Optical fibre cavity for ring-down experiments with low coupling losses

A fibre-cavity design based on highly reflective gold coatings, vapour deposited to the two end faces of a large core multimode waveguide, is presented. In contrast to common 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 commonly very high coupling loss to less than 1%. Accordingly, lower demands on the source power and the sensitivity of the receiver circuit result in lower costs of ring-down sensor approaches. The scope of the paper is on the proof of principle and the characterization of the novel fibre-cavity design. Nevertheless, potential applications are briefly addressed at the end of the paper.