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A system to measure chromatic dispersion in single-mode optical fibers with near point wavelength resolution is described. Differential propagation constant data for the test fiber is collected by using an acousto-optic modulator to frequency-shift laser light passed to an optical heterodyne. Theoretical developments include a frequency-domain calculation technique for interpreting empirical results. The system is proven infeasible, however, due to inherent noise sensitivity / M.S.

Modern engineering structures often incorporate new materials and complex designs for which existing techniques for nondestructive evaluation prove inadequate, especially for dynamic and in-service measurements. At the same time, optical fiber sensors have been identified as an ideal candidate for embedded and attached measurements of material parameters such as strain, temperature, or state of damage. In particular, sensors based on optical fiber modal interference phenomena have been shown to be capable of highly sensitive detection of static and dynamic strain. This work reviews known applications of modal domain sensing to measurement science to date, and discusses the principles behind the method. A general expression for the intensity distribution emerging from a multimode fiber is formulated, covering both few mode and highly multimode fibers, and new expressions for their sensitivity to both radial and axial strain are derived. Optimized multimode fibers are seen to show an intrinsic phase sensitivity which rivals or even surpasses that of the single mode interferometer, especially in the case of applied radial strain. The use of modal domain sensors for real-time ultrasonic wave transduction is described as a particular application to NDE, with experimental results being presented with regard to acoustic emission monitoring as well as the detection and analysis of shock waves due to impact. Finally, optimization schemes and alternatives for such sensors are addressed, and recommendations for future work are raised. / Ph. D.

The single fiber pull-out test was employed to experimentally model the failure of an embedded optical fiber in neat resin. The objective was to evaluate load transfer between resin and fiber through the evaluation of the sensitivity of the single fiber pull-out test to the physical parameters of the polymer matrix. This was accomplished first by appropriately interpreting the load versus extension trace, second by determining the effect of fiber coating and embedding resin on the load carrying ability of the single fiber composite, third by characterizing the fiber/coating/resin system with respect to the physical parameters of the polymer and the failure mechanism of the composite as the cure temperature was varied, and fourth, by correlating the independent parameters of the polymer and fracture data. For the first time, the load versus extension trace generated by such experiments was thoroughly interpreted and mathematically modeled. To this end, the embedding resin was physically characterized through the determination of the glass transition temperature, T_g, the relative change in volume with sample preparation and thus the resulting normal pressure exerted on the embedded fiber by the resin material. The experimental fracture data was quantified by determining the strain energy release rate, SERR, for initiation of crack propagation and, with the consideration of friction, its continuation, as well as the interfacial shear stress, τ, of the bond, and τ associated with debonding and sliding. Based on a series of experiments of varying material parameters, a model material system was chosen: a polyimide coated fiber embedded in uncatalyzed tetraglycidyl-4-4'-diaminodiphenylmethane with 4,4’-diaminodiphenylsulfone. Cure temperatures, T_cure, of 150, 177, 230 and 250°C were employed. The average critical strain energy release rates increased from the 150 to 177 to 230°C sample sets, then decreased for the 250°C sample set. Since the T_g of the fully cured resin is 260°C, these results support the hypothesis of increasing residual stress as a function of T_cure for cure in the vitreous state. In regards to the 250°C cure data set, since T_cure was within T_g - 30°C the internal pressures due to crosslinking were minimized due to cure in a rubber-like state. The residual pressure, independently determined from both the resin characterization and fracture data, increased by a factor of 2.4 with a temperature increase from 150 to 230°C for the two hour cure period. The strain energy release rate and sliding interfacial shear stress of pull-out increased by a factor of 2.54 and 2.1, respectively. The coefficient of friction remained statistically constant at 0.6. Based on this work, it is concluded that the single fiber pull-out test is sensitive to fiber/matrix interactions via the physical parameters of the material system. Also, the failure response of the single fiber composite can be predicted for well characterized matrices. / Ph. D.

High-resolution optical time-domain methods applied to measuring strain in an optical fiber are discussed. The use of this optical time-domain fiber sensor for measuring quasi-distributed strain along a cantilevered beam is experimentally demonstrated. This is accomplished by segmenting the sensor with air-gap sites, allowing reflections to be monitored. Physically looping these fiber segments many times over their interaction regions is shown to improve the sensitivity of the sensor. Also discussed are techniques to improve sensitivity by using a special tap-off coupler to recirculate optical pulses many times through the sensing region. Important in modeling these sensors is determining the photoelastic coefficient, which accounts for the photoelastic and Poisson effects on a strained fiber. The photoelastic coefficient is theoretically modeled using two methods involving waveguide and ray-optics theory. The results of these analyses are compared to experimentally determined values. / Master of Science

A practical fiber optic strain sensor has been developed to measure strains in the range of 0.0 to 2.0 percent strain with a resolution ranging between 10 and 100 microstrain depending on sensor design choices. This intensity based sensor measures strain by monitoring strain induced longitudinal misalignment in a novel fiber interconnection. This interconnection is created by aligning fibers within a segment of hollow core fiber. Related splice loss mechanisms are investigated for their effect on resolution. The effect of gauge length and launch conditions are also investigated. / Master of Science

A novel method of fabricating low reflectance mirrors has been developed based on the photosensitivity of hydrogen-loaded, germanium-doped optical fibers. Using a side-writing technique, point-wise refractive index changes have been induced in the core of Ge-doped optical fibers via ultraviolet light from a high power excimer laser. These refractive index changes cause Fresnel reflections at the boundary of the higher photoinduced index change and the lower index in the unexposed core. The boundary of the two refractive indices may be considered a low reflectance mirror. Several techniques have been explored to characterize the Fresnel reflectors including optical time domain reflectometry (OTDR) and spectral analysis. In-line, optical fiber Fresnel reflectors have applications as internal mirrors in intrinsic Fabry-Perot interferometric (IFPI) sensors and OTDR distributed strain sensors. Photoinduced IFPI sensors have performed well as temperature sensors, strain sensors, and vibration sensors. Improved manufacturing techniques are also discussed for future developments. The photoinduced IFPI sensor is inexpensive to manufacture and involves little skill, in contrast to the labor-intensive fabrication techniques of conventional IFPI sensors. The IFPI sensor has commercial applications in embedded structures, high temperature environments, and situations with large EMI conditions. / Master of Science

Noise and distortion limit the channel capacity and degrade the system performance in AM-VSB SCM CATV Distribution Networks. The use of star couplers and fiber amplifiers is an approach to overcome these problems and meet the increasing demand for large capacity and high quality. The capacity and performance analyses of three different network configurations are compared based on the results of computer modeling. In practice, the best configuration depends on the optical amplifiers and the pump lasers used in the system as well as the requirement on the number of channels and the number of subscribers. It is found that in a two-stage star network the largest improvement in the capacity and performance may be achieved when the optical amplifiers between the stages are individually pumped. For example, a capacity of 200 channels and 572 output nodes may be obtained. There is an optimal length of the erbium-doped fiber amplifier (EDFA) and there is also an optimal position where the EDFA is located between the two star couplers. The modeling is based on an extensive analysis of noise and distortion in the system. A discrepancy in the power behavior of laser relative intensity noise (RIN) is pointed out. A previous analysis on reflection-induced noise is extended to include the effect of quadruply-reflected light and optical amplifiers. Since chirp-dispersion distortion and gain tilt distortion are found to be negligible, clipping distortion is the main limitation on the system capacity. Optical amplifier noise provides an additional limitation to the system. Because the gain and amplified spontaneous noise depend on the input signal power and pump power, the gain and spontaneous emission factor are modeled for the comparison of the different systems. / Master of Science

Fiber optics technology is being used in many applications, both in the military world and in the industrial world. A broad overview of this technology is provided, including a discussion of the fundamentals of fiber operation and component characteristics. Applications of fiber optics in both military and industrial communities is addressed, identifying specific examples in both cases. In addition, market projections and technology trends are discussed for both the military and industrial communities. / Master of Science

Abrasive blasting is a common process for cleaning or roughening the surface of a material prior to the application of a coating. Although the process has been in practice for over 100 years, the lack of a comprehensive understanding of the complex interactions that exist with the process can still yield an inferior surface quality. Subsequently, parts can be rejected at one of many stages of the manufacturing process and/or fail unexpectedly upon deployment. The objective of this work is to evaluate the effect of selected input parameters on the characteristics of the blasted surface characteristics so that a more useful control strategy can be implemented. To characterize surface roughness, mechanical profilometry was used to collect average roughness parameter, Ra. Decreasing blast distance from 6” to 4” gave ΔRa = +0.22 µm and from 8” to 6” gave ΔRa = +0.22 µm. Increasing blast pressure from 42 psi to 60 psi decreased the Ra by 0.33 µm. Media pulsation reduced Ra by 0.56 µm and the use of new media reduced Ra by 0.47 µm. Although blasting under the same conditions and operator on different days led to ΔRa due to shorter blast times, there was no statistically significant variance in Ra attributed to blasting on different days. Conversely, a ΔRa = +0.46 µm was observed upon blasting samples with different cabinets. No significant ΔRa was found when switching between straight and Venturi nozzles or when using different operators. Furthermore, the feasibility of fiber optic sensing technologies was investigated as potential tools to provide real time feedback to the blast machine operator in terms of substrate temperature. Decreasing the blast distance from 6” to 4” led to ΔT = +9.2 °C, while decreasing the blast angle to 45° gave ΔT= -11.6 °C for 304 stainless steel substrates. Furthermore, increasing the blast pressure from 40 psi to 50 psi gave ΔT= +15.3 °C and changing from 50 psi to 60 psi gave ΔT= +9.9 °C. The blast distance change from 8” to 6” resulted in ΔT = +9.8 °C in thin stainless steel substrate temperature. The effects of substrate thickness or shape were evaluated, giving ΔT= +7.4 °C at 8” distance, ΔT= +20.2 °C at 60 psi pressure, and ΔT= -15.2 °C at 45° blasting when comparing thin stainless steel against 304 stainless steel (thick) temperatures. No significant ΔT in means was found when going from 6” to 8” distance on 304 stainless steel, 40 psi and 60 psi blasting of thin SS, as well as angled and perpendicular blasting of thin SS. Comparing thick 304 and thin stainless steel substrates at a 6” blast distance gave no significant ΔT. / Master of Science