Supplementary active optics for illumination within an adaptive optics system

Abstract

The use of additional adaptive optics to manipulate the beacon in an adaptive optics system for the human eye opens up many opportunities. Possibilities include better beacon creation, correction of gross aberrations that the beam acquires as it passes into the eye, symbiotic beams to guide or locate a probe or medical beam, and examination of the higher order aberrations experienced by the beam as it enters the eye. We describe the use of active optics for creation of the beacon in an adaptive optics system that incorporates a fundus imaging arm. In describing the possibilities for beacon shaping and steps that may be employed to use this to create a sharper or more versatile beacon, we determine the effects of such shaping on the performance of the system. Results are presented for both model and human eyes. The beacon shaping incorporates Bessel beam creation with tip-tilt and defocus correction from a mask on a spatial light modulator. Visual feedback from images of the beacon as it appears at the retina is used to refine it.

Improved fixation quality provided by a Bessel beacon in an adaptive optics system

“…Part (b) shows the spot from the Flat beacon. Both the Bessel and Flat beacons occupy the full entrance pupil (6 mm), both having uniform magnitude across the SLM, just differing by virtue of the Bessel phase . The wide pupil results in a very tight point spread function on the retina.…”

“…The experiment was performed using an adaptive optics system based upon the pyramid wavefront sensor (PWS), which has been described in detail elsewhere . A schematic is shown in Figure .…”

“…The Bessel beacon required for the work described in this paper was created using a spatial light modulator (SLM) placed in a conjugate pupil plane, enabling application of appropriate phase masks to the incoming light. This process was described in detail by Lambert et al . Here we adopt the terminology for the probe beam formed with the SLM with no spatial phase, β = 0 pixel −1 , as ‘Flat’, and with a radial phase of β = 0.1 pixel −1 , as ‘Bessel’.…”

“…We are investigating a new type of probe beam in order to better control the quality of scatter from the retina, and be less affected by aberrations on the path into the eye than by the traditional solution where we can exert control over its focus depth, and one which subjects can fixate upon in a more stable way. We believe that a Bessel beam employed as the probe may provide some of these desirable qualities …”

“…The use of the illumination within the AO system itself would seem an ideal approach to extending the fixation time, and for evaluating the normal and abnormal deviations from fixation‐like ocular flutter, micro‐saccades, and pathological causes. The added benefits to controlling the illumination are described elsewhere …”