Coherent Differential Absorption Lidar for Combined Measurement of Wind and Trace Atmospheric Gases
Date of Award
Doctor of Philosophy (PhD)
Electrical & Computer Engineering
Amin N. Dharamsi
Karl H. Schoenbach
A lidar system was developed for making combined range-resolved measurements of wind speed and direction, water vapor concentration, and carbon dioxide concentration in the atmosphere. This lidar combines the coherent Doppler technique for wind detection and the differential absorption lidar (DIAL) technique to provide a multifunctional capability. DIAL and coherent lidars have traditionally been thought of and implemented as separate instruments, but the research reported here has shown a demonstration of combining the coherent and DIAL techniques into a single instrument using solid-state lasers. The lasers used are of Ho:Tm:YLF, which operates at a wavelength of 2 μm. This wavelength is a further advantage to the lidar, as this wavelength offers a much higher level of eyesafety than shorter wavelengths conventionally used for DIAL.
Two generations are lidars are described, with the first design making combined measurement of wind and water vapor. Wind speed measurements are shown of a precision better than 1 m/s, making it useful for many meteorological applications. Water vapor concentration measurements were of 86% accuracy, requiring improvement for scientific applications. This preliminary experiment revealed the largest source of error in concentration measurement to be a lack of stability in the wavelength of the laser. This problem was solved by implementing a means to precisely control the continuous-wave laser that injection seeds a pulsed laser. The finely tunable Ho:Tm:YLF laser was stabilized to absorption lines of both carbon dioxide and water vapor using a wavelength modulation technique. Long-term stabilization to within 13.5 MHz of absorption line center is shown, representing the first frequency-stabilized laser at or within 500 run of 2μm wavelength. Results are presented on injection seeding a pulsed Ho:Tm:YLF laser to impart the tunability and stabilization to the pulsed laser output.
The stabilized laser system was incorporated into a second-generation coherent DIAL to make a combined measurement of wind and carbon dioxide concentration. The DIAL measurement accuracy of concentration was improved to 29%, and designs are suggested for a further reduction in error. The absorption lines around 2-μm have recently become of great interest for a high-accuracy measurements of carbon dioxide for studies in the global carbon cycle, and the lidar demonstration and laser technology presented here are enabling first steps to meet scientific needs for carbon dioxide profiling.
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Koch, Grady J..
"Coherent Differential Absorption Lidar for Combined Measurement of Wind and Trace Atmospheric Gases"
(2001). Doctor of Philosophy (PhD), Dissertation, Electrical & Computer Engineering, Old Dominion University, DOI: 10.25777/091s-3t36
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