A study of transmission properties in tapered graded-index fibre

We investigated the transmission properties of tapered fibres both experimentally with simulations in order to select the appropriate fibre for the high-resolution spectrograph EXOhSPEC. The work focuses on testing commercial produced graded-index (GI) and custom made graded-index (GI) multimode fibres and compares these with a 10 μm step-index (SI) conventional fibre. The performance of a spliced taper package (SP) is also investigated. All tests are performed with coherent light at 635 nm. The main focus of the investigation was to determine the factors that optimised the light transmission by matching the numerical aperture to the fibre and minimising misalignment. We characterised how the transmission through the fibre depends on the effective numerical aperture (NAeff ) or input beam numerical aperture. The results suggest that under optimal coupling conditions, the transmission of a Custom GI taper is better than the 10 μm SI fibre and the commercial Thorlabs taper over the range of NAeff from 0.02 to 0.25. In addition, we characterised a spliced taper package (SP) which contained a tapered fibre spliced to untapered fibre. Additionally a comparison of transmission via two coupling methods was investigated in the work. The results indicate splicing a large core GI fibre onto a tapered fibre is an appropriate technique for the fibre link of a fibre-fed spectrograph.