Thin film planar waveguides were originally introduced in microwave engineering. The spectroscopist began to use such waveguides as tools to solve chemical characterization problems since Harrick and Fahrenfort introduced the Attenuated Total'Reflection (ATR) in the early 1960's. Today, planar waveguides are playing an important and ever-increasing role in modem chemistry. In this thesis, a novel design for a refractometer involving the application of a thin film planar waveguide and coupling prism was demonstrated. This device shows the feasibility of refractive index measurements in a flowing stream. Therefore, an online detector for High Performance Liquid Chromatography (HPLC) was chosen as the vehicle to test out the concepts. The research works were devoted to studies of waveguide properties and flow dynamics in a chromatographic situation. It was found that microscale detection is possible. The sensitivity can be enhanced by using the highest propagation mode as the probe, and by selecting a proper refractive index liquid as the solvent carrier. A description of the investigation and the various factors involved in designing and optimizing a planar waveguide for refractive index detection is included. The results provide guidelines for the device as a realistic analytical detector. / Ph. D.

Wavelength Division Multiplexed (WDM) based broadband fiber optic networks offer an attractive approach to achieve extremely high throughputs while employing moderate speed electronics. Passive optical filters play a crucial role in such networks serving the function of combining and separating multiple wavelengths. Single and multiple cavity F-P filters are analyzed with particular emphasis on their bandwidth and tuning range. Fabry-Perot (F-P) filter tuning commonly employs piezoelectric techniques. A new tuning technique based on a F-P filter fabricated with a fiber having an electro-optic cladding is proposed. A novel F-P filter structure employing optical feedback is proposed and analyzed. The application of F-P filters in an incoherent multiwavelength star network and the associated concerns involving crosstalk and dispersion are evaluated. A comparative error performance analysis of one and two F-P filter receivers in a BFSK transmission system is performed. An overview of numerous multiwavelength network and switch proposals along with their associated merits and demerits is given. / Master of Science

This dissertation presents the exploration of two fiber optics techniques involving high power pulse laser delivery. The first research topic is "Embedded Active Fiber Optic Sensing Network for Structural Health Monitoring in Harsh Environments", which uses the fiber delivered pulse laser for acoustic generation. The second research topic is "Fiber Optics Guided Laser Micromachining", which uses the fiber delivered pulse laser for material ablation. The objective of the first research topic is to develop a first-of-a-kind technology for remote fiber optic generation and detection of acoustic waves for structural health monitoring in harsh environments. Three different acoustic generation mechanisms were studied in detail, including laser induced plasma breakdown (LIB), Erbium-doped fiber laser absorption, and metal laser absorption. By comparing the performance of the acoustic generation units built based on these three mechanisms, the metal laser absorption method was selected to build a complete fiber optic structure health monitoring (FO-SHM) system. Based on the simulation results of elastic wave propagation and fiber Bragg grating acoustic pulse detection, an FO-SHM sensing system was designed and built. This system was first tested on an aluminum piece in the room temperature range and successfully demonstrated its capability of multi-parameter monitoring and multi-point sensing. With additional studies, the upgraded FO-SHM element was successfully demonstrated at high temperatures up to 600oC on P-91 high temperature steels. During the studies of high power pulse laser delivery, it was discovered that with proper laser-to-fiber coupling, the output laser from a multimode fiber can directly ablate materials around the fiber tip. Therefore, it is possible to use a fiber-guided laser beam instead of free space laser beams for micromachining, and this solves the aspect ratio limitation rooted in a traditional laser beam micromachining method. In this dissertation, this Guided Laser MicroMachining (GLMM) concept was developed and experimentally demonstrated by applying it to high aspect ratio micro-drilling. It was achieved that an aspect ratio of 40 on aluminum and an aspect ratio of 100 on PET, with a hole diameter less than 200 um. / PHD / This dissertation presents two research topics both related to high power laser and fiber optic. The first topic studies the application of using optical fiber and high power laser for ultrasonic non-destructive evaluation. The general idea is to use fiber optic to remotely generate and monitor ultrasonic waves on a workpiece. Due to the fact that there are no electronic components involved in the sensing part of the system, this system can work at high temperature and is unsusceptible to EMI. The second topic studies the usage of optical fiber in high aspect ratio micromachining. The key concept is to use a fiber tip and the output high power laser as a "drilling tip", which eliminate the aspect ratio limitation rooted in the traditional free-space laser micromachining method. With this concept and a demonstrative micromachining system, we achieved record-breaking aspect ratio on both aluminum and plastic.

This dissertation addresses several theoretical issues in low-finesse fiber optic Fabry-Perot Interferometric (FPI) sensors. The work is divided into two levels: modeling of the sensors, and signal processing based on White-Light-Interferometry (WLI). In the first chapter, the technical background of the low-finesse FPI sensor is briefly reviewed and the problems to be solved are highlighted. A model for low finesse Extrinsic FPI (EFPI) is developed in Chapter 2. The theory is experimentally proven using both single-mode and multimode fiber based EFPIs. The fringe visibility and the additional phase in the spectrum are found to be strongly influenced by the optical path difference (OPD), the output spatial power distribution and the working wavelength; however they are not directly related to the light coherence. In Chapter 3, the Single-Multi-Single-mode Intrinsic FPI (SMS-IFPI) is theoretically and experimentally studied. Reflectivity, cavity refocusing, and the additional phase in the sensor spectrum are modeled. The multiplexing capacity of the sensor is dramatically increased by promoting light refocusing. Similar to EFPIs, wave-front distortion generates an additional phase in the interference spectrogram. The resultant non-constant phase plays an important role in causing abrupt jumps in the demodulated OPD. WLI-based signal processing of the low-finesse FP sensor is studied in Chapter 4. The lower bounds of the OPD estimation are calculated, the bounds are applied to evaluate OPD demodulation algorithms. Two types of algorithms (TYPE I & II) are studied and compared. The TYPE I estimations suffice if the requirement for resolution is relatively low. TYPE II estimation has dramatically reduced error, however, at the expense of potential demodulation jumps. If the additional phase is reliably dependent on OPD, it can be calibrated to minimize the occurrence of such jumps. In Chapter 5, the work is summarized and suggestions for future studies are given. / Ph. D.

Automated techniques to acquire weight and traffic data are indispensable to effective management and maintenance of the vast network of highways. Weigh-In-Motion (WIM) systems have the potential to greatly reduce the cost and improve the accuracy associated with weight data collection. The existing WIM systems utilizing piezoelectric cables have been shown to result in rather large random errors of up to 12% and need to be installed at permanent sites. In addition, the exponential decay of the output signal of the piezoelectric cable with time can cause complications in signal processing and possibly further errors. In this thesis, a fiber-optic sensor is proposed which measures the pressure generated by the weight of a vehicle. The system consists of a pneumatic tube filled with an incompressible fluid, a rubber pad embedding the tube, a diaphragm to convert pressure into displacement, and an optical displacement sensor. A prototype of the proposed sensor is designed, manufactured and tested in the laboratory. Both piezoelectric cable and the optical sensor are tested under varying load-frequencies~ It is shown that the piezoelectric cable sensor shows considerable dependence on the load frequency, whereas the response of the proposed system is much less frequency dependent and, unlike the piezoelectric cable has a waveform similar to that of the applied load. This latter property can significantly reduce the difficulties associated with signal processing. Besides, the linearity of response over the range of loads applied is better than that of the piezoelectric cable. This implies that the proposed fiber-optic sensor with its high rate of accuracy can be implemented under conditions where piezoelectric sensor does not deliver accurate results. For example, when equally loaded axles at different vehicle speeds and axle configurations are used. / Master of Science

For the past 40 years, acoustic sensing has been a major avenue for the growth of interfero- metric fiber-optic sensors. Fiber-optic acoustic sensors have found uses in military, commer- cial, and medical applications. An interferometric fiber-optic acoustic sensor is presented utilizing the Michelson interferometer configuration with Faraday mirrors to eliminate po- larization fading. A 3 × 3 coupler is used as the beamsplitting component, and a symmetric demodulation algorithm is applied to recover the phase signal. This sensor has a theoretical resolution of 5.5 pico-strains and room to improve. Such improvements are discussed in the conclusion. / Master of Science

Side-polished fiber allows access to the evanescent field propagating in the cladding of a few-mode fiber. This cladding mode is analyzed and experimentally validated to further the design of a novel class of fiber optic devices. To do this, specific modes are excited in the polished fiber using a phase-only spatial light modulator to determine spatial mode distribution. Each mode is excited and compared to the expected field distribution and to confirm that higher order modes can propagate through side-polished fiber. Based on each mode’s distribution, a side-polished fiber can be designed so that perturbations on the polished portion of the fiber effect each mode independently. By carefully analyzing the effects of identical perturbations on each mode, it is determined that each mode can be isolated based on the geometry of the polished fiber and careful alignment of the mode field. This research has the potential to advance the development of novel fiber-based sensors and communications devices utilizing mode-based interferometry and mode multiplexing. / M.S. / Fiber optic devices have seen significant advancement since the realization of the laser and low-loss optical fiber. Modern day fiber optics are commonly utilized for high-bandwidth communications and specialized sensing applications. Utilizing multiple modes, or wave distributions, in a fiber provides significant advantages towards increasing bandwidth for communications and provides potential for more accurate sensing techniques. Significant research has been conducted in both the sensing and communication field, but mode-domain devices have the capability to significantly advance the field of fiber optic devices. This thesis demonstrates the potential for side-polished fiber geometry to effect each mode independently, thus allowing side-polished fiber to be utilized for realizing novel devices such as multiplexing devices and fiber optic sensors.

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

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

The presence of low frequency cutoffs in the forward path of the information signal leads to inter-symbol interference (ISI) and degradation of the signal to noise ratio at the sampling instant in digital on-off keying (OOK) systems. The low frequency cutoffs occur as a result of the presence of power separation filters in a line wire system and gain instability of APD's to D.C. in fiber optic systems. Also, it is easier to design amplifiers that do not extend to D.C. The ISI which manifests itself in the form of baseline wander can cause appreciable degradation in the signal to noise ratio. This thesis investigates two ways of combating the baseline wander problem. They are quantized feedback and line coding schemes. A detailed performance evaluation of quantized feedback scheme is performed. An overview of line coding schemes is given and some specific codes are evaluated in terms of S/N degradation at the receiver. / Master of Science