FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

used, a front lens too large to he feasible. Therefore, it can be concluded
that the étendue will be limited by the viewed area and the front optics size
(= 0.06 mm for example chosen) and that there is no need to increase further
that of the interferometer (29).
We evaluate below the sensitivity of such a system. We assume a beam splitter
with 50% reflexion and transmission and neglect any other reflexion losses in
the interferometer. We also assume isotropic scattering, a surface
displacement U cos(2irfut + ) with fu ^> Av and quantum noise limited
detection. Then it can be shown that the signal-to-noise ratio is given by:
is / iN = 4(fu/Av) (U/X) JIT iL (1-A) Ü n/(hv a S)' (6)
where II is the laser power, E is the étendue, A is the absorption
coefficient Oi. the surface and S is the viewed area. Taking II = 10 W, A =
0, X = 1.06 pm, fu = 2.5 MHz, Av = 25 MHz, B =10 MHz, n = 0.5, E = 0.06 mm 2 ,
S = area corresponding to a 1/4 inch in diameter, we find a detection limit of
0.2Â. This limit is generally appropriate for the detection of laser
generated ultrasound on a metal surface. For more absorbing surfaces, such as
carbon fibers composites, a higher power may be required. One should note
some improvement with a narrower bandwidth Au/2 and a higher étendue E,
although there are limitations on the physical size permissible for such a
system. The detection limit calculated above assumed a single frequency laser
source which is very stable, both in amplitude and frequency. In practice,
the laser should be stabilized with respect to the interferometer and the
fluctuations of amplitude and frequency should be compensated by dividing or
substracting electronically the signal coming from the sample with a signal
representative of these fluctuations (28).
V EXPERIMENTAL RESULTS OBTAINED WITH A VELOCITY INTERFEROMETER
CANMET and IMRI are engaged in the development of an ultrasound laser
generating and receiving system. We present below some results obtained with
a preliminary version of this system.
Fig. 7a shows the present experimental setup, which uses as sample a half inch
steel plate. The generating laser is a Q-switch Nd-YAG laser (made by
Lasermetrics, typically half a Joule multimodcs, 10 to 20 ns pulses), which is
focused with a 2 m focal length lens on thfî plate. Ultrasonic displacements
are detected in the present setup from the opposite side of the plate. Fig.
7b shows the echos obtained from multiple reflexions in the plate. The noise
observed on the picture is quantum noise, except the one at the beginning of
the trace which originates from electromagnetic interference from the laser.
The ultrasonic displacement for the generating conditions used is unipolar,
but since the interferometer detects (as seen above in first approximation)
the optical frequency change and the surface velocity, each echo appears as a