Secrecy Capacities in Space-Division Multiplexed Fiber Optic Communication Systems

The spatial diversity offered by space-division multiplexed (SDM) fiber optic transmission systems can not only be used to increase system capacity, but also to achieve physical-layer security against fiber tapping attacks. In this paper, we examine the information-theoretic security of optical multiple-input-multiple-output (MIMO) SDM by evaluating the trade-off between the achievable information rate and the confidentiality for different channel dynamics. In particular, we provide problem formulations for secure communication over these channels and study three types of secrecy capacities: outage capacity, average capacity, and outage-free capacity, each serving as a performance metric for a coding strategy tailored to a specific type of MIMO-SDM channel. We also assess the impact of key system parameters, such as the number of modes, the mode-dependent loss (MDL), and the signal-to-noise (SNR) ratio, on the various secrecy capacities. Our results indicate that, with a proper design of channel codes that balance information rate and security, an SDM system has the potential of offering confidential data transmission at a rate that could be orders of magnitude higher than what can be achieved through other means of encryption. Moreover, we show that MDL, unavoidably induced by fiber tapping, can allow information-theoretic security even if the SNR of the eavesdropping channel is better than that of the legitimate receiver.