Nanostructured optical fibre tapers and related applications

In the last decade, optical fibre tapers have attracted considerable interest because they offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These distinctive features have been exploited in a wealth of applications ranging from telecommunication devices to sensors, from optical manipulation to high-Q resonators. Nanostructures on the optical fibre tapers are very promising since the size of the device can be extremely small. With the development of nanostructuring methods, sub-wavelength feature sizes have been achieved. In this thesis, nanostructured optical fibre tapers and some related applications are discussed.Light confinement is limited by diffraction and the minimum spot size is related to the light wavelength. In this thesis, light confinement in two and three dimensions is proposed and achieved with two typologies of nanostructured optical fibre tapers. The first group of devices exploits plasmons excited at the optical fibre tips to obtain high transmissivity, and confine light to a sub-wavelength dimension. Optical fibre tips were designed according to numerical simulations and coated by a layer of gold; an extremely small aperture was then opened at the tip apex. The experimental characterization and simulation results showed their improved transmission efficiency (higher than 10-2) and thermal expansion measurements showed no shape changes could be detected within the accuracy of the system (~2 nm) for 9 mW injected powers. Effective confinements to 10 nm or smaller can be envisaged by decreasing the aperture size and slope angle. Application of this small spot size source can include scanning near-field optical microscope, optical recording, photolithography and bio-sensing.The second group achieves three dimensional light confinement exploiting a Fabry-Perot microcavity formed by a microfibre grating similar to those used in distributed feedback lasers. Microfibres were patterned using a Focused Ion Beam (FIB) system. In this structure, the microcavity provides longitudinal light confinement, whereas air dielectric guiding by the microfibre provides diffraction limited confinement in the other two dimensions. Due to the high refractive index contrast between silica and air, strong reflection can be obtained by only dozens of notches. This device can be used for a wide range of applications, e.g. sensing and triggered single-photon sources.Light confinement in nanostructured optical fibre tapers was exploited in a micrometric thermometer. A compact thermometer based on a broadband microfibre coupler tip showed a dynamic range spanning from room temperature to 1511ºC with a response time of tens of microseconds. This is the highest temperature measured with a silica optical fibre device. An average sensitivity of 11.96 pm/ºC was achieved for a coupler tip with ~2.5 µm diameter. A resolution of 0.66ºC was achieved for a coupler tip diameter of ~12.6 µm. Better resolution can be achieved with smaller size microfibre coupler tips.Optical fibre tapers are commonly used to couple light to selected resonator modes. Here FIB was used to inscribe microgrooves on optical Bottleneck Microresonator (BMR) surfaces to excite selected whispering gallery modes. By monitoring the transmission spectrum of the optical fibre taper, substantial spectral clean-up was obtained in appropriately scarred BMRs. Single high-Q mode operation can be achieved by either using two asymmetrical perpendicular scars or placing the grooves closer to the BMR centre, providing the potential for high performance sensors and other optical devices.Finally, strong three dimensional localization has been achieved in Plasmonic Slot Nano-Resonators (PSNRs) embedded in a gold-coated optical fibre tapers. Different shapes PSNRs, embedded in thin gold metal film coated plasmonic microfibre, were numerically investigated. The intensity enhancement (in excess of 106) and the resonance wavelength depend on both the PSNR and microfibre dimensions. Theoretically and experimentally, the transversal excitation of a rectangular PSNR embedded in a thin gold film coated plasmonic fibre tip was discussed for the first time, and showed high localization and strong enhancement (7.24×103). This device can find a wide range of applications such as surface-enhanced Raman scattering, optical filtering, spectroscopy and bio-sensing

Surface Plasmon Polaritons Based Nanophotonic Devices and their Applications

Guiding light at the nanoscale shows significant importance for the next generation on-chip information technology, as it could enable dramatic miniaturization of integrated photonic circuits (IPCs). Conventional dielectric photonic devices cannot be used to realize highly IPCs due to the diffraction limit. A promising solution to this challenge is to explore surface plasmon polaritons (SPPs) based waveguides, which can guide light beyond the diffraction limit. Therefore, a deeper understanding of SPPs based waveguides is developed in this thesis, as a basis for the design of novel plasmonic devices which are essential for building IPCs. However inherent ohmic loss in plasmonic waveguide is very large and the loss will be further increased by reducing the size of IPCs, which limited their applications. Thus the critical challenge for SPPs waveguide is how to increase the propagation length, in the meanwhile maintain a tight mode confinement. Thus several novel plasmonic waveguides are developed to address the challenge of improving the propagation length while maintaining very tight mode confinement. SPPs are exceptionally sensitive to the dielectric properties near the metal surface because of their highly localized fields at the metal surface, offering substantial potential for effective sensing applications. Therefore the application for plasmonic sensing is also explored in the thesis. Several new plasmonic biosensors are designed to achieve a high sensitivity and simultaneous measurements of multiple parameters

Novel Specialty Optical Fibers and Applications

Novel Specialty Optical Fibers and Applications focuses on the latest developments in specialty fiber technology and its applications. The aim of this reprint is to provide an overview of specialty optical fibers in terms of their technological developments and applications. Contributions include:1. Specialty fibers composed of special materials for new functionalities and applications in new spectral windows.2. Hollow-core fiber-based applications.3. Functionalized fibers.4. Structurally engineered fibers.5. Specialty fibers for distributed fiber sensors.6. Specialty fibers for communications

Fiber inline pressure and acoustic sensor fabricated with femtosecond laser

Pressure and acoustic measurements are required in many industrial applications such as down-hole oil well monitoring, structural heath monitoring, engine monitoring, study of aerodynamics, etc. Conventional sensors are difficult to apply due to the high temperature, electromagnetic-interference noise and limited space in such environments. Fiber optic sensors have been developed since the last century and have proved themselves good candidates in such harsh environment. This dissertation aims to design, develop and demonstrate miniaturized fiber pressure/acoustic sensors for harsh environment applications through femtosecond laser fabrication. Working towards this objective, the dissertation explored two types of fiber inline microsensors fabricated by femtosecond laser: an extrinsic Fabry-Perot interferometric (EFPI) sensor with silica diaphragm for pressure/acoustic sensing, and an intrinisic Fabry-Perot interferometer (IFPI) for temperature sensing. The scope of the dissertation work consists of device design, device modeling/simulation, laser fabrication system setups, signal processing method development and sensor performance evaluation and demonstration. This research work provides theoretical and experimental evidences that the femtosecond laser fabrication technique is a valid tool to fabricate miniaturized fiber optic pressure and temperature sensors which possess advantages over currently developed sensors --Abstract, page iii

Femtosecond Laser Micromachining of Advanced Fiber Optic Sensors and Devices

Research and development in photonic micro/nano structures functioned as sensors and devices have experienced significant growth in recent years, fueled by their broad applications in the fields of physical, chemical and biological quantities. Compared with conventional sensors with bulky assemblies, recent process in femtosecond (fs) laser three-dimensional (3D) micro- and even nano-scale micromachining technique has been proven an effective and flexible way for one-step fabrication of assembly-free micro devices and structures in various transparent materials, such as fused silica and single crystal sapphire materials. When used for fabrication, fs laser has many unique characteristics, such as negligible cracks, minimal heat-affected-zone, low recast, high precision, and the capability of embedded 3D fabrication, compared with conventional long pulse lasers. The merits of this advanced manufacturing technique enable the unique opportunity to fabricate integrated sensors with improved robustness, enriched functionality, enhanced intelligence, and unprecedented performance. Recently, fiber optic sensors have been widely used for energy, defense, environmental, biomedical and industry sensing applications. In addition to the well-known advantages of miniaturized in size, high sensitivity, simple to fabricate, immunity to electromagnetic interference (EMI) and resistance to corrosion, all-optical fiber sensors are becoming more and more desirable when designed with characteristics of assembly free and operation in the reflection configuration. In particular, all-optical fiber sensor is a good candidate to address the monitoring needs within extreme environment conditions, such as high temperature, high pressure, toxic/corrosive/erosive atmosphere, and large strain/stress. In addition, assembly-free, advanced fiber optic sensors and devices are also needed in optofluidic systems for chemical/biomedical sensing applications and polarization manipulation in optical systems. Different fs laser micromachining techniques were investigated for different purposes, such as fs laser direct ablating, fs laser irradiation with chemical etching (FLICE) and laser induced stresses. A series of high performance assembly-free, all-optical fiber sensor probes operated in a reflection configuration were proposed and fabricated. Meanwhile, several significant sensing measurements (e.g., high temperature, high pressure, refractive index variation, and molecule identification) of the proposed sensors were demonstrated in this dissertation as well. In addition to the probe based fiber optic sensors, stress induced birefringence was also created in the commercial optical fibers using fs laser induced stresses technique, resulting in several advanced polarization dependent devices, including a fiber inline quarter waveplate and a fiber inline polarizer based on the long period fiber grating (LPFG) structure

Distributed On-chip Brillouin Sensing: Toward Sub-mm Spatial Resolution

Stimulated Brillouin scattering (SBS) involves nonlinear interaction of an optical wave with material, which under the phase-matching condition results in generation of an acoustic wave. In turns, part of the optical wave is scattered by the acoustic wave through an inelastic scattering process. SBS enables unique applications in optical fibers and more recently in on-chip photonic waveguides, ranging from RF-signal processing to lasing, frequency combs, RF sources, and light storage. Harnessing on-chip SBS paves the way to photonic integration by enabling powerful functionalities in an integrated, scalable, energy-efficient and potentially CMOS-compatible platform. In this thesis, we explore the possibility of enabling SBS in a silicon-based platform by designing, fabricating and characterizing a hybrid silicon-chalcogenide waveguide, which shows significant improvement in terms of nonlinear losses and SBS gain compared to a standard silicon waveguide. The SBS response in photonic waveguides including the silicon-chalcogenide platform is subject to spectral broadening which influences the quality of the devices whose performance are relying on the narrow linewidth of SBS. The spectral broadening is mainly due to structural non-uniformities along the waveguides which affect the local SBS response and consequently deteriorates the strength of the integrated SBS response. Therefore, characterizing those waveguides is of great importance. To address this issue, we employed the principle of distributed SBS sensing to monitor the on-chip waveguides. However, since the waveguides length is on the order of cm and mm, the spatial resolution of the distributed technique needs to be very high, preferably in the sub-mm regime, which is the main goal of this thesis

Refractometric platforms for label-free biochemical sensing

Optical fiber technology, which is well-known for having revolutionized the telecommunications industry, is currently proving to have important roles in applications such as sensing, biomedicine and industry. The sensing technology based on optical fibers has attracted considerable interest due to its unique features. High sensitivity, immunity to electromagnetic interferences, chemically and biologically inert, small size, and capability forin-situ, real-time, remote, anddistributedsensingaresomeofthemost appealing characteristics that motivate a growing scientific community. The principle behind optical fiber sensing is the interaction between radiation traveling into the optical fiber and the parameter of interest. The measurand acts over the material, the waveguide or, depending on the sensing structure, directly in the optical signal and this action will infer variations in one or few parameters of the radiation such as: intensity, wavelength, frequency, phase or polarization. This work fits within this area, specifically on fiber optic refractometric sensors for label-free biochemical sensing. It is presented a review about the most relevant works in this field, results associated with the study, development and characterization of few types of optical fiber sensors based on fiber Bragg gratings, long period gratings, multimode interference and fibertapers. Also, adifferentialinterrogationsystem, basedonwhitelight interferometry is exposed for high resolution refractive index sensing. In all cases, it was also a main objective of the developed work to evaluate the potential application of the new sensing structures and systems researched, particularly in the context of food industry and environmental monitoring.A tecnologia da fibra ótica, bem conhecida por ter revolucionado a indústria das telecomunicações, atualmente está a desempenhar um papel importante em outros campos como o do sensoriamento, a biomedicina e a indústria. A tecnologia de sensores baseados em fibras óticas tem sido alvo de considerável interesse devido às suas características únicas. Elevada sensibilidade, imunidade a interferências eletromagnéticas, química e biologicamente inerte, tamanho e peso reduzido, e capacidade sensoriamento remoto, distribuído, in-situ e em tempo real; são são alguns dos benefícios mais relevantes que motivam a uma crescente comunidade científica. O princípio por trás dos sensores de fibra ótica, é a interação entre a radiação que viaja no interior da fibra óptica e parâmetro de interesse. O mensurandoatuasobreomaterial, guiadeondaou, dependendodaestruturasensoradiretamentenosinalótico,resultandonaalteraçãodeumaou mais propriedades da radiação, tais como a sua intensidade, comprimento de onda, frequência, fase ou polarização. Este trabalho enquadra-se nesta área, especificamente no ramo dos sensores de fibra ótica refratométricos para medição direta (sem utilização de indicadores de cor) de parâmetros bioquímicos. Inicialmente é apresentadaumarevisãodosostrabalhosmaisrelevantesnestedomíniocientífico; seguidamente e exposto o conjunto de sistemas sensores desenvolvidos e caracterizados, baseados em diversas estruturas como redes de difração, tapers e dispositivos de interferência intermodal. Adicionalmente, é tambémdescritoumsistemadeleituradiferencialcombaseeminterferometria de luz branca, desenvolvido para medição de índice de refração com elevada resolução. Em todos os trabalhos, um dos principais objetivos foi avaliar o potencial de aplicação das novas estruturas e sistemas de sensoriamento desenvolvidos, em particular no contexto da indústria alimentar e monitorização ambiental.Fundação para a Ciência e Tecnologia SFRH/BD/63758/200

Hollow Core Photonic Bragg Fibers for Industrial Sensing Applications

Le rôle de plus en plus important des senseurs optiques dans une multitude d’applications scientifiques et industrielles, incluant la détection biologique, le diagnostic médical, l’industrie alimentaire, le contrôle des procédés et le monitoring environnemental, a mené à un regain de vitalité dans les efforts de recherche et développement dans ce domaine. Pour ces senseurs, la fibre optique peut être une technologie prometteuse en raison de ses nombreux avantages tels que la portabilité, la protection face l’interférence électromagnétique, la possibilité de les utiliser dans des environnements explosifs, ou encore celle d’avoir une mesure quantitative et qualitative continue. Aujourd’hui, une vaste gamme de senseurs à fibre optique a été proposée et développée. Cependant, la plupart de ces senseurs fonctionnent sur la base d’un couplage évanescent des modes de réflexion totale interne (RTI) à proximité de l’analyte. Ceci comporte plusieurs désavantages, tels que le faible chevauchement du mode avec l’analyte, une distance de sensibilité plus petite, le besoin d’apporter des modifications complexes à la fibre, ainsi qu’une faible robustesse mécanique de la fibre. Afin de surmonter ces limitations et simplifier l’implémentation pratique, dans cette thèse, nous proposons l’utilisation de fibres de Bragg à coeur creux opérant dans des plages fréquentielles distinctes (le visible et les térahertz) pour effectuer de la réfractométrie dans des analytes liquides et de surface. Nous mènerons des analyses théoriques et des caractérisations expérimentales des guides d’onde proposés et nous étudierons leur potentiel d’applications dans une variété de domaines industriels. Dans la première partie de la thèse, nous explorerons la capacité d’utiliser des fibres de Bragg à coeur creux dans le visible pour simultanément détecter la partie réelle et imaginaire de l’indice de réfraction d’analytes liquides. Ce senseur de Bragg est constitué d’un large coeur creux (diamètre d’environ 0.7 mm) entouré de couches alternantes de polymethyl methacrylate (PMMA)et de polystyrènse (PS) qui agissent essentiellement comme des réflecteurs de Bragg. Nous utiliserons ce senseur pour le monitoring de la concentration de liquides de refroidissement commerciaux. La stratégie de détection s’appuie sur une double mesure de la position spectrale du centre de la bande interdite et sa transmission en amplitude. Les deux mesures sont hautement sensibles à l’indice de réfraction de l’analyte qui sera introduit dans le coeur creux du guide d’onde. Ceci permettra de déterminer la concentration des liquides de refroidissement.----------Abstract The expanding role of optical sensors in numerous scientific and industrial applications，including biosensing, medical diagnostics, food industry, process control, and environmental monitoring, has led to a growth of research and development efforts in this field. Optical fibers can be used as a very promising platform for these applications, due to many appealing properties such as compactness, high degrees of integration, safety in explosive environments and potential to provide real time and remote analysis. To date, a wide range of fiber-optic sensors have been proposed and developed. Most of these sensors, however, use an evanescent coupling of total internal reflection guided modes to the test analytes, which typically suffers from many disadvantages, such as poor modal overlap with the analytes, limited probing length, as well as complicated fiber modifications and poor mechanical robustness in fiber structures. In order to circumvent these limitations and simplify the practical sensing implementation, in this thesis, we study using hollow-core Bragg fiber sensors operating in different frequency ranges (i.e., visible and terahertz range) for bulk refractometry of liquid analytes and surface sensing applications. We then carry out the theoretical and experimental characterizations of the proposed sensors, and study their applications in various industrial fields. In the first part of the thesis, we explore the capability of using the hollow-core Bragg fibers operating in the visible range to simultaneously monitor both the real and imaginary parts of liquid analyte refractive index. The Bragg fiber sensor features a large hollow core (diameter: ~0.7mm) surrounded by an alternating polymethyl methacrylate (PMMA)/polystyrene (PS) multilayer as a Bragg reflector. We then apply this fiber sensor to monitor the concentrations of various commercial cooling oils. The sensing strategy relies on a two-channel sensing modality that simultaneously interrogates the bandgap center position of the Bragg fiber as well as the fiber transmission amplitude at the bandgap center. Both measurands are highly sensitive to the complex refractive index of the analyte filled in the fiber core, thus enabling efficient determination and cross correlation with the concentration of cooling oils. The presented fiber sensor inherently integrates optical detection with microfluidics without any fiber modifications, thus allowing for real time monitoring of the concentrations of many industrial fluids, such as heat transfer fluids, sawing fluids, and other industrial dilutions with sub-1%v accuracy