Aiming at the detection of benzene concentration in the atmosphere, a power-modulation scheme with laser wavenumber tuning is proposed in mid-infrared integrated-path differential absorption (IPDA) lidar. Based on the differential absorption lidar (DIAL) theory, we design the configuration of power-modulation IPDA lidar. Its central measurement and reference wavenumber are selected as 3090.3cm-1 and 3137.7cm-1, and the power-modulation scheme of IPDA lidar is proposed through interband cascade laser (ICL) current-driving characteristic. The simulation model of power-modulation mid-infrared IPDA lidar is constructed, and then its theoretical analysis is carried out. Furthermore, the retrieval algorithm for benzene concentration is presented, and then the concentration precision as 1/f noise with different frequencies is analyzed with FFT retrievals of 1000. The detection performance of lidar such as signal-to-noise ratio (SNR) and relative error for path length and visibility is compared between direct current (DC) driving and altering current (AC) driving, i.e. power-modulation scheme. The results show that the uncertainty of concentration is consistent with the effect of 1/f noise, that the SNR increases with the increase of visibility or the decrease of path length and the relative error decreases with the increase of visibility or the decrease of path length, and that the SNR will increase with the increase in frequency, and the relative error will decrease under the same path length and visibility. Therefore, the power-modulation mid-infrared IPDA lidar can be more effective for probing benzene concentration.
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.
Salinity is one of the important physical parameters in oceanography. Current methods for remote sensing of ocean salinity are based on the monotropic function between Raman spectra and ocean salinity, while the effect of the ocean temperature, which influences the measurement of salinity, is neglected. Therefore, this paper proposes a method for fine inversion of salinity combining Brillouin and Raman spectra of water. The Raman spectra of solutions containing a single solute and mixed solutions were detected, and the effect of solutions containing different solutes on the spectra was analyzed. The experimental results revealed the variation in the low- and high-frequency spectral intensities of the Raman spectra with salinity and temperature. The Raman spectra of seawater were modeled as a function of temperature and salinity using the low- and high-frequency area ratios. Brillouin scattering spectra are also related to ocean temperature and salinity, changes in temperature and salinity will affect the frequency shift and line-width of Brillouin scattering spectra. Based on the Brillouin scattering theory, the relationship between frequency shift and line-width with ocean temperature and salinity was analyzed. With high spectra resolution lidar as the detection technique, the Brillouin frequency shift, line-width and Raman spectra of ocean could be detected simultaneously by using multi-beam interferometry, photon correlation spectroscopy, and Raman spectra detection techniques. A high precision inversion model of ocean salinity can be obtained by using a multi-source data fusion. This research will provide reliable data for the study of global climates and ecosystems and improve the accuracy of marine disaster warnings and marine weather forecasting.
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.
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