JPH0593925A - Optical control device - Google Patents

Abstract

PURPOSE:To always stably obtain the characteristics of the optical control device as designed at a good yield. CONSTITUTION:This optical control device has a 1st layer 10 provided on the rear surface of a dielectric crystal substrate 1. The strain generated between the dielectric crystal substrate 1 and a buffer layer 6 by L differences in the coefft. of thermal expansion and optical elastic constants is removed by providing this layer. Then, the optical waveguide characteristics maintain the characteristics when the optical waveguide is formed on the dielectric crystal substrate. The designed characteristics are thus obtd. at a good yield.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical control device for performing optical modulation and switching, and more particularly to a waveguide type optical control device for performing control using an optical waveguide provided in a substrate.

[0002]

2. Description of the Related Art As the practical use of optical communication systems progresses,
Further, there is a demand for sophisticated systems having large capacity and multiple functions, and it is necessary to add new functions such as generation of higher-level optical signals, switching of optical transmission lines, and exchange.
In the current practical system, the optical signal is obtained by directly modulating the injection current of the semiconductor laser or the light emitting diode. However, direct modulation is difficult to achieve high-speed modulation of about 10 GHz or more due to effects such as relaxation oscillation in direct modulation. However, there is a drawback that it is difficult to apply to the coherent optical transmission system because of the wavelength variation. As a means for solving this, there is a method of using an external modulator, and in particular, a waveguide type optical modulator formed by a large optical current formed in a substrate is characterized by small size, high efficiency and high speed.

On the other hand, an optical switch is used as a means for switching the optical transmission line and obtaining the network switching function. The optical switch currently in practical use mechanically moves prisms, mirrors, fibers, etc.
There are drawbacks such as low speed, insufficient reliability, large shape, and unsuitable for matrix formation. What is being developed as a means for solving this is still a waveguide type optical switch using an optical waveguide, which has features such as high speed, multi-element integration, and high reliability. In particular, the one using a ferroelectric material such as lithium niobate (LiNbO ₃ ) crystal is characterized by low light absorption and low loss and high efficiency because it has a large electro-optical effect. There have been conventionally reported various types of optical control elements such as a directional coupler type optical modulator / switch, a total reflection type optical switch, and a Mach-Zehnder type optical modulator. When such a waveguide type optical control element is applied to an actual optical communication system, reproducibility of operating characteristics, that is, high yield of devices, is essential for practical use, in addition to basic performance such as low loss and high speed. Is.

[0004]

However, in the conventional waveguide type optical control device, sufficient reproducibility of operating characteristics has not been obtained. FIG. 3 is a plan view of a directional coupling type optical switch as an example of a conventional optical control device (a).
And a sectional view (b) is shown. In FIG. 3A, band-shaped optical waveguides 2 and 3 formed by diffusing titanium to have a refractive index higher than that of the substrate are formed on a lithium niobate crystal substrate 1 cut out perpendicularly to the Z axis. Cage, optical waveguide 2
And 3 are close to each other by about several μm in the central part of the substrate,
The directional coupler 4 is formed. Also, the directional coupler 4
The control electrode 5 is formed on the optical waveguide forming the structure via a buffer layer 6 for preventing light absorption by the control electrode. FIG. 3B shows an optical waveguide 2, which is a part of the directional coupler 4.
3 shows a sectional view perpendicular to FIG.

In FIG. 3, the incident light 7 that has entered the optical waveguide 2 gradually propagates through the portion of the directional coupler 4 so that the light energy gradually shifts to the optical waveguide 3 that is close to it, and after passing through the directional coupler 4. Almost 100% of the energy is transferred to the optical waveguide 3 and becomes emitted 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 control electrode changes due to the electro-optic effect, and a phase velocity mismatch occurs between the waveguide modes propagating in the optical waveguides 2 and 3. , The joining state between the two changes.

The control electrode 5 is formed by etching the control electrode film other than the control electrode by a lithographic method using a mask after forming the control electrode film. It is known that when the buffer layer film and the control electrode film are formed, strain is generated in the substrate due to the difference in thermal expansion coefficient between the deposited films and the underlying material and the difference in elastic constant such as Poisson's ratio. In this state, generally, the amount of strain that is present during film formation is uniformly distributed over the entire dielectric crystal substrate, so even if the absolute value of the refractive index of the optical waveguide and the dielectric crystal substrate changes, The refractive index difference with respect to the body crystal does not change before and after film formation. Therefore, the optical waveguide characteristic retains the characteristic when the optical waveguide is formed on the dielectric crystal substrate, and does not change the coupling state of the directional coupler. However, since the control electrode portion formed by subsequently etching the control electrode film is elastically discontinuous, the strain that fluctuates when the control electrode is formed is nonuniformly concentrated and localized in the vicinity of the control electrode. This distortion causes the refractive index of the ferroelectric crystal substrate to change due to the piezo effect and the photoelastic effect. Therefore, the change in the refractive index also affects the vicinity of the optical waveguide formed near the control electrode to change the optical waveguide characteristics. As a result, since the coupling state of the directional coupler changes, there is a problem that the optical waveguide coupling state as designed cannot be obtained with good reproducibility. The amount of change in the bonding state varies depending on the batch of buffer layer film and control electrode film formation.

An object of the present invention is to provide an optical control device which can obtain the characteristics as designed with a high yield and always stable, except for the above-mentioned drawbacks of the conventional optical control device.

[0008]

According to a first aspect of the invention, an optical waveguide formed on a dielectric crystal substrate having an electro-optical effect, a buffer layer deposited on the surface of the dielectric crystal substrate, and a buffer layer on the buffer layer are provided. In a light control device including a control electrode provided in the vicinity of the optical waveguide, a first layer having a stress on the dielectric crystal substrate equal to that of the buffer layer is formed on the back surface of the dielectric crystal substrate. It is characterized by being deposited on.

[0009]

In the light control device of the first invention, the first layer is formed on the back surface of the dielectric crystal substrate. By properly selecting the film thickness of the first layer, it is possible to cancel the stress at the interface caused by the difference in thermal expansion coefficient and photoelastic constant between the buffer layer and the dielectric crystal substrate.

In the control device of the second invention, the second layer having the same shape as the control electrode is formed on the first layer of the control device of the first invention. This second
This layer can cancel out the non-uniform stress localized near the electrodes when the control electrodes are formed.

As a result, the change in the refractive index due to the distortion of the dielectric crystal near the optical waveguide is suppressed. Therefore, the optical waveguide characteristic retains the characteristic when the optical waveguide is formed on the dielectric crystal substrate, and does not change the coupling state of the directional coupler.

As described above, the light control device of the present invention can obtain the characteristics as designed as compared with the conventional one in good yield and can always be stably obtained.

[0013]

1 is a plan view (a) and a sectional view (b) of a directional coupler type optical switch which is an embodiment of an optical control device according to the first invention. As in the example of FIG. 3, titanium was deposited on the Z-plate lithium niobate crystal substrate 1 in an amount of 900 to 1
3-10μ formed by thermal diffusion at 100 ° C for several hours
The optical waveguides 2 and 3 of about m are formed, and both optical waveguides are close to each other up to several μm in the central portion of the substrate to form the directional coupler 4. The control electrode 5 is formed on the buffer layer 6 via the buffer layer 6. However, at this time, the difference from FIG. 3 is that the back surface of the Z-plate lithium niobate crystal substrate 1 is
That is, the first layer 10 having the same material and film thickness as the buffer layer 6 is formed.

It is desired that the buffer layer 6 has a smaller refractive index and less light absorption than the lithium niobate crystal substrate 1, and SiO ₂ , SiON, MgF ₂ , Si ₃
N ₄ , Al ₂ O ₃ or the like is used. The manufacturing method includes a sputtering method, a CVD method, an evaporation method, a spin coating method, and the like.

The buffer layer 6 and the first layer 10 need not be made of the same material as long as the layer thickness and the like are adjusted so that the stresses on the dielectric crystal substrate 1 become equal.

FIG. 2 is a sectional view of a directional coupler optical switch which is an embodiment of the optical control device according to the second invention. Similar to the example of FIG. 1, Z plate lithium niobate crystal substrate 1
Optical waveguides 2 and 3 of about 3 to 10 μm formed by thermally diffusing titanium at 900 to 1100 ° C. for several hours are formed on the top of the optical waveguide. To form the directional coupler 4. The control electrode 5 is formed on the front surface via the buffer layer 6, and the layer 10 of 1 is formed on the back surface. Here, the difference from FIG. 1 is that a second layer 11 having the same material and film thickness as the control electrode 5 and having the same shape is formed on the first layer 10. ..

As the control electrode 5, Au, Al, Pt, T
A metal such as i or Ag or a transparent electrode such as ITO or ZnO is used, and is not limited. As a method for forming the electrodes, after a mask pattern is formed by using a lithography method, a dry etching method such as an ion beam etching method, a reactive ion etching method, or a reactive ion beam etching method, or an etching solution is used. In addition to the conventional method of etching using a chemical wet etching method, there is a method of forming directly using a focused ion beam etching method or the like.

[0018]

By applying the light control device of the present invention, the characteristics as designed can be obtained with good yield and always stable, as compared with the conventional light control device.

[Brief description of drawings]

FIG. 1A is a plan view showing an example of a light control device according to the first invention. (B) Sectional drawing which shows an example of the light control device by 1st invention.

FIG. 2A is a plan view showing an example of a light control device according to a second invention. (B) Sectional drawing which shows an example of the light control device by 2nd invention.

FIG. 3A is a plan view showing an example of a conventional light control device. (B) Sectional drawing which shows an example of the conventional light control device.

[Explanation of symbols]

 1 Lithium Niobate Crystal Substrate 2,3 Optical Waveguide 4 Directional Coupler 5 Control Electrode 6 Buffer Layer 7 Incident Light 8,9 Emitted Light 10 First Layer 11 Second Layer

Claims (2)

[Claims] 1. An optical waveguide formed on a dielectric crystal substrate having an electro-optical effect, a buffer layer deposited on the surface of the dielectric crystal substrate, and provided on the buffer layer in the vicinity of the optical waveguide. In the light control device including a control electrode, a first layer having a stress on the dielectric crystal substrate equal to that of the buffer layer is provided on the back surface of the dielectric crystal substrate. device. 2. The light control device according to claim 1, wherein a second layer having the same shape as the control electrode is formed on the first layer formed on the back surface of the light control device. Control device.