M.Ing. / Please refer to full text to view abstract

Excess fluoride in groundwater used for drinking can pose serious health hazards, especially in poor, rural areas of the developing world lacking water treatment. The World Health Organization recommends a maximum fluoride contaminant level of 1.5 mg/L in drinking water [1]. Over 200 million people in low- and middle-income countries currently drink groundwater over that limit [2]. Current field detection of fluoride typically uses HACH kits, with several groups developing smartphone based alternatives [3]. These methods are based on colorimetry. The HACH kit is limiting because appropriate training is required, results are sensitive to competing ion contamination and chlorine, the glassware must be clean, and repetition is needed to ensure reliability [4]. The use of a smartphone for in-field detection of fluoride is promising and takes a strong step towards quick, easy, reliable, and portable fluoride detection. Our research takes the concept of a portable device one step further by using a fundamentally different, and simpler, mode of detection. We have demonstrated the use of optical fibers as an alternative, non-colourimetric fluoride detection method. The tip of a single mode optical fiber is coated with a thin film of Al and is immersed in an aqueous fluoride solution. The reaction between fluoride and the Al coating changes internal reflection proportional to fluoride concentration which is measured by a photodetector as an output voltage. We made great steps in optimizing the methods, materials, and code required for this sensor. Additionally, we built a device to allow approximate standardization of Al thickness as a function of the distance from the target and time of sputtering. We established the best practical thickness of Al coating, improved repeatability between sputter deposition events, and implemented an optical switch into the experimental set-up. / Graduate / 2021-07-28

The polarization-maintaining properties of single crystal sapphire fibers are investigated and a polarimetric optical sensor for high temperature measurements is designed. The polarization-maintaining properties of single crystal sapphire fibers are investigated experimentally for different modal power distributions and different fiber lengths. Experimental results indicate that linearly polarized light launched along one of the principal axes of the birefringence can be partially maintained. The polarization-maintaining ability (PMA) has been measured to be 6 dB and 3 dB for 7 cm and 32 cm long sapphire fibers, respectively. The temperature coefficient of the differential phase delay between the orthogonal polarization modes has been measured to be 0.0208 rad / °C m. A resolution of 2 °C has been obtained over the measurement range of 25 °C to 800 °C. It is also observed that the PMA is strongly dependent on the mode-coupling parameter. The design of the polarimetric sapphire fiber sensor for high temperature measurements is based on the properties of withstanding high temperatures, polarization preservation and temperature dependence of phase delay. Since the temperature dependence of phase delay is proportional to the fiber length, consideration of a similar strain sensor is expected in the near future. / Master of Science

Fiber optic sensors offer many advantages over conventional sensors, including; small size, low weight, high strength and durability. Standard silica optical fibers are limited by the material properties of silica. Temperatures above 700°C and other harsh environments are incompatible with standard optical fiber sensors. Sapphire fiber sensors offer another option for fiber optic sensing. Sapphire fibers are limited by the material properties of sapphire, which include high melting point, extreme hardness and imperviousness to chemical reactions and radiation. These properties coupled with the advantages of conventional fiber optic sensing make sapphire optical fiber sensors a good candidate for sensing requirements in harsh environments. We investigate the potential for the use of sapphire fibers as sensors. Two sensors are developed based on widely different techniques. Results from preliminary tests of each are given. / Master of Science

The polarimetric behavior of conventional dual mode, elliptical-core, optical fiber sensors is investigated. To compliment the experimental results, a novel linearly polarized modal analysis of elliptical-core geometries is performed. The method numerically finds solutions of a waveguide equation described by Mathieu differential equations. To move beyond conventional dual mode fiber sensing applications, selected placement of the dual mode fiber endpoints upon a flexible structure is investigated. Modal filtering behavior will result if the endpoints are chosen to span two antinodes of the underlying structure. To truly achieve signal processing capabilities, spatially weighting the sensitivity of the sensor becomes necessary. Experimental results are described and future applications are proposed. / Master of Science

A survey of research in nonlinear optical fibers is given. Important background concepts are introduced and explained. Present and future applications of nonlinear optical fibers arc reviewed. A mathematical model of a nonlinear optical fiber is developed using a coupled-mode theory approach, and methods of solving nonlinear partial differential equations arc discussed. A detailed history of research in the field is given, and recommendations for future research are made. / Master of Science

A novel coupler measurement station and technique for manufacturing fused biconical tapered multi-port multimode couplers with improved uniformity among ports is presented. Improvement in the uniformity of the couplers is achieved by diffusion of the minimum taper region of the coupler. The phenomenon of dopant transport from the core to the cladding (or from the cladding to the core) at high temperatures is known as diffusion. Diffusion of a germanium-doped core results in the germanium dopants migrating from the core into the cladding thus increasing the effective diameter of the core which is accompanied by a decrease in the refractive index. The cores of individual fibers in the minimum taper region are a few micrometers apart. Diffusion thus results in a minimum taper region that has approximately uniform refractive index leading to better uniformity among ports. The experimental setup and results of diffusion tests on multimode fiber with different core/cladding ratios are presented. A coupler station that is capable of making bi- directional measurements of the coupler is demonstrated successfully. The coupler station also enables a user to dynamically monitor the ports of the coupler during manufacture, and can be adapted to manufacture star couplers ranging in size from two to sixteen fibers. / Master of Science

Microbends are axial distortions on the optical fiber that have a spatial wavelength small enough to effect coupling between guided and radiation or cladding modes. The magnitude of this wavelength-dependent coupling is a function of the nature and the number of microdefonnations. Since these periodic perturbations lead to an attenuation in signal level, they are avoided in fiber-based communication systems. However, controlled induction and signal processing of microbending losses has led to the fabrication of novel optical fiber~based sensors, devices, and components. A systematic study of microbending effects in singlemode optical fibers is presented in this thesis. The theoretical analysis is based on the coupling between the fundamental LP₀₁ mode to discrete cladding modes. An algorithm is developed to characterize optical attenuation as a function of the spatial period of the microbend defonnation. Optical attenuation peaks are described in terms of central wavelength, amplitude and spectral width. An excellent correlation is shown between the experimental results and the theoretical predictions, with nominal errors less than 2.5%. The algorithm developed may be used with any commercially available singlemode fiber, and any kind of microbend de former apparatus, provided the microbend defonnation function â ±(z) is known accurately. Based on the above analysis, a wavelength-tunable fiber polarizer is proposed and demonstrated. The polarizer is fabricated by inducing a periodic perturbation on a high birefringence singlemode optical fiber. The fiber thus exhibits polarization· selective attenuation characteristics. The operating wavelength is shown to be tunable by changing the spatial period of the defonnation. A polarization extinction ratio of 25 dB is obtained with an attenuation of 1.3 dB, at an operating wave length of 1177 nm. / Master of Science

Many commercial and laboratory applications of fiber optics require an ability to manipulate, change and control the state of polarization of light. This is usually accomplished with bulk-optic wave plates which are inherently narrowband, bulky, and often require careful manual tuning and adjustments. H.C. Huang has recently proposed that a variably spun birefringent fiber with spin rate slowly varying from zero to very fast (or vice versa) will transform the state of polarization (SOP) from linear to circular (and vice versa). The most remarkable feature of a Huang fiber is that the transformation from linear to circular polarization is independent of wavelength over entire single mode range of the fiber. In this thesis, using simulations and experiments we explore the properties of such a fiber element and investigate its similarities and differences as compared to a bulk-optic quarter-wave plate. In the simulations, we modeled the Huang fiber as a stack of birefringent waveplates and used this model verify the theoretically predicted polarization transformation behavior and the wideband nature of the Huang fiber. We analyzed the dependence of the polarization transformation by this device on various structural parameters and showed that while the fiber has loose tolerances with respect to the fiber length and the spin variation, it has a strong dependence on the maximum spin rate at the high-spun end. We investigated the PMD characteristics of the Huang fiber for short pulse applications. Using simulations we also verified that two such quarter wave-transforming fibers can be appropriately cascaded to obtain half-wave and full-wave transformation, analogous to bulk optics half wave and full wave plate respectively. In the experiments we studied the polarization transformation behavior of a Huang fiber sample when it is excited by different input states of polarization both from the un-spun end and the high-spun end. We found that the results from the experiments strongly support the simulation results. The experiment was performed at 1310 nm and 1550 nm to verify the wideband nature of the Huang fiber sample. We found that the Huang fiber indeed performs the prescribed state of polarization transformation over a wide band. / Master of Science

A time domain chromatic dispersion measurement system for single-mode optical fibers is presented for easy measurements in both the laboratory and the field. This technique uses a relative group delay of a band of light at slightly different wavelengths from a single pulsed laser diode to determine chromatic dispersion and the zero chromatic dispersion (ZCD) wavelength of the fiber. Time domain dispersion measurements find the group delay directly, eliminating the need for a vector voltmeter and the added phase calibration and group delay calculations needed in most frequency (phase) domain techniques. With the use of a monochromator as a bandpass filter, a number of group delay data points can be taken throughout the spectral width of a single laser diode. Since the group delay data are relative measurements, access is needed only to the output of a length of fiber. This makes this time domain technique especially well suited for field use. Using the modified Sellmeier equation, only three group delay measurements need to be taken to find the zero chromatic dispersion (ZCD) wavelength and a highly accurate approximation to the chromatic dispersion curve. The modified Sellmeier equation coefficients are found by a simple BASIC program, eliminating the need for curve-fitting and numerical differentiation routines. The desired chromatic dispersion curve and ZCD wavelength are found by straightforward differentiation. / M.S.