Ultrashort laser pulse retinal damage mechanisms and their impact on thresholds

Ultrashort laser pulses have been adapted for use in a variety of applications from micromachining of dielectrics to atmospheric spectrochemistry and multiphoton microscopy. These lasers emit almost exclusively in the retinal hazard wavelength regime, making them potential sources for accidental vision loss, but also candidates for biomedical applications where precise alteration of tissues is an objective. The present article reviews the mechanisms for damaging the retina at the threshold for the lowest energy, where any change in tissue is barely perceptible. For laser pulses between several picoseconds and 10 μs, the threshold retinal damage is caused by microbubble formation around melanosomes in the retinal pigment epithelium (RPE). Below 1 ns, both stress confinement in melanosomes and self-focusing reduce the threshold for damage as measured in corneal radiant exposure, although the mechanism for damage is the same. Below several picoseconds, laser-induced breakdown produces intra-retinal damage, sparing the RPE at threshold levels. These mechanisms have been determined in the past decade and provide an understanding of trends in retinal damage with variation in laser parameters, but also elucidate potential techniques for producing precise alteration to tissues.