Chemical and physical side effects at application of ultrashort laser pulses for intrastromal refractive surgery

Focusing femtosecond (fs) laser pulses into transparent media, such as corn eal tissue, leads to optical breakdown, generation of a micro-plasma and, t hus, a cutting effect inside the tissue-provided the intensity at the focus exceeds a threshold which depends on the pulse duration. For fs pulses, th e corresponding threshold pulse energy is reduced by some orders of magnitu de compared with ns or ps pulses. At a low pulse energy, thermal and mechan ical damage to surrounding tissue is minimized, enabling a highly precise c utting effect inside the tissue while leaving anterior layers unaltered. In the following, investigations concerning aspects of the interaction of u ltrashort laser pulses with aqueous media are presented: in particular, det ection of pressure transients and bubble formation. For the first time-to o ur knowledge-the content of the resulting gas bubbles was analysed by gas c hromatography, giving evidence of molecular hydrogen. Secondly, the potential of three-dimensional cutting effects within the cor neal stroma for refractive surgery applications was evaluated in vitro on f reshly enucleated porcine eyeglobes. Laser pulses with a duration of 200-30 0 fs and energies of 1.5-600 mu J were provided by a mode-locked Ti:sapphir e laser with subsequent chirped pulse amplification.