Laser Interferometer With Harmonic Contrast Demodulation for Nanometer Distance Measurement

Abstract

High-precision 3-D topography is essential for surface profile detection in chips, precision optical lenses, and other components. A nanometer-scale laser interferometric distance sensor serves as a key component in such applications. In this kind of sensor, displacement of the target or the measurement position change induces phase shifts in the laser interference signal. Piezoelectric ceramic transducers (PZTs) are commonly used as modulators, but their lifespan, linearity, and frequency response—key factors determining the sensor’s performance—are closely tied to the modulation depth. This article introduces a harmonic contrast (HC) method to demodulate phase changes with high speed, minimal modulation depth, and single-channel detection. For scenarios involving nonstandard phase modulation functions, such as those influenced by loaded PZTs, a novel calibration approach is proposed. This method enables precise calibration without relying on expensive nanometer-precision multistep mirrors, thereby reducing the dependence on stringent modulation depth and linearity requirements. As a result, the same modulator can achieve an extended lifespan and higher sensing frequencies, making it more suitable for industrial applications. Experimental results demonstrate a resolution of 0.7 nm for step displacement signals, showcasing the good performance of the proposed scheme.