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www.lasos.com
Compact housing
Exceptional high coherence length
Made for spectroscopy, holography, metrology
S
single frequency operation
640 nm
up to 200 m W
February 10-12, 2015
The Moscone Center,
San Francisco, CA, USA
Come visit us at booth
No. 5327
See us at Photonics West, Booth 212
sensing methods presented here are based on light scattering
and optical time-domain reflectometer (OTDR) technology.
The three types of light scattering used in distributed sens-
ing applications are Rayleigh, Brillouin, and Raman scattering. Figure 1 summarizes the three different scattering effects.
Each type of scattering phenomenon offers advantages and
disadvantages for distributed sensing:
•;Rayleigh;scattering is elastic, that is, the scattered photon
has the same energy as the signal photon, and therefore occurs at the same wavelength as the light signal. Of the three
scattering effects, Rayleigh provides the strongest scattering absorption signal. Most DAS systems, where the sig-
nal-to-noise ratio (SNR) is very important, are based on
Rayleigh scattering.
•;Brillouin;scattering is an inelastic scattering process in-
volving the interaction of photons with acoustic phonons.
In Brillouin scattering, the intensity and frequency of the
Stokes (red-shifted) and Anti-Stokes (blue-shifted) signals
depend on temperature and strain. The wavelengths of the
Brillouin Stokes and Anti-Stokes peaks occur close to the in-
put signal, so sophisticated filtering methods are needed to
filter the Rayleigh signal and improve SNR. Since Brillouin
scattering is both temperature- and strain-dependent, most
DTSS systems are based on Brillouin scattering.
•;Raman;scattering is an inelastic scattering process involving the interaction of photons with molecular vibrational
and rotational transitions. The intensity ratio of the Raman
Stokes (temperature-insensitive) to the Anti-Stokes (temper-
ature-dependent) peaks depends on temperature. The large
wavelength separation between the Raman Stokes and Anti-
Stokes peaks makes it easy to filter the Rayleigh scatter-
ing and achieve good SNR. Because of the ease of filtering
noise and the insensitivity to strain, most DTS systems are
based on Raman scattering. However, the signal intensity
from Raman scattering is the weakest of the three scattering processes. To increase the Raman signal intensity, it is
necessary to use higher-input pulse signals, and therefore
multimode fibers are preferred over single-mode fibers for
Raman-based DTS systems.
In all three distributed sensing technologies, the system con-
figuration is similar. A laser source is used to launch a pulse
of signal light into the sensing optical fiber. As the light pulse
travels along the length of the fiber, a small amount of the signal is backscattered by Rayleigh, Brillouin, and/or Raman scattering effects. The backscattered light is filtered for the specific
wavelengths and is analyzed to determine the temperature and/
or strain. A spectrum of the backscattered light can be generated along the entire length of the fiber and, based on the time
