Wolfgang Riede

Lightcraft experiments in Germany

Vertical flight and pendulum experiments have been carried out with a simple paraboloid type lightcraft in the air-breathing mode. Pulsed laser energy of up to 240 J/pulse was delivered from a highly reproducible e-beam sustained CO2-laser at repetition rates up to 45 Hz. The lightcraft mass was varied in the range between 22 and 55 g. An average thrust of 1.1 N has been derived from the flight data and the highest impulse coupling coefficient found in the pendulum experiments was 33.3(DOT)10-5 Ns/J. A double shock wave was detected that leaves the thruster exit and an attempt was made to model the thrust, using a modification of Sedovs similarity solution for a blast wave. Finally, the propulsion requirements for the launch of a 10 kg mass into low Earth orbit are presented.

Comparative lightcraft impulse measurements

The impulse coupling coefficients of two radically different laser propulsion thruster concepts (lightcrafts), each 10 cm in diameter, have been measured under equal conditions using two different pendulum test stands. One test stand and one lightcraft of toroidal shape were provided by the U.S. Air Force Research Laboratory. The other test stand and a bell shaped (i.e. a paraboloid) lightcraft were those of the German Aerospace Center (DLR). All experiments employed the DLR electron-beam sustained, pulsed CO2 laser with pulse energies up to 400 J. The laser was operated with two configurations: 1) a stable resonator (flat beam profile); and, 2) an unstable resonator (ring shaped beam profile). A first series of experiments was carried out in the open laboratory environment. Propellant, therefore, was either the surrounding air alone, or Delrin as an added solid propellant. The coupling coefficient was determined as a function of the laser pulse energy. In a second series, the same experiments were repeated at various reduced pressure levels with the German lightcraft suspended in a vacuum vessel. This simulates the conditions of a transitional flight from within the atmosphere to outer space. As an additional parameter the specific mass consumption of Delrin (gram/Joule) was measured for each parameter set, allowing the determination of the average exhaust velocity in vacuum.

Vacuum laser damage test bench

This work summarizes the results from an extensive test campaign in which space-based laser optics were qualified for the upcoming ESA ADM-Aeolus mission. 14 different types of optical components from different suppliers were tested at the Nd:YAG laser wavelength according to the ISO standard 11 254 - 2 for multiple pulse testing. A new technique based on transient pressure sensing was developed to monitor the occurrence of damage on a sample surface exposed to a vacuum environment. Parallel testing of reference samples showed a distinct degradation under vacuum compared to atmospheric or pressurized environment. For all samples tested we found a typical behavior in the characteristic damage curves attained: A sharp drop in LIDT for small pulse numbers followed by a smooth decrease for larger pulse numbers (laser fatigue effect).

Ultrashort pulse damage of Si and Ge semiconductors

An experimental and theoretical investigation of ultrashort pulse damage thresholds of Si and Ge semiconductors has been carried out. As the source of laser radiation, a commercial sub picosecond Ti:Sapphire laser system has been used. It produces laser pulses of 0.5 mJ pulse energy at 1 kHz repetition rate, providing a Gaussian-like beam profile. Compressor tuning allowed for varying the pulse duration from 150 fs to 5.5 ps. The laser damage thresholds were measured in air and for this pulse duration range. The damage morphologies were investigated with various microscopic inspection techniques like Nomarski DIC, atomic force and white light interference microscopy.

Analysis of the air – vacuum effect in dielectric coatings

As a consequence of the ongoing interest for deployment of laser systems into space, optical coatings have to be developed which allow for reliable long term operation under vacuum conditions. Extensive laser damage tests for space qualification of laser optics have recently been performed at the DLR and LZH laser damage test facilities in the IR, VIS, and UV spectral range within the ESA-ALADIN (Atmospheric Laser Doppler Instrument) test campaign. These tests have consistently revealed the degradation of the LIDT values for e-beam evaporated dielectric coatings under vacuum environment, which occurred independently of wavelength and type of coating (HR or AR) and other parameters. Dense coatings like IAD-based coatings, on the other hand, did not show this effect. Water desorption and diffusion processes seem to mediate the degradation under vacuum exposure.

Laser-induced hydrocarbon contamination in vacuum

We investigated laser-induced deposition processes on BK7 substrates under the influence of pulsed Q-switched Nd:YAG laser radiation, starting from small toluene partial pressures in a background vacuum environment. The composition and structure of the deposit was analyzed using microscopic methods like Nomarski DIC, dark-field and white-light interference microscopy, TEM, EDX and XPS. We found a distinct threshold for deposition built-up dependant on the partial pressure of toluene (0.2 J/cm2 at 0.1 mbar, 0.8 J/cm2 at 0.01 mbar toluene). The deposits strictly followed the spherical geometry of the laser spot. No deposit accumulated on MgF2 AR coated BK7 samples even at high toluene partial pressures. The onset of deposit was accompanied by periodic surface ripples formation. EDX and XPS analysis showed a carbon-like layer which strongly absorbed the 1 μm laser radiation. The typical number of shots applied was 50 000. In addition, long term lifetime tests of more than 5 Mio. shots per site were run.

Laser qualification testing of space optics

Laser optics being used in space laser systems are usually exposed to high vacuum conditions under the absence of air or oxygen. In the past, several space-based laser missions have suffered from anomalous performance loss or even failure after short operation times. To mitigate the risks involved with long-term operational conditions, a laser damage test bench has been developed and is operated at the German Aerospace Center (DLR) to test laser optics in the IR, VIS, and in the UV spectral range. The testing is performed under application oriented conditions, i.e. under high-vacuum using dry pump systems. The main goal of the test campaign is to identify the critical components in terms of their laser damage threshold for very high pulse numbers applied per site. Characteristic damage curves according to ISO 11254 are evaluated for each component under investigation for up to 10 000 shots per site. The characteristic damage curves are used for the estimation of the performance at very high pulse numbers.

Results of a round-robin experiment in multiple-pulse LIDT measurement with ultrashort pulses

For the development of standard measurement procedures in optics characterization, comparative measurement campaigns (Round-robin experiments) are indispensable. Within the framework of the CHOCLAB project in the mid-90s, several international Round-robins were successfully performed qualifying procedures for e. g. 1 on 1-LIDT, laser-calorimetry and total scattering. During the recent years, the demand for single pulse damage investigations has been overtaken by the more practically relevant S on 1-LIDT. In contrast to the industrial needs, the comparability of the multiple-pulse LIDT has not been proven by Round-robin experiments up to now. As a consequence of the current research activities on the interaction of ultra-short pulses with matter as well as industrial applications, numerous fs-laser systems become available in universities and research institutes. Furthermore, special problems for damage testing may be expected because of the intrinsic effects connected with the interaction of ultrashort pulses with optical materials. Therefore, a Round-robin experiment on S on 1-damage testing utilizing fs-pulses was conducted within the framework of the EUREKA-project CHOCLAB II. For this experiment, seven parties investigated different types of mirrors and windows. Most of the partners were guided by the International Standard ISO 11254-2, but one partner employed his own damage testing technique. In this presentation, the results of this comparative experiment are compiled demonstrating the problems induced by special effects of damage testing in the ultra-short pulse regime.

Laser-induced contamination on space optics

Operation of high fluence pulsed laser systems in space imposes various risks to optical components involved. Volatile organic components are omnipresent in vacuum vessels housing space-borne laser systems and can be the source for selective contamination of optics. Laser systems may respond very sensitively to absorption increases of their multiple optical surfaces leading to inacceptable transmission losses and system degradation. In the recent past, thorough and long term laser tests, performed at the optics qualification laboratories at DLR and at ESTEC using space relevant and model substances, have revealed the onset, the built-up, and the later stages of the deposition process. It was found that these deposits tend to accumulate preferably on the laser footprint area of the optic. Observed thicknesses are on the order of several tens of nanometers, which can be sufficient to induce noticeable absorption. Sensitive techniques for insitu and ex-situ monitoring of these molecular contaminative effects under vacuum conditions were developed and are applied successfully. They are summarized in this paper, along with the phenomena, which are significant for the appearance of deposits. In addition, adverse conditions, which are favorable for provoking deposits, are communicated. Finally, mitigative and preventive methods are discussed.

Growth mechanisms for laser induced contamination on space optics in vacuum

We have investigated the growth mechanisms for laser induced contamination of space optics in vacuum, particularly during the early stages of the deposit formation. Experiments have been performed in vacuum to study the influence of the environmental conditions and the condition of the optical surface, using a variety of physical and chemical techniques. In particular, different methods of conditioning the surface prior to irradiation and cleaning the surface after irradiation have been tested.