In-plane Sensitive Electronic Speckle Pattern Interferometer using a Diffractive Holographic Optical Element

We describe a student project in electronic speckle pattern interferometry. The project includes holographic recording of diffraction gratings in thick, self-processing photopolymer layers made from off-the-shelf chemicals. The gratings are employed in a simple electronic speckle pattern interferometer to measure in-plane rotation

A New Versatile Electronic Speckle Pattern Interferometer For Vibration Measurements

Electronic speckle pattern interferometry (ESPI) has been widely used for vibration amplitude and phase measurements. Conventional ESPI systems are bulk and expensive and need careful alignment of all the optical components which is a time consuming task. To overcome these problems alternative compact ESPI systems were developed using fibre-optical components or holographic optical elements (HOEs). The fibre-optic based ESPI systems suffer from random phase fluctuations induced by environmental temperature changes. Hence HOEs can be used as more powerful alternative optical elements to design ESPI systems. The time average ESPI method is widely used for vibration studies. The time average method combined with phase stepping can be used for automatic vibration measurements. Using this technique higher vibration amplitudes cannot be measured because fringe patterns follow Bessel function intensity distribution. To overcome this problem an alternative technique can be used by modulating the phase of the reference beam in an unbalanced interferometer. This thesis reports a novel ESPI system for vibration measurements by combining use of holographic optical elements (HOEs) and optical path length modulation (reference beam phase modulation). The optical path length modulation is implemented using laser diode wavelength (frequency) modulation. Different HOE based ESPI systems are reported in this thesis using either a single HOE or dual HOE. This thesis examines performance of different HOE based ESPI systems that are sensitive to out-of-plane displacement components using laser diodes operating either in the near infrared or visible electromagnetic spectrum. Vibration modes of a circular metal plate clamped at the edges of a loud speaker and a circular metal plate driven by a piezoelectric actuator (PZT) were studied using a single RHOE based ESPI system and a hybrid (transmission HOE with a partially reflecting mirror) HOE based ESPI system respectively using a near infrared laser diode (763nm). Optical path length modulation technique was implemented using a laser diode operating in visible electromagnetic spectrum (658nm). Vibration mode patterns of a circular metal plate driven by a PZT actuator were obtained using both single RHOE and dual HOE based ESPI systems. Using optical path length modulation technique in a dual HOE based ESPI system detailed phase and amplitude maps of a circular metal plate driven by a PZT actuator are obtained. The dual HOE based ESPI system was also used for measuring roations of a circular metal plate mounted on a mirror mount. In conclusion we have developed a compact HOE based ESPI system to conduct vibration measurements. A few potential future developments are also suggested at the end of the thesis

근안 홀로그래픽 디스플레이의 개선에 관한 연구: 폼 팩터, 시약 각, 스페클 노이즈

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2018. 2. 이병호.This dissertation presents solutions targeting on the three fundamental issues on near-eye holography: 1. Form factor, 2. Field of view (FOV), and 3. Speckle noise. For the form factor reduction of the holographic display, a multifunctional holographic optical element (HOE) is developed to replace the beam splitter and the eyepiece lens of the conventional holographic display. As a result, it enables us to build a compact see-through holographic display for augmented reality (AR). The feasibility of using HOE in the holographic display is verified by analyzing some optical characteristics of the HOE, based on the Kogelniks coupled-wave theory. In addition, a compensation method is introduced to compensate the wavefront aberration caused by use of the HOE. To build a wide FOV holographic head mounted display, I proposed two methods to increase the space bandwidth product (SBP) which can widen the FOV without losing eyebox size. In each optical path of +1st, 0th, -1st diffraction order of a spatial light modulator (SLM), I place a shutter synchronized with the SLM to display different holograms by temporally dividing the frames of the SLM. The second proposed method is called polarization selective holography which is a brand new hologram calculation and encoding method. In this work, an SLM can modulate waves in different modes through different states of wave retardation and polarization optical elements. Two sets of wave plates and polarizers with optimized states are placed in the path of +1st and -1st different diffraction orders of an SLM, so that the SLM reconstructs two different holograms via each path. As a result, the system achieves a two-fold increase in terms of SBP for a single SLM. Furthermore, a curved hologram can provide wider FOV comparing to a planar one, when they have the same SBP. A curved surface hologram can provide wider FOV. Thus, curved holographic display has a great potential to the near-eye holography. Since the curved surface SLM has not been developed yet, as an example, I also proposed a numerical method of spherical hologram computation of real objects in this chapter. For the speckle noise issue, some analyses are carried out to compare the quality of holographic reconstruction images using different light source such as a light-emitting diode (LED) and a laser diode (LD). Both of them have pros and cons. The main advantage of use of the LED in the holographic display is it can reconstruct the holographic image with significantly less speckle noise compared to laser or LD. However, an inherent problem caused by use of the LED is that it is difficult to reconstruct images located far away from the SLM. This problem is mitigated using a magnifier optical system. As a result, I demonstrate an LED based near-eye holographic display system not only to reduce the speckle noise but also to provide sufficient expressible depth range.Chapter 1 Introduction 1 1.1 Overview of near-eye displays 1 1.2 Motivation of this dissertation 7 1.3 Scope and organization 9 Chapter 2 Form factor reduction of near-eye holography using a multifunctional holographic optical element 11 2.1 Introduction of form factor issues in near-eye holographic display 11 2.2 Introduction of volume hologram 13 2.2.1 Diffraction efficiency: Coupled-wave theory 14 2.2.2 Condition to be met for using holographic optical element in holographic display 19 2.3 Multifunctional holographic optcal element fabrication and application 27 2.3.1 Design of mirror-lens holographic optical element 27 2.3.2 Fabrication of mirror-lens holographic optical element 30 2.3.3 System implementation and experimental results 33 2.4 Wavefront aberration compensation method for holographic optical element 37 2.5 Summary and discussion 44 Chapter 3 Field of view expansion of near-eye holographic display 45 3.1 Overview of field of view expansion for near-eyeholography 45 3.1.1 Investigation on constrains in expanding field of view for near-eye holography 45 3.1.2 Previous space bandwidth product enhancement methods for holographic display 50 3.2 Temporal multiplexing of high-order diffraction guided by holographic attenuating mirror 53 3.2.1 Design of holographic attenuating mirror 54 3.2.2 System imprelentation and expperiment results 60 3.3 Polarization selective holography 67 3.3.1 Principle of polarization selective holography 67 3.3.2 Amplitude combination map optimization using genetic algorithm 71 3.3.3 Simulation and experimental results 73 3.4 Synthesis of computer generated spherical hologram 76 3.4.1 Spatial spectral bandwidth of spherical hologram 76 3.4.2 Spherical hologram calculation of real objects 81 3.4.3 Simulation and experimental results 88 3.5 Summary and discussion 95 Chapter 4 Light source selection for near-eye holography 97 4.1 Coherence of light source 97 4.1.1 Temporal coherence 97 4.1.2 Spatial coherence 98 4.2 Image sharpness for holographic display 101 4.3 Ideal optical design for LED based near-eye holography 105 4.4 Summary and discussion 108 Chapter 5 Conclusion 109 Bibliography 111 Appendix 121Docto

Recent Advances and Future Trends in Nanophotonics

Nanophotonics has emerged as a multidisciplinary frontier of science and engineering. Due to its high potential to contribute to breakthroughs in many areas of technology, nanophotonics is capturing the interest of many researchers from different fields. This Special Issue of Applied Sciences on “Recent advances and future trends in nanophotonics” aims to give an overview on the latest developments in nanophotonics and its roles in different application domains. Topics of discussion include, but are not limited to, the exploration of new directions of nanophotonic science and technology that enable technological breakthroughs in high-impact areas mainly regarding diffraction elements, detection, imaging, spectroscopy, optical communications, and computing

Optics and Fluid Dynamics Department annual progress report for 2001

research within three scientific programmes: (1) laser systems and optical materials, (2) optical diagnostics and information processing and (3) plasma and fluid dynamics. The department has core competences in: optical sensors, optical materials, optical storage, biooptics, numerical modelling and information processing, non-linear dynamics and fusion plasma physics. The research is supported by several EU programmes, including EURATOM, by Danish research councils and by industry. A summary of the activities in 2001 is presented. ISBN 87-550-2993-0 (Internet

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

Holography

Holography - Basic Principles and Contemporary Applications is a collection of fifteen chapters, describing the basic principles of holography and some recent innovative developments in the field. The book is divided into three sections. The first, Understanding Holography, presents the principles of hologram recording illustrated with practical examples. A comprehensive review of diffraction in volume gratings and holograms is also presented. The second section, Contemporary Holographic Applications, is concerned with advanced applications of holography including sensors, holographic gratings, white-light viewable holographic stereograms. The third section of the book Digital Holography is devoted to digital hologram coding and digital holographic microscopy

An optical distance sensor : tilt robust differential confocal measurement with mm range and nm uncertainty

Compared with conventional high-end optical systems, application of freeform optics offers many advantages. Their widespread use, however, is held back by the lack of a suitable measurement method.The NANOMEFOS project aims at realizing a universal freeform measurement machine to fill that void.The principle of operation of this machine requires a novel sensor for surface distance measurement, the development and realization of which is the objective of the work presented in this thesis. The sensor must enable non-contact, absolute distance measurement of surfaces with reflectivities from 3.5% to 99% over 5 mm range, with 1 nm resolution and a 2s measurement uncertainty of 10 nm for surfaces perpendicular to the measurement direction and 35 nm for surfaces with tilts up to 5°. To meet these requirements, a dual-stage design is proposed: a primary measurement system tracks the surface under test by translating its object lens, while the secondary measurement system measures the displacement of this object lens. After an assessment of various measurement principles through comparison of characteristics inherent to their principle of operation and the possibilities for adaptation, the differential confocal measurement has been selected as the primary measurement method. Interferometry is used as secondary measurement method. To allow for correction of tilt dependent error through calibration, a third measurement system has been added, which measures through which part of the aperture the light returns. An analytical model of the differential confocal measurement principle has been derived to enable optimization. To gain experience with differential confocal measurement, a demonstrator has been built, which has resulted in insights and design rules for prototype development. The models show satisfactory agreement with the experimental results generated using the demonstrator, thus building confidence that the models can be applied as design and optimization tools. Various properties that characterize the performance of a differential confocal measurement system have been identified. Their dependence on the design parameters has been studied through simulations based on the models. The results of this study are applied to optimize the sensor for use in NANOMEFOS. An optical system has been designed in which the interferometer and the differential confocal systems are integrated in a compact design. The optical path of the differential confocal system has been folded using prisms and mirrors so that it can be realized within the allotted volume envelope. For the same reason, many components are adapted from commercially available parts or are custom made. An optomechanical and mechatronic design has been made around the optical system. A custom focusing unit has been designed that comprises a guidance mechanism and actuator to enable tracking of the surface. To achieve a low measurement uncertainty, it aims at accurate motion, high bandwidth and low dissipation. The lateral position of the guidance reproduces within 20 nm and from the frequency response, it is expected that a control bandwidth of at least 800 Hz can be realized. Power dissipation depends on the form of the freeform surface and is a few mW for most expected trajectories. Partly custom electronics are used for signal processing, and to drive the laser and the focusing unit. Control strategies for interferometer nulling, focus locking and surface tracking have been developed, implemented and tested. Various tests have been performed on the system to evaluate the performance. Calibrations must be carried out to achieve the required measurement uncertainty. One calibration is based on a new method to measure tilt dependency of distance sensors. The sensor realized has 5 mm measurement range, -2.5 µm to 1.5 µm tracking range, sub-nanometer resolution, and a small-signal bandwidth of 150 kHz. Using the test results, the 2s measurement uncertainty after calibration is estimated to be 4.2 nm for measurement of rotationally symmetric surfaces, 21 nm for measurement of medium freeform surfaces and 34 nm for measurement of heavily freeform surfaces. To test the performance of the machine with the sensor integrated, measurements of a tilted flat have been carried out. In these measurements, a tilted flat serves as a reference freeform with known surface form. The measurements demonstrate the reduction of tilt dependent error using the new calibration method. A tilt robust, single point distance sensor with millimeter range and nanometer uncertainty has been developed, realized and tested. It is installed in the freeform measurement machine for which it has been developed and is currently used for the measurement of optical surfaces

Ultrasound field measurement and modelling for non-destructive testing

Ultrasonic Testing (UT) is one of most important methods of Non-destructive Testing (NDT) and ultrasonic waves can be generated and detected by means of numeric methods. This thesis focuses on piezoelectric transducers and Electromagnetic Acoustical Transducer (EMAT). For testing with piezoelectric transducer (also called a probe), couplant has to be applied between the test material and the probe allowing for ultrasonic waves generated in the probe by an active piezoelectric crystal to propagate into the testing material. The couplant can be water, mineral oil, or gel, dependent on the applications concerned and material compatibility, for example, water for immersion tests or automatated inspections. No two piezoelectric transducer designs are identical in terms of their frequency range, beam propagation characteristics and directionality. The shape, dimensions, backing and matching of the transducer to the pulse generator together play a major role in the generation of the ultrasonic waves. Furthermore, the quantitative interpretation of pulse echo data obtained when such transducers are used in nondestructive evaluation (NDE) requires a complete knowledge of the ultrasonic field transmitted. Ultrasonic fields of circular probes have been studied experimentally using a miniature probe and theoretically with models developed to predict ultrasonic field of such a probe. Good agreement has been observed. In a fluid, the ultrasonic field generated by a circular piezoelectric transducer can be described in terms of a combination of locally plane waves that radiate in the geometric region straight ahead of the active transducer element and edge waves radiating from the rim of the element. When a piezoelectric transducer is directly mounted onto a solid material, the ultrasonic field includes locally plane longitudinal waves, edge longitudinal waves and mode-converted edge shear waves. Both cases can be studied using miniature piezoelectric probes. For electrically conductive materials, EMATs can be used for generation by means of Lorentz force or magnetostriction or both, and detection. EMAT technique is non-contact and couplant free and can work at high temperature. These attributes make it ideal for inspection in extreme conditions, such as high temperature, high speed, rough surface, etc. This thesis focuses on Lorentz force generation. The main disadvantages of an EMAT detector are its lower sensitivity compared to a piezoelectric probe and it is not straightforward to miniaturise the device to operate as a point sensor for the range of wavelengths of interest here. Therefore, optimal EMAT design is extremely important for successful EMAT application. Ultrasound may be generated without presence of external magnetic field as excitation electric current provides magnetic field as it induces eddy currents in the material under test, which creates Lorentz forces for ultrasonic generation. Where external magnetic field is applied, EMATs have to be designed correctly to achieve enhanced efficiency. As an example, Rayleigh wave EMAT generation has been studied. It is found that where external magnetic field is applied, constructive or destructive effects have been observed, which is understood dependent on direction of the external magnetic fields applied relevant to electric current direction. Optical interferometer to measure the true normal displacement of the solid surface with a resolution in the order of nanometres, but it is much more complex than an EMAT and a piezoelectric probe and requires an optically flat surface. The receiving EMAT detector measures particle velocity. By careful design, in-plane or out-of-plane (or both) velocities can be chosen for detection. This capability is very useful for the detection of longitudinal waves, shear waves, Rayleigh waves or Lamb waves efficiently. The ultrasonic pulse-echo technique has been widely used in ultrasonic NDT. Ultrasonic pulse-echo responses and ultrasonic field signals are not the same. Typically, edge waves are rarely seen in a pulse echo response because the plane waves that are normal to the major face of the active crystal of the same probe are nearly in phase to constructively result in a significant signal whilst edge waves arrive at the active crystal in different directions and different phases cancelling each other and destructively producing only a small signal that is barely observable. As an example of ultrasonic pulse-echo application, weak bond evaluation, has been performed. Weak bond evaluation has always been a challenge. As an example of practical applications, this study has evaluated Integrated Circuit packaging in electronic industry using scanning acoustical microscopy. The relationship among resulting ultrasonic C-scan images, destructive mechanical failure measurement, degradation cycles have been observed. The result is promising indicating the SAM is a very useful tool for weak bond evaluation. Ultrasonic field measurement using a miniature probe and specially design EMAT is very important to characterize and standardize a probe. Such a technique can also find its applications in defect detection and categorization, which has not been considered in this study