Multi-point laser coherent detection system and its applications in experimental mechanics

Laser Doppler vibrometry(LDV) is a precise and non-contact optical interferometry used to measure vibrations of structures and machine components. LDV can only provide a point-wise measurement, or a scanning measurement via moving the laser beam rapidly onto the vibrating object which is assumed to be invariant in the scanning course. Consequently, LDV is usually impractical to do measurement on transient events. In this paper, a new self-synchronized multipoint LDV is proposed. The multiple laser beams are separated from one laser source, and different frequency shifts are introduced into these beams by a combination of acousto-optic modulators. The laser beams are projected on different points, and the reflected beams interfere with a common reference beam. The interference light intensity signal is recorded by a single photodetector. This multipoint LDV has the flexibility to measure the vibration of different points on various surfaces. In this study, two applications in experimental mechanics area are presented. Firstly, the proposed system is used to measure the resonant frequencies of structure in a shock test. Secondly, The proposed multi-point LDV is also used to measure the mode shape of a beam with an artificial crack. Compared with the original vibration mode shape, the crack location can be identified easily.