CA1227061A - Sensor using fiber optic interferometer - Google Patents
Sensor using fiber optic interferometerInfo
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- CA1227061A CA1227061A CA000506760A CA506760A CA1227061A CA 1227061 A CA1227061 A CA 1227061A CA 000506760 A CA000506760 A CA 000506760A CA 506760 A CA506760 A CA 506760A CA 1227061 A CA1227061 A CA 1227061A
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Abstract
SENSOR USING FIBER OPTIC INTERFEROMETER
Abstract of the Disclosure A closed loop optical fiber interferometer is used in sensing a quantity, Q, by applying a time varying or modulated measure of, Q, asymmetrically to the closed loop (24) and detecting phase shift betwen two counterpropagating optical signals in the closed loop.
The closed loop (24 ) can be used as the sensing element or a separate sensor (68, 70) can develop a time varying signal which is then applied to the closed loop interferometer.
Abstract of the Disclosure A closed loop optical fiber interferometer is used in sensing a quantity, Q, by applying a time varying or modulated measure of, Q, asymmetrically to the closed loop (24) and detecting phase shift betwen two counterpropagating optical signals in the closed loop.
The closed loop (24 ) can be used as the sensing element or a separate sensor (68, 70) can develop a time varying signal which is then applied to the closed loop interferometer.
Description
7~
SENSOR USING FIBER INTERFEROMETER
Back~round of the Invention Thi.s invention rela~es g~nerally ~o closed lo~p or ~Sagnac" fiber optic interferometers~ ~nd, mor~
particularly; the invention relates to ~ensors for environmental quantities which c~n affect fiber optic waveguides in fiber optic interferomet~rs.
InterferQmeters work on the principal of phas~ change betw~en tw~ coherent ~ignals which are ~nterfered ~ogether. Closed loop int~rferomet~rs measure the ~elative phase chan~e on a ~global~ ~cale, i~e., th~y ~easur2 the overall relative phase change ~etween two counterpropagating light ~ignals in ~ ~ingle loop of fiber.
The optic~l fiber loops of ~nterferomet~r~ are 8ensitive to a large number of ~nvirvnmental ~f~ects su~h as t~mperature, ~coustic pr~ssure, vibratl~n, mot~on, and electric and magnetic fields. These ~xt~rnal phenomena can change the optical transmi~siol1 characteri~tics of the ~iber ~uch a~ ~y changing the birefringence ~n the fiber or the ~eometric path length or the velocity o~
propagat~on o~ ht ~iynals through the ~iber, ~hich can result ~n changes in the amplitude, phase, or polariz~ti~n of lighS propagatin~ thereShrou~h- This ~ens~tivlty to env~ronmental effect~ means that fiber ~nterferometex 30 loops may function ~s ~en50r elements ~ n~ as th~
desired quant~ty to be sensed ean be isolated from th~
otller environmental quant~t~ss to ffhlch the fiber 3seln5it~Ye a A distinction ~hould be ~ade ~n the r~ader's mind ~etwe~n ~nvironmental ef~ects which ~re ~r2ciprocal~
~.e.~ effects ~hich ~o no~ caus~ ~ phase ~hift bstween ~.
I~ t ~he counterpropagating waves) and environmental effects which are "non-reciprocal" (i.e., ~hose effects which do cause a phase shift between counterpropagating waves).
~dditionally, reciprocal and non-reciprocal effects may be thou~ht of either as being on a ~global ~cale~, wherein the effect of the environmental phenomena i5 considered with respec~ to the loop as a whole, or as be$ng on a ~differential scale~, wherein ~he phenomena is considered only w;th respect to a localiæed, tiny segment of ~he f~ber. Certain environmental effects such as rotation and the Faraday effect cause non-reciprvcal effects on the differential ~cale in that they will cause a relative phase change betwsen counterpropagating light s}gnals travelling through the same tiny piece of fiber. These non-reciprocal differential effects are rumulative 50 t~at the resultinS~ ~ffect on ~ global ~cale is slso non-reciprocal. Other environmental effects, such ~
pressure, are reciprocal on the differential scale in that th~y cause no net relative phase change ~etween ~ounterpropagating light s;~nals travelling through the same tiny piece of fiber. On a global ~cale, the cumulative effect of these reciprocal diff~rential ~fects is ordinarily also reciprocal. Sucl~ reciprocal e~fect~ on ~hc differential scale, therefore, ~annot be ~ea~urad by 8 25 c}osed loop interfero~eter unless ~ome way is devised to-mak~ these eff~cts n~n-reciprocal on the ~lobal s~ale even though th~y are reciprocal on the diff~r~ntial scale~
In the prior art, the closed loop inter~ero~e~er is typically used to ~ense non-re~iprocal di~ferent~al effects, such as rotation. When o~erated with ~ polarizer which limits the optical paths of ~he couterpropagating light to ~ single on8 of the pola~ization ~odes, as disclosed in copending Canadian Application 5erial No. 399,776 flled March 30~ 1982~ tha closed loop interferometer i5 '7(~
basically stable and, on a global scale/ is relatively i~sensitive ~o environmental effects o~her than non-reciprvcal, differential effects, such as motion. In such interferometers, any reciprscal~ differential environmental phenomenal such as temperature or prsssure, affects he fiber optical light transmission ch3racteristics of both cvunterpropaga~ing liyht signals ~ubstantially equally ~nd, ther~fore, results in little or no relative phase change between the counterpropagatin~
signalS- The pres2nt inventi~n teaches a elo~ed loop interferometer configuration which senses differential reciprocal effects, ~uch as accoustic pressure, by making their effect on the ~lobal scale non reciprocal~
Summa~y of the Dîsclosure The present invention is directed to use of th~ closed l~op optical inte~erometer to ~ense environmental phenomena which normally produee re~iprocal e~Eects on an infinitesimal length of fiber by causing them to produce non-reciprocal effects. Briefly, this is accomplished by ~aking the ~ensed phenomenon spatially asymetrical and non-uniform with r~spsct t~ the closed loop. The fundamental principal uhich ~s utilized in the three bas~c emb~diments disclosed herein is to cause the quantity Q ~o~
be ~ensed to be applied to the cl~ed loop of the interferometer ~uch ~hat Q is time varying and is ~pplle~
3t ~ point that offset from the center of the path around the loop ~i~e., a rapidly time Yarying Q is applisd at a point on th~ path such that one oF the counterpropagatin~
l~ht 6ignals has farther to travel to ~he d~tector than the other. As u5ed herein, the term ~counterpropagatin~
waves~ l~r the term ~counterpropagatln~ light si~nal~"
means collectively both of the light si9nals travelling in opposite directions in the loop of the interferom~ter~
The term ~cs:~unterpropagating wave" ~or light signal) means 71~
only the light wave travelling in the loop in the direction opposite the direction of travel for tha acoustic wave.
~ special case of the fundamental principal of making the quantity to be sensed spatially asymmetrical and non-uniform with respect to its effect on the two counterpropagating light signals is disclosed in the first two embodiments disclosed because it is easier to ~rasp the basic principle by these two examples. These two 1~ embodiments are sensors for acoust.ic waves which utilize coils of fiber optic waveguide that are specially wound so that the effect of the acoustic wave is non-uniform on the ~wo counterpropagating light waves. That is, the loop is wound and arranged with respect to the wave ~o be sensed 1~ such that the subject wave has a uniform efect on ~he co-propagating light si~nal, i.e~, the light signal traveling in the same dirçction as the subject wave, but has a non-uniform effect on the counterpropagating light sisnal~
This is done in the first embodiment by winding the coil such that the axial velocity of the co-propagating ligh~
wave along the longitudinal axis of the coil matches the Yelocity of propagation of the acoustic wave along the longitudinal a~i~. In the second ~mbodim~nt~ the _, undemental principle is utilized by winding two coils separated in space by 1/4 of the wavelen~th of th~-acoustic wave with ths si2e of the coil diameter and the number of turns selected sueh that the transit time for the light si~nals through eaeh coil is lJ4 of the period of the acoustic wave, The length of each coil is al~o selected such that the coil occupies a very small space compared to the wavelength of the acoustic wave such that the coil is affected substantially ~unio~mly by the portion of the acoustic wave by which it is enY~loped.
In the preferred embodiment, the quantity Q to b~
35 sensed i5 applied to the lc~op of the interferometer ~uch that it i time varying and spatially asymmetrical,~
Because the quantity Q is applied ~symmetrically to the loop, variations in Q will affect each of the counterpropagating wave differently thereby producing ~
phase difference therebetweenu However, if Q is a slowly varying ~uantity such as temperature, ~he value of Q may not change appreciably between ~he arrival times of the two counterpropagating waves at the point on the loop where Q is applied, so that each wave is af~ected substantially the same by Q. In such a case, the quantity Q is modulated onto a bias or carrier frequency which is rapidly ~ime varying by a ~onvential modulator and applied to the loop at a point which is offset from the geometric of the path between th2 light source and the detector. In the preferred embodi~ent, the modulated Q ,i.e., q(t), is applied to the loop with a phase modulator which stretchss the fiber in proportion to the amplitude of q~t).
Importantly, the invention utilizes the inherent ~ta~ility and quie~er operations of the closed loop interferometer to sense reciprocal quantities by causing
SENSOR USING FIBER INTERFEROMETER
Back~round of the Invention Thi.s invention rela~es g~nerally ~o closed lo~p or ~Sagnac" fiber optic interferometers~ ~nd, mor~
particularly; the invention relates to ~ensors for environmental quantities which c~n affect fiber optic waveguides in fiber optic interferomet~rs.
InterferQmeters work on the principal of phas~ change betw~en tw~ coherent ~ignals which are ~nterfered ~ogether. Closed loop int~rferomet~rs measure the ~elative phase chan~e on a ~global~ ~cale, i~e., th~y ~easur2 the overall relative phase change ~etween two counterpropagating light ~ignals in ~ ~ingle loop of fiber.
The optic~l fiber loops of ~nterferomet~r~ are 8ensitive to a large number of ~nvirvnmental ~f~ects su~h as t~mperature, ~coustic pr~ssure, vibratl~n, mot~on, and electric and magnetic fields. These ~xt~rnal phenomena can change the optical transmi~siol1 characteri~tics of the ~iber ~uch a~ ~y changing the birefringence ~n the fiber or the ~eometric path length or the velocity o~
propagat~on o~ ht ~iynals through the ~iber, ~hich can result ~n changes in the amplitude, phase, or polariz~ti~n of lighS propagatin~ thereShrou~h- This ~ens~tivlty to env~ronmental effect~ means that fiber ~nterferometex 30 loops may function ~s ~en50r elements ~ n~ as th~
desired quant~ty to be sensed ean be isolated from th~
otller environmental quant~t~ss to ffhlch the fiber 3seln5it~Ye a A distinction ~hould be ~ade ~n the r~ader's mind ~etwe~n ~nvironmental ef~ects which ~re ~r2ciprocal~
~.e.~ effects ~hich ~o no~ caus~ ~ phase ~hift bstween ~.
I~ t ~he counterpropagating waves) and environmental effects which are "non-reciprocal" (i.e., ~hose effects which do cause a phase shift between counterpropagating waves).
~dditionally, reciprocal and non-reciprocal effects may be thou~ht of either as being on a ~global ~cale~, wherein the effect of the environmental phenomena i5 considered with respec~ to the loop as a whole, or as be$ng on a ~differential scale~, wherein ~he phenomena is considered only w;th respect to a localiæed, tiny segment of ~he f~ber. Certain environmental effects such as rotation and the Faraday effect cause non-reciprvcal effects on the differential ~cale in that they will cause a relative phase change betwsen counterpropagating light s}gnals travelling through the same tiny piece of fiber. These non-reciprocal differential effects are rumulative 50 t~at the resultinS~ ~ffect on ~ global ~cale is slso non-reciprocal. Other environmental effects, such ~
pressure, are reciprocal on the differential scale in that th~y cause no net relative phase change ~etween ~ounterpropagating light s;~nals travelling through the same tiny piece of fiber. On a global ~cale, the cumulative effect of these reciprocal diff~rential ~fects is ordinarily also reciprocal. Sucl~ reciprocal e~fect~ on ~hc differential scale, therefore, ~annot be ~ea~urad by 8 25 c}osed loop interfero~eter unless ~ome way is devised to-mak~ these eff~cts n~n-reciprocal on the ~lobal s~ale even though th~y are reciprocal on the diff~r~ntial scale~
In the prior art, the closed loop inter~ero~e~er is typically used to ~ense non-re~iprocal di~ferent~al effects, such as rotation. When o~erated with ~ polarizer which limits the optical paths of ~he couterpropagating light to ~ single on8 of the pola~ization ~odes, as disclosed in copending Canadian Application 5erial No. 399,776 flled March 30~ 1982~ tha closed loop interferometer i5 '7(~
basically stable and, on a global scale/ is relatively i~sensitive ~o environmental effects o~her than non-reciprvcal, differential effects, such as motion. In such interferometers, any reciprscal~ differential environmental phenomenal such as temperature or prsssure, affects he fiber optical light transmission ch3racteristics of both cvunterpropaga~ing liyht signals ~ubstantially equally ~nd, ther~fore, results in little or no relative phase change between the counterpropagatin~
signalS- The pres2nt inventi~n teaches a elo~ed loop interferometer configuration which senses differential reciprocal effects, ~uch as accoustic pressure, by making their effect on the ~lobal scale non reciprocal~
Summa~y of the Dîsclosure The present invention is directed to use of th~ closed l~op optical inte~erometer to ~ense environmental phenomena which normally produee re~iprocal e~Eects on an infinitesimal length of fiber by causing them to produce non-reciprocal effects. Briefly, this is accomplished by ~aking the ~ensed phenomenon spatially asymetrical and non-uniform with r~spsct t~ the closed loop. The fundamental principal uhich ~s utilized in the three bas~c emb~diments disclosed herein is to cause the quantity Q ~o~
be ~ensed to be applied to the cl~ed loop of the interferometer ~uch ~hat Q is time varying and is ~pplle~
3t ~ point that offset from the center of the path around the loop ~i~e., a rapidly time Yarying Q is applisd at a point on th~ path such that one oF the counterpropagatin~
l~ht 6ignals has farther to travel to ~he d~tector than the other. As u5ed herein, the term ~counterpropagatin~
waves~ l~r the term ~counterpropagatln~ light si~nal~"
means collectively both of the light si9nals travelling in opposite directions in the loop of the interferom~ter~
The term ~cs:~unterpropagating wave" ~or light signal) means 71~
only the light wave travelling in the loop in the direction opposite the direction of travel for tha acoustic wave.
~ special case of the fundamental principal of making the quantity to be sensed spatially asymmetrical and non-uniform with respect to its effect on the two counterpropagating light signals is disclosed in the first two embodiments disclosed because it is easier to ~rasp the basic principle by these two examples. These two 1~ embodiments are sensors for acoust.ic waves which utilize coils of fiber optic waveguide that are specially wound so that the effect of the acoustic wave is non-uniform on the ~wo counterpropagating light waves. That is, the loop is wound and arranged with respect to the wave ~o be sensed 1~ such that the subject wave has a uniform efect on ~he co-propagating light si~nal, i.e~, the light signal traveling in the same dirçction as the subject wave, but has a non-uniform effect on the counterpropagating light sisnal~
This is done in the first embodiment by winding the coil such that the axial velocity of the co-propagating ligh~
wave along the longitudinal axis of the coil matches the Yelocity of propagation of the acoustic wave along the longitudinal a~i~. In the second ~mbodim~nt~ the _, undemental principle is utilized by winding two coils separated in space by 1/4 of the wavelen~th of th~-acoustic wave with ths si2e of the coil diameter and the number of turns selected sueh that the transit time for the light si~nals through eaeh coil is lJ4 of the period of the acoustic wave, The length of each coil is al~o selected such that the coil occupies a very small space compared to the wavelength of the acoustic wave such that the coil is affected substantially ~unio~mly by the portion of the acoustic wave by which it is enY~loped.
In the preferred embodiment, the quantity Q to b~
35 sensed i5 applied to the lc~op of the interferometer ~uch that it i time varying and spatially asymmetrical,~
Because the quantity Q is applied ~symmetrically to the loop, variations in Q will affect each of the counterpropagating wave differently thereby producing ~
phase difference therebetweenu However, if Q is a slowly varying ~uantity such as temperature, ~he value of Q may not change appreciably between ~he arrival times of the two counterpropagating waves at the point on the loop where Q is applied, so that each wave is af~ected substantially the same by Q. In such a case, the quantity Q is modulated onto a bias or carrier frequency which is rapidly ~ime varying by a ~onvential modulator and applied to the loop at a point which is offset from the geometric of the path between th2 light source and the detector. In the preferred embodi~ent, the modulated Q ,i.e., q(t), is applied to the loop with a phase modulator which stretchss the fiber in proportion to the amplitude of q~t).
Importantly, the invention utilizes the inherent ~ta~ility and quie~er operations of the closed loop interferometer to sense reciprocal quantities by causing
2~ them to produce nonreciprocal efects. The result is a quieter, more sensitive sensor which is better ~ble to distinguish the effects of the desired quantity to be sen~ed from the effects of undesired quant;ties.
~ The delay line ~unction of the closed loop increases 25 the sensitivity to the quantity Q and allows a lower modulation frequency for the quantity Q when the invention is being used to sense s~owly varying quantities such as - temperature.
~ccordingly, an object of the invention is a sensor which utilizes a clo~ed loop iber optic interferometer.
$he invention and objects and features thereof wlll be more readily apparent from the ~follo~ing detailed description and appe~ded claims when studied in light of t~e drawings f ~`3.~
Brief Descr ~ the D _ lngs Fi~ure l i5 a schema~ic of a closed loop fiber up~ic~l interferometer ~uch as is used in a rotation ~nsor.
Figure 2 is a ~ch~matic of an a~oustic sensor ~mploying a closed loop fiber optic interf~rometer in accordance with one embodiment of the invention, Pigure 3 is ~ schematic of an acoustic ~ensor employing a clo~ed loop iber optic in~erferometer in accordance with another embodiment of the invention.
- 10 ~igure 4 is a plot of acoustic waves in the sensor of Figure 3.
Fi~ure ~ is a schematic of a ~ensor which utilizes a closed loop fiber optic interferometer in accordance with the preferred embodiment of the invention.
1~ Detailed Descri tion of the Preerred Embodiment P ~_ _ _ eferring now to the drawings, Figure l is a simplified schematic of a closed loop fiber optic interferometer ~uch as i~ us2d in the rotation ~ensor disclosed in copending Canadian Application Serial No.
399,776, filed on March 30, 1982. A light source lO
generates a beam of light which is transmitted through an optical fiber 1~ and polarizer 13 to a fiber optic directional coupler 14. A directi~nal coupler could also bP used to perform the beam splitting func~ion~ The ~5 light beam is split into two waves which propayate through the closed loop 16 in opposite directions as indicated by the arrows. After propagating through the loop 16 the two beams are recombined and pass through the optical fiber 12 to the polarizer 13 and then to a second fiber optic directional coupler 18 which directs the combined signals to a L
detector 20. The detector responds to the intensity of ~he combined waves which ~epends on the differences in phase between the ~wo counterpropagating waves and provides an output signal indicative of the phase shift~
Since the rotation of ~he fiber coil has a nonreciprocal effect on the counterpropagating waves, the detec~or gives a measure of the ro~ation. While not shown in Figure 1, the closed loop interometer may also employ directional couplersr polarization controllers, phase modulators, and 10 ; lock-in amplifiers as described in the co-pending application and as described by ~ergh, Lefevre and Shaw in "All-single-mode fiber-optic Gyroscope with Long Term Stability", Optics Letters, Vol 6, No~ 10, Oct. 1981 and in "All-Single _ mode ~iber~Optic Gyroscope", Optics 1~ , Vol. 6, NoO 4, April 1~81.
The interferometer can take the form of an all fiber integrated device in which all components are ~onstructed in a single monomode fiber. The p~ase modulators , if used, can provide bias which increases sensitivity and linearity of response.
Examples of normally differentially reciprocal effeets which do not strongly affect th~ interferometer of Figure - 1 are temperature, force, ~tress, pressure, displacement, strain, vibration and acoustic waves.
~5 Figure 2 is a schematic of an acoustic wave sensor employing a closed loop interferometer in accordance with one embodiment of the in~entivn. The acoustic wave sensor is similar in configuration to the rotation sensor of Figure 1. However, in thi~ embQdiment, the coil 24 is wound in an ext~nded helical pattern having a longitudional aXi5 ~X~ lying along the direction of tr~v~l of an acoustic wa~e 2~ to be sensed.
The sensor compri~es a closed 1GOP f iber optic waveguide 24 which is coiled into a number Gf turns. The ~, 35 diameter, D, o the turns, the number of th~ turns, N, and the spacing of the turns alon~ the longitudinal axis x are selected to meet certain criteria. That criteria is that ~he acoustic wave 26 ~o bs sensed must travel down the longitudional axis x o the coil at the same velocity ~s that of a co-propagating optical signal traveling in the coil 24 from the ~ end to the B end would have. That is, as the light wave travels around the turns in the coil, it works its way toward the B end at a certain axial rate as if the light wave were climbing up the threads of a very finely threaded screw. This axial velocity or the light wave is much slo~er than the actual velocity of propagation in the fiber because there are many ~urns and eacih is approximately parallel to the planes 28 of the acoustic wave. Acoustic waves have planes of acoustic compression and rarifaction which are orthogonal to the 1~ direction of propagation of the acoustic wave. The planes 28~ for a sound wave, represent peaks of higher pressure medium in the acoustic wave ,an~ the spaces 30 represent areas of lower pressure medium.
In the embodiment of Figure 2, the coil is confi~ured to maximize the overall global, non~reciprocal effect of the acoustic wave on the coil. The coil is conf igured with respect to the acoustic wave such that the effect of the acoustic wave on the coil i5 timç ~aryin~ and spatially non-unifon~ so as t~ cause a relative pha~e shift ~etween the counterpropagating waves. The diameters of ~he coil and the numbsr of turns are selected such that an optical ~ave travels the length of the coi~ ~4 in a tim~ Tl which is e~al to the time, Tar for an acoustic wave to travel the axial l~n~th ~f helical coil Z4, Accordingly, ~ wave propagating through the coil from left . to ri~ht experiences a constant pressure from the acoustic wave, whereas an optical wave trave~ling from ri3ht to left experiencss a continually Yarying effect~
That is, the nu~ber and diameter of spacing of the . 35 turns in the coil ~4 are selected such that as a light wave enters the A end o the fiber, its speed of travel z~
down the x axis ~oward the ~ end matches the speed of propagation of ~he acou~tic wave 26 along the x axis toward the B end. Thus, if a Ipulse or burst of li~ht - enters the A end of the coil 24 when the A end i5 enveloped by the higher pressure region ~f a wave peak 28 of the acoustic wave; the pulse or burst of light will continually travel in $iber which is subjected to the same pressure from the ~ame part o~ the ac~ustic wave 28, This pressure changes the ge~metric path length that the co-propa9atin9 wave travels from A to ~ because each segment of fiber that the light wave traYels ;n is e~ually stretched or compressed by the pressure effects from the traveling acoustic wave. The situation is not the same for the counterpropagating wave however. The counterpropagating light wave traveling from the B end to the A end first encounters segments of fiber that are compressed by the peaks 28 of the acoustic wave and then encounters fiber segments that are stretched by the troughs 30. The geometric path that ~he counterpropagating light wave travels is thus alternately made longer then shorter by the acoustic wave. If the length L, of the coil 24 is an integer multipl~ of the wavelength of the acoustic waYe~ the net path length change for the counterpropagating wave will be zero~ The _ 25 result is a net phase shift betw~en the tw~
counterpropagating li~ht waves which is linearly pr~portional to the a~plitude of the acoustic wave 26 and to the len~th, L, of the coil 24. That isl for lon~er coil lengths, L, the acoustic wave has a greater interaction distance with the fi~er in which the co prop~gating light wave is traveling~ The total increa~e in ~ath len~th, and the resulting ph~e ~h~ft, ther~for~
increases with greater interaction lengthr L.
~ he gr~atest diref~tivity or discrimination of the coil 24 results whe~ the coil len~th, L~ is chos~n to be ~ome integer multiple of the acoustic wavelength because the ~7(~
net phase shift is then ~ero for the counterpropagating wave, i.e., the wave whi^h is travelling in the fiber in an axial direction opposite the direc~ion of propagation of ~he acoustic wave, resulting in the best performance of the sensor. However, L, can be any non-neglible fraction of the wavelength of the acollstic wave which is ~ufficient to create a discernible relative phase shift betwen ~he counterpropagating waves which is indicative of the amplitude of the acoustic wave. The frequency for an acoustic wave of unknown frequency propagating with a known velocity coul~ be determined by usin~ a variable length delay line and tuning the line for zero phase shift in the counterpropagating wave.
The count~rpropaga~ing light waves i~ the loop 24 are produced by a light source 32 coupling light waves into a fiber 34 which guides the waves through a polarizer 35 to a fiber optic directional coupler 38 coupled to the tWG
ends of the coil 24. The light source 32 can have a coherence length which is very short. That is, as long as the coherenee length of the source exceeds $he differential path length through the coil caused ~y the acoustic wave, the two counterpropagating waves will still be coherent after travsling the length of the coil, and they can be successfully mer~ed to interfere with each other. The interference is n~cessary to be able to determine the amount of the relatiYe phase shi f t.
The fiher 34 is, in the preferred embodiment, a monomode fiber.
The polari~er 36 is bidirectional~ and can be ~diusted ~o pass only li~ht of a predetermined polarization. The polarizer 36, is important in maintainin~ the reciprocal operational characteric of the closed~loop in~erferometer as i5 known in the art- The polarizer insure~ that the light going into the loop is in ~nly one poiari~atlon.
Any birefringenee in the fiber of the loop will co~ple part of th~ ~nergy to the orthogonal polarization mode ()6~
where the propagation velocity is diferent from the velocity in the selected polarization. If ~he birefringence is not symmetric about the center of ~he geometric path, the counterpropagating waves will travel differing distances at the new velocities and w;ll ~uffer different phase shifts. The polarizer only allows light of one polarization to ~nter the fiber and blocks all returning light energy in polarization modes other than ~he i~put polar;zation mode. Thus, the detector will only see light energy from the components of the counterpropagating waves which travelled through the loop in the selected polarization mode, and there will be no phase shift since ~th counterpropagating signals will have travelled the s~e geometric path length in the same polarizat~on mode, i.e~ at the ~ame velocity.
The directional c~upler 38 splits the po~arized ligh~
wave ~ravelling toward the loop from the polarizer 36 into two coherent light ~es and couples each wave into one end of the coil 24~ SD that they counterpropagate. A beam splitter could be used in place of the directional coupler 38~ but the direc~ional coupler is the preferred ~m~odiment ~ince the fiber is continuous from the source 32 through the coupler and bac~ to the detector. In contrast; a beam splitter joins separate ~ibers, the ends o~ which generate reflected waves .. These reflected waves bounce back into h~ input fiber and int~rfere with the incoming light waves and can cau5e undesirable standing waves~ The use of ~eam splitters is not a problem in terms o~ re~lections where pulsed operation is used. In pulsed operation, tbe source 32 provides pulses separated ~y dark periods. By adjusting the length of f~ber in the syst~m and the pulse separation an~d duration, it possible to causa the reflected signals to arrlve at the "~ ~etector during dar~ periods betwesn the initial pulse an~
the returning combioed pulse from the counterpropagating pulses.
6~
The fiber optic directional coupler 38 also recombinas the counterpropagating light signals after they have passed through the loop. The combined signal then is coupled into the input fiber 34 and passes through the polarizer 36 on i~s way ~o a photodet~ctor 40. Between the polarizer 36 and the source 32, a second fiber optic directional coupler 42 is coupled to the input fiber 34.
This second cvupler 42 diverts part of the energy from the combined signal returning from the loop 24 into the input o the photodetector 40. The pho~odetector then generates a signal proportional to the intensity of the combined signal which can be interpreted to determine the relative phase shift~ This is known in the art.
In the preferred embodiment, a polarization controller 44 is placed in th~ loop 24 to control the polarization of light signals traveling in the loop. This is desirable to prevent "signal fading" caused by the interaction of the polari2er 36 with bireringence zones in the loop 24. ~or - ~xample, if the polarizer only allQws vertically polarized ~ight in and out of the loop, and birefringence in the loop couples all the energy from the counterpropagating light signals into the horizontal polarization mode, then the polarizer 36 would block all output light and ~he detector would register a zero outp~t. The structure of polarization controllers is known in the art. These devices are capable of taking input light at one polarization and shifting it to output light at any other selected polarization.
By properly adjusting the polarization controller ~4 it is possible to insure that at least some of the lig~t~
and preferably most of the light, ~xits the loop in the polarization passed by the polarizer~ 36. That i5~ the polarization controller can be used to maximize the output s~gnal ~rom the de~ector. This prevents snvir~nmentally caused bireEringence and blrefringence in the fiber structure caused by re5idual stress from manu~aeturin~
processes from causing signal fadeout. The time varying and spatially non-uniform effect of the acoustic wave on counterpropagating optical waves in coil 24 varies the intensity of the recombined optical wave which can then be detected as a measure of the ampli~ude of the acoustic wave~
One possible use for the embodiment described in Figures 2 and 3 i5 as a direction finder for acoustic waves. This can be done by turning the loop longitudinal axis until the output signal is a maximum.
Since the length of the coil is directly proportional to the amount of total phase shift in the co-propagating light, the sensitivity of the sensor can be increased by increasing L, especially in increments of the wavelength of ~he acoustic wave 26.
_ Figure 3 is a schematic diagram ~f an acoustic sensor employing a closed loop interferometer in accordance with another embodiment of the invention. In this embodiment the coil 46 is separated into two separate, spaced groups of coils designated Cl and C , one consisting of the first contiguous turns and the other consisting of the last ~2 contîguous turns of the optical fiber. Th~ two ~roupings of coils ar~ displacecl along the x ax~s with ~respect to each other by a distance, d, such that the 25 planes of the two 9roups of coils remain mutuall~
parallel. The displacement, d, between the two yroups of coils is chosen, in the preferred embodiment, to be an odd multiple of one quarter waYeleng~h of the acoustic wave shown generally at 47. The total len~th of the fiber used to form the two groups of coils is such ~hat the optical transit time through both ~oil5 is one-hal the period of th~ acoustic wavP fre~uency. In other embodiments, the distance d could be some 4ther fraction of the wavelength of the acoustic wave if the sizes of the coils Cl and C2 are adjusted so that the optical transit times are the same fraction of the period of the acoustic wave that the distance d is to the wavelength of the ~coustic wave.
However, in these embodiments, ~he sensor i5 not as directionally selective. That is, in these alterna~ive embodiments, the sensor will not discriminate as ~harply between acoustic waves whose direction oE propagation is parallel to the longitudin~l axis x of the coils ~nd acoustic waves whose direction o propagation i5 not parallel to the longitudinal axis of the coil.
The result of the foregoing structure i~ to cause a 1~ relative phase shift between counterpropagatng light waves when an acoustic wave vf th~ specified wavelength is propagating down the x axis~ This phase shift is caused by the acoustic wave pressures acting on the counterpropagating light w~ves in a non-reciprocal fashion 1~ where such a wave would normally cause a reciprocal effect ~ an interferometer structure like the device of Figure 1.
This non-reciprocal effect is illustrated in the plots of Figure 4 sh~wing t~e acoustic waves in the first set of coils Cl and in the second set of coils C2 at times tl and t2D In the preferred embodiment the distance d between the two groups of coils is selected to be 1/4 of the acoustic wave wavelength. The coils Cl and C2 are sized such that the optical transit time therethrough is equal to 1/4 of the period o~ the acoustic wave. The coils ar~
also sized such that their lengths Ll and L2 are small compared to the wavelength of the acoustic wave~ Again, ~s in the embodiment of Figure 1, the coils have ~
longitudinal axis x which is arranged to be parallel to 30 the axis of propagation of the acoustic wave for maximum ~ensitiYity.
The curves 48 and 50 represent~ the acoust~c wave separated by l/4th of the wavelength at the two instants in t~me tl and t2 which instants in time are 5eparated by 1/4 of the period of the acoustic wave. That is, the curve 48 represents the situation at th~ time tl, i.e~ it 7~
shows how the coils Cl and C2 are enveloped by the peaks and troughs of acoustic pressure at the time tl where the peaks and troughs of pre~sure are represented by the areas 52 and 54 respectively. Likewise, the curve 50 shows how the coils Cl and C2 are enveloped by the acous~ic wave at the point in time t2 1/4 period later. At the time tl, the coil Cl is experiencing a substantially uniform acoustic pressure throughout the coil at the pressure level illustrated at 56. This results from the fact that the planes of the individual loops of the coil Cl are substantially parallel to the planes 60 of ~h~ acoustic wave 47 and from the fact that the length Ll of the coil Cl is small compared to the acoustic wavelength. The same - observations are true for the coil C2 envelope~ in the acoustic wave at the point 58 with the additional comment ~hat the pressure at 58 is equal to the pressure at 5~
This is because the coils Cl and C2 are separated by 1/4 of the acoustic wavelength and the instant of time tl happens to be a time when the wave peak is exactly centered between the coils, At some other later or earlier time, this would not be the case. The principal at work in the embodiment of Figure 3 holds true for any time t however.
The positive compression effects acting uniformly throughout the coils ~1 and C2, chan~e the geometric path length uniformly in both the ~oils Cl an~ C2. The co-propagating light signal in the coil Cl and ~he counterpropagating li~ht signal in the coil C2 at the time tl thus experie~ce e~ual changas in their geomstric path ; 30 lengths and, therefore, the transit tim~ for each li~ht ~i~nal throu~h the whole loop is chan~ed. Bec~u~ the c~ils Cl and C2 are sized such ~hat i~t takes each of the counterpropagating light signals 1~4 period of the acoustic wave to propagate through each coil, the co-- 35 propagatin~ ht signal in Cl stays in the ooil Cl until the time t2, at which time it exits on the fiber ~egment :' 62. An extremely short time later this co-propaga~in0 light signal enters the coil C2 which is immersed in the acoustic wave as shown at 64 on curve 50 in Fiyure 4~B).
Likewise, the counterpropagating wave in the coil C2 ~t the time tl stays in C2 until the time t2 and then travels through the segment 62 and enters the coil Cl. ~hus, at the time t2, the coils Cl and C2 are immersed in acoustic pressure at the corresponding amplitudes as shown by p~ints 64 and 66, respectively.
1~ ~he co-propagating light signal which was in coil group Cl at time tl and experienced the pressure at 56 in Figure 4(A) is now in coil group C2 at ~ime t2 and experiences the same positive compression effects from the acoustic wave 47 as shown at S4 in Figure 4(B~. These compressive effects change the geometric path length through the coil C2 in the same amount and in the same sign as occurred in the coil Cl at the time tl~ Thus the transit time through the co;l C2 is changed and is of the same sign as the change in transit time experienced in the 2D coil Cl at the time tl. The ~verall change in transit time for the co-propagating signal is additive and non zero therefore.
However, the counterpropagating light signal which was in coil group C2 at tl experiences the pres~ure at 58 in Figure 4(A) and then travels to the left and is ln co~l Cl-at time t2 where it experiences an equal and opposite pressurs illustrated at 66 in Figure 4~B). Thust the counterpropagating light signal experiences a path length change in the coil Cl which is equ 1 and opposit~ to the pa~h length change it experienced in ths coil C2l Thus~
the chànge in transit time caus~d by the coil C2 is cancelled by the chan~e in transit tim~ caused by the coil Cl, and the counterpropagating light si~nal will have a net change in transit time through the coil 46 of zero.
In ~ummary, the efect on the wave propagatin~ from coil Cl to C2 is re~nforcing while the effect on the 7~36~
~ignal propagating ~rom coil C2 to coil Cl is offsetting. Accordingl~, the counterpropagating waves contain a relative phase difference when recombined after propagating through the two groups o~ coils. This phase difference is then detected and can be interpreted linearly ~o determine the amplitude of the acoustic wave.
If the optical transit time through each coil and the separation of the coils is other than 1/4 period and 1~4 wavelength respectively, the sensor will still work to sense the presence of wave~ but the counterpropagating wave will not have a net zero transit time change The response of the sensor in this case will be non-linear.
Both the sensors of Figure 2 and 3 have a certain bandwidth in which they are useful~ The eenter frequency Of çach bandwidth is the frequency having the wavelength which causes a net zero phase shift in the counterpropagating light signal. At other wavelengths, the response falls off s~mewhatbecause of various ef~ects~
In the embodiments of Figures 2 and 3~ the optical coil interferometer also functions as the sensor for the measured ~coustical wave. ~urther, both embodiments require that the coils be designed for a particular frequency o~ acoustic wave for optimal operation unless a variable coil length can be achieved such as by an ~5 infinitely variable delay line as is known in t~e art~
The details of the structure of delay lines that could be used to ~ive infinitely variable delays are given in an article entitled ~Fibre-Optic Variable Delay Lines~
published in Electronics Letters of November 11, 1982, 30 Vol. 18~ No. 23, at pages 99g-1000. Such variable delay lines could be used for the coil 24 ln Figure ~ or the coil-- Cl and C~ in Figure 3 to change ~he cent~r frequ~ncy of the sensor bandwidth~
Figure S is a schematic diagram o another embodim~nt of a sensor which utilizes the optîcal coil solely ~s an interferometer and delay line- In this embodiment, the (3~i~
-1~
quantity to be sensed, designated as Q, is applied to a modulator 68 which generates a time varying signal, q(t) consisting of either a signal q(t) which varies directly with the sensed quantity Q or a carrier signal having ~
high frequency which is modulated by the ~uantity Q. Any suitable structure for the modulator 50 will suffic~ for purposes of the invention, and the signal q(t) could be the quantity Q i~self or an electrical signal proportional thereto.
The signal q~t) is applied to a transducer 70 which can be a polari~ation controller or any other device which can affect the optical transmission properties of the fiber at the location of the transducer 70. In the preferred embodiment, the signal q ~t) is a carrier or bias frequency amplitude modulated with the quantity Q.
ID the preferred embodiment, the transducer 70 i5 a phase modulator. The transducer must be located at a spot that is offset from the geometric center on the geometric path of the loop. The si~nal q(ti must be time varyin~ at ~
rate which is more rapid than the rate of change that is naturally filtered out by the structure and operation of closed loop fiber optic interferometers. The siynal q~t~
exerts a spatially non-uniform effect on the coil 72 as will be explained more fully below.
The structural details of the transducer 70 and the modulator S8 are not critical to the invention~ Tha transducer 70 can be any device which can cause a change in the birefringence of the fiber at the location of the ~ransducer 70 or elsewhere such as a device to put bending or other stress on th~ fiber in response to q ~t).
Further, the transducer 70 can be any device which can change the velocity of li~ht prop~gating through the location of the transducer 70. Such a Yelocity modulating device could be any structure to convert q~t) to a physicial change in the fiber transverse dimension at the location of the transducer. This changes the velocity of ;
, 3L,~r~
propagation of light signals throu~h the region of changed transverse dimension by ~he waveguide effect~ In another embodiment~ the transducer 70 could be a polari2ation controller for changing the polarization of the counterpropa~ating light signals at the location of the transducer 70 in response to g(t).
In the preferred embodiment, the transducer 70 i~ a phase modulator which converts the signal q(t) to c~anges in the path length of the fiber optic wave~uide ~t the location of the transducer 70 in the loop. That is, the fiber is ~tretched in the transducer 70 in accordance with the amplitude of the ~ignal q(t~. A ~uitable structure for such a phase modulator is a drum of piezoelectric material with the fiber of the coil 72 wound around the 15 drum. A driving electrical signal such as the signal q(t~
th n is applied to the piezoelectric material and causes it ~o expand and contract radially in accordance wi~h the magnitude of the driving signal, This radial expansion causes the fiber wrapped around the drum to be stre~ched.
20In an alternative embodiment, the signal g~t) could be used to control the adjustment ~f a p~larization controller as the transducer 70. The structur~l details of a polari2ation controller that might be adapted to perform such a function in all thr~e emb~diments are found in an article entitled nSingle Mode Fibre Fr~cti~nal Wav~
-. Devices and Polarization Control:Lers" by H.C. Lefevre published in Electronics Letters, VQ1; 16, NO. 20 and in _ V.S. Patent Number 4,~89,090 issued 3une 21, 1~83.
30The function and structural details of the source 32t the coupler 42J the ~etector 40, the pol~rizer 36, th~
fiber optic directional directional co~plers 38 and 4~ ~nd the polarization controller 44 ~re the same in th~
embodiment of Fi9ure 5 as in the embodiments of Figures 2 35 and 3. Beam splitt~rs could be used in the three embodiments but the reflections cau~ed by these beam ~20-~plitters would render pulsed o~eration more desirable as discussed above with reference to the embodiment of Figur~
2. ~iber optic directional couplers are the preferred embodiment in all three embodiments of the invention. The details of the structure and operation of the polarizer 36 are given in an article entitled rSingle Mode ~iber Optic PolarizerW published in ~ics Letters, Vol. 5, No. ll ~t pp. 479-81 in N~vember~ 19B0. The ~e~ails of ghe structure and operation of the directional cvuplers 42 and 38 are given in an article entitled "Analysis of a Tunable Single Mode Optical Fiber Coupler" by Digonnet and Shaw published in the IE~E Journal of Quantum Electronics, Vol.
QE-18, No. 4, pp~ 746-754 dated April, 1982. The details arP also disclosed in Canadian Application Serial No.
411,079 filed on September 9, 1982, and Canadian Application Serial No. 375,214 filed April 10, 1981.
The operation of the embodiment of Figure 5 is as follows. The light source 32 supplies a light signal which is coupled into the single mod~ optical fiber 34 and ~uided through the directional coupler 42 and the polarlzer 36 to the directional coupler 38. The polarizer 36 causes only light having a selected polarization to be passed through the polarizer regardless of whether the light is travelling toward the lo~p 72 or away from thë
loop 72. The directional coupler 38 splits the light signal into tw~ coherent li9ht siynals counterpr~pa~ating around the loop and combines the returning counterpropagating li~ht ~ignals into one light si~nal travelling toward the source 32 in the fiber 34. The polarization controller 44 allows selection o the pol~rizatlon for light travellin~ in`the loop in either direction. The coupler 42 diverts p2rt of the lig~t trav211ing toward the source into the input of the photodetector 40 where it i5 converted to an ~lectrical ~ignal based upon its amplitude ~uared, i.~, its 6~.
intensity. OE course the amplitude of the combined signals which have counterpropagated in the loop 72 depends upon ~he rela~.ive phase shift between these signals caused by the optical conditions in the loop. The counterpropagating light signals in the loop ~2 have their relative phases shifted by the action of the transducer 70. The quantity Q can be either a rapidly changin~
quantity or a slowly changing quantity. If the quantity Q
is rapidly changing, it can be applied directly to the transducer 70 which can convert it to a time varying shift in the path length of the fiber at the transducer 70 or a change in the birefringence of the fiber at the transducer 70.
If the transducer 70 changes the path length in the transducer 70 at a rapid, time varying rate, the c4unterpropagating light signals travel different path lengths through the lo~p and end up shifted in phase relative to each other~ This is because they arrive at the transducer 70 at different times due to its off center location. Because the path length through the transducer 70 will be different at the two different times when the counterpropagating light signals arriYe at the transducer, _ the two counterpropagating signals will have two different transit times through the 1CQP 72. They will th~r~fore arrive at the directional coupler 38 at different times and thus be shifted in phase relative to each other.
The combined signal will have an amplitude dependent upon the rela~ive phase shift. The amplitude o~ the combined light signal can be interpreted to find the 3Q quantity Q because the relative phase shift is linearly related to Q.
If Q is a slowly varying differ~entially reciprocal ~uantity, such as temperature or pressure, Q will have to be m~dulated upon a rapidly time varying carrier si~nal ; 35 applied asymmetrically because, as ~s well known in the art, closed loop fiber optic interferometers are '' ordinarily not sensitive to slowly time varying reclprocal e~fects. In factt this is the great advantage of closed loop interferometers because this quality makes them very guiet and stable.
When Q is modulated onto 8 bias or c~rrier frequency sueh as by ~mplitude or frequency modulation, and the transducer 70 is a phase modulator, the above discussion still holds true. If amplitude modulation is used, the path length in the transducer will be changing sinusoidally at the bias or carrier frequency but the amplitude of the path length swinys will vary with Q.
Therefore the relative phase ~hift caused by the transducer 70 will vary sinusoidally. Further, the amplitude of the sinusoidal relative phase hift ,i.e., the amplitude envelope of the sinusoidal ~wings ln relativs phase shift will be the q~antity Q. The envelope will vary over ~ime as Q varies over time.
If the transducer changes the birefringence of the fiber a~c the location of the transducer 70, the relative 2~ phase shift is caused by coupling of the light from one polarization mode to the ~ther. Birefringence is the property of optical fibers which causes light propagating in them in different polari7ation modes ~o propagate at different velocities. If the transducer 70 is changing the birefringence of the fibcr ~t the location of ~he transducer 70 in proportion to the signal q(t) or Q, the ransducer wilI be changing the relative phase shift between the cvunterpropagating signals. This is because the transducer will be controlling the amount of snergy trav~lling in the slow mode versus the amount of ~nargy travelling in the ~as~ mode~ That is, when light travelling in a waveguide in one~ polarization mode encounters birefringence, some of the light energy will ~e coupled into the ortho90nal polarizaton mode and will travel thereafter in the diff~rent velocity f~r the new polarization mode. If the amount of birefringence i5 a ~7(~
function of q(t), then the quantity Q can be determined by the amount of energy in the coLInterpropagating light signals propagating in the orthogonal polarization mode from the polarization ~stablished by the polarizer 36.
This can b~ determined from the amplitude of ~he sombined signal from the counterpropagating signals in the orthogonal polarization mode. In the embodiment of Figure 5 to determine the amplitude of the combine signal from the orthogonal mode, another directional coupler would have to be added between the polarizer and ths coupler to direct ~ome of the returning energy to a polarizer tuned to the orthogonal mode and another photodetector would have be coupled to the output of the second polarizer.
The method of sensing acoustic or other waves comprises guiding counterpropagating light signals through a coil of a closed loop fiber optic interferometer which is wound and arranged in a predetermined way. That predetermined way is to arr~nge the coil so that the subject aeoustic or other wave propagates down the longitudinal axis of the coil ~t the same rate that a co~
propagating light waYe progresses down the longitudinal axis of the coil. The phase difference is then detected between the counterpropagatin0 light signals in th~ coil as the subject wave propagates throu~h it.
More specifically the method ccmprises generatin~ a light siynal and polarizing the light signal in a ~slected polarization. The polarized light is Shen split into two li~ht signals which are then counterpropa~ated through the helically c~iled loop of a closed loop fiber optio interferometer which loop is arrang~d with its longitudinal axis parallel to the direction of propa~ation of the subject wave. The loop is co~led and si~ed such that the co-propagatin9 light 5i~nal which travel~ through ths fiber is substantially uniformly affect~d in terms of phase shit by the subject wave while the counterpropagating wave which travels throu~h the fiber is 06~
-2~--not uniformly affected in terms of phase shif~ by the subject wave. Th~ phase ~iference is then detected.
A method for sensing a phenomenon or quantity O
consists of guiding counterpropagating li~ht signals throuyh a coil of closed loop fiber optic interferometer. The quantity ~ is sensed by a modulator which converts the quantity into a sign~l q(t) which varies with ~. The signal q(t) may be Q itself or an electrical signal varying linearly with Q in some applications. The signal q(t) is applied to the coil ~f the fiber optic interferometer at a location offset from the center of the geometric path of the coil such that ~he optical transmission characteristics of the fiber are altered at that location. The rel~tive phase shift between the counterpropagating light signals is then detected.
The si~nal ~(t~ can be applied so as to alter the birefringence in proportion to q(t) at the loeation of application o~ q~t) or it can be applied so as to alter ~U the velocity of propagation in the fiber in accord with q(t) at the location of application. Further q(t) can be applied such that the polari7,ation of light passing through the location in the coil where q~t~ is applied is altered proportionally to q (t~. The preferred m~thod is 2~ to alter the geometric path length of one count~rpropagating signal relative to the other in accord with q~t).
All these methQds should include the ~tep o~
polari~in~ the liyht of the counterpropa~ating light signals in the same prior to entry into the coil and ~iltering out all returning counterpropagating light not of the same polarization as the pol~rization of the counterprop~gating light signals coupled into the loop.
This coupling should be done prior to detecting the relative phase shift between the returning counterpropagatin~ light signals.
7(3 All these methoàs can also include the step of controlling and adjusting the polarization o~ ~he counterpropagating light signal~ in the loop to any selected polarization.
All ~he methods of ~ensing the quantity Q can also include the ~tep of modulating the quantity Q onto a bias or carrier freguency to generate a ~i~nal q~t~ bef~re applying q(t) to the loop.
The quantity could be applied directly to the ~ransducer and the rate of change sensed. ~or example, the quantity Q may ~e temperature of an environment and the signal q(t) would be a time varying measure o~ th~
temperature. . The ~ignal ~(t1 is then applied asymmetrically to one or more small segments in the 1~ opticsl coil to a~fect the interferometer and thus senerate a detectable variation in the recombined ~ptical signals. ln addition to the devices described abo~e, the transducer 70 may also be electro-optical, piezoelectric, or ma~netostrictive.
There has been described several embodiments of a sensor which either includes or utilizes a closed loop optical interferometer for detecting ~ sensed quantity, Q, In ~ach of the embodiments, the sensed quantity is spati~lly non-uniform with respect to the clo5~d loop.
The entire coil ean be empl~yed a5 the sensor elemen~ or the ~uant~ty can be applied at one or more points asymmetrically located on the coil.
Although the invention has been describ~d in terms of preferred embodiment, it will be apparent ~o those skilled in the art that numerous modificatiuns can ~e made without departing from the spirit and scope vf the claims appended hereto. Such modifications ~are in~ended to be includsd within the ~cope of the claims.
~ The delay line ~unction of the closed loop increases 25 the sensitivity to the quantity Q and allows a lower modulation frequency for the quantity Q when the invention is being used to sense s~owly varying quantities such as - temperature.
~ccordingly, an object of the invention is a sensor which utilizes a clo~ed loop iber optic interferometer.
$he invention and objects and features thereof wlll be more readily apparent from the ~follo~ing detailed description and appe~ded claims when studied in light of t~e drawings f ~`3.~
Brief Descr ~ the D _ lngs Fi~ure l i5 a schema~ic of a closed loop fiber up~ic~l interferometer ~uch as is used in a rotation ~nsor.
Figure 2 is a ~ch~matic of an a~oustic sensor ~mploying a closed loop fiber optic interf~rometer in accordance with one embodiment of the invention, Pigure 3 is ~ schematic of an acoustic ~ensor employing a clo~ed loop iber optic in~erferometer in accordance with another embodiment of the invention.
- 10 ~igure 4 is a plot of acoustic waves in the sensor of Figure 3.
Fi~ure ~ is a schematic of a ~ensor which utilizes a closed loop fiber optic interferometer in accordance with the preferred embodiment of the invention.
1~ Detailed Descri tion of the Preerred Embodiment P ~_ _ _ eferring now to the drawings, Figure l is a simplified schematic of a closed loop fiber optic interferometer ~uch as i~ us2d in the rotation ~ensor disclosed in copending Canadian Application Serial No.
399,776, filed on March 30, 1982. A light source lO
generates a beam of light which is transmitted through an optical fiber 1~ and polarizer 13 to a fiber optic directional coupler 14. A directi~nal coupler could also bP used to perform the beam splitting func~ion~ The ~5 light beam is split into two waves which propayate through the closed loop 16 in opposite directions as indicated by the arrows. After propagating through the loop 16 the two beams are recombined and pass through the optical fiber 12 to the polarizer 13 and then to a second fiber optic directional coupler 18 which directs the combined signals to a L
detector 20. The detector responds to the intensity of ~he combined waves which ~epends on the differences in phase between the ~wo counterpropagating waves and provides an output signal indicative of the phase shift~
Since the rotation of ~he fiber coil has a nonreciprocal effect on the counterpropagating waves, the detec~or gives a measure of the ro~ation. While not shown in Figure 1, the closed loop interometer may also employ directional couplersr polarization controllers, phase modulators, and 10 ; lock-in amplifiers as described in the co-pending application and as described by ~ergh, Lefevre and Shaw in "All-single-mode fiber-optic Gyroscope with Long Term Stability", Optics Letters, Vol 6, No~ 10, Oct. 1981 and in "All-Single _ mode ~iber~Optic Gyroscope", Optics 1~ , Vol. 6, NoO 4, April 1~81.
The interferometer can take the form of an all fiber integrated device in which all components are ~onstructed in a single monomode fiber. The p~ase modulators , if used, can provide bias which increases sensitivity and linearity of response.
Examples of normally differentially reciprocal effeets which do not strongly affect th~ interferometer of Figure - 1 are temperature, force, ~tress, pressure, displacement, strain, vibration and acoustic waves.
~5 Figure 2 is a schematic of an acoustic wave sensor employing a closed loop interferometer in accordance with one embodiment of the in~entivn. The acoustic wave sensor is similar in configuration to the rotation sensor of Figure 1. However, in thi~ embQdiment, the coil 24 is wound in an ext~nded helical pattern having a longitudional aXi5 ~X~ lying along the direction of tr~v~l of an acoustic wa~e 2~ to be sensed.
The sensor compri~es a closed 1GOP f iber optic waveguide 24 which is coiled into a number Gf turns. The ~, 35 diameter, D, o the turns, the number of th~ turns, N, and the spacing of the turns alon~ the longitudinal axis x are selected to meet certain criteria. That criteria is that ~he acoustic wave 26 ~o bs sensed must travel down the longitudional axis x o the coil at the same velocity ~s that of a co-propagating optical signal traveling in the coil 24 from the ~ end to the B end would have. That is, as the light wave travels around the turns in the coil, it works its way toward the B end at a certain axial rate as if the light wave were climbing up the threads of a very finely threaded screw. This axial velocity or the light wave is much slo~er than the actual velocity of propagation in the fiber because there are many ~urns and eacih is approximately parallel to the planes 28 of the acoustic wave. Acoustic waves have planes of acoustic compression and rarifaction which are orthogonal to the 1~ direction of propagation of the acoustic wave. The planes 28~ for a sound wave, represent peaks of higher pressure medium in the acoustic wave ,an~ the spaces 30 represent areas of lower pressure medium.
In the embodiment of Figure 2, the coil is confi~ured to maximize the overall global, non~reciprocal effect of the acoustic wave on the coil. The coil is conf igured with respect to the acoustic wave such that the effect of the acoustic wave on the coil i5 timç ~aryin~ and spatially non-unifon~ so as t~ cause a relative pha~e shift ~etween the counterpropagating waves. The diameters of ~he coil and the numbsr of turns are selected such that an optical ~ave travels the length of the coi~ ~4 in a tim~ Tl which is e~al to the time, Tar for an acoustic wave to travel the axial l~n~th ~f helical coil Z4, Accordingly, ~ wave propagating through the coil from left . to ri~ht experiences a constant pressure from the acoustic wave, whereas an optical wave trave~ling from ri3ht to left experiencss a continually Yarying effect~
That is, the nu~ber and diameter of spacing of the . 35 turns in the coil ~4 are selected such that as a light wave enters the A end o the fiber, its speed of travel z~
down the x axis ~oward the ~ end matches the speed of propagation of ~he acou~tic wave 26 along the x axis toward the B end. Thus, if a Ipulse or burst of li~ht - enters the A end of the coil 24 when the A end i5 enveloped by the higher pressure region ~f a wave peak 28 of the acoustic wave; the pulse or burst of light will continually travel in $iber which is subjected to the same pressure from the ~ame part o~ the ac~ustic wave 28, This pressure changes the ge~metric path length that the co-propa9atin9 wave travels from A to ~ because each segment of fiber that the light wave traYels ;n is e~ually stretched or compressed by the pressure effects from the traveling acoustic wave. The situation is not the same for the counterpropagating wave however. The counterpropagating light wave traveling from the B end to the A end first encounters segments of fiber that are compressed by the peaks 28 of the acoustic wave and then encounters fiber segments that are stretched by the troughs 30. The geometric path that ~he counterpropagating light wave travels is thus alternately made longer then shorter by the acoustic wave. If the length L, of the coil 24 is an integer multipl~ of the wavelength of the acoustic waYe~ the net path length change for the counterpropagating wave will be zero~ The _ 25 result is a net phase shift betw~en the tw~
counterpropagating li~ht waves which is linearly pr~portional to the a~plitude of the acoustic wave 26 and to the len~th, L, of the coil 24. That isl for lon~er coil lengths, L, the acoustic wave has a greater interaction distance with the fi~er in which the co prop~gating light wave is traveling~ The total increa~e in ~ath len~th, and the resulting ph~e ~h~ft, ther~for~
increases with greater interaction lengthr L.
~ he gr~atest diref~tivity or discrimination of the coil 24 results whe~ the coil len~th, L~ is chos~n to be ~ome integer multiple of the acoustic wavelength because the ~7(~
net phase shift is then ~ero for the counterpropagating wave, i.e., the wave whi^h is travelling in the fiber in an axial direction opposite the direc~ion of propagation of ~he acoustic wave, resulting in the best performance of the sensor. However, L, can be any non-neglible fraction of the wavelength of the acollstic wave which is ~ufficient to create a discernible relative phase shift betwen ~he counterpropagating waves which is indicative of the amplitude of the acoustic wave. The frequency for an acoustic wave of unknown frequency propagating with a known velocity coul~ be determined by usin~ a variable length delay line and tuning the line for zero phase shift in the counterpropagating wave.
The count~rpropaga~ing light waves i~ the loop 24 are produced by a light source 32 coupling light waves into a fiber 34 which guides the waves through a polarizer 35 to a fiber optic directional coupler 38 coupled to the tWG
ends of the coil 24. The light source 32 can have a coherence length which is very short. That is, as long as the coherenee length of the source exceeds $he differential path length through the coil caused ~y the acoustic wave, the two counterpropagating waves will still be coherent after travsling the length of the coil, and they can be successfully mer~ed to interfere with each other. The interference is n~cessary to be able to determine the amount of the relatiYe phase shi f t.
The fiher 34 is, in the preferred embodiment, a monomode fiber.
The polari~er 36 is bidirectional~ and can be ~diusted ~o pass only li~ht of a predetermined polarization. The polarizer 36, is important in maintainin~ the reciprocal operational characteric of the closed~loop in~erferometer as i5 known in the art- The polarizer insure~ that the light going into the loop is in ~nly one poiari~atlon.
Any birefringenee in the fiber of the loop will co~ple part of th~ ~nergy to the orthogonal polarization mode ()6~
where the propagation velocity is diferent from the velocity in the selected polarization. If ~he birefringence is not symmetric about the center of ~he geometric path, the counterpropagating waves will travel differing distances at the new velocities and w;ll ~uffer different phase shifts. The polarizer only allows light of one polarization to ~nter the fiber and blocks all returning light energy in polarization modes other than ~he i~put polar;zation mode. Thus, the detector will only see light energy from the components of the counterpropagating waves which travelled through the loop in the selected polarization mode, and there will be no phase shift since ~th counterpropagating signals will have travelled the s~e geometric path length in the same polarizat~on mode, i.e~ at the ~ame velocity.
The directional c~upler 38 splits the po~arized ligh~
wave ~ravelling toward the loop from the polarizer 36 into two coherent light ~es and couples each wave into one end of the coil 24~ SD that they counterpropagate. A beam splitter could be used in place of the directional coupler 38~ but the direc~ional coupler is the preferred ~m~odiment ~ince the fiber is continuous from the source 32 through the coupler and bac~ to the detector. In contrast; a beam splitter joins separate ~ibers, the ends o~ which generate reflected waves .. These reflected waves bounce back into h~ input fiber and int~rfere with the incoming light waves and can cau5e undesirable standing waves~ The use of ~eam splitters is not a problem in terms o~ re~lections where pulsed operation is used. In pulsed operation, tbe source 32 provides pulses separated ~y dark periods. By adjusting the length of f~ber in the syst~m and the pulse separation an~d duration, it possible to causa the reflected signals to arrlve at the "~ ~etector during dar~ periods betwesn the initial pulse an~
the returning combioed pulse from the counterpropagating pulses.
6~
The fiber optic directional coupler 38 also recombinas the counterpropagating light signals after they have passed through the loop. The combined signal then is coupled into the input fiber 34 and passes through the polarizer 36 on i~s way ~o a photodet~ctor 40. Between the polarizer 36 and the source 32, a second fiber optic directional coupler 42 is coupled to the input fiber 34.
This second cvupler 42 diverts part of the energy from the combined signal returning from the loop 24 into the input o the photodetector 40. The pho~odetector then generates a signal proportional to the intensity of the combined signal which can be interpreted to determine the relative phase shift~ This is known in the art.
In the preferred embodiment, a polarization controller 44 is placed in th~ loop 24 to control the polarization of light signals traveling in the loop. This is desirable to prevent "signal fading" caused by the interaction of the polari2er 36 with bireringence zones in the loop 24. ~or - ~xample, if the polarizer only allQws vertically polarized ~ight in and out of the loop, and birefringence in the loop couples all the energy from the counterpropagating light signals into the horizontal polarization mode, then the polarizer 36 would block all output light and ~he detector would register a zero outp~t. The structure of polarization controllers is known in the art. These devices are capable of taking input light at one polarization and shifting it to output light at any other selected polarization.
By properly adjusting the polarization controller ~4 it is possible to insure that at least some of the lig~t~
and preferably most of the light, ~xits the loop in the polarization passed by the polarizer~ 36. That i5~ the polarization controller can be used to maximize the output s~gnal ~rom the de~ector. This prevents snvir~nmentally caused bireEringence and blrefringence in the fiber structure caused by re5idual stress from manu~aeturin~
processes from causing signal fadeout. The time varying and spatially non-uniform effect of the acoustic wave on counterpropagating optical waves in coil 24 varies the intensity of the recombined optical wave which can then be detected as a measure of the ampli~ude of the acoustic wave~
One possible use for the embodiment described in Figures 2 and 3 i5 as a direction finder for acoustic waves. This can be done by turning the loop longitudinal axis until the output signal is a maximum.
Since the length of the coil is directly proportional to the amount of total phase shift in the co-propagating light, the sensitivity of the sensor can be increased by increasing L, especially in increments of the wavelength of ~he acoustic wave 26.
_ Figure 3 is a schematic diagram ~f an acoustic sensor employing a closed loop interferometer in accordance with another embodiment of the invention. In this embodiment the coil 46 is separated into two separate, spaced groups of coils designated Cl and C , one consisting of the first contiguous turns and the other consisting of the last ~2 contîguous turns of the optical fiber. Th~ two ~roupings of coils ar~ displacecl along the x ax~s with ~respect to each other by a distance, d, such that the 25 planes of the two 9roups of coils remain mutuall~
parallel. The displacement, d, between the two yroups of coils is chosen, in the preferred embodiment, to be an odd multiple of one quarter waYeleng~h of the acoustic wave shown generally at 47. The total len~th of the fiber used to form the two groups of coils is such ~hat the optical transit time through both ~oil5 is one-hal the period of th~ acoustic wavP fre~uency. In other embodiments, the distance d could be some 4ther fraction of the wavelength of the acoustic wave if the sizes of the coils Cl and C2 are adjusted so that the optical transit times are the same fraction of the period of the acoustic wave that the distance d is to the wavelength of the ~coustic wave.
However, in these embodiments, ~he sensor i5 not as directionally selective. That is, in these alterna~ive embodiments, the sensor will not discriminate as ~harply between acoustic waves whose direction oE propagation is parallel to the longitudin~l axis x of the coils ~nd acoustic waves whose direction o propagation i5 not parallel to the longitudinal axis of the coil.
The result of the foregoing structure i~ to cause a 1~ relative phase shift between counterpropagatng light waves when an acoustic wave vf th~ specified wavelength is propagating down the x axis~ This phase shift is caused by the acoustic wave pressures acting on the counterpropagating light w~ves in a non-reciprocal fashion 1~ where such a wave would normally cause a reciprocal effect ~ an interferometer structure like the device of Figure 1.
This non-reciprocal effect is illustrated in the plots of Figure 4 sh~wing t~e acoustic waves in the first set of coils Cl and in the second set of coils C2 at times tl and t2D In the preferred embodiment the distance d between the two groups of coils is selected to be 1/4 of the acoustic wave wavelength. The coils Cl and C2 are sized such that the optical transit time therethrough is equal to 1/4 of the period o~ the acoustic wave. The coils ar~
also sized such that their lengths Ll and L2 are small compared to the wavelength of the acoustic wave~ Again, ~s in the embodiment of Figure 1, the coils have ~
longitudinal axis x which is arranged to be parallel to 30 the axis of propagation of the acoustic wave for maximum ~ensitiYity.
The curves 48 and 50 represent~ the acoust~c wave separated by l/4th of the wavelength at the two instants in t~me tl and t2 which instants in time are 5eparated by 1/4 of the period of the acoustic wave. That is, the curve 48 represents the situation at th~ time tl, i.e~ it 7~
shows how the coils Cl and C2 are enveloped by the peaks and troughs of acoustic pressure at the time tl where the peaks and troughs of pre~sure are represented by the areas 52 and 54 respectively. Likewise, the curve 50 shows how the coils Cl and C2 are enveloped by the acous~ic wave at the point in time t2 1/4 period later. At the time tl, the coil Cl is experiencing a substantially uniform acoustic pressure throughout the coil at the pressure level illustrated at 56. This results from the fact that the planes of the individual loops of the coil Cl are substantially parallel to the planes 60 of ~h~ acoustic wave 47 and from the fact that the length Ll of the coil Cl is small compared to the acoustic wavelength. The same - observations are true for the coil C2 envelope~ in the acoustic wave at the point 58 with the additional comment ~hat the pressure at 58 is equal to the pressure at 5~
This is because the coils Cl and C2 are separated by 1/4 of the acoustic wavelength and the instant of time tl happens to be a time when the wave peak is exactly centered between the coils, At some other later or earlier time, this would not be the case. The principal at work in the embodiment of Figure 3 holds true for any time t however.
The positive compression effects acting uniformly throughout the coils ~1 and C2, chan~e the geometric path length uniformly in both the ~oils Cl an~ C2. The co-propagating light signal in the coil Cl and ~he counterpropagating li~ht signal in the coil C2 at the time tl thus experie~ce e~ual changas in their geomstric path ; 30 lengths and, therefore, the transit tim~ for each li~ht ~i~nal throu~h the whole loop is chan~ed. Bec~u~ the c~ils Cl and C2 are sized such ~hat i~t takes each of the counterpropagating light signals 1~4 period of the acoustic wave to propagate through each coil, the co-- 35 propagatin~ ht signal in Cl stays in the ooil Cl until the time t2, at which time it exits on the fiber ~egment :' 62. An extremely short time later this co-propaga~in0 light signal enters the coil C2 which is immersed in the acoustic wave as shown at 64 on curve 50 in Fiyure 4~B).
Likewise, the counterpropagating wave in the coil C2 ~t the time tl stays in C2 until the time t2 and then travels through the segment 62 and enters the coil Cl. ~hus, at the time t2, the coils Cl and C2 are immersed in acoustic pressure at the corresponding amplitudes as shown by p~ints 64 and 66, respectively.
1~ ~he co-propagating light signal which was in coil group Cl at time tl and experienced the pressure at 56 in Figure 4(A) is now in coil group C2 at ~ime t2 and experiences the same positive compression effects from the acoustic wave 47 as shown at S4 in Figure 4(B~. These compressive effects change the geometric path length through the coil C2 in the same amount and in the same sign as occurred in the coil Cl at the time tl~ Thus the transit time through the co;l C2 is changed and is of the same sign as the change in transit time experienced in the 2D coil Cl at the time tl. The ~verall change in transit time for the co-propagating signal is additive and non zero therefore.
However, the counterpropagating light signal which was in coil group C2 at tl experiences the pres~ure at 58 in Figure 4(A) and then travels to the left and is ln co~l Cl-at time t2 where it experiences an equal and opposite pressurs illustrated at 66 in Figure 4~B). Thust the counterpropagating light signal experiences a path length change in the coil Cl which is equ 1 and opposit~ to the pa~h length change it experienced in ths coil C2l Thus~
the chànge in transit time caus~d by the coil C2 is cancelled by the chan~e in transit tim~ caused by the coil Cl, and the counterpropagating light si~nal will have a net change in transit time through the coil 46 of zero.
In ~ummary, the efect on the wave propagatin~ from coil Cl to C2 is re~nforcing while the effect on the 7~36~
~ignal propagating ~rom coil C2 to coil Cl is offsetting. Accordingl~, the counterpropagating waves contain a relative phase difference when recombined after propagating through the two groups o~ coils. This phase difference is then detected and can be interpreted linearly ~o determine the amplitude of the acoustic wave.
If the optical transit time through each coil and the separation of the coils is other than 1/4 period and 1~4 wavelength respectively, the sensor will still work to sense the presence of wave~ but the counterpropagating wave will not have a net zero transit time change The response of the sensor in this case will be non-linear.
Both the sensors of Figure 2 and 3 have a certain bandwidth in which they are useful~ The eenter frequency Of çach bandwidth is the frequency having the wavelength which causes a net zero phase shift in the counterpropagating light signal. At other wavelengths, the response falls off s~mewhatbecause of various ef~ects~
In the embodiments of Figures 2 and 3~ the optical coil interferometer also functions as the sensor for the measured ~coustical wave. ~urther, both embodiments require that the coils be designed for a particular frequency o~ acoustic wave for optimal operation unless a variable coil length can be achieved such as by an ~5 infinitely variable delay line as is known in t~e art~
The details of the structure of delay lines that could be used to ~ive infinitely variable delays are given in an article entitled ~Fibre-Optic Variable Delay Lines~
published in Electronics Letters of November 11, 1982, 30 Vol. 18~ No. 23, at pages 99g-1000. Such variable delay lines could be used for the coil 24 ln Figure ~ or the coil-- Cl and C~ in Figure 3 to change ~he cent~r frequ~ncy of the sensor bandwidth~
Figure S is a schematic diagram o another embodim~nt of a sensor which utilizes the optîcal coil solely ~s an interferometer and delay line- In this embodiment, the (3~i~
-1~
quantity to be sensed, designated as Q, is applied to a modulator 68 which generates a time varying signal, q(t) consisting of either a signal q(t) which varies directly with the sensed quantity Q or a carrier signal having ~
high frequency which is modulated by the ~uantity Q. Any suitable structure for the modulator 50 will suffic~ for purposes of the invention, and the signal q(t) could be the quantity Q i~self or an electrical signal proportional thereto.
The signal q~t) is applied to a transducer 70 which can be a polari~ation controller or any other device which can affect the optical transmission properties of the fiber at the location of the transducer 70. In the preferred embodiment, the signal q ~t) is a carrier or bias frequency amplitude modulated with the quantity Q.
ID the preferred embodiment, the transducer 70 i5 a phase modulator. The transducer must be located at a spot that is offset from the geometric center on the geometric path of the loop. The si~nal q(ti must be time varyin~ at ~
rate which is more rapid than the rate of change that is naturally filtered out by the structure and operation of closed loop fiber optic interferometers. The siynal q~t~
exerts a spatially non-uniform effect on the coil 72 as will be explained more fully below.
The structural details of the transducer 70 and the modulator S8 are not critical to the invention~ Tha transducer 70 can be any device which can cause a change in the birefringence of the fiber at the location of the ~ransducer 70 or elsewhere such as a device to put bending or other stress on th~ fiber in response to q ~t).
Further, the transducer 70 can be any device which can change the velocity of li~ht prop~gating through the location of the transducer 70. Such a Yelocity modulating device could be any structure to convert q~t) to a physicial change in the fiber transverse dimension at the location of the transducer. This changes the velocity of ;
, 3L,~r~
propagation of light signals throu~h the region of changed transverse dimension by ~he waveguide effect~ In another embodiment~ the transducer 70 could be a polari2ation controller for changing the polarization of the counterpropa~ating light signals at the location of the transducer 70 in response to g(t).
In the preferred embodiment, the transducer 70 i~ a phase modulator which converts the signal q(t) to c~anges in the path length of the fiber optic wave~uide ~t the location of the transducer 70 in the loop. That is, the fiber is ~tretched in the transducer 70 in accordance with the amplitude of the ~ignal q(t~. A ~uitable structure for such a phase modulator is a drum of piezoelectric material with the fiber of the coil 72 wound around the 15 drum. A driving electrical signal such as the signal q(t~
th n is applied to the piezoelectric material and causes it ~o expand and contract radially in accordance wi~h the magnitude of the driving signal, This radial expansion causes the fiber wrapped around the drum to be stre~ched.
20In an alternative embodiment, the signal g~t) could be used to control the adjustment ~f a p~larization controller as the transducer 70. The structur~l details of a polari2ation controller that might be adapted to perform such a function in all thr~e emb~diments are found in an article entitled nSingle Mode Fibre Fr~cti~nal Wav~
-. Devices and Polarization Control:Lers" by H.C. Lefevre published in Electronics Letters, VQ1; 16, NO. 20 and in _ V.S. Patent Number 4,~89,090 issued 3une 21, 1~83.
30The function and structural details of the source 32t the coupler 42J the ~etector 40, the pol~rizer 36, th~
fiber optic directional directional co~plers 38 and 4~ ~nd the polarization controller 44 ~re the same in th~
embodiment of Fi9ure 5 as in the embodiments of Figures 2 35 and 3. Beam splitt~rs could be used in the three embodiments but the reflections cau~ed by these beam ~20-~plitters would render pulsed o~eration more desirable as discussed above with reference to the embodiment of Figur~
2. ~iber optic directional couplers are the preferred embodiment in all three embodiments of the invention. The details of the structure and operation of the polarizer 36 are given in an article entitled rSingle Mode ~iber Optic PolarizerW published in ~ics Letters, Vol. 5, No. ll ~t pp. 479-81 in N~vember~ 19B0. The ~e~ails of ghe structure and operation of the directional cvuplers 42 and 38 are given in an article entitled "Analysis of a Tunable Single Mode Optical Fiber Coupler" by Digonnet and Shaw published in the IE~E Journal of Quantum Electronics, Vol.
QE-18, No. 4, pp~ 746-754 dated April, 1982. The details arP also disclosed in Canadian Application Serial No.
411,079 filed on September 9, 1982, and Canadian Application Serial No. 375,214 filed April 10, 1981.
The operation of the embodiment of Figure 5 is as follows. The light source 32 supplies a light signal which is coupled into the single mod~ optical fiber 34 and ~uided through the directional coupler 42 and the polarlzer 36 to the directional coupler 38. The polarizer 36 causes only light having a selected polarization to be passed through the polarizer regardless of whether the light is travelling toward the lo~p 72 or away from thë
loop 72. The directional coupler 38 splits the light signal into tw~ coherent li9ht siynals counterpr~pa~ating around the loop and combines the returning counterpropagating li~ht ~ignals into one light si~nal travelling toward the source 32 in the fiber 34. The polarization controller 44 allows selection o the pol~rizatlon for light travellin~ in`the loop in either direction. The coupler 42 diverts p2rt of the lig~t trav211ing toward the source into the input of the photodetector 40 where it i5 converted to an ~lectrical ~ignal based upon its amplitude ~uared, i.~, its 6~.
intensity. OE course the amplitude of the combined signals which have counterpropagated in the loop 72 depends upon ~he rela~.ive phase shift between these signals caused by the optical conditions in the loop. The counterpropagating light signals in the loop ~2 have their relative phases shifted by the action of the transducer 70. The quantity Q can be either a rapidly changin~
quantity or a slowly changing quantity. If the quantity Q
is rapidly changing, it can be applied directly to the transducer 70 which can convert it to a time varying shift in the path length of the fiber at the transducer 70 or a change in the birefringence of the fiber at the transducer 70.
If the transducer 70 changes the path length in the transducer 70 at a rapid, time varying rate, the c4unterpropagating light signals travel different path lengths through the lo~p and end up shifted in phase relative to each other~ This is because they arrive at the transducer 70 at different times due to its off center location. Because the path length through the transducer 70 will be different at the two different times when the counterpropagating light signals arriYe at the transducer, _ the two counterpropagating signals will have two different transit times through the 1CQP 72. They will th~r~fore arrive at the directional coupler 38 at different times and thus be shifted in phase relative to each other.
The combined signal will have an amplitude dependent upon the rela~ive phase shift. The amplitude o~ the combined light signal can be interpreted to find the 3Q quantity Q because the relative phase shift is linearly related to Q.
If Q is a slowly varying differ~entially reciprocal ~uantity, such as temperature or pressure, Q will have to be m~dulated upon a rapidly time varying carrier si~nal ; 35 applied asymmetrically because, as ~s well known in the art, closed loop fiber optic interferometers are '' ordinarily not sensitive to slowly time varying reclprocal e~fects. In factt this is the great advantage of closed loop interferometers because this quality makes them very guiet and stable.
When Q is modulated onto 8 bias or c~rrier frequency sueh as by ~mplitude or frequency modulation, and the transducer 70 is a phase modulator, the above discussion still holds true. If amplitude modulation is used, the path length in the transducer will be changing sinusoidally at the bias or carrier frequency but the amplitude of the path length swinys will vary with Q.
Therefore the relative phase ~hift caused by the transducer 70 will vary sinusoidally. Further, the amplitude of the sinusoidal relative phase hift ,i.e., the amplitude envelope of the sinusoidal ~wings ln relativs phase shift will be the q~antity Q. The envelope will vary over ~ime as Q varies over time.
If the transducer changes the birefringence of the fiber a~c the location of the transducer 70, the relative 2~ phase shift is caused by coupling of the light from one polarization mode to the ~ther. Birefringence is the property of optical fibers which causes light propagating in them in different polari7ation modes ~o propagate at different velocities. If the transducer 70 is changing the birefringence of the fibcr ~t the location of ~he transducer 70 in proportion to the signal q(t) or Q, the ransducer wilI be changing the relative phase shift between the cvunterpropagating signals. This is because the transducer will be controlling the amount of snergy trav~lling in the slow mode versus the amount of ~nargy travelling in the ~as~ mode~ That is, when light travelling in a waveguide in one~ polarization mode encounters birefringence, some of the light energy will ~e coupled into the ortho90nal polarizaton mode and will travel thereafter in the diff~rent velocity f~r the new polarization mode. If the amount of birefringence i5 a ~7(~
function of q(t), then the quantity Q can be determined by the amount of energy in the coLInterpropagating light signals propagating in the orthogonal polarization mode from the polarization ~stablished by the polarizer 36.
This can b~ determined from the amplitude of ~he sombined signal from the counterpropagating signals in the orthogonal polarization mode. In the embodiment of Figure 5 to determine the amplitude of the combine signal from the orthogonal mode, another directional coupler would have to be added between the polarizer and ths coupler to direct ~ome of the returning energy to a polarizer tuned to the orthogonal mode and another photodetector would have be coupled to the output of the second polarizer.
The method of sensing acoustic or other waves comprises guiding counterpropagating light signals through a coil of a closed loop fiber optic interferometer which is wound and arranged in a predetermined way. That predetermined way is to arr~nge the coil so that the subject aeoustic or other wave propagates down the longitudinal axis of the coil ~t the same rate that a co~
propagating light waYe progresses down the longitudinal axis of the coil. The phase difference is then detected between the counterpropagatin0 light signals in th~ coil as the subject wave propagates throu~h it.
More specifically the method ccmprises generatin~ a light siynal and polarizing the light signal in a ~slected polarization. The polarized light is Shen split into two li~ht signals which are then counterpropa~ated through the helically c~iled loop of a closed loop fiber optio interferometer which loop is arrang~d with its longitudinal axis parallel to the direction of propa~ation of the subject wave. The loop is co~led and si~ed such that the co-propagatin9 light 5i~nal which travel~ through ths fiber is substantially uniformly affect~d in terms of phase shit by the subject wave while the counterpropagating wave which travels throu~h the fiber is 06~
-2~--not uniformly affected in terms of phase shif~ by the subject wave. Th~ phase ~iference is then detected.
A method for sensing a phenomenon or quantity O
consists of guiding counterpropagating li~ht signals throuyh a coil of closed loop fiber optic interferometer. The quantity ~ is sensed by a modulator which converts the quantity into a sign~l q(t) which varies with ~. The signal q(t) may be Q itself or an electrical signal varying linearly with Q in some applications. The signal q(t) is applied to the coil ~f the fiber optic interferometer at a location offset from the center of the geometric path of the coil such that ~he optical transmission characteristics of the fiber are altered at that location. The rel~tive phase shift between the counterpropagating light signals is then detected.
The si~nal ~(t~ can be applied so as to alter the birefringence in proportion to q(t) at the loeation of application o~ q~t) or it can be applied so as to alter ~U the velocity of propagation in the fiber in accord with q(t) at the location of application. Further q(t) can be applied such that the polari7,ation of light passing through the location in the coil where q~t~ is applied is altered proportionally to q (t~. The preferred m~thod is 2~ to alter the geometric path length of one count~rpropagating signal relative to the other in accord with q~t).
All these methQds should include the ~tep o~
polari~in~ the liyht of the counterpropa~ating light signals in the same prior to entry into the coil and ~iltering out all returning counterpropagating light not of the same polarization as the pol~rization of the counterprop~gating light signals coupled into the loop.
This coupling should be done prior to detecting the relative phase shift between the returning counterpropagatin~ light signals.
7(3 All these methoàs can also include the step of controlling and adjusting the polarization o~ ~he counterpropagating light signal~ in the loop to any selected polarization.
All ~he methods of ~ensing the quantity Q can also include the ~tep of modulating the quantity Q onto a bias or carrier freguency to generate a ~i~nal q~t~ bef~re applying q(t) to the loop.
The quantity could be applied directly to the ~ransducer and the rate of change sensed. ~or example, the quantity Q may ~e temperature of an environment and the signal q(t) would be a time varying measure o~ th~
temperature. . The ~ignal ~(t1 is then applied asymmetrically to one or more small segments in the 1~ opticsl coil to a~fect the interferometer and thus senerate a detectable variation in the recombined ~ptical signals. ln addition to the devices described abo~e, the transducer 70 may also be electro-optical, piezoelectric, or ma~netostrictive.
There has been described several embodiments of a sensor which either includes or utilizes a closed loop optical interferometer for detecting ~ sensed quantity, Q, In ~ach of the embodiments, the sensed quantity is spati~lly non-uniform with respect to the clo5~d loop.
The entire coil ean be empl~yed a5 the sensor elemen~ or the ~uant~ty can be applied at one or more points asymmetrically located on the coil.
Although the invention has been describ~d in terms of preferred embodiment, it will be apparent ~o those skilled in the art that numerous modificatiuns can ~e made without departing from the spirit and scope vf the claims appended hereto. Such modifications ~are in~ended to be includsd within the ~cope of the claims.
Claims (34)
1. An apparatus for sensing, comprising:
an interferometer having a coil of optical fiber;
a modulator for sensing a quantity, Q, and for responsively generating a signal q(t);
a transducer for converting the signal q(t) to a corresponding change in the optical transmission properties of said coil.
an interferometer having a coil of optical fiber;
a modulator for sensing a quantity, Q, and for responsively generating a signal q(t);
a transducer for converting the signal q(t) to a corresponding change in the optical transmission properties of said coil.
2. An apparatus as defined in Claim 1, further comprising a light source for causing counterpropagating first and second light signals to propagate in said coil, and wherein said transducer is located at a location on said coil which is offset from the geometric center of the optical path formed by said coil.
3. An apparatus as defined in Claim 2, further comprising a polarizer for causing said light source to couple light of a selected polarization into said coil.
4. An apparatus as defined in Claim 2 wherein said signal q(t) is the quantity Q modulated onto a bias signal having a predetermined relationship with the optical transit time of said counterpropagating light signals around said coil.
5. An apparatus as defined in Claim 4 wherein said predetermined relationship is such that said signal q(t) has a different amplitude at the different times when said first and second light signals arrive at said transducer location.
6. An apparatus as defined in Claim 4 wherein said transducer is a phase modulator which stretches the fiber optic waveguide in the coil at the location of the transducer such that each of said first and second light signals travels a different optical path around said coil than each would travel in the absence of said signal q(t).
7. An apparatus as defined in Claim 6 wherein said predetermined relationship is such that said phase modulator stretches said fiber by different amounts at the different times when said first and second light signals arrive at said transducer, thereby causing a phase shift of said first light signal relative to said second light signal.
8. An apparatus as defined in Claim 7 further including a detector which senses said relative phase shift.
9. An apparatus as defined in Claim 4, wherein Q is rapidly time varying and q(t) is directly proportional to Q such that q(t) has a different amplitude at the different times said first and second light signals arrive at said transducer.
10. An apparatus as defined in Claim 9 wherein said transducer changes the birefringence of said fiber in response to q(t).
11. An apparatus as defined in Claim 9 wherein said transducer changes the velocity of propagation of said first and second light signals in response to q(t).
12. An apparatus as defined in Claim g wherein said transducer shifts the relative phase of said first and second light signals in response to q(t).
13. A method of sensing a quantity, Q, utilizing an optical fiber interferometer comprised of a loop of optical fiber which forms an optical path having an optical path length, said method comprising:
generating a signal q(t) in response to quantity Q;
converting the signal q(t) to a corresponding change in the optical properties of the loop at a location on said loop which is offset from the midpoint of said optical path length.
14. A method of sensing a quantity, Q, as defined by
generating a signal q(t) in response to quantity Q;
converting the signal q(t) to a corresponding change in the optical properties of the loop at a location on said loop which is offset from the midpoint of said optical path length.
14. A method of sensing a quantity, Q, as defined by
Claim 14, wherein the step of generating a signal, q(t), comprises the step of modulating the quantity Q onto a bias signal to produce the signal q(t).
15. An apparatus comprising a closed loop optical interferometer for sensing a quantity which produces reciprocal effects on finite lengths of optical fiber, further comprising:
a closed loop of optical fiber for propagating first and second light waves in opposite directions about said loop, said loop arranged such that, in use, one of said counterpropagating light waves is affected differently by the sensed quantity than the other of the counterpropagating light waves, thereby making the loop nonreciprocal with respect to the sensed quantity to cause a phase shift between said counterpropagating light waves;
a light source for generating said light waves;
a coupler for splitting a light signal from said source into said first and second light waves and for optically coupling said first and second light waves to said loop such that said first and second light waves counterpropagate through said loop; and a detector for detecting the phase difference between said first and second counterpropagating light waves.
a closed loop of optical fiber for propagating first and second light waves in opposite directions about said loop, said loop arranged such that, in use, one of said counterpropagating light waves is affected differently by the sensed quantity than the other of the counterpropagating light waves, thereby making the loop nonreciprocal with respect to the sensed quantity to cause a phase shift between said counterpropagating light waves;
a light source for generating said light waves;
a coupler for splitting a light signal from said source into said first and second light waves and for optically coupling said first and second light waves to said loop such that said first and second light waves counterpropagate through said loop; and a detector for detecting the phase difference between said first and second counterpropagating light waves.
16. An apparatus, as defined in Claim 15, wherein said detector detects said phase difference by detecting the intensity of a combined signal resulting from the interference of said first and second counterpropagating light waves.
17. An apparatus, as defined in Claim 15, wherein said source generates light of a selected polarization.
18. An apparatus, as defined in Claim 15, wherein said source and detector are coupled to a polarizer which prevents substantially all light of other than a selected polarization from passing in either direction.
19. An apparatus, as defined in either of Claims 17 or 18, wherein said loop includes a polarization controller to control the polarization of said first and second counterpropagating light waves.
20. An apparatus, as defined in Claim 15, wherein said loop has plural turns to form a coil.
21. An apparatus, as defined in Claim 15, further comprising:
a modulator for sensing a quantity, Q, and transforming Q into a time varying signal g(t); and a transducer for converting the signal q(t) to a corresponding change in the optical transmission properties of said optical fiber loop.
a modulator for sensing a quantity, Q, and transforming Q into a time varying signal g(t); and a transducer for converting the signal q(t) to a corresponding change in the optical transmission properties of said optical fiber loop.
22. An apparatus, as defined in Claim 21, wherein said transducer comprises a phase modulator located at a location on said loop which is offset from the center of the optical path formed by said loop such that the detected phase difference between said counterpropagating light waves varies in response to the quantity Q.
23. An apparatus, as defined in Claim 21, further comprising a polarizer for causing said light source to couple light of a selected polarization into said loop.
24. An apparatus, as defined in Claim 21, wherein said signal q(t) is the quantity Q modulated onto a bias signal.
25. An apparatus, as defined in Claim 21, wherein said signal q(t) varies in amplitude.
26. An apparatus, as defined in Claim 21, wherein said transducer causes phase modulation by stretching the optical fiber such that each of said first and second light waves travels a different optical path around said loop than each would travel in the absence of said signal q(t).
27, An apparatus, as defined in Claim 21, wherein Q
is rapidly time varying and q(t) is directly proportional to Q.
is rapidly time varying and q(t) is directly proportional to Q.
28. An apparatus, as defined in Claim 21, wherein said transducer changes the birefringence of said fiber in response to q(t),
29. An apparatus, as defined in Claim 21, wherein said transducer changes the velocity of propagation of said first and second light waves in response to q(t).
30. An apparatus, as defined in Claim 21, wherein said transducer shifts the relative phase of said first and second light waves in response to q(t).
31. An apparatus, as defined in Claim 23, wherein said transducer changes the birefringence of said fiber in response to q(t), thereby causing coupling of a portion of said light wave to the polarization mode orthogonal to said selected polarization mode.
32. An apparatus, as defined in Claim 31, further comprising a detector which independently detects light of said selected polarization mode and said orthogonal polarization mode.
33. A method of sensing a quantity which produces reciprocal effects on finite lengths of optical fiber utilizing an optical fiber interferometer comprised of a loop of optical fiber which forms an optical path having an optical path length, a light source for producing light, a coupling device for splitting the light into first and second counterpropagating light waves which traverse said optical path in opposite directions, and a detector for detecting the phase difference between the light waves after traversing the optical path length, said method comprising the steps of:
exposing said counterpropagating light waves to the quantity to be sensed such that said first counterpropagating light wave is affected differently than said second counterpropagating light wave thereby making the interferometer nonreciprocal with respect to the sensed quantity to cause a phase shift between said counterpropagating waves which is indicative of the quantity to be sensed; and detecting the phase shift between the counterpropagating waves.
exposing said counterpropagating light waves to the quantity to be sensed such that said first counterpropagating light wave is affected differently than said second counterpropagating light wave thereby making the interferometer nonreciprocal with respect to the sensed quantity to cause a phase shift between said counterpropagating waves which is indicative of the quantity to be sensed; and detecting the phase shift between the counterpropagating waves.
34. A method of sensing a quantity as defined in Claim 33, further comprising the steps of:
generating a signal q(t) in response to the quantity being sensed; and converting the signal q(t) to a corresponding change in the optical properties of the loop at a location on said loop which is offset from the midpoint of said optical path length such that said phase difference varies in response to the quantity Q.
generating a signal q(t) in response to the quantity being sensed; and converting the signal q(t) to a corresponding change in the optical properties of the loop at a location on said loop which is offset from the midpoint of said optical path length such that said phase difference varies in response to the quantity Q.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000506760A CA1227061A (en) | 1982-04-14 | 1986-04-15 | Sensor using fiber optic interferometer |
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US36842282A | 1982-04-14 | 1982-04-14 | |
| US368,422 | 1982-04-14 | ||
| CA000425748A CA1207551A (en) | 1982-04-14 | 1983-04-13 | Sensor using fiber optic interferometer |
| US564,998 | 1983-12-13 | ||
| CA000506760A CA1227061A (en) | 1982-04-14 | 1986-04-15 | Sensor using fiber optic interferometer |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000425748A Division CA1207551A (en) | 1982-04-14 | 1983-04-13 | Sensor using fiber optic interferometer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1227061A true CA1227061A (en) | 1987-09-22 |
Family
ID=25669998
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000506760A Expired CA1227061A (en) | 1982-04-14 | 1986-04-15 | Sensor using fiber optic interferometer |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1227061A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115508448A (en) * | 2022-11-17 | 2022-12-23 | 南京理工大学 | High-spatial-resolution ultrasonic field detection method based on optical fiber common-path interference |
-
1986
- 1986-04-15 CA CA000506760A patent/CA1227061A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115508448A (en) * | 2022-11-17 | 2022-12-23 | 南京理工大学 | High-spatial-resolution ultrasonic field detection method based on optical fiber common-path interference |
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