Journal Article

Characterization and potential for reducing optical resonances in Fourier transform infrared spectrometers of the Network for the Detection of Atmospheric Composition Change (NDACC)

Griffith, David W. T., Heikkinen, Pauli, Hase, Frank, Sussmann, Ralf, Morino, Isamu, Rettinger, Markus, Jones, Nicholas, Mellqvist, Johan, Grutter, Michel, Stremme, Wolfgang, Smale, Dan, Robinson, John, Hannigan, James W., Palm, Mathias, Velazco, Voltaire A., Czurlok, Denis, Servais, Christian, Raffalski, Uwe, Lutsch, Erik, Makarova, Maria, Colebatch, Orfeo, Schneider, Matthias, Strong, Kimberly, Garcia, Omaira, Jeseck, Pascal, Nagahama, Tomoo, Blumenstock, Thomas, Kivi, Rigel, Imhasin, Hamud K., Keens, Axel, Ortega, Ivan, Notholt, Justus, Té, Yao

2021

Overview

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.

• Publisher: Copernicus GmbH,European Geosciences Union,Copernicus Gesellschaften,Copernicus Publications
• Subject: Air / Air gaps / Amplitude / Amplitudes / Astrophysics / Atmospheric chemistry / Atmospheric composition / Beam splitters / Cadmium / Cadmium tellurides / Channeling / Detection / Detectors / Domains / Earth sciences & physical geography / Fourier transforms / FTIR spectrometers / Gases / Indium / Indium antimonide / Infrared detectors / Infrared spectra / Infrared spectrometers / Infrared spectroscopy / Investigations / Laboratories / Mercury / Mercury cadmium tellurides / Optical components / Optical resonance / Optical thickness / Optics / Physical, chemical, mathematical & earth Sciences / Physics / Physique, chimie, mathématiques & sciences de la terre / Sciences de la terre & géographie physique / Sensors / Spectra / Spectrometer / Spectrometers / Spectrum analysis / Substrates