Monolithic Q-switched and mode-locked lasers using liquid crystal modulators

Abstract

Recent innovations in the area of liquid crystals have led to the development of passive electro-optic liquid crystal transducers that enables a new approach to compact, monolithic, and even portable Q-switched and mode-locked lasers. My Ph.D. study concentrates on the liquid crystal cells with a thin gap filled with nano-scale pitch ferroelectric smectic-C* liquid crystals, of which the molecular arrangement can be continuously deformed by the electric fields, giving rise to the "deformed helix" mode of operation. The deformation of molecular helical structure gives rise to the electric field controlled birefringence and orientation of the optical axis, which illustrates the possibility of using the liquid crystal modulator as an active intracavity loss modulator in laser applications.

This thesis presents the basic liquid crystal physics, design, modeling approaches, and characterisation of liquid crystal modulator in great detail for readers to understand its basic principles. To theoretically study its performance when applied to the 1 µm Q-switched lasers, the numerical model was developed. The numerical model predicted that the pulse width can be reduced to 17 ns and the peak power can be enhanced to 1.15 kW, when the absorbed pump power, the external optical loss, and the temperature are optimised. To utilise this novel technique in the industry, the University of New South Wales, Macquarie University, and Lastek started the collaboration for the development of a commercial prototype of compact actively Q-switched lasers in 2017. Later, the operation wavelength range was extended further to 1.88 µm in Tm3+-doped ZBLAN waveguide chip lasers. The experimental results demonstrated that the shortest pulse duration was around 31 ns, which provided the peak power of 1.3 kW at the repetition rate of 200 Hz. Additionally, all-in-one fibre lasers operating at 1.55 µm were designed and experimentally developed in an all-fibre setup. The proposed all-in-one fibre lasers support CW, Q-switched, AM, and FM mode-locked operation modes, and the mode can be easily switched by applying different electrical signals.

The simulation and experimental results illustrate the feasibility of utilising liquid crystal modulators in lasers, nonlinear applications. Additionally, utilising this novel optical technology for the development of the new generation of neural interfaces, that can record activation potentials at different positions, is another ambitious goal to achieve in the future.