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OPTRA
developed a compact, ruggedized, and very
versatile Fourier transform infrared (FTIR) modulator during our work
on the Joint Services Lightweight Standoff Chemical Agent Detector (JSLSCAD) program.
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Our “J-Series” modulator is a miniature
Michelson interferometer (Figure 1) that can be used in a host of
configurations and applications. Energy that passes through the interferometer
receives an amplitude modulation with a wavelength-dependent frequency caused
by the movement of one mirror relative to the other. Because the modulation frequency is
wavelength-dependent, a Fourier transform on the measured signal yields
spectra. The spectral range is
determined by the optical elements as well as the sampling parameters of the
“interferogram”. Spectral resolution is
equal to the reciprocal of the maximum stroke length of the moving mirror.
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| Figure 1 - Fourier Transform Spectroscopy |
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In its current form, this J-Series modulator
is capable of resolving the 7 to 14 mm spectral range to
as high as 2 cm-1, however, the spectral range can be extended or
changed by changing the interferometer beamsplitter and the sampling
parameters. This rugged modulator is ideal for field
applications, as it has successfully undergone rigorous testing for operation
over a temperature range of -40 to +65ºC and vibration levels associated with a
spectrum of military ground, air, and water vehicles. The J-Series modulator (Figure 2) can be
configured for active, passive, or imaging measurements employing cooled or
uncooled detectors.
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| Figure 2 - Joint Service Lightweight Standoff Chemical Agent Detector (JSLSCAD) Modulator Assembly |
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Figure 3 shows the J-Series configured for
passive IR spectroscopy. In this
instance, the temperature contrast between the chemical plume and the
background against which it is being observed results in either spectral
absorption bands at the resonant frequencies of the molecule if the plume is
colder than the background or spectral emission bands if the plume is warmer than
the background. The strength of the
bands is proportional to the concentration of the chemical as well as the depth
of the plume.
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| Figure 3 - Fourier Transform Spectroscopy |
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This measurement can be made with a cooled
mercury cadmium telluride (MCT) detector or an uncooled pyroelectric (deuterated
L-alanine triglycine sulfate [DLATGS]) or bolometer detector, depending on the sensitivity
requirements of the measurement. In
general, passive IR is a very convenient means of detecting a chemical plume
because the measurement is single ended (i.e. there is no requirement to set up
a mirror at the far side of the plume) and no sampling of the chemical is
required. The working standoff range can
be as long as a kilometer, depending on the size of the chemical plume. The shortcomings are that the measurement is
not quantitative without knowledge of the depth of the plume, and the plume
effectively disappears when the temperature contrast between the plume and
background goes to zero, regardless of the concentration.
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| Figure 4a - Fourier Transform Spectroscopy |
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Figure 4a and 4b show the J-Series modulator
configured for active IR measurements using a bistatic and monostatic
arrangement, respectively. The active
approach uses an IR source either remotely located (bistatic) or internally
located in the instrument (monostatic) to create a large temperature contrast
between the effective background and the chemical such that the spectral
resonance bands always appear in absorption.
The monostatic configuration (Figure 4b) requires use of a mirror or
retroreflector array but tends to have better noise rejection since the source
is frequency encoded before leaving the instrument. Active measurements can be done over ranges
of meters to kilometers. Both
configurations can use either the cooled MCT or uncooled pyroelectric
detector. Active IR measurements tend to
be more sensitive than passive and do not suffer the zero degree temperature
contrast problem, however, the set up is more involved than passive IR.
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| Figure 4b - Fourier Transform Spectroscopy |
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| Figure 5 - Fourier Transform Spectroscopy |
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Figure
5 shows the J-Series in a final configuration using a focal plane array (FPA)
in place of the single element detector.
The resulting hyperspectral FTIR spectrometer can be used for spatially
resolved passive IR spectral measurements where the application requires not
only identification of the compound but also its location. The resulting dataset is a hyperspectral cube
with spectral slices of the two-dimensional image, the number of which is
determined by the spectral resolution and sampling parameters of the FTIR. Hyperspectral measurements can be done with
cooled MCT FPAs or uncooled microbolometer FPAs. This type of measurement can also be made
active by employing a remotely or internally located IR source.
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