WO2011109895A1 - Ranging michelson interferometric sensor with compound termination - Google Patents

Abstract

There is provided several configurations of a ranging Michelson interferometric sensor having two single made waveguides, such as optical fibers, acting as a sense arm and a reference arm. Both fibers are connected to an interferometric splitter/combiner at a first end. Each of the fibers is terminated at a second end by a compound termination. Each compound termination consists of two FRMs coupled to the corresponding fiber, one by a delay line. A detection zone for the sensor is defined to extend substantially between the interferometric splitter/combiner and the compound terminations.

Description

RANGING MICHELSON INTERFEROMETRIC SENSOR WITH

COM POUND TERMINATION

RELATED APPLICATIONS

The present disclosure claims priority from United States provisional patent application number 61/313,433 entitled "Ranging Michelson Interferometer Based Fiber Optic Sensor" filed March 12, 2010 by Harman, which is incorporated by reference in its entirety herein. TECHNICAL FIELD

The present disclosure relates to optical fiber sensors and more particularly, to a ranging Michelson interferometric sensor with compound termination .

INTRODUCTION

Outdoor perimeter security sensors are used around high value resources to detect the entry and exit of personnel. In respect of nuclear power plants, VIP residences and military bases, the emphasis is on detecting intruders. In respect of prisons, the emphasis is on detecting escaping prisoners. In addition to outdoor perimeter security applications, such sensors may be used in other distributed sensor applications. These sensors may be based on copper cable-based and optical fiber optic implementations. Optical fiber sensors have a number of inherent advantages over copper-based sensors. A significant advantage is their immunity to electromagnetic interference (EMI) and all forms of radio- frequency interference (RFI). Additionally, optica! fiber sensors have relatively low signal attenuation . Thus, they facilitate implementation of long sensors. Moreover, these characteristics permit the signal processing to be housed indoors out of typically harsh outdoor environments. When the electronics are kept indoors, the supply of outdoor power and data networks to support the sensor may be avoided. Fiber optic perimeter security sensors may be classified as falling within one of two basic categories, namely block-type fiber sensors and ranging fiber optic sensors.

Block sensors are traditionally less expensive than ranging fiber optic sensors and compete with very inexpensive copper cabie-based

technologies. Some block sensors have indoor electronics with multiple fiber lead-in lines from perimeter zones to be monitored, while others maintain the electronics on the perimeter itself.

Ranging fiber optic sensors have conventionally been characterized as having very sophisticated equipment and signal processing. As a result, they tend to be only cost competitive for very long perimeters as they make use of very sophisticated equipment, such as very expensive !ight sources or polarization controllers or both, and signal processing. When used on very long perimeters, the vulnerability of such equipment and signal processing to a cut fiber may prove to be an unacceptable risk.

Ranging fiber optic sensors may be grouped generally into two categories. Optical Time Domain Reflectometry (OTDR) sensors rely on measuring round trip time-of-flight (TOF) between a processor and a disturbance and, in some respects, is analogous, in the optical domain, to time domain reflectometry (TDR) techniques in the electrical domain using copper-based sensors.

OTDR sensors provide precise location of a disturbance but are relatively insensitive to motion of the sensor cable and are very expensive to implement. In general, they do not function well as a fence-mounted perimeter security sensor. Counter-propagating interferometers are the second broad category of ranging fiber optic sensors. There have been a number of advances in fiber optic interferometry over the years. The widespread use of fiber optics in the telecommunications industry has led to the development of inexpensive components, including without limitation, wavelength division multiplexing (WDM) splitters / combiners, to provide a cost-effective means of multiplexing multiple signals of different wavelengths on a single fiber. Other advances include modulation techniques to generate a n effective heterodyne response, mechanisms to measure the phase of an optical response and the use of 3x3 and 4x4 splitter/combiners to directly measure phase of the optical response, One of the earliest known such sensors is described in UK Patent No. 1 508 676 and entitled "Fibre Optic Acoustic Monitoring Arrangement" published on January 12, 1978 issued to Ramsay on May 24, 1978. Ramsay discloses an approach in which a difference in time of arrival of the effects of a disturbance in two counter-propagating signals is measured to provide a range to the disturbance.

Counter-propagating interferometer sensors in use at present are

conventionally based on Mach-Zehnder interferometers, in which a difference in time of arrivals of the effects of a disturbance in two counter-propagating signals is measured, to locate the disturbance. When used in fence-mounted scenarios, such sensors may determine the location of the disturbance to within 5-20 m, usually with in a few milliseconds of time. Their sensitivity to motion of the sensor cable and brief detection window mea ns that it is un likely that two or more disturbances will occur simultaneously, such that one disturbance prevents detection of the other disturbance. As a result, such sensors can effectively detect multiple "simultaneous" disturbances.

When used as a counter-propagating interferometric sensor, Mach-Zehnder interferometers face the problem of polarization-induced fading . This problem is typically dealt with by employing active compensation techniques using expensive polarization controllers. One side effect of doing so is that the sensor in fact pauses momentarily to adjust the polarization , Were a disturbance to occur at the precise instant that the polarization is being adjusted, the sensor would not be in a condition to detect the disturbance, leading to a potential vulnerability to intrusion.

PCT Patent Application WO/220006/001868 filed by Optellios, Inc. , naming as inventors Patel et a/, ("Patel No. 1"), entitled "Phase Responsive Optical Fiber Sensor", published on or about January 5, 2006 as PCT International Publication No. WO/2006/001868, discloses the location of a disturbance along an elongated optical waveguide that may be determined by measuring different propagation times for a disturbance induced phase variation to propagate from the disturbance up to opposite phase responsive receivers at ends of bidirectional signal paths. Each receiver has a coupler that functions as a beam combiner and also as a beam splitter for inserting the opposite signal. On the receiving end, the coupler provides two detectors with mutually independent phase related signal values. These values are processed and mapped to phase angles, from which relative phase angles versus time are derived for each opposite signal pair.

Patel No. 1 describes as its primary objective, a means of avoiding "the detrimental effects of polarization induced fading and phase shift". Patel No. 1 describes computational techniques to measure the phase-induced changes in the presence of polarization changes. In all of the embodiments described by Patel No. 1, signals are applied to, and received from, each end of the sensor cable or waveguide to achieve the counter-propagating effect that underlies the waveguide's use as a ranging sensor.

In each of the described embodiments, either an active polarization controller or other expensive polarization maintaining equipment is introduced as part of the sensor.

The problem associated with polarization-induced fading is known. With the exception of Michelson interferometers, most solutions to polarization- induced fading involve the use of polarization controllers or costly

polarization scramblers. In either case, schemes to continually adjust the polarization are employed in conjunction with such equipment.

With Michelson interferometers, one promising technology to avoid the implementation of such costiy equipment and complicated techniques is the use of a Faraday Rotational Mirror (FRM). An FRM is a low-cost passive device that correctly compensates for variations in the state of polarization (SOP) of light introduced into a single-mode fiber by thermal and mechanical perturbations. The FRM takes advantage of the Faraday effect, which describes the non-reciprocal rotation of a signal's polarization as it passes through an optical medium within a magnetic field. When positioned at the end of an optical fiber, the FRM rotates a signal's SOP twice, by 45° each time. The first rotation takes place when the light enters the FRM and the second rotation takes place when the light is reflected back into the fiber. The combination of the two rotations results in a total reflection of 90° of the SOP with respect to the original signal. The rotations, combined with a reversal of the polarization state's handedness when the signal is reflected at the mirror, yields an SOP that is perpendicular to the original signal, irrespective of its SOP. In this way, any SOP fluctuations that may occur anywhere along the fiber may be compensated and their undesirable effects passively neutralized. Low-cost FRMs are now widely available.

In fiber optic detectors employing Michelson interferometers terminated at one end by an FRM, typically each of two fibre arms of the interferometer is terminated with a single FRM at the distal end thereof. One arm is typically referred to as the sense arm and the other as the reference arm. A few micrometers' change in relative length of the two arms of the interferometer as a result of a disturbance, creates a cycle in the interferometric output. Such interferometers may be used to detect the presence of an intruder. To date, however, it does not appear that any arrangement for adapting such detectors to provide a ranging capability, short of employing a plurality of oppositely oriented interferometers to provide a counter-propagating signal capability, has been disclosed .

An example of such arrangement of oppositely oriented interferometers is PCT International Patent Application No. PCT/US2007/077101 filed by Opteliios, Inc., naming as inventors Patel er a/. ("Patel No. 2), entitled "Detection and Location of Boundary Intrusion, Using Composite Variable Derived from Phase Measurements" and published on or about March 6, 2008 as PCT International Publication No. WO/2008/027959. Patel No. 2 discloses a disturbance, such as vibration from human activity, located along a fiber optic waveguide configuration with two interferometers of the same or different types, such as Mach-Zehnder, Sagnac, and Michelson interferometers. Carrier signals from a source are split at the interferometer inputs and re-combined at the outputs after propagating through the detection zone, where phase variations are induced by the disturbance. Phase responsive receivers detect phase relationships between the carrier signals over time. A processor combines the phase relationships into composite signals according to equations that differ for different

interferometer configurations, with a time lag between or a ratio of the composite signals representing the location of the disturbance. The detected and composite values are unbounded, permitting phase displacement to exceed the carrier period and allowing disturbances of variable magnitudes to be located.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present disclosure will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which:

FIGURE 1 is a block diagram illustrating example fiber optic and analog components of a ranging Michelson interfero metric sensor with compound termination in accordance with an example embodiment of the present disclosure; FIGURE 2 is a graph of interferometric response as a function of time illustrating example inward and outward bound phase response profiles corresponding to an example disturbance at a specific location along the ranging Michelson interferometric sensor with compound termination of Figure 1 ; FIGURE 3 is a graph of example inward and outward bound phase response profiles for illustration of operation of a "ratio of areas" technique for determining the time delay between such profiles obtained using the ranging Michelson interferometric sensor with compound termination of Figure 1 ; FIGURE 4 is a block diagram of an example embodiment of the ranging Michelson interferometric sensor with compound termination of Figure 1, configured to use Modulation with Coherence Length Multiplexing (MCLM) employing first and second harmonic terms; FIGURE 5 is a block diagram of an example embodiment of the ranging Michelson interferometric sensor with compound termination configured to use MCLM with first harmonic terms and orthogonal modulation phase;

FIGURE 6 is a block diagram of an example embodiment of the ranging Michelson interferometric sensor with compound termination configured to use Wavelength Division Multiplexing (WDM);

FIGURE 7 is a block diagram of an example embodiment of the ranging Michelson interferometric sensor with compound termination configured to use Time Domain Multiplexing (TDM); and

FIGURE 8 is a flow chart showing example actions to be performed by a processor in the ranging Michelson interferometric sensor with compound termination of Figure 1.

DESCRIPTION

The present disclosure provides a plurality of example embodiments of a ranging Michelson interferometric sensor having two single mode waveguides, such as optical fibers, acting as a sense arm and a reference arm. Both fibers are connected to an interferometric splitter/combiner at a first end. Each of the fibers is terminated at a second end by a compound termination. Each compound termination consists of two FRMs coupled to the corresponding fiber, one by a delay line. The use of FRMs in the compound terminations ensures that the sensor is substantially devoid of any polarization fading effects.

A detection zone for the sensor is defined to extend substantially between the interferometric splitter/combiner and the compound terminations. The interferometric splitter/combiner may be connected to at least one signal source, such as a laser source, and to a processing module by a lead- in cable that permits both the signal source and the processing module to be housed indoors, so that even if the sensor is employed as an outdoor perimeter security sensor, the only outdoor components are relatively hardy fiber optic cables and passive components. Accordingly, the sensor is substantially immune to EMI and F interference conditions. Moreover, the simplicity of the structure of the disclosed appa ratus may result in a lower cost implementation and reduced installation and ma intenance costs. A portion of a signal, such as light from the signal source, is fed into each of the sense a nd reference arms of the interferometer by the interferometric splitter/combiner, propagates along each of the sense and reference fibers connected thereto, until reflected by the FRMs in the corresponding compound termination, back along the sense and reference arm, as the case may be, to the interferometric splitter/combiner where they are recombined. The combined signal propagates back to at least one optical detector in the processing module along the lead-in cable. The optical detector measures the intensity of the combined response, which is a measure of the relative phase of the light signals returned to the interferometric splitter/combiner on the sense and reference arms.

When the sensor is disturbed within the detection zone, the length of the sense arm is changed relative to the reference arm at the point of disturbance. This change in relative length affects the optical signal propagating past the point of the disturbance in both directions in a substantially identical manner. The optical signal propagating past the point of the disturbance toward the interferometric splitter/combiner is designated the inward bound signal ( X ) while the optical signal propagating past the point of disturbance away from the interferometric splitter/combiner is designated the outward bound sig nal ( F ). The change due to the disturbance will be detected in the inward bound signal before it will be detected in the outward bound signal since the outward bound signal travels to the compound termination, is reflected by both FRMs and traverses the entire detection zone on its way back to the interferometric splitter/combiner. Thus, measurement of the time delay between the change to the inward bound signal response profile ( X ) and the change to the outward bound signal response profile ( K ) is indicative of the location of the disturbance along the detection zone. If the disturbance is located adjacent to the compound terminations, the time delay will be at a minimum, If the disturbance is located adjacent to the interferometric splitter/combiner, the time delay will be at a maximum . Knowledge of the speed of light through the fiber medium and the length of the detection zone permits calculation of the location of the disturbance along the detection zone from the measured ti me delay.

The structure of the compound termination facilitates the separation, and independent determ ination, of inward ( X ) and outward [ Y ) bound phase response profiles. Those FRMs in each of the compound terminations 150 that are connected to the corresponding arm of the interferometer with minimal delay comprise a first FRM or a first pair of FRMs 153, hereinafter "first ( pair)" and those FRMs in each of the compound terminations 150 that are connected to the corresponding arm of the interferometer through the delay lines comprise a second pair of FRMs 154. Thus the inward bound response profiles ( XI , X2 ) corresponding to each of the first (pair) and second pair of FRMs will arrive at the interferometric splitter/combiner at substantially the same time, while the outward bound response profile ( Yl ) corresponding to the first (pair of) FRM(s) 153 will arrive at the

interferometric splitter/combiner 130 at a later point, but before the outward bound response profile ( Y2 ) corresponding to the second pair of FRMs 154, by a time equivalent to the ti me for light to traverse twice the delay line length.

The disclosed example embodiments employ multiplexing techniques used in the multiplexing of arrays of interferometers, including without limitation, Modulation with Coherence Length Multiplexing ( MCLM), which is a form of Frequency Domain Multiplexing (FDM), Wavelength Division Multiplexing (WDM) and Time Division Multiplexing (TDM), to differentiate the inward ( X ) and outward ( 7 ) bound response profiles by ta king advantage of the separate outward bound response profiles corresponding to the first (pair of) FRM(s) 153 ( 71 ) and the second pair of FRMs 154 ( 72 ). In an example MCLM embodiment, the output of a single optical signal source is modulated by a continuous wave (CW) tone. The output of an optical detector in the processing module is related to the dimensions of the compound termination, in particular the length of the delay iine. As a result, selecting an optical source such that the optical path difference (OPD) between the first (pair) and second pair of FRMs is short relative to the coherence length of the optical source and long relative to the OPD of the delay line within each compound termination minimizes noise from

disturbances in the delay line. Such disturbances may be further reduced by careful packaging of the compound termination . In one exam ple MCLM embodiment, the first and second harmonic terms of a pseudo-heterodyne process isolates modulated responses from the first (pair) and second pair of FRMs. In another example MCLM embodiment, the first harmonic terms of the pseudo-heterodyne process and orthogonal modulation responses from the first (pair) and second pair of FRMs are employed.

In an example WDM embodiment, the signal sou rce comprises two laser sources operating at respective first and second wavelengths , Within each compound termination, termination couplers, such as wavelength division multiplexers, are employed to direct the first wavelength to the FRM in the first (pair) and the second wavelength to the FRM in the second pair.

Because both wavelengths appear at the interferometric splitter/combiner, utilizing a detector coupler, such as a wavelength division m ultiplexer, at a detector output of the interferometric splitter/combiner permits the interference pattern from the first (pair of) FRM(s), corresponding to the first wavelength, to be channeled to a first detector, independent of the interference pattern from the second pair of FRMs, corresponding to the second wavelength, which is channeled to a second detector.

In an example TDM embodiment, the signal source is pulse modulated with a duty cycle of less than 50% and with a repetition period that is substantially equal to the delay imposed in the delay line within each compound termination. This configuration ensures that the reflections from the first and second pairs of FRMS are time multiplexed.

The present disclosure will now be described in detail for the purposes of illustration only, in conjunction with certain embodiments shown in the enclosed drawings.

Referring to Figure 1, there is shown a block diagram illustrating example fiber optic and analog components of a ranging Micheison interferometric sensor with compound termination, in accordance with an example embodiment of the present disclosure. The sensor, shown generally at 100, comprises a processing module 110, a lead-in cable 120, an interferometric splitter/combiner 130, a sensor cable 140 and a pair of compound terminations 150.

Processing module 110 is coupled to one end of lead-in cable 120. Lead-in cable 120 is connected at its other end to interferometric splitter/combiner 130. Interferometric splitter/combiner 130 is coupled to sensor cable 140 at a first proximate end. The distal end of each of the sensor cable 140 is coupled to the compound terminations 150.

In some example embodiments, processing module 110 is housed indoors, while interferometric spiitter/combiner 130, sensor cable 140 and compound terminations 150 are housed outdoors. In some example embodiments, interferometric splitter/combiner 130 and sensor cable 140 are fastened to a structure such as a fence defining a detection zone or perimeter. In some example embodiments, the compound terminations 150 are sealed in a watertight enclosure and buried underground. Processing module 110 comprises at least one signal source 111 such as a laser source, at least one optical receiver 113 and a digital signal processor (DSP) 119.

The at least one laser source 111 generates an optical signal that is transmitted along a laser input fiber 121 of lead-in cable 120 to an input 131 of the interferometric splitter/combiner 130. In some example embodiments, the at least one laser source 111 may generate an optical signal having an optical wavelength near 1550 nm. The at least one laser source 111 may, in some example embodiments, comprise an optical isolator (not shown) .

In some example embodiments, a modulation signal may be communicated along a signal line 419 (Figures 4-5) from DSP 119 to the laser source 111 to modulate the optical signal. In some example embodiments, the optical signal is modulated by a continuous wave tone. In some example embodiments, the optical signal may be modulated by a modulator 711 (Figure 7) under control of a modulation signal sent along a signal line 719 from DSP 119. In some example embodiments, the modulator 711 may perform pulse length modulation. In some example embodiments, the modulation signal may have a duty cycle of substantially less than 50%.

Each of the at (east one optical receivers 113 comprises an optical detector 114, a band-pass filter 115 and an analog to digital converter (ADC) 116 coupled in series. The optical detector 114 of each optical receiver 113 is coupled by at least one detector output fiber 122 of lead-in cable 120 to a corresponding detector output 132 of the interferometric splitter/combiner 130. In some example embodiments, a receiver coupler, such as a wavelength division multiplexer 617 (Figure 6), or other splitter/combiner may be interposed between the at least one detector output fiber 122 of lead-in cable 120 and the optical receiver 113 in order to feed a plurality of optical receivers 113. The optical detector 114 detects the amplitude of the optical signals summed in the interferometric splitter/combiner 130 and passes it onto the band-pass filter 115 for processing.

The band-pass filter 115 band li mits the detected phase response profile to reduce noise components and forwards the band-limited profile to the ADC 116. In some example embodiments, the band-pass filter 115 is configured to remove components having a frequency substantially below 100Hz and su bstantially a bove 500kHz.

The ADC 116 converts the analog ba nd-limited phase response profile to digital form and forwards it to the DSP 119 for processing.

Thus, processor 110 includes all of the active circuitry and devices employed by the sensor 100 to drive the sensor cable 140, process the data returned therefrom and to generate an alarm with location information when the sensor cable 140 is disturbed. Lead-in cable 120 comprises at least two waveguides, such as optical fibers 121, 122 and provides data coupling between the processing module 110 and respectively, the input 131 and at least one detector output 132 of the interferometric splitter/combiner 130. Laser input fiber 121 connects the output of laser source 111 to input 131 of the interferometric

splitter/combiner 130. Where a plurality of laser sources 111 are employed (Figure 6), an input coupler, such as a splitter/combiner 618 or a wavelength division multiplexer may be interposed between the optical sources 111 and input fiber 121 to combine the optica l signals generated by the optical sources 111 into a single signal for presentation to input 131 of the interferometric splitter/combiner 130.

The optical detector 114 of each of the optical receivers 113 is coupled by at least one detector output fiber 122 to a corresponding detector output 132 of interferometric splitter/combiner 130 , In some example

embodiments, there is a plurality of detector outputs 132 (Figures 5-7) . In such cases, there is a detector output fiber 122 corresponding to each of the detector outputs 132.

Lead-in cabie 120 permits the processing module 110 to be housed indoors and provides coupling between the processing module 110 and the interferometric splitter/combiner 130, which defines a starting point for the detection zone of the sensor 100.

Interferometric splitter/combiner 130 comprises an input 131 and at least one detector output 132, on a processor side of interferometric

splitter/combiner 130 and a sense terminal 133 and a reference terminal 134 on a sensor side of interferometric splitter/combiner 130. In the example embodiment shown, the interferometric splitter/combiner 130 is a 2x2 coupler with one detector output 132 and one sense terminal 133. In some example embodiments, the interferometric splitter/combiner 130 may be a higher order (3x3 or 4x4 or more) coupler, with respectively two- and three-detector outputs 132, which may be a detector output designated

132, or unused output or terminal, designated 132U, which are terminated in such a way so as to substantially prevent reflections. In some example embodiments, the remaining terminal(s) on the sensor side of

interferometric splitter/combiner 130, designated 132U, are terminated in such a way so as to substantially prevent reflections.

Input 131 is coupled by laser input fiber 121 to the output of laser source 111 and permits the optical signal to be introduced to the interferometric splitter/combiner 130.

At least one detector output 132 is connected by a corresponding detector output fiber 122 to at least one optical detector 114.

The sense terminal 133 and the reference terminal 134 are connected to corresponding waveguides, such as optical sense arm 141and optical reference arml42 of the sensor cable 140 to form respective sense and reference arms of a ichelson interferometer. The optical signal provided at input 131 is split equally between sense terminal 133 and reference terminal 134. Likewise, the optical signals returned along sense arm 141 and reference arm 142 through terminals 133 and 134 are combined and the resultant phase response profile is forwarded along detector output(s) 132 to the processing module 110.

As with all interferometers, where the optical signals are completely in- phase, the resulting response profile is the sum of the combined signals and when completely out-of-phase, the resulting response profile is the difference of the combined signals. The sensor cable 140 comprises at least two waveguides, such as optical fibers, designated the sense arm 141 and the reference arm 142. In some example embodiments, the sense arm 141 and the reference arm 142 are packaged together. In some example embodiments, they are packaged separately. In some example embodiments, sensor cable 140 may comprise at least one additional waveguides, such as one or more optical fiber(s) (not shown) for communication or other purpose unrelated to the present disclosure. The functionality of such additional fiber(s) may be secured by the sense arm 141 and the reference arm 142 as disclosed herein.

The sense arm 141 is coupled at one proximate end to the sense terminal 133. The reference arm 142 is coupled at one end to the reference terminal 134. Each of the sense arm 141 and the reference arm 142 is terminated at a second distal end by a compound termination 150.

The sensor cable 140 comprising the sense arm 141 and the reference arm 142 extending between the interferometric spiitter/combiner 130 and the compound terminations 150 defines the extent of the detection zone for the sensor 100, through which people cutting through or climbing over a fence on which the sense arm 141 or the reference arm 142 or both is mounted, may be detected and located.

Thus, when a person cuts through, or climbs over, the fence, one or both sense arm 141and reference arm 142 of the sensor cable 140 is disturbed. The minute motion of the sensor 100 causes the length of the sense arm 141 to vary relative to the length of the reference arm 142 at the point of the disturbance. While this change in length is very small (typically around 10 μ ιη for a cut and up to 100 μ πι for an intruder climbing the fence), it is large in comparison to the wavelength (typically around 1550 nm) of the optical signal. As a result, the optical signals returning along the sense arm 141 and the reference arm 142 create an interference response at the output of the interferometric splitter/combiner 130.

When modulation is employed, the baseband or harmonic output or both of optical detector(s) 114 may be processed to derive a phase response, which is typically measured in radians. In some example embodiments, a higher order (from 3x3 to 4x4 or more) interferometric splitter/combiner 130 is employed with a plurality of optical detectors 140 to obtain the phase response, with or without the use of modulation. Once the amplitude response(s) at the output of optical detector(s) have been converted to phase, they represent the optical phase angle between the signals on the sense arm 141 and the reference arm 142 inside the interferometric splitter/combiner 130.

For purposes of illustration, the location of the disturbance is indicated, by way of non-limiting illustration only, by a vertical arrow and designated by reference numeral 101.

Each of the compound terminations 150 comprises a termination coupler such as a splitter/combiner 151, a delay line 152 and one or more FRMs 153, 154. For ease of identification, each of the compound terminations 150 will be designated with a suffix denoting whether it is associated with the sense arm 141 ("S") or the reference arm 142 ("R"). Thus, compound termination 150S corresponds to the sense arm 141 and compound termination 150R corresponds to the reference arm 142. Where no suffix is employed, it is understood that either compound termination 150 or both, is being referred to. The suffixes "S" and "R" may also be applied to the components of compound terminations 150. Each of the example embodiments described herein contemplate two compound terminations 150, each having two FRMs 153, 154. In some example embodiments (not shown), one component of the compound terminations 150 could dispense with the first FRM 153, and in some example embodiments, the termination splitter/combiner 151 as well.

Conceivably, in some example embodiments, each compound termination 150 could have first FRM 153 but one of the compound terminations 150 could dispense with the second FRM 154, and in some example

embodiments, the termination splitter/combiner 151 as well. In the present disclosure, input 155S of the termination splitter/combiner 151S is connected to the distal end of sense terminal 133 and input 155R of the termination splitter/combiner 151R is connected to the distal end of reference terminal 134.

A first FRM 153 is coupled, after a first transmission delay, which, in some example embodiments, substantially zero to a terminal 156 of the termination splitter/combiner 151. A second FRM 154 is coupled to a remaining terminal 157 of the termination splitter/combiner 151, but with the delay line 152 having an associated second transmission delay that is greater than the first transmission delay, extending between them. Thus, FRMs 153S and 153R form a first pair of FRMs, respectively designated SI and Rl, and FRMs 154S and 154R form a second pair of FRMs, respectively designated S2 and R2.

Despite their different configuration as described above, each of the FRMs 153, 154 function substantially identically as a phase conjugate mirror by creating a total phase delay of 90° with a change in polarization handedness as it reflects an optical signal incident on it back along the same course, to ensure that when the light arrives back at the interferometric

splitter/combiner 130, it does so with the same polarization as it had when it left the interferometric splitter/combiner 130, irrespective of any polarization changes that may have occurred along the sense arm 141 or the reference arm 142 or both . Computation of Disturbance Location from Inward Bound ( X ) and Outward Bound ( Y Phase Response Profiles

The change induced at the disturbance 101 affects the optical signal propagating past the point of disturbance 101 in both directions. As such, the change in phase to an outward bound signal (defined as propagating from the point of disturbance 101 toward the compound terminations 150 and designated Y ) is substantially identical to that on an inward bound signal (defined as propagating from the point of disturbance 101 toward the interferometric splitter/combiner 130 and designated X ). However, there is a substantial difference in time of arrival of the two changes at the interferometric splitter/combiner 130.

For purposes of illustration, we consider the components of the phase response. The disturbance 101 may be defined as causing a change of phase φ(ί) at the point of disturbance 101, As such, the components of the phase response at the interferometric splitter/combiner 130 may be defined in terms of the sum of components φ(ι - τ) , which represent the phase response corresponding to the traversal of a path along a fiber portion that takes the optical signal a time delay τ to traverse. The value of τ thus depends upon the length of the corresponding path, as weil as the velocity of propagation of light inside the sense arms 141, 142 (typically, the velocity of propagation within a fiber is considered to be 68.13% of the velocity of propagation of light in free space ( c = 2.998 x 10^s m/s)).

Assuming for the purposes of the present discussion that the first pair of FRMs 153S, 153R (collectively referred to as 153) are collocated and that the second pair of FRMs 154S, 154R (collectively referred to as 154) are also collocated but at the end of delay lines 152S, 152R respectively, since there are four FRMs (two in each of the two compound terminations 150) that reflect light in the compound terminations 150, there are four paths to consider, namely: Ρχι: The path taken in the inward bound direction X corresponding to the first pair of FRMs 153; ΡΥΙ'. The path taken in the outward bound direction Y corresponding to the first pair of FR s 153;

P z ' The path taken in the inward bound direction X corresponding to the second pair of FRMs 154; and

The path taken in the outward bound direction Y correspond to the second pair of FRMs 154.

These paths are shown in Figure 1 and a timing d iagram is included with Figure 1 below sensor cable 140 and compound terminations 150, for purposes of illustration . The propagation time for the change in phase in the inward bound signal from the disturbance 101 to reach the interferometric splitter/combiner 130 for the first pair of FRMs 153 and the second pair of FRMs 154 is T_XI = T_X2 = T_x . The propagation time for the change in phase in the outward bound signal from the disturbance 101 to reach the first pair of FRMs 153 is T_y . The propagation time for the change in phase to traverse the delay lines 152 is T_D . Thus, the total time delay inside the sensor cable 140 is r = r_x + 7], .

Moreover, the propagation time for the change In phase in the outward bound signal from the disturbance 101 to reach the interferometric splitter/combiner 130 for the first pai r of FRMs 153 is T_Y] = T + T_Y and the propagation time for the change in phase in the outward bound signal from the disturbance 101 to reach the interferometric splitter/combiner 130 for the second pair of FRMs 154 is T_Y2 = T + T_r + 2T₀ .

Thus, the reflection from the first (pair of) FRM(s) 153 contains two components, namely the change to the inward bound light and the change to the outward bound light, and generates a phase response of: ί) = Φχ ( + η ( (1) Similarly, the reflection from the second pair of FRMs 154 contains two components, namely the change to the inward bound light and the change the outward bound light, and generates a phase response of: φ₂(ί) = φ_χ(ί) + φ_γ1(ί) (2) Where

ΦΛ = φ {ι-τ_χ) (3) φ (/) = φ (t-Ty -T), and (4) φ_ϊ2(ΐ) φ «-Τ_γ ~Τ~2Τ_β) (5)

The output of interferometric splitter/combiner 130 includes both φ{ί) and φ₂(ι). Assuming that these phase responses may be separately determined, they may be manipulated to locate the disturbance 101 along the detection zone. From equations (1) and (2), it follows that the changes in phase to the inward bound and outward bound responses over the time delay period T_D are φ_χ(() and Δ^ ( respectively, where:

These difference responses may be integrated to derive the total change in phase to the inward bound and outward bound responses, designated phase response profiles φ_χ(ί) 210 (Figure 2) and φ_Υ(ί) 220 (Figure 2). Phase is typically measured as the angle of a phasor rotating about the origin of the in-phase (/ ) and quadrature-phase (Q) plane, where the instantaneous phase is the arctangent of the ratio [J/Q]. The frequency of the response may be considered as the rate of change of the phasor rotation radians I sec

{viz, Hz = ) and may in some example embodiments be in the

2n

range of 5-500 kHz for disturbances to the fence.

Thus, the true phase response measured by φ_χ (ι) and φ_γ (ι) may in some example embodiments involve a large number of cycles of the phasor rotation in the I - Q plane and may have a maximum excursion in the range of 5-500 radians with a duration ranging from a few milliseconds to a several seconds, when the phase is tracked through the conventional 2π a m biguity.

Turning now to Figure 2, there are shown example phase response profiles φ_χ (ί) 210 and φ_ϊ (ί) 220 as a function of time (i n ms.) to an example disturbance at a specific location along the detection zone. The vertical axis is expressed in terms of phase angle (either or both of degrees or radians; both are shown) and substantially corresponds to a physical change in relative length of the sense arm 141 and the reference arm 142 at the point of disturbance 101. It may be seen that the inward bound phase response profile φ_χ( 210 and the outward bound phase response profile φ_γ {ι) 220 are substantially identical in shape but the outward bound phase response profile φ_γ ί) 220 is delayed in time relative to the inward bound phase response profile φ_χ {ί) 210. The time delay T_May between the two response profiles is directly proportional to the distance of the disturbance 101 from the compound terminations 150 and is a maximum of 2T when the disturbance 101 is located at the interfero metric splitter/combiner 130 and a minimum of zero when disturbance 101 is at the com pound terminations 150.

For a sensor cable 140 of length L , the location of the disturbance 101 relative to the interferometric splitter/combiner 130 is therefore provided by:

T,

Location = L (8)

2T While conceptually, the determination of the location of the disturbance 101 may be thought of as an analog process, in some example embodiments, such as that shown by non-limiting illustration in Figure 1, the computation may be performed digitally. Th us, the output of the at least one optical detector 114 may be passed through band-pass filter 115 to ADC 116, resulting in sequences of sampled data for processing by DSP 119.

Measurement of T_May

There are a number of ways to measure the time delay between two substantially identical waveforms such as T_May between inward bound phase response profile φ_χ (ί) 210 a nd outwa rd bound phase response profile φ_γ (ί) 220. These include, without limitation, direct time delay measurement between the two waveforms on a point by point basis, performing a least squares fit in a cross-correlation process and employing the ratio of areas.

While each of these mechanisms works satisfactorily, in some example embodiments, the ratio of areas technique, illustrated by way of non-limiting example in Figure 3, may provide an accurate result with minimal com putation, especially in light of the possibility of noisy data.

Trace 310 represents a first waveform, which, by way of non-limiting example in the scenario under consideration, may be the inward bound phase response profile φ_χ (ί) 210. Trace 320 represents a second waveform, which, by way of non-limiting example in the scenario under consideration, may be the outward bound phase response profile φ_γ (ί) 220. Trace 320 represents the first waveform, delayed in time, by an a mount that, by way of non-limiting example in the scenario under consideration, may be 27^" . By design, trace 320 lies between traces 310 and 330. A sampling speed is selected such that the selected time delay 2T is an integer number N sample periods.

Under the ratio of areas technique, two areas, A\ 340 and A2 350, corresponding respectively to the difference between traces 310 and 320, and between traces 320 and 330, are computed. By way of non-limiting example, in the scenario under consideration :

From these areas, the time delay may be calculated as the ratio of the first area A\ 340 to the sum of the areas Al 340 and A2 350 :

Reduction of False Alarm Rate

The number of false positive alarms (identified detections that do not correspond to an intruder) may in some example embodiments be reduced by establishing rules, incl uding without limitation, stipulating that a predetermined number (by way of non-limiting example, 2-5) of events be declared within a given time window.

In some exa mple embodiments, it may be further stipulated that the predetermined number of events be substantially at or near a common location to be counted .

In some example embodiments, stipulating such rules may result in a significant reduction in the false alarm rate of the sensor 100.

Demultiplexing Phase Responses from each Compound Termination

The foregoing analysis assumes that the phase responses ^, (ί) and φ₂ (ί) corresponding to each compound termination 150 may be separately determined. In some example embodiments, therefore, certain multiplexing schemes are implemented to facilitate the separate determination of these phase responses. These include, without limitation, the following : • Modulation with Coherence Length Multiplexing (MCLM ) employing first and second harmonic terms;

• Modulation with Coherence Length Multiplexing (MCLM ) employing first harmonic terms and orthogonal modulation phase; · Wavelength Division Multiplexing (WDM ) ; a nd

• Time Division Multiplexing (TDM) .

The separation of these responses, which a re presented to the detection output(s) 132 of the interferometric splitter/combiner 130 in combined form, may thus be thought of as a demultiplexing problem. Exa mple embodiments corresponding to each of these multiplexing schemes and the analysis leading to the separation of the phase responses (/) and φ_τ {ί) corresponding to each compound termination 150 will now be described. It should be noted that the analysis described therein is lengthy but relatively straightforward, making use of well-known trigonometric and other mathematical principles. In some instances, only certain results are set out, with the understanding that those skilled in the art will be able to discern the intermediate results skipped over for the sa ke of brevity.

Modulation with Coherence Length Multiplexing (MCLM) employing first and second harmonic terms

Referring now to Figure 4, there is shown a block diagram of an example embodiment of the sensor 100, configured to use Modulation with

Coherence Length M ultiplexing (MCLM) employing first and second ha rmonic terms.

Figure 4 differs from Figure 1 in part because the current supplied to the laser source 111 is modulated at a radian frequency ω_ιη under control of the

DSP 119 along signal line 419. Thus, the momentary frequency ω(ή at the input to the interferometric splitter/combiner 130 is : ω{1) = ω₀ + Δ<υ cos(&>„, /) . (12)

A number of path length dimensional parameters are shown with Figure 4 below sensor cable 140 and compound terminations 150, for purposes of illustration . The path lengths refer to the length of the fiber optic path between the interferometric splitter/combiner 130 to each of the four FRMs in the compound terminations 150 :

Lsi : The length of the fiber optic path between the interferometric splitter/combiner 130 and FRM SI 153S;

L_S2 : The length of the fiber optic path between the interferometric splitter/combiner 130 and FRM S2 154S;

LRI : The length of the fiber optic path between the interferometric splitter/combiner 130 and FRM Rl 153R; and

LR2 : The length of the fiber optic path between the interferometric splitter/combiner 130 and FRM R2 154R.

Knowing the propagation velocity of light within the fibers gives rise to corresponding propagation times T_S1, T_S2, T_Ri and T_R2.

The phase angles of the four reflected signals from the four FRMs (SI 153S, S2 154S, Rl 153R and R2 154R) at the interferometric splitter/combiner 130 may be expressed as:

Solving these integrals one finds : ,(/)=<¾ T_sl + [sin{¾ (/ + r_vl)}-sin ] , ( 14) ,( = ώ¾ Γ_¥2 + [sin{i¾ (⁽ + T_S2)}-sm(0,„ t) ] , (15)

[^sinR, (' ^+Tm)}-^Sm' (e⁾,,, ], and (16)

^«2 (0 = ^ωο ^Tm +— [^sinR, (/ + T_l(2 ) } - sm(0)_m I) J . (17)

CO...

Thus, the combined signal E(t) at interferometric splitter/combiner 130 is:

where _,/ = .

The intensity of light In(f) at the output of interferometer splitter/combiner 130 as measured by the optical detector 114 is:

and hence:

In ) =

Of the resulting 16 terms in this product, there are four like terms ((SI 153S, SI 153S), (S2154S, 52 154S), (Rl 153R, Rl 153R) and (R2 154R, R2 154R)) for which the arguments cancel, resulting in a constant factor of 4. This leaves twelve terms, which can be paired to generate six cosine terms, representing the six possible interferometric responses:

In_t ({ ) = 2 cos {ft)₀ (T_s I - T_R ,

T ¹ si - T n\ T_xl +r_m

- 2 sin^T., - T_m )}sin J sin O, COS co A l + Y (21)

(22)

(23)

Jn_n (/)

(24)

,(/)

and

(25)

In_n(t) = 2 cos{o)₀(r_;<, -7',₍₂) -2sin{iy₀(7_;i -T,_t2

(26) Of these six terms, the first, /«,(/) relates to the response from the first pair of FRMs (SI 153S, Rl 153R) and the second, In₂(t) relates to the response from the second pair of FRMs (S2154S, R2154R).

These are the terms that define the phase responses and φ₂(ή , where

-T_Kl ) due to chang es in the length Lsi relative to the length L_R1, and phase response φ₂ (() comes from changes in ω₀(Τ_Χ2 -T_R2) due to changes in the length L_S2 relative to the length L_R2.

Equations (21)-(26) for the six components of the intensity have the same general form, namely an optical phase term multiplied by a term of the form cos[ cos(¾ /+ 7^')] or sin[Ccos(i¾, t + T)] , where the constant C is in fact a function :

The terms cos[Ccos(<¾, / - 7^')] and sin[Ccos(<¾, /+ T)] are Generating

Functions with associated Bessel Function Series of integer order. Since the function is not a function of time, it can be viewed as a constant that depends upon the difference in propagation time between the two FRMs 153, 154 associated with the particular intensity term in question.

The third /«₁₂(/) and fourth //¾, (/) terms of the six relate to the interference of the reflections from (SI 153S, R2154R) and (S2154S, Rl 153R). These terms may be m gths L_Si, L_Si, L_Si and l_si such that the terms are integer multiples

of 2π radians.

As a result, the intensity at the detector output 132 of the interferometric splitter/combiner 130 comprises only the first two /«, (/) , /¾(/) and the last two In (() , In₂₂(t) terms. These terms are rich in harmonics due to the modulation of the laser source 111 by ω_ω. The first and second harmonic terms may be expressed in terms of the Bessel Functions Series of the first J,(z) and second J₂(z) orders. The first harmonic terms are:

(28)

(29)

and

(30)

where a "bar" is placed over the component label to indicate the first harmonic term, which represents the pseudo-heterodyne response about the modulation frequency co_m.

The second harmonic terms are:

/«,(/) = 4 cos {^( } J .∞*{2ω„1 + ω„{Τ₈ +Τη)}, (32) co„,

In₂(l) = 4cos{ ₂(/)} J A sin co_n -cos{2 + „,(Ts₂ +T_K2)}, (33) a>...

In_n(t) = 4 - s2

cos{<z$_n( } J₂ j—— sini co„. ^>∞s{2a> + <a_m(T_Si +T_S2)},

co„

(34) and

2&ω Τ,.

In_n (t) = 4 cos{<t>₂₂ (t) } sin a, COS {lcoj + co^ + T^ )} , (35)

2 J where a "double bar" is placed over the com ponent label to indicate the second harmonic term, which represents the pseudo-heterodyne response about the modulation frequency 2a>_m .

The terms 7«, (r) and 7/7, (7) , as defined in equations ( 28) and (32)_ respectively, relate respectively to the first and second harmonic responses to a disturbance 101 along the sensor cable 140 coming from the first pair of FRMs (SI 153S, Rl 153R). The terms In₂ (t) and In₂ ( ) , as defined in equations (29) and (33)_ respectively, relate respectively to the first and second harmonic responses to a disturbance 101 along the sensor cable 140 coming from the second pair of FRMs (S2 154S, R2 154R) .

The terms In_n (i) and In {t) , as defined in equations (30) and (34)_ respectively, relate respectively to the first and second harmonic responses to a disturbance of the delay line 157S inside compound termination 150S terminating sense arm 141 coming from FRMs (S 1153S, S2 154S) . These terms do not respond to disturbances 101 of sensor cable 140.

The terms In₂₂ (i) and Λ¾₂ ( , as defined in equations (31 ) and (35)_ respectively, relate respectively to the first and second harmonic responses to a disturbance of the delay line 157R inside compound termination 150R terminating reference arm 142 coming from FRMs (R1153 R, R2 154R) . These terms do not respond to disturbances 101 of sensor cable 140.

The first harmonic response to disturbance 101 on sensor cable 140 at the detector output 132 of optical detector 114 is : Q(t)=In_i(l) + In₂(() , (36) which is equivalent to:

2(0 -sin{&( }]cosH,/ + 0} (37)

This pseudo-heterodyned response is shifted to baseband by mixing with cos{ty,_Hr + 0} and low-pass filtered to remove the upper cross-products of the mixing process. Thus, we find:

The second harmonic response to disturbance 101 on sensor cable 140 at the detector output 132 of optical detector 114 is: I(t)=ln,(t) + In₂(t)_f (39) which is equivalent to:

2Δω

/(,) = 4 j₂ [cos^ (0} + cos{ (t)}]cos{2a>

(40)

This pseudo-heterodyned response Is shifted to baseband by mixing with cos{2oj + 2Θ} and low-pass filtered to remove the upper cross-products of the mixing process. Thus, we find:

Adjusting the modulation depth Δω, so that J,{c} =J₂{c} and defining φ_ί (i) = 2tan 00) , and (42)

1( . i¾(i) = 2cos -1 y/(o² ÷g(o² (43)

it follows that:

4⁽ = , and (44)

2

&( (45)

2 The responses ,(ί) and <^₂(/) may then be used to compute the inward φ_χ (ή 210 and outward bound responses φ_γ (ι) 220, and ultimately, determine the location of the disturbance 101, as described above in non- limiting fashion.

As discussed above, the "in-termination" terms In_n{t) , /«, , (/) , In₂₂(t) , and In₂₂(t) as defined in equations (30), (34), (31) and (35) respectively, do not respond to disturbance 101 along sensor cable 140. However, they do contribute to the noise in performing the computation of (/) and <f>₂(t) .

Hence, in some example embodiments, the compound terminations 150 may be packaged so as to dampen any phase angle responses to effects such as thermal changes or mechanical motion. This may be achieved in some example embodiments by potting the compound terminations 150 and burying them beside the fence so that the responses are dampened and have frequency content below the response to a disturbance 101 along sensor cable 140. Even so, additional noise may be present in some example embodiments, if the cancellation of the two "cross" terms ln_n {t) and In_2] (t) , as defined in equations (23) and (24) are respectively not perfect. In both cases, the noise may, in some example embodiments, be further reduced by selecting a laser source 111 having a coherence length that is much greater than the optica! path length differences (OPD) associated with the first pair of FRMs 153 and second pair of FRMs 154, and much less than the length of delay lines 152. In this way, the responses from the residual "cross" terms In (t) and In₂ (() , as defined in equations (23) and (24) respectively, and the "in-termination" terms In (t) , In_u t) , ¾ (/) , and

/¾ (*) , as defined in eq uations (30), (34), (31) and (35) respectively, may be substantially reduced in a manner such as is known for coherence length multiplexed fiber optic interferometers.

Assuming a modulation frequency of 2 M Hz, and a velocity of propagation of light in the fiber of 68.13% of the velocity of propagation of light in free space, example dimensions associated with an MCLM embodiment employing first and second harmonic terms may be : L_Si = L meters,

L_R1 =L+ 1 meters,

meters, and

L_R2=L+2,043.9 meters, where L is the length of sensor cable 140. This example configuration provides a i m optical path difference in the first pair of FRMs 153 and in the second pair of FRMs 154 and in excess of 2 km in the delay line 157. With this in mind, a laser source 111 having an example coherence length of 100 m would thus provide noise reduction through Coherence Length M ultiplexing. Modulation with Coherence Length Multiplexing (MCLM) employing first harmonic terms and orthogonal modulation phase

Referring now to Figure 5, there is shown a block diagram of an example embodiment of the ranging Michelson interferometric sensor with compound termination of Figure 1, configured to use MCLM with first harmonic terms and orthogonal modulation phase. Conceptually, this embodiment differs from the embodiment shown in Figure 4 in that only the first harmonic term is used and orthogonal modulation phase is used to separate ^, (7) from φ₂ ( · Structurally, Figure 5 differs from Figure 1 in a number of respects. First, the current supplied to the laser source 111 is modulated at a radian frequency ω_ηι under control of the digital signal processor 119 along signal line 419, such as is described in respect of Figure 4.

Second, the iead-in cable is a three-fiber cable comprising laser input fiber 121, coupling laser source 111 to input 131 of interferometric

splitter/combiner 530 and detector output fibers 122A and 122B,

respectively coupling detector outputs, designated 132A and 132B of interferometric splitter/combiner 530, to optical receivers 113A and 113B respectively. Third, the interferometric splitter/combiner 530 is a higher order (by way of non-limiting example 3x3) coupler as opposed to a 2x2 coupler, comprising input 131, detector outputs 132A and 132B, sense terminal 133, reference terminal 134 and unused terminal 133U. As before, input 131 is coupled to laser input fiber 121, sense terminal 133 is coupled to sense arm 141 and reference terminal 134 is coupled to reference arm 142. Unused terminal 133U is terminated in such a way so as to substantially prevent reflections. Detector outputs 132A and 132B are respectively coupled to fibers 122A and 122B.

Fourth, there are two optical receivers, respectively designated 113A and 113B. Each of the optical receivers 113 comprise a corresponding optical detector 114, band-pass filter 115 and ADC 116 is coupled in series, with the ADC 116 coupled to DSP 119. Each of optical detectors 114A and

114B are connected to a corresponding detector output fiber, respectively designated 122A and 122B of lead-in cable 420. In the example embodiment of Figure 5, path lengths L_sl , L _{7 I} L_m and L

T ~T

are selected such that ω \——— is an integer multiple of 2π radians, for

(τ ~T Λ

example, in non-limiting fashion, 2k π . Further, ω„, — — is set equal

V 2 J

to 2 (k + \)π, thereby eliminating the "cross" terms In_n{t) and /«₂₁(/), as defined in equations (23) and (24).

Moreover, path lengths L_s , L , L_m and L_ll2 are adjusted such that:

T_sl -T_t r i,n

-ω (46)

2

This results in

Under these conditions, the output of optical detector 114A is:

and

demonstrating that the modulation on InA_t(t) is orthogonal to the modulation on InA₂(t) The replacement of the 2x2 interferometric splitter/combiner 130 with a 3x3 interferometric splitter/combiner 530 provides a nominal optical phase displacement of Ί π Ι Ί radians ( 120°) between outputs a long the laser input fiber 121, and detector output fibers 122A and 122B of lead-in cable 420, which also correspond to input 131 and detector outputs 132A and 132B of interferometric splitter/combiner 530 as described above. Accordingly, the output of optical detector 114B is :

and

From the foregoing, the combined first harmonic signal output of optical detector 114A for a disturbance 101 occurring along sensor cable 140 is

InA_\ (t) + InA₂{l) .

Mixing this with cosjoj,,,/ + θ) and sin{iy„/ + #} and low-pass filtering the result

(to remove the upper cross products of the mixing response), results in the following baseband phase term responses for the first pair of FRMs 153 and the second pair of FRMs 154 respectively :

and

(53)

Similarly, the combined first harmonic signal output of optical detector 114B for the same disturbance 101 occurring along sensor cable 140 is

InB, {i) + InB₂ {l) .

Mixing this with co${ o_mi + e\ and ήη{ω_ιηί + θ} and low-pass filtering the result (to remove the upper cross products of the mixing response), results in the following baseband phase term responses for the first pair of FRMs 153 and the second pair of FRMs 154 respectively:

and

The phase terms A (f) , A₂(() , B_t (t) and B₂ (t) may be converted to In-phase ( / ) and Quadrature-phase ( Q ) phase terms by applying the fol lowing :

(56)

and

(57)

where γ = 2 π /3 radians.

In some example embodiments, the phasor responses from the first pair of FRMs 153 and from the second pair of FRMs 154 may be obtained directly (as opposed to applying Equations (56) and (57)) by employing a 4x4 splitter/combiner (not shown) in place of the 3x3 splitter/combiner 530. With a 4x4 splitter/combiner (not shown), the nominal phase angle between ports is π 12 (90°), but because there are two unused detector outputs 132U rather than only one, such computational simplification comes at the cost of slight signal loss.

However obtained, the responses φ^ή and φ₂ (() may then be computed from :

and

(59)

7₂( from which the inward bound response φ_χ (ΐ) 210, and outward bound responses φ_γ {ΐ) 220, may be computed as described above, and ultimately the iocation of the disturbance 101 may be determined, also as described above in non-limiting fashion.

As discussed above, the "in-termination" terms /«,,(/) , In (l) , ln₂₂(t) and

In₂₂(t) , as defined in equations (30), (34), (31) and (35) respectively, do not respond to disturbance 101 on sensor cable 140. However, they do contribute to the noise in performing the computation of φ (ί) and φ₂( · Hence, in some example embodiments, the compound terminations 150 may be packaged so as to dampen any phase angle responses to effects, such as thermal changes or mechanical motion. This may be achieved in some example embodiments by potting the compound terminations 150 and burying them beside the fence so that the responses are dampened and have frequency content below the response to a disturbance 101 along the sensor cable 140. Even so, additional noise may be present in some example embodiments, if the cancellation of the two "cross" terms In_n (() and In_2] (t) , as defined in equations (23) and (24) respectively, is not perfect.

In both cases, the noise may, in some example embodiments, be further reduced by selecting a laser source 111 having a coherence length that is much greater than the optical path differences (OPD) associated with the first pair and second pair of FRMs 153, 154 respectively, and much less than the length of delay lines 157. In this way, the responses from the residual "cross" terms In_n {t) and In_2] (t) , as defined in equations (23) and

(24) respectively, and the "in-termination" terms In_u (t) , In_u {t) , In₂₂ {t) and Λ¾( , as defined in equations (30), (34), (31) and (35) respectively, may be sig nificantly reduced in the same manner such as is known for coherence length multiplexed fiber optic interferometers.

Assuming a modulation frequency of 4 MHz and a velocity of propagation of light in the fi ber of 68.13% of the velocity of propagation of light in free space, exa mple dimensions associated with the described MCLM embodiment employing first harmonic terms and orthogonal modulation phase may be:

Lsi = L m ;

L_S2 = L + 2,056.77 m ; and L_R2 = L + 2,043.9 m .

This example configuration provides a 12.77 m optical path difference in the first pair of FRMs 153 and in the second pair of FRMs 154 and in excess of 2 km in the delay line 157. With this in mind, a laser source 111 having a coherence length of 100 m would provide further noise reduction through Coherence Length Multiplexing as discussed above. Wavelength Division Multiplexing (WDM )

Referring now to Figure 6, there is shown a block diagram of an example embodiment of the sensor 100, configured to use Wavelength Division M ultiplexing (WDM). Figure 6 differs from Figure 1 in a number of respects. First, a plurality of laser sou rces 111a, 111b are employed. This is facilitated by interposing an input coupler such as a splitter/combiner 618 or wavelength division multiplexer between the laser sources 111a, 111b and laser input fiber 121 of lead-in cable 520. Each of the laser sources 111a, 111b have a different optical wavelength, designated X_a. and X_h respectively, with optical radian frequencies designated ω_α and a>_h respectively. Each of the laser sources

111a, 111b are coupled by corresponding fibers 611a, 611b to an input, respectively designated 618a, 618b of input splitter/combiner 618. Input sp!itter/combiner 618 has two arms, one of which, designated 618u, is unused and terminated in such a way so as to substantially prevent reflections and the other of which, designated 618d, is coupled to laser input fiber 121. This causes the two optical wavelengths X_a. and X_h to be added together and presented to the input 131 of the interfero metric splitter/combiner 130 so that the combined optical signal may be split into two equal intensity signals for propagation along sense arm 141 and reference arm 142 respectively, such that each of the signals propagating a long sense arm 141 and reference arm 142 have components of both optical radian frequencies ω_α and <¾ .

Second, the compound terminations 650 are modified in that the

termination splitter/combiner 151 is repiaced by a termination coupler in the form of a wavelength division multiplexer 651. The input of the termination wavelength division multiplexer 651 is designated 655, is coupled to the dista l end of a fiber of the sensor cable 140 (in the case of termination wavelength division multiplexer 651S, sense arm 141 and in the case of termination wavelength division multiplexer 651R, reference arm 142). Output 656 of the termination wavelength division multiplexer 651 is coupled to FRM 153 and output 657 of the termination wavelength division multiplexer 651 is coupled to FRM 154 through delay line 152. This causes wavelength ω_α to be directed only to FRM 153 and causes wavelength <¾ to be directed only to FRM 154 (in some example embodiments, the wavelengths may be reversed, so long as the wavelength / FRM allocation is consistent between both compound terminations 650S, 650R). When the single wavelength signal is reflected by an FRM 153, 154, it is combined with the signal of the other wavelength by the corresponding termination wavelength division multiplexer 651 such that each of the signals propagating back along sensor cable 140 again have components of both optical radian frequencies ω_α and co_h .

Third, lead-in cable 520 comprises a three-fiber cable, such as is described in connection with Figure 5.

Fourth, the interferometric splitter/combiner 530 is a higher order (by way of non-limiting exa mple 3x3) splitter/combiner such as is described in connection with Figure 5.

Fifth, there are four optical receivers, respectively designated 113Aa, 113Ab, 113Ba and 113Bb. Each of the optical receivers 113 comprise a corresponding optical detector 114, band-pass filter 115 and ADC 116 coupled in series, with the ADC 116 coupled to DSP 119. Each of optical detectors 114Aa and 114Ab are coupled, respectively by fibers 622Aa, 622Ab to a corresponding output of a receiver in the form of a wavelength division multiplexer 617A and each of optical detectors 114Ba and 114Bb a re coupled, respectively by fibers 622Ba, 622Bb to a corresponding output of a receiver coupler in the form of a wavelength division multiplexer 617B. The input, to receiver wavelength division multiplexer 617 is coupled to a corresponding detector output fiber, respectively designated 122A and

122B of lead-in cable 520. This causes wavelength ω_α to be directed only to one fiber (622Aa or 622Ba as the case may be) and ca uses wavelength co_b to be directed only to one fiber (622Ab or 622Bb as the case may be). Thus, the example WDM embodiment illustrated by Figure 6 differs from the example MCLM approaches illustrated by Figures 4 and 5 in that it employs two laser sources 111a, 111b, an input splitter/combiner 618 and a number of wavelength division multiplexers 651S, 651R, 617A and 617B to separate the ^, (/) and φ₂ (() responses instead of employing modulation.

The outputs of ADCs 116Aa and 116Ba represent the response along detector output fibers 122A and 122B, respectively, of the reflections from the first pair of FRMs 153 and the outputs of ADCs 116Ab and 116Bb represent the response along detector output fibers 122A and 122B, respectively, of the reflections from the second pair of FRMs 154.

Since the optical phase of the response along detector output fibers 122A and 122B, corresponding to detector outputs 132A and 132B respectively of the 3x3 interferometric splitter/combiner 530, is nominally 2π / 3 radians (120°) out of phase, the phasor responses from the first pair of FRMs 153 and from the second pair of FRMs 154 may be computed from equations (56) and (57).

In some example embodiments, the phasor responses from the first pair of FRMs 153 and from the second pair of FRMs 154 may be obtained directly (as opposed to applying Equations (56) and (57)) by employing a 4x4 splitter/combiner (not shown) in place of the 3x3 splitter/combiner 530.

With a 4x4 splitter/combiner (not shown), the nominal phase angle between ports is n il (90°), but because there are two unused detector outputs

132U rather than only one, such computational simplification comes at the cost of slight signal loss. However obtained, the responses φ {ί) and φ₂(ι) may then be used to compute the inward and outward bound responses φ_χ (ι) 210 and φ_γ (ί) 220, and ultimately, determine the location of the disturbance 101, as described above in non-limiting fashion. However, in processing the phase data, the phase responses may be adjusted to a constant angular basis. This may be accomplished by multiplying one of the phase responses (by way of non-limiting example, that corresponding to co_a) by the appropriate ratio (in the non-limiting example, by — ) .

In the WDM embodiment described in Figure 6, there are no "cross" terms In_l2(t) and In^(t) , as defined in equations (23) and (24) respectively, or "in- termination " terms In_u(i), In_n(t) , In₂₂(t) and /¾(/) , as defined in equations (30), (34), (31) and (35) respectively, since the laser sources 111a and 111b are separated in wavelength. As a result, there are substantially no constraints imposed on the dimensions L_s L_S2, L_R and L_R2.

In some exa mple embodiments, it may be beneficial to make L_Si-L_m and

L_s2 =L_in, in order to minimize effects of phase noise from the laser sources

111a, 111b. In some example embodiments, a delay line 152 length of L_S2-L_sl -L_Rl~L_m = 2,000 m has been found to work well.

In some example embodiments, the selection of wavelengths used for laser sources 111a, 111b is the result of a design decision taking into account a cost / performance trade-off. In some example embodiments, lasers 111a, lllb having widely spaced waveiengths, such as, by way of non-limiting example, «f 1310 and 1550 nm may be employed. In this scenario, dual band splitter/combiners 618, 651S, 651R, 622A and 622B and dual band FRMs 153S, 153R, 154S and 154R will be employed.

In some example embodiments, cost may be reduced by employing wavelength division multiplexer couplers 618, 651S, 651R, 622A and

622B and FRMs 153S, 153R, 154S and 154R by employing laser sources 111a, lllb having closely spaced wavelengths of 1540 and 1560 nm corresponding to Dense WDM ( DWDM ) technologies, which may be accom modated by such com ponents. Time Division Multiplexing (TDM)

Referring now to Figure 7, there is shown a block diagram of an example embodiment of the sensor 100, configured to use Time Domain Multiplexing (TDM) . Figure 7 differs from Figure 1 in a number of respects. First, the single laser source 111 is coupled to a modulator 711 under control of a modulation signal from DSP 119 along signal line 719 to generate a pulse- modulated optical signal. The modulation signal is configured to ensure that the duty cycle of the pulse modulation is less than 50% and the pulse modulation period is set to be substantially equal to an integer divisor of the time delay corresponding to delay line 520. In some example embodiments, a duty cycle of substantially 40% may be applied .

Second, lead-in cable 520 comprises a three-fiber cable, such as is described in connection with Figure 5. Third, the interferometric splitter/combiner 530 is a higher order (by way of non-limiting example 3x3) splitter/combiner such as is described in connection with Figure 5.

Fourth, there are two optical receivers, respectively designated 113A and 113B. Each of the optical receivers 113 comprise a corresponding optical detector 114, band-pass filter 115 and ADC 116 coupled in series, with the ADC 116 coupled to DSP 119. Each of optical detectors 114A and 114B are coupled to a corresponding detector output fiber, respectively designated 122A and 122B of lead-in cable 520.

Since the pulse modulation period is substantially equal to an integer divisor of the time delay corresponding to delay line 520, the modulation pulse applied to the optical signal will not be simultaneously seen by the second pair of FRMs 154, which is coupled to the sensor cable 140 by delay line 152 and by the first pair of FRMs 153, which is not so coupled. Thus, when the pulse modulation optical signal is reflected by the FRMs 153, 154 and returned along the sensor cable 140, the pulses reflected by the first pair of FRMs 153 are time-multiplexed with the pulses reflected by the second pair of FRMs 154, as illustrated in non-limiting format below the compound terminations 150 on Figure 7 (in respect of compound termination 150R only).

Accordingly the interference pattern between the signal arriving on the sense arm 141 and between the signal arriving on the reference arm 142 alternates between that corresponding to the first pair of FRMs 153 and the second pair of FRMs 154.

The outputs of ADCs 116A and 116B represent the response along detector output fibers 122A and 122B, respectively, of the reflections from the first pair of FRMs 153 and the reflectors from the second pair of FRMs 154.

Because the pulse modulation is controlled by, and thus known to, DSP 119, the response corresponding to the reflections from the first pair of FRMs 153 may be distinguished from the response corresponding to the reflections from the second pair of FRMs 154.

Since the optical phase of the response along detector output fibers 122A and 122B, corresponding to detector outputs 132A and 132B respectively of the 3x3 interferometric splitter/combiner 530, is nominally 2π Ι > radians (120°) out of phase, the phasor responses from the first pair of FRMs 153 and from the second pair of FRMs 154 may be computed from equations (56) and (57).

In some example embodiments, the phasor responses from the first pair of FRMs 153 and from the second pair of FRMs 154 may be obtained directly (as opposed to applying Equations (56) and (57)) by employing a 4x4 splitter/combiner (not shown) in place of the 3x3 splitter/combiner 530. With a 4x4 splitter/combiner (not shown), the nominal phase angle between ports \s n i l (90°), but because there are two unused detector outputs 132U rather than only one, such computational simplification comes at the cost of slight signal loss.

In some example embodiments, the phasor response from the first pair of F s 153 and from the second pair of FR s 154 may be derived from the first and second harmonics at the detector output though the use of a tone modulation.

However obtained, the responses and φ₂(ΐ) may then be used to compute the inward and outward bound responses φ_χ (ί) 210 and φ_γ {ι)

220, and ultimately, determine the location of the disturbance 101, as described above in non-limiting fashion.

In the TDM embodiment described in Figure 7, there are no "cross" //¾(/) and 1η_2λ {ί) , as defined in equations (23) and (24) respectively, or "in- termination" terms In_u(t) , «,,(/) , In_n{t) and In₂₂(i) , as defined in equations (30), (34), (31) and (35) respectively, since the reflections from the first pair of FRMs 153 and from the second pair of FRMs 154 are separated in time. As a result, there are substantially no constraints imposed on the dimensions L_{S] I} L_{S2 I} L_m and L_R2. In some example embodiments, it may be appropriate to select dimensions so as to optimize the signal to noise ratio (SNR) associated with the process. In some example embodiments, it may be beneficial to make L_Si -L_m and L_S2 = L_R2 , in order to minimize effects of phase noise from the laser sources 111a, 111b. In some example embodiments, a delay line 152 length of ?2 - _s'i -Lm - L^ =2,000 m has been found to work well.

Turning now to Figure 8, there is shown a flowchart showing example actions to be performed by a processor in sensor 100. At action 810, the DSP 119 processes the phase responses detected at the optical detector(s) 114, and described above in non-limiting fashion. At action 820, the DSP 119 determines the inward bound phase response profile φ_χ(ί) 210 as described above in non-limiting fashion. At action 830, the DSP 119 determines the outward bound phase response profile φ_γ {ί) 220 as described above in non-limiting fashion. At action 840, the DSP 119 determines the location of the disturbance 101 along sensor cable 140, by comparing relative delays between the inward bound phase response profile φ_χ (() 210 and the outward bound response profile φ_γ (ι) 220, as discussed above in non-limiting fashion.

It will be apparent that various modifications and variations may be made to the em bodiments disclosed herein, consistent with the present disclosure, without departing from the spirit and scope of the present disclosure.

The four embodiments disclosed herein are non-limiting illustrations of ways in which the present disclosure may be implemented . Judicious selection of multiplexing technique (including without limitation MCLM, WDM, TDM, or any combination of any of them) and a mechanism for measuring phase (including without limitation modulation, use of the first harmonic term, use of the first and second harmonic term, use of a 3x3 splitter/combiner, use of a 4x4 splitter/combiner, or any combination of any of them) may permit a variety of permutations and combinations to be applied .

In each of the disclosed embodiments, a pair of compound terminations 150 employing a tota l of four FRMs 153, 154 is disclosed. In some example embodiments, similar effects may be achieved with as few as three FRMs or more than four FRMs.

While the application of the disclosed embodiments is shown in respect of outdoor perimeter security, and in particular use of the sensor 100 with the sensor cable 140 mounted on a perimeter fence, those having ordinary skil l in the relevant art may appreciate other applications for the sensor 100. Without limiting the generality of the foregoing, the sensor cable 140 of the sensor 100 may be buried around a perimeter to detect and locate intruders from seismic activity generated by footsteps or vehicle traffic, located on or in proximity to pipelines to locate potential saboteurs or may form part of a bundle of fiber optic communications cables to locate attempts to intercept communications along such cables by accessing the cables.

In the WDM embodiment described in non-limiting fashion in Figure 6, termination couplers 65 IS, 651R and receiver couplers 617A, 617B are implemented as wavelength division multiplexers. Conceivably, example embodiments of the present disclosure, whether or not in a WDM

embodiment, may replace or supplement one of more of such wavelength division multiplexers with a coupler such as a splitter/combiner (not shown). By the same token, input coupler 618, which is implemented as a splitter/combiner in the WDM embodiment described in non-limiting fashion in Figure 6. Conceivably, example embodiments of the present disclosure may replace or supplement such splitter/combiner with a coupler such as a wavelength division multiplexer (not shown). In the foregoing disclosure, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present disclosure.

While example embodiments are disclosed, this is not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present disclosure and it is to be further understood that numerous changes covering alternatives, modifications and equivalents may be made without straying from the scope of the present disclosure, as defined by the appended claims. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. Also, techniques, systems, subsystems and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other examples of changes, substitutions, and alterations are easily ascertainable and could be made without departing from the spirit and scope disclosed herein.

In particular, features from one or more of the above-described

embodiments may be selected to create alternative embodiments comprised of a sub-combination of features which may not be explicitly described above. In addition, features from one or more of the above-described embodiments may be selected and combined to create alternative embodiments comprised of a combination of features which may not be explicitly described above. Features suitable for such combinations and subcombinations would be readily apparent upon review of the present application as a whole. The subject matter described herein and in the recited claims intends to cover and embrace all suitable changes in technology. In some instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present disclosure with unnecessary detail. All statements herein reciting principles, aspects and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

Thus, for example, it will be appreciated that block diagrams reproduced herein can represent conceptual views of illustrative components embodying the principles of the technology. Similarly, it will be appreciated that any flow charts, state transition diagrams, pseudocode, and the like represent various processes, which may be substantially represented in computer- readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. The present disclosure can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combination thereof.

Apparatus of the disclosure can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and methods and actions can be performed by a programmable processor executing a program of instructions to perform functions of the disclosure by operating on input data and generating output.

The disclosure can be implemented advantageously on a programmable system including at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language, if desired; and in any case, the language can be a compiled or interpreted language. Further, the foregoing description of one or more specific embodiments does not limit the implementation of the disclosure to any particular computer programming language, operating system, system architecture or device architecture.

The processor executes instructions, codes, computer programs, scripts which it accesses from hard disk, floppy disk, optical disk (these various disk based systems may all be considered secondary storage), ROM, RAM, or the network connectivity devices. Multiple processors may be present. Thus, while instructions may be discussed as executed by a processor, the instructions may be executed simultaneously, serially, or otherwise executed by one or multiple processors. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. The functions of the various elements including functional blocks labelled as "modules", "processors" or "controllers" may be provided through the use of dedicated hardware, as well as hardware capable of executing software in association with appropriate software with sufficient processing power, memory resources, and network throughput capability to handle the necessary workload placed upon it. Moreover, explicit use of the term "module", "processor" or "controller" should not be construed to refer exclusively to hardware capable of executing software, and may include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random access memory (RAM) and non-volatile storage.

Suitable processors include, by way of example, both general and specific microprocessors. Generally, a processor will receive instructions and data from a read-only memory or a random access memory. Generally, a computer will include one or more mass storage devices for storing data file; such devices include magnetic disks and cards, such as internal hard disks, and removable disks and cards; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of volatile and non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; CD-ROM and DVD- ROM disks; and buffer circuits such as latches or flip flops. Any of the foregoing can be supplemented by, or incorporated in ASICs (application- specific integrated circuits), FPGAs (field-programmable gate arrays) or DSPs (digital signal processors).

Examples of such types of computer are programmable processing systems suitable for implementing or performing the apparatus or methods of the disclosure. The system may comprise a processor, (which may be referred to as a central processor unit or CPU), which may be implemented as one or more CPU chips, and that is in communication with memory devices including secondary storage, read only memory (ROM), a random access memory, a hard drive controller, or an input/output devices or controllers, and network connectivity devices, coupled by a processor bus. Secondary storage is typically comprised of one or more disk drives or tape drives and is used for non-volatile storage of data and as an over-ffow data storage device if RAM is not large enough to hold al! working data.

Secondary storage may be used to store programs which are loaded into RAM when such programs are selected for execution. The ROM is used to store instructions and perhaps data which are read during program execution. ROM is a non-volatile memory device which typically has a small memory capacity relative to the larger memory capacity of secondary storage. The RAM is used to store volatile data and perhaps to store instructions. Access to both ROM and RAM is typically faster than to secondary storage.

I/O devices may include printers, video monitors, liquid crystal displays (LCDs), touch screen displays, keyboards, keypads, switches, dials, mice, track balls, voice recognizers, card readers, paper tape readers, or other well-known input devices.

The network connectivity devices may take the form of modems, modem banks, ethernet cards, universal serial bus (USB) interface cards, serial interfaces, token ring cards, fiber distributed data interface (FDDI) cards, wireless local area network (WLAN) cards, radio transceiver cards such as code division multiple access (CDMA) or global system for mobile

communications (GSM ) radio transceiver cards, and other well-known network devices. These network connectivity devices may enable the processor to communicate with an Internet or one or more intranets. The network connectivity devices may also include one or more transmitter and receivers for wirelessly or otherwise transmitting and receiving signal as are well known . With such a network connection, it is contemplated that the processor might receive information from the network, or might output information to the network in the course of performing the above-described method steps. Such information, which is often represented as data or a sequence of instructions to be executed using the processor for example, may be received from and outputted to the network, for example, in the form of a computer data baseband signal or signal embodied in a carrier wave. The baseband signal or signal embodied in the carrier wave generated by the network connectivity devices may propagate in or on the surface of electrical conductors, in coaxial cables, in waveguides, in optical media, for example optical fiber, or in the air or free space. The information contained in the baseband signal or signal embedded in the carrier wave may be ordered according to different sequences, as may be desirable for either processing or generating the information or transmitting or receiving the information. The baseband signal or signal embedded in the carrier wave, or other types of signals currently used or hereafter developed, referred to herein as the transmission medium, may be generated according to several well known methods.

Certain terms are used throughout to refer to particular components.

Manufacturers may refer to a component by different names. Use of a particular term or name is not intended to distinguish between components that differ in name but not in function.

The terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to". The terms "example" and "exemplary" are used simply to identify instances for illustrative purposes and should not be interpreted as limiting the scope of the invention to the stated instances. In particular, the term "exemplary" should not be interpreted to denote or confer any laudatory, beneficial or other quality to the expression with which it is used, whether in terms of design, performance or otherwise.

Directional terms such as "upward", "downward", "left" and "right" are used to refer to directions in the drawings to which reference is made unless otherwise stated. Similarly, words such as "inward" and "outward" are used to refer to directions toward and away from, respectively, the geometric center of a device, area or volume or designated parts thereof. Moreover, all dimensions described herein are intended solely to be by way of example for purposes of illustrating certain embodiments and are not intended to limit the scope of the disclosure to any embodiments that may depart from such dimensions as may be specified.

The terms "couple" or "communicate" in any form are intended to mean either a direct connection or indirect connection through some interface, device, intermediate component or connection, whether electrically, mechanically, chemically, or otherwise.

References in the singular form include the plural and vice versa, unless otherwise noted. The purpose of the Abstract is to enable the relevant patent office or the public generally, skill in the art who are not familiar with patent or legal terms or phraseology, to quickly determine from a cursory inspection the nature of the technical disclosure. The Abstract is neither intended to define the scope of this disclosure, which is measured by its claims, nor is it intended to be limiting as to the scope of this disclosure in any way.

One embodiment of the present disclosure is a ranging sensor for locating a disturbance along a detection zone, comprising : an interfero metric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and with the reference terminal; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in

communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms; and a processor for: processing the phase response detected at the at least one receiver, determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

Another embodiment of the present disclosure is a compound termination for a ranging sensor for locating a disturbance along a detection zone, the sensor comprising : an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal, first and second arms in communication at one end thereof with the sense terminal and the reference terminal, a signal source for injecting a signal into the input, for distribution across the first and second arms, two of the compound terminations, one terminating each of the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one compound termination comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms, and a processor for: processing the phase response detected at the at least one receiver, determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the

interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

Another embodiment of the present disclosure is a processor for a ranging sensor for locating a disturbance along a detection zone, the sensor comprising : an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and with the reference terminal; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated waveguide and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in communication with a

corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms; and the processor for: processing the phase response detected at the at least one receiver; determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations aiong the first and second arms and being reflected by at least one FR back along the first and second arms to the

interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles,

Another embodiment of the present disclosure is a method for locating a disturbance aiong a detection zone, comprising the actions of: processing a phase response detected at least one receiver of a ranging sensor comprising : an interferometric splitter/ combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and the reference terminal; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by !t along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having a second transmission delay that is greater than the first transmission delay; the at least one receiver in communication with a corresponding detector output for detecting and processing the phase response corresponding to a signal travelling along the first and second arms; determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

Another embodiment of the present disclosure is a computer program product comprising: a computer readable medium, and stored on the computer readable medium, computer-readable and computer-executable instructions in a processor of a ranging sensor for locating a disturbance along a detection zone, comprising: an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and with the reference terminal ; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second waveguides at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday rotational mirror (FRM) for reflecting a signal portion received by it along the associated waveguide after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms; and a processor; which when executed, cause the processor to: process the phase response detected at the at least one receiver, determine from the phase response : an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interfere metric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reffected by at least one FRM back along the first and second arms to the

interferometric splitter/combiner, and determine a location of the

disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

Other embodiments consistent with the present disclosure will be apparent from consideration of the specification and the practice of the disclosure disclosed herein. Accordingly the specification and the embodiments disclosed therein are to be considered examples only, with a true scope and spirit of the disclosure being disclosed by the following numbered claims:

Claims

WHAT IS CLAIMED IS:

1. A ranging sensor for locating a disturbance along a detection zone, comprising : an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and with the reference terminal; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms; and a processor for: processing the phase response detected at the at least one receiver, determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the

interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

2. A sensor according to claim 1, wherein the outward bound phase response profile has aspects corresponding to reflection by each of the first and second FRMs, the aspects corresponding to the second FRM being delayed in time relative to the aspects corresponding to the first FRM by substantially twice a difference between the second and first transmission delay, and the processor is structured to differentiate the outward bound phase response profile from the inward bound phase response profile, which does not have aspects corresponding to reflection by each of the first and second FRMs.

3. A sensor according to claim 1, further comprising a lead-in cable comprising an input waveguide for coupling the signal source to the input and at least one detector waveguide for coupling at least one detector output to at least one receiver.

4. A sensor according to claim 1, wherein the interferometric

splitter/combiner is a 2x2 coupler.

5. A sensor according to claim 1, wherein the interferometric

splitter/combiner is a coupler of order of at least 3, having two detector outputs to obtain In-phase and Quadrature-phase components therefrom.

6. A sensor according to claim 5, wherein the interferometric

splitter/combiner is a coupler of order 4, having at least two detector outputs to obtain In-phase and Quadrature-phase components directly therefrom.

7. A sensor according to claim 5, wherein at least one unused terminal of the interferometric splitter/combiner is terminated to substantially prevent reflections therealong .

8. A sensor according to claim 1, wherein at least one of the first and second arms is coupled to an object to be protected, selected from a group consisting of a perimeter fence, a wail, a pipeline, a cable or any

combination of any of these.

9. A sensor according to claim 1, wherein at least one of the first and second arms is buried within a ground surface along a perimeter to be monitored.

10. A sensor according to claim 1, wherein the first and second arms have a common package.

11. A sensor according to claim 9, wherein the first and second arms are packaged with at least one other arm.

12. A sensor according to claim 1, wherein the signal source is a laser.

13. A sensor according to claim 1, wherein the signal source is provided with a modulation signal by the processor for modulating the signal.

14. A sensor according to claim 13, wherein the signal source comprises a modulator for receiving the modulation signal and applying it to the signal before injecting it into the input.

15. A sensor according to claim 13, wherein the modulation signal corresponds to a modulation scheme selected from a group consisting of continuous wave modulation, pulse modulation having a pulse duty cycle substantially less than 50% and a pulse modulation period substantially equal to an integer divisor of the difference between the associated second transmission delay of the delay line and the associated first transmission delay and any combination of any of these.

16. A sensor according to claim 1, wherein the signal source comprises a plurality of signal sources for different signals and an input coupler for combining the signals into a common signal for injecting into the input.

17. A sensor according to claim 16, wherein the input coupler is selected from a group consisting of a splitter/combiner, a wavelength division multiplexer and any combination of any of these.

18. A sensor according to claim 1, wherein the second FRM of each of the first and second compound terminations have a common associated second transmission delay.

19. A sensor according to claim 18, wherein the first FRM of each of the first and second compound terminations have a common associated first transmission delay.

20. A sensor according to claim 1, wherein the second FRM of the first compound termination has an associated second transmission delay that is longer than the associated second transmission delay of the second FRM of the second compound termination.

21. A sensor according to claim 20, wherein the first and second compound terminations each comprise respective first and second FRMs.

22. A sensor according to claim 21, wherein the first FRM of the first compound termination has an associated first transmission delay that is shorter than the associated first transmission delay of the second compound termination .

23. A sensor according to claim 22, wherein an absolute value of a difference between the associated first transmission delays of the first FRMs of the first and second compound terminations is substantially equal to an absolute value of a difference between the associated second transmission delays of the second FRMs of the first and second compound terminations.

24. A sensor according to claim 21, wherein the first and second FRMs of the first and second compound terminations are positioned relative to the interferometric splitter/combiner such that reflections from the first FRM of the first compound termination and from the second FRM of the second compound termination and reflections from the second FRM on the first compound termination and the first FRM on the second compound termination do not generate an interferometric output at the interferometric splitter/combiner.

25. A sensor according to claim 21, wherein the first and second FRMs of the first and second compound terminations are positioned relative to the interferometric splitter/combiner such that reflections from the first FRM of the first and second compound terminations are orthogonal to reflections from the second FRM of the first and second compound terminations.

26. A sensor according to claim 1, wherein at least one of the compound terminations is packaged so as to substantially dampen phase angle responses to an effect selected from a group consisting of thermal changes, mechanical motion and any combination of any of these.

27. A sensor according to claim 1, wherein at least one of the compound terminations comprises a termination coupler for coupling the associated arm to the first FRM and to the delay line coupled to the second FRM.

28. A sensor according to claim 27, wherein the termination coupler is selected from a group consisting of a wavelength division multiplexer, a splitter/combiner and any combination of any of these.

29. A sensor according to claim 1, wherein a coherence length of the signal source is greater than an optical path length difference (OPD) associated with the first and second FRMs of the first and second compound terminations.

30. A sensor according to claim 29, wherein the coherence length is less than a length of the delay line.

31. A sensor according to claim 1, wherein the at least one receiver comprises a detector coupled to the corresponding detector output.

32. A sensor according to claim 31, wherein the detector is coupled to a corresponding detector output by a receiver coupler.

33. A sensor according to claim 32, wherein the receiver coupler is selected from a group consisting of a wavelength division multiplexer, a splitter/combiner and any combination of any of these.

34. A sensor according to claim 31, wherein the at least one receiver comprises an analog to digital converter (ADC) coupled to the processor.

35. A sensor according to claim 34, wherein the at ieast one receiver comprises a band-pass filter coupled to the detector and the ADC.

36. A sensor according to claim 35, wherein the band-pass filter has a pass band extending from substantially 100 Hz to substantially 500 kHz.

37. A sensor according to claim 1, wherein the processor comprises a digital signal processor (DSP).

38. A sensor according to claim 1, wherein the processor determines the inward bound and outward bound phase response profiles from a multiplexing technique employed in the sensor.

39. A sensor according to claim 38, wherein the multiplexing technique is selected from a group consisting of coherence length multiplexing, wavelength division multiplexing, time division multiplexing and any combination of any of these.

40. A sensor according to claim 1, wherein the processor determines the inward bound and outward bound phase response profiles by applying a pseudo-heterodyne process to isolate modulated responses from the first and second FRMs of the first and second compound terminations.

41. A sensor according to claim 40, wherein the processor determines the inward and outward bound phase response profiles by applying first harmonic terms of the pseudo-heterodyne process.

42. A sensor according to claim 41, wherein the processor determines the inward and outward bound phase response profiles by applying second harmonic terms of the pseudo-heterodyne process.

43. A sensor according to claim 41, wherein the processor determines the inward and outward bound phase response profiles by applying orthogonal modulation responses from the first and second FRMs of the first and second compound terminations.

44. A sensor according to claim 1, wherein the processor determines the inward bound and outward bound phase response profiles by restricting a first wavelength component of the signal to the FRMs of the first and second compound terminations and by restricting a second wavelength component of the signal to the second FRMs of the first and second compound terminations.

45. A sensor according to claim 1, wherein the processor applies a modulation signal to the signal source having a duty cycle substantially less than 50% and a period substantially equal to a difference between the second transmission delay associated with the delay line and the first transmission delay and determines the inward bound and outward bound phase response profiles by monitoring the signal detected by the at least one receiver and determining whether a signal response corresponds to the FRMs of the first and second compound terminations or to the second FRMs of the first and second compound terminations.

46. A sensor according to claim 1, wherein the arm is selected from a group consisting of an optical fiber, a cable and any combination of any of these.

47. A compound termination for a ranging sensor for locating a disturbance along a detection zone, the sensor comprising : an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal, first and second arms in communication at one end thereof with the sense terminal and the reference terminal, a signal source for injecting a signal into the input, for distribution across the first and second arms, two of the compound terminations, one terminating each of the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one compound termination comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms, and a processor for: processing the phase response detected at the at least one receiver, determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

48. A processor for a ranging sensor for locating a disturbance along a detection zone, the sensor comprising : an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and with the reference terminal; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated waveguide and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at ieast one receiver in communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms; and the processor for: processing the phase response detected at the at Ieast one receiver; determining from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reflected by at Ieast one F M back along the first and second arms to the

interferometric splitter/combiner, and determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

49. A method for locating a disturbance along a detection zone, comprising the actions of: processing a phase response detected at Ieast one receiver of a ranging sensor comprising: an interferometric splitter/combiner having an input, at ieast one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and the reference terminal; a signal source for Injecting a signal into the input, for distribution across the first and second arms;

first and second compound terminations for terminating the first and second arms at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday Rotational Mirror (FRM) for reflecting a signal portion received by it along the associated arm after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having a second transmission delay that is greater than the first transmission delay;

the at least one receiver in communication with a corresponding detector output for detecting and processing the phase response corresponding to a signal travelling along the first and second arms;

determining from the phase response:

an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric spNtter/combiner along the first and second arms, and

an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the

interferometric splitter/combiner, and

determining a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.

50. A computer program product comprising : a computer readable medium, and stored on the computer readable medium, computer-readable and computer-executable instructions in a processor of a ranging sensor for locating a disturbance along a detection zone, comprising: an interferometric splitter/combiner having an input, at least one detector output, a sense terminal and a reference terminal; first and second arms in communication at one end thereof with the sense terminal and with the reference terminal; a signal source for injecting a signal into the input, for distribution across the first and second arms; first and second compound terminations for terminating the first and second waveguides at respective second ends thereof, the interferometric splitter/combiner and the compound terminations defining limits of the detection zone, at least one of the first and second compound terminations comprising a first Faraday rotational mirror (FR ) for reflecting a signal portion received by it along the associated waveguide after an associated first transmission delay and a second FRM for reflecting a signal portion received by it along the associated arm and along a delay line coupled to the second FRM and having an associated second transmission delay that is greater than the first transmission delay; at least one receiver in communication with a corresponding detector output for detecting and processing a phase response corresponding to a signal travelling along the first and second arms; and a processor; which when executed, cause the processor to: process the phase response detected at the at least one receiver, determine from the phase response: an inward bound phase response profile corresponding to a signal travelling directly from the disturbance to the interferometric splitter/combiner along the first and second arms, and an outward bound phase response profile corresponding to a signal travelling directly from the disturbance to the first and second compound terminations along the first and second arms and being reflected by at least one FRM back along the first and second arms to the

interferometric splitter/combiner, and determine a location of the disturbance from a comparison of a relative delay between the inward bound and outward bound phase response profiles.