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
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.
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.
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.
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.
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.
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.
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.