The optical challenge of two-colour lunar laser ranging at MéO station

This article describes recent optical developments for the Nd:YAG laser at the MéO station, which regularly carries out L unar/ S atellite Laser Ranging ( L LR/ S LR) observations for fundamental physics and space geodesy. MéO (registred as GRSM 7845 in the International Laser Ranging Service network) ranges to the Moon since 1981 with lasers undergoing continuous development for improved reliability. Since 2012, a 150 ps pulsed laser version has provided reliable green LLR. Infrared LLR followed from 2015 and two-colour ranging started recently. This work focuses on the recent optical improvements to this laser to achieve this two-colour capability. There, the Paraxial Gaussian Beam tool of the Zemax OpticStudio software models the evolution of the beam shape throughout its propagation inside the station. This enhanced optical set-up drastically improved the lifetime of the biggest rod of the amplification chain, cancelled an annoying thermal pointing drift and supressed infrared vignetting. In 2023, these developments provided 91 two-colour LLR observations for lunar heights greater than 32°. Finally, the text suggests two perspectives for achieving two-colour LLR for Moon heights below 32°.