Present analog optical fiber multi-channel video transmission systems are very sensitive to laser nonlinearities and are consequently limited in the optical modulation depth (OMD) that may be used. This, in turn limits the power budget achievable, signal-to-noise ratio, and the channel capacity. In this dissertation a new analog transmission technique for multi-channel TV transmission on fiber USIng frequency modulation/pulse amplitude modulation/time division multiplexing (FM/TDM) is described and compared with present digital and analog systems. Parameters for the proposed system are selected and the relationship between the performance and parameter values is discussed. Analysis and simulations indicate that the proposed system has a very low sensitivity to nonlinearities and is similar to that of digital systems, and much better than current Frequency Modulated/Frequency Division Multiplexed (FM/FDM) systems. This permits the use of higher OMD (as high as in digital systems), which results in achieving a high signal-to-noise ratio and a large power budget. Analysis of the number of channels as a function of adjacent channel intersymbol interference indicates that the proposed system has a better spectral efficiency than present analog systems. Simulations are also used to predict the performance of the proposed system with laser diodes poorer than the ones presently used for multi-channel analog systems. Considerably poorer lasers may be used while achieving acceptable transmission quality. Finally, carrier-to-noise penalty caused by timing errors and jitter effects are analyzed. / Ph. D.

This thesis demonstrates the use of fiber optic methods for nondestructive testing of composite materials and aluminum specimens using the acousto-ultrasound approach. A noncontact method using a hybrid interferometer is devised for measuring absolute surface acoustic wave (SAW) amplitudes. The J1..J4 spectrum analysis technique is used for calibrating the piezoelectric transducer cylinder (PZT) and JO/J2 spectrum analysis technique is used for demodulating the SAW signal from the interferometer. An extrinsic Fabry-Perot interferometric (EFPI) sensor is utilized for sensing acoustic emission, measuring speed and attenuation in aluminum and composite specimens. A broadband preamplifier is designed for amplifying signals from the EFPI sensor. Theoretical and practical minimum detectable air gap change of an EFPI sensor are calculated for the system. The directional sensitivity of the EFPI sensor to SAW is studied. / Master of Science

Optical resonators, which confine light by resonant recirculation, serve as the basis for a wide variety of optical components. Though they appear in many geometric forms, the most effective of optical resonators show axial symmetry in at least one dimension. A popular variation that finds broad application is the dielectric sphere. Acclaimed for their high quality (Q) factor and small modal volume, spheres owe credit of these attractive features to their support of whispering gallery mode (WGM) resonances. The sensitivity of a resonance's frequency and Q to strain, temperature, and other parameters of the surrounding medium can be the basis for ultracompact modulators and sensors. Physically, WGMs are special optical modes which can be understood as light rays that orbit the equator of the sphere guided by total internal reflection. Like a smooth stone can be skipped along the surface of a pond, light can be confined to the inside of a sphere by successive reflections. To best excite WGMs, the source light should initially trace a line tangent to the sphere's circumference. But incorporating a tiny sphere with such nanometric tolerances into a practical sensor structure has its challenges and the prospects for microsphere applications have suffered because of the plight of this problem. The work in this dissertation details the fabrication and function of three new "press fit" spherical resonators. These etched fiber micro-devices were developed to meet the demand for a robust, self-integrated means of coupling light between an optical fiber and WGMs in a microsphere resonator. The etching processes have been tuned to enable secure storage of a microsphere while also providing efficient excitation and interrogation of WGMs. Furthermore, the methods have been designed to be staightforward, quick, and repeatable. Using standard etchants on common polarization-maintaining fiber with readily purchased microspheres, the press fit resonators demonstrated here can be batch-fabricated and assembled. The press fit spherical resonator offers an alignment-free and conveniently pigtailed WGM coupler that has great potential for bio-science sensing applications and studies of resonant bispheres. / Ph. D.

This study reports on the behavior of silica based optical fibers in a hydrogen environment at high temperatures. The hydrogen response in the form of optical loss in the wavelength range of 1000-2500 nm of a germanium doped graded index 50/125 graded index fiber was examined in the temperature range of 20–800 °C. When the fiber was exposed to hydrogen at 800 °C two absorption bands appeared: ~1390 nm assigned to the first overtone of the hydroxyl stretch and ~2200 nm band with complex assignments including the combination mode of the fundamental hydroxyl stretch with SiO4 tetrahedral vibrations and the combination mode of SiOH bend and stretch. The growth rate of the 1390 nm band fits the solution to the diffusion equation in cylindrical coordinates while the 2200 nm band does not. Absorption for both bands persisted as the fiber is cooled to room temperature. Temperature dependent behavior was observed in that as temperature increases from room temperature, the absorption intensity decreases and band shifts slightly to longer wavelengths. Temperature dependence is repeatable and reversible. However, if no hydrogen is present in the environment at temperatures greater than 700 °C, the 1390 nm band will permanently decrease in intensity, while the 2200 nm band does not change. Changes in the structure of the glass appear to be causing this temperature dependent behavior. Other necessary conditions for structural changes to cause this temperature dependent behavior are examined. / Master of Science

The developing field of smart structures and skins provides an application to which fiber optic sensors bring unique capabilities and benefits. The primary cost in a network of fiber sensors is in the sources, receivers and associated hardware and can be prohibitive for a large number of sensors. Multiplexing of sensors based on spread spectrum techniques offers an efficient and cost effective solution to this limitation. The system hardware developed in this research work is capable of the real time monitoring of a four sensor network. Experimental results with embedded and attached Extrinsic Fabry Perot Interferometers (EFPI) are presented. The system can be used to multiplex any type of sensor which translates the measurand into intensity variations of the light. A measure of the system efficiency is obtained using crosstalk measurements. A suppression of 40 dB has been obtained between the desired sensor signal and the interference. The effect on system performance has been observed by varying system parameters such as code length and separation between codes. Highly sensitive embedded interferometric sensors have been used in a multimeasurand environment to measure temperature and strain. A solution to the inherent 'near-far' problem in an optical COMA system has been proposed and results for the same are presented. A novel WDM/CDM hybrid (Wavelength Division Multiplexing/Code Dhtision Multiplexing) scheme has been proposed to increase the light intensity at the detector thereby increasing the number of sensors in the system. Methods to optimize and upgrade the system are discussed. / Master of Science

All-fiber optical sensors have established themselves in the market for a decade. Among different categories of these sensors is the intensity-based type whose advantages make it suitable for many practical applications. Frustrated Total Internal Reflection (FTIR) is a phenomenon that shows a great potential to be used as a transduction mechanism in this type of sensors. Although physical interpretation of the phenomenon itself is rather complicated, it can be mathematically formulated in a non-extensive manner. The first attempt is to build a physical understanding of the phenomenon as well as a simple mathematical model based on planar waves. Then, it will be expanded it to waves inside a waveguide structure like optical fibers in a concrete way based on computer simulations and actual experiments, a task that has never been done before. This would make it possible to address the practical issues involved afterwards. Again, the obtained experimental results as well as simulations help analyze advantages and disadvantages of our basic structure and move on to design a practically viable all-fiber FTIR sensor. A prototype vacuum pressure sensor has been designed and tested to show the potential in this category of optical sensors. Some other applications are also briefly discussed to give us a good feeling of the restrictive problems and future prospective of the ongoing related research. / Master of Science

The combined effect of group velocity dispersion (GVD) and nonlinearity limits the transmission distance of fiber optic communication systems. By designing a circuit that can model this effect and simulating it using the Block Oriented Systems Simulator (BOSS), we can study the pulse distortion and broadening that may occur. The nonlinearity considered here is self-phase modulation (SPM). A loop system is designed where each pass of the system corresponds to 200 km of fiber. The system mainly consists of a Butterworth bandpass filter which models the dispersion in the fiber and a VCO that imposes a phase modulation on the input signal thus modeling the intensity dependent phase shift due to SPM. By changing various parameters of the circuit, the dispersion limited distance, the nonlinearity limited distance and other related factors can be altered. With D=-0.04 ps/km.nm, the effect of changing the nonlinear limited distance and thus the nonlinear effect is studied and simulated. The results are compared with a simple theoretical model for pulse broadening that has appeared in the literature. Although the simulation results exhibit the same qualitative behavior as the theory, there are significant quantitative differences. The reason for these differences are discussed. / Master of Science

In order to develop more compact optical fiber sensing systems, modal filtering can be performed in-line by using micro-optical devices. Two such devices are a laterally offset few-moded optical fiber mechanical splice and a modal conversion optical fiber coupler. A third device, the air-gap splice used with multimode fibers, can examine the modal content of an optical fiber. A basic theoretical understanding on how these devices operate is reviewed. A splice loss calculation for few-moded optical fibers is presented. Applications of the asymmetrical few-moded mechanical optical fiber splice, the modal conversion coupler, and the air-gap splice are discussed. / Master of Science

Our motivation for this work is based on the need to monitor the cure and inservice health of composite materials. We describe the continuation of an effort to design a multi-functional fiber optic sensor which can be embedded in polymeric composite laminates for monitoring the degree of cure during its fabrication, as well as internal composite strains occurring post-cure.3 In short, this dual-purpose sensor combines the characteristics of a Fresnel reflectometer with those of the extrinsic Fabry-Perot interferometer. For monitoring cure, a broadband source is used so the output intensity of the sensor is amplitude-modulated as the refractive index of the composite is increased during the polymerization process. Post-cure, a coherent light source is implemented so a. sinusoidal variation of the output signal occurs when strains within the composite cause the sensor output to be phase-modulated. We demonstrate the measurement of refractive index with the Fresnel reflectometer/EFPL and test it as an embedded refractive index monitor. Our experimental results demonstrate that the refractive index of 5-minute epoxy increases by approximately 2 % during the cure process. In addition, the sensor can be used as an interferometer to measure internal composite strains, where the phase difference between consecutive fringe peaks is one-half the wavelength of the source. / Master of Science

Monitoring accurate temperature and pressure profiles in harsh environments is currently in high demand in aerospace gas turbine engines and nuclear reactor simulators. Having the ability to measure both quantities continuously over a region, without thermal coupling, using a sensor with a small size (envelope) is also highly desirable. Currently available MEMS (microelectromechanical systems) provide effective small scale pressure and temperature measurement devices, however, they have only been shown to be effective up to 600C and lack the ability to perform distributed measurements unless combined with fiber-optic techniques. In general, fiber-optics provide many advantages over electrical based sensors and are the ideal choice for high temperature regimes and distributed sensing. In this thesis, preliminary designs and suggested future work are presented for a sensor built within an 3.175 mm radius envelope and capable of distributed pressure and temperature sensing up to temperatures reaching 800C. Finite element analysis via ANSYS, along with analytical verification models have been used for the design evolution. Diaphragm based designs, seem to provide easy fabrication methods and good sensitivity, however, for this design to be realized at high temperature operation, a robust bonding method must be chosen to avoid unwanted deformation due to misfit strains. / Master of Science