A novel method for selective generation of single‐mode acoustic waves in multimode media has been developed using a laser beam scanned at the phase velocity of a specified mode. In dispersive media, the acoustic frequency can be varied by changing the scanning velocity. The number of carriers in the generated wave packet is proportional to the difference between the phase and the group velocities. These features were experimentally verified in the fundamental symmetric and asymmetric Lamb waves on an aluminum plate generated by a long‐pulse Nd:YAG laser. Applications to anisotropy and thickness measurements are discussed.

Selective excitation of single‐mode acoustic waves by phase velocity scanning of a laser beam

Critical issues are examined in the application of laser generation and detection of ultrasound to the inspection of large area air-frame composites. Among these issues are surface roughness, signal-to-noise ratio, insensitivity to the path length between the part and detector, and wide band vs. narrow-band generation. Demonstrated is the feasibility of transmission C-scans of 150 ply Gr/epoxy panels containing simulated delaminations. Waveforms having a signal-to-noise ratio of greater than 40 dB in 15-mm thick Gr/epoxy were obtained with generating laser powers well below the ablation limit for the graphite epoxy. Detection was shot-noise limited with a detector noise figure of about 5–10 dB.

Laser-based ultrasonics on gr/epoxy composite:

Both the thermoelastic generation of ultrasound using a laser and the interferometric detection of ultrasound a re well known. The extension of these techniques to the formation of arrays of sources and receivers will be described. These techniques a r e of particular interest for t he rapid inspection of materials without t he need for con tac t or immersion of the material. W e will report t he results of experiments t o study the ultrasonic source characterist ics as a function of laser spot s ize and shape, and as a function of the surface boundary constraints. The requirements of t he detector such as the sensitivity, bandwidth, acceptance angle and suitability for use in a n array will be described. Results of a n analysis of the suitability of different interferometers will be presented.

Synthesis of an Ultrasonic Array Using Laser-Based Techniques

Although the photoacoustic effect is typically generated by pulsed or amplitude modulated optical beams, it is clear from examination of the wave equation for pressure that motion of an optical source in space will result in the production of sound as well. Here, the properties of the photoacoustic effect generated by moving sources in one dimension are investigated. The cases of a moving Gaussian beam, an oscillating delta function source, and an accelerating Gaussian optical sources are reported. The salient feature of one-dimensional sources in the linear acoustic limit is that the amplitude of the beam increases in time without bound.

Photoacoustic effect generated by moving optical sources: Motion in one dimension

Generation of negative group velocity Lamb waves by a moving laser source.

Optoacoustic antennas moving in a liquid were produced experimentally and investigated. Strong acoustic waves were generated when the velocity of the antenna was equal to the velocity of sound in the investigated liquid.

https://iopscience.iop.org/article/10.1070/QE1978v008n02ABEH008545/pdf

Experimental investigation of the acoustic field of a moving optoacoustic antenna

Measurements were made of the intensity of acoustic waves generated in water by YAG laser radiation with monochromatic intensity modulation. The results obtained were in good agreement with theoretical calculations.

F V Bunkin

Papers

Experimental investigation of the acoustic field of a moving optoacoustic antenna

Generation of monochromatic sound in water as a result of absorption of laser radiation