Space qualification of laser optics

Abstract

The Institute of Technical Physics at DLR Stuttgart is operating a laboratory where the laser damage threshold of optical components can be tested under pulsed irradiation. The damage thresholds are being evaluated in accordance with the international standard EN ISO 11254. All tests are carried out in an application oriented manner and at common laser wavelengths in the IR (1064 nm), visible (532 nm), and UV (355 nm / 266 nm) spectral range. Apart from single shot tests (EN ISO 11254-1), which are only relevant for basic scientific investigations, the more practical multipulse damage tests are performed according to the standard EN ISO 11254-2. The results of the multipulse investigations can be used for scaling of damage thresholds to very large pulse numbers or very long irradiation periods. Various types of optics with different coating designs (e.g. anti reflective coatings, high reflective coatings) under different angles of incidence (0°, 45°, 56.5° etc.) can be handled in the setup. The tests can be performed at the DLR laboratory under ambient conditions, or, as an option, under vacuum. The extension to vacuum testing was due to the ongoing requests of European companies and the European Space Agency (ESA) for vacuum laser optics qualification in view of upcoming space-laser missions. Such long term operations of laser systems under vacuum with a mission duration of several years lead to very stringent requirements of precision and longevity of the utilized optical components. Consequently, the qualification is rather complex and elaborate. Several high vacuum and ultra high vacuum chambers are used for testing which allow for realistic test conditions. A main result of the tests run so far was that not all types of coatings can be used for long term vacuum applications. Two independent methods are used to detect optical damage under vacuum. On the one hand the standard scatter monitoring is applied, and on the other hand a DLR patented procedure called transient pressure sensing. The available vacuum chambers are also suitable for the injection and control of contaminants like silicones, polyurethanes, and epoxides at smallest concentrations. This feature allows the investigation of contamination effects, as the outgassing of volatile components in a vacuum chamber is inevitable. Under laser exposure, these substances can decompose and accumulate on the surface of the optic. This can possibly lead to a noticeable increase in the surface absorption, and implicate a partial or complete failure of the whole system. Laser-induced fluorescence imaging is successfully used as an online technique for monitoring of the deposits. This method turned out to be very sensitive allowing the detection of deposits with nanometer thickness.