Liquid crystal resonator as a nonlinear reflector for passive Q-switching and mode locking

We carry out a theoretical study of a Fabry-Perot resonator with a central layer of nematic liquid crystal as a nonlinear polarization rotation (NPR) device for passive Q-switching and mode locking. We develop a self-consistent procedure for calculating the device dynamical response as a function of the input light intensity and polarization. We show how a positive feedback effect for the optical reorientation of the liquid crystal director makes the resonator abruptly switch to high-reflectivity state. We demonstrate that the time-delay between the pulse initiation and the resonator switch can be avoided by tilted liquid crystal molequles at the resonator boundaries. We show that such a NPR optical device is suitable for Q-switching operation with mu s pulses and hundreds of kHz repetition rates. Further decrease of pulse duration and increase of the pulse repetition rate is possible by either increasing the pulse power or by increasing the Fabry-Perot resonator finesse. Finally, we show that the same Fabry-Perot resonator can be used for Q-switching and mode locking at tens of MHz repetition rates and ns pulse durations by using an electro-optic uniaxial-biaxial transition as an alternative to NPR in order to reduce pulse duration and increase repetition rate.