The three-dimensional shapes of microscopic objects are becoming increasingly important for battlespace CBRNE
sensing. Potential applications of microscopic 3D shape observations include characterization of biological weapon
particles and manufacturing of micromechanical components. Aerosol signatures of stand-off lidar systems, using
elastic backscatter or polarization, are dictated by the aerosol particle shapes and sizes that must be well characterized in
the lab. A low-cost, fast instrument for 3D surface shape microscopy will be a valuable point sensor for biological
particle sensing applications. Both the cost and imaging durations of traditional techniques such as confocal
microscopes, atomic force microscopes, and electron scanning microscopes are too high.
We investigated the feasibility of a low-cost, fast interferometric technique for imaging the 3D surface shape of
microscopic objects at frame rates limited only by the camera in the system. The system operates at two laser
wavelengths producing two fringe images collected simultaneously by a digital camera, and a specialized algorithm we
developed reconstructs the surface map of the microscopic object. The current implementation assembled to test the
concept and develop the new 3D reconstruction algorithm has 0.25 micron resolution in the x and y directions, and
about 0.1 micron accuracy in the z direction, as tested on a microscopic glass test object manufactured with etching
techniques. We describe the interferometric instrument, present the reconstruction algorithm, and discuss further
development.
KEYWORDS: Micromirrors, Mirrors, Microelectromechanical systems, Oxides, Digital signal processing, Transmitters, Space operations, CMOS cameras, Charge-coupled devices, Imaging systems
MEMX Corporation in collaboration with Johns Hopkins University Applied Physics Laboratory (JHU/APL) has developed micro-mirror technology applicable to free-space multi-access optical communications terminals. Based on their previously developed micro-electro-mechanical systems (MEMS) optical switches, these new units are being evaluated for applications on spacecraft. These devices must operate within very accurate digitally-controlled pointing and tracking subsystems, which are an essential adjunct to the long-haul optical communication channels that would be operated potentially from geosynchronous earth orbit (GEO) to ground. For such spacecraft applications high-powered laser diodes are likely be the required transmitter. Coupled with their potential operation in a vacuum or at partial atmospheric pressures, MEMS mirror shape stability and fabrication tolerances are of key concern to a system designer. To this end we have measured the performance of preliminary micro-mirror units in terms of angular jitter, focal spot stability, and open and closed-loop response versus laser transmitter power in both ambient air and at low partial pressures. We will describe the fabrication process as well as the experimental test configurations and results in the context of optical beamsteering. We will also discuss the applicability and scalability of this technology to multi-access terminals.
KEYWORDS: Mirrors, Composites, Telescopes, Digital signal processing, Space telescopes, Space operations, Signal processing, Actuators, Control systems, Telescope design
The National Aeronautics and Space Administration (NASA) is planning future deep space missions requiring space-based imaging reconnaissance of planets and recovery of imagery from these missions via optical communications. Both applications have similar requirements that can be met by a common aperture. The Johns Hopkins University Applied Physics Laboratory in collaboration with commercial and academic partners is developing a new approach to deploying and controlling large aperture (meter-class) optical telescopes on spacecraft that can be rapidly launched and deployed. The deployment mechanism uses flexible longeron struts to deploy the secondary. The active control system uses a fiber-coupled laser array near the focal plane that reflects four collimated laser beams off of the periphery of the secondary to four equally-disposed quad cell sensors at the periphery of the primary to correct secondary-to-primary misalignments and enable motion compensation. We describe a compensation technique that uses tip/tilt and piston actuators for quasi-static bias correction and dynamic motion compensation. We also describe preliminary optical tests using a commercial Schmidt-Cassegrain telescope in lieu of an ultra-lightweight composite Cassegrain, which is under development by Composite Mirror Applications, Inc. Finite element and ray trace modeling results for a 40 cm composite telescope design will also be described.
Most spectrometers used for field measurements are based on a diffraction grating to disperse light. Not only are they expensive, but they have some design setbacks as well. A prism does not have the resolving power of a grating, however, enhanced dispersion can be obtained using only a prism and a convex hyperbolic mirror. In this study, we present the design of a prism-based spectrometer, and an analytical model that represents its function. Simulations based on this model and experimental results show that a spectral sampling of 0.1 nm can be accomplished with this design. In addition to dispersion, we address the issue of linearity in the spectral domain and we present simulated and experimental results. Our design should to be cheaper, more lightweight and more light-efficient than typical grating spectrometers.
In the design of optical systems, simple straightforward requirements are often complicated by unusual and unique constraints. In this particular case a design mapping a 20° square field of view onto a CCD sensor is complicated by the requirement that the wide field of view must not vignette through a narrow-diameter, finite-length cylindrical aperture. Furthermore, the design must use off-the-shelf optics available from any major vendor. The imaging system is designed to operate in the near IR. The 20° square field of view must pass through a 20.32mm diameter, 40mm long cylindrical tube without vignetting. This constraint prohibits the use of a simple achromat whose back focal length would place the image within the cylindrical tube. Two design approaches are discussed, a Keplerian telescope with a field lens, and a reverse telephoto system. Matlab programs have been written that evaluate the first-order optical principles to arrive at a design solution space. Representative solutions are then evaluated in Zemax using the built-in lens catalog to select appropriate lenses. The results show the advantages and limitations of each particular design approach.
Analytical results and tradeoffs are reported for an aerosol lidar system that is intended to serve as a referee during testing of standoff bio-aerosol detection systems. The lidar system is still under development by Dugway Proving Grounds -- results from the operational system are not included in this paper. The recommended configuration of the lidar system is to use a 1064 nm lidar in elastic mode to measure the concentration of the aerosol, and a 355 nm excitation to measure the fluorescence of the bio-aerosol. Both of these measurements are important in scoring the performance of the systems that will be tested at DPG. Performance tradeoffs and predictions are presented primarily for the elastic mode lidar. The elastic mode lidar is designed to make measurements out to ranges of approximately 15 km. The UV fluorescence mode of operation is intended to support discrimination of bio-aerosols from non-biological aerosols, and is only required to operate at a range of 1 km. The optical design of the proposed telescope supports dual wavelength operation, allows for effective TV camera imaging for test and alignment support, and tailors the optical overlap function for the UV and near IR lidar to optimize the performance of both subsystems.
Corner detection is an essential feature extraction step in many image understanding applications including aerial image analysis and manufactured part inspection. Available corner detectors require the user to set critical manual thresholds, degrade under significant noise levels, or introduce high computational complexity. We present a nonlinear corner detection algorithm that does not require prior image information or any threshold to be set by the user. It provides 100% correct corner detection and fewer than 1 false positive corner per image when the contrast to noise ratio of the image is 6 or more, under Gaussian white noise.
In this presentation, I present a novel phase-shifting interferometer/ellipsometer. The uniqueness arises from the fact that this study is the consolidation of four distinct ideas drawn from the field of optics and the field of statistics. A conventional four-step phase-shifting interferometer is modified to allow for both TE and TM polarized measurements. Maximum-likelihood estimation theory is then used to extract the three parameters of interest, namely the real and imaginary components of the complex index of refraction and the surface profile. Finally, Cramer-Rao lower bounds serve as a quantitative means of assessing the particular system design at the task of estimating the three parameters in question. I will demonstrate the feasibility of the proposed technique with a Mach-Zehnder prototype, and show how three system parameters, namely the incident amplitude and the relationship between the TE and TM polarized light in terms of amplitude and phase, affect the performance. I also show how quantization of the measured irradiance affects the performance.
A novel interferometer based upon a conventional phase-shifting design is presented. The aim is to introduce the capability of measuring both the surface profile and the complex index of refraction of the test surface. Maximum-likelihood estimation theory and Cramer-Rao lower bounds are introduced and shown to be an effective means of extracting the complex index and surface profile parameters from the measured data and quantitatively assessing the performance. As the design parameters are optimized, the results are shown to improve and approach the theoretical performance limit.
A sensor array for surface curvature and roughness inspection is investigated. The sensor is based on the use of a computer-generated hologram as an analyser of surface curvature and roughness. The sensor can be exploited to estimate the optical surface roughness of irregularly curved surfaces. A contrast parameter to characterize the optical surface roughness is introduced.
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