High-speed subnanometre interferometry using an improved three-mode heterodyne interferometer

Abstract

Previously, the authors have developed a subnanometre heterodyne interferometric system known as a three-mode heterodyne interferometer (TMHI). By converting the target displacement into twice the phase variation of a conventional interferometer, the TMHI has achieved position resolution of 0.044 nm even with a single-pass interferometer. However, the heterodyne beat frequency in TMHI is quite low, of the order of several hundred kilohertz, which limits the measurable target speed to several tens of millimetres per second. In this paper, we present a method to extend the measurable target speed of the TMHI without increasing the heterodyne beat frequency of a light source. Keeping the excellent position resolution of the TMHI, the proposed method provides sufficient response to high speed movement of the target. The system comprises a pair of TMHI and utilizes two conjugate signals generated by them selectively according to the direction of movement of the target. Possible errors in switching the two signals are analysed by introducing an error zone model. The switching function was demonstrated using the mechanical vibration of the interferometer and proved to be free from erroneous counts. It was also confirmed that the system can respond at a speed that exceeds that determined by the beat frequency.