Measurement of surface figure and optical thickness variation of a thin parallel plate in wavelength-scanned interferometry with minimized laser tuning range

Wavelength-scanning interferometry allows the simultaneous measurement of the surface profile and the optical thickness variation of a parallel plate. Previously we have derived two error-compensating algorithms for the detection of the fundamental and third harmonic frequencies. This requires a certain fixed ratio of the interferometer's air gap width to the optical thickness of the parallel plate. As the test plate becomes thinner, so does the air gap of the interferometer, and the wavelength-tuning range eventually becomes insufficient to give the necessary phase shift. By swapping the detection frequencies of the two algorithms, the phase-shift step can be augmented threefold compared with the previous interval. The resultant scheme allows a three-times larger air gap and hence requires only one third of the wavelength-tuning range compared with the previous scheme. Measurements of a BK7 plate of 1 mm thickness and a ZnSe plate of 5 mm thickness in a Fizeau interferometer showed residual errors caused by nonlinear wavelength-scanning and higher order multiple-reflections.