We investigate the optical properties of graded-index tapered fibres designed for efficient coupling of telescope light into a spectrographic instrument. Mode field measurements and COMSOL simulations are presented for 5:1 graded-index tapers with final core diameters of 10–12.5 um. For a given initial fibre core size, the change in refractive index across the fibre core means that the mode field diameter in graded-index tapers is less than half that of step-index fibres. This allows smaller taper ratios in graded-index fibres to achieve the same mode field diameter. Measurements and simulations of mode field diameter agree well for coherent light. Simulated transmission values are lower than experimental results, likely due to limitations in mode summation and coupling assumptions. Overall, graded-index tapers achieve the target mode field diameter with significantly smaller physical taper ratios enabling high transmission efficiency.
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Choochalerm et al. (2026) studied this question.
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