The effect of smoke spatial distribution on pulse laser echo characteristics

A low false alarm rate in smoke has always been chasing after pulse laser ranging system. Smoke distributes particularly random in space so that different smoke distributions produce distinct interference echoes even if they are under same attenuation degree. Based on the theory of single scattering, in this paper the echo-element superposition model is established to study the relationship between smoke backscattering pulse and smoke thickness under the condition of same attenuation but different spatial distribution. The analysis shows that at a constant threshold, the pulse width of backscattered echoes in smoke are first broadened logarithmically by the thickness of the smoke and then compressed to zero with a negative exponential relationship, while a maximum width appears in the process; the peak of the backscattered echoes gradually decreases with the inverse of smoke thickness. In order to validate the model, the Monte Carlo method for photon tracing is used to simulate the model. The simulation results are in good agreement with the theoretical analysis. At the same time, an experimental device is set up in which pulse laser is transmitted in the smoke environment. Experimental results show that with the detection distance, one-way attenuation, transmitted pulse width of 25m, 60%, 20ns respectively, the pulse width of target echoes remains basically unchanged while smoke thickness varies, and the peak value and pulse width of the backscattering pulse are consisted with theoretical analysis within an error of 5%. This results can be used for laser ranging system in low-visibility environment for interference suppression and reducing false alarm rate, so as to improve system stability and anti-interference ability.

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