Novel Fiber-Optical Refractometric Sensor Employing Hemispherically-Shaped Detection Element

Transcription

1 Novel Fier-Optil Refrtometri Sensor Employing Hemispherilly-Shped Detetion Element SERGEI KHOTIAINTSEV, VLADIMIR SVIRID Deprtment of Eletril Engineering, Fulty of Engineering Ntionl Autonomous University of Mexio Ciudd Universitri,. p , MEXICO, D. F. PIETER L. SWART Fulty of Engineering Rnd Afrikns University Auklnd Prk 2006, Johnnesurg PO Box 524, Auklnd Prk 2006 REPUBLIC OF SOUTH AFRICA Astrt: - We present the theoretil nd experimentl dt on the performne of the novel fier-opti refrtometri sensor tht employs the hemispheril optil detetion element. We exploit the sensitivity of internl refletion of light on the element s spheril surfe to the refrtive index of the surrounding medium, severl internl refletions in series, nd the foussing properties of the onve spheril surfe to hieve the enhned sensitivity nd redue the intrinsi optil loss in the sensor. We show tht the trnsmission funtion of the new sensor n esily e modified in the wide intervl of the refrtive index of the surrounding medium. This new sensor my find pplitions in different instruments. Key-Words:- Optoeletronis, instrumenttion, refrtive index mesurement, liquid level mesurement. 1 Introdution Optil refrtometry is reltively simple nd hep method of ssessing the refrtive index of liquids. Optil refrtometry lso llows the disrimintion etween vrious fluids, the disrimintion etween ir nd fluid, the mesurement of the level of liquids in tnks nd reservoirs, the detetion of fluid leks in tnks nd pipes, nd the detetion of moisture et. The re of optil refrtometry enefitted from the development of vrious fier-optil trnsduers. Typilly, suh trnsduers employ the internl refletion t the trnsduer-fluid interfe, whih is fluid-dependent. This llows ess to the refrtive index of fluids. However, the trnsduers tht re simple in struture usully feture exessive optil loss nd low sensitivity to the refrtive index [1]. On the other hnd, more sophistited strutures, whih re proposed for refrtometri trnsduers hve improved sensitivity ut require omplex frition tehnology [2], [3]. In this work we present novel devie onsisting of hemispherilly-shped glss element integrted with pir of optil fiers. We investigte the role of the devie s geometril prmeters nd mteril onstnts on the trnsduer trnsmission funtion nd its non-linerity. In ddition, we investigte the possiility of otining oth the liner nd the inrytype hrteristis for the sme hemispheril optil element y hnging the distne etween the optil fiers, nd y hnging some of their optil nd geometril prmeters. The proposed refrtometri trnsduer is shown in its generi form in Fig. 1. The hemispheril optil element 1 or the rdius R is integrted with pir of optil fiers 2 nd 3 with ore dimeter D. These fiers serve s the optil outlets of the refrtometri trnsduer. They onnet the element with the remote light soure nd light intensity meter (not shown in Fig. 1). The fier position with respet to the element

2 is seleted in suh wy tht the fiers re optilly 1 2 I 1 Fluid ( n) D Glss ( n e ) L O Z R Fig. 1. Refrtometri trnsduer onsisting of the hemispheril glss element (1) integrted with the trnsmitting (2) nd reeiving (3) fier. oupled together vi the totl internl refletion t the element s hemispheril surfe when the element is in the ir. The oupling is, however, sensitive to the refrtive index of the surrounding medium. In this trnsduer, the rnge of ngles of inidene of the rys of the input optil em on the onve spheril surfe is smller thn in the se of flt or onil surfe. This permits the seuring of the qusi-optimum ngle of inidene (slightly lrger thn the ritil one) for the mjor prt of the rys of the optil em nd, hene, the inresed sensitivity to the refrtive index of the surrounding medium. In ddition, the foussing properties of the onve spheril surfe llow for etter optil oupling nd redued intrinsi optil loss in the trnsduer. 2 Prolem Formultion We now onsider the hemispheril element of the trnsprent dieletri mteril with the refrtive index n e equl to tht of the fier ore. (Unless otherwise stted, we ssume n e =1.45.) Two multimode fiers hve identil prmeters: ore dimeter D nd numeril perture in the ir NA. These fiers re t right ngles to the element s flt surfe nd re pled symmetrilly with respet to the ommon system xis Z t the distne L from the xis. (In the following, we use the dimensionless geometril prmeters: d=d/r nd l=l/r.) For the nlysis of the trnsduer properties, we introdue the reltive trnsmission funtion T*(n) = [I 2 (n)/i 1 ]/[I 2ir / I 1 ] = I 2 (n)/i 2ir (1) L I 2 D 3 X where I 1 is the intensity of the input light (i.e. the light lunhed into the element from the trnsmitting fier), I 2 (n) is the light intensity t the trnsduer output (i.e. the light epted y the reeiving fier: this quntity is funtion of the refrtive index of the surrounding medium), nd I 2ir is the light intensity t the trnsduer output when the surrounding medium is ir. 3 Prolem Solution We ssessed the trnsduer s reltive trnsmission funtion T*(n) through numeril ry tring employing the three-dimensionl mthemtil model of the element. The hemispheril surfe of the element ws represented y the nlytil eqution of the sphere. The model ounted for the fier ore dimeter, em divergene nd fier distne from the system xis. We ssumed uniform light intensity distriution ross the em, monohromti, nonpolrised nd non-oherent light. We employed elementry rys for the modeling of the em. The ry s strting point nd the ngle to the fier xis were ssigned t rndom y employing the Monte- Crlo method. The lgorithm ounted for severl seril internl refletions from the element s hemispheril surfe. The Freznel refletion oeffiient ws lulted t eh refletion point nd the resulting intensity ws determined for eh ry. The element trnsmission T ws found y integrting the ontriutions of ll the rys, whih rehed the reeiving fier ore within the limits of its liner nd ngulr perture. The prmeters were: the dimensionless fier distne from the element xis l, the dimensionless fier ore dimeter d, the refrtive index of the element mteril n e, nd the fier numeril perture in the ir NA. (In the ry tring, we employed the em divergene in the fier insted of the NA.) The trnsmission T ws lulted for vrious n vlues rnging from n=1 to n=1.44, nd then the trnsmission funtion T(n) ws determined. It ws found tht the form of the trnsmission funtion depended on the trnsduer geometry, the optil fire prmeters nd the refrtive index of the element. The intrinsi optil loss ws typilly within the rnge of 2-9 db, providing the fire position llowed for two or more seril internl refletions of the light from the hemispheril surfe. The lulted reltive trnsmission funtion T*(n) is presented in Figures 2-4. The digrms shown in Fig. 2 demonstrte the possiility of modifying the trnsduer sensitivity y hnging the optil fier ore dimeter d. Curve () hs reltively lrge nd steep qusi-liner prt tht is suitle for the

3 refrtive index mesurement in the nlogue form. In ontrst to this, urve () represents inry-type hrteristi tht is suitle for the disrimintion etween two fluids with different refrtive index. Reltive trnsmission T* d = 1 d = 2 d = Refrtive index n Fig. 2. Trnsduer reltive trnsmission T* vs. the refrtive index of the surrounding medium n under NA =, l = 7, n e= 1.55, d = 1 ( ), d = 2 ( ), d = 5 ( ). The digrms shown in Fig. 3 demonstrte the possiility of hnging the position of the trnsmission hrteristi on the n-xis over wide rnge. This hnge is hieved y hnging one single geometril prmeter - position l of the optil fiers with respet to the element s xis. The digrms showing the influene of the refrtive index of element mteril n e on the form of funtion T*(n) re presented in Fig. 4. In ll three ses,, nd, funtion T*(n) is qusi-liner in the lrge intervl of n. A hnge in the refrtive index of the element mteril results in displement of the qusi-liner prt of funtion T*(n) long the n-xis. The mount of the shift is diretly relted to n e. Thus, it is possile to shift funtion T*(n) smoothly long the n-xis through the proper seletion of the refrtive index of the element mteril. In the experiment, we employed reltively lrge hemispheril optil element (with rdius R=30 mm nd refrtive index n=1,47), nd two identil optil fier undles of ore dimeter 2 mm nd numeril perture NA of We used field stops to hieve light ems of smller dimeter nd redued numeril perture. We ssessed the trnsmission hrteristi s funtion of devie prmeter l for severl liquid sustnes with different refrtive index in the Reltive trnsmission T* Reltive trnsmission T* l = 0.71 l = 7 l = Refrtive index n Fig. 3. Trnsduer reltive trnsmission T* vs. the refrtive indexof the surrounding medium n under NA =, d = 5, n e = 1.45, l = 0.71 ( ), l = 7 ( ), l = 0.92 ( ) Refrtive index n n e= 1.53 n e= 1.55 n e= 1.57 Fig. 4. Trnsduer reltive trnsmission T* vs. the refrtive index of the surrounding medium n under NA =, d= 5, l = 7, ne = 1.53 ( ), n = 1.55 ( ), n = 1.57 ( ). e e intervl n=1.328 (methnol) to n=1,470 (glyerine). The experimentl dt show good orreltion with the results otined through the numeril nlysis. The experimentl trnsmission urve is similr in form to the predited one. Also, the trnsduer sensitivity to the refrtive index of the surrounding medium oinides with the predited sensitivity. The proposed optil refrtometri trnsduer hs mny potentil pplitions in optoeletroni sensors of different quntities. One of the pplitions is 1.4

4 illustrted in Fig 5 where lrge rry of optil refrtometri trnsduers is employed for liquid the speifitions of the most demnding industril users. Fig 5. Shemti digrm of the disrete liquid level sensor employing the hemispherilly-shped optil refrtometri trnsduers. level mesurement. The trnsduer rry is interrogted y mens of speil omined sptil nd time-division multiplexing sheme operting in the level-trking mode. In this sheme, the interrogtion of ny prtiulr trnsduer is possile under seletive tivtion of the orresponding optoeletroni soure-detetor pir. The eletroni iruitry of the multiplexed rry orresponds in struture to the speil level-trking opertionl lgorithm of the sensor. This disrete-type sensor fetures lrge mesurement spn, high ury nd fst response time. Also, it n operte in wide temperture rnge. Due to the optil priniple of mesurement, the sensor rry is sprk nd explosion proof. Beuse of these fetures, this sensor n meet 4 Conlusions The proposed hemispherilly-shped trnsprent dieletri element, s desried in this work, n serve s n optil mplitude-type sensor for the refrtive index of the surrounding medium. It employs the liquid-dependent internl refletion of light on the spheril surfe of the element tht is in ontt with the surrounding medium of interest. In the hemispherilly-shped element, we employ two or more internl refletions in series. This sensor n operte in wide rnge of the refrtive index of the surrounding medium. It hs redued intrinsi optil loss (from 2 db to 9 db, typilly), moderte non-linerity (3-8%, depending on the output spn) nd severl other useful properties. In prtiulr, the form of the trnsduer trnsmission hrteristi, the input spn, nd the opertionl rnge n e modified y hnging some of the geometril prmeters or mteril onstnts of the devie. These inlude the dimeter of the optil fier ore, the numeril perture of the optil fier, the fier position with respet to the element s xis, nd the refrtive index of the element mteril. We demonstrte the possiility of otining the trnsduer trnsmission hrteristi of two different types: the liner nd the inry. The liner hrteristi is the most suitle one in the se of refrtive index mesurement. In ontrst to this, the inry-type hrteristi is preferle in the se of liquid detetion nd disrimintion etween two fluids, espeilly in situtions where the differene etween the refrtive index of two fluids is smll. The numeril experiment shows tht oth the liner nd the inry-type hrteristis n e otined for the sme hemispheril optil element y hnging the distne etween the optil fiers, nd y hnging some of their optil nd geometril prmeters. Chnging some geometril prmeters of the devie n shift the trnsmission hrteristis of either of the types mentioned ove long the n- xis in wide rnge. It is very importnt tht these modifitions do not result in ny signifint inrese in the devie intrinsi optil loss nd the nonlinerity of its trnsmission hrteristi. The possiility of simple modifition of the trnsduer trnsmission hrteristi llows the diverse speifitions of vrious users to e met. Hemispheril optil elements re esy to frite in lrge quntities. These properties mke them very

5 promising for vrious optil refrtometri sensors. The potentil pplitions inlude the refrtometry of liquid medi, fier-optil detetors of fluids, nd disrete multi-point liquid level guges. This work ws prtilly supported y the CONACyT (Mexio) under the Reserh Projet A, the Ntionl Autonomous University of Mexio under the Reserh Projet PAPIIT / DGPA IN113799, nd the Centre of Optil Communitions, Rnd Afrikns University, Repuli of South Afri. Referenes: [1] V. Svirid nd S. Khotiintsev, Primry trnsduers of disrete fier-opti level guges, Mesurement Tehniques, Vol.33, No.7, 1990, pp [2] V. Svirid, V. De Leon, nd S. Khotiintsev, A prototype fier-opti disrete level-sensor for liquid propne - utne, IEICE Trnstions on Eletronis, Vol.E83-C, No.3, 2000, pp [3] V. De Leon nd S. Khotiintsev, Mirooptil proloidl - shped sensing elements for refrtometri pplitions, Instrumenttion nd Development, Vol.4, No.1, 1999, pp