Impact of Random Fiber Coupling Efficiency on Secrecy of FSO-Fiber Communication Systems

This letter investigates the physical layer security of free-space optical-fiber converged (FSO-FC) communication systems under realistic channel impairments, including atmospheric turbulence, transmitter-receiver pointing errors, and the statistical nature of fiber coupling efficiency (FCE). We derive series-based analytical expressions for the generalized secrecy outage probability, average fractional equivocation, and average information leakage rate. These expressions clearly characterize how random FCE affects secrecy performance under joint channel impairments. Numerical results show that including random FCE enhances the secrecy performance of an FSO-FC system by approximately 86%. The impact of atmospheric turbulence, pointing errors, aperture diameter, average FCE, and lens focal length on secrecy of the proposed FSO-FC system is also examined.