Remote Sensing of Temperature and Speed of Sound in the Ocean


The project is based on the Brillouin shift of light scattered in water. The frequency shift is due to the Doppler shift of light scattered by spontaneous density fluctuations in the water. Hence, it is dependent on the sound velocity, which itself is a function of index of refraction, salinity and temperature. The index of refraction is, of course, also a (well-known) function of wavelength, salinity and temperature. Using historical data for salinity and known functional dependences for sound velocity and index of refraction, it ispossible to extract the temperature from the measurement of the Brillouin shift (Fry <em>et al.</em>, 1996).

The Brilloun shift can be explained as scattering from sound waves (density fluctuations) in the water; in the experiment the direction of the incoming and scattered light is 180o (left);
schematic representation of a Brillouin spectrum of the back scattered light; the central peak is due to hydrosols only. The typical Brillouin shift for ocean water is in the order of 7.5 GHz (middle); laboratory Brillouin spectrum as measured with a scanning etalon (right).


The Ramp-Fire Technique. Cavity Setup. The photo diode detects an interference pattern generated by the seed radiation which ''probes'' the cavity as the cavity length is ramped by means of the piezo element.
Signal detected by the photo diode while the piezo is ramped. The Q-switch is fired when a maximum in the signal is detected.

Techniques developed by Dr. Fry (ramp and fire technique) make it possible to build powerful Nd:YAG lasers which produce Fourier transform limited pulses even in acoustically and mechanically noisy environments - a requirement essential to practical applications.

An alternative approach is the use of a Opens internal link in current windowfiber amplifier. This is the path that we are following.


The challenge is to devise a detection scheme which fulfills three simultaneous requirements: it must be (1) capable of resolving the small Brillouin frequency shifts, (2) fast in order to provide the time resolution necessary to measure the depth profiles of the temperature and (3) rugged as it should be used in acoustically noisy environments.

Ideally suited are edge filters based on the absorption lines of molecules such as two isotopes of the iodine molecule or FADOF filters (see below).

Principle of an Edge Filter. These are relatively insensitive to small frequency drifts of the laser.

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Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834 (Fax)




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