Coherent modal engineering: a new perspective on fiber splice optimization

In this work, we introduce a coherent modal engineering approach to optimize single-mode to multimode doped fiber splices. By systematically varying the arc duration and analyzing higher-order mode excitation using the S² technique, we reveal that the optimal splicing condition does not correspond to matched mode field diameters at the splice interface, as traditionally assumed. Instead, measurements of the thermally diffused refractive index profiles show that the multimode fiber undergoes a longitudinally non-adiabatic index transition, leading to coupling from the fundamental mode (LP₀₁) into higher-order modes (particularly LP₀₂). We demonstrate that the optimal arc duration creates a deliberate mode-field-diameter mismatch that excites LP₀₂ with a tailored amplitude and phase—precisely configured to destructively interfere with the higher-order-mode content generated in the non-adiabatic region. This controlled interference mechanism forms the basis of a new splice optimization paradigm that prioritizes modal coherence and beating over geometric matching.