Stephen L. Browne

Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites

We present here results using two novel adaptive optic elements, an electro-static membrane mirror, and a dual frequency nematic liquid crystal. These devices have the advantage of low cost, low power consumption, and compact size. Possible applications of the devices are astronomical adaptive optics, laser beam control, laser cavity mode control, and real time holography. Field experiments were performed on the Air Force Research Laboratory, Directed Energy Directorates 3.67 meter AMOS telescope on Maui, Hawaii.

Theory and laboratory demonstrations on the use of a nematic liquid-crystal phase modulator for controlled turbulence generation and adaptive optics

We discuss the use of liquid-crystal phase modulators (LCPMs) both as a repeatable disturbance test source and as an adaptive optics corrector. LCPMs have the potential to induce controlled, repeatable, dynamic aberrations into optical systems at low cost, low complexity, and high flexibility. Because they are programmable and can be operated as transmissive elements, they can easily be inserted into the optical path of an adaptive optics system and used to generate a disturbance test source. When used as wave-front correctors they act as a piston-only segmented mirror and have a number of advantages. These include low operating power requirements, relatively low cost, and compact size. Laboratory experiments with a Meadowlark LCPM are presented. We first describe use of the LCPM as a repeatable disturbance generator for testing adaptive optics systems. We then describe a closed-loop adaptive optics system using the LCPM as the wave-front corrector. The adaptive optics system includes a Shack-Hartmann wave-front sensor operated with a zonal control algorithm.

Characterization and control of a multielement dual-frequency liquid-crystal device for high-speed adaptive optical wave-front correction

Multielement nematic liquid-crystal devices have been used by others and ourselves for closed-loop adaptive control of optical wave-front distortions. Until recently the phase retardance of available devices could be controlled rapidly in only one direction. The phase retardance of the dual-frequency device can be controlled rapidly in both directions. Understanding the dynamics of the phase retardance change is critical to the development of a high-speed control algorithm. We describe measurements and experiments leading to the closed-loop control of a multielement dual-frequency liquid-crystal adaptive optic.

Long-range laser illuminated imaging: analysis and experimental demonstrations

David DaytonApplied Technology Associates1900 Randolph SEAlbuquerque, New Mexico 87106E-mail: dayton@aptec.comSteve BrowneThe Optical Sciences Company1341 S. Sunkist Ave.Anaheim, California 92806John GonglewskiAir Force Research LaboratoryDirected Energy DirectorateKirtland AFB, New Mexico 87117Steve SandvenApplied Technology Associates1900 Randolph SEAlbuquerque, New Mexico 87106Joe GallegosBoeing North AmericaKirtland AFB, New Mexico 87117Mike ShilkoITT Advanced Engineering Division6400 Uptown Blvd.Albuquerque, New Mexico 87110Abstract. We demonstrate the utility of laser-illuminated imaging forhigh-resolution clandestine nighttime surveillance from a simulated air-borne platform at standoff ranges in excess of 20 km. In order to reducethe necessary per-pulse laser energy required for illumination at suchlong ranges, and to mitigate atmospheric turbulence effects on imageresolution, we have investigated a unique multiframe postprocessingtechnique. It is shown that in the presence of atmospheric turbulenceand coherent speckle effects, this approach can produce superior resultsto conventional scene flood illumination.

MEMS adaptive optics for high-resolution imaging of low Earth-orbit satellites

We present here results using two novel adaptive optic elements, an electro-static membrane mirror built by OKO Technologies, and a dual frequency multi-segment nematic liquid crystal built by Meadowlark Optics. These devices have the advantage of low cost, low power consumption, and compact size. The total cost for these adaptive optics elements is hundreds of dollars per actuator as compared to a cost of thousands of dollars per actuator for conventional adaptive optics. Field experiments were performed on the Air Force Research Laboratory 3.67 meter telescope on Maui, Hawaii, with the aperture stopped down to 1.15 meters. It is believed that this is the first ever experimental demonstration of these two devices for adaptive correction of images of satellites. Recently, the control electronics for the liquid crystal device were rebuilt and we were able to increase the closed loop bandwidth from 40 to 80 Hz.

ADONIS: daylight imaging through atmospheric turbulence

The AMOS daylight optical near-infrared imaging system, acronym ADONIS, is a sensor system designed for collecting satellite images under daylight conditions and employing speckle post-processing for enhancement of the resulting images. This paper presents our solution (the ADONIS system) to the daylight observation problem by first establishing the issues related to radiometry, daylight detection, and incoherent speckle imaging. System design resolution optimization results are presented. ADONIS imaging results and conclusions based on these results also are presented.

MEMS adaptive optics: field demonstration

We present here results using two novel active optic elements, an electro-static membrane mirror, and a dual frequency nematic liquid crystal. These devices have the advantage of low cost, low power consumption, and compact size. Possible applications of the devices are astronomical adaptive optics, laser beam control, laser cavity mode control, and real time holography. Field experiments were performed on the Air Force Research Laboratory 3.6 meter telescope on Maui, Hawaii.

State of the art in liquid crystal technologies for wavefront compensation: an AFRL perspective

The idea of using liquid crystal as adaptive optics components has been proposed by several authors. In recent years a vigorous research effort has been carried out, and it si still flourishing, in several countries. Mainly the research and experimental work has been concentrated in US, U.K. and Russia. There are several reasons why liquid crystal may represent a valid alternative to the traditional deformable mirror technology that has been used for the past two decades or so. The main attractiveness of LC resides in the cost. Current deformable mirror technology has a range of price going from 2K to 15K per channel. LC technology promises to be at least a couple of orders of magnitude cheaper. Other reasons are connected with reliability, low power consumption and with a huge technological momentum based on a wide variety of industrial applications. IN this paper I present some of the experimental results of a 5 years, on going, research effort at the Air Force Research Lab. Most of the work has been on the development of suitable devices with extremely high optical quality, individually addressable pixels, fast switching time. The bulk of the work has been concentrated in the arena of the untwisted nematic material. However new devices are now under development using dual-frequency nematic material and high tilt angle ferroelectric material.

Multisegment spatial light modulators for the simulation of Kolmogorov turbulence

We discuss the use of Liquid Crystal Phase Modulators (LCPM) as a repeatable disturbance test source for use with adaptive optics systems. LCPMs have the potential to induce controlled, repeatable, dynamic aberrations into optical systems at low cost, low complexity, and high flexibility. Since they are programmable, and can be operated as transmissive elements, they can easily be inserted into the optical path of an adaptive optics system and used to generate a disturbance test source. Laboratory experiments with a Meadowlark LCPM are presented.

Increased bandwidth liquid crystal MEMS adaptive optics system

In previous work we demonstrated a nematic liquid crystal MEMS adaptive optics system for observation of low earth orbit satellites. However the closed loop bandwidth was limited to 40 Hz due to latency in the interface electronics between the control computer and the device driver. This bandwidth is marginal for compensation of atmospheric turbulence effects, where the Greenwood frequency is often in excess of 100 Hz. Recently the interface has been redesigned and as a result we have been able to nearly double the bandwidth. In this paper we describe laboratory experiments with the faster system.