Study On Characteristics Of Seven-core Photonic Crystal Fiber

Photonic crystal fiber(PCF) is a perfect combination of photonic crystal and traditional fiber, which is formed by introducing line defect in two-dimensional photonic crystal. The cladding layer of photonic crystal fiber consists of silica and air holes which are arranged periodically. With the rapid development of photonic crystal fiber technology, PCF gradually becomes one of the research hotspots in the field of optical communication because of its unique cladding layer structure and peculiar optical properties. According to the number of cores, PCF can be divided into two categories: single-core photonic crystal fiber and multi-core photonic crystal fiber. The mode area of multi-core photonic crystal fiber and the transmission capacity of multi-core photonic crystal is bigger when compared with that of single-core photonic crystal fiber. At same time, multi-core photonic crystal fiber has coupling properties. So multi-core photonic crystal fiber has a broad application prospect in the field of high-power fiber lasers, fiber couplers and fiber communication systems. In this thesis, study on seven-core photonic crystal fiber mainly focuses on the following aspects:It analyzes the supermode distribution in seven-core photonic crystal fiber. According to the characteristics of the supermode distribution, a calculation method of the intermodal dispersion(IMD) between two random supermodes in seven-core photonic crystal fiber is obtained. With doping low refractive index materials in the cores, the coupling characteristics of the fiber is changed and the zero intermodal dispersion between two random supermodes is achieved at two common communication wavelengths:1.31mm and 1.55mm. At last, we systematically study the characteristics of zero IMD wavelength by changing the refractive index of the cores, the displacement of the air-holes between the cores and the diameter of the cladding air holes.Also, a kind of large-mode-area seven-core photonic crystal fibers is designed by doping high refractive index materials in different annual regions in the cores of a seven-core photonic crystal fiber. A mode distribution with a small peak power and a large mode area is obtained in the photonic crystal fiber with doped cores. The maximum mode area of the photonic crystal fiber with doped cores can be up to 3732um2, We show that the mode area of the photonic crystal fiber with doped cores is bigger than that of photonic crystal fibers with other structures; We also show that the confinement losses of the photonic crystal fiber with doped cores is as less as 5.66 ′10-9d B/m when the mode area of the photonic crystal fibers with doped cores reaches the maximum while the confinement losses of the photonic crystal fibers with other structures are 0.06 d B/m. Therefore this kind of seven-core PCF designed can realize large mode area and low confinement loss. It can be expected that the thermal damage threshold of high power fiber lasers will be increased, the nonlinear effects of high power fiber lasers will be reduced and the laser beam with high power and high beam quality will be obtained when the photonic crystal fiber with doped cores is used as the basic matrix fiber of high power fiber lasers.