Lidar, Manaus-AM

The UV Raman Lidar is operational on the ACONVEX site since July 2011. It uses a Quantel CFR-400 Nd-YAG laser at 355 nm with 95 mJ per pulse and 10 Hz repetition rate. Beam is expanded by 3 and final laser divergence is 0.25 mrad. The optical system uses a bi-axial setup with a 400 mm separation between the cassegrain telescope and the laser axis. The telescope’s primary mirror has 400 mm diameter, while the secondary has a diameter of 90 mm. Focal length is 4000 mm resulting in a f/10 system. An iris is used at the focal plane which gives a field of view of 1.75 mrad and an initial overlap at 85 m and full overlap at 450 m.

No fiber optics are used and light passing through the iris goes directly in the optical detection box. Interferomet- ric filters separate the elastic back scattered signal and the inelastic signals due to the raman cross-section of N2 (387 nm) and H2 O (408 nm) which are read collected in different photo-multiplier-tubes. Signals from 355 and 387 nm are recorded in analog and photon-count modes, while 408 nm only in photon count. The optical system was designed to give an uniform signal on the cathode surface almost independent of height of the detected sig- nal. A neutral density filter is used to attenuate the elas- tic signal avoiding saturation, and a good signal to noise ratio (S/N) is found above 15 km depending on the at- mospheric conditions. The N2 channel, 1-min average signals have good S/N up to 15 km but only during night time. For the H2 O channel, 1-min average signals have good S/N only up to 6 km during night time.

The system is fully automated and includes a clock-controlled shutter to cover the telescope and laser quartz windows from direct sunlight exposure between 11 am and 2 pm local time. As a backup system, a 10 mm shutter is positioned just above the iris and kept in its light-blocking position by a coil mechanism. Interlocks are connected to the UPS and to the sun light sensor, a small telescope with a 10o field of view around the large telescope axis. Environmental conditions effects on the electronics are minimized by continuously running an air conditioning and a dehumidifier inside the instrumentation cabinet.