Diode-pumped alkali metal vapor lasers (DPAL) offer significant promise for high average power. The DPAL system has high gain and will high output coupling and an unstable resonator to achieve excellent beam quality. We analyze the Rb-He system using average equations for the pump, laser and populations, including amplified spontaneous emission. We extend the formulation to include flow and temperature release and study its effects on the laser efficiency and beam quality. The design and analysis of the DPAL resonator and the influence of spatial variations in gain medium on far field beam quality are developed. A systematic study of the influence of gain medium aberrations, flow geometry, and resonator design on far field beam quality is reported. The relative advantages of longitudinal and transverse flow geometries to beam quality are evaluated. Finally, coupling of the pump and laser radiation fields is dramatic in the DPAL system. The standard approaches to merging CFD analysis of the gain medium with wave optics resonator simulations will require new techniques.
This paper discusses computational modeling and experimental demonstration of a Fresnel zone light field spectral imaging system. This type of system couples an axial dispersion binary diffractive optic with a light field detector design providing a snapshot spectral imaging capability. The computational model was validated experimentally and provides excellent predictions of potential system capabilities. Additionally, the experimentally demonstrated prototype was able to digitally refocus monochromatic images by wavelength across greater than a 100 nm bandwidth. Through simulation, the demonstrated system was approximated to have a full range from ∼400 to 800 nm at close to a 15-nm spectral sampling interval. We also demonstrated experimentally the capability of resolving between and processing two different spectral signatures in a single snapshot. The type of system demonstrated here offers substantial new capability as an optically simple, snapshot spectral imager. The experimental proof of concept and computational model set the stage for further development of these types of systems.
Through numerical simulation, we have demonstrated a novel snapshot spectral imaging concept using binary diffractive optics. Binary diffractive optics, such as Fresnel zone plates (FZP) or photon sieves, can be used as the single optical element in a spectral imager that conducts both imaging and dispersion. In previous demonstrations of spectral imaging with diffractive optics, the detector array was physically translated along the optic axis to measure different image formation planes. In this new concept the wavelength-dependent images are constructed synthetically, by using integral photography concepts commonly applied to light field (plenoptic) cameras. Light field cameras use computational digital refocusing methods after exposure to make images at different object distances. Our concept refocuses to make images at different wavelengths instead of different object distances. The simulations in this study demonstrate this concept for an imager designed with a FZP. Monochromatic light from planar sources is propagated through the system to a measurement plane using wave optics in the Fresnel approximation. Simple images, placed at optical infinity, are illuminated by monochromatic sources and then digitally refocused to show different spectral bins. We show the formation of distinct images from different objects, illuminated by monochromatic sources in the VIS/NIR spectrum. Additionally, this concept could easily be applied to imaging in the MWIR and LWIR ranges. In conclusion, this new type of imager offers a rugged and simple optical design for snapshot spectral imaging and warrants further development.
The high gain Diode Pumped Alkali Laser (DPAL) system will require an unstable resonator with high Fresnel number and high output coupling to achieve excellent beam quality. Coupling of the diode pump and laser radiation fields is dramatic in the DPAL system. Merging flow field analysis of the gain medium with wave optics resonator simulations requires new techniques. We develop a wave-optics simulation of confocal, positive-branch unstable resonators for the DPAL gain media to assess the limitations on far field beam quality. The design and analysis of the DPAL resonator and the influence of spatial variations in gain medium on far field beam quality are developed. The relative advantages of longitudinal and transverse flow geometries to beam quality are evaluated. A systematic study of the influence of gain medium aberrations, flow geometry, magnification, and resonator design on far field beam quality is reported.
Proc. SPIE. 9828, Airborne Intelligence, Surveillance, Reconnaissance (ISR) Systems and Applications XIII
KEYWORDS: Target detection, Signal to noise ratio, Detection and tracking algorithms, Data modeling, Cameras, Sensors, Data processing, Algorithm development, Signal detection, Digital breast tomosynthesis
Increases in the number of cameras deployed, frame rate, and detector array sizes have led to a dramatic increase in the volume of motion imagery data that is collected. Without a corresponding increase in analytical manpower, much of the data is not analyzed to full potential. This creates a need for fast, automated, and robust methods for detecting signals of interest. Current approaches fall into two categories: detect-before-track (DBT), which are fast but often poor at detecting dim targets, and track-before-detect (TBD) methods which can offer better performance but are typically much slower. This research seeks to contribute to the near real time detection of low SNR, unresolved moving targets through an extension of earlier work on higher order moments anomaly detection, a method that exploits both spatial and temporal information but is still computationally efficient and massively parallelizable. It was found that intelligent selection of parameters can improve probability of detection by as much as 25% compared to earlier work with higherorder moments. The present method can reduce detection thresholds by 40% compared to the Reed-Xiaoli anomaly detector for low SNR targets (for a given probability of detection and false alarm).
Chromotomography is a form of hyperspectral imaging that uses a prism to simultaneously record spectral and spatial information, like a slitless spectrometer. The prism is rotated to provide multiple projections of the 3D data cube on the 2D detector array. Tomographic reconstruction methods are then used to estimate the hyperspectral data cube from the projections. This type of system can collect hyperspectral imagery from fast transient events, but suffers from reconstruction artifacts due to the limited-angle problem. Several algorithms have been proposed in the literature to improve reconstruction, including filtered backprojection, projection onto convex sets, subspace constraint, and split- Bregman iteration. Here we present the first direct comparison of multiple methods against a variety of simulatedtargets. Results are compared based on both image quality and spectral accuracy of the reconstruction, where previous literature has emphasized imaging only. In addition, new algorithms and HSI quality metrics are proposed. We find the quality of the results depend strongly on the spatial and spectral content of the scene, and no single algorithm is consistently superior over a broad range of scenes.
Aircraft coatings degrade over time, but aging can be difficult to detect before failure and delamination. We present a method to evaluate aircraft coatings in situ using infrared diffuse reflectance spectra. This method can detect and classify coating degradation much earlier than visual inspection. The method has been tested on two different types of coatings that were artificially aged in an autoclave. Spectra were measured using a hand-held diffuse reflectance infrared Fourier transform spectrometer (DRIFTS). One set of 72 samples can be classified as either aged or unaged with 100% accuracy. A second sample set contained samples that had been artificially aged for 0, 24, 48 or 96 hours. Several classification methods are compared, with accuracy better than 98% possible.
Chromotomography is a form of hyperspectral imaging that utilizes a spinning diffractive element to resolve a rapidly
evolving scene. The system captures both spatial dimensions and the spectral dimension at the same time. Advanced
algorithms take the recorded dispersed images and use them to construct the data cube in which each reconstructed
image is the recorded scene at a specific wavelength. A simulation tool has been developed which uses Zemax to
accurately trace rays through real or proposed optical systems. The simulation is used here to explore the limitations of
tomographic reconstruction in both idealized and aberrated imaging systems. Results of the study show the accuracy of
reconstructed images depends upon the content of the original target scene, the number of projections measured, and the
angle through which the prism is rotated. For cases studied here, 20 projections are sufficient to achieve image quality
99.51% of the max value. Reconstructed image quality degrades with aberrations, but no worse than equivalent
A fieldable hyperspectral chromotomographic imager has been developed at the Air Force Institute of Technology to refine component requirements for a space-based system. The imager uses a high speed visible band camera behind a direct-vision prism to simultaneously record two spatial dimensions and the spectral dimension. Capturing all three dimensions simultaneously allows for the hyperspectral imaging of transient events. The prism multiplexes the spectral and spatial information, so a tomographic reconstruction algorithm is required to separate hyperspectral channels. The fixed dispersion of the prism limits the available projections, leading to artifacts in the reconstruction which limit the image quality and spectrometric accuracy of the reconstructions. The amount of degradation is highly dependent on the content of the scene. Experiments were conducted to characterize the image and spectral quality as a function of spatial, spectral, and temporal complexity. We find that in general, image quality degrades as the source bandwidth increases. Spectra estimated from the reconstructed data cube are generally best for point-like sources, and can be highly inaccurate for extended scenes. In other words, the spatial accuracy varies inversely with the spectral width, and the spectral accuracy varies inversely with the spatial width. Experiment results also demonstrate the ability to reconstruct hyperspectral images from transient combustion events.
An instrument for monocular passive ranging based on atmospheric oxygen absorption near 762 nm has been designed,
built and deployed to track emissive targets. An intensified CCD array is coupled to variable band pass liquid crystal
filter and 3.5 - 8.8 degree field of view optics. The system was first deployed for a ground test viewing a static jet engine
in afterburner at ranges of 0.35 - 4.8 km, establishing a range error of 15%. The instrument was also flight tested in a C-12 imaging an the exhaust plume of another aircraft afterburner at ranges up to 11 km.
Atmospheric oxygen absorption bands in observed spectra of boost phase missiles can be used to accurately
estimate range from sensor to target. One method is to compare observed values of band averaged absorption
to radiative transfer models. This is most effective using bands where there is a single absorbing species. This
work compares spectral attenuation of two oxygen absorption bands in the near-infrared (NIR) and visible (Vis)
spectrum, centered at 762 nm and 690 nm, to passively determine range. Spectra were observed from a static test
of a full-scale solid rocket motor at a 900m range. The NIR O2 band provided range estimates accurate to within
3%, while the Vis O2 band had a range error of 15%. A Falcon 9 rocket launch at an initial range of 13km
was also tracked and observed for 90 seconds after ignition. The NIR O2 band provided in-flight range estimates
accurate to within 2% error for the first 30 seconds of tracked observation. The Vis O2 band also provided
accurate range estimates with an error of approximately 4%. Rocket plumes are expected to be significantly
brighter at longer wavelengths, but absorption in the NIR band is nearly ten times stronger than the Vis band,
causing saturation at shorter path lengths. An atmospheric band is considered saturated when all the in-band
frequencies emitted from the rocket plume are absorbed before reaching the sensor.
Methods of estimating range to an emissive target based on the depth of an atmospheric absorption band are presented.
The present work uses measurements of the CO2 absorption band centered at 2.0 μm where signal-to-background ratios
are maximum for many applications. Observed spectra are compared to model spectra to estimate range. Spectral
regions with minimal attenuation are used to estimate source parameters in order to isolate atmospheric transmission.
The spectra of 21 high explosive events were used to test this technique. A simple technique treating the fireball as a blackbody consistently underestimated true range by approximately 13%. A more realistic source model using some order-of-magnitude assumptions of fireball composition reduces range error to 3%. The technique produces accurate results without requiring detailed knowledge of source parameters or atmospheric conditions.
A field deployable hyperspectral imager utilizing chromotomography (CT), with a direct vision prism (DVP)
as the dispersive element, has been constructed at the Air Force Institute of Technology (AFIT). A "shift and
add" reconstruction algorithm was used to resolve spectral and spatial content of the collected data. The AFIT
instrument is currently the fastest known imaging DVP based hyperspectral CT instrument of its type and is
a prototype for a space-based system. The imager captured images at rates up to 900 frames per second (fps)
and acquired data cube information in 55 ms, during testing. This instrument has the ability to capture spatial
and spectral data of static and transient scenes. During testing, the imager captured spectral data of a rapidly
evolving scene (a firecracker detonation) lasting approximately 0.12 s. Spectral results included potassium and
sodium emission lines present during the explosion and an absorption feature as the fireball extinguishes. Spatial
and spectral reconstruction of a scene in which an explosion occurs during the middle of the collection period
is also presented in this paper. The instrument is capable of acquiring data required to identify, classify and
characterize transient battlespace events, such as explosions.
The depth of absorption bands in observed spectra of distant, bright sources can be used to estimate range to the source. Previous efforts in this area relied on Beer's Law to estimate range from observations of infrared CO2 bands, with disappointing results. A modified approach is presented that uses band models and observations of the O2 absorption band near 762 nm. This band is spectrally isolated from other atmospheric bands, which enables direct estimation of molecular absorption from observed intensity. Range is estimated by comparing observed values of band-average absorption, (see manuscript), against predicted curves derived from either historical data or model predictions. Accuracy of better than 0.5% has been verified in short-range (up to 3km) experiments using a Fourier transform interferometer at 1cm-1 resolution. A conceptual design is described for a small, affordable passive ranging sensor suitable for use on tactical aircraft for missile attack warning and time-to-impact estimation. Models are used to extrapolate experimental results (using 1 cm-1 resolution data) to analyze expected performance of this filter-based system.
Highly-energetic targets such as rocket plumes and detonation fireballs are difficult to locate and track effectively with active sensors that rely on reflection, but traditional passive sensors cannot determine range. Development of a passive ranging sensor will enable accurate target location and tracking while simultaneously improving covertness. Recent work is presented on development of a passive sensor to estimate range using spectroscopic measurements of atmospheric absorption. In particular, advantages of measuring absorption on the O 2(bX) transition near 762nm are discussed. Theoretical predictions are compared with experimental results to verify model performance at short ranges (up to 200m). Range accuracy better than 1% has been demonstrated using a Fourier transform spectrometer at short range. Extension of the theory to long range (in excess of 100km) is also discussed.
We are studying ways to improve the performance of evanescent wave biosensors for use in detecting chemical and biological agents. We show a beam-propagation simulation that is used to determine the optimum fiber profile to achieve the desired propagation parameters. The model parameters can then be used to fabricate polymer fibers using an in-house fiber drawing apparatus. We also demonstrate a simple method of comparing the optical performance of different waveguides for use in such sensors.
A compact portable Fourier transform spectrometer is described. The simplicity of the instrument design and monolithic structure of the interferometer make it suitable for use in satellites or man-portable terrestrial measurements. Conceptual designs for hyperspectral imaging applications in the near-UV through near-IR, and in the mid-IR are presented. Imaging and spectral performance predictions are given using ray and beam tracing software to synthesize multispectral interferograms.
The collection of spatial-spectral data of space objects is unique from most applications of spectral imaging technology. Space object identification requires collection of multiple frames of relatively low intensity, fast moving targets. The objective is to collect a single aggregate reflectance spectrum over the satellite and correlate this to orientation and solar phase angle. Higher statistical signal-to-noise can be achieved by averaging over the spatial dimension of a data frame. Blur was introduced by the tracking conditions in only the spatial dimension due to the design of the instrument. This blur allows some minimization of camera noise that cannot be removed through conventional techniques.
Pulsed photodissociation of iodine monobromide at 532 nm provides a high yield of spin-orbit excited atomic bromine. Near resonant electronic-to-vibrational energy transfer from Br(2P1/2) to NO(v equals 2) is rapid, k equals 2.4 X 10-12 cm3/molecule-s, and selective, with a branching ratio of NO(v equals 2) of 0.89 +/- 0.21. An NO(v equals 2 yields 1) laser operating at 5.4 microns was demonstrated at NO pressures from 0.1 - 1.4 Torr. Temporal profiles were obtained as a function of IBr and No pressures and photolysis energy to analyze laser gain, threshold, and efficiency. The threshold photolysis pump energy was 25 mJ/pulse. Lasing pulses were delayed by 150 ns from photodissociation and persisted for 100 - 200 ns. Device efficiency is limited by NO V yields V relaxation, and the maximum observed NO laser energy was 0.01 mJ for 85 mJ photolysis energy. Comparison to similar Br(2P1/2 yields 2P3/2) and Br(2P1/2)/CO2(101 - yields 100) is provided.