Shock wave and cavitation bubble dynamics during photodisruption in ocular media and their dependence on the pulse duration

Abstract

Shock waves and cavitation bubbles generated by optical breakdown may strongly influence the surgical effect of photodisruptive lasers. We have investigated the shock wave and cavitation bubble effects of femtosecond and picosecond laser pulses generated during photodisruption in corneal tissue and water. Laser pulses with 150 fs duration at approximately 620 nm wavelength have been focused into both corneal tissue and water to create optical breakdown. Pulses with durations of 20 ps have been applied for comparative studies. Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles. Femtosecond pulse engender rapidly decaying shock waves in both materials. The spatial range of shock waves induced by femtosecond laser pulses is considerably smaller than that of shock waves induced by picosecond optical breakdown. Cavitation bubbles excited by femtosecond pulses are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses. In corneal tissue intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses. The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage. Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers. This indicates a potential benefit from the application of femtosecond laser technology to intraocular microsurgery.