Designs of a Planar Waveguide Solar Concentrator

Solar energy, especially through the use of photovoltaic cells, is a promising sustainable energy source for human race. III-V multijunction photovoltaic cells with over 40% confirmed efficiency are among the best candidates for next generation solar cells. However, due to their complex fabrication process, these solar cells are currently too expensive for terrestrial 1 sun use. By using solar concentrators to replace sunlight collection area with cheap materials, total system cost is reduced and cell efficiency is increased. As a result, solar concentrators are viewed as an indispensable part in today’s multijunction photovoltaic cell systems. A novel planar waveguide solar concentrator is proposed in this work. Comparing to conventional solar concentrators, a waveguide is used to output homogenized light onto photovoltaic cells at its end surface. Such a planar structure is potentially easy to fabricate and is possible for novel sun tracking methods. It also benefits in terms of cell connections and heat management. The basic lens array-waveguide structure with the use of a tapered waveguide as a secondary concentrator shows over 90% efficiency under 800 geometric concentration under ideal cases. Optimizations are applied to the lens array, the couplers, and the secondary concentrator. The optimized structure has <1% geometry loss under 1000 geometric concentration and acceptance angles of 0.5˚~0.7˚ depending on the orientations due to structure asymmetry, which is verified by ZEMAX. As an integral part, solar tracking methods are reviewed and a two-axis tracking method realized by using a single-axis tracker and lateral translations is studied. Lateral translation is used for adjusting positions for seasonal sun movement. It has two-dimensional x-y tracking instead of horizontal movement x-only. A prototype system of 50 geometric concentration with >75% optical efficiency in simulation and >65% efficiency in experiment is presented as a practical example of the proposed tracking method

편광 다중화를 이용하여 향상된 기능을 제공하는 도파관 기반의 근안 디스플레이

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2021. 2. 이병호.This dissertation presents the studies on the optical design method that enhances the display performance of see-through waveguide-based near-eye displays (WNEDs) using the polarization multiplexing technique. The studies focus on the strategies to improve the crucial display performances without compromising a small form factor, the most attractive merit of the WNEDs. To achieve this goal, thin and lightweight polarization-dependent optical elements are devised and employed in the WNED structure. The polarization-dependent devices can allow multiple optical functions or optical paths depending on the polarization state of the input beam, which can break through the limitation of the waveguide system with the polarization multiplexing. To realize the function-selective eyepiece for AR applications, the proposed devices should operate as an optically transparent window for the real scene while performing specific optical functions for the virtual image. The proposed devices are manufactured in a combination structure in which polarization-dependent optical elements are stacked. The total thickness of the stacked structure is about 1 mm, and it can be attached to the waveguide surface without conspicuously increasing the form factor of the optical system. Using the proposed polarization-dependent devices, the author proposes three types of novel WNED systems with enhanced performance. First, the author suggests a compact WNED with dual focal planes. Conventional WNEDs have an inherent limitation that the focal plane of the virtual image is at an infinite distance because they extract a stream of collimated light at the out-coupler. By using the polarization-dependent eyepiece lens, an additional focal plane can be generated with the polarization multiplexing in addition to infinity depth. The proposed configuration can provide comfortable AR environments by alleviating visual fatigue caused by vergence-accommodation conflict. Second, the novel WNED configuration with extended field-of-view (FOV) is presented. In the WNEDs, the maximum allowable FOV is determined by the material properties of the diffraction optics and the substrate. By using the polarization-dependent steering combiner, the FOV can be extended up to two times, which can provide more immersive AR experiences. In addition, this dissertation demonstrates that the distortion for the real scene caused by the stacked structure cannot severely disturb the image quality, considering the acuity of human vision. Lastly, the author presents a retinal projection-based WNED with switchable viewpoints by simultaneously adopting the polarization-dependent lens and grating. The proposed system can convert the viewpoint according to the position of the eye pupil without mechanical movement. The polarization-dependent viewpoint switching can resolve the inherent problem of a narrow eyebox in retinal projection displays without employing the bulky optics for mechanical movement. In conclusion, the dissertation presents the practical optical design and detailed analysis for enhanced WNED based on the polarization multiplexing technique through various simulations and experiments. The proposed approaches are expected to be utilized as an innovative solution for compact wearable displays.본 박사학위 논문에서는 편광 다중화 기법을 이용하여 도파관 기반의 증강현실 근안 디스플레이의 성능을 향상시키는 광학 설계 및 이에 대한 분석에 대해 논의한다. 본 연구는 도파관 기반 근안 디스플레이의 가장 큰 장점인 소형 폼 팩터를 유지하면서 디스플레이 성능을 개선하는 것에 중점을 둔다. 이를 위해 기존 광학 소자에 비해 매우 가볍고 얇은 편광 의존형 결합기 광학 소자가 새롭게 제안되며, 이는 입사광의 편광 상태에 따라 독립적인 광 경로 제어를 가능케 하여 편광 다중화를 통해 향상된 성능을 제공 할 수 있다. 또한 실제 영상의 빛은 그대로 투과 시킴으로써 증강현실을 위한 소자로 활용 가능하다. 본 연구에서 제안하는 편광 의존형 결합기 광학 소자는 기하학적 위상(geometric phase, GP)에 기반하여 동작한다. GP 기반의 광학소자가 서로 직교하는 원형 편광 입사광에 대해 상보적인 기능을 수행하는 것을 이용하여, 두 개 이상의 GP 소자와 편광 제어를 위한 광학 필름들을 중첩 시킴으로써 증강현실 결합기 광학 소자를 구현할 수 있다. 이들 광학소자는 매우 얇기 때문에, 본 연구에서 제작된 편광 의존형 결합기 광학 소자의 총 두께는 1 mm 수준으로 폼 팩터 제약으로부터 자유롭다. 또한 평평한 필름 형태이므로, 평판형 도파관에 부착하기 쉽다는 이점을 지닌다. 고안된 편광 의존형 결합기 광학 소자를 사용하여 세 가지 유형의 새로운 도파관 기반의 근안 디스플레이 구조를 제안한다. 첫 번째는 입사광의 편광 상태에 따라 투명 광학 창 또는 오목 렌즈로 작동하는 편광 의존형 결합기 렌즈를 적용하여 가상 영상에 대해 이중 초점면을 제공하는 시스템이다. 제안된 구조는 기존의 도파관 기반 근안 디스플레이가 무한대 위치에 단일 초점면을 제공함으로써 발생하는 시각적 피로 및 흐릿한 증강현실 영상의 문제를 완화할 수 있다. 두 번째로는 입사광의 편광 상태에 따라 광 경로를 좌측 또는 우측으로 제어할 수 있는 편광 격자를 활용하여 가상 영상의 시야각을 기존보다 최대 2배까지 확장할 수 있는 시스템을 제안한다. 이는 단일 도파관 기반 근안 디스플레이에서 영상 결합기 (imaging combiner)로 활용되는 회절 소자의 설계 변수에 의해 제한되는 시야각 한계점을 돌파할 수 있는 구조로 컴팩트한 폼 팩터로 더욱 몰입감 있는 대화면 증강현실 영상을 제공할 수 있다. 마지막으로 위에서 제안된 두 가지 편광 의존형 광학 소자를 모두 사용하여 시점 전환이 가능한 도파관 기반의 망막 투사형 디스플레이 구조를 제안한다. 편광 다중화를 통해 다중 초점들을 선택적으로 활성화함으로써, 확장된 시청영역을 제공하는 동시에 동공 크기 변화 또는 움직임에 의한 이중 영상 문제를 완화할 수 있다. 또한 기계적 움직임 없이 시점 간의 고속 전환이 가능하다는 장점을 지니고 있다. 본 박사학위 논문에서 제시한 편광 다중화를 활용한 새로운 결합기 광학 소자 및 광학 구조들은 도파관 기반 근안 디스플레이의 향상된 성능을 제공하는 해결책 및 새로운 가능성으로 제시할 수 있을 것이라 기대된다.Abstract i Contents iii List of Tables vi List of Figures vii Chapter. 1 Introduction 1 1.1 Augmented reality near-eye display 1 1.2 Key performance parameters of near-eye displays 4 1.3 Basic scheme of waveguide-based near-eye displays 22 1.4 Motivation and purpose of this dissertation 33 1.5 Scope and organization 37 Chapter 2 Dual-focal waveguide-based near-eye display using polarization-dependent combiner lens 39 2.1 Introduction 39 2.2 Optical design for polarization-dependent combiner lens 42 2.2.1 Design and principle of polarization-dependent combiner lens 42 2.2.2 Prototype implementation 48 2.3 Waveguide-based augmented reality near-eye display with dual-focal plane using polarization-dependent combiner lens 51 2.3.1 Implementation of the prototype and experimental results 51 2.3.2 Performance analysis and discussion 57 2.4 Conclusion 69 Chapter 3 Extended-field-of-view waveguide-based near-eye display via polarization-dependent steering combiner 70 3.1 Introduction 70 3.2 Optical design for polarization-dependent steering combiner 73 3.2.1 Principle of polarization grating 73 3.2.2 Principle of polarization-dependent steering combiner 76 3.2.3 Analysis and verification experiment for real-scene distortion 77 3.3 Waveguide-based augmented reality near-eye display with extended-field-of-view 81 3.3.1 Field-of-view for volume grating based waveguide technique 81 3.3.2 Implementation of the prototype and experimental results 84 3.3.3 Performances analysis and discussion 87 3.4 Conclusion 92 Chapter 4 Viewpoint switchable retinal-projection-based near-eye display with waveguide configuration 93 4.1 Introduction 93 4.2 Polarization-dependent switchable eyebox 97 4.2.1 Optical devices for polarization-dependent switching of viewpoints 97 4.2.2 System configuration for proposed method 100 4.2.3 Design of waveguide and imaging combiner 105 4.3 Compact retinal projection-based near-eye display with switchable viewpoints via waveguide configuration 114 4.3.1 Implementation of the prototype and experimental results 114 4.3.2 Performance analysis and discussion 118 4.4 Conclusion 122 Chapter 5. Conclusion 123 Bibliography 127 Appendix 135Docto

Designs of a Planar Waveguide Solar Concentrator

Solar energy, especially through the use of photovoltaic cells, is a promising sustainable energy source for human race. III-V multijunction photovoltaic cells with over 40% confirmed efficiency are among the best candidates for next generation solar cells. However, due to their complex fabrication process, these solar cells are currently too expensive for terrestrial 1 sun use. By using solar concentrators to replace sunlight collection area with cheap materials, total system cost is reduced and cell efficiency is increased. As a result, solar concentrators are viewed as an indispensable part in today’s multijunction photovoltaic cell systems. A novel planar waveguide solar concentrator is proposed in this work. Comparing to conventional solar concentrators, a waveguide is used to output homogenized light onto photovoltaic cells at its end surface. Such a planar structure is potentially easy to fabricate and is possible for novel sun tracking methods. It also benefits in terms of cell connections and heat management. The basic lens array-waveguide structure with the use of a tapered waveguide as a secondary concentrator shows over 90% efficiency under 800 geometric concentration under ideal cases. Optimizations are applied to the lens array, the couplers, and the secondary concentrator. The optimized structure has <1% geometry loss under 1000 geometric concentration and acceptance angles of 0.5˚~0.7˚ depending on the orientations due to structure asymmetry, which is verified by ZEMAX. As an integral part, solar tracking methods are reviewed and a two-axis tracking method realized by using a single-axis tracker and lateral translations is studied. Lateral translation is used for adjusting positions for seasonal sun movement. It has two-dimensional x-y tracking instead of horizontal movement x-only. A prototype system of 50 geometric concentration with >75% optical efficiency in simulation and >65% efficiency in experiment is presented as a practical example of the proposed tracking method

Integrated Optical Interferometers with Micromachined Diaphragms for Pressure Sensing

Optical pressure sensors have been fabricated which use an integrated optical channel waveguide that is part of an interferometer to measure the pressure-induced strain in a micromachined silicon diaphragm. A silicon substrate is etched from the back of the wafer leaving a rectangular diaphragm. On the opposite side of the wafer, ring resonator and Mach-Zehnder interferometers are formed with optical channel waveguides made from a low pressure chemical vapor deposited film of silicon oxynitride. The interferometer's phase is altered by pressure-induced stress in a channel segment positioned over the long edge of the diaphragm. The phase change in the ring resonator is monitored using a link-insensitive swept frequency laser diode, while in the Mach-Zehnder it is determined using a broad band super luminescent diode with subsequent wavelength separation. The ring resonator was found to be highly temperature sensitive, while the Mach-Zehnder, which had a smaller optical path length difference, was proportionally less so. The quasi-TM mode was more sensitive to pressure, in accord with calculations. Waveguide and sensor theory, sensitivity calculations, a fabrication sequence, and experimental results are presented

Planar waveguides obtained on commercial glass substrates by sol-gel and laser irradiation methods

The aim of the thesis is the fundamental study, design, fabrication and characterisation of photonic structures for spatial optic and, particulary, the interconnexion of optical devices. The research explored technologies and substrates for the fabrication of photonic structures based on the guided propogation of light and its application to research and development of integrated optical devices and improving the functionality of communication systems, which realises intelligent optical operations based on Fourier spatial transformed and image formation properties.At the same time, aims to explore new technologies for fabrication ofphotonic structures; which are repeatibility and not contaminants; and the product of well-defined charactericstics and low price

Development of microfabricated optical chemical sensor platforms using polymer processing technology

This work describes the design and fabrication of enhanced polymer waveguide platforms for absorption-based optical chemical sensors and the use of soft lithographic techniques for the fabrication of optical sensor chips. The design of the enhanced polymer waveguide platforms was based on a previously reported theoretical model that was verified experimentally in this work. The platforms were fabricated by micro-injection moulding and subsequently coated with sol-gelderived sensing layers doped with a colorimetric indicator compound. The sensor response to both gaseous ammonia and solution pH was examined using a LEDbased prototype sensor head. Soft lithographic patterning techniques, based on the use of a poly(dimethylsiloxane) (PDMS) patterning element, were employed to produce a variety of sol-gel-based structures with applications in optical sensing. These included discrete sensor spots, surface corrugation grating couplers and ridge waveguides. As a proof of principle, these techniques were applied to the development of an integrated optical oxygen sensor based on the quenching of fluorescence from a sol-gel-encapsulated ruthenium complex that was deposited as a sensor spot onto a ridge waveguide. This work highlights the feasibility of using rapid prototyping technology to fabricate sensitive, mass-producible sensor platforms that employ generic configurations, thereby facilitating their use in a broad range of applications

Bending Loss Mitigation by Surface Plasmon Resonance

Surface plasmon resonance can be used to confine a wave within a thin metal film. The resultant wave is very well-confined by the extreme refractive index difference between the metal and the ambient medium. Such confinement can be used to guide waves under extreme conditions such as subwavelength channels or through extremely tight bends where radiation losses would normally dissipate the wave. A nichrome thin film was deposited and etched as a shadow alongside a series of multi-mode SU-8 slab waveguides with extremely sharp angled bends. Light from a Helium-Neon laser was coupled into these waveguides and the power transmitted was measured and compared to a sample without a nichrome thin film. A total attenuation of signal was found in the non-metallic sample, while a steady signal was successfully transmitted through the sample with the thin film