Frequency dependence of laser ultrasonic SAW phase velocities measurements Chunhui Li a,b , Shaozhen Song a,b , Guangying Guan a,b , Ruikang K. Wang b , Zhihong Huang a,⇑ a School of Engineering, Physics and Mathematics, University of Dundee, Dundee DD1 4HN, Scotland, UK b Department of Bioengineering, University of Washington, 3720 15th Ave. NE, Seattle, WA 98195, USA article info Article history: Received 1 September 2011 Received in revised form 29 May 2012 Accepted 29 May 2012 Available online 9 June 2012 Keywords: Laser ultrasonics Surface acoustic wave (SAW) Phase velocity Frequency content Young’s modulus abstract Advances in the field of laser ultrasonics have opened up new possibilities in applications in many areas. This paper verifies the relationship between phase velocities of different materials, including hard solid and soft solid, and the frequency range of SAW signal. We propose a novel approach that utilizes a low coherence interferometer to detect the laser-induced surface acoustic waves (SAWs). A Nd:YAG focused laser line-source is applied to steel, iron, plastic plates and a 3.5% agar–agar phantom. The generated SAW signals are detected by a time domain low coherence interferometry system. SAW phase velocity disper- sion curves were calculated, from which the elasticity of the specimens was evaluated. The relationship between frequency content and phase velocities was analyzed. We show that the experimental results agreed well with those of the theoretical expectations. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction Laser ultrasonics (LUS) is a remote, non-contact technique that uses a short pulsed laser to excite surface acoustic waves (SAWs) (dominated by Rayleigh waves) to characterize the mechanical properties of material by means of measuring the phase velocity dispersion curves of SAW. SAW-based technology has been used in industry applications such as analyzing the surface structure, compositions, geometry, roughness, plainness and elastic proper- ties of metallic specimens [1–7]. When a material is illuminated with a short laser pulse, the absorption of laser energy results a rapid increase in temperature of the irradiated volume that in turn causes a rapid thermal expan- sion. The result is the generation of ultrasonic waves that propa- gate within the material, including SAW. The phase velocity of SAW at different frequency is dependent on the elastic and geo- metric properties of the material [8]. In isotropic homogeneous material, the surface wave velocity c can be approximated as [1]: c ¼ 0:87 þ 1:12v 1 þ v E 2qð1 þ v Þ   1 2 ð1Þ where E is the Young’s modulus, v is the Poisson’s ratio, and q is the density of material. To detect the laser-induced SAW, the most common method is to employ contact ultrasound transducers. But the selection of proper operating frequency range for ultrasound probes is typically a problem in fabrication procedures, as a typical SAW signal has a broad bandwidth. In general, the maximum frequency of a SAW signal is related to the phase velocity of material [9]: f max ¼ 2 ffiffiffi 2 p c pr 0 ð2Þ where r 0 is the radius of laser pulse and c is the velocity of Rayleigh wave. However, no experimental data was found in literature in val- idating this equation. This paper verifies the relationship between laser-generated SAW frequency range and phase velocity, which could be greatly helpful for the development and fabrication of ultrasonic transduc- ers employed as receivers of SAW in laser ultrasonics technology. A 532 nm Nd:YAG focused laser line-source was applied to three different kinds of hard solid materials including steel, iron, plastic plate. This paper also demonstrates that a laser-generated SAW phase velocity dispersion technique can be used to evaluate the mechanical properties of soft materials (tissue-mimicking phan- tom). The generated SAW signals from different materials were de- tected by a low coherence interferometry system, with different receiving points. Dispersion phase velocity curves and frequency mapping then were calculated to obtain the relationships as well as the elastic properties of different specimen. The relationship be- tween Young’s modulus and frequency range of SAW signal were concluded. 2. System configuration The system set up for generation and detection of laser-induced SAW is shown in Fig. 1. Briefly, the system includes two main 0041-624X/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2012.05.009 ⇑ Corresponding author. E-mail address: z.y.huang@dundee.ac.uk (Z. Huang). Ultrasonics 53 (2013) 191–195 Contents lists available at SciVerse ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras