Optical Vortex Chiral Filtering And Conversion Based On Microstructure Optical Fiber

Owing to its unique spiral phase and orbital angular momentum(OAM),optical vortex beams have attracted wide attention in optical communications.Specifically,as a new multiplexing technology,OAM mode multiplexing is expected to greatly improve capacity of optical communication and spectral efficiency.There are many studies on short-distance OAM optical communication based on free space,while long-distance OAM optical communication inevitably involves optical fibers in which OAM modes modulate.In recent years,twisted Microstructure Optical Fibers(MOF)have become an important means for vortex beam regulation due to their flexibility of foundation design and diversity of functions,especially,structured chiral optical fibers play an important role in the regulation of vortex beam.The different MOFs designed in previous chiral selection studies are only applicable to first-order OAM modes and their bandwidth is generally very narrow,in this paper,we will design the structure of MOF for high-order vortex beam.Because the vortex beams include circularly polarized and linearly polarized OAM modes,two different MOFs are designed to control the chirality of high-order circularly polarized and linearly polarized OAM modes,respectively.One is helical single-ring photonic crystal fibers for chiral selective filtering of broadband OAM modes,the other one is polarization-maintaining fibers for chiral conversion of linearly polarized OAM modes.These fibers can also be used for chiral purification,vortex mode conversion and noise suppression in vortex optical communication.Chiral selective filtering of circularly polarized OAM modes is studied in twisted helically single-ring microstructure optical fibers(HS-MOF).HS-MOF achieves chiral correlation filter mode based on mode leakage loss,which is different from the reflective vortex chiral filter mode of high helically twisted optical fibers.Left-handed and right-handed OAM modes have different transmission losses when they are propagated in a specific twist rate fiber,and the power difference between them increases significantly through a specific transmission distance.Finally,the OAM modes with certain topological charge(depending on the helical direction of the optical fiber)are filtered out,and the OAM modes with opposite signs are scattered.Besides the general first-order radial OAM modes,HS-MOF can also make better chiral correlation selection for higher-order radial OAM modes.It is worth noting that the filter bandwidth of this kind of optical fiber is very wide,covering four communication bands near 1.55 ?m.The chiral conversion of linear polarized orbital angular momentum mode(LP-OAM)is studied in side hole polarization maintaining fiber(SH-PMF).Previous studies only focus on the first-order LP-OAM mode conversion.In this paper,we will design related optical fibers to study chiral conversion mechanism of higher-order modes and propose the optimization scheme of optical fiber structure.SH-PMF consists of an air-core fiber and two low refractive index side holes.The existence of two low refractive index side holes can break the circular symmetry of the fiber and cause mode birefringence between LP odd and even modes.The chiral conversion of LP-OAM mode is realized by controlling the phase difference of linear polarization even and odd modes with appropriate SH-PMF length.