IEEE Geoscience and Remote Sensing Magazine - June 2019 - 80

17 cm

Slit
FPA
Grating

ZnSe Corrector
(a)
ZnSe Lens

Slit
FPA
10 cm
(b)
FIGURE 7. (a) A schematic of the Aerospace Corporation's MAKO

imaging spectrometer [12]. (b) A schematic of the Jet Propulsion
Laboratory's HyTES imaging spectrometer [13]. The dimensions are
approximate. The small size of both facilitates cooling to reduce
thermal emissions, with MAKO operated at approximately 15 K and
HyTES at approximately 100 K. ZnSe: zinc selenide.

Focusing
Lens

Etalon

FPA

B
(a)

M2′

2d
M1

d
(b)

Beam
Splitter

FIGURE 8. (a) The schematic of a Fabry-Pérot interferometer with

the etalon spacing l and the on-axis incident wavefront focused into
a point at the FPA's center. The dashed lines illustrate the field of
view of the instrument. (b) The schematic of a Michelson interferometer composed of a stationary mirror M1, a moving mirror M2,
and a beam splitter. The path difference for the recombined ray
is d = 2d cos i. To the right is the spatial bullseye pattern for the
relative mirror positions. The input and output of the Michelson are
collimated with a focusing optic applied to record the interferogram
at a detector element.
80

[14], and examples of spectral sensors that employ Michelson interferometers are given in [15] and [16]; a commercial
airborne version of the latter is given in [17].
There are advantages to the forms, with the Fabry-Pérot
being extremely compact to help facilitate cooling and the
Michelson producing a modulated signal that is relatively
simple to recover from a constant background. Both interferometers use unpowered optical elements, and the aberration control is limited to the telescope. Neither shows the
level of spatial-spectral uniformity that is possible from a
dispersive sensor, with the spectral resolution varying according to the field position. Interferometric forms also
require spectral prefiltering to mitigate their free spectral
range and aliasing properties.
FOCAL PLANE ARRAYS
The remaining challenges, from an instrument perspective, with all LWIR imaging spectrometers are the FPAs.
A variety of FPAs have been employed, including CMOS
readout integrated circuit (ROIC) arrays with mercury
cadmium telluride (MCT) as the detector material, quantum-well IR photodetectors also hybridized with CMOS
ROICs, and silicon-arsenic- (Si-As)-blocked impurity band
(Si-As BIB) arrays [18]. Historically, BIB arrays have had
superior spatial uniformity and operability compared to
those of MCT, with quantum efficiencies between 7 and
14 µm ranging from roughly 0.6 to 0.95, and have been
preferred by some researchers even though they require
cooling using liquid helium as a cryogen to approximately
10 K [19]. Quantum well infrared photodetector (QWIP)
arrays have the advantage of high spatial uniformity and
a relatively high operating temperature. Previously, they
have been limited by a narrow spectral band range that
has been overcome using multiple stacks tuned to different spectral ranges. HyTES employs a QWIP detector with
a range from 7.5 to 12 nm. Advancements in MCT array
manufacturing have significantly improved their performance to quantum efficiencies as high as 90% for antireflection coated devices and nonuniformity below 2% [20],
[21]. MCT detectors are often cooled to operating temperatures of approximately 40 to 50 K. Detector linearity is
also important because it simplifies the calibration process. The noise characteristics of FPAs are critical to the
overall performance of the imaging spectrometer driving
the cooling requirements.
CHALLENGES
The state of the art for LWIR imaging spectrometers has
been developed and demonstrated at a high level. These
instruments will always be challenging to manufacture
because of their alignment tolerances and temperature
control requirements. There are two areas where significant progress remains. The size, weight, and power
(SWaP) requirements are substantial with these systems,
particularly dispersive designs mounted in dewars and
cooled with either liquid cryogen or cryocoolers. Progress
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

JUNE 2019

IEEE Geoscience and Remote Sensing Magazine - June 2019

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