Computational Depth-resolved Imaging and Metrology

In this thesis, the main research challenge boils down to extracting 3D spatial information of an object from 2D measurements using light. Our goal is to achieve depth-resolved tomographic imaging of transparent or semi-transparent 3D objects, and to perform topography characterization of rough surfaces. The essential tool we used is computational imaging, where depending on the experimental scheme, often indirect measurements are taken, and tailored algorithms are employed to perform image reconstructions. The computational imaging approach enables us to relax the hardware requirement of an imaging system, which is essential when using light in the EUV and x-ray regimes, where high-quality optics are not readily available. In this thesis, visible and infrared light sources are used, where computational imaging also offers several advantages. First of all, it often leads to a simple, flexible imaging system with low cost. In the case of a lensless configuration, where no lenses are involved in the final image-forming stage between the object and the detector, aberration-free image reconstructions can be obtained. More importantly, computational imaging provides quantitative reconstructions of scalar electric fields, enabling phase imaging, numerical refocus, as well as 3D imaging

Roadmap on holography

From its inception holography has proven an extremely productive and attractive area of research. While specific technical applications give rise to 'hot topics', and three-dimensional (3D) visualisation comes in and out of fashion, the core principals involved continue to lead to exciting innovations in a wide range of areas. We humbly submit that it is impossible, in any journal document of this type, to fully reflect current and potential activity; however, our valiant contributors have produced a series of documents that go no small way to neatly capture progress across a wide range of core activities. As editors we have attempted to spread our net wide in order to illustrate the breadth of international activity. In relation to this we believe we have been at least partially successful

Multi-Wavelength Polarising Interferometer for In-Process Metrology

Micro-scale and nano-scale surfaces are now fabricated to serve in many fields: from optics needed in telescope/microscope imaging to semiconductors integrated in electronic devices such as smart phones and micro sensors. The production of these surfaces is inspiring the development of new metrology instrumentation that can not only ensure the quality but also optimise the manufacturing process. However, the state-of-the-art offline metrology instruments suffer from a main limitation, namely the inability to operate in the manufacture environment. The industry evolution requires in-process and metric metrology instrumentation that can provide rich surface information within harsh manufacture environment. The specification of such instruments has to be non-destructive, fast, and highly accurate; such instruments have to be combined with a production line. Interferometers offer non-destructive and parallel fashion measurement with nanometre accuracy. A well-established phase-shift interferometer (PSI) is widely used for high measurement accuracy; however, it has two limitations. Firstly, the height difference between two adjacent points on the sample should be smaller than quarter of wavelength (Λ/4), and secondly, a PSI is slow and not suitable for in-process measurements, if a mechanical scanning is used for phase shifting. In order to utilise PSI for in-process measurements, data capturing at single exposure should be used to overcome the environmental disturbances and advanced phase unwrapping methods need to be employed to extend the measurement range beyond (Λ/4). This research aimed to develop a multi-wavelength polarising phase-shift interferometer (MPI) for surface measurement and to investigate the possibility of its use for in-process metrology applications. The target specifications of the proposed instrument are as follows: a vertical measurement range greater than (Λ/4) (i.e. greater than 1 μm) with the resolution of a single wavelength interferometer (i.e. less than 10 nm). The MPI requires no mechanical scanning to obtain the phase shift with an extended measurement range using a single shot technique. This represents an improvement over the conventional single wavelength interferometer in terms of the measurement range and speed. The methodology followed to achieve this study’s aims included reviewing the literature and implementing proof-of-concept experiments using mechanical and non-mechanical methods to acquire phase-shifted colour interferograms, hence determining algorithms for fringe analysis. Finally a novel MPI setup using polarisation technique and Red-Green-Blue (RGB) illumination source was developed that can be used for in-process measurement with extended range. An acousto-optics tuneable filter (AOTF) was successfully employed to simultaneously provide RGB wavelengths with approximately 2 nm linewidth. Several fringe analyses and phase unwrapping algorithms, such as fringe order and best-match methods, were explored to retrieve areal surfaces. Colour crosstalk between cameras’ pixels was also investigated. It was found that the crosstalk is significant. A mathematical model and AOTF tuning capability were used to achieve minimum crosstalk. A spatial two dimensional image filtration was used to enhance the interferograms, hence signal-to-noise improvement. The proposed MPI has successfully measured samples (from 40 nm to 4 μm) with few nanometres accuracy and with single exposure (less than 0.3 second). This MPI has the potential for use in the measurement of surfaces produced by ultra-fast manufacturing such as roll-to-roll (R2R) manufacturing process. In the R2R process, structured surfaces are fabricated on large-area substrates (on the scale of several metres squared) at high speed exceeding several meters per minute. As such, MPI can be potentially used to measure moving surfaces within the manufacturing environment at speed limited only to the single exposure of the cameras

LASER Tech Briefs, September 1993

This edition of LASER Tech briefs contains a feature on photonics. The other topics include: Electronic Components and Circuits. Electronic Systems, Physical Sciences, Materials, Computer Programs, Mechanics, Machinery, Fabrication Technology, Mathematics and Information Sciences, Life Sciences and books and reports

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

The quantitative analysis of transonic flows by holographic interferometry

This thesis explores the feasibility of routine transonic flow analysis by holographic interferometry. Holography is potentially an important quantitative flow diagnostic, because whole-field data is acquired non-intrusively without the use of particle seeding. Holographic recording geometries are assessed and an image plane specular illumination configuration is shown to reduce speckle noise and maximise the depth-of-field of the reconstructed images. Initially, a NACA 0012 aerofoil is wind tunnel tested to investigate the analysis of two-dimensional flows. A method is developed for extracting whole-field density data from the reconstructed interferograms. Fringe analysis errors axe quantified using a combination of experimental and computer generated imagery. The results are compared quantitatively with a laminar boundary layer Navier-Stokes computational fluid dynamics (CFD) prediction. Agreement of the data is excellent, except in the separated wake where the experimental boundary layer has undergone turbulent transition. A second wind tunnel test, on a cone-cylinder model, demonstrates the feasibility of recording multi-directional interferometric projections using holographic optical elements (HOE’s). The prototype system is highly compact and combines the versatility of diffractive elements with the efficiency of refractive components. The processed interferograms are compared to an integrated Euler CFD prediction and it is shown that the experimental shock cone is elliptical due to flow confinement. Tomographic reconstruction algorithms are reviewed for analysing density projections of a three-dimensional flow. Algebraic reconstruction methods are studied in greater detail, because they produce accurate results when the data is ill-posed. The performance of these algorithms is assessed using CFD input data and it is shown that a reconstruction accuracy of approximately 1% may be obtained when sixteen projections are recorded over a viewing angle of ±58°. The effect of noise on the data is also quantified and methods are suggested for visualising and reconstructing obstructed flow regions