Phase modulation self-mixing interferometry by using rotated optical flat

A self-mixing interferometry based on a simple and effective phase modulation is proposed. The core of the technique is the use of an optical flat for phase modulation. The optical flat is placed in the laser external cavity. By rotating the optical flat, the phase is modulated because the rotation of the optical flat changes the optical path length of the laser beam in the external cavity. With the phase modulation, accurate micro-displacement and vibration measurement can be done. We experimentally demonstrate that when the measurement range is less than 2000 run, the maximum measurement error of the proposed technique is less than 9 nm. The vibration displacement of a loudspeaker is approximately linear with the driving voltage in the range of 0(similar to)80 mV. The proposed technique has the advantages of large modulation range, high precision, simple operation and low cost. It can potentially be used in self-mixing interferometry system widely.