High Spectral Resolution Lidar

Important note: we are currently refurbishing the lidar, a brand new system will be operational early 2024

High Spectral Resolution Lidar

The LEANDRE New Generation (LNG) airborne Lidar system is based on a two-wave interferometry [Mach–Zehnder Interferometer (MZI)] to provide both the determination of optical parameters of aerosol and clouds and along-sight wind in the troposphere (Bruneau et al., 2003, 2015). The instrument has been developed by LATMOS and DT-INSU. It operates in a direct detection mode (measurement of the backscattered light intensity), which has the advantage of relying on both particulate and molecular scattering, and allows extended ranges and capabilities.

The direct detection technique has been chosen for space observations of winds for the ADM-AEOLUS space mission using UV-Fabry Perot Interferometer, and comparisons have shown that wind measurements were in very good agreement between the two techniques as well as with theoretical performance (Bruneau et al., 2004). The design of the MZI is well adapted to the quantification of aerosol and cloud properties, as well as Line-of-sight wind measurements on particles.

The HSR analysis within LNG performed by a Mach–Zehnder interferometer allows phase and intensity analysis simultaneously. In contrast to conventional HSR devices, our approach is not to separate the molecular and particulate signals in two distinct channels but to determine the interference contrast given by the backscattered light, which is linked to the backscatter ratio. The contrast is unambiguously determined by the signals given by four detection channels in phase quadrature, whatever the spectral positioning of the laser frequency with regard to the transmission function of the MZI. This way no frequency stabilization is necessary either for the laser or the interferometer. Calibration is performed using laser signal injected in the MZI.

The signals, in phase quadrature, are delivered by the four detectors. The analysis allows us to retrieve intensity and phase shift of the backscattered signal with respect to the laser emission.  As a result, backscattering signals are derived at the three emitted wavelengths and the HSR signals allow one to separate the attenuated particulate and molecular backscattering, as for ADM-Aeolus.

Wavelength (nm)Energy (mJ)Divergence (mrd)
355 nm linear polarization~400.15
Main characteristics of the emitter (before 2024)

Wavelength (nm)Field of view (mrd)Spectral bandwidth (nm)Detector
355 nm parallel0.59PM
355 nm cross0.59PM
Main characteristics of the receiver (before 2024)

The repetition rate of the emitter (a flashlamp-pumped Nd:YAG Q-switched oscillator) is 20 Hz (= 50 ms) and the spot separation distance  is 10 meters assuming an aircraft speed of 200 m/s (Falcon 20). The digitization of the output from each detector is performed on 16 bits at a 25-MHz sampling rate leading to a vertical resolution of about 5 meters. The 5 meters resolution is due to the acquisition rate. The real resolution of the instrument is about 37m (due to receiver bandwidth).