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

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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.