|Publication number||US20060163457 A1|
|Application number||US 11/311,009|
|Publication date||Jul 27, 2006|
|Filing date||Dec 19, 2005|
|Priority date||Jan 11, 2005|
|Also published as||CA2593628A1, CA2593628C, CN100526925C, CN101142506A, EP1836515A1, EP1836515A4, EP2071374A2, EP2071374A3, US7499177, US20080291462, WO2006074502A1|
|Publication number||11311009, 311009, US 2006/0163457 A1, US 2006/163457 A1, US 20060163457 A1, US 20060163457A1, US 2006163457 A1, US 2006163457A1, US-A1-20060163457, US-A1-2006163457, US2006/0163457A1, US2006/163457A1, US20060163457 A1, US20060163457A1, US2006163457 A1, US2006163457A1|
|Inventors||Jim Katsifolis, Edward Tapanes, Lee McIntosh|
|Original Assignee||Future Fibre Technologies Pty Ltd|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (10), Classifications (12), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to a method and apparatus for locating events, such as intrusions into a secured premises, or breakdown or other events associated with structures, so that the location of the event can be determined.
Apparatus and method for locating events are disclosed in our U.S. Pat. Nos. 6,621,947 and 6,778,717. The contents of these two patents are incorporated into this specification by this reference.
The system used in the above-identified U.S. patents utilises a Mach Zehnder (MZ) interferometer in which counter-propagating signals are provided. By measuring the time difference of perturbed signals caused by an event, the location of the event along the sensing device formed by the Mach Zehnder interferometer can be determined.
Thus, when an event perturbs the MZ sensor portion of the system, the difference in the arrival time of the counter-propagating signals at the detectors can be used to calculate the exact location of the perturbation on the MZ sensor. This type of sensor can be applied to perimeter or infrastructure security applications, with typical sensing lengths exceeding 50 km.
The object of the invention is to improve the system and method disclosed in the above-identified patents so that a more exact location of the event can be provided.
The invention provides an apparatus for detecting and locating disturbances, comprising:
The invention also provides a method for detecting and locating disturbances, comprising:
The invention also provides an improved method for detecting and locating disturbances affecting an optical system including at least one optical waveguide extending along at least one detection zone at which a disturbance can occur, thereby affecting optical signals propagating along counter-propagating optical channels from at least one light source to a detector, wherein at least two beams are separated from the at least one light source and coupled into each of the counter-propagating optical channels, and an effect of the disturbance is detected after the beams have traversed the detection zone and a time difference is determined for calculating a location of the disturbance in the detection zone, wherein the improvement comprises:
The invention still further provides an apparatus for locating the position of an event, comprising:
By matching the amplitude and phase of the counter-propagating signals, output fringes at the detector are produced which are easily detected and therefore the time difference between receipt of the two modified counter-propagating detectors can be accurately recorded to thereby accurately determine the location of the event. This also improves the sensitivity of the system and method.
In the preferred embodiment of the invention, the input polarisation states of the counter-propagating signals are controlled to achieve maximum output fringes. However, in other embodiments, polarisation states which lead to amplitude and phase matched outputs, but with sub-maximum fringe visibilities can also be utilised.
Preferably the control unit comprises the detector means, a polarisation controller for each of the counter-propagating signals and the light source.
Preferably the detector means comprises a first detector for one of the counter-propagating signals and a second detector for the other of the counter-propagating signals.
Preferably the light source comprises a laser light source having bragg gratings and an adjuster for controlling the bragg gratings to thereby alter the wavelength of the light signal output from the laser for producing the counter-propagating signals.
Preferably the control unit includes a processor for receiving outputs from the detectors and for processing the outputs to indicate an event and to determine the location of the event.
In one embodiment the processor is coupled to a polarisation control driver and the polarisation control driver is coupled to the polarisation controllers for controlling the controllers to thereby set the polarisation of the signals supplied from the light source to the first and second arms of the Mach Zehnder interferometer to in turn set the polarisation of the counter-propagating signals.
Preferably the detectors are connected to a Mach Zehnder output monitor for monitoring the counter-propagating signals detected by the detectors so that when the modified counter-propagating detectors are detected by the detector, the MZ output determines detection of those signals by the detectors for processing by the processor.
In one embodiment of the invention the first arm of the Mach Zehnder interferometer is of different length than the second arm of the Mach Zehnder interferometer so that the first and second arms have a length mismatch, the control unit further comprising a dither signal producing element for controlling the light source to wavelength dither the output from the light source to produce a dither in the phase difference between the MZ arms, in turn which produces artificial fringes at the drifting output of the MZ.
Preferably the dither signal element dithers the phase difference between the MZ arms by at least 360°, to produce artificial fringes, so that the drifting output of the Mach Zehnder's operating point always displays its true fringe visibility.
The invention also provides a method of locating an event comprising the steps of:
Preferably the polarisation states of the counter-propagating signals provide amplitude and phase-matched counter-propagating signals from the waveguide which achieve maximum output fringes. However, in other embodiments, the control of the polarisation states may be such that phase matched sub-maximum fringes are provided.
Preferably the step of controlling the polarisation states comprises randomly changing the input polarisation states of the counter-propagating signals whilst monitoring the counter-propagating optical signals output from the Mach Zehnder interferometer to detect a substantially zero state of intensities, or maximum state of intensities of the counter-propagating signals, and selecting the input polarisations which provide the substantially zero or substantially maximum intensities.
In one embodiment fringes for determining the polarisation states are artificially created.
Preferably the artificially created fringes are created by dithering or modulating the wavelength of the light source and providing a path length mismatch between the first and second arms of the Mach Zehnder interferometer.
In one embodiment the step of controlling the polarisation states comprises controlling the polarisation controllers to thereby set the input polarisation state of the signals supplied from the light source to each input of the bidirectional Mach Zehnder interferometer to provide phase matched counter-propagating output signals.
Preferably the wavelength of the laser source is dithered by an amount which leads to the dithering of the phase difference between the MZ arms by 360°, to produce artificial fringes, so that with a drifting operating point, the Mach Zehnder's counterpropagating outputs always display their true fringe visibility.
Preferred embodiments will be described, by way of example, with reference to the accompanying drawings, in which:
With reference to
As shown in
With reference to
The counterpropagating outputs of the bi-directional MZ will also drift and vary in the same fashion. For each direction, there will also be two input polarisation states for which the MZ outputs will achieve a maximum output fringe. Although the choice of either one of these input polarisation states achieves a maximum output fringe, and thus a maximum sensitivity for a conventional MZ, in the case of a bi-directional MZ used to locate events, the choice of input polarisation state for each direction carries an important significance. For this discussion, it will be assumed that only one MZ output for each direction is used (CWout and CCWout).
Given that there are two possible input polarisation states for each direction which achieves maximum fringe visibility, then there are four possible pairings of counter-propagating input polarisation states which will simultaneously achieve maximum output fringe visibilities for both directions.
The bi-directional MZ 10 shown in
For the CW propagation direction there are the two possible input polarisation states which give maximum output fringes—SOP1a and SOP1b. Equally, for the CCW propagation direction, the two possible input polarisation states which give maximum output fringes are SOP2a and SOP2b. These polarisation states can be represented on a Poincare sphere as shown in
Although there are 4 possible pairings which will simultaneously lead to maximum fringes at both counterpropagating outputs CWout and CCWout of the bidirectional MZ (SOP1a and SOP2a, or SOP1a and SOP2b, or SOP1b and SOP2a, or SOP1b and SOP2b,), only two of these pairings will lead to the outputs that have both maximum fringe visibility and are exactly matched in phase.
For the example shown in
This phase and amplitude matching condition is important for the Locator system, as it will allow for the most accurate location of events on the sensing cable to be determined. This means that it is essential that there is no time difference between the counter-propagating drifting MZ output signals when the MZ sensor is in the rest state (no disturbance). If the counter-propagating outputs are not matched in phase, then this will lead to the introduction of an error in the time difference calculation and thus the calculation of the location.
The achievement of counter-propagating, phase-matched maximum fringe outputs leads to two important results with respect to the system. It allows for accurate locating of events, as well as maximum sensitivity of the bi-directional MZ.
However, input polarisation states which lead to amplitude and phase matched counter-propagating outputs are not limited only to the input polarisation states which achieve maximum output fringes. There is also a plurality of other input polarisation state pairs which also lead to amplitude and phase matched outputs, but with sub-maximum fringe visibilities. For example, it is possible to adjust both polarisation controllers 43 and 44 such that the fringe visibility of both outputs is identical and less than the theoretical maximum of 100%, but phase matched. Although a reduction in fringe visibility will lead to a reduction in sensitivity of the bidirectional MZ 10, as long as the fringe visibility is kept relatively high (for example >75%), it is still possible for the system to calculate accurate locations whilst maintaining an acceptable level of sensitivity.
The variation in fringe visibility of the MZ output for each direction in the bi-directional MZ can be plotted on a Poincare sphere to show the relationship between input polarisation states and MZ output fringe visibility. A typical response is shown in
The two unique input polarisation states which lead to a maximum fringe visibility form two opposite ‘poles’ on the sphere, SOPCW1 and SOPCW2 (
The position of opposing maximum fringe visibility poles, and therefore the latitudinal and equatorial belts, will vary according to the birefringence of the bi-directional MZ system, namely the input lead 12 and MZ arms 14 and 15 for the CW direction. This can be thought of as a rotation of the fringe visibility poles and latitudinal belts around the sphere. The minimum fringe visibility is not necessarily always zero, as would be expected in an ideal MZ 10, but can be non-zero. The actual value of the minimum fringe visibility will also vary with the birefringence of the MZ system 10 for that direction. So in summary, a change in birefringence in the MZ system 10, which for the CW direction can include a change in the birefringence in the input lead length 12, and/or MZ sensor arms 14 and 15, can cause not only the fringe visibility poles and latitudinal belts to rotate for each direction, but can also change the range of fringe visibilities possible. Importantly, though, the maximum fringe visibility always approaches unity, irrespective of the birefringence of the MZ system.
Looking at the CCW direction of propagation (shown in
The optical fibre cables that make up the apparatus of the preferred embodiments are actually installed in a variety of environments where they will be subjected to fluctuating and random conditions such as wind, rain, mechanical vibrations, stress and strain, and temperature variations. As mentioned earlier, these effects can vary the birefringence of the optical fibre in the cables, which in turn can change the fringe visibilities of both Locator MZ outputs through the polarisation induced fringe fading (PIFF) effect. So, in a realistic installation, where environmental factors will cause random birefringence changes along the fibres of the Locator system, the fringe visibilities of the respective MZ output intensities can vary randomly with time.
In the apparatus of the preferred embodiments, it is necessary to search and find the input polarisation states for the CW and CCW directions of the bidirectional MZ 10 which correspond to both MZ outputs having the same fringe visibility and being phase matched. One way this could be done is by monitoring the two Locator MZ outputs whilst scrambling the polarisation controllers. A number of scrambling algorithms could be used as long as they achieve the coverage of most of the possible input polarisation states in a relatively short time.
Once these input polarisation states are found, they need to be set to achieve amplitude and phase matched MZ output intensities. To keep the MZ outputs in the amplitude and phase matched condition, it is also necessary to continue adjusting the input polarisation states to compensate for any PIFF that may lead to non-matching counter propagating fringe visibilities, and therefore non-phase matched MZ outputs. This requires knowledge of the actual fringe visibility of the counter-propagating outputs of the bidirectional MZ.
For an apparatus which is using a CW laser as its source, it is not possible to continuously monitor the fringe visibilities of the two MZ outputs, especially in the absence of disturbances. This is because the time taken for the MZ output intensities to go through a full fringe amplitude excursion will vary with time and will be a function of the random phase fluctuations in both arms 14 and 15 of the MZ 10, as well as the PIFF due to the random birefringence changes in the fibres along the length of the bi-directional MZ system.
However, it is possible to determine that a maximum fringe state exists for either of the MZ output intensities if they move through or very close to a zero or maximum intensity level. This is because the zero or maximum level intensities are unique to a maximum fringe visibility. So, for a bidirectional MZ system with polarisation controllers at the inputs of the bi-directional MZ, as shown in
Given that there are 2 possible input polarisation states that simultaneously give a maximum fringe visibility for each direction in the bidirectional MZ, and that only two out of the 4 possible pairings of counter-propagating input polarisation states will give phase matched MZ outputs, it is necessary to check that the chosen two polarisation states yield phase matched MZ outputs. This can be done by simply monitoring the MZ outputs for a predetermined time. If they are not in phase, then polarisation scrambling can be used to find two input polarisation states, and their corresponding maximum fringe outputs, to continue to search for phase matching.
Once the phase matched maximum fringe states are found and set, a tracking algorithm can be used to continue to keep the MZ outputs in a phase matched condition by adjusting the voltage drives to the individual plates of both polarisation controllers accordingly.
This technique will be described in detail with reference to
A more direct technique would involve continuously monitoring the fringe visibilities of the MZ outputs. This requires that fringes are created artificially in the system.
Fringes can be artificially created in the MZ 10 by using a transducer in one of the sensing arms to modulate the phase of the light propagating through the fibre. However, for an event location system where it is preferable that the sensing cables are totally passive, this is not a practical solution.
Another technique for stimulating fringes in a fibre MZ is to modulate or dither the wavelength of the laser source 16. As long as there is a path length mismatch between the MZ arms 14 and 15, then the modulation in optical wavelength (which can also be expressed as an optical frequency) will lead to the creation of fringes. This comes about due to the wavelength dependent phase difference between the MZ arms caused by the path length mismatch. For a Mach Zehnder 10 with a path length mismatch ΔL, the phase difference Δφ between the arms can be expressed by:
where nco is the refractive index of the fibre core, c is the speed of light in a vacuum, and Δv is the laser's optical frequency change. In the case of a bidirectional MZ, as is described in
For a MZ whose operating point is at quadrature, a full fringe excursion can be achieved for a given path length mismatch by modulating the laser source's frequency/wavelength by an amount which results in Δφ=πc. For a typical fibre core index nco=1.46, a path length mismatch ΔL=1 m, and a full fringe Δφ=πc, will give an optical frequency dither of
For a centre wavelength of 1550 nm, this corresponds to a wavelength dither of −0.8 μm.
One of the simplest ways to modulate the wavelength of a standard laser diode is to modulate the drive current to the laser. These types of lasers however do not normally have a high enough coherence to be suitable for the applications discussed herein.
The pumped fibre laser source 16 requires a mechanical modulation of the fibre laser's cavity, or fibre Bragg gratings to achieve wavelength modulation. This can be achieved by using either a temperature tuning approach, or a mechanical piezo tuning approach using a piezo transducer (PZT). Since temperature tuning is very slow, the piezo tuning method is more suited to such a laser in order to achieve the wavelength dithering or modulation. In order to use the dithering of the laser wavelength to continuously monitor the fringe visibility of the MZ, it is necessary to create at least 2 full artificial fringes per cycle of PZT modulation. This requirement is determined by the fact that, as mentioned earlier, the operating point of the MZ drifts in and out of quadrature with time, and the creation of only one full fringe, that is, Δφ=180°, would not be sufficient to continuously show the true fringe visibility. This is illustrated in FIG.
However, in a real MZ 10, the MZ's output operating point drifts in and out of quadrature. This is illustrated in
If, however, the dithering is used to achieve at least 360° of phase modulation at all times, the true fringe visibility of the stimulated fringes can be continuously monitored, irrespective of the drift of the MZ output's operating point. This is illustrated in
If a 360° phase modulation (or more) is used, that is 2 fringes per cycle of modulation are stimulated, this will ensure that the true fringe visibility will always be measurable, irrespective of the drift in the MZ output. This will essentially produce higher harmonics in the stimulated fringes. As the MZ output operating point drifts to the left or the right of quadrature, it will cause higher harmonics of the dither frequency (2nd, 3rd, 4th, etc.) together with the fundamental dither frequency to be present in the stimulated fringes.
To make sure that the stimulated fringes do not interfere with the fringes created by the events which are to be sensed by the apparatus, it is important for the frequency of the stimulated fringes to be in a frequency range well outside that of the event signals detected by the apparatus. For example, in a typical installation, where the frequency range of interest may be 0-20 kHz, the fundamental frequency of the stimulated fringes should be higher, eg. 50 kHz.
As is shown in
To find the input polarisation states which produce matched outputs at the detectors 40 and 50, the polarisation controllers 43 and 44 can be scrambled to randomly change the input polarisation states such that as many different states as possible are covered in as short a time as possible. By continuously monitoring the outputs at the detectors 40 and 50, the polarisation states corresponding to the outputs passing a zero or near zero level, or maximum are stored in the microprocessor 62. When the output reaches a zero or maximum level, the corresponding input polarisation state is considered to be such that it achieves a maximum fringe visibility for that output. When a suitable number of input polarisation states are stored, the scrambling is stopped. The outputs from the arms 14 and 15 and received by the detectors 40 and 50 are then compared for combinations of input polarisation states and the degree of phase matching between the counter-propagating outputs is determined. When the degree of phase matching is above a predetermined acceptable level, the corresponding input polarisation states for which the degree of phase matching was acceptable are set to maintain the phase matched outputs detected by the detectors 40 and 50. If an acceptable degree of phase matching is not reached, the scrambling and comparing procedure described above is repeated again until acceptable degrees of phase matching is achieved.
The polarisation controllers 43 and 44 are driven by PC driver 60 so as to continuously change the polarisation of the signals in the fibres 37 and 38 and therefore provided to the couplers C2 and C3 as the counter-propagating inputs to the arms 14 and 15.
When the required input polarisation states which achieve phase matched maximum fringe visibility at the outputs detected by the detectors 40 and 50 are found, these required input polarisation states are set and the outputs detected by the detectors 40 and 50 are continuously monitored and the micro-processor 62 adjusts the polarisation controllers via the PC driver 63 to maintain the phase matched condition.
The output monitor 60 determines an event by passing the signals detected by the detectors 40 and 50 through a band pass filter having for example a bandwidth of from 1 kHz to 20 kHz (which is the expected frequency of an actual event or perturbation to the apparatus which needs to be detected). The arrival of modified propagating signals within this bandwidth and the time difference between receipt of the counter-propagating signals enables the event to be recognised and also the location of the event to be determined.
Thus, the band pass filtered signals are provided from the monitor 60 to the processor 62 for determining the location of the event.
In order to set the polarisation states, the complete output signal from the detectors 40 and 50 relating to both counter-propagating signals is received at the monitor 60. This is essentially the raw signal from both detectors 40 and 50 and that signal is low pass filtered and used to search for maximum fringes during the polarisation scrambling by detecting zero or maximum intensity levels. When the maximum fringes have been located, the processor 62 also checks for phase alignment. When the desired polarisation states are controlled, these are continuously fed to the PC driver and in turn, the PC driver drives the polarisation controllers 43 and 44 to maintain those polarisation states during use of the system. Monitoring can be performed continuously or intermittently to ensure that the required polarisation states are maintained.
In this embodiment the laser 16 is a diode pumped bragg grating base doped fibre laser. To dither the wavelength of the laser 16, a piezoelectric transducer (not shown) is used, for example, on the internal bragg gratings in the fibre laser to modulate the output wavelength of the laser 16.
To create the artificial fringes, a dither signal which has a frequency above the event frequency of the perturbations which are expected to be provided to the Mach Zehnder 10 and sensed by the Mach Zehnder 10 is applied to the laser 16 from dither signal source 70. This dithers the wavelength of the laser and effectively creates fringes whose frequency consists of the dither frequency and harmonics of the dither frequency (as has been described in detail with reference to FIGS. 10 to 12).
By using the suitable path length mismatch ΔL previously described and adjusting the amplitude of the dithering, continuous fringes are created at the outputs of the Mach Zehnder 10 and which are supplied to the detectors 40 and 50. The outputs which are received by the detectors 40 and 50 will be composed of the dither frequency as well as harmonics of the dither frequency. A fringe visibility monitor 80 is connected to the detectors 40 and 50 for detecting the artificial fringes and determining the fringe visibility for each direction. The frequency range of the artificial fringes is above the event signal frequency range caused by a perturbation. Microprocessor 62 uses a suitable control algorithm, such as a simulated annealing control algorithm, to search and adjust the input polarisation controllers PCcw and PCccw via driver 60 so that the stimulated artificial fringes are at a maximum visibility. The phase matching between the stimulated fringes is also detected by the microprocessor 62 and again, once a suitable input polarisation state from each of the controllers is achieved, that polarisation state is set. A control algorithm is used to adjust the input polarisation controllers PCcw and PCccw to counteract any PIFF, so that the phase matched maximum fringe visibility condition is maintained.
In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise”, or variations such as “comprises” or “comprising”, is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.
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|US7817279||Feb 1, 2007||Oct 19, 2010||British Telecommunications Public Limited Company||Sensing a disturbance|
|US7952720 *||May 31, 2011||Future Fibre Technology Pty Ltd||Apparatus and method for using a counter-propagating signal method for locating events|
|US7961331 *||Feb 1, 2007||Jun 14, 2011||British Telecommunications Public Limited Company||Sensing a disturbance along an optical path|
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|US20130208283 *||Sep 21, 2011||Aug 15, 2013||Fiber Sensys, Inc.||Variable sensitivity interferometer systems|
|US20130222810 *||Nov 2, 2011||Aug 29, 2013||Herve Lefevre||Apolarized interferometric system, and apolarized interferometric measurement method|
|WO2012018272A1||Aug 2, 2010||Feb 9, 2012||Unitelco - Engenharia E Construção Em Telecomunicações, S.A.||Remote monitoring system for network integrity, and respective operation method|
|U.S. Classification||250/227.14, 250/227.17, 250/227.19|
|International Classification||G01J1/42, G01J4/00, G01J1/04|
|Cooperative Classification||G08B13/186, G01M11/39, G02B6/29352, G02B6/2793|
|European Classification||G01M11/39, G08B13/186|
|Mar 30, 2006||AS||Assignment|
Owner name: FUTURE FIBRE TECHNOLOGIES PTY LTD, AUSTRALIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:KATSIFOLIS, JIM;TAPANES, EDWARD E.;MCINTOSH, LEE J.;REEL/FRAME:017393/0087;SIGNING DATES FROM 20051218 TO 20051219