Nonlinear Optical Interactions in the Crystalline Lens and Implications for a Presbyopia Cure

Abstract

Purpose: : Micromachining the crystalline lens of the eye with infrared (IR) femtosecond laser pulses has been proposed as a "cure" for presbyopia. It has previously been predicted that the gradient refractive index (GRIN) lens structure may produce self-focusing in the crystalline lens at powers lower than in homogeneous media. We wished to investigate the thresholds for creation of visible light in the crystalline lens from short infrared pulses, as a signature of the possible underlying nonlinear processes including self focusing and fluorescence from multiphoton absorption or ionization.

Methods: : We focused femtosecond IR lasers of different pulse widths through different powered lenses into a small cell of water or saline. We recorded the power level at which white light was first observed by the naked eye on a card placed behind the cell. We then placed an in vitro crystalline lens (4 bovine and 4 human) into the cell so that the light focus was within the lens. We determined the power level at which white light was seen from above, as a beam through the lens, and when flashes were observed on the card. We recorded the light, visible from above with a cooled camera fitted with an IR blocking filter.

Results: : White light was transmitted through bovine lenses at energies slightly below the threshold for its production in water. In the human lens, white light was seen from above but was not transmitted through the lens. This occurred at energies up to a factor of 8 below the appearance of white light in water.

Conclusions: : Production of visible light in crystalline lenses from femtosecond pulses is species dependent. The GRIN in the bovine lens appears to lower the threshold for self-focusing below that for water. In the human lens, it appears that either multiphoton absorption or multiphoton ionization results in fluorescence at power levels below those for self-focusing. When studying the interaction of femtosecond laser pulses with the crystalline lens, more than one multiphoton process needs to be considered.

Keywords: presbyopia • optical properties • protein structure/function