Measuring large step heights by variable synthetic wavelength interferometry

Many mechanical and optical components contain step features whose surface height changes far exceed the optical wavelength. Therefore, this work presents an interferometer based on variable synthetic wavelength interferometry (VSWI) and differential heterodyne configuration to measure large step heights directly and unambiguously. This largely common-path configuration can substantially reduce the influence of environmental disturbances, which are the main sources of error in the VSWI. Only one external cavity diode laser (ECDL) is employed to synthesize a series of synthetic wavelengths in descending order. The wavelengths are combinations of the varied wavelengths and the initial wavelength of the ECDL. In contrast to wavelength scanning interferometry, this method does not require the laser wavelength to be continuously tuned. The step height is sequentially measured at these synthetic wavelengths and a lock-in amplifier resolves the corresponding synthetic fractional fringes. The step height is determined following a succession of optical path difference calculations, in terms of the synthetic wavelengths and measured synthetic fractional fringes. Three known step heights, verified by a gauge block interferometer, were used to confirm the performance of the proposed system. The results reveal that the uncertainty in the measurement is approximately 80 nm when the measured height is up to 25 mm.