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DIRECTED
ENERGY
PROFESSIONAL
SOCIETY
Journal of Directed Energy
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Volume 2, Number 4 |
Fall 2007 |
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The papers listed below constitute Volume 2, Number 4 of the Journal of Directed Energy.
Print copies of this, and other issues of the Journal of Directed Energy are available through the
DEPS online store.
Access complete technical paper(s) through links in the paper titles.
Aero-Optical Measurements Using High-Bandwidth Two-Dimensional Wavefront Sensor Array
(300 KB)
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
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Low-Cost Experiment to Measure Optical Turbulence Between Two Buildings
(1,150 KB)
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
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Kalman Estimation of Anisoplanatic Zernike Tilt
(750 KB)
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 ?rad.
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
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Aperture Effects in Aero-Optics and Beam Control
(1,250 KB)
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
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Comparison of Climatological Optical Turbulence Profiles to Standard, Statistical, and Numerical Models using HELEEOS
(1,050 KB)
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
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Expected Worldwide, Low-Altitude Laser Performance in the Presence of Common Atmospheric Obscurants
(850 KB)
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 ?m 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
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Volume 2, Number 4, Journal of Directed Energy
Last updated: 3 September 2017
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