Frequency-scanning interferometry for depth mapping using the Fabry-Perot cavity as a reference with compensation for nonlinear optical frequency scanning

High-precision measurements of the surface depth map represent a challenging task in many industrial sectors. In this study, we present a simple frequency scanning interferometry (FSI) system with the use of a single external cavity diode laser (ECDL) for depth map measurements of a surface with large-stepped structures. It is well known that the precision of the basic FSI is limited by the nonlinear optical frequency scanning of the tunable laser source. We propose herein a novel, computationally efficient method to compensate for the optical frequency scanning nonlinearities based on the use of the Fabry-Perot cavity combined with auxiliary interferometry to precisely obtain the optical frequency in real time. The depth map of a sample with a stepped structure was measured in the experiment and the experimental results validated the proposed measurement scheme.