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

Losses induced in a single-mode fiber due to periodic spatial deformations along the fiber axis have been investigated spectrally. Deformation losses in several single-mode fibers were found to be strongly wavelength dependent. This wavelength dependence was characterized by narrow attenuation bands in the transmission spectra of periodically deformed fibers. The attenuation bands were shown to shift as the spatial deformation period was varied. The amplitude and location of the attenuation bands were recorded as the number and amplitude of spatial deformations on the fiber were varied. The backreflected spectra of deformed fibers were also studied. Applications of wavelength dependent losses in periodically perturbed single-mode fibers are proposed. In particular, a self-referenced, intensity-based, fiber sensor is suggested. A distributed sensor is proposed using several sets of deformations with different periods induced on the same fiber to measure the same physical measurand at multiple locations along the length of the fiber. Based on this scheme, a multiplexed fiber sensor is envisioned, capable of measuring different physical variables on the same fiber. Optical notch filters can be made by using the notch-like characteristic of each attenuation band. Methods to induce permanent, periodic, axial deformations on an optical fiber are discussed. Spectral investigation of two-mode fibers with photo-induced refractive index changes is suggested. The proposed investigations would eliminate the need for external deformers to perturb the fiber and yield an intrinsic fiber sensor for sensing different physical variables. / Master of Science

This thesis describes the implementation of a mechanical strain sensor which uses concentric core optical fiber waveguide as the sensing element. When this particular type of fiber is strained, a transfer of optical power occurs between propagating modes in the two concentric cylindrical cores of the fiber. This strain-induced redistribution of optical power may be detected at the output end of the fiber using either two separated optical detectors or a two dimensional detector array. The calibrated strain sensitivity of the sensor is reported and suggestions for continuing research are discussed. / M.S.

High resolution Optical Time Domain Reflectometry (OTDR) measurements have recently allowed spatial resolutions of less than one millimeter. These capabilities indicate that OTDR techniques may be suitable for non-conventional applications such as the determination of fiber strain. This thesis presents an investigation of how high resolution OTDR techniques can be used in such applications. The concept of fiber segmentation via partially reflective optical splices for local strain measurements is discussed both from a theoretical and practical standpoint. Experimental results demonstrating the feasibility of such local strain measurements are also given. Another part of this investigation considers the practical details of the proposed strain measurement technique, addressing such topics as launching conditions, and environmental factors. Possible applications of the local strain measurement techniques, such as two- and three-dimensional stress analysis, and strain determination of fiber optic cables, are also presented. These applications also include the development of small, easy to manufacture elastomeric optical splices, which were shown to yield acceptable performance < 0.2 dB losses) for multimode fibers. / Master of Science

Optical fiber systems have been developed during the past twenty-five years with primary applications in long distance, high speed digital information communication. Optical fiber sensors have also been developed over the past fifteen years for the nondestructive inspection and evaluation of materials used in the aerospace, energy, transportation, and medical industries. Optical fibers may be used as the field-sensitive elements in sensors for the measurement of environmental parameters such as displacement, strain, temperature, vibration, chemical concentrations and electromagnetic fields. Their advantages for such measurements include 1) an inherent immunity to electromagnetic interference (EMD), 2) avoidance of ground loops, 3) the capability of responding to a wide variety of measurands, 4) excellent resolution, 5) the avoidance of sparks, especially important for applications within explosive environments, and 6) operation at temperatures of approximately 800°C for silica waveguides and above 1900°C for sapphire waveguides. Phase-modulated optical fiber sensors have been shown to possess the highest sensitivities to a given measurand. This dissertation describes several novel phase-modulated optical fiber sensors. The sensors described include a strain gage, a temperature sensor for applications up to and including 1700°C, a displacement sensor with sub-Angstrom resolution, and a vibration mode filter. For each sensing concept, a description and method of operation is given, followed by fabrication methods and experimental results. / Ph. D.

Changes in heat content are almost universally associated with chemical reactions. Thermometry as an analytical tool has been extensively researched and developed. Finding solutions to problems involving thermal isolation, specificity, sensitivity, and cross-sensitivity remain as active areas of interest. Fiberoptic interferometers, which use phase phenomena associated with propagating light, are extremely sensitive to heat. This research has focused on a special "birefringent" optical fiber. Two channels of information are generated in this single-fiber interferometer which correspond to the heat evolved from a catalytic reaction isolated on the fiber surface. Because of the unique transduction mechanism associated with the "birefringent" optical fiber, this device is sensitive to heat and remarkably insensitive to pressure. Details of the characterization and development of the birefringent optical fiber into a useful analytical probe are presented. / Ph. D.

A method of using optical fiber to measure strain and correct for the effects of temperature is proposed. A means of measuring apparent strain is given, pure temperature is measured using Fresnel-backscatter based Optical Time Domain Reflectometry, and a method for combining the two measurements to obtain a measurement of mechanically-induced strain alone is developed. The background, theory and experimental results that demonstrate the feasibility of such a system are presented and the results are compared with the performance of existing fiber-based means of measuring temperature. Experiments on several OTDR-addressed, intensity-based optical temperature sensors are performed and a method for manufacturing small air gap splices for use in measuring strain at several places along an optical fiber are presented. / Master of Science

Axial strain effects in multimode fibers are studied. A few-mode fiber is mounted on a tensile testing machine and strained at various speeds. The output of a monochromatic light source passing through it is monitored and recorded. Relations are noted between the light output the magnitude of tension and the rate of the applied axial strain. Flexural behavior of the optical fiber at various tensions is also studied by monitoring the modal output pattern. Relations are compiled to serve as a beginning to model these and other related modal effects. A theoretical background is also suggested to explain the observed effects. / Master of Science

The concept of incorporating optical fiber delay lines as signal processors is investigated. These signal processors are discrete-time filters implemented using optical fiber delay lines in the form of optical fiber couplers advantages of such a filter include the ability to eliminate the need for electro-optical conversion or at least postpone the conversion to a point where the electronics may be simplified. In addition, these optical signal processors exhibit all the advantages afforded traditional optical fiber systems. Furthermore, these devices allow high-speed signal processing rates greater than 1GHZ which rival the performance of the most advanced electronic counterparts available today. In this thesis, optical signal processors are investigated in detail. First, the fundamental signal processing concepts and optical component background is discussed for the analysis of the devices are developed. Subsequently, various configurations of optical signal processors are discussed and novel computer models for these configurations using Z-transform theory are presented. Lastly, experimental verification is presented of the rudimentary models and a unique approach is presented that will allow the creation of optical signal processors with a flexibility never achievable before. / Master of Science