Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer (LDV) targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation

Complex beam profiles for laser annealing of thin-film CdTe photovoltaics

Within the family of thin-film photovoltaics (PV), cadmium telluride (CdTe) has the fastest growing market share due to its high efficiencies and low cost. However, as with other PV technologies, the energy required to manufacture the panels is excessive, encompassing high environmental impact and manufacturing energy payback times of the order of 2-3 years. As part of the manufacturing process, the panels are annealed at temperatures of approximately 400°C for 30 minutes, which is inherently inefficient. Laser heating has previously been investigated as an alternative process for thin film annealing, due to its advantages with regard to its ability to localise heat treatment, anneal selectively and its short processing time. In this investigation, results focussing on improvements to the laser-based annealing process, designed to mitigate panel damage by excessive thermal gradients, are presented. Simulations of various laser beam profiles are created in COMSOL and used to demonstrate the benefit of laser beam shaping for thin film annealing processes. An enabling technology for this, the holographic optical element (HOE), is then used to experimentally demonstrate the redistribution of laser beam energy into an optimal profile for annealing, eliminating thermal concentrations

Eye hazards of laser 'pointers' in perspective

Eight years ago media coverage of incidents involving laser pointers in which individuals claimed to have suffered eye damage resulted in a perspective being published in this journal.1 The final sentence concluded ‘laser pointers, pens or key rings if used appropriately are not an eye hazard, and even if used inappropriately will not cause permanent eye damage’. This statement has been supported by the finding that until recently no irreversible eye injuries had been reported for a period of almost 15 years other than those caused by deliberate and prolonged viewing of laser beams.2 During this time period pointers have been freely available with an estimated 500 000 to c1.2 million laser pointers in circulation

Visualising scattering underwater acoustic fields using laser Doppler vibrometry

Analysis of acoustic wavefronts are important for a number of engineering design, communication and healthrelated reasons, and it is very desirable to be able to understand the interaction of acoustic fields and energy with obstructions. Experimental analysis of acoustic wavefronts in water has traditionally been completed with single or arrays of piezoelectric or magnetostrictive transducers or hydrophones. These have been very successful, but the presence of transducers within the acoustic region can in some circumstances be undesirable. The research reported here, describes the novel application of scanning laser Doppler vibrometry to the analysis of underwater acoustic wavefronts, impinging on circular cross section obstructions. The results demonstrate that this new non-invasive acoustics measurement technique can successfully visualise and measure reflected acoustic fields, diffraction and refraction effects

Deformation analysis of aircraft wheels using a speckle shearing interferometer

Speckle shearing interferometry has developed over the last three decades and is used as a non-destructive analysis tool for aerospace engineering.The primary role of the technique has traditionally been for the examination of defects in fibre composite structural materials, but more recent attention has considered strain and displacement measurement.Furtherm ore, aerospace regulatory bodies such as the UK Civil Aviation Authority are demanding better understanding and traceability of the technique. The novel work reported here has investigated the use of a Michelson-based speckle shearing interferometer for the analysis of compression-loaded aircraft wheels and tyres.Whole- field deformation data have been obtained for repeated compression loading of a BAe146 main wheel, providing displacement derivative and displacement data.The quality of the measurement technique has been ascertained through the analysis of the repeatability and reproducibility data, with correlating data being produced using contact displacement transducers

The role of light in measuring ocular biomechanics

The cornea is a highly specialised tissue with a unique set of biomechanical properties determined by its complex structure. The maintenance of these mechanical properties is fundamental to maintain clear vision as the cornea provides the majority of the focussing power of the eye. Changes to the biomechanics of the cornea can occur during ageing, disease, and trauma, or as a result of surgery. Recently there has been increased interest in the mechanical properties of the cornea as knowledge of these properties has significant implications for the improvement of current ocular treatments including PRK and LASIK, and for the diagnosis and tracking of corneal diseases and therapy such as keratoconus and crosslinking. Biomechanics are also important for the development of artificial corneal replacements. This paper describes the use of a novel, non-destructive lateral electronic speckle pattern shearing interferometer (ESPSI). The data generated via this technique give a full-field view of the mechanical response of the cornea under simulated physiological loading conditions, and enables strain and displacement to be determined in three planes. The technique allows corneal stiffness to be quantified and enables changes and non-homogeneities that occur due to surgery or disease to be detected.Eye advance online publication, 15 January 2016; doi:10.1038/eye.2015.263

The use of holographic optical elements (HOE's) to investigate the use of a flat irradiance profile in the control of heat absorption in wire-fed laser cladding

This work investigates the use of holographic optical elements (HOE's) to control the applied heat profile and thermal absorption of a wire used for laser cladding. The two thermal distributions compared were a circular beam with a Gaussian heat profile (Gaussian beam) and a square beam with a flat profile (pedestal beam). Heat absorption calculations between these were carried out to show the potential differences in absorption, with empirical results created to show how the differences in absorption affect the cladding properties. Micrographs of the clad cross-sections were created using optical microscopy and were analysed with respect to wetting angle and clad dilution. These results were compared to an alternative method of applying an even beam profile; enlarging the Gaussian beam relative to the wire diameter. The results showed that the use of a HOE to create a more even beam profile gave superior wetting behaviour and less dilution

Using wire shaping techniques and holographic optics to optimise deposition characteristics in wire-based laser cladding

In laser cladding, the potential benefits of wire feeding are considerable. Typical problems with the use of powder, such as gas entrapment, sub-100% material density and low deposition rate are all avoided with the use of wire. However, the use of a powder-based source material is the industry standard, with wire-based deposition generally regarded as an academic curiosity. This is because, although wire-based methods have been shown to be capable of superior quality results, the wire-based process is more difficult to control. In this work, the potential for wire shaping techniques, combined with existing holographic optical element knowledge, is investigated in order to further improve the processing characteristics. Experiments with pre-placed wire showed the ability of shaped wire to provide uniformity of wire melting compared with standard round wire, giving reduced power density requirements and superior control of clad track dilution. When feeding with flat wire, the resulting clad tracks showed a greater level of quality consistency and became less sensitive to alterations in processing conditions. In addition, a 22% increase in deposition rate was achieved. Stacking of multiple layers demonstrated the ability to create fully dense, three-dimensional structures, with directional metallurgical grain growth and uniform chemical structure

Selective laser melting using holographic beam manipulation: influence of polypropylene molecular weight

Purpose – There is a requirement to match selective laser melting (SLM) technologies to a wider range of polymeric materials, since the existing market for SLM powders is dominated by polyamide PA12. Drivers include the tailoring of physical properties to individual applications, or cost reduction. Polypropylene (PP) currently has limited use in SLM, so the potential use of PP materials of varying molecular weight (Mw) is explored here. Design / methodology / approach – PP polymers of differing molecular weight were characterised using a range of analytical techniques, including DSC, thermogravimetric analysis (TGA), rotational rheometry and real-time hot-stage (optical) microscopy. Findings – The techniques are sufficiently sensitive to distinguish molecular weight effects, notably in terms of material viscosity. The stable sintering region for SLM has been defined clearly. Some success was achieved in melting parts using some grades of PP, including higher molecular weight grades, which potentially offer improved mechanical performance. Research limitations / implications – The range of techniques (DSC, OIT and TGA) form an effective analytical package with which to consider new polymeric materials for SLM. Practical implications – High-MW PP polymers, in tape or powder form, have potential use in SLM processes, providing scope to enhance part properties in future. Originality / value - This is believed to be the first in-depth study noting the influence of PP molecular weight on important physical performance in a proprietary SLM process, using holographic beam manipulation (HBM)

Applying laser cutting techniques through horology for teaching effective STEM in design and technology

This paper explores the pedagogy underpinning the use of laser manufacturing methods for the teaching of science, technology, engineering and mathematics (STEM) at key stage 3 design and technology. Clock making (horology) has been a popular project in design and technology (D&T) found in many schools, typically it focuses on aesthetical design elements. This paper describes a new project, which has been developed to enhance the STEM content of a horology project through advanced utilisation of laser cutting machinery. It allows pupils to produce their own products from self-made mechanical timing mechanisms. The central aim is to strengthen the application of the underlying technology of mechanisms and the manufacturing capability of laser cutting technology in D&T. Trials with schools have shown success in gaining pupils’ interest in STEM and provided feedback to improve the project. It has highlighted limits when delivering the engineering and maths content with teachers from non-technology backgrounds. The paper discusses this limitation through subject pedagogy, categorisation of teacher knowledge, and teaching effectiveness through experiential and problem-based learning approaches