1,721,067 research outputs found
Multiwavelength operation of an erbium-doped fiber laser by using in-fiber comb filters
No full textVarious approaches has been tried to achieve simultaneous multiple-wavelength lasing by using both semiconductor and fiber gain media. Recently, an Er-doped fiber laser was demonstrated with four simultaneous wavelengths around 1600 nm by using a length of multimode fiber as a comb filter in a ring cavity. In this paper we report the use of two alternative in-fiber comb filters in a ring cavity producing a lasing comb of more than eight wavelengths around 1530 nm
Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters
No full textSimultaneous continuous wave multiwavelength operation of an erbium-doped fiber laser has been demonstrated using two types of recently developed in-fiber grating comb filters. The lasing wavelengths and linewidths were determined by the comb filter, which was inserted in a ring cavity with a single inhomogenously broadened gain medium
Embedding low loss polymer optical fibre Bragg gratings:two different approaches
Full text linkIn this paper, we present two different ways to embed polymer fibre Bragg gratings (FBGs) into polymer matrices. In the first experiment, we embedded the FBG into a 3D printed polymer structure, whereas in the second experiment, the coating was polymerized around the fibre. In both cases, the response of the grating was unchanged, without any loss or distortion of the FBG signal compared with the bare fibre response. The design of the polymer coating was optimised for the measurement of a single measurand. We highlighted two possible applications: surface bend deformation monitoring and improved-sensitivity temperature sensing
Acoustic emission detection using optical fibre sensors for aerospace applications
Full text linkStructural Health Monitoring (SHM) ensures the structural health and safety of critical structures covering a wide range of application areas. This thesis presents novel, low-cost and good-performance fibre Bragg grating (FBG) based systems for detection of Acoustic Emission (AE) in aircraft structures, which is a part of SHM. Importantly a key aim, during the design of these systems, was to produce systems that were sufficiently small to install in an aircraft for lifetime monitoring. Two important techniques for monitoring high frequency AE that were developed as a part of this research were, Quadrature recombination technique and Active tracking technique. Active tracking technique was used extensively and was further developed to overcome the limitations that were observed while testing it at several test facilities and with different optical fibre sensors. This system was able to eliminate any low frequency spectrum shift due to environmental perturbation and keeps the sensor always working at optimum operation point. This is highly desirable in harsh industrial and operationally active environments. Experimental work carried out in the laboratory has proved that such systems can be used for high frequency detection and have capability to detect up to 600 kHz. However, the range of frequency depends upon the requirement and design of the interrogation system as the system can be altered accordingly for different applications. Several optical fibre configurations for wavelength detection were designed during the course of this work along with industrial partners. Fibre Bragg grating Fabry-Perot (FBG-FP) sensors have shown higher sensitivity and usability than the uniform FBGs to be used with such system. This was shown experimentally. The author is certain that further research will lead to development of a commercially marketable product and the use of active tracking systems can be extended in areas of healthcare, civil infrastructure monitoring etc. where it can be deployed. Finally, the AE detection system has been developed to aerospace requirements and was tested at NDT & Testing Technology test facility based at Airbus, Filton, UK on A350 testing panels
Optical fibre sensors and their applications in the industrial weighing and aerospace industries
Full text linkThis thesis presents the design, fabrication and testing of novel grating based Optical Fibre Sensor (OFS) systems being interrogated using “off the shelf” interrogation systems, with the eventual development of marketable commercial systems at the forefront of the research. Both in the industrial weighing and aerospace industries, there has been a drive to investigate the feasibility of using optical fibre sensors being deployed where traditionally their electrical or mechanical counterparts would traditionally have been. Already, in the industrial weighing industry, commercial operators are deploying OFS-based Weigh-In-Motion (WIM) systems. Likewise, in the aerospace industry, OFS have been deployed to monitor such parameters as load history, impact detection, structural damage, overload detection, centre of gravity and the determination of blade shape. Based on the intrinsic properties of fibre Bragg gratings (FBGs) and Long Period Fibre Gratings (LPFGs), a number of novel OFS-based systems have been realised. Experimental work has shown that in the case of static industrial weighing, FBGs can be integrated with current commercial products and used to detect applied loads. The work has also shown that embedding FBGs in e-glass, to form a sensing patch, can result in said patches being bonded to rail track, forming the basis of an FBG-based WIM system. The results obtained have been sufficiently encouraging to the industrial partner that this work will be progressed beyond the scope of the work presented in this thesis. Likewise, and to the best of the author’s knowledge, a novel Bragg grating based systems for aircraft fuel parameter sensing has been presented. FBG-based pressure sensors have been shown to demonstrate good sensitivity, linearity and repeatability, whilst LPFG-based systems have demonstrated a far greater sensitivity when compared to FBGs, as well the advantage of being potentially able to detect causes of fuel adulteration based on their sensitivity to refractive index (RI). In the case of the LPFG-based system, considerable work remains to be done on the mechanical strengthening to improve its survivability in a live aircraft fuel tank environment. The FBG system has already been developed to an aerospace compliant prototype and is due to be tested at the fuel testing facility based at Airbus, Filton, UK. It is envisaged by the author that in both application areas, continued research in this area will lead to the eventual development of marketable commercial products
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Femtosecond Laser Inscribed Calibration Phantoms for Optical Coherence Tomography
Full text linkThe thesis describes work using a femtosecond laser to fabricate high precision microstructures as multipurpose 3D OCT calibration phantoms that can be used to quantitatively characterise and calibrate across different OCT systems. The research has been focused on three areas in developing the OCT calibration phantom - laser power characterisation, fabrication optimisation and novel multipurpose OCT phantom development and phantom application. For the laser power characterisation process, a detailed in-depth study was performed investigating the correlation between laser power and inscription size. Test phantoms were inscribed with different laser pulse energies. The results showed that with increased laser pulse energy, both the linewidth and the cross-inscription height were increased. The critical power of self-focusing was exceeded when the laser power was around 63% of the total laser average output power. As the non-linear effect caused unexpected control of the inscription size along the axial direction, a laser power range below the self-focusing threshold was selected to inscribe the phantoms which was still enough to extend the design to reach greater depths. The phantom fabrication optimisation with a layer-by-layer inscription method has allowed inscriptions at a greater depth up to 2mm whilst keeping the inscription size uniform and consistent for all depths. For the inscription method, the laser power was reset at beginning of the inscription for each layer which allowed a customised option to be made to get the desired inscription size that can be kept consistent and uniform for all depths. In order to address the challenges of inscribing non-planar samples, a series of multipurpose of OCT phantoms were designed and fabricated. A non-planar phantom was initially proposed that comprised of a grid-like pattern inscribed inside a planoconvex lens as the planar side provided a standard non-distorted image and the curved side provided a distorted image which can be used to detect the scanning errors and the post-processing algorithms of the OCT system for distortion correction. However, the grid-like pattern required a highly degree of alignment under the OCT system. Subsequently, a circle-like pattern was proposed to overcome this angular issue with a landmark layer consisting of a series of radial lines inscribed at the top of the test pattern to guide the location. To investigate the impact of OCT image distortion, a planar phantom was designed and used as a reference to correct the distortion due to the scanning errors caused by the OCT system itself. With a known reference, a phantom-based distortion correction method is possible with the reduction of the scan spacing error by 82% which extends the potential use of the OCT phantoms beyond a qualitive measurement tool
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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