Spectral performance analysis of the Fizeau interferometer onboard ESA's Aeolus wind lidar satellite

Abstract

This paper presents an extensive investigation of the signal fringe profile for the Fizeau interferometer used in the first spaceborne wind lidar Aeolus, and considers the fundamental implications for the wind measurement accuracy in Aeolus and future systems. The early Aeolus design phase considered that the basic fringe would be made up of a Fizeau instrumental component of ≈ 100 MHz (FWHM), folded with the laser pulse spectral width of ≈ 50 MHz (FWHM), both of Lorentzian form. Fringe anomalies observed before the mission and related to surface defects in the interferometer plates, triggered the development of wave-optic methods for analysis of the fringe formation. These methods, herein described in an instructional Appendix, were subsequently found to be essential for rigorous modelling of complex fringes for different physical and optical arrangements. Initial signal returns from Aeolus suggested that the Fizeau fringe profile was in fact broadened with a large Gaussian component. The laser pulse was subsequently shown to have a profile close to Gaussian of ≈ 45 MHz (FWHM) and thus provided a partial contribution. However, detailed examination of experimental Aeolus fringes constructed from ground return signals, showed a large Gaussian component up to ≈ 130 MHz (FWHM). Wave-optic modelling established that Fizeau "aperture broadening", of this form and magnitude, would be generated for the input signal beam of 500 µrad field of view (FOV) set at large angles of incidence (AOI) of 300 µrad. These findings have strong implications for fringe shift and wind measurement accuracy, as given in the quantum limited Cramér-Rao expression and the paramount importance of minimising line width. Extensive modelling and simulation for the broadened profiles calculated above, shows good agreement with measured Aeolus global wind measurement accuracies, and indicates that loss of signal could be due to beam clipping at the field stop for such large AOI. It is established that optimisation of the present Aeolus Fizeau parameters could lead to a factor of 2.5 improvement in wind measurement precision. Future upgrades of the Fizeau interferometer and the laser within reasonable parameters, suggest the potential for an factor of 7.6 improvement on the present performance.