Ruth Mackey

Correction of ocular and atmospheric wavefronts: a comparison of the performance of various deformable mirrors

The main applications of adaptive optics are the correction of the effects of atmospheric turbulence on ground-based telescopes and the correction of ocular aberrations in retinal imaging and visual simulation. The requirements for the wavefront corrector, usually a deformable mirror, will depend on the statistics of the aberrations to be corrected; here we compare the spatial statistics of wavefront aberrations expected in these two applications. We also use measured influence functions and numerical simulations to compare the performance of eight commercially available deformable mirrors for these tasks. The performance is studied as a function of the size of the optical pupil relative to the actuated area of the mirrors and as a function of the number of modes corrected. In the ocular case it is found that, with the exception of segmented mirrors, the performance is greatly enhanced by having a ring of actuators outside the optical pupil, as this improves the correction of the pupil edge. The effect is much smaller in the case of Kolmogorov wavefronts. It is also found that a high Strehl ratio can be obtained in the ocular case with a relatively low number of actuators if the stroke is sufficient. Increasing the number of actuators has more importance in the Kolmogorov case, even for the relatively weak turbulence considered here.

Characterisation of MEMs mirrors for use in atmospheric and ocular wavefront correction

The Applied Optics group at the National University of Ireland, Galway, is engaged in research into various aspects of the application of adaptive optics to both ocular and atmospheric wavefront correction. A large number of commercially available deformable mirrors have been selected by the group for AO experiments, and these mirrors have been carefully characterised to determine their suitability for these tasks. In this paper we describe the approach we have used in characterising deformable mirrors and present results for several MEMs mirrors, including membrane mirrors from AgilOptics and Flexible Optical BV, a segmented micromirror from IrisAO and a 140-actuator mirror from Boston micromachines.

Branch point detection and correction using the branch point potential method

Branch points have been shown to cause problems for adaptive optics (AO) systems which attempt to correct for atmospheric distortion over mid-to-long range horizontal paths. Where branch points (or singularities) occur, the phase of the optical wavefront is undefined and cannot be reconstructed by conventional wavefront reconstruction techniques. Branch points occur in pairs of opposite sign (or rotation) and are joined by wavefront dislocations called branch cuts, which have a 2π jump in phase across them. The aim of the project is to construct a branch point sensitive wavefront reconstructor using a Shack Hartmann wavefront sensor which can be used on a 3km line-of-sight (LOS) free space optical (FSO) communications system currently being tested within our group. The first step in our method is to detect the positions of singularities using the branch point potential method first proposed by LeBigot and Wild. The most common zonal reconstruction method used (the least squares reconstructor) is not sensitive to branch points and different methods are being investigated for this part of the project. Results for the detection of singularities using the branch point potential method in simulations are shown here. Some early results for the reconstruction of branch point affected wavefronts are also presented.

Towards a high-speed adaptive optics system for strong turbulence correction

We present the optical design of a laboratory demonstrator for a low- order adaptive optics system with possible application to improving the performance of a free-space optical communication system. The initial design includes a Shack-Hartmann wavefront sensor with high-speed CMOS camera and a 37-element membrane mirror.

Wavefront sensing and adaptive optics in strong turbulence

When light propagates through the atmosphere the fluctuating refractive index caused by temperature gradients, humidity fluctuations and the wind mixing of air cause the phase of the optical field to be corrupted. In strong turbulence, over horizontal paths or at large zenith angles, the phase aberration is converted to intensity variation (scintillation) as interference within the beam and diffraction effects produce the peaks and zeros of a speckle-like pattern. At the zeros of intensity the phase becomes indeterminate as both the real and imaginary parts of the field go to zero. The wavefront is no longer continuous but contains dislocations along lines connecting phase singularities of opposite rotation. Conventional adaptive optics techniques of wavefront sensing and wavefront reconstruction do not account for discontinuous phase functions and hence can only conjugate an averaged, continuous wavefront. We are developing an adaptive optics system that can cope with dislocations in the phase function for potential use in a line-of-sight optical communications link. Using a ferroelectric liquid crystal spatial light modulator (FLC SLM) to generate dynamic atmospheric phase screens in the laboratory, we simulate strong scintillation conditions where high densities of phase singularities exist in order to compare wavefront sensors for tolerance to scintillation and accuracy of wavefront recovery.

Experimental detection of phase singularities using a Shack-Hartmann wavefront sensor

Phase singularities have been shown to cause one of the major problems for adaptive optics (AO) systems which attempt to correct for distortion caused by the atmosphere in line of sight free space optical communications over mid-to-long range horizontal paths. Phase singularities occur at intensity nulls in the cross-section of the laser beam at the receiver. When the light intensity drops to zero at these points the phase of the optical wavefront is undefined. Phase singularities occur in pairs of opposite sign (or rotation) and are joined by a wave dislocation, called a branch cut, with a corresponding 2π radian jump in the phase. It is this 2π jump which causes difficulties for common AO techniques. To negate the effect of the phase singularities they must be detected and then taken into account in the wavefront reconstruction. This is something not done by most of the zonal reconstruction algorithms commonly used in atmospheric turbulence correction. An experimental set up has been built and is used in the laboratory to examine the detection of phase singularities in atmospheric turbulence. This consists of a turbulence generator using a spatial light modulator (SLM) to mimic the atmosphere and a Shack-Hartmann wavefront sensor as the receiver. The branch point potential method for phase singularity detection is then implemented in post processing to locate the position of the phase singularities. Phase singularity detection can now be practiced under different conditions in a controlled manner. Some results of phase singularity detection from this experimental setup are shown.

Adaptive optics compensation over a 3 km near horizontal path

We present results of adaptive optics compensation at the receiver of a 3km optical link using a beacon laser operating at 635nm. The laser is transmitted from the roof of a seven-storey building over a near horizontal path towards a 127 mm optical receiver located on the second-floor of the Applied Optics Group at the National University of Ireland, Galway. The wavefront of the scintillated beam is measured using a Shack-Hartmann wavefront sensor (SHWFS) with high-speed CMOS camera capable of frame rates greater than 1kHz. The strength of turbulence is determined from the fluctuations in differential angle-of-arrival in the wavefront sensor measurements and from the degree of scintillation in the pupil plane. Adaptive optics compensation is applied using a tip-tilt mirror and 37 channel membrane mirror and controlled using a single desktop computer. The performance of the adaptive optics system in real turbulence is compared with the performance of the system in a controlled laboratory environment, where turbulence is generated using a liquid crystal spatial light modulator.

Study of turbulence effects for a free-space optical link over water

In this paper we report on measurements of atmospheric turbulence effects arising from water air interaction. The aim of this study is to aid in the design of a free-space optical relay system to facilitate longer line-of-sight distances between relay buoys in a large expanse of water. Analysis of turbulence statistics will provide the basis for adaptive optics solutions to improve the relay signal strength affected by scintillation and beam wander. We report on experiments determining the isokinetic angle using an array of broadband incoherent sources of variable angular separation on the order of 0.1 mrad to 2.8 mrad. The experimental setup consists of a 5 inch telescope with high speed CMOS camera observing over a distance of 300 m close at a height of 1.5 m above the water surface. As part of the turbulence characterisation we experimentally estimate the relative image motion of angle-ofarrival fluctuations and perform other time series analysis. Analysis of the image motion requires new techniques due to the extended nature of the source. We explore different centroiding algorithms and surface fitting techniques.