Cdng Irinh kinh tdng Gido su Viin sy Nguyin Vdn Hiiu nhdn dip Gido su Viin sy 70 tuoi_
NGHIEN Ciru iTNG DUNG MODULE PHAT, THU, KHUECH DAI QUANG (SOA) VA CACH TLT SOI QUANG TRONG THONG TIN QUANG
BAO MAT DU^OC CHE TAO TAI VIEN KHOA HOC VAT LIEU
• • • • • *
Vu Van Luc', Bernard Journet^ Dinh Xuan Quyen^ Luong Vu Hai Nam , Vu Doan Mien', Vu Thj Nghiem' va Pham Van Trudng'
1) Vien Khoa hgc Vat lieu, Vien Khoa hgc va Cdng nghe Viet Nam, 18 Hoang Qudc Viet, Cau Gidy, Ha Noi; E-mail: wkicf./ inib..ncst,acATi 2) d'Alembert Institute, ENS Cachan, 61 Ave Pdt Wilson 94230 Cachan, France.
Tom tdt: Su bao mat thdng tin va he thong tin quang bao mat dang dugc dac biet quan tam nghien cim d nhieu qudc gia phat trien tren the gidi bdi vi chiing rat can thilt cho an ninh, qudc phdng, he thing lanh dao nha nude va lien qudc gia. Trong cdng trinh nay, chung toi dua ra mot sd cac kit qua nghien cim ciia de tai hgp tac khoa hgc giira Phong laser ban ddn, Vien Khoa hgc Vat lieu va phdng SATIE, Vien De L'Amberd, Dai hgc Su pham Cachan vl thdng tin quang bao mat va he thdng tin quang nay. Dac biet la cac ket qua nghien cim sir dung cac module quang hgc va linh kien quang dien tii' thdng dung nhat cho ky thuat thdng tin quang chuan qudc te hien nay dl thay thi cac linh kien dien khac trong he thdng tin quang bao mat, nliam quang hoa hoan loan he thdng tin nay. He nay sir dung cac laser module DFB la ngudn phat quang va cac module khuech dai quang ban dan sdng chay (SOA) de bien doi budc song tao ra hai budc sdng X: va X,2 gan nhau nhat de giam thieu anh hudng tan sac trong sgi quang. Mot sdng lam kenh truyen tin hieu va sdng kia lam ma hda va dugc truyen tren cimg mot sgi quang. Trudc khi den dau thu hai sdng dugc tach bang each tii' sgi quang va dugc thu bdi photodiode APD (tin hieu ma hda) va PIN (tin hieu khda). Cd the coi day la he thi nghiem ve bao mat thdng tin hoan loan quang, de tich hgp vdi cac he thdng tin quang khac va cd the irng dung trong thuc te.
GIOI THIEU
Da cd rat nhieu phuang phap de bao mat cac thdng tin tren dudng truyen. Khi cac he thdng tin quang dau tien dugc sir dyng, ngoai viec nang cao tdc do truyen, tang khoang each dudng truyen, ngudi ta cho rang day chinh la he thdng tin cd tinh bao mat cao. Tuy nhien eiing vdi sy cai tien, phat trien ciia cac he thdng tin quang thi cung xudt hien cac thiet bi ed the lay tin hieu tren dudng truyen quang ma ca ben truyen Ian ben nhan khdng phat hien dugc ke ca cac tuyen thdng tin quang qudc gia hoac qudc tl. Vi vay, hien nay tren the gidi dac biet la cac qudc gia phat trien cd rat nhieu cac nghien ciru ve bao mat thdng lin quang. Mdt trong eae ket qua nghien ciru ve van de nay la da xay dyng dugc mdt he thi nghiem thdng tin quang bao mat don photon. He nay sir dyng hai kenh lin bieu ciing truyin tren mdt sgi quang, mdt kenh truyen dir lieu va mdt kenh Iam
TuyIn tap bao cao ky niem 15 nam thanh lap Vien Khoa hgc Vat lieu - Ha Noi, 6/2008 khoa quang bgc. Kenh ma khoa boat ddng d viing sdng dai ban kenh dii: lieu mdt kliocing sao cho do tri giii:a hai kenh dir lieu va khoa quang hgc dat 18 fs/kni. Tin hieu ma khoa se den dau thu trudc va dugc coi nhu la ma khda, va dau thu phai cd bd giai ma khda mdi dgc dugc dir lieu da truyen. Nhu vay, dii cd lay dugc lin hieu tren dirdng truyen nhirng khdng cd mat khau tbi cung khdng dgc dugc dii lieu.
CO SO LY THUYET
Ma hda lugng tir dugc coi la giai phap ly tudng de bao mat dy lieu tren dudng truyen. Theo tinh loan ly thuyet, cac khda lugng tir bao mat cd dugc khi sir dyng ngudn phat dan photon [1]. Tuy nhien, tren thi trudng chua cd cac ngudn don photon, vi vay trong hau het cac he thir nghiein thdng tin bao mat ngudi ta sir dyng ngudn phat laser xung yeu. Van de ciia he nay la tim ra phuong phap truyen ddng thdi hai tin hieu khda lugng tir (quantum keys signal) va tin hieu dir lieu theo thir ty sao cho cac tin hieu nay ddng bd ldi nhiil khi den dau thu. Gia thiet dat ra rang, de tin hieu truyen tren sgi quang theo thir ty nhu mong mudn, cd the sir dung bai sgi quang rieng biet cho bai tin bieu [3,4,5]. Tuy nhien, hieu irng tan sac cua sgi quang ed the anh hirdng len cac kenh lin hieu anh hirdng den sy ddng bd tai ldi ra cita dudng truyen. Nen viec truyen ddng thdi hai tin hieu khda lugng tir va tin bieu dir lieu tren ciing mdt sgi quang dugc coi la giai phap kha thi va dugc sir dung trong cac nghien ctiu ve thdng tin quang bao mat khi dat dugc sy tre giira hai tin hieu dii nhd. Sy tre tin hieu klii truyen tren sgi quang do tan sac dugc cho bdi phuong trinh sau:
Ar dv
dX„ Al, L d'n - F) -rr7 ^K
e aF, hoac Ar L ,2
a n d2.l 0 J
AA„
V "-0 J
(1) Bdng I; Su phu thude dig bir&c sdng vdo tdn sdc ciig sgi quang
Ao(pin) 1,30 1,55
n(AJ
1,44726325 1,44438815
dn , -K
dh
-0,01125037 -0,01189888
d'n , -2^
-0,00055057 -0,00416491
Vdi su phu thude dugc chi ra trong bang 1, sir dyng cdng thirc 1 ta cd the tinh dugc do tre ciia tin hieu do tan sdc anh sang cua sgi quang vdi mdi viing sdng nhu sau:
1. Vdi ngudn laser phat d 'viing budc sdng A.o=1550nin, do rdng phd la 2 nm thi d'n
dXl 0,004 pin"^ (tin hieu tre tren dudng truyen la 20ps/km). Va neu do chenh lech giira hai budc A/lg = 2 nm, ta cd Ar = 40 ps/km
2. Khi tinh them sy anh hudng cua nhiet do len do md rdng vach phd ta cd
Cdng trinh kinh Idng Gido su Viin sy Nguyin Vdn Hieu nhdn dip Gido su Viin sy 70 tiioi_
vdi >.o=l,5pm va V(A„) = . ( A J - A „ - ^ thi N(1550) = 1,46283141 va dK
Ar = r(l550)-r(l300) = -[v(l550)-V(l300)]= 3,14x1 (ns) c
h a y L = 1 km, Ar = 3,14 ns v a L = 100 km, Ar = 314 ns
3. Vdi ngudn laser d viing sdng 1,5 pm cd do rdng phd lOOMHz ta cd v = j trong do civ = -^dA. Tir bilu thirc nay ta tim dugc bieu thiic tinh do chenh lech budc sdng ciia
F
hai tin hieu (3). Av=-^AA hoac AA = — Av (3) X' X- c
Ap dung cac tinh loan ly thuylt tren vdi mdt he truyin thir nghiem vdi cac gia tri nhu d'n
sau
L = 1 km; c = 3.10" km/s; X^ =1,55 pm;
dK
-0,00416491 pm"^; Av = 100 MHz thi:
, F- (l,55.10^
^x = — ^v = ^ - ^ c 3.10'
100.10' = 0,8.10" pm va AT = - - A „ ^ A A „ = 1.72.10-"s/m
c dX'^
Nhu vay, do chenh lech (tre) cita hai tin hieu khi truyen tren sgi quang do tan sac la 3.2 ns/km, va vdi dudng truyin 100 km thi do tre se la 320 ns. Dya vao cac tinh toan ly thuylt tren, ta cd the tim cac ngudn quang de lam tin hieu dir lieu hay tin hieu ma hda (khda) mdt each chit dgng va phii hgp de thiet ke he thir nghiein truyen thdng tin quang bao mat nay.
T H U C NGHIEM
He thuc nghiem truyen thong tin quang bao mat He truyen thdng tin quang bao
mat don photon dugc chia thanh 2 phan : phat va thu dugc chi ra tren hinb 1. Phan phat sir dyng ngudn laser phat don mdt, ngudn nay dugc lao ra bai budc sdng sat nhau cd sy chenh lech rat nhd de loai trir sy khdng ddng bg khi hai tin hieu den dau thu va anh hudng tan sac ciia sgi khi dugc truyen tren ciing mdt sgi quang. Phan thu gdm mdt
Hinh 1. Sa dd nguyen ly ciia hi thdng tin quang bdo mdt
TuyIn tap bao cao ky niem 15 nam thanh lap Vien Khoa hgc Vat heu - Ha Noi, 6/2008 lgc quang, bd ddu thu loai PIN photodiode de thu tin hieu kenh dii lieu va bd ddu thu APD (photodiode Avalance) thu va giai lin hieu khoa.
A. Phan phat
/. Phuong phdp bien doi budc sdnh de tgo hai nguon sdng co budc song gdn nhau nhdt
De cd dugc hai kenh tin hieu vdi do chenh lech budc sdng nhd (cd ps/km), cac nghien ciru tren tbi gidi thudng sir dyng mdt ngudn laser va tacb ngudn sang nay thanh hai ngudn sang cd budc sdng rat gan nhau bang bd bien dieu quang-am (acouslo-optic modulation) (hinh 2).
Bd bien dieu nay cd the tach mdt ngudn quang thimh hai ngudn quang vdi do chenh lech budc sdng Ax ^ 0,002 nm. Tuy nhien, nd la mdt bd
bien ddi bang dien rat cdng kenh, khd ghep ndi Hinh 2. bd bien dieu qugng - dm vdi sgi quang va khdng pbii hgp cho irng dung (gcoiisto -optic modulation) thyc le. Vi vay, lir cac ket qua nghien ciru hgp
tcic ve van de niiy ciia phdng tbi nghiem SATIE, Trudng Dai hgc Cachan va phdng Laser biin dan, Vien Khoa hgc Vat lieu, chiing tdi da sir dyng hai laser DFB lam ngudn phat quang va cau hinh Mazhender (XGM) tren cd sd khuech dai quang de tao hai ngudn quang cd budc sdng rat gan nhau. So dd he dugc chi ra tren hinh 3, cac module laser va khuech dai quang (dugc ghep ndi va ddng gdi tai phdng laser ban dan tren co sd cac chip laser va khuech dai nhap ngoai) sir dyng trong he dugc chi ra tren hinb 4.
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111 l!l 1.. 1
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111 l i . ' , • , ,
q - p . To hgp quan
— O J - ^
e S().\
Chia quang
Loc va suy hao
^ 1 1 0 0 ^
V ^^
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\ \_^ 0 0
( Ji_n_ F^
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Hmh 3. He bien doi huvc song tren ca so module khuech dgi (SOA) theo cdu hinh XGM
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7 b , : •
Hinh 4. Cdc linh FFn quang sir dung trong hi; a) Module khuech dgi SOA vd b) Laser module DFB
Cong irinh kinh Idng Gido su Viin sy Nguyin Vdn Hiiu nhdn dip Gido su Viin sy 70 tiioi_
Ngudn laser 1 cd budc sdng A., hoat ddng d cbl do ddng xung cd djnh, laser 2 boat ddng d chi do lien tyc cd budc sdng Xj. Hai ngudn laser nay dugc dua ddng th^di vao SOA nhd bd id bgp quang. Khi ddng bom qua SOA din gia tri ddng nhat dinh kliuech dai quang lam viec nhu mdt bd bien ddi budc sdng. Cac ket qua thyc nghiem da chi ra rang, ngudn laser hoat ddng d cbl do lien tyc cd Xi se bi biln ddi thanh ngudn cd budc song A,i+A?. va ed dang gidng nhu ngudn laser 1 (twin wavelength). Tuy nhien do sir dyng hai ngudn laser khac nhau nen khi budc sdng ciia ngudn laser 2 bi bien ddi se nam ve phia sdng ngan hoac sdng dai cua ngudn laser I (A.|) vdi khoang each nhat dinh. Theo tinh toan ly thuylt, dl tin hieu du: lieu (A.,) va tin hieu khoa Ckj^Xi+AF) dugc ddng bd khi den dau thu tbi khoang each giira hai budc sdng nay phai dii nhd (cd femto giay/km). De giai quyll vdn dl nay chung tdi da diing hai phuang phap: thay ddi ddng boat ddng hoac nhiet do lam viec ciia mot trong hai ngudn.
a. Thay doi dong hogt dgng cua laser 2
De loai trir sy dich b u d c sdng ciia cac ngudn quang do sy thay ddi nhiel do, cac ngudn laser va
khuech dai quang deu d u g c dn djnh nhiet bang pin t'-'^'^", Peltier. Dal ddng lam viec cho SOA va laser 1 d gia ;.'.',Ml'r'"' tri cd dinh, thay ddi d d n g b o m mdt chieu qua laser 2.
Kel qua sy bien ddi b u d c sdng cita SOA va sy dich 2"—• ——• • chuyen budc sdng theo ddng b a m ciia laser 2 dugc • i„ = i'.».
chi ra trong hinh 5. f;:!";™™
Khi ddng mdt chieu (CW) qua laser 2 la 8,3 mA ._ ^ thi b u d c sdng ciia laser 2 nam d phia sdng ngan ciia
laser 1 (hinh 3B) vdi do chenh lech (hay do che) giira bai budc sdng la : AX, = X1-X2 = 0 , I 3 n m . Tiep tuc tang ddng bom mdt chieu qua laser 2 den 20 m A (hinh 3D), ta nhan thdy b u d c sdng ciia laser 1 (A,|) khdng thay ddi nhung b u d c sdng eiia laser 2 (X,2) dieh dan ve phia sdng dai va vugt qua b u d c sdng laser 1. Xet d ddng b a m 17 mA (binh 3C), do chenh lech budc sdng cua hai laser la AX = 0.35 nm vdi
^ = 0.04;7/«/^ Do dich pha cita S O A phy thuoc
vao sy thay ddi mat do hat tai trong m i l n tich cyc Hinh 5. Su dich phd cita edc bang cach thay ddi ddng qua SOA hoac cdng suat ngudn laser d ldi ra eiia quang bom ldi vao. Neu mat do cdng sudt quang cdu hinh XGM tren eo sd bom vao tai ddng I, la A,; (dat sy bao hoa khulch SOA khi ddng bom eiia dai), tbi N (sd bat lai) trong SOA se thay doi ddn tdi ^'^^^'' '^'^y ^di
viec tang cac hire xa kich thich. Keo theo dd, pha (j)
va cdng suat quang ra thay ddi. Nhu vay cd thi thi thay doi cdng sudt quang ra bdng each thay ddi cdng suat quang bom vao dl dat dugc sy biln ddi anh sang va sy dich chuyin budc sdng bien ddi theo ddng bom (hay mat do quang bom vao SOA [2,7]. Vay, nlu thay
Of)
2 H nil c
~i a- ca
=: B
': ^
•|". 1 = 3 l l m A
\
539 1540 154 B-dc song, nm
TuyIn tap bao cao ky niem 15 nam thanh lap Vien Khoa hgc Vat lieu - Ha Noi, 6/2008 ddi ddng bom qua laser den 0,1 mA, ta cd the thu dugc sy dich chuyen cua budc sdng din 0,004 nm va cd ibl tao ra hai budc sdng cd sy chenli lech AA, « 4.10'^ nm.
h. Thay ddi nhiet do ldm viec cua laser 2
Trong phuang phap nay, ngudn laser va SOA boat dgng d ddng cd dinh nhung nliiet do lam viec eiia laser 2 dugc thay ddi. Ket qua cung chi ra rang khi nhiet do cua ngudn bom thay ddi ciing lam thay ddi do dai budc sdng ciia tin hieu da biln ddi. C) 20''C, budc sdng ciia laser 2 nam d phia sdng ngan ciia laser 1 vdi do chenh lech AA. = Xi-Xx = 0.235 nm. Khi nhiet do lam viec ciia nd tang 10 do thi budc sdng ciia hai ngudn laser nay triing nhau hoan loan va chi can tang them 1 do, budc sdng eiia laser 2 vugt qua budc sdng laser 1 va AA = X.-X^ = 0.065 nm vdi ^Vo^ ciia laser 2 la 0 . 0 7 5 W edng bai phuang phap tren, chiing ta cd the tao ra dugc bai budc sdng giong nhau cd AX < 0.13 nm (theo linh toan AX cd the dat « 10" nm) bang khuech dai quang ban dan tren cau hinh XGM. Day la ket qua nghien ciru tien bg rat Idn so vdi cac nghien cim trudc day va da sir dung loan cac linh kien quang thdng dung rat thuan tien de tich hgp vdi nhau khi xay dyng he thdng lin quang chuan.
2. Nguon laser yeu
De cd do bao mat cao bang phuang phap ma hoa lugng tir can cd eae ngudn don photon tuy nhien cac ngudn nay khdng cd tren thi trudng vi vay, chiing tdi da sir dung ngudn laser xung yeu. De cd ngudn laser xung yeu dat den sd photon rat thap, chiing tdi da sir dung ngudn bam xung ngan ( x < 10ns) de bom cho laser 1 va suy bao quang. Tinh toan ly thuyel vdi mdt be thir nghiem vdi cac gia trj sau [3] :
Cdng suat quang bom vao : Po=l inW=lmJ/s Dc) rdng xung : Ax=10ns=10' s, WO=PO*AT
Nang lugng ciia mdt photon: E=hv=hc/A.
Sd photon tren mdt xung: n=Wo/E
PaAr , , 10"'X 10 10''X 1.55 10"' „ ,^7
" A hay n = ~ 8 . 1 0 ' he " 6.62 10"''X 3 1 0 '
Nhu vay, dl so photon tren mdt xung giam xudng < 10 photon thi can phai suy hao quang den 70 dB.
Sau khi cd dugc hai tin hieu gidng nhau bang sy bien ddi quang, ldi ra ciia khuech dcai quang dugc chia ihimb bai dirdng. Mdt dudng dugc dua vao dau thu pin photodiodde, mdt dudng dugc dua qua lgc quang va suy hao quang de cd the thu dugc xung quang ylu ed A,| (gdn din don photon). Sau dd, hai tin hieu lai dugc dua vao mdt dudng quang nhd bd coupler quang vdi do chenh lech budc sdng dit nhd nhu da ndi d tren.
B. Phan thu
a. Tdch tin hieu khoa vd du lieu
Tin hieu vii ma khoa sau khi truyen tren cimg mdt sgi quang vdi mot quang dudng nhdt dinh dugc dua den dau thu va dugc chia ddi (chia cdng suat) nhd bd chia quang 1x2
Cdng trinh kinh tdng Gido su Vien sy Nguyin Vdn Hieu nhdn dip Gido su Vijn sy 70 tudi
(hinh 5). Kenh thir nhdt ciia chia quang gdm tin hieu dir lieu va khoa dugc thu bdi dau thu
photodiode PIN va chuyin thanh —^^z' o - ^ tin hieu dien nhu binh thudng rdi
dugc dua vao khuech dai tin hieu dien. Kenh chia quang thu hai
ciing gdm tin hieu dii lieu va , < i '
khoa. Tuy nhien trong trudng hgp ^ " ^ ^' ^^ ^^ ^^ '^^^ ^^'^^ '"""S va thu tm ^^^
Chung ta cdn giir lai xung quang ^'^" ^^^^ ^""^ " ^ ^ ' ' " ^'^'" ^^ ^'^'"
chira tin hieu nen trudc khi dua den dau thu phai lgc bd kenh ma khoa. Trong mot sd nghien cim tren the gidi, ngudi ta thudng sir dyng cac budng cdng hudng Faby - Pero de tach va triet lieu xung quang chira ma khoa va giiJ lai xung mang tin hieu dir lieu. Tren co sd cac ket qua nghien ciru da dat dugc, chiing tdi nhan thay cd the sir dyng each tir sgi quang de loc lay tin hieu dir lieu de dang, thuan tien han ma ket qua van dang tin cay [8,9].
Viec che tao each tiJ sgi quang boat ddng (cho qua tin hieu) trong viing sdng nhu mong mudn vdi do rdng phd rat hep phii hgp cho he thdng tin nay la khdng khd va thyc hien dugc d Viet Nam. Phd truyen qua cua mdt trong cac each tir chiing tdi da sir dung trong he thdng dugc chi ra trong hinh 6.
^: —T; .. T]-"'l:i^: •' Ej Vol cac gia tri: LI = I556.756 nm, L2=l556.860 nm thi do rgng phd truyen qua cua each tir la AL = L2-L1 = 0.104 nm. Nhu vay, khi sir dyng each tir nay de lgc kenh chira tin hieu va kenh chira khoa vdi sy chenh lech budc sdng ciia hai kenh nay trong khoang AT
~ 112 ps khdng anh hudng den sy ddng bo ciia hai tin hieu nay tai dau thu.
Ngoai ra sti dung each tir lam bg lgc quang trong trudng hgp nay rat thuan Igi vi nd phii hgp vdi cac linh kien quang khac trong he thdng tin chuan.
••0.il;rr. ;6r.s:Hlj 1
r
11
Hinh 6. Phd truyen qua ciia cdch tir sgi
quangb. Thu tin hieu nho
Cac ddu thu don gian tren ca sd PIN photodiode cd thi sir dung dl thu tin hieu khda.
De thu cac xung laser yeu, chung tdi da sir dyng ddu thu APD tren co sd vat lieu InGaAs
ciia hang Fujitsu FPD5W1KS vdi thi danh thiing Fg;^ =^48V. Ddu thu nay cd thi lam
viec d che do Geiger mode: khi chua cd xung anh sang din, ddu thu boat ddng d thi thdp
ban the danh Ihiing Vgj^, ngay khi cd xung din thi tai ddu thu dugc nang len cao hon thi
danh thimg K^/^. Sau khi tat mot chu ky cua xung, thi dugc giam di va hieu img thac lu
ciia dau thu cung dirng lai nhu mdt mach dap tdt. Nhu vay, ddu thu APD se lam viec d
TuyIn tap bao cao ky niem 15 nam thanh lap Vien Khoa hgc Vat lieu - Ha Ndi, 6/2008 trang thai an toan ban va ludn san sang cho mdt chu ky nhan xung mdi. Cd hai loai nicach dap tdt sir dung cho dau thu APD la mach dap tat chii dgng va bi dgng [10, 11, 12].
De giam ddng tdi khi dem photon, dau Ihu can hoat ddng d nhiet do thap. Viol bg khulch dai dien trg giiip cho dau thu APD dugc thiet ke d hai trang thai ciia Peltier. Bang each nay, dau thu APD cd the boat ddng d 0°C. Bd khuech dai trg giiip dau thu dya tren hai trang thai ciia mach OPA846 SMC cd he sd khuech dai tdng dat 43 dB vdi bang thdng 110 MHz (binh 7). Tren ca sd be dau thu APD boat ddng d nhiet do thap, da thu dugc cac khda lugng tir va dugc chi ra tren hinli 8
0 20
bl
Hinh 8. Tin hieu khda ban ddu (xung fPinh 7. Anh chup module thu AP vd mdu do) vdtin hieu tai ldi ra ciia ddu mgch khuech dgi cho ddu thu thu (xung mdu xanh)
Tren ca sd hgp ike nghien ciai giua hai phdng thi nghiem, ke qua xay dyng he thi nghiem ve thdng tin quang bao mat tren co sd laser module, khuech dai quang ban dan, each tir lgc va suy hao quang va dau thu tin hieu nhd tdc do cao dugc chi ra tren hinh 9.
Cd the ndi day la mot he thdng tin quang bao mat vdi xung laser yeu (den dan photon) hoan toan quang.
K E T LUAN
Trong cdng trinh nay, nhdm tac gia da dua ra be thir nghiein thdng tin quang bao mat boat ddng d viing sdng 1550 nm tren co' sd cac linh kien quang dimg trong ky thuat thdng tin quang chuan qudc te hien cd. Bang bieu irng bien ddi budc sdng ciia khuech dai quang ban dan (SOA) da tao dugc hai budc sdng gidng nhcau cd do chenh lech < 0.13 nm, dii nhd de ddng bd dugc tai dau thu va khdng bj anh hudng ciia tan sac dudng truyen. Sir dung each tir
sgi quang de Igc (tach) tin hieu khoa mot each thuan Igi va de dang. De dam bao tinh bao mat, chimg tdi da sir dung ngudn laser xung yeu de tam thdi thay the ngudn phat don
Hinh 9. Anh chup hi thdng tin quang bdo mat
Cdng trinh kinh tdng Gido su Viin sy Nguyin Vdn Hiiu nhdn dip Gido su Vien .sy 70 tudi
photon (se cd trong tuong lai). Vi vay, kit qua nghien cuu nay se ddng gdp budc dau cho ITnh vyc thdng tin quang bao mat don photon.
Loi cam on. Cdng trinh da dugc thyc hien tren co sd kinh phi tir Chuang trinh nghien cim Co ban nha nude ma sd 407906 va cac chuang trinh hgp tac khoa bgc nam 2006- 2007 ma so "CNRS/VAST 15649/17615" giira Phdng Laser ban din, Vien Khoa bgc Vat lieu va Phdng SATIE, Vien De L'Amberd, Trudng Dai hgc Su pham Cachan, Phap. Cac tac gia xin chan thanh cam an Lanh dao Vien Khoa hgc Vat lieu da ddng vien va tao dieu kien thuan Igi trong qua trinh nghien cim.
TAI LIEU THAM KHAO
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