We discuss the relative merits of passive and active fiber cavities for ring-down. Ring-down times of were recently demonstrated in passive cavities, but operation is restricted to weak evanescent wave interaction. We report on active cavities with amplifiers used for loss compensation, permitting the use of open-path micro-optic cells. Ring-down times of tens of microseconds can readily be achieved and extended to several hundred microseconds in gain-clamped cavities, but relaxation oscillations and system drift impose limits on accuracy and repeatability. A cavity enhancement of 2 orders of magnitude is realistically possible, and sensitivity may be further enhanced if ring-down is combined with established spectroscopic methods.
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