Deformable mirrors using polyvinylidene fluoride (PVDF) membranes in a bimorph configuration have been previously
investigated. Kratos Defense and Security Solutions, in partnership with Advanced Optical Systems, Inc. and NeXolve,
Inc., have been evaluating the utility of unimorph PVDF films for fabrication of deformable mirrors. Actuation using a
unimorph film is achieved by creating a gradient in the piezoelectric response of the film through a proprietary process.
This eliminates the requirement to bond multiple films and improves the optical quality of the films. To assist in the
development and design of the films, a multiphysics design tool has been developed by tightly integrating several
commercial software packages. This tool has then been used to model the performance of the films and extract
significant material parameters. This paper reports on the initial modeling results and characterization of this novel
Polyimides are attractive mirror candidate materials due to their low mass, solar radiation resistance, and cryogenic
flexibility. However, polyimides exhibit high coefficients of thermal expansion (CTE) values (40-70 ppm/K), inducing
image distortion from CTE mismatch. Additionally, the temperature of large aperture (10 m) membranes is not
uniformly controlled in space, further increasing image distortion from anisotropic deformations. The CTE of the MSRS
Novastrat polyimide line was adjusted to exhibit CTEs between -16 ppm/K and 53 ppm/K, including 0 ppm/K, 10
ppm/K, 17 ppm/K, and 25 ppm/K corresponding with CTE matches of graphite/epoxy, carbon steel, copper, and
aluminum (respectively). The development of these CTE-matched membranes is presented, as well as the effect of the
CTE adjustment on the mechanical properties.
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.
TRS is developing new actuators based on single crystal piezoelectric materials such as Pb(Zn<sub>1/3</sub>Nb<sub>2/3</sub>)<sub>1-x</sub>Ti<sub>x</sub>O<sub>3</sub> (PZN-PT) and Pb(Mg<sub>1/3</sub>Nb<sub>2/3</sub>)<sub>x-1</sub>Ti<sub>x</sub>O<sub>3</sub> (PMN-PT) which exhibit very high piezoelectric coefficients (d<sub>33</sub> = 1800-2200 pC/N) and electromechanical coupling factors (k<sub>33</sub> > 0.9), respectively, for a variety of applications, including active vibration damping, active flow control, high precision positioning, ultrasonic motors, deformable mirrors, and adaptive optics. The d<sub>32</sub> cut crystal plate actuators showed d<sub>32</sub> ~ -1600 pC/N, inter-digital electroded (IDE) plate actuators showed effective d<sub>33</sub> ~ 1100 pC/N. Single crystal stack actuators with stroke of 10 μm-100 μm were developed and tested at both room temperature and cryogenic temperatures. Flextensional single crystal piezoelectric actuators with either stack driver or plate driver were developed with stroke 70 μm - > 250 μm. For large stroke cryogenic actuation (> 1mm), a single crystal piezomotor was developed and tested at temperature of 77 K-300K and stroke of > 10mm and step resolution of 20 nm were achieved. In order to demonstrate the significance of developed single crystal actuators, modeling on single crystal piezoelectric deformable mirrors and helicopter flap control using single crystal actuators were conducted and the modeling results show that more than 20 wavelength wavefront error could be corrected by using the single crystal deformable mirrors and +/- 5.8 ° flap deflection will be obtained for a 36" flap using single crystal stack actuators.
To maximize the cost-effectiveness of the Mars Laser Communication Demonstration (MLCD), the project is pursuing the use of ground-based astronomical telescopes as large-aperture optical receiving antennae. To facilitate communication as the spacecraft approaches solar conjunction, a large membrane filter is being considered to reject approximately 95% of the sun’s power, while efficiently admitting light at the 1060 nm signal wavelength. Through the use of this filter and some additional facility modifications, the problems of thermally-induced telescope aberrations and dangerous focusing of solar power can effectively be mitigated. The use of a membrane filter is expected to be cost competitive, introduce less scattered light, and provide more flexibility in placement and operations than alternative approaches. This paper addresses the initial design of the filter and preparation of test samples to evaluate candidate materials.
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.
Nickel Titanium (NiTi) film shape memory alloy (SMA) is integrated with space-qualified polymer and mesh materials for potential use as deployment mechanisms and actuation of flexible space apertures. SMA thin film is successfully applied to Astromesh metal mesh, Kapton, Upilex, and CP-1 polymer films. Sputter deposition of NiTi onto the substrate is used to validate the material system process and demonstrate the NiTi deployment capability. Although successful, the relatively high processing temperatures required to crystallize NiTi onto the substrates requires care. A second approach is demonstrated that deposits NiTi onto a silicon substrate, followed by coating the NiTi with the desired polymer, e.g. CP-1. Micro-electro-mechanical (MEMS) processing steps are then used to remove the silicon substrate beneath the NiTi, thus freeing up the composite membrane (i.e. NiTi + CP-1). Using MEMS fabrication techniques, a hot-shaped small dome shape structure is shaped into the NiTi before deposition of the CP-1 polymer. Activation of the integrated SMA/CP-1 produces deformation of this composite structure without damage. The test articles demonstrate the feasibility to both grossly deploy and locally actuate space-qualified polymer materials.
Significant advances have been achieved in manufacturing optical quality membrane materials with surface quality suitable for use as first surface mirrors. These materials have been used to fabricate test articles demonstrating diffraction limited performance in the laboratory environment. These mirrors are supported using heavy rigid fixtures and pressure forces to tension the membrane. A lighter weight system is required to transition the membrane mirror technology to space hardware applications. Using electrostatic forces to tension and figure the membrane is one promising approach to developing a flight weight membrane mirror system. This paper discusses the design and testing of an experimental membrane mirror system that was developed to evaluate the potential areal density, figure accuracy and stability of a lightweight electrostatically figured mirror manufactured from precision cast optical quality membrane material.
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 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.
SRS Technologies has made significant strides in the research and development of ultra-lightweight membrane optics for future imaging applications while conducting work with NASA Marshall Space Flight Center and the Air Force Research Lab. Thin film mirrors have been manufactured using surface replication casting of CP1, a polyimide material developed specifically for space applications. In the course of such efforts processing and manufacturing techniques have been developed to produce polyimide membranes with surface roughness below 1.5 nanometers rms and sub-wavelength thickness variation for both curved and flat membranes. This has led to the production of membranes optically flat to (lambda) /13 ((lambda) equals 633 nm) and curved membranes with figure error on the order of microns over half-meter diameters. This accuracy places such membranes within the demonstrated correctable range of several advanced wavefront correction technologies.
The Integrated Optical Design Analysis (IODA) program is a software tool being developed to support concurrent engineering design of complex optical systems. IODA provides seamless data fusion between thermal, structural, and optical models used to design the system. The software architecture was developed by reviewing current design processes and developing software to automate the existing procedures. IODA significantly reduces the design iteration cycle time and eliminates many potential sources of error.