TP ON BU CHNH VIN THNG VIT NAM HC VIN CNG NGH BU CHNH VIN THNG C S THNH PH H CH MINH CHUYN THNG TIN QUANG:

QUN L TN SC Phng php b tn sc bng DCF Gio vin hng dn: Th.S Phm Quc Hp Nhm SV thc hin : Nhm 3 Lp: 07VTA2+3 Thnh Ph H Ch Minh Thng 6 - 2011 CHUYN THNG TIN QUANG: QUN L TN SC Phng php b tn sc bng DCF Gio vin hng dn : Th.S Phm Quc Hp Nhm SV thc hin : Nhm 3 Lp: 07VTA2+3 Danh sch thnh vin nhm 3 thc hin chuyn : STTH v tnM s Sinh vinLp 1Nguyn Hong Lm40716013307VTA3 2inh Tun Anh 40716011107VTA3 3V Trng i40716011707VTA3 4Trng Nhm Thanh40716009407VTA2 5Trn Quang Nht Huy40716012907VTA3 6Nguyn Vn Linh 407160134 07VTA3 7Nguyn Hu Sn40716014907VTA3 8Nguyn Thanh Tm40716015107VTA3 9L T Ph M 40716013907VTA3 10Trn Ngc Sn40716009007VTA2 MC LC CHNG 1: GII THIU CHUYN ...................................................................................... 1 CHNG 2: KHI NIM TN SC, CC LOI TN SC,NH HNG CA TN SC TRONG THNG TIN QUANG............................................... 4 2.1 Khi nim tn sc........................................................................................................................ 4 2.2 Cc loi tn sc............................................................................................................................ 5 2.2.1 Tn sc mode (Modal Dispersion)..................................................................................... 6 2.2.2 Tn sc trong si n mode ............................................................................................... 9 2.2.2.1 Tn sc vn tc nhm GVD...................................................................................... 10 2.2.2.2 Tn sc vt liu .......................................................................................................... 12 2.2.2.3 Tn sc ng dn sng ................................................................................................ 13 2.2.2.4 Tn sc bc cao hn................................................................................................... 16 2.2.2.5 Tn sc phn cc mode PMD................................................................................... 17 2.3 nh hng ca tn sc trong h thng thng tin quang.................................................... 19 CHNG 3: CC PHNG PHP B TN SC................................................................... 22 3.1 S cn thit ca vic qun l tn sc........................................................................................ 22 3.2 K thut b tn sc trc (Precompensation).......................................................................... 24 3.3 K thut b tn sc trn ng dy (In-line)........................................................................... 25 3.3.1 B tn sc bng si quang DCF........................................................................................ 25 3.3.2 B tn sc bng b lc quang............................................................................................ 26 3.3.3 B tn sc bng tn hiu lin hp pha OPC...................................................................... 27 3.3.4 B tn sc bng cch t Bragg.......................................................................................... 27 3.4 K thut b sau (Post compensation)....................................................................................... 28 CHNG 4: PHNG PHP B TN SC BNG DCF...................................................... 30 4.1 C s ton hc........................................................................................................................... 30 4.2 Cc thng s k thut ca si quang DCF............................................................................... 31 4.3 B tn sc n knh .................................................................................................................. 33 4.4 Kt qu th nghim b tn sc bng DCF............................................................................... 36 4.5 B b tn sc ca hng JDSU................................................................................................... 36 CHNG 5: KT LUN.............................................................................................................. 39 TI LIU THAM KHO............................................................................................................... 40 Nhm 3 B tn sc bng DCFPage 1 1 CHNG 1: GII THIU CHUYN Vin thng Vit Nam ang c nhng bc pht trin mnh m v vt bc, tr thnh mttrongnhngngnhkinhtkthutminhn,nggpmnhmvo s phttrin kinh t x hi ni chung ca t nc. Ngnh Vin thng cng l ngnh i tin phong v t c nhng thnh cng ln trong vic mang chung i nh x ngi bng vic Tp on vin thng Qun i Viettel m rng u t kinh doanh cc th trng ngoi nc nhLo,Campuchia,Mozambique,Haiti,PeruvsptilUcraina,Argentina, Cuba.VaitrcangnhVinThngrtquantrngtrongcvicphttrinkinhtv m bo an ninh Quc phng. Cuc sng cng ngycngc nng cao, nhu cutrao ithng tin cngvic v gii tr ca con ngi ngy cng cao, khng ch n gin l vic truyn thoi truyn thng, nhu cu truyn d liu cng ngy cng i hi v bng thng v dung lng ng truyn. Trong Vin thng, h thng truyn dn c hai dng l v tuyn v hu tuyn. Do h thng v tuyn c nhng hn ch c th nn truyn dn hu tuyn vn l hnh thc truyn dn hiu qu v quan trng nht. Trong truyn dn hu tuyn s dng cp ng v cp quang. Cp ngkhng th m trch c bng thng v dung lng ng truyn rt ln t nhu cu ca con ngi. Truyn thng si quang ra i nh du mt bc pht trin mnh m ca cng ngh truyn dn, vi nhng u im ni tri nh bng thng ln, tc cao, suy hao thptruyn dn quangtrthnh cngnghtruyn dnchnh trong cc ng dng tc cao v mng truyn dn ng trc. Truyn dn thng tin quang bng cp si quang t khi ra di n nay tri qua nhiu thi k pht trin. Ban u l si quang a mode c suy hao cao, vi c ly truyn dn vi km n si quang n mode c suy hao thp vi c ly truyn dn tng ln hng chc, thm ch hng trm km. Cp quang l gii php u tin cho h thng vin thng ng di v quc t c tc truyn dn cao v rt cao, s dng trn t lin v vt i dng. Tuynhin. hthngthngtinquangdunglngln sgpphi3vn lncn quan tm nh : Suy hao, tn sc v hiu ng phi tuyn, lm gim cht lng v c ly truyn dn ca h thng. Vn suy hao c th c gii quyt n gin bng vic s dng cc b khuchiquangEDFAtrongmngWDM.Ngynay,huhtcchthngtruyndn quangcthitkhotng trong bng C,vng bcsng1530nm 1565nm,bi u im suy hao rt thp trong vng ny. Cc hiu ng phi tuyn c th b qua i vi cc h thng thng tin quang hot ng mc cng sut va phi khong vi mW vi tc bit ln n 2,5 Gbps. Tuy nhin cc tc bit cao hn nh 10 Gbps th chng ta phi xem xt cc nh hng ca hiu ngphi tuyn. Cc nh hng ca cc hiu ng phituyn c th gim i khi s dng si quang c dintch li hiu dng ln. V vy vn tn sc l vn ln nht ca cc h thng thng tin quang. Khi chng ta s dng cc b khuch i quang EDFA b suy hao th n li gia tng s tn sc, mt b khuch i quang khng khi phc tn hiu c khuch i thnh tn hiu gc ban u. Do tn sc c tch ly qua cc b khuch i lm gim kh nng truyn tn hiu. Nhm 3 B tn sc bng DCFPage 2 2 Nhchngtabit,nhhngcatnscntcbittruyntiacasi quang nh sau: max14tBr BrDs =Hay 14 .tBrd Lssuy ra 14tBr Ld s Vi tdl tn sc ca si quang. T cng thc trn ta d dng nhn thy mi quan h nghch o gia tc bit v c ly truyn dn khi mt si quang c tn sc l td . Do tngtc bit truyn dn v tng c ly truyn dn th phi gim thng s tnsc td xungmcthpnhtcth.cnhiumhnhiukhintnscc nghincusut thp nin 1990 hn chtcng catn sc trong hthng thng tin quang.Trongbichuynnysgiithiuvccphngphpbtnscc nghin cu t trc n nay. C th phn loi thnh 3 phng php b tn sc gm k thut b trc (Precompensation), k thut b tn sc trn ng dy (In-line) v k thut b sau (postcompensation).Trongkthutbtrnngdycncccloinh :btnsc bng si quangDCF,bng blc quang, ccht Bragg hay bng tn hiu quanglin hp pha. Trong khi hai phng php b trc v b sau t ra kh hn ch khi chiu di tuyn quang ln, i hi phi b tn sc theo tng chng th phng php b tn sc trn ng truyn t ra rt hiu qu v c s dng kh ph bin trong thc t.Mc d phng php dng si b tn sc (DCF) b hn ch v suy hao ghp kh ln, cng sut trong si quang phi m bo nh cc hiu ng phi tuyn khng xy ra v gi thnh lp t cao, tuy nhin DCF vn c s dng rng ri v di bc sng hot ng rng hay khc phc thi gian tr nhm tt, c bit l tnh n gin ca phng php ny. Do chuyn ca Nhm 3 quyt nh chn DCF l phng php b tn sc nghin cu v tho lun chi tit trong chng 4 ca cun bo co. Trong k thut s dng DCFny,tnscdngtrongsitruyndncbvitnscmcaDCFxentrn ngtruyn. DCF cthitktrong mtmodulekchthcnh gnv c nhiu mc chn la tn sc. Yu cu phi c i vi DCF l suy hao xen thp, suy hao ph thuc phn ccthp,tnhphituynthp.DCFcngphicngknhlinhchokchthc module nh li bi v module DCF nn chim mt khng gian nh. Bi bo co c chia thnh 5 chng nh sau: - Chng 1: Gii thiu Nhm 3 B tn sc bng DCFPage 3 3 - Chng2:Khinimtnsc,ccloitnscvnh hngcatn sc trong cc h thng thng tin quang - Chng 3: Gii thiu v cc loi k thut b tn sc trc, b tn sc sau hay b tn sc ng dy - Chng 4: M t v phng php b tn sc s dng si quang DCF, ccthngskthutcaDCFvmodulebtnscDCFangctrnth trng. - Chng 5: Kt lun bi bo co, u v khuyt im ca phng php s dng DCF. Nhm 3 B tn sc bng DCFPage 4 4 CHNG2:KHINIMTNSC,CCLOITNSC, NH HNG CA TN SC TRONG THNG TIN QUANG 2.1 Khi nim tn sc: Tn sc l hin tng tn hiu quang truyn qua si quang b gin ra. Nu xung gin ra ln hn chu k bt s dn ti s chng lp gia cc bt k cn nhau. Kt qu l u thu khng nhn din c bt 1 hay bt 0 c truyn i u pht, dn ti b quyt nh trong u thu s quyt nh sai, v khi t s BER tng ln, t s S/N gim v cht lng h thng gim. Hnh di y minh ha cho s m rng xung do tn sc. Gi D l tn sc tng cng ca si quang, n v l giy (s). Khi D c xc nh bi: 2 2i oT T D =trong Ti, T0 ln lt l rng ti im mt na cng sut cc i ca xung ng vo v ng ra ca si quang (n v l s). tn sc qua mi km si quang c tnh bng ns/km hoc ps/km. i vi loi tn sc ph thuc vo b rng ph ca ngun quang th lc n v c tnh l ps/km-nm. L i o Po Po/2 Minh ha s m rng xung do tn sc Nhm 3 B tn sc bng DCFPage 5 5 Tn sc gy ra tng BER 2.2 Cc loi tn sc: Trong Thng Tin Quang ngi ta chia ra thnh 3 loi tn sc nh sau: tn sc mode, tn sc phn cc mode v tn sc sc th (trong tn sc sc th bao gm tn sc ng dn sngvtn sc vt liu).Khi sitruyn dn lamode (tc loi si quang cth truyn cnglcnhiumodesngkhcnhautrongli)thtacttcccloitnscnitrn. Nhngkhicngnghchtosiphttrinth sinmoderaivnkhcphc ctn sc mode ca siamode. Tuy nhin,v bn cht chit sutSilica lphthuc vo bc sng, hn na ngun pht khng th phtra nh sngn sc (nh sng ch c mt bc sng) m lmt chm tia sng vi mt rng ph no . Chnh v th trong si n mode vn cn tn ti tn sc, l tn sc phn cc mode v tn sc sc th. Ngy nay,vicngnghchtophttrinmnhmngitachtoracccloisi quangmicmctn scgimngk.Nhng si nyc dnglpttrongcc mng mi cn tc bt cao v c ly ln. Sau y ta s tm hiu khi nim c bn v cc loi tn sc trong si quang: Hnh sau y m t s cc loi tn sc trong si quang. Nhm 3 B tn sc bng DCFPage 6 6 2.2.1 Tn sc mode (Modal Dispersion): Mt mode sng c th c xem l mt trng thi truyn n nh ca nh sng trong si quang. Khi truyn trong si quang, nh sng i theo nhiu ng khc nhau, trng thi n nh ca cc ng ny c gi l nhng Mode sng. C th hnh dung gn ng mt mode sng ng vi mt tia sng. Tnscmodeldonnglngcanhsngbphntnthnhnhiumode.Mi modelitruynvivntcnhmkhcnhau,nnthigiantruynnuthucacc mode khc nhau l khc nhau gy ra tn sc. R rng ta thy tn sc mode ch tn ti sia mode, do mun loi b tn sc mode th ta phi s dng si n mode. V vy khi xt n tn sc mode ta ch xt si a mode. Nhtabit,khus(NA)biudinkhnngthunhsngcasiquang. Khu s cng ln th cng d hng nh sng vo si quang.Nhvy tac cm gic nh khu s cng ln th cng tt. Nhng iu ny l khng ng, c mt tr ngi khin ta khng th tng khu s ln. hiu c iu ny ta hy xem xt cc mode trong si quang. Sthtlnhsngchcthtruyntrongsiquangnhmttphpcanhng lung sng hoc nhngtiasngring l. Nicch khc,nu ta ckh nng nhn vo si Tn sc tng Tn sc modeTn sc phn cc mode Tn sc sc th Tn sc vt liuTn sc ng dn sng Si n mode Si a mode S cc loi tn sc trong si quang. Nhm 3 B tn sc bng DCFPage 7 7 quang ta s thy mt tp hp nhng lung sng truyn vi gcobin thin t 0 n co nh c minh ha hnh sau: Nhng lung sng khc nhau c gi l nhng mode. Ta phn bit cc mode bng gc truyn ca chng, hay nh s th t ch nhng mode ring bit. Nguyn tc l: gc truyn ca mode cng nh th s th t ca mode cng thp. Nh vy mode truyn dc theo tm si l mode 0 (hay cn gi l mode c bn) v mode truyn gc truyn ti hn (co ) l mode c s th t ln nht c th ca si quang. Nhiu mode c th cng tn ti trong si quang, v si quang c nhiu mode truyn c gi l si a mode.S lng mode: s lng mode ca si quang ph thuc vo c tnh quang v hnh hc ca si . Nu ng knh li cng ln, li cng cha c nhiu mode sng. V bc sng nh sng cng ngn th si quang cng cha c nhiu mode sng. Nu khu s cnglnth slngmode sngsithu ccngnhiu.Nhvycthktlunls lng mode sng trong si quang t l thun vi ng knh si (d), khu s(NA) v t l nghch vi bc sng nh sng s dng ( ). Gi V l tn s chun ha, ta c:NAdVt=vi 2221n n NA =th s lng mode c tnh nh sau: 22VN =( i vi si SI ), 42VN =(i vi si GI) Nh vy ta thy i vi si a mode khi lung sng pht ra t ngun quang i vo si quang chia thnh mt tp hp mode. Trong si, cng sut quang tng cng c mang bi nhiu mode ring l, v ti u ra nhng phn nh hp li thnh lung ra vi cng sut ca n. Hnh sau s minh ha cho vn trn (vi 4 mode lm v d). n1 li n2 lp bc n2 lp bc 1o2o 3ocoCch thc cc lung sng tng ng vi cc mode i trong si quang Nhm 3 B tn sc bng DCFPage 8 8 T hnh trn ta thy rng xung tn hiu sau si quang c bt u bng mode 1 v kt thc bng mode 4. Do tr v thi gian gia cc mode nn xung tn hiu b gin ra (T0 > Ti). Tn sc mode trong si SI Pi = Pi1 + Pi2 + Pi3 + Pi4 Pi1 Pi2 Pi3 Pi4 Po = Po1 + Po2 + Po3 + Po4

l1 l2 l3 l4 t1 t2 t3 t4 Pi t1 t2 t3 t4 Po1 Po2 Po3 Po4 Ti To > Ti Cch thc cng sut quang c mang bi cc mode truyn trong si quang v gy tn sc Nhm 3 B tn sc bng DCFPage 9 9 Tn sc mode trong si GI 2.2.2 Tn sc trong si n mode: Nh ta xt phn trn, tn sc mode l nguyn nhn ch yu gy ra s hn ch tc bt trong h thng Thng Tin Quang s dng si a mode. iu ny khng c ngha l trong si a mode ch c tn sc mode, m n cn chu nh hng ca nhiu loi tn sc khc. Tuy nhin do tn sc mode c nh hng ln hn c nn ta ch xt tn sc mode trong si amode.khc phc tn scmode ngi ta ch to raloi si quang ch truyn mtmode sng, si quang nh th cgi lsi nmode (SMF-SingleModeFiber). R rng ta thy si n mode khc phc c hon ton tn sc mode. V thtctruyndnc ci thin ngkv tng c cly thng tin. Tuy nhin v si n mode vn c ch to t Silica nn n s cn chu nh hng ca cc loi tnsckhcnhtnscscthvtnscphnccmode.Trongtnscscthl nguyn nhn chnh gy hn ch tc bt. By gita si kho sthintngtnsc scthtrong si quang. y khng mt tnhtng qutkhi ta xttn sc scth trong sin mode. Cth ni nguyn nhn su xa ca tn sc sc th l do b pht quang (LED, LAZER) khng pht ra nh sng n sc (nh sng ch c mt bc sng), m n pht ra mt chm tia sng c bc sng trung tm (ti cng sut pht cc i) v cc bc sng bin, hay cn gi l rng ph ngun pht. Tc l ngun pht pht ra nh sng nm trong mt di tn (di bc sng). M nh ta Nhm 3 B tn sc bng DCFPage 1 10 0 bit th chit sut ca silm t Silical mt hmphthuc vo bc sng (haytn s), nnvn tclantruyn ca ccthnh phntn skhc nhau lkhc nhau,vnph thuc vo bc sng theo cng thc sau:) ( / n c v = . Tnscscthchainguynnhnsinhran:Thnhtnhtabitlccthnh phntnskhcnhaudichuynvivntckhcnhau,vtnscdonguynnhnny ngi ta gi l tn sc vt liu, y l nguyn nhn ch yu ca tn sc sc th. Tuy nhin cn c thnh phn tn sc th hai l tn sc ng dn sng, m nguyn nhn sinh ra n l do nng lng nh sng truyn i c mt phn trong li v mt phn trong lp bc. S phn b nng lng gia li v lp bc l mt hm ca bc sng, c th l nu bc sng di hn th nng lng trong lp bc nhiu hn. Nh vy nu bc sng thay i, s phn b nng lng s thay i v kt qu l h s lan truyn|cng thay i. y chnh l s gii thch cho tn sc ng dn sng. Bn cnh tn sc sc th v tn sc phn cc mode, trong si quang cn tn ti mt loi tn sc khng km phn quan trng l tn sc vn tc nhm. Nh ta bit, u im chnh ca si n mode l khng c tn sc mode bi v nng lng a vo xung ch c chuyn ch bi mt mode n duy nht. Tuy nhin s m rng xung khng bin mt hon ton, v vn tc nhm ca mode c bn th ph thuc vo tn s do tn sc sc th. Kt qu l cc thnh phn ph khc nhau ca xung truyn c vn tc nhm khc nhau mt cht, v tn sc c nguyn nhn nh trn c gi l tn sc vn tc nhm. By gi ta i kho st nh hng ca cc loi tn sc nu trn trong si n mode. 2.2.2.1 Tn sc vn tc nhm (GVD Group Velocity Dispersion) Xt si n mode c chiu di L. Mt thnh phn ph ring bit ti bc sng c tn s gc l es n ng ra cui si quang sau mt tr v mt thi gian l gvLT =trong gv l vn tc nhm v c nh ngha l: [1] 1) (=e|ddvg(2.4-1) Bngcchpdngc n k n /0e | = = ( | lhngslantruyn)vophngtrnh(2.4-1) ta c th tnh c g gn c v / =trong gn l chit sut nhm v c cho bi: (2.4-2) ) / ( e e d n d n ng+ = Nhm 3 B tn sc bng DCFPage 1 11 1 S ph thuc vo tn s ca vn tc nhm dn n s m rng xung, n gin l bi v cc thnh phn ph khc nhau ca xung b tn sc trong sut s lan truyn v khng n ng ra ca si cng mt lc. Nue A l rng ph ca xung th phm vi m rng xung i vi si c chiu di L c cho bi: e | ee|eeeeA = A = A||.|

\|= A = A222LddLvLddddTTg(2.4-3) trongphngtrnh(2.4-1)csdngbini.Thams 2 22/e | | d d =c gi l tham s tn sc vn tc nhm . N quyt nh xung quang b m rng bao nhiu khi lan truyn trong si. Trong mt s hthng ThngTin Quang,stri ravmt tn se Acquyt nhbidiccbcsng A phtbingunquang.Thngthngtasdng A thay choe A . Bng cch s dng t e / . 2 c =v( ) t e A = A2/ . 2 c . Phng trnh (2.4-3) c th c vit li: A = A||.|

\|= A DLvLddTg (2.4-4) trong :

22. 2 1|tcv ddDg =||.|

\|= (2.4-5) D c gi l h s tn sc v c n v l ps/km-nm. [1] nh hng ca tn sc ln tc bt B c th c c tnh bng cch s dng iu kin1 . < AT B .BngcchsdngT A trongphngtrnh(2.4-4)thiukinnytr thnh: 1 . . < A D L B(2.4-6) Phng trnh ny cho ta mt c tnh v tch B.L ca si quang n mode. i vi siquangnmodechunthDtnginhtrongvngbcsngm u 3 , 1 (Dxpx 1ps/km-nm) . i vi Lazer bn dn c rng ph A t 2 n 4nm th gi tr B.L c th vt qu 100Gbps-km . Trong thc t ta c cc h thng hot ng ti bc sngm u 3 , 1 c tc bt l 2Gbps vi khong cch trm lp t 40 n 50km. Gi tr B.L ca si n mode cthvtqu1Tbps-kmkhiLazerbndnnmodecsdnggim A di 1nm. Nhm 3 B tn sc bng DCFPage 1 12 2 H s tn sc D thay i ng k khi bc sng lm vic dch ra khim u 3 , 1 . S ph thucvobcsngcaDldosphthucvotnscachitsutmoden .T phng trnh (2.4-5) D c th c vit li:||.|

\|+ =||.|

\| =222 222 1 . 2eee te tdn ddn dv dd cDg(2.4-7) trongphngtrnh(2.4-2)csdngbini.Ngitachngminh c D c th c vit nh l tng ca hai s hng: [1] D = DM + DW( 2.4-8) trong DM l tn sc vt liu, DW l tn sc ng dn sng v c cho bi: e tddnc ddnDg gM2 221 2= = (2.4-9)

+A =dVVb dddndVVb VdnnDg gW) ( ) ( 22222222e e t(2.4-10) yn2glchit sutnhmcalpbc,Vltn schunha,blhngslan truyn chun ha,A ls chnh lch chitsut tng igialiv lp bc.Trong cc phng trnh t (2.4-8) n (2.4-10) th tham sA c xem l c lp vi tn s. 2..2.2.2 Tn sc vt liu (Material Dispersion) Tn sc vt liu xy ra do chit sut ca Silica (nguyn liu c s dng ch to si quang) thay i theo tn s quange (tc ph thuc vo bc sng tn hiu). Hnh sau y s chothy s phthucvo bc sng ca chit sut (n) v chit sut nhm (ng) trong di t 0,5 m un 1,6 m ui vi si Silica nng chy. Nhm 3 B tn sc bng DCFPage 1 13 3 Tn sc vt liu (DM) c lin h vi dc ca ng bi cng thc (2.4-9) nh sau: ) (1 ddncDgM =Mv dng/d = 0 ti bc sng1,276 m unn DM = 0ti ZD =1,276 m u (ZD c gi l bc sng tn sc 0). H s tn sc DM m khi < ZDv dng khi > ZD . Trong di bc sng t 1,25 n 1,66 m uth DM c th c xp x bng cng thc [1]: ) / 1 ( 122 ZD MD ~ (2.4-11) Lu : ZDch bng 1,276 m ui vi si Silica thun khit. Gi tr ca ZDc th thay i trong di t 1,27 m un 1,29 m ui vi si quang m li v lp bc c pha tp cht thay ichit sut. Bcsngtn sc 0(ZD ) ca si quang cng ph thucvo bn knh li (a) v bc nhy chit sut ( A) ca si quang . 2.2.2.3 Tn sc ng dn sng (Waveguide Dispersion) Trong si a mode, tn sc ng dn sng l mt phn nh trong tn sc tng, do thng thy thut ng tn sc sc th v tn sc cht liu c th s dng hon chuyn cho nhau khi xt si a mode [1]. Nhng i vi si n mode th tn sc ng dn sng li l mt thnh phn tn sc quan trng. Tn sc vt liu v tn sc ng dn sng ph thuc ln nhau v do ta phi xt chng cng nhau [1]. Do xp x nn ta c th b qua s ph thuc xt ring chng. Bc sng ( m u ) Chit sut S thay i ca chit sut n v chit sut nhm ng theo bc sng ca Silica nng chy. Nhm 3 B tn sc bng DCFPage 1 14 4 Tn sc ng dn sng xut hin l do nh sng c truyn bi cu trc l si quang. C ch gy ra tn sc ng dn sng trong si n mode nh sau: Sau khi i vo si quang, mt xung nh sng mang thng tin s c phn b gia li v lp bc nh c minh ha hnh sau: Hai thnh phn nh sng trong li v lp bc truyn vi vn tc khc nhau (do li v lp bc c chit sut khc nhau), nn n cui si quang vo cc thi im khc nhau gy ra tn sc. Thnhtrntathytnscng dnsngphthucvo sphn btrngmode gialiv lp bc,tc phthuc vong knh catrngmode(MFDModeField Diameter) m MFD li ph thuc vo bc sng, do tn sc ng dn sng l ph thuc vo bc sng. Tn sc ng dn sng (DW) c tnh nh trong phng trnh (2.4-10) v ph thuc vo tham s V ( tn s chun ha ) ca si.

+A =dVVb dddndVVb VdnnDg gW) ( ) ( 22222222e e t Hnh sau cho thy s thay i cadV Vb d / ) (v 2 2/ ) ( dV Vb Vdtheo V Core Diameter MFD n2 n1 ( ) % 4 , 0 /2 1< = A n n ntypical S phn b cng nh sng trong si n mode. MFD l ng knh trng mode. Nhm 3 B tn sc bng DCFPage 1 15 5 T hnhta thy cdV Vb d / ) (v 2 2/ ) ( dV Vb Vdu dng nntheo cngthc (2.4-10) th DW m trong ton b di bc sng t 0 n 1,6 m u . HnhsauyschothyDM,DWvtngcachngD=DM+DWcamtsi quang n mode thng thng. Tn sc [ ps/(km-nm)] Bc sng ( m u ) Tn sc tng cng D v s phn b tng i ca tn sc cht liu ( DM) v tn sc ng dn sng ( DW) ca si n mode thng. Bc sng tn sc 0 (ZD ) dch n gi tr cao hn nh s phn b ng dn sng Cng ( Magnitude ) Tn s chun ha V S thay i ca b ( tn s lan truyn chun ha ) v o hmca n d(Vb)/dV v V[d2(Vb)/dV2] theo tham s V Nhm 3 B tn sc bng DCFPage 1 16 6 Ta thy tn sc ng dn sng (DW) lm cho bc sng tn sc 0 (ZD ) dch khong 30 n40nmtn sc tng (D) bng 0 gn bc sng 1,31 m u .Ngoi ra, tn sc ng dn sng cn lm gim tn sc tng t gi tr tn sc vt liu (DM) trong di bc sng t 1,3 m u n1,6 m u .GitrtiubiucaDlt15n18ps/(km-nm)gnbcsng 1,55 m u . Khi D ln s hn ch hot ng ca h thng ti bc sng 1,55 m u . V DW ph thuc vo tham s si nh bn knh li a v s chnh lch chit sutA nn ta c th thit k si bc sng tn sc 0 dch n ln cn gi tr 1,55 m u . Si nh th gi l si dch tn sc. Ta cn c th ghp s phn b ng dn sng D tng i nh qua mt di bc sng t 1,3 m un 1,6 m u , si loi ny gi l si san bng tn sc. Hnh sau y cho thy cc v d tiu biu v s ph thuc bc sng ca D i vi si chun (si thng), si dch tn sc v si san bng tn sc. 2.2.2.4 Tn sc bc cao hn (Higher-Order Dispersion) T phng trnh (2.4-6) ta thy gi tr B.L ca si n mode c th tng ln n v cng khi hot ng ti bc sng tn sc 0 (ZD ) v D=0. Tuy nhin cc nh hng tn sc khng bin mt hon ton ti bc sng tn sc 0. Cc xung quang vn b tri rng do nh hng ca tn sc bc cao hn. Tn sc tng (D) khng th bng 0 ti tt c cc bc sng trong di ph ca mt xung tp trung ti bc sng tn sc 0. R rng l s ph thuc vo bcsng caD sng mtvai tr quantrng trong smrngxung.Cc nh hng Bc sng (m u ) Tn sc [ps/(km-nm)] S ph thuc vo bc sng ca h s tn sc D i vi si chun, si dch tn sc v si san bng tn sc. Nhm 3 B tn sc bng DCFPage 1 17 7 ca tn sc bc cao hn b chi phi bi dc tn sc d dD S / = , tham s S cn c gi l tham s tn sc vi phn. Gi tr ca dc tn sc (S) ng mt vai tr quan trng trong vic thit k cc h thng WDM hin nay. V S > 0 i vi hu ht cc loi si, cc knh khc nhau c gi tr GVD hi khc nhau. iu ny gy kh khn cho vic b tn sc tt c cc knh cng mt lc. gii quyt vn ny, cc loi si quang mi c pht trin, chng c S khng nhng nh ( dc tn sc gim) m cn m (si tn sc ngc). Bng sau cho ta danh sch cc c tnh ca cc loi si hin c trn th trng. c tnh ca mt s loi si quang c tnh thng mi trn th trng: T phng trnh (2.4-6) ta thy gii hn tc bt ca knh hot ng ti bc sng tn sc 0 (ZD ) l v cng ln. Tuy nhin khng ng nh th, v S s tr thnh nhn t gii hn trong trng hp ny. Chng ta c th c tnh tc bt gii hn bng vic ch l i vi ngun c rng ph Agi tr hiu dng ca h s tn sc tr thnh A = . S D[1] v phng trnh (2.4-6) tr thnh : 1 ) .( . .2< A S L B (2.4-12) iviLazerbndnamodec A =2nmvmtsiquangdchtnscc S=0,05ps/(km-nm2)tibcsng1,55 m u thB.Lt5Tbps-km(gitrnyscci thin nu dng Lazer bn dn n mode). 2.2.2.5Tn sc phn cc mode (Polarization Mode Dispersion) Ngun gc ca s m rng xung trong trng hp ny c lin quan n s khc x hailn(Birefringence)casi(Bm= y xn n ; y xn n , lnltlchitsutmodecacc mode phn cc trc giao). S khng i xng trn ca li to ra s phn x hai ln do chit sut mode ng vi cc thnh phn phn cc trc giao ca mode c bn l khc nhau. Nu xung ngvo kch c hai thnh phn phn ccthntr nn rng hn do hai thnh phn Nhm 3 B tn sc bng DCFPage 1 18 8 tnsc dctheo sicvn tc nhm khcnhau. Hintng nygi ltn sc phncc mode (PMD). Trong nhng si c Bm l hng s (v d si duy tr phn cc) s m rng xung c tnh t tr v mt thi gian gia hai trng thi phn cc trong sut qu trnh lan truyn xung lT A. i vi si c chiu di L thT Ac cho bi: ) (1 1 1| | | A = = = A L LvLvLTy xgy gx (2.4-13) trong x, y dng ch hai mode phn cc trc giao; 1| Ac lin h vi chnh lch vn tc nhm ca hai trng thi phn cc. Phng trnh (2.4-1) c s dng c s lin h gia vg vi hng s lan truyn| , vL T / Al i lng nh gi PMD. i vi si duy tr phn cc thL T / Aln (khong 1ns/km) khi hai thnh phn c kch bng nhau ti ng vo ca si, nhng c th gim n 0 nh a nh sng dc mt trc chnh . i vi si thng th hi khc v Birefringence thay i dc theo chiu di si mt cch ngu nhin.i vi mt xung quang, trng thi phn cc cn khc i vi cc thnh phn ph khc nhau ca xung. Trng thi phn cc cui cng khng l s quan tm i vi hu ht cc h thng Thng Tin Quang v Photodetector dng trong b thu khng nhy vi trng thi phn cc tr khi s dng tch sng Coherent. Vn nh hng n cc h thng nh th ny khng phi l trng thi phn cc ngu nhin nhng xung li b m rng do s thay i ngu nhin ca Birefringence. Mtxungquangkhngcphnccdctheohaitrngthichnhchialmhai phn lan truyn vi tc khc nhau. tr nhm vi saiT Aln nht i vi hai trng thi phn cc chnh. Minh ha tn sc phn cc mode Nhm 3 B tn sc bng DCFPage 1 19 9 2.3nh hng ca tn sc trong h thng thng tin quang: Khi sdngEDFAtrntuynthvnsuyhaocgiiquyt,clytruyn dncnnglnrrt, nhngtngtnsccngtngln.Do ,liyu cuphigii quyt vn tn sc, nu khng, khng th thc hin c vic truyn thng tin tc cao v truyn dn c ly di. By gi, nh hng ca hiu ng tn sc si quang li l mt yu t hn ch ch yu, nht l i vi h thng tc cao li cng th hin r rt. V d, si quang G. 652 tn sc tc 2,5 Gbit/s c ly b hn ch khong 928 km, nu tc tng ln 10 Gbit/s th c ly truyn dn b hn ch ch cn 58 km.Bng 2.4 ni v c ly b hn ch bi tn sc khi khng c trm lp. Bn cht ca tn sc l s gin rng xung tn hiu khi truyn dn trn si quang. Tn scnhhngnccsinmodebaogm:tnscvntcnhm,tnscphncc mode, tn sc bc cao v tn sc dn sng. C ly b hn ch bi tn sc khi khng c trm lp (tr s l thuyt). Tc 1550 nm (G.652) 1550 nm (G.655) 1310 nm (G.652) 2,5Gbit/s 928 km4528 km6400 km 10 Gbit/s 58 km283 km400 km 20 Gbit/s 14,5 km70 km100 km 40 Gbit/s 3,6 km 18 km 25 km Ngun: Cng ngh ghp knh quang WDM-Phng Vn Lng Phngphpxc nhnhhngcatnsc nhthngthngtin quangthng qua tnh ton qu cng sut h thng PB bng vic thit k di tuyn c thit lp theo Cng thc (2.12). Nhm 3 B tn sc bng DCFPage 2 20 0 ( ) ( ) ( ) ,B t s M P D C C S SP P t P G NF P P P N l N l dB = + (2.12) Trong : Pt(t): cng sut tn hiu pht c tnh c nh hng chirp phi tuyn. G: khuch i ca cc b EDFA. PM: cng sut d phng ca h thng. PD: cng sut tn tht do tn sc, y chnh l cng sut tng ng do nng lng ph ca xung tn hiu b dn ra ngoi khe thi gian nh sinh ra. PP: cng sut n b lng cng sut mt mt. PS(G,NF): nhy thu c tnh c nh hng ca b khuch i v nhiu ca EDFA. NS v NC l s mi hn v s mi ni quang. Hnh sau y l kt qu tnh ton tm thy s mt mt cng sut ca h thng do tn sc gy ra i vi h thng 1 Gbit/s v 2,5 Gbit/s. Kt qu cho thy h thng 2,5 Gbit/s b tn tht cng sut h thng nhiu hn so vi h thng 1 Gbit/s vi cng mt gi tr tn sc. i vi tc bt 2,5 Gbit/s, cng sut b mt mt l khng ng k khi tn sc nh hn 100ps/nm. Tuy nhin, mt mt cng sut tng dn dn trong khong t 100ps/nmn 200ps/nm. Khi tn sc vt qu 200ps/nm, lng cng sut quang b tn tht rt nhanh. gi tr 300ps/nm th cng sut mt mt ln ti 4dB. Trong khi tc 1 Gbit/s, s mt mt cng sut trong trng hp ny gn bng 0 khi tn sc nh hn 100ps/nm, ti gi tr760ps/nmthicngsutmtmtl4dB.Nhvy,rrngrngtnsclmgiihnng k tc truyn dn ca h thng thng tin quang. Nhm 3 B tn sc bng DCFPage 2 21 1 Hnh 3 l kt qu tnh ton xcnh lng cng sut tn hiu b mt mt cho cc h thng thng tin quang 2,5Gbit/s vi cc Laser c ph rng 0,1 nm, 0,2 nm, v 0,3 nm. T kt qu thu c c th thy rng nhn chung, lng cng sut b tn tht s tng nhanh khi c ly truyn dn tng. Nh vy, mc d cc h thng cng b mt cng sut khi c ly truyn dn tng. Tuy nhin, cc ngun pht khc nhau s gy ra s mt tn hiu cng khc nhau. Khi rng ph ngun pht l 0,1 nm,mt mt cng sut xy ra l 4,5 dB khi c ly truyn dn di 52 km. i vi cc h thng c ph ngun pht pht rng 0,2 nm v 0,3 nm, cng sut b tn tht tng ng l 4,5 dB ti 26 km v 6,2 dB ti c ly ch cn 18 km. Khi tng c ly vt qu cc gi tr trn, tn hiu b mt qu ln v khng th thit k c h thng thc tin k c c s dngkhuchiquang si.ylcchthng s dngsiG.652chtnsc D=18ps/km.nm. Nh vy, trong hthng thng tin quang, ngoi suyhao quang,c nhiu tham s tng tc ti tn sc si gy nh hng ti h thng v lm mting k lng cng sut tn hiu. l cc tham s: C ly truyn dn, tham s tn sc, tc bt ca h thng v ph ngun pht. Nhm 3 B tn sc bng DCFPage 2 22 2 CHNG3:CCPHNGPHPBTNSCTRONG THNG TIN QUANG 3.1 S cn thit ca vic qun l tn sc: Nh chng ta xem xt Chng 2 nh hng ca tn sc c tc ng rt ln n chtlnghthngthngtinquangnichungvhthngthngtinquangtccao WDM ni ring. Tn sc gy ra hin tng dn rng xung, gy mo tn hiu, lm tng cc libitxyra,nhhnglnntctruyn dnca hthng.Cthgimthiunh hng ca tn sc vn tc nhm ( GVD ) nu s dng cc laser ph hp v khi cng gn vi bc sng tn sc zero ZD ca si quang. Tuy nhin khng phi lc no cng kt hp gia bc sng hot ng vi ZD. C th ly v d v cc h thng trn mt t thuc th h th ba hotng gn bc sng = 1,55 m v s dng ccthit b pht quang laze hi tipphntn(DFB).Nhnghthngnhvynhnchungusdngmnglicp quangc xy dngtrong nhng nm1980baogmhn 50 triu kmchiu di ca si n mode tiu chun vi ZD 1,31 m. Do tham s tn scD 16 ps/(km.nm) trong di bc sng 1,55 m ca si n mode, do GVD s hn ch tnh nng khi tc bit vtqu 2Gb/s.iubintrctiplazeDFB,chngta cth s dngphngtrnh: B.L.|D|. < 1/4 ( vi l rng ph ngun RMS trong ca ngun ) c tnh khong cch truyn dn ti a nh sau:L < (4B|D|s )1 ( 3.1 ) Trong , s lrngRMS ca ph xung c dnrngngk bi dch tn s. Khi s dng D = 16 ps/(km.nm) v s = 0,15 nm trong phng trnh 3.1, cc h thng sngnhsnghotngtc2,5Gb/subgiihnmcL42km.Thct, nhng h thng nh th thng s dng cc thit b ti to in t nm cch nhau 40 km, v khng tn dng c tnh kh dng ca cc bkhuchi quang. Ngoi ra, tc bit ca chng cng khng th vt qu 2,5 Gb/s v l do c ly ca thit b ti to l qu nh c th m bo kh thi v mt kinh t. Tnh nng ca h thng c th c ci thin ng k bng cch s dng thit b iu chngoi,nhvycthtrnhc hintng dnrng phdodchtns.Cchny c ng dng cc loi my pht laze DFB thng mi vi mt biu ch tch hp gn lin.Vis= 0 hnch chn chkhimypht sdngloisitiuchun.Khong cch truyn gii hn khi s c tnh nh sau: Nhm 3 B tn sc bng DCFPage 2 23 3 06 2333222=cccc+cctAtA izA | | L < (16|2|B2)1( 3.2 ) Trong 2 l h s GVD lin quan ti D. Nu thay gi tr 2 = -20 ps2/km, ti bc sng 1,55 m, th L < 500 km v t tc 2,5 Gb/s. Mc d c ci thin ng k so vi laze DFB iu ch trc tip, nhng gii hn tn sc ny vn ng c lu tm khi cc thit b khuch i ng truyn c s dng b suy hao. Hn na, nu tc bit t 10 Gb/s, khong cch truyn gii hn GVD s gim xung ch cn 30 km, y l mc qu thp cc thit b khuch i quang c th c s dng trong vic thit k cc h thng bc sng nh sng nh vy. T phng trnh 3.2 ta c th thy rng gi tr GVD tng i ln ca si n mode tiu chun s lm hn ch tnh nng ca cc h thng 1,55 m c thit k s dng trong mng li vin thng hin c khi tc bit t 10 Gb/s hoc cao hn na. c mt s m hnh qun l tn sc c a ra gii quyt vn mang tnh thc tin ny. Nn tng ca nhng m hnh l kh n gin v c th hiu c da trn phng trnh lan truyn c vit nh sau: ( 3.3 ) Trong A l bin bao xung. Nhng nh hng ca tn sc bc ba c k hiu l 3. Trong thc t, k hiu ny c th s khng c s dng nu |2| ln hn 0,1 ps2/km. Gii phng trnh (3.3) s c kt qu nh sau : Xt khi 3 = 0, ta c kt qu nh sau:

=t 21) , ( t z A (0,) exp e e e | d t i zi)2(22 ( 3. 5 ) Trong , (0, ) l khai trin Fourier ca A (0, t). S suy bin gy ra tn sc ca tn hiu quang l do h s pha t gi tr exp(i 2z/2) bi cc thnh phn ph ca xungtrong qutrnhlan truyn trong si. Tt c ccm hnh qun l tn sc u c gng loi b h s pha ny nhm phc hi li cc tn hiu u vo, c p dng trong my pht, my thu hoc dc theo ng truyn si quang. Nhm 3 B tn sc bng DCFPage 2 24 4 3.2 K thut b tn sc trc (Precompensation): Kthutnydatrnnguynlchunglsactnhxungngvocabpht trc khi a vo si, l thay i bin ph ca xung ng vo( ) e , 0~Anh sau: ( ) ( ) |.|

\| 22. .2exp . , 0~, 0~e | e e LiA A (3.2-1) trongL lchiu di si. Khitn sc vntc nhm(GVD)s cb chnh xc v xung vn gi nguyn dng ti ng ra. K thut ny gm c k thut Prechirp, k thut m ha Novel v k thut Prechirp phi tuyn.iu ch FM v AM tn hiu quang cng mt lc th khng cn thit i vi vic b tn sc, nn ngi ta dng kha dch tn (FSK) cho vic truyn tn hiu. Tn hiu FSK c thc hin bng vic chuyn mch bc sng ca Lazer lch nhau mt lng l Agia bt 0 v bt 1. Hai bc sng s lan truyn trong si vi tc hi khc nhau. Thi gian tr gia bt 1 v bt 0 ph thuc vo vic dch bc sng Av c tnh theo cng thc: A = A . .L D Tv ta chn Asao choB T / 1 = A . S tr ny to ra tn hiu quang ba mc ti u thu. Hnh sau y cho ta thy s tr hon mt bt to ra tn hiu quang ba mc nh th no. Do tn sc ca si nn tn hiu FSK s chuyn sang tn hiu c bin c iu ch. V tn hiu ny c th c gii m u thu nh s dng b tch phn in kt hp vi mch quyt nh bt. B tn sc dng m ha FSK:a) Tn s quang v cng sut tn hiu pht; b) Tn s v cng sut tn hiu thu v d liu in c gii m. Nhm 3 B tn sc bng DCFPage 2 25 5 Mt phng php khc l m ha cp nh phn c th lm gim bng thng ca tn hiu cn 50%.Trong phng phpm hany, hai btktip nhau trong chui nh phn gp li hnh thnh mt m cp nh phn ba mc tc bt ch bng mt na. V tn sc vn tc nhm (GVD) ph thuc vo rng bng ca tn hiu, nn khong cch truyn c th tng nh gim bng tn ca tn hiu. Tcbt10Gbpssdngmhacpnhphnthclytruyndncthtng thm 30 n 40km so vi m ha nh phn. M ha cp nh phn c th kt hp vi k thut lch tn trc. Trong thc t truyn c tn hiu tc bt 10Gbps qua c ly 160km si thng nh kt hp m ha cp nh phn vi b iu ch ngoi c C > 0. 3.3 K thut b tn sc trn ng dy (In-line) : 3.3.1 B tn sc bng si quang DCF: Cckthutbtnsctrccthtngkhongcchlngpinhngn khng thch hp cho cc h thng c c ly ln, trong tn sc vn tc nhm (GVD) phi c b dc theo ng dy truyn dn theo chu k. Cc k thut b trn cc h thng nh th ny phi m bo ch ton quang, t trn si (ch khng phi t trn b pht hay b thu). V mt loi si c bit n l si b tn sc. Trong thc t nng cp cc h thng thng tin quang s dng si chun hin c, ngitathmvomtonsibtnsc(vichiudit6n8km)iviccb khuch i quang t cch nhau 60n 80km.Si b tn scs b tn scvn tc nhm (GVD), trong khi b khuch i s m ng nhim v b suy hao cho si.NgitathngsdngsiDCFkthpviccbkhuchiOA(thngs dng b EDFA) b tn sc trn tuyn quang, v ty vo v tr t DCF m c cc kiu b nh sau: Nhm 3 B tn sc bng DCFPage 2 26 6 S dng si DCF trn tuyn quang Chi tit v si DCF s c ni r chng sau. 3.3.2 B tn sc bng b lc quang: Nguyn l ca phng php ny nh sau: gi s hm truyn t ca b lc quang l H(e) th nh hng ca n i vi pha ca xung tn hiu c th hin bi cng thc ( ) ( ) ( )2121, 0,2j L tA L t A H e de ee e et = Bng cch trin khai pha ca H(e) theo chui Taylor v gi s hng bc hai s c: ( ) ( ) ( ) ( )20 1 21exp exp2H H i H i e e o e e o o e o e| |= ~ + + |\ . y mm mdd ooe= ,m=0,1,...cclngtitn ssngmangquange0.Pha hng s o0 v tr thi gian o1 s khng nh hng n dng xung v c th b qua. Pha ph do si sinh ra c b bng cch chn b lc quang sao cho c 2 2L o | = khi xung c th c phc hi hon ton ( ) ( ) ( ) ( )1, , exp2A L T H A L i T d e e e et= Cnhiuloiblc quangchmtruyntph hpviyu cunyvcth csdnglmcc b btn sctrong cbitnibtlccloiblccutrc bung cng hng Fabry Perot v b lc giao thoa k Mach Zehnder. Nhm 3 B tn sc bng DCFPage 2 27 7 3.3.3 B tn sc bng tn hiu quang lin hp pha OPC: K thut OPC (Optical phase conjugation) lin hp pha quang i hi 1 phn t quang phi tuyn m c th to ra tn hiu pha lin hp. Thng thng ngi ta dng phng php trn 4 bc sng (FWM- Four wave mixing) trong vng phi tuyn, v bn thn si quang t n l mt mi trng phi tuyn, (cch n gin l dng mt si quang di vi km c thit k mt cch c bit ti a hiu ng FWM). Tim nng ca k thut OPC c chng minh mt th nghim 1999 vi 1 b kt hp c bn FWM c s dng b tn sc vn tc nhm GVD tc 40 Gb/s trn chiu di 140 km si quang tiu chun. Hu ht cc cuc th nghim v b tn sc c nghin cu trn khong cch truyn l vi trm km. i vi ng truyn di hn n t ra vn k thut OPC c th b tn sc vn tc nhm GVD cho chiu di ln n hng ngn km si quang m c dng cc b khuch i b suy hao c hay khng. Trong 1 th nghim m phng, tn hiu tc 10 Gb/s c th truyn trn 6000 km trong khi ch s dng cng sut trung b?nh di mc 3mW gim hiu ng phi tuyn si quang. Trong 1 nghin cu khc cho thy b khuch i ng 1 vai tr quan trng. Vi khong cch truyn trn 9000 km c th thc hin c bng cch gi cc b khuch i cho mi on 40 km. S la chn bc sng hot ng, cbitlbcsngtn sckhng c nghathen cht.Trongvngtn scdthng, Cng sut ca tn hiu bin i tun hon dc chiu di si quang . iu ny c thdn ti vic to ra cc (di bin) sideband do hin tng bt n iu ch Tnhkhngnnhnycthctrnhnuthngstnsctngiln (D>10ps/(km-nm)).yltrnghp ivi si quang chun bcsnggn1.55m. Hin nhin rng khong cch truyn ti a lun l vn quan trng i vi nhiu hang sn xut,nhk thuttrn 4bc sng FWM,khong cch dng bkhuch i cthgim xung di 3000 km. 3.3.4 B tn sc bng cch t Bragg: Nguyn l ch to si cch t b tn sc da trn iu kin phn x Bragg: 2n A =Trong : n = 1, 2, 3, ... : bc ca cch t : bc sng nh sng Nhm 3 B tn sc bng DCFPage 2 28 8 Nguyn l phng php b tn sc bng cch t si Bragg Siccht Braggc ch tobng cch dngtia t ngoi chiu quamtmt n nh sng vo si quang n mode chun to ra cc vng c chit sut khc nhau phn b dc theo chiu di Z ca si. b li tn sc vn tc nhm GVD, chu k quang ca cch tcchtosaochonAgimdctheodicanchoraGVDchun(|2>0). Trongsiquangnmodetiuchun,ccthnh phntnscao caxung slantruyn nhanh hn ccthnh phntn sthp.Vbc sng Bragg gimdc theo di ccht cho nn cc thnh phn tn s cao s di chuyn thm vo cch t trc khi c phn x v phi chu tr nhiu hn cc thnh phn tn s thp. Nh vy tr tng i c xut hin do cch t s b li GVD do si v b c tn sc si. Tham s tn sc Dg ca cch t c di Lg c xc nh bng mi lin h sau:

R g gT D L = ATrongTRlthigianivngbntrongcchtvAlssaikhcvcc bc sng Bragg ti hai u ca cch t. V 2ggLTct=cho nntn sc cchtc cho bi biu thc sau : 2RggT nDL c = =A A Vic s dng si cch t Bragg b tn sc c rt nhiu u im, do n c s dng rng ri trn thc t. u im u tin phi k n l kch thc ca n kh khim tn trong khi b c lng tn sc ln, d dng trong vic ghp ni vi si quang v suy hao xen nh. Tuy nhin n cng cmt nhc iml cn s nnhv nhit cao do ch mt thay i nh v chiu di si cch t cng c th lm thayi hon ton c tnh b tn sc ca n. 3.4 K thut b sau (post compensation): Cc k thut trong min in c th c dng b tn sc vn tc nhm (GVD) trongmythu.Taddngcnbngnhhngcatnscbngkthutinnusi quang hot ng nh mt h thng tuyn tnh. Cch t Bragg Tn hiu quang vo Tn hiu quang ra Nhm 3 B tn sc bng DCFPage 2 29 9 Vic b s d dng hn nu b thu Heterodyne c s dng tch tn hiu: b thu ny u tin chuyn tn hiu quang thnh tn hiu vi ba ti tn s trung tnIFev vn gi thngtinvbinv pha.Mtblcthngdi hotngtnsvibavipng xung c hm truyn t l: () ( )

=2. exp22Li HIF| e e etrong L l chiu di si. B lc ny s khi phc li dng tn hiu ban u ca tn hiu nhn c. K thut ny thc t nht i vi h thng Coherent. Nhngta bitl b thuCoherent thng khng thc t(vmt s l do) sovi b tch sng trc tip, v mch in tuyn tnh khng th b GVD trong trng hp ny. Vn l do thng tin v pha b mt khi tch sng trc tip do b tch sng ch p ng bin quang. Khi mt k thut cn bng khng tuyn tnh c th c s dng. Mt phng php khc na lvic quyt nh mt bt cthc hin saukhi xem xt dng sng (dng tng t) qua mt khong nhiu bt xung quanh bt ta cn quyt nh. Kh khn ca k thut ny l i hi mch in logich hot ng ti tc bt ca tn hiu v tnh phc tp ca n tng theo hmm caslngbtqua n (s lng bt mmt xung quang b tri rng ra do GVD). Kthutcnbngvmtinthngbgiihndotcbttcthpv khong cch truyn ngn. Mt k thut cn bng quang in da trn mt b lc ngang (transversal filter) cara.Trongkthutnybchiacngsuttimythuchiatnhiuquangnhn cthnhnhiunhnh,ccnhnhctrkhcnhau.Tnhiuquangtrnminhnh c chuyn sang dng in nh s dng cc photodetector c nhy c th thay i, v tngdnginquangcsdngchomchquytnhbt.Kthutnycthtng khong cch truyn dn ln gp ba ln i vi h thng hot ng tc 5Gbps. Nhm 3 B tn sc bng DCFPage 3 30 0 CHNG 4: PHNG PHP B TN SC BNG SI DCF 4.1 C s ton hc: Ta xem xt trng hp mi xung quang c truyn qua hai phn si: phn th nht l qua si n mode , phn th hai l si b tn sc. Phng trnh truyn sng trong si quang: ( ) ( ) ( )

+ = e e | | e etd t i L LiA t L A2 22 1 2122exp , 0~21, (3.1) trong ) , 0 ( e l bin i Fourier ca bin xung,L=L1+L2 v j 2|l h s GVD i vi phn c chiu di si Lj (j=1,2) L1: chiu di si quang n mode. L2: chiu di si DCF. Nu si b tn sc c chn sao cho s hng v pha c cha 2ebin mt th xung s c khi phc thnh hnh dng ban u ti cui si b tn sc. Vy iu kin b tn sc hon ton l:02 22 1 21= + L L | |hay 02 2 1 1= + L D L D (3.2) Vytphngtrnhtrntathylsibtnscphictnscvntcnhm (GVD) ti bc sng 1,55m l D2 phi nh hn 0 v D1 i vi si chun ln hn 0. V chiu di si b tn sc phi tha: 1212LDDL = (3.3) L2 phi cng nh cng tt nn suy ra D2 gi tr tuyt i cng ln cng tt. S dng si DCF b tn sc Nhm 3 B tn sc bng DCFPage 3 31 1 4.2 Cc thng s k thut ca si quang DCF: K thut b bng cch s dng cc b b tn sc quang in t c th tng khong cch truyn ln 2 ln, tuy nhin n li khng ph hp vi cc h thng ng di, h thng ny yu cu GVD phi c b lin tc theo chu k dc theo ng truyn. c bit trong cc h thng ton quang vic s dng cc b b tn sc quang in t l khng ph hp. V thngitanghramtsiquangcbitgilsiquangbtnsc(DCF: Dispersion Compensating Fiber). Vic s dng si DCF cho cc h thng ton quang c th b GVDmtcch ng k nu cngsut quangtrung b?nh cgi nhth cc hiu ng phi tuyn bn trong si l khng ng k. Vic s dng si DCF b tn sc ht sc n gin l ch cn t si DCF xen vo gia, do c im ca si DCF l c tn sc m, ngha l khi tn hiu xung nh sng i qua si ny th xung nh sng s co li dn, hin tng ny ngc vi si SMF xung nh sng b gin ra do tn sc, do vic xung nh sng b gin ra gii quyt c. Nhm 3 B tn sc bng DCFPage 3 32 2 Si quang DCF phi c h s tn sc GVD(Group-Velocity Dispersion ) 1,55m l D2 0. Hn na chiu di si quang cng c la chn tha iu kin :L2 = - (D1/D2) L1 Trong thc t ngi ta c gng chn L2 nh nht nu c th, trng hp ny xy khi si DCF c gi tr m D2 rt nh (hay |D2 | rt ln). H s tn sc D c gi tr-420ps /(nm.km) bc sng 1550nm v thayi nhiu cc bc sng khc. y l mt c tnh quan trng cho php b tn sc bng rng. Ni mtcchtng qut siDCFcthitkDtngtheo bcsng.S phthucvo bc sng ca h s tn sc D l mt c tnh quan trng DCF c th hot ng trong cc h thng WDM Ph tn sc ca si DCF y l phng php n gin nht qun l tn sc trong cc h thng WDM dung lng cao vi s lng knh ln ngha ca ng bao tn sc c hiu t iu kin :D1L1+D2L2 = 0 Nhng ch p dng cho mt knh, khi c nhiu knh tha mn cho tt c cc knh th cn iu kin : D1(n)L1+D2(n)L2=0 Trong :n l bc sng ca knh th n D1 tng vi bc sng tng cho c hai si chun v si dch tn sc, kt qu l tn sc tch ly D1L1 l khc nhau cho mi knh. Nu cng mt DCF phi lm vic cho tt c cc knh, ng bao tn sc ca n nn m v c gi tr tha mn cho tt c cc knh. ng bao tn sc ca DCF:

Nhm 3 B tn sc bng DCFPage 3 33 3 Trong :S l ng bao tn sc (ps/(nm2km) T s S/D l ng bao tn sc quan h (1/nm) Vi si chun D~16ps/nm.km v S~0.05ps/nm.km, t s ny khong 0.003nm-1.V th, cho DCF vi D~- 100ps/nm.km thng bao tn sc khong -0.3 ps/nm2km. Bng tham s ca si DCF v SMF : b tn sc cho mt khong cch 90 Km cho si G.652 ta cn phi s dng xp x 9 Km DCF. Thng s suy hao in hnh ca DCF thng thng gp ti trn 3 ln so vi si quang chun,h s tn sc phn ccmode PMDcng gpi, do din tch hiu dng ca DCF l v cng nh. 4.3 B tn sc n knh: Mt gii php cho vic nng cp cc h thng quang hin ti bng cch s dng cc si chun cng thm cc b DCF (~10km) v cc b khuch i quang t cch nhau 50 Nhm 3 B tn sc bng DCFPage 3 34 4 60km. DCF b tn sc vn tc nhm trong khi khuch i b suy hao. y c xem nh kthutqunltn sc chuk.Ng cakthut nylstrnlngiaGVDmv dng trong mt chu k sau cho tn sc tng cng trn chu k ny gn bng 0. Mch n gin nht l s dng 2 si c tn sc i nghch v chiu di tn sc trung bnh. D = (D1L1 + D2L2) / Lm Trong D1 l h s tn sc ca si SMF D2 l h s tn sc ca si DCF L1 l chiu di si SMF L2 l chiu di si DCF Lm = L1 + L2 l chu k b tn sc Nu D gn bng khng, tn sc c b hon ton trn chu k. Chiu di Lm thng c chn lkhong cch cakhuchi. Chiu di DCFc chn tha iu kin L2=-D1L1/D2 vic b tn sc l hon ton (D=0). Trong thc t nng cp cc h thng thng tin quang s dng si chun hin c, ngi ta thm vo mt on si b tn sc (vi chiu di t 6 n 8km si b tn sc) i vi cc b khuch i quang t cch nhau 60 n 80km. Si b tn sc b tn sc vn tc nhm (GVD), trong khi b khuch i s m ng nhim v b suy hao cho si. K thut ny rt hay nhng n c hai vn cn quan tm l: th nht, suy hao xen ca mt on si b tn sc thng qu 5dB (6 n 8km si b tn sc). Suy hao xen ny c th b nh vic tng li b khuch i, nhng ta phi tr gi bng vic tng nhiu t pht c khuch i (ASE); th hai l ng knh mode ca si b tn sc tng i nh, din tch mode hiu dng ch khong 20 m2. Khi cng quang trong si b tn sc ln th? cc hiu ng phi tuyn tng ng k. Nhng vn gp phi nh trn i vi si b tn sc ta c th gii quyt bng cch dngsi hai modec thit k vigitrV caohn 2,5. Cc si nyc suyhao ging nh si n mode nhng h s tn sc D i vi mode bc cao hn c gi tr m ln hn, D= -770ps/(km-nm) o c i vi cc si c li ellip. Mt km si b tn sc ny c thbtnscvntcnhm(GVD)qua40kmsitruyndn,v suyhaothmtnhcho ton tuyn nh (n gin v chiu di ca si b tn sc cn nh hn). Vic s dng si btn sc c haimode th cn phi cthit b chuyni mode, thit b ny c kh nng chuyn nng lng t mode c bn sang mode bc cao hn. Nhiu thit b nh th c pht trin v n c tnh tng hp vi mng. Hn na n li c suy hao xen vo nh. Mt yu cu i vi b chuyn i mode l n phi khng c nhy vi s phn cc v phi hot ng vi bng thng rng. Nhm 3 B tn sc bng DCFPage 3 35 5 Hu ht cc thit b chuyn i mode trong thc t l n dng si c hai mode cng vimt ccht si,ccht ny dng ghpgia hai mode. Chukccht c chn tng ng vi s khc nhau gia v ch s mode gia hai mode lnh sau: .Cch t ny cn c gi l cch t si chu k di. NgitathngsdngsiDCFkthpviccbkhuchiOA(thngs dng b EDFA) b tn sc trn tuyn quang, v ty vo v tr t DCF m c cc kiu b nh sau: a)B trc: b) B sau: c) B i xng n . / o = A Nhm 3 B tn sc bng DCFPage 3 36 6 c) B i xng 4.4 Cc kt qu th nghim b tn sc bng DCF: Nhiu cucth nghimtrong sut nhng nm 90 nhmthc hin vic sdng li ch ca si b tn sc i vi h thng WDM. Nm 1995 mt th nghim i vi h thng gm8knhvikhongcchmiknhl1,6nm,miknhhotngtitcbtl 20Gb/s truyn quac clil232km sichun bngcch sdng nhiu si btn sc. Tn sc cn li i vi mi knh tng i nh (khong 100ps/nm i vi ton khong lp) bi v tt c cc knh c b ng thi bi cc si b tn sc. Trong mt th nghim vo nm 2001, si b tn sc bng rng c s dng truyn tn hiu WDM 1Tb/s (gm 101 knh, mi knh hot ng ti tc 10Gb/s) qua c li 9000km. Dung lng cao nht l 11Tb/s c thc hin nh s dng si tn sc ngc trong mt th nghim, n truyn 273 knh, mi knh hot ng ti tc 40Gb/s qua bng tn C, L v S mt cch ng thi ( rng bng ton b hn 100nm). 4.5 B b tn sc ca hng JDSU: Module b tn sc ca hng JDSU: WRDCM-20/30/40/50/60 Nhm 3 B tn sc bng DCFPage 3 37 7 Module b tn sc ca hng JDSU: WRDCM-20/30/40/50/60 Chc nng chnh : -B tn sc vi si quang n mode chun (G.652) -Tng thch vi bng C ca knh quang DWDM -Suy hao xen thp -C th b tn sc vi cc khong cch khc nhau -C th s dng b trc v b sau -C th s dng b cho n knh hoc a knh Cc WRDCM cung cp b tn sc m cho h thng truyn dn DWDM v c th s dng b tn sc vi sin mode chun trn ton b bng C. Khi kt hp vi ccb chuyn i bc sng v b khuch i th WRDCM s cung cp mt gii php vn chuyn n gin, tin cyvchi ph thp.Cc WRDCM-20/30/40/50/60c s dng cho btn sc v c th t trc hoc sau b khuch i. Cc WRDCM c th h tr n knh hoc a knh cho tc 2,5Gb/s v 10Gb/s. Cc thng s k thut: Sn phmWRDCM-20WRDCM-30WRDCM-40WRDCM-50WRDCM-60 c tnh quang hc Suy hao xen (dB) (1530-2.73.34.04.35.4 Nhm 3 B tn sc bng DCFPage 3 38 8 1565nm) Tn sc 2% (ps/nm) 1550nm -325-487-650-812-974 PMD (ps)0.290.360.420.450.51 Kt ni quang v cc loi si LC/PC vch ngn c im mi trng MinMax Nhit mi trng xung quanh -50C55oC Nhit bo qun -40oC85oC m tng i5%95% Nhm 3 B tn sc bng DCFPage 3 39 9 CHNG 5: KT LUN Chuyn Thng Tin Quang ca nhm 3 trnh by tng qut v cc khi nim tn sc, cc loi tn sc, nh hng ca tn sc trong h thng thng tin quang. Chuyn cng gii thiu hu ht cc phng php b tn sc nh b trc, b sau hay b ng dy v tp trung nghin cu v k thut b tn sc bng si DCF.Cc phng php b tn sc trc v b tn sc sau c nhng hn ch khi chiu di tuyn quang ln, chnh v vy phng php b tn sc ng dy c s dng ph bin nhtv hiu qunhttrongthct, nibtnhtlhaiphngphp sdngDCFv s dng cch t Bragg.Vi vic btn sc cho tuyn quang s dngsi DCF c mtvi khuytim, do ng knh trng mode ca si DCF nh, do mt cng sut trong si ln l nguyn nhn ca hiu ng phi tuyn, ng thi suy hao ca DCF cng kh ln v vic s dng kt hp vi cc OA truyn trn tuyn quang ng di s lm tng nhiu ASE v nhc im caDCF lkhicn b mt lngtn sc lnth phi cn chiu di si tl thunvi lng tn sc , dn n cng knh v kh khn cho vic thi cng, lp t. Khi phng php dngsibtn sc(DCF)bhn chv suyhaoghpkhln, cng sut trong si quang phi m bo nh cc hiu ng phi tuyn khng xy ra v githnh lptcao,thphngphp dngccht Braggtrakhhiu qukhin khc phc c nhng nhc im ca phng php dng si DCF. Hin ti k thut b tn sc bng cch t Bragg ang l ch nghin cu hp dn v nhiu tnhuvit ca n nh: kch thc nhgn (nmhontontrong mt si quang), ph hp cho h thng quang a knh, c kh nng iu chnh b tn sc mt cch linh ng. Tuy nhin phng php DCF vn s dng ph bin v tnh n gin ca n. Nhm 3 B tn sc bng DCFPage 4 40 0 TI LIU THAM KHO: 1.Phm Quc Hp. Slide Bi ging Thng Tin Quang 1. Hc vin Cng ngh Bu Chnh Vin Thng TPHCM, 2007. 2.LQucCng,VnVitEm,PhmQucHp.HthngThngtin Quang 1. NXB Thng Tin v Truyn Thng, 2009. 3.L Quc Cng, Vn Vit Em, Phm Quc Hp, Nguyn Hunh Minh Tm.HThngThngtinQuang2.NXBThngtinvTruynThng, 2009. 4.VNgcThanhTng.LunVnTtNghip:Btnscbngccht Bragg. D04VTA1, Hc vin Cng Ngh Bu Chnh Vin Thng TPHCM, 2008. 5.Hunh Thanh Tun. Lun Vn Tt Nghip: B tn sc bng si DCF trong hthngtruyndnquangtccao.D06VTH1,HcvinCngNgh Bu Chnh Vin Thng TPHCM, 2008. 6.Nguyn Trn Anh Tun, Phm Minh T. Chuyn iu khin tn sc. H cao hc CH09DT2, Hc vin Cng Ngh Bu Chnh Vin Thng TPHCM, 2010. 7.inh Th Thu Phong, V Vn San.Xc nh nh hng ca tn sc trong hthngthngtinQuangtccao.Hinghnghincukhoahc HCVNBCVT, 2005. 8.G.Keiser.OpticalFiberCommunications.Thirdedition,McGraw-Hill, 2000. 9.G.P.Agawal.FiberOpticCommunicationSystems.Secondedition,John Wiley & Son, 1997. 10.DjafarK.Mynbaev,LowellL.Scheiner.FiberOpticCommunications Technology. Prentice Hall, 2000. 11.JDSU.WaveReadyDispersionCompensationModuleWRDCM-20/30/40/50/60, 2009.