JPS6327681B2 - - Google Patents

Description

[Detailed Description of the Invention] In recent years, research and development of optical IC elements having active functions such as optical waveguides and optical modulation have become active. Optical waveguides can be realized by forming a single crystal thin film layer with a high refractive index on a substrate, and methods for creating these include sputtering method, liquid phase epitaxial method (LPE method), electric field diffusion method, back diffusion method, etc. is being attempted.

Lithium niobate (LiNbO ₃ ) has a higher refractive index than lithium tantalate (LiTaO ₃ ), and the lattice constants of these two are almost the same, so
Optical waveguides made of LiNbO ₃ thin films grown on LiTaO ₃ substrates have been extensively studied.
However, as is well known, LiNbO ₃ is extremely susceptible to optical damage, and especially when laser light is confined in a narrow area such as in an optical waveguide, the density of the laser light increases and optical damage becomes extremely likely to occur. Therefore, in order to operate LiNbO ₃ as a functional element such as an optical modulation element, the intensity of the laser light used is limited, which poses a practical obstacle.

On the other hand, research by the present inventors has shown that LiTaO ₃ has a much higher threshold for photodamage than LiNbO ₃ .
It has been found that there is almost no problem with the practical laser light density. Therefore, LiTaO ₃ is considered to be the favorite optical IC material, and recently
Attempts using LiTaO ₃ have begun. As an example, an attempt has been made to form a high refractive index layer by diffusing impurities such as copper into LiTaO ₃ using a thermal diffusion method to form an optical waveguide. However, the optical waveguide created by this impurity diffusion essentially has (1) a gentle gradient in the refractive index distribution;
As an optical waveguide, it has disadvantages such as (2) increased absorption of laser light and (3) susceptibility to optical damage.
Moreover, it can be said that this is an unfavorable characteristic of an optical modulator using this.

The present invention provides a new optical waveguide made of LiTaO ₃ with a uniform refractive index distribution and free from these drawbacks. Hereinafter, one embodiment will be described in detail with reference to the drawings. The figure is a schematic diagram showing the structure of an optical waveguide according to the present invention. Excellent crystallinity,
Furthermore, it is generally known that the LPE method is superior to the diffusion method, sputtering method, etc. as a method for creating a thin film with a uniform refractive index. LiTaO ₃ thin film
In order to create by LPE method, LiTaO ₃ is used as a substrate.
The conditions are that the refractive index is smaller than that of the substrate, that the lattice constant is very close to that of the substrate, and that the melting point of the substrate is considerably higher than that of the substrate grown in the liquid phase. However, a suitable substrate that satisfies this condition has not yet been found, and therefore the LPE method is used.
It seems that no LiTaO ₃ thin film has been created.

An object of the present invention is to provide an optical waveguide that includes a substrate having the above-mentioned characteristics as a part of its constituent elements. First, as a substrate, vanadium pentoxide (V ₂ O ₅ ) is deposited on LiTaO ₃ single crystal 1 by sputtering method, and V ₂ O ₅ is diffused into LiTaO ₃ by heat treatment to form a 3-6 μm thick layer.
A layer 2 formed of a composition consisting of V ₂ O ₅ and LiTaO ₃ was used. This layer 2 has a refractive index of 1× for ordinary and extraordinary rays compared to LiTaO ₃ units.
It was found that it becomes about 10 ^-3 times smaller. Moreover, the thickness of this layer could be limited by the heat treatment temperature and time. Using the thus newly formed composition layer 2 as a substrate, a LiTaO ₃ thin film 3 was grown by the LPE method. At this time, an appropriate flux was used to make the melting point of the liquid phase of LiTaO ₃ lower than that of the substrate. this
It was confirmed that the LiTaO ₃ thin film created by the LPE method has a higher refractive index than the substrate, and that when the laser beam 5 is introduced using the prism 4, it is guided through the LiTaO ₃ thin film 3. It was also found to be resistant to photodamage. It is also possible to provide the LiTaO ₃ thin film with an appropriate electrode function to function as an optical modulator.

As detailed above, the optical waveguide newly provided by the present invention consisting of LiTaO ₃ 1 - composition 2 made of LiTaO ₃ and V ₂ O ₅ - LiTaO ₃ thin film 3 has properties that are extremely resistant to optical damage. . The main purpose of the present invention is to provide a new structure of an optical waveguide,
Although not necessarily involved in the manufacturing method, in order to take advantage of the features of this structure and create a good optical waveguide, the LPE method is optimal, and the diffusion method and sputtering method are inappropriate.

[Brief explanation of drawings]

The figure is a schematic diagram for explaining the structure of the optical waveguide obtained by the present invention, in which 1 is a LiTaO ₃ single crystal, 2 is a
is a composition consisting of V ₂ O ₅ and LiTaO ₃ , 3 is LiTaO ₃
thin film, 4 is a prism for introducing laser light,
5 is an incident laser beam.

Claims (1)

[Claims] 1. A lithium tantalate single crystal substrate, a 3-6 μm thick low refractive index diffusion layer made of vanadium pentoxide and lithium tantalate formed by diffusing vanadium pentoxide from the surface of the substrate, and a low refractive index diffusion layer on the diffusion layer. 1. An optical waveguide characterized by having a three-layer structure comprising a lithium tantalate single crystal layer epitaxially grown.