Assessment of a Rubidium ESFADOF Edge-Filter as Receiver for a Brillouin-Lidar Capable of Remotely Measuring Oceanic Temperature Profiles

• Main Author: Popescu, Alexandru Lucian
• Format: Others
• Language: English; en
• Published: 2010
• Online Access: http://tuprints.ulb.tu-darmstadt.de/2122/1/PhD_Thesis_Popescu-Final.pdf; Popescu, Alexandru Lucian <http://tuprints.ulb.tu-darmstadt.de/view/person/Popescu=3AAlexandru_Lucian=3A=3A.html> : Assessment of a Rubidium ESFADOF Edge-Filter as Receiver for a Brillouin-Lidar Capable of Remotely Measuring Oceanic Temperature Profiles. Technische Universität, Darmstadt [Ph.D. Thesis], (2010)

Global and local climate changes affect nature and mankind. Forecasts of these processes on global and local scales rely on a thorough understanding of the underlying physics through accurate data. In this context, the knowledge of the temperature profile of the upper-ocean mixed layer is relevant in oceanography, weather forecasts and climate studies and can be correlated to other parameters, such as concentrations of nutrients, oxygen and CO2. Currently, only in-situ techniques are available, such that a remote sensing application for the measurement of oceanic temperature profiles is highly desirable. Such a system would deliver accurate, cost effective and area wide data, which could be used to improve current models and forecasts within many domains of oceanography and climatology. However, only recent progress in laser and receiver technology made a remote sensing solution feasible. When employing the lidar principle, an airborne compatible system based on the detection of the temperature dependent frequency shift of the Brillouin-scattering becomes feasible. Laser pulses are fired into the ocean and the Brillouin-scattering imprints the temperature information on the backscattered light. An appropriate detector on board an aircraft extracts the temperature and correlates it to the time of flight of the laser pulses. As a result, a three-dimensional temperature profile of the upper-ocean mixed layer is extracted. Measuring the very small frequency shift of the Brillouin-scattering is the main challenge of this project. The shift varies from 6.8~GHz-7.8 GHz for water temperatures between 0°C and 40°C, when injecting laser pulses at a wavelength of 543 nm. The employment of spectrally narrow edge-filters converts the frequency measurement into an intensity measurement. As compact, robust and light weight devices, these filters are in particular suited for an airborne implementation. This work demonstrates that Excited State Faraday Anomalous Dispersion Optical Filters (ESFADOFs) are such high resolution optical edge-filters. They deliver the desired edge-filter characteristics, when operated around the Rubidium 5P3/2 -> 5D5/2 transition (543~nm), and transmission changes of up to 24% within a few GHz were demonstrated. In addition, fundamental investigations of the ESFADOF transmissions are presented. They result in distinct operational limits, due to radiation trapping, energy-pooling and plasma formation. Together with the scalability of these devices, their implementation as the Brillouin-lidar detector is addressed.