VTechWorks Repository :: Browsing by Author "Murphy, Kent A."

Browsing by Author "Murphy, Kent A."

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Analysis of Thermally Diffused Single Mode Optical Fiber Couplers
Velayudhan, Nirmalkumar (Virginia Tech, 1994-12-22)
The phenomenon of dopant diffusion as a viable means of coupler fabrication is investigated. It is well known that the diffusion of dopants can improve the uniformity of multimode star couplers manufactured by the fused biconical taper technique. The theoretical basis for the same phenomenon in a single mode coupler is developed, on the basis of the theory of diffusion and the Gaussian approximation for circular fibers. A novel technique to manufacture and design single mode optical fiber couplers with a minimization of the manufacturing complexity is demonstrated. Traditionally fused biconical tapered couplers have been manufactured by twisting, fusing and elongating optical fibers at elevated temperatures. Usually, high temperature oxy-hydrogen flames are used for such purposes and some degree of skill is needed for a human operator. The complexity of control procedures for automation of the process is greatly increased by the fact that the tapering process is an integral part of the feedback loop. This can be eliminated if a constant tension is maintained on the fibers in the heating process while heat is applied uniformly from a source such as a platinum wire furnace. Since the refractive index differentials responsible for the guiding phenomenon at optical frequencies are directly dependent on concentration of dopants like fluorine and germania, radial diffusion of such dopants causes the fiber cores that are heated in a platinum wire furnace to come closer together. Such proximity leads to the phenomenon of evanescent field interaction or coupling of optical power from one arm of the coupler to the other. The time evolution of the coupling process can be predicted in theory. While initial results are promising, the ability to automate the manufacture of couplers will be successful only after greater control over the variables is established. It is the intention of this work to understand the physics behind the mechanism as well as to prove the feasibility of modeling real world phenomena under controlled conditions.
Application of a Fabry-Perot interferometer for measuring machining forces in turning operations
Hansbrough, Andrew K. (Virginia Tech, 1993-05-05)
The FP interferometer was found to be feasible for detecting changes in machining forces. The fiber optic sensor was able to detect increases in strain corresponding to force increases detected by a dynamometer. The FP interferometer system must progress in several ways. A better data acquisition and data analysis system must be developed. A robust sensor must be made to withstand the harsh environment of machining. Also a method for eliminating the affects of thermal strain must be created. Finally, the placement of the FP sensor must also be determined. The FP has the potential to effectively monitor machining forces without affecting the rigidity of a turning operation setup.
Atomic Clock Augmentation For Receivers Using the Global Positioning System
Kline, Paul A. (Virginia Tech, 1997-02-07)
For receivers using the Global Positioning System (GPS), it is standard procedure to treat the receiver clock bias from GPS time as an unknown. This requires four range measurements to the satellites in order to solve for three dimensional position and clock offset. If the receiver clock could be synchronized with GPS time, the extra range measurement would not be necessary. To achieve this synchronization, a stable frequency reference must be incorporated into the GPS user set. This concept is known as clock aiding or clock augmentation of GPS receivers. Clock augmentation increases the availability of the navigation function because only three GPS satellites are required. Also, it is shown that clock augmentation improves vertical accuracy by reducing the vertical dilution of precision (VDOP), which is a unitless multiplier that translates range measurement error into vertical position error. This improvement in vertical accuracy is particularly beneficial for applications involving final approach and landing of aircraft using GPS, because GPS typically provides better horizontal accuracy than vertical accuracy. The benefits of atomic clock augmentation are limited by factors that cause a loss of synchronization either between the receiver and GPS time, or between ground station and airborne receivers processing GPS data in differential mode (DGPS). Among the error sources that cause a clock offset are antenna rotation, hardware drifts due to temperature variations, and relativistic effects for GPS receivers on moving platforms. Antenna rotation and temperature effects are addressed and supported by experimental data. It is shown that two particular relativity terms thought to be missing from GPS receiver algorithms are not evident in data collected during a flight test experiment. Upon addressing the error sources, the dissertation concludes with analysis of DGPS data collected during a flight test at the Federal Aviation Administration (FAA) Tech Center in Atlantic City, during which external rubidium oscillators were used by airborne (Boeing 757-B) and ground station GPS receivers. A new method of clock modeling is introduced, and this clock model is used to demonstrate the improvement in vertical accuracy, as well as three-satellite navigation.
Chemical sensing applications of fiber optics
Nagarajan, Anjana (Virginia Tech, 1994)
A sensing method that can monitor metallic structures continuously would eventually produce safer metallic structures as well as a more efficient and economic way to monitor corrosion. A secondary focus of this research is the implementation of a fiber optic sensor to measure refractive indices of unknown solutions. The surface plasmon sensor, interrogated with white light resulted in attenuations of light at different wavelengths when solutions of different refractive indices were introduced. This sensor has been shown to respond to the three configurations of polished single mode and multimode, as well as the unpolished multimode sensors. The sensitivity calculated was comparable with the sensitivity of the Kretschmann arrangement The transmissive aluminum-clad fiber sensor was shown to be effective in providing a response to the process of corrosion. Varying lengths of aluminum-clad fiber was spliced to acrylate multimode fiber and different wavelengths of sources were used to test the sensor in a bath of NaOH. The results were similar and reproducible. A tapered sensor configuration was attempted and proved to be very useful.
A chemical sensor based on surface plasmon resonance on surface modified optical fibers
Bender, William John Havercamp (Virginia Tech, 1993-02-12)
A sensor is described which utilizes the phenomenon of surface plasmon resonance to detect changes in refractive index of chemical or biochemical samples applied to a surface modified optical fiber. The sensor is constructed by polishing a short section of the lateral surface of an optical fiber to its evanescent field surrounding the fiber core. One or more thin films are applied to the polished section of the fiber to produce the sensing element. One of the films is the metal silver, which acts as the support for the surface plasmon. Under the proper conditions, TM polarized energy propagating in the fiber can be coupled to a surface plasmon electromagnetic mode on the metal film. This coupling depends on the wavelength, the nature of the fiber, the refractive index and thickness of the thin films applied to the fiber, and the refractive index of a chemical sample in contact with the modified surface. The fiber to plasmon coupling is seen as a large attenuation of the light reaching the distal terminus of the fiber.
Code Division Multiplexing of Fiber Optic and Microelectromechanical Systems (MEMS) Sensors
Jacobson, Carl P. (Virginia Tech, 2000-02-22)
Multiplexing has evolved over the years from Emile Baudot's method of transmitting six simultaneous telegraph signals over one wire to the high-speed mixed-signal communications systems that are now commonplace. The evolution started with multiplexing identical information sources, such as plain old telephone service (POTS) devices. Recently, however, methods to combine signals from different information sources, such as telephone and video signals for example, have required new approaches to the development of software and hardware, and fundamental changes in the way we envision the basic block diagrams of communication systems. The importance of multiplexing cannot be overstated. To say that much of the current economic and technological progress worldwide is due in part to mixed-signal communications systems would not be incorrect. Along the vein of advancing the state-of-the-art, this dissertation research addresses a new area of multiplexing by taking a novel approach to network different-type sensors using software and signal processing. Two different sensor types were selected, fiber optics and MEMS, and were networked using code division multiplexing. The experimentation showed that the interconnection of these sensors using code division multiplexing was feasible and that the mixed signal demultiplexing software unique to this research allowed the disparate signals to be discerned. An analysis of an expanded system was performed with the results showing that the ultimate number of sensors that could be multiplexed with this technique ranges from the hundreds into the millions, depending on the specific design parameters used. Predictions about next-next generation systems using the techniques developed in the research are presented.
A Comprehensive Dynamic Model of the Column Flotation Unit Operation
Cruz, Eva Brunilda (Virginia Tech, 1997-07-31)
The core of this project was the development of a column flotation dynamic model that can reasonably predict the changes in the concentrations of all solid and bubble species, along the full column height. A dynamic model of a process is normally composed of a set of partial or ordinary differential equations that describe the state of the process at any given time or position inside the system volume. Such equations can be obtained from fundamental material and/or energy balances, or from phenomenological derivations based on knowledge about the behavior of the system. A phenomenological approach referred to as population balance modeling was employed here. Initially, a two-phase model was formulated, which represents the behavior of the gas phase in a frother solution. The column was viewed as consisting of three main regions: a collection region, a stabilized froth and a draining froth. Experiments were carried out, based on conductivity techniques, for obtaining empirical data for model validation and parameter estimation. After testing the two-phase model, the equations for the solid species were derived. Consideration of the effects of bubble loading, slurry density and slurry viscosity on bubble rise velocity and, therefore, on air fraction is included in the model. Bubble coalescence in the froth is represented as a rate phenomenon characterized by a series of coalescence efficiency rate parameters. Auxiliary equations that help describe the settling of free particles, the buoyancy of air bubbles, and the processes of attachment and detachment, were also developed and incorporated into the model. The detachment of solids from the bubbles in the froth zones was attributed to coalescence, and it was assumed to be proportional to the net loss of bubble surface area. Almost all parameters needed to solve the model equations are readily available. The set of differential equations that comprise the model can be solved numerically by applying finite difference approximation techniques. An iteration has to be performed, which involves calculating the product flowrate at steady state, modifying the tailings rate and solving the model again until a mass balance is satisfied.
Controlled dopant diffusion for fiber optic coupler
(United States Patent and Trademark Office, 1998-09-15)
Uniformity of optical coupling of optical elements such as couplers and splitters is improved by heat treatment which causes dopants in the core of an optical fiber to diffuse into material from the cladding layer of the optical fibers from which the optical element is formed, resulting in a substantially homogeneous interior region of the star coupler or splitter. Increased lossiness of the optical element thus formed may be limited by termination of the heat treatment before dopant diffusion reaches equilibrium throughout the fibers so that a portion of the cladding layer of the fibers remains surrounding the substantially homogeneous region where the fibers have been fused together. Dopant diffusion is constrained to a substantially radial direction in each fiber by uniformity of heating over a region where at least two fibers are twisted together. Thus dopant diffusion is highly repeatable and can be readily regulated to provide highly selective, wavelength-dependent coupling between fibers, particularly for multiplexing and demultiplexing applications.
Design of a Fiber Optic Sensor Array for in Vitro Monitoring of Cellular Processes
West, Douglas (Virginia Tech, 1998-02-26)
Current analysis of the life and death cycles of in vitro cellular systems is based on visual observation methods relying upon morphological changes monitored using a microscope. Data collected from these techniques are not as precise as scientists desire them to be. The methods are discontinuous, indirect, costly, and time and labor intensive. The human element plays a significant part in error propagation as individual style of the researcher lends to skewing the data. Experimental results will differ greatly from laboratory to laboratory just because the methods of monitoring cellular activity are not standardized. The researcher uses experience to determine the best way to collect data quickly and "accurately" according to his or her definition. There is a great need not only to standardize data collection processes, but also to eliminate human error induced by lack of experience or fatigue. This research proposes a fiber optic based monitoring system as a possible solution to eliminate a number of problems with current cellular data collection methods and to increase the data collection rate tremendously since the process could be automated.
Design, Analysis, and Initial Testing of a Fiber-Optic Shear Gage for 3D, High-Temperature Flows
Orr, Matthew William (Virginia Tech, 2004-01-21)
This investigation concerns the design, analysis, and initial testing of a new, two-component wall shear gage for 3D, high-temperature flows. This gage is a direct-measuring, non-nulling design with a round head surrounded by a small gap. Two flexure wheels are used to allow small motions of the floating head. Fiber-optic displacement sensors measure how far the polished faces of counterweights on the wheels move in relation to a fixed housing as the primary measurement system. No viscous damping was required. The gage has both fiber-optic instrumentation and strain gages mounted on the flexures for validation of the newer fiber optics. The sensor is constructed of Haynes 230, a high-temperature nickel alloy. The gage housing is made of 316 stainless steel. All components of the gage in pure fiber-optic form can survive to a temperature of 1073 K. The bonding methods of the backup strain gages limit their maximum temperature to 473 K. The dynamic range of the gage is from 0-500 Pa (0-10g) and higher shears can be measured by changing the floating head size. Extensive use of finite element modeling was critical to the design and analysis of the gage. Static structural, modal, and thermal analyses were performed on the flexures using the ANSYS finite element package. Static finite element analysis predicted the response of the flexures to a given load, and static calibrations using a direct force method confirmed these results. Finite element modal analysis results were within 16.4% for the first mode and within 30% for the second mode when compared with the experimentally determined modes. Vibration characteristics of the gage were determined from experimental free vibration data after the gage was subjected to an impulse. Uncertainties in the finished geometry make this level of error acceptable. A transient thermal analysis examined the effects of a very high heat flux on the exposed head of the gage. The 100,000 W/m2 heat flux used in this analysis is typical of a value in a scramjet engine. The gage can survive for 10 minutes and operate for 3 minutes before a 10% loss in flexure stiffness occurs under these conditions. Repeated cold-flow wind tunnel tests at Mach 2.4 with a stagnation pressure from 3.7-8.2 atm (55-120 psia) and ambient stagnation temperature (Re=6.6x107/m) and Mach 4.0 with a stagnation pressure from 10.2-12.2 atm (150-180 psia) and ambient stagnation temperature (Re=7.4x107/m) were performed in the Virginia Tech Supersonic Wind Tunnel. Some of these tests had the gage intentionally misaligned by 25o to create a virtual 3D flow in this nominally 2D facility. Experimental results gave excellent agreement with semi-empirical prediction methods for both the aligned and 25o experiments. This fiber-optic skin friction gage operated successfully without viscous damping. These tests in the supersonic wind tunnel validated this wall shear gage design concept.
Development of Fiber Optic Aerodynamic Sensors for High Reynolds Number Supersonic Flows
Pulliam, Wade Joseph (Virginia Tech, 2000-02-02)
The purpose of the project was to examine fiber optic sensors for the measurement of pressure, skin friction, temperature, and heat flux in high Reynolds number, supersonic flow. Using a standard fiber optic signal conditioning unit (specifically a broadband interferometric system using spectra), the work centered around determining under what conditions these sensors will work effectively and quantifying the total system limitations. An interferometric-based, fiber optic skin friction sensor was developed for the measurement of wall shear stress in complex, supersonic flows. This sensor type was tested successfully in laminar, incompressible flow, and supersonic flow up to Mach 1.92, Mach 2.4 and 3.0 flow, in which the sensor operated with varying success. A micromachined, fiber optic pressure sensor was also tested in these supersonic conditions, also with varying success. The accurate operation of these sensors was found to be tied to the flow conditions and the fiber optic, signal processing system. A correlation was found to exist between the energy of the flow, either through its dynamic pressure or through external disturbances such as shocks or separation, and the noise in the signals, expressed by the variance of the gap estimate, for the pressure and skin friction sensors in these flows. The energy of the flow couples with the mechanical properties of the sensor reducing the fringe contrast of the signal used by the optical signal processing system to determine a gap estimate. As the energy of the flow is increased and the sensor is excited, the fringe contrast is reduced. A practical limit of a normalized fringe contrast of 0.10 was found for producing accurate gap estimates in real flows. A consequence is that there is a limit to the dynamic pressure of the flow for the sensors to operate accurately, which is demonstrated by the varying success of the supersonic wind tunnel tests. This correlation is sensor specific, meaning that sensors can be designed to operate successfully in any flow. Also, the signal processing system, which forms the other end of the total system, could be improved to allow accurate measurements with the current sensors.
Digital spectral analysis and adaptive processing techniques for phase modulated optical fiber sensors
Tran, Tuan A. (Virginia Tech, 1996)
The objective of this work is to investigate new signal processing techniques for optical fiber sensors that utilize the phase information of the electromagnetic field. Research concentrated on Fourier transform spectroscopy as a means for capturing wavelength encoded information from the fiber sensor. Classical spectral analysis utilizing the Fourier transform as a mathematical foundation for relating a time or space signal to its frequency-domain representation was shown to be inadequate for mitigating the bias errors caused by harmonic distortions. A modified spectral estimation algorithm is presented to overcome some of the practical issues while maintaining the high spectral resolution characteristic of the classical technique. This research also showed that unlike in free-space propagation, an optical signal propagating through a fiber waveguide, even over short distances, can experience significant phase modulation noise. A number of chromatic distortion mechanisms including modal interference, mode coupling due to periodic perturbations such as microdeformation and macrobends, and mode field diameter variations are addressed. We treated these issues by employing both theoretical simulation and experimental data. Coupled-mode formalism based upon approximated field solutions is used in the theoretical analysis. An extensive error analysis was also performed to determine how waveguide and noise distortion affect the performance of the spectral estimation algorithm.
Distributed fiber-optic strain and temperature sensors using photoinduced bragg gratings
Froggatt, Mark E. (Virginia Tech, 1995-02-05)
Much of the analytical and computational work necessary for the development of distributed fiber-optic strain sensors using photo induced Bragg gratings is presented. The one dimensional wave equation is solved for a slowly varying sinusoidal modulation of the index of refraction. The solution is found to take the form of a fourier transform for low reflectivity Â«15%) gratings. As a result, the process can be inverted, and if the phase and amplitude of the reflected light can be measured over frequency, the phase and amplitude of the bragg grating as a function of length can be computed using the inverse fourier transform. These results are computationally verified, and then further analysis of critical engineering parameters is carried out. A measurement system and procedure are described. A method of writing long, low-reflectivity bragg grating is proposed.
A dual wavelength fiber optic strain sensing system
Malik, Asif (Virginia Tech, 1996)
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.
Effect Of External Index Of Refraction on Multimode Fiber Couplers
Wang, G. Z.; Murphy, Kent A.; Claus, Richard O. (Optical Society of America, 1995)
The dependence of the performance of fused-taper multimode fiber couplers on the refractive index of the material surrounding the taper region has been investigated both theoretically and experimentally. It has been identified that for a 2 x 2 multimode fiber coupler there is a range of output-power-coupling ratios for which the effect of the external refractive index is negligible. When the coupler is tapered beyond this region, the performance becomes dependent on the external index of refraction and lossy. To analyze the multimode coupler-loss mechanism, we develop a two-dimensional ray-optics model that incorporates trapped cladding-mode loss and core-mode loss through frustrated total internal reflection. Computer-simulation results support the experimental observations. Related issues such as coupler fabrication and packaging are also discussed. (C) 1995 Optical Society of America
Electronic Signal Processing in an Optical Fiber-Based Magnetometer
Ranade, Jaydeep (Virginia Tech, 1997-09-11)
Conventional Extrinsic Fabry-Perot Interferometric (EFPI) sensors have been used to measure a variety of physical parameters like temperature, pressure and strain. A modified version of this geometry is successfully implemented to measure low-level magnetic field densities. A complete optical fiber based magnetometer system consisting of sensor head, laser source, opto-electronics, signal demodulation circuitry and display is designed, fabricated and tested. Various electronic signal demodulation schemes are discussed. The Quadrature Phase Shifted EFPI signal demodulation scheme, used in the system, is studied in detail. The scheme is designed and implemented in both analog and digital domain and the advantages and drawbacks of each type are presented. Primary limitations in the measurement of extremely low-level magnetic field densities are discussed and methods to overcome them using electronic signal processing schemes are suggested for future consideration.
Extrinsic Fabry-Perot Interferometer System Using Wavelength Modulated Source
Meller, Scott A. (Virginia Tech, 1996-12-04)
Interferometric optical fiber sensors have proved many orders of magnitude more sensitive than their electrical counterparts, but they suffer from limitations in signal demodulation caused by phase ambiguity and complex fringe counting when the output phase difference exceeds one fringe period. Various signal demodulation methods have been developed to overcome some of the these drawbacks with limited success. This thesis proposes a new measurement system for the extrinsic Fabry-Perot interferometer (EFPI) sensor. Using a wavelength modulated source and a novel extended-gap EFPI, some of the limitations of interferometric signal demodulation are overcome. By scanning the output wavelength of a multilongitudinal mode laser diode through current modulation, the EFPI sensor signal is scanned through multiple fringes. Gap movement is then unambiguously determined by monitoring the phase of the multiple fringe pattern.
Fiber optic grating-based weighted, two-mode fiber sensors
(United States Patent and Trademark Office, 1993-05-04)
Two-mode, elliptical-core optic fibers with a permanent photo-induced index change are used as sensors with sensitivity varying as a function of length. The optic fiber sensors act as vibrational-mode filters thereby performing initial signal processing of the sensor signal. The sensors are based on photo-induced refractive index changes. These refractive index changes affect the differential phase modulation between the LP.sub.01 and the LP.sub.11.sup.even modes. The change in beat-length is dependent on the amount of strain induced in the fiber while the grating is being formed. The pattern is thus varied along the length of the fiber by straining the fiber in a specific fashion while the grating is being written. This changes the sensitivity, of the sensor along its length. By choosing an appropriate weighting function in the manufacture of the sensor, it is possible to implement vibrational-mode analysis, vibrational-mode filtering and other functions that are critical in control system applications.
A fiber optic interferometer for measuring sub-micrometer displacements of ciliary bundles
Barrett, Matthew Donald (Virginia Tech, 1995-03-18)
The inner ear contains cells with ciliary bundles that have been identified as sites of mechanoelectrical transduction; they take a mechanical stimuli and convert it to an electrical response. The ciliary bundles vary structurally within the organs of the inner ear; this structural difference may play a role in the mechanical properties of each bundle. A relationship between the structure and the mechanics of the ciliary bundle can be found by studying structurally diverse bundles. To explore this relationship, a system was designed to mechanically stimulate the ciliary bundles in normal physiological range and measure their displacement. An extrinsic Fabry-Perot interferometer (EFPI) was developed to measure the response of a ciliary bundle that is subjected to a force applied by a glass whisker. 'Imitation bundles', similar in stiffness to living ciliary bundles, were made to test the system. The stiffness of an 'imitation bundle' was first determined by suspending styrene beads from its tip and optically measuring the resultant displacement. Then the EFPI was also used to determine the stiffness. The EFPI compared well to the stiffness found using the styrene beads; the largest difference between the two methods was 130/0. The EFPI was also tested in water to ensure its operation in the tissue environment; this test was successful in that it was able to measure displacements in a bundle's normal physiological range. With both of the tests showing good results, we conclude that our system can be used to measure the stiffness of the ciliary bundles located in the inner ear.
Fiberoptic Temperature Sensors Based on Differential Spectral Transmittance Reflectivity And Multiplexed Sensing Systems
Wang, A.; Wang, G. Z.; Murphy, Kent A.; Claus, Richard O. (Optical Society of America, 1995-07-01)
A concept for optical temperature sensing based on the differential spectral reflectivity/transmittance from a multilayer dielectric edge filter is described and demonstrated. Two wavelengths, lambda(1) and lambda(2), from the spectrum of a broadband light source are selected so that they are located on the sloped and flat regions of the reflection or transmission spectrum of the filter, respectively. As temperature variations shift the reflection or transmission spectrum of the filter, they change the output power of the light at lambda(1), but the output power of the light at lambda(2) is insensitive to the shift and therefore to the temperature variation. The temperature information can be extracted from the ratio of the light powers at lambda(1) to the light at lambda(2). This ratio is immune to changes in the output power of the light source, fiber losses induced by microbending, and hence modal-power distribution fluctuations. The best resolution of 0.2 degrees C has been obtained over a range of 30-120 degrees C. Based on such a basic temperature-sensing concept, a wavelength-division-multiplexed, temperature-sensing system is constructed by cascading three sensing-edge filters that have different cutoff wavelengths along a multimode fiber. The signals from the three sensors are resolved by detecting the correspondent outputs at different wavelengths.

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