rus
Issue #1/2015
V.Grishachev
Leakage Of Channel Information Based On The Spurious Crosstalk (Modulation) In An Optical Fiber
Leakage Of Channel Information Based On The Spurious Crosstalk (Modulation) In An Optical Fiber
In the continuation of the review some elements of optic cable systems are considered as source to create the possible information leakage channel. It was shown that the leakage signal power is determined by the power of probing signal. Protection is proposed to build on the basis of screening, filtering, noise and detection spurious modulation.
Теги: eavesdropping talks optical cable systems spurious light modulation in optical fiber оптические кабельные системы паразитные модуляции света в оптоволокне подслушивание переговоров
Spurious Acoustic Modulation[2]
Acoustic fields refer to one of the main sources of information concerning the informatization object in the form of confidential conversation, spurious sound waves accompanying different processes. All these aspects highlight them against the background of other physical fields as one of the main channels of information leakage. Let us discuss the occurrence of spurious modulations of light flux on the cable optical inhomogeneities in more detail [4]. The sound wave is periodic process of elastic disturbances in the medium at the approximate frequency range of 10 Hz to 20 kHz. In air it is elastic compression-extension with the wavelength of 34 m to 1.7 sm. 1.0 kHz shall be assumed to be the typical frequency with the wavelength of 34.0 sm, so the typical length of field homogeneity shall be Λ=17.0 sm. In this case, majority of optical inhomogeneities is l<<Λ and therefore the interaction process can be deemed spatially homogeneous.
Sound wave has the mechanical action on fiber, response of which is characterized by the elastic properties of optical cable and defect in it. Let us designate some degree of freedom inside the defect through x which influences on the coefficient of backscattered radiation βr and varies upon the external action of acoustic field – acoustic pressure δp. Pressure variation in the wave δp causes the variation of parameter by δx and, in turn, changes the power of backscattered radiation by δβr. Modulation depth is determined by the relative variations of the coefficient βr as follows
m = δβr / βr = δx / x0,
where x0 is some typical unperturbed value of shift in oscillating system with the linear dependence βr~x.
The main action of acoustic field consists in the mechanical action on defect, its compression-extension by the variable air pressure. Defect as the mechanical system has resonant response to the mechanical effect with the set of its own frequencies {2pfn} and relevant resonance frequencies. Then, the dependence of modulation depth on the frequency of external effect for the nth resonance will have the following form
mn = δ xn / x0 = ( xn / x0 ) · ( fn γ / π ) / ( fn 2 – f 2 + i f γ / π ),
here xn = S δ p0 / 4 π M γ fn is the resonance amplitude at the frequency fn and amplitude of acoustic pressure δp0 for optical defect with the weight M upon the cross-section area S and coefficient of mechanical oscillation damping γ.
The total modulation depth m in the form of response to the broadband action of acoustic field is defined as the mean square value from mn at the whole frequency range which depends on the action itself.
Releasable Connection
In the infrastructure of cable network, the releasable connections provide the development of information system via the accumulation of additional elements through connectors and adapters. It generates additional hazards connected with the leakage of verbal information through the spurious modulations caused by acoustic field in connectors. Acoustic wave from the sound source affecting the connector changes the distance between fibers and causes the angular and radial displacement of connected fibers. Formation of the backscattered radiation is connected with the reflection from thin layer which occurs upon the contact of two fibers (Fig. 6). Fabry-Perot interferometer represents the model of such system on the thin layer with the thickness d and refraction index n0. In this approximation, coefficient of backscattered radiation is determined on the basis of Airy’s formula for multi-beam interference on reflection
βr = [ 4R · sin 2 ( Δϕ / 2 ) ] / [ ( 1 – R)2 + 4R · sin 2 ( Δ ϕ / 2 ) ],
where R is the coefficient of reflection of the layer edges, Δϕ = ( 4π/λ ) · n0 · d · cos A is the difference of phases of adjacent reflected beams, λ is the light wavelength, A is the angle of light incidence on layer. In the ideal case of normal incidence (A=0) on flat slab (without wedging) with the thickness which is less than the light wavelength (d<<λ) and low reflection value (R<<1) for the refraction index n0=1.0 (air space) we will obtain the following
βr ≈ 160 R ( d / λ )2.
And the modulation depth caused by the external acoustic action will be determined on the basis of the following expression
m = 2 δd / d0 ≤ 2dm / d0,
where d0 is the distance between fibers in unperturbed state, δd is the perturbation and dm~δp0 is the resonant axial displacement of fibers.
The suggested model does not take into account the angular and radial displacement of fibers which introduce additional spurious modulations and angular divergence of the light incident on the reflecting layer. But assumed approximation allows evaluating the hazard of this information leakage channel. Firstly, it should be noted that the reflection βr depends on the wavelength and this fact allows increasing the leakage signal power through the reduction of the wavelength of probing radiation and at the same time the modulation depth will not change. Secondly, if we assume that the wavelength of probing radiation is λ=1 μm the resonant axial displacement of fibers can be estimated as dm<<d0<<λ. In reality, at the acoustic pressure δp0=2·10–2 Pa (60 dB SPL), if we assume that fiber with the wavelength about l≈0.02 m, density ρ=M/Sl=2,1·103 kg/m 3 (fused quartz) forms the oscillating system which has the resonance frequency f0 and γ<<f0 (it is suggested that γ=f0/20π and it corresponds to the quality factor Q=20π 2) then
dm = C / f02,
where C = Q δp0 / 4π 2 ρ l ≈ 2,4 · 10–3 m·Hz 2 is the constant in approximation for this releasable connection. In this case, for the resonance frequency f0=500 Hz we will receive the resonant axial displacement of fibers dm≈0,01 mm and the modulation depth can be estimated m≤20% in case of estimation of the distance between fibers in unperturbed state d0≈0,1 μm. With the power of backscattered light 10·lg (Pr/P0) = –50 dB for the single-mode fiber, we will obtain that the power of leakage optical signal is 10·lg (δPr/P0) ≤-57 dB, and it can be fully registered. In case of decrease of estimation of the distance between fibers in unperturbed state to the value which is less by the order (d0≈0,01 μm, case of good treatment of contacting surfaces), we will obtain the power of leakage optical signal 10·lg (δPr/P0) ≤-50 dB with the modulation depth m≤100%. In case of releasable connection of multiple-mode fibers, the reflection index will increase by two orders and the modulation depth will grow too.
In the discussed model, modulations of backscattered and transmitted light flux caused by acoustic field refer to the connected processes. Growth of the reflection from contact results in the growth of loss on transmission so |δPp|=|δPr|. Hence, the depth of modulation of transmitted radiation
m~= ( βr / βp ) m << m.
Thus, the depth of modulation of transmitted radiation is considerably lower than the depth of reflected radiation (i. e. m~= ( βr / βp ) m << m.<<m) and it can be explained by the fact that the power of radiation which transmitted through the contact is considerably higher than the power of reflected radiation and their variations by the absolute value are equal (i. e. βr<<βp). Such result shows that the reflectometry methods of formation of acoustic information leakage channel are more effective in comparison with the registration of spurious modulations of transmitting light flux.
Thus, the efficiency estimation of acoustic information leakage channel by modulation depth shows great danger of such eavesdropping. It should be noted that the spurious modulation has significant noise component which is connected with possible distortions due to the process nonlinearity, with several modulation mechanisms, with light flux divergence so the leakage signal will be characterized by much lower total efficiency of recognition of speech tenor or sound meaning.
Comparison with Experiment
Experimental studies of light modulation depth on the optical contact of multi-mode fibers performed earlier prove the specified estimations [5]. Studies were performed on the experimental facility (Fig. 7) consisting of optical distribution frame, in which SC-SC adapter of multi-mode fibers and microphone for the control of sound pressure level (SPL) were located. Stabilized laser radiation arrived to SC-SC adapter and it was registered at the exit by the photodetector with selective nanovoltmeter or integrating voltmeter. Acoustic field with the spectrum of white noise was formed in the optical distribution frame and it allowed excitement of all mechanical resonances in the optical contact of multi-mode fibers. White noise formed the spurious modulations of light intensity which were registered by photodetector. Application of the selective nanovoltmeter made it possible to perform the spectral studies of light modulations with the band width of 25 dB of octave.
Study results are given in the form of diagrams of spectral dependence of modulation depth (Fig. 8) for the transmission m~= ( βr / βp ) m << m. (f) at set SPL of white noise and dependence (Fig. 9) of modulation depth on sound pressure level m~= ( βr / βp ) m << m. (SPL) for the laser wavelengths λ=850 nm and 632.8 nm. It can be obtained from the first diagram that this SC-SC adapter has the modulation depth m~= ( βr / βp ) m << m. ≈0,9 ppm (1 ppm=10–6) at the resonance frequency f0≈3 kHz. If we assume that (βr/βp) =40 dB and it corresponds to the ratio of the powers of radiation reflected and transmitted through the contact of multi-mode fibers, then at the resonance frequency f0≈3 kHz we will obtain the estimation of fiber resonance displacement dm≈0.0003 μm and modulation depth m~= ( βr / βp ) m << m. ≈0,6 ppm. It corresponds to the depth of modulation of reflected radiation m≤0,6%. Comparing the experimental and theoretical values m~= ( βr / βp ) m << m. we have the matching of orders and the difference by 1.5 times can be connected with the failure to take into account other mechanisms of spurious modulations. The second diagram confirms the influence of probing radiation wavelength on the modulation depth and with the growth of SPL of white noise the saturation is observed, which corresponds to the saturation of mechanical oscillation system. But the main conclusion consists in the fact that the total depth of radiation modulation for the transmission of the contact of multi-mode fibers connected with the contributions along the whole spectrum exceeds the maximum modulation depth at resonance frequency by four orders. Therefore, in case of adoption of the reflectometry registration method the depth of modulation of reflected radiation can reach 100%.
Methods of Acoustic Information Protection [4–8]
Site protection from the leakage of acoustic information through fiber optical communications duplicates the standard methods of cable system protection in many aspects but they have peculiarities. The structure of leakage channel and physical principles of light modulation are very important.
Structure of Acoustic Information Leakage Channel
Action of acoustic field on fiber optical communications causes the undesirable modulation of light flux which is spread in cable systems and goes far beyond the boundaries of protected zone, where it can be registered by infringer. Thus, confidential information connected with acoustic fields at informatization site can be received by infringer. In the suggested structure of leakage channel three main elements can be highlighted:
Light modulation in optical fiber caused by acoustic fields from information source;
Signal propagation in optical cable systems outside the boundaries of protected zone;
Probing and registration of leakage signal by infringer.
Every element of leakage channel has its weak and strong points in the protection of information confidentiality which make it possible to construct effective protection. Protection can be constructed on the basis of the following requirements:
Minimization of spurious crosstalk (modulations) in optical cable system;
Restriction of leakage signal output outside the boundaries of protected zone;
Prevention or detection of illegal connection to the optical network.
Methods of cable system shielding from external physical effects and fields, filtration and noise pollution of leakage signal, detection of spurious modulations and probing attempts can be used for these purposes.
Further discussion of protection methods for optical cable communication systems is connected with the fact that these systems are the most widespread and used more frequently in comparison with fiber optical measuring systems, security systems and interfaces. But the specified arguments are still valid for all types of application of optical cable.
Acoustic Shielding of Optical Cable
Acoustic shielding of optical cable is represented by sound insulation, reduction of acoustic contact of external acoustic field with optical cable through the use of sound absorbing/reflecting materials, application of high-quality optical cable, cable removal from sound source. Mounting of the structured cable system in office and building is performed using the cable trunks, cable trays, switching distribution frame, terminal elements etc. in the points of optical cable where all types of inhomogeneities are formed. The main part is unavoidable by the network structure. And the other part of inhomogeneities can be created by infringer for the required period of time through different effects and then the optical homogeneity is quickly restored therefore the acoustic shielding is not efficient without constant control of the state of structured cable system. The basic recommendations for shielding consist in the following: quality of the cable which is located near the sources of confidential information must be the highest; network topology includes minimum number of bands with the largest radius; all switching elements are removed, otherwise terminators with acoustic shielding must be installed on them.
Filtration and Noise Pollution of Information Leakage Signal [6, 7]
The method implies the elimination of spurious modulations and addition of the noise to desired signal with the help of special equipment. This method refers to the active methods and as a rule requires the direct introduction of intermediate active equipment to the optical network which contradicts the concept of the technology of passive optical networks. In some cases, protection device might not be the part of optical network directly, for example, when the noise pollution is performed by the direct effect of external noise physical field on the cable. In this case the effect is switched on only at the required points of time and with the required power. Capability to apply filtration for any network including the network which is unknown on spurious modulations is the distinctive feature of filtration.
Detection of Spurious Crosstalk (Modulations) and Probing
Radiations [4, 8]
Protection of acoustic information against leakages through the optical cable network upon the spurious acoustic light modulations can be constructed on the basis of threat detection. Any threat of information leakage is connected with two factors: (1) availability of effective spurious modulation in the area where it is eliminated; (2) availability of probing radiations. The first peculiarity indicates the potential of such leakage and the second one indicates the threat implementation. Peculiarity of the verbal information leakage consists in the fact that the source of such information is located near the terminal network equipment, as a rule. It allows combining the transceiver with the protection device when the analysis of light flux in the network is performed in the terminal transceiver and according to the facts of availability of spurious modulation and probing radiations the conclusion on the eavesdropping is drawn.
Conclusions
Physical principles of spurious modulations of light in optical fiber caused by the external physical fields and acoustic wave, in particular, are discussed in the paper. Performed estimation of possible modulation depth of acoustic information leakage signal upon the reflection from releasable connection shows the significant danger connected with eavesdropping through the fiber optical communications. Detection of eavesdropping threat by the monitoring of light flux in optical cable is highlighted among the protection methods.
Acoustic fields refer to one of the main sources of information concerning the informatization object in the form of confidential conversation, spurious sound waves accompanying different processes. All these aspects highlight them against the background of other physical fields as one of the main channels of information leakage. Let us discuss the occurrence of spurious modulations of light flux on the cable optical inhomogeneities in more detail [4]. The sound wave is periodic process of elastic disturbances in the medium at the approximate frequency range of 10 Hz to 20 kHz. In air it is elastic compression-extension with the wavelength of 34 m to 1.7 sm. 1.0 kHz shall be assumed to be the typical frequency with the wavelength of 34.0 sm, so the typical length of field homogeneity shall be Λ=17.0 sm. In this case, majority of optical inhomogeneities is l<<Λ and therefore the interaction process can be deemed spatially homogeneous.
Sound wave has the mechanical action on fiber, response of which is characterized by the elastic properties of optical cable and defect in it. Let us designate some degree of freedom inside the defect through x which influences on the coefficient of backscattered radiation βr and varies upon the external action of acoustic field – acoustic pressure δp. Pressure variation in the wave δp causes the variation of parameter by δx and, in turn, changes the power of backscattered radiation by δβr. Modulation depth is determined by the relative variations of the coefficient βr as follows
m = δβr / βr = δx / x0,
where x0 is some typical unperturbed value of shift in oscillating system with the linear dependence βr~x.
The main action of acoustic field consists in the mechanical action on defect, its compression-extension by the variable air pressure. Defect as the mechanical system has resonant response to the mechanical effect with the set of its own frequencies {2pfn} and relevant resonance frequencies. Then, the dependence of modulation depth on the frequency of external effect for the nth resonance will have the following form
mn = δ xn / x0 = ( xn / x0 ) · ( fn γ / π ) / ( fn 2 – f 2 + i f γ / π ),
here xn = S δ p0 / 4 π M γ fn is the resonance amplitude at the frequency fn and amplitude of acoustic pressure δp0 for optical defect with the weight M upon the cross-section area S and coefficient of mechanical oscillation damping γ.
The total modulation depth m in the form of response to the broadband action of acoustic field is defined as the mean square value from mn at the whole frequency range which depends on the action itself.
Releasable Connection
In the infrastructure of cable network, the releasable connections provide the development of information system via the accumulation of additional elements through connectors and adapters. It generates additional hazards connected with the leakage of verbal information through the spurious modulations caused by acoustic field in connectors. Acoustic wave from the sound source affecting the connector changes the distance between fibers and causes the angular and radial displacement of connected fibers. Formation of the backscattered radiation is connected with the reflection from thin layer which occurs upon the contact of two fibers (Fig. 6). Fabry-Perot interferometer represents the model of such system on the thin layer with the thickness d and refraction index n0. In this approximation, coefficient of backscattered radiation is determined on the basis of Airy’s formula for multi-beam interference on reflection
βr = [ 4R · sin 2 ( Δϕ / 2 ) ] / [ ( 1 – R)2 + 4R · sin 2 ( Δ ϕ / 2 ) ],
where R is the coefficient of reflection of the layer edges, Δϕ = ( 4π/λ ) · n0 · d · cos A is the difference of phases of adjacent reflected beams, λ is the light wavelength, A is the angle of light incidence on layer. In the ideal case of normal incidence (A=0) on flat slab (without wedging) with the thickness which is less than the light wavelength (d<<λ) and low reflection value (R<<1) for the refraction index n0=1.0 (air space) we will obtain the following
βr ≈ 160 R ( d / λ )2.
And the modulation depth caused by the external acoustic action will be determined on the basis of the following expression
m = 2 δd / d0 ≤ 2dm / d0,
where d0 is the distance between fibers in unperturbed state, δd is the perturbation and dm~δp0 is the resonant axial displacement of fibers.
The suggested model does not take into account the angular and radial displacement of fibers which introduce additional spurious modulations and angular divergence of the light incident on the reflecting layer. But assumed approximation allows evaluating the hazard of this information leakage channel. Firstly, it should be noted that the reflection βr depends on the wavelength and this fact allows increasing the leakage signal power through the reduction of the wavelength of probing radiation and at the same time the modulation depth will not change. Secondly, if we assume that the wavelength of probing radiation is λ=1 μm the resonant axial displacement of fibers can be estimated as dm<<d0<<λ. In reality, at the acoustic pressure δp0=2·10–2 Pa (60 dB SPL), if we assume that fiber with the wavelength about l≈0.02 m, density ρ=M/Sl=2,1·103 kg/m 3 (fused quartz) forms the oscillating system which has the resonance frequency f0 and γ<<f0 (it is suggested that γ=f0/20π and it corresponds to the quality factor Q=20π 2) then
dm = C / f02,
where C = Q δp0 / 4π 2 ρ l ≈ 2,4 · 10–3 m·Hz 2 is the constant in approximation for this releasable connection. In this case, for the resonance frequency f0=500 Hz we will receive the resonant axial displacement of fibers dm≈0,01 mm and the modulation depth can be estimated m≤20% in case of estimation of the distance between fibers in unperturbed state d0≈0,1 μm. With the power of backscattered light 10·lg (Pr/P0) = –50 dB for the single-mode fiber, we will obtain that the power of leakage optical signal is 10·lg (δPr/P0) ≤-57 dB, and it can be fully registered. In case of decrease of estimation of the distance between fibers in unperturbed state to the value which is less by the order (d0≈0,01 μm, case of good treatment of contacting surfaces), we will obtain the power of leakage optical signal 10·lg (δPr/P0) ≤-50 dB with the modulation depth m≤100%. In case of releasable connection of multiple-mode fibers, the reflection index will increase by two orders and the modulation depth will grow too.
In the discussed model, modulations of backscattered and transmitted light flux caused by acoustic field refer to the connected processes. Growth of the reflection from contact results in the growth of loss on transmission so |δPp|=|δPr|. Hence, the depth of modulation of transmitted radiation
m~= ( βr / βp ) m << m.
Thus, the depth of modulation of transmitted radiation is considerably lower than the depth of reflected radiation (i. e. m~= ( βr / βp ) m << m.<<m) and it can be explained by the fact that the power of radiation which transmitted through the contact is considerably higher than the power of reflected radiation and their variations by the absolute value are equal (i. e. βr<<βp). Such result shows that the reflectometry methods of formation of acoustic information leakage channel are more effective in comparison with the registration of spurious modulations of transmitting light flux.
Thus, the efficiency estimation of acoustic information leakage channel by modulation depth shows great danger of such eavesdropping. It should be noted that the spurious modulation has significant noise component which is connected with possible distortions due to the process nonlinearity, with several modulation mechanisms, with light flux divergence so the leakage signal will be characterized by much lower total efficiency of recognition of speech tenor or sound meaning.
Comparison with Experiment
Experimental studies of light modulation depth on the optical contact of multi-mode fibers performed earlier prove the specified estimations [5]. Studies were performed on the experimental facility (Fig. 7) consisting of optical distribution frame, in which SC-SC adapter of multi-mode fibers and microphone for the control of sound pressure level (SPL) were located. Stabilized laser radiation arrived to SC-SC adapter and it was registered at the exit by the photodetector with selective nanovoltmeter or integrating voltmeter. Acoustic field with the spectrum of white noise was formed in the optical distribution frame and it allowed excitement of all mechanical resonances in the optical contact of multi-mode fibers. White noise formed the spurious modulations of light intensity which were registered by photodetector. Application of the selective nanovoltmeter made it possible to perform the spectral studies of light modulations with the band width of 25 dB of octave.
Study results are given in the form of diagrams of spectral dependence of modulation depth (Fig. 8) for the transmission m~= ( βr / βp ) m << m. (f) at set SPL of white noise and dependence (Fig. 9) of modulation depth on sound pressure level m~= ( βr / βp ) m << m. (SPL) for the laser wavelengths λ=850 nm and 632.8 nm. It can be obtained from the first diagram that this SC-SC adapter has the modulation depth m~= ( βr / βp ) m << m. ≈0,9 ppm (1 ppm=10–6) at the resonance frequency f0≈3 kHz. If we assume that (βr/βp) =40 dB and it corresponds to the ratio of the powers of radiation reflected and transmitted through the contact of multi-mode fibers, then at the resonance frequency f0≈3 kHz we will obtain the estimation of fiber resonance displacement dm≈0.0003 μm and modulation depth m~= ( βr / βp ) m << m. ≈0,6 ppm. It corresponds to the depth of modulation of reflected radiation m≤0,6%. Comparing the experimental and theoretical values m~= ( βr / βp ) m << m. we have the matching of orders and the difference by 1.5 times can be connected with the failure to take into account other mechanisms of spurious modulations. The second diagram confirms the influence of probing radiation wavelength on the modulation depth and with the growth of SPL of white noise the saturation is observed, which corresponds to the saturation of mechanical oscillation system. But the main conclusion consists in the fact that the total depth of radiation modulation for the transmission of the contact of multi-mode fibers connected with the contributions along the whole spectrum exceeds the maximum modulation depth at resonance frequency by four orders. Therefore, in case of adoption of the reflectometry registration method the depth of modulation of reflected radiation can reach 100%.
Methods of Acoustic Information Protection [4–8]
Site protection from the leakage of acoustic information through fiber optical communications duplicates the standard methods of cable system protection in many aspects but they have peculiarities. The structure of leakage channel and physical principles of light modulation are very important.
Structure of Acoustic Information Leakage Channel
Action of acoustic field on fiber optical communications causes the undesirable modulation of light flux which is spread in cable systems and goes far beyond the boundaries of protected zone, where it can be registered by infringer. Thus, confidential information connected with acoustic fields at informatization site can be received by infringer. In the suggested structure of leakage channel three main elements can be highlighted:
Light modulation in optical fiber caused by acoustic fields from information source;
Signal propagation in optical cable systems outside the boundaries of protected zone;
Probing and registration of leakage signal by infringer.
Every element of leakage channel has its weak and strong points in the protection of information confidentiality which make it possible to construct effective protection. Protection can be constructed on the basis of the following requirements:
Minimization of spurious crosstalk (modulations) in optical cable system;
Restriction of leakage signal output outside the boundaries of protected zone;
Prevention or detection of illegal connection to the optical network.
Methods of cable system shielding from external physical effects and fields, filtration and noise pollution of leakage signal, detection of spurious modulations and probing attempts can be used for these purposes.
Further discussion of protection methods for optical cable communication systems is connected with the fact that these systems are the most widespread and used more frequently in comparison with fiber optical measuring systems, security systems and interfaces. But the specified arguments are still valid for all types of application of optical cable.
Acoustic Shielding of Optical Cable
Acoustic shielding of optical cable is represented by sound insulation, reduction of acoustic contact of external acoustic field with optical cable through the use of sound absorbing/reflecting materials, application of high-quality optical cable, cable removal from sound source. Mounting of the structured cable system in office and building is performed using the cable trunks, cable trays, switching distribution frame, terminal elements etc. in the points of optical cable where all types of inhomogeneities are formed. The main part is unavoidable by the network structure. And the other part of inhomogeneities can be created by infringer for the required period of time through different effects and then the optical homogeneity is quickly restored therefore the acoustic shielding is not efficient without constant control of the state of structured cable system. The basic recommendations for shielding consist in the following: quality of the cable which is located near the sources of confidential information must be the highest; network topology includes minimum number of bands with the largest radius; all switching elements are removed, otherwise terminators with acoustic shielding must be installed on them.
Filtration and Noise Pollution of Information Leakage Signal [6, 7]
The method implies the elimination of spurious modulations and addition of the noise to desired signal with the help of special equipment. This method refers to the active methods and as a rule requires the direct introduction of intermediate active equipment to the optical network which contradicts the concept of the technology of passive optical networks. In some cases, protection device might not be the part of optical network directly, for example, when the noise pollution is performed by the direct effect of external noise physical field on the cable. In this case the effect is switched on only at the required points of time and with the required power. Capability to apply filtration for any network including the network which is unknown on spurious modulations is the distinctive feature of filtration.
Detection of Spurious Crosstalk (Modulations) and Probing
Radiations [4, 8]
Protection of acoustic information against leakages through the optical cable network upon the spurious acoustic light modulations can be constructed on the basis of threat detection. Any threat of information leakage is connected with two factors: (1) availability of effective spurious modulation in the area where it is eliminated; (2) availability of probing radiations. The first peculiarity indicates the potential of such leakage and the second one indicates the threat implementation. Peculiarity of the verbal information leakage consists in the fact that the source of such information is located near the terminal network equipment, as a rule. It allows combining the transceiver with the protection device when the analysis of light flux in the network is performed in the terminal transceiver and according to the facts of availability of spurious modulation and probing radiations the conclusion on the eavesdropping is drawn.
Conclusions
Physical principles of spurious modulations of light in optical fiber caused by the external physical fields and acoustic wave, in particular, are discussed in the paper. Performed estimation of possible modulation depth of acoustic information leakage signal upon the reflection from releasable connection shows the significant danger connected with eavesdropping through the fiber optical communications. Detection of eavesdropping threat by the monitoring of light flux in optical cable is highlighted among the protection methods.
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