Fast and precision displacement/3D profile measuring interferometers using sinusoidal frequency/phase modulation without lock-in amplifiers

Abstract

A new demodulation method for sinusoidal frequency/phase modulation (SFM/SPM) interferometers, which can be suitable for displacements/3D profiles of targets, is proposed in the paper. Utilizing a laser diode (LD) as the light source and an electric optic modulator (EOM), frequency/phase modulations of the LD light can be achieved by modulating the LD injection current or the EOM. The SFM/SPM interference signal for the displacement measurement is composed by a series of harmonics of the modulation frequency. The two adjacent (2nd and 3rd) harmonics include the displacement information. By using the two harmonics, a Lissajous diagram can be drawn to obtain the displacements. Normally, the two harmonics can be demodulated by two lock-in amplifiers. In this paper, we propose a signal demodulation method to acquire the 2nd and 3rd harmonics without any lock-in amplifiers, for reducing a cost of lock-in amplifiers. In the proposed method, the data acquisition system is synchronized to the modulation frequency such that the sampling points are exactly at the maximum and minimum points of the 2nd and the 3rd harmonics, respectively. By addition/subtraction calculations of the data at the maximum/minimum points, the 2nd and 3rd harmonics can be obtained for drawing the Lissajous diagram. We first show displacement measurements of the target mirror in the SFM interferometer with 20 kHz modulation frequency. Secondly, we show 3D profile (=2D displacement) measurements of the moving target in the modified SFM interferometer using the high speed camera with 128*128 pixels and 180k frames/s. Thirdly, we discuss the feasibility of the proposed method. The authors will introduce experimental results of SPM interferometers at the oral presentation.