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DIRECTED ENERGY PROFESSIONAL SOCIETY

Journal of Directed Energy
Volume 2, Number 4 Fall 2007

The papers listed below constitute Volume 2, Number 4 of the Journal of Directed Energy. Print copies of issues of the Journal of Directed Energy are available through the online store..
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Aero-Optical Measurements Using High-Bandwidth Two-Dimensional Wavefront Sensor Array
D. Cavalieri, D. Wittich, S. Gordeyev, K. Cheung, and E. Jumper, Lawrence Livermore National Laboratory

High-speed compressible turbulent flows around an aircraft change the local index of refraction and impose optical aberrations on an airborne laser beam's wavefronts. These aberrations degrade the laser's beam ability to be focused in the far field, thus reducing its intensity on a target. It can be crucial for communication, interrogation, and targeting or as a directed energy weapon. Fast-evolving convecting turbulent structures that are present in turbulent flows usually exhibit high spatial and temporal bandwidths. Thus, measurement devices with high spatial and temporal resolutions are then required to accurately capture their evolution. Most commercially available two-dimensional wavefront sensors rely on a digital charge-coupled device camera and, although it usually provides an excellent spatial resolution, samplng rates are typically limited to a few hundred frames per second. These rates are far lower than desired sampling frequencies to correctly resolve optical aberrations, which are on the order of tens of thousands of hertz. We have developed a relatively inexpensive complementary analog device to accurately measure optical aberrations at high sampling rates. The device is an analog high-temporal-bandwidth, two-dimensional wavefront Shack-Hartmann sensor. It utilizes an 8 x 8 array of analog position sensing devices, and the sampling rates now are dramatically increased in excess of 78 kHz, with a respectable spatial resolution, ie., 10-mm pitch. The high-bandwidth, two-dimensional wavefront sensor in this current configuration measures wavefronts over 64 subapertures. The device was tested on an acoustically forced, heated-jet facility and compared to a commercially available two-dimensional wavefront sensor with a 33 x 44 subaperture spatial resolution. We intend to show that the high-bandwidth sensor can correctly resolve all of the essential features of the roll-up structure evolution, including the convective nature and amplitude of the optical aberration and pairing of roll-up structures.
KEYWORDS: Aero-optics, OPD, Optical path difference, Wavefront measurements
PAGES 285-296

Low-Cost Experiment to Measure Optical Turbulence Between Two Buildings
Thomas Farrell, David Dixon, Lee Heflinger, Stanley Klyza, and Kenneth Triebes, Pennsylvania State University

Northrop Grumman Space Technology is developing adaptive optics (AO) technology for use in tactical laser weapons systems, as well as other applications. As part of this effort, we are currently building and operating an AO demonstration between the roofs of two buildings on our Space Park campus. To make preliminary AO design estimates, it was necessary to have accurate knowledge of the turbulence strength along the path. Since the campus sits in an uran area with many tall buildings and parking lots, where diurnal heating and local effects can be expected to dominate, predicting the turbulence strength analytically was not practical. Therefore, we devised a low-cost experiment that determines, in near real time, the turbulence strength along the path. Fundamentally, the experiment consists of a set of silvered spherical reflectors at the transmit location, each catching a glint from the sun and sending it to the receiver's location. At the receiver, the apparent tilt angles between pairs of spheres are measured as they change due to the atmospheric turbulence. The variance of these tilt angles is then used to find the turbulence strength. This use of differential tilt to determine turbulence strength is attractive because it is invulnerable to common motion effects due to building or receive telescope jitter. We describe, in detail, the principle of operation of this experiment and our setup. We then present results.
KEYWORDS: Atmospheric turbulence, Coherence length, Laser propagation, Scintillation, Turbulence measurement
PAGES 297-311

Kalman Estimation of Anisoplanatic Zernike Tilt
Todd M. Venema and Juan R. Vasquez, Naval Research Laboratory

Anisoplanatism causes wavefront estimation errors when compensating for atmospheric turbulence of distant, fast-moving objects using wavefronts received from the object to measure the turbulence. An excellent example of this is the case of using adaptive optics for imaging or communication with satellites in low Earth orbit. By the time the light has made a round-trip from the satellite to the ground and back, the satellite will have moved approximately 50 µrad. Linear estimation (extrapolation) of wave front tilt parameters has been shown to mitigate anisoplanatism, providing significant improvement in a noise-free environment. We present Kalman filter estimation in lieu of simple linear estimation and demonstrate the robustness of this new approach.
KEYWORDS: Anisoplanatic estimation, Kalman filter, Laser communication
PAGES 312-324

Aperture Effects in Aero-Optics and Beam Control
John P. Siegenthaler and Eric J. Jumper; University of Missouri-Columbia and other affiliations

In many beam control applications, the control is applied in separate stages through separate devices. Commonly, beam steering and tracking is performed with a flat mirror on a gimbaled mount, and in some applications this is the only form of beam control used. Wavefront correction, if present, is usually performed with a separate system and control loop. It has been known for a few years that there is an upper bound on the frequencies of disturbances that can be corrected with tip-tilt correction alone. This is caused by relations between the size of the beam aperture, the size of the variations in the air that cause aberrations in the beam, and the velocity at which the fluid variations pass through the beam (or the velocity of the beam sweeping through the variations). Variations and structures within the fluid with a length scale larger than the aperture primarily impose a deflection upon a beam. Effects on a scale smaller than the beam diameter manifest as wavefront distortions within the beam. The former can be corrected with a tip-tilt system; the latter cannot. This combined effect of aperture and beam steering correction can be regarded as a filter with a frequency-dependent gain that can be found experimentally or analytically.
KEYWORDS: Beam steering, Correction bandwidths, Tilt Correction, Scaling
PAGES 325-346

Comparison of Climatological Optical Turbulence Profiles to Standard, Statistical, and Numerical Models using HELEEOS
L.E. Gravley, S.T. Fiorino, R.J. Bartell, G.P. Perram, M.J. Krizo, and K.B. Le ; Air Force Institute of Technology

Optical turbulence within Earth's atmosphere plays a significant role in electromagnetic radiation propagation from a high-energy laser (HEL). The index-of-refraction structure constant, C2n, characterizes turbulent spatial fluctuations due to temperature gradients. These changes in the index of refraction affect the phase of the laser wavefront as it propagates through the atmosphere. It is important to characterize this parameter throughout the atmophere, the boundary layer and above, for applications regarding emerging HEL weapons systems. Several ways to include values of optical turbulence in HEL propagation studies include standard and statistical models, physically based numerical models, and climatological compilations of observed values. The purpose here is to quantifiably compare standard, statistical, and numerical models of C2n to climatological values, using the High Energy Laser End-to-End Operational Simulation (HELEEOS), to determine whether each model will yield values similar to that of actual measured optical turbulence data. The study shows that HELEEOS is a powerful tool in atmosheric optical turbulence assessment, because not only of its capability to use standard optical turbulence profiles such as Hufnagel-Valley 5/7 (HV 5/7), but also of its ability to incorporate correlated, climatologically derived turbulence profiles, a technique specifically developed for HELEEOS. The comparative analysis in this research appears to validate the HELEEOS method for correlating climatological C2n to other meteorological parameters. Results illustrate that worldwide Strehl ratio estimates vary more than 10% for tactical low-altitude oblique scenarios using this technique compared to HV 5/7.
KEYWORDS: Climatology, Cn2, Models, Optical turbulence
PAGES 347-362

Expected Worldwide, Low-Altitude Laser Performance in the Presence of Common Atmospheric Obscurants
S.T. Fiorino, R.J. Bartell, M.J. Krizo, and S.J. Cusumano, MIT Lincoln Laboratory and other affiliations

The directed energy modeling and simulation community can make important direct contributions to the joint warfighting community by establishing clear and fully integrated future program requirements. These requirements are best determined via analysis of the expected variability/uncertainty in system performance arising from spatial, spectral, and temporal variations in operating conditions. In the current study the expected performance of laser systems with operationally relevant output powers is assessed at 11 wavelengths between 0.40 and 10.6 µm for a number of widely dispersed locations worldwide. Scenarios evaluated include both up- and down-looking generally oblique engagement geometries over ranges up to 9,000 m in which anticipated clear air aerosols and thin layers of fog, very light rain, and light rain occur. The analysis is conducted for desert and midlatitude conditions and considers seasonal variations (summer and winter) and time-of-day variations for a range of relative humidity percentile conditions. Required dwell time corresponding to select values of probability of desired effect (Pk) is the primary performance metric used in the study. Results indicate that aerosols are the dominant laser propagation attenuators in the atmospheric boundary layer in the absence of clouds and precipitation. Furthermore, the study shows that it is important to realistically model the boundary layer to properly capture the low-altitude aerosol effects on laser propagation traversing that layer.
KEYWORDS: Aerosols, Boundary layer, Climatology, HELEEOS
PAGES 363-375

Volume 2, Number 4, Journal of Directed Energy

 
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