Design And Applications On Photonic Crystal Fibers

Photonic crystal fibers (PCFs), which were first fabricated in 1996, have attracted much attention due to its unique properties compared with the conventional fibers. There are a variety of novel PCF structures, which promotes its application in optical communications, optical sensing, nonlinear optics and many other fields. As the fields of the PCF applications continue to expand, it becomes an important part of the research to design novel high-performance PCFs and expand their applications. With focus on the design and applications on PCFs, researches are carried in the following areas:1. PCFs have been widely used in supercontinuum generation, four wave mixing etc, due to its flexibility in dispersion control. Flat dispersion profile is of great importance for these areas. We propose PCFs and obtain ultra-flattened chromatic dispersion for it. The simulation results show that the 1st air-hole ring decides the trend of the dispersion curve. The 2nd ring mainly decides the dispersion value within the short wavelength range, while the dispersion values within the longer wavelength range depends on the 3rd ring. The 4th and the 5th rings have little effect on the dispersion profile. Based on this law, an optimization procedure is utilized to control dispersion of the C-PCF. A 4-ring PCF and a 5-ring PCF with a dispersion of 0±0.5 ps/nm/km from 1.225 to 1.84?m and from 1.215 to 2.02?m, respectively, are demonstrated. The dispersion slope and the nonlinear coefficient are -1.905×10-5 ps/nm2/km and 13.8 W-1km-1 for the 4-ring PCF,-1.162×10-5 ps/nm2/km and 15.7 W-1km-1 for the 5-ring PCF at 1.55?m wavelength.The optimization procedure is extended to other PCF structures:hexagonal PCFs (H-PCF), square PCFs, octagonal PCFs and 7-hole-missing large-mode-area H-PCFs. Through the procedure that we proposed, it is easy to obtain an ultra-flattened dispersion for these structures. This design procedure is not only simple and universal but also saves lots of computing time and storage space. 2. Highly nonlinear fibers have great application in the wavelength conversion, stimulated Raman scattering etc. We present an all solid index-guiding PCF for highly nonlinearity. The fiber core consists of a Ge-doped area and a F-doped trench-assisted area, while the cladding is composed of F-doped rods. The influence of the concentration of doped germanium, the doping area, the pitch and the size of the cladding holes on the nonlinear coefficient is investigated. Finally, we obtain three PCF structures with highly nonlinearity, low dispersion slope and low confinement loss, with double zero dispersion wavelengths (ZDW), single ZDW and all normal dispersion, respectively. These PCFs surpass the limitation of the dispersion slope of the conventional highly nonlinear fibers and overcomes the shortcoming of the large splice loss of air-hole PCFs with the conventional fibers. They also avoid the large loss and the poor compatibility with the conventional fibers in the multicomponent glasses fibers.3. Many optical active and passive devices are based on the dual-core fibers. To solve the difficulty that it is hard to splice the axial symmetry dual-core fibers in the practical use, a dual-core PCF that has one core in the fiber center is proposed. The linear and nonlinear couplings between the two fiber cores are investigated. The numerical results show the saturation effect of the nonlinear coupling in the dual-core PCF. Then the PCF is applied in the passive mode-locked fiber laser. However, the critical power of the axial symmetry dual-core PCF is too high. Moreover, it is sensitive to the polarization. The influence of the nonlinear coefficient and the coupling length on the critical power is investigated and it is found that the critical power decreases with the increasing of nonlinear coefficient and coupling length. The dual-core PCF with one core in the fiber center is designed. The nonlinear coefficient is improved through the germanium doping in the core area. The influence of the parameters of the pitch, the cladding hole size, the Ge-doped area and the air hole diameter between the two fiber cores on the nonlinear coefficient, coupling length and the dependence on the polarization is investigated. Through the design, a dual-core PCF with polarization independent and significantly reduced critical power is obtained. We also numerically simulate the saturation effect of the nonlinear coupling in this PCF.