caodang.tdt.edu.vncaodang.tdt.edu.vn/lecturer/upload/files/document/Ha-Duy-Hung-06... · i LỜI GIỚI THIỆU Thế kỷ 21 là thế kỷ của công nghệ thông tin. Sự bùng

HC VIN CNG NGH BU CHNH VIN THNG

K THUT

THNG TIN QUANG 2

(Dng cho sinh vin h o to i hc t xa)

Lu hnh ni b

H NI - 2007


K THUT

THNG TIN QUANG 2

Bin son: THS. VN VIT EM

i

LI GII THIU

Th k 21 l th k ca cng ngh thng tin. S bng n ca cc loi hnh dch v thng tin, c bit l s pht trin nhanh chng ca Internet v World Wide Web lm gia tng khng ngng nhu cu v dung lng mng. iu ny i hi phi xy dng v pht trin cc mng quang mi dung lng cao. Cng ngh ghp knh theo bc sng quang (DWDM) l mt gii php hon ho cho php tn dng hu hiu bng thng rng ln ca si quang, nng cao r rt dung lng truyn dn ng thi h gi thnh sn phm. S pht trin ca h thng WDM cng vi cng ngh chuyn mch quang s to nn mt mng thng tin th h mi-mng thng tin ton quang. Trong mng ton quang ny, giao thc IP- giao thc chun cho mng vin thng th h sau (NGN) s c tch hp vi WDM. S tch hp ny s to ra mt kt cu mng trc tip nht, n gin nht, kinh t nht rt thch hp s dng cho c mng ng trc v mng th

Cu trc ca Bi gingbao gm bn chng:

Chng I: H thng thng tin quang WDM. Chng ny trnh by cc nguyn l c bn ca DWDM, kho st chi tit cc hin tng phi tuyn nh hng n cht lng ca h thng WDM, v cc linh kin c s dng cho h thng WDM.

Chng II: Khuch i quang. Chng ny tm hiu nguyn l hot ng ca cc loi khuch i quang, tnh nng v cc ng dng ca chng trong mng truyn dn quang

Chng III: Truyn ti IP/WDM. Chng ny nghin cu v xu hng tch hp IP trn WDM, c bit quan tm n vn nh tuyn v gn bc sng trong mng WDM.

Chng IV: H thng thng tin quang Coherent. Chng ny tm hiu cc nguyn l c bn ca h thng coherent, nhng u im ca n so vi h thng IM/DD v trin vng ca cng ngh ny trong tng lai.

Bi ging c bin son nhm phc v cho cc lp thuc h i hc t xa ca Hc vin Cng ngh Bu chnh Vin thng. Ngoi ra Bi ging cng c th s dng lm ti liu tham kho cho cc sinh vin v k s chuyn ngnh vin thng.

Do khun kh gii hn cng nh tnh ng dng thc t ca ti liu, cc m hnh ton hc c trnh by trong Bi ging ny i khi ch l cc kt qa cui cng v c gii thch, minh ha bng cc ngha vt l c th. hiu thm v vic dn xut v chng minh cc kt qu ny, bn c c th c thm cc ti liu tham kho.

Do tnh cht phc tp cng nh s pht trin nhanh chng ca cng ngh, Bi ging H thng thng tin quang II khng th trnh khi nhng thiu st. Chng ti xin chn thnh cm n tt c cc kin ng gp ca cc bn c hon thin hn Bi ging ny.


Chng 1: H thng thng tin quang WDM

1

CHNG 1

H THNG THNG TIN QUANG WDM

GII THIU

Bc vo thin nin k mi, chng ta chng kin nhiu s thay i quan trng trong nn cng nghip vin thng c nh hng to ln n cuc sng ca chng ta. C nhiu nguyn nhn gy ra s thay i ny:

Trc ht l s gia tng lin tc v dung lng mng. Nhn t chnh cho s gia tng ny l s pht trin nhanh chng ca Internet v World Wide Web. Bn cnh l vic cc nh kinh doanh ngy nay da vo cc mng tc cao thc hin vic kinh doanh ca mnh. Nhng mng ny c dng kt ni cc vn phng trong mt cng ty cng nh gia cc cng ty cho vic giao dch thng mi. Ngoi ra cn c mt s tng quan ln gia vic gia tng nhu cu v gi thnh bng thng ca mng. Cc cng ngh tin tin thnh cng trong vic gim lin tc gi thnh ca bng thng. Vic gim gi thnh ca bng thng ny li lm thc y s pht trin ca nhiu ng dng mi s dng nhiu bng thng v m hnh s dng hiu qu hn. Chu k hi tip dng ny cho thy khng c du hiu gim bt trong mt tng lai gn.

Bi b v ph v s c quyn trong lnh vc vin thng. S bi b s c quyn ny kch thch s cnh tranh trong th trng, iu ny dn n kt qu l gim gi thnh cho nhng ngi s dng v trin khai nhanh hn nhng k thut v dch v mi

S thay i quan trng trong th loi lu lng chim u th trong mng. Ngc li vi lu lng thoi truyn thng, nhiu nhu cu mi da trn d liu ngy cng pht trin. Tuy nhin nhiu mng hin nay c xy dng ch h tr hiu qu cho lu lng thoi, khng phi l d liu. Vic thay i ny l nguyn nhn thc y nhng nh cung cp dch v kim tra li cch thc m h xy dng nn mng, kiu dch v phn phi v trong nhiu trng hp ngay c m hnh kinh doanh ton th ca h.

Nhng nhn t ny dn n s pht trin ca mng quang dung lng cao. Cng ngh p ng vic xy dng cc mng quang dung lng cao ny l cng ngh ghp knh theo bc sng DWDM. Trong chng ny chng ta s tm hiu v h thng thng tin quang WDM, c th s nghin cu:

- Nguyn l ghp knh phn chia theo bc sng quang (WDM).

- Cc yu t nh hng n cht lng ca h thng WDM do s lan truyn ca tn hiu WDM trong si quang, trong tp trung vo vic tm hiu cc hin ng phi tuyn.

- Cc linh kin, phn t trong mng WDM.

- M hnh mng WDM bao gm cc phn t mng, tp vt l, tp logic, v cc k thut chuyn mch bo v


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1.1 NGUYN L GHP KNH THEO BC SNG QUANG (WDM)

1.1.1 Gii thiu chung

S pht trin nhanh chng ca cc m hnh truyn s liu, c bit l Internet lm bng n nhu cu tng bng thng (xem hnh 1.1). Trong bi cnh IP (Internet Protocol) ang ni ln nh l nn tng chung ca mi loi hnh dch v trong tng lai, cc nh cung cp dch v truyn dn bt buc phi xem xt li phng thc truyn dn TDM truyn thng, vn ti u cho truyn thoi nhng li km hiu qu trong vic tn dng bng thng.

1996 1997 1998 1999 2000 2001

250

200

150

100

50

Lu lng

Nam

Thoai

D lieu

Hnh 1.1 Tng quan gia nhu cu truyn thoi v truyn s liu

Tm li, ta phi gii quyt bi ton tng bng thng cho vin thng tng lai. Cc nh cung cp dch v truyn dn bt u xt n ba phng thc truyn dn sau:

Truyn dn ghp phn khng gian SDM (Space Devision Multiplexing): n gin v khng cn s pht trin cng ngh, ch n thun l tng s lng si quang, tc truyn dn vn gi nguyn. Ta c th chn SDM nu trn tuyn truyn dn cn tng bng thng c sn s lng si quang cha dng v khong cch tuyn truyn dn l ngn khng cn dng cc b lp, b khuch i. Nu khong cch l xa, khi chi ph s vt tng do mi h thng lp thm u cn mt s lng b lp, b khuych i... nh h thng c.

Truyn dn ghp phn thi gian TDM (Time Devision Multiplexing): tng tc truyn dn ln trn si quang. Khi tip tc dng phng thc truyn thng ny, ta phi xem xt n hai vn : trc v khi truyn trn si quang. Trc khi chuyn thnh tn hiu quang truyn i, cc linh kin in t c kh nng x l vi tc bit ti a l bao nhiu? Thc t hin nay cho thy, a s cc mng truyn dn, linh kin in t c kh nng p ng tt i vi cc dng tn hiu tc 2.5 Gbps hoc 10 Gbps. Nh vy th cha gii quyt trn vn bi ton tng bng thng. Trong phng th nghim cho cc linh kin hot ng tc 40 Gbps hoc 80 Gbps. TDM c th t c nhng tc cao hn, cc phng php thc hin tch/ghp knh trong min quang, c gi l phn knh thi gian trong min quang (Optical time Division Multiplexing - OTDM) ang c tch cc trin khai. Cc kt qa nghin cu trong phng th nghim cho thy


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OTDM c th ghp c cc lung 10Gbit/s thnh lung 250Gbit/s. Nhng khi , truyn trn si quang s vp phi cc vn nghim trng nh hng n cht lng truyn dn: tn sc sc th, tn sc phn cc, phi tuyn tnh.

Truyn dn ghp phn bc sng WDM (Wavelength Devision Multiplexing): ghp thm nhiu bc sng c th truyn trn mt si quang, khng cn tng tc truyn dn trn mt bc sng. Cng ngh WDM c th mang n gii php hon thin nht trong iu kin cng ngh hin ti. Th nht n vn gi tc x l ca cc linh kin in t mc 10 Gbps, bo m thch hp vi si quang hin ti. Thay vo , cng ngh WDM tng bng thng bng cch tn dng ca s lm vic ca si quang trong khong bc sng 1260 nm n 1675 nm. Khong bc sng ny c chia lm nhiu bng sng hot ng nh minh ho trn bng 1.1. Thot tin, h thng WDM hot ng bng C (do EDFA hot ng trong khong bng sng ny). V sau, EDFA c kh nng hot ng c bng C v bng L nn h thng WDM hin ti dng EDFA c th hot ng c bng C v bng L. Nu theo chun ITU-T, xt khong cch gia cc knh bc sng l 100 Ghz (m bo kh nng chng xuyn nhiu knh trong iu kin cng ngh hin ti), s c 32 knh bc sng hot ng trn mi bng. Nh vy, nu vn gi nguyn tc bit trn mi knh truyn, dng cng ngh WDM cng lm tng bng thng truyn trn mt si quang ln 64 ln !

Bng 1.1 S phn chia cc bng sng

Bng sng M t

Phm vi bc sng (nm)

Bng O Original 1260 n 1360 Bng E Extended 1360 n 1460 Bng S Short 1460 n 1530 Bng C Conventional 1530 n 1565 Bng L Long 1565 n 1625 Bng U Ultra-long 1625 n 1675

1.1.2 S khi tng qut

a) nh ngha

Ghp knh theo bc sng WDM (Wavelength Devision Multiplexing) l cng ngh trong mt si quang ng thi truyn dn nhiu bc sng tn hiu quang. u pht, nhiu tn hiu quang c bc sng khc nhau c t hp li (ghp knh) truyn i trn mt si quang. u thu, tn hiu t hp c phn gii ra (tch knh), khi phc li tn hiu gc ri a vo cc u cui khc nhau.

b) S chc nng

Nh minh ho trn hnh 1.2, m bo vic truyn nhn nhiu bc sng trn mt si quang, h thng WDM phi thc hin cc chc nng sau:

Pht tn hiu: Trong h thng WDM, ngun pht quang c dng l laser. Hin ti c mt s loi ngun pht nh: Laser iu chnh c bc sng (Tunable Laser), Laser a bc sng (Multiwavelength Laser)... Yu cu i vi ngun pht laser l phi c


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rng ph hp, bc sng pht ra n nh, mc cng sut pht nh, bc sng trung tm, rng ph, rng chirp phi nm trong gii hn cho php.

MUX

Tx1

TxN

Tx2

EDFA EDFA

Phat tn hieu Ghep tn hieu

Khuech ai tn hieu

Truyen tn hieu tren si quang

Khuech ai tn hieu

Tach tn hieu Thu tn hieu

DEMUX

Rx1

RxN

Rx2

Hnh 1.2 S chc nng h thng WDM

Ghp/tch tn hiu: Ghp tn hiu WDM l s kt hp mt s ngun sng khc nhau thnh mt lung tn hiu nh sng tng hp truyn dn qua si quang. Tch tn hiu WDM l s phn chia lung nh sng tng hp thnh cc tn hiu nh sng ring r ti mi cng u ra b tch. Hin ti c cc b tch/ghp tn hiu WDM nh: b lc mng mng in mi, cch t Bragg si, cch t nhiu x, linh kin quang t hp AWG, b lc Fabry-Perot... Khi xt n cc b tch/ghp WDM, ta phi xt cc tham s nh: khong cch gia cc knh, rng bng tn ca cc knh bc sng, bc sng trung tm ca knh, mc xuyn m gia cc knh, tnh ng u ca knh, suy hao xen, suy hao phn x Bragg, xuyn m u gn u xa...

Truyn dn tn hiu: Qu trnh truyn dn tn hiu trong si quang chu s nh hng ca nhiu yu t: suy hao si quang, tn sc, cc hiu ng phi tuyn, vn lin quan n khuch i tn hiu ... Mi vn k trn u ph thuc rt nhiu vo yu t si quang (loi si quang, cht lng si...) m ta s xt c th trong phn 1.2.

Khuch i tn hiu: H thng WDM hin ti ch yu s dng b khuch i quang si EDFA (Erbium-Doped Fiber Amplifier). Tuy nhin b khuch i Raman hin nay cng c s dng trn thc t. C ba ch khuch i: khuch i cng sut, khuch i ng v tin khuch i. Khi dng b khuch i EDFA cho h thng WDM phi m bo cc yu cu sau:

- li khuch i ng u i vi tt c cc knh bc sng (mc chnh lch khng qu 1 dB).

- S thay i s lng knh bc sng lm vic khng c gy nh hng n mc cng sut u ra ca cc knh.

- C kh nng pht hin s chnh lch mc cng sut u vo iu chnh li cc h s khuch i nhm m bo c tuyn khuch i l bng phng i vi tt c cc knh.

Thu tn hiu: Thu tn hiu trong cc h thng WDM cng s dng cc b tch sng quang nh trong h thng thng tin quang thng thng: PIN, APD.


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c) Phn loi h thng WDM

MUX

Tx1

TxN

Tx2

N,...,3,2,1

N,...,3,2,1 EDFA EDFA

MUX

Tx1

TxN

Tx2

DEMUX

Rx1

RxN

Rx2

i,...,3,2,1

N),...,2i(),1i( ++EDFA EDFA

DEMUX

Rx1

RxN

Rx2

He thong WDM n hng

He thong WDM song hng

Hnh 1.3 H thng ghp bc sng n hng v song hng

H thng WDM v c bn chia lm hai loi: h thng n hng v song hng nh minh ho trn hnh 1.3. H thng n hng ch truyn theo mt chiu trn si quang. Do vy, truyn thng tin gia hai im cn hai si quang. H thng WDM song hng, ngc li, truyn hai chiu trn mt si quang nn ch cn 1 si quang c th trao i thng tin gia 2 im.

C hai h thng u c nhng u nhc im ring. Gi s rng cng ngh hin ti ch cho php truyn N bc sng trn mt si quang, so snh hai h thng ta thy:

Xt v dung lng, h thng n hng c kh nng cung cp dung lng cao gp i so vi h thng song hng. Ngc li, s si quang cn dng gp i so vi h thng song hng.

Khi s c t cp xy ra, h thng song hng khng cn n c ch chuyn mch bo v t ng APS (Automatic Protection-Switching) v c hai u ca lin kt u c kh nng nhn bit s c mt cch tc thi.

ng v kha cnh thit k mng, h thng song hng kh thit k hn v cn phi xt thm cc yu t nh: vn xuyn nhiu do c nhiu bc sng hn trn mt si quang, m bo nh tuyn v phn b bc sng sao cho hai chiu trn si quang khng dng chung mt bc sng.

Cc b khuch i trong h thng song hng thng c cu trc phc tp hn trong h thng n hng. Tuy nhin, do s bc sng khuch i trong h thng song hng gim theo mi chiu nn h thng song hng, cc b khuych i s cho cng sut quang ng ra ln hn so vi h thng n hng.

1.1.3 c im ca h thng WDM

Thc t nghin cu v trin khai WDM rt ra c nhng u nhc im ca cng ngh WDM nh sau:


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u im ca cng ngh WDM:

- Tng bng thng truyn trn si quang s ln tng ng s bc sng c ghp vo truyn trn mt si quang.

- Tnh trong sut: Do cng ngh WDM thuc kin trc lp mng vt l nn n c th h tr cc nh dng s liu v thoi nh: ATM, Gigabit Ethernet, ESCON, chuyn mch knh, IP ...

- Kh nng m rng: Nhng tin b trong cng ngh WDM ha hn tng bng thng truyn trn si quang ln n hng Tbps, p ng nhu cu m rng mng nhiu cp khc nhau.

- Hin ti, ch c duy nht cng ngh WDM l cho php xy dng m hnh mng truyn ti quang OTN (Optical Transport Network) gip truyn ti trong sut nhiu loi hnh dch v, qun l mng hiu qu, nh tuyn linh ng ...

Nhc im ca cng ngh WDM:

- Vn cha khai thc ht bng tn hot ng c th ca si quang (ch mi tn dng c bng C v bng L).

- Qu trnh khai thc, bo dng phc tp hn gp nhiu ln.

- Nu h thng si quang ang s dng l si DSF theo chun G.653 th rt kh trin khai WDM v xut hin hin tng trn bn bc sng kh gay gt.

1.1.4 Li ITU

Vic chun ho cc b bc sng dng cho cc mng WDM l ht sc cn thit v n bo m cho cc thit b ca cc nh cung cp khc nhau u c sn xut theo cng mt tiu chun, v u lm vic tng thch vi nhau.

Khi chun ho bc sng, vn cn phi xem xt u tin l l khong cch gia cc knh phi da theo tn s hay bc sng. Khong cch tn s bng nhau s lm cho khong cch bc sng hi khc nhau. Khng c mt tiu chun k thut no c u tin la chn trong trng hp ny. Trong khuyn co ITU-G.692 cc knh cch nhau mt khong 50 GHz (tng ng vi khong cch bc sng l 0.4nm) vi tn s trung tm danh nh l 193.1THz (1552.52 nm). Tn s ny gia di thng ca si quang 1.55m v b khuch i quang si EDFA (xem hnh 1.4). Khong cch ny ph hp vi kh nng phn gii ca cc b MUX/DEMUX hin nay, n nh tn s ca cc b laser, MUX/DEMUX,... Khi cng ngh hon thin hn khong cch ny s phi gim i.

Mt vn khc, kh khn hn l chn la mt b bc sng tiu chun bo m cho cc h thng cho 4, 8, 16, v 32 bc sng hot ng tng thch vi nhau bi v cc nh sn xut u c cc cu hnh knh ti u ring v cc k hoch nng cp h thng t t knh ln nhiu knh khc nhau. ITU chun ho (ITU G.959) b 16 bc sng bt u t tn s 192.1 THz, rng 200GHz mi bn cho giao din a knh gia cc thit b WDM.

Cui cng l phi lu khng ch bo bo m cc tn s trung tm m cn phi bo m lch tn s ti a cho php. i vi f 200 GHz, ITU quy nh lch tn s l khng vt


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qu f /5 GHz. Vi f =50 GHz v f =100 GHz th n thi im ny ITU vn cha chun ho.

Hnh 1.4 Li bc sng theo ITU

1.2 CC YU T NH HNG N CHT LNG CA H THNG WDM

C 3 yu t c bn ca si quang nh hng n kh nng ca cc h thng thng tin quang, bao gm:

- Suy hao

- Tn sc

- Hin tng phi tuyn xy ra trong si quang.

Tuy nhin, i vi cc h thng khc nhau th mc nh hng ca cc yu t ny cng khc nhau. V d:

i vi cc h thng c ly ngn, dung lng thp th yu t ch yu cn quan tm l suy hao.

i vi cc h thng tc cao, c ly tng i ln th yu t ch yu cn quan tm l suy hao v tn sc.

i vi cc h thng c ly di v dung lng rt ln th ngoi 2 yu t trn cn phi xem xt n c cc hiu ng phi tuyn.

Hin tng suy hao v tn sc c trnh by chi tit trong bi ging H thng thng tin quang 1.Trong phn ny s tp trung trnh by v cc hin tng phi tuyn.

1.2.1 Tng quan v cc hiu ng phi tuyn

a) nh ngha

Hiu ng quang c gi l phi tuyn nu cc tham s ca n ph thuc vo cng nh sng (cng sut). Cc hin tng phi tuyn c th b qua i vi cc h thng thng tin quang hot ng mc cng sut va phi (vi mW) vi tc bit ln n 2.5 Gbps. Tuy nhin, tc bit cao hn nh 10 Gbps v cao hn v/hay mc cng sut truyn dn ln, vic xt cc


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hiu ng phi tuyn l rt quan trng. Trong cc h thng WDM, cc hiu ng phi tuyn c th tr nn quan trng thm ch cng sut v tc bit va phi.

Cc hiu ng phi tuyn c th chia ra lm 2 loi. Loi th nht pht sinh do tc ng qua li gia cc sng nh sng vi cc phonon (rung ng phn t) trong mi trng silica- mt trong nhiu loi hiu ng tn x m chng ta xem xt l tn x Rayleigh. Hai hiu ng chnh trong loi ny l tn x do kch thch Brillouin (SBS) v tn x do kch thch Raman (SRS).

Loi th hai sinh ra do s ph thuc ca chit sut vo cng in trng hot ng, t l vi bnh phng bin in trng. Cc hiu ng phi tuyn quan trng trong loi ny l hiu ng t iu pha (SPM - Self-Phase Modulation), hiu ng iu ch xuyn pha (CPM - Cross-Phase Modulation) v hiu ng trn 4 bc sng (FWM - Four-Wave Mixing). Loi hiu ng ny c gi l hiu ng Kerr.

Trong cc hiu ng tn x phi tuyn, nng lng t mt sng nh sng c chuyn sang mt sng nh sng khc c bc sng di hn (hoc nng lng thp hn). Nng lng mt i b hp th bi cc dao ng phn t hoc cc phonon (loi phonon lin quan n s khc nhau gia SBS v SRS). Sng th hai c gi l sng Stokes. Sng th nht c th gi l sng bm (Pump) gy ra s khuch i sng Stokes. Khi sng bm truyn trong si quang, n b mt nng lng v sng Stokes nhn thm nng lng. Trong trng hp SBS, sng bm l sng tn hiu v sng Stokes l sng khng mong mun c to ra do qu trnh tn x. Trong trng hp SRS, sng bm l sng c nng lng cao v sng Stokes l sng tn hiu c khuch i t sng bm.

Ni chung, cc hiu ng tn x c c trng bi h s li g, c o bng m/w (meters per watt) v rng ph f (i vi li tng ng) v cng sut ngng Pth ca nh sng ti - mc cng sut m ti suy hao do tn x l 3 dB, tc l mt na cng sut trn ton b di si quang. H s li l mt i lng ch cng ca hiu ng phi tuyn.

Trong trng hp t iu pha SPM, cc xung truyn b hin tng chirp (tn s xung truyn i thay i theo thi gian). iu ny lm cho h s chirp (chirped factor) tr nn ng k cc mc nng lng cao. S c mt ca hin tng chirp lm cho hiu ng gin xung do tn sc mu tng ln. Do vy, chirp xy ra do SPM (SPM induced chirp) c th gy tng gin xung do tn sc mu trong h thng. i vi cc h thng tc bit cao, chirp do SPM c th lm tng mt cch ng k gin xung do tn sc mu thm ch cc mc cng sut va phi. nh hng ca SPM khng ch ph thuc vo du tham s GVD (Group Velocity Dispersion) m cn ph thuc vo chiu di ca h thng.

Trong h thng WDM a knh, chirp xy ra trong mt knh ph thuc vo s thay i chit sut theo cng ca cc knh khc. Hiu ng ny c gi l hiu ng iu ch xuyn pha (CPM - Cross-Phase Modulation). Khi xem xt hin tng chirp trong mt knh do s thay i chit sut theo cng ca chnh knh , ta gi l hiu ng ny SPM.

Trong cc h thng WDM, mt hiu ng quan trng khc l hiu ng trn bn bc sng. Nu h thng WDM bao gm cc tn s f1, f2,,fn, hiu ng trn bn bc sng sinh ra cc tn hiu ti cc tn s nh l 2fi - fj, v fi + fj - fk. Cc tn hiu mi ny gy ra xuyn knh (crosstalk) vi cc tn hiu c sn h thng. Xuyn knh ny nh hng c bit nghim trng khi khong cch gia cc knh hp. Vic gim tn sc mu lm tng xuyn knh gy ra bi hiu ng


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trn bn bc sng. V vy, h thng s dng si quang dch chuyn tn sc chu nh hng ca hiu ng trn bn bc sng nhiu hn l h thng s dng si n mt.

b) Chiu di v din tch hiu dng

S tc ng phi tuyn ph thuc vo c ly truyn dn v mt ct ngang ca si quang. Tuyn cng di, s tc ng qua li gia nh sng v vt liu si quang cng ln v nh hng ca phi tuyn cng xu. Tuy nhin, khi tn hiu lan truyn trong si quang, cng sut ca tn hiu gim i do suy hao ca si quang. V vy, hu ht cc hiu ng phi tuyn xy ra ngay trong khong u ca si quang v gim i khi tn hiu lan truyn.

M hnh ca nh hng ny c th rt phc tp. Trong thc t, c th s dng mt m hnh n gin vi gi thuyt nng lng khng thay i qua mt di hiu dng c nh Leff. Gi s Pin l cng sut truyn trong si quang v P(z)=Pine-z l cng sut ti im z trn tuyn, vi l h s suy hao. L c k hiu l chiu di thc ca tuyn. Chiu di hiu dng ca tuyn c k hiu l Leff c nh ngha nh sau [1]:

=

=L

zeffin dzzPLP

0

)( (1.1)

Do ,

L

effeL = 1 (1.2)

Thng th tuyn di do L >>1/ cho nn Leff 1/. Chng hn nh = 0.22 dB/km ti = 1,55 m v, ta c Leff ~ 20 km.

nh hng ca phi tuyn cng tng theo cng nng lng trong si. Vi mt cng sut cho trc, cng t l nghch vi din tch li. Do nng lng phn b khng ng u theo mt ct ngang ca si, do thun tin khi nim din tch vng li hiu dng Aeff

Chiu di tuyn

(a)

L Leff

Pin Pin

(b)

Chiu di tuyn

Cn

g sut

Cn

g sut

Hnh 1.5 Tnh chiu di truyn dn hiu dng. (a) Phn b cng sut c trng dc theo chiu di L ca tuyn. Cng sut nh l Pin. (b) Phn b cng sut gi nh dc theo tuyn n di hiu dng Leff. Chiu di Leff c chn sao cho din tch ca vng di ng cong trong (a) bng din tch ca hnh vung trong (b).


10

(effective cross-sectional area) thng c s dng (xem hnh 1.6). Din tch ny lin quan n din tch thc ca li A v phn b trn mt ct ngang ca mt c bn F(r,) nh sau [1]:

[ ] =

r

reff

rdrdrF

rdrdrFA

4

22

),(

),( (1.3)

vi r v l ta cc.

Thng c th nh gi gi tr Aeff thng qua cng thc n gin hn [2]

Aeff w02 (1.4)

Vi w0 l bn knh trng mt.

Din tch hiu dng nh c nh ngha trn c mt ngha l s ph thuc ca hu ht cc hiu ng phi tuyn c th c biu din bng din tch hiu dng i vi mt c bn truyn trong si quang cho trc. Chng hn nh cng hiu dng ca mt xung c th c tnh Ieff = P/Aeff, vi P l cng sut xung, tnh ton nh hng ca cc hiu ng phi tuyn nh SPM c xem xt di y. Din tch hiu dng ca SMF khong 85 m2 v ca DSF khong 50 m2. Cc si quang b tn sc c din tch hiu dng nh hn v do c nh hng phi tuyn ln hn.

1.2.2 Tn x do kch thch Brillouin

Trong trng hp SBS, cc phonon lin quan n s tc ng tn x l cc phonon m hc v s tng tc ny xy ra trn di tn hp f = 20 MHz bc sng 1550 nm. Sng bm v sng Stokes truyn theo hai hng ngc nhau. Do , SBS khng gy ra bt k tc ng qua li no gia cc bc sng khc nhau khi m khong cch bc sng ln hn 20 MHz (l trng hp c trng cho WDM). Tuy nhin, SBS cng c th to nn s mo kh quan trng trong mt knh n l. SBS to ra li theo hng ngc li vi hng lan truyn tn hiu, ni cch khc l hng v pha ngun. V vy, n lm suy gim tn hiu c truyn cng nh to ra mt tn hiu

Cng

Bn knh

(a)

Cng

(Aeff/)1/2 Bn knh

(b)

Hnh 1.6 Din tch mt ct ngang hiu dng. (a) Phn b c trng cng tn hiu dc theo bn knh ca si quang. (b) Phn b cng gi nh tng c vi (a) cho thy cng khc khng ch trong vng Aeff xung quang tm si quang


11

c cng mnh v hng pht, nn phi dng mt b cch ly bo v. H s li SBS gB xp x 4x10-11 m/W, khng ph thuc vo bc sng.

Cng sut ngng cho SBS c th tnh bng cng thc sau [2]:

PthSBS=21Aeff/gBLeff (1.5)

Vi gB l li ca SBS. Nh ni trong mc (2.3.1) Leff1/ nn

PthSBS=(21Aeff)/gB (1.6)

Ga tr c trng ca gB 5x10-11 m/W ti = 1550nm. Ly =0.046 1/km = 0.2dB/Km v Aeff=55m2, tnh c PthSBS = 8mW cho mt knh.

Mt cch khc tnh cng sut ngng ny l [3]:

PthSBS=4.4x10-3d22f watts (1.7)

Vi d: ng knh li si quang (m)

: bc sng hot ng (m)

: h s suy hao (dB/Km)

f: rng ph ca ngun quang (GHz)

1.2.3 Tn x do kch thch Raman

Nu a vo trong si quang hai hay nhiu tn hiu c bc sng khc nhau th SRS gy ra s chuyn nng lng t cc knh c bc sng thp sang cc knh c bc sng cao hn (xem hnh 1.7). S chuyn nng lng t knh tn hiu c bc sng thp sang knh tn hiu c bc sng cao l mt hiu ng c bn lm c s cho khuch i quang v laser. Nng lng ca photon bc sng l hc/ vi h l hng s Planck (6.63x10-34 Js). Do , photon ca bc sng thp c nng lng cao hn. S chuyn nng lng t tn hiu bc sng thp sang tn hiu bc sng cao tng ng vi s sinh ra cc photon nng lng thp t cc photon nng lng cao hn.

Khng ging nh SBS, SRS l mt hiu ng bng rng. Hnh 1.8 cho thy li l mt hm ca khong cch bc sng. Gi tr nh ca h s li gR xp x 6x10-14 m/W bc sng 1550 nm nh hn nhiu so vi li ca SBS. Tuy nhin, cc knh cch nhau n 15 THz (125 nm) s b tc ng ca SRS. SRS gy nh hng trn c hng truyn v hng ngc li. Mc

Si quang 1234 1234

Hnh 1.7 nh hng ca SRS. Nng lng t knh bc sng thp c chuyn sang knh bc sng cao hn.


12

SRS gia cc knh trong h thng WDM nh hng xu cho h thng, SRS c th c dng khuch i h thng.

Cng sut ngng cho SRS c th tnh bng cng thc sau [2]:

PthSRS=16Aeff/gRLeff=(16Aeff)/gR (1.8)

Ga tr c trng ca gR 1x10-13 m/W ti = 1550nm. Ly = 0.046 1/km = 0.2dB/Km v Aeff=55m2, tnh c PthSRS = 405mW cho mt knh. Con s ny cho thy c th b qua SRS trong h thng n knh

Mt cch khc tnh cng sut ngng ny l [3]:

PthSRS=5.9x10-2d2 watts (1.9)

Vi d: ng knh li si quang (m)

: bc sng hot ng (m)

: h s suy hao (dB/Km)

1.2.4 Lan truyn trong mi trng phi tuyn

Theo [1,2] i vi mi trng tuyn tnh, vector phn cc P c mi lin h vi in trng tc ng E nh sau:

),r(),r(),r( 0 tEttP e= (1.10)

Hnh 1.8 H s li SRS l hm ca khong cch knh.

0 10 20 30 40

1

2

3

4

7

6

5

Khong cch knh (THz)

H

s

li R

aman

(x10

-14 m

/W)


13

Vi e l cm in (electric susceptibility). Mi lin h gia e vi chit sut ca mi trng n c th biu din bng:

e1n += (1.11)

Cc hiu ng khc x phi tuyn xy ra l do s ph thuc ca cm in vo cng in trng E. Trong si quang, cc hin tng phi tuyn c th xem xt thng qua biu thc gn ng sau:

)t,r(E)t,r(E)t,r()t,r(P)t,r(P)t,r(P 3)3(e0e0NLL +=+= (1.12)

S hng th nht trong cng thc (1.12) l phn cc in mi tuyn tnh, cn s hng th hai l phn cc in mi phi tuyn.

Chit sut khc x ca mi trng s l:

n(,E)=nL ()+nNLE2 (1.13)

Vi l tn s gc ca nh sng. S hng th nht trong cng thc (1.13) l chit sut tuyn tnh v l nguyn nhn gy ra tn sc vt liu. S hng th hai th hin hiu ng phi tuyn bi v n t l vi cng nh sng I=0.50cnE2. H s khc x phi tuyn:

nNL=3/8ne(3) (1.14)

C th biu din s khc x phi tuyn theo mt cch khc:

n = nL ()+nNL(P/Aeff) (1.15)

Vi P l cng sut nh sng. i vi si quang silica, thng nNL=3.2x10-20 m2/W v Aeff55m2. Gi s P=1mW th nNL(P/Aeff) =5.8x10-9. y l phn phi tuyn ca chit sut trong iu kin bnh thng. Chit sut ca silica vo khong 1,45 cho nn r rng trong trng hp ny cc hiu ng phi tuyn c th b qua.

Hng s lan truyn pha =n/c cng phi ph thuc vo E2. S ph thuc ny c th biu din nh sau:

=nL/c + (3/8cn)e(3)E2 (1.16)

C th biu din hng s pha phi tuyn theo mt cch khc:

= L + NLP (1.17)

Vi L l thnh phn tuyn tnh ca hng s lan truyn pha v NL=(2/)nNL/Aeff l h s lan truyn pha phi tuyn. Gi s nNL =3.2x10-20 m2/W, Aeff=55m2, v =1550nm, khi NL=2.35x10-3 1/m.W. Li gi s P=1mW, phn phi tuyn trong hng s lan truyn pha vo khong 10-6. Lu NLP = (2/)nNL(P/Aeff) v vy:


14

= L + (2/)nNL(P/Aeff) (1.18)

1.2.5 Hiu ng t iu pha SPM

Xt mt h thng n knh, nh sng nh l sng EM c dng (ch xt phn thc):

E(z,t)=E0e-zcos(t-z)

c tnh theo cng thc (1.16) nn s c dch pha b xung. dch pha ny c tnh bng:

effinNL

L

0

L

0NLL LPdz)z(Pdz)( === (1.19)

C th biu din s ph thuc ny theo mt cch khc:

=(3/8cn)e(3)E2Leff (1.20)

Da vo cng thc (1.19) v (1.20) c th gii thch c ch SPM nh sau: dch pha phi tuyn ca sng mang quang thay i theo thi gian l do cng (cng sut) ca xung nh sng thay i theo thi gian. S thay i ny l do s thay i ca cng sut u vo Pin(t) v/hay s bin thin theo thi gian ca bin xung khi xung lan truyn dc theo si quang. Do tr thnh mt hm s theo thi gian (t). Theo nh ngha o hm ca dch pha d(t)/dt0 biu din s thay i ca tn s. S thay i tn s ny c gi l chirping. By gi r v sao hiu ng ny c gi l t iu pha: iu ch ny l s thay i tn s xy ra do s dch pha gy ra bi chnh xung nh sng.

Trong tn sc mu, cc bc sng khc nhau (cc tn s) lan truyn theo cc vn tc khc nhau. Nh vy xung mang cc tn s khc nhau khi lan truyn s gin ra. R rng SPM gy ra gin xung thng qua tn sc mu. Cn lu mt u im ca SPM l: khi cng sut lan truyn cao, khong u si quang, SPM c th nn xung. Tuy nhin khi xung lan truyn xa hn, xung s b gin nhiu hn. Hin tng nn xung ny c th s dng b tn sc.

Cc hiu ng phi tuyn thng c nh gi qua cc gii hn cng sut cho h thng thng tin. nh hng ca SPM l ti thiu, dch pha phi tuyn phi rt nh tc l


15

CMP nh hng n cht lng truyn dn thng qua c ch ging nh SPM: tn s chirping v tn sc mu. CPM c th nh hng n h thng mnh hn SPM v h s 2 trong cng thc (1.21).

1.2.7 Hiu ng trn bn bc sng

Trong h thngWDM s dng cc tn s gc 1.n, s ph thuc ca chit sut vo cng (cng sut) khng ch gy ra s dch pha trong mi knh m cn sinh ra tn s mi nh l 2i-j v i+j-k. Hin tng ny gi l hin tng trn bn bc sng (FWM_Four-wave Mixing). Tri vi SPM v CPM ch c nh hng i vi cc h thng tc bit cao, hiu ng trn bn bc sng khng ph thuc vo tc bit m ph thuc cht ch vo khong cch knh v tn sc mu ca si. Gim khong cch knh lm tng nh hng ca hiu ng trn bn bc sng v vic gim tn sc mu cng vy. Do , cc nh hng ca FWM phi c xem xt ngay c cc h thng tc va phi khi khong cch knh gn nhau v/hoc khi s dng si dch chuyn tn sc.

hiu cc nh hng ca hiu ng trn bn bc sng, ta xt mt tn hiu WDM l tng ca n sng phng n sc. Trng in ca tn hiu ny c th c vit mt cch n gin:

=

=n

iiii ztEtrE

1

)cos(),(

Theo cng thc (1.12), phn cc in mi phi tuyn c th c vit

)ztcos(E)ztcos(E)ztcos(E)t,r(P kkkjjjn

1i

n

1j

n

1kiii

)3(e0NL =

= = =

)ztcos(EEE2E4

3iii

n

1i ijji

2i

)3(e0

+

=

=

(1.22)

=

+n

1iii

3i

)3(e0 )z3t3cos(E

4 (1.23)

=

+n

1i ijjijij

2i

)3(e0 )z)2(t)2cos((EE

43 (1.24)

=

++

+n

1i ijjijij

2i

)3(e0 )z)2(t)2cos((EE

43

(1.25)

= > >

+

n

1i ij jkkji

)3(e0 EEE

46

( )z)(t)cos(( kjikji ++++ (1.26)

)z)(t)cos(( kjikji +++ (1.27)

)z)(t)cos(( kjikji +++ (1.28)

))z)(t)cos(( kjikji + (1.29)


16

Nh vy, cm in phi tuyn ca si quang to ra cc trng mi (cc sng mi) tn s ijk (i, j, k c th ging nhau). Hin tng ny gi l hiu ng trn bn bc sng. Nguyn nhn l do cc tn s i, j, k t hp vi nhau to ra bc sng th t tn s ijk. i vi khong cch tn s bng nhau v mt s la chn i, j, k no , bc sng th t nh hng n i. V d, cho khong cch tn s vi 1, 2, 3 l cc tn s lin tip, ngha l 2= 1+ v 3= 1 + 2 s c 1-2+3 = 2 v 22-1=3.

Biu thc (1.22) cho thy nh hng ca SPM v CPM m ta cp n trong mc 1.2.5 v 1.2.6, cc biu thc (1.23), (1.24) v (1.25) c th b qua do khng c s ng pha. Di cc iu kin thch hp, c th tha mn iu kin ng pha i vi cc biu thc cn li, l tt c cc dng i+j-k, i,j k (i, j c th ging nhau). Chng hn, nu cc bc sng trong h thng WDM l gn nhau hoc phn b gn vng tn sc khng ca si, th gn nh l khng i trn cc tn s ny v iu kin ng pha gn nh l c tha mn. Khi cc iu kin ny tha mn, cng sut ca cc tn s mi c to ra c th phi quan tm n.

C th xc nh gi tr ca hiu ng trn bn bc sng dng i+j-k, i,j k n gin nh sau.

nh ngha ijk=i+j-k, v h s suy bin (Degeneracy Factor)

=

=jiji

dijk ,6,3

Phn cc in mi phi tuyn ti ijk c th c vit

))()cos((4

),()3(

0 ztEEEdtzP kjikjikjiijkeNLijk

++= (1.30)

Gi s rng cc tn hiu quang lan truyn nh cc sng phng qua din tch li hiu dng Aeff trong si (xem hnh 1.6), theo (1.25) c th biu din cng sut ca tn hiu tn s ijk sau khi lan truyn qua khong cch L trong si l

22)3(

8LPPP

cnAd

P kjieffeff

eijkijkijk

=

y, Pi, Pj, Pk l cc cng sut ng vo ti cc tn s i, j, k. Ch rng chit sut n c thay th bi chit sut hiu dng neff ca mt c s. S dng chit sut phi tuyn nNL, c th vit

22

3LPPP

cAdn

P kjieff

ijkNLijkijk

=

(1.31)

Hy xem xt mt s v d c th. Gi s rng mi tn hiu quang cc tn s i, j, k c cng sut 1mW v din tch li hiu dng ca si quang l Aeff=50m2. Cng gi s i j, dijk=6. S dng nNL= 3.0x10-8m2/W, khong cch lan truyn L = 20 km. Cng sut Pijk ca tn hiu tn s gc ijk sinh ra do hiu ng trn bn bc sng khong 9.5W. Gi tr ny ch thp hn cng sut ca tn hiu 1mW khong 20 dB. Trong h thng WDM, nu c knh khc cng tn s ijk, hiu ng trn bn bc sng c th gy nhiu knh ny.


17

Thc t, cc tn hiu to bi hiu ng trn bn bc sng c nng lng thp do khng c s ng pha hon ton v suy gim suy hao si.

1.3 CC LINH KIN TRONG KIN TRONG H THNG WDM Cc linh kin c s dng trong cc mng quang hin i bao gm cc b ghp/tch

(couplers), b pht laser (lasers), b tch quang (photodetectors), b khuch i quang (optical amplifiers), b chuyn mch quang (optical switches), b lc (filters) v b ghp/tch knh (multiplexers). Mc ny s tp trung xem xt nguyn l hot ng ca cc linh kin nu trn. i vi mi linh kin trc tin s a ra m hnh m t n gin sau l cc m hnh ton hc chi tit. Bn c c th b qua phn m t ton hc nu thy cha cn thit tham kho. Ring b khuch i quang s c xem xt ring chng 2. Phn ny cng khng trnh by b pht laser. Bn c c th tm hiu b pht laser trong H thng thng tin quang I.

1.3.1 B ghp/tch tn hiu (Coupler)

a) nh ngha

B ghp/tch tn hiu (Coupler) l thit b quang dng kt hp cc tn hiu truyn n t cc si quang khc nhau. Nu coupler ch cho php nh sng truyn qua n theo mt chiu, ta gi l coupler c hng (directional coupler). Nu n cho php nh sng i theo 2 chiu, ta gi l coupler song hng (bidirectional coupler).

Coupler thng dng nht l coupler FBT (Fused Binconical Taper). Coupler ny c ch to bng cch t 2 si quang cnh nhau, sau va nung chy chng kt hp vi nhau va ko dn ra to thnh mt vng ghp (coupling region). Mt coupler 2 x 2 c trng bi t s ghp (0


18

b) Nguyn l hot ng

Khi hai si quang c t cnh nhau, nh sng s c ghp t si ny sang si kia v ngc li. l do qu trnh truyn mt nh sng trn si quang qua vng ghp s khc so vi truyn trn si quang n. Khi , ton b nh sng thuc mt si quang s c ghp hon ton sang si quang ghp vi n, phn nh sng ny li tip tc c ghp ngc tr li sang si quang ban u theo mt chu k tun hon khp kn. Kt qu ta c cng trng in t u ra ca b ghp Eo1, Eo2 c tnh theo cng trng in t u vo Ei1, Ei2 theo cng thc [1]:

=

)()(

)cos()sin()sin()cos(

)()(

2

1

02

01

fEfE

llilil

efEfE

i

ili

(1.32)

Trong :

l h s pha ca s truyn nh sng trong si quang.

l H s ghp. ph thuc vo chiu rng ca si quang, chit sut ca li si v n khong cch gn nhau ca hai si quang khi thc hin nung chy.

Nu ch cho nh sng vo ng 1 (cho Ei2=0), khi cng thc (1.32) c vit li l:

)()cos()( 101 fElefE ili = (1.33)

)()sin()( 1)2/(

02 fEleefE iili = (1.34)

Ta nhn xt rng 2 u ng ra c s lch pha /2. Cng trong iu kin ny, ta tnh c hm truyn t cng sut:

3 dB couplers

Hnh 1.10 Coupler hnh sao vi 8 ng vo v 8 ng ra c hnh thnh t cc coupler 3dB. Cng sut t mt ng vo c chia u cho cc ng ra

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2


19

=

)l(sin)l(cos

)f(T)f(T

2

2

12

11 (1.35)

y hm truyn t cng sut Tij c nh ngha:

2ii

2

ojij

E

ET =

T cng thc (1.35) c coupler 3 dB di coupler phi c chn sau cho ( ) 4/12 += kl vi k l s khng m.

c) Cc thng s c bn

B coupler WDM c c trng bi cc thng s sau [2]:

Suy hao vt mc Pex (Excess Loss): c nh ngha:

Coupler2 x 2

2

34

P1in

P1out

P

P

P

Hnh 1.11 Cc thng s c trng ca coupler

= i

jjex P/Plog10)dB(P (1.36)

y Pj: cng sut ti ng ra j,

Pi: cng sut ti ng vo.

Theo hnh 1.11, Pex c tnh:

Pex(dB) = -10 log[(P2+P3)/P1] (1.36a)

Suy hao xen IL (Insertion Loss): l t s ca cng sut tn hiu ng ra so vi ng vo ti mt bc sng c th. Suy hao xen l suy hao m coupler thm vo ng vo v ng ra.

IL12(dB) = -10 log[P2/P1] (1.37)

T s ghp CR (Coupling Ratio): c nh ngha


20

CR(dB) = -10 log[P2/(P2+P3)] (1.38-a)

CR c th c biu din theo % :

( )[ ] 100(%) 322 += PPPCR (1.38-b)

D thy

exPCRIL += (1.39)

Tnh ng nht U (Uniformity): c trng cho coupler dng trong trng hp chia i cng sut (50:50). H s ny ch ng nht gia 2 nhnh ca coupler (bng 0 trong trng hp coupler l tng).

U(dB) = ILmax ILmin = 10 log[P3/P2] (1.40)

Suy hao do phn cc PDL (Polarization-dependent Loss): l dao ng ln nht ca suy hao xen do s thay i phn cc nh sng u vo. Thng ch s ny khng vt qu 0.15 dB.

Tnh nh hng D (Directivity): l phn cng sut tn hiu ng vo xut hin ti ng ra khng mong mun.

D(dB) = -10 log[P4/P1] (1.41)

Xuyn knh u gn (near-end crosstalk): dng nh gi tnh nh hng

[ ])()(log10)( 1113 PPdBNEC = (1.42)

Suy hao phn hi RL (Return Loss): c nh ngha

RL(dB) = -10 log[P1out/P1in] (1.43)

cch ly (Isolator): dng nh gi phn nh sng trn mt ng b ngn khng t n mt ng khc. V d 1 l bc sng truyn t cng 1 n cng 2, truyn n cng 4 l khng mong mun. Tng t 2 truyn t cng 1 n cng 4, truyn n cng 2 l khng mong mun. Khi cch ly c nh ngha nh sau:

( ) ( )[ ]( ) ( )[ ]212221

111441

log10)(log10)(

PPdBIPPdBI

==

(1.44)

d) ng dng

Coupler l linh kin quang linh hot v c th cho nhiu ng dng khc nhau:B coupler vi t s ghp 1 c dng trch mt phn nh tn hiu quang, phc v cho mc ch gim st.


21

Coupler cn l b phn c bn to nn cc thnh phn quang khc, chng hn nh: cc b chuyn mch tnh, cc b iu ch, b giao thoa Mach-Zehnder MZI... MZI c th c ch to hot ng nh b lc, MUX/DEMUX, chuyn mch v b chuyn i bc sng.

Thc hin ghp/tch bc sng trn si quang. Nh iu chnh chiu di ghp thch hp khi ch to, coupler 2 x 2 ghp 50:50 phn b cng sut nh sng t mt u vo ra lm 2 phn bng nhau 2 ng ra. Coupler ny cn c gi l coupler 3 dB, ng dng ph bin nht. T coupler 3 dB, c th to nn b coupler n x n ghp n tn hiu khc nhau vo mt si quang.

1.3.2 B isolator/circulator

a) nh ngha

Coupler v phn ln cc linh kin quang th ng khc l cc thit b thun ngc (reciprocal) theo ngha thit b s hot ng cng mt kiu nu o ng vo v ng ra vi nhau. Isolator l thit b khng thun ngc (nonreciprocal). N ch truyn nh sng qua n theo mt chiu v ngn khng cho truyn theo chiu ngc li. N c dng ti u ra ca cc thit b quang (b khuch i, ngun pht laser) ngn qu trnh phn x ngc tr li cc thit b , gy nhiu v h hi thit b. Hai tham s chnh ca Isolator l suy hao xen v cch ly.

Circulator cng thc hin chc nng tng t nh b Isolator nhng n thng c nhiu cng, thng l 3 hoc 4 ca. Chnh v s tng ng gia hai loi thit b, ta s ch trnh by hot ng ca b Isolator m thi.

Cong 1

Con

g 2

Con

g 3

Cong 1

Con

g 2

Con

g 4

Cong 3

(a) (b)

ISOLATOR

321 ,, 321 ,,

4

(c)

ISOLATOR

Hnh 1.12 (a) S khi ca b Circulator 3 ca. (b) S khi ca b Circulator 4 ca. (c) S khi ca b Isolator.

b) Nguyn l hot ng

Trng thi phn cc ca nh sng truyn trong si quang c nh ngha l chiu phn

cc ca vector cng trng E nm trn mt phng vung gc vi phng truyn nh sng trong si. Ta gi l phn cc ngang v phn cc dc. Qu trnh truyn nh sng trong si quang l s kt hp tuyn tnh truyn cc sng phn cc ngang v phn cc dc.

M hnh n gin ca b Isolator c minh ha nh trong hnh 1.13 (a). Gi s nh sng truyn l phn cc dc, truyn n b phn cc (Polarizer), b ny thc hin chc nng ch cho sng phn cc dc i qua, khng cho sng phn cc ngang i qua. Sau b phn cc l b quay pha Faraday, thc hin quay pha 45o theo chiu kim ng h khng phn bit chiu nh sng n. Tip theo l b phn cc, b ny thc hin chc nng ch cho sng phn cc 45o i qua. Nh vy, b Isolator ta xt ch cho php sng phn cc dc i qua theo chiu t tri sang phi. Trong


22

trng hp sng phn x theo chiu ngc li, nu sng qua c b phn cc th hai, qua tip theo b quay pha Faraday, th cng khng th qua c b phn cc th nht (do lc ny sng phn cc ngang).

Bo phan cc

Bo quay pha Faraday

Bo phan cc

Anh sang i vaoAnh sang phan xaKhoa

Anh sang i ra

Anh sang i vao

Anh sang phan xa

Anh sang i ra

SWP

Bo quay pha Faraday

SWPMat phang 2/

(a)

(b)

Hnh 1.13 (a) Cu to b Isolator khi nh sng vo phn cc dc. (b) Cu to b Isolator khi nh sng vo phn cc bt k.

Trn thc t th sng truyn trong si quang lun l s kt hp tuyn tnh ca cc trng thi phn cc ngang v dc nn thit k b Isolator phc tp hn. M hnh thu nh c trnh by trong hnh 1.13 (b).

nh sng truyn trong si quang vi trng thi phn cc bt k c a n b tch/ghp trng thi phn cc SWP (Spatial Walk-off Polarizer), tch thnh hai dng tn hiu phn cc dc v ngang theo hai ng c lp nhau. Tip theo, n b quay pha Faraday, quay pha 45o theo chiu kim ng h. Mt phng /2 (Half-wave plate) thc hin quay pha 45o theo chiu kim ng h i vi tn hiu truyn t tri sang phi, quay pha 45o theo chiu ngc kim ng h theo chiu truyn ngc li. Cui cng, tn hiu hai nhnh c kt hp li nh b SWP th hai. Nu theo chiu ngc li, hai b SWP s kh ln nhau. nh sng truyn qua b SWP th hai, qua b quay pha Faraday s khng th kt hp li c ti b SWP th nht nh minh ha trn hnh (1.13).

c) ng dng

B Isolator v Circulator c nhng ng dng sau:

B Isolator thng ng trc u ra b khuch i quang hoc ngun pht laser ngn nh sng phn x ngc tr li thit b gy nhiu v c th lm h thit b.

B Circulator c dng nh mt b phn ch to phn t xen rt quang OADM.

1.3.3 B lc quang

a) Tng quan


23

nh ngha

B lc l thit b ch cho php mt knh bc sng i qua, kha i vi tt c cc knh bc sng khc. Nguyn l c bn nht ca b lc l s giao thoa gia cc tn hiu, bc sng hot ng ca b lc s c cng pha nhiu ln khi i qua n, cc knh bc sng khc, ngc li, s b trit tiu v pha. Tu thuc vo kh nng iu chnh knh bc sng hot ng, ngi ta chia b lc lm hai loi: b lc c nh (fixed filter) v b lc iu chnh c (tunable filter). Hnh 1.14 l s khi b lc c nh v b lc iu chnh c.

N21 ...,,, )Nk1(,k Bo loc kBo loc ieuchnh c

1 2 N )Nm1(m

(a) (b)

Hnh 1.14 S khi ca b lc. (a) B lc c nh bc sng k. (b) B lc c th iu chnh bc sng c trong khang .

Yu cu i vi b lc

Hin nay, c rt nhiu cng ngh ch to b lc. Tuy nhin, yu cu chung i vi tt c cc cng ngh l:

B lc tt phi c gi tr suy hao xen IL thp.

B lc phi khng ph thuc nhiu vo trng thi phn cc ca tn hiu a vo.

Di thng hot ng ca b lc phi khng nhy cm vi s thay i nhit ca mi trng. B lc phi m bo trong khong nhit hot ng (thng l khong 100o C), dch di thng hot ng phi khng vt qu khong cch gia hai knh bc sng hot ng gn nht.

Khi ng dng ghp ni tip nhiu b lc trong h thng WDM, bng thng hot ng s b thu hp li. hn ch ti a iu ny, cc b lc phi c hm truyn t trong khong bc sng hot ng l bng phng.

Hm truyn t ca b lc phi c dc ln trnh giao nhau phn vt ca hai bc sng ln cn, gy xuyn nhiu gia cc knh.

Gim chi ph sn xut. Vn ny li ph thuc vo cng ngh ch to. Tuy nhin, khi vn ny t ln hng u th ta s c hai la chn. Th nht l dng cng ngh ng dn sng, cho php sn xut trn nhng vi mch tch hp quang (b li hot ng ph thuc vo trng thi phn cc ca sng quang). Th hai l dng cng ngh sn xut cc thit b thun quang, tuy kh khn trong tch hp mch nhng c nhiu u im l: khng ph thuc vo trng thi phn cc ca sng quang, ghp sng t si quang vo thit b d dng.


24

0

-10

-20

-30

-40

1533.47 1534.25

0.5 dB o rong bang thong

Bang chan

20 d

B

o

cach

liVung xuyen nhieu

)nm(

Suy

hao

them

vao

(dB

)

0

-10

-20

-30

-40

o gn song

1540.56 )nm(

Suy

hao

them

vao

(dB

)

(a) (b)

Hnh 1.15 (a) Cc thng s c trng ca b lc. (b) gn sng ca b lc.

b) Thng s c bn

Hnh 1.15 minh ha cc c tnh c trng cho mt b lc, cc c tnh c nh ngha nh sau:

Bc sng trung tm: phi l bc sng tun theo tiu chun ITU-T

rng bng thng (Pass Bandwidth): l rng ca hm truyn t ti mc suy hao xen cch nh 0.5 dB. Trong mt s trng hp, ngi ta cn c th xt bng thng i qua 1 dB, 3 dB. c tnh ny rt quan trng v laser trong trng hp khng l tng ch pht tn hiu c bc sng dao ng nht nh so vi bc sng trung tm c qui nh theo chun ITU-T.

rng bng chn (Stop Bandwidth): l rng ca hm truyn t ti mc suy hao xen cch nh 20 dB. Di chn ca b lc phi cng nh cng tt trnh hin tng xuyn nhiu gia cc knh.

cch li (Isolation): ch cng sut ca mt knh bc sng xuyn nhiu sang cc knh bc sng ln cn.

gn sng (Ripple): l chnh lch nh-nh trong phm vi mt knh bc sng.

H s s dng bng thng BUF (Bandwidth-utilization Factor): l t s ca rng knh truyn LW (Linewidth) ca nh sng c truyn i so vi nh sng phn x ti mt mc suy hao xc nh. B lc l tng phi c BUF = 1. Trn thc t, khi IL = -25 dB th BUF 0.4.

Nu b lc thuc loi c th iu chnh bc sng c, n cn c thm cc c tnh na nh l:

Khong iu chnh bc sng ng: l khong bc sng m trong phm vi hot ng ca b lc.

S knh bc sng c th x l: l t l khong iu chnh bc sng ng trn khong cch gia cc knh bc sng.

Thi gian iu chnh: Thi gian iu chnh gia cc knh bc sng hot ng khc nhau.


25

T l nn bin SSR (Sidelobe Suppression Ratio): l khong cch gia gi tr cng sut nh so vi gi tr cng sut ln nht bin.

phn gii: l dch bc sng nh nht b lc c th nhn bit c.

c) Cch t

Cch t dng m t cc thit b m hot ng ca n da trn hin tng giao thoa gia cc tn hiu quang xut pht t cng mt ngun quang nhng c lch pha tng i vi nhau. Phn bit vi cch t l vt chun (etalon) l thit b nhiu tn hiu quang c to ra nh mt hc cng hng (single cavity) lp li cc tia i ngang qua n.

Sng nh sng c lan truyn theo hng z vi tn s gc l v hng s pha l s c dch pha l (t-z). Do dch pha tng i gia hai sng pht sinh t mt ngun c th c to ra bng cch cho chng truyn qua hai ng khc nhau.

Trong WDM cch t c dng nh l mt b tch knh tch cc bc sng hoc nh l mt b ghp knh kt hp cc bc sng.

Hnh 1.16 l hai v d v cch t: trn mt phng cch t (grating plane), cc khe (slit) c cch u nhau. Khong cch gia hai khe k cn gi l pitch. Do cc khe nh nn theo hin tng nhiu x (diffraction) nh sng truyn qua cc khe ny s lan to ra mi hng. Trn mt phng nh x (imaging plane) s quang st c hin tng giao thoa cng hng (constructive interference) v trit tiu cc bc sng ti cc im khc nhau, cch t ny c gi l cch t nhiu x (diffraction grating).

Hnh 1.16(a) l cch t truyn dn (transmission gratings), 1.16(b) l cch t phn x (reflection gratings).

2

1d2d1

i

1 + 2

Mt phng cch t

Mt phng nh x

(a)

2

1d1i

1 + 2

Mt phng cch t

Mt phng nh x

(b)

Hnh 1.16: (a) Cch t truyn dn v (b) Cch t phn x


26

Nguyn l hot ng

Theo hnh 1.17 ta c S chnh di gia cc tia khc x ti gc d vi cc khe k cn

l )]sin()[sin(aCDAB di = .Giao thoa xy dng (constructive interference) xy ra khi

a[sin(i)-sin(d)]=m (1.45)

Vi m: bc ca cch t. Khi i=0 c th vit li (3.14) nh sau

asin(d)=m (1.45a)

Trn thc t, nng lng tp trung ti bc 0 khi i= d i vi mi bc sng. Nng lng nh sng ti bc 0 l v ch v cc bc sng khng c tch ri. Do cn thit k cc cch t khc gi l blazing (xem hnh 1.18).

d) Cch t Bragg

nh ngha

Hnh 1.18 Cch t blazing vi gc blaze . Nng lng ca giao thoa ti i tng ng vi gc blaze l cc i

ai

D

C

B

A

Mt phng cch t

n mt phng nh x

dT ngun

Hnh 1.17 Nguyn tc hot ng ca cch t truyn dn. Cch t phn x hot ng tng t. S chnh lch di gia cc tia khc x ti gc d vi cc khe k cn l: )]sin()[sin(aCDAB di =


27

Cch t Bragg c s dng rng ri trong h thng thng tin quang. Mi s bin i tun hon trong mi trng truyn sng (thng l bin i tun hon chit sut mi trng) u c th hnh thnh cch t Bragg.

Nguyn l hot ng

Xt hai sng truyn theo hai chiu ngc nhau vi h s pha l 0, 1. Nng lng ca tn hiu ny c ghp sang tn hiu kia nu chng tho mn iu kin v pha:

=2

10 (1.46)

Trong : l chu k cch t.

Trong cch t Bragg, nng lng ca sng truyn theo hng n c ghp vo sng phn x tng ng truyn theo hng ngc li. Xt sng c h s pha 0 truyn theo chiu t tri sang phi. Nng lng ca sng ny s c ghp vo sng tn x ca n theo chiu ngc li (c cng bc sng vi sng ti) nu tho mn iu kin v pha:

==22)( 000 (1.47)

Gi 0=2neff/0 , 0 l bc sng nh sng ti v neff l gi tr chit sut hiu dng ca si quang hoc ng dn sng (vt liu lm cch t Bragg). Khi , iu kin phn x c vit li l:

0 = 2neff (1.48)

Cng thc trn gi l iu kin Bragg. Trong , 0 c gi l sng Bragg.

Hnh 1.19 (a) minh ha c ch hot ng ca phn x Bragg. l mt si quang hot ng theo c ch phn x Bragg. Chit sut tng i ca li si quang c lm bin i tun hon dc theo chiu di ca si ng vai tr nh cch t Bragg. Sng truyn trong si quang v n c phn x li theo mi chu k cch t. Cc sng phn x s cng pha vi nhau nu bc sng tun theo iu kin Bragg ta trnh by trn.

Hnh 1.19 (c) v (d) l rng ph cng sut ca sng phn x i vi hai trng hp cch t: cch t ng nht v cch t gim dn. Cch t gim dn (apodized grating) l trng hp ch to chit sut tng i sao cho cng xa trung tm cch t, s khc bit v chit sut cng gim. Dng cch t gim dn s gim c cng sut ca sng phn x ln cn, nhng i li phi chu bng thng hot ng tng ln. Theo th, ta cng thy rng cng xa bc sng Bragg, ph ca sng phn x cng gim. C ngha l khi hot ng vi cc knh bc sng cch nhau mt khong cch nht nh, ch c bc sng Bragg l phn x tr li khi truyn qua cch t Bragg, cc bc sng khc s truyn i xuyn qua.


28

Cach t trong si quang chiet suat ong nhat Cach t trong si quang chiet suat giam dan

0-2-4 -2 -4-40

-30

-20

-10

0

Pho

cong

suat

son

g ph

an x

a (d

B)

Pho

cong

suat

son

g ph

an x

a (d

B)

-40

-30

-20

-10

0

0-2-4 -2 -4

(a) (b)

(c) (d)

Hnh 1.19 (a) Cch t Bragg trong si quang chit sut ng nht. (b) Cch t Bragg trong si quang chit sut gim dn. (c) Ph cng sut phn x ca cch t ng nht. (d) Ph cng sut phn x ca cch t gim dn. l rng ca di thng v l khong cch gia bc sng nh v im phn x ti tiu u tin trong trng hp mt ct chit sut ng nht. t l nghch vi chiu di cch t. l lch bc sng so vi bc sng ng pha.

ng dng ca cch t Bragg

Cch t Bragg l nguyn l c bn dng trong cng ngh ch to b lc, b ghp xen/rt quang, dng b suy hao tn sc.

ng dng ch to b lc c th iu chnh quang-m hc.

Ngoi ra, i vi lnh vc khuych i quang, cch t Bragg cn cho nhiu ng dng quan trng nh: n nh li, cn bng li cho EDFA.

e) B lc cch t kiu si quang

nh ngha

Cch t Bragg kiu si quang l mt on si quang nhy vi nh sng, c ch to bng cch dng tia cc tm UV (Ultra-violet) chiu vo lm thay i mt cch tun hon chit sut bn trong li. S thay i chit sut trong li si ch cn rt nh (khong 10-4) cng to ra cch t Bragg. B lc cch t Bragg kiu si quang c phn lm hai loi: cch t chu k ngn v cch t chu k di. Cch t chu k ngn c chu k cch t tng ng vi bc sng hot ng (khong 5m). Trong khi cch t chu k di c chu k cch t ln hn


29

nhiu ln so vi bc sng hot ng (khong vi trm 5m n vi mm). B lc Bragg kiu si quang cng c th l b lc c nh hoc b lc iu chnh c.

Nguyn l hot ng ca cch t chu k ngn

Nguyn l hot ng ca b lc Bragg kiu si quang hon ton tng t nh ta cp phn trn. Bng cch to s thay i tun hon chit sut trong li si quang. Qu trnh truyn sng trong si quang qua nhng min chit sut khc nhau khi tr nn nghim ng i vi iu kin Bragg. Khi truyn trong si quang c cch t Bragg ho, ch c bc sng = B s c phn x tr li v cng pha vi nhau, cng pha vi sng ti, lm tng cng sng phn x. Cc bc sng khc s truyn xuyn qua hoc phn x tr li khng ng k do trit pha vi nhau. Bc sng Bragg B ca b lc c tnh t cng thc:

B = 2neff (1.49)

Trong : neff l chit sut tng i ca li si.

l chu k cch t Bragg.

Nguyn l hot ng ca cch t chu k di

Nguyn l hot ng ca b lc Bragg kiu si quang chu k di c khc so vi loi chu k ngn. Trong loi cch t chu k ngn m ta xt trn, khi bc sng truyn trong li si l B, sng phn x tr v s c ghp cng pha vi nhau v cng pha vi sng ti. Tt c qu trnh ch din ra trong li si quang. i vi cch t chu k di, sng truyn trong phn lp ph ngoi li si theo chiu i s c ghp cng pha vi sng truyn trong phn li si cng chiu. iu kin c s ghp cng pha gia phn mode sng truyn trong li v phn mode sng truyn trong lp v l:

=2p

cl (1.50)

Trong : l h s pha ca mode sng truyn trong li.

pcl l h s pha ca mode sng bc p truyn trong lp v.

Thng th hiu s gia hai hng s lan truyn ny rt nh nn s tr nn rt ln vic ghp nng lng c th xy ra. Gi tr ny thng vo khong vi trm micrometers. (Lu i vi cch t si Bragg hiu s gia hng s lan truyn ca mode ti v mode phn x l rt ln nn chu k cch t s rt nh). Do ta c mi lin h gia h s pha v chit sut tng i:

= effn2

(1.51)

Nn t cng thc (1.50), ta suy ra c:

)nn( peffeff = (1.52)


30

Nh vy khi bit c neff, peffn ta c th ch to mt cch t gi tr mt cch hp l sao cho vic ghp nng lng xy ra ngoi di bc sng mong mun. Cch t trong trng hp ny hot ng nh mt b suy hao theo bc sng.

B lc Bragg kiu si quang c th iu chnh c

T cng thc (1.49) v (1.52), ta thy ta ch cn thay i chu k cch t l c th thay i bc sng hot ng ca b lc Bragg kiu si quang. Trn thc t, thay i chu k cch t ngi ta thng dng 2 cch: dng nhit hoc dng sc cng. S thay i bc sng hot ng ca b lc c xc nh l mt hm theo chiu di cch t (L) v nhit (T).

TCxLL += )/108()/(8.0 06 (1.53)

ng dng ca b lc cch t Bragg kiu si quang

Cch t Bragg kiu si quang l thit b thun quang nn c nhng u im ca h thit b ny nh: suy hao thp, d phi ghp tn hiu t si quang vo thit b, khng nhy vi s phn cc, h s nhit thp. Nh cc c tnh cho nhiu ng dng khc nhau:

To ngun laser bng cch ghp 2 b lc cch t Bragg si quang cng bc sng hot ng vi nhau to thnh hc cng hng, ri dng mt ngun laser pht bc sng lin tc cho chiu vo. Ngun laser to bng phng php ny cn c th iu chnh sng ra ch mode-locked.

n nh bc sng: tn dng tnh chn lc chnh xc bc sng ca b lc Bragg kiu si n nh sng bm bc sng 980 nm. Dng kt hp vi 2 coupler nh minh ha trn hnh 1.20 (a). Mch hi tip so snh gia sng qua b lc v sng khng qua b lc iu chnh tr li ngun laser DBR.

B tn sc bng cch dng cch t chu k gim dn (Chirped Grating). Khi nh sng vo si quang kiu ny, nhng bc sng khc nhau s phn x nhng im khc nhau, ng i khc nhau, dn n b c tn sc nu cu hnh thch hp. Minh ha trn hnh 1.20 (b).

L thnh phn quan trng trong vic ch to cc b xen/rt, kt hp vi b Circulator. Minh ha trn hnh 1.20 (c).

B lc cch t Bragg chu k di c th ng vai tr nh cc b lc bng (bng gm nhiu knh bc sng) rt hiu qu, ng dng cn bng li khi dng b khuych i EDFA.


31

1

2

)( 12


32

Suy ra, khong cch gia 2 knh bc sng lin tip l:

N+1-N = x2/2l (1.55)

Trong : x l bc sng nh ca b lc trong mi trng c chit sut nx v x = /nx vi l bc sng nh sng tronb chn khng.

Hm truyn t cng sut ca b lc Fabry-Perot c xc nh l [1]:

]))2sin(12(1[

)1

1()(

2

2

fRR

RA

fTFP

+

= (1.56)

Cng thc (1.56) c th biu din theo bc sng nh sau:

]))/nl2sin(R1R2(1[

)R1

A1()(T

2

2

FP

+

= (1.56a)

Trong : A l suy hao do hp th ca gng R l phn x ca gng, c tnh l t s cng sut sng phn x so vi sng n. l l chiu di ca khoang cng hng. =nl/c vi c l vn tc nh sng

Ta thy rng )(TFP l hm tun hon theo , chu k ca n c nh ngha l khong ph t do FSR (Free Spectral Range):

FSR = c/2nl (1.57)

Bng thng 3 dB ca b lc Fabry-Perot ti mi nh ca hm truyn t cng sut c k hiu l FWHM. Khi suy hao trong b lc b qua (A=0), FWHM c tnh t cng thc:

FWHM = )1)(ln2

(RRc

(1.58)

Thng s c trng cho b lc l mn F (Finesse), c nh ngha nh sau:

F=R1

RFWHM

FSR

= (1.59)

F ch s bc sng m b lc c th phc v.

Hnh 1.22 l hm truyn t ca b lc Fabry-Perot khi A = 0, R = 0.75, 0.9 v 0.99. Ta c th thy rng R cng ln th kh nng chng xuyn nhiu gia cc knh ca b lc cng gim.


33

FSR

Ham

truy

en

at c

ong

suat

(dB

)

-40

-30

-20

-10

0

FSRf0-1 1

R=0.99

R=0.90

R=0.75

Hnh 1.22 Hm truyn t cng sut ca b lc Fabry-Perot.

B lc Fabry-Perot iu chnh c

T cng thc (1.54) xc nh bc sng hot ng ca b lc Fabry-Perot, ta thy c th thay i bc sng hot ng bng cch thay i chit sut n ca khoang cng hng hoc thay i chiu di l ca khoang. Thay i chiu di l bng cch p gia mt trn v mt di ca khoang mt cp in cc lm bng gm. Thay i in p gia hai in cc s lm thay i chiu di ca khoang. Tuy nhin, khi di chuyn hai gng, vn gi cho hai gng song song nhau rt kh thc hin. Do vy, c ch ny khng t c tnh chnh xc cao. C ch thc hin c minh ha nh trn hnh 1.23.

Mt phng php khc l thay i n bng cch dng tinh th lng in-t lp y khoang cng hng. Chit sut ca tinh th lng in-t s thay i khi c dng in i qua. Phng php ny cng c nhc im l cc cht tinh th lng in-t thng d bin ng theo nhit .

ien cc gom

ien cc gom

Si quang Si quang

l

C che gi cho 2 gng cuabo loc song song

Hnh 1.23 Dng in cc thay i bc sng hot ng ca b lc Fabry-Perot

ng dng ca b lc Fabry-Perot


34

B lc Fabry-Perot cng l thit b thun quang nn kh nng ng dng kh phong ph. Tuy nhin, kh nng vt tri so vi cc thit b lc kiu khc l h s F ca b lc Fabry-Perot kh ln (n 2000), cng vi kh nng iu chnh bc sng linh ng nn thng dng trong phng th nghim kim tra, o lng cc thit b quang khc.

g) B lc a khoang mng mng in mi (TFMF)

nh ngha

B lc mng mng TFF (Thin-film Filter) cng l mt dng ca giao thao k Fabry-Perot, trong cc gng bao quang hc cng hng c hin thc bng nhiu lp mng mng in mi c th phn x c. B lc ny l b lc di thng ch cho mt bc sng nht nh i qua v phn x tt c cc bc sng cn li.

B lc a khoang mng mng in mi (TFMF) gm nhiu hc cng hng cch nhau bng cc mng mng in mi phn x nh minh ha trong hnh 1.24 (a). S hc cng hng cng nhiu th hm truyn t cng sut c nh cng phng trong di thng v c dc cng ng (hnh 1.24).

Gng gom nhieu lpmang mong ien moi

Hoc

con

g h

ng

1

Hoc

con

g h

ng

2

Hoc

con

g h

ng

3

3 hoc cong hng2 hoc cong hng1 hoc cong hng

10.998 1.002-40

-30

-20

-10

0

Ham

truy

en

at c

ong

suat

(dB

)

a) b)

0

Hnh 1.24 B lc a khoang mng mng TFMF (Thin-film Multicavity Filter). (a) Cu to b lc gm c 3 khoang cng hng. (b) Hm truyn t cng sut i vi cc trng hp gm: mt, hai, ba khoang cng hng.

ng dng ca b lc TFMF

B lc TFMF c nhiu u im nh: hm truyn t c nh bng phng, dc cao, thit b hot ng n nh vi nhit , suy hao thp v t b nh hng bi s thay i trng thi phn cc ca tn hiu nn hin nay b lc loi ny c ng dng rng ri. Mt ng dng tiu biu nht l to b tch bc sng (DEMUX), thc hin vi 8 bc sng, nh minh ha trn hnh 1.25).


35

h) B lc Mach-Zehnder

nh ngha

B lc Mach-Zehnder l mt loi giao thoa k. Sng i vo b lc c phn thnh nhiu ng khc nhau, sau cho giao thoa vi nhau. MZI thng c sn xut da trn cc mch tch hp quang v thng gm cc couple 3 dB c ni vi nhau bng cc ng c cc di khc nhau (hnh 1.26).

Hnh 1.26: (a) B lc MZI c to thnh bng cch kt ni cc couplers nh hng 3 dB. (b) S khi ca MZI. l l lch v ng i gia hai nhnh (c) S khi ca MZI bn tng s dng cc b sng khc nhau mi tng.

Output 2

Output 1

Input 2

Input 1

lch ng i L

(a)

MZI (L)

(b)

Input 2

Input 1

Output 2

Output 1

Output 2 Input 2

Input 1MZI

(2L) MZI

(4L) MZI

(8L) MZI (L)

(c)

Output 1

Lens 1, 2 8

Si quang

B lc bng hp

Si quang

Lens

Lens

Lens

Lens

Lens

Lens

Lens

Lens

2

4

6

8

1

3

5

7

Si quang

thy tinh

Hnh 1.25: B lc ghp/tch knh c to t cc b lc mng mng in mi


36

Nguyn l hot ng

Trn hnh (1.26), gi s tn hiu i vo ng s 1, sau b coupler u tin, cng sut c chia u hai ng ra nhng c lch pha /2. lch v ng i L lm cho lch pha tng thm L nhnh di. Ti coupler th hai, tnh hiu nhnh di i vo nhnh trn v li tr hn nhnh trn l /2. lch pha tng i tng cng nhnh trn l /2+L +/2. Tng t tn hiu t nhnh trn i vo nhnh di nhnh di th lch pha tng i tng cng l /2+L -/2=L. Nu L=k vi k l l th cc tn hiu ng ra th nht c cng ng pha cn ng ra th hai s trit tiu ln nhau do ngc pha. Do tn hiu s c truyn vo t ng vo th nht ra ng ra th nht. Nu k chn th tn hiu s truyn t ng vo th nht n ng ra th hai.

Hm truyn t cng sut trong trng hp ch c ng vo 1 l tch cc:

=

)2/L(cos)2/L(sin

)f(T)f(T

2

2

12

11 (1.60)

Trn thc t, tng dc ca hm truyn t cng sut, b lc Mach-Zehnder thng c mc ni tip vi nhau. Tuy nhin, nu mc ni tip nhiu b lc s dn n suy hao thm vo tng.

B lc Mach-Zehnder c th iu chnh c

B lc Mach-Zenhder c th iu chnh c c cu trc i xng gia 2 nhnh trn v nhnh di ca ng dn sng. iu chnh bc sng hot ng bng cch iu chnh chit sut tng i ca mt nhnh. Phng php thay i chit sut tng i c th l dng nhit hoc to ng dn sng bng cc cht in-t (chng hn nh LiNbO3), sau p in p ln lm thay i chit sut tng i ca cht .

ng dng ca b lc Mach-Zehnder

B lc Mach-Zehnder thng c ng dng ch to cc b ghp/tch knh. Xt trng hp MZI n. Vi L c nh, ng vo 1 tch cc, MZI s ng vai tr l b tch knh 1x2 nu cc bc sng c chn trng vi cc nh ca hm truyn t. C th nu =2neff/ th bc sng ng vo i phi c chn sao cho neffL/i=mi/2 vi mi l s nguyn dng. Nu mi l l th i s xut hin ng ra th nht v hm truyn t cng sut trong trng hp ny l sin2(mi/2)=1, cn nu mi l chn th i s xut hin ng ra th hai v hm truyn t cng sut trong trng hp ny l cos2(mi/2)=1. V MZI l thit b thun nghch nn khi cc ng vo v ra i ch cho nhau n s l mt b ghp knh 2x1. c b tch knh 1xn vi n l ly tha ca 2 cn ni chui (n-1) MZI (hnh 1.26c). Tuy nhin so vi TFMF th chui MZI c cht lng km hn: di thng khng phng v vng chuyn tip khng dc. Cc b ghp/tch knh dung lng cao hn thng dng cng ngh tin tin hn m ta s xt phn sau.

i) B lc cch t ng dn sng sng ma trn (AWG)

nh ngha


37

AWG l trng hp tng qut ca b lc giao thoa Mach-Zehnder (hnh 1.27). B lc ny bao gm hai b coupler nhiu cng (multiport coupler) c kt ni vi nhau bng mt ma trn ng dn sng (array of waveguides). AWG c th c xem nh l mt thit b mt tn hiu c nhn bn ln thnh mt lot cc tn hiu vi cc lch pha tng i khc nhau rng c cng li vi nhau.

AWG c th c s dng nh l b ghp/tch knh 1xn. So vi chui MZI, AWG c tn hao thp, di thng phng, v d dng c sn xut da trn cc mch quang t hp (integrated optic substrate). AWG cn c th c s dng nh l mt b kt ni cho bc sng (wavelength crossconnect). Tuy nhin b kt ni cho trong trng hp ny khng c kh nng t nh tuyn (xem hnh 1.28).

Nguyn l hot ng

Xem xt AWG trn hnh (1.27). S ng vo v ng ra ca AWG l n. Coupler th nht c kch thc l nxm, cn coupler th hai c kch thc l mxn. Hai coupler ny c ni vi nhau bng m ng dn sng, c gi l cc ng dn sng dng ma trn. di ca cc ng dn sng c chn sau cho lch v chiu di gia hai ng dn sng k cn l L. Coupler u tin chia tn hiu thnh m phn. lch pha gia cc phn ny ph thuc vo khong cch m tn hiu t ng vo n mt ng dn sng.

K hiu dikin l lch v ng i (tng i vi bt k mt ng dn sng ng vo no v bt k mt ng dn sng ma trn no) gia ng dn sng ng vo i v ng dn sng ma trn k. Gi s rng ng dn sng ma trn k c di ng dn ln hn ng dn sng k-1 mt

Cc ng dn sng ng vo

Coupler Ng vo

Ma trn ng dn sng

Coupler ng ra

Cc ng dn sng ng ra

Hnh 1.27 Cch t ng dn sng ma trn

Cch t ng dn sng ma trn

14

13

12

11 ,,,

24

23

22

21 ,,,

34

33

32

31 ,,,

44

43

42

41 ,,,

44

33

22

11 ,,,

34

23

12

41 ,,,

24

13

42

31 ,,,

14

43

32

21 ,,,

Hnh 1.28 Mu b kt ni cho c to t AWG


38

khong L. Tng t k hiu dkjout l lch v ng i (tng i vi bt k mt ng dn sng ma trn no v bt k mt ng dn sng ng ra no) gia ng dn sng ma trn k v ng dn sng ng ra j. Khi pha tng i ca cc tn hiu t ng vo i n ng ra j i qua m ng khc nhau c cho bi

( ) m,....,1k,dnLkndn2 outkj12inik1ijk =++

= (1.61)

y n1 l chit sut khc x ca cc coupler ng vo v ng ra, n2 l chit sut khc x ca cc ng dn sng ma trn. T ng vo i, nhng bc sng lm cho ijk khc nhau mt bi s ca 2 th s c cng ng pha ti ng ra j.

Gi s cc coupler ng vo v ng ra c thit k sao cho

outi

outi

outik

ini

ini

inik

kdd

kdd

+=

+=

(1.62)

th pha tng i ca cc tn hiu c th vit li nh sau

( ) ( ) m,....,1k,nLnnk2dndn2 outj12ini1outj1ini1ijk =++

++

= (1.63)

Cu trc ny c gi l cu trc vng Rowland (hnh 1.29)

Nu bc sng ng vo i tho iu kin:

=++ pnLnn outj12ini1 (1.64)

vi p l mt s nguyn dng th s c cng ng pha ng ra j.

i vi trng hp AWG s dng nh mt b tch knh th tt c cc bc sng c a n cng mt ng vo i. Do nu cc bc sng 1,2,...,n ca h thng WDM tha iu kin:

Hnh 1.29 Vng Rowland dng to cc coupler cho AWG

R

R Cc ng dn sng ng vo

Cc ng dn sng ma trn


39

joutj12

ini1 pnLnn =++ (1.65)

cho mt p no th cc bc sng ny s c tch bi AWG. Lu iin v L cn thit xc nh chnh xc t hp cc bc sng c phn knh, khong cch ti thiu gia cc bc sng khng ph thuc vo hai tham s m ch yu ph thuc vo jout.

Nu c bc sng j tha iu kin:

'j

outj12

ini1 )1p(nLnn +=++ (1.66)

th c hai bc sng j v j u c tch ra ng ra j. Nh vy AWG c p ng tun hon v tt c cc bc sng phi cng nm trong mt FSR.

j) B lc quang m iu chnh c (AOTF)

nh ngha

B lc quang-m AOTF (Acousto-Optic Tunable) l tiu biu cho h thit b m cng ngh ch to kt hp gia m thanh v nh sng. Dng sng m thanh to cch t Bragg trong ng dn sng, cc cch t ny thc hin chc nng la chn bc sng. Trong iu kin cng ngh hin ti, b lc AOTF l mt trong nhng thit b duy nht c kh nng iu chnh la chn nhiu bc sng cng mt lc. Kh nng ny gip cho b lc l linh kin ch cht ch to cc b kt ni cho bc sng.

Nguyn l hot ng

Xem v d ca mt AOTF trn hnh 1.30. AOTF l mt ng dn sng c to thnh t vt liu khc x kp v ch h tr cc mode TE v TM bc thp nht (v d lm bng Ti trn nn LiNbO3). Gi s nng lng nh sng ng vo l TE mode. B phn cc ng vo (input polarizer) ch chn nng lng nh sng trong mode TM c b tr 2 u cui ca ng dn sng.

B to sng m (Acoustic transducer) to ra sng m b mt SAW (Surface Acoustic Wave) lan truyn dc theo hoc ngc chiu vi hng truyn dn ca nh sng. Kt qu ca s lan truyn ny l mt ca mi trng thay i mt cch tun hon. Chu k ca s thay i mt ny bng vi bc sng ca sng m. S thay i mt mt cch tun hon ny ng vai tr nh l mt cch t Bragg.

Nu cc h s chit sut nTE v nTM ca cc mode TE v TM tha iu kin Bragg:

TE B phn cc

TMNg vo Ng ra

B chuyn i sng m Sng m hc

Hnh 1.30 Mt AOTF n gin.


40

=

1nn TETM (1.67)

th nh sng s c ghp t mt mode ny n mt mode khc. Nng lng nh sng trong mt di ph hp xung quanh bc sng tha iu kin phn x Bragg s b chuyn i t TE sang TM mode. Nh vy thit b ny ng vai tr nh mt b lc bng hp khi ng vo ch c nng lng nh sng trong mode TE v ng ra ch c nng lng nh sng trong mode TM l c chn (xem hnh 1.30).

Trong LiNbO3, mode TE v TM c chnh lch v chit sut n=0.07. iu kin phn x Bragg c th vit li:

( )n= (1.68)

au vao 1

au vao 2

au ra 1

au ra 2

n21 ,...,,

Bo phan ccngo vao

Bo phancc ngo ra

TE+TM

TE

TM

TE+TM

TM

TE

1

n32 ,...,,

Bo phan chuyen oinang lng song am

Tn hieu RF

Hnh 1.31 B lc quang-m c th iu chnh c AOTF (Acousto-optic Tunable Filter).

Vi mt bc sng m hc thch hp AOTF c th chn ra bc sng ph hp. V d chn c bc sng =1550nm vi n=0.07 th bc sng m hc vo khong =22m. Vn tc m thanh trong LiNbO3 l 3.75 km/s do tn s RF s l khong 170 MHz. B AOTF trn hnh 1.30 l thit b ph thuc vo phn cc. Hnh 1.31 l AOTF khng ph thuc vo phn cc.

Hm truyn t cng sut

( )( ) ( )

( )2

22

/21

/212/sinT

+

+

= (1.69)

Vi =-0 vi 0 l bc sng tha iu kin Bragg

=02/ln s o rng di thng ca b lc vi l l chiu di ca b lc.

Hm truyn t c v trn hnh (1.32). Nh vy l quyt nh rng ca di thng. C th chng minh c FWHM0.8. Nh vy b lc cng di di thng cng hp. Tuy nhin lu rng tc hiu chnh cng t l thun vi l v tc ny c xc nh bi thi gian cn thit sng m truyn qua ht chiu di ca b lc.


41

0 2 4-2-4

-30

-20

-10

0

Ham

truy

en

at c

ong

suat

(dB

)

Hnh 1.32 Hm truyn t cng sut ca b lc AOTF

ng dng ca b lc AOTF

AOTF l b kt ni cho bc sng. Nu cp cho AOTF nhiu sng m hc ng thi th nhiu bc sng quang c th tha iu kin Bragg ng thi. Do vi mt AOTF c th thc hin vic hon i cc bc sng gia hai cng mt cch ng thi (hnh 1.33 b). Nh vy AOTF c th thc hin vic nh tuyn bc sng ng d dng bng thay i bc sng m hc. Tuy nhin AOTF kh c ng dng nhiu trn thc t v nhiu xuyn knh ln ng thi c khong cch knh hp cho DWDM th AOTF phi c chiu di ln nn kh sn xut

B lc quang m iu chnh c

12

11 ,

22

21 ,

12

21 ,

22

11 ,

RF1

(a)

B lc quang m iu chnh c

14

13

12

11 ,,,

24

23

22

21 ,,,

24

13

12

21 ,,,

14

23

22

11 ,,,

RF1 ,RF4

(b)

Hnh 1.33 B kt ni cho bc sng c to t cc b lc AOTF


42

1.3.4 B ghp/tch knh bc sng

B ghp/tch knh bc sng, cng vi b kt ni cho quang, l thit b quan trng nht cu thnh nn h thng WDM. Khi dng kt hp vi b kt ni cho quang OXC (Optical Crossconnect) s hnh thnh nn mng truyn ti quang, c kh nng truyn ti ng thi v trong sut mi loi hnh dch v, m cng ngh hin nay ang hng ti. Tuy nhin, trong khi thit b v cng ngh chuyn mch quang nhn chung vn cn ang mc nghin cu ti cc phng th nghim th cc thit ghp/tch knh bc sng c thng mi ho rng ri. V cng ngh ch to, cng ngh ch to b lc (m c trnh by chi tit phn 1.3.3) v cng ngh ch to b tch/ghp knh hon ton ging nhau. Ch khc l b lc thng ch c mt bc sng hot ng, cn b tch/ghp knh hot ng trn nhiu knh bc sng lin tc. B lc chnh l phn t c bn cu to nn b ghp/tch knh nn phn ny ta s khng i su vo cng ngh ch to na.

a) nh ngha

Nguyn l hot ng ca b MUX/DEMUX cng tng t nh b Coupler. Tuy nhin, b Coupler/Splitter thc hin ghp tch tn hiu c cng bc sng, cn b MUX/DEMUX thc hin ghp tch tn hiu cc bc sng khc nhau. S khi b MUX/DEMUX cho trong hnh 1.34 (a) v (b).

b) c tnh

B MUX/DEMUX thng c m t theo nhng thng s sau:

Suy hao xen (Insertion Loss): m t phn coupler.

S lng knh x l: l s lng knh bc sng u vo v u ra ca b ghp/tch knh. Thng s ny c trng cho dung lng ca thit b.

Bc sng trung tm: Cc bc sng trung tm phi tun theo chun ca ITU-T m bo vn tng thch.

Bng thng: l rng ph (linewidth) ca knh bc sng trn thc t. Bng thng thng c tnh l rng ca hm truyn t cng sut cc mc cch nh 1dB, 3dB, 20dB.

Gi tr ln nht suy hao xen: c tnh l khong cch nh nht gia nh ca hm truyn t cng sut ca mt knh bc sng no so vi mc IL=0 (dB) (minh ha trn 1.34 (c)).

chnh lch suy hao xen vo gia cc knh: c tnh l hiu ca gi tr ln nht v nh nht suy hao xen vo gia cc knh bc sng.


43

1

2

n

n21 ,...,, n21 ,...,,

1

2

n

MUX DEMUX

0.8 nm

Khoang cach giacac kenh bc song

Suy hao themvao cc aiBW 1dB

BW 3dB

BW 20dB

-40

-30

-20

-10

0

1540.56 1541.35 )nm(

Suy

hao

them

vao

(dB

)

(a) (b)

(c)

Hnh 1.34 B tch/ghp knh bc sng quang. (a) S khi b ghp knh bc sng (MUX). (b) S khi b tch knh bc sng (DEMUX). (c) Cc thng s c trng ca b MUX/DEMUX.

c) Ghp tng to b ghp knh dung lng cao

Nhu cu v dung lng ngy cng cao trong khi cng ngh ch to si quang vn cn nhng gii hn nht nh, lm hn ch tc truyn dn mt knh bc sng. iu i hi phi tng s lng knh bc sng truyn trn mt si quang, c ngha l s lng knh m b MUX/DEMUX x l phi tng ln. Trong khi cng ngh ch to phn t c bn vn cn gii hn mt khong s lng knh bc sng c th x l nht nh th gii php ghp tng c xem l gii php hp l nht.

Ghp tng ni tip n knh (Serial)

Trong phng php ny thc hin ghp n tng, mi tng ch thc hin ghp/tch mt knh bc sng. Mt v d tiu biu l b DEMUX 8 knh bc sng, ch to t cc phn t c bn l b lc TFMF nh trnh by trong hnh (1.25).

u im ln nht ca phng php ghp tng ni tip l s bc sng x l c th thay i linh ng bng cch thm/bt s b lc ghp vo (pay as you grow).

Nhc im chnh l ch c th tng ln n mt s lng bc sng no m thi, do suy hao xen s tng gn nh tuyn tnh vi s lng b lc thm vo.


44

Bang 1 Bang 2 Bang 3 Bang 4

1 4 5 8 9 12 13 16

Bang 1

Bang 2

Bang 3

Bang 4

1

4

5

8

9

12

13

16

1 3 5 7 9 111315

2 4 6 8 10121416

1 3 5 7 9 111315

2 4 6 8 10121416

13

15

24

16

DEMUX tang 1 DEMUX tang 2 DEMUX tang 1 DEMUX tang 2

(a) (b)

Hnh 1.35 Ghp tng tng dung lng ghp/tch cc knh bc sng. (a) Ghp tng theo tng bng sng. (b) Ghp tng an xen chn l.

Ghp mt tng (Single-Stage)

Tt c cc bc sng u c tch ng thi trong mt tng duy nht. V d cho cu trc ny l b lc AWG (hnh 1.27).

u im ca phng php ny suy hao xen nh, tnh ng nht ca suy hao tt hn.

Nhc im l s knh c tch b hn ch do cng ngh sn xut AWG.

Ghp tng theo tng bng sng (Multistage Banding)

Phng php ny thc hin vi n/m tng, mi tng thc hin ghp/tch m knh bc sng thuc cng mt bng sng (thng thng m=4 hoc m=8, s lng knh bc sng trong mt bng sng l do nh sn xut thit b qui nh) xem hnh (1.35 a). Nh vy, i hi b MUX/DEMUX tng u phi c di bc sng hot ng rt rng.

u im: hn ch c suy hao thm vo m ln so vi phng php ghp tng ni tip. Cu trc ny c th m rng thm nhiu tng. Cu trc c dng mun nn tng cui cng c th ch dng mt bng

Nhc im: ph phm ti nguyn bc sng do phi cha khong cch rng gia cc bng sng.

Ghp tng an xen chn l

C th p dng vi hai tng hoc nhiu hn. Tng u lm nhim v ghp/tch cc knh bc sng chn, l ra lm thnh hai bng, a n tng hai. Tip theo, tng hai c th thc hin chc nng tng t nh tng mt hoc thc hin ghp/tch ring ra thnh cc knh ring l (xem hnh 1.35 b).


45

u im ca phng php ny l cng v tng cui, khng cn phi dng cc b lc c chnh xc cao do khong cch gia cc knh bc sng cn x l tng sau s cng rng ra.

1.3.5 B chuyn mch quang

a) Cc b chuyn mch n (Single Switch)

Theo chc nng, cc b chuyn mch n thng c chia lm hai loi: on/off v chuyn tip (passing). B chuyn mch on/off (1x1) s cho php/hoc khng cho php tn hiu nh sng i qua (hnh 1.36(a)). Chuyn mch chuyn tip 1x2 hng tn hiu nh sng t si quang th nht sang si quang th hai hoc sang si quang th ba (hnh 2.3(b)). Cu hnh chuyn mch 1x2 trong hnh 1.36(b) l cu hnh chuyn mch n gin nht. Cc b chuyn mch chuyn tip 1xN n mang tnh thng mi cao. B chuyn mch chuyn tip 2x2 c th kt ni hai si quang ny vi hai si quang khc. B chuyn mch chuyn tip 2x2 c th c hai trng thi: trng thi kt ni thng (bypass/bar) v trng thi kt ni cho (cross/inserted) (hnh 1.36(c)). Thut ng khng nghn dng ch mt b chuyn mch c th kt ni bt k ng vo n bt k ng ra. Hnh 1.36(d) trnh by b chuyn mch 2x2 c nghn v b chuyn mch ny ch c th kt ni t si quang 1 n si quang 4.

Hnh 1.36 Cc loi cu hnh chuyn mch quang: (a) chuyn mch On/Off (1x1); (b) chuyn mch chuyn tip (1x2) (khng nghn); (d) chuyn mch 2x2 c nghn.

Nhiu v d n gin v cc ng dng ca b chuyn mch khng nh tm quan trng ca cc b chuyn mch trong h thng thng tin si quang nh sau:

Cc chuyn mch on/off c dng lm cc u pht v u thu cch ly trong thit b o th. B chuyn mch 1x2 cho php la chn knh v c dng cho chuyn mch bo v ( nh hng li lu lng khi si quang b t).

3

(a)

4

2

3

2 1 2

4

1

1 1

2

3 1

(b)

(c)

3 4

1

(d)


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Cc chuyn mch 1xN c dng kim tra v o cc linh kin quang, kim tra t xa cc h thng thng tin si quang.

Cc chuyn mch 2x2 c dng kt ni thng nt trong cc mng quang. Chuyn mch 2x2 thng c ng dng trong cc mng FDDI. Khi mt trm no b h hng hoc b mt ngun, b chuyn mch s t ng thay i sang trng thi nghn, do m bo lung lu lng khng b nh hng khi nt b h hng.

Hnh 1.37 Nguyn l hot ng ca cc b chuyn mch quang: (a) chuyn mch on/off dng SOA; (b) b ghp ng dn sng ch to t LiNBO3; (c) chuyn mch dng lng knh chuyn ng; (d) chuyn mch dng gng hnh cu; (e) chuyn mch bng cch di chuyn si quang; (f) chuyn mch ng dng hiu ng FTIR; (g) chuyn mch quang-nhit s dng b giao thoa Mach-Zehnder.

1

GRIN lens

GRIN lens

d)

V

SOA

a)

c)

1

2

3

V1 2

3

b)

1

2

3

12

3

e)

Switching plate

32

GRINlens

f) Phase shifter

3

21

g)


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Cc chuyn mch n c ch to theo cc kiu kha (latching) hoc khng kha (nonlatching). Loi kha s gi nguyn trng thi (v tr) ca chuyn mch nu ngun b mt.

Nguyn l hot ng ca cc chuyn mch n in hnh thng da trn: in-quang, quang c hc, nhit quang. Hnh 1.37 trnh by mt s nguyn l hot ng ca b chuyn mch. Nu chuyn mch thin p ca mt b khuch i quang bn dn (SOA- Semiconductor Optical Amplifier) sang hai trng thi on/off th to ra c mt chuyn mch in-quang (hnh 1.37(a)) v mt SOA ch khuch i nh sng khi n c phn cc v hp th nh sng khi n khng c phn cc. Hnh 1.37(b) trnh by mt v d khc ca chuyn mch EO, trong t s ghp ca b ghp ng dn sng ph thuc vo in p p vo. Thay i t s ghp bng cch dng LiNbO3 ch to lp trn cng ca b ghp v LiNbO3 l loi nhin liu c ch s khc x thay i theo gi tr ca in p p vo. Chuyn mch quang c hc hot ng da trn chuyn ng c hc ca cc linh kin quang. V d, di chuyn lng knh trong hnh 1.37(c) theo chiu dc s cho php chuyn mch mt tn hiu quang t si 2 sang si 3. Ta cng c th t c kt qu tng t khi t nh ln tr mt gng hnh cu (hnh 1.37(d)). Cc thu knh GRIN (graded-index) s lm cho vic chuyn tip nh sng ghp t/vo si quang c d dng hn. Hnh 1.37(e) trnh by v d v chuyn mch quang. Mt cp si quang ng vo chuyn t v tr ny sang v tr khc thc hin vic chuyn mch tn hiu quang.

Nguyn l ca vic lm mt tc dng ca ton b nh sng phn x bn trong (FTIR- Frustration of Total Internal Reflection) c ng dng ch to b chuyn mch c trnh by trong hnh 1.34(f). Nhc li l mt phn nh sng s truyn qua mi trng khc x khc ngay c khi khng c nh sng phn x bn trong. Ta gi nh sng c truyn qua ny l sng suy bin (evanescent wave). Nh hiu ng ny, khi l chuyn mch c gn vi lng knh th s to ra nh sng phn x bn trong. Khi l chuyn mch (switching plate) tin ti gn lng knh, th ton b nh sng phn x bn trong b mt tc dng, kt qu l chm nh sng phn x s di chuyn dn dn sang hng khc. Do , bng cch di chuyn l chuyn mch hng ti lng knh, ta c th nh hng tn hiu nh sng sang si 3. Khi l chuyn mch khng tip xc vi lng knh, s xy ra hin tng phn x nh sng bn trong v tn hiu quang i vo si 2. Ch l qu trnh chuyn ng c hc ca cc linh kin quang xy ra rt nhanh, rt t v chc chn. Nn hin nay, cc b chuyn mch quang c hc ang c s dng ph bin nht.

Hnh 1.37(g) trnh by v d v chuyn mch quang nhit. Mt b giao thoa Mach- Zehnder kt hp vi mt b dch pha gn trong mi nhnh giao thoa. Bng hi nng, ta c th iu khin s lng b dch pha, ngha l c th nh hng tn hiu quang sang si 2 hoc si 3. Cc chuyn mch quang nhit c tc chuyn mch nhanh hn so vi cc b chuyn mch quang c hc, v quan trng nht l, chng c th c thc hin theo cng ngh trng thi rn planar (planar solid-state) nh cc ma trn chuyn mch ln.

Mt s tham s chnh quy nh c tnh ca cc b chuyn mch:

T s tt m (extinction ratio): th hin c tnh ca b chuyn mch on/off. y l t s gia nng lng nh sng khi chuyn mch trng thi on v nng lng nh sng khi chuyn mch trng thi off. Gi tr ny cng cao cng tt, thng nm trong khong t 45 n 50 dB.

Suy hao xen (insertion loss): l n v o cng sut suy hao do b chuyn mch gy ra thng c gi tr khong 0.5 dB.


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Nhiu xuyn m (crosstalk): t s gia cng sut ng ra c to ra bi ng vo mong mun v cng sut ng ra c to ra bi ng vo khng mong mun. Gi tr ny cng cao cng tt, thng khong 80 dB.

Thi gian chuyn mch (switching time): l tham s rt quan trng. Khi s dng cc b lc hiu chnh c, thi gian chuyn mch yu cu ph thuc vo cc ng dng ca chuyn mch. i vi cc mng chuyn mch knh ngy nay, thi gian chuyn mch khong c s, thm ch c ms, nhng i vi cc mng quang chuyn mch gi, thi gian chuyn mch ch khong vi ns, thm ch khong s. Cc b chuyn mch n quang-c hc v quang-nhit c thi gian chuyn mch nm trong khong t 2 n 20 ms, trong khi cc b chuyn mch n quang-in c thi gian chuyn mch c ns.

Ngoi nhng tham s k ra trn y, trong ti liu tham kho c tnh ca phn t chuyn mch cn c mt s tham s nh di bc sng hot ng, PDL, v nhit phng.

b) Cc khi chuyn mch quang ln (multistage/large optical switch)

Cc khi chuyn mch quang vi s lng cng t vi trm n vi ngn ang c nghin cu cho h thng mng quang th h tip theo. Khi thit k cc khi chuyn mch quang ln, cn quan tm n cc vn sau:

S lng ca cc phn t chuyn mch cn thit: cc b chuyn mch ln c to thnh t cc phn t chuyn mch theo nhiu cch khc nhau, nh s trnh by bn di. Chi ph v phc tp ca khi chuyn mch ph thuc vo s phn t chuyn mch c yu cu, cch ng gi, ghp ni, phng php ch to v iu khin.

Tnh ng nht ca suy hao: cc b chuyn mch c th to ra suy hao khc nhau cho tng kt ni khc nhau ca ng vo v ng ra. Khi chuyn mch cng ln th s khc nhau v suy hao cng nhiu. nh gi tnh ng nht ca suy hao bng cch xem xt s phn t chuyn mch ti thiu v ti a trn ng dn quang i vi tng kt ni ng vo/ra khc nhau.

S im ni cho trong khi chuyn mch: thng s ny c bit quan trng trong vic ch to cc khi chuyn mch quang. Mt s khi chuyn mch quang c tch hp t nhiu b chuyn mch trn mt mch duy nht. Khng ging nh trong cc mch in tch hp (IC), , cc kt ni gia nhiu linh kin khc nhau c th nm trn nhiu lp, trong cc mch quang tch hp, tt c cc kt ni u c to ra trn mt lp duy nht bng cc ng dn sng. Nu cc ng dn ca hai ng dn sng ct nhau (to ra im ni cho) th s xy ra cc hiu ng khng mong mun nh suy hao cng sut v hin tng nhiu xuyn m. hin tng suy hao cng sut v nhiu xuyn m khng gy nh hng n khi chuyn mch th phi ti thiu ha hoc hn ch hon ton cc im ct nhau ny.

Cc c tnh nghn: v chc nng, c th chia khi chuyn mch thnh hai loi: nghn v khng nghn. Khi chuyn mch gi l khng nghn khi mt cng ng vo no ang ri c th kt ni vi bt k ng ra no cng ang ri. V th, mt khi chuyn mch khng nghn c kh nng thc hin mi kt ni t ng vo n ng ra. Nu trong khi chuyn mch c mt s kt ni khng th thc hin c, th khi chuyn mch ny c gi xem l c nghn. Phn ln cc ng dng u yu cu chuyn mch khng nghn. Vi chuyn mch khng nghn c th phn thnh hai loi l: chuyn mch khng nghn theo


49

ngha rng (wide-sense nonblocking), v chuyn mch khng nghn theo ngha hp (strict-sense non-blocking). Theo ngha rng, bt k ng vo no cha c s dng cng c th kt ni vi bt k ng ra no cng cha c s dng m khng cn phi nh tuyn li cc kt ni ang tn ti; khi chuyn mch dng ny s dng cc thut ton nh tuyn c trng nh tuyn cho cc kt ni hin c sao cho m bo khng xy ra nghn cho cc kt ni tip sau . Theo ngha hp, bt k ng vo no cha c s dng cng c kt ni vi bt k ng ra no cng cha c s dng m khng cn quan tm n trng thi ca cc kt ni trc trong khi chuyn mch.

Mt khi chuyn mch khng nghn yu cu vic nh tuyn li cho cc kt ni m bo thuc tnh khng nghn c gi l khi chuyn mch khng nghn sp xp li (rearrangeably non-blocking switch). Vic nh tuyn li cc kt ni c th hoc khng th c chp nhn cn ty thuc vo ng dng v chc chn cc kt ni s b ngt trong mt khong thi gian no khi chng c chuyn mch sang ng dn khc. So vi cc cu trc chuyn mch khng nghn theo ngha rng, u im ca cc cu trc chuyn mch khng nghn sp xp li l s dng cng t cc b chuyn mch nh th kch thc ca khi chuyn mch cng ln. Tuy nhin, khi cc cu trc khng nghn sp xp li s dng cng t b chuyn mch nh th thut ton iu khin thit lp kt ni ca chng cng phc tp, nhng ni chung vi cng ngh vi x l p dng trong khi chuyn mch ngy nay, th y l vn nh, khng quan trng. Nhc im ln nht ca cc khi chuyn mch khng nghn sp xp li l khng th phc v cho cc ng dng khng cho php ngt cc kt ni ang tn ti, thm ch trong khong thi gian cc ngn khi cn thit lp mt kt ni mi.

R rng, khi thit k mt khi chuyn mch dung lng ln th khng th cng lc t c s ti u i vi tt c cc thng s k trn. Tu thuc vo ng dng ca khi chuyn mch trn thc t mt thng s c th c u tin hn cc thng s khc.

Bng sau y trnh by s so snh gia cc cu trc chuyn mch khc nhau, trong cu trc Spanke dng cc b chuyn mch 1n, cc cu trc cn li u hnh thnh t b chuyn mch 22.

Bng1.2 So snh gia cc cu trc chuyn mch khc nhau.

Loi khng nghn S b chuyn mch Suy hao ln nht Suy hao nh nht

Crossbar Wide-sense n2 2n-1 1

Clos Strict-sense 5.124 n 525 n 3

Spanke Strict-sense 2n 2 2

Benes Rearrangeable )1log2(

2 2nn 1log2 2 n

2log2n-1

Spanke-Benes

Rearrangeable )1(

2nn

n 2n


50

Cu trc Crossbar

Hnh 1.38 trnh by cu trc ca khi chuyn mch crossbar 4x4. Khi chuyn mch ny dng 16 phn t chuyn mch 2x2, v cc kt ni gia ng vo v ng ra c thc hin bng cch b tr thch hp cc phn t chuyn mch 2x2. V d, kt ni gia ng vo 1 v ng ra 3, cn phi sp xp cc phn t chuyn mch 2x2 nh trong hnh 2.5. Cn nhiu ng dn khc i t ng vo 1 n ng ra 3 nhng ng dn trong hnh l ng dn thch hp nht c chn da trn thut ton nh tuyn s dng trong khi chuyn mch.

Hnh 1.38 Khi chuyn mch 4x4 dng 16 b chuyn mch 2x2.

Cu trc crossbar l cu trc ca loi chuyn mch khng nghn theo ngha rng (wide-sense). kt ni t ng vo i n ng ra j, ng dn c chn s i qua cc phn t chuyn mch 2x2 trn hng i cho n khi n i n ct j, sau i qua cc phn t chuyn mch trn ct j cho n khi n i n ng ra j. Do , cc phn t chuyn mch trn ng dn ny hng i v ct j phi c t cc v tr thch hp c th to ra kt ni ny. Theo quy tc nh tuyn kt ni nh trn th khi chuyn mch s khng b nghn v khng yu cu phi nh tuyn li cc kt ni ang tn ti.

Tm li, mt cu trc crossbar n(n cn phi c n2 phn t chuyn mch 2x2. Chiu di ng dn ngn nht l 1 v chiu di ng dn di nht l 2n-1, v y l mt trong nhng nhc im chnh ca cu trc crossbar. Chuyn mch ny lun tn ti cc im ni cho nhau.

Cu trc Clos

Cu trc Clos l loi chuyn mch khng nghn theo ngha hp (strict- sense nonblocking) v c s dng rng ri trong thc t xy dng cc khi chuyn mch

2 1

1

2

3

4

Inpu

ts

Outputs

3 4


51

c vi s lng cng rt ln. Hnh 1.39 trnh by cu trc chuyn mch Clos ba tng c 1024 cng. Mt khi chuyn mch n(n c xy dng nh sau: dng ba tham s l m, k, v p. Cho n=mk. Tng u tin v tng th ba c k phn t chuyn mch (mxp). Tng th hai (tng gia) c p phn t chuyn mch (kxk). Mi phn t trong s k phn t chuyn mch ca tng u tin c kt ni vi tt c cc phn t chuyn mch ca tng th hai (mi phn t chuyn mch ca tng u tin c p ng ra. Mi ng ra c kt ni vi mt ng vo ca tng phn t chuyn mch khc nhau ca tng th hai). Tng t nh th, mi phn t trong s k phn t chuyn mch ca tng th ba c kt ni vi tt c cc phn t chuyn mch ca tng th hai. Trng hp p 2m-1, th khi chuyn mch loi ny hon ton khng b nghn.

ti thiu ha chi ph ca khi chuyn mch, chn p = 2m-1. Thng thng, tng khi chuyn mch ring trong mi tng l cc khi chuyn mch crossbar. Do , mi b chuyn mch mx(2m- 1) cn m(2m-1) phn t chuyn mch 2x2, v mi b chuyn mch (kxk) trong tng th hai cn k2 phn t chuyn mch 2x2. Tng s phn t chuyn mch cn thit l:

2km(2m-1) + (2

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