Aiming at the real-time detection of toluene concentration in the atmosphere, an integrated-path differential absorption (IPDA) lidar is proposed based on inter-band cascade lasers. Since the C-H bond of toluene has a slowly-changing absorption spectrum in the mid-infrared band, this IPDA lidar is designed using 2935.5cm-1 and 3192.0cm-1 as the operating wavelength by considering the influence of the main interfering gases such as H2O, CH4, and HCl. A spectroscopic system with a mid-infrared diffraction grating is configured to realize synchronous detection of dual-wavelength received signals. A retrieval algorithm and its error analysis model for atmospheric toluene concentration are presented. And then the performance of lidar is analyzed and discussed under the conditions of different visibilities, path lengths, and water vapor concentrations by combining with the mid-latitude standard atmospheric model. These results show that the relative error of toluene concentration is less than 10% within the concentration range of 20ppb to 10ppm under the condition of atmospheric visibility of 5km, path length of 1.6km, and the water vapor concentration of less than 0.4%. This IPDA lidar can provide an effective scheme for real-time detection of atmospheric toluene concentration.
Owing to the enhancement effect of backscattering for laser propagation through turbulent atmosphere, a comparison between theoretical analysis and model simulation based on random phase screen is conducted, and then a preliminary experiment observation is carried out for the enhanced backscattering effect for laser propagation in turbulence atmospheric. Firstly, from the generalized Huygens-Fresnel principle and the mutual interference effect, the light intensity expression and the backscattering enhancement coefficient are derived for the backscattering enhancement effect of laser propagation through turbulence atmospheric to the reflecting screen. The backscattering enhancement coefficient based on theoretical analysis increases as laser transmission distance increases, whereas it decreases to 1 with the increase of the distance between receiver and transmitter. Furthermore, random phase screens are respectively established for turbulence through the power spectrum inversion method, Zernike polynomial method and low-high frequency combination method and are compared with the theoretical values. Then the laser propagation through turbulent atmosphere is simulated to verify that the laser propagation through turbulence atmospheric possesses backscattering enhancement effect, and the influence of the factors on the backscattering enhancement effect is analyzed. Simulations show that there is a saturation effect of the enhanced backscattering, and its value is approximately 1.4. While the atmospheric refractive index structure constant is 10 -13, the enhanced backscattering effect gradually reach saturation at the laser propagating to 600m. Whereas the enhanced backscattering effect gradually reach saturation with the atmospheric refractive index structure constant of 10 -14 at the distance of 1500m. The enhanced backscattering effect of laser propagation through turbulent atmosphere is preliminarily observed through an experimental system. The results show that the backscattering enhancement effect is positively correlated with the change of the wind speed in the area where the laser passes through, decreases as the temperature drop, and increases with the increase of the laser propagating distance, and that the backscattering enhancement coefficient is less than 1.4.
High-spectral-resolution-lidar (HSRL) can realize the retrieval with high accuracy of aerosol optical properties without the assumption of atmospheric conditions, in which the key technique is to construct a spectral discriminator for separating aerosol Mie scattering signals and molecular Rayleigh scattering signals. Both Mach-Zehnder interferometer and Fabry-Perot interferometer have been developed and applied in the HSRLs, as they can provide the fine spectral structure in signal separation. In this paper, we make the performance comparison between the Mach-Zehnder interferometer and Fabry-Perot interferometer from the points of view of single-frequency HSRL and multi-longitudinalmode HSRL. The simulation results show that the design parameters of Mach-Zehnder interferometer and Fabry-Perot interferometer are different as the spectral discriminator in the design of HSRL to separate aerosol Mie scattering signals and molecular Rayleigh scattering signals. Meanwhile, to match the free spectral range of optical interferometer with the laser longitudinal mode interval in the multi-longitudinal-mode HSRL, Mach-Zehnder interferometer is a better choice compared with Fabry-Perot interferometer, although both can provide the periodic spectral transmittance functions.
High-spectral-resolution lidar plays an important role in the measurement of aerosol distributions and variations for understanding the influence on the local environment. The multi-mode high-spectral-resolution lidar is a new type of high-spectral-resolution lidar for fine detection of aerosol optical properties, which uses the multilongitudinal-mode pulsed laser rather than the single longitudinal mode laser. Considering the Mie-Rayleigh scattering signals excited by the multi-longitudinal-mode laser have the periodic characteristics, the tunable MachZehnder interferometer is selected as the optical discriminator in the construction of multi-mode high-spectralresolution lidar. In order to minimize the effect of divergence angle on the transmittance function, a filedcompensated Mach-Zehnder interferometer is designed, which has the complementary outputs to realize the transmission and suppression of Mie scattering signals respectively. The optimal optical path difference of the MachZehnder interferometer should be twice of the cavity length of the Nd:YAG pulsed laser. The capability of the multimode high-spectral-resolution-lidar has been verified using the standard atmospheric model, the simulation results show that the proposed system can achieve the accurate measurement of aerosol optical properties up to 10km.
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