Remote Sensing of Temperature and Speed of Sound in the Ocean

Present Status

Preliminary measurements of the Brillouin scattering as a function of salinity and temperature using an etalon as the frequency analysing tool have been successfully performed. These measurements were performed using the bulk Nd:YAG system at Texas A&M as well as the fiber amplifier at TU Darmstadt.

Using the bulk Nd:YAG system we analyzed the linewidth of the Brillouin scattered contributions as a function of temperature and salinity. A prototype sensor in the laboratory has been set up and is calibrated using iodine transitions as molecular edge filters.

Using our fiber amplifier and a Fabry-Perot etalon we have measured Brilloiun spectra as a function of temperature and have, for the first time, measured a temperature profile in the laboratory using our test setup, which consists of two chambers mounted horizontally and held at different temperatures.

In the next step we have combined our ESFADOF setup as receiver with the fiber amplifier. We were able to detect a temperature profile in our test setup using this system.

The next steps are:

increasing the sensitivity of the system
ruggedize the laser setup
field test

Current Investigations

Photograph of our laboratory test ocean.

Within the Graduiertenkolleg 1114, funded by the DFG we are working towards a system enabling a first field test:

developing a compact, high repetition rate laser system for the transmitter
We are investigating the possibilities of a pulsed fiber amplifier as a replacement for the bulk Nd:YAG laser. Advantages are higher flexibility in terms of wavelength, which is desirable for the FADOF (see below) and weight. A disadvantage is the lower output power which is compensated for by the higher repetition rate.
FADOF system as an edge filter
We are making rapid progress towards the implementation of an ESFADOF (Excited State Faraday Anomalous Dispersion Optical Filter) as a robust, sensitive edge-filter.
Making first time resolved measurements
We set up a 5m long test ocean in our laboratory which is long enough for time resolved measurements. The tank is segmented such that temperature and salinity gradients can be applied. First temperature profile measurements have been successful.