| In this paper, we focus on two types of optical devices for all optical network: optical switch and slow light. The whole paper is divided into two parts. First,we design and analyze all-optical switches based on fiber parametric devices, which enhance the switching scalability compared to the traditional devices in literatures. Second, we propose an asymmetric photonic crystal (PC) waveguide used for slow light transmission, which is characterized by both low group velocity and reduced dispersion.This dissertation is organized as follows:Chapter 1 introduces the development of optical switching system and its key role in present high-speed communication networks. We employ fiber parametric amplifiers (FPA) to realize efficient all-optical switching. We utilize four-sideband model to analyze three four-wave mixing processes in dual-pump FPAs. We take into account polarization effects to eliminate the crosstalk issues between input signals. Then, we briefly describe the definition, theoretical basis, classification and analyzing methods of photonic crystals. Last, we introduce slow-light techniques, especially photonic-crystal approach, and the unresolved problems in present slow-mode research. In Chapter 2, we propose a novel 2x2 wavelength-convertible optical switch based on dual-pump fiber parametric devices: one is driven by linearly parallel pumps, and the other one by perpendicular pumps. Theoretical analysis is made on the polarization effects on the switching performance of the two devices. The result predicts that two incident signals which are positioned symmetrically with respect to one pump can be switched independently, with judicious combinations of the relative pump-signal polarization states. Simulation results show that the scheme can achieve crosstalk-free packet switching with acceptable extinction ratios for both signals. Besides, future applications of this novel scheme in high-speed switching nodes are discussed.In Chapter 3, an asymmetric photonic crystal (PC) waveguide is proposed for slow light transmission. A row of air holes is removed to form a line-defect waveguide, and the lateral symmetry of the waveguide is broken by shifting the holes in the bottom PC cladding along the waveguide axis. Two structural parameters are carefully adjusted: the amount of shift compared with the array of holes in the top cladding, and the radius of the holes closest to the waveguide core in the shifted PC cladding. In the asymmetric waveguide, it is possible to obtain flat band modes with low group velocity (c/50) and low dispersion (on the order of 104 ps2/km) over a signal bandwidth of 10GHz. The delay-bandwidth product (DBP) of the proposed slow-light device is analyzed and compared with the DBP of the PC waveguides reported in literatures. We find that our structure yields a significant increase in DBP, and improves the effective bandwidth in which we can obtain slow modes with both low group index and vanishing dispersion. Then in Chapter 4, we consider reducing the group velocity of slow mode as our main optimizing goal. In this case, we adjust another two structural parameters, including the radius of the basic air holes and the width of the line defect. We flatten the slow-mode dispersion curve, exclude the extreme points from the flat band, and avoid the unacceptably large dispersion induced by extreme points. We obtain an ultra-flat band with an inflection point corresponding to a slow-light mode with reduced distortion. We achieve two waveguides with different widths: W1.00, and W1.05. The former can generate ultra-low group velocity, which remains well below 0.0045c in normalized frequency width 0.00018 (2πc/a). The latter provides wider transmission window 0.0006 (2πc/a) and reduced dispersion, at the cost of increased group velocity 0.0038c~0.012c. We can select different waveguides to satisfy specific demands. We utilize the finite difference time domain method (FDTD) to verify the inflection-point ultra-slow wave. |
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