Research On Enhanced Nonlinear Effect And Transmission Performance Of Optical Fiber By Exploring Slow Light

The study of fiber optics has been a growing field of telecommunications for several decades. Following the progress in fabrication technology, low loss optical fibers have revolutionized the field of optical fiber communications, and stimulated the advancement of nonlinear fiber optics. A new class of optical structures known as photonic crystals has generated tremendous research interest in recent years. They possess optical properties not present in any naturally occurring material, which can be precisely tailored for specific applications. Of particular interest is also the study of the nonlinear optical properties of photonic crystals, which could lead to a number of important fundamental and technological advances in the field of nonlinear fiber optics including parametric amplification. In order to enhance the nonlinearity of the photonic crystals, phenomena in optical nanostructures exhibiting slow group velocities are recently explored. The ability to slow down and delay optical pulses is an intriguing physical phenomenon with significant applications, such as in telecommunications. However, typical slow-light systems exhibit dispersion, which distorts the pulses on propagation, leading to loss of information, thereby limiting the bandwidth. Finding a scheme to eliminate dispersion is essential for the slow yet very short light pulses that carry the information to transmit in future telecommunications systems.This dissertation studies the important nonlinear process for photonic systems: parametric four-wave mixing and dispersionless transmission slow light, which could lead to a number of important fundamental and technological advances in the field of nonlinear fiber optics. This dissertation is organized as follows:Chapter 1 first introduces various types of fiber nonlinearities and presents fundamental theories of Fiber-optical parametric amplifier (FOPA), focusing on FOPA in a one-pump configuration. The remainder of this chapter introduces basic concepts of photonic crystal. These concepts and theories are necessary for the following research work.Chapter 2 presents a numerical simulation on the WDM transmission system employing fiber-optic parametric amplifier (FOPA) cascades based on one-pump FOPA module including Raman Effect. The end-to-end equalization scheme is applied to optimize the system features in terms of proper output powers and signal-to-noise ratios for all the channels. By comparing the results generated by different lengths of fiber span, we come to the optimal span length to achieve the best transmission performance. Furthermore, a comparison is made among the WDM transmission systems employing different inline amplifiers, namely, FOPA, EDFA and FRA. FOPA demonstrated its advantage over the other two in an ideal system.Chapter 3 designs a multi-defect nonlinear superstructure fiber with double periodicities and studies its third-harmonic generation by using FDTD method. Based on the numerical results and theoretical analysis, we observe an enhancement of third-harmonic intensity over the conventional fiber whose structure is normally uniform along the fiber length.In Chapter 4, the superstructure fiber designed in Chapter 3 is used to serve as the gain medium of optical parametric amplification. The parametric amplification process in the superstructure fiber is modeled under some general approximations and also numerically calculated by using FDTD method. Lastly, a pair of tapers serving as the input/output ports is coupled with the nonlinear fiber to reduce the serious Bragg reflection experienced by slow light.In Chapter 5, another design for the superstructure fiber is presented which would be better suited for fabrication. By adjusting defect cavity width and period, slow light could be found on the defect mode. By introducing nonlinearity, the pulse can travel slowly and also remain undistorted by the formation of a soliton. It is shown in this chapter that the delay can be tuned.Chapter 6 summarizes the results of this work. Future research topics for the superstructure fiber-based optical amplifier pumped by the slow light are proposed. In this dissertation, both theoretical derivation and simulation results have shown that the signal amplification can be enhanced at frequencies where the local field intensities are increased or the slow group velocities of the modes are greatly slowed down on the band edges to offset the negative effect raised by large wave vector mismatching. The signal gain generated by the superstructure fiber of our design is much higher than the ideal conventional fiber and photonic bandgap fiber of the same length. Moreover, its signal gain curve is observed to contain multiple gain peaks, which does not arise with the conventional fiber. That might be of interest in some applications where parts of the spectrum could be used to make filters with gain.