Distributed optical sensing systems and methods

Abstract

A distributed optical detection system comprising: a broadband optical source; and a phase and amplitude receiver for measuring phases and amplitudes of distributed backscattered signals from a sensing medium. Methods of quantitatively sensing optical path length changes along a sensing medium in a distributed manner are also disclosed.

Claims

1. A distributed optical sensing system comprising: a sensing medium; an optical source for generating an optical output (OO); a means for separating the OO into at least two OO portions; a first optical delay means adapted to delay a first OO portion, named Delayed Optical Output Signal (DelayOOS), by a delay time, τ.sub.1, with respect to a further OO portion, named Direct Optical Output Signal (DirectOOS); a means for directing the DelayOOS and the DirectOOS to the sensing medium; a means for receiving distributed backscatter signals from the sensing medium generated by the DelayOOS and the DirectOOS; a means for separating the received backscatter signals into at least two backscattered signal portions; a second optical delay means adapted to delay a first backscattered signal portion, named Delayed Backscatter Signal (DelayBS), by a delay time, τ.sub.2, with respect to a further backscattered signal portion, named Direct Backscatter Signal (DirectBS), wherein: one portion of the distributed backscatter generated by DirectOOS, having been received from the sensing medium, provides a portion of the DelayBS, named Direct-DelayBS; and one portion of the distributed backscatter generated by DelayOOS, having been received from the sensing medium, provides a portion of one or more DirectBS, named Delay-DirectBS; an optical receiver configured for measurement of the phase difference between Direct-DelayBS and one or more Delay-DirectBS with full phase quadrature determination (without ambiguity in a range of 2*π radians) to determine optical path length changes along the sensing medium; and an analysis processor configured to receive measured signals from the optical receiver and analysis of the measured signals thereby to infer physical changes in the sensing medium from the sensed optical path length changes.

2. The system of claim 1, wherein the optical receiver is a phase and amplitude receiver adapted for measuring both amplitude and phase without ambiguity in a range of 2*π radians.

3. The system of claim 1, wherein the optical source is selected from one of an incoherent or low-coherence broadband optical source.

4. The system of claim 1, wherein the optical source is selected from one of a multi-wavelength or partially coherent optical source.

5. The system of claim 1, wherein the optical source has a coherence time, τ.sub.coh and the delay of the first and second delay means satisfy the relation |τ.sub.1−τ.sub.2|<a τ.sub.coh, where a is between about 1 and about 100.

6. The system of claim 1, further comprising a modulator adapted to modulate at least a portion of the light generated by the optical source.

7. The system of claim 6, wherein the modulator is adapted to modulate the intensity, phase, frequency or polarisation of either the OO or the DirectOOS and/or DelayOOS prior to directing the output signals to the sensing medium.

8. The system of claim 1, wherein the first optical delay means and the second optical delay means are common, identical, and/or having approximately equal delay.

9. The system of claim 1, wherein the sensing medium is an optical fiber or one of a gas, liquid, water, sea water or atmospheric medium.

10. The system of claim 1, further comprising: a plurality of sensing mediums; frequency selection means for selecting a plurality of frequency bands within each of the DirectOOS and DelayOOS, each selected frequency band being directed to a selected medium; a plurality of receiving means for receiving distributed backscatter from each selected optical medium combined to produce DelayBS and DirectBS; frequency selection means for selecting a plurality of frequency bands within each of the Direct-DelayBS and Delay-DirectBS, each selected frequency band being directed to a selected phase and amplitude receiver; and a plurality of phase and amplitude receivers adapted to measure differences in amplitude and phase of the received optical signals in the selected frequency bands to determine optical path length changes along each selected mediums.

11. A method of sensing optical path length changes in a sensing medium in a distributed manner comprising the steps of: providing an optical source for generating an optical output (OO); separating the OO into at least two portions; providing a first optical delay means adapted to delay a first OO portion, named Delayed Optical Output Signal (DelayOOS), by a delay time, τ.sub.1, with respect to a further OO portion, named Direct Optical Output Signal (DirectOOS); directing the DelayOOS and the DirectOOS into the sensing medium; receiving distributed backscatter signals from the sensing medium generated by DelayOOS and DirectOOS; separating the received distributed backscatter into at least two portions; providing a second optical delay means adapted to delay a first backscattered signal portion, named Delayed Backscatter Signal (DelayBS), by a delay time, τ.sub.2, with respect to a further backscattered signal portion, named Direct Backscatter Signal (DirectBS), wherein: one portion of the distributed backscatter generated by DirectOOS, having been received from the sensing medium, provides a portion of the DelayBS, named Direct-DelayBS; and one portion of the distributed backscatter generated by DelayOOS, having been received from the sensing medium, provides a portion of one or more DirectBS, named Delay-DirectBS; and measuring with full phase quadrature determination (without ambiguity in a range of 2*π radians) the phase difference between Direct-DelayBS and Delay-DirectBS to determine optical path length changes along the sensing medium.

12. The method of claim 11, wherein measuring with full phase quadrature determination is performed with a phase and amplitude receiver capable of measuring both amplitude and phase without ambiguity in a range of 2*π radians.

13. The method of claim 11, wherein the optical source is an incoherent or low-coherence broadband optical source.

14. The method of claim 11, wherein the optical source is either a multi-wavelength or a partially coherent optical source.

15. The method of claim 11, wherein said distributed backscatter is due to Rayleigh backscatter in an optical fiber.

16. The method of claim 11, wherein said distributed backscatter is due to backscatter in a non-guiding sensing medium such atmosphere, gasses, fluids, water or a marine environment.

17. The method of claim 11, wherein the optical source is an intensity modulated optical source or pulsed optical source.

18. The method of claim 11, further comprising the step of modulating either the OO or the DirectOOS and/or DelayOOS prior to directing the output signals to the sensing medium.

19. The method of claim 11, further comprising, prior to directing the output signals into the sensing medium, providing combining means for combining the DelayOOS and the at least one DirectOOS.

20. The method of claim 11, wherein the first optical delay means and the second optical delay means are common, identical, and/or having approximately equal delay.

21. The method of claim 18, further comprising the steps of: determining the locations of the optical path length changes using the travel time of light in the sensing medium and the modulation scheme; quantitatively determining optical path length changes in a distributed manner using the measured phase; and inferring one or more physical changes in the sensing medium from the determined optical path length changes.

22. The method of claim 18, wherein the method of determining the locations of said optical path length changes or said physical parameter involves: a numerical deconvolution between a complex signal from a phase and amplitude receiver and a known or measured modulation; and/or a numerical cross-correlation between the complex signal from a phase and amplitude receiver and a known or measured modulation.

23. The method of claim 18, wherein the step of modulating comprises either modulating the intensity, modulating the amplitude, modulating the frequency, modulating the phase, or modulating the polarization of the optical signals.

24. The method of claim 11, wherein a time gating device is used to prevent light from entering the sensing medium at unwanted times.

25. The method of claim 11, further comprising the steps of: providing a first frequency selection means to: selecting a plurality of frequency bands within each of the DirectOOS and DelayOOS; directing each pair of optical signals in each selected frequency band to a selected one of a plurality of sensing mediums; receiving distributed backscatter optical signals from each selected optical medium; combining each of the received distributed backscatter optical signals to produce DirectBS and DelayBS; providing a second frequency selection means for selecting a plurality of frequency bands with Direct-DelayBS and Delay-DirectBS; and directing signals within each selected plurality of frequency bands to a selected one of a plurality of phase and amplitude receivers to measure the relative phase difference between Direct-DelayBS and Delay-DirectBS in each selected frequency band to determine optical path length changes in each of the plurality of sensing mediums.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Notwithstanding any other forms which may fall within the scope of the present invention, a preferred embodiment/preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic block-diagram representation of the DAS systems and methods disclosed herein;

(3) FIG. 2 shows a schematic representation of the arrival times of distributed backscatter optical signals from the sensing medium in the presently disclosed systems and methods;

(4) FIG. 3 shows a schematic block-diagram representation of the DAS systems and methods disclosed herein in a common path configuration;

(5) FIG. 4 shows a schematic diagram showing the photonic components of a particular arrangement of the sensing system disclosed herein;

(6) FIG. 5 shows a schematic diagram showing the photonic components of a further arrangement of the sensing system disclosed herein having a common path configuration;

(7) FIG. 6 shows a schematic diagram showing the photonic components of a particular arrangement of the sensing system disclosed herein;

(8) FIG. 7 shows a schematic diagram showing the photonic components of a further arrangement of the sensing system disclosed herein having a common path configuration;

(9) FIG. 8 shows the photonic components of a further arrangement of the sensing system disclosed herein in the common-path embodiment, using all output ports of a 3×3 coupler as phase and amplitude receiver;

(10) FIG. 9 shows the photonic components of a of a further arrangement of the sensing system disclosed herein in the common-path embodiment with polarization beam splitter;

(11) FIG. 10 shows an example detected amplitude trace for a pulsed DAS system according to the systems and methods disclosed herein;

(12) FIG. 11 shows an example of the measured phase vs. position along the sensing fiber with an vibrating fibre stretcher at 430 m for testing purposes according to the systems and methods disclosed herein;

(13) FIG. 12 shows an example of the computed strain rate vs. position along the sensing fiber and vs. time with a vibrating fibre stretcher at 430 m for testing purposes according to the systems and methods disclosed herein;

(14) FIG. 13 shows the photonic components of a further arrangement of the sensing system disclosed herein where amplified spontaneous emission from one optical amplifier is used as the source;

(15) FIG. 14 depicts a method of sensing distributed backscatter signals as disclosed herein;

(16) FIG. 15 shows the photonic components of a further arrangement of the sensing system disclosed herein in the common-path embodiment, using all output ports of a 3×3 coupler as phase and amplitude receiver;

(17) FIG. 16 depicts a method of processing and analysis of amplitudes and phases as disclosed herein;

(18) FIG. 17 shows a schematic block-diagram representation of the DAS systems and methods disclosed herein in a common path configuration with 2 independent optical paths to the sensing medium;

(19) FIG. 18 depicts an arrangement of the systems and FIGS. 18A to 18C depict further arrangements of the systems depicted above with methods of directing outgoing signals into a sensing medium without recombining in a common outward path and receiving backscatter signals from the sensing medium as disclosed herein; and

(20) FIG. 19 depicts a further arrangement of the systems depicted above with the addition of frequency selective multiplexing/demultiplexing and frequency directing components for detecting path length changes on multiple sensing mediums.

DETAILED DESCRIPTION

(21) It should be noted in the following description that like or the same reference numerals in different embodiments denote the same or similar features.

(22) Disclosed herein are systems and methods for quantitative distributed acoustic sensing (DAS) systems for quantitative distributed measurement of optical path length changes along a sensing medium such as, for example an optically transparent medium such as an optical fiber.

(23) The DAS system disclosed herein comprises a broadband optical source and phase and amplitude measurement capable of accurately measuring phase. The broadband optical source may have a coherence time, τ.sub.coh and, interchangeably, a coherence length, l.sub.coh, defined as the optical path length corresponding to propagation in a medium for a time equal to the coherence time. There is no in-principle lower bound on the coherence length of the source when used in the systems and methods disclosed herein. Coherence time, τ.sub.coh, is calculated by dividing the coherence length, l.sub.coh, by the velocity of light in a medium, and is approximately given by the relation

(24) ${\tau }_{coh}\approx \frac{1}{\Delta v}\approx \frac{{\lambda }^2}{c\Delta \lambda },$ where λ is the central wavelength of the source, Δv and Δλ is the spectral linewidth of the source in units of frequency and wavelength respectively, and c is the speed of light in vacuum.

(25) Multiple physical parameters may be sensed by the consequential optical path length changes they create in a medium coupled thereto. An example of phenomena which are capable of inducing optical path length changes in coupled media may include: longitudinal strain, transverse strain, acoustic waves, seismic waves, vibration, motion, bending, torsion, temperature, optical delay or chemical composition. Any other physical parameter having a mechanism which induces elongation and/or refractive index change and/or deformation along an optical path can also be sensed. Optical path length changes can also occur by the movement of scattering/reflecting particles in the sensing medium. The systems and methods disclosed herein utilise an intensity-modulated broadband light source in conjunction with delays and phase and amplitude measurement for accurate phase measurement on the distributed backscatter from a sensing medium such as, for example an optical fibre (referred to herein as a sensing fiber). Possible mechanisms for light returning from the sensing optical path back to the system include: Rayleigh backscatter; Mie backscatter; discrete reflections (intentional and non-intentional, such as faults or connector joins); Bragg grating reflections; scattering particles in solids such as dopants in crystals and glasses; scattering particles in liquids such as cells or slit in water; or scattering particles in gasses such as atmospheric aerosols in air.

(26) Possible optical paths for the sensing medium used in conjunction with the presently disclosed systems and methods, or used for implementation of the optical delay means disclosed herein, include: single mode fiber; multi-mode fiber; multi core fiber; polarization maintaining fiber; photonic crystal; photonic bandgap fiber; fiber with liquid or gas filled core; planar waveguides any of which can be fabricated from any suitable material; or free space propagation in a backscattering medium (gas, liquid or solid) which may include air or water.

(27) In the systems disclosed herein the output signal generated by the optical source is split into, for example, two portions which are then directed to IRDP where one portion of the output signal is time delayed (thus producing a delayed output signal) with respect to the second portion of the output signal which is not delayed (a direct output signal). The delayed output signal and the direct output signal are then directed to a sensing medium such that the delayed and direct output signals propagate through the sensing medium and thus subject changes in the sensing medium due to external disturbances. The delayed output signal and the direct output signal may optionally be combined onto a common optical path and and/or a common polarization prior to the sensing medium. Possible methods of separating light into multiple paths as would be appreciated by those skilled addressee for implementation of suitable IRDP with differing delays may include: optical couplers (2×2, 3×3, or M×N); beam splitter; polarizing beam splitter; switch (e.g. LCOS, holograms, MEMs or electro-optic); acousto-optic modulator; optical filter; partial reflector; or birefringence.

(28) As they propagate through the sensing medium, the delayed and direct output signals are each scattered by the sensing medium in a distributed manned along the propagation direction of the output signals, and a portion of the scattered output signals propagates directly backward in the reverse propagation direction to the forward propagating signals. The backward propagating (or backscattered) light from the output signals is collected by the DAS system for analysis of any external disturbances causing optical path length changes in the sensing medium.

(29) FIG. 1 shows a conceptual schematic block-diagram representation of a DAS system according to the present invention.

(30) Output light 150 from optical source 101 is optionally modulated by modulator 103. In alternative arrangements, the optical source 101 is a pulsed optical source. In still further arrangements optical source 101 may have modulator 103 incorporated therewith. The source is preferably modulated within in the laser or before the separation, but alternatively can be modulated anywhere before entering the sensing medium (such as a sensing fiber) 160 i.e. the light in the forward-propagating path 130 is modulated at any point between the source 101 and sensing medium 160. Modulator 103 may be adapted to modulate any one or more of the intensity, frequency, phase or polarisation of the light in the forward-propagating path 130 of system 100 prior to sensing medium 160. Pulses or coded modulation are examples of possible modulation schemes which can be used. Possible alternative means for modulation as would be appreciated by the skilled addressee may include, for example: electro-optic modulators; acousto-optic modulators; optical switches; direct source modulation; and saturable absorbers. Possible modulation schemes may include: pulsing; pseudo-random coding; simplex Code; Golay Code; linear frequency chirp; or Barker Code.

(31) If a modulator is placed after splitter 105 of system 100 than it can be adapted to act on either the delayed 151 or direct 153 outgoing portions without acting on the other outgoing portions, then the system 100 can function in a similar way to the case where the modulator is placed before the splitter 105.

(32) If more than one modulators are used to act on the two outgoing portions 151 and 153 separately, then these modulators would preferably act on the portions before the delays and act in unison. If modulators act on the outgoing portions after the delays but before the combiner 111a, then the modulators would preferably use the same modulation pattern with a delay equal to the optical delay.

(33) If a modulator is used after the combiner 111a and before the sensing medium 160, then the modulation would preferably repeat its modulation pattern after a time period equal to the optical delay.

(34) Possible broadband optical sources may include: multimode lasers e.g. Fabry-Perot laser, single mode lasers e.g. DFB laser; spontaneous emission or amplified spontaneous emission (ASE), e.g. EDFA and SOA; superluminescent diodes (SLED); supercontinuum sources; mode locked lasers; amplitude modulated sources; frequency modulated sources; swept frequency sources; phase-shift keying lasers; phase modulated sources; natural light; fluorescence or phosphorescence; any optical filtered light source listed above as would be appreciated by the skill addressee; or any combination of the above sources. In particular arrangements the coherence length of the source is less than the spatial resolution of the system 100.

(35) In the following discussion, the modulator is described as an intensity modulator for example purposes only. The skilled addressee, however, would readily appreciate that intensity modulation can readily be substituted with modulation of either the phase, polarisation or frequency of the light generated by the optical source 101.

(36) In particular arrangements of system 100, the optical paths between each element thereof are provided by optical fiber. In alternate arrangements, the optical paths between elements may be free space. Light 150 from source 101 is next directed to optical splitter 105 where it is separated into at least 2 portions of output light 152 and 153 which are respectively directed to a first Intentional Relative Delay Path (IRDP) 106a comprising separated optical paths (for example, separate optical fibres). The first IRDP 106a comprises a first optical delay means 107a which, for example may be an optical fiber delay line of predetermined length in order to impart a known delay time, τ.sub.1, onto a first portion 152 of the split output signal and thereby to generate a delayed output signal 151. In further discussion, the split portion 153 of the optical output 150 which bypasses first optical delay means 107a is referred to as direct output signal 153. The delayed output signal 151 and direct output signal 153 are both then recombined in combiner 111a onto a common forward-propagating optical path 171 and then directed to the sensing medium 160, for example an optical fiber adapted for sensing one or more parameters capable of inducing optical path length changes on the sensing media, 160. In particular arrangements, the difference delay imparted on the separated signals 151 and 153 in the IRDP 106a is preferably (although not necessarily) longer than the round-trip time of light in the sensing medium 160.

(37) Distributed backscatter induced by the forward propagating light in the sensing medium 160 and propagating backwards along the same optical path in sensing medium 160 is collected by system 100 and initially separated from the forward-propagating path 130 in system 100 into a backward-propagating path 135 by backscatter receiving means 115. Backscatter receiving means 115 may be an optical circulator or similar which a) receives forward-propagating light from forward propagating path 130 and directs it to sensing medium 160 and b) receives backward-propagating backscatter light from sensing medium 160 and directs it to backward-propagating path 135. The backscattered optical signals 161 and 163 received by the system 100 comprise a first backscattered return signal 161 arising from backscattering in the sensing medium 160 of the delayed output signal 151 and a second backscattered return signal 163 arising from backscattering in the sensing medium 160 of the direct output signal 153. The backscattered signals 161 and 163 each propagate on backward-propagating path 135 and are each split into at least two backscatter signal portions on separate optical paths by splitter 111b, and the split backscatter signal portions are directed to a second IRDP 106b. The second IRDP 106b comprises a second optical delay means 107b which, for example may be an optical fiber delay line of predetermined length in order to impart a known delay time, τ.sub.2, onto a first portion of each of the backscatter return signals 161 and 163.

(38) The second optical delay means 107b is adapted to delay at least one portion of each the received backscatter signals 161 and 163 on the return path with respect to the other return optical paths by a predetermined delay time, τ.sub.2, thereby to produce a delayed backscatter signal portion of each of the backscatter signals 161 and 163 and at least one direct backscatter signal portion of each of the backscatter signals 161 and 163, wherein the delay of the first (τ.sub.1) and second (τ.sub.2) delay means satisfy the relation |τ.sub.1−τ.sub.2|<a τ.sub.coh, the coherence time of optical source 101, wherein multiplication factor, a, may be between 1 and about 100. In some instances, intentionally designing the system with τ.sub.1≠τ.sub.2 can be advantageous for phase and amplitude measurement, as described below. The return backscatter signals are then each directed to phase and amplitude receiver 131.

(39) According to the optical pathways of system 100 described above, the phase and amplitude receiver 131 receives a plurality of signals including: a Direct+Direct signal comprising a backward propagating signal 161a arising from the direct output signal 151 having bypassed the second optical delay means 107b (Signal 1); a Delay+Direct signal comprising a backward propagating signal 163a arising from the delayed forward-propagating output signal 153 having bypassed the second optical delay means (Signal 2); a Direct+Delay signal comprising a backward propagating signal 161b arising from the direct output signal 151 having been transmitted through the second optical delay means 107b (Signal 3); and a Delay+Delay signal comprising a backward propagating signal 163b arising from the delayed output signal 153 having been transmitted through the second optical delay means (Signal 4).

(40) Signal 2 163a and Signal 3 161b arrive at the receiver approximately at the same time to permit Signal 2 163a to interfere with Signal 3 161b thereby to generate an interference signal 170 at the output of receiver 131 adapted to provide a measure of the optical path differences between Signal 2 and Signal 3 which is indicative of a path length differences in the sensing medium 160 caused by an external disturbance.

(41) The detected signals are recorded and stored in storage 133 and analysed by analysis processor 135 to calculate the effective path length changes in the sensing medium 160 caused by external disturbances.

(42) System 100 further comprises optional amplifiers 113a and 113b respectively for 113a) optical amplification of the outbound forward propagating optical signals 151 and 153 prior to launching into the sensing medium (e.g. sensing fibre) 160; and 113b) optical amplification of the received backscatter signals 161 and 163.

(43) In this way, broadband light which travels coherently throughout the system 100 and within the sensing medium 160, can travel nearly equal optical path lengths between the source and the phase and amplitude receiver (the so called “white-light” interference condition), regardless of the optical frequency and regardless of the location along the sensing fiber where back-reflection has occurred. This condition produces an electronically measurable interference signal at the phase and amplitude receiver 131 between the broadband distributed backscatter which returns from the sensing medium 160 at different times. The relative phase of the delayed and non-delayed backscatter signals 163a and 161b (arising from the direct output signal and the delayed output signal respectively), as measured at a phase and amplitude receiver 131, contains the primary information required to accurately determine the changes in the optical path length that occur in the sensing medium within the delay period induced by the first (forward-propagating) IRDP and first optical delay means 107a. The amplitude of the detected signal, as measured at phase and amplitude receiver 131, can be used to estimate the quality of the phase information, as discussed in the signal model derivation below, and be used to improve the spatial resolution and sensitivity of the distributed sensing by the use of coding schemes.

(44) Possible phase and amplitude measurement methods may include: frequency shifting (e.g. acousto-optic frequency shifting) the Direct+Delay and/or Delay+Direct signals and complex demodulation to recover phase; 3×3 coupler or M×N coupler where M≥2 or N≥3; frequency sweeping and Hilbert transform for recovery of phase; phase modulator receiving phase through time multiplexing of a changing phase shift; arrangement of waveplates within a multiport interferometer; use of a spectrometer (e.g. grating) or optical filters and detecting intensity or performing phase and amplitude measurement in different frequency bands; Interference with a local oscillator; or any polarization-diverse (dual-polarization) version of the above as would be appreciated by the skilled addressee. When the first (τ1) and second (τ.sub.2) delays are not equal (τ.sub.1≠τ.sub.2) it would be expected that the phase is not uniform throughout the optical spectrum, in which case the phase and amplitude measurement could be performed by using this fact, or performed separately on different frequency bands. This could have advantages, including: improving manufacture simplicity and costs and compensating for dispersion in the sensing medium. Performing phase and amplitude measurement in different frequency bands could also enable distributed sensing on multiple sensing media by using a frequency demultiplexer to direct outgoing signals to different sensing media and receiving the returning backscatter signals with a frequency multiplexer.

(45) Possible arrangements of first and second optical delay means 107a and 107b as would be appreciated by the skilled addressee may include such means of delaying light as: optical fiber delay line; optical beam delay line (e.g. free space); will optical cavities; recirculating loop; or electromagnetically induced transparency.

(46) Preferably, the splitter and the phase and amplitude receiver could comprise of common devices, as shown in FIG. 8, FIG. 13 or FIG. 15.

(47) Referring now to FIG. 14, there is depicted a method of sensing distributed backscatter signals including a method 1400 of quantitatively sensing optical path length changes along a sensing medium in a distributed manner. Method 1400 comprises the step of providing 1401 a broadband optical source for generating an optical output, the optical source having a coherence time, τ.sub.coh. Method 1400 further comprises the step of separating 1403 the optical output into at least two portions and directing 1405 each separated portion to an outward independent optical path. Separation may be performed by spatial separation or separation into polarization components, but is not limited to these. An independent polarization is considered an independent optical path. Method 1400 further comprises providing 1407 a first optical delay means in a first of said optical paths. The first optical delay means is adapted to delay at least one portion of the optical output on the first forward optical path with respect to the other independent forward optical paths by a predetermined delay time, τ.sub.1 thereby to produce a delayed output signal and at least one direct output signal. Method 1400 optionally comprises providing an optical re-combiner for recombining 1409 the delayed optical signal and at least one direct output signals on to a common forward optical path and directing 1411 the output signals into the sensing medium, whether recombined or not. Method 1400 further comprises providing 1420 a modulator for modulating the optical output generated by the optical source. Modulating step 1420 may be performed anywhere between steps 1401 and 1411. The modulator may be adapted to modulate any one or more of the intensity, frequency, phase or polarisation of the light in the forward-propagating path 130 prior to the sensing medium 160.

(48) Method 1400 further comprises providing 1413 a receiving means for receiving backscatter signals from the sensing medium. Method 1400 further comprises separating 1415 the backscatter signals into at least two independent return paths. Separation may be performed by spatial separation or separation into polarization components, but is not limited to these. An independent polarization is considered an independent optical path. Method 1400 further comprises providing 1417 a second optical delay means in a first of the return paths. The second optical delay means is adapted to delay at least one portion of the received backscatter signals on the first return path with respect to the other return optical paths by a predetermined delay time, τ.sub.2 thereby to produce a delayed backscatter signal and at least one direct backscatter signal.

(49) The delay of the first and second delay means satisfy the relation |τ.sub.1−τ.sub.2|<a τ.sub.coh wherein multiplication factor, a, may be between 1 and about 100. The first and second delay means may be a common device or a common optical path. Method 1400 further comprises receiving 1419 the delayed and direct backscatter signals with a phase and amplitude receiver adapted to measure the relative phase difference between the separated delayed and direct backscatter signals. An analysis processor 135 can be used on the measured amplitudes and phases to perform the measurement of optical path length changes in the sensing medium in a distributed manner.

(50) Returning now to FIG. 2, there is shown a timeline depiction of the arrival times of distributed backscatter signals onto the phase and amplitude receiver 131 from the sensing medium in the presently disclosed systems and methods such as system 100 with a modulated optical source 101. Where the modulated source is an intensity modulated source, the depth of the intensity modulation is preferably greater than 50% up to 100% (for example the output from a pulsed optical source). For clarity, the arrival times in FIG. 2 are described with reference to a pulsed optical source. Soon after the pulse 200 is generated, light which has traveled through the direct paths (i.e. bypassing the first optical delay means 107a in the forward propagating direction as well as the second optical delay means 107b in the backward propagating direction) before and after the sensing medium (optical fibre) 160 arrives at the phase and amplitude receiver 131. It is spread through time due to the distributed backscatter. If the delays are longer than the round-trip time of light in the sensing medium 160, the light which arrives at receiver 131 has traveled first through the direct path then through the delayed path (i.e. Signal 3 161b) will arrive at the receiver without overlap with signal 1 161a and signal 4 163b. Signal 3 161b will interfere coherently with the light which has first traveled through the delayed path then the direct path i.e. Signal 2 163a, which arrives nearly synchronously with Signal 3 161b. The relative phase of Signal 3 161b and Signal 2 163a is used for the present method of sensing. The light which travels through both delayed paths (i.e. Signal 4 163b) then follows. The process is repeated for subsequent pulses 205 and 206.

(51) FIG. 3 shows a conceptual schematic block-diagram representation of a DAS system 200 according to a further arrangement of the present invention in a common-path configuration. In FIG. 3 common reference numerals are used to designate like elements as compared with FIG. 1.

(52) A common-path embodiment, as illustrated in FIG. 3, is where the delays before and after the sensing medium 160 are achieved using a common optical path (for example a waveguide such as an optical fiber though the polarization, direction or timing may be different. Alternatively, a common path embodiment can have completely identical optical paths for the delays before and after the sensing medium 160. Such embodiments greatly simplify the manufacturing costs and complexity of the system, since the change in delays between two or more separate IRDP or optical delay means with respect to such parameters as temperature, pressure or ageing, do not need to be accounted or compensated for in the system (either physically, electronically or in signal processing) to ensure that the difference in delay remains less than the coherence time of the source in accordance with the system that methods disclosed herein. In the common path embodiments (where the delays are static), the phase measured at the phase and amplitude receiver would be close to zero when the sensing fiber is also static. Any change in optical path length within the sensing medium 160 would result in proportional non-zero phase. Without a common path embodiment, and where long lengths of different optical fiber are used for delays before and after the sensing medium, care should be taken to ensure that the fibers optical path lengths are manufactured to within the coherence length of the source (which can be as short as τ.sub.coh˜100 micrometers), and will remain so over the operating temperature range of the system (for example 10° C. to 50° C.).

(53) In the common-path arrangement of FIG. 3 the forward-propagating IRDP with optical delay means 107a and the backward-propagating IRDP with optical delay means 107b are replaced with a single IRDP which is common to both forward- and backward propagating paths with a single common optical delay means 207. System 200 additionally comprises a plurality of optical circulators or couplers 206a and 206b to: (a) receive light from optical source 101 and direct it through the IRDP in the forward propagating direction where the two split portions of the output signal are combined in hybrid combiner/splitter 211 before being launched into the sensing medium 160 to generate backscatter signals; and (b) receive the backward propagating backscattered light from the sensing medium 160 (e.g. optical fiber sensing medium) where each received backscatter signal is split into two portions by combiner/splitter 211, and each of the split backscatter signals passing through common-path IRDP in the backward-propagating direction and directed by circulators 206a and 206b to phase and amplitude receiver 131. Again the detected signals are recorded and stored in storage 133 and analysed by analysis processor 135 to calculate the distributed path length changes in the sensing medium 160 caused by external disturbances.

(54) System 200 further comprises optional amplifier 213 for optical application of the forward propagating optical output signals and the backward propagating received optical backscatter signals from medium 160.

(55) FIGS. 4 and 5 show schematic layouts of dual path (see for example, FIG. 1) and common-path (see for example, FIG. 3) optical systems 400 and 500 in common system nomenclature in further arrangements of systems 100 and 200 respectively.

(56) FIGS. 6 and 7 show schematic layouts of dual path (see for example, FIG. 1) and common-path (see for example, FIG. 3) optical systems 600 and 700 in common system nomenclature in further arrangements of systems 100 and 200 respectively. FIGS. 6 and 7 are configured utilising polarisation modifying Faraday mirrors in the interferometer arms of each arrangement. A Faraday mirror returns light with its polarisation rotated by 90° with respect to the polarisation of the input light. As would be appreciated by the person skilled in the art, the Faraday mirrors thereby serve to compensate for any uncontrolled and/or random change in the polarization state of light which has traveled through long lengths of optical fibre. In this way, long lengths of standard single mode optical fibre, which is less costly than polarization maintaining fibre, can be used for delaying optical signals while ensuring a fixed relationship between the input and output states of polarization.

(57) FIG. 8 shows the photonic components of a further arrangement 800 of the sensing system disclosed herein in a common-path embodiment, using all output ports of a 3×3 coupler 810 as a phase and amplitude receiver as would be appreciated by the person skilled in the art. If we represent the three detected output signals from an ideal 3×3 coupler by I.sub.1, I.sub.2 and I.sub.3, then the real part of the complex interference signal can be determined by the linear combination (I.sub.1+I.sub.2−2*I.sub.3) and the imaginary part of the complex signal can be determined by the linear combination (I.sub.1−I.sub.2). A time gate or modulator 801 may be used to prevent return signal from the sensing fibre from being directed back to the sensing fibre with the outgoing signals.

(58) FIG. 9 shows the photonic components of a further arrangement of the sensing system disclosed herein in the common-path embodiment with polarization beam splitter 908. The polarization states of the light travelling within different parts of the system are illustrated by the vectors and the action of various components on those polarization states can be understood by a person skilled in the art. The purpose of this arrangement is to utilize polarization to ensure that the majority of the light 901 entering the Intentional Relative Delay Path (IRDP) 902 is directed towards the sensing fibre by the action of the polarization beam splitter and Faraday mirrors. The forward-propagating light 903 entering into the sensing medium 160, in this arrangement, an optical fibre. Furthermore, this arrangement also ensures that the majority of backscattered light 904, which is backscattered by the sensing fibre and which enters the IRDP 902 is directed towards the phase and amplitude receiver 910 by same action of the polarization beam splitter and Faraday mirrors 906 and 907. The light which is directed towards to the phase and amplitude receiver and its associated polarization states are labelled 905. In this arrangement, the two orthogonal polarizations in a single fibre act as independent optical paths.

(59) Experimental validation of the present invention is provided in FIGS. 10 to 12. FIG. 10 shows the amplitude of the signal from a phase and amplitude receiver vs. time for a pulsed scheme detecting distributed backscatter radiation from a sensing medium comprising a telecommunications-grade optical fibre of approximately 800 m in length. The first portion 1001 of the detected signal corresponds to detected backscatter signals 161a arising from the direct output signal 151 having bypassed the second optical delay means 107b (Signal 1). The last portion 1003 of the detected signal corresponds to detected backscatter signals 163b arising from the delayed output signal 153 having been transmitted through the second optical delay means (Signal 4). The central portion 1005 of the detected signal corresponds to detected backscatter signal 163a arising from the delayed forward-propagating output signal 153 having bypassed the second optical delay means (Signal 2) and also from backscatter signals 161b the direct output signal 151 having been transmitted through the second optical delay means 107b (Signal 3) and thus is the portion of the detected signal in which interference between Signal 2 and Signal 3 occurs at the receiver and which is used for analysis of the optical path length changes in the sensing medium.

(60) In this example, the pulse length of pulsed light output generated by the optical source is 100 ns, the pulse repetition rate of the source output is 20 kHz and the delay is from the IRDP used in the experimental system is 15 microseconds. The data in the present example was acquired using the common path setup illustrated in FIG. 15. The coherence length of the source in the present example is less than 0.05 mm corresponding to a bandwidth of approximately 4 THz. A fiber stretcher with single frequency tone is placed in the sensing fiber to provide a vibration signal which modifies the path length of the sensing fibre at that location and is located approximately 430 m from the input end of the sensing fiber.

(61) FIG. 11 is a graph of the detected phase of the backscattered signal from a phase and amplitude receiver vs. position along the sensing fiber. FIG. 12 shows the computed strain rate vs. position and vs. time. The strain, ε, is computed from the phase gradient,

(62) $\frac{\mathrm{\Delta \varphi }}{\Delta x},$
using the formula

(63) $\mathrm{.Math.}=\frac{\Delta \varphi }{\Delta x}\frac{\lambda }{4\pi n\gamma },$
where λ is the central wavelength of the source, n is the refractive index of the fibre and γ=0.78 is an elasto-optic coefficient which quantifies the change in the refractive index of an optical fiber caused by variation in the length of the fibre length in response to mechanical strain. The position 1011 of the external disturbance to the sensing fibre (the fiber stretcher) can readily be observed in the detected signal at a distance of 430 m along the fibre.

(64) FIG. 11 also exhibits nonlinearity in the sensing fiber that is manifested in the detected signal by an upward slope in the detected phase with respect to distance indicating the regular deterministic nature of the effects of nonlinearity in the sensing medium in the present systems and methods. This effect has been attributed to the Kerr non-linearity, in which the refractive index of the fibre is momentarily modified by the intensity of the light in the fibre. If so desired, the effects of the nonlinearity in the detected signal can readily be corrected for either by equalizing the power in the delayed and direct paths, or in the analysis by removing a constant offset, as would be appreciated by the skilled addressee. Experimental results using the arrangement in FIG. 5 and with peak optical powers exceeding 1 W have not shown any negative impacts associated with the non-linear effects of modulation instability nor stimulated Brillouin scattering, which are known have detrimental effects on coherent optical fibre sensing systems at much lower peak powers (peak optical power).

(65) The system 1300 in FIG. 13 has a similar arrangement to system 800 in FIG. 8, utilising a 3×3 coupler 1310 and can operate in 2 distinct modes. In the first mode of operation, the broadband amplified spontaneous emission (ASE) from the amplifier 1301 functions as the optical source for the system. This ASE can be modulated by an intensity modulator 1302 prior to entering the IRDP 1305 and then is directed towards the sensing fibre. A second modulator/time gate 1312 serves as a time gate to allow this modulated ASE into the sensing fibre but no other unwanted light (e.g. backscattered light from the sensing fibre would be prevented from re-circulating in the system). In the second mode of operation, the modulator 1302 is not required and the continuous ASE from amplifier 1301 enters the IRDP and is then modulated at the intensity modulator 1312 prior to being directed towards the sensing fibre. The intensity modulator 1312 can also serve to prevent unwanted light from entering the sensing fibre in this mode of operation. In either mode of operation, the modulators 1302 and 1312 may be incorporated directly in the amplifiers 1301 and 1311 (respectively), through direct modulation of the amplifier gain.

(66) FIG. 15 shows a schematic layout of a common-path (cf. FIG. 3) optical system 1500 in common system nomenclature in further arrangements of systems 100, 200 and 300 respectively. System 1500, in a similar manner to system 800 of FIG. 8, uses all three optical signal outputs on the returning paths of a 3×3 coupler 1510 as a phase and amplitude receiver as would be appreciated by the person skilled in the art.

(67) In FIG. 16, there is depicted a method 1600 of analysis and processing as would be implemented in the analysis processor 135 of FIG. 1). With reference to system e of FIG. 1, method 1600 comprises the steps of: reducing system noise 1601 through means such as filtering of electronic signals; Constructing a complex signal 1602 using the amplitudes and phases measured by phase and amplitude receiver 131; applying an process/algorithm 1603 of deconvolution, cross-correlation, de-coding, spiking, chromatic dispersion compensation, polarization dispersion compensation or nonlinearity compensation on the complex signal to compensate for the known or measured modulation applied to the output signals 151 and 153 or the propagation properties of the medium these signals traversed; computing the phase of the resulting complex signal 1604; taking phase differences or phase gradients 1605 to compute the optical path length change or a physical parameter of the sensing medium 160 such as, for example, strain (where sensing medium 160 comprises an optical fibre) as a function of time and position; filtering or post-processing 1606 as required for a given application; applying known methods of automated interpretation or classification 1607 as required for a given application; displaying 1608 and/or storing 1609 the resulting sensing data; and generating an alert 1610 to a user based on predefined criteria and as required for a given application.

(68) FIG. 17 shows a conceptual schematic block-diagram representation of a DAS system 1700 according to a further arrangement of the present invention in a common-path configuration. In FIG. 17, common reference numerals are used to designate like elements as compared with FIG. 1 and FIG. 3. System 1700 shows the delayed output signal 151 and the direct output signal 153 being directed to the sensing medium 160 without an intermediate step of recombining the output signals onto a common forward optical path.

(69) FIG. 18 shows a conceptual schematic block-diagram representation 1800 of output signals directed to the sensing medium without a step of recombining the output signals onto a common forward optical path. In FIG. 18 common reference numerals are used to designate like elements as compared with FIG. 17. FIGS. 18A to 18C respectively show three example arrangements 1801, 1802 and 1803 of photonic implementations of system 1800. Arrangement 1802 shows an example where the 2 output and 2 return paths from the sensing medium may not be separated spatially, but rather are separated and independent due to orthogonal polarizations, and not necessarily linear polarization states.

(70) FIG. 19 depicts a further embodiment 1900 of the systems disclosed above, where the forward propagating path 130 comprises splitter and delay means (not shown) similar to that depicted in FIG. 1. Before being directed to sensing medium 160, however, frequency demultiplexer/multiplexer 1920 is provided to split the forward propagating (direct and delayed) optical signals into a plurality of frequency bands, and to direct each pair of forward propagating signals, in each frequency band to a selected one of a plurality of sensing mediums 160.

(71) Frequency multiplexer/demultiplexer (Mux/Demux) 1920 is further adapted to receive backward propagating backscatter signals from each sensing medium 160 and direct the pairs of signals from each sensing medium 160 onto return path 135 similar to return path as shown in FIG. 1. System 1900 further comprises frequency demultiplexer 1930 to direct selected frequency bands from the backward-propagating signals onto a corresponding plurality of phase and amplitude receivers 131 adapted to measure differences in amplitude phase of the received optical signals in the selected frequency bands to determine optical path length changes along each selected mediums 160 in a distributed manner to infer physical changes in the selected mediums 160. As shown in FIG. 3, the forward and return paths 130 and 135 can share a common IRDP. In some embodiments, it may be advantageous to direct the backward-propagating signals in multiple frequencies bands to one phase and amplitude receiver 131, in which case the number of sensing mediums is more than the number of phase and amplitude receivers. In other embodiments, it may be advantageous to separate the backward-propagating signals from one sensing medium into multiple frequencies bands and direct the light in each frequency band to separate phase and amplitude receivers, in which case the number of sensing mediums is less than the number of phase and amplitude receivers.

(72) Derivation of the Signal Model

(73) Using the coupled mode equations for coupling between forward and back propagating modes in an optical medium specifically, an optical fiber, Froggatt and Moore (M. Froggatt and J. Moore, “High-spatial-resolution distributed strain measurement in optical fiber with Rayleigh scatter”, Appl. Opt., vol. 37, no. 10, pp. 1735-1740, 1998.) have derived the following expression:

(74) $\begin{array}{cc}R\left(\beta \right)=\frac{\beta E_0\left(\beta \right)}{2i}{\int }_{-\infty }^{\infty }\frac{\mathrm{.Math.}\left(z\right)-{\mathrm{.Math.}}_{co}}{{\mathrm{.Math.}}_{co}}{e}^{i2\beta z}dz& \left(1\right)\end{array}$
where R(β) is the complex amplitude of the Rayleigh (non-frequency shifted) backscattered wave; ε.sub.co is the permittivity of the fibre core; ε(z)−ε.sub.co is the random variation of the permittivity of the fibre core; We assume (z)−ε.sub.co ≡0 outside the sensing fibre; β is the propagation constant in the waveguide; and E.sub.0(β) is the complex amplitude of the exciting field at z=0.

(75) This result shows that the complex amplitude of the backscattered field from the random permittivity fluctuation is the spatial Fourier transform of the permittivity fluctuation evaluated at twice the special frequency of the exciting field.

(76) The following substitutions can then be made:
z=v.sub.pτ/2, where τ is the 2-way (phase velocity) travel time; and v.sub.p=c/n.sub.eff is the phase velocity; S(ω)=R(β(ω)) is the signal in the frequency domain; E(ω)=E.sub.0(β(ω)) is the launch field in the frequency domain; and β=ω/v.sub.p is the propagation constant,
to give:

(77) $S\left(\omega \right)=E\left(\omega \right)\left(-i\omega \right){\int }_{-\infty }^{\infty }\frac{1}{4}\frac{\mathrm{.Math.}\left(\frac{v_p\tau }{2}\right)-{\mathrm{.Math.}}_{co}}{{\mathrm{.Math.}}_{co}}{e}^{i\omega \tau }d\tau$
Now we define:

(78) $y\left(\tau \right)=\frac{1}{4}\frac{\mathrm{.Math.}\left(\frac{v_p\tau }{2}\right)-{\mathrm{.Math.}}_{co}}{{\mathrm{.Math.}}_{co}}$
And using Fourier transforms formulations:
Y(ω)=[y(τ)]
to get:
S(ω)=E(ω)(−iω)Y(ω).
Or, equivalently, the signal model can be rewritten in concise notation as:
S(ω)=E(ω)G(ω)
s(τ)=e(τ)*g(τ)  (2)
where * denotes convolution G(ω)=(−iω)Y(ω) has been substituted g(τ) is interpreted as the impulse response function of the fibre in the time domain. s(τ) is the signal in the time domain e(τ) is the launch field in the time domain g(τ), s(τ) and e(τ) are related to G(ω), S (ω) and E(ω) by Fourier transforms.
Using the identity:

(79) ${ℱ}^{-1}\left[\left(-i\omega \right)Y\left(\omega \right)\right]=\frac{dy\left(\tau \right)}{d\tau }$
we find:

(80) $\begin{array}{c}g\left(\tau \right)=\frac{dy\left(\tau \right)}{d\tau }\\ =\frac{1}{4{\mathrm{.Math.}}_{\mathrm{co}}}\frac{d\mathrm{.Math.}\left(\frac{v_p\tau }{2}\right)}{d\tau }\\ =\frac{v_p}{8{\mathrm{.Math.}}_{\mathrm{co}}}\frac{d\mathrm{.Math.}\left(z\right)}{\mathrm{dz}}\end{array}$

(81) To determine the interference between 2 backscattered fields on the same fibre, we consider the geometry depicted in FIG. 1. In this geometry, there are two backscattered signal fields at the phase and amplitude receiver:
s(t)=e(t)*g(t)+e(t−T)*{tilde over (g)}(t)
s(t−{tilde over (T)})=e(t−{tilde over (T)})*g(t)+e(t−T−{tilde over (T)})*{tilde over (g)}(t) where {tilde over (g)}(t) is the impulse response function of the fibre after a delay T. It represents the perturbation applied to g (t) which is to be sensed.
Consider the interference term:

(82) $s\left(t\right)s^{*}\left(t-T\right)=\left(e\left(t\right)*g\left(t\right)\right){\left(e\left(t-\tilde{T}\right)*g\left(t\right)\right)}^{*}+\left(e\left(t\right)*g\left(t\right)\right){\left(e\left(t-T-\tilde{T}\right)*\tilde{g}\left(t\right)\right)}^{*}+\left(e\left(t-\tilde{T}\right)*\tilde{g}\left(t\right)\right){\left(e\left(t-\tilde{T}\right)*q\left(t\right)\right)}^{*}+\left(e\left(t-T\right)*\tilde{g}\left(t\right)\right){\left(e\left(t-T-\tilde{T}\right)*\tilde{g}\left(t\right)\right)}^{*}$

(83) Through time, frequency or polarization multiplexing, terms 1, 2 and 4 can be forced to equal zero, leaving the only the 3rd term:
s(t)s*(t−T)=(e(t−T)*{tilde over (g)}(t))(e(t−{tilde over (T)})*g(t))*

(84) Substituting: e.sub.τ(t)=e(t−T); {tilde over (g)}(t)=g(t+ρ(t)), i.e. a very small deformation has occurred to g(t) after a time T, equivalent to a position dependent shift/dilation; ΔT=T−T;
we get

(85) 0$s\left(t\right)s^{*}\left(t-T\right)=\left(e_T\left(t\right)*g\left(t+\rho \left(t\right)\right)\right){\left(e_T\left(t+\Delta T\right)*g\left(t\right)\right)}^{*}=\underset{t^{\prime }}{\int }{e}_T\left(t-t^{\prime }\right)g\left(t^{\prime }+\rho \left(t^{\prime }\right)\right){\mathrm{dt}}^{\prime }\underset{t^{″}}{\int }{e}_T^{*}\left(t-t^{″}-\Delta T\right)g^{*}\left(t^{″}\right){\mathrm{dt}}^{″}=\underset{t^{″}}{\int }\underset{t^{\prime }}{\int }{e}_T\left(t-t^{\prime }\right)e_T^{*}\left(t-t^{″}-\Delta T\right)g\left(t^{\prime }+\rho \left(t^{\prime }\right)\right)g^{*}\left(t^{″}\right){\mathrm{dt}}^{\prime }{\mathrm{dt}}^{″}$

(86) Then, substitute t.sup.#=t′+ρ(t′) to get:

(87) $=\underset{t^{″}}{\int }\underset{t^{\prime }}{\int }{e}_T\left(t-t^{\#}+\rho \left(t^{\prime }\right)\right)e_T^{*}\left(t-t^{″}-\Delta T\right)g\left(t^{\#}\right)g^{*}\left(t^{″}\right){\mathrm{dt}}^{\prime }{\mathrm{dt}}^{″}$${\mathrm{dt}}^{\#}={\mathrm{dt}}^{\prime \left(1+\frac{d\rho \left(t^{\prime }\right)}{{\mathrm{dt}}^{\prime }}\right)}\cong {\mathrm{dt}}^{\prime }\mathrm{since}\frac{d\rho \left(t^{\prime }\right)}{{\mathrm{dt}}^{\prime }}<<1,\mathrm{and}$$\rho \left(t^{\prime }\right)\cong \rho \left(t^{\#}\right)$

(88) Therefore:

(89) $=\underset{t^{″}}{\int }\underset{t^{\#}}{\int }{e}_T\left(t-t^{\#}+\rho \left(t^{\#}\right)\right)e_T^{*}\left(t-t^{″}-\Delta T\right)g\left(t^{\#}\right)g^{*}\left(t^{″}\right){\mathrm{dt}}^{\#}{\mathrm{dt}}^{″}$

(90) Substituting t′=t.sup.#:

(91) $=\underset{t^{″}}{\int }\underset{t^{\prime }}{\int }{e}_T\left(t-t\text{'}+\rho \left(t^{\prime }\right)\right)e_T^{*}\left(t-t^{″}-\Delta T\right)g\left(t^{\prime }\right)g^{*}\left(t^{″}\right){\mathrm{dt}}^{\prime }{\mathrm{dt}}^{″}$

(92) When sampled at time t.sub.j with sampling time Δt:

(93) $d\left(t_j\right)=\frac{1}{\Delta t}\underset{t_j}{\overset{t_j+\Delta t}{\int }}s\left(t\right)s^{*}\left(t-T\right)\mathrm{dt}$$d\left(t_j\right)=\underset{t^{″}}{\int }\underset{t^{\prime }}{\int }g\left(t^{\prime }\right)g^{*}\left(t^{″}\right)\frac{1}{\Delta t}\underset{t_j}{\overset{t_j+\Delta t}{\int }}{e}_T\left(t-t^{\prime }+\rho \left(t^{\prime }\right)\right)e_T^{*}\left(t-t^{″}-\Delta T\right)\mathrm{dt}{\mathrm{dt}}^{\prime }{\mathrm{dt}}^{″}$

(94) Assuming the following Stochastic model for the impulse response function:

(95) $\frac{1}{\Delta T}\underset{t_j}{\overset{t_j+\Delta t}{\int }}{e}_T\left(t-t^{\prime }+\rho \left(t^{\prime }\right)\right)e_T^{*}\left(t-t^{″}-\Delta T\right)\mathrm{dt}=\frac{1}{\Delta t}\underset{t_j}{\overset{t_j+\Delta t}{\int }}{e}_T\left(t-t^{″}-\Delta T+t^{″}-t^{\prime }+\Delta T+\rho \left(t^{\prime }\right)\right)e_T^{*}\left(t-t^{″}-\Delta T\right)\mathrm{dt}\approx I\left(t_j-t^{″}-\Delta T\right)\delta \left(t^{″}-t^{\prime }\right)e^{i{\omega }_0\left(t^{″}-t^{\prime }+\Delta T+\rho \left(t^{\prime }\right)\right)}$

(96) Where the detected intensity is given by:

(97) $I\left(t_j-t^{″}-\Delta T\right)=\frac{1}{\Delta t}{\int }_{t_j}^{t_j+\Delta t}{e}_T\left(t-t^{″}-\Delta T\right)e_T^{*}\left(t-t^{″}-\Delta T\right)dt;$
we get:

(98) $d\left(t_j\right)\approx \underset{t^{″}}{\int }\underset{t^{\prime }}{\int }g\left(t^{\prime }\right)g^{*}\left(t^{″}\right)I\left(t_j-t^{″}-\Delta T\right)\delta \left(t^{″}-t^{\prime }\right)e^{i{\omega }_0\left(t^{″}-t^{\prime }+\Delta T+\rho \left(t^{\prime }\right)\right)}{\mathrm{dt}}^{\prime }{\mathrm{dt}}^{″}$$\mathrm{or}\text{:}$$d\left(t_j\right)\approx \underset{t^{\prime }}{\int }g\left(t^{\prime }\right)g^{*}\left(t^{\prime }\right)I\left(t_j-t^{\prime }-\Delta T\right)e^{i{\omega }_0\left(\Delta T+\rho \left(t^{\prime }\right)\right)}{\mathrm{dt}}^{\prime }$

(99) Or equivalently, in convolution notation:
d(t.sub.j)≈I(t.sub.j−ΔT)*(e.sup.iω.sup.0.sup.(ΔT+ρ(t.sup.j.sup.))|g(t.sub.j)|.sup.2)

(100) Thus, to determine the change in optical path length, ρ(t.sub.j), a deconvolution between the intensity modulation, I(t.sub.j−ΔT), and the recorded data, d(tj), is applied
[I(t.sub.j−ΔT)*].sup.−1d(t.sub.j)≈e.sup.iω.sup.0.sup.(ΔT+ρ(t.sup.j.sup.))|g(t.sub.j)|.sup.2.

(101) And therefore:

(102) $\rho \left(t_j\right)\approx \frac{\mathrm{phase}\left[{I\left(t_j-\Delta T\right)}^{*-1}d\left(t_j\right)\right]}{{\omega }_0}-\Delta T.$

(103) Or alternatively:

(104) $\rho \left(t_j\right)\approx \frac{\mathrm{phase}\left[{ℱ}^{-1}\left[ℱ\left[d\left(t_j\right)\right]/ℱ\left[I\left(t_j-\Delta T\right)\right]\right]\right]}{{\omega }_0}-\Delta T.$

(105) Cross-correlation can form an approximate deconvolution, particularly if ||[I(t.sub.j)]|.sup.2≈constant, and the cross-correlation is written as:

(106) 0$\rho \left(t_j\right)\approx \frac{\mathrm{phase}\left[{ℱ}^{-1}\left[ℱ\left[d\left(t_j\right)\right]{ℱ}^{*}\left[I\left(t_j-\Delta T\right)\right]\right]\right]}{{\omega }_0}-\Delta T.$

(107) If ΔT is larger than the coherence length, then:
d(t.sub.j)≈∫.sub.t″∫.sub.t′g(t′)g*(t″)I(t.sub.j−t″−ΔT)δ(t″−t′+ΔT)e.sup.iω.sup.0.sup.(t″−t′+ΔT+ρ(t′))dt′dt″;
d(t.sub.j)≈∫.sub.t′g(t′)g*(t′−ΔT)I(t.sub.j−t′ΔT)e.sup.iω.sup.0.sup.(ρ(t′))dt′.

(108) Or equivalently, in convolution notation:
d(t.sub.j)≈I(t.sub.j−ΔT)*(e.sup.iω.sup.0.sup.ρ(t.sup.j.sup.)g(t.sub.j)g*(t.sub.j−ΔT))=I(t.sub.j−ΔT)*(e.sup.iω.sup.0.sup.ρ(t.sup.j.sup.)e.sup.iψ(t.sup.j.sup.)|g(t.sub.j)g*(t.sub.j−ΔT)|)

(109) If g*=ĝ represents a different sensing medium (e.g. optical fibre), then
d(t.sub.j)≈∫.sub.t″∫.sub.t′g(t′)ĝ(t″)I(t.sub.j−t″−ΔT)δ(t″−t′+ΔT)e.sup.iω.sup.0.sup.(t″−t′+ΔT+ρ(t′))dt′dt″;
d(t.sub.j)≈∫.sub.t′g(t′)ĝ(t′−ΔT)I(t.sub.j−t′ΔT)e.sup.iω.sup.0.sup.(ρ(t′))dt′.

(110) Or equivalently, in convolution notation:
d(t.sub.j)≈I(t.sub.j−ΔT)*(e.sup.iω.sup.0.sup.ρ(t.sup.j.sup.)g(t.sub.j)ĝ(t.sub.j−ΔT))=I(t.sub.j−ΔT)*(e.sup.iω.sup.0.sup.ρ(t.sup.j.sup.)e.sup.iψ(t.sup.j.sup.)g(t.sub.j)ĝ(t.sub.j−ΔT)|)
Analysis of the Detected Signal

(111) The derivation above shows that the change in optical path length, ρ(t), induced in the sensing medium, e.g. an optical fiber, is given by the expression:

(112) $\rho \left(t\right)\approx \frac{\mathrm{phase}\left[{ℱ}^{-1}\left[ℱ\left[d\left(t\right)\right]/ℱ\left[I\left(t-\Delta T\right)\right]\right]\right]}{{\omega }_0}-\Delta T$
where: t is the sample time; z=v t/2 is the position along the fiber; v is the velocity of light in the fiber; d(t) is the complex signal from the phase and amplitude receiver; I(t) is the modulated laser intensity; denotes the Fourier transform; .sup.−1 denotes the inverse Fourier transform; ω.sub.0 is the central frequency of the source; and ΔT is the difference between the delays before and after the sensing fiber.

(113) Thus the change in optical path length can be determined by a deconvolution between the complex signal from the phase and amplitude receiver and the modulated laser intensity. A cross-correlation can be applied instead of a deconvolution if |[I(t.sub.j)]|.sup.2≈constant.

(114) In the case where the intensity modulation is pulsed, the change in optical path length can be determined directly from the phase of the complex signal, as measured at the phase and amplitude receiver:

(115) $\rho \left(t\right)\approx \frac{\mathrm{phase}\left[d\left(t\right)\right]}{{\omega }_0}-\Delta T.$

Applications

(116) Possible uses of the systems and methods disclosed herein may include: distributed acoustic sensing on fiber or waveguides; surface vibrometry; distributed acoustic LIDAR in air and atmosphere; wind velocity measurement (anemometry); distributed acoustic LIDAR in water; distributed acoustic LIDAR in pipeline fluids; Vertical seismic profiling in boreholes and wells; Marine streamers for seismic exploration; Land seismic sensors for seismic exploration; Permanent seismic monitoring arrays for repeat seismic imaging and inversion; Passive seismic monitoring, such as earthquake monitoring, micro-seismic monitoring and induced seismicity related to underground fluid injection or production; Monitoring of mine wall stability, such as microseismic monitoring and caving; Monitoring of dam stability, such as water dam induced seismicity and stiffness of tailings dams; Pipeline monitoring, such a leak detection and tampering; Perimeter and security monitoring/surveillance, such as intrusion detection; Infrastructure monitoring, such as strain and vibration control on bridges, tunnels, buildings and wind turbines; Vehicle structure monitoring, such as strain and vibration control in car, aeroplanes and ships; Flow measurement, such as metering flow in pipelines; Geotechnical surveys, such as surface wave inversion for of near surface shear wave velocity; Air movement profiling, such as atmospheric profiling, wind chamber profiling and around air vehicles; Water movement profiling, such as profiling oceanic currents, river flow and around marine vehicles; Medical devices, such as body strain sensors and blood flow measurements; Monitoring of telecommunication networks, such as disturbance and faults; Traffic and vehicle flow monitoring, such as roads, rail and boats; audio recording; and Fire monitoring, such as in tunnels and infrastructure.

Advantages

(117) As will be appreciated by the skilled addressee from the disclosure herein, the systems and methods disclosed herein overcome limitations inherent in the systems taught by existing distributed acoustic sensing systems, including the advantages set out below.

(118) Direct phase and amplitude measurement allows accurate determination of the rate and magnitude of optical path length changes in the sensing fiber with very high sensitivity. This also allows for a wider range of applications, such as machine condition monitoring, which are quantitative and beyond basic disturbance detection for security alerting. As a demonstration of sensitivity and fidelity, the systems disclosed herein experiments have been demonstrated to be able to acoustically record normal human voice and play-back the recorded audio with fidelity comparable to a microphone recording.

(119) Direct phase and amplitude measurement provides the ability to unambiguously distinguish between amplitude and phase changes of interfering light, which (in practical operation) overcomes erroneous measurements of the optical path length change, where such errors may be induced by attenuation effects, splices, connectors, non-linear effects (including the Kerr non-linear effect) in optical fibres.

(120) The measurement of phase in full-quadrature (2π range) and subsequent unwrapping eliminates any π range ambiguity which would result in errors in sign and interpretation of a physical quantity, for example, causing ambiguity between compression and tension. Furthermore, it extends dynamic range of the systems disclosed herein by a factor of 2.

(121) Direct phase and amplitude measurement overcomes the problem of the systems disclosed herein being prone to drifting into a state of total insensitivity during practical operation. This state of insensitivity occurs near specific phase values where a small phase-change does not produce a measurable interference-intensity change. A 3×3 coupler can be used just to create a relative phase bias of 120° between the optical fields on its output ports, thereby using the phase bias to select a higher slope region of the coupler's transfer-function in order to improve the sensitivity when the magnitude of the disturbance is small. However, this solution is not reliable in practical operation, as the bias requirements would drift with such factors as time; position along the fibre sensing medium; system parameters and environmental conditions.

(122) Direct phase and amplitude measurement allows for coding and/or modulation schemes to be used with the systems disclosed herein to improve the system performance, including improving sensitivity and extending measurement range.

(123) Direct phase and amplitude measurement avoids source intensity-noise corrupting the phase measurement and thereby the disturbance signal, which otherwise limits the signal-to-noise discrimination of the system.

(124) Intentional intensity modulation of the source enables accurate distributed sensing in a sensing medium such as, for example, a sensing fiber. The systems and methods disclosed herein are not limited to the detection and location of a single disturbance in the sensing medium only and allow for disturbances to be categorised based on position, time or frequency.

(125) The systems and methods disclosed herein also overcome the limitations inherent in existing c-OTDR and c-OFDR systems, for example: use of a broadband source significantly reduces cost, complexity and robustness of the system, without introducing phase noise, degrading sensitivity or limiting measurement range.

(126) The systems and methods disclosed herein intrinsically and simultaneously average the distributed sensing signal for all optical frequencies present in the broadband source. This is a key advantage when compared to existing narrowband/coherent c-OTDR and c-OFDR systems, as it greatly improves sensitivity, accuracy and linearity by eliminating issues associated with amplitude fading vs. position along the sensing fiber. The phenomenon of amplitude fading in narrowband/coherent c-OTDR and c-OFDR is fundamental to the random nature of Rayleigh backscatter, which results in random amplitudes and phases at each location along the fibre. This phenomenon is analogous to speckle on rough surfaces when illuminated with coherent light. At locations where the backscatter amplitude happens to be close to zero, the corresponding phase at these locations cannot be accurately determined or may be undefined when the amplitude is exactly equal to zero. Furthermore, near-zero amplitudes ‘appear’ as a sign change in the signal, which produces highly non-linear errors in any estimated phase change. These phase errors are a large and fundamental source of error in c-OTDR and c-OFDR DAS systems, which would then require the use of multiple narrowband, highly coherent laser sources in the system to overcome the limitations of amplitude fading; achieved by the exploiting the fact that each distinct laser wavelength produces a different, but random, realization of amplitudes and phases (i.e. a different speckle pattern). Averaging of phases with an amplitude-weighting, performed in digital signal processing, can reduce phase errors in the system since the phases measured at locations where the amplitude is near zero are largely ignored. The systems and methods disclosed herein utilize a sufficiently broadband source to ensure that there are no locations along the sensing fiber where backscatter amplitude may randomly occur near zero. This is achieved since, at each optical frequency, the phases at each location are not random (although the amplitudes are random), rather the phase at each location is proportional to the optical path length change (the desired measurement) and the delay difference. Therefore, each frequency of the optical source constructively contributes to the amplitude and phase at each location. The random amplitude pattern is then washed-out analogously to the elimination of speckle on a rough surface when using a sufficiently broadband source.

(127) Use of a broadband optical source raises the optical power that can be used for sensing, since the power-threshold for unwanted non-linear effects is higher compared to coherent sources. Nonlinear effects with improved thresholds include; stimulated Brillouin scattering, four-wave mixing and modulation instability. Indeed, the systems and methods disclosed herein can be used even in the presence of strong nonlinear effects within the sensing medium as the effect of the nonlinear mechanism on the backscattered signal is deterministic in the low coherent regime and thus can be corrected for in the analysis of the coherently detected signal.

(128) Arrangements of the systems and methods disclosed herein exhibit superior stability and robustness in uncontrolled environments and in the presence of vibration noise sources. This can be very important for reliable performance in outdoor applications with large machinery, such as seismic monitoring at oil-rigs and mine sites.

(129) Generally, in c-OTDR and c-OFDR systems, phase noise in the source is unwanted, as it increases overall noise and therefore reduces the signal-to-noise of the system. This encourages the use of narrower-band and more highly coherent laser sources. The systems and methods disclosed herein teaches contrary to this situation. That is, simulations of systems and methods disclosed herein indicate that increasing the source bandwidth (linewidth) and encouraging greater incoherence (less coherence i.e. shorter coherence times and lengths) in the optical source will actually reduce noise and improve the signal-to-noise ratio contrary to conventional thinking. This is due to the fact that the incoherent interference between backscattered light returning from different sections of the sensing medium which are separated by more than the coherence length will create a background electronic (beating) noise which is spread out over the entire bandwidth of the source. Since this noise can have an electronic bandwidth larger than 1 THz, it can be very effectively filtered and removed from the signal.

(130) Pulsing of the source has advantages over continuous wave sources which can “blind” the system during practical operation where strong reflections are present in the sensing medium. This is a common occurrence in optical fibre links involving 1 or more connectors or devices, which potentially makes existing distributed acoustic sensing systems unusable in a many practical cases.

(131) Pulsing of the source improves signal-to-noise discrimination of the systems disclosed herein when the Direct+Direct and Delayed+Delayed backscatter signals arrive at different times to the Direct+Delayed and Delayed+Direct signals.

(132) Distributed sensing can be achieved with the systems disclosed herein even in the limitation of having access to only one end of the sensing fiber, as opposed to Sagnac-type systems which requires access to both ends of the sensing fibre. This provides advantages in borehole and well applications, or other applications with constrained access.

(133) Methods of polarization management presented herein avoid major inaccuracies caused by polarization stability of the laser, or polarization mode dispersion within optical components or the sensing medium.

(134) The systems disclosed herein allow for implementation with optical fibre components, bulk optic components, micro-optics components and/or planar waveguide technologies for greater versatility in the implementation of the systems for many varied sensing applications.

(135) Multiple sensing fibres (or available sensing media) can be connected to the system by means of wavelength division multiplexing since the broadband source has a plurality of optical wavelengths that can be used for independent sensing.

Interpretation

In Accordance With

(136) As described herein, ‘in accordance with’ may also mean ‘as a function of’ and is not necessarily limited to the integers specified in relation thereto.

Embodiments/Arrangements

(137) Reference throughout this specification to “one embodiment”, “an embodiment”, “one arrangement” or “an arrangement” means that a particular feature, structure or characteristic described in connection with the embodiment/arrangement is included in at least one embodiment/arrangement of the present invention. Thus, appearances of the phrases “in one embodiment/arrangement” or “in an embodiment/arrangement” in various places throughout this specification are not necessarily all referring to the same embodiment/arrangement, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments/arrangements.

(138) Similarly it should be appreciated that in the above description of example embodiments/arrangements of the invention, various features of the invention are sometimes grouped together in a single embodiment/arrangement, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment/arrangement. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment/arrangement of this invention.

(139) Furthermore, while some embodiments/arrangements described herein include some but not other features included in other embodiments/arrangements, combinations of features of different embodiments/arrangements are meant to be within the scope of the invention, and form different embodiments/arrangements, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments/arrangements can be used in any combination.

SPECIFIC DETAILS

(140) In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Terminology

(141) In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as “forward”, “rearward”, “radially”, “peripherally”, “upwardly”, “downwardly”, and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms.

Different Instances of Objects

(142) As used herein, unless otherwise specified the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

Scope of Invention

(143) Thus, while there has been described what are believed to be the preferred arrangements of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

(144) Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

INDUSTRIAL APPLICABILITY

(145) It is apparent from the above, that the arrangements described are applicable to the mobile device industries, specifically for methods and systems for distributing digital media via mobile devices.

(146) It will be appreciated that the methods/apparatus/devices/systems described/illustrated above at least substantially provide improved systems and methods for quantitative distributed measurement of optical path length changes in an optically transparent medium.

(147) The systems and methods described herein, and/or shown in the drawings, are presented by way of example only and are not limiting as to the scope of the invention. Unless otherwise specifically stated, individual aspects and components of the systems and methods described herein may be modified, or may have been substituted therefore known equivalents, or as yet unknown substitutes such as may be developed in the future or such as may be found to be acceptable substitutes in the future. The systems and methods described herein may also be modified for a variety of applications while remaining within the scope and spirit of the claimed invention, since the range of potential applications is great, and since it is intended that the present systems and methods described herein be adaptable to many such variations.