JPH04122915A - Optical waveguide device - Google Patents

Abstract

PURPOSE:To prevent irregular strain from being applied to an optical waveguide which is formed on the surface of a ferroelectric crystal substrate by forming the same dielectric film with a buffer layer even on the reverse surface of the ferro-electric crystal substrate under the same conditions as the buffer layers. CONSTITUTION:On the surface of the lithium niobate crystal substrate 10, titanium (Ti) is diffused at 900 - 1,100 deg.C for several hours to form optical waveguides 11 and 12 which are about 3 - 10 mum deep, and both the optical waveguides 11 and 12 are put close to several mum at the center part of the substrate 10 to constitute a directional coupler 13. On the optical waveguides 11 and 12, electrodes 15 are formed across the buffer layer 13 so as to prevent light absorption by the electrodes. On the reverse surface of the substrate 10, on the other hand, the same dielectric film 16 with the buffer layer 14 is formed under the same conditions as the buffer layer 14.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention relates to optical waveguides formed on ferroelectric crystal substrates having electro-optic effects, and in particular to optical waveguides used for optical modulation, optical path switching, etc. Regarding wave path devices.

[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 order to satisfy such requirements, development of waveguide type optical modulators and optical switches constructed by optical waveguides formed on the surface of a substrate is underway. These devices have the advantages of being small, fast, highly efficient, and capable of integrating multiple elements.

In particular, those using ferroelectric materials such as lithium niobate (LiNbO2) crystals have the characteristics of low light absorption, low loss, and high efficiency due to their large electro-optic effect. Various types of optical waveguide devices such as directional coupling type optical modulators, optical switches, and total internal reflection type optical switches have been reported.

When applying such optical waveguide devices to actual optical communication systems, basic performance such as low loss and high speed, as well as operational stability, long-term reliability, ease of manufacture, and stability are practically essential. It is.

FIGS. 2A and 2B show a plan view and a cross-sectional view of a directional coupling type optical switch as an example of a conventional guided waveguide device. In FIG. 2(A), strip-shaped optical waveguides 2 and 3 are made by diffusing titanium onto the surface of a lithium niobate crystal substrate 1 cut perpendicularly to the Z-axis so that the refractive index is larger than that of the substrate.
The optical waveguides 2 and 3 are close to each other by several μm in the center of the substrate 1, and the directional coupler 4
It consists of Further, on the optical waveguide 2.3 constituting the directional coupler 4, an electrode 5 is formed with a buffer layer 6 interposed therebetween for preventing light absorption by the electrode. Figure 2 (B
) shows a sectional view perpendicular to the optical waveguide 2.3 of the directional coupler 4 part.

In FIG. 2(A), incident light 7 that has entered the optical waveguide 2
As it propagates through the directional coupler 4, the optical energy gradually transfers to the adjacent optical waveguide 3, and after passing through the directional coupler 4, almost 100% of the energy is transferred to the leading waveguide 3 and the output light It becomes 8. On the other hand, when a voltage is applied to the electrode 5, the optical waveguide 2.3 under the electrode 5 is
The refractive index of the optical waveguide 2.3 changes, a phase velocity mismatch occurs between the waveguide modes propagating in the optical waveguide 2.3, the coupling state between them changes, and the output light 9 is emitted.

[Problem to be solved by the invention]

However, in conventional optical waveguide devices, ferroelectric crystals (
When forming an optical waveguide type directional coupler 4 by thermally diffusing metal (e.g., titanium) on a substrate 1 (e.g., lithium niobate), and forming a buffer layer 6 to prevent absorption of light by the electrodes 5, In order to strengthen the light confinement in the leading waveguide 2.3, silicon dioxide (αS), which has a small thermal expansion coefficient, is used as a dielectric material whose refractive index is much smaller than that of the optical waveguide 2.3.
02-4.3 x 10-6) is used. Therefore, when lithium niobate is used as the substrate 1,
The thermal expansion coefficient of this substrate 1 is αLiNb○315.4
10-6, there is a large difference from the coefficient of thermal expansion of silicon dioxide (αSi○2-4.3X 10-6) which is the buffer layer 6. For this reason, when forming a silicon dioxide film or
When heat is applied after film formation, distortion is added to the leading waveguide portion of the directional coupler 4, causing phase velocity mismatch between the waveguide modes propagating in the two optical waveguides 2.3, and polarization. The state becomes different, and the two leading waveguides 2.
The bonding state between the three will change. Therefore, the optical characteristics of the optical waveguide device after the formation of the buffer layer 6 are significantly different from the characteristics before the formation of the buffer layer 6, resulting in a problem that an optical waveguide device with stable optical characteristics cannot be obtained.

An object of the present invention was made in view of the above-mentioned problems, and is to provide an optical waveguide device having stable optical characteristics.

[Means to solve the problem]

In order to achieve the above object, the present invention includes an optical waveguide formed on the surface of a ferroelectric crystal substrate having an electro-optic effect, and a buffer layer made of a dielectric material provided in the vicinity of the optical waveguide. and an electrode that changes the refractive index of the guiding waveguide by controlling an electric field, the same dielectric film as the buffer layer is also provided on the back surface of the ferroelectric crystal substrate as a buffer layer. It is characterized by being formed under the same conditions.

[Effect]

As described above, according to the present invention, the same dielectric film as the buffer layer is formed on the back surface of the ferroelectric crystal substrate under the same conditions as the buffer layer. A stress of the same magnitude as the stress due to the difference in thermal expansion coefficient between the ferroelectric crystal substrate and the buffer layer is also generated on the back surface of the ferroelectric crystal substrate. Therefore, the stress applied to the front and back surfaces of the ferroelectric crystal substrate is balanced, and the substrate is not distorted.

Therefore, since non-uniform strain is not applied to the optical waveguide formed on the surface of the ferroelectric crystal substrate, an optical waveguide device with stable optical characteristics can be obtained.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIGS. 1(A) and 1(B) are a plan view and a sectional view of a directional coupler type optical switch, which is an embodiment of the guiding waveguide device according to the present invention. Lithium niobate (LiNbO2)
Titanium (T1) is applied on the surface of the crystal substrate 10 from 900 to 110%.
Optical waveguides 11 and 12 with a depth of about 3 to 10 μm are formed by thermal diffusion at about 0°C for several hours, and the substrate 10
The optical waveguides 11 and 12 are located close to each other within several μm to form a directional coupler 13 at the center. Furthermore, an electrode 1 is placed on the optical waveguide 11.12 constituting the directional coupler 13 via a buffer layer 14 to prevent light absorption by the electrode.
5 is formed.

On the other hand, on the back surface of the lithium niobate crystal substrate 10, the same dielectric film 16 as the buffer layer 14 is formed under the same conditions as the buffer layer 14, and the material has a refractive index lower than that of the lithium niobate crystal substrate 10. Low silicon dioxide (S1
02) is used. Since the dielectric film 16 must be formed under the same film forming conditions as the buffer layer 14, a method of forming the dielectric film 16 and the buffer layer 14 at the same time may be used.

Now, to explain the operation of the present directional coupling type optical switch, as the incident light 17 that has entered the optical waveguide 11 propagates through the directional coupler 13, the optical energy is gradually transferred to the optical waveguide 12 that is close to it. After passing through the directional coupler 13, almost 100% of the energy is transferred to the leading waveguide 12 and becomes the emitted light 18. On the other hand, when a voltage is applied to the electrode 15, the electro-optical effect causes the optical waveguides 11 and 12 below the electrode]
The refractive index of the optical waveguides 11 and 12 changes, phase velocity mismatch occurs between the waveguide modes propagating in the optical waveguides 11 and 12, the coupling state between them changes, and the output light 19 is emitted.

〔Effect of the invention〕

As explained above, according to the present invention, there is provided an optical waveguide formed on the surface of a ferroelectric crystal substrate having an electro-optic effect, and an electrode provided in the vicinity of the optical waveguide via a buffer layer made of a dielectric material. In the optical waveguide device configured with
Stress of the same magnitude as the stress generated on the front surface of the ferroelectric crystal substrate due to the difference in coefficient of thermal expansion between the ferroelectric crystal substrate and the buffer layer during formation of the buffer layer is also generated on the back surface of the ferroelectric crystal substrate. Therefore, the stress applied to the front and back surfaces of the ferroelectric crystal substrate is balanced, and the substrate is not distorted. Therefore, since no non-uniform strain is applied to the optical waveguide formed on the surface of the ferroelectric crystal substrate, an optical waveguide device with stable optical characteristics can be obtained. In particular, the effect is remarkable in a directionally coupled optical waveguide device, and in a directionally coupled optical switch, the variation in branching ratio caused by strain can be reduced from 50% to 5% or less.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIGS. 1(A) and 1(B) are a plan view and a sectional view showing an embodiment of an optical waveguide device according to the present invention, FIG. 2(A),
(B) is a plan view and a cross-sectional view showing an example of a conventional optical waveguide device. 10...Lithium niobate crystal substrate, 11.12
...... Optical waveguide, 13 ... Directional coupler, 14 ... Buffer layer, 15 ... Electrode, 16 ... Dielectric film .

Claims (1)

[Claims] An optical waveguide is formed on the surface of a ferroelectric crystal substrate having an electro-optic effect, and a buffer layer made of a dielectric is provided in the vicinity of the optical waveguide, and the refractive index of the optical waveguide is changed by controlling the electric field. An optical waveguide device comprising an electrode for changing the ferroelectric crystal substrate, wherein the same dielectric film as the buffer layer is formed on the back surface of the ferroelectric crystal substrate under the same conditions as the buffer layer. wave path device.