Peter G. LoPresti

Dynamics of picosecond laser‐induced density, temperature, and flow‐reorientation effects in the mesophases of liquid crystals

A detailed theoretical and experimental study of laser‐induced density and temperature changes, and flow‐reorientation effects in the nematic and smectic phases of liquid crystals is presented. Using picosecond lasers, the initial nanosecond dynamics of the photoinduced density waves, temperature buildup, and relaxations are temporally resolved. The experimentally observed relaxation phenomena and time scales are in good agreement with the theoretical expressions obtained by analytical solutions of the coupled hydrodynamical equations describing these fundamental mechanisms. Our new measurement and theory provide a quantitative account of the relative contribution from the electrostrictive and thermoelastic contributions that had not been presented in previous studies. Our study of the smectic phase has conclusively established the mechanism for the formation of erasable and permanent grating effects under short‐laser‐pulse excitation as laser‐induced electrostrictive and thermoelastic effects.

Handbook of neuroprosthetic methods

INTRODUCTION Introduction to Neuroprosthetics Warren E. Finn, Oklahoma State University Center for Health Sciences Peter G. LoPresti, University of Tulsa NEURONS AND NEURON MODELING Neuron Excitability: Membrane Ion Channels Steven Barnes, Dalhousie University Neuron Modeling Frank Ratty, Vienna University of Technology Robert Greenberg, Second Sight LLC Suzanne Resatz, Vienna University of Technology STIMULATING AND RECORDING OF NERVES AND NEURONS Stimulating Neural Activity James Weiland, Doheny Retina Institute, University of Southern California Extracellular Electrical Stimulation of Central Neurons: Quantitative Studies Dongchul Lee and Cameron McIntyre, John Hopkins University Warren Grill, Case Western Reserve University Semiconductor-Based Implantable Microsystems Wentai Liu, North Carolina State University Silicon Micro-electrodes for Extracellular Recording Jamille Hetke and David Anderson, University of Michigan PROCESSING NEURAL SIGNALS Wavelet Methods in Biomedical Signal Processing Kevin Englehart, Philip Parker, and Bernard Hudgins, University of New Brunswick Neuroprosthetic Device Design Donald Russell, Carleton University PROSTHETIC SYSTEMS Implantable Electronic Otologic Devices for Hearing Rehabilitation Kenneth Dormer, University of Oklahoma Health Science Center Visual Neuroprostheses David J. Warren and Richard A. Normann, University of Utah Motor Prostheses Richard T. Lauer, Cleveland Department of Veterans Affairs Medical Center P. Hunter Peckham, Case Western Reserve University EMERGING TECHNOLOGIES Neurotechnology: Microelectronics Danny Banks, ICT Centre, UK Biomolecular Electronics Michael C. Petty and C. Pearson, University of Durham Appendix: Summary of Computer Programs for Neuroprosthetic Analysis and Design

Enlargement of Beam Coverage in FSO Mobile Network

Pointing, acquisition and tracking of a free-space optical node in a mobile network experiencing misalignment due to adverse factors including vibration, motion and atmospheric turbulence requires a different approach than traditional free-space optical transceivers. A recent fiber-bundle approach for beam steering at the transmitter was investigated to provide continuous beam coverage at the receiver without the application of mechanical devices. Utilizing multiple fibers-lenses sets at the receiver was also proposed to enhance the tolerance of optical link misalignment. This paper investigates the feasibility of applying multiple fibers with a single lens at the transmitter and multiple fiber-lenses sets at the receiver to improve the transmission performance on tolerating link misalignment. Two series of laboratorial experiments were implemented to compare the optical link robustness for different transceiver setups as the fiber-lens distance (zfl) was varied to modify the beam divergence: multi-fiber transmitter working with single-fiber-lens receiver and multi-fiber transmitter working with multi-fiber-lens receiver. The results showed that using multiple fibers in both the transmitter and the receiver reveals significant potential to maximize the up time of the link.

Free-space optical wavelength diversity scheme for fog mitigation in a ground-to-unmanned-aerial-vehicle communications link

Atmospheric weather conditions adversely affect the performance of free-space optical communications systems. Fog present in the atmosphere has the largest impact on free-space optical systems. We use simulation techniques to investigate the performance of a slant-path, wavelength-diversified free-space optical link between a ground station and an unmanned aerial vehicle in the presence of radiation fog. The free-space optical link is configured to operate using the simultaneous transmission of three wavelengths, 0.85, 1.55, and 10 µm, connecting a ground station to an unmanned aerial vehicle operating at either a 4- or 8-km altitude. The link is further analyzed by combining the multiple carrier wavelengths into either an equal-gain diversity scheme or a selective diversity scheme. The simulation results are obtained using ALTM and PcModWin simulation software from Ontar Corporation. The results obtained demonstrate that the use of either an equal-gain diversity scheme or a selective diversity scheme enables the deployment of a ground-to-air free-space optical communications link. Furthermore, a selective diversity scheme demonstrates a received power approximately three times greater than an equal-gain diversity scheme.

Thresholding-Based Optimal Detection of Wireless Optical Signals

The receiver design for a high-speed free-space (wireless) optics (FSO) signal is necessarily highly complex when channel state information (CSI) is not available. Currently, although most approaches provide high detection performance in terms of bit error, receiver design is difficult to implement. This paper proposes two practical thresholding-based detection schemes, which offer significant improvement to receiver throughput on a computational load basis when CSI is not available. The first is based on a simple maximum likelihood (ML) function where the bit error rate (BER) is the same as conventional symbol-by-symbol detection. This method, however, causes a loss of BER performance. The second uses the aid of pilot-symbol-assisted modulation (PSAM) to modify the ML function when channel coefficients are temporally correlated. While numerical analysis based on this method shows that the BER performance in a log-normally distributed fading channel is very close to detection achieved with perfect CSI, the receiver suffers from increased complexity. If random processes for fading and noise are assumed as stationary and given that the detection threshold is quickly calculated and applied during a given period, such complexity of PSAM-based and symbol-by-symbol detection methods can be reduced.

BER analysis of optical wireless signals through lognormal fading channels with perfect CSI

Due to inconsistent atmospheric conditions, scattering and scintillation of free space optical (FSO) signal can occur, thus negatively influencing the received signal intensity. The channel is usually modeled as a normalized fading coefficient with additive Gaussian noise. Optimal detection of the received signal is designed based on a decision rule, e.g., Maximum Likelihood (ML), assuming the receiver knows the noise statistics and fading correlation of the channel. This paper briefly deals with analysis on bit error rate (BER) of a wireless optical signal passing through a lognormally distributed fading channel, when perfect knowledge of channel state information (CSI) at the receiver side is available. Two approaches will be presented to provide closed-form expressions for BER. One uses Gauss-Hermite quadrature approximation and the other one is based on power series. While numerical analysis shows a very small approximation error when the Gauss-Hermite approach is considered, the power series approach does not uses any approximation.

Availability Modeling of FSO/RF Mesh Networks through Turbulence-Induced Fading Channels

Even if a line-of-sight condition of Free Space Optics (FSO) is satisfied, atmospheric-induced fading, scattering, and attenuation may severely deteriorate the availability of the communication link. This argument is true except in reconfigurable FSO ad hoc networks, a path reconfiguration scheme replaces a severed FSO link with an operational one. Reconfigurability, as a property of our hybrid FSO/RF (Radio Frequency) work in progress, provides connection reliability and network throughput. The traffic can be directed to a different FSO link or even an RF link as backup. Hence, node failure and outage probabilities due to link failure will be reduced, thus resulting in a higher availability of nodes. Mathematical investigation and statistical consideration of availability and capacity of FSO/RF ad hoc mesh network is the focus of this paper. We assume normalized scintillation fading channels in our analysis. We apply different availability cases of channel state information to derive closed-form expressions for system capacity.

Circular MIMO FSO Nodes With Transmit Selection and Receive Generalized Selection Diversity

Mobile free space optical (FSO) nodes have been characterized as a way to resolve movement and misalignment incompatibilities that are inherent in optical nodes. By introducing multibranch angular diversity mobility to FSO nodes, connection reliability is enhanced. In this paper, receiving branches are assumed to be circularly placed on a single plane, and transmitting branches are positioned on a different platform. A coverage of 360 in this unbalanced branch configuration is achieved with the deployment of diversity combining, wherein the transmitter follows a transmit selection diversity rule. Channel turbulence is modeled as weak lognormal with spatially correlated fading samples. On the receiver side, selection combining (SC), equal gain combining (EGC), maximum ratio combining (MRC), and threshold generalized SC (T-GSC) schemes serve as candidates for combining purposes. Statistical discussion relative to branch signals and combiner output is presented, and bit error performance and outage probability are numerically evaluated. When compared with MRC, T-GSC was found to have lower estimation error. The T-GSC scheme additionally demonstrated overall superior performance over SC and EGC for mobile FSO.

On the Capacity of Hybrid FSO/RF Links

Hybrid Free Space Optics (FSO)/Radio Frequency (RF) communication systems have emerged as a way to improve network performance by providing enhanced availability and reliability. In an effort to mitigate individual drawbacks in the optical link during adverse weather conditions, network traffic flows simultaneously between channels. Based on the Shannon-Hartley theorem, channel capacity is dependent, among others, on both the signal to noise ratio (SNR) and channel bandwidth. As such, combined link throughput may be affected by channel state conditions. Because atmosphere turbulence can be modeled as a time-varying fading channel, capacity analysis can be investigated. In this paper, authors apply various availability scenarios of channel state information (CSI) on the optical link to derive closed-form expressions for combined link capacity. Numerical simulation provides a comparison to the results.

An Electrical Engineering Summer Academy for Middle School and High School Students

An Electrical Engineering Summer Academy for Pre-College Students was held at the University of Tulsa, Tulsa, OK, during the summers of 2007 and 2008. The Academy participants included students having just completed 7th to 11th grade and teachers from middle school through high school. The students and teachers participated in team-building, professional development, and technical activities designed to teach them about the engineering profession and the field of electrical engineering. Activities included laboratories in electrical circuits, designing an electric car, soldering, a field trip, and discussion about ethics. Students worked in two- and four-person teams and made presentations on their experiences. The Academy was evaluated using formal assessment instruments and faculty observations. Both individual activities and the overall program were evaluated. The evaluation of the 2007 Academy revealed ratings of 4.5 or greater out of 5 on most aspects of the Academy, though some areas indicated a need for improvement, such as clarity of written materials and the availability of additional material for advanced students who finished early. Improvement in knowledge was demonstrated on over 50% of the questions on a survey administered at the beginning and again at the end of the Academy. Changes were proposed and implemented for the 2008 Academy in response to the assessment data. The impact of these changes and the lessons learned in the process are also presented.