Design And Fabrication Of Novel Broadband Photonic Crystal Fibers

With development of the current networks towards ultra high bit rate and large capacity all-optical network, it is urgent to improve the performance of key optical devices to support the future all-optical network. The study of new materials and novel structures offer an effective ways to achieve high-speed, low cost and low-power broadband optical devices.This dissertation is supported by the State Key Development Program for Basic Research of China "Theory and key technology of fiber optic devices for optical switching network" and the National Natural Science Foundation of China "A series of ultra-broadband dual-core photonic crystal fiber and its related devices". It is mainly focus on the theoretical and experimental investigations on the design and fabrication of the broadband polarization-independent dual-core photonic crystal fiber (PCF) and single-polarization single-mode PCF due to the flexibility structure design and controllable features and other outstanding features of PCFs. The main achievements of the dissertation are listed below:1. A combining model for rapid analysis and monitoring the fiber properties during the PCF fabrication process is proposed by using the digital image processing technology and full-vector finite element method. This model can be used not only to analyze the properties of PCF samples in the fabrication process but also to reduce efficiently the impact of the typical fabrication defects on the fiber characteristics and deviations of the actual properties from the design properties. The correctness of this model is verified by analysis results of two kinds of commercial PCFs. We use this method to monitor the fabrication process of the actual PCF in real time. Experimental results demonstrate that this model can improve the fabrication efficiency and quality of PCFs.2. Based on analysis of the dependence of the fiber structure parameters on the fiber broadband characteristics and polarization-independent characteristics, an optimized broadband polarization-independent dual-core PCF is proposed. The mode field area of the proposed PCF can match with that of and the single mode fiber. The distance of the two cores are far from each other to easily design fiber coupler due to adopting a independent center modulation region. Two fiber directional couplers with coupling ratio of50:50and90:10are designed, respectively, which cover the all optical communication band over the wavelength range of1.225-1.675μm. The achieved coupling ratio error is less than1%and the difference of the coupling ratio between the two polarizations is less than0.5%for50:50coupler. For90:10coupler, the coupling ratio of error is less than0.2%and the difference of the coupling ratio between the two polarizations is less than0.1%.3. A novel broadband polarization-independent dual-core PCF is proposed from the view of the fabrication feasibility and application. The small air holes arranged in the linear lattice are adopted instead of the elliptical air hole in the central modulation region to improve the fiber fabrication feasibili-ty. By optimizing the fiber structure parameters, the wavelength-independent and polarization-independent characteristics can be implemented in the wave-length range of1.26-1.625μm. Two fiber directional couplers with coupling ratio of50:50and90:10are designed, respectively, covering the all optical communication band. The splitting ratio errors of50:50and90:10coupler are less than1%and0.2%, respectively and the two polarization states error are both less than0.2%. It is confirmed from the fabrication experiment that the novel structure with the array of small holes is easier to fabricate than that with elliptical air hole.4. Based on the coupling theory, a novel single-polarization single-mode PCF is proposed. The fiber can realized the single-polarization single-mode property in the wavelength range from0.8μm tol.8μm, covering the operating band-width of1000nm, The confinement loss is less than ldB/km in the low-loss core region and up to100dB/km in the high-loss core region. Mode field distribution has the Gaussian profile that low splice loss of0.0079dB can be realized between this fiber and the SMF at the wavelength of1.55μm.5. Based on the mode cutoff method, a novel square lattice single-polarization single-mode PCF is proposed and fabricated, which realized the single-mode operation in the wavelength range from1.3μm to1.7μm over all-optical com-munication band wavelength range. The confinement loss is about the0.01d-B/km for the low-loss polarization state and up to100dB/km for the high-loss polarization state. Through modeling the properties of the actual PCF sam- ple, the effective modal field area is about91/μm2at the wavelength of1.55μm and the splice loss with SMF is only0.0051dB. The results show that the novel square lattice single-polarization single-mode PCF has the advantage of easy splice with SMF over other PCFs which have been reported to realize single-polarization single-mode with mode cutoff method.