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Instrument Design and Technology | Cover Story | Winter 2007
Adaptive Optics Ready for Prime Time
by Paul Bierden and Steven Menn, Boston Micromachines Corp.

Introduction and Motivation
Originally introduced in the 1950s as a concept for improving astronomical imaging by correcting atmospheric aberration, it took nearly two decades for adaptive optics (AO) technology to catch up with theory and starlight to hit the first AO system. Years later, major telescopes around the world are now equipped with this expensive imaging technology. While these AO enabled telescopes were being developed, technological advances of the last decade in CCD cameras, frame grabbers, and MEMS deformable mirrors have inspired innovative researchers to solve wavefront distortion problems in fields such as microscopy, retinal imaging, and laser communication. These early adopters were rewarded with scientific breakthroughs in their respective fields. Now, with proven applications and mature, affordable components, AO is poised for widespread use in a myriad of optical fields.

A major strength of AO is its application versatility. Many of the world's major telescopes such as the European Southern Observatory in Chile and the Keck Observatory in Hawaii rely on AO to remove wavefront distortion caused by atmospheric turbulence in order to provide clear images of stars and extra-solar planets. Biological researchers have integrated AO into microscopes to correct wavefront aberrations introduced by tissue and thus extracting vital information from biological specimens. Vision science researchers are using AO in their efforts to detect eye disease before its onset and begin earlier treatment . Finally, laser applications such as laser beam shaping for free space communication and laser machining has been successfully demonstrated with AO.

Adaptive Optics Fundamentals
Although AO technology has advanced since its conception, its three main components have remained constant: a wavefront sensor to measure distortion, a wavefront corrector to compensate for the distortion and a control system to calculate the required correction and necessary shape to apply to the corrector. Figure 1a shows a schematic diagram of the system with each of these elements.

light corrected with adaptive optics
Figure 1a

Wavefront Sensors and Controller
The most common wavefront sensor used is called a Shack-Hartman Wavefront Sensor. This sensor by splits light into a number of small beams using an array of miniature lenses, (called lenslets). The light from each of these lenslets is focused onto a CCD camera. As the portion of the wavefront hitting the lenslet is aberrated, the focused spot on the CCD camera moves. Through simple geometry using the displacement of the focused spot and the focal length of the lenslet, the local tilt of the wavefront is calculated, as seen in Figure 1b. Shack-Hartman sensors are used most commonly given their simplicity and manufacturability. There are other techniques for wavefront sensing such as curvature and pyramid sensors but these are not as widely used. The control system is typically a computer running control algorithm software. Upon receiving wavefront information from the wavefront sensor, the controller calculates the appropriate shape to compensate the wavefront and sends that information to the wavefront corrector.

Wavefront correction done by a shack hartman sensor
Figure 1b

Deformable Mirrors
After the aberration is measured, it needs to be corrected, which brings us to the true adaptive element in AO, the wavefront corrector. The most prevalent technology used for this function is a deformable mirror (DM), a thin, flexible reflective layer whose shape is controlled through a variety of mechanisms, based on different competing technologies. The selection criteria for a DM are application based. The fundamental specifications for DM systems are resolution, spatial frequency, speed, stroke, and surface finish. The resolution is determined by the number of actuators in the mirror array and ranges from 19 actuators for an entry level membrane DM up to 4000 actuators for a MEMS DM used for astronomy . Spatial resolution is a measure of how complex a wavefront the DM is capable of correcting. Spatial resolution is determined by actuator count, as well as inter-actuator coupling. Speed is based on the architecture and material properties of the DM. Stroke is a measure of maximum actuator deflection. Stroke and resolution present a significant tradeoff. Low resolution bimorph and ferromagnetic DMs can have a stroke as high as 50 µm, but are not suitable for applications that call for correction of more than simple, low order aberrations. Most microscopic, vision science, and laser shaping applications require 1 to 4 µm of stroke, which is achievable with high resolution MEMS DMs.

MEMS deformable mirrors are currently the most commonly used in many AO applications given their versatility, maturity of technology, and the high resolution wavefront correction that they afford. Using advanced, inexpensive manufacturing technology, MEMS DMs performance strengths are based inherent to micromachining: 1. large actuator arrays create high resolution wavefront correction, 2. advanced microstructures allow minimal influence between adjacent actuators for high frequency surfaces 3. optimized design enables rapid wavefront shaping for high speed applications. The challenges of the early days of MEMS DMs in creating flat surfaces have been largely overcome , and now technological developments push for higher actuator counts and greater maximum stroke.

Bimorph deformable mirrors are made by creating a membrane surface by connecting a piezoelectric material with another material. Electrodes are patterned on the piezoelectric layer. A localized voltage is applied to the piezoelectric layer, thereby expanding one layer with respect to the next and creating a localized membrane curvature. Bimorph are capable of high stroke, but are not able to correct high order wavefront aberration due to high coupling between adjacent pixels. The ability to put dielectric coatings on bimorphs makes them well-suited for high-energy laser applications.

Membrane deformable mirrors employ a simple membrane layer similar to a drum with an electrode pattern underneath. When electrodes are charged, the membrane deflects electrostatically. The simple architecture makes for relatively inexpensive fabrication, but like bimorph DMs, membrane DMs have high interactuator coupling resulting in limited spatial frequency.

Ferromagnetic deformable mirrors have recently been developed for adaptive optics. These devices are capable of high stroke of membrane DMs and have the low interactuator coupling of MEMS DMs, but are limited by their complex and expensive manufacturing process. As a result, they are limited to low actuator counts of 52 pixels makes them viable for high amplitude, low resolution wavefront correction. However, their ferromagnetic nature of these devices limits frame rates to 100s of hertz, making them impractical for many communication and astronomical applications.

Piezoelectric DMs were the first widely used for astronomy. These macroscale deformable mirrors are driven by individual piezoelectric stacks giving them the low interactuator coupling just like MEMS and ferromagnetic DMs. Their large size, in the 100s of millimeters, and high pricing, in the hundreds of thousands to several millions of dollars, made piezoelectric impractical for most applications.

Adaptive Optics Applications
Vision Science
Leading vision scientists believe that someday the human retina will be a window into human health. The ability to resolve individual retina cells or photoreceptors and ocular blood flow through microscopic vasculature will allow scientists to monitor changes in patient health. This holds promise to noninvasively detect, diagnose, and treat the leading eye pathologies such as glaucoma, diabetic retinopathy, and age related macular degeneration. To date, ultra-high resolution images of the retina have not been achievable due to imperfections of the eye itself, causing wavefront distortions.

AO corrects the wavefront distortions introduced by the cornea and crystalline lens, and has enabled increased contrast levels and unprecedented retinal resolution levels. The two primary techniques that employ AO for eye imaging are confocal scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT). Confocal SLO works by focusing laser light on the retina, and creating an image through scanning. Without AO, the best achievable resolution levels are in the 5-10 µm scale which doesn't resolve individual cells that are about 3 µm. AO can achieve 1 µm resolution levels and can produce detailed images of photoreceptor cells, as seen in Figure 3. In OCT, an interferometric imaging technique that creates 3D scanned images. Figure 4 shows cross sectional images obtained with and without an adaptive optics spectral domain OCT (AO-SDOCT) system by Hammer, et. al, of Physical Sciences, Inc. The left image is with AO turned off, and the right image is with AO turned on. In the OCT images taken with adaptive optics, the external limiting membrane, shown by the arrow, is better resolved as are capillaries and structures in other retinal layers.

 
Figure 3a   Figure 3b

 
Figure 4a   Figure 4b

Microscopy
In biological microscopy in vivo imaging is critical since living tissue is much more relevant for studying cellular processes. A major obstacle is the amount of light that can illuminate the tissue without damaging the sample. AO increases not only resolution, but also signal strength and contrast level. These enhancements afforded by AO allow for deep-tissue imaging in vivo.

Dr. Benjamin Potsaid, Research Scientist at the Center for Automation Technologies and Systems at Rensselaer Polytechnic Institute, found an opportunity to solve a problem present in all high-power microscopes. The tradeoff between magnification and field of view poses a constant challenge to researchers looking at larger samples under higher magnification. An existing solution uses a fast scanning microscope stage that patches together an image mosaic. However, for many samples, the moving mechanics disturbs the image. Another solution uses a fast scanning mirror instead of a moving stage. This requires expensive and complex optics to overcome blurring caused by light passing through an off-axis optical path. Potsaid's team created the Adaptive Optic Scanning Microscope to compensate for aberrations caused by optical imperfections and greatly reduce the cost of a high powered, wide field of view scanning microscope. The simplification in optics can be seen in Figure 5.

Figure 5

Laser Applications
A different push in AO has been in the field of long range laser communication. Free-space optical communication holds potential for a new method for data transmittal without the needs for wires or fibers. There are a number of commercial systems that currently provide this, but they are limited to a range of ~1km. When sending data over longer distances, atmospheric turbulence will limit the achievable data rate. AO allows for the compensation of this atmospheric distortion and will provide a high-speed long range data link.

AO Resources
There are a number of organizations dedicated to supporting AO research and education. These centers provide an excellent starting point for scientists and engineers seeking to integrate AO into their optical instrumentation. Based at the University of California, Santa Cruz, the Center for Adaptive Optics (CfAO) was founded in 1999 with a mission "to advance and disseminate the technology of adaptive optics in service to science, health care, industry, and education". University of Arizona hosts a similar center which focuses on astronomical imaging. Working in partnership with the Steward Observatory, the Center for Astronomical Adaptive Optics (CAAO) works to enhance the resolution capabilities of large ground-based telescopes.

Future of AO
Medical Advances
AO holds promise to change the way doctors diagnose disease. Recent, highly publicized research by Prof. Tien Wong, Director of The Retinal Vascular Imaging Centre, University of Melbourne, Australia has shown a link from retinal vasculature damage to coronary heart disease , stroke , and diabetes. His work uses new retinal imaging technology to predict diabetes, stroke, heart disease, hypertension and other risk factors. According to Wong, an increase in imaging resolution provided by AO may address a major limitation with the prediction of cardiovascular disease using imaging from standard retinal photography in its inability to detect subtle changes, on the scale of a few microns, between healthy and diseased vessels. "We know these changes exist," Wong says, "but are detectable only by averaging large populations, and not in an individual." This will allow doctors to detect subtle changes in cell attrition and microvasculature on an individual patient to non-invasively diagnose the diseases that cause the most hospitalization and death worldwide.

Towards Commercialization
In the second half of the last century, inventive scientists developed a theory for a cumbersome, expensive optical correction technique that would someday be improved. Industrious engineers and early adopting researchers worked in parallel: simultaneously refining technology while finding applications beyond astronomy: in retinas, communication signals, and cancer cells. This culmination of mature, affordable technology and proven applications has paved a path to commercialization. AO experts such as Benjamin Potsaid believe that in the next five years, adaptive optics will be an enabling technology in biological research, medical diagnostics, and high precision laser manufacturing. In the five years that follow, Potsaid believes "AO will be a standard component in an optical engineer's toolbox just as polarizers and beam splitters are today."

1. Yuhua Zhang, Siddharth Poonja, Austin Roorda, "MEMS-based adaptive optics scanning laser ophthalmoscopy", Optics Letters; Vol. 31, No. 9, 2006
2. Steven Cornelissen, Paul Bierden, Thomas Bifano, "Development of a 4096 Element MEMS Continuous Membrane Deformable Mirror for High Contrast Astronomical Imaging", Proc. of SPIE, Vol. 6306, 2006.
3. Thomas Bifano, Julie Perreault, Paul Bierden, "A micromachined deformable mirror for optical wavefront compensation", Proc. of SPIE Vol. 4124 2000.
4. Benjamin Potsaid, Yves Bellouard, and John T. Wen, "Adaptive Scanning Optical Microscope (ASOM): A multidisciplinary optical microscope design for large field of view and high resolution imaging". Optics Express, Vol. 13, No. 17, 2005.
5. ien Y. Wong, Wayne Rosamond, Patricia P. Chang, David J. Couper, A. Richey Sharrett, Larry D. Hubbard, Aaron R. Folsom, Ronald Klein, "Retinopathy and risk of congestive heart failure". JAMA 2005; 293:63-69.
6. Tien Yin Wong, Ronald Klein, A. Richey Sharrett, David J. Couper, Barbara E. K. Klein, Duan-Ping Liao, Larry D. Hubbard, Thomas H. Mosley, "Cerebral white matter lesion, retinopathy and risk of clinical stroke: The Atherosclerosis Risk in the Communities Study". JAMA 2002;288:67-74.

Meet the authors
Paul Bierden is president and CEO of Boston Micromachines Corp. in Watertown, Mass.
Steven Menn is director of product marketing at Boston Micromachines Corp.