(1) LASER GENERATION OF "DIRECTED" ULTRASOUND IN SOLIDS USING SPATIAL AND TEMPORAL BEAM MODULATION

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1 LASER GENERATON OF "DRECTED" ULTRASOUND N SOLDS USNG SPATAL AND TEMPORAL BEAM MODULATON James W. Wagner Andrew D. W. McKie James B. Spicer and John B. Deaton, Jr. The Johns Hopkins University Center for Nondestructive Evaluation Baltimore, Maryland NTRODUCTON Laser based methods for generation and detection of ultrasound are well established laboratory tools[l]. Since only beams of light interact with the surface of an object, laser ultrasonic methods are potentially non-contacting and remote and may be used in applications involving hazardous environments or unusual component geometries. However, for use in the field as a nondestructive testing tool, or in the factory as a sensor for process control, laser ultrasonic methods suffer by comparison with more conventional contact transducer techniques with regard to their generation efficiency and sensitivity. n an effort to improve the overall sensitivity of laser ultrasonic systems, schemes for temporally and spatially modulating the laser generation source have been investigated. The means by which modulation of the laser source may improve which overall sensitivity can be understood by considering the f a c t r s determine the detection signal-to-noise ratio. For a detection system whose sensitivity is shot noise limited, the signal-to-noise ratio varies as a function of surface displacement, 6, optical power on the detector, P, and overall system bandwidth, B, according to the relation given below[2]. This shot noise limited condition is normally assumed to hold for detection systems such as optical beam deflection and interferometry. Laser power, surface reflectivity, and the efficiency of the receiving optical system determine the amount of optical power, P, which ultimately reaches the system detector. The surface displacement, 6, and signal bandwidth, however, may be affected directly by proper modulation of the laser generating source. t has been shown previously that a reduction in signal bandwidth, even for a single point laser source, can be achieved by repetitive pulsing of the source laser[3). Similar bandwidth reduction has been obtained by projecting an array of closely spaced line sources on the surface of a material[4,5]. For a (1) Review of Progress in Quantitative Nondestructive Evaluation, Vol. 9 Edited by D.O. Thompson and D.E. Chimenti Plenum Press, New York,

2 detecting position located off-epicenter, such an array produces an acoustic signal approximating a toneburst for each single pulse of the source laser. Since the acoustic signal is in the form of a toneburst, rather than that of a single spike normally associated with the longitudinal component of acoustic signals generated by a single Q switched laser pulse, the acoustic signal bandwidth is considerably reduced. By correspondingly narrowing the bandwidth, B, of the optical detection system, a commensurate improvement in signal-to-noise ratio may be obtained as predicted by Equation 1. Furthermore, repetitive pulsing of the laser array source at the appropriate frequency should result in considerable enhancement of the amplitude of the toneburst signal. A resulting increase in the measured surface displacement, 6, will contribute to still greater improvement of overall signal-to-noise ratio. SOURCE LASER MODULATON An array of cylindrical lens elements was used to produce experimentally a laser source consisting of an array of parallel lines. The number of source array elements was determined by the diameter of the incident beam. A varifocal lens system was implemented in order that the spacing between the array lines could be controlled. From Figure 1, it can be seen that the arrival of the acoustic signal at a point, P, and at an angle,. off-epicenter will consist initially of a series of longitudinal pulses arriving in sequence - first from the right-most line source and then sequentially through the left-most line source. f the angle,. and source pulse duration are selected appropriately, the longitudinal pulse from one source can be timed to end just as the longitudinal pulse from the adjacent source arrives. The resulting signal, shown in Figure 2a for a single laser pulse incident on the array, shows a strong frequency component near 13 MHz (Figure 2b). This signal is obtained for an off-axis angle of 8, a 9-line array with a.48 mm line separation, and a laser pulse duration of -25 nanoseconds. Further enhancement of signal amplitude may be obtained by repetitively pulsing the source laser at a 13 MHz repetition rate. NCDENT LASER LNE SOURCES Figure 1. Linear Laser Array Source 488

3 a b '? 1:: ::::: z E-o l>l ;::;! l>l u j a.. <Jl i :: 1. ::>.5 ;:j '? 2 :::;:::15 l>l t: G TME /(J.Ls) FREQUENCY /(MHz) 4 Figure 2. a) Acoustic signal received at 8 off-epicenter for a single laser pulse incident on a lenticular array. b) Frequency spectrum of signal shown in Figure 2a. Unfortunately, conventional intracavity modulation schemes, including Q switching, cavity dumping, and mode locking, do not normally produce pulse repetition rates in the 5 to 1 MHz range. For a Nd:YAG laser, for example, Q-switching may be used for repetition frequencies up to -5 khz. Cavity dumping may extend that frequency range to a few megahertz. Conversely, passive mode locking of a pulsed Nd:YAG laser cavity produces frequencies typically around 15 MHz. n order that repetition frequencies over the range of interest for acoustic wave generation may be produced, passive mode locking of a folded cavity Nd:YAG laser[6] has been implemented. The cavity permits generation of a 13 MHz mode locked pulse train using a total optical cavity length of about ll meters with a physical length of under 1.5 meters. With repetitive pulsing at the proper frequency, it should be possible to reinforce the acoustic signal from a given line source by superposition with an adjacent line source resulting from subsequent laser pulses in the mode-locked train. The effect is in some ways similar to that obtained through constructive reinforcement from a phased antenna array. ACOUSTC DRECTVTY With a laser array source operating in some ways analogous to that of a phased antenna array, the prospects for directing a beam of ultrasound in a material simply by manipulating the array spatial and temporal characteristics will be considered. n fact, narrowband directivity of the acoustic signal has been demonstrated[4]. However, such directivity must be understood as being significantly distinct from acoustic energy directivity which may be obtained using an array of piezoelectric transducers or electromagnetic wave directivity obtained with phased antenna arrays. This distinction is clarified with the following discussion. Consider the experiment detailed in Figure 3. n this case, a Q switched Nd:YAG laser beam passes through a lenticular array and varifocal lens system and impinges on the flat surface of an aluminum 489

4 Beomspl(tter Aluminum alloy hemicylinder Trigger pulse Signal processing f _ / system Figure 3. Experimental configuration. hemicylinder. The curved surface of the hemicylinder is polished to permit sensitive detection of surface vibrations using a path-stabilized Michelson interferometer[7]. The experimental arrangement is such that detection may be performed over a range of angles from (on epicenter) to approaching 9". Thus, for a given array pattern and laser pulse duration, the directivity of the generated acoustic signal may be measured directly. The acoustic waveforms resulting from a 1 mm array spacing and 9 ns laser pulse duration are shown in Figure 4. The flat surface of the hemicylinder was coated with black ink between each laser pulse in order to enhance the acoustic signal amplitudes by the rapid evaporation of the surface coating[8]. The acoustic signals generated using this surface modification were reproducible. Owing to this surface modification, a significant pulse was observed at " as shown in Figure 4. As measurements progressed off-epicenter, the pulse width broadened with fine detail beginning to appear so that at 4" and 6", considerable depth of modulation was obtained. From the corresponding amplitude spectra, one can see that considerable frequency content at 1 MHz existed on-epicenter and again at 4" off-epicenter. This effect is even more evident at 6 where a strong frequency component was observed at 7.5 MHz. The total energy at any angle, however, followed the directivity pattern predicted for an evaporative source of comparable dimension to that of the laser array source[9]. n other words, the broadband directivity obtained using this linear array source was unaltered from that associated with a raw laser beam having the same spatial extent. This result is consistent with the fact that the acoustic field resulting from an array source is simply the superposition of a series of monophasic longitudinal wave pulses of common polarity. That being the case, there exists no possibility for complete destructive interference at any angle, so that the total acoustic energy delivered at any angle is unaltered from what one would expect of radiation from a single raw beam source. Although energy directivity in a classical sense may not be obtained by a laser array source, considerable narrowband directivity 49

5 s :: e 15...,... c e g, degrees u cs -a is ' ; Time (ps) r , 5.,..zo.o degroos e.oo a.oo 1. Time (i.:s) e , 5. j lt r ; : ; ' " 1 " ' 1 ' - -,.,.,.,....,...,..,..,.. M " T. o.oo Time (j.s) ,. -a -...-,.=4. de9rce:1.. }.! ' J '.,! r l r W -., q.,. " ' H - 1,.,.,. t + l ', Time (J.s) p=oo.o degrms! u , i 9'==6. degrees \.A... ' _ " f ' ' ' M 5. * " o o l " f l : - ", f ' f o " i ' _ o o W,.,.,. ' P? : ; : o o,.,...,.,.,., ,..ao.o degrms s ::1 1Z5. E =a. 'o.f-i!;;..5,.>:oo"'""'.,no,..o..o,.,...s.ono..tt1"'!:2!1.oo Figure 4. Acoustic waveforms and spectra which result for a single pulsed laser array source with a lmm line spacing over a range of angles, - 491

8.s..> c " e "'. is 11 s Time ( }. < ),.... dog ree3 1. :2. -.------------------. 1. ll -1. Q. i5... c.. 4) 8,.-2. r e < o o.oo + - - - - - - 1 1. '! /! W, N t - - - - - - - - - o o + r -. o o a.oo a.

6 8.s..> c " e "'. is 11 s Time ( }. < ),.... dog ree3 1. : ll -1. Q. i5... c.. 4) 8,.-2. r e < o o.oo '! /! W, N t o o + r -. o o a.oo a.oo Time (}.<:s) 2.., , 1. ll -1. 'ii. i5 c 8 " g a J j J ' M :2. -1-nrrrTTTTT"T"TT"On-T",...rrrrrrrn,.-ri 1. ll -1. i c 6 ". -oo e.oo e.oo 1 o.oo 12. Time (}.<s) """"( - T T T T T " T " T T ' O n " " T " T T T T T " T T M T n r T T r r l Time (}.<s), ,..ao.o dag,.s , i5-2. -h-t't""tt'"r't'ttt"1"tt"ttt"tt1rt'tttt"t't"ortttt1"tt"rn""ri Figure 5. Time (}.<s) Bandpass filtering at lomhz of the acoustic signals in Figure Figure 6. Narrowband energy directivity of laser array source. has been observed. n other words, a given frequency component may be directed by varying the array source characeristics. For example, consider Figure 5 which is the result of 1 MHz bandpass filtering of the time signals shown in Figure 4. Note that onepicenter and at 4, a strong toneburst at 1 MHz is observed which is not seen at other offepicentral angles. The energy within these narrowband signals is plotted as a function of propagation angle in Figure 6. Those data indicated by triangles show the directivity at 1 MHz for a 1 mm line spacing. The square symbols indicate the directivity at 1 MHz for an increase in array line spacing to 1.14 mm. Note that the angle of maximum energy propagation at

7 1 MHz initially at 4 is shifted to a value of about 3. Thus, by altering the array characteristics, it is possible to "steer" a laser generated acoustic beam of a given frequency. DSCUSSON AND CONCLUSON While narrowband acoustic signals may be generated using a linear array source, the directivity of the resulting energy must.be qualified. That is to say, total energy directivity using laser arrays cannot be obtained in the same sense that phased antenna arrays or contact ultrasonic arrays may be used to direct radiated energy. Specifically for laser-generated ultrasonic longitudinal displacements, only constructive reinforcement of specific wave features may be obtained. This work has shown that narrowband directivity of laser generated tonebursts can be obtained. Related work using fiber-optic delay lines to produce phased, point-source arrays has shown that an enhanced longitudinal pulse may be generated and directed as a function of element spacing[lo]. Still, the total acoustic energy propagated in any direction appears to behave according to the radiation pattern which one would expect for a raw laser beam whose dimensions are comparable to that of the array source. Thus, only selected features such as acoustic energy at a given frequency, or acoustic pulse amplitude, may be directed or steered. ACKNOWLEDGEMENTS This work is supported in part by the Lockheed Missiles and Space Co. and General Electric Corporate Research and Development. REFERENCES 1. C.B. Scruby, Some Applications of Laser Ultrasound, Ultrasonics, Vol. 27, (1989). 2. J.W. Wagner and J.B. Spicer, Theoretical Noise-Limited Sensitivity of Classical nterferometry, J. Opt. Soc. Am. B Vol (1987). 3. J.W. Wagner, J.B. Deaton, Jr J.B. Spicer, Generation of Ultrasound by Repetitively Q-switching a Pulsed Nd:YAG Laser, Appl. Opt Vol. 27 (22), (1988). 4. A.D.W. McKie, J.W. Wagner, J.B. Spicer, J.B. Deaton, Jr Narrowing the Bandwidth of Generated Ultrasound by Laser llumination of Aluminum with an Array Source, Ultrasonics nternational '89 (Madrid, Spain), Butterworths, London, 1989 (n Press). 5. A.D.W. McKie, J.W. Wagner, J.B. Spicer, C.M. Penney, Laser Generation of Narrowband and Directed Ultrasound, to be published in Ultrasonics. 6. N.H. Schiller et al, Compact Nd:GLASS Mode-Locked Laser with Variable Cavity Length from 5 to 12m, Applied Optics, 28 (5), (1989). 7. R.J. Dewhurst, C. Edwards, A.D.W. McKie, and S.B. Palmer, Comparative Study of Wideband Ultrasonic Transducers, Ultrasonics, Vol. 25, (1987). 8. D.A. Hutchins, R.J. Dewhurst, S.B. Palmer, Laser Generated Ultrasound at Modified Metal Surfaces, Ultrasonics, Vol. 19, (1981). 493

8 9. D.A. Hutchins, R.J. Dewhurst, and S.B. Palmer, Directivity Patterns of Generated Ultrasound in Aluminum, J. Acoust. Soc. Am Vol. 7, (1981). 1. J.A. Vogel, A.J.A. Bruinsma, A.J. Berkhout, Beamsteering of Laser Generated Ultrasound, Ultrasonics nternational '87 (London, UK), Butterworths, London, (1987-). 494

AN ACTIVELY-STABILIZED FIBER-OPTIC INTERFEROMETER FOR

AN ACTIVELY-STABILIZED FIBER-OPTIC INTERFEROMETER FOR LASER-ULTRASONIC FLAW DETECTION S.G. Pierce, R.E. Corbett*, and RJ. Dewhurst Department of Instrumentation and Analytical Science UMIST P.O. Box 88