Expansivity of Fused Quartz Glass Measured Within $$6 \times 10^{-10}\,\text{K}^{-1}$$

A method is described to measure the thermal expansion coefficient of fused quartz glass. The measurement principle is to monitor the change in resonance frequency of a Fabry-Perot cavity as its temperature changes; the Fabry-Perot cavity is made from fused quartz glass. The standard uncertainty in the measurement was less than 0.6 (nm·m^-1)·K^-1, or 0.15 %. The limit on performance is arguably uncertainty in the reflection phase-shift temperature dependence, because neither thermooptic nor thermal expansion coefficients of thin-film coatings are reliably known. However, several other uncertainty contributors are at the same level of magnitude, and so any improvement in performance would entail significant effort. Furthermore, measurements of three different samples revealed that material inhomogeneity leads to differences in the effective thermal expansion coefficient of fused quartz; inhomogeneity in thermal expansion among samples is 24 times larger than the measurement uncertainty in a single sample.