Enhanced vibration measurement using differential dynamic holographic interferometry with dual photorefractive crystals

A differential dynamic holographic interferometry based on dual BSO photorefractive crystals without the need for an external electric field is studied theoretically and experimentally in this work. The proposed system allows for simultaneous measurement of in-plane and out-of-plane vibrations by using a dual-crystal symmetrical optical path and corresponding differential processing. By adjusting the polarization state of the reference beam to introduce an additional phase shift of $\pi /2$, high sensitivity linear demodulation of small phase-intensity vibration signals can be achieved without an external electric field on the crystals. By ensuring the polarity of this additional phase shift the same or opposite in two symmetrical interference optical paths, dual-path differential signals corresponding to in-plane or out-of-plane components can be obtained. The signal noise can be effectively suppressed and the sensitivity is improved by the differential processing of the dual-path signals, by which the high-sensitivity detection of high-frequency micro vibrations can be achieved. Both the numerical analysis and experiments have been carried out to confirm the feasibility of the proposed method, demonstrating its ability to accurately detect vibrations without the risk associated with high voltage. Furthermore, a built-in phase modulator is added in the system to simulate out-of-plane vibration, which serves to calibrate the symmetry and accuracy of the system. The proposed system provides a powerful and reliable tool for contactless detection both in-plane and out-of-plane components of high-frequency micro vibrations.