High-temperature laser interferometer for thermal expansion and optical-length measurements

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Abstract
A simple laser interferometer has been utilized to measure the thermal expansion and optical‐length variation with temperature of highly reactive fluorides at high temperatures. The interferometer is a modification of the Jamin interferometer in which a laser beam is split into two parallel beams which traverse nearly identical paths and are reflected from the sample and a reference mirror. The reflected beams recombine to produce an interference pattern. The displacement of the front sample surface, relative to the reference mirror at the back sample surface, allows the thermal expansion to be measured. The sample and reference mirrors are made reflecting by depositing a platinum film on the surface. The same arrangement of beam splitters has been used to measure the variation in optical length with temperature. In this case, the sample is not coated with platinum. Measurements of thermal expansion have been made up to 1000 K above which the platinum film becomes unstable. Optical lengths of various fluorides have been measured up to 1100 K. The platinum is not observed to diffuse into divalent metal fluorides although these samples oxidize readily at certain high temperatures. The advantages and disadvantages of this method are discussed.