Optical Fiber Sensors for Temperature and Strain Measurement

Optical fiber sensors have already been developed from the experimental stage to practical applications in the past 20 years. There is no doubt that this technology can bring a wealth of applications, ranging from sensors in medical industry, aerospace and wind-energy industries, through to distributed sensors in oil and gas industry. Among a large amount of physical and chemical parameters which optical fiber sensors could measure, temperature and strain are the most widely studied. This thesis presents several low-cost optical fiber sensor configurations primarily for temperature and strain measurement. Several basic optical fiber components which are good candidates as optical fiber sensors are used in our experiments, such as fiber Bragg gratings (FBGs), multimode fibers (MMFs), small-core dispersion compensation fibers (SCDCFs), high-birefringence fiber loop mirrors (HBFLMs), and polarization-maintaining photonic crystal fibers (PMPCFs). Temperature and strain cross sensitivity is a crucial issue when designing high performance optical fiber sensors, since most of the sensing components are both sensitive to temperature and strain. This would introduce an error when measuring each of them independently. We developed several schemes to overcome this problem by cascading an FBG and a section of MMF, inserting an FBG into an HBFLM, and space division multiplexing two HBFLMs. By measuring the wavelength shifts of the two independent components' spectra in each scheme, simultaneous measurement of temperature and strain could be achieved. However, all the above schemes need optical spectrum analyzers to monitor the spectral information, which increases the cost of the system and limits the operation speed. In order to avoid using optical spectrum analyzers, we use an intensity-based interrogation method with MMFs and HBFLMs as edge filters. By measuring power ratio changes, instead of monitoring spectra shifts, simultaneous measurement of temperature and strain could be realized with a low cost and high speed. The resolutions of the above five configurations are between 0.26 - 1.2 ^oC in temperature and 9.21 - 29.5 με in strain, which are sufficient for certain applications. We also investigate the sensing applications with the SCDCF. Since the cutoff wavelength of this kind of fiber is around 1663 nm, which makes it naturally an MMF in the wavelength range of 1550 nm. By slightly offsetting the core of the SCDCF with respect to that of the standard single-mode fiber (SMF), a high extinction ratio could be achieved with almost 9 dB. When a lateral force (lateral strain) applied on the SCDCF, extinction ratio will decrease. The change of the extinction ratio is almost independent of temperature variation. The measured extinction ratio change has a good quadratic relationship with respect to applied lateral force. This feature could be used to measure lateral force (lateral strain). In addition, we also use this feature to realize simultaneous measurement of both the longitudinal strain and lateral strain, since the applied longitudinal strain results in the whole spectrum shift. Moreover, a miniature high temperature sensor could also be made using the SCDCF. One end of a 4-mm long SCDCF is spliced directly to SMF with the other end cleaved. By monitoring the reflection spectrum of the SCDCF, temperature information could be obtained. This sensing head is very compact and could realize high temperature measurement up to 600 ^oC. Recently, a kind of PMPCF has been found to have very small responses to temperature change. This offers an opportunity to measure other parameters without considering temperature influence. We construct a compact 7-mm long transmission-type sensor with this kind of PMPCF. The interference spectrum generated by the coupling of cladding modes and core mode is obtained by slightly offsetting the PMPCF core to SMF core. The experiment shows that the interference spectrum is almost unchanged within the temperature range of 25-60 ^oC. The presented sensor has the potential to be used to measure strain and refractive index in the normal environment without temperature discrimination for practical applications

Refractive-index-sensing optical comb based on photonic radio-frequency conversion with intracavity multi-mode interference fiber sensor

Optical frequency combs (OFCs) have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology. However, if the utility of OFCs can be further expanded beyond their application by exploiting new aspects of OFCs, this will lead to new developments in optical metrology and instrumentation. Here, we report a fiber sensing application of OFCs based on a coherent link between the optical and radio frequencies, enabling high-precision refractive index measurement based on frequency measurement in radio-frequency (RF) region. Our technique encodes a refractive index change of a liquid sample into a repetition frequency of OFC by a combination of an intracavity multi-mode-interference fiber sensor and wavelength dispersion of a cavity fiber. Then, the change in refractive index is read out by measuring the repetition frequency in RF region based on a frequency standard. Use of an OFC as a photonic RF converter will lead to the development of new applications in high-precision fiber sensing with the help of functional fiber sensors and precise RF measurement

Contribution to the development of new photonic systems for fiber optic sensing applications

En este trabajo de doctorado se presentan nuevos sistemas y subsistemas de sensores de fibra óptica. Así, se proponen y desarrollan nuevas técnicas, componentes y tecnologías basadas en láseres de fibra con espejos distribuidos (random), fibras de cristal fotónico, estructuras de luz lenta, multiplexores de inserción y extracción (add and drop), conmutadores tele-alimentados por luz, reflectometría óptica tanto en el dominio del tiempo como de la frecuencia o filtros ópticos reconfigurables. También se han demostrado nuevas aplicaciones para estructuras de sensores tradicionales y técnicas de medida ya conocidas. Todas ellas dirigidas a la mejora del funcionamiento de los actuales transductores, redes de sensores y aplicaciones de monitorización de salud estructural. De este modo, y en primer lugar, se han desarrollado nuevos transductores puntuales. En concreto, dos sensores interferométricos basados en fibras de cristal fotónico y otro basado en una estructura resonante en anillo. También se han realizado diferentes redes de sensores utilizando OTDRs comerciales. Por un lado, se han multiplexado diferentes sensores utilizando una red en forma de bus y, por el otro, se ha interrogado de manera remota un sensor FLM/LPG a una distancia de 253 km sin necesidad de amplificación. Se han estudiado láseres basados en efecto de realimentación distribuida random (RDFB) para su uso en interrogación de sensores. Para ello, se han demostrado dos nuevos láseres multi-longitud de onda y también, por primera vez, se ha modulado un laser random. Después, se han demostrado experimentalmente varias redes de sensores de fibra óptica teniendo en cuenta los principales desafíos que estas presentan: multiplexar varios sensores en una misma red y permitir su monitorización de manera remota. En primer lugar, se han multiplexado sensores basados en la modulación de la intensidad óptica utilizando técnicas de multiplexación en dominio del tiempo. En segundo lugar, se han multiplexado sensores basados en fibras de cristal fotónico. En tercer lugar, se presentan tres nuevos métodos para la medida remota de sensores. Por último, se incluye la demostración de un conmutador de fibra óptica tele-alimentado a través de luz. Éste se utiliza en tres redes diferentes para añadir robustez e incrementar la versatilidad en la multiplexación. Finalmente, se han realizado tres pruebas de campo para aplicaciones de monitorización de salud estructural.In this PhD work, different new photonic systems and subsystems for fiber optic sensing are presented. The aim of this thesis has been to contribute to the fiber optic sensors field using modern techniques, components and technologies such as random fiber lasers, photonic crystal fibers, slow light structures, add and drop multiplexers, powered by light switches, optical frequency and time domain reflectometry or reconfigurable optical filters, among others. New applications of traditional sensing structures or techniques have been also demonstrated. All of them focused on improving the performance of current sensors transducers, multiplexing networks and structural health monitoring applications. Thus, new point transducers have been developed: two of them are interferometric sensors based on photonic crystal fibers; and another one is based on a fiber ring resonator structure. Fiber optic sensor networks using commercial OTDRs have been also explored. On the one hand, different sensors have been successfully multiplexed in the same bus network. And, on the other hand, a FLM/LPG sensor was remotely interrogated at a distance of 253 km without using amplification. Random distributed feedback (RDFB) lasers have been explored for sensors interrogation. Two multi-wavelength Raman fiber lasers suitable for sensors interrogation have been demonstrated. Also, a random fiber laser has been internally modulated for the first time. Then, some experimental demonstrations of fiber optic sensors networks have been carried out taking into account the principal challenges they pose: multiplexing a number of optical sensors in a single networks, and enabling the possibility of remote sensing. Firstly, intensity sensors using TDM technology have been multiplexed. Secondly, PCF sensors have been successfully multiplexed. Thirdly, three new approaches to enable remote sensing are presented. Finally, a remote powered by light fiber optic switch have been included in three networks in order to add robustness and multiplexing versatility.Este trabajo se ha llevado a cabo gracias a las aportaciones económicas recibidas de los siguientes organismos, entre otros: - Secretaría 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. - Universidad Pública de Navarra mediante las ayudas a tesis doctorares. - Acción Europea COST- TD1001: Novel and Reliable Optical Fibre Sensor Systems for Future Security and Safety Applications (OFSeSa) - También se ha recibido financiación del Proyecto de Investigación de la Secretaría de Estado de Investigación, Desarrollo e Innovación, Ministerio de Economía y Competitividad de España TEC2013-47264-C2-2-R, de Innocampus, del Proyecto Europeo SUDOE-ECOAL-Intereg Project ECOAL-MGT y de los Fondos FEDER.Programa Oficial de Doctorado en Tecnologías de las Comunicaciones (RD 1393/2007)Komunikazioen Teknologietako Doktoretza Programa Ofiziala (ED 1393/2007