Research On Macro-bending Loss Of Circular Photonic Crystal Fiber Transmitting Orbital Angular Momentum Modes

Recently,orbital angular momentum(OAM)has inspired much interest because of its unique spatial profile of the light intensity and phase.OAM can be used to increase the transmission capacity and spectral efficiency in optical communication system.In order to achieve the stable transmission of OAM mode,some kinds of special fibers are designed to support the OAM modes.Our previous works have proposed a family of circular photonic crystal fibers(C-PCFs)family which can support the transmission of OAM modes and the C-PCFs are a kind of more potential candidate for OAM fibers.However,bending is inevitable in practical applications,and that will increase the nonlinear effect and impact the modes number excited of fibers,thereby decrease the transmission capacity.On the other hand,fibers are intentionally bent to make some special optical devices,such as sensors,multiplexers(couplers)and so on.Therefore,as a fundamental characteristic,fiber performance under bending is an indispensable property in optical devices and communications.There have been some researchers who studied the macro-bending characteristics of OAM fiber,but the bending characteristics of the C-PCF have not been investigated.In this paper,the transmission performance of the bending C-PCF is investigated.For the special structure of the C-PCF,we establish two kinds of bending model and research macro-bending characteristics of the C-PCF.Furthermore,the comparison is presented between the C-PCF and ring-core fiber(RCF)with the similar OAM modes.The research are as follows:(1)The bending model and relevant theory of the normal single mode fiber are investigated.On this basis,the two bending models for the special structure of the C-PCF are established and we verify the correctness of the two models through cross validations.(2)Using the bending models,we investigate the transmission performance of the C-PCF under the macro-bending from the mode field characteristics,loss,intra-mode walk-off and inter-mode crosstalk of OAM modes.It shows that the bending tolerance of the C-PCF for the above four aspects is 2mm,5mm,9mm and 9mm curvature radius,respectively.And the 9 mm radius of curvature can be as the final bending tolerance of the C-PCF.In addition,we find that phases of all OAM modes are not sensitive to bending and higher order OAM modes show better bending resistance.(3)The comparison of transmission performance is presented between the C-PCF and ring-core fiber(RCF)with the similar OAM modes under the same bending case.We give the evidences that the C-PCF possesses an obvious better performance immune to bending than that of the RCF through the aspects of OAM mode field characteristic,number of modes excited and intra-mode walk-off.