Atmospheric fogs create degraded visual environments, making it difficult to recover optical information from our surroundings. We have developed a low-SWaP technique which characterizes these environments using an f-theta lens to capture the angular scattering profile of a pencil beam passed through a fog. These measurements are then compared to data taken in tandem by conventional characterization techniques (optical transmission, bulk scattering coefficient, etc.). We present this angular scattering measurement as a low-SWaP alternative to current degraded visual environment characterization techniques to provide real-time data for implementation with signal recovery algorithms.
Event-based sensors are a novel sensing technology which capture the dynamics of a scene via pixel-level change detection. This technology operates with high speed (>10 kHz), low latency (10 μs), low power consumption (<1 W), and high dynamic range (120 dB). Compared to conventional, frame-based architectures that consistently report data for each pixel at a given frame rate, event-based sensor pixels only report data if a change in pixel intensity occurred. This affords the possibility of dramatically reducing the data reported in bandwidth-limited environments (e.g., remote sensing) and thus, the data needed to be processed while still recovering significant events. Degraded visual environments, such as those generated by fog, often hinder situational awareness by decreasing optical resolution and transmission range via random scattering of light. To respond to this challenge, we present the deployment of an event-based sensor in a controlled, experimentally generated, well-characterized degraded visual environment (a fog analogue), for detection of a modulated signal and comparison of data collected from an event-based sensor and from a traditional framing sensor.
Atmospheric fog is a common degraded visual environment (DVE) that reduces sensing and imaging range and resolution in complex ways not fully encapsulated by traditional metrics. As such, better physical models are required to describe imaging systems in a fog environment. We have developed a tabletop fog chamber capable of creating repeatable fog-like environments for controlled experimentation of optical systems within this common DVE. We present measurement of transmission coefficients and droplet size distribution in a multiple scattering regime using this chamber.
Degraded visual environments like fog pose a major challenge to safety and security because light is scattered by tiny particles. We show that by interpreting the scattered light it is possible to detect, localize, and characterize objects normally hidden in fog. First, a computationally efficient light transport model is presented that accounts for the light reflected and blocked by an opaque object. Then, statistical detection is demonstrated for a specified false alarm rate using the Neyman-Pearson lemma. Finally, object localization and characterization are implemented using the maximum likelihood estimate. These capabilities are being tested at the Sandia National Laboratory Fog Chamber Facility.
Dangerous materials present in factories and military combat locations, can cause negative effects to the human body and can be life threatening. Due to this, a portable, easily maintained, and robust sensor is required to detect CWA’s, TIC’s, and TIM’s. We present a method to grow MOFs on quartz crystal microbalances (QCM’s) for sensitive, selective detection of CWA’s. Our next step is to test the sensitivity and selectivity of the MOF to dimethyl methylphosphonate (DMMP) when under varying environmental conditions.
This communication reports progress towards the development of computational sensing and imaging methods that utilize highly scattered light to extract information at greater depths in degraded visual environments like fog for improved situational awareness. As light propagates through fog, information is lost due to random scattering and absorption by micrometer sized water droplets. Computational diffuse optical imaging shows promise for interpreting the detected scattered light, enabling greater depth penetration than current methods. Developing this capability requires verification and validation of diffusion models of light propagation in fog. We report models that were developed and compared to experimental data captured at the Sandia National Laboratory Fog Chamber facility. The diffusion approximation to the radiative transfer equation was found to predict light propagation in fog under the appropriate conditions.
Degraded visual environments are a cause of problems for surveillance systems and other sensors due to the reduction in contrast, range, and signal. Fog is a concern because of the frequency of its formation along our coastlines; disrupting border security and surveillance. Sandia has created a Fog Facility for the characterization and testing of optical and other systems. We will present a comparison of our generated fogs to the measured naturally occurring fogs reported in the literature and an overview of Sandia’s work using this facility to investigate ways to enhance perception through degraded visual environments.
Exposure to chemicals in everyday life is now more prevalent than ever. Air and water pollution can be delivery mechanisms for toxins, carcinogens, and other chemicals of interest (COI). A compact, multiplexed, chemical sensor with high responsivity and selectivity is desperately needed. We demonstrate the integration of unique Zr-based metal organic frameworks (MOFs) with a plasmonic transducer to demonstrate a nanoscale optical sensor that is both highly sensitive and selective to the presence of COI. MOFs are a product of coordination chemistry where a central ion is surrounded by a group of ligands resulting in a thin-film with nano- to micro-porosity, ultra-high surface area, and precise structural tunability. These properties make MOFs an ideal candidate for gaseous chemical sensing, however, transduction of a signal which probes changes in MOF films has been difficult. Plasmonic sensors have performed well in many sensing environments, but have had limited success detecting gaseous chemical analytes at low levels. This is due, in part, to the volume of molecules required to interact with the functionalized surface and produce a detectable shift in plasmonic resonance frequency. The fusion of a highly porous thin-film layer with an efficient plasmonic transduction platform is investigated and summarized. We will discuss the integration and characterization of the MOF/plasmonic sensor and summarize our results which show, upon exposure to COI, small changes in optical characteristics of the MOF layer are effectively transduced by observing shifts in plasmonic resonance.
Fog is a commonly occurring degraded visual environment which disrupts air traffic, ground traffic, and security imaging systems. For many application of interest, spatial resolution is required to identify elements of the scene. However, studying the effects of fog on resolution degradation is difficult because the composition of naturally occurring fogs is variable, and data collection is reliant on changing weather conditions. For our study, we used the Sandia National Laboratories fog facility to generate repeatable characterized fog conditions. Sandia’s well characterized fog generation allowed us to relate the resolution degradation of active and passive long-wave infrared (LWIR) imagers to the properties of fog. Additionally, the fogs we generated were denser than naturally occurring fogs. This allowed for testing of long range imaging in the shorter optical pathlengths obtainable in a laboratory environment.
In this presentation, we experimentally investigate the resolution degradation of LWIR wavelengths in realistic fog droplet sizes. Transmission of LWIR wavelengths has been studied extensively in literature. To date however, there are few experimental results quantifying the resolution degradation for LWIR imagery in fog. We present experimental results on resolution degradation for both passive and active LWIR systems. The degradation of passive imaging was measured using 37˚C blackbody with a slant edge resolution targets. The active imaging resolution degradation was measured using a polarized CO2 laser reflecting off a set of bar targets. We found that the relationship between meteorological optical range and resolution degradation was more complicated than described purely by attenuation.
Heavy fogs and other highly scattering environments pose a challenge for many commercial and national security sensing systems. Current autonomous systems rely on a range of optical sensors for guidance and remote sensing that can be degraded by highly scattering environments. In our previous and on-going simulation work, we have shown polarized light can increase signal or range through a scattering environment such as fog. Specifically, we have shown circularly polarized light maintains its polarized signal through a larger number of scattering events and thus range, better than linearly polarized light. In this work we present design and testing results of active polarization imagers at short-wave infrared and visible wavelengths. We explore multiple polarimetric configurations for the imager, focusing on linear and circular polarization states. Testing of the imager was performed in the Sandia Fog Facility. The Sandia Fog Facility is a 180 ft. by 10 ft. chamber that can create fog-like conditions for optical testing. This facility offers a repeatable fog scattering environment ideally suited to test the imager’s performance in fog conditions. We show that circular polarized imagers can penetrate fog better than linear polarized imagers.
Degraded visual environments remain a concern for national security applications that require continuous imaging. Fog, due to its prevalence in coastal regions, interrupts surveillance, harbor security, and transportation with notable economic impact. Fog reduces visibility by scattering ambient light and active illumination thereby obscuring the environment and limiting operational capacity. Here we will present our work on two major capabilities for testing polarized light transport in a fog like condition caused by water droplet aerosols. Sandia utilizes a fully polarimetric Monte Carlo simulation to predict transmission performance in varying degraded optical conditions. We have extended this capability to cover the conditions that occur in our second capability that allows for repeatable testing of optical systems in a man-made fog analogue. This capability is a facility for performing optical propagations experiments in a known, stable, and controlled environment where fog can be made on demand. This facility is a 180 ft. by 10 ft. by 10 ft. chamber with temperature control with 80 agricultural spray nozzles. We will discuss the characterization of the fog and instrumentation used for the characterization. We will summarize and present new results from the work performed under the internally funded research program that developed Sandia’s fog capabilities. This includes a short-wave infrared snapshot imaging polarimeter for enhanced contrast in degraded visual environments and investigations of the degradation of image quality in the long-wave infrared waveband with passive and active illumination.
The scattering of light in fog is a complex problem that affects imaging in many ways. Typically, imaging device performance in fog is attributed solely to reduced visibility measured as light extinction from scattering events. We present a quantitative analysis of resolution degradation in the long-wave infrared regime. Our analysis is based on the calculation of the modulation transfer function from the edge response of a slant edge blackbody target in known fog conditions. We show higher spatial frequencies attenuate more than low spatial frequencies with increasing fog thickness. These results demonstrate that image blurring, in addition to extinction, contributes to degraded performance of imaging devices in fog environments.
Degraded visual environments are a cause of problems for surveillance systems and other sensors due to the reduction in contrast, range, and signal. Fog is a concern because of the frequency of its formation along our coastlines; disrupting border security, shipping, surveillance, and sometimes causing deadly accidents. Fog reduces visibility by scattering ambient/active illumination light obscuring the environment and limiting operational capability. Sandia has created a fog facility for the characterization and testing of optical and other systems. This facility is a 180 ft. by 10 ft. by 10 ft. chamber with temperature control that can be filled with a fog-like aerosol using 64 agricultural spray nozzles. We will discuss the physical formation of fog and how that is affected by the environmental controls at our disposal. We have recently made several improvements to the facility including temperature control and will present the results of these improvements on the aerosol conditions. We will discuss the characterization of the fog and instrumentation used for the characterization. In addition, we will present preliminary results from work at Sandia, that leveraged this facility to investigate using polarized light to enhance the range of optical systems in fog conditions. This capability provides a platform for performing optical propagations experiments in a known, stable, and controlled environment where fog can be made on demand.
Heavy fogs and other highly scattering environments pose a challenge for many commercial and national security sensing systems. Current autonomous systems rely on a range of optical sensors for guidance and remote sensing that can be degraded by highly scattering environments. In our previous and on-going simulation work, we have shown polarized light can increase signal or range through a scattering environment such as fog. Specifically, we have shown circularly polarized light maintains its polarized signal through a larger number of scattering events and thus range, better than linearly polarized light. In this work we present an active polarization imager in the short-wave infrared. We explore multiple polarimetric configurations for the imager, focusing on linear and circular polarization states. We also describe initial testing of the imager in the Sandia Fog Facility. The Sandia Fog Facility is a 180 ft. by 10 ft. chamber that can create fog-like conditions for optical testing. This facility offers a repeatable fog scattering environment ideally suited to test the imager’s performance in fog conditions.
We report on the design, modeling, calibration, and experimental results of a LWIR, spectrally and temporally resolved broad band bi-directional reflectance distribution function measuring device. The system is built using a commercial Fourier transform infrared spectrometer, which presents challenges due to relatively low power output compared to laser based methods. The instrument is designed with a sample area that is oriented normal to gravity, making the device suitable for measuring loose powder materials, liquids, or other samples that can be difficult to measure in a vertical orientation. The team built a radiometric model designed to understand the trade space available for various design choices as well as to predict instrument success at measuring the target materials. The radiometric model was built by using the output of commercial non sequential raytracing tools combined with a scripted simulation of the interferometer. The trade space identified in this analysis will be presented.
The design was based on moving periscopes with custom off axis parabolas to focus the light onto the sample. The system assembly and alignment will be discussed. The calibration method used for the sensor will be detailed, and preliminary measurements from this research sensor will be presented.
Ultrafast optical microscopy is an important tool for examining fundamental phenomena in semiconductor nanowires with high temporal and spatial resolution. Here, we used this technique to study carrier dynamics in single GaN/InGaN core−shell nonpolar multiple quantum well nanowires. We find that intraband carrier−carrier scattering is the main channel governing carrier capture, while subsequent carrier relaxation is dominated by three-carrier Auger recombination at higher densities and bimolecular recombina tion at lower densities. The Auger constants in these nanowires are approximately 2 orders of magnitude lower than in planar InGaN multiple quantum wells, highlighting their potential for future light-emitting devices.
Degraded visual environments are a serious concern for modern sensing and surveillance systems. Fog is of interest due to the frequency of its formation along our coastlines disrupting border security and surveillance. Fog presents hurdles in intelligence and reconnaissance by preventing data collection with optical systems for extended periods. We will present recent results from our work in operating optical systems in our controlled fog experimental chamber. This facility is a 180-foot-long, 10-foot-wide, and 10-foot-tall structure that has over 60 spray nozzles to achieve uniform aerosol coverage with various particle size, distributions, and densities. We will discuss the physical formation of fog in nature and how our generated fog compares. In addition, we will discuss fog distributions and characterization techniques. We will investigate the biases of different methods and discuss the different techniques that are appropriate for realistic environments. Finally, we will compare the data obtained from our characterization studies against accepted models (e.g., MODTRAN) and validate the usage of this unique capability as a controlled experimental realization of natural fog formations. By proving the capability, we will enable the testing and validation of future fog penetrating optical systems and providing a platform for performing optical propagation experimentation in a known, stable, and controlled environment.
Scattering environment conditions, such as fog, pose a challenge for many detection and surveillance active sensing operations in both ground and air platforms. For example, current autonomous vehicles rely on a range of optical sensors that are affected by degraded visual environments. Real-world fog conditions can vary widely depending on the location and environmental conditions during its creation. In our previous work we have shown benefits for increasing signal and range through scattering environments such as fog utilizing polarized light, specifically circular polarization. In this work we investigate the effect of changing fog particle sizes and distributions on polarization persistence for both circularly and linearly polarized light via simulation. We present polarization tracking Monte Carlo results for a range of realistic monodisperse particle sizes as well as varying particle size distributions as a model of scattering environments. We systematically vary the monodisperse particle size, mean particle size of a distribution, particle size distribution width, and number of distribution lobes (bi-modal), as they affect polarized light transmission through a scattering environment. We show that circular polarization signal persists better than linear polarization signal for most variations of the particle distribution parameters.
We present experimental and simulation results for a laboratory-based forward-scattering environment, where 1 μm diameter polystyrene spheres are suspended in water to model the optical scattering properties of fog. Circular polarization maintains its degree of polarization better than linear polarization as the optical thickness of the scattering environment increases. Both simulation and experiment quantify circular polarization’s superior persistence, compared to that of linear polarization, and show that it is much less affected by variations in the field of view and collection area of the optical system. Our experimental environment’s lateral extent was physically finite, causing a significant difference between measured and simulated degree of polarization values for incident linearly polarized light, but not for circularly polarized light. Through simulation we demonstrate that circular polarization is less susceptible to the finite environmental extent as well as the collection optic’s limiting configuration.
We present simulation results that show circularly polarized light persists through scattering environments better than linearly polarized light. Specifically, we show persistence is enhanced through many scattering events in an environment with a size parameter representative of advection fog at infrared wavelengths. Utilizing polarization tracking Monte Carlo simulations we show a larger persistence benefit for circular polarization versus linear polarization for both forward and backscattered photons. We show the evolution of the incident polarization states after various scattering events which highlight the mechanism leading to circular polarization’s superior persistence.
Although planar heterostructures dominate current optoelectronic architectures, 1D nanowires and nanorods have distinct
and advantageous properties that may enable higher efficiency, longer wavelength, and cheaper devices. We have
developed a top-down approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable
height, pitch and diameter. This approach avoids many of the limitations of bottom-up synthesis methods. In addition to
GaN nanorods, the fabrication and characterization of both axial and radial-type GaN/InGaN nanorod LEDs have been
achieved. The precise control over nanorod geometry achiveable by this technique also enables single-mode single
nanowire lasing with linewidths of less than 0.1 nm and low lasing thresholds of ~250kW/cm2.
We demonstrate chemical/biological sensor arrays based on high quality factor evanescent microring waveguide
resonators in a process that is compatible with CMOS fabrication, glass microfluidic integration, and robust surface
chemistry ligand attachment. We cancel out any fluctuations due to liquid temperature variations through a
differential dual sensor design. Using laser locking servo techniques we attain detection sensitivities in the ng/ml
range. This combination of silicon photonic sensors, robust packaging, high sensitivity and arrayed design is capable
of providing a platform for multiplexed chem-bio sensing of molecules suspended in solution.
We present the theory of operation along with detailed device designs and initial experimental results of a new class of
uncooled thermal detectors. The detectors, termed microphotonic thermal detectors, are based on the thermo-optic effect
in high quality factor (Q) micrometer-scale optical resonators. Microphotonic thermal detectors do not suffer from
Johnson noise, do not require metallic connections to the sensing element, do not suffer from charge trapping effects,
and have responsivities orders of magnitude larger than microbolometer-based thermal detectors. For these reasons,
microphotonic thermal detectors have the potential to reach thermal phonon noise limited performance.
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