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

Integrated Additive and Subtractive Manufacturing of Glass Photonic Sensors for Harsh Environment Applications

Research and development in advanced manufacturing for sensors and devices fabrication is continuously changing the world, assisting to giving sensing solutions in the physical, chemical and biological fields. Specifically, many modern engineered systems are designed to operate under extreme conditions such as high temperature, high pressure, corrosion/erosion, strong electromagnetic interference, heavy load, long reaching distance, limited space, etc. Very often, these extreme conditions not only degrade the performance of the system but also impose risks of catastrophic failures and severe consequences. To perform reliably under these harsh conditions, the materials and components need to be properly monitored and the systems need to be optimally controlled. However, most existing sensing technologies are insufficient to work reliably under these harsh conditions. Innovations in sensor design, fabrication and packaging are needed to address the technological challenges and bridge the capability gaps. Optical fiber sensors have been widely researched and developed for energy, defense, environmental, biochemical and industry sensing applications. In general, optical fiber sensors have a number of well-known advantages such as miniature in size, high sensitivity, long reaching distance, capability of multiplexing and immunity to electromagnetic interference (EMI). In addition, optical fiber sensors are capable of operating under extreme environment conditions, such as high temperature, high pressure, and toxic/corrosive/erosive atmospheres. However, optical fiber sensors are also fragile and easy to break. It has been a challenging task to fabricate and package optical fiber sensors with predicable performance and desired reliability under harsh conditions. The latest advancements in high precision laser micromachining and three-dimensional (3D) printing techniques have opened a window of opportunity to manufacture new photonic structures and integrated sensing devices that deliver unprecedented performance. Consequently, the optical sensor field has quietly gone through a revolutionary transition from the traditional discrete bulk optics to today’s devices and structures with enhanced functionalities and improved robustness for harsh environment applications. Driven by the needs for sensors capable of operating in harsh environments, integrated additive and subtractive manufacturing (IASM) for glass photonics sensor fabrication process has been proposed and developed. In this dissertation, a series of high-performance optical fiber sensors were proposed and fabricated. In addition, several significant sensing measurements (e.g., pressure, temperature, refractive index variation) of the proposed sensors and structures with enhanced robustness were demonstrated in this dissertation. To realize measurement of above parameters, different working principles were studied, including mechanical deflection, light-material interaction and utilizing properties of fluidics. The sensing performance of the fabricated sensors and structures were characterized to demonstrate the capabilities of the developed IASM process on advanced manufacturing of glass photonic sensors with specific geometry and functions, and the realization for information integrated manufacturing purpose

Investigations towards the development of a novel multimodal fibre optic sensor for oil and gas applications.

Oil and gas (O&G) explorations are moving into deeper zones of earth, causing serious safety concerns. Hence, sensing of critical multiple parameters like high pressure, high temperature (HPHT), chemicals, etc., are required at longer distances. Traditional electrical sensors operate less effectively under these extreme environmental conditions and are susceptible to electromagnetic interference (EMI). Compared to electrical sensors, fibre optic sensors offer several advantages like immunity to EMI, electrical isolation, ability to operate in harsh environmental conditions and freedom from corrosion. Existing fibre optical sensors in the O&G industry, based on step index single mode fibres (SMF), offer limited performance, as they operate within a narrow wavelength window. A novel multimodal sensor configuration, based on photonic crystal fibre (PCF) and utilising a multiwavelength approach, is proposed for the first time for O&G applications. This thesis reports computational and experimental investigations into the new multimodal sensing methodology, integrating both optical phase-change and spectral-change based approaches, needed for multi-parameter sensing. It includes investigations to improve the signal-to-noise ratio (SNR) by enhancing the signal intensity attained through structural, material and positional optimisations of the sensors. Waveguide related, computational investigations on PCF were carried out on different fibre optic core-cladding structures, material infiltrations and material doping to improve the signal intensity from the multimodal sensors for better SNR. COMSOL Multiphysics simulations indicated that structural and material modifications of the PCF have significant effects on light propagation characteristics. The propagation characteristics of the PCF were improved by modifying the geometrical parameters, and microstructuring the fibre core and cladding. Studies carried out on liquid crystal PCF (LCPCF) identified its enhanced mode confinement characteristics and wavelength tenability features (from visible to near infrared), which can be utilised for multi-wavelength applications. Enhancing core refractive index of the PCF improved the electric field confinements and thereby the signal intensity. Doping rare earth elements into the PCF core increases its refractive index and also provides additional spectroscopic features (photoluminescence and Raman), leading to a scope for multi-point, multimodal sensors. Investigations were carried out on PCF-FBG (Fibre Bragg grating) hybrid configuration, analysing their capabilities for optical phase-change based, multipoint, multi-parameter sensing. Computational investigations were carried out using MATLAB software, to study the effect of various fibre grating parameters. These studies helped in improving understanding of the FBG reflectivity-bandwidth characteristics, for tuning the number of sensors that can be accommodated within the same sensing fibre and enhancing the reflected signal for improved SNR. A new approach of FBG sensor positioning has been experimentally evaluated, to improve its strain sensitivity for structural health monitoring (SHM) of O&G structures. Further, experimental investigations were carried out on FBGs for sensing multiple parameters like temperature, strain (both tensile and compressive) and acoustic signals. Various spectroscopic investigations were carried out to identify the scope of rare earth doping within the PCF for photoluminescence and Raman spectroscopy based multimodal sensors. Rare earth doped glasses (Tb, Dy, Yb, Er, Ce and Ho) were developed using melt-quench approach and excitation- photoluminescence emission studies were carried out. The studies identified that photoluminescence signal intensity increases with rare earth concentration up to an optimum value and it can be further improved by tuning the excitation source characteristics. Photoluminescence based temperature studies were carried out using the rare earth doped glasses to identify their suitability for O&G high temperature conditions. Raman spectroscopic investigations were carried out on rare earth (Tb) doped glasses, developed using both melt-quench and sol-gel based approaches. Effect of 785 nm laser excitation on Raman signatures and suitability of rare earth doped materials for fibre-based Raman distributed temperature sensing (DTS) were also studied. Finally, a novel multimodal fibre optic sensor configuration is proposed for the O&G applications, consisting of rare earth doped photonic crystal fibre integrating Bragg gratings and operating in multiple wavelength regimes in a multiplexed fashion. The integrated sensor combination is expected to overcome the limitations of existing sensors with regards to SNR, sensing range and multimodal sensing capability

Study on fibre optic sensors embedded into metallic structures by selective laser melting

Additive Manufacturing, which builds components layer by layer, opens up exciting possibilities to integrate sophisticated internal features and functionalities such as fibre optic sensors directly into the heart of a metal component. This can create truly smart structures for deployment in harsh environments. The innovative and multidisciplinary study conducted in this thesis demonstrates the feasibility to integrate fibre optics sensors with thin, protective nickel coatings (outer diameter ~350 μm) into stainless steel (SS 316) coupons by Selective Laser Melting technology (SLM). Different concepts for fibre embedment by SLM are investigated. The concepts differ in which way the fibre is positioned and how the powder is deposited and solidified by the laser in respect to the optical fibre. Only one approach is found suitable to reliably and repeatable encapsulate fibres whilst preserving their structural integrity and optical properties. In that approach SS 316 components are manufactured using SLM, incorporating U-shaped grooves with dimensions suitable to hold nickel coated optical fibres. Coated optical fibres containing Fibre Bragg Gratings (FBG) for strain and temperature sensing are placed in the groove. Melting subsequent powder layers on top of the FBGs fuses the fibre’s metallic jacket to the steel and completes the integration of the fibre sensor into the steel structure. Cross-sectional microscopy analysis of the fabricated components, together with analysis of fibre optic sensors’ behaviour during fabrication, indicates proper stress and strain transfer between coated fibre and added SS 316 material. During the SLM process embedded Fibre Bragg grating (FBG) sensors provide in-situ temperature measurements and potentially allow measuring the build-up of residual stresses. Subsequently, FBG sensors embedded into SS 316 structures using our approach follow elastic and plastic deformations of the SS 316 component, with a resolution of better than 3 pm*μɛ-1. Temperature measurements are also conducted with a precision of 3 pm*K-1. Such embedded fibre sensors can also be used to high temperatures of up to ~400 °C. However, at elevated temperatures issues arise from the significantly larger thermal expansion coefficient of SS 316, leading to delamination of the more rapidly expanding metal from the glass. Rapid thermal expansion of the metal also leads to high axial stresses within the glass exceeding the fibres tensile strength and ultimately leading to structural damage of the optical fibre

Fabrication and Sensing Applications of Special Microstructured Optical Fibers

This chapter presents the fabrication of the special microstructured optical fibers (MOFs) and the development of sensing applications based on the fabricated fibers. Particularly, several types of MOFs including birefringent and photosensitive fibers will be introduced. To fabricate the special MOFs, the stack-and-draw technique is employed to introduce asymmetrical stress distribution in the fibers. The microstructure of MOFs includes conventional hexagonal assembles, large-air hole structures, as well as suspended microfibers. The birefringence of MOFs can reach up to 10−2 by designing the air hole structure properly. Fiber Bragg gratings as well as Sagnac interferometers are developed based on the fabricated special MOFs to conduct sensing measurement. Various sensing applications based on MOFs are introduced