Fourier Spectrum Techniques For Characterization Of Spatial Noise In Imaging Arrays

Noise performance in imaging arrays is often specified simply by the variance of the pixel levels. In this paper a more complete characterization technique is presented, based on the spatial-frequency power spectrum of the noise data on the detectors. This is seen to provide additional information for cases in which the noise spectrum is nonwhite. Experimental data demonstrate the nonwhite nature of the spectrum under certain conditions, especially resulting from spatial correlation of detector nonuniformity and from sampling-lattice-related artifacts in the data

Classification of imaging spectrometers for remote sensing applications

The continuing development of new and fundamentally different classes of imaging spectrometers has increased the complexity of the field of imaging spectrometry. The rapid pace at which new terminology is introduced to describe the new types of imaging spectrometers sometimes leads to confusion, particularly in discussions of the relative merits of the different types. In some cases, multiple different terms are commonly used to describe the same fundamental approach, and it is not always clear when these terms are synonymous. Other terminology in common use is overly broad. When a single term may encompass instruments that operate in fundamentally different ways, important distinctions may be obscured. In the interest of clarifying the terminology used in imaging spectrometry, we present a comprehensive system for classification of imaging spectrometers based on two fundamental properties: the method by which they scan the object spatially, and the method by which they obtain spectral information

Limiting aspect ratios of Sagnac interferometers

Any two-beam interferometer may be employed as a Fourier transform spectrometer. The two most commonly used for Fourier transform spectrometry are the Michelson interferometer and the Sagnac interferometer, the relative merits of which have been discussed in the literature. Typically, it is the interferometer that limits the acceptable range of angles for the input beam, and this maximum acceptance angle in turn limits the etendue, and hence limits the responsivity of the instrument when viewing an extended source. In designs where the interferometer is in a diverging or converging beam, the allowable range of input angles limits the focal ratio of the instrument, while in designs where the beam is collimated through the interferometer, this effect limits the field-of-view of the instrument. In a Michelson, it is a loss of fringe contrast that limits the range of acceptance angles; a limitation that is discussed in many general texts on optics. A Sagnac, however, suffers no such loss of contrast as the range of acceptance angles is increased. The maximum acceptance angle for a Sagnac is instead limited by vignetting, caused by the geometry of the interferometer. The limitation for a Sagnac has an origin and behavior entirely different from that found for a Michelson, and has not been previously discussed in the literature. It is therefore important to understand this limitation when designing a Sagnac interferometer for Fourier transform spectrometry. This vignetting limitation may be quantified by an aspect ratio, which we define as the ratio of the separation of the entrance and exit apertures to the width of these apertures in the plane of the interferometer. To facilitate the design of Sagnac interferometers for Fourier transform spectrometry, we discuss the limitations on the aspect ratio and derive equations for the limiting aspect ratios for nine variations of the Sagnac interferometer

Broadband infrared meanderline reflective quarter-wave plate

We present a novel reflective quarter-wave plate comprised of subwavelength meanderline elements. The device is operational over the long-wave infrared (LWIR) spectrum, with significant spectral and angular bandwidths. Power reflection is approximately 70% over the majority of the LWIR. Efficient conversion from a 45 linear polarization state into circular polarization is demonstrated from finite-element electromagnetic simulations and from broadband polarimetric measurements

Birefringence and transmission of an antireflection-coated sulfur-free cadmium selenide Wollaston prism at 30 K

We present a determination of the change with temperature and wavelength of the degree of birefringence of a cold (similar to 30K) Wollaston prism constructed from antireflection (AR)-coated sulfur-free cadmium selenide (CdSe). We compare the normalized birefringence for the material to that estimated by the Sellmeier-4 formula and to previously published measurements of a warm sample of sulfur-free CdSe. Finally, we measure the transmission as a function of wavelength

Spectral modification of array truncation effects in infrared frequency selective surfaces

AbstractInfrared frequency selective surfaces (FSS) are widely used in quasi-infinite, planar configurations; however, applications for finite arrays exist as well. Here, a square loop infrared FSS was designed with an infinite array resonance near 10μm when illuminated at 60° off-normal. Along with the quasi-infinite array, a patterned area containing finite arrays of 7×7 square loops of this design was fabricated and characterized to have a resonance which was blue-shifted due to the effects of truncation. To counteract the effects of truncation, two geometrically modified arrays of 7×7 square-loop elements were designed and fabricated to shift the resonant wavelength approximately back to that of the infinite array

Near- and far-field measurements of phase-ramped frequency selective surfaces at infrared wavelengths

Near- and far-field measurements of phase-ramped loop and patch structures are presented and compared to simulations. The far-field deflection measurements show that the phase-ramped structures can deflect a beam away from specular reflection, consistent with simulations. Scattering scanning near-field optical microscopy of the elements comprising the phase ramped structures reveals part of the underlying near-field phase contribution that dictates the far-field deflection, which correlates with the far-field phase behavior that was expected. These measurements provide insight into the resonances, coupling, and spatial phase variation among phase-ramped frequency selective surface (FSS) elements, which are important for the performance of FSS reflectarrays

Broadband circularly-polarized infrared emission from multilayer metamaterials

Development of a 2D metamaterial that preferentially emits broadband circularly-polarized (CP) infrared radiation is hindered by the fact that orthogonal electric-field components are uncorrelated at the surface of the thermal emitter, a consequence of the fluctuation-dissipation theorem. We achieve broadband CP thermal emission by fabricating a meanderline quarter-wave retarder on a transparent thermal-isolation layer. Behind this isolation layer, in thermal contact with the emitter, is a wire-grid polarizer. Along with an unavoidable linear polarized radiation characteristic from the meanderline, we measured a degree of circular polarization (DOCP) of 28%, averaged over the 8- to 12 mu m band