中文摘要：

提出了三种基于少模光纤的全光纤熔融模式选择耦合器.根据模式匹配原理采用单模光纤与少模光纤熔融连接方式,运用耦合模理论及光束传播法模拟分析了模式选择耦合器的结构参数对模式选择及耦合特性的影响,实现了单模光纤中基模到少模光纤中不同阶模式的转换,以满足不同的应用需求.实验上以2×2熔融光纤耦合器为例,采用对称和非对称熔融拉锥方式,分别实现了从基模到LP_（11）,LP_（21）模式的转换.实验结果表明所得到的LP_（11）,LP_（21）模式在1530—1560 nm的波长带宽范围内均有较高的模式纯净度,且模式耦合效率高于80%,与理论模拟结果基本一致.

英文摘要：

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.LP_（01） mode is selectively converted into the LP_（11）,LP_（21）,LP_（02） 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 LP_（01） and the mixed high order modes.Moreover,conversions from the LP_（01） 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 LP_（11） or LP_（21） 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.