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NASA NTRS1993

Smart focal-plane technology for micro-instruments and micro-rovers

It is inevitable that micro-instruments and micro-rovers for space exploration will contain one or more focal-plane arrays for imaging, spectroscopy, or navigation. In this paper, we explore the state-of-the-art in focal-plane technology for visible sensors. Also discussed is present research activity in advanced focal-plane technology with particular emphasis on the development of smart sensors. The paper concludes with a discussion of possible future directions for the advancement of the technology.

Fossum, Eric R.↗

NASA NTRS1992

Smart Focal-Plane Technology for Micro-Instruments and Micro-Rovers

It is inevitable that micro-instruments and micro-rovers for space exploration will contain one or more focal-plane arrays for imaging spectroscopy or navigation. In this paper, we explore the state-if-the-art in focal-plane technology for visible sensors. Also discussed is present research activity in advanced focal-plane technology with particular emphasis on the development of smart sensors. The paper concludes with a discussion of possible future directions for the advancement of the technology.

Fossum, Eric R.↗

NASA NTRS2004

Variable Shadow Screens for Imaging Optical Devices

Variable shadow screens have been proposed for reducing the apparent brightnesses of very bright light sources relative to other sources within the fields of view of diverse imaging optical devices, including video and film cameras and optical devices for imaging directly into the human eye. In other words, variable shadow screens would increase the effective dynamic ranges of such devices. Traditionally, imaging sensors are protected against excessive brightness by use of dark filters and/or reduction of iris diameters. These traditional means do not increase dynamic range; they reduce the ability to view or image dimmer features of an image because they reduce the brightness of all parts of an image by the same factor. On the other hand, a variable shadow screen would darken only the excessively bright parts of an image. For example, dim objects in a field of view that included the setting Sun or bright headlights could be seen more readily in a picture taken through a variable shadow screen than in a picture of the same scene taken through a dark filter or a narrowed iris. The figure depicts one of many potential variations of the basic concept of the variable shadow screen. The shadow screen would be a normally transparent liquid-crystal matrix placed in front of a focal-plane array of photodetectors in a charge-coupled-device video camera. The shadow screen would be placed far enough from the focal plane so as not to disrupt the focal-plane image to an unacceptable degree, yet close enough so that the out-of-focus shadows cast by the screen would still be effective in darkening the brightest parts of the image. The image detected by the photodetector array itself would be used as feedback to drive the variable shadow screen: The video output of the camera would be processed by suitable analog and/or digital electronic circuitry to generate a negative partial version of the image to be impressed on the shadow screen. The parts of the shadow screen in front of those parts of the image with brightness below a specified threshold would be left transparent; the parts of the shadow screen in front of those parts of the image where the brightness exceeded the threshold would be darkened by an amount that would increase with the excess above the threshold.

Lu, Ed↗

DOE OSTI2023

Snapshot multispectral imaging using a diffractive optical network

Abstract Multispectral imaging has been used for numerous applications in e.g., environmental monitoring, aerospace, defense, and biomedicine. Here, we present a diffractive optical network-based multispectral imaging system trained using deep learning to create a virtual spectral filter array at the output image field-of-view. This diffractive multispectral imager performs spatially-coherent imaging over a large spectrum, and at the same time, routes a pre-determined set of spectral channels onto an array of pixels at the output plane, converting a monochrome focal-plane array or image sensor into a multispectral imaging device without any spectral filters or image recovery algorithms. Furthermore, the spectral responsivity of this diffractive multispectral imager is not sensitive to input polarization states. Through numerical simulations, we present different diffractive network designs that achieve snapshot multispectral imaging with 4, 9 and 16 unique spectral bands within the visible spectrum, based on passive spatially-structured diffractive surfaces, with a compact design that axially spans ~72 λ m , where λ m is the mean wavelength of the spectral band of interest. Moreover, we experimentally demonstrate a diffractive multispectral imager based on a 3D-printed diffractive network that creates at its output image plane a spatially repeating virtual spectral filter array with 2 × 2 = 4 unique bands at terahertz spectrum. Due to their compact form factor and computation-free, power-efficient and polarization-insensitive forward operation, diffractive multispectral imagers can be transformative for various imaging and sensing applications and be used at different parts of the electromagnetic spectrum where high-density and wide-area multispectral pixel arrays are not widely available.

36 MATERIALS SCIENCE↗

NASA NTRS1981

1.0 to 2.5 micrometer short wavelength infrared /SWIR/ linear array technology for low background applications

A design concept is presented for a hybrid linear focal plane array operating in the 1.0 to 2.5 micron band under low background conditions such as encountered in spaceborne missions. The concept is based on direct gate coupling of the input from a photovoltaic mercury-cadmium telluride detector into a CCD multiplexer. Theoretical analysis indicates a specific detectivity at peak wavelength on the order of 10 to the 12th cm sq rt Hz/W when the system is operating at 200 K. Values of the voltage responsivity, specific detectivity and the relative voltage noise levels of the coupled system obtained in experimental studies with direct-gate-coupled 4.0-micron detectors support the theoretical analysis performed for the case of system detectivity limited by the CCD 1/f noise.

Stobie, J.↗

NASA NTRS1992

Implementation of a noise reduction circuit for spaceflight IR spectrometers

The paper discusses the implementation and analysis of a correlated triple sampling circuit using analog subtractor/integrators. The software and test setup for noise measurements are also described. The correlation circuitry is part of the signal chain for a 256-element InSb line array used in the Visible and Infrared Mapping Spectrometer. Using a focal-plane array (FPA) simulator, system noise measurements of 0.7 DN are obtained. A test setup for FPA/SPE (signal processing electronics) characterization along with noise measurements is demonstrated.

Ramirez, L.↗

NASA NTRS2007

Measurement and Simulation of the Variation in Proton-Induced Energy Deposition in Large Silicon Diode Arrays

The proton induced charge deposition in a well characterized silicon P-i-N focal plane array is analyzed with Monte Carlo based simulations. These simulations include all physical processes, together with pile up, to accurately describe the experimental data. Simulation results reveal important high energy events not easily detected through experiment due to low statistics. The effects of each physical mechanism on the device response is shown for a single proton energy as well as a full proton space flux.

Howe, Christina L.↗

NASA NTRS2003

Metal Side Reflectors for Trapping Light in QWIPs

Focal-plane arrays of quantum-well infrared photodetectors (QWIPs) equipped with both light-coupling diffraction gratings and metal side reflectors have been proposed, and prototypes are expected to be fabricated soon. The purpose served by the metal side reflectors is to increase quantum efficiency by helping to trap light in the photosensitive material of each pixel. The reasons for using diffraction gratings were discussed in several prior NASA Tech Briefs articles. To recapitulate: In an array of QWIPs, the quantum-well layers are typically oriented parallel to the focal plane and therefore perpendicular or nearly perpendicular to the direction of incidence of infrared light. By virtue of the applicable quantum selection rules, light polarized parallel to the focal plane (as normally incident light is) cannot excite charge carriers and, hence, cannot be detected. Diffraction gratings scatter normally or nearly normally incident light into directions more nearly parallel to the focal plane, so that a significant portion of the light attains a component of polarization normal to the focal plane and, hence, can excite charge carriers. Unfortunately, light scattered in directions parallel or nearly parallel to the focal plane can escape sideways from the QWIP of a given pixel, as illustrated in Figure 1. The escaped light has made only a single pass through the interior photosensitive volume of the QWIP. The quantum efficiency of the QWIP would be increased by trapping light so that it makes multiple passes through the photosensitive volume. As shown in Figure 2, the sides of the QWIP of each pixel would be coated with gold to reflect escaping light back into the interior.

Gunapala, Sarath↗

NASA NTRS1982

Imaging spectrometer - An advanced multispectral imaging concept

The concept of an imaging spectrometer, which is being studied as a potential Space Shuttle experiment, is evaluated as a 'push-broom' imager that includes a spectrometer to disperse each line of imaging information into its spectral components. Using this instrument, the dispersed energy falls upon a two-dimensional focal plane array that detects both spatial and spectral information. As the line field of view is advanced over the earth by the motion of the spacecraft, the focal plane is read out constantly, which produces 'push-broom' images at multiple wavelengths. Ground instantaneous fields of view of 10 m in the visual and 20 m in the infrared are provided by the system, at a spectral resolution of 20 nm over the range from 0.4-2.5 microns. The system utilizes a triple-pass Schmidt optical system with a mosaic focal plane. A subset of the data stream is selected and encoded for transmission by the use of onboard processing.

Wellman, J. B.↗

NASA NTRS2006

Initial On-Orbit Spatial Resolution Characterization of OrbView-3 Panchromatic Images

Characterization was conducted under the Memorandum of Understanding among Orbital Sciences Corp., ORBIMAGE, Inc., and NASA Applied Sciences Directorate. Acquired five OrbView-3 panchromatic images of the permanent Stennis Space Center edge targets painted on a concrete surface. Each image is available at two processing levels: Georaw and Basic. Georaw is an intermediate image in which individual pixels are aligned by a nominal shift in the along-scan direction to adjust for the staggered layout of the panchromatic detectors along the focal plane array. Georaw images are engineering data and are not delivered to customers. The Basic product includes a cubic interpolation to align the pixels better along the focal plane and to correct for sensor artifacts, such as smile and attitude smoothing. This product retains satellite geometry - no rectification is performed. Processing of the characterized images did not include image sharpening, which is applied by default to OrbView-3 image products delivered by ORBIMAGE to customers. Edge responses were extracted from images of tilted edges in two directions: along-scan and cross-scan. Each edge response was approximated with a superposition of three sigmoidal functions through a nonlinear least-squares curve-fitting. Line Spread Functions (LSF) were derived by differentiation of the analytical approximation. Modulation Transfer Functions (MTF) were obtained after applying the discrete Fourier transform to the LSF.

Blonski, Slawomir↗

NASA NTRS2013

PHyTIR - A Prototype Thermal Infrared Radiometer

This paper describes the PHyTIR (Prototype HyspIRI Thermal Infrared Radiometer) instrument, which is the engineering model for the proposed HyspIRI (Hyperspectral Infrared Imager) earth observing instrument. The HyspIRI mission would be comprised of the HyspIRI TIR (Thermal Infrared Imager), and a VSWIR (Visible Short-Wave Infra-Red Imaging Spectrometer). Both instruments would be used to address key science questions related to the earth's carbon cycle, ecosystems, climate, and solid earth properties. Data gathering of volcanic activities, earthquakes, wildfires, water use and availability, urbanization, and land surface compositions and changes, would aid the predictions and evaluations of such events and the impact they create. Even though the proposed technology for the HyspIRI imager is mature, the PHyTIR prototype is needed to advance the technology levels for several of the instrument's key components, and to reduce risks, in particular to validate 1) the higher sensitivity, spatial resolution, and higher throughput required for this focal plane array, 2) the pointing accuracy, 2) the characteristics of several spectral channels, and 4) the use of ambient temperature optics. The PHyTIR telescope consists of the focal plane assembly that is housed within a cold housing located inside a vacuum enclosure; all mounted to a bulkhead, and an optical train that consists of 3 powered mirrors; extending to both sides of the bulkhead. A yoke connects the telescope to a scan mirror. The rotating mirror enables to scan- a large track on the ground. This structure is supported by kinematic mounts, linking the telescope assembly to a base plate that would also become the spacecraft interface for HyspIRI. The focal plane's cooling units are also mounted to the base plate, as is an overall enclosure that has two viewing ports with large exterior baffles, shielding the focal plane from incoming stray light. PHyTIR's electronics is distributed inside and near the vacuum enclosure, and in a nearby rack. The data acquisition technique would be to take measurements over a 51deg wide swath in the cross spacecraft velocity direction, which is brought into view through the rotating scan mirror. A landscape mosaic thus can be assembled by overlaying rows of measurements. The paper briefly outlines the proposed HyspIRI mission and its data acquisition technique; it then describes the prototype PHyTIR instrument.

Space Instrument↗

NASA NTRS1989

Future directions for NICMOS arrays

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) for the Hubble Space Telescope (HST) requires focal plane arrays of 256x256 pixels for both its cameras and its spectrometers. The new arrays, developed by the Rockwell Corporation for NICMOS, have 40 microns pixels of HgCdTe bump bonded to a switched MOSFET readout. Expected read noise and dark current for the arrays at 60 K are 30 e and 1 e/sec. respectively. The basis for these numbers is previous experience with 128x128 arrays.

Thompson, R.↗

NASA NTRS1991

Characterization of NbN films and tunnel junctions

Properties of NbN films and NbN/MgO/NbN tunnel junctions are discussed. NbN junctions are being developed for use in high-frequency, SIS quasiparticle mixers. To properly design mixer circuits, junction and film properties need to be characterized. The specific capacitance of NbN/MgO/NbN junctions has been measured as a function of the product of the normal-state resistance and the junction area (RnA), and it is found to vary by more than a factor of two (35-85 fF/sq microns) over the range of RnA measured (1000-50 ohm sq microns). This variation is important because the specific capacitance determines the RC speed of the tunnel junction at a given RnA value. The magnetic penetration depth of NbN films deposited under different conditions is also measured. The magnetic penetration depth affects the design of microstrip line used in RF tuning circuits. Control of the magnetic penetration depth is necessary to fabricate reproducible tuning circuits. Additionally, the critical current uniformity for arrays of 100 junctions has been measured. Junction uniformity will affect the design of focal-plane arrays of SIS mixers. Finally, the relevance of these measurements to the design of Josephson electronics is discussed.

Stern, J. A.↗

NASA NTRS2004

High density pixel array

A pixel array device is fabricated by a laser micro-milling method under strict process control conditions. The device has an array of pixels bonded together with an adhesive filling the grooves between adjacent pixels. The array is fabricated by moving a substrate relative to a laser beam of predetermined intensity at a controlled, constant velocity along a predetermined path defining a set of grooves between adjacent pixels so that a predetermined laser flux per unit area is applied to the material, and repeating the movement for a plurality of passes of the laser beam until the grooves are ablated to a desired depth. The substrate is of an ultrasonic transducer material in one example for fabrication of a 2D ultrasonic phase array transducer. A substrate of phosphor material is used to fabricate an X-ray focal plane array detector.

Wiener-Avnear, Eliezer↗

NASA NTRS2003

High density pixel array and laser micro-milling method for fabricating array

Wiener-Avnear, Eliezer↗

NASA NTRS1991

Digital Signal Combiner For Receiving-Antenna Feed Array

Proposed digital system combines digitized baseband samples of signals received by array of feeds in large Cassegrainian paraboloidal-dish antenna. Signals received by subantennas in focal-plane array of large dish antenna combined by system compensating in real time for distortions of wave fronts caused by mechanical distortions of antenna. Signal-combining system combines baseband output signals of receiver front ends according to complex weighting parameters estimated from signals themselves. Intended to compensate in real time for degradation of signal-to-noise ratio by vibrational, gravitational, and wind-induced deformations of antenna structure.

Vilnrotter, Victor A.↗

NASA NTRS1990

Detector array evaluation and figures of merit

The commonly used methods to evaluate the performance of a two-dimensional focal-plane array using charge transfer devices are reviewed. Two figures of merit that attempt to combine quantum efficiency, read noise and dark-current generation into a single parameter are discussed. The figures of merit are suggested as possible alternatives to the D asterisk.

Dereniak, Eustace L.↗

NASA NTRS2001

Novel Multiplexing Technique for Detector and Mixer Arrays

Future submillimeter and far-infrared space telescopes will require large-format (many 1000's of elements) imaging detector arrays to perform state-of-the-art astronomical observations. A crucial issue related to a focal plane array is a readout scheme which is compatible with large numbers of cryogenically-cooled (typically < 1 K) detectors elements. When the number of elements becomes of the order of thousands, the physical layout for individual readout amplifiers becomes nearly impossible to realize for practical systems. Another important concern is the large number of wires leading to a 0.1-0.3 K platform. In the case of superconducting transition edge sensors (TES), a scheme for time-division multiplexing of SQUID read-out amplifiers has been recently demonstrated. In this scheme the number of SQUIDs is equal to the number (N) of the detectors, but only one SQUID is turned on at a time. The SQUIDs are connected in series in each column of the array, so the number of wires leading to the amplifiers can be reduced, but it is still of the order of N. Another approach uses a frequency domain multiplexing scheme of the bolometer array. The bolometers are biased with ac currents whose frequencies are individual for each element and are much higher than the bolometer bandwidth. The output signals are connected in series in a summing loop which is coupled to a single SQUID amplifier. The total number of channels depends on the ratio between the SQUID bandwidth and the bolometer bandwidth and can be at least 100 according to the authors. An important concern about this technique is a contribution of the out-of-band Johnson noise which multiplies by factor N(exp 1/2) for each frequency channel. We propose a novel solution for large format arrays based on the Hadamard transform coding technique which requires only one amplifier to read out the entire array of potentially many 1000's of elements and uses approximately 10 wires between the cold stage and room temperature electronics. This can significantly reduce the complexity of the readout circuits.

Karasik, Boris S.↗