SPIE Photonics West: LASE
For the spaceborne laser-altimeter (BELA) of ESA's BepiColombo mission a master-oscillator-power-amplifier system (MOPA) is presented. The specified system-requirement is a pulsed laser source with a nearly diffraction limited beam (M2 15 \%) laser setup is required. Stable operation at a temperature range of at least 25 K for the MOPA system and 15 K for the pump diodes has to be guaranteed. Both oscillator and amplifiers are longitudinally pumped by fiber coupled QCW laser diodes. The performance of a longitudinal pumped system is, because of the longer absorption path, less sensitive to pump wavelength variations due to temperature changes of the laser pump diodes. The pump-pulse duration of 200 μs represents as a trade-off between output energy and efficiency of the whole system. The Nd:YAG oscillator was passively Q-switched with Cr4+:YAG crystal as a saturable absorber. With 100 W of peak pump power a nearly diffraction limited (M2≈ 1.2) laser pulse with a duration of 2.8 ns and a pulse energy of 2.4 mJ was generated. The output beam of the oscillator was amplified in a two stage amplifier. A maximum of 62 mJ pulse energy was achieved by pumping each crystal with a peak pump power of 600 W.