Milind Mahajan

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.

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.

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.

High power liquid crystal spatial light modulators

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.

Adaptive suppression of optical jitter with a new liquid crystal beam steering device

This paper presents a new approach to closed-loop control of optical jitter with a new liquid crystal beam steering device. In contrast to conventional fast steering mirrors, where the laser beam is reflected of the controlled mirror surface, the transmissive liquid crystal beam steering device optically redirects the laser beam. The new device has no moving parts and requires low operating power. This research suggest the new device can replace the fast steering mirrors in a variety of electro-optic systems. The functionality of the transmissive liquid crystal beam steering device along with the analysis of real-time adaptive control experiments are described in this paper. The experimental results show that the new liquid crystal beam steering device can reject disturbances with an LTI feedback controller, and that the disturbance rejection capability can be improved significantly with feedforward adaptive control.

Adaptive control in the presence of quantization and saturation: Application to laser beam steering by a liquid crystal device

This paper presents an investigation of adaptive control for a system with control quantization and magnitude and rate saturations. The application is laser beam steering by a liquid crystal device. The effects of using a linear time-invariant feedback loop along with the adaptive controller are discussed, as well as the effects of using nonlinear and linear plant models in the adaptive controller. In contrast to conventional fast steering mirrors, the liquid crystal device optically redirects the laser beam and has no moving parts or structural resonance, which often requires a stabilizing feedback controller prior to the adaptive controller. Simulation and experimental results demonstrate the effects on the adaptive control performance of choices of linear or nonlinear, and open or closed-loop plant models, and show which choices yield best performance.