Estimation of required parameters for detection of small smoke plumes by lidar at 1.54��m (original) (raw)

2000, Appl Phys B Lasers Opt

Parameters of eyesafe lidar at 1.54 µm for detection of small plumes of smoke from burning wood or oil have been evaluated. It was assumed that a diode-pumped solid-state Er:glass laser at 1.54 µm or a Nd:YAG laser with a Raman cell or optical-parametric oscillator is used as a light source and that detection of backscattered light is performed with an avalanche photodiode. Ash and soot particle size distributions were taken from experiments. A backscattering coefficient at 1.54 µm for various source of smoke was estimated. In computing the laser energy, range between lidar and smoke, receiver optics diameter, fuel mass burned in unit time, fire source radius, laser pulse duration and visibility were varied. Results of the computations enabled estimation of the required laser energy, which ranges from 0.05 to 1400 mJ depending on the parameters. PACS: 42.68.Wt; 92.60.Mt Detection of smoke from sources such as power plants, factories, forest fires, etc., was one of the first practical applications of lidar . Smoke contains a large number of small particles of ash or soot, leading to a large backscattering efficiency and consequently favourable conditions for lidar application. The use of lidar increased considerably during the following years, and this technology is now widely used . In general the first lidars used ruby or Nd:YAG lasers (wavelength 694 and 1064 nm, respectively), which are dangerous to the eye. They were applied to the investigation of plumes, which have a great output of smoke. However the modern tendency in lidar is to use lasers with eyesafe wavelengths (at 1.54 or ≈ 2.1 µm [5, 6]) and to increase its sensitivity, allowing increasingly small sources of smoke to be detected . It is obvious that detection of the early stages of a fire, when smoke plumes are tenuous, allows the means required to extinguish the fire and fire damage to be minimized. For eyesafety, lidars with a wavelength within the range ≈ 1.54 µm are increasingly being used. It is possible in this lidar to use solid-state Er:glass lasers at 1.54 µm, Nd:YAG lasers with a Raman cell at 1.54 µm [9], and Nd:YAG lasers with opticalparametric oscillators (OPO) . Detection of smoke is pos-sible by direct or heterodyne detection mode . But it is known [5] that OPO leads to frequency instability, so in this case it is preferable to use lidar with direct-detection mode.