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Fabrication of Gratings in Mid-Infrared Compatible Fibres via Femtosecond Laser Direct Inscription

We report on research into the fabrication of different types of gratings into optical fibres that are transparent at mid-infrared wavelengths. In particular, we focus on direct femtosecond laser inscription of fibre-Bragg gratings with parallel as well as with tilted grating planes into zirconium and indium fluoride fibres. It is shown that post-annealing of the gratings can lead to an increase or decrease in reflectivity depending on the fabrication method and that even a relatively low concentration of rare-earth ions that are present in active fibres can have a big influence on the inscription process.Further, the feasibility investigating the physical mechanisms that underpin the induced refractive index modulation by utilising argon-beam cross-sectional polishing of the inscribed fibres followed by spatially resolved Raman microscopy and electron probe microanalysis, is discussed.