Song, Changbo (2017) Development of the Spectral Analysis System for a Spaceborne High Spectral Resolution Lidar. [Tesi di dottorato]

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Aerosol particles are an important component of the atmosphere. Most of the aerosol mass suspended in the atmosphere resides within the PBL, which is the atmospheric layer directly above the ground. Atmospheric aerosols affect air quality and, in turn, human and ecosystem well-being (WHO, 2013a), and they also play an important role in the Earth’s climate system (IPCC, 2013). In fact, PM (Particles Mater) pollution is probably the most urgent issue in air quality regulation worldwide, and at the same time it represents one of the biggest sources of uncertainty in current climate simulations. Therefore, vertically resolved measurements of physical and optical properties of aerosol particles are of great interest, and height-resolved observations of these parameters can only be carried out with lidar techniques. The lidar technique has proved to be effective to measure the vertical profile of aerosol optical properties with high vertical and temporal resolution. Spaceborne lidars are capable of mapping vertical distributions of aerosol over globe spatial regions in a short amount of time. For existing spaceborne lidars, such as Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), an assumption of aerosol extinction-to-backscatter ratio is needed to retrieve aerosol optical properties. To measure the vertical profile of aerosol extinction without assumptions of the aerosol extinction-to-backscatter ratio, High Spectral Resolution Lidar (HSRL) technique has been employed due to the advantage of day and night measurements compared to Raman lidar. A spectral analysis system developed for a spaceborne HSRL has been implemented and is presented in this thesis. The spectral analysis system is based on the combination of an interference filter, a planar Fabry-Perot interferometer (PFPI) background filter, and a confocal Fabry-Perot interferometer (CFPI) high spectral resolution filter. By the comparison of a molecular absorption filter and three kinds of interferometer filters, the CFPI has been adopted for the high spectral resolution filter of the spaceborne HSRL. Unlike gas absorption cells, CFPI has no problem of leak and vapor formation, which would make the gas absorption cells useless. Compared to other interferometers, the CFPI can provide high spectral resolution and a large étendue simultaneously, and is much less sensitive to alignment errors and vibrations. A frequency-locking subsystem is used to lock the center transmission wavelength of the spectral analysis system with the wavelength of the emitted laser. The developed two-stage frequency-locking technique is a novel technique and can be used whether the locking laser is a pulsed laser or a continuous laser. The frequency-locking subsystem of the spectral analysis system has been designed and realized. The tests show that it is a robust apparatus, with very good stability. The parameter requirements of the spectral analysis system have been obtained by a simulation of the spaceborne HSRL. All the components of the spectral analysis system have been designed and realized. After being assembled, the spectral analysis system has been tested by using a narrow linewith continuous laser. The test results show that transmission and reflection at the wavelength of 532nm are close to the theoretical value, when the central wavelength of the spectral analysis system is locked to the wavelength of the laser source by the developed frequency-locking system. The error sources that affect the accuracy of aerosol optical properties are analyzed. The results show that the detector noise is the dominant source of error. Further analysis also shows that the relative error of the retrieved aerosol and molecular signals are more sensitive to the error of the CFPI peak transmission than to the error of the CFPI bandwidth.

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