Pulsed laser sources utilising liquid crystals as intracavity modulators

A range of techniques for fabricating optical waveguides in actively doped media has previously been developed. Macquarie University has specialised in the femtosecond laser direct-write technique, a flexible method that enables the inscription of three-dimensional structures into transparent media. By inscribing depressed-cladding waveguides into doped fluoride glass chips, continuous-wave (CW) lasers with extraordinarily large fundamental mode-field diameters have been realised. However, in order to generate optical pulses with high peak-power levels, an actively controlled intracavity loss modulator would be required. While the use of bulk acousto-optic modulators has been demonstrated, the waveguide chip laser architecture is best utilised with a modulator that is compatible with a monolithic and integrated design. In the present work, a novel type of a liquid-crystal based transducer was utilised as an intracavity loss modulator that can be integrated onto the laser chip, opening up the possibility of realising monolithic miniaturised actively Q-switched high peak-power laser sources. This liquid crystal cell was initially developed by our collaborator in the context of distributed sensor networks and therefore its switching behaviour on a microsecond to sub-microsecond timescale had to be studied first. Next, the cell was employed as an active Q-switch in an ytterbium glass waveguide laser, resulting in peak power levels approaching 100 W. Further,the use of this technology in laser chips based on polarisation-maintaining waveguides in crystalline materials was investigated in the visible spectral region, and this approach was also extended towards the mid-infrared spectral region with the aim of utilising nonlinear optical effects in chalcogenide fibres to generate broadband radiation in the important mid-infrared fingerprint region of the optical spectrum. Finally, the feasibility of using the liquid-crystal modulator as a mode-locking device in fibre lasers was investigated and a fibre laser that can be switched between mode-locked, Q-switched and CW operation was demonstrated.