Fibre Bragg Gratings in Few-Mode and Multimode Fibres

Fibre Bragg gratings (FBGs) have found several uses in astronomical instrumentation such as spectral filtering and spectroscopic analysis. FBGs are conventionally inscribed in single-mode fibres where spectral responses are easily modelled and desired features are readily fabricated. Generally, light is coupled into single-mode fibres from multimode fibres using photonic lanterns, which are currently only available in a limited wavelength range, or by accepting a significant coupling loss. Fabrication of FBGs directly in multimode fibres has the potential to reduce coupling losses and output a more versatile beam post-filtering by the FBG. We explore the properties of FBGs fabricated in multimode and few-mode fibres using direct laser writing, discuss the options for their modelling, and consider potential applications for astronomical instrumentation.