The Generation And Regulation Of Orbital Angular Momentum Mode Based On Few-mode Fiber Grating

Orbital Angular Momentum?OAM?beam is a kind of vortex beams with a symmetrical spiral phase distribution along the axis of the beam.The central of the beam has phase singularities.Therefore,OAM modes have great potential applications in many fields such as Orbital communication,particle manipulation,imaging technology and life sciences.In this paper,the orbital angular momentum generation and control method has been studied by using long period fiber grating?LPFG?and chiral fiber grating?CFG?.The main work of this thesis are summarized as follows:First,we propose and demonstrate a method for generating l=±1-order OAM modes by tuning a polarization controller?PC?to vary the input-light polarization of long-period fiber gratings?LPFGs?in a four-mode fiber?FMF?.The four-mode fiber long period grating?FMF-LPFG?exhibit asymmetric refractive index modulation,inscribed by periodically etching grooves on the side of the FMF using a high-frequency CO₂ laser beam.These OAM_±1 modes were generated within a broad wavelength range from 1530 to 1630 nm.The OAM mode was detected using space-free interference between light propagating through the LPFG in the FMF and a reference beam after a distance of 50 m.The LPFG,as a temperature sensor,has a temperature sensitivity of³8.6 pm/°C at the resonant wavelength of 1625 nm.OAM temperature characteristics were also investigated and the helical interference pattern exhibited only minor intensity changes with increasing temperature.The polarization-dependent loss?PDL?of the LPFG was also measured.Second,a novel polarization-insensitive OAM generator utilizing a CFG was proposed.The CFG exhibited a coupling efficiency of more than 99%and was produced by twisting a fused FMF during hydrogen-oxygen flame heating.The purity of the l=±1-order OAM modes,excited by a left-or right-handed CFG?LCFG or RCFG?,were higher than 93%due to helical index modulation.This helical phase can be preserved for input light of varying polarization states,indicating polarization independence.No coupling was observed between the l=-1-and l=+1-order OAM modes in this experiment,suggesting OAM chirality was determined only by the helicity of the CFG.Additionally,the polarization state of these excited OAM modes was the same as that of the input light.The resulting high-purity OAM generator is stable,polarization-independent,and a viable candidate for future applications of OAM.Finally,We proposed and improved the spatial OAM mode test system and purity analysis system.Spatial devices can effectively eliminate the influence of fiber disturbance on the experiment.Due to the devices have a certain distance with each other,it can be used to test polarization-related physical quantities by adding polarizers and half-wave plates and other optical components can also be added to the system.Therefore,the system has been playing an important part in OAM mode test with flexibile,convenience and stable.