Nonlinear optical frequency conversion with KTP and BiBO crystals for lasers in space

Within ESA’s ADM-Aeolus and EarthCARE missions Doppler-wind Lidar systems will be operated in the Earth’s orbit to measure global wind profiles. The active instrument will be based on a Nd:YAG laser, frequency tripled by nonlinear optical crystals. Different crystals are therefore to compare and qualify in regard of their space acceptability. A dedicated set-up to measure the maximum conversion efficiencies and its stability during longterm operation for KTP crystals (SHG) and BiBO crystals (SHG and THG) is presented in this work. In order to detect gray-tracking and its influence on thermal lensing in situ, measurements with a Shack-Hartmann sensor and a co-aligned HeNe laser were performed. Conversion efficiencies were 76±3 % at SHG for KTP and BiBO crystals and 48±2 % at THG with a combination of two BiBO crystals. During longterm experiments of 60 million laser pulses, conversion efficiencies were demonstrated to be stable over time (±1 % at SHG with KTP and ±2 % at THG with BiBO). The occurrence of gray-tracking was detected in the KTP crystal and the resulting thermal lensing with an exponential saturation over time was observed in situ.