,The Florida Specifier.
WATER TEST CELL.
L A ER(8MH g or A,)
I l B""'I 1 1AMPLFER(S)1
The Laser/Heterodyn Experimental Setup would measure the frequency shift and
spectral line width of light scattered from the test volume. Water salinity and
temperature are determined from these parameters.
separates the effects of water
temperature and salinity.
Langley Research Center,
A proposed visible-light laser/
heterodyne receiver would remotely
measure the temperature and salinity
of subsurface water. ts operation is
based on the acoustioptical scatter-
ing of light by sound waves.
The thermal motion of water
molecules gives rise to high-fre-
quency sound waves, which interact
with light through the mechanism of
Brillouin scatering. Monochromatic
laser light scattered by the water un-
dergoes a frequency shiM proportion
to the temperature and salinity the
scattering volume. The frequency
shifts are in the megahertz-to-
gigahertz range and fall within the re-
sponse bandwidth of heterodyne
ture marine remotesensingpro-
grams. The concept is also applicable
to environmental monitoring systems
since water temperature and salinity
greatly affect aquatic life.
This wort was done by Daniel J.
Jobson, Carl L Faes, and Slephen J.
Katrberg of Langley Research
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rodyne receiver wou remotely
re the temperature alinity
of su rface water. Its oper is
based the acoustic/optical er-
ing of lig y sound waves.
The f al motion of wat
molecules Qes rise to high-fre-
quency sou waves, which interact
with light thro the mechanism of
Brillouin scatterin. Monochromatic
laser light scatter the water un-
dergoes a frequency proportional
to the temperature alinity of the
scattering volume. frequency
shifts are in the -to-
gigahertz rage and fall the e-
sponse bandwidth of he
rmceivers. -------\ --
As shown in the figure, light scat-
tered by the water is combined with
the incident laser light through beam-
splitting optics, and the combination is
incident on a photomixer. The photo-
mixer signals are processed through
amplifier/filter/square-law detector cir-
cuitry and fed to a recorder that dis-
plays an intensity-vs.-frequency
graph. This graph is the spectrum of
the Brillouin scattering. The frequency
shift and spectral line width indicate
the temperature and salinity of the
The new technique departs from
eardir techniques in that it is based on
Brillouin scattering, alone and uses a
measurement of the line width of the
Brillouin line in addition to the amount
of frequency shift of the Brillouin line
unambiguously to 'separate tempera-
ture and salinity effects. The high
special resolution required for the fre-
quency shift and line width measure-
ments is best obtained by using
heterodyne technology. The spatial
resolution and depth information on
salinity and temperature obtained by
optical heterodyne detection are po-
tentialy superior to those obtained by
current mote-sensing techniques.
These improvements are inherent to
the wavelength of visible light and the
heterodyne measuremment process,.
where angular fields of view are set by
the coherence property of light.
Application of this concept is fore-
seen .in current research on energy
conversion from ocean currents pro-
duced by thermal gradients and on fu-
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