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

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Conclusion and Outlook
The present work is dedicated to the development and the thorough assessment of
Rubidium based excited state Faraday anomalous dispersion optical filters, which
allow, when operating them as edge–filter receiver within a Brillouin–lidar remote
sensing application, an area wide measurement of the temperature profile of the
oceanic water column.
Chapter 2 discusses the working principle and the system requirements of this
mobile remote sensing application. Emphasis is put on the Brillouin–scattering
and in particular on the temperature and salinity dependence of its spectral profile,
which is crucial for the implementation of the proposed lidar remote sensing
technique. The working principle is based on a system of equations (cf.
Eqs. 2.9, 2.12, 2.13, 2.23), whose known temperature and salinity dependence allows
the deduction of both parameters by the measurement of the Brillouin–shift
ν B and the Brillouin–width δν B . The latter can be extracted from the spectral
profile of the Brillouin–backscatter. Briefly speaking, a temperature range of 0 ◦ C
to 40 ◦ C has to be resolved and salinities between 30 ‰ and 40 ‰ have to be expected.
These values translate into a Brillouin–shift of ±6.8−7.8 GHz, when employing
green laser pulses at a wavelength of 543 nm. The Brillouin–width varies
between 1.66 GHz and 0.39 GHz for the same temperature range and wavelength.
However, as the measurement of the Brillouin–width δν B within a mobile remote
sensing application is extremely challenging, the lack of this observable has to be
compensated by the knowledge of the salinity parameter. Direct measurements of
oceanic salinities are mostly restricted to fixed locations and hence not applicable
for a remote sensing system. As Sec. 2.2.2 discusses and as extensive studies of
Fry et al. show, the salinity parameter can be procured by relying on historical
data, while including local and seasonal variations. As this data delivers the salinity
parameter with an uncertainty of 1 ‰, a temperature accuracy of 0.5 ◦ C can
be achieved. An increase of the knowledge of the salinity parameter, improves
the temperature deduction. The precise knowledge of the salinity considerably
decreases the temperature uncertainty by one order of magnitude to 0.06 ◦ C, when
the Brillouin–shift is known with an accuracy of 1 MHz [72].
In addition, the discussion of Sec. 2.2.3 revealed that the temperature and
salinity dependence of the Brillouin–width has to be included for a thorough assessment
of the detection system. This is particularly important, when employ-