Anh T. La

Development of a 1K x 1K GaAs QWIP Far IR Imaging Array

In the on-going evolution of GaAs Quantum Well Infrared Photodetectors (QWIPs) we have developed a 1,024 x 1,024 (1K x 1K), 8.4-9 μm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using the Rockwell TCM 8050 silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). The finished hybrid is thinned at the Jet Propulsion Lab. Prior to this development the largest format array was a 512 x 640 FPA. We have integrated the 1K x 1K array into an imaging camera system and performed tests over the 40K-90K temperature range achieving BLIP performance at an operating temperature of 76K (f/2 camera system). The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, well-established GaAs processing capabilities. In this paper we will present the first results of our 1K x 1K QWIP array development including fabrication methodology, test data and our imaging results.

Logarithmically variable infrared etalon filters

Due to high launch vehicle costs, space instrumentation designers are constantly pressured to decrease weight and increase reliability of flight hardware. To meet these needs in a spectrometer, the infrared products team at Optical Coating Laboratory, Inc. (OCLI) and the NASA Goddard Space Flight Center (GSFC) have developed an infrared logarithmically variable filter for use in NASAs Pluto Fast Fly-by instrument. The filter and diode array combination replaces the multiple optical elements in conventional spectrometers, resulting in lower instrumentation weight and complexity with no moving parts. The choice of logarithmic rather than linear profile yields constant resolving power on every pixel of the array. Filters were produced in which the center wavelength varied from 1.0-1.581 micrometers , and 1.581-2.5 micrometers over a distance of 1.024 cm. Bandwidth was 0.3% FWHM and overall transmittance ranged from 30% to 50%. This paper discusses the major issues and tradeoffs in the design, manufacture, and testing of the filters. Measurement techniques are presented and comparisons are made between theoretical and measured performance of bandwidth, transmittance, and spectral profile.

The QWIP focal plane assembly for NASA's Landsat Data Continuity Mission

The Thermal Infrared Sensor (TIRS) is a QWIP based instrument intended to supplement the Operational Land Imager (OLI) for the Landsat Data Continuity Mission (LDCM) [1]. The TIRS instrument is a dual channel far infrared imager with the two bands centered at 10.8μm and 12.0μm. The focal plane assembly (FPA) consists of three 640x512 GaAs Quantum Well Infrared Photodetector (QWIP) arrays precisely mounted to a silicon carrier substrate that is mounted on an invar baseplate. The two spectral bands are defined by bandpass filters mounted in close proximity to the detector surfaces. The focal plane operating temperature is 43K. The QWIP arrays are hybridized to Indigo ISC9803 readout integrated circuits (ROICs). Two varieties of QWIP detector arrays are being developed for this project, a corrugated surface structure QWIP and a grating surface structure QWIP. This paper will describe the TIRS system noise equivalent temperature difference sensitivity as it affects the QWIP focal plane performance requirements: spectral response, dark current, conversion efficiency, read noise, temperature stability, pixel uniformity, optical crosstalk and pixel yield. Additional mechanical constraints as well as qualification through Technology Readiness Level 6 (TRL 6) will also be discussed.

An advanced airborne multisensor imaging system for fast mapping and change detection applications

The advanced airborne multisensor imaging system (AAMIS) has been developed for a light fixed wing aircraft. It integrates a suite of state-of-the-art electro-optical (EO), thermal, hyperspectral, and Lidar imaging instrument packages for simultaneous active ranging and passive imaging that covers the electromagnetic (EM) spectrum of the visible and near infrared range and the long-wave infrared range. AAMIS has been tested for todays fast mapping and change detection needs. It demonstrates leading performance in providing comprehensive geometric and geophysical aerial image products with high spatial, spectral, radiometric, temporal and range resolutions. High resolution innovative data products collected by AAMIS sensors are presented. These include 3.5 cm resolution orthomosaics for a complete 15 km times 25 km large area coverage, 1 ft resolution hyperspectral images with contiguous 10 nm spectral resolution for the 410-820 nm range, 0.02 K resolution thermal images in a large 1 k by 1 k frame video format, and 20 cm range accuracy 3D Lidar mapping products.

Large-format and multispectral QWIP infrared focal plane arrays

The next generation of infrared (IR) focal plane arrays (FPAs) will need to be a significant improvement in capability over those used in present-day second generation FLIRs. The Armys Future Combat System requires that the range for target identification be greater than the range of detection for an opposing sensor. To accomplish this mission, the number of pixels on the target must be considerably larger than that possible with 2nd generation FLIR. Therefore, the 3rd generation FLIR will need to be a large format staring FPA with more than 1000 pixels on each side. In addition, a multi-spectral capability will be required to allow operability in challenging ambient environments, discriminate targets from decoys, and to take advantage of the smaller diffraction blur in the MWIR for enhanced image resolution. We report on laboratory measurements of a large format (1024 x 1024 pixels) single-color LWIR IR FPA made using the corrugated quantum well infrared photodetector (QWIP) structure by the ARL/NASA team. The pixel pitch is 18 μm and the spectral response peaks at 8.8 μm with a 9.2 μm cutoff. We report on recent results using a MWIR/LWIR QWIP FPA to image the boost phase of a launch vehicle for missile defense applications and a LWIR/LWIR FPA designed specifically for detecting the disturbed soil associated with buried land mines. Finally, we report on the fabrication of a new read-out integrated circuit (ROIC) specifically designed for multi-spectral operation.

Development of 256x256 GaN ultraviolet imaging arrays

We have successfully developed a prototype 256 X 256 photoconductive GaN ultraviolet (UV) imaging array. The array, with its pixels (30 X 30 micrometer2) indium bump bonded to a Lockheed Martin Fairchild Systems LT9601 readout integrated circuit, is highly sensitive to ultraviolet light below 365 nm with a sharp reduction in response to visible and infrared light. The array was installed into a custom designed UV camera utilizing a Nikon UV lens with all the off-chip electronics interfaced to an automatic computer controlled system. To the best of our knowledge, this is the first reported UV array camera based on the nitride materials.

Hyperspectral sensing using the Linear Etalon Imaging Spectral Array

The Linear Etalon Imaging Spectral Array (LEISA) represents a new class of hyperspectral cameras which use non- dispersive thin film filters as wavelength selective elements. The simplicity and versatility of these instruments make them attractive for spaceflight use. LEISA currently operates in the shortwave IR spectral region, but the design is adaptable to operation at wavelengths from visible to longwave IR.

Fabrication and evaluation of 11.2- and 16.2-μm cutoff C-QWIP arrays

Corrugated quantum well IR photodetector (QWIP) focal plane arrays (FPAs) with cutoff wavelength of 11.2 and 16.2 micrometers were fabricated and tested. Each detector array has 256 X 256 pixel elements, indium bumped to a direct injection readout circuit manufactured by Rockwell Science Center. The rest of the supporting electronics were designed and built in-house to provide biases and clock functions to the FPAs. IR imageries with good aesthetic attributes were obtained from both FPAs. For the 11.2 micrometers FPA, background limited IR performance (BLIP) was obtained at 63 K under F/2 optics, consistent with the test results of a large area detector. This operating temperature is substantially higher than the grating coupled arrays with comparable cutoff wavelengths. On the other hand, the optics of the present camera were not optimized for wavelengths beyond 14 micrometers . As a result, the BLIP temperature for the 16.2 micrometers FPA, observed to be 38 K, was somewhat lower than the expected 42 K from the single detector characterization. Despite the reduced detector volume of a C-QWIP structure, the measured internal quantum efficiency remains to be high, being 20.5 percent and 25.4 percent at 2 V bias for the 11.2 micrometers and the 16.2 micrometers FPA, respectively.

Development of a 1 megapixel long IR QWIP focal plane array

In the rapid development of GaAs Quantum Well Infrared Photodetectors (QWIPs) we have fabricated a 1,024 x 1,024 (1K x 1K), 8-12 μm infrared focal plane array (FPA). This focal plane array is a hybrid using an L3 Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to the 1 megapixel GaAs QWIP. This effort was a collaboration of engineers at the Goddard Space Flight Center (GSFC), the Army Research Laboratory (ARL) and L3 Cincinnati Electronics (L3). We have integrated this focal plane into an SE-IR based imaging camera system and performed tests over the 55K-77K temperature range. As in previous developments the ease of fabrication of the GaAs array continues to be a valuable asset. The overall focal plane development costs are currently dominated by the costs associated with the silicon readout/hybridization. The GaAs array fabrication is based on a high yield, well-established GaAs processing capability. The broadband long wavelength response of this array combined with markedly improved quantum efficiency is of particular value in science applications where spectroscopy is required. One of the features of GaAs QWIP technology is the ability to precisely design and fabricate arrays responsive to a particular IR spectral region but the spectral response is typically only a few tenths of a micrometer wide limiting the spectral information content. By broadening the spectral response of this device the applications for imaging and spectroscopy are substantially increased. In this paper we will present the latest results of our corrugated 1K x 1K, 8-12 μm infrared focal plane array development including fabrication methodology, test data and experiments.

Order-sorting filter transmittance measured with an array detector

The simultaneous measurement of the spectrally and spatially variant transmittance of a linear variable order-sorting filter in a manner that closely resembles its conditions of actual use is described. The transmittance of a prototype order-sorting filter was measured in the 400- to 880-nm wavelength region by illuminating it with the output beam of a spectrophotometer while the filter was attached to the front of a 30 x 32 pixel silicon array detector. The filter was designed to be used in the output beam of a grating spectrometer to prevent the dispersal of higher diffracted orders onto an array detector. Areas of the filter that were spatially matched to the corresponding detector pixel column had measured peak transmittances of about 90 percent that were uniform to within +/- 1.5 percent along a given column. Transmittances for incident wavelengths shorter than the desired bandpass, corresponding to the order overlap region, were measured in the 0.003 range. Line spread function measurements made with the array detector indicated no significant beam spreading caused by inserting the filter into the beam.