|
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 does not need a mirror as a reflector to be
mounted on the target; 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 set-up of this experiment is shown on the left. 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+df. 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 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 the speed
of the target. |
