Transmission properties of microcoils based on twisted birefringent fibre

►Transmission spectrum of birefringent microcoils was modelled. ►Twisting the fibre between turns couples the orthogonal modes together. ►Coupling from y to x mode is favoured, or vice versa, depending on twist direction. ►Extinction ratio of resonances can be reduced or enhanced by 10%. ►Spectrum contains bands where output is largely polarised in one mode only.