Energy transfer between the interacting waves in a distributed Brillouin sensor can result in a distorted measurement of
the local Brillouin gain spectrum, leading to systematic error. We demonstrate here that this behavior can be fully and
precisely modeled, and an excellent quantitative agreement is found with experimental tests. Strict guidelines can be
enunciated from this description to make the impact of depletion negligible, for any type and any length of fiber.
The impact of Raman scattering and modulation instability is studied in Brillouin time-domain analysis systems. It turns
out to be very detrimental for long-range sensing as a result of the extended interaction length combined to the high
pump peak pulse power. The conditions under which these effects limit the sensing range are determined and the
modeling is very well confirmed by experimental results.
The energy transfer between the two interacting optical waves in a distributed sensor based on stimulated Brillouin
scattering can lead to a non-uniform spectral distribution of the pumping power after a long propagation. This results in a
spectrally distorted gain that biases the determination of the maximum gain frequency. A quantitative analytical model
gives an expression for the tolerable pump power change keeping the maximum bias within a given accuracy.
Optical fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability
for long-range distributed strain and temperature measurements. The fibre is used as sensing element and a value for
temperature and/or strain can be obtained from any point along the fibre. While classical configurations have practically
a spatial resolution limited by the phonon lifetime to 1 meter, novel approaches have been demonstrated these past years
that can overcome this limit. This can be achieved either by the prior activation of the acoustic wave by a long lasting
pre-pumping signal, leading to the optimized configuration using Brillouin echoes, or by probing a classically generated
steady acoustic wave using a ultra-short pulse propagating in the orthogonal polarization of a highly birefringent fibre.
These novel configurations can offer spatial resolutions in the centimetre range, while preserving the full accuracy on the
determination of temperature and strain.
The spectral broadening of the pump pulse through self phase modulation in a time domain distributed Brillouin sensor is
demonstrated to have a non-negligible detrimental effect, leading to a doubling of the effective gain linewidth after some
20 km in standard conditions. The theoretical modeling is fully confirmed by experimental results.
Photonic Crystal Fibers (PCF) play a crucial role for fundamental investigations such as acousto-optical interactions
as well as for applications, such as distributed sensors. One limiting factor for these experiments is the
fiber inhomogeneity owing to the drawing process. In this paper we study the effect of structural irregularities on
both the backward and forward Brillouin scattering by comparing two PCFs drawn with different parameters, in
order to minimize diameter fluctuations. We fully characterize their Brillouin properties including the backward
Brillouin spectrum, the Brillouin threshold, a distributed measurement along the fibers and polarized Guided
Acoustic Wave Brillouin Scattering (GAWBS). In the Brillouin spectrum we observe a single peak as in a singlemode
fiber whereas former investigations have often shown a multiple peak spectrum in PCFs with small core.
The theoretical and experimental values for the Brillouin threshold are in good agreement, which results from
the single peak spectrum. By using a Brillouin echoes distributed sensing system (BEDS), we also investigate
the Brillouin spectrum along the fiber with a high spatial resolution of 30 cm. Our results reveal a clear-cut
difference between the distributed measurements in the two fibers and confirm the previous experiments. In the
same way the GAWBS allows us to estimate the uniformity of the fibers. The spectra show a main peak at about
750 MHz, in accordance with theoretical simulations of the acoustic mode and of the elasto-optical coefficient.
The fiber inhomogeneity impacts on the stability and the quality factor of the measured GAWBS spectra. We
finally show that the peak frequency of the trapped acoustic mode is more related to the optical effective area
rather than the core diameter of the PCF. Thus measuring the main GAWBS peak can be applied for the precise
measurement of the effective area of PCFs.
In Geotechnical Engineering, progressive failure in soil-structure interaction is one of the least understood problems. It is
difficult to study this phenomenon at laboratory scale, because of the large amount of strain gages required per unit
length/area of the structure, which would interfere with the mechanical properties of both the structure and the soil. The
recently developed Brillouin Echo Distributed Sensor (BEDS) technology overcomes this dilemma by distributed
readings and 5cm spatial resolution. A laboratory pullout testing program has been carried out to verify applicability of
BEDS for the study of progressive failure in the soil-structure interaction.
KEYWORDS: Sensors, Signal detection, Spatial resolution, Fiber optics sensors, Optical resolution, Clocks, Oscilloscopes, Optical fibers, Tellurium, Signal to noise ratio
A technique developed to acquire fast optical signals using low frequency detection and acquisition is presented here. It
is based on optical sampling that creates a replica of the fast signal on a much slower time scale by a simple strobe effect.
High bandwidth detection and acquisition is totally suppressed leading to a better response and a substantial cost
reduction. The performance is illustrated by comparative measurements using a Brillouin high resolution distributed
fibre sensor.
A novel configuration has been developed to optimize the response of Brillouin echoes for distributed fibre sensing.
Fully resolved measurements of the Brillouin frequency shift of a 5cm spot perturbation have been performed using a
500 ps (5cm) pulse width. The linewidth of the measured Brillouin gain spectrum remains comparable to the intrinsic
linewidth for any pulse width. The high accuracy and inherent stability of the technique have been successfully verified.
The absorption of light by a gas molecule has been measured comparatively using light propagating in normal conditions
and in a slow light regime. The experiment is designed to make the 2 measurements possible without modifying the
interaction conditions, so that the sole effect of slow light is unambiguously observed. A 26% group velocity reduction
induced by stimulated Brillouin scattering in a gas-filled microstructured fiber caused no observable change in the
measured absorption, so that it is proved that material slow light does not enhance Beer-Lambert absorption and has a
null impact on gas sensing or spectroscopic applications.
A simple physical description of the nonlinear optical interaction based on Brillouin echoes is presented. This technique
makes potentially possible distributed Brillouin sensing down to centimeter spatial resolution while preserving the
narrowband feature of the natural Brillouin gain spectrum. Experimental conditions for the generation of Brillouin
echoes are described and demonstrations of distributed measurements using a 1 ns (10 cm) pulse are presented.
We propose a novel configuration for a Brillouin distributed sensor based on Brillouin optical time domain analysis. This
new configuration eliminates many intensity noise issues found in previous schemes. Resolution of 3.5 m all over a
47 km single-mode fibre was achieved and resolution down to 30 cm in a few kilometre fibre. Noise reduction makes
possible measurements with a 16 times averaging.
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