This thesis presents a comprehensive investigation into the determination of optical and dynamical properties of turbid media, with the ultimate goal of developing a non-invasive, portable, and potentially wearable interferometric diffuse optics system for cerebral blood flow (CBF) measurement... ... Read more

Due to the trend towards minimization of optical space instruments, in combination with ever increasing performance requirements, their optical and mechanical design are becoming more and more intertwined. As a result, it is invaluable to have insights into how mechanical and thermomechanical disturbances influence the system’s optical performance already in early stages of the design process. In this work, a differential ray tracer is implemented into a topology optimization framework, allowing for direct optical performance analysis of optical instruments under disturbance loads. The gradient information of the optical performance is provided by semi-automatic differentiation. A wide range of optics can be used including freeform optics, that have become frequently used in state-of-the-art instruments. The method has been verified and shows a strong agreement with ZEMAX. To demonstrate the advantages of the STOP-based computational design workflow, a fully coupled opto-thermo-mechanical topology optimization is performed on a case inspired by CHAPS-D, a hyperspectral air pollution sensor. The results show the potential of direct optical performance analysis in topology optimization, creating a structure that maintains optical performance at or above nominal performance, while highlighting challenges posed by the optimization process. ... Read more

Accurate measurement of blood flow in deep tissue is essential for diagnosing and monitoring a wide range of clinical conditions, yet current imaging methods are often invasive, limited to snapshots, or impractical for continuous bedside use. Diffuse correlation spectroscopy (DCS) offers a non-invasive optical alternative, measuring blood flow dynamics through temporal fluctuations in scattered light. However, modelling DCS signals remains computationally challenging, particularly when accounting for structured flow and tissue complexity.
This thesis investigates a coupled simulation framework that integrates particle dynamics from COMSOL with optical propagation computed via a modified Born series solver (WaveSim). The framework was used to test three validation cases: temperature-dependent Brownian motion, laminar flow, and parameter extraction through exponential fitting of the autocorrelation function.
The results demonstrate that the framework captures the qualitative behaviour of DCS. Brownian motion produced linear mean squared displacement (MSD) curves, with faster optical decorrelation at higher temperatures, while laminar flow introduced quadratic MSD growth and accelerated decay. However, quantitative agreement with theoretical scaling was not achieved. A key limitation was identified as a pixel-quantisation effect: particle displacements per frame were typically far smaller than the 0.2 μm WaveSim pixel size, rendering most motion invisible to the solver. This suppressed decorrelation, distorted temperature and flow scaling, and prevented unbiased parameter extraction. Additional constraints included limited statistical averaging, absence of phase tracking, reliance on decay rates without correlation diffusion equation (CDE) modelling, and high computational costs.
The findings confirm that the COMSOL–Born framework provides proof of concept for simulating DCS but requires significant refinement for quantitative accuracy. Future work should focus on subpixel rendering, improved temporal sampling, integration of CDE solutions, and domain decomposition. With these improvements, the framework has the potential to advance real-time, non-invasive monitoring of blood flow in clinical settings. i ... Read more

Cerebral blood flow (CBF) is a valuable physiological parameter for the diagnosis of brain function and disease. A number of optical techniques have been developed to estimate CBF non-invasively with high spatial and temporal resolutions with the benefit that no contrast dyes or surgical procedures are required.

In this project, we present a novel coherence-gated iSVS approach that leverages a low-cost, low-coherence laser diode to selectively filter out photons from superficial layers—eliminating the need for long SD separations. This strategy dramatically improves light throughput and spatial resolution while reducing system cost. Our method offers a practical and scalable solution for high-resolution, depth-sensitive CBF imaging.

To validate the system, experiments were performed using tissue-mimicking phantoms, where a programmable pump generated controlled flow patterns. The results demonstrate the technique’s potential for high-resolution, real-time assessment of cerebral hemodynamics, offering a promising tool for both clinical and research applications. ... Read more

Satellite based imaging spectrometry is an important tool for earth observation. Most instruments use dispersive optics such as gratings to obtain spectral information.
This demands a certain minimal instrument size to provide the necessary optical path length. Filter based spectrometry allows smaller optical systems, but traditional filter designs generally only use fractions of the incoming light. Spectral filters based on photonic crystals show the potential for instruments with high total transmission while enabling further miniaturization. Development of this concept requires experimental investigation on the influence of the filter-detector distance on the spectral characteristics of the photonic crystals.
This work shows the development and demonstration of an experimental design that allows transmission measurements of filters as close as 40 μm from the detector.
Through combining the interference originating from the filter and detector surfaces forming a Fabry-Perot cavity with the spectral imaging data, 6-DoF position measurement of the filter is achieved. The design and measurement concept are demonstrated through a set of validation measurements, followed by transmission measurements on a filter sample over the 50-100 μm range with 5 μm spacing.
... Read more

This thesis aims to characterize the photopolymer resins designed for the 3D printing of Gradient Refractive Index (GRIN) lenses. This thesis focuses explicitly on preparing UV-curable inks doped with nanoparticles that are custom-synthesized for sophisticated optical applications. Four different ink formulations have been studied, including epoxy-based and PMMA-based, together with two commercial UV-curable resins, which are a combination of acrylates and epoxies. Spectroscopy testing of these inks is conducted to study their optical performance for properties like the transmittance and absorbance over the UV, visible light, and NIR regions. The results show that it is possible to obtain significantly different behavior in the optical properties of materials compared to the inks without nanoparticles, offering a promising pathway for developing GRIN lenses with superior characteristics. This research also reports on some modifications performed on an Elegoo Mars 4 DLP printer to control a projector and the DMD device for arbitrary exposure times, further contributing to the development of high-performance optical devices through additive manufacturing. ... Read more

PRobe far-Infrared Mission for Astrophysics (PRIMA) is a cryogenically cooled NASA mission aimed at making hyperspectral observations in the far infrared. The Netherlands Institute for Space Research (SRON) will design and manufacture a Linear Variable Filter (LVF) for use within PRIMA’s optics. In the design and manufacture of the LVF it is essential to characterize its optical properties. This is done by scanning a laser beam over the LVF’s surface while at 4.2K within a cryostat by means of a cryogenic positioning system. This research is tasked with designing that cryogenic positioning system; which must be capable of 2 degree-of-freedom planar motion to scan the the 40 × 20 mm surface of the LVF, with steps of 25 µm and an uncertainty in LVF position of ±2.5 µm. Additionally, the mechanism must fit within the cryostat, and must be able to bring the LVF to a position that is out of the laser beam, resulting in a total horizontal range of motion of 57.5mm. Due to the large range of motion, high accuracy requirements, low temperature, and limited available space, off-the-shelf solutions are not viable. A custom, highly integrated, solution must be developed. The far-infrared and cryogenic context requires that heat dissipation is kept to a minimum, and has consequences for the applicability
of standard components such as ball bearings.

The design process begins with divergent concept generation. Through the course of two downselects, concepts of lesser promise are eliminated. The final concept consists of a 5-bar mechanism with a parallelogram to constrain the rotation of the LVF. It is driven by two stepper motors. Through the use of a MATLAB model, the lengths of the links that define the mechanism are optimized. A Monte Carlo tolerance analysis is performed to calculate the expected performance of the manufactured mechanism. Lastly, the theoretical mechanism is materialized as a CAD model.

The final design of the mechanism is capable of scanning the 40 × 20 mm surface of the LVF with steps of 25 µm or smaller; meeting the target requirement. The worst-case position uncertainty of of the LVF is approximately ±5 µm; twice larger than the target requirement. The estimated heat dissipation is twice the targeted requirement, at 19.4 mW. The detent torque of the recommended stepper motor-gearbox combination provides a safety factor of 12.6 over the maximum torque required by the mechanism. ... Read more

Linear Variable Filters (LVFs) are integral to future space missions aimed at measuring the spectral features of distant galaxies. Given the varying red-shifts (shift of light to longer wavelengths as an object moves away from the observer) of these galaxies, an LVF with a resolving power of approximately 10 can effectively identify, classify, and image large populations of galaxies, facilitating studies of cosmic evolution through hyperspectral imaging (HSI) techniques. HSI captures detailed spectral information across a wide range of wavelengths for each pixel in an image, enabling material identification and analysis. Since the galaxies to be studied are very faint, hyperspectral imagers need to be equipped with highly sensitive detectors, cryogenically cooled conditions, and filters with high transmission efficiency. This thesis investigates LVFs based on thin membranes, focusing on materials like silicon nitride (Si3N4) and silicon carbide (SiC). The primary objective is to model these thin and pre-stressed membranes using COMSOL Multiphysics to simulate the thermal expansion and mechanical behaviour of membranes in cryogenic environments. Experimental validation is conducted using Digital Holography Microscopy (DHM) and Laser Doppler Vibrometry (LDV) to ensure the accuracy and consistency of the simulations. The model enables the simulation of membranes with varying dimensions, improving the accuracy of behavioural predictions thereby eliminating the need for physical prototypes for analytical purposes, significantly reducing cost and time. Overall, this research bridges the gap between theoretical models and practical applications of LVFs in space. Enhancing the understanding of material behaviour under extreme conditions advances the development of reliable LVFs for spaceborne optical instruments, offering valuable insights into their durability and performance. ... Read more

A significant focus of current and next-generation astronomical research is the study of exoplanets and the formation of habitable conditions in planetary systems. This requires high-performance spectrom- eters operating in the far-infrared (FIR) range, with the Fabry-Pérot etalon being the state-of-the-art solution. This thesis, a collaboration between TU Delft and the Netherlands Institute for Space Research (SRON), focused on identifying and overcoming the major performance limitations of the Fabry-Pérot. The performance metrics were spectral resolving power and transmission. The major performance lim- itation was identified to be diffraction, which is particularly relevant in the FIR range. A two-part model was developed based on Gaussian beam propagation and the Huygens-Fresnel principle. Gaus- sian beam propagation is a well-established method that takes diffraction effects into account but is limited to the paraxial approximation, which quickly breaks down in the FIR. The novel application of the Huygens-Fresnel principle to Fabry-Pérot propagation modeling accounts for diffraction effects and remains accurate with large beam divergence caused by diffraction. The Gaussian beam approach was used to validate initial results from the Huygens-Fresnel approach. The effect of diffraction on critical device dimensions, such as cavity length, beam waist, finite aperture, and mirror tilt was investigated. Additionally, an experiment design was proposed to further validate the model. This included research into the design of metal mesh mirrors and silicon-air distributed Bragg reflector mirrors using COMSOL and MATLAB simulations. The successful results of the Huygens-Fresnel model provide new insights into the constraints on critical dimensions of the Fabry-Pérot etalon due to diffraction effects. These insights can be used to propose high-performance Fabry-Pérots designs suitable for the exploration of exoplanets. ... Read more

Protoplanetary disks (PPDs) have gained substantial attention due to their crucial role in planet formation. Observing their features in the far-infrared (FIR) spectrum has been challenging due to limitations in current optics and detectors, with proposed missions requiring a spectrometer with a spectral resolving power on the order of magnitude of 100,000. A promising solution is the virtually imaged phased array (VIPA), which could surpass the commonly used Fabry-Perot etalon and diffraction grating spectrometers. However, due to their novelty, VIPAs have seen limited applications in space and lack a numerically efficient model, preventing confi- dent recommendation for FIR spectroscopy. In this project, a one-dimensional (1D) analytical VIPA model with a numerically efficient implementation using Gaussian beam formalism was developed, showing close alignment with an advanced 2D model by Hu et al. (2015) based on the angular spectrum of plane waves. The new model demonstrated potential for VIPAs to meet the spectral resolving power required for space missions and showed the possibility to be further expanded to 2D for higher accuracy, while retaining high efficiency. An experimental setup for verifying the simulation results was proposed. The interplay between various device parameters, coupled with the numerous degrees of freedom required for aligning the optical setup, presented significant experimental challenges. Additionally, investigations into the design of inductive metal mesh mir- rors, supported by simulations in COMSOL and MATLAB, uncovered several difficulties that were thoroughly explored. The analytical VIPA model based on Gaussian beam propagation shows promise as a method for optimizing a VIPA for high-resolution far-infrared spectroscopy but further research and experimental validation are necessary to fully establish its efficiency and effectiveness. The project was a collaboration between TU Delft and the Netherlands Institute for Space Research (SRON). ... Read more

This study presents a novel approach for evaluating the effects of dynamic disturbances on optical performance using sensitivity analysis. The computation of optical performance for perturbed optical systems is too costly with state-of-the-art analysis software when applied in the use case for the optimization of the optomechanical interface, and therefore using a simplified model, based on ray transfer matrices, describing the most critical phenomena provides a solution. The proposed analytical framework employs a Taylor expansion for the merit function, incorporating both the Jacobian and Hessian matrices, to reduce computation time in transient analysis. The effect of small perturbations on the merit function is found to be accurately described by the approximation when solely the Jacobian is included, for larger displacement fields the approximation deviates significantly without the inclusion of the Hessian. Next to this, the definition of a grating matrix is appended to the framework to facilitate the analysis of a larger set of systems. For transient analysis, the proposed framework exhibits a remarkable improvement in computation time, with minimal degradation in accuracy for paraxial systems. All findings hold relevance for the effect of rigid body displacements in coupled mechanical-optical analyses and further optimization of this coupling under disturbed conditions. ... Read more

Adaptive optics (AO) are essential in ground-based telescopes to correct atmospheric distortions and achieve high-resolution imaging; adaptive secondary mirrors (ASMs) integrate deformable mirrors directly into telescope optics but typically require a wavefront sensor (WFS) for calibration and flattening. When a WFS is unavailable or nonfunctional, alternative methods are needed. This thesis explores the feasibility of flattening the convex ASM of the UH-88 telescope without a WFS, using only a natural guide star and focal plane imaging. By developing a numerical model of the UH-88 system—including the deformable mirror, Mauna Kea atmospheric aberrations, and focal plane camera characteristics—we employed an image metric based on the second moment of intensity to evaluate image quality, adjusting Zernike mode coefficients via the Nelder-Mead simplex optimization algorithm to control mirror shape. Experimental validation using a laboratory setup that replicated key aspects of the UH-88 system confirmed that the ASM could be flattened to within 100 nm RMS surface error, meeting passive mirror operation requirements. The flattening was achieved within a 10-minute timeframe using only focal plane images of a natural guide star distorted by atmospheric turbulence. This study demonstrates that flattening a convex ASM without a WFS is feasible using focal plane image metric optimization, offering a practical solution when the WFS is unavailable or calibrated commands are outdated, thereby ensuring continued high-quality telescope operation. ... Read more

Cerebral oxygen saturation is an important indicator that reflects the oxygen metabolism of the brain tissues in such patients. The utilization of near-infrared spectroscopy (NIRS) enables the identification of the oxygen saturation levels in nearby tissues by analyzing the distinct oxygenation states of hemoglobin-oxyhemoglobin (HbO2) and the distinctive molecular spectrum of deoxygenated hemoglobin (Hb). It is helpful to realize the objective of noninvasive continuous detection on cerebral oxygen saturation because near-infrared radiation can directly penetrate the skull to obtain the characteristics of the average oxygen saturation of the brain tissues. This also allows a noninvasive way to monitor blood flow in the brain. A frequency comb is a pulsed laser in the time domain and a sequence of discrete, uniformly spaced frequency lines in the frequency domain. Applying the frequency comb to NIRS has the potential to develop a high-speed and fs-level time-of-flight resolution blood flow measurement system. The objective of this thesis is to utilize a chip-based optical frequency comb as an illuminating source in order to conduct time-of-flight measurements of a single target, employing a Michelson interference experiment. The establishment of the time of flight in the single-layer case and the investigation of the non-ambiguity range form the fundamental basis for future experiments multi-layer. ... Read more

The investigation of the interaction between lasers and brain tissue holds significant theoretical and practical significance in the field of brain physiology studies. A two-dimensional finite element-based simulation model was constructed using the commercial finite element simulation software COMSOL Multiphysics to investigate the propagation of light and photon correlation in tissues. The simulations included static and dynamic conditions of light propagation in tissue. In both cases this diffuse light propagation itself was studied along with photon correlation using the solution of the diffusion equation. Subsequently, the obtained simulation results are assessed and summarized by considering the outcomes of data fitting. The findings indicate that the accuracy of simulating light propagation is higher for larger sizes, although satisfactory results can be achieved for correlation propagation simulations in the small size range. The simulation results remain unaffected by the refinement of the mesh, provided that the fundamental criteria for mesh size are satisfied. ... Read more

This work delves into designing and implementing an optimized self-centering lens mounting technique for the collimator and coupler lenses in the Universal Light Source (ULS) by ASML. The ULS aims to meet the growing demand for smaller logic nodes and higher throughput in optical metrology sensors. To achieve precise alignment and stability, the light source utilizes a supercontinuum spectral shaping method based on a highly non-linear hollow core anti-resonance photonic crystal fiber filled with a high-pressure noble gas.
This research identifies the limitations of the existing lens mounting design in terms of manufacturing, stability, and adhesive usage. To address these challenges, a self-centering design is proposed, leveraging the geometrical relationship between thread angle, barrel and retaining ring diameters, and lens clearance. By optimizing these parameters, the centering error is substantially reduced, demonstrating a considerable improvement in centering precision compared to the current method. Additionally, thermal effects are mitigated through a symmetrical design and local flexibility, reducing the impact of temperature fluctuations during operation and transportation.
An analytical assessment of five different collimator and coupler lens mount concepts are presented to identify an optimal design meeting specific operating temperature, stresses, eigenfrequency, and manufacturability criteria. Numerical analysis is employed to evaluate the performance of each design concept relative to the currently employed mounting designs.
The resulting optimized self-centering coupler and collimator lens mount exhibits centering errors influenced by various tolerances. These tolerances have a deviation of ±0.2% for the lens radial surface, ±1° for the thread angle, and ±0.2 mm for the remaining parameters. The calculated centering errors for coupler lens and collimator lens 1 and 2 are 6.27 μm, 8.24 μm, and 6.98 μm, respectively. However, it is crucial to acknowledge that these results are purely theoretical and necessitate validation through experimental data to ascertain their accuracy.
Overall, this optimized self-centering technique offers enhanced stability, relaxed manufacturing tolerances, and eliminates the need for adhesives when compared to the current flexure design. These significant findings contribute to the advancement of optomechanical engineering and provide valuable insights into the implementation of self-centering mounts, thereby enhancing precision while relaxing manufacturing requirements in optomechanical mounting.
... Read more

There is a growing demand for precision and high-quality optical objectives and lenses due to their numerous advantages and uses. The market for high precision objective lenses has tremendous potential for development and is a sector that is expanding quickly.

Measuring the aberrations of optical systems is an essential step in the fabrication of high precision optical components. However, when working at the cutting-edge of technology, it is increasingly difficult to provide trustworthy measurements as the used metrology instrument has to be of comparable or higher precision. This poses a major problem especially when working with high numerical aperture (NA) optics.

In this thesis, we will analyze and quantify the measurement uncertainty of a Twyman-Green interferometer used for lens testing of high-NA microscope objectives.

To quantify the measurement uncertainty of the interferometer, various sources of uncertainty that affect the accuracy and precision of the measurements are considered. These include environmental and instrumentation factors such as incorrect phase-stepping, laser instability, camera noise, stray light, photon shot noise, effects of mid-spatial frequencies originating in the optical reference, as well as computational shortcomings such as: incorrect phase unwrapping, polynomial fit errors, incorrect pupil scaling and edge detection.

By carefully analyzing these individual sources of uncertainty and their impact, we determine the overall measurement uncertainty of the interferometer and provide an assessment of its accuracy through Monte Carlo simulations, where the introduced uncertainties are obtained from real measurement data. The uncertainty analysis procedure described in this paper is a useful tool that can also be applied to different types of interferometers by taking proper considerations into account. ... Read more

Acoustic levitation is an novel method that could lend itself very useful to fast and precise transportation of small objects. To verify the acoustic field and the modelling assumptions, the acoustic field must be visualised and quantified. Different methods to visualise a pressure gradient are investigated and rated against design requirements. These requirements are: 1. difficulty, as the setup should be buildable within the period of a MSc. thesis. 2. Time per measurement and 3. measurement sensitivity. It is decided that using a double coincidence spherical mirror schlieren setup is most suitable measurement method. To capture the high speed sphenomena of ultrasound acoustic waves in air, the schlieren light source is pulsed and matched to the frequency of the acoustics to ’freeze’ the acoustic wave in the air. Parameters that influence the schlieren image are identified and studied. These parameters are duty cycle, cut-off, ISO, light source phase delay and light source frequency. A horizontal cut-off is determined as the best, while it is determined that the other parameters can be adjusted dependent on the acoustic field. The acoustic field generated by double phased array is observed. Trap locations and types can be identified using the schlieren system. Quantification of the schlieren images has been attempted but without satisfactory results, mainly due to an interesting observation regarding a correlation between the camera focus distance and the pressure gradient. The schlieren system is a useful research tool and successful at visualising
the pressure gradient under the assumption that it is constant over the optical axis. Further research is needed to for quantification and to assess the impact of the observation. ... Read more

inPhocal, a startup company specialized in laser marking, developed a novel optical module to make the depth of focus more than 50mm, and wants to replace ink marking by this laser marking system in the production line when marking information on products. Distortion will always happen when marking 2D patterns onto 3D objects with laser, so deforming the 2D patterns beforehand to make marked pattern keep its shape is necessary. This process is called shape correction. Nowadays, the way to do this shape correction is very complicated and time- consuming. This thesis is focused on providing a new method that can do shape correction easily and fast. In this research, Non-Uniform Rational B-Spline (NURBS) is used in reconstructing the 3D surfaces and the shape correction will be based on these NURBS surfaces. With this NURBS-based shape correction, the pattern can be mapped on to 3D object fast without distortion. ... Read more

Flatness is an important surface tolerance requirement during the manufacturing of a part of an ASML machine. Flatness can be measured by either measuring the variation in height of the surface in one measurement (a plane-wise measurement) or by measuring the height on multiple xy-coordinates on this surface (point-wise measurement). Point-wise distance measurement sensors tend to have small physical dimensions, while having good specifications on resolution perpendicular to the measured surface. During a separate research, a gap is found in performing a point-wise flatness measurement while accounting for undesired stage deviations. The stage deviations are in the order of
micrometers, while the sensor accuracy is in the order of nanometers. The performance of a point-wise flatness measurement is therefore in a negative sense dominated by the stage inaccuracies rather than sensor performance. During this research the gap is filled by designing, building and testing a feasibility demonstrator which uses reference sensors to account for stage inaccuracies. By performing a live correction with either 1 or 3 reference sensors, the stage inaccuracy (in z, Rx and Ry) can be compensated for. The measured stage wobble for this specific setup is 3.2 µm, which is reduced to 0.13 µm when using three reference sensors. The method of using reference sensors for flatness measurements is shown to work, mainly when using 3 reference sensors.
... Read more