Nonlinear propagation of short pulses in the eye

At or above a threshold in peak pulse power that is easily accessible by laser sources available today, a short light pulse can undergo tremendous changes as it propagates from the cornea to the retina. The spot size, the pulse shape, the peak intensity, and the spectral content of the pulse can all undergo changes not predicted with either linear or stationery nonlinear propagation models. Such changes play an important role in understanding the mechanisms by which energy is deposited in ocular tissues in the short-pulse regime. Understanding breakdown, tissue damage or other absorption processes in the eye hinges on understanding the details of the delivery of the light energy. In this paper we develop a simple model which contains the basic short-pulse propagation processes relevant to the optical system of the eye. This model assumes that propagation through the anterior elements of the eye such as the cornea and lens is linear, and that deviations from linear propagation become significant as the pulse propagates through the vitreous humor. Such a model provides information about the irradiance at or near the retina. In the simplest nonlinear media, short pulses propagate under the influence of diffraction, group velocity dispersion, and cubic nonlinearity. The vitreous humor, having optical properties that are very similar to water, is normally dispersive throughout the visible region and becomes anomalous only above 1.1 μm. Normal dispersion tends to inhibit self focusing and broaden the spectrum. As a result, increased input energies are required to cause breakdown. Anomalous dispersion acts in the opposite way, promoting self focusing and localizing the delivery of the pulse energy both in space and time. The Raman response can play an important role in the dynamics even for short pulse propagation. While the Raman effect tends to reduce the time averaged intensity, it causes a temporal modulation that breaks-up the pulse, delocalizing the delivery of energy to the affected tissue. At the same time, the peak intensity can be significantly higher. Further extensions to the basic propagation model and their limitations will also be discussed.