Carvel E. Holton

Colloidal quantum dots entrained in microstructured optical fibers

We present the initial results of entraining colloidal quantum dots emitting at wavelengths from 0.5um through 1.2um, in various micro-structured optical fibers. Conventional and non-conventional, micro-structured optical fibers fabricated at Virginia Tech’s Fiber & ElectroOptics Research Center (FEORC) have been combined with semiconductor, colloidal quantum dots fabricated by the VT Advanced Biomedical Center (VTabc). The results are presented primarily in the form of visual verification and analysis of entrainment phenomena, for a cross-section of colloidal dot and micro-structured fiber forms. Unique optical, electro-optical and material properties resulting from the combinations are visibly suggested in the results. Core/clad/free space propagation properties and effects of emitted and absorbed light fields are observed to be dependent on the structure, aspect ratio and materials of the fibers as well as the properties of the colloidal quantum dots. Basic spectral data on representative free-space materials will be presented in the current paper. The presentation will explore in passing, the research options available to such quantum dot-fiber combinations, including advanced sensors, sources and filters.

Dynamic model for the wheel-rail contact friction

Accurately estimating the coefficient of friction (CoF) is essential in modelling railroad dynamics, reducing maintenance costs, and increasing safety in rail operations. The typical assumption of a constant CoF is widely used in theoretical studies; however, it has been noticed that the CoF is not constant, but rather depends on various dynamic parameters and instantaneous conditions. In this paper, we present a newly developed three-dimensional nonlinear CoF model for the dry rail condition and test the CoF variation using this model with estimated dynamic parameters. The wheel–rail is modelled as a mass–spring–damper system to simulate the basic wheel–rail dynamics. Although relatively simple, this model is considered sufficient for the purpose of this study. Simulations are performed at a train speed of 20 m/s using rail roughness as an excitation source. The model captures the CoF extremes and illustrates its nonlinear behaviour and instantaneous dependence on several structural and dynamic parameters.

Stochastic Analysis of the Wheel-Rail Contact Friction Using the Polynomial Chaos Theory

The coefficient of friction (CoF) is a very important factor for designing, operating, and maintaining the wheel-rail system. In the real world, accurate estimation of the CoF at the wheel-rail interface is difficult due to the effects of various uncertain parameters, e.g., wheel and rail materials, rail roughness, contact patch size, and so on. In this study, a stochastic analysis using polynomial chaos (poly-chaos) theory is performed with the newly developed 3D dry CoF model at the wheel-rail contact. The wheel-rail system is modeled as a mass-spring-damper system. Stochastic analyses with one uncertainty, combinations of two uncertainties, and a combination of three uncertainties are performed. The probability density function (PDF) results for stick CoF, slip CoF, and combined (total) CoF are presented. The stochastic analysis results show that the total CoF PDF before 1 s is dominantly affected by the stick phenomenon, whereas the slip dominantly influences the total CoF PDF after 1 s. The CoF PDFs obtained from simulations with combinations of two and three uncertain parameters have wider PDF ranges than those obtained for only one uncertain parameter. The current work demonstrates that the CoF is strongly affected by the stochastic variation of dynamic parameters. Thus, the PDF distribution of the CoF could play a very important role in the design of the wheel-rail system.

Fiber optics in meteorological instrumentation suites

Standard meteorological sensors and sensor suites used for weather and environmental monitoring are currently based primarily on electronic instrumentation that is frequently susceptible to destruction and/or interruption from natural (e.g. lightning) and man-made sources of Electromagnetic Interference (EMI). The cost of replacement or shielding of these systems is high in terms of frequency of replacement and the incipient capital cost. Sensors based on optical fibers have been developed in sufficient variety as to allow the development of full meteorological instrumentation suitess based on individual or multiplexed optical fiber sensors. Examples of sensing functions which can be implemented using optical fibers include: wine speed (cup anemometers & Doppler lidars), wind direction (vanes & lidars), temperature, humidity, barometric pressure, accumulated precipitation and precipitation rate (fiber lidar). Suites of such sensors are capable of using little or no electronics in the environmentally exposed regions, substantially reducing system EMI susceptibility and adding functional capability. The current presentation seeks to explore options available in such meteorological suites and examine the issues in their design and deployment. Performance data on several newer fiber sensors suitable to meteorological use will be presented and discussed.

Performance of optical fiber sensors embedded in polymer matrix composites for 15 years

The objective of this paper is to describe the performance of optical fiber sensors that have been embedded within polymer matrix composites (PMCs) for more than 15 years. This paper was included in a session concerning the history of sensors for smart materials and structures, and was intended as an overview of early work in this area. Our group at Virginia Tech has been involved in the use of embedded fiber sensors since 1978 when R. Claus, then at the NASA Langley Research Center, helped embed sensors in PMCs to monitor cure and post-cure strain and temperature. Our oldest surviving cross-ply laminate composite specimen with embedded fiber sensors dates from 1982, and was fabricated on campus using a hot platen press. We have recently physically examined this specimen to study possible degradation of the material in the vicinity of the embedded fiber elements, and interrogated the embedded sensors using intensity, modal, interferometric and time-of-flight measurement systems. The basic conclusions of this work thus far are 1) the sensor fibers are still functional, 2) the sensor leads have not been sheared off the specimen after 15 years of use, 3) the composite specimen has not delaminated or otherwise showed signs of degradation, and 4) problems concerning the motion of sensor elements within curing systems, the interconnect problem, and cross-sensitivities that were difficulties in the early 1980s remain key issues today.

Patterned alignment liquid crystal diffractive spatial light modulators and devices

Recent advances in diffractive liquid crystal (LC) modulation structures for projection display service have generated diffractive spatial light modulator (SLM) devices capable of high performance in transmission and/or reflection. Patterning of the LC domain alignments has resulted in the generation of near perfect phase diffraction grating structures in the lC material, which can be controlled by a single electrode or by an array of segmented electrodes, such as an array of pixels. Consequently, diffractive artifacts from inter-electrode gaps can be eliminated and/or suppressed in a wider spectrum of applications with enhanced performance relative to previous structures. Transmissive diffractive LC modulators suitable for display or switch applications with modulation efficiencies in excess of 90 percent and contrast ratios of greater than 1500:1 in un-polarized light have been demonstrated. Operation of this type of modulator in reflection has also been verified and demonstrated. Comparison of patterned alignment, LC device performance with MEMS and other diffractive structures will be given. Extensions of these techniques to devices which include beam scanners and controllable diffractive optics will be proposed.

OPTICAL FIBER MAGNETIC FIELD SENSORS AND SIGNAL PROCESSING FOR VEHICLE DETECTION AND CLASSIFICATION

A miniaturized optical fiber magnetometer has been designed, assembled and evaluated for the detection and classification of vehicles. The sensor element consists of a Fabry-Perot cavity formed between the parallel ends of a high quantity cylindrical metallic glass ribbon and a single mode optical fiber, both held int a hollow tube with an inner diameter several microns larger than the ribbon and the fiber. This sensor head is then potted in a small-diameter, rugged, nonmagnetic housing to allow handling and installation into a protected section of parking lot or other environment to enable vehicle signature detection and analysis. The minimum detectable magnetic field is on the order of 100 nT at dc. The sensor has been used to evaluate the potential detection and analysis of vehicle signatures. When material in a vehicle passes nearby the sensor element, it perturbs the magnetic field and produces a complex output signal dependent upon the shape of the ferrous material in the vehicle, its distance and orientation with respect to the sensor element, and the sped of the vehicle. We have also considered the use of wavelet methods to allow the processing of such data, because it allows variations in differential phasing corresponding to varying vehicles speeds.

Wheel-Rail Dynamic Model and Stochastic Analysis of the Friction in the Contact Patch

The coefficient of friction (CoF) is one of the most important parameters for characterizing the contact between the wheel and the rail. The assumption of a constant CoF is still used in most theoretical studies, although experimental work indicates that the CoF depends on material and dynamic parameters. In the real world, accurate estimation of the CoF is not simple due to various uncertainties. In this paper we present a new 3D nonlinear dry CoF model at the wheel-rail contact. In addition, a stochastic analysis using the polynomial chaos theory is performed with the CoF model. The maximum amplitude of rail roughness and the lateral displacement of the wheel are considered as uncertain parameters in this study. One of the novelties in this study is that our CoF model captures the maximum CoF value (an initial peak) when the wheel starts to move. The stochastic analysis results show that the CoF probability density function (PDF) of a combination of two uncertain parameters has wider PDF ranges than the PDF obtained for only one uncertain parameter. The current work demonstrates that the CoF is strongly affected by the stochastic variation of dynamic parameters. In reality, the CoF is critical to rail tractive performance and efficiency. Thus, the PDF distribution of the CoF must be accounted for in the design of the wheel-rail system.Copyright

An Empirical Approach to Modeling the Friction Coefficient for Wheel-Rail Contact

The coefficient of friction (CoF) is one of the most important parameters for the contact between the wheel and the rail. Accurate estimation or measurement of the CoF has a very important role, both in terms of modeling the train dynamics and in terms of reducing operational costs in the long-term. For ease of implementation, since the nature of the wheel-rail contact dynamics is very complex, the assumption of a constant CoF is still used in most theoretical studies. Nevertheless, experimental work indicates that the CoF depends on dynamic changes in various wheel-rail conditions, like sliding velocity, contact patch shape and size for stick and sliding region, wheel and rail geometry, wheel vibration, rail surface roughness and/or lubrication, etc. In this paper we present the proposed equation to model the nonlinear dry friction coefficient at the wheel-rail contact. The friction coefficient is calculated at the three different values for change in the damping ratio while maintaining all the other conditions the same. As expected, the analysis performed to estimate the dry friction coefficient based on the proposed equation and using NUCARS® simulation results shows that the coefficient of friction has a highly nonlinear dependence on its parameters.Copyright