Power scaling of Tm:YLF-pumped Ho:YAG lasers

Abstract

Reported in this thesis are methodologies for power-scaling Ho:YAG lasers for both the continuous-wave (CW) and Q-switched regime. Advancement in Ho:YAG laser power is made possible through the improvements in output power of the Tm:YLF pump lasers. The slab geometry provides good thermal management characteristics which helps to overcome the limitations set by the thermally-induced stress fracture when scaling output power of Tm:YLF lasers. In conjunction with the use of novel holmium cavity designs that allow the use of slab pump lasers, high-power Ho:YAG lasers are realised.

Conflicting requirements on the thulium doping concentration in Tm:YLF provided motivation for the preliminary experiments of laser performance characterisation to find the optimum doping concentration. The optimum doping concentration is defined as that which gives the highest output power before fracture. These experiments show that 2 at.% is the optimum thulium doping concentration, which is then used with the slab geometry to demonstrate 100 W of high M2 thulium output.

Two different Ho:YAG cavity designs are detailed that make use of the power-scaled Tm:YLF slab pump lasers. An intra-cavity side-pumping scheme involves the Tm:YLF slab laser side-pumping a Ho:YAG slab laser within its own cavity. This simple and compact resonator design allows efficient operation of the holmium laser. An output power of 13 W was obtained at an incident diode power of 200 W with an M2 of 140 x 1.7. The slope efficiency was nearly 16 % with respect to the diode incident power, with a lasing wavelength of 2097 nm. The second cavity design used for the Ho:YAG laser is an external end-pumping arrangement. A pump-guiding rod was employed in order to minimise the pump size, and thus threshold, and allow the use of spatially multiplexed pump lasers. A maximum output power of 38 W was achieved at an incident thulium pump power of ∼70 W with an M2 of 18.3 x 3.3. The slope efficiency was 61 % with respect to the thulium incident power, and the lasing wavelength was 2090 nm. Pulse energies as high as 14 mJ were achieved when Q-switched at 20 Hz, with a pulse duration of 17 ns. Further future improvements in output power and beam quality are also discussed.

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