A real-time nonlinear error measurement method with picometer accuracy and free from target motion state

Abstract

Nonlinear error measurement technology is critical for the development of heterodyne laser interferometer. The existing periodic nonlinear error measurement methods is limited in measuring the nonlinear error of the heterodyne interferometer with double-direction Doppler frequency shift during the measured target non-constant velocity motion. To solve the problem, this paper presents a new method of measuring the nonlinearity in picometer level based on the double-channels orthogonal demodulation. A pair of orthogonal signals generated by FPGA multiply with the two output signals of the interferometer, respectively. And two pairs of beat frequency signals are obtained through low pass filtering, which are then mutually multiplicative and obtain the sine and cosine components containing the overall nonlinear errors of the interferometer by mathematical operations. At length the nonlinear errors are obtained through them. The experiments show that the method can measure the periodic nonlinear error in real-time free from target motion state and the measurement accuracy is in picometer level.