Basic Concept

The laser vibrometer allows the high precision and non-intrusive measurement of the movement of a target. In a common interferometer, a mirror must be attached to the target which reflects the light back into the interferometer. The outstanding property of the laser vibrometer is the fact that it directly focuses the laser beam to the target. It therefore belongs to the important class of non-intrusive working measuring instruments. The heart of the set-up is formed by a Mach-Zehnder Interferometer. The frequency of one of the two beams is shifted by an acousto optic modulator in order to apply the heterodyne fringe detection technique. In contrast to the homodyne technique, the subsequent signal amplifiers are AC coupled allowing a much higher gain in a simpler way. Due to its non-intrusive operation and high precision, the laser vibrometer has found a lot of applications in industrial applications.

Experimental Set-up

The beam of the laser is divided into two beams at the beam splitter cube A. One part is reflected and the other part is transmitted. Both beams have the same frequency, namely the one of the laser (fo). The reflected beam passes the acousto optic modulator (AOM) whereby its frequency is changed to fo+fD. At the beam splitter cube B, this beam is directed back and finally hits the photodetector (PD). The beam which is transmitted at the beam splitter cube A also transmits the beam splitter cube B and is guided within a fibre to the telescope (FT). The focused beam hits the vibrating target. The frequency of the back beam is superimposed by the doppler frequency, caused by the vibration of the object. The frequency of the returning radiation is therefore fo ± fD. Both beams are combined at B and hit the photodetector (PD). Due to its non-linear characteristic, the photodetector produces the difference of both frequencies df ± fD. To obtain the desired doppler frequency, which is proportional to the speed of the target, this frequency is mixed with the modulation frequency of the AOM by the mixer. The subsequent photo diode amplifier (PDA) conditions the signal in such a way that it fits the needs of further monitoring or recording.The amplitude of the doppler frequency finally gives the time resolved translation and the frequency itself gives the speed of the target.