Hybrid-mode-locked Ce:LiCAF lasers
This research project entitled “cerium lasers: ultraviolet, ultrafast, and ultracool” was introduced at the MQ Photonics Research Centre in Macquarie University with the aim of advancing ultrafast ultraviolet lasers based on Ce:LiCAF. This resulted in a new modelocking scheme for Ce:LiCAF lasers, a modified cross-correlation technique to measure the duration of the generated pulses, and characterisation of the spectral behaviour of the laser system.
Combining a synchronous pumping scheme with Kerr-lens mode-locking led to the first hybrid mode-locking scheme within a Ce:LiCAF laser. Compared with previous results, the addition of this latter mode-locking mechanism decreased the generated pulse duration by more than a factor of 60, from 6 ps to 91 fs. This duration was measured by modifying asynchronous cross-correlation with a simple microscope cover-slip time reference introduced into half of the reference beam profile, dubbed split-beam asynchronous cross-correlation. This new approach determines the measurement time-scale more quickly, on the order of 104, reducing uncertainty. The intra-cavity dispersion of the Ce:LiCAF laser was calculated, and measured experimentally, which enabled Kerr-lens mode-locking. The phenomenon of dispersion tuning was characterised and utilised to determine the amount of intra-cavity dispersion, and as a guide for identifying the zero point, at which there is minimal tuning. Additional dispersive elements were used to dispersion-tune over a range of more than 5 nm in the region of 290 nm.
The slope efficiency of the hybrid-mode-locked Ce:LiCAF laser was measured to be 9%, with a threshold of 1.0W. A low-transmission plane output coupler was used to achieve this, which also consolidated the output power from the previous four beams, via a near- Brewster’s-angle plate output coupler, into a single beam for the first time. The low threshold achieved using this output coupler enabled the first demonstration of un-chopped operation from the mode-locked Ce:LiCAF laser.