372 research outputs found
Phase-measuring time-gated BOCDA
We demonstrate a simple scheme that allows performing distributed Brillouin phase spectrum (BPS) measurements with very high spatial resolution (~7 cm) over long (~4.7 km) optical fibers. This is achieved by inserting a Sagnac interferometer (SI) in a Brillouin optical correlation domain analysis (BOCDA) configuration. Over its already-presented time-domain equivalent (SI-BOTDA), this approach reduces the main source of noise (coherent backscatter noise) thanks to the low-coherence nature of the used signals. On the other hand, over the most usual schemes used for distributed BPS measurements, this implementation presents the key advantage of not requiring high-bandwidth detection or complex modulation while reaching unprecedented values of spatial resolution and number of resolved points for this type of measurement. Thanks to the linear dependence of the BPS feature around the Brillouin frequency shift, this scheme could also have the advantage of requiring shorter scanning ranges than amplitude-based configurations.European CommissionMinisterio de Economía y CompetitividadComunidad de Madri
Simple Method for the Elimination of Polarization Noise in BOTDA Using Balanced Detection and Orthogonal Probe Sidebands
Polarization noise arises in Brillouin optical timedomain
analysis due to the strong polarization sensitivity of stimulated
Brillouin scattering. To avoid this noise, it turns out to be
indispensable to perform some kind of polarization scrambling,
either in the pump pulse, the probe signal or both. This is usually
achieved using polarization scrambling/switching systems, which,
being mechanical, tend to be not as robust as it would be desirable.
In this paper, we propose a completely passive system,with no moving
parts, to eliminate polarization noise in a BOTDA. It is based
on the use of passive depolarization of the pump pulse together
with balanced detection among orthogonally polarized Stokes and
anti-Stokes bands of the probe signal. The setup requires no alignment
readjustment over time and provides a performance similar
to a conventional BOTDA using scrambler.Ministerio de Economía y CompetitividadComunidad de MadridEuropean Commissio
Balanced detection in Brillouin optical time domain analysis
We propose the use of balanced detection in Brillouin Optical Time Domain Analysis (BOTDA) sensors. Balanced detection can be effectively accomplished among the Stokes and anti-Stokes bands in the probe signal. This type of detection leads to a doubling of the trace amplitude and at least a √2 increase in signal to noise ratio over the conventional configuration. Moreover, it leads to a complete cancellation of the common-mode noise in the probe signal, including relative intensity noise in Raman-assisted configurations. We show all these benefits both theoretically and experimentallyMinisterio de Ciencia e InnovaciónComunidad de MadridEuropean Commissio
Modulation instability-induced fading in phase-sensitive optical time-domain reflectometry
Phase-sensitive optical time-domain reflectometry (?OTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. We show in this Letter that modulation instability (MI) can induce a position-dependent signal fading in long-range ?OTDR over conventional optical fibers. This fading leads to a complete masking of the interference signal recorded at certain positions and therefore to a sensitivity loss at these positions. We illustrate this effect both theoretically and experimentally. While this effect is detrimental in the context of distributed vibration analysis using ?OTDR, we also believe that the technique provides a clear and insightful way to evidence the Fermi?Pasta?Ulam recurrence associated with the MI process
Wide range group delay tuning in lossy fiber ring resonators
22nd International Conference on Optical Fiber Sensors, OFS 2012, Beijing, China, 15-19 Oct. 2012We demonstrate theoretically and experimentally that a wide-range tuning of group delay values can be achieved in a\ud
lossy fiber ring resonator. The tuning mechanism relies simply on varying the loss/coupling ratio in the resonator. This\ud
simple structure may be used advantageously in different regimes for many sensing configurations, both for achieving\ud
extremely high sensitivity enhancements (by working close to critical coupling, where the group index becomes\ud
extremely large) or suppression of undesired refractive index effects (e.g. Kerr effect), by working in the under-coupled\ud
regime
Unexpected non-local effects in dual-probe-sideband BOTDA
Until now, non-local effects in dual-probe-sideband Brillouin Optical Time Domain Analysis (BOTDA) systems have been considered negligible if the probe power is below the Stimulated Brillouin Scattering (SBS) threshold. In this paper, we show the appearance of non-local effects even below the SBS threshold. The pump pulse experiences a frequencydependent spectral deformation that affects the readout process differently in the gain and loss configurations. The main conclusion of our study is that the measurements in gain configuration are more robust to this non-local effect than the loss configuration. These results are of particular interest for manufacturers of long-range BOTDA systems.European CommissionMinisterio de Economía y CompetitividadComunidad de Madri
Non-local effects in dual-probe-sideband Brillouin optical time domain analysis
According to recent models, non-local effects in dual-probesideband
Brillouin Optical Time Domain Analysis (BOTDA) systems
should be essentially negligible whenever the probe power is below the
Stimulated Brillouin Scattering (SBS) threshold. This paper shows that
actually there appear non-local effects in this type of systems before the
SBS threshold. To explain these effects it is necessary to take into account a
full spectral description of the SBS process. The pump pulse experiences a
frequency-dependent spectral deformation that affects the readout process
differently in the gain and loss configurations. This paper provides a simple
analytical model of this phenomenon, which is validated against compelling
experimental data, showing good agreement. The main conclusion of our
study is that the measurements in gain configuration are more robust to this
non-local effect than the loss configuration. Experimental and theoretical
results show that, for a total probe wave power of ~1 mW (500 μW on each
sideband), there is an up-shifting of ~1 MHz in the Brillouin Frequency
Shift (BFS) retrieved from the Brillouin Loss Spectrum, whereas the BFS
extracted from the measured Brillouin Gain Spectrum is up-shifted only
~0.6 MHz. These results are of particular interest for manufacturers of longrange
BOTDA systems.European CommissionMinisterio de Economía y CompetitividadComunidad de MadridUniversidad de Alcal
Sidelobe apodization in optical pulse compression reflectometry for fiber optic distributed acoustic sensing
We demonstrate a technique to reduce the sidelobes in optical
pulse compression reflectometry for distributed acoustic
sensing. The technique is based on using a Gaussian
probe pulse with linear frequency modulation. This is
shown to improve the sidelobe suppression by 13 dB compared
to the use of square pulses without any significant
penalty in terms of spatial resolution. In addition, a 2.25 dB
enhancement in signal-to-noise ratio is calculated compared
to the use of receiver-side windowing. The method is tested
by measuring 700 Hz vibrations with a 140 nϵ amplitude
at the end of a 50 km fiber sensing link with 34 cm spatial
resolution, giving a record 147,058 spatially resolved
points.European CommissionMinisterio de Economía y CompetitividadComunidad de Madri
Turning a low Q fiber resonator into a high-sensitivity displacement sensor using slow light concepts
High-Q resonators have been widely used for sensing purposes. High Q factors normally lead to sharp spectral peaks which accordingly provide a strong sensitivity in spectral interrogation methods. In this work we employ a low-Q ring resonator to develop a high sensitivity sub-micrometric resolution displacement sensor. We use the slow-light effects occurring close to the critical coupling regime to achieve high sensitivity in the device. By tuning the losses in the cavity close to the critical coupling, extremely high group delay variations can be achieved, which in turn introduce strong enhancements of the absorption of the structure. We first validate the concept using an Optical Vector Analyzer (OVA) and then we propose a simple functional scheme for achieving a low-cost interrogation of this kind of sensors.European CommissionMinisterio de Ciencia e InnovaciónComunidad de Madri
Cova de sa Balma des Quartó (Manacor, Mallorca)
The survey and description of a new cave recently explored in the vicinity of s’Estany
d’en Mas (Manacor, Mallorca) is presented in this paper. The total development of the cave
is 270 m, featuring a wide spacious chamber with a rich decoration of speleothems, including
an abundance of straw stalactites of remarkable length. The cave seems to have been used
as a burial place in prehistoric times
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