Laser ultrasonics

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Laser-ultrasonics uses lasers to generate and detect ultrasonic waves. [1] It is a non-contact technique used to measure materials thickness, detect flaws and carry out materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.

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Ultrasound generation by laser

The generation lasers are short pulse (from tens of nanoseconds to femtoseconds) and high peak power lasers. Common lasers used for ultrasound generation are solid state Q-Switched Nd:YAG and gas lasers (CO2 or Excimers). The physical principle is of thermal expansion (also called thermoelastic regime) or ablation. In the thermoelastic regime, the ultrasound is generated by the sudden thermal expansion due to the heating of a tiny surface of the material by the laser pulse. If the laser power is sufficient to heat the surface above the material boiling point, some material is evaporated (typically some nanometres) and ultrasound is generated by the recoil effect of the expanding material evaporated. In the ablation regime, a plasma is often formed above the material surface and its expansion can make a substantial contribution to the ultrasonic generation. consequently the emissivity patterns and modal content are different for the two different mechanisms.

The frequency content of the generated ultrasound is partially determined by the frequency content of the laser pulses with shorter pulses giving higher frequencies. For very high frequency generation (up to 100sGHz) femtosecond lasers are used often in a pump-probe configuration with the detection system (see picosecond ultrasonics).

Historically, fundamental research into the nature of laser-ultrasonics was started in 1979, by Richard J Dewhurst and Stuart B Palmer. They set up a new laboratory in the Department of Applied Physics, University of Hull. Dewhurst provided the laser-matter expertise and Palmer the ultrasound expertise. Investigations were directed towards the development of a scientific insight into physical processes converting laser-matter interaction into ultrasound. The studies were also aimed at assessing the characteristics of the ultrasound propagating from the near field into the far field. Importantly, quantitative measurements were performed between 1979 and 1982. [2] [3] [4] [5] In solids, the measurements included amplitudes of longitudinal and shear waves in absolute terms. Ultrasound generation by a laser pulse for both the thermoelastic regime and the transition to the plasma regime was examined. [5] By comparing measurements with theoretical predictions, a description of the magnitude and direction of stresses leading to ultrasonic generation was presented for the first time. It led to the proposition that laser-generated ultrasound could be regarded as a standard acoustic source. [6] [7] [8] Additionally, they showed that surface modification can sometimes be used to amplify the magnitude of ultrasonic signals. [9]

Their research also included the first quantitative studies of laser induced Rayleigh waves, which can dominate ultrasonic surface waves. In studies beyond 1982, surface waves were shown to have a potential use in non-destructive testing. One type of investigation included surface–breaking crack depth estimations in metals, using artificial cracks. Crack sizing was demonstrated, using wideband laser-ultrasonics. Findings were first reported at a Royal Society meeting in London [10] with detailed publications elsewhere. [11] [12] [13]

Important features of laser ultrasonics were summarised in 1990. [1]

Ultrasound detection by laser

For scientific investigations in the early 1980s, Michelson interferometers were exploited. They were capable of measuring ultrasonic signals quantitatively, in typical ranges of 20nm down to 5pm. They possessed a broadband frequency response, up to about 50MHz. Unfortunately, for good signals, they required samples that had polished surfaces. They suffered from serious sensitivity loss when used on rough industrial surfaces. A significant breakthrough for the application of laser ultrasonics came in 1986, when the first optical interferometer capable of reasonable detection sensitivity on rough industrial surfaces was demonstrated. Monchalin et al. [14] [15] at the National Research Council of Canada in Boucherville showed that a Fabry–Pérot interferometer system could assess optical speckle returning from rough surfaces. It provided the impetus for the translation of laser ultrasonics into industrial applications.

Today, ultrasound waves may be detected optically by a variety of techniques. Most techniques use continuous or long pulse (typically of tens of microseconds) lasers but some use short pulses to down convert very high frequencies to DC in a classic pump-probe configuration with the generation. Some techniques (notably conventional Fabry–Pérot detectors) require high frequency stability and this usually implies long coherence length. Common detection techniques include: interferometry (homodyne or heterodyne [16] or Fabry–Pérot) [15] and optical beam deflection (GCLAD) or knife edge detection. [17]

With GCLAD, [18] (Gas-coupled laser acoustic detection), a laser beam is passed through a region where one wants to measure or record the acoustic changes. The ultrasound waves create changes in the air's index of refraction. When the laser encounters these changes, the beam slightly deflects and displaces to a new course. This change is detected and converted to an electric signal by a custom-built photodetector. This enables high sensitivity detection of ultrasound on rough surfaces for frequencies up to 10 MHz.

In practice the choice of technique is often determined by the physical optics and the sample (surface) condition. Many techniques fail to work well on rough surfaces (e.g. simple interferometers) and there are many different schemes to overcome this problem. For instance, photorefractive crystals and four wave mixing are used in an interferometer to compensate for the effects of surface roughness. These techniques are usually expensive in terms of monetary cost and in terms of light budget (thus requiring more laser power to achieve the same signal to noise under ideal conditions).

At low to moderate frequencies (say < 1 GHz), the mechanism for detection is the movement of the surface of the sample. At high frequencies (say >1 GHz), other mechanisms may come into play (for instance modulation of the sample refractive index with stress).

Under ideal circumstances most detection techniques can be considered theoretically as interferometers and, as such, their ultimate sensitivities are all roughly equal. This is because, in all these techniques, interferometry is used to linearize the detection transfer function and when linearized, maximum sensitivity is achieved. Under these conditions, photon shot noise dominates the sensitivity and this is fundamental to all the optical detection techniques. However, the ultimate limit is determined by the phonon shot noise. Since the phonon frequency is many orders of magnitude lower than the photon frequency, the ultimate sensitivity of ultrasonic detection can be much higher. The usual method for increasing the sensitivity of optical detection is to use more optical power. However, the shot noise limited SNR is proportional to the square root of the total detection power. Thus, increasing optical power has limited effect, and damaging power levels are easily reached before achieving an adequate SNR. Consequently, optical detection frequent has lower SNR than non-optical contacting techniques. Optical generation (at least in the firmly thermodynamic regime) is proportional to the optical power used and it is generally more efficient to improve the generation rather than the detection (again the limit is the damage threshold).

Techniques like CHOTs (cheap optical transducers) can overcome the limit of optical detection sensitivity by passively amplifying the amplitude of vibration before optical detection and can result in an increase in sensitivity by several orders of magnitude.

Ultrasonic laser technique operation

Ultrasonic laser set-up Ultrasonic laser.png
Ultrasonic laser set-up

The "Laser Ultrasonic" technique is part of those measurement techniques known as "non-destructive techniques or NDT", that is, methods which do not change the state of measurand itself. Laser ultrasonics is a contactless ultrasonic inspection technique based on excitation and ultrasound measurement using two lasers. A laser pulse is directed onto the sample under test and the interaction with the surface generates an ultrasonic pulse that propagates through the material. The reading of the vibrations produced by the ultrasounds can be subsequently measured by the self-mixing vibrometer: [19] the high performance of the instrument makes it suitable for an accurate measurement of the ultrasonic wave and therefore for a modeling of the characteristics of the sample. When the laser beam hits the surface of the material, its behavior may vary according to the power of the laser used. In the case of high power, there is a real "ablation" or "vaporization" of the material at the point of incidence between the laser and the surface: this causes the disappearance of a small portion of material and a small recall force, due to compression longitudinal, which would be the origin of the ultrasonic wave. This longitudinal wave tends to propagate in the normal direction to the surface of the material, regardless of the angle of incidence of the laser: this would allow to accurately estimate the thickness of the material, knowing the speed of propagation of the wave, without worrying about the angle of incidence. The use of a high power laser, with consequent vaporization of the material, is the optimal way to obtain an ultrasonic response from the object. However, to fall within the scope of non-destructive measurements, it is preferred to avoid this phenomenon by using low power lasers. In this case, the generation of ultrasound takes place thanks to the local overheating of the point of incidence of the laser: the cause of wave generation is now the thermal expansion of the material. In this way there is both the generation of waves longitudinal, similarly to the previous case, and the generation of transverse waves, whose angle with the normal direction to the surface depends on the material. After a few moments the thermal energy dissipates, leaving the surface intact: in this way the measurement is repeatable an infinite number of times (assuming the use of a material sufficiently resistant to thermal stresses) and non-destructive, as required in almost all areas of application of this technology. The movement of the object causes a shift in the phase of the signal, which cannot be identified directly by an optical receiver: to do this it is first necessary to transform the phase modulation into an amplitude modulation (in this case, in a modulation of luminous intensity ). [19] Ultrasound detection can therefore be divided into 3 steps: the conversion from ultrasound to phase-modulated optical signal, the transition from phase modulation to amplitude and finally the reading of the amplitude modulated signal with consequent conversion into an electrical signal.

Industrial applications

Well established applications of laser-ultrasonics are composite inspections for the aerospace industry and on-line hot tube thickness measurements for the metallurgical industry. [20] Optical generation and detection of ultrasound offers scanning techniques to produce ultrasonic images known as B- and C-scans, and for TOFD (time-of-flight-diffraction) studies. One of the first demonstrations on small defects (as small as 3mm x 3mm) in composites was demonstrated by Dewhurst and Shan in 1993, [21] for which they were awarded an outstanding paper award by the American Society for Non-Destructive Testing in 1994. This was also the time when significant developments on composite examinations were developed from the National Research Council of Canada [22] [23] and elsewhere. A wide range of applications have since been described in the literature. [24]

Related Research Articles

<span class="mw-page-title-main">Ultrasound</span> Sound waves with frequencies above the human hearing range

Ultrasound is sound with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, including ultrasound. Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz.

<span class="mw-page-title-main">Interferometry</span> Measurement method using interference of waves

Interferometry is a technique which uses the interference of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy, quantum mechanics, nuclear and particle physics, plasma physics, remote sensing, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms.

<span class="mw-page-title-main">Elastography</span> Any of several imaging modalities that map degrees of soft-tissue elasticity and stiffness

Elastography is any of a class of medical imaging modalities that map the elastic properties and stiffness of soft tissue. The main idea is that whether the tissue is hard or soft will give diagnostic information about the presence or status of disease. For example, cancerous tumours will often be harder than the surrounding tissue, and diseased livers are stiffer than healthy ones.

<span class="mw-page-title-main">Time of flight</span> Timing of substance within a medium

Time of flight (ToF) is the measurement of the time taken by an object, particle or wave to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a way to learn about the particle or medium's properties. The traveling object may be detected directly or indirectly.

<span class="mw-page-title-main">Terahertz time-domain spectroscopy</span>

In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on both the amplitude and the phase of the terahertz radiation.

<span class="mw-page-title-main">Ultrasonic testing</span> Non-destructive material testing using ultrasonic waves

Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.

<span class="mw-page-title-main">Ultrasonic transducer</span> Acoustic sensor

Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound.

Picosecond ultrasonics is a type of ultrasonics that uses ultra-high frequency ultrasound generated by ultrashort light pulses. It is a non-destructive technique in which picosecond acoustic pulses penetrate into thin films or nanostructures to reveal internal features such as film thickness as well as cracks, delaminations and voids. It can also be used to probe liquids. The technique is also referred to as picosecond laser ultrasonics or laser picosecond acoustics.

<span class="mw-page-title-main">Electromagnetic acoustic transducer</span>

Electromagnetic acoustic transducer (EMAT) is a transducer for non-contact acoustic wave generation and reception in conducting materials. Its effect is based on electromagnetic mechanisms, which do not need direct coupling with the surface of the material. Due to this couplant-free feature, EMATs are particularly useful in harsh, i.e., hot, cold, clean, or dry environments. EMATs are suitable to generate all kinds of waves in metallic and/or magnetostrictive materials. Depending on the design and orientation of coils and magnets, shear horizontal (SH) bulk wave mode, surface wave, plate waves such as SH and Lamb waves, and all sorts of other bulk and guided-wave modes can be excited. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries, in which they are typically used for nondestructive testing (NDT) of metallic structures.

Stuart Palmer FREng, also known as S. B. Palmer, is the Honorary Secretary of the Institute of Physics, and was the Deputy Vice-Chancellor of the University of Warwick between 1999-2009. He is an emeritus professor of physics at Warwick who has worked in Condensed Matter Physics and Engineering Physics and has extensively exploited the technique of ultrasound.

Acoustic microscopy is microscopy that employs very high or ultra high frequency ultrasound. Acoustic microscopes operate non-destructively and penetrate most solid materials to make visible images of internal features, including defects such as cracks, delaminations and voids.

The photoacoustic Doppler effect is a type of Doppler effect that occurs when an intensity modulated light wave induces a photoacoustic wave on moving particles with a specific frequency. The observed frequency shift is a good indicator of the velocity of the illuminated moving particles. A potential biomedical application is measuring blood flow.

Ultrasound-modulated optical tomography (UOT), also known as Acousto-Optic Tomography (AOT), is a hybrid imaging modality that combines light and sound; it is a form of tomography involving ultrasound. It is used in imaging of biological soft tissues and has potential applications for early cancer detection. As a hybrid modality which uses both light and sound, UOT provides some of the best features of both: the use of light provides strong contrast and sensitivity ; these two features are derived from the optical component of UOT. The use of ultrasound allows for high resolution, as well as a high imaging depth. However, the difficulty of tackling the two fundamental problems with UOT have caused UOT to evolve relatively slowly; most work in the field is limited to theoretical simulations or phantom / sample studies.

<span class="mw-page-title-main">SRAS</span>

SRAS a non-destructive acoustic microscopy microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide information about the structure and crystallographic orientation of the material. Traditionally, the information provided by SRAS has been acquired by using diffraction techniques in electron microscopy - such as EBSD. The technique was patented in 2005, EP patent 1910815.

Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which length, distance, or range can be measured. The most commonly used approaches are the rulers, followed by transit-time methods and the interferometer methods based upon the speed of light.

<span class="mw-page-title-main">Atomic force acoustic microscopy</span>

Atomic force acoustic microscopy (AFAM) is a type of scanning probe microscopy (SPM). It is a combination of acoustics and atomic force microscopy. The principal difference between AFAM and other forms of SPM is the addition of a transducer at the bottom of the sample which induces longitudinal out-of-plane vibrations in the specimen. These vibrations are sensed by a cantilever and tip called a probe. The figure shown here is the clear schematic of AFAM principle here B is the magnified version of the tip and sample placed on the transducer and tip having some optical coating generally gold coating to reflect the laser light on to the photodiode.

A common-path interferometer is a class of interferometers in which the reference beam and sample beams travel along the same path. Examples include the Sagnac interferometer, Zernike phase-contrast interferometer, and the point diffraction interferometer. A common-path interferometer is generally more robust to environmental vibrations than a "double-path interferometer" such as the Michelson interferometer or the Mach–Zehnder interferometer. Although travelling along the same path, the reference and sample beams may travel along opposite directions, or they may travel along the same direction but with the same or different polarization.

Reflectometry uses the reflection of waves at surfaces and interfaces to detect or characterize objects.

Active thermography is an advanced nondestructive testing procedure, which uses a thermography measurement of a tested material thermal response after its external excitation. This principle can be used also for non-contact infrared non-destructive testing (IRNDT) of materials.

<span class="mw-page-title-main">Photoacoustic microscopy</span>

Photoacoustic microscopy is an imaging method based on the photoacoustic effect and is a subset of photoacoustic tomography. Photoacoustic microscopy takes advantage of the local temperature rise that occurs as a result of light absorption in tissue. Using a nanosecond pulsed laser beam, tissues undergo thermoelastic expansion, resulting in the release of a wide-band acoustic wave that can be detected using a high-frequency ultrasound transducer. Since ultrasonic scattering in tissue is weaker than optical scattering, photoacoustic microscopy is capable of achieving high-resolution images at greater depths than conventional microscopy methods. Furthermore, photoacoustic microscopy is especially useful in the field of biomedical imaging due to its scalability. By adjusting the optical and acoustic foci, lateral resolution may be optimized for the desired imaging depth.

References

  1. 1 2 C.B. Scruby and L.E. Drain, Laser Ultrasonics, (Adam Hilger: Bristol), 1990.
  2. ↑ C.B. Scruby, R.J. Dewhurst, D.A. Hutchins and S.B. Palmer, "Quantitative studies of thermally-generated elastic waves in laser-irradiated metals", J. Appl. Phys., 51, 6210-6216, 1980.
  3. ↑ D.A. Hutchins, R.J. Dewhurst and S.B. Palmer, "Laser generation as a standard acoustic source in metals", Appl. Phys. Lett., 38, 677-679, 1981.
  4. ↑ D.A. Hutchins, R.J. Dewhurst and S.B. Palmer, "Directivity patterns of laser-generated ultrasound in aluminium",J. Acoustic Soc. Amer., 70, 1362-1369, 1981.
  5. 1 2 R.J. Dewhurst, D.A. Hutchins, S.B. Palmer and C.B. Scruby, "Quantitative measurements of laser-generated acoustic waveforms", J. Appl. Phys., 53, 4064-4071, 1982.
  6. ↑ A.M. Aindow, R.J. Dewhurst, D.A. Hutchins and S.B. Palmer, "Characteristics of a laser-generated acoustic source in metals", Proc of SPIE, 236, 478-485, 1981.
  7. ↑ D.A. Hutchins, R.J. Dewhurst and S.B. Palmer, "Laser generation as a standard acoustic source in metals", Appl. Phys. Lett., 38, 677-679, 1981.
  8. ↑ C.B. Scruby, H.N.G. Wadley, R.J. Dewhurst, D.A. Hutchins and S.B. Palmer, "A laser-generated standard acoustic emission source", Materials Evaluation, 39, 1250-1254, 1981
  9. ↑ D.A. Hutchins, R.J. Dewhurst and S.B. Palmer, "Laser-generated ultrasound at modified metal surfaces", Ultrasonics, 19, 103-108, 1981.
  10. ↑ J.A. Cooper, R.J. Dewhurst and S.B. Palmer, "Characterisation of surface breaking defects in metals using laser-generated ultrasound", Phil. Trans. Roy. Soc., London, Series A, 320, 319-328, 1986.
  11. ↑ J.A. Cooper, R.A. Crosbie, R.J. Dewhurst, A.D.W. McKie and S.B. Palmer, "Surface acoustic wave interactions with cracks and slots: A non-contacting study using lasers", IEEE Trans. in Ultrasonics, Ferroelectrics and Frequency Control, UFFC-33, 462-470, 1986.
  12. ↑ R.J. Dewhurst, C. Edwards and S.B. Palmer, "Non-contact detection of surface-breaking cracks using a laser acoustic source and an electromagnetic acoustic receiver", Appl. Phys. Lett., 49, 374-376, 1986.
  13. ↑ R.J. Dewhurst, A.D.W. McKie and S.B. Palmer, "Further evidence for two-component surface acoustic wave reflections from surface-breaking slots" Appl. Phys. Lett., 49, 1694-1695, 1986.
  14. ↑ J.-P Monchalin, "Optical detection of ultrasound", IEEE Trans. Sonics, Ultrasonics, Freq. Control, UFFC-33, 485-499, 1986.
  15. 1 2 J.-P. Monchalin and R. He'on, 'Laser Generation and Optical Detection with a Confocal Fabry-Perot Interferometer,' Materials Evaluation, Vol. 44, 1986, p. 1232
  16. ↑ J.W. Wagner and J.B. Spicer, 'Theoretical Noise-Limited Sensitivity of Classical Interferometry,' Journal of the Optical Society of America B, Vol. 4, no. 8, p. 1316, 1987.
  17. ↑ L. Noui and R.J. Dewhurst, "A beam deflection technique for photoacoustic Lamb wave measurements", Photoacoustic and Photothermal Phenomena II, Ed. J.C. Murphy et al., Springer Series in Optical Sciences, Springer-Verlag, 62, 278-281, 1990.
  18. ↑ J.N. Caron, Y. Yang, J.B. Mehl, and K.V. Steiner, ``Gas coupled laser acoustic detection for ultrasound inspection of composite materials", Materials Evaluation, Vol. 58, No. 5, 2001, p. 667.
  19. 1 2 Norgia, M; Bandi, F; Pesatori, A; Donati, S (May 2019). "High-sensitivity vibrometer based on FM self-mixing interferometry". Journal of Physics: Conference Series. 1249 (1): 012020. Bibcode:2019JPhCS1249a2020N. doi: 10.1088/1742-6596/1249/1/012020 . ISSN   1742-6588.
  20. ↑ J.P. Monchalin, "Laser-ultrasonics: from the laboratory to industry," Review of Progress in Quantitative Nondestructive Evaluation, 23A, eds. D. O. Thompson and D. E. Chimenti, AIP Conference Proceedings, vol. 700, American Institute of Physics, Melville, NY, pp. 3–31 (2004).
  21. ↑ R.J. Dewhurst, R. He and Q. Shan, "Defect visualisation in carbon fibre composite using laser-ultrasound", Materials Evaluation, 51, 935-940, 1993.
  22. ↑ C. Padioleau, P. Bouchard, R. Héon, J.-P. Monchalin, F.H. Chang, T.E. Drake, and K.I. McRae, "Laser ultrasonic inspection of graphite epoxy laminates", in Review of Progress in Quantitative Nondestructive Evaluation, Vol., 12, eds. D. O. Thompson and D. E. Chimenti, Plenum, New-York, pp. 1345-1352, 1993.
  23. ↑ F.H. Chang, T.E. Drake, M.A. Osterkamp, R.S. Prowant, J.-P. Monchalin, R. Héon, P. Bouchard, C. Padioleau, D.A. Froom, W. Frazier, and J.P. Barton, "Laser ultrasonic inspection of honeycomb aircraft structures" in Review of Progress in Quantitative Nondestructive Evaluation, vol. 12, eds. D. O. Thompson and D. E. Chimenti, Plenum, New-York, pp. 611-616, 1993
  24. ↑ J.-.P. Monchalin, "Laser-ultrasonics: Principles and Industrial Applications", Nondestructive Evaluation of Materials, Vol 17, ASM Handbook, to be published 2018
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