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

Fiber optic sensors fabricated from standard silica fibers have many advantages over conventional sensors like small size, portability, durability and immunity to electromagnetic fields. Unfortunately, these sensors are not suitable for use in harsh environments where the temperatures are greater than 700°C and large working stresses are involved. Sapphire fiber-based sensors present an attractive alternative for use in such environments. The material properties of sapphire like high melting point, extreme hardness and relative imperviousness to chemical reactions, coupled with the advantages of optical fiber sensing, enhance the performance of these sensors for rugged use. Unfortunately, commercial sapphire fiber that is currently available has higher optical attenuation than silica fiber and is costlier. 0, it is prudent to use a small length of sapphire fiber as a sensor head, which is then spliced to a standard singlen10de silica fiber which acts a lead-in/lead-out fiber to the sapphire sensor head. This thesis investigates possible splicing techniques to fabricate such a sensor set-up. Comparative results from experiments performed on splices that have been obtained by each of these techniques, are presented. Furthermore, two different sensor configurations using a sapphire fiber, spliced to a silica fiber, are developed, and the results of preliminary tests are presented. / Master of Science

The performance of an optical fiber depends to a great extent on the conditions of the process being used to draw the fiber from a glass preform. To get fibers with higher strength and lower transmission loss, the parameters of the draw process must be optimized and the geometric uniformity of the fiber must be maintained with high precision. To increase the geometric uniformity of the fiber the online fiber diameter measuring accuracy must be improved. The coating concentricity and uniformity also play an important role in fabricating quality fiber. As the fiber drawing speed goes higher and higher, which is the trend of modern fiber fabrication processes, the uniformity of fiber diameter and coating concentricity becomes even tougher to maintain. The main objective of this research has been to understand and develop an optical fiber drawing process that produces fiber with improved dimensional uniformity, higher strength and lower transmission loss. Particular emphasis has been given to the accurate measurement of on-line fiber diameter, which is the first and most important step in obtaining uniform fiber. / Master of Science

Interference between the modes of an optical fiber results in specific intensity patterns which can be modulated as a function of disturbances in the optical fiber system. These modulation effects are a direct result of the difference in propagation constants of the constituent modes. In this presentation it is shown how the modulated intensity patterns created by the interference of specific mode groups in few-mode optical fibers (V < 5.0) can be used to detect strain. A detailed discussion of the modal phenomena responsible for the observed strain induced pattern modulation is given and it is shown that strain detection sensitivities on the order of 10-9 can be expected. Data taken during the evaluation of an actual experimental strain detection system based on the developed theory is also presented. / Master of Science

Interference between the modes of an optical fiber generates specific mode (intensity) patterns which get modulated by disturbances in the optical fiber system. Mode-mode interference has been analyzed from first principles and a model based on differential phase modulation presented. Mode-mode interference effects such as intensity modulation of the mode patterns are directly related to differential phase modulation between modes which arises due to the difference between the propagation constants of the constituent modes. Practical implementation of modal methods involves selective launching of modes and processing of the output pattern to demodulate the information. Axial strain has been chosen as the modulating mechanism in experiments designed to quantify mode-mode interference effects. Quasi-statically varying strain as well as vibrational strain was used to study 'dc' and 'ac' mechanisms. Specific mode combinations have been excited and their radiation patterns identified. Mode pattern changes have been described. Experimental observations and results correlate very well with analysis. / Master of Science

Single-mode optical fiber has been widely used not only in long-haul, high speed digital communication systems but also in sensing applications because of its inherent immunity to electromagnetic interference, low transmission loss, wide bandwidth, small size, and light weight. Birefringence is an important parameter of the optical fiber, determining bandwidth in fiber optic telecommunications, and resolution in fiber optic sensors. This thesis describes and demonstrates permanent photo-induced rotation of the principal axes of birefringence in Ge-doped circular-core low-birefringence single-mode optical fiber. Light from a linearly polarized Ar⁺ laser at a wavelength of 488 nm was launched into the fiber along one of the principal axes of the initial birefringence in the fiber. Rotation of the principal axes was observed after several hours of exposure. This observation helps one to understand the mechanism of the photo-induced effects in Ge-doped optical fiber, and to discover possible ways to control the birefringence in the fiber by means of photo exposure. / Master of Science

This thesis evaluates a new approach for effectively increasing the dynamic range of optical fiber links for transporting RF signals. This new approach, called the Dynamic Range Enhancement Technique (DRET), is key to connecting remotely located microcell base station antenna sites to a centralized base station via optical fiber while maintaining a good dynamic range. This thesis examines the causes and characteristics of distortion introduced by the optical fiber link and describes and compares the performance of the DRET with other dynamic range enhancing techniques using computer simulation. In most instances, the DRET is shown to be superior both in reducing harmonic and intermodulation distortion compared to other dynamic range enhancement techniques. The DRET has several advantages over automatic gain control (AGC) including stability and the absence of parasitic modulation on weak signals caused by fading or transient high power signals. / Master of Science

The physical and optical properties of sapphire fiber has been investigated in an effort to create a high temperature optical fiber sensor. Sapphire fiber demonstrates high optical attenuation. This attenuation is very sensitive to injection conditions, and roughly proportional to the cube of the fiber length. The loss was found to be largely due to surface scattering, which causes the fiber to deviate from a perfect cylindrical waveguide. Because of the high optical losses (and high cost) of sapphire fiber, it is desirable to fashion a splice between the sapphire and an inexpensive, low-loss silica fiber so that sapphire is only used in the sensor head. The great physical disparities between sapphire and silica make this a challenging proposition. One solution demonstrated here is the sapphire capillary tube splice, in which the two fibers are aligned in a sapphire capillary tube and bound together with alumino-silicate glass. Sapphire fiber optical sensors cannot use standard interferometric techniques used with silica fibers because sapphire fibers are not clad, making a strongly guiding, highly multimode waveguide that introduces a great deal of modal distortion to interferometric signals. Consequently a simple intensity-based sensor was developed and tested using sapphire. More exotic intensity-based sensors are explored with their applicability to a sapphire fiber sensor head. / Master of Science

In December of 1987 tests were conducted in the Virginia Tech Stability Wind Tunnel on a full-scale model of a fiber optic mortar projectile. The desired model configurations were sting-mounted on the Stability Tunnel STO-1 strain gauge balance. The sting was mounted on a streamlined vertical pylon which provided remote rotation in both pitch and yaw while maintaining the center of the balance along the tunnel centerline. The model inputs included the six-component force and moment data in body coordinates and the pressure data from the five pressure taps located within the model. The tunnel inputs were the static temperature, static pressure, and dynamic pressure. The angle of attack and yaw angle were input manually by the tunnel operator. The data analysis for the preliminary test program was intended to define the aerodynamic qualities of various components and configurations to aid in the redesign of the projectile. / Master of Science / incomplete_metadata