Optic and acoustic detection of laser-induced optical breakdown in DDFP

Abstract

Laser induced optical breakdown (LIOB) or laser induced cavitation (LIC) in water has been investigated widely. Several patterns of LIC bubble are involved. However, LIOB in water needs relatively high laser intensity and extremely short pulse length in femtosecond. In the study, low intensity laser in nanoseconds was utilized in the LIOB in dodecafluoropentane (DDFP), and then the optical and acoustic detection were performed to reveal the characteristics of bubble dynamics and acoustic signal emitted from the cavitation site. LIOB was realized in the confocal system of laser, acoustic detection and microscopic imaging. A single pulse laser of 521 nm wavelength, with a 3-5 ns pulse width and average power of 50 μJ, was employed in the experiment after being focused on 200μm cellulose tube by 40× and 0.8 numerical aperture objective lens. High speed camera was used to acquire the images during LIOB, bubble formation and collapse. Passive acoustic detection (PCD) was performed by 10 MHz focused transducer connected via an amplifier to a high speed digitizer. The spectrum analysis and joint time-frequency analysis (JTFA) were performed to show the characteristics of LIOB in DDFP. Comparing to LIOB in water, focused laser at lower intensity could induce optical breakdown in DDFP liquid with longer bubble life time. Three patterns were observed and the difference is closely related with the circumstance temperature. The life time of bubbles correspond to their maximum radius. The original temperature is closely related with the cavitation forming time. In acoustic detection, significant RF signal were recorded by PCD when LIOB occurred. Its spectrum analysis and joint time-frequency analysis revealed LIOB happened in the duration of 3 μs, and the spectrum of LIOB signal was mainly distributed between 0-12MHz, with characteristics of specific frequencies of n×f. These characteristics of LIOB bubble in DDFP gives information for analyzing LIOB, suggesting acoustically monitored LIOB has potential as an important tool in diagnosis and in vivo.