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Adaptive Optics (AO) is most well known for its success in the field of astronomy where near diff raction limited performance has been achieved from terrestrial telescopes—but AO is also solving problems in retinal imaging, vision, free-space communications, laser systems and more. So far, each user group is originating their own system from scratch, with obvious expertise requirements and impact on cost and time. Our consortium is working on a low-cost, high performance plug-and-play toolkit for AO which will provide capability for all of the areas mentioned and more—moving adaptive optics out of the laboratory and into industrial and medical applications.
Adaptive optics
An adaptive optics system consists of three elements: a wavefront sensor,
a wavefront corrector and a control system. The wavefront sensor—for
example an interferometer— provides a measure of the timevarying
aberrations that need to be corrected. The wavefront corrector, which
is typically a deformable mirror or maybe a LC device, is adjusted to
compensate the measured aberration. Finally a control system is required
to link the sensor and corrector.
The example above is a closed loop configuration since the wavefront sensor measures the residual wavefront error after correction by the mirror.
An experimental compact ophthalmic wavefront sensor developed by the Photonics
Group at Imperial College London for the study of the optical quality
of the eye of premature babies.
The growth of adaptive optics
There are situations where conventional optical systems are limited by the dynamic nature of the problem that they are trying to solve. An optical system that can adjust itself dynamically to cope with, for example, changing optical aberrations is needed – in other words an adaptive optics system. A good example is in ground-based astronomy where the dynamic aberrations due to turbulence in the Earth’s atmosphere are compensated for in real time with adaptive optics. The results are impressive, with enormous improvements in resolution – recovering close to diffraction limited performance [1].
Though initially developed for compensating the effects of atmospheric turbulence in vertical paths for telescopes, adaptive optics is now employed in a range of fi elds: from retinal imaging to high power lasers. Adaptive optics technology with the use of wavefront sensors has recently been applied to the eye, for laser refractive surgery and for high resolution retinal imaging. A narrow optical beam is directed into the eye to form a spot on the retina and the refl ected light is used to measure the aberrations that degrade the optical quality of the eye. There are a number of ways this information can be used: improving the resolution of retinal images using deconvolution techniques or guiding corneal laser ablation for refractive surgery or spectacle prescription. Alternatively, the measurements can be used in real time to correct the aberrations with an adaptive mirror. In this case, the performance of retinal imaging instruments such as fundus cameras and laser scanning ophthalmoscopes (LSO) can be pushed to resolve individual photoreceptors [2]. The method is now also being applied to new techniques such as optical coherence tomography, were it off ers improvements in signal and resolution [3].
Adaptive optics may also be useful for remote sensing and free-space line of sight communications channels. Here, an optical beam propagated through the atmosphere to measure environmental parameters or transmit information, for example, is disrupted by atmospheric turbulence. This results in intensity fade or signal loss. The eff ects are especially bad near the ground where turbulence eff ects are strongest. In these situations the adaptive optics system has to operate at high speed – with correction bandwidths in the kHz range.
The requirements for each of these applications are very diverse – e.g., for the eye, low-light levels and eye-safety are of paramount importance, whereas for atmospheric laser propagation, high-speed wavefront correction, and the ability to operate with intensity scintillation are important. The types and strengths of the aberrations to be corrected vary considerably between applications. So far, each research laboratory or company has had to develop their own AO system, often using some commercially available components, such as deformable mirrors or spatial light modulators. Considerable eff ort is often spent re-developing and understanding the intricacies and subtleties of the adaptive optics before the real work of applying it to the particular application starts.
To address this problem, we are developing a public interface specification for a complete modular adaptive optics system, and a set of exemplar low-cost components, with modules for wavefront sensing, correction and control. Users can construct a system with this set of modules or use a subset: either way it is our aim to massively simplify the process in designing and implementing AO systems.
Bimorph Mirrors
The bimorph mirror in its simplest and most widely used form consists
of a piezoceramic disc bonded to a rigid substrate. The figure below shows
a schematic cross section of such a device.
Application of a voltage across the piezoceramic layer produces an in-plane
expansion/contraction of this layer causing the mirror to bend or flex
in the same way as a bimetallic strip when heat is applied. Compared to
other deformable mirror technologies such as membrane mirrors, bimorph
mirror fabrication uses lower cost components and involves fewer and much
simpler processes.
The finished product is physically far more robust and the specification
can be tailored to meet particular requirements by making simple changes
to thefabrication process. Thus the bimorph mirror design can be optimised
for a particular application with minimal impact on cost, yield or reliability.
The design parameters such as pupil size and electrode layout are determined
by consideration of the wavefront correction required for the specific
application together with the boundary conditions imposed by the mounting
scheme. The figure below shows a surface plot from a 45-channel bimorph
mirror, manufactured at Imperial College, with the electrodes biased alternately
+ve or –ve in the 1st and 3rd rings.
Bimporph mirror under test at Imperial College London
Plug-and-play adaptive optics
There are a number of different types of wavefront sensor, namely, Shack-Hartmann, curvature, pyramid, and lateral and radial shearing interferometers, the choice and design of which can be optimised for the particular application. The most widely used and well understood is the Shack-Hartmann, consisting of a lenslet array in front of an image detector. It is also one of the most versatile and as such will be the primary wavefront sensor module for the toolkit (with diff erent detector options for diff erent sensitivity/speed requirements).
There are also several different wavefront corrector technologies available covering a broad range in terms of specifi cation and cost. In particular, spatial light modulators, membrane mirrors, MEMS mirrors and bimorph mirrors have all been widely utilised. Bimorph mirrors offer high performance with large deformation strokes- particularly important in applications such as retinal imaging. Bimorph mirrors and drivers will form part of the toolkit. The flexibility of the bimorph technology will allow a range of mirror configurations to be offered for different types of applications.
The core of the AO toolkit is the control unit, which will be a stand-alone system containing the electronics to read and process data from the wavefront sensor and calculate the signals to send to the deformable mirror, although the same output signals can be used to drive other technologies like spatial light modulators. By avoiding the limitations of general purpose PC platforms, this approach will off er signifi cant performance advantages – vital for high-speed atmospheric correction.
At the heart of the toolkit approach is an open, freely available specification. So although the toolkit will be complete, it will also allow each user to pick and choose from the modules, and facilitate the integration of outside components if required. We welcome all suggestions for this project and are very interested to capture requirements that we may not be aware of.
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