Fluorescence-based temperature measurement in laser-induced vapor bubbles

Abstract

A fluorescence-based temperature probe (optrode) was designed to measure temperature within laser-induced vapor bubbles. The optrode consisted of a 400-micrometer optical fiber with a rhodamine B-doped polyurethane film attached to the fiber tip. The film exhibited a fluorescence decay time of 4 ns, and the measured fluorescence yield was temperature dependent in the range from 20 to 110 degrees Celsius. A Ho:YAG laser ((lambda) equals 2.12 micrometer, (tau) <SUB>p</SUB> equals 250 microseconds) with a pulse energy of 256 plus or minus 7 mJ delivered through a fiber was used to produce the vapor bubbles. The bubbles reached maximum expansion 200 microseconds after the onset of the laser pulse and had an average lifetime of 350 microseconds. The temperature was measured by positioning the optrode within the vapor bubble and exciting its fluorescent film-coated tip with a nitrogen dye laser ((lambda) equals 540 nm, (tau) <SUB>p</SUB> equals 500 ps). At maximum expansion, the temperatures in the vapor bubbles were approximately 61 degrees Celsius, indicating sub-atmospheric saturated vapor pressures. The vapor pressure at maximum expansion was confirmed by Rayleigh's equation.