The LIDAR group designed and achieved a 2-µm Heterodyne Doppler Differential Differential Absorption Lidar for simultaneous CO2 mixing ratio and radial velocity measurements in the troposphere. The instrumental challenges were to make absolute range-resolved CO2 mixing ratio measurements in the atmospheric boundary-layer using aerosols as diffuse targets with no bias in the signal processing and spectroscopic calculations and a standard deviation lower than 1 %.
The EMIL (Experimental Mid-Infrared Lidar) system was first developed in 1994 by Didier Bruneau & Jacques Pelon (LATMOS) for wind measurement. The system was modified in 2003 to add the DIAL ability and was then called LIDIA (Lidar Doppler DIAL). At present it is installed at LMD in Palaiseau.
Fig. 1: Didier Bruneau (left) & Fabien Gibert (right) operating the 2.05 µm Heterodyne Doppler Lidar for wind velocity & CO2 density measurements (2004).
The EMIL-LIDIA system is based on a injection seeded Tm, Ho:YLF oscillator pumped by a 500 mJ- 10 Hz Alexandrite laser. The 2-µm oscillator delivers on- and off-line 10 mJ – 140 ns – 2.5 MHz pulses (Gibert et al., Appl. Opt., 2006). An a posteriori method using a photoacoustic cell technique is used to control the laser line positionning (Gibert et al, Appl. Spect., 2007c) (Fig. 2)
Fig. 2: 2-µm HeD-DiAL transmitter block diagram and instrumental specifications. BS, beam splitter, LF, Lyot filter, OAM, opto-acoustic modulator, BE, beam expander, PAC, photo-acoustic cell, PC, Pockels cell. D1-4 are InGaAs photodiodes.