Effect of twist on fused couplers

The effect of twist on fused couplers is discussed theoretically. We find that the twist, large or small, introduced unintentionally in fused couplers may deteriorate the extinction ratio of polarization splitting when devices are used as polarization beam splitters, and that the modulation period of the spectral response of fused couplers can be tuned by controlling the twist rate when they are used as wavelength filters.