Fiber-based multi-beam laser Doppler vibrometer for measuring transient vibrations

Abstract

This article presents a novel fiber-based multi-beam laser Doppler vibrometer (LDV). In this design, a single wavelength laser source at 1550 nm combined with several acousto-optic modulators (AOM) form the transmitter head of the LDV. At the receiver side, one single high-speed photo-detector is employed, instead of multiple detectors according to other reported multi-beam laser Doppler vibrometer.1, 2 Utilization of spatial encoding technique allows us to produce transmitted laser beams with different frequency shifts. In this work, a laser source passes through a sequence of totally four AOMs at different regimes, producing a 4×5 laser beam matrix which is then sent onto different points of vibrating targets for measurement. The backscattered light signals are collected back into a single mode fiber by a fiber collimator and combined with a common reference beam. This mixture of optical signals passes through an Erbium Doped Fiber Amplifier (EDFA) before it is detected by a high-speed fiber-based InGaAs photo-detector. With a digital demodulation algorithm implemented in Labview, the phase variations and thus the vibrations of different testing points can be extracted separately from their corresponding frequency bands. The experimental results show it is possible to do a precise vibration measurement on twenty testing points simultaneously using this novel multi-beam LDV.