Compact high repetition rate mid-infrared solid-state lasers
Thorburn, Fiona Elizabeth
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The work in this thesis focuses on the development of novel mid-infrared laser sources which are compact, robust and suitable for the generation of ultrafast pulses with GHz pulse repetition frequencies. The motivation behind the work is to overcome the challenges associated with many of the mid-infrared ultrafast sources used at present. The novel laser gain medium Erbium doped Gallium Lanthanum Sulphide glass has been explored for use as a compact mid-infrared emission source. Ultrafast Laser Inscription was utilised to inscribe waveguides and under pumping conditions the waveguides emitted fluorescence at ~ 2.73 µm. The fluorescence intensity increased with increasing dopant concentration. The propagation loss of these waveguides was measured to be ~ 1.83 dBcm-1 and an IR waveguide amplifier was demonstrated exhibiting a maximum gain of 5.4 dBcm-1 at 1538 nm. These results indicate ways in which the cavity should be optimised for mid-infrared lasing operation. High repetition rate pulse generation from a Holmium doped YAG waveguide laser has been investigated using a graphene based saturable absorber. Depressed cladding waveguides were fabricated with Ultrafast Laser Inscription; these were subsequently employed in a compact quasi-monolithic laser resonator. Initially the laser operated in the CW regime resulting in a maximum laser output power of 1.78 W with a slope efficiency of 16% at 2.09 µm. In pulsed operation the laser generated pulses in a Q switched modelocked regime with an average output power of 170 mW and slope efficiency of 6.8%. The modelocked pulses were emitted with a high pulse repetition rate of 5.9 GHz. A room temperature Kerr-lens modelocked Chromium doped Zinc Selenide laser emitting sub-40-fs mid-infrared pulses is presented. The active crystal in this laser has been treated by hot isostatic pressing. Ultrafast pulses were generated with a pulse duration of 37 fs centred at 2388 nm. The laser emitted stable modelocked pulses at a repetition rate of 182 MHz with a maximum average output power of 144 mW. This is the shortest pulse width demonstrated to date from polycrystalline Cr:ZnSe indicating that hot isostatic pressing is beneficial for the generation of mid-infrared ultrafast pulses in Transition metal doped II – VI semiconductors.