JPH04172316A - Wave guide type light control device - Google Patents

Abstract

PURPOSE:To facilitate manufacture and obtain the low operation voltage and high speed operation by making the thickness of a buffer layer under one electrode, larger at the part separated from a light wave guide part in comparison with the part close to the optical waveguide. CONSTITUTION:The wave guide type light control device is equipped with the optical waveguides 3 - 5 formed on a crystal substrate 1 which possesses the photoelectric effect, buffer layer 16 formed on the optical waveguides 3 - 6, and at least a pair of electrodes 7 and 8 a part of which is installed on the buffer layer 16. The phase speed of the microwave is increased by increasing the thickness of the buffer layer 16 under the electrodes 7 and 8 at the part separated from the optical waveguides 3 - 5, and the phase speed of the microwave is made close to that of the light wave. At this time, the buffer layer 16 is kept thin at the part close to the optical waveguides 3 - 5, i.e., at the part generating an electric field in the optical waveguide. Accordingly, the increase of the operation voltage is small, and the phase speed matching of the microwave and light wave and the low voltage operation can be satisfied at the same time.

Description

[Detailed Description of the Invention] [Industrial Application Field] 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 control device that performs control using a guide waveguide provided in a substrate. Regarding devices.

[Conventional technology]

As the practical use of optical communication systems progresses, advanced systems with even higher capacity and multiple functions are required. Such systems require the addition of new functions such as generation of higher-speed optical signals and switching and switching of optical transmission lines.

In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers or light emitting diodes, but direct modulation requires several GHz due to effects such as relaxation oscillation.
It has drawbacks such as difficulty in high-speed modulation of Z or higher, and difficulty in applying it to a coherent optical transmission system due to wavelength fluctuations. One way to solve this problem is to use an external optical modulator. As the optical modulator used in this method, a waveguide type optical modulator constituted by an optical waveguide formed in a substrate is particularly suitable because it has the features of small size, high efficiency, and high speed operation.

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 practical use mechanically move prisms, mirrors, fibers, etc., and have the disadvantages of slow speed, insufficient reliability, and large size making them unsuitable for matrix formation. A waveguide type optical switch is a means to solve this problem. This waveguide optical switch has features such as high-speed operation, integration of multiple elements, and high reliability. In particular, those using ferroelectric materials such as lithium niobate (SiNbOs) crystals have features such as low light absorption and low loss, and high efficiency because they have a large electro-optic effect. Conventionally, various types of optical control devices such as directional coupling type optical modulators or switches, total internal reflection type optical switches, etc. have been reported.

FIG. 4 shows a perspective view of a branching interference type optical modulator as an example of a conventional optical control technology. In Figure 4, lithium niobate (L, i N b○
3) A strip-shaped optical waveguide 2 on the incident side formed by diffusing titanium on a crystal substrate 1 to have a refractive index larger than that of the substrate, and an optical waveguide # having a length of about several mm to 30 mm branched from the optical waveguide 2.
13 and 4, and an optical waveguide 5 on the output side where the optical waveguides 3.4 and 3.4 merge together to form a branching interferometer. Further, a ground electrode 7 is formed on the optical waveguide 3 and a signal electrode 8 is formed on the optical waveguide 4 via a buffer layer 6 for preventing light absorption by the electrodes, and the output end of the signal electrode 8 is connected to the line. It is terminated with a resistor R6 close to impedance.

In FIG. 4, the energy of light 9 incident on the optical waveguide 2 is split by branching, and after passing through the optical waveguides 3 and 4, it merges into the optical waveguide 5. At this time, if the lights that have passed through the optical waveguides 3 and 4 merge in the same phase, the loss will be small and the output light 10 will have a large amount of light, but if the lights that have passed through the optical waveguides 3 and 4 are in opposite phases to each other, A large loss occurs at the merging portion, and the amount of output light 10 is small. Therefore, depending on the magnitude of the voltage applied to the signal electrode 8, the refractive index of the optical waveguide under the electrode changes due to the electro-optic effect, and the phase of the light passing through it changes, so an optical output corresponding to the applied voltage can be obtained. The output light 10 is modulated. Further, in FIG. 3, the signal electrode 8. The impedance between the ground electrodes 7 is made close to the impedance of the input electric signal line (usually 50Ω), and the electrodes are terminated with a resistor RO close to that impedance to create a distributed constant, that is, a traveling waveform electrode, thereby achieving a wide band. I know.

[Problem to be solved by the invention]

It is difficult for ordinary drive circuits to output high voltages at high speeds, and optical control devices are required to operate at as low an operating voltage as possible. In the branching interference type optical modulator and directional coupling type optical switch shown in FIG. 4, it is necessary to increase the element length in order to lower the voltage. However, in such conventional waveguide-type optical control devices, the phase velocity of the modulated electric signal, the microwave, and the light wave in the waveguide are significantly different, so the electric field cannot be effectively applied over a long element length, limiting the speed. It will be done. In order to bring the phase velocity of microwaves and light waves closer to each other, it has been reported that there is a method of increasing the thickness of a buffer layer with a small dielectric constant, or a method of increasing the velocity of microwaves by thickening an electrode. This method requires a high operating voltage, and the latter method has the disadvantage of being extremely difficult to manufacture.

An object of the present invention is to provide a waveguide type optical control device that is easy to manufacture, requires low operating voltage, and can operate at high speed.

[Means to solve the problem]

According to the present invention, the optical waveguide is formed on a crystal substrate having an electro-optic effect, a buffer layer is formed on the optical waveguide, and at least a pair of electrodes are disposed at least in part on the buffer. and the buffer layer under at least one of the pair of electrodes is thicker in a portion away from the leading waveguide than in a portion close to the optical waveguide. a wave-type optical control device; an optical waveguide formed on a crystal substrate having an electro-optic effect; at least a pair of electrodes formed on the optical waveguide; A waveguide type optical control device is obtained, which includes a small dielectric film and an electrode film formed on the dielectric film and electrically connected to one of the pair of electrodes.

[Effect]

In the waveguide type optical control device of the present invention, by increasing the thickness of the buffer layer under the electrode in the portion away from the optical waveguide, the phase velocity of the microwave is increased and brought closer to the phase velocity of the light wave. At this time, in the present invention, since the buffer layer remains thin in a portion close to the optical waveguide, that is, in a portion where an electric field is generated in the optical waveguide, the increase in operating voltage is small.

In the case of a traveling wave electrode, the area of the entire electrode is usually larger in the part away from the optical waveguide, so making the buffer layer thicker there can have a sufficient effect on the phase velocity of the microwave. , it can simultaneously satisfy both microwave and light wave phase velocity matching and low voltage operation. Further, the thickness of the buffer layer can be easily changed by a method such as etching, and manufacturing is easy because it does not require high precision unlike an electrode pattern.

In another waveguide type optical control device of the present invention, a dielectric film with a small dielectric constant is provided on the signal electrode, and an electrode film connected to the ground electrode is further provided on the dielectric film, so that the signal electrode and the electrode By propagating at least a portion of the microwave energy applied to the signal electrode between the films, the phase velocity of the microwave is reduced and phase velocity matching with the light wave is obtained. By selecting the thickness and dielectric constant of the dielectric film, the thickness of the buffer layer and the thickness of the electrode can be made thin, so that a low operating voltage is possible and manufacturing is easy.

〔Example〕

FIG. 1 shows a branching interference type optical modulator which is an embodiment of the waveguide type optical control device according to the present invention, with FIG. 11(a) showing a perspective view and FIG. 1(b) showing a sectional view. Similar to the example in FIG.
Width 5-12 formed by thermal diffusion at a temperature of 00℃ for several hours
μm, optical waveguide 2, 3, 4.5 with a depth of about 3 to 10 μm
Established! A branching interferometer is formed. A buffer layer 16 made of a 5i02 film is formed on the optical waveguide, and a signal electrode 8 with a width of about 10 to 30 μm is formed on the buffer layer 16, and a signal electrode 8 with a width of about 100 to 30 μm is formed on the buffer layer 16.
A ground electrode 7 of 1000 μm is installed. here,
The thickness of the buffer layer 16 is t on the optical waveguides 3 and 4,
= Q, 3 to 1.0 μm, and outside of that, t2 = 2.0 to 10.0 μm. Conventional third
In the optical control device shown in the figure, the thickness of the buffer layer 6 is uniform and 0.
When the buffer layer is thick, the modulation band is wide but the modulation voltage is high; when the buffer layer is thin, the modulation voltage is low but the modulation band is narrow, and characteristics that satisfy both simultaneously cannot be obtained. Not yet.

On the other hand, in this example, since the buffer layer on the optical waveguide is thin, an electric field is effectively applied to the optical waveguide, and the modulation voltage is small.
On the other hand, the microwave of the modulation signal propagates more through the thicker portion of the buffer layer, and its phase velocity approaches that of light waves, so a wide modulation band can be obtained.

FIG. 2 shows a cross-sectional view of another embodiment of the waveguide type optical control device according to the present invention.

Figure 2(a) shows a LiNb0. 3 is a cross-sectional view of a branching interference type optical modulator formed on a crystal substrate 1. In this embodiment, since an electric field component in a direction parallel to the substrate surface is used, a branched optical waveguide 3.4 is Signal electrode 8. A ground electrode 7 is installed. In this case, a buffer layer for preventing light absorption is not necessarily required, so in this embodiment, the thickness of the buffer layer is set to 0 in the vicinity of the optical waveguide 3.4, and the thick buffer layer 26 is used in a portion away from there. has been established.

Furthermore, if the width of the signal electrode 8 is narrow, the buffer layer below it may be entirely removed as shown in FIG. 2(b).

FIG. 3 shows a branching interference type optical modulator which is an embodiment of the waveguide type optical control device according to the present invention, with FIG. 3(a) showing a plan view and FIG. 3(b) showing a sectional view. Similar to the example in Fig. 1, titanium is placed on the LiNbO3 substrate 1 at an angle of 900 to 1100°.
A width of 5 to 12 μm formed by thermal diffusion at a temperature of C for several hours.
, optical waveguides 2, 3, 4.5 having a depth of approximately 3 to 10 μm are installed to form a branching interferometer. On the optical waveguide, a buffer layer 6 made of SiO
A ground electrode 7 of .mu.m is installed. Here, the thickness of the buffer layer 6 is 0.5 to 3.0 μm, and is -like. Furthermore, on the signal electrode 8, a thickness of 5 to 500 ml of SiC2 film is formed.
A 20 μm dielectric film 20 is coated, and an electrode film 21 is placed thereon, one end of which is connected to the ground type $i7.

In this embodiment, the thickness of the buffer layer can be made thin, so a low operating voltage can be obtained, and the thickness of the electrode can also be the usual 1 to 4 .mu.m, making it easy to manufacture.

The above describes the case where the present invention is applied to a branching interference type optical modulator, but one or more pairs of electrodes are installed in the vicinity of a leading waveguide, such as a directional coupling type switch, an optical modulator, or a crossing type optical switch. It goes without saying that the present invention can be applied to light-controlled technology.

〔Effect of the invention〕

As described above, the waveguide type optical control device of the present invention is easy to manufacture, and provides low operating voltage and high speed operation.

[Brief explanation of drawings]

FIGS. 1, 2, and 3 are diagrams showing an embodiment of a waveguide type optical control device according to the present invention, and FIG. 4 is a diagram showing an example of a conventional waveguide type optical control device. In the figure, 1 is a LiNb○2 crystal substrate, 2°3.4.
5 is an optical waveguide, 6, 16, 26 are buffer layers, 7 is a ground electrode, 8... signal electrodes.

Claims (1)

[Claims] 1. An optical waveguide formed on a crystal substrate having an electro-optic effect, a buffer layer formed on the optical waveguide, and at least a pair of at least a portion of which is disposed on the buffer. comprising an electrode, and the buffer layer under at least one of the pair of electrodes is thicker in a portion away from the optical waveguide than in a portion close to the optical waveguide. Waveguide type optical control device. 2. An optical waveguide formed on a crystal substrate having an electro-optic effect, at least a pair of electrodes formed on the optical waveguide,
A waveguide composed of a dielectric film formed on the electrode and having a dielectric constant smaller than that of the crystal substrate, and an electrode film formed on the dielectric film and electrically connected to one of the pair of electrodes. type light control device.