Time-resolved measurements of shock-wave emission and cavitation-bubble generation in intraocular laser surgery with ps- and ns-pulses

Abstract

Intraocular microsurgery relies on plasma generation with subsequent shock wave emission and cavitation bubble formation. To asses the potentials of photodisruption with picosecond pulses in comparison to the clinical techniques presently used, the shock wave characteristics and the bubble expansion after optical breakdown with picosecond (30 ps) and nanosecond (6 ns) Nd:YAG laser pulses were investigated by time-resolved photography. Frequency doubled light from the laser pulses was optically delayed and used as the illumination source for photography.The shock wave position and bubble wall position were determined as a function of time. From the slope of the r(t) curves, the shock wave and bubble wall velocities were derived, and then the shock wave pressure p(r) was calculated from the shock velocity. The optical breakdown threshold was 15 μJ for the ps-pulses and 200 μJ for the ns-pulses. The initial shock pressures were 13 kbar and 17 kbar after ps-pulses with an energy of 50 μJ and 1 mJ, respectively, 24 kbar after a 1 mJ ns-pulse, and 71 kbar after a 10 mJ ns-pulse. After formation of the shock front, the shock pressure decayed approximately proportional to 1/r 2. The maximum expansion velocity of the cavitation bubble was 390 m/s after a 50 μJ ps-pulse, 780 m/s after a 1 mJ ps-pulse, 1850 m/s after a 1 mJ ns-pulse, and 2500 m/s after a 10 mJ ns-pulse The lower threshold for plasma formation with ps-pulses allows a reduction of the laser pulse energy which is accompanied by a considerable decrease of the disruptive shock wave and cavitation effects and an increase of the surgical precision.