J.N. Caron, "Displacement and Deflection of an Optical Beam by Airborne Ultrasound," to be published in Review of Progress in Quantitative Nondestructive Evaluation , ed. by D.O. Thompson and D.E. Chimenti, AIP, Vol. 27, 2008. 

Gas-Coupled Laser Acoustic Detection (GCLAD) enables laser-based sensing of ultrasound from a
solid without contact of the surface, and independent of the optical properties of the solid surface.  A probe laser beam, directed parallel to the surface, intercepts the ultrasound wave after transmission to air.  A split-cell position-sensitive photodetector senses changes in the optical beam path created by the disturbance. The interaction between the probe beam and acoustic field has typically been modeled as creating a deflection in the beam. To first order, sensitivity to deflection improves by increasing the distance from the interaction point to the photodetector.   This paper describes this interaction as a combination of displacement and deflection of the optical beam. Displacement occurs when the beam is deflected twice by the acoustic field such that the probe beam is  translated perpendicular to the optical axis.   Experiments show that the sensitivity of the displacement response is comparable to the deflection response.  Sensing the displacement can significantly decrease the system's dependence of length. This enables the miniaturization of the GCLAD technique.

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J.N. Caron, "Progress towards a portable laser-based ultrasound sensor using gas-coupled laser acoustic detection," Review of Progress in Quantitative Nondestructive Evaluation , Vol. 24, 2005.

Gas-Coupled Laser Acoustic Detection (GCLAD) has proven to be a viable alternative to interferometric detection of ultrasound for noncontact inspection of materials. Unlike other laser-based detection techniques, GCLAD operates independently of the optical properties of the sample surface. Instead, the probe laser intercepts the ultrasound wave after it has been transmitted to air. The concept is being researched as part of an efficient, ultrasound sensor, with hangar-to-hangar portability, for interrogating flight-critical aircraft structural supports. Areas of active research include improving system sensitivity and extending the frequency response out to 10 MHz. Research to this point has shown that higher frequency waveforms can be detected using this technique and provide good sensitivity. Well-resolved waveforms have been detected in the test sample at 2.25 MHz. More research is necessary to reach the goal of detecting the signal from a 10 MHz signal. Improvements in the electronic, optical and signal processing methods are being considered.

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J.N. Caron, "Multiple-beam detection using Gas-coupled Laser Acoustic Detection," Review of Progress in Quantitative Nondestructive Evaluation, vol 20, 2000.

A novel laser-based technique for the detection of ultrasound radiated from solid materials has been developed.  In this approach, a probe beam is directed parallel to the surface of a sample.  Ultrasonic waves in the solid are detected when an acoustic wave is radiated from the surface into the ambient air, where the density variations cause a beam deflection.  Because the laser beam is not reflected from the sample surface, the technique is not dependent upon the surface optical properties of the material under investigation.  It is particularly useful for testing graphite/polymer composites and other materials with poorly reflecting surfaces.  Gas-coupled laser acoustic detection (GCLAD) has been used to record well-resolved through-transmission and surface-acoustic waveforms in various materials.  GCLAD has also been incorporated into a C-scanning system where it has been used to image subsurface flaws in graphite/polymer composite panels.  Recent studies have investigated the inspection of curved surfaces. To this end, the flanges and corner of an angled graphite-reinforced composite panel were scanned using this technique.  In addition, the prospect of using surface acoustic waves (SAWs) for the interrogation of the skins on multi-layer materials has also been studied.  Using GCLAD, Lamb and Rayleigh waves have been detected in composites, polymers, thin metal films, and metal plates.

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J.N. Caron, Y. Yang, J.B. Mehl, and K.V. Steiner, ``Gas coupled laser acoustic detection for ultrasound inspection of composite materials, Vol. 58, No. 5, 2001, p. 667.

A novel laser-based technique for the detection of ultrasound radiated from solid materials has been developed.  In this approach, a probe beam is directed parallel to the surface of a sample.  Ultrasonic waves in the solid are detected when an acoustic wave is radiated from the surface into the ambient air, where the density variations cause a beam deflection.  Because the laser beam is not reflected from the sample surface, the technique is not dependent upon the surface optical properties of the material under investigation.  It is particularly useful for testing graphite/polymer composites and other materials with poorly reflecting surfaces.  Gas-coupled laser acoustic detection (GCLAD) has been used to record well-resolved through-transmission and surface-acoustic waveforms in various materials.  GCLAD has also been incorporated into a C-scanning system where it has been used to image subsurface flaws in graphite/polymer composite panels.  Recent studies have investigated the inspection of curved surfaces. To this end, the flanges and corner of an angled graphite-reinforced composite panel were scanned using this technique.  In addition, the prospect of using surface acoustic waves (SAWs) for the interrogation of the skins on multi-layer materials has also been studied.  Using GCLAD, Lamb and Rayleigh waves have been detected in composites, polymers, thin metal films, and metal plates.

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J.N Caron, Y. Yang, J.B. Mehl and K.V. Steiner, "Gas-coupled Laser Acoustic Detection at Ultrasonic and Audible Frequencies," Review of Scientific Instruments, vol 69(8), 1998, p. 2912.

Airborne acoustic waves have been detected by a laser-beam deflection technique in both the ultrasonic and audio frequency ranges. For ultrasonic applications, a probe beam is directed parallel to the surface of a sample. Ultrasonic waves in the solid are detected when an acoustic wave is radiated from the surface into the ambient air, where the density variations cause a beam deflection. Gas-coupled laser acoustic detection  GCLAD! has been used to record well-resolved through-transmission and surface-acoustic wave forms in various materials. GCLAD has also been incorporated into a C-scanning system where it has been used to image subsurface flaws in graphite/polymer composite panels. Because the laser beam is not reflected from the sample surface, the technique is not dependent upon the surface optical properties of the material under investigation. It is particularly useful for testing graphite/polymer composites and other materials with rough surfaces. The beam-deflection technique has been tested quantitatively in the kHz frequency range by passing a probe beam through a cylindrical resonator. The acoustic spectrum of the resonator was measured from 4 to 13.5 kHz by scanning the frequency of a source and recording the acoustic field with both a microphone and the beam-deflection system. The acoustic fields of the lower-frequency modes are well known and enable both qualitative and quantitative tests of the beam-deflection technique. Measurements on the lowest-frequency plane-wave mode were used for absolute calibration of the microphone. The noise level of the beam-deflection measurements at 4.3 kHz was found to be 0.05 nrad~rms!, corresponding to an acoustic pressure of 0.005 Pa~rms.

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J.N. Caron, Y. Yang, J.B. Mehl and K.V. Steiner, "Thermoelastic and Ablative Laser Generation of Ultrasonic Waveforms in Graphite/Polymer Composite Materials," submitted to the Journal for Applied Physics for publication, but was never published, May 1998.

A laser-based ultrasonic system was used to study thermoelastic and ablative ultrasonic laser generation mechanisms in graphite/polymer  composite structures.  Ultrasonic waveforms were generated in 16-layer quasi-isotropic AS-4/PEEK composite and 8-layer thick AS-4/PEKK [0/90]_{2S} composite panels.  Waveforms generated onone side of the samples were observed on the opposite side using a confocal Fabry-Perot (CFP) based detection system.  The waveforms, as functions of the generation-laser power density, show that there are two distinct generation mechanisms.  Below a well-defined threshold power density, the observed signals were proportional to the power density and are assumed to be generated thermoelastically.  Above the threshold the observed waveforms are a superposition of a thermoelastic (TE) and ablatic (AB) waveforms.  The relative amplitudes of the TE and AB components were determined as a function of power density by fitting a theoretical model to the data.  The ablation threshold was independently observed through photodetection of the light radiated by the ablation plume.  Further experimentation partially characterized the directivity of the generation mechanisms for the graphite/PEEK composite panel.