LALINET

(Not rainproof yet, Remote operation: Not required, Inside the Air Force Campus where hotel service is provided)

The DIAL technique requires two emitter lasers. For the wavelength absorbed by ozone, an excimer (XeCl) laser emitting at 308 nm with a repetition rate in the range 1–100 Hz, maximum energy per pulse of 300 mJ is used. The reference wavelength is produced by the third harmonic of a Nd–YAG laser at 355 nm, 30 Hz repetition rate and 130 mJ maximum energy. The backscattered photons are collected by four Newtonian f/2 telescopes of 50 cm diameter with parabolic aluminized surfaces of 48 cm diameter. This produces a total reception area of ~7238 cm2. At the focus of each telescope an optical fiber of 0.94 mm effective diameter, 0.22 ± 0.02 numerical aperture, 0.27 db km-1 attenuation (@ 308 nm) is placed. The other end of the fiber is positioned at the focus of a quartz lens placed inside a spectrometric box. A mechanical chopper is in the optic path of the signal collected by the telescopes. It has rotating velocity of 150 Hz and its role is to block the strong lidar signals originating from the lower part of the atmosphere. The wavelength separation is performed by a plane holographic diffraction grating of 3600 grooves per mm with 40% transmittance at 300 nm. Six channels are detected simultaneously. Four of them correspond to the elastic backscattering photons at both laser wavelengths in high-energy and low-energy mode. Additionally, the radiations due to the Raman scattering by nitrogen molecules of the 308 and 355 nm radiations are detected. In all the channels, Hamamatsu photomultipliers type H6780-03 adapted for photon counting are used for signal detection. The signals are amplified by six amplifiers with 250 MHz bandwidth and amplification factor of 10 and 20 for the high-level and low-level channels respectively, and 30 for the Raman channels. The acquisition of the signals is made by a system developed at Service d’Aéronomie, Institute Pierre Simon Laplace, Paris, France, in six photo counting modules of 300 MHz bandwidth each.