Determination of Refractive Indices and Linear Coefficients of Thermal Expansion of Silicate Glasses Containing Titanium Oxides

Abstract

A growing attention has been paid to development of ‘athermal glass’, which is a material showing no temperature dependence in its optical path length and is expected to be used in optical devices for the optical fibre transmission system. The athermal characteristic is usually evaluated using the dependence of optical path length (S) on temperature (T) expressed by the following equation:

where l, n and α are the geometrical length, refractive index and linear coefficient of thermal expansion of a sample, respectively. In the present work, the refractive index and liner coefficient of thermal expansion have been determined for silicate glasses containing titanium oxides in the temperature range from room temperature to about 673 K, respectively, using ellipsometry and the apparatus with image analysis equipment. The values of nα and temperature coefficient of n ranged from 1.289×10⁻⁵ to 3.345×10⁻⁵ K⁻¹ and from 0.270×10⁻⁵ to 1.467×10⁻⁵ K⁻¹, respectively, depending on the glass composition. As a consequence, it has been found that additions of titanium oxides do not have good effects on the athermal characteristic. However, only 80SiO₂–5TiO₂–15Na₂O (on a mole basis) glass has shown almost the same degree of athermal characteristic as SiO₂ glass, and it is suggested that 80SiO₂–5TiO₂–15Na₂O glass has more advantages in practice due to its lower melting temperature than SiO₂.