Characterization and potential for reducing optical resonances in Fourier transform infrared spectrometers of the Network for the Detection of Atmospheric Composition Change (NDACC)
2021
Abstract. Although optical components in Fourier transform infrared (FTIR) spectrometers
are preferably wedged, in practice, infrared spectra typically suffer from the
effects of optical resonances (“channeling”) affecting the retrieval of
weakly absorbing gases. This study investigates the level of channeling of
each FTIR spectrometer within the Network for the Detection of Atmospheric
Composition Change (NDACC). Dedicated spectra were recorded by more than
20 NDACC FTIR spectrometers using a laboratory mid-infrared source and two
detectors. In the indium antimonide (InSb) detector domain (1900–5000 cm−1 ), we found
that the amplitude of the most pronounced channeling frequency amounts to
0.1 ‰ to 2.0 ‰ of the spectral background level,
with a mean of ( 0.68±0.48 ) ‰ and a median of
0.60 ‰ . In the mercury cadmium telluride (HgCdTe) detector domain
(700–1300 cm−1 ), we find even stronger effects, with the largest
amplitude ranging from 0.3 ‰ to 21 ‰ with a mean
of ( 2.45±4.50 ) ‰ and a median of 1.2 ‰ . For
both detectors, the leading channeling frequencies are 0.9 and 0.11 or
0.23 cm−1 in most spectrometers. The observed spectral frequencies
of 0.11 and 0.23 cm−1 correspond to the optical thickness of the
beam splitter substrate. The 0.9 cm−1 channeling is caused by the
air gap in between the beam splitter and compensator plate. Since the air gap
is a significant source of channeling and the corresponding amplitude differs
strongly between spectrometers, we propose new beam splitters with the wedge
of the air gap increased to at least 0.8 ∘ . We tested the insertion of
spacers in a beam splitter's air gap to demonstrate that increasing the wedge
of the air gap decreases the 0.9 cm−1 channeling amplitude
significantly. A wedge of the air gap of 0.8 ∘ reduces the
channeling amplitude by about 50 % , while a wedge of about
2 ∘ removes the 0.9 cm−1 channeling completely. This study
shows the potential for reducing channeling in the FTIR spectrometers operated
by the NDACC, thereby increasing the quality of recorded spectra across the
network.
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