Modelling and validation of slit-homogenizers for space-based imaging spectrometers

Increasing recent demands for climate awareness require accurate measurement and monitoring of the atmospheric composition. A suitable method for this is the measurement via spaceborne passive imaging spectrometers. The analysis of Earth radiance spectra allows to determine the concentration of absorbing gases in the atmosphere. The measurement accuracy of recent and future spaceborne imaging spectrometers with a high spectral and spatial resolution suffers from the inhomogeneity of the radiance of the observed Earth scene. The scene heterogeneity creates a pseudo-random deformation of the instrument's spectral response function (ISRF), which is the direct link between the forward radiative transfer model, used to retrieve the atmospheric state, and the radiance measured by the instrument. Consequently, heterogeneous scenes will degrade the precision of the retrieved column concentration of the atmospheric components. The present thesis aims to investigate a hardware solution called slit homogenizer (SH), which is used to mitigate the impact of heterogeneous scenes. Several new design considerations, performance predictions and further implications of employing a SH are presented. First, a mirror-based 1D-slit homogenization solution (1DSH) that homogenizes the scene in along-track direction is investigated. To this end, the performance of the 1DSH is modelled by a comprehensive propagation model based on scalar diffraction theory. In the context of the Sentinel-5/UVNS mission, the specific mission requirements on spectral accuracy with respect to applicable heterogeneous scene test cases are evaluated for the 1DSH. Second, a next-generation SH concept based on waveguides is presented. In the context of the upcoming \ce{CO2} Monitoring Mission (CO2M), a slit concept based on rectangular optical fibres is investigated. In this concept, the scene heterogeneity is scrambled in both along-track direction and across-track direction and is therefore referred to as 2D-Slit Homogenizer (2DSH). Particular emphasis is put on the study of the influence of the length and arrangement of the fibres on the scrambling performance. A specific bending routine is proposed to homogenize the far field after the fibre, which further stabilizes the spectral performance. Replacing a classical slit by a 2DSH also changes other than spectral aspects of the instrument like modified signal-to-noise ratios or co-registration schemes. Finally, using the 1DSH as an example, the impact of the stabilized spectral performance in the UV-VIS spectral range on the accuracy of the retrieved \ce{NO2} column density is investigated. For this, the artificially generated heterogeneous Earth radiance scene is propagated through a model of the Sentinel-5/UVNS instrument. The instrument propagation was performed for an instrument equipped with a SH and one with a classical slit. The resulting deviation in the retrieved column density from the ground truth is compared for both cases. The result of the enhanced retrieval accuracy underlines the advantages of employing a SH in spaceborne imaging spectrometers.Die zunehmenden aktuellen Anforderungen an die Erfassung wichtiger Klimaparameter erfordern eine genaue Messung und Überwachung der atmosphärischen Zusammensetzung. Eine geeignete Methode hierfür ist die Messung mit weltraumgestützten passiven abbildenden Spektrometern. Mit Hilfe der von der Erde reflektierten Spektren lässt sich die Konzentration der absorbierenden Gase in der Atmosphäre bestimmen. Die Messgenauigkeit aktueller und künftiger weltraumgestützter abbildender Spektrometer mit hoher spektraler und räumlicher Auflösung leidet unter der Heterogenität der Radianz der beobachteten Erdszene. Die Heterogenität der Szene führt zu einer pseudo-zufälligen Deformierung der spektralen Pulsantwort des Instruments (ISRF), die das Strahlungstransportmodell, das zur Rekonstruktion der atmosphärischen Parameter verwendet wird, mit der vom Instrument gemessenen Strahldichte verknüpft. Infolgedessen beeinträchtigen heterogene Szenen die Genauigkeit der ermittelten Säulenkonzentrationen der atmosphärischen Komponenten. In der vorliegenden Arbeit soll eine Hardware-Lösung namens Slit Homogenizer (SH) untersucht werden, die verwendet wird, um die Auswirkungen heterogener Szenen abzuschwächen. In dieser Arbeit werden mehrere neue Designüberlegungen, Effizienzvorhersagen und weitere Implikationen des Einsatzes eines SH vorgestellt. Zunächst wird eine spiegelbasierte 1D-Slit Homogenizer (1DSH) Lösung untersucht, die die Erdszene entlang der Flugrichtung des Satelliten homogenisiert. Dazu wird die Effizienz des 1DSH durch ein umfassendes Propagationsmodell auf der Grundlage skalarer Beugungstheorie modelliert. Im Kontext der Sentinel-5/UVNS-Mission werden die spezifischen Anforderungen der Mission an die spektrale Genauigkeit in Bezug auf mehrere relevenate heterogenen Szenen für den 1DSH evaluiert. Anschließend wird ein weiteres SH-Konzept vorgestellt, das auf optischen Wellenleitern basiert. Im Kontext der bevorstehenden \ce{CO2} Monitoring Mission (CO2M) wird ein Spaltkonzept auf der Basis von rechteckigen optischen Fasern untersucht. Dieses Konzept mischt die Heterogenität der Szene sowohl entlang der Flugrichtung des Satelliten als auch senkrecht dazu und wird daher als 2D-Slit Homogenizer (2DSH) bezeichnet. Insbesondere wird der Einfluss von Länge und Anordnung der Fasern auf die Homogenisierung der Szenen untersucht. Es wird eine spezielle Krümmung der Fasern vorgeschlagen, um das Fernfeld nach der Faser zu homogenisieren, was die spektrale Pulsantwort des Instruments weiter stabilisiert. Das Ersetzen eines klassischen Spalts durch einen 2DSH führt zu weiteren Konsequenzen für das Instrument, wie z.B. veränderte Signal-Rausch-Verhältnisse oder neue Methoden der co-Registrierung. Als letztes wird am Beispiel des 1DSH die Auswirkung der stabilisierten spektralen Leistung des Instruments auf die Genauigkeit der berechneten \ce{NO2} Säulen-konzentration im UV-VIS-Wellenlängenkanal untersucht. Zu diesem Zweck wird eine künstlich erzeugte heterogene Erdszene durch ein Modell des Sentinel-5/UVNS-Instruments propagiert. Die Instrumentenpropagation wurde für ein mit SH ausgestattetes Instrument und ein Instrument mit einem klassischen Spalt durchgeführt. Die aus der heterogenen Szene resultierende Abweichung der berechneten Säulenkonzentration von der tatsächlichen Säulenkonzentration wird für beide Fälle verglichen. Das Ergebnis der verbesserten Übereinstimmung zwischen berechneter und tatsächlicher Säulenkonzentration für ein Instrument mit SH unterstreicht die Vorteile der Verwendung eines SH in weltraumgestützten abbildenden Spektrometern

Pluggable Optical Connector Interfaces for Electro-Optical Circuit Boards

A study is hereby presented on system embedded photonic interconnect technologies, which would address the communications bottleneck in modern exascale data centre systems driven by exponentially rising consumption of digital information and the associated complexity of intra-data centre network management along with dwindling data storage capacities. It is proposed that this bottleneck be addressed by adopting within the system electro-optical printed circuit boards (OPCBs), on which conventional electrical layers provide power distribution and static or low speed signaling, but high speed signals are conveyed by optical channels on separate embedded optical layers. One crucial prerequisite towards adopting OPCBs in modern data storage and switch systems is a reliable method of optically connecting peripheral cards and devices within the system to an OPCB backplane or motherboard in a pluggable manner. However the large mechanical misalignment tolerances between connecting cards and devices inherent to such systems are contrasted by the small sizes of optical waveguides required to support optical communication at the speeds defined by prevailing communication protocols. An innovative approach is therefore required to decouple the contrasting mechanical tolerances in the electrical and optical domains in the system in order to enable reliable pluggable optical connectivity. This thesis presents the design, development and characterisation of a suite of new optical waveguide connector interface solutions for electro-optical printed circuit boards (OPCBs) based on embedded planar polymer waveguides and planar glass waveguides. The technologies described include waveguide receptacles allowing parallel fibre connectors to be connected directly to OPCB embedded planar waveguides and board-to-board connectors with embedded parallel optical transceivers allowing daughtercards to be orthogonally connected to an OPCB backplane. For OPCBs based on embedded planar polymer waveguides and embedded planar glass waveguides, a complete demonstration platform was designed and developed to evaluate the connector interfaces and the associated embedded optical interconnect. Furthermore a large portfolio of intellectual property comprising 19 patents and patent applications was generated during the course of this study, spanning the field of OPCBs, optical waveguides, optical connectors, optical assembly and system embedded optical interconnects

Design, measurement and analysis of multimode light guides and waveguides for display systems and optical backplane interconnections

The aim of the research in this thesis was to design and model multimode lightguides for optimising visible light for liquid crystal display systems and to design, model and experimentally test infrared light propagation within polymer multimode waveguides as board-to-board interconnects for high data rate communication. Ray tracing models the behaviour of a novel LCD colour separating backlight to optimize its efficiency by establishing the optimum dimensions and position for a unique micro-mirror array within the light guide. The output efficiency increased by 38.2% compared to the case without the embedded mirror array. A novel simulation technique combined a model of liquid crystal director orientation and a non-sequential ray tracing program was used first time to compute the reflected intensity from a LCOS device for a rear projection TV system. The performance of the LCOS display was characterised by computing the contrast ratio over a ±15° viewing cone. Photolithographically manufactured embedded multimode waveguides made from acrylate Truemode® polymer are characterized by measuring the optical transmission loss of key waveguide components including. straight, bend and crossing. Design rules derived from the experimental measurement were used to optimize optical PCB (OPCB) layout. A most compact and complex optical interconnects layout up-to-date for data centres, including parallel straight waveguide sections, cascaded 90° bends and waveguide crossing other than 90° angles, was designed, tested and used in an optic-electrical demonstration platform to convey a 10.3 Gb/s data. A further new method for reducing the end facet roughness and so the coupling loss, by curing a thin layer of core material at the end of the waveguide facet to cover the roughness fluctuations, was proposed and successfully demonstrated giving the best results reported to date resulting in an improvement of 2.8 dB which was better than the results obtained by using index matching fluid

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

High precision laser radar tracking device

This thesis explores a relatively new Solid Silver Thin Film Source technology, for the implementation of a novel High Precision Laser Radar Tracking device. The process which consists of a Ag+-Na+ ion exchange, is designed in two steps. It utilizes an initial electric field-aided ion exchange step for a predeposition, and a subsequent second diffusion step to force the profile latitude necessary for optimization of the device. While the entire project of implementing this device, consists of analyzing, processing, polishing and testing, this thesis covers only the process aspect in detail. The success achieved by obtaining the required Power Coupling Ratio curve on a Simple Coupler, demonstrates a novel integrated optic multimode feed for a Monopulse LIDAR application

Laser ablation of polymer waveguide and embedded mirror for optically-enabled printed circuit boards (OEPCB)

Due to their inherent BW capacity, optical interconnect (OI) offers a means of replacement to BW limited copper as bottlenecks begin to appear within the various interconnect levels of electronics systems. Low-cost optically enabled printed circuit boards are a key milestone on many electronics roadmaps, e.g. iNEMI. Current OI solutions found in industry are based upon optical fibres and are capable of providing a suitable platform for inter-board applications especially on the backplane. However, to allow component assembly onto high BW interconnects, an integral requirement for intra-board applications, optically enabled printed circuit boards containing waveguides are essential. Major barriers to the deployment of optical printed circuit boards include the compatibility of the technique, the cost of acquiring OI and the optical power budget. The purpose of this PhD research programme is to explore suitable techniques to address these barriers, primarily by means of laser material processing using UV and IR source lasers namely 248 nm KrF Excimer, 355 nm UV Nd:YAG and 10.6 µm IR CO2. The use of these three main lasers, the trio of which dominates most PCB production assembly, provides underpinning drive for the deployment of this technology into the industry at a very low cost without the need for any additional system or system modification. It further provides trade-offs among the suitable candidates in terms of processing speed, cost and quality of waveguides that could be achieved. This thesis presents the context of the research and the underlying governing science, i.e. theoretical analysis, involving laser-matter interactions. Experimental investigation of thermal (or pyrolitic) and bond-breaking (or photolytic) nature of laser ablation was studied in relation to each of the chosen lasers with regression analysis used to explain the experimental results. Optimal parameters necessary for achieving minimum Heat Affected Zone (HAZ) and surface/wall roughness were explored, both of which are key to achieving low loss waveguides. While photochemical dominance – a function of wavelength and pulse duration – is desired in laser ablation of photopolymers, the author has been able to find out that photothermallyprocessed materials, for example at 10.6 µm, can also provide desirable waveguides. Although there are literature information detailing the effect of certain parameters such as fluence, pulse repetition rate, pulse duration and wavelength among others, in relation to the etch rate of different materials, the machining of new materials requires new data to be obtained. In fact various models are available to try to explain the laser-matter interaction in a mathematical way, but these cannot be taken universally as they are deficient to general applications. For this reason, experimental optimisation appears to be the logical way forward at this stage of the research and thus requiring material-system characterisation to be conducted for each case thereby forming an integral achievement of this research. In this work, laser ablation of a single-layer optical polymer (Truemode™) multimode waveguides were successfully demonstrated using the aforementioned chosen lasers, thus providing opportunities for rapid deployment of OI to the PCB manufacturing industry. Truemode™ was chosen as it provides a very low absorption loss value < 0.04 dB/cm at 850 nm datacom wavelength used for VSR interconnections – a key to optical power budget – and its compatibility with current PCB fabrication processes. A wet-Truemode™ formulation was used which required that optical polymer layer on an FR4 substrate be formed using spin coating and then UV-cured in a nitrogen oxygen-free chamber. Layer thickness, chiefly influenced by spinning speed and duration, was studied in order to meet the optical layer thickness requirement for multimode (typically > 9 µm) waveguides. Two alternative polymers, namely polysiloxane-based photopolymer (OE4140 and OE 4141) from Dow Corning and PMMA, were sparingly utilized at some point in the research, mainly during laser machining using UV Nd:YAG and CO2 lasers. While Excimer laser was widely considered for polymer waveguide due to its high quality potential, the successful fabrication at 10.6 µm IR and 355 nm UV wavelengths and at relatively low propagation loss at datacom wavelength of 850 nm (estimated to be < 1.5 dB/cm) were unprecedented. The author considered further reduction in the optical loss by looking at the effect of fluence, power, pulse repetition rate, speed and optical density on the achievable propagation but found no direct relationship between these parameters; it is therefore concluded that process optimisation is the best practice. In addition, a novel in-plane 45-degree coupling mirror fabrication using Excimer laser ablation was demonstrated for the first time, which was considered to be vital for communication between chips (or other suitable components) at board-level

A practical review on the measurement tools for cellular adhesion force

Cell cell and cell matrix adhesions are fundamental in all multicellular organisms. They play a key role in cellular growth, differentiation, pattern formation and migration. Cell-cell adhesion is substantial in the immune response, pathogen host interactions, and tumor development. The success of tissue engineering and stem cell implantations strongly depends on the fine control of live cell adhesion on the surface of natural or biomimetic scaffolds. Therefore, the quantitative and precise measurement of the adhesion strength of living cells is critical, not only in basic research but in modern technologies, too. Several techniques have been developed or are under development to quantify cell adhesion. All of them have their pros and cons, which has to be carefully considered before the experiments and interpretation of the recorded data. Current review provides a guide to choose the appropriate technique to answer a specific biological question or to complete a biomedical test by measuring cell adhesion