Coupling characteristics between two conical micro/nano fibers: simulation and experiment

The coupling characteristics of conical micro/nano fibers (CMNFs) are investigated using numerical simulations and experiments. Distinct from uniform micro/nano fibers (UMNFs), the coupling efficiency not only depends on the overlapping length between two CMNFs but also the tapering angle of the CMNFs. With the increase of overlapping length, the coupling efficiency gradually converges to a stable value, with its convergence speed determined by the angle of the CMNFs. Experimental result shows the convergent coupling efficiency can be >90%. The spectral response of the coupler shows a "box-shape" profile with a 3-dB bandwidth of 2 nm, resembling a flat-top bandpass filter. And experimental results also show the coupling characteristics of CMNFs are overlapping length and taper angle dependent, which verify the simulation conclusions.