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

(1)原理：自混频激光二极管（SM-LD）发射的光束直接照射到振动的扬声器表面时，得到的多普勒拍频信号呈锯齿形波形。节拍信号中的每一个波对应半个波长位移（0.93毫微米）。因此，通过分别根据速度的正负符号添加或减去由每个锯齿的锋利边缘产生的连续计数脉冲序列，成功地计数了数字表面位移，并由方向识别电路检测到。(2)精度：在200Hz正弦振动下，测量到的数字位移幅值为10mm ~ 140mm，与理论值吻合较好，误差在几个百分点以内，锯齿状波形清晰锐利，但随着锯齿状波形清晰度的降低，测量到的位移减小到理论值的40% ~ 90%。(3)可测范围：振动频率和速度的可测范围分别为7.6Hz-40kHz和6.7mm/s-163mm/s。位移振幅的可测上限在10Hz时为1200mum， 500Hz时为40mum，下限在4mum左右。在使用光纤的SM-LD的情况下，由于锯齿的清晰度较低，测量范围大大缩小，测量位移减少到理论值的50%左右。(4)同时测量：使用每根光纤的两个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方法获得两个振动波形之间的相位差。(5)雨滴排斥镜的振动测量：不带光纤的SM-LD可以测量到42.97kHz共振频率下0.6 mm -0.8mum的位移幅值，带光纤的SM-LD则无法测量。

(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.