Appl Phys B DOI 10.1007/s00340-009-3365-7 The airborne multi-wavelength water vapor differential absorption lidar WALES: system design and performance M. Wirth · A. Fix · P. Mahnke · H. Schwarzer · F. Schrandt · G. Ehret Received: 30 October 2008 / Revised version: 17 December 2008 © Springer-Verlag 2009 Abstract A high-performance airborne water vapor differ- ential absorption lidar has been developed during the past years. This system uses a four-wavelength/three-absorption line measurement scheme in the 935 nm H 2 O absorption band to cover the whole troposphere and lower stratosphere simultaneously. Additional high spectral resolution aerosol and depolarization channels allow precise aerosol charac- terization. This system is intended to demonstrate a future space-borne instrument. For the first time, it realizes an out- put power of up to 12 W at a high wall-plug efficiency us- ing diode-pumped solid-state lasers and nonlinear conver- sion techniques. Special attention was given to a rugged op- tical layout. This paper describes the system layout and tech- nical realization. Key performance parameters are given for the different subsystems. PACS 42.65.Yj · 42.68.Wt · 92.60.Jq 1 Introduction The primary objective of the project WALES (derived from WAter vapor Lidar Experiment in Space) of the DLR M. Wirth ( ) · A. Fix · G. Ehret Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Münchner Str. 20, 82234 Wessling, Germany e-mail: martin.wirth@dlr.de P. Mahnke Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Technische Physik, Stuttgart, Germany H. Schwarzer · F. Schrandt Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Robotik und Mechatronik, Berlin, Germany (Deutsches Zentrum für Luft- und Raumfahrt) was the preparation of a space-borne mission to overcome the short- comings of radio-sondes and passive satellite sensors in mapping the global water vapor distribution. While the for- mer do not cover the globe uniformly and do not provide reliable water vapor observations in the upper troposphere and lower stratosphere, the latter suffer from insufficient vertical resolution and accuracy [1]. In contrast, a space- borne multi-wavelength H 2 O-DIfferential Absorption Lidar (DIAL) could provide global water vapor observations suit- able for a reliable assessment of its temporal and spatial evo- lution. These data would lead to an improved description of climate processes in general circulation models (GCMs) and to benefits in numerical weather prediction (NWP) [2]. The methodology of DIAL has been developed during the late 1960s and 1970s, and a large number of studies ap- peared in the following years, see [3, 4] for a review of the principle of measurement and [5, 6] for an overview of ex- isting systems. First proposals for a space-borne H 2 O-DIAL dating back to the 1980s and 1990s [6, 7] suffered from a high power-aperture product, driving the system costs and lacking coverage of the upper troposphere. To overcome these problems, a measurement scheme was developed at DLR that uses four wavelengths in the 935 nm absorption band of H 2 O, each one especially adapted to a restricted alti- tude range of the atmosphere. Figure 1 shows the absorption cross section of water vapor in this wavelength region near ground and at 10 km altitude and indicates possible lines for DIAL measurements. In this way, relatively large absorption coefficients can be chosen, which allow for short averaging times even at high noise levels, thus lowering the system’s power-aperture product considerably [2, 9–11]. One major step undertaken during recent years to vali- date the four-wavelength concept was the realization of an airborne demonstrator. This instrument not only implements