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/cm<sup>2</sup>. 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.
The European Space Agency is developing its first spaceborne LIDAR for global monitoring of wind velocities. ALADIN, to be launched on board ADMAeolus in 2008, is a pulsed Nd:YAG laser with about 120 mJ of pulse energy at 355 nm and a repetition rate of 100 Hz during bursts. Within the projected mission duration of three years, this gives a lifetime requirement of close to 5 billion pulses. <p> </p>While laser-induced damage thresholds of optics in vacuum (possibly contaminated by small amounts of organic compounds) can differ from atmospheric conditions, their damage behaviour is generally poorly understood. The European Space Agency has therefore established a test campaign to measure the power handling of all the instrument optics with several European laboratories participating. <p> </p>In the Optics and Opto-Electronics laboratory at ESTEC, a laser-induced damage threshold (LIDT) test facility has been set up with a 50 Hz Nd:YAG test laser. The pulse energy is 700 mJ at 1064 nm. This allows us to recreate the laser pulse conditions to which the ALADIN optics will be exposed. The flattop beam profile of the test laser irradiates the optics with uniform fluences and relatively large spots (up to 1mm across) at damaging intensities. <p> </p>Damage tests are performed with up to 1 million pulses per test spot according to the S-on-1 test ISO-11254 standard, requiring typically 10 days to test one sample. With such extended tests, we can predict the laser-induced damage threshold over the ALADIN lifetime with improved accuracy.
An extended qualification program has proven the quality of a previously evaluated semiconductor laser diode, which is intended to be used in a subsystem for the GAIA mission. We report on results of several reliability tests performed in subgroups. The requirements of the procurement specification with respect to reliability and desired manufacturing processes were confirmed. This is an example for successful collaboration between component supplier, system integrator and payload responsible party.
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.
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.
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.
In the context of lifetime of optics in laser systems in space, spot size dependencies of laser induced damage thresholds have been investigated. The measurements were performed with two different Nd:YAG laser systems at 1064 nm with pulse durations of 8 and 50 ns and repetitition rates of 10Hz and 3kHz, respectively. The effective beam diameter was varied in the range of a few microns up to some tens of millimetres using several focussing optics. In preparation of the experimental analysis, simulations have been performed to determine the difference of the linear evaluation algorithm and a more sophisticated theoretical description of the damage threshold. Correspondence of simulated and experimental results should reveal information concerning the applicability of small spot sizes to standardized damage tests.
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/cm<sup>2</sup> at 0.1 mbar, 0.8 J/cm<sup>2</sup> at 0.01 mbar toluene). The deposits strictly followed the spherical geometry of the laser spot. No deposit accumulated on MgF<sub>2</sub> 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.