New digital vibrometer with high accuracy using self-mixing type LDV

We propose a new compact digital vibrometer using a self-mixing laser Doppler velocimeter (SM-LDV) with a new measurement circuit. The Doppler beat signal obtained from a self-mixing laser diode (SM-LD) has a sawtooth-like waveform, in which every sawtooth corresponds to a displacement of a half light wavelength. The new measurement circuit can always, discriminate precisely a positive or a negative displacement by comparing a rise time with a fall time of a sawtooth employing a magnitude comparator. The measurement accuracy for the vibrational displacement is greatly increased within 0.5 percent even for a degraded sawtooth. We can obtain a vibrational displacement waveform as well as its frequency spectrum by FFT. Since the new digital vibrometer with high accuracy can be made compact and inexpensive, it is useful for vibration analysis of many kinds of vibrating bodies.