Research Of Acoustics Induced By Pulsed CO₂ Laser In Water

Since the output energy and peak power of the TEA-CO₂ pulse laser are high, its wavelength is 10.6μm which is the atmospheric window, absorption coefficience of air to CO₂ laser is low and absorption coefficience of water to CO₂ laser is high, photoacoustic detection using TEA-CO₂ pulse laser for remote sensing in ocean obtains more and more attentions. Invetigation of laser acoustics induced by TEA-CO₂ pulse laser in water is practical and important in military and remote detection. This paper investigates the characteristics of laser acoustic signal induced by pulsed CO₂ laser in water.Firstly, the waveform, amplitude, laser acoustic energy conversion efficiency, spectra of thermoelastic regime, induced by the three common waveforms of CO₂ laser, e.g., an initial gain-switched spike of approximately several tens ns FWHM (full width at half-maximum) in duration and followed by a several us tail, as well as their relations to the laser waveforms and laser waveform parameters, were investigated theoretically in this paper. Some valuable results are obtained. Thermoelastic laser acoustic signal is demermined by the first derivative of laser waveform; the acoustic signal is contributed to the combination effects of the spike peak and the tail of the laser pulse, the waveform amplitude and profile are mainly determined by the duration and the shape of the spike, high frequence singals are induced by the spike, while the low frequence singals are induced by the tail. The role of the tail becomes important when its relative intensity to the peak increased or its duration been near the duration of the peak.The flux density of vapor for different liquid temperature and theoretical model for solving optoacoustic induced by surface and body vaporization mechanisms are proposed. Characteristic of laser acoustics of vaporization and relation with the parameters of laser pulse and medium are discussed. The mechanism of breakdown and laser acoustic characteristics is researched. Waveforms and spectra of laser acoustics induced by vaporization and laser breakdown are discussed according to the theoretical results, and conversion efficiency is also acquired.Using attenuation model, e.g., absorption coefficient proportional to square of frequency, the transmission and propagation of the laser acoustics in water were theoretically analyzed. The effect of attenuation on the laser acoustics induced by different mechanisms is studied in detail. The results show that the thermoelastic acoustic signals with higher frequency attenuate significantly, while laser acoustic signals with frequency under 50 kHz attenuate slowly, which are induced by vaporization and breakdown of water.The parameters of CO₂ laser pulsed were changed by choosing different lasers and working voltage, composition of the hybrid gas or pressure of the working gas. The experimental study of laser acoustic induced by pulsed CO₂ laser has been performed in this thesis. The influence of the laser parameters on the characteristics of the acoustic signals were investigated in detail. By comparing the difference of the waveforms and the spectra if there was an object at the bottom of the container, the scheme of detecting object in water is proposed.By analyzing the photos taken by ICCD, the movement veolocity of the steam is got and the mechanism of the laser acoustics can be deduced.By solving salt in water as seawater, the influence of salinity on laser acoustics is studied experimentally.The results show that the amplitude and peak frequency of the acoustic signals decrease with the increase of the salt concentration.These results can provide theoretical and experimental proof for application of laser acoustics in under-water object detecting.