Mn hc: Cc chuyn thng tin quang Tiu lun: B pht quang Gio vin hng dn: Ph Gio s Bi Trung Hiu Nhm thc hin: Nhm 15 Lp H08VT2

PHN CNG LM BO CO

STT 1

H V Tn Dng Trng Bng

Phn Lm Bo Co 1. NGUYN L CHUNG V BIN I QUANG IN 1.1- Mc nng lng 1.2- Cc nguyn l bin i quang in 1.3. Vng nng lng 2. B PHT QUANG 2.1. S khi b pht quang 2.2. Chc nng cc khi 2.3. Hot ng ca h thng 2.4. Mch pht iu bin cng trc tip 2.5. B iu ch ngoi

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Nguyn c Tn (Nhm trng)

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Nguyn Quang Hip

Tiu lun: B PHT QUANG MC LC 1. NGUYN L CHUNG V BIN I QUANG IN..............................................31.1. Gii thiu h thng thng tin quang in hnh ...................................................................3 1.2. Nguyn l chung v bin i quang in ..........................................................................4 1.2.1. Mc nng lng (Enrgy Level )...................................................................................4 1.2.2- Cc nguyn l bin i quang in..............................................................................6 1.2.3. Vng nng lng (Energy Band):.................................................................................7 1.2.4. Ngun quang bn dn (Semiconductor Light Source).................................................8

2. B PHT QUANG.....................................................................................................102.1. S khi...........................................................................................................................10 2.2. Chc nng cc khi: ..........................................................................................................10 2.3. Hot ng ca h thng:.....................................................................................................10 2.4. Mch pht iu bin cng trc tip............................................................................11 2.5 B iu ch ngoi................................................................................................................13

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Tiu lun: B PHT QUANG 1. NGUYN L CHUNG V BIN I QUANG IN 1.1. Gii thiu h thng thng tin quang in hnh S khi c bn h thng thng tin quang

Hnh 1.1 M hnh ca mt h thng thng tin quang M hnh c bn ca mt h thng thng tin quang. Ni chung, tn hiu in t my in thoi, t cc thit b u cui, s liu hoc Fax c a n b E/O chuyn thnh tn hiu quang, sau gi vo cp quang. Khi truyn qua si quang, cng sut tn hiu (nh sng) b suy yu dn v dng sng b rng ra. Khi truyn ti u bn kia si quang, tn hiu ny c a vo b O/E to li tn hiu in, khi phc li nguyn dng nh ban u m my in thoi, s liu v Fax gi i. Cu trc c bn ca mt h thng thng tin quang c th c m t n gin gm: - B pht quang. - B thu quang. - Mi trng truyn dn l cp si quang. Nu c ly thng tin qu di th trn tuyn c th c mt hoc nhiu trm lp (Repeater) Chc nng cc khi: Khi E/O: b pht quang c nhim v nhn tn hiu in a n, bin tn hiu in thnh tn hiu quang, v a tn hiu quang ny ln ng truyn (si quang). l chc nng chnh ca khi E/O b pht quang. Thng ngi ta gi khi E/O l ngun quang. Hin nay linh kin c s dng lm ngun quang l LED v LASER. Khi O/E: khi tn hiu quang truyn n u thu, tn hiu quang ny s c thu nhn v bin tr li thnh tn hiu in nh u pht. l chc nng ca khi O/E b thu quang. Cc linh kin hin nay c s dng lm chc nng ny l PIN v APD, v chng thng c gi l linh kin tch sng quang (photo-detector). Trm lp: khi truyn trn si quang, cng sut tn hiu quang b suy yu dn (do si quang c suy hao). Nu c ly thng tin qu di th tn hiu quang ny c th3

Tiu lun: B PHT QUANG khng n c u thu hoc n u thu vi cng sut cn rt thp u thu khng nhn bit c, lc ny ta phi s dng trm lp (hay cn gi l trm tip vn). Chc nng chnh ca trm lp l thu nhn tn hiu quang suy yu, ti to chng tr li thnh tn hiu in. Sau sa dng tn hiu in ny, khuch i tn hiu sa dng, chuyn i tn hiu khuch i thnh tn hiu quang. V cui cng a tn hiu quang ny ln ng truyn truyn tip n u thu. Nh vy, tn hiu ng vo v ng ra ca trm lp u dng quang, v trong trm lp c c khi O/E v E/O. 1.2. Nguyn l chung v bin i quang in 1.2.1. Mc nng lng (Enrgy Level ) Qu trnh bin i nng lng in thnh nh sng v ngc li trong cc linh kin bin i quang in c th c gii thch da trn tnh cht ht ca nh sng v tnh cht lng t ca vt cht. Theo tnh cht ht ca nh sng,bao gm nhiu ht photon. Mi photon mang mt nng lng nht nh c xc nh bng cng thc sau : Eph = hf Trong : h l hng s Plank (h= 6,625x10-34J.s); f l tn s ca photon nh sng. Nng lng ca nh sng bng tng nng lng ca cc photon (Nph): Enh sng = Nph x Eph Theo tnh cht lng t ca vt cht, vt cht c cu to t cc nguyn t. Mi nguyn t gm c mt ht nhn, mang in tch dng, c bao quanh bi cc in t (electron), mang in tch m. Cc in t ny quay quanh ht nhn theo cc qu o n nh (hnh ) v mang mt mc nng lng nht nh. Cc mc nng lng ny l khng lin tc. Mt in t ch c th mang mt trong cc mc nng lng ri rc ny. Khi in t thay i trng thi nng lng th n s chuyn t qu o ny sang qu o khc khi quay quanh ht nhn. Trng thi nng lng ca in t trong mt nguyn t c minh ho qua biu mc nng lng (energy level diagram).

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Tiu lun: B PHT QUANG Hnh 1.2 Biu mc nng lng (energy level diagram) Trong biu mc nng lng, trng thi nng lng thp nht ca in t c gi l trng thi nn (ground state) E0. y l trng thi n nh ca in t. Cc trng thi nng lng cao hn ca in t c gi l cc trng thi kch thch (excited state) c biu din bng cc mc nng lng E1, E2, E3, Cc mc nng lng ny khng lin tc v cch nhau mt khong nng lng c gi l di cm nng lng (energy gap) Eij = Ej Ei (i,j = 0,1,2,3 ). Nh vy, nng lng mt in t c th l E0, E1, E2 ch khng nm gia cc mc nng lng ny. Mt in t cc trng thi nng lng khc nhau ph thuc vo nhit v c th c biu din bng hm phn b Boltzmann [2]: Ni = N0eexp[Ei / k BT ] Trong , Ni v N0 l mt in t mc nng lng Ei v mc nng lng nn E0, Ei l chnh lch nng lng gia Ei v E0; kB= 1,38.10-23 (J/oK) l hng s Boltzmann, T l nhit tuyt i (oK). Phn b nng lng xy ra khi cn bng nhit

Hnh 1.3 Phn b mt in t Boltmann khi cn bng v nhit Mt in t cc mc nng lng khc nhau n nh khi cn bng v nhit v ng thi vi qu trnh hp th nng lng ca in t (lm tng mt in t cc mc nng lng cao v gim mt in t mc nn) l qu trnh tr v trng thi nng lng nn ban u ca cc in t khi cc mc nng lng cao. Hin tng ny xy ra v trng thi nn l trng thi nng lng bn vng ca in t. Cc in t khi cc trng thi nng lng kch thch lun c xu hng chuyn v cc trng thi nng lng thp hn sau mt khong thi gian duy tr trng thi kch thch. Khong thi gian ny c gi l thi gian sng (lifetime) ca in t. Thi gian sng thay i ty theo loi vt cht, thng thng trong khong vi nano giy n vi micro giy. Khi in t chuyn t trng thi nng lng cao xung trng thi nng lng thp hn, theo nh lut bo tan nng lng, n s gii phng mt phn nng lng bng ng 5

Tiu lun: B PHT QUANG chnh lch gia hai mc nng lng. Nng lng c gii ta ny c th di dng nhit hoc nh sng. Qu trnh ny din ra t nhin v ngu nhin v y l bn cht t nhin ca vt cht. Khi in t chuyn t trng thi nng lng cao xung trng thi nng lng thp hn, theo nh lut bo tan nng lng, n s gii phng mt phn nng lng bng ng chnh lch gia hai mc nng lng. Nng lng c gii ta ny c th di dng nhit hoc nh sng. Qu trnh ny din ra t nhin v ngu nhin v y l bn cht t nhin ca vt cht. 1.2.2- Cc nguyn l bin i quang in Nguyn l bin i quang in trong thng tin quang c thc hin da trn 3 hin tng c minh ho nh trn hnh 1.4

Hnh1.4 Cc hin tng bin i quang in ,(a). Hp th ;(b) pht x t pht (c). pht x kch thch Hin tng hp th (absorption), hnh 1.4.a, xy ra khi mt photon c nng lng hf b hp th bi mt in t trng thi nng lng thp E1. Qu trnh ny ch xy ra khi nng lng hf ca photon bng vi chnh lch nng lng gia mc nng lng cao v mc nng lng thp ca in t (Eg = E2 E1). Khi xy ra hin tng hp th, in t s nhn nng lng t photon v chuyn ln trng thi nng lng cao. Hin tng pht x t pht (spontaneous emission), hnh 1.4.b, xy ra khi mt in t chuyn trng thi nng lng t mc nng lng cao E2 xung mc nng lng thp E1 v pht ra mt nng lng Eg= E2 E1 di dng mt photon nh sng. Qu trnh ny xy ra mt cch t nhin v trng thi nng lng cao E2 khng phi l trng thi nng lng bn vng ca in t. Hin tng pht x kch thch (stimulated emision), hnh 1.4.c, xy ra khi mt in t ang trng thi nng lng cao E2 b kch thch bi mt photon c nng lng hf bng vi chnh lch nng lng gia trng thi nng lng cao v trng thi nng lng thp ca in t (Eg= E2 E1). Khi , in t s chuyn t trng thi nng lng cao xung trng thi nng lng thp hn v to ra mt photon c nng lng bng vi nng lng ca photon kch thch ban u. Nh vy, t mt photon ban u sau khi khi xy ra hin tng pht x kch thch s to ra hai photon (photon ban u v photon mi c to ra). Photon mi c to ra c c im: cng tn s, cng pha,6

Tiu lun: B PHT QUANG cng phn cc v cng hng truyn vi photon kch thch ban u. y l cc c im ca tnh kt hp (coherent) ca nh sng. Do vy, nh sng do hin tng pht x kch thch to ra c tnh kt hp. Hin tng pht x t pht xy ra t nhin do cc in t lun c khuynh hng chuyn t trng thi nng lng cao xung trng thi n nh c nng lng thp hn. Cc photon nh sng do hin tng pht x t pht to ra mt cch ngu nhin theo thi gian v khng gian. Do , pha, tn s, hng truyn cng nh phn cc ca sng nh sng c to ra cng ngu nhin. V vy, nh sng do hin tng pht x t pht to ra khng c tnh kt hp. Hnh 1.5 minh ho cc trng thi thay i nng lng ca in t khi xy ra 3 hin tng bin i quang in nu trn.

Hnh 1.5 Minh ho cc trng thi chuyn i nng lng gia in t v photon Cc linh kin bin i quang in dng trong thng tin quang s da vo mt trong cc hin tng ny thc hin qu trnh bin i quang in theo chc nng ca tng loi linh kin. V d nh: - Hp th l nguyn l bin i tn hiu nh sng thnh tn hiu in trong cc linh kin tch sng quang (photodetector). - Pht x t pht l nguyn l bin i tn hiu in thnh tn hiu nh sng xy ra trong LED (Light Emitting Diode). V vy, nh sng do LED pht ra khng c tnh kt hp. - Pht x kch thch l nguyn l bin i tn hiu in thnh tn hiu quang xy ra trong Laser (Light Amplification by Stimulated Emission of Radiation). Hin tng ny cng l nguyn l khuch i tn hiu nh sng xy ra trong cc b khuch i quang (Optical Amplifier). nh sng uc to ra sau Laser cng nh cc b khuch i quang c tnh kt hp. Trong t nhin, c ba hin tng ny xy ra ng thi vi nhau. Qu trnh hp th nng lng photon (hin tng hp th) v pht x photon (hin tng pht x t pht v pht x kch thch) cn bng vi nhau trong iu kin cn bng v nhit (Khi , phn b in t cc mc nng lng khc nhau l n nh theo phn b Boltzmann). 1.2.3. Vng nng lng (Energy Band):

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Tiu lun: B PHT QUANG Trong cht bn dn (cng nh trong cht rn ni chung), cc mc nng lng vn ri rc nhng chng rt l gn nhau v c xem nh mt vng nng lng hn l mt nhm cc mc nng lng. Khi nim ny c th c minh ho trong hnh 1.6 khi mt khong lin tc ca vng nng lng khi c phng to ln s cho thy n c cu to bi cc mc nng lng ri rc nhau .

Hnh 1.6 Vng nng lng ca cht bn dn Trong cht bn dn, cc in t phn b trong hai vng nng lng tch bit nhau c gi l: vng ha tr (valence band) v vng dn (conduction band). Vng ha tr l vng nng lng c nng lng thp v l vng nng lng bn vng ca in t. Cc in t lun c xu hng chuyn v vng ha tr sau mt khong thi gian sng vng dn. Vng dn l vng nng lng cao hn ca cc eletron. S dn in ca cht bn dn c thc hin bi cc in t nm vng dn ny. Qu trnh bin i quang in xy ra trong cht bn dn cng c gii thch da trn ba hin tng: hp th, pht x t pht v pht x kch thch nh trong biu mc nng lng. Ngha l, iu kin mt in t c th chuyn t trng thi nng lng thp (vng ha tr) sang trng thi nng lng cao (vng dn) l: nng lng m in t nhn c phi bng vi chnh lch nng lng gia hai vng nng lng ha tr v vng dn. Nu nng lng c cung cp khng bng vi bt k chnh lch nng lng no gia hai vng nng lng ny th qu trnh hp th cng nh pht x kch thch s khng xy ra. 1.2.4. Ngun quang bn dn (Semiconductor Light Source) Ngun quang l linh kin bin i tn hiu in thnh tn hiu nh sng c cng sut t l vi dng in chy qua n. C hai loi ngun quang c s dng trong thng tin quang: - Diode pht quang LED (Light Emitting Diode) - Laser (Light Amplification by Stimulated Emission of Radiation) Cc yu cu i vi mt ngun quang s dng trong h thng thng tin quang l: - C kch thuc nh tng ng vi si quang c th ghp nh sng vo trong8

Tiu lun: B PHT QUANG si quang. L tng, nh sng ng ra ca ngun quang phi c tnh nh hng cao. - Thu nhn tn hiu in ng vo mt cch chnh xc gim s mo dng v nhiu ln tn hiu. L tng, ngun quang phi tuyn tnh. - Pht ra nh sng c bc sng ph hp vi vng bc sng m si quang c suy hao thp v tn sc thp, ng thi linh kin thu quang hot ng hiu qu ti cc bc sng ny. - C kh nng iu ch tn hiu mt cch n gin (v d nh iu ch trc tip) trn di tn rng tri di t tn s m thanh ti di tn gigahezt. - Hiu sut ghp quang tt gim suy hao ghp t ngun vo trong si quang. - rng ph hp gim tn sc trong si quang. - Duy tr mc cng sut ng ra n nh v khng b nh hng nhiu bi cc yu t mi trng bn ngoi. - Gi thnh thp,c tin cy cao cnh tranh vi cc k thut truyn dn khc. Loi ngun quang c s dng trong thng tin quang l cc loi ngun quang bn dn v c th p ng c cc yu cu trn. Tuy nhin, khng phi cht bn dn no cng c s dng ch to ngun quang trong thng tin quang. c th c s dng trong thng tin quang, cc cht bn dn cn phi c di cm nng lng trc tip v rng ca di cm nng lng ph hp sao cho c th to ra nh sng c bc sng nm trong vng bc sng hot ng ca thng tin quang. ng lng ca l trng (nm vng ha tr) [1], [3]. Khi , ng lng ca in t c bo tan. Nh vy c th thy rng, iu kin v bo tan ng lng khi xy ra qu trnh bin i quang in ch t c vi cc cht bn dn c di cm trc tip. Khi , nng lng c pht ra khi cc in t chuyn t trng thi nng lng cao (vng dn) sang trng thi nng lng thp (vng ha tr) s to ra cc photon. Vi hiu sut pht x nh sng (pht x t pht v pht x kch thch) ln, cc cht bn dn c di cm trc tip c th to ra cc ngun quang c cng sut pht quang ln, hiu qu. Ngc li, i vi cc cht bn dn c di cm nng lng gin tip, cc nng lng c to ra do qu trnh chuyn trng thi nng lng ca in t s pht ra di dng phonon, khng pht x (nonradiation). Nng lng ny c th l nng lng nhit hay dao ng ca cc phn t. Cht bn dn c s dng ch to ngun quang cn phi c: di cm trc tip v nng lng chnh lch gia vng dn v vng ha tr phi ph hp c th to ra bc sng nm trong cc ca s bc sng hot ng trong thng tin quang.

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Tiu lun: B PHT QUANG 2. B PHT QUANG 2.1. S khi D liu Xung ng h Bin i d liu iu khin Laser

iu khin nh thin v iu ch

iu khin nhit

Lm mt

PD LD

Ghp cp quang knh quang

Hnh 2.1: S khi b pht quang in hnh trong truyn dn s 2.2. Chc nng cc khi: - Khi bin i d liu: M ha tn hiu d liu dng in u vo, bin i tn hiu d liu in t dng song song sang dng m NRZ ni tip. - Khi iu khin Laser: Cung cp dng in v in p phn cc cho Laser Diot. - Khi iu khin nh thin v iu ch: gi cho Diot Lazer hot ng n nh ng thi gim st v cnh bo cng sut u ra. - Khi iu khin nhit : Gim st v cnh bo v nhit ca Laser Diot ng thi iu khin b lm mt cho Laser Diot. - Khi ghp knh quang: ghp cc knh tn hiu quang, ngoi ra n cn phc v cho mc ch gim st knh quang. - Laser Diot: bin i tn hiu in sang tn hiu quang nh sng. - Photodiot: nhn mt phn tn hiu quang t Laser Diot bin i sang tn hiu in. 2.3. Hot ng ca h thng: Tn hiu d liu in u vo c a vo b bin i d liu nh vo xung kch Clock. Ti y d liu c bin i v dng ph hp cung cp cho mch iu khin Laser to ra dng phn cc iu khin Laser Diot. Tn hiu in c a ti Laser Diot bin i thnh tn hiu quang v a ln ng truyn cp quang.10

Tiu lun: B PHT QUANG Trong qu trnh bin i in/ quang, nhit bn trong khi thay i s lm thay i cc tnh cht ca ngun quang nh bc sng pht quang v cng sut pht quang. Do hn ch iu ny, trong b pht quang s dng Laser Diot ngi ta s dng mch iu khin cng sut dng Photodiot n nh cng sut quang u ra ca b pht quang. Photodiot s thu mt phn nh sng pht ra t Laser Diot to ra dng quang in (t l vi cng sut pht quang) iu khin dng nh thin v cng sut tc thi u vo. iu ny lm cho tn hiu quang u ra c gi tr cng sut pht n nh. Ngoi ra khi nhit tng ln, b iu khin nhit s nhn tn hiu v truyn sang b phn lm mt cho Laser. 2.4. Mch pht iu bin cng trc tip Mt mch pht quang iu bin cng c biu din trn hnh sau.

Hnh 2.2: Mch pht quang s dng LD in hnh Mch pht quang iu bin cng l s kt hp ca mch iu khin v mch iu ch tn hiu. Hot ng ca mch pht quang iu bin cng c th c phn tch da trn hot ng ca mch iu khin v mch iu ch tn hiu.

Hnh 2.3: Mch iu khin

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Tiu lun: B PHT QUANG

Hnh 2.4: Mch iu ch tn hiu Tc dng linh kin - Mch iu khin: R: in tr hi tip cho mch khuch i thut ton. OP: IC khuch i thut ton. VRBias : chit p iu chnh in p iu khin. Q4: Q4 kt hp vi OP dng khuch i thut ton, n nh dng qua LD. R1 : hi tip m dng in 1 chiu n nh im lm vic. LD: Laser diode pht tia sng laze PD: photodiode thu mt phn tn hiu ca LD iu khin dng nh thin v iu khin cng sut tc thi. - Mch iu ch tn hiu: Q1, Q2: to thnh mch khuch i vi sai. Q3: cp ngun dng n nh cho mch vi sai. R2, VRdrive: l phn t b nhit, n nh nhit cho Q3 Hot ng: Mch iu khin c chc nng bin i ngun in p t b bin i d liu v dng dng in cung cp dng phn cc cho LD. V dng phn cc cho LD c to ra cn phi rt n nh vi dng in ngng c th truyn tn hiu d liu khng b li. Trong mch iu khin , in p iu khin Vbias l in p ng vo ca OP, dng in chy qua in tr R ch ph thuc vo in p ng vo m khng ph thuc vo in p tr ti ca LD. Do , bng cch thay i Vbias , ngi ta c th iu khin dng phn cc Ibias.khi nhit thay i, vic n nh cng sut Quang ng ra ca LD c12

Tiu lun: B PHT QUANG thc hin bi tn hiu hi tip t PD. PD ny thu nh sng t laser pht ra v to ra dng quang in t l vi cng sut pht quang ca laser. V vy khi cng sut ng thay i, do s thay i ca nhit , dng quang in s thay i lm cho dng in phn cc Ibias cng thay i theo nhm b li nhng thay i trong cng sut quang ca laser. Trong mch iu ch , qu trnh iu ch c iu khin bi dng phn cc qua laser. Chc nng chnh ca mch l cung cp dng phn cc cc i cho laser, d liu pht c a vo cc B ca Q1, cc B ca Q2 c c nh bi ngun phn cc VBB. Khi tn hiu ng vo ln hn VBB, Q1 dn Q2 tt, dng qua LD gim lm cho LD ngng pht. Ngc li, khi tn hiu ng vo nh hn VBB, Q1 tt Q2 dn , dng qua LD tng ln lm LD pht sng. Q3 ng vai tr cung cp ngun dng n nh cho mch vi sai Q1 v Q2. Q4 kt hp vi mch hi tip dng khuch i thut ton n nh dng qua LD di tc ng ca nhit cng nh cung cp tn hiu cho vic gim st nhit lm vic ca LD phc v cng vic cnh bo v bo dng cho b pht quang 2.5 B iu ch ngoi - S khi b iu ch ngoi

Hnh 2.5: S khi b iu ch ngoi - Chc nng cc khi: PD: photodiode thu mt phn tn hiu ca LD a ti cc in t li tia laser. LD: Laser diode pht tia sng laser. Laser-driving electronics: cc in t li tia laser iu khin cng sut pht ca LD. Extemal modulator: b iu ch ngoi. Interface electronics: cc giao din in t. - Nguyn l hot ng : nh sng do laser pht ra di dng sng lin tc CW light cp cho b iu ch ngoi, mt khc PD thu mt phn nh sng do LD pht ra a ti khi li tia laser, khi ny c nhim v gim st v iu khin cng sut pht ca LD, Do tc dng ca nhit13

Tiu lun: B PHT QUANG lm cho LD pht khng n nh dn n tn hiu quang c pht ra khng chnh xc, v vy khi li tia laser iu khin sao cho LD pht n nh. nh sng CW c a ti b iu ch ngoi , khi ny c nhim v iu ch tn hiu ph hp v c a n khi tip theo. ng thi tn hiu d liu c a vo khi giao din in t , khi ny l thit b gim st, bo hiu v iu khin cho b iu ch ngoi nhm iu ch nh sng thch hp.

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Tiu lun: B PHT QUANG KT LUN Trn y l nguyn l bin i quang in, s khi hot ng ca b pht quang vi mt mch pht quang s dng Laser diot in hnh. Do nhng hn ch v thi gian v nng lc bn thn c hn nn bi tiu lun cn c nhiu thiu st. Nhm chng Em rt mong nhn c nhng kin ng gp ca thy c gio v cc bn bi tiu lun c th y v hon thin hn. Nhm chng Em xin chn thnh cm n!

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