Fiber Eavesdropping Using Tapers in Standard and Trench-Assisted Single-Mode Fibers

We investigate taper tapping in three types of single-mode fibers using simulations and experiments: Standard single-mode fiber (SMF), bend-resistant single-mode fiber (BR-SMF) with a shallow index trench, and bend-insensitive single-mode fiber (BI-SMF) with a deep index trench. Both trench-assisted fibers demonstrate a larger mode field expansion than the standard single-mode fiber, making them more susceptible to taper-tapping than standard single-mode fibers. Tapering both SSMF and BR-SMF fibers to as little as $\mathrm {25~\mu m}$ diameter results in a high information extraction efficiency for an eavesdropper ( $\mathrm {\geq -20~dB}$ ) while maintaining low loss for the legitimate channel ( $\mathrm {\leq 1~dB}$ ), underlining the vulnerability of all fiber types to taper tapping. We also use optical time-domain reflectometry (OTDR) to identify changes in the fiber before and after tapering. Changes in the back reflected power of $\mathrm {\leq 0.5~dB}$ and $\mathrm {\leq 0.25~dB}$ are observed in $\mathrm {25~\mu m}$ tapers in SMF and BR-SMF respectively.