of this page or responsible for its content.
/nist3:/usr/kjw/jres106-5/Pfick-01
Volume 106, Number 5, SeptemberOctober 2001
Journal of Research of the National Institute of Standards and Technology
[J. Res. Natl. Inst. Stand. Technol. 106, 833841 (2001)]
Precision Ultrasonic Wave Measurements
With Simple Equipment
Volume 106
Number 5
SeptemberOctober 2001
Steven E. Fick
National Institute of Standards and
Technology,
Gaithersburg, MD 20899-8221
and C. Harvey Palmer
Johns Hopkins University,
Baltimore, MD
steven.fick@nist.gov
We describe the design and construction of
a relatively simple, inexpensive laser in-
terferometer system for accurate measure-
ments of ultrasonic surface displacement
waveforms in reasonably friendly environ-
ments. We show how analysis of a single
waveform can provide both the calibration
constant required for absolute measure-
ments and an estimate of the uncertainty of
these measurements. We demonstrate the
performance of this interferometer by
measuring ultrasonic waveforms gener-
ated by a novel conical-element ultrasonic
transducer.
Key words: displacement; inexpensive;
interferometer; precision; ultrasonic wave.
Accepted: July 19, 2001
Available online: http://www.nist.gov/jres
1.
Introduction
Ultrasonic methods are now widely used for many
purposes: academic, industrial, and medical. For many
applications, simple detection suffices to determine the
time intervals between pulses. For other uses, such as
the determination of material constants, accurate sur-
face displacement waveform measurements may be
needed. A variety of systems [1-3] have been shown to
yield highly accurate waveform information and to have
high sensitivity, even under adverse ambient conditions.
Because they typically involve elaborate apparatus: con-
focal Fabry-Perot interferometers [4,5], photorefractive
materials [6], high power laser generators and high
power laser detectors, these systems can be very expen-
sive, and can themselves add hazards to the working
environment.
Our system, designed for use in reasonably benign
environments found in many laboratories, uses relatively
inexpensive equipment to yield the desired surface dis-
placement waveform information. It is effectively a point
source/point receiver arrangement [7,8]. The ultrasonic
wave source is a small conical piezoelectric transducer
(designed as an acoustic emission sensor), and the dis-
placement detector is a low power (1 mW) laser interfer-
ometer of special design. These, together with associ-
ated electronics, a digital oscilloscope, and a computer,
compose the system. With it we have been able to obtain
high quality, quantitative waveforms.
2.
Piezoelectric Source Transducer
Our unusual piezoelectric transducer source was de-
veloped at the National Bureau of Standards (NBS)
[now National Institute of Standards and Technology
(NIST)] for use as an acoustic emission sensor [9]. Typ-
ical commercial transducers have sensitive areas 10 mm
to 25 mm or more in diameter, a scale useful for detect-
ing or generating plane acoustic waves. The NBS de-
sign, on the other hand, is optimized for detection of the
highly curved wave fronts characteristic of small buried
833 Volume 106, Number 5, SeptemberOctober 2001
Journal of Research of the National Institute of Standards and Technology
acoustic emission sources. For this purpose, its sensing
area is very smallonly 0.7 mm in diameter. It can be
used effectively as a point receiver because the diameter
of the sensing area is small compared to most of the
wavelengths to be measured.
The design, illustrated in Fig. 1, incorporates a trun-
cated, conical, lead-zirconate-titanate (PZT) piezoelec-
tric element mounted directly on a large brass block
using hard solder. The tip of the element is equipped
with a nickel-plated electrode. The specimen itself, if
metallic, is used as one of the electrodes of the trans-
ducer. With nonmetallic specimens, a thin strip of alu-
minum foil is interposed between the specimen and the
transducer element to provide electrical contact. In ei-
ther case, the effect of the grounded electrode on the
incident elastic wave is much less than that of the wear
plate which covers and protects the grounded electrode
in transducers of conventional design. The brass block,
which substitutes for the usual backing material, is pro-
vided with two nylon feet, which, together with the
piezoelectric element, provide three-point kinematic
support. The weight of the block ensures good contact
with the specimen.
The overall design clearly makes the transducer rather
delicate and hence not very useful for most commercial
applications. However, for acoustic emission sensing in
the laboratory and for ultrasonic wave generation, the
device has been found to be quite useful.
For the experiments reported here, the transducer was
used as a source excited by the 750 V exponential pulse
waveform shown in Fig. l. The excitation pulse was
generated by a vacuum tube amplifier driven by a sim-
ple exponential pulse generator circuit.
3.
Interferometer
The classic Michelson interferometer design, in
which the sample and reference paths are at right angles
and well separated, is especially sensitive to small de-
flections of the base plate. Furthermore, the presence of
air, which under standard conditions has a refractive
index of about 1.00029 [10], introduces roughly an extra
45 (slightly shorter) wavelengths in a 100 mm path.
Even minute temperature or pressure fluctuations cause
the interference fringes to shift significantly. To measure
ultrasonic wave details as small as one five thousandth
of an optical wavelength, a better design is needed; we
used a new interferometer design which was much more
satisfactory.
The basic design of our instrument has been de-
scribed in some detail previously [11]. The essential
optical layout is shown in Fig. 2. It features a more-or-
less in-line arrangement of both reference and sample
beams. The expanded input laser beam is focused by the
large lens through the beam splitter plate, BS, onto the
specimen. The reference beam is reflected by the beam
splitter to focus on the small mirror left of the beam
splitter. The focused spot size on the specimen is about
0.02 mm diameter, much smaller than the shortest ultra-
sonic wavelengths to be measured. If necessary, the spot
could be made much smaller by using a lens with
Fig. 1. Piezoelectric source transducer and excitation voltage waveform.
834 Volume 106, Number 5, SeptemberOctober 2001
Journal of Research of the National Institute of Standards and Technology
Fig. 2. Basic optical system system of the interferometer.
shorter focal length. Thus the instrument acts as a point
receiver, and neither flatness of the specimen surface
nor a high quality optical polish are essential for good
results. Non-reflecting specimens were also studied by
cementing a tiny mirror on the surface, as explained
below. For simplicity, Fig. 2 does not show a small
device that redirects the horizontal interferometer
beams 90 up or down for probing horizontal surfaces.
The interferometer components are mounted on a
long, rigid aluminum U-channel. The fringes are stable,
to first order, against any bending of the aluminum base
in either the Y or Z directions, or twisting along the X
direction, because both sample and reference beams are
similarly affected. In addition, with the sample and ref-
erence beam parallel over most of their lengths, most
small atmospheric changes tend to affect both optical
paths about equally.
The interferometer, specimen mount, associated mea-
suring and positioning equipment, pulser, and other
components were installed on a heavy, magnetic tabletop
originally used for holographic demonstrations. The
tabletop in turn was supported by four air-filled inner
tubes, which damped out vibrations as low as about 5
Hz. The resulting anti-vibration table in turn was set on
a heavy lab bench top supported by four water-filled
inner tubes to further damp building vibrations. This
homemade arrangement was inexpensive and very ef-
fective in suppressing building vibrations.
An advantage of our design over the Michelson de-
sign is that virtually no light from either the specimen or
the reference mirror is returned to the laser. This isola-
tion removes a potential source of instability in the laser
resulting from variable feedback of different amplitude
and phase. A more expensive option is to the use a
Faraday rotator to isolate the laser.
The use of two photodetectors yields an improvement
best explained by first considering how the photodetec-
tor output signals for single and dual detector interfer-
ometers are similar. For both designs, the output voltage
of an individual photodetector can be considered to be
the sum of two components: (1) a voltage which is
directly proportional to incident optical power (laser
power reduced by static losses) but independent of the
path difference between reference and sample beams,
and (2) a voltage which is determined by both the laser
power and the path difference.
Both the path-independent and the path-dependent
signal components are affected by variations in laser
power. Both signal components can therefore compro-
mise the performance of an interferometer of either
design if the frequency range of laser power fluctuations
overlaps the frequency range of the signals of interest.