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Item type:Publication, Access status: without files , Auto-generating ageing self-tests with hardware in the loop for commercial microcontrollersMicrocontrollers in critical systems require monitoring of hardware ageing to avoid unexpected malfunctioning. We previously introduced a deployable timing-based method with self-tests for ageing monitoring in commercial microcontrollers that relies solely on user-level information. While effective, manually constructing computing payloads for system-level tests can limit scaling and coverage of hardware-specific critical paths or subsystems. In this paper, we complement our technique with an automated generation of the self-test firmware. We explore several approaches for generation using hardware in the loop (HiL) evaluation on commercial microcontrollers, including a random generation and evolutionary strategies. In the evolutionary approach, we investigate two mutation mechanisms: random selection, and contextual bandits. Our evaluations on real hardware and a surrogate model show the impact of evaluation budget and algorithm selection on the effectiveness of automatic self-test generation. Results reveal that random generation is well suited to reduced HiL explorations, while evolutionary strategies benefit from extended evaluations. Our Hil-based technique is able to produce firmwares with up to double the degradation sensitivity as the functional tests provided by the vendor.Publicationtype: Conference Paper16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Artifact-suppressing reconstruction for X-ray near-field holography(2026); X-ray near-field holography is an excellent tool for imaging objects with a resolution in the nanometer range. Full-field measurements with a lens-free microscopic measurement setup allow holograms of objects to be recorded at multiple magnifications. Unfortunately, these holograms can only be measured as an integral over a certain period of time. The phase information of the hologram is lost and only the intensities are recorded, commonly known as the phase problem. To recover the phase information, a spatial support and/or the recording of several holograms at different detector distances are usually used as additional information in reconstruction approaches. However, both constraints are only feasible for a very closed set of experiments, require time consuming optimization of hyper-parameters and a lot of computation time. Avoiding both techniques is a necessity to enable in-situ/operando measurements but, in doing so, reconstruction artifacts appear. Another challenge is the Autofocus problem. For the reconstruction of holographic data, the numerical focus of the measurement setup is part of the forward model and must be known very precisely. If the numerical focus is inaccurately embedded in the forward model of the reconstruction algorithm, it will generate artifacts or result in blurred reconstructed images. In practice, it is often difficult to accurately determine the Fresnel number for a good reconstruction. In the following work, the origin of possible artifacts as described above is investigated in detail and countermeasures are developed by adjusting the raw data preprocessing, by adjusting the reconstruction and by an automatic optimization of the forward model. The first part of this thesis addresses reconstruction artifacts with respect to a reference method, a projected gradient decent approach, using a Nesterov accelerated gradient. The reconstruction artifacts have the following identified causes: Truncation artifacts are caused by the limited field of view of the detector. The real hologram is cut off and as a result, the acquired image contains inconsistent areas at the borders. Mirroring the hologram in all directions in combination with a modified window function is derived as a countermeasure to reduce the impact of truncation edges. Low frequency artifacts are identified to be mainly caused by a global normalization error in the empty beam normalization or flat-field-correction. The normalization is corrected by introducing a constant offset parameter into the reconstruction. Other artifacts are overestimation artifacts, mainly caused by the Nesterov acceleration. The forward model amplifies this effect, which is sensitive to high spatial frequencies. To reduce the overestimation of the phase and absorption values, a regularization of the absorption values is applied, which is sufficient for both parameters simultaneously. The insensitivity of the forward model to low spatial frequencies leads to weak reconstruction artifacts. These are therefore reconstructed slowly since they have a small impact on the reconstruction loss. A newly introduced frequency-dependent Nesterov momentum and iterations on multiple grids explicitly accelerate the reconstruction of low frequencies. Both techniques also reduce an overestimation of high spatial frequencies. The combination of all described methods is summarized in a new artifact-suppressing reconstruction method (ASRM). Compared to the reference algorithm, it suppresses all artifacts mentioned above and reduces the reconstruction time significantly. The second part of the thesis proposes a solution to the autofocus problem with an approach to automatically optimize the Fresnel number. The approach consists of three problems to be solved. An error metric is required that quantifies the distance between the estimated forward model and the correct one. A global optimization has to be defined, which has to be solved by an appropriate solver. As an error metric, a model fit error (MFE) metric is derived from data inconsistencies that are introduced when a non-negative electron density constraint is applied to a defocused wave-field. The novel error metric is evaluated by simulation and experimental results and finally compared with other error metrics from the literature. On simulated data, the MFE metric performs similarly to comparative error metrics and significantly better on experimental data. For an automated solution of the autofocus problem, a global optimization problem with respect to the numerical focus is formulated. The expected uncertainty of the setup geometry is added as a constraint to the target function. Since the gradients of the target function are not trivially accessible, a gradient-less downhill simplex method is chosen as a solver. As fine tuning, the parameters for the ASRM algorithm are adjusted so that the MFE behaves optimally.Publicationtype: Doctoral Thesis64 59 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Efficient optical coating design using an autoencoder-based neural network model(IOP Publishing, 2025-11-12); ; ; ; Optical thin-film coatings are integral to modern photonics and in particular to ultrafast lasers, providing precise control of dispersion and reflectivity, thus enabling tailored pulse shaping. Designing these coatings represents an inverse problem, requiring the mapping of desired optical properties to physical designs, a task that poses major challenges for traditional heuristic methods, which often rely on time-consuming, expert-guided iteration and can be constrained by the choice of an initial starting point. Here, we present an artificial intelligence (AI) framework for optical thin-film coating design that accelerates the design process, achieving excellent performance characteristics without expert intervention. We discuss our AI approach and demonstrate the capabilities of our algorithm by designing a complex broadband high-reflectivity mirror with state-of-the-art performance characteristics including a group delay dispersion covering a spectral range of 940 nm to 1120 nm.Publicationtype: Journal Article25 44 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Stereo hard X-ray ptychography(Optica Publishing Group, 2025-05-23); ; ; ; Hard X-ray ptychography has strongly developed during the last decade, enabling one to visualize structural properties of materials at high spatial resolution. By combining it with multi-slicing or tomographic techniques, optically thick samples can be investigated in 3D. Nevertheless, the depth resolution in multi-slicing is often limited to several micrometers by the ptychographic optical system and a full laminographic or tomographic investigation may be hindered by experimental constraints of limited space or acquisition time. Here, we introduce a stereoscopic imaging system using two inclined nanofocused X-ray beams to illuminate a sample at varying angles at the same time. Similar to human vision, adding these stereoscopic views results in considerably improved in-depth resolution beyond the current limits of pure 2D imaging systems. This is especially promising for experimental applications requiring bulky sample environments.Publicationtype: Journal Article28 37 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Impact of seismic activities on the optical synchronization system of the European X-ray free-electron laser(Cambridge University Press (CUP), 2025-03-05); ; ; ; This study investigates the influence of seismic activities on the optical synchronization system of the European X-ray Free-Electron Laser. We analyze the controller I/O data of phase-locked-loops in length-stabilized links, focusing on the response to earthquakes, ocean-generated microseism and civilization noise. By comparing the controller data with external data, we were able to identify disturbances and their effects on the control signals. Our results show that seismic events influence the stability of the phase-locked loops. Even earthquakes that are approximately 5000 km away cause remarkable fluctuations in the in-loop control signals. Ocean-generated microseism in particular has an enormous influence on the in-loop control signals due to its constant presence. The optical synchronization system is so highly sensitive that it can even identify vibrations caused by civilization, such as road traffic or major events like concerts or sport events. The phase-locked loops manages to eliminate more than 99 % of the existing interference.Publicationtype: Journal Article23 39 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fast perfekt: regression-based refinement of fast simulation(2025-02-14); ; ; ; The availability of precise and accurate simulation is a limiting factor for interpreting and forecasting data in many fields of science and engineering. Often, one or more distinct simulation software applications are developed, each with a relative advantage in accuracy or speed. The quality of insights extracted from the data stands to increase if the accuracy of faster, more economical simulation could be improved to parity or near parity with more resource-intensive but accurate simulation. We present Fast Perfekt, a machine-learned regression to refine the output of fast simulation that employs residual neural networks. A deterministic morphing model is trained using a unique schedule that makes use of the ensemble loss function MMD, with the option of an additional pair-based loss function such as the MSE. We explore this methodology in the context of an abstract analytical model and in terms of a realistic particle physics application featuring jet properties in hadron collisions at the CERN Large Hadron Collider. The refinement makes maximum use of existing domain knowledge, and introduces minimal computational overhead to production.Publicationtype: Journal Article27 47 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Neural network method for dielectric optical coating design(EDP Sciences, 2024-10-18); ; ; ; We use neural networks to address the challenge of deriving dielectric coating designs from the desired optical properties. We show that our trained neural network can automatically design common laser mirror coatings types efficiently.Publicationtype: Journal Article50 44 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A review of techniques for ageing detection and monitoring on embedded systems(Association for Computing Machinery (ACM), 2024-10-07); ; ; ; Embedded digital devices are progressively deployed in dependable or safety-critical systems. These devices undergo significant hardware ageing, particularly in harsh environments. This increases their likelihood of failure. It is crucial to understand ageing processes and to detect hardware degradation early for guaranteeing system dependability. In this survey, we review the core ageing mechanisms, and identify and categorize general working principles of ageing detection and monitoring techniques for Commercial-Off-The-Shelf (COTS) components that are prevalent in embedded systems: Field Programmable Gate Arrays (FPGAs), microcontrollers, Systems-on-Chips (SoCs), and their power supplies. From our review, we find that online techniques are more widely applied on FPGAs than on other components, and see a rising trend towards machine learning application for analysing hardware ageing. Based on the reviewed literature, we identify research opportunities and potential directions of interest in the field. With this work, we intend to facilitate future research by systematically presenting all main approaches in a concise way.Publicationtype: Journal Article26 66 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Deformation dynamics of nanopores upon water imbibition(National Acad. of Sciences, 2024-09-17); ; ; ; Capillarity-driven transport in nanoporous solids is ubiquitous in nature and is of increasing importance for the functionality of modern liquid-infused engineering materials. During imbibition, highly curved menisci are driven by negative Laplace pressures of several hundred atmospheres, exerting an enormous contractile load on an increasing portion of the porous matrix. Due to the challenge of simultaneously monitoring imbibition and deformation with high spatial resolution, the resulting coupling of solid elasticity to liquid capillarity has remained largely unexplored. Here, we study water imbibition in mesoporous silica using optical imaging, gravimetry, and high-resolution dilatometry. In contrast to an expected Laplace pressure-induced contraction, we find a square-root-of-time expansion and an additional abrupt length increase when the menisci reach the top surface. The final expansion is absent when we stop the imbibition front inside the porous medium in a dynamic imbibition-evaporation equilibrium, as is typical for water transport and transpiration in plants. These peculiar deformation behaviors are validated by single-nanopore molecular dynamics simulations and described by a continuum model that highlights the importance of expansive surface stresses at the pore walls (Bangham effect) and the buildup or release of contractile Laplace pressures as nanoscale menisci collectively advance, arrest, or disappear. Our model predicts that these observations are valid not only for water imbibition in silica, but for any imbibition process in nanopores, regardless of the liquid/solid combination. This also suggests that simple deformation measurements can be used to quantify surface stresses and Laplace pressures or transport in a wide variety of natural and artificial porous media.Publicationtype: Journal Article75 52 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Studying the degradation of propagation delay on FPGAs at the European XFELAn increasing number of unhardened commercial-off-the-shelf embedded devices are deployed under harsh operating conditions and in highly-dependable systems. Due to the mechanisms of hardware degradation that affect these devices, ageing detection and monitoring are crucial to prevent critical failures. In this paper, we empirically study the propagation delay of 298 naturally-aged FPGA devices that are deployed in the European XFEL particle accelerator. Based on in-field measurements, we find that operational devices show significantly slower switching frequencies than unused chips, and that increased gamma and neutron radiation doses correlate with increased hardware degradation. Furthermore, we demonstrate the feasibility of developing machine learning models that estimate the switching frequencies of the devices based on historical and environmental data.Publicationtype: Conference Paper16