Self-Mixing Laser interferometry

Laser vibrometry is a well established technique that allows remote and contactless measurement of the vibration of a solid target. It relies on the coherence properties of a laser beam, and on the high sensitivity of the coherent detection exploited in a Michelson interferometer, which permits us to detect the very small echo signal backscattered by a rough diffusing surface.

Laser vibrometry has been demonstrated and successfully used in a variety of scientific and industrial applications, where high sensitivity and low invasiveness are of importance, e.g. modal analysis, vibration and noise testing, characterization of loudspeakers and piezoceramic transducers.

The standard approach to optical measurement of small vibrations is laser Doppler velocimetry (LDV), on which a number of commercial products have been developed. The operating principle is that of conventional Michelson and Mach–Zehnder interferometers. The measuring arm projects light from a He–Ne laser onto a vibrating target, the backscattered light undergoes a Doppler frequency shift proportional to the target velocity and it is then coherently detected at the instrument side. Unlike displacement measuring interferometers, the interferometric signal is usually processed by extracting the Doppler beat frequency through a frequency demodulator, so as to obtain an output signal proportional to the instantaneous target velocity.

The self-mixing configuration allows for a practical set-up that is simpler by far than conventional laser vibrometer schemes. Light from a single-longitudinal-mode LD is simply projected onto a reflective or diffusive target, and a small fraction of the backreflected or backscattered light is allowed to re-enter the laser cavity, thus generating a modulation of both the amplitude and the frequency of the lasing field. It turns out that the power emitted by the LD is modulated. By analysing this modulation it is possible to reconstruct the target displacement (along the laser direction).