Performance of a Distributed Simultaneous Strain and Temperature Sensor Based on a Fabry-Perot Laser Diode and a Dual-Stage FBG Optical Demultiplexer

A simultaneous strain and temperature measurement method using a Fabry-Perot laser diode (FP-LD) and a dual-stage fiber Bragg grating (FBG) optical demultiplexer was applied to a distributed sensor system based on Brillouin optical time domain reflectometry (BOTDR). By using a Kalman filter, we improved the performance of the FP-LD based OTDR, and decreased the noise using the dual-stage FBG optical demultiplexer. Applying the two developed components to the BOTDR system and using a temperature compensating algorithm, we successfully demonstrated the simultaneous measurement of strain and temperature distributions under various experimental conditions. The observed errors in the temperature and strain measured using the developed sensing system were 0.6 °C and 50 με, and the spatial resolution was 1 m, respectively

Arrayed Waveguide Grating-Based Interrogation System for Safety Applications and High-Speed Measurements

This thesis is focused on the design of two interrogation systems for Fiber Bragg Grating (FBG) sensors based on the Wavelength Domain Multiplexing (WDM) by means of the Arrayed Waveguide Grating (AWG) device. The FBG sensors have been employed in a large number of environments thanks to their intrinsic characteristics. To design a measurement system based on the Fiber Optic Sensor (FOS) technology, it is mandatory to make use of an optoelectronic system with the aim to "read" the wavelength shifting performed by the sensors. This latter is named interrogation system and, actually, sets a limit on the employability of the FBG sensors, due to its cost, design complexity and low reliability in some contests. For this reasons, the researchers are constantly looking on new technologies for the design of innovative interrogation systems. The AWG device seems to provide characteristics which cannot be reached with other devices and, due to its passivity, gives the possibility to increase the system speed to let the FBG sensors to be employed also for the detection of high-speed phenomena. Furthermore, thanks to the robustness and reliability of AWG device, is possible to turn an interrogation system into a full analog monitoring system employable in a safety scenario, such as industrial processes or other kind of environments, in which digital processing does not ensure enough reliability

Multiplexed optical fibre sensors for civil engineering applications

Fibre-optic sensors have been the focus of a lot of research, but their associated high cost has stifled their transferral from the laboratory to real world applications. This thesis addresses the issue of multiplexing, a technology that would lower the cost per unit sensor of a sensor system dramatically. An overview of the current state of research of, and the principles behind, multiplexed sensor networks is given. A new scheme of multiplexing, designated W*DM, is developed and implemented for a fibre Bragg grating (FBG) optical fibre sensor network. Using harmonic analysis, multiplexing is performed in the domain dual to that of the wavelength domain of a sensor. This scheme for multiplexing is compatible with the most commonly used existing schemes of WDM and TDM and thus offers an expansion over, and a resultant cost decrease from, the sensor systems currently in use. This research covered a theoretical development of the scheme, a proof of principle, simulated and experimental analysis of the performance of the multiplexed system, investigation into sensor design requirements and related issues, fabrication of the sensors according to the requirements of the scheme and the successful multiplexing of eight devices (thus offering an eightfold increase over current network capacities) using this scheme. Extensions of this scheme to other fibre sensors such as Long Period Gratings (LPGs) and blazed gratings were also investigated. Two LPGs having a moiré structure were successfully multiplexed and it was shown that a blazed Fabry Perot grating could be used as a tuneable dual strain/refractive index sensor. In performing these tests, it was discovered that moiré LPGs exhibited a unique thermal switching behaviour, hereto unseen. Finally the application of fibre sensors to the civil engineering field was investigated. The skill of embedding optical fibre in concrete was painstakingly developed and the thermal properties of concrete were investigated using these sensors. Field tests for the structural health monitoring of a road bridge made from a novel concrete material were performed. The phenomenon of shrinkage, creep and cracking in concrete was investigated showing the potential for optical fibre sensors to be used as a viable research tool for the civil engineer

Investigation into Smart Multifunctional Optical System-On-A-Chip Sensor Platform and Its Applications in Optical Wireless Sensor Networks

Wireless sensor networks (WSNs) have been widely used in various applications to acquire distributed information through cooperative efforts of sensor nodes. Most of the sensor nodes used in WSNs are based on mechanical or electrical sensing mechanisms, which are susceptible to electromagnetic interference (EMI) and can hardly be used in harsh environments. Although these disadvantages of conventional sensor nodes can be overcome by employing optical sensing methods, traditional optical systems are usually bulky and expensive, which can hardly be implemented in WSNs. Recently, the emerging technologies of silicon photonics and photonic crystal promise a solution of integrating a complete optical system through a complementary metal-oxide-semiconductor (CMOS) process. However, such an integration still remains a challenge. The overall objective of this dissertation work is to develop a smart multifunctional optical system-on-a-chip (SOC) sensor platform capable of both phase modulation and wavelength tuningfor heterogeneous sensing, and implement this platform in a sensor node to achieve an optical WSN for various applications, including those in harsh environments. The contributions of this dissertation work are summarized as follows. i)A smart multifunctional optical SOC sensor platform for heterogeneous sensing has beendeveloped for the first time. This platform can be used to perform phase modulation and demodulation in a low coherence interferometric configuration or wavelength tuning in a spectrum sensing configuration.The multifunctional optical sensor platform is developed through hybrid integration of a light source, an optical modulator, and multiple photodetectors. As the key component of the SOC platform, two types of modulators, namely, the opto-mechanical and electro-optical modulators, are investigated. For the first time, interrogating different types of heterogeneous sensors, including various Fabry-Perot (FP) sensors and fiber Bragg grating (FBG) sensors, with a single SOC sensor platform, is demonstrated. ii)Enhanced understanding of the principles of the multifunctional optical platform withanopto-mechanical modulator has been achieved.As a representative of opto-mechanical modulators, a microelectromechanical systems (MEMS) based FP tunable filter is thoroughly investigated through mechanical and optical modeling. The FP tunable filter is studied for both phase modulation and wavelength tuning, and design guidelines are developed based on the modeling and parametric studies. It is found that the MEMS tunable filter can achieve a large modulation depth, but it suffers from a trade-off between modulation depth and speed. iii) A novel silicon electro-optical modulator based on microring structures for optical phase modulation and wavelength tuning has been designed. To overcome the limitations of the opto-mechanical modulators including low modulation speed and mechanical instability, a CMOS compatible high speed electro-optical silicon modulator is designed, which combines microring and photonic crystal structures for phase modulation in interferometric sensors and makes use of two cascaded microrings for wavelength tuning in sensors that require spectrum domain signal processing. iv)A novel optical SOC WSN node has been developed. The optical SOC sensor platform and the associated electric circuit are integrated with a conventional WSN module to achieve an optical WSN node, enabling optical WSNs for various applications. v) A novel cross-axial dual-cavity FP sensor has been developed for simultaneous pressure and temperature sensing.Across-axial sensor is useful in measuring static pressures without picking up dynamic pressures in the presence of surface flows. The dual-cavity sensing structure is used for both temperature and pressure measurements without the need for another temperature sensor for temperature drift compensation. This sensor can be used in moderate to high temperature environments, which demonstrates the potential of using the optical WSN sensor node in a harsh environment

Impact detection techniques using fibre-optic sensors for aerospace & defence

Impact detection techniques are developed for application in the aerospace and defence industries. Optical fibre sensors hold great promise for structural health monitoring systems and methods of interrogating fibre Bragg gratings (FBG) are investigated given the need for dynamic strain capture and multiplexed sensors. An arrayed waveguide grating based interrogator is developed. The relationships between key performance indicators, such as strain range and linearity of response, and parameters such as the FBG length and spectral width are determined. It was found that the inclusion of a semiconductor optical amplifier could increase the signal-to-noise ratio by ~300% as the system moves to its least sensitive. An alternative interrogator is investigated utilising two wave mixing in erbium-doped fibre in order to create an adaptive system insensitive to quasistatic strain and temperature drifts. Dynamic strain sensing was demonstrated at 200 Hz which remained functional while undergoing a temperature shift of 8.5 °C. In addition, software techniques are investigated for locating impact events on a curved composite structure using both time-of-flight triangulation and neural networks. A feature characteristic of composite damage creation is identified in dynamic signals captured during impact. An algorithm is developed which successfully distinguishes between signals characteristic of a non-damaging impact with those from a damaging impact with a classification accuracy of 93 – 96%. Finally, a demonstrator system is produced to exhibit some of the techniques developed in this thesis

Development of Novel Fiber Optic Humidity Sensors and Their Derived Applications

The main focus of this thesis is on the design and development of novel fiber optic devices for relative humidity (RH) sensing with emphasis on high sensitivity, a wide humidity range, low temperature dependence, fast response time and good stability.Novel RH sensors based on fiber bends are fabricated by coating the surface of the buffer stripped bent fiber with selected hygroscopic materials such as Polyethylene oxide or Agarose. It is shown that the Polyethylene oxide coated device has a high sensitivity in a narrow RH range while the Agarose coated fiber bend shows a linear RH sensitivity in a wide RH range. Both of these sensors demonstrate a fast response (in the order of milliseconds) to RH variations. The limitations of fiber bend based humidity sensors are also discussed in the thesis. A novel RH sensor based on a reflection type photonic crystal fiber interferometer (PCFI) is presented which does not rely on the use of any hygroscopic material. The operating principle of a PCFI sensor based on the adsorption and desorption of water vapour at the silica-air interface within the PCF capillaries is discussed. The demonstrated sensor shows a good RH sensitivity in the higher RH range. Furthermore this RH sensor is almost temperature independent and can also be used in a high temperature and high pressure environment for humidity sensing.In order to improve the sensitivity of a reflection type PCFI over a wider RH range an alternative sensor is developed by infiltrating the microholes of the PCF with the hygroscopic material Agarose. The demonstrated novel sensor has a good sensitivity, a fast response time and a compact size. The temperature dependence of the device is also investigated. A novel hybrid device based on Agarose infiltrated PCFI interacting with a fiber Bragg grating is also presented which can simultaneously measure RH and temperature.A novel RH sensor based on a transmission type photonic crystal fiber interferometer coated with Agarose is also presented and discussed. This structure is used to study the effect of Agarose coating thickness in such a sensor on the RH sensitivity. It is demonstrated that the RH sensitivity of the sensor has a significant dependence on the thickness of the coating. An experimental method is also demonstrated to select an optimum coating thickness to achieve the highest sensitivity for a given RH sensing range. The sensor with the highest demonstrated sensitivity shows a linear response in the RH ranges of 40-80 % and 80-95 % with a sensitivity of 0.57 nm/%RH and 1.43 nm/%RH respectively.Finally, a comparison of the four RH sensing devices is presented, based on their size, operating range, RH sensitivity, temperature dependence and response time, in the context of selecting suitable devices for end-user applications. Two examples of applications are presented: dew sensing and breathing monitoring. The reflection type PCFI which does not use any hygroscopic material is selected for dew sensing and the dew response of the device is presented and discussed. Finally a novel breathing sensor based on the Agarose infiltrated PCFI is developed, which due to its immunity to interference from electric and magnetic fields, is suitable for breath monitoring of patients during medical procedures such as a magnetic resonance imaging scan

Time-Domain Fiber Loop Ringdown Sensor and Sensor Network

Optical fibers have been mostly used in fiber optic communications, imaging optics, sensing technology, etc. Fiber optic sensors have gained increasing attention for scientific and structural health monitoring (SHM) applications. In this study, fiber loop ringdown (FLRD) sensors were fabricated for scientific, SHM, and sensor networking applications. FLRD biosensors were fabricated for both bulk refractive index (RI)- and surface RI-based DNA sensing and one type of bacteria sensing. Furthermore, the effect of glucose oxidase (GOD) immobilization at the sensor head on sensor performance was evaluated for both glucose and synthetic urine solutions with glucose concentration between 0.1% and 10%. Detection sensitivities of the glucose sensors were achieved as low as 0.05%. For chemical sensing, heavy water, ranging from 97% to 10%, and several elemental solutions were monitored by using the FLRD chemical sensors. Bulk indexbased FLRD sensing showed that trace elements can be detected in deionized water. For physical sensing, water and cracking sensors were fabricated and embedded into concrete. A partially-etched single-mode fiber (SMF) was embedded into a concrete bar for water monitoring while a bare SMF without any treatment was directly embedded into another concrete bar for monitoring cracks. Furthermore, detection sensitivities of water and crack sensors were investigated as 10 ml water and 0.5 mm surface crack width, respectively Additionally fiber loop ringdowniber Bragg grating temperature sensors were developed in the laboratory; two sensor units for water, crack, and temperature sensing were deployed into a concrete cube in a US Department of Energy test bed (Miami, FL). Multi-sensor applications in a real concrete structure were accomplished by testing the six FLRD sensors. As a final stage, a sensor network was assembled by multiplexing two or three FLRD sensors in series and parallel. Additionally, two FLRD sensors were combined in series and parallel by using a 2×1 micro-electromechanical system optical switch to control sensors individually. For both configurations, contributions of each sensor to two or three coupled signals were simulated theoretically. Results show that numerous FLRD sensors can be connected in different configurations, and a sensor network can be built up for multiunction sensing applications