The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound

Armstrong, I.

Cleary, A.

Culshaw, B.

McKee, C.

Pierce, S.G.

Thursby, G.J.

Veres, Istvan

English

Name not available

Low power laser generated ultrasound : signal processing for time domain data acquisition

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryCleary, A. and Veres, Istvan and Thursby, G.J. and McKee, C. and Armstrong, I. and Pierce, S.G.and Culshaw, B. (2010) Low power laser generated ultrasound : signal processing for time domaindata acquisition. In: Laser Ultrasonics 2010, 2010-07-05 - 2010-07-08, Bordeaux, France.Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/            Cleary, A. and Veres, Istvan and Thursby, G.J. and McKee, C. and Armstrong, I. and Pierce, S.G. and Culshaw, B. (2010) Low power laser generated ultrasound: signal processing for time domain data acquisition. In: Laser Ultrasonics 2010, 5-8 July 2010, Bordeaux, France.   http://strathprints.strath.ac.uk/27793/     Strathprints is designed to allow users to access the research output of the University of Strathclyde. Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk) and the content of this paper for research or study, educational, or not-for-profit purposes without prior permission or charge. You may freely distribute the url (http://strathprints.strath.ac.uk) of the Strathprints website.    Any correspondence concerning this service should be sent to The Strathprints Administrator: eprints@cis.strath.ac.uk Low power laser generated ultrasound: signal processing for time domain data acquisitionA Cleary1, I Veres2, G Thursby1, C McKee1, I Armstrong1, S G Pierce2, B Culshaw11Centre for Microsystems and Photonics, Department of Electronic and ElectricalEngineering, University of Strathclyde, Glasgow, G1 1XW2Centre for Ultrasonic Engineering, Department of Electronic and Electrical Engineering,University of Strathclyde, Glasgow, G1 1XWEmail: alison.cleary@eee.strath.ac.ukAbstract. The use of low power modulated laser diode systems has previously been established as a  suitable  methodfor non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering  control  of  further  parameterssuch as the frequency content or signal shape. In addition, pseudo-random binary sequences  (PRBS)  can  be  used  toimprove the detected impulse response. Here we compare two sequences, the  m-sequence  and  the  Golay  code,  anddiscuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound.IntroductionThe use of low power continuous wave laser diodes for generating ultrasound  is  an  established  technique[1,2]. In contrast to  the  high  peak  power,  short  pulse  lasers  more  commonly  used  in  laser  generatedultrasound [3,4], laser diodes offer the option of distributing the excitation energy over a longer pulse  or  asinusoid to help avoid ablation damage to the sample.The excited ultrasonic waves have very low amplitude and for the case of sinusoidal excitation,  lock-indetection is required; the  excitation  frequency  is  stepped  through  the  range  of  interest.  For  pulsed  ortoneburst  excitation  however,  lock-in   detection   is   unsuitable   and   the   data   is   typically   averagedand captured on an oscilloscope. In this case,  signal  processing  or  coded  excitation  for  noise  reductionbecomes attractive.One such method of noise reduction is to use  PRBS  coded  pulses  as  excitation,  as  demonstrated  byPierce [5,6] who used 850 nm optical excitation and interferometric detection. Madaras and  Anastasi  havealso demonstrated a similar method, using a contact transducer for detection [7]. Here, we show how codedexcitation can be used in conjunction with a laser  diode  at  1550  nm,  where  an  EDFA  can  be  used  foroptical amplification.PRBS codingThe two types of sequence described here are the m-sequence and the Golay code [8,9]; they use a  pseudo-random train of pulses rather than a single repeated pulse. M-sequences consist  of  a  pseudo-random  trainof 2N-1 pulses of magnitude 1 or -1,  where  N  is  an  integer.  The  autocorrelation  function  is  an  almostperfect delta function with magnitude 2N-1. However, as optical pulses cannot be negative,  the  pulse  trainmust be modified to be a set of 1’s and zeroes  [10,11].  This  modification  influences  the  zero  frequencycomponent and reduces the amplitude of the autocorrelation function by a factor of four.  Golay  codes  aremore complex, but the autocorrelation function is perfect. They are a set of two complementary codes  withmagnitudes of 1 or -1. The two codes are then decomposed into two further series  that  consist  of  1’s  andzeroes, which are suitable for optical excitation. Each code is sent separately to the sample  excitation,  andthen   the   four   measured   responses   are   combined   together   through   cross-correlation    [9,13].    Itsautocorrelation function has magnitude 2x(2N).By cross-correlating the ultrasonic signal with the original pulse stream  that  was  sent  to  the  sample,  theunique properties of the  autocorrelation  functions  mean  that  the  final  recovered  signal  is  the  impulseresponse with increased amplitude. Therefore, the PRBS coding acts as a type of smart averaging  where  itis not necessary to wait for each individual pulse response to decay before  inputting  the  next,  and  so  theacquisition time required for averaging is considerably reduced.Experimental setupThe optical excitation consisted of a distributed feedback (DFB) laser diode which was modulated  througha bias tee with an arbitrary function generator, and amplified using an EDFA. The  output  power  from  theEDFA was 1 W, and the power incident on the sample  was  approximately  0.8  W.  The  output  from  theEDFA was focussed onto the sample surface using a single element lens,  and  the  smallest  1/e2  spot  sizewas measured to be 30 µm. Figure 1 shows the experimental setup.|[pic]                                                           ||Figure 1. Experimental setup, where AFG is arbitrary function   ||generator, LDC is laser diode controller, BT is bias tee, LD is ||laser diode and EDFA is erbium doped fibre amplifier.           |  Detection  was  with  a  Polytec  OFV303  vibrometer,  using  a  displacement  decoder  with  20  MHzmaximum  bandwidth.  The  output  from  the  vibrometer  was   sent   directly   to   an   Agilent   Infiniiumoscilloscope. The sample under test was a  12.5  µm  thick  brass  plate  which  was  mounted  vertically  toenable the detection beam to be placed on the opposite side of the plate from the excitation. The source anddetection spots were aligned to be within approximately 1 mm of each other, but some  drift  was  observedduring the course of the experiments.M-sequence excitation was provided by sequences of length  2047  (211-1)  and  8191  (213-1)  bits,  andGolay codes of length 2048  bits  were  used.  Single  pulse  excitation  was  also  used  for  the  purpose  ofcomparing responses from the coded sequences. All measurements were averaged on the oscilloscope 4096times.ResultsM-sequencesA set of measurements under identical excitation and detection conditions were recorded for  an  individualpulse at 200  kHz  and  m-sequences  of  length  2047  and  8191  bits,  again  at  200  kHz.  Control  of  thefrequency content of the  pulses  is  achieved  by  varying  the  width  of  the  individual  pulses  within  thesequence. This means that every element (1 or 0) in the m-sequence has a width  of  (1/200  kHz)  or  5  µs.Figure 2 shows a segment of the 2047 bit input m-sequence as recorded on the  oscilloscope,  and  figure  3shows the response from two different length m-sequences after cross-correlation.|                                               |                                         ||Figure 2. Segment of the 2047 bit m-sequence   |Figure 3. Response from two different    ||used as input to the DFB laser diode.          |length m-sequence measurements           |The  increase   in   maximum   amplitude   between   the   two   sequences   is   approximately   fourfold,corresponding to the increase in the number of bits in the m-sequence.The frequency response of the 8191 bit m-sequence measurement  at  200  kHz  is  shown  in  Figure  4,showing that the first null occurs at 200 kHz, and that most of the energy is concentrated in the frequenciesup to 200 kHz.SNR enhancementIf the noise level can be significantly reduced by using coded  excitation,  there  is  a  chance  to  detect  thesmall displacement signals  associated  with  low  power  laser  generated  ultrasound.  The  expected  SNRenhancement with respect to a single pulse measurement for a perfect m-sequence is ?(2N-1) and for  Golaycodes ?(2N) [12,13]. This would correspond to enhancements of 45 and 90 for m-sequences of 2047 (211-1)and 8191 (213-1) bits respectively. As we have used imperfect m-sequences with pulses of 1  and  zero,  theexpected SNR is reduced by a factor of four. An increase by a factor of 11 for the 2047 bit  and  23  for  the8191 bit m-sequence with respect to the single pulse measurement is then anticipated. The maximum  SNRwas measured by taking the maximum signal amplitude and dividing by  the  average  noise  value  in  eachcase. In practice, we found the maximum SNR was improved by factors  of  20  and  36  for  the  2047  and8191 bit sequences.Golay codesThe SNR obtained with Golay code excitation should be better than that seen with  the  m-sequences,  as  aperfect Golay code has a larger autocorrelation function. Figure 5 shows an example of  a  2048  bit  Golaycode measurement at 200  kHz  and  that  of  a  single  200  kHz  pulse  response.  The  SNR  improvementbetween these measurements was expected to be ?(2N) which is 45, but was measured as  102.  The  greaterthan expected improvement seen is thought to be due to errors in the noise estimation.Golay codes are also more suitable than m-sequences for the case where the pulses are used as a  triggerfor a more complex waveform such as a toneburst, as they  consist  only  of  pulses  of  magnitude  ?0,  andtherefore the autocorrelation function in practice allows  a  more  complete  reconstruction  of  the  originalsignal. This is beyond the scope of this paper, but will be described in a future communication.|                                          |                                          ||Figure 4. Fourier transform of an 8191 bit|Figure 5. Response to 200 kHz pulse, and a||m-sequence response.                      |2048 bit Golay code excitation.           |ConclusionsWe have shown experimentally how both m-sequences and Golay codes can be used in the laser generationof ultrasound. M-sequences codes have to be modified from their original form for optical  excitation,  thusreducing the SNR enhancement in comparison to an equivalent length Golay code. The SNR in the case  ofboth m-sequences and Golay codes was improved  by  increasing  the  length  of  the  coded  sequence  andcorresponded reasonably well to theoretical expectations.AcknowledgementsThe authors wish to thank Richard Leach and Nigel Jennett from NPL, and the EPSRC for funding.References1] Balogun O and Murray T W 2006 J. Appl. Phys. 10 0349022] Bramhavar S, Pouet B and Murray T W 2009 Appl. Phys. Lett. 94 1141023] Davies S J, Edwards C, Taylor GS and Palmer S B 1993 J. Phys. D: Appl. Phys. 26 3294] Sorazu B, Culshaw B and Pierce S G 2005 Proc. SPIE 5758 1775] Pierce S G, Culshaw B and Q Shan 1998 Appl. Phys. Lett. 72 (9) 10306] Pierce S G and Culshaw B 1998 IEE Proc. Sci Meas. Technol. 145 (5) 2447]  Madaras  E  I  and  Anastasi  R  F  2000  Review  of  Progress  in  Quantitative  Non  DestructiveEvaluation 19 509 3038] Dixon R C 1994 Spread Spectrum Systems  with  Commercial  Applications  (New  York:  Wiley)p609] Golay M J E 1961 IRE Trans. Inf. Theory 7 8210] Thursby G, Culshaw B, Pierce S G, Cleary A, McKee C and Veres I  2009 Proc. SPIE 729311] Cleary A,  Veres I, Thursby G, McKee C, Pierce S G and Culshaw B 2010 Review of Progress  inQuantitative Non Destructive Evaluation 29 1211 27112] Schroeder M R 1979 J. Acoust. Soc. Am. 66 (2) 49713] Foster S 1986 Proc. IEEE Int. Conf. On Acoustics, Speech and Sig. Proc.11 929---------------------------------------scopeAFGsampleLDLDCvibrometerEDFABT

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Low power laser generated ultrasound : signal processing for time domain data acquisition

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