Profile measurement of optically rough surfaces by fiber-optic interferometry

An instrument for measurement of the profiles of optically rough surfaces is described. The sensor is a robust and compact Fizeau interferometer, in which one reflection is derived from the test surface. A laser-diode source and a photodetector communicate with the sensor through an optical fiber. The optical output is demodulated with a phase-stepping algorithm, achieved by frequency modulating the source. The measured horizontal resolution was 7 μm, and the noise-limited vertical resolution was $0.3\hspace{0.17em}\text{nm}/\sqrt{\text{Hz}}$.