CA1242909A - Fiber optic directional coupler - Google Patents

Abstract

ABSTRACT
Apparatus is disclosed for superimposing a modulating waveform on a fiber optic transmission signal. The apparatus includes means for coupling the transmission signal from a first single mode optical fiber to a second single mode optical fiber and means for physically moving the first and second single mode optical fibers relative to each other in accordance with the modulating waveform. The coupling means preferably comprises means for mounting the optical fibers adjacent to one another to place the core of each fiber within the evanescent field of the other to form an interaction region in which evanescent field coupling occurs. The fibers preferably gradually converge and diverge at the interaction region. Each fiber preferably has a flat oval surface at the interaction region, which surfaces at least partially overlap, said moving means being a transducer vary-ing the degree of the overlap in accordance with the modulating waveform.

Description

This invention pertains generally to fiber optic systems and, more particularly, to apparatus for superimposing a modulating waveform on a fiber optic transmission signal.
This invention is divided from Canadian Patent Application Serial No. 375,214, filed April 10, 19~1.
It is an object of the present invention to provide improved apparatus for superimposing a modulating waveform on a fiber optic transmission signal. According to the present invention, such apparatus comprises an optical coupler for coupling the transmission signal from a first single mode optical fiber to a second single mode optical fiber, and at least one driver for physically moving the first and second single mode optical fibers relative to each other in accordance with the modulating waveform.
Preferably, the coupler comprises at least one block for mounting the optical fibers adjacent to one another to place the core of each fiber within the evanescent field of the other to ~orm an interaction region in which evanescent field coupling occurs.
Advantageously, the fibers gradually converge and diverge at the interaction region. At least one of the drivers preferably laterally displaces the optical fibers.
In one preferred embodiment, each of the fibers has a flat oval surface at the interaction region, which surfaces at least partially overlap, with at least one driver varying the degree of the overlap in accordance with the modulating waveform. At least one of the drivers may comprise a transducer.

The present invention also discloses a method of manufacturing an apparatus for superimposing a modulating waveform on a fiber optic transmission signal, comprising the steps of coupling the transmission signal from a first single mode optical fiber to a second single mode optical fiber by means of an optical coupler, and providing at least one driver for physically moving the first and second single mode optical fibers relative to each other in accordance with the modulating waveform.
Preferably, this method comprises the additional step of mountiny the optical fibers adjacent to one another on at leask one block so as to place the core of each fiber within the evanescent field of the other to form an interaction region in which evanescent ~ield coupling occurs.

'~`' -la-In the accompanying drawings:
Figure 1 is a centerline sectional view, somewhat schematic, of one embodiment of a coupling device according to the invention.
Fiyure 2 is an enlarged fra~nentary cross-sectional view, somewhat schematic, taken along line 2-2 in Figure 1.
Figure 3 is an enlarged fraymentary view, somewhat schematic, taken alony line 3-3 in Figure 1.
Figure 4 is an isometric view of one of the blocXs on which the fiber optic strands are mounted in the embodiment of F.igure 1.
Figure 5 is a side elevational view, somewhat schematic, of a second embodiment of a coupling device according to the invention.
Figure 6 is a cross-sectional view, somewhat schematic, taken along line 6-6 in Figure 5.
Figure 7 is a block diagram of a dynamically variable fiber optic coupler utilizing a coupling device of the type shown in Figures 1 4.
As illustrated in Figures 1-4, the coupler 10 includes two strands 11 of a single mode fiber optic material. Each strand comprises a single ~iber of quartz glass which is doped to have a central core portion 12 and an outer claddiny 13. For single mode operation, the core typically has a diameter on the order of 10-15 microns or less, and the claddiny has a d.iameter on the ~RD-2132:bb order of 125 microns. In Figure 1 the diameter of the strands is exaggerated for clarity of illustration, and in Figures 2-3 the diameter of the core is likewise exaggerated.
Strands 11 are affixed to bases or blocks 16 having optically flat confronting faces or surfaces 17. The strands are mounted in slots 13 which open through the confronting faces, and they extend along generally parallel, intersecting paths defined by the inner or bot-tom walls 19 of the slots. The primary func-tion of the bases is to hold the strands, and the bases can be fabricated of any suitable rigid material. In one presently preferred embodiment, the bases comprise generally rectangular blocks of fused quartz glass approximately one inch long, one inch wide and one-quarter inch thick, and slots 18 are aligned with the sagital planes of the blocks. In this embodiment, the fiber optic strands are secured in the slots by suitable cement 21 such as epoxy glue. One advantage of the fused quartz blocks is that they have a coefficient of thermal exp.lnsioll similar to th;lt oE glass fi.bcrs and this advantage is particularly important if the hloc~s and f;l~crs arc subjectcd to any heat treatment durillg the manufacturillg proccss. Anotllcr suitable material for the blocks is silicon l~hicll also h.ls exccllcllt thcrmal proncrties for this al~plication.
Slots 18 are deeper -T.oward the edges oE thc blocks thall toward -the center. I~ith one of the blocks mouTlted on the other in an inverted posi.tion both the bottom walls of the slots ancl the stran(ls mounted in the slo-ts converge tow.lrd the centers and diverge toward the edges o-f the blocks. In the embodi-ment illustrated bottom walls 19 are arcuately curved along their length bu-t they can have any other suitable contour preEerably one which provides gradual convergence and divergence of the fiber optic strands with no sharp bends or abrupt ch mges in di.rection. In the sc~ematic illustration o~ the drawings the bottom ~alls are illustrated as being flat in cross-section. ~lowever they may be curved or have any other desired cross-section~
Toward the centers of the blocks tllc~ depth of slots l~ is less tllall the diameter of strands 11 and the outer porti.ons of the fiber optic ma-terial are removed evenly with surface 17. At -the edges o-E the blocks the depth of the slots is preferably at least as great as the diameter of the strands so that none of the claddi.ng is removecl at these points. Tll-ls the amount of f;.ber optic removed increases gradually Erom zero towlrd the edges of the blocks to a maximuTn toward the centers of the blocks. Remov.ll oE -the materi;ll permits each core to be positioned wi-tll;ll the ev.lnescent f;clcl oE the othcr wllereby light is tr;lnsfeIre~l between the two ribc15. II1C evallescellt Eields extend into the claclding an(l dec-rease rLII~;(I1Y witll dist.ll-lce outside the core in Wl1;~11 tl1CY Or~ tC. 'IhC t l~ercd renlov3l oE m.lteIi;ll erl.lbles the E;bers to converge alld ~livcrgc gradu.lll~ and tllis is iml~OIT;lnt in avoidirnl backwar(l _ ~ _ ,3~

reflcction and e~cess loss of the incidcnt light cncr~y.
Appl;cants have discovcred that thc amount of matcrial removcd must be carefully controlled to-provide propcr coupling between the fiber optic strands. If too little cladding is removed, the strands cannot be brought close enougll togethcr, and insuEficitnt couplillg l~ill resul-t. I-F too much ma-tcrial is rcmc)~cd, the prop~gation charactcristics of the fibers will be altcred, and improper operation will rcsult, e.g. back reflcction and loss o-f light energy. I~'hen the spacing between the core portions oF the strancls is within a certain predetermined "critical zone", however, each of the strands receives a significant portion of tlle evanesccnt energy from the other strand, and optimum coupling is achieved without thc undesirable effects associated with removal of too little or too much of thc fiber optic material.
The eYtent of tlle critical ~one for a particular coup]er is dependent upon a nwnber of interrelated factors such as the parameters of the fiber it-self and the geometr~ of the coupler, and ~ith single mode fiber optic strands having a step indeY gradient, the critical ~onc can be qui-te narrow. In a single modc coupler of the t~pe shown in ~igures 1-4, for eYal!lple7 tlle recl-lircd center-to-center spacing bet~;een thc strands at the center of the coupler is typically less than a few (e.g. 2-3) core diametcrs.
An interaction region 23 is formcd at the junction of thc str.mds, and in this region light is trans~errccl betweel1 thc two strands. The amount oE ligl1t transfcrred is depelldcllt upOI1 thc pro.Yilnity and oricntatioll of the cores, as wcll as the lcngth of thc rcgiorl of -intcr;1ctio~ 'hc lcngtll o~ that rcg;ol-l is, ;n turl1, del-cndclll ul)oll the raclills Or curvaturc o~ bottom walls 19 and thc spacing ~ctwecn -thc corcs. In onc prcscn-tly prcfcrrccl emboclilncnt employil1( an ed~c-to-cclgc COl'C '.p.lCillg 01~ thc orclcr of m;lgnitu~lc o~ -thc corc dialllctcr, thc ra-lills o~ culv;1turc is on tl~c orclcl o~ 1 mctcr, allCI thc intcr-S

act;on region is approxi;mcltely ~.5 millimcters long. ~itll thcsc dimcIlsiolls~
the light ma~cs only onc transfcr bet\~een the strancls aS it travels -throu~
thc interaction region. ~lo~evcr, if desired, a longcr intc-nction re~ion carl be ernploycd, in ~hich casc the light will transfer bac~ and forth bet~cen thc t~.~o s-trands as it travels throuIIh the region. These addi.tion.Il transfers can yrovide increasccl sensitivity to motion for some ty~es of sw-itclling, c.g.
translat;on or acoustic. If desirei, the length of thc -in-tcnaction rcgioIl can be increased ~ithout incre.Ising the n~ImI)er of trans:fers if tlle separation betl~een the cores is incrcased by a corresponding amolInt.
A filrn of fl~Iid (not shown) is provided bet~een the confronting surfaces of block 16. rhis fluid serves the dual function of ma-tching refract-ive indexes and preventing the optically flat s~Irfaces o-E-the bloc~s from becoming permanently locked together.
The amount of coupling betI~een the fibers is adjustcd by changing the relative positions and/or orientations of the fiber cores in the intcr--action regio]l. The prirnary adjustlIlent is provided by translat;ng the bloc~s in a direction perpcndicular to thc axis of the -fibers. Additional adjustmcIlts ean be made by translating thc hloc~s in a direction parallel to thc fibcr axis and by rotating -the bloc~s about an a.xis perpendicular to the fiber a~is.
One of the bloc~s can be mountcd in a fixed position, and thc othcr can bc mounted on a carriage having microIlleter scre~s for ma~ing thc translatioIlal and rotational adjustlllellts.
The colIpler l1.IS -foul ports labelled A-D :in lig-Ire 1, ~ith ports A, B at opposite ends of one of tlIC fibcrs aIld ~orts C, D at opposite cnds of tlIe othcr fibcr. ~ln tllc follo~ing disc~Issioll, it is ass~lmc(I th;It input li'ht of suitablc ~iavclcIlgt]l (c.g. I ]C microIl) is aI)l)Iic(I to port A. Ihis ligllt passcs thro-IgIl thc co~IpIcl alld is dcIivcrcd to pOlt B all(l/or port D, dcl)cndill;

upon thc couplin~ ratio for \~lIich ~he coupler is sct The cocfEicicnt of coupling is dcfined as the ratio oE poI~er at output port n to tlIe po~-~cr at inpu-t port A In onc ecample of a couplcr having the dimensioIls g;ven above as much as S5~o of tl-e input po~cr at port A has bcen obscrvcd to be delivered to port D. In principle, ho~ever lOO!o coup]ing is possiblc, and tlle amount of coupling can be adjusted to any dcsired valuc betl3een zcro and thc lOO~o maximum Ihus thc coupler has a high widely adjustablc coefficient of coupling.
The coupler also has a very lo-~ throughput loss and very good direc-tivity. The throughput loss in the above ecample is less thall 0 2 db Tl-e dircctivity is defined as the ratio of the po~:cr at port D to thc pc,;:er at port C ~itl- the input applicd to port A ~ith this couplcr the po~-~er at port D is greater than 60 db above the po~er at port C. Thus substantially all of the po~ier applied to input port .~ is delivered to the output ports B
and D.
Thc coupler also llas e~cellent polarization rcsponse in tllat it ~
pass light of any po1arizatioll almost cqually ~ell Thus thc characterist;cs of the coupler are substantially indcI)cndcIlt of polarizatioll In onc presently pre~Eerrcd method of Inanu-Eacture, the coupler of Figurcs 1-4 is made by first gr;nding thc opposite faccs 17, 26 of bloc~s 16 flat and parallel Slots 18 arc thcll cut throu-h faccs 17 to a uniEorm depth grcatcr thall thc d;amcter oE thc fibcr optic strancIs Thc bottom ~alls of thc slots arc then shlpc(l to provicIe thc ~Icsircd contour Thc sl~apiIl~ is plcrcr.lbly such th;lt thc cIcI~th oE thc slots at thc edges of thc bloc~s is at lcast onc I~alf of a fibcr COI`C diaIllcter grcater than thc dcpth at tlIc centcrs of thc slots Oncc the slots h;lvc bccll Ec)llilcd, cpo.~y gluc 21 is placcd thcrci an l strancls 11 are placed in the slots ~ th the glue ~eigl ts are then attacllcd to the ends o-F the strands to tension the str.lllcls and dra~ them tightly ag.lillst thc bottom l/alls of the slot; Ille entire asseml-l~ is thell hcatcd in an ovcn to eure the glue typ;cally at a tem!-erat-lre on the order of 7nc for abo~lt 10 hollrs l~ith the epo~y glue it is very important that the heat be ;-pplicd and remove l gradu;llly in order to prevent breal;age of thc fibers ~ithin thc slots l`his ean be aecomr)lished hy plaein7g the bloeks in the oven before it is energized and leaving them in the oven until it has eooled clo~ill to room temperature after the lleatillg process hllen the lleating is completed the ~eigllts are removed to release thc tension in tlle Eibers Once the fibers have been mounted in the slots faces 1 are lar)ped parallel to faces 26 until they intersect tlle eladding oF the ~Eibers forming elongated oval shaved flat surfaees on the outer sides of the fibers rhe ~idths of these oval shaped areas arc measured to determille -the positions of the fibers relativc to b]ock surfaces 26 and thc!reafter thesc surfaces serve as refcrenees for loeating the core portions o E tllc fibers By measuring the lengtlls of the oval sllapecl areas at different depths of euts tlle radii of eurvature of tlle fibers ean he deterlllined The lapping proecess eontinlles until tlle eladding llas heen removed to ~.ithill about 3 mierons of the desired distanee from tlle eores as determined by direet measulelllellt of the tllicliness oE tlle bloel~s. The final three mierons are remove l 1 y polishillg The pol;shed surflccs of the bloclis are then placed togetl~er l~l;th the cut-a~ ay portions of the f-il-ers rac;~l,,7 C;lCh o-tl-er The eonElollting ~Eaee;
of the bloelis are se})al;lte(l by a distance on the or Icr of n.5 nlicroll 0l less and optieal uil ;s -introduecd betl~eell the blocl~s ly eapillaly aetio r~l tlle ellll)o lilllellt 1~ l ctofol~ Ic~ l il)c~(l slots lS ;1l ~ folllle~ c~ltti into the surE;Ices of the blocl~s Tt s!lolll.l I-e ullclc~l-stoo(l ho~cver tl-at the slots eall bc formcd by otllcr mcans sucli as buil(ling up areas on the bloclis or joining tl.lo or more blocks toge-ther and that the slots can have othcr shapes e.g. V-grooves. Lilie~./;.se -tecllniques othcr thall cementing :nigll-t be employed to boncl the fi.bers to the bloc'is e.g. indiulll boncling. Similarly the material ean bc rcmoved from thc. bloelis and the claddillg by othcr suital-lc techniques sueh as etclling and photolit}-ograplly.
In the cmbodimellt o-f ~igurcs 5-6 a plurality of -fiber optie strancls 31 similar to strands 11 are a:EEi~ced to bascs or bloel;s 32 and posi.tioned to provide a plurality of interaction rcgions 33 bct-~een eorresponding pairs of 10 . the s-trands on the t-~o bloclis. In this embocliment thc eorcs of the fibcrs arc designated by the referenee numeral 36 and the eladdings ar- designated 37.
As illustrated bases 3Z eomprise gencrally reetangular bloelis of quartz or other suitable rigid m~terial having eonfron-ting faees or surfaces 38 and outer faces or surfaces 39. The eentral portions 41 of surfaces 38 are planar and parallel to surfaces 39 and to~ard tllc~ edges o E the bloc}is surfaces 38 eurve a-~/ay from central portions 41. The lengtll of intcraetion regions 33 is determincd by the length of plallar ecntral portions 41 and the radius of eurvature oE the end portions of surfaees 38 as ~cll as the eorc spaeing of the fibcrs.
I~ibcrs 31 arc mounted on surEaccs 38 and a~ffi.Yecl thcreto by suit.lblc means suel~ as epo~y gluc 42 or otl~cr suital)1c eemellt. As in thc cmbodilllent of l~igulcs 1-4 tllc m;ltcI;.I] on thc outcr si.des of thc fi.bers ;s removcd L~radually from zcro -to~ard the cdgci of thc bloelis to ~ithill thc eritie.ll zone to~ard thc eentcls of t!~c bloelis by lal~l)illg alld l!oLisl~ing ;n a cl;.rcetion parallel to surEaccs 3S 3".
Operatioll ancl usc oE the emboclilllellt oE l'igures 5-G is sim;lar to _ 9 c~

that of the couplcr oE ~igures 1-4, and the amoullt o~ coul)ling l~et~ecn thc aligned pairs of strands OTI thc t~o blocks can be adjustcd by transl;ltion and rotatioll of the bloclis In OllC' presently ~referred method of manufacture for the cou~ler of Fig~lres 5-6, the surfaces of bloclis 32 are first gro~lnd Elat and par.lllel Thereafter, the desirecl curvat-lres are formed to~ard the outer edges o~
sur-faccs 3~ The glue is then applied to the contoured surfaces, and tlle fiber optic strands are placed on the blocks and pressed against surfaces 3g ~.~hile thc~ glue cures If heating is required to cure the glue, thc heat should be applied and removed gradually to avoicl breali.ge of thc fibers 1~'11en tilC glue is cured, the material on the ou-ter sid~s of the strands is removed bv lapping and po]ishi]lg in a direction parallel to surfaces 3S, 39. The distance bet~veen the lapped and polished surfaccs and the cores of the fibers is determined by measurelTIent of the distances bet~een these surfaccs and outer surfaces 39. Re1noval of thc material in tllis mallner provides a gradual tapering of the fibe-rs into and out of thc interactioll regiolls In this embodiment, tlle adjace1lt fibers on each blocli provide lateral support for each other and serve as a guide in the grinding alld polishillg steps l~'hen the desired amount of material has been removed, the bloc~s are superposed ~ith surfaces 35 facing each other and the correspo11dirng strands on the tl~O bloclis aligned to form a plurality of couplcl pails ~s illustrated in lig~lre 7, a coul)l;ng device 10 of the type l1ereto-' ~fore described cal1 be ~Itil;~ed to provide dyl1;llllic;llly var-i;ll)le coupling in a fiber optic systelll In this elllbodilllel1t, suital)le tr;lllsd~lcers or clrivers 51, 52 are conllectecl to ~Ipper blocli lG fol trallslatillg th;lt blocli bacli allcl fortll relative to thc lo~.~cr t~loc~ a1ong a~cs 5~, 5~1 in directioll~ 1-erpel1-1ic~l1;lr alld pala11cl to thc a.~cs of thc ~it~CIs. J\ Sillli t;ll' tr;lnsducer or drivel 56 is also ~ f~J~ ~?~

conncctcd to tllc uppcr bloc~ for moving th.lt block alollg all axis 57 in a dircctioll perpclldicul;lr to a~es 53, 54 to vary tllc sy;lcing l)et~eell thc blocks A fourth transcluccr 5S providcs rclative rotation of the blocks about axis 57 By varying the rclativc yositions ancl/or oricntations of the blocks and t?-e fiber cores in the interaction region, one or more of the -transducers vary the amoullt of cou~ling bet~een thc ~ibers in accordancc ~ith sign,lls ayl-licd to the transducers. These trallsducels can be Or any suitable clesign, including pie70electric transducers and other knowll electro-mecllallical transducers 'rhe system illustrated in ~igure 7 can be employed as a variable coul)ler in wllicll the coefficient oE couplillg is controlled 'by voltages or other suitable control signals applied to the trinsducers Tlle system can also E-mction as a mod-llator if a time varying voltage or other suitable modulation signal is applied to the transducers

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for superimposing a modulating waveform on a fiber optic transmission signal, comprising:
an optical coupler for coupling said transmission signal from a first single mode optical fiber to a second single mode optical fiber; and at least one driver for physically moving said first and second single mode optical fibers relative to each other in accordance with said modulating waveform.

2. An apparatus as defined by Claim 1, wherein said coupler comprises at least one block for mounting said optical fibers adjacent to one another to place the core of each fiber within the evanescent field of the other to form an interaction region in which evanescent field coupling occurs.

3. An apparatus as defined by Claim 2, wherein said fibers gradually converge and diverge at said interaction region.

4. An apparatus as defined by Claim 1, wherein said at least one driver laterally displaces said optical fibers.

5. An apparatus as defined by Claim 2, wherein each of said fibers has a flat oval surface at said interaction region, which surfaces at last partially overlap, said at least one driver varying the degree of said overlap in accordance with said modulating waveform.

6. An apparatus as defined in Claim 5, wherein said at least one driver comprises a transducer.

7. A method of manufacturing an apparatus for superimposing a modulating waveform on a fiber optic transmission signal, comprising the steps of:
coupling said transmission signal from a first single mode optical fiber to a second single mode optical fiber by means of an optical coupler; and providing at least one driver for physically moving said first and second single mode optical fibers relative to each other in accordance with said modulating waveform.

8. A method of manufacturing an apparatus for superimposing a modulating waveform as defined in Claim 7, further comprising the step of mounting said optical fibers adjacent to one another on at least one block so as to place the core of each fiber within the evanescent field of the other to form an interaction region in which evanescent field coupling occurs.