Multipoint temperature-independent vector bending sensing with coupled-core fibers

36784 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Mul ipoin Tempe a u e-Independen Vec o Bending Sensing Wi h Coupled-Co e Fibe s Ma co A. Con e as-Te an , Amaia Be ganza , Flo ian Lindne , Jö g Bie lich, Ka in Wond aczek, and Joel Villa o o , Senio Membe , IEEE Abs ac —Fibe op ic bending sensing has po en ial use in indus ial and medical applica ions. Thus, so a , se - e al con igu a ions ha e been epo ed wi h ha end, bu he s a e-o - he-a senso s a e ei he complex, empe a u e dependen , o canno be mul iplexed easily. To ci cum en hese impo an limi a ions, we ha e de eloped a sensing pla o m based on an asymme ic coupled-co e op ical ibe ha is combined wi h con en ional B agg g a ings. The asymme ic ibe was designed wi h h ee co es a anged in an equila e al iangle. The said ibe suppo s supe modes ha su e d as ic changes when i is ben . Consequen ly, he e lec ion o a B agg g a ing insc ibed close o he asymme - ic ibe changes d as ically, bu i s wa eleng h posi ion is no al e ed. We demons a e expe imen ally ha ou sensing pla o m allows he de elopmen o highly sensi i e bend- ing senso s ha ha e impo an p ac ical asse s. The la e include compac ness and simple ab ica ion, capabili y o dis inguishing he di ec ion o bending and simul aneous de ec ion o empe a u e and bending. Mo eo e , he senso s can be mul iplexed easily and can be in e oga ed wi h comme cially a ailable ibe op ic senso ead ou uni s. Index Te ms—Bending senso s, coupled-co e ibe s (CCFs), ibe B agg g a ing (FBG), mode in e e ome e s, mul i- co e ibe s, op ical ibe senso s, supe modes. I. INTRODUCTION THE e ec s o bending on he ligh guided by an op ical ibe a ac ed esea ch in e es since he ea ly days o Recei ed 30 Augus 2024; accep ed 7 Sep embe 2024. Da e o pub- lica ion 18 Sep embe 2024; da e o cu en e sion 14 No embe 2024. This wo k was suppo ed in pa by MCIN/AEI/10.13039/501100011033, “ERDF A Way o Making Eu ope,” unde G an PID2023-152763NB- I00, G an PDC2022-133885-I00, G an PID2021-122505OB-C31, and G an TED2021-129959B-C21; in pa by Eu opean Union “Nex Gen- e a ion EU/PRTR;” in pa by he Gobie no Vasco/Eusko Jau la i za unde P ojec IT1452-22; and in pa by Ge man Fede al Minis y o Educa ion and Resea ch (BMBF): “RUBIN-QUANTIFISENS-TP11: Special y Fibe s and Fibe -Based Componen s o Omni unc ional Fibe Senso Sys ems” unde G an FKZ: 03RU1U071J. The associa e edi o coo dina ing he e iew o his a icle and app o ing i o publica ion was P o . A naldo Gomes Leal-Junio . (Co esponding au ho : Joel Villa o o.) Ma co A. Con e as-Te an is wi h he Depa men o Elec- onic Enginee ing, Enginee ing Di ision Campus I apua o-Salamanca, Uni e sidad de Guanajua o, Salamanca 36885, Mexico (e-mail: ma.con e as e an@ug o.mx). Amaia Be ganza is wi h he Depa men o Applied Ma hema ics, Uni e si y o he Basque Coun y UPV/EHU, 48013 Bilbao, Spain (e-mail: [email p o ec ed]). Flo ian Lindne , Jö g Bie lich, and Ka in Wond aczek a e wi h he Leibniz Ins i u e o Pho onic Technology (Leibniz IPHT), 07745 Jena, Ge many (e-mail: lo [email p o ec ed]; joe g.bie lich@ leibniz-iph .de; [email p o ec ed]). Joel Villa o o is wi h he Depa men o Communica ions Enginee ing, Uni e si y o he Basque Coun y UPV/EHU, 48013 Bilbao, Spain, and also wi h IKERBASQUE, Basque Founda ion o Science, 48011 Bilbao, Spain (e-mail: [email p o ec ed]). Digi al Objec Iden i ie 10.1109/JSEN.2024.3459623 ibe op ics echnology. I was soon demons a ed ha bending o cu a u e applied on an op ical ibe changes i s e ac i e index p o ile due o he di e en mechanical and pho og aph elas ic p ope ies o he co e (o co es) and cladding [1]. Consequen ly, pu e bending al e s he p o ile o dis ibu ion o he modes guided by an op ical ibe and he eby al e ing i s ansmission p ope ies [2]. Thus, by analyzing such ans- mission p ope ies, i is possible o know how much an op ical ibe has been ben . In p inciple, any ype o op ical ibe o combina ions o di e en ibe s can be used o de elop a bending senso , see, o example, [3]. Howe e , in many cases, i is also impo an o know he bending di ec ion along wi h (o independen o ) o he pa ame e s, such as empe a u e and s ain. Ideally, a ibe op ic bending senso mus be compac , cos e ec i e, and simple o in e oga e. To achie e all hese desi able ea- u es, he op ical ibe senso communi y has p oposed se e al con igu a ions. Fo ins ance, he changes o mode bea ing o speckle pa e ns in mul imode ibe s can be exploi ed o de elop bending senso [4],[5],[6],[7]. Howe e , hese senso s a e a ec ed by empe a u e; hus, addi ional senso s o a mechanism o compensa e he e ec o empe a u e may be necessa y in p ac ical applica ions. In addi ion, he mul iplexing o mul imode ibe bend senso s is complex. Long-pe iod g a ings can be used o de ice compac bending senso s, see, o example, [8],[9],[10],[11]. The disad an age © 2024 The Au ho s. This wo k is licensed unde a C ea i e Commons A ibu ion-NonComme cial-NoDe i a i es 4.0 License. Fo mo e in o ma ion, see h ps://c ea i ecommons.o g/licenses/by-nc-nd/4.0/ CONTRERAS-TERAN e al.: MULTIPOINT TEMPERATURE-INDEPENDENT VECTOR BENDING SENSING WITH CCFs 36785 o hese g a ings is hei concu en sensi i i y o bending, s ain, and empe a u e. On he o he hand, hei b oad and mul idip ansmission spec um complica es hei mul iplex- ing. Eccen ic B agg g a ings, i.e., g a ings ha a e insc ibed o -axis inside he ibe co e, o e mul iple possibili ies o sensing bending and i s di ec ion as well as empe a u e [12], [13],[14],[15],[16],[17]. Howe e , he insc ip ion o eccen ic B agg g a ings equi es ca e ul posi ioning o a mic oscopic op ical ibe co e wi h espec o a pe iodic UV o em osecond lase pa e n. Mo eo e , eccen ic g a ings can exci e cladding o highe o de modes, which may lead o a mul ipeak e lec ion o ansmission spec um [12],[14], [16]. In addi ion, hese g a ings ha e concu en sensi i i y o s ain, bending, and empe a u e. This imposes he use o sophis ica ed da a p ocessing o decouple all hese pa ame e s. Op ical ibe s wi h ellip ical co es [18] o wi h mo e han one co e a e a good al e na i e o de ice senso s ha can dis inguish he bending di ec ion as well as he bending deg ee. The co es can be uncoupled, i.e., hey a e isola ed om each o he [9],[10],[19],[20] o can be coupled [21],[22],[23], [24]. The disad an age o senso s based on ibe s wi h isola ed co es includes he use o expensi e an-in/ou de ices o in e oga e each indi idual co e and he necessi y o o ien he co es wi h espec o he bending di ec ion. This complica es he p ac ical applica ion o such senso s. Bending senso s buil wi h coupled-co e op ical ibe s a e based on supe mode in e e ence and a e sensi i e o empe a u e and s ain [25]. The mul iplexing o supe mode in e e ome e s, placed in se ies o in pa allel, is no s aigh o wa d as hei ansmis- sion o e lec ion spec a a e mul iplied o added up [26], [27],[28]. The limi a ions and d awbacks o he s a e-o - he-a ibe op ic bending senso s ep esen oppo uni ies o de elop inno- a i e pla o ms ha allow he de elopmen o compac , di ec ion-sensi i e bending senso s, ideally ha a e indepen- den o empe a u e and ha can be mul iplexed easily. Thus, in his wo k, we p opose a simple s uc u e ha is buil wi h a sho segmen o asymme ic coupled-co e ibe (CCF) and con en ional ibe B agg g a ings (FBGs). The leng h o he segmen o he asymme ic ibe and he B agg wa eleng h o he g a ings a e chosen o minimize he inse ion losses and o simpli y he mul iplexing o he senso s. I is demons a ed expe imen ally ha he sensing a chi ec u e he e p oposed allows he de elopmen o bending senso s ha can dis inguish he bending di ec ion and ampli ude; in addi ion, hey can moni o bending and empe a u e simul aneously and can be mul iplexed easily. Mo eo e , hei in e oga ion can be ca ied ou wi h well-es ablished FBG in e oga o s. We belie e ha all he dis inc i e ea u es o ou de ices may be appealing in a a ie y o p ac ical applica ions. II. MATERIAL AND METHODS A CCF wi h an asymme ic co e dis ibu ion was designed a he Uni e si y o he Basque Coun y (Spain) and ab i- ca ed a he Leibniz Ins i u e o Pho onic Technology (Jena, Ge many). The CCF was designed wi h h ee iden ical pho o- sensi i e co es embedded in a pu e silica cladding; one co e was in he geome ical cen e o he CCF and o he wo co es Fig. 1. (a) C oss sec ion o he designed CCF. (b) Pho og aph o he on iew o he p e o m; he h ee co es a e ma ked in ed. Two SEM images o he ab ica ed CCF a e shown in (c) and (d). we e o cen e . The h ee co es we e a anged in an equila e al iangle [see Fig. 1(a)]. To ab ica e such an op ical ibe , he modi ied chemical apo deposi ion (MCVD) me hod was used o p epa e a Ge-doped co e ma e ial. Fi s , he p e o m wi h he pho osensi i e co es was ab ica ed and d awn o ods o a diame e o 1 mm. Second, a hexagonally s acked p e o m was a anged consis ing o pu e silica ods o di e en diame e s su ounding he h ee pho osensi i e ods. All he co es we e loca ed a he a ge posi ion by means o he so- called s ack-and-d aw me hod [29]. In Fig. 1(b), we show a pho og aph o he on iew o he s acked p e o m. The h ee co es a e ma ked in ed o isualiza ion pu poses. Finally, he p e o m was inse ed in o a silica cladding F300 ube and consolida ed p io o d awing o inal ibe . The ab ica ed CCF was inspec ed wi h a scanning elec on mic oscope (SEM). Fig. 1(c) and (d) shows he images o he b igh shining backsca e ed elec ons (BSE) o he Ge-doped co es in he da ke SiO2-glass ma ix. To isualize he shape o he co es, an e ching o he ibe end ace was done. The sligh ly hexagonal shape o he co es and he laye s uc u e o he deposi ion a e isible in Fig. 1(d). All he co es o he CCF we e designed o ha e simila nume ical ape u e (NA) o a s anda d single-mode ibe (SMF); 0.13 a 1550 nm. The diame e o he ibe wi h he pho osensi i e co e and he SiO2-cladding was 125 µm, while he diame e o he s anda d ac yla e coa ing o he CCF was 250 µm. The a e age diame e o each co e o he ab ica ed CCF was 8.80 µm (±0.025 µm) and he a e age cen e - o-cen e sepa a ion be ween neighbo co es was 13.21 µm (±0.06 µm). The CCF wi h he abo e co e a angemen allows o de ise highly sensi i e senso s ha can dis inguished he di ec ion o bending independen ly o he co e’s o ien a ion wi h espec o he bending as p e ious publica ions sugges [22],[23]. The asymme ic CCF suppo s h ee pola iza ion-degene a e supe modes; howe e , i ligh is launched in he cen al co e o he CCF, wi h a s anda d SMF, o example, only wo supe modes can be exci ed in he CCF [22]. In Fig. 2, we show a mic og aph o he ab ica ed CCF and he 2-D p o iles 36786 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Fig. 2. (a) Mic og aph o he c oss sec ion o he ab ica ed CCF. Fo isualiza ion pu poses, he ibe was illumina ed la e ally wi h whi e ligh . (b) and (c) Two-dimensional p o iles o wo supe modes exci ed in he CCF. In bo h cases, he wa eleng h was 1550 nm and he simula ion a ea was 55 ×55 µm2. Fig. 3. (a) Ske ch o he senso a chi ec u e placed on a empe a u e- con olled pla e. FBG is he ibe B agg g a ing; Lis he leng h o he CCF segmen . The bending coo dina e sys em is x,y.(b) Diag amma ic ep esen a ion o he senso s in e oga ion; S1–S4 mean senso s 1–4, espec i ely. o he exci ed supe modes in such ibe . The p o iles o he supe modes we e analyzed wi h he ini e di e ence me hod using he comme cial simula ion so wa e (Ansys Lume ical MODE) assuming an exci a ion wa eleng h o 1550 nm. I can be no ed ha o bo h supe modes, he cen al co e o he CCF has a good po ion o guided ligh . The p oposed sensing a chi ec u e, bending se up, and sen- so in e oga ion a e shown schema ically in Fig. 3(a) and (b). The s uc u e o he senso consis s o a segmen o leng h Lo he asymme ic CCF usion spliced wi h a con en ional SMF (SMF-28 om Co ning). A e he CCF, a comme cial FBG was usion spliced. In o he wo ds, he sensing a chi ec u e is an SMF–CCF–FBG s uc u e. To cu he segmen s o CCF wi h high p ecision, we used an au oma ic la ge diame e ibe clea e (model CT-105 om Fujiku a). Be o e clea ing he ibe s, he p o ec ing polyme coa ings we e emo ed; hen, he ba e ibe segmen s we e cleaned wi h alcohol. The CCF was usion spliced wi h s anda d SMF wi h a special y ibe splice (model FSM-100P+ om Fujiku a). The de aul p og ams se in he machine we e used bu wi h he cladding alignmen mode. In his manne , he cen al co e o he CCF and he unique co e o he SMF we e aligned and in di ec physical con ac . This splicing p ocess ensu es ha ligh is launched in he cen al co e o he CCF. FBGs wi h di e en B agg wa eleng hs, all wi h a leng h o 5 mm and e lec i i y ci ca 40%, we e pu chased om B-Sens (Belgium). In some expe imen s, comme cial ibe e o e- lec o s (model P5-SMF28ER-P01-1 om Tho labs) we e used. Such e o e lec o s ope a e in he 1260–1625-nm wa e- leng h ange. To cha ac e ize and es ou de ices, we used a b oadband supe luminescen ligh -emi ing diode (model SLD1005S om Tho labs) d i en by homemade elec onics Fig. 4. Maximum o he in e e ence pa e n as a unc ion o he leng h o he CCF obse ed in h ee se s o SMF +CCF +SMF s uc u es. The inse g aph shows he e lec ion spec a o he samples buil wi h di e en leng hs o CCF. In all cases, a ibe e o e lec o was used o measu e he spec a. and an op ical spec um analyze (model AQ6360 om Yokogawa). We also used a ou -channel FBG in e oga o (Hype ion Si155 om LUNA Inno a ions). The wa eleng h ange in which we cha ac e ized ou de ices was om 1480 o 1620 nm. III. RESULTS AND DISCUSSION The p oposed senso a chi ec u e was designed o ope a e in e lec ion mode; i is ske ched in Fig. 3(a). I can be no ed ha such a senso a chi ec u e is compac and simple o assemble; only wo usion splices a e equi ed. The inse ion losses o he SMF–CCF–SMF s uc u es (wi hou FBG) we e minimal, less han 0.3 dB due o he good ma ch be ween he NA and dimensions o he CCF and hose o he SMF. The ole o he CCF leng h in he pe o mance o he de ices was in es iga ed i s as i is known ha an SMF–CCF–SMF s uc u e is a supe mode in e e ome e [21],[25],[30]. Thus, we ab ica ed h ee se s o samples wi h di e en leng hs o CCF anging om 9 o 13 mm in s eps o 0.5 mm. Each supe mode in e e ome e was cha ac e ized indi idually in e lec ion mode wi h he con igu a ion shown in Fig. 3(b); FBGs we e no used in his cha ac e iza ion. Ins ead, we used he comme cial ibe e o e lec o men ioned abo e. Ligh om a b oadband sou ce was launched o he in e e ome e s by means o a ibe couple ; he e lec ed ligh was analyzed wi h he spec um analyze desc ibed in p e ious pa ag aphs. The no malized in e e ence pa e ns o some samples a e shown in he inse g aph o Fig. 4. The igu e also displays he peak wa eleng h (maximum o he in e e ence pa e n) as a unc ion o he leng h o CCF. I can be concluded om Fig. 4 ha o sho segmen s o CCF, he posi ion o he maximum o he in e e ence pa e n depends linea ly on he CCF leng h. The samples wi h longe segmen s o CCF (L>20 mm) we e also ab ica ed ( esul s no shown). In hese cases, he in e e ence pa e ns exhibi ed mo e han one maximum. The SMF–CCF–SMF s uc u e is sensi i e o bending bu also o empe a u e and s ain as all hese pa ame e s pe u b CONTRERAS-TERAN e al.: MULTIPOINT TEMPERATURE-INDEPENDENT VECTOR BENDING SENSING WITH CCFs 36787 Fig. 5. Re lec ion spec a obse ed when an SMF +CCF +FBG s uc u e (L=11.60 mm) was ben in he +x-di ec ion ( op g aph) and −x-di ec ion (bo om g aph) acco ding o he coo dina e sys em o Fig. 1(d). In all cases, he empe a u e was 21.5 ◦C. The alues in he g aphs a e he bending angles in deg ees. The e ical do ed lines show he wa eleng h posi ion o he g a ing. he in e e ing supe modes o he CCF [21],[30],[31]. Thus, a single in e e ence pa e n canno p o ide in o ma ion o all such pa ame e s. In addi ion, i se e al SMF–CCF–SMF s uc u es—buil wi h di e en leng hs o CCF—a e placed in se ies o in pa allel, he esul ing spec a can be he mul iplica ion o sum o he indi idual spec a [26],[27],[28]. Thus, i would be di icul (i no impossible) o dis inguish he e ec o bending and empe a u e on an in e e ome e when i is mul iplexed wi h o he s. To sol e he abo e issues, we usion spliced an FBG wi h sui able B agg wa eleng h (λB)a e he segmen o CCF. The ibe a e he B agg g a ing was scisso s cu ; hus, he inal con igu a ion had an SMF–CCF–FBG s uc u e as i is shown schema ically in Fig. 3(a). The leng h o CCF and λBcanno be a bi a y as a ce ain wa eleng hs, he e lec ion spec um o a supe mode in e e ome e is ze o o minimal (see he inse g aph o Fig. 4). Thus, he maximum o he in e e ence pa e n was made o coincide wi h λBo he B agg g a ing. This equi emen does no add complexi y o he sensing a chi ec u e as he leng h o CCF and he B agg wa eleng h o he g a ing can be con olled wi h accu acy. The e o e, we ab ica ed se e al SMF–CCF–FBG s uc u es wi h hese condi ions, and hei pe o mance o sensing bending and empe a u e was in es iga ed. An SMF–CCF–FBG s uc u e (L=11.60 mm and λB= 1550 nm) was ab ica ed and placed in can ile e -like posi ion on a empe a u e-con olled b eadboa d (model PTC1/M om Tho labs), as shown schema ically in Fig. 3(a). The SMF– CCF–FBG s uc u e was immobilized in he SMF and he CCF–FBG segmen was ben wi h a ansla ion s age. The em- pe a u e s abili y o he b eadboa d was 0.1 ◦C in he 15 ◦C–45 ◦C ange. The CCF–FBG segmen was ben in a plane pa allel o he b eadboa d in he +xand −xdi ec ions acco ding o he coo dina e sys em shown in Fig. 3(a). The o ien a ion o he co es o he CCF wi h espec o he di ec ion o bending was a bi a y because i is no c ucial as epo ed Fig. 6. (a) Re lec ion spec a o an SMF +CCF +FBG s uc u e (L=11.60 mm) when i was ben in he +x-di ec ion a di e en em- pe a u es. (b) Co esponding e lec ion spec a o he same s uc u e when i was ben in he −x-di ec ion. In each empe a u e, he s eps o he bending angles we e as hose desc ibed in Fig. 5. in a p e ious publica ion [23]. The esul s o ou expe imen s a cons an empe a u e (21.5 ◦C) a e shown in Fig. 5. The e lec ion o he SMF–CCF–FBG s uc u e exhibi ed a single peak whose wa eleng h posi ion can be deno ed as λB. No e om Fig. 5 ha he heigh o he e lec ion peak inc eases o dec eases depending on he di ec ion o bending bu i s wa eleng h posi ion is no al e ed. This means ha a cons an empe a u e, only he e lec i i y o he FBG changes in p opo ion o he bending applied o he SMF–CCF–FBG s uc u e. The esul s shown in Fig. 5can be explained as ollows. In a ben CCF wi h asymme ic co e a angemen , he cen al co e and he o -cen e co es expe ience di e en s ess [22], [23],[32]. As a consequence, he p o ile and he p opaga ion cons an s o he in e e ing supe modes change. These changes gi e ise o a shi o he in e e ence pa e n. Hence, he amoun o ligh ha eaches and e lec s om he FBG a e he CCF changes d as ically. As ou CCF and FBGs a e sensi i e o empe a u e, we ca - ied ou bending expe imen s a di e en empe a u es. Ou 36788 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Fig. 7. Calib a ion cu es ob ained om he spec a shown in Fig. 6. The g aph shows he no malized e lec ion o he SMF +CCF +FBG s uc u e as a unc ion o he bending angle a di e en empe a u es. The inse plo shows he wa eleng h posi ion o he FBG as a unc ion o empe a u e. esul s a e summa ized in Fig. 6. The measu ed ange, s eps, and di ec ions o bending a each empe a u e we e he same as ha shown in Fig. 5. The g aphs displayed in Fig. 6(a) and (b) show ha he e lec i i y was well as he posi ion o λB changes. The changes o e lec i i y o he SMF–CCF–FBG s uc u e ela i e o he unben one as a unc ion o he bending angle a e shown in Fig. 7. The bending sensi i i ies in he +x and −xwe e ound o be 0.1326/◦and 0.1382/◦, espec i ely. The igu e also shows he posi ion o λBas a unc ion o empe a u e. The empe a u e sensi i i y was ound o be 8.6 pm/◦C. The esul s shown in Fig. 7sugges ha by moni o ing he e olu ion o a single e lec ion peak o an SMF–CCF–FBG s uc u e, i is possible o know he bending deg ee, he di ec ion o bending, as well as empe a u e. This is possible because he e lec i i y o he s uc u e is no a ec ed by empe a u e. The esul s shown in Fig. 7also sugges ha by moni o ing he e lec i i y o an SMF–CCF–FBG s uc u e, i seems easible o ca y ou empe a u e-independen bending sensing. Based on he ea u es o he SMF–CCF–FBG s uc u es discussed abo e, we in es iga ed hei mul iplexing. To his end, we ab ica ed ou de ices wi h he ollowing leng hs o CCF: 10.8, 11.60, 12.5, and 16 mm. The maximum o he in e e ence pa e ns o such samples (be o e splicing he FBGs) we e loca ed a 1570, 1550, 1535, and 1525 nm, espec- i ely. Thus, ou B agg g a ings wi h λB ha coincided wi h such maxima we e used. The ou samples we e hen placed in pa allel and in e oga ed wi h he con igu a ion shown in Fig. 3(b). We induced bending in wo de ices deno ed as S1 (λB=1525 nm) and S2 (λB=1535 nm) in he +xand −x di ec ions and he o he wo de ices (deno ed as S3 and S4) we e no ben . The expe imen s we e ca ied ou a cons an empe a u e. The esul s o ou expe imen s a e summa ized in Figs. 8and 9.Fig. 8shows ha he e lec i i y o he de ices S1 and S2 ha we e subjec ed o bending inc eased Fig. 8. Re lec ion spec a obse ed when ou de ices we e se in pa allel and in e oga ed, as shown in Fig. 3(b). Bending was applied on senso s S1 and S2 (λB=1524.68 nm and λB=1534.62 nm) and he o he s (S3 and S4) we e no ben . The alues shown in he ame a e bending angles in deg ees. Fig. 9. Re lec ion changes o di e en bending angles obse ed in ou SMF +CCF +FBG s uc u es se in pa allel when bending in he +x- and −x-di ec ions, acco ding o Fig. 1(d), was applied o wo o hem. The e lec ion changes a e ela i e o unben s uc u es. The solid lines a e linea i ings o he da a. The c osses a e e o ba s in all cases. The measu emen s we e ca ied ou a oom empe a u e. o dec eased in p opo ion o he bending deg ee, bu he e lec i i y o he o he de ices (S3 and S4) was unal e ed. No e also ha he wa eleng h posi ions o he e lec ion o he ou samples did no change when wo SMF–CCF–FBG s uc u es we e subjec ed o bending. The calib a ion cu e ob ained wi h he spec a shown in Fig. 8is displayed in Fig. 9. I can be no ed ha one de ice subjec ed o bending has no e ec on he o he s. Thus, i can be concluded ha mul ipoin bending sensing wi h SMF–CCF–FBG s uc u es in e oga ed wi h comme cially a ailable ins umen a ion is s aigh o wa d. The ad an age in his case is ha a con- en ional FBG in e oga o can be used. Mino modi ica ions in he so wa e o such ins umen s can be equi ed o ack simul aneously he wa eleng h posi ion and he ela i e e lec- i i y o na owband and well-de ined e lec ion peaks. CONTRERAS-TERAN e al.: MULTIPOINT TEMPERATURE-INDEPENDENT VECTOR BENDING SENSING WITH CCFs 36789 IV. CONCLUSION In his wo k, we ha e epo ed on a simple sensing pla o m composed by a segmen o a ew millime e s o asymme ic coupled-co e op ical ibe usion spliced o con en ional SMF and an FBG. The sensing pla o m may allow he de elopmen o bending senso s ha can ha e he ollowing p ac ical asse s: capabili y o moni o ing he bending deg ee along wi h empe a u e (o independen o i ) as well as he di ec ion o bending. Mo eo e , mul ipoin bending sensing is easible; addi ionally, he in e oga ion o he senso s can be ca ied ou wi h comme cially a ailable FBG ead ou uni s. The pe o mance o he de ices epo ed he e is due o he co e a angemen o he CCF as well as on he selec ion o i s leng h and he B agg wa eleng h o he g a ings. The op ical p ope ies o a ben CCF make he e lec i i y o a B agg g a ing loca ed a e such ibe change in p opo ion o he bending angle and di ec ion. The compac de ices p oposed he e along wi h he ad an- ages o op ical ibe echnology can make hem a ac i e in se e al applica ions, whe e i is impo an o moni o bending deg ee and di ec ion along wi h empe a u e in mul iple poin s. O he senso s can be de ised wi h ou pla o m. Fo example, senso s o moni o il , inclina ion, ib a ions (pe iodic bend- ing), and impac ( as bending), among o he s, can be easily de eloped. REFERENCES [1] K. Nagano, S. Kawakami, and S. Nishida, “Change o he e ac i e index in an op ical ibe due o ex e nal o ces,” Appl. Op ., ol. 17, no. 13, pp. 2080–2085, 1978. [2] R. T. Sche me , “Mode scalabili y in ben op ical ibe s,” Op . 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Schus e e al., “Ma e ial and echnology ends in ibe op ics,” Ad . Op . Technol., ol. 3, no. 4, pp. 447–468, Aug. 2014. [30] J. Villa o o e al., “Accu a e s ain sensing based on supe -mode in e - e ence in s ongly coupled mul i-co e op ical ib es,” Sci. Rep., ol. 7, no. 1, Jun. 2017, A . no. 4451. [31] J. E. An onio-Lopez, Z. S. Ezna eh, and P. LiKamWa, “Mul ico e ibe senso o high- empe a u e applica ions up o 1000 ◦C,” Op . Le ., ol. 39, no. 15, pp. 4309–4312, 2014. [32] J. A. Flo es-B a o, J. Mad igal, J. Zubia, S. Sales, and J. Villa o o, “Coupled-co e ibe B agg g a ings o low-cos sensing,” Sci. Rep., ol. 12, no. 1, Jan. 2022, A . no. 1280. Ma co A. Con e as-Te an ecei ed he M.Eng. deg ee om he Uni e si y o Guanajua o, Salamanca, Mexico, in 2021, whe e he is cu - en ly pu suing he Ph.D. deg ee wi h he Engi- nee ing Di ision campus I apua o-Salamanca (DICIS). His esea ch ocuses on he de elopmen o in e e ome ic senso s and op ical ibe lase s o applica ions in indus ial, biosensing, and eal-li e scena ios. 36790 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Amaia Be ganza ecei ed he M.Sc. deg ee in elecommunica ions enginee ing and he Ph.D. deg ee om he Uni e si y o he Basque Coun- y UPV/EHU, Bilbao, Spain, in 2004 and 2014, espec i ely. F om ha yea o mid-2007, she wo ked wi h Gamesa Eólica and a GFI No e, Zamudio, Spain, pa icipa ing in di e en enginee ing p ojec s. F om 2007 o 2011, she had a Ph.D. Fellowship a he Applied Pho onics G oup (APG), Depa men o Elec onics and Telecom- munica ions, ETSI o Bilbao. As o 2011, she began o each a he Depa men o Applied Ma hema ics, School o Enginee ing o Bilbao, while con inuing wi h he esea ch wo k a he APG G oup. She has pa icipa ed in nume ous esea ch p ojec s, wo kshops, and na ional and in e na ional con e ences. She has published nume ous esea ch pape s. Besides, she has egula ly aken pa in a ious scien i ic dis- semina ion ac i i ies. He cu en scien i ic in e es s include modeling, simula ion, and cha ac e iza ion o op ical ibe s and hei use in di e en applica ions. Flo ian Lindne ecei ed he Ph.D. deg ee om he F ied ich-Schille Uni e si y o Jena, Jena, Ge many, in 2016, wi h a ocus on he modi ied chemical apo deposi ion (MCVD) echnology. He has been an Enginee o Ma e ial Science wi h he Depa men o Fibe Op ics, Leibniz Ins i u e o Pho onic Technology Jena (IPHT), Jena, Ge many, since 2008. F om 2004 o 2011, he s udied ma e ial science a he F ied ich-Schille Uni e si y Jena. He is engaged in he de elopmen o ma e ials and gas phase deposi ion me hods o he p epa a ion o special y op ical ibe s. His cu en esea ch ocuses on he inco po a ion o RE and o he elemen s in silica glass wi h di e en doping and p epa a ion me hods o he ealiza ion o special ibe s design. Jö g Bie lich ecei ed he Ph.D. deg ee om he Uni e si y o Technology “Be gakademie” F eibe g, F eibe g, Ge many, in 2008, ocused on supe conduc ing ce amic composi es. He wo ked as a G adua e Enginee a he Depa men o Magne ics, Leibniz-IPHT, Jena, Ge many, om 2001 o 2008. Since 2008, he has been wo king a he Depa men o Fibe Op ics, Leibniz-IPHT, in he ield o Fibe D awing Technologies and he De elopmen o Special Op ical Fibe s. Since 2021, he has been engaged in he p epa a ion and cha ac e iza ion o mic os uc u ed ibe s based on high silica and o he glass ma e ials a he Compe ence Cen e o Special y Op ical Fibe s, Leibniz-IPHT. Ka in Wond aczek ecei ed he Ph.D. deg ee om he Claus hal Uni e si y o Technology, Claus hal-Zelle eld, Ge many, in 2005. She has been wo king a he Co ning Eu opean Resea ch Cen e (CEEC) in Fon ainebleau/F ance and A e a NP GmbH, E langen/DE p io joining Leibniz IPHT, Jena, Ge many, since 2012, and s a ing esea ch on op ical ibe ab ica ion. Now, she is he Head o he Wo k G oup “Op ical Fibe Ma e ials and S uc u es” ocusing on he syn hesis o high-pu i y, high-pe o mance ma e ials o special y op ical ibe s. Joel Villa o o (Senio Membe , IEEE) ecei ed he M.Sc. and Ph.D. deg ees in op ics om he INAOE-Ins i u o Nacional de As o ísica, Óp ica y Elec ónica, Puebla, Mexico, in 1995 and 1999, espec i ely. He is cu en ly an Ike basque Resea ch P o- esso wi h he Facul y o Enginee ing o Bilbao, Uni e si y o he Basque Coun y UPV/EHU, Bilbao, Spain. He has nine in e na ional pa en s and mo e han 93 jou nal and 74 con e ence pape s o his c edi . He has been SPIE and an OPTICA ellow, since 2020. His esea ch in e es s include he de el- opmen o ad anced plasmonic and in e e ome ic senso s based on con en ional and special y op ical ibe s, applica ions o such senso s in eal-li e scena ios, and de elopmen o op ical senso s based on nanoma e ials o applica ions in he biomedical, en i onmen , and ene gy sec o s.