TNG QUAN V CC LINH KIN I. H THNG THNG TIN QUANG WDM 1. S khi chc nng h thng WDM a. Khi nim: Wavelength Devision Multiplexing) l cng ngh trong mt si quang ng thi truyn dn nhiu bc sng tn hiu quang. b) S chc nng: m bo vic truyn nhn nhiu bc sng trn mt si quang, h thng WDM phi thc hin cc chc nng sau: Pht tn hiu: Trong h thng WDM, ngun pht quang c dng l laser.Yu cu i vi ngun pht laser l phi c rng ph hp, bc sng pht ra n nh, mc cng sut pht nh, bc sng trung tm, rng ph, rng chirp phi nm trong gii hn cho php.

- Ghp/tch tn hiu: Ghp tn hiu WDM l s kt hp mt s ngun sng khc nhau thnh mt lung tn hiu nh sng tng hp truyn dn qua si quang. Tch tn hiu WDM l s phn chia lung nh sng tng hp thnh cc tn hiu nh sng ring r ti mi cng u ra b tch. - Truyn dn tn hiu: Qu trnh truyn dn tn hiu trong si quang chu s nh hng ca nhiu yu t: suy hao si quang, tn sc, cc hiu ng phi tuyn, vn lin quan n khuch i tn hiu ... - Khuch i tn hiu: H thng WDM hin ti ch yu s dng b khuch i quang si EDFA (Erbium-Doped Fiber Amplifier) C ba ch khuch i: khuch i cng sut, khuch i ng v tin khuch i. Khi dng b khuch i EDFA cho h thng WDM phi m bo cc yu cu sau: - li khuch i ng u i vi tt c cc knh bc sng (mc chnh lch khng qu 1 dB). - S thay i s lng knh bc sng lm vic khng c gy nh hng n mc cng sut u ra ca cc knh. - C kh nng pht hin s chnh lch mc cng sut u vo iu chnh li cc h s khuch i nhm m bo c tuyn khuch i l bng phng i vi tt c cc knh. - Thu tn hiu: Thu tn hiu trong cc h thng WDM cng s dng cc b tch sng quang nh trong h thng thng tin quang thng thng: PIN, APD. c. Phn loi h thng WDM

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- H thng WDM v c bn chia lm hai loi: h thng n hng v song hng: + H thng n hng ch truyn theo mt chiu trn si quang. Do vy, truyn thng tin gia hai im cn hai si quang. H thng WDM song hng, ngc li, truyn hai chiu trn mt si quang nn ch cn 1 si quang c th trao i thng tin gia 2 im. + Xt v dung lng, h thng n hng c kh nng cung cp dung lng cao gp i so vi h thng song hng. Ngc li, s si quang cn dng gp i so vi h thng song hng. + ng v kha cnh thit k mng, h thng song hng kh thit k hn v cn phi thm cc yu t nh: vn xuyn nhiu do c nhiu bc sng hn trn mt si qua m bo nh tuyn v phn b bc sng sao cho hai chiu trn si quang khng d chung mt bc sng. + Cc b khuch i trong h thng song hng thng c cu trc phc tp hn trong h n hng. 2. Cc yu t nh hng n cht lng ca h thng WDM C 3 yu t c bn ca si quang nh hng n kh nng ca cc h thng thng tin quang, bao gm: - Suy hao Suy hao trn si quang ng mt vai tr rt quan trng trong vic thit k h thng, l tham s xc nh khong cch gia pha pht v pha thu - Tn sc Tn sc l dn xung nh sng truyn trong si quang do vn tc nhm khc nhau ca cc bc sng khc nhau cha trong thnh phn ca ph ngun pht2

- Hin tng phi tuyn xy ra trong si quang. Hiu ng quang c gi l phi tuyn nu cc tham s ca n ph thuc vo cng nh sng (cng sut) Trong h thng thng tin quang, cc hiu ng phi tuyn s xy ra khi cng sut tn hiu trong si quang vt qu mt gii hn ca h thng WDM th mc cng sut ny thp hn nhiu so vi cc h thng n knh. Cc hiu ng phi tuyn nh hng n cht lng ca h thng WDM c th chia thnh hai loi l hiu ng tn x v hiu ng Kerr (khc x). Tuy nhin, i vi cc h thng khc nhau th mc nh hng ca cc yu t ny cng khc nhau. 3. Cc linh kin trong h thng WDM Cc linh kin c s dng trong cc mng quang hin i bao gm cc b ghp/tch (couplers), b pht laser (lasers), b tch quang (photodetectors), b khuch i quang (optical amplifiers), b chuyn mch quang (opticalswitches), b lc (filters) v b ghp/tch knh (multiplexers). 4. Mng WDM S pht trin ca cc h thng WDM nhn chung c th chia lm ba giai on: - H thng WDM th h 1: H thng WDM im-im vi cc trm xen/r trn tuyn phi s dng cc thit b MUX/DEMUX tch/ghp tt c cc bc sng. - H thng WDM th h 2: H thng WDM im-a im vi cc trm xen/r trn tuyn l cc OADM cho php tch trc tip bc sng cn xen/r. - H thng WDM th h 3: Mng quang WDM hon ton vi cc thit b chuyn mch v nh tuyn bc sng. Kin trc mng phi p ng c cc yu cu sau: -Ti s dng bc sng - Chuyn i bc sng - Trong sut - Chuyn mch knh - Kh nng tn ti khi mng gp s c (Surviability) - M hnh ng quang 5. u nhc im ca cng ngh WDM: So vi h thng truyn dn n knh quang, h thng WDM c nhng u im ni tri:

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+ Dung lng truyn dn ln, h thng WDM c dung lng truyn dn ln hn nhiu so vi h thng TDM. Hin nay h thng c tng dung lng 200Gb/s c thc nhim thnh cng vi WDM 80 bc sng, mi bc sng mang tn hiu TDM 2,5Gb/s. + Loi b yu cu kht khe cng nh nhng kh khn gp phi vi h thng TDM n knh tc cao. Khng ging nh TDM phi tng tc s liu khi lu lng truyn dn tng, WDM ch cn mang vi tn hiu, mi tn hiu ng vi mi bc sng ring (knh quang). + p ng linh hot vic nng cp dung lng h thng, k thut WDM cho php tng dung lng ca mng hin c m khng cn phi lp t thm si quang. Vic nng cp dung lng n gin l cm thm card mi trong khi h thng vn hot ng. + Qun l bng tn v cu hnh mm do, linh hot nh vic nh tuyn v phn b bc sng trong mng WDM nn c kh nng qun l hiu qu bng tn truyn dn v cu hnh li dch v mng trong chu k sng ca h thng. + Ngoi ra cn ng dng truyn nhiu chng trnh truyn hnh cht lng cao, c ly di. + Gim chi ph u t mi. Bn cnh nhng u im trn, h thng WDM cn c nhng hn ch: + Dung lng h thng cn nh, cha khai thc trit bng tn rng ln ca si quang. + Chi ph cho khai thc, bo dng tng do c nhiu h thng cng hot ng II. SI QUANG G655 1. Nhng kiu si quang C hai kiu si quang : Multimode v Singlemode ( hoc gi l Monomode ) . Nhng kiu ny i din cho kiu nh sng c truyn bn trong si li . Multimode : ng knh ca li Multimode ln hn ng knh li si Singlemode . iu ny lm cho nh sng truyn theo nhiu phng thc khc nhau , c ngha l nh sng i qua si li bng nhiu ng ch khng phi ch s dng mt ng nh si Singlemode . ng knh si Multimode c kch thc t 50 micron - ti 100 micron ( kiu thng mi thng thngc ng knh 50, 62.5 , 100 micron) v c ng knh lp sn ph ( cladding ) 125 micron . Si Multimode c phn bit : Graded-index v Step-index , tu thuc vo ch s khc x gia li l lp sn ph . Graded-index c s thay i dn gia li v lp sn ph . Step-index ch s khc x thay i theo tng on ( tng bc ) . Si Step-index c th truyn d liu ti 50Mbps , trong khi si Graded index c th truyn d liu ti tc 1Gbps . Si Multimode cn c gi l MMF c s dng trong truyn d liu vi c li ngn . Singlemode : si Singlemode dng truyn d liu vi c li di , nhng chng yu cu cc Connector c chnh xc cao v thit b t tin . Trong kiu si ny nh sng ch c th i theo mt ng nht nh bn trong si li . Li c ng knh 7 micron v 10 micron , lp sn ph c ng knh 125 micron . Do c hai si kiu Multimode v4

Singlemode u c cng ng knh nhng ch c ng knh si li l khc nhau . C ba kiu ca si Singlemode : non dispersion-shifted fiber (NDSF), dispersion-shifted fiber (DSF) and non zero-dispersion-shifted fibers (NZ-DSF). ITU a ra mt lot chun phn loi kiu ca si Multimode v Singlemode : ITU G.651 : si Graded-index Multimode c ng knh li 50-micron v ng knh lp sn ph 125-micron ITU G.652 : si Non dispersion-shifted fiber (NDSF) Singlemode dng cho bc sng 1130nm h tr cho c li xa : 1,000 km vi tc 2.5 Gbps, 60 km vi tc 10 Gbps v 3 km (1.86 miles) vi tc 40 Gbps. ITU G.653 : si Dispersion-shifted (DSF) Singlemode ITU G.655 : si Non-zero dispersion-shifted (NZ-DSF) Singlemode . N ht vi khong cch xa 6,000 km vi tc 2.5 Gbps, 400 km vi tc 10 Gbps v 25 km (15.5 miles) vi tc 40 Gbps. Ch : Si quang truyn nh sng bc sng m chng ta khng nhn thy bng mt thng . u im ca G.655: - Si quang G655 thch hp cho h thng DWDM, lm tng dung lng truyn dn. - Si quang G655 thch hp cho h thng truyn dn ng di WDM dung lng cao. - tn sc dng ca si G655 trnh vic trn ln 4 bc sng quang. - Vng hiu dng cao ca si G655 (vn nh hn si SMF) lm gim thiu cc hiu ng phi tuyn. - Erbium Doped Fiber Amplifier (EDFA) khuch i cc tn hiu quang trong ca s C, iu ny l tng cho loi si quang NZDS (non-zero dispersion-shifted). 2. Li th ca si quang G655 so vi G652 - Tuyn truyn dn s dng loi si single mode thng thng (SMF) cn b b tn sc si quang (DCF) vi c ly ln hn. iu dn ti vic tng tn sc do hiu ng phi tuyn ca b DCF. suy hao cao ca DCF dn ti tng cao suy hao cng sut ca mng. - Cc b laser iu ch trc tip DFB gi r vi dung sai tn sc cao hn c th c s dng do tn sc thp ca loi si quang G655. - S thc thi h thng cao nht. - tn sc thp c li cho tuyn ng di, tc truyn d liu cao v mt knh ti a. - Kh nng bng thng tng lai (ti 10Gbps) - dc tn sc thp gip cho ghp knh bc sng quang cao nht - Gim thiu s module b tn sc (DCM) - Phi tuyn thp lm gim s trn ln 4 sng quang. - D lp t, suy hao mi hn thp nht v nng sut hn ni cao nht. 3.Khi nim v tn sc Tn sc l dn xung nh sng truyn trong si quang do vn tc nhm khc nhau ca cc bc sng khc nhau cha trong thnh phn ca ph ngun pht. H s tn sc l tn sc tnh cho mt n v b rng ph ca ngun pht v mt n v chiu di ca si v thng tnh bng ps/(nm.km)Trng hp l tng l t c h s tn sc bng 0. Si c khuyn ngh trongG.652 c h s tn sc nh nht ti 1310 nm khong 1,2 ps/nm.km, trong khi si dchchuyn tn sc c khuyn ngh trong G.653 li c h s tn sc nh nht ti 1550nmkhong 2,5ps/nm.km.Mc d si G.652 c tn sc thp5

hn nhng suy hao qua 1 khong cch ti bc sng 1550nm li gp khong 2 ln so vi ti bc sng 1310nm. Si G.653 c c tnh suy hao thp hn ti bc sng 1550nm m vn c c tnh tn sc tt ti bc sng ny.Tuy nhin, si G.653 khng c khuyn ngh cho cc h thng WDM. Khi cc mccng sut quang v cc khong cch khng trm lp trong cc h thng WDM th cc nh hng phi tuyn nh hiu ng trn 4 bc sng (FWM) c th tc ng n h thng.FWM c th sinh ra cc bc sng khc nhau trong mng v cc bc sng ny tng tc vi nhau. c tnh tn sc rt thp trong cc si dch chuyn tn sc (G.653) m bo rng mi quan h pha- bc sng s c duy tr dc chiu di si. Tuy nhin, c tnh ny li lm pht trin nhanh chng cc hiu ng phi tuyn. Cc hiu ng quang phi tuyn ny dn n s ra i ca si tn sc dch chuyn khc 0-si G.655. Si G.655 clng tn sc nh trong vng bc sng 1550nm. Lng tn sc nh ny s khin cho mi quan h bc sngpha thay i lin tc, do ngn chn s pht trin cacc hiu ng phi tuyn. III. CU TRC C IM CA LASER 1. Khi nim ca laser: Laser (c l la-de) l tn vit tt ca cm t Light Amplification by Stimulated Emission of Radiation trong Ting anh v c ngha l "khuch i nh sng bng pht x kch thch. C th ni laze l mt ngun sng pht ra mt chm sng c tnh n sc, tnh nh hng, tnh kt hp rt cao v cng ln. Chm sng pht ra c gi l tia laze. Ta s hiu r tt c cc c im ny khi nghin cu cc nguyn tc tc ng ca laze. Nm 1958 cc nh bc hc Nga, M nghin cu c lp nhau a ch to thnh cng Laze u tin. D l ngun pht sng mi pht ra chum sng gi l laze, c c im khc vi chum sng thong thng: i - Tia laze c tnh n sc ca tn s nh sng do laze pht ra cao. sai lch tng ch c th bng 10-15. - Tia laze l chm sng kt hp (cc photon trong chum c cng tn s v cng pha). - Tia laze l cmmsng song song (c tnh nh hng cao). - Tia laze c cng ln. Chng hn tia laze rubu (hng ngc) c cng 106 W/cm2. Nh vy: Laze l ngun sng pht ra chum sng song song, kt hp c tnh n sc cao v c cng ln. - Nguyn tc pht quang ca laze da trn ng dng hin tng pht x cm ng. 2. Cu to : Nguyn l cu to chung ca mt my laser gm c: bung cng hng cha hot cht laser, ngun nui v h thng dn quang. Trong bung cng hng vi hot cht laser l b phn ch yu. Bung cng hng cha hot cht laser, l mt cht c bit c kh nng khuych i nh sng bng pht x cng bc to ra laser. Khi 1 photon ti va chm vo hot cht ny th ko theo l 1 photon khc bt ra bay theo cng hng vi photon ti. Mt khc bung cng hng c 2 mt chn hai u, mt mt phn x ton phn cc photon khi bay ti, mt kia cho mt phn photon qua mt phn phn x li lm cho cc ht photon va chm lin tc vo hot cht laser nhiu ln to mt photon ln. V th cng chm laser6

c khuc i ln nhiu ln. Tnh cht ca laser ph thuc vo hot cht , do ngi ta cn c vo hot cht phn loi laser.

Cu to c bn v c ch hot ng ca laser: 1) Bung cng hng (vng b kch thch) 2) Ngun nui (nng lng bm vo vng b kch thch) 3) gng phn x ton phn 4)gng bn m 5) tia laser 3. Cu to my pht laser, bao gm cc b phn chnh: -Hot cht laser : L mi trng cha cc hot cht c kh nng bc x laser khi c kch hot bng mt ngun nng lng. - Ngun nui: L ngun nng lng duy tr hot ng ca mi trng hot cht laser, gi cho hot cht lun lun trng thi c s phn t mc B nhiu hn mc A. -- Bung cng hng: Bao gm 1 gng phn x ton phn v 1 gng bn m ( phn x t 70% n 99%). Bung cng hng cho php ngun sng kch thch cht nhiu ln v chm tia sng bc x s c khuych i v chn lc qua gng phn x ton phn v gng m cho n khi n nh pht ra chm sng laser. 4. Phn loi laser: Ty theo loi hot cht laser ta s thu c cc tia laser vi tn gi khc nhau: - Laser rn c mi trng hot cht th rn. C hng trm loi nh Laser Ruby, Laser YAG, Laser bn dn, Laser thy tinh, vv. - Laser lng c mi trng hot cht th lng. C 50 loi khc nhau. Cc hot cht th lng c mu s cho ta laser mu, l nhng laser rt thng dng hin nay. - Laser kh c mi trng hot cht th kh . Cng c hn trm loi kh c dng lm hot cht laser nh laser CO2 , laser heli-Neon, laser Argon, vv . Ngi ta cn phn loi theo tnh cht nh: laser nng nh laser CO2, Argon) v laser lnh (nh Laser He-Ne, Laser hng ngoi). 5. Cc tnh cht ca tia laser: - n sc cao Laser l chm nh sng m cc tia sng ca n c mc chnh lch bc sng nh nht, so vi cc chm sng n sc khc. S chnh lch bc sng ny cn gi l ph nh sng ca chm nh sng.7

V d nhin l ph cng hp th n sc ca chm sng cng cao. Trc khi c laser cc nh vt l to c cc chm nh sng n sc c chnh lch bc sng t 1Ao n 10nm, nhng s dng trong nghin cu khoa hc. Trong khi mc chnh lch bc sng ca chm nh sng laser c th ti 0,1 Ao. Tnh cht ny rt quan trng v hiu qu tc dng ca laser khi tng tc vi vt cht, vi cc t chc sinh hc ph thuc vo n sc ny. - nh hng cao Khc vi cc ngun sng khc, cc tia sng Laser c chn lc ch pht ra nhng tia vung gc vi gng, nn hu nh song song vi nhau (hay ni theo ngn ng vt l l gc m gia cc tia l rt nh). Nh vy, laser c nh hng l tng, c th chiu i rt xa, n mc ngi ta c th dng laser o nhng khong cch trong v tr. - Mt ph ( chi) rt cao chi ca ngun sng c tnh bng cch chia cng sut ca chm sng cho rng ca ph. V rng ca ph Laser rt nh nn laser c tp trung cc tia sng rt cao, hay ni cch khc l chi rt cao so vi cc ngui sng khc. V d: laser c cng sut thp l laser He-Ne cng c chi gp hng vn ln chi ca nh sng mt tri. Nhng laser c cng sut ln c chi cao gp hng triu ln mt tri. - Cng sut ca laser Ty loi laser m c ngui sng cng sut khc nhau. C nhng loi laser cng sut mnh tng ng cng sut 1 vn nh my in 1 triu KW. Nhng ngun laser cng sut mnh c th s dng trong cng nghip nng nh khoan ct vt liu, hay ch to cc loi v kh, kh ti qun s. Cc loi laser s dng trong y hc l nhng laser c cng sut thp nh laser He Ne cng sut ch khong t 2MW n 10MW. 6. Nhng cng dng ca laser: Ngy nay, ngi ta ch to ra c gn 500 loi laser khc nhau, ng dng trong rt nhiu lnh vc: -o c nhng khong cch cc ln, nh trong nghnh thin vn (o khong cach t tri t n cc hnh tinh v khong cch gia cc hnh tinh trong v tr). -Thit lp dn ng nh cc loi bom, tn la c dn ng bng laser. -Thng tin lin lc . - Cng nghip nng : hn ct kim loi -Cng nghip ch to v kh -Cng nghip ch to v kh -Ci to ging -Trong y hc (chun on v iu tr bnh, sn sc thm m) . 7. S khc nhau gia LED v Laser : C mt s s khc nhau, nhng c hai thit b hot ng trn nguyn tc cng c electron d tha trong ban nhc dn ca cht bn dn, v sp xp n recombine electron vi l trong mt thi trang x, to ra nh sng trong tin trnh.im khc bit laser?Trong mt n LED, cc recombine electron mt cch ngu nhin v c t chc.H cho ra nh sng bi nhng g c gi l pht thi t pht, m ch n gin c ngha l thi gian chnh xc v8

ni m mt photon i ra ca in thoi l n tng in t c nhn, v nhng iu xy ra trong mt cch ngu nhin. C mt cch khc, trong mt kch thch in t c th pht ra mt photon tuy nhin.Nu mt lnh vc ca nh sng (hoc mt tp cc photon) xy ra c i qua ca mt in t ti mt nh nc nng lng cao, m trng nh sng c th gy ra cc in t pht ra mt photon b sung thng qua mt qu trnh c gi l kch thch kh thi.Cc lnh vc photon kch thch in t pht ra nng lng ca n nh l mt photon thm, m i ra trong giai on vi cc lnh vc kch thch.y l s khc bit ln gia nh sng khng lin lac (nhng g n t mt n LED hoc n pin a) v mch lc m nh sng n t laser.Vi nh sng mch lc, tt c cc lnh vc in kt hp vi mi phonon l tt c cc giai on chnh xc.Tnh mch lc ny l nhng g cho php chng ti gi mt tia laser trong tp trung cht ch, v cho php n i du lch mt khong cch ln m khng c s phn k nhiu hay ly lan ra ngoi. Led c: + tin cy cao, vn hnh n nh v bn b . +Kch thc my nh gn, do khng b hn ch gc ti nh tia Laser . +Cht lng bn in sc nt hn. laser n sc v c c nng lng tp trung .... LED khng n sc .. -S khc bit chnh gia laser v n LED l hot ng ca kh thi nng lng.Trong trng hp n LED, n to ra c da trn nhng pht thi Spontaneous.Tuy nhin, laser da vo khuyn khch pht thi - Laser n n t pht x kch thch ca photon trong mt diode (nh). N (Nh tn gi ca n cho thy - khuch i nh sng bng pht x kch thch ca bc x) c nhiu quyn lc hn. y l mch lc trong t nhin.,n l tn km. - LED N n t pht thi Spontaneous ca photon trong mt diode (nh). N c t nng lng. N l ch yu hoc khng lin lac noncoherent.,n l r hn. 8. CWLASER: Laser diode cho ngi chi a CD v my nghe nhc DVD Red cao nng lng laser diodes CW Laser Diodes (Red) M t: Cc diode laser c in pht ra trong phm vi quang ph mu t 635 nm n 680 nm. Ty thuc vo lnh vc ng dng v cc thng s k thut cn thit, chng ti c th cung cp cho bn vi mt diode laser ph hp vi mt di cng sut t 3 mW n 4 mW. 655 nm tia laser it cho th trng i chng Th trng my nghe nhc CD v DVD lm cho n c th cung cp cht lng cao 655 it laser nm vi mt di cng sut t 3 mW n 50 mW vi gi thp. H cng thng c s dng trong o lng v cng ngh cm bin. 635 nm tia laser didoes cho con tr laser v h thng hin th it laser pht ra 635 nm c s dng ch yu trong cc con tr laser v h thng hin th. Mt ngi l gn gp ba ln nhy cm hn vi it laser bc sng ny so vi cc it laser ti 655 nm vi cng sut tng ng. APC tia laser it vi in t a tch hp APC tia laser it c in t a c tch hp vo mt 5,6 mm 3,3 mm hay nh c9

kh sng to bi v h lm cho n c th thit k cc con tr laser rt nh v nh gn v laser cng nghip. Powerpul tia laser diodes Diode laser mnh nht ca chng ti pht ra 4 W. ny sc rng a ch diode laser l cc k hiu qu (0,8 W / A) v c mt s ph thuc bc sng / nhit thp. Cc bc sng nh cao pht ra trong khong t 631 nm n 635 nm. Diode ny c s dng trong liu php quang ng, hnh nh y t v h thng chn on, hin th tia laser DPSS, chiu sng, v qu trnh kim sot. CW Laser Diodes Blue laser diode CW Laser Diodes (Blue) it laser mu xanh vi cng sut u ra cao cw l 200 mW cho mt tc ghi rt cao. M t CS4052003x, c thit k cho cc ng dng trong lu tr quang hc, s dng cng ngh tng trng tinh th, m ban u c pht trin cho hng ngoi, it laser mu , v mu xanh da tri vi nng lng thp. Chip laser mi c pht trin c mt cu trc c thit k c bit v mt in cw mc cao l 200 mW cho tc ghi cao nht c th. Ngoi ra, thnh phn ny iu hnh rt di tui th 10.000 gi. N c mt tiu chun nh c ng knh l 3,8 mm v l ph hp vi nhit hot ng ln n +80 C. Phin bn t mnh m vi mt sn lng in quang hc ca 20 mW - 85 mW v 405 nm 450 nm cng c sn. NIR laser bn dn cho cm bin v cng ngh o lng CW Laser Diodes (NIR) New NIR laser diodes t 785 nm n 980 nm M t Chng ti c it laser tiu chun vi 3 n 500 mW ca nng lng cho phm vi quang ph NIR t 785 n 1600 nm. Trong thc t, phn ln cc it laser c bn trong phm vi bc sng pht ra 785, 808, 1310, v 1550 nm. Th trng i chng nh my nghe a CD cho php truy cp vo it laser cht lng cao pht ra 785 nm v sn xut 3-200 mW quyn lc gi c hp l. Nhng it laser c s dng trong o lng v cng ngh cm bin. IV. B IU CH NGOI 1. H thng quang coherent

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DE (Drive Electronic): khi ny thc hin khuch i tn hiu ng vo nhm to tn hiu c mc ph hp vi cc khi pha sau. CWL (Continuous Wave Laser): y l b dao ng quang s dng laser bn dn c rng ph hp pht ra nh sng lin tc c bc sng 1. LC (laser control): khi ny nhm n nh bc sng pht ra ca b dao ng quang. MOD (Modulator): y l khi iu ch quang, s dng k thut iu ch ngoi to ra tn hiu iu ch dng ASK (Amplitude Shitf Keying), FSK (Frequency Shitf Keying), PSK (Phase Shitf Keying) hay PolSK (Polarization Shitf Keying ).

2. Cc dng iu ch ngoi u im quan trong ca k thut tch song coherent l ch c bin v pha ca tn hiu quang thu c c th tch ra v o. c im ny m ra kh nng gi thng tin bng cch iu ch bin hoc pha hoc l tn s ca sng mang quang.Trong HT thng tin quang phng php c s dng iu ch l ASK,PSK v FSK l c s dng rng di. a. ASK:(iu bin) -ASK c th to ra bng cch iu ch trc tip dng kch cho laser. Tuy nhin k thut ny c vn l khng duy tr c s n nh tn s ng ra khi thay i dng kch, s thay i ny khong 200MHz/mA. -Dng 2 bin khc nhau ca sng mang biu din 0 v 1 (thng thng mt bin bng 0) -S dng mt tn s sng mang duy nht -Phng php ny ch ph hp trong truyn s liu tc thp (~1200bps trn knh truyn thoi) -Tn s ca tn hiu sng mang c dng ph thuc vo chun giao tip ang c s dng -K thut c dng trong cp quang Nu gi tn hiu s c iu ch l b(t) v tn hiu trng pht ra t laser bn dn l eS(t) c tn s gc S, ta c:

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B l bng thng ca tn hiu c iu ch b(t).

Tn hiu eS(t) sau khi lan truyn trn si quang v ti u thu s lch pha vi tn hiu pht l S (gi s b qua suy hao v bin ca tn hiu eS(t)), biu thc (4.1) c th vit li di dng sau: eS(t) = b(t)Emcos(St + S) (4.3) khi phc tn hiu di nn, chng ta c hai cch sau. Cch th nht l ta nhn tn hiu eS(t) vi cos(St + S) l tn hiu c to ra t b dao ng ni, ta c: b(t)Em[cos(St + S)]2= b(t)Em[1+cos(2St + 2S)] = b(t)Em + b(t)Emcos(2St + 2S)12

(4.4)

Nh vy tn hiu di nn b(t) xut hin. Vi cch ny i hi chng ta phi to c

tnhiu dao ng ni b thu c cng tn s v cng pha vi tn hiu ti.Cn cch th hai l ta bnh phng tn hiu eS(t), biu thc (4.3) tr thnh: [b(t)Emcos(St + S)]= [b(t)Em]2[1+cos(2St + 2S)] = [b(t)Em]2 + [b(t)Em]2 cos(2St + 2S)] (4.5) Sau cho tn hiu ny qua b lc loi b thnh phn tn s 2S ta s thu c tn hiu di nn b(t). b. FSK: (iu tn) -S dng hai tn s sng mang: tn s cao tng ng mc 0,tn s thp tng ng mc1. - t li hn so vi ASK - c s dng truyn d liu tc 1200bps hay thp hn trn mng in thoi -C th dng tn s cao (3-30MHz) truyn trn sng radio hoc cp ng trc Dng nhiu hn 2 tn s.Bng thng c dng hiu qu hn,Kh nng li nhiu hn,Mi phn t tn hiu biu din nhiu hn 1 bit d liu c tnh thay i tn s ca iu ch trc tip trn laser c th p dng cho h thng thng tin quang coherent FSK bng rng. C th l i vi tn s iu ch trn 1MHz th s thay i tn s l t 100 n 500MHz/mA K thut iu ch ngoi cng c th p dng cho FSK bng cch s dng cch t Bragg hoc b giao thoa Mach-Zehnder MZI Trong dng iu ch FSK, thng tin c truyn trn sng mang c tn s S theo biu thc (4.1). Vi dng iu ch ny, ng bao sng mang khng thay i, cn tn s S c hai gi tr l (S - ) v (S + ) ty thuc tn hiu pht i l bit 0 hay bit 1. Do biu thc ton hc biu din dng iu ch FSK c dng: eS(t) = Emcos[St + b(t)2f] vi (4.6)

f = /2 gi l lch tn Nu gi 2f l lch tn nh - nh th khuch i tn s

i lng

c gi l h s

Khi = 0,5 th khi ny iu ch FSK c gi l MSK (Minimum Shift-Keying). Dng ph cng sut c dng nh hnh 4.5, ph b nn cht nn dng iu ch ny rt hp dn cho cc h thng tc cao.

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Khi = (0,5 0,7) th dng iu ch ny c gi l CPFSK (Continuous Phase Frequency Shift-Keying) hay cn gi l iu ch lch tn hp. Dng ph cng sut ca n b nn rt cht. Khi >> 1 th c coi l iu ch FSK lch tn rng. Dng ph cng sut c dng nh hnh 4.6, ph ca n c chia lm hai thnh phn tp trung xung quanh (fS - f) v (fS + f).

Nh vy rng bng tn tng rt rng, do kiu ny khng ph hp vi h thng tc cao. Thc t ch p dng cho h thng n gin, r tin. Cn trng hp 1 thc tn khng quan tm v tn s ca tn hiu khng l hng s trong khi iu ch. c. PSK (iu pha) iu ch dch pha tn hiu quang cng c p dng cho h thng thng tin quang coherent. Khi tn s ca laser c iu chnh chnh xc vi tn s tn hiu iu ch, lc ny quan h pha gia tn hiu ng ra vi tn hiu iu ch l 0. c c s thay i quan h pha l /2 th ta phi iu chnh li tn s ca laser. Trong dng iu ch PSK, cc bit tin 0 v 1 c truyn ti i thng qua s thay i pha ca tn hiu quang trong khi bin v tn s khng thay i. Biu thc ton hc biu din dng iu ch PSK nh sau: eS(t) = Emcos[St + b(t)] vi14

(4.8)

Vi gi tr ca b(t) ta nhn thy pha ca tn hiu iu ch nhn hai gi tr l 0 v . Dng ph ca tn hiu PSK cng ging nh ASK nhng c vch ph sng mang. C th s dng phng php tch sng homodyne v heterodyne. Tuy vy gii iu ch ca n rt phc tp nn thc t t dng. i vi dng iu ch PSK, tch sng coherent cn duy tr cng quang khng i v ton b thng tin c th b mt nu nh tn hiu quang c tch sng trc tip m khng trn n vi tn hiu dao ng ni. iu ch DPSK (Differential PSK) l dng iu ch m thng tin c m ho theo s chnh lch v pha gia hai bit k cn. Chng hn, nu k biu din cho pha ca bit th k th lch pha = k - k-1 s thay i l hoc 0 ph thuc vo bit th k l bit 1 hay bit 0. u im ca iu ch DPSK l tn hiu pht c th c gii iu ch thnh cng cho cho n khi pha ca sng mang duy tr n nh trong khong thi gian hai bit.

V. B TCH SNG PHOTODIODE PIN (Passive Intricsic Negative): 1. Cu trc c bn ca PIN Gm cc vng bn dn P-N gia l mt lp t dn I rt mng, khng pha tp cht hoc pha tp cht rt t nn khng c in t t do nn c in tr rt ln. Khi diode PIN c phn cc ngc th diode khng c dng, ch c dng ngc rt nh gi l dng ti. Khi nh sng i vo photondiode nu mt photon sinh ra ti lp P, I, N mt cp in t v l trng. Cc in t th b ht v min N v c in p dng, cn cc l trng th v pha

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min P c in p m nh in trng ngoi. Tt c cc phn t mang in ny sinh ra mt ngoi mt dng in v in p. Qu trnh pht ra cc cp in t v l trng cn gi l ht mang quang

IpL trng

RL: in tr ti Tn hiu ra

P

i in t - n - in t

Vng cm

Vng dn

hv Eg

n

L trng

Vng ha tr

Vng ngho

S vng nng lng ca photodiode PIN

Lung dng to ra trn ti gi l photon, tn s v cng dng ny ph thuc vo nh sng u vo. Tuy nhin di ca xung nh sng a vo phi ln hn thi gian tri tdtd = d vs

cn thit cc phn t mang in chy qua vng tri vi rng d (trong min i).

Vs: Tc tri bo haVs = 10 4 10 5 m / s

Photodiode: y l b t c h s n g q u a n g t h n g d n g n h t c s d n g . c i m c a c c Photodiode PIN l c thi gian p ng nhanh v hiu sut lng t cao. N khng c kh nng khuch i dng quang in nhng n li trnh c s khuch i nhiu.

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Kh nng thm nhp nh sng ph thuc vo b dy lp P . nh sng c bc sng cng dy cng d thm nhp vo bn dn. 2. Nguyn l hot ng - Nguyn l hot ng ca Photodiode PIN da trn hiu ng quang in. Khi chiu mt photon c nng lng ln hn nng lng vng cm vo b mt bn dn ca Photodiode th qu trnh hp th photon xy ra. Khi hp th mt photon, mt in t c kch thch t vng ha tr ln vng dn li mt trong vng ha tr mt l trng, ta ni photon to ra mt cp in t v l trng . Cc cp in t - l trng ny c sinh ra trong vng ngho. Khi c in trngt vo linh kin, s c s chuyn ri cc in tch v hai cc to ra dng in mch ngoi, dng in nyc gi l dng quang in. Bnh thng mt photon ch c th to ra mt cp in t - l trng, vi gi thit hiu sut lng t bng 1, ngha l vi mt lng photon xc nh ch c th to ra mt dng in xc nh. Tuy nhin trong thc t khng c nh vy v nh sng cn b tn hao do nhiu yu t trong c yu t phn x b mt. - Nh vy kh nng thm nhp ca nh sng vo lp bn dn thay di theo bc sng. V vy, lp bn dn P khng c qu dy. Min I cng dy th hiu sut lngt cng ln, v xc sut to ra cc cp in t v l trng tng ln theo dy ca min ny v do cc photon c nhiu kh nng tip xc vi cc nguyn t hn. Tuy nhin,nu di min I cao th thi gian tri ca cc ht ny di hn, xung nh sng a vocng phi tng ln tng ng vi thi gian tri tng. iu ny khin cho p ng v bng tn iu bin b hn ch. Do , rng ca min I khng c qu ln v nh th tc bt s b gim i. Ta phi chn di min I rng m bo iukin nht nh l hp th ht photon trong vng ngho v khng nh hng thi gian tri. - Tm li PIN hot ng da trn nguyn l hp th nh sng bin i tn hiu quang thu vo thnh dng tn hiu in. 3. c tnh ca PIN - c tnh ca Photodiode thng c c trng bi h s p ng (cn gi l nhy ca ngun thu) v hiu sut lng t .17

- c tuyn V-I ca photodiode Si c R= 0,5 A/W c v nh hnh trn. - Dng ti sinh ra do nhit to ra cc ht mang in , do n cn c tn gi l dng r phn cc ngc. - Nu c tn hiu quang nh th dng photon nh nn c th khng pht hin c tn hiu ny v dng photon nh b dng ti che lp mt. 4. u im nhc im PIN u im: - so vi APD th dng nhiu ca Pin t hn do dng ti nh - t nhy vi nhit - p phn cc thp thng < 20V - Chi ph ca PIN thp - n nh cao Nhc im: - Diode PIN c tp m ni nh, song khng c kh nng khuch i dng in nn khng nng cao nhy my thu. - Cc diode PIN ch hot ng vi tc bit khng ln n vi ba trm Mbit/s VI. B LC THNG THP BESSEL 1. Khi nim. - B lc Bessel l mt loi b lc tuyn tnh vi ti a bng phng chm tr nhm (ti a phn ng giai on tuyn tnh). B lc Bessel thng c s dng trong cc h thng m thanh cho . Tng t cc b lc Bessel c c trng bi s tr hon nhm gn nh lin tc trn ton b cc di thng, do bo tn hnh dng sng tn hiu trong di thng lc. - Mt Bessel b lc thng thp c c trng bi chc nng chuyn giao :

n (s) l mt o ngc a thc Bessel m t cc b lc c tn ca n v 0 l mt tn s la chn cung cp cho cc tn s ct mong mun. B lc c mt s chm tr tn s thp nhm 1 / 0.18

2. a thc Bessel

- Cc a thc Bessel o ngc c cho bi:

ni

V d : - Chc nng chuyn giao cho mt trt t th ba (ba cc) Bessel qua b lc thp, bnh thng c s chm tr nhm n v,

- Cc gc r ca cc a thc mu s ca b lc cc, bao gm mt cc thc ti s = -2,3222, v mt cp phc tp lin hp cc ti s = -1,8389 1,7544 j, m mu trn. T s 15 c chn cung cp cho mt tng ca 1 ti DC (s = 0). t c sau c

Giai on

S chm tr nhm

M rng chui Taylor ca s chm tr nhm19

- Lu rng hai iu khon trong 2 v 4 l bng khng, kt qu l mt s chm tr nhm rt phng = 0. y l s lng ln nht t ng c th c thit lp khng, k t khi c tng cng bn h s trong cc a thc Bessel th ba , i hi bn phng trnh c xc nh. Mt phng trnh xc nh c c thng nht ti = 0 v th hai xc nh rng tng c s khng = , li hai phng trnh xc nh hai nhim k trong vic m rng hng lot bng khng. y l mt ti sn chung ca s chm tr nhm cho mt b lc Bessel n n u tin - 1 iu khon trong vic m rng lot cc s chm tr nhm s c khng, do ti a ha phng ca s chm tr nhm ti = 0.

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