Orbital Angualr Momentum Generators Based On Chiral Fiber Gratings

The orbital angular momentum?OAM?beam,with a helical wavefront,annular intensity profile,phase singularity,has been widely used in the fields of optical communication,optical trapping,microscopy imaging and material processing.Currently,generation and propagation of the OAM beams in optical fiber has become an attractive approach.These chiarl fiber gratings and twisted photonic crystal fibers have been successfully fabricated using stress-induced method,posting-processing techique?CO₂ laser,arc discharge?and directly form a fiber drawing tower.Based on the work of long-period fiber grating?LPFG?in thin-core using CO₂ laser,we used hydrogen-oxygen flame as the heat source to fabricate the helical long period fiber gratings?H-LPFGs?and the twisted photonic crystal fiber?PCF?,which has realized the generation of the OAM beam.The main contents of the thesis are summarized as follows:1.A CO₂ laser irradiation system based on scanning technique was designed and demonstrated.And we experimentally demonstrated the fabrication of asymmetric long period fiber gratings in thin core fiber by use of focused CO₂ laser irradiation system.The proposed device exhibits a high extinction ratio of over 25 dB at the resonant wavelength and a narrowed 3d B-bandwidth of only 8.7 nm,which is nearly one order of magnitude smaller than that of long period fiber gratings in conventional single mode fibers.It also exhibits high polarization-dependent loss of over 20 d B at resonant wavelength.The temperature and external refractive index sensitivity of the proposed structure are measured to be 46 pm/?C within a temperature range from 25to 100?C,and 1047.3 nm/RIU within the refractive index range from 1.400 to 1.440,respectively.The temperature induced error is⁸%for refractive index measurement.Such long period fiber gratings may find potential applications of highly sensitive refractive index sensors in the fields of chemical and biomedical sensing.3.A high-efficiency grating fabrication method was,for the first time,demonstrated to inscribe H-LPFGs in small numbers by means of twisting a single mode fiber during hydrogen-oxygen flame heating and then cutting the helical fiber into in series of sections.Each section of the helical fiber was a desired LPFG whose resonant wavelength,i.e.grating pitch,can be changed by adjusting the twist rate of the helical fiber.Moreover,the response of H-LPFGs to torsion,axial strain,temperature,and surrounding refractive index?RI?were investigated.4.The H-LPFG inscribed in a standard SMF could be used to generate OAM modes,i.e.OAM₊₁ mode,with a purity of 91%and a conversion efficiency of 87%within a large wavelength range from more than the cutoff wavelength of a SMF,which is highly advantageous to all-fiber optical communications based on the OAM mode-division multiplexing technique.The H-LPFGs had a clear annular shape beam profiles in the center and anti-clockwise spiral interference patterns.5.High-order OAM modes,i.e.,OAM₊₅ and OAM₊₆,were generated and demonstrated experimentally by twisting a solid-core hexagonal PCF during hydrogen-oxygen flame heating.The resonance dips,Dip₁ and Dip₂,in the transmission spectrum,correspond to the sixth-and fifth-order OAM mode,respectively,generated by the helical PCF.The mode pattern of the Dip₁ is the ring-shaped with six-fold symmetry which are localized between the first and second ring of air-holes in the cladding.Leaky orbital resonances in the cladding depend strongly on the twist rate and length of the helical PCF.Moreover,the generated OAM₊₆ mode could be a polarized mode.The secret of the successful observation of high-order modes is that leaky orbital resonances in the twisted PCF cladding have a high coupling efficiency of more than 20 dB.In addtion,the response of the helical PCF to torsion and axial strain were investigated.