Image blur due to aerosols and correlation with aerosol morphology and optical propertiesByrd, Patricia; Goertzen, Aaron; Bose-Pillai, Santasri; Keefer, Kevin; Fiorino, Steven; Wilson, Benjamin
doi: 10.1117/12.3028355pmid: N/A
Small angle scattering by relatively large atmospheric cloud/fog water droplets and ice crystals can cause significant contrast reduction and blurring of imagery. While this effect is quite well explained and verified in field experiments and sensor models, the extent to which aerosols, especially those of quite prevalent anthropogenic fine/ultra-fine/coarse mode play a role in image degradation remains to this date, a controversial topic. In this work, the contribution of aerosols to image blur is revisited but with special focus on field data collected with a relatively large variety of ambient aerosol characterization and optical instrumentation. Ambient particulate/aerosol morphology and optical properties and trends are correlated with collected imagery using instruments including nano-class condensation particle counters, and a nephelometer. Images were captured by a visible camera at different times of the day over a 450 m path. We quantified the blurring in these images through evaluation of the Modulation Transfer Function (MTF). The MTF of the imaging system was characterized through a short-range experiment in the laboratory and turbulence MTF was computed independently from the turbulence-induced motion of features in the images. The aerosol MTF was extracted by dividing the overall MTF by the turbulence and imager MTFs.
Profiling RATS generated turbulence with SMASHOesch, Denis W.; Sexauer, Michael S.; Horst, Samuel B.; Afanador, Shyheim N.; Beason, Melissa K.
doi: 10.1117/12.3027218pmid: N/A
The Reflective Atmospheric Turbulence Simulator (RATS) in the Air Force Research Laboratory’s Beam Control Laboratory is used to impart realistic distortions to a propagating wavefront for testing in the advancement of adaptive optical technologies. Here RATS is being used to simulate turbulence conditions over which the Small Mobile Atmospheric Sensing Hartmann (SMASH) system has typically operated. An optical clone of the SMASH system installed on the optical bench behind RATS measures the imparted optical disturbance and makes an estimate of the turbulence profile. The results are compared with the profile calculated based on the configuration of the RATS system.
Preliminary aero-mechanical jitter analysis of hemispherical turretsBukowski, Timothy J.; Rennie, R. Mark; Gordeyev, Stanislav; Kemnetz, Matthew
doi: 10.1117/12.3028193pmid: N/A
This work presents preliminary results on aero-mechanical jitter of a hemispherical optical turret. A simplified geometry with a hemispherical shell and optics-holding canister was designed to reduce degrees of freedom and provide better insight into fundamental physics. Modal analysis of the turret and mounting plate to the wind tunnel, performed using finite element analysis (FEA), revealed significant plate displacements in the lowest frequency modes. Three mounting plate thicknesses (1/4”, 1/2”, and 1”) were tested. Wind tunnel tests at the University of Notre Dame’s White Field Mach 0.6 wind tunnel assessed turret vibrations at speeds from Mach 0.2 to 0.5, using accelerometers and Shack-Hartmann tilt sensors. Two scenarios were tested: one with the turret inside the tunnel exposed to the flow, and another with the turret attached outside of the wind tunnel so that it is only excited by the base motion of the wind tunnel. The 1/4” plate showed tilt measurements ranging from 30 to 190 microradians when exposed to flow, compared to 10 to 50 microradians in the baseline case. The 1/2” and 1” plates exhibited lower tilts and less distinction between flow and baseline conditions. Overall, the simplified turret only had about three vibration modes affecting tilt, with strong spatial agreement between experimental and FEA modal patterns.
Novel methods for dynamic simulations of optical propagation through atmospheric turbulenceLuna, Kevin; Cubillos, Max; Jimenez, Edwin
doi: 10.1117/12.3028123pmid: N/A
We review our recently developed computationally-efficient methods for optical propagation and phase screen generation, and apply these techniques to the split-step propagation through turbulence. Specifically, we demonstrate the sinc method for optical propagation and phase screen generation provides accurate results for a wide range of propagation distances without the restrictive sampling requirements of Fourier methods. Beyond phase screen generation, the sinc method for phase screen generation lends itself to an efficient extension algorithm that maintains ergodicity of the random fields - a crucial detail for dynamic simulations. We verify the performance of the sinc method for static and dynamic split-step propagation of a Gaussian beam through turbulence as modeled by a modified von Karman spectrum, and we compare it to propagation with the angular spectrum method (ASM). In particular we evaluate the accuracy and robustness of the sinc method for split-step simulations through computation of second and fourth order statistics of the propagated field wherein the ASM breaks down due to artificial periodicity at the same conditions.
Simulating speckle fields in deep turbulence via wave optics: angular spectrum method versus sinc-basis propagationBanet, Matthias T.; Luna, Kevin; Fienup, James R.
doi: 10.1117/12.3027682pmid: N/A
Wave optics simulations of speckle fields can be particularly prone to aliasing when using the angular spectrum propagator. We examined several methods of mitigating aliasing, which include larger guard bands, output plane windowing, absorbing boundaries, and an alternative propagator known as the sinc-basis propagator. We compared the angular spectrum propagator to the sinc-basis propagator and found that, while absorbing boundaries greatly assisted the angular spectrum propagator, the sinc-basis propagator always achieved lower root-mean-squared errors for a given array size due to the non-periodicity of the sinc basis. We examined the computation time as a function of the number of pixels and the root-mean-squared error associated with each of the propagators. Direct comparisons on same array size configurations primarily indicated that the relative wall clock time between the two methods depended highly on the core count of the machine. For all machines tested at the same pixel number, the sinc-basis propagator was generally faster up to a machine dependent pixel threshold, after which the angular spectrum propagator was faster. For machines with more parallelization, this threshold was higher and the speed-up of the sinc-basis propagator relative to the angular spectrum method was larger. It was found that the sinc-basis propagator usually has comparable to shorter computation times than the angular spectrum method to achieve the same threshold error in simulations on the computers tested.
Scintillation-induced centroid jitter: analytical solutionsMitchell, Eric W.; Burrell, Derek J.; Hyde, Milo W.; Driggers, Ronald G.; Spencer, Mark F.
doi: 10.1117/12.3027710pmid: N/A
Atmospheric induced amplitude fluctuations, known as scintillation, impose limitations on active tracking and wavefront-sensing performance over near-horizontal propagation paths. These sensors typically use centroid tracking to estimate the aperture-averaged phase gradient (G-tilt). G-tilt, in practice, is a phase-only measurement, whereas centroid tracking includes both the phase and amplitude components. For a nonuniform beam, centroid tracking will measure the irradiance-weighted average phase gradient (C-tilt). In a closed-loop system, the angular position of the centroid is used to conjugate tilt and reduce system jitter. Of particular interest are the effects of scintillation on the estimation of G-tilt from the centroid angular position. Scintillation will cause an error in the estimation of the G-tilt, and this error can be quantified by the noise-equivalent angle (NEA). The two main objectives of this work are to formulate a closed-form expression for (1) the NEA due to scintillation, and (2) the difference between G-tilt and C-tilt in the weak-to-moderate scintillation regime. The derived solutions are based on the first-order Rytov approximation. As such, the difference will be quantified by deriving a mean-squared error between the desired measurement (G-tilt) and the estimator (C-tilt).
Wavefront sensing requirements for high-speed AO in deep turbulenceDave, Harshil; Ahn, Edwin; Gibson, Steve
doi: 10.1117/12.3027000pmid: N/A
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.
Modern bandwidth requirements in active trackingBurrell, Derek J.
doi: 10.1117/12.3027950pmid: N/A
For several decades, the Tyler frequency has provided the tracking community with a reliable estimate of the bandwidth required to track an object through turbulence consistently. Specifically, it determines a 3-dB bandwidth at which the expected one-axis/one-sigma residual G or Z tilt equals the diffraction angle of a given system. That analysis, however, arrives at tractable solutions by operating in continuous rather than discrete time. Furthermore, it assumes only a first-order lowpass filter as the dynamic controller model. This paper extends Tyler’s original treatment to address each of these potential limitations by analyzing digital rather than analog controllers and generalizing beyond a single-pole transfer function for higher-order control. It further identifies an additional bandwidth constraint from image-plane speckle noise associated with coherent illumination. At its most severe, speckle can reduce precision to the point of becoming stringent than turbulence as a limiting factor in tracking performance. Reducing the sample rate can then allow for speckle averaging, which in turn leads to improvements in track precision and ultimately buys back performance. This trade space poses an optimization problem, with proposed solutions in the form of modified bandwidth requirements that depend upon system diffraction angle, object angular velocity and speckle contrast ratio. Validation from wave-optics simulations and computer-aided control system design informs the analytical tools developed here and demonstrates their applicability to modern challenges in active tracking.
Simulation of wave propagation through turbulent media utilizing the split-step beam propagation method incorporating light attenuation due to aerosol scatterPawlowski, Barry; Wells, Jonathan D.; Pinzhoffer, Kevin
doi: 10.1117/12.3027715pmid: N/A
Wave optics simulations study the effect of beam propagation through the atmosphere; one notable method is the split-step beam propagation method. Current atmospheric propagation software utilizes an alternating series of Fresnel propagation and phase accumulation methods through screens which are statistically representative of the atmospheric turbulence along a line-of-sight path. From this assumption the scintillation parameter along the laser path relies on an atmospheric structure parameter, 𝐶𝑛2, and is typically measured using a path averaged scintillometer. While this works well for links established in static atmospheric conditions in harsher environments such as high precipitation rates, heavy fog, or clouds demand a more rigorous approach. Thus, current software typically over-predicts beam performance of links in harsh conditions. This work proposes a model in which scattering is computed from a “first principles” approach, i.e. the full Mie series is calculated at several locations along the simulated beam path. The scattering results are then combined with the traditional split-step beam propagation method through a correction factor to provide a model of attenuation on beam performance. Results show that the addition of the extinction efficiency factor reduced the overall intensity on target by a significant margin as compared to the traditional split-step beam propagation method. More work is needed to verify the utilization of the Mie scattering extinction along with the log-amplitude variance typically utilized to model the turbulence phase.