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.

The purpose of this study is to characterize the plasma spray deposits used for attaching intrinsic Fabry-Perot interferometric fiber optic strain sensors. The deposits must maintain adhesion at elevated temperatures without distorting the sensors' signals. Two different material systems were tested and modeled, a nickel based alloy and yttria-stabilized zirconia. The material properties of the deposits and the thermal stresses in the system were evaluated to determine attachment lifetime of the sensors. The encapsulated sensors' signals were collected before and after plasma spraying and at elevated temperatures. The material properties of the deposits were evaluated by electron microscopy, energy dispersive x-ray spectroscopy, scratch testing, thermal fatigue testing, and nanoindentation. The thermal stresses were evaluated by Raman spectroscopy and from finite element analysis in COMSOL® Multiphysics®. Several of the sensors broke during encapsulation due to the plasma spray processing conditions and the signals experienced distortion at elevated temperatures. The sensors can be treated to remove this interference to allow for this deposit attachment. The nickel based alloy's ductility and lamellar microstructure allowed for non catastrophic relaxation mechanisms to relieve induced thermal stresses. The yttria stabilized zirconia failed catastrophically at elevated temperatures due its lack of compliance to mismatches in thermal expansion. A high melting point metallic deposit, similar to the nickel based alloy, is desirable for fiber optic sensor attachment due to its ductility, thermal expansion, and dominant relaxation mechanisms. The processing conditions may need to be optimized to allow for the sensors' protection during encapsulation. / Master of Science

In this thesis, the optical communication systems for millimeter-scale sensing and communication devises known as "Smart Dust" are described and analyzed. A smart dust element is a self-contained sensing and communication system that can be combined into roughly a cubic-millimeter mote to perform integrated, massively distributed sensor networks. The suitable passive optical and fiber-optic communication systems will be selected for the further performance design and analysis based on the requirements for implementing these systems. Based on the communication link designs of the free-space passive optical and fiber-optic communication systems, the simulations for link performance will be performed. / Master of Science

Gas Sensing Has Become a Very Important and Attractive Technique Because of Its Various Applications, Such as in the Increasingly Concerning Case of Environmental Issues, Automobile Emission Detection, Natural Gas Leakage Detection, Etc. It Also Has Significant Applications in Industries, Such as Safety and Health Monitoring in Underground Mines. Among Those Sensing Areas, Fiber-optic Sensors Have Drawn Considerable Attention Because of Its Small Size, Light Weight, High Sensitivity, and Remote Sensing Capability. However, Current Fiber-optic Gas Sensing Techniques Have Several Limitations on Their Potential for Multiplexed or Distributed Sensing Due to Difficulties Such as High Complexity or Large Loss. To Accomplish the Goal for Multiplexed Gas Sensing, an Off-centered Core Fiber Design Is Investigated. The Eccentric Core Can Reduce Attenuation, Keep Mechanical Strength, and Lower Fabrication Cost. To Verify the Feasibility of the Design, Fiber Field Distribution Is First Studied in Simulation, Which Will Be Discussed in Detail in Chapter 2. Then Two Fiber Samples with a Length of 10 Cm and 40 Cm Are Prepared and Placed in a Custom Methane Sensing System for Gas Absorption Testing, Which Is Detailed in Chapter 3. From Etching Analysis, Localized Surface Defects Are Found as the Main Reason for Power Loss. Performance Such as Detection Resolution and Sensitivity Are Investigated. In Chapter 4, Theoretical Evaluations Have Been Conducted for Multiplexed Sensors Performances Using the Off-centered Core Fiber to Study the Impact Fiber Parameters on Sensing System Design. The Conclusion and Summary Are Presented in Chapter 5. / Master of Science / Gas Sensing Has Become a Very Important and Attractive Technique Because of Its Various Applications, Such as in the Increasingly Concerning Case of Environmental Issues, Automobile Emission Detection, Natural Gas Leakage Detection, Etc. It Also Has Significant Applications in Industries, Such as Safety and Health Monitoring in Underground Mines. Among Those Sensing Areas, Fiber-optic Sensors Have Drawn Considerable Attention Because of Its Small Size, Light Weight, High Sensitivity, and Remote Sensing Capability. However, Current Fiber-optic Gas Sensing Techniques Have Several Limitations on Their Potential for Long Distance Distributed Sensing Due to Difficulties Such as High Fabrication Complexity. In This Work, a Fiber-optic Gas Sensor with Special Structure Was Designed. The Sensor Can Reduce Attenuation, Keep Mechanical Strength, and Lower Fabrication Cost. To Verify the Feasibility of the Design, Theory Analysis and Simulation Were Conducted, Which Will Be Discussed in Detail in Chapter 2. Then Two Samples with a Length of 10 Cm and 40 Cm Were Prepared and Placed in a Custom Methane Sensing System for Testing. And Their Performance Such as Sensitivity Is Investigated. In Chapter 4, Theoretical Evaluations Have Been Conducted for Multiplexed Sensors Performances Evaluation to Study the Impact Fiber Parameters on Sensing System Design. The Conclusion and Summary Are Presented in Chapter 5.

A majority of underground coal mine disasters in the United States are due to explosions. Current atmospheric monitoring system (AMS) practices in the US could be enhanced to facilitate data sharing and learning of the entire work force. With the inclusion of additional atmospheric monitoring and data collecting, meaningful analysis can be realized and shared with the workforce. AMS data can be utilized to advance the understanding of underground atmospheres for the entire workforce along with adding to the knowledge base for preventative planning. An AMS interface ADAMAS is suggested to facilitate this conglomeration and sharing of the data visually, so that it can be quickly processed and applied in their daily decisions. An emerging sensor technology for underground mining, fiber optics is explored and tested in emergency, or fire and explosion situations. The fiber optic methane sensor performed well in smoke only showing a slow in response time due to soot on the filter. The ADAMAS interface was tested in a large population of underground coal miners. The population varied in age, job, classification, and experience. They all primarily found it to be easy to use and helpful to them. Concerns arose when asked how this will facilitate an improved relationship with regulatory agencies. There is trepidation when it comes to additional atmospheric information sharing, that it may not be used advance understanding of mine atmospheres. The AMS data collected is individual to each mine site but can assist in the understanding of underground atmosphere as a whole. Moving forward, regulatory bodies should use this as a stepping point to consider how this information can be used to advance the field of mine ventilation and also the health and safety of the miner. / Ph. D. / Many accidents in underground coal mines in the United States are due to explosions. Explosions occur when there is a spark created in an atmosphere that contains an explosive mixture of methane and oxygen. Current monitoring in the United States standardly follows what regulators stipulate. It is suggested and tested that we use additional atmospheric monitors, including fiber optic technology, to monitor the atmosphere, trend the data and share it with the mining workforce. Shown is current trends for atmospheric monitoring systems (AMS) and out suggestion for increased monitoring and using an interface called ADAMAS for trending and sharing data visually in graphs and locations on maps. A novel sensing technology a fiber optic methane sensor was also tested in smoke conditions for its applicability in underground mines during an emergency. Both the interface and the fiber optic sensor were successful in testing. Miners found the interface easy to use and informative. The fiber optic sensor was successful sensing methane even in smoke environments although the response time of the sensor decreased.

Yes / We have read with interest the recent article by Sponsel et al.1 There is much evidence that glaucomatous damage occurs at the optic nerve head,2 and therefore we were surprised by the authors' conjecture that there may be a central mechanism that preserves the binocular visual field in advanced glaucoma.

Yes / Purpose: We evaluated variability and conviction in tracing paths of retinal nerve fiber bundles (RNFBs) in retinal images, and compared traced paths to a computational model that produces anatomically-customized structure–function maps. Methods: Ten retinal images were overlaid with 24-2 visual field locations. Eight clinicians and 6 naïve observers traced RNFBs from each location to the optic nerve head (ONH), recording their best estimate and certain range of insertion. Three clinicians and 2 naïve observers traced RNFBs in 3 images, 3 times, 7 to 19 days apart. The model predicted 10° ONH sectors relating to each location. Variability and repeatability in best estimates, certain range width, and differences between best estimates and model-predictions were evaluated. Results: Median between-observer variability in best estimates was 27° (interquartile range [IQR] 20°–38°) for clinicians and 33° (IQR 22°–50°) for naïve observers. Median certain range width was 30° (IQR 14°–45°) for clinicians and 75° (IQR 45°–180°) for naïve observers. Median repeatability was 10° (IQR 5°–20°) for clinicians and 15° (IQR 10°–29°) for naïve observers. All measures were worse further from the ONH. Systematic differences between model predictions and best estimates were negligible; median absolute differences were 17° (IQR 9°–30°) for clinicians and 20° (IQR 10°–36°) for naïve observers. Larger departures from the model coincided with greater variability in tracing. Conclusions: Concordance between the model and RNFB tracing was good, and greatest where tracing variability was lowest. When RNFB tracing is used for structure–function mapping, variability should be considered.

Yes / Purpose: We have developed customized maps that relate visual field and optic nerve head (ONH) regions according to individual anatomy. In this study, we aimed to determine feasible map resolution for research use, and to make a principled recommendation of sector size for clinical applications. Methods: Measurement variability in fovea–ONH distance and angle was estimated from 10 repeat OCT scans of 10 healthy people. Errors in estimating axial length from refractive error were determined from published data. Structure–function maps were generated, and customized to varied clinically-plausible anatomical parameters. For each parameter set (n = 210), 200 maps were generated by sampling from measurement/estimation error distributions. Mapped 1° sectors at each visual field location from each parameter set were normalized to difference from their mean. Variation (90% ranges) in normalized mapped sectors represents the precision of individualized maps. Results: Standard deviations of repeated measures of fovea–ONH distance and angle were 61 μm and 0.97° (coefficients of variation 1.3% and 12.0%, respectively). Neither measure varied systematically with mean (Spearmans's ρ = 0.26, P = 0.47 for distance, ρ = −0.31, P = 0.39 for angle). Variation (90% ranges) in normalized mapped sectors varied across the visual field and ranged from 3° to 18° when axial length was measured accurately, and from 6° to 32° when axial length was estimated from refractive error. Conclusions: The 90% ranges represent the minimum feasible ONH sector size at each visual field location. For clinical use an easily interpretable scheme of 30° sectors is suggested.

No / Purpose To determine whether the structure–function relationship in glaucoma can be strengthened by using more precise structural and functional measurements combined with individualized structure–function maps and custom sector selection on the optic nerve head (ONH). Design Cross-sectional study. Participants One eye of each of 23 participants with glaucoma. Methods Participants were tested twice. Visual fields were collected on a high-resolution 3° × 3° grid (164 locations) using a Zippy Estimation by Sequential Testing test procedure with uniform prior probability to improve the accuracy and precision of scotoma characterization relative to standard methods. Retinal nerve fiber layer (RNFL) thickness was measured using spectral-domain optical coherence tomography (OCT; 4 scans, 2 per visit) with manual removal of blood vessels. Individualized maps, based on biometric data, were used. To customize the areas of the ONH and visual field to correlate, we chose a 30° sector centered on the largest defect shown by OCT and chose visual field locations using the individualized maps. Baseline structure–function correlations were calculated between 24-2 locations (n = 52) of the first tested visual field and RNFL thickness from 1 OCT scan, using the sectors of the Garway-Heath map. We added additional data (averaged visual field and OCT, additional 106 visual field locations and OCT without blood vessels, individualized map, and customized sector) and recomputed the correlations. Main Outcome Measures Spearman correlation between structure and function. Results The highest baseline correlation was 0.52 (95% confidence interval [CI], 0.13–0.78) in the superior temporal ONH sector. Improved measurements increased the correlation marginally to 0.58 (95% CI, 0.21–0.81). Applying the individualized map to the large, predefined ONH sectors did not improve the correlation; however, using the individualized map with the single 30° ONH sector resulted in a large increase in correlation to 0.77 (95% CI, 0.47–0.92). Conclusions Using more precise visual field and OCT measurements did not improve structure–function correlation in our cohort, but customizing the ONH sector and its associated visual field points substantially improved correlation. We suggest using customized ONH sectors mapped to individually relevant visual field locations to unmask localized structural and functional loss.

The extrinsic Fabry-Perot interferometer (EFPI) has been extensively used as a strain sensor in various applications. However, like other interferometric sensors, the EFPI suffers from ambiguity in detecting directional changes of the applied perturbation, when the operating point is at a maxima or a minima on the transfer function curve. Different methods, or sensor configurations have been proposed to solve this problem. This thesis investigates the use of dual wavelength interferometry to overcome this limitation. Possible systems configurations based on dual wavelength interferometry were considered, and the comprehensive design and implementation of a dual laser time division multiplexed (TOM) system based is presented. The system operates by alternately pulse modulating two laser diodes, which are closely spaced in center wavelength. Although the strain rate measurement capability of the system is dependent primarily on the speed of its hardware and the accuracy of its software, it is shown that it can be considerably enhanced by employing digital signal processing techniques. / Master of Science