Research
Brillouin

Remote Sensing of Temperature and Speed of Sound in the Ocean

We are working on the realization of a Brillouin LIDAR system for the remote air-plane based measurement of water temperature in the ocean.

The system will for the first time provide the capability of highly accurate remote sensing of temperature and sound velocity profiles in the ocean. There presently does not exist a remote sensing technique that can achieve the required accuracy of at least 0.1 K. This new approach will be very sensitive and large areas can be surveyed in short periods of time – a requirement vital to large scale measurement campaigns required for long term climate surveys and weather forecasting such as hurricane movements.

Artist rendition of a completed system

The goal of this project is the development of a LIDAR transceiver capable of measuring the water temperature up to a depth of 100 m in clear ocean. The basic principle used is the precise measurement of the small frequency shift of the laser radiation due to Brillouin scattering in the water.

Currently water temperatures of the ocean are measured by satellite (surface only) or by in-situ techniques such as XBTs (expandable bathythermographs), buoys or gliders.

Therefore, a remote technique would be highly desirable. In particular as the date could provide valuable input in such areas as meteology, ocean-atmosphere interaction and oceanography.

Early on there have been proposals to measure the temperature of the ocean remotely based on either Raman (Leonard) or Brillouin-scattering (J. L. Guagliardo). Practical realizations of such systems, however, failed due to principal problems or due to the absence of laser systems fulfilling the requirements in terms of linewidth and stability. Only recent developments in the area of Opens internal link in current windowinjection seeding of pulsed lasers have changed this situation (Henderson et al.(1986); Fry et al (1991)).

Exact accuracy estimates for our proposal of measuring the temperature profiles based on Brillouin scattering exist (Fry et al., 1996). First laboratory versions of our Brillouin LIDAR have been successfully demonstrated to measure the linewidth of the Brillouin lines as a function of temperature (Fry et al., 2002). Depth resolved measurements without real-time detection has been performed (Schorstein et al., 2009).

In recent developments we have demonstrated a edge filter with high transmission (Rudolf et al., 2012) and have implemnted a full laboratory version of our approach (A. Rudolf et al., 2014).

  • D. A. Leonard et al., Appl. Opt. 18, 1782 (1979)
  • J. L. Guagliardo, Dufilho, Rev. Sci. Instrum. 51, 79 (1980)
  • S. W. Henderson, E.H. Yuen, E.S. Fry, Opt. Lett. 11, 715 (1986)
  • E. S. Fry, Q. Hu, X. Li, Appl. Opt. 30, 1015 (1991)
  • E. S. Fry, Y. Emery, L. Quan, J. Katz, Appl. Opt. 36, 6887 (1996)
  • E. S. Fry, J. Katz, D. Liu, Th. Walther, J. of Mod. Opt. 49, 411 (2002)
  • K. Schorstein, A. Popescu, M. Göbel, Th. Walther; Sensors 8, 5820-5831 (2008)
  • A. Rudolf, Th. Walther; Optics Letter 37 4477-4479 (2012)
  • A. Rudolf, Th. Walther; Opt. Eng. 53(5) 051407 (2014)
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
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

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).

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

Principle – transmitter

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.

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.
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.
ESFADOF spectrum in Rb
ESFADOF spectrum in Rb

Principle – receiver

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

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
Laboratory test ocean
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.

Latest results

Transmitter System

A five-stage, frequency doubled Yb-doped fiber amplifier has been set up. For 10 ns pulses we can generate more than 0.5 mJ/pulse energies in the green spectral range.

Receiver System

Our approach for an edge filter has been successful. By the use of a modified Halbach cylinder we were able to generate an ESFADOF with the edges at the correct spacing and peak transmission up to 80%.

First temperature profile measurement in our testing setup. So far the results are not taken in real time
First temperature profile measurement in our testing setup. So far the results are not taken in real time
ESFADOF spectrum in Rb.
ESFADOF spectrum in Rb.