Phng php b tn sc trong mng thng tin quang tc cao dng quang si
cch t Bragg chu k thay i tuyn tnh
Phng php b tn sc trong mng thng tin quang tc cao dng quang si
cch t Bragg chu k thay i tuyn tnh
Hin nay c mt hng nghin cu mi s dng cc quang si c li l cc cch t
Bragg b tn sc. u im ca phng php ny l thit b c kch thc nh gn, ch to
n gin, v hot ng rt c hiu qu. Bi vit ny xin trnh by v phng php b tn
sc trong quang si n mode dng quang si cch t Bragg chu k thay i tuyn
tnh (linear chirped Fiber Bragg Grating).
ThS. H Huy ThnhGii thiuTn sc trong quang si n mode l mt trong
nhng hin tng vt l nh hng nghim trng n cht lng ca mng thng tin quang
tc cao dng bc sng nh sng vng ca s 1550nm. N lm tng t l li bit, gii
hn tc hoc khong cch truyn ca mng. xy dng hoc nng cp nhng mng thng
tin quang (OTDM, DWDM, OCDMA) kch thc ln (vi nghn km), tc cao (vi
chc Gbit/s) th mt vn quan trng phi gii quyt l gim ti thiu tn sc
trong si cp quang.
Mt s gii php khc phc hin tng tn sc ang c p dng hin nay trn th
gii l s dng cc cp quang thng tin c tn sc ti thiu
(dispersion-shifted fibers) hoc cp quang c kh nng b tn sc
(dispension-compensating fibers). Tuy nhin cc phng php ny c mt nhc
im l chi ph ln do gi thnh cp quang c bit ny t hn nhiu so vi cp
quang thng tin thng thng. Ngoi ra phi tnh n chi ph thay th ton b cc
cp quang thng thng lp t trc hoc lp t thm nhng on cp quang b tn sc
kh di (c vi km cp quang b tn sc b cho vi chc km cp quang thng). Hin
nay c mt hng nghin cu mi s dng cc quang si c li l cc cch t Bragg b
tn sc. u im ca phng php ny l thit b c kch thc nh gn, ch to n gin, v
hot ng rt c hiu qu. Bi vit ny xin trnh by v phng php b tn sc trong
quang si n mode dng quang si cch t Bragg chu k thay i tuyn tnh
(linear chirped Fiber Bragg Grating).
Hin tng tn sc trong si cp quangHin tng mt xung nh sng b gin rng
ra v mt thi gian sau mt qung ng truyn nht nh trong si cp quang c gi
l hin tng tn sc trong si cp quang. C ba ngun gy nn hin tng tn sc
l:
- Tn sc vt liu
- Tn sc dn sng
- Tr nhm
i vi cc bc sng trong phm vi 1550nm th tn sc vt liu l nguyn nhn
chnh gy nn hin tng tn sc. Tn sc vt liu sinh ra l do trong mt si cp
quang, vn tc nh sng cng nh chit xut ca quang si l mt hm s ca bc sng
nh sng tn hiu. Hnh v 1 biu din s thay i ca vn tc nhm ca mt xung nh
sng i vi cc bc sng khc nhau trong mt si cp quang thng tin n mode
thng thng.
Hnh 1. S thay i ca vn tc nhm theo bc sng trong quang si n mt
thng thngTrn hnh v 1, chng ta nhn thy ti cc bc sng vng ca s 1550nm,
vn tc nhm t l nghch vi bc sng ca nh sng. Nh chng ta bit, trn thc t
khng th c mt ngun sng n sc tuyt i, mi ngun sng u c mt rng ph nht
nh. Gi s mt xung nh sng c bc sng trung tm ti 1550nm, rng ph0truyn
qua mt si cp quang n mode. Cc thnh phn bc sng di hn ca xung s chuyn
chm hn cc thnh phn bc sng ngn hn. Nh vy, sau mt qung ng truyn di,
rng xung s b ko gin ra ti mc hai xung k tip nhau s b chn ln nhau
(hnh 2). Hu qu l thit b u thu s khng th phn bit c 2 xung ring bit.
thit b thu c tn hiu xung, ngi ta phi gim tc truyn hoc rt ngn khong
cch gia bn pht v bn thu.
Hnh 2. Hu qu ca tn sc i vi tc truyn ca mnga) xung ti u pht b)
xung thu c ti u thu v thit b thu khng th phn bit c hai xung k tipB
tn sc bng quang si cch t Bragg chu k bin i tuyn tnhQuang si cch t
Bragg chu k bin i tuyn tnh l mt si quang n mode c mt on li c ghi
nhng cch t c chu k thay i mt cch tuyn tnh dc theo chiu di ca quang
si.
(z) = 0 + 1(z)0l chu k im bt u ca on cch t,1l s thay i tuyn tnh
dc theo chiu di ca on cch t.
Ti v tr z trn on cch t Bragg, mt sng nh sng s b phn x ngc li nu
bc sng ca n tho mn cng thc:
B(z) = 2neff(z)(z)B(z) l bc sng Bragg ti v tr z tng ng vi chu k
cch t(z).
c tnh ca quang si cch t Bragg chu k bin i l ti nhng v tr tng ng
vi chu k di hn s phn x nhng nh sng c bc sng di hn.
Gi s xung tn hiu i vo u c chu k di hn ca on cch t nh hnh v 3.
Khi , nhng bc sng di hn s b phn x gn phn u ca on cch t hn. Ni cch
khc, nhng bc sng ngn hn s phi i mt qung ng xa hn trong on cch t trc
khi chng c phn x ngc li. Kt qu l mt khong thi gian tr d s c to ra
gia thnh phn bc sng ngn so vi thnh phn bc sng di.
Trong cng thc trn d l khong thi gian tr, neffl chit xut hiu dng,
L l di on cch t Bragg, c l vn tc nh sng trong chn khng,cl hiu s gia
bc sng b phn x u on cch t (thnh phn bc sng di nht) so vi bc sng b
phn x cui on cch t (thnh phn ngn nht).
Hnh 3. Nguyn l b tn sc ca quang si cch t Bagg chu k bin iy l hin
tng ngc vi hin tng tn sc v l nguyn l ca thit b b tn sc trong mng
thng tin quang dng cch t Bragg chu k thay i tuyn tnh.
Hnh v 4 l mt m hnh c bn ca thit b b tn sc dng cch t Bragg chu k
thay i tuyn tnh.
Hnh 4. M hnh c bn ca thit b b tn sc dng cch t Bagg chu k thay i
tuyn tnhTrn hnh v circulator l thit b ghp ni quang ch cho nh sng i
ln lt cc cng 1, 2, 3 theo chiu kim ng h. Mt xung b gin rng sau khi
c khuch i s i qua mt circulator ti on cch t Bragg c chu k bin i nh
hnh v. Ti on cch t, thnh phn bc sng ngn ti trc do tn sc s phi i thm
qung ng na trc khi c phn x ngc li ti thit b u thu. Trong khi , thnh
phn bc sng di hn, n chm hn do b tn sc, s c phn x ngay khi ti cch t
Bragg. Kt qu l xung tn hiu sau khi i qua thit b b c co li. Tnh ton
hp l cc s liu v di on cch t Bragg, hm thay i ca chu k cc cch t
L(z), ngi ta c th thu c xung nh sng c rng nh u pht.
Ngi ta chng minh rng mt on cch t Bragg di 5,7cm c th b cho 100km
quang si truyn thng c tn sc 17ps/nm km dng bc sng 1550nm, rng ph
0,2 nm.
Kt lunHin nay phng php ng dng quang si cch t Bragg ch to cc thit
b b tn sc trong mng thng tin quang ang l mt ch nghin cu rt hp dn ti
cc trung tm nghin cu v photonics v cng ngh quang si trn th gii. u
im ni tri ca phng php ny l thit b ch to rt n gin, kch thc nh (nm
hon ton trong mt si quang), hon ton tng thch vi mng cp quang nn tn
hao ghp ni rt nh, c kh nng iu chnh b tn sc.
Xu hng nghin cu tip hin nay l cc phng php apodization gim gn tr
nhm, nghin cu cc cch t Bragg c chu k bin i phi tuyn khng nhng b tn
sc vt liu m cn b c tn sc phn cc v tn sc mc cao (xy ra khi truyn tc
ln hn 100 Gbps).
Ti liu tham kho[1] Ajoy Ghatak, K. Thyagarajan"Introduction to
Fiber Optics", Cambridge University Press 1998.
[2] Bacelos, S., M.N.Zervas, and R.I.Laming,"Characteristics of
chirped fiber gratings for dispersion compensation,"Optical Fiber
Technology, 1996, pp. 213-215
[3] Eggleton, B.J., et. al."Implications of fiber grating
dispersion for WDM lighwave systems", Journal of Lightwave
technology, Vol.9, 1997, pp. 1403-1405.
[4] Andreas Othonos, Kyriacos Kalli,"Fiber Bragg Gratings
Fundamentals and Applications in Telecommunications and Sensing",
Artech House 1999.
[5] V Vn San"K thut thng tin quang", Nh xut bn KHKT 12/1997, Tng
cng ty Bu Chnh Vin Thng Vit Nam.
