Bruce K. Winker

Liquid crystal beam directors for airborne free-space optical communications

Liquid crystal beam directors for airborne free-space optical communications are discussed and compared with several types of purely mechanical beam directors (i.e., gimbals) in terms of performance, size, weight and power (SWaP). Beam director requirements for initial laser pointing and link acquisition are described. The fine and coarse steering capabilities of liquid crystal optical phased arrays and emerging liquid crystal wide-angle steering technologies are discussed, with particular emphasis on agility and tracking smoothness.

Switchable optical phased prism arrays for beam steering

The Wollaston prism with large deflection angle usually has small cross section size, which constrains its application in beam steering. This paper investigates the possibility of assembling the prisms together to increase the cross section size. Single-layer- and double-layer- assembled Wollaston prisms are investigated. The compression ratio and transmission ratio associated with the diffraction efficiency of assembled prisms are calculated and formulated.

Compact 4 cm aperture transmissive liquid crystal optical phased array for free-space optical communications

There is a critical need for high bandwidth, high availability free-space optical communication links between the battlefield and the global information grid. Compact large aperture transceivers with low size, weight and power (SWaP) are needed to initiate and maintain communication links involving airborne platforms. The transceiver optical beam director typically contains fine and coarse steering stages. Existing beam director technology is based on electro-mechanical gimbaled mirrors with large SWaP that hinders deployment on many airborne platforms. To address the need for compact beam directors, we designed, fabricated, and tested an optical phased array (OPA) based on electro-optic dual frequency liquid crystal technology. This OPA has a transmissive architecture that enables a lower system SWaP, as compared to conventional reflective OPA. It has an 8 μm pixel pitch and steers over a 2.5° field of regard in one dimension at 1.55 μm. Two such OPAs can be stacked to steer in two dimensions. It has four independently addressable 1 cm x 4 cm regions arranged in a linear array to produce a continuous 4 cm x 4 cm aperture. The device incorporates novel addressing schemes to reduce the number of control channels by over an order of magnitude compared to conventional OPA addressing methods. It also utilizes proprietary low-loss transparent conductive TransconTM film for low optical absorption in the infrared. The OPA uses a custom multi-channel controller circuit operating at a 500 Hz frame rate. We present results on OPA design, fabrication, and optical performance on steering.

Liquid Crystal Digital Beam Steering Device Based on Decoupled Birefringent Deflector and Polarization Rotator

ABSTRACT We describe digital beam deflectors (DBDs) based on liquid crystals. Each stage of the device comprises polarization rotator and birefringent prism deflector. We used prisms made of the uniaxial smectic A (SmA) liquid crystal and a solid yttrium orthovanadate crystal. SmA prisms have high birefringence and can be constructed in a variety of shapes. We address the challenges of uniform alignment of SmA, such as elimination of focal conic domains. Rotation of linear polarization is achieved by electrically switched twisted nematic or electrically controlled birefringent cell. We describe a four-stage DBD steering a normally incident laser beam within ±56 mrad range with 8 mrad steps.

Wavefront control with a spatial light modulator containing dual-frequency liquid crystal

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.

Dual frequency liquid crystal devices for infrared electro-optical applications

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.

Voltage calibration of dual-frequency liquid crystal devices for infrared beam steering applications

This paper addresses the use of liquid crystal devices for electro-optic infrared laser beam steering, such as liquid crystal optical phased arrays (OPAs) and digital beam deflectors (DBDs). In these devices, voltages are synchronously applied to different liquid crystal pixels to steer light, either by diffraction and/or refraction using birefringent prisms. Dual frequency liquid crystals provide an order of magnitude higher speed as compared to conventional nematic liquid crystals, at the cost of more complex addressing algorithms and control circuits. In order to optimize the optical performance of a liquid crystal device, the control voltages must be calibrated. This procedure involves adjustment of the control voltages while monitoring the optical efficiency, and must be done for both steady-state phase levels as well as transitions between phase levels. Manual voltage calibration is unrealistically time consuming for multi-channel beam steering devices. Computer based calibration algorithms for dual frequency liquid crystal devices are discussed, and results are presented for both steady state and dynamic voltage calibration procedures.

Computing the liquid crystal director field in optical phased arrays

Optical phased arrays using birefringent liquid crystals can efficiently steer a laser communication beam to a small angle. The algorithm presented derives a thickness for the liquid crystal cell and computes a sequence of voltages for the device electrodes that best deflects the beam.

Gray-scale and contrast compensator for LCDs using obliquely oriented anisotropic network

Birefringent optical compensators containing layers with substantially inclined optic axes can improve not only the contrast but also the gray scale and chromatic stability of 90 degrees twisted nematic LCDs over a large field of view. We present the detailed architecture of such a compensator. It consists of multiple birefringent layers, including one with an in-plane optic axis, one with its optic axis normal to the plane, and two with optic axes inclined at about 40 degrees from the plane. The in-plane and inclined layers are fabricated by photopolymerization of oriented liquid crystal monomers to form anisotropic networks. The precise thicknesses and azimuthal orientations of the various layers are determined by computer optimization. Laboratory measurements of compensated display units show good contrast, gray level, and chromatic stability over a large field of view. The performance is suitable for demanding avionics applications. These compensators are currently being fabricated at the Rockwell Science Center.

Liquid Crystal Tunable Polarization Filter for Target Detection Applications

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.