Demonstration of 100 GHz electrically tunable liquid-crystal Bragg gratings for application in dynamic optical networks.

We demonstrate liquid-crystal-based integrated optical devices with >140 GHz electrical tuning for potential applications in dynamic optical networks. Bragg wavelength tuning covering five 25 GHz wavelength-division multiplexing channel spacing has been achieved with 170 V (peak-to-peak) sinusoidal voltages applied across electropatterned indium tin oxide-covered glass electrodes placed 60 microm apart. This tunability range was limited only by the initial grating strength and supply voltage level. We also observed two distinct threshold behaviors that manifest during increase of supply voltage, resulting in a hysteresis in the tuning curve for both TE and TM input light.

Line defects and temperature effects in liquid crystal tunable planar Bragg gratings.

Liquid crystal tunable planar Bragg Gratings produced by Direct UV Writing are capable of wavelength tuning of over 100GHz. However, such devices exhibit non-linear tuning curves with threshold points and hysteresis. We show that these effects are due to the formation of disclination structures in the liquid crystal and discuss the role of electrode defects and sample temperature on wavelength tuning.

Control of topological defects in microstructured liquid crystal cells.

We study how the propagation of light inside recently developed micro-structured cells, can be actively tuned by polarising the nanoscale defects in the nematic liquid crystals they contain. Our 'planar-spherical' cells are formed by assembling a planar and a gold-coated hemispherical micro-mirror. Optical reflection images of the back-reflected polarised light show a remarkable change of symmetry as a function of the voltage applied to the cell. Theoretical models of the alignment of the liquid crystal within the cell indicate that the constraints imposed on the liquid crystal by the cell geometry and by the applied electric field induces the formation of defects. Their motion under the effect of the applied electric field is responsible for the change of symmetry of the back-reflected light. Furthermore, experimental measurements of the relaxation time of the back-reflected intensity indicate that the motion of the defect in our micro-structured cells is much faster than in equivalent planar cells.