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 postannealing 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.