Darryl P. Greenwood

Bandwidth specification for adaptive optics systems

A simplified expression for the bandwidth of an adaptive optics system is found to depend on a weighted path integral of the turbulence strength, where the weighting is transverse wind velocity to the 5/3 power. The wave-front corrector is conservatively assumed to match the phase perfectly, at least spatially, if not temporally. For the case of astronomical imaging from a mountaintop observatory, the necessary bandwidth is found to be less than 200 Hz.

Power spectra requirements for wave-front-compensative systems*

The phase degradation of an optical wave front distorted by turbulence in the propagation medium may be corrected in a piecewise-linear fashion by using an array of small circular mirrors. An option in the correction scheme is to compensate for overall tilt separately. We have evaluated power spectra and variances of the piston and tilt motions of the mirror segments as well as the motion of the overall tilt corrector. The form of the spectra for any propagation medium is an aperture integral of the product of the phase-difference power spectrum, describing the medium, and a generalized transfer function, representing the aperture and its segments. In the case of low-power atmospheric propagation, the necessary propagation results are linear in turbulence strength; hence the path may be sectioned into a large number of thin slices. A set of standard curves is found to represent a generalized slice, and the differential contributions may be summed to represent any propagation path. The standard curves, further modeled in terms of power-law dependencies, are practical for use on a desk calculator.

Optical Techniques for Detecting and Identifying Biological-Warfare Agents

Rapid and accurate detection and identification of biological agents is an objective of various national security programs. Detection in general is difficult owing to natural clutter and anticipated low concentrations of subject material. Typical detection architectures comprise a nonspecific trigger, a rapid identifier, and a confirming step, often in a laboratory. High-confidence identification must be made prior to taking action, though this must be traded against regrets stemming from delay. Sensing requirements are best established by positing plausible scenarios, two of which are suggested herein. Modern technologies include the use of elastic scatter and ultraviolet laser-induced fluorescence for triggering and standoff detection. Optical and nonoptical techniques are used routinely in analyzing clinical samples used to confirm infection and illness resulting from a biological attack. Today, environmental sensing serves at best as an alert to medical authorities for possible action, which would include sample collection and detailed analysis. This paper surveys the state of the art of sensing at all levels.

Mutual coherence function of a wave front corrected by zonal adaptive optics

The mutual coherence function (MCF) of a wave propagated through atmospheric turbulence and corrected by zonal adaptive optics is derived for a segmented corrector with either piston only or piston with a local tilt fit. We begin with an intuitive approach which is based on replacing the outer scale of turbulence in the uncorrected MCF with a quantity proportional to the segment size. This approach is especially useful in deriving asymptotes. A rigorous approach provides the full curves, and reduces to a single curve for piston only and a single curve for piston with tilt, after proper parametrization. The effect of amplitude fluctuations is also considered, since scintillation limits the ultimate degree of correction as the segment size goes to zero. Although the analysis is for a segmented corrector, the results apply fairly well to continuous corrector with actuator spacing equal to d/√3, where d is the segment diameter.

Tracking turbulence-induced tilt errors with shared and adjacent apertures*†

Tracking apertures which are on axis, off axis, and annular with respect to the pointer optics are considered in terms of their effectiveness in canceling atmospheric turbulence-induced wave-front tilt errors. The off-axis tracker is found to be the least effective, whereras the annular configuration is least sensitive to the wind profile and slewing conditions. The key to minimizing the centroid wander in the focal plane is the proper setting of the low-pass cutoff frequency of the tracking servo. That setting is based on wind velocity, slew rate, and aperture diameters. A too-high setting of the cutoff frequency can actually degrade tracker performance when the tracking aperture is small.

Proposed form for the atmospheric turbulence spatial spectrum at large scales.

The performance of large optical systems can be strongly influenced by the behavior of large-scale atmospheric turbulence. Previous optical phase difference data diverged from theoretical predictions based on von Kármáns model. To establish a better model microthermal data over large spatial scales were collected at two sites with few terrain inhomogeneities, in unstable midday conditions. Spatial structure function and spectra are developed. A model spatial spectrum is fit to the data with parameters to set two power law dependencies and the transition rate between them. By working with spatial spectral data the assumption of frozen flow is avoided. The optimum spatial spectrum is Kolmogorov-like in the inertial range. Temporal spectral data compare favorably with the new model, which better describes the transition of turbulence scaling from small scales to large.

History of Adaptive Optics Development at the MIT Lincoln Laboratory

For more than two decades Lincoln Laboratory has been a major participant in adaptive-optic research and has performed seminal experiments in atmospheric compensation, including the first thermal-blooming compensation of a high-energy laser, the first compensation of a laser beam propagating from ground to space, and the first compensation using a synthetic beacon. In this paper we briefly review more than 20 years of Lincoln Laboratory work in the field.

Performance projections for laser beam power to space

We examine the requirements placed on an adaptive-optics system used to compensate atmospheric effects in propagating high-power lasers from ground to space. The particular application involves energy transfer from a ground station to a satellite. Our analysis explores performance associated with various beacon configurations, including satellite-based beacons, beacons in the lead-ahead direction, and synthetic beacons. Other system parameters are adjusted as well, including number of actuators in the deformable mirror and bandwidth of the servo system. We show that with an optimized system design it is possible to achieve collection efficiencies of 10-50 percent over zenith angles as great as 70 deg.