Andrea M. Brown

Analysis of link performance for the FOENEX laser communications system

A series of experiments were conducted to validate the performance of the free-space optical communications (FSOC) subsystem under DARPAs FOENEX program. Over six days, bidirectional links at ranges of 10 and 17 km were characterized during different periods of the day to evaluate link performance. This paper will present the test configuration, evaluate performance of the FSOC subsystem against a variety of characterization approaches, and discuss the impact of the results, particularly with regards to the optical terminals. Finally, this paper will summarize the impact of turbulence conditions on the FSOC subsystem and present methods for estimating performance under different link distances and turbulence conditions.

Optical material characterization through BSDF measurement and analysis

The optical scattering signature and the absorbance of a material are of interest in a variety of engineering applications, particularly for those pertaining to optical remote sensing. The John Hopkins University Applied Physics Laboratory has developed an experimental capability to measure in-plane bidirectional scattering distribution functions to retrieve optical properties of materials. These measurements are supported at high angular resolution with wavelengths that span the ultra-violet to the long-wave infrared. Models have been developed to fit Lambertian, diffuse, near-specular, and specular scattering at a range of incident angles. Useful material properties can then be determined through analysis of the modeled BSDF. Optical characterization results are shown for a variety of materials, including paints, metals, optical windows, and leaves.

Developing a broad spectrum atmospheric aerosol characterization for remote sensing platforms over desert regions

Remotely sensed imagery of targets embedded in Earth’s atmosphere requires characterization of aerosols between the space-borne sensor and ground to accurately analyze observed target signatures. The impact of aerosol microphysical properties on retrieved atmospheric radiances has been shown to negatively affect the accuracy of remotely sensed data collects. Temporally and regionally specific meteorological conditions require exact site atmospheric characterization, involving extensive and timely observations. We present a novel methodology which fuses White Sands New Mexico regional aerosol micro pulse lidar (MPL) observations with sun photometer direct and diffuse products for broad-wavelength (visible – longwave infrared) input into the radiative transfer model MODTRAN5. Resulting radiances are compared with those retreived from the NASA Aqua MODIS instrument.

Temperature dependent BRDF facility

Applications involving space based instrumentation and aerodynamically heated surfaces often require knowledge of the bi-directional reflectance distribution function (BRDF) of an exposed surface at high temperature. Addressing this need, the Johns Hopkins University Applied Physics Laboratory (JHU/APL) developed a BRDF facility that features a multiple–port vacuum chamber, multiple laser sources covering the spectral range from the longwave infrared to the ultraviolet, imaging pyrometry and laser heated samples. Laser heating eliminates stray light that would otherwise be seen from a furnace and requires minimal sample support structure, allowing low thermal conduction loss to be obtained, which is especially important at high temperatures. The goal is to measure the BRDF of ceramic-coated surfaces at temperatures in excess of 1000°C in a low background environment. Most ceramic samples are near blackbody in the longwave infrared, thus pyrometry using a LWIR camera can be very effective and accurate.

Sensitivity of the polarization ratio method to aerosol concentration

A multiwavelength, multistatic optical scattering instrument is being developed to characterize spherical aerosols. This instrument uses 405 nm (blue), 532 nm (green) and 655 nm (red) diode lasers and two CCD imagers to measure the angular distribution of light scattered from aerosols. The incident light is polarized parallel or perpendicular to the scattering plane; the scattered intensity is measured at backscatter angles ranging from 120° to 170° by CCD imagers. The phase function for each polarization is used to form the polarization ratio, which is used to characterize the aerosols. This method has proven to be a reliable way to characterize spherical aerosols by matching the measured polarization ratio with the polarization ratio calculated by the Mie scattering equations. This method is used to determine the number density, size distribution, and index of refraction of the aerosols. The sensitivity of the polarization ratio to particle concentration is explored using a narrow distribution of one micron polystyrene beads in a chamber. The aerosol concentration is found via an inversion technique that is based on Mie calculations. This study provides the basis for transitioning this instrument to measure multiple particle size ranges and concentrations for common aerosols in an outdoor environment.

A General BRDF/BSDF Model Including Out-of-Plane Dependence

A semi-empirical reflectance/scatterance model has evolved over the years to represent a diverse set of materials from coated substrates to optical windows. This model separates the BRDF/BSDF into four basic components, specular, near-specular, diffuse, and Lambertian (random diffuse) terms. The specular and near-specular components employ a Gaussian phase function and the Fresnel power reflection coefficient. The Lambertian component uses Kubelka-Munk theory for the total integrated reflectance and transmittance. The model features wavelength, angle, and full hemispherical dependencies. It is applied to a variety of samples, from painted surfaces to transparent windows, with good success. This parameterized modeling approach is attractive because algorithms that use the model can be computationally efficient. Previous work has only considered in-plane effects. The present paper now explicitly takes into account the out-of-plane contribution and improves the total integrated factors.

Initial test of MITA/DIMM with an operational CBP system

The MITA (Motion Imagery Task Analyzer) project was conceived by CBP OA (Customs and Border Protection - Office of Acquisition) and executed by JHU/APL (Johns Hopkins University/Applied Physics Laboratory) and CERDEC NVESD MSD (Communications and Electronics Research Development Engineering Command Night Vision and Electronic Sensors Directorate Modeling and Simulation Division). The intent was to develop an efficient methodology whereby imaging system performance could be quickly and objectively characterized in a field setting. The initial design, development, and testing spanned a period of approximately 18 months with the initial project coming to a conclusion after testing of the MITA system in June 2017 with a fielded CBP system. The NVESD contribution to MITA was thermally heated target resolution boards deployed to support a range close to the sensor and, when possible, at range with the targets of interest. JHU/APL developed a laser DIMM (Differential Image Motion Monitor) system designed to measure the optical turbulence present along the line of sight of the imaging system during the time of image collection. The imagery collected of the target board was processed to calculate the in situ system resolution. This in situ imaging system resolution and the time-correlated turbulence measured by the DIMM system were used in NV-IPM (Night Vision Integrated Performance Model) to calculate the theoretical imaging system performance. Overall, this proves the MITA concept feasible. However, MITA is still in the initial phases of development and requires further verification and validation to ensure accuracy and reliability of both the instrument and the imaging system performance predictions.

Lasercomm system development for high-bandwidth terrestrial communications

In recent years, various terrestrial free-space optical (FSO) communications systems have been demonstrated to achieve high-bandwidth communications between mobile platforms. The terminal architectures fall into three general categories: (1) single aperture systems with tip/tilt control, (2) multi-aperture system with tip/tilt control, and (3) single aperture systems with tip/tilt control and higher order adaptive optics correction. Terrestrial modem approaches generally use direct detection receivers because they provide high bandwidth capability (0.1-10 Gbps) without the complexity of coherent detection. Modems are often augmented with a mix of forward error correction (FEC), interleaving, and/or retransmission for improved data transport. This paper will present a terminal and modem architecture for a low-SWAP FSO communications system that enables robust, high-bandwidth communications under highly scintillated links as found in terrestrial applications such as air-to-air, air-to-surface, and surface-to-surface links.

Scatter properties of polycrystalline YAG in the visible and near-infrared

Yttrium Aluminum Garnet (YAG) is an important laser host material. Ideal host materials have low loss at the laser transition frequency. This becomes more important as the gain length increases or a low gain transition is of interest. Unfortunately, single crystal YAG suffers from relatively high scatter caused by strain induced index of refraction variations generated by the growth method. For this reason polycrystalline YAG has been developed with virtually no strain. Furthermore, this material can be doped with concentrations that vary spatially. This can provide a tremendous advantage in matching the gain volume to the mode volume in a laser. However, because of the grain boundaries and porosity, polycrystalline materials have scatter loss. Angle resolved, in-plane scatter measurements of polycrystalline YAG and Nd:YAG are reported from 405 to 1064 nm. This covers the range of interest for laser operation but also with enough bandwidth to derive a physical understanding of the scatter mechanisms. A model is also applied to provide physical insight and interpolation and meaningful extrapolation of the experimental results.

Models to support active sensing of biological aerosol clouds

Elastic backscatter LIght Detection And Ranging (LIDAR) is a promising approach for stand-off detection of biological aerosol clouds. Comprehensive models that explain the scattering behavior from the aerosol cloud are needed to understand and predict the scattering signatures of biological aerosols under varying atmospheric conditions and against different aerosol backgrounds. Elastic signatures are dependent on many parameters of the aerosol cloud, with two major components being the size distribution and refractive index of the aerosols. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been in a unique position to measure the size distributions of released biological simulant clouds using a wide assortment of aerosol characterization systems that are available on the commercial market. In conjunction with the size distribution measurements, JHU/APL has also been making a dedicated effort to properly measure the refractive indices of the released materials using a thin-film absorption technique and laboratory characterization of the released materials. Intimate knowledge of the size distributions and refractive indices of the biological aerosols provides JHU/APL with powerful tools to build elastic scattering models, with the purpose of understanding, and ultimately, predicting the active signatures of biological clouds.