Common-path heterodyne self-mixing interferometry with polarization and frequency multiplexing.

A heterodyne Nd:YVO microchip laser self-mixing interferometry based on frequency and polarization multiplexing has been demonstrated. By using two orthogonally polarized lights to measure the measurement and reference target, the effect of the acousto-optic crystal thermal creep and air disturbance in interference light path is eliminated. In addition, the measurement error caused by the difference between two identical microchip lasers is compensated for by shifted frequency multiplexing technique. A rate equations model with multi-channel frequency-shifted feedback is established for interpreting the principle of the interferometry. Due to the ultrahigh sensitivity of the microchip laser, the target used in the experiment is a non-cooperative object which is different from the targets in conventional Michelson interferometers. Under typical room conditions, the short-term resolution is better than 2.5 nm, and the long-term zero drift is less than 60 nm within 7 h. The result shows that this self-mixing interferometry system is feasible and robust in the field of displacement measurement.