Chuong 5 he thong thong tin quang

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Transcript of Chuong 5 he thong thong tin quang

  • 1. Chng 7: H thng thng tin quang Chng 7 H THNG THNG TIN QUANG7.1. Cu to v nguyn l si quang: Si quang cu to gm 1 rut li (core) chit sut n1 v v (Cladding) chitsut n2 tho mn n1 > n2, nh s phn x ton phn xut hin ti b mt phn cchny. Hnh 7.1. Si quang S khc bit gia n1 v n2 rt b (c o/oo ), tia sng i thng v tia phn x c di qung ng v thi gian truyn chnh lch khng nhiu Hnh 7.2. S bin thin chit sut si quang 166
  • 2. Chng 7: H thng thng tin quang Ngi ta c gng gim s chnh lch ny nh hn na, bng cch s dng siquang c chit sut bin thin lin tc ( dng parabol - chit sut gradient) Si quang c ng knh li rt b ( 2 8m). s chnh lch chiu di truynthng v phn x khng ng k, ta gi l si n mode (Single mode) Si quang a mode (Multimode) c ng knh li c 25 100 m s chu modng sng nhiu hn , do thi gian truyn khc nhau ca nhiu nhm sng, nn hnch tc truyn thng tin Vt liu lm si quang c 2 loi Plastic v glass Si quang c sp xp trong b c gi l cp quang v c s dng truyn tn hiu trong khong cch rt xa. Cp quang c cu to gmli : Trung tm phn chiu ca si quang ni nh sng io : Vt cht quang bn ngoi bao bc li m phn x nh sng tr li vo li.lp bc u tin : Lp ph do bn ngoi bo v si khng b hng v m tjacket: Nhiu si quang c t trong b gi l cp quang. Nhng b ny c bov bi lp ph bn ngoi c gi l jacket. PHN LOI SI QUANG si quang thch anh phn loi theo vt liu in mi si thu tinh a vt liu si quang bng nha si quang n mt phn loi theo mt truyn lan si quang a mt phn loi theo chit sut khc si quang chit sut bc x si quang chit sut bin i u Trong mng vin thng , si quang thu tinh thch anh c s dng nhiu biv n c kh nng cho sn phm c suy hao thp v cc c tnh truyn dn n nhtrong thi gian di. phn loi theo mt lan truyn: theo mt lan truyn c chia lm hai loi.mt l si quang n mt (c gi l loi SM ) loi si ny ch cho mt mt lantruyn. loi si th hai l loi a mt, cho nhiu mt lan truyn. 167
  • 3. Chng 7: H thng thng tin quang phn loi theo phn b ch s khc x: si quang SI (stepped index ) Si quang GI (graded index ) Si quang SM (Single mode ) Hnh 7.3. ng i cu nh sng qua si quang Multimode stepped index (chit sut bc): Li ln (100 m ), cc tia to xung nh sng c th i theo nhiu ng khc nhau trong li: thng, zig-zagti im n s nhn cc chm tia ring l, v vy xung d b mo dng. Multimode graded index (chit sut lin tc): Li c ch s khc x gim dn t trong ra ngoi cladding. Cc tia gn trc truyn chm hn cc tia gn cladding. Cc tia theo ng cong thay v zig-zag. Cc chm tia ti im hi t, v vy xung t b mo dng. Single mode (n mode): Li nh (8 m hay nh hn), h s thay i khc x thay i t li ra cladding t hn multimode. Cc tia truyn theo phng song song trc. Xung nhn c hi t tt, t mo dng. c im : Pht: Mt it pht sng (LED) hoc laser truyn d liu xung nh sng vocp quang. Nhn: s dng cm ng quang chuyn xung nh sng ngc thnh data. Cp quang ch truyn sng nh sng (khng truyn tn hiu in) nn nhanh,khng b nhiu v b nghe trm. suy gim thp hn cc loi cp ng nn c th ti cc tn hiu i xa hngngn km. Ci t i hi phi c chuyn mn nht nh Cp quang v cc thit b i km rt t tin so vi cc loi cp ng .Cc thng s tiu chun . Si quang chit sut Cc ch s Si quang n mt bin i 168
  • 4. Chng 7: H thng thng tin quang Bc sng s dng 0.85m 1.3 m 1.3 m 1.55 m ng knh li 50 m +/- 6% ------------------- ng knh v 125 m +/- 2.4% 125 m +/- 2.4%`T l ng tm hoc s 6% hoc t hn 0.5 - 3 m ng tmT l khng trn ca li 6% hoc t hn -----------T l khng trn ca v 2% hoc t hn 2% hoc t hn 169
  • 5. Chng 7: H thng thng tin quang7.2. Truyn s bng cp quang: Cc tn hiu in cn dng n cc . Cc b ng b clock ni thu l rt cn thit. Do bng thng si quang rt rng , nn tc thng tin ph thuc nhiu vo tc chuyn i in quang , t ph thuc vo knh truyn . Thng thng c 2 dng m ng truyn. M nh phn n gin nhng khng gi c ng b bit khi thng tin c dng chui bit ging nhau . khc phc , ngi ta ngu nhin ho (Scrambling) tn hiu ni pht tin . Ti ni thu , ngi ta gii ngu nhin (Descrambling) khi phc tn hiu tr li. M CMI ( Coded Mark Inversion) ph hp vi truyn tc cao trn si quang . M CMI c kh nng ng b rt cao , nhng c ph rng hn. Hnh 7.4. Code Mark Inversion CMI7.3. Tc ng truyn ca h thng quang- hin tng tn sc: Hnh 7.5. Dng xung Thi gian ln l thi gian c tnh t 10% 90% bin 170
  • 6. Chng 7: H thng thng tin quang Ni pht : LED c T = 5 15 ns, LD T = 0,1 2 ns Ni thu : PIN , APD T = 1 4 ns Thi gian ln ton h thng l: 2 2 2 2 T HeThong = 1,1. TPhat + TModal + TVat lieu + TThu Tpht l thi gian ln ca diode pht TThu l thi gian ln ca diode thu TModal l do tn x mode dc theo si quang (n mode , a mode ) TVt liu l do s tn x bc sng ca vt liu si quang (Chit sut ca si quang ph thuc vo bc sng ) Gi t l rng xung iu ch , thng thng The thong = 0,35.t Tc bit ng truyn l Rmax ~ 1 / The thongTn sc trong si quang Tn sc trong si quang gy ra s dn rng xung nh sng khi truyn.7.3.1. Tn sc mode do Si truyn nhiu mode Mi mode truyn c vn tc nhm khc nhau (mi mode c hng s lan truyn khc nhau) Hnh 7.6. Lch thi gian truyn gy ra tn sc mode 171
  • 7. Chng 7: H thng thng tin quang7.3.2. Tn sc vt liu (bc sng) Nguyn nhn: nh sng truyn trong si quang khng n sc m c rng ph xc nh. Tc lan truyn ca cc thnh phn ph l khc nhau (do chit sut l hm ca bc sng) Cc thnh phn ph c thi gian truyn lch nhau gy ra tn sc vt liu Hnh 7.7. Quan h gia tn sc vt liu vi bc sng7.3.3. Di thng B trn ng truyn quang Di thng ng truyn quang B l di tn s ca tn hiu sao cho cng sutquang bin thin 3 dB ( hay cng sut in tng ng bin thin 6 dB ) Di thng ng truyn quang B ph thuc chiu di d si quang v loi siquangNhn xt Di thng B si quang n mode theo qui lut 1/d , n rng hn nhiu so vidi thng ca si quang a mode Vi si quang a mode , di thng B ph thuc vo chiu di si quang d, dos tn x ca sng quang trong mi trng truyn. N theo qui lut 1/d nu d < d ti hn;theo qui lut 1 chia cho cn bc 2 ca d nu d > d ti hnCh Tc bit max truyn trn si quang l D = 1,25B 172
  • 8. Chng 7: H thng thng tin quang7.4. Cc b chuyn i in quang Cc b chuyn i in quang thng thng l LED, diode Laser (LD) Cc b chuyn i quang in thng thng l photodiode PIN , photodiode thcl APD ( Avalanche photodiode ) Hnh 7.8. H thng thng tin quang n gin Hnh 7.9. c tnh ca Laser diode 173
  • 9. Chng 7: H thng thng tin quangHnh 7.10. c tnh ca photo diode 174
  • 10. Chng 7: H thng thng tin quang7.5. Suy hao si quangSuy hao thun si quang ph thuc bc sng Suy hao thun si quang khong vi dB/Km Suy hao thun ca si quang cn ph thuc vo bc sng nh sng nh sng vng hng ngoi (0,7 1,6 m ) c suy hao nh hn nhiu so vi nh sng kh kin (0,4 0,7 m )C 3 vng bc sng vng hng ngoi cho suy hao cc tiu Cc h si quang c s dng bc sng 0,85 m Vng bc sng 1,3 m Cho suy hao nh hn so vi 0,85 m Vng bc sng 1,55 m ang c nghin cu a vo thc t p ng tn s tn hiu trn si quang Vi cng s suy hao thun nh nhau, Tn hiu mang thng tin c tn s cng cao cng chu suy hao nhiu trn ng truyn si quang Hnh 7.11. Suy hao thun si quang ph thuc bc sngCc nguyn nhn khc gy suy hao : Suy hao do hp th Suy hao do tn x Suy hao do un cong 175
  • 11. Chng 7: H thng thng ti