XPRIMNT-V letptic ffect Aim: T stud the lectptic effect i LiNbO cstal Appaatus: A He-Ne lase, pai f plaise with gaduated scales, LiNbO cstal i a hlde, phtdetect, digital multimete / pwe mete. Itducti: letptics Mdulat: The chage i the efactive ide iduced i a cstal the applicati f a electic field which affects the state f plaiati f a light beam ppagati thugh a cats is w as electptic effect. If the chages i the efactive idices ae pptial t the applied electic field, it is called as Pcel effect liea electptics effect. If the chages i the efactive ide ae pptial t the squae f the applied vltage, the effect i w as Ke effect quadatic electptics effect. I this secti we will l discuss abut the Pcel effect. The electptic effect is empled i maig the electptic mdulat. Opeati f electptic mdulats is based the piciple f iduced biefigece. Pcels cell is such device which ca pduce ctllable biefigece b applig vltage t the cell. The cell ctais uiaial cstal, which becmes biaial whe electic field is applied. If ew aes iduced b the applied electic field ae at 45 t the plae f plaiati f the icidet beam, the icidet beam is split it tw equal thagall plaied cmpets. The ppagate i a cstal with diffeet velcities because f iduced diffeet efacti idees. Iduced biefigece ( e - ) is pptial t the applied electic field ad the cstal legth l it gives ctlled phase etadati Γ betwee these cmpets, which is used f the icidet beam π Γ l λ mdulati whee: λ - wavelegth f the ptical beam. whee: λ - wavelegth f the ptical beam. Vltage sesitivit f the Pcels cell is give b half-wave vltage (V λ/ ). This is the vltage equied f phase etadati Γ f 8. Ofte used quate wave vltage equals V λ/4 ½ V λ/ ad it is vltage equied f Γ 9. Thee ae l tw basic desigs f Pcels cells: Pcels cell with lgitudial electptic effect, emplig a cstal i which electic field is applied alg the diecti f the ptical beam. Pcels cell with tasvese electptic effect, utiliig a cstal i which electic field is pepedicula t the
diecti f the ptical beam.f this tpe f mdulat the half-wave vltage is ivesal pptial t the cstal legth ad diectl pptial t the distace betwee electdes. Lithiumibate Pcels cells The Lithiumibate Pcels Cells ae used i the lase applicatis f medicie, idust ad eseach. The Pcels cells ae specified f Q-switch applicatis iside a lase esat f mdulatig f a lase beam utside the esat. Lithiumibate is a ve suitable cstal f the use i Pcel Cells because f its special ppeties. The mst imptat phsical chaacteistics ae the lage electptical cefficiet, the taspaec i the spectal age fm 4m up t 45m, ad a high meltig pit f 5 C. ecause f the hadess ad islubilit the cstal des t eed a additial ptecti. Data f the LiNbO Pcels Cells Mateial LiNbO Spectal age 4m 45 m Tasmissi with AR catig: 98 % Rise time: app. s The: Cstal Optics I a aistpic dielectic media D..() The dielectic tes () Rtate the tes such that (,,) becmes the picipal aes (diagalisati) () F a uiaial cstal. F a biaial cstal,, all ae diffeet. The Optical idicati Ide f ellipsid Repeset tes as a ellipsid a b c
which ca be witte as a D..(4) c (,, ) b i.e. M. Ivese f equati () D (5) whee..(6) quati (5) ca be witte as D.. D (7) ( D. ) Reaage D D.. (8) D whee (,, ) Cmpaig equati (4) ad (8) we get a b c The equati f ellipsid becmes Nw eplace The we get The f lectptics ffect The liea chage i the efactive idices due t electic field (,, ) ) i the ( abita diecti ij j (,, )..(9) i j The ide f ellipsid ca be witte as ()
4 6 6 6 5 5 5 4 4 4.() F LiNbO cstal: ad e. If the applied field is alg the Y diecti the,. S the lectptics tes f the LiNbO 5 5 e.() Right had side f the abve equati ca be simplified t 5 5.() Cmpaig equati () ad () e
5 5 The ppagati f light is i the Z diecti, the ide ellipsid css-secti f X-Y plae mal t Z. S put. The we get Substitutig the -cefficiets fm the abve equatis ( ) ( ) ( ) ( ) The simplified fm is Assume that the chage i efactive ide due t the electic field ad Thus ( ) S we get The cmpets f the icidets electic fields ( ) i t φ ω cs whee the phase φ ( ) i t φ ω cs whee the phase φ The phase diffeece
π φ φ φ λ V φ l d ( ) F a half vltage φ π, l ad π λ which π V l π λ d λd Thus the half vltage V. F LiNbO.4 - m/v,.97 l ad λ6.8m l5mm; d.mm. X-ais Y- ais Aalise Detect Lase Plaise Optic Ais (Z-ais) Mete Fig.5(a): Schematic diagam f a lectptic setup V Icidet light Y 45 X Phase diffeece betwee X, Y Fig.5(b): Ppagati f light thugh the cstal (Tasvese OP) 6
Pcedue:. Yu will eed a beam f plae plaised light fm the He-Ne lase. Detemie the plaiati f the lase light b placig a piece f sheet plaie i the beam, muted the agula tati mut. Diect the beam emegig fm the plaie it the pwe mete detect with digital multimete. Rtate the plaie thugh 6 ad bseve the vaiati f the pwe mete. If the beam is plae plaied, the eadig f the pwe mete shuld decease t e at tw psitis 8 apat. At agles that ae 9 fm the psitis whee the itesit is e., the itesit will ise t a maimum. I this case, whe the lase utput is plae plaied, it ma be used as it is. If the light passig thugh the plaie des t decease t e at tw psitis 8 apat, it is t plae plaied. It ma be uplaied(itesit des t chage as the plaie is tated) patiall plaied(itesit ges thugh tw miima 8 apat, but the miima ae geate tha e). Diect the beam thugh the plaie. The beam emegig shuld be plae plaied. Chec it agai, usig the sheet plaie, t be sue.. Nw aage the elect-ptic set up as shw Fig.5(a).. The fist plaie will t be equied if the lase was igiall plae plaied. Caefull alig the cstal i the hlde s that the light beam passes accuatel alg the ais f the device ad emeges with miimum lss f itesit. Mut the secd plaie (aalse) a agula tati mut s that the beam passes thugh it ad etes the detect with digital mete pwe mete. Rtate the aale util the light tasmitted thugh it is at a miimum. The miimum shuld be ea e. 4. Net cect the DC pwe suppl t the electical cects the case f the cstal hlde (Fig.5(b)). Tu the pwe suppl ad gaduall icease the vltage fm e i steps f Vlts. Obseve the icease i the light eachig the detect with digital mete pwe mete. As u icease the vltage i steps, ecd the vltage ad the pwe mete/ digital multimete set i dc vltage mde) eadig at each step. The pwe mete/ digital multimete eadig shuld icease with iceasig vltage at fist. The a maimum eadig will be eached ad, with futhe icease i vltage, the pwe mete eadig shuld begi t decease. Whe u have eached this pit, u ma temiate this pat f the measuemet. The value f the vltage at which the pwe is maimum is the half-wave vltage. 5. Plt the pwe mete/ digital multimete eadig as a fucti f the applied vltage. The gaph btaied is as shw. 7
Tasmitted Itesit (a.u) 8 6 4 V π 4 9 Applied Vltage (V) 4 Fig.(c): Vaiati f tasmitted itesit as a fucti f applied vltage. (t t the scale) 6. Detemie the value f half-wave vltage (Fig.5(c)). Detemie the ecitati ati f the mdulat. The eticti ati is the pwe mete eadig whe the applied vltage is equal t the half- wave vltage divided b the pwe mete eadig whe the vltage is e. 7. Use the plt f pwe vesus applied vltage t deive the biefigece as a fucti f applied vltage. Use the equati: φ π I t I Si whee φ l λ t detemie as a fucti f vltage V. Hee I is the mete eadig at the half-wave vltage V /, L is the legth f the cstal, ad l is the wavelegth (6.8m). Use the value specified f the legth f the cstal. Questis. What is the electptic effect? Hw ca the amplitude f the sigal be mdulated?. What is the phsical meaig f the half vltagev π?. A tasvese electptic mdulat with LiNbO cstal is peatig at a wavelegth λ55m. The cstals has legth lcm ad width d.5cm. The electptics cefficiet ( ) ad the efactive ide ( ) f the cstal ae.4 - m/v ad.9 espectivel. Calculate (a) the phase diffeece betwee the emeget field cmpets with applied vltage V, (b) the additial phase diffeece betwee the emeget field cmpets with VV, ad (c) the halfwave vltage V π f the cstal. (.9, e.) 8
Refeeces:. Optical electics, Yaiv. Optical Phsics, Lips ad Lips (Cambidge Uivesit Pess) Optics, Hecht (Peas ducati Asia) 9