Abstract

Detection of smoke was one of the first practical applications of lidar Smoke contains a large number of small particles, leading to a large backscattering efficiency. The first lidars used ruby or Nd:YAG lasers, which are dangerous to the eye However the modem tendency in lidar is to use lasers with eyesafe wavelengths at ~1 54 or ≈2 1 or ≈10.6 μm. In particular, lidars with a wavelength within the range ≈1 54 μm are increasingly being used [1]. In our study we will perform computations for a lidar with direct detection mode, at a wavelength at 1.54 μm which is intended for the detection of smoke plumes originating from burning wood or oil. It is assumed that the fuel (wood or oil) is situated at ground level and is in the form of a circle with diameter varying in the range 1 3-3.5 m and burning rate in the range 0.3-4 kg/s. To find parameters burning thermodynamic calculations was performed. The buoyant plume of burning products was described by slender flow equations. An approximate numerical solution of these equations was found on the assumption that the velocity and temperature profiles are Gaussian. The particle emission factor for the burning of wood is within the range 1% and the average density of ash is equal to 1.0 g/cm³. These values correspond to a concentration of ash in the gas products of burning C_pb≈0 3 g/m³ During the burning of oil the smoke yield is ≈12%. A proper soot concentration in this case in the gas products of burning is C_pb≈ 1 45g/m³. The particle size distribution of products of burning wood and oil from literature was used. The backscattering coefficient β is calculated taking into account backscattering cross section, complex refraction index, and particle size distribution. For further computations for burning wood and oil we will use the values 6 6*10⁻³ m⁻¹sr⁻¹ and 2.5* 10⁻² m⁻¹sr⁻¹, respectively. The terms of postdetection electrical-signal power, thermal-noise power, power of electronic postdetection amplifier noise, detector dark-current-noise power, signal shot-noise power, end background illumination shot-noise power appear in the equation for the signal-to-noise ratio (SNR). Rearrangement of this equation gives the equation for the required laser energy E₁.

© 2000 IEEE