Information Leakage Channel Based On Spurious Crosstalk (Modulation) In An Optical Fiber
Currently, fiber optical technologies have widespread application in many areas of human activities [1, 2]. One of the most effective applications consists in the use of optical cable as the transport medium in many communication systems from telecommunications to subscriber access. Such technologies as passive optical networks (PON), optical structured cabling systems (SCS) make it possible to implement many broadband services, for example, service for the cable transmission of audio, video and data to subscriber – Triple Play; network convergence.
Other type of application is connected with fiber-optic sensors (FOS) and distributed measuring networks which are used in the technological monitoring of condition of buildings, strictures and ecological monitoring of natural and artificial objects. Implementations in the area of security and protection are connected with the measuring and transport capabilities of fiber optics. They include fiber optical systems of site protection and use of fiber optical communication links in observation systems etc.
Such widespread occurrence of fiber optical technologies is connected with the advantages of fiber optical transport medium in comparison with the copper medium for the information transmission. Medium of optical transport is represented by dielectric (fused quartz, glasses, optical plastics) but the fused quartz, which physical properties determine its advantages, is mainly used in the optical cable. On one hand, its great mechanical strength, durability, nonsusceptibility to electromagnetic fields, and low light absorption refer to its advantages. On the other hand, installation and operation technologies repeat the capabilities of copper cable in many aspects. As a result, communication systems received the new transport medium with unique properties and standard installation.
Thus, fiber optical technologies for information transmission and measurements have widespread application in many areas and replace copper cable systems improving the main characteristics. However, new technologies generate problems for the provision of information security which are connected with the newness of transport system and low exploration degree of physical principles of the formation of information leakage channels. Security services are mainly focused on the traffic protection in fiber optical telecommunications, interception of which can cause the maximum damage. But besides the traffic threat, the optical cable systems for office and house have the danger of information confidentiality inside sites due to the measuring capabilities of optical cable. This paper is devoted to the explanation of similar threats.
Information Leakage through Fiber Optical Communications
Measuring Capabilities of Fiber Optics
Infiltration of fiber optical technologies into the office and house results in the fact that optical cable and fiber are located near human being, operating equipment, office internal environment which generate the physical fields influencing on the light passage in fiber. Intruder has the opportunity to register similar spurious crosstalk from the surrounding physical fields and analyzing them receive the access to the confidential information circulating at the site. Generalized scheme of information leakage channel can be shown in the following form (Fig. 1): physical field as the information carrier affects the light flux in fiber, causes the modulation of light flux at field frequencies and as a result the intruder receives access to the confidential information using the probing radiation in optical fiber. Information leakage channel can be implemented according to the optical scheme for the passage or reflection of light from the inhomogeneity point and for the implementation intruder can use his/her own probing radiation and light flux of legal information signal.
External physical fields form the spurious crosstalk in fiber and cause the modulation of light wave parameters. Modulation type and depth determine the efficiency of information leakage channel. Amplitude, phase, frequency and polarization modulation can occur in fiber and its depth depends on the parameters of probing radiation such as intensity, spectral composition, polarization, degree of monochromaticity and coherence. The light phase, variation of which is registered on the basis of interference methods with the accuracy of more than 1 ppm (10–6), has the highest sensitivity to the external influence and it allows measuring the mechanical effects on fiber and acoustic fields near it. Polarization modulations are mainly connected with the action of surrounding electromagnetic fields and spectrum variations are connected with nonlinear scattering which depends on temperature.
Thus, any external effect on fiber and surrounding physical fields have the spurious influence on the process of light propagation which can be detected by the special technical devices. Difficulty of spurious crosstalk registration is compensated by the importance of received confidential information.
Optical Reflectometry in Information Accumulation (Fig. 1)
Methods of optical reflectometry (Optical Time Domain Reflectometry, OTDR), which are widely used in measuring technology and monitoring systems of fiber optical communication links, refer to the main methods of spurious crosstalk registration . This method consists in the fiber probing by pulse or continuous optical radiation and registration of backscattered radiation or radiation generated as a result of the interaction of probing radiation with optical inhomogeneities, defects of optical fiber and directed to the opposite way (backward) relative to the probing radiation, which is connected with the phenomena of light scattering, reflection and generation (reradiation). Optical scheme for reflection allows determining the location of studied section of optical cable by the delay time of backscattered radiation (response) τ, and this fact is very important for the formation of information leakage channel. Cable distance to the observation point is calculated as follows:
L = τ · c / 2 n,
where c is the light speed in vacuum, n is the index of fiber refraction. Backscattered radiation carries the information on the condition of studied cable section which is subjected to the spurious crosstalk by surrounding physical fields and effects.
In the measurement technology, the radiation with various spectral composition, coherence and polarization is used for probing. Selection of the parameters of probing radiation and registration methods allows receiving the information on fiber condition and processes in fiber-surrounding medium. In particular, the time reflectometry is used for the control of such parameters of optical network as loss, defects etc.; Brillouin reflectometers (Brillouin Optical Time Domain Reflectometer, BOTDR) based on spontaneous and stimulated Mandelstam-Brillouin scattering make it possible to register the tension distribution and temperature distribution along the optical cable; coherent optical reflectometry (Coherent Optical Time Domain Reflectometer, COTDR) is applied in the systems of vibration-acoustic control.
Inhomogeneities of Fiber Optical Network in Information Leakage
Efficiency of optical reflectometry methods is considerably improved with the formation of backscattered radiation in cable from the optical inhomogeneities, defects or fiber sections with the local variation of refraction index, coefficients of absorption and scattering which can be compared with the probing radiation wavelength by their values. Availability of optical inhomogeneity results in the growth of backscattered radiation power and increases the fiber sensitivity to external effects and physical fields. The special sections with hypersensitivity are formed in fiber optical measuring systems, for example, on the basis of fiber Bragg grating (FBG) which allows measuring the deformations, temperature with high accuracy, in other cases optical fibers with hypersensitivity to electromagnetic fields (optical fibers with large value of Verdet, Kerr, Pockels constants) are used etc. Similar structures can be created in the regular optical structured cable systems but with considerably lower sensitivity to the external fields. Generally, any inhomogeneity causes the growth of influence of external effects on the parameters of light flux in fiber and this fact can be used for the formation of side information leakage channel. Increase of probing radiation power causes the growth of backscattered radiation which makes it possible to measure more distant cable sections with lower requirements to the registration equipment.
Topology and Infrastructure of Fiber Optical Communications in Information Leakage Channel
Cable location in building, office and use of infrastructure elements for installation have great impact on the efficiency of fiber optical cable as distributed measurement system. If cable of one subsystem is installed near the sources of side fields forming the informative signals, it is necessary to take into account the potential of spurious crosstalk on light flux in cable. Undesirable modulations can be considerably intensified with the use of breakout boxes, cable trunks without screening from external physical fields. For example, acoustic fields can resonate with the structural elements of cable system increasing the acoustic contact of external field with fiber. Analogous phenomena occur upon the action of electromagnetic, thermal and other external fields. Cable placement inside the boxes, their structure and location in building have great impact for undesirable spurious crosstalk on optical fiber.
Characteristic of Optical Inhomogeneities
in Information Leakage Channel
Classification of Optical Inhomogeneities (Defects)
Role of optical inhomogeneities in the formation of confidential information leakage channels via undesirable, spurious crosstalk in fiber can be very significant and therefore it is necessary to analyze the types of inhomogeneities and their characteristics. All inhomogeneities can be divided into three groups.
Internal optical inhomogeneities (Fig.2) of optical fiber are connected with the existing defects formed during the production of fiber and cable, installation of cable infrastructure and following operation. Internal defects in the form of local scattering centers, internal cracks, internal stresses, nonideality of fiber shape etc. Distinctive feature of this type of inhomogeneity is its dependence on the quality of used cable, quality of installation and operation.
Infrastructure optical inhomogeneities and defects (Fig.3) are connected with the selection of topology and structure of cable system. Switching nodes, releasable and welded connections, cable bending and laying up, installation fastening of cable and cable trunks etc. refer to this category. Each element is characterized by its own backscattered radiation and loss of light propagation which considerably depend on used fabrication methods and methods of cable installation, allocation, mounting.
Induced optical inhomogeneities and defects (Fig.4) are caused by inconstant external effects and fields which have natural or artificial origin. They can be divided into two types: mechanical effects on optical cable and effect of external physical fields. The first type includes different bends, stretching-compression, kink, to which the cable can be subjected for the purposes of improvement of sensitivity to external informative fields. The second type includes acoustic fields, constant electromagnetic fields, thermal effect, radiation etc. It should be noted that simultaneous action of two fields, one of which carries the informative signal, creates the situation when the first effect initiates the growth of backscattered radiation and the second one modulates it. Both fields can have the same character but be divided by any parameter, for example, frequency. It allows dividing two effects and extracting the informative signal from the common leakage signal.
Parameters of Inhomogeneities and Physical Field in Information Leakage Channel
Information leakage signal in leakage channel is formed by the physical field and effect, which is informative (Fig. 5). The physical field is characterized by some force parameter G which has the constant component G0 = <G> and variable component δG, that <δG> = 0. Thus, the physical field can be represented in the form G = G0 + δG which contains the informative signal with the modulation depth
g = δG / G0.
The typical dimensions of field inhomogeneity can be designated through Λ.
Geometrics of optical inhomogeneity in fiber are described by the typical dimensions l and fiber length in it L. Ratio between them: l ≤ L. For example, the length of twisted fiber is much lower than its diameter. Ratio between the typical dimensions of fiber inhomogeneity and field homogeneity is very important: l ≤ Λ, in this case it can be considered that the defect is located under the homogeneous effect of physical field and the interference processes between various fiber parts can be neglected.
Optical characteristics of fiber inhomogeneity are shown in the decrease of propagating radiation power and formation of backscattered radiation which can be obtained through the loss factor
βp = Pp / P0
and coefficient of backscattered radiation
βr = Pr / P0.
Here, P0 is the probing radiation power, Pp is the power of radiation which passed inhomogeneity, Pr is the radiation scattered/reflected/reradiated in the direction which is reverse to the probing one. It is understandable from the general concepts that overall loss includes the loss connected with the formation of backscattered radiation, so βp (βr) >βr. In case of small variation of loss it can be stated that
δβp ~ δβr.
This is important conclusion because in some cases it can replace the analysis of backscattered radiations by the analysis of loss power and it can considerably simplify the calculations.
External variable effect of physical field δG with the modulation depth g causes the variation of backscattered radiation power δPr with the modulation depth
m = δPr / Pr = δβr / βr.
Thus, the optical inhomogeneity can be characterized by the absolute sensitivity (δPr/δG) or standardized sensitivity
η = m / g.
Power of backscattered radiation is the information leakage signal because it carries the confidential information on the processes near the optical cable. The absolute value of modulated part of backscattered radiation proceeding from the introduced definitions is expressed by the formula
δPr = ( P0 · βr · η ) · g.
Obtained expression shows that the leakage signal power is determined by the power of probing signal. And the statement concerning the complete dependence of backscattered radiation part on optical inhomogeneity properties is valid. In some cases this dependence becomes nonlinear, for example, for Brillouin reflectometry but in any event this is the main parameter for the leakage signal formation. In the formula, the second factor is determined by the defect properties and the third one depends on the modulation parameters of studied cable section, its sensitivity to the external field. Their value can be increased through the additional effect or selection of the parameters of probing radiation which is the most sensitive to this type of crosstalk.
In analogous manner, the modulation depth of passing radiation loss can be introduced
m~= δPp / Pp = δβp / βp.
which in general case will be proportional to the depth of backscattered radiation modulation m.
To be continued...