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Author

Tuan-Kay Lim

Date of Award

9-1974

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Supervisor

Professor B. K. Garside

Abstract

The major purpose of the work described in this thesis is the development of a theoretical model for the mode-locked continuous (CW) and quasi-continuous lasers using a suitable saturable absorber, and the direct testing of the applicability of this model to some important systems.

In essence, the development of the theoretical model consists of three parts. First, the dynamical equations governing the laser operation are derived employing a semi-classical approach, where both the gain and absorber media are treated in the density matrix formalism and propagation of the electromagnetic radiation is governed by Maxwell's equations. Second, the possibility of achieving mode-locking is investigated by examining the stability of the time-invariant solutions of the dynamical equations against small perturbations. Finally, a finite difference approximation is employed as a means of solving the laser dynamical equations so that steady-state pulses (SSP) can be generated within the mode-locking regime.

A systematic study of the general behaviour of CW mode-locked lasers is undertaken. Such an investigation helps to provide an overall understanding of the physical mechanisms controlling the mode-locking process. In particular, the atomic coherent effects of the interactions between the radiation field and the active media are studied. Furthermore, the general procedure and techniques employed for the numerical analysis form the basic mathematical tool for handling specific systems.

The behaviour of mode-locked CW and flashlamp-pumped (FLP) dye lasers is investigated. It is verified directly that picosecond pulses can be generated from a CW dye laser with an absorber whose recovery time is several hundred picoseconds. Further, it is shown that under typical experimental conditions, the FLP dye lasers can be operated in the quasi-continuous regime where the output pulses are close to being SSP. Numerical results concerning the build-up process of the SSP are in good agreement with experimental observations.

As an integral part of our work, the interrelation between mode-locking and relaxation oscillations (passive Q-switching) is also discussed. It is shown that the passive Q-switching instability solutions follow as a special case of the analysis employed for saturable absorber mode-locking. The impact of the presence of passive Q-switching instabilities on the prediction of mode-locking instability regions is investigated. As a particular result of a study of the transition from a mode-locked to a passive Q-switched regime, it is suggested that relaxation oscillations from FLP dye lasers and mode-locking in suitable semiconductor lasers may be obtainable.

Finally, a number of suggestions on further research are proposed.

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