Грачев Г.Н.   Miroshnichenko I.B.   Смирнов А.Л.   Стаценко П.А.   Tischcenko V.N.   Berezutskiy A.  

Effect of the average power and pulse repetition rate of laser radiation on the spectrum and localization of intense sound produced by an optical pulsating discharge in the air

Reporter: Miroshnichenko I.B.

Earlier, the authors showed that an optical pulsed discharge (OPD) produced by pulsed-periodic (PP) laser radiation in air or on the surface of solids, forms in the surrounding space a sound whose power reaches ~ 10-20% of the power of the supplied radiation. With the use of trains of pulses, the spectrum can be rearranged in the infra-ultrasound frequency range. Laboratory experiments were carried out at a small distance of ~ 30 cm, where the surrounding acoustic background has little effect on measurements.
The effect of absorption in air on the characteristics of the sound produced by the OPD at an arbitrary power and the repetition frequency of the pulses of the PP laser radiation was studied. A model is proposed which makes it possible to determine the intensity and structure of the sound spectrum depending on the distance to the OPD. The model was confirmed in experiments where the OPD was created by PP CO2 laser - an average power of W ~ 2 kW, a repetition rate of f ~ 50 kHz, a pulse duration of ~ 1 μs. It is shown that spectrum structure depends on W and in the zone of intense sound localization (RL) (more than 120 dB) and contains the fundamental line at frequency f and its overtones or one f or simultaneously the line f and a strong line at the train repetition rate F <<f.
For example, the effect of the frequency f and sound absorption will be shown for the PP radiation with an average power of W ~ 10 kW: at f <10 kHz, the spectrum contains the fundamental line at frequency f and a large number of overtones and does not change at a distance RL ~ 10 m. At f ~ 30 kHz, RL ~ 6 m, overtones are absorbed at a distance of ~9 m. At f ~ 50 ÷ 150 kHz - RL ~ 4 ÷ 2 m, the initial spectrum contains one line.
Thus, the spectrum and radius of localization of the intense sound produced by the OPD depends not only on the power, but also on the pulse repetition rate of laser radiation.

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