Secured 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 contents of which are incorporated herein by reference.

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

[0003] An intruder may penetrate and steal information from an optical fiber transmission line by bending one or more segments of the fiber or stretching, e.g., thinning, one or more segments of the fiber through the application of heat. Doing so may enable reading and interpreting signal energy that escapes from the bent or stretched fiber. Although other methods exist for intercepting information from optical fibers, interception based on bending or stretching the fiber can be easy to implement, effective, and difficult to detect. Interception of valuable data transmitted over the global optical fiber infrastructure is a threat to every major industrial and government organization, especially large organizations that use multiple facilities. Although these organizations may be able to protect the optical fiber cables within their own facilities, they generally have much less control over the optical fiber cable links between their facilities.

[0004] The vulnerability of optical fibers to interception, especially by bending or stretching the fiber, renders many organizations susceptible to data theft. Such data theft may lead to the disclosure of confidential information, thereby causing harm to entities whose data is being transported through such intercepted optical fibers. In the business context, the disclosure of information such as marketing strategies or development techniques may ultimately result in loss of profits. In the political context, the disclosure of information regarding national secrets may endanger the lives of citizens. Existing solutions are inadequate to detect or prevent data exfiltration using optical fiber interception.

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

[0006] An optical time domain reflectometer (OTDR) is a known tool for characterizing, monitoring, and troubleshooting fibers. OTDRs usually work by sending laser pulses of different widths and monitoring their reflections as received at the pulse sending end of the fiber. OTDRs can pinpoint the location of faults in fiber links, and OTDRs find and characterize both reflective and non-reflective events in optical fibers. Thus, OTDRs can be used to detect bends introduced in a fiber link after the fiber link has been established by comparison with a previous OTDR trace before the fiber was bent, e.g., a trace made when the link is first installed. OTDRs can be used to test in-service fibers, i.e., fibers carrying data intended to be delivered to a destination, by running test pulses at wavelength channels different from those used to carry the data intended to be delivered.

[0007] Unfortunately, in highly secure government communications, it is often desired that the optical data signal not be modified by the transceiver. If made a requirement, such a preference would mean that the fiber carrying the secure communications would also not be able to carry the pulses required by the OTDR.

[0008] As is also known, the technique used by OTDR suffers from so-called "dead zones", which are areas behind where the reflection event occurred that cannot be seen by the OTDR. Such dead zones often occur over large distances at the beginning of the fiber when trying to probe very long optical fibers. This is because when trying to probe very long optical fibers, it is necessary to send out a lot of power in order to be able to see the conditions at the end of the fiber. When a lot of optical power is sent out, the pulse width of the sent 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 the reduced resolution can extend to hundreds of meters. As a result, the hundreds of meters between the receiver, where the reflected pulse can be seen, and the sent pulse hides faults near the sender.

[0009] If there is a fault near the launch point, it can also create large reflections that saturate and overload the receiver. This length of fiber is also called the "dead zone" because the fault is hidden in the length close to the OTDR. The receiver needs a lot of time to recover from the saturation. Depending on the OTDR design, wavelength, and magnitude, the OTDR can take, for example, 500 meters or more to fully recover from such a fault near the launch point.

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

[0011] Moreover, due to government preferences or requirements, additional signals often should not be coupled into fibers carrying secure data not intended for delivery to their destination, and further, such outgoing fibers become additional insecure tamper points.

[0012] It would therefore be advantageous to provide a solution that overcomes the deficiencies of the prior art. Summary of the Invention [Means for solving the problem]

[0013] A summary of some exemplary embodiments of the present disclosure follows below. This summary is provided for the convenience of the reader to provide a basic understanding of such embodiments, and does not completely define the breadth of the present disclosure. This summary is not a detailed overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments or to delineate 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 description that is presented later. For convenience, the term "several embodiments" may be used herein to refer to a single embodiment or multiple embodiments of the present disclosure.

[0014] Disclosed embodiments include a system for securing communications over optical fiber, comprising a transmit spatial multiplexer configured to couple optical signals of a plurality of optical signals into spatial paths of a plurality of spatial paths of an optical fiber, each spatial path capable of carrying an optical signal, at least a first optical signal of the plurality of optical signals being an optically modulated version of a desired sequence of information intended for transfer over the optical fiber, at least a first optical signal of the plurality of optical signals being coupled into a first spatial path of the plurality of spatial paths, at least a second optical signal of the plurality of optical signals being an optical chaff signal, at least a second optical signal of the plurality of optical signals being coupled into a second spatial path of the plurality of spatial paths that is different from the first spatial path, and at least a third optical signal of the plurality of optical signals being an optical signal for use by an optical time domain reflectometer (OTDR), whereby eavesdropping along the fiber cannot determine the desired sequence of information being transmitted.

[0015] Disclosed embodiments also include a method for protecting information transmitted over an optical fiber having multiple spatial paths, the method including coupling each of a set of optical signals into at least one spatial path of the multiple spatial paths, at least one of the set of optical signals being an optically modulated version of a desired sequence of information intended for transfer over the optical fiber, at least a first optical signal of the multiple optical signals being coupled into the first spatial path of the multiple spatial paths, at least a second optical signal of the set of optical signals being an optical chaff signal, at least a second optical signal of the multiple optical signals being coupled 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 being an optical signal for use by an optical time domain reflectometer (OTDR).

[0016] Disclosed embodiments also include a terminal equipment for securing communications over an optical fiber, the terminal equipment comprising a transmit spatial multiplexer configured to couple a plurality of optical signals into respective spatial paths of a plurality of spatial paths of the optical fiber, each spatial path capable of carrying an optical signal, at least one of the plurality of optical signals being an optically modulated version of a desired sequence of information intended to be transferred over the optical fiber, and at least one of the plurality of optical signals being an optical chaff signal multiplexed with an optical signal for use by an optical time domain reflectometer (OTDR), such that eavesdropping along the fiber cannot determine the desired sequence of information being transmitted.

[0017] Disclosed embodiments also include a system for securing communications over an optical fiber, the system comprising a receive spatial demultiplexer coupleable at one end to a plurality of spatial paths of the optical fiber, at least two of the plurality of spatial paths carrying optical signals, the optical signal of a first of the at least two spatial paths including at least an optically modulated version of a desired sequence of information intended to be received from the optical fiber, and the optical signal of a second of the at least two spatial paths different from the first of the at least two spatial paths including an optical chaff signal, the receive spatial demultiplexer configured to receive a signal from at least one of the plurality of spatial paths for use by an optical time domain reflectometer (OTDR) and to provide the received signal to the OTDR for use by the OTDR. [Brief description of the drawings]

[0018] [Figure 1] 1 is a block diagram of a secure fiber link system according to one embodiment. [Figure 2A] FIG. 1 is a block diagram of an illustrative embodiment of a chaff source that uses amplified spontaneous emission (ASE). [Figure 2B] FIG. 1 is a block diagram of an illustrative embodiment of a chaff source using ASE. [Figure 2C] FIG. 1 is a block diagram of an illustrative embodiment of a chaff source using ASE. [Figure 2D] FIG. 1 shows an illustrative embodiment of a chaff generator using a copy of a chaff signal and an optional delay line. [Diagram 3] 13 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] FIG. 1 shows an illustrative embodiment in which the terminal is contained in a secure box. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] It is important to note that the embodiments disclosed herein are merely illustrative of the many advantageous uses of the innovative teachings herein. In general, statements made herein of the present application do not necessarily limit any of the various claimed embodiments. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be pluralized without loss of generality and vice versa. In the figures, like numerals refer to like parts throughout the several views.

[0020] The terms "true signal", "true data", "information signal", "true data signal" and "data signal" are used interchangeably 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 carry true data.

[0021] In one embodiment, a secure fiber link system is configured to allow transmission of data to an intended user while simultaneously making the signal obscured or uninterpretable to an eavesdropper eavesdropping anywhere along the entire link other than at the intended receiver, by overpowering the information signal with interfering signal energy at such interception points, as well as allowing detection of such interference through the use of an optical time domain reflectometer (OTDR), intercepting, moving, or the like of an optical cable.

[0022] For this purpose, a secure fiber link system transmits in a first spatial path of a fiber cable, e.g., a core of a multi-core fiber, a desired sequence of information in the form of a first "regular" or true signal, or a set of true signals, usually intended to be transferred between legitimate users located at the ends of the link. Also, at least one other spatial path of the fiber cable, e.g., a different core of a multi-core fiber, carries a chaff signal, and at least one spatial path carries a signal that can be monitored by an OTDR to detect changes in those signals that may result from an act of fiber eavesdropping. The OTDR signal may be carried in a dedicated core or may be wavelength multiplexed with one or more true signals or with one or more chaff signals. This allows the detection of eavesdropping or tampering at the ends of the fiber optic link system.

[0023] Chaff signals, which are interfering signals not needed to carry the true signal, may be applied to the link at one or both ends of the link. Similarly, OTDR signals may be applied at one or both ends of the link.

[0024] In one embodiment, the secure fiber link system is configured to provide physical security for a data signal propagating through at least one of a plurality of spatial paths of the optical fiber in parallel with at least one chaff signal propagating through another of the plurality of spatial paths of the optical fiber that is combined, e.g., multiplexed, such as using frequency division multiplexing, at least sometimes with a signal for use by the OTDR.

[0025] In an embodiment using multiple spatial paths to respectively carry the respective chaff signals, two or more of the spatial paths carrying the chaff signals may carry multiplexed OTDR signals. In such an embodiment, the OTDR signal may be carried continuously on one or more of the spatial paths carrying the chaff signals, or the OTDR signal may be time multiplexed between the various spatial paths carrying the chaff signals, or a combination of both may be used.

[0026] In embodiments of the present disclosure, the spatial paths carrying the data and chaff signals may be the cores of a multicore fiber.

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

[0028] As mentioned above, eavesdropping on a fiber link can be the act of altering the fiber, for example by applying a 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 in the fiber to leak out of the fiber in a way that it can be detected, for example, by a hacker who is trying to steal information being transmitted on the cable.

[0029] Embodiments of the secure fiber link system may be compatible with modern data rates, formats, and telecommunications protocols, as well as signal wavelength provisioning, e.g., wavelength division multiplexing (WDM). The disclosed embodiments may also be agnostic to future equipment and protocols.

[0030] FIG. 1 illustrates a block diagram of an exemplary secure fiber link system 100 according to one embodiment. System 100 includes transceiver terminals 110-1 and 110-2 connected via optical links, e.g., optical fibers 150-1 and 150-2, each of which may be, e.g., multicore, multimode, or few-mode fiber, preferably enabling spatial division multiplexing. Note that optical fibers 150-1 and 150-2 are shown as separate fibers for purposes of illustration, but may be implemented as different spatial paths that are part of the same physical optical fiber. As such, 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 terminals 110, may include 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, respectively. For clarity and educational purposes, but not limitation, FIG. 1 is described hereinafter with respect to various optical signals being transmitted from terminal 110-1 and received at terminal 110-2. However, those skilled in the art will readily recognize that such optical signals may be transmitted from terminal 110-2 and received at terminal 110-1 as well without departing from the disclosed embodiments. Thus, unless specifically indicated otherwise, it should be understood that any description of any particular component of terminal 110-1 applies mutatis mutandis to its corresponding and similarly designated components of terminal 110-2, and vice versa. It should be noted that not every feature illustrated for terminal 110 need be implemented in every embodiment of the terminal, and that other embodiments may include other features not illustrated in FIG.

[0032] 1 shows an embodiment in which the terminal 110-1 is supplied with an electrical input data signal 101-1, in particular the electrical input data signal 101-1 is received at a transmitter 121-1. The electrical input data signal 101-1 is converted into an optical input data signal 102-1 by the transmitter 121-1 acting as an optical-electronic-optical (OEO) converter. In another embodiment of the invention, instead of the electrical input data signal 101-1 being supplied to the terminal 110-1 and being converted into an optical input data signal 102-1 by the transmitter 121-1, the optical input data signal 102-1 can be directly supplied to the terminal 110-1 as an input data source, i.e. as a true signal, in particular to a spatial multiplexer 180-1. In one such embodiment, eg, by requirement or preference, optical input data signal 102-1 is not to be modified by terminal 110-1, and thus transmitter 121-1 is not to be used.

[0033] The terminal 110-1 transmits and receives 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 eavesdropping on the true signal, is generated in the terminal 110-1 and transmitted via the optical fiber 150-1 together with the true optical input data signal 102-1 as described above. In the terminal 110-1, the chaff signal can be generated by the chaff generator 140-1, or the chaff signal received via the fiber 150-2 from the terminal 110-2 can be reused to propagate along the optical fiber 150-1. In this latter case, the output connection of the chaff reuse module 141-1 takes the place of the output connection of the chaff generator 140-1 shown in dashed lines in FIG. 1. Although one skilled in the art may use any configuration for generating a suitable chaff signal, an exemplary chaff generator that may be used as the chaff generator 140-1 or 140-2 is shown in FIG. 2 and described further below.

[0034] In accordance with the principles of the present disclosure, an OTDR, e.g., OTDR 151-1 in terminal 110-1, generates a signal for use in detecting problems with one of the optical fibers 150 to which it is coupled, e.g., optical fiber 150-1, and the location of such problems. One such problem may 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, e.g., transmitter 152-1, which is typically incorporated within OTDR 151-1.

[0035] Also typically incorporated in an OTDR, e.g., OTDR 151-1, is a receiver and signal processor 153, e.g., receiver and signal processor 153-1, and a coupler 154, e.g., coupler 154-1. Coupler 154-1 is typically arranged such that in a first direction it couples a signal from transmitter 152-1 to an outgoing cable 155, e.g., outgoing cable 155-1, and at the same time in a second direction it couples a signal received from outgoing cable 155-1 to receiver and signal processor 153-1. In the second direction, coupler 155-1 may also act as a filter to block reflections of chaff signals.

[0036] The transmitter 152-1 is typically a high power laser transmitter that transmits pulses of light to be sent down one of the optical fibers 150. Backscattered and reflected light returns from one of the optical fibers 150 to each OTDR 151. At the terminal 110-1, such backscattered and reflected light is directed by coupler 154-1 to a receiver and signal processor 153-1. The OTDR signal from the 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 near simultaneous launch of copies of the same OTDR signal to multiple spatial paths of the optical fiber 150. It is useful to do so since it is typically not required to identify which one of the fiber paths, for example which one of the cores of a multi-core fiber, is affected by eavesdropping or tampering. The various splitters may be passive splitters, but need not be.

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

[0038] Although the chaff signal and the data signal are typically arranged to propagate in the same direction in each of the fibers 150, the OTDR delivery cable 155-1 may be coupled to either the fiber 150-1 or 150-2 or to both simultaneously via a space or wavelength multiplexer or both. In particular, the OTDR signal provided by the 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 signals and chaff signals, or may be provided on its own spatial path without being multiplexed with any other signals. Among these various possibilities, FIG. 1 shows the case where the signal from the OTDR delivery cable 155-1 is split using a splitter 158-1 to be finally provided to both the fibers 150-1 and 150-2 via wavelength multiplexing with the chaff signals 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 be capable of independently directing an optical signal along the entire length of the multicore fiber. The individual cores may be single mode or multimode at the signal wavelength being transmitted.

[0040] In one embodiment, the combined optical chaff signal and the OTDR signal are fed into at least one spatial path of the optical fiber 150-1, e.g., one core thereof when the optical fiber 150-1 is a multicore fiber, while the true signal is fed into a different spatial path of the optical fiber 150-1.

[0041] In one embodiment, instead of the OTDR 151-2 detecting interference with, e.g., tapping, and possibly locating, the optical fiber 150-2 through the use of backscattered and reflected light, a device in the terminal 110-1, e.g., an OTDR type device such as OTDR 151-1, may be used to determine that interference with the optical fiber 150-2 is present. This may be accomplished through the use of an optional splitter 157-1. For example, the splitter 157-1 may replicate the OTDR signal provided by the OTDR 151-1 of the terminal 110-1 and provide it to the fiber 150-2 spliced ​​to the terminal 110-1. The reflection of the OTDR signal from the optical fiber 150-2 is then processed, e.g., by the receiver and signal processor 153-1. Similarly, for example, splitter 157-2 can replicate the OTDR signal provided by OTDR 151-2 in terminal 110-2 and provide that signal to fiber 150-1 spliced ​​to terminal 110-2 for processing, for example, by receiver and signal processor 153-2.

[0042] In another embodiment of the present disclosure, detection of interference with optical fiber 150-1 may occur through cooperation of OTDR 151-1 and an OTDR type device, such as OTDR 151-2, within terminal 110-2.

[0043] In one embodiment, at least one chaff signal multiplexed with the OTDR signal, as well as at least one data signal, are coupled into respective cores, or channels, of an optical fiber at the transmitter side.

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

[0045] The true data signal or any chaff signal that is desired to be extracted, for example for use as described below, can be obtained in the same way by using a receiving spatial demultiplexer, for example one of the spatial demultiplexers 190-1 and 190-2, which can be composed of individual demultiplexers. Such demultiplexers can be couplers that operate inversely for this purpose, or any other known or developed demultiplexers. Such couplers can also be operated in both directions to couple a signal to one of the optical fibers 150 to which they are coupled. Alternatively, the fiber core carrying the true data signal can simply be extended alone into the receiver.

[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 arranged to be of sufficient optical intensity to lower the signal-to-noise-ratio (OSNR), or equivalently to raise the bit error rate (BER), that would be observed by an eavesdropper at an eavesdropper placed at any position along one of the optical fibers 150, to a level such that the information obtained by the eavesdropper is uninterpretable, e.g., the information carried in the data signal cannot be recovered at the eavesdropper.

[0047] At the receiver, only the true signal needs to be recovered. For that purpose, only the cores carrying the true signal may be coupled to the receiver 130-2, which may include an optical to electrical converter, e.g., one or more photodiodes. In another embodiment, the true signal may be conveyed for further processing in optical form. Advantageously, at the receiver side of the multicore fibers 150-1 and 150-2, the intended recipient is enabled to obtain the data carried by the true signal regardless of the chaff signal.

[0048] An OTDR signal 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 discussed in more detail below, which prevent separation of the true data signal from the resulting combined signal, which is a combination of the true data and chaff signals, obtained by tapping the optical fiber.

[0050] A secure fiber system provides protection along the entire length of the optical fiber without the need for costly guards or encasements along the optical fiber. This reduces installation and security maintenance costs, especially with long optical fiber runs. In addition, encryption can be used for true signals, but it is not required. Advantageously, not using encryption increases the available bandwidth for data transmission that would otherwise often be consumed by data encryption.

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

[0052] 2A illustrates an exemplary chaff source 200-A implemented according to one embodiment. In this embodiment, the chaff source includes an optical amplifier (OA) 210 and operates as an amplified spontaneous emission (ASE) generator. The OA 210 may be, for example, any of the following: 1) a semiconductor amplifier; 2) a Raman amplifier; 3) a doped optical fiber amplifier without a signal source as an input, e.g., an erbium doped amplifier; and 4) the like. The spontaneous emission from the OA 210 may be amplified to high intensity.

[0053] 2B illustrates 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 OA 225. The optical filter 220 is located at 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 of FIG. 3, the optical filter 225 flattens the output of the chaff source 200-B to include at least the data signal spectrum 320.

[0054] 2C illustrates 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, i.e., the chaff signal, received from the optical filter 240.

[0055] Separate chaff sources, such as those shown in Figures 2A-2C, may be used within the chaff generator 140 to generate the respective chaff signals provided 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 using a splitter 260 into multiple copies to obtain various chaff signals, as shown in Figure 2D. These copies of the chaff signal may be delayed relative to each other, for example using optical fiber delay lines 270-1 to 270-N, to decorrelate them.

[0056] In one embodiment, the chaff signals can be made overwhelmingly strong so that the level of intercepted optical energy exceeds the dynamic range of the detectors in the interceptor's listening equipment. In this case, there may be no need to structure the format of the chaff signals since their full power would simply overwhelm the listening equipment.

[0057] The OSNR of the data channel seen by the intended recipient at one of the receivers 130 is essentially not degraded by the presence of the chaff signal in the other cores of the multicore fiber, as long as there is substantially no leakage into the core carrying the data channel. The OSNR seen by the intended recipient is given by the ratio of the signal power detected in the data channel to the total noise power detected:

number

[0058] In one embodiment, the monitoring signal from one of the OTDRs 151 is combined, e.g., multiplexed, with at least one of the chaff signals before the chaff signal is fed into a core of the optical fiber 150 when the optical fiber 150 is a multi-core fiber. In that way, the combined OTDR and chaff signal is fed into at least one core of one of the multi-core fibers 150. The OTDR signal can be used to detect tampering in the cable, e.g., tapping and its location, by reflection of the signal back to one of the OTDRs 151 that generated it, or by being received by an OTDR or similar detector at the opposite end of the optical fiber.

[0059] 4 shows an illustrative embodiment of the present disclosure in which an embodiment of a terminal, such as terminal 110-1 (FIG. 1), is contained within a secure box 400. In the embodiment of FIG. 4, a true data signal is provided to input 460 as an optical signal.

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

[0061] A monitoring signal 407 from an OTDR, e.g., OTDR 151-1, which may be provided, for example, from a transmitter 121-N (FIG. 1), is coupled to a second port 415 of the 2:N optical coupler 459-1 via an outgoing fiber 157-1, which may be arranged, for example, as a fiber coil. For clarity, the transmitter, receiver and signal processor, as well as couplers that may be included in the OTDR 151-1 as shown in FIG. 1, are not shown in FIG. 4. The 2:N optical coupler 459-1 combines the optical chaff signal and the OTDR monitoring signal and generates N copies of the combined, e.g., multiplexed, chaff and OTDR monitoring signal, each of which is provided to a respective one of each of the output ports 405-1 to 405-N.

[0062] The 2:N optical coupler 459-1 may be implemented, for example, using 1) fiber, 2) integrated optical circuits, 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 be configured as a tree of 1:2 and 2:2 couplers as known in the art. A delay difference, for example as shown in FIG. 2D, may be introduced at the 2:N coupler outputs 405-1 to 405-N before sending the decorrelated combination of OTDR and chaff signals to the individual cores 403-1 to 403-N of the multi-core optical fiber. A data signal may be sent to the central core 403-N+1, for example as an optical signal 460 provided directly to the box 400 from outside.

[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, including cores 403-1 through 403-N, of multi-core fiber 150.

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

[0065] The chaff signal, whether or not combined with the OTDR signal, is coupled to individual channels of the multi-core fiber 150 using, for example, each of the couplers 421-1 to 421-N. In addition, the coupler 421-N+1 provides an optical path that couples the input data source 460 to the core 403-N+1. Such couplers may be, for example, 1) lensed fiber-based couplers, 2) tapered glass fiber couplers, 3) free-space bulk optic couplers, or 4) any other known or developed couplers and combinations thereof. The couplers may also include fiber and free-space paths. Collectively, these couplers are transmit spatial multiplexers 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 FIG. 4, in other embodiments multiple true data signals may be used, each fed to a unique respective one of the cores 403.

[0067] Although FIG. 4 shows the use of each core not used to carry a true data signal as carrying one of the combined chaff and supervisory signals 405, it is only necessary that one of the cores carry one of the combined chaff and supervisory signals output from one of the output ports 405. The other cores may carry other signals or none at all. For example, only chaff signals may be carried. 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 need not 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 need to be used.

[0068] According to one aspect of the present disclosure, the known OTDR so-called "dead zone" may be disposed to be substantially contained within the secure box 400. This may be accomplished in one embodiment of the present disclosure by providing a length of the delivery fiber 155-1 sufficient so that the delivery fiber 155-1 extends substantially 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 provided to substantially include the OTDR dead zone and the length of the delivery 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 delivery fiber 155-1, 2:N coupler 459-1, interconnects, and the combined length of the optical fiber 150 within the secure box 400 is provided to be sufficiently long to extend substantially the length of the OTDR dead zone. Advantageously, according to this aspect of the present disclosure, the portion of the optical fiber 150 that extends beyond the secure box 400 falls within a zone that may be monitored for tampering by the OTDR 151. Thus, the OTDR signal can be used to detect tampering, such as bending or interception, and its location, for example, at any point along substantially the entire portion of the optical fiber 150 that is outside the secure box 400, while those portions of the link from the OTDR 151 to the exit point of the optical fiber 150 that are within a dead zone and therefore cannot be monitored by the OTDR 151 are certainly within the secure box 400 and cannot be tampered with or otherwise intercepted.

[0069] In an embodiment of the present disclosure, the length of fiber length within the secure box 400 may range from 10 meters to 100 meters such that the entire dead zone of the OTDR 151 is contained within the secure box 400.

[0070] In one embodiment of the present disclosure, secure box 400 may be a secure enclosure compliant with Committee on National Security Systems (CNSSI) 7003 for Protected Distribution Systems (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, can be used to couple at least one of each of the individual chaff signals with the OTDR monitoring signal. Each individual chaff signal can be generated individually, or they can be copies of a single chaff signal, for example, through the use of a 1:M coupler, where M is an integer equal to or greater than 2, or a combination of such approaches, for example one or more generated individually and at least two copies produced by the coupler.

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

[0073] The OSNR seen by an eavesdropper tapping the optical fiber is given by:

number

[0074] As mentioned above, secure optical fiber systems may use multicore optical fibers. Such fibers are prepared having a set of cores that run parallel along the length of the multicore fiber. Optical signals may propagate independently in each core. The cores may be sized to accommodate single mode, few mode, and multimode fibers. As such, multicore fibers allow the use of space division multiplexing as well as wavelength division multiplexing and time division multiplexing.

[0075] Multicore fibers can be made with a variety of core geometries, including, but not limited to, concentric cores as well as concentric refractive index layers within the fiber creating individual cores arranged in a variety of cross-sectional configurations. These configurations include, but are not limited to, linear, circular, hexagonal, rectangular, etc.

[0076] The individual cores of a multicore fiber used in a secure fiber link system may be the same as or different from one another. In various embodiments, one or more of the cores may be bend sensitive while one or more of the cores may be bend insensitive. A bend insensitive core may be created using an air-core refractive index groove or ring surrounding the signal carrying core that limits the amount of light that can escape the fiber when the fiber is bent.

[0077] In one embodiment, the center core of the multicore fiber carries the true signal and is a bend-sensitive core, while one or more outer cores, at least one of which carries the chaff and at least one OTDR signal, are of a bend-insensitive type. Upon interception by an eavesdropper, the energy in the signal channel will drop and is detectable by an OTDR at at least one of the terminals at the end of the link. The location of the interception can also be detected. In a similar embodiment, if there are M cores, M≧1, carrying the desired signal and N cores, N≧1, carrying the chaff, and in the multicore fiber, at least one of the cores carrying the chaff also carries an OTDR signal, so that the total cores in the multicore fiber are ≧M+N, then the M signal cores can be of the bend-sensitive type and the N chaff cores can be of the bend-insensitive type.

[0078] In another embodiment, the center core of the multicore fiber, carrying the true signal, is bend insensitive and any outer cores carrying chaff signals, at least one of which also carries an OTDR signal, are of a bend sensitive type. This improves the OSNR advantage of the secure fiber link system when eavesdropped on by an eavesdropper, since more energy escapes from the chaff core than the true signal core. In a similar embodiment, if there are M cores, M≧1, carrying true signals and N cores, N≧1, carrying chaff signals in the multicore fiber, and at least one of the cores carrying the chaff signal also carries an OTDR signal, such that the total number of cores in the multicore fiber is≧M+N, then the M true signal carrying cores may be of a bend insensitive type and the N cores carrying chaff may be of a bend sensitive type.

[0079] Desirably, the chaff signal is uncorrelated with the true data signal and has a bandwidth at least as wide as that used for the data signal being transmitted over the fiber link. The chaff signal should also be strong enough to lower the OSNR or equivalently raise the BER of an eavesdropper tapping the fiber link.

[0080] In other embodiments, instead of multicore fiber, any transmission medium in which multiple, independent, information-bearing optical signals can propagate simultaneously may be used, such as few-mode fiber and multimode fiber, such that space division multiplexing techniques are used.

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

[0082] In a typical embodiment, the fiber optic medium should have properties that maximize the effectiveness of the anti-eavesdropping capabilities of the system while not impeding the transfer of legitimate information between system users: the relevant properties of the fiber are the outcoupling efficiency (dB) of the true and chaff channels through bends or stretches, the attenuation (dB / m) of the chaff and true signal channels along the fiber link, and the crosstalk (dB) between the chaff and true signal channels. 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 seen by an eavesdropper. The strength of the true and chaff signals depends on the strength of the corresponding sources, 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 an eavesdropper from the fiber link at the point of eavesdropping must be strong enough compared to the true signal energy so that the OSNR observed by the eavesdropper is low enough, and therefore the bit error rate is high enough to prevent the eavesdropper from extracting useful information from the true signal. At the same time, the OSNR observed by each intended receiver, e.g., one of receivers 130-1 or 130-2, to which the true signal is normally coupled, must be high enough so that the intended receiver can extract all the information from the true signal.

[0083] If the crosstalk per unit length of fiber between the chaff and true signal channels, γ (dB / m), is significant, then this can severely limit data transfer along the regular signal channels. It can also become easier to tap the fiber, since both the signal and chaff channels will contain signal information.

[0084] Desirably, crosstalk within the fiber between the chaff channels and the true data signal channels, some of which may be carrying the OTDR signal, should be minimized.

[0085] In one embodiment, a station reflector may be used at the end of the multicore fiber core carrying the chaff signal so that the chaff signal generated at the central office may be "reused" on the cable. This may eliminate the need to generate the chaff signal at the receiver's premises, thereby reducing the equipment required at the customer's premises. This may be advantageous for fiber-to-the-home, where this embodiment minimizes the equipment required at the customer's residence. This aids in network management in that all chaff signal generation may be at the central office, making repairs easier and less invasive for the customer at home. A reflector may be placed at the end of the multicore fiber link that reflects only the chaff channel and not the true signal core. An alternative is to use a multichannel fan-out coupler and then terminate each individual chaff fan-out channel with a connector that has a reflector that will reflect the amplified spontaneous emission energy back through the multichannel fan-out coupler into the multicore fiber. The signal channel would not be terminated at the reflector. Such a reflector may be considered to be an implementation of a chaff generator, for example, one of the chaff generators 140.

[0086] In one embodiment, one or more of the non-true signaling cores may be used to transmit light used to transmit power from the central station to the receiving station, where a photocell will convert the light energy into electrical energy that can be used to run the receiving station, or that can be stored in a battery if one is used in the receiver. This allows functionality such as that provided by telephone systems that operated for many years, where wired telephones were powered without the need for power from the local power company. Optionally, the chaff reuse module 141 may be used in this regard to convert such received light, which may be in the form of a received chaff signal with or without an OTDR signal multiplexed therewith, into electrical power. Thus, at the discretion of the implementer, the chaff reuse module 141 may reuse the chaff signal in the form of electrical power, as a chaff signal as described above, or a combination of electrical power and 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 the terminal 110, the power required to supply the terminal 110 can simply be reduced by the amount of power generated from the conversion of the chaff signal. It should be noted that in the absence of implementation of the functionality provided by the wavelength multiplexer 181, e.g., without the reuse of the received chaff signal as a chaff signal, the chaff signal multiplexed with the OTDR signal can be provided directly to the chaff reuse module 141 for conversion to power.

[0087] Although shown in the illustrative embodiment of FIG. 1 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 of the cores of the 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. Cores of the remaining cores of the single optical fiber may be used to carry a chaff signal, one or more of which may be multiplexed with a signal for use by an OTDR.

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

[0089] Any electronics required for the various embodiments disclosed herein, for example for the OTDR or for processing the received signal, may be implemented in hardware or as a combination of firmware and / or software executing in hardware. Furthermore, software may be implemented as a program tangibly embodied 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 ("CPU"), memory, and input / output interfaces may be suitably used. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be part of the microinstruction code or part of the application program, or any combination thereof, that may be executed by a CPU, whether or not such a computer or processor is explicitly shown. In addition, various other peripheral units may be connected to the computer platform, such as additional data storage devices and printing devices. Furthermore, a non-transitory computer readable medium is any computer readable medium except a transitory propagating signal.

[0090] It should be understood that references to elements herein using designations such as "first", "second", etc., generally do not limit the quantity or order of those elements. Instead, these designations are generally used herein as a convenient way of distinguishing between multiple elements or instances of an element. Thus, references to first and second elements do not imply that only two elements may be used therein or that the first element must precede the second element in any manner. 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" or "one or more of A, B, or C" or "at least one of the group consisting of A, B, and C" as used in the present specification or claims mean "A or B or C, or any combination of these elements." For example, the 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, and so on.

[0091] All examples and conditional language recited herein are intended for educational purposes to aid the reader in understanding the principles of the disclosed embodiments and concepts contributed by the inventors to advance the art, and should not be construed as limitations to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and examples of the disclosed embodiments, as well as specific examples thereof, are intended to encompass structural and functional equivalents thereof. In addition, such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

Claims

1. 1. A system for securing communications over optical fiber, comprising: a transmit spatial multiplexer configured to couple optical signals of a plurality of optical signals into spatial paths of a plurality of spatial paths of an optical fiber, each of the spatial paths being capable of carrying an optical signal; at least 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 the at least a first optical signal of the plurality of optical signals is coupled into a first spatial path of the plurality of spatial paths; At least a second optical signal of the plurality of optical signals is an optical chaff signal, and the at least a second optical signal of the plurality of optical signals is coupled into a second spatial path of the plurality of spatial paths, the second spatial path being different from the first spatial path; at least a third optical signal of the plurality of optical signals is an optical signal for use by an optical time domain reflectometer (OTDR); A system whereby eavesdropping along the fiber cannot determine the desired sequence of information being transmitted.

2. 2. The system of claim 1, wherein the optical signal for use by an OTDR is wavelength multiplexed with the optical chaff signal and coupled therewith by the spatial multiplexer onto the second spatial path.

3. 2. The system of claim 1, wherein the optical signal for use by an OTDR is provided to a third spatial path of the plurality of spatial paths, the third spatial path being distinct from the first and second spatial paths.

4. 2. The system of claim 1, wherein the optical fiber is a multicore fiber, the first spatial path of the plurality of spatial paths is a first core of the multicore fiber, and the second spatial path of the plurality of spatial paths is a second core of the multicore fiber.

5. The system of claim 4 , wherein the at least one optical signal for use by an OTDR is coupled into a third core of the multicore fiber.

6. 5. The system of claim 4, wherein the optical signal for use by an OTDR is wavelength multiplexed with at least one of the first and second optical signals and is at least coupled into the same core as the at least one of the first and second optical signals with which it is multiplexed.

7. 2. The system of claim 1, further comprising a chaff signal generator configured to generate the chaff signal such that the chaff signal occupies substantially the same bandwidth as the optically modulated version of the desired sequence of information.

8. The system of claim 1 , wherein the optical signal for use by an optical time domain reflectometer is at a frequency different from a frequency of the chaff signal.

9. an optical coupler for coupling together the chaff signal and the optical signal for use by an optical time domain reflectometer; an optical filter coupled to the spatial path along which the chaff signal and the signal for use by an optical time domain reflectometer are coupled to block reflections of the chaff signal back to the transmit spatial multiplexer; The system of claim 1 further comprising:

10. the transmit spatial multiplexer; a chaff signal generator configured to generate the chaff signal such that the chaff signal occupies substantially the same bandwidth as the optically modulated version of the desired sequence of information; an optical time domain reflectometer configured to generate the optical signal for its own use and to detect changes to a state of the optical fiber relative to a previous state of the optical fiber; a delivery fiber coupled at a first end thereof to the optical time domain reflectometer; The system of claim 1 further comprising a secure enclosure containing:

11. 11. The system of claim 10, wherein the delivery fiber is coupled at a second end thereof to an optical path that provides the optical signal to the spatial multiplexer for use by an OTDR.

12. 11. The system of claim 10, wherein a length of an optical path traveled within the secure enclosure by the optical signal for use by the OTDR is long enough to prevent an OTDR dead zone at the point where the optical signal for use by the OTDR exits the secure enclosure.

13. 13. The system of claim 12, wherein the length of the optical path traveled within the secure enclosure by the optical signal for use by the OTDR is determined as a function of at least one of the group consisting of the length of the optical fiber and the power of the optical signal for use by the OTDR.

14. 13. The system of claim 12, wherein the length of the optical path traveled within the secure enclosure by the optical signal for use by the OTDR is substantially equal to a length of the delivery fiber.

15. an optical coupler for coupling together the chaff signal and the optical signal for use by an optical time domain reflectometer; an optical filter coupled to the delivery fiber to block reflections of the chaff signal from being received by the optical time domain reflectometer; The system of claim 10 further comprising within the secure enclosure.

16. 2. The system of claim 1, further comprising a photocell located at an end of the optical fiber remote from the transmit spatial multiplexer, the photodetector coupled to receive and convert the optical chaff signal into electrical energy.

17. 17. The system of claim 16, wherein the electrical energy is used to at least partially power a receiver coupled to the optical fiber at the remote end.

18. 1. A method for protecting information transmitted over an optical fiber having multiple spatial paths, comprising: Coupling each of a set of optical signals into at least one of the plurality of spatial paths. Including, at least one of the set of optical signals is an optically modulated version of a desired sequence of information intended to be transferred over the optical fiber, and at least a first optical signal of the plurality of optical signals is coupled into a first spatial path of the plurality of spatial paths; at least a second optical signal of the set of optical signals is an optical chaff signal, and the at least a second optical signal of the plurality of optical signals is coupled into a second spatial path of the plurality of spatial paths, the second spatial path being different from the first spatial path; at least a third optical signal of the plurality of optical signals is an optical signal for use by an optical time domain reflectometer (OTDR); method.

19. 20. The system of claim 18, wherein the optical signal for use by an OTDR is wavelength multiplexed with the optical chaff signal and concatenated therewith onto the second spatial path of the plurality of spatial paths.

20. 20. The system of claim 18, wherein the optical signal for use by an OTDR is wavelength multiplexed with the optical signal carrying a desired sequence of information and coupled therewith by the spatial multiplexer to be placed on the first spatial path of the plurality of spatial paths.

21. 20. The method of claim 18, wherein the at least one optical chaff signal occupies an optical frequency range that is substantially the same as an optical frequency range occupied by the optically modulated version of the desired sequence of information.

22. operating an optical amplifier to generate said at least one optical chaff signal by amplifying spontaneous emission; 20. The method of claim 18, further comprising:

23. 19. The method of claim 18, wherein the optical power of the at least one optical chaff signal when coupled into the optical path is such that a signal representing the optically modulated version of the desired sequence of information is strong enough to enable the desired sequence of information to be determined therefrom, and such that the signal cannot be obtained upon interception at any point along the optical fiber due to interference caused by the presence of the optical chaff signal at the interception point.

24. determining at an end of the fiber where the coupling is performed whether there is an interception of the optical fiber based on the returned optical energy of the optical signal for use by an OTDR; 20. The method of claim 18, further comprising:

25. determining, when an interception of the optical fiber is detected, the location of the interception at the end of the fiber where the coupling is performed based on the returned optical energy of the optical signal for use by an OTDR; 20. The method of claim 18, further comprising:

26. determining, at an end of the fiber opposite to the end at which the coupling is performed, whether there is an interception of the optical fiber based on the energy of the optical signal for use by an OTDR received at the end of the optical fiber opposite to the end at which the coupling is performed; 20. The method of claim 18, further comprising:

27. determining, when an interception of the optical fiber is detected, the location of the interception at the end of the optical fiber opposite the end at which the coupling is performed based on the energy of the optical signal received at the opposite end of the fiber for use by an OTDR; 20. The method of claim 18, further comprising:

28. A terminal device for securing communications over optical fibers, comprising: a transmit spatial multiplexer configured to couple a plurality of optical signals into respective spatial paths of a plurality of spatial paths of an optical fiber, each of said spatial paths being capable of carrying an optical signal; at least one 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; at least one of the plurality of optical signals is an optical chaff signal multiplexed with an optical signal for use by an optical time domain reflectometer (OTDR); whereby an eavesdropper along the fiber cannot determine the desired sequence of information being transmitted. Terminal equipment.

29. 30. The terminal equipment of claim 28, wherein the optical fiber is a multicore fiber, and an optically modulated version of the desired sequence of information is coupled by the transmit spatial multiplexer into a first core of the multicore fiber, and the at least one optical chaff signal multiplexed with the optical signal for use by the optical time domain reflectometer is coupled into a second core of the multicore fiber.

30. 1. A system for securing communications over optical fiber, comprising: a receive spatial demultiplexer coupleable at an end of the optical fiber into a plurality of spatial paths of the optical fiber, at least two of the plurality of spatial paths carrying optical signals; an optical signal in a first spatial path of the at least two spatial paths including at least an optically modulated version of a desired sequence of information intended to be received from the optical fiber; an optical signal of a second spatial path of the at least two spatial paths different from the first spatial path of the at least two spatial paths includes an optical chaff signal; the receive spatial demultiplexer is configured to receive a signal from at least one of the plurality of spatial paths for use by an optical time domain reflectometer (OTDR) and to provide the received signal to the OTDR for use by the OTDR. system.

31. 31. The system of claim 30, wherein the signal for use by an OTDR originates from the one end of the optical fiber.

32. 33. The system of claim 32, wherein the signal for use by an OTDR originates from an end of the optical fiber opposite the one end of the optical fiber.

33. 31. The system of claim 30, wherein the chaff signal originates from the end of the optical fiber and the signal for use by an OTDR originates from an end of the optical fiber opposite the end of the optical fiber.

34. 31. The system of claim 30, wherein the chaff signal travels in the optical fiber in a direction opposite to the signal for use by an OTDR.

35. 31. The system of claim 30, wherein the receive spatial demultiplexer is further arranged to provide as an output the optically modulated version of a desired sequence of information.

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