RESUMEN
Trapped in the stringent adiabatic transmission condition of high-order modes, low-loss fused biconical taper mode selective coupler (FBT-MSC) has long been challenging to achieve. We identify the adiabatic predicament of high-order modes to stem from the rapid variation of the eigenmode field diameter, which is caused by the large core-cladding diameter difference of few-mode fiber (FMF). We demonstrate that introducing a positive-index inner cladding in FMF is an effective approach to address this predicament. The optimized FMF can be used as dedicated fiber for FBT-MSC fabrication, and exhibits good compatibility with the original fibers, which is critical for the wide adoption of MSC. As an example, we add inner cladding in a step-index FMF to achieve excellent adiabatic high-order mode characteristics. The optimized fiber is used to manufacture ultra-low-loss 5-LP MSC. The insertion losses of the fabricated LP01, LP11, LP21, LP02 and LP12 MSCs are 0.13â dB at 1541â nm, 0.02â dB at 1553â nm, 0.08â dB at 1538â nm, 0.20â dB at 1523â nm, and 0.15â dB at 1539â nm, respectively, with smoothly varying insertion loss across the wavelength domain. Additional loss is less than 0.20â dB from 1465.00â nm to 1639.31â nm, and the 90% conversion bandwidth exceeds 68.03â nm, 166.68â nm, 174.31â nm, 132.83â nm, and 84.17â nm, respectively. MSCs are manufactured using commercial equipment and a standardized process that takes just 15 minutes, making them a potential candidate for low-cost batch manufacturing in a space division multiplexing system.