CC BY
14LM-I-13
UV-laser induced contamination on space optics
F. R. Wagner1, G. Gebrayel El Reaidy12, D. Faye2 and J.-Y. Natoli1
1- Aix Marseille Univ, CNRS, Centrale Marseille, Institut Fresnel, Marseille, France 2- Centre National d'Etudes Spatiales, 18, Avenue E. Belin — 31401 Toulouse Cedex 9, France
frank. w agner@fresnel._ fr
Laser Induced Contamination (LIC) is a phenomenon that takes place under multi-pulse laser irradiation and causes damage to optical surfaces. It was first mentioned in 1993 when working with a sealed Nd:YAG laser [1]. Today LIC is mostly a problem for high power photonics in vacuum, especially when working in the UV. In particular, LIC was identified as a major problem for operating lasers on satellites and just lately led to a strongly reduced lifetime of the 355nm laser of the ESA Aeolus mission (Aladin instrument) [2]. The development of the Aladin instrument triggered in fact an important research effort concerning LIC [3].
The physical and chemical processes behind LIC are the same as those exploited by Laser Induced Chemical Vapor Deposition (LI-CVD) of carbonaceous thin films on the optical elements: Polymers outgas volatile organic molecules that cover all surrounding surfaces including the optical elements. The laser radiation interacts directly or indirectly with the adsorbed molecules and cracks them. Some of the less volatile products bind strongly to the optical surface increasing its optical absorption coefficient. The enhanced optical absorption may lead to catastrophic laser induced damage of the optics (creating micro cracks) or simply decrease the performance of the optical system (especially in resonators).
Building on the former project oriented knowledge, we led some more fundamental investigations on the LIC process and characterized the LIC deposits.
A new setup was built, in which toluene vapor or the outgassing of 3M EC2216 epoxy glue were used as contaminants. An experimental protocol was established staying close to the one given by the DLR Stuttgart [4] and good reproducibility was demonstrated in terms of deposit morphology (measured by WLIM white light profilometer) and transmission loss of the 355 nm 13 ns laser pulses [5]. The optical properties of the LIC deposits were then analyzed using quantitative optical path difference microscopy combined with WLIM thickness data and it was shown that the deposit is not yet very absorbing in the first stage of the process [6]. When creating a LIC deposit on bare fused silica substrates, a slight anti-reflective coating effect can even be observed [5].
Laser induced fluorescence images easily allow to detect the deposit onset [3] and the transition from the first to the second stage of the process, i.e. the start of the crater formation in the initially bump shaped deposit. The average thickness growth rate during the first stage can thus be analyzed as a function of laser fluence and contamination conditions and the results be compared to CVD knowledge.
Finally, the lateral growth of the LIC deposit around the irradiated region shows that the process includes an important thermal component. Lateral crater growth is presumably driven by energy being absorbed in the center of the crater and subsequent heat conduction from the center towards the edges.
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[2] https://www.esa.int/Our_Activities/Observing_the_Earth/Aeolus/Second_laser_boosts_Aeolus_power, Second laser boosts Aeolus power, last accessed on 2021-05-27, European Space Agency, (2019).
[3] H. Schröder, S. Borgmann, W. Riede, and D. Wernham, Investigation of laser induced deposit formation under space conditions, in
International Conference on Space Optics — ICSO 2008, Proc SPIE, vol. 10566, p. 105661K, (2017).
[4] W. Riede, H. Schroeder, G. Bataviciute, D. Wernham, A. Tighe, F. Pettazzi, and J. Alves, Laser-induced contamination on space optics,
in Laser-Induced Damage In Optical Materials: 2011, Proc SPIE, vol. 8190, p. 81901E, (2011).
[5] G. G. El Reaidy, F. R. Wagner, D. Faye, and J.-Y. Natoli, Study of the first stages of laser-induced contamination, Optical Engineering, vol. 57, p. 121903, (2018).
[6] F. R. Wagner, G. Gebrayel El Reaidy, D. Faye, and J.-Y. Natoli, Laser induced deposits in contaminated vacuum environment: Optical properties and lateral growth, Optics & Laser Technology, vol. 122, p. 105889, (2020).
