环绕空气孔结构的双模大模场面积多芯光纤的特性分析

Dual-mode large-mode-area multi-core fiber with circularly arranged airhole cores

将多芯光纤与无芯空气孔结构结合,设计了一种具有大模场面积的十九芯双模光纤结构.该结构由位于中心的5根常规纤芯及环绕其周围的14根空气纤芯按正六边形排布构成,能实现稳定的双模传输,其基模有效模场面积的最小值约为285.10μm^2.系统地分析了影响模式传输特性和模式有效模场面积的结构参数:纤芯间距、相对折射率差和纤芯大小.通过对这三个参数的优化,在双模传输的条件下,增大基模的有效模场面积.此外,具有大模场面积的多芯双模光纤结构具有良好的抗弯曲特性,基模弯曲损耗小于5×10^(-5)dB/m.该结构还具有制作简单、设计灵活等优点,适用于高功率光纤激光器和光纤放大器.

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 5conventional 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 LPn 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 HE11modes 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 A,refractive index difference between core and cladding An,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 HEu 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.