Non-linear optical crystals are key components for frequency conversion in many laser systems, such as, for example, in the ESA ADM-Aeolus mission. The objective is to measure wind profiles from space using LIDAR methods. To this end, the emission wavelength of an Nd:YAG laser in the infrared range at 1064 nanometres must be converted with high efficiency to 355 nanometres in the ultraviolet range. This is done by the use of a frequency conversion module consisting of two non-linear crystals.
In this project, the Institute of Technical Physics has carried out extensive studies with different crystals for frequency doubling and tripling (LBO, BBO, BiBO, KTP) to identify the most suitable crystal varieties for use in space, and to provide a conversion efficiency of about 30 percent for the generation of laser radiation in the ultraviolet range. For this purpose, in addition to long-term efficiency measurements, simulations of frequency conversion are also performed. On this basis, the influence of various parameters on the conversion process, such as the crystal length and the beam quality of the laser, can be determined. The crystals must also be investigated prior to use in space to determine whether their optical properties are adversely affected by cosmic rays. For this purpose, the crystals were exposed to high-energy proton and gamma radiation in several stages.