CA2780396A1 - Fiber optic interferometric perimeter security apparatus and method - Google Patents

Abstract

A fiber optic interferometric perimeter security apparatus is provided for detection and location of a disturbance on a sensor cable caused by an intruder attempt to climb over, or cut through, a perimeter fence-mounted sensor cable or walk over a hidden perimeter defined by a buried sensor cable. A single modulated laser source and optical time delay elements which have been strategically located in the optical paths of optical sensors comprising two back-to-back Michelson interferometer and a sensor cable are used to generate pseudo-IF responses from each Michelson interferometer. The interferometer responses are used to isolate desired in-phase and quadrature-phase responses while suppressing undesired inherent interferometer responses. The in-phase and quadrature-phase responses each comprise a distortion signal wherein the distortion may be represented by an inherent range cosine filter (RCF) which is a function of the range (i.e. distance) along the cable of the disturbance.
Digital signal processing is applied to the isolated in-phase and quadrature-phase response signals and to corresponding inferential RCFs to form a bridge, and iterative bridge measurements are performed using different range bins (i.e. distances to partitions dividing the length of the cable) for the inferential bridges. The location of the disturbance over a brief window in time is determined to be in the range bin which balances the bridge measurement. If desired, the location of the disturbance within that range bin may be determined by further processing using a tangent function and interpolation.

Description

FIBER OPTIC INTERFEROMETRIC PERIMETER
SECURITY APPARATUS AND METHOD
s FIELD OF THE INVENTION
The invention is in the field of perimeter security for detection and location of intruders as they attempt to climb over, or cut through, a perimeter fence-mounted sensor cable or walk over a hidden perimeter defined by a buried sensor cable. More specifically, the invention relates to a fiber optic interferometric perimeter security apparatus and method.
BACKGROUND
A number of different types of outdoor perimeter security apparatus are known and used to detect and locate intruders in many different applications. These include prisons, VIP residences, military bases, nuclear facilities, chemical sites, petro chemical sites, oil and gas pipelines, critical resource depots, borders, etc. The ability to reliably detect and locate intruders on such perimeters is a critical step in securing such sites from vandals, burglars, terrorists, illegal immigrants, drug traffickers, etc.
To be effective the intrusion detection sensors of such apparatus must provide a timely and reliable alarm annunciation to a response force in order to apprehend the intruder.
The performance of outdoor intrusion detection systems is measured in terms of the Probability of Detection (PD), the Nuisance Alarm Rate (NAR) and the False Alarm Rate (FAR). In many cases the perimeter is defined by a fence such as a chain link fence and a sensor cable of the security apparatus is affixed to the fence to detect a person attempting to climb over or cut through the fence. In other situations such as at VIP
residences, it may be preferred to bury the sensor cable to establish a hidden secured perimeter and provide a covert means of detecting intruders. In recent times sensors have been introduced that both detect and precisely locate on the sensor cable a disturbance caused by an intrusion. The ability to locate the intruder assists in the assessment of the alarm and the dispatch of a response force. Perhaps even more importantly the ability to locate an alarm has been proven to reduce the NAR
and FAR
while preserving the PD.
Fiber optic perimeter security sensors have an advantage over copper based sensors in that they are not sensitive to radio frequency interference (RFI) or electromagnetic interference (EMI) because all electronics components thereof are located indoors and only fiber and passive fiber components are located outdoors on the perimeter to be secured. The relatively low cost of the fiber sensor cable is attractive for use over long perimeters but the relatively high cost of the signal processing equipment required for ranging fiber optic sensors has substantially limited their usage to very long perimeters where the cost per unit length of the sensor is competitive with other technologies.
There are numerous ranging fiber optic outdoor perimeters security sensors that are able to function over very long lengths such as 50 to 100 kilometers (km).
However, the majority of the market for outdoor perimeter security sensors is for use on perimeters less than a few km where the currently available ranging fiber optic sensor products are not cost competitive. An objective of the present invention is to provide a ranging fiber optic sensor that can be commercially realized at a competitive price for shorter perimeters.
Moreover, although the cost per meter of such long sensors may be low, they have a significant vulnerability. If the cable is cut, or the processor fails, a very long length of perimeter security becomes inoperable and unsecured. To avoid such vulnerability, it is preferable to use a series of shorter length security apparatus and this adds to the marketplace need for a ranging fiber optic sensor that is cost effective in short lengths.
Existing ranging fiber optic sensors for outdoor perimeter security applications operate according to several different technologies. For example, some use optical time domain ref lectometer (OTDR) and polarization-optical time domain ref lectometer (POTDR) based sensors that trace their roots back to the work of Dr. Henry Taylor at Texas A&M
as described in US patent 5,194,847. These sensors are divided into two groups;

distributed fiber optic sensors and quasi-distributed fiber optic sensors. The distributed fiber optic sensors typically rely one, or more, of what is commonly referred to in this technical field as Rayleigh, Raman or Brillouin backscatter from the fiber.
The amount of light reflected in these sensors from a disturbance of the sensor cable can be extremely small thereby requiring a lot of signal integration to reliably detect an intruder. The quasi-distributed fiber optic sensors typically use arrays of backscatter devices such as what is commonly referred to as Fiber Bragg Gratings or mechanical mechanisms along the length of the sensor cable to increase the amount of light reflected from a disturbance applied to the sensor cable caused by an intruder. The processing 1.0 electronics associated both of the types of sensors are very expensive.
Hence, they are not cost competitive with copper perimeter security apparatus for lengths of less than 7 km. Also, they have largely been applied to buried line applications where the signature is of low enough frequency that it can be integrated to achieve an adequate signal-to-noise ratio (SNR).
There are a number of different types of fiber optic sensors used in outdoor perimeter security apparatus based on various interferometric technologies such as those commonly referred to as Sagnac, Mach Zehnder and Michelson interferometers which are well known in the optical technologies. Sagnac, Mach Zehnder and Michelson-type interferometers have all been used, or at least proposed for use, in a back-to-back arrangement in which the three different types of interferometers share many features.
On the other hand a Michelson interferometer with its Faraday Rotational Mirror (FRMs) terminations is different in two fundamental aspects. First, the FRMs solve a problem of polarization induced fading associated with the other types of interferometers without need for costly polarization controllers and polarization scramblers. Second, every disturbance applied to the sensor cable, which forms two arms of the interferometer, is seen twice; once as the light propagates to the FRMs and again as the light propagates back from the FRMs.

Perhaps the earliest interferometric sensors are described in GB patent 1,497,995 by Melvin Ramsay filed in April 1976, US patent numbers 4,787,741, 4,898,468, 4,976,507 and US 5,402,231 by Eric Udd of McDonnell Douglas Corporation occurring in the late 1980s and 1995, and US 5,355,208 by Brian Crawford et al of Mason and Hanger National Inc. in October 1994. The distributed fiber optic sensing system of US

5,355,208 is based on Sagnac interferometers (Figure 2 of this patent shows a back-to-back pair of Sagnac interferometers) and locates a disturbance applied to a length of the sensor cable using counter propagating beams and time measurements of the delay caused by propagation from the disturbance to each end of the sensor cable. US
5,402,231 describes a fiber optic sensing system which uses a back-to-back pair of Sagnac interferometers and wavelength division multiplexing (WDM) of optical source signals to separate responses obtained from the two Sagnac interferometers.
The responses from the two Sagnac interferometers are summed together to determine the relative amplitude of the sensed disturbance effect and compared to determine its position on the optical path.
Other known sensors use back-to-back Mach Zehnder interferometers in a similar manner to detect and locate intruders, examples of which are described in US

6,621,947 and US 6,778,717 by Edward Tapanes of Future Fiber Technologies Inc.
which correlate frequency domain responses obtained from two Mach Zehnder sensors.
Further examples are provided by US 7,139,476 and US 7,725,026 by Jayantilal Patel et al of Optellios Inc. which use various measurements of time delay between unwrapped phase response signal obtained from two similar back-to-back Mach Zehnder interferometers to detect and locate intruders. Similarly, US patent application no. 12/438,877 by Patel et al. describes two fiber optic interferometer based sensors for detecting and locating a disturbance by an intruder which are based on many different combinations of Sagnac, Mach Zehnder and Michelson interferometers and use a concept of composite variable signals whereby responses are obtained from two interferometers and combined in specific ways to create composite variable response signals from which the disturbance location is determined by making a time delay measurement between two unwrapped phase responses. However, unlike the other interferometer combinations described in this publication, the composite variable signal responses obtained from the back-to-back Michelson interferometer sensor are not based only on a change in relative phase but, instead, are more complex variables based on a change in relative phase over a round trip time. This presents a disadvantage of the described back-to-back Michelson sensor because, in effect, it high pass filters the responses and complicates the subsequent signal processing required to detect and locate a disturbance. Moreover, it uses two frequencies and WDM
to separate the responses of the two Michelson interferometers, so it disadvantageously requires the use of two laser sources which increases its implementation cost.
In addition, it is subject to a problem of drift between the two laser source frequencies which produces noise in the detection process.
US patent application no. 61/313,433 by Harman et al of Senstar Corporation describes a related Michelson interferometer sensor which uses a compound termination to provide two Michelson interferometer responses of the same orientation. In similar manner to the response signal processing described in US patent application no.
12/438,877, the sensor described in this application measures a time delay between two unwrapped phase responses. Therefore, it too is a function of both the change in relative phase and a round trip time associated with a delay line in the compound termination which, in effect, high pass filters the response and complicates the subsequent signal processing required to detect and locate a disturbance.
Therefore, there is a need for an improved fiber optic interferometer sensor which avoids the relatively high costs associated with multiple laser sources and complicated signal processing.
BRIEF SUMMARY OF INVENTION
The present invention provides perimeter security apparatus which, advantageously, requires only a single modulated laser source to detect and locate disturbances along a distributed fiber optic sensor cable. It utilizes two back-to-back Michelson interferometers with Faraday Rotational Mirror (FRM) terminations each including a time delay element that is strategically located in the optical path to create an optical path difference (OPD) which optimizes the response signals to essentially provide a positive frequency response from one Michelson interferometer, a negative frequency response from the second Michelson interferometer and only a base band response from the undesired Mach Zehnder interferometer that is inherent to the design of the back-to-back Michelson interferometers. The modulation of the laser source is used to extract the complex responses from the two Michelson interferometers while suppressing the undesired Mach Zehnder interferometer response. The FRM terminations in the back-to-back Michelson interferometers avoid any problem of polarization induced fading problem so there is no need for a polarization controller or a polarization scrambler such as there would be for other types of interferometers.
Through providing better performance at a lower cost the ranging stereo Michelson fiber optic sensor of the present invention is able to provide the benefits of fiber optic sensing to shorter length applications on a more competitive basis with traditional perimeter security apparatus using copper.
In accordance with the invention, perimeter security apparatus is provided for use with a laser source providing two identical frequency modulated optical source signals. The apparatus detects a disturbance applied to a fiber optic sensor cable extending along the perimeter. It also determines a range bin along the length of the sensor cable in which the disturbance is located, wherein each range bin corresponds to a distance along the extended sensor cable when partitioned into a predetermined number of range bins with the total number of range bins corresponding to the length of the extended sensor cable. The apparatus includes two fiber optic Michelson interferometric sensors. The first sensor comprises a first input for receiving a first one of the identical modulated optical source signals, first and second fibers, splitter/combiner's, a first optical time delay element in the first fiber and Faraday rotational mirror terminations terminating each fiber. The second sensor comprises a second input for receiving a second one of the identical modulated optical source signals, first and second fibers, splitter/combiners, a second optical time delay element in the second fiber and Faraday rotational mirror terminations terminating each fiber. A
fiber optic sensor cable comprises first and second fibers for connecting at one end to the first and second fibers, respectively, of the first Michelson interferometric sensor and at the other end to the second and first fibers, respectively, of the second Michelson interferometric sensor wherein the first and second sensors and sensor cable are configured to form, when connected, first and second Michelson interferometers in a back-to-back configuration, the first and second fibers of the Michelson sensors and cable being common to both sensors and defining first and second optical paths of the first and second Michelson interferometers, respectively.
The first and second source signals produce first and second optical output signals from the first and second Michelson interferometers, respectively. The first and second optical time delay elements are located in the first and second optical paths, respectively, to create a predetermined optical path difference in each of the first and second optical paths producing a complex optical response signal. The complex optical response signal comprises a positive pseudo-IF first output response signal by the first Michelson interferometer and a negative pseudo-IF second output response signal by the second Michelson interferometer, while suppressing a response of a Mach Zehnder interferometer inherent to the back-to-back configuration of the first and second Michelson interferometers. The disturbance produces in the pseudo-1F output response signals a distortion which is subject to representation by a predetermined first mathematical function (a Range Cosine Function) dependent upon a range along the sensor cable to the disturbance.
First optical detector and converter components detect and convert the first output response signal to an electronic first output digital signal. Second optical detector and converter components detect and convert the second output response signal to an electronic second output digital signal.
One or more digital signal processors process the first and second output digital signals.
The first and second pseudo-IF output response signals are down converted to base band in-phase and quadrature-phase distortion signals for each of the Michelson interferometers. The in-phase and quadrature-phase distortion signals are used to form each half of one half of a bridge and the first predetermined mathematical function is used to produce complex inferential signal components for each half of the other half of the bridge. Iterative bridge measurements are performed, with each successive iteration using the next range bin of the range bins to produce the inferential signal components, until a bridge measurement determines that the bridge is balanced.
The disturbance is located in the range bin which results in the balanced bridge.
The predetermined first mathematical function is applied in the frequency domain using 1.0 complex fast Fourier transforms (FFT). The apparatus may also be applied to locate where, in that range bin the disturbance is located. Iterative bridge measurements are performed for range bins neighboring the range bin used for the balanced bridge and interpolation is applied to those bridge measurements for neighboring range bins using a predetermined second mathematical function (a tangent function).
Advantageously, the present invention requires only a single laser source. It does so by using a modulated laser source with output split between two Michelson-type interferometers with two strategically located optical time delay elements to provide an optical path difference (OPD) between the two optical paths (arms) of the Michelson interferometer. The modulation is used to separate the two Michelson interferometer responses while excluding an undesired, inherent Mach Zehnder signal response component and to derive the complex quadrature-phase and in-phase response components.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram showing an overview of the modules of a stereo Michelson interferometric fiber optic perimeter security apparatus in accordance with the present invention.

FIG. 2 is a schematic diagram provided for reference to describe the basic elements of a Michelson-type interferometer with Faraday Rotational Mirrors as utilized in the present invention.
FIG. 3 is a graph on which the amplitude of the Bessel Functions of the First Kind;
JO(C), J1 (C), and J2(C), are plotted as a function of the argument C along with the desired operating point.
FIG. 4 is a schematic diagram showing the modules of FIG. 1 in greater detail.
FIG. 5 is a diagram showing a conceptual illustration of the primary two Michelson and the undesired Mach Zehnder interferometers which are inherent to the stereo Michelson interferometric fiber optic perimeter apparatus of the present invention.
FIG. 6 is a functional block diagram showing the processing functions performed by the digital signal processing module of FIG. 1.
FIG. 7 is a schematic illustration showing the inputs and outputs of the basic complex fast Fourier transform (FFT) operation of the digital signal processing module of FIG. 6.
FIG. 8 is a schematic illustration showing the bridge measurement of the digital signal processing module of FIG. 6.

DETAILED DESCRIPTION OF INVENTION
The basic modules of a stereo Michelson interferometric fiber optic perimeter security apparatus in accordance with the present invention are shown in FIG. 1. A
processor unit 1 is typically housed indoors. The processor unit 1 includes a laser source 10, a splitter 11 (illustrated as an interferometric splitter/combiner with a terminated/unused port 90), optical detectors 13A and 13B and a digital signal processing module 14.
Lead-in cable 2 connects processor unit 1 to an optical interferometric sensor module 3A. Lead-in cable 2 comprises optical fiber feeder lines 44 and 42' which connect laser source 10 to each of the stereo Michelson interferometers of optical interferometric sensor modules 3A and 3B. A fiber optic sensor cable 4 connects sensor 3A to sensor 3B. Sensor cable 4 comprises fiber optic feeder lines 42 and 43 which send the light source input signal to, and return the output response signal from, the Michelson interferometer of sensor 3B. In addition, sensor cable 4 includes a fiber optic sense line 40 and a fiber optic reference line 41 which provide the common arms of the stereo (alternately referred to herein as back-to-back) Michelson interferometers provided by the sensor modules 3A and 3B. Since the sense and reference lines 40, 41 are shared by the two Michelson interferometers they each see the same disturbance but the stereo signal responses to the disturbance, produced by them, are distorted relative to each other due to the inherent functioning of a Michelson interferometer.
A disturbance 5 caused by an intruder is applied to the sensor cable 4 which is attached to a fence (not illustrated) mounted around the perimeter of the area to be secured (or, alternatively for a different embodiment could be buried below the surface of such perimeter so that a seismic signature is produced in the sensor cable when an intruder walks over it). Lead-in cable 2 is insensitive to disturbances. All active components of the perimeter security apparatus are in processor unit 1 with only passive components outdoors thereby providing a perimeter security apparatus that is immune to EMI and RFI.
A characteristic of a Michelson interferometer is that it sees a disturbance twice. This manifests as a filter effect on the response of the interferometer and enables the response to be modeled or represented mathematically by a predetermined function.
The two Michelson interferometers of sensors 3A and 3B produce a stereo signal response. As demonstrated by the mathematical analysis set out in the following, a disturbance applied to sensor cable 4 produces a complex signal response.
Digital signal processing is used to implement a bridge. One half of the bridge is formed by the two sensor response signals, each component of which may be represented by the inherent RCF function, alternately referred to as the RCF filter and RCF
filter function. The other half of the bridge is formed as inferred (i.e.
theoretical) responses The range associated with each step of the iterative bridge measurement is referred to as a range bin. The range bin in which the bridge output is closest to zero is the range bin in which the disturbance is located and is referred to herein as the disturbance Frequency domain digital signal processing is used for the bridge measurement in the illustrated embodiment. However, it is to be understood that an alternate embodiment of the invention might instead use time domain digital signal processing to implement a bridge type measurement.
During an intrusion attempt the fiber optic sensor cable 4 is disturbed due to an impact to, or motion of, the fence to which it is attached. The two Michelson interferometers of sensors 3A and 3B share the same sense and reference fibers 40, 41 but are of opposite orientations. The single laser source 10 and splitter 11 launch light in one direction along the sensor cable as part of the first Michelson interferometer of sensor 3A and, simultaneously, in the opposite direction as part of the second Michelson interferometer of sensor 3B. The sense and reference fibers 40, 41 are terminated in Faraday rotational mirrors (FRMs) 30A, 31A and 30B, 31B for each of the two Michelson interferometers. The use of FRMs in the terminations of the Michelson interferometers solves the problem of polarization induced fading that affects other interferometers.
The disturbance 5 of the sensor cable 4 caused by an intruder changes the length of the sense fiber 40 relative to the reference fiber 41 at the point of the disturbance 5. This change in relative length causes the two Michelson interferometers to produce a wide band signal frequency response on lead-in cable 2 feeder lines 45 and 43' which are connected to the outputs of the Michelson interferometers. Characteristics of the resulting two wide band signal frequency responses are used to determine that a disturbance 5 caused by an intrusion attempt is in progress and to identify the location along the sensor cable 4 where the disturbance/intrusion attempt is occurring.
The frequency (wavelength) of the laser source 10 that drives the Michelson interferometers of the two sensors 3A, 3B is modulated. Relatively small optical delay lines 34A, 34B are strategically located in the sense and reference lines 40, 41 so that the split modulated laser source signals produce symmetric output signal responses that are rich in harmonics on each of the Michelson interferometers. The output signal responses at the modulation frequency, and at twice the modulation frequency, are used to measure the complex frequency response associated with the disturbance using two complex Fast Fourier Transforms (FFT).
s Even though the two Michelson interferometers respond to the same disturbance the complex frequency response seen by the two Michelson interferometers differ.
The difference is described herein as the Range Cosine Filter (RCF) effect. The location of the disturbance relative to the FRMs of each of the Michelson interferometers produces a unique ROE effect. Digital signal processing is performed on the measured complex frequency responses to effect a bridge measurement of the disturbance seen by each interferometer and from this the location of the disturbance along the length of the sensor cable 4 is determined.
If desired, it is possible to use the same laser source to simultaneously support a second set of stereo fiber optic sensor responses, for purposes of redundancy and improve fault tolerance, whereby a second sensor cable and second set of sensor modules are installed parallel to the first. For such an embodiment, in normal operation the signal responses resulting from the two sets of sensor cables and modules can be integrated together to enhance the overall sensor performance. Further, when used on a fence such dual operation could be used to distinguish between different types of disturbances by determining signal response differences generated by, say, a "cut" and a "climb". In buried applications it could be used to determine the direction and speed of crossing. In any case, if either sensor cable is cut the interferometric sensors would continue to operate but with the performance characteristics of a single sensor cable.
Still further, for other applications such a sensor cable redundancy could be used to divide the security perimeter into two independent lengths in order to better suit the particular site requirement.
Michelson interferometers with Faraday Rotational Mirror (FRM) terminations are used to provide a cost effective means to avoid polarization induced fading. In all other types of interferometers such as Sagnac, Mach Zehnder and Michelson with normal mirror termination polarization induced fading is a severe problem that typically is resolved through the use of relatively costly polarization controllers or polarization scramblers. It can also be solved by use of polarization maintaining fiber in the sensor cable but such a solution would add considerably to the cost of implementation.
The single modulated laser source 10 used to drive the stereo Michelson sensors 3A, 3B advantageously separates the stereo signal responses of the two Michelson interferometers while minimizing the effect of unwanted responses, to effectively suppress or reject those unwanted responses, as detailed more fully in the following.
The modulation provides what is referred to hereinafter as a "pseudo-intermediate frequency (IF)" for each of the complex time domain responses from the stereo Michelson interferometers. The meaning and reason for choosing to use the term "pseudo-IF" here draws upon the readily apparent analogies between this modulation/demodulation structure and those of a conventional heterodyne radio receiver, as explained more fully below. In a demodulation process performed by the digital signal processor 18 the in-phase and quadrature-phase responses from the two Michelson interferometers are retrieved. The optical time delay elements 34A, 34B are symmetrically located in the interferometric sensors 3A, 3B to create symmetrical responses, a positive frequency response in one Michelson interferometer and a negative frequency response in the other. At the same time it suppresses an undesired response that is created from an undesired primary Mach Zehnder interferometer that is inherent to the design of sensors 3A and 3B in addition to the primary stereo Michelson interferometers. As will be appreciated by one skilled in the art, the resulting phasor response obtained from the sensor A and B responses (i.e. the combination of sensor modules 3A, 3B and sensor cable 4) may, to advantage, be represented by alternative complex components to suit particular processing requirements. More specifically, rectangular or polar co-ordinates, in either of the time and frequencies domains, may be used to represent and process the phasor response of the sensor.
The term "pseudo-IF" used herein refers to the higher frequency signal onto which the sensor response signal is superimposed. By way of explanation of the choice of this term, it is arbitrarily chosen for purposes of description and in view of certain analogies of the subject sensor response signal to the IF of a conventional heterodyne receiver which persons skilled in the art will readily recognize. Here, the information of interest, being the response to a disturbance on the sensor cable, is superimposed as response modulation on a higher frequency signal and is retrieved by beating that higher frequency down to base band. For convenience, that higher frequency signal on which the response signal of interest is superimposed is referred to as the "pseudo-IF".
However, unlike a conventional heterodyne receiver, due to the nature of the response modulation process (see the following discussion using Bessel Functions) the quadrature-phase component is at the first harmonic and the in-phase component is at the second harmonic.
Numerous additional multipath interferometers are also inherent to this design but responses from such higher order multipath Mach Zehnder and Michelson interferometers are significantly attenuated by the design and the use of modulation and the multiple passes through the various splitter/combiner devices. As will be readily understood by one who is skilled in the art, residual responses from even order multipath Michelson interferometers produce the same range information as the primary Michelson interferometers and residual responses from odd order multipath Michelson interferometers, that could add noise to the ranging process, are substantially attenuated due the number of splitter/combiners involved and to the very long optical path length differences involved and through a selection of a laser source with a coherence length that is less than the length of the sensor cable 4.
The analog stereo output signal responses 45, 43' from the opposing Michelson interferometers of sensors 3A, 3B are detected 13A,13B, low pass filtered 15A, 15B, converted from analog to digital signals 16A, 16B and then digitally processed by a field programmable logic array (FPGA) 17 which applies two complex Fast Fourier Transform (FFT) routines. Each of the stereo Michelson interferometer frequency domain signal responses includes a data sequence associated with the disturbance 5 that is distorted by an inherent Range Cosine Filter (RCF). This disturbance-associated data sequence is alternately referred to herein as a sound bite of data because it is a 1024 point (or as long as the FFT) sample segment of the signal and is, roughly, in the audio frequency band or high in the illustrated embodiment. The RCF effect provides information as to how the sound bite is distorted in the frequency domain as a function of the range of the disturbance to the FRMs of the interferometer in question.
The RCF
effect increases with the frequency content of the disturbance and with the range to the FRMs. The present invention exploits the differences in the stereo frequency responses of the two Michelson interferometers due to the RCF effect to locate the disturbance 5 along the length of the sensor cable 4. Although it is common to find in some textbooks lo and technical papers a statement that one of the advantages of a Michelson interferometer over a Mach Zehnder interferometer is that it is twice as sensitive. While this is true it overlooks the RCF effect and hence the two times increase in sensitivity is only true for short Michelson sensors or for low frequencies and when the disturbance is in proximity to the FRMS.
Next, the stereo complex frequency domain signal produced by the FPGA 17 are used to form two arms of a measurement bridge used to determine the location of the disturbance 5. Unlike the RCFs which are inherent in the design (i.e. in the arms of each Michelson interferometer), digital signal processing 18 is used to establish the bridge structure and perform the bridge measurement. By digitally creating inferential range cosine filters (RCFs) and range sine filters (RSFs) and using them as two of the arms of a bridge, with the complex signal produced by the FPGA 17 used for the other two arms, the inferentially created arms of the digital side of the bridge can be adjusted to balance the bridge and in doing so locate the disturbance 5 along the length of the sensor cable 4. The adjustments are performed iteratively until balancing is effected.
The use of a Wheatstone Bridge to measure resistance is taught in most basic measurement courses in Electrical Engineering. These courses typically describe the advantage of a bridge measurement as being that the balance point is valid regardless of the supply voltage. This is true of all types of bridges including radio frequency bridges. In the present invention the use of the bridge measurement means that the balance point is not significantly affected by amplitude or phase noise on the laser source. This means that one can use a less costly laser source than might be used in other types of interferometric sensors.
In order to understand the operation of the present invention it is necessary to appreciate the workings of a Michelson interferometer with Faraday Rotational Mirrors and the process of using modulation to generate complex responses (which may be represented in the frequency or time domain, as desired) to a disturbance applied to sense fiber of such Michelson interferometer. In the following a review of these workings is undertaken and an analysis of the modulation process from which several important equations describing the process result.
A basic Michelson interferometer is illustrated in FIG. 2. A laser source 10 sends a frequency modulated light signal to a 2x2 splitter/combiner 35 where it splits (50/50) onto a sense line 40 and a reference line 41. With the exception of an optical path difference (OPD) produced from an optical time delay element 34 the sense line 40 and the reference line 41, forming the arms of the interferometer, are of equal optical path length. The two light signals propagate along the two arms of the interferometer to Faraday rotational mirrors (FRMs) 30 and 31 at the ends of the lines 40, 41.
The light is zo reflected by the mirrors back along the arms of the interferometer returning to the 2x2 splitter/combiner 35 where they are summed together and exit the 2x2 splitter/combiner 35 to arrive at optical detector 13 which measures the intensity of the combined responses.
The interferometric action occurs in the summation process of the 2x2 splitter/combiner 35. When the light signals are "in-phase" they add and when they are "out-of-phase"
they subtract. This means that the intensity of light measured by the detector 13, in effect, measures the relative phase of the light returned on sense line 40 to that returned on reference line 41.

The basic interferometric action just described is common to all interferometers including Mach Zehnder, Sagnac and Michelson Interferometers. A problem of "polarization induced fading" can occur in all such interferometers. The summation process occurring in the splitter/combiner depends on relative phase and relative polarization. Hence for the intensity output of the optical detector to accurately measure the relative phase of the two input signals they must have the same polarization. If the polarization of the two signals happens to be orthogonal then the output will be zero regardless of the phase. A problem is created by the fact that as light propagates along the fiber its polarization changes in a somewhat random manner and these changes vary with time and temperature. The effects of "polarization induced fading"
can be minimized through the use of polarization controllers or polarization scramblers but these add cost and complexity to the interferometer. In addition it is often necessary to briefly interrupt the detection process while the polarization is adjusted.
While perhaps unlikely, this could make the sensor vulnerable to not detecting a disturbance caused by an intruder. The FRMs provide an inexpensive alternative to polarization controllers or polarization scramblers for the Michelson interferometer and they allow the sensor to operate without interruption.
A person skilled in the art will understand how FRMs work and there are numerous papers and articles describing such workings. The end result is that the Faraday rotators in front of the mirrors ensure that the light reflected from the FRM
has a polarization that is 90 degrees from that of the light entering the FRM. This means that the light arriving back at the 2x2 splitter/combiner always has the same polarization as when it left thereby overcoming the effects of "polarization induced fading".
Sense line 40 and reference line 41 are embedded in a sensor cable 4 which is affixed to the fence structure. When the fence fabric is cut by an intruder or when the fence fabric is deformed by an intruder climbing on the fence the sensor cable is flexed and the length of sense line 40 changes relative to the length of reference line 41 at the point of the disturbance thereby generating a signal at the output of optical detector 13.

This is the basic mechanism whereby a Michelson Interferometer with FRMs detects intruders.
In order to use modulation to measure the frequency response at the output of optical detector 13 an optical path difference (OPD) 34 is added to sense line 40 by inserting a time delay element 34. This insures that when the frequency of the light source changes the number of wavelengths in the total path length in sense line 40 plus OPD
34 is different from that in reference line 41. This generates a pseudo-intermediate frequency (pseudo-IF) output at the modulation frequency and at twice the modulation frequency that allows one to usefully measure the in-phase and quadrature-phase of the output of the optical detector. As will be recognized by persons skilled in the art, it is necessary that the light source have a coherence length well in excess of the OPD.
In single mode fiber, light typically propagates at 68.13% the speed of light in free space. It is this velocity that determines the time taken to propagate to the FRMs 30 and 31 and back from the 2x2 splitter/combiner 35. Let us refer to the propagation time in sense line 40 plus the OPD 34 as Ts and the time in reference line 41 as TR .
The momentary frequency at the input to splitter/combiner 35 is assumed to be:
Cd(t)= + AO)cos(0)mt) where wo is the optical carrier frequency of the light, kis the modulation frequency and Am is the modulation depth. All frequencies expressed herein are in "radians /
second".
A fundamental characteristic of this classic frequency modulated wave is that the frequency deviation em is proportional to the peak amplitude of the modulating signal, and is independent of the modulation frequency. The phase angle of the signal arriving back at splitter/combiner 35 reflected from FRM 30 on sense line 40 can be expressed as:

r+7.5 (t)= Sw(t')de and the phase angle of signal arriving back at splitter/combiner 35 reflected from the FRM 31 on the reference line 41 can be expressed as;
t+TR
OR (t) = 1(0(0 de It follows that Act) f OS (0= coo(Ts)+¨isin[com (t +Ts)]--sin[Wnit]l tom and r (t) = (00(R) a- sin[ok (t + TR )1¨sin[conzt]
Vn I
These equations describe the phase of the signals that are summed together in splitter/combiner 35 as a function of time. The summed signal can be expressed as;
E(t)= ei[41-1-00)1 eikv+0R(191 where j = jiT (the imaginary operator). The intensity of light at the output of optical detector 13 is In(t) =1E (Or = E(t) E(t)*
where (*) is the complex conjugate operator. It then follows that the intensity of the light at the output of optical detector 13 can be expressed as:
In(t)= 2+ 2 cos{ Os (t)-0R(t) Using this equation and the phase terms previously defined along with some trigonometric identities it follows that In(t)= 2+ 2 cos[wo (Ts ¨TR)1COS{¨AW Sin{co,n[Ts ¨TR]}COS{COm(t +[Ts +TR])}}

Aw ¨ 2 sin [wo(Ts ¨TR) jsin{¨wm sin{w. [ Ts ¨2 TR1}COS{W t TS +TR
[
There are two Bessel function expansions on page 361 of the "Handbook of Mathematical Functions" by Abramowitz and Stegun (being a reference text that is well known to, and commonly used by, persons skilled in the art) that are particularly helpful in interpreting this equation 9.1.44 cos[ z cos(9) ]= Jo (z)+ 2 Ec-ok .12k(Z)COS[2k k=1 9.1.45 sin[ z cos(9) 1= 21 (¨ J2k,i (Z)COS[(2 k + 1) 0]
k=0 Equations (9.1.44) and (9.1.45) describe the generating functions of the associated series of Bessel Function of the First Kind.
After considerable manipulation of equations and low pass filtering to remove the higher harmonics it can be shown that the intensity of light at the output of optical detector 13 is In(t)= 2 + 2 Jo [ C]cos[woTA
¨ 4J, [ dcos[ con, + TA sin[cooTA
¨4 J2[C]cos[2 coni(t+T)] cos[cooTA
where the argument of the Bessel Functions is defined as C= AC sinf wmT
co,õ L 2 j and the time delay associated with OPD 34 is T.
J2(C)respectively. All three Bessel Functions share the same argument C.
From the equation for the intensity of light at the output of optical detector 13 it is clear that the first term involving Jo(C) is at base band, the second term involving J1(C)isthe modulation of the harmonic at twice the modulation frequency, 2m In order to understand the significance of the Bessel functions; Jo(C), J1(C), .12 (C) and FIG. 3. There are three important observations to make:
J(C)=J2(C)=O when C=0 J1(C)= J2 (C)= 0.46235 when C=2.630 J 2(C)=¨ 0.46235 when C=-2.630 C=0 when T=0 sign(C)=sign(TA) TA then quadrature term of the optical phase output depends on the sign of TA
and has the same amplitude as the in-phase term.

In the complex time domain the sign of the quadrature-phase component (Q) defines the direction of rotation of the time domain phasor. Hence when Tõ is positive the phasor rotates in a positive direction and when Tõ is negative the phasor rotates in a negative direction. In the complex frequency domain this relates to positive and negative frequencies.
One could repeat the same analysis for a Mach Zehnder interferometer. Similar to the Michelson case, it can be shown there are zero first and second harmonic term outputs lo if one selects C=0 which would relate to a zero OPD (Tõ =0) . In other words a Mach Zehnder interferometer with zero OPD does not produce any first or second harmonics.
When the sensor cable is disturbed, sense line 40 changes in length relative to reference line 41 at the location of the disturbance 5. This relative change in length occurs over a short length (a few meters) of the sensor cable. In the complex time domain the relative change in length, d(t) , creates a complex time domain response of r(t)= I r(t)+ iQr(t) where j=../ , /r(t)=C cos( 1 d(t)) and Q,.(t). c sin( 1 d(t)) 2 Al) 2 a the intensity of the light, C is not altered appreciably by the disturbance the intensity of the light but the phase of the light is changed in proportionality to d(t)divided by the wavelength of the light A).
In general r(r)is cyclical in nature and it tends to decay in magnitude with time as the sense and reference lines return to their static position following the disturbance. The peak change is typically many wavelengths (say 20 to 30 wavelengths) in amplitude. It is a very random function since it depends on the positioning of the fibers in the cable, the amplitude of the disturbance of and the coupling of the disturbance to the fibers. In practice no two disturbances will be the same.

Taking the Fourier Transform of the complex time domain function r(t)the complex frequency domain response is R(6)= Er(t) dt In the complex frequency domain a typical disturbance has a broad spectrum of frequency components ranging from 10 kHz to 700 kHz. Complex frequency response, R(co), describes the disturbance 5 at the point of the disturbance. Since one does not measure the response at the point of the disturbance, R(co)may seem somewhat academic but it is important since it forms the common point of reference between the two Michelson responses.
In the Michelson interferometer the disturbance affects the light first as it propagates towards the FRMs and then again as it propagates back from the FRMs. If one selects the time that the light reaches the FRMs as the time point of reference, the first disturbance occurs T e seconds before the light arrives at the FRMs and the second disturbance occurs Te seconds after the light arrives at the FRMs. Hence in the complex frequency domain the measured effect of the disturbance can be expressed as:
Re(co)=1eJ + R(0)) =2 coskoT1 R(co)=2cos(--a)t)R(co) where v is the velocity of propagation in the fiber lines. The range cosine filter (RCF) defined herein as:
RCF(w1)=2 cos(---a)t) distorts the true frequency response R(o) as shown by Re (0= RCF(cot ) R(m) and the distortion depends upon the distance between the disturbance 5 and FRMs 30 and 31 defined as 1. The RCF only affects the amplitude of the frequency components and not the phase of these components.

The stereo fiber optic sensors 3A, 36 and sensor cable 4 of the illustrated embodiment, illustrated in FIG. 4, are described in the following in terms of three primary interferometers viz. Michelson A (formed by sensor A), Michelson B (formed by sensor B) and an inherent Mach Zehnder. The forgoing analysis provides the basis for why the Michelson A interferometer produces a positive response in the complex frequency domain, the Michelson B interferometer produces a negative response in the complex frequency domain and the Mach Zehnder interferometer's outputs at the first and second harmonics are suppressed. It also defined the RCF effects relating to the Michelson interferometers.
In FIG.1 and FIG.4 several splitter/combiner ports are not used. These unused ports are fitted with reflection-less terminations denoted by a "dot" on the device lead.
In FIG. 4 laser source 10 is modulated at a) radians per second by FPGA 17 over line 12 about a carrier frequency of cao radians per second. The modulated output of laser 10 is split 50:50 in splitter/combiner 11 to be sent to identical interferometric sensors 3A
and 36, of which the A and B notation refers to Michelson interferometers A
and 13, respectively. Interferometric splitter/combiner 35A associated with Michelson A
interferometer is contained in sensor 3A and splitter/combiner 356 associated with Michelson B interferometer is contained in sensor 36. Likewise, time delay element 34A
associated with Michelson A interferometer is contained in sensor 3A and time delay element 34B associated with Michelson B interferometer is contained in sensor 36.
Time delay elements 34A and 346 create OPDs of equal length. Time delay element 34A adds to sense line 40 and time delay element 34B adds to reference line 41.
In the illustrated embodiment an OPD of 5 meters is used in order to ensure that the OPD relating to the inherent undesired Mach Zehnder interferometer 52 is nominally of zero length. As will be recognized by persons skilled in the art, in practice the relative length of the fibers 40 and 41 in cable 4 will vary over the wide range of temperatures that the sensor cable 4 may be subjected to and, in turn, it is difficult to realize a perfect zero OPD. Therefore, the OPDs produced by the time delay elements 34A, 346 need to be significantly longer - about an order of magnitude greater, than the worst case variation in the Mach Zehnder OPD to ensure adequate suppression of the Mach Zehnder response.
Splitter/combiner 32A splits the signal arriving on sense line 40 equally (50:50) between FRM 30A and splitter/combiner 35B. Likewise splitter/combiner 33A splits the signal arriving on reference line 41 equally (50:50) between FRM 31A and splitter/combiner 35B. Splitter/combiner 32B splits the signal arriving on sense line 40 equally (50:50) between FRM 30B and splitter/combiner 35A. Likewise splitter/combiner 33A
splits the lo signal arriving on reference line 41 equally (50:50) between FRM 31B and splitter/combiner 35k Throughout, unused/terminated inputs and outputs of the splitter/combiner components are identified by reference numeral 90. Feeder lines 42 and 43 carry the light signals to and from sensor 3B and processor unit 1. The feeder lines 42, 43 simply pass through sensor 3A for convenience.
Sensor cable 4 comprises four fibers viz, feeder lines 42, 43, sense line 40 and reference line 41. In practice, there may well be additional fibers included in sensor cable 4 for other purposes but only these four fibers are used by the stereo sensors 3A, 3B.
Lead-in cable 2 similarly comprises four fibers viz, feeder lines 42 and 43, carrying input and output signals, respectively, to and from sensor 38 and feeder lines 44 and 45 carrying input and output signals, respectively, to and from sensor 3A.
Because lead-in cable 2 does not include sense line 40 or reference line 41 it is not sensitive to motion.
The response from Michelson A interferometer arrives at optical detector 13A
on feeder line 45. The light being transmitted into Michelson A interferometer is sent from splitter/combiner 11 in processor 1 to splitter/combiner 35A in sensor 3A on feeder line 44. The response from Michelson B interferometer arrives at optical detector 13B on feeder line 43'. The light being transmitted into Michelson B interferometer is sent from splitter/combiner 11 in processor 1 to splitter/combiner 35B in sensor 3B on feeder line 42.
While not shown in FIG.4 optical isolators are connected in front of optical detectors 13A and 13B to prevent reflections from the optical detectors from re-entering the interferometers. Likewise laser source 11 must include an optical isolator to prevent reflections from the laser source from re-entering the interferometers.
The outputs of optical detectors 13A and 13B are passed through analog low pass lo filters 15A, 15B to remove the third and higher harmonics before the signals are applied to analog to digital converters (ADCs) 16A and 16B. The digital signals produced by ADCs 16A and 16B are passed to a field programmable logic array (FPGA) 17 where the high speed digital signal processing (DSP) is performed. In addition, modulation frequency, co,,,, is generated by FPGA 17 and transmitted on line 12 to laser source 10.
Digital lines connect FPGA 17 to a computer (PC) 18 where slower speed DSP is performed. To further reduce the cost of the perimeter security apparatus the signal processing performed by PC 18 could be implemented in an embedded microprocessor inside FPGA 17 thereby avoiding the cost of PC 18.
In the end, disturbance 5 applied to the sensor cable 4 is detected and located and reported as an alarm with location coordinates as an output 6 of PC 18.
The three primary interferometers that share sense line 40 and reference line 41 are illustrated in FIG. 5. While shown as three separate interferometers in FIG. 5 it will be understood by the skilled reader that they are all inherently present in the stereo fiber optic sensors 3A, 3B with sense cable 4 as shown in FIG. 4. They are illustrated separately in FIG. 5 simply as a convenient means of explaining the operation of the stereo fiber optic sensors 3A, 3B and sense cable 4.

Referring to FIG. 5, Michelson A interferometer 50 includes splitter/combiner 35A, time delay element 34A, sense line 40, reference line 41, FRM 30A and FRM 31A.
Michelson B interferometer 51 includes splitter/combiner 35B, time delay element 34B, sense line 40, reference line 41, FRM 30B and FRM 31B. The Mach Zehnder interferometer 52 shown in FIG. 5 includes; splitter/combiner 35A, OPD 34A, sense line 40, reference line 41, OPD 34B and splitter combiner 35B. Splitter/combiners 32A, 32B
and 33A, 32B have been omitted to explain the operation of the interferometers. They do not inhibit the operation of Michelson A or B interferometers, or Mach Zehnder interferometer, but they do add attenuation of the optical signals.
1.0 From FIG. 5 the back-to-back nature of the Michelson A and B interferometers, 50 and 51, is apparent. Each of the Michelson interferometers is similar to the basic Michelson interferometer shown in FIG. 2 and functions as previously described. With time delay element 34A in sense line 40 the Michelson A interferometer 50 produces a positive response in the complex frequency domain. With the time delay element 34B in reference line 41 the Michelson B interferometer 51 produces a negative response in the complex frequency domain.
Because the OPDs created by time delay elements 34A, 34B are the same length there is effectively zero OPD in the Mach Zehnder interferometer 52. As described previously having a zero OPD means that the first and second harmonic response signals that would otherwise be produced by the inherent Mach Zehnder interferometer 52 are suppressed leaving only the Michelson A and B first and second harmonic signal responses.
From FIG. 4 one can envisage more than the three primary interferometers.
Multipath interferometers are created when one considers multiple reflections between and FRM 30B interfering the normal operation of the primary interferometers.
Likewise multipath interferometers are created when one considers multiple reflections between FRM 31A and FRM 31B interfering the normal operation of the primary interferometers.
In addition the multiple reflections between FRM 30A and FRM 30B can interfere with the multiple reflections between FRM 31A and FRM 31B. It is important that the responses from these multipath interferometers be relatively small compared to those of the desired primary Michelson interferometers 50 and 51.
Splitter/combiners 32A, 32B, 33A and 33B attenuate the optical signals passing through them by 3dB for each pass. This means that for Michelson A and B
interferometers 50 and 51 the signal is down by 12 dB. For the Mach Zehnder interferometer 52 it is down by 6 dB. This means that the null provided at the near zero OPD for the Mach Zehnder interferometer 52 must overcome 6 dB and, preferably, at least another 12 dB.
It is anticipated that there will be some mismatch between the length of the sense and reference lines 40 and 41over time and temperature. As indicated above, the time delay elements 34A, 34B must be about an order of magnitude longer than the largest difference between 40 and 41 to achieve the desired suppression of the Mach Zehnder response.
There are multipath Mach Zehnder interferometers. These are attenuated by splitter/combiners 32A, 32B, 33A and 33B and they experience the same zero OPD

suppression as the primary Mach Zehnder and are of little concern.
Responses from higher order multipath Michelson interferometers are also attenuated by splitter/combiners 32A, 32B, 33A and 33B. As indicated above, the residual responses from even order multipath Michelson interferometers produce the same range information as the primary Michelson interferometers, and the residual responses from odd order multipath Michelson interferometers that could add noise to the ranging process are substantially attenuated due to the very long optical path length differences involved and through the selection of a laser source with a coherence length that is less than the length of the sensor cable.
Disturbance 5 affects all three primary interferometers. In the case of Michelson A
interferometer 50 disturbance 5 is P meters from FRMs 30A and 31A and L-t meters from splitter/combiner 35A. In the case of Michelson B interferometer 51 disturbance 5 is L-t. meters from FRMs 30B and 31B and t meters from splitter/combiner 35B.
The sensor cable 4 is L meters long.
The objective of the DSP 18 is to determine length which defines the location of disturbance 5 along the length of sensor cable 4. The key to achieving this objective is in recognizing that Michelson interferometers 50 and 51 respond to the same disturbance and that the measured complex frequency domain responses from the Michelson A interferometer 50 is RCF[cot]R(co) and from Michelson B
interferometer 51 is RCF[co(L-OJR(co).
1.0 While the present invention can be adapted to address numerous applications the specific embodiment described herein is particularly tailored to the outdoor perimeter security requirements associated with detecting intruders who attempt to cut through, or climb over, a chain link fence. The following design parameters are optimized for this specific application. A standard laser wavelength of 1310 nanometers with a coherence length in the order of 100 meters is selected. A modulation frequency of 2 MHz ( tom =27t fõ,)and a digitization rate of 12 MHz are used. This is designed to accommodate the harmonic at twice the modulation frequency (4 MHz) and the response modulation carried on this harmonic. Anti-aliasing analog low pass filters 13A
and 13B should have a corner frequency of approximately 5 MHz. The 12 MHz sample rate means that the outputs of ADCs 16A and 16B are updated every 83 nanoseconds.
The digital signal processing described herein is applied to the data stream generated by ADCs 16A and 16B .
The digital signal processing aspects of the present invention are illustrated in FIG.6.
For purposes of explanation, it is convenient to use analog symbols to describe the DSP
operation. The first digital processing step is to add a digital delay line to the responses from Michelson A interferometer 50. The purpose of this digital delay is to effectively match the analog delay in receiving the response for Michelson interferometer 51. In particular an integer number of sample delay periods (a multiple of the 83 nanosecond sample period) is selected to match the propagation time in feeder line 43 that brings the Michelson B interferometer 51 response back to sensor 3B.
The Michelson A interferometer quadrature-phase (QA) response is derived by performing mixing operation 60 of the optical detector output response with the modulation frequency, cen , as provided by FPGA 17 on line 12 and low pass filtering the output with a corner frequency of approximately 1.4 MHz to remove the upper cross products in digital low pass filter 66. Low pass filter 66 is also used to decimate the processing rate from 12 MHz to 1 MHz.
The Michelson B interferometer quadrature-phase (QB) response is derived by performing mixing operation 61 of the optical detector output response with the modulation frequency, aim, as provided by FPGA 17 on line12 and low pass filtering the output with a corner frequency of approximately 1.4 MHz to remove the upper cross products in digital low pass filter 65. Low pass filter 65 is also used to decimate the processing rate from 12 MHz to 1 MHz.
The Michelson A interferometer in-phase (IA) response is derived by performing mixing operation 62 of the optical detector output response with twice the modulation frequency, 20Jõõ as provided by FPGA 17 low pass filtering the output with a corner frequency of approximately 1.4 MHz to remove the upper cross products in digital low pass filter 64. Low pass filter 64 is also used to decimate the processing rate from 12 MHz to 1 MHz.
The Michelson B interferometer in-phase (119 response is derived by performing mixing operation 63 of the optical detector output response with twice the modulation frequency, 20Jn, as provided by FPGA 17 and low pass filtering the output with a corner frequency of approximately 1.4 MHz to remove the upper cross products in digital low pass filter 67. Low pass filter 67 is also used to decimate the processing rate from 12 i MHz to 1 MHz. Inverter 69 is included in the quadrature response to invert the sign of the complex frequency response from Michelson B interferometer.
The complex time response for Michelson A interferometer is passed to complex FFT
70A and the complex time response for Michelson B interferometer is passed to complex FFT 70B.
The operation of the complex FFT process performed by each complex FFT 70A, 70B is illustrated in FIG.7. A radix-2 complex FFT is used for maximum computational lo efficiency. In the specific illustrated embodiment of the invention a 1024 point complex FFT is used (i.e. H =1024). With a 1 MHz input rate the sound bite processed by the complex FFT is 1.024 milliseconds long. Since it is a complex FFT the Nyquist frequency associated with the FFT is 1 MHz which is adequate for the anticipated 10 kHz to 700 kHz response spectrum. There are 1024 output frequency bins spaced apart at 976 Hz.
For notational purposes the sound bites are identified by the subscript "K'.
This subscript is incremented every 1.024 milliseconds. The individual time and frequency samples associated with each complex FFT operation are denoted by the subscript "h"
where h=0,1,2õH ¨1.
As shown in FIG. 7 the H real and imaginary input variables are labeled as the conventional / and ()while the real and imaginary outputs of the complex FFT
are labeled a and b. The H frequency bins describe frequencies from DC tofNyqutst = The Nyquist frequency, Livquist, is determined by the sampling rate. Complex FFTs with real and imaginary inputs have H inputs and H outputs unlike FFTs that have only real inputs and have only H/2 frequency outputs.
Having delayed the Michelson A interferometer input to arrive at the same time as the Michelson B interferometer the two complex frequency outputs are of exactly the same disturbance but with the responses distorted by the two inherent Range Cosine Filters as RCF[cot]R(co) and RCF[co(L-0]R(M.
The digital signal processing performed for a bridge measurement of the location of the disturbance is illustrated in FIG. 8. The left hand side of the bridge is optical in nature and it is inherent to the back-to-back Michelson interferometers. The top left branch 80 of the bridge represents the measured output of the Michelson A
interferometer. The lower branch 81 the bridge represents the measured output of the Michelson B
interferometer. The RCF is inherent in the measured outputs and the filter functions ic shown in 80 and 81 are included only as means of explaining the operation of the bridge.
The complex frequency components of these measurements can be interpreted as aA(K+h) bA(K+h) = RCF[C0h ij [a Ki_h flic+1,1 h=
0,1,...,H ¨1 aB(K+h)+ jbAK,h)=RCFkoh(L¨)] [4K+h+ il3K+11] h =0,1,...,H ¨1 where [aK+h+./flki-h] h=0,1,...,H-1 are the complex frequency components relating to the disturbance R(co). The complex frequency domain response at the point of the disturbance, R(w)provides a common point of reference. In the above equations, R(), is represented by the H complex variables [alc+h +./fix+h] h=0,1,...,H ¨1 which would be the complex frequency component outputs of a complex FFT had one been able to capture the response data at the point of the disturbance and perform a similar complex FFT on said data. This describes the inherent RFC responses 80 and 81 from the two Michelson interferometers.
The right hand side of the bridge is implemented digitally. There are two parts to the right hand side of the bridge; one based on inferential RCF computations and the other on inferential RSF computations. The bridge configuration and measurements provide the mechanism by which the perimeter security apparatus locates the disturbance 5 on the sensor cable 4. By digital signal processing, different bridge arms are configured to include specific RCFs for each of a number of pre-determined range bins and each such configuration is tested to determine whether the disturbance is in that particular range bin. This is determined when a particular set of bridge arms balance the bridge because such state of balance means that the disturbance is located in that particular range bin.
The length of the sensor cable 4 extended along the perimeter is partitioned into a predetermined number (N) of range bins. Thus, each range bin of the predetermined number of range bins corresponds to the distance along the length of the sensor cable of that range bin, with the end of the Nth range bin corresponding to the end of the sensor cable. The range bins are denoted by the subscript "n" where n=0,1,2õN-1.
The number of range bins selected for use will depend on the length of sensor cable 4 and the application of the sensor apparatus. A typical range bin length is 20 meters.
Hence, for a sensor cable 4 that is 2 km long, with 20 meter long range bins the number of range bins used for the digital bridge processing is 100 i.e.: N=100. The range (i.e.:
distance) along the cable to each range bin is denoted by 7õ .
In classical radar, range is the line of sight distance from the transceiver which together with azimuth determines location. In this case the azimuth measure is replaced by the knowledge of where the sensor cable is installed and range is the linear distance along the sensor cable as it follows the perimeter of the site around corners and up and down hills. With this modified definition of range the concept of range bins has a similar meaning as they do in classical radar. In other words the distance along the sensor cable is measured in discrete elements called range bins. Since the computational burden on the signal processor increases significantly with the number of range bins one typically utilizes relatively large range bins such as the 20 meter range bin length in the preferred embodiment.
The first step in implementing the bridge measurement is to compute the inferential cosine parameters:
aCAK,õ,õ+ j bCAK.õ, = RCF[coh(L¨ 7)] [aAK,h+ j bkch] 82 and aCB Kjoõ, + j bCBK,h,,,= RCFkoh rn] [aB 1 ch + j bB K ,h] 83 for h=0,1õH ¨land n=0,1õN-1. The "C' in the parameter name relates to the multiplication by the cosine function and the "S" in the parameter name relates to the multiplication by the sine function. The length 7 is the distance from range bin "n" to FRM 30A and 31A.
Subtracting one result from the other in adder 86 and after some manipulation it follows that the "bridge sine function" is S(õh),õ = (aCA(K,h),õ¨ aCB(K+0,n)+ j (bCA(K4.0,¨ bCB(K +0,n) = 'Ph sin av (yõ
¨0 [
where Th= 2sin[--1c L][ach + j filch]
V
A range sine filter (RCF) is defined as RSF(o)t)=2 sin(PP) v and it is used to compute the inferential sine parameters:
aSAK A, + j bSAch, = RSF[coh(L¨ yn)] [aAlch+ j bAK A] 84 and aSBK,h,+ j bSI3(K+h),n = RSFfroh yn] [aB 1 ch + j bBK,h1 85 for h=0,1õH ¨land n=0,1õN ¨1 .
, Adding one result to the other in adder 87 and after some manipulation it follows that "bridge cosine function" is C Kh,n --= (aSAK,h,õ +aS/3,,h,õ)+ j(bSAK +bSAK+0,n)=1Ph cos a'h (rn t) , Based on the bridge sine and cosine functions the "bridge tangent function" is defined as TAN K õ SK,,, (aCAK h aCB, ch,õ) + (bCAK,h,õ - bCB K'h) = tan[oa (y. -0]
. ='= ' ' " C chm (aSAKAõ+ aSBK.õ,õ)+ j (bSAK + bSBK.h,) While the RCF and RSF filters change the amplitude of the various frequency components they do not change the phase angle of the components hence the phase 1.0 angles of the numerator equals that of the denominator. This enables one to simplify the computation aCS K h n+bCSK nn TAN K An= c _____ = tan --(rn-e)]
where aCSKAn =(aCkch,n ¨ aCB K,h,õ)(aSAK,h,n aSBK,h,n) bCSKhR = (bCAK,h.õ bCBK,h,n)(bSAK,hm bSBK An) and MS K,h,n= max{ LaSAKhfl + aSBK,h,õ)2 (bSAK,h,õ bASBK,h,n )2 I AlSnlin for h=0,1õH¨land n=0,1õN-1.
A minimum bound MS,,õõ is imposed to avoid a division by zero. This does not affect the bridge measurement since when balanced the denominator term MS(K+h),n is maximized.

The function TANK,,,, describes the tangent of the angle --Aa) (71õ for each of the H
frequency outputs of the FFT and for each of the N range bins in terms of the H outputs of the complex FFT.
When the inferred range bin xi is close to the location of the disturbance, , the angle ¨t(yõ ¨ e) becomes small for all frequencies and the TANK,,,, function can be approximated by the angle.
In order to locate the nearest range bin to the location of the disturbance the parameter NSK,õ is defined as H ¨I
NS K,, = El aCSKhfl bCS K ,h,õ1 h=0 The disturbance is located in range bin nx where NSK,,,, is a minimum.
From this computation one knows where the disturbance is along the sensor cable to the nearest range bin. In many cases it is desirable to determine the range of the disturbance more accurately than to the nearest range bin. While one could do so by increasing the number of range bins and hence reducing the length of each range bin this comes at a significant computational burden. Even still the range accuracy is limited to the range bin size in this approach.
A more efficient means of determining the precise range associated with a disturbance involves the use of the TANK,,,, function and linear interpolation. The TAN K
,h,n function is computed for range bins nx-2,nx-1, nx, nx+1 and nx-i-2 and the precise location where the TANK,,,, function goes to zero is determined using linear interpolation of the data about range bin nx . At this point the bridge is balanced and the disturbance has been precisely located.
In summary the inferential side of the bridge scans through all range bins to find the range bins that bring the bridge near to its balance point. The precise location of the disturbance is then determined using linear interpolation of the TAN K,h,n data to find the exact location where the bridge is balanced.
The denominator term MSch,h is a measure of the magnitude of the frequency components for each of the H frequency bins at range bin nx. Once located to the nearest range bin, nx, it is useful to compute the weighted average energy over all H
frequency bins.
H-I
EKja .EwhmsK,h,õõ
/7=-0 where weighting parameters wh h=0,1,....,H¨lare selected to roughly match filter the expected frequency response of the anticipated disturbance. While only one set of weighting function are presented in this description of the stereo Michelson fiber optic sensor it is anticipated that in some applications two or more weighting functions may be used to optimize the sensor performance. For example one set of weighting functions may be used to filter the anticipated lower frequency components associated with a person climbing on a fence while another set of weighting functions may be used to filter the anticipated higher frequency components associated with a person attempting to cut through a fence. Different weighting functions will be used when the sensor is buried in the ground and one is detecting walking or running intruders.
The weighted average energy, Ef 0,õ is used to update a response range bin high pass filter. For range bin "me where the intruder has been located the range bin high pass filter equation is = r{ - E(K¨I),nx and for all other range bins RK,n= F RK¨I,n This represents a digital single pole low pass filter for each range bin response that is updated for every sound bite when Kis incremented. The filter constant F is selected 1.0 to match the nature of the disturbance. For the detection of a person cutting the fence F is selected to accommodate a shorter duration disturbance than would be selected for the detection of a person climbing on a fence. Likewise different filter constant F would be used when the sensor cable is buried in the ground.
For a fence disturbance application the high pass filter should have a time constant of approximately 50 milliseconds. With the filter updated every millisecond a filter constant of F=0.0198 is appropriate.
The cycle of data collection, computation of the complex FFT and execution of the bridge measurement is repeated for every sound bite of data as window counter Kis incremented. In practice it is anticipated that K increments approximately once every millisecond.
Each time that the range buffer is updated the energy in each range bin buffer is compared to a threshold for that particular range bin. When the energy exceeds the threshold an "Event" is declared in the particular range bin. If desired the precise disturbance location derived with every bridge measurement and be averaged for each range bin to provide a more precise Event location.

As with many fence disturbance sensors an "Alarm" is only declared when there have been P Events detected at, or near, the same location within a given time window.
Typically P is selected to be 2, 3, 4, or 5. A typical time window for the Event count is 3 to 8 minutes.
While the embodiment used to describe the present invention is optimized for a fence mounted outdoor perimeter security apparatus, the various parameters described herein for the illustrated embodiment, such as laser wavelength, laser coherence length, modulation frequency, sampling rate, size of complex FFTs, number of range bins and 1.0 the filter constants, can be adjusted appropriately by a person skilled in the art to accommodate many other applications.
Applications in which the sensor cable is buried is just one other example.
The protection of oil and gas pipelines is another application. A further example of an 15 application is for securing fiber optic communications lines. It is well known that data can be extracted from fiber optic cables buy a number of nefarious means. This usually requires some manipulation of the cable. By including stereo fiber optic sensors according to the present invention on the same cable this can be prevented.
The present invention provides a very cost effective means of detecting and locating such 20 attempts to steal valuable data.
The securing of data lines has an application in outdoor security. In many high security sites it is necessary to utilize a number of sensor technologies on the perimeter as well as video cameras for assessment purposes. These additional devices require the use of 25 a secure data network around the perimeter. Including the stereo fiber optic sensors described herein on the data network cable, with data network fibers in the sensor cable, provides a cost effective solution to this requirement.
In the case of perimeter security it may be desirable in high security applications to 30 install redundant sensors. Having parallel but spaced apart stereo fiber optic sensors provides redundant operation as well as improved performance through integration of sensor data.
The invention has been described in a specific embodiment that should be considered as illustrative only and not limiting of the invention. Reference should be made to the claims to determine the scope of the invention.

Claims (14)

1. Perimeter security apparatus for use with a laser source providing two identical frequency modulated optical source signals for detecting a disturbance applied to a fiber optic sensor cable extending along the perimeter and/or determining a range bin along the length of the sensor cable in which the disturbance is located, wherein each range bin corresponds to a distance along the extended sensor cable when partitioned into a predetermined number of range bins with the total number of range bins corresponding to the length of the extended sensor cable, the apparatus comprising:
(a) a first fiber optic Michelson interferometric sensor comprising a first input for receiving a first one of the identical modulated optical source signals, first and second fibers, splitter/combiners, a first optical time delay element in the first fiber and Faraday rotational mirror terminations terminating each fiber;
(b) a second fiber optic Michelson interferometric sensor comprising a second input for receiving a second one of the identical modulated optical source signals, first and second fibers, splitter/combiner's, a second optical time delay element in the second fiber and Faraday rotational mirror terminations terminating each fiber;
(c) the fiber optic sensor cable comprising first and second fibers for connecting at one end to the first and second fibers, respectively, of the first Michelson interferometric sensor and at the other end to the second and first fibers, respectively, of the second Michelson interferometric sensor wherein the first and second Michelson interferometric sensors and sensor cable are configured to form, when connected, first and second Michelson interferometers in a back-to-back configuration, the first and second fibers of the Michelson sensors and cable being common to both the first and second Michelson interferometers and defining first and second optical paths of the first and second Michelson interferometers, respectively; the first and second source signals producing first and second optical output signals from the first and second Michelson interferometers, respectively; the first and second optical time delay elements being located in the first and second optical paths, respectively, to create a predetermined optical path difference in each of the first and second optical paths producing a complex optical response signal comprising a positive pseudo-IF first output response signal by the first Michelson interferometer and a negative pseudo-IF second output response signal by the second Michelson interferometer, while suppressing a response of a Mach Zehnder interferometer inherent to the back-to-back configuration of the first and second Michelson interferometers; the disturbance producing in the pseudo-IF
output response signals a distortion which is subject to representation by a predetermined first mathematical function dependent upon a range along the sensor cable to the disturbance;
(d) first and second optical detector and converter components, the first optical detector and converter components for detecting and converting the first output response signal to an electronic first output digital signal and the second optical detector and converter components for detecting and converting the second output response signal to an electronic second output digital signal; and, (e) one or more digital signal processors for processing the first and second output digital signals;
wherein the one or more digital signal processors down converts the first and second pseudo-IF output response signals to produce base band in-phase and quadrature-phase distortion signals for each of the Michelson interferometers; and, wherein the one or more digital signal processors use the in-phase and quadrature-phase distortion signals to form each half of one half of a bridge and the first predetermined mathematical function to produce complex inferential signal components for each half of the other half of the bridge, and perform iterative bridge measurements, with each successive iteration using the next range bin of the range bins to produce the inferential signal components, until a bridge measurement determines that the bridge is balanced and thereby determine that the disturbance is located in the range bin used for the balanced bridge.

2. Apparatus according to claim 1 wherein the one or more digital signal processors apply the predetermined first mathematical function in the frequency domain using complex fast Fourier transforms (FFT).

3. Apparatus according to claim 1 or 2, further determining the range along the sensor cable to the disturbance, wherein the one or more digital signal processors process the iterative bridge measurements for range bins neighboring the range bin used for the balanced bridge and perform interpolation to those bridge measurements for neighboring range bins using a predetermined second mathematical function.

4. Apparatus according to any one of claims 1 to 3 wherein the predetermined first mathematical function is the Range Cosine Function (RCF):
where we is the argument of the RCF function;
co is the radian frequency of the component being considered;
v is the velocity of propagation; and, is the distance between the disturbance and the Faraday rotational mirrors of the first Michelson interferometer.

5. Apparatus according to claim 4 wherein the predetermined second mathematical function is the tangent function:
where K refers to the sample period;
h refers to the FFT sample whereby h = 0 - (H-1) and H is the FFT sample rate;

n is the range bin number whereby n = 0 - (N-1) and N is the total number of range bins making up the length of the sensor cable;

.omega. h is the radian frequency of the h th frequency component;
.NU. is the velocity of propagation;
.gamma. n is the distance along the sensor cable to the nth range bin ; and, ~ is the distance between the disturbance and the Faraday rotational mirrors of the first Michelson interferometer.

6. Apparatus according to any one of claims 1 to 5, further comprising first and second optical isolators for connecting in front of the first and second optical detector components, respectively.

7. Apparatus according to any one of claims 1 to 6 wherein the optical source comprises an optical isolator.

8. A method for detecting a disturbance applied to a fiber optic sensor cable extending along the perimeter and/or determining a range bin along the length of the sensor cable in which the disturbance is located, wherein each range bin corresponds to a distance along the extended sensor cable when partitioned into a predetermined number of range bins with the total number of range bins corresponding to the length of the extended sensor cable, the method comprising:
(a) providing two identical frequency modulated optical laser source signals;
(b) providing a first fiber optic Michelson interferometric sensor comprising a first input receiving a first one of the identical modulated optical source signals, first and second fibers, splitter/combiner's, a first optical time delay element in the first fiber and Faraday rotational mirror terminations terminating each fiber;
(c) providing a second fiber optic Michelson interferometric sensor comprising a second input receiving a second one of the identical modulated optical source signals, first and second fibers, splitter/combiner's, a second optical time delay element in the second fiber and Faraday rotational mirror terminations terminating each fiber;

(d) providing a fiber optic sensor cable comprising first and second fibers and connecting one end of the first and second fibers of the sensor cable to the first and second fibers, respectively, of the first Michelson interferometric sensor and the other end to the second and first fibers, respectively, of the second Michelson interferometric sensor to form first and second Michelson interferometers in a back-to-back configuration, the first and second fibers of the Michelson sensors and cable being common to both the first and second Michelson interferometers and defining first and second optical paths of the first and second Michelson interferometers, respectively;
whereby the first and second source signals produce first and second optical output signals from the first and second Michelson interferometers, respectively; the first and second optical time delay elements being located in the first and second optical paths, respectively, to create a predetermined optical path difference in each of the first and second optical paths which produces a complex optical response signal comprising a positive pseudo-IF first output response signal by the first Michelson interferometer and a negative pseudo-IF second output response signal by the second Michelson interferometer, while suppressing a response of a Mach Zehnder interferometer inherent to the back-to-back configuration of the first and second Michelson interferometers; the disturbance producing in the pseudo-IF output response signals a distortion which is subject to representation by a predetermined first mathematical function dependent upon a range along the sensor cable to the disturbance;
(e) providing first and second optical detector and converter components, the first optical detector and converter components detecting and converting the first output response signal to an electronic first output digital signal and the second optical detector and converter components detecting and converting the second output response signal to an electronic second output digital signal; and, (e) digitally processing the first and second output digital signals, the digital processing comprising: down converting the first and second pseudo-IF output response signals to produce base band in-phase and quadrature-phase distortion signals for each of the Michelson interferometers; using the in-phase and quadrature-phase distortion signals to form each half of one half of a bridge and the predetermined first mathematical function to produce complex inferential distortion signal components for each half of the other half of the bridge; performing iterative bridge measurements, with each successive iteration using the next range bin of the range bins to produce the inferential signal components, until a bridge measurement determines that the bridge is balanced, whereupon the disturbance is determined to be located in the range bin used for the balanced bridge.

9. A method according to claim 8 wherein the predetermined first mathematical function is applied in the frequency domain using complex fast Fourier transforms (FFT).

10. A method according to claim 8 or 9, further determining the range along the sensor cable to the disturbance, wherein the digital processing includes performing the iterative bridge measurements for range bins neighboring the range bin used for the balanced bridge and performing interpolation to those bridge measurements for neighboring range bins using a predetermined second mathematical function.

11. A method according to any one of claims 8 to 10 wherein the predetermined first mathematical function is the Range Cosine Function (RCF):
where .omega.~ is the argument of the RCF function;
.omega. is the radian frequency of the component being considered;
v is the velocity of propagation; and, ~ is the distance between the disturbance and the Faraday rotational mirrors of the first Michelson interferometer.

12. A method according to claim 11 wherein the predetermined second mathematical function is the tangent function:

where K refers to the sample period;
h refers to the FFT sample whereby h = 0 - (H-1) and H is the FFT sample rate;

n is the range bin number whereby n = 0 - (N-1) and N is the total number of range bins making up the length of the sensor cable;
.omega. h is the radian frequency of the h th frequency component;
v is the velocity of propagation;
.gamma. n is the distance along the sensor cable to the nth range bin ; and, ~ is the distance between the disturbance and the Faraday rotational mirrors of the first Michelson interferometer.

13. A method according to any one of claims 8 to 12, connecting in front of the first and second optical detector components, first and second optical isolators, respectively.

14. A method according to any one of claims 8 to 13 wherein the optical laser source signals have been isolated.