Self-mixing laser diode vibrometer

The principle and the experimental realization of a new type of laser vibrometer based on the self-mixing interference effect in a laser diode are presented. The self-mixing configuration allows for a practical set-up that is simpler by far than conventional laser vibrometer schemes. The vibrometer relies on locking of the system to half the interferometric fringe, and on active phase-nulling by wavelength modulation. This allows an extended dynamic range to be achieved, whilst retaining a good sensitivity to sub-wavelength vibrations. We have designed and built a prototype of the vibrometer that can operate on nearly any kind of rough surface, covering the 0.1 Hz-70 kHz frequency range of vibration. The noise floor is less than 100 pm Hz(-1/2) and the maximum measurable vibration amplitude is 180 mum peak to peak. The proposed method can find application in modal analysis and noise and vibration measurements in industrial and scientific environments.