Interaction of Modulated Optical Fields with Saturable Media and its Application to Laser Instability

by

Lloyd W. Hillman
(1984)

This thesis presents a series of theoretical and experimental studies on the dynamics of modulated optical fields interacting with saturable media. The total modulated field is decomposed into a weak probe field (i.e. the frequency sidebands) and a strong pump field. The interaction of the pump field with the medium actively modifies the absorption of the probe field. Two of the more interesting consequences of this interaction are: (1) in a homogeneously broadened absorber, a dip appears in the spectral profile of the probe field when its frequency is near that of the pump field, and (2) that a homogeneously broadened ring laser is intrinsically unstable when it is pumped far above threshold. Although similar phenomena occur for inhomogeneously broadened media, this thesis reports the first observations of these effects for the case of homogeneously broadened media.

A semiclassical formalism is developed that describes the propagation of a weak probe beam through a homogeneously broadened two-level medium in the presence of a strong, collinear, near-resonant pump beam. The principal new result of this theory is the demonstration that the self-consistent treatment of the coupling of the two single-frequency sidebands, which are the probe field, leads to two special linear combinations of the sidebands, which we call the "natural modes." In extended media, all injected probe fields asymptotically transform into a bichromatic natural mode. It is also shown that this general theoretical framework is well-suited for the analysis of AM/FM saturation spectroscopy, nearly degenerate four-wave mixing, laser stability, and optical bistability.

Experimental verifications of the theoretical predictions are presented for both an absorber and an inverted medium. By using amplitude modulation sidebands as a probe, we measured a spectral hole of width 37 Hz (HWHM) in the green absorption band of ruby. We also report observation of multiple instabilities in a cw-driven ring dye laser. The power in the cavity displays a series of discontinuities with hysteretic cycles as a function of the pumping power. The output spectrum bifurcates into two components whose separation is the Rabi frequency on the first branch and a sub-multiple of the Rabi frequency on the second branch. We present theoretical results which show that localized gain maxima for two strong fields exist and are stable at sub-multiples of the Rabi frequency.

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