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Using Multivariate Statistics

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

Approaches to learning

Biomechanics and wound healing in the cornea

We describe an electronic speckle pattern interferometer devised to measure orthogonal in-plane strain components simultaneously and automatically. The interferometer is based on an earlier system that measured two in-plane displacement components simultaneously. Automatic displacement analysis is achieved using a phase-stepping technique. The surface strain is then calculated by numerical differentiation of the displacement data. Simultaneous measurement of the two displacement components is important for shear strain measurement because data from the two in-plane views must be combined. Results are presented for a cantilever loaded at its free end.

An electronic speckle pattern interferometer for two-dimensional strain measurement

An approach to the assessment of fumes produced in laser material processing is presented. The subsequent design of the fume containment system and sampling strategy is detailed. Polyvinyl chloride (PVC) has been selected for investigation and the complexity resulting from the plethora of additives is discussed. Qualitative results from the preliminary studies of CO 2 laser cutting of PVC are presented together with an appraisal of the problems involved in extending this to quantitative analysis. Benzene, hydrogen chloride (HCl) and methyl methacrylate were found to be the dominant emissions.

Fumes generated during laser processing of polyvinyl chloride (PVC)

Geometrically qualified ESPI vibration analysis of an engine

Within the family of thin-film photovoltaics (PV), cadmium telluride 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 min, 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 localize heat treatment, anneal selectively, and its short processing time. In this investigation, results focusing 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, is then used to experimentally demonstrate the redistribution of laser beam energy into an optimal profile for annealing, eliminating thermal concentrations.Within the family of thin-film photovoltaics (PV), cadmium telluride 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 min, 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 localize heat treatment, anneal selectively, and its short processing time. In this investigation, results focusing on improvements to the laser-based annealing process, designed to mitigate panel damage by excessive thermal gradients, are presented. Simulations of var...

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

Due to their high speed and versatility, laser processing systems are now commonplace in many industrial production lines. However, as the need to reduce the environmental impact from the manufacturing industry becomes more urgent, there is the opportunity to evaluate laser processing systems to identify opportunities to improve energy efficiencies and thus reduce their carbon footprint. While other researchers have studied laser processing, the majority of previous work on laser systems has focused on the beam–material interaction, overlooking the whole system viewpoint and the significance of support equipment. In this work, a methodical approach is taken to design a set of energy modelling terminologies and develop a structured power metering system for laser systems. A 300 W fibre laser welding system is used to demonstrate the application of the power characterization system by utilizing a purpose-built power meter. The laser is broken down according to sub-system, with each part analysed separately to give a complete overall power analysis, including all auxiliary units. The results show that the greatest opportunities for efficiency improvements lie in the auxiliary units that support the laser devices as these were responsible for a majority of the electrical draw; 63.1% when the laser was operated at 240 W, and increasing as the beam power reduced. The remaining power draw was largely apportioned to electrical supply inefficiencies. In this work, the laser device delivered a maximum of 6% of the total system power. The implications of these results on laser processing system design are then discussed as is the suitability of the characterization process for use by industry on a range of specific laser processing systems.

/pdf/characterisation-of-laser-system-power-draws-in-materials-2i5iiymg2o.pdf

John R. Tyrer

Papers

An electronic speckle pattern interferometer for two-dimensional strain measurement

Fumes generated during laser processing of polyvinyl chloride (PVC)

Geometrically qualified ESPI vibration analysis of an engine

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

Characterisation of laser system power draws in materials processing