Carolyn M. Tewksbury-Christle

Aggregate behavior of branch points - persistent pairs

Light propagating through atmospheric turbulence acquires spatial and temporal phase variations. For strong enough turbulence conditions, interference from these phase variations within the optical wave can produce branch points; positions of zero amplitude. Under the assumption of a layered turbulence model, our previous work has shown that these branch points can be used to estimate the number and velocities of atmospheric layers. Key to this previous demonstration was the property of branch point persistence. Branch points from a single turbulence layer persist in time and through additional layers. In this paper we extend persistence to include branch point pairs. We develop an algorithm for isolating persistent pairs and show that through experimental data that they exist through time and through additional turbulence.

Aggregate behavior of branch points: characterization in wave optical simulation

Abstract. Atmospheric turbulence imparts phase distortions on propagating optical waves. These distortions couple into amplitude fluctuations at the pupil of a telescope, which, for strong enough phase distortions, produce zeros in the amplitude called branch points. In our earlier work, we presented the case that branch points can be utilized as a source of information on the turbulent atmosphere. Using our bench-top data, we have demonstrated several properties of branch points including motion, density, persistence and separation. We have shown that the pupil plane motion of subsets of branch points scales to the velocities of atmospheric turbulence layers and identifies the number of branch point producing layers. We have identified empirical relationships for density and separation as functions of the strength and altitude for a single layer. All of this work has been done using a bench-top adaptive optics system utilizing a two-layer atmospheric turbulence simulator. In this paper, we use simulations to verify these previous results by showing that all of these branch point properties follow similar behaviors in independently anchored wave optics simulations.

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.

The aggregate behavior of branch points: a proposal for an atmospheric turbulence layer sensor

We propose a sensor that measures the number, strength, altitude and velocity of atmospheric turbulence layers. Recent research has shown that pupil plane branch points contain four independent and measureable parameters and that these four parameters can be used to estimate four independent turbulence layer parameters--number, strength, altitude and velocity--for each atmospheric turbulence layer. Here, we summarized previous results and then demonstrate how these results allow for construction of a turbulence layer sensor.

The aggregate behavior of branch points: the use of branch point pairing to generate a hidden phase for closed-loop AO

Recent research has shown that branch points, as they appear in astronomical applications, have a rich collective behavior, showing, in particular, that branch point pairs have a well-defined, non-stoichastic velocity, and that once a branch point pairs location is measured, it can be tracked in open-loop adaptive optics operation. The research presented here uses this new information as a priori knowledge in closed-loop AO. Specifically, an algorithm was developed that measures branch point location and velocity at time tk and then uses this to estimate the phase contribution at time tk+n, giving it an effective memory of where branch points appear and allowing it to determine more accurately between real branch points and noise. The output of the new algorithm is used as a second input to the DM control law. Results of initial closed-loop AO tests will be presented.

The Aggregate Behavior of Branch Points - Modeling Parameters

This paper continues a series discussing branch points. In creating a wave optical model of our experiments, we found unexpected dependencies in the branch point distributions. This paper discusses these dependencies.

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.

The aggregate behavior of branch points: verification in wave optical simulation II

Branch points form from interference within a propagating wave due to phase differences imparted by atmospheric turbulence. In the companion paper, we demonstrated that the characteristics of density and separation found in our experimental work are reproducible in an independent wave optical simulation. In this paper, we expand on this demonstration to include the measurement of the number and velocities of branch point producing turbulence layers as well as the existence of persistent pairs in pupil plane measurements. Together these two papers verify our previous experimental results on pupil plane branch point measurements.

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.