Laser Self-Mixing Interferometer Based on Multiple Reflections and Phase-Modulation Technique

An improved method combining multiple reflections with the phase-modulation technique (MR-PM) is proposed to construct a self-mixing interferometer with high accuracy. The phase modulation is performed by using an electro-optic modulator that is placed in the external cavity. To broaden the harmonic components spectrum of the self-mixing signal, the multiple-reflection technique is employed. By extracting orthogonal signals from the spectrum, phase demodulation is implemented to realize displacement reconstruction. The principle and signal processing approach are described in detail. A series of simulations and experiments indicate that the measurement accuracy of the system can be effectively improved with the increase in reflection times. The vibration with an amplitude of 44 nm has been proved to be measurable with a reconstruction error less than 3 nm. Due to the advantages of high accuracy and broad measurement range, the proposed method will play a significant role in the field of non-contact nanometer vibration measurement.