Novel passive fibre-cavity design for ring-down experiments using a multimode optical waveguide

Abstract

A novel fibre-cavity design based on highly reflective gold coatings, vapor-deposited to the two end faces of a 400um 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 coupling losses from generally almost 100 to less than 1 . Since the decay function of the back and forth reflected pulses is acquired through the same micro hole, a compact bi-directional module can be used for pulse transmission and acquisition, consisting of a low power uncooled laser source and a fast photodiode detector. By choosing the cavity length to be longer than the pulse width, wavelength tuning of the pulses can be omitted resulting in a simplified hardware setup. Thus, the novel fibre-cavity design facilitates ring-down experiments and considerably reduces the cost of the associated sensor applications.