J.N. Caron. "Focused ultrasound sensing in water using a GCLAD system," the 5th International Symposium on Laser-Ultrasonics and Advanced Sensing, July 4-8, 2016, in Linz Austria.
Gas-coupled Laser Acoustic Detection (GCLAD) has been developed as a technique to sense airborne ultrasound waves by passing a laser beam through the acoustic disturbance. Fluctuations in the medium's index of refraction divert the beam from the original path. A position-sensitive photodetector receives the beam and senses the change created by the acousto-optic interaction. The method has proven to be a simple and effective method for sensing ultrasound that has been transmitted from materials and is suitable for applications in materials characterization and non-destructive evaluation.
By passing the laser beam through the glass windows of a water tank, the same method can be used for immersion detection of ultrasound. In this study, a 10~MHz immersed focused transducer was used to generate ultrasound in an immersed test sample. A laser beam is directed through the tank twice to capture the ultrasound before and after it passes through the material. The arrangement is used to directly compare the sensitivity of the immersed GCLAD arrangement with a commercial transducer. Ultrasound C-scans were created to produce individual spatial resolution measurements for the vertical and horizontal directions.
J. L. Johnson, K. van Wijk, J. N. Caron, and M. Timmerman, "Photoacoustic and ultrasound imaging with a gas-coupled laser acoustic line detector," Proc. SPIE 9708, Photons Plus Ultrasound: Imaging and Sensing 2016, 970814 (March 15, 2016); doi:10.1117/12.2211800.
Conventional contacting transducers are highly sensitive and readily available for ultrasonic and photoacoustic imaging. On the other hand, optical detection can be advantageous when a small sensor footprint, large bandwidth and no contact are essential. However, most optical methods utilizing interferometry or Doppler vibrometry rely on the reflection of light from the object. We present a non-contact detection method for photoacoustic and ultrasound imaging--termed Gas-Coupled Laser Acoustic Detection (GCLAD)--that does not involve surface reflectivity. GCLAD measures the displacement along a line in the air parallel to the object. Information about point displacements along the line is lost with this method, but resolution is increased over techniques that utilize finite point-detectors when used as an integrating line detector. In this proceeding, we present a formula for quantifying surface displacement remotely with GCLAD. We will validate this result by comparison with a commercial vibrometer. Finally, we will present two-dimensional imaging results using GCLAD as a line detector for photoacoustic and laser-ultrasound imaging. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
J.N. Caron, "Immersion Detection of Ultrasound using a GCLAD system," Laser Ultrasonics Symposium, Evanston, Illinois, USA, June 29, 2015. (Presentation and Paper)
When an optical beam passes through a medium with acoustic disturbances, fluctuations in the medium's index of refraction divert the beam from the original path. As such, an acoustic wave can be detected by directing a laser beam through the disturbance and sensing the path changes with a position-sensitive photodetector. Depending on the optical components used, the detector can sense the deflection or displacement portion of the diversion. In a gas medium, this technique is designated Gas-coupled Laser Acoustic Detection (GCLAD) and has been used to sense audio-frequency and ultrasonic waves. The method has proven to be a relatively simple and effective method for sensing ultrasound that has been transmitted from materials for applications in materials characterization and nondestructive evaluation.
Here we present research where the GCLAD system is used to detect ultrasonic waves in water. As with noncontact transducers, the advantage of using water as the medium is the comparatively low attenuation of higher ultrasound frequencies. This produces stronger signals at lower frequencies and enables detection of higher frequencies that would be difficult to sense in air. Recent modelling has shown that this approach produces broadband sensitivity, allowing detection from a few hundred Hertz to 20~MHz for a 2.5~cm travel distance.
GCLAD also allows the acoustic wave to be detected undisturbed at different travel distances by repeatedly passing the beam through the medium. The presence of the laser beam does not have any affect on the acoustic wave. This characteristic is used to sense the ultrasound wave both before and after passing through a material. The second waveform can be then be calibrated per shot using the amplitude of the first wave. Frequency changes in wave, beyond the attenuation from travel through water, can be attributed to the material itself, producing additional information about the material characteristics.
J.N. Caron, G.P. DiComo, "Continuous Laser Generation of Ultrasound," Laser Ultrasonics Symposium, Evanston, Illinois, USA, June 29, 2015. (Poster and Paper)
Pulsed lasers have been used to generate ultrasound in materials for the purposes of nondestructive evaluation. The deposited energy heats the localized area rapidly, producing an ultrasonic wave by thermoelastic expansion. Detection of the ultrasound provides information about the material, such as thickness, strength, and the presence of defects.
Alternatively, a continuous-wave laser can be scanned across the sample surface using a fast steering mirror or rotating polygon mirror. As the optical beam moves across the surface, the deposited energy creates thermoelastic expansion along the scanning line. With sufficient energy and scanning speed, a ultrasonic wavefront is generated.
Continuous Laser Generation of Ultrasound has the potential to scan an object several orders of magnitude faster than pulsed laser generation. To create detectable ultrasound with frequencies near 1 MHz, pulsed lasers with over 100 mJ per pulse and pulse widths of a few nanoseconds are required. Commercially available lasers with sufficient power and pulse width are limited to repetition rates between 10 and 400 Hz. A meter-long B-scan with millimeter resolution would at best take 2.5 seconds without signal averaging. A squared meter C-scan with millimeter resolution would take between 42 minutes and 28 hours. By contrast, a CLGU system could scan a single line in less than a millisecond and perform the full C-Scan in less than a second.
J.N. Caron and G.P. DiComo, "Frequency Response of Optical Beam Deflection by Ultrasound in Water," Applied optics, 53(32), 7677-7683.
Acoustic waveforms create fluctuations in the index of refraction of the medium. An optical beam passing through the disturbance will be deflected or displaced from the original path. The acoustic wave can be detected by sending a laser through the disturbance and sense the path changes of the beam with a position-sensitive photodetector. This paper presents a model of this interaction in water to predict the sensitivity and frequency response. The model demonstrates that the frequency response of the system is broadband, allowing detection from a few hundred Hertz to 20~MHz. This technique has potential use for underwater acoustic sensing and the ultrasonic inspection of materials.
J.N. Caron and P Kunapareddy, "Atypical Applications for Gas-coupled Laser Acoustic Detection," presented on June 25, 2013, at the 3rd International Symposium on Laser Ultrasonics and Advanced Sensing, in Tokahoma, Japan.
Gas-coupled laser acoustic detection (GCLAD) was primarily developed to sense laser-generated ultrasound in composite materials. In a typical setup, a laser beam is directed parallel to the material surface. Radiated ultrasound waves deflect or displace the probe beam resulting from changes in the air's index of refraction. A position-sensitive photodetector senses the beam movement, and produces a signal proportional to the ultrasound wave. In this paper, we discuss three applications of GCLAD that take advantage of the unique detection characteristics. Directivity patterns of ultrasound amplitude in water demonstrate the use of GCLAD as a directional hydrophone. We also demonstrate the sensing of waveforms from a gelatin. The gelatin mimics ultrasound propagation through skin tissues. Lastly, we show how GCLAD can be used as a line receiver for continuous laser generation of ultrasound. CLGU may enable ultrasound scanning at rates that are orders of magnitude faster than current methods.
James N. Caron, Gregory P. DiComo, and Sergei Nikitin, "Generation of ultrasound in materials using continuous-wave lasers," Opt. Lett. 37, 830-832 (2012).
Generating and detecting ultrasound is a standard method of nondestructive evaluation of materials. Pulsed lasers are used to generate ultrasound remotely in situations that prohibit the use of contact transducers. The scanning rate is limited by the repetition rates of the pulsed lasers, ranging between 10 and 100 Hz for lasers with sufficient pulse widths and energies. Alternately, a high-power continuous-wave laser can be scanned across the surface, creating an ultrasonic wavefront. Since generation is continuous, the scanning rate can be as much as 4 orders of magnitude higher than with pulsed lasers. This paper introduces the concept, comparing the theoretical scanning speed with generation by pulsed laser.
J.N. Caron, "Displacement and Deflection Sensitivity of Gas-coupled Laser Acoustic Detection," to be published in Laser Ultrasonics, 2008.
Ultrasound radiated from a surface can change the path of an optical beam, directed through the acoustic field and parallel to the surface, through acousto-optic interaction. Sensing of the beam motion with a position-sensitive detector produces a simple but effective non-contact ultrasound detector, designated Gas-coupled Laser Acoustic Detection, or GCLAD. Recent research has shown that the received signal is a combination of the deflection and displacement of beam. The technique proved capable of detecting displacements of the beam, created by a transducer-generated airborne ultrasound wave, of less than a micrometer. Deflections were recorded that measured less than a microradian. The presented work estimates the sensitivity of GCLAD to an ultrasonic surface displacement. The results are compared to the sensitivities of more standard ultrasound detection methods.
J.N. Caron, "Displacement and Deflection of an Optical Beam by Airborne Ultrasound," published in Review of Progress in Quantitative Nondestructive Evaluation, ed. by D.O. Thompson and D.E. Chimenti, AIP, Vol. 27A, 2008, p. 247.
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.
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.
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.
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.
J.N Caron, Y. Yang, J.B. Mehl and K.V. Steiner, "Gas-coupled Laser Acoustic Detection at Ultrasonic and Audio 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.
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.
