Applications utilizing free space beam propagation over long distances in the atmosphere require active sensors and beam shaping. New wavefront sensor designs promise improved performance in deep turbulence, but comprehensive comparisons of modern wavefront sensor designs within Adaptive Optics (AO) loops have yet to reveal winning system-level designs for adaptive optics systems capable of correcting deep turbulence. Here, we attempt to shed light on the problem using a comprehensive wave optics model to evaluate a least-squares based and interferometric-based wavefront sensing techniques, namely the Shack-Hartmann wavefront sensor and pupil plane off-axis digital holography in combination with optimal and adaptive predictive control. The Shack Hartmann wavefront sensor has been an established wavefront sensor that provides a measurement of the wavefront through fast measurements of the wavefront gradient and least squares reconstruction. Interferometric techniques such as digital holography provide higher resolution wavefront reconstruction and improved performance with strong turbulence but with stricter laser requirements and larger computation time. For an optimal AO design in a given application, there is a trade-off between reconstructed wavefront resolution and speed. In this paper, we use wave-optics simulation to qualitatively discuss the upper bounds of AO in deep turbulence, spatial resolution limitations of Shack-Hartmann and Digital Holography wavefront sensors. We show preliminary results of closed-loop AO performance in dynamic deep turbulence, inclusive of wind and limited spatial resolution. Additionally, we show a preliminary analysis of using predictive control to improve the temporal performance of an AO loop and compensate for latencies due to hardware.
Digital holography and image sharpening have been used increasingly in recent years for wavefront sensing and imaging. Compared to conventional imaging and wavefront sensing techniques, digital holography and image sharpening require significantly fewer and simpler optical components to retrieve the complex field (i.e., both the amplitude and phase) and produce a focused image from an estimate of the phase aberrations present in the imaging system. A drawback for digital holography in real-time applications, such as wavefront sensing for high energy laser systems and high-speed imaging for target tracking systems, is the fact that digital holography and image sharpening are computationally intensive, requiring iterative virtual wavefront propagation to optimize sharpness criteria. Recently, it was shown that minimum variance wavefront prediction can be integrated with digital holography and image sharpening to reduce significantly the large number of costly sharpening iterations required to achieve near-optimal wavefront correction. This paper demonstrates further gains in computational efficiency with a new subspace sharpening method in conjunction with predictive dynamic digital holography for real-time applications. The method sharpens local regions of interest in an image plane by parallel independent wavefront correction on reduced-dimension subspaces of the complex field in a pupil plane. Results in this paper from wave-optics simulations show that the new subspace method produces results comparable to that from conventional global and local sharpening, and that subspace wavefront estimation and sharpening coupled with wavefront prediction achieves order-of-magnitude increases in processing speed.
We evaluate a novel wavefront sensing technique using digital holography. This technique divides the aperture plane into multiple subapertures with a unique digital holographic image created in each subaperture. This reduces the complexity and allows for parallelization of the total wavefront computation. This is a key result, as the associated decrease in the processing latency will increase the available bandwidth of an adaptive optics system. We demonstrate this technique through laboratory testing and show that the wavefront sensor can accurately measure phase degradations on a sample target image.
Future adaptive optics systems will require advanced predictive control algorithms that mitigate loop latency by forecasting disturbances. In many applications, such as laser communications terminals, synthesizing such controllers is challenging due to the non-stationary nature of the disturbance statistics over long periods. We present initial experimental results using a new, multichannel, adaptive control algorithm applied on the Integrated Optical System, an extreme AO system under development for the Laser Communications Relay Demonstration. The adaptive controller implicitly tracks disturbance statistics, and provides broadband wavefront control without the need for open loop downtime. The results illustrate the improved disturbance rejection capabilities of the controller compared to a traditional integrator, even when the exogenous disturbance statistics evolve.
Digital holography holds several advantages over conventional imaging and wavefront sensing, chief among these being significantly fewer and simpler optical components and the retrieval of complex field. Consequently, many imaging and sensing applications including microscopy and optical tweezing have turned to using digital holography. A significant obstacle for digital holography in real-time applications, such as wavefront sensing for high energy laser systems and high speed imaging for target racking, is the fact that digital holography is computationally intensive; it requires iterative virtual wavefront propagation and hill-climbing to optimize some sharpness criteria. It has been shown recently that minimum-variance wavefront prediction can be integrated with digital holography and image sharpening to reduce significantly large number of costly sharpening iterations required to achieve near-optimal wavefront correction. This paper demonstrates further gains in computational efficiency with localized sharpening in conjunction with predictive dynamic digital holography for real-time applications. The method optimizes sharpness of local regions in a detector plane by parallel independent wavefront correction on reduced-dimension subspaces of the complex field in a spectral plane.
Digital holography has received recent attention for many imaging and sensing applications, including imaging through turbulent and turbid media, adaptive optics, three dimensional projective display technology and optical tweezing. A significant obstacle for digital holography in real-time applications, such as wavefront sensing for high energy laser systems and high speed imaging for target tracking, is the fact that digital holography is computationally intensive; it requires iterative virtual wavefront propagation and hill-climbing to optimize some sharpness criteria. This paper demonstrates real-time methods for digital holography based on approaches developed recently at UCLA for optimal and adaptive identification, prediction, and control of optical wavefronts. The methods presented integrate minimum variance wavefront prediction into digital holography schemes to short-circuit the computationally intensive algorithms for iterative propagation of virtual wavefronts and hill climbing for sharpness optimization.
The beam control system of a high energy laser (HEL) application can typically experience error amplification due to disturbance measurements that are associated with the non-common path of the optical train setup. In order to address this error, conventional schemes require offline identification or a calibration process to determine the non-common path error portion of a measured sequence that contains both common and non-common path disturbances. However, not only is it a challenging to model the properties of the non-common path disturbance alone but also a stationary model may not guarantee consistent jitter control performance and repeated calibration may be necessary. The paper first attempts to classify the non-common path error problem into two categories where the designer is only given one measurement or two measurements available for real-time processing. For the latter case, an adaptive correlated pre-filter is introduced here to provide in situ determination of the non-common path disturbance through an adaptive correlation procedure. Contrasting features and advantages of this algorithm will be demonstrated alongside a baseline approach of utilizing notch filters to bypass the non-common portion of the combined sequence.
A state-space disturbance model and associated prediction filter for aero-optical wavefronts are described. The model is computed by system identification from a sequence of wavefronts measured in an airborne laboratory. Estimates of the statistics and flow velocity of the wavefront data are shown and can be computed from the matrices in the state-space model without returning to the original data. Numerical results compare velocity values and power spectra computed from the identified state-space model with those computed from the aero-optical data.
A new method for adaptive prediction and correction of wavefront errors in adaptive optics (AO) is introduced. The new method is based on receding-horizon control design and an adaptive lattice filter. Experimental results presented illustrate the capability of the new adaptive controller to predict and correct aero-optical wavefronts derived from recent flight-test data. The experimental results compare the performance of the new adaptive controller the performance of a minimum-variance adaptive controller previously used in AO. These results demonstrate the reduced sensitivity of the receding-horizon adaptive controller to high-frequency sensor noise.
This paper introduces a new method for adaptive prediction and correction of wavefront errors in adaptive
optics. The new method is based on receding-horizon control design and an adaptive lattice filter. Experimental
results presented in the paper illustrate the capability of the new adaptive controller to predict and correct
aero-optical wavefronts derived from recent flight-test data. The experimental results compare the performance
of the new adaptive controller the performance of a minimum-variance adaptive controller previously used in
adaptive optics. These results demonstrate the reduced sensitivity of the receding-horizon adaptive controller to
high-frequency sensor noise.
This paper compares an optimal linear time-invariant controller and an adaptive controller for prediction and
control of aero-optical wavefronts derived from recent flight-test data. Both control methods have the form of
multichannel prediction filters that capture the statistics of the aero-optical turbulence to mitigate latency in
the adaptive optics loop. Experimental results show the improvement in wavefront correction achieved by both
prediction methods. Altering the flow characteristics of the disturbance wavefront during the control process
illustrates the ability of the adaptive controller to track changes in the aberration statistics.
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
KEYWORDS: Adaptive control, Digital signal processing, Telescopes, Digital filtering, Electroluminescence, Cameras, Line of sight stabilization, Missiles, Feedback control, Optical filters
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.
This paper presents results from an adaptive optics experiment in which an adaptive control loop augments a
classical adaptive optics feedback loop. A membrane deformable mirror is used for wavefront correction, and
a set of frequency-weighted modes based on the actuator geometry are used to define the control channels for
the adaptive controller. In the adaptive optics experiment, the wavefront sensor in the control loop is a three
step phase shifting self-referencing interferometer. The corrected laser beam is imaged by a diagnostic CCD
camera. The effect of atmospheric turbulence is simulated in the experiment by a sequence of wavefronts that
is generated by a WaveTrain adaptive optics model and added to the laser beam by a spatial light modulator.
The experimental results show the improved closed-loop wavefront errors and diagnostic images produced by the
adaptive control loop as compared to the classical adaptive optics loop.
KEYWORDS: Sensors, Adaptive control, Mirrors, Digital filtering, Control systems, Microelectromechanical systems, Computing systems, Actuators, Error control coding, Linear filtering
In recent experimental research on adaptive control of jitter in laser beams, sufficiently high levels of high
frequency sensor noise have produced high frequency spikes in the output errors. The phenomenon has been
observed for both adaptive and high bandwidth linear-time-invariant (LTI) control loops. Recently, the source
of the problem has been discovered to be saturation associated with the MEMS fast steering mirror used as
the control actuator. Results in this paper demonstrate that the spikes in the output error are eliminated by a
recently developed frequency-weighting method for the tuning signal used to determine adaptive control gains.
The method places more weight on jitter in frequency ranges where large sensor noise otherwise produces the
unwanted response. The frequency-weighted adaptive control loop is based on a recursive least squares lattice
filter that implicitly identifies the disturbance statistics from real-time sensor data. The adaptive controller
achieves both fast adaptation and true minimum variance steady state performance. Results from an experiment
with a MEMS fast steering mirror used in current free space optical communications systems illustrate suppression
of jitter with simultaneous multiple bandwidths produced by multiple jitter sources.
This paper presents a new approach to closed-loop control of optical jitter with a new liquid crystal beam steering
device. In contrast to conventional fast steering mirrors, where the laser beam is reflected of the controlled mirror
surface, the transmissive liquid crystal beam steering device optically redirects the laser beam. The new device
has no moving parts and requires low operating power. This research suggest the new device can replace the fast
steering mirrors in a variety of electro-optic systems. The functionality of the transmissive liquid crystal beam
steering device along with the analysis of real-time adaptive control experiments are described in this paper. The
experimental results show that the new liquid crystal beam steering device can reject disturbances with an LTI
feedback controller, and that the disturbance rejection capability can be improved significantly with feedforward
adaptive control.
This paper presents an adaptive control scheme for adaptive optics. Adaptive compensation is needed in many adaptive optics applications because wind velocities and the strength of atmospheric turbulence can change rapidly, rendering classical fixed-gain reconstruction algorithms far from optimal. The paper also presents a method for generating frequency-weighted deformable-mirror modes, which are important for optimal performance of the adaptive control scheme. The performance of the adaptive control scheme is illustrated by simulations of a directed-energy system with a high-energy laser propagating through extended turbulence.
We present an adaptive control scheme for laser-beam steering by a two-axis microelectromechanical systems (MEMS) fast steering mirror. Disturbances in the laser beam are rejected by a µ-synthesis feedback controller augmented by the adaptive control loop, which determines control gains that are optimal for the current disturbance acting on the laser beam. The variable-order adaptive controller is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results demonstrate that the adaptive controller significantly extends the disturbance-rejection bandwidth achieved by the feedback controller alone. The experimental results also illustrate the value of the variable-order capability of the adaptive controller.
This paper presents new results on adaptive control of jitter in laser beams. Experimental results illustrate the capability of a recently developed method for variable-order adaptive control reduce jitter in bandwidths well beyond the bandwidth of linear time invariant control systems. The adaptive control loop is based on recursive least squares lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. The adaptive controller achieves both fast adaptation and true minimum variance steady state performance. The main innovation in this paper is frequency weighting in the adaptive control loop to emphasize the relative importance of jitter in particular bandwidths and mitigate the effects of high-frequency sensor noise. Results from an experiment with a MEMS fast steering mirror used in current free space optical communications systems illustrate suppression of jitter with simultaneous multiple bandwidths produced by multiple jitter sources.
This paper discusses the application of adaptive control methods in the Atmospheric Simulation and Adaptiveoptics
Laboratory Testbed at the Starfire Optical Range at the Air Force Research Laboratory, Kirtland AFB.
Adaptive compensation is useful in adaptive optics applications where the wavefronts vary significantly from one
frame to the next or where wind velocities and the strength of atmospheric turbulence change rapidly, rendering
classical fixed-gain reconstruction algorithms far from optimal. The experimental results illustrate the capability
of the adaptive control scheme to increase Strehl ratios and reduce jitter.
This paper presents an adaptive control scheme for laser-beam steering by a two-axis MEMS tilt mirror used in current free-space optical communications systems. In the control scheme presented here, disturbances in the laser beam are rejected by a high-performance linear time-invariant feedback controller augmented by the adaptive control loop, which determines control gains that are optimal for the current disturbance acting on the laser beam. The variable-order adaptive control loop is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results are presented to demonstrate the effectiveness of the adaptive controller for rejecting multi-bandwidth jitter. These results demonstrate that the adaptive loop significantly extends the jitter-rejection bandwidth achieved by the feedback controller alone.
KEYWORDS: Data modeling, Systems modeling, Wave propagation, System identification, Sensors, Databases, Damage detection, Aerospace engineering, Matrices, Structural health monitoring
Development of efficient tools to successfully localize and characterize hidden damage in critical structural components is an important task in the design and construction of structural health monitoring systems in aging as well as new structures. In this paper two methodologies for damage identification and localization will be presented. The first is an automatic numerical scheme using a state space system identification approach and the second is based on certain damage correlation indices associated with changes in the frequency response of the structure in presence of flaws. In each case, the structure is to be instrumented with an array of sensors to record its dynamic response including vibration and wave propagation effects. To determine the type and location of an unknown defect, the sensor data detected is used to identify a new system, which then is compared to a database of state-space models to find the nearest match. The second method deals with the definition of a set of damage correlation indices obtained from the frequency response analysis of the structure. Two types of indices have been considered. The first uses the correlation between the responses of the defect free and damaged structure at the same point, and the second uses correlation at two different points. The potential application of the general approach in developing health monitoring systems in defects-critical structures is discussed.
This paper concerns a class of adaptive optics problems in which phase distortions due to atmospheric turbulence are corrected by adaptive wavefront reconstruction with a deformable mirror - i.e., the control loop that drives the mirror adapts in real time to time-varying atmospheric conditions, as opposed to the linear time-invariant control loops use din conventional 'adaptive optics.' The basic problem is posed here as an adaptive disturbance rejection problem with many channels. The solution given here is to augment a linear time-invariant feedback control loop with an adaptive feedforward control loop based on a multichannel adaptive lattice filter. Simulation results are presented for a one-meter telescope with both single-layer and two- layer atmospheric turbulence profiles. These results demonstrate the significant improvement in imagin resolution produced by the adaptive control loop.
KEYWORDS: Intelligence systems, Control systems, Smart structures, Systems modeling, Matrices, Digital filtering, Data modeling, Linear filtering, Actuators, Motion models
Active disturbance rejection to minimize optical path length error is illustrated by experimental results from the JPL Phase B Test Bed, which incorporates an interferometric sensor and a controllable trolley mounted on a flexible truss structure. The controller actively isolates the optical instruments from structural vibrations induced by external disturbances consisting of linear combinations of sinusoidal signals.
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