In situ interference measurements of glass substrate temperature

We describe the use of optical interference with a small HeNe laser for the measurement of glass substrate temperatures. The measurement is insensitive to window transmission changes and requires only the monitoring of the beam reflected from the back of the substrate while the glass is heated or cooled from a known reference temperature. We find that the technique readily yields a temperature measurement sensitive to ±10 C; however, good accuracy requires not only knowledge of the thermal expansion coefficient but also data on the temperature dependent changes in the index of refraction good to one part in 10⁶. Since such information generally does not exist in the literature, we have obtained the data up to 400 C for four glasses: fused silica, Corning 7059, NEG 0A-2, and LOF soda-lime. Analysis of the data yields the direct temperature derivative as well as the indirect density derivative of the index of refraction