Optical cross connects (OXC) are optical switches that re-route optical channels for the purpose of telecoms network management and restoration. They allow light from each fibre in an input fibre array to be routed in a controlled fashion to any fibre in an output fibre array. The component used for selecting the optical paths between the arrays can be made using LC based reconfigurable phase gratings (or holograms) which route the light signals appropriate for each channel by diffraction through free space. Alternatively optical add/drop multiplexers (OADM) can be used with WDM (Wavelength Division Multiplexing) to selectively route light of a particular wavelength out of the main WDM signal beam (drop) or introduce data at a new wavelength to the main signal beam (add). These are most efficiently realised using the dispersive properties of phase gratings. For efficient use of available light and/or available bandwidth each grating pixel should be capable of analogue operation preferably with a phase excursion in excess of 2pi radians at wavelengths around 1550nm. The preferred embodiment is LCoS (Liquid Crystal on Silicon). The implications of this with regard to suitable liquid crystal electro-optic effects are considered for a range of electro-optic effects and data is presented to support the conclusions reached. The main factors considered are the switching speed and diffraction efficiency. The switching speed is limited by the available voltages and the need for rather thick cells, even with material of high birefringence, to achieve the desired phase excursion at infrared wavelengths. The diffraction efficiency is dependent on artefacts associated with the pixel size and gaps between them. When gaps are included fringing fields can result in a considerably increased time intervals for switching to complete compared with a continuous pixel. The conclusion reached was that the fastest suitable electro-optic effects are associated with nematic liquid crystals with positive dielectric anisotropy. With some reservations related to the splay to bend transition pi-cells were found to operate fastest even though they must be operated in the bend mode and thus cells with approximately twice the thickness of those not subject to this limitation are needed (ref). However the reduced level of artefacts associated with diffraction using vertically aligned devices with negative dielectric anisotropy materials meant that their diffraction efficiency was greater. Improved (faster)negative materials therefore are desirable.

ref: J. Rao, A.B. Davey, H. Xu, W.A. Crossland, “Pi-Cells for Telecoms”, Mol. Cryst .Liq. Cryst., 434, 97-105, (2005).