Improving the Hufnagel-Andrews-Phillips refractive index structure parameter model using turbulent intensity.

This paper proposes a modification of the Hufnagel-Andrews-Phillips (HAP) Refractive Index Structure Parameter model that will better characterize the HAP profile against experimental data using the turbulent intensity, which is the ratio of wind speed variance to the average wind speed-squared, and Korean Refractive Index Parameter yearly statistics, Comparisons between this modified HAP model, the Critical Laser Enhancing Atmospheric Research 1 (CLEAR 1) profile model and several of the data sets are made. These comparisons highlight that this new model offers a more consistent representation of the averaged experimental data profiles than the CLEAR 1 model did. In addition, comparisons between this model and various experimental data set reported in the literature will show good agreement between the model and averaged data, and reasonable agreement with non-averaged data sets. This improved model should prove useful in system link budget estimates and atmospheric research.

Probability density function estimation for filament creation in lossy, turbulent, nonlinear media.

Optical Kerr effects induced by the propagation of high peak-power laser beams through real atmospheres have been a topic of interest to the nonlinear optics community for several decades. Previous work has focused on estimating the Filamentation Onset Distance (FOD) in real atmospheres but not its statistical variance. This paper describes two ad hoc engineering models for predicting the FOD Probability Density Function (PDF) for lossy, turbulent, nonlinear media. Specifically, these models characterize the FOD variation with turbulence. One model uses a log-normal PDF with mean and variance proportional to the Rytov Variance. The other uses a gamma PDF employing the same mean and variance equations. These two PDFs will be compared to previous computer simulation results. Both show reasonable agreement between PDF predictions and computer simulation results for long-range filamentation. In fact, both give similar results, and there is no preference given to the data comparisons presented.

Thermoelectric Energy Harvesting for Implantable Medical Devices.

A thermoelectric energy harvesting system is proposed to supplement the primary power supply of an implantable medical device. A low-power synchronous boost converter capable of boosting 10mV input voltage to 1V output voltage is designed in a 180nm CMOS process. To increase the charging speed, a maximum power transfer scheme is utilized. The harvester delivers up to 10µW power with a minimum temperature gradient of 1-degree, which results in a power efficiency over 40%.Clinical Relevance- The use of this technology could enhance the longevity of battery-operated implanted medical devices by reducing the possibility of invasive battery replacement surgery.

Optical Kerr effect field measurements and ad hoc engineering model comparisons.

Optical Kerr effects induced by the propagation of high peak-power laser beams through real atmospheres have been a topic of interest to the nonlinear optics community for several decades. This paper proposes a new analytical model for predicting the filamentation/light channel onset distance in real atmospheres based on modulation instability model considerations. The normalized intensity increases exponentially as the beam propagates through the medium. It is hypothesized that this growth can be modeled as a weighted ratio of the Gaussian beam diameter at range to the lateral coherence radius and can be used to set the power ratio for an absorbing, turbulent, nonlinear media to estimate the beam collapse distance. Comparison of onset distance predictions with those found from computer simulation and deduced from field experiments will be presented. In addition, this model will be used with an analytical approach to quantify the expected radius of light channels resulting from self-focusing both with and without the production of a plasma filament. Finally, this paper will describe a set of 1.5-micron, variable focal length USPL field experiments. Comparisons of theoretical radius calculations to measurements from field experiments will be presented.

Adaptive optics model characterizing turbulence mitigation for free space optical communications link budgets.

Free-space optical communications (FSOC) is becoming an important option for both atmospheric and space-based high data rate networks. Long-range, mobile FSOC links in the former environments must mitigate the effects of turbulence if they are to provide reliable, high link availability under cloud-free atmospheric conditions. Adaptive optics (AO) has been proposed as one means of reducing link degradation in turbulence, but field validated AO performance models are few and not definitive. These models are important if credible FSOC high performance links are expected to be deployed using AO systems. This paper reviews the Strehl ratio for the Andrews AO model and provides comparisons between predicted AO-based FSOC link budgets and measured link performance derived from several field trials. These results suggest that AO systems perform well under weak turbulence or short range under 10 km, but only offer limited tip/tilt gain in moderate turbulence and no gain under high/saturation regime turbulence at longer ranges. Long range links close to the ground in high moderate turbulence may degrade AO performance further.

Engineering equations for the filamentation collapse distance in lossy, turbulent, nonlinear media.

The propagation of high peak-power laser beams in real atmospheres has been an active research area for a couple of decades. Atmospheric turbulence and loss will induce decreases in the filamentation self-focusing collapse distance as the refractive index structure parameter and volume extinction coefficient, respectively, increase. This paper provides a validated analytical method for predicting the filamentation onset distance in lossy, turbulent, nonlinear media. It is based on a modification of Petrishchev's and Marburger theories. It postulates that the ratio of the peak power to critical power at range in turbulence is modified by a multiplicative, rather than additive, gain factor. Specifically, this new approach modifies the Petrishchev's turbulence equation to create the required multiplicative factor. This is necessary to create the shortened filamentation onset distance that occurs when a laser beam propagates through the cited nonlinear medium. This equation then is used with the Marburger distance and the Karr et al loss equations to yield the filamentation onset distance estimate in lossy, turbulent, nonlinear environment. Theory validation is done against two independent sets of computer simulation results. One comes from the NRL's HELCAP software and the other from MZA's Wave Train modeling software package. This paper also shows that once the zero-turbulence onset distance is set based on link loss, the addition of turbulence creates essentially the same PDFs at similar median distances for each loss case. This result had not been previously reported. This is the first quantitative comparison between closed form equations and computer simulation results characterizing filament generation in a lossy, turbulent, nonlinear medium.

Engineering equation for filamentation self-focusing collapse distance in atmospheric turbulence.

The propagation of high peak-power laser beams in real atmospheres will be affected by both linear and nonlinear effects contained therein. Atmospheric turbulence usually will induce decreases in the filamentation self-focusing collapse distance for refractive index structure parameter increases. This paper provided the first validated analytical equation for predicting the nonlinear self-focusing collapse distance based on a modification of Petrishchev's and Marburger's theories. It shows that the estimate of the peak power to critical power at range in turbulence is modified to be the product of the transmitted peak power to critical power ratio times a multiplicative factor derived from Petrishchev's turbulence equations. This estimate is used in the Marburger distance equation to yield a predicted self-focusing collapse distance. This approach was compared to previous NRL's HELCAP computer simulation results and showed good agreement. The HELCAP simulations capability has shown good agreement between its results and a previously published laboratory-scale experiment. The analytical approach in this paper may provide a guide for further numerical simulations, more formal theoretical developments and field experiments.

Calculating RF-Induced Voltages for Implanted Medical Devices in MRI Using Computational Human Models.

Despite its import as a diagnostic tool, patients with active implantable medical devices (AIMDs) are generally denied access to magnetic resonance imaging (MRI). The complexity of MRI environments stems from a multiplicity of fields and numerous scan parameters. In order to perform a risk assessment for RF-induced malfunction, manufacturers perform electromagnetic simulations using computational human models (CHMs) to calculate RF induced energy at the AIMD ports. This work explores the impact of the CHMs on the calculation of RF-induced voltages at the RF antenna port for cardiovascular implantable electronic devices (CIEDs).

Engineering equations for characterizing nonlinear laser intensity propagation in air with loss: erratum.

We present an erratum regarding the x-axis label in several figures, and one equation citation correction.

Engineering equations for characterizing non-linear laser intensity propagation in air with loss.

The propagation of high peak-power laser beams in real atmospheres will be affected at long range by both linear and nonlinear effects contained therein. Arguably, J. H. Marburger is associated with the mathematical characterization of this phenomenon. This paper provides a validated set of engineering equations for characterizing the self-focusing distance from a laser beam propagating through non-turbulent air with, and without, loss as well as three source configurations: (1) no lens, (2) converging lens and (3) diverging lens. The validation was done against wave-optics simulation results. Some validated equations follow Marburger completely, but others do not, requiring modification of the original theory. Our results can provide a guide for numerical simulations and field experiments.

Virtual Humans for Implantable Device Safety Assessment in MRI: Mitigating Magnetic Resonance Imaging Hazards for Implanted Medical Devices.

Magnetic resonance imaging (MRI) is the preferred modality for soft tissue imaging because of its nonionizing radiation and lack of contrast agent. Due to interactions between the MR system and active implantable medical devices (AIMDs), patients with implants such as pacemakers are generally denied access to MRI, which presents a detriment to that population. It has been estimated that 50-75% of patients with a cardiac device were denied access to MRI scanning and, moreover, that 17% of pacemaker patients need an MRI within 12 months of implantation [1]. In recent years, AIMD manufacturers, such as Biotronik, have assessed the conditional safety of devices in MRI.

SAW-LC coupled resonator wideband VCO for medical telemetry.

Wireless links with implantable devices can help in real-time monitoring of symptoms, irregularities, implanted device efficacy and their reconfiguration. We present the design of a low-power wideband voltage controlled oscillator (VCO) to facilitate implantable wireless telemetry. A coupled SAW-LC resonator design combines high Q and spectral purity of a SAW element and tunability of an LC-tank. The designed 2.7 MHz bandwidth VCO is suitable for full duplex communication protocols in the MedRadio band with frequency agility and higher duty cycles, in conformance with FCC regulations. Output power at the fundamental frequency is above -7.5 dBm for a wide range of load impedances. Output power load insensitivity provides a wide margin for selection of communication system parameters, variability in device placement, temporal variation in tissue properties and flexibility for implants at different locations (e.g. heart, gastrointestinal tract, brain). The maximum output power variation in the entire 2.7 MHz band is limited to 1.3 dBm. Sensitivity of oscillation frequency to loading can be addressed by individual device calibration. The small size, component count and low DC power consumption (1.9 V, ~1.95 mW) is favorable for including in a miniaturized and integrated design assembly with a battery-powered implanted device.

MR conditional safety assessment of implanted medical devices: advantages of computational human phantoms.

Until recently, patients with active implantable medical devices (AIMDs) have been contraindicated for magnetic resonance imaging (MRI). Current efforts to demonstrate safety of these devices separate the interaction of the device and MRI into several hazards. For several of these hazards, computational human phantoms (CHPs) are used to provide conservative conditions for a risk-based analysis. This use of CHPs for the identification of conservative conditions provides a substantial benefit to the assessment of MR conditional safety over experimental techniques, as the evaluation of millions of test cases is possible in simulation, but impractical (due to economic constraints) and, in some cases, unethical for an experimental effort.

Statistical detection of resolved targets in background clutter using optical/infrared imagery.

The use of optics to detect targets has been around for a long time. Early attempts at automatic target detection assumed target plus noise, which means that the targets were small compared to the pixel field of view and therefore unresolved. However, the advent of advanced focal plane technology has resulted in optical systems that can provide highly resolved target images. The intent of this paper is to develop a general solution for the detection of resolved targets in background clutter. We recognize that resolved targets obscure any background clutter that would have been visible if the targets were absent. An optimum detection algorithm is derived that compares a test statistic to a threshold and decides a target is present if the statistic is less than the threshold. We find that the detection performance depends upon (1) the apparent contrast rather than the signal to noise ratio and (2) is highly dependent on the background clutter to common system noise ratio. In fact, the target can still be detected even when the target contrast goes to zero provided the background clutter is greater than the common system noise. Computer simulations are shown to validate the theoretical detection and false alarm probabilities. The findings in this paper should be useful to engineers and scientists designing electro-optical and infrared sensors for finding resolved targets immersed in background-cluttered images.

High-sensitivity DPSK receiver for high-bandwidth free-space optical communication links.

A high-sensitivity modem and high-dynamic range optical automatic gain controller (OAGC) have been developed to provide maximum link margin and to overcome the dynamic nature of free-space optical links. A sensitivity of -48.9 dBm (10 photons per bit) at 10 Gbps was achieved employing a return-to-zero differential phase shift keying based modem and a commercial Reed-Solomon forward error correction system. Low-noise optical gain was provided by an OAGC with a noise figure of 4.1 dB (including system required input loses) and a dynamic range of greater than 60 dB.

Free-space optical communications link budget estimation.

This paper describes a new methodology of estimating free-space optical communications link budgets to be expected in conditions of severe turbulence. The approach is derived from observing that the ability of an adaptive optics (AO) system to compensate turbulence along a path is limited by the transmitter and receiver Rayleigh range, proportional to the diameter of the optics squared and inverse of the wavelength of light utilized. The method uses the Fried parameter computed over the range outside of the transmitter and receiver Rayleigh ranges, to calculate the Strehl ratios that yield a reasonable prediction of the light impinging on the receiving telescope aperture and the power coupling into the fiber. Comparisons will be given between theory and field measurements. These comparisons show that AO is most effective within the Rayleigh ranges, or when an atmospheric gradient is present, and lesser so when the total range is much greater than the sum of the Rayleigh ranges.

Probability of Interference between LP-LDC and LBT MICS implants in a medical care facility.

The latest generation of medical implants incorporate RF telemetry to facilitate communication of patient data to the patient's physician. Regulatory agencies have enabled medical implant telemetry by allocating RF spectrum in the 402-405 MHz band. The first generation of regulations mandated the use of a Listen-Before-Talk (LBT) protocol. Most of these regulators, recognizing the need for expanded services, are modifying the regulations to allow a Low-Power Low-Duty-Cycle (LP-LDC) protocol as an alternative access method. Medical implant device manufacturers incorporate mitigation techniques to maintain communication in the face of expected link impairments. Designers must understand the expected operational environment and probability of interference in order to incorporate appropriate levels of such mitigation techniques. In this study the authors use the SEAMCAT-3 modeling tool to examine the probability of interference between LP-LDC and LBT medical implants in a medical care facility, where a high density of implants using either protocol can be expected. This study shows that because LP-LDC transmitters operate with very low power and low duty cycle, they can safely coexist with LBT devices, with extremely low probability of interference. Furthermore, with appropriate mitigation techniques, the probability of any harmful interference is virtually non-existent.