BUSCADOR RECOLECTA

In this study, a new method for increasing simultaneously the sensitivity and interrogation stability of fiber-optic intensity sensors is presented. The configuration is based on a double-coupler ring resonator. On one side, the attenuation sensitivity of the sensor is enhanced by placing it inside a ring resonator. On the other side, the stability of the interrogation method is widely improved by creating an external lasing ring resonator with matched spectral response. The combination of the wavelength selective response of the ring resonator and optical amplification from Er-doped fiber amplifier is used to create different fiber lasers. The resulting lasers present an impressive sensitivity enhancement and a dynamic range increment up to 15 dB. Simultaneously, the instability of the system is reduced to ±0.05 dB. In comparison with systems based on tunable laser interrogation, our method is >100 times more stable and also simpler, as no wavelength tunable elements are required., Ministerio de Economía y Competitividad, European Commission

24th International Conference on Optical Fibre Sensors (OFS 24), Curitiba, Brazil, Sept. 28-Oct. 2, 2015., In this work, we present a random laser sensor system for increasing simultaneously the sensitivity and interrogation stability of a fiber-optic displacement sensor. The system is based on a random distributed fiber laser modulated by a double-coupler ring resonator within which is placed the sensor. This allows to increase the sensitivity of the sensor and also its interrogation stability. The experimental results show a dynamic range increment (and consequently its sensitivity) of 10dB. Moreover, by using this laser-based interrogation system, the instability is reduced to 0.04 dB. In addition, wavelength filtering elements are not required, simplifying the system., Ministerio de Economía y Competitividad, European Commission

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 Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

We demonstrate a simple technique to provide conventional Brillouin optical time-domain analysis sensors with mitigation for pump pulse attenuation. The technique is based on operating the sensor in loss configuration so that energy is transferred from the probe wave to the pump pulse that becomes amplified as it counter-propagates with the probe wave. Furthermore, the optical frequency of the probe wave is modulated along the fiber so that the pump pulse experiences a flat total gain spectrum that equally amplifies all the spectral components of the pulse, hence, preventing distortion. This frequency modulation of the probe brings additional advantages because it provides increased tolerance to non-local effects and to spontaneous Brillouin scattering noise, so that a probe power above the Brillouin threshold of the fiber can be safely deployed, hence, increasing the signal-to-noise ratio of the measurement. The method is experimentally demonstrated in a 100-km fiber link, obtaining a measurement uncertainty of 1 MHz at the worst-contrast position., The authors wish to acknowledge the financial support from the Spanish Ministerio de Economía y Competitividad through project TEC2013-47264-C2-2-R, FEDER funds and the Universidad Pública de Navarra.

We demonstrate the extension of the measurement range of
Brillouin optical time-domain analysis (BOTDA) sensors using a distributed
Brillouin amplifier (DBA). The technique is based on injecting a DBA
pump wave in the fiber to generate an additional Brillouin interaction
that amplifies the BOTDA pump pulses and compensates optical fiber
attenuation. This amplification does not introduce any significant noise to
the BOTDA’s probe wave due to the inherent directionality of the Brillouin
gain. Additionally, we deploy a differential pulse-width pair measurement
method to avoid measurement errors due to the interplay between the
self-phase modulation effect and the changes in the temporal shape of the
pulses induced by the transient behavior of Brillouin gain. Experimental
proof-of-concept results in a 50-km fiber link demonstrate full compensa-
tion of the fiber’s attenuation with no penalty on the signal-to-noise ratio of
the detected signal., The authors wish to acknowledge the financial support from the Spanish Ministerio de
Economía y Competitividad through project TEC2013-47264-C2-2-R, FEDER funds and the
Universidad Pública de Navarra.

A novel optical fiber installation method was explored for asphalt monitoring. Glassfiber polymer encapsulated SMF was installed in the intermediate and surface layers in order to study the strain sensitivity with a distributed strain interrogator., Financial support from Spanish CICyT project TEC2013-47264-C2-2- and FEDER funds is acknowledged.

Distributed fiber optic sensors (DFOS) are becoming an increasingly used technology
to monitor the integrity of structures. This is due to the fact that this technology can
be embedded within the structure and provide distributed information of several relevant
parameters for the structure, such as stress, temperature or strain. In DFOS the fiber itself
is the transducer, and the measurement of a given parameter is provided continuously along
the fiber at a particular spatial resolution, without blind spots. This is the main advantage
of DFOS compared to other sensing technologies, the fact that DFOS provide information
of a given parameter over thousands or hundreds of thousands of positions along the
optical fiber. Conversely, other sensing technologies only give information over the specific
points where they are installed, that is to say, they are point sensors. This characteristic
of distributed fiber sensors makes them enormously interesting when many points of a
structure need to be monitored. In this case, a single distributed fiber sensor can replace
many point sensors, which considerably reduces the cost per sensing point when monitoring
large structures. In addition, due to the properties of the optical fiber, these sensors have
a better performance compared to other kind of sensors. Among other important features,
DFOS present a low signal loss, electromagnetic interference immunity, remote sensing and
multiplexing capabilities, light weight, and are chemically passive, which make them a very
attractive technology for field measurements. Therefore, DFOS have the added advantage
of being electrically, magnetically, and chemically passive, so that can be placed in harsh
environments, such as nuclear plants or areas with gas concentration, where, due to the
possibility of a short circuit, electronic sensors cannot be placed. Al these characteristics
make this technology unique.
Among the different types of DFOS, those based on stimulated Brillouin scattering,
and more specifically, those that build upon the Brillouin optical time-domain analysis
(BOTDA) technique, are one of the most promising. The main characteristic that makes
BOTDA sensors as promising, is the ability to perform distributed strain and temperature
measurements over long distances at high spatial resolution. For the functioning of the
sensor, the general interaction that takes place in the BOTDA technique involves two
optical waves: a continuous wave probe and a counter-propagating pump pulse. The
performance of the sensor response is limited, among others, by the maximum optical
power of both waves that can be injected into the fiber. In this way, the main research
line in BOTDA sensors is focused on the study of the physical limitations of the technique
as well as the development of solutions to these constraints. Another important line relies
on the simplification of the sensor setup so as to reduce the complexity and the cost of
the sensor. This thesis dissertation contributes to the development of BOTDA sensors by
means of different contributions in these two research lines.
Several theoretical and experimental studies have been conducted to accurately determine
the main limits to the sensor performance in terms of the maximum optical power
of the pump and probe waves that can be used. One of the most important limitation
in BOTDA sensors is the onset of non-local effects, which limits the maximum pump and
probe waves power that can be injected in the fiber, and hence, the signal-to-noise ratio (SNR) at the receiver is worsen. The so-called non-local effects generate measurement
errors, because the Brillouin spectra measured at distant locations depend on the interaction
at previous positions in the fiber. In this research line, we have examined the effects
caused by the limited extinction ratio (ER) of the pump pulse, finding that, among other
impairments, it leads to the onset of a new non-local effect originated in the depletion of
the pedestal of the pump wave. In addition, it has been found that the pedestal deformation
caused by the transient response of erbium-doped fiber amplifiers, which are typically
deployed to amplify the pump pulse, also constrains the performance of the sensor. Another
contribution is the study of the techniques presented in the literature to mitigate the
impairments caused by second-order non-local effects, which cause a frequency-dependent
spectral deformation of the pulse. The findings of this study show that these techniques
are only applicable when the Brillouin frequency shift (BFS) of the fiber is uniform, which
is hard to find in real applications. Lastly, another subject of study is the limitations of
the pump and probe optical power in coded-pump wave BOTDA configurations. We have
observed that, in addition to some known limitations, there are two important restrictions
that have to be taken into account: the onset of non-local effects and the non-linear amplification
of the probe wave, both generated by the successive gain induced by the multiple
pulses of the coded-pump wave.
As a consequence of the findings of these studies, BOTDA configurations intended to
solve these limitations have also been proposed during the thesis work. A technique to
mitigate the constraints induced by the limited ER of the pump pulse has been presented.
This method is based on adding a dithering to the optical source used to generate the
two waves involved in the BOTDA sensor, so that the optical wavelength of both signals
is modulated. In this way, the Brillouin interaction between the pedestal and the probe
wavefronts become uncorrelated, and hence, the influence of the pedestal is greatly reduced.
Another contribution is a technique focused on completely overcome the onset of
second-order non-local effects. This method is based on continuously tracking the BFS
distribution of the fiber, which combined with the probe-dithering method, has allowed,
to the best of our knowledge, to inject the highest demonstrated probe wave power in a
BOTDA sensor to date. In addition, in order to improve the SNR of the sensor, a novel
BOTDA sensor has been proposed. This analyzer combines mono-color cyclic coding and
probe-dithering techniques, so that the impairments caused by a coded pump wave are
reduced, and hence, it is possible to increase the optical power and consequently enhance
the sensing distance range.
Finally, a novel simplified BOTDA sensor has been presented, which relies on passive
optical filtering of the spectral components generated in a single optical source. In this
way, the sensor setup is simplified reducing the number of optical devices, and therefore,
the cost of the sensor is also reduced. This BOTDA configuration has been shown to have
a performance comparable to more complex setups., Nafarroako Unibertsitate Publikoari, ikertzaileak prestatzeko dirulaguntza, atzerriko egonaldia
zein bestelako laguntza moten bidez; Espainiako Ekonomia eta Lehiakortasun Ministerioari,
TEC2013-47264-C2-2-R eta TEC2016-76021-C2-1-R proiektuen bidez; Europako
Eskualde Garapenerako Funtsari (FEDER funds)., Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011), Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011)

We demonstrate the relaxation of the stringent requirements placed on the pulse extinction ratio in long-range Brillouin optical time-domain analysis sensors (BOTDA) by modulating the wavelength of the laser source that is used to generate both pump and probe waves. This modulation makes the counter-propagating pulse pedestal and probe waves to become correlated only at certain locations in the fiber, thus reducing the gain experienced by the probe wave, which is precisely the process that limits the performance in long-range BOTDAs. Proof-of-concept experimental results in a 20-km sensing link demonstrate a 6-dB reduction of the required modulator extinction ratio., The authors acknowledge the support of the Spanish Ministerio de Economía y Competitividad project TEC2013-47264-
C2-2-R, FEDER funds and the Universidad Pública de Navarra.

Brillouin denbora-eremu optikoaren analisian (BOTDA) oinarritutako sentsoreak estruktura handien (120 km luzerako estrukturak) deformazioa eta tenperatura neurtzeko gaitasuna dute, metro bateko bereizmen espazialarekin. Lan honetan, sentsorearen merkatu-prezioa handitu gabe bereizmen espazial txikiagoa lortzeko beharrezkoa litzatekeen teknika erabiltzeak dituen efektu kaltegarriak aztertu egin dira. Izan ere, pultsu azkar bat sortzeko beharrezkoak diren Mach-Zehnder modulagailu elektrooptikoek iraungitze-ratio (ER) txikia dute. Horiek horrela, aztertu egin da pultsatutako seinalea sortzeko sorgailuak duen ERaren eragina BOTDA sentsoreen prestazioetan, akats handiak sortzen direla ondorioztatuz., Distributed Brillouin optical time domain analysis (BOTDA) sensors can provide high precision distributed measurements of the strain and temperature profile over extremely large structures (structures
of 120 km) with a meter spatial resolution. In this work, we present the detrimental effect that would
imply the use of the necessary technique to achieve a lower spatial resolution, without increasing
the price of the sensor. Since the Mach-Zehnder electro-optical modulators that are required to perform a fast pulse have a low extinction ratio (ER). Therefore, we have studied the consequences of
using a pulse generator device with a low ER on BOTDA sensors, concluding that it generates large errors., Lan honen egileek eskerrak eman nahi dizkiete Espainiako Zientzia eta Berrikuntza Ministerioari
(TEC2013-47264-C2-2-R proiektuaren bitartez), EEGF funtsei eta Nafarroako Unibertsitate Publikoari
emandako laguntza ekonomikoengatik.

A new concept for the fabrication of integrated microring
resonators, requiring only one single epitaxial growth and two single-side
lithographic steps, is proposed in what is the simplest fabrication scheme
for vertical microrings published to date. The approach is based on two
vertically stacked phase matched core layers. The effect of bus waveguide,
coupling region and ring structure parameters is theoretically analyzed.
Numerical calculations predict high performance devices with quality
factors of over 10000. The scheme can feature both active and passive
regions, allowing the fabrication of microring lasers., This research was supported by the Spanish Government project TEC2013-47264-C2-2-R and
FEDER funds.

In this work, a new application of random distributed feedback lasers to fiber optic sensing has been presented. The particular properties of these lasers, such as the lack of longitudinal modes and high stability, have been exploited to monitor transversal load using a phase-shifted fiber Bragg grating (PS-FBG), obtaining a resolution of 1g and a sensitivity of 3.95GHz/Kg. Due to the PS-FBG birefringence and the load-interrelated transmission lines generated by the PS-FBG along the orthogonal polarization directions, the beating of the two emission lines generated in the laser can be monitored in the electrical domain. As a result, transversal load applied on the sensor can be measured., Financial support from the Spanish Comisión Interministerial de Ciencia y Tecnología within project TEC2013-47264-
C2-2-R and the FEDER funds are acknowledged.

In this work, a preliminary study of the behavior of two different interferometric fiber optic sensors and two different wavelength selective fiber optic sensors is performed. A photonic cristal fiber Fabry-Pérot interferometer, a Sagnac interferometer, a commercial fiber Bragg grating (FBG) and a π-phase shifted fiber Bragg grating interrogated in a random distributed feedback fiber laser are analyzed. A comparison of their sensitivities and resolutions is carried out to analyze their performance as sensors for cryogenic temperatures, taking into account their advantages and drawbacks., Financial support from the Spanish Comisión Interministerial de Ciencia y Tecnología within projects TEC2013-47264-C2-2-R and TEC2016-76021-C2-1-R and the FEDER funds are acknowledged.

In this work, narrow-band emission lines are generated by means of two random distributed feedback fiber laser schemes. Spectral line-widths as narrow as 3.2 pm have been measured, which significantly improves previous reported results. The laser is analyzed with the aim of obtaining a spectral line-width as narrow as possible. Additionally a variation of this setup for multi-wavelength operation is also validated. Both
schemes present a simple topology that use a combination of phase-shifted fiber Bragg gratings and regular fiber Bragg gratings as filtering elements., This work was supported by the Spanish Government projects TEC2013-47264-C2-2-R and INNOCAMPUS.

In this paper, a new sensor system for relative humidity measurements based on a SnO2 sputtering deposition on a microstructured optical fiber (MOF) low-finesse Fabry-Pérot (FP) sensing head is presented and characterized. The interrogation of the sensing head is carried out by monitoring the Fast Fourier Transform phase variations of the FP interference frequency. This method is low-sensitive to signal amplitude variations and also avoids the necessity of tracking the evolution of peaks and valleys in the spectrum. The sensor is operated within a wide humidity range (20%-90% relative humidity) with a maximum sensitivity achieved of 0.14rad/%. The measurement method uses a commercial optical interrogator as the only active element, this compact solution allows real time analysis of the data., We also thank the Spanish Government projects TEC 2013-47264-C2-2-R, TEC2013-43679-R Innocampus and the INTERREG SUDOE Project SOE3/P2/P714 as well as to the FEDER Funds.

En esta tesis se presenta diferentes técnicas que contribuyen en la mejora de DOFS basados en dispersión Brillouin y Rayleigh. Todas estas técnicas han sido demostradas experimentalmente mostrando resultados prometedores. Con estas nuevas técnicas, los sensores BOTDA pueden llegar a medir más lejos y con más precisión. Además, las técnicas encaradas a mejorar las prestaciones basadas en dispersión Rayleigh coherente permiten mejorar la SNR de la señal, reducir el fading que afecta a la señal medida, así como incrementar notablemente la máxima amplitud de la perturbación que se puede medir., In summary, in this thesis different techniques to improve the performance DOFS based on Brillouin and Rayleigh scattering have been presented and experimentally demonstrated. These new techniques permit to increase the sensing range and accuracy of BOTDA measurements and to enhance the SNR and the fading tolerance of OTDR sensors., Financial support of three institutions: a) Public University of Navarre, b) fons FEDER, and c) Spanish Ministry of Economy and Competitiveness (project TEC2013-47264-C2-2-R)., Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011), Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011)

In this paper, a theoretical and experimental study of two interferometric sensor multiplexing schemes has been carried out by means of the fast Fourier transform (FFT) analysis. This work addresses one of the main drawbacks of photonic crystal fiber (PCF) sensors, that is, its multiplexing capability. Using a commercial optical interrogator combined with a simple FFT measurement technique, the simultaneous real-time monitoring of several PCF sensors is achieved. A theoretical analysis has been performed where simulations matched with the experimental results. For the experimental verification, highly birefringent (HiBi) fiber sections that operated as sensing elements were multiplexed and tested in two configurations. Due to the FFT analysis, both multiplexing schemes can be properly interrogated by monitoring the FFT phase change at the characteristic spatial-frequency of each sensor. For this purpose a commercial interrogator and a custom Matlab program were used for computing the FFT and for monitoring the FFT phase change in real-time (1 Hz)., This work was supported by the Spanish Government projects TEC2010-20224-C02-01, TEC2013-47264-C2-2-R, INNOCAMPUS, by the European SUDOE-Interreg Project ECOALMGT and the FEDER funds.

We demonstrate, for the first time to our knowledge, a Brillouin optical time domain analysis
(BOTDA) sensor that is able to operate with a probe power larger than the Brillouin threshold of the
deployed sensing fiber and that is free from detrimental non-local effects. The technique is based
on a dual-probe-sideband setup in which an optical frequency modulation of the probe waves along
the fiber is introduced. This makes the optical frequency of the Brillouin interactions induced by each
probe wave on the pump to vary along the fiber so that two broadband Brillouin gain and loss spectra
that perfectly compensate are created. As a consequence, the pulse spectral components remain
undistorted avoiding non-local effects. Therefore, a very large probe power can be injected, which
improves the signal-to-noise ratio in detection for long-range BOTDA. Moreover, the probe power can
even exceed the Brillouin threshold limit due to their frequency modulation, which reduces the effective
amplification of spontaneous Brillouin scattering in the fiber. Experiments demonstrate the technique
in a 50-km sensing link in which 8 dBm of probe power is injected., This work was
supported by the Universidad Pública de Navarra, by the Universidad de Cantabria, and by the
Spanish Ministerio de Ciencia e Innovación through project TEC2013-47264-C2 and Feder funds.

This paper presents a new sensor system for vibration and relative humidity measurements based on its interaction with the evanescent field of a microwire. The interrogation of the sensing head is carried out by monitoring the fast Fourier transform (FFT) phase of one of the FFT peaks of the microwire transmission signal. This technique is not dependent of the signal amplitude and also eludes the requisite of tracking the wavelength evolution in the spectrum, which can be a handicap when there are multiple interference frequency components with different sensitivities. The point sensor is able to measure a wide humidity range (20%-70% relative humidity) with a maximum sensitivity reached of 0.14πrad/% relative humidity. This microwire sensor is also operated within a frequency range from 320 to 1300 Hz with a sensitivity of around 0.0051 nm -1 /Hz. Finally, due to the system uses an optical interrogator as unique active element, the system presents a cost-effective feature., This work was supported in part by the Spanish Comision Interministerial de Ciencia y Tecnología within project TEC2013-47264-C2-2-R and SUDOE ECOAL-MGT and FEDER funds from the European Union.

In this work, we present and demonstrate a novel sensor system for simultaneous measurement of strain and temperature through a unique combination of a long period grating (LPG) and a fiber laser based on a fiber Bragg grating (FBG). In order to achieve this, a new erbium doped fiber laser (EDFL) structure is created, showing an optical signal-to-noise ratio (OSNR) of 55 dB and a peak power measured on the OSA between -5 and 0 dBm. The strain and the temperature information can be obtained by using a unique emission line through monitoring both the fiber laser wavelength shift and the change of the power level, both of which showing a clear linear behavior., This work was supported by the by the Spanish Government projects TEC2010-20224-C02-01, TEC2013-47264-C2-2-R, the FEDER funds and by the European COST action TD1001.

We report on two previously unknown non-local effects that have been found to impair
Brillouin optical time-domain analysis (BOTDA) sensors that deploy limited extinction ratio
(ER) pump pulses. The first one originates in the increased depletion of the pedestal of the pump
pulses by the amplified probe wave, which in turn entails a reduced amplification of the probe
and a measurement distortion. The second effect is due to the interplay between the transient
response of the erbium-doped fiber amplifiers (EDFA) that are normally deployed to amplify the
pump and the pedestal of the pump pulses. The EDFA amplification modifies the pedestal that
follows the pulses in such a way that it also leads to a distortion of the measured gain spectra
after normalization. Both effects are shown to lead to non-local effects in the measurements
that have similar characteristics to those induced by pump pulse depletion. In fact, the total
depletion factor for calculations of the Brillouin frequency shift (BFS) error in BOTDA sensors
is shown to be the addition of the depletion factors linked to the pump pulse as well as the
pedestal. A theoretical model is developed to analyze both effects by numerical simulation.
Furthermore, the effects are investigated experimentally in long-range BOTDA sensors. The
pedestal depletion effect is shown to severely constrain the probe power as well as the minimum
ER of the pulses that can be deployed in BOTDA sensors. For instance, it is shown that, in
a long-range dual-probe BOTDA, an ER higher that 32-dB, which is above that provided by
standard electro-optic modulators (EOM), is necessary to be able to deploy a probe power of
-3 dBm, which is the theoretical limit for that type of sensors. Even more severe can be the
limitation due to the depletion effect induced by the EDFA transient response. It is found that the
impairments brought by this effect are independent of the probe power, hence setting an ultimate
limit for the BOTDA sensor performance. Experimentally, a long-range BOTDA deploying a
26-dB ER EOM and a conventional EDFA is shown to exhibit a BFS error higher than 1 MHz
even for very small probe power., Spanish Ministerio de Economía y Competitividad (TEC2013-47264-C2-2-R and TEC2016-
76021-C2-1-R); FEDER funds; Universidad Pública de Navarra; German Research Foundation
(DFG SCHN 716/13-1).

In this work, a new method for increasing simultaneously the sensitivity and interrogation stability of fiberoptic intensity sensors is presented. The configuration is based on a double-coupler ring resonator. On one side, the attenuation sensitivity of the sensor is enhanced by placing it inside a ring resonator. On the other side, the stability of the interrogation method is widely improved by creating an external lasing ring
resonator with matched spectral response. The combination of the wavelength selective response of the ring resonator and optical amplification from Er-doped fiber amplifier is used to create different fiber lasers. The resulting lasers present an impressive sensitivity enhancement and a dynamic range increment up to 15 dB. Simultaneously, the instability of the system is reduced to ±0.05 dB. In comparison with systems based
on tunable laser interrogation, our method is >100 times more stable and also simpler, as no wavelength tunable elements are required., The authors would like to acknowledge the financial support from the Spanish Ministerio de Educación y Ciencia through projects TEC2010-20224-C02-01, TEC2013-47264-C2-2-R, TEC2012-37958-C02-01 and Innocampus; from the European FEDER funding, ECOAL-MGT SUDOE project and from the European Research Council through Starting Grant U-FINE (Grant No. 307441).

In this paper, a random distributed feedback fiber laser is proposed as a multiplexing scheme for ultralong range measurements (up to 200 km). Optical fiber sensors are time and wavelength multiplexed overcoming one of the main limitations of long-range sensing setups, which is their limited multiplexing capability. The direct modulation of the laser's cavity allows the interrogation of sensors by measuring the reflected power for different wavelengths and distances. Fiber Bragg gratings placed at different fiber locations and wavelengths have been interrogated in two different sensor networks. In addition, in order to improve the performance of the system, some features have been analyzed., This work was supported by the Spanish Government project TEC2013-47264-C2-2-R and the FEDER funds.

We present a simplified configuration for distributed Brillouin optical time-domain analysis sensors that aims to reduce the cost of the sensor by reducing the number of components required for the generation of the two optical waves involved in the sensing process. Te technique is based on obtaining the pump and probe waves by passive optical filtering of the spectral components generated in a single optical source that is driven by a pulsed RF signal. Te optical source is a compact laser with integrated electroabsorption modulator and the optical filters are based on fiber Bragg gratings. Proof-of-concept experiments demonstrate 1m spatial resolution over a 20km sensing fiber with a 0.9MHz precision in the measurement of the Brillouin frequency shiſt, a performance similar to that of much more complex setups. Furthermore, we discuss the factors limiting the sensor performance, which are basically related to residual spectral components in the filtering process., The authors wish to acknowledge the financial support from
the Spanish Ministerio de Ciencia e Innovación through the Project TEC2013-47264-C2-2-R, FEDER funds, and Universidad
Pública de Navarra.

Desde la irrupción de la fibra óptica en las telecomunicaciones, han surgido multitud de desarrollos basados en sus propiedades. Así, en las últimas décadas, la rápida evolución de los sistemas basados en fibra óptica, así como la aparición en el mercado de dispositivos como los amplificadores ópticos, los láseres de semiconductor de alta potencia, y otros elementos, ha hecho avanzar considerablemente la investigación en este campo. Además de la utilización de la fibra óptica en telecomunicaciones, han surgido nuevas aplicaciones, generando nuevas familias de dispositivos. Dos de ellas particularmente interesantes, son los láseres de fibra óptica y los sensores de fibra óptica. Ambas familias pueden combinarse de manera que sensores y redes de sensores de fibra óptica pueden interrogarse por medio de láseres de fibra óptica. En este marco de investigación se centra esta tesis, donde se han desarrollado estructuras avanzadas de fibra óptica para láseres y sensores. El objetivo ha sido mejorar las propiedades de los láseres de fibra, sensores y sus sistemas de monitorización, así como abordar los diferentes problemas que éstos presentan actualmente. Los retos encontrados, así como los objetivos de cada sistema, varían en función del tipo de sistema desarrollado. De esta forma, la primera parte de este trabajo ha sido dedicada al desarrollo de láseres de fibra óptica, buscando mejorar sus propiedades en términos de estabilidad, relación señal ruido, modos longitudinales, múltiples líneas de emisión y anchura de las mismas. Se han abordado diferentes tipos de configuraciones clásicas de láseres en anillo y de cavidad lineal. También se han realizado estudios de láseres de cavidad distribuida mediante dispersión Rayleigh (random). Estos láseres han suscitado gran interés en los últimos años dado que no presentan modos longitudinales. Por otro lado en el ámbito de la monitorización de sensores, se han desarrollado transductores ópticos basados en fibras micro-estructuradas, aprovechando las ventajas que éstas presentan en comparación con las fibras ópticas estándar. Finalmente se han desarrollado diferentes configuraciones de sistemas sensores con el objetivo de contribuir a la mejora de alguno de sus parámetros, como su alcance, capacidad de multiplexación, sensibilidad, rango dinámico y estabilidad de los sistemas sensores., Optical fiber technology as well as its rapidly evolution, has attracted the scientific community attention to develop lasers, fiber optic transducers and sensor networks. In this regard, the appearance in the last decades of commercial devices such as optical amplifiers, semiconductor high power lasers and others, has contributed to the rise of this technology. In this framework, a contribution to the improvement of the performance of fiber lasers and fiber optic sensors is presented in this PhD. work, taking into account their main merits and addressing their principal challenges. The challenges as well as the objectives addressed in each design vary depending on the application. In this way, the first part of the thesis has been dedicated to the development of optical fiber lasers. And it is focused on improving their properties in terms of stability, signal noise ratio, longitudinal modes, multiwavelength emission and bandwidth of the emission lines. Different classical configurations based on ring and linear cavity lasers have been addressed. Also, random cavity lasers, which have attracted great interest in recent years due to their longitudinal modeless behavior, have been studied. On the other hand, in the field of sensor monitoring, optical transducers based on micro-structured fibers have been developed, exploiting their advantages in comparison with standard optical fibers. Finally different optical sensor networks have been developed, whose main goal have been the improvement of features such as monitoring distance, multiplexing skill, sensitivity, dynamic range and stability., Este trabajo se ha llevado a cabo gracias a las aportaciones económicas recibidas
de los siguientes organismos y proyectos:
- Universidad Pública de Navarra mediante la ayuda de investigación
predoctoral, la ayuda para movilidad y las ayudas complementarias a
tesis doctorales.
- Secretaria de Estado de Investigación, Desarrollo e Innovación, Ministerio
de Economía y Competitividad de España a través del programa de
Formación del Personal Investigador y asociado al proyecto de investigación
TEC2010-20224-C02-01.
- Proyecto de investigación TEC2013-47264-C2-2-R a través del Fondo
Europeo de Desarrollo Regional (FEDER).
- Acción Europea COST- TD1001: Novel and Reliable Optical Fibre Sensor
Systems for Future Security and Safety Applications (OFSeSa)
- Proyecto INTERREG SUDOE Project “ECOAL-MGT - Ecological Management
of Coal Waste Piles, SOE3/P2/P714”.
- Ministerio de Economía y Competitividad de España a través del Proyecto
campus de excelencia-Iberus (Innocampus).
- Grupo de investigación Comunicaciones Ópticas y Aplicaciones Electrónicas
de la Universidad Pública de Navarra mediante la financiación
de material y participación en congresos., Programa Oficial de Doctorado en Tecnologías de las Comunicaciones (RD 1393/2007), Komunikazioen Teknologietako Doktoretza Programa Ofiziala (ED 1393/2007)

We investigate the deleterious effect and the error in Brillouin optical time-domain analyzers induced by the combination of a low extinction ratio pulse generation with the transient behavior of erbium-doped fiber amplifiers., The authors wish to acknowledge the financial support of the German Research Foundation (DFG SCHN 716/13-1),
Spanish Ministerio de Economía y Competitividad (TEC2016-76021-C2-1-R, TEC2013-47264-C2-2-R) and FEDER.

We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent detection. It is found that the large Brillouin gain that comes from the use of high power pulses induces a narrowing of the RF phase-shift spectrum that is measured in these sensors. This narrowing leads to a Brillouin frequency shift measurement error when the sensor is configured for dynamic measurements. However, the effect has been found to be less significant than that observed in dynamic slope-assisted BOTDA sensors based on amplitude., The authors wish to acknowledge the financial support from the Spanish Ministerio de Economía y Competitividad
through the project TEC2013-47264-C2-2-R, FEDER funds and Universidad Pública de Navarra.

In this work, a new all-polarization maintaining (PM) fiber loop mirror interferometer is proposed and validated as temperature and mechanical vibration sensor. The scheme employs the arms of a PM coupler as communication fibers, fused with a relative angle of 45° to the sensing fiber. The length of the arms is equal so their contribution in canceled, obtaining a total transfer function exclusively defined by the sensing fiber. The capabilities of the system as sensor are tested, achieving mechanical vibration and temperature sensing without crosstalk between measurands. In this manner, vibration frequencies up to 1.5 kHz have been monitored using a commercial interrogator with a scan rate of 1 Hz and a technique based on the fast Fourier transform. Additionally, the immunity of the setup to external perturbations in the communication fibers is studied and compared to the conventional approach., This work was supported in part by the Spanish Comisión Interministerial de Ciencia y Tecnología within projects TEC2013-47264-C2-2-R and TEC2016-76021-C2-1-R and in part by the Institute of Smart Cities by means of a postdoctoral fellowship.

In this work, we demonstrate a stable fiber sensing system for remote temperature measurements, where the sensing element is an array of four fiber Bragg gratings (FBGs) and sensor interrogation is achieved with a multi-wavelength erbium fiber ring laser. By introducing a feedback fiber loop in a fiber ring cavity, four laser emission lines were obtained simultaneously in single-longitudinal mode operation (SLM). The power instability obtained was lower than 0.5 dB with an optical signal-to-noise ratio (OSNR) higher than 50 dB for all the emitted wavelengths. The application of this system for remote temperature measurements has been demonstrated even though the SLM regime cannot be preserved., This work was supported by the Spanish Ministerio de Ciencia y Tecnologia through project TEC2013-47264-C2-2-R and the INTERREG SUDOE Project “ECOAL-MGT-Ecological Management of Coal Waste Piles, SOE3/P2/P714”.

In this work, two all-fiber loop mirrors using a clover microstructured fiber for the simultaneous measurement of temperature and strain are presented. The sensing heads are formed by a short piece of clover microstructured fiber with 35 mm and 89 mm length respectively. The geometry of the fiber allowed observing different interferences created by the microstructured fiber core section. Different sensitivities to
temperature and strain were obtained and, using a matrix method, it is possible to discriminate both physical parameters. Resolutions of ±2ºC and ±11 μ for the first structure and ±2.3ºC and ±18 μ for the second one, for temperature and strain, respectively, were attained., This work was supported in part by the European COST action TD1001, FEDER founds and the Spanish Government project TEC2013-47264-C2-2-R1.

We demonstrate the extension of the measurement range of Brillouin optical time-domain analysis (BOTDA)
sensors using a distributed Brillouin amplifier (DBA). The technique is based on injecting a DBA pump wave in
the fiber to generate an additional Brillouin interaction that amplifies the BOTDA pump pulses. Furthermore,
the differential pulse-width pair method is used to counteract the detrimental effects of the DBA amplification
on the temporal shape of the pulses. Experimental proof-of-concept results in a 50-km fiber link demonstrate
full compensation of the fiber’s attenuation with no penalty on the signal-to-noise ratio of the detected probe
wave., The authors acknowledge support from the Spanish Ministerio de Economa y Competitividad project TEC2013-47264-C2-2-R,
FEDER funds and the Universidad Pública de Navarra.

In this paper, a new multipoint optical fiber system for relative humidity measurements based on Sn02-FP (Fabry-Pérot) sensing heads and an optical interrogator as single active device is presented and characterized. The interrogation of the sensing heads is carried out by monitoring the Fast Fourier Transform phase variations of the FP (Fabry-Pérot) interference frequencies. This method allows to multiplex several sensors with different wavelength spacing interference pattern. The sensors operate within a wide humidity range (20%-90% relative humidity) with low crosstalk between them. Five sensing heads have been measured using two different channels of the optical interrogator. The availability of four channels in the interrogator allows to multiplex a higher number of sensors, reducing proportionally the cost of each sensing point., The authors thank the Spanish Government projects TEC 2013-47264-C2-2-R, TEC2016-76021-C2-1-R, TEC2013-
43679-R Innocampus and the INTERREG SUDOE Project SOE3/P2/P714 as well as to the FEDER Funds.

A hybrid Fabry-Pérot cavity sensing head based on a four-bridge microstructured fiber is characterized for temperature sensing. The characterization of this cavity is performed numerically and experimentally in the L-band. The sensing head output signal presents a linear variation with temperature changes, showing a sensitivity of 12.5 pm/°C. Moreover, this Fabry-Pérot cavity exhibits good sensitivity to polarization changes and high stability over time., The authors are grateful to the Spanish Government projects TEC 2013-47264-C2-2-R, Innocampus and the INTERREG SUDOE Project “ECOAL-MGT—Ecological Management of Coal Waste Piles, SOE3/P2/P714”, as well as to the FEDER Funds.

This work presents an experimental demonstration of a scheme based on an internally modulated fiber ring laser for high-sensitivity temperature sensing. The attained temperature resolution has been as low as ± 2pm even when a commercial FBG with a sensitivity of 10 pm/°C was used. Thus, a fivefold improvement in the temperature sensor resolution can be achieved when compared to a simple FBG interrogation scheme. In addition to this, the measuring range could be selected only by changing the frequency modulation of the fiber ring laser. This technology also allows to triple the photodiode bandwidth unambiguously when temperature or strain measurements are carried out, which is a remarkable achievement in term of cost reduction., This work was supported by the Spanish Government projects TEC2013-47264-C2-2-R and TEC2014-58048-C2-1-P.

Recently, random mirrors have been proposed as a method to create fiber laser cavities. This kind of cavity is based on cooperative Rayleigh scattering, which is generated along a fiber due to the material inhomogeneities presented in that fiber. In this work, basics of Random fiber lasers and different demonstrated lasing sensors systems for interrogating arrays of optical fiber sensors are shown. These systems use different kinds of amplification and cavities schemes and can interrogate optical fiber sensors located up to 225 km away., The authors would like to thank the financial support from Spanish Comisión Interministerial de Ciencia y Tecnología within projects TEC2016-76021-C2-1-R and TEC2013-47264-C2-2-R, Campus Iberus and FEDER funds from the European Union.

In this paper, the experimental study and characterization of a novel real-time switchable multi-wavelength fiber laser has been carried out. Two different gain materials, such as a 50 km SMF and a 2.5 km DCF fibers were characterized and compared, respectively. The MWFL can generate any wavelength combination with an emission lines distance of 50, 100 and 200 GHz fitting the ITU grid specifications. By using both Er-doped fiber and Raman amplification, a ~30 nm wide lasing window at the C band can be utilized to create up to 30 different lasing wavelengths into the ITU Grid, that can be switched automatically and in real-time
when desired. Utilization of such a laser for versatile interrogation of different sensing networks is also shown., This work was supported by the Spanish Government projects TEC2010-20224-C02-01, TEC2013-47264-C2-2-R, INNOCAMPUS and by the European SUDOE-Interreg Project ECOALMGT, and FEDER funds.

In this paper, a new optical fiber sensor for relative humidity measurements is presented and characterized. The sensor is
based on a SnO2 sputtering deposition on a microstructured optical fiber (MOF) low-finesse Fabry-Pérot (FP) sensing
head. The feasibility of the device as a breathing sensor is also experimentally demonstrated. The interrogation of the
sensing head is carried out by monitoring the Fast Fourier Transform phase variations of the FP interference frequency.
This method substitutes the necessity of tracking the optical spectrum peaks or valleys, which can be a handicap when noise or multiple contributions are present: therefore, it is low-sensitive to noise and to artifacts signal amplitude. The
sensor shows a linear behavior in a wide relative humidity range (20%–90% relative humidity) in which the sensitivity is
0.14 rad/%; the maximum observed instability is 0.007 rad, whereas the highest hysteresis is 5% RH. The cross correlation
with temperature is also considered and a method to lower its influence is proposed. For human breathing measurement,
the registered rising and recovery times are 370 ms and 380 ms respectively., The authors are grateful to A. Ortigosa, D. Erro, Dr. M. Bravo and Dr. R.A. Perez-Herrera. We also thank the Spanish
Government projects TEC2013-47264-C2-2-R, TEC 2016-76021-C2-1-R, TEC2016-78047-R, TEC2016-79367-C2-2-R,
Innocampus and the Cost Action MP 1401, as well as to the AEI/FEDER Funds.

In this work, the performance of five different fiber optic sensors at cryogenic temperatures
has been analyzed. A photonic crystal fiber Fabry-Pérot interferometer, two Sagnac interferometers,
a commercial fiber Bragg grating (FBG), and a -phase shifted fiber Bragg grating interrogated in
In this work, the performance of five different fiber optic sensors at cryogenic temperatures has been analyzed. A photonic crystal fiber Fabry-Pérot interferometer, two Sagnac interferometers, a commercial fiber Bragg grating (FBG), and a π-phase shifted fiber Bragg grating interrogated in a random distributed feedback fiber laser have been studied. Their sensitivities and resolutions as sensors for cryogenic temperatures have been compared regarding their advantages and disadvantages. Additionally, the results have been compared with the given by a commercial optical backscatter reflectometer that allowed for distributed temperature measurements of a single mode fiber., Financial support from the Spanish Comisión Interministerial de Ciencia y Tecnología within
projects TEC2013-47264-C2-2-R and TEC2016-76021-C2-1-R, Innocampus, Cost action MP1401 and the FEDER
funds is acknowledged. Also the support of the Institute of Smart Cities by means of a postdoctoral fellowship.

The international research community has been nurturing photonic technologies over the years, aiming to develop cost effective solutions for a number of applications. Optical fibers offer an efficient solution for sensing and communication fields. Fiber optic sensors present appealing characteristics that make them very attractive when compared with conventional electric sensors such as immunity to electromagnetic interferences, as well as unfavorable environments, small size, ability for multiplexing and remote sensing. Another important aspect when using optical fibers is the ability to simultaneously use them as sensors and communication channels.
The primary motivation for this PhD work was the study and development of new structures based in photonic crystal fibers. The main idea was to take advantage of the know-how of our group in fiber-optic communications and photonic sensing, as well as the recent advances in the sensing area to develop new structures based on the platform of photonic crystal fibers., The work presented in this thesis was accomplished due to different financial supports, as well as co-work relationships inside of the university and outside during research times developed abroad. Due to the financial support obtained, thanks should be pay to: Spanish Government project TEC 2013-47264-C2-2-R; Innocampus and the INTERREG SUDOE Project “ECOAL-MGT-Ecological Management of Coal Waste Piles, SOE3/P2/P714”; As well as FEDER founds; Spanish Government project TEC 2016-76021-C2-1-R; Cost action MP1401;, Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011), Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011)

The combustion of coal wastes resulting from mining is of particular environmental concern, and the importance of proper management involving real-time assessment of their status and identification of probable evolution scenarios is recognized. Continuous monitoring of the combustion temperature and emission levels of certain gases allows for the possibility of planning corrective actions to minimize their negative impact on the surroundings. Optical fiber technology is well suited to this purpose and here we describe the main attributes and results obtained from a fiber optic sensing system projected to gather data on distributed temperature and gas emissions in these harsh environments., This work was supported by the project ECOAL—MGT—SOE3/P2/P714, Ecological Management of Coal Waste Piles in Combustion, SUDOE—Interreg IV B. The author Joana Ribeiro benefited from a post-doctoral scholarship financed by the Portuguese Science and Technology Foundation, FCT (Ref: SFRH/BPD/76230/2011). The authors acknowledge the funding provided by the Institute of Earth Sciences under contracts UID/GEO/04683/2013 with FCT and COMPETE POCI-01-0145-FEDER, and also by the Spanish Ministry of Economy via project TEC2013-47264-C2-2-R. The work of Sonia Martín-Lopez was supported by the Spanish Ministry of Science and Innovation through a “Ramón y Cajal” Contract. Miguel González Herráez acknowledges the support of the European Research Council through Starting Grant U-FINE (Grant No. 307441).

In this work, two all polarization-maintaining (PM) highbirefringence
(Hi-Bi) fiber loop mirrors (FLM) which are immune to
external polarization perturbations are validated both theoretically and
experimentally. Simplified and stable versions of classical FLMs were
attained using a PM-coupler and by fusing the different Hi-Bi fiber sections
with an adequate rotation angle between them. Since the polarization states
are fixed along the whole fiber loop, no polarization controllers are needed.
This simplifies the operation and increases the stability of the systems,
which were also validated as ultra-high resolution sensors, experimentally
obtaining a resolution of 6.2·10−4 °C without averaging., This work was supported by the Spanish Government project TEC2013-47264-C2-2-R and
the FEDER funds.

Two interrogation techniques for plasmonic tilted fiber Bragg grating sensors are reported and experimentally tested. Typical interrogation methods are usually based on tracking the wavelength shift of the most sensitive cladding mode, but for biosensing applications, spectrometer-based methods can be replaced by more efficient solutions. The proposed techniques thus rely on the measurement of the induced changes in optical power. The first one consists of a properly polarized tunable laser source set to emit at the wavelength of the sensor most sensitive mode and an optical power meter to measure the transmitted response. For the second method, a uniform fiber Bragg grating is photo-inscribed beyond the sensor in such a way that its central wavelength matches the sensor most sensitive mode, acting as an optical filter. Using a LED source, light reflected backwards by this grating is partially attenuated when passing through the sensor due to plasmon wave excitation and the power changes are quantified once again with an optical power meter. A performance analysis of the techniques is carried out and they both result competitive interrogation solutions. The work thus focuses on the development of cost-effective alternatives for monitoring this kind of biosensors in practical situations., Á. González-Vila is supported by the F.R.S.-FNRS through a FRIA grant. C. Caucheteur is supported by the F.R.S.-
FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting
Independent Grant PROSPER (grant agreement No. 280161 – http://www.umons.ac.be/erc-prosper), from the Spanish
Comisión Interministerial de Ciencia y Tecnología within projects TEC2016-76021-C2-1-R and TEC2013-47264-C2-2-
R and from SUDOE ECOAL-MGT and FEDER funds from the European Union.

This work presents an all-fiber wavelength converter based on a highly Er-doped fiber laser, which has been experimentally demonstrated. Frequencies from several kilohertz up to tens of gigahertz have been investigated. Making use of the gain competition in the amplifying medium, the wavelength conversion can be carried out over not only one lasing wavelength, but over several. These analyses have also been carried out for more than one all-fiber ring structure, demonstrating the viability of this wavelength converter. To the best of our knowledge, this is the first time an Er-doped fiber wavelength converter has been validated., This work was supported by the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT TEC2013-47264-C2 and TEC2016-76021-C2) and FEDER Funds.

We present two techniques that provide an extension of the dynamic range of coherent Brillouin optical time-domain analysis (BOTDA) sensors for dynamic measurements. This type of BOTDA sensors rely on self-heterodyne detection of a phase-modulated probe wave and the dynamic range for fast measurements is limited to the linear region of the RF phase-shift spectrum measured. The first method for range extension that we introduce is based on launching pump pulses containing multiple frequency components. This makes the Brillouin spectra generated by each component to overlap, providing a wider linear region of the detected RF phase-shift spectrum and allowing to measure larger Brillouin frequency shift variations. The second method relies on shortening the length of the pump pulses, which leads to the broadening of the detected RF spectra. The theoretical fundamentals of both range enhancing techniques are presented. Moreover, we experimentally demonstrate that they provide a threefold to fourfold enhancement in the dynamic range. Finally, the factors limiting their performance are determined: for the multi-frequency pump pulse technique, it is the worsening of Kerr non-linear effects due to the simultaneous propagation of multiple spectral components in the fiber, and, for the pulse-shortening method, it is the signal-to-noise ratio penalty linked to the reduction of the magnitude of the Brillouin interaction., This work was supported in part by the Universidad Pública de Navarra, in part by the Feder funds, and in part by the Spanish Ministerio de Economía y Competitividad through Project TEC2013-47264-C2-2-R and Project TEC2016-76021-C2-1-R.

We introduce a Brillouin optical time domain analysis (BOTDA) sensor deploying a novel polarization diversity technique based on the use of two orthogonal pump pulses which simultaneously interact with a phase modulated probe wave. The orthogonallity of the two pump pulses guarantees that two complementary Brillouin interactions take place at each position of the fiber, so that polarization independent measurements are performed throughout the fiber even if no averaging is applied. This feature can be exploited in dynamic distributed BOTDA sensors to reduce the measurement time, as no extra averaging is required to compensate the polarization dependence of Brillouin interaction. Proof of concept experiments demonstrate the feasibility of the technique by employing a completely passive scheme to generate the orthogonal pump pulses. Furthermore, the technique is stable and easy to implement, making it a perfect candidate for practical sensor implementations., The authors wish to
acknowledge the financial support from the Universidad Pública de Navarra
and Spanish Ministerio de Ciencia e Innovación through the project
TEC2013–47264–C2–2–R.

Trabajo presentado a IEEE Sensors 2014, celebrado en Valencia (España) del 2 al 5 de noviembre de 2014., We demonstrate, for the first time to our knowledge, the extension of the measurement range of Brillouin distributed sensors (BDS) by using a distributed Brillouin amplifier (DBA) to compensate the attenuation of the sensor's pump pulses. The technique is based on injecting an extra DBA pump wave in the fiber to generate an additional Brillouin interaction to that used for sensing. Moreover, the bandwidth of the DBA is tailored to fit that of the pump pulses by deploying a wavelength modulation of the DBA pump that is synchronized to the pump. Experimental proof of concept results demonstrate a fivefold enhancement of the measurement range of a BDS, from 10.36 km to more than 50 km in this particular case. Moreover, it is found that the use of the DBA does not introduce any significant penalty in the detection signal to noise ratio, highlighting the potential of the technique to provide much larger sensing lengths., The authors wish to acknowledge the financial support from
the Universidad Pública de Navarra and Spanish Ministerio de
Ciencia e Innovación through the projects TEC2010-20224-
C02-01 and TEC2013-47264-C2-2-R.

We demonstrate a simple technique to provide conventional Brillouin optical time-domain analysis sensor with compensation for pump pulse attenuation and tolerance to non-local effects. The technique is based on operating the sensor in loss configuration so that energy is transferred from the probe wave to the pump pulse that becomes amplified as it counter-propagates with the probe wave. Furthermore, the optical frequency of the probe wave is modulated along the fiber so that the pump pulse experiences a flat gain spectrum that equally amplifies all the spectral components of the pulse, hence, preventing distortion. The method is experimentally demonstrated in a 100-km fiber link, obtaining a measurement uncertainty of 1 MHz at the worst-contrast position., The authors wish to acknowledge the financial support from the Spanish Ministerio de Economía y Competitividad
through project TEC2013-47264-C2-2-R, FEDER funds and the Universidad Pública de Navarra.

Brillouin optical time-domain analysis (BOTDA) sensors have great potential to provide distributed measurements of temperature and strain over large structures with high spatial resolution and measurement precision. However, their performance ultimately depends on the amount of probe and pump pulse power that can be injected into the sensing fiber, which determines the signal-to-noise ratio of the detected measurement signal. The probe wave power is constrained by the generation of noise induced by spontaneous Brillouin scattering and at lower power by the so-called non-local effects. In this work, we focus on the latter. We review the physical origins of non-local effects and analyze the performance impairments that they bring. In addition, we discuss the different methods that have been proposed to counteract these effects comparing their relative merits and ultimate performance. Particularly, we focus on a technique that we have devised to compensate non-local effects which is based on introducing an optical frequency modulation or dithering to the probe wave. This method is shown to provide a comprehensive solution to most of the impairments associated with non-local effects and also to enable some side benefits, such as amplification of the pump pulses to compensate the attenuation of the fiber., The authors wish to acknowledge the financial support of the Spanish Ministerio de Economía y Competitividad through the projects TEC2013-47264-C2-2-R and TEC2016- 76021-C2-1-R, FEDER funds and the Universidad Pública de Navarra.

We demonstrate a simple technique to enhance the signal-to-noise ratio (SNR) in Brillouin optical time-domain analysis sensors by the addition of gain and loss processes. The technique is based on the shift of the pump pulse optical frequency in a double-sideband probe system, so that the gain and loss processes take place at different frequencies. In this manner, the loss and the gain do not cancel each other out, and it makes possible to take advantage of both informations at the same time, obtaining an improvement of 3 dB on the SNR. Furthermore, the technique does not need an optical filtering, so that larger improvement on SNR and a simplification of the setup are obtained. The method is experimentally demonstrated in a 101 km fiber spool, obtaining a measurement uncertainty of 2.6 MHz (2σ) at the worst-contrast position for 2 m spatial resolution. This leads, to the best of our knowledge, to the highest figure-of-merit in a BOTDA without using coding or raman amplification., The authors wish to acknowledge the financial support of the Spanish Ministerio de Economía y Competitividad
through the projects TEC2013-47264- C2-2-R and TEC2016-76021-C2-1-R, FEDER funds and the Universidad Pública de Navarra.

We study the performance limits of mono-color cyclic coding applied to Brillouin optical time-domain analysis (BOTDA) sensors that use probe wave dithering. BOTDA analyzers with dithering of the probe use a dual-probe-sideband setup in which an optical frequency modulation of the probe waves along the fiber is introduced. This avoids non-local effects while keeping the Brillouin threshold at its highest level, thus preventing the spontaneous Brillouin scattering from generating noise in the deployed sensing fiber. In these conditions, it is possible to introduce an unprecedented high probe power into the sensing fiber, which leads to an enhancement of the signal-to-noise ratio (SNR) and consequently to a performance improvement of the analyzer. The addition of cyclic coding in these set-ups can further increase the SNR and accordingly enhance the performance. However, this unprecedented probe power levels that can be employed result in the appearance of detrimental effects in the measurement that had not previously been observed in other BOTDA set-ups. In this work, we analyze the distortion in the decoding process and the errors in the measurement that this distortion causes, due to three factors: the power difference of the successive pulses of a code sequence, the appearance of first-order non-local effects and the non-linear amplification of the probe wave that results when using mono-color cyclic coding of the pump pulses. We apply the results of this study to demonstrate the performance enhancement that can be achieved in a long-range dithered dual-probe BOTDA. A 164-km fiber-loop is measured with 1-m spatial resolution, obtaining 3-MHz Brillouin frequency shift measurement precision at the worst contrast location. To the best of our knowledge, this is the longest sensing distance achieved with a BOTDA sensor using mono-color cyclic coding., H. Iribas and A. Loayssa wish to acknowledge the financial support of the Spanish Ministerio
de Economía y Competitividad through the projects TEC2013-47264-C2-2-R and TEC2016-
76021-C2-1-R, FEDER funds and the Universidad Pública de Navarra.

In this paper, we present the results of a new pH sensor based on a polyaniline (PAni) coating on the surface of a tilted fiber Bragg grating. The pH-sensitive PAni was deposited by in situ chemical oxidative polymerization. The performance of the fabricated pH sensor was tested and the obtained pH values were compared with the results obtained using a pH meter device. It was found that the sensor exhibits response to pH changes in the range of 2-12, achieving a sensitivity of 46 pm/pH with a maximum error due to the hysteresis effect of ±1.14 pH. The main advantages of this PAni-TFBG pH sensor are biochemical compatibility, temperature independence, long-term stability and remote real-time multipoint sensing features. This type of sensor could be used for biochemical applications, pipeline corrosion monitoring or remote-multipoint measurements., Á. González-Vila is supported by the F.R.S.-FNRS through a FRIA grant. C. Caucheteur is supported by the F.R.S.-
FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting
Independent Grant PROSPER (grant agreement No. 280161 – http://www.umons.ac.be/erc-prosper), from the Spanish
Comisión Interministerial de Ciencia y Tecnología within projects TEC2016-76021-C2-1-R and TEC2013-47264-C2-2-
R and from SUDOE ECOAL-MGT and FEDER funds from the European Union. The authors also acknowledge the
financial support of CAPTINDOOR WBGREEN programs (Walloon Region of Belgium).