Direct optical observation of disclination effects in active photonic devices

Liquid crystals (LC) are increasingly finding uses in fields outside of display optics. Their strong electro-optic response can be used in applications such as tunable photonic devices, for example, to make tunable planar Bragg gratings. While Bragg gratings are well known as fixed wavelength reflectors, the application of a liquid crystal can convert these fixed reflectors into tunable filter elements, with potential applications in telecommunications networks [1]. We have previously demonstrated such devices, and have shown the capability to tune the Bragg peak through adjacent DWDM channels [2]. These devices contained channel waveguides and gratings defined by laser writing techniques. The Bragg peak is tuned by applying an electric field across the device, which causes a change in the LC refractive index, which modifies the modal index of an exposed Bragg grating. The tuning curves for these devices exhibit hysteresis depending on whether the field amplitude is being increased or decreased. In this work we report investigation of the liquid crystal behaviour by direct observation of polarization state and physical changes in the LC. We will also highlight new interaction geometries and show that careful control of modal interactions can be used to optimize the available tuning performance.