raman-va-edfa
DESCRIPTION
EDFATRANSCRIPT
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CHNG III KHUYT I QUANG TRONG H THNG DWDM
Khi Nim Khuch i Quang, Cc Thng S Khuyt i Quang. Khi Nim Khuch i Quang.
Nguyn l khuch i quang trong cc b khuch i quang c thc hin da trn hin tng pht x kch thch v khng c s cng hng xy ra trong qu trnh khuch i.
Hin tng pht x kch thch (stimulated emission) l mt trong ba hin tng bin i quang in c ng dng trong thng tin quang.
Hin tng pht x kch thch xy ra khi mt in t trng thi nng lng cao E2 b kch thch bi mt photon c nng lng hf12 bng vi chnh lch nng lng gia trng thi nng lng cao v trng thi nng lng thp ca in t (Eg = E2 - E1).
Khi in t s chuyn t trng thi nng lng cao xung trng thi nng lng thp hn v to ra mt photon c nng lng bng vi nng lng ca photon kch thch ban u. Nh vy, t mt photon ban u sau khi xy ra hin tng pht x kch thch s to ra hai photon (photon ban u v photon mi c to ra) c cng phng truyn, cng phn cc, cng pha v cng tn s. Hay ni cch khc, qu trnh khuch i nh sng c thc hin. Hin tng ny c ng dng trong cc b khuch i bn dn (OSA) v khuch i quang si (OFA). Hin tng pht x kch thch cng c ng dng trong vic ch to laser. Tuy nhin, im khc bit chnh gia laser v cc b khuch i quang l trong cc b khuch i quang khng xy ra hin tng hi tip v cng hng. V nu xy ra qu trnh hi tip v cng hng nh trong laser, b khuch i quang s to ra cc nh sng kt hp ca ring n cho d khng c tn hiu quang ng vo. Ngun nh sng ny c xem l nhiu xy ra trong b khuch i. Nh vy, khuch i quang c th lm tng cng sut tn hiu nh sng c a vo ng vo b khuch i nhng khng to ra tn hiu quang kt hp ca ring n ng ra.
Hin tng hp th (absorption) xy ra khi mt photon c nng lng hf12 b hp th bi mt in t chuyn trng thi nng lng trng thi t mc nng lng cao E2 xung mc nng lng thp E1 v pht ra mt nng lng Eg = E2 - E1 di dng mt photon nh sng. Qu trnh ny xy ra mt cch t nhin v trng thi nng lng cao E2 khng phi l trng thi nng lng bn vng ca lng t. Sau mt khong thi gian c gi l khong thi gian sng (life time)ca in t mc nng lng cao, cc in t s t ng chuyn v mc trng thi thp hn (trng thi nng lng bn vng). Ty theo loi vt liu khc nhau th thi gian sng ca in t s khc nhau.
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/)(1
)(2
22
0
0
sPPTww
gwg
Cho d hin tng pht x t pht to ra photon nh sng, nhng trong khuch i quang, pht x t pht khng to ra li khuch i. Nguyn nhn ny l do hin tng ny xy ra mt cch t pht khng ph thuc vo tn hiu nh sng a vo b khuch i. Nu khng c nh sng tn hiu a vo, vn c nng lng nh sng c to ra ng ra ca b khuch i. Ngoi nh sng do pht x t pht to ra khng c tnh kt hp nh hin tng pht x kch thch. Do vy, pht x t pht c xem l nguyn nhn chnh gy nhiu trong cc b khuch i quang. Loi nhiu ny c gi l nhiu pht x t pht c khuch i ASE (Amplified Spontaneous Emission noise)
Cc Tham S Chnh Ca Khuch i Quang
DWDM thc hin ghp mt s lng ln cc bc sng quang c iu ch trn mt si quang. Nhng knh quang trong h thng DWDM thng nm trong mt ca s bc sng, ch yu l 1550nm v mi trng ng dng trong h thng ny thng l mng ng trc, c ly truyn dn di v c dung lng truyn dn ln. Ging nh bt c mt cng ngh no, DWDM cng tn ti nhng gii hn v nhng vn k thut. Mt s cc tham s c trng: suy hao, nhiu xuyn knh, s knh bc sng, b rng ph ngun pht, qu cng sut, tn sc v nh hng ca cc hiu ng phi tuyn.
li
Hu ht cc b khuch i quang u c thc hin thng qua hiu ng bc x kch thch. Khuch i t c khi b khuch i quang thc hin bm quang, hay bm in o ln mt . Nhn chung khuch i quang khng ch ph thuc vo bc sng truyn m cn ph thuc vo cng bm, mt ht c trong vt liu. Chng ta coi vt liu l ng nht, ta c c phng trnh sau:
(3.1)
Trong :
g0: gi tr nh ca li
: tn s ca tn hiu quang ti
0: tn s truyn trung tm
P: cng sut ca tn hiu c khuch i
Ps: cng sut bo ha
T2: thi gian hi phc phn cc (thng nh hn 1ps)
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in
out
P
PG
gP dP
dP
Gi Pin, Pout ln lt l cng sut u vo v u ra ca b khuch i. Vy th h s khuch i l:
(3.2)
Mt khc ta li c cng thc sau:
(3.3)
Suy ra:
(3.4)
Vi P(z) l cng sut tn hiu ti v tr z so vi u vo
Gi s khong rng ca b khuch i l L, khi Pout = P(L). Suy ra h s khuch i ca tn hiu quang c di L l
(3.5)
D dng nhn thy rng, g() t gi tr ln nht ti = 0 nn G() cng t gi tr ln nht ti 0. V gi tr hai h s ny u gim khi ( 0)/T2 tng.
Ta c biu sau:
)exp()( gzPzP in
Lg
in
L
in
out eP
P
P
PG )()(
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Hnh 3.1 Mi tng quan h s khuch i
Bng Thng Li
Bng tn li c nh ngha lg = 2/T2 hay l :
(3.6)
Nh vy, nu g ~ 5THz vi b khuch i quang bn dn c T2 = 60fs. B khuch i bng rng thch hp vi cc h thng vin thng thng tin quang, v li ca c bng tn gn nh l hng s, thm ch c khi l tn hiu a knh. Bng tn khuch i g c nh ngha l mt FWHM, v lin quan ti g theo cng thc sau:
(3.7)
Vi G0 = exp(g0L)
D dng nhn thy, bng tn khuch i nh hn bng tn li, v s khc bit ny cn ty thuc vo li khuch i.
Cng Sut Ng Ra Bo Ha.
li bo ha
20 )( T
2
1
2 Tv gg
2/1
0 )2/ln(
2ln
Gvv gA
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bo ha ca li ph thuc vo gi tr g(). D dng nhn thy rng, khi P tin n Ps th gi tr g gim dn, ng thi h s khuch i G cng gim theo tng ca cng sut tn hiu. Chng ta coi gi tr nh xy ra khi = 0.
(3.8)
Xt phng trnh vi chiu di b khuch i l L, v coi P0 = Pinv P(L) = GPin = Pout, t ta c phng trnh:
(3.9)
D dng nhn thy G bt u gim dn t gi tr nh G0 khi gi tr Pout t gn ti gi tr cng sut bo ha Ps
Cng sut u ra chun ho Pout/Ps
Hnh 3.2 S ph thuc ca cng sut ra (theo Ps)theo li G (theo G0)
Cng sut ra bo ha
sPP
Pg
dz
dP
/10
)1
exp(0s
out
P
P
G
GGG
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Chng ta xem xt n cng sut ng ra bo ha, l cng sut ln nht to c
cng ra, k hiu l soutP . C th nhn thy rng, gi tr li ny t c khi li
khuch i gim t 2dB n 3dB, tng ng vi gi tr G = G0/2. Khi ta c cng thc:
(3.10)
H S Nhiu
Nguyn l ca b khuch i l da trn nguyn l bc x kch thch. Nhng trong qu trnh khuch i c rt nhiu cc in t ht thi gian sng, chuyn i t mc nng lng cao xung mc nng lng thp, hay t gii dn sang gii ha tr, y chnh l bc x t pht. Bc x ny khi c phng cng lung in t, s gy nh hng ln bin v pha ca tn hiu. Hin tng ny gi l hin tng nhiu bc x t pht ASE. Nh vy cng sut ca ra gm c cng sut vo khuch i v cng sut pht x t pht:
Pout = G.Pin + PASE (3.11)
nh hng nhiu i vi b khuch i quang c biu din bi h s nhiu NF, m t s suy gim t s tn hiu trn tp ti u ra v u vo :
(3.12)
ss
out PG
GP
2
2ln
0
0
out
in
SNR
SNRNF
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Hay NF = SNRin(dB) SNRout(dB) (3.13)
Ngi ta cng chng minh c rng, gi tr hng s nhiu tnh c th theo cng thc sau:
(3.14)
H s nhiu NF ca b khuch i cng nh cng tt, v gi tr nh nht c th t c l 3dB. Ti gi tr ny chng ta gi l gi tr lng t.
Cc K Thut Khuch i Quang. Trong mt b khuch i quang, qu trnh khuch i nh sng c din ra
trong mt mi trng c gi l vng tch cc (active medium). Cc tn hiu quang c khuch i trong vng tch cc vi li ln hay nh ty thuc vo nng lng c cung cp t mt ngun bn ngoi gi chung l ngun bm (Pump Source). Cc ngun bm ny c tnh cht nh th no ty thuc vo loi khuch i quang hay ni cch khc l ty thuc vo cu to ca vng tch cc
Hnh 3.3 M hnh tng qut ca mt b khuch i quang Ty theo cu to ca vng tch cc, c th chia khuch i quang thnh 2 loi
chnh: - Khuch i quang bn dn SOA - Khuch i quang si OFA
Khuch i quang bn dn OSA (Optical Semicondutor Aplifier)
- Vng tch cc c cu to t vt liu bn dn - Cu trc ca vng tch cc SOA tng t nh vng tch cc ca laser bn dn. im khc bit gia SOA v laser l SOA hot ng trng thi mc ngng pht x
spsp nGGnNF 2/)1(2
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- Ngun cung cp nng lng khuch i tn hiu quang l dng in Khuch i quang si OFA (Optical Fiber Amplifier)
- Vng tch cc l si quang pha t him. Do , OFA cn c gi l DFA (Doped-Fiber Amplifier)
- Ngun bm l nng lng nh sng c cung cp bi laser c bc sng pht quang nh hn bc sng ca tn hiu cn khuch i.
- Ty theo loi t him c pha trong li si quang, bc sng bm ca ngun bm v vng nh sng c khuch i ca OFA s thay i. Mt s loi OFA tiu biu:
EDFA (Erbium-Doped Fiber Amplifier): 1530nm 1565nm PDFA (Praseodymium-Doped Fiber Amplifier): 1280nm 1340nm TDFA (Thulium-Doped Fiber Ampifier): 1440nm 1520nm NDFA (Neodymium-Doped Fiber Amplifier): 900nm, 1065nm hoc 1400nm
Trong cc loi OFA ny, EDFA c s dng ph bin hin nay v c nhiu u im v c tnh k thut so vi SOA v c vng nh sng khuch i (1530nm 1565nm) thch hp vi gii tn hot ng ca h thng ghp knh theo bc sng mt cao DWDM (Dense Wavelength Division Mutiplexing).
LI DN CA CH TRNG N EDFA
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H THNG THNG TIN QUANG S DNG B KHUCH EDFA
B Khuch i EDFA
Cu to c bn ca EDFA
Hnh 3.4 Mt ct ngang ca mt loi si quang pha ion Erbium EDFA c thnh phn chnh gm mt on ngn cp quang c li pha tp khong
0,1% Erbium. Erbium l mt nguyn t t him c tnh nng quang tch cc. on si pha tp Erbium c k hiu l EDF (Erbium - Doper Fiber) thng c chiu di khong 10 - 20m. Ngoi ra EDFA cn c mt laser bm cung cp nng lng cho on EDF, mt b ghp bc sng WDM ghp bc sng nh sng tn hiu v bc sng nh sng bm vo on EDF v b phn cch hn ch nh sng phn x t h thng.
Cu trc tiu biu ca b EDFA :
Hnh 3.5 Cu to ca mt EDFA
Nguyn l hot ng ca EDFA
Biu mc nng lng ca ion Erbium c m t nh hnh v 3.5. Er3+ trngthi khng b bt k tn hiu quang no kch thch, mc nng lng thp nht, khi bm quang ht Erbium hp th nng lng ri chuyn tip ln mc nng lng
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cao hn. Quang bm vo c bc sng khc nhau, cc mc nng lng cao c ht chuyn ln mc nng lng cao hn. Quang bm vo c bc sng khc nhau, cc mc nng lng cao c ht chuyn ln cng khc nhau. S dch chuyn in t t mc nng lng cao ny xung mc nng lng c bn pht ra photon, photon ny bc x c th l do hin tng bc x t pht (s phn hy t nhin ca cc ion m khng c bt c mt ng tc no chen vo) hay kch thch (do s c mt ca cc photon c cha nng lng bng nng lng dch chuyn, kch thch s pht x v to ra photon t l vi s photon ca chm sng). Trong qu trnh bc x kch thch, n to ra s photon cng pha cng hng vi photon ti, nh vy l to ra c qu trnh khuch i trong EDFA.
Bc x t pht to ra cc photon cng pha v hng ngu nhin, iu ny gy ra nhiu trong EDFA gi l nhiu do bc x t pht c khuch i (ASE). Tuy nhin thi gian sng ca cc in t mc nng lng cao khong 10ms m bo thay v nhiu bc x gy ra do bc x t pht th hu ht cc ion Erbium i khuch i tn hiu bng bc x t kch thch.
Hnh 3.6 Gin nng lng Erbium
Nhiu v li ca EDFA li ca EDFA li: l c im u tin chng ta nh gi mt b khuch i. Chng ta tho lun c im ny i vi mt si quang tch cc. nh ngha li l t s gia cng sut nh sng ra v cng sut nh sng vo:
(3.15)
in
out
P
PGain
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Khi cng sut o bng watt. Thng thng chng ta o li bng dB, iu ny c ngha:
(3.16)
Nu chng ta nhn li hnh 1.3c chng ta s nhn ra rng cng sut ra bao gm
cng sut tn hiu ra v nhiu. Kt qu chng ta cn tch cng sut nhiu khi tnh ton li:
(3.17)
Hay
(3.18)
li ca EDFA hin i ngy nay c gi tr trong khong 20dB n 40dB, da vo chc nng ca n, ngi ta c th thit k chng nh b khuch i cng sut, khuch i ng dy hay b tin khuch i. S Bo Ha Li
li c ph thuc vo cng sut ca tn hiu vo, da vo iu sau: mt tn hiu cng sut cao ngha l c mt s lng ln cc photon trn mt n v thi gian s i vo si quang trn Erbium. Cc photon ny s kch thch mt s lng v cng ln cc photon chuyn dch t mc trung gian (mc 2) xung mc thp (mc 1). iu ny c ngha mc nng lng trung gian s nhanh chng mt i cc photon. Hay ni cch khc th cng sut nh sng vo cng ln th mt ti mc trung gian cng thp. Nhng theo nguyn l ca hin tng pht x kch thch, li l tng ng vi s khc nhau v mt mc 2 v mc 1. T , lm ngho mc 2 c ngha l lm gim li. Hin tng ny c gi l s bo ha li.
S bo ha li l mt c tnh quan trong ca EDFA, c bit trong khuch i cng sut, ni m tn hiu vo hu nh n trc tip t b pht quang do rt ln. S bo ha li xc nh cng sut ra mt cch ln nht, thng c gi l cng sut ra bo ha m EDFA c th t c.
Cng sut bm Cng sut bm vo cng cao, s lng ln cc ion Erbium t mc nng lng thp
s c kch thch ln mc trung gian v li cao hn cho EDFA. Nhng chng ta khng nn ngh vy, cng sut bm cao s qut sch tt c cc ion t mc nng lng thp v lm cho s nghch o nng chuyn sang trng thi bo ha.
Chng ta cng cn phi tnh ton rng mt b khuch i EDFA khuch i bao nhiu knh ng thi v cng sut bm c chia cho tt c cc bc sng khuch i. Nhiu bc sng c ghp cho tuyn truyn dn, nhiu cng sut bm c yu cu tha mn hot ng ca EDFA.
Nhiu ca EDFA Nhiu l mt thng s quan trng th hai ca mt b khuch i quang (hay bt k
loi khuch i no). Chng ta thy rng li ca mt b EDFA thng c s
in
out10 P
Plog10Gain(dB)
in
out
PP
Gain ASEP
in
out10 P
Plog10Gain(dB) ASE
P
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dng khuch i bc sng xc nh. Do , rt quan trng tnh ton li v nhiu trong bng thng ca mt knh ring bit. Nhiu quan trng trong EDFA c gi l nhiu pht x t pht (Amplifier Spontanoues noise_AES). S gim st tn hiu thc s n t bin i tn hiu to ra giao thoa nhiu vi nhiu, giao thoa nhiu vi tn hiu. Tuy nhin s giao thoa nhiu vi nhiu c th d dng loi b bng cch s dng b lc bng hp. Thc s cc b EDFA thng bao gm mt b lc ASE (ASE filter) lm nhim v ny. S thit hi n t s giao thoa nhiu vi tn hiu. Nhiu nh th khng th b lc bi v n nm trong bng thng ca tn hiu. y l phn ng gp thc s vo h s nhiu ca mt b EDFA
Hnh 3.7 Cc loi giao thoa nhiu vi nhiu v nhiu vi tn hiu
H s nhiu da vo giao thoa tn hiu vi nhiu c cho bi:
(3.19)
Trong :
S: bc sng tn hiu : nng lng cu knh ring G: li cu b khuch i BW: bng thng ca b khuch i
Ta cng bit tng cng sut trung bnh ca nhiu pht x t pht c khuch i l:
(3.20)
)(.. s
sASEn BWGhf
PF
BWGhfnP spASE ...2
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Nsp : h s pht x t pht hay h s nghch o nng Ta c th d dng tnh c h s nhiu ca b EDFA:
(3.21)
Ta thy i vi b khuch i l tng khi nsp = 1, SNR ca mt tn hiu c
khuch i gim hai ln (3dB). EDFA khuch ai tn hiu nhng cng lm tng mc nhiu. Thng thng gi tr nsp thay i t 1.4 n 4 nn Fn thay i t 3.5dB n 9dB cho EDFA
Yu cu i vi ngun bm
Bc sng bm
nh sng bm c th c s dng ti cc bc sng khc nhau 650nm(4F9/2), 800 nm(4I9/2), 980nm (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 tr v vng nn v pht x ra photon nh sang.
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 cc bc sng 1480 nm hoc bm gin tip bc sng 980nm.
Hin nay, bm bc sng 1480nm 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 dung bm cho khong cch di v trnh lm nhiu tn hiu.
Bng 3.1 So snh hai bc sng bm 980nm v 1480 nm
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 ion erbium
spn nF .2
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c cy vo si dy l c gii hn. Ngoi ra, khi cng sut bm tng ln th h s nhiu s gim.
Hng bm
B khuch i EDFA c th c bm theo ba cch:
- Bm thun (codirectional pumping): ngun bm c bm cng chiu vi hng truyn tn hiu - Bm ngc (counterdirectional pumping): ngun bm c bm ngc chiu vi hng truyn tn hiu. - Bm hai chiu (dual pumping): s dng hai ngun bm v c theo hai chiu ngc nhau.
Mt EDFA c bm bng mt ngun bm c th cung cp cng sut u ra cc i khong +16dBm trong vng bo ha hoc h s nhiu t 5 ~ 6dB trong vng tn hiu nh. C hai bc sng bm c s dng ng thi c th cung cp cng sut u ra cao hn. Mt EDFA c bm kp c th cung cp cng sut ra ti +26 dBm trong vng cng sut bm cao nht c th t c. Hnh di y th hin mt EDFA c bm kp.
Hnh 3.8 Cu hnh b khuch i EDFA c bm kp.
c tnh ca b khuch i EDFA Cc c tnh c bn ca EDFA l c tnh tng ch, c tnh cng sut ra v c
tnh m.
c tnh tng ch (c tnh khuch i)
c tnh tng ch biu th kh nng khuch i ca b khuch i, nh ngha l t s gia cng sut ra v cng sut vo.
H s khuch i ph thuc vo cng sut v bc sng bm:
Nhiu kt qu nghin cu cho kt lun rng h s khuch i ph thuc vo cng sut v bc sng bm v nu t bc sng bm ti 980nm v 1480nm l cho hiu qu cao nht.
-
Cng sut bm (mW)
Hnh 3.9 H s khuch i l mt hm ca cng sut bm vi 14m chiu di ca si Silico Al-Ge pha tp Erbium c bm ti bc sng 980nm v 1480nm
Theo hnh 3.9 h s khuch i ca EDFA c kh nng khuch i cao nht khi tn hiu bc sng 1530nm v 1550nm.
Nhn xt:
+ Vi cng sut bm cao, bc sng 980nm s cho h s khuch i cao hn so vi bc sng bm 1480nm, iu ny c ngha l ti bc sng 980nm s t c s nghch o mt cao hn so vi bc sng 1480nm.
+ Vi cng sut bm cao, h s khuch i i vi bc sng 1530nm cao hn so vi bc sng 1550nm.
H s khuch i ph thuc vo chiu di si v phng thc bm:
Hnh 3.10 biu th mi quan h v chiu di si quang.
-
Hnh 3.10 (a) H s khuch i tn hiu ti 1530nm v 1550nm, bc sng bm 980nm v 1480nm vi cng sut bm l 40 mW v cng sut tn hiu in l -
40dB
Hnh 3.10(b) H s khuch i tn hiu ti 1530nm v 1550nm, bc sng bm 980nm v 1480nm vi cng sut bm l 10 mW v cng sut tn hiu in l -40dB
Lc u kh nng tng ch tng ln khi chiu di si quang tng, nhng sau khi si quang di qu di nht nh, tng ch s gim dn, vy c mt di nht nh t c kh nng khuch i ti a.
Ngoi ra h s khuch i cn ph thuc vo phng thc bm l cng chiu hay ngc chiu vi tn hiu nh hnh v 3.11.
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Hnh 3.11 H s khuch i ph thuc vo chiu di si v phng thc bm
c tnh tp m nhiu
Trong si pha tp Erbium, cc photon bc x t pht c pha v hng ngu nhin. Mt s photon bc x t pht c gi li cc mode ca si quang, lan truyn dc theo li si v c khuch i thnh cc ngun tp m nh hng n tn hiu quang. Tp m ca EDFA ch yu c 4 loi:
- Tp m tn ht ca tn hiu quang;
- Tp m tn ht bc x t pht b khuch i (ASE);
- Tp m phch gia quang ph ASE v tn hiu;
- Tp m phch gia cc quang ph ASE.
Trong 4 tp m trn c 2 loi tp m th 3 v th 4 c nh hng ln nht, c bit tp m th 3 l nhn t quang trng quyt nh tnh nng ca EDFA.
Ngoi ra cn c nhiu bn c ngun gc pht sinh thi gian n ca cc photon ti b tch quang khng ging nhau v do tc chuyn ng v s lng cc ht ti in qua tip gip P-N ca b tch quang thay i ngu nhin theo thi gian. B khuch i quang t trc diode tch quang nn n l mt trong cc ngun sinh ra nhiu bn.
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Hnh 3.12 H s tp m EDFA Hnh 3.12 cho thy nh hng ca cc phng thc bm khc nhau i vi h s
tp m, khi si quang trn Erbium tng i di th h s tp m khi bm ngc chiu cao hn vi khi bm cng chiu.
Nhiu ti ng ra b khuch i quang:
Trong cc h thng thng tin nh sng dng cc b khuch i quang, tn hiu quang c bin i sang tn hiu in cui ng truyn. Cc b tch sng s bin i cc photon thnh electron, pht x t pht tn ti trong b khuch i quang s gy ra s gia tng i vi tn hiu in, c xem nh l nhiu, n hon ton ngu nhin ch khng cha thng tin.
c tnh cng sut ra
Vi b khuch i quang l tng khng k cng sut vo cao bao nhiu, tn hiu quang c khuch i theo t l nh nhau. Nhng thc t khng phi nh vy, khi cng sut vo tng ln, bc x b kch thch tng nhanh, gim s ht chuyn ng ngc li, quang bc x b kch thch yu i, dn n bo ha tng ch, cng sut pht c xu hng n nh. Bo ha tng ch l c tnh l h s khuch i gim khi tn hiu vo tng. c tnh bo ha tng ch l c tnh v cng quan trng do ccng sut u ra ca b khuch i lin quan n c ly truyn dn v c ly trm lp hoc lm tng s u ra trong cu hnh phn phi si quang. Hnh 3.10 th hin mi quan h gia u vo v u ra trong EDFA.
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Hnh 3.13 Mi quan h gia u vo v u ra ca EDFA Cc h thng thng tin si quang ng di c th s dng chui EDFA trong
truyn dn. C ly truyn dn c th t c xa hn nh s dng cc EDFA, c nhiu thp v khuch i cao.
Vic s dng chui EDFA thay th cc trm lp trong h thng thng tin si quang ng di, c bit i vi h thng cp quang bin, l rt ph hp cho c im hn hn ca n. Vn t ra y l vic tnh ton v tr t cc EDFA v nh hng ca nhiu tch ly t nhng EDFA ny ti c tnh BER ti my thu.
Cc ng dng PA, BA, LA
Tin khuch i (PA)
Hnh 3.14 S tuyn s dng EDFA lm tin khuch i Trong :
Ptx l cng sut pht
Ps l cng sut u vo pin
Pin l cng sut quang u vo b khuch i
Pout l cng sut quang u ra
L l khong cch cho nh hnh v
tot l h s suy hao trung bnh ca si quang (ln)
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Tin khuch i l thit b khuch i quang si c nhiu rt thp c s dng trc tip trc b thu quang nng cao nhy thu, thng lm vic trng thi tn hiu nh. H s khuch i ca PA hot ng trong ch tuyn tnh vi cng sut li vo ti mc cng sut bm v bc sng tn hiu cho trc.
Cc chc nng OAM cho cc b PA c th c iu hnh cng chung vi b thu quang.
Ngoi ra, la chn PA khi thit k h thng phi tnh n cc tham s ch tiu:
- Di cng sut quang vo. - Di cng sut pht ra. - Di bc sng cng tc.
Khuch i cng sut (BA)
Hnh 3.15 S tuyn s dng EDFA lm tin khuch i Khuch i cng sut l thit b khuch i quang si c cng sut bo ha cao
c s dng trc tip ngay sau b pht quang lm tng cng mc cng sut tn hiu. Thit b BA khng cn yu cu nghim ngt v nhiu v lc quang.
Cc chc nng OMA cho BA c th c iu hnh chung vi b pht quang. BA c cng sut li ra tng i cao nn nhiu ASE sut hin tt yu do qu trnh bc x t pht cc phton trong OFA. Cc thnh phn nhiu ngoi bng tn hiu c th c b qua nh b lc quang.
Tuy nhin vic ng dng nhiu b BA c th gy ln cc tnh cht phi tuyn trong cc si dn quang. Cc ch tiu cn la chn khi nh gi v thit k h thng l:
- Di cng sut pht vo b BA - Di cng sut pht ra t b BA. - Di bc sng cng tc. - Mc nhiu NF(tnh theo dB). - H s khuch i - Suy hao phn x li vo.
Vic s dng kt hp gia b khuch i cng sut v tin khuch i l rt hp dn cho cc nh khai thc, c bit l trong trng hp m trong cc v tr trung gian vi cc thit b tch cc l khng th hoc khng nn lp t, chng hn nh h thng cp quang bin. Trong bt c trng hp no th cng t trm trung gian trn
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ng truyn th cng d dng bo dng cho cc nh khai thc v nng cao tin cy ca cc tuyn tryn dn ln rt nhiu.
Khuch i ng truyn (LA)
Hnh 3.16 S tuyn s dng EDFA lm khuch i ng truyn Trong : Ptx l cng sut pht.
Ps l cng sut u vo PIN.
PIN cng sut quang u vo b khuch i.
Pout cng sut quang u ra.
Khong cch cho nh hnh v.
tot l h s suy hao trung bnh ca si quang(ln)
Khuch i ng truyn l thit b khuch i quang si c tp m thp c s dng nh thit b th ng trn ng truyn tng khong cch gia hai trm lp. Trong mng thu bao quang theo cu hnh im - a im, khuch i ng truyn c s dng b suy hao gia cc im nhnh. Khuch i ng truyn c th thay th mt vi hoc tt cc trm lp thng thng trn cc tuyn ng trc. N c u im lm gim thit b trn cc tuyn truyn dn, n ch ng vai tr b suy hao tn hiu( cn cc trm lp th cn c kh nng sa mo tn hiu).
Trn cc h thng truyn dn c s dng nhiu b LA cn phi c cc knh thng tin ring bit cnh bo v gim st cc b iu khin, cc b khuch i ng truyn xa.
V mt l thuyt th c th ko di khong cch tuyn truyn dn ln hng nghn km bng cch lp t u n cc b LA vi khong cch tnh ton trc. Tuy nhin c qu nhiu b LA dn n cht lng h thng s b suy gim do hiu ng tch ly, hiu ng phn cc tn sc v cc hiu ng phi tuyn khc.
Cc ch tiu nh gi, thit k LA:
Di cng sut pht vo b LA. Di cng sut pht ra b LA Cng sut quang bo ha: Cng sut quang pht ra ca tn hiu ca LA vi h
s khuch i b gim i 3dB so vi h s khuch i tuyn tnh ti bc sng tn hiu.
PsPin PoutPtx
My ph t My thu
L1,tot1L2,tot2
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Di bc sng cng tc. Mc nhiu NF. Mc cng sut nhiu. Suy hao phn x li vo. H s khuch i tuyn tnh: H s khuch i ca LA hot ng trong ch
tuyn tnh, trong cng sut pht ra ph thuc tuyn tnh vo cng sut li vo, ti mc cng sut bm v bc sng tnh hiu cho trc.
Tn sc mode phn cc: tr nhm cc i gia cc trng thi phn cc bt k lan truyn qua b LA.
Cc s ng dng ca EDFA
Theo s phn loi thit b quang si OFA nh trnh by phn trn (gm BA, LA, PA) cng vi s tham chiu v vic lp t EDFA trn ng truyn, ta c cu hnh ng dng OFA theo khuyn ngh G.611 ca CCITT theo cc s ng dng nh sau:
- Tx + BA + Rx
- Tx + PA + Rx
- Tx + LA + Rx
- Tx + BA + PA + Rx
- Tx + BA + LA + Rx
- Tx + LA + PA + Rx
- Tx + BA + LA + PA + Rx (*)
Trong : Tx l b pht quang
Rx l b thu quang
LA c th l mt hay nhiu b khuch i ng dy
BA l b khuch i cng suet
PA l b tin khuch i LI DN N RAMAN
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H THNG THNG TIN QUANG S DNG B KHUCH I RAMAN
Tn x nh sng. Khi nh sng truyn qua mi trng vt cht trong sut th phn ln nh sng
truyn thng v mt phn nh s b tn x. Mi trng c th gy ra nhiu loi tn x trong in hnh l tn x Rayleigh, tn x Brillouin, tn x Raman Tu thuc vo loi vt cht, nh sng, iu kin mi trng m mi loi tn x xy ra khc nhau.
Tn x Rayleigh l qu trnh tn x n hi, tn s nh sng tn x bng tn s nh sng ti. Trng thi ca cc phn t vt cht do tn x Rayleigh khng thay i sau khi nh sng truyn qua. Ngc li, tn x Brillouin v tn x Ramman l cc qu trnh tn x khng n hi, cc nguyn t b kch thch khi c nh sng i qua v tn s nh sng tn x b dch chuyn so vi tn s ca nh sng ti.
a-Qu trnh tn x n hi b-Qu trnh tn x khng n hi.
Hnh 3.17 Qu trnh tn x nh sng Qu trnh tn x khng n hi c s tham gia ca cc phonon. Trong qu trnh
ny cc phonon c th sinh ra hoc b hp th. Mc thay i tn s ca nh sng tn x so vi nh sng ti bng vi tn s ca phonon. Tn x Brilloin lin quan n cc phonon m hc cn tn x Raman lin quan n cc phonon quang hc. Do nh sng tn x Raman c mc dch chuyn tn s ln hn nh sng tn x Brilloin. Nu nh sng tn x c tn s nh hn nh sng ti th nh sng tn x c gi l nh sng Stoke v qu trnh tn x c gi l tn x Stoke. Ngc li, nu nh sng tn x c tn s ln hn nh sng ti th nh sng tn x c gi l nh sng phn Stoke v qu trnh tn x c gi l tn x phn Stoke. Vi tn x khng n hi, n v o
dch tn ca nh sng tn x l (rad/s) hoc l cm vi ( l dch chuyn tn 1c
v2
v
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s theo cm , l dch chuyn tn s theo rad/s v c l vn tc ca nh sng trong chn khng theo cm/s).
Cc hiu ng tn x s lm gii hn cng sut quang ln nht c th truyn trong si. Trong h thng WDM tn x l nguyn nhn gy nhiu gia cc knh. Tuy nhin tn x Raman cng c ng dng trong cc b khuch i quang Raman nhng bc sng m b khuch i quang EDFA khng ph hp. Hiu ng tn x Brilloin l nguyn l trong cc b cm ng o nhit mi trng ti nhng ni m b cm ng in khng ph hp.
Tn s
Hnh 3.18 Tn s ca nh sng tn x.
Tn x Raman Tn x Raman c phn chia thnh hai loi: Tn x Raman t pht
(Spontaneous Raman Scattering) v tn x Raman kch thch (Stimulated Raman Scattering). Hiu ng tn x Raman t pht c d on bi Smekal vo nm 1923 v n nm 1928 c Raman ch ra bng thc nghim.
Trong qu trnh tn x Raman t pht, nh sng ti tng tc vi mi trng lm sinh ra cc photon. Tu thuc vo bn cht ca mi trng cc photon sinh ra s c tn s ln hn hoc nh hn tn s ca nh sng ti.
Gin nng lng ca qu trnh tn x Raman c th hin trn (hnh 3.19). Electron s chuyn t trng thi khi u (trng thi c bn) ln trng thi o (trng thi kch thch) khi hp th mt photon c nng lng bng hiu nng lng gia trng thi o v trng thi khi u. Khi chuyn t trng thi o v trng thi cui electron s pht x mt photon c nng lng bng hiu nng lng trng thi o v trng thi cui.
Nu nh trng thi khi u c nng lng thp hn nng lng ca trng thi cui, tn s photon pht x s nh hn tn s nh sng ti v qu trnh tn x to ra nh sng Stoke.
1
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Hnh 3.19 Gin nng lng qu trnh tn x Raman. (a)Tn x Stoke (b)Tn x phn Stoke.
Gi s , ln lt l tn s ca nh sng ti v nh sng tn x, l tn s
phonon c sinh ra. Khi theo nh lut bo ton chuyn ho nng lng th
- .
Ngc li nu trng thi cui c nng lng thp hn th qu trnh tn x to ra nh sng phn Stoke c tn s , chch lch gia mc nng lng trng thi
khi u v trng thi cui chnh l nng lng ca mt phonon. Thc t, tn x phn Stoke thng yu hn tn x Stoke.
Tn s ca cc photon tn x c xc nh bng tn s dao ng ca nguyn t. Vi thu tinh, quang ph ca nh sng tn x Raman gm nhiu thnh phn tn s khc nhau l do cc nguyn t trong thu tinh dao ng trong mt khong tn s rt rng.
Hiu ng tn x Raman t pht c ng dng xc nh cu trc ca cc phn t, thnh phn cu to v loi lin kt trong cc phn t .
Khi ta tng dn cng sut bm u vo th cng sut ca sng Stoke cng tng dn. Nhng nu nh cng sut sng bm vt qu mt gi tr xc nh th cng sut sng Stoke s tng ln rt nhanh theo hm m. Nguyn nhn gy ra hin tng ny l qu trnh tn x Raman kch thch.
Nguyn l hot ng b khuch i Raman. Khuch i Raman da trn hin tng tn x Raman kch thch (Stimulated
Raman Scattering). Tn x Raman kch thch l hin tng mt nguyn t hp th nng lng ca mt photon, sau to ra mt photon c nng lng khc. V vy, tn x Raman kch thch c nh ngha l hin tng photon th cp c sinh ra do ngun bn ngoi.
c khuch i Raman th phi to ra s nghch o nng . iu ny t c bng cch cung cp nng lng cho cc nguyn t ca si quang t mt laser bm c bc sng thp hn bc sng ca tn hiu. Khi , cc nguyn t ca si
1 2
2
1
12
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quang s hp th nng lng bm c nng lng cao(bc sng ngn) v chuyn ln mc cao hn. Khi c tn hiu n, n s kch thch cc nguyn t ang mc nng lng cao chuyn sang trng thi nng lng thp hn v gii phng ra mt nng lng di dng photon nh sng c cng bc sng (di hn bc sng bm) v cng pha vi tn hiu n. Do , tn hiu c khuch i bng hiu ng tn x Raman kch thch.
Hnh 3.20 S chuyn nng lng trong khuch i Raman Da trn gin nng lng trn, tn s nh sng bm fbm v tn s nh sng
c khuch i fkhuch i c tnh nh sau
fbm= E3-E1 (3.22)
fkhuch i= (E2-E1)/h (3.23)
Trong : h l hng s Plank;E1, E2, E3 l nng lng ca cc trng thi nng lng cao(transition state), trng thi nng lng trung gian (vibrationstate) v trng thi nng lng thp (groundstate) ca cc nguyn t trong si quang.
Trong khuch i Raman, tn hiu quang c khuch i dc theo ton b chiu di ca si quang silic bnh thng. Cu trc ca mt b khuch i Raman c minh ha trong hnh 3.21.
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Hnh 3.21 Cu trc b khuch i Raman - Si quang: l ni xy ra qu trnh khuch i. Si quang ny cng l si
quang truyn tn hiu nh si SMF, DSF.. - B ghp (Coupler): dng ghp cc bc sng tn hiu vo sng bm - Laser bm (Pump Laser): dng cung cp nng lng cho cc nguyn
t ca si quang chuyn ln trng thi kch thch gip to ra s nghch o nng
- B cch ly (Isolator): t hai u ca b khuch i quang ngn chn tn hiu phn x hai u b khuch i. ng thi n cng gip loi tr nhiu ASE theo hng ngc v pha u vo c th gy nh hng n tn hiu u vo.
C mt s cu hnh bm ph bin cho khuch i quang Raman nh: C th dng cch bm cng hng tc l bm cng chiu vi chiu tn hiu. Cng c th bm ngc hng tc l bm ngc vi hng truyn tn hiu.
Hnh 3.22 Cu hnh bm cng hng
Hnh 3.23 Cu hnh bm ngc hng
Cc thng s k thut.
li (Gain).
li ca mt b khuch i quang l t s gia cng sut quang ng ra chia cho cng sut quang ng vo.
Pump
Fiber
Isolator
Signal
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G=
(3.24)
G(dB) = 10.log [
]
Trong :
G: li tn hiu ca b khuch i quang;
inP , outP : cng sut tn hiu nh sng ng vo v ng ra ca b khuch i
quang(mW)
li l mt thng s quan trng ca b khuch i. N c trng cho kh nng khuch i cng sut nh sng ca b khuch i. Tuy vy, li ca mt b khuh i b gii hn bi cc c ch bo ho li. iu ny lm gii hn cng sut quang ra cc i ca b khuch i quang.
Bng thng li.
li ca b khuch i quang khng bng nhau cho tt c cc tn s ca tn hiu quang vo. Nu o li G ca cc tn hiu quang vi cc tn s khc nhau, mt p ng tn s quang ca b khuch i G(f) s t c. y chnh l ph li ca b khuch i quang.
Bng thng li ca b khuch i quang Bo c xc nh bi im -3dB so vi li nh ca b khuch i. Gi tr Bo xc nh bng thng ca cc tn hiu c th c truyn bi mt b khuch i quang. Do , nh hng n hot ng ca cc h thng thng tin quang khi s dng chng nh cc b lp hay b tin khuch i.
Cng sut ng ra bo ho (Saturation Output Power).
Khi cn hot ng ch tn hiu nh, cng sut quang ng ra s tng tuyn
tnh vi cng sut quang ng vo theo h s li G: outP =G. inP . Tuy nhin, cng
sut ng ra khng th tng mi c bng thc nghim, ngi ta thy rng trong tt c
cc b khuch i quang, khi cng sut ng vo inP tng n mt mc no G bt
u gim. Kt qu l cng sut ra khng cn tng tuyn tnh vi tn hiu ng ra na m t trng thi bo ho. S thay i ca tn hiu quang ng ra so vi cng sut quang ng vo c minh ho nh trong hnh di:
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Hnh 3.24 a) Cng sut ng ra theo cng sut theo cng sut ng vo b) li khuch i quang theo cng sut quang ng ra
Hnh b biu din s bin i ca li ca tn hiu G theo cng sut quang ng
ra outP . Cng sut ng ra ti im li gim 3dB cn c gi l cng sut ng ra
bo ho ,sat outP
Cng sut ra bo ho ,sat outP ca mt b khuch i quang cho bit cng sut ng
ra ln nht m b khuch i quang c th hot ng c. Thng thng, mt b khuch i quang c li cao s c cng sut ra bo ho cao bi v s nghch o nng cao c th c duy tr trong mt di cng sut vo v ra rng.
H s nhiu.
Cng ging nh cc h thng thng tin quang khc, b khuch i ny cng c nhiu. Nguyn l ca b khuch i l da trn nguyn l bc x kch thch. Nhng trong qu trnh khuch i, c rt nhiu cc in t ht thi gian sng, chuyn i t mc nng lng cao xung mc nng lng thp, hay t di dn sang di ho tr, y chnh l bc x t pht. Bc x ny, khi c phng cng lung in t, s gy nh hng ln bin v pha ca tn hiu. Hin tng ny c gi l hin tng nhiu bc x t pht ASE. Do vy, cng sut ca ra gm c cng sut vo khuch i v cng sut bc x t pht:
Pout = G.Pin + PASE (3.25)
nh hng nhiu i vi b khuch i quang c biu din bi h s nhiu NF (Noise Figure). H s ny m t s suy gim t s tn hiu trn tp ti u ra v u vo. H s NF c cho bi cng thc sau:
NF =
(3.26)
Hay NF = SNRin(dB) SNRout(dB) (3.27)
Trong : SNRin v SNRout ln lt l t s tn hiu trn nhiu ti ng vo v ng ra ca b khuch i.
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H s nhiu NF ca b khuch i cng nh th cng tt. Gi tr nh nht ca NF c th t c l 3dB. Nhng b khuch i tha mn h s nhiu ti thiu ny c gi l ang hot ng gii hn lng t.
Ngun nhiu trong b khuch i Raman C 4 ngun nhiu c bn trong k thut Raman, l:
Nhiu tn x Rayleigh kp DRS N tng ng vi 2 hin tng tn x n (mt tn x phn x v mt tn x
ti) do tnh cht khng ng nht ca b vi hin th bng thu tinh tng hp. Khuch i bc x t pht(ASE) truyn trong hung nghch s c phn x li trong hng thun nh DRS v nhng kt qu thu c l do kch thch Raman phn tn to nn. iu ny gp phn lm cho ASE phn x rt nhiu ln, n s lm gim t s S/N. Hn th na, DRS gy ra rt nhiu tuyn nhiu giao thoa v iu cng lm gim SNR. DRS tng ng vi chiu di ca si v h s tng ch bn trong si, v th n l iu ht sc quan trng trong k thut Raman bi trn di ca si quang, nht l nhng ni c chiu di vi km th l yu cu in hnh. ng trn quan im thc t, h s tng ch ca DRS gii hn trn mt tng c khong xp x 10 n 15 dB. b khuch i t c h s tng ch cao hn th s dng bin php cch ly gia cc
tng vi nhau trong b khuch i. V d, mt b khuch i Raman phn tn 30 dB c gii thiu vi 2 tng khuch i cho nhiu nh hn 5.5 dB.
Nhiu do thi gian sng ca electron ti trng thi kch thch ngn
Ngun nhiu th hai xut hin trong thi gian sng rt ngn (c khong 3 n 6 fs) ca bc x Raman trng thi cao hn (bc x t trng thi thp ln trng thi cao). Kt qu ca hin tng gn nh xy ra tc th ny l s ghp ni ca dao ng kch thch ti tn hiu. Cch thng dng trnh ghp ni bt li ny l to mt b kch thch v mt b m truyn tn hiu, n s c tc dng hu hiu khi m u cho thi gian sng trng thi cao m tng ng vi thi gian truyn trong ton b si. Nu s dng b m truyn tn hiu v b kch thch th Laser kch thch s khng b kch thch. iu khin chng c cng rt nh, bi vy mi c gi l cng nhiu tng i (RIN). V d, b m truyn kch thch s dng Laser Diode Fabry-Perot thay v s dng LD c b lc quang n nh.
Nhiu pht x t pht ASE
Ngun nhiu ch yu th ba trong k thut Raman thng l ASE. in hnh l cc mc cng sut trung bnh, vi tn hiu ASE lun c nhiu tng hp tn ti trn n - gi l nhiu tng ASE. Rt may l b khuch i Raman vn c nhiu thp i vi nh hng t tn hiu ASE tng bi v h thng Raman lun hot ng nh l mt h thng o hon ton. V d, cng thc tnh mt ph cng sut ASE :
SASE () = (G 1).h.N2/(N2 N1) (3.28)
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V cng thc tnh dng nhiu l :
NF = 1/G.[ 2.SASE()/ h + 1 ] (3.29)
Vi N2 l mu trng thi cao hn, N1 trng thi thp. Vi k thut Raman th t s N2/(N2 N1) lun gii hn xung quanh mc 1. Chng hn k thut EDFAs th t s ny thng ln hn 1. Trong mt si EDFAs th gii hn t s ny ch ngang bng mc 1 i vi b khuch i o hon ton trn ton b chiu di si quang thu. Mt khc, t khi k thut Raman s dng si quang tuyn di, phn mt mt th ng rt nh ca si thu cng cn c tnh thm vo khi xt ti dng nhiu. Nhng d sao, b khuch i Raman phn tn vi nhiu khong 4.2 dB cng c xt n.
Nhiu do bc sng nh sng bm v nh sng tn hiu gn nhau
N xut hin khi mt thanh t b kch thch bi nhiu quang to ra khi bc sng ca tn hiu c ph ca n c khuch i gn bng vi bc sng kch thch c s dng khi khuch i. Ni mt cch khc th nhit phng hay nhit trong thang my, c mt mu nhit khin cho cc thanh t trong si thy tinh c th t ng kch hot t cc b to sng kch thch, theo m n to thm nhiu cho tn hiu gn vi bc sng kch thch. N cho thy rng iu ny c th dn n vic tng nhiu ln ti 3 dB i vi tn hiu c bc sng gn bng bc sng kch thch.
Phn loi cc b khuch i Raman
Khuch i Raman phn b DRA (Distributed Raman Amplifier)
Hnh 3.25 Khuch i tp trung (a) v khuch i phn b (b) Vi b khuch i Raman phn b DRA, nh sng bm c phn b tri di
trong si quang. DRA tn dng si quang sn c trong mng nh mt phng tin khuch i tn hiu v nh vy nh sng s c khuch i ng u dc theo si quang trn mt khong cch ln (Vi cc b khuch i DRA, thng thng nh sng bm c cng sut cao c bm theo hng ngc kt hp vi cc b khuch i
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tp trung khc nh cc b khuch i quang si pha t him EDFA. u im chnh ca DRA l ci thin t s tn hiu trn nhiu SNR v gim tnh phi tuyn.
Hnh 3.26 Cng sut tn hiu trong h thng s dng DRA Hnh 3.26 biu din mc cng sut nh sng tn hiu theo khong cch ca h
thng khuch i theo chu k. nh hnh rng ca tng ng vi cc im khuch i tp trung. ng nt t l biu din cng sut nh sng tn hiu trong h thng ch s dng cc b khuch i tp trung vi tng ch cao. ng cong trn hnh tng ng vi cng sut nh sng tn hiu trong trng hp s dng b khuch i DRA kt hp vi b khuch i quang tp trung c tng ch nh. Khi s dng DRA mc cng sut tn hiu dc theo si quang s ng u hn. Nu kt hp cc b khuch tp trung mc nh sng tn hiu nh khng qu ln. Nh vy s trnh c cc hiu ng phi tuyn. ng thi mc cng sut nh sng tn hiu cng khng xung thp qu do nh hng ca suy hao do t s SNR c ci thin. T s SNR cao tng ng vi kh nng tng khong cch gia cc b khuch i hoc tng dung lng ca knh tn hiu. Khong cch gia cc b khuch i quang tp trung thng khong 80 km, bng cch s dng DRA hiu nng ca h thng tng ng vi s dng khuch i quang tp trung vi khong cch gia chng l 35 n 38 km .
Ngoi kh nng tng khong cch gia cc b khuch i hoc tng tc bit DRA cn c s dng trong h thng WDM gim khong cch gia cc knh hoc hot ng ti bc sng tn sc khng.
Bn cnh cc u im va nu, khuch i Raman tp trung cng c mt s nhc im:
- Si quang c chiu di hiu dng thp Leff c xc nh t h s suy hao ca si. Trong cc b khuch i DRA chiu di hiu dng ca si quang thng nh hn 40 km. Chiu di hiu dng thp lm gim kh nng tng khong cch gia cc b khuch i.
-
- DRA c cng sut nh sng bm rt cao, v d ti u ho mc nhiu cng sut nh sng bm vi si dch tn sc khong 580 mW v 1.28 W vi si n mode chun. Vi mc cng sut nh sng bm cao nh vy cc thit b quang nh connector rt d b h hi.
- DRA rt nhy cm vi cc iu kin mi trng nh nhit , m v s thay i c hc.
- Mt vn ng c quan tm khc i vi DRA l nhiu tn x Rayleigh kp. Cc b khuch i DRA thng c nhiu DRS cao hn so vi cc b khuch i Raman tp trung khi s dng cng loi si v c chiu di si nh nhau.
- Nhng vn trn lm gim tnh u vit ca DRA. Tuy nhin do li ch t t s SNR v gim hiu ng phi tuyn ca DRA l rt ln nn DRA c s dng kh rng ri trong cc h thng c ly di.
Khuch i Raman tp trung LRA (Lumped Raman Amplifier)
Hnh 3.27 Khuch i Raman tp trung. B khuch i Raman tp trung LRA l mt khi n. Trong b khuch i
Raman tp trung tt c cng sut nh sng bm c tp trung trong mt khi. Hnh 3.27 l mt th d kt ni b khuch i Raman tp trung trong h thng thng tin quang. Trong s trn nh sng bm c gi trong b khuch i bng cc b cch ly xung quanh b khuch i vi chiu di si tng ch Raman khong vi km. Nh vy khc vi b khuch i Raman phn b nh sng bm khng i vo si quang t bn ngoi b khuch i.
c im ng lu nht ca khuch i Raman tp trung l kh nng s dng di bc sng mi m ti cc di bng ny EDFA khng th hot ng.
B khuch i quang lai ghp Raman/EDFA
nh sng bm
Laser bm
nh sng tn hiu c
Si quang khuch i Raman
khuch i
B cch ly quang
Coupler
-
Hnh 3.28 Khuch i quang lai ghp EDFA/Raman. Khuch i quang Raman phn b DRA c th c s dng kt hp vi cc b
khuch i tp trung khc in hnh trong s l kt hp vi b khuch i EDFA hnh thnh b khuch i quang lai ghp Raman/EDFA. Loi khuch i quang ny c th thay th b khuch i EDFA trong khuch i Raman phn b ng vai tr ca mt b khuch i tp m nh (tin khuch i).
So snh b khuch i EDFA v Ramman
S dng khuch i Raman trong mng vin thng l hiu qu thp so vi EDFA. Tuy nhin khi tc bit v tng s knh tng ln th khuch i Raman tr nn hp dn hn. Tng ch ca khuch i Raman ln hn khi cng sut bm ln, iu ny c p ng bi cc h thng trong tng lai.
Trong h thng thng tin quang th h mi, khuch i Raman s chim u th v hiu qu nh sng bm hn khi so snh vi EDFA c nh sng bm 1480nm
PCE=(Pout Pin)/Ppump x 100% ca EDFA nh sng bm 1480nm v khuch i Raman vi cng sut vo 200mW.
Hnh 3.29: Hiu sut chuyn i cng sut ca Raman v EDFA
Hi
u su
t c
huy
n
i cn
g su
t (%
)
Cng sut bm
-
Loi thit b
Khuch i laser FP-LD
Khuch i quang si
Khuch i Raman
Khuch i Brillouin
Khuch i Laser TW-LD
Nguyn l Bc x t nghch o tch ly mi trng
Bc x t nghch o tch ly mi trng
Tn x Raman c kch thch
Tn x Brillouin c kch thch
Bc x t nghch o tch ly mi trng
Cng sut boha li ra(dBm)
8
11
20
-
9
Bng tn khuch i
(1-3)Ghz
(0,5-4)Thz
1Thz
50 Mhz
>5 Thz
Mc tp m (6-9) dB (3-5) dB - - 5,2 dB Suy hao ghp vo si
ln
nh
nh
nh
ln
Phn cc tn hiu
TE-mode c lp Tn hiu/bm
Tn hiu/bm
TE-mode
H s khuch i
(25-30)dB
(40-50)dB
~50dB
~30dB
(20-30)dB
Dng/cng sut bm
10mA
20-100mW ~vi W ~vi W ~100mW
Loithit b Khuch i laser FP-LD
Khuch i quang si
Khuch i Raman
Khuch i Brillouin
Khuch i Laser TW-LD
Nguyn l Bc x t nghch o tch ly mi trng
Bc x t nghch o tch ly mi trng
Tn x Raman c kch thch
Tn xBrillouin c kch thch
Bc x t nghch o tch ly mi trng
Cng sut boha li ra(dBm)
8
11
20
-
9
Bng tn khuch i
(1-3)Ghz
(0,5-4)Thz
1Thz
50 Mhz
>5 Thz
Mc tp m (6-9) dB (3-5) dB - - 5,2 dB Suy hao ghp vo si
ln nh nh nh ln
Phn cc tn hiu
TE-mode c lp Tnhiu/bm Tn hiu/bm
TE-mode
H s khuch i
(25-30)dB
(40-50)dB
~50dB
~30dB
(20-30)dB
Dng/cng 10mA 20-100mW ~vi W ~vi W ~100mW
-
Bng 3.2 Bng so snh cc loi b khuch i
sut bm