Accuracy of analytical expressions for Rayleigh backscattered crosstalk in bidirectional multi-core fiber transmissions

Use of bidirectional multi-core fiber (MCF) transmissions is attractive as it suppresses inter-core crosstalk by propagating the signals in opposite directions between nearest neighboring cores; therefore, signal quality and/or core density is improved compared with unidirectional transmissions where all the signals propagate in the same direction across the cores. In long-haul bidirectional MCF transmissions, Rayleigh backscattering is one of the major causes of inter-core crosstalk (XT). In this paper, we derive approximate expressions for Rayleigh backscattered XT and discuss the accuracy of the approximations. The derived expressions revealed that the reduction from conventional co-propagating XT to backscattered XT depends approximately only on wavelength, refractive index, effective area, the Rayleigh scattering component in propagation loss, and span loss.