Abstract

The steady-state distribution of light in a turbid tissue can be treated by diffusion theory. However, when laser pulse durations shorten into the picosecond (ps) and femtosecond (fs) range, the light does not have time to either become diffuse or achieve a steady-state distribution before absorption takes place. The propagation of ps and fs laser pulses is illustrated using Monte Carlo simulations. Two issues are addressed: (1) the time course of noninvasive reflectance measurements can specify the tissue optical properties in vivo, and (2) transient light distributions influence the distribution of two-photon chemistry in turbid tissues. (1) The total reflectance from a homogeneous turbid tissue is uniquely related to the dimensionless ratio of scattering to absorption. The time course of backscattered reflectance escaping the tissue allows a specification of the mean free path in the tissue in units of cm⁻¹. Together, these measurements specify the tissue optical properties. (2) Two types of two-photon chemistry are considered: (i) simultaneous absorption of two photons and (ii) an initial absorption event which yields an excited state that absorbs a second photon. The duration of the laser pulse affects the spatial distribution of two-photon chemistry.

© 1988 Optical Society of America