Paul Allenspacher

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.

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.

Risk mitigation in spaceborne lasers

Spaceborne lidars carry much promise for Earth observation and interplanetary missions to measure atmospheric parameters (wind velocity, optical extinction or species concentrations) and planet topologies. As the first European lidar mission, the European Space Agency is developing a Doppler wind lidar, ALADIN, to be launched on board ADM-Aeolus in 2008. ALADIN is a pulsed laser, emitting about 120 mJ of pulse energy in the UV. The mission duration is envisaged to be three years, which corresponds to several billion emitted pulses, thus imposing very stringent criteria on the longevity of the system. Laser-induced damage is one of the most significant issues here, in particular since laser-induced damage in space vacuum is still poorly understood. The European Space Agency has therefore established a test campaign to measure the power handling of all the instrument optics with laboratories in Germany, Italy, the Netherlands, the United Kingdom and France participating. Measurements are conducted at three wavelengths (1064nm, 532nm and 355nm) and with the introduction of several contaminants. The presentation covers laser-induced damage risk mitigation, the ESA test campaign and some test results.

An empirical investigation of the laser survivability curve

In this paper, we report on the first steps in an empirical investigation into the nature of the laser survivability curve. The laser survivability curve is the onset threshold as a function of shot number. This empirical investigation is motivated by the desire to design a universal procedure for the measurement of the so-called S on 1 damage threshold. Analysis is carried on the test results for first results from a large set of planned measurements from identical samples produced for this investigation. The sample set and test conditions are discussed. A pair of measurements, one taken at atmospheric pressure and one at vacuum are introduced and analyzed as an example. Interim observations on the nature of the laser survivability curve, and its determination to be used in the remainder of this investigation based on this initial look, are presented at the conclusion of this paper.

S on 1 testing of AR and HR designs at 1064nm

In this paper, we present test results and involved procedures of a comprehensive test campaign for S on 1 testing of laser optics with large test areas allowing the generation of a profound test database for further analysis. This database will serve as a starting point for an empirical study of the lifetime of laser optics, which will be discussed in companion paper somewhere in these proceedings. The optics are designed to operate as anti-reflective or high-reflective components at the respective test wavelengths for 0° angle-of-incidence. Both, coatings and substrates of 2.0 inch diameter are produced from the same batches to be as identical as possible. There were two different coating technologies used, e-beam and IAD e-beam, to explore a possible effect of the coating process on the long term laser irradiation behavior. The laser damage test bench is operated with a laser source delivering laser pulses in a single longitudinal mode at a repetition frequency of 100 Hz. The beam profile is of a Gaussian-shape and of high spatial quality at the fundamental Nd:YAG laser wavelength with a pulse duration of 3.5 ns at 1064 nm. Typical beam diameters on the samples were 400 μm, and usually more than 500 test sites are irradiated in one test to achieve statistical significance. The laser test procedure itself is adapted from the ISO standard 11254-2 for multiple pulse irradiations, and the LIDT evaluation is done accordingly.