R. R. Boye

Comparison of subpixel image registration algorithms

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.

Demonstration of polarization-independent resonant subwavelength grating filter arrays

We demonstrate a two-dimensional (2D) polarization-independent resonant subwavelength grating (RSG) in a filter array. RSGs, also called guided mode resonant filters, are traditionally one-dimensional gratings; however, this leads to TE and TM resonances at different wavelengths and with different spectral shape. A 2D grating can remove the polarization dependence at normal incidence, while maintaining the desirable RSG properties of high reflectivity, narrow passband, and low sidebands without ripple. We designed and fabricated 2D gratings with near-identical responses for both polarizations at normal incidence in the telecommunication band. Ninety percent reflectivity is achieved at the resonant wavelengths.

Fabrication and measurement of wideband achromatic waveplates for the mid-infrared region using subwavelength features

Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-µm spectral region. 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. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.

Polarimetric imaging cross talk effects from glue separation between FPA and micropolarizer arrays at the MWIR

We have numerically and experimentally determined the effect of crosstalk from adjacent gold wiregrid micropolarizer pixels in a midwave infrared (MWIR) focal plane array (FPA). Fabrication of a snapshot polarization-imaging device involves gluing a micropolarizer array substrate on top of an FPA. We evaluated several arrays of super-cells of four pixelated polarizers by modeling the near fields behind the devices. Each polarizer in the super-cell is oriented to allow solving three Stokes parameters by themselves or four Stokes parameters in conjunction with a birefringent waveplate. In addition, we fabricated sets of super-cells for determining optimum polarizer-FPA separation. Modeling and empirical data indicate cross talk between the adjacent pixels at several microns after crossing the polarizer plane. Cross talk between adjacent pixels increases uncertainty in the measured polarization states of a scene of interest. Data shows that the extinction ratio will decrease by 17% when moving the FPA from 0.5 &mgr;m to 1.0 &mgr;m away from the polarizer. These changes in extinction ratio are important given that typical glue separation is approximately 10 &mgr;m.

Fabrication and testing of finite aperture polarizers for determination of edge termination effects on polarimetric imaging applications at midwave infrared

Polarimetric imaging applications at the 2-to-5-µm or midwave infrared range use large pixel-count focal plane arrays (FPAs) with small pixel sizes. We report on the design, fabrication, and characterization of micropolarizers for the 2-to-5-µm regime. These micropolarizers will be used in conjunction with a FPA 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, because their aperture dimension varies between three and five waves. We are interested in understanding the effect on the extinction ratio due to variations in the edge terminations of a polarizer with a small aperture. Edge terminations are the spaces between the first and last wire with the perimeter of the aperture of the polarizer. To verify these effects, we fabricated micropolarizers with apertures of 5 to 20 µm and with termination edge spaces of one-quarter and three-quarters of the wiregrids period. The devices measured extinction ratios from 50:1 for the smallest aperture to 200:1 for the largest. Simulations and measurements show that the extinction ratio is larger for the smaller termination edge spacing.

Pixilated wideband achromatic waveplates fabricated for the mid IR using subwavelength features

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.

Edge termination effects on finite aperture polarizers for polarimetric imaging applications at mid wave IR

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.

Active resonant subwavelength grating for scannerless range imaging sensors.

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.

Angular sensitivity of guided mode resonant filters in classical and conical mounts

The angular sensitivity of guided mode resonant filters (GMRF) is well known. While at times useful for angle tuning of the response, this sensitivity can also be a major detriment as angular changes of tenths of a degree can shift the wavelength response in a narrow bandwidth device by an amount greater than the width of the resonance peak. We identify geometries where the resonance is more angularly stable, demonstrating high reflectivity at the design wavelength for several degrees in both azimuth and inclination angular directions with virtually no change in lineshape of the response. The investigation of GMRFs in both classical and conical mounts through simulation using rigorous coupled wave analysis reveals that there are preferred mounts for greater angular tolerance. We simulate a grating at telecom wavelengths using a design that we have previously fabricated. The identical grating placed in different mounts can exhibit angular tolerances that differ by well over an order of magnitude (60x). The most commonly used classical mount has a much more sensitive angular tolerance than does the conical mount. The lineshape of the resonant response shows only negligible changes across the angular band. The angular band for the sample grating is simulated to be several degrees in the conical mount as opposed to a tenth of a degree in the classical mount. We could thus expand the application space for narrow-band GMRFs into areas where angular tolerance cannot be controlled to the degree that we have believed required in the past.

Micro ion frequency standard

We are developing a highly miniaturized trapped ion clock to probe the 12.6 GHz hyperfine transition in the 171Yb+ ion. The clock development is being funded by the Integrated Micro Primary Atomic Clock Technology (IMPACT) program from DARPA where the stated goals are to develop a clock that consumes 50 mW of power, has a size of 5 cm3, and has a long-term frequency stability of 10-14 at one month. One of the significant challenges will be to develop miniature single-frequency lasers at 369 nm and 935 nm and the optical systems to deliver light to the ions and to collect ion fluorescence on a detector.