Analytical analysis of sensitivity of optical waveguide sensor

Transcription

1 MultCraft Internatonal Journal of Engneerng, Scence and Technology Vol. 3, No. 3, 0, pp INTERNATIONAL JOURNAL OF ENGINEERING, SCIENCE AND TECHNOLOGY 0 MultCraft Lmted. All rghts reserved Analytcal analyss of senstvty of optcal wavegude sensor Alka Verma, Y. Prajapat, S. Ayub, J.P. San, V. Sngh * Department of Electroncs and Communcaton Engneerng, BIET, Jhans, INDIA Department of Physcs, BHU Varanas, INDIA * Correspondng Author: e-mal: yogendrapra@gmal.com, Tel Abstract In ths artcle, we carred out analytcal analyss of senstvty and mode feld of optcal wavegude structure by use of effectve ndex method. Ths structures as predcted have extended mode whch could nteract wth the surroundng analyses n a much better way than the commonly used EWS. Keywords: Sensor, Evanescent wave, Integrated fber sensor, Nano-technology.. Introducton Optcal sensors utlze the modfcaton of measurands to optcal propertes such as ntensty, phase, frequency, and polarzaton of an nput optcal sgnal. Optcal chemcal sensors have mmunty to electromagnetc nterference, have no danger of gnton, and are compatble wth fber networks for use n remote spectroscopy and dstrbuted sensng. Such knds of sensor are useful for hghly senstve analyss and montorng of hazardous envronments and remote sensng (Brecht and Gaugltz, 997. Demands for optcal sensors have tremendously ncreased over the years due to ssues concernng envronmental polluton and other bohazards. In the recent years, optcal sensors have attracted consderable attenton, especally n the applcaton of bochemcal speces detecton. They have excellent advantages such as good compactness and robustness, mmunty to electromagnetc nterference, hgh senstvty, shorter response tme, low cost, and hgh compatblty wth fber optc networks (Passaro et al., 007. However, optcal fber-based systems do not seem promsng wth respect to fabrcaton, effcency and mnaturzaton. However, planar wavegude-based platforms employng evanescent wave sensng technques have shown tremendous mprovement (Burke et al., 006 and evanescent wave sensors have proven to be hghly senstve (Brandenburg et al., 995; Helmers et al., 995; Pandraud et al., 000. Thus, an optcal sensor wth good detecton scheme, senstvty and low cost s needed. The major contrbutons of ths paper are as follows: Evanescent wave sensng (EWS technque for ammona (gaseous detecton utlzng a optcal wavegude structure. Effectve refractve ndex s a crucal parameter for analyzng the senstvty; The rest of ths paper s organzed as follows. Secton provdes the prncple and basc theory of optcal sensors and the modal feld and senstvty, and ts valdaton s presented. In sectons 3, results and dscusson of the optcal sensors and compare t other fber sensors. Fnally, Secton 4 concludes the paper.. Theoretcal Detal Evanescent waves are those waves whch penetrate nto the claddng or the regon where lght s reflected off the surface. In optcal termnology, when total nternal reflecton occurs at an angle greater than the crtcal angle, the snusodal waves reflect off from an nterface, and the waves whch penetrate nto, are called the evanescent waves (Qng et al., 999. Fg. shows the evanescent feld decayng exponentally nto the cover regon. The above mentoned mechansm s used n sensng, when the analytes or speces to be detected are n contact or are made to nteract wth the evanescent feld (whch decay exponentally from the surface of the wavegude, forms the bass for evanescent wave sensng. The operatng wavelength s selected such that the

2 37 Verma et al. / Internatonal Journal of Engneerng, Scence and Technology, Vol. 3, No. 3, 0, pp analytes show peak absorpton, so that there s a change at the output ether n the ntensty or power or any other parameter, when measured usng a detector. Chemcally selectve layer Evanescent Feld W Wavegude Substrate Fg. : Sngle mode wavegude wth Evanescent wave Penetraton. Generally, effectve refractve ndex N eff of a guded mode s consdered to be most crucal physcal parameter. It s because n ths paper for optcal sensor, a chemcally selectve layer or adlayer s deposted on the wavegude surface to bnd the analyte. The evanescent feld senses the changes n the refractve ndex caused by the layer, thus nducng an effectve refractve ndex change (Lukosz et al., 995; Campbell et al., 999. The dffculty faced by most evanescent wave sensor archtectures les n the lack of strong nteracton between the analytes (target speces and the evanescent feld. Ths s prmarly due to the use of strongly confned wavegude desgns (Whaley et al., 007. So we utlze the concept of small optcal mode area structure n core, where a nanoscale core regon s used, whch results n a large and dstended mode. These structures have a very small percentage (usually less than % of the optcal mode confned n the gudng materal. These large modes are suffcent to provde a strong nteracton wth the analytes, as the feld s pushed out of the core regon, enhancng the feld nteracton wth the analytes. All the upper and lower claddng layer are ar, whle core s delectrc or dependng upon applcaton. In ths paper used planer symmetrc wavegude shown n fg.. ar n3.7 d n n w n Fg. : Planer wavegude structure. Bref Revew of Effectve Index for a Symmetrc Planar Wavegude structure: From (Koshba, 973, we understand that a wavegude mode s a transverse feld pattern whose ampltude and polarzaton profle remans constant along the longtudnal drecton. The electrc and magnetc felds of a mode can be wrtten as ( β z ωt E( r, t Em ( x, y e ( β z ω t ( H( r, t H m ( x, y e Where m s the mode ndex, Em( x, y and Hm( x, y are the mode feld patterns and β s the propagaton constant of the mode. Usng the basc Maxwell Equatons, we obtan the TE and TM modes respectvely. Ey + ( k 0 β Ey 0 ( fortemod es ( Hy ε Hy + ( k 0 β Hy ( fortm mod es ε ω where k0 n ( x c A guded mode s one n whch the power s retaned nsde the core as the wave, s perfectly reflected off both the nterfaces. Ths mode can exst only f t satsfes a transverse resonance condton, such that the reflected wave has constructve nterference wth tself. The transverse component (x-component of the wave vector nsde the core s h k 0 cosθ, where θ s the angle of ncdence, k0 π n λ,,,3 and the longtudnal component (z-component β k 0 snθ. Smlarly, the transverse

3 38 Verma et al. / Internatonal Journal of Engneerng, Scence and Technology, Vol. 3, No. 3, 0, pp components for the substrate and cover regons can be defned as h k 0 cosθ, and h 3 k 03 snθ, where k 0, k 0 and k 03 are the propagaton constants n the respectve regons. Applyng boundary condton at n /n and n /n 3 the nterface, and usng the above equaton (, we obtan the egen value equaton for TE and TM mode respectvely. V h d hd mπ tan, 0,,...( mod m forte e hd hd V h mπ n d n hd tan, m 0,,...( fortm mod e The equatons above mentoned are solved usng Matlab, graphcally by plottng left hand sde and rght hand sde, as a functon of (hd. The solutons gve the value of β, and hence the effectve ndex, Neff, could be calculated from the relaton Neff λβ/π.. General Expresson for senstvty: For a planer optcal wavegude consstng of charge free, homogeneous, sotropc meda, senstvty s defned as the rate of change of the modal effectve permttvty, relatve to the cover permttvty, where εn. However, n case of optcal detecton, we are more nterested n the phase of a lght wave than delectrc constant. Hence the sensor senstvty s redefned as the rate of change of effectve ndex relatve to ts cover ndex change, n case of homogeneous sensng and n case of surface sensng, t s relatve to the change n the flm thckness. Usng the varatonal theorem as mentoned n (Dell Olo et al., 007, for delectrc wavegudes, we obtan the equaton of senstvty for quas TE and quas TM modes as the 3 TE Q+ k0ωna ( na a + na a STE 3 N k0ωn a ( na + na + aa ( a+ a followng (3 TM TM a( A A TM STM A + N ( + k0ω a ( a+ a where TE TE TE TE Q 4aa A n 4aa A n + aa aa + aa aa a n N and a N n A TE nc, c c + hk f c s 4 4 f + fn c 4 3 fn c nc c cn ( c + nc TM A, hk0 ( c + f nc ( s + f n s n f ( cn f + fn c c ( sn f + fn s s c n nc, s n ns, f n n, f where k 0 s the vacuum wave number or propagaton constant, n s the effectve ndex obtaned for the structure wth heght h and n s the effectve ndex obtaned for the structure wth heght h, (as shown n the Fgure. 3. Results and Dscussons In order to get senstvty of the above proposed wavegude, we have to solve the equaton (3 numercally. For ths purpose we have chosen the followng parameters: Regon conssts of a core ndex n 3.6 and n n 3, wth a wdth of 00nm, whle regons and 3, have the smlar ndex dstrbuton, but the wdth s 50nm each usng wavelength.55μm. For the structure proposed here, β β 3. After obtanng the three effectve ndces, we treat the structure wth n Neff and n n 3 Neff or Neff3.Now, we apply the above procedure agan to calculate the propagaton constant for the fnal structure, havng a y- dependence on the y-component for the TE lke mode or the electrc feld Ey and the magnetc feld Hy for TM mode n fgure3. Frstly, the three regons are analyzed n the x-drecton (vertcal, the correspondng effectve ndces are obtaned and those

4 39 Verma et al. / Internatonal Journal of Engneerng, Scence and Technology, Vol. 3, No. 3, 0, pp effectve ndces are consdered to get a fnal structure n the y-drecton (horzontal, wth a wdth of 3μm. Ths fnal structure s agan a three slab symmetrc wavegude as mentoned before, but for ths fnal structure, to obtan a TE- lke mode, we consder the TM feld of the vertcal structure; and for TM-lke mode, t s the TE feld of the ntal vertcal structure. The Neff obtaned from the analytcal calculatons are.6856 and.00 for TE mode and TM mode respectvely. In the case of homogeneous Electrc feld of Core Magnetc Magnetc feld feld of of Core Core Ey Hy Thckness of core Thckness of core (a Fg.3. Feld dstrbuton (a Electrc feld (b Magnetc feld sensng, we obtaned the senstvty of TE modes have and TM modes have Ths result shows that TE modes more senstvty to TM modes. 4. Conclusons The purpose of ths paper s to ntroduce small optcal mode area structure n core based on Evanescent wave sensng n Integrated Optcs (IO sensors. Ths structures as predcted have extended mode whch could nteract wth the surroundng analyses n a much better way than the commonly used EWS. The shortcomngs of EWS are overcome by the ntroducton of the small optcal mode area structure technque. The modal analyss of small optcal mode area structure n core structures used n both gaseous sensng s studed. References Brecht A. and Gaugltz G., 997. Recent developments n optcal transducers for chemcal or bochemcal applcatons, Sens. Actuators Vol. B38-39, pp. 7. Brandenburg A., Raner Edelhauser, Tobas Werner, Huaru He, and Otto S. Wolfbes, 995. Ammona Detecton va Integrated Optcal Evanescent Wave Sensors, Mkrochm. Acta., Vol., pp Campbell D.P., Moore J.L., Cobb J.M., Hartman N.F., Schneder B.H. and Venugopal M.G., 999. Optcal system-on-a-chp for chemcal and bochemcal sensng: The chemstry, Chemcal, Bochemcal, and Envronmental Fbre Sensors X, Vol. 3540, pp Conor S. Burke, Ondrej Strank, Helen M. McEvoy, and Bran D. MacCrath, Planar Optcal Sensors and Evanescent Wave Effects, Optcal Chemcal Sensors, pp. 93-5, 006. De-Ku Qng and Ichrou Yamaguch Analyss of the senstvty of optcal wavegude chemcal sensors for TM modes by the group-ndex method Vol. 6, No. 9/September 999/J. Opt. Soc. Am. B.. Dell Olo F., Passaro V. M. N. and De Leonards F., 007. Senstvty analyss of rb wavegudes for ntegrated optcal sensors, /07, IEEE. Hakon Helmers, Perre Greco, Rolf Rustad, Rochd Kherrat, Gerard Bouver, and Perre Benech, Performance of a compact, hybrd optcal evanescent-wave sensor for chemcal and bologcal applcatons, Appled Optcs, vol. 35, no. 4,Feb 996. Koshba, M Optcal wavegude analyss, New York, McGraw-Hll. Lukosz W., 995. Integrated optcal chemcal and drect bochemcal sensors, Sensors and Actuators B, vol. 9, pp Pandraud G., T. M. Koster, C. Gu, M. Djkstra, A. van den Berg, and P. V.Lambeck, Evanescent wave sensng: new features for detecton n small volumes, Sensors and Actuators, vol. 85, pp. 58-6, 000. Passaro V. M. N., Dell Olo F., Casamassma B., and De Leonards F., 007. Guded wave optcal bosensors, Sensors, Vol. 7, pp (b

5 40 Verma et al. / Internatonal Journal of Engneerng, Scence and Technology, Vol. 3, No. 3, 0, pp Whaley R. D. Jr., Kwakernaak M. H., Khalfn V. B., Lpp S. A., Chan W. K., An H, and Abeles J. H., 007. Observaton of low optcal overlap mode propagaton n nanoscale ndum phosphde membrane wavegudes, Appl. Phys. Lett., Vol. 90, pp Bographcal notes Alka Verma s pursung M.Tech n Dgtal Communcaton at BundelKhand Insttute of Engneerng and Technology, Jhans. Areas of nterest are Optcal Communcaton, optcal wavegude sensor and Optoelectroncs. Y. K. Prajapat receved Ph.D. from U.P.Techncal Unversty, Lucknow, Inda n 00. He s a Assstant Professor n the Department of Electroncs and Communcaton Engneerng, BundelKhand Insttute of Engneerng and Technology Jhans, Inda. Hs research nterests nclude Opto-electroncs, Optcal and photonc wavegudes and optcal fber sensors. He has publshed more than seven papers n referred nternatonal journals. He has also presented more than eght research artcles n natonal and nternatonal and conferences. He s also revewer of two Internatonal repute journals. Dr. Shahanaz Ayub s an Assocate Professor n the Electroncs & Communcaton Engneerng Department of Bundelkhand Insttute of Engneerng & Technology, Jhans, UP, Inda. She has twelve years of work experence whch ncludes Industral, Research & Teachng. Her area of research ncludes Bomedcal Sgnal smulaton & analyss, Antennas, Optcal & Photonc Wavegude. She has wdely worked wth Mcrocontrollers, uses artfcal ntellgence technques. She has publshed thrteen research papers n Internatonal journals, presented papers n twenty three natonal & nternatonal conferences. Dr.J. P. San s a Professor n the Department of Electroncs and Communcaton Engneerng, Bundelkhand Insttute of Engneerng and Technology Jhans, Inda. He has more than 5 years of experence n teachng, research and admnstratve work. Hs current area of research ncludes Mult-crtera Decson-Makng, Neural Networks, and Optcal & Photonc wavegudes. He has publshed more than twenty fve papers n referred nternatonal journals. He has also presented more than forty research artcles n natonal and nternatonal conferences. He s currently workng as a Prncpal of Madan Mohan Malavya Engneerng College, Gorakhpur, Inda on deputaton. Dr. V.Sngh He s a Assstant Professor n the Department of appled Physcs, Banaras Hndu Unversty Varanas, Inda.. He has more than 0 years of experence n teachng, research. Hs research nterests nclude Opto-electroncs, Optcal and photonc wavegudes, optcal fber sensors. He has publshed more than ffty fve papers n referred nternatonal journals. He has also presented more than twenty research artcles n natonal and nternatonal and conferences. He s currently dealng wth few projects sponsored by government of Inda. Receved January 0 Accepted March 0 Fnal acceptance n revsed form Aprl 0