James W. Wagner

Experimental evaluation of enhanced generation of ultrasonic waves using an array of laser sources

Abstract An array of ten pulsed Nd: YAG lasers was constructed in order to study the effects of generating ultrasound with an array of laser sources. The laser system permitted the spatial and temporal control of the firing of the individual lasers in the array necessary for the production of both narrow-band ultrasonic signals and phased array single pulses. The increase in sensitivity of a laser ultrasonic system associated with the generation of narrow-band and phased array acoustic waves is discussed theoretically and verified experimentally for surface and bulk wave generation. The ultrasonic signals were generated in aluminium samples of various thicknesses and with source laser power densities consistent with generation in the thermoelastic regime, thus causing no damage to the surface of the specimens. The signals were detected using a path stabilized Michelson interferometer. In the narrow-band case, the waveforms were digitally filtered in order to take advantage of the reduced spectral range of the generated acoustic energy. A significant increase in the sensitivity of the laser ultrasonic system, consistent with theoretical predictions, was observed in both the narrow-band and phased array cases.

Quantitative theory for laser ultrasonic waves in a thin plate

Numerical inversion of the Hankel–Laplace transform has been performed for the case of ultrasonic displacements in an infinite, homogeneous, isotropic plate which is excited thermoelastically by a laser pulse. Values for the elastic moduli and the plate thickness may be extracted when the calculated displacements are compared directly to those obtained experimentally. Previous authors have demonstrated methods for determining the elastic modulus in thick plates; this letter shows that using a different method for the development of the theory allows similar modulus determinations to be made for thin as well as thick plates.

Laser generation of narrow-band and directed ultrasound

Abstract An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element.

Dynamic imaging microellipsometry

Dynamic imaging microellipsometry (DIM) is a rapid full-field imaging technique for high resolution studies of thin-films. The DIM concept is based on radiometric polarizer, compensator, specimen, analyzer (PCSA) ellipsometry combined with video and image processing techniques. The theoretical basis for this approach is developed using the Jones vector and matrix formalism. Basic systems design is presented with error model predictions of ellipsometric accuracies better than 0.1° for full-field ψ and Δ images captured in a few seconds with spatial resolution under 10 microns.

Dynamic imaging microellipsometry: theory, system design, and feasibility demonstration

Dynamic imaging microellipsometry (DIM) is a new rapid full-field imaging technique for high spatial resolution studies of thin films. The DIM concept is based on radiometric polarizer, compensator, specimen, and analyzer ellipsometry combined with video and image processing techniques. The theoretical basis for this approach is developed using the Jones vector and matrix formalisms. Basic systems design is presented with error model predictions of ellipsometric accuracies better than 0.1 degrees for full-field Delta and psi images captured in a few seconds with spatial resolution under 10 microm. Initial feasibility tests have demonstrated interframe discriminations of 0.36 degrees for Delta and 0.082 degrees for psi.

Laser generation of acoustic waves in the ablative regime

A practical model of acoustic wave generation by a pulsed laser source in the ablative regime is presented. The pressure exerted on the surface during Q-switched laser heating is calculated through a finite difference solution of the vaporization problem. The epicentral displacement is found through summation of the displacement field induced by the vaporization process with that caused by thermoelastic expansion. The model is restricted to the weakly ablative regime in the absence of a backing gas. The results are compared to the epicentral displacements generated in aluminum samples under rough vacuum conditions at generating wavelengths of 532 and 1064 nm. The waveforms compare well over a limited irradiance range. The effects of rough vacuum conditions on the generated acoustic signals are also examined and compared to signals generated in the presence of a backing gas. The divergence in the shape and amplitude of these signals observed under highly ablative conditions is discussed.

Generation of ultrasound by repetitively Q-switching a pulsed Nd:YAG laser

Repetitively Q-switching a Nd:YAG laser during a single flashlamp pulse has been used successfully to generate a train of acoustic pulses with a repetition rate as high as 53 kHz. The spectral content of this multiple-pulse ultrasonic signal is significantly narrower in bandwidth than that of a single pulse. A corresponding reduction in the detection system bandwidth results in a marked improvement in detection sensitivity.

Generation of narrow‐band ultrasound with a long cavity mode‐locked Nd:YAG laser

A passively mode‐locked, flashlamp‐pumped Nd:YAG laser with a cavity length of 11.19 m has been developed to study the noncontact generation of narrow‐band ultrasound. The individual mode‐locked pulses acted as separate sources of ultrasound, producing a train of acoustic pulses with a repetition rate of about 13.4 MHz. The ultrasound was generated in an aluminum sample and remotely detected with a path stabilized Michelson interferometer. The energy in the multiple pulse acoustic signal was confined to a considerably reduced spectral range compared with that in a single pulse.

Sensitivity enhancement in laser ultrasonics using a versatile laser array system

Past investigators have predicted and, in limited cases, demonstrated the potential utility of using temporally and spatially modulated laser array sources as a means of increasing the signal‐to‐noise ratio in laser ultrasonic systems without causing the surface damage characteristic of a single high‐power laser point source. In an effort to develop a practical and flexible laser array source, a laser system has been designed and implemented which uses a single Nd:YAG pulsed laser and an optical delay system in which the laser pulse passes repetitively through a White cell cavity being sampled by a custom beamsplitter after each pass. Up to ten spatially separated light beams exit from the system each time a single laser pulse is introduced. Time separation between the distinct pulses can be adjusted over a range from 28 to 170 ns, corresponding to a pulse repetition rate from 6 to 36 MHz. Since individual control of the beam paths is possible, one has the flexibility to implement either a single‐element,...

Modulated laser array sources for generation of narrowband and directed ultrasound

Patterned illuminating sources and appropriate time modulation may be used to enhance certain features of laser-generated acoustic waves in time or frequency as a function of direction. Steerable, narrowband, “toneburst” ultrasonic signals have been generated from a single laser pulse which was spatially modulated by transmission through a lenticular array. In addition, narrowband toneburst ultrasonic waves have been generated from a mode-locked laser pulse train providing spectral narrowing of the laser-ultrasonic signal. Both temporal and spatial modulation of the laser pulse improve the signal-to-noise ratio for laser ultrasonics.