Novel multimode fibre-cavity for ring-down experiments

A novel fibre-cavity design based on highly reflective gold coatings, vapor-deposited to the two end faces of a 400 um multimode waveguide, is presented. In contrast to common passive fibre-cavity approaches, the laser pulses are not coupled through the reflective coatings into the cavity but through a micro hole in one of the fibre end faces, which reduces the coupling losses from generally almost 100% to less than 1%. An OTDR-graph is presented as proof of principle and the influence of the cross section ratio of the remaining reflective area after milling and the overall fibre core cross section is theoretically evaluated.