Study On The Design Of Photonic Crystal Fiber For The Applications Of Beam Shaping

Photonic crystal fibers (PCFs) have found many applications in optical communica-tions and optical sensing fields as novel photonic devices due to their highly adjustablestructure design and unique optical properties. Compared with the conventional fibers,PCFs can obtain a lot of excellent properties through adjusting the structure parameter-s. More wide applications are expected if other mediums are introduced into the PCFsor other materials are used to fabricate PCFs. In this thesis, several kinds of PCFs withone single core or multicore which can be used in the field of beam shaping are proposed.Generations of high quality hollow beam (HB) and flat-top beam (FTB) are hopeful usingthese PCFs. The finite element method (FEM) is employed to analyze the basic proper-ties of the proposed PCFs, including the effective mode area, chromatic dispersion, andconfinement loss, et al. The main contents are described as follows:Firstly, a simple silica PCF whose air holes are all arranged in a shape of ring isproposed. The analysis procedure of the FEM is demonstrated in detail. And the guid-ing modes of the PCF are analyzed based on the method. Through modulating the size,amount and arrangements of the air holes in cladding of the PCF, HBs with differentsizes are obtained. This kind of PCF is useful for the development of the all-fiber opticaltweezers.Secondly, PCF with an annular core, whose air holes are all arranged in a shapeof hexagon, is designed. HB in a shape of hexagon can be obtained in this fiber. Basiccharacteristicsofthe PCFareanalyzed, includingthe effectivemodearea, dispersion, andconfinementloss, etal. ComparedwithanotherordinaryPCF,itpresentsalargereffectivemode area, which opens a new way for the design of large mode area fibers, and a moreflattened dispersion. Mode coupling between the fundamental mode and the higher ordercore modes and between the fundamental mode and the cladding modes are numericallyanalyzed.Finally, based on the generation conditions of the FTB, a large mode area PCF withfour cores is designed. When the germanium dioxide is properly introduced into the silicacores, aflattopmodeisobtained. Themaximumeffectivemodeareaachievedinthefiberis3395.52μm~2. Thecombinationofflattenedfundamentalmodeandlargemodeareawilleffectively increase not only the threshold of nonlinear effects but also the threshold of the thermal damage in the high power fiber laser and amplifier.