Điều khiển tán sắc

7/30/2019 iu khin tn sc

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HC VIN CNG NGH BU CHNH VIN THNGKHOA QUC T V O TO SAU I HC

-----------oOo----------

N MN HC

THNG TIN QUANG NNG CAO

H CAO HCNGNH IN T- VIN THNG

CHUYN

IU KHIN TN SC

GVHD: TS. L Quc CngHVTH: Nguyn Trn Anh Tun

Phm Minh TLp: CH09T2

TP.H CH MINH, NM 2010


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HC VIN CNG NGH BU CHNH VIN THNGKHOA QUC T V O TO SAU I HC

-----------oOo----------

N MN HC

THNG TIN QUANG NNG CAO

H CAO HCNGNH IN T- VIN THNG

CHUYN

IU KHIN TN SC

GV hng dn: TS. L Quc CngSinh vin thc hin: Nguyn Trn Anh Tun

Phm Minh TLp: CH09T2

TP.H CH MINH, NM 2010


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Kthutiu khin tn sc

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MC LC

DANH MC HNH V........................................................................................................ 2DANH MC TVIT TT ................................................................................................ 3IU KHIN TN SC ...................................................................................................... 6

1. CN THIT PHI IU KHIN TN SC .......................................................... 62. M HNH B TRC (PRECOMPENSATION) ................................................... 9

2.1 K thut Prechirp ................................................................................................ 92.2 K thut m ha Novel: .................................................................................... 122.3 K thut Prechirp phi tuyn: ............................................................................. 14

3. K THUT B SAU .............................................................................................. 164. SI QUANG B TN SC ................................................................................... 175. B LC QUANG .................................................................................................... 196. CCH T SI QUANG BRAGG (Fiber Bragg Gratings).................................... 22

6.1 Chu k cch tng nht (Uniform-Period Gratings) ..................................... 236.2 Chirped Fiber Gratings: (Cch t si quang Chirped) ..................................... 266.3 B ghp mode Chirped (chirped mode couplers) ............................................ 29

7. LIN HP PHA QUANG OPC .............................................................................. 307.1 Nguyn l hot ng: ........................................................................................ 307.2 B tn sc bng tiu ch pha (Compensation of Self-Phase Modulation ) .. 317.3 Tn hiu lin hp pha (Phase-conjugated Signal): ............................................ 33

8. H THNG QUANG NG DI: ..................................................................... 378.1 L thuyt cs: ................................................................................................ 398.2 Hiu ng tng tc phi tuyn ng knh (Intrachannel Nonlinear Effects):.... 41

9. H THNG QUANG DUNG LNG CAO ......................................................... 439.1 B tn sc bng rng : ...................................................................................... 439.2 B tn sc iu khin c (Tunable Dispersion Compensation) .................... 46


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9.3 iu khin tn sc bng thnh phn tn sc bc cao : ...................................... 489.4 B tn sc phn cc mode PMD ....................................................................... 50

TI LIU THAM KHO ............................................................................................... 54

DANH MC HNH V

Hnh 1.1: th quan h tc truyn theo chiu di si quang tng ng vi b rng

ph ca ngun i-t pht quang bng 0, 1 v 5 nm .............................................................. 7

Hnh 2.1: th m t gin rng xung theo khong cch truyn vi xung u vo lxung chirp Gauss trong trng hp 2>0 ........................................................................... 10Hnh 2.2: M hnh k thut prechirp dng b tn sc ................................................... 11Hnh 2.3: B tn sc s dng m ha FSK ........................................................................ 12Hnh 2.4: th cng sng theo thi gian tn hiu 16 Gbps khong cch truyn 70km

s dng si quang tiu chun c v khng c s dng k thut nn tn sc ...................... 14Hnh 2.5: th quan h gia khong cch truyn b gii hn do tn sc GVD v mc

cng sut truyn trung bnh ................................................................................................. 15Hnh 4.1: M hnh si DCF hai mode s dng cch t chu k di .................................... 19Hnh 5.1: M hnh kt hp gia b lc quang v khuych i quang. .............................. 20Hnh 5.2 : M hnh b lc quang s dng giao thoa Mach-Zehnder. ............................... 21Hnh 6.1 : ln(a) v pha(b) ca h s phn x cch t si quang ng nht vi gL

=2 v gL =3 ....................................................................................................................... 23Hnh 6.2: Tn sc vn tc nhm GVD . M t hm 2

g theo thng s tng ng vi

cc gi tr ca h s trong khong 1-10 .......................................................................... 24Hnh 6.3: Tn hiu pht (ng lin nt) v tr( ng chm) , hm ca bc sng cho

cch tng nht (z) thay i t 0-6 trn chiu di cch t 11cm ...................... 25Hnh 6.4 Cch t quang Chirped dng b tn sc a/ chit sut n(z) theo chiu di cch t

b/ h s phn xtn s thp v cao ti nhng vng khc nhau trong cch t ................. 27Hnh 6.5: H s phn x v thi gian tr trong cch t quang Chirped tuyn tnh ........... 27

1cm


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vi bng thng 0.12nm ....................................................................................................... 27Hnh 6.6: S b tn sc bng cch dng 2 b lc pht fiber base transmission filter . 29Hnh 7.1: Th nghim b tn sc trong o khong gia ph trn 21 km chiu di si

quang ................................................................................................................................... 34Hnh 8.1: Vng lp quang dng pht tn hiu tc 10 Gb/s trn khong cch

10.000 km si quang chun s dng SCF. .......................................................................... 37Hnh 9.1 : M hnh ghp tng cch t b tn sc trong h thng WDM ...................... 44Hnh 9.2 : Ph phn x v th tn sc theo in p t ca phng php gradient nhit

............................................................................................................................................. 47Hnh 9.3 : Dng xung ng ra khi truyn vi khong cch 300km khi khng .................... 49v c dng si dch tn sc ................................................................................................. 49Hnh 9.4: M hnh b tn sc PMD quang v in ............................................................ 51Hnh 9.5: B tn sc iu chnh c s dng cch t quang chirp khc x kp .............. 52Hnh 9.6: th quan h gia h s mrng xung v gi tr DGD trung bnh. ................ 53

DANH MC TVIT TTASE Amplifier Spontaneous Emission Bc x t pht khuch i

ADM Add Drop Multiplexer Thit b xen r

BER Bit Error Ratio T l li bt

CW Continuous Wave Sng lin tc

DBR Distributed Bragg Reflector Phn x phn b Bragg

DCF Dispersion-Compensating Fiber Si quang b tn sc

DDF Dispersion-Decreasing Fiber Si quang gim tn sc

DGP Differential Group Delay Tr nhmDM Dispersion-managed Qun l tn sc

DWDM Dense Wavelength-Division

Multiplexing

Ghp knh phn chia theo

bc sng mt cao

EDFA Erbium-Doped Fiber Amplifier B khuych i quang pha

trn Erbium

FM Frequency Modulation iu ch tn s

FP FabryPerot Mt loi khoang cng

hng

FRASL Fiber Raman Soliton Laser Laser quang Raman Soliton


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FWHM Full-Width at Half-Maximum rng ti na cc i

FWM Four-Wave Mixing Trn 4 bc sng

GVD Group-Velocity Dispersion Tn sc vn tc nhm

LED Light Emitted Diode Ngun pht dng LED

MZ MachZehnder Mt loi b lcNTE Network Terminal Equipment Thit bu cui mng

NLS Nonlinear Schrodinger Schrodinger phi tuyn

NOLM Nonlinear Optical-Loop Mirror Gng quang vng phi

tuyn

NRZ Nonreturn to Zero M NRZ

NSE Nonlinear Schrodinger Equation Phng trnh Schrodinger

NSDSF Nonzero-Dispersion-Shifted Fiber Si quang dch tn sc

OA Optical Amplifier Khuch i quang

OAT Optically amplified transmitter B pht khuch i quangOADM Optical Add/Drop Multiplexer B r bc sng quang

OBA Optical Booster Amplifier B khuch i m quang

OAR Optically amplified receiver B thu khuch i quang

OC Optical Channel Knh quang

ODM Optical Demultiplexer Tch bc sng quang

OF Optical Fiber Si quang

OFC Optical Fiber Cable Cp si quang

OM Optical Multiplexer Ghp bc sng quang

OMUX Optical MUX B ghp knh quang

OPA Optical Preamplifier B tin khuch i quang

OPU Optical Preamplification Unit Khi tin khuch i quang

ORX Optical Receiver B thu quang

OSC Optical Transmission Section Knh gim st quang

OTX Optical Transmitter B pht quang

PIM Polarization-Interleaved multiplexing Ghp xen knh phn cc

PMD Polarization-Mode Dispersion Tn sc phn cc mode

RZ Return to Zero M RZSAGCM Separate Absorption, Grading,

Charge, and Multiplication

S hp th, pha trn, ph tn

v khuch i ring bit

SNR Signal-To-Noise Ratio T s tn hiu trn nhiu

SOA Semiconductor Optical Amplifier B khuch i quang bn

dn

SPM Self-Phase Modulation Tiu ch pha

SRS Stimulated Raman Scattering Tn x kch thch Raman

SSFS Soliton Self-frequency Shift Dch tn s Soliton

TOD Third-Order Dispersion Tn sc bc 3TW Traveling Wave Sng Traveling


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WDM Wavelength-Division Multiplexing Ghp knh phn chia theo

bc sng

WADM Wavelength Division Multiplexing

Access

Mng s dng k thut a

truy nhp ghp knh theo

bc sngXPM Cross-Phase Modulation iu ch xuyn pha


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IU KHIN TN SC

Suy hao quang khng cn l gii hn ln nht trong cc h thng thng tin quang, thay

vo trong cc h thng thng tin quang hin i gii hn thng gp nht l do tn sc

v cc hiu ng phi tuyn gy nn. Suy hao quang c gii quyt mt cch d dng bng

cc b khuych i quang tuy nhin i km vi n li lm gia tng tn sc, tri ngc vi

cc b ti to (Regenerator) in t, mt b khuych i quang khng khi phc li tn

hiu c khuych i thnh tn hiu gc ban u. Kt qu l, tn sc tch ly qua cc b

khuych i lm gim kh nng truyn tn hiu. Chnh v l do ny, c nhiu m hnh

iu khin tn sc c nghin cu sut thp nin 1990 hn ch tc ng ca tn sc

trong cc h thng thng tin quang. Trong bi bo co ny s gii thiu mt s k thut

c bit da vo l tnh ca hin tng truyn dn quang ci thin tn sc trong thc t.

mc 1 gii thch s cn thit phi iu khin tn sc. Mc 2 v 3 dnh ton b cho cc

phng thc c dng u pht v u thu iu khin tn sc. mc 1.4 n mc

1.6 gii thiu phng php s dng cc phn t quang tn sc cao trn ng cp quang.

K thut s dng tn hiu quang pha kt hp hay cn c gi l k thut o khong gia

ph (midspan spectral inversion) sc gii thiu mc 7. Mc 8 gii thiu viu

khin tn sc trong cc h thng ng di. Mc 9 tp trung vo cc h thng dung lng

cao nh cc h thng bng rng. K thut b tn sc phn cc mt (PMD) cng sc

cp trong mc ny.

1. CN THIT PHI IU KHIN TN SCTn sc lm gin b rng xung nh sng truyn trong si quang lm gii hn hot ng

ca h thng truyn dn quang. Nh ta bit hiu ng tn sc vn tc nhm (GVD) c

thc ti thiu ha bng la-de c rng ph hp (xem hnh 1.1 quan h gia tc

truyn theo chiu di si quang tng ng vi b rng ph ca ngun i-t pht quang

bng 0, 1 nm v 5 nm) v khng b tn sc bc sng tn sc khng ZD . Tuy nhin,

trong thc t, h thng truyn dn quang thng hot ng bc sng khc vi bc

sng tn sc khng ZD . Mt v d trong thc t l h thng thng tin quang trn b hotng bc sng 1,55 m s dng la-de pht DFB, cc h thng ny s dng cp si


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quang n mt tiu chun vi bc sng tn sc khng ZD l 1,31 m h thng ny

c xy dng trong sut thp nin 1980 Hoa K v c chiu di khong 50 triu km.

Do c tn sc 16 / ti vng bc sng 1,55 m, nn tn sc GVD hn

ch hot ng ca h thng tc 2Gbps.

Hnh 1.1: th quan h tc truyn theo chiu di si quang tngng vi brngphca ngun i-t pht quang bng 0, 1 v 5 nm

i vi la-de pht DFB iu ch trc tip, chng ta c th s dng phng trnh 1.1

c lng khong cch truyn ti a

1(4 )L B D s< (0.1)

Vi s

l gi tr cn trung bnh bnh phng (RMS) b rng ca ph xung b mrng do

tn s chirp (s thay i tn s theo thi gian). H thng c h s tn sc D=16 ps/(km-

nm) v s= 0,15nm hot ng tc B = 2,5 Gbps theo cng thc 1.1 ta c th tnh ra

c L 42 km. V th, i vi cc h thng s dng thit b ti to tn hiu bng in t,

khong cch gia cc b ti to tn hiu vo khong 40km. Hn na vic s dng cc b

ti to tn hiu lm hn ch kh nng tng tc truyn dn ca h thng, bi nu mun

tng tc truyn phi thu nh khong cch gia cc b ti to tn hiu dn n chi ph

u t s tng cao.


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Hot ng ca h thng c thc ci thin ng k bng vic s dng mt biu ch

ngoi trnh c vic m rng ph do tn s chirp. La chn ny c ng dng

vo thc t bng cc b pht s dng la-de DFB vi biu ch ngoi tch hp. Trong

trng hp s=0, khong cch truyn gii hn theo cng thc2 1

2(16 )L B< (0.2)

Vi 2 l h s tn sc vn tc nhm GVD. Nu ta s dng gi tr thng dng ca h s

tn sc vn tc nhm GVD 2= -20ps2/km bc sng 1,55 m, p dng cng thc 1.2 ta

tnh c khong cch truyn L


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2. M HNH B TRC (PRECOMPENSATION) tng thc hin phng php ny l thc hin iu khin tn sc bng cch iu chnh

cc c tnh ca cc xung ng vo b pht trc khi truyn i trong si quang. tng

ny c thc hin da vo phng trnh 1.4. Phng trnh ny s ch cn thnh phnbin ph bin thin (0, )A trong trng hp tn sc GVD c loi b. R rng,

thc hin iu ny bin phu pht phi c b thm mt thnh phn theo biu

thc 2.1 trit tiu tc ng do GVD gy nn:

22(0, ) (0, )exp( / 2)A A i L (1.1)

Vi L l chiu di si quang, GVD sc b mt cch chnh xc v xung vn s gi

c hnh dng ca n u ra ca si quang. Tuy nhin, khng d dng gii quyt vn

ny bng vic thay i bin phu pht theo nh biu thc 2.1 trong thc t. Mt

cch n gin hn, ngi ta s dng hiu ng chirp xung ng vo lm ti thiu ha tc

ng gin rng xung ca tn sc GVD. Do tn s chirp c a vo b pht trc khi

truyn xung, nn k thut ny c gi l k Prechirp.

2.1 Kthut PrechirpCch n gin hiu c nguyn tc hot ng ca k thut Prechirp l da vo l

thuyt truyn xung chirp Gauss trong si quang. Bin ng vo di tc ng ca

hiu ng chirp c biu din bng cng thc 2.2

2

00

1(0, ) exp

2

iC tA t A

T

+ = (1.2)

Vi C l h s chirp, da vo hnh 2.1 ta thy rng vi gi tr C m 2C


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ti a ca h thng l 2 ln khi C=1/ 2 . Do k thut prechirp cn c ti u mt

cch k lng c c gi tr C hp l nht. Trong thc t hnh dng xung ch xp x

xung Gauss, nn k thut prechirp c th gip ci thin c n 2 ln khong cch truyn

nu ti u hp l. Vo khong cui nm 1986, m hnh Super-Gaussian cho vic truyntn hiu NRZ c a ra nhm nng ci thin hn na khong cch truyn dn quang.

Hnh 2.1: th m t gin rng xung theo khong cch truyn vi xungu vo lxung chirp Gauss trong trng hp2>0

K thut prechirp c xem xt trong sut thp nin 1980 s dng cc la-de iu ch trc

tip. Xung chirp do cc la-de ny to ra l do s thay i ch s cm ng sng mang

(carrier-induced index) c m t bng h s tng b rng phC. Khng may l h s

chirp C m (C= - C) i vi cc la-de iu ch trc tip. Do h s2 ca si quang tiu

chun hot ng bc sng 1,55 m cng m, v thiu kin 2.C0 s lm tng thm gin xung do GVD gy ra, v th lm gim khong cch

truyn. V th c mt s m hnh c a ra vo thp nin 1980 da vo vic tm hnh

dng xung pht khc ph hp hn nhm ci thin khong cch truyn .

Trong trng hp s dng b iu ch ngoi, cc xung quang gn nh khng b hin

tng chirp, v th to ra tn hiu chirp ngi ta s dng cc biu tn FM. K thut

prechirp trong trng hp ny to ra tn s chirp vi h s chirp C dng nhm tha iu

kin 2.C


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m hnh cbn, tn s ca la-de DFB to ra u tin c iu tn FM sau c a

vo biu ch ngoi iu ch bin AM. Kt qu l tn hiu quang truyn i c

iu ch c AM v FM. Trn thc t, sng mang quang s dng trong iu ch FM c th

c thc hin bng cch a dng in nh khong 1mA vo la-de DFB.

Hnh 2.2: M hnh kthut prechirp dngb tn sc

hnh 2.2 khi c tn hiu FM s dng sng mang quang theo sau s to ra tn hiu AM

pht ra ngoi, tn hiu ny cha cc xung chirp. Mt s cc chirp c hn ch nh sau.

Gi rng hnh dng xung l Gauss, tn hiu quang c thc vit

[ ]2 20 0 0(0, ) exp( / )exp (1 sin )mE t A t T i t t = + (1.4)

Vi tn s 0 ca xung c iu ch hnh sin ti tn s m vi qu iu ch l .

gn trung tm ca xung sin( )m mt t khi phng trnh 2.4 s trthnh

2

0 00

1(0, ) exp exp( )2iC t

E t A i tT

+

(1.5)

H s chirp C l:

20 02 mC T = (1.6)

Ta thy rng h s chirp c thiu khin c bng cc thng siu tn FM l v m.

iu ch pha ca sng mang quang cng lm cho chirp dng, phng trnh 2.4 c th

c vit li thnh:


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[ ]2 20 0 0(0, ) exp( / )exp cos( )mE t A t T i t i t = + (1.7)

Do s dng cosx1-x2/2. u im ca k thut iu pha l bn thn biu ch ngoi c

th tiu chnh pha ca n. Phng php n gin nht thc hin l s dng biu

ch ngoi c chit sut iu chnh c bng in t, bng cch ny s to ra tn s chirpvi C>0. Vo cui nhng nm 1991 tn hiu 5 Gbps truyn i c 256 km khi s dng

biu ch LiNbO3 cho gi tr C trong khong 0,6 n 0,8. Cc gi tr thc nghim ny

tng ng vi phng trnh 2.3 ca l thuyt truyn xung Gauss. Nhng dng khc ca

cc biu ch bn dn nh biu ch hp thin (Electroabsorption Modulator) hoc

biu ch Mach-Zehnder (MZ) cng to ra cc xung quang c h s chirp C>0, v th ci

thin c kh nng truyn dn tn hiu quang do hn chc tc ng ca tn sc. Vi

s pht trin ca cc la-de DFB cha cc phn t Li n (monoLithically) tch hp trong

cc biu ch hp thin (Electroabsorption Modulator) tip tc lm ci thin rt

nhiu k thut Prechirp trong thc t. Vo nm 1996 tn hiu NRZ 10Gbps truyn i

c khong 100km s dng si quang tiu chun vi nhng biu ch ngoi ny lm

b pht.

2.2 Kthut m ha Novel:

Hnh 2.3: B tn sc sdng m ha FSK

S dng kt hp iu ch AM v FM cho tn hiu quang khng phi l phng php duy

nht b tn sc. Mt phng thc khc c s dng b tn sc l s dng iu ch

kha dch tn FSK truyn dn tn hiu. Tn hiu FSKc to ra bng cch chuyn i

bc sng ca la-de bng mt khong gia cc bit 0 v bit 1 trong khi cng sut pht

khng i. Trong sut qu trnh truyn dc theo si quang, hai bc sng ny truyn i


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vi tc khc nhau. Khong thi gian tr gia bit 0 v bit 1 c xc nh da trn

khong bc sng dch v bng T DL = . dch bc sng c chn sao

cho 1/T B = . Hnh 2.3 ch lm th no mt bit b tr to ra c ba mc tn hiu quang

ti b thu. Xt v mt bn cht, do tn sc trong si quang, tn hiu FSK b chuyn thnhtn hiu biu ch c bin . Tn hiu c gii m ti u thu bng cch s dng b

tch phn in kt hp vi mch quyt nh.

Nhiu thc nghim c thc hin nh gi k thut b tn sc nu trn. Tt c cc

th nghim ny nhm n vic tng khong cch truyn bc sng 1,55m tc 10

Gbps hoc cao hn s dng si quang tiu chun. Vo nm 1994, vic truyn tn hiu 10

Gbps qua mt khong cch 253km s dng si quang tiu chun thc hin c. Cho

n nm 1998, trn si quang tiu chun ngi ta truyn i c tn hiu 40 Gbps vi

khong cch truyn l 86km. R rng so snh vi mc trc, khong cch truyn c

ci thin ng k hn nhiu khi s dng k thut FSK.

Mt cch khc tng khong cch truyn da vo vic truyn tn hiu quang c bng

thng nh hn tc truyn chng hn nh k thut s dng m tt m(On-Off). Mt m

hnh thc hin vic to ra tn hiu quang c bng thng nh hn tc truyn l s

dng m ha nh phn kp (duobinary coding), k thut ny c th lm gim bng thng

n 50%. M hnh n gin nht to m nh phn kp l s dng hai bit lin tip trong

chui bit cng li vi nhau, kt qu l to ra c m nh phn kp ba mc bn tc. Do tc

ng ca GVD ph thuc vo bng thng tn hiu, nn khong cch truyn cng c th

c ci thin do gim bng thng. iu ny c chng t trong thc nghim.

Thc nghim vo nm 1994 so snh hai m hnh nh phn v nh phn kp, mt tn hiu

10 Gbps c th truyn c khong cch 30 n 40 km bng cch thay th m nh phn

thng thng bng m nh phn kp. K thut s dng m nh phn kp c th kt hp

vi k thut Prechirp. Tn hiu 10 Gbps truyn i c 160km trn si quang tiu

chun khi kt hp m nh phn kp vi mt biu ch ngoi to tn s chirp c C>0.

Trong thc t, xut hin hin tng o pha khi tn hiu nh phn kp c to ra, hin

tng ny gip ci thin hot ng ca h thng khi s dng m nh phn kp. Mt m

hnh iu khin tn sc mi c gi l m hnh to dng pha nh phn (phase-shaped

binary), tn dng nhng u im ca hin tng o pha. S dng phng thc truyn

nh phn kp yu cu phi tng t s tn hiu trn nhiu S/N v phi c b gii m u

thu. Mc d c nhng hn ch nh th nhng li ch do n em li gp phn ci thin

ng k hot ng cho cc h thng tc 10Gbps v cao hn.


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2.3 Kthut Prechirp phi tuyn:Mt k thut b tn khc c gi l k thut prechirp phi tuyn c a ra vo nm

1989 bng cch khuych i ng ra b pht bng b khuych i bn dn quang SOA

hot ng ch c li bo ha. ch c li bo ha xy ra cc bin i phthuc thi gian ca mt sng mang, do xut hin hiu ng chirp bn cnh vic

khuych i xung truyn. Hiu ng chirp ph thuc vo dng xung ng vo, v gn nh l

tuyn tnh vi hu ht cc xung. SOA khng nhng khuych i n thun xung truyn

m cn lm cho chirp c thng s C>0. Do xut hin hiu ng chirp ny, xung ng vo c

th b nn li trong si quang c 2


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nh mt phn t khuych i ng dy c th dng b ng thi suy hao si quang v

tn sc GVD.

mi trng phi tuyn cng c th s dng k thut prechirp cho cc xung truyn. mi

trng c chit sut ph thuc cng quang (Intensity-dependent refractive index) sgy ra hiu ng chirp cho cc xung quang thng qua hin tng tiu pha SPM. Do

mt k thut prechirp phi tuyn n gin da trn hin tng ny l s dng ng ra b

pht mt si quang c chit sut ph thuc cng quang vi chiu di ph hp trc

khi a tn hiu quang vo tuyn quang cn truyn. Tn hiu quang si quang thm

vo l:

[ ](0, ) ( ) exp ( )mA t P t i L P t= (1.8)

Vi P(t) l cng sut ca xung, Lm l chiu di ca mi trng phi tuyn v l h s phi

tuyn. Trong trng hp cc xung Gauss c cng thc 2 20 0( ) exp( / )P t P t T= , hiu ng

chirp khi gn nh l tuyn tnh, khi phng trnh 2.8 s xp x bng

2

0 00

1(0, ) exp exp( )

2 miC t

A t P i L PT

+

(1.9)

Vi thng s chirp C=2LmP0 , thng s phi tuyn >0 th thng s chirp C s dng v

th c th thc hin c vic b tn sc.

Hnh 2.5: th quan h gia khong cch truyn b gii hn do tn sc GVD v mccng sut truyn trung bnh


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Kthutiu khin tn sc

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Do>0 i vi cc si quang silica, v th bn thn si quang c thc dng gy ra

hiu ng chirp ln xung. tng ny c a ra vo nm 1986. Bng vic s dng cc

soliton th t cao i qua tng nn u em li nhiu ci thin ng k. Hnh 2.5 th

quan h gia khong cch truyn b gii hn do tn sc GVD v mc cng sut truyntrung bnh h thng 4 v 8 Gbps. th ny ch ra rng hon ton c th tng gp i

khong cch truyn bng cch ti u li mc cng sut trung bnh ca tn hiu ng vo

mc khong 3 mW.

3. K THUT B SAUCc k thut in t c thc dng b tn sc GVD ti b thu. tng ca phng

php ny l mc d tn hiu quang c th b suy bin do GVD, nhng ta c th cn bng

in t tn sc ny ca si quang bng hm truyn tng ng nhm trit tiu thnh phntn sc 2. N c th d dng b tn sc nu s dng b thu Heterodyne nhn dng tn

hiu. B thu Heterodyne u tin s chuyn tn hiu quang thnh tn hiu vi sng tn s

trung tn IF vi y thng tin v bin v pha. B lc thng di (bandpass) vi sng

c p ng xung theo hm truyn:

( )2

IF 2( ) exp / 2H i L = (2.1)

Vi L l chiu di si quang. Tn hiu sc khi phc li u thu tn hiu, kt lun

ny tun theo l thuyt chun ca cc h thng tuyn tnh bng cch s dng phng trnh1.4 vi z=L. Thc nghim vo nm 1992 s dng ng dy microstrip chiu di 31,5 cm

cn bng tn sc, thc nghim ny truyn tn hiu 8 Gbps i mt khong cch 188

km vi si quang tiu chun c tn sc D=18,5 ps/(km-nm). Th nghim vo nm 1993, s

dng k thut pht hin homodyne truyn tn di bn n (single sideband), v kt qu

l tn hiu 6 Gbps c th khi phc c ti u thu cch 270 km s dng si quang

tiu chun. ng truyn vi di (microstrip) c thc thit k b tn sc GVD trn

si quang c chiu di 4900km tc 2,5Gbps.Thng thng ngi ta s dng b cn bng tn sc in t trong thc t nh mt b tch

sng thu trc tip (direct-detection receiver). Mt mch in t tuyn tnh khng th b

tn sc GVD, cc thng tin v pha hon ton b mt trong sut qu trnh tch sng trc

tip. p ng ca tch sng quang chn thun nhn bit cng tn hiu quang. Kt

qu l khng mt k thut tuyn tnh no c th khi phc li tn hiu b gin rng. Tuy

vy, mt vi k thut cn bng phi tuyn c pht trin cho php khi phc li tn

hiu ban u t tn hiu thu b tc ng bi tn sc. phng php u tin "ngng

quyt nh" (Decision threshold) sc gicnh trung tm ca gin mt (eye


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Kthutiu khin tn sc

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diagram), y l gin ph thuc nhiu vo cc bit trc . Mt phng php khc, b

thu s quyt nh bit thu c sau qu trnh kim tra dng tn hiu tng t trn ng

bao bit khong thi gian gia cc bit. Kh khn chnh ca tt c cc k thut ny l cc

mch in t logic phi hot ng tc bit cao v phc tp ca n tng theo hm sm vi s lng cc bit c xung quang b gin rng do GVD. Do , cn bng in t

thng b gii hn l hot ng tc thp v khong cch truyn khng cao.

K thut cn bng quang in t da vo si quang ngang (transversal fiber) c gii

thiu. k thut ny, mt b chia cng sut u thu chua tn hiu quang nhn c

thnh nhiu nhnh. trng dy trn cc nhnh s khc nhau, tn hiu quang trn mi

nhnh ny c chuyn thnh dng quang in bng cc b tch sng quang nhy thay

i v cc dng quang in ny sc cng li vi nhau sau a vo mch quyt

nh. K thut ny c th tng khong cch truyn thm 3 ln i vi cc h thng 5Gbps

4. SI QUANG B TN SCK thut b trc c th tng khong cch truyn ln 2 ln, tuy nhin n li khng ph

hp vi cc h thng ng di, h thng ny yu cu GVD phi c b lin tc theo

chu k dc theo ng truyn. c bit trong cc h thng ton quang vic s dng cc b

b tn sc quang in t l khng ph hp. V th ngi ta nghra mt si quang c

bit gi l si quang b tn sc (DCF : Dispersion Compensating Fiber). Vic s dng siDCF cho cc h thng ton quang c th b GVD mt cch ng k nu cng sut quang

trung bnh c gi nh th cc hiu ng phi tuyn bn trong si l khng ng k.

hiu bn cht vt l ca k thut iu khin tn sc ny, ta nh gi xung quang truyn

i trong hai si quang trong si th hai l si DCF. S dng cng thc 1.4 phng

trnh truyn nh sng quang :

221 1 22 2

1( , ) (0, )exp ( )

2 2

iA L t A L L i t d

+

= +

(3.1)

Vi L=L1+L2 v 2j l thng s GVD ca si quang c chiu di Lj (j=1,2). Nu s dng

si DCF s kh thnh phn pha 2, xung truyn sc khi phc v hnh dng ban u

pha cui ca si DCF. iu kin b tn sc tt nht l 21 1 22 2 0L L + = hoc

1 1 2 2 0D L D L+ = (3.2)

Phng trnh 4.2 ch ra rng si quang DCF phi c h s tn sc GVD 1,55m l D20. Hn na chiu di si quang cng c la chn

tha iu kin :


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Kthutiu khin tn sc

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2 1 2 1( / )L D D L= (3.3)

Trong thc t ngi ta c gng chn L2 nh nht nu c th, trng hp ny xy khi si

DCF c gi tr m D2 rt nh (hay 2D rt ln).

Mc d tng s dng si DCF c a ra vo nhng nm 1980, nhng cho mi sau

ny khi xut hin cc b khuych i quang vo nhng nm 1990 th vic ng dng si

DCF mi c pht trin. C hai hng cbn thit k si DCF. u tin si DCF h

trn mode, nhng n c thit k vi tn s chun ha V nh. Mode cbn c

gii hn quanh mc 1V . Phn nh cc mode cn li c truyn lp bc (cladding),

l ni chit sut nh, ng dn sng lm gia tng thm GVD v kt qu l h s tn sc c

gi tr 100 / ( )D ps km nm . Thit k lm gim lp bc thng c s dng trong

thc t sn xut si DCF. Tuy nhin, si DCF li c suy hao ln do s gia tng suy hao doun cong ( 0,4 0,6 /dB km = ). H s /D thng c s dng v gi l h s phm

cht M ca si DCF. Vo nm 1997, si DCF vi 250 / ( )ps nm dB> c th ch

to c.

Cng vi s xut hin ca si DCF mt gii php thc t ci thin cc h thng quang

mt t l thm vo tuyn si quang tiu chun hin c cc module DCF (vi khong 6-

8km si DCF) kt hp vi cc b khuych i quang cc khong 60-80km. Si DCF b

tn sc GVD trong khi cc b khuch i b li phn suy hao ca si quang. M hnh nycn tn ti hai vn . u tin l suy hao chn ca cc module DCF thng thng

khong 5dB. Suy hao chn c thc b bng cch tng li ca cc b khuych i

tuy nhin vic tng li li lm tng nhiu ASE (nhiu do khuych i bc x t pht).

Vn th hai, lin quan bn knh trng mode nh ca si DCF, n ch xp x~20 m2 .

Khi cng sut quang ln a vo DCF nh cng sut ng vo, cc hiu ng phi tuyn s

tng ng k.

Cc vn lin quan n si DCF c thc gii quyt bng cch s dng si quang haimode c thit k vi tn s chun ha V ca mode c th t cao hn gn im

cutoff hn ( 2, 5V ). Chng hn nh cc si quang c cng suy hao nh si quang n

mode, nhng c thit k h s tn sc D ca mode c th t cao c gi tr m nh v

bng khong -770ps/(km-nm). 1km chiu di ca si DCF c th b GVD cho khong

40km tuyn cp quang, vic thm vo si DCF chiu di nh th cng lm tng ng k

suy hao quang trn ton tuyn.

Vic s dng si DCF hai mode yu cu mt thit b chuyn i mode c kh nng

chuyn i nng lng t mode cbn sang mode c th t cao hn. c mt s thit b


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Kthutiu khin tn sc

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chuyn i mode ton quang c pht trin. Thit b chuyn i mode ton quang c

da trn quan im v tnh tng thch trong mng ton quang, v gim suy hao chn. Mt

yu cu na i vi thit b chuyn i mode l n phi c phn cc mnh v hot ng

vi bng thng rng. Hu ht cc b chuyn i mode trong thc t s dng si quang haimode vi mt cch t quang ghp ni gia hai mode. Chu k cch t c chn sao

cho sai khc h s mode gia hai mode l n tha / n = v thng thng n xp

x 100 m. V cc cch tc gi l cch t quang chu k di. Hnh 4.1 ch ra m hnh

si DCF hai mode vi cch t chu k di.

Hnh 4.1: M hnh si DCF hai mode sdng cch tchu k di

Cc c tnh tn sc o c ca si DCF c ch ra hnh 4.1b. H s tn sc D c gi

tr ( )420ps / km nm bc bc sng 1550nm v thay i nhiu cc bc sng

khc. y l mt c tnh quan trng cho php b tn sc bng rng. Ni mt cch tng

qut si DCF c thit k D tng theo bc sng. S ph thuc vo bc sng ca

h s tn sc D l mt c tnh quan trng DCF c th hot ng trong cc h thng

WDM. Phn ny sc cp mc 9.

5. B LC QUANGNhphn trc cp nu s dng si DCF c chiu di ln hn 5km c th b tn sc

GVD cho khong 50km si quang. Vic thm vo si DCF ny lm gia tng ng k suy

hao ca tuyn quang, iu ny gy nh hng i vi cc ng dng ng di. Chnh v l

do ny, mt vi m hnh quang khc c nghin cu iu khin tc ng ca tn sc.

Hu ht trong s nghin cu mi ny l cc b lc cn bng quang (Optical Equalizing

Filter) . Cc b lc giao thoa (Interferometric Filter) sc gii thiu trong mc ny,

mc tip theo s trnh by v cc cch t quang.

Chc nng ca b lc quang c thc din t qua cng thc 1.4. Tc ng ca GVD

ln tn hiu quang c biu hin thng qua phn t pha 22exp( / 2)i z , hin nhin l b


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Kthutiu khin tn sc

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lc quang c hm truyn s trit tiu phn t pha ny khi phc li tn hiu ban u.

Tuy nhin, khng c b lc quang no c hm truyn ph hp hon ton b tn sc

GVD mt cch chnh xc. Mt s b lc quang c kh nng b tn sc ring bng cch bt

chc hm truyn l tng. Nu b lc c t sau si quang c chiu di L, tn hiuquang sc lc v c thc vit li bng cch s dng biu thc 1.4

22

1( , ) (0, ) ( ) exp

2 2

iA L t A H L i t d

+

=

(5.1)

Bng cch khai trin thnh phn pha H() bng khai truyn Taylorn bc 2 ta c :

[ ] 20 1 21

( ) ( ) exp ( ) ( ) exp ( )2

H H i H i

= + + (5.2)

Vi / ( 0,1,...)m mm d d m = = c nh gi tn s sng mang quang 0. Hng s pha

0 v thi gian tr 1 khng tc ng vo hnh dng xung v c thc b qua. Xung pha

to ra bi si quang c thc b bng cch chn b lc c 2 2L = . Xung sc

khi phc hon ton ch khi ( ) 1H = v phn t th 3 trong khai trin Taylorphng

trnh 5.2 phi m. Hnh 5.1 m t m hnh kt hp gia b lc quang v khuych i

quang b ng thi tn sc GVD v suy hao. Hn na, b lc quang c th gim nhiu

khuych i nu bng thng b lc nh hn bng thng b khuych i.

Hnh 5.1:M hnh kt hp gia b lc quang v khuych i quang.

Cc b lc quang hot ng da trn nguyn l giao thoa, y l nguyn l t nhin ca

nh sng v n rt nhy vi tn s nh sng ng vo v c ng dng trong cc b lc

quang do cc c tnh truyn dn ph thuc vo tn s ca n. Mt v dn gin ca b

lc quang l s dng giao thoa Fabry-Perot. Thc ra ph truyn2

FPH ca giao thoa

Fabry-Perot c thc xc nh thng qua h s khuych i Fabry-Perot phng trnh

5.2b vi G=1 . b tn sc, chng ta cn mt tn sc lp v pha ca hm truyn

H(), c xc nh bng vic nh gi cc thnh phn nh sng truyn gia hai gng.


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Giao thoa Fabry-Perot phn chiu c gi l giao thoa Gires-Tournois, c thit k

c th phn x 100%. Hm truyn t phng trnh 5.3.

(5.2b)

(5.3)

Vi hng s H0c trng cho suy hao tng cng,2

r l h s phn x trc gng v T l

thi gian truyn i v (round-trip) bn trong hc FP. Do ( )FPH l c lp tn s, ch c

ph pha c thay i bi b lc FP. Tuy nhin, thnh phn pha ( ) ca ( )FPH cnhiu im khc bit hn. y l hm tun hon c cc cc ti cc gi tr cng hng FP.

Ti vng ln cn ca mi cc, tn ti mt vng ph m ti s thay i v pha gn nhl hm bc 2. Bng cch khai trin ( ) bng chui Taylor, biu thc 2 s l

2 32 2 (1 ) / (1 )T r r r = + (5.4)

V d vi mt hc FP 2cm vi r=0,8 v 22 2200ps , b lc quang c th b tn sc GVD

cho khong 110km tuyn quang tiu chun. Thc nghim vo nm 1991 s dng cc thit

b ton quang c th truyn tn hiu 8Gbps qua mt khong 130km s dng si quang tiu

chun. Cc h thng ny c suy hao chn khong 8dB v c b bng vic s dng cc

b khuych i quang. Suy hao 6dB l do b ghp quang 3dB s dng chia tn hiuphn x t cc tn hiu ti. Lng suy hao ny c th rt gim xung cn khong 1dB nu

s dng b truyn vng quang (optical circulator), y l mt thit b c ba cng dng

truyn cng sut t mt port n cc port cn li theo vng. Tuy nhin do suy hao cao v

bng thng hp ca cc b lc FP lm gii hn chng trong cc h thng quang thc t.

Hnh 5.2 : M hnh b lc quang sdng giao thoa Mach-Zehnder.


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Kthutiu khin tn sc

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Giao thoa Mach-Zehnder cng c thc ng dng ch to cc b lc quang. Giao

thoa MZ trong si quang c thc to ra bng cch kt ni hai b ghp ni c hng

(directional coupler) mc ni tip nh trong hnh 5.2b. B ghp u tin chia tn hiu ng

vo thnh hai phn bng nhau, hai thnh phn ny s c s sai pha nu c s sai khc vchiu di nhnh ca b ghp. Tn hiu c th thot ra khi mt trong hai cng ng ra ph

thuc vo tn s nh sng v chiu di nhnh ca b ghp. Hm truyn i ti port ng ra

b ghp s l

[ ]1

( ) 1 exp( )2MZ

H i = + (5.5)

Vi l tr gia hai nhnh ca b ghp to giao thoa MZ.

Mt b giao thoa MZ n khng th s dng lm b lc quang m phi c mt chui

nhiu b to giao thoa ghp tng vi nhau to thnh b lc cn bng. B lc nh th c

thc ch to theo kiu mch sng quang phng (planar lightwave ccuit) bng cch s

dng cc ng dn sng thy tinh. Hnh 5.2 a m t cu trc thit b nu trn. Thit b vi

kch thc 52x71mm2 c suy hao l 8dB. N bao gm 12 b ghp ni vi chiu di nhnh

khng ng b ghp tng ni tip vi nhau. Mt u t bng Crm c s dng mt

nhnh ca mi b giao thoa MZ to ra quang nhit nhm iu khin xung quang. u

im chnh ca thit b ny l c tnh cn bng tn sc c thciu khin bng chiu

di nhnh v s lng b giao thoa MZ.Hot ng ca b lc MZ c thc biu din qua hnh 5.2b. Thit bc thit k

cc thnh phn tn s cao sc truyn i nhnh c chiu di ln hn ca b giao thoa

MZ. Kt qu l, chng s tr hn cc thnh phn tn s thp do c truyn cc nhnh

ngn. Hm truyn H() c thc xc nh c qua phn tch mch v c dng

ti u thit k cng nh hot ng ca thit b. Thc nghim vo nm 1994 vi mt mch

sng quang phng c nm b giao thoa MZ to ra tr 836 ps/nm. Thit b ny ch c vi

cm chiu di nhng li c kh nng b tn sc cho khong 50km si quang tiu chun.Hn ch chnh ca thit b ny chnh l bng thng hp ( 10GHz ) v nhy vi phn cc

ng vo. Tuy nhin, vi mt b lc quang lp trnh c th tn sc GVD v bc sng

hot ng c kh nng iu chnh. thit b ny, GVD c th thay i t -1006 n

834ps/nm.

6. CCH T SI QUANG BRAGG (Fiber Bragg Gratings)Cch t Bragg hot ng nh 1 b lc quang ,da trn hin tng bng chn (stopband),

vng tn s m hu ht tt c nhng nh sng ti b phn x li.


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Kthutiu khin tn sc

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2 2 ,0

ng

B B

= =

Bng chn ny c tp trung ch yu ti bc sng Bragg

Vi l chu k cch t, n l chit sut trung bnh. Sng truyn hng ti v hng v

c bc sng gn vi bc sng Bragg v kt qu l to ra 1 h s phn xc lp tn s

cho tia ti trn ton bng thng

Thc cht cch t quang si hot ng nh 1 b lc phn x, c s dng b tn

sc v c xut thc hin t thp nin 80.

6.1Chu k cch tng nht (Uniform-Period Gratings)Xt 1 cch tn gin nht khi n l 1 hm bin i tun hon ( ) cos(2 / )gn z n n z = +

Vi gn l su iu ch. Cch t Bragg c phn tch da trn vic s dng phng

trnh lng mode (coupled-mode equations) m n m t sng truyn hng ti v sngtruyn hng v /dA dz i A i A

f f b = + (6.1)

/b b fdA dz i A i A = (6.2)

Vi f v bA l bin ph ca 2 sng v : (6.3)

l h s kt hp

Phng trnh kt hp c thc gii da trn bn cht tuyn tnh.

Hm truyn t ca cch t: (6.4)

Hnh 6.1 : ln(a) v pha(b) ca h sphn x cch tsi quangng nht vi

gL =2 v gL =3

Vi gL l di cch t.2 2 2

q =

Hnh 6.1 trn cho thy ln v pha ca h s phn x tng ng vi gL =2 v gL =3.

H s phn x cch t gn t 100% khi gL =3

2B

n =

)sin((0)( ) ( ) (0)gb

f B

i qLAH w r wA

= = =


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Bq =

Tuy nhin khi pha gn nh trong vng tuyn tnh, cch t lm cho s tn sc ch tn ti

bn ngoi stop band (bng chn)

Lu rng hng s lan truyn

Thng s tn sc ca cch t quang si c cho bi:

(6.5)

Vi gv l vn tc nhm( Group velocity) ca xung, tn s sng mang

Hnh 6.2: Tn sc vn tc nhm GVD . M t hm 2g theo thng s tngng vi

cc gi tr ca h s trong khong 1-10

Tn sc vn tc nhm (GVD) ca cch t ty thuc vo thng s GVD bt thng tn

s cao hoc vng blue ca bng chn (stop band) khi l tuyt i v tn s sng mang

vt qu tn s Bragg. Ngc li GVD tr li bnh thng ( 2g >0) vng tn s thp

hoc vng red ca bng chn. Vng red ny c th dng b cho vng bt thngca GVD ca si quang tiu chun. V 2

g c th vt qu 1000 .

Mt cch tn di 2 cm c thc dng b GVD cho si quang di 100km. Tuy

nhin tn sc bc 3 ca cch t b suy hao v bin i nhanh . Dng cch t si quang

ng nht b tn sc.

Vn c th gii quyt bng cch s dng k thut lm mn (apodization) ti

ni chit sut khng ng nht ca cch t. Kt qu l z ph thuc vo . Trong thc t

k thut lm mn apodization xy ra 1 cch t nhin khi m tia Gaussian s dng cho

2 /p s cm

2 2 2 3

2 32 2 3/ 2 2 2 5/ 2

sgn( ) / 3 /,

( ) ( )g gg g

v v

= =

0 02 /w =


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php chp nh giao thoa. (khi dng chm tia cc tm phn b Gauss ghi nh cch t.

Vi cc cch t nh trn, h s ln nht gia si v gim dn v 2 u ca si.

Phng php ch to cch t tt hn l lm cho h s k thay i tuyn tnh dc theo ton

chiu di cch t si quang.)Mt cuc th nghim vo 1996 cho thy 1 cch t di 11cm c th b tn sc t c tc

10Gb/s ca tn hiu truyn trn chiu di si quang 100km. H s(z) thay i t

0-6 trn chiu di cch t.

Hnh 6.3: Tn hiu pht (ng lin nt) v tr(ng chm) , hm ca bc sng cho

cch tng nht (z) thay i t 0-6 trn chiu di cch t11cm

c tnh truyn dn ca cch tc tnh ton bng cch gii phng trnh lng mode

bng phng php s hc.

ng cong lin nt ch ra mi lin h gia tr nhm vi o hm ca pha trong phng

trnh [ ] 20 1 21

( ) ( ) exp ( ) ( ) exp ( )2

H H i H i

= + + .

Trong vng bc sng 0.1nm gn vi bc sng 1544.2nm, tr nhm bin i gn nh

tuyn tnh ti t s 2000 ps/nm. iu ny ch ra rng cch t c th b tn sc cho GVD

trn 100 km si quang chun .

Thc vy, cch t b tn sc GVD trn 100km chiu di si quang vi tc tn hiu

10Gb/s ch suy gim 2% cng sut v t c t l li bit BER = . Ni khng c

cch t s c suy hao rt ln.

S dng cch t b tn sc khi bc sng tn hiu trong phm vi vng stop band v

bc sng cch t nh 1 b lc phn x. S lng trng thi ca phng trnh cho chu k

1cm

1cm

910


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cch tng nht khi (z) bin i tuyn tnh t 0 n 12 trn 12 cm chiu di.

iu ny ch ra rng tc tr nhm, tp trung bc sng Bragg , c thc s dng

cho b tn sc nu bc sng ca tn hiu ti trong vng trung tm ca stop band ph

tn hiu bin i tuyn tnh. 1 cch t di 8.1 cm c kh nng b tn sc GVD trn chiudi 257 km si quang chun vi tc 10 Gb/s.

Mc d cch tng nht c s dng cho b tn sc nhng n chu ng c vng

hp ca stop band (nhng vng bng chn ca chng l tng i hp (tiu chun


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Hnh 6.4 Cch tquang Chirped dng b tn sc a/ chit sut n(z) theo chiu di cch tb/ h sphn xtn sthp v cao ti nhng vng khc nhau trong cch t

thc hin cch t chirped c th dng nhiu phng php khc nhau.

iu quan trng l chu k quang n thay i dc theo cch t ( trc z), v do s thay

i c th l hoc chit sut n theo z.

Mt s k thut c cp: Dual-beam holographic.Double-exposure technique, Phase-

mask technique.

ng dng ca cch t quang Chirped vo vic b tn sc c chng minh trong thp

nin 90 vi nhiu th nghim truyn tn hiu.

Hnh 6.5: H sphn x v thi gian trtrong cch tquang Chirped tuyn tnh

vi bng thng 0.12nm

Nm 1994 B tn sc GVD cho chiu di 160 km v tc 10-20Gb/s


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Kthutiu khin tn sc

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Nm 1995 cch t chirped di 12 cm dng b tn sc cho GVD chiu di270 km

Tc 10Gb/s. Sau khong cch truyn tng ln 400 km ch s dng cch t chirped

di 10cm. iu ny c 1 s cch bit so vi khong cch 20km khi khng dng cch t b

tn sc.

Trong hnh 6.5 ch ra h s phn x v tr nhm (quan h vi o hm ca pha )

theo 1 hm ca bc sng cho cch t 10cm vi =0.12nm , tc 10 Gb/s Tn hiu

c b trong vng bng chn ca cch t. Chu k cch t thay i ch 0.008% trn

sut chiu di ca n

S b tn sc hon ton xy ra trong vng ph m o hm pha l tuyn tnh.

dc ca tr nhm( khong 5000 ps/nm) l 1 i lng b tn sc ca cch t.

C th b tn sc GVD cho 400 km vi tc 10Gb/s.

Cch t chu k gim dn cn c lm mn theo cch cc h s ghp to nh khong

gia cch t nhng gim dn v cui. Qu trnh lm mn l cn thit loi b cc gn

sng xy ra trong cch t c h s.

Cch t gD b gii hn bi bng thng m b tn sc vn tc nhm (GVD)

yu cu vi tc bit B. Hn na khi khong cch truyn tng ti cng 1 tc bit th

bng thng tn hiu gim v k thut prechirp c s dng ti u pht. Trong 1 thnghim h thng vo 1996 cho thy k thut prechirp ca tn hiu quang 10Gb/s c kt

hp vi 2 cch t quang chirped, ghp ni tip tng khong cch truyn ln n 537

km. K thut thu hp bng thng cng c kt hp vi cch t. M hnh m ha nh

phn i (duobinary) thu hp bng thng n 50%. Trong 1 th nghim 1996. khong

cch truyn tc Gb/s c mrng n 700 km bng cch s dng cch t chirped

kt hp vi m hnh m nh phn i pha lun phin ( phase-alternating duobinary ). Bng

thng cch t b thu hp xung 0.073 nm, qu hp tc tn hiu 10 Gb/s nhng rng

cho tn hiu nh phn i bng thng hp.

Hn ch chnh ca cch t si quang chu k thay i l chng hot ng ging nh

mt b lc phn x (hot ng da trn nguyn l phn x cc tia sng). Mt b ghp

quang 3-dB thng c s dng cch ly gia sng phn x v sng ti. Tuy nhin,

phi mt 6-dB cho cc suy hao xen (ngha l cc suy hao xen trong b ghp quang 3-dB).

Thng trong thc t ngi ta dng b cch ly quang vng (circulator) do n c th gim

suy hao xen xung di mc 2dB.

/d dw

/d dw


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Kthutiu khin tn sc

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Vi k thut khc cng c s dng. 2 hoc nhiu hn cch t quang c th kt hp vi

b lc pht b tn sc vi mc suy hao tng i thp. Hai hoc nhiu hn cch t

quang c thc kt hp vi nhau to thnh mt b lc pht c bin i thnh 1 b

lc pht bng cch to ra dch pha gia cch tCch t Moierc to thnh bng cch t chng 2 cch t chirped trn cng 1 on

ca si quang, do n cng st gi trnh trong vng bng chn ca n. Bng thng

ca b lc pht ny th tng i hp.

Hnh 6.6: S b tn sc bng cch dng 2 b lc pht fiber base transmission filter

6.3B ghp mode Chirped (chirped mode couplers)Phn ny tp trung vo 2 linh kin quang hot ng nh mt b lc ph hp cho vic b

tn sc. Mt b ghp mode chu k thay i l mt linh kin ton quang c thit k da

trn nguyn l ghp cng hng phn b chu k thay i. tng pha sau b ghp cng

hng phn b chu k thay i tht n gin. Khc vi cch ghp cc sng ti v sng

phn x trong cng mt mode (nh trong cch lm ca cch t si quang), cch t chu k

thay i ghp hai mode sng trong cng mt si quang hai mode. Linh kin ny tng t

nh bi mode sng cp trong mc 4 lin quan n si DCF (si b tn sc) ngoi

tr chu k cch t bin i tuyn tnh dc theo chiu di si quang.

Tn hiu c chuyn t mode cssang mode bc cao hn bi cch t, nhng

cc thnh phn tn s khc nhau s di chuyn vi cc qung ng khc nhau trc khi

c chuyn i mode nhvo c tnh chu k thay i ca cch t v iu dn n

vic ghp hai mode sng li vi nhau (ie. trthnh mode cao hn). Nu chu k cch t

tng dn dc theo chiu di ca b ghp, b ghp c th b c GVD (tn sc vn tc

nhm). Tn hiu vn lan truyn theo hng ti cho n khi c chuyn sang mode cao


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Kthutiu khin tn sc

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hn ca b ghp. Mt b chuyn i mode cch t chu ku c dng chuyn tn

hiu trli mode cs.

Cch lm khc cho cng tng trn l ghp cc mode csca si quang hai li

vi hai li ny l khc nhau. Nu hai li l gn (v khong cch), sng suy bin giacc mode sng s dn n vic chuyn nng lng t li ny sang li kia, tng t nh b

ghp c hng. Khi khong cch gia hai li gim tuyn tnh, s chuyn nng lng xy

ra ti nhiu im khc nhau dc theo si quang, ph thuc vo tn s ca tn hiu lan

truyn. Do vy, mt si quang 2 li vi khong cch ca cc li gim tuyn tnh c th b

c GVD. Linh kin da trn nguyn l ny gi tn hiu lan truyn theo hng ti, mc

d v mt vt l c s chuyn nng lng sang li k cn. Cch thit k ny c thc

tch hp vo cng mt linh kin bng cch dng cc ng dn sng bn dn do cc siu

mode (supermode) sinh ra t vic ghp hai ng dn sng c th iu chnh c mt

lng ln GVD.

7.LIN HP PHA QUANG OPCMc d s dng k thut lin hp pha quang (OPC) cho b tn sc c ngh t 1979

nhng n 1993 k thut ny mi c a vo th nghim. N gy s ch rt ln t.

Ngc li vi nhng g cp trong chng ny, k thut OPC l mt k thut quang phi

tuyn. Trong mc ny s m t ngun gc v tp trung vo ng dng h thng quang trongthc t

7.1Nguyn l hotng:Cc n gin nht hiu mt OPC c th b tn sc vn tc nhm GVD l gii phng

trnh lin hp phc

22

1( , ) (0, )exp( )

2 2

iA z t A w zw iwt dw

= (7.1.1)

Thu c: * 2 * 3 *32

2 30

2 6

iA A A

z t t

=

(7.1.2)

So snh pt (7.1.1) v (7.1.2) cho thy vng pha kt hp A* lan truyn vi h s truyn 2

ca GVD c o du. Nhn thy 1 iu rng, nu min quang c kt hp pha gia lin

kt si quang , s tn sc trn na u v na sau ca lin kt l b nhau. V s hng 3

khng i du khi lin hp pha, OPC khng th b cho tn sc bc 3. y, d dng thy

rng, bng cch gi cc thnh phn bc cao hn trong khai trin Taylorphng trnh


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Kthutiu khin tn sc

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2 3320 1( ) ( ) ( ) ( ) ( )2 3

ww n w w w w

c

= + + + (7.1.3)

khi OPC c th b cho tt c cc thnh phn tn sc bc chn m khng nh hng

n cc thnh phn bc l.

Tnh hiu qu ca lin hp pha quang khong gia ph trong vic b tn sc c thc

kim chng bng cch s dng phng trnh

22

1( , ) (0, ) exp

2 2

iz t A z i t d

+

=

(7.1.4)

Min quang (optical field) trc OPC tm c bng cch s dng z = L/2 phng trnh

ny. S lan truyn ca vng sng kt hp pha A* trong phn sau OPC nh sau:

* * 22

1( , ) ( , ) exp( )

2 2 4

L iA L t A w Lw iwt dw

= (7.1.5)

Vi A* (L/2,) l bin i Fourier ca A* ( L/2,t) v c tnh bi:

* * 22( / 2, ) (0, ) exp( / 4)A L w A w iw L= (7.1.6)

Thay phng trnh (7.1.6) vo (7.1.5), ta c A(L,t)=A*(0,t).

V vy, ngoi tro pha do OPC, vng u vo c phc hi mt cch hon ton, v

dng sng c phc hi nh ban u. V ph ca tn hiu sau OPC tr thnh i xng

vi ph ban u, k thut OPC c cp nh k thut o ph khong gia.

7.2B tn sc bng tiu chpha (Compensation of Self-Phase Modulation )Hin tng phi tuyn SPM lm cho si quang sinh ra hin tng chirp i vi tn hiu

truyn trn si quang

Phn 3 ch ra rng hin tng chirp do SPM c th mang li li ch vi mt thit k thch

hp S thay i ny thng s dng vi vi mt thit k ring bit. Soliton quang hc

cng s dng SPM (Truyn dn quang soliton cng dng hin tng SPM c c mt

su im) Tuy nhin, phn ln h thng thng tin quang , cc hiu ng phi tuyn do

SPM gy ra lm gim cht lng r rt, c bt l khi cc tn hiu truyn i trn khong

cch di phi s dng nhiu b khuych i quang.

Cng ngh OPC khc vi cc m hnh b tn sc khc mt iu quan trng: Trong

nhng iu kin nht nh, n c th b ng thi cho c GVD v SPM. Tnh nng ny

ca OPC c lu vo u nhng nm 1980 v c nghin cu rng ri sau 1993 .

D dng thy l c GVD v SPM u c b mt cch tuyt i m khng c s suyhao quang no. S lan truyn xung trong si quang c suy hao cho bi phng trnh


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Kthutiu khin tn sc

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222

22 2

iA Ai A A A

z t

+ =

(7.2.1)

Vi thnh phn 3 b b qua v l suy hao ca si quang. Khi =0, A* tho cc

phng trnh ging nhau khi ta tnh lin hp phc ca pt (7.1.4). v i z thnh z. Kt

qu l OPC khong gia ph c th b ng thi cho SPM v GVD.

S suy hao c th ph hu 1 thuc tnh quan trng ca khong gia ph OPC. L

do rt d thy l nu chng ta lu rng s thay i pha ca SPM ph thuc vo cng

sut. th pha thay i ln na u ng dn quang hn l na th 2, v OPC khng th

b cho cc hiu ng phi tuyn. Phng trnh (7.2.1) c th dng nghin cu tc ng

ca s suy hao. Bng cch thay:

( , ) ( , )exp( / 2)A z t B z t z= (7.2.2)

Phng trnh. (7.2.1) c th vit li2

222

( )2

iB Bi z B B

z t

+ =

(7.2.3)

Vi ( ) exp( )z z = . Hiu ng ca suy hao quang c th tng ng vi trng hp

khng li nhng vi tham s phi tuyn ph thuc z. Bng cch tnh lin hip phc ca Eq

(7.2.3) v thay i z thnh z, d dng thy s b tn sc ton phn SPM xy ra khi v

ch khi ( ) ( )z L z = iu kin ny khng thc tho mn khi 0

Mt cch na c th gii quyt vn l khuych i tn hiu sau OPC cng

sut tn hiu tr nn bng vi mc cng sut u vo trc khi n n na th 2 ca

ng dn quang. Mc d cch ny lm gim s tc ng ca SPM, nhng n vn khng

th b tn sc ton phn c.

Nguyn nhn c thc hiu rng: s truyn dn ca tn hiu pha kt hp tng

ng vi vic truyn mt tn hiu time-reseverd . V vy. s b SPM ton phn xy ra

khi v ch khi bin thin cng sut i xng quanh im gia ph , v th, ( ) ( )z L z =

phng trnh (7.2.3). S khuych i quang khng uc tho mn trng hp ny. Ntng t vi s b SPM nu cc tn hiu c khuch i cng sut khng thay i

trong 1 khong ln trong mi trng thi khuch i. Tuy nhin, iu ny khng thc t

bi n i hi cc b khuch khong cch gn nhau.

S b tn sc ton phn cho c GVD v SPM c thc tm c bi cch s

dng cc si quang gim tn sc (dispersion decreasing fibers) cho 2 gim trn chiu di

si quang. xem mt m hnh c thc hin nh th no, gi thit rng 2 phng

trnh (7.2.3) l mt hm ca z Bng cch bin i:


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Kthutiu khin tn sc

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0

( )z

z dz =

2c p sk k k=

( ) /j j ck n w w c=

Z D

(7.2.4)

Pt (7.2.3) c th vit li2

2

2( )2

B i B

b i B Bt

+ =

(7.2.5)

Vi 2( ) ( ) / ( )b = C GVD v SPM u c b nu ( ) ( )Lb b = , L l gi

tr ca khi z=L. iu kiu ny c tho mn tng th s gim tn sc ging nh

( )z , 2 ( ) ( ) = v ( ) 1b = .

V suy hao quang lm ( )z gim theo hm m exp( )z c GVD v SPM c th b

chnh xc cc si quang gim tn sc m c GVD gim theo hm m exp( )z Cch

tip cn ny ph bin v c s dng khi cc b khuych i quang c s dng.

7.3Tn hiu lin hp pha (Phase-conjugated Signal):K thut OPC( Optical phase conjugation) lin hp pha quang i hi 1 phn t

quang phi tuyn m c th to ra tn hiu pha lin hp . Thng thng ngi ta dng

phng php trn 4 bc sng (FWM- Four wave mixing) trong vng phi tuyn, v bn

thn si quang t n l mt mi trng phi tuyn, (cch n gin l dng mt si quang

di vi km c thit k mt cch c bit ti a hiu ng FWM)

K thut FWM trong si quang c nghin cu v ng dng rng ri

dng hin tng FWM cn mt ngun bm ti tn s wp c dch t tn s ca tn

hiu sw mt lng nh (~0.5THz) . Si quang phi tuyn to ra tn hiu lin hp pha quang

ti tn s . Min l tha mn iu kin ph hp pha.

Vi l s lng xung quang ti tn s wj

iu kin ph hp gc pha tha mn nu bc sng tn sc khng ca si quang

c chn (trng vi bc sng bm) ng thi vi bc sng bm.

iu ny c thng qua trong 1 cuc th nghim 1993 Ln u tin chng

minh c rng tim nng ca k thut OPC trong vic b tn sc. Trong 1 th nghim

khc 1 tn hiu ( bc sng 1546nm) c kt hp gc pha bng cch s dng FWM cho

23 km si quang vi bc sng pump bm l 1549 nm . Tn hiu tc bit 6-Gb/s

c pht trn 152 km si quang tiu chun vi iu ch FSK. Mt th nghim khc na

l 1 tn hiu 10 Gb/s c th pht trn 360 km.

2c p sw w w= +


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Kthutiu khin tn sc

Trang 34

2 3

2 32p c

c

L

L

+=

+

c c sw w =

K thut OPC khong gia phc thc hin cho 21 km si quang bng cch s

dng tia pump laser m bc sng ca n c cn chnh mt cch chnh xc vi bc

sng tn sc khng. Bc sng tn hiu v pump lch nhau 3.8 nm.

Hnh 7.1: Th nghim b tn sc trongo khong gia phtrn 21 km chiu di siquang

Cch thc hin to ra tn hiu pha lin hp hp. 1 b lc di thng bandpass c s

dng to ra tn hiu pha lin hp .

Nhiu nhn t cn phi c xem xt trong k thut pha kt hp khong gia ph

trong thc t. Trc ht v bc sng tn hiu thay i t sw thnh 2c p sw w w= ti b

to pha lin hp. Thng s tn sc vn tc nhm GVD 2 b thay i. Kt qu l , xy ra

b tn sc ton phn ch khi b kt hp gc quang lch rt nh so vi im gia ca

ng quang. V tr chnh xc pL c thc xc nh bng cch s dng iu kin

Vi L l tng chiu di .

Khai trin 2( )cw thnh chui Taylor ti tn s ,s pw L th tm c:(7.3)

Vi khong dch tn s ca tn hiu bng k thut OPC.

i vi khong dch bc sng chun 6nm, gc pha thay i khong 1%.

Mt nhn t th 2 cn ch l qu trnh trn 4 bc sng (FWM) trong si quang

l nhy phn cc v phn cc ca tn hiu khng c iu khin bi si quang, nthay i ti b to pha kt hp theo cch thc ngu nhin. nh hng ca bin i ngu

2 2( ) ( )( )s p c pw L w L L =


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Kthutiu khin tn sc

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1 2p pw w1 2c p p sw w w w= +

nhin ny tc ng n hiu sut FWM v lm cho k thut FWM khng ph hp vi mc

ch thc t. Tuy nhin s phi hp c th gim bt nhy phn cc. 2 tia pump trc

giao ti bc sng khc nhau v tr i xng ca bc sng tn x khng ca si quang

c s dng. S sp xp ny c 1 thun li khc l sng kt hp gc quang c thcto ra ti tn s ca bn thn tn hiu bng cch chn lc bc sng tn x khng ZD

trng vi tn s tn hiu

T mi quan h: vi .

nhy phn cc ca OPC cng thu c bng cch s dng mt ngun bm

(pump) n kt hp vi cch t si quang v mt thit bi xng kt hp trc giao

(orthoconjugate mirror) nhng thit b ny lm vic trong mode phn x v cn cc sng

kt hp ring bit t tn hiu bng cch dng b ghp 3-dB hoc b cch ly vng. Hiusut tng i thp ca qu trnh OPC trong si quang cng c li. in hnh l hiu sut

chuyn i c di 1% khin n phi khuch i tn hiu pha kt hp .

Thc t suy hao ca pha kt hp vt qu 20 dB. Tuy nhin, qu trnh FWM th

khng phi l 1 qu trnh hiu sut thp. Thc vy, phn tch phng trnh FWM cho thy

rng c tng ln ng k bng cch tng cng sut pupm v gim cng sut tn hiu. N

c th vt qu 100 % mc cng sut v khc bit bc sng pump (bm) . (Cn trnh

s dng ngun bm cng sut cao do nh hng ca hin tng pht x kch thchBrillouin)

Cng sut (ngun bm) pump cao thng c hn ch v SBS. Tuy nhin c th

loi b SBS bng cch iu ch ngun bm ti tn s ~100Mz. Trong 1 th nghim 1994,

35 % hiu sut chuyn i c to ra bng k thut ny.

Qu trnh FWM trong 1 b khuch i quang bn dn (SOA) cng c s dng

to ra tn hiu pha kt hp dng vo vic b tn sc. Cng trnh ny c thc hin

trong 1 cuc th nghim 1993 v chng minh c rng tn hiu truyn tc 2.5 Gb/s.c iu ch bng bng cc laser bn dn truyn trn 100 km chiu di si quang

chun. Sau trong 1 th nghim 1995, tng t truyn c tn hiu vi tc ln

n 40 Gb/s trn chiu di 200 km si quang chun. Kh nng duy tr FWM trong SOA

c xut vo nm 1987v k thut ny c s dng trong chuyn i bc sng. u

im chnh l tn hiu kt hp gc c thc to ra trong 1 thit b di 1 nm. Hiu sut

chuyn i cao hn so vi FWM trong si quang v n c khuch i.


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Bng cch la chn chnh xc sng bm - iu hng tn hiu, thu c hiu sut

chuyn i ln hn 100% ( li thu c cho tn hiu kt hp gc quang) cho k thut

FWM trong SOAs.

ng dn sng phn cc tun hon LiNbO3 c s dng to ra cc b chuyni ph bng rng, tn hiu pha kt hp c to ra bng cch s dng cc qu trnh phi

tuyn bc hai lin tip. Cc qu trnh ny l cc qu trnh ph hp pha (quasi-phase-

matched) nhvo chu k cc ca tinh th. i vi thit b OPC ch b suy hao 7 dB v c

kh nng b tn sc cho 4 knh tn hiu tc 10 Gb/s trn chiu di 150 km si quang

chun. Tim nng ca k thut OPC c chng minh mt th nghim 1999 vi 1 b

kt hp cbn FWM c s dng b tn sc vn tc nhm GVD tc 40 Gb/s trn

chiu di 140 km si quang tiu chun

Hu ht cc cuc th nghim v b tn sc c nghin cu trn khong cch tuyn

l vi trm km. i vi ng truyn di hn n t ra vn k thut OPC c th b tn

sc vn tc nhm GVD cho chiu di ln n hng ngn km si quang m c dng cc

b khuch i b suy hao c hay khng. Trong 1 th nghim m phng, tn hiu tc

10 Gb/s c th truyn trn 6000 km trong khi ch s dng cng sut trung bnh di mc 3

mW gim hiu ng phi tuyn si quang. Trong 1 nghin cu khc cho thy b khuch

i ng 1 vai tr quan trng. Vi khong cch truyn trn 9000 km c th thc hin c

bng cch gi cc b khuch i cho mi on 40 km. S la chn bc sng hot ng ,

c bit l bc sng tn sc khng c ngha then cht. Trong vng tn sc d thng

10 ps/(km-nm)). y l trng hp i vi si quang chun bc sng gn 1.55 .

Hin nhin rng khong cch truyn ti a lun l vn quan trng i vi nhiu hng

sn xut, nh k thut trn 4 bc sng FWM, khong cch dng b khuch i c thgim xung di 3000 km.

S dng k thut OPC cho h thng quang ng di (long haul lightwave system)

i hi s dng cc b khuch i quang v b pha kt hp. 2 phn t quang ny c th

ghp li thnh 1 bng cch s dng cc b khuch i tham s (parametric amplifiers) -

Mch khuch i thng dng linh kin diod bin i in dung, trong in dung thay

i theo chu k ca tn hiu, n khng ch to ra tn hiu pha kt hp trong sut tin trnh

FWM m cn khuch i tn hiu. Phn tch cho h thng quang ng di cho thy tn

hiu vo (20 n 30 ps) c th truyn dc chiu di ln n hng ngn km si quang mc

2

m


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Kthutiu khin tn sc

Trang 37

d tn sc vn tc nhm GVD cao. Tng khong cch truyn c th vt qu 15.000 km

cho si quang tn sc thay i. vi 22 2 /ps km = gn vi bc sng 1,55 . K thut

pha kt hp khng c s dng trong thc t khi cc b khuch i tham s cha thng

dng trong thng mi. Phn k tip tp trung vo k thut thng dng s dng iukhin tn sc trong h thng quang ng di.

8. H THNG QUANG NG DI:Chng ny tp trung vo h thng quang ng di, vic iu khin tn sc s gip cho

vic mrng khong cch truyn t 10km cho n vi trm km. Vn quan trng

L iu khin tn sc nh th no c th s dng c cho h thng ng di vi

Khong cch c th ln n hng ngn km. Nu tn hiu quang c lp li mi 100-200

km. Tt c cc k thut c cp trong chng ny s hot ng tt v hiu ng phituyn khng c tch ly trong h thng ng di. Ngc li nu tn hiu c duy tr

Trong min quang bng cch s dng cc b khuch i chu k th cc hiu ng phi tuyn

nh SPM, XPM(cross phase modulation) v FWM sc gii hn. Do tc ng ca

hiu ng phi tuyn ln ch hot ng ca h thng iu khin tn sc c nghin

cu 1 cch su sc Trong chng ny s tp trung vo h thng quang ng di m bn

iu khin tn sc v suy hao c s dng ng thiPeriodic Dispersion Maps ( Bn

tn sc tun hon)

Hnh 8.1: Vng lp quang dngpht tn hiu tc 10 Gb/s trn khong cch

10.000 km si quang chun sdng SCF.

m


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1 1 2 2( ) / mD D L D L L= +

Khi khng c hiu ng phi tuyn, tng tn sc vn tc nhm GVD tch ly trn chiu di

hng ngn km c thc b ti u thu cui cng ca h thng. L do l mi xung

quang c ti to li nh gc trong thi bit ca h thng tuyn tnh, ngay c khi n c

tri rng trn vi thi bit trc khi GVD c b.S tng tc phi tuyn xung quang cng knh (intrachannel effects), v trn cc knh ln

cn trong h thng WDM lm gim cht lng tn hiu n mc m vic b tn sc GVD

ti pha thu cng khng th thc hin c trong mng ng di.

Mt gii php n gin l k thut periodic dispersion management (iu khin tn sc

tun hon) . tng cbn kh n gin l dng b trn quang vi tn sc vn tc nhm

GVD dng v m trong 1 chu k tng tn sc trn mi chu k gn bng 0. M hnh

n gin nht ch s dng 2 si quang c tn sc ngc nhau v chiu di tn sc trungbnh:

(8.1)

Vi jD l tn sc si quang c chiu di L (j=1,2) v 1 2mL L L= + l chu k ca bn

tn sc. Nu D~0 th tn sc c b trn mi chu k. Chiu di mL l 1 thng s thit k

t do m c thc chn ph hp vi yu cu lm vic ca h thng. Thc t, thng

thng chn mL bng vi khong khuch i amplifier spacing AL . in hnh

80m AL L Km= = cho h thng quang mt t nhng s gim xung cn khong 5 km choh thng di mt bin.

Do s cn nhc v chi ph, cc cuc th nghim s dng 1 vng lp quang lm tn hiu

quay vng nhiu ln ti to cho h thng quang ng di.

Hnh 8.1 ch ra s vng lp quang. N c s dng vi tc 10 Gb/s khong

cch truyn 10.000 km si quang chun vi suy hao chu k v iu khin tn sc . 2 b

chuyn mch quang quyt nh vng tun hon. Chiu di vng lp v s vng xc nh

tng khong cch truyn.Chiu di vng lp chun in hnh l 300-500 km. Chiu di ca

DCF c chn tha mn pt 8.1 v c thit lp 2 1 1 2/L D L D= cho b tn sc ton

phn (D=0)

Mt b lc bandpass quang cng c thm vo bn trong vng lp gim nh hng

ca nhiu khuch i.


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8.1L thuyt cs:nh hng chnh ca hin tng phi tuyn ln ch lm vic ca h thng n knh l

SPM. Nh trnh by trc y s truyn tia quang ca m chui bit quang bn trong

mt h thng qun l iu khin tn sc c khng ch bng phng trnh Schrodingerphi tuyn. pt (7.2.1)

222

22 2

dA A ii A A A

dz t

+ =

(8.2)

Vi s khc bit chnh 2 , v l hm chu k hin ti theo z bi v gi tr khc bit

ca chng gia 2 hoc nhiu on si quang c dng to bn tn sc.

B suy hao ti cc b khuch i c thc bao gm bi vic thay i thng s suy hao

hp l ti nhng vng khuch i

Phng trnh 8.2 lm r nh lng xem xt nghin cu h thng iu khin tn sc.

N v cng hu ch c lng gii hn cui cng trong phng trnh ny

(8.3)

Thay vo Phng trnh 8.2

222

2

( )( ) 0

2

zB Bi z B B

z t

+ =

(8.4)

Vi s thay i cng sut dc si quang iu khin tn sc c bao gm 1 tham s phi

tuyn thay i theo ch k0

( ) exp( ( ) )z

z z dz =

Phn tch chi tit vic thit k mt h thng iu khin tn sc c tht c bng cch

gii phng trnh 8.4 bng phng php bin phn (variational approach). iu ny da

trn nhn xt l mt xung Gauss chirped vn duy tr c dng tuyn tnh mc d bin ,

rng v chirp ca n thay i trong khi lan truyn. Do cc hiu ng phi tuyn l tng

i yu so vi tn sc ti mi on si quang, dng xung c khuynh hng duy tr dng

Gauss. Do ta c th gi s rng xung lan truyn dc theo si quang c dng xung Gausschirped sao cho:

2 2( , ) exp[ (1 ) / 2 ]B z t a iC t T i= + + (8.5)

Vi a l bin , T l rng, l phase. 4 tham s ny thay i theo z. 1 phng php

khc rt hu dng tm s ph thuc z ca cc tham s ny.

Phng trnh 8.4 c thc suy ra t phng trnh Euler-Lagrange :

(8.6)

0

1( , ) ( , )exp ( )

2

z

A z t B z t z dz

=


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Theo nhng phng php khc nhau chng ta c th gii phng trnh tm 4 thng s

a,T,C v . Phng trnh phase c th b qua nu n khng kt hp c vi 3 phng

trnh kia. Phng trnh bin c th kt hp thy rng t hp khng thay i theo z

v lin h vi xung u vo Eo :2

0a T E= . Do Cn phi gii 2 phng trnh sau:

Bng cch t 0 = . Lu rng t s 2 2(1 ) /C T+ lin quan n rng ph ca xung l

hng s trong mi trng truyn dn tuyn tnh

Ta c th thay n bng 1 gi tr ban u2 2

0 0(1 ) /C T+ Vi To v Co l rng v h schirp ca xung vo trc khi n c a vo h thng iu khin tn sc ca si quang.

Hai phng trnh trn c thc gii bng phng php gii tch v theo phng php

tng hp sau:

Phng php ny c v phc tp nhng n d dng cho vic tnh tch phn. Thc t gi tr

ca T v C ti cui ca chu ku tin ( z=Lm) c cho bi:

Vi 22 0/md L T= v 2 l gi tr GVD trung bnh.

D dng thy rng khi 2 0 = th c T v C tr thnh gi tr ban u ti mi chu k ca

bn tn sc

Ging nh chc nng nh 1 b pht tuyn tnh. Khi GVD trung bnh khc 0, T v C thay

i sau mi chu k bn v s pht xung th khng tun hon.

Khi phn phi tuyn khng ng k, cc thng s ca xung khng th trv gi tr ban u

ca n trong b tn sc ton phn (d=0). Trong nhiu cuc th nghim, h thng phi tuyn

lm vic tt nht khi b GVD ch l 90-95% phn tn sc cn li c gi li sau mi

chu k bn . Trn thc t, nu xung vo sao cho 2


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Kthutiu khin tn sc

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Nhn xt ny dn n vic chp nhn dng xung trn cc lin kt si quang c iu khin

tn sc.

Nu bn tn sc c to ra lm gin xung trong phn u v c nn trong phn

th 2 th tc ng ca hiu ng phi tuyn c thc gim ng k. L do nh sau: Cngsut nh ca xung c hn chng k trong phn u bi v s mrng (gin) nhanh

chng ca xung chirped trong khi phn th 2 th chm hn bi v suy hao si quang tch

ly. Nhng lin kt si quang iu khin tn sc nh thc gi l nhng chng truyn

dn ta-tuyn tnh (tc l ch tuyn tnh trn mt on ngn). Kt qu cho trong cng thc

8.9 cng p dng c cho nhng lin kt nh vy. Do cc xung quang tri rng mt cch

ng k ra khi khe bit ca chng vt qua (ln hn) h s ca mi chu k bn , s

chng chp c th lm gim hiu sut h thng khi hiu ng phi tuyn ng k.

Cc hiu ng ny sc xem xt trong mc k tip.

Nu cng sut nh qu ln n mc khng cn duy tr c tnh ta-tuyn tnh, ta phi

gii cc cng thc 8.7 v 8.8 vi s c mt ca cc s hng phi tuyn.

Khng c phng n gii tch no c dng trong trng hp ny. Tuy nhin c th tm

c

0( )mT L T= , 0( )mC L C= (8.11)Phi m bo rng xung c khi phc v dng ban u ti mi chu k bn tn sc.

Cc xung truyn qua h thng iu khin tn sc trong h thng tun hon v c gi l

dispersion-managed solitons sc trnh by trong chng 9.

8.2Hiu ng tng tc phi tuyn ng knh (Intrachannel Nonlinear Effects):Vic xem xt hiu ng phi tuyn rt quan trng trong h thng iu khin tn sc bi v n

c tng ln trong si quang b tn sc DCF v hiu ng gim vng li. S sp xp cc

b khuch i sau DCF l c li v tn hiu yu hiu ng phi tuyn t nh hng .Snh gi 1 cch lc quan v hiu sut ca h thng s dng nhng bn tn sc khc

nhau c nghin cu rt k. Trong 1 th nghim 1994, Mt vng quang di 1000 km bao

gm 31 b khuch i c s dng 3 bn tn sc khc nhau. Khong cch ti a

truyn ln n 12.000 km c thc hin trong trng hp si quang c GVD bnh

thng c b tn sc trong si quang c GVD bt thng ca si quang ng di.

Trong 1 th nghim 1995 , tn hiu 80 Gb/s bao gm 8 knh ghp 10 Gb/s vi khong knh

(channel spacing) c truyn bn trong vng lp quang. Tng khong cch truyn gii

hn 1171 km bi v nhng hiu ng phi tuyn khc nhau.


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B tn sc GVD ton phn trong mi chu k bn tn sc khng phi l gii php tt

nht khi c s hin din ca hiu ng phi tuyn. Phng php s thng c s dng

ti u vic thit k h thng iu khin tn sc. Ni chung vng tn sc vn tc nhm

GVD nn gi tng i ln trit hiu ng phi tuyn, v phi cc tiu tn sc trung bnhtrn tt c cc knh. Trong 1 th nghim vo 1998, tn hiu vn tc 40 Gb/s c truyn

trn khong cch 2000 km si quang chun s dng 1 bn tn sc novel. Sau

khong cch c tng ln 16.500 km ti tc thp hn( 10Gb/s) bng cch t 1 b

khuch i quang ngay sau DCF trong vng lp quang.

V hiu ng phi tuyn l 1 nhn t quan trng. Gii hn ch yu xut pht t vic gin

xung trong si quang chun ca bn tn sc, kt qu l s tc ng ln nhau gia xung

chng chp gn nhau. Nhng hiu ng phi tuyn ny c nghin cu su hn v c

lin h vi hiu ng

Hiu ng Intrachannel (hiu ng tng tc ng knh) phn bit n vi hiu ng phi

tuyn xuyn knh.(Interchannel nonlinear effect) xy ra khi xung 2 knh ln cn c bc

sng khc nhau chng chp min thi gian.

Ngun gc ca hiu ng phi tuyn ng knh c th thy t phng trnh:

222

2

( )( ) 0

2

zB Bi z B B

z t

+ =

Bng cch a 3 xung ln cn , 1 2 3B B B B= + +

Thay vo ta c:

S hng phi tuyn u tin tng ng vi SPM. Hai s hng k tip l kt qu t hiu ng

XPM sinh ra bi hai xung khc nhau. S hng cui cng ging nh FWM.

Mc d thng thng n c xem l hiu ng FWM trong knh (intrachannel) nhng

iu c mt cht thiu chnh xc v c 3 xung c cng bc sng.Tuy nhin thnh phn

ny c th to ra 1 xung mi trong min thi gian. Nhng xung ny xut hin nh 1 xung

ghost. Xung ny c th tc ng n hiu sut h thng ng k nu chng ri vo thi

bit 0 (the 0-bit time slots)

(8.12)

(8.13)

(8.14)


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Hiu ng XPM tng tc ng knh chphase tn hiu nhng lch pha th ph thuc

vo thi gian.

Tc ng ca hiu ng XPM v FWM ln h thng ty thuc vo s la chn bn tn

sc. Ni chung nh gi h thng iu khin tn sc ty thuc vo nhiu thng s thit knh cng sut, khong cch khuch i v v tr ca DCF. Trong 1 th nghim vo nm

2000. mt tn hiu 40 Gb/s c truyn vt i dng, mc d n ch s dng si quang

chun, s dng phng php iu chng bng dy c nghu tin.

Truyn gi tuyn tnh tc 320 Gb/s cng c xc nhn khong cch truyn 200

km m tn sc ca n l 5.7 ps(km-nm) c b bng cch s dng DCF

9. H THNG QUANG DUNG LNG CAOCc h thng thng tin quang WDM hin i s dng mt s lng ln cc knh truyn

lu lng ln n 1Tbps. Vi h thng nh th ny, k thut iu khin tn sc phi c

kh nng hot ng trn bng rng vi tn hiu a knh. mc ny chng ta s bn v k

thut iu khin tn sc trong cc h thng dung lng cao.

9.1B tn sc bng rng :Thng thng tn hiu WDM s dng bng thng khong 30nm hoc cao hn, n l bng

thng tng cng t cc knh n c bng thng khong 0,1nm (ph thuc vo tc bitca tng knh) ca cc knh n. Vi cc knh 10Gbps, thnh phn tn sc th ba khng

ng vai tr ch cht nhi vi cc xung c b rng >10ps. Tuy nhin v cc bc sng

l c lp vi cc gi tr2 v h s tn sc D nn tn sc s c s khc nhau gia cc

knh. Do b tn sc trong cc h thng WDM th m hnh b tn sc phi c th

thc hin c vic b tn sc GVD trn tt c cc knh ng thi. Phng php u tin

s dng mt cch t quang bng rng hoc nhiu cch t quang c bng chn (stop band)

tng ng b tn sc cho cc knh. Mt phng php khc da vo ph WDM bng

cch s dng mt b lc quang vi cc nh chu k truyn tng ng. Mt phng php

khc s dng si DCF cho cc h thng WDM bng cc si quang b tn sc DCF c

thit k ph hp cho vic truyn a knh bng rng.

phng php u tin s dng cch t quang. Mt cch t quang b chirp c th c mt

bng thng dng b rng khong 10nm tng ng vi chiu di cch t. V th cch t

ny c thc s dng trong h thng WDM nu s lng knh nh (thng thng l


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WDM bn knh, mi knh c tc 40Gbps. Khi bng thng tn hiu WDM ln hn, c

th s dng nhiu cch t chirp ghp tng ni tip vi nhau mi cch t phn x mt

knh v b tn sc cho knh . u im ca phng php ny l cc cch t c thc

bin i tng ng vi tn sc GVD trn mi knh. Hnh 9.1 m t m hnh ghp tngcch t trong h thng WDM bn knh. C mi 80km, mt b b tn sc gm 4 cch t

c s dng b tn sc cho tt c cc knh trong khi hai b khuych i quang c

dng b suy hao. Vic mrng s cch t c lm trong thc t. Vo nm 2000,

h thng WDM c mrng ln 32 knh vi bng thng 18nm. Su cch t chirp vi

6nm bng chn c ghp tng b tn sc GVD cho tt c cc knh.

Hnh 9.1 : M hnh ghp tng cch tb tn sc trong h thng WDM

S dng nhiu cch t lm tng tnh cng knh ca b b tn sc khi s lng knh ln

vi bng thng tn hiu khong 30nm . B lc FP vi nhiu nh truyn vi cc nh ph

truyn ca b lc cch nhau theo chu k. B lc nh th c th b tn sc GVD ca tt c

cc knh nu tt c cc knh c phn chia khong cch bng nhau v di ph t do ca

b lc tng ng vi khong cch knh. Khi phi lm vic vi mt lng tn sc ln vic

thit k b lc FP l rt kh khn. Mt loi cch t mi c gi l cch t quang mu(sampled fiber grating) c pht trin gii quyt vn ny. Cch t ny c nhiu

bng chn v d ch to. Thay v l lm mt cch tn di, ngi ta lm nhiu cch t

ngn kt hp li vi nhau v c khong cch ph hp (mi mt cch t ngn ny c gi

l mt mu nn cch t loi ny c gi l cch t quang mu). Khong cch bc sng

gia cc nh phn xc xc nh qua cc chu k mu v c iu khin trong qu

trnh ch to. Hn na nu "mu" b chirp, th c tnh tn sc ca mi nh phn x cng

bnh hng bi lng chirp ny. Cch tu tin c s dng vo nm 1995 b tn

sc ng thi cho hai knh 10 Gbps vi khong cch truyn 240km. Thc nghim vo


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nm 1999 s dng cch t quang mu cho h thng WDM bn knh. Khi s lng knh

tng, n trnn cng kh hn trong vic b tn sc GVD tt c cc knh ng thi.

Vic s dng si DCF dc tn sc m c a ra nh mt gii php n gin nht

iu khin tn sc trong cc h thng WDM dung lng cao vi s lng knh ln. SiDCF loi ny c pht trin v thng mi ha vo nhng nm 1990 v c s dng

trong trong cc h thng WDM dung lng cao. S cn thit ca dc sc m c th

c hiu qua phng trnh 4.2 mc 4 cho knh n. iu kin ny cng phi tha cho

tt c cc knh

1 1 2 2( ) ( ) 0n nD L D L + = (9.1)

Vi n l bc sng ca knh th n. V gi tr gii hn dng ca ng tn sc S hoc

thnh phn tn sc th ba 3, D1 tng theo bc sng c si quang thng v si quang

dch tn sc. Kt qu l tn sc tch ly D1L1 khc nhau gia cc knh. Nu s dng cng

si DCF cho tt c cc knh, dc tn sc ca n s m v c gi tr tha phng trnh

9.1.

Gi tr ( ) ( )j n j j n cD D S = + vi Dj (j=1,2) l gi tr tn sc knh trung tm, dc

tn sc ca si DCF l :

2 1 1 2 1 2 1( / ) ( / )S S L L S D D= = (9.2)

Ta c th s dng phng trnh 4.2 cho knh trung tm. Phng trnh ny ch ra rng h

s S/D c gi l ng dc quan h tn sc (Relative Dispersion Slope), v ging nhau

c hai si quang . i vi si quang tiu chun h s tn sc 16 / ( )D ps km nm v

dc tn sc 20,05 / ( )S ps km nm , h s S/D s vo khong 0,003 nm-1. Do , vi si

DCF c 100 / ( )D ps km nm , dc tn sc s vo khong -0,3ps(km-nm2). i vi

si quang dch tn sc, h s S/D c th vo khong 0,02 nm-1. Vic sn xut si DCF c

dc tn sc ln rt kh khn, si DCF hai mode ch c th c gi tr khong 0,01nm-1

nh trong hnh 4.1. Si quang nh th ny cn c gi l si quang o tn sc (Reverse-

Dispersion Fiber) l si quang c gi tr D v S c gi tro m so vi si quang thng

thng.

Nhiu thc nghim vo nhng nm 1990 nh gi vic s dng si DCF trong h thng

WDM. Thc nghim vo nm 1995, 8 knh vi khong cch knh l 1,6nm, mi knh

hot ng tc 20Gbps truyn i c khong cch 232km trn si quang tiu

chun bng nhiu si DCF. Gi tr tn sc ca mi knh nh v vo khong xp x

100ps/nm, ngoi ra ton b cc knh cng c b tn sc ng thi bi si DCF. Thc


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nghim vo nm 2001, si DCF bng rng c s dng truyn tn hiu quang WDM

1-Tbps (bao gm 101 knh) vi mi knh tc 10Gbps khong cch 9000km. Tc

cao nht c th nghim l 11 Tbps vi 273 knh mi knh c tc 40Gbps trn

ng thi cc bng C, L, S (do bng thng tng cng vo khong hn 100nm).

9.2B tn sc iu khin c (Tunable Dispersion Compensation)Kh c th b tn sc hon ton trn tt c cc knh trong h thng WDM. Vn cn mt

lng nh tn sc tn ti trong cc h thng quang ng di. Trong nhiu th nghim

thc t, k thut b sau c thc hin bng cch thm vo si DCF (hoc cch t quang)

b thu. K thut ny khng ph hp cho cc h thng WDM thng mi v mt s l do.

u tin, lng tn sc khng phi lc no cng c th xc nh mt cch chnh xc do

nhng thay i khng iu khin c trn cc phn ca tuyn quang. Th hai, vic thayi chiu di tuyn li lin quan n c vic phi cu hnh li mng quang. Th ba, khi tc

mt knh n tng ln khong 40Gbps, gi tr tn sc m h thng c th chu ng

c trnn nh v dn n d b tc ng ngay c khi c s thay i nh v mi trng

chng hn nh nhit lm bin ng gi tr tn sc GVD. V nhng l do ny, m hnh

b tn sc iu khin c c a ra cho php iu khin vic b tn sc mi knh

mt cch linh hot.

Mt vi k thut b tn sc iu khin c c pht trin v th nghim trong thct. Hu ht trong s chng u s dng cch t Bragg l loi cch t m tn sc c th

iu chnh bng cch thay i chu k cch t n . mt m hnh, cch tc ch to

vi hiu ng chirp phi tuyn (bc sng Bragg tng phi tuyn dc theo chiu di cch t),

cch t ny c th thay i bng cch ko di cch t bng bin thin in p. Mt m

hnh khc, cch tc ch to khng c c hiu ng chirp tuyn tnh v phi tuyn m s

dng Gradient nhit to ra cc chirp iu khin. Trong c hai trng hp, bng cch

thay i nhit hoc chiu di cch t lm cho h s mode n cng thay i theo dn n

bc sng Bragg cng thay i v bng ( ) 2 ( ) ( )B z n z z = . H s tn sc ca cch t

theo cng thc 6.6 c thay th thnh :

0

2( ) ( )

gL

g

g

d dD n z dz

d c d

= =

(9.3)

Vi g l tr nhm Lg l chiu di cch t. Gi tr Dgbt k bc sng no c th

c thay i bng cch thay i h s mode n (thng qua t nng hoc thay i chiu

di), kt qu l c tnh tn sc c thiu chnh c trong cch t Bragg.


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Phn b qu trnh t ca cch t Bragg yu cu u t rt mng c t ngoi b mt

si quang trong li ca cch t. mng ca u t thay i dc theo chiu di cch

t to ra gradient nhit thng qua qu trnh t khng ng b. Hnh 9.2 biu din ph

phn x v th quan h gia tn sc tng DgLg v in p t ca cch t di 8cm.Cch t ban khng xy ra hiu ng chirp v c ph chn hp nhng n c th dch v m

rng ph chn khi cch t b chirp qua qu trnh t nng khng ng b. Bc sng

Bragg B thay i dc theo cch t v chu k quang ( )n z trnn c lp vi z khi qu

trnh gradient nhit c thit lp dc theo cch t. Tn sc tng cng DgLg c thc

thay i trong di -500 n -2200 ps/nm. Cch t loi ny c thc dng iu khin

tc sc cho cc h thng 10 Gbps.

Hnh 9.2 : Phphn x v th tn sc theo in p t ca phng php gradient nhit

Khi tc ln n 40 Gbps hoc cao hn, cn thit phi gy hiu ng chirp ln cch t

bng chn mrng cho ton b ph tn hiu i qua. Vic s dng qu trnh chirp phi

tuyn c s dng tng kh nng iu khin trn loi thit b c tc cao ny. Ccht chirp c ch to v s dng cho k thut b tn sc iu chnh c tc

160Gbps. Hnh 9.3 th quan h gia nhy thu v tn sc tc 160Gbps khi dng

v khng dng cch t chirp Bragg iu chnh c. Khi khng s dng cch t, nhy

nh nht mc tn sc 91ps/nm do si DCF c gi tr tn sc cnh. nhy thu gim

n 4dB khi gi tr tn sc thay i quanh mc 91ps/nm khong 8ps/nm. Tuy nhin n ch

thay i khong 0,5dB nu s dng k thut b tn sc iu chnh c. Gin mt

mc tn sc 110ps/nm ch ra rng h thng s tr nn khng th hot ng c nu

khng c cch t chirp Bragg bng qu trnh b tn sc iu khin c duy tr mt vn


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m trong gin . Thc nghim s dng xung quang 2ps vi khe bit 6,25ps v tc

truyn l 160Gbps. Tc ng ca thnh phn tn sc th ba trnn rt quan trng i vi

cc xung ngn. Chng ta s tho lun phn tip theo.

9.3iu khin tn sc bng thnh phn tn sc bc cao :Khi tc ca mt knh n ln n 40Gbps, thnh phn tn sc bc ba hoc cao hn tc

ng gy nh hng cho tn hiu quang. Mt v d, mt bit tc 100Gbps ch c b

rng 10ps cn nu l xung quang RZ th b rng xung ch


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Kthutiu khin tn sc

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k c dc tn sc -15,8 ps/nm trn bng thng 170 GHz. B cn bng hn ch bin

ng ln ca tn hiu pha cui v gim rng ca nh chnh t 4,6 n 3,8ps.

Hnh 9.3 : Dng xung ng ra khi truyn vi khong cch 300km khi khngv c dng si dch tn sc

Cc cch t quang chirp thng c s dng trong thc t v tnh ton quang ca mng.

Cc cch t quang di (~1m) c pht trin vo nm 1997. Vo nm 1998, cch t

quang chirp phi tuyn 6nm c s dng b tn sc bc ba khong cch 60km. Vic

ghp tng cc cch t quang c th to ra mt b b tn sc c c tnh tn sc ty v c

kh nng b cc tn sc bc cao. Cch t quang mng ng dn sng (Arrayed-Waveguide

Grating) hoc cch t quang mu cng c th b tn sc cho cc thnh phn tn sc bc

hai v bc ba ng thi. Mc d cch t quang mu chirp phi tuyn c th b tn sc iu

khin c cho nhiu knh ng thi, nhng bng thng ca n b gii hn. Cch t

quang mng ng dn sng kt hp vi b lc pha theo khng gian c th b dc tn sc

trn mt bng thng ln n 8THz vi cc h thng knh 40Gbps. Vic truyn tn hiu

tn hiu 100Gbps vi khong cch 10.000km c kho st bng vic s dng k thut

quang pha kt hp gia (midway optical phase conjugation) kt hp vi b tn sc bc ba.

Mt vi th nghim trong vic truyn knh n vi tc hn 200Gbps c thc

hin. Ngi ta cng tin hnh th nghim vi chu k bit 2ps, nu s dng m RZ th b

rng xung ch c 1ps tng ng vi tc truyn 500Gbps s dng si DCF hoc cch t

quang chirp b tn sc 3 trn bng thng 4nm. Thc nghim vo nm 1996 tn hiu

400Gbps c truyn s dng k thut b tn sc kt hp vi xung pht 0,98ps trong khe

thi gian 2,5ps. Nu khng b thnh phn tn sc bc ba, xung s m rng ra 2,3ps sau

40km truyn v rng chn xung mrng ra n 5-6ps . Vi vic b tn sc tng phnthnh phn tn sc bc ba s khc phc vic chn xung b mrng v b rng xung gim


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xung cn 1,6ps do tn hiu 400Gbps s d dng khi phc u thu. Xung nh sng

hp 0,4ps c dng vo nm 1998 truyn tc 640Gbps. Thc nghim vo nm

2001 nng tc truyn ln 1,28 Tbps bng cch truyn xung c rng 380fs trn

chiu di 70km. Vic truyn thng tin vi xung hp yu cu phi thc hin b ng thithnh phn tn sc bc hai, ba v bc bn. Nu thc hin iu ch sin pha ph hp cc

xung chirp trc khi truyn i qua si quang b tn sc GVD, n c th b c c tn sc

bc ba v bc bn.

9.4B tn sc phn cc mode PMDPMD l tc ng mrng xung quang do cc bin i ngu nhin ca hin tng lng

chit/khc x kp dc theo chiu di si quang. Vic mrng ny cng lm xung b m

rng hn na bn cnh vic dn rng xung do GVD. Cc k thut iu khin tn sc gpphn tc ng ln GVD nhng gn nh khng ng g n nh hng ca PMD. Chnh

v l do ny PMD l mt trong nhng i tng quan tm c bit trong cc h thng iu

khin hin i.

Trc khi xem xt cc k thut s dng cho b tn sc PMD, chng ta tin hnh c

lng cc thng s trong cc h thng khng b tn sc. Gi tr RMS (cn bnh phng

trung bnh) m rng ca xung vi chiu di L c tnh theo cng thc

( )1/22

T PT D L = vi Dp l h s PMD v T l tr tng i theo hai nguynl trng thi phn cc (PSP). K hiu Tbiu th gi tr trung bnh. Gi tr tc thi ca

T dao ng trong mt di rng theo thi gian do nhit v cc thng s mi trng

khc. Nu T ln hn khe bit (bit slot) h thng s ngng hon ton hot ng.

Hot ng ca h thng b gii hn bi PMD c phn loi da trn khi nim xc sut

ngng t xut, xc sut ny phi di gi tr qui nh (thng l 10-5 hay 5 pht/nm).

Xc sut ny c tnh theo T da trn phn b Maxwell. Mt cch tng qut, gi tr

RMS T phi nh hn khe bit TB vi tc bit l B=1/TB. Gi tr nh ca nm trongkhong 0,1 n 0,15 ty theo loi iu ch (RZ, CRZ hoc NRZ) v cc thng s ca xung

ng vo. Vi m hnh 10%, chiu du h thng v tc bit phi tha iu kin

2 2(10 )P

B L D < (9.7)

Trong cc tuyn quang c s dng si quang tiu chun, iu kin 9.7 tr thnh2 4 210 ( )B L Gbps km< nu s dng 1 /PD ps km= . Do si quang yu cu phi b tn

sc PMD tc 10Gbps v khong cch truyn 100km, do trong cc si quang hin

i thng thng h s tn sc PMD 0,1 /PD ps km< . Vi h thng c thit k s


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dng si quang ny th B2L phi mc 106 (Gbps)2-km. Kt qu l, b tn sc PMD

khng cn phi thc hin tc 10Gbps nhng cn phi thc hin tc 40Gbps vi

khong cch truyn 600km. Cc c lng chnh xc hn da vo l thuyt PMD c

pht trin trong nhiu nm gn y.

Hnh 9.4: M hnh b tn sc PMD quang v in

Nh cp trn tn sc phn cc mode hn ch hot ng ca knh n khi tc

bit ln vt xa 10Gbps. b tn sc phn cc mode c nhiu k thut c pht trin

c th phn chia thnh k thut b PMD quang v b PMD in. Hnh 9.4 s b tn sc

PMD quang v b tn sc PMD in. B cn bng PMD in u thu dng sa cc

tc ng ca PMD s dng m hnh lc ngang. Cc b lc chia tn hiu in ( )x t thnh

nhiu nhnh dy r tr v cng kt hp chng ng ra

1

0

( ) ( )N

m

m

y t c x t m

=

= (9.8)

Vi N l tng s nhnh r, l thi gian tr v cm l trng s r ca nhnh th m. Trng s

r c thiu chnh c bng cc thut ton iu khin p ng nng cao cht lng

h thng. Tn hiu li a vo mch in tiu khin thng c da vo trng thi

ng ca mt trong gin mt ti u thu. Do k thut in t khng th hn ch PMD

mt cch hon ton nn n t c nghin cu tip gim tr PMD so vi phng php

nguyn l trng thi phn cc PSP (Principle state of Polarization). K thut ny c thhiu chnh c tt c cc ngun gy thoi ha dn n mt bng ti u thu.

B b tn sc quang cng s dng dy tr, n c thc chn vo theo chu k dc theo

chiu di tuyn quang hoc ch chn vo u thu. Thng thng qu trnh b PMD

quang c chia ra lm hai giai on, u tin tn hiu quang c chia thnh cc nhnh

PSP bng cch s dng biu khin phn cc kt hp theo sau l b chia tia phn cc;

giai on th hai tin hnh kt hp tn hiu cc nhnh PSP trong c tn hiu c

iu chnh trmt nhnh bng dy tr (hnh 9.4). Vng hi tip dng xc nh tn hiuli hiu chnh biu khin phn cc p ng vi cc thay i ca mi trng dc


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ln si quang PSP. Thnh cng ca k thut ny da vo t s L/LPMD vi L l chiu di

si quang, v ( )2

0 /PMD PL T D= l chiu di PMD, T0 l b rng ca xung. Ci thin ng

kc mong ch l a khong cch truyn c di nht c th v t s ny khng

vt qu 4. Do chiu di LPMD vo khong 10.000km vi 0,1 /PD ps km v T0=10psnn cc b b tn sc PMD c th s dng cho cc h thng truyn dn quang xuyn bin

vi tc truyn 10Gbps.

Mt vi k thut quang khc c thc dng b tn sc PMD. Chng hn nh b b

tn sc LiNBO3 based Soleil Babinet hot ng da trn vic iu khin phn cc

cui ng truyn. Cc thit b khc nh tinh th st 2 v 3 khng bn, si quang duy tr

phn cc kp, b lc quang all-pass v cch t quang chirp khc x kp. Hnh 9.5 ch ra

nguyn tc hot ng ca b b tn sc PMD s dng cch t. Do c mt lng ln khc

x kp, hai trng phn cc dc theo hai trc nhanh v chm, n khc so vi bc sng

Bragg v gn dch n bng chn. Kt qu l, n phn x ti v tr khc nhau dc theo cch

t do nhn c gi tr tr nhm khc v v th c th b tn sc PMD, gim tr

nhm. tr ph thuc vo bc sng bi v qu trnh chirp xy ra t nhin trong cch t.

Hn na, n c thiu chnh c thng qua dch chuyn nm khong cch cch t. Cc

b b tn sc PMD iu chnh c rt ph hp cho cc h thng WDM.

Hnh 9.5: B tn sc iu chnh c sdng cch tquang chirp khc x kp

Cc b b tn sc PMD hnh 9.4 v 9.5 ch loi b thnh phn tn sc PMD bc 1. tc

cao, xung quang ngn ph ca chng trnn rng cc thng s PSP khng

th l hng s trn ton b ph xung. Lc ny cc thnh phn tn sc PMD bc cao bt u

c tc ng n h thng, thng cc h thng t 40Gbps cn phi xem xt nh hng cc

ca cc thnh phn tn sc PMD bc cao. Do c nhiu k thut b tn sc c nghin

cu gii quyt vn ny.


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Kthutiu khin tn sc

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Cc b b tn sc bc 1 c thc nh gi qua vic xu

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