Modified dual Mach-Zehnder interferometers with new locating algorithm for intrusion detection

Abstract

A new type of dual Mach-Zehnder interferometer (DMZI) scheme is presented to eliminate the polarization induced fading usually encountered in fiber-optic intrusion detection systems that use DMZIs. With such a new optical scheme, two identical signal waveforms with only a time delay that corresponds to the location of intrusion can always be obtained without using a polarization control loop. In the study, we also present a new algorithm for locating the intrusion-induced disturbance on the fiber. The experiments carried out in our lab have demonstrated that locating errors of <26 m can be obtained with the presented system used for detecting an intrusion point on a 250 meter long fiber cable. Also, the proposed DMZI system has been tested for years without changing any optical components except the laser type and the length of the sensing fiber. Notably, the thresholds for determining the intrusion have never been altered since the laser type was changed in 2018. In addition, detections of the clockwise and counter-clockwise signals have maintained a condition of high interference visibility, and the locating capability has remained at the same level.

Dual Mach–Zehnder Interferometers With Hierarchical Clustering Analysis Method Applied for Positioning the Intrusion

“…However, an intrusion event should not be determined by merely examining the signal waveforms acquired within a particular 10-ms time period or several 10-ms time periods. In a previous work [19], we have investigated the locating capability of the DMZI intrusion sensor by examining 100 pairs of signal waveforms (i.e., CW and CCW signal waveforms) with each pair acquired in a 10-ms time period, where an intrusion event was defined if more than a half of the 100 pairs of signal waveforms were reduced to an intrusion case in which three parameters (i.e., SA, LC, and FR [19]) all exceeded their respective thresholds. Each data point is obtained by averaging the 99 position numbers with each obtained by applying the clustering analysis to a pair of differential signals S(t) and S(t − 2τ 2 ).…”

“…constant over the frequency range of concern in this study. However, the delay time varies due to detection noises and nuisances, as revealed in [19,Figs. 6,7,[9][10][11][12][13][14].…”

“…The DMZI system used in the study is as same as that in [19] and is shown in Fig. 1, where a 1036-m-long fiber cable comprising four single-mode fibers (SMFs) is used as a disturbance sensor.…”

“…2(c) by noting that the calculated position numbers at some frequencies lower than ∼1000 Hz could reach a magnitude of 20 or even larger, in contrast to those obtained at frequencies higher than 2500 Hz. This was the reason why the position numbers at frequencies higher than 2500 Hz were only considered in determining the location of intrusion in our previous work [19]. Meanwhile, nuisances arising from temperature variation, wind, small animals, and so on could be unwanted sources added to the detected signals and result in locating errors.…”

“…We have proposed a Fourier spectral analysis (FSA) method to determine the location of disturbance for a modified DMZI system [19]. In the FSA method, the spectrally dependent locations of disturbance were calculated and the average of the locations over an appropriate spectral band gave the location of disturbance.…”