Preventing the Saturation Phenomenon of Detectors Used in environmental Remote Sensing

摘要

The nowadays environmental issues motivated us to develop a mobile LiDAR system for two applications: 1°/ Early detection of forest fires in the hilly and inaccessible continuous mountains in north Algeria, where the risk factor of a forest fire is higher in summer and 2°/ the study of the atmosphere by monitoring pollutant gases and aerosols emanating either from cement plants in the Algiers agglomeration or from the Sahara. The system, designed in a mobile configuration, enables measurements in different locations. For the first application, we use an eye safe laser wavelength at λ=1.57 μm and an avalanche photodiode (APD) as a detector. For the second application, we use a Nd:YAG laser, followed by a second harmonic generator, that emits at λ=532 nm and a photomultiplier (PMT) as a detector. As the LiDAR often deals with weak and noisy return signals, the signal-to-noise ratio (SNR) is known as the criterion for an effective detection. This parameters needs to be optimized for better performance. That is, when the SNR is low, the detected signal is proportionally amplified and vice versa. In the case of forest fires detection, it may happen that strong signals from nearest objects lead the detector to a saturation status. We suggest a method to handle this issue, based on the control of the detector gain by driving its applied voltage.