Cavitation bubble dynamics and acoustic transient generation in ocular surgery with pulsed neodymium: YAG lasers

The authors have investigated the application of mode-locked and Q-switched Neodymium:YAG (Nd:YAG) lasers in ocular surgery by means of high-speed photography and hydrophone measurements. The incisive effect relies on the optical breakdown at the laser focus. Cavitation bubbles and acoustic transients are thereby generated. Their size and pressure amplitude have been measured at various laser-pulse energies. With a pulse energy of 5 mJ, the bubble is 1.5 to 2.3 mm in diameter and the pressure of the acoustic transient is 9 to 16 bar (130 to 230 psi) at a distance of 18 mm from the focal point. Bubble size and amplitude of the pressure pulse are always approximately 50% higher with a Q-switched laser than with a mode-locked laser. However, the size of the ruptures produced in a polyethylene membrane by the laser pulses is the same for both modes of operation. The energy balance shows that not only mechanical effects, but also thermal mechanisms are responsible for photodisruption.