Towards high-speed liquid crystal electrically tunable planar Bragg gratings for integrated optical networks

Abstract

Liquid crystal-based integrated optical devices offer the potential for high speed and dynamically tunable optical switches in modern telecommunications networks. Here, electrically tunable devices have major advantages over their thermal counterparts, with superior response times and low operating voltages (~100V). Our approach to achieving such devices is to fabricate planar optical waveguides with integrated Bragg gratings via direct UV writing1 into silica-on-silicon samples with evanescent field coupling into a liquid crystal overlay through an etched window (Fig. 1(a)). Such electrically tunable devices work on the principle of shifting the Bragg wavelength by modifying the effective index of a waveguide in a multilayer substrate. Electrically controlled liquid crystal birefringence modifies the waveguide effective index, producing a Bragg wavelength shift. In our early samples, Merck 18523 nematic liquid crystal is used as it has a compatible refractive index to silica (n=1.49 at lambda=1550nm). Homeotropic alignment of the liquid crystal is provided by application of a surfactant layer