Steven T. Fiorino

Comparison of coherent and incoherent laser beam combination for tactical engagements

Abstract. The performance of a multibeam laser system is evaluated for coherent and incoherent beam combination under tactical scenarios. For direct comparison, identical aperture geometries are used for both, coherent or incoherent, combination methods. The analysis assumes a multilaser source coupled with a conventional 0.32 m diameter, on-axis, beam director. Parametric analysis includes variations over residual errors, beam quality, atmospheric effects, and scenario geometry. Analytical solutions from previous results are used to evaluate performance for the vacuum case, providing an upper bound on performance and a backdrop for organizing the multitude of effects as they are analyzed. Wave optics simulations are used for total system performance. Each laser in the array has a wavelength of 1.07 μm, 10 kW (25 kW) output power, and Gaussian exitance profile. Both tracking and full-aperture adaptive optics are modeled. Three tactical engagement geometries, air to surface, surface to air, and surface to surface, are evaluated for slant ranges from 2.5 to 10 km. Two near-median atmospheric profiles were selected based upon worldwide climatological data. The performance metric used is beam propagation efficiency for circular target diameters of 5 and 10 cm.

Methodology for comparing worldwide performance of diverse weight-constrained high energy laser systems

The Air Force Institute of Technologys Center for Directed Energy has developed a software model, the High Energy Laser End-to-End Operational Simulation (HELEEOS), under the sponsorship of the High Energy Laser Joint Technology Office (JTO), to facilitate worldwide comparisons across a broad range of expected engagement scenarios of expected performance of a diverse range of weight-constrained high energy laser system types. HELEEOS has been designed to meet JTOs goals of supporting a broad range of analyses applicable to the operational requirements of all the military services, constraining weapon effectiveness through accurate engineering performance assessments allowing its use as an investment strategy tool, and the establishment of trust among military leaders. HELEEOS is anchored to respected wave optics codes and all significant degradation effects, including thermal blooming and optical turbulence, are represented in the model. The model features operationally oriented performance metrics, e.g. dwell time required to achieve a prescribed probability of kill and effective range. Key features of HELEEOS include estimation of the level of uncertainty in the calculated Pk and generation of interactive nomographs to allow the user to further explore a desired parameter space. Worldwide analyses are enabled at five wavelengths via recently available databases capturing climatological, seasonal, diurnal, and geographical spatial-temporal variability in atmospheric parameters including molecular and aerosol absorption and scattering profiles and optical turbulence strength. Examples are provided of the impact of uncertainty in weight-power relationships, coupled with operating condition variability, on results of performance comparisons between chemical and solid state lasers.

Validation of a UV-to-RF High-Spectral-Resolution Atmospheric Boundary Layer Characterization Tool

AbstractThis paper demonstrates the capability of the Laser Environmental Effects Definition and Reference (LEEDR) model to accurately characterize the meteorological parameters and radiative transfer effects of the atmospheric boundary layer with surface observations or climatological values of temperature, pressure, and humidity (“climatology”). The LEEDR model is a fast-calculating, first-principles, worldwide surface-to-100-km, ultraviolet-to-radio-frequency (UV to RF) wavelength, atmospheric characterization package. In general, LEEDR defines the well-mixed atmospheric boundary layer with a worldwide, probabilistic surface climatology that is based on season and time of day and, then, computes the radiative transfer and propagation effects from the vertical profile of meteorological variables. The LEEDR user can also directly input surface observations. This research compares the LEEDR vertical profiles created from input surface observations or numerical weather prediction (NWP) data with the LEEDR ...

Critical Assessment of Microphysical Assumptions within TRMM Radiometer Rain Profile Algorithm Using Satellite, Aircraft, and Surface Datasets from KWAJEX

Abstract The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager precipitation profile retrieval algorithm (2a12) assumes cloud model–derived vertically distributed microphysics as part of the radiative transfer–controlled inversion process to generate rain-rate estimates. Although this algorithm has been extensively evaluated, none of the evaluation approaches has explicitly examined the underlying microphysical assumptions through a direct intercomparison of the assumed cloud-model microphysics with in situ, three-dimensional microphysical observations. The main scientific objective of this study is to identify and overcome the foremost model-generated microphysical weaknesses in the TRMM 2a12 algorithm through analysis of (a) in situ aircraft microphysical observations; (b) aircraft- and satellite-based passive microwave measurements; (c) ground-, aircraft-, and satellite-based radar measurements; (d) synthesized satellite brightness temperatures and radar reflectivities; (e) radiometer-only pr...

Enhanced, fast-running scaling law model of thermal blooming and turbulence effects on high energy laser propagation

A new scaling law model is presented to rapidly simulate thermal blooming and turbulence effects on high energy laser propagation, producing results approaching the quality normally only available using wave-optics code, but at much faster speed. The model convolves irradiance patterns originating from two distinct scaling law models, one with a proficiency in thermal blooming effects and the other in turbulence. To underscore the power of the new model, results are verified for typical, realistic scenarios by direct comparison with wave optics simulation.

A first principles atmospheric propagation & characterization tool: the laser environmental effects definition and reference (LEEDR)

The Air Force Institute of Technology Center for Directed Energy (AFIT/CDE) has developed a first principles atmospheric propagation and characterization model called the Laser Environmental Effects Definition and Reference or LEEDR. This package enables the creation of profiles of temperature, pressure, water vapor content, optical turbulence, and atmospheric particulates and hydrometeors as they relate to line-by-line layer extinction coefficient magnitude at wavelengths from the UV to the RF. Worldwide seasonal, diurnal, and geographical variability in these parameters is accessed from probability density function (PDF) databases using a variety of recently available resources to include the Extreme and Percentile Environmental Reference Tables (ExPERT), the Master Database for Optical Turbulence Research in Support of the Airborne Laser, and the Global Aerosol Data Set (GADS). GADS provides aerosol constituent number densities on a 5° x 5° grid worldwide. ExPERT mapping software allows the LEEDR operator to choose from specific site or regional upper air data to characterize correlated molecular absorption, aerosol absorption and scattering by percentile. The integration of the Surface Marine Gridded Climatology database, the Advanced Navy Aerosol Model (ANAM), and the Navy Surface Layer Optical Turbulence (NSLOT) model provides worldwide coverage over all ocean regions on a 1° x 1° grid. Molecular scattering is computed based on Rayleigh theory. Molecular absorption effects are computed for the top 13 absorbing species using line strength information from the HITRAN 2004 database in conjunction with a community standard molecular absorption continuum code. Aerosol scattering and absorption are computed with the Wiscombe Mie model. Each atmospheric particulate/hydrometeor is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on laser energy delivered at any wavelength from 0.355 μm to 8.6 m. LEEDR can also produce correlated optical turbulence profiles in percentile format. In addition, probability of cloud free line of sight (CFLOS) for hundreds of land sites worldwide is available in LEEDR. Effects of layers of fog, several types of rain and several types of water droplet and ice clouds can also be considered. In addition to describing some of the underlying theory to the LEEDR calculations, this paper presents graphical results for several different scenarios. These generic scenarios are meant to exemplify how LEEDR enables the physically realistic data capture of atmospheric effects on electromagnetic propagation.

Physical optics solution for the scattering of a partially-coherent wave from a statistically rough material surface.

The scattering of a partially-coherent wave from a statistically rough material surface is investigated via derivation of the scattered field cross-spectral density function. Two forms of the cross-spectral density are derived using the physical optics approximation. The first is applicable to smooth-to-moderately rough surfaces and is a complicated expression of source and surface parameters. Physical insight is gleaned from its analytical form and presented in this work. The second form of the cross-spectral density function is applicable to very rough surfaces and is remarkably physical. Its form is discussed at length and closed-form expressions are derived for the angular spectral degree of coherence and spectral density radii. Furthermore, it is found that, under certain circumstances, the cross-spectral density function maintains a Gaussian Schell-model form. This is consistent with published results applicable only in the paraxial regime. Lastly, the closed-form cross-spectral density functions derived here are rigorously validated with scatterometer measurements and full-wave electromagnetic and physical optics simulations. Good agreement is noted between the analytical predictions and the measured and simulated results.

A Computational Tool for Evaluating THz Imaging Performance in Brownout Conditions at Land Sites Throughout the World

This study quantifies terahertz (THz) or sub-millimeter imaging performance during simulated rotary-wing brownout or whiteout environments based on geographic location and recent/current atmospheric weather conditions. The atmospheric conditions are defined through the Air Force Institute of Technology Center for Directed Energy (AFIT/CDE) Laser Environmental Effects Definition and Reference or LEEDR model. This model enables the creation of vertical profiles of temperature, pressure, water vapor content, optical turbulence, and atmospheric particulates and hydrometeors as they relate to line-by-line layer extinction coefficient magnitude at wavelengths from the UV to the RF. Optical properties and realistic particle size distributions for the brownout and whiteout particulates have been developed for and incorporated into LEEDR for this study. The expected imaging performance is assessed primarily at a wavelength of 454 μm (0.66 THz) in brownout conditions at selected geographically diverse land sites throughout the world. Seasonal and boundary layer variations (summer and winter) and time of day variations for a range of relative humidity percentile conditions are considered to determine optimum employment techniques to exploit or defeat the environmental conditions. Each atmospheric particulate/hydrometeor is evaluated based on its wavelength-dependent forward and off-axis scattering characteristics and absorption effects on the imaging environment. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are used. Most evaluated scenarios are brownout environments over ranges up to 50 meters. At submillimeter wavelengths and the short ranges studied, preliminary results indicate the main source of image degradation in brownout conditions is water vapor content, even with visibility less than 10 m and strong optical turbulence.

A worldwide physics-based high spectral resolution atmospheric characterization and propagation package for UV to RF wavelengths

The Air Force Institute of Technologys Center for Directed Energy (AFIT/CDE) developed the High Energy Laser End-to-End Operational Simulation (HELEEOS) model in part to quantify the performance variance in laser propagation created by the natural environment during dynamic engagements. As such, HELEEOS includes a fast-calculating, first principles, worldwide surface-to-100 km, atmospheric propagation and characterization package. This package enables the creation of profiles of temperature, pressure, water vapor content, optical turbulence, atmospheric particulates and hydrometeors as they relate to line-by-line layer transmission, path and background radiance at wavelengths from the ultraviolet to radio frequencies. Physics-based cloud and precipitation characterizations are coupled with a probability of cloud free line-of-sight algorithm for all possible look angles. HELEEOS was developed under the sponsorship of the High Energy Laser Joint Technology Office. In the current paper an example of a unique high fidelity simulation of a bi-static, time-varying five band multispectral remote observation of laser energy delivered on a test object is presented. The multispectral example emphasizes atmospheric effects using HELEEOS, the interaction of the laser on target and the observed reflectance and subsequent hot spot generated. A model of a sensor suite located on the surface is included to collect the diffuse reflected in-band laser radiation and the emitted radiance of the hot spot in four separate and spatially offset MWIR and LWIR bands. Particular care is taken in modeling the bidirectional reflectivity distribution function (BRDF) of the laser/target interaction to account for both the coupling of energy into the target body and the changes in reflectance as a function of temperature. The architecture supports any platform-target-observer geometry, geographic location, season, and time of day; and it provides for correct contributions of the sky-earth background. The simulation accurately models the thermal response, kinetics, turbulence, base disturbance, diffraction, and signal-to-noise ratios.

Examining the validity of using a Gaussian Schell-model source to model the scattering of a fully coherent Gaussian beam from a rough impedance surface

Abstract. Military applications that use adaptive optics (AO) often require a point source beacon at the target to measure and correct for wavefront aberrations introduced by atmospheric turbulence. However, turbulence prevents the formation of such a point beacon. The extended beacons that are created instead have finite spatial extents and exhibit varying degrees of spatial coherence. Modeling these extended beacons using a Gaussian Schell-model (GSM) form for the autocorrelation function would be a convenient approach due to the analytical tractability of Gaussian functions. We examine the validity of using such a model by evaluating the field scattered from a rough impedance surface using a full-wave computational technique called the method of moments (MoM). The MoM improves the fidelity of the analysis since it captures all the physics of the laser-target interaction, such as masking, shadowing, multiple reflections, etc. Two rough-surface targets with different roughness statistics are analyzed. The simulation results are verified with experimental bidirectional reflectance distribution function measurements. It is seen that for rough surfaces, in general, the scattered-field autocorrelation function is not of a GSM form. However, under certain conditions, modeling an extended beacon as a GSM source is legitimate. This analysis will aid in understanding the behavior of extended beacons and how they affect the overall performance of an AO system.