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.