Incoherent light in tapered graded-index fibre: a study of transmission and modal noise

We investigated the impact of taper length on light transmission through tapered graded-index fibres. We tested commercial fibres from Thorlabs and a custom graded-index fibre using both coherent and incoherent light sources. Our experimental results show optimum performance for taper transition lengths of 25 mm, although our simulations suggest further improvement may be possible for even shorter transition lengths. We also measured the modal noise power fluctuations caused by bending the fibre. Here, we observe that the custom fibre tapers have the highest transmission but suffer from the most modal noise. Accordingly, we find that the commercial graded-index fibre tapers promise practical usage as a beam mode-field converter, as they have lower power fluctuations but retain relatively high transmission if compared to commercial small core step-index fibre.