Safe fiber link system

Abstract

To provide a system and method for securing communication over an optical fiber in which tap along the fiber cannot determine the transmitted desired sequence of information.SOLUTION: In a system 100, a transceiver terminal 110-1 includes a transmit spatial multiplexer that couples optical signals into spatial paths of an optical fiber. Therein: a first optical signal of the plurality of optical signals is an optically modulated version of a desired sequence of information intended to be transferred over the optical fiber and is coupled into the first spatial path of the spatial paths; a second optical signal of the plurality of optical signals is an optical chaff signal and is coupled into a second spatial path different from the first spatial path; and a third optical signal is an optical signal for use by an optical time domain reflectometer (OTDR) coupled into one of the spatial paths of the optical fiber.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application claims the benefit of U.S. Patent Application No. 16 / 190,801, filed Nov. 14, 2018, the content of which is incorporated herein by reference.

[0002] The present disclosure generally relates to optical fiber cables, and more particularly to detecting and preventing eavesdropping on optical fiber cables.

Background Art

[0003] An intruder can gain access to an optical fiber transmission line and steal information by bending one or more segments of the fiber or stretching one or more segments of the fiber via application of heat, e.g., making it thinner. By doing so, it may be possible to read and decipher signal energy leaking from the bent or stretched fiber. There are other ways to eavesdrop on information from optical fibers, but eavesdropping based on the fiber being bent or stretched is easy to implement, effective, and can be difficult to detect. Eavesdropping on valuable data transmitted through the worldwide optical fiber infrastructure is a threat to every major industry and government organization, especially large organizations that use multiple facilities. These organizations may be able to protect the optical fiber cables within their own facilities, but generally have far less control over the optical fiber cable links between their facilities.

[0004] The vulnerability of optical fibers to interception, particularly through bending or stretching, makes them vulnerable to data theft for numerous organizations. Such data theft can lead to the disclosure of confidential information, thereby harming entities whose data is being transported through such intercepted optical fibers. In business contexts, the disclosure of information such as marketing strategies or development technologies can ultimately result in lost profits. In political contexts, the disclosure of information concerning national secrets can endanger the lives of citizens. Existing solutions are insufficient to detect or prevent data extraction using optical fiber interception.

[0005] Data encryption is generally believed to protect data transmitted over fiber links. Such encryption is often implemented using the Advanced Encryption Standard (AES). However, this method ignores the fact that while the data payload is encrypted, the IP header information that guides the data packets across the internet is not. Such headers reveal both the source and destination of each packet, and therefore reveal information about any message sent across the internet.

[0006] An optical time-domain reflectometer (OTDR) is a well-known tool for characterizing, monitoring, and troubleshooting optical fibers. OTDRs typically operate by sending laser pulses of different widths and monitoring their reflections as received at the pulse-transmitting end of the fiber. OTDRs can pinpoint the location of faults within a fiber link, detecting and characterizing both reflected and non-reflected events in an optical fiber. Therefore, an OTDR can be used to detect bends introduced into a fiber link after it has been established by comparing it to previous OTDR traces before the fiber was bent, for example, traces made when the link is first installed. OTDRs can also be used to test a working fiber—that is, a fiber intended to transmit data to a destination—by running test pulses on a different wavelength channel than the one used to transmit the data intended for delivery.

[0007] Unfortunately, in highly secure government communications, it is often desirable that optical data signals not be modified by the transceiver. If this is a requirement, such preference means that the fiber transmitting the secure communication cannot also transmit the pulses required by the OTDR.

[0008] Furthermore, as is well known, the techniques used by OTDRs suffer from a so-called "dead zone," which is the area after the reflection event occurs that cannot be seen by the OTDR. Such dead zones often occur over a large distance at the beginning of a fiber when examining very long optical fibers. This is because, when examining very long optical fibers, a lot of power is required to see the condition at the end of the fiber. When a lot of optical power is sent out, the pulse width of the transmitted optical signal is increased. The use of a large pulse width reduces the resolution of the measurements that can be made by the OTDR, and the result of this reduction in resolution can be hundreds of meters. As a result, faults near the source are masked by hundreds of meters between the receiver, where the reflected pulse can be seen, and the transmitted pulse.

[0009] If a fault occurs near the transmission point, it can also produce large reflections that saturate and overload the receiver. Since the fault is hidden in the length of the fiber close to the OTDR, this length is also called the "dead zone." The receiver requires a significant amount of time to recover from saturation. Depending on the OTDR design, wavelength, and magnitude, the OTDR may require more than 500 meters, for example, to fully recover from such a fault near the transmission point.

[0010] Many OTDR manuals suggest using a transmit fiber outside the OTDR equipment to solve these problems. A transmit fiber is a fiber of a predetermined length placed between the OTDR and the actual fiber being measured, thus providing time for the receiver to stabilize and for pulse width-dependent resolution to be overcome. When a transmit fiber is used, faults near the end of the fiber being measured can be detected by the OTDR. They do not interfere with the actual fiber being measured and are a proven technique for identifying faults within the entire length of the fiber being tested, from its first interface to its last. Therefore, such a transmit fiber is positioned on a spool or in a "transmit box" between the OTDR and the fiber under test to create the appropriate conditions for testing the optical fiber for faults.

[0011] Furthermore, due to government preferences or requirements, additional signals should often not be coupled to fibers carrying secure data not intended for delivery to their destination, and such transmission fibers would become additional insecure points of tampering.

[0012] Therefore, it is advantageous to provide a solution that overcomes the shortcomings of prior art. [Overview of the project] [Means for solving the problem]

[0013] A summary of some exemplary embodiments of this disclosure follows below. This summary is provided for the reader's convenience to provide a basic understanding of such embodiments and does not fully define the scope of this disclosure. This summary is not a detailed overview of all intended embodiments, nor is it intended to identify the major or significant elements of all embodiments or to accurately describe the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed explanations presented later. For convenience, the term "some embodiments" may be used herein to refer to a single or more embodiments of this disclosure.

[0014] The disclosed embodiment includes a system for securing communications over an optical fiber. The system comprises a transmit space multiplexer configured to concatenate optical signals from a plurality of optical signals into a spatial path of a plurality of spatial paths of an optical fiber, each spatial path capable of transmitting an optical signal, wherein at least a first optical signal from the plurality of optical signals is an optically modulated version of information in a desired sequence intended to be transmitted over the optical fiber, and at least a first optical signal from the plurality of optical signals is concatenated into a first spatial path of the plurality of spatial paths, and at least a second optical signal from the plurality of optical signals is an optical chaff signal, and at least a second optical signal from the plurality of spatial paths is concatenated into a second spatial path different from the first spatial path, and at least a third optical signal from the plurality of optical signals is an optical time-domain reflectometer (OTDR) so that interception along the fiber cannot determine the information in the desired sequence being transmitted.

[0015] The disclosed embodiments also include a method for protecting information transmitted over an optical fiber having multiple spatial paths. The method comprises concatenating each of a set of optical signals into at least one of the multiple spatial paths, wherein at least one of the set of optical signals is an optically modulated version of a desired sequence of information intended to be transmitted over the optical fiber, at least a first optical signal of the multiple optical signals is concatenated into a first spatial path of the multiple spatial paths, at least a second optical signal of the set of optical signals is an optical chaff signal, at least a second optical signal of the multiple optical signals is concatenated into a second spatial path of the multiple spatial paths that is different from the first spatial path, and at least a third optical signal of the multiple optical signals is an optical signal for use by an optical time-domain reflectometer (OTDR).

[0016] The disclosed embodiments also include terminal equipment for securing communications over optical fiber. The terminal equipment comprises a transmit space multiplexer configured to link multiple optical signals into each of several spatial paths of an optical fiber, each spatial path capable of carrying an optical signal, at least one of the multiple optical signals being an optically modulated version of a desired sequence of information intended to be transmitted over the optical fiber, and at least one of the multiple optical signals being an optical chaff signal multiplexed with an optical signal for use by an optical time-domain reflectometer (OTDR), thereby preventing interception along the fiber from determining the desired sequence of information being transmitted.

[0017] The disclosed embodiments also include a system for securing communications over an optical fiber. The system comprises a receiving spatial demultiplexer, at one end of which is connectable to a plurality of spatial paths of the optical fiber, wherein at least two of the spatial paths carry optical signals, the optical signal of the first of the at least two spatial paths includes at least an optically modulated version of a desired sequence of information intended to be received from the optical fiber, the optical signal of the second of the at least two spatial paths, distinct from the first of the at least two spatial paths, includes an optical chaff signal, and the receiving spatial demultiplexer is configured to receive signals from at least one of the spatial paths for use by an optical time-domain reflectometer (OTDR) and to supply the received signals to the OTDR for use by the OTDR. [Brief explanation of the drawing]

[0018] [Figure 1] This is a block diagram of a safe fiber link system according to one embodiment. [Figure 2A] This is a block diagram of one embodiment illustrating a chaff source using amplified spontaneous emission (ASE). [Figure 2B] This is a block diagram of one embodiment illustrating the use of ASE in a chaff source. [Figure 2C] This is a block diagram of one embodiment illustrating the use of ASE in a chaff source. [Figure 2D] This figure shows an embodiment illustrating a chaff generator that uses a copy of the chaff signal and an optional delay line. [Figure 3] This is an exemplary plot of the spectrum of an ASE source with an optional filter compared to the spectrum of a data channel according to one embodiment. [Figure 4] This figure shows one example for illustrative purposes, in which the device is contained in a secure box. [Modes for carrying out the invention]

[0019] It is important to note that the embodiments disclosed in this specification are merely examples of the many advantageous uses of the innovative teachings of this specification. In general, the descriptions made in this specification of the application do not necessarily limit any of the embodiments of the various claims. Further, some descriptions may apply to some features of some inventions, but may not apply to others. In general, unless otherwise indicated, elements in the singular may be pluralized without loss of generality, and vice versa. In the figures, like numerals refer to like parts throughout several figures.

[0020] The terms "true signal", "true data", "information signal", "true data signal", and "data signal" are used synonymously to refer to a desired sequence of information intended to be transferred between legitimate users at the ends of a link. A chaff signal is a signal that does not convey true data.

[0021] In one embodiment, a secure fiber link system is configured to enable the transmission of data to an intended user while simultaneously obscuring or making the signal unintelligible to an eavesdropper who may eavesdrop anywhere along the entire link other than the intended receiver, by overwhelming the information signal with interference signal energy at such an eavesdropping point and by enabling the detection of eavesdropping, movement, or similar such interference of the optical cable through the use of an optical time domain reflectometer (OTDR).

[0022] For this purpose, a secure fiber link system transmits a desired sequence of information in the form of a first “legitimate” or true signal, or a set of true signals, typically intended to be transmitted between legitimate users located at the ends of the link, on a first spatial path of the fiber cable, for example, the core of a multicore fiber. Additionally, at least one other spatial path of the fiber cable, for example, a different core of a multicore fiber, carries a chaff signal, and at least one spatial path carries a signal that can be monitored by an OTDR to detect any changes in those signals that may result from an act of fiber interception. The OTDR signal may be transmitted on a dedicated core, or it may be wavelength-multiplexed with one or more true signals or one or more chaff signals. This enables the detection of interception or tampering at the terminals of the fiber optic link system.

[0023] Chaff signals, which are interference signals not required to carry the true signal, can be applied to a link at one or both ends. Similarly, OTDR signals can be applied to a link at one or both ends.

[0024] In one embodiment, a secure fiber link system is configured to provide physical security for a data signal propagating through at least one of multiple spatial paths of an optical fiber in parallel with a combined data signal propagating through at least one of multiple spatial paths of an optical fiber, such as by using frequency division multiplexing, which is multiplexed, for example, to a signal for use by an OTDR at least at times.

[0025] In an embodiment where each chaff signal is transmitted using multiple spatial paths, two or more of the spatial paths transmitting the chaff signals may transmit the multiplexed OTDR signal. In such an embodiment, the OTDR signal may be transmitted continuously in one or more of the spatial paths transmitting the chaff signals, or the OTDR signal may be time-multiplexed among the various spatial paths transmitting the chaff signals, or a combination of both may be used.

[0026] In the embodiments of this disclosure, the spatial path that transmits the data signal and the chaff signal may be the core of a multicore fiber.

[0027] This system is configured to ensure that the intended recipient at the other end of the link receives the true data.

[0028] As mentioned above, intercepting a fiber link can be an act of altering the fiber, for example, by applying physical force to the fiber cable, for example by bending the fiber, or by modifying the fiber, for example by applying heat to allow one or more segments of the fiber to be stretched axially without breaking the fiber. Such techniques cause energy propagating within the fiber to leak out of the fiber so that it can be detected, for example, by a hacker attempting to steal information being transmitted through the cable.

[0029] The embodiments of secure fiber link systems may be compatible with the latest data rates, formats, and telecommunications protocols, as well as signal wavelength provisioning, such as wavelength division multiplexing (WDM). The disclosed embodiments may also be independent of future equipment and protocols.

[0030] Figure 1 shows a block diagram of an exemplary secure fiber link system 100 according to one embodiment. The system 100 includes transceiver terminals 110-1 and 110-2 connected via optical links, for example, optical fibers 150-1 and 150-2, each of which may be a multicore fiber, multimode fiber, or minority-mode fiber, preferably enabling spatial division multiplexing. Note that although optical fibers 150-1 and 150-2 are shown as separate fibers for illustrative purposes, they may be implemented as different spatial paths that are part of the same physical optical fiber. In such a case, optical fibers 150-1 and 150-2 may represent, for example, different optical cores within a multicore fiber.

[0031] Each terminal 110-1 or 110-2, collectively referred to as terminal 110, may include, respectively, at least one transmitter, e.g., transmitter 121-1 or 121-2, at least one receiver, e.g., receiver 130-1 or 130-2, at least one chaff generator, e.g., chaff generator 140-1 or 140-2, and at least one OTDR, e.g., OTDR 151-1 or 151-2. For clarity and educational purposes, but without limitation, Figure 1 is later described herein with respect to various optical signals transmitted from terminal 110-1 and received at terminal 110-2. However, it will be readily apparent to those skilled in the art that such optical signals may also be transmitted from terminal 110-2 and received at terminal 110-1 without departing from the disclosed embodiments. Therefore, unless otherwise specifically instructed, any description of any particular component of terminal 110-1 should be understood to apply to its corresponding part of terminal 110-2 and similarly designated components, with necessary modifications, and vice versa. Note that not all features shown for terminal 110 are required to be implemented in every embodiment of the terminal, and other embodiments may include other features not shown in Figure 1.

[0032] Figure 1 shows one embodiment in which terminal 110-1 is supplied with an electrical input data signal 101-1, specifically, the electrical input data signal 101-1 is received by transmitter 121-1. The electrical input data signal 101-1 is converted to an optical input data signal 102-1 by transmitter 121-1, which functions as an optical-electronic-optical (OEO) converter. In another embodiment of the present invention, instead of the electrical input data signal 101-1 being supplied to terminal 110-1 and converted to an optical input data signal 102-1 by transmitter 121-1, the optical input data signal 102-1 may be supplied directly to terminal 110-1 as an input data source, i.e., as a true signal, specifically, to a spatial multiplexer 180-1. In one such embodiment, for example, depending on requirements or preferences, the optical input data signal 102-1 is not modified by the terminal 110-1, and therefore the transmitter 121-1 is not used.

[0033] Terminals 110-1 transmit and receive signals via optical fibers 150-1 and 150-2, respectively. In one embodiment, a chaff signal, which is a signal used as described herein to cause interference in interception of a true signal, is generated within terminal 110-1 and transmitted via optical fiber 150-1 together with the true optical input data signal 102-1 as described above. Within terminal 110-1, the chaff signal may be generated by a chaff generator 140-1, or a chaff signal received from terminal 110-2 via fiber 150-2 may be reused to propagate along optical fiber 150-1. In the latter case, the output connection of the chaff reuse module 141-1 replaces the output connection of the chaff generator 140-1, which is shown as a dashed line in Figure 1. Those skilled in the art may use any configuration for generating a suitable chaff signal, but exemplary chaff generators that can be used as chaff generators 140-1 or 140-2 are shown in Figure 2 and further described below.

[0034] According to the principles of this disclosure, an OTDR, for example, an OTDR 151-1 in terminal 110-1, generates a signal for use in detecting a problem with one of the optical fibers 150 to which it is connected, for example, optical fiber 150-1, and the location of such a problem. One such problem could be an interception. Thus, the OTDR can pinpoint the location of the interception. The OTDR signal may be transmitted via one of the transmitters 152, for example, a transmitter 152-1, which is typically incorporated within the OTDR 151-1.

[0035] An OTDR, for example, OTDR151-1, typically also incorporates a receiver and signal processor 153, for example, receiver and signal processor 153-1, and a coupler 154, for example, coupler 154-1. Coupler 154-1 is typically configured to couple the signal from transmitter 152-1 to a transmission cable 155, for example, transmission cable 155-1, in a first direction, and simultaneously couple the signal received from transmission cable 155-1 to receiver and signal processor 153-1 in a second direction. In the second direction, coupler 155-1 may also function as a filter to block reflections of the chaff signal.

[0036] Transmitter 152-1 is typically a high-power laser transmitter that transmits pulses of light to be sent downward in one of the optical fibers 150. Backscattered and reflected light return to each OTDR 151 from one of the optical fibers 150. At terminal 110-1, such backscattered and reflected light are directed to receivers and signal processors 153-1 by coupler 154-1. The OTDR signals from transmitter 152-1 may be split or directed by splitter 158, and then further split or directed by splitters 156-1 or 157-1 to allow for nearly simultaneous transmission of copies of the same OTDR signal to multiple spatial paths of optical fiber 150. It is usually not necessary to identify which of the fiber paths, for example, which of the cores of a multicore fiber, is affected by interception or tampering, so it is beneficial to do so. Various splitters may, but do not have to be, passive splitters.

[0037] Alternatively, the splitter 156-1 or 157-1 can be replaced with a switch that can route OTDR pulses sequentially to different spatial paths among the spatial paths and direct their corresponding reflections to the receiver 153-1.

[0038] Chaff and data signals are typically configured to propagate in the same direction within each of the fibers 150, but the OTDR transmission cable 155-1 may be coupled to or both of the fibers 150-1 and 150-2 simultaneously via a spatial or wavelength multiplexer or both. Specifically, the OTDR signal supplied by OTDR 151-1 may be wavelength-multiplexed with one or more true data signals, one or more chaff signals, any desired combination of true data and chaff signals, or it may be supplied to its own spatial path without being multiplexed with any other signals. Among these various possibilities, Figure 1 shows an example in which the signal from the OTDR transmission cable 155-1 is split using a splitter 158-1 and ultimately supplied to both fibers 150-1 and 150-2 via wavelength-multiplexing with the chaff signal in wavelength multiplexers 143-1 and 143-2, followed by spatial multiplexing in spatial multiplexers 180-1 and 181-1.

[0039] In one embodiment, optical fibers 150-1 and 150-2 may be structured as a multicore cable. Each core of the multicore fiber may have the ability to independently guide optical signals along the entire length of the multicore fiber. Each core may be single-mode or multi-mode in terms of the transmitted signal wavelength.

[0040] In one embodiment, the coupled optical chaff signal and OTDR signal are supplied to at least one spatial path of optical fiber 150-1, for example, one core of optical fiber 150-1 when it is a multicore fiber, but the true signal is supplied to a different spatial path of optical fiber 150-1.

[0041] In one embodiment, instead of the OTDR 151-2 detecting interference with the optical fiber 150-2, e.g., interception, and optionally its location, via the use of backscattered and reflected light, an OTDR-type device within terminal 110-1, such as OTDR 151-1, may be used to determine if interference with the optical fiber 150-2 exists. This can be achieved through the use of an optional splitter 157-1. For example, the splitter 157-1 can duplicate the OTDR signal supplied by OTDR 151-1 in terminal 110-1 and supply it to fiber 150-2 joined to terminal 110-1. The reflection of the OTDR signal from optical fiber 150-2 is then processed, for example, by a receiver and signal processor 153-1. Similarly, for example, the splitter 157-2 can duplicate the OTDR signal supplied by the OTDR 151-2 of terminal 110-2 and supply that signal to the fiber 150-1 joined to terminal 110-2 for processing by, for example, the receiver and signal processor 153-2.

[0042] In another embodiment of the present disclosure, detection of interference with optical fiber 150-1 is achieved through the cooperation of OTDR151-1 and an OTDR-type device in terminal 110-2, for example, OTDR151-2.

[0043] In one embodiment, at least one chaff signal, which is multiplexed with the OTDR signal, and at least one data signal are coupled to the respective cores or channels of the optical fiber on the transmitter side.

[0044] The true data and chaff signals, whether coupled with the OTDR signal or not, are coupled using their respective couplers to individual channels of one of the optical fibers 150, for example, to individual cores of optical fiber 150 when optical fiber 150 is a multicore fiber. The couplers used in embodiments of the present invention may be, for example, 1) lensed fiber-based couplers, 2) tapered glass fiber couplers, 3) free-space bulk optical couplers, or 4) any other known or developed couplers. The couplers may also include fibers and free-space paths. Collectively, these couplers constitute a transmit-space multiplexer, for example, one of transmit-space multiplexers 180-1 and 180-2, in that they couple multiple optical signals to each of multiple spatial paths of one of the optical fibers 150.

[0045] For example, the true data signal or any chaff signal that is to be extracted for use as described later can also be obtained by using a receiving spatial demultiplexer, for example, one of spatial demultiplexers 190-1 and 190-2, which may consist of individual demultiplexers. Such a demultiplexer may be a coupler that operates in reverse for this purpose, or any other known or developed demultiplexer. Such couplers may also be operated bidirectionally to couple signals into one of the optical fibers 150 to which they are connected. Alternatively, the fiber core carrying the true data signal may simply be extended into the receiver by itself.

[0046] In one embodiment, the generated chaff signal is uncorrelated with the true data signal. The bandwidth of the chaff signal may be at least as wide as the bandwidth used for the true data signal. Furthermore, the chaff signal may be prepared to have an optical intensity sufficient to lower the signal-to-noise ratio (OSNR) or equivalently increase the bit error rate (BER) to a level that would be observed by an interceptor at an intercept placed at any position along one of the optical fibers 150, such that the information acquired by the interceptor is uninterpretable, for example, the information conveyed in the data signal cannot be recovered in the intercept.

[0047] In the receiver, only the true signal needs to be recovered. For this purpose, only the core that carries the true signal can be coupled to receiver 130-2, which may include an optical-to-electric converter, for example, one or more photodiodes. In another embodiment, the true signal may be transmitted in optical form for further processing. Advantageously, on the receiver side of the multicore fibers 150-1 and 150-2, the intended receiver is able to acquire data transmitted by the true signal regardless of the chaff signal.

[0048] OTDR signals may be available at one or both ends of an optical fiber to detect tampering in the optical fiber, for example, to detect interception and its location.

[0049] In this way, the properties of the chaff signal protect the optical fiber from tampering. As will be discussed in more detail below, such properties prevent the separation of the true data signal from the resulting combined signal, which is a combination of the true data and the chaff signal acquired during interception of the optical fiber.

[0050] A secure fiber system provides protection along the entire length of the fiber without the need for costly guards or enclosures along the fiber. This reduces installation and security maintenance costs, especially in the use of long fiber optic cables. In addition, encryption can be used for true signals, but it is not required. Advantageously, by not using encryption, the bandwidth available for data transmission, which is often consumed otherwise by data encryption, is increased.

[0051] It should be noted that only two terminals 110 and two optical fibers 150 are shown in Figure 1 for the sake of simplification and without limiting the embodiments disclosed. Additional terminals and / or optical fibers may be used without departing from the scope of this disclosure. Wavelength division multiplexing may also be used for both data signals and chaff signals as needed, as long as each data signal to be protected is accompanied by one or more co-propagating chaff signals which preferably occupy at least essentially the same wavelength range as the data signals.

[0052] Figure 2A shows an exemplary chaff source 200-A implemented according to one embodiment. In this embodiment, the chaff source includes an optical amplifier (OA) 210 which operates as an amplified spontaneous emission (ASE) generator. The OA 210 may be, for example, 1) a semiconductor amplifier, 2) a Raman amplifier, 3) a doped optical fiber amplifier without a signal source as input, such as an erbium doped amplifier, and 4) the same. Spontaneous emissions from the OA 210 can be amplified to a high intensity.

[0053] Figure 2B shows an exemplary chaff source 200-B implemented according to another embodiment. In this embodiment, the chaff source 200-B includes an optional optical filter 220 coupled to the OA225. The optical filter 220 is located on the output stage of the chaff generator 200-B to flatten and limit the spectrum over a predefined frequency band. As shown in the exemplary ASE spectrum 310 in Figure 3, the optical filter 225 flattens the output of the chaff source 200-B to include at least the data signal spectrum 320.

[0054] Figure 2C shows an exemplary chaff source 200-C implemented according to another embodiment. In this embodiment, a first optical amplifier 230 is coupled to an optical filter 240 which is coupled to a second optical amplifier 250. The optical amplifier 250 is the output stage of the chaff generator 200-C and is used to amplify the output signal received from the optical filter 240, i.e., the chaff signal.

[0055] Separate chaff sources, for example shown in Figures 2A-2C, may be used within the chaff generator 140 to generate the respective chaff signals supplied therefrom. Alternatively, the output of a master chaff source, which may be one of the chaff sources 200-A, 200-B, or 200-C, may be split into multiple copies using a splitter 260 to obtain various chaff signals, as shown in Figure 2D. These copies of the chaff signals may be delayed relative to each other, for example, using optical fiber delay lines 270-1 to 270-N, to cause them to lose their correlation.

[0056] In one embodiment, the chaff signal can be overwhelmingly strong so that the level of intercepted light energy exceeds the dynamic range of the detector in the interceptor's intercepting equipment. In this case, the total power of the chaff signal may not need to be structured, as it would simply overwhelm the intercepting equipment.

[0057] The OSNR of the data channel observed in one of the receivers 130 by the intended receiver is essentially not degraded by the presence of chaff signals in other cores of the multicore fiber, provided there is no substantial leakage to the cores carrying the data channel. The OSNR observed by the intended receiver is given by the ratio of the signal strength detected in the data channel to the total noise power detected.

number

[0058] In one embodiment, a monitoring signal from one of the OTDRs 151 is coupled, for example, multiplexed, with at least one of the chaff signals before the chaff signal is supplied to the core of the optical fiber 150, when the optical fiber 150 is a multicore fiber. In this way, the coupled OTDR and chaff signals are supplied to at least one core of one of the multicore fibers of the multicore fiber 150. The OTDR signal can be used to detect tampering in the cable, such as interception and its location, by reflection of the signal back to one of the OTDRs 151 that generated it, or by reception by an OTDR or similar detector at the opposite end of the optical fiber.

[0059] Figure 4 shows an embodiment for illustrating the present disclosure in which one embodiment of a terminal, for example terminal 110-1 (Figure 1), is contained within a secure box 400. In the embodiment of Figure 4, the true data signal is supplied to input 460 as an optical signal.

[0060] In Figure 4, the optical chaff signal is generated by a chaff source 411, which may include, for example, one of the chaff generators 200 (Figure 2). The optical chaff signal is supplied, for example, via fiber 417, as the output from the chaff source 411 and coupled to the first port 413 of a 2:N optical coupler 459-1, where N is an integer greater than or equal to 1, but N is often greater than 1.

[0061] For example, a monitoring signal 407 from an OTDR, e.g., OTDR151-1, which may be supplied from transmitter 121-N (Figure 1), is coupled to the second port 415 of a 2:N optical coupler 459-1 via a transmission fiber 157-1, which may be arranged as a fiber coil, for example. For clarity, the transmitter, receiver and signal processor, as well as the coupler which may be included in OTDR151-1 as shown in Figure 1, are not shown in Figure 4. The 2:N optical coupler 459-1 combines the optical chaff signal and the OTDR monitoring signal, generating N copies of the combined, e.g., multiplexed, chaff and OTDR monitoring signals, each of which is provided to each of its respective output ports 405-1 to 405-N.

[0062] The 2:N optical coupler 459-1 can be implemented using, for example, 1) fiber, 2) optical integrated circuit, 3) free-space coupling, or 4) other methods and combinations thereof. In one embodiment of the present invention, the 2:N optical coupler 459-1 may consist of a tree of 1:2 and 2:2 couplers as known in the art. For example, as shown in Figure 2D, a delay difference may be introduced at the 2:N coupler outputs 405-1 to 405-N before sending a combination of the OTDR and chaff signals, with the correlation lost, to the individual cores 403-1 to 403-N of the multicore optical fiber. The data signal may be sent to the central core 403-N+1 as an optical signal 460 provided directly to a box 400 from an external source.

[0063] At least one of the combined chaff and OTDR monitoring signals from output port 405 is provided to at least one of the cores 403 of the multicore fiber 150, including cores 403-1 to 403-N.

[0064] Core 403-N+1 is supplied with the true data signal to be received at the input data source 460. In an embodiment such as that shown in Figure 4, the received true signal is already in a suitable format for transmission over the optical fiber 150, for example, within Core 403-N+1. In this regard, it should be noted that, as mentioned above, there is often a strong preference or requirement on the government side, for example, for highly secure government communications, that the optical data signal is not modified by the transceiver. The configuration shown in Figure 4 is advantageously particularly suitable for such application areas.

[0065] The chaff signals, whether coupled with the OTDR signals or not, are coupled to individual channels of the multicore fiber 150 using, for example, each of the couplers 421-1 to 421-N. In addition, coupler 421-N+1 provides an optical path that connects the input data source 460 to core 403-N+1. Such couplers may be, for example, 1) lensed fiber-based couplers, 2) tapered glass fiber couplers, 3) free-space bulk optical couplers, or 4) any other known or developed couplers and combinations thereof. Couplers may also include fibers and free-space paths. Collectively, these couplers constitute a transmit-space multiplexer in that they couple multiple optical signals to each of the multiple spatial paths of the optical fiber 150.

[0066] Although only a single true data signal is shown in Figure 4, in other embodiments, multiple true data signals may be used, each supplied to its own core among the cores 403.

[0067] Figure 4 shows the use of each core that is not used to transmit a true data signal, but rather to transmit one of the combined chaff and monitoring signals 405. It is simply necessary that one of the cores transmits one of the combined chaff and monitoring signals output from one of the output ports 405. Other cores may transmit other signals, or none at all. For example, only the chaff signal may be transmitted. In practice, the number of cores, the number of chaff and OTDR multiplexed signals, the number of true data signals, and the number of chaff-only signals do not need to be directly related and are at the discretion of the implementer. Not all cores need to be used, and not all chaff or chaff multiplexed with the generated OTDR signal needs to be used.

[0068] According to one aspect of the present disclosure, the known OTDR so-called “dead zone” can be arranged to be substantially contained within the secure box 400. This can be achieved in one embodiment of the present disclosure by ensuring that the length of the transmission fiber 155-1 is sufficient so that the transmission fiber 155-1 extends substantially over the length of the OTDR dead zone. In another embodiment of the present disclosure, the length of the optical fiber 150 contained within the secure box 400 is prepared to substantially include the length of the OTDR dead zone and the transmission fiber 155-1, and the optical path to the optical fiber 150 may be relatively short. In a further embodiment of the present disclosure, the optical path, including the combined length of the transmission fiber 155-1, 2:N coupler 459-1, interconnects, and the optical fiber 150 within the secure box 400, is prepared to be sufficiently long so as to extend substantially over the length of the OTDR dead zone. Advantageously, according to this aspect of the present disclosure, the portion of the optical fiber 150 extending beyond the secure box 400 falls within a zone that can be monitored for tampering by the OTDR 151. Therefore, the OTDR signal can be used to detect tampering such as bending or interception at any point along substantially the entire length of the optical fiber 150 outside the secure box 400, and for example, its location, while those parts of the link from OTDR 151 to the exit point of the optical fiber 150, which are in the dead zone and cannot be monitored by OTDR 151, are certainly inside the secure box 400 and cannot be tampered with or otherwise intercepted.

[0069] In the embodiments of this disclosure, the length of the fiber within the safe box 400 can range from 10 meters to 100 meters, so that the entire dead zone of OTDR151 is contained within the safe box 400.

[0070] In one embodiment of the present disclosure, the secure box 400 may be a secure enclosure conforming to CNSSI (Committee on National Security Systems) 7003 for a Protected Distribution System (PDS).

[0071] In one embodiment of the present disclosure, instead of using the 2:N coupler 459-1, separate fiber couplers, for example, 1:2 and 2:2 fiber couplers, may be used to couple at least one of each of the individual chaff signals with the OTDR monitoring signal. Each individual chaff signal may be generated individually, or they may be copies of a single chaff signal, for example, via the use of a 1:M coupler where M is an integer greater than or equal to 2, or a combination of such methods, for example, at least two, which are one or more individually generated and copies produced by the coupler.

[0072] In another embodiment, the 2:N coupler 459-1 may be a device configured to supply an OTDR signal to different outputs of output 405 at different times, for example. For example, the OTDR signal may be multiplexed with the chaff signal to supply the coupled signal to one of the cores 403-1 to 403-N on a round-robin basis.

[0073] The OSNR observed by an interceptor intercepting an optical fiber is given by:

number

[0074] As mentioned above, a secure optical fiber system can utilize multicore optical fibers. Such fibers are prepared to have a set of cores running parallel to each other along the length of the multicore fiber. Optical signals can propagate independently in each core. The cores can be sized to accommodate single-mode, minority-mode, and multimode fibers. In this way, multicore fibers enable the use of spatial division multiplexing, as well as wavelength division multiplexing and time division multiplexing.

[0075] Multicore fibers include, but are not limited to, concentric cores and concentric refractive index layers within the fiber that make up individual cores with various cross-sectional configurations. These configurations include, but are not limited to, linear, circular, hexagonal, and rectangular shapes.

[0076] In a secure fiber-link system, the individual cores of a multicore fiber may be identical or different from one another. In various embodiments, one or more cores may be sensitive to bending, while one or more cores may be insensitive to bending. Bending-insensitive cores may be made using refractive index grooves or rings of an air core surrounding the signal-transmitting core, which limit the amount of light that can escape from the fiber when the fiber is bent.

[0077] In one embodiment, the center core of a multicore fiber is a bend-sensitive core that transmits the true signal, while at least one of the outer cores transmits the chaff and at least one OTDR signal, and one or more outer cores are of a bend-insensitive type. When intercepted by an interceptor, the energy in the signal channel will decrease and will be detectable by an OTDR at at least one of the terminals at the end of the link. The location of the interception may also be detected. In a similar embodiment, there are M cores M≧1 that transmit the desired signal and N cores N≧1 that transmit the chaff, and in the multicore fiber, at least one of the chaff-transmitting cores also transmits the OTDR signal, so that the total number of cores in the multicore fiber is ≧M+N, then the M signal cores may be of a bend-sensitive type, and the N chaff cores may be of a bend-insensitive type.

[0078] In another embodiment, the center core of a multicore fiber, which transmits the true signal, is bend-insensitive, and any outer cores that transmit the chaff signal, at least one of which also transmits the OTDR signal, are of a bend-sensitive type. Since more energy leaks from the chaff core than from the true signal core, this improves the OSNR advantage of the secure fiber link system when intercepted by an interceptor. In a similar embodiment, if there are M cores M≧1 that transmit the true signal and N cores N≧1 that transmit the chaff signal, and at least one of the chaff signal cores also transmits the OTDR signal, and therefore the total number of cores in the multicore fiber is ≧M+N, then the M true signal-transmitting cores may be of a bend-insensitive type, and the N core-transmitting chaff may be of a bend-sensitive type.

[0079] Ideally, the chaff signal should be uncorrelated with the true data signal and have at least as wide a bandwidth as the one used for the data signal transmitted over the fiber link. The chaff signal should also be strong enough to lower the OSNR or, conversely, increase the BER of an interceptor intercepting the fiber link.

[0080] In other embodiments, as spatial division multiplexing technology is used, instead of multicore fibers, any transmission medium capable of simultaneously propagating multiple independent information-transmitting optical signals, such as few-mode fibers and multimode fibers, may be used.

[0081] Therefore, when anti-interception systems according to various disclosed embodiments are used, the resulting protection will be the same regardless of how the interceptor applies their fiber bending or stretching mechanism. In current secure systems, it is assumed that the chaff and true signal channels can be selectively coupled into the chaff and true signal channels of the fiber at the transmitter end, respectively, and selectively coupled and decoupled at the other end of the link (receiver) with a suitable coupler. Such couplers include lensed fiber-based couplers, tapered glass fiber couplers, polymer-based couplers, and free-space bulk optical couplers.

[0082] In a typical embodiment, the optical fiber medium should have properties that maximize the effectiveness of the system's interception capability and that do not interfere with the legitimate transfer of information between system users: relevant fiber properties are the outcoupling efficiency (dB) of the true and chaff channels through bending or stretching, the attenuation (dB / m) of the chaff and true signal channels along the fiber link, and the crosstalk (dB) between the chaff channel and the true signal channel. Various disclosed embodiments include the relative outcoupling of the chaff signal energy compared to the true signal energy at the point of interception along the link, and the corresponding effect on the OSNR as observed by the interceptor. The intensities of the true and chaff signals depend on the intensity of the corresponding source, the attenuation of the signal in the fiber from the source to the point along the fiber link where the interception occurs, and the outcoupling efficiency of each signal at the point of interception. The essential point is that the total chaff energy extracted by the interceptor from the fiber link at the point of interception must be sufficiently strong compared to the true signal energy so that the OSNR observed by the interceptor is sufficiently low, thereby the bit error rate is sufficiently high to prevent the interceptor from extracting useful information from the true signal. At the same time, the OSNR observed by each intended receiver to which the true signal is properly concatenated, for example, one of receivers 130-1 or 130-2, must be sufficiently high so that the intended receiver can extract all information from the true signal.

[0083] When crosstalk γ (dB / m), which is the crosstalk per unit length of fiber between the chaff and the true signal channel, is significant, this can severely restrict data transfer along the normal signal channel. Since both the signal and chaff channels will contain signal information, intercepting the fiber can also become easier.

[0084] Ideally, crosstalk within the fiber between the chaff channel and the true data signal channel, some of which may be transmitting OTDR signals, should be minimized.

[0085] In one embodiment, station reflectors may be used at the termination of multicore fiber cores that transmit chaff signals, so that the chaff signals generated at the central station can be "reused" in the cable. This eliminates the need to generate chaff signals on the recipient's premises, thereby reducing the equipment required on the customer's premises. This can be advantageous for fiber-to-home, where this embodiment minimizes the equipment required in the customer's home. This helps network management in that all chaff signal generation may be at the central station, making repairs easier and less invasive for the customer at home. Reflectors may be placed at the ends of multicore fiber links that reflect only the chaff channels and not the true signal core. An alternative is to use multichannel fan-out couplers and then terminate individual chaff fan-out channels with connectors that have reflectors that reflect the amplified spontaneously emitted energy through the multichannel fan-out couplers back into the multicore fiber. The signal channels would not be terminated with reflectors. Such a reflector can be considered as an implementation of a chaff generator, for example, one of the chaff generators 140.

[0086] In one embodiment, one or more non-true signal transmission cores may be used to transmit light used to transmit power from a central station to a receiving station, where a photocell will convert the light energy into electrical energy that can be used to power the receiving station, or, if a battery is used in the receiver, electrical energy that can be stored in it. This enables functionality similar to that provided by telephone systems that have been in operation for many years, where the wired telephone was powered without requiring power from a local power company. Optionally, the chaff reuse module 141 may be used in connection therewith to convert such received light into power, which may be in the form of a received chaff signal with or without an OTDR signal multiplexed together. Thus, at the implementer's discretion, the chaff reuse module 141 can reuse the chaff signal in the form of power, as the chaff signal described above, or in a combination of power and the chaff signal. If insufficient power is generated by the chaff reuse module 141, which includes a photocell and generates power from the chaff signal to fully power terminal 110, the power required to supply terminal 110 can simply be reduced by the amount of power generated from the conversion of the chaff signal. Note that if there is no implementation of the functionality provided by the wavelength multiplexer 181, for example, no reuse of the received chaff signal as a chaff signal, the chaff signal multiplexed with the OTDR signal can be supplied directly to the chaff reuse module 141 for conversion into power.

[0087] Although one embodiment for illustrative purposes in Figure 1 is shown as two separate cable optical fibers 150-1 and 150-2, in one embodiment only a single optical fiber may be used while achieving bidirectional transmission. For example, one or more cores of a single optical fiber may be used to transmit a true signal in one direction, while other cores of the single optical fiber may be used to transmit a true signal in the opposite direction. The remaining cores of the single optical fiber may be used to transmit a chaff signal, one or more of which may be multiplexed with a signal for use by the OTDR.

[0088] It should be noted that the disclosed embodiments may be used in conjunction with existing or future configurations to prevent interception or other tampering of optical fibers. Therefore, other means for protecting transmitted data, such as data encryption, patrolling of data lines by guards, intrusion detection and monitoring sensors, and hardening of data lines by embedding them in concrete or steel conduits, may be used in addition to the techniques disclosed herein. However, the use of the disclosed embodiments may reduce or eliminate the need for some or all of those means.

[0089] For the various embodiments disclosed herein, any electronic equipment required, for example, for an OTDR or for processing received signals, may be implemented in hardware or as a combination of firmware and / or software running in hardware. Furthermore, the software may be implemented as a program tangibly realized in a program storage device or computer-readable medium. The program may be uploaded to and executed by a machine having any suitable architecture. A computer platform having hardware such as one or more central processing units ("CPUs"), memory, and input / output interfaces may be appropriately used. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be parts of microinstruction code or application programs, or any combination thereof, that can be executed by a CPU, whether or not such a computer or processor is explicitly indicated. In addition, various other peripheral units may be connected to the computer platform, such as additional data storage devices and printing devices. Furthermore, non-temporary computer-readable medium is any computer-readable medium other than temporary propagation signals.

[0090] It should be understood that references to elements in this specification using names such as “first,” “second,” etc., do not generally limit the number or order of those elements. Rather, these names are generally used herein as a convenient way to distinguish multiple elements or instances of elements. Thus, references to first and second elements do not mean that only two elements may be used therein, or that the first element must precede the second element in any way. Also, unless otherwise stated, a set of elements includes one or more elements. In addition, terms of the form “at least one of A, B, or C,” “one or more of A, B, or C,” “at least one of the group consisting of A, B, and C,” or “at least one of A, B, and C” used herein or in the claims mean “A or B or C, or any combination of these elements.” For example, this term may include A, or B, or C, or A and B, or A and C, or A and B and C, or 2A, or 2B, or 2C, etc.

[0091] All examples and conditional language cited herein are intended for educational purposes to help the reader understand the principles of the disclosed embodiments and the concepts to which the inventors contribute to advancing the art, and should not be considered as limitations to such specifically cited examples and conditions. Furthermore, all descriptions herein that cite the principles, aspects, and embodiments of the disclosed embodiments, as well as specific examples thereof, encompass their structural and functional equivalents. In addition, such equivalents, regardless of structure, include both currently known equivalents and equivalents to be developed in the future, i.e., any elements developed to perform the same function.

Claims

1. A method for making communication via optical fiber secure, A transmit space multiplexer for connecting an optical signal from among multiple optical signals into a spatial path of a multiple spatial path of an optical fiber, wherein each of the spatial paths constitutes a transmit space multiplexer capable of transmitting an optical signal. Here, At least one of the plurality of optical signals is an optically modulated version of a desired sequence of information intended to be transmitted through the optical fiber, and at least one of the plurality of optical signals is coupled in a first spatial path of the plurality of spatial paths. At least the second of the plurality of optical signals is an optical chaff signal, and at least the second of the plurality of optical signals is connected to a second spatial path, which is different from the first spatial path, among the plurality of spatial paths. At least the third of the plurality of optical signals is an optical signal for use by an optical time-domain reflectometer (OTDR), The above method further, An optical coupler is used to couple the optical signal and the optical chaff signal for use by the OTDR. By connecting an optical filter to the spatial path through which the optical signal and the optical chaff signal for use by the OTDR are linked, the reflection of the optical chaff signal received by the transmitting spatial multiplexer is blocked. This includes, A method by which interception along the optical fiber cannot determine the information of the desired sequence being transmitted.

2. The method according to claim 1, wherein the optical signal for use by the OTDR is wavelength multiplexed with the optical chaff signal and coupled thereto by the transmit space multiplexer and placed on the second space path of the plurality of space paths.

3. The method according to claim 1, wherein the optical signal for use by the OTDR is supplied to a third spatial path among the plurality of spatial paths, which is different from the first and second spatial paths.

4. The method according to claim 1, wherein the optical fiber is a multicore fiber, the first spatial path among the plurality of spatial paths is the first core of the multicore fiber, and the second spatial path among the plurality of spatial paths is the second core of the multicore fiber.

5. The method according to claim 4, wherein the at least third optical signal for use by the OTDR is coupled to the third core of the multicore fiber.

6. The method according to claim 4, wherein the optical signal for use by the OTDR is wavelength multiplexed with at least one of the first and second optical signals among the plurality of optical signals, and is coupled to at least the same core as the at least one of the first and second optical signals that are multiplexed.

7. The optical chaff signal is generated such that it occupies substantially the same bandwidth as the optically modulated version of the information of the desired sequence. The method according to claim 1, further comprising the following:

8. The method according to claim 1, wherein the optical signal for use by the OTDR has a frequency different from the frequency of the optical chaff signal.

9. The method according to claim 1, further comprising converting the optical chaff signal received by a photocell located at the end of the optical fiber, away from the transmitting space multiplexer, into electrical energy.

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