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

6 2 Remote Sensing of the Water Column: The Brillouin-Lidar
distance of objects from the observer. However, the employment of optical frequencies
increases the spatial resolution, but allows also to correlate other observables
with the time dependent return signal. The working principle is as follows:
(1) light-pulses are emitted and (2) their interaction with the surrounding medium
and/or targets results in a time dependent return signal; (3) by collecting and evaluating
this signal a profile of the interaction processes results [24]. By investigating
the return signal which emerges from the Brillouin– or Raman–scattering of the
injected light pulses inside the water column, an optically accessible interaction
is found. Both processes rely on the inelastic scattering of light and exhibt a clear
temperature dependance. Hence, by incorporating these processes into an active
lidar measurement scheme, a remote sensing technique results, which extracts
the temperature profile from the water column. Briefly speaking, the Ramanscattering
process probes properties on a molecular scale, such as the temperature
dependent concentration of the monomer and polymer forms of water [25, 26],
while Brillouin-scattering explores the water properties on a larg scale, such as
the temperature dependent speed of sound, which emerges from inelastic scattering
of light on permanently available density fluctuations [27–31]. Both scattering
processes result in a frequency shift of the backscattered signal, which can be exploited
as temperature tracer [32–34]:
1. Raman–scattering: The Raman–scattering imprints a large Stokes–shift of
about 3350 cm -1 with respect to the injected light pulses on the backscattered
signal [33]. This potentially shifts the backscattered signal out of the
narrow spectral absorption minimum of water (380–550 nm [18]), when employing
green laser light. This fact limits the achievable penetration depth of
such a lidar system [26, 33]. In addition, the measurement principle relies crucially
on a highly accurate spectral discrimination of the contributions from the
monomer and polymer forms of water. The latter includes also contributions
from hydrogen bonds to the rovibrational spectra of water, whereas the former
emerges from the bare water molecule. As the monomer– and the polymer–
contributions cover a large frequency span of 2800-3900 cm -1 , they can be discriminated
by selecting two different observation wavelengths. This method
is known as the two-color technique [26, 33]. But, as the Raman–spectrum is
susceptible to daylight, parasitic fluorescence and selective absorption due to
Chlorophyll or dissolved organic matter, a significant reduction in achievable
measurement accuracy results [25, 26, 33]. For the sake of completeness, it has
to be mentioned that the monomer and polymer forms of water show different
amounts of depolarization of the injected polarized laser pulses. This can be
exploited in order to determine their corresponding concentrations and hence
the water temperature [26, 33]. This approach, known as the depolarization
technique, does not require two explicit wavelength discriminations.
2. Brillouin–scattering: The Brillouin-scattering imprints a temperature dependent
frequency shift of approx. ±7–8 GHz symmetrically located around the
injected light pulses on the backscattered signal, when employing green laser
light [30]. This small frequency shift assures high penetration depths, when
operating near the absorption minimum of water [35], but also requires a