Research at CMMPEDevices & applications — Adaptive optics for ophthalmics


Non-invasive optical monitoring techniques are limited by fine microsaccadic eye movements, which create time-varying aberrations and restrict the spatial resolution achievable by current fundus imaging systems. Adaptive optical (AO) systems can overcome these problems by providing closed-loop detection and compensation of these aberrations. Current AO systems are limited by the range, accuracy and noise inherent in the sensing and compensating elements. We have replaced these elements with liquid crystal spatial light modulator (SLM) devices which overcome these problems by increasing the number of active elements by four orders of magnitude.

Figure 2. (a) Ocular aberration from City database, (b) binary encoded ocular aberration, (c) interferometer reconstruction of the aberrated wavefront generated when displayed on an FLC-SLM.

Figure 4. Experiment apparatus used for adaptive optics.




Figure 1. Pin-hole based bias masks for
Zernike mode wavefront detection.

Modal wave front sensing provides an alterative method of detecting optical phase. Ocular aberrations in particular lend themselves to this representation as the number of modes required to describe the ocular wave front are relatively few. One method of detecting aberration modes is through holography. The use of holograms to generate fully complex functions can be traced back to the use of the phase diversity method to generate complex matched filters using computer plotting. This was later superseded by reconfigurable binary, phase only liquid crystal displays which have been used for applications such as phase diversity when applied to the detection of the ocular wave front and an alternative method using biasing aberrations to determine the amplitudes of the aberration modes.

Figure 3. Generated aberrations (left) and the corresponding
images of light passing through them (right).

Holographic phase detection offers several unique advantages over zonal wave front sensing. For example, local opacities in the ocular lens (such as a cataract) cause an irretrievable loss of local information in a zonal sampling scheme, increasing the wave front error. Holographic detection uses information that is in the Fourier plane of the entire beam width, and is therefore more robust against local losses of information. If a SLM is used to display the hologram, then it can rapidly reconfigure in response to the aberration that it is detecting, maximising sensitivity. There is also no limit on the number of modes that can be detected, leading to adaptive optical systems with a high space-bandwidth product. Finally, although the Zernike modal basis is typically chosen to represent the ocular wave front, in principle any orthogonal basis set could be used. Mirror modes could then be used for a 1:1 mapping between the output of the holographic sensor and the input to the mirror, circumventing expensive calculations and increasing closed loop bandwidth.

Further reading

Designing a holographic modal wavefront sensor for the detection of static ocular aberrations
A.D. Corbett, T.D. Wilkinson, J.J. Zhong and L. Diaz-Santana
J.Opt.Soc.Am.A, Vol 24(5), pp.1266-1275, (2007)

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