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Dual-Mirror Adaptive Optics Systems

Photonics Spectra
Jun 2010
Take the Low and High Roads to Imaging Success

Michael R. Feinberg and Paul Bierden, Boston Micromachines Corp.

Researchers worldwide have leveraged advances in microelectromechanical systems (MEMS) mirror technology by using adaptive optics to correct for wavefront aberrations caused by distortion. Now, the next generation of adaptive optics systems is using dual deformable mirror configurations to effectively compensate for a variety of wavefront aberrations. Researchers have opened possibilities in ground-based astronomy and biological imaging that previously were unimaginable.

The right mirror for the job

Deformable mirrors are advanced wavefront control devices that can change shape to correct a distorted incident wavefront. The fundamental specifications for deformable mirror systems are resolution, spatial frequency, speed, stroke and surface finish.

The resolution is determined by the number of actuators in the mirror array, which ranges from 19 for an entry-level membrane-based device to more than 4000 for a MEMS deformable mirror. Spatial resolution is a measure of how complex a wavefront the deformable mirror can correct. This is determined by the number of actuators that control the shape of the mirror and by the mechanical coupling between adjacent actuators. Speed is based on the architecture and material properties of the deformable mirror. Finally, stroke is a measure of maximum actuator deflection.

As the technology stands today, mirrors are most proficient at either low-resolution, low-speed and high-stroke operation, or high-resolution, high-speed and low-stroke operation, identified as “woofer” and “tweeter,” respectively (see table). It is this tradeoff that necessitates a next-generation approach to adaptive optics wavefront correction.

To achieve a higher degree of wavefront correction, newer adaptive optics systems are using both a woofer and tweeter style of deformable mirror. The woofer/tweeter dual-mirror configuration allows for better compensation of large-variance, high-spatial-frequency phase distortion. A simplified layout of such a system is shown in Figure 1.


Figure 1.
Shown is a simplified layout of a typical woofer/tweeter adaptive optics system. In this configuration, incoming light is reflected off the woofer and tweeter mirrors, which are managed by a single control system. The deflection of each mirror is determined by an algorithm that calculates the optimal mirror shapes to compensate for aberrations as measured by a wavefront sensor.


Low-order optical aberrations are the most common and are corrected using a high-stroke, low-resolution mirror (the woofer). This can be accomplished using a membrane-type mirror with a limited number of actuators. High-order aberrations are more complex and require more precision. Most microscopy, vision science and laser shaping applications require 1 to 4 μm of stroke to correct for these aberrations, which is achievable with a high-resolution mirror (the tweeter).

One mirror that can fill the role of the tweeter is a MEMS deformable, which consists of a mirror membrane – either continuous or segmented – supported by an underlying actuator array. Each actuator in the array can be deflected by electrostatic actuation to achieve the desired pattern of deformation.

Scientists have been developing dual deformable mirror woofer/tweeter systems to deploy in adaptive optics applications for retinal imaging and astronomy.

Vision science applications

Leading vision scientists and ophthalmologists believe that the human retina will be a window into human health. Being able to visualize the retina at a cellular level gives researchers the ability to study vasculature and photoreceptor properties and holds promise for earlier diagnosis of the “big three” eye diseases: glaucoma, diabetic retinopathy and age-related macular degeneration.

Wavefront distortions generated in the eye itself prevent generation of useful high-resolution images without the use of adaptive optics, which corrects distortions introduced by the cornea, crystalline lens and vitreous humor. Adaptive optics enables increased contrast levels and unprecedented retinal resolution levels.

In retinal imaging with an adaptive optics scanning laser ophthalmoscope (AOSLO), a dual deformable mirror approach could be used because of the large individual differences in defocus and astigmatism in humans. For the first time, researchers can see critical detail within the retina and can detect changes in the eye significantly earlier than with current diagnostic tools. Earlier detection can enable early treatment that could slow the progression of eye disease – or even prevent it.

Steps are being taken to detect disease through the use of an AOSLO system at Indiana University Bloomington, where researchers Stephen A. Burns, Weiyao Zou and Xiaofeng Qi have developed a real-time zonal control algorithm that uses wavefront slope measurements from a single Shack-Hartmann wavefront sensor to generate control signals for two deformable mirrors.

The procedure will be used to implement a woofer/tweeter dual deformable mirror AOSLO system for in vivo human retinal imaging with a 140-actuator deformable mirror (maximum stroke, 3.5 µm) and a 52-actuator magnetic deformable mirror (maximum stroke, 50 µm). The low-stroke deformable mirror is the tweeter, for correcting the high-order aberrations; the high-stroke deformable mirror is the woofer, for correcting the low-order aberrations. The dual-mirror system effectively removed aberrations in the eye, thereby generating clear images that will be used to study the progression of eye disease in live patients.

Astronomy applications

Adaptive optics also is commonly used on telescopes to remove the effects of atmospheric distortion. When light from a star or another astronomical object enters the Earth’s atmosphere, turbulence distorts the light in various ways, including blurring images. An adaptive optics system tries to correct this by using a wavefront sensor that takes some of the astronomical light for analysis, a deformable mirror that lies in the optical path and a computer that receives input from the sensor. The sensor measures the distortions that the atmosphere has introduced on the timescale of a few milliseconds, and the computer calculates the optimal mirror shape to correct them. The surface of the deformable mirror is reshaped accordingly.

One system under construction that will use a woofer/tweeter design is the Gemini Planet Imager, a next-generation adaptive optics instrument being built for the Gemini telescope by a consortium of US and Canadian institutions. Funded by the Gemini Observatory, a partnership of seven nations, the group’s goal is to image extrasolar planets orbiting nearby stars using a dual deformable mirror system. The Gemini system’s low-actuator-count woofer reduces the residual wavefront error to a level controllable by the finer tweeter, a MEMS deformable mirror with 4096 active elements. First light is projected to be recorded in early 2011.

A second example is the PALM-3000, a high-precision upgrade to the Palomar Adaptive Optics System on the 5.1-m Hale Telescope at Palomar Observatory in Palomar Mountain, Calif. It will use its existing deformable mirror (241 active actuators) as the woofer and a new high-actuator-count deformable mirror (3388 active actuators) as its tweeter. As with the Gemini Planet Imager, first light is projected to be in early 2011.

Dual-mirror control

Two distinct methods for control of dual deformable mirror woofer/tweeter systems have been developed – serial and parallel. Using a serial approach, the aberration is corrected by the woofer first, and the resulting image is sequentially corrected by the tweeter. This is a “best effort” approach and is currently taking a backseat to a more real-time parallel approach. With improved algorithms and faster processing speeds, most applications now take advantage of parallel correction methods. To do this, the algorithm sorts the wavefront aberrations into two groups – one for the woofer correction and one for the tweeter correction – in real time.

A recent algorithm developed for vision science has come from Chaohong Li, Nripun Sredar, Hope Queener, Kevin M. Ivers and Jason Porter at the University of Houston’s College of Optometry in Texas. The success of this technique is shown in Figure 2, which depicts a high-resolution image of photoreceptors in the retina.


Figure 2.
This image shows individual photoreceptors in the retina using an adaptive optics scanning laser ophthalmoscope with woofer/tweeter optical architecture. Courtesy of Kevin M. Ivers, Chaohong Li and Jason Porter, University of Houston College of Optometry.


For astronomical applications, previous work has been done as part of the “Woofer Tweeter Experiment” at the University of Victoria in British Columbia, Canada, and is currently proceeding for the Palomar Observatory’s PALM-3000 adaptive optics upgrade.

Adaptive optics continues to evolve to provide clarity. Using dual deformable mirror devices, biological imaging researchers can look deeper in vivo and astronomers can obtain higher-resolution images of celestial objects, enabling further research into the behavior of other solar systems. The woofer/tweeter optical architecture provides continuous improvement to correct wavefront aberrations and promises to lead to new discoveries in all types of imaging applications.

Meet the authors

Paul Bierden is president and CEO of Boston Micromachines Corp. in Cambridge, Mass. Michael R. Feinberg is the company’s director of product marketing; e-mail: mrf@bostonmicromachines.com.



GLOSSARY
adaptive optics
Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
astronomy
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
mirror
A smooth, highly polished surface, for reflecting light, that may be plane or curved if wanting to focus and or magnify the image formed by the mirror. The actual reflecting surface is usually a thin coating of silver or aluminum on glass.
resolution
1. In optics, the ability of a lens system to reproduce the points, lines and surfaces in an object as separate entities in the image. 2. The minimum adjustment increment effectively achievable by a positioning mechanism. 3. In image processing, the accuracy with which brightness, spatial parameters and frame rate are divided into discrete levels.
retina
1. The photosensitive membrane on the inside of the human eye. 2. A scanning mechanism in optical character generation.
spatial frequency
With a repetitive object such as a series of equispaced lines, the reciprocal of the line spacing in object or image, generally expressed in cycles per millimeter.
vitreous humor
The transparent fluid that fills the portion of the eye between the eye lens and the retina (the posterior chamber).
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