David V. Wick

Active optical zoom system

In order to optically vary the magnification of an imaging system, continuous mechanical zoom lenses require multiple optical elements and use fine mechanical motion to precisely adjust the separations between individual or groups of lenses. By incorporating active elements into the optical design, we have designed and demonstrated imaging systems that are capable of variable optical magnification with no macroscopic moving parts. Changing the effective focal length and magnification of an imaging system can be accomplished by adeptly positioning two or more active optics in the optical design and appropriately adjusting the optical power of those elements. In this application, the active optics (e.g. liquid crystal spatial light modulators or deformable mirrors) serve as variable focal-length lenses. Unfortunately, the range over which currently available devices can operate (i.e. their dynamic range) is relatively small. Therefore, the key to this concept is to create large changes in the effective focal length of the system with very small changes in the focal lengths of individual elements by leveraging the optical power of conventional optical elements surrounding the active optics. By appropriately designing the optical system, these variable focal-length lenses can provide the flexibility necessary to change the overall system focal length, and therefore magnification, that is normally accomplished with mechanical motion in conventional zoom lenses.

Adaptive optical zoom

In order to optically vary the magnification of an imaging system, continuous mechanical zoom lenses require multiple optical elements and use fine mechanical motion to precisely adjust the separations between individual or groups of lenses. By incorporating active elements, such as liquid crystal spatial light modulators or deformable mirrors, into the optical design, we can eliminate the need to change the spacing between lenses and create an imaging system with variable optical magnification that has no macroscopic moving parts.

Foveated imaging demonstration

A wide field-of-view (FOV), theoretically diffraction-limited imaging system is demonstrated using a single positive lens (a singlet), a reflective liquid crystal spatial light modulator (SLM), a turning mirror and a CCD camera. The SLM is used to correct the off-axis aberrations that would otherwise limit the useful FOV of our system. Foveated imaging refers to the variation in spatial resolution across the image caused by using the SLM in this manner.

Liquid crystal based active optics

Liquid crystal spatial light modulators, lenses, and bandpass filters are becoming increasingly capable as material and electronics development continues to improve device performance and reduce fabrication costs. These devices are being utilized in a number of imaging applications in order to improve the performance and flexibility of the system while simultaneously reducing the size and weight compared to a conventional lens. We will present recent progress at Sandia National Laboratories in developing foveated imaging, active optical (aka nonmechanical) zoom, and enhanced multi-spectral imaging systems using liquid crystal devices.

Non-mechanical zoom system

In order to optically vary the magnification of an imaging system, mechanical zoom lenses, such as those found on 35mm cameras, require multiple optical elements and use cams or gears to adjust the spacing between individual or groups of lenses. By incorporating active elements in the optical design, we can eliminate the need to change lens separations and create an imaging system with variable optical magnification that has no macroscopic moving parts.

Active zoom imaging for operationally responsive space

Deployment costs of large aperture systems in space or near-space are directly related to the weight of the system. In order to minimize the weight of conventional primary mirrors and simultaneously achieve an agile system that is capable of a wider field-of-view (FOV) and true optical zoom without macroscopic moving parts, we are proposing a revolutionary alternative to conventional zoom systems where moving lenses/mirrors and gimbals are replaced with lightweight carbon fiber reinforced polymer (CFRP) variable radius-of-curvature mirrors (VRMs) and MEMS deformable mirrors (DMs). CFRP and MEMS DMs can provide a variable effective focal length, generating the flexibility in system magnification that is normally accomplished with mechanical motion. By adjusting the actuation of the CFRP VRM and MEMS DM in concert, the focal lengths of these adjustable elements, and thus the magnification of the whole system, can be changed without macroscopic moving parts on a millisecond time scale. In addition, adding optical tilt and higher order aberration correction will allow us to image off-axis, providing additional flexibility. Sandia National Laboratories, the Naval Research Laboratory, Narrascape, Inc., and Composite Mirror Applications, Inc. are at the forefront of active optics research, leading the development of active systems for foveated imaging, active optical zoom, phase diversity, and actively enhanced multi-spectral imaging. Integrating active elements into an imaging system can simultaneously reduce the size and weight of the system, while increasing capability and flexibility. In this paper, we present recent progress in developing active optical (aka nonmechanical) zoom and MEMS based foveated imaging for active imaging with a focus on the operationally responsive space application.

Transmissive spatial light modulators with high figure-of- merit liquid crystals for foveated imaging applications

Unique liquid crystal (LC) spatial light modulators (SLM) are being developed for foveated imaging systems that provide wide field-of-view (FOV) coverage (±60° in azimuth and elevation) without requiring gimbals or other mechanical scanners. Recently, a transmissive-SLM- based system operating in the visible (532 nm) has been demonstrated. The LC SLM development is addressing implementation issues through the development of high figure-of-merit (FoM) LC materials and transmissive high-resolution SLMs. Transmissive SLM operation allows the foveated imaging configuration to be very compact using a very simple lens system. The reduction in the size, weight and cost of the imaging optics and in data acquisition/processing hardware makes the foveated approach attractive for small platforms such as unmanned airborne vehicles (UAVs) or missile seekers.

Optical testbed for comparative analysis of wavefront sensors

An optical testbed has been developed for the comparative analysis of wavefront sensors based on a modified Mach Zender interferometer design. This system provides simultaneous measurements of the wavefront sensors on the same camera by using a common aberrator. The initial application for this testbed was to evaluate a Shack-Hartmann and Phase Diversity wavefront sensors referenced to a Mach-Zender interferometer. In the current configuration of the testbed, aberrations are controlled using a liquid crystal spatial light modulator, and corrected using a deformable mirror. This testbed has the added benefit of being able to train the deformable mirror against the spatial light modulator and evaluate its ability to compensate the spatial light modulator. In the paper we present results from the wavefront sensors in the optical testbed.

Adaptive optics using MEMS and liquid crystal devices

In the past two decades, the use of adaptive optics has been validated in many different observatories around the world. However, the availability of new technologies like liquid crystal modulators (LCM) or micro-electro-mechanical-systems (MEMS) deformable mirrors (DM) are providing a revolution in the field. These devices are lower in cost and complexity and are opening the door to applications of adaptive optics that are beyond the astronomical use. In this paper we will present a review of our experience with both MEMS and LCM. Both theoretical and experimental results will be presented.

Multi-spectral foveated imaging system

The development of sensors that are smaller, lighter weight and require less bandwidth is critical for the success of space-based and airborne imaging systems. One solution to this problem is foveated imaging, wherein a liquid crystal spatial light modulator is used to selectively enhance resolution in a wide field-of-view imaging system. Selective enhancement decreases the bulk and complexity of the optical train, while simultaneously reducing data transmission and processing requirements. This enhancement is done modulo 2pi, as such it is inherently a monochromatic correction. In this paper, we propose to overcome that limitation by introducing a switchable polarization interference filter, obtaining near diffraction limited performance over the instantaneous field-of-view (IFOV) at the wavelength(s) of interest