fig7 shows a preferred embodiment using two thin layers of liquid crystal 15 to achieve polarisation independent operation the glass plates 16 are coated with a transparent conducting material 17 eg indium tin oxide ito and separated by spacers 18 typically 10 u03bcm in thickness the conducting material is treated such that for no applied field the liquid crystal in layer 1 aligns along the x axis and in layer 2 aligns along the y axis application of an electric field causes the liquid crystal in both layers to align along the z axis resulting in a polarisation independent change in optical path travelled by the beam 19 at an appropriate operating point the optical path or phase change will be close to linearly dependent on applied voltage fig8 shows a single element switchable split beam fourier filter the construction is similar to a standard sbff but the element optical flat 20 can be switched electro mechanically or otherwise from a position out of the beam 21 to a position in the beam 22 as indicated by the arrow which gives the required filter characteristics fig9 shows a split beam mode convertor the only difference from the sbff described above being that the fibres 23 24 support two or more optical modes and the element approximately bisects the beam if fibre which supports two modes lp 01 and lp 11 is used rather than single mode fibre an sbff can be effectively used as a mode convertor converting power in the lp 01 mode.