Abstract
Unstable resonators are an efficient method of generating coherent laser beams of large dimensions.1 The spatial extent of their modes can be matched to the size of the laser active media. The use of variable reflectivity minors (VRMs) can improve the fundamental mode profile and eliminate the detrimental effects (ripples in the near field, secondary lobes in the far field) caused by hard apertures.2 However, a fundamental limitation of unstable resonators is associated with the geometrical magnification M experienced by the laser beam at each round trip; as a consequence, the feedback coefficient of the resonator is equal to 1/M2 according to the geometrical theory. Diffractional effects can increase this value slightly at the expense of a complicated eigenvalue spectrum.1,3
© 1990 Optical Society of America
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