Measuring the thickness of opaque plane-parallel parts using an external cavity diode laser and a double-ended interferometer

摘要

This investigation proposes a method, which is based on an external cavity diode laser (ECDL) and a double-ended interferometer, for directly measuring the thickness of opaque plane-parallel parts. The test part does not need to be wrung on a platen because a ring-like configuration enables the interferometer to view both surfaces of the part simultaneously. The optical path difference (OPD) of the interferometer is sequentially measured at a series of descending synthetic wavelengths to eliminate interference fringe order ambiguity. Every synthetic wavelength is a combination of a varied wavelength and the initial wavelength of the ECDL. The thickness of the part is finally determined following a succession of OPD calculations in terms of synthetic wavelengths and measured synthetic fractional fringes. The uncertainty in the measurement is gradually reduced as the measuring synthetic wavelength is progressively reduced. The results of performance testing indicate that the accuracy is about 0.5 μm for a 10 mm gauge block.