Design of Tunable Multiple- Cavity Filter for Optical Fiber Communication

Transcription

1 ACSIJ Advances n Computer Scence: an Internatonal Journal, Vol., Issue 3, No. 4, July 13 ISSN : Desgn of Tunable Multple- Cavty Flter for Optcal Fber Communcaton Vahdeh Khadem Hossen Ghasem 1, Mohammad Orvatna, Asghar Ebrahm 3 1 Department of Electrcal Engneerng,, Hgh Educaton College, Islamc Azad Unversty Boushehr Branch, Boushehr, Iran vahdeh_khademhossen@yahoo.com Telecommuncaton College /Mnstry Informaton and communcaton Technology(I.C.T), Tehran, Iran 3 Malek Ashtar Unversty of Technology, Tehran,Iran ebrahm@mut.ac.r Abstract In ths paper a new type of cavty flter s nvestgated. The devce s based on a Fabry Perot nterferometer whch employng a cavty and two delectrc Bragg mrrors. The cavty flter uses mult layers of ( SO ) and (ZnS) to produce a mult - layers ZnS/ SO cavty flter operatng n the center wavelength of 155 (nm). Znc sulfde s used n ths paper due to ts hgh refractve ndex, whle SO due to ts low refractve ndex materal. The flter s to be coated on Fused Slca havng the ndex of 1.55 and operates at normal ncdence. The ZnS/ SO thn flm structure was desgned usng the Thn Flm Desgn software (TF Calc). The smulaton results show that the transmttance of the ZnS/ SO flter s about 95% for central wavelength of 155 nm. Ths flter s essental for montorng and reconfgurng optcal networks. Keywords: Multple cavty flter, optcal communcaton, thn flm layer. 1. Introducton Modern optcal communcaton systems reure ever ncreasng transmsson capacty, whch s nowadays provded by wavelength dvson multplexng (WDM). Instead of nstallng new fber lnks, addtonal ndvdually modulated optcal channels are added to already establshed transmsson systems. These operate at fxed optcal freuences proposed by the Internatonal Telecommuncaton Unon (ITU). The typcal channel spacng s, 1 or even 5 GHz, whch corresponds to wavelength steps of 1.6,.8 or.4 nm respectvely. Wth ncreasng channel number, hgh ualty tunable devces become of great mportance for flexble network management. As a result, tunable flters represent key components n a large number of optcal subsystems, such as wavelength selectve add drop multplexers, optcal channel montors and tunable lasers. Non-tunable hgh performance multlayer desgns are prmarly used for fabrcaton of optcal flters. Several concepts exst for realzaton of tunable nterference flters, whch make use of acousto-optc effects [1, ] or are based on ndex tunng wthn lud crystals [3]. Dfferent approaches use fbers wth optcal multlayers deposted on the fber end or gratngs wrtten nto a fber [4 6]. Mcromechancally tunable devces feature dstrbuted Bragg reflectors (DBR) n connecton wth a sngle ar cavty of varable thckness. Fabrcaton of these structures s mostly based on III V semconductors [7 11] whereby tunng s acheved by varyng the dstance between the mrrors.. Optcal flter prncple and structure The present optcal flter s based on a mcro cavty, whch conssts of two coplanar reflectors separated by a sold state materal. Ths structure resembles a classcal Fabry Perot cavty usng dstrbuted Bragg reflectors and s strongly remnscent of a vertcal cavty laser, as shown n fgure Copyrght (c) 13 Advances n Computer Scence: an Internatonal Journal. All Rghts Reserved.

2 ACSIJ Advances n Computer Scence: an Internatonal Journal, Vol., Issue 3, No. 4, July 13 ISSN : Fg 1. The flter structure consstng of a sold-state resonant cavty surrounded by two dstrbuted Bragg reflectors (DBR). Each reflector conssts of λ/4 thck layer pars of hgh and low ndex materals. The structure s transparent for a wavelength defned by the resonance condton wthn the cavty, whle the other part of the spectrum gets reflected The thckness of the central cavty regon s of the order of the wavelength of lght. Plane waves propagatng nsde the cavty can nterfere constructvely and destructvely such that only the resonant wavelengths are transmtted. As mentoned above, a par of Bragg mrrors defnes the resonator; each of these conssts of N pars of two materals wth dfferent refractve ndces. Each layer has an optcal thckness of λ/4, where λ s the Bragg wavelength for whch maxmum mrror reflectance occurs. Maxmum reflectance ncreases wth N and wth the dfference n refractve ndex. The shft n transmsson wavelength s only determned by changes of the optcal thckness of the cavty layer.[1-13] 3. OPTICAL INTERFACES Ths secton ntroduces the bass of the electromagnetc theory and ts applcaton n the calculaton of the behavor of optcal flters based n Fabry-Perot resonant cavtes. 3.1 Prncples of the propagaton of the electromagnetc waves The lght s an electromagnetc wave, and the study of ts propagaton must start wth the Maxwell euatons. Wth the objectve to become ths study not very extensve, t wll be started wth some results of the Maxwell euatons that are vald for lnear meda. A more detaled and rgorous approach of the electromagnetc feld euatons can be found n [14]. 3. Ampltude of the electromagnetc waves An electromagnetc wave can be represented by: j( wt n. k. r) E Ee (1) j( wt n. k. r) H H e where E and H are respectvely ts electrc and magnetc felds, ω s the optcal angular freuency, K s the wave number (π /λ ) and n s the refractve ndex of the propagaton medum. If the propagaton medum s absorbent, the refractve ndex must be substtuted by n jk, where k s the coeffcent of extncton of the medum and j s the complex operator. For a gven wave, the relatonshp between the electrc and magnetc felds s fxed and, n an sotropc medum, s gven by: H ne () where ε and μ are the absolute delectrc and the nducton constants of the medum, respectvely [15], r r 1/( c ) NA where r and r are the relatve delectrc and the nducton constants of the medum, and are the delectrc and the nducton constants of the free space and c s the speed of the lght n vacuum. In an ansotropc meda the euaton ()becomes more complex, snce and are vectors nstead of scalar uanttes [16, 17]. The drectons of the vectors E and H are also related, formng an angle of / rad between them and wth the drecton of propagaton. The drecton of the propagaton s gven by the rule of the rght hand. 3.3 Polarzaton The lght that travels n a gven drecton can have two ndependent polarzatons. The lght ncdent n a surface, formng an angle wth ts normal, can have ether the electrc feld vector or the magnetc feld vector parallel to the plane of ncdence. In the frst case, the polarzaton s p and n the second, the polarzaton s s. In a general way, the electrc feld vector forms an angle wth the plane of ncdence. In ths n case, t can be decomposed n two components, one of polarzaton p and other of polarzaton s, beng: E P E cos cos for P polarzaton E s E sn for S polarzaton In a smlar way, for the magnetc feld vector: H P H cos H S H sn cos The normal refractve ndex n s eual to H/E. In a smlar way, a generalzed refractve ndex u can be defned for each of the polarzatons, such that: u p H P / EP n / cos us H s / Es H cos / E n cos Notce that all the polarzaton states can be decomposed n the p and s components, possbly wth a phase shft between them. Ths turns the followng analyss useful for 49 Copyrght (c) 13 Advances n Computer Scence: an Internatonal Journal. All Rghts Reserved.

3 ACSIJ Advances n Computer Scence: an Internatonal Journal, Vol., Issue 3, No. 4, July 13 ISSN : all the polarzaton states. In a general way, any optc nterface has dfferent propertes for the two polarzatons, except when the ncdent lght s normal to the surface. In ths case, the polarzatons p and s are euvalent. 3.4 Boundary condtons The boundary condtons n each nterface between two dfferent optcal meda derve from the electromagnetc theory and allow establshng the relatonshp between the electromagnetc felds on one sde of the nterface wth the ones on the other sde. These condtons state that the components of E and H parallel to the nterface must have the same value n both sdes. 3.5 Propertes of a thn flm stack In Fgure. a set of thn flms wth nterfaces,. e., 1 flms s shown. 3.6 Optcal path The phase thckness of the flm n the case of the lght wth normal ncdence s gven by: u d g where d s the thckness of the flm and the wavelength of the lght n the free space. The phase thckness represents the varaton n phase that the lght suffers when crosses the flm [19]. In the most general case, where the lght arrves wth an angle, the expresson of the phase thckness s: u d cos g where the angles are determned from the Snell law: u sn u sn u sn The varable u wll be u p n the polarzaton p and u s n the polarzaton s. Notce that, n the case of the flm absorbng lght, u s a complex uantty, what makes a complex number too. 3.7 Euatons n the nterface between two flms Fgure. Thn flm stack. The flm has a wave travelng to the rght, E, and another one travelng to the left, E. They make an angle to the normal of the nterface. The flm has a refractve ndex n and a thckness d (not represented n the fgure). Flm (at the rght sde) s the ext medum and the flm (at the left sde) s the ncdence medum. Usually, the ncdence as well as the ext meda of a flm stack s consttuted by ar. Assumng that the stack of thn flms forms an optcal flter, the beam of ncdent lght s E, the reflected one s E and the transmtted one s E. E s eual to zero, once t s consdered the case n whch there s only one lght source on the left sde of the flter. In the case that several lght sources exst, each one can be analyzed separately, once t s estmated that the optcal medum s lnear [18]. From here, only the values of the electrc felds E, E and E are mportant. All the other felds have nterest only for the attanment of the three last ones. Frst, t s assumed that the phase of the waves s zero n the nterface between the flms and 1. In other words, the phase s zero at the rght sde of the flm. In the nterface between the flm + 1 and, at the left sde of the nterface the felds E1 and E1 are present, and at the jg rght sde are present the felds E jg e and E e, jg jg where e and e represent the phase shfts suffered by the electromagnetc waves along the flm. Usng the boundary condtons descrbed prevously, t comes to the nterface jg E 1 E 1 E jg e + E e (3) It s convenent to defne now: E E 1 E 1 H and H H where E and H are the total felds n the nterface between the flms and 1. Knowng that H ue, then: 1 E ( E H / u ) (4) 1 E ( E H / u ) 5 Copyrght (c) 13 Advances n Computer Scence: an Internatonal Journal. All Rghts Reserved.

4 ACSIJ Advances n Computer Scence: an Internatonal Journal, Vol., Issue 3, No. 4, July 13 ISSN : Then the euaton (3) becomes j E 1 cos ge sn gh (5) u In a smlar way, the followng value for the magnetc feld n the nterface + 1, s obtaned: H 1 ju sn ge cos gh (6) The euatons (5) and (6) can be wrtten n a matrx form E 1 cos g jsn g / u E H 1 ju sn g cos g H Or E 1 E M H 1 H where the matrx M s gven by cos g j sn g / u M (7) ju sn g cos g The matrx M of elements contans all the detals of the flm and relates the values of E and H of one sde of the flm wth the ones of the other sde. It can also be wrtten: In a general way the values of the electrc and magnetc feld n the ext medum, can be related wth the ones n the ncdence medum through the euaton: E E M 1 M... M M1 (8) H H In ths case, t was consdered that g =, reducng M to the dentty matrx. 3.8 Transmsson and absorpton coeffcents Now, t s necessary to calculate the values of the electrc and magnetc felds of the ncdent, reflected and transmtted waves. To do that, t s necessary to go back to the euaton (4)that, wrtten n the matrx form, becomes: E 1 1 1/ u E (9) E u 1 1/ H And E 1 E (1) H u as E Combnng the euatons (8), (9) and (1), comes E E / u 1/ u M 1... M 1 1 E u (11) From euaton (11), the transmsson and reflecton coeffcents are: r E / E t E / E and for the reflectvty and transmssvty comes respectvely u R r E / E u T t E / E (1) u u 3.9 Fabry-Perot optcal flter The Fabry-Perot optcal flter s an optcal nterface consttuted by two flms of hgh refractve ndex placed at both sdes of a flm of low refractve ndex, as t s shown n Fgure. 3. Fg. 3. Fabry-Perot optcal flter. In ths case, the ncdence and the ext meda are consttuted by ar, whose refractve ndex s approxmately eual to 1. As u = u = 1, Euaton (11) becomes: E M H M LM H E (13) E 1 1 where M H and M L represent the flm matrxes of the hgh and low refractve ndex flms respectvely, and are gven by euaton (7). The transmssvty of the flter s then calculated usng euaton (1). By analyzng euatons (7) and (13), t s possble to conclude that the transmssvty of the Fabry-Perot optcal flter depends, among other parameters, on the thckness of the lowrefractve ndex flm. Actually, the resonant condton s acheved when the phase change n the low refractve ndex flm s null,. e. cos = 1 gl. Ths condton s acheved makng g l = k ul, k=, 1,, Therefore, n a Fabry-Perot optcal flter the thckness of the resonance cavty determnes the tuned wavelength. 4. The choce of materals The development of optcal flters for dfferent wavelengths n s mportant for many communcaton 51 Copyrght (c) 13 Advances n Computer Scence: an Internatonal Journal. All Rghts Reserved.

5 ACSIJ Advances n Computer Scence: an Internatonal Journal, Vol., Issue 3, No. 4, July 13 ISSN : nstruments. Currently avalable flters are based on nterference n multlayer stacks (so-called multlayer nterference flters). The materal layer s reured manly for multlayer system have hgh transmttance n []. Takng nto account that thn flms n multlayer nterference coatngs must be dense and have low absorpton n the wavelength range defned for the ntended applcaton [1]. The arrangement of materal on two faces of substrate must be accordng to the value of refractve ndex. Frst materal have hgher refractve ndex than second materal as conseuently the lght wll be suffer many of refracton as Snell s law to obtan the desred wavelength as shown n fgure 4 these wavelengths the flter s allowed to transmt nearly zero as n fgure 5. The flter s to be coated on Fused Slca havng ndex The flter operates at normal ncdence. The most common structure for narrow bandpass flters (mult-cavty band-pass flters) s an optcal thn flm flter consstng of a uarter-wave optcal thck layers for the mrrors and half-wave optcal thck, or multple half-wave optcal thck layers for the spacers. So that the open flter program can be used to desgn ths flter, we use ZnS and SO as the two coatng materals. The layers structures of narrow band pass flter for two desgns can be see below. The characterstcs transmsson vs wavelength clearly seen from fgure 5 whch shows the center to center spacng of the channels n wavelength unts. Fg4:. Reflecton, Transmsson and layers arrangement When the dfference n optcal path length between the rays transmtted at successve reflectons s such that the emergng waves are n the same phase, constructve nterference wll occur and flter wll show the maxmum transmttance value. If ths condton does not hold, the nterference between successve emergng rays wll be destructve and the transmsson wll be relatvely low []. Ths flter desgn consst of two materal. ZnS s hgh ndex layer and SO s low ndex layer. The wavelength range from 1nm to 18nm. The flter s to be coated on Fused Slca havng ndex 1.55 and operates at normal ncdence. 5. Results and dscusson In the present work, these theoretcal desgns have been suggested and ther profles have been fully studed for the vsble regon and near IR regon usng open flter software [3]. The technology of DWDM s one of the most recent and mportant technue n the development of fber optc communcaton technology. The band-pass flter used for detectng lght at range wavelength 1nm-18nm as n desgn below out of Fg 5.Transmton vs wavelength for Proposed optcal Flter 6. Concluson In ths paper, the desgn of an optcal flter at a wavelength of 155 nm s consdered. Ths flter s useful for telecommuncatons ndustry to control the transmsson through fber optc cables. to do ths, at frst the structures of optcal flter s studes. factors nfluencng transmsson spectrum of flter s nvestgated. These factors are thckness of layer and ndex materal of layer. Proposed flter was smulated by usng TFCalc that ndcate transfer coeffcent above 95% n desred spectrum.n comparng wth Ge/MgF flter advantage of desgned flter than prevous flters s determned.havng more transfer coeffcent and havng flat and rectangular shape n t's transmsson spectrum are among t's advantage.ths flter cheaper than Ge/MgF flter because ZnS layer s cheaper than Ge layer.number of layer`s flter and general thckness of flter s lower than Ge/MgF flter. also adjustable power of proposed flter s one of mportant advantage. when the number and thckness of dfferent layer has changed, flter can be adjusted propertes to the desred bandwdth. 5 Copyrght (c) 13 Advances n Computer Scence: an Internatonal Journal. All Rghts Reserved.

6 ACSIJ Advances n Computer Scence: an Internatonal Journal, Vol., Issue 3, No. 4, July 13 ISSN : References [1] Saprel, J., Charssoux, D., Voloshnov, V., & Molchanov, V. ().Tunable acoustooptc flters and eualzers for WDM applcatons. Journal of lghtwave technology, (5), 864. [] Enguang Da ; Demng Wu ; Ansh Xu, (1999),Integrated acoustooptc tunable flter, eualzer, and swtch n WDM communcaton system,proc. SPIE Volume [3] Sneh.A and Johnson K. M., (1996). Hgh-speed contnuously tunable lud crystal flter for WDM networks. Journal of Lghtwave technology, 14, [4] Btteberre J and Lazardès B (1). Bcorn flters wth strong and broad rejecton for sngle-mode wavegudes. Appl Opt 4 (8): [5] Ortega B, Pastor D, Capmany J and Ibsen N (1998).WDM grd tunable flter based on a sampled fbre gratng and a FFP Proc. 4th Eur. Conf. on Optcal Communcaton Telefonca vol 1, 91 [6] Lampersk. J, (). Dscretely tunablemult cavty FFP flter for standardwdm freuency grd Proc. 5th Electronc Components and Technology Conf. pp 157 [7] Le Dantec R., (1999). Hghly tunable and selectve Fabry Perot flter based on InP-ar Bragg mrrors for W.D.M. applcatons, SPIE Proc. 363,, [8], Azz M., ().A new and smple concept of tunable two-chp mcrocavtes for flter applcatons n WDM systems, IEEE Photoncs Technolgy. Lett. 1, 11,, [9] Hubert Halbrtter ; Mchael Azz ; Frank Remenschneder ; Peter M. (3). Mcromachned two-chp, low-cost tunable flters for WDM Messner Volume 4945(3) [1]. M. Strassner, C. Luber, A. Tarraf, N. Chtca, (). tunableconstant bandwdth monolthc Fabry-Perot flter wth a stable cavty desgn for WDM systems, IEEE Photon. Tech. Lett., Vol: 14, no. 11, pp ,. [11] J. Daleden, V. Rangelov, S. Irmer, F. RÃmer, M. Strassner, C. Prott, A. Tarraf, H. Hllmer, () Record tunng range of InP-based multple ar-gap MOEMS flter, Electronc Letters, 38(1), [1] Domash L H. (3), Broadly Tunabel Thn Flm Interference Coatngs: Actve Thn Flm for Telecom Applcatons, Proceedngs of SPIE, vol. 4989, [13]. Iodce, M.; Cocorullo, G.; Della Corte, F.G.; Rendna, I. (). Slcon Fabry Perot flter for WDM systems channels montorng. Optcs Communcatons vol. 183 ssue 5-6, [14] M. Born and E. Wolf, (1999); electromagnetc theory of propagaton, nterference and drecton of lght, Prncples of optcs, Pergamon, Oxford, 9 th edton [15] H. A. Macleod, (1) Thn-Flm Optcal Flters, Insttute of Physcs Publshng, Brstol and Phladelpha, 3rd edton,. [16]. P. Yeh, (1988), Optcal Waves n Layered Meda,Wley, New York [17] I. J. Hodgknson and Q. H. Wu, (1997),Brefrngent Thn Flms and Polarzng Elements, World Scentfc, Sngapore,. [18] D. W. de Lma Montero, G. Vdovn, J. G. Rocha, V. P. Iordanov, M. Y.Loktev and P. M. Sarro, (),Customzed CMOS wavefront sensor, In Proc.SPIE, vol. 4493, 88-99,. [19] D.P. Poenar, (1996) Thn flm color sensors, Ph.D. Thess, Delft Unversty. Press The Netherlands. [] Marc Chrstophersen, Vladmr Kochergn and Phlp R. Swnehart, (4). Porous slcon flters for md-to-far IR range, Proc. of SPIE Vol [1] Xuanje Lu, Xun Ca, Jnshuo Qao, Jfang Mao and Nng Jang, (3) The desgn of ZnS/Ag/ZnS transparent conductve multlayer flms, Thn Sold Flms 441, p 6,. [] G. P`erez, A.M. Bernal-Olva, E. M aruez, J.M. Gonza lez-leal, C. Morant, I. Ge nova, J.F. Trgo and J.M. Sanz, (5). Optcal and structural characterzaton of sngle and multlayer germanum/slcon monoxde systems, Thn Sold Flms 485, p [3] H A Macleod, (1999), Thn-Flm Optcal Flters. London, U.K. Insttute of Physcs Publshng. Vahdeh Khadem Hossen Ghasem receved BSc degree n electroncs engneerng from Islamc Azad Unversty Naeen Branch, n 7, and MS degree n electroncs engneerng from Islamc Azad Unversty Boushehr Branch, n 13. Her current research s n feld of thn flm flter. Mohammad Orvatna receved BSc degree n electroncs engneerng from Isfahan Unversty of Technology, n 1993, and MS degree n electroncs engneerng from Shraz Unversty, n 1996, and PhD degrees n electroncs engneerng from KNT Unversty of Technology, n 3. He s assstant professor n faculty of appled Scence of Informaton and Communcaton Technology (ICT) n Mnstry of ICT, Iran now. Hs current research s n feld of semconductor sensors. Asghar Ebrahm receved BSc degree n physcs from Tehran Unversty, n 1989, and MS degree n physcs from Tehran Unversty, n 1991, and PhD degrees n Mechancal engneerng from Russa Unversty, n1997. He s head of space ndustral n IRAN(SAIRAN). Hs current research s n feld of desgn of space craft. 53 Copyrght (c) 13 Advances n Computer Scence: an Internatonal Journal. All Rghts Reserved.