Biomedical Engineering Reference
In-Depth Information
Fig. 2.15.
The laser-induced breakdown (LIB) phenomenon in a water bath. The
LIB occurred at the bright spot and heat was thought to be generated because of
bubble generation
2.3.3
Ultrasound Generated by Laser-Induced Breakdown
We created LIB in a very small region in water. In order to examine the cha-
racteristics of ultrasound generated by LIB, we carried out an experiment to
measure the waveform and directivity of the ultrasound. Figure 2.16 shows
the experimental arrangement, where the
z
direction is set along the laser
beam and the other directions (
x
and
y
) are normal to the
z
-axis;
x
is ho-
rizontal and
y
is vertical. This time, ultrasound was received with a needle
hydrophone made of polymer, because it has a higher sensitivity than a re-
ceiver made of ceramics. The received signal was amplified and then stored
in a digital storage oscilloscope (DSO) at a sampling frequency of 500 MHz.
Figure 2.17 is an example of an ultrasound waveform. The wave has a
large and sharp peak at 30
s after LIB occurs, and is a sinusoidal wave with
a period of 200 ns. Besides, the small peaks appear in the range of 30-50
μ
s.
The former peak is the wave received from LIB directly. As the pulsed laser
was radiated in this measurement, this figure shows the impulse response of
the measurement system including the hydrophone and the amplifier. This
result demonstrates that ultrasound by LIB has little ringing. On the other
hand, the latter peaks are the waves reflected at the walls of a water bath
or the water surface, and the period of the wave becomes larger as reflected
waves from various points are superimposed.
μ
