Global Tropospheric and Total Ozone Monitoring With a Double-Etalon Fabry-Perot Interferometer
Authors: Larar Allen M. , Drayson S. Roland , Hays Paul B. 
Published: February 2023

Knowledge of the global scale distribution of atmospheric ozone and its temporal variability can only be achieved through implementation of a satellite-based remote sensing technique. Development of a space-based tropospheric ozone remote sensor has remained a difficult task to accomplish due to the challenges associated with such a measurement. Nevertheless, the ability to monitor the global distribution of tropospheric ozone is of much interest for enhancement of scientific understanding as well as to potentially lessen the ill-health impacts associated with exposure to elevated concentrations in the lower atmosphere. Such a capability can be achieved using a satellite-based nadir-viewing device making high spectral resolution measurements with high signal-to-noise ratios;1 this would enable observation in the pressure-broadened wings of strong O3 lines while minimizing the impact of undesirable signal contributions associated with, for example, the terrestrial surface and interfering species. The Fabry-Perot interferometer (FPI) provides high spectral resolution and high throughput capabilities that are essential for this measurement task. The periodic nature of the Fabry-Perot instrument function can be advantageous when observation of periodic spectra is desired.2 However, for most applications, additional optical elements are necessary to reduce the effect of unwanted passbands. This is frequently accomplished using additional Fabry-Perot etalons in a series configuration3 in conjunction with a bandpass filter. This paper will discuss a Fabry-Perot interferometer conceptual instrument design to achieve tropospheric and total ozone monitoring capability from a satellite-based nadir-viewing geometry. The design involves a double-etalon fixed-gap series configuration FPI along with an ultra-narrow bandpass filter to achieve single-order operation with an overall spectral resolution of approximately .068 cm−1. The impact of inter-etalon reflections has been reduced to acceptable levels by placement of a slightly attenuating medium in between the etalons. A passive device is selected for low power consumption; and continuous day/night coverage, independent of solar zenith angle, is enabled by observing within the strong 9.6 μm ozone infrared band. The IR-FPI detection will be performed through implementation of the new Circle to Line Interferometer Optical (CLIO) system4 (U.S. patent #4893003, 1990), developed by researchers at the Space Physics Research Laboratory (SPRL) of the University of Michigan, to accomplish focal plane fringe detection; the CLIO system converts the circular interferometric fringes into a linear pattern which then can be detected by conventional linear array detectors. A multiplex signal advantage is achievable as all necessary frequencies can be measured simultaneously using a multichannel configuration. Through proper selection of channel spectral regions, the FPI optimized for tropospheric O3 measurements can simultaneously observe a stratospheric component and thus the total O3 column abundance. Details of the preliminary instrument design are given by Larar1.