Liquid-crystal technology based filters for WDM over plastic optical fibres

This paper describes the development of notch filters and dichroic beam-splitters, the latter to be a visible 2-channel WDM Multiplexer/Demultiplexer (MUX/DEMUX). Both discrete devices are intended to be used on very short-haul links based on Poly-Methyl-Methacrylate (PMMA) Polymer Optical Fibres (POFs). The devices were firstly developed from commercially available multilayer dielectric stack filters and secondly from engineered Cholesteric Liquid Crystals (CLCs) manufactured by using nematic LC doped with appropriate chiral dopant. In order to have the reflection of above 80% for the desired wavelength and also for non-polarised light approximately 20 layers of CLC with both helicities in a planar Grandjean texture were used. The CLC with left handed helix reflects left circularly polarised light, while the right handed helix reflects the right handed circularly polarised light.