Abstract:
An intrusion detection system for monitoring a premises includes at least one optical cable that houses at least one optical fiber and extends about the premises. Optical time domain reflectometry (OTDR) means is operably coupled to opposite first and second ends of the at least one optical fiber. The OTDR means includes first signal processing circuitry that analyzes the backscatter signal received via the first end of the at least one optical fiber in order to detect an intrusion of the premises, and second signal processing circuitry that analyzes the backscatter signal received via the second end of the at least one optical fiber in order to detect an intrusion of the premises. The redundancy of intrusions decisions made by the first and second signal processing circuitry can be verified. The system preferably further includes means for detecting a break in the at least one fiber, for identifying location of the break, for outputting to a user the location of the break, and for raising an alarm indicating the break.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates broadly to security systems and intrusion detector used therein. More particularly, this invention relates to fiber optic intrusion detectors. 
         [0003]    2. State of the Art 
         [0004]    Intrusion detectors are widely used in security systems to monitor the boundaries of a well-defined area in order to detect the presence, location and motion of people and vehicles. Exemplary applications for such intrusion detectors include the monitoring of the perimeters of national boundaries, military facilities, chemical plants, airports, rail stations and correctional facilities. One of the challenges for these detectors is the need to operate remotely in harsh environments with exposure to wide temperature ranges as well as rain, snow, and dirt. 
         [0005]    Fiber optic sensors have been developed for intrusion detection. The fiber optic sensor has inherent advantages in that the fiber optic sensing element is passive (it does not carry electricity), which is particularly important for facilities with highly combustible materials. The fiber optic sensing element can also span over extended lengths (e.g., tens of kilometers). The fiber optic sensing element is immune to electromagnetic effects that might otherwise damage or interfere with its operation. And the fiber optic sensing element is readily available at competitive prices and in ruggedized cables capable of withstanding harsh environments. 
         [0006]    Fiber optic intrusion detection systems are commercially-available from Future Fibre Technologies Pty Ltd of Mulgrave, Victoria, Australia and Fiber Sensys of Hillsboro, Oreg., USA. The Future Fibre Technologies system operates using a fiber optic loop including a forward path and a return path. The forward path includes two separate optical fibers. The return path includes a single optical fiber. The two optical fibers of the forward path form the arms of an interferometer. Continuous laser light is sent down the two arms of the interferometer. The light returned by the return path is analyzed. If there has been no external interference (motion, sound or vibrations) acting on the two arms of the interferometer, the return light will not change. If there is an external interference acting on the two arms of interferometer, the return light will change and an interference pattern generated. A controller detects this change and will interpret the effect as either an intrusion event or an ambient condition. The Fiber Sensys system injects coherent light into a multimode fiber. The mode of the light disperses along its length and mixes at the fiber&#39;s terminus, resulting in characteristic pattern of light and dark splotches called speckle. The laser speckle is stable as long as the fiber remains immobile, but flickers when the fiber is vibrated due to environmental effects (such as a person or vehicle passing nearby). Intrusion detection is accomplished by analyzing the speckle pattern over time. In either system, a break in the fiber optic sensor completely disables the intrusion detection system. Moreover, either system cannot detect and report the position of the fiber break. 
         [0007]    An alternative approach is proposed in U.S. Pat. No. 5,194,847 to Taylor et al. In the Taylor system, light from a highly-coherent pulsed laser is launched into a sensing optical fiber. As the individual pulses propagate within the fiber in the forward direction, normal Rayleigh scattering causes a proportion of the light to be scattered uniformly, with a small fraction being recaptured by the fiber before it propagates in the reverse direction to the receiver. The coherent (narrow linewidth) nature of the launched pulses ensures that detectable optical interference can take place between the components of the scattered light. The system analyzes the phase changes and corresponding time delays of the backscatter signal in order to collect a spatial distribution of localized disturbances along the sensing fiber. In the static case, the spatial distribution is random but stable. In the dynamic case (which can be caused, for example, by a disturbance by an unauthorized intruder or vehicle), the localized pattern changes. Such changes can be used to indicate the occurrence of an intrusion and the approximate location of the intrusion along the sensing fiber. In this system, a break in the fiber would disable the capability for intrusion detection at points beyond the break. Such limitations hinder the deployment of such systems in critical security applications and provide opportunities for organized groups (terrorists, thieves and other undesirable third parties) to quickly disable these systems. 
         [0008]    Thus, there remains a need in the art for fiber-optic based intrusion detection systems that can operate without interruption in the event that a break occurs in the sensing optical fiber of the system. 
       SUMMARY OF THE INVENTION 
       [0009]    It is therefore an object of the invention to provide a fiber-optic based intrusion detection system that can operate without interruption in the event that a break occurs in the sensing optical fiber of the system. 
         [0010]    It is another object of the invention to provide such a fiber-optic based intrusion detection system that identifies and reports the position of such a break. 
         [0011]    In accord with these objects, which will be discussed in detail below, an intrusion detection system for monitoring a premises includes at least one optical cable that houses at least one optical fiber and extends about the premises. Optical time domain reflectometry (OTDR) means is operably coupled to opposite first and second ends of the at least one optical fiber. The OTDR means includes first signal processing circuitry that analyzes the backscatter signal received via the first end of the at least one optical fiber in order to detect an intrusion of the premises, and second signal processing circuitry that analyzes the backscatter signal received via the second end of the at least one optical fiber in order to detect an intrusion of the premises. The redundancy of intrusions decisions made by the first and second signal processing circuitry can be verified. The system preferably further includes means for detecting a break in the at least one fiber, for identifying location of the break, for outputting to a user the location of the break, and for raising an alarm indicating the break. 
         [0012]    It will be appreciated that the fiber-optic based intrusion detection systems described herein provide continued operation in the event that a break occurs in the sensing optical fiber of the system. Such systems also report the position of such a break. Moreover, the fiber-optic based intrusion detection systems described herein can be used for a wide variety of applications, such as monitoring national boundaries, military facilities, chemical plants, airports, rail stations, correctional facilities, a power cable, a tunnel, a pipeline, a building, or other smart structures. 
         [0013]    According to one embodiment of the invention, the OTDR means includes a laser source for generating optical pulses, an optical detector, and a directional coupler and an optical switch operably coupled between the laser source and an optical fiber pair. The directional coupler and the optical switch cooperate to direct the optical pulses generated by the laser source over the optical fibers of the pair in a time-division-multiplexed manner and to direct scatter that propagates back along the optical fiber pair to the optical detector in a time-division-multiplexed manner. The first signal processing circuitry analyzes the backscatter signal received via the first end of one optical fiber of the pair in order to detect an intrusion of the premises. The second signal processing circuitry analyzes the backscatter signal received via the second end of the other optical fiber of the pair in order to detect an intrusion of the premises. 
         [0014]    According to another embodiment of the invention, the OTDR means includes a first laser source for generating optical pulses, a first optical detector, and a first directional coupler operably coupled between the first laser source and the first end of one optical fiber of an optical fiber pair. The first directional coupler directs optical pulses generated by the first laser source over the one optical fiber and directs scatter that propagates back along the one optical fiber to the first optical detector. The first signal processing circuitry analyzes the backscatter signal received via the one optical fiber in order to detect an intrusion of the premises. The OTDR means also includes a second laser source for generating optical pulses, a second optical detector, and a second directional coupler operably coupled between the second laser source and the second end of the other optical fiber of the optical fiber pair. The second directional coupler directs the optical pulses generated by the second laser source over the other optical fiber and directs scatter that propagates back along the other optical fiber to the second optical detector. The second signal processing circuitry analyzes the backscatter signal received via the other optical fiber in order to detect an intrusion of the premises. 
         [0015]    According to yet another embodiment of the invention, the OTDR means includes a first laser source for generating optical pulses at a first wavelength, a first optical detector, and a first directional coupler operably coupled between the first laser source and the first end of an optical fiber. The first directional coupler directs the optical pulses generated by the first laser source over the optical fiber and directs scatter that propagates back along the optical fiber to the first optical detector. The first signal processing circuitry analyzes the backscatter signal at the first wavelength received via the first end of the optical fiber in order to detect an intrusion of the premises. The OTDR means also includes a second laser source for generating optical pulses at a second wavelength different than the first wavelength, a second optical detector, and a second directional coupler operably coupled between the second laser source and the second end of the optical fiber. The second directional coupler directs the optical pulses generated by the second laser source over the optical fiber and directs scatter that propagates back along the optical fiber to the second optical detector. The second signal processing circuitry analyzes the backscatter signal at the second wavelength received via the second end of the optical fiber in order to detect an intrusion of the premises. 
         [0016]    Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic illustration of a fiber optic intrusion detection system in accordance with a first embodiment of the present invention. 
           [0018]      FIG. 2  is a functional block diagram of exemplary signal processing functionality and control functionality carried out by the fiber optic intrusion detection system of  FIG. 1 . 
           [0019]      FIG. 3  is a schematic illustration of a fiber optic intrusion detection system in accordance with a second embodiment of the present invention. 
           [0020]      FIG. 4  is a functional block diagram of exemplary signal processing functionality and control functionality carried out by the fiber optic intrusion detection system of  FIG. 3 . 
           [0021]      FIG. 5  is a schematic illustration of a fiber optic intrusion detection system in accordance with a third embodiment of the present invention. 
           [0022]      FIG. 6  is a functional block diagram of exemplary signal processing functionality and control functionality carried out by the fiber optic intrusion detection system of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Turning now to  FIG. 1 , an intrusion detection system  10  in accordance with a first embodiment of the present invention includes an optical time domain reflectometer (OTDR) (elements  11 ,  13 ,  15 ,  21 ,  23 ) that injects a series of optical pulses into opposite ends of two optical fibers  17 A,  17 B, and extracts from these same opposite ends light that is scattered back and reflected back from points in the fibers where the index of refraction changes. The backscatter light is measured and stored as a function of time, and analyzed to make an intrusion decision in a fault tolerant manner. 
         [0024]    More particularly, the optical time reflectometer is realized by a pulsed-mode laser source  11  that launches a sequence of highly-coherent light pulses through a directional coupler  13  to an optical switch  15 . The optical switch  15  alternately directs the light pulses generated by the laser source  11  to two optical fibers  17 A,  17 B in a time-division-multiplexed manner. The optical fibers  17 A,  1 B form the sensing element of the system, and are housed in a fiber optic cable  19 , which is deployed about the periphery of the premises  20  that is to be monitored for intrusion detection. This may be along national boundaries, military facilities, chemical plants, airports, rail stations, correctional facilities, a power cable, a tunnel, a pipeline, a building, or other smart structures. For pipelines, the fiber optic cable  19  can be deployed to monitor the pipeline right of way in order to detect construction equipment entering the pipeline right-of-way before it can damage the pipeline. At one end of the fiber optic cable  19 , the fiber optic  17 A is coupled to the optical switch  15  as shown. At the other end of the fiber optic cable  19 , the fiber optic  17 B is coupled to the optical switch  15  as shown. In this configuration, the fiber optic  17 A extends along the periphery of the premises  20  to be monitored in a clockwise direction, and the fiber optic  17 B extends along the periphery of the premises  20  to be monitored in an opposite counter-clockwise direction. As a pulse propagates along either one of the optical fiber  17 A or the optical fiber  17 B, its light is scattered through several mechanisms, including density and composition fluctuations (Rayleigh scattering) as well as molecular and bulk vibrations (Raman and Brillouin scattering, respectively). Some of this scattered light is retained within the respective fiber core and is guided back towards the laser source  11 . This returning light passes through the optical switch  15  to the directional coupler  13 , where it is directed to an optical detector  21 . 
         [0025]    The optical detector  21  converts the received backscatter light into an electrical signal and amplifies the electrical signal for output to a signal processing block  23 . The signal output by the optical detector  21  represents a moving-time-window interference pattern for light backscattered from the optical fiber  17 A and the optical fiber  17 B. Such interference patterns represent the interference of the backscattered light from different parts of the optical fibers  17 A and  17 B. If either one (or both) of the optical fibers  17 A,  17 B is subjected to an impinging acoustic wave (or to pressure) which can be caused, for example, by a disturbance from an unauthorized intruder or vehicle, a localized change in the effective refractive index of the respective optical fiber is induced, which causes a change in such interference patterns at a time corresponding to the location of the disturbance. During the time periods that the optical switch  15  connects to the optical fiber  17 A, the signal processing block  23  converts the signal output by the optical detector  21  into digital form and processes such digital data in a time resolved manner to identify changes in the interference pattern therein and make a decision whether an intrusion has occurred based upon such interference pattern changes. Similarly, during the time periods that the optical switch  15  connects to the optical fiber  17 B, the signal processing block  23  converts the signal output by the optical detector  21  into digital form and processes such digital data in a time resolved manner to identify changes in the interference pattern therein and make a decision whether an intrusion has occurred based upon such interference pattern changes. A system controller  25  receives data from the signal processing block  23  over a data path  27  therebetween. Such data provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. 
         [0026]    During normal operations when an intrusion occurs, the system controller  25  will receive over data path  27  data for such intrusion that results from the processing of interference pattern of optical fiber  17 A as well as data for such intrusion that results from the processing of the interference pattern of optical fiber  17 B. The system controller  25  can possibly verify the redundancy of such data and/or generate one or more alarm signals based on such data. Such alarm signals can be output via data path  29  to trigger an audible alarm (such as an audible alert message or tone played over a loudspeaker or bell), a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the intrusion), and/or any other suitable alarm event. 
         [0027]    The signal processing block  23  (and/or the system controller  25 ) can perform data processing operations that analyze the backscatter signals from the two optical fibers  17 A,  17 B to automatically detect that a break has occurred in one or both of the optical fibers  17 A,  17 B and identify the location of the break. The system controller  25  can generate one or more alarm signals in the event that a break is detected. Such alarm signals can be output via data path  29  to trigger an audible alarm, a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the break), and/or any other suitable alarm event representing the break. Such alarm signals will be derived from the signal processing operations of the backscatter signals that return from each respective optical fiber ( 17 A or  17 B) along its length between the break point and the optical switch  13 . 
         [0028]    The system controller  25  also generates the appropriate timing signals to synchronize the time-division-multiplexed operations of the light source  11 , the optical switch  15  and the signal processor block  23 , which are supplied thereto over control paths  31 A,  31 B and  31 C, respectively. 
         [0029]      FIG. 2  shows an illustrative embodiment of the signal processing block  23  and system controller  25 . The signal processing block  23  includes an analog-to-digital converter section  51  that interfaces to the output of the optical detector  21 . The analog-to-digital converter section  51  samples the electrical signal output from the optical detector  21  at designated sampling rate and converts the samples into digital words, which represent the detected backscatter signals in digital form. Logic  53 A and  53 B stores the digital words generated by the converter section  51  in time bins corresponding to different sections of the two optical fibers  17 A,  17 B in a time-division multiplexed manner. The timing for such storage operations is derived from control signals generated by a timing signal generator block  71  of the system controller  25  and supplied thereto over control path  31 C. The time bins, which are labeled  55   A1 ,  55   A2 , . . .  55   AN  for the optical fiber  17 A and  57   B1 ,  57   B2 , . . .  57   BN  for the optical fiber  17 B, correspond to different lengths of the two optical fibers  17 A,  17 B, respectively. Logic blocks  59   A1 ,  59 A 2 , . . .  59   AN  operate on the backscatter signal data stored in the corresponding time bins  55   A1 ,  55 A 2 , . . .  55   AN  to analyze the interference pattern in each respective time bin over time. Similarly, logic blocks  61   B1 ,  61   B2 , . . .  61   BN  operate on the backscatter signal data stored in the corresponding time bins  57   B1 ,  57   B2 , . . .  57   BN  to analyze the interference pattern in each respective time bin over time. A change in the interference pattern in a time bin indicates some traffic across the perimeter being monitored at the location corresponding to that time bin. In the preferred embodiment, the logic blocks  59   A1 ,  59 A 2 , . . .  59   AN  and the logic blocks  61   B1 ,  61   B2 , . . .  61   BN  analyze the difference between the interference pattern in the corresponding time bin and a steady-state interference pattern for the corresponding time bin. Such differences operations can be based on convolution operations, phase difference operations, FFT operations, filtering operations and/or other operations typically used in optical time-domain reflectometry. Block  63  uses the interference pattern analysis of logic blocks  59   A1 ,  59 A 2 , . . .  59   AN  to make an intrusion decision, which is a decision whether or not an intrusion as occurred. Similarly, block  65  uses the interference pattern analysis of logic blocks  61   B1 ,  61   B2 , . . .  61   BN  to make an intrusion decision. The logic of blocks  63  and  65  may utilize signature analysis to identify the type of intruder, i.e., to distinguish between humans, vehicles, and animals. When either of block  63  or block  65  determine that an intrusion has occurred, data is provided to the system controller  25  over data path  27 . The data provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. 
         [0030]    The system controller  25  receives such data over data path  27  and includes logic block  73  that can possibly verify the redundancy of such data and/or generate one or more alarm signals based upon such data. Such alarm signals can be output via data path  29  to trigger an audible alarm (such as an audible alert message or tone played over a loudspeaker or bell), a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the intrusion), and/or any other suitable alarm event. 
         [0031]    The signal processing block  23  (as part of blocks  59 ,  61 ,  63 ,  65 ) and/or system controller  25  (as part of logic block  73 ) can perform data processing operations that analyze the backscatter signals from the two optical fibers  17 A,  17 B to automatically detect that a break has occurred in one or both of the optical fibers  17 A,  17 B and identify the location of the break. The system controller  25  (as part of logic block  25 ) can generate one or more alarm signals in the event that break is detected. Such alarm signals can be output via data path  29  to trigger an audible alarm, a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the break), and/or any other suitable alarm event representing the break. Such alarm signals will be derived from the signal processing operations of the backscatter signals that return from each respective optical fiber ( 17 A or  17 B) along its length that extends from the break point to the optical switch  13 . 
         [0032]    The system controller  25  also includes timing signal generator block  71  that generates the appropriate timing signals to synchronize the time-division-multiplexed operations of the light source  11 , the optical switch  15  and the signal processor block  23 , which are supplied thereto over control paths  31 A,  31 B and  31 C, respectively. 
         [0033]    Turning now to  FIG. 3 , an intrusion detection system  10 ′ in accordance with a second embodiment of the present invention includes an optical time domain reflectometer (OTDR) (elements  11 A′,  13 A′,  21 A′,  23 A′,  11 B′,  13 B′,  21 B′,  23 B′) that injects a series of optical pulses into opposite ends of two optical fibers  17 A′,  17 B′, and extracts from these same opposite ends light that is scattered back and reflected back from points in the fibers where the index of refraction changes. The backscatter light is measured and stored as a function of time, and analyzed to make an intrusion decision in a fault tolerant manner. 
         [0034]    More particularly, the optical time reflectometer is realized by a first pulsed-mode laser source  11 A′ that launches a sequence of highly-coherent light pulses through a first directional coupler  13 A′ to an optical fiber  17 A′. A second pulsed-mode laser source  11 B′ launches a sequence of light pulses through a second directional coupler  13 B′ to an optical fiber  17 B′. The optical fibers  17 A′,  17 B′ form the sensing element of the system, and are housed in a fiber optic cable  19 ′, which is deployed about the periphery of the premises  20 ′ that it to be monitored for intrusion detection. This may be along national boundaries, military facilities, chemical plants, airports, rail stations, correctional facilities, a power cable, a tunnel, a pipeline, a building, or other smart structures. For pipelines, the fiber optic cable  19 ′ can be deployed to monitor the pipeline right of way in order to detect construction equipment entering the pipeline right-of-way before it can damage the pipeline. At one end of the fiber optic cable  19 ′, the optical fiber  17 A′ is coupled to the first directional coupler  13 A′. At the other end of the fiber optic cable  19 ′, the optical fiber  17 B′ is coupled to the second directional coupler  13 B′ as shown. In this configuration, the fiber optic  17 A′ extends along the periphery of the premises  20 ′ to be monitored in one direction (from left to right), and the fiber optic  17 B extends along the periphery of the premises  20  to be monitored in an opposite direction (from right to left). As a pulse propagates along either one of the optical fiber  17 A′ or the optical fiber  17 B′, its light is scattered through several mechanisms, including density and composition fluctuations (Rayleigh scattering) as well as molecular and bulk vibrations (Raman and Brillouin scattering, respectively). Some of this scattered light is retained within the respective fiber core and is guided back towards the respective laser sources  11 A′,  11 B′. This returning light passes through the respective directional couplers  13 A′,  13 B′, where it is directed to corresponding optical detectors  21 A′,  21 B′. 
         [0035]    The optical detectors  21 A′,  21 B′ each convert the received backscatter light into an electrical signal and amplifies the electrical signal for output to corresponding signal processing blocks  23 A′,  23 B′. The signal output by the optical detectors  21 A′,  21 B′ represents a moving-time-window interference pattern for light backscattered from the optical fiber  17 A′ and the optical fiber  17 B′, respectively. Such interference patterns represent the interference of the backscattered light from different parts of the optical fibers  17 A′ and  17 B′. If either one (or both) of the optical fibers  17 A′,  17 B′ is subjected to an impinging acoustic wave (or to pressure) which can be caused, for example, by a disturbance from an unauthorized intruder or vehicle, a localized change in the effective refractive index of the respective optical fiber is induced, which causes a change in such interference patterns at a time corresponding to the location of the disturbance. The signal processing block  23 A′ converts the signal output by the optical detector  21 A′ into digital form and processes such digital data in a time resolved manner to identify changes in the interference pattern therein and make a decision whether an intrusion has occurred based upon such interference pattern changes. Similarly, the signal processing block  23 B′ converts the signal output by the optical detector  21 B′ into digital form and processes such digital data in a time resolved manner to identify changes in the interference pattern therein and make a decision whether an intrusion has occurred based upon such interference pattern changes. 
         [0036]    System controller  25 B′ receives data from the signal processing block  23 B′ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. System controller  25 A′ receives data from the signal processing block  23 A′ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. System controller  25 A′ communicates such data to the system controller  25 B′ over a communication link therebetween, which can be a wired or wireless communication link. 
         [0037]    During normal operations when an intrusion occurs, the system controller  25 B′ will receive data from signal processing block  23 A′ that results from the processing of interference pattern of optical fiber  17 A′ as well as data from the signal processing block  23 B′ that results from the processing of the interference pattern of optical fiber  17 B′. The system controller  25 B′ can possibly verify the redundancy of such data and/or generate one or more alarm signals based on such data. Such alarm signals can be output to trigger an audible alarm (such as an audible alert message or tone played over a loudspeaker or bell), a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the intrusion), and/or any other suitable alarm event. 
         [0038]    The signal processing blocks  23 A′,  23 B′ (and/or the system controller  25 B′) can perform data processing operations that analyze the backscatter signals from the two optical fibers  17 A′,  17 B′ to automatically detect that a break has occurred in one or both of the optical fibers  17 A′,  17 B′ and identify the location of the break. The system controller  25 B′ can generate one or more alarm signals in the event that a break is detected. Such alarm signals can be output to trigger an audible alarm, a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the break), and/or any other suitable alarm event representing the break. Such alarm signals will be derived from the signal processing operations of the backscatter signals that return from each respective optical fiber ( 17 A′ or  17 B′) along its length between the break point and the respective directional coupler ( 13 A′ or  13 B′). 
         [0039]      FIG. 4  shows an illustrative embodiment of the signal processing block  23 A′ and system controller  25 A′ as well as the signal processing block  23 B′ and system controller  25 B′. The signal processing block  23 A′ includes an analog-to-digital converter section  51 A′ that interfaces to the output of the optical detector  21 A′. The analog-to-digital converter section  51 A′ samples the electrical signal output from the optical detector  21 A′ at a predetermined sample rate and converts the samples into digital words, which represent the detected backscatter signals in digital form. Logic  53 A′ stores the digital words generated by the converter section  51 A′ in time bins corresponding to different sections of the first optical fiber  17 A′. The time bins, which are labeled  55   A1 ′,  55   A2 ′, . . .  55   AN ′ for the optical fiber  17 A′, correspond to different lengths of the first optical fiber  17 A′. Logic blocks  59   A1 ′,  59 A 2 ′, . . .  59   AN ′ operate on the backscatter signal data stored in the corresponding time bins  55   A1 ′,  55 A 2 ′, . . .  55   AN ′ to analyze the interference pattern in each respective time bin over time. A change in the interference pattern in a time bin indicate some traffic across the perimeter being monitored at the location corresponding to that time bin. In the preferred embodiment, the logic blocks  59   A1 ′,  59 A 2 ′, . . .  59   AN ′ analyze the difference between the interference pattern in the corresponding time bin and a steady-state interference pattern for the corresponding time bin. Such differences operations can be based on convolution operations, phase difference operations, FFT operations, filtering operations and/or other operations typically used in optical time-domain reflectometry. Block  63 ′ uses the interference pattern analysis of logic blocks  59   A1 ′,  59 A 2 ′, . . .  59   AN ′ to make an intrusion decision, which is a decision whether or not an intrusion as occurred. 
         [0040]    Similarly, the signal processing block  23 B′ includes an analog-to-digital converter section  51 B′ that interfaces to the output of the optical detector  21 B′. The analog-to-digital converter section  51 B′ samples the electrical signal output from the optical detector  21 B′ at a predetermined sample rate and converts the samples into digital words, which represent the detected backscatter signals in digital form. Logic  53 B′ stores the digital words generated by the converter section  51 B′ in time bins corresponding to different sections of the first optical fiber  17 B′. The time bins, which are labeled  57   B1 ′,  57   B2 ′, . . .  57   BN ′ for the optical fiber  17 B′, correspond to different lengths of the second optical fiber  17 B′. Logic blocks  61   B1 ′,  61   B2 ′, . . .  61   BN ′ operate on the backscatter signal data stored in the corresponding time bins  57   B1 ′,  57   B2 ′, . . .  57   BN ′ to analyze the interference pattern in each respective time bin over time. In the preferred embodiment, the logic blocks  61   B1 ′,  61   B2 ′, . . .  61   BN ′ analyze the difference between the interference pattern in the corresponding time bin and a steady-state interference pattern for the corresponding time bin. Such differences operations can be based on convolution operations, phase difference operations, FFT operations, filtering operations and/or other operations typically used in optical time-domain reflectometry. Block  65 ′ uses the interference pattern analysis of logic blocks  61   B1 ′,  61   B2 ′, . . .  61   BN ′ to make an intrusion decision. The logic of blocks  63 ′ and  65 ′ may utilize signature analysis to identify the type of intruder, i.e., to distinguish between humans, vehicles, and animals. 
         [0041]    When block  63 ′ detects an intrusion, data is provided to the system controller  25 A′ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. System controller  25 A′ communicates such data to the system controller  25 B′ over a communication link therebetween supported by communication interfaces  66 A′ and  66 B′. Similarly, when block  65 ′ detects an intrusion, data is provided to the system controller  25 B′ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. 
         [0042]    The system controller  25 B′ receives such data and includes logic block  73 ′ that can possibly verify the redundancy of such data and/or generate one or more alarm signals based upon such data. Such alarm signals can be output to trigger an audible alarm (such as an audible alert message or tone played over a loudspeaker or bell), a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the intrusion), and/or any other suitable alarm event. 
         [0043]    The signal processing blocks  23 A′,  23 B′ (as part of blocks  59 ′,  61 ′,  63 ′,  65 ′) and/or the system controller  25 B′ (as part of logic block  73 ′) can perform data processing operations that analyze the backscatter signals from the two optical fibers  17 A′,  17 B′ to automatically detect that a break has occurred in one or both of the optical fibers  17 A′,  17 B′ and identify the location of the break. The system controller  25 B′ (as part of block  73 ′) can generate one or more alarm signals in the event that a break is detected. Such alarm signals can be output to trigger an audible alarm, a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the break), and/or any other suitable alarm event representing the break. Such alarm signals will be derived from the signal processing operations of the backscatter signals returned from each respective optical fiber ( 17 A′ or  17 B′) along its length between the break point and the respective directional coupler ( 17 A′ or  17 B′). 
         [0044]    The system controllers  25 A′ and  25 B′ also include respective timing signal generator blocks  71 A′ and  71 B′ that generate the appropriate timing signals to drive the pulsed-mode light sources  11 A′,  11 B′, respectively. 
         [0045]    Turning now to  FIG. 5 , an intrusion detection system  10 ″ in accordance with a third embodiment of the present invention includes an optical time domain reflectometer (OTDR) (elements  11 A″,  13 A″,  21 A″,  23 A″,  11 B″,  13 B″,  21 B″,  23 B″) that injects a series of optical pulses at different wavelengths into opposite ends of an optical fiber  17 ″, and extracts from these same opposite ends light that is scattered back and reflected back from points in the fibers where the index of refraction changes. The backscatter light for the two wavelengths is measured and stored as a function of time, and analyzed to make an intrusion decision in a fault tolerant manner. 
         [0046]    More particularly, the optical time reflectometer is realized by a first pulsed-mode laser source  11 A″ that launches a sequence of highly-coherent light pulses through a first directional coupler  13 A″ to the optical fiber  17 ″. A second pulsed-mode laser source  11 B″ launches a sequence of light pulses through a second directional coupler  13 B″ to the same optical fiber  17 ″. The laser source  11 A″ operates at a first wavelength (λ A ), while the laser source  11 B″ operates at a second wavelength (λ B ) different than the first wavelength (λ A ). The optical fiber  17 ″ forms the sensing element of the system, and is housed in a fiber optic cable  19 ″, which is deployed about the periphery of the premises  20 ″ that it to be monitored for intrusion detection. This may be along national boundaries, military facilities, chemical plants, airports, rail stations, correctional facilities, a power cable, a tunnel, a pipeline, a building, or other smart structures. For pipelines, the fiber optic cable  19 ″ can be deployed to monitor the pipeline right of way in order to detect construction equipment entering the pipeline right-of-way before it can damage the pipeline. At one end of the fiber optic cable  19 ″, the optical fiber  17 ″ is coupled to the first directional coupler  13 A″. At the other end of the fiber optic cable  19 ″, the optical fiber  17 ″ is coupled to the second directional coupler  13 B″ as shown. As a pulse propagates along the optical fiber  17 ″, its light is scattered through several mechanisms, including density and composition fluctuations (Rayleigh scattering) as well as molecular and bulk vibrations (Raman and Brillouin scattering, respectively). Some of this scattered light is retained within the respective fiber core and is guided back towards the respective laser sources  11 A″,  11 B″. This returning light passes through the respective directional couplers  13 A″,  13 B″, where it is directed to corresponding optical detectors  21 A″,  21 B″. 
         [0047]    The optical detectors  21 A″,  21 B″ each convert the received backscatter light into an electrical signal and amplifies the electrical signal for output to corresponding signal processing blocks  23 A″,  23 B″. The signal output by the optical detectors  21 A″ represents a moving-time-window interference pattern for light backscattered from the optical fiber  17 ″ for the first wavelength (λ A ). The signal output by the optical detectors  21 B″ represents a moving-time-window interference pattern for light backscattered from the optical fiber  17 ″ for the second wavelength (λ B ). Such interference patterns represent the interference of the backscattered light from different parts of the optical fiber  17 A″. If the optical fiber  17 ″ is subjected to an impinging acoustic wave (or to pressure) which can be caused, for example, by a disturbance from an unauthorized intruder or vehicle, a localized change in the effective refractive index of the respective optical fiber is induced, which causes a change in such interference patterns at a time corresponding to the location of the disturbance. The signal processing block  23 A″ converts the signal output by the optical detector  21 A″ into digital form and processes such digital data in a time resolved manner to identify changes in the interference pattern for the first wavelength (λ A ) and make a decision whether an intrusion has occurred based upon such interference pattern changes. Similarly, the signal processing block  23 B″ converts the signal output by the optical detector  21 B″ into digital form and processes such digital data in a time resolved manner to identify changes in the interference pattern for the second wavelength (λ B ) and make a decision whether an intrusion has occurred based upon such interference pattern changes. 
         [0048]    System controller  25 B″ receives data from the signal processing block  23 B″ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. System controller  25 A″ receives data from the signal processing block  23 A″ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. System controller  25 A″ communicates such data to the system controller  25 B″ over a communication link therebetween, which can be a wired or wireless communication link. 
         [0049]    During normal operations when an intrusion occurs, the system controller  25 B″ will receive data from signal processing block  23 A″ that results from the processing of the interference pattern for the wavelength λ A  as well as data from the signal processing block  23 B″ that results from the processing of the interference pattern for the wavelength λ B . The system controller  25 B″ can possibly verify the redundancy of such data and/or generate one or more alarm signals based on such data. Such alarm signals can be output to trigger an audible alarm (such as an audible alert message or tone played over a loudspeaker or bell), a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the intrusion), and/or any other suitable alarm event. 
         [0050]    The signal processing blocks  23 A″,  23 B″ (and/or the system controller  25 B″) can perform data processing operations that analyze the backscatter signals for the two wavelengths to automatically detect that a break has occurred in the optical fiber  17 ″ and identify the location of the break. The system controller  25 B″ can generate one or more alarm signals in the event that a break is detected. Such alarm signals can be output to trigger an audible alarm, a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the break), and/or any other suitable alarm event representing the break. Such alarm signals will be derived from the signal processing operations of the backscatter signals returned from the optical fiber  17 ″ along its length between the break point and the respective directional coupler ( 13 A″ or  13 B″). 
         [0051]      FIG. 6  shows an illustrative embodiment of the signal processing block  23 A″ and system controller  25 A″ as well as the signal processing block  23 B″ and system controller  25 B″. The signal processing block  23 A″ includes an analog-to-digital converter section  51 A″ that interfaces to the output of the optical detector  21 A″. The analog-to-digital converter section  51 A″ samples the electrical signal output from the optical detector  21 A″ at a predetermined sample rate and converts the samples into digital words, which represent the detected backscatter signals for the first wavelength λ A  in digital form. Logic  53 A″ stores the digital words generated by the converter section  51 A″ in time bins corresponding to different sections of the optical fiber  17 ″. The time bins, which are labeled  55   A1 ″,  55   A2 ″, . . .  55   AN ″ correspond to different lengths of the optical fiber  17 ″ for the first wavelength λ A . Logic blocks  59   A1 ″,  59 A 2 ″, . . .  59   AN ″ operate on the backscatter signal data stored in the corresponding time bins  55   A1 ″,  55 A 2 ″, . . .  55   AN ″ to analyze the interference pattern in each respective time bin over time. A change in the interference pattern in a time bin indicate some traffic across the perimeter being monitored at the location corresponding to that time bin. In the preferred embodiment, the logic blocks  55   A1 ″,  55   A2 ″, . . .  55   AN ″ analyze the difference between the interference pattern in the corresponding time bin and a steady-state interference pattern for the corresponding time bin. Such differences operations can be based on convolution operations, phase difference operations, FFT operations, filtering operations and/or other operations typically used in optical time-domain reflectometry. Block  63 ″ uses the interference pattern analysis of logic blocks  59   A1 ″,  59 A 2 ″, . . .  59   AN ″ to make an intrusion decision, which is a decision whether or not an intrusion as occurred. 
         [0052]    Similarly, the signal processing block  23 B″ includes an analog-to-digital converter section  51 B″ that interfaces to the output of the optical detector  21 B″. The analog-to-digital converter section  51 B″ samples the electrical signal output from the optical detector  21 B′ at a predetermined sample rate and converts the samples into digital words, which represent the detected backscatter signals for the second wavelength λ B  in digital form. Logic  53 B″ stores the digital words generated by the converter section  51 B″ in time bins corresponding to different sections of the optical fiber  17 ″. The time bins, which are labeled  57   B1 ′,  57   B2 ′, . . .  57   BN ′ correspond to different lengths of the optical fiber  17 ″ for the second wavelength λ B . Logic blocks  61   B1 ″,  61   B2   ″ , . . .  61   BN ″ operate on the backscatter signal data stored in the corresponding time bins  57   B1 ″,  57   B2 ″, . . .  57   BN ″ to analyze the interference pattern in each respective time bin over time. In the preferred embodiment, the logic blocks  61   B1 ″,  61   B2 ″, . . .  61   BN ″ analyze the difference between the interference pattern in the corresponding time bin and a steady-state interference pattern for the corresponding time bin. Such differences operations can be based on convolution operations, phase difference operations, FFT operations, filtering operation and/or other operations typically used in optical time-domain reflectometry. Block  65 ″ uses the interference pattern analysis of logic blocks  61   B1 ″,  61   B2 ″, . . .  61   BN ″ to make an intrusion decision. The logic of blocks  63 ″ and  65 ″ may utilize signature analysis to identify the type of intruder, i.e., to distinguish between humans, vehicles, and animals. 
         [0053]    When block  63 ″ detects an intrusion, data is provided to the system controller  25 A″ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. System controller  25 A″ communicates such data to the system controller  25 B″ over a communication link therebetween supported by communication interfaces  66 A″ and  66 B″. Similarly, when block  65 ″ detects an intrusion, data is provided to the system controller  25 B″ which provides an indication that an intrusion has occurred, a location of such intrusion, and a preferably time stamp corresponding to the time of the intrusion. The system controller  25 B″ receives such data and includes logic block  73 ″ that can possibly verify the redundancy of such data and/or generate one or more alarm signals based on such data. Such alarm signals can be output to trigger an audible alarm (such as an audible alert message or tone played over a loudspeaker or bell), a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the intrusion), and/or any other suitable alarm event. 
         [0054]    The signal processing block  23 A″,  23 B″ (as part of blocks  59 ″,  61 ″,  63 ″,  65 ″) and/or the system controller  25 B″ (as part of logic block  73 ″) can perform data processing operations that analyze the backscatter signals for the two wavelengths to automatically detect that a break has occurred in the optical fiber  17 ″ and identify the location of the break. The system controller  25 B″ can generate one or more alarm signals in the event that a break is detected. Such alarm signals can be output to trigger an audible alarm, a visual alarm (such as an update to a display terminal that provides a visual alarm message and possibly a visual indication of the location of the break), and/or any other suitable alarm event representing the break. Such alarm signals will be derived from the signal processing operations of the backscatter signals returned from the optical fiber  17 ″ along its length between the break point and the respective directional coupler ( 13 A″ or  13 B″). 
         [0055]    The system controllers  25 A″ and  25 B″ also include respective timing signal generator blocks  71 A″ and  71 B″ that generate the appropriate timing signals to drive the pulsed-mode light sources  11 A″,  11 B″, respectively. 
         [0056]    Advantageously, the fiber-optic based intrusion detection systems described herein provide continued operation in the event that a break occurs in the sensing optical fiber of the system. Such systems also report the position of such a break. Moreover, the fiber-optic based intrusion detection systems described herein can be used for a wide variety of applications, such as monitoring national boundaries, military facilities, chemical plants, airports, rail stations, correctional facilities, a power cable, a tunnel, a pipeline, a building, or other smart structures. 
         [0057]    There have been described and illustrated herein several embodiments of a fault tolerant intrusion detection system employing an OTDR subsystem and methods of operating same. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while particular signal processing functions and methodologies for intrusion detection have been disclosed, it will be appreciated that other signal processing functions and methodologies for intrusion detection as well. In addition, while particular system architectures have been disclosed, it will be understood that other system architectures can be used. For example, the signal processing steps and/or control and alarm notification steps as described herein can be carried out by on a single computer processing platform, or on a distributed computer processing platform as is well known. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.