Novel liquid crystal-based integrated optical devices with >140GHz electrical tuning are presented. Initial results with Bragg wavelength tuning covering five 25GHz WDM channel spacing have been achieved with 170V (peak-to-peak) sinusoidal voltages applied across electro-patterned ITO-covered glass electrodes placed 60μm apart. These prototype devices were fabricated using direct UV grating writing, with an evanescent field coupling into a liquid crystal overlay through an etched window. Two distinct threshold conditions are observed, manifesting only during the increase of supply voltage and forming a hysteretic tuning curve. The secondary threshold which takes place at higher voltages has never been reported before. We believe these threshold points are related to the formation and bleaching of disclination lines. Geometric and effective index consideration could not explain the similar tuning behaviour displayed by both TE and TM polarised light.
Second harmonic generation via periodically-poled nonlinear materials offers an efficient means of generating high-quality
visible light at wavelengths that would be otherwise unattainable with traditional laser sources. While this
technology has the potential for implementation in many mass-industrial applications, temperature stability
requirements, often as restrictive as 0.1°C, can make packaging with a pump source problematic. In this work we are
investigating the use of synthesised response PPLN gratings to create crystals that are better suited to visible SHG. Our
route towards addressing this issue is to convert the standard sinc-shaped temperature-tuning response of a uniform
grating to a flat-top temperature tuning function with widths of up to several degrees. We have achieved a
computationally efficient means of designing such gratings with a required temperature tuning profile based on a
simulated annealing algorithm using repeated local changes of grating layout and subsequent Bloembergen-style
analysis of the second harmonic, successive iterations of which quickly lead to the desired temperature tuning profile.
Using our high fidelity poling technique we have fabricated synthesised response PPLN with precise placement of poled
domains in Lithium Niobate based on the designs from our mathematical models. Measurements on these initial devices
provide more than 4°C flat-top temperature stability, albeit with a corresponding loss in operational efficiency. Our aim
is to implement improved designs in magnesium-doped Lithium Niobate for packaging with near-room temperature
diode-based pump sources, as could be applied towards RGB TV and projector applications.