Damage Threshold Testing of Laser Optics

The department of Active Optical Systems has further expertise in the qualification of optical components that are used in space conditions, such as retroreflectors (CCR, Corner Cube Reflectors). According to ISO standards, dielectric laser optics are subjected to standardized laser stress tests in a clean room or studies on laser-induced contamination are carried out.

Damage Threshold Tests

Laser-induced damage to an optical coating

The Institute of Technical Physics at the DLR in Stuttgart is operating a qualification laboratory where the laser vulnerability of optical components can be tested under pulsed irradiation and for operation at high power or in space. Therefore, standardized damage threshold tests are performed, supplemented by raster scans across large areas. Another field of activity is laser induced contamination. Furthermore, conversion efficiency and radiation resistance of non-linear crystals are investigated. These tests are also provided as a service for external customers. Damage threshold 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) spectral range and with pulse lengths of a few nanoseconds. Tests are performed mostly in multipulse mode according to ISO 21254. The results of the multipulse tests can be used for scaling of damage thresholds to very large pulse numbers or very long irradiation times. Various types of optics with different coating designs (e.g. anti reflective or high-reflective coatings) can be handled in the setup at different angles of incidence. But also other optical components, for example, thin disk laser modules have been already successfully tested. The tests can be performed at the DLR laboratory under ambient conditions or optionally under artificial gas atmosphere and vacuum conditions. The extension to vacuum testing was due to the ongoing requests of european companies and European Space Agency (ESA) for vacuum laser optics qualification in view of upcoming space-laser missions. Such long term operations of laser systems in space with a mission duration of several years lead to very stringent requirements of precision and longevity of utilized optical components.

Loading

Laser induced Contamination

Operation of laser systems in space can be tremendously affected by laser-induced contamination. Several NASA missions failed for that reason. Source for laser-induced contamination is outgassing of volatile materials from glues, adhesives, isolation materials or electronic boards. These outgassing molecules could be cracked by interaction with laser radiation and the residuals are deposited on the optics. This occurs particular in case of intensive radiation in the shortwave spectrum (VIS/UV). Laser-induced contamination mainly appears in laser systems which are operated in vacuum, but it can also be a risk for hermetically sealed systems. For mitigation or reduction of this risk, basic research is necessary to investigate the formation and growth of laser-induced depositions. The Institute of Technical Physics runs several ultrahigh vacuum chambers, in which fundamental research as well as screening tests of critical materials are performed.

Contact

Wolfgang Riede

Head of department
German Aerospace Center (DLR)
Institute of Technical Physics
Active Optical Systems
Pfaffenwaldring 38-40, 70569 Stuttgart