螺旋扭曲双包层-三芯光子晶体光纤用于轨道角动量的生成

Helically twisted double-cladding-three-core photonic crystal fiber for generation of orbital angular momentum

针对螺旋扭曲的单包层-少芯光子晶体光纤在生成轨道角动量(orbital angular momentum,OAM)方面存在的不足,首次将三芯和内外空气孔不均匀的双包层结构引入光子晶体光纤,并通过螺旋扭曲实现了高阶OAM模式的生成.该光纤通过引入特殊设计的双包层结构有望降低生成的OAM模式的损耗,而围绕中心呈正三角分布的三个纤芯有望增加生成的OAM模式的数量.在光学变换原理的基础上,通过有限元方法对该光纤进行系统的分析,结果发现,当扭曲率a=7853.98 rad/m时,生成的OAM模式包括“OAM_(–4,1),OAM_(+9,1),OAM_(+10,1),OAM_(+11,1),OAM_(+13,1)”,其中+13阶是目前利用螺旋扭曲光纤生成的OAM模式中最高的阶数,且OAM模式的损耗均小于1.64×10^(–3)d B/m,比已有文献中最低的OAM模式损耗(Napiorkowski M,Urbanczyk W S 2018 Opt.Express 2612131)至少降低2个数量级,以及OAM模式纯度均大于93%.进一步研究表明,轨道角动量的生成依赖于纤芯超模与环形芯模式的共振耦合,而生成的OAM模式阶数的奇偶性与纤芯超模和环形芯模式的极化方向有关.

Aiming at the shortcomings of helically twisted single-cladding-few-core photonic crystal fibers in generating orbital angular momentum(OAM),the double-cladding and three-core structures with non-uniform inner and outer air holes are introduced into a photonic crystal fiber for the first time and the generation of high-order OAM modes through helical twisting is realized.The fiber is expected to reduce the losses of the generated OAM modes by introducing a specially designed double-cladding structure,while the three cores distributed in a regular triangle around the center are expected to increase the number of generated OAM modes.On the basis of optical transformation theory,the optical fiber is systematically analyzed by the finite element method.It is found that with the twist rate a=7853.98 rad/m,the generated OAM modes include“OAM_(-4,1),OAM_(+9,1),OAM_(+10,1),OAM_(+11,1),OAM_(+13,1)”,where+13 is the highest order in the OAM modes currently generated by using helically twisted fibers.And the losses of OAM modes are all less than 1.64×10^(–3)dB/m,which is at least two orders of magnitude lower than the lowest OAM mode loss reported in the existing references(Napiorkowski M,Urbanczyk W S 2018 Opt.Express 2612131),and their purity is greater than 93%.Further studies show that the generation of orbital angular momentum depends on the resonant coupling between the core supermode and the ring-core mode,and the parity of the order of the generated OAM modes is related to the polarization direction of the fiber core supermode and the ring-core mode.