Photo-acoustic induced by Q-Switched Nd : YAG laser detected by PVDF sensor in different liquids

摘要

A Q-switched Nd:YAG laser beam with short pulsed duration of 6 ns and energy 60 mJ at a wavelength 1064 nm was focused via a camera lens into a glass container occupied by liquid samples. The laser pulse focused at a small area creates an optical breakdown followed by plasma formation. This induced shock wave formation followed by acoustic wave in the liquid medium. The acoustic wave generated was detected using a Piezo sensor coupled with preamplifier of 1 GO input impedance. Difference distances of source to transducer were configured. Difference sample solutions such as distilled water, sodium chloride (NaCl) solutions and white grape juice were used. Properties of acoustic wave, such as the attenuation (amplitude voltage), transient pressure, and dominant component frequency were observed. It was found experimentally that spatial extent of the generation breakdown can affect the acoustic signature and Fourier spectrum of photo-acoustic response where the amplitude and the transient pressure of the signals increase when the distance of the source-sensor decrease. The voltage amplitude for white grape juice and distilled water samples at a distance of 10 mm, were 4.57 V and 4.75 V respectively while for the transient pressure, the value were 481.46 kPa and 500.42 kPa respectively. The presence of ions in aqueous solution can also affect its photo-acoustic response due to water molecule association, where OH band strongly attached to hydrogen-bond water molecule and the other part of band deals with free molecules. The attenuation effect in electrolyte solutions (NaCl) was found to be larger, compared to other samples where the amplitude at lower concentration was 0.40V, with pressure of 42.14 kPa. The frequency of photo-acoustic Fourier spectrum for distilled water and white grape juice samples were found at lower frequency ranging from 24 kHz - 60 kHz but for NaCl solutions the frequencies were higher ranging from 44.06 kHz – 144.16 kHz.