High accuracy measurement of power spectral density in middle spatial frequency range of optical surfaces using optical profiler

Abstract

The scattering from residual optical fabrication errors in the middle spatial frequency range limits the resolution and signal-to-noise ratio of optical systems. An optical profiler is a useful tool for characterization of residual surface roughness for middle spatial frequencies. An accurate calibration and measurement strategy of the optical profiler are critical for accurate optical metrology. In this manuscript, the optical profiler is calibrated using two methods. The first one is the median filter method, based on the oblique line power spectral density of a super-smooth surface. The super-smooth surface was considered as a standard and the measured data were calibrated for a normal optical surface. The use of the median filter enables us to extend the effective frequency of the measured data. The second method applies multiple measurement and is built on the primary spectral characteristics of white noise: a random signal with a constant power spectral density and an oblique line power spectral density for a fractal surface. Through multiple measurement, the white noise decreases and more accurate values for the surface height can be obtained. Both methods can extend the ranges of effective spatial frequency and optimize the capability and utilization of the optical profiler.