Ultra-long range Brillouin Optical Time Domain Analysis

Nuestro estilo de vida actual, basado en la industrialización y el consumo, requiere de grandes infraestructuras que puedan dar servicio a todas las necesidades asociadas al entorno: autopistas, líneas ferroviarias de alta velocidad, presas, gasoductos, oleoductos, estaciones de aerogeneración, líneas de alta tensión, etc. Evidentemente, la seguridad de las citadas estructuras es una prioridad ya que decenas, cientos, e incluso miles de vidas pueden estar en riesgo en caso de accidente. Ese creciente requisito en términos de seguridad, está aumentando de manera considerable el desarrollo y aplicación de sensores de fibra óptica en detrimento de los típicos sensores eléctricos. Este reemplazo se debe a ciertas ventajas que tienen los dispositivos basado en fibra óptica: pequeño tamaño y ligereza, menor coste, baja atenuación, capacidad de multiplexación, inmunidad al ruido electromagnético o resistencia a temperaturas extremas. Evidentemente, no todos los sensores de fibra óptica son aplicables a cualquier tipo de estructura o aplicación. En el campo de las grandes infraestructuras (> 25-30 km), los sistemas de fibra basados en la tecnología BOTDA (Brillouin Optical Time Domain Analysis) están tomando cada vez mayor protagonismo debido a sus cualidades en rango de medida (> 50 km) y resolución ( 100 km), donde la fibra tenga que ir y volver a la estación de sensado (> 200 km) sin posibilidad de que la luz sea amplificada por algún elemento intermedio. Además, cada vez se demanda más la posibilidad de monitorizar grandes estructuras (> 100 km) con resoluciones por debajo del metro, lo que dificulta considerablemente la implementación de los sistemas BOTDA. En este trabajo de tesis, se desarrolla un estudio cuidadoso de todos los problemas asociados al incremento de distancia y resolución en sistemas BOTDA y, consecuentemente, se proponen técnicas aplicables para evitarlos. En particular, abordaremos en profundidad los desafíos y las constantes de diseño de los sistemas BOTDA asistidos por Raman. La amplificación Raman en sistemas BOTDA implica un aumento del rango de sensado pero, desgraciadamente, introduce un ruido en la señal detectada denominado RIN (Relative Intensity Noise). Para evitar los problemas asociados a la transferencia de RIN, propondremos diferentes métodos capaces de eliminar parcialmente las características típicas del ruido RIN. Gracias a la aplicación de estas nuevas técnicas y procedimientos de optimización, se demuestra de manera experimental el sensado a lo largo de 100 km con 0,5 metros de resolución y, por primera vez, una medición lineal de 240 km de fibra (120 km de fibra sensible) con una resolución de 5 metros

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

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

Simultaneous gain and phase profile determination on an interferometric BOTDA

24th International Conference on Optical Fibre Sensors, 96343Y, Curitiba, Brazil, September 28, 2015.Up to now, complex (phase and intensity) measurements in Brillouin Optical Time-Domain Analysis (BOTDA) systems required complex phase modulation methods and high-bandwidth (multi-GHz) detection. In this work, we propose a novel technique that is able to retrieve simultaneously both gain/loss and phase characteristics of the Brillouin interaction by just introducing a Sagnac Interferometer (SI) 011 a standard BOTDA sensing scheme. The technique is described analytically and demonstrated experimentally. With this technique, a reliability increase is produced since redundant measurements can be performed.European CommissionMinisterio de Economía y CompetitividadUniversidad de AlcaláComunidad de MadridGeneralitat Valencian

Limits of BOTDA Range Extension Techniques

Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. Particularly, Brillouin Optical Time Domain Analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation or environmental applications. The extension of the measuring range in these sensors has therefore become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results.European CommissionMinisterio de Economía y CompetitividadUniversidad de Alcal

Limits of BOTDA Range Extension Techniques

Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. Particularly, Brillouin Optical Time Domain Analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation or environmental applications. The extension of the measuring range in these sensors has therefore become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results.European CommissionMinisterio de Economía y CompetitividadUniversidad de AlcaláMinisterio de Ciencia e InnovaciónComunidad de Madri

All-optical flip-flop based on dynamic Brillouin gratings

One of the most essential building blocks in modem electronics is the flip-flop. A flip-flop operates bi-stably between two states, remaining at a given output level (high or low level) until a specific input control signal changes. For many years, photonics has attempted to build all-optical flip-flops [1-3]; however, the success of these approaches has usually been limited by the dependence of the bi-stable operation on bit-rate and optical power [1, 2]. Furthermore, in most of the reported demonstrations, the storage time is inherently short. Typically, the figure-of-merit of these devices is measured by the time-bandwidth product, which is defined as the storage time of the device times the available bandwidth. State-of-the-art values are in the order of 10-100.This work proposes a method to generate all-optical flip-flops based on dynamic Brillouin gratings (DBGs) in polarisation maintaining fibres (PMF) [4]. Contrarily to existing approaches, this method can allow extremely long storage times and arbitrarily high bandwidth response. The technique relies on generating a very long, weak DBG along a PMF. The experimental setup here used as a proof-of-concept is shown in Fig. 1(a) (see ref. 4 for details). The DBG is generated by launching two continuous-wave pumps (Pump1 and Pump2) through the opposite sides of a 1 m-long Panda PMF. Both pumps are amplified by Erbium-doped fibre amplifiers (EDFAs) up to 25 dBm and aligned to the fast axis of the PMF. An electro-optic modulator is used to shift the optical frequency of one of the pumps, so that the frequencies fulfil the condition: fPump1 = fPump2 - QB, where QB (=10.8 GHz) is the Brillouin frequency along the fast axis of the PMF. The created DBG acts as the all-optical equivalent of an integrator [4]. To read the DBG, 300 ps pulses with controlled phase are generated (see lower branch in Fig. 1(a)) and launched along the slow axis of the PMF at the probe frequency fProbe = (nfast/nslow)fPump1, where nfast and nslow are the fibre refractive indexes along fast and slow axes. The output state of the flip-flop is changed (i.e. set and reset) using pulses with opposite phases. This flip-flop response is then observed at frequency

Highly-sensitive distributed birefringence measurements based on a two-pulse interrogation of a dynamic Brillouin grating

A method for distributed birefringence measurements is proposed based on the interference pattern generated by the interrogation of a dynamic Brillouin grating (DBG) using two short consecutive optical pulses. Compared to existing DBG interrogation techniques, the method here offers an improved sensitivity to birefringence changes thanks to the interferometric effect generated by the reflections of the two pulses. Experimental results demonstrate the possibility to obtain the longitudinal birefringence profile of a 20 m-long Panda fibre with an accuracy of ~10-8 using 16 averages and 30 cm spatial resolution. The method enables sub-metric and highly-accurate distributed temperature and strain sensing

All-optical flip-flops based on dynamic Brillouin gratings in fibers

A method to generate an all-optical flip-flop is proposed and experimentally demonstrated based on dynamic Brillouin gratings (DBGs) in polarization maintaining fibers. In a fiber with sufficiently uniform birefringence, this flip-flop can provide extremely long storage times and ultra-wide bandwidth. The experimental results demonstrate an all-optical flip-flop operation using phase-modulated pulses of 300 ps and a 1 m long DBG. This has led to a time-bandwidth product of ∼30, being in this proof-of-concept setup mainly limited by the relatively low bandwidth of the used pulses and the short fiber length