One of the problems that arises when restoring the reflectivity of the target surface from the echo pulse of a navigation lidar is the effect of the tilt of the reflecting target surface on the broadening of the echo pulse. In this paper, a model is proposed that describes the broadening of the reflected echo pulse of a short-range lidar at different angles of incidence of the laser beam on the target. This model also takes into account the main parameters of the lidar, the atmosphere, the reflecting surface of the target and its inclination, which allow calculations to be made in a wide range of conditions.
KEYWORDS: LIDAR, Signal attenuation, Mass attenuation coefficient, Particles, Ocean optics, Airborne laser technology, Water, Monte Carlo methods, Scattering, Polarization
Results of experimental determination of the laser radiation extinction coefficient by sea water from the depth profiles of the airborne lidar return signal power by the gradient method for pure and coastal waters at depths from 5 to 50 m are presented. The extinction coefficients have been calculated for the “Makrel” polarization lidar with a wavelength of 532 nm. The proposed method expands the possibilities of remote hydrooptical sensing of the optical characteristics of the subsurface inhomogeneous water layers.
A system of imaging of backscattered lidar pulses based on a SPAD camera developed at the IAO SB RAS is described. The main technical characteristics of the system are presented. Examples of 3 images of a single laser pulse with wavelength of 532 nm recorded by the SPAD camera with imaging array of 64 × 32 pixels from distances of 75, 90, and 285 m are given.
A concept of hyperspectral LIF lidar based on a time-of-flight SPAD camera with high spatial resolution is suggested. Hyperspectral registration of the FIF spectrum of an oil film with the time-of-flight SPAD camera is numerically modeled. A quantitative dependence of the oil concentration on the distance and technical characteristics of the transceiving lidar system is refined.
Results of statistical simulation of laser sensing of water depth in an experimental water pool by the Monte Carlo method are presented in the report. As a rule, experimental pools have limited sizes. The influence of pool walls on the lidar signal characteristics is analyzed. The power of laser radiation with a wavelength of 0.53 m is estimated depending on sizes of the experimental water pool, water extinction index, laser beam divergence angle, and field of view angle of the receiver. The results obtained demonstrate not too great influence of radiation reflection from the pool walls when the pool size exceed 20 m. Hence, this effect can be disregarded for pools of larger sizes.
For statistical estimation of the potential possibilities of the lidar with matrix photodetector placed on board an aircraft, the nonstationary equation of laser sensing of a complex multicomponent sea water medium is solved by the Monte Carlo method. The lidar return power is estimated for various optical sea water characteristics in the presence of solar background radiation. For clear waters and brightness of external background illumination of 50, 1, and 10–3 W/(m2⋅μm⋅sr), the signal/noise ratio (SNR) exceeds 10 to water depths h = 45–50 m. For coastal waters, SNR ≥ 10 for h = 17–24 m, whereas for turbid sea waters, SNR ≥ 10 only to depths h = 8–12 m. Results of statistical simulation have shown that the lidar system with optimal parameters can be used for water sensing to depths of 50 m.
We launched into a development of a new stand-off sensing system that can detect atmospheric and hazardous gases in real atmosphere utilizing THz technology. Narrow line width, <0.1 cm-1, long-wave (mid-IR and THz) source based on difference frequency generation (DFG) in collinear configuration in GaSe0.91S0.09:Al(0.03 at. %) using seeded YAG laser and KTP OPO as pump sources was designed. The low optical loss coefficient and large hardness, together with the simplicity of the processing, make GaSe0.91S0.09:Al(0.03 at. %) as a high-reliable and effective THz-wave generator suitable for out-of-door application. We demonstrate incoherent terahertz wave detection by stand-off room temperature Schottky diodes located at over 110 m using open waveguide formed by multitude HPE lenses, mirrors and/or unpolished reflectors.
The results of numerical simulation of polarization characteristics of terahertz signals from a ground-based remote sensing system in stratus clouds for various models of liquid-droplet clouds are compared. Models of the scattering medium take into account the vertical stratification of the water vapor concentration in the atmosphere. The model of droplet size distribution includes droplets larger than 20 μm in radius. They are referred to as large droplets, while droplets with radius 1<r<20 μm are referred to as small ones. The model of a cloud layer was developed with allowance for a difference in the vertical stratification of the mean radii of the small and large droplets.
The interaction of radiation of the Novosibirsk Free Electron Laser (FEL) at a wavelength of 130 μm in the atmospheric transmission window with a model aerosol cloud having the known droplet size distribution function has been studied experimentally. The experimental findings are compared with theoretical calculations obtained from solution of the lidar equation for the conditions of the experiment.
The paper presents results of numerical statistical simulations of experiments of ground-based sensing of cloud layers by terahertz linearly polarized radiation for certain wavelengths from the atmospheric transparency windows. Summarized results of many years’ field measurements of liquid droplet size distributions in temperate latitudes of the Earth and the distributions obtained by aircraft experiments off Great Britain’s coast are used in the scattering layer models. The models of the scattering medium take into account the vertical stratification of water vapor concentration in the atmosphere and the differences in cloud layer microstructure at the top and the base.
The interaction of high-power terahertz radiation from the Novosibirsk free-electron laser at a wavelength of 130 μm in atmospheric transparency window with a model aerosol cloud having the known droplet size distribution function has been studied experimentally for the first time. The experimental data are compared with theoretical calculations obtained from solution of the lidar equation for conditions of the experiment.
The spectral transparency method has been tested experimentally in the terahertz spectral region. The aerosol particle size distribution spectrum has been reconstructed from measurements of the spectral transparency coefficient by the method of Time-Domain THz spectroscopy. The particle size distribution spectra obtained by the spectral transparency method in the terahertz region and by the method of optical microscopy have been compared.
Time distributions of ground-based LIDAR echo-signals reflected by the bottom of a liquid droplets cloud were calculated with the help of local estimates of Monte Carlo methods for wavelengths from visible to submillimeter range. The calculations are performed for three models of clouds. In the first model the cloud layer consists only of small droplets of radius from 1 to 20 microns, in the second one there are small and large droplets of radius from 1 to 85 microns and in the third one we take into account small, large and supersize droplets. A density of a particle size distribution is generalized on results of cloud microstructure observations in temperate latitudes. Wavelength range for which one should take into account large and supersize droplets in cloud models is found.
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