Pulsed 2 μm fiber lasers for direct and pumping applications in defence and security

摘要

The paper gives an overview on pulsed 2 μm fiber laser technology that has strongly evolved during the last years. In the beginning, 2 μm fiber-amplifier systems have been considered to allow short-pulse generation at high repetition rates for pumping mid-IR conversion elements due to their possibility of generating short pulses by properly-driven laser diodes and successive amplification. However, this scheme turned out to be rather complex in architecture and needed low-phonon fluoride-glass fibers for efficient energy storage. Then a new technology has been developed to provide short pulses directly from a single, Q-switched fiber laser at very high repetition rates with silica fibers. Even taking into account the high phonon energies of these fibers, an efficient operation is possible. These fibers are also much more durable than fluoride fibers and thus better suited for military applications. Thulium-doped and thulium-holmium-codoped fibers are therefore the most promising candidates for pulse generation when some-ten-ns pulses at 100 kHz or more are needed. Average powers of over 32 W have been achieved with pulse durations down to 42 ns at > 100 kHz with Thulium-doped fibers. Tm-Ho codoping can provide longer emission wavelengths towards 2.1 μm, thus allowing direct conversion towards the mid-IR using ZGP crystals. On the Holmium emission, pulses of down to 58 ns have been achieved at average powers exceeding 14 W, being currently only limited by the available pump power. The theory of this short-pulse generation will also be explained. Pumping, e.g., quasi-phase-matched nonlinear GaAs, with a Tm-Ho-doped fiber laser, > 2 W of average power has been achieved in the mid-IR.