High temperature tolerant optical fiber inline microsensors by laser fabrication

Fiber sensors are particularly attractive for harsh environment defined by high temperature, high pressure, corrosive/erosive, and strong electromagnetic interference, where conventional electronic sensors do not have a chance to survive. However, the key issue has been the robustness of the sensor probe (not the fiber itself) mostly due to the problems stemmed from the traditional assembly based approaches used to construct fiber optic sensors. For example, at high temperatures (e.g., above 500°C), the thermal expansion coefficient mismatch between different composited parts has a high chance to lead to sensors\u27 malfunction by breaking the sensor as a result of the excessive thermo-stress building up inside the multi-component sensor structure. To survive the high temperature harsh environment, it is thus highly desired that the sensor probes are made assembly-free. We are proposing to fabricate assembly-free fiber sensor probes by manufacturing various microstructures directly on optical fibers. This dissertation aims to design, develop and demonstrate robust, miniaturized fiber sensor probes for harsh environment applications through assembly-free, laser fabrication. Working towards this objective, the dissertation explored three types of fiber inline microsensors fabricated by two types of laser systems. Using a CO₂ laser, long period fiber grating (LPFG) and core-cladding mode interferometer sensors were fabricated. Using a femto-second laser, an extrinsic Fabry-Perot interferometric (EFPI) sensor with an open cavity was fabricated. 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 laser fabrication technique is a valid tool to fabricate previously undoable miniaturized photonic sensor structures, which can avoid complicated assembly processes and, as a result, enhance robustness, functionality and survivability of the sensor for applications in harsh environments. In addition, a number of novel optical fiber sensor platforms are proposed, studied and demonstrated for sensing and monitoring of various physical and chemical parameters in high temperature harsh environments --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

Advanced optical fibre gratings for nano-structural characterisation and biosensing applications

This thesis presents detailed investigation on the fabrication, spectral characterisation and applications of UV-inscribed optical fibre gratings devices. Of prominent significance is the characterisation of the optical fibre gratings devices with nanoparticles and biological recognition elements for novel developments in the field of optical biosensing. A major contribution detailed in this thesis is the systematic study on fabrication, spectral characterisation and applications of different UV-inscribed in-fibre gratings. Specifically, uniform and apodized Fibre Bragg gratings (FBGs), normal and dual-peak long period fibre gratings (LPFGs), small-angle tilted fibre gratings (S-TFGs) and excessively tilted fibre gratings (Ex-TFGs) are presented. The holographic, phase-mask scanning and point-by-point methods are employed to fabricate these advanced optical fibre gratings using 244nm frequency-doubled Ar+ laser. Particular emphasis is laid on fabrication of dual-peak LPFGs in SMF-28 and thin-cladding single mode fibres of grating periods 140μm and 300μm respectively. Also, Ex-TFGs of different tilt angles are inscribed in single mode fibres using amplitude masks of different periods: 5.0μm, 6.6μm and 25μm. Another important contribution from this study is the nano-structural characterisation of the in-fibre gratings with nanoparticles such as carbon nanotubes (CNT), zinc oxide (ZnO) and gold nanoparticles for power demodulation, sensitivity enhancement and polarisation dependent SPR excitation respectively. Refractive index (RI) sensors based on 81° Ex-TFGs with carbon nanotube (CNT) overlay deposition have been investigated. The CNT, a dark material, with high absorption of light and high RI is responsible for the power demodulation of the attenuation band while the 81°-TFG induces the wavelength shift as the surrounding medium RI changes. Results show high sensitivities of 557.29 nm/RIU and 95.54 dB/RIU for the wavelength shift and power demodulation respectively. Also, nano-deposition of zinc oxide (ZnO) on Ex-TFGs inscribed in two different fibre types has been investigated using dissimilar morphologies (direct ZnO overlay and PSZnO overlay) for enhanced RI sensing. Significant improvement in sensitivity of ~ 21% (~ 522 nm/RIU) is obtained. The polarisation dependence of Au-coated S-TFGs on excitation of surface plasmon resonance (SPR) has also been investigated. Finally, the in-fibre gratings are surface-functionalized with bioreceptor elements such as enzymes (glucose oxidase) and antibody/antigen (Trx, IL-6). Enzyme functionalized biosensor based on dual-peak LPFG has been investigated for sugar concentration level and specific glucose detection and high sensitivities of ~4.67 nm/% and 12.21 ± 0.19 nm/ (mg/ml) are obtained respectively. Also, fibre optic biosensors based on antibodyfunctionalized 81º-TFGs have been presented for label-free specific recognition of interleukin-6 (IL-6) and thioredoxin (Trx) proteins. High saturation values (∧λ max ) of 35.05nm and 33.19nm are obtained respectively. The specificity validation of the biosensors in the presence of other interfering proteins is investigated using human plasma and results show high specificity

Arc-induced long-period fibre gratings : fabrication and their applications in optical communications and sensing

Tese de doutoramento. Ciências de Engenharia. 2006. Faculdade de Engenharia. Universidade do Port

Long period fiber grating, thin coating of graphene and silver nanowires, and corrosion sensing for life-cycle assessment of steel structures

This study aims to develop and validate a compact, integrated lab-on-sensor system for simultaneous measurement of strain, temperature and corrosion-induced mass loss in steel structures and concrete reinforcement elements in order to assess their life-cycle performance. The sensing system operates based on the principle of long period fiber gratings (LPFG) that are responsive to both thermal and mechanical deformation, and the change in refractive index of any medium surrounding the optical fiber. To fabricate a LPFG sensor for strain and temperature measurement, a CO2 laser aided fiber grating system was assembled. To enable mass loss measurement, a low pressure chemical vapor deposition (LPCVD) system was built to synthesize a graphene/silver nanowire composite film as flexible transparent electrode for the electroplating of a thin Fe-C layer on the curve surface of a LPFG sensor. Together with two LPFG sensors in LP06 and LP07 modes for simultaneous strain and temperature measurement, three Fe-C coated LPFG sensors were multiplexed and deployed inside three miniature, coaxial steel tubes to measure critical mass losses through the penetration of tube walls and their corresponding corrosion rates in the life cycle of an instrumented steel component. A Fe-C coated LPFG sensor was submerged in a NaCl solution and calibrated for stress corrosion cracking under three strain levels. The corrosion mechanism of the Fe-C layer was investigated and the distribution of cracks (width, length and spacing) were characterized and correlated with the wavelength change of the sensor. Thermal, loading and accelerated corrosion tests were conducted to validate the functionality, sensitivity, accuracy, and robustness of the proposed sensing system and demonstrate its feasibility in in situ applications --Abstract, page iii

Review of Fiber Optic Sensors for Structural Fire Engineering

Reliable and accurate measurements of temperature and strain in structures subjected to fire can be difficult to obtain using traditional sensing technologies based on electrical signals. Fiber optic sensors, which are based on light signals, solve many of the problems of monitoring structures in high temperature environments; however, they present their own challenges. This paper, which is intended for structural engineers new to fiber optic sensors, reviews various fiber optic sensors that have been used to make measurements in structure fires, including the sensing principles, fabrication, key characteristics, and recently-reported applications. Three categories of fiber optic sensors are reviewed: Grating-based sensors, interferometer sensors, and distributed sensors

Femtosecond laser micro-machined optical fiber based embeddable strain and temperature sensors for structural monitoring

Structural monitoring technology is becoming increasingly important for managing all types of structures. Embedding sensors while constructing new structures or repairing the old ones allows for continual monitoring of structural health thus giving an estimate of remaining utility. Along with being embeddable, miniaturized sensors that are easy to handle are highly sought after in the industry where in-situ monitoring is required in a harsh environment (corrosive atmosphere, high temperatures, high pressure etc.). This dissertation demonstrates the use of femtosecond laser-fabricated Fabry-Perot interferometer (FPI) based optical fiber sensors for embedded applications like structural health monitoring. Two types of Fabry-Perot interferometer sensors, extrinsic FPI and intrinsic FPI, have been designed, developed and demonstrated for strain and temperature monitoring applications. The absence of any movable parts make these sensors easy-to-handle and easy to embed inside a material. These sensors were fabricated using a laboratory integrated femtosecond (fs) laser micromachining system. For the extrinsic Fabry-Perot interferometer (EFPI) design, the fs-laser was used to ablate and remove the material off the fiber end face while for intrinsic Fabry-Perot interferometer (IFPI) design, the laser power was focused inside the fiber on the fiber core to create two microstructures. The scope of the work presented in this dissertation extends to device design, laser based sensor fabrication, sensor performance evaluation and demonstration. Feasibility of using these sensors for embeddable applications was investigated. A new type of material called Bismaleimide (BMI) was used for demonstrating the embeddability of the sensors. Experimental results of strain and temperature testing are presented and discussed. The EFPI sensor has low temperature sensitivity of 0.59 pm/⁰C and a high strain sensitivity of 1.5 pm/µε. The IFPI sensor has the same strain sensitivity as EFPI but is 25 times more sensitive to the temperature. These sensors were tested up to 850 ⁰C in non-embedded condition and they produced a linear response. A hybrid approach combining the EFPI and IFPI sensors was demonstrated for simultaneous measurement of strain and temperature --Abstract, page iii

A progressive collapse evaluation of steel structures in high temperature environment with optical fiber sensors

In the process of a progressive failure of steel structures in a post-earthquake fire, real-time assessment and prediction of structural behaviors are of paramount significance to an emergency evacuation and rescue effort. However, existing measurement technologies cannot provide the needed critical data such as large strains at high temperature. To bridge this gap, a novel optical fiber sensor network and an adaptive multi-scale finite element model (FEM) are proposed and developed in this study. The sensor network consists of long period fiber gratings (LPFG) sensors and extrinsic Fabry-Perot interferometer (EFPI) sensors or their integration. Each sensor is designed with a three-tier structure for an accurate and reliable measurement of large strains and for ease of installation. To maintain a balance between the total cost of computation and instrumentation and the accuracy in numerical simulation, a structure is divided into representative/critical components instrumented densely and the remaining components simulated computationally. The critical components and the remaining were modeled in different scales with fiber elements and beam/plate elements, respectively, so that the material behavior and load information measured from the critical components are representative to the remaining components and can be used to update the temperature distribution of the structure in real time. Sensitivity studies on the number of sensors and the initial selection of an updating temperature parameter were conducted. Both the sensor network and the FEM were validated with laboratory tests of a single-bay, one-story steel frame under simulated post-earthquake fire conditions. The validated FEM was applied to a two-bay, four-story steel building under the 1995 Kobe earthquake excitations. Based on extensive tests and analyses, the proposed sensor can measure a strain of 12% at as high as 800⁰C (1472⁰F) in temperature. Within the application range, the LPFG wavelength and the EFPI gap change linearly with the applied strain and temperature. The proposed updating criterion and algorithm in the adaptive FEM are proven to be effective. The number of sensors is sufficient in engineering applications as long as the sensors can adequately represent the material behavior of the instrumented components. The predicted structural behavior is unaffected by any change in a low temperature range and thus insensitive to the initial selection of the updating parameter --Abstract, page iii