基于飞秒激光加工长周期光栅的全光纤三阶轨道角动量模式的产生  

Generation of all-fiber third-order orbital angular momentum modes based on femtosecond laser processing of long-period grating

作　　者：吴航 陈燎 舒学文 张新亮 Wu Hang;Chen Liao;Shu Xue-Wen;Zhang Xin-Liang(Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information,Huazhong University of Science and Technology,Wuhan 430074,China)

机构地区：[1]华中科技大学武汉光电国家研究中心和光学与电子信息学院,武汉430074

出　　处：《物理学报》2023年第4期74-84,共11页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:61505060,61631166003,61675081,61735006,61927817)资助的课题。

摘　　要：高效地产生相互正交的各阶轨道角动量(orbital angular momentum,OAM)模式具有重要的研究价值.目前全光纤系统中高效地产生高阶轨道角动量模式的方法主要是基于二氧化碳激光器加工的长周期光纤光栅(long period fiber grating,LPFG).然而产生高阶模式的光栅需要强的折射率调制与小的光栅周期,因此二氧化碳激光器高的功率和大的聚焦光斑不利于其刻写的重复性、成功率和延展性.为了解决这一问题,本文首次提出并制作了基于飞秒激光加工的三阶OAM模式转换器,在六模光纤上加工出了非对称的长周期光纤光栅,实验结果表明其在1550 nm附近能将基模转换为三阶的角向线性偏振模式LP31模式,模式转换效率为98%,该模式可进一步被叠加转化为三阶OAM模式.与此同时,在1310 nm附近,该光栅还能够产生角向一阶径向二阶的OAM模式.本文证明了飞秒激光加工提供了一种可用于全光纤系统,具有高重复刻写性的长周期光纤光栅来产生高阶OAM模式的思路.The generation of orbital angular momentum(OAM) modes is very important,for they have a variety of applications such as in optical tweezers,quantum optics,and optical communication systems.Particularly,how can high-order OAM modes be generated efficiently in fibers with the advantage of low cost and compatible with fiber system?The Traditional method for first order to third order OAM is based on long period fiber grating(LPFG) fabricated by carbon dioxide laser.However,high power and large focused spot of carbon dioxide laser are unfavorable for stable and repeatable generation of higher-order OAM,which needs the LPFG with small grating pitch.In order to solve this problem,a third-order OAM mode converter based on femtosecond microfabrication is proposed and fabricated for the first time.With the advantage of 4.4 μm focused spot size near the core,lower power and lower heat absorption efficiency,this method can be more stable and promising.Therefore,we first carry out the mode filed analysis and simulate the intensity and phase profiles of the superposed mode field in LP odd-even mode on different scales and phases patterns to obtain OAM mode.Second,we use the coupled-mode theory to analyze and simulate the transmission spectrum of LPFG,which guides the setting of the grating parameters such as the grating pitch,the depth of modulation and the length of the grating.By experimental verification,an asymmetric modulated long-period fiber grating with a pitch setting to 194 μm is fabricated on a six-mode fiber.The fundamental mode can be converted into the third-order angular linear polarization mode LP31mode with 98% mode conversion efficiency near 1550 nm,and further converted into the OAM±3modes by superposition of the odd and even LP31mode with ±π/2 phase difference.At the same time,this fiber grating can also generate LP12mode with 90% mode conversion efficiency near 1325 nm.Then we can take the same approach to transform LP12mode into OAM modes with angular first-order as well as radial second-order.The e

关 键 词：轨道角动量 光纤光栅 光纤器件 光纤通信 

分 类 号：TN253[电子电信—物理电子学]