Research On Novel Special Optical Fibers And Related Fabrication Technics

Special Optical Fibers are widely used in such fields as optical fiber communications, sensing, physic, material process and military technoloty. Research on erbium-doped polarization-maintaining fibers, side-hole fibers, hole-assisted lightguide fiber and polarization-maintaining photonic crystal fibers are deeply discussed in this dissertation. The work is supported by the National High Technology Research and Development Programs of China, "Rare-earth doped fibers" and "Novel special fibers".The main achievements of the dissertation are listed as follows:1) The 2nd transparent boundary conditions (2nd TBC) are first used in 2D Galerkin vectorial finite-element method (FEM), by which the modes of fibers with arbitrary nonhomogeneous and anisotropic refractive indices distribution can be analyzed. Compared to 1st TBC, the confinement loss (CL) of modes of fibers can be solved more accurately using 2nd TBC. The relative error between results of FEM with 2nd TBC and multipode method (MM) is no more than 10%.2) On the basis of FEM with 2nd TBC, the temperature properties of a single mode PCF are numerically simulated. Then the approximate formulas for the change of effective refractive index n_eff, effective radius R_eff and CL of the PCF with temperature are constructed. The results show when the temperature coefficient of refactive indexξvaries slowly within the concerned temperature range, with the increasing of the temperature, n_eff increases linearly, R_eff and CL decrease linearly. But n_eff, R_eff, and CL change little as temperature increases. The dispersive properties of PCF are not affected by the change of temperature. That means the PCF shows good temperature stability.3) By FEM with 2nd TBC, together with FEM for plane strain model, the stress zones and inherent birefringence of circular-core side-hole fibers (CSHF) and K-type elliptical-core side-hole fibers (KESHF) are analyzed. The distributions ofσ_x,σ_y and Bs in the cross-sections are calculated and explained clearly. The relations among geometric birefringence B_g, stress-induced birefringence B_s and mode birefringence B_m is discussed, and the change of B_g, B_s and B_m change with wavelength and fiber structure parameters is illustrated.4) The current method based on FEM of the plane strain model is improved to calculate the stress at any point in the cross-section of the fiber when arbitrary transverse force is applied. By the improved FEM for plan strain model, the cross-section distribution of pressure-induced birefringence B_of in hole-contain fibers (HCF), including SHF, PCF, PM-PCF and HALF, are calculated. The relationship between sensitivity of B_of and fiber structure parameters are discussed. It is shown that because of diffusion and interference of concentration stress near the boundary of holes in cross-section, the distributions of B_of near the center of HCF are extremely complex. These complex distributions of B_of can be qualitatively explained by the linearly superposed field of concentration stress of separated holes. It's also shown that B_of is proportion to the pressure on fiber boundary and is sensitive to the pressure direction.5) A novel method, the fiber preform goniometric-groove method is first reported, and a set of perfect fabrication technics are developed. By goniometric-groove method, longer PMF perform can be fabricated and more easily the collineation of the centers of two stress-applied parts and the fiber core can be realized than by Pit-in-jacket method. The expected specification for symmetrical characteristic, linearity and surface roughness of the groove are easy to meet. Besides, SHF, twin-core fiber and HALF are fabricated by goniometric-groove method.6) By FEM with 2nd TBC and FEM for plain strain model, according to the property requirements for mode field diameter, cut-off wavelength and mode birefringence, the structure parameters of Panda type Erbium-doped polarization-maintaining fiber (EDPMF) are optimized and the value range of structure parameters for a birefringence no less than 3×10^-4 is given. Partial technology and related equipments of modified chemical vapor deposition (MCVD) and the solution-doped technique are improved. A gas-cooling attachment is installed at the down stream in the high-temperature region of the graphite oven to increase the deposition efficiency and soot layer uniformity in fiber core. The equipment for on-line doping is installed on the MCVD lathe to guarantee the purity in solution-doped process. A six-wheel fiber drawing equipment is developed to restrain twist rotation of the fiber in fiber drawing process. Under the improvement in technics mentioned above, by goniometric-groove method, MCVD and solution-doped technique, two PMEDFs with high quality and low cost are fabricated. The absorption coefficient of Al³⁺-codoped PMEDF at 1530 nm is 9.5 dB/m, both average mode birefringence and group birefringence are 4.76×10^-4, the bit-length at 1550 nm is 3.26 mm. The absorption coefficient of Bi³⁺-Ga³⁺-Al³⁺ codoped PMEDF at 1530 nm is 19.5 dB/m, in the range from 1530 nm to 1560 nm both average mode birefringence and average group birefringence are 1.93×10^-4, the bit-length at 1550 nm is 8.05 mm. The background loss of two PMEDFs at 1200nm is near 30 dB/km.