Measurement of effective thermal expansion coefficient using spectral domain low coherence interferometry

Temperature dependence of the optical path difference in optically transparent materials is governed by both thermal expansion and thermo-optic effects. Combining both effects with a single parameter, referred to as effective thermal expansion coefficient ( α eff ), which measures change in optical path length with temperature. This quantity, α eff , is important for quantitative characterization of the impact of temperature variations as well as thermal effects on the optical components. This work demonstrates the use of a spectral domain low coherence interferometer (SDLCI) in common path configuration, along with phase sensitive analysis, to measure effective thermal expansion coefficient of fused silica and sapphire samples with sub-nanometer scale optical path length sensitivity. The phase based optical path length retrieval methodology is first established with simulated data and then validated experimentally. Experiments were performed for phase sensitive optical path difference measurements with sub nanometer scale repeatability in using 840 nm wavelength in the temperature range of 35–70 ºC. The experimentally measured values of α eff of fused silica and sapphire samples are found to be in agreement with the values reported in literature.