.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical control device that modulates light waves, switches optical paths, etc., and particularly relates to a waveguide type optical device that performs °C control using an optical waveguide provided in a substrate. Regarding control devices.

(Prior Art) As the practical use of optical communication systems progresses, advanced systems with even higher capacity and multiple functions are required. There is a need to add new functions such as generation of faster optical signals and switching and exchanging optical transmission lines. In current practical systems, optical signals are obtained by directly modulating the injected current of a semiconductor laser or light emitting diode, but with direct modulation, it is difficult to achieve high-speed modulation of several α1z or more due to effects such as relaxation oscillation. Coherent optical transmission methods have drawbacks such as difficulty in application due to wavelength 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, an optical switch is used as a means for switching optical transmission lines and providing network switching functions. Optical switches currently in use mechanically move prisms, mirrors, fibers, etc., and have drawbacks such as slow speed, insufficient reliability, and large size. be. What is being developed as a means to solve this problem is a waveguide type optical switch using an optical waveguide, which is capable of high speed and multi-element integration.
It has features such as high reliability. Especially lithium niobate (LiN)
b03) Materials using ferroelectric materials such as crystals have features such as low light absorption and low loss, and high efficiency due to large electro-optic effects, and have traditionally been used for directivity. Various types of optical control elements such as coupled optical modulators or switches and total internal reflection optical switches have been reported.

When applying such a waveguide type optical control element to an actual optical communication system, at the same time as basic performance such as low loss and high speed, operational stability and long-term reliability are particularly important.
It is lacking.

(Problems to be solved by the invention) However, in conventional waveguide type optical control devices, stability and
Sufficient characteristics regarding reliability have not been obtained. FIG. 2 shows a plan view (a) and a side view (b) of a directional coupling type optical switch as an example of a conventional optical control device. Second
In Figure (al), band-shaped optical waveguides 2 and 3 are formed by diffusing titanium on a C lithium niobate crystal substrate 1 to make the refractive index thicker than that of the substrate. They are close to each other by several micrometers in the center of the substrate, and constitute a directional coupler 4. In addition, a 'C control electrode 5 is provided on the optical waveguide constituting the directional coupler 4 via a buffer film 6. Figure 2 (bl is the optical waveguide 2
A cross-sectional view along the line is shown.

In FIG. 2, as the incident light 7 that has entered the optical waveguide 2 propagates through the directional coupler 4, its optical energy gradually transfers to the adjacent optical waveguide 3; Approximately 100 degrees of energy is transferred to the wave path 3 and becomes the emitted light 8. On the other hand, when a voltage is applied to the control electrode 5,
The refractive index of the leading waveguide under the electrode changes due to the electro-optic effect, causing phase velocity mismatch between the waveguide modes propagating in the optical waveguides 2 and 3, and the coupling state between them changes.

As the applied voltage increases, the amount of optical energy transferred from the leading waveguide 2 to 3 decreases, and at a certain voltage value Vs, the incident light 7 has 1oo of optical energy after passing through the directional coupler 4.
q6 returns to the optical waveguide 2. That is, depending on the presence or absence of the voltage 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.
becomes.

Waveguide-type optical control devices using lithium niobate crystals have the greatest electro-optic effect and can be expected to operate at low voltages, and are easy to connect with fibers with low loss.
A combination of a substrate cut perpendicular to the axis, that is, a Z plate, and a polarization mode in the direction perpendicular to the substrate, that is, TM 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 FIG. 2 is conventionally installed to prevent light absorption by the electrode.

The upper 41j buffer film 6 has a refractive index smaller than that of lithium niobate in order to prevent optical energy in the waveguide from seeping into the electrode and to effectively apply an electric field to the lithium niobate crystal, and A transparent material that is insulating and does not absorb light, such as a silicon dioxide (S 1o2) film or an aluminum oxide (At20s) film, is used. It is well known that when such a buffer film is provided, the quality of its insulation greatly affects device stability. In other words, when the buffer film has insufficient insulation properties and low resistivity, when a DC voltage is applied, the charge in the buffer film drifts, and over time, the charge accumulates at the interface between the buffer film, the electrode, and the crystal, and the crystal The voltage applied thereto becomes smaller and the switching characteristics change. In the case of optical switches, this change in operating characteristics over time when applying the C voltage causes crosstalk 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 waveguided optical control devices to actual systems. Attempts have been made to improve the coating method and improve quality by oxygen annealing after coating in order to increase the resistance of the buffer film, but sufficient stability and long-term reliability have not been achieved.

An object of the present invention is to provide an optical control device which eliminates the drawbacks of the conventional optical control devices described above and which can operate stably even when a voltage of 1) C is applied.

(Means for Solving the Problems) An optical control device according to the present invention is constituted by an optical waveguide formed by diffusing metal ions into a crystal substrate, and an electrode installed on the upper part of the optical waveguide. The optical waveguide under the electrode is formed by diffusing first metal ions that increase the refractive index of the crystal substrate and second metal ions that decrease the refractive index of the crystal substrate, and the diffusion of the second metal ions The depth is smaller than the diffusion depth of the first metal ion, and the electrode is in direct contact with the crystal substrate.

(Function) The major difference between the present invention and the conventional method 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 metal ions, the refractive index under the electrode is lowered, reducing the amount of energy of light waves propagating in the optical waveguide seeping out to the substrate surface, and preventing light absorption by the electrodes of the 'I'M mote. There is. As described above, since the present invention does not use a 1j buffer film, it is possible to eliminate instability and deterioration over time caused by conventional buffer films.

(Example) FIG. 1 shows a plan view (a) and a side view (bl) of a directional coupling type optical switch which is an example of a preemptive a+++ device according to the present invention. Titanium, which is the first metal ion, is placed on the lithium oxide crystal substrate 1.
Optical waveguides 12 and 13 with a depth of about 3 to 10 μm formed by thermal diffusion at about 00 to 1100°C for several hours are installed, and both optical waveguides are brought close to each other within a few μm in the center of the substrate for directional coupling. It constitutes a container 14. However, in this embodiment, magnesium, which is the second metal ion, is diffused into the part 10 of the substrate surface where the directional coupler 14 is formed to a depth of about 1 to 2 μm, and the part 10 is The refractive index C is lower than that of the part where magnesium is not diffused. For the diffusion of magnesium, first, after forming the optical waveguide 12.13, magnesium oxide is coated on the substrate surface by a method such as spacing.
When magnesium is used as the second metal ion, the diffusion rate of magnesium is sufficiently higher than that of titanium. The diffusion depth of titanium may be sufficiently smaller than that of titanium, so
An advantage of magnesium diffusion is that the optical waveguide, which has already been formed by diffusing titanium, is hardly affected. Note that the refractive index of a lithium niobate crystal substrate decreases when magnesium is diffused, as reported in the Journal of Applied Physics (J, Appl.
Phys.), Vol. 49, p. 3150.

The basic switch operation of the directional coupling type optical switch of this embodiment is the same as that of the conventional directional coupling type optical switch shown in FIG. The output light is 19 or 18. However, in this embodiment, the buffer film 6 is not installed as described above, and instead, the optical waveguides 12 and 13 under the control room 45 are
Magnesium is diffused on the surface, reducing the refractive index. Therefore, in this embodiment, as in the case where a buffer film is provided, the light energy of the TM mode propagating through the optical waveguide is prevented from seeping out to the substrate surface by the above-mentioned region in which magnesium is diffused, and the control electrode 5 prevents the light energy from seeping into the substrate surface. Excludes absorption. Furthermore, in this embodiment, since no buffer film is used, there is no instability phenomenon when a DC voltage is applied due to imperfections in the buffer film.

Note that the present invention is not limited to the substrate material metal ions used in the above embodiments, but any crystal having an electro-optic effect, such as LiTa0. A crystal or the like can be used, and as the first metal ion, an ion having an effect of increasing the refractive index, such as copper or niobium, can be used.

(Effects of the Invention) As described above, since the optical control device of the present invention does not use a buffer film, stable operation can be obtained compared to conventional optical control devices.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are diagrams showing an example of a light control device according to the present invention, and FIGS. 2(a) and (b) are diagrams showing an example of a conventional light control device, in which (a) is a plane. Figure, (b
) is a side view. In the figure, 1 is a lithium niobate crystal substrate 2, 3, 12.13 is an optical waveguide, 5 is a control electrode, 6 is a buffer film, and 10 is a portion into which the second metal ion is diffused. 5 Iq electric 4 doors ν 1st rgl

Claims (1)

[Claims] In an optical control device that includes an optical waveguide formed by diffusing metal ions into a crystal substrate and an electrode installed above the optical waveguide, the optical waveguide under the electrode has a refractive index that is controlled by the refractive index of the crystal substrate. It is formed by diffusing first metal ions that increase the refractive index of the crystal substrate and second metal ions that decrease the refractive index of the crystal substrate, and the diffusion depth of the second metal ions is equal to the diffusion depth of the first metal ions. 1. A light control device characterized in that the device is smaller than the crystal substrate, and the electrode is in direct contact with the crystal substrate.