Dual-Band Megapixel QWIP Focal Planes for Remote
Sensing Applications
S. D. Gunapala, S. V. Bandara, D. Z. Ting, C. J. Hill, and
J. Nguyen
Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive, Pasadena, CA 91109
Potential uses for light detectors operating in the 3–5
and 8–12 μm wavelength range include ground- and
space-based applications such as night vision,
temperature detection, early warning systems,
navigation, flight control systems, weather monitoring,
as well as security and surveillance. In addition, they can
be used to monitor and measure pollution, relative
humidity profiles, and the distribution of different gases
(such as ozone, carbon monoxide, and nitrous oxide) in
the atmosphere.
This is due to the fact that most of the absorption lines
of gas molecules lie in this IR spectral region. The earth's
atmosphere is opaque to most of the IR. Of its few
transparent windows, the 3–5 and 8–12μm are two of
the clearest. Cameras operating in this wavelength range
and used in ground-based telescopes will be able to see
through the earth's atmosphere, image distant stars and
galaxies, and help in the search for cold objects such as
planets orbiting nearby stars.
Single-band Quantum well infrared photodetectors
(QWIPs) are well known for its ease of fabrication,
ruggedness, pixel-to-pixel uniformity and high pixel
operability. Researchers have already demonstrated
megapixel size single-band QWIP focal plane arrays
(FPAs). QWIP is based on a resonant absorption between
ground state and a quasi-continuum state. The spectral
response of QWIPs are inherently narrow-band and the
typical full-width at half-maximum (FWHM) is about 10%
of the peak wavelength. This makes it suitable for
fabrication of negligible optical cross-talk dual-band
detector arrays.
There are many applications that require MWIR and LWIR
dual-band FPAs. For example, a dual-band FPA camera
would provide the absolute temperature of a target with
unknown emissivity, which is extremely important to the
process of identifying a temperature difference between
missile targets, warheads, and decoys. Dual-band
infrared FPAs can also play many important roles in Earth
and planetary remote sensing, astronomy, etc.
Furthermore, monolithically integrated pixel co-located
simultaneously readable dual-band FPAs eliminate the
beam splitters, filters, moving filter wheels, and rigorous
optical alignment requirements imposed on dual-band
systems based on two separate single-band FPAs or a
broad-band FPA system with filters. Dual-band FPAs also
reduce the mass, volume, and power requirements of
dual-band systems. Due to the inherent properties such as
narrow-band response, wavelength tailorability, and
stability (i.e., low 1/f noise) associated with GaAs based
QWIPs, it is an appropriate detector choice for large
format dual-band infrared FPAs.
Fig. 1. An image taken with the first megapixel simultaneous pixel
co-registered MWIR:LWIR dual-band QWIP camera. The flame in the
MWIR image (left) looks broader due to the detection of heated CO 2
(from cigarette lighter) re-emission in 4.1–4.3-micron band, whereas
the heated CO2 gas does not have any emission line in the LWIR (8–9
microns) band. Thus, the LWIR image shows only thermal signatures
of the flame.
All dual-band QWIP wafers were grown on
semi−insulating 6-inch GaAs substrates using molecular
beam epitaxy. 1024x1024 pixel dual-band QWIP arrays
were fabricated using stepper based lithographic
techniques and hybridized with 1024x1024 pixel silicon
readout integrated circuits via an indium bump−bonding
process. The FPAs were characterized for quantum
efficiency, noise, detectivity (the signal−to−noise ratio
normalized to unit area and bandwidth), noise equivalent
temperature difference (NEΔT), uniformity, and pixel
operability. The peak spectral responsivity of the FPA
was at 4.8 and 8.6 μm with FWHM of 10%.
The experimentally measured NEΔT of an FPA were 29
and 42mK (at 300K background with f/2 optics), for
MWIR and LWIR respectively. The corrected NEΔT nonuniformity of the megapixel dual-band pixel co-located
FPA is about 1%. Video images were taken at a frame
rate of 30Hz. Figure 1 shows an image taken with the
first MW:LW dual-band pixel co-registered
simultaneously readable megapixel QWIP FPA.
An image taken with this megapixel simultaneous pixel
co-registered MW:LW dual-band QWIP FPA is shown in
Fig. 1. The flame in the MWIR image (left) looks broader
due to the detection of heated CO2 (from cigarette
lighter) re-emission in 4.1–4.3-micron band, whereas the
heated CO2 gas does not have any emission line in the
LWIR band. Thus, the LWIR image shows only thermal
signatures of the flame. This CO2 signal clearly shows up
MW to LW ratio frame. In other words one can use 8-9
um channel as a reference channel to detect minute
quantities of CO2. This technique can be easily
extendable to remotely detect other gases and weak
features using simultaneous dual-band infrared imaging .
In summary, we have demonstrated the first MW:LW
pixel co-located dual-band megapixel QWIP FPA and its
potential use to remotely sense minute quantities of
gasses. These dual-band QWIP FPAs have myriad
application in science, medicine, and industry.
ACKNOWLEDGEMENTS
The research described in this publication was carried
out at the Jet Propulsion Laboratory, California Institute
of Technology, under a contract with the National
Aeronautics and Space Administration.