Polarisation dependence of the phase shift of 2×2 singlemode fibre directional couplers

Phase shift variations of 2×2 couplers caused by polarisation changes of the input lightwaves have been investigated. Non-negligible deviations from the ideal phase shift value of 180° occur, especially when unequal states of polarisation are excited at the input ports of the coupler. This effect must be considered in interferometric applications