Ultrashort pulse laser induced bubble creation thresholds in ocular media

The measurement and characterization of laser induced breakdown (LIB) in ocular media for ultrashort (< 1 ns) laser pulses is important in understanding both eye damage mechanisms and various ophthalmic applications. In particular, the American National Standards Institute laser safety standards (ANSI Z136.1-1993) have included only guidance but no definitive safety limits due to lack of both experimental data and quantitative understanding of the damage processed induced by ultrashort pulses. Moreover, LIB needs to be understood fully for the growing number of ophthalmic applications which employ LIB in beneficial ways, such as in capsulotomies and iridotomies. The threshold for gas bubble creation from a plasma induced by 100 fs, 400 fs, and 2.4 ps laser pulses at 0.58 micrometers was determined for various ocular media. Bubble creation was used as the endpoint for indication of LIB at these pulse durations due to the absence of broadband visible light emission (plasma spark) that is normally the indication of LIB at longer pulse durations. In addition, light emitted from the focal region was shown to come from gas breakdown within the bubbles produced by previous pulses when the laser was fired at 10 Hz. The difference in endpoints observed for ultrashort pulses and endpoints observed for longer pulses (> 30 ps) may result from aberrations in the optical setup, in particular the focusing optics. However, the nonlinear phenomena involved may play an important role in the observation of a different type of plasma. The cause and reduction of aberrations and the endpoints for LIB threshold studies are discussed.