J-Series Fourier Transform Infrared Spectrometer Module
OPTRA developed a compact, ruggedized, and very versatile Fourier transform
infrared (FTIR) modulator during our work on the JSLSCAD (Joint Services Lightweight
Standoff Chemical Agent Detector) program. Our “J-Series” modulator is a miniature
Michelson interferometer (Figure 1) that can be used in a host of configurations and
applications.
Figure 1: Michelson Interferometer
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.
In its current form, this J-Series modulator is capable of resolving the 7 to 14 µm 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.
Technical Background: The J-Series Fourier Transform Infrared Spectrometer Module
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The J-Series modulator (Figure 2) can be configured for active, passive, or imaging
measurements employing cooled or uncooled detectors.
Laser diode
reference
assembly
Moving mirror /
flexure
assembly
1.825”
2.725”
Stationary
mirror
assembly
Reference
detector
assembly
1.765”
Exit
aperture
Entrance
aperture
Vibration
isolation
mounts
Figure 2: Photograph of J-Series Modulator
Figure 3: J-Series Configured for Passive IR Spectroscopy
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. 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
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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.
Figure 4a: J-Series Configured for Bistatic Active IR Spectroscopy
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: J-Series Configured for Monostatic Active IR Spectroscopy
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.
Figure 5: J-Series Configured for Hyperspectral Imaging
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