Large-core fibre-cavity design for ring-down experiments

A fibre-cavity design based on highly reflective gold coatings, vapor-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. An estimate of the ringdown time constant of the fibre-cavity is given and the influence of the micro-hole on the reflection loss is discussed.