OPTICAL SWITCH USING TOTAL REFLECTION IN LIQUID CRYSTALS

Transcription

1 7 th teratial Cferece DEVELOPMENT AND APPLCATON SYSTEMS S u c e a v a, m a i a, M a 7 9, OPTCAL SWTCH USNG TOTAL EFLECTON N LQUD CYSTALS Serge DOVGALETS 1, Yulia SEMENOVA, Vasl PYSYAZHNYUK 1 1) Departmet f Autmatics & frmati-measurig Egieerig, Viitsa Natial Techical Uiversit, Khmelitske Shsse 95, 101 Viitsa, Ukraie dvgalets@vstu.viica.ua ) Schl f Electric & Cmmuicatis Egieerig, Dubli stitute f Techlg, Kevi Street, Dubli 8, relad Abstract. Optical switch usig electr-ptical effect i liquid crstals is prpsed ad ivestigated fr icrprati i a switch matri fr ptical etwrks. This device empl ttal reflecti at the brder betwee glass ad ematic liquid crstal. Prttpe f the switch has bee desiged ad its parameters have bee ivestigated. itial results shw the switchig ctrast rati f 4 db, respectivel, with iserti lss abut.9 db fr plaried light. Kewrds: All-ptical matri switch, liquid crstal, ttal reflecti. trducti All-ptical switchig fabrics will be a sigificat cmpet i rder t relieve the capacit limitati f electric-switched etwrks. These devices allw t switch the traffic directl i the ptical dmai, avidig the eed f several ptical electrical cversis. Several techlgies have bee prpsed as cadidates fr ptical switchig. The mst cmm switchig techlg is based ptmechaical switches, where the path f light is switched b sme frm f mechaical mvemet f either fibers r mirrrs. Such switches feature gd scalabilit, hwever, this apprach ivlves mvig parts, ad tpicall has a limited lifetime f up t 10 6 ccles. Sme f the best develped tpes f ptical switch empl liquid crstal (LC) materials due t their etreme sesitivit t applied fields, lw pwer csumpti, lg lifetime ad t their lw cst. Sigle Switch X Matri Several differet phsical mechaisms fr LC switches have bee ivestigated ad tested. Oe LC techlg ivlves plmers ctaiig ematic LC drplets [1]. Ather apprach ivlves chiral smectic A, which has a much faster respse (10μs versus a few ms) []. Sme ferrelectric liquid crstal switchig techlgies are based the ccept f ttal iteral reflecti [3, 4]. Hwever, these appraches are characteried b a high eerg lss ad thus high iserti lss, which limits the practical applicatis f LC ptical switches i ptical etwrks. The mst ppular frm f switchig techlg is a s-called matri switch. We have csidered the matri csistig f sigle switches as a basic structure. This structure ca be subsequetl icrprated it a mdule ctaiig N basic matrices (Fig.1) [5]. this paper the desig f all-ptical switchig device based ttal reflecti i liquid crstals will be preseted. Priciple f perati Optical put Figure 1. Matri spatial switch. Schematic diagram f the ttal iteral reflecti switch is shw i the Figure. Plaar ematic LC laer is sadwiched betwee tw glass prisms with trasparet cductig ad aligig laers. 51

2 B cmig plariati cmig fr the case whe E-cmpet is lig i the plae frmed b the k-vectr ad the LC ptical ais. this wa the LC ptical ais chages its rietati depedig applied electric field value. t gives a pssibilit t var the LC refractive ide i the rage frm LC-OFF t LCθ OFF-state U=0 glass LC 1 ON-state Vltage applied Figure. Ttal reflecti switch structure: 1- glass prisms; - liquid crstal laer The glass ad LC material s refractive idices shuld be adjusted t maitai the switch trasparec i the OFF-state ad ttal reflecti cditi i the ON-state. The aligmet laers shuld prvide a hmgeeus plaar rietati with the directr parallel t the icidet light plariati directi. The required thickess f LC laer shuld be much greater tha the deca distace fr the evaescet wave. Withut a applied vltage the ematic has a refractive ide ( LC-OFF ) ear that f the glass prisms ( g ), s the icidet light passes thrugh the liquid crstal laer. Liquid crstal mlecules chage their rietati while the switch is tured b the applied vltage, s the LC refractive ide becmes smaller tha that f the glass ( LC-ON < g ) t satisf the ttal iteral reflecti cditi, s the iput will be reflected. Let s csider a mchrmatic plae wave prpagatig i a arbitrar directi thrugh ematic liquid crstal; tw idepedet, liearl plaried, prpagati mdes ca eist whse phase velcities are determied b the idices f refracti A ad B B alg the each directi f plariati. The itersecti f the material s ide ellipsid cmig Outgig θ Outgig glass > LC 1 B. Fr ematic liquid crstal these idices deped the surface aligmet cditis (r the teture tpe) ad the stregth ad directi f the electric field applied t the electrdes. Withut a applied electric field the ematic directr is parallel t the electrdes surfaces ad is mdeled as a uiaial crstal with a pricipal ais ellipsid give b: e = 1. () The itersecti f the rmal plae fr a icidet liearl plaried plae wave has idices f refracti give b: = =, (3) A 1/ cs θ si θ B =, (4) e where, e liquid crstal refractig idices, θ - agle betwee the light prpagati directi ad the crstal surface. Thus, fr the light with its E cmpet lig i the plae frmed b the k vectr f the ad the crstal ptical ais, the refractive ide is LC-OFF = B. B Uder a applied electric field liquid crstal mlecules chage their rietati alg the field, s the ematic laer becmes hmetrpicall aliged ad the crstal ais is alg the -ais. Fr a liearl plaried prpagatig thrugh the cell i the ON-state the ide f refracti is give b: B si θ cs θ = e 1/ (5) = 1, (1) ad the plae rmal t the directi f prpagati prduce a ellipse whse majr ad mir semi-aes determie the idices A ad 5

3 ON. As ca be see frm Fig.3, the differece betwee LC-OFF ad LC-ON values depeds bth the LC ptical aistrp ad the agle f iput icidece. A plaar liquid crstal laer ma be csidered as a cvetial Fabri-Pert iterfermeter with a refractive ide LC depedig applied vltage which light trasmittace is give b the frmula: 0 =, (6) 1 M si ( φ) where is the itesit f the light trasmitted b liquid crstal laer; 0 is the itesit f the icidet light; ( θ ) φ = k LC d cs, is the phase differece betwee tw s reflected b the laer; d is the liquid crstal π laer thickess; k = is the wave umber i λ vacuum; λ - radiati wavelegth; θ - agle f refracti at the glass-lc iterface which is give b Sell s law: glass si( θ ) = LC si( θ ), where θ is the agle f icidece; 4 M =, 1 r ad = - itesit f the reflected light r at the glass-liquid crstal iterface related t the icidet itesit, which is give b: 0 si ( θ θ ) = (7a) si ( θ θ ) fr the light with its E cmpet perpedicular t the plae f icidece ad ta ( θ θ ) =, (7b) ta ( θ θ ) fr the case whe vectr E is parallel t the plae f icidece. Fig.4 shws simulated depedece f the switch trasmittace the LC refractive ide ad the icidet agle. Liquid crstal laer ttall reflects the light whe the material s refractive idices ad the agle f icidece satisf the cditi f ttal iteral reflecti: si(θ ). (8) LC glass 1,0 1,75 1,70 1,65 LC-ON LC-OFF 0,5 / 0 LC 1,60 1,55 1, θ, 0 Figure 3. Calculated depedecies f refractig idices the agle f icidece fr OFF ad ON mdes cidet agle, 0 0,0 1,1 1, 1,3 1,4 1,5 1,6 1,7 LC refractive ide Figure 4. Simulated depedece f the ttal reflecti switch trasmittace liquid crstal refractive ide ad the icidet agle f icmig ( glass = 1.67). 53

4 The figure shws that the ttal reflecti switch desig ad peratig mdes are defied b the crrelati betwee the glass prisms material, ematic liquid crstal refractive idices ad the agle f icmig icidece. Thus t achieve a maimum switchig trasmittace chage it is ecessar t make a ptimiati f these parameters. such a wa it is pssible t chse a ptimum icidet agle fr the fied ematic glass cmbiati. t shuld be ted, that the chice f a apprpriate icidet agle defies the glass prisms shape, which shuld be simple ad suitable fr icrpratig the switchig elemet it a matri. The thickess f liquid crstal laer des t sigificatl ifluece the switch perfrmace uless it is clse t the radiati wavelegth value. this case a evaescet field ma reduce the itesit f the reflected. This parameter is mre imprtat frm the drivig vltage pit f view as lg as icreasig f the laer thickess leads t the ctrl vltage grwth. ur eperimet the switch parameters are justified t perate at λ = 633m ad t prvide the agle f reflecti f The glass prisms are rectagular shaped t simplif a matri desig. Eperimetal results The ttal reflecti switch perfrmace was ivestigated i the sample the base f tw glass prisms ( g = ) with trasparet electrdes. The prisms were cated i a cetrifuge with PVA, bth substrates were rubbed ad the aliged atiparallel t each ther. The liquid crstal material (Merck E63, e = , = 1.517, Δ = 0.7 at λ = m) was itrduced it the cell b capillar acti. The thickess f the ematic liquid crstal laer was 5 μm. The eperimetal set-up is shw i Figure 5. The LC cell (ttal reflecti switch) is desiged t perate at 633 m wavelegth f He-Ne laser. Fr rectagular prisms used i measuremets the icidet agle value is 75 0 ad the icidet light plariati is parallel t the directi f the rubbig f the aligig laer. 54 Laser P Geeratr θ LC Switch PD PD Oscillscpe Figure 5. Ttal reflecti switch eperimetal set-up: P plarier, PD phtdide. The ttal reflecti elemet is drive with a biplar square wave sigal with a amplitude frm 0 V t 35 V p-p. The frequec f the drivig sigal equals t 1 kh. A tpical electr-ptic respse fr the ttal reflecti switch is shw i the Figure 6. The measuremets t evaluate the switch perfrmace shw the fllw results - switchig ctrast: 4dB fr trasmittig mde ad 0.5dB fr reflectig mde, iserti lss level:.8db ad switchig time: 0,34ms. The glass parts f bth switches did t have a ati-reflecti catig their surfaces. The iserti lss ca be reduced b use f a 100% plarier, based chlesteric mirrrs. Such a cmpet culd have a high trasmissi, limited l b the ptical precisi ad material rietati defects. Pht dide's respse, V,5,0 1,5 1,0 0,5 trasmitted, reflected 0, Applied vltage, V Figure 6.Tpical electr-ptic respse fr the ttal reflecti switch.

5 Cclusi cclusi, we have described a switchig device usig the ttal reflecti phemea the base f glass prisms ad a ematic liquid crstal laer. Prttpe f sigle switch has bee desiged ad its parameters have bee ivestigated. The first eperimetal results fr a visible radiati idicate that a high switchig ctrast (40dB) ad a satisfactr lw (3dB) iserti lss level ca be achieved fr the give tpe f switch. The advatages f the prpsed desig are simple ad cmpact structure, electric ctrl, lw pwer csumpti ad gd scalabilit. Wrk i prgress icludes miimiig the iserti lss ad crss talk fr icrprati it a switch matri. efereces [1] C.C. Bwle, P.A. Kssrev, G.P. Crawfrd, S.Faris, (001), Variable-wavelegth switchable Bragg gratigs frmed i plmer-dispersed liquid crstals, App. Phs. Lett. 79, 1 p.6574, [] A.Seh ad K. M. Jhs, (1996), Highspeed ctiuusl tuable liquid crstal filter fr WDM etwrks, J. Lightwave Techl. 14, p.1067, [3] N. A. ia ad S. Yua, (1999), ecfigurable wavelegth add-drp filterig based a baa etwrk tplg ad ferrelectric liquid crstal fiber-ptic switches, J. Lightwave Techl. 17, p.1575, [4] J. E. Fuquet, (000), Cmpact ptical crss-cect switch based ttal iteral reflecti i a fluid-ctaiig plaar lightwave circuit, Prc. Optical Fiber Cmmuicati Cf., p. 04, OSA, Washigt, DC, [5] Yu. Semeva, S. M. Dvgalets, Yu. Paari, G. Farrell, Liquid crstal based Optical Switches, Mlecular Crst.& Liquid Crst., (accepted fr publicati). 55