Han Woong Yoo

Automated spherical aberration correction in scanning confocal microscopy

Mismatch between the refractive indexes of immersion media and glass coverslips introduces spherical aberrations in microscopes especially for high numerical aperture objectives. This contribution demonstrates an automated adjustment of the coverslip correction collar in scanning confocal microscopy to compensate for spherical aberrations due to coverslip thickness mismatch. With a motorized coverslip correction collar, the adjustment procedure consists of xz image scans, image processing, correction quality evaluation, the mismatch estimation, and eventually the optimal adjustment of the correction collar. For fast correction with less photodamage, coarse-fine Gaussian fitting algorithms are proposed and evaluated with various specimen for their estimation accuracy. The benefits of the proposed automated correction are demonstrated for various coverslips with biological specimens, showing the optimized resolution of the confocal microscope.

Automated adjustment of aberration correction in scanning confocal microscopy

This contribution demonstrates an automated adjustment of the coverslip correction collar in scanning confocal microscopy to compensate for aberrations caused by coverslip thickness mismatch. An axial image model is derived for filtering the measured axial image to improve the signal to noise ratio. To find the best collar alignment, 70 axial scans equally spaced over the actuation range are recorded and evaluated automatically. The axial scans reveal that different coverslips have a shift of the maximum intensity for optimal imaging along the collar angle. Correction quality such as the maximum intensity or sharpness of the image are examined and used to find the optimal adjustment. The proposed automated correction is demonstrated with two different coverslips and two biological specimen showing the improved resolution of the obtained confocal microscopy images.

High bandwidth deflection readout for atomic force microscopes

This contribution presents the systematic design of a high bandwidth deflection readout mechanism for atomic force microscopes. The widely used optical beam deflection method is revised by adding a focusing lens between the cantilever and the quadrant photodetector (QPD). This allows the utilization of QPDs with a small active area resulting in an increased detection bandwidth due to the reduced junction capacitance. Furthermore the additional lens can compensate a cross talk between a compensating z-movement of the cantilever and the deflection readout. Scaling effects are analyzed to get the optimal spot size for the given geometry of the QPD. The laser power is tuned to maximize the signal to noise ratio without limiting the bandwidth by local saturation effects. The systematic approach results in a measured -3 dB detection bandwidth of 64.5 MHz at a deflection noise density of 62fm/√Hz.

MEMS-based lidar for autonomous driving

Lidar, the acronym of light detection and ranging, has received much attention for the automotive industry as a key component for high level automated driving systems due to their high resolution and highly accurate 3D imaging of the surroundings under various weather conditions. However, the price and resolution of lidar sensors still do not meet the target values for the automotive market to be accepted as a basic sensor for ensuring safe autonomous driving. Recent work has focused on MEMS scanning mirrors as a potential solution for affordable long range lidar systems. This paper discusses current developments and research on MEMS-based lidars. The LiDcAR project is introduced for bringing precise and reliable MEMS-based lidars to enable safe and reliable autonomous driving. As a part of development in this project, a test bench for the characterization and performance evaluation of MEMS mirror is introduced. A recently developed MEMS-based lidar will be evaluated by various levels of tests including field tests based on realistic scenarios, aiming for safe and reliable autonomous driving in future automotive industry.ZusammenfassungLidar, ein Akronym für Light Detection And Ranging, erhielt als Schlüsselkomponente für autonome Fahrsysteme in der Automobilindustrie viel Aufmerksamkeit, da es hochauflösende und hochgenaue 3D-Bilder der Umgebung bei verschiedensten Wetterbedingungen liefert. Derzeit entsprechen die Preise und die Auflösung der kommerziell verfügbaren Lidar-Sensoren jedoch noch nicht den Zielanforderungen, um als Basissensor zur Gewährleistung der Sicherheit während des autonomen Fahrens akzeptiert zu werden. MEMS-basierte Scan-Spiegel waren als potentielle Lösung für ein leistbares weitreichendes Lidar-System Fokus einer kürzlich durchgeführten Arbeit. Diese Arbeit erörtert die derzeitige Entwicklung und Forschung von MEMS-basierten Lidar-Systemen und stellt das LiDcAR-Projekt vor, dessen Ziel es ist, ein präzises und verlässliches MEMS-basiertes Lidar-System für autonomes Fahren zu entwickeln. Als Teil-Entwicklung dieses Projekts wird ein Prüfstand zur Charakterisierung und Evaluierung von MEMS Scan-Spiegeln vorgestellt. Das kürzlich entwickelte MEMS-basierte Lidar-System wird auf verschiedenen Testebenen evaluiert, inklusive Feldtests, basierend auf realistischen Szenarien, mit dem Ziel, in Zukunft sicheres und verlässliches autonomes Fahren zu gewährleisten.

Comparison of modeling-free learning control algorithms for galvanometer scanner's periodic motion

For an accurate and precise periodic scanning motion of a galvanometer scanner, this paper presents iterative learning control (ILC) that is designed and implemented in the frequency domain to compensate for system nonlinearities, such as static friction. For a case that system identification in advance is difficult due to the nonlinearities, the frequency-domain ILC itself incorporates and performs system identification during iterative learning, as modeling-free inversion-based iterative control (IIC). A learning law is derived for a nonlinear system, where the internal system identification is formulated as an estimation problem of a Jacobian matrix that represents the system. In order to find a suitable Jacobian estimation method in the IIC, this paper compares Broydens method and the linear method, as well as the secant method. To decease the algorithms, the IIC is operated only at the harmonic frequencies of the motion trajectory. In the implementation of the modeling-free IIC, the control input update is explicitly separated from the Jacobian estimation, so that the IIC can still decrease the motion error even when the Jacobian estimation is interrupted for stability. The experimental results demonstrate that the secant method is the best of the three for raster scanning due to its fast learning and high tracking performance.

Non-parametric robustness analysis for feedback motion control for a high precision stage with large mass uncertainty

Robust feedback control with large parameter uncertainties in a high precision stage with large sample mass variation is presented. Instead of expressing the nominal plant and its uncertainty by parametric models, they are expressed using the identified frequency response data. The open loop transfer function is mapped onto a Nyquist plot, to assess robust stability. Next, the complementary sensitivity function is derived from this data to describe tracking performance, which is verified by experiments. The advantage is that the parameter estimation step is omitted, without compromising robust stability and performance.

Adaptive optics for confocal laser scanning microscopy with adjustable pinhole

The pinhole plays an important role in confocal laser scanning microscopy (CLSM) for adaptive optics (AO) as well as in imaging, where the size of the pinhole denotes a trade-off between out-of-focus rejection and wavefront distortion. This contribution proposes an AO system for a commercial CLSM with an adjustable square pinhole to cope with such a trade-off. The proposed adjustable pinhole enables to calibrate the AO system and to evaluate the imaging performance. Experimental results with fluorescence beads on the coverslip and at a depth of 40 μm in the human hepatocellular carcinoma cell spheroid demonstrate that the proposed AO system can improve the image quality by the proposed calibration method. The proposed pinhole intensity ratio also indicates the image improvement by the AO correction in intensity as well as resolution.

Intelligente Kameras in der Messtechnik

SummaryDue to their reconfigurability and sensor-near signal processing, smart cameras are ideally suited for applications in optical metrology. This article describes ongoing research activities in this area and demonstrates the efficient implementation of complex algorithms on limited hardware through intelligent systems integration. This enables the realization of fast wavefront-sensors for adaptive optics and real-time strain sensors based on speckle-metrology.ZusammenfassungIntelligente Kameras sind aufgrund ihrer Rekonfigurierbarkeit und sensornahen Signalverarbeitung für Anwendungen in der optischen Messtechnik prädestiniert. Dieser Artikel zeigt anhand konkreter Beispiele aus laufenden Forschungsaktivitäten, dass sich komplexe Algorithmen durch intelligente Systemintegration auch auf eingeschränkter Hardware effizient umsetzen lassen. Dadurch können schnelle Wellenfrontsensoren für die adaptive Optik sowie auf Speckle-Messtechnik basierende Echtzeit-Dehnungssensoren realisiert werden.