FSD-HSO Optimization Algorithm for Closed Fringes Interferogram Demodulation

Due to the physical nature of the interference phenomenon, extracting the phase of an interferogram is a known sinusoidal modulation problem. In order to solve this problem, a new hybrid mathematical optimization model for phase extraction is established. The combination of frequency guide sequential demodulation and harmony search optimization algorithms is used for demodulating closed fringes patterns in order to find the phase of interferogram applications. The proposed algorithm is tested in four sets of different synthetic interferograms, finding a range of average relative error in phase reconstructions of 0.14–0.39 rad. For reference, experimental results are compared with the genetic algorithm optimization technique, obtaining a reduction in the error up to 0.1448 rad. Finally, the proposed algorithm is compared with a very known demodulation algorithm, using a real interferogram, obtaining a relative error of 1.561 rad. Results are shown in patterns with complex fringes distribution

Development of a wafer geometry measuring system : a double sided stitching interferometer

The drive for miniaturization of electrical devices and the increased production size of chips has forced lithographic production techniques to improve continuously. With this the requirements for silicon wafers, which form the basis of chips, have increased continuously as well. To ??nd a cost e??ective solution for the characterization of large diameter double side polished silicon wafers, the development of a measurement machine has been started. The measurement machine should measure the free form ??atness and thickness variations of wafers with a diameter of up to, and possibly beyond, 300 mm. The proposed measurement concept should have the potential to achieve a high throughput and a low measurement uncertainty, while reducing the cost of ownership signi??cantly compared to currently available systems on the market. The designed measurement setup which is described in this thesis is intended to demonstrate the potential of a chosen measurement concept for measuring double side polished silicon wafers. In the innovative measurement concept a double sided stitching Fizeau type interferometer has been adopted. A surface interferometer o??ers the required high accuracy and the scanning principle of a small aperture stitching interferometer allows the use of relatively small and low cost optical components which can measure with a high spatial resolution. The self referencing capability of a double sided Fizeau interferometer is important for achieving high accuracy when measuring thickness variations. In the proposed measurement concept the aperture of a single interferometer is split to measure the frontside and backside ??atness of a wafer simultaneously. The thickness variations can be derived from the measured ??atness measurements. A prototype measurement setup has been designed, built and tested. All major mechanical and optical error sources have been eliminated by using advanced calibration techniques. By using proper measurement principles and advanced software a robust and traceable wafer thickness and ??atness measurement instrument is created for measuring nominally ??at objects. The developed calibration techniques enable low uncertainty measurements to be taken while using relatively low quality optical and mechanical components. Several measurement method have been applied to derive accurate geometrical parameters from the recorded interferograms. Besides the processing of interferograms and development of calibration techniques a surface stitching software package has been developed which combines many subaperture ??atness maps into a large scale ??atness map of the entire wafer

Point-diffraction interferometry for wavefront sensing in adaptive optics

The work presented in this thesis aims at the development and validation of a wavefront sensor concept for adaptive optics (AO) called the pupil-modulated Point-Diffraction Interferometer (m-PDI). The m-PDI belongs to a broader family of wavefront sensors called Point-Diffraction Interferometers (PDIs), which make use of a small pinhole to filter a portion of the incoming light, hence generating a reference beam. This allows them to perform wavefront sensing on temporally incoherent light, such as natural guide stars in the context of astronomical AO. Due to their high sensitivity, PDIs are being developed to address several difficult problems in AO, namely measuring quasi-static aberrations to a high degree of accuracy, the cophasing of segmented apertures, and reaching a high correction regime known as extreme AO. But despite their advantages, they remain limited by their narrow chromatic range, around ∆λ = 2% relative to central bandwidth, and short dynamic range, generally of ±π/2. The purpose of developing the m-PDI is to explore whether this new concept has any ad- vantages regarding these limitations. Indeed, we find that the m-PDI has a maximum chromatic bandwidth of 66% relative to the central wavelength and a dynamic range at least 4 times larger than that of other PDIs. Although the m-PDI concept had been proposed previously, it had not been explored to the extent reached in this manuscript. This thesis presents an initial investigation into the m-PDI, beginning with the development of the theory. Here the theoretical framework is laid out to explain how interference fringes are modulated by the wavefront, how to then demodulate the propa- gating electric field’s phase and then finally how to measure the signal-to-noise ratio (SNR). After building analytical and numerical models, a prototype is designed, built and characterised using CHOUGH, a high-order AO testbed in the lab. This incarnation of the m-PDI is called the Calibration & Alignment Wavefront Sensor (CAWS). The characterisation of the CAWS shows two things. The first one is that the CAWS’ response is approximately flat across its spatial frequency domain. The second one is that its dynamic range decreases at higher frequencies, suggesting that it depends, amongst other things, on the wavefront’s slope. In order to prove that m-PDIs can be used for AO, a control loop is closed using the CAWS and CHOUGH’s deformable mirror, with both monochromatic and broadband light. The results show that the final Strehl ratio increases from 0.2 to 0.66, at a wavelength of 633 nm. The difference in residual aberrations seen separately by the imaging camera and by the CAWS is about 20 nm RMS. This is explained by non-common path aberrations and low order aberrations which are invisible to the CAWS. Finally, the instrument was tested on the CANARY AO bench at the William Herschel Telescope. The CAWS was successful at characterising the quasi- static aberrations of the system and at demodulating the phase of wavefronts produced with the deformable mirror. When demodulating on-sky residual aberrations at the back of CANARY’s single-conjugate AO loop, the SNR remained too low for effective wavefront demodulation, only sporadically in- creasing above 1. These results are not discouraging as the CAWS was only a first prototype and CANARY is not a high-order system, reaching a Strehl ratio of around 0.5% at 675 nm. The lessons and improvements for future de- signs are to increase the diameter of the instrument’s pinhole by at least twice, and deliver it a higher Strehl ratio by moving towards longer wavelengths and employing a higher order AO system

Optical coherence tomography methods using 2-D detector arrays

Optical coherence tomography (OCT) is a non-invasive, non-contact optical technique that allows cross-section imaging of biological tissues with high spatial resolution, high sensitivity and high dynamic range. Standard OCT uses a focused beam to illuminate a point on the target and detects the signal using a single photodetector. To acquire transverse information, transversal scanning of the illumination point is required. Alternatively, multiple OCT channels can be operated in parallel simultaneously; parallel OCT signals are recorded by a two-dimensional (2D) detector array. This approach is known as Parallel-detection OCT. In this thesis, methods, experiments and results using three parallel OCT techniques, including full -field (time-domain) OCT (FF-OCT), full-field swept-source OCT (FF-SS-OCT) and line-field Fourier-domain OCT (LF-FD-OCT), are presented. Several 2D digital cameras of different formats have been used and evaluated in the experiments of different methods. With the LF-FD-OCT method, photography equipment, such as flashtubes and commercial DSLR cameras have been equipped and tested for OCT imaging. The techniques used in FF-OCT and FF-SS-OCT are employed in a novel wavefront sensing technique, which combines OCT methods with a Shack-Hartmann wavefront sensor (SH-WFS). This combination technique is demonstrated capable of measuring depth-resolved wavefront aberrations, which has the potential to extend the applications of SH-WFS in wavefront-guided biomedical imaging techniques

Adaptive Optics Progress

For over four decades there has been continuous progress in adaptive optics technology, theory, and systems development. Recently there also has been an explosion of applications of adaptive optics throughout the fields of communications and medicine in addition to its original uses in astronomy and beam propagation. This volume is a compilation of research and tutorials from a variety of international authors with expertise in theory, engineering, and technology. Eight chapters include discussion of retinal imaging, solar astronomy, wavefront-sensorless adaptive optics systems, liquid crystal wavefront correctors, membrane deformable mirrors, digital adaptive optics, optical vortices, and coupled anisoplanatism

Fiber Bragg Grating Based Sensors and Systems

This book is a collection of papers that originated as a Special Issue, focused on some recent advances related to fiber Bragg grating-based sensors and systems. Conventionally, this book can be divided into three parts: intelligent systems, new types of sensors, and original interrogators. The intelligent systems presented include evaluation of strain transition properties between cast-in FBGs and cast aluminum during uniaxial straining, multi-point strain measurements on a containment vessel, damage detection methods based on long-gauge FBG for highway bridges, evaluation of a coupled sequential approach for rotorcraft landing simulation, wearable hand modules and real-time tracking algorithms for measuring finger joint angles of different hand sizes, and glaze icing detection of 110 kV composite insulators. New types of sensors are reflected in multi-addressed fiber Bragg structures for microwave–photonic sensor systems, its applications in load-sensing wheel hub bearings, and more complex influence in problems of generation of vortex optical beams based on chiral fiber-optic periodic structures. Original interrogators include research in optical designs with curved detectors for FBG interrogation monitors; demonstration of a filterless, multi-point, and temperature-independent FBG dynamical demodulator using pulse-width modulation; and dual wavelength differential detection of FBG sensors with a pulsed DFB laser

Cellular Dynamics and Three-Dimensional Refractive Index Distribution Studied with Quantitative Phase Imaging

We present in this PhD thesis work various applications of digital holographic microscopy (DHM), an imaging technique based on coherent illumination which enables the recovery of the full complex wavefront, i.e. the amplitude and phase of a wave field which interacted with a specimen. The possibility to retrieve the phase information with DHM allows to measure surfaces with nanometric accuracy, or to employ it as an endogenous quantitative signal to assess the morphology of biological specimens. The technique has been developed during the past fifteen years to reach nowadays a mature state, where it can be used routinely for metrology applications for example. We study in this work advanced applications by taking advantage of this technique, while focusing on a specific measurement method of DHM, namely the off-axis configuration, which makes it possible to measure the complex wave field with one-shot capability through spatial encoding, thus enabling real-time detection. In a first part, we develop mathematical methods based on the fundamental model of holographic recording to suppress the so-called zero-order, which consists in intensity terms that coherent detection must suppress for complex wave retrieval. In the particular case of off-axis holography, the zero-order terms usually limit the spatial resolution because of the spatial encoding of the coherent signal. We first develop an iterative method which uses the fundamental relations between coherent and incoherent detection, in order to gradually suppress the zero order terms. In a second stage, we develop a non-iterative filtering method, based on nonlinear operators. The technique is based on the transfer to another filtering space through the use of the logarithm, and enables intrinsic suppression of the zero-order terms. Both methods present the advantage of not relying on any approximation, and are thus general for any off-axis holographic configuration. We show their applicability on various hologram types, and demonstrate that in the context of microscopy, their use can increase the spatial resolution of holography, in order to reach diffraction-limited imaging for any magnification. In a second part, we study potential applications of three-dimensional imaging through coherent detection by employing multiple acquisitions with a new scanning method. The coupling of tomographic reconstruction and quantitative phase imaging showed great potential in various published works, yielding to quantitative 3D refractive index distribution measured within biological specimens, and super-resolution imaging through synthetic aperture formalism. These methods are however still subjects to many issues, in particular due to practical limitations such as mechanical imprecision in the measurement protocols and the availability of flexible reconstruction algorithms. We study a new data acquisition method which eliminates the necessity of any scanning of the illumination pattern or object rotation during the acquisition, providing potentially a more stable acquisition protocol. We present results proving the principle of our approach by measuring the 3D refractive index distribution of pollen grain. In a third part, we applied DHM to the analysis of cell morphology and dynamics, applied in particular to neuronal cells. We couple the phase measurement with widely assessed methods such as dye probing or quantitative wide field fluorescence, in order to derive relevant biological indicators from DHM. Through the interpretation of the phase as an indicator of cell volume regulation, we derive criteria for early label-free cell death detection, where we show that cell monitoring with DHM makes it possible to detect cell non-viability at early stage by measuring deregulation mechanisms. We compare our methods with dyes for cell viability assessment, showing that DHM can detect cell death typically hours before usual dye probing procedures. We also couple the phase signal with intracellular ionic concentration imaging through fluorescence, showing that the phase measured on neuron cultures is intimately linked with ionic homeostasis and in particular transmembrane water movements accompanying ionic currents such as Ca2+ or Na+. We derive typical phase signatures related to the well-known Ca2+ bursts occurring during action potentials in neurons through stimulation with glutamate, one of the major neurotransmitters in the central nervous system

NASA Tech Briefs, June 1996

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