Julie C. Smith

Creation of photonic orbital angular momentum by distributed volume turbulence.

In previous work, we presented theory of how atmospheric turbulence can impart orbital angular momentum to propagating optical waves. In this paper we provide the first experimental demonstration of the detection of orbital angular momentum from distributed volume turbulence through the identification of well-defined, turbulence-induced, optical vortex trails in Shack-Hartmann wave front sensor measurements.

Experimental analysis of perspective elongation effects using a laser guide star in an adaptive-optics system

The use of a laser guidestar (LGS) for the purpose of a beacon in an adaptive-optics (AO) system is prone to perspective elongation effects on the spots of a Shack-Hartmann wavefront sensor. The elongated spots can vary in size over the subapertures and affect the gradient sensitivity of the sensor. The Air Force Research Laboratory (AFRL) has developed a LGS model that outputs gradient gains which represent the effects of an extended beacon on the spots for a Shack-Hartmann wavefront sensor. This paper investigates the application of these gains in an experimental setup in order to both analyze the effects of the variation in those gains due to spot size elongation and to measure the impact on the performance of an AO system.

Understanding the physics of optical deep turbulence at the Earth's boundary layer

The Air Force Research Laboratory (AFRL) is developing and extending a model of the boundary layer that takes, as input, common atmospheric measurements and ground condition parameters, and predicts key parameters of optical turbulence such as strength and inner scale. In order to anchor the model, a field campaign is also being conducted. The campaign will include co-located meteorological instruments and an open loop Hartmann wavefront sensor. Here, a portion of the boundary layer model is discussed: that relevant for the daytime surface layer. A sensitivity analysis of input parameters is presented.

Initial Results from Implementing and Testing a MEMS Adaptive Optics System

This paper is the 3rd in a series of papers discussing characterization of a Micro-Electrical-Mechanical-System (MEMS) deformable mirror in adaptive optics. Here we present a comparison between a conventional adaptive optics system using a Xinetics continuous face sheet deformable mirror with that of segmented MEMS deformable mirror. We intentionally designed the optical layout to mimic that of a conventional adaptive optics system. We present this initial optical layout for the MEMS adaptive optics system and discuss problems incurred with implementing such a layout; also presented is an enhanced optical layout that partially addresses these problems. Closed loop Strehl highlighting the two systems will be shown for each case as well. Finally the performances of both conventional adaptive optics and the MEMS adaptive optics system is presented for a range of adaptive optics parameters pertinent to astronomical adaptive optics leading to a discussion of the possible implication of introducing a MEMS adaptive optics system into the science community.

Dynamic spatial filtering of deformable mirror commands for mitigation of the waffle mode

The conventional adaptive-optics (AO) system configuration consisting of a Shack-Hartmann wavefront sensor using the Fried geometry is prone to an unsensed waffle mode because of an inability to have discrete point reconstruction of the phase at the actuator positions. Techniques that involve filtering and/or projecting out the waffle mode in the reconstructor have been shown to be effective at not allowing the unwanted mode to occur, but come at the cost of also omitting relevant high frequency content from the measured phase. This paper analyzes a technique of sensing the waffle mode in the deformable mirror commands and applying a spatial filter to those commands in order to mitigate for the waffle mode. Directly spatially filtering the deformable mirror commands gives the benefit of maintaining the reconstruction of high frequency phase of interest while having the ability to alleviate for the waffle pattern when it arises.

An experimental study showing the effects on a standard PI controller using a segmented MEMS DM acting as a mod(λ) device

The ASALT lab has been investigating the use of a segmented MEMS DM in adaptive optics systems. One of the anticipated benefits of a segmented device is that in monochromatic light the throw is essentially infinite due to the modulo 2π nature of the device. Earlier work demonstrated how this modulo 2π behavior interacts unexpectedly with a standard proportional integral controller. Here we present experimental data on this effect to include the testbed on which the data was taken and the methodology used to measure the effect.

Experimental analysis of diffraction effects from a segmented MEMS deformable mirror for a closed loop adaptive optics system

Micro-Electro-Machined Systems (MEMS) have been increasingly used as mirrors in place of conventional continuous face sheet deformable mirrors (DM) in adaptive optics (AO) systems. Here we study the diffraction effects introduced into the optical path when a segmented MEMS DM is used to correct for the wavefront aberrations. Diffraction effects are monitored through the intermediate focus plane prior to the wavefront sensor. Low pass spatial filter is used at that plane in order to investigate how the masking of various diffraction orders affects the phase. Measured phase and focal image plane data for various turbulence conditions are presented and analyzed.