Secure fiber optic communication systems and methods

#The present invention relates generally to fiber optical communication networks. More particularly, the present invention relates to fiber optic networks carrying sensitive information such as classified government communications, sensitive financial information, proprietary corporate information, and associated systems and methods for secure transmission where fiber tampering is easily detected.

#Conventionally, fiber optic links use fibers that rely on total internal reflection for confinement of an optical propagation mode. Therefore, the structure of the fibers uses a high refractive index core of approximately five to one hundred microns in diameter, surrounded by a low refractive index cladding. The cladding is present mostly to provide mechanical stability to the fibers. Thus, in a conventional fiber, if the cladding is removed, light is still confined to the high index core suffering very little loss since the surrounding air also has a very low refractive index. Disadvantageously, a conventional fiber optic link can be tapped into quite easily. For example, the fiber may be exposed from its cable, removing an outer polymer protective coating. The glass cladding layer may be thinned via mechanical or chemical methods. A second strand of fiber with removed cladding may be brought in close proximity to the fiber being tapped, and a small fraction of light will be coupled out of the tapped fiber into the second strand. While the light coupled out reduces transmitted light intensity in the tapped fiber, the coupling loss may be extremely small such as on the order of one percent or less. Here, an intruder may monitor the coupled out light. Conversely, the intruder may inject parasitic light into the tapped fiber to disrupt or spoof existing optical carriers. This small one percent or less loss induced in the transmitted light is easily obscured by natural optical power variations associated with thermal and mechanical stresses, optical amplifier noise, etc. Therefore, network operators and end users may be completely unaware of a compromised link.

#Conventional fiber optic security includes various systems and methods. First, optical signal light intensity may be monitored for decreases in power (e.g., one percent or less). Unfortunately, this method has limited resolution capability as intrusion events may be easily obscured by natural system mechanical and thermal perturbations, optical amplifier noise, etc. Thus, low alarm threshold settings will produce many false positives whereas high alarm threshold settings will miss intrusion events. Second, an external probe light may be injected into a fiber optic link. This probe may be conditioned to be less sensitive to natural system perturbations. Disadvantageously, the probe solution requires a large amount of dedicated hardware with a separate deployment required for each segment of a large network. Third, quantum key distribution may be used as a secure communication method. Such a system is very complex and provides only extremely slow kilobit rate communications over short distances (tens of kilometers). Thus, it is only suitable for distributing encoding information (i.e., keys) and not for carrying actual data. Finally, fiber-based strain sensors, such as ones based on Brillouin optical time domain analysis, may be used for intrusion detection. Glass material strain induces a frequency shift in light backscattered by the Brillouin effect allowing for identification of induced fiber strain location. However, this method only indicates that a fiber is being mechanically handled and may provide false alarms in cases where fiber is not mechanically isolated (e.g., aerial cables, subway ducts, etc.).

#The present invention is illustrated and described herein with reference to the various drawings of exemplary embodiments, in which like reference numbers denote like system components, respectively, and in which:

#FIG. 1 is a cross sectional diagram of Hollow-Core Photonic Bandgap Fiber (HC-PBF); and

#FIG. 2 is a network diagram of a secure fiber optic communication system utilizing Hollow-Core Photonic Bandgap Fiber (HC-PBF) according to an exemplary embodiment of the present invention.

#In various exemplary embodiments, the present invention relates to fiber optic networks carrying sensitive information such as classified government communications, sensitive financial information, proprietary corporate information, and associated systems and methods for secure transmission where fiber tampering is easily detected. The present invention provides improved security systems and methods for fiber optic communication links. Specifically, a hollow-core photonic bandgap fiber (HC-PBF) is deployed as a transmission medium. A secure fiber optic communication link is established over the hollow-core photonic bandgap fiber with a monitoring system and method. The monitoring system and method is configured to detect large losses and large spectral variability between wavelengths, each indicative of loss introduced by malicious tapping attempts, i.e. intrusions. Further, the monitoring system and method allow easy differentiation of intrusion relative to normal system variations thereby reducing false positives and missed intrusions.

#Referring to FIG. 1, in an exemplary embodiment, a cross sectional diagram illustrates a close up view of Hollow-Core Photonic Bandgap Fiber (HC-PBF) 10. An inset diagram 12 illustrates a full cross sectional diagram of the HC-PBF 10. The last several years have seen a very rapid development of a new class of optical fiber, such as the HC-PBF 10. Note, the HC-PBF 10 may be referred to as Hollow Core Fiber, Photonic Bandgap Fiber, Photonic Crystal Fiber, or the like. The HC-PBF 10 has a radically different structure and a different optical guiding principle from conventional fiber. FIG. 1 illustrates a typical structure of the exemplary HC-PBF 10. Specifically, the HC-PBF 10 includes various regions 14 of air surrounded by glass 16. As the name implies, the HC-PBF 10 has a hollow core 18 such that optical light is guided mostly in the air-filled center region, i.e. the hollow core 18. The light is confined to the hollow core 18 by surrounding the hollow core 18 with a periodic lattice 20 of additional holes in the glass 16, such that the lattice 20 forms a resonantly reflective structure. This is similar to a stack of layers in a thin-film filter which reflects a specific portion of the spectrum defined by layers' index of refraction and periodicity. Only the light spectrum that satisfies the reflective periodicity of the cladding lattice 20 can propagate in the HC-PBF 10.

#With proper engineering, the HC-PBF 10 can maintain good propagation properties over a large range of wavelengths, including ones used in conventional telecommunication window of 1525 to 1565 nm. In the example of FIG. 1, the lattice 20 includes three concentric cylindrical glass rings surrounding the hollow core 18 with a diameter dcore. The glass 16 includes a thickness of tring, separated by the air regions 14 with a width of tholes and connected by glass bridges tbridge wide. It should also be noted that since light is guided mostly in air in the hollow core 18, this type of fiber is not constrained by the low loss properties of the glass material, but may be engineered for wide range of wavelengths. For example, FIG. 1 is taken from Foroni et al., “Guiding properties of silica/air hollow core Bragg fibers,” J. Lightwave Techn., vol. 23, no. 14, July 2008, pp. 1877-1884.