Abstract This thesis is devoted to methods of analyzing optical spectra obtained from turbid liquids, i.e., liquids that are optically very thick and/or scatter light. Data for spectral analysis were obtained with a new, multifunction spectrophotometer developed for industrial liquid samples. One characteristic of the spectrophotometer is that spectral analysis methods can be implemented into the software. Here, the emphasis was on data inversion methods, particularly the Kramers-Kronig analysis and the maximum entropy method, which can be used to gain information on the wavelength-dependent complex refractive index of liquid samples. Relating to such characteristics as density and colour, the complex refractive index also helps to identify the species that form a liquid. The methods were applied to study the internal reflection of light from the prism-liquid interface of the probe and to analyze surface plasmon resonance spectra. This study provided new methods of investigating the optical properties of relatively difficult objects, like offset inks, and of assessing adhesion forces between ink and the substrate system. Another important part of the thesis was the exploration of spectral analysis methods to obtain optical properties of nanoparticles in a liquid matrix. Bounds for the optical properties of multi-component structures in a liquid were considered with the aid of Wiener bounds.

Abstract This thesis describes the application of Low Coherence Interferometry (LCI) and Optical Coherence Tomography (OCT) in paper measurements. The developed measurement system is a combination of a profilometer and a tomographic imaging device, which makes the construction versatile and applicable in several paper measurement applications. The developed system was first used to measure the surface structure of paper. Different grades of paper were selected to provide maximum variation in surface structure. The results show that the developed system is capable of measuring grades of paper from rough base paper to highly coated photo printing paper. To evaluate the developed system in surface characterization, the roughness parameters of five laboratory-made paper samples measured with the developed system and with a commercial optical profilometer were compared. A linear correlation was found with roughness parameters Ra and Rq. Next, the surface quality of paper was evaluated using LCI, a Diffractive Optical Element Based Glossmeter (DOG), and a commercial glossmeter. The results show linear correlation between Ra and gloss measured with the commercial glossmeter. The roughness Ra and averaged gloss measured with the DOG didn't give such a correlation, but a combination of these techniques provided local properties of gloss and surface structure, which can be used to evaluate the local surface properties of paper. In the next study, determination of the filler content of paper using OCT is discussed. The measurement results show clear correspondence of the slope of the averaged logarithmic fringe signal envelope and the filler content. The last studies focus on 2D and 3D imaging of paper using OCT and begin with imaging of a self-made wood fiber network. The visibility of the fibers was clear. Next, several refractive index matcing agents are studied by means of light transmittance and OCT measurements to find the best possible agent for enhancing the imaging depth of OCT in paper. Benzyl alcohol was found to have the best possible combination of optical, evaporation, and sorption characteristics, and it is applied in 2D and 3D visualizations of copy paper.

M.Phil. / Cataracts have been identified as one of the leading causes of blindness, especially in the developing world. The only presently known effective treatment for this growing problem is surgical removal of the opaque lens followed by replacement with an artificial intra ocular lens. Newer methods have brought greater success, and greater costs. For people in the developing world, these newer methods are not always an option. Together with the increased cost, there is a growing demand because of this worldwide problem. This increased need for surgery has lead to the development of waiting lists in the state funded hospitals. To qualify for a cataract extraction in most state funded hospitals, a best visual acuity of 6/60 is required, compared to the 6/12 to 6/24 levels required in the industrial countries and private practices. With these levels of visual impairment in the developing world, many patients are left functionally blind for long periods of time until cataract extraction can be performed. Older methods such as extra-capsular cataract extraction are still being used in the developing world. This is mostly due to the increased density of the cataracts at the time when the extraction can be performed because of the long waiting time leading to further maturation of the cataract. This method requires a large corneal incision, which is normally closed with nylon sutures. With this method of surgery meticulous wound closure is very important, and in many cases surgically induced astigmatism is one of the unwanted consequences. It was therefore decided, for the purpose of this study, to use autokeratometric data to explore the refractive effects of two different methods of corneal wound closure following planned extra-capsular cataract extraction (ECCE). Astigmatism is a major problem associated with extra capsular cataract extraction, especially when the wound is closed by means of sutures. Studies by Minassian et al. (2001), Jacobi (2003) and Dowler et al. (2000) all show that newer methods of cataract extraction making use of smaller incisions and therefore fewer sutures show faster recovery and less astigmatism. These methods are however mostly restricted to private practice, and therefore potentially unsuited for use in developing countries. The type of material used for wound closure is another very important factor. Depending on the method of suturing wound gape and wound compression can cause increased amounts of astigmatism. Using a method of wound closure that would cause less traction on the cornea could therefore cause less of a problem postoperatively. Tissue adhesives such as Tisseel® fibrin glue could be such an alternative. Studies by Henrick et al. (1987), Kim and Kharod (2007) and Bhatia (2006) show that fibrin glue forms a watertight, non irritating wound while promoting the healing process by the cross linking of collagen fibres.

This thesis proposes and tests an individually customizable model of the human crystalline lens. This model will be crucial in developing both research on the human eye and driving diagnostic tools to help plan and treat optical issues, such as those requiring refractive surgery. This thesis attempts to meet two goals: first, it will determine whether this new lens model can reproduce the major aberrations of real human eyes using a computational framework. Second, it will use clinical information to measure how well this model is able to predict post-operation results in refractive surgery, attempting to meet clinical standards of error. The model of the crystalline lens proposed within this thesis is shown to be valid, as it is able to both reproduce individual patient's optical information, and correctly predicts the optical results of a refractive surgery of an individual human eye within clinical standards of error.

Astronomical instruments dedicated to the study of Cosmic Microwave Background polarization are in need of optics with very low systematic effects such as beam shape and cross-polarization in an optical configuration. With the demand for millimetre wave larger focal planes comprising thousands of pixels, these systematic effects have to be minimal across the whole focal surface. In order to reach the instrument requirements such as resolution, cross-polarization and beam ellipticity, new optical configurations with well-understood components have to be studied. Refractive configurations are of great importance amongst the potential candidates. The aim is to bring the required technology to the same level of maturity that has been achieved with well-understood existing ones. This thesis is focused on the study of such optical components for the W-band spectral domain. Using optical modelling with various software packages, combined with the manufacture and accurate experimental characterization of some prototype components, a better understanding of their performance has been reached. To do so, several test set-ups have been developed. Thanks to these new results, full Radio-Frequency refractive systems can be more reliably conceived.

BACKGROUND: The human eye is an incredible organ, capable of focusing and detecting light that passes through its specialized structures. These organs are responsible for the sensation of vision and are crucial to the ability to function and operate during daily life. Loss of the sense of vision can result in serious deficits that greatly reduce the quality of a person’s life. One of the most prevalent causes of vision impairment in the world is refractive error. Although this is a common and relatively simple problem to fix with modern technology, ophthalmologists are continuously searching for new methods of treating people’s eyes to correct refractive errors and improve vision. There are several popular methods for correcting refractive errors, including traditional glasses, contact lenses, and laser vision correction. Laser vision correction is the most modern method for improving visual acuity deficits caused by refractive errors. The two most favored techniques for laser vision correction are laser assisted in site keratomileusis (LASIK) and photorefractive keratectomy (PRK). The performance of these procedures requires a laser treatment input to be calculated based on the patient’s refractive error. These laser treatment inputs are often determined using a nomogram. Nomograms are mathematical functions utilizing a graphical display to calculate a given value based on a set of parameters. Nomograms are crucial for safe and effective LASIK and PRK procedures and there is much interest in enhancing these algorithms to improve the refractive and visual acuity outcomes. OBJECTIVES: The objective of the study was to determine if a newer, more complex laser vision correction nomogram could generate laser treatment inputs better than another more traditional nomogram in terms of refractive and visual acuity outcomes. METHODS: The study included 109 eyes belonging to 59 patients. Fifty-one eyes belonging to 28 patients were operated on using the Wellington nomogram and 58 eyes belonging to 31 patients were operated on using the Internet-based refractive analysis (IBRA) nomogram. Visual acuity and refractive outcomes were recorded at a six-week follow-up evaluation. Data were analyzed using statistical tests to determine significant values. RESULTS: No statistically significant difference was found between the recorded visual acuity and refractive outcomes of eyes operated on using the Wellington and IBRA nomograms. CONCLUSIONS: The two nomograms were found to perform at equal efficacy and to reach the established standards for safety. Although no significant difference was found between the two nomogram outcomes it is possible that there were variables limiting the external validity of the statistical analysis. With extra time, additional cases, and better sample matching the study could be expanded and developed to provide a more reliable and more representative dataset to elucidate conclusions with greater impact. / 2022-06-07T00:00:00Z

Thesis (MPH.) --University of Limpopo, 2015 / Background: Uncorrected refractive errors remain a public health problem among different population and age groups worldwide, including South Africa. Refractive error has serious visual and functional impacts on those affected. In children, refractive errors may negatively affect the academic pursuits and activities of daily living such as reading. Aims and Objectives: To determine and document the prevalence, types and magnitude together with age and gender differences of refractive errors among primary school children in Motherwell Township, Eastern Cape Province, South Africa. Methods: This was a quantitative, cross sectional refractive error study. Four hundred and twenty one (421) school children aged 7 – 14 years were randomly selected from five randomly selected schools in Motherwell Township, Eastern Cape Province, South Africa. Visual examination which included unaided and aided visual acuity (with LogMAR E chart), non-cycloplegic autorefraction, subjective refraction, internal and external examination of the eye using an ophthalmoscope was conducted. Refractive errors were measured with an autorefractor, refined subjectively and findings presented in spherical form. Hyperopia was defined as a spherical equivalent (SE) of +0.50 D or greater, myopia as spherical equivalent of -0.50 D or greater. A cylindrical power of -0.50 DC (D cylinder) or greater was considered as astigmatism. Results: The prevalence of hyperopia, myopia and astigmatism among the children were 25.2%, 18.7% and 58.0% respectively. Hyperopia ranged from +0.50 to +3.62 D and myopia ranged from -0.50 D to -20.25 D in the right eyes with a mean of -0.17 ± 1.7 D. In the left eye, hyperopia ranged from +0.50 to +2.62 D and myopia ranged from -0.50 to -20.62 D with a mean of -0.12 ± 1.7 D. Astigmatism in the right eyes ranged from -0.50 to -5.50 D with a mean of -0.6 ± 0.52 D and in the left eye ranged from -0.50 to -4.00 D with a mean of -0.6 ± 0.45 D respectively. Association between hyperopia and age was not statistically significant (p = 0.839), also refractive error and gender was statistically insignificant (p = 0.120). Against-the-rule (ATRA) astigmatism (43.4%) was more common, followed by with-the-rule (WTRA) astigmatism (39.0%) and oblique, (all other meridians) (17.6%). There was a significant association between types of astigmatism and age (p = 0.05), more so inter-gender difference in the prevalence of different types of astigmatism was not statistically significant (p = 0.774). Conclusion: The study concludes that refractive error has high prevalence of 43.9% in this children population. Astigmatism (58.0%) was more common followed by hyperopia (25.2%) v and myopia (18.7%). Although hyperopia was not age dependent, there was obvious relationship pattern between female genders and hyperopia in the present study. Population-based vision screening or at least school visual screening in the rural communities of Motherwell Township is, therefore recommended. Vision screening and proper eye examination with appropriate optical compensation will improve the activity of daily living and quality of life of those affected. Key words: Refractive error, hyperopia, myopia, astigmatism, school children

Wurtzite III-nitride semiconductor materials have many technically important applications in optical and electronic devices. As GaN-based visible light-emitting diodes (LEDs) and lasers starts to mature, interest in developing UV devices starts to rise. The search for materials with larger bandgaps and high refractive index contrast in the UV range has inspired multiple studies of BN-based materials and their alloys with traditional III-nitrides. Additionally, alloying III-nitrides with boron can reduce their lattice parameters giving a new option for strain engineering and lattice matching. In this work I investigate the refractive indices of BAlN and BGaN over the entire compositional range using hybrid density functional theory (DFT). An interesting non-linear trend of the refractive index curves appears as boron content is increased in the BAlN and BGaN alloys. The results of this calculation were interpolated and plotted in three dimensions for better visualization. This interpolation gives a 3D dataset that can be used in designing a myriad of devices at all binary and ternary alloy compositions in the BAlGaN system. The interpolated surface was used to find an optimum design for a strain-free, high reflection coefficient and high bandwidth DBR. The performance of this DBR was quantitatively evaluated using finite element simulations. I found that the maximum DBR reflectivity with widest bandwidth for our materials occurs at a lattice parameter of 3.113 Å using the generated 3D dataset. I use the corresponding material pair to simulate a DBR at the wavelength 375 nm in the UVA range. A design with 25 pairs was found to have a peak reflectivity of 99.8%. This design has a predicted bandwidth of 26 nm measured at 90% peak performance. The high reflectivity and wide bandwidth of this lattice-matched design are optimal for UVA VCSEL applications. I have assisted in exploring different metalorganic chemical vapor deposition (MOCVD) techniques, continuous growth and pulsed-flow modulation, to grow and characterize BAlN alloys. Samples grown using continuous flow show better optical quality and are characterized using spectroscopic ellipsometry. The refractive index of samples obtained experimentally is significantly below the predicted value using DFT.

The development of reliable, affordable, and efficient sensors is a key step forward in providing tools for efficient monitoring of critical environmental parameters. Fiber-optic sensors are already widely used in various industrial sensing fields. They have proven themselves reliable in harsh environments and can measure different physical quantities, such as temperature, pressure, strain, refractive index (RI), and humidity. Fiber-optic Mach-Zehnder Interferometer (MZI) is a well-studied optical fiber interferometer that has proven capacity for sensing ambient refractive index. In this dissertation, we present Fiber Bragg grating (FBG) embedded in a microfiber Mach-Zehnder Interferometer designed for sensing temperature and refractive index. The MZI is constructed by splicing a short length of 40-μm-diameter microfiber between standard single mode fibers. A one-millimeter-long FBG is then written in the microfiber using a direct, point-by-point, ultrafast laser inscription method. The microfiber MZI shows only moderate sensitivity to ambient refractive index and temperature changes. In contrast, the microfiber FBG is insensitive to ambient refractive index change, while it exhibits typical sensitivity to temperature variation. These distinct characteristics of the FBG and MZI sensors enable the simultaneous measurement of refractive index and temperature as well as temperature compensation in ambient refractive index measurement. Further, we report the use of a fiber-optic Mach-Zehnder Interferometer to measure core refractive index changes written by femtosecond laser irradiation. The core-offset interferometer was constructed by splicing a lightly misaligned stub of standard single-mode fiber between the device’s lead-in and lead-out optical fibers. When the core refractive index of an in-fiber interferometer is altered, that process changes the phase of the core light. Since the phase of light propagating in the cladding (reference arm) remains unchanged, the transmission fringe pattern of the interferometer undergoes a spectral shift. In the present research, that spectral shift was used to quantify the effective core refractive index change in a standard single-mode fiber. In addition, we designed and developed a custom flame-based tapering machine that is used to fabricate miniaturized Mach–Zehnder interferometers (MZIs) using sharply tapered photonic crystal fiber (PCF). This technique produces sensors capable of highly sensitive ambient refractive index (RI) measurements. The sensor is fabricated by fusion splicing a small stub of PCF between standard single-mode fibers with fully collapsed air holes of the PCF in a splicing region. Tiny flame geometry enables the sharp tapering of the PCF, resulting in a short fiber length and high RI sensitivity. It appears that sharp tapering has a great impact on RI sensitivity enhancement, when compared with methods that decrease taper waist diameter. The tapering technique is further used to construct the Mach-Zehnder Interferometer-based fiber-optic refractive index (RI) sensor by uniformly tapering standard single mode fibers (SMF) for RI measurement. The fabricated MZI device does not require any splicing of fibers and shows excellent RI sensitivity. / Graduate

This Master’s dissertation focuses on exploring optical nonlinearities in IIIV semiconductors. This work covers a range of III-V materials and a few devices. To begin with, optical characterization of Aluminium Gallium Arsenide (AlGaAs) waveguides with enhanced nonlinear optical interactions was carried out. We have experimentally demonstrated wide conversion ranges andhigh conversion efficiencies for four-wavemixing inAlGaAswaveguides with three different geometries. In addition to that, both linear and nonlinear losses in each of these geometries were explored. AlGaAs represents only one compound of the large group of III-V semiconductors. To explore the potentials of other semiconductors compounds of this group for nonlinear optics, it is imperative to have information about refractive indices of different III-V compounds. This refractive index information is only available for some binary compounds in isolated spectral windows. In this thesis, we developed a model capable of predicting the values of the refractive indices of binary, ternary and quaternary III-V semiconductor compounds from the values of their band-gap energies. We compared the value predicted by our proposed model with existing experimental data and it was found not only is the predicted values in good agreement with the known values, but also has a lower error margin when compared to previously reported models. Finally, in quest for more suitable material platform for nonlinear photonic integration at different wavelength ranges, a detailed analysis of other potential III-V compounds not previously explored for photonic integration is presented.