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[ES] La fibra óptica ha supuesto una gran revolución en el mundo de las telecomunicaciones debido a su alta capacidad de transmisión y sus bajas pérdidas. Hoy en día no sería posible transportar la cantidad de tráfico que se genera en internet si no fuera por sis- temas de comunicaciones basados en fibras ópticas. Sin embargo, el número de dispo- sitivos conectados a internet es cada vez mayor, por lo que la capacidad de la fibra óptica estándar de un solo núcleo se puede ver limitada en un futuro no muy lejano. Una forma de aumentar dicha capacidad es utilizar fibras ópticas con varios núcleos. Actualmente existe un gran interés sobre la investigación en este tipo de fibras para aplicaciones de telecomunicaciones, por lo que no es difícil encontrar fibras multinú- cleo comerciales. Aunque el uso más común de la fibra óptica es para telecomunicaciones, también se puede utilizar como sensor. Uno de los métodos más comunes para la implementa- ción de sensores es la inscripción de redes de difracción en fibras ópticas de un solo núcleo. Sin embargo, la inscripción de redes de dirección en fibras de múltiples núcleos abre nuevas líneas de investigación para el desarrollo de sensores avanzados. En esta tesis se ha estudiado distintos tipos de redes de difracción inscritas en una fibra de siete núcleos para su aplicación en la implementación de sensores. En primer lugar, se describe el sistema de fabricación que permite inscribir distintos tipos de redes de difracción en la fibra multinúcleo de forma selectiva, es decir, permite seleccionar en que núcleos se va a inscribir la red. Mediante este sistema se han inscrito redes de periodo largo y posteriormente se han caracterizado como sensor de deformación, tor- sión y curvatura. Después, se han inscrito redes de Bragg inclinadas para aumentar de forma intencionada la diafonía entre los núcleos de la fibra mediante el acoplo de luz entre ellos. Además, se ha demostrado experimentalmente que esta diafonía es sensible a la deformación de la fibra, a la curvatura, a la temperatura y al índice de refracción que rodea la fibra. Por otro lado, se ha demostrado que las redes de Bragg inscritas en fibras multinúcleo se pueden utilizar para implementar sensores de curvatura capaces de operar en entornos radioactivos. Finalmente se han fabricado redes de Bragg rege- neradas capaces de operar a altas temperaturas, estas redes se han caracterizado como sensor de temperatura, deformación y curvatura. / [CAT] La fibra òptica ha suposat una gran revolució en el món de les telecomunicacions a causa de la seua alta capacitat de transmissió i les seues baixes pèrdues. Hui en dia no seria possible transportar la quantitat d'informació que es genera en internet si no fos pels sistemes de comunicacions basats en fibres òptiques. No obstant això, el nombre de dispositius connectats a internet es cada vegada més gran, per la qual cosa la capacitat de la fibra òptica estàndard d'un sol nucli es pot veure limitada en un futur no gaire llunyà. Una manera d'augmentar aquesta capacitat es utilitzar fibres òptiques amb diversos nuclis. Actualment existeix un gran interès sobre la investigació en aquesta mena de fibres per a aplicacions de telecomunicacions, per la qual cosa no es difícil trobar fibres de múltiples nuclis comercials. Encara que l'ús mes comú de la fibra òptica es per a telecomunicacions, també es pot utilitzar com a sensor. Un dels mètodes més comuns per a la implementació de sensors es la inscripció de xarxes de difracció en fibres òptiques d'un sol nucli. No obstant això, la inscripció de xarxes de difracció en fibres de múltiples nuclis obri noves línies d'investigació per al desenvolupament de sensors més complexos. En aquesta tesi s'ha estudiat diferents tipus de xarxes de difracció inscrites en una fibra de set nuclis per a la seua aplicació en la implementació de sensors en fibra òptica. En primer lloc, es descriu el sistema de fabricació de xarxes de difracció que permet inscriure diferents tipus de xarxes de difracció en la fibra de múltiples nuclis de manera selectiva, es a dir, permet seleccionar en que nuclis s'inscriurà la xarxa. Mitjançant aquest sistema s'han inscrit xarxes de període llarg i posteriorment s'han caracteritzat com a sensor de deformació, torsió i curvatura. Després, s'han inscrit xarxes de Bragg inclinades per a augmentar de manera intencionada la diafonia entre els nuclis de la fibra mitjançant l'acoblament de llum entre ells. A més d'això, s'ha demostrat experimentalment que aquesta diafonia es sensible a la deformació de la fibra, a la curvatura, a la temperatura i a l'índex de refracció que envolta la fibra. D'altra banda, s'ha demostrat que les xarxes de Bragg inscrites en fibres múltiples nuclis es poden utilitzar per a implementar sensors de curvatura que poden operar en entorns radioactius. Finalment s'han fabricat xarxes de Bragg regenerades que suporten altes temperatures, aquestes xarxes s'han caracteritzat com a sensor de temperatura, deformació i curvatura. / [EN] Optical fiber has been a great revolution in the world of telecommunications due to its high transmission capacity and low attenuation. Today it would not be possible to transport the amount of traffic that is generated on the Internet without communication systems based on optical fibers. However, the number of devices connected to the Internet is increasing, so the capacity of standard single-core fiber optics may be limited so far in the future. One way to increase this capacity is to use multi-core optical fibers. Nowadays is a great interest in research in this type of fibers for telecommunications applications, so it is not difficult to find commercial multicore fibers. Although the most common use of fiber optics is for telecommunications, it can also be used as a sensor. One of the most common methods for sensor implementation is the inscription of diffraction gratings on single-core optical fibers. However, the enrollment of steering networks in multi-core fibers opens new lines of research for the development of advanced sensors. In this thesis, different types of diffraction gratings inscribed in a seven-core fiber have been studied for their application in the implementation of sensors. In the first place, the diffraction grating manufacturing system is described that allows to inscribe different types of diffraction gratings in the multicore fiber selectively, that is, it allows to select in which cores the grating is going to be inscribed. By means of this system, long-period networks have been inscribed and subsequently they have been characterized as a deformation, torsion, and curvature sensor. Then, slanted Bragg gratings have been inscribed to intentionally increase the crosstalk between the fiber cores by coupling light between them. Furthermore, this crosstalk has been experimentally shown to be sensitive to fiber deformation, curvature, temperature, and the index of refraction surrounding the fiber. On the other hand, it has been shown that Bragg networks inscribed in multicore fibers can be used to implement curvature sensors capable of operating in radioactive environments. Finally, regenerated Bragg nets capable of operating at high temperatures have been manufactured. These nets have been characterized as a temperature, deformation, and curvature sensor. / Agradezco a la Universitat Politècnica de València la beca FPI (PAID-01-18) que me fue concedida para realizar está tesis. / Madrigal Madrigal, J. (2022). Diseño, fabricación y caracterización de sensores basados en fibras ópticas de múltiples núcleos [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/180806

Ao longo dos últimos anos, redes de Bragg em fibras ópticas (FBG, do inglês Fiber Bragg Gratings) vêm sendo frequentemente utilizadas como sensores de deformação e de temperatura. O problema da indistinguibilidade entre esses dois parâmetros físicos, presente durante medidas realizadas por esse tipo de sensor, tem sido bem resolvido com o uso de duas FBGs com comprimentos de onda distintos. Muito embora esse artifício tenha apresentado bons resultados, ele também oferece algumas desvantagens, sendo uma delas a necessidade de duas fontes de luz para diferentes comprimentos de onda. Em virtude disto, este trabalho apresenta um sistema capaz de realizar medidas de temperatura e deformação, simultaneamente, utilizando apenas uma fonte de luz. O método baseia-se na inscrição de duas redes de Bragg com comprimentos de onda próximos (no caso, 810 e 860 nm) na mesma posição da fibra óptica. Apesar de a separação entre os comprimentos de onda das FBGs ser aparentemente pequena (cerca de 50 nm), o sistema respondeu precisamente a variações de deformação e temperatura. Dessa forma, a utilização de apenas uma fonte de luz no sistema é corretamente justificada, uma vez que, com essa alteração, o custo do sistema é substancialmente reduzido. Ademais, o uso de comprimentos de onda em torno de 800 nm também barateia o sistema, pois os CCDs usados neste intervalo espectral são menos onerosos do que aqueles tradicionalmente usados em comprimentos de onda de comunicações ópticas (1,55 μm). / In recent years, fiber Bragg gratings (FBGs) have been frequently used as strain and temperature sensors. Several studies have tackled the problem of distinguishing between these two physical parameters using a dual-wavelength sensor. Although these sensors have shown good results, they have a few drawbacks, one of them being the need for two light sources with different wavelengths. We present an approach for simultaneous strain and temperature sensing which uses only one light source. The method relies on writing FBGs with nearby wavelengths (for instance, at 810 and 860 nm) at the same section of the fiber. Even though the Bragg wavelengths are separated by just a few nanometers (about 50 nm), it is possible to accurately measure variations in strain and temperature. One of the major advantages of this approach is the use of a unique light source, what reduces substantially the system cost. Another advantage is the lower cost of array detectors at 800 nm when compared to those of telecom wavelengths (1,55 μm).

In this dissertation, the rate of the BFD during body armor impact is characterized with fiber Bragg gratings for the first time ever. The depth rate is characterized using a single fiber optic sensor, while the entire shape rate can be characterized using multiple fiber optic sensors. This is done with a final depth accuracy of less than 10% and a timing accuracy of 15% for BFDs as deep as 50 mm and impact event of less than 1 millisecond. The shape sensing method introduced in this dissertation is different from traditional fiber optic sensor shape reconstruction methods in the fact that strain from the kinetic friction regime is used rather than the static friction regime. In other words, information from the fiber optic sensors slipping is used to reconstruct the shape in this work, whereas strain from the fiber optic sensor remaining fixed to a reference is used for typical fiber optic shape sensing purposes.

The different topics of this thesis include high-temperaturestable fiber Bragg gratings, photosensitivity and fiber basedcomponents. Fiber Bragg gratings (FBG) are wavelength dispersiverefractive index structures manufactured through UV exposure ofoptical fibers. Their applications range from WDM filters,dispersion compensators and fiber laser resonators fortelecommunication applications to different types of point ordistributed sensors for a variety of applications. One aim of this thesis has been to study a new type of FBGreferred to as chemical composition grating. These gratingsdiffer from other types of FBG in that their refractive indexstructure is attributed to a change in the chemicalcomposition. Chemical composition gratings have shown to beextremely temperature stable surviving temperatures in excessof 1000 oC. Photosensitivity of pure silica and germanium-dopedcore fibers in the presence of hydroxyl groups has also beenstudied and different types of fiber based components have beendeveloped. The main result of the thesis is a better understanding ofthe underlying mechanism of the formation of chemicalcomposition gratings and their decay behavior at elevatedtemperatures. The refractive index modulation is caused by aperiodic change in the fluorine concentration, which has beenverified through time-of-flight secondary-ion-mass spectrometryand through studies of the decay behavior of chemicalcomposition gratings. A model based on diffusion of dopants hasbeen developed, which successfully predicts the thermal decayat elevated temperatures. Studies of the dynamics of chemicalcomposition grating formation have resulted in a manufacturingtechnique that allows for reproducible gratingfabrication. The main results regarding photosensitivity is a method tosignificantly increase the effect of UV radiation on standardtelecommunications fiber. The method, referred to asOH-flooding, has also been applied to pure-silica core fibersresulting in the first report of strong grating formation insuch fibers. Finally, research into different schemes for developingfiber-based components has resulted in two types of singlefiber integrated Mach-Zehnder interferometers; one passiveinterferometer that can be used as an optical filter and oneactive interferometer controlled with internal metalelectrodes. Keywords:optical fibers, fiber Bragg gratings,photosensitivity, thermal stability, fiber sensors, chemicalcomposition gratings, fiber components, Mach-Zehnderinterferometer, optical switch, optical modulator. / QC 20100607

Fiber Bragg gratings (FBGs) are key elements in modern telecommunication and sensing applications. In optical communication, with the advancement of the Erbium doped fiber amplifier (EDFA), there is a great demand for devices with wavelength tunability over the Erbium gain bandwidth (in particular, for wavelength division multiplexing (WDM) networks). The center wavelength of a FBG can be shifted by means of change of temperature, pressure or mechanical axial strain. The axial strain approach is the best method among all other techniques because it allows relatively large wavelength shifts with high speed. Axial strain of up to 4% will be required to cover the whole EDFA region (more than 40 nm of central wavelength shift). The formation of Bragg grating results in significant reduction in mechanical strength of optical fibers especially in tension. As a result, axial strain of only about 1% can be achieved by mechanical stretching of FBGs. In order to achieve the remaining 3% strain compression of FBGs has to be applied. In this thesis, the design and analysis of a novel device for achieving central wavelength shift are presented. In particular, the device has achieved, for a fiber with 12 mm FBG, a shifting of 46 nm in compression and 10.5 nm in tension with a reflection power loss of less than 0.25 dB and a FWHM bandwidth variation of approximately 0.1 nm. Both variations are well below the Bellcore standards requirement of 0.5 dB for peak reflectivity variation and 0.1 nm for bandwidth variation. The device consists of two fixed and one guiding ferrules. The difficulties associated with compressing the FBG were handled by carefully selecting tolerances and adjustment procedures. The device allows both tension and compression of FBGs, and the use of different FBG lengths and actuators. The effects of glue deformation and bending of the FBG during compression were analyzed in detail. Further, using the piezoelectric transducer (PZT) actuator as a driver, tuning speed of around 1.5nm/ms was achieved.

The focus of this thesis is on the development of sensing devices based on optical fiber sensors, specifically optical Fiber Bragg Gratings (FBG), using laser microfabrication methods. FBG is a type of optical fibers whose spectral response is affected by applied strain and temperature. As a result, it can be calibrated for the measurement of physical parameters manifesting themselves in the changes of strain or temperature. The unique features of optical fiber sensors such as FBGs have encouraged the widespread use of the sensor and the development of optical fiber-based sensing devices for structural measurements, failure diagnostics, thermal measurements, pressure monitoring, etc. These features include light weight, small size, long-term durability, robustness to electromagnetic disturbances, and resistance to corrosion. Despite the encouraging features, there are some limitations and challenges associated with FBGs and their applications. One of the challenges associated with FBGs is the coupling of the effects of strain and temperature in the optical response of the sensors which affects the reliability and accuracy of the measurements. Another limitation of FBGs is insensitivity to the index of refraction of their surrounding medium. In liquids, the index of refraction is a function of concentration. Making FBGs sensitive to the index of refraction and keeping their thermal sensitivity intact enable optical sensors with the capability of the simultaneous measurement of concentration and temperature in liquids. Considering the unique features of FBGs, embedding of the sensors in metal parts for in-situ load monitoring is a cutting-edge research topic. Several industries such as machining tools, aerospace, and automotive industries can benefit from this technology. The metal embedding process is a challenging task, as the thermal decay of UV-written gratings can starts at a temperature of ~200 oC and accelerates at higher temperatures. As a result, the embedding process needs to be performed at low temperatures. The objective of the current thesis is to move forward the existing research front in the area of optical fiber sensors by finding effective solutions to the aforementioned limitations. The approaches consist of modeling, design, and fabrication of new FBG-based sensing devices. State-of-the-art laser microfabrication methods are proposed and implemented for the fabrication of the devices. Two approaches are adopted for the development of the FBG-based sensing devices: the additive method and the subtractive method. In both methods, laser direct microfabrication techniques are utilized. The additive method deals with the deposition of on-fiber metal thin films, and the subtractive method is based on the selective removal of materials from the periphery of optical fibers. To design the sensing devices and analyze the performance of the sensors, an opto-mechanical model of FBGs for thermal and structural monitoring is developed. The model is derived from the photo-elastic and thermo-optic properties of optical fibers. The developed model can be applied to predict the optical responses of a FBG exposed to structural loads and temperature variations with uniform and non-uniform distributions. The model is also extended to obtain optical responses of superstructure FBGs in which a secondary periodicity is induced in the index of refraction along the optical fiber. To address the temperature-strain coupling in FBGs, Superstructure FBGs (SFBG) with on-fiber metal thin films are designed and fabricated. It is shown that SFBGs have the capability of measuring strain and temperature simultaneously. The design of the sensor with on-fiber thin films is carried out by using the developed opto-mechanical model of FBGs. The performance of the sensor in concurrent measurement of strain and temperature is investigated by using a customized test rig. A laser-based Direct Write (DW) method, called Laser-Assisted Maskless Microdeposition (LAMM), is implemented to selectively deposit silver thin films on optical fibers and fabricate the superstructure FBGs. To attain thin films with premium quality, a characterization scheme is designed to study the geometrical, mechanical, and microstructural properties of the thin films in terms of the LAMM process parameters. A FBG, capable of measuring concentration and temperature of liquids is developed, and its performance is tested. Femtosecond laser micromachining is successfully implemented as a subtractive method for the sensor fabrication. For this purpose, periodic micro-grooves are inscribed in the cladding of regular FBGs so as to increase their sensitivity to the concentration of their surrounding liquid while keeping their thermal sensitivity intact. This type of sensors has the potential for applications in biomedical research, in which the in-situ measurement of the properties of biological analytes is required. Another accomplishment of this thesis is the development of FBG sensors embedded in metal parts for structural health monitoring using low temperature embedding processes. In this regard, the opto-mechanical model is extended to predict the optical response of the embedded FBGs. The embedding process involves low temperature casting, on-fiber thin film deposition, and electroplating methods. The performance of the embedded sensors is evaluated in structural loading and thermal cycling.

The focus of this thesis is on the development of sensing devices based on optical fiber sensors, specifically optical Fiber Bragg Gratings (FBG), using laser microfabrication methods. FBG is a type of optical fibers whose spectral response is affected by applied strain and temperature. As a result, it can be calibrated for the measurement of physical parameters manifesting themselves in the changes of strain or temperature. The unique features of optical fiber sensors such as FBGs have encouraged the widespread use of the sensor and the development of optical fiber-based sensing devices for structural measurements, failure diagnostics, thermal measurements, pressure monitoring, etc. These features include light weight, small size, long-term durability, robustness to electromagnetic disturbances, and resistance to corrosion. Despite the encouraging features, there are some limitations and challenges associated with FBGs and their applications. One of the challenges associated with FBGs is the coupling of the effects of strain and temperature in the optical response of the sensors which affects the reliability and accuracy of the measurements. Another limitation of FBGs is insensitivity to the index of refraction of their surrounding medium. In liquids, the index of refraction is a function of concentration. Making FBGs sensitive to the index of refraction and keeping their thermal sensitivity intact enable optical sensors with the capability of the simultaneous measurement of concentration and temperature in liquids. Considering the unique features of FBGs, embedding of the sensors in metal parts for in-situ load monitoring is a cutting-edge research topic. Several industries such as machining tools, aerospace, and automotive industries can benefit from this technology. The metal embedding process is a challenging task, as the thermal decay of UV-written gratings can starts at a temperature of ~200 oC and accelerates at higher temperatures. As a result, the embedding process needs to be performed at low temperatures. The objective of the current thesis is to move forward the existing research front in the area of optical fiber sensors by finding effective solutions to the aforementioned limitations. The approaches consist of modeling, design, and fabrication of new FBG-based sensing devices. State-of-the-art laser microfabrication methods are proposed and implemented for the fabrication of the devices. Two approaches are adopted for the development of the FBG-based sensing devices: the additive method and the subtractive method. In both methods, laser direct microfabrication techniques are utilized. The additive method deals with the deposition of on-fiber metal thin films, and the subtractive method is based on the selective removal of materials from the periphery of optical fibers. To design the sensing devices and analyze the performance of the sensors, an opto-mechanical model of FBGs for thermal and structural monitoring is developed. The model is derived from the photo-elastic and thermo-optic properties of optical fibers. The developed model can be applied to predict the optical responses of a FBG exposed to structural loads and temperature variations with uniform and non-uniform distributions. The model is also extended to obtain optical responses of superstructure FBGs in which a secondary periodicity is induced in the index of refraction along the optical fiber. To address the temperature-strain coupling in FBGs, Superstructure FBGs (SFBG) with on-fiber metal thin films are designed and fabricated. It is shown that SFBGs have the capability of measuring strain and temperature simultaneously. The design of the sensor with on-fiber thin films is carried out by using the developed opto-mechanical model of FBGs. The performance of the sensor in concurrent measurement of strain and temperature is investigated by using a customized test rig. A laser-based Direct Write (DW) method, called Laser-Assisted Maskless Microdeposition (LAMM), is implemented to selectively deposit silver thin films on optical fibers and fabricate the superstructure FBGs. To attain thin films with premium quality, a characterization scheme is designed to study the geometrical, mechanical, and microstructural properties of the thin films in terms of the LAMM process parameters. A FBG, capable of measuring concentration and temperature of liquids is developed, and its performance is tested. Femtosecond laser micromachining is successfully implemented as a subtractive method for the sensor fabrication. For this purpose, periodic micro-grooves are inscribed in the cladding of regular FBGs so as to increase their sensitivity to the concentration of their surrounding liquid while keeping their thermal sensitivity intact. This type of sensors has the potential for applications in biomedical research, in which the in-situ measurement of the properties of biological analytes is required. Another accomplishment of this thesis is the development of FBG sensors embedded in metal parts for structural health monitoring using low temperature embedding processes. In this regard, the opto-mechanical model is extended to predict the optical response of the embedded FBGs. The embedding process involves low temperature casting, on-fiber thin film deposition, and electroplating methods. The performance of the embedded sensors is evaluated in structural loading and thermal cycling.

Fiber Bragg gratings (FBGs) are key elements in modern telecommunication and sensing applications. In optical communication, with the advancement of the Erbium doped fiber amplifier (EDFA), there is a great demand for devices with wavelength tunability over the Erbium gain bandwidth (in particular, for wavelength division multiplexing (WDM) networks). The center wavelength of a FBG can be shifted by means of change of temperature, pressure or mechanical axial strain. The axial strain approach is the best method among all other techniques because it allows relatively large wavelength shifts with high speed. Axial strain of up to 4% will be required to cover the whole EDFA region (more than 40 nm of central wavelength shift). The formation of Bragg grating results in significant reduction in mechanical strength of optical fibers especially in tension. As a result, axial strain of only about 1% can be achieved by mechanical stretching of FBGs. In order to achieve the remaining 3% strain compression of FBGs has to be applied. In this thesis, the design and analysis of a novel device for achieving central wavelength shift are presented. In particular, the device has achieved, for a fiber with 12 mm FBG, a shifting of 46 nm in compression and 10.5 nm in tension with a reflection power loss of less than 0.25 dB and a FWHM bandwidth variation of approximately 0.1 nm. Both variations are well below the Bellcore standards requirement of 0.5 dB for peak reflectivity variation and 0.1 nm for bandwidth variation. The device consists of two fixed and one guiding ferrules. The difficulties associated with compressing the FBG were handled by carefully selecting tolerances and adjustment procedures. The device allows both tension and compression of FBGs, and the use of different FBG lengths and actuators. The effects of glue deformation and bending of the FBG during compression were analyzed in detail. Further, using the piezoelectric transducer (PZT) actuator as a driver, tuning speed of around 1.5nm/ms was achieved.