James N. Caron

Gas-coupled laser acoustic detection at ultrasonic and audio frequencies

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 qua...

Generation of ultrasound in materials using continuous-wave lasers

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.

Blind deconvolution of audio-frequency signals using the self-deconvolving data restoration algorithm

A signal processing algorithm has been developed in which a filter function is extracted from degraded data through mathematical operations. The filter function can be used to restore much of the degraded content of the data through use of a deconvolution process. The operation can be performed without prior knowledge of the detection system, a technique known as blind deconvolution. The extraction process, designated self-deconvolving data reconstruction algorithm, is applied here to audio-frequency signals showing significant qualitative improvement. Degradation arising from the process of electronic recording and reproduction is significantly reduced.

Application of a scattered-light radiometric power meter

The power measurement of high-power continuous-wave laser beams typically calls for the use of water-cooled thermopile power meters. Large thermopile meters have slow response times that can prove insufficient to conduct certain tests, such as determining the influence of atmospheric turbulence on transmitted beam power. To achieve faster response times, we calibrated a digital camera to measure the power level as the optical beam is projected onto a white surface. This scattered-light radiometric power meter saves the expense of purchasing a large area power meter and the required water cooling. In addition, the system can report the power distribution, changes in the position, and the spot size of the beam. This paper presents the theory of the scattered-light radiometric power meter and demonstrates its use during a field test at a 2.2 km optical range.

OVERVIEW: ULTRASOUND SENSING USING GAS-COUPLED LASER ACOUSTIC DETECTION

Abstract 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 detection 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 materials with poorly reflecting surfaces such as graphite/polymer composites. Gascoupled laser acoustic detection (OCLAD) has been used to record well-resolved throughtransmission, Rayleigh, and Lamb waves 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.

Acoustic‐wave detection by optical beam deflection

Audio‐frequency acoustic signals in gases have been detected by measuring the bending of an optical beam directed through the gas to a custom‐built position‐sensitive photodetector. The beam deflection is caused by a transverse acoustic density gradient, and can be calculated when the acoustic field is known. The spectrum of a cylindrical resonator was recorded using both a calibrated microphone embedded in one end of the cylinder, and deflection of a laser beam passing through the center of the cylinder along a diameter. Beam deflection is expected for the first plane‐wave mode of the resonator and other modes having nonzero average transverse density gradients along the beam path. The observed beam deflections were in qualitative agreement with calculations for all modes which could be resolved. For the first plane‐wave mode, the measured and calculated deflections agreed within a few percent. Refinement of this technique could be applied to the absolute calibration of microphones. The beam‐deflection t...

Underwater laser acoustic source control using shaped plasmas

NRL is developing an intense laser acoustic source using underwater shaped plasmas. Recent experiments include near-field acoustic source characterization using high energy lens-focused pulses of a Q-switched Nd:YAG 532 nm laser. The laser-generated plasma evolves into a piston expanding at supersonic speed, which launches an intense shock in the near field. The size and shape of this super-heated piston determines the acoustic waveform and energy spectral density (ESD). We have demonstrated the ability to change the ESD centroid from 15 kHz to a few MHz, with lower frequencies generated using highly elongated plasmas generated by a single laser pulse. We will discuss ongoing laser acoustic source experiments and research plans at NRL involving shaped underwater plasmas, including both demonstrated single-laser-pulse techniques and proposed two-laser-pulse techniques (T. G. Jones, et al., “Two laser generation of extended underwater plasma,” U.S. patent application 13/711,752). Two-laser-pulse acoustic ge...

Continuous laser generation of ultrasound for nondestructive evaluation

Non-contact generation of ultrasound in materials is often accomplished using pulsed lasers. The rapid heating from the laser pulse produces a thermoelastic expansion in the material that produces the ultrasound. Here we introduce an alternative method, designated Continuous Laser Generation of Ultrasound (CLGU). With sufficient power, a continuous-wave laser can be scanned across the surface of the test material creating an ultrasonic wavefront that propagates through the material. Discontinuities in the wavefront indicate defects in the material. CLGU will have the ability to perform ultrasonic C-scans three orders of magnitude faster than pulsed laser generation.

LASER GENERATION AND DETECTION OF SURFACE ACOUSTIC WAVES USING GAS-COUPLED LASER ACOUSTIC DETECTION

Laser generation and detection of ultrasound has the advantage of requiring no mechanical contact with the materials under investigation. We previously reported [1] laser-based measurements on Lamb waves in graphite/polymer composite laminates using a confocal Fabry-Perot interferometer for detection. Related work by other groups includes air-coupled detection of Lamb waves in similar composites using capacitive transducers [2,3] and interferometric detection of Lamb waves in paper [4]. Our earlier work has been extended using Gas-Coupled Laser Acoustic Detection (GCLAD), an economical alternative laser-based method which has the additional advantage that the detection laser beam is not reflected from the sample surface. GCLAD is thus particularly useful for materials with surfaces of poor optical quality. We demonstrate below that the combination of laser generation and GCLAD can be used to obtain well-resolved surface-acoustic waves (SAWs) in a variety of materials, including metals, paper, thin films, and composite pre-preg tape. We also show some preliminary SAW scans obtained with laser generation and GCLAD using metallic samples. Each pixel in the scans represents the strength of a SAW passing through a portion of the sample with an area of about 1 cm2. Scans of this type offer the possibility of economical testing of large sample areas, potentially on-line in a manufacturing environment.

Progress in Gas-Coupled Laser Acoustic Detection for NDE Applications

A laser-based technique which enables noncontact detection of well-resolved ultrasonic waveforms independently of the optical qualities of the sample surface has been developed. [1,2] The technique can be applied to any material, but is particularly useful for evaluation of materials with optically rough or poorly reflecting surfaces.