Assessment of a Rubidium ESFADOF Edge-Filter as ... - tuprints

Abstract
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 CO 2 . 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 5P 3/2 → 8D 5/2 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.