LIDAR is the acronym for Light Detection And Ranging, a remote sensing technique that uses laser light to perform atmospheric measurements.
The basic idea is simple: a pulse of laser light is transmitted into the atmosphere, and the power back-scattered is measured versus time (or round trip distance); the amount of light returned is proportional to the atmospheric density.
The scattered light from each target, opportunely scaled by the square of the target altitude (according to the "Lidar equations") depends, in fact, on the cross-sectional area, the incident light level, and the "scattering efficiency" of a particle with its particular size, shape and composition.
Different types of physical processes in the atmosphere are related to different types of light scattering. The Rayleigh Lidar technique (which is a molecular scattering where the wavelength remains the same after the scattering) is used, for example, to detect the aerosol pollutants or study the thermal structure of the middle atmosphere through the creation of density and temperature profiles.
A Lidar system consists in a transmitter (laser), a receiver (optical telescope) and a extremely sensitive detector (photomultiplier tube).
Source, receiver and detector system
A Q-switched Nd:YAG or a diode pumped Nd:YAG laser are used as transmitters; the Lidar signal is then collected by a telescope and detected by a photomultiplier tube.
The relevant features of the laser beam (power, divergence, pulse duration etc.) and of the receiver are matched, in order to detect the back-scattered light up to 30 Km of distance.
The laser head is fixed over the telescope structure and the laser beam travels in axis with the telescope by means of two folding mirrors: this allows the detection of the light scattered by the closest layers of the atmosphere, because the laser beam is always within the field of view of the telescope.
The Lidar signal collected by the telescope is diaphragmed by a pin-hole mounted in the focal plane, collimated, filtered by an interference filter, refocused, and finally delivered to the photomultiplier tube. The gain of the photomultiplier is modulated with an externally applied waveform: in this way it's possible to lower the initial high intensity background and better detect the signals which come later (which are weaker).
For example, a parabolic waveform can be used to modulate the gain, according to the fact that the back-scattered light scales with the square of the altitude.
The gain modulation of the photomultiplier tube, the Lidar signal acquisition and the initial check of the laser operative conditions are software performed.
Specifications are subject to change without notice.