We present theoretical and experimental results on a slow-light assisted all-fiber configuration that can be used for
efficient sensing of a variety of parameters (pressure, displacement...). In particular, we report here a structure that can be
transformed into a slow-light assisted displacement sensor capable of sub-micrometric resolution values with a
potentially simple intensiometric measurement scheme. The basic element in the structure is a lossy ring resonator tuned
close to the critical coupling regime. In this working regime, the resonator transfer function displays extremely high
group delay values close to the resonances, and, accordingly, a large sensitivity to additional losses. A mechanic
transducer transforms displacement into small additional losses in the ring. This leads to strong variations in the log
transmission of the resonances, which are shown to scale with the group index. This scheme shows orders of magnitude
sensitivity enhancements over a conventional bending-loss configuration. We believe that this structure can be further
developed to provide large sensitivity enhancements to conventional intensiometric fiber sensors.
We demonstrate theoretically and experimentally that a wide-range tuning of group delay values can be achieved in a
lossy fiber ring resonator. The tuning mechanism relies simply on varying the loss/coupling ratio in the resonator. This
simple structure may be used advantageously in different regimes for many sensing configurations, both for achieving
extremely high sensitivity enhancements (by working close to critical coupling, where the group index becomes
extremely large) or suppression of undesired refractive index effects (e.g. Kerr effect), by working in the under-coupled
regime.
In this paper we combine the use of optical pulse coding and seeded second-order Raman amplification to extend the sensing distance of Brillouin optical time-domain analysis (BOTDA) sensors. Using 255-bit Simplex coding, the power levels of the Raman pumps and the Brillouin pump and probe signals were adjusted in order to extend the real physical sensing distance of a BOTDA sensor up to 120 km away from the sensor interrogation unit, employing a 240-km long loop of standard single-mode fiber (SSMF) with no repeater. To the best of our knowledge, this is the first time that distributed measurements are carried out over such a long distance with no active device inserted into the entire sensing loop, constituting a considerable breakthrough in the field.
Temperature or strain sensing in long-range (> 70 km) Brillouin Optical Time Domain Analysis (BOTDA) below 2
meter resolution cannot be trivially achieved due to numerous matters such as fiber attenuation, self-phase modulation,
depletion, resolution-uncertainty trade-offs, etc. In this paper we show that combining Raman assistance and the
Differential Pulse-width Pair (DPP) technique sub-metrical resolution is achievable in a BOTDA over 100 km sensing
range. We successfully demonstrate the detection of a 0.5 meter hot-spot in the position of worst contrast along the fiber.
High sensing resolution in long-distance distributed fiber optic sensors, such as Brillouin Optical Time Domain Analysis
(BOTDA), cannot be trivially achieved due to several issues including self-phase modulation, resolution-uncertainty
trade-offs and fiber attenuation. These problems could be fixed by the use of differential pulse-pair techniques in
combination with Raman amplification. In this work we present a Differential Pulse-width Pair (DPP) Raman-assisted
BOTDA sensor, that can achieve 1 meter resolution in a 100 km range.
Optical frequency combs are a useful tool for measuring reference laser frequency whose uncertainty depends on the
stability and accuracy of the reference clock. The relative uncertainty of the laser frequency measurements in the optical
telecommunication band with the frequency comb technique is estimated around 10-12. In this paper, we present the
development and implementation of a filtering technique on an optical frequency comb based on an Erbium optical fiber
oscillator using Brillouin scattering amplification. This filtering technique allows us to isolate and transmit frequencies
generated by a stabilized optical frequency comb. This method has been developed for the remote comparison of
frequency combs. Finally, we present the characterization of the optical frequency comb and its application to the
calibration of high wavelength resolution meters in the optical telecommunications window. Measurements with
uncertainties under the resolution of the own instrument were achieved using stabilized lasers at molecular absorptions.
The result is a significant improvement of the measurement capability given by the current equipment.
Kerr effect accounts for the change in refractive index of a material with the light intensity and appears in all known
optical materials. In this work we analyze Kerr effect in structured superluminal media (e.g some specific types of
resonators). We show that Kerr effect in these structures can be cancelled or even reversed (in comparison with the Kerr
effect of the material composing the structure) depending on the group index of the structure. We also discuss some
possible realizations of structured superluminal media.
We have developed a long-range Brillouin distributed sensor featuring 100 km measuring distance with 2 meter
resolution. To our knowledge, this is the first time that a high-resolution setup reaches the barrier of 100 km
measurement range. The key improvements with respect to previous configurations are explained.
In this work we present field tests concerning the application of Fiber Bragg Grating (FBG) sensors for the monitoring of
railway traffic. The test campaigns are performed on the Spanish high speed line Madrid-Barcelona, with different types
of trains (S-102 TALGO-BOMBARDIER, S-103 SIEMENS-VELARO and S-120 CAF). We located the FBG sensors
in the rail track at 70 km from Madrid in the country side, where the trains primarily are tested during commercial
operation with maximum speeds between 250-300 km/h. The FBG sensor interrogation system used allows the
simultaneous monitoring of four FBG sensors at 8000 samples/s. The different position of the FBG sensors in relation
with the rail can be used with different purposes such as train identification, axle counting, speed and acceleration
detection, wheel imperfections monitoring and dynamic load calculation.
We have used distributed Raman amplification to extend the measurement distance of a Brillouin Optical Time-Domain
Analysis (BOTDA) sensor. We successfully demonstrate a dynamic range of 75 km with 2 meter spatial resolution.
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.
This paper analyses the advantageous features of supercontinuum (SC) generated from continuous-wave (CW)
excitation. It has been shown that both the generation mechanisms and the temporal and spectral properties of
supercontinuum produced with CW pump lasers are different from those of generated by means of pulsed excitation, in
particular the remarkable smoothness of CW-SC spectra. We show that these unique spectral features stem from the
fission of the partially coherent CW input beam into a train of subpicosecond pulses induced by the modulation
instability (MI). These subpicosecond pulses lead to the formation of optical solitons with inherently random parameters,
which self-frequency shift differently depending on their characteristics. The resulting supercontinuum spectrum is hence
the average of many different soliton spectra, which have suffered different frequency shifts. Different experimental
setups used in our lab are presented and the dependence of the SC generation on the coherence of the fiber laser and the
fiber dispersion profile are shown.
Supercontinuum sources generated by continuous-wave excitation are very promising for many applications as they
present in general higher spectral power density then their pulsed counterparts. On the other hand, the properties of
supercontinuum are very difficult to be controlled as the initial broadening is driven by modulation instability. This latter
one breaks-up the CW radiation into a train of ultra-short pulses whose peak power, spectral length and shape strongly
depend on the power, coherence and noise of the pump and on the fiber properties. In this paper, we present a
preliminary work on the role of chromatic dispersion on supercontinuum spectral broadening in order to study how to
optimize SC spectral width under CW regimes. By means of a home-made tunable high-power laser we induce
supercontinuum generation by pumping at different dispersion values of the fiber. We show that at low injected powers
the wider spectrum is obtained when pumping just above the
zero-dispersion wavelength of the fiber. By contrast, for
higher injected powers, wide and squared-shaped spectra can be obtained when pumping over a larger range of
anomalous dispersion values. These results seem to be very promising for a number of applications requiring smooth,
squared and high-power SC spectral profiles such as optical coherence tomography.
We present a multiwavelength fiber source based on cascaded of four-wave mixing in two semiconductor optical
amplifiers followed by further four-wave mixing in an optical fiber enhanced by Raman amplification. The multiwavelength
source is generated by two initial frequencies detuned 200 GHz and referenced in the absorption lines of the
acetylene 12C2H2, which sweep in frequency keeping the detuning of the lasers constant. With this configuration, we
have achieved a high resolution source with a spectrum of 36 channels centered with adjustable peaks separation. The
source can be employed to interrogate a fiber Bragg grating sensors network and in gas spectroscopy applications.
The generation of a continuous-wave pumped supercontinuum source at 1.3 &mgr;m is described. The device makes use of a
tunable Yb-doped fiber laser, a cascade of fiber Bragg-grating mirrors and a concatenation of standard silica fibers with
stepwise decreasing dispersion. The generated supercontinuum spans from 1280 to 1513 nm, shows and average output
power of 1.34 W and exhibits >0 dBm/nm spectral power density over 200 nm.
Supercontinuum (SC) generation in optical fibers and waveguides is
a phenomenon of increasing interest that has found applications in
fields like time-resolved spectroscopy, ultrashort pulse
compression, multiwavelength optical sources for WDM and optical
frequency metrology. Most of the experiments performed up to now
have been accomplished using femtosecond or picosecond-pulsed
laser sources and special fibers such as highly-nonlinear photonic
crystal fibers. Supercontinuum generation using continuous-wave
laser sources was demonstrated only recently, but the initial
results demonstrate that high power density (>1 mW/nm),
broadband supercontinuums (more than 250 nm) can be achieved with
good long-term stability. In this paper we show different
experimental setups to produce continuous-wave supercontinuums in
optical fibers. We show how the supercontinuum varies depending
upon the pump source used in the experiment. We believe that such
an incoherent source can have very interesting applications in
optical fiber and component characterization, fiber sensing and
optical coherence tomography for biomedical applications. As a
sample application, we show that this source can be used to
measure polarization mode dispersion (PMD) in optical fibers very
accurately and with an extremely large dynamic range (>200 km).
We present a new kind of broadband continuous-wave source which outperforms any other broadband superluminescent or amplified spontaneous emission source both in terms of output spectral density and bandwidth. Our source covers the wavelength band of interest for fiber applications (from 1450 to 1625 nm) and has an output power of approximately 1.3 W. Additionally, it features a good power stability and we believe it might have very interesting applications in fiber sensing, for instance to avoid the need of amplification in the interrogation of remote Bragg gratings or to improve the resolution and dynamic range of optical coherence tomography setups.
Modulation instability can limit the resolution and the range of distributed fibre sensors based on stimulated Brillouin scattering. In this paper we analyse this process and suggest adequate methods to overcome it.
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