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Aviation is among the social sectors most impacted by the COVID-19 pandemic, and at the same time has contributed to the rapid global spread of the SARS-CoV-2 virus. SARS-CoV-2 is one of the coronaviruses that have led to outbreaks such as MERS-CoV in the past. This group of pathogens, as well as others that may be unknown at this time, will continue to challenge our society in the future. In order to be able to react better, a research training group was established at DLR in cooperation with 6 institutes, which will develop interdisciplinary approaches to researching and combating current and future pandemics. Engineers, physicists, software developers, biologists and physicians are working closely together on new concepts and the development of interdisciplinary knowledge in order to better control and contain future pandemics and to be able to react in a more targeted manner. One focus is the reduction of germ contamination in airplanes but also in other means of public transport such as buses and trains. In this review, we provide an overview of the baseline situation and possible approaches to address future pandemic challenges.

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SIGNIFICANCE: Fast and reliable detection of infectious SARS-CoV-2 virus loads is an important issue. Fluorescence spectroscopy is a sensitive tool to do so in clean environments. This presumes a comprehensive knowledge of fluorescence data. AIM: We aim at providing fully featured information on wavelength and time-dependent data of the fluorescence of the SARS-CoV-2 spike protein S1 subunit, its receptor-binding domain (RBD), and the human angiotensin-converting enzyme 2, especially with respect to possible optical detection schemes. APPROACH: Spectrally resolved excitation-emission maps of the involved proteins and measurements of fluorescence lifetimes were recorded for excitations from 220 to 295 nm. The fluorescence decay times were extracted by using a biexponential kinetic approach. The binding process in the SARS-CoV-2 RBD was likewise examined for spectroscopic changes. RESULTS: Distinct spectral features for each protein are pointed out in relevant spectra extracted from the excitation-emission maps. We also identify minor spectroscopic changes under the binding process. The decay times in the biexponential model are found to be ( 2.0 ± 0.1 ) ns and ( 8.6 ± 1.4 ) ns. CONCLUSIONS: Specific material data serve as an important background information for the design of optical detection and testing methods for SARS-CoV-2 loaded media.

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The properties of a laser beam coupled out of a standard unstable laser resonator are heavily dependent on the chosen resonator magnification. A higher magnification results in a higher output coupling and a better beam quality. But in some configurations, an unstable resonator with a low output coupling in combination with a good beam quality is desirable. In order to reduce the output coupling for a particular resonator, magnification fractions of the outcoupled radiation are reflected back into the cavity. In the confocal case, the output mirror consists of a spherical inner section with a high reflectivity and a flat outer section with a partial reflectivity coating. With the application of the unstable resonator with reduced output coupling (URROC), magnification and output coupling can be adjusted independently from each other and it is possible to get a good beam quality and a high power extraction for lasers with a large low gain medium. The feasibility of this resonator design is examined numerically and experimentally with the help of a chemical oxygen iodine laser.

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The application of an off-axis negative-branch unstable resonator to an active medium of rectangular geometry is examined. The presented unstable resonator consists of spherical mirrors and a scraper mirror. The adaptation to the rectangular cross section is performed by the scraper, which takes two different shapes. One shape resembles a rectangular bracket "[" and the other resembles the letter "L." The [ and L configurations correspond to a shift of the optical axis away from the center of the cross section, toward one of the edges or toward one of the corners, respectively. Both scraper setups are examined numerically and experimentally. Experiments are performed with a multikilowatt class chemical oxygen iodine laser. The active medium is characterized by a low amplification coefficient. Measured results of the intensity distribution in the far field and of the phase distribution in the near field are shown for both resonator configurations. Using the same resonator magnification, the setup with the L-shaped scraper has a lower output coupling and, therefore, a higher output power and a slightly higher beam divergence. The L-shaped scraper configuration is able to cover the gain medium completely.

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A double-pass negative-branch hybrid resonator is applied to a 10 kW chemical oxygen iodine laser. The resonator is folded in such a way that the dimension of the stable direction is reduced. The intensity distributions of the near and far fields of the laser beam and the sensitivity against tilts of the output mirror are investigated. A comparison between theory and experiment is performed. It is shown that the folded hybrid resonator provides a better beam quality and therefore a higher power density in the far field than a single-pass hybrid resonator. The sensitivity against tilts of the resonator mirrors in the stable direction is reduced.

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A modified negative branch confocal unstable resonator (MNBUR) was coupled to the chemical oxygen-iodine laser (COIL) device of the German Aerospace Center. It consists of two spherical mirrors and a rectangular scraper for power extraction. Experimentally measured distributions of the near- and far-field intensities and the near-field phase were found in close agreement to numerical calculations. The extracted power came up to approximately 90% of the power as expected for a stable resonator coupled to the same volume of the active medium. The output power revealed a considerable insensitivity towards tilts of the resonator mirrors and the ideal arrangement of the scraper was found to be straightforward by monitoring the near-field distributions of intensity and phase. The beam quality achieved with the MNBUR of an extremely low magnification of only 1.04 was rather poor but nevertheless in accordance with theory. The demonstrated consistency between theory and experiment makes the MNBUR an attractive candidate for lasers that allow for higher magnification. In particular, it promises high brilliance in application to 100 kW class COIL devices, superior to the conventional negative branch confocal unstable resonator.

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A new type of unstable resonator, suitable for a laser with a large medium cross section and a small or median output coupling, is presented. The resonator configuration, a modification of a negative-branch confocal unstable resonator, is numerically investigated. The basis of the theory is the Fresnel-Kirchhoff integral equation, and the calculations describe a passive resonator. With respect to output mirror tilting, the calculations confirm that the modified negative-branch confocal unstable resonator is less sensitive to mirror misalignments than the conventional negative-branch confocal unstable resonator. Furthermore, the modified resonator improves the beam quality in comparison with the conventional unstable resonator.

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A rectangular negative branch off-axis hybrid resonator was coupled to a 10 kW class chemical oxygen-iodine laser. Resonator setup and alignment turned out to be straightforward. The extracted power was 6.6 kW and reached approximately 70% of the power for an optimized stable resonator. The divergence of the emitted laser beam in the unstable direction was lower than two times the diffraction limit. Experimentally measured margins for mirror misalignment were found in close agreement with numerical calculations.