Polarization mode dispersion (PMD) causes significant impairment in high bit-rate optical telecommunications systems. Knowledge of a fibre’s PMD mean value, and the relevant confidence interval, is essential for determining a fibre’s maximum allowable bit-rate. Various methods of confidence interval construction for time series data were tested in this dissertation using simulation. These included the autocovariance-matrix methods as suggested by Box and Jenkins, as well as the more practical and simpler batch means methods. Some of these methods were shown to be significantly better than the standard method of calculating confidence intervals for non time series data. The best of the tested methods were used on actual PMD data. The effect of using polarization scramblers was also tested.

In this study, two polarization effects, namely the state of polarization (SOP) and polarization mode dispersion (PMD) in optical fibre cable are investigated. The change in polarization effects introduces errors in optical fibre communication system. We find that the SOP drifts slowly in buried cables, and rapidly in aerial cables. This is because buried cables are located in a static environment, whereas aerial cables are exposed directly to a dynamic environment. The SOP change in aerial cable shows significant correlation with its environment (the global radiation, temperature and wind). The autocorrelation function (ACF) was not performed in buried cable, since they do not satisfy the ACF assumption, whereas in aerial cable it is found that the ACF of the SOP decorrelates quite quickly during the day. The 50 percent decorrelation time during the day and night are 9.6 and 30.4 minutes, respectively. During the day the properties of the optical fibre change rapidly as a result of the rapidly changing environmental conditions, whereas at night the environmental conditions change relatively slowly. Fast Fourier Transform (FFT) of the SOP fluctuations show discrete peaks, which corresponds to the wind induced vibrational frequency of the cable. The PMD fluctuations for undeployed and deployed aerial optical fibre cable are monitored using the generalized interferometric technique (GINTY). It is found that the PMD measured with polarization scrambling is more scattered but more reliable than the PMD measured without polarization scrambling. This is because the PMD obtained with polarization scrambling is averaged over different input and output (I/O) SOP pairs. For deployed aerial cable, it is found that the PMD measured without polarization scrambling fluctuates rapidly during high wind speed conditions. Furthermore, there is a correlation between the measured PMD and the change in temperature. It is found that the change in temperature has a stronger influence than the wind on the PMD of the optical fibre link.

The work presented in this dissertation is a comparative study of polarization mode dispersion (PMD) measurement performance where the fixed analyzer (FA) technique was built and tested for the first time in South Africa. Techniques involved in the study are: the Jones matrix eigenanalysis (JME), generalised interferometric technique (GINTY) and the FA technique, with a particular focus on the FA technique. The FA PMD measurement technique determines the average differential group delay (DGD) from the transmitted intensity spectrum through a polarizer and has three analysis methods (extrema counting, mean level crossing and Fourier analysis) which were all evaluated. PMD measurements were performed in the laboratory on several different fibre types and in the field on buried deployed Telkom fibre links (28.8 km). The techniques showed good agreement in the measured PMD value, both in the laboratory and field measurements. In particular very good agreement was found between the JME average DGD and the extrema counting analysis PMD value. The GINTY and FA Fourier analysis method also gave very similar PMD values. It was found that the fibre birefringence and the mode coupling manifest in different ways on the intensity spectrum. By using the FA ratio method, the length regimes of the different fibre types were determined. Three characteristics of the FA technique were investigated, namely: wavelength window variation, sampling and input SOP scrambling. It was found that the wavelength window and the PMD are inversely proportional. Correct sampling plays a significant role in determining the correct measured PMD value. Lastly an average PMD value over the PMD values for different input SOPs serves as a better representation of the true PMD value. An additional study showed that the FA technique and a developed Poincaré sphere analysis method agree very well regarding the PMD value.

In this study, the generalized and traditional interferometric techniques (GINTY and TINTY) are used to investigate some of the important aspects of Polarization Mode Dispersion (PMD) phenomenon in optical fibres. Measurements of PMD and chromatic dispersion (CD) are performed on buried optical fibre cable in the Pretoria Telkom’s metropolitan network. The upgradeability of this network was investigated, and it was observed that just over a half of the fibres are upgradeable to 10 Gb.s-1 transmissions. Long-haul aerial network in the Northern Cape was also tested for upgradeability to 10 Gb.s-1. It was found that 41 percent of the fibres tested are upgradeable to 10 Gb.s-1. Long-term monitoring showed that PMD varies rapidly in aerial fibres and, on the other hand, it is relatively stable in buried cables. Investigations on the accuracy of the techniques showed that polarization scrambling is essential for the reduction of the measurements uncertainties. Furthermore, it was observed that TINTY underestimates the single scan PMD distributions. The study on the effect of the change in mode coupling on various fibre configurations was performed, and fibres showed a reduction in PMD after the introduction of mode coupling. Measurements of PMD conducted in the laboratory on cabled fibre with low PMD showed the floor sensitivity of TINTY. Comparison between GINTY and Jones Matrix Eigeanalysis (JME) PMD measurements methods were performed on an emulator, and the results showed a good agreement in the measured PMD.

D.Ing. (Electrical & Electronic Engineering) / Please refer to full text to view abstract

M.Ing. / In recent years, many research and development projects have focused on the study of fibre Bragg gratings. Fibre Bragg gratings have been used in the field of sensors, lasers and communications systems. Commercial products that use fibre Bragg gratings are available. On the other hand, in the field of software development, object-oriented programming techniques are also becoming very popular and powerful. The focus of this work is on solving fibre Bragg grating problems by a simulation program with object-oriented programming techniques. For fibre Bragg grating problems, widely used theories and numerical methods such as the coupled-mode theory and the transfer matrix method will be applied in the analysis, modelling and simulation. The coupled-mode theory is a suitable tool for analysis and for obtaining quantitative information about the spectrum of a fibre Bragg grating. The transfer matrix can be used to solve non-uniform fibre Bragg gratings. Two coupled-mode equations can be obtained and simplified by using the weak waveguide approximation. The spectrum characteristics can be obtained by solving these coupled-mode equations. The optical numerical libraries of fibre Bragg gratings have been built by using object-oriented techniques. The code was realized by C++ and Object Pascal language in the Delphi4, C++ Builder4 and Visual C++6 environment. The compiled binary files and the code of the simulation program are available for both the end user and program developer. This simulation program can be used to analyze the performance of sensors and communication systems that use fibre Bragg gratings. Uniform, chirped, apodized, discrete phase shifted and sampled Bragg gratings have already been simulated by using the direct numerical integration method and the transfer matrix method. The reflected and transmitted spectra, time delay and dispersion of fibre Bragg gratings can be obtained by using this simulation program. At the same time, the maximum reflectivity, 3dB-bandwidth and centre wavelength can also be obtained. This thesis consists of three parts. The first part introduces a suitable theory and modelling that have been used to analyze the characteristics of fibre Bragg gratings. Secondly, the codes of the modelling are realized by the suitable programming languages in different development environments. Finally, this simulation program is utilized to analyse real physical problems with fibre Bragg grating applications.

Terahertz spectroscopy shows promise in applications including quality control, security and medical imaging, but remains limited by slow data acquisition. This also poses an impediment to the study of samples undergoing irreversible transitions, as typical schemes rely on the assumption that results are consistent from pulse to pulse. In this work, we propose a high-speed terahertz detection technique based on chirped-pulse encoding that can enable single-shot measurements up to laser repetition rates in the MHz. An all-normal dispersion optical fiber is used to create a broadband probe spectrum, onto which the terahertz pulse waveform is encoded as a phase modulation. The sampling process makes use of the dispersive Fourier transform, a technique which maps the spectral features of a pulse into the time domain, in this case via the dispersion of a long commercial optical fiber. The elongated pulse can subsequently be detected with a high-speed photodetector and oscilloscope. We show steps toward implementing the technique by characterizing the components required to shape the probe pulse and providing some proof-of-concept measurements. In addition, fiber optic simulation procedures are detailed including complete coupled generalized nonlinear Schrödinger equations to provide insight into polarization effects that occur during highly nonlinear processes such as supercontinuum generation.

Today the world of fiber optics is bigger than ever. What is available to the engineer and the technician is now only apparent to the well informed. Not all engineers and technicians have a background or an education in the field of fiber optics. This report is intended to be an informative preliminary guide to those readers whose interest lies in the field of fiber optics. The advantages of fiber optic systems are discussed and compared to conventional systems. Although a fiber optic system includes transmitters and receivers, this report concentrates primarily on fiber optic connectors and cables. It also includes a look at the characteristics of the fibers themselves. The basis for this report is information obtained from a survey taken from a select number of manufacturers of fiber optic connectors and fiber optic cables. Available fiber optic connectors range from reusable to permanently installed, cheap to expensive, plastic to stainless steel. Some require special tools and preparation including the use of epoxy, whereas others can be used in the field quickly without any special tools or epoxy. Since many connectors, however, require epoxy, this report introduces six different types of epoxies, showing their characteristics and listing their applications. The fiber cable has become a widely appreciated medium for data and information transmission systems. The consumer has many choices from one or two fibers per cable to a bundle fiber cable. The cables have many designs which lead to various applications. These cable designs and their characteristics are summarized and discussed. There are many other optical devices that are available and on the market. Among these devices were not covered in this report, the coupling from one fiber to another (attached side by side) was investigated theoretically. The case where internal reflection in the fiber’s core created an evanescent field in the cladding (this is when the angle of incidence is greater than the critical angle) was emphasized. This shows that the power in the evanescent field is imaginary (reactive), and the angle of refraction is also imaginary. However, when the evanescent field extends on out to the second fiber’s cladding, and continues until it reaches and enters the second fiber’s core, power then exists; thus, optical power was transferred by way of an evanescent field.

This thesis discusses the dispersion of optical pulses in fiber waveguides. Procedures for determining the transfer function of an optical fiber using either time domain measurements or frequency domain measurements are given. A test set is described which uses time domain measurements for dispersion characterization. Complex transfer function data are presented for various fiber lengths as well as optical connectors. Prediction of bandwidths of long fibers from bandwidth/measurements taken on shorter length fibers is discussed. A relation between the bandwidth of cascaded connectors in terms of the bandwidth of a single connector is obtained.

Coupling efficiency an alignment sensitivity models for the coupling of light from a fundamental mode Gaussian light beam into a single mode optical fiber are given. Experimental results are presented which verify the coupling and misalignment predictions of the models. Data were taken at two wavelengths: 0.6328 and 0.8460 micrometers. The light sources were a HeNe laser and a GaAlAs laser respectively. Coupling efficiencies were measured versus beam spot size; and lateral, axial and angular misalignments of the beam and fiber axes.