Measurement of Waveplate Parameters over Entire Clear Aperture Based on Differential Frequency Modulation with Dual Photoelastic Modulators

Abstract

To obtain highly sensitive, accurate, fast, and repeatable measurements of waveplate parameters over an entire clear aperture, a novel measurement method using dual differential frequency photoelastic modulations is proposed. Simple polarimetry is conducted based on two photoelastic modulators, which operate at different frequencies. The fast-axis azimuth and retardance parameters of the waveplate are loaded into the modulation signals. Employing digital phase-locked technology, the fundamental and differential frequency harmonic terms are extracted, and then the two parameters of the waveplate are demodulated. The principle is analyzed, and the measurement system is built for verification experiments. The experimental results reveal that the two parameters of the waveplate are simultaneously measured over the entire clear aperture. The standard deviations of the fast-axis azimuth and retardance are 0.02° and 0.03 nm, respectively, and the maximum relative deviations of the fast-axis azimuth and retardance are 0.6% and 0.06%, respectively. The single-point data measurement time is only 200 ms. The proposed method exhibits high precision and speed, and provides an effective quality inspection and calibration method for waveplates.