Micro Sensors and Actuators Laboratory

Deformable Micromirrors for Adaptive Optics in Vision Science

Vision science for the human eye has made many advances in recent years, with the development of sophisticated imaging systems and correction techniques. For in-vivo examination of the retina, however, it is necessary to use the optics of the eye in reverse, and this means that the image quality that can be achieved is limited by the ocular aberrations.

Adaptive optics (AO) has been identified as the key to correcting these ocular aberrations and has been employed in scanning laser ophthalmoscopes to provide the first real-time images of photoreceptors and blood flow in the living human retina. Adaptive optics was originally developed for ground-based astronomical telescopes to correct wavefront aberrations caused by the atmosphere. An AO system comprises a wavefront sensor to measure the optical aberrations in the incoming wavefront, a control computer and an array of deformable mirrors which correct the outgoing wavefront. The cost and size of the conventional deformable mirror has limited the use of AO in ophthalmology.

The development of deformable mirrors based on micro-electro-mechanical systems (MEMS) technology means that AO-enhanced equipment may soon be widely available in eye clinics. Not only will this provide high resolution imaging (without needing to first paralyze the eye) and early diagnosis and treatment of eye disease, but it should also lead to improvements in prescriptions for eyeglasses and contact lenses and in guiding laser surgery for vision correction. This is because the deflection of the deformable mirror, once set to correct the wavefront aberrations, will provide a more accurate prescription than has previously been possible.

MEMS deformable mirrors (DMs) are batch manufactured on a wafer in a similar way to silicon chips, and are therefore at least an order of magnitude cheaper than the $1,000 USD per channel cost of conventional DMs.

A significant challenge for MEMS DMs is to output a large stroke (range of movement) to provide wavefront correction for both lower and higher order ocular aberrations, particularly at a large pupil sizes (>6 mm) where the effects of the aberrations are most severe. Stroke requirements of 7 – 11 mm or even 12 – 53 mm have been identified. This has posed a problem for MEMS DMs. Existing DMs can only achieve a stroke of a few micrometers.

The current project is to develop MEMS deformable micromirrors which can output a stroke of 10~20 mm, with a fill factor >99%, to fully satisfy the requirements for adaptive optics used in vision science.