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Physical fiber security by the state of polarization change detection

Adrián Tomašov, Petr Dejdar, Tomáš Horváth, Petr Münster

Published May 27, 2022• 5 citations
Publisher

Abstract

The main topic of this paper is the detection state of polarization changes to enhance data security in fiber cable paths. The changes are generated by fiber manipulation or movements, which suggest potential security threats. Our designed system detects these changes using a polarization beam splitter and a pair of photodetectors. The values are subtracted from each other, sampled, and sent for analysis. The software detector applies FFT onto the signal and normalizes the output. The last step compares the sum of the bottom eighth of the spectrum against the threshold.

DOI
10.1117/12.2618490

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The main topic of this paper is the detection state of polarization changes to enhance data security in fiber cable paths. The changes are generated by fiber manipulation or movements, which suggest potential security threats. Our designed system detects these changes using a polarization beam splitter and a pair of photodetectors. The values are subtracted from each other, sampled, and sent for analysis. The software detector applies FFT onto the signal and normalizes the output. The last step compares the sum of the bottom eighth of the spectrum against the threshold.
Polarization state changes in optical fibers provide a valuable tool for enhancing optical network security. This paper uses a robotic arm to compare the polarization sensing capabilities of 1G SFP and 200G CFP2 transceivers under controlled fiber manipulations. Stokes vectors extracted from a 4-diode polarization analyzer are evaluated against a reference signal using mean squared error (MSE). Results show that the 1G system is more sensitive to fiber manipulations, achieving lower MSE overall, but is affected by signal spikes during force application. The 200G system demonstrates greater stability but reduced sensitivity to fine birefringence changes, resulting in higher MSE values.
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.
Optical fiber channels are used as media to transfer the information globally. This paper presents an implementation of a novel procedure using which a secured communication between two parties can be carried out using polarized beam of light over an optical fiber. The paper presents the experimental results obtained of the procedure in the lab environment and a security analysis of the same. It is observed that polarization state of a light pulse cannot be retained as it travels over an optical fiber because of the birefringence phenomenon. Multiple environmental factors such as pressure, vibration, temperature, etc. also add a non-linearity to the birefringence of an optical fiber leading towards an unpredictable polarization state changes over the course of an optical fiber. The proposed procedure helps the receiving party to successfully retrieve the data in the form of a polarization state transmitted by the sending party without having any knowledge about the state of polarization at the transmitting end. The paper also explains an added layer of security the procedure provides to the communicating parties to make it difficult for an adversary to fetch the data being transferred. The proposed system does not depend on the wavelength of the light being used, nor does it depend upon the type of the optical fiber used for the communication. Using this procedure, multiple bits of secured information can be sent over an optical fiber in a single polarized pulse and retrieved at the receiving end, also known as Polarization Shift Keying.
Optical soliton amplitude squeezed state caused by intensity correlation self-phase modulation effect was generated experimentally.The main part of experiment system was asymmetric Sagnac interferometer.Pulsed laser was connected to a coupler with splitting ratio of 90∶10 by PC/FC connector.The coupler and 40-meter photonic crystal fiber composed interference loop.At proper input power,optical soliton amplitude squeezed state could be observed at output terminal of the interferometer.Balanced homodyne detection system composed of 50/50 beam splitter,two low-noise photo detectors,adder/subtracter,and spectrum analyzer was used to detect the squeezed state.Intensity distribution curves of input and output of the Sagnac loop were drawn,and the signal noise curve using shot noise as reference was obtained.The compressibility was 1.6?dB.Analysis on experimental data shows that it's possible to improve compressibility further by choosing proper photonic crystal fiber.