Biosphère Climat

    - Version EN

The COWI lidar is, in its original version (2013), a coherent differential absorption lidar for measuring the absorption of atmospheric CO 2 and wind speed. In its current version (2018) it also makes it possible to measure the absorption of water vapor and integrates a double direct detection (APD HgCdTe detector) and consistent.

The COWI lidar is based on an innovative patented laser source produced in the laboratory [Gibert Brevet 2013; APB, 2014; OL, 2015] . The laser consists of a Holmium doped crystal pumped by a fiber Thulium laser for emission in the near infrared at 2 µm. The cavity is injected, triggered and makes it possible to deliver pulses of duration 50 ns, of energy 10 mJ at a rate of 2 kHz. The laser in fact delivers pulses at two wavelengths, one centered on an absorption line of the probe gas, the other outside to serve as a reference. The system uses consistent detection. Most of the instrument is fiberized. In the spectral domain considered absorption lines of CO 2 and H 2O allow DIAL measurements of the absorption of the two compounds with the same lidar system.


Figure 1   Experimental diagram and specifications of the COWI lidar
The DIAL coherent lidar consists of a pulsed laser source at 2.05 µm, an injection module controlled by a spectral reference system and a coherent detection
EOM : electro-optical modulator; AOM : acousto optical modulator; PID : proportional integral derivative; PDH : Pound Drever Hall; AOFS : acousto optic frequency shifter; TDFA : Thulium doped fiber amplifier; PBS : polarizer beam splitter; HWP : half wave plate; QWP : quarter wave plate; PZT : piezoelectric transducer

Since 2014, the COWI instrument has been integrated into a container to facilitate its implementation in the field. The container is equipped with a scanner which has been developed in the laboratory to perform 3D measurements in the atmosphere of the wind field, CO 2 and H 2 O in the atmosphere. Two in situ stations made up of sonic anemometers and gas analyzer fixed on telescopic masts (<20 m) make it possible to measure pressure, temperature, sensible, latent heat flux and CO 2 at the surface. The whole constitutes an operational “wind, CO 2 and H 2 O” mobile station . First COWI tests in a DIAL H 2 coherent versionO were carried out in vertical and oblique sight. These measurements were compared with in-situ measurements at ground level and by radiosondings. The precision on the dry air mixing ratio of H 2 O is 10% at 1km with a spatial resolution of 100m and temporal resolution of 10s

Figure 2   Left: COWI instrument in LMD test phase.
Right: test of the COWI instrument integrated into the container with the scanner and first measurements of H 2 O.
Profile at 20 ° inclination with respect to the horizontal of (a) CNR (lidar reflectivity at 2 µm), (b) radial wind speed at the two wavelengths (Off: blue, Off: green), (c) optical thickness due to the absorption of H 2 O, (d) absorption coefficient, (e) of dry air mixing ratio of H 2 O compared to ground measurements (★) and maps of (f) CNR, (g)radial wind speed and (h) mixing ratio of H 2 O