Laser interactions with materials: Optimizing the laser source for the generation of acoustic waves in laser ultrasonics applications.

摘要

Signal detectability is arguably the key parameter to be optimized when designing a laser-based system for remote generation and detection of acoustic signals. The acoustic signal generated by a laser source depends on the thermal, optical, and elastic properties of the specimen and on the characteristics of the laser source. For a given materials system, the laser source parameters, including temporal profile, spatial profile, energy, and wavelength, can be chosen such that the signal-to-noise ratio of the detection system is maximized.; It is well known that the amplitude of laser generated acoustic waves can be significantly enhanced by increasing the energy in the generation pulse such that surface ablation occurs. The amplitude of acoustic waves generated in the ablative regime is directly related to the surface vaporization process. In this manuscript, the laser vaporization process in vacuum is modeled using an implicit finite difference technique. The surface pressure resulting from vaporization serves as a source for acoustic wave generation. The acoustic displacements generated by a laser source in the ablative regime are calculated. The calculations are then compared to experimentally measured surface displacements in aluminum specimens. Acoustic wave generation is considered for the limited range of vaporization in which absorption of light in the vapor can be neglected. Processes beyond this point are discussed qualitatively. Good agreement is seen between theory and experiment over a limited irradiance range.; A novel technique for laser ultrasonic system sensitivity increase through spatial modulation of the incident laser source is presented. The method uses a transmission mask to generate linearly frequency modulated (chirped) surface waves. The laser source is extended in space allowing for a large amount of laser energy to be utilized in the generation process before the surface damage threshold is exceeded. The received signal is subsequently processed using a matched filter. The application of a matched filter to a linearly frequency modulated signal leads to the compression of this signal in time. The technique allows for temporal resolution to be maintained while surface damage is avoided.; Temporal modulation of laser sources for the generation of acoustic waves is considered. Methods for controlling the pulse length of a conventional Q-switched laser system are given. The effects of varying incident laser pulse length on the thermoelastic generation of acoustic waves are discussed. For materials exhibiting strong surface absorption, it is found that there exists a pulse length which optimizes the laser generated signal amplitude while avoiding surface damage. A linear systems approach for determination of pulse length effects is presented. This allows for the calculation of acoustic waves with an arbitrary temporal profile based on a theoretically or experimentally determined reference signal. Finally, pulse length effects on two composite specimens are evaluated and the results discussed.