We report a Brillouin-based fully distributed and dynamic monitoring of a strain wave, propagating at a speed of
~4km/sec in a 6m long fiber. Using an audio speaker, a mechanical impulse was introduced to one end of a 6m long
optical fiber, initiating a strain wave propagating towards the other fixed end of the fiber. Employing a simplified
version of the Slope-Assisted Brillouin Optical Time Domain Analysis (SA-BOTDA) technique, the whole length of the
fiber was interrogated every 1μs (before averaging). A dynamic spatially and temporally continuous map of the strain was obtained, from which the wave velocity could be deduced. With a trade-off among sampling rate, range and signal to noise ratio, kHz sampling rates and hundreds of meters of range can be covered with spatial resolutions down to a few centimeters.
We report a 10cm spatial resolution in a Brillouin-based distributed sensing system using two simultaneously launched
gain and loss pump pulses, having slightly different durations. Post-recording subtraction of the probe signal, excited by
the shorter pulse, from the corresponding one, obtained from the longer pump pulse, is no longer required, since it is
done automatically by the overlapping parts of the gain and loss pump pulses. Using a 30ns gain pump pulse and a 29ns
loss pump pulse we were able to improve upon previously published results, achieving a distributed strain measurement
along a standard single mode optical fiber with a spatial resolution of ~10cm. This technique does not broaden the
involved Brillouin gain spectra so that the strain/temperature sensitivity is not compromised.
We present two recently developed new methods for fast and distributed strain/temperature sensing in optical fibers,
based on Brillouin optical time domain analysis (BOTDA). Both methods make use of very fast (<1ns) and controlled
tuning of the optical frequency of a light wave. In the Fast BOTDA (F-BOTDA) method the complete Brillouin gain
spectrum is scanned, while in the Slope-Assisted technique (SA-BOTDA) only a single frequency point on the slope of
the Brillouin gain spectrum is probed. A sensing speed of a few hundred Hz is experimentally demonstrated, limited only
by the fiber length and the need for averaging.
The performance of systems employing Brillouin Dynamic Gratings may suffer from noise generated by the spontaneous
Brillouin processes. It is experimentally demonstrated that spontaneous emissions, accompanying the signal of interest,
contain not only the probe spontaneous Brillouin backscattering but also a significant contribution from the co-propagating
writing pump. For a strong enough writing pump and even moderate probe power levels, the observed noise
is dominated by the pump, exhibiting an average backscattered power more than 30 dB stronger than that of the probe
Employing stimulated Brillouin scattering (SBS), we present a novel method for the quasi-simultaneous distributed
measurement of dynamic strain along an entire Brillouin-inhomogeneous optical fiber. Following classical mapping of
the temporally slowly varying Brillouin gain spectrum (BGS) along the fiber, we use a specially synthesized and
adaptable probe wave to always work on the slope of the local BGS, allowing a single pump pulse to sample fast strain
variations along the entire fiber. Strain vibrations of tens of Hertz and up to 2KHz are demonstrated, simultaneously (i.e.,
using the same pump pulse) measured on two different segments of the fiber, having different static Brillouin shifts.