For the spaceborne laser-altimeter (BELA) of ESA´s Bepi Colombo mission a master-oscillator-power-amplifier system (MOPA) is presented. The system-requirement is a pulsed laser source with a nearly diffraction limited beam (M² < 1.6) that combines high pulse energy of about 50 mJ at less than 10 ns pulsewidth and up to 20 Hz pulse repetition rate with the stringent environmental conditions at space missions. A low-mass (< 1.3 kg) and high optical-to-optical efficiency (> 15 %) laser setup is required. Stable operation at a temperature variation 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 due to the longer absorption path less sensitive to pump wavelength variations resulting from temperature changes of the laser pump diodes. The pump-pulse duration of 200 µs represents a trade-off between output energy and efficiency of the whole system. The Nd:YAG oscillator was passively Q-switched with a Cr 4+ :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.

The onset of parasitic oscillations limits the extraction efficiency and therefore energy scaling of Q-switched lasers. A solid-state laser was end pumped with a fiber-coupled diode laser and operated in q-cw as well as in passively Q-switched operation. For Q-switched operation, we demonstrate the suppression of parasitic oscillations in a core-doped ceramic Nd:YAG laser by Sm:YAG cladding.

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