(12> Ulllted States Patent
`Kersey et al.
`
`US006285806B1
`(10) Patent N0.:
`US 6,285,806 B1
`(45) Date of Patent:
`Sep. 4, 2001
`
`(54) COHERENT REFLECTOMETRIC FIBER
`BRAGG GRATING SENSOR ARRAY
`
`(75) Inventors: Alan D. Kersey, So. Glastonbury, CT
`(US); Anthony Dandridge, Burke, VA
`(US); Sandeep T. V0hra, Crofton, MD
`(Us)
`
`(73) Assignee: The United States of America as
`representedtby the Secretary of the
`Navy> Washlngtom DC (Us)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl' NO‘: 09/093’827
`(22) Filed;
`May 31, 1998
`
`(51) Int. Cl.7 ..................................................... .. G02B 6/00
`U-S- Cl- ............................................... ..
`(58) Field of Search ................ .. 385/12, 13; 250/227.14,
`250/22723
`
`(56)
`
`References Clted
`U'S' PATENT DOCUMENTS
`
`H1626 * 1/1997 Kersey ............................... .. 370/479
`3,633,183 * 1/1972 Cobb .................................. .. 340/173
`4,775,216
`10/1988 Layton -
`477787248
`10/1988 Am“ et al' '
`4,889,986
`12/1989 Kersey et a1. .
`5,144,690
`9/1992 Domash .
`5,208,877
`5/1993 Murphy et a1. .
`5,323,404
`6/1994 Grubb.
`
`206
`CODE v
`GENERATOR
`(PRBS)
`
`7/1995 Narendran .
`5,436,988
`5,973,317 * 10/1999 Hay ............................... .. 250/227.14
`5,987,197 * 11/1999 Kersey ................................. .. 385/24
`
`FOREIGN PATENT DOCUMENTS
`2189880
`11/1987 (GB) .
`2214636
`9/1989 (GB) .
`
`*
`
`.
`
`.
`
`cued by exammer
`Primary Examiner_Teresa M_ Arroyo
`Assistant Examiner—Gioacchio InZirillo
`74 Attorne , A em, or Firm—John J. Karasek; Sall A.
`gerzett
`y g
`y
`
`(57)
`
`ABSTRACT
`
`A ?ber optic sensor array has multiple segments, each
`capable of detecting a physical condition such as an acoustic
`Wave. The segments are separated by Weak re?ectors such as
`?ber optic Bragg gratings. Light from a light source is input
`into the input end of the array Light re?eeted by each of the
`re?ectors has a phase
`representing the effects of the
`physical condition on all of the segments from the input end
`to that re?ector. To address a speci?c re?ector, the return
`light is demultipleXed. This demultipleXing is done by
`modulating the light input into the input end of the array
`W1th a pseudo-random b1t sequence and correlating the
`output with a time-shifted version of the pseudo-random b1t
`sequence to isolate the part of the output caused by that
`re?ecton TO address a Speci?c segment, the phase Shifts
`from tWo adjacent re?ectors are determined. The return light
`can be strengthened by mixing it With a portion of the light
`picked off from the light source.
`
`22 Claims, 3 Drawing Sheets
`
`2/08 2,0
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`
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`
`222
`
`DELAY
`_' CIRCUIT
`
`229
`'\_/
`
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`
`224
`
`226
`
`227
`
`SUBTRACTION P\,2.3i2
`CQRRELAHON
`CIRCUIT _" CIRCUIT
`
`230 J
`
`HALLIBURTON, Exh. 1005, p. 0001
`
`

`

`U.S. Patent
`
`Sep. 4, 2001
`
`Sheet 1 of3
`
`US 6,285,806 B1
`
`Q 93 N2
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`HALLIBURTON, Exh. 1005, p. 0002
`
`

`

`U.S. Patent
`
`Sep. 4, 2001
`
`Sheet 2 of 3
`
`US 6,285,806 B1
`
`232
`
`SUBTRACTION
`
`CIRCUIT
`
`226
`
`227
`
`222
`
`228
`
`229
`
`DELAY
`
`CIRCUIT
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`202
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`206
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`
`HALLIBURTON, Exh. 1005, p. 0003
`
`HALLIBURTON, Exh. 1005, p. 0003
`
`

`

`U.S. Patent
`
`Sep. 4, 2001
`
`Sheet 3 of3
`
`US 6,285,806 B1
`
`206
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`
`M o LAT R
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`F/G. 3
`
`HALLIBURTON, Exh. 1005, p. 0004
`
`

`

`US 6,285,806 B1
`
`1
`COHERENT REFLECTOMETRIC FIBER
`BRAGG GRATING SENSOR ARRAY
`
`FIELD OF THE INVENTION
`
`The present invention is directed to an interferometric
`sensor array Which provides a large number of individually
`addressable sensor locations With high spatial accuracy and
`in particular to such an array as applied for detection of
`acoustic or other vibrations, disturbance or the like.
`
`DESCRIPTION OF RELATED ART
`
`It is knoWn in the art to form a sensor array by providing
`an optical ?ber With multiple sensing segments separated by
`Weakly re?ecting portions such as ?ber Bragg grating re?ec
`tors. The sensing segments undergo a change in refractive
`index in response to a physical condition to be detected, such
`as stress, strain or sound. Typically, one short light pulse is
`sent into the ?ber, and the time delay of the return pulse
`identi?es the Weakly re?ecting portion Which re?ected the
`return pulse. The Weakly re?ecting portions are spaced far
`enough apart that the propagation time betWeen them is at
`least equal to the Width of the short light pulse. Propagation
`time is in turn determined by the speed of light in a ?ber,
`Which is given by c/n, Where c is the speed of light in a
`vacuum, and n is the index of refraction of the ?ber. For
`many commercially available optical ?bers, nz1.5.
`Concepts relating to such sensor arrays are set forth in
`detail in the folloWing references:
`US. Pat. No. 4,775,216 to Layton, Oct. 4, 1988;
`US. Pat. No. 4,778,248 to ArZur et al, Oct. 18, 1988;
`US. Pat. No. 4,889,986 to Kersey et al, Dec. 26, 1989;
`US. Pat. No. 5,144,690 to Domash, Sep. 1, 1992;
`US. Pat. No. 5,208,877 to Murphy et al, May 4, 1993;
`US. Pat. No. 5,323,404 to Grubb, Jun. 21, 1994;
`US. Pat. No. 5,436,988 to Narendran, Jul. 25, 1995;
`UK. Published Patent Application 2,189,880 A to Lamb,
`published Nov. 4, 1987;
`UK. Published Patent Application 2,214,636 A to Lamb,
`published Sep. 6, 1989; and
`H. S. Al-RaWeshidy et al, Spread spectrum technique for
`passive multiplexing of interferometric optical ?ber sensors,
`SPIE Vol. 1314 Fibre Optics 90, pp. 342—7.
`Pseudo-random bit sequences (PRBS’s) are knoWn in
`such arts as radar and code-division multiple-access
`(CDMA) communication systems. An important character
`istic of a PRBS is that it comprises a plurality of segments,
`each of Which can be easily distinguished from the others.
`This characteristic alloWs demultiplexing by correlation.
`The characteristics of PRBS’s have been explored in detail
`in SarWate et al, Crosscorrelation Properties of Pseudoran
`dom and Related Sequences, Proceedings of the IEEE, Vol.
`68, No. 5, May, 1980, pp. 593—620.
`FIG. 1 shoWs a schematic diagram of a knoWn interfero
`metric sensor array using code-division multiplexing. In
`sensor array 100, laser 102 emits coherent light. Pseudo
`random bit sequence (PRBS) generator 104 generates a
`pseudo-random bit sequence, Which is input to modulator
`106. Modulator 106 modulates the coherent light from laser
`102 to produce PRBS optical input 108. PRBS optical input
`108 is input to ?ber 110. Fiber 110 includes N sensors 112-1,
`112-2, 112-3, .
`.
`. , 112-N separated by lengths of ?ber 114-1,
`
`114-2, .
`
`.
`
`.
`
`, 114-(N-1).
`
`, except last sensor
`.
`.
`Each sensor 112-1, 112-2, 112-3, .
`112-N, includes a corresponding ?rst coupler 116-1,
`
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`2
`. , 116-(N-1), Which splits off a portion of the light
`.
`116-2, .
`?ux of PRBS optical input 108 in ?ber 110. In each detector
`112-n, n=1, 2, .
`.
`. , N, the split-off portion of the light enters
`second coupler 118-n, Which divides the ?ux betWeen ?rst
`?ber 120-n and second ?ber 122-n, the ?rst and second
`?bers having equal optical lengths. First ?ber 120 undergoes
`a change in its refractive index When exposed to the condi
`tion to be sensed (e. g., such measurands as an acoustic Wave,
`temperature change, distension because of stress or strain,
`etc.), While second ?ber 122 undergoes no such change. The
`?uxes are recombined in third coupler 124, Where they
`interfere to produce PRBS output signal 130-1, .
`.
`. , 130-N.
`Each PRBS output signal is time-delayed by the total length
`of ?ber betWeen laser 102 and the corresponding third
`coupler 124-n. Fourth coupler 126-n couples the PRBS
`output signal to return ?ber 128. Last sensor 112-N has the
`same construction as the other sensors, except that ?rst
`coupler 116 and last coupler 126 are unnecessary. PRBS
`output signals 130-1, .
`.
`. , 130-N add in return ?ber 128 to
`produce total output 132. Total output 132 is detected by
`detector 134.
`Total output signal 132 must be demultiplexed to rederive
`each of the PRBS output signals. To effect this
`demultiplexing, time delay circuit 136 receives the PRBS
`from PRBS generator 104 and applies a time delay to the
`PRBS corresponding to the time delay of each PRBS output
`signal. The time-delayed PRBS is correlated With the output
`of detector 134 in correlation circuit 138. The result of the
`correlation is applied through loW-pass ?lter (LPF) 140 to
`reduce high frequency noise, and is output at sensor array
`100. Thus, each sensor is addressable.
`HoWever, sensor array 100 has the folloWing draWbacks.
`First, because sensor array 100 requires four couplers for
`each sensor except the last and also requires return ?ber 128,
`sensor array 100 is complicated and expensive to build.
`Second, because of the length of the ?bers required and
`imperfect transmission in any real-World optical ?ber, sensor
`array 100 suffers from a signi?cant loss of light ?ux. A
`particular disadvantage arising from such a loss is a limita
`tion on the number of sensors.
`
`SUMMARY OF THE INVENTION
`
`An object of the invention is to reduce number of sensors
`necessary to do remote sensing, eg of the kind done by the
`apparatus of FIG. 1.
`Another object is to reduce amount of optical ?ber
`necessary to do remote sensing such as is done by the
`apparatus of FIG. 1.
`Another object is to provide an optical ?ber sensor array
`Which has a simple design and is inexpensive to build.
`To achieve these and other objects, the present invention
`concerns an optical system and method employing an optical
`?ber With a plurality of partially re?ective elements, an
`optical source to launch an optical signal into the ?ber, and
`a phase detector disposed effective to determine the phase
`betWeen the optical signal and light re?ected from at least
`one preselected element. By using re?ected light, the inven
`tion requires less optical ?ber for the same number of
`sensors because the invention need not employ an additional
`return line, such as line 128 of FIG. 1. Moreover, because the
`invention uses re?ected light, rather than plural sensor taps
`(e.g. sensors 120-n in FIG. 1), it can dispense With the
`numerous couplers needed in each of these taps, saving on
`hardWare, and the inherent lossyness of such couplers.
`Consequently, the invention provides an improved optical
`budget for the user, permitting a larger number of sensors for
`
`HALLIBURTON, Exh. 1005, p. 0005
`
`

`

`US 6,285,806 B1
`
`3
`the same optical power, and permits one to do so With a
`simpler apparatus using less hardWare.
`These and other objects are further understood from the
`following detailed description of particular embodiments of
`the invention. It is understood, hoWever, that the invention
`is capable of extended application beyond the precise details
`of these embodiments. Changes and modi?cations can be
`made to the embodiments that do not affect the spirit of the
`invention, nor exceed its scope, as expressed in the
`appended claims. The embodiments are described With
`particular reference to the accompanying draWings, Wherein:
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Apreferred embodiment of the present invention Will noW
`be set forth in detail With reference to the draWings, in
`Which:
`FIG. 1 shoWs a schematic diagram of a ?ber sensor array
`according to the prior art; and
`FIG. 2 shoWs a schematic diagram of a ?ber sensor array
`according to the invention.
`FIG. 3 shoWs a schematic diagram of another embodi
`ment of the invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIG. 2 shoWs a schematic diagram of a ?ber sensor array
`200 according to the invention, in Which laser 202 emits
`light having a long coherence length and a narroW Wave
`length range. The light emitted by laser 202 passes through
`coupler 204, Which splits the ?ux into a ?rst portion directed
`to modulator 208, and a second portion 219. The use of
`second portion 219 Will be explained beloW With regard to
`detection. Pulse modulator 208 modulates the ?ux With a
`PRBS generated by PRBS generator 206 to produce PRBS
`optical signal 210. Phase modulator 208 can be an electro
`optical sWitch or the like. The PRBS can be a maximal or
`M-sequence code of the type knoWn in the radar and
`communication arts. PRBS optical signal 210 passes
`through coupler 212 into ?ber 214, Which has a series of
`coils 216-1, 216-2, 216-3, 216-4, 216-5, 216-6, 216-7, .
`.
`. ,
`216-N (of Which FIG. 2 shoWs only coils 1 through 7),
`bounded by Weak Bragg grating re?ectors 218-0, 218-1,
`218-2, 218-3, 218-4, 218-5, 218-6, 218-7, .
`.
`.
`, 218-N (of
`Which FIG. 2 shoWs only gratings 0 through 7). Each coil
`acts as a sensor by undergoing a change in its refractive
`index in accordance With a condition to be sensed (e.g., an
`acoustic Wave, etc., as discussed above). Bragg grating
`re?ectors 218-0, 218-1, 218-2, .
`.
`. can be replaced by
`photo-induced index steps Which provide Weak Fresnel
`re?ections, or by any knoWn type of refractive-index ?ber
`anomaly.
`Each Weak Bragg grating re?ector 218-0, 218-1,
`218-2, .
`.
`. re?ects a small portion of the light ?ux incident
`on it. The sum of the re?ected light ?uxes is received by
`coupler 212 and directed thereby to coupler 220. Coupler
`220 also receives second portion 219 of the light ?ux split
`off by coupler 204. In coupler 220, the second portion of the
`?ux from coupler 204 is coherently mixed With the re?ected
`light from coupler 212. The optics disposed betWeen cou
`plers 204 and 220 effectively constitute a Mach-Zender
`interferometer, With ?ber 214 constituting one arm, and
`optical path 219 constituting the other. Optical signals in the
`tWo arms recombine in output 220, the resultant output
`signal depending on the relative phase betWeen the signals
`from the arms, as is knoWn to those skilled in the art.
`
`4
`Transducers 222, 224 covert the output of coupler 220 to
`electric signals, Which in turn are directed to respective
`inputs of difference ampli?er 226. Ampli?er 226 differen
`tially detects the interferometer’s output, again in a manner
`knoWn to those skilled in the art. The net result of this
`interferometric con?guration is an output 27 representing
`the signal re?ected from Bragg gratings 218, With the optical
`carrier from laser 208 removed. The signal 228, Which
`initially modulates the optical carrier, is fed in via delay 228
`to correlator 230. The dimensions of ?ber 214 are chosen
`such that the round trip optical path from coupler 212 to any
`Bragg grating 216 and back to coupler 212 is longer than any
`change in optical path length along the same round trip path
`due to changes in ?ber segments 216 therealong responsive
`to expected measurands. Thus, no re?ection from any of
`Bragg gratings 218 Will “lap,” or overlap, a re?ection from
`any other. Consequently, signals re?ected from each of the
`Bragg gratings 218 Will arrive at correlator 230 sequentially,
`each Within a knoWn time WindoW uniquely associated With
`a speci?c grating, albeit shifted Within that WindoW by any
`measurand induced change in optical path. The delay
`imposed by member 228 is preferably chosen to correspond
`to the round trip travel time of an optical signal from
`modulator 208 to one selected Bragg grating and back to
`detector 226. The delay imposed by member 228 can thus
`“target” a particular Bragg grating by causing the delayed
`modulation signal 229 to arrive at correlator 230 Within the
`time WindoW associated With that grating. Correlator 230
`performs a correlation over the span of the time WindoW,
`determines in a knoWn manner the time shift betWeen
`signals 227 and 229 Which maximiZes the correlation,
`thereby determining the phase betWeen the tWo signals, from
`Which one can infer the phase shift induced by ?ber seg
`ments 216 Which signal 227 traversed in its round trip Within
`?ber 214.
`As an example, upon light from laser 202 modulated by
`member 206 entering ?ber 214, Bragg grating 218-4 re?ects
`a portion of the input light Which, to get to grating 218-4,
`traverses ?ber segments 216-1, 216-2, .
`.
`.
`, 218-4. Any
`change in optical path length in these segments changes the
`time of ?ight of the light re?ected from grating 218-4, With
`the result that the total phase shift of light re?ected from
`grating 218-4 Will be the sum of the individual phase shifts
`imposed by each of the ?ber segments. Setting the delay of
`member 228 to place delayed modulation signal 229 at
`correlator 230 Within the time WindoW associated With
`grating 218-4 ensures that the demodulated signal re?ected
`from grating 216-4, and the modulation signal delayed by
`member 228, Will both arrive at correlator 230 Within the
`time WindoW. Correlator 230 then determines the phase
`difference betWeen the tWo signals, ie the phase shift Which
`maximiZes the correlation betWeen the tWo signals, thereby
`determining the phase betWeen signals 227 and 229.
`Because one Would knoW a priori What the phase difference
`Would be absent measurand changes in optical path length in
`?ber segments 216-1 .
`.
`. 216-4, one can determine the
`cumulative change in optical path Which has occurred in
`these segments, e.g. responsive to measurands.
`If, in this example, one kneW a priori, the phase shift
`associated With all but one of ?ber segments 216-1 .
`.
`.
`216-4, and one Wished to determine the phase shift of that
`last segment, subtractor 232 preferably Would subtract the
`knoWn phase shifts from the cumulative phase detected by
`correlator 230. Preferably, hoWever, this Would be done in
`the manner illustrated in FIG. 3, Which shoWs apparatus
`identical to that of FIG. 2, except that in place of delay 228
`and correlator 230, there are N delay-correlator pairs
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`HALLIBURTON, Exh. 1005, p. 0006
`
`

`

`US 6,285,806 B1
`
`5
`, N, having
`.
`.
`denominated 228-n and 230-n, n=1, 2, .
`corresponding outputs (1)1, (1)2, .
`.
`.
`, q>N, in which 4),, is the
`cumulative phase induced on a signal re?ected from the nth
`Bragg grating. Subtractor 232 receives this phase
`information, and determines A¢n=¢n—q)n_1, i.e. the phase
`shift induced by ?ber segment 218-n alone.
`Other techniques can be used to isolate a particular sensor.
`For example, the interferometer signal can be demodulated
`by using heterodyne processing by inserting an optical
`frequency shifter in the path of the second portion of the ?uX
`split off by coupler 204. Alternatively, a phase-generated
`carrier (PGC) approach can be used by inserting a phase
`modulator in the path of the second portion of the ?uX.
`A large number of sensors can be interrogated if the
`re?ectivity of the Bragg re?ectors is suf?ciently loW. For
`eXample, if each Bragg re?ector has a re?ectivity of 0.1%,
`more than one hundred re?ectors can be used in series. For
`a 10 mW laser beam launched into the system With an
`effective duty cycle of 50% from the PRBS modulation, the
`average optical poWer at the detectors from the re?ectors is
`~1 MW. Coherent miXing of this signal in coupler 220 With
`a split-off light ?uX of ~1 MW produces a detector signal With
`a shot-noise limited performance of ~10“6 rad/HZ. Rayleigh
`scattering in ~20 m ?ber sections betWeen the re?ectors
`produces a stray re?ection component Whose average poWer
`is ~10 NW. This component also produces a Weak interfering
`component Which is largely masked by the component
`derived by the Bragg re?ectors. The use of loW-re?ectivity
`gratings also reduces crosstalk effects.
`In practice, modulator 206 can be any knoWn optical, or
`electro-optical, modulation device, and delay 228, correlator
`230, subtractor 230, demodulator 220, and detector 226 can
`be any optical or electric-circuit device knoWn to operate as
`above described. The modulation signal is preferably a
`pseudorandom pulse code because such a code Will correlate
`especially poorly With noise, and, as disucssed above, such
`codes are Well knoWn. HoWever, any non-random modula
`tion could in principle be used. In particular, all circuitry
`doWnstream of ?ber 224 could be constituted by a digital
`computer adapted to digitally sample an analog input,
`although differential detection such as is done by ampli?er
`226 is preferred to cancel noise and otherWise increase
`signal to noise performance of the detector. Indeed, the
`choice of hardWare is determined largely by the hardWare’s
`response time compared With optical time of ?ight betWeen
`Bragg gratings 218, choice of Which is Within the skill of
`Workers in this ?eld once otherWise informed by the fore
`going.
`While a preferred embodiment has been set forth, those
`skilled in the art Who have revieWed this disclosure Will
`appreciate that modi?cations can be made Within the scope
`of the invention. In addition to the modi?cations described
`above, several interrogating lasers can be used to address
`arrays of Bragg gratings, Which can have the same Wave
`length sensitivity or different Wavelength sensitivities (such
`as 1.55 pm and 1.2 pm). Also, laser 202 can be replaced by
`a broad-band optical source. Modi?cations disclosed sepa
`rately can be combined Whenever it is technologically
`feasible to do so. Therefore, the invention should be con
`strued as limited only by the appended claims.
`We claim:
`1. A ?ber optic sensor array for detecting a physical
`condition, the ?ber optic sensor array comprising:
`bit sequence generating means for generating a pseudo
`random bit sequence;
`light source means, receiving the pseudo-random bit
`sequence, for emitting a light beam Which is modulated
`in accordance With the pseudo-random bit sequence;
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`an optical ?ber disposed to receive the light beam so that
`the light beam propagates in the optical ?ber in a ?rst
`direction, the optical ?ber comprising a plurality of
`?ber segments disposed in series, each of the plurality
`of ?ber segments having an optical characteristic Which
`varies in accordance With the physical condition, the
`?ber segments being separated by means for Weakly
`re?ecting portions of the light beam to form return
`optical signals Which propagate in the optical ?ber in a
`second direction Which is opposite to the ?rst direction,
`each of the return optical signals representing an effect
`of the physical condition on the light beam;
`photodetecting means, receiving the return optical
`signals, for producing a photodetector output Which
`represents a sum of the return optical signals;
`time delay means, receiving the pseudo-random bit
`sequence, for producing a time-delayed pseudo
`random bit sequence, said time delay means compris
`ing means for producing a plurality of time delayed
`pseudo-random bit sequences, each having a different
`time delay;
`correlating means, receiving the time-delayed pseudo
`random bit sequence and the photodetector output, for
`performing a correlation betWeen the time-delayed
`pseudo-random bit sequence and the photodetector
`output to produce a correlation result representing one
`of the return optical signals, said correlating means
`comprising means for receiving a plurality of time
`delayed pseudo-random bit sequences, for producing a
`plurality of correlations to produce a plurality of cor
`relation results, each representing a different one of the
`return optical signals; and
`subtracting means, receiving the plurality of correlation
`results, for calculating a difference betWeen tWo of the
`return optical signals to determine an effect on the light
`beam by one of the ?ber segments.
`2. A ?ber optic sensor array as in claim 1, Wherein each
`of the re?ecting means comprises a ?ber optic Bragg grating
`formed in the optical ?ber.
`3. A ?ber optic sensor array as in claim 1, Wherein the
`light source means comprises:
`a laser for emitting light; and
`a modulator, receiving the light emitted by the laser and
`the pseudo-random bit sequence, for modulating the
`light emitted by the laser in accordance With the
`pseudo-random bit sequence to produce the light beam.
`4. A ?ber optic sensor array as in claim 3, Wherein the
`modulator comprises an electro-optic sWitch.
`5. A ?ber optic sensor array as in claim 3, Wherein:
`the light source means further comprises a ?rst coupler,
`disposed in a path of the light emitted by the laser
`betWeen the laser and the modulator, for picking off a
`portion of the light emitted by the laser;
`the optical ?ber further comprises a second coupler for
`picking off portions of the return optical signals; and
`the photodetecting means further comprises:
`a third coupler for coherently miXing the portion of the
`light picked off by the ?rst coupler With the portions
`of the return optical signals picked off by the second
`coupler; and
`means for detecting a result of coherent miXing by the
`third coupler.
`6. A ?ber optic sensor array as in claim 5, Wherein:
`the result of coherent miXing comprises tWo outputs of the
`third coupler; and
`
`HALLIBURTON, Exh. 1005, p. 0007
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`US 6,285,806 B1
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`the means for detecting comprises a pair of
`photodetectors, each detecting one of the tWo outputs
`of the third coupler.
`7. A ?ber optic sensor array as in claim 6, Wherein the
`means for detecting further comprises a differential
`ampli?er, receiving outputs of the pair of photodetectors, for
`producing a balanced detector output.
`8. A method of detecting a physical condition, the method
`comprising:
`(a) generating a pseudo-random bit sequence;
`(b) emitting a light beam Which is modulated in accor
`dance With the pseudo-random bit sequence;
`(c) causing the light beam to enter an optical ?ber so that
`the light beam propagates in the optical ?ber in a ?rst
`direction, the optical ?ber comprising a plurality of
`?ber segments disposed in series, each of the plurality
`of ?ber segments having an optical characteristic Which
`varies in accordance With the physical condition, the
`?ber segments being separated by means for Weakly
`re?ecting portions of the light beam to form return
`optical signals Which propagate in the optical ?ber in a
`second direction Which is opposite to the ?rst direction,
`each of the return optical signals representing an effect
`of the physical condition on the light beam;
`(d) receiving the return optical signals and producing a
`photodetector output Which represents a sum of the
`return optical signals;
`(e) producing a time-delayed pseudo-random bit sequence
`by producing a plurality of time-delayed pseudo
`random bit sequences, each having a different time
`delay;
`(f) performing a correlation betWeen the time-delayed
`pseudo-random bit sequence and the photodetector
`output to produce a correlation result representing one
`of the return optical signals by performing a plurality of
`correlations to produce a plurality of correlation results,
`each representing a different one of the return optical
`signals; and
`(g) calculating a difference betWeen tWo of the return
`optical signals to determine an effect on the light beam
`by one of the ?ber segments.
`9. A method as in claim 8, Wherein each of the re?ecting
`means comprises a ?ber optic Bragg grating formed in the
`optical ?ber.
`10. A method as in claim 8, Wherein step (b) comprises:
`(i) emitting light; and
`in accor
`(ii) modulating the light emitted in step (b)
`dance With the pseudo-random bit sequence to produce
`the light beam.
`11. A method as in claim 10, Wherein step (b) (ii)
`comprises modulating the light emitted in step
`With an
`electro-optic sWitch.
`12. A method as in claim 10, Wherein:
`step (b) further comprises picking off a portion of the light
`emitted by the laser; and
`step (d) further comprises:
`(i) picking off portions of the return optical signals from
`the optical ?ber;
`(ii) coherently miXing the portion of the light picked off
`in step (b) With the portions of the return optical
`signals picked off in step (d)(i); and
`(iii) detecting a result of the step of coherently miXing.
`13. A method as in claim 12, Wherein:
`the result of the step of coherently miXing comprises tWo
`outputs; and
`
`8
`step (d)(iii) comprises detecting each of the tWo outputs to
`produce an output signal.
`14. A method as in claim 13, Wherein step (d) further
`comprises producing a balanced detector output in accor
`dance With the output signals.
`15. An optical monitoring system, comprising:
`an optical coupler, said ?ber comprising a plurality of
`partially re?ective and partially transmissive elements;
`an optical source disposed to launch an input signal into
`said ?ber;
`a coupler in said ?ber disposed to receive signal re?ected
`from said elements; and
`a detector effective, responsive to said coupler, to deter
`mine the phase of at least a portion of said signal
`re?ected from at least one of said elements, said
`detector comprising a correlator disposed to correlate
`said input signal With said at least a portion of said
`signal re?ected from said at least one element.
`16. The system of claim 15, Wherein:
`said optical source comprises:
`means for generating an optical carrier; and
`a modulator disposed to impose a modulation signal on
`said carrier;
`Wherein said detector comprises a demodulator dis
`posed to remove said carrier from said at least a
`portion of said signal re?ected from said at least one
`of said elements, effective to produce a demodulated
`signal;
`Wherein said correlator is disposed effective to correlate
`said demodulated signal and said modulation signal.
`17. The system of claim 15, Wherein said at least one of
`said elements is a member of the group consisting of: Bragg
`gratings, Fresnel re?ectors; preselected anomalies in the
`refractive indeX of said ?ber.
`18. The system of claim 15, Wherein said at least one of
`said elements is a Bragg grating.
`19. An optical system comprising:
`an optical ?ber comprising a plurality of partially re?ec
`tive elements;
`an optical source disposed to launch an optical signal into
`said ?ber;
`a phase detector disposed effective to determine the phase
`betWeen said optical signal and light re?ected from a
`preselected one of said elements, said phase detector
`comprising a correlation means; and
`a subtractor means for subtracting knoWn phase shifts
`from a cumulative phase detected by said correlation
`means, thereby detecting a phase shift induced by a
`preselected one of said elements.
`20. A ?ber optic sensor array for detecting a physical
`condition, the ?ber optic sensor array comprising:
`bit sequence generation means for generating a pseudo
`random bit sequence;
`light source means, receiving the pseudo-random bit
`sequence, for emitting a light beam Which is modulated
`in accordance With pseudo-random bit sequence, the
`light source comprising:
`a laser for emitting light;
`a modulator;
`a ?rst coupler, disposed in a path of the light emitted by
`the laser betWeen the laser and the modulator, for
`picking off a portion of the light emitted by the laser;
`an optical ?ber disposed to receive the light beam so that
`the light beam propagates in the optical ?ber in a ?rst
`direction, the optical ?ber comprising a plurality of
`
`HALLIBURTON, Exh. 1005, p. 0008
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`US 6,285,806 B1
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`9
`?ber segments disposed in series, each of the plurality
`of ?ber segments having an optical characteristic Which
`varies in accordance With the physical condition, the
`?ber segments being separated by means of Weakly
`re?ecting portions of the light beam to form return
`optical signals Which propagate in the optical ?ber in a
`second direction Which is opposite to the ?rst direction,
`each of the return optical signals representing an effect
`of the physical condition on the light beam, the optical
`?ber further comprises a second coupler for picking off
`portions of the return optical signals;
`photodetecting means for producing a photodetector out
`put Which represents a sum of the return optical signals,
`the photodetecting means further comprises:
`a third coupler for coherently miXing the portion of the
`light picked off by the ?rst coupler With the portions
`of the return optical signa