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IEEE Photonics Technology Letters

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

Thomas D. Bradley, Menno van den Hout, Besma Kalla, Vincent van Vliet, Marianne Bigot-Astruc, Adrian Amezcua Correa, Pierre Sillard, Gary Weiner, Peter J. Winzer, Chigo M. Okonkwo

Published Jun 5, 2024• 12 citations
Publisher

Abstract

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.

DOI
10.1109/lpt.2024.3410043

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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 as25 μm\mathrm {25~\mu m}25 μmdiameter results in a high information extraction efficiency for an eavesdropper (≥−20 dB\mathrm {\geq -20~dB}≥−20 dB) while maintaining low loss for the legitimate channel (≤1 dB\mathrm {\leq 1~dB}≤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≤0.5 dB\mathrm {\leq 0.5~dB}≤0.5 dBand≤0.25 dB\mathrm {\leq 0.25~dB}≤0.25 dBare observed in25 μm\mathrm {25~\mu m}25 μmtapers in SMF and BR-SMF respectively.
Initially considered secure against interception, fiber-optic networks were subsequently found susceptible to covert tapping, although such attacks require different coupling mechanisms and components than those used for twisted pair and coaxial cables. Fiber-optic networks carry the majority of data and voice traffic worldwide, including backhaul and fronthaul traffic in 4G and 5G radio networks, connections between data centers, and submarine cable systems. Consequently, physical access to fibers for interception has attracted the attention of intelligence agencies and industrial spies. This vulnerability is facilitated by the fact that single-mode optical fibers (SMFs) used in telecommunication networks have evolved relatively little over the last 40 years. Fibers of substantially different designs—anti-resonant hollow core fibers (HCF) and multi-core fibers (MCF)—made their commercial debut around 2023, and their susceptibility to tapping may differ substantially from that of conventional SMFs. New promising fiber monitoring techniques based on polarization measurements have also been demonstrated recently. While established tapping techniques remain feasible for conventional fibers, new fiber designs and monitoring technologies can make covert interception difficult. This review examines physical fiber tapping techniques and their practical feasibility, with particular emphasis on optical power constraints, introduced loss, detectability, and implementation limitations. It also reviews prevention and monitoring approaches, including optical time-domain reflectometry, received-power monitoring, and state-of-polarization (SOP) monitoring, and provides a prospective engineering assessment of the susceptibility of MCFs and HCFs to physical tapping.
In this Letter, we present a scheme for detecting fiber-bending eavesdropping based on feature extraction and machine learning (ML). First, 5-dimensional features from the time-domain signal are extracted from the optical signal, and then a long short-term memory (LSTM) network is applied for eavesdropping and normal event classification. Experimental data are collected from a 60 km single-mode fiber transmission link with eavesdropping implemented by a clip-on coupler. Results show that the proposed scheme achieves a 95.83% detection accuracy. Furthermore, since the scheme focuses on the time-domain waveform of the received optical signal, additional devices and a special link design are not required.
A quantum-enhanced fiber curvature sensor based on single-mode fiber (SMF) taper exploiting intensity-correlated pulse twin beams has been proposed and experimentally realized. The curvature can be measured by intensity difference noise power and degree of intensity difference squeezing which are tailored by the bending-induced loss of one beam’s SMF taper. In addition, the three different waist diameters of SMF tapers, i. e.,52.12 μm52.12~ \boldsymbol {\mu }\text{m}52.12 μm,61.68 μm61.68~ \boldsymbol {\mu }\text{m}61.68 μm, and 67.54μm\boldsymbol {\mu }\text{m}μmrespectively have been prepared using a fusion splicer. Finally, the maximum measured sensitivity enhancement of 4.42 dB/m−1and signal-to-noise ratio enhancement of 2.0 dB have been experimentally implemented through intensity difference measurement technique compared with the corresponding classical counterpart (double-channel light source). In contrast to other curvature sensors, this proposal has the advantages of high sensitivity, ease of fabrication, simple structure, and low cost.
Optical fiber communication is not as secure as generally perceived. There are a number of known methods of extracting or injecting information into a fiber link, while avoiding detection. Few incidents have been reported as a successfully tapped fiber is difficult to detect. In this paper we highlight a number of known fiber tapping methods. We report simulation of optical characteristics of a fiber being tapped by `bend' method and proof of concept with physical experiment. We also presented visualized scenarios in which a resourceful eavesdropper can compromise security of a fiber link with existing technologies. Some measures to prevent fiber tapping or to nullify the significance of information tapped from fiber were also discussed.
Optical fiber communication was widely believed that an optical network was a good way to prevent hacking through wiretaps which intercept network data. However, fiber optic cable is not particularly secure. Various tapping methods can be used to extract data from fiber optic networks with little or no risk of detection. One of the easiest ways of intruding into a fiber optic communication link is creating a bend on the fiber cable. Even small percentage of the transmitted light acquired with the bending method is enough for recovering the data by the attacker. Though there are some techniques and devices that can track the power and intensity inside the fiber cable, these methods cannot detect an attack exploiting small leakage. Therefore, it is critical to determine a range of radii of curvature for which tapping attacks are not detectable. Though there are some studies to quantify the power loss both theoretically and numerically, as of the authors' knowledge, there has not been any experimental validation of those studies. In this respect, an experimental optical setup and a technique relying on the ADC conversion capability of Arduino UNO to measure power loss resulting from different bending radii are proposed in this paper. A lower limit for radius of curvature for an attack to be undetected is obtained and the estimations in the literature for power loss resulting from fiber bending is validated.