Investigation on stability of Optical Parametric Oscillators for LIDAR measurement of methane (CH4) and carbon dioxide (CO2)

Abstract

Within a joint German-French Satellite mission (MERLIN) that aims at measuring global methane concentrations, using the Integrated Path Differential Absorption LIDAR technique, feasibility studies are currently carried out at DLR to identify the most suitable technology for its laser transmitter. Pulsed Optical Parametric Oscillators operating in the near-infrared spectral range and spectrally narrowed by means of injection seeding are a promising candidate. An important parameter to achieve accurate measurement from space is the laser transmitter’s beam pointing stability, which had supposed to be investigated within the scope of this thesis. Therefore, a detection system based on an InGaAs-camera and a Si-camera for the beam analysis of the OPO and the pump laser, respectively, has been devised. Both camera systems are controlled by a specially developed LabView application, which is capable of the required image processing and analysis, such as background correction and centroid calculation. The measurements have been performed in accordance with ISO 11670. The pointing stability of the OPO was measured to be better than 1% of its divergence angle, whereas that of the pump laser is on the order of 5%. The investigations further revealed influences on the pointing accuracy of on-line and off-line pulses from the movement of a piezoelectric transducer that is required to match the OPO cavity to its seed wavelength. A correlation between the beam pointing of the OPO and that of the pump laser could also be revealed. The present thesis therefore provides important information on the design of a laser system for both the space- and airborne remote sensing of column densities of the most important anthropogenic greenhouse gases CH4 and CO2.