Multipoint laser Doppler vibrometry with single detector: principles, implementations, and signal analyses

Abstract

A 20-point laser Doppler vibrometer with single photodetector is presented for noncontact dynamic measurement. A 5×4 beam array with various frequency shifts is generated by a 1.55 μm distributed feedback laser and four acousto-optic devices, and illuminating different points on vibrating objects. The reflected beams are coupled into a single-mode fiber by a pigtailed collimator and interfere with a reference beam. The signal output from a high-speed photodetector is amplified and then digitized by a high-speed analog-to-digital converter with a sampling rate of 1 gigasample per second (1 GS/s). Several methods are introduced to avoid the cross talk among different frequencies and extract the vibration information of 20 points from a one-dimensional signal. Two signal processing algorithms based on Fourier transform and windowed Fourier transform are illustrated to extract the vibration signals at different points. The experimental results are compared with that from a commercial single-point laser vibrometer. The results show simultaneous vibration measurement can be realized on multiple points using a single laser source and a single photodetector.

Applications of windowed Fourier fringe analysis in optical measurement: A review

“…The laser Doppler vibrometry (LDV) is a point-based dynamic measurement technique with much larger measurement range on velocity [43]. The probing beam reflected from a moving object interferes with a reference beam, which generates a 1D fringe pattern.…”

“…Because of the Doppler effect, the local frequency shift of the fringe pattern reflects the velocity of the point being measured. Thus the local frequency must be extracted from the fringe pattern, and the WFR became a choice, so that the velocity was obtained directly and the displacement was obtained by integration [43]. In this work, multipoint LDV with a single detector was highlighted as a feature.…”

Two-dimensional optical coherence tomography for real-time structural dynamical characterization

“…However, these single-point methods require time-consuming scanning of the tested sample [6]. Recently, Fu et al [7][8][9] have employed a single photodetector or high-speed imaging for multi-point laser Doppler vibrometry which allows 20-point measurement. Haist et al [10] used a spatial light modulator-based multipoint vibrometer realizing 14 point independent fast heterodyne interferometry channels for measuring vibrations.…”

“…Speckle interferometry [13][14][15][16], holographic interferometry [6,17,18], heterodyne interferometer [19] and Self-mixing interference [20] enable reliable measurements of mechanical vibrations. However, for these interferometer techniques [1][2][3][4][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], the displacement is obtained from the phase change of the interferometric patterns, providing extremely high depth resolution (down to 10 À 12 m or 1 pm) [21]. However, the detection range is usually limited to half the wavelength of the laser source owing to the 2π phase ambiguity [2].…”

Some discussion on high-speed-imaging-based optical coherent measurement

“…Nyquist sampling theorem has to be satisfied along time axis. Generally, there are two types of techniques: one is based on photodetector [3][4][5] and one is based on high-speed-camera [6][7]. The former technique is mainly for dynamic measurement due to the high sampling rate of photodetector.…”