Theory of Pulse-Shaping by Saturable Optical Materials

As a high-intensity monochromatic light pulse passes through a material containing two-level systems, saturable absorption or amplification of the radiation may occur. The resultant ``shaping'' of the intensity-vs-time curve as a function of distance into the sample is the subject of this paper. A theory is developed based on the Einstein rate equations, and pulse-shape functions are derived which depend on the absorption and loss coefficients of the material, the relaxation time of the systems, the input-pulse intensity and width, the initial ``state-of-pump'', and number density of absorbers in the sample.