Yb-doped femtosecond lasers and their frequency doubling
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Ultralow threshold, compact and highly efficient femtosecond lasers based on Yb³⁺-doped potassium yttrium tungstate (Yb:KYW) and Yb³⁺-doped vanadium yttrium oxide (Yb:YVO 4 ) have been demonstrated within this PhD-research project. For a continuous wave unmode-locked Yb:KYW laser a threshold as low as 101 mW was obtained with a slope efficiency of 74%. By employing a single prism for dispersion control, the laser was tunable between 1012 nm to 1069 nm. When operated in the mode-locked regime, this laser produced transform-limited pulses having durations of 210 fs at a central wavelength of 1044 nm. Stable mode locking was observed for an optimised incident pulse fluence on the SESAM between 140 μJ/cm² to 160 μJ/cm² which was 2-3 times higher than the designed energy pulse fluence of the SESAM (70 μJ/cm²). The employment of several combinations of chirped mirror designs for control of intracavity group velocity dispersion led to excellent results. The threshold for mode locking was satisfied for a pump power of 255 mW where the slope efficiency was measured to be 62%. This is the most efficient SESAM-assisted femtosecond laser yet reported and the highest optical-to-optical efficiency of 37% is exceptional. Transform-limited pulses with durations as short as 90 fs were produced in a spectral region centred on 1052 nm. The success of this research thus represents a good foundation on which to design and build more compact configurations that will incorporate just one chirped mirror for dispersion compensation. A relatively high nonlinear refractive index, n₂ , of 15 x 10⁻¹⁶ cm²/W was measured in Yb:YVO₄ and this affords particular potential for this candidate material in Kerr-lens mode locking. In fact, for operation in the femtosecond domain, the threshold power was 190 mW with a slope efficiency of 26% and near-transform-limited pulses as short as 61fs were generated at a centre wavelength of 1050 nm. The main objectives in developing this type of laser relate to a demonstration of high peak power operation in thin disc laser configurations. The deployment of a diode-pumped Yb:KYW femtosecond laser as a pump source for frequency doubling in a periodically-poled LiTaO₃ crystal was realised. The maximum realized output power of 150 mW corresponded to an impressive second harmonic conversion efficiency of 43%. 225-fs duration green pulses (centred at 525 nm) were generated under the condition of strong focusing in the nonlinear crystal.
Thesis, PhD Doctor of Philosophy
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