Metric adaptive optics systems search over a set of wavefront modes or commands to actuators to
optimize a system performance metric like Strehl ratio or brightness. These systems have been explored
for many decades and have been thought to be unreliable due to local minima in the metric space. It has
been shown that some modes match well with no local minima to a given metric, but they rely on the
ability of a mirror to create reliable replicas of the search modes. We present here a study of the most
common implementation of metric adaptive optics that involves searching over the actuator command
space while evaluating an intensity-based metric. We map an error space relating a common metric to
actuator commands and statistically analyze the error function to determine the quantity and location of the
local minima.
This paper illustrates continuing theoretical and simulation work examining the use of different methods to control a deformable mirror in an adaptive optics system without a wavefront sensor. Two independent techniques, stochastic optimization, or SPGD, and a newly developed functional approximation method, are discussed. Specific results from simulation work performed at SAIC are presented.
Mean irradiance data from a field experiment conducted jointly by the Australian Defence Science and Technology Organisation, the Office of Naval Research, and the University of Central Florida is presented. The experiment was conducted in May 2004 in Adelaide, Australia. The propagation path was characterized by conditions of moderate to strong irradiance fluctuations. The data is compared to existing theoretical results and a new theoretical result developed in this paper. The new theoretical result is based on a modified Rytov method that extends the validity of the Rytov method into moderate to strong irradiance fluctuation conditions.
It is well known that the transmission of an optical signal through the turbulent atmosphere results in random phase fluctuations. In turn, these random phase fluctuations impart a random frequency fluctuation onto the optical signal. As laser radar (lidar) systems rely on the evaluation of micro-Doppler frequency shifts of the reflected optical wave to determine certain target characteristics, it is critical to understand the impact of the atmospheric induced frequency fluctuations. Additionally, lidar systems used for defense applications would typically operate in moderate to strong atmospheric turbulence conditions. Hence, for such applications, it is necessary to develop models describing atmospheric induced frequency fluctuations of an optical wave that are valid in all regimes of optical turbulence. In this paper, we present preliminary results for a model of atmospheric induced frequency fluctuations for the double pass propagation problem in weak optical turbulence conditions and a possible method for extension of these results into moderate to strong turbulence conditions.
The authors have recently developed analytical expressions governing atmospheric induced frequency fluctuations of an optical signal along a horizontal path. Expressions valid in conditions of weak irradiance fluctuations were derived using the Rytov approximation and extended to conditions of moderate to strong irradiance fluctuations via an effective atmospheric spectral model. However, many optical systems, such as coherent ground to satellite communication, imaging, and
astronomical systems, operate in a slant path setting. In this paper, the horizontal path frequency variance results have been extended to slant path scenarios. Integral expressions for one-way slant path, both uplink and downlink, are presented. Additionally graphical results for various operational settings are also provided.
Recently, new theory governing laser beam scintillation was developed for all regimes of optical turbulence. This theory is based on the Rytov approximation but modified with a filter function that eliminates intermediate scale sizes that do not contribute to the refractive and diffractive effects of propagation. This modification extends the validity of the Rytov approximation into moderate to strong regimes as evidenced by the agreement with simulations and experimental data. In this paper we apply this theory to the phase covariance and new expressions governing phase fluctuations are presented. The phase structure function is then compared with previous experimental data.
Recently, a heuristic model for scintillation in moderate to strong turbulence was developed. It is based on the idea of filter functions that eliminate scale sizes that lose their ability to affect a laser beam as it propagates. This approach allows the validity of the Rytov approximation to be extended into moderate to strong turbulence. In this paper, we investigate applying this theory to second order statistics.
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