Dissertations / Theses on the topic 'Fibre optic'

This thesis is concerned with the realisation of an all-fibre acousto-optic device which can be used as a switch, filter, frequency shifter, polarisation controller or amplitude modulator, depending on the configuration. The devices are based on the acousto-optic interaction in fused tapered couplers and as such are completely compatible with fibre networks. For the first time polarisation insensitive devices have been realised by precisely controlling the degree of fibre fusion in an asymmetric coupler. This thesis details the theoretical considerations required to fabricate and operate all-fibre acousto-optic devices. The acousto-optic theory of circular cross-section devices is developed and extended to non-circular cross-sections. In this way all practical devices with non-circular cross-sections can be accurately modelled, and the optical bandwidth, switching speed and polarisation dependence can be tailored to a specific requirement. Polarisation independent operation is a serious obstacle in the development of practical photonic devices. This issue was resolved in this device by using geometric birefringence in the interaction region, produced by fusing the fibres of the null coupler to a precise degree of fusion. Polarisation independent operation is achieved with this technique without compromising other device parameters such as drive power. In order to fabricate these devices with the required uniformity, the tolerance on the manufacturing process was theoretically and experimentally studied. A new coupler rig was designed to operate within these tolerances. The coupler rig was capable of the manufacture of uniform devices both in the degree of fibre fusion and in the area of the device cross-section. The rig was optimised to produce low excess loss devices with high isolation between the output ports. In addition, it was possible to manufacture the length of the uniform tapered region over three times longer than previously possible. The incorporation of the non-circular operational theory enables non-destructive measurements of both the degree of fibre fusion and the device cross-sectional area, anywhere along the tapered region. Unlike competing technologies, geometric birefringence produces polarisation insensitive devices without compromising device parameters. This technique allows the following characteristics to be simultaneously achieved: 1) lower drive power, < 0.25 mW, 2) improved polarisation insensitivity, < 0.01 dB, 3) improved crosstalk, < -40 dB, 4) better excess loss, < 0.001 dB, 5) switching speeds of < 40 µs for a 25 mm long device, and 6) only one fabrication step.

The aim of the research work described in this thesis was to investigate the interrogation of fibre optic sensors using "off the shelf optical components and equipment developed mainly for the telecommunications industry. This provides a cost effective way of bringing fibre optic sensor systems to within the price range of their electro-mechanical counterparts. The research work focuses on the use of an arrayed waveguide grating, an acousto-optic tuneable filter and low-coherence interferometry to measure dynamic strain and displacement using fibre Bragg grating and interferometric sensors. Based on the intrinsic properties of arrayed waveguide gratings and acousto-optic tuneable filters used in conjunction with interferometry, fibre Bragg gratings and interferometric sensors a number of novel fibre optic sensor interrogation systems have been realised. Special single mode fibre, namely, high-birefringence fibre has been employed to implement a dual-beam interrogating interferometer. The first interrogation scheme is based on an optical channel monitor, which is an arrayed waveguide grating with integral photo-detectors providing a number of amplified electrical outputs. It is used to interrogate fibre Bragg grating and interferometric sensors. Using the properties of polarisation maintainability in high-birefringent fibre an interrogating interferometer was realised by winding a length of the fibre around a piezoelectric modulator generating a low-frequency carrier signal. The system was used to interrogate both fibre Bragg grating and interferometric sensors. Finally, the use of an acousto-optic tuneable filter is employed to interrogate fibre Bragg gratings. The device is used to generate a very high frequency carrier signal at the output of an optical interferometer.

Optical fibre-based sensors have gained much attention over the past three decades and are gaining acceptance in many industries such as the aerospace, automotive, chemical processing, biological sensing, and medical industries. The work presented here demonstrates the use of D-shaped fibres as intensity-based fibre optic sensors. The fibres are etched to gain access to their evanescent optical fields and are operated in their "leaky", regimes, i.e., the regime in which optical power leaks out of their cores. The optical power transmission through the etched D-fibre, which constitutes the sensor head, depends on the refractive index of the measurand placed on the sensor head. Thus by using materials whose refractive indices depend on the desired measurands, intensity-based sensors can be realised. A "non-destructive" process for determining the etched cladding thickness of the D-fibre is also developed. Once calibrated for a specific fibre or fibre batch, only one thermo-optic oil, three temperature measurements and a mathematical fitting routine are required to determine the etched cladding thickness with an accuracy better than 0.3 μm. Four types of sensor are demonstrated. A refractive index sensor with a high resolution region and a low resolution region, both of which can be shifted by changing the operating wavelength, is demonstrated with a maximum resolution on the order of 10⁻⁶ refractive index units. Naturally, this leads to the development of a temperature sensor with an adjustable dynamic range and resolution that uses thermo-optic oils. Proof of principle is demonstrated for an electric field sensor using chiral smectic A liquid crystals. A continuous liquid level sensor (that can also be implemented as a discrete level sensor) is also demonstrated using only one optical source, one optical detector and a single length of fibre.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

"Smart Structures" or "Smart Skins" will require structurally integrated sensing systems that can operate in practical situations. Optical sensing techniques are receiving considerable attention for the monitoring of such systems. Single ended polarimetric sensors were utilized with a large dynamic range for strain measurements as surface mounted and embedded strain sensors in composite materials (glass fibre and carbon fibre reinforced polymers). They were also used to monitor the strain and the formation of microcracks in the glue line of carbon fibre reinforced polymer (CFRP) concrete beams. The intrinsic Fabry-Perot was also used as a surface mounted sensor to monitor axial strain of GFRP coupons. Finite Element (FE) modelling was used in order to investigate the stress/strain distributions within the composite material and the embedded optical fibre. The modelling results show excellent agreement with the experimental results and suggest that the soft acrylate coating is debonding, thus reducing the sensor's dynamic range. Actuators and/or Sensors embedded into a host material will disrupt the physical properties of the host. Finite element analysis was used to determine and to minimise the stress concentrations which arise in a "Smart" material system due to the embedded optical fibre sensor. A parametric study was undertaken to determine the theoretical mechanical and thermal properties of the interface coating that minimises the disruption of the polymer composite host material properties due to the optical fibre inclusion. The effects of transverse tensile and thermal loading were studied, and also the residual thermal stress concentrations due to the manufacturing process were taken into consideration. The stress concentrations in the composite host are affected by the dimensions, mechanical and thermal properties of the interface coating. The results show that with careful selection of the interface coating properties die stress concentrations in the host material caused by the optical fibre inclusion can be reduced and be similar to those of the pure host material.

The intrinsic stability of fibre optic based sensing systems offer a platform that is suited to hazardous waste detection in a wide range of environments. Over the last few years Cranfield University has been working on the development of chemical sensors using optical fibres in combination with a group of chemical recognition molecules called calixarenes. Calixarenes semi-selectively with a range of solvents of interest makes them useful for chemical detection systems. This work has primarily been focused on the use of calixarenes in sensing benzene and other hazardous solvents. However, this approach could potentially be expanded for use in a wide range of chemical and even biological recognition systems. The initial aim of this project was to build on the previous work in fibre optic sensing at Cranfield and explore approaches to improve and extend the performance of the sensor system. The project first focused on improving the techniques used in the Langmuir-Blodgett (LB) deposition of calixarenes. Initial studies in this area highlighted one critical experimental error associated with the use of dry Wilhelmy plates to monitor the surface pressure of the Langmuir film. Dry filter paper plates take up to 2 hours to give stable data, with a drift of up to 10% in the measured surface pressure. It is shown that this problem can be avoided by using pre-soaked plates. To provide an alternative to the Wilhelmy plate surface pressure senor, an optical fibre surface pressure sensor was developed, measuring changes in the meniscus forming properties of a liquid. The sensor consists of a tapered single mode silica fibre, mounted with a small curvature and positioned with the tapered region of the fibre immersed in the water. The performance of the fibre optic sensor is comparable with that of the conventional Wilhelmy plate surface pressure sensor showing linearity of greater than 0.9. Following the analysis of the experimental systems used in the construction of the sensors, the project then focused on the chemistry of the materials and their suitability for LB coating. A variety of these materials were spread as Langmuir monolayers and their behavior upon compression measured. Long chain-substituted resorcinarenes gave more stable monolayers than their short chain analogues. The incorporation of long chain surfactants led to large increases in surface area, demonstrating that both resorcinarenes and surfactants are located at the water surface, except for one system where a bilayer structure is potentially formed. Further work on the behavior of the materials involved the alteration of the dipole-dipole interaction of the monolayer materials with the subphase. The modification of this interaction through the introduction of dipole altering additives, including alcohols and hydrogen peroxide, to the aqueous subphase was investigated. The resulting isotherms of the materials showed a reduction in the surface pressure and area per molecule required in order for the monolayer to reach its point of collapse. This ability to shift the point of collapse has application in the optimisation of Langmuir-Blodgett coating of surfaces. Within this project the sensing properties of a fibre sensor were also modelled extensively in order to determine the theoretical sensing limits of a fibre optic vapour sensor. The model showed that the sensing goals of 1ppm originally envisaged for this project were unobtainable due to the low number of gas molecules interacting with the sensor. However, this led to the proposal of a new application of the system in sensing contaminants in water, where the same limitations would not apply. The results for the sensor system tested in water show how significantly more sensitive the system is to toluene contamination in water than it is to toluene vapour. These results demonstrate the utility of the developed system for many pollutant-sensing applications, include crude oil detection.

This thesis describes the novel multicomponent glasses as the basis for fibre optic device and system applications. It is shown that multicomponent glasses are expected to greatly enhance the functionality of the fibre-optical devices and systems by compensating for the limitations of current silica-glass based fibre-devices. Fabrication and characterization of various multicomponent glasses and fibres suitable for specific applications have been investigated. The fabrication of rare-earth-doped multicomponent glasses based on commercial optical-glasses has been tackled by developing a novel doping technique. An intermediate maximum phonon-energy lead-germanate glass suitable for fibre optic applications has been achieved by optimizing the glass composition via the Levin-Block concept. Non-hygroscopic chlorotellurite glass. 60TeO2. 40ZnCl2, and its modifications for fibre structure have been prepared, their intrinsic high thermal-stabilities characterized and intrinsic optical transparency in the mid-IR evaluated. Spectroscopic and structural characterization of rare-earth-doped multi-component glasses and fibres have also been conducted. Radiative and non-radiative processes for rare-earth transitions in glasses were studied with a view to finding materials in which rare-earth transitions are more radiative and less non-radiative. A "crystal-chemistry" model is proposed to evaluate the rare-earth local structure in glass, based on the Modified-Random-Network (MRN) theory in glass structure. The rare-earth spectral behaviour in glasses has been well described and predicted by this model. Using the rare-earth-doped novel multicomponent glass-waveguides has led to a number of fibre and planar-waveguide laser devices having been demonstrated. This includes the most efficient and a high-power Nd3+- fibre laser ever reported at 1.06µm in oxide glasses, the first demonstration of Nd3+- and Er3+- doped monomode planar-waveguide lasers at 1.06µm and 1.54µm and a new class of fibre laser based on rare-earth-doped lead-germanate glass.

Currently several medical imaging techniques are in operation in clinical environments that help the doctors to diagnose diseases before proceeding to treatment. This research investigated and implemented configurations for a medical imaging technique called Optical Coherence Tomography (OCT). The Michelson interferometer has been the principal configuration for OCT systems. For endoscopic OCT applications though such a configuration is susceptible to polarisation induced signal fading due to environmental changes within the sample arm, like fibre bending and temperature fluctuations. The development and testing of a Fizeau interferometer based OCT system demonstrate the elimination of this polarisation induced signal fading as the sample and reference arm of the interferometer experience the same environmental changes. The Fizeau interferometer is formed between the end of the fibre and the sample itself. A secondary processing interferometer is used in this configuration, to compensate the path difference inherent to the Fizeau configuration. Longitudinal resolution is about 19 m in air, and lateral resolution about 23 m. A signal-to-noise ratio of about 80 dB was achieved with this system and several sets of image data were collected from laboratory and biological samples. An OCT system based on a Fizeau interferometer incorporating a coherent fibre imaging bundle was also investigated. Fibres in the bundle are accessed sequentially by a beam focused onto the input face of the bundle, allowing 2D information to be acquired using point detection. Incorporating a fibre optic imaging bundle in Fizeau based interferometer OCT allows mechanical scanning parts to be removed from the sample arm, resulting in a passive probe. Such a configuration can form a compact, robust and “downlead insensitive” OCT system and is a strong candidate for in vivo applications. The performance of leached and wound coherent fibre optic bundles was accessed and an analysis is presented. The longitudinal resolution of the system developed was about 10 m in air, and lateral resolution about 9 m. This thesis demonstrates several sets of image data that were collected from various samples using a Fizeau interferometer based OCT incorporating coherent imaging bundles. 2D image data captured by such a configuration are presented and reported for the first time in this thesis.

There is both commercial and practical demand to extend the applications for which fibre optic sensors are used. This thesis describes the development of fibre optic sensors and systems for security and defence applications, based on established fibre-optic hydrophone I technology. Dynamic acoustic and seismic signals have been detected with sensor arrays, and used to demonstrate the detection of targets. Achievements have been made in the development and characterisation of entirely novel designs of fibre optic accelerometers and microphones, with the demonstration of arrays of these in practical application to intruder detection and the location of distant acoustic sources. Several prototypes of both acoustic and seismic sensors have been developed. Research efforts focused on designing ,,/ . /~nd establishing the perfonnance of these sensors, then refining the strongest designs for incorporation into arrays of sensors. These arrays have been deployed in environments representative of real operational deployment situations, extending the proof of principle established in the laboratory to demonstration in the field. Specifically, six entirely novel· fibre optic microphone designs are presented, concerning both intensity and interferometric schemes, and evaluated for responsivity and suitability to the location of acoustic targets in a battlefield application. The strongest of these is identified to be a foam-backed fibre wrapped mandrel, of simple design and high responsivity, and readily multiplexed into and interrogated with an inter~rometric system. An array of such foam-backed mandrel microphones is also demonstrated in the field and shown to have very good target location capability at ranges in excess of lkm. In other work, an entirely novel design of fibre optic accelerometer is presented, and theresponse of this is characterised in detail, in terms of both responsivity and the impact of the sensor set-up condition on responsivity. A buried array of such accelerometers combined with distributed cable sensors is demonstrated in a field deployment detecting the presence of signals from an intruder in the region of the array.

Cancer is a major health problem in the world today. Almost all cancers have a significantly better chance for therapy and recovery if detected at their early stage. The capability to perform disease diagnosis at an early stage requires high-resolution imaging that can visualise the physiological and morphological changes at a cellular level. However, resolving powers of current medical imaging systems are limited to sub-millimeter sizes. Furthermore, the majority of cancers are associated with morphological and functional alterations of cells in epithelial tissue, currently assessed by invasive and time-consuming biopsy. Optical imaging enables visualisations of tissue microstructures at the level of histology in non-invasive means. Optical imaging is suitable for detecting neoplastic changes with sub-cellular resolution in vivo without the need for biopsy. Nonlinear optical microscopy based on multi-photon absorption and higher harmonic generation has provided spectacular sights into visualisation of cellular events within live tissue due to advantages of an inherent sectioning ability, the relatively deep optical penetration, and the direct visualisation of intrinsic indicators. Two-photon excited uorescence (TPEF) from intrinsic cell components and second harmonic from asymmetric supermolecular structures can provide complementary information regarding functionalities and morphologies in tissue environments, thus enabling premalignant diagnosis by detecting the very earliest changes in cellular structures. During the past sixteen years, nonlinear optical microscopy has evolved from a photonic novelty to a well-established laboratory tool. At present, in vivo imaging and long-term bedside studies by use of nonlinear optical microscopy have been limited due to the fact that the lack of the compact nonlinear optical instrument/imaging technique forces the performance of nonlinear optical microscopy with bulk optics on the bench top. Rapid developments of fibre-optics components in terms of growing functionalities and decreasing sizes provide enormous opportunities for innovation in nonlinear optical microscopy. Fibre-based nonlinear optical endoscopy will be the soul instrumentation to permit the cellular imaging within hollow tissue tracts or solid organs that are inaccessible with a conventional optical microscope. Lots of efforts have been made for development of miniaturised nonlinear optical microscopy. However, there are major challenges remaining to create a nonlinear optical endoscope applicable within internal cavities of a body. First, an excitation laser beam with an ultrashort pulse width should be delivered eciently to a remote place where ecient collection of faint nonlinear optical signals from biological samples is required. Second, laser-scanning mechanisms adopted in such a miniaturised instrumentation should permit size reduction to a millimeter scale and enable fast scanning rates for monitoring biological processes. Finally, the design of a nonlinear optical endoscope based on micro-optics must maintain great exibility and compact size to be incorporated into endoscopes to image internal organs. Although there are obvious diculties, development of fibre-optic nonlinear optical microscopy/endoscopy would be indispensible to innovate conventional nonlinear optical microscopy, and therefore make a significant impact on medical diagnosis. The work conducted in this thesis demonstrates the new capability of nonlinear optical endoscopy based on a single-mode fibre (SMF) coupler or a double-clad photonic crystal fibre (PCF), a microelectromechanical system (MEMS) mirror, and a gradientindex (GRIN) lens. The feasibility of all-fibre nonlinear optical endoscopy is also demonstrated by the further integration of a double-clad PCF coupler. The thesis concentrates on the following key areas in order to exploit and understand the new imaging modality. It has been known from the previous studies that an SMF coupler is suitable for twoii photon excitation by transmitting near infrared illumination and collecting uorescence at visible wavelength as well. Although second harmonic generation (SHG) wavelength is farther away from the designed wavelength of the fibre coupler than that of normal TPEF, it is demonstrated in this thesis that both SHG and TPEF signals can be collected simultaneously and eciently through an SMF coupler with axial resolution of 1.8 um and 2.1 um, respectively. The fibre coupler shows a unique feature of linear polarisation preservation along the birefringent axis over the near infrared and the visible wavelength regions. Therefore, SHG polarisation anisotropy can be potentially extracted for probing the orientation of structural proteins in tissue. Furthermore, this thesis shows the characterisation of nonlinear optical microscopy based on the separation distance of an SMF coupler and a GRIN lens. Consequently, the collection of nonlinear signals has been optimised after the investigation of the intrinsic trade-off between signal level and axial resolution. These phenomena have been theoretically explored in this thesis through formalisation and numerical analysis of the three-dimensional (3D) coherent transfer function for a SHG microscope based on an SMF coupler. It has been discovered that a fibreoptic SHG microscope exhibits the same spatial frequency passband as that of a fibreoptic reection-mode non-uorescence microscope. When the numerical aperture of the fibre is much larger than the convergent angle of the illumination on the fibre aperture, the performance of fibre-optic SHG microscopy behaves as confocal SHG microscopy. Furthermore, it has been shown in both analysis and experiments that axial resolution in fibre-optic SHG microscopy is dependent on the normalised fibre spot size parameters. For a given illumination wavelength, axial resolution has an improvement of approximately 7% compared with TPEF microscopy using an SMF coupler. Although an SMF enables the delivery of a high quality laser beam and an enhanced sectioning capability, the low numerical aperture and the finite core size of an SMF give rise to a restricted sensitivity of a nonlinear optical microscope system. The key innovation demonstrated in this thesis is a significant signal enhancement of a nonlinear optical endoscope by use of a double-clad PCF. This thesis has characterised properties of our custom-designed double-clad PCF in order to construct a 3D nonlinear optical microscope. It has been shown that both the TPEF and SHG signal levels in a PCF-based system that has an optical sectioning property for 3D imaging can be significantly improved by two orders of magnitude in comparison with those in an SMF-based microscope. Furthermore, in contrast with the system using an SMF, simultaneous optimisations of axial resolution and signal level can be obtained by use of double-clad PCFs. More importantly, using a MEMS mirror as the scanning unit and a GRIN lens to produce a fast scanning focal spot, the concept of nonlinear optical endoscopy based on a double-clad PCF, a MEMS mirror and a GRIN lens has been experimentally demonstrated. The ability of the nonlinear optical endoscope to perform high-resolution 3D imaging in deep tissue has also been shown. A novel three-port double-clad PCF coupler has been developed in this thesis to achieve self-alignment and further replace bulk optics for an all-fibre endoscopic system. The double-clad PCF coupler exhibits the property of splitting the laser power as well as the separation of a near infrared single-mode beam from a visible multimode beam, showing advantages for compact nonlinear optical microscopy that cannot be achieved from an SMF coupler. A compact nonlinear optical microscope based on the doubleclad PCF coupler has been constructed in conjunction with a GRIN lens, demonstrating high-resolution 3D TPEF and SHG images with the axial resolution of approximately 10 m. Such a PCF coupler can be useful not only for a fibre-optic nonlinear optical probe but also for double-clad fibre lasers and amplifiers. The work presented in this thesis has led to the possibility of a new imaging device to complement current non-invasive imaging techniques and optical biopsy for cancer detection if an ultrashort-pulsed fibre laser is integrated and the commercialisation of the system is achieved. This technology will enable in vivo visualisations of functional and morphological changes of tissue at the microscopic level rather than direct observations with a traditional instrument at the macroscopic level. One can anticipate the progress in bre-optic nonlinear optical imaging that will propel imaging applications that require both miniaturisation and great functionality.

Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, Centre for Micro-Photonics, 2007.
A thesis submitted for the degree of Doctor of Philosophy, Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, 2007. Typescript. Bibliography: p. 146-162.

An important research topic in engineering and science is the miniaturization of systems and components, where small size and weight, low power consumption and low manufacturing cost can offer new functionalities. In recent years the fields of optics and fluidics have merged to create the area of research known as "optofluidics". The unique properties of fluids (compression, flow, and variable refractive index) can be combined with optical devices to create systems with advantages in optical measuring, communication and imaging areas. Variable optical attenuators (VOAs) are components in optical communication networks for managing optical power levels. This thesis is focused on the design, fabrication and characterization of two novel optofluidics based single-mode fibre VOAs ("continuous fibre" type VOA and "fibre gap" type VOA). The first VOA is constructed from a side-polished optical fibre which is characterized by a sloping shape profile of the external cladding thickness. The fibre is positioned on top of a platform which exploits electrowetting-on-dielectric (EWOD) techniques to move a liquid droplet. By moving a liquid droplet across EWOD platform (and along the polished region of the fibre), optical attenuation can be obtained. The droplet, whose refractive index is equal to or higher than the refractive index of the fibre core, is accessing and leaking radiation from the optical evanescent field of the polished fibre which then modifies the optical attenuation. The level of attenuation depends on the position of the droplet; the attenuation increases as the cladding thickness reduces and the droplet moves closer to the fibre core. The second VOA is a fibre gap device, where a ferrofluid shutter/actuator is located in the gap between lensed single mode optical fibres. The ferrofluid shutter movement is controlled by a magnetic field and changes the light propagation between fibres. The level of attenuation is defined by the shutter position.

Common problems encountered in clinical sensing are those of non-biocompatibility, and slow response time of the device. The latter, also applying to chemical sensors, is possibly due to a lack of understanding of polymer support or membrane properties and hence failure to optimise membranes chosen for specific sensor applications. Hydrogels can be described as polymers which swell in water. In addition to this, the presence of water in the polymer matrix offers some control of biocompatibility. They thus provide a medium through which rapid transport of a sensed species to an incorporated reagent could occur. This work considers the feasibility of such a system, leading to the design and construction of an optical sensor test bed. The development of suitable membrane systems and of suitable coating techniques in order to apply them to the fibre optics is described. Initial results obtained from hydrogel coatings implied that the refractive index change in the polymer matrix, due to a change in water content with pH is the major factor contributing to the sensor response. However the presence of the colourimetric reagent was also altering the output signal obtained. An analysis of factors contributing to the overall response, such as colour change and membrane composition were made on both the test bed, via optical response, and on whole membranes via measurement of water content change. The investigation of coatings with low equilibrium water contents, of less than 10% was carried out and in fact a clearer signal response from the test bed was noted. Again these membranes were suprisingly responding via refractive index change, with the reagent playing a primary role in obtaining a sensible or non-random response, although not in a colourimetric fashion. A photographic study of these coatings revealed some clues as to the physical nature of these coatings and hence partially explained this phenomenon. A study of the transport properties of the most successful membrane, on a coated wire electrode and also on the fibre optic test bed, in a series of test environments, indicated that the reagent was possibly acting as an ion exchanger and hence having a major influence on transport and therefore sensor characteristics.

Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, Centre for Atom Physics an Ultra-fast Spectroscopy, 2008.
A thesis submitted for the degree of Master of Engineering, Centre for Atom Physics an Ultra-fast Spectroscopy, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, 2008. Typescript. Bibliography: p. 143-149.

Long period gratings (LPGs) are formed by inducing a permanent periodic modulation of the refractive index (RI) of the core of an optical fibre. The transmission spectrum of the LPG contains a series of attenuation bands centered at discrete wavelengths. The exact form of the transmission spectrum and the central wavelengths of the attenuation bands, are sensitive to the period and the length of the LPG and to the local environment. The sensitivity of a LPG to a particular measurand is dependant upon the composition of the fibre and upon the order of the cladding mode to which the guided mode is coupled to and is thus different for each attenuation band. Cont/d.

The aim of the project was to develop the Fizeau interferometer configuration to take advantage of the benefits derived from its “downlead insensitivity” to temperature and polarisation fading. This sensing interferometer was investigated and implemented in conjunction with various processing interferometers with the view to achieve optimised performances for Optical Coherence Tomography (OCT) application. A comprehensive theoretical analysis has been carried for Signal-to-Noise ratio of these OCT systems. Balanced detection indenting two detectors with anti-phase signals was also investigated to improve further the SNR. The analysis showed that the SNR (67 dB) of the balanced Fizeau interferometer improved by 30 dB from that of standard Fizeau interferometer implementing a single detector. Experimentally, the best SNR for this configuration was achieved by adding an electronic rectifier based demodulation system for the signal after balanced detection. The OCT systems investigated in this project were developed based on a broadband source operating in the 1550 nm wavelength-band to facilitate improvement in the depth of penetration of light directed into imaged samples. The coherence length or axial resolution in air of the system was 21[micro milli]while the transverse resolution was 18 [micro milli] and focusing depth was 340 [micro milli].

This work is the continuation of previous research by A. Raza and other contributors to the field of fibre optic plethysmography. Plethysmography is defined as the volume estimation of an object according to its external dimensions. fu particular, this technique can be used to produce an estimation of the respiratory volume of a human subject according to the dimension of his chest, measured at the thoracic and abdominal levels. A respiratory plethysmograph simply attempts to deliver a signal being the closest possible estimation of the true respiratory volume, as measured by a spirometer or a pneumotachometer. There are essentially two instrumental approaches to respiratory plethysmography: 1) The Respiratory Inductive Plethysmograph (RIP) estimates the cross section area of the chest by monitoring the variation of inductance in an electrical wire encircling the chest. 2) The Fibre Optic Respiratory Plethysmograph (FORP) sees the contribution of fibre optic sensors to measure the chest's circumference variations. The purposes of the present investigation were to improve the performance of previous FORP prototypes, and to extend its capabilities to cardiac monitoring. Both these targets have been reached, and the new prototype is now demonstrating the potential of plethysmography as a sound investigation technique for both cardiac and respiratory monitoring. Overall, the improved sensor and acquisition system permitted the resolution of details of the plethysmographic waveforms that were beyond the reach of the previous prototype. The new FORP prototype is generally more reliable and more precise, if not less compact. From a medical point of view, research carried out with the new FORP prototype has had two major outcomes: 1) The increased temporal resolution-of the new acquisition system has given us the possibility to precisely measure the phase shifts between the plethysmographic signals, and the spirometric signal. Such measurements have contributed. to producing a better estimation of the spirometric signal, therefore increasing the credibility of the FORP as a non-invasive, respiratory volume monitoring device. 2) The increased amplitude resolution of the new acquisition system, coupled with the better linearity, better precision and smaller hysteresis of the new sensor, has enabled the FORP to detect body circumference variations due to cardiac activity around head, neck, thorax and abdomen of a patient. Observations of heart movements at thoracic level had already been reported with the RIP, the direct analogue of the FORP. The signal processing required by the RIP for such monitoring only permitted offline, Electro-Cardio-Gram (ECG) assisted interrogation of cardiac displacements. However, thanks to better signal processing, the FORP has been made capable of real time cardiac position monitoring, without referencing to a simultaneous ECG signal. The combined impact of this research and previous research by A. Raza and A. Augousti on respiratory gating with the FORP, is potentially important in the field of cardiac imaging with Magnetic Resonance and Computed Tomography scanners. The FORP should permit better synchronisation with cardiac movements, while helping the patient to maintain stable chest position, subsequently increasing the image resolution by limiting motion blur.

The development of fibre-optic interferometric sensor arrays for application in underwater acoustics has been an area of active research since the late 1970's. The technology has reached a level whereby prototype arrays have been successfully demonstrated in sea-trials. However, the recent development of several new technologies may significantly increase the size and performance of these arrays. We demonstrate the potential increase in multiplexed array sizes using architectures based on combining dense wavelength division multiplexing and time division multiplexing. These architectures also include erbium doped fibre amplifiers for post, pre, inline and remote amplification in order to increase the standoff distance between the array and electronics unit. We also theoretical investigate the limitations imposed on the number of sensors that can be multiplexed, due to nonlinear transmission effects in the link fibre in the presence of high optical powers and multiple wavelengths. We also demonstrate novel DFB erbium doped fibre lasers as optical sources. These sources exhibit linewidths significantly narrower than semiconductor DFB lasers, which are currently used in many sensor arrays, and thus may provide a significant improvement in sensor resolution. We investigate the intensity and frequency noise properties of these lasers, their modulation properties and successfully develop intensity noise and frequency noise reduction techniques. We also investigate the potential of fibre-optic acoustic vector sensors and demonstrate fibre-optic flexural disk accelerometers. Finally, we demonstrate polymer coated in-fibre Bragg gratings as pressure and temperature sensors and investigate polymer coatings as a means to increase the acoustic responsivity of fibre laser acoustic sensors.

This thesis deals with the subject of fibre optic magnetic field sensors utilizing iron garnet materials. Such materials exhibit a large Faraday rotation which make them advantageous for application in compact mag­netic field sensors. After an introduction, in which fibre optic sensors and optical methods to measure electric current are reviewed, the original research work is summarized. A system for the measurement of the magneto-optic properties of trans­parent materials is described. Measurement results, showing the influence of temperature, magnetic field direction and sample treatment on the magneto-optical proper­ties of YIG-crystals, are presented. The proper­ties of thin magneto-optical waveguiding films have also been studied using different light coupling methods. Measurement results obtained for holo­graphic grating, prism and edge (end-fire) light coupling to different substituted YIG films are presented. It is shown that the launching method may affect the properties to be measured. The design and performance of several versions of extrinsic guided wave fibre optic magnetic field sensors are then reported. The sensors employ substi­tuted YIG (Yttrium Iron Garnet, Y3Fe5O12) thin film waveguides as sensing elements. Polari­zation maintaining fibres were used as feed and return to provide two signal channels. The signals were combined in a balanced measure­ment system, providing insensitivity to both fluctuations in optical power and loss. Sensors have been made both with separate fibres to guide the light to and from the sensing element and with a single fibre for both functions. The two fibre version, although less ”elegant”, is found to have a better performance. This version also makes it possible to determine both the magnitude and sign of the magnetic field. Measurement results indicate a usable measurement range of at least several mT with a noise equivalent magnetic field level of less than 8 nT/root(Hz). The design and performance of multimode fibre optic magnetic field sensors utilizing the Faraday effect in an epitaxially grown thick (YbTbBi)IG film is also described. This type of sensor is found to be linear over a range from 27 mT to less than 270 nT. Sensor prototypes suitable for current monitoring in high voltage transmission lines have also been developed.
QC 20111209
YIG

Fibre optic chemical sensing has been demonstrated using side-polished single mode optical bre, evanescently coupled to chemically sensitive Langmuir-Blodgett overlay waveguides. The sensors exhibit a channel dropping response centred on a wavelength dependent upon the thickness and refractive index of the overlay waveguide. It has been shown that chemically sensitive organic dyes prove to be suitable materials for forming the overlay waveguide whereas the Langmuir-Blodgett deposition technique provides the required overlay thickness control. A brief optic pH sensor has been demonstrated by depositing a merocyanine dye onto a side-polished optical fibre which exhibits a change in transmission of 9.7 ± 0.8 dB pH" at a working wavelength of 750 n and 4.2 ± 0.5 dB pH`1 at 780 nm, with a shift in the channel dropping centre wavelength of 18.8 ± 0.8 n pH`1. The response time of the sensor to a step change in pH has been measured to be ~ 20 s. The advantage of being able to chemically engineer the organic materials used to form the overlay waveguides has been demonstrated by manufacturing a second pH sensor designed to operate at longer wavelengths using a second, modified merocyanine dye. The overall wavelength response of this second sensor to pH has been shown to be non- linear but a sensitivity of 23.3 ± 0.3 n pH`l is observed over a linear region. Finally, i order to investigate the non-linear response, a computer model based on the Kramers- Kronig relations has been devised and shows good agreement with experimental values. l

The influence of nano-structured overlays on the properties of fibre optic Long Period Gratings (LPGs) has been investigated. The presence of overlays of thickness of order 100 m is shown to significantly alter the response of the LPG to environmental parameters, such as the surrounding refractive index. A LPG with a nano-structured overlay was observed to exhibit enhanced sensitivity to refractive indices lower than silica, and was observed to exhibit sensitivity to refractive indices greater than that of the silica cladding, not observed in the absence of the overlay. A theoretical model predicts the observed behaviour. The observation was exploited to enhance the sensitivity of a LPG based chemical concentration sensor. The response of the transmission spectrum of concatenated long period gratings (LPGs) to ' the deposition of nano-structured overlays was also investigated. Concatenated LPGs act as a Mach-Zehnder interferometer and the phase of the interference fringes that appear within the LPGs° attenuation bands were shown to be highly sensitive to the optical thickness of the overlay, for thicknesses of order 100 mn. Several devices were fabricated with increasing path lengths to demonstrate effect of overlay thickness and refractive index on the transmission spectrum of the LPG.

Fibre optic chemical sensing has been demonstrated using side-polished single mode optical bre, evanescently coupled to chemically sensitive Langmuir-Blodgett overlay waveguides. The sensors exhibit a channel dropping response centred on a wavelength dependent upon the thickness and refractive index of the overlay waveguide. It has been shown that chemically sensitive organic dyes prove to be suitable materials for forming the overlay waveguide whereas the Langmuir-Blodgett deposition technique provides the required overlay thickness control. A brief optic pH sensor has been demonstrated by depositing a merocyanine dye onto a side-polished optical fibre which exhibits a change in transmission of 9.7 ± 0.8 dB pH" at a working wavelength of 750 n and 4.2 ± 0.5 dB pH`1 at 780 nm, with a shift in the channel dropping centre wavelength of 18.8 ± 0.8 n pH`1. The response time of the sensor to a step change in pH has been measured to be ~ 20 s. The advantage of being able to chemically engineer the organic materials used to form the overlay waveguides has been demonstrated by manufacturing a second pH sensor designed to operate at longer wavelengths using a second, modified merocyanine dye. The overall wavelength response of this second sensor to pH has been shown to be non- linear but a sensitivity of 23.3 ± 0.3 n pH`l is observed over a linear region. Finally, i order to investigate the non-linear response, a computer model based on the Kramers- Kronig relations has been devised and shows good agreement with experimental values.