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Operation of the Airborne 355-nm High Spectral Resolution and Doppler Lidar LNG

Abstract : High spectral resolution lidar (HSRL) are known to offer capabilities of separating attenuated aerosol and molecular backscattering so that particle extinction and backscattering can be separately retrieved. UV operation provides high energy in eye-safety conditions. Further to that, it could be important for most meteorological or environmental studies to get wind measurements at the same time. LNG is now the only HSR Doppler Lidar (HSRDL) system capable of this. Results obtained during ground-based and airborne measurements show that the backscatter and extinction coefficients at 355 nm can be measured with a relative precision better than 10% (adjusting altitude and time resolution from 60 m to 240 m and 30s to 2mn, respectively) in aerosol layers of 0.5 10 −6 m −1 sr −1 backscatter coefficient from ground and aircraft. The same relative precision is obtained in cirrus clouds of a 10 −5 m −1 sr −1 backscatter coefficient. The capacity of the system to perform wind velocity measurements has also been demonstrated with precisions in the range of 1 to 2 ms −1 in same conditions. We present the main characteristics and illustrate observational capabilities from ground-based and airborne measurements. I. Instrument The High Spectral Resolution (HSR) lidar technique allows to differentiate molecular and particular scattering thus improving aerosol or cloud extinction and backscatter coefficients retrieval which are critical parameters for environmental and meteorological studies. The first airborne HSR lidar systems were developed at DLR in Europe [1] and NASA in the USA [2 ]. The LNG system is based on the use of a Mach-Zehnder interferometer (MZI) as a spectral discriminator [3,4]. This presents the advantage to not require any frequency locking between the emitter and the interferometer and is moreover compatible with a multimode laser emission [5]. Measurement method, system description and first results can be found in [3,6]. We focus here on new results from ground-based and airborne field experiments presenting both scattering and wind measurements. LNG is a three-wavelength (1064 nm, 532 nm and 355 nm) backscatter lidar with polarization analysis at 355 nm. The HSR capability, based on a MZI, has been added at 355 nm following a previous concept analysis [3]. The lidar is designed to be installed in the Falcon 20 or ATR 42 aircraft of SAFIRE, the French research aircraft service. A. Emitter The emitter is based on a Quantel Nd:YAG Q-Switched oscillator, injection seeded with a CW narrow-line emission. It provides single longitudinal mode pulses with 7 ns duration and a 70 MHz linewidth. The fundamental infrared emission is directed though second and third harmonic generators and then through a beam expander. The emitted energy at 355 nm is about 50 mJ@20 Hz, with a divergence smaller than 0.2 mrd. at the beam expander output. The divergence at 532 and 1064 is larger (4 and 5.5 mrd, respectively, to reduce eye-safety distance to a few hundred meters. B. Receiver The backscattered light is collected by a Cassegrain telescope with a 300 mm aperture. Behind the telescope focus, an optical package (receiving optics, RO) ensures the separation of the wavelengths, the background light spectral and spatial filtering and the parallel and cross polarization separation at 355 nm. The receiver field-of view is increased at 532 and 1064nm, accordingly to emission. This difference in field-of-views allows particle size estimates from multiple scattering analyses. C. MZI design The actual MZI design, detailed in [6], has an optical path difference (OPD) Δ= 20 cm and is field compensated in order to accept the large fiber diameter input without reduction of interference contrast. The MZI optical components and the detectors are mounted in a temperature stabilized box (+/-0.1°C).
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Didier Bruneau, Jacques Pelon, F. Blouzon, Quitterie Cazenave, Hélène Collomb, et al.. Operation of the Airborne 355-nm High Spectral Resolution and Doppler Lidar LNG. EPJ Web of Conferences, EDP Sciences, 2020, The 29th International Laser Radar Conference (ILRC 29), 237, pp.06011. ⟨10.1051/epjconf/202023706011⟩. ⟨insu-02899059⟩

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