Non-Linear Optical Amplification and Absorption.

Abstract

Abstract : This report summarizes the main results of theoretical and experimental investigations on two-photon interaction processes. Theoretical investigations have been directed to the analysis of the evolution of an optical pulse through a pumped medium which operates as a two-photon amplifier. The physical model under consideration consists of a collection of atoms prepared in a state of inversion between two levels of the same parity. An incident pulse with a carrier frequency approximately equal to one-half the atomic transition frequency stimulates an induced nonlinear polarization which oscillates with the same carrier frequency as the incident pulse. The evolution of the coupled atom-field system is described without restriction on the duration of the incident pulse and of the atomic relaxation times. Under coherent propagation conditions, i.e. when the pulse duration is much shorter than the atomic relaxation time, considerable pulse sharpening and power amplification is possible above threshold. In the rate equation limit, a regime where the atomic relaxation times are much shorter than the pulse duration, optimum amplification conditions occur when the frequency of the incident pulse is detuned from its resonance value. The entire range from the coherent to the rate equation limit is explored with the help of hybrid and digital computer simulations. Experimental investigations have lead to the measurement of the two-photon absorption constant in ZnS using a Q-switched ruby laser as the source. The measured value of the absorption constant is 0.013 cm/MW.