Full duplex coherent optical communication with one in-line polarisation controller

Ordinarily full duplex coherent communication requires polarisation control or diversity receivers at both ends and has a 6 dB penalty because of power splitting and combining losses. A novel scheme having only one in-line polarisation controller and no penalties by utilising the reciprocity properties of optical fibre is presented. The large reduction of polarisation complexity is particularly conspicuous for subscriber networks. The feasibility of the scheme was demonstrated by testing the reciprocity of the retardation of 10 km fibre under dynamic conditions with an orthogonal polarisation reflector.