Since the introduction of optical fibers in 1960's in communication systems, researchers have encountered many challenges to improve the signal quality at the receiver as well as transmitting the signal as distant as possible. The former was achieved by employing coherent receivers, which let us use M-array modulation formats, such as QPSK, or QAM, and polarization of the signal. The later is accomplished by the advent of optical amplifiers. Optical amplifiers enable us to compensate for the loss occurred within the fiber optic line, without the need for optical-electrical signal conversion. These amplifiers add noise to the line which interacts with the nonlinearity in the fiber line. This interaction causes phase change in the propagating signal called nonlinear phase noise, which degrades the system performance. In this study we will derive an analytical expression for the linear and nonlinear phase noise variance in dispersion unmanaged fiber optic systems, using a first-order perturbation theory. We use numerical examples to depict the proposed system performance in terms of nonlinear phase noise variance. We will conclude that the nonlinear phase variance in a dispersion unmanaged system is much lower than the corresponding noise variance in a dispersion managed system. We will use this concept and will introduce more dispersion in the line by adding fiber brag gratings (FBGs) throughout the fiber link. Through numerical simulations, we will illustrate the improvement we get by adding FBG in each span. We will show that employing FBG improves the system performance for systems working at symbol rates 5 GBaud, which we get the best improvement to less than 20 GBaud, and beyond 20 GBaud there will be no improvement. Nowadays, telecommunication systems based on fiber optics are working at symbol rates around 28 GBaud. We will introduce new models to reduce the nonlinear phase, by splitting digital back propagation (DBP) between transmitter and receiver, and using optical phase conjugation (OPC) in the line. We will prove that the new proposed models lower the phase noise variance significantly, for single pulses. We will also illustrate numerical examples to validate the betterment they provide in terms of Q-factor. / Thesis / Master of Applied Science (MASc)

Microstructure and photonic crystal fibers with periodic as well as random refractive-index distributions are investigated. Two cases corresponding to fibers with one-dimensional (1D) radial index distributions and two-dimensional (2D) transverse index distributions are considered. For 1D geometries with an arbitrary number of cladding layers, exact analytical solutions of guided modes are obtained using a matrix approach. In this part, for random index distributions, the average transmission properties are calculated and the influence of glass/air ratio on these properties is assessed. Important transmission properties of the fundamental mode, including normalized propagation constant, chromatic dispersion, field distributions, and effective area, are evaluated. For 2D geometries, the numerical techniques, FDTD (Finite-Difference Time-Domain) method and FDM (Finite Difference Method), are utilized. First, structures with periodic index distributions are examined. The investigation is then extended to microstructure optical fibers with random index distributions. Design of 2D microstructure fibers with random air-hole distributions is undertaken with the aim of achieving single-mode guiding property and small effective area. The former is a unique feature of the holey fiber with periodic air-hole arrangement and the latter is a suitable property for nonlinear fiber devices. Measurements of holey fibers with random air-hole distributions constitute an important experimental task of this research. Using a section of a holey fiber fabricated in the draw tower facility at Virginia Tech, measurements of transmission spectra and fiber attenuation are performed. Also, test results for far-field pattern measurements are presented. Another objective of this dissertation is to explore new applications for holey fibers with random or periodic hole distributions. In the course of measuring the holey fibers, it was noticed that robust temperature-insensitive pressure sensors can be made with these fibers. This offers an opportunity for new low-cost and reliable pressure fiber-optic sensors. Incorporating gratings into holey fibers in conjunction with the possibility of dynamic tuning offers desirable characteristics with potential applications in communications and sensing. Injecting gases or liquids in holey fibers with gratings changes their transmission characteristics. These changes may be exploited in designing tunable optical filters for communication applications or making gas/liquid sensor devices. / Ph. D.

Coding is an essential technology for efficient fiber-optic communications and secure quantum communications. In particular, low-density parity-check (LDPC) coding is favoured due to its strong error correction capability and high-throughput implementation feasibility. In fiber-optic communications, it has been realized that advanced high-order modulation formats and soft-decision forward error correction (FEC) such as LDPC codes are the key technologies for the next-generation high-speed optical communications. Therefore, energy-efficient LDPC coding in combination with advanced modulation formats is an important topic that needs to be studied for fiber-optic communications. In secure quantum communications, large-alphabet quantum key distribution (QKD) is becoming attractive recently due to its potential in improving the efficiency of key exchange. To recover the carried information bits, efficient information reconciliation is desirable, for which the use of LDPC coding is essential. In this dissertation, we first explore different efficient LDPC coding schemes for optical transmission of polarization-division multiplexed quadrature-amplitude modulation (QAM) signals. We show that high energy efficiency can be achieved without incurring extra overhead and complexity. We then study the transmission performance of LDPC-coded turbo equalization for QAM signals in a realistic fiber link as well as that of pragmatic turbo equalizers. Further, leveraging the polarization freedom of light, we expand the signal constellation into a four-dimensional (4D) space and evaluate the performance of LDPC-coded 4D signals in terms of transmission reach. Lastly, we study the security of a proposed weak-coherent-state large-alphabet QKD protocol and investigate the information reconciliation efficiency based on LDPC coding.