The laser generation of sound in liquids and gases is reviewed. The sound‐generating mechanisms of laser interaction with matter are discussed with emphasis on the thermoelastic process. The studies on strongly absorbing liquids include detailed theoretical considerations of the thermoelastic sound generation with pulsed lasers. Acoustic waveforms for H2O and D2O are calculated analytically on the basis of a model laser‐pulse shape. Both free and rigid boundaries on the surface of the liquid are considered. Good agreement between theory and experiments with respect to waveforms and amplitudes is obtained. The experiments are performed with a hybrid CO2 laser and piezoelectric or optical detection of the acoustic transients. In view of a present controversy, special emphasis is put on the temperature dependence of the acoustic amplitudes in H2O, D2O, and in aqueous MgSO4 solutions. Good agreement is found between experimental data and a new, pure thermal model which takes heat conduction into account. The distortion of the acoustic waveform during the propagation through the liquid is treated in terms of sound absorption, diffraction, and nonlinear acoustics. A simple experimental method for the determination of Beyer’s nonlinearity parameter B/A is presented. In the last section some characteristics of photoacoustic spectroscopy (PAS) in gaseous media are reviewed. This method has been demonstrated to be highly sensitive. The measurement of absorption coefficients as low as 10− 8 cm− 1 is possible. PA studies on H2O vapor are discussed with new results on line and continuum absorption in the 9–11‐μm wavelength range. Finally, the impact of PAS on trace gas analysis is demonstrated. With PAS the detection of gas concentrations in the ppb range is feasible. The operational characteristics of a stationary CO laser and a mobile CO2 laser‐PAS system are presented, including first results on continuous i n s i t u air pollution monitoring.

Laser generation of acoustic waves in liquids and gases

Application of efficient ultraviolet excimer lasers such as the 248 nm KrF laser to laser fusion requires that long laser pulses be efficiently converted to short pulses at high intensity. The backward Raman amplifier is shown to be a promising candidate for this application. Gain, saturation, and limits to amplifier performance are described. It is shown that pump beams of poor spatial quality may be converted to output beams of high spatial quality. Several common gaseous vibrational Raman scatterers are discussed, and it is shown that a simple KrF-pumped backward Raman amplifier using methane at atmospheric pressure will have a saturation fluence near 1 J/cm2and can produce an output five times as intense as the pump in a ten times shorter pulse with an efficiency of about 50 percent. Design tradeoffs and possible techniques for further improving the performance of such amplifiers are discussed.

Raman pulse compression of excimer lasers for application to laser fusion

Bulk copper is laser etched with 193‐ and 351‐nm excimer radiation. The transition from the thermal to the plasma etch region is studied by measuring the densities and kinetic energies of three copper species (Cu0, Cu+, and Cu2 ) in the etch plume. A unique laser‐induced fluorescence experiment allows these three species to be followed essentially simultaneously as a function of fluence. Three separate types of etching behavior are clearly evident (even within the small fluence range of ∼1–12.5 J/cm2 ); i.e., thermal vaporization of Cu, multiphoton ionization of the Cu vapor, and electron‐atom collision‐induced ionization (breakdown) and dissociation (of Cu2 ).

Cu0, Cu+, and Cu2 from excimer‐ablated copper

The laser-triggered switching (LTS) of high-voltage spark gaps is considered. The basic theory is presented which predicts dependencies of the delay to breakdown and switching jitter on such variables as fill gas mixture and pressure, gap spacing, polarity, and geometry. It is shown that electrical arcs of several metres length can be directed by laser action. A complete set of experiments is reported which adequately support the proposed theory. The performance of LTS is considered and results are reported on multiple gap triggering, multiple channel triggering, triggering of voltages in excess of 3 mV, repetitive switching at rates up to 50 pps with subnanosecond jitter, as well as various geometries, pulse forming demonstrations, and output voltage selection on a Marx generator.

http://iopscience.iop.org/article/10.1088/0022-3727/11/12/001/pdf

The laser triggering of high-voltage switches

Survey of aerodynamic flow control at high speed by energy deposition

A survey is presented of papers published from 1964 through 1971 on the laser spark in gases. The authors consider the mechanisms whereby laser radiation breaks down a gas, the influence of the experimental conditions on the breakdown threshold, and the interaction of the spark plasma with the laser radiation. The parameters of the plasma during the time it is acted upon by the radiation and after the end of the laser pulse are determined. The possibilities of a number of technical applications of a laser spark in gases are discussed.

http://iopscience.iop.org/article/10.1070/PU1974v016n06ABEH004094/pdf

A N Zaĭdel

Papers

Laser spark in gases