chuong 6 cac ung dung mang co ban

B khuch i quang bn dn; Thc mc xin li n h: [email protected]

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Chng 6: CC NG DNG MNG C BNCc ng dng c bn ca cc SOA trong cc h thng truyn thng

quang hc c th c chia thnh ba lnh vc chnh: Postamplifier hoc bkhuch i tng cng tng cng sut laser ca b truyn, khuch ing dy b cho s mt mt trong si quang hoc do qu tr nh lantruyn khc trong mi trng v cc ng truyn ti xa v cc b tinkhuch i ci tin nhy b thu. S hp nht cc b khuch i quanghc vo cc lin kt truyn thng quang hc c th ci tin tnh nng ca hthng v gim gi thnh. Cc SOA cng c th c dng thc hin ccchc nng khc chng hn nh chuyn mch quang hc vn tc cao , chuyni bc sng v pht hin ng dy. Nhiu ng dng chc nng nh thc tho lun trong chng 7.

Trong chng ny chng ta tp trung vo nhng ng dng truynthng ca SOA trong cc ng truyn v cc mng truyn thng quanghc. Chng ta bt u vi mt s tng quan ngn gn v cc vn c bnca pht hin quang hc trc tip v kt hp ri n ngun gc ca l thuytpht hin tn hiu nh sng khuch i. ng dng tip theo ca cc SOAnh mt b khuch i tng cng, b tin khuch i , b khuch ing dy cng c xem xt. Cc tnh cht ca SOA ng dy xp tngc phn tch. Tnh nng ca cc SOA trong cc mng WDM c tholun cng vi cc k thut c tn dng gim nhiu xuyn knh v mogia cc ln iu ch. Cui cng l mt s ch lin quan n vic s dngSOA trong cc h thng truyn tng t.6.1. TNG QUAN V TRUYN THNG QUANG HC KT HPV TRC TIP

Cc h thng truyn thng quang hc c th chia th nh truyn kt hphoc khng kt hp [1-3]. Trong truyn thng khng kt hp ch cng ca tn hiu mang quang hc c iu bin. Ti b thu, tn hiu c phthin trc tip, mt qu trnh ch nhy vi cng tn hiu. Nhng h thngnh th c gi l iu ch theo mt tch sng trc tip (IM-DD) .

Trong nhng h thng kt hp , tn hiu nh sng dao ng cc bc thm vo tn hiu mang quang hc ti trc khi c pht hin. Trongnhng s nh th, pha hoc tn s mang quang hc , cng nh cng ca n c th c pht hin. Trong truyn thng quang hc, thut ng kthp c dng ch bt c h thng no trong laser dao ng cc b


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c thm vo nh sng tn hiu iu bin ti trc khi pht hin, cho dpha v tn s ca tn hiu nh sng ti b b qua trong qu tr nh x l v giiiu bin sau ny.6.1.1. B thu iu ch theo mt tch sng trc tip

S ca my thu IMDD c bn c biu din trong hnh 6.1a.Trong s ny, tn hiu mang quang hc c iu bin cng cpht hin bi photodetector (diode p -i-n hoc photodiode kiu thcl_APD). Dng quang in cui cng c khuch i v c cho qua mtmch quyt nh quyt nh xem mi bit c nhn l mt du hoc khongtrng .

Hai phng php c th c dng quyt nh dng sng nhnc. Phng php u tin lin quan n ly mu dng sng nhn c mi chu k bit, thng gia bit v so snh vi gi tr c ly mu mcngng. Nu gi tr c ly mu nh hn mc ngng, bit nhn c chiu l trng v ngc li. Cch th hai l k thut integrate and dump dng sng nhn c c ly tch phn trn mt chu k bit. Sau ura ca b tch phn c so snh vi mc ngng v mt quyt nh tngt c thc hin nh trong phng php u tin. Phng php u c uim l ti thiu ha cc hiu ng nhiu giao thoa lin k t (ISI) trong khici sau, do vic ly tch phn, c khuynh hng cc tiu ha cc hiu ngnhiu. C hai k thut nhn i hi s to ra mt xung ng h cc b chocc mc ch ng b ha.


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H s cht lng thng thng ca mt b thu s l xc sut li bit(BER). B thu IMDD c th c phn tch nh sau. Chng ta gi s rngs iu bin bt tt (OOK) trong cc khong trng v cc du tng ngc biu din bng cc cng sut u vo bng khng v 2 sP . sP l cngsut c nhn trung bnh gi s rng xc sut truyn qua ca cc xung nhdu hoc khong trng bng nhau . Trng tn hiu u vo nphotodetector c th c biu din l

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( ) 2 cos(2 ) ( )

( ) 0 ( )( ) sE t P t

E tE t

cho xung anh dau

cho mot khoang trong(6.1)

y l tn s mang quang hc v t l thi gian. Dng quang in di tphotodetector t l vi bnh phng ca trng tn hiu nh sng n cly trung bnh trn mt chu k quang hc, tc l

2( )di R E t (6.2)

y ton t < > ch vic ly trung b nh theo thi gian trn mt chu kquang hc v R l p ng ph photodetector ( / )A W cho bi cng thc

eR

h

(6.3)

y l hiu sut lng t detector. Th (6.1) vo (6.2) cho ta c mtdng quang in tn hiu tng ng,

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2 ( )0( )( )

s s

s

I RPs II t

cho xung anh daucho mot khoang trong (6.4)

Trong (6.4), s hng ti 2 b b qua bi v n vt qu bng thng indetector. Cng vi dng quang in tn hiu, cn c nhng dng nhiu docc dng ti detector dI (mt dng quang in tn ti cho d khng c nhsng c pht hin ) vi phng sai 2di , dng tp nhiu ht tn hiu viphng sai 2si (do bn cht lng t ca tn hiu nh sng c pht hin ) vnhiu mch b thu vi phng sai l 2ci . Ngun nhiu sau ch yu l donhiu nhit b thu. Cc phng sai ca nhng dng nhiu ny c th cbiu din l

2

2

2

2

2

4

d e d

s e s

ec

L

i eB I

i eB I

kTFBi

R

(6.5)

y eB l bng thng gi-pht hin ca b thu , F l h s tp nhiu ca bthu , LR l in tr ti v T nhit ca b thu. i vi iu bin OOK ,phng sai dng nhiu ton phn i vi mt du v khong trng l:

2 2 21 12 2 20

2 (2

( )i d c e s

i d c

i i eB IiT i i

cho xung anh dau)cho mot khoang trong

(6.6)


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i vi b thu OOK truyn thng (dng phng php u tin trn), nu gi s rng dng nhiu c hm phn b cng dng Gauss,BER l:

1

2 2

QBER erfc

(6.7)

y tha s Q l1 0

2 21 0i i

S SQ

(6.8)

21 1sS I v 0 0S l cc cng sut tn hiu du v khong trng, erfc l hm sai

b22

( ) 1 ( ) u

x

erfc x erf x e du

(6.9)

y erf l hm sai s , erfc thng xut hin trong biu thc BER ca nhiuloi b thu. th ca erfc v BER theo Q c biu din trong hnh 6.2.i vi nhng gi tr x ln php gn ng sau l hu dng:

2

( ) ,xe

erfc xx

x ln (6.10)

nhy ca b thu c th c nh ngha l cng sut quang hc thutrung bnh hoc cc photon trn bit cn cho mt BER cho trc. Nhng hthng truyn thng quang hc thng c BER < 910 , n i hi rng Q>6 .Trong trng hp l tng, dng ti v nhiu mch c th b qua th:

2 121

s

i e

RPSQ

eB (6.11)


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Nu photodetector c hiu sut lng t duy nht v bng thng 1/e TB B , y TB l tc bit th:

2 2 bQ N (6.12) y / ( )b s TN P h B l s photon trn bit trung bnh. Gi tr ny c gi lgii hn lng t ( hoc gii hn tp nhiu ht ). Mt BER 910 cn

18bN . Nu bc sng tn hiu nhn v tc bit bit, s ny c thc dng thu c cng sut hoc nhy quang hc trung bnh cctiu cn thit ca b thu . Tuy nhin trong cc b thu thc t, hu nh khngth t c gii hn lng t. iu ny l do s nhiu trong mch , thngthng n ln hn nhiu ht nhiu bc v ln, lm ny sinh nhy nmtrong khong vi trm ngn photon trn bit . Phn tch trn cng b qua nhhng ca hin tng tn sc si quang. S tn sc si quang cng dn ns m rng xung v v th n ISI. iu ny lm tng BER v do gim nhy b thu. Nh chng ta s thy trong phn 6.3 .2, nhy ca b thuquang hc IMDD thc t c th c ci tin nhiu bng cch dng b tinkhuch i quang hc. Phn tch chi tit hn v nhy ca b thu quanghc v BER c th c tm thy trong [1-4].6.1.2 S tch sng nht qun

S ca b thu kt hp c bn c biu din trong hnh 6.1(b) .Trong loi b thu ny, mt laser dao ng cc b c thm vo tn hiumang quang hc ti c iu bin. Sau c hai tn hiu c pht hinng thi bng mt photodetector . Dng quang in cui cng c giiiu bin v c x l nhn c tn hiu d liu . C hai thun li chnhca cc h thng truyn thng quang hc kt hp so vi cc h thng IMDDtruyn thng. Trc ht, c th t ti nhy ca ca b thu ln hn (minl l tn hiu v nhiu pha dao ng cc b c gi trong gii hn chophp) v th hai l c th t c s chn lc b thu cao hn nhiu. Thunli u tin c ngha l c th tng khong cch gia cc b chuyn tip tisinh trong cc h thng nht qun so vi cc h thng IM-DD. Tuy nhin,thun li ny khng cn quan trng v s c mt ca cc b khuch ing dy v cc b tin khuch i trong cc h thng IM -DD. iu nyc ngha l s quan tm trong vic thc thi thng mi ca cc h thngtruyn thng quang hc kt hp b m dn, so vi s nhn nhp ca nhnghot ng ny nhng nm 1980 trc s kin b khuch i si quan gng tin cy v hiu sut cao. Thun li th hai c ngha l c th dng ghpknh phn chia tn s quang hc (OFDM) truyn nhiu knh quang hchn vi khong cch tn s rt mnh trn mt si quang. iu ny dn ns tn dng hiu qu hn bng thng si quang. Vic chn knh c thchin bng cch gi pht trong min in. Trong cc h thng IMDD, s


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ghp knh phn chia bc sng (WDM) cng c th c dng tng stn dng bng thng si quang; tuy nhin, trong trng hp ny s chn lcknh c thc hin trc khi pht hin dng cc b lc quang hc hoc btch knh theo bc sng. Khong cch knh ln hn nhiu so vi FDM.Trong bt c trng hp no, vic dng cc h thng IMDD cng vt xacc h thng kt hp. V l do ny, trong phn cn li ca sch chng ta tptrung vo cc h thng IMDD.6.2 KHOA HC THNG K V PHT HIN NH SNG KHUCHI

Vic thm vo pht x t pht (ngha l nhiu ) l mt h qu khngth trnh c ca khuch i nh sng . Vic dng b lc quang hc ti ura ca b khuch i c th gim ng k n hiu ny, tuy nhin khng thtrit tiu n hon ton. Khi tn hiu v nhiu km theo c pht hin biphotodetector, qu trnh pht hin c trng bnh phng lm ny sinh ccdng nhiu phch cng vi tp nhiu ht. Trong phn tch sau y , da trnphng php ca [5], biu thc c rt ra cho thnh phn nhiu ca dngquang in c to ra bi tn hiu khuch i quang hc . Vn cn xemxt c biu din trong hnh 6.3, y tn hiu phn cc tuyn tnh v ASEkm theo c cho qua mt knh phn cc v b lc quang hc trc khipht hin .


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B lc quang hc c hm truyn dng ch nht vi bng thng0B (Hz) xung quanh bc sng tn hiu. Photodetector c ph p ng l R

v bng thng gi - pht hin eB . Chc nng ca knh phn cc l cho tnhiu phn cc tuyn tnh i qua v gim phn na nhiu ASE. iu ny ldo mi photon pht x t pht c th tn ti mt trong hai ch phn ccvung gc vi nhau (gi s rng s phn cc khng nhy vi s khuch iquang hc). Nhiu ASE ti detector ASE c gi s c mt ph cngsut quang hc ng u trng thi phn cc n trn bng thng b lc.Gi thit ny c gi tr khi cc b lc quang hc c dng trong thc t cbng thng hp hn nhiu so vi bng thng ph SOA thng thng. Thnhphn tn hiu ca in trng ca detector c th c biu din l

( ) 2 os( )sig sig sigE t P c t (6.13)

y sigP v sig tng ng l cng sut tn hiu v tn s gc quang hc.Thnh phn ASE ca in trng u vo c th c biu din l

0

0

/2

A/2

( ) 2 os ( 2 )B

sp SE sig ii B

E t c i t

(6.14)

y l khong tn s nh v i l pha ngu nhin. Do in trngc pht hin ton phn l

( ) ( ) ( )sig spE t E t E t (6.15)

Dng quang in cui cng theo (6.2) l2

2 2

( ) ( )

( ) 2 ( ) ( ) ( )

d sig sp

sig sig sp sp

i R E t E t

R E t E t E t E t

(6.16)

S hng u tin v phi ca (6.16) n gin l dng quang in tn hiuc pht hin ( )s inI RP . N l mt qu trnh nhiu ht vi phng sai

22sig shot e sigi eB RP (6.17)

S hng th hai v phi ca (6.16) do s phch ca tn hiu vi ASE . Nc th c vit l:

2 cos(2 )M

sig sp in sp ii M

i R P i t

(6.18) y 0 / (2 )M B v cc s hng xung quanh tn s 2 sig c b

qua. Mi tn s 2 i trong (6.18) c hai thnh phn vi pha ngu nhin.iu ny c ngha l ph cng sut ca sig spi ng u trong khong tn s

0[0, / 2]B vi mt ph cng sut 2( / )A Hz .2 24sig sp sig ASER P (6.19)

V th phng sai ca dng nhiu phch tn hiu - t pht l:


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2 24sig sp sig ASE ei R P B (6.20)

S hng th ba v phi ca (6.16) l do s phch ca nhiu t pht vichnh n v c tnh bng cng thc

2 cos( ) cos( )M M

sig sp ASE i ji M j M

i R

(6.21)

Vi( 2 )

( 2 )i sig i

j sig j

i t

j t

(6.22)

Tnh (6.21) b qua cc s hng vi tn s xung quanh 2 sig , ta c2 2

0 0

os ( )2M M

sp sp ASE i ji j

i R c i j t

(6.23)

S hng ti tn s bng khng thu c khi i=j . C 2M s hng nh th nndng t pht trung bnh l

0sp ASEI R B (6.24)

Dng ny l mt qu trnh nhiu ht vi phng sai2

2sig shot e spi eB I (6.25)

Cc s hng cn li trong (6.23) c th c sp xp theo tn s ca chng.S cc s hng ti tn s l l 2M l , y l l s nguyn nm trongkhong (2M-1) n -1 v trong khong t 1 n 2M+1. Cc s hng vi trtuyt i ca cc tn s bng nhau cng pha v vy mt ph cng sutmt pha 2sp sp ca sp spi m rng t 0 ti 0B vi dng hnh tam gic. Bi v

0B ln hn nhiu so vi eB , chng ta ch cn quan tm n mt ph cngsut gn tn s bng khng v vy

2 2 202sp sp ASER B (6.26)

V th phng sai dng nhiu phch t pht - t pht l:2 2 2

02sp sp ASE ei R B B (6.27)

Phng sai dng quang in ton phn l tng ca cc phng saidng ni trn ngha l

2 2 2 22d sig shot sp shot sig sp sp spi i i i i (6.28)

Trong cc b thu thc t, s phn cc ca tn hiu u vo thngkhng th bit c v vy khng th dng knh phn cc. Trong trng hpny, phng sai dng nhiu ht t pht v phng sai dng phch t pht t pht phi c nhn i. Phng sai dng nhiu phch tn hiu-t phtkhng b nh hng v tn hiu ch c th phch vi nhiu c c ng trng thiphn cc nh chnh n.


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6.2.1. H s tp nhiu tng ng v mt in (electrically equivalentnoise figure) ca b khuch i quang hc

Mt h s phm cht hu dng ca b khuch i quang hc l h stp nhiu tng ng v mt in F, c nh ngha l t s gia ccSNR in u vo v ra ca b khuch i ngha l

( / )

( / )in

out

S NF

S N (6.29)

Cc SNR c tnh ton bng cch gi s rng tn hiu u vo v tnhiu u ra ca b khuch i cng ASE c cho i qua b lc quang hcdi hp trc khi c pht hin bi mt photodetector l tng (ngha lhiu sut lng t bng 1). Trong trng hp ny, ch c cc s hng nhiudng quang in cn c tnh ton l nhiu ht tn hiu v nhiu phch tnhiu - t pht. Nu cng sut tn hiu u vo ca b khuch i l sigP , thSNR u vo tng ng v mt in l

2( )

2sig

in e sig

RPS

N eB RP

(6.30)

SNR tng ng v mt in u ra b khuch i l 2

2

( )

2 4sig

out e sig sig ASE e

GRPS

N eB GRP R GP B

(6.31)

y G l li khuch i. Ly t s ca (6.30) vo (6.31) ta c21 ASEF

G h G

(6.32)

y l tn s tn hiu . T (2.24) , ASE c th c biu din l( 1)ASE spn h G (6.33)

y spn l h s pht x t pht. Nu G>>1 h s tp nhiu tr thnh2 spF n (6.34)

Gi tr cc tiu c th c ca 1spn , v vy h s tp nhiu ca b khuchi quang hc l tng bng hai (ngha l 3dB). Nu b khuch i c hiusut ghp u vo in , h s tp nhiu l

21 ASEinF G h G

(6.35)

H s tp nhiu khng b suy gim do cc mt m t ghp u ra ca bkhuch i. Nu khng dng knh phn cc, h s tp nhiu nh nht l 6dBv s hng th hai v phi ca cng thc (6.35) phi c nhn i6.3. CC KHI LI SOA TRONG CC MNG QUANG HC

Ch rng chng ta phn tch cc tnh cht ca nh sng khuchai c pht hin, chng ta ang v tr khm ph ng dng ca SOA nh


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nhng khi li c bn trong cc h thng truyn thng quang hc. Nhngng dng ny c biu din trong hnh 6.4 l post-amplifier hoc bkhuch i tng cng , b khuch i ng dy v b tin khuch i. ihi chnh ca cc b khuch i quang hc cho cc ng dng nh th clit k trong bng 6.1 .

6.3.1. B khuch i tng cngChc nng ca b khuch i tng c ng l tng tn hiu u vo

cng sut tng i cao trc khi truyn. Cc ng dng chnh ca bkhuch i tng cng c lit k trong bng 6.2. S tng cng cng sutlaser trong b truyn quang hc lm cho c th xy dng cc ng dy ditrung bnh vi khong cch truyn tng. Nhng ng dy nh th n ginbao gm mt si quang gia b truyn v b thu. Bi v iu ny khng linquan n cc thnh phn tch cc trong ng truyn, tin cy v hiu sutc ci tin .

Trong cc ng truyn di, vic dng cc b khuch i tng cngc th tng lng cng sut ng truyn v c th gim s b khuch i


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ng dy hoc s b ti sinh cn thit. Cc b khuch i tng c ng cngc ch trong cc mng phn b nh c biu din trong hnh 6.5, y cnhng s mt mt tch ln hoc s dy l ln (taps). Cc b khuch i tngcng cng cn thit khi i hi khuch i ng thi mt s tn hiu uvo ti cc bc sng khc nhau, nh trng hp truyn WDM. Ti tc bit cao (thng thng ln hn 2.5 Gb/s), cc laser bn dn c iu bintrc tip d b chirp bc sng. y bc sng dao ng laser thay i vidng kch thch. Chirp tng rng ph hiu dng ca laser. iu n y tngtn sc si quang dn n tng ISI v s suy hao ca h thng BER. trnh hiu ng ny, ti tc bit cao, cc laser thng c iu bin ngoi.Tn hao do chn bn trong ca cc b iu bin ngoi c c b bngvic s dng cc b khuch i tng c ng quang hc.

Bi v cng sut tn hiu u vo ca b khuch i tng cngthng cao, b khuch i phi c cng sut b o ha u ra ,o satP cao. iuny lm cho c th t c cng sut tn hiu u ra ca b khuch i caov cng gim cc hiu ng vn do bo ha li. Cc hiu ng vn tng ddi khi cng sut tn hiu u vo ca b khuch i v tc bit tng. Mt

,o satP cao cng cn cho cc ng dng ca b khuch i tng c ng trong hthng truyn WDM [6]. Trong trng hp ny, c tuyn bo ha ca b


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khuch i c xc nh bi cng sut u v o ton phn. iu ny l dobn cht ng nht ca mi trng li khuch i. Mt ,o satP cao gimnhiu xuyn knh. Ph li b khuch i rng cng cn cho s khuchi knh a bc sng. Cc cu trc SOA c th c dng thc hincng sut u ra bo ha cao c m t trong chng 3.

Mt m un SOA tng cng in hnh c biu din trong hnh6.6[7]. M un bao gm mt chip SOA MQW ng dn sng b vut cdn trn mt nn c th iu khin nhit v ghp quang hc vi cc thuknh Aspheric cng vi b cch li u ra , b ly mu chm v photodiode iu khin cng sut u ra. Si quang duy tr s phn cc (PMF) v siquang n mode tiu chun c dng cho cc kt ni u ra v u votng ng .

V d v vic s dng b khuch i tng c ng quang hc tngkhong cch ng truyn trung bnh c biu din trong hnh 6.7 [8].Trong php th h thng, b truyn bao gm mt laser DFB c iu bintrc tip bi mt chui bit gi ngu nhin di (PRBS) 312 1 ti 10Gb/s.Bc sng pht laser l 1310nm, tng ng vi vng tn sc trong vt liucc tiu ca si quang n mode tiu chun. Mt module khuch i tngcng quang hc tip theo laser. SOA c dng trong module l mt thitb MQW vi bn ging lng t ko cng. li chirp l 21dB vi cngsut u ra bo ha l 3dB ca 20 dBm. cc tiu ha cc nh hng canhng phn x bn ngoi, cc b cch ly quang hc c dng c hai phaSOA. Cc b cch ly ch cho qua mt trng thi phn cc n. B cch lyu ra cng chia i ASE t b khuch i. Trong m un, nh sng u v oc tp trung ln b cch ly bng cc thu knh aspheric n lm ny sinhs mt mt do ghp nh hn 2dB. M un cng cha b lm lnh nhit inv mt in tr nhit c th iu khin nhit ca chip SOA. Cc th


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nghim dng laser v m un tng cng ni trn vi b thu photodiode p-i-n cho lng cng sut cc i khong 26.8dB i vi mt cng sut u rakhuch i tng cng trung bnh 15.8dB. Lng cng sut ny bccu 70km si quang n mode. Trong th nghim th ny, ngi ta thyrng li nhy b thu do b khuch i tng cng nh hn 1dB nu cngsut u ra tng cng trung bnh c gi thp hn so vi cng sut boha 1dB ca n khong 15dBm. Lng cng sut cc i thu c ti cngsut u ra cao hn gn 1dB. S mo dng xung in h nh do bo ha ling lc hc ti cng sut hot ng n y c biu din trong hnh 6.7. Khib tin khuch i quang hc c s dng trc khi thu nhn , lng phnphi cng sut tng 39.4dB cho mt chiu di khng lp li tim nng100km ti 10Gb/s.

6.3.2. B tin khuch iChc nng ca b tin khuch i quang hc l tng mc cng sut

ca tn hiu n trc khi nhn hoc gii iu bin thng thng. Tng mccng sut c th tng ng k nhy b thu v v vy tng lng cng sutng truyn. iu ny cho php cc cc ng truyn khng lp li dihn c xy dng. Cc b thu IMDD dng b tin khuch i quang hcnhy hn nhiu so vi b thu IMDD truyn thng dng cc photodiode p-i-nhoc APD. So vi mt APD , n b khuch i quang hc phn phi mt li ln hn v bng thng rng hn. Khng c li ch trong vic dng btin khuch i quang hc trong mt b thu kt hp v tn hiu dao ng ccb c th tng n mc m hiu sut gii hn nhiu ht c th t ti.

S ca b thu quang hc s tin khuch i c biu din tronghnh 6.8. B thu bao gm mt b tin khuch i quang hc vi li ng


14

u G , mt b lc quang hc bng thng 0B , mt photodiode p-i-n vi hiusut lng t v tip theo l mt mch gi pht hin bng thng in eB vmt mch quyt nh. Cc b lc in v quang phi c bng thng rng tnht bng bng thng tn hiu iu bin trnh mo tn hiu. B lc quanghc gim pht x t pht n detector dn n gim s ph thuc ca ASEvo dng nhiu. Nu trng thi phn cc ca tn hiu nh sng n cbit, knh phn cc c th c chn vo gia b tin khuch i vdetector. Knh phn cc cho tn hiu khuch i i qua nh ng loi b ASEphn cc vung gc. iu ny ci tin nhy b thu 3 dB trong gii hnnhiu phch tn hiu - t pht. Tuy nhin trng thi phn cc ca tn hiunh sng thng khng bit c. Cc mch gi-pht hin khuch i v xl dng quang in t detector. Cc loi x l gi-pht hin in hnh nhtl integrate and dump c tho lun trong phn 6.11.

Trong mt b thu quang hc khng c b tin khuch i , dng nhiuquang in chim u th l nhiu mch. Nhiu ny thng ln hn mt nhai bc v ln so vi gii hn nhiu ht ca b thu. Trong mt b thuquang hc IMDD tin khuch i vi bng thng quang hc di hp, dngnhiu quang in chim u th l nhiu phch tn hiu t pht. Gi s rng li khuch i G>>1 , t s tn hiu trn nhiu ca b thu (6.31) l

4sig

out sp e

PS

N n h B

(6.36)

Nhng ci tin in hnh trong SNR so vi b thu truyn thng lnhn 10dB. Dng ca (6.36) ch ra rng , trong gii hn nhiu phch tn hiu -t pht, b thu SNR khng ph thuc vo c li khuch i v bngthng b lc quang hc.

Cng c mt s suy hao trn ng truyn L gia b tin khuch iv detector. iu ny ch yu l do s mt mt b lc quang hc v hiusut ghp n detector quang hc. S mt mt do ghp n y s khng dn nbt k s suy hao no trong hiu sut ca b thu min l tch 1G . iuny ph hp tt vi b thu truyn thng trong li cng sut bng vi s


15

mt mt do ghp. Phn tch chi tit hn v b thu tin khuch i quang hcc cp trong [9-12].

LI CNG SUT DO T S TT QUANG XC NHTrong cc h thng thc t ngun quang hc diode laser thng c

phn cc trn ngng m bo hot ng vn tc cao v gim chirp laser.iu ny c ngha l mt s cng sut quang hc s c a vo trongkhng gian ca dng bit quang hc c truyn, ngha l t s tt quang hckhng v hn. iu ny dn n li cng sut ca b thu. Li cng suttrong trng hp ny c nh ngha l s chnh lch dB gia cc cngsut tn hiu cho cc BER ging nhau i vi t s tt quang xc nh vkhng xc nh r . Li l :

( 1)10log

( 1)

( 1)( 1)10log

( 1)( 1)

r

r

r r

r r

penalty

(6.37)

Li c v trong hnh 6.9 nh hm s theo r. Li nh hn 1dB cn r>10 dBtrong trng hp b thu b gii hn nhiu nhit v r>20 dB trong trng hpb thu b gii hn nhiu tn hiu-t pht. Tuy nhin s nhy nn ca b thusau cao hn ci trc. Trong b thu tin khuch i quang hc , s suy hao ts tt quang hc ch yu l do s bo ha li khuch i.

Minh ha th nghim truyn dng cc b tin khuch i quang hcc biu din trong hnh 6.10 [14]. Th nghim dng ca s si quang 1300nm thu c s truyn t do gn tn sc 10Gb/ s. H thng i hi lngcng sut ln hn s ng trong vng 1550nm do s mt mt si quangtng. Laser truyn c iu bin 10Gb/s bin dng d liu 72 1 PSBSNRZ. D liu c thi gian tng v gim tng ng l 38ns v 72ns vi t stt quang hc 7.8dB. B thu quang hc bao gm mt b khuch i u cuiquang hc 50 v b khuch i in di rng c ghp xoay chiu. Bngthng in ca b khuch i l 10.6GHz. N tng ng vi nhy l -13.7dBm ti 10Gb/s i vi BER bng 910 . tng cng nhy ca bthu, s ca hai b tin khuch i ghp tng SOA MQW 1310nm vi blc di qua quang hc t ti bc sng tn hiu trc khi nhn. li ghptng ton phn ca hai b khuch i l 26.8 dB vi nhy phn cc l 3.4dB. S hin din ca b cch ly quang hc gia hai b khuch i ci tinhiu sut bo ha li so vi mt b khuch i cng li. B cch lyth hai ngn phn x t b lc quang hc ghp vi SOA th hai. D ng blc quang hc bng thng 0.2nm nhy tt nht thu c l -30dBm. Cclin kt truyn c gp 89 km si quang n mode truyn thng vi s mtmt ton phn l 34dB v bc sng tn sc bng khng ti 1305nm. Bcsng tn hiu l 1314.8ns, lm ny sinh li b thu nh 0.3dB.


16

6.3.3.B khuch i ng dy v cc tng khuch iTrong cc h thng truyn thng quang hc hn ch mt mt , cc b

khuch i ng dy c dng b mt mt trong si quang do khcphc c nhu cu ti to quang hc. u im chnh trong cc SOA ngdy l : trong sut vi tc d liu v hnh thc iu bin (trong ch khng bo ho v ti tc bit cao ), tnh hai chiu, kh nng WDM , ch hot ng n gin, tiu tn cng sut thp v rn chc. Hai u im sau cbit quan trng i vi cc thnh phn quang hc c t t xa.


17

H s tp nhiu ca mt tng khuch iH s phm cht tt cho mt tng kh uch i quang hc l h s tp nhiuton phn totF ti u ra ca b khuch i cui cng trong ng truyn. tnh totF , hy xt tng khuch i trong h nh 6.11, n bao gm k b khuchi vi li iG (k c mt mt do ghp) v ch s nhiu ( 1.... )iF i k ctch nhau bi cc ng truyn si quang vi mt mt iL . Hn na, gis rng b lc di hp c tp trung ti bc sng tn hiu c a vo timi u ra ca b khuch i v vy h thng hot ng trong gii hn nhiu


18

phch tn hiu-t pht. Ph thuc vo chiu di ca ng truyn cui cng,SNR thc s ca b thu c th l nhiu phch tn hiu-t pht hoc nhiumch b thu b gii hn. Mi b khuch i c gi s l hot ng trongch khng bo ha.

H s tp nhiu ton phn sau b khuch i th k l

1 2

1 1 1 2

1

( / )

( / )

....

intot

out

kk

i i ki

S NF

S N

FF F

L L G LL G L

(6.38)

y ( / )inS N v ( / )outS N tng ng l cc SNR ca u vo v u raca tng khuch i. Nu chng ta xt tr ng hp khi iL =1, ngha l skhuch i tng hp ca li tn hiu ton phn totG kG , c to ra bimt chui ca b khuch i nh hn vi li G b chia tch bi cc blc quang hc di hp , th

21 ...

ktot k

FFF F

G G (6.39)

Biu thc ny ch ra rng b khuch i li cao , nhiu thp c thc to ra bng cch kt hp mt b khuch i tng u ti n nhiu thp vtip theo sau l cc b khuch i cng sut u ra bo ha cao. K thut nyc bit hu dng khi xy dng cc b tin khuch i quang hc nhiu thpnh trong th nghim truyn trn trong b cch ly gia hai SOA hotng ci tin h s tp nhiu ca b khuch i u tin v tng cng sutu ra bo ha ca b khuch i th hai.

Nu chng ta tip tc xt trng hp chc nng ca mi b khuchi trong tng l b cho s mt mt ca si quang pha trc, ngha l

1i iG L , th t (6.38) h s tp nhiu ton phn ca tng l

1

k

tot i ii

F G F

(6.40)Nu tt c cc iG v iL bng nhau th

totF kGF (6.41)


19

iu ny c ngha l mi b khuch i ng gp ngang nhau v o h s tpnhiu ton phn ca h thng. Nu bit SNR cho php ti b thu th s bkhuch i ghp tng cc i xmak c th c xc nh. Khong cch truynkhng lp li cc i bng xmak l y l l khong cch gia cc b khuchi lin tip nhau. ti a ha khong cch truyn cn phi c cc bkhuch i cng sut u ra bo ha cao, nhiu thp.

Trong mt ng truyn khng c cc b khuch i ng dy, SNRnh mt hm theo khong cch z t b truyn l [16],

2 2

( )4

2

fzs

fz e ee s

L

P eSNR z

kTF BeB P e

R

(6.42)

Trong (6.42) gi s rng detector c hiu sut lng t bng mt. S hngth nht v th hai mu ca (6.42) tng ng l nhiu ht tn hiu v nhiunhit b thu. eF l h s tp nhiu ca b thu, eB l bng thng in, LR l trkhng ti ca photodetector v f l h s mt mt ca si quang. H nh 6.12biu din SNR ca ng truyn khuch i quang hc thc t nh mt hmtheo khong cch so vi ng truyn khng khuch i. SNR ca ngtruyn khuch i quang hc gim rt chm theo khong cch, ng c liSNR ca ng truyn khng khuch i gim rt nhanh. Trong tr ng hpny, khong cch cc i c th c chuyn ti bi mt ng truynkhng khuch i sao cho BER ti b thu nh h n 910 i vi d liu NRZl 145km. i vi ng truyn khuch i , nu SNR l h s duy nhtc tnh n, khong cch truyn ton phn c th khng xc nh. Trongthc t, cc h s khc chng hn nh bo ha b khuch i do ASE, tnsc si quang (c bit l tn sc ), cc hiu ng phn cc v s phi tuynsi quang gii hn khong cch c th c gia cc b ti sinh.

S BO HA NHIU TRONG MT CHUI KHUCH I kha cnh thit k, iu ng quan tm nht trong cc chui khuch

i l s bo ha li do tch ly ca nhiu khuch i. S b o ha khuchi gy ra s ng kn trong th mt h thng dn n s tng BER.Chng ta xt chui b khuch i ca 6.11 vi k b khuch i ging nhauvi li l G cch nhau bi chiu di mt mt si quang l L, y GL=1.Thm vo , ngi ta cng gi s rng cc b lc quang hc c a vou ra ca mi b khuch i. Cc b lc c gi s rng c p ng chnht l tng c t ti bc sng tn hiu. Cng sut nhiu u ra to nphn t b khuch i th k l

02 1k sp outin out

GN kn h B

(6.43)


20

y in v out tng ng l h s mt mt ghp u vo v u ra. Mtcng thc tng t p dng cho nhiu truyn theo h ng ngc li. Nhiutruyn v pha sau ny c th c trit tiu dng cc b cch ly. Tuy nhindng cc b cch ly khng cho php s truyn theo hai h ng v v vycng loi tr vic dng cc k thut pht hin li chng hn nh php ophn x min thi gian quang hc.

Nhiu cng c th c gim ng k bng cch d ng cc b lcquang hc di hp. Tuy nhin, nu bng thng b lc qu hp , bng thngquang hc ca h thng b gii hn v vy ngn cn s tng cng sc chadng cng ngh WDM.

trnh s suy hao nghim trng hiu sut ca h thng, cng sutnhiu u ra ton phn t mi b khuch i trong chui phi nh h n cngsut u ra bo ha ca b khuch i .o satP . V th, s b khuch i cc ic ghp trong ng truyn t vic xem xt bo ha li l:

,max

02 1

o sat

sp outin out

Pk

Gn h B

(6.44)

Nh mt v d, hy xt mt tng cc SOA hot ng trong v ng1550nm vi cc tham s nh sau : li si si 25dB, mt mt do ghpu vo v u ra 3dB v cng sut u ra bo ha 5dBm. Nu mt b lc


21

quang hc di qua bng thng 10nm c dng ti u ra ca mi bkhuch i v gi s rng bo ha do tn hiu truyn c th b qua, th dng(6.44) s b khuch i ghp tng cc i l 15. nh hng chnh xc canhiu SOA trn hiu sut h thng ph thuc vo cc tham s khc nhauchng hn nh t s tt quang hc, chirp, dng iu bin, tc bit v thitk b thu.

Mt th nghim truyn dn n knh minh ha vic dng cc SOAng dy c biu din trong h nh 6.13 [17]. B truyn bao gm mtlaser DFB 1309nm c iu bin trc tip vi b ti sinh tn s 10GHz to ra mt chui cc xung vi rng to n phn na cc i l 40ps. Ti1309nm, s tn sc si quang nh v khong cch truyn cc i ch yu bgii hn do s suy gim si quang. u ra laser c kt ni vi b iubin LiNiO3. u vo in ca b iu bin l 312 1 PRBS to ra mt dngd liu quang hc RZ vi t s tt l 13dB. SOA MQW tng cng cdng tng cng sut truyn trung bnh t 0 n 2dBm. Chiu di truyn l420km vi 12 SOA MQW ng dy, c dng b cho mt mt siquang t cch nhau 38km. Cc b cch ly khng ph thuc phn cc cdng ti u ra ca mi b khuch i. Cc b lc di qua quang hc khngc dng dc theo ng truyn si quang. Ti b thu, tn hiu ckhuch i bi mt b tin khuch i quang hc , ging vi cc b khuchi ng dy v c lc bi mt b lc quang hc di qua 1nm . Sau tn hiu c pht hin bi mt photodiode PIN c theo sau bi mt xungng h in v mch phc hi d liu. B tin khuch i quang hc v blc tng nhy ca b thu t -14dBm ti -31dBm. Li b thu sau 420km l5dB. Trong th nghim ny, gii hn chnh trn khong cch truyn l stch ly ASE trong bng thng b lc quang hc.


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S TRUYN WDMS truyn WDM c th c dng tng sc cha ca cc ng truynsi quang hc. V cc SOA c bng thng li rng, chng c th c tndng khuch i nhiu bc sng. trnh s bo ha li khuch i,cng sut tn hiu u vo phi nh hn cng sut u vo bo ha ,i satP ,ngha l

, ,1

N

k in i satk

P P

(6.45) y N l s bc sng truyn v ,k inP l cng sut tn hiu u vo ca bkhuch i ti bc sng k

V d v th nghim truyn WDM c biu din trong h nh(6.14)[18]. B truyn bao gm 8 laser bung cng h ng ngoi c kthp vi nhau qua mt b ghp 8:1. Cc b c sng nm trong khong1558nm n 1570nm vi khong cch gia cc k nh l 200 GHz. Cc knhc iu bin ngoi ti 20Gbit/s bi mt PRBS 132 1 v c cho qua mtsi quang n mode di 2km to ra cc dng bit c lp. Ba SOA tngcng (thit b vng hot tnh InGaAsP b ko cng dng khi vi cng sutu ra bo ha 12dBm) c dng b cho mt mt ca b ghp v mtmt ca b iu bin. Cc lin kt truyn bao gm bn ng truyn siquang n mode 40km c khuch i bao gm si quang b tn sc. Mtmt bc cu (span loss) l 13dB. nh li ca mi b khuch i ng


23

dy ti 1530nm. li c sn 12 n 14dB t mi b khuch i thch hp b cho s mt mt ng truyn. B thu bao gm hai b tin khuch iSOA gia chng tn hiu c phn knh n 10Gbit/s bi mt b iu binLiNiO3. D liu c phn knh sau c pht hin bi diode p-i-n.Dng s chnh tng, mt SNR quang hc 24dB (trong bng thng phn gii0.1nm) thu c cho tt c cc knh. Tt c cc knh c tha s pht hin Qnm trong khong 17 n 18dB tng ng vi BER < 133.10 . Hnh 6.15 biudin s ph thuc tha s Q vo cng sut phng ton phn (total launchedpower). SNR b suy hao ti cng sut phng (launched power) thp do ASEv ti cng sut phng cao do ISI v nhiu xuyn knh do s t iu bin li (hiu ng vn ca knh) v nhiu xuyn knh.

S GIM NHIU XUYN KNHNhiu xuyn knh c th trnh c bng cch cho SOA hot ng

trong vng tn hiu nh. Tuy nhin, cc b khuch i cn hot ng trongch bo ha t c hiu sut v cng sut u ra cao. Cc k thutkhc nhau c nghin cu gim nhiu xuyn knh[19-23]. Mc tiuchnh ca nhng phng php ny l cc tiu ha s thay i trong mt ht ti in ca b khuch i. Cc SOA kp cht li (ch ng 3 ) c thc dng cho mc ch ny [19-20].


24

Trong [21], k thut ghp knh phn cc c dng gim nhiu xuynknh WDM trong cc SOA. Trong phng php ny , c biu din tronghnh 6.16(a), hai ngun laser c tch thnh hai dng ri sau c iubin bi tn hiu d liu v phn b sung ca n. S phn cc ca tn hiuc iu chnh vung gc vi nha u. Ri sau tn hiu c ghp knhvi s tr hon thch hp sao cho tng vect ca hai dng c gi tr khngi trong khi c khuch i. Ti cui b thu, cc dng phn cc ringc tch knh v cng sut ca mt trong nhng trng thi c ti u ha pht hin v phc hi d liu. K thut ny dn n s ci tin ln tronghiu sut ca h thng nh c biu din trong cc s mt ca hnh6.16(b).

Mt k thut khc c dng gim nhiu xuyn knh l hi tip vpha trc. Trong phng php ny, mt ht ti in ca SOA c gi gi tr khng i bng cch iu khin thit b vi d ng phn cc khng iv tn hiu t l vi tng tr hon v tng c trng s ca cc dng iu khinin ca diode laser ngun tn hiu. Nhng k thut tr n tng i phc tpv khng mang li nhiu li ch.

Trong [23], s truyn 32 knh DWDM qua ba SOA ng dy tc m khng i hi iu khin li cc b. iu ny c thc hinc th do ba yu t : (a) Ti u ha nhy b thu bng iu chnh ngngpht hin ca b thu. (b) Nhng dao ng trong cng sut u vo ton phnca b khuch i gim khi s knh bc sng truyn tng. iu ny l dodng d liu c mang trn mi knh khng tng quan, v vy mt htti in ca b khuch i ch ph thuc vo s thay i cng sut tn hiu


25

u vo ton phn mt cch chm chp. (c) M t knh cha ngoi (khngiu bin) c truyn cng vi nhng knh c iu bin trit tiunhng hiu ng bin ng v cng sut.6.3.4. Cc SOA trong truyn tng t

Ni chung, cc h thng truyn tng t c s rng buc cht ch hnnhiu so vi cc h thng s, iu gii hn ng dng ca cc SOAtrong nhng mng ny [24]. Nh trong trng hp truyn s, nhng hiung suy hao chng hn nh : mo tn hiu v mo gia cc ln iu ch cth gim bng cch cho SOA hot ng ch khng b o ha [25], sdng cc thit b kp cht li [26] hoc nhng k thut iu khin linh c tho lun trn.


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chuong 6 cac ung dung mang co ban

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