New method to make high frequency ultrasound wavesvisible: the sliding light diffractometer

Eugenio Fazio

A diffractometric system is described, geometrically simple, that can reveal high frequency ultrasound wavespropagating in a solid body. The system can be utilized for nondestructive testing, for example, to discoverdefects in steel samples.

1. Introduction

Low energy ultrasound represents one of the mostpopular nondestructive testing methods. The trans-ducers commonly used to generate and to reveal thepulses give many problems when the samples to beanalyzed have some distinctive feature, chiefly a su-perficial roughness, or are at high temperature.

So far optical systems have already been setl-6 sothat ultrasound may be detected at a distance withoutusing any piezoelectric probe. In this way direct con-tact with the sample is avoided, also permitting workwith samples at a high temperature. However allthese interferometric systems need mirrored samples,which sharply cuts back on their use. Starting fromthe necessity of enlarging operational aspects of thesesystems we have evolved a new method by modifyingthe existing knife-edge systems.7 -9

This optical system-we refer to it as a sliding lightdiffractometer-permits working at a distance-withultrasound pulses even on a rough sample. A compari-son with conventional Michelson interferometric sys-tems shows that this method is easier both for configu-ration and operation and gives stronger signals. Itshows ultrasound waves and in some cases their veloci-ty. A full noncontact system can also be assembledusing high power laser beams to generate ultrasoundpulses.10

II. Theory

The sliding light diffractometer exploits the Fresneldiffraction.1" 2 A laser beam slides over a sample thatvibrates in a perpendicular direction from the ultra-sound probe, so that one part of the beam is cut (Fig. 1).

The passage from dark to light on the observationscreen is not abrupt but shaded according to Fresnel'swell-known formula' 3 :

E(xyz) = E exp(ikz) {- C(X) + [ S(X)]}

withx2kx=2z (1)

where E is the incoming field strength, and C (X) and S(X) are the well-known Fresnel functions. An expan-sion series of Fresnel functions can be performed forlarge X, and two different solutions are obtained for x< 0 (dark) and x > 0 (bright region):

I 1 1 1I' 4X 2rk x2

for the dark, andI /kx2 70- = 1-2a cos ( + -II'( 2z 4/

withz z 1a X =-=2k

aXnXFr OX

(2)

(3)

The author is with University of Rome, Energy Department,Physics Section, 14 Via A. Scarpa, 00161 Rome, Italy.

Received 13 November 1987.0003-6935/88/183879-04$02.00/0.© 1988 Optical Society of America.

for the bright region, where I' is the incoming intensityof the laser beam, I is the intensity on the detectingplane at a distance x, z is the distance between thesample and the detecting photodiode (see Fig. 2), and kis the light wavenumber.

The intensity as a function of the coordinate x asderived from Eq. (3) is shown in Fig. 3. If we work withone detector in the bright region, we can use approxi-

15 September 1988 / Vol. 27, No. 18 / APPLIED OPTICS 3879

1/1

\ \ \.

UJv v

xx,

Fig. 1. Principle of the sliding light diffractometer.

LIGHT

_X0E~~~~~~~~~~~~~~

Fig. 2. Sliding light diffractometer geomet

mation (3) for the case of a vibrating samplereplacing x with x + Ax(t):

I z-=1-2~~~~~~~~~~~~~-= 1 - 2

cos{t k[x + Ax(t)]2 + }

x + x(t)

We note that the intensity of the light varieither the maximum and minimum of diffraalong the coordinate x (the term given by thebecause their amplitude is modulated by tthe denominator of the fraction.

We can effect two different kinds of detone case the photodiode is placed behindthat it is possible to detect the fringe displacthis way in formula (4) greater importan(given to the cosine, which makes the wholFig. 3 move along the x axis and consequeivariation of intensity. Therefore it will be rto put the slit at x = x1 (see Fig. 3) whemaximum intensity variation.

In the second case the detector directly rlaser beam, and it will reveal an integrated light intensity. In Eq. (4) the prevailing tthis case be given by the amplification fa,Ax(t)] that around a maximum of a frircontribution much greater than that givensine. In this second case formula (4) becoi

I 1-- = 1- AO

where

2z o(k2 r)2rk 2z 4/

Fig. 3. Diffracted light intensity as a function of the coordinate.

Then it is useful for the detector to be situated toread the intensity value around the first and the sec-ond maxima of the diffraction fringes of Fig. 3. Theexperiment showed that the position of the detectorwith regard to the laser beam is not important if onewishes simply to detect the onset of the surface motion.

Z The change of the mean intensity is so strong that it isenough to detect directly the beam to reveal the signal.Also the detector size is not important.

We have to note that the exact point where the laserbeam is cut by the surface is not known, but it presentsan indetermination as big as the sample is large. Be-

ry. cause the light wavelength is shorter than that of theultrasound, it is possible to assume that the laser beamdoes not follow the shape of the vibrating sample sur-

simply by face but is only cut by the highest vibration displace-ment. On the other hand the vibrational behavior ofthe sample can have a complicated movement too, but

4 we have to pay attention only to the detection of the* (4) ultrasound pulse arrival. From this point of view this

method is not affected by a particular vibration fre-es because quency or by a particular surface movement.actonmove It is worthwhile to point out two aspects of the

cotsme) ort system: The first, perhaps the most important, is thathe term at the system may work without any particular treatment

ection: In of the sample, and consequently this may be rough too.theslit so The second concerns the possibility of developing a

te In method to have vibrational information about a smallementt bn portion of the surface, taking away the indetermina-ce must be tion over the z coordinate. This is possible with par-e profile of ticular sample geometries, for example, cylindrical,ntly gives a where the laser beam can be cut by a known small areanore useful around a point on the sample surface. This secondre there is aspect finds direct use in nondestructive testing of, foreceives the example, tubes, rods, pipes, etc.

value of the Ill. Experimentterm will in The scheme of the experimental apparatus is repre-ctor 1/[x + sented in Fig. 4. The piezoelectric probe is driven byige gives a 5-MHz frequency pulses. This frequency value is se-by the co- lected in order to work far from the characteristic

nes frequencies of the noise (mostly at low frequency) aswell as to have an ultrasound wavelength so that it ispossible to detect defects in the sample with millime-tric resolution.

The amplifier used includes a high-pass filter cut-ting frequencies <0.5 MHz, so that only the signal canpass.

3880 APPLIED OPTICS / Vol. 27, No. 18 / 15 September 1988

- l w

- -

photodiode

laser U

Fig. 4. Block scheme of the diffractometer system.

Y: 250 mV/div X: 6.00 s/div

D RI R2

/ SAMPLE

DEFECT PZT

Fig. 6. Sketch of sound propagation for a sample with a back cavityhole.

Fig. 5. Example of the signal obtained by a prismatic carbon steelsample.

A first set of measurements was made with someprismatic carbon steel samples by using a slit in frontof the detector (the first case we described above). InFig. 5 an example of the signals obtained is shown.The first visible peak on the left-hand side, S peak, isthe starting signal of the pulse given by the probe; thepeak R1 is the reflection of the ultrasound, detected bythe diffractometer, on the opposite surface of the sam-ple.

The third signal, R2, is the second reflection of theultrasound on the same surface. The distance R1-R2is twice the S-R1 distance.

A second set of measurements was aimed at detect-ing the presence of defects. A sample of the sameshape and material as the previous one was used, hav-ing an 8-mm deep hole on the back surface as sketchedin Fig. 6.

In Fig. 7 the signal obtained is shown: Fig. 7(a) givesthe integrated signal, while Fig. 7(b) is the signal atzero average value.

As we can see, another peak is visible before the R2peak, smaller than RI and R2, that comes from thereflection of a part of the ultrasound pulse over thebase of the hole (as sketched in Fig. 6). The distanceD-R2 is just twice the hole depth.

IV. Conclusions

We have described an experimental disposition todetect the arrival of ultrasound waves on the surface ofa sample. This system, at variance with the interfero-metric systems in existance, does not need mirroredsamples. The sliding light diffractometer can work

Fig. 7. Example of the signal obtained from a sample with thegeometry shown in Fig. 6: (a) integrated signal; (b) signal at zero

average value.

with high temperature samples without any direct con-tact, as the piezoelectric probes need. Moreover theexperimental tests have shown the sensitivity of thisoptical system to discover defects in the sample.

Another advantage is low noise. This feature doesnot come from casual factors and/or from the betteroperator skill, but from the simple structure of theapparatus, and from the particular high frequencyused for the ultrasound pulses, as we have employed ahigh-pass filter.

The author is grateful to M. Bertolotti and A. Ferrariof the University of Rome and to G. Canella and M.Taddei of the Centro Sviluppo Materiali for their sup-port. The experimental work was performed at theCentro Sviluppo Materiali, Pomezia (RH), Italy.

References1. L. M. Barker, "Velocity Interferometer Data Reduction," Rev.

Sci. Instrum. 42, 276 (1971).2. L. M. Barker, "Laser Interferometry in Shock-Wave Research,"

Exp. Mech. 12, 209 (May 1972).3. R. J. Clifton, "Analysis of the Velocity Interferometer," J. Appl.

Phys. 41, 5335 (1970).4. L. M. Barker and R. E. Hollenbach, "Interferometer Technique

for Measuring the Dynamic Mechanical Properties of Mater-ials," Rev. Sci. Instrum. 36, 1617 (1965).

5. L. M. Barker and R. E. Hollenbach, "Shock-Wave Studies ofPMMA, Fused Silica, and Sapphire," J. Appl. Phys. 41, 4208(1970).

6. L. M. Barker and R. E. Hollenbach, "Laser Interferometer for

15 September 1988 / Vol. 27, No. 18 / APPLIED OPTICS 3881

Measuring High Velocities of Any Reflecting Surface," J. Appl.Phys. 43, 4669 (1972).

7. G. Birnbaum and G. S. White, Res. Tech. Nondestr. Test. 7,260(1984).

8. R. Adler, A. Korpel, and P. Desmares, "An Instrument for Mak-ing Surface Waves Visible," IEEE Trans. Sonics Ultrason. SU-15, 157 (1968).

9. R. L. Whitman and A. Korpel, "Probing of Acoustic SurfacePerturbations by Coherent Light," Appl. Opt. 8, 1567 (1969).

10. C. B. Scruby, R. J. Dewhurst, D. A. Hutchins, and S. B. Palmer,"Laser Generation of Ultrasound in Metals," Res. Tech. Non-destr. Test. 5, 281 (1984).

11. J. D. Jackson, Classical Electrodynamics (Wiley, New York,1975).

12. M. Born and E. Wolf, Principles of Optics (Pergamon, NewYork, 1975).

13. L. D. Landau and E. M. Lifshitz, Theorie des Champs (Editionsde Moscou-MIR, 1970).

Books continued from page 3863

I like this book because it is straightforward, emphasizing thephysics of telescopes etc. rather than the mathematics of optics. Ifone wants to know the whys and wherefores of ground-based tele-scopes, it is probably found in this book. Do not look for the moreesoteric types of instrument, such as glancing-angle telescopes andspectrometers using corrected holographic gratings, because theyare not included. Apparently there is little need or use for correctedgratings in ground-based astronomy. It was a little disappointingnot to have more information on detectors or information on themanner of supporting large mirrors to preserve their figures.

W. R. HUNTER

Instabilities and Chaos in Quantum Optics. Edited by F. T.ARECCHI and R. G. HARRISON. Springer-Verlag, New York,1987. 253 pp. $59.50.

This book, the thirty-seventh volume in the well-known Springerseries on Synergetics, contains ten chapters on instabilities andchaos in lasing systems. The nonlinear and irregular behavior haslong been a common feature of experimental laser phenomenology,and there is great current interest in periodic and chaotic laserpulsations from both practical and fundamental points of view. Forexample, detailed understanding of lasing systems is an importantprerequisite for the development of new laser systems or scaled-upversions of existing devices.

This book gives a beautiful survey of the present state of the art inthis lively and rapidly growing field and is a useful and much neededaddition to the spate of recent books on quantum optical phenomenaand methods. It is organized into two parts: The first six chaptersdeal with instabilities and chaos in active systems, i.e., laser systemsin which an external energy source (the pump laser) provides theessential ingredient for the system operation. The remaining fourchapters treat instabilities and chaos in passive optical systems, suchas optical bistability.

The book contains a balance between theoretical and experimen-tal surveys, all by authors that have significantly contributed toresearch in this area. Most chapters require the reader to beequipped with decent knowledge of the subject matter (for example,this reviewer could not find a clear-cut definition of passive vs activeoptical systems), while others can be read with little effort by anyinterested student. In particular, the first two chapters by Arecchigive clear access into the subject matter. They contain a primer ondeterministic chaos and a discussion of the modulated laser, a laserwith injected signal or instabilities in a laser with feedback. Some ofthese topics are dealt with in greater detail in the subsequent chap-ters.

In short, this book, of which the printing is excellent, maintainsthe good quality of the series. It surely should be on the shelves ofthose doing research in quantum optics and all those practitioners ofnonequilibrium statistical mechanics who work on instabilities andchaos, including also those who hope to do so.

PETER HANGGI

Semiconductor Interfaces: Formation and Properties.Edited by G. LELAY, J. DERRIEN, and N. BOCCARA. Springer-Verlag, New York, 1987. 389 pp. $49.50.

This volume in the Springer Proceedings in Physics Series, whichis divided into several sections, stems from the workshop of the sametitle held in Les Houches, France, 24 Feb.-6 Mar. 1987. The firstsection, Experimental Study of the Formation of SemiconductorInterfaces, is devoted to the synthesis of the semiconductor inter-faces to be characterized in the rest of the book. The section onStructural Characterization of the Interfaces begins with a paper byG. LeLay on the current understanding of the atomic structure ofselected semiconductor surfaces. The rest of the section is devotedto the description of a variety of techniques which have been used inthe determination of the structure of semiconductor surfaces andinterfaces. The next section dealing with Electronic Properties ofInterfaces is introduced by C. Calandra and F. Manghi with a briefdiscussion of the band structure theory of semiconductor surfacesand interfaces. Other papers in the section are evenly dividedbetween photon approaches to the study of the electronic propertiesof interfaces (such as photoemission) and studies involving electricalmeasurements (such as deep level transient spectroscopy). Opticaland Vibrational Properties of Interfaces is a brief section whose titleis self-explanatory. Finally, Interfaces: Present Status and Per-spectives provides a certain amount of discussion of electronic de-vices whose properties are governed by interfaces.

That the workshop brought together individuals working on awide variety of techniques to characterize interfaces, both structur-ally and electrically, is obvious from the contributions in this vol-ume. The book thus provides a useful overview of the many tech-niques currently being used in semiconductor interface research.Each chapter discusses a different characterization technique withits strengths and (sometimes) weaknesses. Many chapters also citespecific results arising from the technique under consideration. Al-most every chapter provides alengthy list of references. A usefuladdition to the volume would have been a few chapters synthesizingvarious techniques, providing an assessment of such considerationsas ease of use, degree of quantitativeness of the data, and type ofinformation obtained.

A number of chapters are sufficiently detailed and didactic to be ofuse to advanced graduate students. The paper by K. Ploog on theformation of semiconductor interfaces during molecular beam epi-taxy is a particularly good example of such a treatment. Otherpapers, such as that by LeLay on the atomic structure of semicon-ductor surfaces, are qualitative in nature but give copious referencesto the original literature. A large number of the contributions,however, are essentially summary abstracts, which are probably oflimited application to those who do not make use of the references.

JULIA M. PHILLIPS

continued on page 3943

3882 APPLIED OPTICS / Vol. 27, No. 18 / 15 September 1988