Laser-based generation of ultrasound

Pol Masclans Abelló, winter semester 2019/20

The laser-based generation of ultrasonic testing is the physical process in which an ultrasound wave is created in an object using a laser pulse. This technic is based in the principles of photoacoustics and provides multiple applications in the field of non-destructive testing and biomedicine. It enables imaging of opaque systems and determination of material properties and geometry (see also: Laser based Ultrasonic Testing). The major contribution of this method is that it allows for ultrasound generation directly in specimen as well as it can generate short pulses with higher energy, so it is consequently broadband.

Physical principles

Laser

A laser (light amplification by stimulated emission of radiation) is a device that emits coherent light by a process of optic amplification based on the stimulated emission of electromagnetic radiation.

Most of the studies in ultrasound generation have employed a solid-state laser, either ruby or Nd: YAG when working with metals or composite materials ^[1] [2], while a CO₂ laser is usually used in glasses and ceramics. This is because of the different absorption characteristics of the materials at the optical wavelength of the pulsed laser ^[2].

Photoacustics

Photoacoustics is the study of vibrations induced in matter by light. This idea originates from the optoacoustic (also known as photoacoustic) effect, which showed that sound can be excited in a medium that absorbs a variable luminous flux ^[3]. In particular for laser-based ultrasound generation, laser light cause vibrations and thus ultrasound by thermal mechanisms, by the ablation of surface particles or by the laser momentum.

Ultrasound generation

The laser generation of ultrasound takes place at the surface of the solid. There are two main regimes for ultrasound generation, the thermoelastic regime, which apply for lower power densities, and the ablative (also referred as plasma) regime, that occurs in higher power densities ^[6][7]. Some authors also consider a third regime, usually referred as radiation pressure or constrained surface regime, that occurs in intermediate power densities ^[8]. The first regime causes the so called thermoelastic waves, the second regime is responsible for the ablation-induced waves and the last regime, respectively, produces the light pressure induced waves ^[9][10][11]. At the moment, the light pressure induced waves research focuses on optical manipulation and no particular application in ultrasound generation has been found in the literature.

Thermoelastic regime

In this regime, the generation of ultrasound is caused by thermal mechanisms. The elastic waves are generated within the diameter of incidence of the laser by strains arising from thermoelastic expansion and the compression waves are directed in broad lobes centered at 60º.  The equations 1 and 2 show the shapes of the compression and shear waves, which are also plot in figure 1 ^[1].

$//<![CDATA[ \begin{array}{l}u_r \propto \frac {\sin 2\theta_o \sqrt{k^2 - \sin^2\theta_o}} {(k^2 - \sin^2\theta_o)^2 + 4 \sin^2\theta_o \sqrt{1 - \sin^2\theta_o} \sqrt{k^2 - \sin^2\theta_o} }\end{array} //]]>$  (1)

$//<![CDATA[ \begin{array}{l}u_\theta \propto \frac {k \sin 4\theta_o } {k(1 - 2 \sin^2\theta_o)^2 + 4 \sin^2\theta_o \sqrt{1 - \sin^2\theta_o} \sqrt{k^2 - \sin^2\theta_o} }\end{array} //]]>$  (2)

Where,

$//<![CDATA[ \begin{array}{l}u_r =\end{array} //]]>$ amplitude of the compresion wave

$//<![CDATA[ \begin{array}{l}u_\theta =\end{array} //]]>$ amplitude of the shear wave

$//<![CDATA[ \begin{array}{l}\theta_o =\end{array} //]]>$ angle of incidence of ultrasonic waves at surface

$//<![CDATA[ \begin{array}{l}k =\end{array} //]]>$ ratio of compression to shear wave velocity

The amplitudes of the compression, shear and Rayleigh (surface) waves increase linearly with increasing power density. A typical threshold value in metals for the maximum power density of the laser is 50 MW/cm^{2 [1]}.

Ablative source

In the ablative regime, the thermoelastic stresses described above can be neglected and ablation can be simply described as a time-varying force acting normal to the surface. The high energy density of the laser pulse causes the electrons and ions form plasma which expands in the surface of the tested object and at around 5 µm deep, a small ablation pit is formed ^[7]. This makes ablation regime not fully non-destructive, however in some cases the damage can be accepted if the ablation regime is the only possibility to generate ultrasonic waves of sufficient amplitude in a non-contact manner ^[12]. Equations 3 and 4 show the angular dependences of the compression and shear waves, which are plot in figure 2 ^[1].

$//<![CDATA[ \begin{array}{l}u_r \sim \frac {2 k^2\cos \theta_o(k^2-2\sin^2\theta_o)} {(k^2 - \sin^2\theta_o)^2 + 4 \sin^2\theta_o \sqrt{1 - \sin^2\theta_o} \sqrt{k^2 - \sin^2\theta_o} }\end{array} //]]>$ (3)

$//<![CDATA[ \begin{array}{l}u_\theta \sim \frac {\sin2\theta \sqrt{k^2-2\sin^2\theta_0}} {(k^2 - \sin^2\theta_o)^2 + 4 \sin^2\theta_o \sqrt{1 - \sin^2\theta_o} \sqrt{k^2 - \sin^2\theta_o} }\end{array} //]]>$ (4)

Where,

$//<![CDATA[ \begin{array}{l}u_r =\end{array} //]]>$ amplitude of the compresion wave

$//<![CDATA[ \begin{array}{l}u_\theta =\end{array} //]]>$ amplitude of the shear wave

$//<![CDATA[ \begin{array}{l}\theta_o =\end{array} //]]>$ angle of incidence of ultrasonic waves at surface

$//<![CDATA[ \begin{array}{l}k =\end{array} //]]>$ ratio of compression to shear wave velocityLigth-pressure induced waves

Light-pressure induced waves

The light-pressure induced waves are generated by the momentum transfer from a laser to an elastic solid during the light’s reflection from the solid’s surface. The power of light-pressure induced waves is linked to the reflectivity of the solid surface, so the greater its reflectivity is, the more momentum will be transferred and thus, greater waves will be generated. This phenomenon generates ultrasound waves with very low energy, so it is too small for practical applications in non-destructive testing ^[9][10][11].

Uses

The generation of ultrasound with pulsed lasers is used in non-destructive testing for flaw detection and determination of physical properties, as well as in the medical industry for photoacoustic tomography (PAT) and other clinical evaluation systems.

The main uses in non-destructive testing include the inspection of polymer-matrix composites (especially in the aircraft industry), the inspection of complex geometries or measurements on hot products and the thickness determination of microelectronic thin layers ^[13].

Furthermore, the Laser based Ultrasound generation is having a big impact in the medical industry and many new uses have been proposed in recent years. The particularity of this field is that only the thermoelastic regime can be performed, as otherwise it would affect the inspected tissues. Some PAT applications are Breast imaging and cancer detection, skin imaging, cardiovascular and ophthalmologic evaluation, HIFU monitoring and molecular imaging ^{[14] [15].}

References

1. Scruby, C. B., Drain, L.E: Laser Ultrasonics: Techniques and applications. (1990)

2. Blodgeet, D., Baldwin, K.: Laser-Based Ultrasonics, Applications at APL. John Hopkins APL Technical Digest. (2005) vol. 26,1.  

3. Manohar, S., Razansky,D.: Photoacustics: a historical review. Advances in optics and photonics (2016) vol. 8.4, p. 586-617.

4. Biagi, E., Brenci, M., Fontani, S., Masotti, L., & Pieraccini, M.: Photoacoustic generation: optical fiber ultrasonic sources for non-destructive evaluation and clinical diagnosis. Optical review, 4(4), (1997), p. 481-483.

5. Große, C. U.: Einführung in die Zerstörungsfreie Prüfung im Ingenieurwesen. Grundlagen und Anwendungsbeispiele. Version 2019-07-19-Skript_ZfP_70, p. 38-43.

6. Kalms, M., Focke, O., & Kopylow, C. V.: Applications of laser ultrasound NDT methods on composite structures in aerospace industry. In Ninth International Symposium on Laser Metrology International Society for Optics and Photonics. (2008) vol. 7155, p. 71550E.

7. Monchalin, J. P.: Non-contact generation and detection of ultrasound with lasers. In Proceedings of the 16th World Conference on Nondestructive Testing (2004), p. 1-9.

8. Davies, S. J., Edwards, C., Taylor, G. S., & Palmer, S. B.: Laser-generated ultrasound: its properties, mechanisms and multifarious applications. Journal of Physics D: Applied Physics, 26(3), (1993) vol. 329.

9. Požar, Tomaž, and Janez Možina. Measurement of elastic waves induced by the reflection of light. Physical review letters 111.18 (2013): 185501.

10. Požar, Tomaž, Aleš Babnik, and Janez Možina. From laser ultrasonics to optical manipulation. Optics express 23.6 (2015): 7978-7990.

11. Požar, Tomaž, et al. Isolated detection of elastic waves driven by the momentum of light. Nature communications 9.1 (2018): 3340.

12. Green Jr, R. E.: Non-contact ultrasonic techniques. Ultrasonics, (2004) vol 42(1-9), p. 9-16.

13. Monchalin, J. P. Optical and laser NDT: A rising star. Proceedings of 16th WCNDT. 2004.

14. Xu, Minghua, and Lihong V. Wang. Photoacoustic imaging in biomedicine. Review of scientific instruments 77.4 (2006): 041101.

15. Beard, Paul. Biomedical photoacoustic imaging. Interface focus 1.4 (2011): 602-631.