James Michael Wilkes

Holographic compensation of severe dynamic aberrations in membrane-mirror-based telescope systems

Real-time holography compensates for severe aberrations in membrane-mirror based telescope systems. Laboratory demonstrations in both imaging and beam projection have been conducted. Prototype optically addressed liquid-crystal spatial light modulator devices, developed and adapted for this application, are demonstrated with significantly improved diffraction efficiencies.

Optical evaluation of membrane mirrors with curvature

Thin membranes with curvature are investigated as mirror substrates for use in large optical telescopes. These films are mounted on an optically flat circular ring and stretched over a smaller optically flat circular ring where pressure or vacuum is applied to create the doubly curved surface as shown in figure 1. The films may vary in thickness from 20 to 200 microns. This particular experiment examines an aluminum coated 125 micron thick homogeneous, planar, isotropic membrane with a clear aperture of 28 centimeters. The nature of a flexible membrane implies that the surface curvature will result in an assorted array of gross surface figure issues associated with deterministic shape limits, probabilistic imperfections, nonlinear constitutive effects, and long-time- dependent effects. This report will focus on the empirical deterministic shape limits of a doubly curved membrane. Theoretical work on thin films inflated or evacuated into a doubly curved surface has a long history, and remains an active area of research. A number of articles [1,2,3,4,7] include summaries of this history, and offer insight on the deterministic membrane shapes.

Minimum strain requirements for optical membranes

Thin membrane inherently require a certain minimum amount of strain to adequately perform as optical elements. This minimum strain can be established by simultaneously considering the effects of strain on the reflective surface, film thickness variations, and the corrective range of the adaptive optics (AO) scheme. To show how strain and the optimal optical surface are related, 75 and 125-micron thick polyimide films were examined under various strain conditions. Thickness variations were also mapped and correlated. The limits of the AO correction scheme set the films surface topography requirement. Our results will help to partially define an optical quality membrane, which is an important initial step toward the manufacturing of such a film.

Concave membrane mirrors from aspheric to near-parabolic

The surface of an initially planar membrane, which is subsequently subjected to a pressure difference, can be manipulated into a variety of shapes. This report discusses two methods by which optically desirable deterministic shapes might be achieved. The first involves pre-straining of the membrane, a technique which has already been demonstrated to reduce the spherical aberration in such a mirror. However, near-parabolic shapes at low f-numbers appear not to be achievable with this method, i.e., using pressure differences and pre-strain alone. The second technique is a somewhat novel one involving the use of a plunger to translate the central region of the membrane along the optical axis. Preliminary results suggest that attainment of a near-parabolic shape over a substantial area of the membrane may indeed be possible with this method. The experiments described here use an aluminum coated 125 micron thick polyimide membrane with a clear aperture of 11 inches.

Large optically flat membrane mirrors

The feasibility of forming very thin (approximately 100 um), flexible membranes into low-cost, low-mass, large diameter optical elements is being explored. While spherical or parabolic shapes are the ultimate goal for imaging and other light-gathering applications, there are potential applications for large, planar surfaces. Also, knowledge gained while working with planar membranes is being applied to concave structures. Recent efforts have concentrated on measuring and understanding the behavior of currently available materials. This paper discusses experimental results, and describes measurement techniques and membrane materials used. Highlighted are our most recent results on a 11-inch diameter membrane mirror which we measured to be flat to approximately 0.1 um rms.

Tunable membrane mirrors used with real time holography

Space-based inflatable technology is of current interest to NASA, DOD, and in particular to the Air Force Research Laboratory. Potentially large gains in lowering launch costs, through reductions in structural mass and volume are driving this activity. Diverse groups are researching and developing this technology for radio and radar antennae, optical telescopes, and solar power and propulsion applications. Regardless of the use, one common requirement for successful application is the accuracy of the inflated surface figure. This paper gives a very cursory description of the research being performed at the Air Force Research Laboratory in the fields of membrane mirrors and real time holography. First, the article will show a shape modification method for the membrane mirror, achieved through enforced boundary displacements. The membrane mirror shape modification, resulted in moving the inflated membrane shape towards a desired optical profile. Minimization of the optical figure error is further discussed. Next, the optical requirements levied from the membrane mirror to an optically addressed spatial light modulator performing real time holographic correction are discussed. A proposed optical configuration in which a real time holographic optical element could be combined with the membrane mirror to achieve near diffraction limited optical performance is discussed.

Large lightweight optical quality windows and filters

The Air Force Research Laboratory, Directed Energy Directorate, together with SRS Technologies Inc., Huntsville, AL, and Surface Optics Corporation, San Diego, CA, have developed meter-class optical quality membranes with dielectric coatings and custom spectral filtering. The windows range in thickness from 5 to 20 µm and can operate in the visible and the near-infrared. To date the largest membrane manufactured is slightly less than one meter in diameter and its optical thickness variation is on the order of 35 nanometers rms. Surface roughness, optical density, and other optical data will be presented. The intent of this article is to expose this technology to optical designers with the expectation that significant design opportunities for observatories, telescopes, and experiments will result.

Correction of large dynamic aberrations by real-time holography using electro-optical devices and nonlinear optical media

Real-time holography compensates for severely aberrated primary mirrors in large aperture telescope systems. A prototype optically addressed liquid-crystal spatial light modulator device developed for this application is demonstrated with short response times, approximately 600 microsecond(s), and high diffraction efficiencies approaching 40%.

Deployable near-net-shaped membrane optics

The Air Force Research Laboratory is developing a large space-based optical membrane telescope. When this research began a little more than three years ago, the conceptual design was based upon a totally inflatable structure. An inflatable structure has been used for space solar power collection and radio-frequency antennas. To place the development of the membrane optical telescope in perspective, a short history of past inflatables will be presented. The totally inflatable lenticular design used in a variety of space-based applications in radio and radar antennae, solar power for propulsion applications and solar shields is of particular interest. Recently, a new version of a membrane telescope has emerged. Thin membranes on the order of 10 to 100 micrometers thick will be packaged and deployed without using inflation to maintain the surface figure. The move away from a pure inflatable is driven by several factors, including wavelength-level tolerances required of optical telescopes, even when real-time holography is invoked as the adaptive optics correction technique. Issues that led us to de-emphasize an inflatable, lenticular design and concentrate on a near-net shape film using stress coatings and dual boundary edge control are discussed.

Development of a novel liquid crystal spatial light modulator for real-time holography

Deformed-helix ferroelectric liquid-crystal media with high molecular tilt angles have been used in spatial light modulators to make them faster and more efficient. A prototype device developed for real-time holography applications has generated diffraction efficiencies as high as 38 percent and response times as short as 600 microsecond(s) ec.