Design and experimental study of mode selective all-fiber fused mode coupler based on few mode fiber

Three types of all-fiber mode-selection couplers based on fused few-mode fibers (FMFs) are proposed and demonstrated. The specific mode conversions are achieved with appropriate parameters, keeping to the coupling mode theory. LP01 mode is selectively converted into the LP11, LP21, LP02 mode via a 2×2 fused fiber coupler composed of single-mode fiber (SMF) and FMF. By changing the preset parameters in fabrication, mode conversions are also realized between the LP01 and the mixed high order modes. Moreover, conversions from the LP01 mode to other higher order modes are implemented as well in a 3×3 fiber coupler comprising FMF-SMF-FMF structures. Besides, different modes are simultaneously obtained in separated channels to reduce model crosstalk. Distinguished from other techniques, symmetric and asymmetric fused biconical taper are employed in this paper. The 2×2 fiber coupler achieves the conversion from LP01 mode to a single higher order mode such as LP11 or LP21 mode over a broadband spectral range from 1530 nm to 1560 nm. Meanwhile, the mode conversion efficiency exceeding 80% is recorded in experiment, while the insertion loss remains as low as 0.8 dB. Through the comparison with all-fiber mode-selection couplers reported, the relationship between fusion-degree and conversion efficiency is further studied. The experimental results are consistent with the numerical simulations. In addition, the coupler based mode-selection with lower insertion loss and higher conversion efficiency shows potential applications in mode-division multiplexing and sensing systems.