[HTVT2] 3 - He Thong Thong Tin Quang

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H THNG VIN THNG 2 -Bin son:PGS-TS L Tin ThngChng 4:H THNG THNG TIN QUANGChng ny s gii thiu nhng cu trc c bn ca cp quang v gii thch lm th no nh sng truyn trong n - mt mi trng truyn c s dng trong truyn thng quang. So snh vi cc mi trng truyn khc nh khng gian, dy dn, cp quang cho s suy gim nhvmin nhim mnh vinhiu ca sng in t (Electronic Magnetic Interference- EMI). V nhng u im ny m cp quang c s dng nhiu t di y bin cho n lin lc lin i 1 (Interoffice Communications). Gn y, v cht lng tng xng vi gi thnh ca n, cp quang cn c s dng thay th cho cc dy cp truyn thng trong TV cp.T quan im ca h thng vin thng, cp quang l mt knh truyn vin thng m trong nh sng lan truyn i. Ging nh nhng mi trng truyn dn khc, suy gim bin tn hiu v mo l hai thng s quan trng nht trong vic nh gi s gim cht lng tn hiu. Chng ny s ni v s suy gim bin v tn x v chng nh hng n tc v c ly truyn dn nh th no. Chng ny cng ni n cch ci thin cc gii hn ny bng cch gim suy hao v tn x. xc nh c s suy gim v tn x ca cp quang, phi hiu c cc ngun suy gim v nh sng truyn trong cp quang nh th no. gii thch vic nh sng truyn trong cp quang, c quang hnh hc ln quang sng u c s dng. Phn tch chi tit v cc phng trnh truyn sng c trnh khng dng v mc ch ca chng ny ch l minh ho nhng tnh cht quan trng ca vic truyn sng vi nhng cng thc ton n gin.Sau khi gii thch v xc nh s suy gim v tn x, s dn ra nhng gii hn do s suy gim v tn x.Nhng gii hn ny a n bin trn ca khong cch truyn ti mt mt tc truyn cho trc. Khi mt h thng m khong cch truyn b gii hn bi s suy gim th n c gi l h thng gii hn bi suy gim Attenuation limited. Cn chng c gi lhthngbgii hnbi tnxdispersion limited-trongtrnghp khong cch b gii hn bi tn x. Ni chung, cc h thng u l Attenuation limited tc truyn thp v l gii hn bi tn x dispersion limited tc truyn cao.Chng ny xem cp quang nh mt knh truyn tuyn tnh, trong tn hiu c th chng cht c, cc thuc tnh nh chit sut th khng thay i theo tn hiu. 1 Cu trc v cc loi cp quang dn nh sng lan truyn bn trong, mt cp quang c bn c tit din hnh trn nh tronghnh1. Mcdmt cpquangthctthngcnhiulp, ch cli vvbc (cladding) l quan trng i vi s lan truyn nh sng 2 . C li ln v thng lm bng thu tinh silica, li th c chit sut quag cao hn v gi nh sng bn trong.Khi nh sng truyn bn trong si quang, hu ht cng sut ca n c gi trong phn li - c bao bc bi v. V c mt quang (chit sut quang) nh hn mt cht , thng khong mt n vi phn trm. Hu ht si quang c ng knh v khong 125 m. Kch c ca n khng quan trng i vi s truyn nh sng . Bn ngoi lp v l nhiu lp bo v. Cc lp bo v trnh cho b mt si quang b try xc hoc b ct bi cc lc c hc.1.1 Si quang Single mode v Multimode(n kiu v a kiu)Khi sng nh sng truyn bn trong li mt si quang , n c th c nhiu s phn b trng in t (EMF) trn mt ct ngang ca si quang. Mi s phn b m tun theo cc H thng thng tin quang1H THNG VIN THNG 2 -Bin son:PGS-TS L Tin ThngphngtrnhMaxwell viukinbinti mt tipxcv-li gilmt modengang (tranverse mode)Nhiu loi mode ngang c trnh by trong hnh v 4.2. Nh ni, chng c cc phn b trng in t khc nhau trong mt ct ngang ca si quang. Ni chung, cc mode ngang truyn dc si quang vi cc tc khc nhau. Nh c ch ra chng 3, y l kt qu ca s tn x v l khng mong mun. Cp quang m ch cho duy nht mt mode ngang gi l cp quang n mode (single-mode fiber). Cp quang cho php s truyn theo nhiu mode ngang gi l cp a mode (multimode fiber).Mu cht trong vic thit k c c s truyn single-mode l phi c ng knh li nh. Vic ny c th hiu c t s ph thuc ca tn s ct C (cutoff wavelength) vo ng knh li ca si quang. Bc sng ct l bc sng m trong ch c th c duy nht mt mode ngang bn trn. C c biu din bi :

22212n nVaC (4.1)Trong V=2.405 vi si quang kiu ch s bc thang (step-index fiber), a l bn knh li, n1 n2 ln lt l chit sut ca li v v. Cng thc ny cho thy si quang c ng knh cng nh th bc sng ct cng nh.Hnh 1 - Cu trc c bn ca cp quang (a) n mode (b) a modeThng thng, ng knh li l 10 m cho si quang single-mode v 50 m cho si quang multimode. Khi ng knh li ca si quang khng ln hn nhiu so vi bc sng th phn ln cng sut tp trungv.Do , cn phi nh ngha mt thng slmode field diameter(MFD), ngknhtrngmode. Nmna, lbrngcu trng ngang. c bit, n l root mean square (RMS) trung bnh bnh phng ca rng trng nu trng tun theo phn b Gauss. Khi lut phn b ca trng khng phi l Gauss, cc nh ngha MFD l khng duy nht. Khi nim v MFD ny hu ch khi ta mun xc nh coupling loss hoc splicing loss (tn hao do ghp ni). Trong trng hp ny, MFD ph hp thay v ng knh li l mt iu quan trng gim tn hao do ghp ni.V d 1 : ng knh li cho single-mode . Bi v hai bc sng quan trng thng dng trong truyn dn quang l 1.30 m v 1.55 m, bc sng ct nh hn 1.30 m l cn thit truyn single-mode . c bin an ton ln hn, chn bc sng ct l 1.20 m, ng knh nhn tng ng s l

222112n nVaC 8.5 mH thng thng tin quang2H THNG VIN THNG 2 -Bin son:PGS-TS L Tin ThngHnh 2 : Vi v d v cc mode ngang bc thp ca mt si quang ch s bc thang. (a) Thit k mode phn cc tuyn tnh (b) Cc mode thit k c th (c) phn b trng in t (d) phn b trng in thnh phn Ex .1.2 Profile chit sut (Lc t chit sut)Si quang c c chit sut thay i trong v v li. Hai kiu lc t c bn ca chit sut cho si quang single-mode v multimode c v trn hnh 3a v 3b, mt c chit sut u trong li v mt c chit sut tng t t khi i t bin li-v n tm li. Hai kiu si quang ny c gi l step-index (ch s bc thang ) v graded-index(ch s phn cp). Mc ch ca vic s dng cp quang ch s phn cp - cn bng vn tc nhm ca cc mode truyn sng khc nhau gim thiu tn x - s c trnh by sau.i vi si quang single-mode , cn c nhiu kiu profile khc nhau ca chit sut. Vi v d quan trng c v trong hnh 3c-j. hnh th nht (hnh 3c) c lc t ch s bc thang . Cli ln vu c chit sut khng i. MFD nm trong khong 9 n 10 m, v chit sut thay i khong 0.3%. Si quang dng d v e c nhng chit sut khc nhau trong v. Nichung, phnvst lith cchitsut nhnht. Dngcpquangnycgil depressed-cladding (DC) . Ngc li, dng e c gi l matched-cladding (MC). Ba dng si quang ny c s dng trong truyn single-mode vi bc sng khong 1300nm.Spht trin sau ny ca cp quang c thng qua trong tiu chun CCITT G.652 trong bng 1.Bng 1 : khuyn ngh G.652 ca CCITTThng s c tng knh v 125 mng knh trng mode (MFD)Bc sng ct C 9-10 m1100-1280 nmTn hao do cong1db cho 100 vng vi ng knh 7.5 cmTn x3.5 ps/nm km gia 1285 v 1330 nm6 ps/nm km gia 1270 v 1340 nm20 ps/nm km ti 1550 nm dc tn x0.095 ps/nm2 kmH thng thng tin quang3H THNG VIN THNG 2 -Bin son:PGS-TS L Tin ThngTrong hnh 3f, mt ng knh li nh hn (5 m) hoc mt MFD nh hn 7-9 m c sdng.Trong hnh 3g v 3h, mt profile chit sut c hnh dng tam gic c sdng, ng knh li khong 6 m. Trong phn sau ca chng ny ta s tip tc ni v chng, nhng loi si quang c s tn x theo ng dn sng cao hn nn s tn x ton phn l 0 ti 1550 nm. Chng c gi l cp quang dispersion-shifted.Hnh3Profile chit sut cacpquang(a) multimode ch sbc thang, (b) multimode ch sphncp, (c) Match-claddingsingle-mode, (d)-(e) depressed-cladding single-mode, (f)-(h) dch tn sc, (i)-(j) tn sc phngSi quang trong hnh 3i v 3j c profile chit sut ln v xung. Chng c gi l si quangmuticladding(nhiu v) hocdispersion-flattened(tn xphng). ng knh li ca chng nh, khong 6 m. Nhng si quang ny c c tnh tn x phng trong khong bc sng 1300-1550 nm. 1.3 Cp quang i xng tit din khng trnNgoi nhng si quang c ng knh li khc nhau, cn c nhng si quang c li hoc v c tit din khng trn. Nhng cu trc khng trn ny c thit k truyn sng theomodebt i xngtrongmt phngngang. Taskhost chngk hnvocui chng. Cu trc bt i xng ny cho php gim ghp ni (coupling) gia mt sng nh sngtsphncc nysangsphncc khc. V th, chngthngc gi l Polarization maintaining fibers (PMF) - cp quang gi phn cc.H thng thng tin quang4H THNG VIN THNG 2 -Bin son:PGS-TS L Tin Thng2. S suy hao trong si quang v cc gii hn suy haoCp quang khng phi l mt mi trng khng suy hao v khng tn sc. Thc t, khi cp quang c dng cho lin lc cch y 20 nm, tn hao vo khong 20db/km. Hin nay, tn hao c gim xung 0.2 dB/km, mt s ci tin ht sc n tng. Chng ny s trnh by nhng nguyn nhn suy hao khc nhau trong cp quang. gim ti thiu s suy hao ny, bc sng nh sng c dch chuyn t 0.85 m ln 1.3 m v 1.55 m. Khi nim v nhy ca u thu v power budget (qu cng sut) s c trnh by c th hiu c nhng s suy hao ca cp quang gii hn khong cch truyn nh th no.2.1 Cc ngun suy haoC 4 ngun suy hao chnhTn hao do vt liu hp thu : C hai loi tn hao do vt liu hp thu : intrisic v extrisic (ni ti v ngoi lai). Tn hao ni ti gy ra bi cc dao ng ca nguyn t vt liu si quang. Nh trong hnh 4, s hp thu xy ra c hai thang : hng ngoi v t ngoi. Tn hao ngoi lai l do sdaong ca cc nguyn tca cc phn bn ngoi si quang. Mt tn hao ngoi lai quan trng l do nc hoc lin kt O-H m tn s dao ng c bn l 1.1 x 1014 Hz, hoc bc sng 2.8 m. V cc lin kt c th hp thu sng ti ti tn s dao ng c bn v cc hi ca n, s c nhng nh hp th ti bc sng 2.8/(n+1) m. Mt s trong chng l : 1.40, 0.93,v0.70 (n=1,2,3) nh trong hnh 5.Mt nh hp th khc ti 1.24 m l do tng tc gia lin ktO-H v SiO2 ca thu tinh.Hnh 4 Suy hao ni tiTn hao do tn x : c 4 loi tn hao do tn x trong si quang : tn x Rayleigh, Mie, Brillouin, v Raman. Tn x Rayleigh l tn hao do tn x quan trng nht, c th hin trn hnh 6. N t l thun vi 1/4 theo cng thc sau : [ ] km dB cR R/14 (4.2)Trong cr l h s tn x Rayleigh , n v (dB/km). ( m)4 . Cc gi tr o c thc t c cho trong hnh 6 . N thay i trong khong t 0.8 n 1.0 (dB/km). ( m)4 v l mt hm s ca s chnh lch chit sut gia li v v, ca ng knh li v ca loi vt liu pha tp (doped) trong thu tinh. Ni chung, Tn x Rayleigh cng ln khi chnh lch chit sut cng ln.H thng thng tin quang5H THNG VIN THNG 2 -Bin son:PGS-TS L Tin ThngHnh 5 Suy hao do hp th ngoi laiHnh 7 biu din tn hao tng hp ca tn hao do tn x Rayleigh v tn hao do vt liu. Trong , hp th hng ngoi l c th b qua c khi so snh vi tn x Rayleigh. Cng c cc ca s suy hao yu 1.3 m v 1.55 m. V th, hu ht cc ngun quang u hot ng cc bc sng ny gim ti thiu suy hao.V d 2 : Gii hn di ca s suy hao trong cp quang .Ti hai ca s suy hao yu 1.3 m v 1.55 m, c hp th hng ngoi v t ngoi u c th b qua. Do , gii hn di v mt l thuyt ca cp quang c th c tnh ch bng tn x Rayleigh. V d, nu h s tn x Rayleigh l 0.9 (dB/km). ( m)4 :[ ] km dBR/ 32 . 03 . 119 . 04 ;V[ ] km dBR/ 16 . 055 . 119 . 04 Ti =1.3 v 1.55 m. Si quang thc t c suy hao ln lt l : 0.4 v 0.2 dB/km.Tn x Rayleigh v Mie l tn x tuyn tnh, trong mt phn