HỌC VIỆN CÔNG NGHỆ BƯU CHÍNH VIỄN THÔNG KHOA QUỐC TẾ VÀ ĐÀO TẠO SAU ĐẠI HỌC

8/2/2019 HC VIN CNG NGH BU CHNH VIN THNG KHOA QUC T V O TO SAU I HC

1/75

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

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

N MN HC

THNG TIN QUANG NNG CAO

H CAO HC NGNH IN T - VIN THNG

CHUYN

IU KHIN TN SC

GVHD: HVTH: Lp:

TS. L Quc Cng Nguyn Trn Anh Tun Phm Minh T CH09T2

TP.H CH MINH, NM 2010


2/75

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

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

N MN HC

THNG TIN QUANG NNG CAO

H CAO HC NGNH IN T - VIN THNG

CHUYN

IU KHIN TN SC

GV hng dn: Sinh vin thc hin: Lp:

TS. L Quc Cng Nguyn Trn Anh Tun Phm Minh T CH09T2

TP.H CH MINH, NM 2010


3/75

K thut iu khin tn sc

MC LC

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

SC ...................................................................................................... 6 1. CN THIT PHI IU KHIN

TN SC .......................................................... 6 2. M HNH B TRC

(PRECOMPENSATION)................................................... 9 2.1 2.2 2.3 K thut

Prechirp ................................................................................................ 9 K thut m ha

Novel: .................................................................................... 12 K thut Prechirp phi

tuyn: ............................................................................. 14

3. K THUT B SAU .............................................................................................. 16 4. SI QUANG B

TN SC ................................................................................... 17 5. B LC

QUANG .................................................................................................... 19 6. CCH T SI QUANG

BRAGG (Fiber Bragg Gratings).................................... 22 6.1 6.2 6.3 Chu k cch t ng nht (Uniform-

Period Gratings) ..................................... 23 Chirped Fiber Gratings: (Cch t si quang

Chirped)..................................... 26 B ghp mode Chirped (chirped mode

couplers) ............................................ 29

7. LIN HP PHA QUANG OPC .............................................................................. 30 7.1 7.2 7.3 Nguynl hot ng:........................................................................................ 30 B tn sc bng t iu ch pha

(Compensation of Self-Phase Modulation ).. 31 Tn hiu lin hp pha (Phase-conjugated

Signal): ............................................ 33

8. H THNG QUANG NG DI: ..................................................................... 37 8.1 8.2 L thuyt c

s: ................................................................................................ 39 Hiu ng tng tc phi tuyn ng

knh (Intrachannel Nonlinear Effects):.... 41

9. H THNG QUANG DUNG LNG CAO ......................................................... 43 9.1 9.2 B tn sc

bng rng : ...................................................................................... 43 B tn sc iu khin c

(Tunable Dispersion Compensation) .................... 46 Trang 1


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K thut iu khin tn sc 9.3 9.4 iu khin tn sc bng thnh phn tn sc bc

cao : ...................................... 48 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 l xung chirp Gauss trong trng hp

2>0 ........................................................................... 10 Hnh 2.2: M hnh k thut prechirp dng b

tn sc ................................................... 11 Hnh 2.3: B tn sc s dng m ha

FSK ........................................................................ 12 Hnh 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 ...................... 14 Hnh 2.5: th quan h gia khong cch truyn b gii hn do tn sc

GVD v mc cng sut truyn trung bnh ... ..............................................................................................

15 Hnh 4.1: M hnh si DCF hai mode s dng cch t chu k di .................................... 19 Hnh 5.1:

M hnh kt hp gia b lc quang v khuych i quang. .............................. 20 Hnh 5.2 : M hnh b

lc quang s dng giao thoa Mach-Zehnder. ............................... 21 Hnh 6.1 : ln(a) v pha(b) ca h

s phn x cch t si quang ng nht vi

Lg

=2 v Lg =3 ... .................................................................................................................... 23 Hnh 6.2: Tn

sc vn tc nhm GVD . M t hm 2g theo thng s tng ng vi cc gi tr ca h s trong

khong 1-10 .......................................................................... 24 Hnh 6.3: Tn hiu pht (ng lin nt)

v tr( ng chm) , hm ca bc sng cho cch t ng nht

(z)

1

thay i t 0-6 c m


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trn chiu di cch t 11cm ...................... 25

Hnh 6.4 Cch t quang Chirped dng b tn sc a/ chit sut n(z) theo chiu di cch t b/ h s phn

x tn s thp v cao ti nhng vng khc nhau trong cch t ................. 27 Hnh 6.5: H s phn x

v thi gian tr trong cch t quang Chirped tuyn tnh ........... 27 Trang 2


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K thut iu khin tn sc vi bng thng

0.12nm ....................................................................................................... 27 Hnh 6.6: S b tn sc

bng cch dng 2 b lc pht fiber base transmission filter . 29 Hnh 7.1: Th nghim b tn sc trong

o khong gia ph trn 21 km chiu di si

quang ................................................................................................................................... 34 Hnh 8.1:

Vng lp quang dng pht tn hiu tc 10 Gb/s trn khong cch 10.000 km si quang chun sdng SCF. .......................................................................... 37 Hnh 9.1 : M hnh ghp tng cch t b

tn sc trong h thng WDM ...................... 44 Hnh 9.2 : Ph phn x v th tn sc theo in p t

ca phng php gradient

nhit ............................................................................................................................................. 47 Hnh

9.3 : Dng xung ng ra khi truyn vi khong cch 300km khi khng .................... 49 v c dng si dch

tn sc ................................................................................................. 49 Hnh 9.4: M hnh b tn sc

PMD quang v in ............................................................ 51 Hnh 9.5: B tn sc iu chnh c s

dng cch t quang chirp khc x kp.............. 52 Hnh 9.6: th quan h gia h s m rng xung v

gi tr DGD trung bnh. ................ 53

DANH MC T VIT TT

ASE ADM BER CW DBR DCF DDF DGP DM DWDM EDFA FM FP FRASL Amplifier Spontaneous Emission

Add Drop Multiplexer Bit Error Ratio Continuous Wave Distributed Bragg Reflector Dispersion-

Compensating Fiber Dispersion-Decreasing Fiber Differential Group Delay Dispersion-managed Dense

Wavelength-Division Multiplexing Erbium-Doped Fiber Amplifier Frequency Modulation FabryPerot

Fiber Raman Soliton Laser Bc x t pht khuch i Thit b xen r T l li bt Sng lin tc Phn x

phn b Bragg Si quang b tn sc Si quang gim tn sc Tr nhm Qun l tn sc Ghp knh phn

chia theo bc sng mt cao B khuych i quang pha trn Erbium iu ch tn s Mt loi khoang

cng hng Laser quang Raman Soliton Trang 3


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K thut iu khin tn sc FWHM FWM GVD LED MZ NTE NLS NOLM NRZ NSE NSDSF OA OAT OADM

OBA OAR OC ODM OF OFC OM OMUX OPA OPU ORX OSC OTX PIM PMD RZ SAGCM SNR SOA SPM SRS

SSFS TOD TW Full-Width at Half-Maximum Four-Wave Mixing Group-Velocity Dispersion Light Emitted

Diode MachZehnder Network Terminal Equipment Nonlinear Schrodinger Nonlinear Optical-Loop

Mirror Nonreturn to Zero Nonlinear Schrodinger Equation Nonzero-Dispersion-Shifted Fiber Optical

Amplifier Optically amplified transmitter Optical Add/Drop Multiplexer Optical Booster AmplifierOptically amplified receiver Optical Channel Optical Demultiplexer Optical Fiber Optical Fiber Cable

Optical Multiplexer Optical MUX Optical Preamplifier Optical Preamplification Unit Optical Receiver

Optical Transmission Section Optical Transmitter Polarization-Interleaved multiplexing Polarization-

Mode Dispersion Return to Zero Separate Absorption, Grading, Charge, and Multiplication Signal-To-

Noise Ratio Semiconductor Optical Amplifier Self-Phase Modulation Stimulated Raman Scattering

Soliton Self-frequency Shift Third-Order Dispersion Traveling Wave rng ti na cc i Trn 4 bc

sng Tn sc vn tc nhm Ngun pht dng LED Mt loi b lc Thit b u cui mng Schrodinger

phi tuyn Gng quang vng phi tuyn M NRZ Phng trnh Schrodinger Si quang dch tn sc Khuch

i quang B pht khuch i quang B r bc sng quang B khuch i m quang B thu khuch

i quang Knh quang Tch bc sng quang Si quang Cp si quang Ghp bc sng quang B ghp

knh quang B tin khuch i quang Khi tin khuch i quang B thu quang Knh gim st quang B

pht quang Ghp xen knh phn cc Tn sc phn cc mode M RZ S hp th, pha trn, ph tn v

khuch i ring bit T s tn hiu trn nhiu B khuch i quang bn dn T iu ch pha Tn x kch

thch Raman Dch tn s Soliton Tn sc bc 3 Sng Traveling Trang 4


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K thut iu khin tn sc WDM WADM Wavelength-Division Multiplexing Wavelength Division

Multiplexing Access Cross-Phase Modulation Ghp knh phn chia theo bc sng Mng s dng k

thut a truy nhp ghp knh theo bc sng iu ch xuyn pha

XPM

Trang 5


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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 iukhin 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) s c gii thiu mc 7. Mc 8 gii thiu v iu 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 s c cp trong mc ny.

1. CN THIT PHI IU KHIN TN SC

Tn 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 th c 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 hot

ng bc sng 1,55 m s dng la-de pht DFB, cc h thng ny s dng cp si Trang 6


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K thut iu khin tn sc 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 tng ng vi b rng ph ca 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

L < (4B D s )1

Vi

(0.1)

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

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

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

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.

Trang 7


11/75

K thut iu khin tn sc Hot ng ca h thng c th c ci thin ng k bng vic s dng

mt b iu 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 b iu ch ngoi tch hp. Trong trng hp

s =0, khong cch truyn gii hn theo cng thc

L < (16 2 B 2 )1

(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


12/75

A(0, ) exp 2 z 2

i

2

it d

(0.4)

Hm A(0, ) l bin i Fourier ca A(0,t)

Tn sc lm nh hng n tn hiu quang truyn i trong h thng nguyn nhn gy ra l do phn t

pha exp (i 2 z 2 / 2) , xut hin trong qu trnh truyn nh sng trong si quang. Tt c cc m hnh

iu khin tn sc u tm cch trit tiu phn t pha ny tn hiu ng vo c th khi phc c

u thu.

Trang 8


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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 tnhca 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 phn bin ph bin thin A(0, ) trong trng

hp tn sc GVD c loi b. R rng,

thc hin iu ny bin ph u pht phi c b thm mt thnh phn theo biu thc 2.1

trit tiu tc ng do GVD gy nn:

A(0,) A(0, )exp(i2 2 L / 2)

(1.1)

Vi L l chiu di si quang, GVD s c 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

ph u 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 K thut Prechirp

Cch 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

1 + iC t 2 A(0, t ) = A0 exp 2 T0

(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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2 ngha l chu k xung thu c chia cho chu k xung ban u T1/T0= 2 , khong cch

truyn s c xc nh theo cng thc:

L=

C + 1+ C2 LD 1+ C2

(1.3)

2 Vi LD = To / 2 l chiu di tn sc, trong trng hp khng s dng xung chirp Gauss

C=0 khi L=LD. Tuy nhin khong cch L s tng khong 36% khi C=1. Ch rng L


15/75

K thut iu khin tn sc 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, nnk 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 truyn tn 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 xung u vo l xung chirp Gauss

trong trng hp 2>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 ph C. Khng may l h s chirp C m (C= - C) i vi cc la-de iu ch

trc tip. Do h s 2 ca si quang tiu chun hot ng bc sng 1,55 m cng m, v th iu 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

b iu tn FM. K thut prechirp trong trng hp ny to ra tn s chirp vi h s chirp C dng nhmtha iu kin 2.C


16/75

K thut iu khin tn sc m hnh c bn, tn s ca la-de DFB to ra u tin c iu tn FM sau

c a vo b iu 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 k thut prechirp dng b 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 th c vit

E (0, t ) = A0 exp(t 2 / T02 ) exp [ i0 (1 + sin mt )t ]

Vi tn s gn trung tm ca xung sin(mt ) mt khi phng trnh 2.4 s tr thnh

(1.4)

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

1 + iC t

2

E (0, t ) A0 exp

exp(i0t ) 2

T0

(1.5)

H s chirp C l:

C = 2m0T02

(1.6)


17/75

Ta thy rng h s chirp c th iu khin c bng cc thng s iu 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: Trang 11


18/75


E (0, t ) = A0 exp(t 2 / T02 ) exp [ i0t + i cos(mt )]

(1.7)

Do s dng cosx 1-x2/2. u im ca k thut iu pha l bn thn b iu ch ngoi c th t iu

chnh pha ca n. Phng php n gin nht thc hin l s dng b iu ch ngoi c chit sut

iu chnh c bng in t, bng cch ny s to ra tn s chirp vi C>0. Vo cui nhng nm 1991

tn hiu 5 Gbps truyn i c 256 km khi s dng b iu 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 b iu ch bn dn nh b iu ch hp th in (ElectroabsorptionModulator) hoc b iu 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 ch c 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 b iu ch hp th

in (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 b iu ch ngoi ny lm b pht. 2.2 K thut m ha Novel:

Hnh 2.3: B tn sc s dng 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 FSK c 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 Trang 12


19/75

K thut iu khin tn sc 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 T =

1/ 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 thnh tn hiu b iu 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 nynhm 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 b iu 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. Trang 13


20/75

K thut iu khin tn sc 2.3 K thut 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 ph thuc 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


21/75

K thut iu khin tn sc 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)

s gy ra hiu ng chirp cho cc xung quang thng qua hin tng t iu 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 quangcn truyn. Tn hiu quang si quang thm vo l:

A(0, t ) = P (t ) exp [i Lm 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 P(t ) = P0 exp(t 2 / T02 ) , hiu ng chirp khi gn nh l

tuyn tnh, khi phng trnh 2.8 s xp x bng 1 + iC t 2 A(0, t ) P0 exp exp(iLm P0 ) 2 T0 (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 mc cng sut truyn

trung bnh

Trang 15


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K thut iu khin tn sc Do >0 i vi cc si quang silica, v th bn thn si quang c th c

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 truyn trung 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 SAU

Cc k thut in t c th c 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 phn tn 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 H () = exp i ( IF ) 2 L / 2

(2.1)

Vi L l chiu di si quang. Tn hiu s c 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 trnh 1.4 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 th c 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 ch n 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 thutcn 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) s c gi c

nh trung tm ca gin mt (eye Trang 16


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K thut iu khin tn sc 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 s m 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 (transversalfiber) c gii thiu. k thut ny, mt b chia cng sut u thu chua tn hiu quang nhn c

thnh nhiu nhnh. tr ng 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 s c 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 SC

K 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 ngh ra mt si quang c bit gi l si quang b tn sc (DCF : Dispersion

Compensating Fiber). Vic s dng si DCF 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 : A( L, t ) = 1 2

+

A(0, ) exp 2

i

2


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( 21 L1 + 22 L2 ) it 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 s c khi phc v hnh dng ban u pha cui ca si DCF. iu

kin b tn sc tt nht l 21 L1 + 22 L2 = 0 hoc

D1L1 + D2 L2 = 0

(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 : Trang 17


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L2 = ( D1 / D2 ) L1

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

(3.3)

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

Mc d tng s dng si DCF c a ra vo nhng nm 1980, nhng cho mi sau ny khi xut hincc b khuych i quang vo nhng nm 1990 th vic ng dng si DCF mi c pht trin. C hai

hng c bn thit k si DCF. u tin si DCF h tr n mode, nhng n c thit k vi tn s

chun ha V nh. Mode c bn c gii hn quanh mc V 1 . 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 D 100 ps / (km nm) . Thit k lm gim lp bc thng c s dng trong thct sn xut si DCF. Tuy nhin, si DCF li c suy hao ln do s gia tng suy hao do un cong ( = 0, 4

0,6dB / 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 M > 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 68km 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 ny cn

tn ti hai vn . u tin l suy hao chn ca cc module DCF thng thng khong 5dB. Suy hao chn

c th c 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 th c gii quyt bng cch s dng

si quang hai mode c thit k vi tn s chun ha V ca mode c th t cao hn gn im cutoff

hn ( V 2, 5 ). 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 c bn sang mode

c th t cao hn. c mt s thit b Trang 18


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K thut iu khin tn sc 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 hai mode 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 t c gi l cch t quangchu 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 s dng cch t chu 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 bcsng. 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 s c cp mc 9.

5. B LC QUANG

Nh phn 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 cnbng quang (Optical Equalizing Filter) . Cc b lc giao thoa (Interferometric Filter) s c gii thiu

trong mc ny, mc tip theo s trnh by v cc cch t quang. Chc nng ca b lc quang c th c

din t qua cng thc 1.4. Tc ng ca GVD ln tn hiu quang c biu hin thng qua phn t pha

exp(i2 z 2 / 2) , hin nhin l b Trang 19


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K thut iu khin tn sc 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 hiu quang s c lc v c th

c vit li bng cch s dng biu thc 1.4 A( L, t ) = 1 2

+

A(0, )H ( ) exp 2 L 2

i

2

it d

(5.1)

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

1 H () = H () exp [i ()] H () exp i(0 + 1 + 2 2 ) (5.2) 2 Vi m = d m / d m (m= 0,1,...) 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 th c b qua. Xung pha

to ra bi si quang c th c b bng cch chn b lc c 2 = 2 L . Xung s c khi phc hon

ton ch khi H ( ) = 1 v phn t th 3 trong khai trin Taylor phng trnh 5.2 phi m. Hnh 5.1 m


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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 d n gin ca b lc quang l s dng giao thoa Fabry-Perot.

Thc ra ph truyn HFP ca giao thoa Fabry-Perot c th c 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 s c lp v pha ca

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

2

Trang 20


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K thut iu khin tn sc 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 H0 c trng cho suy hao tng cng, r l h s phn x trc gng v T l ph

pha c thay i bi b lc FP. Tuy nhin, thnh phn pha () ca HFP ( ) c thi gian truyn i v

(round-trip) bn trong hc FP. Do H FP ( ) l c lp tn s, ch c nhiu 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 nh l hm bc 2. Bng cch khai trin ( ) bng chui Taylor,

biu thc

2

2 s l

(5.4)

2 = 2T 2 r (1 r ) / (1 + r )3

V d vi mt hc FP 2cm vi r=0,8 v 2 2200 ps 2 , 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 suyhao 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 hiu phn 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 s dng giao thoa Mach-Zehnder.

Trang 21


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K thut iu khin tn sc Giao thoa Mach-Zehnder cng c th c ng dng ch to cc b lc

quang. Giao thoa MZ trong si quang c th c 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 v chiu 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

H MZ ( ) = 1 [1 + exp(i ) ] 2

(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 th c 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 th c iu khin bng chiu di nhnh v s lng b giao thoa MZ. Hot ng

ca b lc MZ c th c biu din qua hnh 5.2b. Thit b c thit k cc thnh phn tn s cao

s c 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 th c xc nh c

qua phn tch mch v c dng ti u thit k cng nh hot ng ca thit b. Thc nghim vonm 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. Tuynhin, 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 mhu ht tt c nhng nh sng ti b phn x li.

Trang 22


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K thut iu khin tn sc Bng chn ny c tp trung ch yu ti bc sng Bragg B = 2n 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 x c 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. 6.1 Chu k cch t ng nht (Uniform-Period Gratings)

Xt 1 cch t n gin nht khi n l 1 hm bin i tun hon n( z ) = n + ng cos(2 z /) Vi ng 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 sng truyn hng v

dA / dz = i A + i A f f b dAb / dz = i Ab i Af

ng 2 2 = , = 0 B B

(6.1) (6.2) (6.3)

Vi A f v Ab l bin ph ca 2 sng v :

l h s kt hp

Phng trnh kt hp c th c gii da trn bn cht tuyn tnh. A (0) i sin( qLg ) Hm truyn t ca

cch t: H ( w) = r ( w) = b = A f (0) B

(6.4)

Hnh 6.1 : ln(a) v pha(b) ca h s phn x cch t si quang ng nht vi Lg =2 v Lg =3


32/75

Vi Lg l di cch t. q 2 = 2 2 Hnh 6.1 trn cho thy ln v pha ca h s phn x tng ng

vi Lg =2 v Lg =3. H s phn x cch t gn t 100% khi Lg =3 Trang 23


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K thut iu khin tn sc 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 = B q Thng s tn sc

ca cch t quang si c cho bi: 2 3 sgn( ) 2 / vg g 3 2 / vg g 2 = 2 , 3 = 2 ( 2 )3/ 2 (

2 )5/ 2

(6.5)

Vi vg l vn tc nhm( Group velocity) ca xung, tn s sng mang w 0 = 2 / 0

Hnh 6.2: Tn sc vn tc nhm GVD . M t hm 2g theo thng s tng ng 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 thng ca GVD ca si quang tiu chun. V 2g c th

vt qu 1000 p s

2

/ cm

.

Mt cch t n di 2 cm c th c 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) k thut lm mn apodization ti nichit sut khng ng nht ca cch t. Kt qu l z ph thuc vo . Trong thc t xy ra 1 cch t

nhin khi m tia Gaussian s dng cho Trang 24


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K thut iu khin tn sc 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 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 c m

1

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

trn chiu di cch t.

Hnh 6.3: Tn hiu pht (ng lin nt) v tr( ng chm) , hm ca bc sng cho 1 cch t ng

nht (z) thay i t 0-6 c m trn chiu di cch t 11cm

c tnh truyn dn ca cch t c 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

2 trnh H () = H () exp [i ()] H () exp i(0 + 1 + 2 ) .

1 2


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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

9 10Gb/s ch suy gim 2% cng sut v t c t l li bit BER = 10 . 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

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1 cch t ng nht khi (z) bin i tuyn tnh t 0 n 12 c m trn 12 cm chiu di.

iu ny ch ra rng tc tr nhm, tp trung bc sng Bragg , c th c s dng cho b tn scnu 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 chiu di 257 km si quang chun vi tc 10

Gb/s. Mc d cch t ng 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 t quang Chirped dng b tn sc a/ chit sut n(z) theo chiu di cch t b/ h s phn

x tn s thp 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, Phasemask 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 s phn x v thi gian tr trong cch t quang Chirped tuyn tnh

vi bng thng 0.12nm Nm 1994 B tn sc GVD cho chiu di 160 km v tc 10-20Gb/s Trang 27


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K thut iu khin tn sc 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 d / dw ) 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 trongvng ph m o hm pha d / dw 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 Dg 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 th nghim 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 m rng 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 ngda 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.

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K thut iu khin tn sc 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 th c 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 t Cch t Moier c to thnh bng cch t chng 2 cch t chirped trn

cng 1 on ca si quang, do n cng s t gi tr nh 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

6.3 B 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 xtrong 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 b i 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 c s sang 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 nh

vo c tnh chu k thay i ca cch t v iu dn n vic ghp hai mode sng li vi nhau (ie.

tr thnh 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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K thut iu khin tn sc hn ca b ghp. Mt b chuyn i mode cch t chu k u c dng

chuyn tn hiu tr li mode c s. Cch lm khc cho cng tng trn l ghp cc mode c s ca si

quang hai li vi hai li ny l khc nhau. Nu hai li l gn (v khong cch), sng suy bin gia cc

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 cchca 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 th

c 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 OPC

Mc 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 trong thc t 7.1 Nguyn l hot ng: Cc n gin nht

hiu mt OPC c th b tn sc vn tc nhm GVD l gii phng trnh lin hp phc 1 A( z , t ) = 2 Thu

c:

A* i 2 2 A* 3 3 A* =0 z 2 t 2 6 t 3

A(0, w) exp( 2 zw

2

i

2

iwt )dw

(7.1.1)


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(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 co 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 Taylor phng trnh

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w 0 + 1 (w) + 2 (w)2 + 3 (w)3 (7.1.3) c 2 3 khi OPC c th b cho tt c cc thnh phn

tn sc bc chn m khng nh hng (w) = n (w)

n cc thnh phn bc l. Tnh hiu qu ca lin hp pha quang khong gia ph trong vic b tn sc

c th c kim chng bng cch s dng phng trnh

1 i 2 (7.1.4) A(0, ) exp 2 2 z i t d 2 Min quang (optical field) trc OPCtm c bng cch s dng z = L/2 phng trnh A( z , t ) =

+

ny. S lan truyn ca vng sng kt hp pha A* trong phn sau OPC nh sau: A* ( L, t ) = 1 2

A ( 2 , w) exp( 4 Lw

* 2

L

i

2

iwt )dw


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(7.1.5)

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

A* ( L / 2, w) = A* (0, w) exp( iw2 2 L / 4)

(7.1.6)

Thay phng trnh (7.1.6) vo (7.1.5), ta c A(L,t)=A*(0,t). V vy, ngoi tr o 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.

7.2 B tn sc bng t iu ch pha (Compensation of Self-Phase Modulation ) Hin tng phi tuynSPM 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 s u 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 suy hao

quang no. S lan truyn xung trong si quang c suy hao cho bi phng trnh Trang 31


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A i 2 2 A 2 (7.2.1) + = i A A A 2 2 t 2 z 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:

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

Phng trnh. (7.2.1) c th vit li

B i 2 2 B 2 + = i ( z ) B B 2 z 2 t

(7.2.2)

(7.2.3)

Vi ( z) = exp( 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 th

c 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 th c 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 ixng 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. N tng 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 th c tm c bi cch s dng cc si quang gim tn sc (dispersion decreasing fibers) cho


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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: Trang 32


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= ( z )dz

0

z

(7.2.4)

Pt (7.2.3) c th vit li

B i 2 B 2 + b( ) 2 = i B B 2 t

(7.2.5)

Vi b( ) = 2 ( ) / ( ) C GVD v SPM u c b nu b( ) = b( L ) , L l gi tr ca khi z=L.

iu kiu ny c tho mn t ng th s gim tn sc ging nh

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

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. 7.3 Tn 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 ngin 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 ws mt lng nh (~0.5THz) . Si quang phi tuyn to

ra tn hiu lin hp pha quang ti tn s wc = 2wp + ws . Min l tha mn iu kin ph hp pha. Vi k j

= n(wj )wc / c l s lng xung quang ti tn s wj iu kin ph hp gc pha tha mn nu bc sng

tn sc khng Z D c chn (trng vi bc sng bm) ng thi vi bc sng bm. iu ny c


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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. ca si quang

kc = 2k p ks

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K thut iu khin tn sc K thut OPC khong gia ph c 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 trong o khong gia ph trn 21 km chiu di si quang

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 ws thnh wc = 2wp ws 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 Lp c th c xc

nh bng cch s dng iu kin

2 (ws )Lp = 2 (wc )(L Lp )

Vi L l tng chiu di .

Khai trin 2 ( wc ) thnh chui Taylor ti tn s ws , Lp th tm c:

Lp L

=

2 + c 3 22 + c 3

(7.3)

Vi c = w c w s 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


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(FWM) trong si quang l nhy phn cc v phn cc ca tn hiu khng c iu khin bi si quang,

n thay i ti b to pha kt hp theo cch thc ngu nhin. nh hng ca bin i ngu Trang 34


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K thut iu khin tn sc 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 th c to 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: wc

= w p1 + w p 2 ws vi .w p 1 w p 2

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 b i 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. Hiu sut 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 vgim 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 thch Brillouin)

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 th c 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.

Trang 35


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K thut iu khin tn sc 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 chuyn i 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-phasematched) nh vo 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 trnchiu di 150 km si quang chun. Tim nng ca k thut OPC c chng minh mt th nghim

1999 vi 1 b kt hp c bn 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 2 10 ps/(km-nm)). y l

trng hp i vi si quang chun bc sng gn 1.55 m . Tnh khng n nh ny c th c trnh

nu thng s tn sc tng i ln

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 th gim xung di 3000 km. S

dng k thut OPC cho h thng quang ng di (long haul lightwave system) i hi s dng cc bkhuch 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 Trang

36


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K thut iu khin tn sc 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 2 = 2 ps 2 / km gn vi bc sng 1,55 m . 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 iu khin tn sc trong h

thng quang ng di.

8. H THNG QUANG NG DI:

Chng ny tp trung vo h thng quang ng di, v ic iu khin tn sc s gip cho vic m rng

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 phi tuyn 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 s c 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 dng pht tn hiu tc 10 Gb/s trn khong cch 10.000 km si quang

chun s dng SCF.

Trang 37


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K thut iu khin tn sc Khi khng c hiu ng phi tuyn, tng tn sc vn tc nhm GVD tch ly

trn chiu di hng ngn km c th c 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 thutperiodic dispersion management (iu khin tn sc tun hon) . tng c bn 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 trung bnh:

D = ( D 1 L1 + D 2 L 2 ) / L m

(8.1)

Vi D j l tn sc si quang c chiu di Lj (j=1,2) v Lm = L1 + L2 l chu k ca bn tn sc. Nu D~0

th tn sc c b trn mi chu k. Chiu di Lm l 1 thng s thit k t do m c th c chn

ph hp vi yu cu lm vic ca h thng. Thc t, thng thng chn Lm bng vi khong khuch i

amplifier spacing LA . in hnh

Lm = LA = 80 Km cho h thng quang mt t nhng s gim xung cn khong 5 km cho

h 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 L2 = D1 L1 / D2 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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K thut iu khin tn sc 8.1 L thuyt c s: 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

Schrodinger dA 2 2 A i 2 (8.2) phi tuyn. pt (7.2.1) i + A A = A 2 dz 2 t 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 th c bao gm bi victhay 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

1z A( z , t ) = B( z, t ) exp ( z )dz 20 Thay vo Phng trnh 8.2i

(8.3)

B 2 ( z ) 2 B 2 (8.4) + ( z) B B = 0 2 z 2 t 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 k ( z ) = exp( ( z ) dz )

0

z

Phn tch chi tit vic thit k mt h thng iu khin tn sc c th t 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 trongkhi 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 Gauss chirped sao cho:

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


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(8.5)

Vi a l bin , T l rng, l phase. 4 tham s ny thay i theo z. 1 phng php khc rt hudng tm s ph thuc z ca cc tham s ny. Phng trnh 8.4 c th c suy ra t phng trnh

Euler-Lagrange : (8.6)

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K thut iu khin tn sc 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 2

trnh kia. Phng trnh bin c th kt hp thy rng t hp a T khng thay i theo z v lin h vi

xung u vo Eo : a 2T = E0 . Do Cn phi gii 2 phng trnh sau: (8.7) (8.8) Bng cch t = 0 .

Lu rng t s (1 + C 2 ) / T 2 lin quan n rng ph ca xung l hng s trong mi trng truyn

dn tuyn tnh

2 Ta c th thay n bng 1 gi tr ban u (1 + C0 ) / T02 Vi To v Co l rng v h s

chirp ca xung vo trc khi n c a vo h thng iu khin tn sc ca si quang. Hai phng

trnh trn c th c gii bng phng php gii tch v theo phng php tng hp sau: (8.9) 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 k u tin ( z=Lm) c cho bi: (8.10) Vi d = 2 Lm / T02 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

tr v 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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K thut iu khin tn sc 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 th c gim ng k. L do nh sau: Cng

sut nh ca xung c hn ch ng k trong phn u bi v s m rng (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 th c gi l nhng chng truyn dn ta-tuyn tnh (tc l ch tuyntnh 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 s c 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

T ( Lm ) = T0 ,

C ( Lm ) = C0

(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 s

c trnh by trong chng 9. 8.2 Hiu 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 . S nh 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 truynbn trong vng lp quang. Tng khong cch truyn gii hn 1171 km bi v nhng hiu ng phi tuyn

khc nhau. Trang 41


58/75

K thut iu khin tn sc 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 bnh trn 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:

B 2 ( z ) 2 B 2 + ( z) B B = 0 i 2 2 t z Bng cch a 3 xung ln cn , B = B1 + B2 + B3 Thay vo ta

c:

(8.12) (8.13) (8.14) 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)

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K thut iu khin tn sc Hiu ng XPM tng tc ng knh ch phase 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 k

nh 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

ch ng b ng dy c ngh u tin. Truyn gi tuyn tnh tc 320 Gb/s cng c xcnhn 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 CAO

Cc 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. 9.1 B 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 bit ca tng knh) ca cc knh n. Vi cc knh 10Gbps, thnh phn tn sc th ba

khng ng vai tr ch cht nh i vi cc xung c b rng >10ps. Tuy nhin v cc bc sng l c lp

vi cc gi tr 2 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 th c s dng trong h thng WDM nu s lng knh nh (thng thng l


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K thut iu khin tn sc 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 th c bin i tng

ng vi tn sc GVD trn mi knh. Hnh 9.1 m t m hnh ghp tng cch 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 m rng s cch t c lm trongthc t. Vo nm 2000, h thng WDM c m rng 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 t b 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 theochu 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 t n 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 x c 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 b nh hng bi lng chirp ny. Cch t u tin c s dng vo nm

1995 b tn sc ng thi cho hai knh 10 Gbps vi khong cch truyn 240km. Thc nghim voTrang 44


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K thut iu khin tn sc nm 1999 s dng cch t quang mu cho h thng WDM bn knh. Khi s

lng knh tng, n tr nn 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. Si DCF 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 kinny cng phi tha cho tt c cc knh

D1 (n ) L1 + D2 (n ) L2 = 0

Vi

(9.1)

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 D j (n ) = D j + S j (n c ) vi Dj

(j=1,2) l gi tr tn sc knh trung tm, dc tn sc ca si DCF l :

S2 = S1 ( L1 / L2 ) = S1 ( D2 / D1 )

(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 D 16 ps / ( km nm ) v dc tn sc S 0, 05 ps / (km nm2 ) , h

s S/D s vo khong 0,003 nm-1. Do , vi si DCF c D 100 ps / (km nm) , dc tn sc s vo


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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

(ReverseDispersion Fiber) l si quang c gi tr D v S c gi tr o 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 Trang 45


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K thut iu khin tn sc 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). 9.2 B 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 thct, 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 thay i 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 tr nn 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 thc t. 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 t c 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 t c 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 B ( z) = 2n( z)( z) . H s tn sc ca cch t theo cng thc 6.6 c thay th thnh :

Lg 2 d = Dg ( ) = n( z )dz d c d 0

d g

(9.3)

Vi g l tr nhm Lg l chiu di cch t. Gi tr Dg bt 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 th iu chnh c trong cch t Bragg.

Trang 46


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K thut iu khin tn sc 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 )

tr nn c lp vi z khi qu

trnh gradient nhit c thit lp dc theo cch t. Tn sc tng cng DgLg c th c thay i trong

di -500 n -2200 ps/nm. Cch t loi ny c th c dng iu khin tc sc cho cc h thng 10

Gbps.

Hnh 9.2 : Ph phn 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 m

rng 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. Cch t 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 c nh. 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 Trang 47


65/75

K thut iu khin tn sc 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 tr nn rt quan trng i vi cc

xung ngn. Chng ta s tho lun phn tip theo. 9.3 iu 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


66/75

(9.6)

iu kin ny gn ging vi phng trnh 9.2 xc nh mc trc cho si DCF trong

Vi h thng n knh, bng thng tn hiu nh khong 4nm tc c th t 500Gbps v tha

iu kin 9.5. Yu cu ny cng c th c p ng d dng cc b lc quang v cch t chirp quang.

Trong trng hp b lc quang. Mch sng quang phng da trn nhiu b lc giao thoa MZ d dng

p ng c yu cu nu trn do kh nng c th lp trnh c vn c ca cc b lc. Mt thc

nghim vi b lc c thit

Trang 48


67/75

K thut iu khin tn sc 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 khng v 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 lnn 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 m rng ra

n 5-6ps . Vi vic b tn sc tng phn thnh phn tn sc bc ba s khc phc vic chn xung b m

rng v b rng xung gim Trang 49


68/75

K thut iu khin tn sc 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 thi thnh 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. 9.4 B tn sc phn cc mode PMD PMD ltc ng m rng xung quang do cc bin i ngu nhin ca hin tng lng chit/khc x kp dc

theo chiu di si quang. Vic m rng ny cng lm xung b m rng hn na bn cnh vic dn rng

xung do GVD. Cc k thut iu khin tn sc gp phn 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

T ( T )

2 1/2

= DP L vi Dp l h s PMD v T l tr tng i theo hai nguyn

l trng thi phn cc (PSP). K hiu T biu 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 trong khong 0,1 n 0,15 ty theo loi iu ch (RZ, CRZ hoc NRZ) v cc thng s caxung ng vo. Vi m hnh 10%, chiu du h thng v tc bit phi tha iu kin

B2 L < (10DP )2


69/75

(9.7)

Trong cc tuyn

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