Brett E. Bagwell

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.

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.

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

Performance of a MEMS reflective wavefront sensor

An all reflective Shack Hartmann style wavefront sensor has been developed using a Sandia National Laboratory segmented Micro-Electro-Mechanical (MEM) deformable mirror. This wavefront sensor is presently being explored for use with adaptive optics systems at the Naval Prototype Optical Interferometer and other experimental adaptive systems within the Naval Research Laboratory. The 61 MEM mirror segments are constructed in a hexagonal array and each segment can be constructed with either flat or optically powered surfaces. The later allows each mirror segment to bring its subaperture of light to a focus on an imaging array, creating an array of spots similar to a Shack Hartmann. Each mirror segment has tip, tilt and piston functionality to control the position of the focused spot such that measurement of the applied voltage can be used to drive a deformable mirror. As the system is reflective and each segment is controllable, this wavefront sensor avoids the light loss associated with refractive optics and has larger dynamic range than traditional Shack Hartmann wavefront sensors. This wavefront sensor can detect large magnitude aberrations up to and beyond where the focused spots overlap, due to the ability to dither each focused spot. Previous publications reported on this novel new technique and the electrical specifications, while this paper reports on experiments and analysis of the open-loop performance, including repeatability and linearity measurements. The suitability of using the MEM deformable mirror as a high dynamic range reflective wavefront sensor will be discussed and compared to current wavefront sensors and future work will be discussed.

Adaptive polymer lens for rapid zoom shortwave infrared imaging applications

Abstract. This work demonstrates the use of adaptive polymer lenses (APLs) for short-wavelength infrared (SWIR) applications. First, we present a push-button adaptive optical zoom system for variable magnification with a SWIR focal plane array. We then present a push-button, variable divergence, SWIR laser system for pointing and illumination. Last, we outline a system that combines the two: an SWIR adaptive zoom coupled with an APL-enhanced designator/illuminator. The result would allow a user to toggle between different fields of view (magnification), while optimizing illumination (beam divergence) for each field of view. This could be critical for situational awareness and target identification/designation in tactical applications.

Adaptive Imaging for ISR Applications.

Imaging intelligence is hindered by the diametrically opposed needs of high resolution and wide area surveillance. Multi-Gigapixel focal plane arrays are one solution, but we have successfully demonstrated adaptive imaging systems as an alternative. Article not available.

Actuation for deformable thin-shelled composite mirrors

Thin-shelled composite mirrors have been recently proposed as both deformable mirrors for aberration correction and as variable radius-of-curvature mirrors for adaptive optical zoom. The requirements on actuation far surpass those for other MEMS or micro-machined deformable mirrors. We will discuss recent progress on developing the actuation for these mirrors, as well as potential applications.

Large aperture adaptive doublet polymer lens for imaging applications

We report a full design process-finite element modeling, fabrication, and characterization-of adaptive doublet polymer lenses. A first-order model was developed and used to design fluidic doublets, analogous to their glass counterparts. Two constant-volume fluidic chambers were enclosed by three flexible membranes, resulting in a variable focal length doublet with a clear aperture of 19.0 mm. Chromatic focal shift was then used to compare numerical modeling to experimentally measured results over a positive focal length range of 55-200 mm (f/2.89 to f/10.5).

Iris imaging system with adaptive optical elements

Iris recognition utilizes distinct patterns found in the human iris to perform identification. Image acquisition is a critical first step toward successful operation of iris recognition systems. However, the quality of iris images required by standard iris recognition algorithms puts stringent constraints on the imaging systems, which results in a constrained capture volume. We have incorporated adaptive optical elements to expand the capture volume of a 3-m stand-off iris recognition system.