The advanced imagers team at JHU APL and ECE has been advocating and developing a new class of sensor systems
that address key system level performance bottlenecks but are sufficiently flexible to allow optimization of associated
cost and size, weight, and power (SWaP) for different applications and missions. A primary component of this approach
is the innovative system-on-chip architecture: Flexible Readout and Integration Sensors (FRIS). This paper reports on
the development and testing of a prototype based on the FRIS concept. It will include the architecture, a summary of test
results to date relevant to the hostile fire detection challenge. For this application, this prototype demonstrates the
potential for this concept to yield the smallest SWaP and lowest cost imaging solution with a low false alarm rate. In
addition, a specific solution based on the visible band is proposed. Similar performance and SWaP gains are expected for
other wavebands such as SWIR, MWIR, and LWIR and/or other applications like persistent surveillance for critical
infrastructure and border control in addition to unattended sensors.
KEYWORDS: Calibration, Image quality, Image processing, Sensors, Data modeling, Image enhancement, Temperature metrology, Black bodies, Signal to noise ratio, Near infrared
Ground-based high-resolution, calibrated, near-infrared (NIR) imagery of the Space Shuttle STS-134 Endeavour during
reentry has been obtained as part of NASA's HYTHIRM (Hypersonic Thermodynamic InfraRed Measurements) project.
The long-range optical sensor package called MARS (Mobile Aerospace Reconnaissance System) was positioned in
advance to acquire and track part of the shuttle re-entry. Imagery was acquired during a few minutes, with the best
imagery being processed when the shuttle was at 133 kft at Mach 5.8. This paper describes the processing of the NIR
imagery, building upon earlier work from the airborne imagery collections of several prior shuttle missions. Our goal is
to calculate the temperature distribution of the shuttle's bottom surface as accurately as possible, considering both
random and systematic errors, while maintaining all physical features in the imagery, especially local intensity
variations. The processing areas described are: 1) radiometric calibration, 2) improvement of image quality, 3)
atmospheric compensation, and 4) conversion to temperature. The computed temperature image will be shown, as well
as comparisons with thermocouples at different positions on the shuttle. A discussion of the uncertainties of the
temperature estimates using the NIR imagery is also given.
KEYWORDS: Near infrared, Sensors, Potassium, Alkali metals, Doppler effect, Signal to noise ratio, Imaging systems, Silicon, Signal attenuation, Absorption
Utilization of Near-Infrared (NIR) spectral features in a muzzle flash will allow for small arms detection using low cost
silicon (Si)-based imagers. Detection of a small arms muzzle flash in a particular wavelength region is dependent on the
intensity of that emission, the efficiency of source emission transmission through the atmosphere, and the relative
intensity of the background scene. The NIR muzzle flash signature exists in the relatively large Si spectral response
wavelength region of 300 nm-1100 nm, which allows for use of commercial-off-the-shelf (COTS) Si-based detectors.
The alkali metal origin of the NIR spectral features in the 7.62 × 39-mm round muzzle flash is discussed, and the basis
for the spectral bandwidth is examined, using a calculated Voigt profile. This report will introduce a model of the 7.62 ×
39-mm NIR muzzle flash signature based on predicted source characteristics. Atmospheric limitations based on NIR
spectral regions are investigated in relation to the NIR muzzle flash signature. A simple signal-to-clutter ratio (SCR)
metric is used to predict sensor performance based on a model of radiance for the source and solar background and pixel
registered image subtraction.
Normal incidence InAs/In0.15Ga0.85As dots-in-a-well detectors operating at T=78K with λcut-off ~8.2 μm and a spectral width (Δλ/λ) of 35% are reported. The peak at 7.2 μm is attributed to the bound-to-bound transitions between the ground state of the dot and the states within the InGaAs well. A broad shoulder around 5 μm, which is attributed to the bound to continuum transition, is also observed. Calibrated blackbody measurements at a device temperature of 78K yield a peak responsivity of 3.58 A/W (Vb=-1V), peak detectivity= 2.7x109cmHz1/2/W (Vb=-0.3V), conversion efficiency of 57% and a gain ~25.
Mid- and far-infrared detectors operating at elevated temperatures (T>150K) are critical for imaging applications. In(Ga)As/GaAs quantum dots, grown by self-organized epitaxy, are an important material for the design and fabrication of high-temperature infrared photodetectors. Quantum dot infrared photodetectors (QDIPs) allow normal-incidence operation, in addition to low dark currents and multispectral response. The long intersubband relaxation time of electrons in quantum dots improves the responsivity of the detectors, contributing to better high-temperature performance. These devices also exhibit photoconductive gain. The characteristics of state-of-the-art lateral and vertical QDIPs will be described. We have achieved peak responsivity for wavelengths ranging from 3.7-18micrometers . We have also obtained extremely low dark currents (Idark=27pA,T=100K,Vbias=0.5V), high detectivities (D*=2.9x108 cmHz½/W, T=100K, Vbias=0.2V), and high operating temperatures (T=150K) for these quantum-dot detectors. The excellent performance of these devices at low bias voltages indicates the compatibility of high temperature QDIPs with commercially available silicon read- out circuits for imaging focal plane arrays. These results, as well as infrared imaging with QDIP arrays, will be described and discussed.
This paper provides the design details for a new two-color quantum-well infrared photodetector for use in advanced thermal imaging. The single pixel experimental data used in the evolution of this design is discussed in significant detail. Using the knowledge gained from extensive measurements comparing miniband transport with bound-to-quasi-bound QWIPs and also double and triple coupled-well QWIPs with the more conventional single-well QWIPs, a two-color design is presented. The detector design provides for a lattice-matched growth and superior response in the MWIR spectral region, while keeping the dark currents from the LWIR portion of the detector to a minimum. The architecture will allow for the MWIR and LWIR stacks to be biased separately and also for the signals to be read independently.
We report on results of laboratory and field tests of dual- band MWIR/LWIR focal plane arrays (FPAs) produced under the Army Research Laboratory's Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system and by BAE Systems using QWIP technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640 X 480-pixel single-color read-out integrated circuit (ROIC) to produce a dual-band 320 X 240 pixel array. The MWIR and LWIR pixels are co-located and have a high fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using MBE-grown III-V materials. The LWIR section consisted of GaAs quantum wells and AlGaAs barriers and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two color operation) which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The relative advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.
The U.S. Army Research Laboratory (ARL) is currently investigating unique self-mixing detectors for ladar systems. These detectors have the ability to internally detect and down-convert light signals that are amplitude modulated at ultra-high frequencies (UHF). ARL is also investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver because the UHF mixing is done internal to the detector, thereby reducing both the cost and complexity of the system and enhancing its range capability. This fits well with ARL's goal of developing low-cost, high-speed line array ladars for submunition applications and extremely low-cost, single pixel ladars for ranging applications. Several candidate detectors have been investigated for this application, with metal-semiconductor-metal (MSM) detectors showing the most promise. This paper discusses the requirements for a self-mixing detector, characterization measurements from several candidate detectors and experimental results from their insertion in a laboratory FM/cw ladar.
We report on the results of laboratory and field tests on a pixel-registered, 2-color MWIR/LWIR 256 X 256 QWIP FPA with simultaneous integrating capability. The FPA studied contained stacked QWIP structures with spectral peaks at 5.1 micrometer and 9.0 micrometer. Normally incident radiation was coupled into the devices using a diffraction grating designed to operate in both spectral bands. Each pixel is connected to the read-out integrated circuit by three bumps to permit the application of separate bias levels to each QWIP stack and allow simultaneous integration of the signal current in each band. We found the FPA to have high pixel operability, well balanced response, good imaging performance, high optical fill factor, and low spectral crosstalk. We present data on measurements of the noise-equivalent temperature difference of the FPA in both bands as functions of temperature and bias. The FPA data are compared to single-pixel data taken on devices from the same wafer. We also present data on the sensitivity of this FPA to polarized light. It is found that the LWIR portion of the device is very sensitive to the direction of polarization of the incident light. The MWIR part of the device is relatively insensitive to the polarization. In addition, imagery was taken with this FPA of military targets in the field. Image fusion techniques were applied to the resulting images.
Intersubband carrier transitions in either the conduction or valence bands of III-V semiconductors are important for the design of a new generation of optoelectronic devices. These transitions have been successfully used to demonstrate the operation of a new class of infrared lasers and detectors. In this paper, we discuss the use of intersubband transitions in quantum dot nanostructures for infrared sensing. The quantum dot structures in our work allow the detection of normal-incidence light. This is promising for the design of future focal plane arrays that do not require a grating structure to scatter incident light into the correct polarization for detection via intersubband transitions. The quantum dot infrared photodetectors in this work also exhibit an intrinsic photovoltaic effect. Both photoconductive and photovoltaic operation has been demonstrated at low temperatures (40 K) with responsivities on the order of tens of mA/W for bias voltages less than 0.5 V. Detectivities ranging from 2 X 108 cmHz1/2/W to 7 X 109 cmHz1/2/W have been measured in devices operating in either the photovoltaic or photoconductive mode. We have demonstrated that the quantum dot structures have the capability to detect infrared light in the 9 to 13 micrometers spectral band.
Infrared sensor technology is critical to many commercial and military defense applications. Traditionally, cooled infrared material systems such as indium antimonide, platinum silicide, mercury cadmium telluride, and arsenic doped silicon (Si:As) have dominated infrared detection. Improvement in surveillance sensors and interceptor seekers requires large size, highly uniform, and multicolor IR focal plane arrays involving medium wave, long wave, and very long wave IR regions. Among the competing technologies are the quantum well infrared photodetectors based on lattice matched or strained III-V material systems. This paper discusses cooled IR technology with emphasis on QWIP and MCT. Details will be given concerning device physics, material growth, device fabrication, device performance, and cost effectiveness for LWIR, VLWIR, and multicolor focal plane array applications.
We describe here a study of Mini-Band Transport (MBT) Quantum Well Infrared Photodetector (QWIP) samples in which the binding energy of photoexcited electrons is systematically varied. Each sample has been characterized electrically and radiometrically. Results are reported for variation of absorption spectra, spectral response, IV characteristics, blackbody responsivity, detector noise, and detectivity vs. binding energy for dark current limited mode of operation.
Quantum well infrared photodetector (QWIP) technology has developed rapidly in the past decade culminating in the demonstration of large format focal plane arrays. Most of the efforts so far have been on tactical applications in which an increased operating temperature is the major objective. For strategic applications with a cold background and a faint target, low temperature operation is required. Under these conditions, improving the conversion efficiency (quantum efficiency times gain) is very important for QWIPs to collect sufficient signal. Simplified QWIP (S-QWIP) structures with increased optical gains have been demonstrated. In this presentation, experimental results of several S-QWIPs will be given. The properties of simplified QWIPs will be examined at low temperatures with a low background and a faint target. Results of a computer simulation with an unresolved target will be discussed.
Intersubband transitions in GaAs/(Al,Ga)As quantum wells have been successfully used in the design of novel infrared detectors for over a decade now. Both conduction- and valence-band based detectors have been investigated. In general, the conduction-band based detectors fabricated from direct gap GaAs/(Al,Ga)As heterostructures are not sensitive to normal-incidence light. This is a consequence of the quantum mechanical rules that govern light absorption in these structures. In order to detect normal-incidence light, a grating structure which scatters the incident light into higher order, transverse magnetic modes is used. To avoid the use of gratings, research is being carried out in (In,Ga,Al)As/(Al,Ga)As conduction-band quantum well structures that can absorb normal-incidence light. This paper reviews recent progress in such detectors.
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