Demonstration of a compact polarisation based two-dimensional multimode optical fibre space switch as optical fibre sensor multiplexer

Abstract

We constructed two polarisation based space division switches, using nematic liquid crystal variable retarders (LCVR) or ferro-electric liquid crystal polarisation rotators (FLC). Our systems have the potential of establishing a bandwidth transparent relation between 1 input and 9 output multimode optical fibres. [1] To shift a polarised light beam from the input to one of the nine possible outputs, we use a combination of commercially available large aperture LCVRs or FLCs and birefringent calcite [CaCO3] crystals (BC). This yields easily controllable modules for unidirectional beam displacement. Since each LCVR or FLC only needs two control voltage amplitudes, one for zero phase retardance and one for π phase retardance the complete system can be controlled with only four bits. The input light is provided through a GRIN collimating lens terminated singlemode optical fibre, whereas the light exiting the last BC is coupled in 1 out of 9 possible output fibres located in a two-dimensional PMMA (PolyMethyl MethAcrylate) fibre array holder fabricated with deep proton lithography. This technique allows to position the fibres with a ± 2 pm accuracy. The compact system construction is depicted in figure 1. The maximum frequency at which the LCVRs can be driven and hence, at which switching between two fibres is possible, is on the order of 30 Hz. Using a Nippon Sheet Glass Planar Microlens (DI-PML®) array to couple light exiting the last BC in the optical fibres located in the PMMA array holder, we obtain the losses and cross-talk ratios given in table 1.