The axial twisting of a fused-tapered single-mode fiber coupler causes optical decóupling, which can change the power-splitting ratio of the coupler. Twisting of the coupling region causes a redistribution of the modal fields away from the coupler axis as a result of an effective refractive-index change. A simple analytical expression is derived that describes this decoupling effect. An environmentally stable package is implemented that allows a coupler's response to be tuned through all possible values.