Mode-locked pumped continuous wave Ce:LiCAF lasers
thesisposted on 2022-03-29, 00:15 authored by Barbara Wellmann
At the MQ Photonics Research Centre of Macquarie University the project "Unlocking the Ultraviolet" was launched to establish a new laser platform based on Ce:LiCAF for continuous wave (CW) and ultrafast pulsed operations in the ultraviolet spectral region. The work presented in this PhD thesis was part of this project, with the main aim to develop a deep understanding of the dynamics in Ce:LiCAF lasers that are pumped by a mode-locked source. As a result, a novel mode of operation for CW solid-state lasers was identified and implemented successfully. When setting up a mode-locked pumped laser system in a way that the normalized cavity length (that is the ratio between the length of the pump resonator and length of the laser cavity) is a rational number, ‘rational-harmonic mode-locking’ is obtained and short laser pulses are generated. In contrast, when the cavity length is detuned away from resonances, modulated continuous output is emitted. Such asynchronous pumping has never been substantially investigated before in solid-state lasers and led to the first ever CW Ce:LiCAF laser. The optimal normalized laser cavity length for asynchronously pumped CW Ce:LiCAF lasers was calculated to be 0.3964, which in the present case is a distance of 120 mm detuned from the third harmonic of the 78.75 MHz pump. The generated CW output featured a residual modulation of only 1% for frequencies below 1 GHz and 4% on faster time scales. The slope efficiency was 20% and the laser threshold 1.37W. In the set-up used, the transition between CW behaviour and rational-harmonic mode-locking could be achieved easily by adjusting the length of the laser cavity, and thus in addition to the CW output, mode-locked output with pulse repetition rates up to 1.1 GHz was achieved. Both mode-locked and CW laser output generation have been explored experimentally and theoretically by modelling the laser resonator based on the laser rate equations. Birefringent tuning of the CW Ce:LiCAF laser using single and multiple MgF2 Brewster plates has also been investigated. Depending on the thickness of the MgF2 plates used, continuous tuning over a range of up to 13 nm from 284.5 nm to 297.5 nm with a full width at half maximum linewidth of 50 GHz was achieved. By combining MgF2 plates with etalons, the linewidthof the laser was narrowed down to 2.7 GHz.