F2-laser-based fabrication of Fabry-Perot cavities in fused silica and sapphire optical fibres

The majority of the reported applications on 157 nm ablation of fused silica and sapphire were concerned with surface micromachining, where the depth of ablation ranges from few hundreds of nanometres to several micrometres. The control of surface morphology and wall-angle of the microstructures was found to be difficult with increasing ablation depth. In this paper, the performance of a Kohler-based illumination in conjunction with a low-numerical aperture imaging system for micromachining cavities through the diameter of standard silica optical fibres and sapphire fibres is presented. Roughness of wall surface and tapering of cavities at different energy densities have been investigated. Nearparallel walled cavities micromachined in silica fibres produced Fabry-Perot fringes as a result of interference of light undergoing Fresnel reflections at the cavity walls. A technique to control the ablation depth of cavities in optical fibres via in-situ monitoring of laser-induced fluorescence is developed.