The SPU-Lidar is a multiwavelength Raman LIDAR operated by the Lasers Environmental Applications Research Group at the Center for Lasers and Applications (CLA), Nuclear and Energy Research Institute (IPEN) in São Paulo (−23◦ 56’ S, 46◦74’ W, 740 m above sea level). It is a monoestatic coaxial system, pointed vertically to the zenith and using a commercialized Nd:YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) laser by Quantel, model Brilliant B, with a fundamental wavelength of 1064 nm, and generating second and third harmonics, 532 nm and 355 nm, respectively, at a repetition rate of 10 Hz. The output energy per pulse is 850 mJ for 1064 nm, 400 mJ for 532 nm and 230 mJ for 355 nm. The pulse duration is 6 ± 2 ns. The laser beam has an average diameter of 9 mm and is directed to a beam expander (expands the three wavelengths), which increases the beam diameter about 5 times, with a divergence of less than 0.1 mrad. The expanded laser beam is directed to the atmosphere through a second set of mirrors. A 30 cm diameter telescope (Focal length of 1.5 m) is used to collect the backscattered laser light. The telescope’s field of view (FOV) can be adjusted and once a desirable value of 0.1 mrad is reached, using a small diaphragm. The system is currently used with a fixed FOV of 0.1 mrad, which permits a full overlap between the telescope FOV and the laser beam at altitudes higher than 500 m above the ground level. This FOV value, in accordance with the detection electronics, permits the probing of the atmosphere up to the free troposphere. The detection box collects six different wavelengths and separates them into 6 different channels. The elastic 355 nm and the corresponding shifted Raman signals: nitrogen 387 nm, and water vapor, 408 nm; the elastic 532 nm and the corresponding shifted Raman Nitrogen signal at 530 nm using a combination of high-pass and low-pass filters, and the elastic channel at 1064 nm. Each splitted beam is directed to narrowbands spectral interference filters (532±1nm FWMH, 355±1 nm FWMH, 387±0.25nm FWMH, 408±0.25 nm FWMH, 530±0.5 nm FWMH and 1064±1 nm FWMH) and then directed to photomultipliers tubes (PMTs). R7400 photomultiplier tubes from Hamamatsu are used for all channels, except for 607 and 660 nm, where R9880U-20 are used. The PMT signals are digitized by a transient recorder TR 20-80/160 for 532 nm, TR 20-160 for 355, 387 and 408 nm, TR 20-40 for 607 and 660 nm, all supplied by LICEL. They are recorded in both analog and photoncounting mode. Corrections of background noise and the dark current are applied before analysis. The current vertical spatial resolution is 7 meters and temporal resolution of 2 or 60 seconds.

The lidar is based on a tripled Nd:YAG laser with a 20 Hz repetition rate, and a pulse energy of 70 mJ at 355 nm. A 30 cm aperture Newtonian telescope, collects the backscattered light. The lidar contains a separate detection module with a three - channel module (TCM) for detection of elastic backscatter at 355 nm, rotational Raman backscatter at 353.9 nm and methane vibrational Raman scattering at 396 nm. The optical fiber can be switched between the detection modules for specific tasks. The optical signal from the fiber is collimated in the TCM by a silica lens and the collimated beam is separated for the spectral components by two dichroic mirrors. The spectral components are isolated by the interference filters and detected by R9880 PMTs. The outputs of the detectors are recorded at 7.5 m range resolution using Licel transient recorders that incorporate both analog and photon-counting electronics. The full geometrical overlap of the laser beam and the telescope field of view (FOV) is achieved at approximately 500 m height for 1.0 mrad the field of view used.