Adaptive Laser Compensation for Aero-Optics and Atmospheric Disturbances

Abstract

Performance of adaptive optics (AO) compensation for beam control is quantified via wave-optical propagation, sensing, and control methods using wind tunnel measurements 1 of aero-optical disturbances in addition to Kolmogorov turbulence distributed over a laser path. For Kolmogorov turbulence the residual phase variance scales as (fG/f 3dB ) 5/3 , where fG is the Greenwood frequency for the propagation path 2 and f 3dB is the error-rejection bandwidth of the classical AO control. It is shown that the residual phase variance with classical AO control, when normalized to the open-loop, scales as (fA/f 3dB ) γ , where γ is an arbitrary power and fA is a characteristic frequency of the aero-optical disturbance determined from a linear fit of the compensation data with increasing bandwidth. AO system latency degrades performance, especially with high-bandwidth control. When operating at a fixed but modest sampling frequency with appreciable latency, AO compensation performance can be significantly enhanced by application of an adaptive control augmentation based on lattice filtering of the residual wavefront sensor gradients, as was implemented in the wave-optics simulations. A classical controller operating at 200 Hz bandwidth with > 400 µsec latency has a limited ability to compensate the aero-optical and free-stream disturbances. By application of the adaptive feed-forward control, laser peak irradiance is shown to increase by a factor of 2.5 or more compared to classical AO feedback control. Strehl ratio degradation associated with wavefront aberrations other than tilt (higher-order)

Measurement of Beacon Anisoplanatism Through a Two-Dimensional Weakly-Compressible Shear Layer

“…As a final comment, it should be noted that correction of the beacon measurements using the MMSE technique relies on the availability of a simultaneously-measured set of beacon and reference wavefronts in order to evaluate the anisoplanatism of the beacon beam and construct the estimation matrix; this requirement is apparent from the definition of the B matrix, Eq. (8). For the results presented in this paper, separate sets of beacon and reference wavefronts were acquired for each test condition (streamwise location and forcing frequency), and a distinct estimation matrix A was computed and used to correct the beacon anisoplanatism.…”

“…This is an intrinsic property of the aero-optical flow for which no theoretical expressions exist. However, insight into the nature of this quantity was obtained from an analysis of laboratory wind tunnel data collected by the University of Notre Dame [1] and employed in other aero-optics efforts [7]. These data showed that for a near-field …”

“…An alternate approach to mitigating the anisoplanatic degradation associated with a beacon located at L b /L a ~ 2 is the use of optimal estimation. Optimal modal compensation [8,9] is a method that offers considerable advantage and flexibility to the problem of aero-optics focal anisoplanatism. In summary, this technique uses a linear estimation matrix A applied to a modal measurement set c m consisting of N modes from one or more reference beacons to yield an estimate for the desired correction:…”

Efficacy of predictive wavefront control for compensating aero-optical aberrations

“…However, recent computational fluid dynamics (CFD) simulations and measurements from wind tunnel and flight experiments have shown that aero-optical aberrations have sufficiently high spatial and temporal bandwidth to sometimes exceed the limits of conventional AO control systems. For greater look-back angles, where separated shear layers exhibit coherent vortical structures with correspondingly large density (and hence index of refraction) variations, 6 conventional AO may be insufficient, 7 and a more advanced AO control technique 8,9 and/or flow control 10 may be required. Furthermore, the air flow around and over turrets with flat and conformal windows is complicated, consisting of several interacting flow phenomena such as horn vortices, necklace vortices, flow separation, shear layer formation, Kelvin-Helmholtz vortices, separation bubbles, recirculation regions, turbulent wakes, and von Kármán vortex shedding.…”

Conformal phased array aero-optical modeling and compensation

“…[12][13][14] The analysis assumes the use of a classical discrete integrator control law where the control commands c at time t kþ1 are given by 4.2, we follow the standard error rejection modeling approach we have applied previously to other aero-optical data.…”