KEYWORDS: Scintillation, Signal to noise ratio, Staring arrays, Wavefront sensors, Digital holography, Digital recording, Holograms, Signal detection, Optical engineering, Wave propagation
This paper develops wave-optics simulations which explore the estimation accuracy of digital-holographic detection for wavefront sensing in the presence of distributed-volume or “deep” turbulence and detection noise. Specifically, the analysis models spherical-wave propagation through varying deep-turbulence conditions along a horizontal propagation path and formulates the field-estimated Strehl ratio as a function of the diffraction-limited sampling quotient and signal-to-noise ratio. Such results will allow the reader to assess the number of pixels, pixel field of view, pixel-well depth, and read-noise standard deviation needed from a focal-plane array when using digital-holographic detection in the off-axis image plane recording geometry for deep-turbulence wavefront sensing.
Digital holography in the pupil-plane recording geometry shows promise as a wavefront sensor for use in adaptive-optics systems. Because current wavefront sensors suffer from decreased performance in the presence of turbulence and thermal blooming, there is a need for a more robust wavefront sensor in such distributed-volume atmospheric conditions. Digital holography fulfills this roll by accurately estimating the wrapped phase of the complex optical field after propagation through the atmosphere to the pupil plane of an optical system. This paper examines wave-optics simulations of spherical-wave propagation through both turbulence and thermal blooming; it also quantifies the performance of digital holography as a wavefront sensor by generating field-estimated Strehl ratios as a function of the number of pixels in the detector array, the Rytov number, and the Fried coherence diameter. Altogether the results indicate that digital holography wavefront sensing in the pupil-plane recording geometry is a valid and accurate method for estimating the wrapped phase of the complex optical field in the presence of distributed-volume atmospheric aberrations.
Small MEMS (Micro-Electro-Mechanical Systems) deformable mirror (DM) technology is of great interest to the adaptive optics (AO) community. These new, MEMS-DM's are being considered for many conventional AO applications since they posses some advantages over conventional DM's. The MEMS-DM technology is driven by the expectation of achieving improved performance with lower costs, low electrical power, high number of actuators, high production rates, and large reductions in structural mass and volume. In addition to the imaging community, the directed energy community is also interested in taking advantages of some of the characteristics which MEMS-DM's offer.
The Air Force Research Laboratory has undertaken the challenge of developing and analyzing several continuous face-sheet MEMS-DM's designs. This paper proposes a simple controls loop computer model for a typical continuous reflective face-sheet MEMS-DM. There are a variety of MEMS-DM's which are being developed for the Air Force Research Laboratory. Data from a typical continuous reflective face-sheet MEMS-DM has been acquired and analyzed. From this data a model for a typical MEMS-DM device could be developed and verified. This MEMS-DM's data can be compared to previous design models which were developed in previous articles. In addition a feedback, closed adaptive optics loop can be designed and analyzed. The research goal is to get a realistic idea of the behavior and performance of these new, innovative devices.
An innovative adaptive optic is discussed that provides a range of capabilities unavailable with either existing, or newly reported, research devices. It is believed that this device will be inexpensive and uncomplicated to construct and operate, with a large correction range that should dramatically relax the static and dynamic structural tolerances of a telescope. As the areal density of a telescope primary is reduced, the optimal optical figure and the structural stiffness are inherently compromised and this phenomenon will require a responsive, range-enhanced wavefront corrector. In addition to correcting for the aberrations in such innovative primary mirrors, sufficient throw remains to provide non-mechanical steering to dramatically improve the Field of regard. Time dependent changes such as thermal disturbances can also be accommodated. The proposed adaptive optic will overcome some of the issues facing conventional deformable mirrors, as well as current and proposed MEMS-based deformable mirrors and liquid crystal based adaptive optics. Such a device is scalable to meter diameter apertures, eliminates high actuation voltages with minimal power consumption, provides long throw optical path correction, provides polychromatic dispersion free operation, dramatically reduces the effects of adjacent actuator influence, and provides a nearly 100% useful aperture. This article will reveal top-level details of the proposed construction and include portions of a static, dynamic, and residual aberration analysis. This device will enable certain designs previously conceived by visionaries in the optical community.
Small Micro-Electro-Mechanical Systems (MEMS) deformable mirror (DM) technology is of great interest to the adaptive optics (AO) community. These MEMS-DM's are being considered for many conventional AO applications since they posses some advantages over conventional DM's. The MEMS-DM technology is driven by the expectation of achieving improved performance with lower costs, low electrical power, high number of actuators, high production rates, and large reductions in structural mass and volume. In addition to the imaging community, the directed energy community is also interested in taking advantage of the characteristics which MEMS-DM's offer.
Unlike imaging, the optical fill-factor of a high-energy laser DM, has to be essentially 100 percent! Many modern MEMS-DM designs consist of small, lightweight, segmented mirrors that can be precisely controlled. For high-energy laser applications, the MEMS DM's should have a continuous reflective face-sheet with no gaps. This continuous reflective face-sheet must include high-energy laser coatings, which render the face sheet very stiff. This is a new challenge for MEMS-DM's, which has not previously been addressed. The Air Force Research Laboratory has proposed to meet this challenge with several continuous face-sheet high-energy laser MEMS-DM's designs. This paper will give a generic description of a MEMS-DM computer model. The research goal is to develop a MEMS-DM model for closed loop control of a high-energy laser, MEMS-DM adaptive optics application.
The use of thin-film membranes is of considerable interest for lightweight mirror applications. The low areal density makes them ideal for large aperture imaging applications. One type of setup looked into in the past has been the lenticular design, which consists of a clear canopy attached to a reflective film that uses positive pressure to set the curvature of the mirror. One drawback to this concept has been the fact that too much error was introduced during the pass through the canopy due to material inhomogeneities and poor optical properties. This is no longer an issue thanks to developments over the past several years in the field of optical-quality polymer development. Thin-films (< 24 microns) can now be routinely made with surface roughness, thickness variation, and very good transmission properties well within specification for many visible and IR applications. The next step in this developmental process has been maintaining a prescribed figure in the mirror. This paper summarizes the current efforts in fabricating and testing a 1-meter class lenticular membrane mirror system utilizing active boundary control and stress-coating applications to form a usable aperture for visible imaging applications.
Large aperture optical quality primary mirrors have been developed which are extremely lightweight (areal densities less than 1kg/m2) made from stretched reflective polymer membranes. However, aberrations induced by boundary support errors and pressurization of a flat membrane do not produce a perfect parabolic shape. Modeling studies have shown that active boundary control can be very effective in correcting certain types of figure errors typically seen in membrane mirrors. This paper validates these design studies by applying boundary control on a 0.25-meter pressure augmented membrane mirror (PAMM). The 0.25 meter PAMM was fabricated as a pathfinder for a larger prototype. A combination of displacement actuators and electrostatic force actuators were used to control the shape of the mirror. A varied thickness stress coating prescription was developed by a SRS/AFRL team using nonlinear membrane theory. Based on modeled data, the stress coating should force the membrane into a parabolic shape when pressurized, as opposed to a spherically aberrated shape characteristic of a pressurized flat membrane. Test data from the 0.25-meter PAMM proved that the varied thickness stress coating allows for a better shape than the uniform coating.
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.
Materials and processes have been developed for production of polymer membranes with optical quality surface characteristics. These materials have been successfully used to manufacture large, high quality, ultra lightweight, optical flats for beam splitters, lens covers and other applications. These materials can potentially be used to develop large aperture primary mirrors with areal densities less than 1kg/m2. However, for curved mirrors it is more difficult to establish and maintain desired optical figure from the initial packaged configuration. This paper describes design analysis being performed to support fabrication of a membrane mirror test article. Modeling was performed to evaluate the effectiveness of several different boundary control concepts for correcting different types of figure aberrations. Analyses of different combinations of boundary displacement actuators, electrostatic force actuators, and pressure are presented.
This paper continues the work of a previous paper by investigating the concept of an initially parabolic membrane that is "flattened" due to intrinsic stresses and then re-inflated. The pressure to obtain zero apex displacement is determined, and root mean square (RMS) error is calculated. The RMS error is then minimized by varying the pressure. We also investigate the frequency response of a support ring connected to such a membrane. It is shown that the compressive loads applied by the membrane do not change the natural frequencies of the support ring appreciably.
There is a significant amount of research devoted to developing materials and processes for spaceborne mirrors. Carbon fiber mirrors and advanced ceramic mirrors such as SiC are being developed. These materials provide excellent stiffness to weight ratios and thermal stability. The principal problem with using these lightweight materials for mirrors is the difficulty of polishing the surface to achieve the required optical quality finish. Carbon fiber mirrors also suffer from fiber print through and ceramic mirrors are difficult and costly to polish due to the material hardness and porosity. SRS has been developing processes for depositing a very thin, optical-quality membrane layer of space-qualified polymer onto the surface of a mirror still in a rough-polished state to eliminate the need for expensive and time consuming final surface finishing of lightweight mirrors. By flow casting a polymer onto the surface, remaining peaks and valleys are filled in resulting in an extremely smooth surface. Initial research has shown that the membrane mirror surface can have a significantly better surface finish than the casting substrate, thus eliminating the need for costly final polishing.
Previous research has demonstrated the feasibility of manufacturing polymer membranes with surfaces suitable for use as optical elements on scales up to 1.5 meters. These membranes have optical surface finishes characterized by a roughness of 1.2 nanometers (rms) and mid spatial frequency figure errors (caused by thickness variations) of approximately 350 nanometers-adequate for many optical applications. With optical quality membranes fabrication demonstrated, the next technical challenges that must be met before large-aperture, ultra-light membrane mirrors can be practically achieved are to develop (1) light-weight deployable support structures, (2) the ability to control the global figure of large optical quality membranes, and (3) an improved understanding of the effects of membrane material properties (e.g., material in-homogeneities, coatings, and boundary conditions) on global figure.
The work reported herein further characterizes several key system properties and their effects on optical aberrations. This analysis helps establish technical requirements for membrane optical systems and provides additional insight required to optimize deployable support structures capable of providing passive figure control for membrane optical elements. The results are also used to investigate the need for an electrostatic control system that can actively control the figure of a large membrane mirror.
The Directed Energy Directorate is developing technologies for large space-based optical membrane telescopes. The goal is to develop technologies that will enable 20-meter or greater diameter telescopes, with areal densities of less than 1 kilogram per square meter. The challenge of building these precise structures is reduced by employing a diffractive wavefront controller, which will significantly relax the structural tolerances normally associated with conventional optical systems. A significant portion of the corrector's range and bandwidth can be consumed by structural disturbances. This survey will describe the relationship between the structural dynamics of a highly compliant, 11 inch diameter, planar optical aperture and the diffractive wavefront controller's ability to compensate for the resulting wavefront error. This overview should give the optical physicist and the opto-mechanical engineer a starting point to communicate system design and research needs.
There is a considerable and growing interest in the use of thin membrane mirrors for optical and infrared systems in space. The possibility of very large, monolithic light-collecting apertures with extremely low areal densities is strong motivation for developing the various technologies that might allow the realization of such systems. During the course of working toward space deployable, large mirror systems, we are also aware of possible ground-based applications of membrane optical elements. This paper briefly discusses some of our recent work and progress with membrane mirror elements, and possible future directions.
The Directed Energy Directorate is developing a large space-based optical membrane telescope. The goal is to develop technologies that will enable 20-meter, or greater, diameter telescopes, with areal densities of less than 1 kilogram per square meter. The challenges include the development of a new material process that dramatically improves the optical quality of available films, choosing a process that is conceivably scalable to these larger diameters, and designing new structural concepts to meet surface accuracy requirements and areal density restrictions. A significant part of the realization of these goals relies on the development of a stress-coated net-shape film. A stress-coated net-shape film is a bilaminate system comprised of a pre-shaped polymer substrate coated with a compressive dielectric coating. This article is restricted to a discussion of surface data information on a 40-centimeter diameter, 10 tm thick, uncoated net-shape film. Passively forming these films to a near final shape (i.e. net-shape) will reduce the force, power, and range burden of the actuation system required to acquire and maintain the optical figure. Additionally, passively maintaining the form of these film structures will reduce the stiffness requirements of the supporting structure. The union of the polymer substrate and dielectric coating is still under development and will be reported on at a later date.
The Air Force Research Laboratory (AFRL) is exploring the feasibility of large-aperture, deployable, space-based membrane telescopes operating in the visible and/or near- infrared spectral regions. One of the near-term goals of this work is to develop an understanding of available and achievable membrane materials, specifically concentrating on practical techniques to form large aperture membranes with the necessary surface quality and economy. 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. This totally inflatable lenticular design is simple and relatively easy to demonstrate, but maintaining inflation during an extended lifetime in near-earth orbit may not be feasible. Recently, a new concept for a membrane telescope has emerged which does not depend on sustained inflation during operation. Thin membranes on the order of 10 to 100 micrometer thick will be packaged and deployed, maintaining their surface figure by means other than inflation. Given the fact that the sub- wavelength level surface tolerances required of imaging telescopes will probably not be practical with a membrane- based telescope, such systems will probably rely on real-time holography or some other wavefront correction or compensation technique. We will discuss the primary experimental work ongoing in the AFRL Membrane Mirror Laboratory, and in doing so, some of the issues relevant to demonstrating a practical, large-aperture membrane mirror system.
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.
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.
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.
Thin flexible membranes with curvature are gaining favor as lightweight optical components. The Surveillance Technologies Branch of the Air Force Research Laboratory has demonstrated that membranes can yield a near diffraction limited image when combined with real-time holography as the wavefront correction method. Researchers at the University of Strathclyde in Glasgow, Scotland are using large membrane mirrors in a volumetric imaging project, while others at the Vavilov Laser Physics Institute in Saint Petersburg, Russia are investigating the use of real-time holography to correct membrane mirror aberrations. Existing membranes, however, have not been designed with optical imaging as the intended application. Thus, there is a need to design and construct optical quality 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.
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.
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.
The Multipurpose Multiple Telescope Testbed is described, and initial tests are discussed. After the optical quality of individual telescopes was established with interferometric tests, the cophasing and image superpositioning accuracy of the array were measured using star tests. Point spread functions were calculated with a physical optics code. Preliminary star tests using two of the four telescopes are presented and compared with the predicted pattern. It is concluded that the preliminary results obtained lend credence to the given method of controlling lateral pupil geometry and to previous calculations of optical aberration and optical alignment tolerances.
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