may thu quang hc

8/3/2019 May Thu Quang Hc

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TKMH :H THNG THNG TIN QUANG GVHD: PHM HNG QUN

MC LCI. LI GII THIUII. B TCH SNG PH-T-I T P-I-N

1. B tch sng ph-t-i t p-i-n2. Thi gian p ng v dng ph t vng tri ca b tch sng quang

a.Thi gian p ngb. Dng ph t vng tri

3. Ph t i t thc APD4. Vt liu ch to ph t i t

III.T S TN HIU TRN NHIU CA B TCH SNG QUANG1. Cc ngun nhiu ca b tch sng quang2. T s tn hiu trn nhiu

IV. B THU QUANG1. Cu hnh b thu quang

2. Cc ngun li trong b thu quangV. NHY CA B THU QUANG

1. Gii thiu v nhy ca b thu quang2. nhy thu quang v t s li bit ca b thu quang

a. T s li bit trong b thu quangb. nhy thu ca b thu quangc. Cc tham s nh hng n nhy ca b thu quang

VI. CU TRC MCH B THU QUANG1. Cc mch khch i FET tr khng cao

2. Cc b khch i transistor lng cc tr khng cao3. B khch i h dn ngc4. c tnh b thu quang c mch tch hp

VII.CC MY THU QUANG HIN NAY

*****************

NHM 3 -1- K THUT VIN THNG K47


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I. GII THIU- Thit b thu quang, hay cn gi l b thu quang, l mt trong nhng

b phn quan trng nht trong h thng thng tin quang v n v tr saucng ca t chc h thng truyn dn ni m thit b ny thu nhn mi ctnh tc ng trn ton tuyn a ti, v cng v th cho nn hot ng can c lin quan trc tip ti cht lng ton b h thng truyn dn. Chcnng chnh ca n l bin i tn hiu quang thu c thnh tn hiu in.Thit b thu quang cn c nhy thu cao, p ng nhanh, nhiu thp, githnh h v bo m tin cy cao.

- Ti b thu quang, sng tn hiu quang t pha pht i ti, c bini thnh tn hiu in, ri c khuych i v phc hi tr li thnh tnhiu cng dng nh u vo thit b pht quang. Tn hiu quang c bini thnh tn hiu in ti b bin i quang in (O/E). B bin i quangin thng l mt b tch sng ph-t-it. y l mt b tch sng theolut bnh phng v n bin i cng sut quang thu c trc tip thnhdng in (dng photo ti u ra ca n). V th m b thu kiu ny cgi l b thu tch sng trc tip DD (Direct Detection). Tn hiu quang t

pha pht i vo si quang s d b suy hao dn, v mo tng ln theo di c ly truyn dn do tc ng ca tn x, hp th v tn sc trong si dnquang. V vy, b thu quang phi lm vic trong iu kin gp nhiu cc yut tc ng. vic thit k thit b thu quang s kh khn hn rt nhiu so vi

thit b pht quang. B thu phi m bo thu c tn hiu rt yu, b mo vphi tch c cc thnh phn nhiu kh ln so vi tn hiu. trong cc b thuquang s thc t, tn hiu in yu thu c ti u ra b tch song s ckhuych i, cn bng ti cc b khuych i in v b cn bng tngng. Cui cng, tn hiu s c phc hi ti mch quyt nh.

- c c mt tuyn truyn dn di vi tc bit ln, b thu quangcn phi tha mn nhng yu cu chnh sau y:

+ C t s tn hiu trn nhiu SNR (Signal to Noise Ration) ln v nhy thu cao.+ Hot ng trong iu kin tn hiu c bng tn ln.

II.B TCH SNG PH-T-IT1. B Tch Sng Ph-t-it p-i-n- y l b tch sng quang c s dng rng ri nht. Mt ph-t-it

thng thng c cu trc gm cc vng p v n cch nhau bi mt vng i. thit b hot ng th phi cp mt thin p ngc cho n. Trong ch hot



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ng bnh thng, thin p ngc ln c t ct ngang thit b chovng bn trong m bo hon ton tri c cc ht mang.

Hnh 1 :Mch in v s vng nng lng cho ph-t- i t p-i-n-Do cu trc c bn bn trong ca n, lp i nm gia c tr khng

cao v hu ht in p t vo phn ngang ca n. Kt qu l c mt intrng ln tn ti trong lp i. Khi c mt photon i ti m mang mt nnglng ln hn (hoc bng) vi nng lng vng cm ca vt liu bn dndng ch to ph-t-it,photon ny c th b ra nng lng ca n vkch thch mt in t t vng ha tr sang vng dn. Qu trnh ny s pht

ra cc cp in t-l trng t do, cc cp ny ch yu c pht ra trongvng tri v c gi l cc ht mang photo hoc cc in t photo.

-S pht nh sng c hp th trong vt liu tng ng vi mt hmm sau y:

P(x) = Pin(1-e- s ()x) (2.1)



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s l h s hp th ti bc sng , Pin l mc cng sut quang ti ph-t-it v P(x) l cng sut quang c hp th c ly x.H s hp th ph thuc vo mt vi loi vt liu bn dn thng

c s dng ch to ph-t-it. Bc sng ct pha trn c m ti bng khng c gi l bc sng ct, t vt liu c th c s dngcho b tch sng khi c< . Bc sng ct trn c xc nh t nng lngdi cm Eg ca vt liu. Khi Eg c din gii bng n v in-Von(eV) v

c c tnh bng micrmet ( m ) th ta c:

gg

cEE

hc 24.1== (2.2)

Vi c l vn tc nh sng, h l hng s Planck. Bc sng ct ca Sivo khong 1,06 m v ca Ge vo khong 1,6 m . i vi bc sng dihn, nng lng ca photon khng kch thch in t t vng ha trsang vng dn. i vi bc sng ngn hn, p ng photo s ct xung v

th cc gi tr ca s ti bc sng ngn hn l rt ln. Trong trng hpny cc photo c hp th rt gn vi b mt ca b tch sng, ni m thigian ti hp ca cc cp in t-l trng l rt ngn.

Nu nh vng tri c rng w, cng sut tng c hp th c ly ws l:

P(w) = Pin(1 - e- ws ) (2.3)Khi tnh n s phn x Rf ti li vo b mt ph-t-it, dng

photo ban u Ip c t s hp th cng sut cng thc 2.3 c vit nhsau:

PI = hve

inP ( wse 1 )( fR1 ) (2.4)

e l in tch in t, hv l nng lng photon, h = 6.625 x 10-34 Js lhng s Planck, v v l tn s ca sng nh sng.

Khi ni n cc tham s ca ph-t-it, c hai tham s rt quan trngphi k n. Th nht, mi mt ph-t-it c mt hiu sut lng t can. Hiu sut lng t l t s gia cc cp in t-l trng c pht ra vs cc photon mang nng lng hv i ti v c vit nh sau:

hvPeI

in

p

/

/

= (2.5)

Vi Ip l dng photo c gi tr trung bnh, dng ny c pht ra t cngsut quang trung bnh Pin trong ph-t-it. Khi c 100 photo n ph-t-it c th to ra t 40 n 95 cp in t-l trng. Vy ta c th ni rng

ph-t-it c hiu sut lng t t 40 n 95%. c c hiu sut lng



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t cao, vng tri phi dy hu ht cc photon i ti c hp th trongvng ny.

Tham s th hai l h s chuyn i dng photo R cn gi l p ng R.c tnh ca ph-t-it c c trng bi p ng R ny:

=R hve

P

I

in

p

= (2.6)Tham s ny rt hay c s dng n v n c trng cho dng photo

pht ra trn mt n v cng sut quang.C hiu sut lng t v R u ph thuc vo di cm vt liu, bc

sng cng tc, dy ca cc vng p, n, i ca ph-t-it. Khi thit b cvng tri dy, hiu sut lng t ca n s cao. Tuy nhin, vng tri cngdy th th cc ht mang photo pht ra cng phi cn thi gian tri di hnngang qua tip gip phn cc ngc. V thi gian tri ca cc ht mang xcnh tc p ng ca ph-t-it, nn cn phi dung ha gia hiu sut

lng t v tc p ng. i vi cc bn dn c vng cm gin tip nhSi v Ge, rng vng tri w nm trong khong 20-50 m m bo hiusut lng t hp l. Bng tn ca cc ph-t-it b gii hn do thi gianchuyn dch tng i lu. i vi cc ph-t-it c ch to t vt liuc di cm trc tip nh vt liu ghp InGaAs, rng vng tri w c thnh ti 3-5 m , v v th m rng bng c ci thin.

Hiu sut lng t ca ph-t-it thng l c lp vi mc cng sut vo b tch sng ti nng lng photon cho. Vy th p ng l mthm tuyn tnh ca cng sut quang. iu c ngha l dng photo IP s t

l trc tip vi cng sut quang Pin i vo ph-t-it, vy th p ng R lhng s ti bc sng hoc gi tr hv cho. Cui cng p ng R l mthm ca bc sng v vt liu ph-t-it. Tuy nhin h s lng t khngl mt hng s mi bc sng, v n thay i theo nng lng photon.

c tnh ca cc ph-t-it p-i-n c th c ci thin ng k bngcch s dng cu trc d th kp. Tng t nh cu trc ca lazer LD bndn, lp i gia c kp gia cc lp cht bn dn khc nhau p v n vidi cm c chn sao cho nh sng c hp th ch trong lp i. Cutrc ph-t-it kiu ny thng s dng InGaAs lm lp gia v InP lmlp p v n bao quanh. Lp InGaAs gia s hp th mnh bc sng

vng 1.3-1.6 m . Mt trc thng c ph bng lp cch in ph hp gim phn x ti mc nh nht. Hiu sut lng t hu ht l t c gnnh 100% t InGaAs vi dy 4-5 m . Cc ph-t-it s dng InGaAs lhon ton ph hp cho cc b thu quang thc t trong cc h thng thng tinquang.



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0

0 .2

0 .4

0 .6

0 .8

1 .0

0 .7 0 .9 1 .1 1 . 3 1 . 5 1 . 7

I n G a A s

9 0%

7 0%

5 0%

3 0%

G a

S i

1 0 %

B u o c S o n g

1 0 %

Dapung

A / W

Hnh 2: Hiu sut lng t v p ng l cc hm s ca bc sng vicc vt liu lm ph-t- i t khc nhau

Bng 4.1 Cc c tnh ca cc ph-t-it p-i-n tiu biuPh-t-it p-i-n cng c pht trin vi cc n lc ca cng ngh tintin. T 1990 n nay, cc c gng ng n i theo hng pht trin cc

ph-t-it p-i-n tc cao c kh nng hot ng trn 10Gbit/s. Nm 1995,cc ph-t-it c ra mt vi bng tn 110GHz. Mt vi k thut c nghin cu ci thin hiu sut ca ph-t-it tc cao.Hc cng


Tham s n v Si Ge InGaAsBc sng p ng RHiu sut lng t

Dng ti IdThi gian ln TrBng tn fThin p Vb

mA/W

%nAns

GHzV

0,4-1,10,4-0,679-901-100,5-10,3-0,650-100

0,8-1,80,5-0,750-5550-5000,1-0,50,5-36-10

1,0-1,70,6-0,960-701-200,05-0,51-55-6


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hng Fabry-Perot (FP) c to xung quanh cu trc p-i-n l lm tnghiu sut lng t. Ph-t-it hin nay c th t c hiu sut lng tgn 100% l nh vic to thnh cng mt gng trong hc FP bng bin

php to di phn x Bragg trong lp AlGaAs/AlAs. Cu trc ny c th chophp to c cc ph-t-it vi bng tn rt cao trn 100GHz m vn chiu sut lng t cao.

Mt cch khc to ra cc ph-t-it tc cao l s dng mt ngdn sng quang ghp cnh tn hiu quang. Tri vi lazer bn dn, ng dnsong ny c th c lm rng h tr cc mode ngang nhm ci thinhiu sut ghp. Hiu sut lng t y c th t gn 100% ngay c khi clp hp th cc mng, v bng tn c th t ti 110GHz bng cch phngto ng dn song mushroom-meza. c tnh ca cc ph-t-it cn cth c ci thin hn na bng cch phng to cu trc in cc nhm htr cho cc sng in lan truyn vi tr khng ph hp trnh cc phn x.

Cc ph-t-it nh vy c gi l cc ph-t-it sng chy. Cc ph-t-it lm t GaAs da trn cu trc ny c th c bng tn trn 170GHz vhiu sut lng t t trn 50% khi m ng dn sng rng m1 v trkhng c tnh l 50.

2.Thi gian p ng v dng photo vng tri ca b tch sng quanga. Thi gian p ng

- Thi gian p ng l mt yu t quan trng ca b tch sng quang xcnh kh nng lm vic ca b thu quang vi cc tc khc nhau ca hthng truyn dn. Thi gian p ng ca ph-t-it cng vi cc mchin u ra ca n ph thuc ch yu vo 3 yu t sau:

+ Thi gian chuyn dch ca cc ht mang photo trong vng tri+ Thi gian khuych tn ca cc ht mang photo c pht tn ra bn

ngoi vng tri.+ Hng s thi gian RC ca ph-to-it v cc mch in lin quan ca

n.- Cc tham s c lin quan ca ph-t-it i vi cc tham s ny l h

s hp th s, rng vng tri w, cc in dung tip gip ph-t-it vng v, in dung b kkhuch i in, in tr ti b tch sng, in tru vo b khuch i, in tr ni tip ca ph-t-it. Trong cc b tch

sng thc t, in tr ni tip thng rt nh v c th b qua khi so viin tr ti v in tr u vo b khuch i.

- Trc ht ta xem xt thi gian dch chuyn ca cc ht mang phototrong vng tri. Thi gian chuyn dch cha cc ht mang photo s lm hnch tc p ng ca ph-t-it. N l thi gian cn thit i vi cc htmang photo i ngang qua vng tri. Thi gian chuyn dch ny ph thuc



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vo vn tc tri ht mang v rng vng tri. Nu ta gi thi gian chuyndch la Tt, vn tc tri ht mang l Vd v rng vng tri l w th ta c:

tTdV

w= ( 2.7)

- Trong thc t, trng in trong vng tri nhn chung l ln cc ht mang t c vn tc gii hn tn x ca chng. i vi Si, tc ln nht ca cc in t l 8.4 x 106 cm/s v ca l trng l 4.4 x 106 cm/skhi cng in trng mc 2 x 104 V/cm. Ph-t-it Silic tc caoc rng vng tri in hnh l 10 nn c gii hn thi gian p ng vokhong 0,1ns.

1 0 %

9 0 %

H n h 4 . 1 2

T h o i g i a nTr Tr

9 0 %

1 0 %

Dapungdienappho-to-d

iot

Hnh 3 : p ng ph-t it cho xung quang u vo

- Qu trnh khuych tn l chm so vi s tri ca cc ht mang trongvng c in trng cao. V vy, c c ph-t-it tc cao, cc htmang photo cn phi c pht ra vng tri hoc gn vi vng ny chothi gian khuch tn l t hn hoc bng vi thi gian tri ht mang. nhhng ca thi gian khuych tn di c th c th c xem xt thng quathi gian p ng ph-t-it. Thi gian p ng ph-t-it c m t

bng thi gian ln v xung ca u ra b tch sng khi b tch sng tip



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cn mt xung tn hiu quang u vo. Trong hnh 4.3a, chng ta gi thigian ln Tr thng c tnh bt u t 10% n 90% ca sn trc (snln) ca xung u ra, thi gian xung Tf thng c xc nh t im 90%n 10% ca sn sau (sn xung) ca xung u ra. cc ph-t-itngho hon ton, thi gian ln Trv thi gian xung Tf l gn nh nhau.Tuy nhin chng c th khc nhau cc mc in p thp, ni m ph-t-it khng hon ton ngho, v sau thi gian thu nhn, photon mi bt utham gia vo thi gian ln. Thi gian p ng in hnh ca ph-t-itngho mt phn c m t nh hnh 4.3b. Cc ht mang nhanh cho phpu ra thit b ln ti 50% gi tr ln nht khong 1 nm. Nhng cc htmang chm gy ra tr kh lu trc khi u ra tin ti gi tr ln nht ca n.

Hnh 4 :p ng xung ca ph-t-iot vi cc tham s tch sng khc

nhau- t c hiu sut lng t cao, rng vng tri phi ln hn,

sao cho hu ht nh sng c hp th. p ng i vi xung u vo hnhch nht ca ph-t-it in dung thp c c m t hnh 4.4b. Thigian ln v xung ca ph-t-it theo xung u vo hon ton tt. Nu indung ph-t-it ln hn, thi gian p ng s b hn ch bi thi gian png RC ca in tr ti RL v in dung ph-t-it. p ng ny nh hnh4.4c. Nu vng tri qu hp, bt k mt ht mang no c to ra trong vtliu khng ngho s phi khuch tn tr li vng tri trc khi chng cthu nhn. Nh vy, thit b c cc vng tri rt mng s c hng phn rathnh cc thnh phn p ng chm v p ng nhanh nh hnh 4.4d. Thnh

phn p ng nhanh thi gian ln s ng vi cc ht mang c pht ratrong vng tri, nhng tri li thnh phn chm xut hin do s khuych tncc ht mang c to ra trong c ly Ln bt u t cnh ca vng tri. cui xung quang, cc ht mang trong vng tri c thu nhn rt nhanh,iu ny to ra thnh phn p ng nhanh trong thi gian xung. S



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khuch tn cc ht mang trong c ly Ln ca cnh vng tri xut hin nhmt ui xung xung chm. Cng nh vy, nu w qu mng, in dung tipgip s tr ln qu mc. in dung tip gip Cj c cho l:

jC =w

As (2.8)

Vi s l hng s in mi ca vt liu bn dn, bng, trong 0ks lhng s cch in bn dn, 0 = 8.8542 x 10-12 F/m l hng s in mi khng gian t do, v A l din tch vng khuch tn.

- S vt qu mc ny s lm tng hng s thi gian RC gii hn thigian p ng b tch song. Vi dy vng hp th nm trong khong gia

s1 v s

2 th s dung ha c p ng tn s cao vi hiu sut lng t

cao. Nu RT l tng ca cc in tr u vo v ti b khuych i, v CT ltng cc in dung ph-t-it v b khuych i, m s c m t trong

s phn phn tch v nhiu sau ny, th b tch sng c th c coi nhmt b lc thng thp RC vi bng thng l:

TT

cCR

B2

1= ( 2.9)

b. Dng photo vng tri- Khi tho lun v b tch sng quang, ph-t-it phi thu tn hiu quangrt yu bin i n thnh tn hiu in. Tn hiu quang i vo thit bthng qua lp p v to ra cc cp in t-l trng v n c hp th trongvt liu bn dn. Cc cp in t-l trng y m c pht trong vngtri hoc trong di khuych tn ca n, s c phn cch bi mi trng

in p ngc, t dn n dng in chy trong mch ngoi v c s triht mang ngang qua vng tri.- V cc ht mang in tch chy qua vt liu, mt s cc cp in t-l trngs ti hp v ri bin mt. Trung bnh th cc ht mang in tch s chuynng vi mt c ly l Ln di vi cc in t v Lp i vi cc l trng. C lyny c gi l di khuych tn. Thi gian hon thnh s ti hp giain t v l trng c gi l tui th ca ht mang v c k hiu tngng bng v. Cc tui th v di khuych tn c mi quan h nh sau: nnn DL = v ppp DL = (2.10)

Vi Dn v Dp tng ng l cc h s khuych tn in t v l trng, cdin gii bng n v centimeter bnh phng trn giy. cc iu kintrng thi bn vng, mt dng tng Jtot chy qua vng tri l: didrtot JJJ += (2.11)Vi Jdrl mt dng tri sinh ra t cc ht mang bn trong vng tri, v

Jdi l mt dng khuych tn c t cc ht mang c to ra ngoi vngtri trong khi bn dn (trong cc vng p v n) v khuych tn vo tip gip



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phn cc ngc. V lp p b mt ca ph-t-it p-i-n thng l rt mng,dng khuych tn c xc nh ch yu bi s khuych tn l trng tvng n. Cui cng, G. Keiser a ra kt qu nh sau:

n

p

n

ps

w

tot

D

Dep

L

eeJ

s

00

1

1 +

+

=

(2.12)

Vi Pn0 l mt l trng cn bng. S hng c lin quan Pn0 thng ln nh dng photo c pht ra t l vi dng photon 0.

3.Pht-it thc APDTt c cc b tch sng lun i hi mt dng ti thiu no hot

ng mt cch tin cy. Dng ny c chuyn thnh cng sut ti thiuthng qua quan h Pin = Ip / R. V vy, cc b tch sng c p ng R ln vcn thit v chng yu cu mt cng sut quang nh hn l . Chng ta bitrng pht-it p-i-n, v n c thit k cho ra s b khuch i (tng

ch) dng bn trong. Sau khi bin i cc photo thnh cc in t photo, nkhuch i ngay dng photo bn trong n trc khi dng ny i vo mchkhuch i din tip sau v iu ny lm tng mc tn hiu, dn ti nhythu c tng ln ng k. thu c hiu ng nhn bn trong, cc htmang quang s c tng dn nng lng ti mc ln ion ha cc int xung quang do va chm vi chng. Cc in t xung quang c y tvng ha tr ti vng dn, ri to ra cc cp in t-l trng mi sn sngdn in. cc ht mang mi c to ra ny s tip tc c gia tc nh intrng cao v li c th pht ra cc cp in t l trng khc. Hin tng

ny gi l hiu ng thc. Qu trnh thc dn ti lm tng dng in t photo.i vi pht it Si , ngng trng in cn thit thu c s nhnl mc 105 V/cm.

Cu trc thng dng ca mt photodiot thc APD c th m t nh hnh 5.N c cu to gm c vt liu loi p in tr sut cao dt lm lp epitaxinn p. Sau ngi ta khuch tn hoc cy lp n+ (loi n pha tp nng ). Haivng cch nhau bi mt in trng thp (ni m cc photon c hp thv cc ht mang quang tri theo chiu phn cc ca n) v mt vng trngin cao (ni m cc ht mang c gia tc v chu qu trnh nhn). i viSi, cht kch tp vng ny thng tng ng l Bo hoc pht pho. Cu

trc nh vy thng c gi l cu trc cn xuyn p+ipn+. Lp I (hay ) cbn l lp vt liu t dn c pha tp mt cht p.

V hot ng ca APD c th hiu nh sau. Khi c mt thin p phn ccngc nh t vo APD, hu ht cc in th ri ngay qua lp tip gip pn+.Vng tri s m rng theo s tng ca thin p cho ti mt gi tr in pno m trng in nh ti tip gip pn+ vo khong 5 n 10% di



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trng in cn thit gy ra hiu ng thc. Ti thi im ny, vng trich cn xuyn ti vng t dn (va ti mc nh thng vng i).

T phn tch trn ta thy r rng rng cc photo- i t thc khuch iti ch dng ph t tn hiu ban u trc khi i vo mch u vo ca bkhuch i din. V dng photo c nhn trc khi nhiu nhit pht sinh mch in, cho nn n lm tng nhy thu. qu trnh nhn ht mangc xy ra ng lc, cc ht mang pht ra dng photo phi i qua vng nic in trng rt cao. Di vng in trng cao ny, mt in t c giatc c th nng lng pht ra cc cp in t l trng mi. iu ny cngha rng n ion ha cc in t bao quanh trong vng ha tr do va chmvi chng. C ch nhn ht mang nh vy c gi l s ion ha do vachm. Cc ht mang mi c to ra cng c th c gia tc bng intrng cao, khi thu c nng lng, chng c th to ra tip qu trnhion ha va chm. Nh vy, kt qu thc ca qu trnh ion ha do va chm c

ngha rng, ch n thun mt in t ban u c pht thong qua qu trnhhp th photo s to ra nhiu in t l trng th cp.

T r u o n g d ie n t o it h ie u d u o c y e uc a u d e i o n h o a

S u p h a n b o t r u o n g d i e nA

TruongDien

V u n g th a c

p+ i p n+

H a p t h u K h e c h d a i

H n h 4 . 1 2

Hnh 5. S phn b trng in v cu trc APD cn xuyn SilicCu trc ca APD trong hnh 4.5 l mt cu trc cn xuyn RAPD tiu

biu c nhiu tri rt nh. Trong APD, s trung bnh cc cp in t - l



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trng c pht ra trong mt n v di di chuyn c gi l h s ionha (hay tc ion ha). Nhn chung, hu ht cc vt liu u th hin skhc nhau v h s ion ha in t v h s ion ha l trng . c tnh

b tch sng quang xc nh bi t s kA = / . Gi tr s ca v sph thuc vo vt liu cht bn dn v trng in m n gia tc cho ccin t v l trng. Trong thc t, ch c Silic l c s khc nhau ng kgia cc h s ion ha in t v l trng.

H s nhn M cho tt c cc ht mang c pht ra trong photo- it APDc cho l:

MP

IM

I=

Vi IM l gi tr trung bnh ca dng tng u ra c nhn v Ip ldng photo ban u cha c nhn m xc nh trong cng thc trn.Trong thc t v khng phi mi cp ht mang c pht ra trong photo-

it u cng c nhn cho nn c ch nhn thc l mt qu trnh thng k.V vy gi tr M c xc nh s phi c din gii bng mt gi tr trung

bnh.S tng ch dng i vi cc bc sng khc nhau s ph thuc vo in

p phn cc, v d loi RAPD Silic c c tnh tng ch dng ph thuc voin p phn cc nh hnh 4.6. S ph thuc ca tng ch vo bc sngkch thch l thuc tnh ca qu trnh thc do cc in t v l trng gy rakhi hu ht nh sng c hp th trong vng pn+ gip vi b mt b tchsng. iu ny cn c lu c bit i vi cc bc sng ngn v y

phn ln cng sut quang c hp th gn vi b mt hn l cc bc sngdi hn. V Si, h s ion ha i vi cc l trng l nh hn i vi ccin t, cho nn s tng ch dng tng b gim i cc bc sng ngn.



14/45


1 0 0 2 0 0 3 0 0 4 0 0

50

1 00

20 0

50 0

1 0 0 0

20

10

6

2

0

D i e n a p

56 879 9

1 0 6 0 n m

A P D s i l ic n+ppp+

B u o c s o n g

52 3

47 6

V

Hnh 6. Khuch i dng ca RAPD silic vi cc bc sng khc nhau.Do c s khuch i dng, p ng ca APD c tng bng h s nhn

M v c vit nh sau:

APDe

R M RM hv

= =

R cng c th gi l p ng thun nht. Bng tn ni ti ca APD phthuc vo h s nhn M. iu ny c th c din gii bng vic ch rng, thi gian chuyn dch Tt i vi APD khng di hn nh thi gian

cho cng thc 2.7, nhng thc t li tng ng k bi l n gin l do cs pht ra v la chn cc cp in t l trng th cp m n sinh ra mtthi gian ph s khuch i ca APD s gim ti tn s cao l do s tng vthi gian chuyn dch lm gii hn bng tn.

Nhn chung, silic l vt liu c s khc nhau ng k gia h s ion hain t v l trng. V h s ion ha i vi l trng l nh hn i vi int trong vt liu ny nn khuch i dng tng l b gim ti bc sngngn.

Bng 4.2 so snh cc c tnh hot ng ca cc photo it thc loi Si,

Ge v InGaAs. Bng 1: Cc c tnh ca photo-it thc APD ph bin

Tham s n v Si Ge InGaSaBc sng p ng RAPDH s nhn M

m

A/W-

0,4-1,180-130100-500

0,8-1,83-3050-200

1,0-1,75-2010-40



15/45


H s k kADng ti IdThi gian ln TrBng tn f

Thin p Vb

-nAnsGHz

V

0,02-0,050.1-10.1- 20,2-1,0

200-500

0,7-1,050-5000,5-0,80,4-0,7

20-40

0,5-0,71-50,1-0,51-3

20-30Trong bng 1 v kA i vi silic l rt nh hn 1, nn cc photo- it thc

silic c s dng cho ra cc c tnh tt hn. Cc photo-it ny rtthng dng cho cc h thng thong tin quang bc sng 0,85 m ti tc

bit ~ 100 Mbit/s. i vi cc h thng nm bc song t 1,3 -1,6 m , ccphoto-it thc APD loi Ge v InGaAs nn c s dng v cc vt liuny hp th nh sng trong vng bc sng di. Cc photo-it thc loiAPD c ci tin c thit k ph hp. loi photo-it thc tiu biu lloi s dng cu trc d th vi lp InP cho vng khuch i. v vng hp

th (lp InGaAs loi i) v vng nhn (lp InP loi n ) l cch nhau, cu trcny c gi l SAM. Trong cu trc ny c s lch nhau kh ln v vngcm gia InP c Eg = 1,35 v InGaAs co Eg = 0,75. do bc nhy vng hatr vo khong 0,4 cho nn cc l trng pht ra trong vng InGaAs b mc liti tip gip d th v chng b chm ng k trc khi tin ti vng khuchi (lp InP). Photo-it thc APD nh vy s c p ng cc k chm v

bng tn li tng i hp. gii quyt vn ny, cu trc SAM photo-it thc c ci tin bng cch s dng them mt lp khc t gia cclp hp th v nhn; n c vng cm l trung gian gia cc vng cm ca

lp InP v InGaAs. Lp ny c lm bng hn hp bn vt liu InGaAsPcng nh vt liu ch to laser bn dn. cu trc ny c gi l SAGM. Nc bng tn khuch i l: MB= 70GHz vi M=12.

Cu trc APD thnh cng nht cho loi InGaAs l s dng cu trc siuli i vi vng nhn. Trong cu trc ny, vng nhn dy 231nm gm c11 chu k ging lng t loi InAlGaAs dy 9nm, c cch nhau bi cclp chn dy 12nm. Lp m loi InP cch vng hp th InGaAs bng vngnhn siu li. APD loi ny c bng tn 15GHz vi h s nhn M=10.

photo it kiu ny c th p ng cho cc h thng thng tin quang hotng ti ch bit ti 30Gbit/s.

4.Vt liu ch to ph t-itTrong phn tho lun v cu trc ca ph t-it trn, chng ta bit

v h s hp th s v p ng R ca thit b. H s hp th s ca cc vtliu bn dn thay i ln theo bc sng nh c ch ra hnh 7. p ngca b tch sng c xc nh ch yu bi cu trc b tch sng v loi vtliu c s dng. Trong cc ph t-it thc t, vt liu c nng lng dicm hi t hn nng lng ca photon ti bc sng di nht m ta quan tm



16/45


c th cho ra c p ng tt nht v hiu sut lng t cao nht. Ngoivic bo m c c hiu sut v p ng tt, cu trc nh vy cng gic dng ti ca ph t-it thp.

Trong vng ph t 800 nm n 900 nm, mt s cc vt liu khc nhaunh Si, Ge, GaAs c th c s dng lm ph t-it. Tuy nhin, Si l vtliu ph hp nht v c dng cho nhiu loi thit b tch sng quang. Vtliu Si c tp m thp nht, nh vy n cho ra c nhy thu ca b thucao. Hn th na, cng ngh pht trin vt liu ny t c mccao.

bc sng di hn, t trn 1100 nm n 1700 nm, s dng vt liu Sil khng ph hp. p ng ca n qu thp s dng ch to ph t-it,cc photon trong cc bc sng di ny khng c nng lng kchthch mt in t ngang qua c di cm 1,17 eV ca vt liu. Trong vngny, mt s cc vt liu c dng trin khai cc b tch sng quang nh

Ge, InP, InGaAsP, InGaAs, GaAlSb v GaAb. i vi cc ph t-it p-i-nv APD hot ng bc sng di, InGaAs l vt liu ph hp nht v n chp th nh sng vi bc sng di ti 1650 nm. Gemanium Ge cng l vtliu dng cho b tch sng bc sng di. H s hp th ca n ln ti 104

cm-1 trn vng bc sng t 1100 nm n 1550 nm. Vi cc c tnh ny,vt liu Ge c th c s dng lm ph t-it l tng cho cc h thngthng tin quang bc sng di. Nhng ng tic l Ge li c mt s nhcim ca n. V di cm ca Ge hp hn di cm ca Si, dng ti khi lcao hn nhiu, iu ny lm hn ch tng ch trong ph t-it thc APD.Hn na, Ge li c h s nhiu tri cao trong hiu ng nhn thc do t l hs ion ha ht mang ch l 2.



17/45


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- B tch sng quang cn c hiu sut lng t cao pht ra cng suttn hiu ln.

- Nhiu ca b tch sng quang v b khuch i in phi cng thpcng tt.

Trc ht, ta hy xem xt cc ngun nhiu trong b tch sng quang.1. Cc ngun nhiu trong b tch sng quang a. Cc ngun nhiu ca b tch sng p-i-n

Cc ngun nhiu trong b tch sng quang bao gm nhiu b tch sngsinh ra t bn cht thng k ca qu trnh bin i photon thnh in t vnhiu nhit c lin quan ti cc mch khuch i in trong b thu. Trongcc b tch sng quang thc t, hiu sut lng t ca ph t-it thngt ti gi tr ln nht ca n. V th, cc dng nhiu l cc yu t chnh xcnh t s tn hiu trn nhiu ca b tch sng quang trong cc h thngthng tin quang, chng ta s xem xt dng photo v cc loi nhiu khc nhau

v cc quan h tng tc ca tham s ny.Ta hy kho st mt mu b thu quang s dng tch sng ph t-it p-

i-n n gin c th hin nh hnh 8. Trong hnh ny, c b thu n ginv mch tng ng ca n c m t. y Rs l mt in tr ni tipc gi tr rt nh, Cd l in dung tng, n gm in dung ghp ni v indung ng v thit b, RL l in tr ti ca b tch sng, thng thng thRL >> Rs. Trong b thu quang, cc mch khuch i in ng mt vai trrt quan trng. n gin, c th c b khuch i chnh sau b tinkhuch i. B tin khuch i cng l mt b khuch i quan trng nhttrong b thu v n trc tip tham gia vo t s tn hiu trn nhiu SNR.Trong hnh 8, C v Rs tng ng l in dung v in tr u vo ca mchkhuch i in pha sau b tch sng quang.



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Hnh 8. M hnh n gin ca mt b thu tch sng quang v mch tngng ca n.

Khi c mt cng sut quang c iu ch P(t) i vo ph t-it,dng photo c pht ra tng ng vi tn hiu quang c hiu nh sau:

i(t) = RP(t) =Dng photo ny bao gm dng mt chiu Ip l dng photo trung bnhc c t cng sut tn hiu quang, v dng tn hiu is(t) c lin quan tinhiu lng t (hay cn gi l nhiu bn) c dng tn hiu trung bnh bnh

phng = . Quan h Ip = RPin s vn cn c duy tr nu nh Ipl mt dng trung bnh. Tuy nhin, nhiu in c sinh ra t s bin ngdng lm nh hng ti c tnh b thu. thu c t s tn hiu trnnhiu SNR ca b thu quang, vic xem xt cc dng nhiu khc nhau l mtvn quan trng xc nh xem SNR l cao hay thp. Trong b tch sngquang, cc nhiu c bn l nhiu lng t, nhiu dng ti v nhiu dng r

b mt.- Nhiu lng t pht sinh do bn cht thng k ca qu trnh tch sng

v tnh la chn ca cc in t photo khi c tn hiu quang i ti b tchsng quang. Nhiu lng t bt ngun t s khng chc chn v thi giann ca cc in t hoc cc photon ti b tch sng. Tnh thng k nyc th hin theo mt qu trnh Poisson. i vi b tch sng p-i-n, s

bin i nhiu lng t trong bng tn hiu dng Be (hoc cn gi l bngtn in) l gi tr bnh phng trung bnh ca dng nhiu lng t c cho

bi:

s2

= (ish2

) = 2eIpBeGi tr ny t l vi gi tr bnh phng ca dng photo Ip.- Nhiu dng ti c pht ra t vt liu khi ca ph t-it. Nhiu

dng ti ph t-it l dng m n lun xut hin ngay c khi khng c nhsng i ti ph t-it. Dng ti ny chy qua mch thin p ca thit b. Vn c pht ra t vt liu khi cho nn n tng theo cc in t v (hoc) ltrng c pht theo nhit trong tip gip pn ca ph t-it, v n cngc gi l dng ti khi idb. S bin i nhiu ca dng ny c vit nhsau:

db2 = (idb2) = 2eIdBeVi Id l dng ti khi ban u ca b tch sng.- Dng r b mt ph t-it isl cng c xem nh l dng ti b mt,

hoc cho n gin th gi l dng r. Dng ny ph thuc vo s khimkhuyt, in tch b mt, mc sch ca b mt v in p nh thin. Bini nhiu ca dng r nh sau:

sl2 = (isl2) = 2eIlBe



20/45


Vi Il l dng r r ban u. lm gim dng r, cch tt nht l sdng cu trc mc ring bo v, iu ny lm r cc dng r cho in trti.

- Mt ngun nhiu na m n tham gia vo nhiu tng trong b thuquang l dng nhiu nhit ir. Dng nhiu ny c mt nh hng ng k tit s tn hiu trn nhiu ca b thu quang. n gin, chng ta gi thit trkhng u vo ca mch khuch i in b thu l ln hn nhiu so vi intr ti. iu ny dn ti nhiu nhit ca n nh hn nhiu nhiu nhit tin tr ti RL. N c ngha rng dng nhiu nhit c pht t in tr ti

b tch sng l chnh. Bin i nhiu c cho nh sau:r2 = (ir2) = (4kBT/RL)FBe

Vi kB l hng s Boltzman, T l nhit tuyt i (Kenvil). K hiu F lh s nhiu c khuch i, n th hin mt h s m nhiu nhit tngdo s dng cc in tr khc nhau trong mch tin khuch i v khuch i

chnh.T nhng thnh phn nhiu trn, cng sut nhiu tng c vit l:

2 = (i2total) = 2eBe(Ip + Id + Il) + (4kBT/RL)FBeb. Cc ngun nhiu ca b tch sng APD

i vi b tch sng APD, cc ph t-it thc thc hin khuch ibn trong dng tn hiu ban u bng h s nhn M trc khi i ti mchkhuch i in pha sau. y l nguyn nhn ca s tng t s tn hiu trnnhiu trong b thu quang APD. Nhn chung, cc b thu quang ph t-itthc cho ra t s tn hiu trn nhiu cao hn so vi SNR b thu p-i-n vicng mt cng sut quang u vo. Vi mc cng sut quang ti l Pin, dng

ban u IP c vit l:IP= RMPin = RAPDPin

Vi RAPD l p ng ca ph t-it thc, n c nhn ln M so viph t-it p-i-n. V th dng tn hiu trung bnh bnh phng cvit cho APD l:

(Is2) = [Ip2(t)]M2

Vy th t s tn hiu trn nhiu hy vng s c tng thm h s M2

nu nh cc nhiu b thu khng b nh hng t c ch khuch i bntrong ca ph t-it APD. Nhng ng tic l nhiu ca b thu ph t-it

thc cng tng, v t s tn hiu trn nhiu ca b thu li b gim i.Da vo vic tho lun i vi nhiu, b tch sng p-i-n trn, ta c

th tm thy cc thnh phn nhiu ca b tch sng quang APD. Nhiulng t hoc nhiu bn v nhiu dng ti s tng M2 ln. Trong khi cnlu rng, qu trnh nhn thc l mt qu trnh hiu ng khi, dng r bmt isl khng b nh hng t khuch i thc. Nhiu nhit cng khng chunh hng t hiu ng thc v n c ngun gc pht sinh t cc thnh phn



21/45


in khng c lin quan ti ph t-it thc. y khng ging nh trnghp ca nhiu lng t. Do ta c th vit nhiu tng ca b tch sngAPD nh sau: 2APD = (i2APDtotal) = 2eBe(IP+ Id)M2FA + 2eBeIl+ (4kBT/RL)

Vi FA l h s nhiu tri ca APD ph thuc vo M[1], n c lin quanti bn cht ngu nhin ca qu trnh thc. Qua nghin cu, ngi ta tm rac:

FA = kAM + (1 kA)(2 1/M)T cc kt qu thc nghim, FA c coi l xp x bng: FA ~ Mx

Tham s x l phn t gam, v gi tr ca n bng 0,3 i vi ph t-itthc Silic, bng 0,7 i vi InGaAs, v bng 1 i vi Germanium.

1 1 0 1 0 0 1 0 0 0

1 0

1 0 0

kA= 10 .50 .2

0 .10 . 0 5

0 . 0 2

0 . 0 1

0 . 0 0 5

0

K h e c h d a i M c u a A P D

HesonhieutroiF

A

1

Hnh 4.9. S ph thuc ca h s nhiu tri vo M ca APD.

2. T s tn hiu trn nhiu

T dng tn hiu v cc dng nhiu c xc nh nh trn, t s tnhiu trn nhiu SNR (hay cn gi l t s trn nhiu in eSNR) ca b thuph t-it c xc nh l:

SNR = =



22/45


Trn thc t, ngi ta s dng s bin i cng sut nh l bnhphng ca dng in, vy th t l SNR cho b thu ph t-it p-i-n cvit l:

SNRpin =

Trong gii hn nhiu nhit, s l nh hn nhiu T (s> T), lc

ny ta c:

SNRpin = =

Trong cc b thu quang thc t s dng b tch sng p-i-n, nhiu tri lnhiu nhit iT. V b tch sng c in tr ti tch sng, n sinh ra dngnhiu nhit kh ln to ra nhiu nhit. Trong nhiu b thu quang, dngnhiu nhit hu dng ln gp 20 ln dng nhiu lng t hiu dng, vkhong 100 ln dng ti hu dng. Nhiu dng ti idb v dng r b mt islcn c th c kt hp li v gi chung l nhiu dng ti. V vy ta c thgi thay th Id bi (Id + Il) trong mt s cc trng hp tin cho vic xemxt cc ngun nhiu chung ca b thu. Ngoi ra, nu vic thit k b thukhng c lm cn thn, cc phn t tch cc ca mch khuch i c thto ra dng nhiu khuch i iamp.

Tng t, ta c th vit t s tn hiu trn nhiu cho b thu quang ph

t-it thc APD nh sau:SNRAPD =

Nh phn tch v nhiu phn trc cho ph t-it p-i-n, dng tiId c kt hp t Id + Il, cho nn s2 = 2e(Ip + Id)Be, ta vit cho b thu quangAPD nh sau:

2sAPD = 2eBe(RPin + Id)M2FATrong gii hn nhiu nhit, s l nh hn nhiu T (s> T), lc ny ta c:

SNRAPD = =

V t s tn hiu trn nhiu SNR b gim i vi h nhiu tri FA so vib thu ph t-it p-i-n.



23/45


IV. B THU QUANG 1. Cu hnh b thu quang

B thu quang trong h thng thng tin quang bao gm b tch sngquang,b khch i in v mch x l tn hiu.Cc thnh phn thit b nytrong b thu c cc chc nng tng ng l bin i nng lng tn hiuquang yu i vo ph t iot thnh tn hiu in v ri khch i tn hiu nyti mc ln n c th c x l bng cc mch in theo sau.Nh cp trn,phn khch i in ca b thu quang c th bao gm mchtin khch i v mch khch i chnh,ta s xt b thu vi cu hnhny.Hn na hu ht cc h thng truyn dn quang s dng dng iu chs nn ti liu ny s tp trung vo phn tch b thu quang s.Cu hnhtiu chun ca b thu quang s dng ph t iot c th hin hnh 10.Ccthnh phn tiu biu ca b thu ny thng c sp xp theo th t banhm: phn trc b thu,knh tuyn tnh v phn hi phc tn hiu s.

PDhv Khch i Chnh

Tn hiu s ra

RL

Front- end b thu Clock ra

Hnh 10 Cu hnh b thu quang s tiu biuTrong phn u ca chng ny chng ti cng gii thiu v b tchsng vi cc thnh phn nhiu ca n t c t s tn hiu trnnhiu.Tuy nhin cn phi nhn mnh rng t s tn hiu trn nhiu ineSNR ti b tch sng quang l ch nh gi cht lng ca tn hiuthu c c p ng c tiu chun h thng cho hay khng,nh trongtrng hp truyn dn analog c bn cho thng tin ting ni v video.Nhngi vi cc h thng truyn dn s t s tn hiu trn nhiu in t bn thnn khng l mt hnh nh mang ngha c gi tr.iu ny c th c chra bng vic xem xt nguyn l c bn ca b tch sng s.Trong bi cnh

b thu quang s,cc nhiu v mo tin hiu khc nhau s khng th trnh khi


Quytnh

ClockExt

Pre

- am

p


24/45


25/45


khi phc tn hiu v cho ra chui tn hiu s.Tn hiu clock phi ca vo u vo quyt nh xc nh cc thi gian ly mu.Nh vy,thit k b thu quang ta xem xt 3 thnh phn chnh cu trc nn thit b nhsau:

Front- endFront-end ca b thu quang bao gm mt ph t iot v mt b tinkhch i.Ph t iot bin i lung bit nh sng thnh tn hiu in thay itheo thi gian.B tin khch i in c vai tr khch i tn hiu in choqu trnh x l tip sau.Vic thit k front-end i hi s tng xng giatc bit v nhy thu.Kh nng ca b thu quang c p ng c choh thng thng tin quang c tc bit cao v c ly xa khng ph thuc phnln vo vic thit k front-end thu.Khi s dng in tr ti RL c gi trln,in p u vo mch tin khch i c th tng v khi ta c b front-end tr khng cao.Gi tr in tr RL ln c th gim nhiu nhit v do

tng c nhy thu.Nu nh khng qu quan tm n nhy thu ta cth gim RL tng bng tn h thng,v khi ta c b front-end tr khngthp.

Knh tuyn tnhKnh tuyn tnh trong b thu quang bao gm b khch i chnh v

mch cn bng.B khch i chnh thng l b khch i c khch icao.Mch cn bng ngay sau b khch i thng l b lc sharp-tn stuyn tnh.N c s dng lm gim nh hng ca mo v ISI tnhiu.Trong mt s cc trng hp mch cn bng c th c s dng ch

hiu chnh p ng tn s in ca b tch sng quang v b khchi. khch i ca b khch i c iu khin mt cch t ng giihn in p u ra trung bnh ti mc c nh bt k cng sut quang trung

bnh n b thu nh th no.B lc thng thp s to xung in p. tnh ton vi cc xung ch nht c gi t pha pht,c th m t nhsau.Nu P(t) l cng sut quang thu c trn b tch sng th chui xung snh phn trn b tch sng c th c din gii bng biu thc sau:

=

=n

bpn nTthbtP )()(

Vi bn l tham s bin th hin digit tn hiu th n n c th ly gi tr1 v 0; Tb l chu k bit v hp(t) l dng xung b thu m l dng chomi t.Nu ta gi thit xung u vo ph t iot khng m,h(t) c chunha c vng bng 1 th:

1)( =

dtthp

Dng u ra trung bnh t photo-iot ti im t c th vit nh sau:



26/45


== )()()( bpn nTthbRtPhv

eti

Vi R l p ng ca photo-diot, khi s dng APD th thay bng RAPDDng ny c khuch i v c lc cho ra in p trung bnh ti u

ra ca mch cn bng c cho bi tch chp ca dng vi p ng xung bkhuch i. in p ny c vit nh sau:)()()()( ththtARPtV eqBout =

Vi A l khuch i ca b khuch i,hB(t) l p ng xung ca mch thinp, heq(t) l p ng xung mch cn bng , v * l k hiu tch chp. da vohnh 4.11, hB(t) c cho bi dng chuyn i Fourier ngc ca hm truynt mch thin p HB(f) v c vit nh sau:

( )[ ] ( ) dfefHfHFth ftjBBB

21)(

==

Vi F l k hiu thut ton dng chuyn i Fourier. Hm truyn t dngthin p HB(f) n gin l in khng ca t hp mc song song ca Rb,Ra,Ca v Cd c vit l:

( )fCj

R

fHB21

1

+=

ViLa RRR

111+= v da CCC +=

Tng ng nh biu thc (4.32), in p trung bnh u ra mch cnbng c vit nh sau:

=

=n

boutnout nTthbtV )()(

y: ( ) ( )ththtARhth eqBpout = )()( L dng ca xung c khuch i v lc c lp . dng chuyn i Fourierca biu thc (4-40) c th c vit nh l:

( ) ( )fHfHfARHethfHeqBp

ftj

outout)()()( 2 ==

Vi Hp(f) l hm chuyn i ca dng xung thu c hp(t),Heq(f) l hmtruyn t ca b cn bng.

Khi phc tn hiuPhn khi phc tn hiu s ca b thu quang bao gm mch quyt nh

v mch hi phc clock.Mch quyt nh so snh tn hiu ra t knh tuyntnh vi mc ngng ti cc thi im ly mu c xc nh bi mch hi

phc clock,v quyt nh xem tn hiu c tng ng vi bit 1 hoc 0hay khng.Thi im ly mu tt nht s ng vi v tr m trong s khc



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nhau v mc tn hiu gia cc bit 0 v 1 l ln nht.iu ny c thc xc nh t hnh mt c to bng vic xp chng cc chui xungin di 2-3 bit trong chm bit trn nh vi nhau.Thi im ly mu ttnht tng ng vi m mt ln nht.V nhiu tn ti mi b thu,chonn s lun lun c mt xc sut nht nh m bit c th c nhn dngmt cch khng chnh xc ti mch quyt nh.Tuy nhin b thu quang s cth c gi tr t s li bit rt nh l do mch quyt nh thng c thit k hot ng theo cch nh vy.

2. Cc ngun li trong b thu quangNhn chung ,vic thit k b thu quang l phc tp hn nhiu so vi thit k

b pht quang v b thu phi tch sng cc tn hiu yu v b mo dng,riquyt nh xem tn hiu s no c pht ti t pha pht.Qu tnh tchsng trong b tch sng quang ca b thu quang phi chu nh hng t ccnhiu khc nhau v cc tc ng khc c lin quan ti tch sng tn hiu

trong thit b.Cc ngun nhiu khng mong mun ny xut pht t nhiuyu t.Cc ngun nhiu c th n t ngoi h thng nh nhiu kh quyn,vnhiu ca thit b no ,hoc c t bn trong bn thn h thng.Trong tiliu ny ta xem xt nhiu bn trong ca h thng.Nhiu ny c pht ra tnhng s khng n nh t pht ca dng in hoc in p trong mchin.Nh phn tch trn ,nhiu lng t v nhiu nhit l hai nhiu tritrong b thu quang.Tc n ngu nhin ca cc ph tn l nguyn nhnsinh ra nhiu lng t trong b tch sng quang.Nhiu ny lun ph thucvo mc tn hiu ,vy th n rt quan trng cho b thu s dng ph t iot p-i-n thng c cc mc u vo quang ln.c bit khi s dng b tch sngqang APD,nhiu lng t cn tng ln do bn cht thng k ca qu trnhnhn thc;v mc nhiu c th tng theo h s nhn M.Nhiu nhit sinh ra tin tr ti ca b tch sng v cc mch in khch i trong b thuquang.Loi nhiu ny s tri khi b tch sng ph t iot p-i-n c t s tnhiu trn nhiu SRN thp.V nhiu nhit l nhiu c bn chtGaussian,chng c th c hn ch nh p dng cc gii php k thutchun.

Dng photon ban u c pht t ph t iot l mt qu trnh Poissonbin i theo thi gian c t s n ngu nhin ca cc photon ti b tch

sng.Khi c mt cng sut tn hiu quang P(t) ti b tch sng quang, strung bnh cc cp in t-l trng c pht trong thi gian ccho nh sau:

( )hv

EdttP

hvN

== 0

Vi l hiu sut lng t,hv l nng lng photon,v E l nng lng thuc trong khong thi gian .Trong b tu quang thc t cc cp in t-l



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trng thc s m c pht ra s dao ng quanh s trung bnh theo phn bPoisson.V th xc sut Pr(m) m m l cp in t-l trng c pht trongkhong s l:

( )!m

eNmP

Nm

r

=

Trn thc t khng th d bo chnh xc c bao nhiu cp in t-l trng lc pht ra t mt cng sut quang cho i n ph t iot. l ngungc ca nhiu lng t.

Mt ngun nhiu khc pht sinh t s gin xung trong si quang,v loinhiu ny gi l nhiu giao thoa gia cc k t ISI.Mi mt xung tn hiuthng c c trng mt khe thi gian nht nh,khi xung ny c phti,hu ht nng lng ca xung c th i ti b thu quang trong khe thigian tng ng.V cc xung b gin tri trong qu trnh lan truyn theo si,mt phn nng lng c pht s dn dn sang cc khe thi gian ln

cn.S c mt ca nng lng ny khe thi gian ln cn s lm cho tnhiu giao thoa, v y l giao thoa gia cc k t.V. NHY THU CA B THU QUANG

1. Gii thiu v nhy thu quang nhy ca b thu quang l yu t quang trng nht nh gi kh nng vcht lng ca h thng thng tin si quang. nhy thu cao cho php thitk h thng thng tin quang c tc d cao v c ly xa. nhy ca b thuquang b tc ng trc tip t t s tn hiu trn nhiu SNR ca b tch sngquang.T s tn hiu trn nhiu cng cao th b thu quang cng c nhycao hn.Trong b thu quang, nhy thu cao hn c ngha l mc cng sutthu c cng nh i vi cng mt ch tiu cht lng truyn dn.Nh tho lun trn,t s tn hiu trn nhiu in eSNR ti b tch sng ch nh gi xem liu cht lng ca tn hiu thu c trong trng hp truyndn analog video hoc ting ni c p ng c tiu chun h thng chohay khng.Cn i vi cc h thng thong tin s,bn thn eSNR ca b tchsng khng l mt hnh nh c gi tr.iu ny c th ch ra bng vic phntch nguyn l c bn ca b tch sng s.

Trong thng tin truyn dn s cc tn hiu quang c to bi cc k hiunh im v khong trng ,c truyn ti bng nh sng iu

ch.Cc k t ny thng c gi l 1 hoc 0 tng ng.i vi ccc tnh ca h thng truyn dn quang s, nhy ca b thu quang lunlun b rang but bi t s li bit BER. nhy phi cn c th hin bimt cng sut quang c th thu c cng nh cng tt m vn m boc t s li bit cho.Cc khi nim v nhy thu quang c xc nhnhn chung l tha mn vi cc iu kin v quan h gia cng sut thuc v BER.Cc gi tr ca BER khc nhau l ty thuc vo cc yu cu v



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nh hng v gi ha l khng c xc nh ring r v chng l nhngvn ca nhng nh khai thc mng li v nh ch to cung cp thit

b.D phng tiu biu gia thi im bt u v kt thc ca tui th cho bthu hot ng nhit danh nh,tng ng trng hp xu nht mongmun l t 2 n 4 dB.

Nh vy ta c th hiu rng nhy ca b thu quang c xc nh lmc cng sut quang trung bnh thu c nh nht c th chp nhn cti im tham chiu trn si quang ngay trc b ni quang pha thu mvn duy tr c mt t l li bit BER nh trc.

2. nhy thu v t s li bt ca b thu quanga. T s li bt trong b thu quang

B thu quang trong h thng thng tin quang s iu bincng .tch sng trc tip IM-DD gm mt b tch sng mang, bkhuch i in ,mch cn bng va mch quyt nh .khi i ti u vo

photo-diot, tn hiu quang c bin i thnh tn hiu in tng ng.v tnhiu in sau khi tch sng l rt yu,n c khuch i ti mch tinkhuch i v ri c lc qua mch cn bng .sau tn hiu ny c sosnh vi mc ngng ti mch quyt nh thu c xung mong mun (bt1 hoc 0) trong khe thi gian ca n.trong trng hp l tng,khi 1 cthu, tn hiu u ra Vout(t) phi thp hn ngng tuy nhin, trong b thuquang thc t cc nhiu khc nhau v s giao thoa gia cc xung ln cn cth gy ra s trch khi gi tr trung bnh ca Vout(t),v xy ra li trong hthng. ta nh li rng t s tn hiu trn nhiu ca b tch sng quang nhnchung l khng hon ton c ngha gi tr cho tch sng s. hnh nh cgi tr cho tch sng y l xc sut li c lin quang n mc tn hiu cho v cng sut nhiu.ni dung xc sut nhiu y s cho kh nng phngon xem c bao nhiu bt trung bnh c th c truyn thnh cng quamt h thng truyn dn trn mt khong thi gian cho.

xc nh c t s xc nh xy ra,ta c th s dung cch tip cnchung bng cch chia s cc xung li xy ra N, trn tng s cc xung

N, trong khong thi gian xc nh t.N c gi l t s li BER:

BER =

T s li bt BER ph thuc vo t s tn hiu trn nhiu SNR (hayeSNR).nh vy,vi BER=2.10-6 s tng ng vi trung bnh c 2 li xy ratrong mt triu bit. xc nh BER ti u thu ,s phn b xc xut tn hiuti u ra ca mch cn bng l rt quang trng v iu y quyt nh rng

bt 1 hay 0 l c gi i. Vi l do nh vy ta xem ch ny thong qua hthng thng tin s vi cc xung 1 v 0.hnh 4.13 m t c ch tn hiu


Ne

Nt


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dao ng bt thng bi mt mch quyt nh m n ly mu ti hng squyt nh tD c xc nh thng qua s phc hi xung ng h.Ga tr lymu I s dao ng t bit sang bt xung quanh gi tr trung bnh )hocty thuc vo bt tng ng l 1 hay 0 trong trm bit. Mch quyt nhs so snh gi tr ly mu vi gi tr ngng ID v gi n l bt 0 nu I ID .Li xy ra nu nh I > ID i vi bt 0 donhiu b thu sinh ra v li cng xy ra vi bt 1 nu nh I < I D.C haingun nhiu c th gp li bi xc sut li gi l t s li bit BER v cxc nh nh sau: BER=p(0)P(1/0) + p(1)P(/0/1) (a)

Vi p(1/0) th hin mt xc sut c iu kin vi mch quyt nh in(tch ra) nhm k t 1khi 0 c pht t pha pht ti ,v p(1/0) biu thxc sut c iu kin i vi mch quyt nh in pht hin nhm k t 0khi 1c gi ti. cc s hng P(0) v p(1)biu th xc sut rng 0v 1

l c pht tng ng .trong trng hp khi m s cc bt N trong chui tnhiu l ln (n>=2.1015), vic truyn dn 0 v 1c coi l ngang nhau

p(0)=p(1)=0.5; khi t s li bit BER c vit li nh sau:

BER=

iu quang trng l la chn ngng quyt nh m ti BER c gitr nh nht.v tnh thng k ca in p u ra ti thi im ly mu l rt

phc tp,tnh ton chnh xc l rt kh .hin ti c vi phng php xpx khc nhau c p dng tnh ton c tnh ca b thu quang nh

phn.phng php n gin nht l da tren tnh xp x Gaussian.trongPhng php ny c gi thit rng ,khi c mt chui cc xung quang c

bit ,in p u ra mch cn bng Vout(t) l bin ngu nhin Gaussian.V th tnh ton xc sut li ,ta ch cn bit s chch tiu chun v trung bnhca in p tn hiu u ra mch cn bng.


P(1/0)+P(0/1)

2


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n

L

eBT F

R

TBK42 =

V tng ca hai bin i ngu nhin Gaussian cng l mt bin i ngunhin Gaussian,cho nn gi tr ly mu c ngu nhin Gaussian,cho nn

gi tr c ly mu I c mt xc sut Gaussian vi bin i TS222

+=Trong vic phn tch ny,nu v l cc dng trung bnh, 02 v

12

, l cc bin i nhiu tng ng t cc dng photo trung bnh lin quanti cc bt 0 v 1,v ID l mc quyt nh ,th xc sut li c iu kin

p(1/0) v p(0/1)c xc nh nh sau :

P(1/0)=( )

22

1

2

)(exp

2

1

0

0

02

2

0

0

>< v Q0 =0

0

>< ii

H s Q c th vit nh sau:

Q= 20

2

1

01

+

>< ii



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Khi2

, 0101ii

ID+

== m n ng vi thit lp ngng quyt nh

gia,t s li bit BER vi s thit lp ti u mc ngng quyt nh cvit l

BER=

22

1 Qerfc

y p dng hm erfc(x)=1- erf(x) l hm li ph ,trong erf(x) lhm li. Hm erf(x) c vit di dng chui nh sau:

erf(x) = ( )12

12!

)1(2 +

+

nn

xnn

BER c vit nh sau :

BER=

++

+

=0

22

)12()!1(

1

22

2

222exp

2

1

n

n

nn

Q

Q

QQ

Q

BER

2exp

2

12Q

Q

Hnh 14 l kt qu tnh ton m phng ng cong BER nh l mthm s ca h s Q .Ta thy BER s c ci thin khi Q tng vtr nn nhhn 10-12 khi Q>7 . nhy thu ng vi cng sut quang trung bnh i vi

Q=6 s c BER=10-9

.


LOG(

BER)

2 3 4 5 6 7 8 9

-1-2

-3-4-5-6-7-8-9

-10-11-12-13-14-15-16

---------{1/Q(2 )1/2 }exp(-Q2/2)0.5 erfc(Q/21/2 )


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Hnh 14: T l li bit BER l mt hm ca h s Qb. nhy thu ca b thu quang

Nh bit rng nhy thu cng sut quang trung bnh thu cnh nht c th chp nhn c vi BER cho.B thu quang hot ng tincy vi mt t s li bit c gi tr xc lp.khi tnh ton nhy thu quang,cng sut quang n photo-dit l mt yu t quang trng .V th h s Q cquang h vi cng sut quang n b tch sng ,v y cng l mc tiu ca

phn ny. B thu quang pht-dit p-i-n

n gin ha vic tnh ton, trc ht chng ta hy xc nh trongtrng hp b thu quang pht-it p-i-n.ta hy xt trong trng hp githit rng bt 0 khng mang cng sut quang sao cho p0=0,vy th=0.Nu gi l cng sut quang trung bnh ca c cc bt 1v0,v c th vit nh sau:

=

Trong khi cng sut Pt ti cc bt 1 c lin quan ti trong btch sng quang p-i-n nh sau:

= RP1 =2R (vi P0=0 )Cc bin i nhiu RMS l 1va 0 bao gm cc thnh phn ca nhiulng t v nhiu nhit.v vy chng c th c vit nh sau:

TS22

1 += v T =0

B qua thnh phn dng ti ,cc bin i nhiu c th vit nh sau: erecS BPRe )2(2

2 >

may thu quang hc

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