In the framework of the ADM-Aeolus satellite mission, successful test campaigns have been performed in ESTEC’s laser laboratory, and the efficiency of several mitigation techniques against Laser-Induced Contamination (LIC) have been demonstrated for the ALADIN laser. These techniques include the standard contamination control methods of materials identification with particular tendency to cause LIC, reduction of the outgassing of organic materials by vacuum bake-out and shielding of optical surfaces from the contamination sources. Also novel mitigation methods such as in-situ cleaning via partial pressures, or the usage of molecular absorbers were demonstrated. In this context, a number of highly sensitive optical measurement techniques have been developed and tested to detect and monitor LIC deposits at nanometre level.
Many different environmental factors can have an effect on optical coating durability for space applications. This includes in-orbit effects such as vacuum exposure, UV radiation, particle radiation, atomic oxygen, thermal cycling, contamination and orbital debris, as well as ground based effects such as cleaning, contamination and humidity .
In this paper some basic investigations about laser-induced contamination are reported. As contamination materials pure
aromatic hydrocarbons (naphthalene and anthracene) were used. A particular focus of the tests was to investigate the
impact of laser-induced contamination on damage threshold. Onset and evolution of deposit formation and damage were
observed in-situ by laser-induced fluorescence and transmission monitoring. As optical samples uncoated fused silica
substrates and AR and HR coated optics with different coating morphology, depending on coating process (e-beam,
magnetron sputtering) were investigated. Ex-situ characterization of deposits and damage morphology was performed by
differential interference contrast, fluorescence, and atomic force microscopy. The tests were run with pulsed UV light at
355 nm. Partial pressure of contamination material in the range of 10<sup>-4</sup> mbar induced a drastic reduction of laser damage threshold compared to values obtained without contamination.
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.
Laser-induced contamination (LIC) is a phenomenon that can lead to the degradation of the properties of optical
components in vacuum due to the formation of deposits in the area irradiated by a laser beam. The deposit growth is
proposed to be the result of photochemical and photothermal mechanisms triggered by the interaction of UV laser
radiation and outgassing species from polymeric materials on the surface of the optics. In the framework of ESA's ADM-Aeolus
satellite mission, a successful test campaign has been performed, which has demonstrated the efficiency of
several mitigation techniques against LIC for the ALADIN laser. These include the standard contamination control
methods of identification of materials with particular propensity to cause LIC, reduction of the outgassing of organic
materials by vacuum bakeout and shielding of optical surfaces from contamination sources as well as novel methods
such as in-situ cleaning. These methods are now being applied at satellite level in order to guarantee the success of the
mission. The subject of this paper is to summarise the various mitigation techniques from the large number of studies
that have been performed and is applicable to any use of high power pulsed lasers in vacuum in the presence of organic
Photon-induced contamination of optical surfaces is a major obstacle for space-bound laser applications. At
Laser-Laboratorium Göttingen, a setup was developed that allows monitoring transmission, reflection and fluorescence of
optical components under well-controlled vacuum conditions, in order to assess their possible optical
degradation due to radiation-induced contaminant deposition in orbit. In cooperation with the European Space Agency
ESA optical elements for the ADM-Aelolus mission were investigated. In order to perform global wind-profile
observation based on Doppler-LIDAR, the satellite ADM-Aelolus will be launched in 2011 and injected into an orbit 400
km above Earth's surface. ADM-Aeolus will be the first satellite ever that is equipped with a UV-laser (emitting at a
wavelength of 355 nm) and a reflector telescope.
For both high-reflecting mirrors and an anti-reflective coated windows long-term irradiation tests (up to 500 million laser
pulses per test run) were performed at a base pressure < 10<sup>-9</sup> mbar, using a XeF excimer laser (λ=351 nm, repetition rate
1kHz). At this, samples of polymers used inside the satellite (insulators for cabling, adhesives, etc.) were installed into
the chamber, and the interaction of their degassing with the sample surfaces under laser irradiation was investigated.
Optical degradation associated with contaminant adsorption was detected on the irradiated sample sites as a function of
various parameters, including pulse repetition rate, view factor and coating material
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.