Laser Zentrum Hannover

Currently planned concepts for laser-based fusion systems critically rely on high-quality optical coatings for the creation and transport of the required short and ultra-short pulses. Both the deposition on the required large surfaces and characterization of these coatings currently pose challenges that need to be addressed to enable the efficient production of optics for future laser-based inertial confinement fusion (ICF) power plants. These challenges are currently investigated at LZH in several research projects: A novel concept for high-quality ion-beam sputtering coatings will enable the homogeneous coating of optics with diameters of up to two meters. Furthermore, as part of the grants “PriFUSIO” and “SHARP” in the German fusion energy program “Fusion2040,” the LZH develops innovative approaches for high-damage coatings as well as their characterization in large-area format on optics with diameters of up to 400mm. The contribution will present the current state of both projects, including current results on coating homogeneity, design approaches, and laser-induced damage threshold (LIDT) measurements in the PriFUSIO project.

BackgroundSymptomatic vitreous opacities (SVO) are a condition that can significantly affect patientsʼ quality of life. This online survey aims to contribute to the scientific understanding of symptomatic vitreous opacities, by investigating the subjective burden and individual perception of floaters, using a novel tool for field of view reconstruction. In addition, correlations with demographic data and refractive status will be analysed to identify patterns and potential influencing factors.MethodsWe introduce a VO-related online survey covering patient education, experience with floaters and treatment, refraction error and eye diseases, quality of life, and a new sketching tool to simulate the individual field of view with VO. The data were collected between August 2022 and November 2023 and are analysed for correlations; participantsʼ answers are compared to the sketching tool results. We used linear regression with gradient descent to test whether a subjective suffering index from the questionnaire can be matched with an objective suffering index from the new sketching tool.ResultsA total of 1502 people participated in the survey, of whom, 1384 reconstructed their field of view by positioning a total of 10,571 VO. There was no robust correlation between the sketched floaters and the subjective degree of suffering. Translucent VO types are more frequent in younger attendees. The dataset shows a higher prevalence of myopic participants and a lower prevalence of hyperopes with VO compared to global data.ConclusionWe were able to collect the largest dataset to date quantifying individual experience with floaters. As evidenced by the large number of participants with SVO and individual remarks on their psychological burden, greater effort is needed to improve the care for floater patients.

Mode-locked fiber lasers using frequency shifted feedback (FSF) for pulse formation relies on the interplay between the frequency shift of an acousto-optic modulator (AOM), Kerr nonlinearity in optical fibers and optical filtering effect to prevent CW operation leading to a pulsed output [1]. To ensure minimal cavity losses, the AOM is typically operated with maximum possible transmission. However, many FSF lasers reported to date suffer from relatively high pulse intensity fluctuations of several percent. In this work, we propose taking advantage of the variability of AOM transmission in controlling cavity dynamics. Following the procedure hereafter, it was possible to investigate and minimize the pulse intensity fluctuations in a Tm-doped FSF fiber laser.

A time-dependent numerical laser model was developed, which is affirmed by experimental results of two different pulsed Alexandrite laser systems, a Q-switched and a cavity-dumped Q-switched system. The model incorporates temperature-dependent Alexandrite transitions and measured data (beam caustic of the pump) to generate conditions similar to those in the laboratory. Comparisons were made for the first time between the simulated and experimental results, and excellent agreement was found in output energy, pulse duration, temporal shape, and laser wavelength.

Gain dynamics play a crucial role in the design of laser oscillators and amplifiers. In order to expand the understanding of gain dynamics into the spectral 2 μm region, this study presents the experimental characterization of transfer functions in thulium-doped fiber amplifiers associated with the 3F4 → 3H6 transition that governs emission in this spectral range. We investigate the transfer functions regarding magnitude and phase for out-of-band pumping at 790 nm by populating the 3H4 level. The results are in particular useful to determine the effective frequency for power scaled thulium-doped fiber amplifiers based double-clad geometry.

A method of determining the thermal properties of thin-film filter materials from spectrophotometric measurements is proposed. Its feasibility is shown by extracting the material properties from simulated transmission spectra imprinted with real world measurement errors.

The paper investigates purely dielectric broadband mirrors as an alternative to metallic optics for space applications. Designs, materials, and production for different space missions are presented, as well as results and advantages over metallic coatings.

Abstract 3D printing is one of the key technologies in space exploration. The disparity in gravitational forces between Earth and space presents both challenges and opportunities with regard to material handling. This article examines the potential of employing ultrasonic levitation as a handling tool for substrate-free additive manufacturing processes in microgravity environments. Through preliminary experiments, we demonstrate the feasibility of manipulating polymer powders using acoustic fields while concurrently melting the levitated material. Subsequent experiments conducted in our drop tower facility confirm our ability to manipulate particles with acoustic traps under microgravity conditions. Building upon these findings, we outline plans to further advance our research using an expanded acoustic levitation system capable of three-dimensional object manipulation. Our objectives include moving and orienting large components beyond the wavelength limit in microgravity, manipulating granular raw material while melting it in proximity to the print part, and achieving a semi-continuous fusion of print material with the print part. Therefore, we present an intelligent control strategy based on the results of a digital twin simulation. Furthermore, we utilize a stereo camera combined with computer vision as feedback for the control system to ensure precise handling of the manipulated objects and particles. This study represents a significant advance toward the realization of efficient substrate-free additive manufacturing processes in microgravity environments, with potential applications for in-space manufacturing. Ultimately, this could result in long-term space missions becoming less reliant on supply deliveries, thus reducing cost and additionally enabling faster response to unforeseen issues.