I HC QUC GIA TP. H CH MINH TRNG H CNG NGH THNG TIN

----------

TIU LUN

TM HIU PHNG PHP GHP BC SNG MT CAO

-DWDM-

Gio vin hng dn: Th.S Chu Tin Dng

Nhm SV thc hin: 1. Nguyn Hong Dng 2. L Trng Trng 3. Nguyn Chnh Tn

Nha Trang, 09/2015

1

LI M U

Th k 21 l th k ca cng ngh thng tin. S bng n ca cc loi hnh dch v

thng tin, c bit l s pht trin nhanh chng ca Internet v World Wide Web lm

gia tng khng ngng nhu cu v dung lng mng. iu ny i hi phi xy dng

v pht trin cc mng quang mi dung lng cao. Cng ngh ghp knh theo bc

sng quang (DWDM) l mt gii php hon ho cho php tn dng hu hiu bng

thng rng ln ca si quang, nng cao r rt dung lng truyn dn ng thi h gi

thnh sn phm. S pht trin ca h thng WDM cng vi cng ngh chuyn mch

quang s to nn mt mng thng tin th h mi-mng thng tin ton quang. Trong

mng ton quang ny, giao thc IP- giao thc chun cho mng vin thng th h sau

(NGN) s c tch hp vi WDM. S tch hp ny s to ra mt kt cu mng trc tip

nht, n gin nht, kinh t nht rt thch hp s dng cho c mng ng trc v mng

th.

2

Phn 1

TNG QUAN V CNG NGH DWDM V C S

K THUT GHP KNH THEO BC SNG

DWMD (Dense Wavelength Division Multiplexer - Ghp Knh Theo

Bc Sng Mt Cao)

gii quyt vn bng thng v pht trin h thng a dch v trn

cng mt mng, cng ngh DWDM thc hin ghp nhiu bc sng trn

cng mt si quang. Vi vic tng s bc sng ghp trn mt si quang mt

cch ng k so vi cng ngh WDM trc y, im ni bt ca DWDM

chnh l kh nng cho php truyn trn si quang mt lu lng khng l ln

ti hng Terabits/s. Tuy nhin, t c iu ny mt cch c hiu qu th

h thng DWDM c nhng yu cu rt c bit i vi cc chc nng quang

nh: linh hot cao, kt cu u cho nhanh, cc b lc v ngun laser phi

c kh nng iu hng, cc b thu phi c tp m thp v nhy cao.

Cc h thng DWDM hin nay lm vic trn cc knh bc sng theo

khuyn ngh ca ITU-T dnh cho DWDM. Nhiu bc sng ghp trn mt

si quang mang li s linh hot v mm do cho c cc dch v v bng

thng. Mi knh bc sng c th truyn ti mt loi lu lng khc nhau

nh SONET/SDH trn mt knh, ATM trn mt knh khc, tn hiu thoi

TDM hay Internet trn mt knh khc na.

1.1. K THUT GHP BC SNG QUANG

Trong h thng n knh, khi tc ng truyn t n mc no

, ngi ta thy cc hn ch ca cc mch in t trong vic nng cao tc

cng nh ko di c ly truyn dn. Thm vo , chi ph cho cc gii php

trn tuyn truyn dn rt tn km v cu trc h thng kh phc tp. Do ,

k thut ghp knh quang ra i nhm khc phc c nhng hn ch trn.

TIENTypewriterComparing SDHvs DWDM

3

MUX

DEMUX

Cc phn t quang trong h thng thit b s ng vai tr ch o trong vic

thay th hot ng ca cc phn t in cc v tr xung yu i hi k thut

x l tn hiu nhanh.

V l thuyt, ta c th lm tng ng k dung lng truyn dn ca h

thng bng cch truyn ng thi nhiu tn hiu quang trn cng mt si nu

cc ngun pht c ph cch nhau mt cch hp l v u thu c th thu c

cc tn hiu quang ring bit nu phn thu c b tch bc sng, y chnh l

c s k thut ghp bc sng.

1.2. NGUYN L C BN CA GHP BC SNG QUANG

Nguyn l c bn ca ghp bc sng quang c th minh ha nh hnh

1.1. Gi s c cc ngun pht quang lm vic cc bc sng 1,2,,n.

Cc tn hiu quang lm vic cc bc sng khc nhau ny s c ghp vo

cng mt si dn quang. Cc tn hiu c bc sng khc nhau c ghp li

pha pht nh b ghp knh, b ghp bc sng phi m bo c suy hao nh

v tn hiu sau khi ghp s c truyn dc theo si n pha thu. Cc b

tch sng quang khc nhau pha u thu s nhn li cc lung tn hiu vi

cc bc sng ring r ny sau khi chng qua b gii ghp bc sng.

Si quang

I1( 1 ) O1( 1 )

In( n ) O( 1

n

) I( 1

n

)

On( n

)

Hnh 1.1: M t tuyn thng tin quang c ghp bc sng

TIENHighlight

TIENTypewriterWhat is WDM?

TIENTypewriter

4

c im ni bt ca h thng ghp knh theo bc sng l tn dng

hu hiu ngun ti nguyn bng thng rng trong khu vc tn hao thp ca si

quang n mode, nng cao r rt dung lng truyn dn ca h thng, ng

thi h gi thnh ca knh dch v xung mc thp nht.

H thng WDM da trn c s tim nng bng tn ca si quang

truyn i nhiu bc sng nh sng khc nhau, iu thit yu lc ny l vic

truyn ng thi nhiu bc sng cng mt lc khng gy nhiu ln nhau.

Mi bc sng i din cho mt knh quang trong si quang. Cng ngh

WDM pht trin theo xu hng m s ring r bc sng ca knh c th l

mt phn rt nh ca 1nm hay 10-9 m, iu ny dn n cc h thng ghp

knh theo bc sng mt cao DWDM. Cc thnh phn thit b trc kia ch

c kh nng x l t 4 - 16 knh, mi knh h tr lung d liu ng b tc

2.5Gbps cho tn hiu mng quang phn cp s ng b (SDH/SONET). Cc nh

cung cp WDM sm pht trin cc thit b nhm cho vic truyn nhiu hn

cc knh quang. Cc h thng vi hng trm knh gi y sn

sng c a vo s dng, cung cp mt tc d liu kt hp hng trm

Gbps v tin ti t tc Tbps truyn trn mt si n.

C hai phng n thit lp h thng truyn dn s dng ghp bc

sng quang WDM.

Truyn dn mt chiu trn hai si.

WDM mt chiu l tt c cc knh quang cng trn mt si quang

truyn dn theo cng mt chiu, u pht mang cc tn hiu c bc

sng khc nhau v iu ch 1,2,,n thng qua b ghp knh t

hp li vi nhau v truyn dn mt chiu trn mt si quang. V cc tn

hiu c mang thng qua cc bc sng khc nhau, do s khng ln

ln. u thu, b tch knh quang tch nhng tn hiu c bc sng

khc nhau, hon thnh truyn dn tn hiu quang, hng ngc li

truyn dn qua mt si quang khc, nguyn l ging nh trn.

5

1 My pht

quang

My pht

quang

n

My thu

quang

My thu

quang

1

My thu

quang My pht

quang 1, 2 ....... n My thu

quang

n

My pht

quang

B khuch i si

quang

B ghp/tch knh

B ghp/tch

knh O O

My pht

quang My thu

quang n

+1

n+1, n+2 .......

2n

My pht

quang My thu

quang

2n

B khuch

O B tch knh

i si quang

B ghp knh

O

1

1. .

.

n

My thu

1. quang . .

n My thu quang

B tch knh

1, 2 .......

n

B O khuch O

i si quang

B ghp knh

1 . . .

n

1

My pht 1 quang .

.

.

My pht n

n quang

1, 2 ....... n

Hnh 1.2: S truyn dn mt chiu trn hai si quang

Truyn dn hai chiu trn mt si.

WDM hai chiu l knh quang trn mi si cng truyn dn theo hai

chiu khc nhau, dng cc bc sng tch ri nhau thng tin hai chiu

(song cng).

H thng WDM song hng yu cu pht trin v ng dng cao hn,

c c cu phc tp hn i hi yu cu k thut cc k nghim ngt. C rt

nhiu vn cn lu nh phn x quang, xuyn m gia cc knh, mc

in ca cng sut truyn dn. pha pht, cc thit b ghp knh phi c suy

hao nh t mi ngun quang ti u ra ca b ghp knh. pha thu, cc b

tch sng quang phi nhy vi di rng ca cc bc sng quang. Khi thc

hin tch knh cn phi cch ly knh quang tht tt vi cc bc sng khc

bng cch thit k cc b tch knh tht chnh xc, di lm vic n nh. Do

s dng b khuch i quang hai chiu nn h thng song hng gim c

s lng b khuch i v tit kim c si quang. 1 .

. .

. .

.

n n

1 1. . . . . .

n n

Hnh 1.3: S truyn dn hai chiu trn mt si quang

6

V nguyn l, bt k mt b ghp bc sng no cng c th c

dng lm b gii ghp bc sng. Nh vy, iu n gin l Multiplexer

trong trng hp ny thng s dng dng chung tng thch cho b

ghp v b gii ghp, tr trng hp cn thit phn bit hai thit b ny.

Ngi ta chia loi thit b ghp bc sng quang thnh ba loi: Cc b

ghp (MUX), cc b gii ghp (DEMUX) v cc b ghp v gii ghp hn

hp (MUX - DEMUX). Cc b MUX v DEMUX c dng cho cc phng

n truyn dn song hng.

Phn loi cc b ghp bc sng trong k thut ghp bc sng:

Hnh 1.4: Phn loi cc b ghp bc sng quang

Cc b ghp bc sng th ng c s dng hin nay thng l cc

b vi quang hc (micro-optic) v b ghp si kiu dn sng (guided wave

fiber coupler). Mi loi u c u nhc im.

Cc b vi quang hc thng i hi h thng ghp ni cc thu knh

ghp vo si quang. Cc kh khn trong vic nh v v ghp ni lm hn ch

cc c tnh k thut, c bit l i vi cc si n mode. Tuy nhin, vic s

dng cc b vi quang hc cho php la chn c tnh ca b lc rng ri hn.

Cc b ghp si t chu nh hng ca cc kh khn nu trn nhng li

b hn ch trong vic la chn cc c tnh cn c ca b lc, chng hn nh

mc bng phng ca bng thng.

Cc b ghp

bc sng

Th ng Tch cc

Cc b thu

pht nhiu

bc sng

Vi quang Ghp si

Cc thit b

khc

Quang t hp

Tn sc B lc

Phi tuyn Giao thoa Phn cc

Tn x vt liu Cch t

7

C 3 tiu chun c bn xc nh c tnh ca b ghp bc sng:

Suy hao xen

Xuyn m

rng ph ca knh

Suy hao xen

Suy hao xen y c xc nh nh lng tn hao cng sut trn tuyn

truyn dn quang do vic thm vo cc b ghp bc sng. Khc vi cc coupler

thng thng, y suy hao xen c xem xt i vi tng bc sng:

Lk = -10log O(k)/Ik(k) MUX

Li = -10log Oi(i)/I(i) DEMUX

Trong :

I(i), O(k) l cng sut tn hiu c ghp trn ng chung.

Ik(k) l cng sut tn hiu bc sng k i vo ca th k ca b

ghp, tn hiu ny c pht t ngun pht quang th k.

Oi(i) l cng sut tn hiu bc sng k i khi cng th i ca b tch.

Suy hao ny bao gm suy hao sinh ra ti cc im ghp ni ca b

ghp bc sng m nguyn nhn ch yu l do hp th hoc do phn x. Mc

nh hng tng i ca hai ngun suy hao trn h thng cn ty thuc

vo loi cng ngh c la chn ch to b ghp bc sng.

Xuyn m

Xuyn m l s d tn hiu t knh ny sang knh kia. N lm tng nn

nhiu, do vy lm gim t s tn hiu trn nhiu SNR. Hin tng ny

c sinh ra do cc yu t sau:

Do cc c tnh ca b lc khng hon thin.

Do phn x hay hi t khng hon ton lm cc tia sng b tn mt.

Do ph ca cc ngun pht chng ln ln nhau.

Do cc hiu ng phi tuyn xy ra khi a cng sut cao vo

si quang.

8

Trong mt b gii ghp knh l tng, s khng c s d cng sut tn

hiu t knh th i c bc sng i sang knh khc c bc sng khc vi i.

Nhng trong thc t lun tn ti mt mc xuyn m no v lm gim cht

lng truyn dn ca thit b. Kh nng tch cc knh khc nhau c

din gii bng suy hao xen v c tnh bng dB nh sau:

Di(i) = -10log Ui(k)/I(k)

Trong : Ui(k) l lng tn hiu khng mong mun bc sng k do

c s d tn hiu ca th i sang, m ng ra th ch c tn hiu bc sng i.

Trong thit b ghp - gii hn hp, vic xc nh suy hao xen knh cng

c p dng nh b gii ghp. trng hp ny phi xem xt c hai loi

xuyn knh. Xuyn knh u xa l do cc knh khc c ghp i vo ng

truyn gy ra, v d nh I(k) sinh ra Ui(k). Xuyn knh u gn l do cc

knh khc u vo sinh ra, n c ghp bn trong thit b nh Ui(i). Khi

cho cc sn phm, cc nh ch to cng phi cho bit suy hao knh i vi

tng thit b.

rng ph ca knh:

rng ph ca knh l di bc sng dnh cho mi knh. rng

ny phi m bo trnh nhiu gia cc knh. rng ph gia cc knh

ty thuc vo tng ngun pht. ng vi cc tc truyn dn khc nhau s

c rng ph mi knh l khc nhau v chnh xc khc nhau.

rng knh dch tn

25 GHz 5 GHz

50 GHz 10 GHz

100 GHz 20 GHz

200 GHz 50 GHz

Bng 1.1: rng ph ca knh

9

1.3. CC THAM S CHNH TRONG DWDM

DWDM thc hin ghp mt s lng ln cc bc sng quang c

iu ch trn mt si quang. Nhng knh quang trong h thng DWDM

thng nm trong mt ca s bc sng, ch yu l 1550 nm v mi trng

ng dng h thng ny thng l mng ng trc, c ly truyn dn di v c

dung lng truyn dn ln. Ging nh bt c mt cng ngh no, DWDM

cng tn ti nhng gii hn v nhng vn k thut. Trong chng ny,

chng ta s xem xt mt s tham s nh: suy hao, nhiu xuyn knh, s knh

bc sng, b rng ph ngun pht, qu cng sut, tn sc v nh hng ca

cc hiu ng phi tuyn.

1.3.1. Suy hao ca si quang

Suy hao trong si quang ng mt vai tr rt quan trng trong vic thit

k h thng. Suy hao si c tnh bng t s gia cng sut cui si quang

P2 ca si dn quang di L(km) vi cng sut a vo si quang P1. Nu gi

l h s suy hao ca si th:

P A(dB)= 10log

1

P2 (1.1)

dB / kmAdB

Lkm

(1.2)

Cc nguyn nhn chnh gy suy hao trong si quang l: Suy hao do hp

th nh sng, trong c hp th t ngoi v hp th hng ngoi. Hp th

ch yu do hp th in t, hp th tp cht v hp th vt liu. Ngoi ra, cn

phi k n suy hao do ghp ngun quang vo si quang, suy hao do mi hn,

suy hao do un cong si v suy hao do tn x do tnh khng ng nht quang

hc ca li si gy ra. C 3 loi suy hao do tn x c bn ca li si quang l

tn x Rayleigh, tn x Brillouin v tn x Raman.

TIENHighlight

TIENTypewriter20 B Tran Hung Dao

10

1.3.2. S knh bc sng

Mt trong nhng vn quan trng l h thng s dng bao nhiu knh

bc sng v s knh cc i h thng c th s dng c. S knh bc

sng s dng ph thuc vo:

Kh nng ca cng ngh i vi cc thnh phn quang nh:

Kh nng bng tn ca si quang.

Kh nng tch/ghp cc knh bc sng.

Khong cch gia cc knh gm cc yu t sau:

Tc truyn dn ca tng knh.

Qu cng sut quang.

nh hng ca hiu ng phi tuyn.

rng ph ca ngun pht.

Kh nng tch/ghp ca h thng DWDM.

Mc d ca s truyn dn ti vng bc sng 1550 nm c rng

khong 100 nm, nhng do di khuch i ca cc b khuch i quang ch c

rng khong 35 nm (theo quy nh ca ITU - T th di khuch i ny l t bc

sng 1530 nm n 1565 nm i vi bng C; hoc bng L t 1570 nm n

1603 nm) nn trong thc t, cc h thng DWDM khng th tn dng ht bng

tn ca si quang.

Gi l khong cch gia cc knh bc sng th tng ng ta c: f c./ 2

Nh vy, ti bc sng = 1550 nm, vi = 35 nm th

f = 4,37.1012 Hz. Gi s tc truyn dn ca mi knh bc sng

l 2.5Gbps th theo nh ngha Nyquist, ph c s ca tn hiu l 2 x 2,5 =

5Gbps th s knh bc sng cc i c th t c N = f /5 = 874 knh

trong di bng tn khuch i quang. y l s knh tnh theo l thuyt, tuy

nhin, vi mt knh cng ln th i hi cc thnh phn quang trn tuyn

phi c cht lng cng cao. trnh xuyn m gia cc knh ny cn c b

pht n nh v mt b lc quang c kh nng chn lc bc sng cao. Bt k

s dch tn no ca ngun pht cng c th lm dn ph sang knh ln cn.

11

Da vo kh nng cng ngh hin nay, ITU - T a ra quy nh v

khong cch gia cc knh bc sng l 100 GHz (0,8 nm) hoc 50 GHz (0,4

nm) vi chun tn s l 193,1 THz.

Vi cng ngh hin nay, DWDM ch yu s dng di bng tn C (1530

- 1560)nm v bng L (1560 - 1600)nm.

1.3.3. rng ph ca ngun pht

Vic chn rng ph ca ngun pht nhm m bo cho cc knh

bc sng hot ng mt cch c lp nhau, ni khc i l trnh hin tng

chng ph pha thu gia cc knh ln cn. Khong cch gia nhng knh

ny ph thuc vo c tnh ca cc thit b nh MUX/DEMUX, b lc,

dung sai cng nh mc n nh ca cc thit b ny.

V bn cht, vic ghp cc bc sng khc nhau trn cng mt si

quang l da trn nguyn tc ghp knh theo tn s. Cc knh khc nhau lm

vic cc knh tn s khc nhau trong cng bng thng ca si quang. Theo l

thuyt, bng thng ca si quang rt rng nn s lng knh bc sng ghp

c rt ln ( c 2 ca s truyn dn). Tuy nhin, trong thc t, cc h thng

WDM thng i lin vi cc b khuch i quang si v lm vic ch ca s

bc sng 1550 nm. V vy, bng tn ca si quang b gii hn bi bng tn

ca b khuch i. Nh vy, mt vn t ra khi ghp l khong cch gia

cc bc sng phi tha mn c yu cu trnh cng ph ca cc knh ln cn

pha thu. Khong cch ny ph thuc vo c tnh ph ca ngun pht v

cc nh hng khc nhau trn ng truyn nh tn sc si, hiu ng phi

tuyn

Mt cch l tng, c th xem h thng DWDM nh l s xp chng

ca cc h thng truyn dn n knh khi khong cch gia cc knh bc

sng ln v cng sut pht hp l. Mi quan h gia ph cng sut pha thu

vi ph cng sut ngun pht c th hin bi tham s c trng cho gin

ph, k hiu , bng tn tn hiu B v b tn sc D. Nu gi l h s c

trng cho s tng tc gia ngun pht v si quang, ta c biu thc:

12

= B.D.RMS (1.4)

Trong : B l rng bng tn tn hiu truyn dn.

D l tn sc tng ng khong cch truyn dn.

RMSl gin rng ph.

1.3.4. Qu cng sut

Trong mi trng truyn dn cp si quang, qu cng sut l mt yu t

rt quan trng nhm m bo cho h thng hot ng bnh thng. Mc ch

ca qu cng sut l bo m cng sut n my thu ln duy tr hot

ng tin cy trong sut thi gian sng ca h thng.

Suy hao cng sut trn ton tuyn bao gm: suy hao trn si dn quang,

trn cc b ni quang v ti cc mi hn. Tng suy hao trn ton tuyn nhn

c t cc phn b suy hao lin tip ca tng phn t trn tuyn. Suy hao

ca tng phn t c tnh:

A(dB) = 10log

(1.5)

Trong : P1, P2 l cc cng sut quang u vo v u ra ca phn t.

Ngoi cc suy hao do cc phn t trn tuyn quang gy ra nh nu

trn, ta cn phi c mt lng cng sut quang d phng cho tui th ca cc

thnh phn, s thay i nhit v cc suy hao tng ln cc thnh phn. D

phng cho tuyn thng thng t 6 - 8 dB. Chnh v vy m qu cng sut

ca tuyn c th xem nh l cng sut tng PT nm gia ngun pht quang v

b tch sng quang. Suy hao tng ny bao gm suy hao si, suy hao b ni

quang, suy hao mi hn v d phng cho h thng.

Nu gi PS l cng sut quang ca ngun pht c a vo u ghp

si v PR l nhy ca b thu quang th:

PT = PS - PR= 2lC + f.L + d phng h thng (1.6)

13

Trong : lC l suy hao b ni quang f

l suy hao si

L l c ly truyn dn

y, suy hao do mi hn lSP c gn vo trong suy hao si n

gin php tnh.

1.3.5. Tn sc

Khi truyn dn tn hiu s dc theo si quang, xut hin hin tng

gin xung u thu. Thm ch trong mt s trng hp cc xung ln cn

ln nhau, khi khng phn bit c cc xung vi nhau na, gy mo tn

hiu khi ti sinh.

S d c hin tng mo ny l do tn sc bn trong mode v hiu

ng gia cc mode gy ra.

Tn sc bn trong mode bao gm tn sc vt liu v tn sc dn sng.

Tn sc vt liu do ch s chit sut ca vt liu li ph thuc vo bc

sng to nn. N gy ra s ph thuc ca bc sng vo vn tc nhm

ca bt k mode no.

Tn sc dn sng ph thuc vo thit k si v hng s lan truyn mode

. N thng c b qua trong si a mode nhng li cn quan tm

trong si n mode. Gi l tn sc dn sng v hin tng ny thng

xy ra trong cc ng dn sng k c sng cao tn v siu cao tn.

Tn sc gia cc mode

Tn sc ny ch nh hng n cc si a mode, n sinh ra do c nhiu

ng khc nhau (cc mode khc nhau) m mt tia sng c th truyn lan

trong si a mode dn n tia sng truyn qua nhng quang l khc nhau,

lm cho xung truyn dn b gin rng ra, tn sc ny ph thuc vo kch

thc ca si quang, c bit ph thuc vo ng knh ca li si.

Cc phng php lm gim thiu s nh hng ca tn sc n h

thng DWDM tc cao c dng khuch i EDFA gm: lm hp b

rng ph ca ngun pht hoc s dng cc phng php b tn sc nh:

S dng si quang c h s tn sc nh.

14

B tn sc bng phng php t dch pha SPM.

B tn sc bng cc thnh phn b tn sc th ng.

B tn sc bng si DCF.

B tn sc bng cc modul DCM s dng cch t si Bragg.

Cc h thng truyn dn TDM cng nh WDM b nh hng nhiu hn

i vi mt loi tn sc khc, khi tng tc truyn dn ca h thng cn

phi quan tm n nh hng ca tn sc mode phn cc (PMD). nh

hng ny thng c b qua i vi h thng tc thp.

Khi nim tn sc mode phn cc PMD

Tn sc mode phn cc PMD l mt thuc tnh c bn ca si quang

n mode v cc thnh phn hp thnh trong nng lng tn hiu

bt k bc sng no cng c phn tch thnh 2 mode phn cc trc

giao c vn tc truyn khc nhau. Do vn tc ca hai mode chnh lch

nhau i cht nn thi gian truyn qua cng khong cch l khc nhau

v c gi l s tr nhm (DGD). V vy, PMD s lm gin rng xung

tn hiu gy nn suy gim dung lng truyn dn. V phng din ny,

nh hng ca tn sc mode phn cc cng ging nh nh hng ca

tn sc. Tuy nhin, c mt im khc bit ln l: tn sc l mt hin

tng tng i n nh, trong khi , PMD ca si n mode bt k

bc sng no cng l khng n nh. Ngoi nhng nh hng trn cn

phi k n suy hao ph thuc phn cc (PLD) ca cc thnh phn hp

thnh. PLD phn bit s thay i phn cc trong thnh phn cng

c tch ra t tn hiu mong mun thng qua s suy hao trng thi

phn cc c chn lc.

Tn sc mode phn cc c tnh theo cng thc:

PMDtotal = K.L1/2

(1.7)

Trong : PMDtotal l tn sc phn cc ca si quang (ps) K

l h s tn sc phn cc (ps/km1/2)

L l chiu di ca si (km)

15

Nguyn nhn ca tn sc phn cc

Do cu trc khng hon ho ca si quang cng nh cc thnh phn

quang hp thnh nn c s khc bit v chit sut i vi cp trng thi

phn cc trc giao, c gi l s lng chit. S khc bit chit sut

s sinh ra chnh lch thi gian truyn sng trong cc mode phn cc

ny. Trong cc si n mode, hin tng ny bt ngun t s khng

trn hoc ovan ca li si theo 2 cch: ng dn sng ovan (vn c tnh

lng chit) v trng lc cng c hc to nn bi li ovan gm c c

lng chit ph. Nhn chung, nh hng ca ng dn sng ovan c vai

tr ln trong si PMD thp.

S lng chit ca cc vt liu trong sut ging nhau nh thch anh

c to ra t cu trc tinh th cn xng. V nh vy, PMD trong cc

thnh phn quang c th sinh ra t s lng chit ca cc thnh phn

con trong cc thnh phn quang hp thnh. Tn hiu truyn trn cc

ng song song nhau c di quang khc nhau cng sinh ra hin

tng tr nhm.

S phn cc trong si c trng cho lng chit do lc c hc. Nhiu

phn t khng phi l thy tinh c cho vo trong lp v ca si nn

li xut hin trng lc khng i xng nhau dc theo chiu di si.

Khi nh sng phn cc b ghp trong mt on si ny th trng in

u ra ca nh sng u vo c phn tch thnh 2 modul phn cc

trc giao vi tc truyn khc nhau. Cc modul phn cc c duy

tr dc theo si v nng lng ca chng s khng b ghp.

Ngoi nhng nguyn nhn trn, lng chit cn sinh bi s un cong

ca si. S un cong ny lm thay i mt phn t ca cu trc si,

lm cho h s khc x mt i xng. Tuy nhin, lng chit do un

cong khng phi l nguyn nhn ch yu sinh ra PMD.

1.3.6. Vn nh hng ca cc hiu ng phi tuyn

i vi h thng thng tin si quang, cng sut quang khng ln, si

quang c tnh nng truyn dn tuyn tnh. Sau khi dng EDFA, cng sut

16

quang tng ln, trong iu kin nht nh si quang s th hin c tnh truyn

dn phi tuyn, hn ch rt ln tnh nng ca b khuch i EDFA v c ly

truyn dn di khng c chuyn tip.

Nhn chung, c th chia hiu ng phi tuyn thnh 2 loi:

Hiu ng tn x: bao gm tn x do kch thch Raman (SRS) v tn

x do kch thch Brillouin (SBS).

Hiu ng lin quan n chit sut ph thuc vo cng sut quang:

bao gm hiu ng t iu ch pha (SPM), iu ch pha cho(XPM)

v trn bn bc sng (FWM).

1.3.6.1. Hiu ng tn x Raman SRS

Hiu ng Raman l kt qu ca qu trnh tn x khng n hi m

trong photon ca nh sng ti chuyn mt phn nng lng ca mnh cho

dao ng c hc ca cc phn t cu thnh mi trng truyn dn v phn

nng lng cn li c pht x thnh nh sng c bc sng ln hn bc

sng ca nh sng ti (nh sng vi bc sng mi ny c gi l nh sng

stoke). Khi nh sng tn hiu truyn trong si quang c cng ln, qu

trnh ny tr thnh qu trnh kch thch (c gi l SRS) m trong nh

sng tn hiu ng vai tr sng (gi l bm Raman) lm cho phn nng lng

ca tn hiu c truyn ti bc sng stoke.

Nu gi PS(L) l cng sut ca bc sng stoke trong si quang th:

(1.8)

8

Trong : P0 l cng sut a vo si ti bc sng tn hiu gr l h

s khuch i Raman

L l khong cch nh sng lan truyn tong si quang Seff l din tch vng

li hiu dng

K c trng cho mi quan h v phn cc gia tn hiu, bc sng stoke v phn

cc ca si. i vi si thng thng th K 2.

17

T y c th tnh ton mc cng sut P0 m ti hiu ng SRS nh

hng ln n h thng, c gi l ngng Raman (Pth0) (Pth

0 l cng sut

ca tn hiu u vo m ng vi n cng sut bc sng stoke v bc sng

tn hiu ti u ra l bng nhau).

T y, ngi ta tnh ton c rng, i vi h thng n knh,

hiu ng SRS c th nh hng n cht lng h thng th mc cng sut

phi ln hn 1W (nu nh h thng khng s dng khuch i quang trn

ng truyn). Tuy nhin, trong h thng WDM th mc cng sut ny s

thp hn nhiu v c hin tng khuch i i vi cc bc sng ln, trong

khi cng sut ca cc knh c bc sng ngn hn li b gim i (do

chuyn mt phn nng lng cho cc bc sng ln) lm suy gim h s

SNR, nh hng n cht lng h thng. m bo suy gim SNR khng

nh hn 0,5 dB th mc cng sut ca tng knh phi tha mn:

P <

N N 1Leff P <

Trong : N l s knh bc sng f l khong cch gia cc knh bc sng.

Nh vy, trong h thng WDM, hiu ng ny cng hn ch s knh bc

sng, khong cch gia cc knh, cng sut ca tng knh v tng chiu di ca

h thng. Hn na, nu nh bc sng mi to ra li trng vi knh tn hiu

th hiu ng ny cng gy s nhiu gia cc knh.

f

Pth0 (

) (1.9)

18

1.3.6.2. Hiu ng tn x Brillouin SBS

Tn x Brillouin c hiu nh l s iu ch nh sng ca nng lng

nhit trong vt liu. Photon nh sng vo si quang chu s tng tc khng

tuyn tnh v to ra nng lng rung ng trong thy tinh cng nh to ra nh

sng tn x. dch tn s v cng tn x bin i theo hm ca gc tn

x, vi gi tr ln nht l ngc vi hng truyn v nh nht bng khng

theo hng truyn. Do , tn x Brillouin ch yu v hng ngc hng v

ngun v xa b thu, v vy lm gim cng sut nh sng ti b thu. Mc cng

sut m ti tn x Brillouin tr nn ng k trong si quang n mode tun

theo cng thc:

PB=17,6 x 10-3

x a2 x 2 x x (1.11)

Trong : PB l mc cng sut (W) cn thit tn x Brillouin tr nn

ng k.

a l bn knh si quang (m)

l bc sng ca ngun pht (m)

l suy hao ca si quang (dB/km)

l rng ph ca ngun (GHz)

Hiu ng SBS l hiu ng tng t nh hiu ng SRS, tc l c s to

thnh ca bc sng stoke vi bc sng di hn bc sng ca nh sng ti.

im khc nhau chnh ca hai hiu ng ny l: hiu ng SBS lin quan n

cc photon m hc, cn hiu ng SRS lin quan n cc photon quang. Chnh

do s khc bit ny m hai hiu ng c nhng nh hng khc nhau n h

thng WDM. Trong hiu ng ny, mt nh sng b tn x do cc photon m

hc v lm cho phn nh sng b tn x ny dch ti bc sng di hn (tng

ng vi dch tn l khong 11 GHz ti bc sng 1550 nm). Tuy nhin,

ch c phn nh sng b tn x theo chiu ngc tr li (tc l ngc chiu

vi chiu ca tn hiu) mi c th truyn i trong si quang. V vy, trong

h thng WDM khi tt c cc knh u cng truyn theo mt hng th hiu

ng SBS khng gy xuyn m gia cc knh.

19

Trong tt c cc hiu ng phi tuyn th ngng cng sut xy ra hiu

ng SBS l thp nht, ch vi mV. Tuy nhin, do hiu ng SBS gim t l vi

VB/VLaser (VB l bng tn khuch i Brillouin,VLaser l rng

ph ca laser) v bng tn khuch i Brillouin l rt hp (ch khong 10 100

MHz) nn hiu ng ny cng kh xy ra. Ch cc h thng vi ngun pht

c rng ph rt hp mi c th b nh hng ca hiu ng SBS. Ngi ta

tnh ton c mc cng sut ngng i vi hiu ng SBS nh sau:

Pth = 21

Trong : g l h s khuch i Brillouin Aeff l vng li hiu dng

k: c trng cho mi quan h v phn cc gia tn hiu, bc

sng stoke v phn cc ca si. i vi h thng thng

thng th k 2.

VB l bng tn khuch i Brillouin

VP l rng ph ca tn hiu

Nh vy, hiu ng SBS s nh hng n mc cng sut ca tng knh

v khong cch gia cc knh trong h thng WDM. Hiu ng ny khng ph

thuc vo s knh ca h thng.

1.3.6.3. Hiu ng t iu ch pha SPM

Hiu ng SPM thuc loi hiu ng Kerr, tc l hiu ng trong chit

sut ca mi trng truyn dn thay i theo cng nh sng truyn trong :

n = n0 + nNL= n0 + n2 E 2

(1.13)

Trong : n0 l chit sut tuyn tnh

n2 l h s chit sut phi tuyn (n2 = 1,22.10-22

(V/m)2 i vi

si Silic)

E l trng quang

20

Hin tng ny to nn s dch pha phi tuyn NL ca trng quang

khi lan truyn trong si quang. Gi s b qua suy hao quang th sau khong

cch L, pha ca trng quang s l:

i vi trng quang c cng khng i hiu ng SPM ch lm

quay pha ca trng quang, do t nh hng n cht lng ca h thng.

Tuy nhin, i vi cc trng quang c cng thay i th pha phi tuyn

NL s thay i theo thi gian. S thay i theo thi gian ny cng c ngha l

trong xung tn hiu s tn ti nhiu tn s quang khc vi tn s trung tm v0

mt gi tr l vNL, vi:

Hin tng ny cn gi l hin tng dch tn phi tuyn lm cho sn

sau ca xung dch n tn s vv0. iu ny c ngha l ph ca tn hiu b gin trong qu trnh truyn.

Trong h thng WDM, c bit khi khong cch gia cc knh gn nhau, hin

tng gin ph do SPM c th dn n giao thoa gy nhiu gia cc knh.

Hn na, nu xt n nh hng ca tn sc th thy dng xung b bin i

dc theo si. Nu gi D l h s tn sc ca si th:

- Vi D < 0: thnh phn tn s cao s lan truyn nhanh hn thnh phn tn

s thp. Do , xung b gin ra.

- Vi D > 0: thnh phn tn s cao s lan truyn chm hn thnh phn tn s

thp lm cho xung b co li (nguyn l truyn dn Soliton). Tuy nhin,

vic to ra Soliton phi c kim sot, nu khng s c hin tng lc

u xung co li, sau li gin ra rt nhanh.

21

1.3.6.4. Hiu ng iu ch pha cho XPM

i vi h thng WDM, h s chit sut ti mt bc sng no

khng ch ph thuc vo cng ca sng m cn ph thuc vo cng

ca bc sng khc lan truyn trong si. Trong trng hp ny, chit sut

phi tuyn ng vi bc sng th i s l:

Trong : N l tng s knh quang

Ei, Ej l cng trng quang ca bc sng th i, j

XPM c quan h n phng thc iu ch, khi iu ch PSK th nh

hng n tnh nng ca h thng l ln nht. C th thng qua vic gim

cng sut ca cc knh tn hiu gim XPM. Ngoi ra, XPM khng ch ph

thuc vo cng sut ca knh tn hiu m cn ph thuc vo s knh tn hiu.

S knh tn hiu cng nhiu th nh hng ca XPM cng ln.

1.3.6.5. Hiu ng trn bn bc sng FWM

Hin tng chit sut phi tuyn cn gy ra mt hiu ng khc trong si

n mode, l hiu ng FWM. Trong hiu ng ny, 2 hoc 3 sng quang

vi cc tn s khc nhau s tng tc vi nhau to ra cc thnh phn tn s

mi. Tng tc ny c th xut hin gia cc bc sng ca tn hiu trong h

thng WDM, hoc gia bc sng tn hiu vi tp m ASE ca cc b

khuch i quang, cng nh gia mode chnh hoc mode bn ca mt knh

tn hiu. Gi s c 3 bc sng vi tn s i, j,k th t hp tn s

mi to ra s l nhng tn s ijk tha mn:

ijk = i + j - k

Theo quan im c lng t, hiu ng FWM l hiu ng m trong

c s ph hy photon mt s bc sng v to ra mt s photon cc bc

sng mi sao cho vn bo ton v nng lng v ng lng.

22

Nu gi Pijk(L) l cng sut ca bc sng ijk trong si quang th:

Trong : l hiu sut ca qu trnh FWM

c l vn tc nh sng trong chn khng Seff

l din tch vng li hiu dng

Pi, Pj, Pk l cng sut tng ng ca bc sng i, j, k,

(3) l cm phi tuyn bc 3

Hiu sut ca qu trnh FWM ph thuc vo iu kin ph hp

v pha. Hiu ng FWM xy ra mnh ch khi iu kin ny c tha mn (tc

l ng lng photon c bo ton). V mt ton hc th iu ny c th

biu th nh sau:

(ijk ) = (i) + (j) - (k)

V trong si quang tn ti tn sc nn iu kin ph hp v pha rt kh

xy ra. Tuy nhin, vi mi trng truyn dn l loi si c tn sc thp v

khong cch truyn dn tng i ln v cc knh gn nhau th iu kin ny

c th coi l xp x t c.

Do vic to ra cc tn s mi l t hp ca cc tn s tn hiu nn hiu

ng FWM s lm gim cng sut ca cc knh tn hiu trong h thng WDM.

Hn na, nu khong cch gia cc knh l bng nhau th nhng tn s mi

c to ra c th ri vo cc knh tn hiu, gy xuyn m gia cc knh, lm

suy gim cht lng ca h thng.

S suy gim cng sut s lm cho dng hnh mt ca tn hiu u thu

b thu hp li nn s lm gim cht lng BER ca h thng. V cc h thng

WDM ch yu lm vic ca s bc sng 1550 nm v do tn sc ca si

quang n mode thng thng (si G.652) ti ca s ny l khong 18 ps/nm

km, cn tn sc ca si tn sc dch chuyn (si G.653) l 0 (< 3 ps.nm), do

h thng WDM lm vic trn si n mode chun thng thng (SSMF)

23

s t b nh hng bi hi ng FWM hn h thng WDM lm vic trn si

tn sc dch chuyn (DSF).

nh hng ca hiu ng FWM cng ln nu nh khong cch gia cc

knh trong h thng WDM cng nh cng nh khi khong cch truyn dn v

mc cng sut ca mi knh ln. V vy, hiu ng FWM s hn ch dung

lng v c ly truyn dn ca h thng WDM.

1.3.7. Di bc sng lm vic ca DWDM

Si quang thch anh c 3 ca s suy hao thp 860 nm, 1310 nm v

1550 nm, trong ti ca s 1550 nm c tnh suy hao ca si quang l nh

nht, ca s ny c p dng truyn dn tn hiu SDH vi khong cch

ngn v di. Hn th na, cc b khuch i quang EDFA s dng hin nay

c c tnh li kh bng phng trong ca s ny, bi vy y l ca s hot

ng rt tt ca h thng DWDM. Cc bc sng lm vic trong ca s

1550nm c chia thnh 3 di: bng S, bng C v bng L.

Hnh 1.5 : S phn chia di bc sng lm vic ti ca s 1550 nm

Trong :

Bng S (1460 nm 1530 nm): hin ti, di bc sng lm vic ca

b khuch i quang EDFA thuc bng C v L. Do , bng S

khng s dng trong h thng DWDM.

Bng C (1530 1565 nm): y l di bc sng lm vic ca cc h

thng DWDM s dng 40 bc sng (khong cch gia cc bc

sng l 100 GHz), h thng DWDM s dng 80 bc sng (khong

cch gia cc bc sng l 50 GHz) v h thng SDH.

24

Bng L (1565 1625 nm): y l di bc sng lm vic ca cc h

thng DWDM s dng 80 bc sng (khong cch gia cc bc

sng l 50 GHz).

Da trn s lng knh c ghp v khong cch gia cc knh trong

h thng DWDM, cc bc sng lm vic trong h thng DWDM c

th l: 40 bc sng, 80 bc sng v 160 bc sng.

H thng DWDM ghp 40 bc sng

Di bc sng lm vic: bng C (1530 1565 nm)

Di tn s: 192.1 THz 196.0 THz

Khong cch gia cc knh: 100 GHz

Central frequency offset: 20 GHz (ti tc thp

hn 2.5Gbps) v 12.5GHz (ti tc 10Gbps)

25

Phn 2

CC THNH PHN C BN CA MNG DWDM

2.1. CU TRC TRUYN DN C BN CA MNG DWDM

H thng DWDM thc hin ghp bc sng danh nh khc nhau (tng

ng vi cc tn hiu knh quang ring l) thnh mt chm sng v c truyn

dn trn mt si, trong mi knh quang mang dch v khc nhau.

Cu trc c bn ca h thng DWDM gm cc thnh phn chnh sau:

B sa dng tn hiu

Cc b tch ghp knh quang OMUX, ODMUX

Cc b khuch i quang si EDFA

Cc b xen/r quang OADM

Cc modul b tn sc DCM

B kt ni cho quang OXC

2.2. KHI PHT P QUANG OTU

OTU (Optical Transponder Unit) l thit b c s dng thc hin

sa dng tn hiu. N chuyn i nhng tn hiu ca cc knh quang u vo

pha Client side thnh cc tn hiu quang chun theo khuyn ngh G.692 ca

ITU-T c th truyn trn h thng DWDM.

Nguyn l hot ng:

OTU thc hin vic chuyn i quang - in vi cc tn hiu quang a

vo ghp knh theo khuyn ngh G.957 v thc hin ti to tn hiu, khi

phc nh thi v khi phc d liu i vi cc tn hiu quang c

chuyn i thnh in. Sau thc hin chuyn i in - quang a ra tn

hiu knh quang DWDM m c bc sng, tn sc v cng sut pht

quang theo chun G.692.

26

O

D

M

U

X

OTUG

O

A

OTUG

O

D

M

U

X

OTUR

O

A

OTUR

OTUT

OTUT

G.692

E/O

Tn hiu quang u

ra

G.957

Tn hiu quang u

vo

O/E

Ti to dng tn hiu, khi phc nh thi

Hnh 2.1: Nguyn l ca b thu pht quang OTU

Sau khi chuyn i O/E nu ch thc hin ti to dng tn hiu, khi

phc nh thi (thc hin chc nng 2R: Reshape, Retiming) th OTU thc

hin chc nng sa dng tn hiu cho truyn dn khong cch ngn.

Nu sau khi chuyn i O/E, OTU x l ti to dng tn hiu, khi

phc nh thi v khi phc d liu (chc nng 3R: Reshape, Retiming,

Regenerator) c thc hin th OTU c chc nng ca mt b repeater.

Phn loi v ng dng:

Ph thuc vo v tr ca OTU trong mng DWDM m OTU c th

c chia lm 3 loi: OTUT (OUT Transmitter), OTUR (OUT Receiver) v

OTUG (OUT Generrator). ng dng ca chng trong h thng nh hnh v:

Hnh 2.2: V tr ca b chuyn i bc sng OTU trong h thng

OTUT (OTU u pht): t gia cc thit b ca khch hng v

OMUX. Thc hin chuyn i tn hiu quang t pha khch hng thnh tn

hiu quang u ra theo chun G.692 ri a vo OMUX. Loi OTU ny

khng ch thc hin chc nng chuyn i O/E v E/O m cn thc hin vic

ti to dng tn hiu, khi phc nh thi (chc nng 2R) v c chc nng tm

byte B1 (byte gim st li bit tng on lp).

O

M

U

X

O

M

U

X

27

OTUR (OTU u thu): t gia ODMUX v cc thit b ca khch

hng. Tn hiu quang u ra t ODMUX n OUTR phi tun theo chun

G.692. Loi OUT ny c chc nng ging nh OTUT, n thc hin sa dng

tn hiu, chc nng 2R v tm kim byte B1.

OTUG (OTU chuyn tip): t gia OMUX v ODMUX. Tn hiu u

vo v u ra ca OTUG phi tun theo chun G.692. OTU loi ny khng

ch c chc nng chuyn i O/E, E/O m cn c chc nng ti to li dng

tn hiu, khi phc nh thi v chc nng khi phc d liu (chc nng 3R).

V vy, OTUG tng ng nh mt b lp thng thng v cng c chc

nng tm byte B1.

2.3. B GII/GHP KNH QUANG

Gi s cc ngun pht quang lm vic cc bc sng khc nhau 1,

2,..., n. Cc tn hiu quang cc bc sng khc nhau ny s c ghp

vo cng mt si quang pha pht. B ghp knh theo bc sng phi m

bo c suy hao nh v tn hiu sau khi c ghp s c truyn dc theo

si ti pha thu. Ti bn thu, qua b gii ghp knh (ODMUX), tn hiu

s c tch ra thnh cc bc sng ring r v n cc b thu thch hp.

C 3 loi WDM chnh thc c s dng l:

S dng b lc in mi mng mng (Dielectric thin film

filters - DTF) dng lc cc bc sng ring bit trong di

bc sng c th cng nh d dng thch ng trong vic

truyn dn mt chiu hoc hai chiu. B lc DTF c s

dng ghp bc sng ti hai ca s.

S dng cch t nhiu x dng ghp v tch nhiu tn hiu

trong cng mt ca s.

Ghp nh hng theo phng php hn si.

28

2.3.1. Phng php ghp knh s dng b lc mng mng

B lc in mi s dng trong ghp knh quang hot ng da trn

nguyn tc phn x tn hiu mt di ph ny v cho phn ph cn li i qua.

Phn t c bn thc hin ghp knh theo bc sng l b lc in

mi giao thoa, n c cu trc a lp gm cc mng mng c ch s chit sut

cao v thp t xen k nhau. Hu ht cc b lc giao thoa lm vic da trn

nguyn l bung cng hng Fabry - Perot, gm 2 gng phn x thnh phn

t song song cch nhau bi mt lp in mi trong sut.

Nguyn l hot ng ca n nh sau: Khi chm tia sng chm vo thit

b, cc hin tng giao thoa s to ra nhng phn x nhiu ln trong khoang

cng hng. Nu b dy ca lp m l s nguyn ln ca na bc sng nh

sng th giao thoa xp chng xy ra v cng sut quang ca bc sng t gi

tr cc i. Cc tia nh sng ca cc bc sng khc vi bc sng cng

hng phn x trn vn, ch c mt bc sng i qua b lc. Gng phn x

l cc lp thy tinh nm trn lp m trong sut.

1, 2, ...., n

1

2, ...., n B lc

Hnh 2.3: B tch bc sng dng b lc

Theo c tnh ph th th c th phn cc b lc giao thoa thnh:

B lc thng thp hoc thng cao c bc sng ct c.

B lc thng di c bc sng trung tm 0 v rng di .

Cc b lc thng thp hoc thng cao thng s dng ghp hoc

tch 2 bc sng khc nhau, chng hn 850 nm v 1310 nm hoc 1310 nm v

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1550 nm. Loi b lc nh vy thch hp cho ngun quang c di ph rng

(LED). B lc thng di c s dng trong WDM khi ngun quang c ph

hp (LD). i vi b lc thng di c mt vi yu cu, l dc sn

ng cong hm truyn t phi ln trnh xuyn m gia cc knh k

nhau, mt khc rng gii c dung sai cho php phng dch

bc sng trung tm ca ngun quang do nhit d thay i.

Chit sut thp Chit sut cao

Lp phn cch trong sut

Hnh 2.4: Cu to b lc

2.3.2. Mt s thit b tch knh dng b lc in mi mng mng

B tch 2 bc sng:

Do thit b ghp v thit b tch bc sng c cu trc thun - nghch,

ngha l gia b ghp v b tch ch thay i cng vo v cng ra, cho nn

trong phn ny ch yu ch trnh by cu trc v hot ng ca cc b tch

bc sng.

Thu knh Knh lc

Hnh 2.5: B tch 2 knh dng thu knh phng v b lc

1, 2

1

Si quang

2

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Lng knh Grin (1/4 P)

B lc

1, 2

1

2

Hnh 2.6: B tch 2 knh dng b lc v lng knh Grin

Cu trc c bn ca b gii ghp 2 knh nh hnh 2.5, trong khi

vic thc hin trn thc t cu trc ny ch n gin nh hnh 2.6. Cc phn

t chun trc v hi t l cc lng knh Grin - rod 1/4 bc. B lc c thit

k pht i 1 v phn x 2 s c t gia 2 lng knh. Cc thit b gii

ghp ny c sn trn th trng thng mi v c s dng rng ri cc h

thng thng tin quang s dng cc ngun pht LED bc sng 850 nm v

1300 nm hoc s dng cc ngun pht ph hp ca cc t chc bc sng

nh 800 nm v 830 nm; 800 nm v 890 nm;, vi suy hao nh hn 3 dB

(cho mi cp) v suy hao xuyn knh cao hn 25 dB.

B tch ln hn hai bc sng:

Thit b ny s dng cc b lc ni tip nhau v mi b lc cho i qua

mt bc sng v phn x cc bc sng cn li.

1, 2,......, n

B lc (1)

1

2

3

4

Hnh 2.7: Cu to c bn ca b lc nhiu bc sng

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Trong thc t, thit b tch nhiu bc sng ngoi cc b lc cn c

thu knh, cc si quang,Hnh v l b tch 5 bc sng dng thu knh

Grin v khi thy tinh trong sut.

1,......, 5 2 4

Si quang

Lng knh GRIN

Khi thy tinh trong sut

B lc

1 3 5

Hnh 2.8: B tch knh vi quang nhiu knh trn thc t

i khi c th thc hin to ra b tch knh m khng cn s dng n

cc phn t trc chun, thit b khng c thu knh m cc b giao thoa y

c t trn tng on mt cch thch hp v u si c mi nhn.

1 2 3

Si quang

1,......, 4 4

B lc

Hnh 2.9: Cu trc c bn ca b tch knh s dng b lc gn trc tip vo si

2.3.3. Phng php ghp knh s dng cch t nhiu x

Do nhc im khng tch c cc tia sng c bc sng gn nhau

nn thu knh ngy nay hu nh khng c s dng na, thay vo ngi

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ta s dng cch t nhiu x l ch yu. Cc b tch/ghp bc sng s dng

b lc khng th thc hin c khi s lng knh ln v s lng bc sng

st gn nhau. Thun li chnh ca cch t l c th nhiu x ng thi tt c

cc bc sng v c th thc hin c vi cc b tch ghp nhiu knh.

Nguyn l c bn ca ghp bc sng s dng cch t da trn hin tng

gc nhiu x ph thuc vo bc sng nh sng.

Cch t nhiu x l mt thit b quang th ng, nhiu x chm sng

ti theo cc hng khc nhau ty theo gc ti ca chm sng trn b mt cch

t, bc sng ca nh sng ti, cc c tnh thit k ca cch t, khong cch

gia cc rnh (chu k cch t), gc ca rnh cch t . Trn 1mm ca cch t

c hng chc hay hng ngn rnh nh, s rnh trn mt n v chiu di ca

cch t c gi l hng s cch t.

Ngi ta ch to cch t bng cch dng mt mi kim cng nhn rch

nhng ng song song cch u trn mt tm thy tinh phng. Ch b rch

c tc dng nh nhng chn sng, ch cn li c tc dng nh nhng khe

sng. Cch t thu c bng cch gi l cch t truyn x. Cch t truyn

x ngy nay dng rt t v mi kim cng mau mn khi rch ln thy tinh

hoc thch anh, khin cho cc khe cui khng cn rng nh khe u. Thay

vo ngi ta s dng cch t phn x bng cch rch ln b mt kim loi

ph trn thy tinh, to ra cc chn sng, phn cn li phn x nh sng c tc

dng nh cc khe. Cc lp kim loi ph thng l bc hoc nhm, kh mm

nn mi kim cng rt lu mn.

Ty theo cc bc sng khc nhau m cch t nhiu x nh sng theo

cc hng khc nhau. Do vy, chm tia ti vi nhiu bc sng khc nhau s

c tch ra theo chiu hng ty thuc vo bc sng. Ngc li cc nh

sng n sc t cc hng khc nhau cng c th c ghp li thnh mt

chm sng truyn theo cng mt hng.

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Thu knh hi t Cch t

2.3.4. Cc b tch ghp bc sng s dng cch t

Ni chung, cc b ghp knh hoc tch knh s dng cch t gm 3

phn chnh: cc phn t vo v ra (l mng si hoc mt si truyn dn vi

cc thnh phn thu - pht), phn t hi t quang, phn t tn sc gc.

1

1,2

2

a)

1

1,2

2

b)

Lng knh Grin

Cch t

Hnh 2.10: B tch Littrow: a) B tch bc sng dng thu knh hi t, b) B tch bc

sng dng thu knh Grin

Hnh 2.10 a,b l b tch Littrow vi cu trc c bn v cu trc thc t s

dng thu knh Grin ca b tch 2 knh. Trong cu hnh ny, c tn hiu nh

sng i vo v nh sng i ra khi b ghp ch s dng mt thu knh, dng thu

knh chun trc hoc thu knh Grin.

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KT LUN

Tn dng c phn ln bng thng ca si quang, to ra c dung

lng truyn dn ln. Cng ngh DWDM cho php s dng ton b ti

nguyn bng thng rt ln ca si quang (khong 25THz) nng cao

dung lng truyn dn ca h thng.

Khong cch truyn dn xa bng cch s dng cng ngh khuch i quang si EDFA.

Cho php truy nhp nhiu loi hnh dch v: cc bc sng trong h

thng DWDM c lp nhau, do c kh nng truyn nhiu loi hnh

dch v trn cng mt cp si quang nh: SDH, GE hay ATM

Hn ch c s si quang cn s dng: h thng DWDM ghp nhiu

bc sng trn mt si quang nn tit kim c rt nhiu cp quang,

t c th gim c cho ph xy dng ng dy.

Kh nng nng cp v m rng d dng.

linh hot cao, mng kinh t v n nh.

DWDM vi nhng u th v mt cng ngh tr thnh mt phng tin

ti u v k thut cng nh kinh t m rng dung lng truyn dn si

quang mt cch nhanh chng v qun l hiu qu h thng. DWDM p

ng c hon ton cc dch v bng rng trn mng v l tin xy

dng mng ton quang trong tng lai. Cc c ch bo v v khi phc d

liu trn h thng DWDM ch yu da trn nn tng WDM. Nhng trong

thc t khng phi thit b trn h thng DWDM cng ging hon ton vi

thit b trn h thng WDM, do vy ta phi ch trong qu trnh xy dng cc

c ch bo v v khi phc d liu trn h thng DWDM.