JPS6032843B2 - Method for forming lithium tantalate optical waveguide - 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 waveguide 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, liquid phase epitaxial method (LPE method), electric field diffusion method, and back diffusion method. etc. are being attempted. Lithium niobate (LiNb03) is lithium tantalate (LITa03)
), and since these two have almost the same lattice constant, the L grown on the LITa03 substrate
Optical waveguides made of iNb03 thin films have been extensively studied.

However, as is well known, LiNb03 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 laser light increases, making optical damage extremely likely to occur. Therefore, in order to operate LiNb03 as a functional element such as a light modulation element, the intensity of the laser light used is limited, which is a practical obstacle. On the other hand, LITa03 has a much higher optical damage threshold than LINO03, and it has been found that there is almost no problem with practical laser light density.

Therefore, it is thought that LITa03 is the most suitable material for light, and recently attempts have been made to use LITa03. As an example, an attempt has been made to form a high refractive index layer by diffusing impurities such as copper into LITa03 using a thermal diffusion method to form an optical waveguide. However, optical waveguides created by this impurity diffusion essentially have the following problems: (1) They have a gentle slope in their refractive index distribution, (2) Increased absorption of laser light, and are susceptible to beam damage. It has drawbacks, and it can be said that it has unfavorable characteristics as an optical waveguide and as an optical modulator using it. The present invention provides a new method for forming an optical waveguide structure made of LITa03 with a uniform refractive index distribution that does not have these drawbacks.

Hereinafter, one embodiment will be described in detail with reference to the drawings. In order to use LITa03 as an optical waveguide,
The refractive index of the substrate must be smaller than that of LITa03. Therefore, we searched for a substrate with a similar lattice constant and a lower refractive index than LITaQ. Magnesium oxide (M
When the Curie temperature of lithium tantalate grown by the pulling method with the addition of 1 mol % of g○) was measured, it was approximately 40° C. higher than that of lithium tantalate without additives.

By the way, it has been revealed that the Curie temperature of lithium tantalate grown from a melt containing excess Li increases as the Li content increases (Y. Fujino,
Day. Tsuya and K. Sugb ratio, Fen fertilizer
ecbics magazine, volume 2, pages 113-117, 1971
It is also known that the refractive index for extraordinary rays decreases as the Li component increases. (AABal
lmon, day. J. Uvi ship tein, C. D. Cap
ioa other day. Brown, J. Am. Ceram. S skin. Magazine 5 Choto, pages 657-659, 1967). Therefore, the fact that the Curie temperature increased due to the addition of Mg0 was inferred from the fact that the addition of Mg0 would increase the refractive index. Therefore, spots 93A and 6328A were observed for LITa03 single crystals grown by adding Mg○.
When the refractive index was measured for wavelengths of , it was found that the refractive index decreased for extraordinary and ordinary rays, as shown in the figure.

The relational expression of the refractive index change with respect to the doping concentration is as follows: If the amount of change for the extraordinary ray and the ordinary ray is △rudder and △no, and the doping concentration is C, then △ne2 - 0.001C and △no
2-0.0008C, and decreases almost proportionally. For example, when 5 mol of Mg○ is added, the refractive index is approximately 0.005.
Decrease. Therefore, if more Mg○ is added, the decrease in the refractive index becomes more remarkable.
When grown with the above addition, the crystals cracked and the crystallinity deteriorated significantly, making them unusable. Furthermore, when the polarization coefficient of Mg○ was determined by atomic absorption spectrometry, it was approximately 1.0, and the concentration added was approximately equal to the concentration within the crystal. Therefore, a LITaQ film was created by the LPE method using LITa03 doped with M as a substrate.

Using the well-known V2Q as the flux, V
Approximately 12 from the melt of 205:LITa03=80:20
When laser light was introduced into the LITa03 film grown at 00qo using a prism, it was confirmed that the laser light was guided through the LITa03 thin film. It was also found that no photodamage occurred. As detailed above, the Mg○ doped LITa○ back substrate newly provided by the present invention and the LITa○
Optical waveguides made of Q thin films are extremely resistant to optical damage.

[Brief explanation of the drawing]

The figure shows the relationship between the refractive index and the concentration of Mn added thereto.

Claims (1)

[Claims] 1. Grow a lithium tantalate single crystal to which 10 mol% or less of magnesium oxide has been added by a pulling method, mold this single crystal as a substrate, and form a lithium tantalate film on this substrate by a liquid phase epitaxial method. A method for forming a lithium tantalate optical waveguide, characterized by: