Measurement of Mode Field Expansion in Tapered Space-Division Multiplexing Fibers

We focus on experimental mode field diameter (MFD) evolution in tapered single-mode (SMF) and few-mode fibers (FMF) with simulations supporting the trends and including a simulation-only case for a three-core coupled-core fiber (CCF3). Experimental measurements on an FMF supporting ten linearly polarized modes, tapered down to 1.84 μm, show close agreement with simulations and confirm the onset of evanescent mode field expansion. Simulations extended to sub-micrometer diameters predict rapid MFD growth below ≈2μm for several modes. For CCF3, simulations show supermode evolution into LP-like modes under tapering, with increasing MFD at small waist diameters.