Study On Guiding Charicteristics Of Large Mode Area Fibers

High power fiber lasers have incited particular interest as efficient, compact lasers with good beam quality for a variety of applications in industry processing, medical instruments and national defense. However, a further power scaling of fiber lasers is mainly limited by various nonlinear effects in fibers, and the obvious way to reduce the nonlinearity and increase the power is to use large mode area (LMA) fibers. With the core size increasing, the obstacles for high power fiber lasers are multi-mode operation and bend-induced mode distortion. So it is very significant to investigate and design bend-resistant, single-mode large mode area fibers to further enhance the power of fiber lasers.The main contents and key creation points of this dissertation are as follows:1. By investigating the waveguiding characteristics of gain-guided fibers, the cutoff conditions of different modes as function of gain-coefficient and index step are obtained. The effect of the resonator mirror reflectivity on gain-guided and index-antiguided fiber lasers has been discussed. The mode field distributions varying with gain coefficient and negative index step, and the mode confinement capability in gain-guided fibers have also been investigated, which is highly instructive for designing large mode area fiber with core size as large as several hundred meters.2. Through analyzing the cutoff properties of fundamental mode and second mode by effective index method, effective area method and confinement loss method, the phase diagrams of cut-off mode, single mode and multi-mode for photonic crystal fibers are obtained, and the comparison among the three methods also has been made. A novel octagonal photonic crystal fiber is designed, and compared with hexagonal photonic crystal fiber, which has more circular field distribution, wider single mode operation region and has advantage in designing dispersion-compensating fiber.3. In this paper, the characteristics of bent large mode area photonic crystal fibers are investigated comprehensively. Numerical results show that in bent large mode area photonic crystal fibers, the mode field distributions is deformed, and the normalized propagation constant and the effective mode area reduce steadily. The propagation character of bent photonic crystal fibers is determined by normalized propagation number and normalized bend radius, which allows the behavior of bent photonic crystal fibers to be broadly extended. Meanwhile, the bending loss oscillation is observed, and the results obtained from finite difference method show good agreement with analysis method.4. The transmission spectra and cutoff wavelength condition for the 1-D planar model analog of the all-solid photonic crystal fibers with high index inclusions are presented. Then based on plane wave method and finite element method, the bandgap, mode cutoff, and loss properties are studied in theory. An all-solid photonic bandgap fiber based on an array of rings doped with high-index material are designed, and the calculated results present that for a specific structure, the confinement loss and the critical bend radius are reduced simultaneously in some bands by increasing the inner diameter of ring, which provids a useful guide for large mode area fibers with low loss.5. The mode distributions and coupling performances of photonic crystal fibers with two cores arranged in three different ways and two-core all-solid photonic crystal fibers with high index inclusions are modeled. A comparison of coupling properties between a close-to-hexagonal air-holes two-core hollow core photonic crystal fiber and an ideal circular air-holes two-core hollow core photonic crystal fiber are performanced. The coupling characteristics and the sensitivity of coupling length to fiber structure parameters of three-core photonic crystal fibers are investigated in theory, which is instructive for coherent beam combining technique.