ung dung cua he thong wdm - Tại 123doc.vn

7/27/2019 ung dung cua he thong wdm - Ti 123doc.vn

1/82

n tt nghip i hc

MC LC

MC LC.....................................................................................i

CHNG I..................................................................................2

TNG QUAN V H THNG WDM...............................................2CC THNH PHN TRONG H THNG WDM.............................14

MT S VN CNG NGH THEN CHT...............................56

CHNG IV.............................................................................67

NG DNG CA H THNG WDM............................................67

4.1 NG DNG WDM TRONG MNG TRUYN DN...................67

Nguyn Tnnh Chung D2001VT i


2/82

n tt nghip i hc Chng 1. Tng quan v h thng WDM

CHNG I

TNG QUAN V H THNG WDM

1.1 Gii thiu chungTrong nhng nm gn y, s pht trin ca cc dch v thoi v phi thoi

m c bit l Internet cng nh mt s dch v khc to ra mt s bng n

nhu cu v dung lng. iu ny t ln vai nhng nh cung cp dch v ng

trc nhng kh khn v thch thc mi. K thut ghp knh theo min thi gian

TDM gii quyt phn no cc yu cu trn nhng vn cn rt hn ch. Trong

thc t, tc ca tn hiu TDM thng nh hn hoc bng 10Gb/s. Do nh

hng ca hin tng tn sc, hiu ng phi tuyn trong si quang v tc ca

cc thnh phn in t nn khi tng tc bit ca mt knh TDM ln qu gii

hn ny, cht lng h thng khng m bo. thch ng vi s tng trng

khng ngng v tho mn yu cu tnh linh hot ca mng, cc cng ngh

truyn dn khc nhau c nghin cu, trin khai th nghim v a vo ng

dng, trong s phi k n cng ngh WDM, OTDM, Soliton Phng

php ghp knh theo bc sng WDM(Wavelength Division Multiplexing)

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

si quang n mode. Ghp knh theo bc sng WDM nng cao dung lng

truyn dn ca h thng m khng cn phi tng tc ca tng knh trn mi

bc sng. Do , WDM chnh l gii php tin tin trong k thut thng tin

quang, p ng c nhu cu truyn dn v c nhng yu cu v cht lng

truyn dn ca h thng.

1.1.1 Khi qut v WDM

Trong h thng WDM, tn hiu in ca tng knh quang c iu ch vi

cc sng mang quang khc nhau. Sau , chng c ghp li v truyn trn

cng mt si quang n u thu. Pha thu thc hin qu trnh tch tn hiu quang

thnh cc knh quang ring bit c bc sng khc nhau. Mi knh ny c

a n mt my thu ring. Cng ngh WDM cho php khai thc c tim

nng bng thng to ln ca si quang. V d, hng trm knh 10Gb/s c th

Nguyn Tnnh Chung D2001VT 2


3/82


truyn trn cng mt si quang. Khong cch gia cc knh khong 50GHz.

Di y l mt tnh ton cho thy s hp dn ca cng ngh WDM:

Hnh 1.1 ch ra hai ca s truyn dn 1,3 v 1,5 ca si quang. Mi ca s

c bng thng truyn dn(suy hao thp) ca si quang l rt ln; Ch vi ringca s quang 1550 nm th di bc sng c th s dng l 1500 nm 1600 nm,

tng ng vi di tn rng c 12,5 THz !.

S dng cho tc truyn tin c 10 Gbps th ch cn s dng mt phn rt

nh trong bng tn truyn dn ny. R rng, c th thy dung lng yu cu c

hng trm Gbps l hon ton nm trong kh nng ca h thng WDM. Thm vo

, h thng cn rt mm do khi c cc phn t nh b tch ghp quang, b ni

cho quang, chuyn mch quang, cc b lc quang thc hin la chn knh ng

hoc tnh

Khi nim v WDM c bit n t nhng nm 1980, khi m h thng

quang c bt u thng mi ha. Dng n gin nht ca WDM l truyn

hai knh tn hiu trn hai ca s khc nhau. V d, truyn trn hai bc sng1,3m v 1,55m. Khi , khong cch gia cc knh l 250nm. Sau ,

khong cch gia cc knh gim dn i. Nm 1990, khong cch gia cc knh

ch cn nh hn 0,1nm. Trong sut thp k 90, h thng WDM c nhiu

nc trn th gii quan tm nghin cu. Hin nay, k thut ghp knh theo bc

sng c ng dng nhiu nc trn th gii. nc ta, Tng cng ty bu

chnh vin thng Vit Nam quyt nh nng cp tuyn truyn dn Bc Nam bng

gii php ghp knh theo bc sng.

Nguyn Tnnh Chung D2001VT

0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,41,5 1,6

1,51,6

Hnh 1.1. Bng tn truyn dn ca si quang l rt ln!

[dB] Bng tn ca s 1550nm

[m]

Ph camt ngun

quang

3


4/82


1.1.2 Nguyn l hot ng ca h thng tch/ghp knh quang

S khi tng qut ca mt h thng truyn dn quang n hng ghp

knh theo bc sng c m t nh hnh 1.2.

Ti mi b pht, tn hiu in ca mi knh quang c iu ch vi sng

mang quang c rng ph rt hp. Tn hiu quang ti u ra ca mi b pht

c bc sng khc nhau l n ..., 21 . Cc knh quang ny c ghp vi nhau

nh b ghp knh quang OMUX v truyn trn mt si quang duy nht n u

thu. Yu cu ca b ghp knh l phi c suy hao nh m bo tn hiu ti

u ra ca b ghp t b suy hao, gia cc knh c khong bo v nht nh

trnh gy nhiu sang nhau. Ti pha thu, b ODMUX thc hin qu trnh tch

tn hiu thu c thnh cc knh khc nhau. Mi knh ny tng ng vi mt

bc sng. Mi knh c a n mt u thu ring. trnh xuyn nhiu

gia cc knh, yu cu thit k b gii ghp tht chnh xc.


Tx

Tx

OMUX ODMUX

Rx

Rx

1 2, ,... n

Siquang

1

n

1

n

1 1

n n

Hnh 1.2 S khi h thng WDM n hng

Tx

Tx

Rx

Rx

MUX/

DMU

X

MUX/

DMU

X

Rx

Rx

Tx

Tx

1

n

1

1

1

n

n

n

n .......21

nn 21......

+

1

n

n2

1+n

1

n

1+n

n2

Hnh 1.3 S khi h thng WDM hai hng

4


5/82


Phn trn trnh by phng n truyn dn ghp bc sng quang mt

hng, tc l tn hiu c ghp ti mt u v tch ti u kia, tn hiu truyn

trn si quang theo mt hng. Ngoi ra ngi ta c th thc hin truyn dn

ghp bc sng quang hai hng trn cng mt si quang nh hnh 1.3.Trong h thng truyn dn hai hng, n knh quang c bc sng 1n

c ghp li v truyn i theo mt hng, n knh quang khc c bc sng

n+12n c ghp li v truyn i theo hng ngc li trn cng si quang.

Phng php ny yu cu rt nghim ngt v rng ph ca tng knh v cht

lng ca b tch knh.

Trong h thng m cc bc sng ca cc knh quang cch xa nhau,thng thuc cc ca s khc nhau, c gi l ghp tha SWDM (Sparse

Wavelength Division Multiplexing). H thng c khong cch gia cc knh

quang rt nh, cc knh quang c bc sng gn nhau c gi l h thng ghp

knh mt cao DWDM (Dense Wavelength Division Multiplexing). Khi

vn tr nn phc tp hn nhiu v yu cu v cht lng cc thnh phn

trong h thng quang rt cao.

1.1.3 c im ca h thng WDM1.1.3.1 Tn dng ti nguyn

Cng ngh WDM tn dng ti nguyn bng thng truyn dn to ln ca si

quang, lm cho dung lng truyn dn ca si quang so vi truyn dn bc sng

n tng t vi ln ti hng trm ln, t tng dung lng ca si quang, h gi

thnh h thng. Hin nay, di tn truyn dn c suy hao thp ca si quang mi

ch c s dng mt phn rt nh. Nu ng dng cng ngh WDM th hiu qu

tn dng bng tn si quang trong vn truyn dn qu l ht sc to ln.

Dng cng ngh WDM c th ghp N bc sng truyn dn trong si

quang n mode v c th truyn dn hon ton song cng. Do vy, khi truyn

dn thng tin ng di vi dung lng ln, c th tit kim s lng ln si

quang. Thm vo l kh nng m rng dung lng cho h thng quang

xy dng. Ch cn h thng c c d cng sut tng i ln th c th tng

thm dung lng m khng cn thay i nhiu i vi h thng c.



6/82


1.1.3.2 ng thi truyn dn nhiu tn hiu

V trong cng ngh WDM s dng cc bc sng c lp vi nhau, do

c th truyn dn nhng tn hiu c c tnh hon ton khc nhau, thc hin vic

tng hp v phn chia cc dch v vin thng, bao gm tn hiu s v tn hiutng t, tn hiu PDH v tn hiu SDH, truyn dn tn hiu a phng tin

(thoi, s liu, ho, nh ng).

1.1.3.3 Nhiu ng dng

Cn c vo nhu cu, cng ngh WDM c th c rt nhiu ng dng nh

trong mng ng trc, mng phn phi kiu qung b, mng cc b nhiu

ng, nhiu a ch, bi th n rt quan trng trong cc ng dng mng.

1.1.3.4 Gim yu cu siu cao tc i vi linh kin

Tc truyn dn tng ln khng ngng do vy m tc x l tng ng

ca nhiu linh kin quang in tng ln theo nhng khng p ng c . S

dng cng ngh WDM c th gim yu cu qu cao v tc i vi linh kin

m vn c th p ng dung lng ln.

1.1.3.5 Knh truyn dn IP

Ghp knh bc sng i vi khun dng s liu l trong sut, tc l

khng h c quan h g vi tc ca tn hiu v phng thc iu ch tn hiu

xt trn phng din in. Ghp knh bc sng cng l bin php m rng v

pht trin mng l tng, l cch thun tin a vo dch v bng rng mi

(v d nh IP). Ch cn dng thm mt bc sng l c th tng thm mt

dch v mi hoc dung lng mi mong mun .

1.2 Mt s tham s k thut trong h thng WDMH thng WDM c mt s tham s chnh, l khong cch knh, s knh

ghp, suy hao xen, suy hao xuyn knh, rng knh. Trong , ba tham s suy

hao xen, suy hao xuyn knh, rng knh l ba tham s m t c tnh ca b

ghp/tch knh.

1.2.1 Suy hao xen

Suy hao xen c xc nh l lng cng sut tn hao sinh ra trong tuyntruyn dn quang do tuyn c thm cc thit b tch/ghp knh quang. Suy hao



7/82


ny bao gm suy hao do cc im ni ghp thit b WDM vi si v suy hao do

bn thn thit b ghp gy ra. V vy, trong thc t ngi thit k tuyn phi tnh

cho vi dB mi u. Suy hao xen c din gii tng t nh suy hao i vi

cc b tch/ghp hn hp (MUX/DMUX) nhng cn lu trong WDM l xtcho mt bc sng c trng. Suy hao xen c xc nh nh sau:

- i vi OMUX:

)(

)(lg10

ii

i

iI

OL

= (dB) (1-1)

- i vi ODMUX

)()(

lg10i

ii

iI

O

L

= (dB) (1-2)

Trong : I(i) v O(i) tng ng l cng sut cc tn hiu quang ti u

vo v u ra b ODMUX v b OMUX.

Ii(i) l cng sut tn hiu ti u vo th i cab ghp

Oi(i) l cng sut tn hiu ti u ra th i ca b tch

Tham s suy hao xen lun c cc nh ch to cho bit i vi tng knhquang ca thit b.

1.2.2 Suy hao xuyn knh

Khi thc hin ghp cc knh quang c bc sng khc nhau truyn trn

cng mt si quang th mt phn tn hiu ca knh ny ghp sang vng ph ca

knh khc. Do khi tch knh s c s r cng sut tn hiu t knh th i c

bc sng th i c bc sng i sang cc knh c bc sng khc vi i.Ngy

c trong trng hp ghp knh hon ho, cc b tch ghp thc t lun c hin

tng r cng sut tn hiu t mt knh sang knh khc.Hin tng ny gi l

xuyn knh .



8/82


Hnh 1.4 Xunh m trong h trong h thung

Trong thc t lun tn ti hin tng xuyn knh v lm gim cht lng

truyn dn. Ngi ta a ra tham s suy hao xuyn knh c trng cho kh

nng tch cc knh khc nhau v c tnh bng dB nh sau:

- i vi b tch knh:

1,

( )( ) 10log

( )

n

i k

k k i

i

i

UD

I

=

=

(dB) (1_3)

Trong trng hp l tng, ti ca ra th i ch c bc sng i, nhng do

c hin tng xuyn knh, ti ca ra th i c tn hiu r t cc knh khc. U i(k)

v Pi (j ) l cng sut tn hiu khng mong mun bc sng k v j ti ca

ra th i. Trong thit b ghp/tch knh hn hp, vic xc nh suy hao xuynknh cng c p dng nh b tch knh. trng hp ny phi xem xt c

hai loi xuyn knh, xuyn knh u gn v xuyn knh u xa (hnh 1.4).

Xuyn knh u xa l do cc knh khc c ghp i vo ng truyn gy

ra. V d, Ui(k) l xuyn nhiu do knh quang c bc sng k ti u ra th i.

Xuyn knh u gn l do cc knh khc u vo sinh ra. V d, P i (j ) l

xuyn nhiu do knh Ij (j ) gy ra trn knh ra th i. Khi a ra sn phm, cc

nh ch to cng phi cho bit suy hao knh i vi tng knh ca thit b.



9/82


1.2.3 rng knh v khong cch knh

rng knh (i ) l di bc sng c nh ra cho tng knh quang.

rng knh bng tng rng ph ca ngun v khong phng v ca knh.

Bc sng trung tm ca ngun quang phi nm gia di bc sng ny.Khong cch

knh l khong cch gia hai bc sng trung tm ca hai knh quang k nhau.

Nu rng knh ca cc knh quang bng nhau th rng knh chnh bng

khong cch knh.Khong cch ti thiu gia cc knh b gii hn bi xuyn nhiu gia cc

knh. Khong cch gia cc knh thng ln hn bn ln tc bit. Nu ngun

pht quang l cc diode laser th cc rng knh yu cu khong nm m

bo khng b xuyn nhiu gia cc knh do s bt n ca cc ngun pht gy ra.

i vi ngun pht quang l cc diode pht quang LED, yu cu rng knh

phi ln hn 10 n 20 ln, v rng ph ca loi ngun ny rng hn.

1.2.4 S lng knh

Trong h thng WDM, mi bc sng c coi tng ng vi mt knh

quang. S lng knh bng s lng cc bc sng c ghp li truyn trn

cng mt si quang n u thu. Tham s ny ph thuc vo rng bng tn

truyn dn ca si quang m h thng s dng v rng knh.

Gi s cc knh quang cn truyn trong h thng c rng ph bng nhau

v bng i(nm) tnh mc 3 dB. Nh trn ni, trnh hin tng xuyn


Hnh 1.5. Khong cch knh v rng knh

Khong cch knh

rng knh

P

1 2 3

9


10/82


knh cn c mt khong bo v gia chng. t khong bo v ny l

i(nm). Khi , rng knh ca mt knh quang l:

i = i + i (1-4)

Gi s bng thng s dng cho h thng WDM l . Khi s lng

knh ti a ca h thng WDM l:

+

=

=iii

n

'''(1-5)

Trong cc h thng in, cht lng b lc rt tt nn khong bo v

(i) thng nh hn rng ph ca knh. Nhng trong h thng quang, do

hn ch ca b lc nn khong bo v (i) yu cu rt ln. i thng c

yu cu ln gp bn ln i. Theo khuyn ngh ca ITU-T, rng knh

khong 100GHz. Hin nay mt s nc sn xut c h thng thng tin

quang WDM c i = 50GHz m vn m bo cht lng.

Bng thng s dng cho h thng WDM () nm trong vng ca s suy

hao thp ca si quang. Bng thng ny cng c tnh mc suy hao 3 dB.

Trong thc t ta khng th s dng c ton b di bng ny, c rt nhiu yu

t hn ch vic s dng ton b ca s suy hao thp. V d, khi trn tuyn c s

dng b khuch i quang. B khuch i c di khuch i nh. iu ny gii

hn s lng knh truyn trn si quang. Bng thng ca EDFA thng l

3035nm, ngay c khi s dng cc cng ngh lm phng ph khuch i. Mt

s yu t khc cng hn ch s lng knh, l n nh v kh nng iu

chnh ca laser, s suy gim ca tn hiu trong qu trnh truyn dn gy ra bi

hiu ng phi tuyn, nhiu xuyn knh.

Trong cc h thng WDM s lng knh cng ln th dung lng truyn

dn cng tng, nhng h thng cng tr nn phc tp, yu cu cht lng ca

cc thnh phn trong h thng cao hn v phi c bit quan tm n hin tng

xuyn knh, suy hao do thit b WDM gy ra. i vi mt dung lng nh

trc, vic tng s lng knh s lm gim s si s dng hoc tc truyn

dn nn. y l mi tng quan cn quan tm khi p dng xy dng hoc nng

cp cc tuyn truyn dn quang.



11/82


1.3 ng dng WDM

Ni chung WDM c ng dng rt nhiu cho cc tuyn truyn dn

cp si quang ng di trong lc a, xuyn lc a, cc tuyn quang bin ni

vng cng nh lin vng, cc tuyn quang xuyn chu lc N c la chnnh mt gii php duy nht m cho hiu qu cao c v mt k thut cng nh

kinh t.

C th k ra rt nhiu tuyn truyn dn cp si quang ng dng cng

ngh WDM trn th gii (SEA-ME-WE 3, SAT, NPC). Di y l mt s

tuyn truyn dn cp si quang in hnh c s dng WDM trong khu vc Chu :

BIMPP-GUAM: D n tuyn thng tin quang ny nhm kt ni Brunei,

Indonesia, Malaysia, Philippines, qun o Palau v Guam (Hoa K). Tuyn

truyn dn ny hot ng nh mt mng khu vc v c th d phng cho cc

mng ln cn nh mng ni vng Malaysia, mng SEA-ME-WE 3 m rng.

p ng mc tiu ny, ngi ta phn b mt s bc sng m bo vic d

phng v mt s bc sng phc v cc kt ni gia cc quc gia trn tuyn.

Trn hnh 1.6 l s tuyn ni ca BIMPP-GUAM .

SEA-ME-WE 3 v phn m rng: Tuyn thng tin quang ny ni t Chu

u xuyn qua a Trung Hi ti ng Nam (hnh 1.7). N c 6 trm u cui

tri di trn 38.000 km (vi hn 20.000 km bin), c 27 im xen r (ADM) v

hn 40 im cp b. Dung lng ca tuyn ny l 8 x 2,5 Gbps, s dng cng

ngh WDM truyn ng thi 8 bc sng (mi bc sng mang dung lng 2,5


Palau (Qun o Caroline)

Guam(USA)

Indonesia

Malaysia

Brunei

Philippine

Hnh 1.6 Tuyn thng tin quang BIMPP-GUAM

11


12/82


Gbps) trn hai i si quang. Phn m rng ca SEA-ME-WE 3 trin khai ti

Chu nhm kt ni hn 10 quc gia t Singapore ti Nht Bn v Hn Quc

vi cc nhnh xen r l Malaysia, Brunei, Vit Nam, Hng Kng, Ma Cao, i

Loan, Trung Quc v Philippines.

ASIA-GUAM: D n ny kt ni Trung Quc, Hn Quc v Nht Bn ti

Guam (hnh 1.8). Trong tuyn thng tin ny mi quc gia Trung Quc, Hn

Quc v Nht Bn s dng mt bc sng ring vi dung lng 2,5 Gbps nhmm bo s truy cp c lp ti Bc M. ng thi, tuyn ny c th c thm

mt s bc sng ph d phng cho truyn thng tin SEA-ME-WE 3.

NORTH PACIFIC CABLE 2 (NPC2): D n ny kt ni Nht Bn, Hn

Quc, Trung Quc v Lin bang Nga ti Bc M (hnh 1.6).


Hnh 1.7 Tuyn thng tin quang SEA-ME-WE 3 v phn m rng

Keoja

Okinawa

Thng hi

Ma caoHngcng

Shantou

Fengshan

Toucheng

Batangas

Brunei

nng

Mersing

Singapore

Phn m rngSEA-ME-WE

3

SEA-ME-WE3

Guam(USA)

Nht BnTrung Quc

Hn Quc

Hnh 1.8 Tuyn thng tin quang ASIA-GUAM

12


13/82



Ti Bc M

Nht Bn

Lin bang nga

Hn Quc

Trung Quc

Hnh 1.9 Tuyn thng tin quang NPC2

13


14/82

n tt nghip i hc Chng 2. Cc thnh phn trong h thng WDM

CHNG 2

CC THNH PHN TRONG H THNG WDM

Gii thiu chung

Cc thnh phn trong h thng WDM cn phi c chun ha. Thng

thng cc knh khc nhau (bc sng khc nhau) trong h thng WDM phi

c truyn dn nh nhau trong ton tuyn quang. iu ny c ngha l cc

thnh phn trong mng quang (ngun quang, b ghp, b tch, b khuch i v

bn thn si quang) phi c tnh nng mong mun, t cht lng v p ng

yu cu ca h thng. Cc tham s ca thit b nh suy hao xen, suy hao tr v,

tn sc, phn cc phi c o trn ton di bng thng ca h thng WDM.Cc thnh phn quang c th c o kim tt ti nh my, nhng khi lp t

vo h thng, cc tnh nng ca n li b suy gim i, hoc cc tnh nng ca cc

thnh phn khc nhau tc ng qua li nh hng ln nhau. Kt qu l cht

lng ca h thng khng m bo ngay c khi cc thnh phn ring l ca h

thng hot ng tt. Nh vy cc thnh phn thit b phi tng thch vi nhau

v c la chn cn thn khi a vo lp t h thng. Phn sau y s trnh

by cc thnh phn c bn ca h thng thng tin quang WDM.

2.1 B pht quang

Trc y cc b pht quang bao gm thit b in v thit b quang ring

bit, nhng hin nay b pht quang c hnh thnh t vic tch hp cc mch

tch hp v laser diode.Vic b xung thm cc vi mch tch hp mt cao cho

php b pht quang p ng c cc yu cu v iu ch tc cao v ci

thin c tin cy ca thit b. Khi nhn t quan im thit k h thng ngita thng coi chng nh mt hp en c chc nng chuyn i in sang quang.

Hin ti c hai loi ngun quang dang c ng dng rng ri l laser phn b

hi tip DFB (Distributed Feedback Laser) v laser phn b phn x Bragg DBR

(Distributed Bragg Reflector Laser). Ngoi ra laser si quang pha tp cht him

cng ang c nghin cu, cc ngun quang ny c u im l ph hp v

n nh tn s cao. B pht quang c ngun pht quang ph hp c ngha rt

ln, c bit trong h thng ghp knh quang WDM.



15/82


16/82


- Nhiu thp: C rt nhiu loi nhiu laser bao gm: nhiu cnh tranh

mode, nhiu pha, nhiu MPN... Nhiu thp rt quan trng t c mc BER

thp trong truyn thng s, m bo cht lng dch v tt.

Trn c s cc yu cu ni trn, ngi ta tin hnh nghin cu, trin khaithc nghim v a vo ng dng cc loi ngun quang m c th p ng c

phn no hoc ton b cc yu cu kht khe . Cc loi ngun quang c

trnh by chi tit trong phn (2.1.2), (2.1.3) di y.

2.1.2 Nguyn l Bragg

Phn x Bragg l trn mt tip gip ca 2 loi phng tin c phn x

mang tnh chu k, khi c chiu quang s xut hin phn x chu k, phn xny gi l phn x Bragg. Bn thn mt giao nhau c hnh dng khc nhau: hnh

sin hoc khng sin (nh hnh vung gc, hnh tam gic...).

Trong hnh, ta thy:

A+B=m u (2.1)

Trong m l s nguyn chn

A l chu k cch t

u = /n l bc sng trong cht mi gii

l bc sng quang trong khng kh

n l h s khc x tng ng.

Bin i mt cht ta c th c:

A(1+sin)= m u (2.2)


Aa

Aa

B

B

1 2

1

1

Hnh 2.1. Nguyn l phn x Bragg

16


17/82


Cng thc (2.2) gi l iu kin phn x Bragg. ngha vt l ca cng

thc l: i vi A v nht nh, khi c mt u tng ng th sng quang c

bc sng u s can thip cng vi sng quang phn x .

Da vo nguyn l ny m ngi ta ch to ra cc loi laser c th chomode dc n duy nht. l laser DFB v DBR.

2.1.3 LASER hi tip phn b (DFB)

a)Kt cu: Laser DFB khng phi da vo khoang F-P dao ng kch

quang m da vo li quang phn b khong cch theo chiu dc u nhau,

hnh thnh phi ghp quang. Chu k A ca li quang gi l c ly li. Kt cu

chu k ny nm lun trong hc cng hngLaser. Hnh 2.2 th hin mt ct dcca laser ny.

Hnh 2.2: Mt ct dc ca Laser DBF

b) Nguyn l: Khi c dng in vo laser, trong khu c ngun phc hp

in t- l trng bc x ra nng lng tng ng vi quang t. Nhng quang t

ny b mt si li quang b mt lp c ngun phn x, ging nh hnh 2.1,

ch khc l trong s phn b phn hi Bragg ca laser DFB c = /2, cng

thc (2.2) bin thnh:

A =2

nm (2.3)



18/82


Tn hiu quang c bc sng tho mn cng thc trn mi c phn x

mnh v c khuch i ln, cc bc sng khc khng tho mn cng thc

trn th s b dp v khng pht x. Cng thc (2.1) gi l iu kin phn b

phn hi. Thng thng m = 1, khi n = 2A c gi l bc sng Bragg.Vi loi LASER ny, cc mode bn b trit khong 30 40 dB so vi mode

chnh.

c) c im: DFB so snh vi LASER F-P c 2 u im sau y:

- Dao ng n mode dc di hp: Do chu k cch t(A) trong b pht

quang DBF rt nh,nn hnh thnh khoang cng hng kiu nh, i vi

bc sng c tnh la chon tt lm cho gii hn tng ch ca mode chnh vmode bin tng i ln, t c di ph ngun quang rt hp so vi

laser F-P.

- Tnh n nh ca bc sng tt: V li quang trong laser DFB c

gip cho vic cht trn bc sng cho trc, tri nhit ca 0,8 o / C , tt

hn nhiu so vi LASER khoang F-P.

2.1.4 LASER phn b phn x Bragg (DBR)a) Kt cu: Kt cu laser phn b phn x Bragg (DBR- Distributed Bragg

Reflection) v kt cu ca laser DFB cn bn khc nhau (xem hnh 2.3). Ch

khc nhau l n c kt cu li phn x nm ngoi hc cng hng. Vi s khc

bit ny, phn iu khin hc cng hng LASER v phn iu khin tn s

theo nguyn l Bragg l hon ton c lp.


Hnh2..3. Kt cu LASER DBR

Phn phn x BraggVng hot tnh

Hng dc

iu khin hccng hng

iukhin tn

s

18


19/82


b) Nguyn l: Nguyn l lm vic ca laser DBR cng da vo nguyn l

phn x Bragg v tho mn chnh xc cng thc (2.3), c im cng c bn

ging laser DFB. ch c mt s im khc bit nh cn lu , l: (i)vt liuch to ca laser DBR l kh khn hn laser DFB v n khng nht thit i hi

s ghp cng sut gia cc vng th ng v vng tch cc; (ii) c tnh ph

thuc nhit th khc nhau, khi nhit tng th trong lasser DBR c s

chuyn i t mode ny qua mode khc cn vi laser DFB th th hin c tnh

n nh nhit trong mt di rng .

2.2 B tch quang v b ghp quang

Tn hiu ti u ra mi b pht mt bc sng ring, nhng tia sng ny

c ghp li v truyn vo si quang. Thit b thc hin chc nng ny gi l

b ghp (Multiplexer hay MUX). Ngc li, pha thu c mt thit b thc hin

tch tn hiu quang thu c thnh cc knh quang c bc sng khc nhau

a n mi b thu quang ring bit. Thit b ny gi l b tch quang

(DeMultiplexer hay DMUX). y l cc thnh phn ct yu ca h thng thng

tin quang WDM v c chia thnh 2 nhm ln.Nhm da trn nguyn l giaothoa v nhm da trn nguyn l tn sc gc.V mt nguyn l bt k b ghp

bc sng no cng c th lm b tnh bc sng v ngc li.

B tch quang v b ghp quang c cu to t b lc quang v coupler

quang. Cc b lc hin nay hay c s dng gm b lc cng hng Fabry-

Perot, b lc giao thoa k Mach-Zender, b lc cch t Bragg... Nguyn l v

c tnh ca cc b lc ny s c trnh by cc phn di y.

2.2.1 B lc Mach-Zender

2.2.1.1 Cu trc v nguyn l hot ng ca b lc Mach-Zender

B lc Mach-Zender lm vic da trn s giao thoa ca hai tia sng xut

pht t cng mt ngun v i theo hai ng khc nhau. Hnh 2.4 l cu trc

ca b lc Mach-Zender. B lc c cu to t 2 coupler 3dB 2x2 v hai

nhnh ng dn sng. Nhnh trn c lp thm b quay pha 90o v b tr. Cu trc

ny to ra kh nng iu chnh bc sng cu b lc.



20/82

Coupler 3dB

A AL/2

B nung nng

Nn Si

B nung nngLi siquang

Nn Si

Hnh 2.5 Cu to b lc Mach-Zender c sn xut theocng ngh tch hp quang


Nguyn l hot ng ca b lc nh sau. Tn hiu WDM c a vocng 1 ca b lc. Khi qua coupler 3dB th nht, nh sng c tch ra thnh

hai lung i vo hai nhnh khc nhau ca thit b. Sau , chng c kt hp

li ti coupler 3dB th hai. Tn hiu WDM gm nhiu bc sng khc nhau. Khi

i qua b tr, mi bc sng c mt tr ring. Kt qu l ti coupler 3dB th

hai, hai tn hiu i theo hai nhnh khc nhau c lch pha l mt hm ph

thuc vo bc sng. Ti bc sng c lch pha ny bng 0 s c cng sut

ra ln nht. Cc bc sng khc c lch pha khc 0 cho cng sut u ra nh.

Trong thc t thit b lc quang Mach-Zender thng c ch to t si

quang hoc da trn cng ngh tch hp quang si. Hnh 2.5 ch ra mt b lc

quang Mach-Zender c ch to da trn cng ngh tch hp si quang. B lc

quang c to thnh t hai coupler quang 3dB v hai ng dn sng quang. Tt

c c tch hp trn nn Silic. Qu trnh tch hp ng dn sng SiO2 trn nn

Silic s c miu t chi tit phn sau.


2

l/v

Tr

Coupler

3dB2x2

Coupler

3dB2x2

12

34

[P] [D] [Q]

Hnh 2.4 Cu trc b lc Mach-Zender

20


21/82


2.2.1.2 Mt s c tnh ca b lc quang Mach-Zender

B lc quang Mach-Zender gm 3 thnh phn mc ni tip nhau l coupler

3dB u vo 2 nhnh ng dn sng v coupler 3dB u ra. Gi [P], [D], [Q] ln

lt l ma trn truyn tng ng vi 3 thnh phn trn. Nh vy ma trn truyn

t ca b lc l:

31 32

41 42

( ) ( )

( ) ( )

H f H f

H f H f =[P][D][Q] =

=21 101 1

1 10 12 2

j fj je

j j

+ ++ + =

=2 2

2 2

( 1) 11

2 1 ( 1)

j f j f

j f j f

j e e

j e j e

++

(2-4)

Trong l thi gian tr do b to tr gy ra:

. effLL L n

v c c

= = = (2-5)

Trong nhiu ng dng, b lc Mach-Zender ch s dng mt u vo, v d

u vo 1. Khi hm truyn t vector trng in t ca b lc l:

231

241

( ) ( 1)1

( ) 2 1

j f

j f

H f j e

H f j e

=

+(2-6)

Hm truyn t cng sut ca b lc:

2

23131

2241 41

( )( ) sin ( )

( ) cos ( )( )

H fP f f

P f fH f

= = (2-7)

Nh vy hm truyn t cng sut t cng 1 n cng 4 v 3 l :

T41(f) = P41(f)= cos2( f )=cos2(effLf

c

) (2-8)



22/82


T31(f)= P31(f) = sin2( f ) = sin2(effLf

c

) (2-9)

B lc quang Mach-Zender c hm truyn t tun hon, di ph t do

FSR tho mn:

. . effL

FSRc

=

.eff

c cFSR

L L n= =

Khi tn hiu quang n gm hai knh quang c tn s l f1, f2, mi u ra

3 hoc 4 ch c tn hiu ca mt knh quang th f1, f2phi tho mn:

1

effL

f kc

=

2(2 1)

2eff

Lf k

c

= +

Do : 1 2( ) 2eff

Lf f

c

=


0 1 2 3 4 5 6 7 8 9 10 11

12 13 14f (A)

m=1

m=2

m=3

m=M=4

(B)

(C)

Hnh 2.6 Chui Mach-Zender (M=4)

f

f

f

f

f

f22


23/82




24/82


Khong cch gia 2 tn s b lc ra c xc nh bi:

2 2. .

FSR cf

L n = =

(2-10)

Trong thc t, h thng WDM gm nhiu hn hai knh quang. Ngi ta

mc ni tng nhiu b lc Mach-Zender 2x2 to ra b lc mong mun.

Trng hp cn tch 1 knh quang t chm sng N knh (N=2M) th mc M

b lc Mach-Zender 2x2 ni tip. Gi s hm truyn t ca b lc Mach-

Zender th m (m=1 M) l:

Tm(f) = cos2(,eff m

Lf c

) = cos2( mf )

Trong ,eff mL l chnh lch v chiu di quang hc gia 2 nhnh ca

b lc Mach-Zender th m v mL tho mn:

2 .m mv

Lf

= hay

1

2 .m m f =

(2-11)

Trong : f l khong cch gia cc knh ca h thng.

Hm truyn t ca b lc chung l:

T(f) =1

M

m=

Tm(f) =2

1

cos ( )M

m

m

f =

(2-12)

p dng cng thc:

cos(x) =sin(2x)/2sin(x)

Ta c:

T(f) =2

sin( / )

sin( / )

f f

N f N f

(2-13)

T cng thc (2.13), T(f) l hm tun hon vi chu k l N f . Do , di

ph t do ca b lc l:

FSR = N f (2-14)

Hnh 2.6 l trng hp ring M=4 v cn tch mt knh quang t lung

tn hiu WDM 16 knh. T hnh v cho ta thy, khi qua tng lc th nht cc



25/82


knh 0,2,4,6,8,10,12,14 c cho qua. Khi qua tng lc th hai cc knh

0,4,8,12 c cho qua. Khi qua tng lc th ba cc knh 0 v 8 c cho qua.

V khi qua tng lc th 4 ch c duy nht knh 0 c cho qua. Hm truyn t

cng sut ca h thng c biu din nh hnh 2.6C.

Hnh 2.7 l th hm truyn cng sut ca thit b lc gm nhiu b lc

Mach-Zender c ni tng trong trng hp M=2, 3, 4.


Hnh 2.7 th hm truyn cng sut ca thit b lc gmnhiu b lc Mach-Zender mc ni tng

Cng vo

Hnh 2.8 B lc 1 knh t 128 knh

25


26/82


Hin nay cng ngh tch hp quang rt pht trin, c th ch to chui

Mach-Zender rt n gin bng cch tch hp ng dn sng SiO2 trn nn Silic.

Hnh 2.9 ch ra thit b lc cho h thng gm 128 knh quang s dng cng

ngh ny. Thit b gm 7 b lc quang Mach-Zender 2x2 mc ni tip, c gi

tr L tng ng l: 10mm, 5mm, 2,5mm, 1,25mm, 0,63mm, 0,31mm, 0,16mm.

Cc b lc c tch hp trn nn Silic c kch thc l 50mmx60mm. B lc

ny c xuyn nhiu khong 13dB.

Khi cn thit k b tch knh quang cho h thng WDM c N knh(N=2M), ta cn s dng N-1 b lc Mach-Zender 2x2 , ghp vi nhau thnh hnh

cy M tng. Ti tng th m (m=1 M) cn 2m-1 b tch knh c chnh lch

chiu di quang hc gia 2 nhnh si quang l:

2m mnc

Lf

=

Hnh 2.9 ch ra v d M=2.


f

f3

f

1

2f f1

f

3

2f f2

f4

f2

f4

f1f2

f3

f4

B lc MZ(a)

Hnh 2.9 Cc b lc c ghp hnh cy M=2tng

26


27/82


2.2.1.3 B lc quang kh chnh

a. Gii thiu chung

B lc quang kh chnh thc hin chc nng la chn bc sng mong

mun ti my thu. Di bng thng b lc c th thay i nh h thng iu

chnh ngoi. Bng thng ca b lc kh chnh phi ln cho qua tn hiu

mong mun, nhng cng phi hp trnh xuyn nhiu t cc knh khc.

Cc yu cu i vi b lc quang kh chnh l:

1. Di iu chnh rng s knh c la chn l ln

2. Xuyn nhiu gia cc knh phi nh nht

3. Tc iu chnh ln

4. C kh nng n nh khi cc iu kin mi trng

xung quanh thay i (nhit , m...)

5. Gi thnh thp

b. B lc quang Mach-Zender kh chnh


f1

f2

f3

f4

frequency

MZ[f]

MZ[2f]

Chui MZ

f

(b)

27


28/82


Theo phn trc hm truyn t ca b lc Mach-Zender l :

T(f)=cos2(f.Leff/c) = cos2(2f /c. nL/2)

B lc s a ra knh c bc sng tho mn:

.n L k

=

Do :

=n.L /k (2-15)

Trong kl s nguyn dng.

Do , bc sng trung tm ca b lc l mt hm ca chit sut ng dnsng n v chnh lch chiu di hai ng dn sng L. V L l c nh nn

mun to thnh b lc kh chnh, cn thay i chit sut ng dn sng.

C 2 phng php lm thay i chit sut ca ng dn sng quang l nh

vo hiu ng nhit quang v nh vo hiu ng in quang.

Hnh 2.5 l cu trc ca b lc Mach-Zender kh chnh lm vic da vo

hiu ng nhit quang. Thit b ny do Oda et at (1990) v Takato (1990) snxut. Mt nhnh si quang ca b lc c lp thm b lm nng Cr mng

mng. Khi t in p vo hai u b nung nng, b nung nng tng nhit .

Nhit ny cung cp cho nhnh si quang v lm bin i chit sut si

quang. di quang hc ca nhnh ny tng ln mt lng:

dn/dT.L.T (2-16)

Trong : dn/dTl hng s nhit quang ca ng dn sng

L l chiu di ca b nung nng

T nhit tng ln

Vi SiO2 th dn/dT = 1x10-5

V d, mt b lc c L = 10mm, T=7,8oC, chiu di quang hc tng ln

0,78cm. Ti bc sng =1550 nm th pha tng ln tng ng l . Nng lng

kch thch pha tng ln l 0,5W. Thi gian p ng nng pha hoc h



29/82


pha khong 1 2ms. Tc p ng ca b lc Mach-Zender tng i chm v

b gii hn bi thi gian khuych tn nhit t in cc sang cht nn. Tc

ny qu chm v khng p ng c cho h thng chuyn mch gi. Trong

thit b tch knh gm nhiu b lc Mach-Zender th vic iu khin rt khkhn, vic thit lp nhit trn mt tng ph thuc vo nhit ca cc tng

trc trong chui. Cht nn Si c tc dng nh mt b tn nhit, n nh nhit

cho thit b.

B lc Mach-Zender kh chnh da trn hiu ng in quang c sn xut

t cc tinh th c in quang. Tc l chit sut c kh nng thay i khi in p

cung cp thay i. Thi gian iu chnh ca thit b b gii hn bi tc in t.

Do thi gian iu chnh b lc in quang nhanh hn b lc da trn hiu ngnhit quang. Tc iu chnh c vi ns, nhng di iu chnh rt nh c 16 nm.

2.2.2B lc Fabry-Perot

2.2.2.1 M u

Cu trc ca b lc Fabry-Perot bao gm mt khoang cng hng c to

thnh t 2 gng phn x t song song. Mt thit b lc quang thc t cn c

si quang u vo, si quang u ra v h thng thu knh. Ti cc thnh phnny c th gy ra suy hao nh hng n cht lng thit b. Song trong mc

ny ta ch tp trung vo khoang cng hng. Hnh 2.10 ch ra cu trc c bn

ca b lc Fabry-Perot.

Trong hnh v ch a ra mt mt ca lp thu tinh to nn gng phn x

ca khoang cng hng. Mt cn li ca hai lp thu tinh ny cng c th gy

phn x v to ra cng hng ti cc bc sng khng mong mun. trnh

hin tng ny, t hai mt ngoi ca hai lp thu tinh ny hi nghing so vi

trc thng ng hoc bc ngoi bng mt lp chng phn x.

nh sng t si quang c a vo khoang cng hng. Sau mt s ln

phn x lin tip trn hai b mt gng ca khoang cng hng, nh sng c

a n u ra. Gi x l chiu di khoang cng hng, nu bc sng tho

mn:

x=i/2n=ic/2nf (2-17)



30/82


Trong n l chit sut khoang cng hng

i l s nguyn v c gi l bc cng hng

th knh tn hiu ti bc sng c cng hng. Cng sut u ra ca knh

ny rt ln. Vi cc knh quang c bc sng khng tho mn iu kin ny th

cng sut b gim.

2.2.2.2 Mt s tham s ca b lc Fabry-Perot

* Hm truyn t

Gi s hai gng c h s phn x bng nhau v bng R. Do cng

trng ca tn hiu phn x bng R ln cng trng ca nh sng ti. Gi

s suy hao do hp th khi nh sng i t si quang u vo n khoang bng suy

hao do hp th khi nh sng i t khoang cng hng n si quang u ra v

bngA

GiEi l bin trng nh sng u vo, khi qua gng th nht cng

trng l 1 . iA R E . Phn cng sut cn li ti b mt di dng nhit hoc

do phn x. Gi s khi nh sng truyn qua khoang cng hng khng b suy

hao, v b tr pha mt lng bng x (vi l h s truyn).

Khi nh sng n gng G2, ln trng 1 . iA R E .e-jx. Mt phn nh

sng c ln trng (1-A-R)Ei.e-jxc a n u ra, phn cn li c ln

trng 1 . iA R E . R .e-jx b phn x tr li. Qu trnh tip tc nh vy v ta

c chm nh sng u ra th hai c ln trng l

(1-A-R)R.Eie-j3x . Chm nh sng tip tc phn x trn hai mt gng v

cho cng sut n si quang u ra gim dn. Cng sut tn hiu ti ln ra th

m:

(1-A-R).Rm-1.Ei.e-j (2m-1)x


x=

(A)

30


31/82



(B)

.e-jx

..e-jx

..e-j2x

.R.e-j2x

.R.e-j3x

x

Ei

ER

G1

G2

(1-A-R) Eie-jx

(1-A-R)REie-j3x

Hnh 2.10 B lc Fabry - Perot

x/v

x/v

Eo1 A R 1 A R

R R

Ei

ER

B nhn B tr B nhn

(C)

B nhn

B tr

B nhn

31


32/82


H s truyn ca khoang cng hng c xc nh theo cng thc sau:

=2n/=2f.(n/c) hay x=2f(xn/c)=2f (2-19)

Hm truyn t phc ca ln trng l:

H(f) = Eo(f)/Ei(f)=(1-A-R)e-j2f0m

= Rme-4jnf

VR


33/82


T(f)=

122

2 41 1 sin

1 1 2

A R f

R R

+ (2-23)

th biu din hm truyn t ca b lc c ch ra trn hnh 3.11

Di ph t do FSR v mn F

Hm truyn t ca b loc Fabry-Perot l hm tun hon. Di ph t do

FSR bng chu k ca hm v c xc nh nh sau:

4

2

FSR = suy raFSR=1/2=c/2nx (2-24)

mn ca b lc Fabry-Perot c xc nh bng t s ca FSR v

rng ph na cng sut HPBW ca b lc (cn gi l rng ph 3dB). HPBW

ca b lc c trng cho hp ca nh hm truyn t v c xc nh nh

sau:

HPBW = c/2nx.(1-R)/ R (2-25)

mn c quan h cht ch vi s knh ti a m b lc c kh nng la

chn:

F=FSR/HPBW=(. R)/(1-R) (2-26)

Mt b lc Fabry-Perot n c s knh ti a l 0,65F. Khi mun tng s

lng knh, ngi ta c nhiu hng ci thin khc nhau. V d, dng b lc a

khoang cng hng hoc b lc a ng (multipass).

T cng thc (2-23), (2-26) ta c:

T(f) =

12 2

2 41 1 sin1 2

A F fR

+ (2-27)

ln ca T(f) ti nh cng hng:

T(f)=2

11

A

R

(2-28)

T s gia gi tr ln nht v nh nht ca b lc l:

2

21 Fc

= +

(2-29)



34/82


T cng thc (2-26) F tng ln (tc l s lng knh m b lc c kh

nng iu chnh c tng ln) th h s phn x R ca gng phi ln. Ngoi ra

trnh mt mt do suy hao hoc phn x th hai gng rt phng v t song

song nhau.2.2.2.3 B lc Fabry-Perot kh chnh

Theo cng thc (2-23), nh hm truyn t ti v tr c tn s tho mn:

4

2

fm

= = (mZ)

hay 2 . .c Ln

mc

=

hay2Ln

m= (2-30)

Theo cng thc (2-30) c 2 cch iu chnh bng thng ca b lc l

iu chnh L hoc n. Khi cn iu chnh t bc sng sang mt bc sng

mi +, cn iu chnh tch Ln mt lng tho mn:

2 2 ( )Ln Ln Ln

+ = =

+ hay

.Ln

Ln

= (2-31)

*iu chnh bng thng b lc bng cch iu chnh L

Trong trng hp cn iu chnh L, ngi ta thng s dng tinh th p

in v ngun in ngoi.


Hnh 2-12 Cu trc b lc Fabry-Perot kh chnh

Tinh th p inL

34


35/82


B lc Fabry-Perot kh chnh dng tinh th p in thng hay s dng c

cu trc nh hnh 2-12. Khoang cng hng c to thnh t hai u si

quang t gn nhau. Hai u si quang ng vai tr l hai gng phn x. Hailp tinh th p in c gn vo hai u si quang nh hnh v.

Khi tng in p t vo hai lp tinh th p in mt lng v, th chiu

di lp tinh th p in tng ln mt lng L, xc nh theo cng thc:

L=kL.v (2-32)

Trong : kl hng s vt liu [v-1]

L chiu di ca khoang cng hng

T cng thc (2-31) v (2-32) v gi thit n c nh ta c :

/=L/L=k.v (2-33)

hay1 1

. .f

vk k f

= = ( 2-34)

Nh vy khi mun thay i bc sng trung tm b lc t ln + thcn tng in t vo tinh th p in mt lng v tho mn cng thc (2-34).

V d, b lc quang Fabry-Perot kh chnh c khong cch gia cc knh l

=1nm, bc sng trung tm ca b lc trc khi iu chnh =1,5m. in

p t vo tinh th cn thay i mt lng:

v=/k=1/(1500.10-2)=6,67(V)

* iu chnh bng thng b lc bng cch iu chnh chit sut khoangcng hng n.

Trong trng hp cn iu chnh chit sut n ca khoang cng hng,

ngi ta s dng cc vt liu in quang lp y khoang cng hng. Cc vt

liu ny c chit sut ph thuc vo in p t vo. Khi cn thay i bng

thng b lc, ch cn thay i in p t vo lp vt liu.

Vt liu in quang hay c s dng trong thc t l tinh th lng. Vt

liu ny ging vt liu trong h thng hin th tinh th lng. Tinh th lng c



36/82


tnh cht d hng nn chit sut ca n c kh nng thay i khi thay i in

p t vo.

B lc Fabry-Perot tinh th lng l mt cng ngh mi. Thit k b lc

tng t nh b lc thng, nhng khoang cng hng ca n lm t tinh thlng. Thi gian iu chnh ca b lc c s , di thng iu chnh khong 30

40nm. B lc ny c cng sut tiu th thp v gi thnh h. Tc ca b lc

ln, c kh nng ng dng trong chuyn mch gi tc cao ca mng

WDM qung b v la chn.

2.2.2.4 Mc ni tng cc b lc Fabry-Perot

Khi s lng knh trong h thng WDM tng ln (do nhu cu dung lngtng), yu cu mn F hiu dng ca b lc tng ln. C nhiu phng php

tng F ca b lc. Th nht, ta ci thin cht lng thit b nh iu chnh hai

gng b lc rt song song nhau v tng h s phn x R ca gng. Th hai c

th mc ni tng nhiu b lc lin tip. Phng n th hai hiu qu v n gin

hn phng n th nht.

Trong phn ny ta s nghin cu cc phng php thc hin trong phng

n hai, gm dng nhiu khoang cng hng v cho tia sng qua cng mtkhoang cng hng nhiu ln. Hnh 3.13A ch ra ng i ca tia sng qua h

thng b lc. Tia sng c i qua cng mt khoang cng hng hai ln. Do

hm truyn t cng sut ca b lc ny bng bnh phng hm truyn t cng

sut ca b lc n.

T(f)=

21

222 4

1 1 sin

1 1 2

A R f

R R

+

(2-35)


(A)

in

out

(B)

in

Hnh 2.13 Mc ni tng , (A) two-pass(B) two-cavity 36


37/82


Trong phng php dng nhiu khoang cng hng mc ni tip (hnh

3.13B), mn ca 2 khoang cng hng c la chn bng nhau v bng F,

rng ph t do FSR t l k:l (trong k, l l cc s nguyn v (k,l)=1). Khi

mn F tng ng ca b lc l F = max (k,l)Fo.

Hnh 2.14 l v d cho trng hp k=3 v l=4. Hnh2.14A l c tnh

truyn t cng sut ca mt khoang cng hng ly lm chun ( i chiu)

c F=10. Hnh2.14B l c tnh truyn t ca khoang cng hng th nht, c

FB=F=10, FSBB=1/3.FSR. Hnh 2.14C l c tnh truyn t cng sut cakhoang cng hng th 2 c Fc=F=10, FSRc=1/4.FSR. B lc gm 2 khoang

cng hng mc ni tip c hm truyn t cng sut bng tch hai hm truyn

t cng sut thnh phn. Hnh 2.14D l hm truyn t cng sut ca b lc.

T hnh v ta nhn thy rng bng thng ca b lc bng rng bng

thng ca khoang cng hng th hai, nhng FSR tng ln 4 ln so vi khoang

cng hng th hai. Do , s mn F ca b lc F=4F=max(3,4)F.


i=0 1 2 3 4 56 7

j=0 1 2 3 4 5 6 78

T(f)

f

f

f

(A)

(B)

(C)

TB(f)

TC(f)

Hnh 2.14 c tnh ph b lc Fabry Perot hai khoangf

(D)TD(f)

37


38/82


Trong trng hp ni tng nhiu khoang cng hng ta cn phn bit 2

trng hp:

- Khi k,l l hai s nguyn lin tip ((k,l) vn m bo nguyn t cng nhau)

th b lc c gi l b lc hai khoang vernier (nh v d trn)

- Khi k,l chnh lch nhau rt ln th b lc c gi l b lc hai

khoang Coarse-Fine. Hnh 2.15 ch ra mt v d v b lc loi ny c k=1, l=4.

B lc th nht ng vai tr l b lc th, b lc th hai ng vai tr l b lc

tinh.

2.2.3 B tch knh trong min khng gian

B tch knh trong min khng gian c cu to t cc thnh phn c bn

l cch t v lng knh. Lng knh lm vic da trn nguyn l tn sc gc. Cch

t hot ng da trn s giao thoa gia cc tia phn x (hoc nhiu x) trn mt


f

f

(A)

j=0 1 2 3 4 5 6 78 9

f

(B)

(C)

Hnh 2.15 Ph ca b lc hai khoang Coarse-fine

TB(f)

TC(f)

TA(f)

38


39/82


cu trc c chu k. C ba loi cch t c bn l cch t nhiu x, cch t phn x

Bragg v cch t Bragg quang m.

2.2.3.1 Dng lng knh lm phn t tn sc gc

Trong giai on u ca k thut WDM ngi ta thng hay s dng lng

knh lm phn t tn sc gc (hnh 2.16). Do chit sut ph thuc vo bc sng

nh sng nn chm tia sng c nhiu bc sng khc nhau u vo s b lng

knh phn tch thnh cc tia sng n sc khc nhau theo cc hng khc nhau u ra. Theo nh lut Snel:

ir

Ax

dn

d

di

cos/cos

sin

= (2-36)

Lng knh c mc tn sc thp, kh tch c cc bc sng gn nhau.

V vy, ngi ta ch c th s dng lng knh trong trng hp tch cc bc

sng hai ca s truyn dn khc nhau (v d hai bc sng 1300nm v 1550

nm). Do , hin nay lng knh khng c s dng na. Thay vo ngi tas dng cch t lm phn t tn sc gc.

Lng knh c mc tn sc thp, kh tch c cc bc sng gn nhau.

V vy, ngi ta ch c th s dng lng knh trong trng hp tch cc bc

sng hai ca s truyn dn khc nhau (v d hai bc sng 1300nm v 1550

nm). Do , hin nay lng knh khng c s dng na. Thay vo ngi ta

s dng cch t lm phn t tn sc gc.


r ii

Hnh 2.16 Tn sc dng lng knh

39


40/82


2.2.3.2 Cch t nhiu x

a. Gii thiu

Cch t c cu to gm nhiu rnh (nh rng ca), c khc bng dng

c kim cng, trn b mt ca cc rnh ny c ph mt lp phn x, s lngrnh c th ln ti vi nghn trn 1mm (gi l mt in ca cch t). Cch t c

kh nng truyn hoc tn x nh sng theo nhng hng nht nh tu thuc vo

bc sng ca nh sng ti. Gc tn x ph thuc vo khong cch rnh (gi l

bc cch t), gc ti v bc sng ca nh sng ti.

Cng ging nh lng knh, nh sng khng n sc u vo, sau khi i qua

lng knh s c tch thnh cc tia n sc u ra theo cc gc khc nhau.

Khc vi lng knh, cch t nhiu x cho phn gii gia cc gc nhiu x

cc bc sng khc nhau ln hn.

Ti mt bc sng c duy nht mt hng ra c cng sut ra ln nht. Cc

bc sng khc nhau c hng ra khc nhau. Do ta c th tch c mt

bc sng ra mong mun bng cch t si quang ti cc v tr c gc n nh

sng thch hp.b) Nguyn l v cc tham s ca cch t nhiu x

Hnh 3.18 minh ho s phn x nh sng ca mt tia sng bc sng xc

nh. Theo hnh v chnh lch v pha gia hai tia phn x ti hai rnh cch t

k nhau l:

2(sin sin )

= (2-37)

Trong : l gc ti


1 2, ,... n 1

2 n

Hnh 2.17 S dng cch t tch bc sng

40


41/82


l gc nhiu x

Gi s cch t c N rnh phn x. Cng sut u ra ti gc nhiu x t l

vi:

X=2

1

Njm

m

e

= =

2

sin( . / 2)

sin( / 2)

N

(2-37)

T cng thc (2-37), ta thy rng X l mt hm tun hon theo v c

chu k l 2 , Xmax= N2 ti 2 m = . Mt khc theo cng thc (2-37), l mt

hm ca bc sng . Do tn ti nhiu bc sng cho cng sut u ra ln

nht ti cng gc. Khong cch gia cc bc sng ny l rng ph t do ca

b lc (FSR). Nh vy FSR bng khong cch gia cc bc sng m ti

thay i mt gc 2 . Khi hai bc sng cng nm trong mt khong ti FSR

th chng phn x cc gc phn x khc nhau.Gi m l bc sng m ti 2 m = . Theo cng thc (2-36) ta c:

m =2 (sin sin ) (sin sin )

2 m m

= (2-38)

Di ph t do bc m l khong cch gia 2 bc sng m v 1m + , v c

xc nh nh sau:

FSRm= 1m + - m = (sin sin )( 1)m m

+ =1

1m m

m m + =

+ (2-39)


Hnh 2.18Cch t nhiu x

41


42/82


Cng thc (2-39) ch ra rng bc nhiu x cng ln th FSR cng nh.

xc nh phn gii ph ca b lc ta xem xt hai knh bc sng m ,

m + . Gi s 2 bc sng ny c cng sut ln nht ti cc bc sng c gc

khc x l v + . Ta c:

(sin sin )

(sin sin( ))

m

m

m

m

= + + =

suy ra ( )sin sin 2 sin2 2

cosm m

= + = +

v rt nh nn cos( )m

hay: .cos

m

=

(2-40)

Trong m l bc nhiu x.

Hnh 2.20 ch ra th phn b cng sut theo gc bc x ti hai bc sng

v + . Theo tiu chun v phn gii Rayleigh, chnh lch gia

hai nh nhiu x phi bng chnh lch gia hai hng c gc nhiu x nh

nht (hnh 2.20). Do , vi mt bc sng xc nh th bng chnh

lch gia hai gc nhiu x c X t gi tr ln nht v nh nht u tin.


( , )X + ( , )X

Hnh 2.20 Cch t Bragg kh chnh

42


43/82


Theo cng thc (2-37), X t gi tr ln nht ti =0 v X t gi tr nh

nht u tin ti 2 /N = Thay vo cng thc(2-36)ta c:

2

0 (sin sin ).m

=

2 2

(sin sin( )).m

N

= +

Do :

[ ]2 2 2

sin sin( ) 2cos( )sin( )2 2

m mN

= + = +

V rt nh nn :

1

. .cosm

N

=

vy :

cos 1

.m N

= (2-41)

Kt hp cng thc (2-40) v (2-41), ta c tch c cc knh quang

c khong cch knh phi tho mn :

1

.m m m

m

FSR

mN N

+ > = (1-42)

Theo cng thc (2-42), mun tng phn gii ph ca b tch knh, ta c

th tng Nhoc gim FSR. Tng s knh quang m cch t c kh nng phn

tch l:

Nch= mFSR 18dB,i vi trng hp a knh th P nh hn v

gim khi s knh khi s knh truyn dn tng), sn ln ca xung b dch v

pha bc sng dI c hiu ng SPM v hin tng ny gi l chirp phi tuyn.

i vi cc si quang theo tiu chun G.652, G.653 s dng trn tuyn thchirp phi tuyn ny ngc vi chirp tuyn tnh. Xung s b chirp mt lng

bng tng hai chirp trn. Nh vy trong trng hp ny xung phi chu mt

lng chirp bng chirp tuyn tnh tr i chirp phi tuyn nn xung dng nh

c hiu ng SPM b chirp do tn sc gy ra.

Nhng l lun trn c th nghim li bng thc nghim truyn xung

dng Gauss rng xung l 20ps, cng sut nh l 50mw. Kt qu th nghim

c trnh by hnh 3.1.Hnh ny gm 3 ng biu din rng xung theo

thi gian truyn trong 3 trng hp tn sc khc nhau v ta c nhn xt:trng

hp si c tn sc m (ng nt t) th u tin xung co li sau li b dn

rng ra rt nhanh,nhanh hn c trng hp tn sc dng.Vi si c h s tn

sc D=-1ps/nm.km th ti khong cch c 10km rng xung t cc tiu (c

15 ps) nhng khong cch 60 km th rng xung t 40 ps. i vi tn sc

dng (ng lin nt tng ng loi si G.652 v si G.653)th u tin xung

li v t cc tiu sau mt khong truyn dn dI c 60 km.Nu tip tc truyn

thi xung s b dn rng ra do cc hiu ng tn sc thng thng.Vy xung th

truyn ngng hi tuyn mt cng sut no c th loi b hon ton tn

sc.



59/82

n tt nghip i hc Chng 3. Mt s vn cng ngh thencht.

Hnh 3.1: S dn xung do nh hng ca hiu ng SPM

b) nh gi kh nng ng dng ca phng php SPM.

Trong thc t ng dng hiu ng SPM vo vic b tn sc cho xung

truyn dn phI s dng k thut phn phi tn sc tc l kt hp c si tn sc

dng v si tn sc m trn tuyn. Hnh 3.2 l mt v d phn phi tn sc

c th truyn dn trn khong di. y khong cch gia cc b khuych i

l 90 km,bao gm 60 km si tn sc m (D=-1,75 ps/nm.km) v 30 km si tn

sc dng (D=2 ps/nm.km ) tc truyn dn thng thng th xung s dn ti

rng 75 ps.

Hnh 3.2: Phng phn phi tn sc

*u im ca phng php b tn sc iu ch t dch pha(SPM):

- Tng ng k khong cch trm lp nn gim s trm lp trn tuyn. Yu

t ny cng gp phn lm gim gi thnh thit b trn tuyn.

- Cho php tn dng s si G.652 c sn trn tuyn



60/82


*Nhc im ca phng php b tn sc iu ch t dch pha (SPM)

- Dng xung yu cu l RZ trong khi hin nay dng xung ang s dng l

NRZ. Nh vy mun s dng k thut b tn sc ny th phI thay dng xung

ang truyn trn tuyn.

- C th xy ra hin tng nn xung khng mong mun do d b b qu

- Phng php ny yu cu rng ph lase phI tt.

3.2.3 Phng php b tn sc PDC

a) Nguyn l ca phng php b tn sc PDC: Nguyn l c bn ca phng

php ny l s dng cc phn t b tn sc, chng hn nh dng cc si c

thit k c bit hoc cch t si, b kt hp pha. Hin nay phn ln cc b b

tn sc m thnh phn c bn l si quang c th b c gi tr c 1500ps

bc sng 1550nm. Sau y l phng php b tn sc bng phn t th ng

in hnh: b bng b kt hp pha quang (OPC).

Trong phng php ny OPC s b cho c tn sc dng v tn sc m.

Qu trnh b tn sc khng ph thuc vo qu trnh iu bin quang.

Hnh 3.3 phng php b tn sc OPC

Hnh 3.3 minh ho tn sc c b. Tn hiu Es c cng sut quang P1

c pht vo on si c di L1, tn sc D1 v h s phi tuyn 1 . B b

pha s thc hin bin i h thnh sng kt hp pha Ec ( ). Ec c cng sut

quang P2 c pht dc theo on si c di L2, tn sc D2 v h s phi

tuyn2 . Tn sc s c b khi t s gia tn sc v cng hiu ng Kerr

quang ti cc v tr L1 v L2 bng nhau.


)()(

)(

)()(

)(

2222

22

1111

11

LpLy

LD

LpLY

LD=

60


61/82


b) nh gi kh nng s dng ca k thut OPC b tn sc

* u im:

- Thit b b tn sc hon ton th ng.

- B c khong tn sc ln.

* Nhc im:

- Suy hao ca b b tn sc ln v ph thuc vo khong tn sc phi b

- Phi gim st cng sut tn hiu truyn trnh cc hin tng hiu ng.

3.2.4 Phng php b tn sc bng Pre-chirp (dch tn trc)

a) Nguyn l ca phng php b tn sc Pre-chirp.

Nguyn l ca phng php ny thc hin dch ph trong khong thi

gian ca xung quang. Ni cch khc, Pre-chirp l s sp t lai bc sng sao

cho nh sng c bc sng di hn bc sng trung bnh tp trung sn ln

v nh sng c bc sng ngn hn bc sng trung bnh tp trung sn

xung ca xung tn hiu pht. Khi tn hiu truyn trong si cc bc sng dI

hn s b dch chuyn nhiu hn. Do vy nu chn khong cch truyn hp lth xung s khng b gin u thu, tc l trnh c nh hng ca tn sc.

b) nh gi kh nng s dng ca phng php ny.

Phng php ny c nhc im l ch b c tn sc trong mt khong

nh. Hn na s dng phng php ny i hi k thut pha pht cao.

Pre-chirp thng c kt hp trong u pht b mt phn chirp do ngun

pht gy ra, do n phi kt hp vi mt s phng php b tn sc nh

PDC, DST Cho tn hiu truyn trn tuyn th mi b hon ton tn sc gy ra

trn tuyn.

3.3 nh hng ca hiu ng phi tuyn n truyn dn

Khi cng sut trong si quang nh th si quang c xem l mi trng

tuyn tnh,tnh phi tuyn ca si quang(ch yu do chit sut) c th b

qua.Tuy nhin khi cng sut trong si quang vt qu mc no ,tnh phi

tuyn ca si quang s nh hng ti qu trnh truyn dn tn hiu trong si



62/82


quang.Khi xut hin hiu ng phi tuyn,ngha l suy hao v chit sut ph

thuc cng sut tn hiu quang trong si.Hiu ng phi tuyn s gy ra mt s

hin tng nh:xuyn m gia cc knh,suy gim mc tn hiu ca tng knh

dn n suy gim t s S/N vv...3.3.1 Cc hiu ng phi tuyn trong h thng WDM

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

- Hiu ng tn x:bao gm cc hiu ng tn x Raman(SRS)v hiu ng

tn x Brillouin(SRB).

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

gm cc hiu ng t iu ch dch pha(SPM),iu ch pha cho(XPM)v trnbn bc sng(FWM).

*Hiu ng Raman (SRS): l qu trnh tn x khng n hi, trong

photon 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, phn nng lng cn li

c pht x thnh nh sng c bc sng di hn bc sng ti. Bc sng

mi ny c gi l bc sng Stoke. Khi nh sng truyn trong si c cng

ln th qu trnh ny tr thnh qu trnh kch thch, trong nh sng ng

vai tr sng bm lm cho phn nng lng ca tn hiu chuyn vo bc sng

Stoke. Trong ghp knh theo bc sng, hiu ng SRS hn ch s knh ghp,

khong cch gia cc bc sng, cng sut pht mi knh v c li truyn dn

ca h thng. Ngoi ra, nu bc sng Stoke trng vi cc bc sng tn hiu

s gy ra xuyn m.

Hiu ng SRS t l thun vi cng sut nh sng P v t l nghch vi hiudng ca li si quang. Nu gi SRS l nh hng ca hiu ng SRS th SRS ~

P/Aeff

*Hiu ng SBS: cng tng t nh hiu ng SRS. Ngha l t n cng

hnh thnh cc bc sng Stoke. im khc nhau gia hai hiu ng ny l hiu

ng SBS lin quan ti cc photon m, cn hiu ng SRS li lin quan n cc

photon quang. Do c s khc nhau m chng c nh hng khc nhau n

h thng ghp bc sng quang. Trong tn x SBS, mt phn nh sng b tn



63/82


x do cc photon m v dch ti bc sng di hn. Phn nh sng tn x

ngc truyn theo si quang v nu cc bc sng ca tn hiu chnh trong

WDM cng truyn theo mt hng th hiu ng SBS khng gy xuyn m gia

cc knh. Tt nhin, nu truyn hai hng cc bc sng trn cng mt siquang th nh hng ca SBS l ng k.

nh hng ca SBS cng t l thun vi cng sut tn hiu nh sng v t

l nghch vi Aeff ca li si. Nu gi SBS l nh hng ca hiu ng SBS th

SBS ~ P/Aeff.

*Hiu ngBrillouin (SPM): l hin tng khi cng quang a vo

thay i, hiu sut khc x ca si quang cng bin i theo, gy s bin ipha ca sng quang. Sau khi kt hp vi tn sc ca si quang s dn n tn

ph dn rng v tch lu theo s tng ln ca chiu di. S bin i cng sut

quang cng nhanh th bin i tn s quang cng cng ln, gy nh hng ln

ti h thng tc cao.

Hiu ng SPM l s ph thuc chit sut ca li si vo cng nh

sng truyn trong .

n = no + n2I (3.1)

Vi no l chit sut tuyn tnh.

n2 l h s chit sut phi tuyn (n2 = 3,2.10-20 m2/W

v ph thuc vo bc sng )

I l cng quang.

Khi cng quang ln th chit sut ph thuc vo cng quangtc thi. Nu nh sng truyn trong si di L th dch pha ca tn hiu quang s

b sung thm mt lng l:

NL(t) = 2n2LI(t)/ (3.2)

o hm phng trnh theo thi gian ta nhn c

(t) = d/dt = (2n2L/)(dI/dt) (3.3)



64/82


Biu thc 3.2, 3 .3 cho bit dch pha ph thuc vo cng nh sng v

hin tng ny gi l t iu ch pha. N cn c xem nh hin tng t iu

ch tn b xung, hay cn gi l chirp.

i vi trng quang c cng khng i, hiu ng SPM ch lm quaypha ca trng quang do t nh hng n cht lng ca h thng. Tuy

nhin, i vi cc trng quang c cng thay i (nh cc xung trong h

thng thng tin s) th pha phi tuyn s thay i theo thi gian nh cng thc

1.3. S thay i theo thi gian cng c ngha l trong xung tn hiu s tn ti

nhiu tn s quang khc vi tn s trung tm mt gi tr l

f = (-1/2

)(d

/dt) (3.4)

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

xung dch n tn s nh hn tn s trung tm v sn trc dch n tn s

ln hn tn s trung tm. iu ny c ngha l ph ca tn hiu b dn rng ra

trong qu trnh truyn. i vi h thng WDM, khi cc knh quang qu gn

nhau th hin tng dn ph do SPM gy ra s dn ti giao thoa v nhiu gia

cc knh. nh hng ca hiu ng SPM cng t l thun vi cng sut quang v

t l nghch vi tit din hiu dng ca si quang.

*Hiu ng iu ch pha cho(XPM):i vi h thng WDM th chit sut

ti mt bc sng no ph thuc khng nhng vo cng quang ca

bc sng m cn ph thuc vo cng quang ca cc bc sng khc.

Trong trng hp ny, chit sut phi tuyn ng vi bc sng th i s l:

nNL = n2(Ii+ 2N IiIj) (3.5)

Trong N l tng s knh quang.

Ii l cng ca bc sng th i ang xt

Ij l cng quang ca bc sng thj ln cn.

Trong phng trnh 3.5 ta thy: S hng th nht ng vi hiu ng SPM,

s hng th hai tng ng vi hiu ng XPM. Nu gi s cng sut ca cc

knh l nh nhau th nh hng ca hiu ng XPM s gp 2N ln hiu ng



65/82


SPM. Do vy, trong h thng WDM s dng si n mode thng thng c th

gim hiu ng XPM bng cch tng khong cch gia cc bc sng ghp.

*Hiu ng trn bn bc sng (FWM): Trong h thng WDM, chit

sut ph thuc cng quang khng ch gy ra hin tng t iu pha m cngy ra hin tng trn bn sng. Trong hiu ng ny, 2 hoc 3 sng quang c

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

2f1 f2, f1 + f2 f3, 2f2 f3,... y gi l trn bn bc sng, ngha l t cc tn

s ban u to ra tn s th t c t hp l t cc tn s trn v nm st cc tn

s .

Do vic to ra tn s mi t cc tn s c nn cng sut cc tn s c s

gim i ng k. Hn na, khi khong cch cc knh l bng nhau th cc tn

s mi uc to ra s ri vo mt trong nhng knh tn hiu v gy nhiu, lm

suy gim cht lng h thng.nh hng ca hiu ng FWM cng ln nu nh

khong cch gia cc knh trong h thngWDM 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 li truyn dn. Nu gi FWM l nh

hng ca hiu ng FWM th FWM ~ P2

/(Aeff2

D2

). vi D l h s tn sc ca siquang. Tng s cc thnh phn mi c to ra c th tnh nh sau:

M = (N3 N2)/2 (3.6)

Trong N l s knh ban u.

3.3.2 Gii php khc phc hiu ng phi tuyn ca si quang

T phn tch trn ta thy c cc gii php khc phc sau:

+ La chn si quang ph hp.

+Gim cng sut pht ca cc knh...

Khi s knh tn hiu khong cnh t mi n vi chc knh th nh

hng ca hiu ng FWM l chnh.V mc tiu t ra l gim nh hng ca

hiu ng FWM.C 3 phng n:

a)phng n to khong cnh gia cc knh tn hiu bng nhau:



66/82


Phng n ny c khong cnh gia cc knh bc sng bng nhau v s

dng si quang NZ-DSF(G.655).Nh vy va gim nh hng ca FWM,h

tr truyn dn nhiu knh,li lm cho tn sc vng bc sng 1550 nm khng

qu ln(1-6ps/nm.km),thun li cho vic truyn tn hiu tc 10Gbit/s.Phng n ny s lm nn tng cho cng ngh DWDM.

b)phng n to khong cnh gia cc knh tn hiu khng bng nhau:

Phng n ny lm cho thnhphn tn s mi sinh ra bi hiu ng FWM

phn ln bn ngoi bng thng ca b lc.T c th gim c rt nhiu

nh hng ca hiu ng FWM,nng cao cng sut quang ti a ca cc knh.

c)Phng n to mt phn khong cnh gia cc knh tn hiu bng nhauKhuyt im chnh ca phng n b l chim dng bng tn qu ln,gp

1,8ln so vi phng n c.V bng tn ca h thng thc t l c hn,do

phng n b khng thch hp vi h thng WDM c s knh bc sng tng

i nhiu.T ngi ta ngh ra phng n to mt phn khong cnh gia cc

knh tn hiu bng nhau.Phng n ny chim bng tn ca h thng ch bng

58% phng n b,v vy s lng bc sng c th ghp tng ln rt nhiu.

3.4 bng phng ca tng ch b khuych i quang si

EDFA s dng trong h thng WDM c mt yu cu c bit, l tng

ch bng phng.Nh bit cc b EDFA c tng ch khng ng u ,do

cc bc sng khc nhau s c tng ch vi cc h s khc nhau. Qua nhiu

tng khuych i ,sai lch ca tng ch tch lu li. iu s lm gim cp

nghim trng SNR ca cc tn hiu c bc sng vng h s tng ch

thp.Mt khc n cnh lm tng hiu ng phi tuyn ca cc tn hiu c bcsng vng c h s tng ch cao.Tm li lm gim cht lng ca h thng.

Do mc tiu t ra l cn ch to cc b EDFA c h s tng ch phi tng

i bng phng,m bo chnh lch tng ch trn cc knh nm trong phm vi

cho php.

3.5 Tch lu tp m khi dng b khuych i quang EDFA nhiu tng

V l lun, h s tp m ca b khuch i EDFA c th t ti 3 dB(Thng th trong thc t l c 4 6 dB). iu nyc ngha l qua khuch i



67/82


EDFA, SNR xu i t nht 1 ti 2 ln. Mc xu i ca SNR c quan h ti

s lng b khuch i EDFA nhiu tng v khong cch on si quang gia

cc b khuch i vi nhau. Khong cch ca on si quang gia cc b

khuch i cng ln th s xu i ca SNR cng nghim trng. Do khi xcnh tng c li truyn dn khng c b chuyn tip th cn lm cho di ca

chng nh hn 120 km (tng ng vi suy hao 33 dB) nhm m bo tnh

nng truyn dn v mc SNR.

CHNG IV

NG DNG CA H THNG WDM4.1 NG DNG WDM TRONG MNG TRUYN DN

4.1.1 Tuyn truyn dn im - im dung lng cao

Trong tuyn thng tin im-im ng di, tc ca mt knh thng

nh hn 10Gb/s. WDM c vai tr tng dung lng tuyn truyn dn. Hnh 4.1

ch ra s khi mt tuyn thng tin WDM. Mi knh tn hiu in c iu

ch vi mt sng mang ring. Sau chng c ghp li v truyn n uthu. Gi s c N knh vi tc B1,B2,...,BN c truyn ng thi qua si

quang c chiu di L. Tch tng s tc v khong cchBL oc xc inh nh

sau:

BL= (B1 +B2+...+BN)L (4-1)


Siquang

1

T

xT

x

Tx

R

xR

x

Rx

2

n

1

2

n

Hnh 4.1 Tuyn thng tin quang WDM im -im

n hng dung lng cao

MUX

DMUX

67


68/82


Khi tc ca cc knh bng nhau, dung lng ca h thng tng ln N ln.

Nm 1985, BL t n 1,37Tb/s-km bc sng 1500nm bng cch ghp 10

knh 2Gb/s, chiu di tuyn 68,3 km, khong cch knh 1,35nm. Do tn sc, gitrBL ca mt h thng n knh ch nh hn 1Tb/s-km.

Dung lng ca mt tuyn WDM ph thuc vo khong cch gia cc knh

trong min bc sng. Khong cch ti thiu gia cc knh b gii hn bi

xuyn nhiu gia cc knh. Khong cch gia cc knh thng ln hn bn ln

tc bt. ca s 1500nm, vng suy hao thp ca si quang khong 120nm

(hnh 1.1). V d, i vi cc knh c tc 2,5Gb/s, khong cch ti thiugia cc knh l 10GHz hoc 0,1nm, ta c th ghp 1200 knh trn bng thng

120nm. Do , tng dung lng ln ti 3Tb/s. Nu mt tuyn thng tin khng

s dng trm lp in hoc b khuch i quang th khong cch truyn dn l

80km,BL hiu dng l 240Tb/s-km. Trong mt h thng quang n knh c

kh nng truyn tc bit 2,5 Gb/s, chiu di tuyn l 80 km thBL l 0,2 Tb/s-

km.

Trong thc t, c rt nhiu yu t hn ch vic s dng ton b ca s

suy hao thp ca si quang. V d, b khuch i quang c di khuych i

ng u thp, iu ny gii hn s lng knh truyn trn si quang. Bng

thng ca EDFA thng t 30 n 35 nm, ngay c khi s dung cng ngh lm

phng h s khuych i. Mt s yu t khc cng lm hn ch s lng knh

l n nh v kh nng iu chnh ca laser phn b hi tip, s suy gim tn

hiu trong qu trnh truyn dn gy ra bi hiu ng phi tuyn, nhiu xuyn knh

trong qu trnh ghp knh.

Hin nay c rt nhiu ng dng ca h thng thng tin quang WDM

im-im dung lng cao cp thc nghim. Da vo khong cch truyn,

c th chia h thng WDM thc nghim thnh hai loi, loi c khong cch

truyn c 100 km v loi trn 1000 km.

Nm 1985 tn ti h thng tin quang WDM thc nghim gm 10 knh

2Gb/s v khong cch truyn khng dng trm lp l 68,3 km. Nm 1995 h



69/82


thng thng tin quang WDM dung lng 340 Gb/s ra i bng cch ghp 17

knh tc 20 Gb/s v khong cch truyn 150 km. Nhng ngay trong nm ny

c ba h thng WDM thc nghim khc ra i vi dung lng trn 1 Tb/s.

Th nht l h thng WDM 55 knh, khong cch gia cc knh 0,8 nm, tc mi knh 20 Gb/s v khong cch truyn 150 km, s dng 2 b khuych i.

Do , dung lng h thng l 1,1 Tb/s v BL bng 165 Tb/s-km. Th hai l h

thng 50 knh, mi knh c tc 20 Gb/s, khong cch truyn l 55 km. Th

ba l h thng ghp 10 knh 100Gb/s vi khong cch truyn 55 km. H thng

ny s dung k thut WDM kt hp vi TDM (mi knh 100 Gb/s c to

thnh bng cch ghp 10 knh 10Gb/s theo k thut TDM). n cui nm 1996

xut hin h thng c dung lng 2,64 Tb/s bng cch ghp 132 knh,khong cch knh 0.27 nm. Bng 4.1 lit k mt s h thng WDM thc

nghim trc nm 1995.

Bng 4.1 Mt s h thng WDM thc nghim

S knh

N

Tc bit

B(Gb/s

)

Dung ng

N.B(Gb/s)

Khong cch

L (km)NBL

(Tb/s-km)

10 100 1000 40 40

16 10 160 531 85

32 10 320 640 205

32 5 160 9300 1488

50 20 1000 55 55

55 20 1100 150 165

132 20 2640 120 317

Nhm th hai trong h thng WDM thc nghim c khong cch truyn

trn 1000km. Nhm ny c chia lm hai loi, mng ng thng v mng

vng. Nm 1994 tuyn quang ng thng 40 Gb/s vi khong cch truyn

1420 km ra i. H thng ny c to thnh t vic ghp 16 knh 2,5 Gb/sv c khong cch gia hai b khuych i l 100km. Tip theo l hng lot



70/82


cc h thng mi ra i c dung lng v khong cch truyn tng ln. Trong

mt th nghim to ra h thng 20 Gb/s (ghp 8 knh 2,5 Gb/s), khong cch

truyn 6000 km, khong cch gia hai b khuych i l 75 km. Mt th

nghim khc cng to ra h thng 40 Gb/s (ghp 8 knh 5 Gb/s), khong cchtruyn 4500 km v s dng m RZ ci thin t s tn hiu trn tp m. Nm

1996 cho ra i h thng WDM thc nghim dung lng 320 Gb/s (ghp 16

knh 20 Gb/s) di tuyn 531 km. i vi loi loop vng si quang tn ti

h thng 100 Gb/s (ghp 20 knh 5 Gb/s) truyn vt i dng vi khong

cch 9100 km. H thng ny c s dng thm k thut ngu nhin ho phn cc

v sa li trc.

Nh vy vi s ra i ca h thng WDM, dung lng ca h thng thng

tin quang tng ln rt nhiu. Mt s h thng WDM c trin khai trong

thc t. Nm 1995, h thng WDM 4 knh, mi knh c tc 2,5 Gb/s

c thng mi ho. n nm 1996, h thng WDM dung lng 40 Gb/s

(ghp 16 knh 2,5 Gb/s) bt u c thng mi ho. H thng ny s dng

di bng tn 12nm bc sng 1550 nm v khong cch gia cc knh 0,8 nm.

n nm 1997 tn ti tuyn thng tin quang 80 Gb/s tip l h thng 160

Gb/s (ghp 16 knh 10 Gb/s). K thut WDM to ra mt cuc cch mng

trong h thng thng tin quang.

4.1.2 Mng qung b

Trong mng qung b, nhiu knh tn hiu c ghp li v truyn ti

mt nhm thu bao. Ti mi thu bao, b thu thc hin chc nng la chn mt

knh thch hp thng qua vic tch knh. Mng truyn hnh cp CATV l mt

v d ca mng qung b. Nhiu knh truyn hnh c ghp li v a ti mtbus quang chung, sau phn phi ti tng thu bao ring bit. Hnh 4.2 l s

ca mng WDM qung b s dng sao qung b. Mi knh tn hiu c

iu ch vi mt bc sng ring. Cc knh ny c ghp vi nhau nh mt

coupler quang th ng v c phn phi ng u ti tt c cc b thu. Mi

thu bao nhn c ton b dung lng ca h thng v la chn mt knh

thch hp. Mng ny cn c gi l mng pht qung b v thu la chn. Nu

h thng thng tin quang s dng k thut tch sng coherent th pha thu c th



71/82


la chn knh bng cch thay i bng thng b lc trung tn hoc iu chnh

tn s dao ng ni. Nu h thng s dng k thut tch sng trc tip th ta

dng cc b lc quang la chn bc sng cn thit.

Hin nay tn ti rt nhiu ng dng ca mng qung b cp th nghim.Gn y c mt th nghim v sao qung b 8x8 phn phi 7 knh tn hiu,

khong cch cc knh 15 nm. Mi b thu c mt b lc kh chnh bng thng

10 nm v di iu chnh 400 nm. Ngoi ra cn mt s th nghim khc s dng

k thut tch sng coherent, thc hin ghp 10 knh quang 70Gb/s, v khong

cch knh 6GHz. Th nghim ny s dng sao qung b 128 x128 a tn

hiu ti nhiu thu bao khc nhau.

Trong mng qung b, c hai vn cn c quan tm l suy hao phn bv suy hao xen. Suy hao phn b l do tn hiu trn ng truyn c chia u

ti tt c cc thu bao. Mi thu bao nhn mt phn cng sut ca tn hiu tng.

Suy hao phn b tng khiNtng, do n gii hnNnh hn 100 nu khng

s dng cc b khuch i quang b suy hao. i vi sao qung b Nx N,

suy hao phn b gim v cng sut tn hiu m mi thu bao nhn c l:

PR =(PT /N) (1- ) log2N (1-CL) (4-2)

Trong :PT l cng sut truyn trung bnh ca tn hiu


1

1

2

.

.N

2

N

1,

2,...

N

Bpht

Bthu khchnh

Saoqung b

Hnh 4.2 Mng WDM qung b hnh sao

71


72/82


l suy hao xen ca mi coupler cu to nn sao qung b

CLl cc loi suy hao khc nh suy hao si quang, suy hao connector

Cng sut nhn cPRphi ln hn nhy my thuPrec .Precc xc

nh nh sau:

Prec= NPhB (4-3)

Trong :NP l s lng photon trung bnh trn mt bit ti mc cng sut

vo bng nhy my thu.

B l tc bit ca mi knh tn hiu.

T cng thc (4-2),(4-3) v gi thitPR = Prec ta c:

( ) )1(1 2log

L

N

P

T ChN

PBN

=

(4-4)

T cng thc (4-4) ta c th d bo dung lng ca mng qung b. Trong

my thu coherentNP = 1000, cng sut truyn ln nht PT = 1mW, bc sng

1550 nm nng lng photon h= 0,8 eV. Gi s suy hao coupler v cc suy hao

khc u bng khng. Khi , B.N = 78 Tb/s. Trong trng hp tch sng trctipNP =100 th B.N= 7,8 Tb/s. Trong thc t, gi tr B.Nthng nh hn v

lun tn ti suy hao coupler, mt s suy hao khc v cng sut pht quang cng

nh hn.

Nguyn Tnnh Chung D2001VTHnh 4.3 th biu din s ph thuc ca BN

vo s lng knh N

72


73/82


Hnh 4.3 ch ra s ph thuc ca B.Nvo s lng knhNca hai phng

php tch sng coherent v tch sng trc tip. B.N= 10 Tb/s trong trng hp

l tng khng c suy hao ca coupler quang (ng 1). Khi ly = 0.05 (suy

hao ca coupler bng 0,2dB) ta c ng 2. Hai ng cong 3 v 4 biu din

B.Ntrong trng hp thc t. Cc ng t nt biu din B.Nvi cc tc

bit c nh. T th ta c, khi B =622Mb/s v tch sng trc tip th B.N=

1Tb/s,N=1600. Khi tch sng coherent thB.N= 3 Tb/s vN=4800.

Ta c th khc phc cc nh hng ca suy hao phn b bng cch s

dng cc b khuych i quang. Nm 1990 c mt th nghim s dng hai b

khuych i quang EDFA trong mng qung b, mng ny c dung lng

khong 39,5 triu thu bao. B khuych i c chc nng b phn nng lng

b suy hao do phn b, m bo cng sut tn hiu ti mi thu bao l ln.

Ngoi ra, cn mt h thng th nghim na c s thu bao l 43,8 triu, dunglng ca h thng 39,81Gb/s v khong cch truyn dn l 507km.

4.2 ng dng ca WDM trong mng a truy nhp

4.2.1 M u

Trong thc t c rt nhiu kiu a truy nhp khc nhau nh TDMA,

CDMA, FDMA, WDMA. Cc k thut ny da vo ti nguyn thi gian, khng

gian, tn s v bc sng phn phi tn hiu t trm thu ti trm pht. MngWDM a truy nhp cn gi l mng quang a truy nhp theo bc sng

(WDMA). im khc bit ln nht gia mng quang a truy nhp v mng

qung b l mng a truy nhp c kh nng p ng truy nhp song hng ca

mt thu bao bt k. Mi thu bao c th thu/pht tn hiu t/ti bt k mt thu

bao khc.

Trong mng WDMA, bng thng rng ca si quang c chia thnh cc

khong nh, mi khong ny mang mt knh quang ring bit. Cc knh ny



74/82


c truyn ng thi trn cng mt si quang. Khong cch ti thiu gia hai

bc sng c 0,4 n 0,8 nm. Mi bc sng ny c th mang mt knh tn

hiu c tc ln ti Gb/s. Hnh 4.4 m t s khi ca mt mng truyn dn

quang a truy nhp phn chia theo bc sng.

Cc knh quang t cc nt khc nhau c ghp vi nhau nh mt coupler

quang N xN. B ghp trn tt c cc tn hiu n v chia u cng sut ti ccb thu. Kt qu l, tn hiu t tt c cc cng vo c th c thu t cc cng ra

bt k. H thng c s chia s bc sng, t mt nt bt k c th thu c knh

chung trong mi trng chia s.

Tt c cc knh c pht vo mi trng chia s (coupler sao) v mi

nt thu tn hiu ca tt c cc nt cn li trn mng. Mi trng chia s c cu

trc mng sao, bus hoc mng ring, kt ni tt c cc nt vi nhau. Mi nt thu

tn hiu mong mun bng phng php tch sng trc tip hoc coherent. Mi


Hnh 4.4 S khi mng truyn dn quang

a truy nhp phn chia theo bc sng

Node 1

Node 3

Node 2

NodeN

CouplerNxN

74


75/82


nt thu/pht tn s c nh hoc thay i. Do , vic trin khai mng WDMA

i hi cc thnh phn quang c kh nng iu chnh bc sng nh laser pht

kh chnh, b lc quang kh chnh. Cc thnh phn ny xy dng nn cc b

thu pht quang c kh nng iu chnh bc sng mi nt mng.Mng WDMA c hai vn cn c quan tm. l tc iu chnh

bc sng v giao thc mng. Tc iu chnh bc sng phi nhanh p

ng yu cu ca mng, c bit trong mng chuyn mch gi. Ngoi ra, giao

thc c trin khai trong mng phi m bo kt ni ngang hng cc knh tn

hiu khc nhau. Mng WDMA c hai cu hnh c bn l WDMA n chng

(single hop) v WDMA a chng (multi hop).

4.2.2 Mng WDMA n chng

Trong mng WDMA n chng mi nt u c kh nng kt ni trc tip

n tt c cc nt khc. D liu c pht i di dng nh sng v c

truyn trc tip n nt ch m khng phi chuyn v dng tn hiu in.

mt gi d liu c truyn, trc ht n c pht vo mng nh mt laser

pht. Ti nt ch b thu quang phi iu chnh bc sng sao cho trng vi

bc sng pht. Khi , gi tin c truyn qua mng ti nt ch. MngWDMA n chng c th chia lm hai loi, pht qung b thu la chn v

mng WDMA nh tuyn theo bc sng.

Trong mng WDMA n chng pht qung b thu la chn, d liu ti

cc nt pht ra c ghp li v pht qung b ti tt cc nt khc trong mng.

Pha thu la chn knh tn hiu mong mun da vo bc sng. C bn loi

mng WDMA pht qung b thu la chn, pht thay i thu c nh (TT-FR),

pht c nh thu thay i (FT-TR), c pht v thu cng thay i (TT-TR) v c

pht v thu c dnh (FT-FR). Mng WDMA n chng s dng cc b nh

tuyn theo bc sng hoc ma trn chuyn mch khng gian bc sng.

Trong thc t, c rt nhiu ng dng ca mng WDMA n chng cp

thc nghim hoc thc t. Mt s ng dng ca mng WDMA l mng

quang th ng PON (telephone PON v Broad PON), mng Lambdanet, mng

quang th ng PPL, mng Rainbow. Mng Lambdanet l mng quang u tinc trin khai cp th nghim. Cu trc mng ny c cho nh hnh 4.5



76/82


Mng Lambdanet l mng pht qung b thu la chn hnh sao FT-FR.

Trong mng ny s dng mt coupler hnh sao N xN phn phi tn hiu ti

tt c cc nt. Mi nt c mt b pht ring bc sng xc nh v N b thu

hot ng N bc sng khc nhau (N l s ngi s dng hay s nt), mi ntnhn ton b lu lng ca mng. Do , mng ny khng b tc nghn m

truyn tn hiu trong sut khng ph thuc vo tc bit v phng php iu

ch. Nhng ngi s dng khc nhau c th truyn tn hiu c tc bit khc

nhau v dng iu ch khc nhau, c th truyn tn hiu s hoc tn hiu tng

t. V vy mng Lambdanet rt linh hot v thch hp cho nhiu loi ng dng

khc nhau. Ta c th truyn tn hiu thoi trong cng mt c quan. Nm 1987

c mt th nghim v mng 18 knh tc bit mt knh l 1,5 Gb/s, dung lngh thng l 27 Gb/s. Mi knh c th truyn khong cch 57,8 km. Nhc

im ca mng Lambdanet l s lng ngi s dng b gii hn bi s lng

bc sng. Ngoi ra, mi nt cn c rt nhiu b thu (bng s lng ngi s

dng). Do rt tn km khi u t phn cng cho h thng

Khi s dng b thu kh chnh vo mng Lambdanet lm gim gi thnh v

phc tp ca h thng. Mng nh vy c gi l mng Rainbow. Mng nyc kh nng kt ni 32 nt, mi knh c tc 1Gb/s v khong cch truyn t

10 n 20 km.

Mng s dng coupler sao th ng (hnh 4.5) kt ni nhiu my tnh.

B lc quang kh chnh lc ra bc sng thch hp cho mi nt. Hn ch ca

mng Rainbow l tc iu chnh ca b thu chm., khng th p ng cho

mng chuyn mch gi. Cc mng WDM s dng coupler sao th ng c

gi l mng quang th ng (PON). Mi nt thu ton b lu lng. PON mang

tn hiu quang ti tn nh. Hnh 4.6 ch ra s khi ca mng vng quang th

ng, bc sng quang c dng nh tuyn tn hiu trn mng vng ni

ht. Trm trung tm c N b pht cc bc sng 1,, 2... N v N b thu hot

ng cc bc sng N+1,, N+2... 2N (N l s thu bao). Ti mi thu bao thu

v pht cc bc sng ring. Trm xa ghp tn hiu t cc thu bao v gi ti


Coupler

Sao NxN

G

iaodinin

Las

er Tx

1Bthu

WDM

DEMUX

Giaodinin

Laser Tx

2B

thu

WD

MDEMUX

Nt

4

Nt

3

Nt

N

Hnh 4.5 Cu trc mng Lambdanet

Nt 1

Nt 2

76


77/82


trm trung tm. Trm xa l thit b th ng nn tn t chi ph bo dng. B

chuyn mch trm xa nh tuyn tn hiu da vo bc sng.

Nm 1996, mng vng ni ht c thit k nh l mt mng quang th

ng PON. Mc ch ca vic ny l cung cp kh nng truy nhp bng thngrng ca mi khch hng v phn pht tn hiu hnh nh video, d liu theo yu

cu m vn m bo gi thnh thp. Ngoi ra ngi ta cn s dng cng ngh

ct ph LED to ra cc bc sang cho h thng WDM


Hnh 4.6 S khi ca mng vng quang

th ng ni ht

Trm trungtm

1

SW

I

TCH

N

Bpht

MUX/DMUX

N

B thu

MUX/

DMUX

T

hubao 1

Thu

bao 2

Thu

baoN

N+1

2

N+2

N2N

Trm xa

Siquang

77


78/82


4.2.3 Mng WDMA a chng

Trong mng a chng, mt knh quang c chuyn i t mt nt phi

c chuyn qua mt s nt trung gian. Mi nt thu/pht quang ca mng

WDMA a chng c mt s b thu pht quang c th thu pht mt vi bc

sng nht nh.

Kt ni trc tip xy ra khi bc sng c nh trc ti nt ch trng

vi mt trong nhng bc sng c nh trc nt pht. Kt ni gia hai nt

bt k c nh tuyn qua cc nt trung gian. Ti mi nt trung gian, d liu

c chuyn thnh tn hiu in. a ch ch ca gi c gii m v x l

di dng in. Sau , gi tin li c chuyn thnh tn hiu quang c bc

sng thch hp chuyn ti nt ch hay nt trung gian k tip. Qu trnh c

lp li cho ti ch. Do vy, gi tin c chuyn qua nhiu chng, gi tin phi

qua mt s nt trung gian trc khi i ti nt ch cui cng. S lng nt

trung gian ph thuc vo thit k v qui m mng.

Hnh 4.7 ch ra mt v d ca mng a chng c 8 nt. Gi s gi tin c

truyn t nt 1 ti nt 3. Ta khng th thit lp c kt ni trc tip gia hai

nt ny. Do , gi tin phi qua mt s nt trung gian. Gi s nt 1 pht i gi

tin bc sng 2 , gi tin c chuyn ti nt 6. Ti nt 6 c qu trnh bin

i quang in, s dng bc sng 11 truyn ti nt 3. Hng i ny gm

hai chng. Ngoi ra, gi tin c th i theo cc hng khc vi s chng ln hn

hoc bng.

Mt u im c bn ca mng a chng so vi mng n chng l khng

nht thit phi c thit b iu khin knh v mi nt mng hot ng nh mt

trm lp v c nhim v quyt nh nhn gi tin hay chuyn gi tin i tip ti

cc nt khc trong mng. Kt ni trong mng a chng c thc hin linh hot

bng nhiu cch khc nhau, bng nhng con ng khc nhau. Chnh v vy,


T R

T R

TR

T

R

T

R

R

T

R

T

TR

1, 2

9, 13

Coupler

saoNxN

9

,10

1,

5

3, 4

10, 14

5, 6

11, 15

7, 6

12, 16

2, 6

11, 10

13, 14

3, 7

15, 16

8, 4

11

2

Nt 1

Nt 2

Nt 8

Nt 4

Nt 6

Nt 5Hnh 4.7 Mt mng a chng 8 nt

78


79/82


mng a chng gim c hin tng tc nghn ng truyn so vi mng n

chng.

Hin nay c mt s cu trc ca mng WDMA a chng. V d, mng a

chng c cu trc hnh a din. Cu trc ny thng c s dng ghp

nhiu b x l ca mt my tnh ln. Trong mng a chng 8 nt th 8 nt ny

nm 8 gc ca hnh lp phng. Trong mng N nt (N =2 m) th mi nt ct ti mt nh ca hnh a din. Mi nt c ni trc tip vi m nt khc.

Khi truyn tn hiu t mt chng ti mt chng khc qua nhiu nht m chng.

S lng chng trung bnh phi qua l m/2. Mi nt cn c m b thu. Ta c th

s dng mt s k thut khc gim s b thu. Nhng khi s chng trung

bnh li tng ln.

Mt s mng WDMA a chng c trin khai cp thc nghim

nh mng Teranet hoc Starnet.


N

N

79


80/82


Mng Teranet l mng thc nghim c trin khai trng i hc

Columbia di dng truyn mch t bo ATM 1Gb/s hoc chuyn mch knh

1Gb/s. Hnh 4.8 m t s khi chc nng ca mt nt trong mng Teranet.

Thit b thu quang ca mt nt gm b lc quang kh chnh t trc mt

photodiode v mt b lc trung tn c tn s c nh. Thit b thu quang l laser

DFB c bc sng nh trc. Thit b chuyn mch l ma trn chuyn mch

in 3 x3, tc cng vo ra l 1 Gb/s. Ma trn chuyn mch nh tuyn gi tin

ti mt cng ra thch hp tu thuc vo a ch ch ca gi tin.

Mng Starnet l mng LAN WDMA Coherent c xy dng trng i

hc Standford h thng ny h tr c mng WDMA n chng chuyn mchknh v a chng chuyn mch gi 100Mb/s. Mi nt c mt b pht v hai b

thu. H thng hot ng bc sng trung tm 1319nm, si quang n mode,

khong cch truyn 2km, cng sut d tr 10dB, t l li bit 10-9

4.3 ng dng ca WDM trong mng chuyn mch quang

Hin nay, s pht trin ca cc dch v thoi v phi thoi m c bit l

Internet cng nh mt s dch v bng rng khc to ra s bng n nhu cuv dung lng. K thut WDM ra i gii quyt c vn trn. Nh phn


1

1

1

2

Chuynmch

ATM3x3

1,

2,...

N

1

2

T ura ca nt

khc

1,

2,...

N

1 , 2,... N 1, 2,... N

1 , 2,... N 1, 2,... N

1 , 2,... N 1, 2,... N

nu vo cant khc

Biu chRF khchnh

LaserDBF

3

4

Hnh 4.8 S khi chc nng ca mt nt trongmng

ung dung cua he thong wdm - Tại 123doc.vn

Documents

Transcript of ung dung cua he thong wdm - Tại 123doc.vn