Image processing offers a potential to simplify an optical system by shifting some of the imaging burden from lenses to the more cost effective electronics. Wavefront coding using a cubic phase plate combined with image processing can extend the system's depth of focus, reducing many of the focus-related aberrations as well as material related chromatic aberrations. However, the optimal design process and physical limitations of wavefront coding systems with respect to first-order optical parameters and noise are not well documented. We examined image quality of simulated and experimental wavefront coded images before and after reconstruction in the presence of noise. Challenges in the implementation of cubic phase in an optical system are discussed. In particular, we found that limitations must be placed on system noise, aperture, field of view and bandwidth to develop a robust wavefront coded system.
We propose a superresolution technique to resolve dense clusters of blinking emitters. The method relies on two basic assumptions: the emitters are statistically independent, and a model of the imaging system is known. We numerically analyze the performance limits of the method as a function of the emitter density and the noise level. Numerical simulations show that five closely packed emitters can be resolved and localized to a precision of 17nm. The experimental resolution of five quantum dots located within a diffraction limited spot confirms the applicability of this approach.
We describe the design of pixelated filter arrays for hyperspectral monitoring of CO2 and H2O absorption in the midwave infrared (centered at 4.25μm and 5.15μm, respectively) using resonant subwavelength gratings (RSGs), also called guided-mode resonant filters (GMRFs). For each gas, a hyperspectral filter array of very narrowband filters is designed that spans the absorption band on a single substrate. A pixelated geometry allows for direct registration of filter pixels to focal plane array (FPA) sensor pixels and for non-scanning data collection. The design process for narrowband, low-sideband reflective and transmissive filters within fabrication limitations will be discussed.
This project investigates the fundamental imaging capability of an optic with a physical thickness substantially less than 1 mm. The analysis assumes that post-processing can overcome certain restrictions such as detector pixel size and image degradation due to aberrations. A first order optical analysis quickly reveals the limitations of even an ideal thin lens to provide sufficient image resolution and provides the justification for pursuing an annular design. Some straightforward examples clearly show the potential of this approach. The tradeoffs associated with annular designs, specifically field of view limitations and reduced mid-level spatial frequencies, are discussed and their impact on the imaging performance evaluated using several imaging examples. Additionally, issues such as detector acceptance angle and the need to balance aberrations with resolution are included in the analysis. With these restrictions, the final results present an excellent approximation of the expected performance of the lens designs presented.
This project demonstrates the feasibility of a novel imager with a thickness measured in microns rather than inches. Traditional imaging systems, i.e. cameras, cannot provide both the necessary resolution and innocuous form factor required in many data acquisition applications. Designing an imaging system with an extremely thin form factor (less than 1 mm) immediately presents several technical challenges. For instance, the thickness of the optical lens must be reduced drastically from currently available lenses. Additionally, the image circle is reduced by a factor equal to the reduction in focal length. This translates to fewer detector pixels across the image. To reduce the optical total track requires the use of specialized micro-optics and the required resolution necessitates the use of a new imaging modality. While a single thin imager will not produce the desired output, several thin imagers can be multiplexed and their low resolution (LR) outputs used together in post-processing to produce a high resolution (HR) image. The utility of an Iterative Back Projection (IBP) algorithm has been successfully demonstrated for performing the required post-processing. Advanced fabrication of a thin lens was also demonstrated and experimental results using this lens as well as commercially available lenses are presented.
Research into the use of multiframe superresolution has led to the development of algorithms for providing images with enhanced resolution using several lower resolution copies. An integral component of these algorithms is the determination of the registration of each of the low resolution images to a reference image. Without this information, no resolution enhancement can be attained. We have endeavored to find a suitable method for registering severely undersampled images by comparing several approaches. To test the algorithms, an ideal image is input to a simulated image formation program, creating several undersampled images with known geometric transformations. The registration algorithms are then applied to the set of low resolution images and the estimated registration parameters compared to the actual values. This investigation is limited to monochromatic images (extension to color images is not difficult) and only considers global geometric transformations. Each registration approach will be reviewed and evaluated with respect to the accuracy of the estimated registration parameters as well as the computational complexity required. In addition, the effects of image content, specifically spatial frequency content, as well as the immunity of the registration algorithms to noise will be discussed.
Conductive polymers have become an extremely useful class of materials for many optical applications. We have developed an electrochemical growth method for depositing highly conductive (~100 S/cm) polypyrrole. Additionally, we have adapted advanced fabrication methods for use with the polypyrrole resulting in gratings with submicron features. This conductive polymer micro-wire grid provides an optical polarizer with unique properties. When the polymer is exposed to ionizing radiation, its conductivity is affected and the polarization properties of the device, specifically the extinction ratio, change in a corresponding manner. This change in polarization properties can be determined by optically interrogating the device, possibly from a remote location. The result is a passive radiation-sensitive sensor with very low optical visibility. The ability to interrogate the device from a safe standoff distance provides a device useful in potentially dangerous environments. Also, the passive nature of the device make it applicable in applications where external power is not available. We will review the polymer deposition, fabrication methods and device design and modeling. The characterization of the polymer's sensitivity to ionizing radiation and optical testing of infrared polarizers before and after irradiation will also be presented. These experimental results will highlight the usefulness of the conductive infrared polarizer to many security and monitoring applications.
We present the design and initial fabrication for a wavelength-agile, high-speed modulator that enables a long-term vision for the THz Scannerless Range Imaging (SRI) sensor. This modulator takes the place of the currently utilized SRI micro-channel plate which is limited to photocathode sensitive wavelengths (primarily in the visible and near-IR regimes). The new component is an active Resonant Subwavelength Grating (RSG). An RSG functions as an extremely narrow wavelength and angular band reflector, or mode selector. Theoretical studies predict that the infinite, laterally-extended RSG can reflect 100% of the resonant light while transmitting the balance of the other wavelengths. Previous experimental realization of these remarkable predictions has been impacted primarily by fabrication challenges. Even so, we have demonstrated large-area (1.0mm) passive RSG reflectivity as high as 100.2%, normalized to deposited gold. In this work, we transform the passive RSG design into an active laser-line modulator.
Conductive polymers have become an extremely useful class of materials for many optical applications. Additionally, advanced fabrication methods have led to the development of metal based micro-wiregrid polarizers utilizing submicron features. Adapting these fabrication approaches for use with polymer materials leads to optical polarizers with unique properties. The patterning of conductive polymers with the small features required for wiregrid polarizers leads to several challenges. First, the deposition of the polymer must provide a layer thick enough to provide a polarizer with a useful extinction ratio that also has high conductivity and environmental stability. Two deposition approaches have been investigated, spin coating and electrochemical growth, and results of this work will be presented. Also, the polymers considered here are not compatible with basic photoresist processes. Various tactics have been examined to overcome this difficulty including the use of hard bakes of the polymer, protective overcoats and patterned growth. The adaptations required for successfully patterning the polymer will be reviewed. Finally, fabricated devices will be shown and their optical characterization presented.
In this late-start LDRD, we will present a design for a wavelength-agile, high-speed modulator that enables a long-term vision for the THz Scannerless Range Imaging (SRI) sensor. It takes the place of the currently-utilized SRI micro-channel plate which is limited to photocathode sensitive wavelengths (primarily in the visible and near-IR regimes). Two of Sandia's successful technologies--subwavelength diffractive optics and THz sources and detectors--are poised to extend the capabilities of the SRI sensor. The goal is to drastically broaden the SRI's sensing waveband--all the way to the THz regime--so the sensor can see through image-obscuring, scattering environments like smoke and dust. Surface properties, such as reflectivity, emissivity, and scattering roughness, vary greatly with the illuminating wavelength. Thus, objects that are difficult to image at the SRI sensor's present near-IR wavelengths may be imaged more easily at the considerably longer THz wavelengths (0.1 to 1mm). The proposed component is an active Resonant Subwavelength Grating (RSG). Sandia invested considerable effort on a passive RSG two years ago, which resulted in a highly-efficient (reflectivity greater than gold), wavelength-specific reflector. For this late-start LDRD proposal, we will transform the passive RSG design into an active laser-line reflector.
The goal of this project is to fabricate a four-state pixelated subwavelength optical device that enables mid-wave infrared (MWIR) or long-wave infrared (LWIR) snapshot polarimetric imaging. The polarization information can help to classify imaged materials and identify objects of interest for numerous remote sensing and military applications. While traditional, sequential polarimetric imaging produces scenes with polarization information through a series of assembled images, snapshot polarimetric imaging collects the spatial distribution of all four Stokes parameters simultaneously. In this way any noise due to scene movement from one frame to the next is eliminated. We fabricated several arrays of subwavelength components for MWIR polarization imaging applications. Each pixel unit of the array consists of four elements. These elements are micropolarizers with three or four different polarizing axis orientations. The fourth element sometimes has a micro birefringent waveplate on the top of one of the micropolarizers. The linear micropolarizers were fabricated by patterning nano-scale metallic grids on a transparent substrate. A large area birefringent waveplate was fabricated by deeply etching a subwavelength structure into a dielectric substrate. The principle of making linear micropolarizers for long wavelengths is based upon strong anisotropic absorption of light in the nano-metallic grid structures. The nano-metallic grid structures are patterned with different orientations; therefore, the micropolarizers have different polarization axes. The birefringent waveplate is a deeply etched dielectric one-dimensional subwavelength grating; therefore two orthogonally polarized waves have different phase delays. Finally, in this project, we investigated the near field and diffractive effects of the subwavelength element apertures upon detection. The fabricated pixelated polarizers had a measured extinction ratios larger than 100:1 for pixel sizes in the order of 15 {micro}m by 15 {micro}m that exceed by 7 times previously reported devices. The fabricated birefringent diffractive waveplates had a total variation of phase delay rms of 9.41 degrees with an average delay of 80.6 degrees across the MWIR spectral region. We found that diffraction effects change the requirement for separation between focal plane arrays (FPA) micropolarizer arrays and birefringent waveplates arrays, originally in the order of hundreds of microns (which are the typical substrate thickness) to a few microns or less. This new requirement leads us to propose new approaches to fabricate these devices.
Polarimetric imaging applications at the 2 to 5 μm or Mid-Wave Infrared (MWIR) range use large pixel-count focal plane arrays (FPA) with small pixel size. This project is centered in designing, fabricating and testing micropolarizers that work in that wavelength regime and intended for that type of FPAs. The micro-polarizers will be used in conjunction with a FPA in snapshot mode and will be in the near field of the imaging device. The pixel pitches for some commercial FPAs are small enough that the finite apertures of the polarizing devices may significantly affect their performance given that their aperture size varies between 3 and 5 waves. We are interested in understanding the effect on extinction ratio due to variations in the edge terminations of a polarizer with a small aperture. Edge terminations are the spaces between the first or last wire with the perimeter of the aperture of the polarizer. While this parameter has negligible effects on a larger polarizer, it will be significant for apertures that are about 3 to 5 waves. We will present data that indicates significant variation in performance due to edge terminations.
Subwavelength diffractive features etched into a substrate lead to form birefringence that can be utilized to produce polarization sensitive elements such as waveplates. Using etched features allows for the development of pixilated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Typically, the main drawback from using diffractive devices is their high sensitivity to wavelength. Taking advantage of the dispersion of the form birefringence, diffractive waveplates with good achromatic characteristics can be designed. We will report on diffractive waveplates designed for minimal phase retardation error across the 2-5 micron spectral regime. The required fabrication processes of the sub-wavelength feature sizes will be discussed as well as the achromatic performance and transmission efficiency of final devices. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. In addition, the effect of the finite size of the apertures of the pixilated devices is of particular interest since they are designed to be used in conjunction with a detector array. The influence of small aperture sizes will also be investigated.
We report here on an effort to design and fabricate a polarization splitter that utilizes form-birefringence to disperse an input beam as a function of polarization content as well as wavelength spectrum. Our approach is unique in the polarization beam splitting geometry and the potential for tailoring the polarized beams' phase fronts to correct aberrations or add focusing power. A first cut design could be realized with a chirped duty cycle grating at a single etch depth. However, this approach presents a considerable fabrication obstacle since etch depths are a strong function of feature size, or grating period. We fabricated a period of 1.0 micron form-birefringent component, with a nominal depth of 1.7 microns, in GaAs using a CAIBE system with a 2-inch ion beam source diameter. The gas flows, ion energy, and sample temperature were all optimized to yield the desired etch profile.
Optoelectronic microsystems are more and more prevalent as researchers seek to increase transmission bandwidths, implement electrical isolation, enhance security, or take advantage of sensitive optical sensing methods. Board level photonic integration techniques continue to improve, but photonic microsystems and fiber interfaces remain problematic, especially upon size reduction. Optical fiber is unmatched as a transmission medium for distances ranging from tens of centimeters to kilometers. The difficulty with using optical fiber is the small size of the core (approximately 9 {micro}m for the core of single mode telecommunications fiber) and the tight requirement on spot size and input numerical aperture (NA). Coupling to devices such as vertical cavity emitting lasers (VCSELs) and photodetectors presents further difficulties since these elements work in a plane orthogonal to the electronics board and typically require additional optics. This leads to the need for a packaging solution that can incorporate dissimilar materials while maintaining the tight alignment tolerances required by the optics. Over the course of this LDRD project, we have examined the capabilities of components such as VCSELs and photodetectors for high-speed operation and investigated the alignment tolerances required by the optical system. A solder reflow process has been developed to help fulfill these packaging requirements and the results of that work are presented here.