We report a new familiy of polarimetric imaging cameras based on tunable liquid crystal components. Our camera designs use a dual frequency liquid crystal tunable filter that rotates the polarization of incoming light, in front of a single linear polarizer. The unique features of this approach include fast switching speed, high transmission throughput, no mechanical moving parts, broad bandwidth, high contrast ratio, wide viewing angle, and compact/monolithic architecture. This paper discusses these tunable liquid crystal polarimetric imaging camera architectures (time division, amplitude division), the benefits of our design, the analysis of laboratory and field data, and the applicability of polarization signatures in imaging.
Many natural materials produce polarization signatures, but man-made objects, typically having more planar or smoother
surfaces, tend to produce relatively strong polarization signatures. These signatures, when used in combination with
other means, can significantly aid in the detection of man-made objects. To explore the utility of polarization signatures
for target detection applications we have developed a new type of polarimetric imaging sensor based on tunable liquid
crystal components. Current state-of-the-art polarimetric sensors employ numerous types of imaging polarimeters, the
most common of which are aperture division, micropolarizer, and rotating polarizer/analyzer. Our design uses an
electronically tunable device that rotates the polarization of incoming light followed by a single fixed oriented linear
polarizer. Its unique features include: 1) sub-millisecond response time switching speed, 2) ~75% transmission
throughput, 3) no loss of sensor resolution, 4) zero mechanical moving parts, 5) broadband (~75% of center wavelength),
6) ~100:1 contrast ratio, 7) wide acceptance angle (±10°), and 8) compact and monolithic architecture (~10 inch3). This
paper summarizes our tunable liquid crystal polarimetric imaging sensor architecture, benefits of our design, analysis of
laboratory and field data, and the applicability of polarization signatures in target detection applications.
A scalable wavefront control approach based upon proven liquid crystal (LC) spatial light modulator (SLM) technology was extended for potential use in high-energy near-infrared laser applications. With use of an ultra-low absorption transparent conductor in the LC SLM and materials with better physical properties, the laser power handling capability of the device was improved. The experimental results are reported regarding a LC SLM functioning as a wavefront control device under illumination of a kilo-watt laser source. Compared to conventional deformable mirrors, this non-mechanical wavefront control approach offers substantial improvements in speed (bandwidth), resolution, power consumption and system weight/volume, and the zero-coupling between pixels enables a fast feed-forward wavefront correction scheme.
Characterization and calibration process for a liquid crystal (LC) spatial light modulator (SLM) containing dual frequency liquid crystal is described. Special care was taken when dealing with LC cell gap non-uniformity and defect pixels. The calibration results were fed into a closed loop control algorithm to demonstrate correction of wavefront distortions. The performance characteristics of the device were reported. Substantial improvements were made in speed (bandwidth), resolution, power consumption and system weight/volume.
A versatile, scalable wavefront control approach based upon proven liquid crystal (LC) spatial light modulator (SLM) technology was extended for potential use in high-energy near-infrared laser applications. The reflective LC SLM module demonstrated has a two-inch diameter active aperture with 812 pixels. Using an ultra-low absorption transparent conductor in the LC SLM, a high laser damage threshold was demonstrated. Novel dual frequency liquid crystal materials and addressing schemes were implemented to achieve fast switching speed (<1ms at 1.31 microns). Combining this LCSLM with a novel wavefront sensing method, a closed loop wavefront controller is being demonstrated. Compared to conventional deformable mirrors, this non-mechanical wavefront control approach offers substantial improvements in speed (bandwidth), resolution, power consumption and system weight/volume.
A dual frequency liquid crystal (DFLC) can be field-driven towards its unperturbed state, which dramatically reduces the overall electro-optical response time. DFLC materials with sub-millisecond switching speed are being used in infrared electro-optical devices at wavelengths up to 3 microns. The performance of devices such as tunable half-wave plates and optical phased arrays in agile beam steering devices, and wavefront controllers for adaptive optics are described. Device issues discussed include drive schemes, field of view, reflective direct drive backplane, infrared-transparent conductors, and antireflection coatings.