Physical Layer Security of Space-Division Multiplexed Fiber-Optic Communication Systems in the Presence of Multiple Eavesdroppers

In this paper, we examine the information-theoretic security of multiple-input-multiple-output space-division multiplexed (MIMO-SDM) fiber-optic communication systems in the presence of multiple eavesdroppers. In particular, we analyze the achievable secrecy rate for the two special cases that reflect different capabilities of the eavesdroppers: independent eavesdroppers who do not share received information and colluding (cooperating) eavesdroppers who can combine and jointly process the received signals. Our results show that MIMO-SDM systems are robust against multiple independent tapping attacks, in the sense that the average achievable secrecy rate is strictly positive even with an infinite number of eavesdroppers. Though extremely difficult to implement in practice, if all the eavesdroppers could cooperate coherently, the average secrecy rate decreases quickly with the number of eavesdroppers. As such, to counter the colluding eavesdroppers, a combination of much higher SNR for the legitimate receiver and higher mode-dependent loss (MDL) for the eavesdroppers is needed.