Dual-band Focal Plane Array Research Articles

Phase grating design for a dual-band snapshot imaging spectrometer.

Infrared spectral features have proved useful in the identification of threat objects. Dual-band focal-plane arrays (FPAs) have been developed in which each pixel consists of superimposed midwave and long-wave photodetectors [Dyer and Tidrow, Conference on Infrared Detectors and Focal Plane Arrays (SPIE, Bellingham, Wash., 1999), pp. 434-440]. Combining dual-band FPAs with imaging spectrometers capable of interband hyperspectral resolution greatly improves spatial target discrimination. The computed-tomography imaging spectrometer (CTIS) [Descour and Dereniak, Appl. Opt. 34, 4817-4826 (1995)] has proved effective in producing hyperspectral images in a single spectral region. Coupling the CTIS with a dual-band detector can produce two hyperspectral data cubes simultaneously. We describe the design of two-dimensional, surface-relief, computer-generated hologram dispersers that permit image information in these two bands simultaneously.

IMAGING PERFORMANCE OF ADVANCED QWIP FOCAL PLANE ARRAYS

Since the first demonstration of the quantum well infrared photodetector (QWIP) in the 1980s, there has been much progress in the application of QWIPs to the production infrared (IR) imaging systems. At this time, focal plane arrays (FPAs) made from QWIPs are readily available for insertion in IR cameras with formats as large as 640 × 480 pixels. Several organizations now have commercially available IR camera systems using QWIPs. In spite of the low single-pixel quantum efficiency relative to MCT, excellent IR imagery has been demonstrated with large format (640 × 480 pixels) single-band and moderate format (256 × 256 pixels) dual-band FPAs. With a large-format staring FPA, one can integrate the signal current for a relatively long time to produce images of similar quality to that from a scanned line array run at the same frame rate. In fact, it can be shown that due to the nature of the noise in a QWIP device, the noise performance of a QWIP FPA can be better than that of MCT FPA as long as the conversion efficiency (the product of the quantum efficiency and the photoconductive gain) is high enough for the read-out integrated circuit (ROIC) integration capacitor to be filled in a frame time. In this chapter the results of laboratory and field tests on large-format single-color QWIP FPAs operating in the LWIR band and dual-band FPAs operating in both the MWIR and LWIR bands simultaneously will be shown. Single-color and dual-band arrays will be shown to give excellent imaging performance and that dual-band FPAs offer unique capabilities to investigate the phenomenology of targets and backgrounds. The performance of the FPAs will be presented from a system performance perspective over a wide range of operating conditions (temperature, bias, integration time, etc.). Results of measurements of noise-equivalent temperature difference (NEΔT), minimum resolvable temperature difference (MRTD measured as a function of target spatial frequency), responsivity, and dark current will be reported. Imagery collected in the field will show the utility of large-format LWIR FPAs for increasing the range at which targets can be identified over previous-generation scanning imagers. Dual-band imagery collected using a QWIP FPA will show how such an array as part of a future imaging system may be able to exploit differences in the IR signatures of targets and backgrounds in the MWIR and LWIR bands to enhance the visibility of targets in cluttered environments. We also show how such an array can be used to make accurate remote temperature measurements. Finally, we will compare the performance of state-of-the-art FPAs made from QWIPs and MCT.

Laboratory and field imaging test results on single-color and dual-band QWIP focal plane arrays

We report on recent laboratory and field measurements on quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). The results of laboratory measurements of imaging performance such as noise-equivalent temperature difference (NE ΔT), minimum resolvable temperature, conversion efficiency, uniformity of response and dark current and their dependence on operating temperature are presented on large format (640×480 pixels) single-color long-wavelength infrared (LWIR) and dual-band (256×256 pixels) mid-wavelength infrared (MWIR)/LWIR arrays. We found that under optimum operating conditions (65 K and 1 V bias), the NE ΔT of the LWIR FPA was approximately 30 mK and was limited by the optics and noise from the closed-cycle cooler. We will show field imagery of military targets taken with this FPA. In addition, field imagery taken with the LWIR QWIP camera of a large-transient event will be compared with that taken with an InSb MWIR camera. We will also show imagery of a recent total eclipse of the moon. We will also present laboratory results on a simultaneously integrating, pixel-registered dual-band FPA showing excellent operability and response uniformity with NE ΔT of approximately 30 mK in both bands ( T=60 K). The dual-band FPA was also taken to the field and we will present imagery of various targets acquired with this FPA.