An optical fibre device comprises an optical fibre null coupler (10) formed of two or more optical fibres (20, 30) fused together at a coupling region, at least a part of the coupling region being mechanically twisted substantially about a longitudinal axis of the coupling region. The twisting can alleviate the polarisation dependance observed in untwisted null coupler devices. In other embodiments of the invention, substantially polarization-insensitive operation of a null coupler acousto-optic tunable filter is obtained by simultaneously applying two acoustic waves.The two waves provide phase-matched coupling for each of the individual eigen-polarization states. The polarization dependence of the device to excitation at one of the acoustic frequencies leads to two different filter responses, one corresponding to each polarisation. One of these two responses will be at higher optical wavelength than the other, and if a particular input polarisation gives rise to, say, the higher optical wavelength filter response of the pair at a particular acoustic frequency it will do so at other acoustic frequencies. So, if two acoustic frequencies are applied, the acoustic frequencies can be selected so that the higher wavelength response of the two filter responses from one acoustic frequency overlies in optical wavelength the lower wavelength response of the two filter responses from the other acoustic frequency, then the combination of the two overlying filter responses (which relate to the two different input polarisations) provides a substantially polarisation independent filter response.