The present invention relates to a perimeter security system for intrusion detection, using optical fibers having Fiber Bragg Gratings (FBGs).
U.S. Pat. No. 7,385,506 granted to Shibata et al. discloses an optical-fiber-based perimeter security system. An optical fiber having Fiber Bragg Gratings (FBGs) is laid out at the perimeter of an area to be secured so that intrusion stretches the optical fiber and the FBGs in the optical fiber. FBGs reflect narrowband optical power around a center wavelength while transmitting optical power at other wavelengths. If the FBGs in the optical fiber are stretched, the center wavelength of the reflected narrowband optical power is shifted towards longer wavelengths. The magnitude of the wavelength shift is commensurate with the magnitude of the stretch. The wavelength shifts are converted into a time-varying electrical signal. The electrical signal is processed by a pattern recognition device for differentiating between true intrusion on the one hand, and false alarms due to wind and other environmental noise on the other hand. The required pattern recognition device may incur a substantial implementation effort.
The present invention avoids the disadvantage of the prior art.
According to the invention, there is provided a perimeter security system. The system comprises a sensing optical fiber for laying out tautly at the perimeter of an area to be secured. The sensing fiber includes at least one sensing Fiber Bragg Grating (FBG). The system further comprises a source of broadband optical power and means for launching the broadband optical power into the proximal end of the sensing fiber. The distal end of the sensing fiber is optically terminated. The sensing FBG reflects narrowband optical power back to the proximal end of the sensing fiber. When the sensing fiber, and hence the sensing FBG, is stretched, the center wavelength of the reflected narrowband optical power shifts towards longer wavelengths. The system yet further comprises receiving and detecting means responsive to the reflected narrowband optical power with the longer center wavelength. The system is thus responsive to an intrusion which causes a stretch of the sensing fiber.
Advantageously, the sensing fiber has a loose buffer coating for isolating the sensing fiber and the sensing FBG from nuisance disturbances and noise such as vibrations caused by wind.
The drawings illustrate the preferred embodiments by way of example only.
The following descriptions describe the preferred embodiments of the invention by way of example only.
In normal use, the narrowband optical power with the center wavelength λS which is reflected back from the sensing FBG 11 to port 32 exits the circulator 30 at port 33, travels in the reference fiber 15 through the reference FBG 17 to the optical termination 19. No optical power is reflected back to port 33, and hence, no optical power exits port 34 and no optical power is detected by the optical power detector 21.
However, if an intruder stretches the sensing fiber 5, and hence stretches the sensing FBG 11, the center wavelength of the narrowband optical power reflected by the sensing FBG 11 is shifted towards longer wavelengths. If the shifted center wavelength of the reflected narrowband optical power equals the center wavelength λR of the reference FBG 17, the reference FBG 17 reflects the narrowband optical power back to port 33 which then exits port 34 and is detected by the optical power detector 21. Thus, detection of optical power by the optical power detector 21 indicates intrusion causing a stretch of the sensing fiber 5.
Advantageously, the sensing fiber 15 has a loose buffer coating for isolating the sensing fiber 15 and the sensing FBG 11 from nuisance disturbances and noise such as vibrations caused by wind. More advantageously, the loose buffer coating is weather-proof.
Advantageously, the reference fiber 15 including the reference FBG 17 is exposed to the same ambient temperature as the sensing fiber 5 including the sensing FBG11 for cancelling out the temperature dependencies of the center wavelengths of reflection λR, λS of the reference FBG 17 and the sensing FBG 11, respectively.
Advantageously, the sensing fiber 5 includes multiple sensing FBGs 11 spaced apart along the length of the sensing fiber 5 to increase sensitivity to intrusion causing a stretch of the sensing fiber 5, which in turn allows for long sensing fibers 5 while maintaining sensitivity to such intrusion.
Advantageously, trip wires 10 such as common fishing lines are attached to the sensing fiber 5 and fixed to the ground with stakes 12. An intruder on foot may trip the wires 10 thereby stretching the sensing fiber 15. The trip wires 10 thus provide enhanced intruder detection.
Advantageously, an enclosure houses the broadband optical source 1, the optical circulator 30, and the optical power detector 21.
In normal use, broadband optical power with wavelengths that are not reflected by the sensing FBG 11 travels to and out of the distal end of the sensing fiber 5, and optical power is detected by the optical power detector 23. However, a cut of the sensing fiber 5 results in no optical power being detected by the optical power detector 23. Intrusion causing a stretch of the sensing fiber 5 is detected by optical power being detected by power detector 21, as in the first embodiment 101.
Advantageously, the sensing fiber 5 is looped back so that the optical power detector 23 can be housed in the same enclosure that houses the broadband optical source 1, the optical circulator 30, and the optical power detector 21.
Note that whereas the embodiment 101 of
In normal use, optical power is thus detected by the optical power detector 230. However, an intrusion causing a cut of the sensing fiber 5 disrupts the path of the narrowband optical power reflected by the fiber cut sensing FBG 25. The optical power detected by the optical power detector 230 is greatly reduced in case of a cut of the sensing fiber 5.
Advantageously, the broadband optical source 1, the optical circulator 30, the optical splitter 27, the optical band-passes 28, 29, and the optical power detectors 210, 230 are housed in an enclosure. Note that in embodiment 103, the distal end of the sensing fiber 5 does not need to be looped back to the enclosure, as is advantageously done in embodiment 102.
Advantageously, the reference fiber 15 including the fiber cut reference FBG 18 is exposed to the same ambient temperature as the sensing fiber 5 including the fiber cut sensing FBG 25 for cancelling out the temperature dependencies of the center wavelengths of reflection of the FBGs 18 and 25.
Note that whereas the embodiment 101 of
In normal use, the center wavelengths of reflection λS1, λS2 of the sensing FBGs 111, 112, respectively, are not shifted. There are thus no reflections from the reference FBGs 171, 172, and hence, no optical power is detected by the optical power detectors 211, 212.
However, if the sensing fiber 5 in ZONE 1 is stretched, optical power is detected by the optical power detector 211. If the sensing fiber 5 in ZONE 2 is stretched, optical power is detected by the optical power detector 212. Intrusions causing stretches of the sensing fiber 5 are thus separately detected according to zone.
Note that fiber cut detection capability can be added to the multi-zone embodiment 104 of
Advantageously, an optical spectrum analyzer may be used in lieu of the 2-way optical-splitter-band-pass-detector-bank 272. An optical power peak displayed at wavelength λR1 detects a stretch of the sensing fiber 5 in ZONE 1. An optical power peak displayed at wavelength λR2 detects a stretch of the sensing fiber 5 in ZONE 2.
Advantageously, the sensing fiber 5 has multiple zones ZONE 1, ZONE 2, . . . ZONE N. Each zone has at least one sensing FBG, FBG 111, FBG 112, . . . FBG 11N with center wavelengths of reflection λS1, λS2, . . . λSN, respectively. Correspondingly, the reference fiber 15 has N reference FBGs, FBG 171, FBG 172, . . . FBG 17N with center wavelengths of reflection λR1, λR2, . . . λRN, respectively. The center wavelengths of reflection of the reference FBGs λR1, λR2, . . . λRN are about 1 nm longer than the center wavelengths of reflection of the sensing FBGs λS1, λS2, . . . λSN, respectively. The narrowband optical powers at wavelengths λR1, λR2, . . . λRN exiting port 34 are separately detected by an N-way optical-splitter-band-pass-detector-bank which has an N-way optical splitter, and N combinations of optical band-pass and optical power detectors.
In normal use, none of the center wavelengths of reflection λS1, λS2, . . . λSN of the sensing FBGs 111, 112, . . . 11N, respectively, of the sensing fiber 5 is shifted, and hence, no optical power is detected in any of the optical power detectors 211, 212, . . . 21N. However, a stretch in a particular zone of the sensing fiber 5 is detected by optical power being detected by the corresponding optical power detector.
Advantageously, an optical spectrum analyzer may be used in lieu of the N-way optical-splitter-band-pass-detector-bank. An optical power peak displayed at wavelength λRX detects a stretch of the sensing fiber 5 in ZONE X, where ZONE X can be any one of the zones of the sensing fiber 5.
Advantageously, any one of the zones of the sensing fiber 5 may have multiple FBGs spaced apart along the length of the zone to increase sensitivity to intrusion causing a stretch of the sensing fiber 5, which in turn allows for long zones while maintaining sensitivity to such intrusion. A sensing fiber 5 with long zones would require fewer zones, and FBGs with fewer center wavelengths of reflection. Such a sensing fiber 5 having FBGs with fewer center wavelengths of reflection is easier to manufacture. Moreover, the fewer center wavelengths of reflection can be spectrally spaced further apart for ease of detection.
Advantageously, the sensing fiber 5 is loosely looped between zones. The fiber loops 50 prevent any stretches in a zone of the sensing fiber 5 from propagating to a neighboring zone. Moreover, sufficient lengths of fiber may be looped in the fiber loops 50 to provide for fiber restoration in case of a cut of the sensing fiber 5.
In normal use, broadband optical power with wavelengths not reflected by the sensing FBGs 111, . . . , 117 are detected by the optical power detector 237 and ‘optical power detected by optical power detector 237 ’ is communicated to the system computer 70. The center wavelengths of reflection of the sensing FBGs 111, . . . , 117 are not shifted, there are no reflections from the reference FBGs 171, . . . , 177, no optical powers are detected by the 7-way optical-bank 277, and ‘no optical powers detected by the 7-way optical-bank 277 ’ is communicated to the system computer 70.
However, if either one of the sensing fibers 5, 8 or both are stretched in an intruded zone, the shifted center wavelength of reflection of the FBG or FBGs in that zone is detected by the 7-way optical-bank 277 and the zone of intrusion is communicated to the system computer 70.
An intrusion causing a cut of either one of the sensing fibers 5, 8 or both is indicated by a significant drop of optical power or no optical power being detected by the optical power detector 237. The cut is communicated to the system computer 70 which toggles the optical bypass switch 60 to the bypass mode, shown as the dashed line in
Advantageously, the optical powers exiting the distal ends of the sensing fibers 5, 8 may be detected with separate optical power detectors for separate detection of which one of the two sensing fibers 5, 8 has been cut.
In normal use, the narrowband optical powers reflected from the fiber cut sensing FBGs 257 travel to and are reflected by the fiber cut reference FBG 187, and travel further to and are detected by an optical spectrum analyzer 80. However, a cut of either one of the sensing fibers 5, 8 or both causes at least a significant drop of the optical power detected by the optical spectrum analyzer 80. The cut is communicated to the system computer 70 which toggles the optical bypass switch 60 to route the narrowband optical powers reflected by the sensing FBGs 111, . . . , 117 to the optical spectrum analyzer 80. Any significant drops of optical power at center wavelengths of reflection of FBGs beyond the cut indicate the zone of intrusion causing the cut.
Similar to the preceding embodiments, in normal use the embodiment 106 shows no narrowband optical power being detected by the optical spectrum analyzer 80 due to the sensing FBGs 111, . . . , 117 and the reference FBGs 171, . . . , 177. However, intrusion in a zone causing a stretch of the sensing FBG or FBGs in that zone shows up at the optical spectrum analyzer 80 as optical power being present at the center wavelength of reflection of the corresponding one of the reference FBGs 171, . . . , 177, thereby locating the zone of intrusion causing the stretch.
A person skilled in the art will have by now appreciated the full scope of the invention. In particular, the scope of the invention is not limited to the preferred embodiments described by way of example in the above.
This application claims the benefit of U.S. Provisional Application No. 61/183,569, filed Jun. 3, 2009
Number | Date | Country | |
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61183569 | Jun 2009 | US |