Laser safety is starting to play an increasingly important role, especially when the laser is used as a tool. Passive laser safety systems quickly reach their limits and, in some cases, provide inadequate protection. To counteract this, various active systems have been developed. Flexible and especially textile-protective materials pose a special challenge. The market still lacks personal protective equipment (PPE) for active laser safety. Covering these materials with solar cells as large-area optical detectors offers a promising possibility. In this work, an active laser protection fabric with amorphous silicon solar cells is presented as a large-scale sensor for continuous wave and pulsed lasers (down to ns). First, the fabric and the solar cells were examined separately for irradiation behavior and damage. Laser irradiation was performed at wavelengths of 245, 355, 532, and 808 nm. The solar cell sensors were then applied directly to the laser protection fabric. The damage and destruction behavior of the active laser protection system was investigated. The results show that the basic safety function of the solar cell is still preserved when the locally damaged or destroyed area is irradiated again. A simple automatic shutdown system was used to demonstrate active laser protection within 50 ms.

The combustion of single liquid oxygen droplets in gaseous hydrogen is investigated experimentally under microgravity conditions to shed light on spray combustion processes in rocket engines. Using a drop tower apparatus, experiments are performed varying the ambient pressure between 0.1-5.7 MPa, which corresponds to a reduced pressure of oxygen p(r) between 0.02-1.12. The combustion is investigated using high-speed shadowgraph imaging to track the droplet shape and OH-chemiluminescence to identify the flame zone. At low pressures (p(r) < 0 . 15 ), the droplet shape is found to change significantly during combustion likely due to the formation of a water ice layer around the droplet. Small jets of oxygen appear to break out of this ice layer, leading to an observed increase in linear and angular momentum of the droplet. At higher pressures, the visible effect of ice formation near the droplet surface decreases. The combustion process at different pressures in the subcritical and the supercritical regime is compared and discussed. The pressure has a limited influence on the flame standoff ratio, whereas it influences the burning rate constant substantially. Specifically, the experimental data suggest a maximum of the burning rate constant near the critical pressure, which is consistent with several experiments on hydrocarbon droplet combustion. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The induction times of pairs of droplets of n-decane and n-tetradecane have been investigated experimentally in microgravity. The results were compared to the induction times of single droplets. N-decane is a well matching model fuel for synthetic kerosene while n-tetradecane is well matching the ignition behavior of synthetic diesel fuel. All droplets had a diameter of 0.8 mm. The experiments were carried out at pressures of 0.3 and 0.5 MPa and in the temperature range of 650 to 850 K. The spacing of the droplet pairs was 1, 2, 4 and 6 mm (center distance). The LIF on formaldehyde was applied. Formaldehyde is the stable product of the cool flame reaction and is formed parallel to the OH-radical whose concentration in the cool flame is too low for detection. As formaldehyde is immediately decomposed at hot ignition, the whole ignition process is displayed by applying this technique. The results show that for both fuels the induction times of a pair of 1 mm distance are slightly longer than for a single droplet. Beyond 1 mm the induction times drop below that of a single droplet. The 6 mm spacing showed the shortest induction times. With widening gap between the droplets, the instant of hot ignition moves from near the droplets interspace to in between the droplets and moves back to an arbitrary outside location at 6 mm gap. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

To investigate the fundamentals of liquid oxygen droplet combustion in hydrogen under microgravity conditions, a drop tower apparatus has been developed. In a cryogenic combustion chamber cooled with liquid nitrogen, single oxygen droplets with a diameter of 1 mm are suspended at the tip of a thin suspender. Ignition is accomplished after microgravity conditions are reached by a near-infrared laser, which generates a plasma spark positioned in the immediate vicinity of the oxygen droplet. The subsequent combustion is investigated with various optical diagnostics. Shadowgraph imaging is used to determine the initial droplet size and the droplet diameter regression with high temporal resolution. In addition, the position and diameter of the flame are determined by OH chemiluminescence imaging. The oxygen/hydrogen combustion at two different pressure levels is reported. At a pressure of 0.1 MPa, irregular structures appear to form on the droplet surface during the combustion process, which lasts 137 ms before the droplet is completely consumed. The formation of these irregular structures is consistent with the condensation and freezing of water vapor in the cold-temperature region near the droplet surface. At a higher pressure of 4 MPa (reduced pressure pr,O2 =0.79), the water ice formation is significantly reduced due to the shorter burning time of 66 ms and the closer proximity of the flame to the droplet surface. The measured burning constant is k0.1 = 5.5 mm2/s at 0.1 MPa and k4 = 7.1 mm2/s at 4 MPa, respectively. This increase with increasing pressure is consistent with the smaller flame standoff distance.

The interaction between two neighboring n-decane droplets during the autoignition process in air was experimentally investigated under microgravity conditions in the drop tower Bremen. The initial droplet diameter was 0.8 mm in all experiments. Single droplets and droplet pairs with a center distance of 1, 2, 4 and 6 mm were investigated. The air temperature was varied between 650 and 850 K with an increment of 25 K. The examined pressure conditions were 0.3 and 0.5 MPa. This results in a total number of 90 drop experiments. The ignition process was observed applying the laser induced fluorescence (LIF) on formaldehyde with high temporal and spatial resolution. Both, the cool flame process and the hot flame ignition could be measured regarding induction times and the temporal and spatial evolution of the formaldehyde formation.

We describe a specially designed diode pumped solid state laser system based on the disk laser architecture for combustion diagnostics under microgravity (μg) conditions at the drop tower in Bremen. The two-stage oscillator-amplifier-system provides an excellent beam profile (TEM00) at narrowband operation (Δλ < 1 pm) and is tunable from 1018 nm to 1052 nm. The laser repetition rate of up to 4 kHz at pulse durations of 10 ns enables the tracking of processes on a millisecond time scale. Depending on the specific issue it is possible to convert the output radiation up to the fourth harmonic around 257 nm. The very compact laser system is integrated in a slightly modified drop capsule and withstands decelerations of up to 50 g (>11 ms). At first the concept of the two-stage disk laser is briefly explained, followed by a detailed description of the disk laser adaption to the drop tower requirements with special focus on the intended use under μg conditions. In order to demonstrate the capabilities of the capsule laser as a tool for μg combustion diagnostics, we finally present an investigation of the precursor-reactions before the droplet ignition using 2D imaging of the Laser Induced Fluorescence of formaldehyde.

For special applications in spectroscopy, tunable single-frequency lasers are required to excite selectively relevant molecules. Amplified Yb:YAG disk lasers provide one opportunity for such lasers with a number of advantageous properties. Nevertheless, changing the wavelength from shot to shot at kHz repetition rates - desired e. g. for background subtraction or two-wavelength methods - remains challenging.We present results from two approaches, which in combination allow for fast wavelength switching of the oscillator and for extension of the tunability range of the laser system. For wavelength switching high voltage (some kV) is applied to a special birefringent filter (Lyot filter). Polarization rotation induced by the electric field yields losses at the wavelength emitted without voltage:the laser emits at a "new" wavelength with the highest gain. This new wavelength is determined by multiples of the free spectral range of the intra-cavity etalon used for single-frequency operation.The second stage of the laser system comprises an Yb:YAG regenerative amplifier. To ensure that parasitic lasing of this laser at the gain maximum is suppressed effectively, an additional birefringent filter is inserted into the amplifier. Adjusting this filter suppresses parasitic lasing and extends the tunability range of the system by a factor of more than 4.

We investigate the characteristics of a thin disk laser system based on the master oscillator-power amplifier design. The amplifier emits parasitic laser oscillation (PL) when the wavelength of the tunable narrow-bandwidth seed pulse is detuned more than 4 nm from the gain maximum at 1030 nm. For suppression of this unwanted PL, a birefringent filter (Lyot filter) was inserted into the amplifier cavity in order to generate wavelength selective losses, especially at the gain maximum of the Yb:YAG crystal. It is shown that the tunability range of the laser system can be extended by a factor of 4, when a properly chosen Lyot filter is applied and its angle of rotation is adapted to the seed wavelength of the system.