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Institute of Optics


Nonlinear Optical Systems Interacting with Amplitude-Modulated Optical Fields


Stephen H. Chakmakjian

This thesis is concerned with the interaction of amplitude-modulated (AM) optical fields with various nonlinear systems. An experimental and theoretical analysis of three distinct nonlinear systems is treated: two-level atoms interacting with a 100% AM field: four-level laser amplifier with an AM pump intensity: a multimode dye laser with an AM pump intensity.

A 100% AM field is the limiting case of strong modulation in which the energy at the carrier frequency is completely supressed, and only the modulation sidebands remain. The interaction of such an optical field with an ensemble of radiatively broadened two-level atoms (an optically-pumped sodium atomic beam is used) yields a complicated series of parametric resonances when both the Rabi frequency and the modulation frequency are large compared with the atomic-transition linewidth. The time-averaged fluorescence, and therefore, the absorption of energy exhibits parametric-resonant enhancement whenever the modulation frequency is equal to a subharmonic of the Rabi frequency.

Population oscillations in a multilevel laser amplifier are studied using weak amplitude-modulation spectroscopy. Two laser fields are applied to a four level laser amplifier (alexandrite is used as the amplifier). The intensity of the laser tuned to the pump-transition frequency is weakly modulated, while the modulated gain experienced by a second laser, tuned to the inverted transition, is measured. Amplitude-modulation spectroscopy is used to determine the temperature increase due to thermal relaxations within the crystal.

The near-threshold behavior of multimode cw dye lasers with an AM pump intensity is studied for several dye-laser cavity configurations. The intensity of the argon pump beam is weakly modulated and the modulation spectrum of the laser intensity is studied. Critical slowing down of the response of the laser intensity is observed. The behavior of multimode lasers are compared with the predictions of single-mode four-level laser theory. Also, the modulation spectrum of the fluorescent intensity is studied. The phenomenon of non-adiabatic gain clamping is discussed in relation to the results. Furthermore, the absorption spectrum of the modulation in the intensity of the argon pump beam is studied to complete a modulation-energy-balance analysis.

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