Frequency comb based absolute frequency reference design for future spaceborne multi species differential absorption lidar systems for green-house gases monitoring
Résumé
The Lidar Emitter and Multi-species greenhouse gases Observation iNstrument (LEMON) is a novel Differential Absorption Lidar (DIAL) sensor concept for greenhouse gases and water vapor measurements from space.1,2 It is based on a versatile transmitter allowing for addressing various absorption lines of different molecules. This highly flexible emitter design requires a universal frequency referencing scheme. Here we present a concept employing a 1 GHz frequency comb, which allows the absolute referencing over a spectral range from 0.95 μm to 1.15 μm. By using an intermediate frequency doubling stage, this allows for DIAL measurements on CO2, H2O/HDO, and CH4 in the 2 μm range. Absolute referencing is obtained by using a GPS disciplined oscillator as the common time base for frequency measurements. The concept of the LEMON Frequency Reference UnIT (FRUIT) is designed to match the requirements of the vibration loads associated with airborne operation to allow implementation on the airborne demonstrator for LEMON. In addition, the requirements for a future space development are considered in the design. For example, radiation critical items have been identified and radiation tested within the project and a compact wavemeter design has been implemented.