Estimation of optical turbulence characteristics from Shack Hartmann wavefront sensor measurements
Author(s):
Terry J. Brennan;
David C. Mann
Show Abstract
In the field of optical propagation through the atmosphere a knowledge of optical turbulence strength and
other key statistical parameters is crucial for performance prediction and system design. This work presents
techniques for reliably estimating the most essential parameters of optical turbulence, namely r0, the Fried
coherence length, fG, the Greenwood frequency, and l0, the inner scale of turbulence from Shack-Hartmann
wavefront sensor measurements. The earliest approaches for estimating r0 were based on MTF measurements.1
The MTF approach requires accurate calibration and stability of the system MTF which is often problematic.
Astronomers have used differential motion and scintillation to measure seeing conditions.2-5 Others have used
the slope structure function estimated from a Hartmann wavefront sensor principally for r0 estimation.6
We have shown that the slope discrepancy7 or rotational component of the slopes can be used effectively in
turbulence estimates.8 The techniques we describe here can be used to estimate r0, fG, and l0. The inner scale
estimate is based on the assumption of the Hill spectrum for refractive index fluctuations.9-11 A high resolution,
high frame rate, mobile sensor has been developed to utilize these estimation techniques. Section 2 describes the
estimation techniques. Results from field measurement campaigns will be presented in Section 3.
Shack-Hartmann wavefront sensing performance evaluation for active correction of the Large Synoptic Survey Telescope (LSST)
Author(s):
K. L. Baker;
L. G. Seppala
Show Abstract
The Large Synoptic Survey Telescope will map out the dark matter in the universe and is scheduled to see "first light" in
2014. This telescope will require active correction of its mirrors to remove the aberrations that arise from changing
gravitational force vectors and from thermal drifts in the telescope during observational runs. In this article we present a
comprehensive evaluation of a Shack-Hartmann wavefront sensor and reconstruction algorithm which is capable of
meeting the unique challenges associated with this wide field-of-view survey telescope. The advantages of this technique
over other potential wavefront sensing technologies are discussed and the potential problems encountered with this
approach are analyzed and solutions to these problems presented.
The aggregate behavior of branch points: altitude and strength of atmospheric turbulence layers
Author(s):
Denis W. Oesch;
Darryl J. Sanchez;
Carolyn M. Tewksbury-Christle;
Patrick R. Kelly
Show Abstract
In earlier work we have shown that pupil plane branch points carry information about the conditions of the
atmospheric turbulence. Experiments in the Atmospheric Simulation and Adaptive-optic Laboratory Test-bed
(ASALT) at the Air Force Research Laboratory, Directed Energy Directorate's Starfire Optical Range have
shown that branch points can provide the number and velocity of turbulence layers. Here we demonstrate that
these measurements can further be used to estimate the turbulence layers' altitude and strength. This work is
the culmination of research demonstrating that a methodology exists for identification of the number, altitude,
strength, and velocity of atmospheric turbulence layers.
Impact of spatial resolution on thermal blooming phase compensation instability
Author(s):
Mark F. Spencer;
Salvatore J. Cusumano;
Jason D. Schmidt;
Steven T. Fiorino
Show Abstract
Phase compensation instability (PCI) is the time-dependent development of spatial perturbations that occur within
thermally bloomed high-energy laser (HEL) beams. These types of spatial perturbations act as local hot spots that create
small negative lenses within the HEL beam. Closed-loop adaptive optics (AO) corrects for these spatial perturbations by
applying small positive-lens phase compensations, which only increases the strength of the local hot spots and leads to
runaway in the adaptive-optics servo. This study uses a straightforward wave-optics code to model horizontal
propagation with the effects of thermal blooming for a focused Gaussian beam. The strength of the thermal blooming
effects is characterized using the classic dimensionless distortion number. A nominal AO system is used to mitigate
phase distortions accumulated from thermal blooming. Parameters within the AO system, such as the number of
actuators on the deformable mirror and the resolution of the wavefront sensor, are varied to determine the impact of
spatial resolution in the development of the PCI. A discussion is given on the potential use of control theory to diminish
the effects of the PCI.
New phasor reconstruction methods for speckle imaging
Author(s):
Gregory C. Dente;
Michael L. Tilton
Show Abstract
We will develop and then compare object spectrum phasor reconstruction results for several
speckle imaging approaches. Each phasor reconstruction algorithm results from minimizing a
very naturally defined weighted-least-squares error function. Once we pick a phasor-based error
function, the remaining steps in our algorithms are developed by setting the error function
variation, with respect to each phasor element, to zero. The resulting coupled nonlinear
equations for the minimum error phasor array are then solved iteratively. In the applications, we
will compare and contrast three implementations: 1) Knox-Thompson; 2) Bispectrum, using
only two bispectrum planes; 3) Bispectrum, using four bispectrum planes. In each application
of the three approaches, we first calculate the modulus of the object spectrum using a Wiener-
Helstrom filter to remove the speckle transfer function. The methods then differ only in their
object spectrum phasor reconstructions. In the simulations, we will implement all three methods
on a simple object at low photon-per-frame light levels. Next, we will apply the methods to a
complex extended object.
Adaptive jitter control for tracker line of sight stabilization
Author(s):
Steve Gibson;
Tsu-Chin Tsao;
Dan Herrick;
Christopher Beairsto;
Ronnie Grimes;
Todd Harper;
Jeff Radtke;
Benito Roybal;
Jay Spray;
Stephen Squires;
Dave Tellez;
Michael Thurston
Show Abstract
A field test experiment on a range tracking telescope at the U. S. Army's White Sands Missile Range is exploring
the use of recently developed adaptive control methods to minimize track loop jitter. Gimbal and platform
vibration are the main sources of jitter in the experiments, although atmospheric turbulence also is a factor. In
initial experiments, the adaptive controller reduced the track loop jitter significantly in frequency ranges beyond
the bandwidth of the existing track loop. This paper presents some of the initial experimental results along with
analysis of the performance of the adaptive control loop. The paper also describes the adaptive control scheme,
its implementation on the WSMR telescope and the system identification required for adaptive control.
Evaluation of polymer membrane deformable mirrors for high peak power laser machining applications
Author(s):
Justin D. Mansell;
Brian G. Henderson;
Gideon Robertson
Show Abstract
Polymer membrane deformable mirrors offer a low-cost alternative to conventional technology in a wide variety of
adaptive optics or laser beam shaping applications. In this paper we evaluate the suitability of two different kinds of
polymer membrane deformable mirrors for laser machining. We present results showing that a 12.5-mm diameter
nitrocellulose membrane fails near 400 microns of motion. We present results from a demonstration of a high peak
power beam shaping and show a new compact laser beam shaping system using a polymer membrane deformable
mirror. We evaluate the effect of Q-switched laser radiation on polymer membranes at 355nm and 1060nm.
A method of generating atmospheric turbulence with a liquid crystal spatial light modulator
Author(s):
Christopher C. Wilcox;
Freddie Santiago;
Ty Martinez;
Jonathan R. Andrews;
Sergio R. Restaino;
Melissa Corley;
Scott W. Teare;
Brij N. Agrawal
Show Abstract
The Naval Research Laboratory has developed a new method for generating atmospheric turbulence and a testbed that
simulates its aberrations far more inexpensively and with greater fidelity using a Liquid Crystal (LC) Spatial Light
Modulator (SLM) than many other methods. This system allows the simulation of atmospheric seeing conditions ranging
from very poor to very good and different algorithms may be easily employed on the device for comparison. These
simulations can be dynamically generated and modified very quickly and easily. In addition, many models for simulating
turbulence often neglect temporal transitions along with different seeing conditions. Using the statistically independent
set of Karhunen-Loeve polynomials in conjunction with Kolmogorov statistics in this model provides an accurate spatial
and temporal model for simulating turbulence. An added benefit to using a LC SLM is its low cost; and multiple devices
can be used to simulate multiple layers of turbulence in a laboratory environment. Current testing with using multiple LC
SLMs is under investigation at the Naval Research Laboratory and the Naval Postgraduate School.
Speed enhancements for a 489-actuator, piston-tip-tilt segment, MEMS DM system
Author(s):
Michael A. Helmbrecht;
Marc Besse;
Carl J. Kempf;
Min He
Show Abstract
Iris AO has been developing a 489-actuator, 163 piston-tip-tilt segment, deformable mirror system controlled with a
personal computer. The system includes the MEMS-based DM, drive electronics, and a precision factory-calibrated
position controller. The position controller implements both position limiting to keep DM segments within the safe
operating region and calculates the actuator voltages that correspond to desired DM piston, tip, and tilt positions. This
paper describes recent speed enhancements and benchmarking results for the 489-actuator deformable mirror system.
Benchmarking showed an execution time of 157.5 μs from the start of the DM piston/tip/tilt (PTT) position controller
operation to when the last bit was output from the computer interface card to the DM drive electronics. Initial testing of
an asynchronous write operation for the computer interface card shows that the PTT controller function can return within
5 μs of a data transfer, thereby shortening the processor time required for a DM to an estimated 74.4 μs. All aspects that
give rise to latencies and bandwidth are presented herein, namely: 1) PTT controller safe-operating-point limiting and
voltage calculations; 2) computer interface and DAC latencies; 3) drive electronics bandwidth, and 4) DM bandwidth.
High efficiency quasi-ternary design for nonmechanical beam-steering utilizing polarization gratings
Author(s):
Jihwan Kim;
Matthew N. Miskiewicz;
Steve Serati;
Michael J. Escuti
Show Abstract
We introduce and demonstrate a quasi-ternary nonmechanical beam steering design based on Polarization Gratings
(PGs). That uses a single wave plate and N PGs to generate 2(N+1)-1 steering angles. When compared to
conventional binary (2N) or ternary (3N) liquid crystal PG steering designs, this technique uses fewer elements
arranged in a simpler configuration to obtain the same number of steering angles. This advantageous property
can be achieved by selecting proper diffraction angles and alignment of the PGs. Due to fewer elements per
stage, losses due to electrode absorption and Fresnel reflections are reduced, thereby increasing the overall steering
efficiency. Using this approach, we demonstrate a four-stage (N = 4) quasi-ternary beam steering device
that achieves 52° Field Of Regard (FOR) with 1.7° resolution (31 steering angles) at 1550 nm wavelength.
Na layer variability and implications for LGS adaptive optics: determination, analysis and impact on AO correction
Author(s):
Katharine J. Jones
Show Abstract
The concept of the laser guide star was proposed by Foy and Labeyrie, 1985. The Laser Guide Stars (LGS) depend on the abundance and distribution of sodium in the mesosphere. The mesopheric sodium often appears to consist of more than one layer, each of which exhibits time variation in density and altitude. The non-zero thickness and finite range of the layers results in elongation of the LGS defocus on extremely large telescopes such as TMT and VLT. Na variability will be examined for determination, analysis and impact on Adaptive Optics aberration correction.
SWIR air glow mapping of the night sky
Author(s):
Michael M. Myers;
David C. Dayton;
John D. Gonglewski;
Gregory Fertig;
Jeff Allen;
Rudolf Nolasco;
Dennis Burns;
Ishan Mons
Show Abstract
It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km
altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band of wave
length 0.9 to 1.7 μm. Numerous studies of these phenomena have demonstrated that the irradiance shows significant
temporal and spatial variations in the night sky. Changes in weather patterns, seasons, sun angle, moonlight, etc
have the propensity to alter the SWIR air glow irradiance pattern. By performing multiple SWIR measurements a
mosaic representation of the celestial hemisphere was constructed and used to investigate these variations over time
and space. The experimental setup consisted of two sensors, an InGaAs SWIR detector and a visible astronomical
camera, co-located and bore sighted on an AZ-EL gimbal. This gimbal was programmed to view most of the sky
using forty five discrete azimuth and elevation locations. The dwell time at each location was 30 seconds with a
total cycle time of less than 30 minutes. The visible astronomical camera collected image data simultaneous with
the SWIR camera in order to distinguish SWIR patterns from clouds. Data was reduced through batch processing
producing polar representations of the sky irradiance as a function of azimuth, elevation, and time. These spatiotemporal
variations in the irradiance, both short and long term, can be used to validate and calibrate physical models
of atmospheric chemistry and turbulence. In this paper we describe our experimental setup and present some results
of our measurements made over several months in a rural marine environment on the Islands of Kauai and Maui
Hawaii.
SWIR sky glow imaging for detection of turbulence in the upper atmosphere
Author(s):
David Dayton;
Rudy Nolasco;
Jeff Allen;
Mike Myers;
John Gonglewski;
Gregory Fertig;
Dennis Burns;
Ishan Mons
Show Abstract
It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km
altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band between
0.9 and 1.7 μm wave length. This has been demonstrated as an effective illumination source for night time imaging
applications. It addition it has been shown that observation of the spatial and temporal variations of the
illumination can be used to characterize atmospheric tidal wave actions in the sky glow region. These spatiotemporal
variations manifest themselves as traveling wave patterns whose period and velocity are related to the
wind velocity at 85 km as well as the turbulence induced by atmospheric vertical instabilities. Ground to space
observation systems especially those employing adaptive optics are adversely affected by high altitude turbulence
and winds. In this paper we propose the use of sky glow observations to predict and characterize image system
degradation due to upper atmosphere turbulence.
Fabrication and simulation of large-scale MEMS deformable mirror for wave front active control
Author(s):
Po-Yu Lin;
Hsin-Ta Hsieh;
Guo-Dung John Su
Show Abstract
Adaptive optics (AO) is a technology which improves the performance of optical systems by reducing the effects of
rapidly changing optical distortion. Wave front active control by combining wave front sensor and MEMS deformable
mirrors which made of polyimide thin film actuated by electrostatic force is one possible solution. Wave front sensor
detects the image and aberration could be described with Zernike polynomials, and the distorted wave-front is corrected
by deformable mirror. Combining these two technologies, we fabricate a large-scale MEMS deformable mirror with a
20mm diameter circular opening and 67 hexagonal actuation electrodes in this thesis. Moreover, we use commercial
software, Ansys, to simulate the deformation behavior of the membrane with different electrodes applied and give some
device parameter tuning for versatile application. We measure the maximum stoke is 39 um as 195 volts applied to 67
electrodes. Due to the large-scale of our thin membrane, resonant frequency is around 8 Hz. Besides, we also discuss
some possible ways to improve device characteristics and we think deformable mirror has a good potential for wave
front active control based on our experiment results.