COMPACT LASER DOPPLER VIBROMETER FOR SIMULTANEOUS MEASUREMENT OF MULTIPOINTS UTILIZING SELF-MIXING LASER DIODE WITH OPTICAL FIBER

(1)原理：自混合激光二极管(SM-LD)发射的光束直接照射到振动扬声器表面时，产生的多普勒拍频信号呈锯齿形。差拍信号中的每一波对应于半波长位移(0.93um)。因此，通过分别根据速度的正或负符号加或减从每个锯齿的锐边产生的连续计数脉冲串，成功地计算出数字表面位移，该脉冲串由方向识别电路检测。(2)精度：当锯齿状波形清晰而尖锐时，在200 Hz正弦振动时，测量的数字位移幅度为10um-140um，与理论值在几个百分点的误差内符合良好，然而，测量范围：振动频率和速度的测量范围分别为7.6 Hz~40 kHz和6.7 mm/S~163 mm/S。在10赫兹和500赫兹，位移幅度的可测量上限分别为1200微米和40微米，下限约为4微米。在带有光纤的SM-LD的情况下，由于锯齿锐度低，测量范围大大减小，被测位移减小到理论值的50%左右。(4)同时测量：在每根光纤上用两个SM-LD同时测量扬声器上两个不同点的两个振动波形，并通过FFT方法得到两个振动波形之间的相位差。(5)雨滴拒光镜的振动测量：用一个没有光纤的SM-LD可以测量到共振频率为42.97 kHz的位移幅度0.6um-0.8um，但在具有光纤的SM-LD的情况下不能。

(1)Principle : The Doppler beat signal obtained from a self-mixing laser diode (SM-LD) has a sawtooth waveform when the emitted light beam is directly illuminated onto a vibrating loudspeaker surface. Every one wave in the beat signal corresponds to a half wavelength displacement (0.93 mum). Therefore, the digital surface displacement has been successfully counted by adding or subtracting a successive counting pulse train produced from the sharp edge of the each sawtooth in accordance with the positive or negative sign of the velocity, respectively, which is detected by a direction discrimination circuit.(2) Accuracy : The measured digital displacement amplitude, 10mum-140mum at 200Hz sinusoidal vibration, agreed well with the theoretical value within a few percent erro, when the sawtooth-like waveform is clear and sharp, however, the measured displacement was reduced to 40% - 90% of the theorelical value in accordance with the decrease of the sharpness of the sawtooth.(3) Measurable Range : The measurable range for vibration frequency and velocity was 7.6Hz-40kHz and 6.7mm/s-163mm/s, respectively. The measurable upper limit of displacement amplitude was 1200mum at 10Hz and 40mum at 500Hz, and the lower limit was around 4mum. In case of an SM-LD with an optical fiber the measurable range was greatly reduced, and the measured displacement was reduced to around 50% of the theoretical value, because the sharpness of the sawtooth was low.(4) Simultaneous Measurement : Two vibration waveforms at two different points on the loudspeaker were simultaneously measured using two SM-LD's with each optical fiber, and the phase difference between the two vibration waveforms was obtained by an FFT method.(5) Vibration Measurement of Rain-Drop Repelling Mirror : The displacement amplitude 0.6mum-0.8mum at a resonance frequency 42.97kHz could be measured using an SM-LD without an optical fiber, but could not in case of an SM-LD with a fiber.