Displacement measurement using direct phase determination method for heterodyne interferometers

抄録

Heterodyne interferometers are widely utilized for precision displacement measurement applications such as nanotechnology, precision metrology and manufacturing, semiconductor manufacturing because of their advantages of high resolution, direct traceability to the definition of meter, and high noise-immunity. The commercial heterodyne interferometers typically use the pulse-counting phase meter with several mrad resolutions to measure the displacement, therefore achieving the sub-nanometer order of length resolution. In this paper, the direct phase determination (DPD) method based on the phase lock loop (PLL) technique with null method is proposed. The proposed method uses two real sinusoidal heterodyne signals, the reference signal and the measurement signal. The reference signal will be phase-shifted to any phase by a numerical phase meter. The measurement signal will be multiplied with the phase-shifted reference signal, and then be low-pass-filtered to create an error signal. By using PLL technique with null method, the error signal becomes null and the shifted-phase of the reference signal is equal to the phase change of the measurement signal. Consequently, the phase shift between two signals will be accumulated by an integrator, corresponding to the displacement measurement.In the paper, we will introduce the principle of the DPD method and some experimental results. The results show the proposed method may have a resolution of 0.1 mrad order.