Optical Fiber Interferometric Sensors

The contributions presented in this book series portray the advances of the research in the field of interferometric photonic technology and its novel applications. The wide scope explored by the range of different contributions intends to provide a synopsis of the current research trends and the state of the art in this field, covering recent technological improvements, new production methodologies and emerging applications, for researchers coming from different fields of science and industry. The manuscripts published in the Special issue, and re-printed in this book series, report on topics that range from interferometric sensors for thickness and dynamic displacement measurement, up to pulse wave and spirometry applications

High-resolution 3D printing enabled, minimally invasive fibre optic sensing and imaging probes

Minimally invasive surgical procedures have become more favourable to their traditional surgical counterparts due to their reduced risks, faster recovery times and decreased trauma. Despite this, there are still some limitations involved with these procedures, such as the spatial confinement of operating through small incisions and the intrinsic lack of visual or tactile feedback. Specialised tools and imaging equipment are required to overcome these issues. Providing better feedback to surgeons is a key area of research to enhance the outcomes and safety profiles of minimally invasive procedures. This thesis is centred on the development of new microfabrication methods to create novel fibre optic imaging and sensing probes that could ultimately be used for improving the guidance of minimally invasive surgeries. Several themes emerged in this process. The first theme involved the use and optimisation of high-resolution 3D injection of polymers as sacrificial layers onto which parylene-C was deposited. One outcome from this theme was a series of miniaturised parylene-C based membranes to create fibre optic pressure sensors for physiological pressure measurements and for ultrasound reception. The pressure sensor sensitivity was found to vary from 0.02 to 0.14 radians/mmHg, as the thickness of parylene was decreased from 2 to 0.5 μm. The ultrasound receivers were characterised and exhibited a noise equivalent pressure (NEP) value of ~100 Pa (an order of magnitude improvement compared to similarly sized piezoelectric hydrophones). A second theme employed high-resolution 3D printing to create microstructures of polydimethylsiloxane (PDMS) and subsequently formed nanocomposites, to create microscale acoustic hologram structures. This theme included the development of innovative manufacturing processes such as printing directly onto optical fibres, micro moulding and precise deposition which enabled the creation of such devices. These microstructures were investigated for reducing the divergence of photoacoustically-generated ultrasound beams. Taken together, the developments in this thesis pave the way for 3D microfabricated polymer-based fibre optic sensors that could find broad clinical utility in minimally invasive procedures

Optical Fibre Sensors applied to condition and structural monitoring for the marine and rail transport sectors

This thesis reports the development of a suite of FBG-based optical fibre sensors for non-destructive testing (NDT) and illustrating their potential for several specific industrial applications in the marine and railway sectors. These arose from work driven by the needs of project collaborators from these industries and are intended to be illustrative of the wider potential applications that optical fibre sensors have for measurements in different industrial sectors. The research has involved the development of new sensor system designs to meet these needs, building as they do upon a comprehensive review of NDT technologies and solutions, discussed in some detail. In this research for the marine sector, a single FBG-based acoustic sensor was specifically developed and evaluated and compared with the performance of conventional sensors. To do so, a metal plate to which the sensors were fixed was excited with a sonotrode, at a resonant frequency of 19.5 kHz. The signal reflecting that acoustic excitation was captured by the FBG sensors designed and implemented and their performance has been shown to be comparable with that from conventional, industry-standard piezoelectric transducers (PZTs). Preliminary work undertaken for the sponsors then lead to the further development of an acoustic sensor array comprising of 3 FBGs, which was subsequently validated against co-located PZTs which all were installed on a glass plate and excited in an industry-standard way, through the acoustic signal from a 0.2 g steel ball dropped onto the plate. When signals were analysed and compared, the positive comparative performance outcomes from the sensors used enabled further the design and implementation of instrumentation for a marine lifting surface using a different array, designed comprising 4 FBG-based acoustic sensors. Extensive tests on the smart marine lifting surface created were undertaken under water with a sonotrode set at 26 kHz as an excitation source. Based on the arrival time of acoustic signals captured by each grating and the use of triangulation method, the location of the excitation source could thus be determined, to meet the needs of the industrial sponsor and show good agreement with the outputs of conventional sensor systems. In parallel with the above, a further new industrial application of FBG-based sensor arrays was developed for a major player in the field, for the first time successfully instrumenting a railway current-collecting pantograph to allow reliable, remote in situ monitoring of key parameters: the contact force and contact location of the pantograph against the catenary. The optical fibre sensor approach has been shown to be an excellent means of measurement whose performance can be extrapolated to situations where the train is driven at high speeds up to 125 mph and powered from a high voltage line at 25 kV, in this design taking full advantages of the immunity of the optical fibre sensors to electromagnetic interference. In this research, key technical performance challenges were addressed and successfully overcome, including the temperature compensation needed for ‘all-weather’ performance, due to the intrinsic cross-sensitivity problems of using a FBG-based design being been fully addressed. This ensures the accurate measurement of the contact force/location between the pantograph and the catenary under all weathers. The research concludes by considering future directions for the work in these and other industry sectors