Proc. SPIE. 9640, Remote Sensing of Clouds and the Atmosphere XX
KEYWORDS: Signal to noise ratio, Avalanche photodetectors, Atmospheric laser remote sensing, LIDAR, Sensors, Remote sensing, Interference (communication), Signal detection, Camera shutters, Fast packet switching
The nowadays environmental issues motivated us to develop a mobile LiDAR system for two applications: 1°/ the prevention against forest fires in the hilly and inaccessible continuous mountainous forests in north Algeria, where the risk factor of a forest fire is high in summer and 2°/ the study of the atmosphere by monitoring pollutant species and sand aerosols emanating either from cement plants in the Algiers agglomeration and the Sahara. The system mobility 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 an Nd:YAG laser followed by a second harmonic generator that emits at λ=532 nm and a photomultiplier (PMT) as a detector. The LiDAR is an active technique that often deals with weak return signals. The latter come with an intensity proportional to 1⁄R2, where R is the range from the target to the detector. The signal-to-noise ratio (SNR), as an effective detection criterion, needs to be optimized towards better performance. That is, when the SNR is low, the detected signal is increased 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 that is base on the control of the detector gain by driving its applied voltage.
KEYWORDS: Signal to noise ratio, Sensors, LIDAR, Avalanche photodiodes, Photomultipliers, Avalanche photodetectors, Signal detection, Interference (communication), Environmental sensing, Camera shutters
We deal with two types of detectors utilized in LiDAR systems that remotely sense the environment. One is a photomultiplier, often used to detect visible light. The second is an avalanche photodiode used to sense the infrared. In this contribution, we explicitly formulate the signal-to-noise ratio and relate it to the applied polarizing voltage. Then, we show how to control the applied voltage for better performance of the detector.
As well known, rectangular Hermite-Gaussian modes as well as cylindrical Laguerre-Gaussian modes do form an orthogonal set endowed with shape-invariance properties. So they can form an expansion basis for many propagation-invariant wave fields in the paraxial regime. Especially for those coupled from laser resonators or oscillating inside. In this contribution, is addressed the determination of the sufficient number of modes in the expansion for both sets and in different aperturing kinds: hard, soft and moderate.