Design And Characteristics Of Highly Birefringent Photonic Crystal Fiber

Since successfully fabricated in1996for the first time, photonic crystal fibers (PCF)have received wodespread attentions from a lot of researchers all over the world due tothose unique and excellent properties that can’t be realized in conventional fibers. Atpresent, applications of PCFs have expanded into many important research domains, suchas optical communications, optical sensing, photoelectric detection, optical fiber lasers andmedical devices, etc. With application fields gradually extending, to develop highperformance PCFs and novel devices based on PCFs has become an urgent demand.Therefore, carrying out theoretical studies on PCF characteristics and applications is ofgreat academic values and wide application prospects.Focusing on index-guiding highly birefringent PCFs, propagation properties of severealnovel PCF designs are thoroughly investigated theoretically by employing multipolemethod. Two novel dual core PCF polarization splitters are presented and the full-vectorfinite element method is adopted to analyze and optimize the structures, and main researchcontents are listed below:Firstly, PCF structures with the hexagonal cladding and elliptical cores are proposedand impacts of air hole sizes and the core’s ellipticity on birefringence are analyzed, andresults indicate claddings of these designs are highly symmetrical so birefringence ismainly caused by elliptical cores. Effects of hole sizes on birefringence are trivial butellipticity of the core has great impacts. With ellipticity of3:2or2:1, birefringence is onthe order of magnitude of10-3; Structures with air holes in elliptical configuration arepresented and effects of air hole sizes, the number of air holes and ellipticity onbirefringence are investigated, and results show that symmetry of these structures is higherthan that of structures with triangular lattice and elliptical cores. With ellipticity of4:3,birefringence of these structures is on the order of10-3. Meanwhile, asymmetry ofelliptical claddings is mostly determined by ellipticity of air holes’ distribution, the largerthe ellipticity is, the more asymmetrical these structures are and the higher birefringence is;Three squeezed hexagonal PCF designs are proposed and impacts of air hole sizes andhole pitch on birefringence are analyzed. Results indicate hole pitch plays a moreimportant role on birefringence than hole sizes, and decreasing the hole pitch can enhancethe asymmetry of the ocre and cladding, which leads to higher birefringence. By selecting proper structural parameters, the highest birefringence of1.544102can be realized atthe wavelength of1.55μm.Secondly, using multipole method, numerical analysis on dispersion, effective modalarea and nonlinear coefficient of PCFs with elliptical claddings, squeezed hexagonalcladdings and panda-like squeezed hexagonal claddings is carried out. As for the ellipticalcladding structres, impacts of the number of air hole rings, hole sizes and ellipticity ondispersion, effective modal area and nonlinear coefficient are analyzed; As for thesqueezed hexagonal structures, hole size and hole pitch both affect those transmissionproperties, and results with different hole pitches are compared. Results show thatstructural parameters affect all those properties, leading to normal and oscillatingdistributions, zero dispersion points in multiple communication wavebands and largenegative dispersion values. Moreover, structures with higher asymmetry have strongernonlinearity, for example, the nonlinear coefficient of panda-like squeezed hexagonaldesign is around69.54W-1km-1at the wavelength of1.55μm.Finally, based on the modal interference principle, two novel dual core PCF polarizationsplitters are proposed. The first one’s structure is a combination of octagonal andrectangular claddings, and its fundamental modes’ distribution and impacts of parameterson birefringence and coupling length are analyzed by using full vector finite elementmethod. After optimization the normalized power and extinction ratio of this design arenumerically investigated, moreover, the extinction ratio at1.55μm is around-50.5dB, andthe bandwidth at the extinction ratio of-10dB is170nm,60nm at the extinction ratio of-20dB. The second one is made of soft glass SF6with circular and elliptical air holesarranged in rectangular lattice, and by selecting proper structural parameters a281μm longpolarization splitter is realized. The extinction ratio at1.55μm is around-45.42dB, andthe bandwidth at the extinction ratio of-10dB is90nm,32nm at the extinction ratio of-20dB.