Large mode-size fibre Fabry-Perot interferometers

We show that fibre-coupled open resonator Fabry-Perot interferometers can be made with reduced insertion loss due to diffraction by using fibre with slightly larger mode size. For a 50% increase in mode size, the insertion loss is reduced by 3–4dB for a 7μm-long resonator at 1.5μm wavelength.