Continuous Laser Interference Thermometry of Transparent Plates

A method for quasi-continuous measurements of the nonstationary temperature of plane–parallel plates by using optical Fabry–Perot resonators is discussed. The temperature was earlier determined from interferograms only for time intervals corresponding to the extrema of the reflected or transmitted light intensities, because the deviations of the shapes of plates (polished single crystals and glasses) from the perfect plane-parallel shape distort the geometry of the resonators. It is shown that, for plates with a small surface reflectivity (R< 0.1), when the Fabry–Perot resonances are described by the two-beam interference approximation, thermometry with an arbitrary number of readings n>> 1 within a single interference fringe is implementable. Measurements with 20 temperature readings within each fringe were performed for K-8 glass in a gas-discharge plasma.