Splice optimisation between dissimilar fibres in the presence of dopant diffusion

In this work, we optimize singlemode-to-multimode doped fiber splices by varying the arc duration and monitoring HOM excitation, using the standard S² technique. By measuring the diffused Refractive Index Profile (RIP) distributions around the splice point, contrary to current belief, we show that the optimum arc duration does not corresponds to RIP that match the MFDs on either side of the splice point. Furthermore, we show that the MM doped fiber diffused RIP is longitudinally non-adiabatic and results in additional power transfer from the excited FM (LP₀₁) to HOMs (LP₀₂). As a result, we find that the optimum arc duration corresponds to a substantial MFD mismatch which results in an initial excitation of the HOM (LP₀₂) with the appropriate magnitude and relative phase to nullify the corresponding HOM power generated in the non-adiabatic region.