Optical systems that rapidly change their configuration have many uses. In 1953, the astronomer H. W. Babcock first suggested using a deformable optic with a wavefront sensor and feedback control to measure and correct dynamic aberrations due to turbulence in the atmosphere.1 This technique was thereafter known as adaptive optics. Over the ensuing 25 years, major developments in adaptive optics focused on correcting atmospheric turbulence for astronomical telescopes and spatial inhomogeneities in laser cavities. Success in these areas led to other applications, including microscopy, directed energy, and ophthalmology.2
The same dynamic elements that are used in adaptive optics can be applied in an open-loop configuration (i.e., without a wavefront sensor) to vary the optical properties of an imaging system. In this case, the optical properties of the system are understood and do not vary with time. Consequently, real-time feedback is not necessary to effectively operate these systems. Early examples of such optical devices were patented in the late 1800s, when researchers attempted to mimic the biological processes of animal vision. Lenses for eyeglasses were developed that contained variable focal lengths, providing the wearer with adaptive accommodation for both near and distant viewing. In 1940, Robert Graham was the first to suggest using these lenses in cameras to enable autofocusing.3
Figure 1. Foveated imaging lens developed under the Defense Advanced Research Projects Agency Bio-Optic Synthetic Systems program. The foveated lens is 40% the length and has one-tenth the volume of glass of a comparable conventional wide-angle lens. The lens houses a liquid crystal spatial light modulator (SLM, developed for Sandia by Boulder Nonlinear Systems Inc., the University of Central Florida, and Narrascape) to dynamically correct aberrations at any field point.
Now, recent developments in device technologies are spurring new applications. For example, liquid crystal4 and electrowetting lenses5, 6 have been developed for autofocus and zoom applications. Microelectromechanical system (MEMS) mirrors7 were developed for displays, and as low-cost alternatives to other deformable mirror technologies.
We at Sandia National Laboratories have led several recent efforts to incorporate active devices in open-loop architectures to improve the capabilities of imaging systems for defense and security. By integrating electronically controllable elements into conventional lenses and reflective telescopes, Sandia has improved the field-of-view (FOV), magnification, and spectral properties of various imaging systems. Ultimately, the use of active components will also reduce their size, weight, and power requirements.
Foveated imaging systems use an active element to replace bulky fisheye lenses for wide-FOV applications.8–10 This technology was first developed under the Defense Advanced Research Projects Agency (DARPA) Bio-Optic Synthetic Systems program and integrates a spatially varying dynamic element to correct off-axis aberrations for use in small unmanned aerial systems. This approach increases the usable FOV and in some cases is preferable to supplementing the number of glass lenses.11
Figure 2. Example of an 8× zoom system using variable focal length lenses. There is no longitudinal motion or additional optics flipped into the optical train, as is currently done in conventional zoom systems.
The method is carried out using a custom spatial light modulator (SLM) developed for Sandia by Boulder Nonlinear Systems, the University of Central Florida, and Narrascape. One limitation of foveated imaging systems is that SLMs can only correct aberrations at a particular field angle, which results in diffraction-limited performance only over a small area with a resulting lower resolution in the periphery. The method is flexible, as the area of high resolution can be moved anywhere within the FOV in a few milliseconds. The human fovea (the part of the human eye responsible for sharp vision) works in a similar way. Scanning the area of high resolution is also possible, and will allow a high-resolution image to be constructed throughout at a reduced refresh rate. The lens used to construct this system provides a decrease in size and weight, which may justify the trade-off in quality imaging over the entire FOV.
This same concept has been used to improve imaging in a wide FOV microscope that was recently developed by Thorlabs12 with the Rensselaer Polytechnic Institute. This new ‘adaptive optical scanning microscope’ uses a Boston Micromachines13 MEMS mirror to correct the field-dependent aberrations associated with high numerical apertures without the use of a wavefront sensor.
Another imaging process, called adaptive optical zoom,14,15 uses multiple active elements to change the magnification of an imaging system. Adaptive optical zoom takes advantage of small changes in the focal length of individual elements to change the magnification rather than moving lenses along the optical axis, as is done in a conventional zoom lens. Recent breakthroughs in device development have shown larger zoom ratios with this method. For example, Figure 2 shows an adaptive zoom demonstration where a magnification of 8× is achieved by changing the focal length of two variable lenses.16
Using this method of adaptive optical zoom, variable magnification can be achieved within milliseconds with very little mechanical motion. We have thus far implemented this concept with deformable mirrors, adaptive lenses, and with pixilated spatial light modulators.17, 18 Using an imaging system with individual pixels, optical tilt and higher-order corrections can be added to change the area of magnification within the FOV. By adding a small tilt angle, combined with the required off-axis aberration correction, the system can magnify any area of interest within the wider FOV without moving the entire optical system. This may eventually reduce the need for gimbaled, or multiple camera systems, commonly used for acquisition and tracking.
Finally, we have shown that active optics can be used to vary the spectral properties of an imaging system.18 Previous work19, 20 used liquid crystal technology to create a non-mechanical, variable spectral filter. We have incorporated variable bandpass filters with active optics into a compact multi-spectral imaging system. Such a system is significantly simpler than a conventional wide optical bandwidth system since a lens would not be required to maintain diffraction-limited performance over the entire spectral range simultaneously. This passband can be varied in real time such that the individual monochromatic images at successively longer wavelengths are stored and combined later to present a wide-bandwidth, fused image.
As the dynamic range and wavefront characteristics of new devices improve in the future, applications for adaptive imaging will become even broader. These novel active optic devices, used in conjunction with new techniques in computational imaging, may prove to be even more beneficial. Currently, Sandia is working with Composite Mirror Applications Inc. and the Naval Research Laboratory to develop ultralight, thin-shelled composite mirrors that can vary their radius of curvature.21 In the future we plan to show that an 8-inch-diameter telescope can maintain a relatively wide FOV and, by changing the radius of curvature of these mirrors, can change magnification by 10× or more.
In addition to the many people who have supported these efforts, the authors would like to thank Grant Soehnel and Brian Clark at Sandia for their extraordinary efforts. We also gratefully acknowledge the support of the Office of Naval Research (Michael Duncan and Keith Krapels), DARPA (Len Buckley and Steve Wax), and the Lab Directed Research and Development program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
David V. Wick, Brett E. Bagwell
Sandia National Laboratories
David Wick is an optical engineer. His current interests involve the integration of active or adaptive optics into conventional imaging systems in order to improve performance and reduce size, weight, and power requirements. He has 15 years' experience, over 50 technical articles, and three patents. He is a past member of the Board of Directors of SPIE and serves as a subject matter expert to various entities within the Department of Defense.
Brett Bagwell is an optical engineer. His current area of research is active optics for military applications. He has an extensive background using MEMS, liquid crystal devices, adaptive polymer lenses, microfluidic lenses, and other active optical elements to improve imaging capability. He graduated with a BS in physics from the US Military Academy and an MS and PhD in optical sciences from the University of Arizona. He is currently a member of SPIE, past Chair of the SPIE Scholarship Committee, and serves as a subject matter expert to various entities within the Department of Defense.
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3. R. Graham, A variable focus lens and its uses, J. Opt. Soc. Am. 30, pp. 560, 1940.
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8. D. V. Wick, et al., Foveated imaging demonstration, Opt. Express 10, pp. 60-65, 2002.
9. D. V. Wick, Ty Martinez, M. A. Kramer, Wide field-of-view imaging system using a spatial light modulator, US Patent 6,421,185, 2001.
10.D. V. Wick, Ty Martinez, Wide field-of-view imaging system using a reflective spatial light modulator, US Patent 6,473,241, 2001.
11. J. Harriman, et al., Transmissive spatial light modulators with high figure-of-merit liquid crystals for foveated imaging applications, Proc. SPIE
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Provider of novel polarization measurement and control solutions. Accessed 12 September 2011.
14. D. V. Wick, Active optical zoom system, US Patent 6,977,777, 2005.
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21. Christopher C. Wilcox, David V. Wick, Grant Soehnel, Actuation for deformable thin-shelled composite mirrors, Proc. SPIE 8031
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