Research
Brillouin LIDAR

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 simultaneous and depth-resolved measurement of water temperature and salinity in the ocean.

The system will for the first time provide the capability of highly accurate remote sensing of temperature, salinity 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 since the first few hundred meters of the ocean are in constant heat and CO2 exchange with the atmosphere. 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 data could provide valuable input in such areas as meteorology, 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 developments in the area of injection 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).

We have demonstrated an edge filter with high transmission (Rudolf et al., 2012) and have implemented a full laboratory version of our approach with a temperature resolution of 0.07°C (A. Rudolf et al., 2014). A first field test in the Mediterranean sea demonstrated the suitability of the system in a realistic environment. With this knowledge further improvements were made on the system, making it less sensitive to outer disruptions.

Furthermore an empirical relation between the spectral range of the Brillouin-scattering and the temperature and salinity of ocean water was developed which now enables the system to simultaneously measure the ocean temperature and salinity.

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 is possible to extract the temperature from the measurement of the Brillouin shift (Fry et al., 1996).

An alternative approach is the use of a fiber amplifier. This is the path that we are following.

The system consists of mainly three parts, the source for the pulse laser light, the receiving optics for the scattered light and the detector unit. The infrared light of a DFB (distributed feedback laser) in a MOFA (master oscillator fiber amplifier with two Yb fibres) setup is cut in transform limited ns-pulses by an AOM. The pulsed laser light is then amplified in four consecutive fiber amplifier stages. The infrared light is frequency doubled to reach the desired wavelength of 543nm. This laser light can be used to obtain the Brillouin scattering in a probe chamber. The backscattered light is guided through a mirror telescope and then detected. The detection is performed by an edge filter, the so called ESFADOF (excited state Faraday anomalous dispersion optical filter) to detect only the desired Brillouin-scattered light.

Next steps

  • Complete splicing of the seed source and more parts of the system making it less sensitive to outer disruptions
  • Second ESFADOF detector unit
  • Demonstration of the simultaneous measurement of the ocean temperature and salinity in the lab as well as in a field test
Schematic setup of the whole system of the Brillouin-LIDAR system
Schematic setup of the whole system of the Brillouin-LIDAR system
Experimental setup to determine the spectral width of the Brillouin-scattering
Experimental setup to determine the spectral width of the Brillouin-scattering

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 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

Generally speaking, a Faraday anomalous dispersion optical filter (FADOF) consists of an atomic vapor cell placed between two crossed polarizers. An homogenous, constant magnetic field of the strength B is applied parallel to the optical axis of the gas cell. Two effects govern the transmission of such a filter: the absorption of the atomic vapor and the polarization rotation of the incident light. The polarization rotation originates from the Zeeman splitting of the atomic transitions, which causes a difference in index of refraction for right and left circular polarized light. The latter effect is similar to the regular Faraday effect, but is much enhanced in the vicinity of absorption lines(1). The degree of rotation is strongly wavelength dependent and thus the crossed polarizers in combination with the gas cell act as sensitive transmission filters(2).

FADOF systems have been considered in the literature mostly as daylight suppression filters for use in communication(3). Since off resonant light is blocked by crossed polarizers FADOFs typically show passbands of a few GHz only and a high out-of-band suppression(3). Other advantages of FADOF systems include a large field of view and their relative insensitivity to vibrations. A possible disadvantage is the restriction to the vicinity of atomic transitions. However, FADOF systems of excited states have been implemented(4,5). These works are of particular interests as some of the alkali have excited state transitions within the range of frequency doubled Nd:YLF or Nd:YAG lasers.

In remote sensing applications such as in our Brillouin project often small frequency shifts must be detected. An established method for this task is the use of edge filters. The basic idea is to provide steep transmission edges of the filter in the regions of interest. Small shifts in the frequency of the signal will thus yield a large change in transmission. After calibration of the system, the determination of the frequency shift is therefore transfered to an intensity measurement.

Currently we are evaluating the potential of FADOF systems as frequency analyzing tools for our Opens internal link in current windowBrillouin project. By setting the temperature of the gas cell and appropriately tuning the magnetic field FADOF systems are capable of producing steep transition edges at the desired frequency separation. The first step was to set up a FADOF experiment, work out the theory and compare the experiment with the theoretical prediction. The agreement between theory and experiment is excellent. Therefore, we are now working on implementing an excited state FADOF (ESFADOF) system in order to evaluate the FADOF's potential as an edge filter in the 532-nm wavelength range.

References

  1. D. Macaluso and O. Corbino, C.R. Acad. Sci. 127, 548 (1898).
  2. Y. Ohman, Stockholm Obs. Ann. 19, 3 (1956).
  3. D. Dick and T. Shay, Opt. Lett. 16, 867 (1991).
  4. T. Shay, in Proceedings of the IEEE Lasers and Electro-Optical Society's Annual Meeting (1993), pp. 359-360.
  5. R. Billmers, S. Gayen, M. Squicciarini, V. Contarino, W. Scharpf, and D. Alloca, Opt. Lett. 20, 106 (1995).
  6. C. Korb, B. Gentry, and C. Weng, Appl. Opt. 31, 4202 (1992).
ESFADOF spectrum in Rb.
ESFADOF spectrum in Rb.
Laboratory test ocean
Laboratory test ocean
  • Complete splicing of the seed source and more parts of the system making it less sensitive to outer disruptions
  • Second ESFADOF Detector unit
  • Demonstration of the simultaneous measurement of the ocean temperature and salinity in the lab as well as in a field test
The emperical relation between the spectral width of the Brillouin-scattering and the salinity.
The emperical relation between the spectral width of the Brillouin-scattering and the salinity.

Recently we were able to show an empirical correlation between the spectral width of the Brillouin-scattering and the salinity of the water. This will enable us to measure not only the temperature but simultaneous the salinity of the ocean water.

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

  • Daniel Koestel, Thomas Walther
    The Brillouin linewidth in water as a function of temperature
    and salinity: the missing empirical relationship
    Applied Physics B (2024) 130:53 (opens in new tab)
  • A. Rudolf, Th. Walther; Opt. Eng. 53(5) 051407 (2014)
  • A. Rudolf, Th. Walther; Optics Letter 37 4477-4479 (2012)
  • K. Schorstein, A. Popescu, M. Göbel, Th. Walther; Sensors 8, 5820-5831 (2008)
  • E. S. Fry, J. Katz, D. Liu, Th. Walther, J. of Mod. Opt. 49, 411 (2002)
  • E. S. Fry, Y. Emery, L. Quan, J. Katz, Appl. Opt. 36, 6887 (1996)
  • E. S. Fry, Q. Hu, X. Li, Appl. Opt. 30, 1015 (1991)
  • S. W. Henderson, E.H. Yuen, E.S. Fry, Opt. Lett. 11, 715 (1986)
  • J. L. Guagliardo, Dufilho, Rev. Sci. Instrum. 51, 79 (1980)
  • D. A. Leonard et al., Appl. Opt. 18, 1782 (1979)