Keywords: 
• 多芯光纤 /
• 双模特性 /
• 大模场面积 /
• 弯曲损耗

Abstract: Multi-core fiber has aroused considerable interest as one of potential candidates for space division multiplexing that provides an additional freedom degree to increase optical fiber capacity to overcome the transmission bottleneck of current single-mode fiber optical networks.Few-mode fiber is also under intense study as a means to achieve space division multiplexing.We propose a novel dual-mode large-mode-area multi-core fiber (DMLMAMCF),which uses multi-core structure to realize few-mode condition when pursuing large mode-area.The proposed fiber consists of 5 conventional silica-based cores in the center region and 14 air hole cores surrounding the center cores.The outer circle with 12 air hole cores,which function similarly to the fluorine doping region in the bend-insensitive fiber,can mitigate the bending loss when keeping large mode area.The symmetrically distributed two cores on both sides of the center core in central region can reduce the half second-order LP11 mode consisting of two degenerate HE11 modes,TE01 mode,two degenerate HE21 modes and TM01 mode,thus leading to the remaining four vector modes,i.e.two degenerate HE11 modes and two degenerate HE21 modes.That is the reason why we call it strict dual-mode.We focus on large-mode-area properties and bending characteristics of the dual-mode.The influence of structural parameters that include corepitch Λ,refractive index difference between core and cladding △n,and fiber core radius a,on mode characteristics and mode area of HE11 mode and HE21 mode is investigated in detail.The results reveal that it is helpful to increase the effective area of fundamental mode when we increase the value of corepitch,reduce the refractive index and fiber core radius.The effective mode area of HE11 is about 285.10 μm2 under the strict dual-mode condition.In addition,the relationship between bending loss and bending radius,and the relationship between effective mode area and bending radius of two modes are both investigated.For the HE11 mode,the least bending loss is about 5 × 10-5 dB/m while the least effective mode area with bending radius larger than 0.6 m is about 285.10 μm2.The HE21 mode is more sensitive to bend effect.The least bending loss is about 0.028 dB/m and the effective mode area is larger than 280.00 μm2 except for resonant coupling points.Large effective areas of both modes with low bending loss can be realized.Larger effective mode area with larger corepitch,appropriate refractive index difference and fiber core radius can be achieved.This fiber may find its usage in high power fiber lasers and amplifiers.