DSP-based CW lidars for clouds and aerosol
This thesis aims at exploring the limits of low peak-power, low-cost, all-semiconductor, compact lidars (laser radars) for range-resolved remote sensing of atmospheric aerosols and clouds. The systems investigated are based on laser diodes for emitter, avalanche photodiodes for the receiver, and dig...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2020 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/670294 |
| Acceso en línea: | http://hdl.handle.net/10803/670294 https://dx.doi.org/10.5821/dissertation-2117-334956 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria de la telecomunicació 621.3 68 |
| Sumario: | This thesis aims at exploring the limits of low peak-power, low-cost, all-semiconductor, compact lidars (laser radars) for range-resolved remote sensing of atmospheric aerosols and clouds. The systems investigated are based on laser diodes for emitter, avalanche photodiodes for the receiver, and digital signal processors for processing the returns from the atmosphere and the system control. The studied systems are built on the use of M-sequences and other derivated ones, which allows retrieving range-dependent information from the returning echoes, while having almost the same number of ones and zeros to maximize the transmitted average power for a limited peak power. The use of digital processors to generate the modulating sequences and to process the lidar returns allows for systems with agile reconfiguration capabilities and the use of very long sequences while keeping the processing time virtually negligible compared with the time employed to transmit the laser pulse-modulated output and receive the echoes from the atmosphere (the measuring time). The theorical developments are tested and demonstrated by the experimental results obtained with two lidar prototypes designed and build according the theoretical framework. The detection of cloud bases up to 8km with transmission peak powers of 125 mW in the near (almost visible) infrared (785 nm wavelength), with spatial resolution of 70 to 100 m and time resolution from 30 seconds to 4 minutes, has been achieved. Aerosols at lower altitudes are also observed in conditions of high aerosol load. In addition to further developments in low-cost lidar systems for detections of particles suspended in the atmosphere, the technical developments of this work pave also the way to the design of low-power systems por open path, range-resolved detection of gases, for example in industrial environments. After an introductory chapter, chapter 2 reviews the basic principles of lidar systems and focuses on continuous-wave systems with pseudorandom sequence modulation of the transmitted power and the constraints set by the requirement of a low peak-power transmitter. Chapter 3 reviews different types of power-modulating digital sequences that can be used, with the constraint of average 50% duty cycle, selects the best ones to be investigated in the experimental study, and unravels some of the issues associated to possible non-linear behavior of the photoreceiver. Chapter 4 presents the theoretical performance of the system, which is compared to a model that takes into account the technical constraints and non-idealities of the system building blocks. Chapter 5 sets out the complete design of the prototypes that have been built and tested, which are presented in chapter 6. Chapter 7 presents the tests and field results obtained with the prototypes built according to the design discussed in chapter 5. Chapter 8 presents the conclusions and outlines possible lines opened by the thesis work. Two appendices (chapter 9) develop further details on the detrimental effect of non-linear behavior in the receiver chain, and practical aspects of the alignment procedure between the lidar transmitter and the receiving optics. |
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