QmagiQ LLC, has recently completed building and testing high operability two-color Quantum Well Infrared Photodetector (QWIP) focal plane arrays (FPAs). The 320 x 256 format dual-band FPAs feature 40-micron pixels of spatially registered QWIP detectors based on III-V materials. The vertically stacked detectors in this specific midwave/longwave (MW/LW) design are tuned to absorb in the respective 4-5 and 8-9 micron spectral ranges. The ISC0006 Readout Integrated Circuit (ROIC) developed by FLIR Systems Inc. and used in these FPAs features direct injection (DI) input circuitry for high charge storage with each unit cell containing dual integration capacitors, allowing simultaneous scene sampling and readout for the two distinct wavelength bands. Initial FPAs feature pixel operabilities better than 99%. Focal plane array test results and sample images will be presented.
Infrared Imaging sensors operating in the 3 - 5 um (MW) and 8 - 12 um (LW) spectral bands have long since been traded against one another with respect to mission utility, sensor performance, and system viability (cost factors, technology maturity, etc.). Over the past decade, staring InSb detectors have matured to a refined level (high performance, moderate cost, high yields) and have been used extensively by IR sensor integrators throughout the industry. By the same token, 2-D LW QWIP-based FPA's are fast becoming a viable alternative to traditional LW-scanned technology systems, offering the benefits of mid and large format staring sensor resolution with good sensitivity (even for modest optical F/#'s). With the commercialization of QWIP technology, system viability is rapidly increasing, revealing the need for serious system trade assessments and field measurements to enable the best use of this emerging, complementary detector technology. This paper presents a top-level technical comparison of these two sensor technologies and their use in surveillance/night vision system applications. A variety of technical considerations are discussed to help end users be cognizant of the extent of the trade space that exists between MW and LW staring sensor selection with specific focus on performance comparisons for small, compact militarized IR thermal imaging sensors (including handheld, man-portable and small gimbal products) employing each detector technology in context to various surveillance missions. Application areas include: ground, airborne, and maritime surveillance. Field data is also provided to support the conclusions drawn from these comparisons.
Spatial distributions of hole trap sites on a quasipixel level in InSb arrays for SIRTF are examined. The dependence of flux, fluence, and applied bias on image latency is investigated, and experimental results are presented and discussed. Models of linearity and capacitance are compared with experimental results. We find increasing the depletion width in a light exposed pixel by larger reverse biasing decreases the trapped charge (or latency) in that pixel by factors of approximately 3. Assumed pixel geometries lead to an apparent spatial density of active trap sites that falls quickly with distance from the implants.
SIRTF requires detector arrays with extremely high sensitivity, limited only by the background irradiance. Especially critical is the near infrared spectral region around 3 micrometers , where the detector current due to the zodiacal background is a minimum. IRAC has two near infrared detector channels centered at 3.6 and 4.5 micrometers . We have developed InSb arrays for these channels that operate with dark currents of < 0.2 e/s and multiply-sampled noise of approximately 7 e at 200 s exposure. With these specifications the zodiacal background limited requirements has been easily met. In addition, the detector quantum efficiency of the InSb devices exceeds 90% over the IRAC wavelength range, they are radiation hard, and they exhibit excellent photometric accuracy and stability. Residual images have been minimized. The Raytheon 256 X 256 InSb arrays incorporate a specially developed (for SIRTF) multiplexer and high-grade InSb material.
We have developed a new information-content based look-up table technique for the fast computation of near- monochromatic atmospheric transmittances in the infrared that is well suited for nadir viewing satellite and airplane observations. It allow a user to quickly compute near- monochromatic radiances using a very simple algorithm that is easily ported to many machine architectures. Radiative transfer based on look-up tables of monochromatic absorption coefficients could speed calculations, but they are impractical due to their large size and the need to interpolate long wavenumber vectors in temperature and pressure. We use a singular value decomposition to transform monochromatic look-up tables of absorption coefficients into a compressed representation that is almost 100 times smaller. Moreover, temperature and pressure interpolations can be performed in this compressed representation, resulting in significant savings in computation times and computer I/O. We start with the line-by-line computation of a set of tables of absorption coefficients for each relevant gas. Each 25 wavenumber table has 10,000 wavenumber points and 1,100 temperature/pressure layers. For water vapor we add an extra dimension to these tables that spans 5 water vapor profiles to provide variability in the self-broadening of water vapor spectral lines. On average we need 37 basis vectors for water, 12 for carbon dioxide, and 6 for each of the other required gases in order to reproduce the absorption coefficient tables to an accuracy equivalent to a nadir-viewing monochromatic brightness temperature error of 0.1K.