Helmut Schröder

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.

Investigation of laser induced deposit formation under space conditions

In this paper comprehensive investigations of laser induced deposit formation are reported. In a high vacuum chamber (p < 10-6 mbar) different space relevant materials containing epoxy, silicone and polyurethane based components were tested under space conditions. The experiments were performed with a pulsed Nd:YAG laser with peak fluences up to 2.5 J/cm2 at 355 nm wavelength and 3 ns pulse width. Additional tests were performed with an UV cw laser diode at 375 nm and 10 mW mean power. The onset and growth of the deposits was monitored in-situ and online by UV induced fluorescence imaging. The influence of roughness, temperature and chemical composition of the optical surface on the deposition process was investigated. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was used for chemical characterization of the deposits. Furthermore the influence of deposits on the UV-transmission of the optics was estimated.

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.

Non-linear optical frequency conversion crystals for space applications

Reliable, long term operation of high-power laser systems in the Earth orbit is not a straightforward task as the space environment entails various risks for optical surfaces and bulk materials. The increased operational risk is, among others, due to the presence of high energy radiation penetrating the metallic shielding of satellites and inducing absorption centers in the bulk of optical components, and vacuum exposure which can deteriorate coating performance. Comprehensive testing for analyzing high-energy radiation effects and mitigation procedures were performed on a set of frequency conversion crystals and are discussed in this paper. In addition to a general resistance to space environmental effects, the frequency conversion crystals were subjected to a comparative analysis on optimum third harmonic efficiency, starting from pulsed 1064 nm laser radiation, with the goal of exceeding a value of 30%. Concomitant modeling supported the selection of crystal parameters and the definition of crystal dimensions.

In situ observation of UV-laser-induced deposit formation by fluorescence measurement

We investigated the formation of UV laser induced deposits on uncoated and coated fused silica optics under vacuum conditions in presence of outgassing materials. As contamination samples epoxy, silicone and polyurethane containing materials were used. To realize low partial pressures of the contaminants in the gas phase they were slightly heated (40°C). The formation of the depositions was monitored in situ and online by detecting the fluorescence emission of the deposits, excited by the UV laser beam. The influence of different optical coatings on the deposit formation was studied. By analysing the surface profiles of the deposits, growth rates were estimated. Time-of-flight secondary ion mass spectroscopy was used for chemical characterization of the deposits.

Investigation of UV laser induced depositions on optics under space conditions in presence of outgassing materials

We have investigated the formation of UV laser induced deposits on uncoated fused silica optics under simulated space conditions in presence of outgassing materials at 30°C and 100°C. We used a frequency tripled Nd:YAG laser with 355 nm wavelength, 3 ns pulse length and 100 Hz repetition rate. Optics were exposed to fluence values in the range of 0.5 – 1.0 J/cm2. As contamination samples epoxy, silicone and polyurethane containing materials were used. The depositions were monitored online and in-situ by measuring the fluorescence intensity distribution with CCD cameras, where the UV laser beam itself served as excitation source for fluorescence emission. This method allows for a very sensitive detection of the onset of deposit formation. Contaminant layers with a thickness down to 20 nm can be consistently detected. The influence of water on the formation of deposits was investigated. Time-of-flight secondary ion mass spectroscopy (ToFSIMS) was used for chemical characterization of the deposits.

Fluorescence monitoring of organic deposits

In this paper, we present the continued joint effort of ESA/ESTEC and DLR laser laboratories of improving the fluorescence monitoring technique towards a quantitative means for analysis of UV laser-induced deposit formation on optical samples in vacuum. In addition, a separate low power UV fluorescence excitation light source was implemented into the system allowing the investigation of laser-induced deposition occurring during irradiation of optics with IR and VIS light beams.

In-situ laser-induced contamination monitoring using long-distance microscopy

Operating high power space-based laser systems in the visible and UV range is problematic due to laser-induced contamination (LIC). In this paper LIC growth on high-reflective (HR) coated optics is investigated for UV irradiation of 355 nm with naphthalene as contamination material in the range of 10-5 mbar. The investigated HR optics were coated by different processes: electron beam deposition (EBD), magnetron sputtering (MS) or ion beam sputtering (IBS). In-situ observation of contamination induced damage was performed using a long distance microscope. Additionally the onset and evolution of deposit formation and contamination induced damage of optical samples was observed by in-situ laserinduced fluorescence and reflection monitoring. Ex-situ characterization of deposits and damage morphology was performed by differential interference contrast and fluorescence microscopy. It was found that contamination induced a drastic reduction of laser damage threshold compared to values obtained without contamination. Contamination deposit and damage formation was strongest on IBS followed by MS and smallest on EBD.

Enhancement of contamination growth and damage by absorption centers under UV irradiation

Contamination plays a major role in lifetime of vacuum optics. Several efforts have been made to derive suitable models for lifetime prediction in laser-induced contamination related optical breakdown. But the broad spectrum of potential contaminants present in the various applications with their very specific contamination mechanisms complicates the derivation of universal optics degradation models. As one possible contamination initiation process, the impact of optical absorption on the laser-induced contamination and resulting optical breakdown is studied in this work. A set of specifically prepared samples using nanometer sized gold particles embedded in dense IBS anti-reflecting coatings is exposed to radiation of 355nm in low pressure naphthalene atmosphere. Even though the artificial defects are not in direct contact with the contaminant, their influence on the long-term optics performance in dependence on the particle concentration in the coating is evident. In the presence of naphthalene, the artificial nano-defects cause a significantly accelerated degradation compared to reference samples without those defects or in absence of the contaminant. For this specific type of contaminant, a correlation of the optical absorption and long-term durability is derived.

Testing of laser optical components for space-based laser systems

In this paper we address the qualification of optical components used in space based laser systems with special emphasis on nonlinear optical crystals, performed at the DLR optics qualification laboratory. The motivation for this activity is due to upcoming ESA space laser missions like ADM-Aeolus, EarthCARE, or BEPI Colombo. The ADM-Aeolus satellite is operating a Doppler wind LIDAR laser source named ALADIN (Atmospheric Laser Doppler Instrument), and will be the first system to be launched, scheduled in 2011. ALADIN aims at measuring wind profiles on a global scale as required by the climatology and meteorology fraternities. In all these missions, high performance laser systems rely on the efficient and long term stable operation of passive and active optical components.