Thong Tin Quang Soi

8/8/2019 Thong Tin Quang Soi

1/93

Gio trnh Quang - in t& Thng tin quang PGS.TS. Phm Vn Hi

- 1 -

CHNG

GII THIU CHUNG V QUANG -IN TV

THNG TIN QUANG SI

M U

T khi laser ra i trong nhng nm 60 ca th k XX, ngnh Quang -in t

pht trin cc k mnh m v c ng dng trong hu ht cc thit b t qun

s, cng nghip cho n dn dng. Tuy nhin, lnh vc ng dng c hiu qu

nht cc kt qu nghin cu ca quang -in t chnh l thng tin v cm bin.

Ti Vit nam, chuyn ngnh quang-in t c nghin cu v ging dy timt s Vin nghin cu v ti cc Trng i hc t nhng nm 60-70 ca th

k trc. Mt s kt qu nghin cu v quang-in t ti Vit nam gp phn

ng k trong cuc khng chin chng M cu nc, trong vic trin khai c

hiu qu cao mng thng tin cp quang v tip thu cng ngh cao t bn ngoi

trong qu trnh cng nghip ho, hin i ho v hi nhp quc t hin nay.

Mc d c nghin cu t lu, nhng ngnh cng nghip quang-in t

hin i ca Vit nam vn cha c pht trin ng vi nhu cu chung ca

quc gia, hu ht cc linh kin quang -in t u phi nhp t bn ngoi lmcho g thnh cc thit b quang -in t ch to trong nc rt kh cnh tranh

trn th trng. Ngoi ra, kinh ph duy tr v bo hnh cc thit b quang-in

t , c bit l cc thit b thng tin quang hin c, cng cn rt cao.

Gio trnh ny trnh by csvt l ca cc linh kin quang -in t ng

dng trong thng tin quang v cm bin quang gm: mi trng truyn dn tn

hiu quang, linh kin pht tn hiu quang, linh kin thu v x l tn hiu quang,

thit b khuch i quang. Ngoi ra, gio trnh trnh by slc h thng truyn

thu tn hiu quang l cs pht trin cc linh kin v thit b quang -in t

ng dng trong thng tinv cm bin.

1. Lch s pht trin h thng thng tin

Cc mc pht trin quan trng :


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Nm 1866-1876: ng thng tin telex v telephone vi tc truyn

thu cvi chc KHz.

Nm 1940: h thng cp ng trc c 300 ng thoi vi tc

truyn thu 3MHz

Hn ch ca cp ng trc: suy hao tn hiu ph thuc tn s, vi tc ln hn 10MHz suy hao tng vt.

Nm 1948: h thng viba (siu cao tn) c sng mang n 4GHz vi

tc bt t 100 Mb/s. H thng viba hin i c tc bt 274Mb/s c lp

t nm 1975.

Hn ch ca h thng viba: cn cc trm lp trn khong cch ngn, tc

bt hn ch bi tn s sng mang.

Thng s quan trng ca h thng thng tin l thng s khong cch x tc

bt (BL), trong B l tc bt v L l khong cch cc trm lp.

Thng s BL cho h thng viba t 100 (Mb/s)-km trong nhng nm 1970.

Nm 1960:

o Si dn quang c nghin cu ch to t thu tinh silica ( SiO2),

h s suy hao quang khong 1000 dB/km.

o Laser bn dn c nghin cu ch to, dng ngng rt cao, nhit

lm vic thp, thi gian sng ngn.

Nm 1970:o H s suy hao si dn quang bng thu tinh gim xung cn 20

dB/km ti bc sng =1m

o Laser bn dn GaAs lm vic ti nhit phng, dng ngng

gim xung cn vi chc mili-ampere i vi laser cng tc di. Trong nhng

nm 70, h thng thng tin quang si bt u pht trin.

Nm 1980: h thng thng tin quang u tin c a vo hot ng.

Bc sng laser trong vng 0.8m (GaAlAs/GaAs), tc bt 45Mb/s, khong

cch lp 10km. ngha pht trin h thng nm trong khong cch lp lnhn h thng viba v do gim gi thnh lp t v duy tr. Trong nhng nm

ny, laser bn dn InGaAs/InP c bc sng pht ti 1310 nm c ch to kh

hon thin. Bc sng 1310 nm c suy hao trong si quang khong 1dB/km

(1980) v h s tn sc cc tiu l i tng nghin cu ng dng rt mnh

trong thi gian ny cho h thng thng tin quang.


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u nhng nm 80 (1982-1983): h thng Thng tin quang th h

s dng laser 1310 nm bt u c s dng. Thi gian u, tc bt ch t

100Mb/s do s dng si a mt. Khi si n mt c a vo s dng, tc

bt c tng ln rt cao. Nm 1987, h thng thng tin quang 1310 nm c

tc bt 1,7Gb/s vi khong cch lp 50km tr thnh hng ho thng mi.H s suy hao trong si ~ 0,5dB/km t i b c sng 1310 nm.

Nm 1990 h thng thng tin quang th h s dng laser bn dn

bc sng 1550 nm (Laser InGaAsP) c suy hao trong si quang c

0,2dB/km c thng mi ho. 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 1550 nm

kh cao (16-18ps/km.nm), do hn ch khong cch lp ca h thng 1550

nm mc d qu cng sut quang cn bo m di hn. c trng khong cch

lp ca th h l 60-70km ti tc bt 2,5Gb/s. Khi s dng cc loi si

quang b tn sc kiu dch tn sc (DSF) hoc lm phng tn sc (DFF), khong

cch lp s tng ln hn 100km.

Th h th V ca thng tin cp quang l s dng khuch i quang

tng khong cch lp v ghp nhiu bc sng trong mt si quang tng tc

bt trong mt si quang .

Khuch i quang si pha tp Erbium (EDFA) c kh nng b cng sut

cho suy hao quang trong cc khong cch ln hn 100km. EDFA c nghincu thnh cng trong phng th nghim 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 21.000km v 5Gb/s

trn khong cch 14.300km.

Nm 1996 h thng thng tin cp quang di bin c tc 5Gb/s trn

khong cch 11.300km s dng EDFA c a vo s dng, nm 1997 h

thng u c tn FLAG c tc bt 5Gb/s v khong cch 27.000km

a vo hot ng. H thng cp quang vng quanh chu Phi (Africa One) ckhong cch 35.000km cng c lp t trong nm 1997.

S dng cng ngh ghp nhiu bc sng trn mt si quang (WDM)

lm tng dung lng thng tin quang mt cch ng k. Khuch i quang

EDFA c th khuch i ton b cc bc sng quang trong di 1525-1575nm

m khng cn phi tch tng bc sng. Trong nm 1996 th nghim tuyn


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truyn dn 20 bc sng quang vi tc bt ca tng bc sng l 5Gb/s trn

khong cch 9100km. Tc bt ca tuyn t 100Gb/s v BL l 910 (Tb/s)-

km.

Trong nm 2000 h thng TPC-6 xuyn i Ty Dng c tc bt

100Gb/s c a vo hot ng.C th tng kt rng BL ca h thng tng rt nhanh t 1Gb/s - km

n 900Tb/s km ch trong vng 25 nm.

Th h th V ca h thng thng tin quang da trn c s gii quyt

vn tn sc trong si quang. Khuch i quang gii quyt rt hon ho s

suy hao quang trong si nhng khng gii quyt c vn tn sc. Rt nhiu

phng nb tr tn sc c pht trin v s dng, tuy nhin s gii quyt

dt im vn tn sc trong si quang s da trn hiu ng Soliton quang.

Hiu ng Soliton quang l hiu ng phi tuyn trong si quang. Chng 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 tn hiu 10Gb/s trn khong cch

35.000km v 15Gb/s trn khong cch 24.000km. Nm 1996 h thng WDM

c 7 bc sng truyn Soliton trn khong cch 9400km vi tc d bt 70Gb/s.

Vn ca Soliton ang gp phi hin nay l ngun pht tn hiu quang

vi xung cc ngn c pic giy c n nh cao cha c hon thin s

dng trong h thng.

2. Nguyn l c bn ca h thng thng tin

2.1. H thng thng tin tn hiu tng t v tn hiu s

Tn hiu tng t: lin tc theo thi gian, bin thay i.

Tn hiu s: gi tr gin on, ch c 2 mc 0 v 1 gi l bit (binary

digit). Mi bit c khong thi gian TB gi l chu k bit (hoc bit slot).

i lng B ch s bit trong 1 giy gi l tc bit:-1BB = T

Tn hiu tng t v s u c trng bi bng tn ca chng. Bng tn

c o bng dung lng ph ca tn hiu v biu th vng ph tn s ca tn

hiu.

Tn hiu tng t c th bin i thnh tn hiu s bng k thut ly mu

trong khong thi gian ca tn hiu.


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Tc ly mu c xc nh bng bng tn f ca tn hiu tng t. Theo

Nyquist, tn s ly mu fs 2f. Bc u ca qu trnh l ly tng t ti tn s

bn phi. Ga tr mu c th ly bt k gi tr no trong khong 0 A Amax,

trong Amax l bin cc i ca tn hiu tng t. Chia Amax cho M khong

gin on, mi gi tr mu c ly s tng ng vi mt gi tr gin on nyv ta c M gi tr mu. Tp m ca tn hiu tng t k hiu l AN (cng tp

m rms), ta s c M > Amax/AN v Amax/AN l vng ng lc ca tn hiu.

T s tn hiu trn tp m c tnh bng biu thc:

SNR = 20 log10 maxN

AA

(1.1)

trong Amax, AN l bin (dng hoc th) ca tn hiu.

Cc gi tr mu c th chuyn i thnh cc tn hiu s (digital format) theo

mt k thut chuyn i thch hp. Trong thng tin quang si, k thut chuyn

Ly mu

Dng bt s

001010 1101111111 01

Tn hiu Analog

t

t

t

Hnh 1.1. S tc ly mu cho tn hiu s


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i ph bin l iu ch xung m ho (PCM: Pulse-Code Modulation). M nh

phn c s dng chuyn i gi tr mu thnh bit 1 v 0. S lng bit

m cn thit m ho mi mu lin h vi s mc ca gi tr mu M theo quy

lut sau:

M = 2m m = log2M (1.2)

Tc bit trong PCM s l:

B=m. f s (2f)log2M (1.3)

Ta c: M > Amax/AN

v : SNR = 20 log10max

N

A

A

log210 3.33

Vy : B > (f/3)SNR (1.4)

SNR c n v l decibel (dB).

Xt chuyn i sang tn hiu s:

Tn hiu m thanh in thoi c tn s trong di 0.3-3.4KHz, ta c f =3,1KHz, vi iu kin SNR = 30dB ta c tc bt B > 31Kb/s. Trn thc t

tn hiu s m thanh thoi B = 64Kb/s, tc ly mu fs = 8KHz (khe thi

gian ly mu 125m) v mi mu c c trng bi 8 bit.

Tn hiu video: f= 4MHz ; SNR = 50dB Bmin = 66Mb/s.

Trn thc t tn hiu hnh s cn tc bt 100Mb/s hoc ln hn.

2.2. Trn knh

Knh thoi s hot ng ti tc bt l 64Kb/s, mi ng truyn quangc th truyn vi tc hng trm Mb/s v vy cn phi trn knh truyn

ng thi nhiu knh thoi. C 2 cch trn knh c bn: trn theo phn chia

thi gian (Time-Division multipexing: TDM) v trn theo phn chia tn s

(Frequency-Divion multiplexing: FDM).

Trn knh theo TDM:


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Nu mi knh thoi nn xung n 3s, ta c th trn 5 knh theo k thut TDM

( xem hnh 2).

Trn knh FDM: Mi knh c truyn ti bng tn s sng mang ring bit.

FDM tch hp vi c tn hiu tng t v s. Nu hiu theo ngha rng trong

quang hc FDM cng l WDM ti ph quang.

TDM c s dng ph bin trong k thut c phn cp (digital hierarchies)

Hin nay c cc phng thc trn knh sau:Bc M v Nht: trn 24 knh thoi vi tc 1,544Mb/s (DS-1)

Chu u: trn 30 knh thoi vi tc 2,048Mb/s

Tc bt ln hn tch s lng bt nhn vi 64Kb/s l do phi nhi thm cc

bt kim tra tch knh ti u thu.

15 s

3s

Bin

5 1 2 3 4 5 1 2 3 Thi gian

Hnh 1. 2. Slot 15s cho 1 knh thoi ti tc bit c 64kb/s trn 5 knh

Bin

Knh 1 Knh 2 Knh 3

f1 f2 f3

Tn s

Hnh 1.3. Phn b knh theo tn s FDM


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Cc tc bt 1,544Mb/s M, Nht v 2,048Mb/s Chu u l mcphn cp

s . Cc mcphn cp tip theo nhn c bng cch nhn vi 4 t cc mc

phn cp gn k.

Th d: Phn cp :

DS-2 c tc bt 6,321Mb/s M, Nht v 8,448Mb/s Chu uCc phn cp bc 5: 396Mb/s M-Nht v 565Mb/s Chu u

Trong nhng nm 80, h thng tiu chun mi gi l Mng quang ng

b (Synchronous optical networkSONET) c thit lp. Tiu chun ny

da trn c s cu trc ng b khung cho cc tn hiu truyn TDM. Khi xy

dng c s ca SONET c tc bit 51,84Mb/s (OC-1 : Optical Carrier-1) v

khi xy dng c s ca SDH l 155,52Mb/s (STM-1: Synchronous Transport

Module -1 )

Bng 1 : Tc bit ca SONET/SDH

Sonet SDH B (Mb/s) S knh

OC-1 51,84 672

OC-3 STM-1 155,52 2016

OC-12 STM-4 622,08 8064

OC-48 STM-16 2418,32 32256

OC-192 STM-64 9953,28 129024

2.3. Phng thc iu ch

C 2 phng thc iu ch bt thng tin quang: RZ (v 0) v NRZ (khng v 0).

Th d c dng bit s 010110 c m theo phng thc RZ v NRZ


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RZ xung quang i din cho bt 1 ngn hn bt slot v tr v 0 trc khi htchu k. Bng tn gp i tc bt.

NRZ xung quang bao ht bit slot v khng tr v 0 gia cc bt 1 lin

tip. Bng tn bng tc bit.

RZ l phng thc iu ch tin li cho h thng thng tin Soliton.

Xt phng trnh sng quang trc khi iu ch:

( ) cos( )o

E t eA t (1.5)

E(t) - vect in trng

e - vect n v ca phn cc

A - bin

o - tn s sng in trng

Ta c th chn: iu ch theo bin A (Amplitude modulation-AM), iu

ch theo tn s o (Frequency modulation-FM) hoc iu ch theo pha (Phase

modulation-PM). iu ch tn hiu s c th p dng: iu ch kho dch bin

(Amplitude Shift Keying-ASK) iu ch kho dch tn s (FSK) v iu chkho dch pha (PSK) tng ng vi dch bin , tn s v pha ca sng

mang gia 2 mc ca tn hiu s nh phn. K thut n gin nht l thay i

cng tn hiu gia hai mc, mt mc trong l 0 gi l kho tt-m

(on-off KeyingOOK). a s h thng thng tin quang p dng OOK kt hp

vi PCM.

Tn hiu RZ

0 1 0 1 1 0

Tn hiu NRZ

Thi gian

Thi gian

Hnh 1.4. S miu ch RZ (trn) v NRZ (di)


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3. H thng thng tin quang

H thng thng tin quang gm phn pht tn hiu quang, mi trng truyn dn

knh quang v phn thu v x l tn hiu quang.

Mi trng truyn dn quang c chia lm 2 loi: Truyn trong si quang vtruyn trong khng gian. Tuyn truyn dn quang chia ra 2 loi tuyn truyn

dn ng di v tuyn ngn. S phn chia ny ph thuc vo khong cch

tuyn dn. Tuyn ng di c dung lng rt ln gi l ng trc (trunk

line) v c cc b lp. Tuyn ngn p dng cho cc tuyn ni ht v ni gia

cc thnh ph.

3.1. Cc linh kin ca h thng thng tin quang

Pht quang: l ngun pht quang gm it pht quang (LED) hoc laser bn

dn (LD) c ghp vi b iu ch v b ghp knh quang. iu ch in

quang c chia lm 2 loi: iu ch trc tip DIM (Direct Intensity

Modulation) hc iu ch lm 2 loi: iu ch ngoi EM (External Modulation).

Cng sut quang vo trong tuyn truyn dn l thng s quang trong cho thit

ng truyn quang. Cng sut quang thng c tnh bng mW hoc bng

dBm:

P(dBm) = 10 log10

( Po

/1mW) (1.6)

Ta thy: 1 m W tng ng 0 dBm, v 1W tng ng vi -30dBm.

Thu quang: l cc b thu quang bng phtdit v chuyn i tn hiu quang

thnh tn hiu in ban u. B thu quang bao gm b ghp knh quang v linh

Pht tn hiuquang

Mi trngtruyn dn

quang

Thu tn hiuquang

Tn hiu raTn hiu vo

Hnh1. 5. S khi ng truyn thng tin


11/93


- 1 1 -

kin thu, photodiode v b gii iu ch. B gii iu ch c thit k thch

hp vi phng thc iu ch ca h thng. Cc phng thc FSK v PSK

c p dng cho cc h thng thng tin quang kt hp, trong c cc b

heterodyne v homodyne. Thng dng nht l cc b gii iu ch trc tip 1-

0 gi l iu ch cng vi tch sng trc tip (IM/DD).Thng s quang trng ca b thu l nhy u thu (Receiver sesitivity) lin

quan n t s bt (Bit-error rateBER). BER~10-9 cho cc h thng thng tin

di Gb/s, ~10-11 cho cc h thng bc Gb/s v c th ln n 10-14 cho cc h

thng dung lng cc ln. Cc thng s tp m ca b thu nh hng n cht

lng thng tin v BER. Tp m nhit, tp m khuch i v tp m lng t l

nhng thnh phn nh hng ln n thng s ca b thu quang.

Cc thnh phn ca h thng thng tin quang bao gm:

o Khuch i quang: khuch i bn dn, khuch i si quang.o B chia-ghp knh quang (coupler): bng si hoc bng linh kin quang

hc.

o B iu ch v chuyn mch quang: bng tinh th phi tuyn quang hoc

bng cu hnh si quang kiu giao thoa k.

o B cch ly quang ch cho nh sng i qua 1 chiu.

o B phn cc quang (to phn cc, kim sot phn cc) bng tinh th si

quang hoc phi hp si quang-in trng.

o B lc quang bng cu trc giao thoa k Fabry-Perot hoc bng cch t

Cc linh kin v thnh phn h thng thng tin quang s ln lt c nghin

cu cc chng sau.


12/93


- 1 2 -

CHNG II.

SI QUANG

Si quang l mi trng truyn dn quang l tng da trn hiu ng phn

x ton phn trong si quang. Si quang hin i c h s suy hao quang rt nhtrong cc ca s 0,85; 1,3 v 1,5m nh sau: ~ 3dB/km cho vng 0,85m ~

0,4dB/km cho 1,3m v 0,2dB/km cho 1,5m. Mt s si quang c bit c

tnh cht duy tr phn cc quang, khuch i tn hiu quang hoc b tr tch sc

quang.

1. Cu trc si quang theo quang hnh

Si quang l cc ng hnh tr c cu trc li dn quang v lp bc vi chit sut

nh hn lp li (n1> n2). Cu trc li-bc ca si quang c chia thnh 2 loi:

- Chit sut nhy bc (Step index).

- Chit sut thay i tun t (Graded index).

1.1. Cu trc chit sut nhy bc (Step-index)

Tia sng vo si quang theo gc r so vi trc i xng ca si quang. Theo

nh lut khc x ta c:

nosini = n1sinr (2.1)

trong no : chit sut khng kh

n1 : chit sut li si quang

Trong si quang, tia sng ch phn x ton phn khi gc ti ca n n bin

phn cch li-bc p ng iu kin:

sin c n2 / n1 (2.2)

trong n1 - chit sut lp li; n2 - chit sut lpbc

V bo v

r

Bc

Li

n1n2 n0

n1n2 n0

ir

n

Lp bc n2Li

Tia dn

Hnh 2.1. Mt ct ca si quang (tri) v s tnh khu s NA


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- 1 3 -

iu kin phn x ton phn:2

1

arcsinc

n

n (2.3)

Ta thy: r= 2 c

; trong c

c tnh theo (2.3).

Thay vo (2.1) ta c:2 2

0 1 1 2sin os ( )i cn n c n n (2.4)

Tng t nh trong l thuyt thu knh, nosini gi l khu s (NA) ca si

quang:

12 2 21 2NA = (n - n ) (2.5)

Vi trng hp n1 n2, ta c th phn chia:

(n12n2

2)1/2 = [(n1+n2)(n1n2)]1/2=n1

1/ 2

1 2

1

2n n

n

= n1(2)

1/2 (2.6)

vi1 2

1

n n

n

- lch chit sut t i

v n = n1n2 - lch chit sut tuyt i

NA = n1 (2) 1/2 (2.7)

T (2.7) ta thy ln s cho h s ghp quang cao. Tuy nhin do tn sc

gia cc mt trong si quang tng cao khi NA tng s khng c li cho truyn

thng tin dng s trong si. Xt tn sc gia cc mt, ta thy mt xung nh sng

s m rng trong qu trnh truyn dn do quang trnh ca cc tia khc nhau. T

hnh v ta thy quang trnh ngn nht do cc tia c gc ti i=0, v di quang

trnh chnh bng di si quang. Quang trnh di nht do cc tia c gc ti itho mn iu kin phn x ton phn ti hn theo (2.4). di ca quang trnh

ny s lsin

c

L

, vi L l di si quang. Tc lan truyn nh sng trong li

si l1

cV

n , vy thi gian tr gia tia truyn thng v tia phn x ti hn s l:


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- 1 4 -

t

21 1

2sin c

n nL LL

c c n

(2.8)

Thi gian tr ny lm xung quang b dn n trong qu trnh truyn trong si.

Tc bt B v t lin h vi nhau theo quy lut: t phi nh hn chu k

bt (bit slot) ca chui xung :

t < TB trong TB1

B , ta c t.B


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vi a - bn knh li si.

Khi ln, si s c cu trc chit sut nhy bc, = 2 si c chit sut thay i

dngparabol.

Hnh 2.2 : Mt ct ca siquang c cu trc chit sut thay i tun t

Qu o ca cc tia bin c tnh theo cng thc:

2

2

1d dn

dz n d

(2.12)

Trong z l trc lan truyn quang (trc si quang), l khong cch tnh t

trc quang.

Vi = 2,


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T st

L

, trong t l tr ln nht trong si quang c di L, ph thuc

vo h s . Tnh ton chi tit cho thy: n1=1,5; = 0,01 tn sc gia cc mt

nh nht khi = 2(1-) v ch ph thuc vo nh sau:

21

8nt

c (2.14)

V ta bit: t < 1B

vy BL

Thong Tin Quang Soi

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