Near- and far-field laser modes produced by confocal unstable resonators. I. Free-space and saturable gain media

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Abstract
Optical modes produced in positive branch confocal unstable resonators have been obtained using the method developed by Rensch (1974). The limitations and advantages of using a finite difference scheme are highlighted by examination of the modes produced under free-space conditions. A typical gain medium produced by an air-breathing gas dynamic laser is then modelled and introduced into the resonator code. The effect of magnification on medium saturation and also the power extracted is then investigated. In all the examples used far-field conditions are also obtained by Fourier transforming the extracted near-field mode. A visual display of the three-dimensional effects is presented by hidden line plots produced on a calcom plotter.