Theoretical And Experimental Researches On Photonic Crystal Fibers Nonlinearity And Polarization Demultiplexing Technologies

The main research works of this dissertation are supported by the National Basic Research Program of China (2010CB327605), the National Natural Science Foundation of China (61077049), the China Postdoctoral Science Foundation funded project (20100470259), Program for New Century Excellent Talents in University of China (NCET-08-0736), the Fundamental Research Funds for the Central Universities (BUPT2009RC0401, BUPT2009RC0405, BUPT2009RC0410) and the111Project of China (B07005).Photonic crystal fibers (PCFs) have plenty of unique physical and optical properties and possess dramatic flexibility in microstructures design. PCFs have provided a novel optical waveguide platform for designing advanced photonic devices, and have various applications in the fields of telecommunications, optoelectronics and nonlinear fiber optics. Because of PCFs’controllable nonlinearity properties which are obtained from the microstructures in the fibers, the related researches in designing optoelectronic devices for Dense Wavelength Division Multiplexing (DWDM) system with high degree of integration have become a research hotspot. In this dissertation, the generation and application of supercontinuums (SCs) in PCFs, and the tunable slow and fast light device based on asymmetric dual-core fiber couplers in PCFs are studied theoretically and experimentally. In recent years, as the rapid development of the application of the multimedia service, the optical transport networks are forced to face the pressure of expanding and upgrading the bandwidth and capacity. Polarization multiplexing technologies can improve the spectral efficiency of the telecommunication system under low cost. Therefore, it has become the research hotspot in the field of optical communication. The automatic demultiplexing solution in optical domain using the power of low frequency in electric field domain is researched by theoretical analysis, systematic simulation and experiment. The main contents and achievements are as follows.1. We have experimentally realized compressing the temporal width of the picosecond pulse and broadening the bandwidth of the SCs generated in an all-normal dispersion photonic crystal fiber (ANDi PCF) by using standard single-mode fiber (SSMF) as high-order soliton compressor. The measured SC’s-20dB bandwidth is broadened from84.2nm to277.1nm by using a20m long SSMF compressing1.6ps hyperbolic-Secant shaped (Sech) pulses without enhancing fiber input power.2. An analytical expression for calculating proper fiber input power to generate the narrowest pulse temporal width in a SSMF and the broadest SC’s bandwidth in an ANDi PCF is derived from an empirical formula which depicts the pulse optimal compression length. The numerical results show that the deviation of the optimal compression position becomes large as the fiber length increases. In order to limit the error of the calculated optimal compression length within one meter, the power level and fiber parameters used in the formula should make the soliton order in SSMF is larger than two.3. The effects of the fiber parameters and pump conditions on the spectral flatness of the SCs generated in the ANDi PCF have been investigated theoretically. We have proved that in an ANDi PCF, the self-phase modulation (SPM) determines the generated SC’s bandwidth and the four-wave mixing (FWM) process combining with dispersion dominantly affects the flatness of the spectral profile. The dispersion coefficients β2and β4influence the FWM conversion efficiency and the β2, plays the major role in the SC flatly broadening. The dispersion coefficient β3determines the symmetry of the output SC’s spectral profile and pulse temporal profile. The numerical results show that the flatness of the generated SC could be improved by increasing the pulse original positive chirp, while the improvements are restricted by the fiber length.4. In order to improve the spectral flatness of the generated SC by enhancing the FWM process efficiency during the SC’s generation, a double-pass Littman-Metcalf optical bandwidth filter (OBPF) has been demonstrated originally to make pulses obtain steepened temporal edges. The pump with1.60ps Sech pulses is used in the experiment and the results show that the pulse steepest temporal profile is achieved by using the Littman-Metcalf OBPF with3.5nm bandwidth. Compared with the SC generated from the original1.60ps Sech pulses, the flattest SC is obtained by injecting the4nm filtered pulses into the ANDi PCF and the flatness of the generated SC has been improved by0.22dB. The numerical results show that as the pulse’s FWHM increases from1.0ps to6.0ps, the filter bandwidth corresponding to the pulse steepest temporal profile decreases from4.5nm to2.0nm. We have found that the filter bandwidth for generating the flattest SC is about0.5nm larger than the bandwidth for generating the steepest pulse. This feature can be applied to Sech pulse ranges from1.0ps to6.0ps. Without altering other experimental conditions, the generated SC’s bandwidth within12nm～85nm can be continuously changed by adjusting the Littman-Metcalf OBPF bandwidth from1nm to7nm.5. An asymmetric dual-core fiber coupler is designed based on PCFs. The left core of this PCF coupler is constructed by doping silicon on the inner surface of one air hole to form a ring structure, and the right core is created by omitting seven air holes to serve as a silica core. The mechanisms of the modes coupling between the two nonidentical cores of the asymmetric dual-core PCF coupler are investigated theoretically. The effects of the coupler structural parameters on the effective refractive index, group velocity dispersion (GVD) and the pulse propagation properties of the individual guided modes and the supermodes are analyzed numerically. The coupler parameters of the linearly coupling among the two individual modes at1550nm are obtained.6. The numerical results show that the all-optical control of the pulse’s velocity can be realized under nonlinear conditions. The generation of time delay (slow light) and advance (fast light) of2.0ps for soliton pulse with2.0ps temporal width can be realized in the asymmetric dual-core PCF coupler within1cm length. The numerical results show that the "fast light" phenomenon of the nonlinear coupler will not significantly alter the pulse shape and the10.0ps advance for soliton pulse can be obtained within5cm length without obvious distortion. On the contrary, the pulse shape is sensitive to the PCF coupler’s "slow light" phenomenon. As a result of the chirps generated form the SPM and normal dispersion, the pulse temporal profile will be broadened and steepened after the2.0ps time delay.7. An automatic polarization demultiplexer based on direct detection technology in optical domain has been designed and implemented. The polarization demultiplexing solution in optical domain using the low frequency power in electric field domain for system with NRZ-OOK and RZ-OOK modulation formats have been investigated theoretically. The feasibility of this method has been verified by systematic simulation and experiment.