Mobile and high-spectral-resolution Fabry–Pérot interferometer spectrographs for atmospheric remote sensing
Authors: Kuhn Jonas , Bobrowski Nicole , Wagner Thomas , Platt Ulrich 
Published: December 2021

Abstract. Grating spectrographs (GS) are presently widely in use for atmospheric trace
gas remote sensing in the ultraviolet (UV) and visible spectral range
(e.g. differential optical absorption spectroscopy, DOAS). For typical DOAS
applications, GSs have a spectral resolution of about 0.5 nm,
corresponding to a resolving power R (ratio of operating wavelength to
spectral resolution) of approximately 1000. This is sufficient to quantify the
vibro-electronic spectral structure of the absorption of many trace gases with
good accuracy and further allows for mobile (i.e. compact and stable)
instrumentation. However, a much higher resolving power (R≈105, i.e. a spectral
resolution of about the width of an individual rotational absorption line)
would facilitate the measurement of further trace gases (e.g. OH radicals),
significantly reduce cross interferences due to other absorption and
scattering processes, and provide enhanced sensitivity. Despite these major
advantages, only very few atmospheric studies with high-resolution GSs are
reported, mostly because increasing the resolving power of a GS leads to
largely reduced light throughput and mobility. However, for many environmental
studies, light throughput and mobility of measurement equipment are central
limiting factors, for instance when absorption spectroscopy is applied to
quantify reactive trace gases in remote areas (e.g. volcanoes) or from airborne or space-borne platforms. For more than a century, Fabry–Pérot interferometers (FPIs) have been
successfully used for high-resolution spectroscopy in many scientific fields
where they are known for their superior light throughput. However, except for
a few studies, FPIs have hardly received any attention in atmospheric trace gas
remote sensing, despite their advantages. We propose different high-resolution
FPI spectrograph implementations and compare their light throughput and
mobility to GSs with the same resolving power. We find that nowadays mobile
high-resolution FPI spectrographs can have a more than 2 orders of magnitude
higher light throughput than their immobile high-resolution GS
counterparts. Compared with moderate-resolution GSs (as routinely used for
DOAS), an FPI spectrograph reaches a 250 times higher spectral resolution while
the signal-to-noise ratio (SNR) is reduced by only a factor of 10. Using a
first compact prototype of a high-resolution FPI spectrograph (R≈148 000, <8 L, <5 kg), we demonstrate that these
expectations are realistic. Using mobile and high-resolution FPI spectrographs could have a large impact
on atmospheric near-UV to near-infrared (NIR) remote sensing. Applications include the
enhancement of the sensitivity and selectivity of absorption measurements of many
atmospheric trace gases and their isotopologues, the direct quantification of
OH radicals in the troposphere, high-resolution O2 measurements
for radiative transfer and aerosol studies, and solar-induced chlorophyll
fluorescence quantification using Fraunhofer lines.