Determination of refractive indices and linear coefficients of thermal expansion for development of athermal glass

Recently, there has been a growing importance of development of 'athermal glass' having 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 by temperature dependence of optical path length, (1/1)·(dS/dT) (1 : geometrical length, S: optical path length, T : temperature), which is the summation of nα and dn/dT (n: refractive index, α: linear coefficient of thermal expansion). 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, using ellipsometry and utilizing the sessile drop method. The values of nα and temperature coefficient of n ranged from 1.289×10-5 K- 1 to 3.345×10-5 K-1 and from 0.270×10-5 K-1 to 1.467×10-5 K-1, respectively, depending on thelass composition. Consequently, only 80SiO ₂-5TiO₂-15Na₂O glass has shown almost the same degree of athermal characteristic as SiO₂ glass, having more advantages in practice due to its lower melting temperature than SiO ₂.