CHNG 1

CHNG 1

TNG QUAN V H THNG THNG TIN QUANG

1.1 Gii thiu chng

Trong chng ny s trnh by mt cch chung nht v tng quan h thng thng tin si quang, v lch s ra i v qu trnh phat trin cua h thng thng tin quang, v cc giai on pht trin lin quan n khoa hc cng ngh, nhng u nhc im cua h thng thng tin si quang, nhng nguyn l c bn ca h thng truyn dn quang.

1.2 Tng quan

H thng thng tin c hiu mt cch n gin l mt h thng truyn thng tin t ni ny n ni khc. Khong cch gia cc ni ny c th t vi trm mt n vi trm kilmt thm ch hng trm ngn kilmt vt qua i dng. Thng tin c th truyn thng qua cc sng in vi cc di tn s khc nhau. H thng thng tin quang l mt h thng thng tin bng nh sng v s dng cc si quang truyn thng tin. Thng tin truyn i trong h thng thng tin quang c thc hin tn s sng mang cao trong vng nhn thy hoc vng hng ngoi gn ca ph sng in t. Cng vi s pht trin ca x hi th nhu cu ca con ngi i vi thng tin ngy cng cao. p ng c nhng nhu cu , i hi mng vin thng phi c dung lng ln, tc cao... Cc mng li ang dn dn bc l ra nhng yu im v tc , dung lng, bng thng... Mt khc, my nm gn y do dch v thng tin pht trin nhanh chng, thch ng vi s pht trin khng ngng ca dung lng truyn dn thng tin, th h thng thng tin quang ra i t khng nh c chnh mnh.

1.2.1 Lch s phat trin cua h thng thng tin quang.

T xa xa con ngi bit tuyn tn hiu thng tin bng khi la, cc hnh thc...

Qua qu trnh pht trin cc hnh thc truyn thng tin ngy cng pht trin trnn phong ph v hin i hn. Ngy nay, thng tin quang ni ln nh l mt ht hng thng tin tin tin bc nht, n c pht trin rt nhanh trn mng li vin thng nhiu quc gia trn th gii trong c Vit Nam. Thng tin quang l k thut truyn tinbng nh sng. Ti my pht, thng tin di dng tn hiu in c chuyn thnh tn hiu nh sng v pht vo ng truyn (si quang). Ti my thu, tn hiu quang s c bin i ngc li thnh tn hiu in. V mt nguyn tc h hng thng tin quang cng tng t nh cc h thng thng tin khc bao gm cc thnh phn c bn nh trong hnh 1.1

Nh vy, vi vic pht minh ra Laser lm ngun pht quang m ra mt thi k mi c ngha rt to ln vo nm 1960 v bng khuyn ngh ca Kao v Hockham nm 1966 v vic ch to ra si quang c tn tht thp. Bn nm sau, Kapron ch to ra c si quang trong sut c suy hao truyn dn khong 20dB/km. Cho ti u nhng nm 1980, cc h thng thng tin si quang c ph bin kh rng ri vi vng bc sng lm vic 1300nm v 1500nm. Nhng nm gn y si quang at ti suy hao rt nho (a gn at c gia tri suy hao theo tinh toan ly thuyt la 0,14 dB/km cua si n mode). Ngy nay cua khoa hoc ky thut, cho n nay h thng thng tin quang tr thanh mang toan cu ni gia cac quc gia, cac chu luc khac nhau, vic s dung cng ngh ghep knh phn chia theo bc song cho phep truyn tai vi dung lng ln, s dung cng ngh EDFA cho phep khuych ai trc tip tin hiu quang ma khng cn chuyn i quang-in nhm nng cao hiu sut truyn dn.

1.2.2 Cac giai oan phat trin cua h thng thng tin si quang

Trn hinh 1.1[1]. Biu din s phat trin cua tich s dung lng vi khoang cach BL qua cac giai oan khac nhau t nm 1975 en nm 2000 c thc hin thng qua nghim [1]. Giai oan nghin cu u tin v cac h thng thng tin si quang c bt u khoang nm 1975. qua trinh phat trin trn 25 nm t nm 75 n nm 2000 c chia lam cac th h khac nhau. ng thng biu din tc tng trng BL gp i hng nm. mi th h BL luc u tng, sau o bao hoa khi cng ngh at n im chin mui. Mi mt th h co mt thay i c ban gop phn hoan thin cht lng h thng sau nay

Hinh 1.1 S phat trin cua tich s BL cua h thng thn tin si quang qua cac giai oan

.

Th h u tin hoat ng bc song 800 nm va s dung Laser ban dn GaAs. Sau giai oan th nghim (1977-1979), cac h thng c trin khai vao nm 1980 hoat ng vi tc 45 Mb/s va khoang lp 45 km. Gia tri nay ln hn rt nhiu so vi khoang lp 1 km cua h thng cap ng truc. y la ng lc quan trong cho phep cac nha thit k giam c gia thanh lp t va bao tri cho cac tram lp. Trong nhng nm 1970, khoang lp tng ln ang k vi h thng lam vic bc song 1300nm, tng ng vi tn hao si 1dB/km. Hn na tai bc song nay si tan sc cc tiu. Cac u im nay thuc y vic phat trin cac Laser ban dn va b tach quang hoat ng bc song 1300nm.[1]

u nhng nm 1980, h thng thng tin quang th h th 2 s dng Laser vi bc sng 1330nm c a vo s dng. Thi gian u tc bt B ch t 100Mb/s do bi tan sc trong si a mode. Khi si n mode c a vo s dng, tc bt c tng ln rt cao. Nm 1987 h thng thng tin quang vi bc song = 1330nm, at n tc 1,7 Gb/s, khoang lp 50 km c sn xut v a ra th trng vi suy hao ca si ~ 0,5 dB/km.

Nm 1990 h thng thng tin quang th h th 3 s dng Laser bn dn bc sng 1550nm vi suy hao trong si quang c 0,2dB/km c thng mi ha. Tc bt t n 2,5Gb/s v sau t n 10Gb/s. Tuy nhin h s tn sc trong si quang ti bc sng 1550nm li kh cao (16-18ps/nm.km) do hn ch khong cch trm lp ca h thng mc d cng sut quang cn cho php truyn xa hn. c trng khong cch ca th h th 3 l 60 - 70 km ti tc 2,5 Gb/s. giai on ny s dng cc cng ngh b tn sc nh kiu dch tn sc (DSF) hoc lm phng tn sc (DFF) tng khong cch lp, c th ln n 100km.

Th h thng tin quang th 4 s dng khuch i quang tng khong lp v k thut ghp knh phn chia theo bc sng (WDM) trong mt si quang tng dung lng truyn dn. Khuch i quang pha tp Erbium (EDFA) c kh nng b cho suy hao quang trong cch khong cch ln hn 100km. EDFA c nghin cu thnh cng trong phng th nghim vo nm 1987 v tr thnh thng phm nm 1990. Nm 1991 ln u tin h thng thng tin quang c EDFA c th nghim truyn tn hiu s tc 2,5Gb/s trn khong cch 21000km v 5 Gb/s trn khong cch 14300 km. Vic phat trin WDM nhm nng cao dung lng yu cu khuch ai quang phai co bng thng u rng co th khuch ai tt ca cac knh trong WDM. Nm 2000 mt h thng thc nghim at c dung lng 3,28 Gb/s gm 82 knh, mi kng co tc bit 40 Gb/s, truyn trn khoang cach 3000 km, tng ng vi tich BL khoang 10000 (Tb/s).km. Chi trong 1 nm dung lng c nng ln trn 11 Tb/s (273 knh, mi knh co tc bit 40 Gb/s, khquaoang cac 117 km). Cui nm 2000 cac h thng ng truc trn mt t a at c dung lng 1,6 Tb/s.

Th h th 5 ca h thng thng tin quang da trn c s gii quyt vn tn sc trong si quang. Khuch i quang gii quyt hon ho suy hao quang si nhng khng gii quyt c vn tn sc. C nhiu phng n b tn sc nhng phng n c tnh kh dng cao nht l da trn hiu ng Soliton quang. Hiu ng Soliton quang l mt hiu ng phi tuyn trong si quang, n da trn c s tng tc b tr tn sc ca cc thnh phn quang trong mt xung quang cc ngn c truyn trong si quang khng c suy hao.Nm 1994 h Soliton th nghim truyn dn tn hiu 10Gb/s trn khong cch 35000km v 15Gb/s trn khong cch 24000km. Bt u t nm 1996 h thng WDM Soliton kt hp vic quan ly tan sc. Trong nm 2000, mt h thng thc nghim a thanh cng vi vic truyn 27 knh WDM, tc mi knh 20 Gb/s, khoang cach 9000 km s dng s khuch ai

Nhng nm gn y ngay cang nhiu h thng xuyn quc gia va xuyn ai dng co dung lng ln, ni nhiu quc gia khac nhau a c trin khai xy dng, a vao hoat ng va khai thac nhm ap ng nhu cu gia tng nhanh chong, a dang cac loai hinh dich vu, thng tin lin lac toan cu. SEA-ME-WE-3 va SEA-ME-WE-4 la hai trong rt nhiu h thng nh vy.

Hnh 1.2 H thng cp quang xuyn bin SEA-ME-WE-3 [1]

SEA-ME-WE-3 la mt h thng cap quang xuyn bin dai nht th gii c hoan thanh vao cui nm 2000 [10]. c keo dai t phia Bc nc c cho n Nht Ban va Uc, co chiu dai khoang 39000km co 39 tram cp b. Vit Nam tham gia vao tram cp b s 30 Hoa Hai, thanh ph a Nng, nh c m ta Hinh (1.2 ). y la h thng ghep knh theo bc song (WDM) s dung cng ngh phn cp s ng b (SDH) tng dung lng. n thang 5 nm 2007 h thng s dung 2 cp si, mi cp si co 48 knh vi tc mi knh truyn la 10 Gb/s; at dung lng 960 Gb/s (0,96 Tb/s).

Tip theo SEA-ME-WE-3 la h thng SEA-ME-WE-4 c trin khai xy dng va hoan thanh thang 12 nm 2005 [11]. H thng nay dai 18800 km ni gia cac nc ng Nam A n Trung ng mt vai nc Chu Phi va Chu u vi 17 im cp b. Hai h thng c d inh b sung cho nhau nhng hoat ng c lp SEA-ME-WE-4, truyn tai thng tin thoai, Internet, Multimedia va ngh ghep knh theo bc song mt cao (DWDM), s dung 2 cp si quang, mi cp si truyn 64 knh, tc mi knh la 10 Gb/s nn dung lng hin nay cua h thng la 1,28 Tb/s.

Hnh 1.3 H thng cp quang xuyn bin SEA-ME-WE-4 [1].

1.3 Nhng u im, nhc im ca h thng thng tin si quang.

1.3.1 Nhng u im:

Vic truyn tn hiu trong si quang c cc u im chnh sau:

+ Dung lng cc ln. Cap quang co th truyn tai tin hiu co tn s cao hn rt nhiu so vi cap ng truc va thng tin v tuyn. Bng thng (BW) gp khoang 10000 ln so vi thng tin vi ba (BW thng c chon khoang vai % tn s song mang (f), f anh ang c 200 THz, f vi ba c 20 GHz).

+ Tn hao rt thp: Tn hao si quang co th at 0,2 dB/km, so vi cap ng truc t 10 300 dB/km.

+ Kich thc nho, gon nhe: Trong mt dy cap quang co th co nhiu si quang nh trn (Hinh1.4) ng kinh si quang nho trong lng nhe thun tin cho vic lp t, bao dng si quang.

Hnh 1.4 Cp si quang dng di bin

+ Tinh chng nhiu cao: Si quang c cu tao t thuy tinh xit (trong o SiO2 la loai xit thng dung nht tao ra si) nn khng bi anh hng cua in t trng bn ngoai va cac loai nhiu t cac nha may in nguyn t, sm set....

+ Gia thanh thp: Vi thach anh la nguyn liu chinh san xut si quang co sn trong thin nhin. So vi kim loai thi ngun nguyn liu nay di dao hn.

+ Khoang cach truyn ln: Kt hp kha nng khuch ai cua cac b khuch ai quang trn ng truyn cung vi s suy hao thp va nhay thu cao cua cac may thu cho phep tng khoang cach cc ln.

+ Tc cao, hiu sut ln: Cac kha nng iu ch tc cao, kich thc nho, hiu sut bin i quang in cao.

+ Kha nng truyn tin hiu vi cac bc song khac nhau: Thng tin si quang cung cho phep truyn ng thi cac tin hiu co bc song khac nhau. c tinh nay gop phn rt ln lam tng dung lng truyn dn.

1.3.2 Nhc im:

Bn cnh nhng u im th vn cn nhng nhc im sau:

- Cc b phn bin i quang - in t tin, kh ch to, kh nng ghp ni vi si quang kh khn.

- Vic ghp ni si quang i hi thit b c kh c chnh xc cao. Cc khp ca u ghp ni khng tng hp hay c vt nt trn si quang l nguyn nhn gy ra suy hao.

- Vic ghp cc knh truyn quang hc gp nhiu kh khn.

- Khi gim kch thc ng truyn dn th d lp t v bo dng, gim chi ph u ni nhng li gp kh khn khi u ni.

1.4 H thng truyn dn quang

1.4.1 Cc h thng thng tin

Hnh 1.5 Mng thng tin si quang ghp knh theo bc sng vi cc thnh phn c bn

Mt h thng thng tin quang hay h thng thng tin nh sng l mt h thng lin lc s dng nh sng mang thng tin si quang. H thng thng tin l h thng im im, cn mng thng tin c nhiu tuyn c ni vi nhau bng cc b ghp hoc cc b chuyn mch, cho php lin lc t trm ny n trm khc. Hinh 1.5 minh ha mng thng tin si quang ghp knh theo bc sng vi cc thnh phn c bn ca n. Mng cung cp cc ng quang ti ngi s dng l cc u cui SDH (SONET) hay cc b nh tuyn IP.

Mt h thng thng tin lin lc im im gm ba phn c bn l phn pht, mi trng truyn dn v my thu. Phn pht c nhim v bin i tn hiu vo thnh tn hiu ph hp vi mi trng truyn. i vi h thng thng tin si quang th mi trng truyn ny l cp si quang. Mt yu t quan trng nh hng xu n cht lng tn hiu truyn dn l nhiu. Nhiu c th xut hin trong ba phn ca h thng. Khc vi cc h thng thng tin min in nh h thng thng tin vi ba, v tinh, trong h thng thng tin si quang nhiu t mi trng bn ngoi tc ng vo si l khng ng k. Tuy nhin, nhiu t ngun pht v my thu ln lm nh hng n cht lng tn hiu, khi nhiu t cc ngun ny ln, tn hiu c th b mo dng, t l li bt (BER) tng ln.

1.4.2 S nguyn l ca h thng thng tin si quang

im khc nhau c bn ca h thng thng tin quang v h thng vi ba di tn s sng dng mang thng tin khc nhau. Cc tn s sng mang quang tiu biu khong 200THz , trong khi di tn sng mang vi ba l khong vi GHz n vi chc GHz. Do dung lng thng tin t l thun vi tn s sng mang nn sng nh sng c th mang dung lng thng tin gp khong 10.000 ln so vi sng vi ba. iu ny c gii thch l bng thng ca sng mang iu ch c rng bng vi phn trm ca tn s sng mang. Chng hn, nu cho rng tiu biu l 1% th bng thng ca h thng thng tin quang l khong 2 THz, trong khi ca vi ba c th t khong 200 MHz. Do , thng tin quang c nh gi l h thng truyn dn v phn phi y tim nng nhm p ng nhu cu a dng trong tng lai ca x hi.

Thng thng th tn gi ca mt h thng thng tin gn lin vi mi trng truyn dn. V vy m i vi h thng thng tin si quang th mi trng truyn dn chnh l cp si quang. S nguyn l hot ng ca h thng c m t nh hnh di.[1]

Hnh 1.6 S nguyn l ca h thng thng tin si quang

Tn hiu in c a vo b bin i in-quang (E/O) bin thnh tn hiu quang ( y chnh l thit b u cui pht quang ). Sau tn hiu quang mang thng tin ny c a vo si dn quang truyn n pha thu ( c th n qua Trm lp khuyt i). pha thu thc hin ngc li, bin i tn hiu quang thnh tn hiu i n nh b bin i quang-in (O/E)- y l thit b u cui thu quang.

1.4.3 c im ca h thng thng tin si quang

H thng thng si tin quang c chia thnh 2 loi l h thng di v ngn ph thuc vo khong cch truyn dn so vi khong cch tiu biu (khong 100km). H thng di thng l cc tuyn ng trc trn mt t c dung lng ln ni gia cc thnh ph, cc quc gia hoc cc tuyn cp quang bin xuyn i dng. H thng ngn thng l cc tuyn v cc vng lp trong thnh ph c dung lng thp hn v khong cch di 10 km. p ng vi cc loi hnh thng tin s a dng, tc cao, nhiu thu bao nh hin nay v trong tng lai, cc h thng ny cng thc hin cc k thut ghp knh v khuch i quang nh h thng ng trc.

Da vo phng php iu ch th c th chia h thng thng tin si quang thnh hai h thng: H thng u ch cng tch sng trc tip (IM-DD) v h thng Coherence. Trong h thng IM-DD s dng phng php iu ch s ASK, cn h thng Coherence s dng thm cc phng php iu ch s khc nh: FSK, PSK, iu ch phn cc PoLSK...

H thng thng tin si quang IM-DD hin nay c s dng ph bin hn, bao gm cc thnh phn chnh l phn pht quang, si dn quang v phn thu quang c biu din nh hnh sau:[1]

Phn pht quang c cu to t ngun pht tn hiu quang v cc mch in iu khin lin kt nhau; bao gm cc phn t m ho, iu khin, ngun pht quang. Ngun pht quang s dng diode pht quang LED hoc LD, cc loi ny to ra tn hiu quang ra tng ng vi s thay i ca dng iu bin vo. Tuy nhin LED ph hp vi h thng c c li ngn, dung lng thp, cn Laser Diode dnh cho cc h thng c khon cch truyn dn di v dung lng cao. Tn hiu in u vo dng s hoc tng t, thit b pht s bin i thnh tn hiu quang tng ng.

Mi trng truyn dn l cp si quang. Cc loi si c s dng l si n mode hay a mode. Khi lp t tuyn quang, tham s quan trng cn ch n l suy hao tn hiu trn si theo bc sng. c tuyn suy hao ca si quang theo bc sng c th hin nh hnh 1.8.[3]

Hnh 1.8 Suy hao tn hiu trong si quang theo bc sng qua cc giai on khc nhau

Theo nh hnh trn th c 3 vng bc sng c s dng ln lc theo thi gian trong h thng thng tin si quang: l vng 850nm, 1300nm, 1550nm gi l vng ca s th nht, th hai v th ba tng ng. Giai on u ca s th nht c s dng. Sau , nh cng ngh ch to si quang pht trin mnh, suy hao si quang ca s th hai gim nh (khong 0,5 dB/km) cho php tng khong cch truyn dn. n cui thp nin 70, ngi ta s dng si vng ca s th ba vi tn hao tiu biu trong si l khong 0,2dB/km.

Cc trm lp hoc cc khuch i quang: Khi h thng cn truyn ti thng tin vi khong cch ln do suy hao truyn dn gia ngun pht v ngun thu ln. Do cn phi lp t thm cc trm khuch i b li suy hao sao cho my thu c cng sut cn thit tn hiu thu c m bo c yu cu li bt BER cho trc. Hin nay, hu ht cc trm lp u c thay th bng cc b khuch i quang do n cng knh, cn h thng cung cp ngun phc tp, bng thng quang b hn ch v phi thc hin chuyn i quang-in, in quang.

Phn thu quang bao gm cc khi tch quang, cc mch khuch i, iu khin gii m. Cc b tch quang thng c s dng l Photodiode PIN hoc diode thc quang ADP, c hai loi u c hiu sut lm vic cao v tc chuyn i nhanh.

Ngoi ra cn nhiu thnh phn to nn h thng truyn dn hiu qu, chng hn cc b xen r knh, b chia quang, b ni quang...

1.5 Kt lun chng

Qua tm hiu ni dung chng 1, n c ci nhn bao qut v h thng thng tin quang. Ta thy h thng thng tin quang ngy cng c s dng rng ri vi nhng u th ni bt m cc h thng khc khng c c v c tnh k thut v hiu qu kinh t. Tuy nhin, nh gi s thnh cng ca mt h thng khng th khng ni n vai tr ca si quang v cp quang, vn ny s c trnh by c th chng sau.

Ni tn hiu cn truyn i

Thit b pht

Mi trng truyn dn

Ni tn hiu n

Hnh 1.1: Cc thnh phn c bn ca mt h thng thng tin

Thit b thu

Cp quang

Tn hiu

vo

Tn hiu

ra

Bin i

E/O

Bin i

O/E

Mch iu

khin

Ngun pht

quang

phn pht quang

Pht

quang

Mch in

Thu

quang

Trm lp

Khuch i

quang

Mythu

quang

Chuyn i

tn hiu

Phn thu quang

Khuch i

B ni quang

mi hn si

cc thit b

khc

mi hn si

chia quang

B ni quang

tn

hiu in vo

tn

hiu in ra

Hnh 1.7 S khi ca h thng truyn dn thng tin si quang IM-DD [2]