Fiber optical loop mirror with a symmetrical coupler and a quarter-wave retarder plate in the loop

In this paper we report an experimental method to vary the transmission through a fiber optical loop mirror with a symmetrical coupler, highly twisted fiber and a quarter-wave (QW) retarder plate in the loop. We demonstrate that as the QW retarder plate is rotated it is possible to adjust the transmission over a wide range (from around 2% to around 82%) and that by adjusting the twist in the loop nearly ideal operation can be achieved. We provide a detailed theoretical analysis of the operating characteristics.