CA1296919C - Distributed sensor array and method using a pulsed signal source - Google Patents

Abstract

DISTRIBUTED SENSOR ARRAY AND METHOD
USING A PULSED SIGNAL SOURCE

ABSTRACT OF THE DISCLOSURE
A distributed sensor system using pulsed optical signals optionally produced by a short coherence length source to provide a phase difference output signal representative of conditions affecting A selected sensor.
In one preferred embodiment, an array of fiber-optical sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light pulses transmitted through the sensors are multiplexed onto a return arm of the ladder.
The multiplexed signals are received by an optical fiber compensating interferometer which coheretly couples portions of adjacent multiplexed light signals to produce a phase difference signal representing conditions influencing selected sensors. In other preferred embodiments, the system is configured to define a plurality of adjacent Mach-Zehnder interferometers which provide output signal pairs which coherently couple to yield a phase difference signal directly representing the environmental effects on a particular sensor. Functional equivalents of the Mach-Zehnder interferometer configurations comprise configuration including adjacent modulation interferometers. A phase and amplitude modulation technique and apparatus (Figure 8) is disclosure for providing heterodyned output signals from the distributed sensor system.

abstract

Description

1296~19 STANF.7lA
DISTRIBUTED SENSOR ARRA~ AND HETHOD
- USINC A PULSED SIGNAL SOURCE

~ackground Or the In~ention The present invention relates to riber-optic sensors, and particularly to distributed riber-optic sensor arrays ~hich utilize time di~ision multiplexing in their ¦ operation.
Over the past re~ years, riber-optlc devices have 10 1 been actively studied and developed rOr u~e in various sensing applications ln a uide range of rields. One reason ror this interest is the sensitiYity Or optical ribers to envlronmental conditions ~hich surround them.
~or example, ractors such as temperature, pressure, and a~o~stical ~a~es directly arrect the li~ht transmittin~
characteristics Or optical fiber. These chanses in the optical riber produce a change in the phase Or light slgnals traYeling in the riber. Thus, a measure~ent Or the chan6e in phase Or optical si~nals ~hich have been transmitted thrcugh that riber is representa;iYe Or chan~es in thcse enYironmental conaitions ~hich ha~e ~rrected the rl~er.

129~919 Recently, particular efrorts have been directed to the development Or systems having sensors or~anized in arrays, so that a number Or sensors can utilize light from a single source, and provide environmental information at a common detection location. Ideally, such an array would consist Or a fiber input bus which would carry light to a set of sensors. Each sensor would imprint inrormation - about the environment to this optical carrier. An output riber bus would then collect this inrormation and bring it back to a central processing location, where information obtained from any selected one of the sensors could be readily identified and analyzed.
The goal Or these development erforts is to produce sensor arrays which could be used for specific applications such as monitoring rapidly changing environmental conditions. For example, such sensor zrrays could be used to detect acoustic waves in order to determine the source location and acoustical characteristics Or those waves. For many such applications, it may be necessary to space the arrays over a relatively large area. In these situations, the replacement Or electrical lines by fiber optics, for example, would overcome problems such as electrical pickup, cable weight, and safety hazards associated vith the use Or those electrical lines. Even when the sensor is used in limited space, the removal Or electronics and bulk optics components generally should provide improved system perrormance due to reduced noise. On the other hand, replacement of long electrical lines by optical fibers creates a problem in preventing or removing any influence Or environmental conditions on the non-sensor portions Or the system. This, thererore, beccmes an important desi6n consideration.
or course, the pri~,ary design consi_eration in cc~elopin6 a sensor array i5 the metho~ by which 129~;919 information from each sensor can be separated for individual identirication from among all Or the inrormation arriving at the central processing location on the single data stream. Distributed sensing systems developed previously have generally applied one Or two approaches for separating inrormation Or an individual sensor from a single data stream.
One approach which has been used for separating each sensor's inrormation rrom the sinsle data stream has been to rrequency-division multiplex the sensor outputs, in the manner described by I. P. Giles, D. Uttam, B. Culshaw, and D. E. N. Davies, "Coherent Optical-Fibre Sensors h'ith Modulated Laser Sources," Electronics Letters, Vol. 19, Page 14, (1~83). This approach is accomplished by lS rrequency ramping the optical source and arranging the array geometry so that the transit time Or the li~ht from the source to a sensor and back to the central location is unique for each sensor. In this case, the array output is mixed with the source's present output, thereby producing a unique central rrequency ror each sensor. The environmental information is carried in the sidebands about this central rrequency.
One particular problem with the above-described system involves the "fly back" period when the periodic ramp signal is reset rrom its maximum to its minimum position. This fly back period comprises a time ~hen system operation may not occur, since no ramp si6nal is present, and no meaningrul results would be produced.
This places some limit on the rate at which environmental conditions may change and still be reliably mGnitored by the sensor system.
Anothcr problem ~ith this rarticular s;stem is that ~he n"mber Or sensors t~hich ma~ be used in the array or the rrequcltcy ran6e Or the si6nals to be de~ected are limit~d based on the ran6e Or FM frequencie5 ~hich are ~ ~ ~ ", rr ~

utilized in the ramp signal, and on the period Or the ramp si&nal. More specirically, since a dirferent central rrequency is produced for each sensor, the amount Or dirference between each such central rrequency and the overall ranBe of frequencies within which these central rrequercies are contained dictates the number Or sensors which may be utilized. Equivalently, the number of sensors, together with the overall range Or frequencies determine the maximum difrerence between central frequencies, and hence the maximum environmental frequencies which may be detected. The range Or frequencies is, of course, determined by the slope and period Or the ramp signal.
These sensor conrigurations are also li~lited in the distance from the optical source uhich a given sensor may be positioned, not only due to the limitations based on the coherence len~th Or the optical source, but also based on the fact that as the sensor is moved further rrom the optical source, the path length dirference between adjacent optical paths becomes very large.
Another approach which has been used for separating each sensor's inrormation rrom the single data stream comprises time-division multiplexing Or the sensor outputs, as is described by M.L. Henning et 21., "Optical ~ibre Hydrophones with Down lead Insensitivity," I.E.E.
Conrerence Publication 221, pages 23-27, (April 19~3). In time-division multiplexing, the optical input most &enerally is pulsed so that the input signal cor,pri~es a pulse waveror~. In the interrerometric pu'sed system described by L:enning et al., the input li&ht is pulsed tuice uith a particular delay between the two pulses.
~his delay is determined by the Eeometry Or ~he sensor, and in particular by the relative delay bet~een the two arms Or the interrometer comprising the sensor.
Specirically~ the optical input pulses cc runicated J~Z~6919 through each sensor are mixed and placed on the output fiber by each of the sensors at a different time. By controlling the relative position of the sensors, interleaving Or the pulse signals may be accomplished as the signals are multiplexed from the sensors onto a return fiber bus. These interleaved pulse signals are then carried back to the central processing loc2tion uhere demultiplexing and further signal processing occur.
One Or the problems with these types Or systems is that they generally have required use of an optical source having a coherence length which is longer th2n the path length difference between adjacent signal p2ths. The long coherence length is necessary in order to have the light from adjacent paths interrere. The interference creates an intensity modulation which is proportional to the phase modulation created in the light by the environment. In addition, the two pulses which are launched into the sensor array are generated from the source at different _ times. The result Or mixing light uhich oriEinates from the source at different times is phase induced intensity noise. Such source phase induced noise ray create a limitation to the sensitivity of a sensor in such a system. Another limitation with these types of devices is that they measure only the difference betueen the sensors, and do not provide a means for measuring the environmental efrects on a selected sensor by itself.
Based on the above, it would be an i~;portant improvement in the art to provide a sensing system and technique ror multiplexing a plurality of re~ote sensors without being subject to the abc~e-identified restrictions. ~hus, the system should option211y be free of sensor spacing limitations, and experience little degr2dation Or the signals carried thereon due to laser phase-induced intensity no~se. Such a eyste~ should pro~ide ror op~r2tion uithout requiring use of ele.Lrcnics 12~6919 or ac~1ve devices ln the envlronmental 8enc~ng region.
~he ystom ~ho~l~ provide f~r maximiz-d duty cycle operatlon to lncr~ase the e~ici~ncy an~ potentl~l applications of the sy~tem. Pr~erably, ~uch a ~y6tem shoul~ permit u~e o~ any of ~ wide rangs o~ optlcal source~, includln~ short or mo~ex~te, a~ w~ll as long, coherenc~ l~ngth ~urce~ where the coherenc~ length 16 gr~t~r than or ~gual to ab~ut one centi~ater, ana 6houl~
~e both l~ple and economi~l to pr~uce and use ln actual appllcati~n.

~he present inv-ntlon compr~se~ a dlstrlbuted ~ensor sy~tem ~nd m~thod whlch preferably usec ~ chort coherence length llght source for accompllshing multiplexing o~ an array of f1b~r-optic ~ensor~. More ~peci~ically, the ~en~or syGtem o~ the p~e~ent lnventlon ganer~lly compr~e~
an optical c~urce wh~ch ~ opt~o~lly coupl~d t~ provldR
pulsed optlc~l D~gnals tG A ~l~et optlcal waveguide- An envlronment~lly aenslt$ve wavegulde egment i8 DptiCally coupl~ t~ th- fir~t opt~c~l wav-gu~de, ~id ensitive segment in~lusnc~ng ~ptic~ nal~ propa~a~ing wl~hin the 6en~tlve ~Qg~en~ ln rs~p~n~ t~ par~icular ~nvironmental condition6, A second optlcal wa~guld~ i~ optically ooupled to t~e ~lrst o~tical wav~guidQ in a con~iguratlon euch th~t the cec~n~ optlcal w~veguide ~nd ~he environmen~lly ens~tiv~ wavegui~e egment ~orm at least po2tion~ 0~ ~rms o2 a ~lr~t unbal~nced lnterferometer A
cecond unb~lanc~d ~nterrerom~ter i~ ~ptically coupl~d to the flr~t u~al~nce~ ~nter~Qrometer ~or roc~lvlng optlc~l 3~ ~ignal~ rrom th~ ~ir~t ~ntor~romat-r The ~econd inter~erometer prov$d~ a pair o~ opt~c~l path~ havlng an opt~cal path length dlffsrenc~ which ~ub~tQntially matches an op~lcal p~th ~ength dlf~erence de~lne~ by the fir~t lnter~rom~t-r, uch that th~ ~econd ln~erferometer 5 comblne~ optlc~ ignel- roc-lv-d rrom th~ r~ rst l'Z'~9~l9 lnter~rometer to ~orm ~ ph~e ~ erence output sig~al repre~en~ive o nvlron~ental ln~luence on th~
cnv~ronmentally ~nsl~ive waveguld~ segment.
~h~ ~rm~ o~ the ~r~t un~alancea interf~rometer form ~ir~t and ~con~ llgh~ path~ in that int~r~erometer.
Likewl~e, arm~ of th~ ~econd unbal~nce~ interferometer rorm third ~nd ~our~h llqht paths. Th~ ~lr~t and ~econd interferometer~ ~r~ con~lgur~d ~o that th~ di~far~nce ln opt~cal path lon~th~ b~tween t~e ~lr~t and second l~ght path~ ~n the ~lr6t lnter~erom~ter $~ ~ub6tantlally equal to th~ oreno~ ln optlc~l p~th length~ b~tw-~n the ~ir~t and ~cond l~ght path~ ln the s~c~nd int~rforom~ter.
Slnce the optical path l-ngth diffarence~ are ubetant~ally the ame, th~ ~-cond lnter~erometer 18 a~le to combin~ tho optical ~l~nals thereln ~o that cign~l~
whlch hav~ trav~r~ed llgh~ path~ which are ub~t~ntinlly qual ~n length, or whlch placo the ~ignal~ into appropriat~ pha~ rslA~on~hip, c~n be coherently coupled ~y oombining them at an output Or tho ~econd lnt~r~-rometer.
T~e en~lronmentall~ ~ns~tl~e w~vegu~de seg~ent comprice~ a een~lng regl~n ~n whlch optlc~l 6lgn~l#
trAv~l~ng therQln are lnfluen¢ed ~y an ~nviron~ental effect. This in~luence 18 evide~ced by a ohange ln pha6e o~ th~ opt~oæl ~lgnal pr~pagatin~ ~n the ~en~ing region.
~ccord~ngly, t~e slgn~l~ whlch ~re cohcrently co~pled at the o~tp~t o~ th- ~eeond lnt~r~oromet-r ~orm ~ pha~e ~l$~erenc~ output ~n~l which 1~ repre~nt~tive o~
environmental ~n~lu~nca on the ~nvlr~n~en~lly sen~it~v~
wav~gulde egm-nt.
El~ctro~lc detector~ ~xe pr~er~bly optlcally coupled to re~Qlv- the oomb~ned optlcal cl~nal~ ~rom the second ~nt-r~-rometer. Th~ d~tector~ are typlcally lntsrconnected to other lnformatlon proce6~1n~ devices for _7_~.

~9~.9 ~onltoring ~n~ ~alu~t~ng the partlcular nvlron~ental condit~ons whlc~ hav~ be~n ~stocte~.
In on- pr-~rre~ embodlment, th- lnventlon co~prises a S Np~r~ l" conflgur~tlon ln ~hleh an optlcal ~ource such ehort coh~r-nce length l~sQr launch-e pul6ed optlcal i~n~l~ lnto ~ lngl~-~ode Slb-r-optlc lnput bu6 Srom whenc- th- ignal~ ~re ~lstr~buted to a plurallty o~

-~a-~,20C,~

optical fibers or other components such as electronic transducers CDmpriSing sensors which are each optically connected at one terminal to the input riber-optic bus, and which are connected at their other terminal to a r iber-optic output bus, thereby forming a ladder conriguration. The pulses are timed such that the return pulses rrom the sensors do not overlap with each other or with pulses from the next sampling of the array.
Each sensor imprints environmental information onto the light passing therethrough in the form of modifications to the optical phase. Light from each of the sensors is optically coupled onto the fiber-optic return bus. The difference between the lengths Or each of the light paths defined by the input fiber-optic bus, an individual sensor, and the riber-optic return bus is much ereater than the coherence length Or the optical source, so that intensity modulation does not occur uhen the light from each sensor is collected onto the fiber-optic return _ bus A Mach-Zehnder interferometer is constructed on the - return bus to receive the signals coupled rrom the sensors. The arms Or the Mach-Zehnder interferometer are Or different lengths, with the difrerence in the arm lengths being equal to the difrerence in path lengths between each two adjacent sensors. Consequently, the interferometer causes mixing of the outputs Or adjacent sensors and produces an output signal corresponding to the dirrerence in phase bet~een signals passing throush the adjacent sensors. This difference inrormation relates directly to the en~ironmental conditions ~hich inrluenced the particular sensor. A frequency shifter can be placed in one arm Or the ~:ach-Zehnder interrero~eter to produce a heterodyned output.
In another preferred embodiment, the abo~e-described system is Fodirled by locating en~ironmentally sensiti~e 12~

regions defining sensors pn a portion Or the fiber-optic input bus between each pair of rungs in the ladder structure Or the system. This configuration minimizes the required number of optical components. In addition, because the sensors are positioned on the input bus, or optionally on the output bus, no additional delay line is needed to sep2rate the pulses from adjacent sensors. In addition, in this embodiment, every pulse from the optical source except the rirst and last pulse can provide information from a sensor, thereby permitting optimization Or the output duty cycle. Again, the path length difference for optical signals traversing paths between adjacent sensors is equal to the path length difference between the arms in the sensing interrerometer positioned on the return bus.
In still another pre~erred embodiment, a ~ach-Zehnder interferometer is positioned on each rung of the ladder structure Or the system. Again, the len6ths of the arms _ of each Or the sensing interrerometers on the run6s correspond in difference by an amount which substzntially matches the arm length difference Or the compensating interferometer. ln this system, the pulse co~.municated from the optical source produces two pulses from each sensing interferometer, for transmission on the return bus to the compensating interferometer. Accordingly, the pair of signals rrom a given sensing interferometer are caused to constructively interfere at the output of the compensating interferometer, producing an a~plitude modulation. Photodetectors at the output Or the compensating interrerometer may monitor the a~plitude modulation corresponding to the phase modulation of the given sensor, and produce a si~nal representatiYe Or the environmental conditions ~hich inrluenced that sen~or.
ln each confi~uration Or the invention, the compensating interferometer can also be located on the lZ'~

input bus between the optical signal source and the sensing regions. In this configuration, the two optical signals produced by the compensating interferometer from each single optical pulse signal received from the source are combined by the sensing interferometers to provide a coherently coupled signal on the return path. This coherently coupled signal is received by the detector and ?rocessed in the same manner as if the si~nal were received from the compensating interferometer in the configurations described above.
In still another preferred embodiment of the system, the confi~uration described above can be modified by cutting each waveguide in the run in half, and depositing a reflective mirror on the cut end of the wavesuide to reflect optical signals back into the waveguide. In this configuration, adjacent waveguides defining rungs form, in conjunction with the interconnecting portion Or the input waveguide, a Michelson interferometer. If sensin~ regions are located on the input waveguide, the length of each region is reduced by half since the light reflected on the - input bus from each interfomerter will pass through the - sensing region twice, once when coming from the source and once when returnin~. The returning signals are transmitted to a compensating interferometer and processed as with the other embodiments described above. This configuration minimizes the amount of optical fiber needed to form the sensor but has some loss in efficiency due to the use Or an additional optical coupler to transmit rerlected li6ht returning on the input wave~uide to the compensating interferometer.
Each Or the confi&urations of the present in~ention are lead insensitive since the si&nals are carried on a common riber except while in the sensor or compensating interferometer. Thus, environmental shielding is needed 3~ only on the co-pensating interferGmeter in order to obt~in lZ~f ~.9 signals which, if proper techniques are used to avoid signal fading, directly reflect changes in the selected sensor.
The present invention also includes a novel apparatus and technique which may be utilized in several Or the configurations Or the invention to provide a heterodyne-like output signal, without the use of an optical rreguency shifter. In heterodyning, the frequency Or the signal is shifted so that the information contained by the signal is carried on sideband frequencies Or the resulting non-zero center frequency. Heterodyning is desirable since it o-ercomes the problem of signal fading due to low frequency environmental influences on the fiber. In addition, the heterodyned signal can be readily evaluated by use of conventional electronic equipment such as spectrum analyzers, FM demodulators or phase detectors.
The present invention avoids the use Or a frequency shifter for heterodyning by providing a phase modulator in - the receiver portion of the sensor, together with a signal ~ processing technique for turning the resulting phase modulated signal into a frequency shirted electrcnic signal.
The phase modulator is operated at a freQuency much higher than that Or the signal in the sensor. A switching component, such as a gate, is used to modulate the electronic signal from the optical detector, in a m2nner synchronized tG the operation Or the phase modulator.
Thus, the output signal from the receiver effectively multlplies the detected signal by a sguâre ~ave at the higher modulation freguency, mixing the harmonics Or that modulation ~re4uency in the signal. Since odd ana eYen harmonics never simultaneously fade, it is possible to eliminate signal fading by mixing the t~o harmonics as described. h'hen the modulation amplitude Or the p~ase modulator and the synchronization of the g2te are adjusted 12~f fr!~ ~9 appropriately, the output signal will contain a heterodyne-like signal around one Or the modulation frequency sidebands.
The distributed array sensors Or the present invention comprise a system and technique for multiplexing remote sensors hhich is accurate, and which permits detection of rapidly changing environmental conditions which influence the sensors. The invention permits use Or optical sources having a short coherence length~, thereby including a wide ranBe Or commercially available lasers which are less expensive and more compact and rugged than those having longer coherence lengths. Of course, the invention is not limited to use Or such short coherence length lasers, but may use any optical source haYing a coherence length which is greater than or equal to about one centimeter. Further, the invention preferably accomplishes its purpose in an all fiber-optic configuration, eliminating unnecessary bulk optic components which degrade system performance by reducing reliability and increasing system loss and complexity.
- The system is configured to be lead insensitive, permitting use of long lines for carrying optical si~nals to and rrOm connection with each unbalanced interferometer, without the need for en~ironmental shielding of those lines. The inYention also includes a technique for efrectively heterodynin~ the output sisnal, which remo~es the need of frequency shifters in the compensating interferometer, thereby ~urther reducin6 the cost and increasing the accuracy o~ the sensing system.
Brief Description Or the Drawings Fi6ure 1 is a schematic drawing Or one preferred embodiment of a distributed fiber-optic sensor systcm of the present in~ention.
Fi6ure ~ is a sectional Yiew Or one e~odiment Or a riber-optic directional coupler ror use in the distributed ser.sor system of the present in~ention.

12~f~

Figure 3 is a schematic drawing of another prererred embodiment Or the distributed sensor system Or the present invention, illustrating sensors positioned on the input waveguide of the system.
Figure 4 comprises another preferred embodi~ent Or the distributed sensor system of the present invention, illustrating sensors comprising Mach-Zehnder interferometers positioned on each rung of the ladder confi&uration of the invention.
1Q Figure 5 is a schematic dra~ing Or still another preferred embodiment Or the distributed sensor system Or the present invention, utilizing a confi6uration rorming Michelson interferometers connected to the input waveguide.
Figure 6 is a schematic drawing of another preferred embsdiment Or the invention, providing another configuration Or Michelson interrerometers connected to the input waveguide.
Figure 7 is a schematic drawing Or a further prererred embodiment Or the invention, wherein the compensating interferometer is positioned on the input waveguide.
Figure 8 is a schematic drawing of one embodiment Or the distributed sensor system, illustrating a technique ror rrequency shirting the output signal rrom distributed sensors using phase modulators and gates.
Figure 9 is a schematic drawing Or a simplified ~er~ion Or an embodiment Or the coherent distributed sensor system Or the present invention.
Figure 10 is a graphical presentation Or minimum detectable modulation as a function Or sisnal rreQuency, illustrating results of system sensitivity evaluations for input optical signals at selected pulse rre~uencies and uith optical and electronic s~itches.

lZ~?~

Figure 11 is a perspective view of one embodiment Or a fiber-optic polari7ation controller ~or use in the distributed sensor Or the present invention.
Detailed Description of the Preferred Embodiment -The invention is best understood by reference to the figures ~herein like parts are designated with like numerals throughout.
Figure 1 illustrates one preferred embodiment Or the invention comprising a sensor array system for monitoring environmental conditions influencing a plurality Or distributed sensors. A light source 100, such as a laser diode, which preferably has a short coherence length is utilized in this embodiment. To produce the desired pulsed optical signal output, the light source 100 can comprise either a continuous wave laser which is electronically or mechanically pulsed, or a self-pulsed laser.
Coherence length means the length over which signal _ interference ef~ects may be obtained. Those skilled in the art will appreciate that the coherence length (Lc) may - be defined, for at least some types Or laser sources, by the ~ollowing relationship:

g ~ L (1 where: 2~r = optical bandwidth at 1/2m2ximum power; and ~g - group velocity Or light ~n an optical riber.
Thus, from Equation (1) it becomes apparent that the coherence length increases as spectral pur,ty Or the laser improves. It ~ill also be appreciated by those in the technology that, in comparison to the prior art systems requiring lon~er coherence length sources, a sensor system which can utilize short coherence length signal sources comprises a versatile system ln which any of a large number Or laser light sources may be used, including relatlvely inexpenslve and compact diode lcsers.

12~919 In the embodiment shown, the light source 100 comprises an Aluminum Gallium Arsenide (AlGaAs) laser which produces light having a wavelength on the order Or about 820 nm. By specific example, the light source 100 may comprise a model ~LP1400 laser diode, commerciaily aYailable rrom Hitachi Limited, 6-2, 2-Cho~,e Otemshi Chiyoda-Ku, Tokyo 100, Japan.
The light source 100 Or Figure 1 is optically coupled to an optical fiber comprising fiber-optic input bus 102. Positioned upon input bus 102 are a plurality Or dirèctional couplers 108a, lOBb, ... 108n ~hich couple some Or the optical power to a plurality Or optical ribers 110a, 110b, ... 110n ~hich are each optically connected to one of the directional couplers 108. The basis for selecting the locations of couplers 108 on input bus 102 will be explained more fully subsequently.
In the illustrated embodi~ent, the directional couplers 108 are Or the same type as other directional couplers utilized in the sensor system. One preferred ~ embodiment of a directional coupler which may be used in the system is disclosed subsequently herein, and is described in detail in U.S. Patent No. 4,493,528 entitled "Fiber-Optic Directional Coupler" and ~.S. Patent No.
4,536 058 entitled l'Fiber-Optic Directional Couplern, both of said patents being assi~ned to the assignee Or the present invention.

The optical fibers 110a, 110b, ... 110n each haYe a first end exter,ding throu~h ports of a corre pondin6 optical coupler 108a, 108b, ..... 108n. She optical ribers 110 co~,prise fiber-optic sensors ~hich are positioned in the en~ironment so as to be sensitive to, and inrluenced by, changes in the environmental conditions surroundin~
the sensors lloc or course in this, as uell as substantially all ot~er e~.bodiments Or the in~en~ion, r,~.9 device~ such as transducers could be connected to the optical fibers in the system and be utilized as sensors 110 for responding to environmental efrects by influencing the flow Or light through those optical fibers. For example, an acoustic transducer could be connected to an optical riber 110 to increase acoustic sensitivity of that r iber.
The second end Or each of the sensors 110 passes through one Or a plurality Or directional couplers 112a, 112b, ... 112n. Couplers 112 are positioned at selected locations on a riber-optic return bus 114, bringinB the sensors 110 into optical coupling relationship with the return bus 114. It will be appreciated that the above-described relationship defines a ladder net~ork for the sensor arm of the sensing system.
The optical source of Figure 1 is pulsed to producean input pulse 201 which is distributed to the Yarious sensors 110 via input bus 102 and directional couplers 108a - 10~n. As the pulse 201 traYels down line 102 and is distributed to the various sensors 110, a string Or pulses 203 is produced on return bus 114 with each pulse in the string coming from a difrerent sensor 110. The spacing between each pulse in the string 203 is based upon the optical path difference bet~een adjacent sensors 25 110. Thus, the first pulse in the string will correspond to the pulse which was communicated through sensor 110a, since this optical pulse had the shortest travel time between the light source 100 and ~he return bus 114.
~ ike~lse, the second optical pulse corresponds to the pulse pro~ided from sensor 110b, since this pulse had the next shortest optlcal path iength from the ~i6ht source 100 to return bus 114. ~he sp3cing Or the pulses in thls embodiment is not based on the coherence length Or the optical source since this pulsed system is not coherence dependent. Thererore, an optical source Or any Or a broad range Or coherence len6ths may be used in this embodiment.

lZ~91!~

or course, the pulse length Or the pulses rrom the light source 100 should be adjusted so that the return pulses rrom the sensors do not overlap with each other.
Further> the pulses rrom light source 100 should be timed so that the return pulses from the sensors do not overlap with pulses rrom the next sampling of the array. ~or example, if the pulse length from light source 100 ~ere too long, the length of the pulse communicated from sensor 110a onto return bus 114 may be such that the tail of the pulse would not be placed on bus 114 at coupler 112a before the leading edge of the pulse from sensor 110b passes through coupler 112a on return bus 114. Likewise, if the timinB Or the pulses from light source 100 is too close together, the output pulse rrom sensor 110a corresponding to the second pulse rrom the li3ht source could be placed on the return bus 114 before the output pulse rrom sensor 110n corresponding to the first pulse rrom light source 100 passes couplers 112a on the return bus 114. In either Or these situations, it would be virtually impossible ror a detector, receiving the pulses from the return bus 114, to determine which sensor those pulses had been received from.
The string of pulses 203 is transmitted along riber-optic return bus 114 to the input Or a ~.ach-Zehnder interferometer 200 ~hich is comprised of a pair Or directional couplers 202 and 204 positioned on the riber-optic return bus 11~ so as to define a first arm 206 between the couplers. A second len~th o~ optical fiber 208 is secured at either of its ends in the couplers 202 and 204 so as to define a second arm Or the interrerometer betheen couplers 202 and 204. The dirference in optical path lengths Or arms 206 and 208 should substantially equal the difference bet~een optical path len~ths Or successi~e sensors. 0ptionally, arm 206 coud be Or a lenth greater than arm 208 by an amount ~hich ~z~ 9 substantially equals the dirference between the optical path lengths Or successive sensors.
With the arm lengths chosen as described above, the pulses pass through interferometer 200 such that the portion of the rirst pulse rrom string 203 which traverses the longer arm 208 reaches coupler 204 at substantially the same time as does the portion Or the second pulse from string 203 which traverses the shorter arm 206. Likewise, the portion Or that second pulse which traverses arm 210 arrives at the coupler 204 substantially at the same time as does the portion Or the third signal from string 203 whi ch traverses arm 206. Thus, it is seen that the interferometer 200 ~ill cause mixing in the optical coupler 204 of the output signals from adjacent sensors.
The mixed signal which is output rrom coupler 204 is communicated to a detector 212 positioned on that portion of fiber 208 ~hich extends beyond coupler 204.
Optionally, detector 212 could be posltioned on the portion Or fiber 114 which continues beyond coupler 204 rrom riber 206. Still rurther, t~in detectors 212 could be positioned one each on the detector locations just descri bed.
The detector 212 receives the mixed signal, ~hich represents the gradi ent Or the environmental paraDleter influencing the related sensor. One preferred embodiment Or a detector for use in the system of the present invention may comprise a model MFOD2401~ detector preamplirier, commercially available rrom l~;otorola Semiconductors, Phoenix, Arizona. or course, connected to 30 the output Or the detector 212 for each Or the embodiments Or the invention is appropriate measuring equip~ent (not sho~n) Or a type ~hi ch is generally used in the techno!o6y ror monitoring and e~aluating such optical output signals.
In the e~bodiments illustrated here~n, the optic21 35 path length dirferences bet~een adjacent optical paths are ~Z~ lg made to conform with the optical path length difrerence ln interferometer 200. By so doing, and with proper spacing of the paths andior proper timing Or p~lses rrom the optical source, all pulses returning from the sensing regions can be processed through a single interferometer 200. Howe~er, ir the difrerence between adjacent optical path lengths in the sensing region is not substantially the same as the path length difference Or interrerOmeter 200, then other interrerometers can be optically coupled to return bus 114 to define path length dirferences ~hich correspond ~ith those other path length difrerences in the sensor region. Such additional interferometers could be optically coupled either in parallel or series configuration with interferometer 200 on ~a~eSuide 114.
System performance is Breatly enhanced; and undesirable conditions such as phase induced intensity noise are minimized by tbe ability to closely match the optical path length difrerence in compensating interferometer 200 ~ith the optical path length difrerence between optical signals to be combined in the interrerometer 20~. Yarious techniques and systems are generally known in the lndustry ror matching these optical path length differences. One prererred embDdiment Or a method and system for accomplishing thi~ close matching is disclosed in Tur, et ~l., Electronics Le~ters, Vol. 22, No.
15, Jul~ 17, 1986.
In the embodiment of F~8~re 1, as ~ell as ln the other embodiments described herein; a freq~ency shifter (not shown) may optionaily be positioned in the s~stem, S such as on one arm of the compensating interferometer.

~z~

The frequency shirter is utilized to shift the fre~uency of the optical signal and, therefore, to "heterodyne" the matched optical signal detected by detector 212. By heterodynin~, the phase modulated output signal enters the detector 212 as a comparatively lou frequency phase modulation Or a hi6her rrequency arplitude modulated optical signal. Heterodyning provides a method by uhich lower frequency environmental effects can be prevented from reducing the system's sensitivity to small signals in the desired frequency range. Thus, those environmental effects in the desired frequency range can more readily be identified.
One preferred embodiment of a frequency shifter ~hich may be utilized for this purpose is a Bragg Cell modulator, which is uell-known in the technology, many types of which are commercially available. Such frequency shifters are comprised of bulk optics, which are inserted into the system by separating the fiber. Light is coupled to and from such bulk optic devices by lenses. It becomes apparent that the use of bulk optics such as Bragg Cells ror frequency shifters increases the system loss and reduces overall efriciency and quality of performance.
Another technique for accomplishing heterodyning in the coherence distributed sensor Or the present inventior"
without the necessity of frequency shifters and uithout experiencing the losses involved with the use Or bulk optics, is described in detail subsequently uith reference to Figure 8.
With respect to the coupling of light signals in the present invention, a more detailed description Or a prererred riber-optic directional coupler which may comprise couplers 108 and 112, for exarple, Gay be provided by reference to Fieure 2. Specifically, this coupler comprlses tuo optical fiber strands labeled 150a and ~50b in Fi~ure 2 Or a sin~le mode riber-optic r~aterial f ~

having a portion of the cladding remoYed from one side thereof. The two strands 150a and 150b are mounted in respective arcuate slots 152a and 152b, formed in respective blocks 153a and 153b. The strands 150a and 150b are positioned with the portions Or the strznds uhere the cladding has been removed in close-spaced relationship, to form a region Or interaction 154 in which the light is transferred between the core portions Or the strands. The amount Or material removed is such that the core portion Or each strand 150a and 150b is within the evanescent field of the other. The center-to-center spacing between the strands at the center Or the coupler is typically less than about 2 to 3 core diameters.
It is important to note that the light transferred between the strands at the reeion Or interaction 154 is directional. That is, substantially all Or the light applied to input port A is delivered to the output ports B
and D without contra-directional coupling to port C.
Likewise, substantially all Or the light applied to input port C is delivered to the output ports B and D. Further, - this dir~ctivity is symmetrical. Thus, light supplied to either input port B or input port D is deli~ered to the output ports A and C. Moreover, the coupler is essentially nondiscriminatory uith respect to polarizations, and thus preserves the polarization cf the coupled light. Thus, for example, if a light bea~ haYing a ~ertical polarization is input to port A, the light coupled from port A tv port D, as well as the light passing straight through from port A to port B, uill remain ~ertically polarlzed.
From the roreEoing, it can be seen that the coupler may function as a bezm splitter to divide the applied light into tuo optical paths, as is accomplished by coupler 104 Or Fi6ure 1.

lZ~

In the embodiment shown in Figure 2, the coupler has a coupling efriciency which may be varied based on the positioning Or the fibers with respect to each other. As used herein, the term "coupling efficiency" is defined as the power ratio of the coupled power to the total output power, expressed as a percent. For example, referring to Figure 2, if light is applied to port A, the coupling efficiency would be equal to the ratio of the power at port D to the sum of the power output at ports B and D.
In the distributed sensor Or the present invention, careful adjustment Or the relative path lengths and of the coupling efficiencies Or the couplers is required to achieYe optimum efficiency and performance.
Another preferred embodiment Or the distributed 15sensor system is illustrated in Figure 3. In this embodiment, a light source 100 such as a continuous wave optical laser is optically connected to an optical ~ate 101 for producing pulsed optical signals. The optical gate 101 is optically connected Yia a line 103 to a fiber-- - 20optic input bus 102. Secured in spaced relationship along input bus 102 are a plurality of optical couplers 108a, 108b, 108c, ... 108n which optically connect bus 102 to a plurality Or sensors 110a, 110b, 110c, ... 110n which themselves are optically coupled via a plurality Or optical couplers 112a, 112b, 112c, .. 112n to a riber-optic return bus i14. This configuration corresponds to the ladder network of the sensor arm Or the embodiment illustrated in l~igure ~.
Portions of the input bus 102 between sets Or 30 couplers 108 are configured in a coil arrangement ~enerally indicated at 105. The coils 105 comprise delay lines for definin6 the difference in si~nal path length tra~eled by optical signals passing in adjacent sensor arms 110. In addition, at least a portiDn Or each Or the 35 colls 105 is not enviror,mentally shielde~, thereby forming ~Z~

a sensor located on the input bus. By positioning the sensors and delay lines on the input bus, the number Or optical components required in this ladder portion Or the structure is minimized. Also, by positioning the sensors on the input bus, every pulse on the return bus 114 except the first and last pulse corresponding to a given pulse rrom the light source can provide inrormation rrom an individual sensor so that differences in phase between adjacent pulses on return bus 114 derine environmental conditions influencing a particuiar sensor. In contrast to the embodiment illustrated in Figure 1, uhich provides a measure of the difference in the environment betueen sensors 110a and 110b, for example, the embodiment illustrated in Figure 3 provides a direct measure Or the environment at sensor 105.
For e~ample, when an optical pulse 203 having traveled the optical path through arm 110b is combined with a pulse 203 having traveled the optical path through - arm 110a, the phase dirference signal provided rrom coupler 204 will directly relate to the environmental errect influencing the pulse which passed through the sensing region 105 between couplers 108a and 108b. This occurs because both combined pulses traveled a common optical path through any other sensing regions in their path on waveguide 102, however, only one sisnal in this pair passed through the sensor between couplers 108a and 108b. Thererore, the phase change caused in that si~nal uhlle passing through that sensor uill be directly evidenced by the phase difrerence signal rrom coupler 204.
By providing sensor inrormation for a 61ven sensor on all but the first and last pulses on the return bus 114, as lndicated above, the output duty c~cle can be sub~tantlally optimized ln the configuration Or ~igure 3. ~hls is accomplished by spacing the sensors 110 uith respect to each cther so that the optical p3th lensth lZ~913 difrerences between adjacent sensors are substantially the same for each sensor pair. Thus, all but the rirst and last signal rrom a pulse 201 can be used in the compensating inteferometer 200 to deYelop sensor information. This duty cycle can be further optimized by timing the optical pulse signal rrom the light source 100 so that the resulting pulse signal placed on return bus 114 from rung 110a immediately follows the passage of the pulse on bus 114 rrom arm 110n, and is synchronized therewith, thereby reducing the minimum time between transmission Or pulses rrom the otpical source.
Return bus 114 is connected to provide the optical signals traveling therein to a Mach-Zehnder interferometer 200 which corresponds to the interrerometer described ~ith rererence to Figure 1. As with the configuration of Figure 1, the adjacent signals communicated to interrerometer 200 from return bus 114 are mixed to provide an output from coupler 204 ~hich is communicated to a detector 212. The mixed signal comprises a phase difference signal which represents the environmental parameter inrluencing the sensor through which only one of the adjacent signals has passed.
As with the configuration o~ Figure 1, the difrerence in optlcal path lengths traveled by the optical signal bet~een adjacent sensors should substantially equal the path length difrerence of the arms 206 and 208 Or the interrerometer 200.
In operation, a li~ht source 100 such as a laser diode communicates a continuous wave optical signal to the optical gate 101. Cate 101 gates the optical-sisnal to produce a series Or pulsed output signals at a rate and signal length which will avoid the sisnal overlappin6 problemQ described earlier. One such optical pulse si6nal i~ illustrated at 201. The optical pulse 201 is communicated via lens 103 into the input bus 102. As the lZ~i919 optical signal 201 continues to travel down input bus 102, portions Or it are transmitted via couplers 108 through sensors 110 and couplers 112 to the fiber-optic return bus 114 in the manner preYiously described with respect ~o Figure 1. The signals returning on bus 114 comprise a signal train illustrated at 203, with the signals spaced at intervals such that interference will occur between adjacent signals at coupler 204, rollowing transmission Or the signals through the interferometer 200.
Optionally, a frequency shifter (not sho~n) may be included in the riber delay line 206 or 208 Or the embodiment of Figure 3, thereby providing a heterodyned signal as was previously discussed with respect to Fi~ure 1.
~ Another preferred embodiment Or the distributed sensor system Or the present invention can be described by reference to Figure 4. The system Or Figure 4 is configured substantially identically to the system Or Figure 3, except that the portions Or input bus 102 between adjacent couplers 108 comprise only delay lines - generally indicated at 107. These portions are not exposed to environmental inrluence, as was the case in the device Or Figure 3. The senslng is accomplished in the illustrated embodiment by constructing ~ach-Zehnder interrerometers such as t~ose illustrated generally at lOga, lO9b, 109c, ... 109n on the rungs 110 of the ladder .
conriguration.
Y.ore specirically, each rung 110 comprises a rirst optical coupler llla, 111b, lllC, ... 111n, and a second optical coupler 113a, 113b, 113c, ...... 113n, ~hich derine the length Or a rirst arm 115a, 115b, 115c, ... 115n Or the interrerometer 109. Also connected at opposite ends to the couplers 111 and 113 on each rung 110 is a second arm Or the interrerometer comprising a sensin~ arm 117a, 117b, 117c, .... 117n.

~2~

Preferably, the optical path length dirrerence between the arms in each of the sensing interrerometers 109 is substantially identical to the optical path length difrerence between the arms of the com.pensating interrerometer 200.
In operation, a series Or pulsed optical signals such as the pulse signal illustrated at 201 is produced and transmitted into the input bus 102 in the ~anner previously described with respect to Figures 1 and 3. As the optical signal 201 travels down the input bus 102, a portion of the signal is coupled via couplers 108 into the rungs 110. The signal in each Or the rungs 110 is communicated through the interrerometer 109 Or that rung, with the result that a portion Or the signal is communicated through the first arm 115, with another portion going through the second, sensing arm 117. ~he output rrom each interrerometer is communicated via couplers 112 onto the return bus 114. This output comprises a pair Or optical pulse signals 205a, 205b, 205c, ..... 205n ror each signal communicated into the sensing interrerometer 10g. Each pair of optical pulse signals 205 travels down output bus 114 and is received in the compensating interrerometer 200.
With the optical path length difrerence Or the interrerometer 200 matched to each Or the path length dirrerences Or the interrerometers 109, the pair Or optical pulse signals 205 produce a pair Or signals uhich are caused to constructively interrere in coupler 204, to produce a phase difrerence signal on the output Or coupler 204. This phase difrerence output is representative Or the phase dir~erence Or the palr Or signals 205 arter traverslng the separate arms Or their particular interferometer 109. Accordingly, the phase d1rrerence signal indicates the environmental efrect on the particular sensor arm 117 Or the interferometer 109 ~hich ~296919 influenced the phase Or the signal passing therethrough.
~he phase difference signal rrom coupler 204 is communicated to the detector 212, from uhence it is communicated to processing equipment for use in evaluating the environmental parameter which influenced the sensing arm of the interferometer 109 through which the signal pair traveled.
Because each rung 110 contains its own ~ach-Zehnder interrerometer, and so long as the rungs are spaced sufriciently and signal pulses are separated properly to prevent signal overlap on the return bus 114, the monitoring of environmental efrects on a particular sensing interferometer 109 is accomplished uithout sensing involvement of signals from any other sensing interrerometer 109. As a result, there is essentially no restriction on the length Or the input bus 102, or the output bus 114, between adjacent rungs of the system.
Accordingly, a distributed sensor such as that disclosed with reference to Figure 4 rinds particular value in applications requiring sensors to be located at selected points which may not be equidistant rrom one another, and uhich may be at extended distances from the light source 100 or rrom the compensating interferometer 200.
It is noted that in the system Or Figure 4, like the arrangement or the other embodiments disclosed herein, the optical signal is a pulsed signal. Therefore, the pos~tioning Or the couplers 108 and rungs 110 is not dependent upon the source coherence length Or the light source. However, the pulses from light source 100 should be tlmed such that the pulses returning- to the compensating interferometer 200 do not overlap each other, nor interfere uith pulses produced by the next pulse from li~ht source 100.
Rererrin6 no~ to Figure ~, another embodiment Or the device uhich is the equivalent Or the embodiment of Fi~ure lZ9~919 3 may be described. Specirically, like the embodiment Or Figure 3, the system illustrated in Figure 5 includes a light source 100 which can comprise a self-pulsed laser ror producing a pulsed optical signal, or which can comprise a continuous wave laser optically connected to an optical gate 101 for electronically or mechanically gatin6 the optical signal to produce a pulsed optical output.
~he pulsed optical signal is communicated via lens 103 to an optical riber input bus 102.
A rirst optical coupler 104 is positioned on input bus 102 and is connected to one end Or an optical fiber comprising a return bus 207 such that optical signals are coupled between the input bus 102 and the return bus 2~7 15through coupler 104. Return bus 207 is optically connected at its other end to a Mach-Zehnder interrerometer 200 which corresponds in conriguration to the interrerometer 200 previously described with respect to the embodiments Or Figures l, 3 and 4. The output Or the interrerometer 200 is also connected to a detector 212 in the manner previously described.
- Also positioned at selected locations on input bus 102 are a plurality Or optical couplers 108a, 108b, 108c, ... 108n. Couplers 108 are each connected to a first end Or an optical riber 220a, 220b9 220c, ... 220n. ~he other end Or each Or the optical fibers 220 is optically connected to a reflectiYe mirror 222a, 222b, 222c, ...
222n. Rerlective mirrors 222 are positioned in a conri6uration so as to directly rerlect li~ht travelin6 dounward throu~h optical ribers 220 back in the opposite dlrectlon throu~h ribers 220. Such mirrors may comprise metal or other rerlectiYe m~aterial ~hich ls deposited dlrectly on the fiber end. ~he optical ribers 220 are approxlmately 1/2 as long as the rungs 110 Or Figure 3.
Thu~, the total optical path traversed by an optical 35~16nal enterin6 one Or the optical ribers 220 and thcn -2~-lZ~ I9 rerlected back through the optical fiber 220 would be substantially the same distance as the optical path of light traveling through the corresponding rung 110 Or the embodiment of Figure 3.
At positions between adjacent optical couplers 108, the input bus 102 i9 configured to form a delay line uhich creates an optical path of a desired length. At least a portion Or each delay line is exposed to the environment or is otherwise configured to be susceptible to environmental influence, so as to define a sensor for imprinting environmental information on optical signals traveling ~ithin the sensing portion of the input bus 102. The optical path length defined by the delay line 224 is selected to create an optical path length difference between paths of optical signals reflected through adjacent optical fibers 220 which corresponds to the optical path length difrerence between the arms Or the compensating interferometer 200.
Accordingly, since the signal reflected from mirror 222 through fiber 220 tra~els the delay and sensing - portion twice~ the length of each delay coil 224 as uell as the length Or each portion Or bus 102 which is exposed to enYironmental influence should be approximately 1/2 the length of the corresponding delay coils and sensor regions 105 of Figure 3. The number Or optical fibers 220 and delay portions 224 are selected based upon the number of difrerent locations to be monitored, or the number Or en~ironmental sensors to be monitored.
The relationship between the adjacent optical fibers 220 ln this conriguration corresponds to a ~;ichelson lnterrerometer. For exzmple, the portion Or the device Or Figure 5 derined by optical coupler 108a, optical fiber 220a, and rerlecti~e mirror 222a, in combination ~ith Input bus 102, delay line and sensor 224, and optical riber 220b and associated mirror 222b define a lZ~

conventional Michelson interferometer. Thus, optical signals rerlected from the optical ribers 220 are communicated back onto input bus 102 and then through coupler 104 onto the free end 235 Or return bus 207 to produce a series Or optical signals similar to those produced on return bus 114 Or Figure 3.
More specifically, in operation of the device Or Figure 5, an optical pulse 201 is communicated rrom the light source 100 and optical gate 101 through lens 103 to the optical input bus 102. Although a portion Or the optical signal 201 is communicated via coupler 104 onto return bus 207, the remainder Or the optical signal 201 travels down input bus 102 and is partially coupled in each Or the couplers 108 onto its associated optical fiber 220. In each case, the signal on optical fiber 220 is reflected by the reflective mirror 222 so as to return throu~h the optical riber 220 and again be coupled through coupler 108 onto the inpuS bus 102. These rerlected signals travel along input bus 102 toward the optical signal source 100 and are partially coupled through coupler 104 onto return bus 207. As a result, a strin6 o~
optical pulses 203 are communicated rrom the system onto the return bus 207. Since each Or the optical ribers 220 are hal~ the length Or the rungs 110 Or Fi6ure 3, and since the delay and sensor portions 224 are half as long a~ the corresponding delay and sensor portions 105 Or Figure 3, the optical path of each Or the signals 203 on return bus 207 will correspond to the path len~th traveled by the optical pulse signals 203 Or Figure 3.
The optical pulse si6nals 203 are communicated from return bus 207 into interrerometer 200 and p.ocessed in the manner previously described ith respect to Fi~ure 3. The resultin~ signal detected by detector 212 rro~ the output Or interferGmeter 200 pro~ides a phase ~i'rerence sisnal representative oS the environmental influence on the optical sensor located between the two adjacent optical ribers 220 traveled by the two optical signals combined in the coupler 204 of interferometer 200.
It will be appreciated that the embodiment Or Figure accomplishes substantially the same runction and operation as the ladder confi6uration Or the device Or Figure 3, but does it without the use Or a return bus connected to each Or a plurality of ladder-type rungs.
Howe~er, the embodiment Or Figure 5 does cause the optical signals to each pass through one additional coupler 104, since the input signal passes through this coupler after being coupled onto the input bus 102, and the signals reflected rrom the optical ribers 220 also pass through that coupler 10~. As a result, the embodiment Or Figure S
experiences about a 6dB loss in perrormance as compared to the embodiment Or Figure 3.
The embodiment Or the invention illustrated in Figure 4 can also be modified by replacing the return bus 114 and sensing Mach-Zehnder interferometers with equivalent Michelson interferometers. The embodiment ror accomplishing this can best be described by rererence to Figure 6. As with the previous embodiments Or the invention, this system also utilizes a pulsed light source 100 which can comprise either a continuous wave laser ~hich is electronically or mechanically pulsed by means such as an optical gate 101, or through use Or a self-pulsed laser. In either conri6uration, the pulsed optical signal is communicated from the self-pulsed optical laser 100, or the optical gate 101 through a lens 103 to an optical input fiber 102. Positloned on the input riber is an optical coupler ~04 ~hich is connected to one end Or an optical riber return bus 207. ~eturn bus 207 is optically connected to a compensating in~erferometer 200 ana then to an optlcal detector 212 ~hich are substantially identical to the corresponding interferometer 200 and àe~ec~or 212 of the embodiments Or the invention described pre~ioucly.

lZ~q~

Also positioned on optical input bus 102 are a plurality Or optical couplers 108a, 108b, 108c, ...
lO~n. Each of the optical couplers is connected so as to couple optical signals from the input bus 102 to an end Or an optical fiber 220a, 220b, 220c, ... 220n. Positioned on optical ribers 220 is an optical coupler 2213, 221b, 221c, ... 221n, coupling light between the optical riber 220 and the end of a sensing arm 227a, 227b, 227c, ...
227n of a Michelson interferometer. At least a portion of the sensing arm 227 Or each Michelson interferometer is exposed to environmental influences, so as to sense those influences by changing the optical phase Or light traveling within the arm 227 in response to environmental influences. The other arm of the Michelson interferometer comprises a continuation of the optical riber 220, and is illustrated at 225a, 225b, 225c, ... 225n. The other end of each Or arms 225 and 227 is terminated by a connection to reflective mirrors 222, which are configured to reflect light received rrom arms 225 or 227 back into those respective arms.
A portion of the input bus 102 betueen pairs Or couplers 108 may extend to ~hatever length is necessary in order to position the sensors in their desired sensin~
locations.
To make the system of Figure 6 equivalent to the embodiment illustrated in Figure 4, the length o~ sensing interferometer arms 225 and 227 should be approximately 1/2 the length of corresponding sensing interferometer arms 115 and 117, respectively, of Figurc q. In this configuration, the optical signals transmitted into the arms 225 and 227 of the sensing Michelson interrerometers and reflected back tnrough those arms ~ill have tra~eled substantially the equiYalent path length as in thc correspondin6 Mach-Zehnder interrerometers 109 Or Figurc 4.

lZ~9~C3 In operation, the light source 100 and optical gate 101 communicate via lens 103 a pulsed optical signal 201 onto the optical input bus 102. A portion Or the optical pulse signal 201 is communicated through coupler 104 on to the free end 235 on return bus 207. The remaining portion Or pulse 201 travels doun bus 102 and portions Or this signal are coupled in each Or the couplers 108 onto optical fibers 220 and into the associated ~.ichelson interferometers defined by arms 225 and 227. The signals rerlected from those Michelson interferometers are again coupled via couplers 1D8 onto the input bus 102, with each interrerometer producing a pair of optical pulse signals 205a, 205b, 205c, ... 205n for transmission onto input bus 102. The optical signals 205 are coupled in optical 15 coupler 104 onto return bus 207, rrom whence they pass into the Mach-Zehnder interrerometer 200 and are processed - in the manner previously described with respect to Figure 4. As with the system Or Figure ~, the optical signal detected by detector 212 comprises a phase difrerence ~ 20 signal representative Or the environmental parameters inrluencing the sensing arm 227 Or the Michelson interrerometer which produced the pair of optical signals 205 c~rrently combined in the output rrom interrerometer 200.
As with the embodiment Or the device illustrated in F~gure 4, the embodiment Or Figure 6 permits positioning Or the sensing interrerometers at any desired location such that spacing between adjacent couplers 108 is sufriciently great so that optical signals 205 coupled 30 onto bus 102 by a given coupler 10~ do not o~erlap or interrere ~ith optical si~nals coupled onto bus 208 by other optical couplers 108O
The presence Or coupler 104 on the input bus 102 comprises one additional coupler uhich the optlcal signals must pass through as compared to the embodi~ent Or Figure lZ~369~.9 b. Accordingly, the system Or Figure 6 sufrers approximately a 6dB loss in perrormance as compared to the performance Or the embodiment of Figure 4.
In the embodiments described thus far, the compensating interferometer 200 is optically coupled to the return bus 114. However, the coupling interrerometer 200 could also be optically coupled to the input bus 102, between the optical signal source 100 and the first optical coupler on the bus 102. That configuration would produce substantially the same results as the configurations described above.
One example Or an embodiment Or the device which includes the compensating interferometer 200 positioned on the input bus 102 may be described by reference to Figure 7. The e~bodiment of Fieure 7 comprises the embodiment illustrated in Figure 3, with the compensating interferometer 200 repositioned such that the optical signal communicated through lens 103 is coupled into the input of interrerometer 200 at coupler 202. The signal from interferometer 200 is optically coupled in coupler 204 to the optical input bus 102.
With the compensating interrerometer 200 located on the input bus 102 in Figure 7, the return bus 114 is directly coupled to detector 212. Except for the chanses described above, no further modifications Or the device as conrigured in Figure 3 are included in the embodiment Or Figure 7.
In operation, an optical pulse 201 is communicated from the light source 100 and optionally the optical ~ate 101 via lens 103 to the input Or coupler 202. A portion of optical signal 20~ is coupled into ar~ 208 Or interrerometer 200, uhile the remainder Or pulse 201 travels through arm 20~ o~ the interferometer. ~he pulses from arms 208 and 206 are coupled on to input bus 102 in coupler 204. The result Or this couplins p^ocess is a ~296919 pair Or pulsed optical signals 241 which traYel down input bus 102 in the same manner as signal 201 doeR in the embodiment illustrated in Figure 3. Specifically, a portion Or each Or the pair Or pulsed optical signals 241 is coupled in each Or the optical couplers 108 into each Or the arms 110. Since the optical path length traveled by signals ~hich propagate through adjacent arm~ 110 corresponds to the optical path length dirrerence Or interrerometer 2QO, the pair of signals 241 returning on bus 114 from propagation through adjacent arms such as 11Oa and 11Ob are combined in their associated common coupler, such as coupler 112a. As a result of this coupling, those portions Or the pulsed signals 241 which have traveled the same optical path lengths are coherently coupled to form an output signal 243.
Signal 243 comprises a phase difrerence signal representing the environmental efrects which influenced the phase Or the portion of signal 241 propagating through ~ the sensor region 105 Or the optical path derined through arm 110. As is the case in the embodiment Or Figure 3, only one Or the signals ~hich are coherently coupled to form the phase di~rerence signal 243 have traveled through the sensing region 105. Accordingly, the phase dirrerence value Or signal 243 $s representati~e Or those environmental effects influencing the sensor region 105.
The phase difrerence signal 243 is communicated ~ia return bus 114 to detector 212 and processed in the sa~e manner as the phase dirference signal generated by coupler 204 in Figure 3.
The results produced by the embodiment Or Fi&ure 7, ~ith compensating interrerometer 200 positioned on the lnput bus 102 are substantially identical to the results obtained rrom the embodiment Or the system illustrated in ~igure 3, with the sensing interrerometer 200 positioned on the output bus 114. Like~ise, subst2ntially identical ~Z96~g results are obtained from the other embodiments Or the invention, when the compensating interrerometer 200 is positioned on the input bus 102, rather than on the output bus 114.
In each Or the confi&urations Or the present invention, the compensating interferometer 200 is preferably shielded from environmental conditions which may influence the phase of light waves being transmitted therethrough. In addition, such environmental shielding can be used on the non-sensing arms 115 and 225 Or the embodiments of Figure~ 4 and 6, respectively, as well as on the non-sensing portions of sensing arms Or all Or the embodiments particularly when these arms are very long.
Use Or shielding as described above will increase the sensitivity of the system, although such shielding is not required for system operability. ~o other shielding for this purpose is required in these systems, since the systems are environmentally insensitive except in those ~ portions Or a system where signals to be combined in the 2~ compensating interferometer are traveling in dirferent paths. This insensitivity is due to the fact that in optical signals in the system which are communicated along a common path, environmental influences arrecting the light signals in the common path ~ill not produce any changes in the phase difrerence between the light signals in those paths. Changes in phase difrerence will only occur uhen the light is traveling in dirferent paths, and then only ln the sensors and portions Or those difrerent paths which are arrected by influences such as en~lronmental effects.
Each Or the configurations Or the lnvention described herein co~prise a representatlve embodiQent Or the inventlon. It will be appreciated that these conri6uratlons can be expanded as necessary by adding rurthcr couplers and associated sensin6 sections in the repet~tive confi6urations ~llustrated.

~.z9G~

Based on the above description, it becomes apparent that each of the embodiment~ Or the invention disclosed herein derines a distributed sensor system which is lead insensitive, and therefore requires only a minimum amount of enYiron~ental shielding. These conrigurations also describe an all fiber-optic sensor system which is rree rrom both source phase-induced intensity noise and crosstalk between sensors.
It will be noted that each sensor has a rree end rrom which light may escape. Although this introduces loss, it is not a serious problem since, even ror a large number Or sensors, power loss can be kept relatively modest by properly selecting the coupling constants Or the directional couplers. The method ror selecting these 1~ coupling constants is explained in detail hereinarter.
Selecting Coupling Coefricients An issue relevant to the design Or a coherence multiplexed distributed sensor system, is the proper selection Or coupling coerficients ror the various directional couplers used in the system. As used herein, - the term "coupling coefricient" is defined as the power ratio Or the coupled power to the total output power. For example, rererring to Figure 2, ir light is applied to port A, the coupling coerficient would be equal to the ratio Or the power at port D to the sum Or the output at ports B and D.
The determination Or the coupling coerricients may be based ~n part on the intuiti~e requirement that all sensors experiencing equal environmental modulation amplitudes should return signals Or comparable strength to the central processing location.
Using the embodIment Or Figure 1 as an example, assume that there are N sensors 110. Number the sensors uith an index j running rrom 1 to N, startin~ ~ith j - 1 3S for the sensor closest to the light source 100 and to the lZ96~9 compensating interferometer 200. Let the power coupling coerricient for the couplers 108 and 112 associated wi~h sensor j be kj, so that a fractional portion kj Or the total power is transrerred between the two fibers in the coupler, and an amount Or power 1-kj passes straight through the coupler, without being coupled. It is noted the couplers at the ends Or a given optical riber sensor should be identical.
It is assumed for simplicity that light must couple across fibers in the couplers 108 in order to get from the input bus 1~2 to a sensing fiber 11~ and back to the return bus 114, although the situation could just as well be reversed. Light returning from sensor j will have sufrered loss rrom couplers 1 through j on both the input bus 102 and the return bus 114. Couplers 1 through j-1 will have a transmission 1~kq ror both the input and return couplers 108 and 112, respectively, and the two couplers at sensor j will have a transmission kj. Hence, the power returning from sensor j to the receivers 120 is given by 2 j-1 j,return Pinkj D (l~kq)2 (2) where Pin is the power being sent to the sensor array.
g ~1,return Pj,return it is round that the coupling coefricients are related by kjt1 ~ kj/(l-k;), or equlvalently, k = ~ 1 (3) The last sensor does not really require 2ny couplers slnce no pouer is needed ror the later sensors; hence one can set kn ~ 1. To~ether wlth the recurslor, relation just iZ~6~

derived, this implies that the coupling coefficlent ror the couplers Or sensor ~ is just k G 1 (4) ; N~

This, in turn, means that the total transmission P~ retUrn~Pin is the same for every sensor, as expected, and is equal to 1/N2. The ractors Or 1/N appears because the input power has to be split up among N sensors.
The Pseudo-Heterodyne Technique For Preventin~ Si6nal Fading Signal rading is a signiricant problem ror all Mach-Zehnder type sensors. One solution to this problem is to heterodyne the signal by introducing a rrequency shirterinto one arm Or the receiver, in the manrer described preYiously with respect to the embodiment Or Fi~ure l.
While conventional heterodyning provides one method - 20 ror avoiding signal rading and ~or distinguishing between si~nals in the desired rrequency range and lower frequency environmental efrects, this approach has the disadvantage that it requlres the use Or frequency shifters, hich orten comprise bulk optics de~ices. Such de~ices can be bulky, increase system loss, degrade erriciency, and can be costly.
An easier and less expensi~e method to avoid si6nal rading is a pseudo-heterodyne technique uhich requires no bulk optic devices in the optical path Or the sensor system. ~he techniq~e is derined in connection uith its application in a riber-opt1c ~yroscope in B. Y. Kim and H.
J. Shau, "Phase-Readlng All-Fiber-Optic Gyroscope,"
Optical Letters, Vol. 9, Pa6e 378, (1984). ~he technicue ls also disclosed in connection ~ith its application in a ,. .

~29~919 fiber-optic gyroscope in corresponding Canadian Application Serial No. 479,798 filed Aprll ~3, 1985.
The application Or the technique to the distributed sensor of the present invention can be described by reference to Figure 8. The sensing system optically coupled to the modulating system Or Figure 8 can tO correspond to any Or the systems illustrated in Figures 1, 3, 4, 5 and 6. This technique also holds for the conriguration of Figure 7, wherein the optical signals are recei~ed by detector 212 directly from return bus 114.
Thus, only the compensating interrerometer uhich is optically connected to those sensing system is specirically illustrated in Fi~ure 8.
In particular, a 1 to N switch 300 is optically connected on its input side to the output Or detector 2t2. Switch 300 runctions essentially as a multiplexer in response to incoming signals, such that uhen a neu signal is received rrom detector 212, the suitch 300 increments to the next channel location uhich corresponds uith the particular sensor ~hose en~ironmental inrormation is represented by the signal rrom detector 212. Thus, lr the signal inrormation rrom detector 212 ls representati~e or the sensor inrormation communicated through a particular arm, such as 110b Or ~igure 3, then the suitch 30D uill output this inrormation on channel 2. Likeuise, as the next lnrormation is recei~ed rrom detector 212, relating to the si~nal communicated rrom arm l10C, the suitch 300 uill mo~e to the next channel and transmit the information through corresponding channel 3. One prererred emb~diment Or a 1 to N suitch uhich ~ay be utilized in conjunction uith the present in-ention is a CMOS 40668 ~ET suitch manuractured by ~ational Semiconductor.

iZ96919 Each Or the output channels from switch 300 is connected to an identical equipment configuration.
Accordingly, the 1 to N switch is optically connected via each Or its channels to a low pass AC amplirier 302a, ...
302n, which itself is connected to a gate circuit 304a, ... 304n for producing a square wave signal on a periodic basis. The output Or gate 304 is electronically connected to a spectrum analyzer 306a, ... 306n, rOr use in identifying side bands around a harmonic of a modulation frequency in order to monitor the phase shift in the sensor 105 at a particular rreQuency. Alternatively, an FM demodulator may be used instead Or the spectrum analyzer.
The gates 304 are also connected to a signal generator 308 uhich produces a sinusoidal signal at a modulation rrequency fm. This signal controls periodic production Or the square wave in the gate circuit 304.
The signal generator 308 is also connected to a phase _ modulator 310 which is positioned in optical communication 20 with the arm 208 Or compensating interferometer 200. The - phase modulator 310 is controlled by the signal generator 308 uhich causes the phase modulator to produce a phase modùlation signal at the modulation rrequency rm.
Since the equipment connected to each output channel rrom suitch 300 runctions ln an identical manner, the operation of the equipment on a single channel uill be described ror example purpose~. It is noted that the optical sisnals in the system Or Figure 8, propagate and interrere ln the manner previously described uith respect 30 to the embodiment Or Figure~ 1, 3, 4, 5 and 6 except as is otheruise indicated belou. The description herein al~o applies to the embodiment Or Figure 7, uherein the phase modulator 310 is positioned on arm 208 Or the interrerometer 200 or input bus 102. Specirically, the light ln arm 208 is phase modulated by the phase modu1ator 310, which is driven at a modulation rrequency corresponding to the operating rrequency of generator 308. As a result, the intensity Or the output signal rrom coupler 204 which is received by detector 212 is modulated, and the resulting electrical output signal from detector 212 contains components at the phase modulation frequency fm and its harmonics, as indicated by the following equation:

I(t) = C[1 ~ cos(~msin ~mt + ~asin ~at ~ ~e)]
CD
[- ~ (~m) 2 ~1J2n(~m)cS 2n~mt~ cos(~a5in ~at ~
~D
- ~? ~ J2n~ m)sin (2n-1) ~mt~sin (~asin ~at ~ ~e)~

uhere C is a constant;
Jn denotes the nth order Bessel function;
~mis the amplitude Or the phase modulation betueen - the light ~aves in arms 206 and 208 due to the phase modulator 310;
~m= 2nfm ;
~a is the amplitude of the phase dirference between the light waves in arms 206 and 208 produced by external acoustic signals;
~a 2nfa ; and ~els the amplitude of the phase difre.ence between the light waves in the arms 428 and 554 produced by slou changes in the environment.
Equation 5 indicates that the output from detector 212 contalns te.ms .ncluding: cos(~asin ~at ~ ~e) and sln(~aSln ~at ~e) Ho~e~er, these ccsine and sine elements are at difrerent rrequencies. Ir t~ese si6nals were at the ~z~e frequency, ~ith their phases in i29~ 9 quadrature, well-kno~n trigonometrlc rules could be applied so that the signals could be added directly to obtain a single, low rrequency, sinusoidal signal whose phase corresponds to (~asin ~at ~e). Such a relationship can be achieved in the system Or Figure 8 through use Or amplitude modulation. Amplitude modulation simply involves making the amplitude Or the electrical output signal from detector 212 vary in accordance uith the amplitude Or a modulating signal.
When the output signal rrom detector 212 is amplitude modulated by a modulating signal having a rrequency uhich is an odd multiple Or the phase modulation rrequency (fm) (uhich is also the difrerence rrequency between adjacent harmonics), then each component Or the output signal rrom detector 212 which is a harmonic f fm becomes partially translated into the rrequencies Or its harmonic neighbors. In other ~ords, through amplitude modulation in this manner, sideband frequencies are created at harmonics Or the phase modulation rrequency. The sideband rrequencies are combined ~lth the component Or the output signal at the corresponding rrequency, and are readily identified by use Or a spectrum analyzer.
~hese and other characteristics Or amplitude modulation are generally known to those skilled in the art and are described ln detall in F. G. Stremler, Introduction to_ Communication Systems, Addison-~'esley, (1979). Subject matter of particular relevance at this point is set forth on pages 191-260 of the Stremler text.
Based on the abo~e, lt ~lll be appreciated that a ~lnusoidal amplitude modulation at a rrequency rm uill transrer energy out Or each harmonic frequency component and into the nearest harmonlc rrequPncy nei~hbors. ~o preYent lnterrerence in the present ~ensor s~stem, lt is desirable that rm be ~.uc~ treater than fa (the frequenc Or acoustlc signals ~hich are being detected).

. ~
~ 43 lZ9~

In operation, the optical signal in arm 208 Or compensating interreromeSer 200 is phase modulated at a ~requency which is controlled by rrequency generztor 308. As indicated above, the rrequency Or generator 308 is selected ~o that rm (the modulation frequency) is much greater than the acoustical rrequency (fa) The signal rrom arm 208 passes through coupler 204 where it is mixed uith the signal rrom arm 206, producing an intensity modulated signal due to the phase difference which is passed to detector 212. From detector 212, the intensity modulated signal due to the phase difrerence is communicated through the 1 to N switch to amplifier 3û2 where the signal is amplified and then transmitted to the conventional electronic gate 304.
Gate 304 functions in response to a signal recei~ed rrom the rrequency generator 308, causing Bate 304 to produce a square wave amplitude modulation or the signal received rrom amplirier 302. When modulated at the appropriate phase with respect to the AC detector current, 20 and with th~ appropriate choice Or a~m, the amplitude modulated signal Or this embodiment may be derined as cos(n~mt ~ (~aSin ~at ~ ~e)) ~ ith respect to the appropriate phase and amplitude for modulation, lt is noted that due to the trigonometric 5 relationships betueen the waverorms in coupler 204, amplitude modulation at e~en harmonics Or Sm would not produce coupling bet~een adJacent harmonic frequencies.
Rather, amplitude modulation at even harmonics Or Sm uould result in the e~en harmonics coupllng -with even harmonics, 30 and odd harmonics coupling uith odd harmonics. This sltuation is generally understood by those skilled in the art, and the basls ror this condition may be :;;ore rully understood with reference to the Stremler text. These problems are avoided if amplitude modulation at the odd 35 harmonics is utilized.

lZ969~9 ~ he output from gate 304 is communicated to the spectrum analyzer 306 for processing. It is noted that spectrum analyzers 306 incorporate a band pass filter for selecting and analyzing particular components Or signals. If such a band pass filter, centered on 2~m, were placed on the output Or the gate, and if the amplitude Or the phase modulation ~ were chosen appropriately, the filter would pass a signal Or the form:

V(t)= k x ~Jo(~a)COS(2~mt-~e) O~
+ ~ J2n(~a)[cos (2(~1~m-nwa)t-A~e)~cos (2(~m~n~"a)t-a~e) ~

.s + ~ J2n ~ a)[cos((2~1~m-(2n-~ a)t-~e)-cos((2~m~(2n~ a)t-~e)]}

(6) ~ where k is a constant which does not influence the identirication and evaluation Or phase shirts occurring in the sensor at particular rrequencies.
By putting the demodulated signal from the gate 304 into the spectrum analyzer 306, the height cr the Bessel runctlon sidebands around the second harmonic Or the modulation rrequency can be measured by use Or techniques uhich are uell-~no~n in the technology to give the phase ~hirt in the sensor at a particular rrequency.
Alternatively, rOr a complicated signal, an FM demodulator could be used. In that case, the measured signal uould be the derivative Or the phase rather than the actual phase, or alternatively, an integrator could be used to produce a measured signal representative Or the actual phase.
Optionally, gatinB Or the system Or Figure 8 could be accompllshed ~ptically rather than electrically by ~5 util~lng at least one optical Eate, such as a shutter, ~29~919 positioned bet~een coupler 204 and detector 212, or on any riber in the system comprising an optical path ~here all light signals tra~el, such as input bus 102 between the li~ht source 1~0 and optical coupler 108a, Or Figure~ 1, 3, 4, 5 or 6, or optical fiber return bus 114 bet~een couplers 112a and 202 Or the embodiments Or those figures. Ir the Bate 304 were positioned distant rrom the detector 212, the Bate should be controlled by a delay signal at a rrequency rm so that the light traveling within the system uould be amplitude modulated at the rm rrequency, in appropriate phase with the distant ate. In all other respects, the use of optical gating uould pro~ide a result substantially identical to that described in connection with electrical 6ating.
A simplified version or the embodiment Or Figure 3, including the synthetic heterodyning conriguration Or Figure 8 was constructed and tested to evaluate its perrormance. The or~anization Or this simplirled Yersion may be described along with the results Or the e~aluation, by rererence to Figure 9. In the embodiment Or Figure 9, the light source 100 comprised an essentially 3ingle-mode laser diode comprising a ~Hitachi HLP 1400 emltting a continuous wave 820 nm light. This light ~as communicated through a lens 4D0 and a conventional optical isolator 402 to an acousto-optic Bra8g cell 404 corresponding to optical 8ate 101 of Figure 3, uith a 35 nsec rise time.
The Bragg sell 404 uas used, instead Or direct modulation Or the input current to the laser, in order to a~oid modulation Or the laser spectrum.
The 100 nsec uide pulses uere communicated through a lens 406 to the end Or an optical input bus 408, corresponding to bus 102 Or Fi~ure 3.
The optical pulses uere transmitted throu6h a first rlber-optic Yach-Zehnder interrerometer 410 ~a~lns a first arm 412 corres~onding to arm 110a Or Figure 3. Likeuise, Trademark _ 12969~9 the arm 414 Or interferometer 410 corresponds to the optical path length derined between coupler 108a and 112a, for the signal passing through arm 11Ob Or the embodiment Or Figure 3. Interrerometer 410 additionally included a phase modulator 434 in optical contact with the arm 414 Or that interferometer. Phase modulator 434 was provided to simulate an acoustic signal. A signal generator ror producing the phase modulation in modulator 434 is illustrated at 442. ~he phase modulator 434, as well as all other phase modulators in the system correspond to the type described previously herein.
Interferometer 410 also included polarization controllers generally indicated at 446. These manually adjustable polarization controllers were used to overcome polarization induced signal-rading ror the associated sensor. The polarization controllers correspond to those which will be described hereafter. Optionally, polarization preserving or polarizing fiber can be used to _form the optical riber waveguides in the system, removing the need ror polarization controllers 446.
Optical couplers 430 and 432 were positioned on the input bus 408 to couple optical signals between that input bus and the arms Or the interferometer 410. Couplers 430 and 432, as uell as all other couplers in the system, comprised tunable directional couplers Or the type described previously herein.
From the output Or interferometer 410 the optical signals were communicated to another interferomeSer 420 which corresponds to the interferometer 200 of Fi~ure ô.
30Particularly, 2rm 422 Or Figure 9 corresponds to arm 206 Or the interrerometer 200 Or Figure 8. ~ikewise, arm 424 Or interrerometer 420 Or Figure 9 coFresponds to arm 20S
of the interferometer 200 Or Figure 8. ~he optical interferometer ~20 compri~ed optical coupler~ 436 and 43 ~or couplin6 optlcal sisr.als received rrom interreromete-410 between arms 422 and 424 of the interrerometer 420.
In addition, a phase modulator 440 was positioned in optical contact to the arm 424 of the interrerometer 420. The phase modulator 440 was used to generate relatively high frequency modulation at the rate Or approximately 30 kHz for the synthetic-heterodyne demodulation technique employed avoid signal-fading caused by phase drift.
The optical path dirference between the arms in interrerometer 410 corresponds to the optical path difrerence between the arms of the interrerometer 420. In the experimental case, this relative optical path length time delay betueen the arms in each interrerometer was approximately 230 ns.
15Matching of the path imbalances of the two interrerometers is important. To insure that phase-induced intensity noise does not arise to a measurable level on the signal pulse, it is necessary to match the _ optical path length dirrerence Or the sensing interferometer 410 and Or the compensating interferometer 420 to each other so that the amount Or mismatch is less than a small rractiOn Or the coherence length Or the light source 100. The accuracy with which riber lengths can be matched constitutes a practical limit to the coherence 2 length Or the optical source which may be used in these distributed sensor arrays. Measurement Or the path difrerences in the e~aluation system Or ~igure 9 uas accomplished by amplitude modulating a laser diode and determining the characteristic rrequency Or the rlltering Or each Y.ach-2ehnder individually.
In order to equalize the path difrerences, a technique for taking small lengths Or riber rrom one arm Or an interrerometer uas required. This ~as acco~plished by using capill~r~ tubes to hold the fibers ror splicing, and ~hen ~rinding do~n and resplicing the ca~illary tubes 1%~919 containing the ribers ~hen adjustment of the length ~as required. The capillary tu~es uere polished at an angle to minimize rerlertion back into the laser, ~hich would arfect the laser spectrum. The isolator 402 ~as placed between the laser and the Bragg cell to rurther decrease rerlections. All optical ribers in the system comprise ~Corning single-mode sensor (high N.A.) riber. The signal ~enerator for producing the modulation in phase modulator 440 is illustrated at 444. Interrerometer 420 also included manually adjustabie polarization controllers generally indicated at 44B, for o~ercoming polarization induced signal fading rOr the associated sensor.
The output rrom irterferometer 420 was communicated via a lens 450 to a Bragg cell 452 which ~as provided to optically sample signal pulses rrom the series Or output pulses received rrom interrerometer 420. This Bragg cell 4~2 was connected ~ia a delay line 454 to a pulse generator 456. Pulse generator 456 ~as also connected to - 8ragg cell 404. ~hus, the pulse generator 456 runctioned to cause operation Or both ~ragg cell 404 and 452. She BraBg cell 452 was pulsed synchronously with BraBg cell 404 in order to extract only the signal pulse.
The signal current receiYed through lens 4~0 in Bragg cell 452 was communlcated to a detector 458 corresponding to detector 212 Or ~igure 8. The signal current rrom detector 458 was transmitted through a narro~ band uidth (about 300 kHz) AC amplirler 460 corresponding to amplirier 302 Or Figure 8, and into an electronic 6ate 462 corresponding to gate 304 Or Figure 8. The gate 462 ~as connected ~ia delay line 464 to the signal generator 444, to synchronize the ~ate 462 to the phase modulation si~nal rrom generator 444. ~he output Or the gate 462 ~as proYlded to a spec~rum analyzer 466 corresponding to spectrum analyzer 306 o~ F1~ure 8.

* Trademark 129~9~9 With the amplitude of the phase modulation ror the synthetic heterodyne demodulation adjusted to be about 2.8 radians, the phase Or the second harmonic signal received in the spectrum analyzer 466 rrom gate 462 reflected the optical phase difrerence between the interrering optlcal waves. The magnitude of this signal is independent Or the optical phase difrerence, leading to a constant sensitivity.
The minimum detectable phase shift in the sensor was ascertained by measuring the signal-to-noise ratio displayed on the spectrum analyzer for a small known phase modulation amplitude from the sensor. To calibrate the phase modulation amplitude induced by phase modulator 434 on the optical signal, the voltage corresponding to 3.83 radians was measured at each signal frequency. Voltage at this level nulled the rirst Bessel functi~n side band.
~he sensor sensitivity was measured with the repetition rate Or the optical input pulses set at 1.46 MHz, corresponding to 3 times the optical path length difrerence in the interrerometers 410 and 420. In this condition, no pulse was generated which contained phase-induced intensity noise. With the repetition rate Or the optical input pulses set at 2.18 ~Hz, corresponding to 2 times the optical path length difrerence Or the interferometers 410 and 420, the non-signal bearing pulses, emitted at dirferent times from the source, overlapped and generated phase-induced intensity noise in the pulses which were discarded.
In both sets Or measurements, the sensitiYity Or the s~stem was below ~0 ~rad/~Hz o~er a broad range Or rrequencies. The results Or the rirst set Or measurements are plotted as O's in Figure 9. Like~ise, results Or the second set Or measurements are plotted using the s~mbol X
in Figure 9.

12g691~ ' The fact that there was no signiricant dirference in the sensitivity of the system in the tuo cases demonstrates that the signal pulse is well separated rrom the pulse which contains phase-induced intensity noise.
The sensitivity was round to be limited by the electronic noise in the signal processing electronics 460 and 462.
In another set Or measure~ents, an electronic switch was substituted for the second Bragg cell 452.
Sensitivity was again measured in this conriguration, with 1 the results indicated as ~'s in Figure 9. These results indicate that there was no significant difference in the sensitivity resulting from the two types Or gating.
The Polarization Controllers 446, 448 One type of polarization controller suitable for use in the sensor system Or the present invention, such as the embodiment of Figure 8, is illustrated in Figure 10. The controller includes a base 570 on which a plurality of upright blocks 572a through 572d are mounted. Between adjacent ones Or the blocks 572, spools 574a through 574c ~ 20 are tangentially mounted on sharts 576a through 576c, respectively. The sharts 576 are axially aligned ~ith each other and are rotatably mounted bet~een the blocks 572. The spDols 574 are generally cylindrical and are p~sitioned tangentially to the sharts 576.
A segment Or optical fiber 510 extends through axial bores in the sharts 576 and is wrapped about each Or the spools 574 to rorm three coils 578a through 578c. The radii Or the coils 578 are `such that the fiber 510 is stressed to rorm a birerringent medium in each Or the coils 578. The three coils 578a through 578c may be rotated independently Or each other about the axis Or the sharts 574a through 574c, respectively, to adjust the birerringence Or the fiber 510 and, thus to control the polarlzatlon of the light passin3 through the riber 510.

The diameter and number Or turns in the coils 578 are such that the outer coils 578a and 57~c provide a spatial delay of one-quarter wavelength, while the ~entral coil 578d provides a spatial delay Or one-halr wavelength. The quarter wavelength coils 578a and 578c control the elipticity Or the polarization, and the half wavelength coil 578d controls the direction Or polarization. This provides a rull range of adjustment of the polarization Or the light propagating through the fiber 510.
It will be understood, however, that the polarization controller may be modified to provide only the two quarter wave coils 578a and 57~c, since the direction of polarization (otherwise provided by the central coil 578b) may be controlled indirectly through proper adjustment of the elipticity of polarization by means Or the t~o quarter wave coils 578a and 57~c. Accordingly, the polarization controllers 551 and 557 are shown in Figure 10 as including only the two quarter wave coils 578a and 578c.
Since this configuration reduces the overall size of the ~ - 20 controllers 551 and 557, it may be advantageous ror certain applications Or the present invention involving space limitations.
Thus, the polarization controllers ~51 and 557 provide means for establishing, maintaining and controlling the polari2ation Or the light within arms Or the interferometers, such a~ arms 117 o~ interferometers 109 Or Figure 4, and arm 208 Or the compensating inteferometer 2D0.
Summary In summary, not only does the invention described herein comprise a slgniricant improvement over the prior art in monltoring environmental conditions at a plurality of locations by use Or an optical source ~hich optionally has a short coherence len6th, but it also oYe-comes o~her long-existent problems in the Industry by (1) providing a lZ9f~

system Or all-passive remote sensors permitting high duty cycle time-domain addressing while not requiring highly coherent light sources; (2) providing distributed sensor array systems ~hich are free from signal fading, source phase-induced intensity noise, crosstalk between sensors, and downlead senstitivity; (3) providing such systems which permit accurate sensing at remote locations without environmental shielding Or the leads; ~4) providing for heterodyning Or optical signals in a straightforward, economic, and optionally all-riber-optic manner uhich produces accurate and easily analyzed inrormation signals for identifying environmental influences afrecting the sensors; and ~5) providing the option Or all-riber-optic sensor array systems, which do not require the use Or bulk optics or Or electronic equipment at the sensor sites.
The invention may be embodied in other speciric forms uithout departing rrOm its spirit or essential characteristic~. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope Or the invention is, thererore, indicated by the appended claims rather than by the foregoing description. All changes ~hich come within the meaninB and range of equivalency Or the claims are to be embraced withln their scope.

Claims (28)

1. An apparatus for remotely sensing environmental effects comprising:
a source of pulsed optical signals;
a first optical waveguide optically coupled to the signal source;
an environmentally sensitive waveguide segment optically coupled to the first optical waveguide, said sensitive segment influencing optical signals propagating within said sensitive segment in response to particular environmental conditions;
a second optical waveguide optically coupled to the first optical waveguide in a configuration such that the second optical waveguide and the environmentally sensitive waveguide segment form at least potions of arms of a first unbalanced interferometer: and a second unbalanced interferometer optically coupled to the first unbalance interferometer for receiving optical signals from said first interferometer, wherein the second interferometer provides a pair of optical paths having an optical path length difference which substantially matches an optical path length difference defined by the first interferometer, to form a phase difference output signal representative of environmental influence on the environmentally sensitive waveguide segment.

2. An apparatus for remotely sensing environmental effects as defined in claim 1, wherein the environmentally sensitive waveguide segment comprises a portion of the first optical waveguide.

3. An apparatus for remotely sensing environmental effects as defined in claim 2, wherein the first interferometer comprises a third optical waveguide optically coupled to the first waveguide at a location such that the environmentally sensitive waveguide is located between the coupling locations locations of the second and third waveguides.

4. An apparatus for remotely sensing environmental effects as defined in claim 3, further comprising a fourth optical waveguide optically coupled to extend between ends of the second and third waveguides which are opposite from the ends coupled to the first waveguide, and wherein the first interferometer defines a first optical path through the second waveguide, and a second optical path through the environmentally sensitive waveguide segment, and the third and fourth waveguides.

5. An apparatus for remotely sensing environmental effects as defined in claim 4, wherein the optimal signals combined in the second interferometer include only one optical signal which propagated through the environmentally sensitive waveguide segment, thereby making the phase difference output signal directly representative of environmental conditions influencing said optical signal as it propagated through said environmentally sensitive waveguide segment.

6. An apparatus for remotely sensing environmental effects as defined in claim 1, wherein the source or pulsed optical signals comprises a short coherence length light source.

7. An apparatus for remotely sensing environmental effects comprising:
a source or pulsed optical signals;
means defining a first optical path of a first length for carrying a portion of a pulsed optical signal;
means defining a second optical path or a second length which is different than the first length for carrying another portion of said pulsed optical signal;
means defining a third optical path for carrying optical signals received from the first and second paths;
means defining a fourth optical path for carrying optical signals received from the third optical path;
means defining a fifth optical path of a length differing from a length or the fourth optical path in an amount substantially equal to the difference between said first and second lengths for carrying optical signals received from the third optical path;
wherein at least a portion of at least one or the means defining the first, second, fourth and fifth optical paths comprises a selected sensing region which is sensitive to an environmental effect and influences optical signals propagating in said sensing region in response to said environmental effect; and means for coherently combining optical signals from the fourth and fifth optical paths, wherein only one of said coherently combined optical signals has propagated through said selected sending region, thereby providing an output signal representative of the environmental effect which influenced the optical signal that propagated through the selected sensing region.

8. An apparatus for remotely sensing environmental effects as defined in claim 7, wherein the source of pulsed optical signals comprises a short coherence length light source.

9. An apparatus for remotely sensing environmental effects as defined in claim 7, further comprising means for controlling the source of pulsed optical signals so that optical signals received on the third optical path from a first optical pulse from said signal source do not interfere with optical signals received on the third optical path from a second optical pulse from said signal source.

10. An apparatus for remotely sensing environmental effects as defined in claim 9, further comprising means for synchronizing production of said pulsed optical signals so that a last signal reaching the fourth and fifth optical paths from a first pulsed optical signal is spaced from a first signal reaching the fourth and fifth optical paths from a second pulsed optical signal by an amount permitting said last and first signals to be combined in said combining means to obtain a phase difference output signal between those signals and to provide for a substantially continuous duty cycle in apparatus operation.

11. An apparatus for remotely sensing environmental effects as defined in claim 7, wherein the means defining first and second optical paths comprise arms of an interferometer which receives a single optical pulse signal from the signal source. and which provides a corresponding pair of optical pulse signals to the third optical path;
wherein the means defining fourth and fifth optical paths comprise arms of another interferometer, with a portion or one or said arms comprising the sensing region; and wherein the combining means comprises an optical coupler joining an output end of the arms of said another interferometer to provide a coherently coupled, pulsed output signal comprising the coherent combination of the pair of optical pulse signals and representing environmental effects which influenced the optical signal that propagated through the selected sensing region.

12. An apparatus for remotely sensing environmental conditions as defined in claim 7, further comprising:
a detector optically coupled to the second interferometer for forming an output which corresponds to the phase difference of the coherently coupled light; and a circuit for amplitude modulating the output to produce a first signal having selected harmonics which contain both sine and cosine components of the output, thereby providing for analysis or the output signal of identify environmental effects influencing the first and second light paths.

13. An apparatus for remotely sensing environmental conditions as defined in claim 12 further comprising:
a signal generator for providing a phase conditions signal at a selected modulation frequency;
a phase modulator, responsive to the signal generator for phase modulating the light waves in one of the first, second, fourth and fifth optical paths at the selected modulation frequency; and wherein the circuit functions to amplitude modulate the output at the selected modulation frequency.

14. An apparatus for remotely sensing environmental effects comprising:
a source of pulsed optical signals;
a first optical waveguide optically coupled to the signal source, with at least one portion of the first optical waveguide comprising a sensing region which is sensitive to an environmental effect and which influences optical signals propagating in said sensing region in response to said environmental effect;
a second optical waveguide optically coupled at one end to the first optical waveguide;
a third optical waveguide optically coupled at one end to the first optical waveguide at a location on the first optical waveguide separated from the coupling location of the second optical waveguide by the sensing region;
a fourth optical waveguide optically coupled to other ends of said second and third optical waveguides, such that the first, second, third and fourth waveguide define a first optical interferometer forming a first optical signal path carrying a portion of a pulsed optical signal from /

the first waveguide through said second waveguide to the fourth waveguide and a second optical signal path carrying another portion or said pulsed optical signal through the sensing region and the third waveguide to the fourth waveguide; and a second optical interferometer optically coupled to receive the pulsed optical signals from the fourth waveguide, wherein the second optical interferometer includes waveguides defining third and fourth optical signal paths for carrying portions of each of said pulsed optical signals, and wherein the third and fourth optical signal path length difference is substantially equal to the first and second optical signal path length difference so that pulsed optical signals in the second interferometer which have traveled substantially identical path lengths are coherently coupled to provide an optical output signal representative of the environmental effects which influenced the optical signal that propagated through the sensing region.

15. An apparatus for remotely sensing environmental effects as defined in claim 14, wherein the optical output signal comprises the phase difference of the coherently coupled signals, said phase difference representative of environmental effects influencing that optical signal which propagated through the sensing region.

16. An apparatus for remotely sensing environmental effects as defined in claim 14, further comprising:
a plurality of sensing regions located in spaced relation in the first optical waveguide, each of said sensing regions being sensitive to an environmental effect so as to influence optical signals propagating in said sensing region in response to said environmental effect;
a plurality of optical waveguide segments, with each said optical waveguide segment coupled at one end to the first optical waveguide at locations separated by at least one sensing region from other optical waveguide segments respectively comprise optical waveguide coupled at its other end to the fourth optical waveguide, whereby each adjacent pair of optical waveguide segments respectively comprise at least a portion of a pair of arms of an unbalanced optical interferometer defining an optical path length difference which substantially matches the path length difference of the first and second optical signal paths so that optical signals from each of the unbalanced optical interferometers are combined, at different times for each interferometer, in the second optical interferometer to form phase difference signals representative of environmental influence on the sensing region of the corresponding unbalanced interferometer.

17. An apparatus for remotely sensing environmental conditions as defined in claim 14, further comprising:
a detector optically coupled to the second interferometer, said detector forming an output which corresponds to the phase difference of the coherently coupled light; and a circuit for amplitude modulating the output to produce a first signal having selected harmonics which contain both sine and cosine components of the output, thereby providing for analysis of the output signal to identify environmental effects influencing the first and second light paths.

18. An apparatus for remotely sensing environmental conditions as defined in claim 17 further comprising:
a signal generator for providing a phase modulation signal generator for providing a phase a phase modulator, responsive to the signal generator for phase modulating the light waves in the second interferometer at the selected modulation frequency; and wherein the circuit functions to amplitude modulate the output at the selected modulation frequency.

19. An apparatus for remotely sensing environmental effects as defined in claim 14, wherein the source of pulsed optical signals comprises a short coherence length light source.

20. A distributed sensor system comprising;
a source of pulsed optical signals;
a plurality of fiber-optic sensing interferometers, each said sensing interferometer defining a pair of optical paths, and at least a portion of each said interferometer having light transmission characteristics which vary in response to environmental conditions, with each said sensing interferometer having input and output terminals which are optically coupled together to form a ladder network, said input terminals being optically coupled to the light source; and at least one fiber-optic compensating interferometer optically coupled to the output terminals of the sensing interferometers and defining a pair of optical paths whose optical path length difference substantially matches an optical path length difference of the pair of optical paths in a selected sensing interferometer, such that portions of a pulsed optical signal transmitted from the optical signal source through the sensor system will interferometer providing an optical signal representative of conditions causing change in light transmission characteristics of said selected sensing interferometer.

21. A distributed sensor system as defined in claim 20, wherein the light source comprises an optical source having a short coherence length.

22. A distributed sensor system as defined in claim 20, wherein the path length difference between the pair of optical paths in each sensing interferometer substantially equals the optical path length difference between the pair of optical paths in the compensating interferometer.

23. An apparatus for remotely sensing environmental effects comprising:
a source of pulsed optical signals;
a first optical waveguide optically coupled to the signal source, with at least one portion of the optical waveguide comprising a sensing region which is sensitive to an environmental effect and which influences optical signals propagating in said sensing region in response to said environmental effect;
second and third optical waveguides optically coupled at one end on either side of the sensing region to the first optical waveguide;
a fourth optical waveguide optically coupled to other ends of said second and third optical waveguides, such that the first, second, third and fourth waveguides define a first optical interferometer forming a first otical signal path carrying a portion of a pulsed optical signal from the first waveguide through said second waveguide to the fourth waveguide and a second optical signal path carrying another portion of said pulsed optical signal through the sensing region and the third waveguide to the fourth waveguide;
means optically coupled to the other ends of each of said second and third optical waveguides for reflecting optical signals from said second and third waveguide back into said second and thirds waveguide, such that the first, second and third waveguides and reflecting means define a first optical interferometer forming a first optical signal path carrying a potion of a pulsed optical signal from the first optical waveguide into the second optical waveguide until reflected back into the first waveguide, and forming a second optical signal path carrying a portion or said pulsed optical signal through the sensing region and into the third waveguide until reflected back through the sensing region into the first waveguide; and a second optical interferometer optically coupled to receive the reflected pulsed optical signals from the first waveguide, wherein the second optical interferometer includes waveguides defining third and fourth optical signal paths for carrying portions of each or said reflected, pulsed optical signals, and wherein the third and fourth signal paths are substantially equal in length, respectively, to the first and second optical signal paths so that pulsed optical signals in the second interferometer which have traveled substantially equal path lengths are coherently coupled to provide an optical output signal representative of the environmental effects which influenced the optical signal that propagated through the sensing region,

24. A distributed sensor system as defined in claim 23 wherein the first interferometer is configured to define a Michelson interferometer and the second interferometer is configured to define a Mach-Zehnder interferometer.

25. An apparatus for remotely sensing environmental effects as defined in claim 23 wherein the source or pulsed optical signals comprise a short coherence length light source.

26. A distributed sensor system comprising:
a source of pulsed optical signals;
a first optical waveguide optically coupled to the pulsed optical signal source;
at least one Michelson interferometer defining first and second optical paths and optically coupled to the first optical waveguide to receive at least a portion of a pulsed optical signal and to return a corresponding pair of pulsed optical signals to the first optical waveguide; and another optical interferometer optically coupled to receive said pair of pulsed optical signals from the first waveguide, wherein said another interferometer includes waveguides defining third and fourth optical signal paths for carrying portions of said pair of optical signals, and wherein the third and fourth signal paths are substantially equal in length, respectively, to the first and second optical signal paths so that pulsed optical signals in said another interferometer which have traveled substantially equal path lengths are coherently coupled to provide an optical output signal representative of the environmental effects which influenced the optical signal that propagated through the sensing region.

27. A distributed sensor system as defined in claim 26 wherein said another interferometer comprises a Mach-Zehnder interferometer.

28. An apparatus for remotely sensing environmental effects as defined in claim 26 wherein the source of pulsed optical signals comprise a short coherence length light source.