RU2506701C1 - Method of detecting unauthorised signal tapping from single-mode optical fibres - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to methods of monitoring fibre-optic lines based on single-mode optical fibres and can be used as a method of separating local defects formed through unauthorised tapping from local defects caused by permanent optical connections. The method involves measuring loss reflectograms depending on the length of the optical fibre from each of its terminals using a backscattering method at the operating wavelength and at a wavelength larger than the operating wavelength. Areas with local defects are determined on each reflectogram and areas with local defects on all reflectograms are compared. Unauthorised tapping is indicated by presence of a reflected signal and by the value of direct losses at the longer wavelength being higher than that at the operating wavelength. When measuring reflectograms at different terminals, direct losses at each wavelength must be equal.

EFFECT: high efficiency of detecting unauthorised tapping in single-mode optical fibres.

4 dwg

Description

The invention relates to methods for monitoring fiber-optic transmission lines (FOCL) based on single-mode optical fibers (OM OM) and can be used as a method for separating local defects formed by unauthorized taps from local defects caused by one-piece optical connections.

The well-known "Method for detecting sections of a fiber optic transmission line with increased lateral radiation" (see Russian patent No. 2252405, published in BI No. 14 dated 05/20/2005). The method is the closest in technical essence to the claimed detection method and is therefore selected as a prototype. The method consists in the fact that using the backscattering method (MOR), a loss trace is obtained in the OM OF the FOCL depending on the length by which the sections with local introduced losses are determined, which measure the amount of introduced losses and the distance from the beginning of the line to the section with entered losses. The maximum permissible value of the introduced losses is determined by the formulas: for conventional fiber-optic transmission systems

α _{l max} = -10lg [1-QW _pore 10 ^{-0.1α (L-1)} / (K _p W _p )],

for systems with quantum noise

α _{l max} = -10lg [1-Q (2еВI _p ) ^1/2 10 ^{-0,05α (L-1)} / K _p (W _p MS) ^1/2 ],

where W _then is the sensitivity threshold of the interception receiver at a signal-to-noise ratio of 1;

Q is the maximum allowable signal-to-noise ratio for the reconnaissance receiver, ensuring the impossibility of intercepting information;

K _p - transmission coefficient, showing what part of the radiation lost in the fiber optic line fell on the input pole of the interception receiver;

W _p - the power of the optical signal at the input pole of the receiver of the fiber optic transmission system, providing the required transmission quality;

α is the value of the introduced losses in the fiber optic line, the average in the area (L-1);

L is the length of the fiber optic transmission line;

1 - distance from the transmitter to the place with the measured value of the local introduced losses;

e is the electron charge;

B - frequency band of transmitted information signals;

I _p - the integral of Person;

S is the transfer characteristic of the photodetector of the receiver of the fiber optic system,

compare the measured value of the introduced losses with the calculated maximum allowable value, determine the areas with increased lateral radiation when the measured value exceeds the maximum allowable value.

The disadvantage of the above method is the inability to determine the unauthorized signal taps with OM OM in contrast to its welded joints, especially after the installation of the fiber optic link.

The technical task to be solved is the creation of a method for detecting and localizing unauthorized taps (“bookmarks”) in the optical fiber after installing a fiber optic link with the help of MPAs, which can be created before the protection system of the fiber-optic transmission system (FOTS) of restricted access information is turned on.

Achievable technical result is to increase the efficiency of detection of “bookmarks” in the OM of OM using MPAs by measuring the dependence of losses on the length of OM at two wavelengths and from different poles of the line.

To achieve a technical result in the method for detecting unauthorized signal taps from single-mode optical fibers, which consists in the fact that using the backscattering method at the working wavelength, one loss trace is measured depending on the length of the optical fiber from one of its poles, and areas with local defects are determined , new is that they additionally measure reflectograms from both poles of the optical fiber at a wavelength longer than the working length, and from the other pole of the optical fiber and at the working wavelength, they compare the areas with local defects in all reflectograms, detect unauthorized signal taps by the presence of the back-reflected signal or in the case when the magnitude of direct losses at a longer wavelength is greater than at the working wavelength, while measuring reflectograms from different poles, direct losses at each wavelength should be equal to each other.

A new set of essential features in the present method allows to increase the efficiency of monitoring OM OM from "bookmarks" using MPA by measuring the dependence of losses on the length of OM at two wavelengths and from different poles of the line.

Figure 1 shows the trace of OM OM with a local defect having a large reflected signal.

Figure 2 presents the trace of OM OM with welded joints, measured at two wavelengths of 1.31 and 1.55 μm.

Figure 3 shows the traces of the same bend OM OM, measured at two wavelengths of 1.31 and 1.55 μm.

Figure 4 presents the trace of OM OM with welded joints, measured from opposite optical poles.

The inventive method works as follows. The connection of photodetector devices (FPU) for the removal and registration of optical signals can be carried out in two ways: using a detachable optical connector (taps with an optical connector) or using a local bend of an optical fiber (or another method without breaking the optical fiber).

Any connection to the FPU using optical connectors inevitably leads to the appearance of an air gap at the connection point and, accordingly, to the appearance of a reflected signal on the trace. Figure 1 presents the defect with the tap signal from the OB using a detachable optical connection.

Connecting to the FP FP using a bend or other method without breaking the OB leads to the fact that a local defect is formed only with direct losses. The size of the mode spot in an OM OM having a Gaussian shape increases with increasing wavelength. This leads to the fact that when an optical signal is output without rupture of the optical radiation, the radiation from the “tail” of the mode spot is primarily output. It turns out that with any non-discontinuous method of output from a larger mode spot, a greater amount of side radiation comes out. For example, FIG. 2 shows traces of the same bend of OM OM, measured at two wavelengths of 1.31 μm and 1.55 μm. Figure 2 shows that the magnitude of direct losses at a wavelength of 1.55 microns exceeds the magnitude of direct losses at a wavelength of 1.31 microns by 20 times. In welded joints, on the contrary, direct losses at two wavelengths are approximately equal. Figure 3 shows traces of the same OM OM with welded joints, measured at two wavelengths.

Welded joints are joints of OM segments having different parameters (within the tolerances defined by the standards), which leads to the fact that the sizes of mode spots in the joint do not match. Therefore, the losses in the welded joint do not coincide when measuring from different sides, not only in magnitude, but also in sign (the effect of "amplification" of the signal). Therefore, in the standards, the measurement of losses by reflectograms is calculated as the arithmetic mean value when measuring from both poles of the OM. Figure 4 shows the traces of OM OM, measured on both sides at a wavelength of 1.31 μm. When measuring a local defect without rupturing the OM, the losses measured from different sides coincide.

To confirm the operability of the method, measurements were carried out using a FOD - 7005 OM OM microreflectometer with welded joints and bends at the wavelengths of 1.31 and 1.55 μm, which are presented in figures 1-4.

Claims (1)

A method for detecting unauthorized signal taps from single-mode optical fibers, which consists in the fact that using the backscattering method at the working wavelength, one loss reflectogram is measured depending on the length of the optical fiber from one of its poles, areas with local defects are determined, characterized in that additionally measure reflectograms from both poles of the optical fiber at a wavelength longer than the working length, and from the other pole of the optical fiber at the working wavelength, compare areas with local defects in all reflectograms, they detect unauthorized signal taps by the presence of a back-reflected signal or in the case when the magnitude of direct losses at a longer wavelength is greater than at the working wavelength, while direct losses at each pole are measured from different poles wavelength should be equal to each other.

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