Chuyen de Quang 2_ban In

7/31/2019 Chuyen de Quang 2_ban In

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Tiu lun Cc chuyn thng tin quang

GVHD: Th.S Cao H ng Sn1 SVTH: Nhm 18 Lp L10CQVT12

MC LC

LI NI U ................................................................................................................ 2

CC TVIT TT ....................................................................................................... 3DANH MC CC HNH V ........................................................................................ 3

DANH MC CC BNG BIU ................................................................................... 4

DANH SCH SINH VIN THC HIN ...................................................................... 4

CHNG 1: CU TRC V NGUYN L HOT NG B KHUCH I

EDFA .............................................................................................................................. 5

1.1. Cu trc EDFA .................................................................................................. 5

1.2. Nguyn l khuch i trong EDFA. .................................................................. 6

1.2.1. Gin phn bnng lng ca Er3+. ....................................................... 6

1.2.2. Nguyn l hot ng ca EDFA ................................................................ 8

1.3. Cc yu cu i vi ngun bm...................................................................... 10

1.3.1. Bc sng bm......................................................................................... 10

1.3.2. Cng sut bm. ......................................................................................... 12

1.3.3. Hng bm............................................................................................... 12

CHNG 2: CC THNG S CA B KHUCH I EDFA ............................. 14

2.1. Phtng ch v bng tn ................................................................................. 14

2.2. H s khuch i ............................................................................................. 15

2.3. Tng ch bo ha ............................................................................................. 16

2.4. Ph ASE (Amplified Spontaneous Emission) ................................................. 17

2.5. Thng s tp m .............................................................................................. 18

2.6. Cc ng dng ca khuch i EDFA trong mng truyn dn quang .............. 21

KT LUN ................................................................................................................... 24

TI LIU THAM KHO ............................................................................................ 24


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LI NI U

Th k 21, k nguyn vi s pht trin v cng mnh m ca cng ngh thng

tin ni chung v k thut vin thng ni ring. Nhu cu dch v vin thng pht trin

rt nhanh to ra p lc ngy cng cao i vi tng dung lng thng tin.

Cng vi s pht trin ca k thut chuyn mch, k thut truyn dn cng

khng ngng t c nhng thnh tu to ln, c bit l k thut truyn dn trn mi

trngcp si quang. Tng lai cp si quang c s dng rng ri trn mng vin

thng v c coi nh l mt mi trng truyn dn l tng m khng c mt mi

trng truyn dn no c th thay th c. Cc h thng thng tin quang vi u im

bng thng rng, c li xa, khng b nh hng ca nhiu v kh nng bo mt cao ph

hp vi cc tuyn thng tin xuyn lc a ng trc, trung k v c tim nng to ln

trong vic thc hin ccchc nng ca mng ni ht vi cc cu trc linh hot v p

ng mi loi hnh dch v hin ti v tng lai.

Gn y thc hin thnh cng vic khuch i trc tip tn hiu quang m

khng cn phi thng qua bt k mt qu trnh bin i v inno. K thut mi ny

cho php khc phc c nhiu hn ch ca trm lp nh: hn ch v bng tn truyndn, cu trc phc tp lm cho k thut truyn dn trn cp si quang cng t r tnh

u vit ca n.

ng trc vn ny em chn ti Tm hiu v b khuch i quang

si pha trn ERBIUM EDFA bo co cho mn hc Cc chuyn thng tin

quang.

Em xin cm n thy Cao Hng Sn gip em hon thnh bi bo co ny.

Do hn ch v thi gian v kin thc nn khng trnh khi thiu st cng nh nhm

ln, em mong nhn c kin ng gp bi bo co c hon thin hn. Em xin

chn thnh cm n.

H Ni ngy 23 thng 7 nm 2012


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CC TVIT TT

Vit tt Ting Anh Ngha

EDFA Erbium Doped Fiber Amplifier B khuch i quang si pha trn

Erbium

EDF Erbium Doped Fiber Si quang pha ion t him Erbium

FWHM Full Width at Half Maximum rng phna bin

ASE Amplified Spontaneous

Emission

Pht x t pht c khuch i

NF Noise Figure H s tp m

SNR Signal Noise Ratio T s tn hiu trn tp m

DANH MC CC HNH V

Hnh 1.1: Cu trc tng qut mt b khuch i EDFA ................................................ 5

Hnh 1.2: Mt ct ngang ca mt loi si quang pha ion Erbium .................................. 5

Hnh 1.3: Gin phn bnng lng ca ion Er3+ trong si silica ............................ 7

Hnh 1.4: Ph hp th (absorption spectrum) v ph li (gain spectrum) ca EDFA

c li pha Ge ................................................................................................................... 8

Hnh 1.5: Qu trnh khuch i tn hiu xy ra ............................................................. 9

Hnh 1.6: Cu hnh bm thun ...................................................................................... 12

Hnh 1.7: Cu hnh bm ngc..................................................................................... 12

Hnh 1.8: Cu hnh bm hai chiu ................................................................................ 13

Hnh 2.1: Ph ASE vi cng sut bm l 4, 6, 8, 15 v 20mW ................................... 17

Hnh 2.2: S ph thuc ca thng s tp m theo: (a) tng ch v (b) cng sut bm . 21

Hnh 2.3: Khuch i cng sut .................................................................................... 21

Hnh 2.4: Khuch i trn tuyn ................................................................................... 22

Hnh 2.5: Khuch i cng sut .................................................................................... 23


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DANH MC CC BNG BIU

Bng 1.1: So snh hai bc sng bm 980nm v 1480nm .......................................... 11

DANH SCH SINH VIN THC HIN

1. o Xun Bch2. Bi Cng Anh Dng3. Nguyn Xun Sn


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CHNG 1: CU TRC V NGUYN L HOT NG B

KHUCH I EDFA

1.1. Cu trc EDFACu trc ca mt b khuch i quang si pha trn Erbium EDFA (Erbium-

Doped Fiber Amplifier) c minh ha trong hnh 1.1. Trong bao gm:

- Si quang pha ion t him Erbium EDF (Erbium Doped Fiber): l ni xy raqu trnh khuch i (vng tch cc) ca EDFA. Cu to ca si quang pha ion

Er3+ c minh ha nh trn hnh 1.2.

Hnh 1.1: Cu trc tng qut mt b khuch i EDFA

Hnh 1.2: Mt ct ngang ca mt loi si quang pha ion Erbium


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Trong , vng li trung tm (c ng knh t 3 -6 m) ca EDF c pha

trn ion Er3+ l ni c cng sng bm v tn hiu cao nht. Vic pha cc ion

Er3+ trong vng ny cung cp s chng lp ca nng lng bm v tn hiu vi cc

ion erbium ln nht dn n s khuch i tt hn. Lp bc (cladding) c chit sutthp hn bao quanh vng li. Lp ph (coating) bo v bao quanh si quang to bn

knh si quang tng cng l 250 m. Lp ph ny c chit sut ln hn so vi lp bc

dng loi b bt k nh sng khng mong mun no lan truyn trong si quang.

Nu khng kn cht pha erbium, cu trc EDF ging nh si n mode chun trong

vin thng. Ngoi ra, EDF cn c ch to bng cc bng cc loi vt liu khc nh

si thy tinh flouride (flouride-based glass fiber) hoc si quang thy tinh a vt liu

(multicomponent glass fiber).

- Laser bm (pumping laser): cung cp nng lng nh sng to ra trng thinghch o nng trong vng tch cc. Laser bm pht ra nh sng c bc

sng 980nm hoc 1480nm.

- WDM Coupler: Ghp tn hiu quang cn khuch i v nh sng tlaser bmvo trong si quang. Loi coupler c s dng l WDM coupler cho php

ghp cc tn hiu c bc sng 980/1550nm hoc 1480/1550nm.- B cch ly quang (Optical isolator): ngn khng cho tn hiu quang c

khuch i phn xngc vpha u pht hoc cc tn hiu quang trn ng

truyn phn xngc v EDFA.

1.2. Nguyn l khuch i trong EDFA.1.2.1. Gin phn bnng lng ca Er3+.

Gin phn b nng lng ca Er3+ trong si silica c minh ha trong

hnh 1.3. Theo , cc ion Er3+ c th tn ti nhiu vng nng lng khc nhau c

k hiu: 4I15/2 ,4I13/2 ,

4I11/2 ,4I9/2 ,

4F9/2,4S9/2 ,

4H11/2. Trong :

- Vng 4I15/2 c mc nng lng thp nht, c gi l vng nn (ground-stateband).

- Vng 4I13/2c gi l vng gi bn (mestable band) v cc ion Er3+ c thigian sng (lifetime) ti vng ny lu (khong 10ms) trc khi chuyn xung


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vng nn. Thi gian sng ny thay i ty theo loi tp cht c pha trong li

ca EDF.

- Vng 4I11/2 , 4I9/2 , 4F9/2, 4S9/2 , 4H11/2 l cc vng nng lng cao, c gi lvng kch thch hay vng bm (pumping band). Thi gian cc ion Er3+ ctrng thi nng lng trong cc vng ny rt ngn (khong 1s)

Hnh 1.3: Gin phn bnng lng ca ion Er3+ trong si silica

S chuyn i nng lng ca cc ion Er3+ c th xy ra trong cc trng hp

sau:

- Khi cc ion Er3+ vng nn nhn mt mc nng lng bng chnh lchnng lng gia vng nn v vng nng lng cao hn, chng s chuyn lnvng c mc nng lng cao hn (s hp thnng lng).

- Khi cc ion Er3+ chuyn tcc vng nng lng cao xung vng nng lngthp hn s xy ra hai trng hp sau:

Phn r khng bc x(nonradiative decay): nng lng c gii phngdi dng photon to ra sdao ng phn t trong si quang.

Pht x nh sng (radiation): nng lng c gii phng di dngphoton.


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chnh lch nng lng gia vng gi bn (4I13/2) v vng nn4I15/2:

- 0.775eV (tng ng vi nng lng ca photon c bc sng 1600nm) tnh ty vng gi bn n nh ca vng nn.

- 0.814eV (1527 nm) tnh ty vng gi bn n y ca vng nn.- 0.841 eV (1477nm) tnh tnh vng gi bn n y ca vng nn.

Mt phn b nng lng ca cc ion Er3+ trong vng gi bn khng u

nhau: cc ion Er3+ c khuynh hng tp trung nhiu cc mc nng lng thp. iu

ny dn n khnng hp th v pht x photon ca ion Erbium thay i theo bc

sng. Ph hp th (absortion spectrum) v ph li (gain spectrum) ca EDFA c li

pha Ge c biu din trn hnh 1.4

Hnh 1.4: Ph hp th (absorption spectrum) v ph li (gain spectrum) ca EDFA

c li pha Ge

1.2.2. Nguyn l hot ng ca EDFANguyn l khuch i ca EDFA c da trn hin tng pht x kch thch.

Qu trnh khuch i tn hiu quang trong EDFA c thc thc hin theo cc

bc nh sau (xem hnh 1.5):

Khi s dng ngun bm laser 980nm, cc ion Er3+vng nn s hp thnng

lng tcc photon (c nng lng Ephoton =1.27eV) v chuyn ln trng thi nng

lng cao hn vng bm (pumping band) (1)


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Ti vng bm, cc ion Er3+ phn r khng bc x rt nhanh (khong 1s) v

chuyn xung vng gi bn (2)

Khi s dng ngun bm laser 1480nm, cc ion Er3+ vng nn s hp th

nng lng tcc photon (c nng lng Ephoton=0.841eV) v chuyn sang trng thi

nng lng cao hn nh ca vng gi bn (3)

Cc ion Er3+ trong vng gi bn lun c khuynh hng chuyn xung vng

nng lng thp (vng c mt in t cao) (4)

Sau khong thi gian sng (khong 10ms), nu khng c kch thch bi cc

photon c nng lng thch hp (pht x kch thch) cc ion Er3+ s chuyn sang trng

thi nng lng thp hn vng nn v pht x ra photon (pht x t pht) (5).

Hnh 1.5: Qu trnh khuch i tn hiu xy ra EDFA vi hai bc sng bm 980 nm

v 1480nm

Khi cho tn hiu nh sng i vo EDFA, s xy ra ng thi hai hin tng sau:

- Cc photon tn hiu b hp th bi cc ion Er3+vng nn (6). Tn hiu nhsng b suy hao

- Cc photon tn hiu kch thch cc ion Er3+vng gi bn (7). Hin tng phtx kch thch xy ra. Khi , cc ion Er3+ b kch thch s chuyn trng thi nng

lng t mc nng lng cao vng gi bn xung mc nng lng thp


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vng nn v pht x ra photon mi c cng hng truyn, cng phn cc, cng

pha v cng bc sng. Tn hiu nh sng c khuch i.

rng gia vng gi bn v vng nn cho php s pht x kch thch (khuch

i) xy ra trong khong bc sng 1530 nm1565nm. y cng l vng bc snghot ng ca EDFA. li khuch i gim nhanh chng ti cc bc sng ln hn

1565 nm v bng 0 dB ti bc sng 1616 nm.

1.3. Cc yu cu i vi ngun bm.1.3.1. Bc sng bm.

Vi cc vng nng lng ca Er3+nh sng bm c thc s dng ti cc

bc sng khc nhau 650 nm (4F9/2), 800 nm (4I9/2), 980 nm (

4I11/2), 1480 nm (4I13/2).

Tuy nhin, khi bc sng bm cng ngn th cc ion Er3+ phi tri qua nhiu giai on

chuyn i nng lng trc khi trv vng nn v pht x ra photon nh sng. Do

, hiu sut bm khng cao, nng lng bm s b hao ph qua vic to ra cc

phonon thay v photon. V vy, trn thc t, nh sng bm s dng cho EDFA ch

c s dng ti hai bc sng 980nm v 1480nm.

Trong EDFA, iu kin c khuch i tn hiu l t c s nghch o

nng bng cch s dng ngun bm bm cc ion erbium ln trng thi kch

thch. C hai cch thc hin qu trnh ny: bm trc tip ti bc sng 1480 nm hoc

bm gin tip bc sng 980 nm.

- Phng php bm gin tip (bm 980 nm): Trong trng h p ny, ionerbium lin tc c chuyn tip tvng nng lng 4I15/2 thp ln vng nng

lng cao 4I11/2 sau cc ion s phn r xung vng4I13/2 nhng khng pht

x. T vng ny, khi c nh sng kch thch th cc ion s pht xbc sng

mong mun (t1550 n 1600 nm) khi chuyn tvng nng lng4

I13/2 xungvng 4I15/2. y chnh l h thng ba mc. Thi gian sng ca ion erbium

mc 4I11/2 khong 1s trong khi 4I13/2 th ti 10ms. Vi thi gian sng di,

vng 4I15/2c gi l vng n nh. V vy, cc ion c bm ln mc cao,

sau nhanh chng ri xung vng 4I13/2 v tn ti trong mt khong thi

gian tng i di to nn s nghch o v nng .

- Vi phng php bm trc tip (1480 nm): cc ion erbium ch hot ng tronghai vng nng lng 4I13/2 v 4I15/2 . y l h thng 2 mc. Cc ion erbium


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lin tc c chuyn tvng nng lng nn 4I15/2 ln vng nng lng kch

thch 4I13/2 nhnng lng bm. V thi gian tn ti mc ny di nn chng

tch ly ti y to ra s nghch o nng .

Ngun bm c hiu qu cao chai bc sng 980 v 1480 nm. c h skhuch i hn 20 dB th ch cn to ra ngun bm c cng sut nh hn 5 mW,

nhng vn cn phi c ngun bm t10 n 100 mW m bo cho cng sut ra

ln.

Ch s nhiu lng t gii hn l 3 dB t c bc sng 980 nm. i vi

bc sng 1480 nm th ch s nhiu l vo khong 4 dB v tit din ngang pht x

ti1480 nm cao hn ti 980 nm v s bc x kch thch do ngun bm gii hn s

nghch o tch lu ti 1480nm. Do , bc sng bm 980 nm c ng dng cho

cc b khuch i tp m thp. H s li ti bc sng bm 980 nm cao hn t i

1480 nm ti cng cng sut bm. Do , t c cng mt h s li th cng

sut bm ti 1480 nm phi cao hn ti 980 nm. V cng sut bm 1480 nm ln hn

nn cng sut ng ra ln hn, do bm bc sng 1480nm c ng dng cho cc

b khuch i cng sut. Ngoi ra, bc sng bm 1480 nm c truyn trong si

quang vi suy hao thp. Do , ngun bm laser c tht xa b khuch i.

Hin nay, bm bc sng 1480 nm c s dng rng ri hn v chng sn c

hn v tin cy cao hn. tin cy l c im quan trng i vi laser bm v n

dng bm cho khong cch di v trnh lm nhiu tn hiu. Cc thit b khuch

i cng sut i hi cng sut bm cao nht v n nh ca chng l mu cht

trong qu trnh nghin cu pht trin chng. Nu tng c n nh ca laser c

bc sng 980 nm th c th chng sc chn lm ngun bm. Mt sEDFA c

bm ti chai bc sng tn dng u im ca chai bc sng.Bc sng bm 980nm 1400nm

Tnh cht :- li- li cng sut bm- Suy hao cng sut bm- H s nhiu

Cao hnThp hnCao hnThp hn

Thp hnCao hnThp hnCao hn

ng dng Tin khuch i Khuch i cng sut

Bng 1.1: So snh hai bc sng bm 980nm v 1480nm


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1.3.2. Cng sut bm.Cng sut bm cng ln th s c nhiu ion erbium b kch thch trao i

nng lng vi tn hiu cn khuch i v s lm cho h s khuch i tng ln. Tuy

nhin, h s khuch i khng th tng mi theo cng sut bm v s lng cc ionerbium c cy vo si l c gii hn. Ngoi ra, khi cng sut bm tng ln th h s

nhiu s gim.

1.3.3. Hng bm.B khuch i EDFA c thc bm theo ba cch:

a) Bm thun (codirectional pumping):

Hnh 1.6: Cu hnh bm thunNgun bm c bm cng chiu vi hng truyn tn hiu. Hng bm thun

c u im nhiu thp v nhiu kh nhy cm vi li m li tn hiu cao nht khi

cng sut tn hiu vo thp nht.

b) Bm ngc (counterdirectional pumping):

Hnh 1.7: Cu hnh bm ngc


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Ngun bm c bm ngc chiu vi hng truyn tn hiu. u im ca cu

hnh ny l cng sut quang li ra cao, khng bnh hng ca hp thngc v rt

tin li trong vic tng cng cng sut tn hiu vo tuyn cp quang. Nhc im ca

cu hnh ny l tp m kh ln do khuch i bc x t pht (ASE AmplifedSpontaneous Emission).

c) Bm hai chiu (dual pumping):

Hnh 1.8: Cu hnh bm hai chiu

Khi kt hp c hai cu hnh trn ta sc c cc u im ni bt nh cng

sut quang li ra cao v tp m tng i thp.


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CHNG 2: CC THNG S CA B KHUCH I EDFA

2.1. Phtng ch v bng tnH s tng ch quang trong mi trng khuch i ph thuc vo tn s (hay

bc sng) ca tn hiu quang ti v cng chm nh sng khuch i ti cc im

khc nhau ca b khuch i quang. n gin, chng ta gi thit mi trng

khuch i l mt h hai mc nng lng m rng ng nht. H s tng ch g()

trong mi trng khuch i c tnh bng biu thc sau:

() ( ) Trong :

: l gi trtng ch cc i : l tn s ca tn hiu quang ti

: l tn s chuyn di nguyn tP : l cng sut quang ca chm sng ti

: l cng sut bo ha ca b khuch i, ph thuc vo cc thng s ca mitrng khuch i nh T1 thi gian pht x hunh quang (flourescence time) v tit

din dch chuyn ca nguyn t kch thch. T2 l thi gian phc hi lng cc (dipolerelaxtion time) thng c gi tr rt nh (T2


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Bng tn tng ch c xc nh bng rng ca ph tng ch g() ti im

gia gi tr cc i (FWHMFull Width at Half Maximum).

Vi phLorentz, bng tn tng ch c cho bi: hay:

2.2. H s khuch i

H s khuch i G l t s gia cng sut tn hiu li ra trn tn hiu li vo,

v c tnh theo n v dB.

() ( )Ngi ta c th tnh h s khuch i quang theo cch khc: Tn hiu v bm

truyn dc theo chiu di si, xt phn bnng lng theo trc z. Khi tn hiu vo b

khuch i nh ta c phn bnng lng bm dc theo si xem nh ng u, khi

cng sut tn hiu quang tng theo hm m.

P(z) l cng sut quang ti ta z tnh t li vo b khuch i:

()()()

Vi () , . Khi z = L ( di b khuch i) th:( ) ()()Vy : () ()H s khuch i quang trong mi trng khuch i ph thuc vo tn s ca

tn hiu quang ti, cng chm sng khuch i ti cc im khc nhau ca b

khuch i quang v chiu di si quang pha tp Er3+.

Bng tn khuch i:

[ ( )]

trong : ()(bng tn khuch i ti im gia cc i G()


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2.3. Tng ch bo haTa c tng ch () () trong cng thc ny g() gim khi P

tng gn bng Ps v h s khuch i G gim khi tng cng sut quang ca tn hiu ti,hin tng ny gi l tng ch bo ha.

Xt trng hp tn s nh sng ti , t thay biu thc ca g() tphng trnh vo phng trnh ta sthu c s bin thin cng sut tn hiu dc theo

si quang pha tp Er3+nh sau:

S dng cc iu kin ( ) v ( ) . Ta c:

(

)

Ta thy h s khuch i G gim t gi trcha bo ha G0 (gi tr cc i) khi

cng sut tn hiu ra Pout gn bng vi Ps .

Cng sut ca tn hiu ra bo ha c xc nh khi G gim mt na (tcl gim i 3dB) so vi G0 (G=G0/2). Ta c:

Cng sut ra bo ha nhhn cng sut bo ha khong 30%.. Thc

vy, qua thc nghim ngi ta thy do G0 >>2 (G0 =1000 hay 30dB) nn

() . Chng ta khng nn nhm ln gia

vi

l mt thng s bn

trong ca mt b EDFA khng ph thuc vo cng sut vo, ra hay di si khuch

i.

Cng sut ra bo ha l mt thng s quan trng ca b EDFA. Khi thit k cc

b khuch i quang, ta thng tnh ton mc khuch i bo ha, gim thiu

nng lng d tha trong si. V nng lng d tha s to nn nhiu do khuch i

bc x t pht (Amplified Spontaneous EmissionASE).


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2.4. Ph ASE (Amplified Spontaneous Emission)Cc ion kch thch mc laser trn vn c chuyn di t pht v mc di v

pht bc x t pht trong vng khuch i, bc x ny khng c tnh kt hp vi

photon tn hiu, v c khuch i ln v tr thnh ngun nhiu ng k trong

khuch i quang si.

Khi c tn hiu li vo b khuch i, li ra lun c nhiu do ASE lm xu i

cht lng ca tn hiu. Vi chbm thch hp v tn hiu vo ln th nhiu

ASE s yu i.

Ph ca khuch i bc x t pht ASE c dng gn ging vi ph ca h s

khuch i nn n cung cp cc thng tin c ch vcc c tnh ca b EDFA. Dng

ph ASE s ph thuc theo cng sut bm, c bit l ti v tr bc sng 1530nm.

Ban u khi cng sut bm thp, nghch o tch ly cn yu th nh ph ASE ti

bc sng 1530nm rt thp do xc sut hp th v bc x l gn nhau (theo ph hp

th v bc x ca ion Er3+). Khi tng dng bm, nghch o tch ly ngy cng tng

ln v t ti chbo ha th nh ph ASE ti vtr bc sng 1530nm s cao dn

ln v khi ti ch bo ha th n t cc i ti v tr cao nht. iu ny l do tich bo ha, xc sut bc x ti bc sng 1530nm l ln nht.

Hnh 2.1: Ph ASE vi cng sut bm l 4, 6, 8, 15 v 20mW

(a) ng hng v (b) ngc hng


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Hnh 2.1 cho thy s thay i dng ph ASE theo cng sut bm. Vi ASE

ng hng, ta c th thy r c s dch nh ph ca ASE ti v tr bc sng

1530nm nhng vi ASE ngc hng th li khc. Vi ASE ngc hng, ch cn

mt cng sut bm rt nhcng c th to c nghch o tch ly kh tt ti usi. Chnh v vy nh phASE ngc hng ti bc sng 1530nm u nh cao.

Ph ASE s yu i khi cng tn hiu quang trong si mnh ln. V vy

im cn lu khi thit k b khuch i EDFA cn thit k sao cho n hot ng

ch gn bo ha.

Khi c tn hiu li vo b khuch i, li ra lun c nhiu do ASE lm xu i

cht lng ca tn hiu. Vi chbm thch hp v tn hiu vo ln th nhiu

ASE s yu i. chnh l thng s ti u cn quan tm khi tnh ton v kho st.

Vi cc ngun nhiu trong mch in t c nhiu cch lc b nhiu, nhng

i vi ngun nhiu ASE ta khng c bin php lc bi c. c bit nu trn

tuyn c qu nhiu b khuch i EDFA th nhiu ASE stch ly dn v tn hiu s

khng dng c na.

2.5. Thng s tp mEDFA l thit bt trn ng truyn tn hiu, nn yu cu khng c lm

mo dng tn hiu v pht sinh tp m ln, h s khuch i khng ph thuc nhiu

vo bc sng v phn cc ca tn hiu vo.

Trong thng tin quang thnh phn nhiu ch yu do cc b khuch i quang,

vi khuch i quang si pha tp Er3+ thnh phn nhiu ch yu l do ASE v s thay

i cng quang li ra ca b khuch i. Thnh phn ASE l do bn cht vt l

ca h thng, sn nh cng quang pht ra do cht lng ca blaser bm.Tp m ca b khuch i c nh gi qua thng s hnh nh nhiu (Noise

FigureNF) theo biu thc sau:

()()Trong SNR l t s tn hiu trn tp m (Signal Noise Ratio SNR), c

tnh theo cng sut in hnh thnh do tn hiu quang chuyn thnh dng in. Ni


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chung ph thuc vo cc thng s ca bthu quang, c bit l tp m nhit trongb thu quang.

Xt b khuch i c h s khuch i G: Ta c t s tn hiu trn tp m li vo l:

()

Trong :

: l dng quang in trung bnh : l t stng thch ca bthu quang l tng vi

(): l tp m shot khi dng ti

h : l hng s Planck (h = 6,625.1034Js)

: l tn s tn hiu quang.

: l bng tn b thu. tnh SNR li ra ca b khuch i ta cn tnh thm thnh phn tp m do

pht x ngu nhin vo tp m b thu. Mt ph ca tp m pht x ngu nhin cm

ng (spontaneous-emission induced noise) l mt hng s (tp m trng) c th tnh

theo cng thc sau: () ( ) Trong c gi l yu t pht x ngu nhin hay yu to mt c

cho bi:

Vi N1 v N2 ln lt l mt nguyn ttrng thi c bn v trng thi kch

thch. Pht x ngu nhin ng gp thm nhiu lon vo cng sut khuch i vpht x ngu nhin lun i km vi tn hiu khuch i.

Dng quang in do pht x ngu nhin ng gp l :()

Trong l cng sut pht x ngu nhin trong bng tn khuchi v l lch pha ngu nhin. Gi thit cc ngun tp m khc c b qua,

sthay i dng quang in s l: ()()Trong c gi tr trung bnh l


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T s SNR ca tn hiu ra (tn hiu khuch i):

()

()

()()

Ta s c hnh nh nhiu ca b khuch i l:

() () ( )

Do tp m trong cc b khuch i quang ch yu sinh ra do khuch i bc x

t pht ASE, ta c:

( )

Thay vo ta c th tnh NF bng biu thc sau:

Hay

()(

)

Trong , l cng sut ASE, l nng lng photon, G l tng ch ca bkhuch i v l di thng tng ng vi rng ph ca tn hiu.

Ngi ta xc nh trong vng tng ch cao G >> 1, thng s tp m lun ln

hn mt gi trxc nh gi l gii hn lng t. Gii hn nh nht ny bng 3dB.

Nh vy, thng s tp m t l nghch vi tng ch ca b khuch i. Khi h s

khuch i cng ln th tp m cng nh. iu ny tng ng vi vic khi chng tatng cng sut bm th tp m s nh dn.


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Hnh 2.2: S ph thuc ca thng s tp m theo: (a) tng ch v (b) cng sut bm

Tuy nhin, khi cng sut bm vt qu ngng xc nh bo ha v tng ch G

t cc i th thng s tp m tng rt nhanh. Khi ch bo ha, cng sut bm

d tha s chlm tng nhiu ASE m khng lm khuch i tn hiu thm na. Chnh

v vy vic xc nh cng sut bm bo ha l rt cn thit, n va gip trnh lng ph

ngun bm, va lm gim thiu ti a tp m.

2.6. Cc ng dng ca khuch i EDFA trong mng truyn dn quanga) Khuch i cng sut:

Hnh 2.3: Khuch i cng sut

Khuch i cng sut tng cng cng sut quang vo ng truyn. Trong

trng hp ny, EDFA c cng sut bo ha cao v c t ngay sau ngun pht

nhm khuch i cng sut tn hiu truyn i. B khuch i ny c s dng trong

trng hp ngun quang pht i b hn ch v mt cng sut. Ta c tht c cng

sut pht quang ln d dng nhcc bEDFA. Tuy nhin, trong trng hp EDFA c

h s khuch i ln th c th dn ti hiu ng phi tuyn si lm mt mt cng sut

h thng. c tnh phi tuyn si trthnh vn ng lu cho tt c cc h thng c


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dung lng cao v cc tuyn khng tram lp c ly xa. Cch b tr ny gim nh cng

sut pht ca laser iu ch tn hiu.

b) Khuch i trn tuyn

Hnh 2.4: Khuch i trn tuyn

Vi ng truyn tc thp nh hng ca tn sc khng ln, tham s cn

quan tm i vi h thng truyn dn l suy hao ca si. Khi truyn dn thng tin trnnhng khong cch rt di, nu nh ch s dng b khuch i cng sut hay b tin

khuch i skhng m bo. Trong trng hp ny ngi ta phi s dng cc b

EDFA lm cc b khuch i trn tuyn. Cc b khuch i trn tuyn sc b tr

thnh chui cch u nhau trn ton tuyn. Vi cc b khuch i trn tuyn, cc xung

nh sng c th lan truyn m khng b bin dng, duy tr c mc cng sut ti

thiu nhm khc phc nh hng ca cc hiu ng quang phi tuyn. V ASE sinh ra

trong cc b EDFA v tham gia vo h s khuch i nn ta cn duy tr ti u vo ca

tng b EDFA mt cng sut tn hiu ln. V mt l thuyt ta c th s dng cc

b khuch i trn tuyn ko di khong cch truyn dn ln n hng nghn km.

Tuy nhin, nu ta s dng nhiu b khuch i trn tuyn th cht lng truyn dn

ca h thng s suy gim do tp m tch ly, do cc hiu ng phn cc, tn sc tch ly

v cc hiu ng phi tuyn khc.

Mt trong cc u im ca cc b khuch i EDFA c s dng cho lp ttrn tuyn l n c lp t d dng v rt d bo tr.

Cc b khuch i trn tuyn c thtnh ton t v tr thun tin trn

tuyn thng tin, diu khin hot ng v cung cp ngun in. Mt strng

hp nh cp quang trong lng bin, ngi ta cng nghin cu bm cho EDFA t xa

nhlaser bc sng 1480nm.


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c) Tin khuch i

Hnh 2.5: Tin khuch i

B tin khuch i thc hin khuch i pha trc u thu. N khuch i

nhng tn hiu c cng sut nhngay trc u thu quang nng cao nhy u

thu. Tn hiu trc khi vo b khuch i ny thng l yu v bsuy hao trn ng

truyn. Khi cng sut quang n u thu tn hiu qu yu di ngng hot ng ca

u thu tn hiu, ngi ta cng dng khuch i quang EDFA. Thc t, khi EDFAc thng mi trn th trng, ngi ta ng dng n c hiu qu cho cc tuyn

thng tin tc cao nhiu Gigabite v c ly xa khng trm lp. Vi cc ng dng nh

vy, h thng thng tin yu cu c cng sut pht ln v nhy u thu cao. i vi

EDFA loi ny, chng ta khng cn quan tm n hin tng bo ha. Tuy nhin tn

hiu trc khi a vo EDFA bao gm c tp m do bc x tpht c khuch i

(ASE) nn cn gim ti a cng sut tn hiu vo nhm m bo t s tn hiu trn tp

m (SNRSignal Noise Ratio)


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KT LUN

Vi nhng tnh nng vt tri ca mnh, b khuch i quang EDFA l mt

thnh phn khng th thiu trong cc h thng thng tin quang. Khi kt hp b khuch

i EDFA vi k thut ghp knh theo bc sng, lc ny s to ra mt mng truyn

dn kh hon ho. EDFA s gip cho WDM pht huy ti a u im ca n.

TI LIU THAM KHO

1. K thut thng tin quang 2Th.S Nguyn Vn Vit Em.2. Khuch i quang si v kh nng ng dng trn mng vin thngV Tun

Lm.

3. G. Keiser. Optical Fiber Communications . Third edition, McGraw-Hill, 2000.

Chuyen de Quang 2_ban In

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