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

Abstract

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.