Natural fiber span reflectometer providing a virtual differential signal sensing array capability

• US 7,274,441 B2
• Filed: 02/07/2005
• Issued: 09/25/2007
• Est. Priority Date: 08/06/2004
• Status: Active Grant

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: The United States of America As Represented By The Secretary of Agriculture
• Inventors: Payton, Robert Michael
• Primary Examiner(s): Nguyen; Tu T.
• Application Number:
  US11/056,631
• Publication Number:
  US 20060028637A1
• Time in Patent Office: 960 Days
• Field of Search: None
• US Class Current: 356/73.1
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01M 11/3118 using coded light-pulse sequences
• CPC Class Codes: G01M 11/3172 Reflectometers detecting the back-scattered light in the frequency-domain, e.g. OFDR, FMCW, heterodyne detection
• CPC Class Codes: G08B 13/186 using light guides, e.g. optical fibres

Abstract

A CW lightwave modulated by a continuously reiterated pseudorandom (PN) code sequence is launched into an end of a span of ordinary optical fiber cable. Portions of the launched lightwave back propagate to the launch end from a continuum of locations along the span because of innate fiber properties including Rayleigh scattering. This is picked off the launch end and heterodyned producing a r.f. beat signal. The r.f. beat signal is processed by a plurality (which can be thousands) of correlator pseudonoise code sequence demodulation and phase demodulator units operated in different delay time relationships to the timing base of the reiterated modulation sequences. Pairs of outputs of the units are connected to respective substractor circuits, each providing a signal representative of a differential signal between acoustic, or other forms of, signals incident the bounds of virtual increments of the span.

Claims

1. A time-domain reflectometer for sensing at a desired set of n spaced sensing positions along an optical fiber span, said sensing positions being for sensing a type of external physical signal having the property of inducing light path changes within the optical fiber span at regions there along where the signal is coupled to the span, comprising:

an optical fiber span having a first end which concurrently serves as both the interrogation signal input end and the back propagating signal output end for purposes of reflectometry, and having a second remote end;

a first light source for producing a coherent carrier lightwave signal of a first predetermined wavelength;

a binary pseudonoise code sequence modulator modulating said carrier signal for producing a pseudonoise code sequence modulated interrogation lightwave signal which continuously reiterates the binary pseudonoise code sequence, the reiterated sequences being executed in a fixed relationship to a predetermined timing base;

a lightwave heterodyner having first and second inputs for receiving a primary signal and a local oscillator signal, respectively, and operative to produce the beat frequencies of their respective frequencies;

a lightwave directional coupler having a first port which receives said binary pseudonoise code sequence modulated interrogation lightwave, a second port coupled to said first end of said optical fiber span, and a third port coupled to said primary signal input of the heterodyner;

said directional coupler coupling said binary pseudonoise code sequence modulated interrogation lightwave to said second port where it is launched in a forwardly propagating direction along said optical fiber span causing the return to said second port of a composite back-propagating lightwave which is a summation of lightwave back-propagations from a continuum of locations along the length of the span, said composite back-propagating lightwave signal comprising a summation of multiple components including

a first signal component comprising the summation of portions of the said pseudonoise code sequence modulated interrogation lightwave signal which the innate properties of the optical fiber cause to backpropagate at a continuum of locations along the span;

a second signal component comprising the modulation of said first signal component caused by longitudinal components of optical path changes induced into said span at a continuum of locations along said span by external physical signals, said second signal component further including a corresponding set of subcomponents comprising the modulation of said first signal component by optical path changes caused by said external signals at the respective sensing positions;

said directional coupler coupling said composite back-propagating lightwave to said third port where it is applied to said first input of the heterodyner;

a second light source coupled to said second input of the lightwave heterodyner, said second light source producing a coherent local oscillator lightwave signal in phase locked relation to said carrier lightwave signal, said local oscillator signal being of a second predetermined wavelength which differs from the first predetermined wavelength by an amount of difference small enough to produce at the output of the heterodyner a radio frequency (r.f.) composite difference beat signal, but by an amount large enough to cause said r.f. composite difference beat signal to have sufficient bandwidth to cause it to include r.f. counterparts of signal components and subcomponents of said composite back propagating lightwave signal;

said r.f. composite difference beat signal being coupled to an n-way splitter providing a corresponding set of n output channels, each transmitting said r.f. composite difference beat signal;

a corresponding set of n correlation-type binary pseudonoise code sequence demodulators having their respective inputs connected to the corresponding output channels of said n-way splitter through a corresponding set of time delay circuits which respectively provide a corresponding set of predetermined time delays in relation to said predetermined timing base of the binary pseudonoise code sequence modulator, to establish said n desired sensing positions along said optical fiber span;

said set of correlation-type binary pseudonoise code sequence demodulators serving to conjunctively temporally and spatially de-multiplex said r.f. composite difference beat signal to provide at their respective outputs r.f. counterparts of the subcomponents of said second signal component of said composite back-propagating lightwave signal caused by changes in the optical path within said optical fiber span induced by external physical signals respectively coupled to the corresponding sensing positions;

a corresponding set of n phase demodulators for transforming each respective r.f. counterpart of said second component into a substantially linear signal representative of radian phase of the corresponding lightwave subcomponent signal; and

at least one subtracter circuit having a first and a second input for respectively receiving outputs from a selected first and a selected second of said set of n phase demodulators, to produce a differential phase signal which is representative of the difference between the radian phases of the r.f. counterparts of two subcomponents of said second component of the composite back-propagating lightwave signal, which two subcomponents are caused by external signals at sensing positions along said optical fiber span established by the time delay circuits respectively associated with said selected first and said selected second phase demodulators.

2. The reflectometer of claim 1, further including:

said at least one subtracter circuit comprises a plurality of subtracter circuits; and

a phase signal routing and switching network operative to route to the first input of each subtracter circuit of the plurality an output from one of said set of n phase demodulators and to route to the second input of each subtracter circuit an output from another of said set of n phase demodulators, to enable the respective subtracter circuits to produce outputs of the said form of differential phase signal between two subcomponents caused by external signals at sensing positions along said optical fiber span established by the time-delay circuits associated with outputs of the respective one and another phase demodulator routed to each subtracter circuit.

3. The reflectometer of claim 2, wherein said phase signal routing and switching is adapted to enable an operator to make selective choices of routing of outputs from individual demodulators of said set of n phase demodulators to the respective inputs of the respective subtracter circuits of said plurality of subtracter circuits, concurrently with ongoing operation of the reflectometer.

4. The reflectometer of claim 1, wherein:

said innate properties of the said optical fiber material include the generation of Rayleigh optical scattering effects at a continuum of locations along said optical fiber span in response to said forwardly propagating binary pseudonoise code sequence modulated interrogation lightwave.

5. The reflectometer of claim 1, wherein said type of external physical signal which induces light path changes in said optical fiber span is an acoustic pressure wave signal.

6. The reflectometer of claim 5, wherein:

said optical fiber span is an acoustic security alarm perimeter monitoring line buried at a predetermined depth beneath the surface of the ground;

said type of external physical signal is caused by the vibration of the ground surface by movement of an object thereon; and

said set of n sensing positions along the line form a virtual array of n geophones which respectively produce substantially linear signals respectively representative of the vibrations of the surface of the ground at corresponding sensing positions.

7. The reflectometer of claim 5, wherein:

said optical fiber span is of a length L; and

said first light source is a laser having the performance capability to generate a lightwave signal with sufficient phase stability to substantially retain coherency in propagation along said optical fiber span for a distance at least equal to 2L.

8. The reflectometer of claim 7, wherein:

said the length L of said optical fiber span is at least 5.0 km.

9. The reflectometer of claim 5, wherein said optical fiber span comprises a single-mode fiber optic cable.

10. The reflectometer of claim 5, wherein:

said optical fiber span is a perimeter monitoring line of a security system buried beneath the surface of the ground;

said set of n sensing positions along the span serving as perimeter line monitoring stations;

said outputs of said corresponding set of n phase demodulators providing substantially linear monitoring signals representative of ground vibrations caused by objects moving on the surface of the ground near said optical fiber line.

11. The reflectometer of claim 10, wherein the range of depths of burial of the optical fiber span beneath the surface of the ground is of six inches to the order of one foot.

12. The reflectometer of claim 5, wherein said optical fiber span comprises a fiber optic cable of the polarization preserving type.

13. The reflectometer of claim 5, wherein:

said optical fiber span has a coating made of a thermoplastic material having the combined characteristics of a low Young'"'"'s modulus and a Poisson'"'"'s ratio below that of natural rubber; and

said coating serving to enhance the longitudinal component of strain variation derived from an acoustic wave signal whose wave front is incident to the span from a direction at least in part having a lateral component in the direction along which the wave front propagates.

14. The reflectometer of claim 5, wherein each of:

(i) said coherent carrier lightwave signal;

(ii) said coherent local oscillator lightwave signal;

(iii) said spread spectrum modulated interrogation lightwave signal;

(iv) said composite back-propagating lightwave signal;

(v) said radio frequency (r.f.) composite difference beat signal; and

(vi) each counterpart of said r.f. counterpart of the subcomponents of said second signal component of said composite back-propagating lightwave signal, is a continuous wave (CW) signal.

15. The reflectometer of claim 1, wherein:

said lightwave heterodyner is of the photodetector type.

16. The reflectometer of claim 15, wherein:

said lightwave heterodyner of the photodetector type is a balanced optical detector circuit including a matched pair of photodetectors with the composite back-propagating lightwave signal applied to each photodetector of the pair; and

said balanced optical detection circuit produces said r.f. composite difference beat signal as a differential current from the matched pair of photodetectors.

17. The reflectometer of claim 1, wherein the continuously reiterated binary pseudonoise code sequences are binary pseudonoise sequences wherein shifts between binary states of the signal alternatingly shift the radian phase of the carrier between substantially 0°

and substantially 180°

.

18. The reflectometer of claim 17, wherein said reference phase signal produced by said fixed frequency oscillator is used in establishing the phase locked relationship between the local oscillator lightwave signal and the carrier lightwave signal.

19. The reflectometer of claim 1, wherein said pseudonoise code sequence is a pseudorandom number (PN) code sequence generated by a shift-register type PRN code generator.

20. The reflectometer of claim 1, further comprising:

a fixed frequency reference oscillator which produces a reference phase signal;

wherein each phase demodulator including an I &

Q quadrature demodulator having a first input for receiving said reference phase signal and a second input for receiving an r.f. counterpart of the corresponding subcomponent of said second signal component of said composite back-propagating lightwave signal, said I &

Q demodulator being operative to derive from said reference phase signal an interim in phase signal and an interim quadrature phase signal and to split the signal received at its second input and mix one part thereof with the interim in phase signal and another part thereof with the interim quadrature phase signal to provide a pair of output signals; and

each phase demodulator further including a phase detector having a pair of inputs for receiving respectively one and the other of said outputs of the I &

Q demodulator and operative to provide at the output of the phase demodulator said signal representative of the radian phase of the respective subcomponent of said set of n subcomponents.

21. The reflectometer of claim 1, wherein:

a time period TP is required for forward propagation of said autocorrelatable spectrum spreading signal from the output of the source of the spectrum spreading signal to where said first light source is modulated, and then for the forward propagation of the derivative spread spectrum modulated interrogation lightwave signal to the second remote end of the fiber optical span, plus the time period required for the back propagation of a subcomponent of said composite back-propagating CW lightwave signal produced at the remote end of the span to the input of the heterodyner, and then for the back propagation of the derivative counterpart subcomponent of the r.f. composite difference beat signal from the output of the heterdyner to the input of a corresponding de-spreader and de-multiplexer of said set of n de-spreader and de-multiplexers; and

the temporal length of a single autocorrelatable spectrum spreading signal sequence of the continuously reiterated code sequences is one of one and the other of less than the time period TP, and greater than the time period TP.

22. The reflectometer of claim 1, wherein said type of external physical signal which induces light path changes in said optical fiber span is a selected one of a group consisting of:

(i) a seismic signal wherein with the media which couples the signal to said optical fiber span includes at least in part the ground in which the fiber optic span is buried;

(ii) an underwater sound signal wherein the media which couples the signal to said optical fiber span includes at least in part a body of water in which the fiber optic span is immersed;

(iii) an electromagnetic force field coupled to the optical fiber span;

(iv) a signal comprising temperature variations coupled to the optical fiber span; and

(v) at least one microphonic signal which is coupled to said optical fiber span at an at least one of said set of n sensing positions along the optical fiber span.

23. A system wherein, at respective span increment sensing stations of a plurality of span increment sensing stations along a span of optical fiber, the system senses the phase differential between input signals respectively incident upon the bounds of each station, for signals of a type having a property of inducing light path changes at regions of the span influenced by such signals, said system comprising:

means for illuminating the span with a CW optical signal and for retrieving back-propagating portions of the illumination back propagating from a continuum of locations along the span;

means for modulating said CW optical signal in accordance with a reiterative autocorrelatable form of modulation with respect to a reference timing base;

means for picking off from said retrieved back-propagating portions of the illumination a radio frequency (r.f.) counterpart of the retrieved modulated CW optical signal;

means for performing a second plurality of autocorrelation detection processes upon said r.f. counterpart of the retrieved back-propagated modulated CW optical signals, said second plurality corresponding to the number of bounds of sensing stations, said second plurality of autocorrelation detection processes being performed in different timed relationships relative the reference timing base determined by propagation distances which the modulated CW optical signal travels to the respective bound;

means for detecting the radian phase of each input signal at each respective timed relationship by which said autocorrelation detection means performs processing for a respective bound, said detection means performing the radian phase detection of each input signal in phase locked relationship to said CW optical signal; and

means for determining the difference between radian phases of the signals at the respective timed relationships associated with each pair of bounds of each span increment sensing station.

24. Signal sensing array apparatus, wherein at respective span increment sensing stations of a first plurality of span increment sensing stations along a span of optical fiber the apparatus senses phase differentials between input signals respectively incident the bounds of each span increment sensing station, the input signals being of a type having a property of inducing light path changes at regions of the span influenced by such signals, said apparatus comprising:

an optical wave network comprising a transmitter laser and a lightwave directional coupler, said network being operative to illuminate an optical fiber span with a CW optical signal and to retrieve portions of the illumination back-propagating from a continuum of locations along the fiber span;

a modulator operative to modulate the CW optical signal in accordance with a reiterative autocorrelable form of modulation code;

a heterodyner which receives said retrieved, back-propagated, portions of illumination and derives therefrom a radio frequency (r.f.) counterpart there of; and

a phases-of-input-signals detector for determining the phase of input signal at each bound of each span increment sensing station of said first plurality of span increment sensing stations, said phases-of-input-signals detector being operative in respective timed relationships of a second plurality of timed relationships corresponding to the bounds of the sensing stations, said timed relationships being with respect to said reiterative autocorrelable form of modulation code, said phases-of-input-signals detector including;

a corresponding second plurality of autocorrelation detectors which respectively detect temporal portions of the r.f. counterparts of the retrieved, back-propagated, portions of said illumination that occur in the timed relationships corresponding to the bounds of each span increment sensing station and detect electronic representations of the optical input signals incident the respective bounds;

a corresponding second plurality of phase demodulators which are operative in phase locked relationships with said CW optical signal and which respectively receive said detected electronic representations of the optical input signals; and

a subtracter network coupled to receive the outputs of said second plurality of phase demodulators and which is operative to determine the differences between the phase demodulator outputs of each pair of phase demodulator outputs which correspond to a pair of bounds of each span increment sensing station of the first plurality of said stations.