58722586-Đồ-an-viễn-thong-nhom-5

8/2/2019 58722586--an-vin-thong-nhom-5

1/59

n: H Thng Thng Tin Quang

LI NI U

T khi ra i cho n nay, cp si quang chng t c nhng u im vttri so vi cc phng thc truyn dn khc nh suy hao thp, cho php ko ditrm lp, trng lng nh, kch thc nh d lp t , hon ton cch in, khng bcan nhiu bi trng in t, vt liu ch to sn c trong t nhin (SiO2), gi thngthp Chnh v th h thng truyn dn thng tin cp quang hin nay ang chim mtt l rt ln nc ta .

n ny nhm em thc hin nhm tm hiu mt cch y v r rng v victhit k, trin khai cp quang cng nh cc phn t trong h thng truyn dn quang.

n ca chng em c th hon thnh v kt qu thu c ny l do s lm victch cc ca cc thnh vin trong nhm c bit l s hng dn v ch bo tn tnhca cc thy c trong khoav c bit l thy Nguyn L Cng ngi trc tiphng dn nhm 5 t lc bt u lm n n khi n hon thnh.

n c chia lm 4 chng

Chng 1 : Tng quan v h thng thng tin quang

Chng 2 : Cc phn t quang th ng

Chng 3 : Cc phn t quang tch cc

Chng 4 : Bi ton tnh ton mt tuyn thng tin quang thc t

Nhm 5 chng em xin chn thnh cm n Thy !

Nhm 5 : Lp 2_TVT1 3

8/2/2019 58722586--an-vin-thong-nhom-5

2/59


MC LC

CHNG 1: TNG QUAN V H THNG THNG TIN QUANG 7

1.1. Gii Thiu Chung . 7

1.1.1. M hnh h thng thng tin quang. 7

1.1.2. Nguyn l hot ng ca h thng thng tin quang.. 8

1.1.3. u im ca h thng thng tin quang.. 9

1.2. Phn loi cc phn t quang in trong thng tin quang 9

1.2.1. Cc phn t th ng... 10

1.2.2. Cc phn t tch cc 11

CHNG 2: CC PHN T QUANG TH NG... 122.1. C s vt l chung cho cc phn t th ng.. 12

2.1.1 S phn x v khc x nh sng.. 12

2.1.2. nh lut Snell 13

2.1.3. Phn cc nh sng.. 14

2.2. Si quang 16

2.2.1. Cu trc si quang.. 16

2.2.2. Phn loi si quang. 16

2.2.2.1. Si n mode(SM). 16

Nhm 5 : Lp 2_TVT1 4

8/2/2019 58722586--an-vin-thong-nhom-5

3/59

n: H Thng Thng Tin Quang2.2.2.2. Si a mode chit sut nhy bc(MM-SI)... 17

2.2.2.3.Si a mode chit sut bin i (MM - GI) 18

2.2.3. Cc tham s nh hng ti truyn lan trong si quang.. 19

2.2.3.1. Suy hao 19

2.2.3.2 Tn sc 21

2.3. Coupler quang 25

2.4. B lc quang 26

2.4.1. Chc nng ca cc b lc 26

2.4.2. c im, tham s ca b lc. 26

2.4.3 B lc quang 27

2.4.3.2 B lc cch t Bragg si. 272.4.4. B Isolator v Circulator 28

CHNG 3: CC PHN T TCH CC. 29

3.1. C s vt l chung ca cc phn t tch cc 29

3.1.1. Cc khi nim vt l bn dn... 29

3.1.1.1 Lp tip gip p-n. 29

3.1.2. Cc qu trnh c trng trong vt l bn dn. 31

3.1.2.1. Qu trnh hp th v pht x... 31

3.1.2.2 Trng thi o mt . 32

3.2. Ngun quang... 33

3.2.1. it pht quang... 33

Nhm 5 : Lp 2_TVT1 5

8/2/2019 58722586--an-vin-thong-nhom-5

4/59

n: H Thng Thng Tin Quang3.2.1.1 Cu trc LED.. 34

3.2.1.2 Nguyn l hot ng ca LED 34

3.2.1.3. ng dng ca LED. 36

3.2.2. Ngun pht laser (Light Amplication By Stimulate Emission of Radiation ) 36

3.2.2.1 Nguyn l hot ng... 36

3.3. B thu quang37

3.3.1. Photodiode PIN.. 38

3.3.1.1. Cu trc ca PIN. 38

3.3.1.2. Nguyn l hot ng... 38

3.3.1.3 c tnh ca PIN. 403.3.2. Photodiode quang thc APD... 41

3.3.2.1. Cu trc ca APD 41

3.3.2.2. Nguyn l hot ng.. 41

3.3.2.3. c trng ca APD. 42

3.4 B khuch i. 44

3.4.1. B khuch i quang bn dn 44

3.4.1.1. Cu trc b SOA 44

3.4.1.2. Cc thng s ca b khuch i SOA 45

3.4.2.1. Cu trc v nguyn l hot ng ca b EDFA. 47

3.4.2.2. c tnh ca b EDFA 48

3.5. B chuyn i bc sng 50

Nhm 5 : Lp 2_TVT1 6

8/2/2019 58722586--an-vin-thong-nhom-5

5/59

n: H Thng Thng Tin Quang3.5.1. B chuyn i bc sng quang in.. 50

3.5.2. B chuyn i bc sng dng cch t quang 50

3.5.3 B chuyn i bc sng dng b trn song.. 51

CHNG 4: THIT K TUYN THNG TIN QUANG 52

4.1. Khi qut.. 52

4.2. Thit k mt tuyn thng tin quang.. 53

4.2.1. Bc 1: Tnh c ly gii hn bi cng sut.. 54

4.2.2. Bc 2: C ly gii hn do di thng 57

4.2.3 Bi ton c th. 58

Nhm 5 : Lp 2_TVT1 7

8/2/2019 58722586--an-vin-thong-nhom-5

6/59

n: H Thng Thng Tin QuangCHNG 1

TNG QUAN V H THNG THNG TIN QUANG

1.1. GII THIU CHUNGH thng thng tin c hiu mt cch n gin l mt h thng truyn thng

tin t ni ny n ni khc. Khong cch gia cc ni ny c th t vi trm mt nvi trm kilmt thm ch hng trm ngn kilmt vt qua i dng. Thng tin cth truyn thng qua cc sng in vi cc di tn s khc nhau. H thng thng tinquang l mt h thng thng tin bng nh sng v s dng cc si quang truynthng tin. Thng tin truyn i trong h thng thng tin quang c thc hin tn ssng mang cao trong vng nhn thy hoc vng hng ngoi gn ca ph sng in t.

1.1.1. M hnh h thng thng tin quang truyn thng tin gia cc vng khc nhau, h thng thng tin quang cng cn

phi c m hnh truyn tin c bn nh ch ra trong hnh 1.1, v n nay m hnh chungny vn c p dng. Trong m hnh ny, tn hiu cn truyn i s c pht vo mitrng truyn dn tng ng, v u thu s thu li tn hiu cn truyn. Nh vy tnhiu c thng tin t ni gi tn hiu i ti ni nhn tn hiu n. Thng tin quangc t chc h thng cng nh cc h thng thng tin khc, v th m thnh phn c

bn ca h thng thng tin quang cng nh m hnh chung, tuy nhin mi trngtruyn dn y chnh l si quang. Do si quang s thc hin truyn nh sng cmang tn hiu thng tin t pha pht ti pha thu.

Mt h thng thng tin quang bao gm cc thnh phn c bn: Phn pht quang,si quang, v phn thu quang.

Phn pht quang c cu to t ngun pht tn hiu quang v cc mch in iukhin. Cc mch iu khin c th l b iu ch ngoi hay cc b kch thch ty thucvo cc k thut iu bin. Ngun pht quang to ra sng mang tn s quang, cn ccmch iu khin bin i tn hiu thng tin thnh dng tn hiu ph hp iu khinngun sng theo tn hiu mang tin. C hai loi ngun sng c dng ph bin trongthng tin quang l LED (Light Emitting Diode) v LASER (Laser Diode).

Si quang l mi trng truyn dn trong thng tin quang. So vi mi trngtruyn dn khc nh mi trng khng kh trong thng tin v tuyn v mi trng cp

Nhm 5 : Lp 2_TVT1 8

Ni phttn hiu i

Thit bpht

Mi trngtruyn dn

Ni tnhiu n

Thit bthu

Hnh 1.1 M hnh truyn thng tin vi cc thnh phn c bn.

8/2/2019 58722586--an-vin-thong-nhom-5

7/59

n: H Thng Thng Tin Quangkim loi th truyn dn bng si quang c nhiu u im ni bt l : hu nh khngchu nh hng ca mi trng ngoi, bng tn truyn dn ln, v suy hao thp. Vinhng u im , cng vi nhiu tin b trong lnh vc thng tin quang, si quang c s dng trong cc h thng truyn ng di, h thng vt i dng. Chngva p ng c khong cch va p ng c dung lng truyn dn cho php

thc hin cc mng thng tin tc cao. Si quang c 3 loi chnh l : si quang amode chit sut nhy bc, si a mode chit sut bin i v si quang n mode. Tythuc vo h thng m loi si quang no c s dng, tuy nhin hin nay cc hthng thng s dng si n mode truyn dn v u im ca loi si ny.

Phn thu quang c chc nng chuyn tn hiu quang thu c thnh tn hiubng tn c s ban u. N bao gm b tch sng quang v cc mch x l in. Btch sng quang thng s dng cc photodiode nh PIN v APD. Cc mch x l tnhiu in ny c th bao gm cc mch khuch i, lc v mch ti sinh.

1.1.2. Nguyn l hot ng ca h thng thng tin quang

Ngay t thi k khai sinh, h thng thng tin s dng nguyn l truyn thngtin theo m hnh chung nh hnh 1.1 trn. Nguyn l ny thc hin vic truyn thngtin t pha pht qua mi trng si quang v cui cng n pha thu. Ti mi phn tnhiu thng tin c bin i nh sau :

Pha pht : Ngun tn hiu thng tin nh ting ni, hnh nh, d liu sau khic x l tr thnh tn hiu in (c th dng tng t hoc s) s c a n b

pht quang (c th l ngun quang). Cc tn hiu in a vo b pht quang c

iu ch quang theo nhiu phng php iu bin khc nhau (iu bin trc tipcng nh sng hay iu bin gin tip) thu c tn hiu quang. Tn hiu quangny s c ghp vo si quang truyn i ti pha thu.

Mi trng si quang: L mi trng truyn dn nh sng (tn hiu c iuch quang) t u pht ti u thu. Trong qu trnh truyn dn ny, do c tnh quanghc ca nh sng v si quang m tn hiu quang b suy gim (suy hao v tn sc). Cly truyn dn cng di th nh sng b suy gim cng mnh, iu ny dn n kh khnkhi khi phc tn hiu pha thu. Do vy, trn tuyn truyn dn thng tin quang,

thng c cc b khuch i tn hiu quang v cc trm lp nhm ti to li tn hiu bsuy gim trn ng truyn.

Pha thu : Tn hiu thu c t mi trng truyn dn s c b thu quang tipnhn. Ti y, tn hiu quang s c bin i ngc tr li thnh tn hiu in nh tnhiu pht ban u. Cui cng ta thu c tn hiu cn thng tin.

1.1.3. u im ca h thng thng tin quang

H thng thng tin quang s dng mi trng truyn dn l si quang nn hthng c nhng u im hn cc h thng truyn thng s dng cp ng hay hthng thng tin v tuyn trc y, l :

Nhm 5 : Lp 2_TVT1 9

8/2/2019 58722586--an-vin-thong-nhom-5

8/59

n: H Thng Thng Tin Quang Dung lng truyn dn ln : Trong h thng thng tin si quang, bng tn

truyn dn ca si quang l rt ln (hng ngn THz) cho php pht trin cch thng WDM dung lng ln. So vi truyn dn v tuyn hay truyn dndng cp kim loi th truyn dn si quang cho dung lng ln hn nhiu.

Suy hao thp : Suy hao truyn dn ca si quang tng i nh, c bit ltrong vng ca s 1300nm v 1550nm. Suy hao nh nn si quang c thcho php truyn dn bng rng, tc ln hn rt nhiu so vi cp kim loicng chi ph xy dngs mng.

Khng chu nh hng ca mi trng bn ngoi : Bi vt liu ca siquang cch in, khng chu nh hng ca cc yu t nh in t trngnn khng b nhiu in t

tin cy : Tn hiu truyn trong si quang hu nh khng chu nh

hng ca mi trng bn ngoi, khng gy nhiu ra ngoi cng nh sxuyn m gia cc si quang. Do si quang thc t cho cht lngtruyn dn rt tt vi tin cy cao, tnh bo mt cng cao hn so vitruyn dn v tuyn v cp kim loi.

Chi ph thp : V vt liu ch to si quang sn c, ng thi si li nh hncp kim loi v c th un cong, lp t d dng v t b h hng do cc yut thin nhin tc ng (nh nng, ma) nn h thng c th tit kimc chi ph xy dng.

Thng tin si quang c nhiu u im t si quang em li tuy nhin si quangcng tn ti mt s nhc im nh kh ch to, hn ni phc tp v si quang rt b,v rt d t gy.

1.2. PHN LOI CC PHN T QUANG IN TRONG THNG TINQUANG

Mt h thng thng tin quang c cu thnh t rt nhiu phn t quang in khcnhau. Mt tuyn thng tin quang c th bao gm cc phn t nh th hin trn hnh1.3.

Cc phn t ny c nhiu c tnh, chc nng, tc hot ng v v tr khcnhau. Ty thuc vo yu cu ca h thng c s dng m cc phn t ny c sdng cho chc nng no hay v tr no trn h thng.

phn loi cc phn t quang in trong h thng thng tin quang ta c nhiutiu ch phn loi nh: c im. v tr, chc nng hay ng dng Da vo cim hot ng ca cc phn t quang in trong h thng thng tin quang c th chiathnh hai nhm l cc phn t th ng v cc phn t tch cc.

Nhm 5 : Lp 2_TVT1 10

Tnhi

uvo

Tn

hiu

ra

S

i

dnquang

B phtquang B ni

quang

Mi hn si

B chiaquang

MchinPht

quan

gKhuchi quang

u

thu

quang

Chuyn

i

tn hiu

Mchi

ukhin

Ngunpht

quang

B thu quang

Khuch i

Trm lp Thuquang

Hnh 1.3 Cc thnh phn trong mt tuyn thng tin quang.

8/2/2019 58722586--an-vin-thong-nhom-5

9/59


1.2.1. Cc phn t th ng

Cc phn t th ng l cc phn t quang hot ng khi c chm sng truyn quan. Phn t th ng hot ng khng cn ngun kch thch, n ch n thun bin icc tn hiu trong min quang m khng c s chuyn i sang min in. Nhngc im ny dn n v nguyn l hot ng cc phn t th ng ch yu da vocu trc quang hnh ca chnh bn thn chng, v tun theo cc nh lut hay ccnguyn l nh sng. Cc phn t th ng c nhng u im v cu trc, v tr lp t,v ng dng nh :

D dng lp t bt k v tr no trn h thng v khng cn c ngun cung cphot ng i km theo.

n gin v cu trc.

D dng bo tr.

An ton v in cho ngi s dng.

Tuy vy chng c nhng nhc im so vi phn t tch cc chnh l th ngv cu hnh nn kh nng thay i, iu chnh hot ng km, khng linh hot. Chtlng hot ng ca cc phn t th ng cng ph thuc vo vt liu v cng nghch to ca bn thn thit b nh cc vn v suy hao hay tn sc ca cc phn t thng. Cng ngh cng pht trin th kh nng ca cc phn t th ng cng cao.

Cc phn t th ng trong h thng thng tin quang bao gm :

Si quang, cp quang

Coupler quang


8/2/2019 58722586--an-vin-thong-nhom-5

10/59

n: H Thng Thng Tin Quang Cc b lc quang

B cch ly quang

B b tn tc

1.2.2. Cc phn t tch cc

Cc phn t tch cc l cc phn t quang in hot ng da theo vo tnh chtht ca nh sng v c s vt l bn dn. Khi hot ng, cc phn t tch cc da vokch thch in ngoi bin i tn hiu m n cn x l. Do vy khc vi cc phnt th ng, hot ng c cc phn t cn ngun kch thch. iu ny dn nyu cu ca phn t tch cc phc tp hn cc phn t th ng nh : v tr lp t, cch bo dng chng qu p ca ngun, yu cu an ton v in Tuy nhin cc

phn t tch cc c th iu chnh hiu qu hot ng khi thay i ngun cung cp.

Cc phn t tch cc bao gm : Ngun quang

B tch quang

B khuch i quang

Chuyn i bc sng

CHNG 2

CC PHN T QUANG TH NG

2.1. C S VT L CHUNG CHO CC PHN T TH NG


8/2/2019 58722586--an-vin-thong-nhom-5

11/59

n: H Thng Thng Tin QuangPhn t th ng ch n thun bin i cc tn hiu trong min quang m khng

c s chuyn i sang min in. Do vy c s vt l chung cho cc phn t th ngl vt l quang hnh.

2.1.1 S phn x v khc x nh sng

Hin tng khc x v phn x nh sng c xem xt trong trng hp c haimi trng khc nhau v ch s chit sut. Khi nh sng i t mt mi trng trongsut ny n mt mi trng trong sut khc th nh sng s thay i hng truynca chng ti ranh gii phn cch gia hai mi trng. Nh vy c hai kh nng xyra :

nh sng b i hng quay ngc tr li

nh sng c pht tip vo mi trng trong sut th 2.

Cc tia sng khi qua vng ranh gii gia hai mi trng b thay i hng nhngc th tip tc i vo mi trng chit sut mi th ta ni tia b khc x. Cn cc tiasng khi qua ranh gii ny li quay ngc tr li mi trng ban u th ta ni tia

b phn x. Hnh 2.1 m t qu trnh khc x v phn x nh sng qua hai mi trngtrong sut vi chit sut mi trng th nht n1 ln hn chit sut mi trng th hain2.

Trong : i l gc ti gc hp gia php tuyn ca mt phn cch hai mi trngvi tia ti. r l gc khc x - gc to bi php tuyn ca mt phn cch hai mitrng vi tia khc x.

hnh 2.1, chit sut n1 > n2 cho nn gc ti i nh hn gc khc x r (hnh2.1a). Khi gc ti ln dn ti mt gi tr gc ti c to ra tia khc x nm song songvi ranh gii phn cch hai mi trng, lc y c c gi l gc ti hn (nh hnh2.1b).

Nhm 5 : Lp 2_TVT1

Tiakhc x

Tiati

Tiaphnx

Hnh 2.1 S khc x v phn x nh sng ca vi gc ti khc nhau.

r

r

i

n2

n1

i

i=

c

a) b) c)

13

8/2/2019 58722586--an-vin-thong-nhom-5

12/59

n: H Thng Thng Tin Quang2.1.2. nh lut Snell

nh lut Snell pht biu : T l gia sin gc ti v khc x s lun l mt hng

s. Tia khc x lun nm trong cng mt phng vi tia ti v sin gc khc x ( r) phthuc vo sin gc ti (i) nh sau :

1

2

sinsin

nn

r

i = = a (hng s). (2-3)

Trong : n1, n2 l chit sut ca hai mi trng vt liu m nh sng i qua.

Khi mt tia sng ti c gi tr gc ln hn gc ti hn th nh sng b phn x honton li mi trng u ti mt phng phn cch hai mi trng. Lc ny ta gi lhin tng phn x ton phn (Total Internal Reflection). Hnh 2.1c minh ha qutrnh phn x ton phn - TIR.

Nh vy c th nu ra iu kin xy ra hin tng phn x ton phn l :

Cc tia sng phi i t mi trng c ch s chit sut ln hn sang mi trngc ch s chit sut nh hn.

Gc ti ca tia sng phi ln hn gc ti hn c =arcsin (n2 /n1).

nh lut khc x v phn x nh sng trn l nguyn l c bn p dng cho victruyn tn hiu nh sng trong si dn quang s dng trong thng tin quang. Trong sidn quang, cc tn hiu nh sng kt hp c lan truyn da vo hin tng phn xton phn, iu ny c th gii thch nh sau:

Xt nh sng truyn qua cc mi trng vi ng bin song song (ng thytinh). Cc mi trng ny c chit sut nh sau : chit sut mi trng u tin v mitrng cui cng bng nhau (cng l khng kh - n1), nhng khc vi mi trng trunggian (l thy tinh - n2 >n1).

- Khi nh sng ti mi trng u tin vi mt gc ti thch hp (gi s 1

8/2/2019 58722586--an-vin-thong-nhom-5

13/59


- Khi ngun sng t trong mi trng thy tinh th c mt s tia sng di khingun ti bin gii phn cch gia thy tinh v khng kh. Nu gc ti ca tia nh

hn gc ti hn c th n s b khc x v i ra khi mi trng thy tinh. Ngcli gc ti ln hn gc ti hn th s c s phn x ton phn trong mi trngthy tinh (nh hnh 2.3). Hn na, cc mt ca khi thy tinh song song vi nhaunn cc tia sng ti b mt s phn x bn trong ng vi cng mt gc bng gcti. Cc tia phn x s phn x lin tip trong thnh ng cho n khi t ti imcui ca ng. Ta c s truyn dn nh sng trong ng thy tinh.

.

2.1.3. Phn cc nh sngS phn cc c nh ngha thng qua in trng. Trong m t bi hm phc,

vect in trng ny c th c vit di dng sau :

E(z,t)= Re[Aexxp(it-ikz)] (2-4)

Trong A l vect phc trong mt phng xy.

Chng ta kho st hai thnh phn Ex v Ey nh sau :

Ex =[Acos(t- kz+x )] v Ey =[Acos(t- kz+y )] (2-5)

i lng A c th biu th dng sau : A = x Axexp(ix) + y Ayexp(iy)

Trong : Ax v Ay l cc s thc dng.


Tiasng

i

Tia phn x

n1

n2

n1

Hnh 2.3 Tia sng i trong ng thy tinh

y 'x '

x

Hnh 2.5 Phn cc thng thng ca nh sng theo elip c trcx v y lch mt gc .

y

8/2/2019 58722586--an-vin-thong-nhom-5

14/59


Sau khi bin i bng cch s dng tnh cht cc hm lng gic cc phng trnh2-4 v 2-5 ta c :

2sin

cos2 =+ yx

yxy

y

x

x EEAAA

E

A

Ev = x - y (2-6)

Phng trnh ny l phng trnh elip v c th kt lun sng nh sng trongtrng hp thng thng l c phn cc elip. Trc ca elip thng phi l trc x, y mlch i mt gc nh hnh 2.5. Gi tr ca gc c th xc nh c nh sau : tg

(2)= 222

yx

yx

AA

AA

cos . V t cc gi tr khc nhau ca ta c cc phn cc khc nhau

ca sng nh sng nh hnh 2.6. Nh trong hnh 2.6 cc dng phn cc : tuyn tnh,trn v elip i vi mt s sng truyn khc nhau.

2.2. SI QUANG

2.2.1. Cu trc si quang

Si quang c cu trc nh mt ng dn sng hnh tr bao gm phn li v lp vbao bc xung quanh li, c hai u lm t vt liu trong sut nh thy tinh hoc cht


=/4 = /2 = 3/4 =

=-3/4 = -/2 = -/4 = 0

Hnh 2.6 Cc trng thi phn cc i vi mt s sng truyn khc nhau

8/2/2019 58722586--an-vin-thong-nhom-5

15/59

n: H Thng Thng Tin Quangdo. Lp li thng c chit sut cao hn lp v bn ngoi, iu ny cung cp c chhng qu trnh truyn lan nh sng vo bn trong li.

Ngoi ra bo v si ngi ta dng mt lp bao bc bo v bn ngoi thnglm t vt liu polyme (nh hnh 2.7). Lp cht do ny nhm ngn chn cc tc ngc hc v bc si thnh cp.

Li (n1)

Hnh2.7Cutrcsi quang

V( n2) Lpbov

Thng thng ng knh li si quang l rt nh khong t 10 50 m, cnng knh v l 125 m. Do vy si quang c kch thc rt nh. Khi bc cc lp,

bo v th ng knh ca si mi t c t 200 900m.

2.2.2. Phn loi si quang

Si quang c rt nhiu loi khc nhau, ty thuc vo vic s dng v cch phnloi m ta c cc loi si quang khc nhau. Theo s phn b chit sut trong li singi ta chia si quang thnh si chit sut nhy bc (Step Index) v si chit sut

bin i (Graded Index). Si chit sut bc c phn b chit sut trong li khng itrong khi si chit sut bin i c chit sut li phn b gim dn t trong ra ngoi.

Ngi ta cn phn si quang thnh hai loi : si n mode (Single mode) si a

mode (Multi mode). Si a mode l si cho php truyn dn nhiu mode trong n, cnsi n mode l si ch cho php mt mode truyn dn trong n. (Vi mi mt model mt mu cc ng sng trng in v trng t c lp i lp li dc theo si cc khong cch tng ng vi bc sng).

Ngoi ra si cn c phn theo vt liu nh si thy tinh v si plastic. Hay ccloi si tin tin hin nay mi sn xut nh si duy tr phn cc v si dch tn sc.

Tuy vy trong thc t ngi ta thng xt cc loi si quang sau : Si a modechit sut nhy bc (MM-SI), si a mode chit sut bin i (MM-GI) v si n

mode (SM). 2.2.2.1. Si n mode(SM)

Si n mode l si ch cho php truyn dn mt mode trong n nhng kh nngv bng thng ca si kh ln (khong 40GHz). Si quang n mode ph hp i vih thng ng trc vi gi thnh thp. Mc d giai on u, si SM mi ch s dngtrong vng ca s 1300nm, nhng chng cng c th hot ng hiu qu trong vngca s 1550nm i vi cc h thng ghp knh theo thi gian TDM v ghp knh theo

bc sng WDM.

Cu trc si SM nh hnh 2.8


8/2/2019 58722586--an-vin-thong-nhom-5

16/59

n: H Thng Thng Tin QuangLi

VLplt

Lpbov

Chskhcx

Hnh2.8Cutrcsi nmode

Si n mode c li rt nh thng khong t 8 10 m. Kch thc nythng nh hn so vi bc sng nh sng c s dng rt nhiu. Thng th 20%nh sng c truyn vo si n mode b khc x ra ngoi v.

u im ca si n mode l ch ghp mt mode nn khng c tn sc mode bng

tn ca si tng ln. Tuy nhin, kh ghp nh sng vo si. 2.2.2.2. Si a mode chit sut nhy bc(MM-SI)

c im ca si MM-SI l kch thc ln, ng knh li thng l 50m. Sithng dng trong h thng truyn dn c c ly ngn vi bng thng si khong20MHz.

Cu trc mt ct chit sut c m t nh trong hnh 2.9.

a

a

n1

n2

V

Li

Hnh 2.9 Cu trc s i a mode nhy bc

Trong si MM - SI, chit sut li v v to thnh dng hnh bc thang. Thngthng, si c ch to vi chit sut v nho hn 10% so vi chit sut li.

Khu s (NA) ca si c trng cho kh nng nhn tia sng c tnh nh biuthc 2-7 :

NA= n1(2)1/2 (2-7)

Trong : =1

21

n

nn l chnh lch chit sut tng i gia li v v.

V ch s chit sut trong si MM-SI l khng thay i dc theo si nn khu s ca MM-SI cng l hng s.


8/2/2019 58722586--an-vin-thong-nhom-5

17/59

n: H Thng Thng Tin QuangS lng mode trong si a mode ph thuc vo tn s chun ha V ca si nh

cng thc 2-8:

M=V2/2 vi V=

NAa.2(2-8)

u im ca si a mode chit sut nhy bc l ch s chit sut ca v v likhng i, do tc truyn khng i. Tuy nhin do qung ng truyn dn cacc mode khc nhau nn c th gy nn hin tng tn sc mode.

2.2.2.3. Si a mode chit sut bin i (MM - GI)

c im kch thc ca si cng ging nh si MM-SI, tuy nhin si li c chs chit sut ca li thay i. S bin i ca ch s chit sut li c m t nhtrong cng thc 2-9.

n2(r)=

( )

=

8/2/2019 58722586--an-vin-thong-nhom-5

18/59

n: H Thng Thng Tin Quang 2.2.3. Cc tham s nh hng ti truyn lan trong si quang

Trong qu trnh truyn sng t pha pht n pha thu, tn hiu c th b thay irt nhiu. Do vy ti pha thu tn hiu khng c nh mong mun. S suy gim vcht lng tn hiu do rt nhiu yu t gy ra. Mt trong nhng yu t quan trng l

tham s gy nh hng ti truyn dn trong si quang. Ta xt cc tham s sau. 2.2.3.1. Suy hao

A. Khi Nim

Suy hao l thng s c lin quan n s thay i cng sut quang trong qa trnhlan truyn. Tham s suy hao c th c xc nh theo nh lut Beer :

Pdz

dP= trong l h s suy hao. (2-12)

P P - dPdz

L

z

Hnh 2.11 Cng sut vo ra ca tn hiu truyn qua s i quang v i di L

Bin i cng thc 2-12 ta c cng sut truyn ti khong cch L :

P(L) = P(0)exp(- L) hay)(

)0(ln

1

LP

P

L= (2-13)

Trong : P(0) tng ng cng sut vo u si Pin

P(L) tng ng cng sut ra si c chiu di L (Pout)

n v ca l m-1 hoc km-1

Theo n v dB th ta c : 343,4lg10

=in

out

dBP

P

L(2-14)

Trong thng tin quang c khi n v cng sut c tnh theo n v dBm nn hs suy hao c th tnh theo cng thc :

L

PP outindB

=

B.Nguyn nhn v cc loi suy hao

Suy hao trong si quang c nhiu nguyn nhn nhng nguyn nhn c bn gy suyhao trong si quang l do cc suy hao do hp th, do tn x v do b un cong si.

- Suy hao do hp th: Bn cht nh sng l cc ht photon, m si quang cng lvt rn c cu trc mng tinh th, nn cc in hay in t u nt mng c th hp

th photon khi nh sng truyn qua si quang. S hp th ny ph thuc vo bc


8/2/2019 58722586--an-vin-thong-nhom-5

19/59

n: H Thng Thng Tin Quangsng v bn cht ca vt liu hp th nh cc tp cht trong si hay vt liu ch tosi. C th, trong qu trnh sn xut si quang c rt nhiu tp cht nh cc in kimloi (Fe,Cu, Cr) hoc cc in OH-. Cc in ny gy nn cc nh hp th ti bc

sng chnh l 2,7 m v cc nh sng ph nh 0,94 m; 1,24 m; 1,39 m gynh hng n sng lan truyn trong si.

Bn cnh , bn thn vt liu chnh lm nn si quang l thy tinh cng gy nn

cc di hp th l hp th cc tm ch bc sng < 0,4 m v hp th hng ngoich bc sng >7 m nh hnh 2.13. Tuy nhin vi cng ngh hin i ngy nay,ngi ta c th gim thiu c s hp th bng cch loi tr cc tp cht hnh thnhtrong qu trnh sn xut (c bit l in OH-).

- Suy hao do tn x : Tn x l kt qu ca nhng khuyt tt hay nhiu lan trongsi v cu trc vi m ca si. Tn x suy ra t nhng thay i v cu trc phn t vnguyn t ca thy tinh hay t nhng thay i v mt v thnh phn si. Nhngthay i ny do qu trnh sn xut si to ra. N gy nn s thay i v chit sut dnn thay i s phn x ca tia sng ti nhng im trn li si m ta c th gi l cctm tn x. Xt hnh 2.12 sau :

V

LiTia t i

Hnh 2.12 S t n x

Gc lan truyn ca tia sng ti giao din li v v c nhng thay i lm thay itia c khc x theo ng dn mi v khng xy ra hin tng phn x ni ton

phn (TIR), iu ny gy gim lng nh sng c lan truyn dc theo li si. C hailoi tn x chnh l : Tn x Rayleigh v tn x Mie, trong tn x Rayleigh rt quantrng. Nguyn nhn ca n l do s khng ng nht ca thy tinh v thnh phn v

mt . iu ny gy nn s thng ging v ch s chit sut v dn n suy gimcng sut bc sng theo cng thc sau : R=C/4 vi hng s C nm trong di 0,70,9 dB/km v ph thuc vo cu trc si. Cn tn x Mie l tn x xy ra ti nhng nikhng ng nht, nh nhng im c khuyt tt trong cu trc si hay s khng ngu ca ch s chit sut v bt kh to ra trong qu trnh sn xut. Tuy nhin ta c thcoi tn x Mie l khng ng k bng cch ch trng ti qu trnh sn xut gimthiu cc nguyn nhn gy tn x.

Nhng suy gim bi s tn x l mt qu trnh tuyn tnh, bi n khng gy ra sdch tn, bc sng trc v sau tn x khng thay i.


8/2/2019 58722586--an-vin-thong-nhom-5

20/59


Hnh 2.13 Cc ph suy hao do hp th v tn x trong si quang [9].

- Suy hao do un cong si: y l nhng suy hao do s un cong v thay i v

bn knh cong ca si. C hai loi suy hao do un cong l : suy hao do un cong cnh v suy hao do un cong c ln. Suy hao do un cong c ln xy ra khi bn knhcong ca si gim. Ban u bn knh cong ca si ln hn bn knh si. Khi si b uncong th gc lan truyn s thay i dn n mt s tia sng khng cn m bo iukin phn x ton phn v dn n gim s lng tia sng truyn trong li si. Do khi bn knh cong gim th mc suy hao s tng. Bn knh cong cho php l Rc = a/NA.Trong thc t yu cu bn knh cong phi ln hn bn knh cong cho php suy haokhng vt qu 0,1dB.

Suy hao do un cong c nh l do cc un cong c bn knh cong nh theo trc

si xut hin do trong qu trnh ci t, o kim hay thit lp c cc lc tc ng lnsi quang lm si b mo dng v thay i cc gc lan truyn ca cc tia sng. nhsng s b mt mt ra ngoi v si. Ngoi ra n cn gy ra qu trnh ghp cp mode.

2.2.3.2 Tn sc

A.Khi nim

Tn sc trong thng tin quang xt v mt thi gian l s dn rng xung nh sngkhi lan truyn trong si quang nh hnh 2.14.

H s tn sc c xc nh theo cng thc :

L

TD

= (ps/nm.km) trong T = ra - vo (ps/nm)

L

vo

raSi quangHnh 2.14 S t n sc

Tn sc c th c gii thch bi nh sng truyn trong si quang c th coi l tphp ca nhiu thnh phn v d c th coi l cc thnh phn trong bin i Fourier ca


8/2/2019 58722586--an-vin-thong-nhom-5

21/59

n: H Thng Thng Tin Quangn hay tng cc mode truyn. Cc thnh phn ny c tn s khc nhau zmode truyndn vi vn tc nhm l tc m ti nng lng trong mode ring bit lantruyn dc theo si. Ta c :

Vn tc nhm :

d

dvg = v tr nhm l :

dK

d

cvg

11 == (2-15)

Trong : l h s lan truyn c gi tr = n/c

K l h s sng c gi tr K = /c.

B. Nguyn nhn v cc loi tn sc

C bn nguyn nhn chnh gy ra tn sc v cng l cc loi tn sc ch yu l : Tn sc vt liu, tn sc ng dn sng, tn sc mode v tn sc phn cc mode.

- Tn sc mode: Nguyn nhn chnh l do trong si c nhiu mode truyn dn, ccmode li c tc truyn dn khc nhau, nn thi gian truyn dn trong si cng khcnhau, v xy ra hin tng tn sc. Loi tn sc ny ch xy ra trong si a mode. Tnsc mode ph thuc vo kch thc si c th l bn knh li si a mode. Tia kinhtuyn truyn trong cc si a mode (chit sut nhy bc v bin i) s i theo ccng khc nhau vi qung ng khc nhau. Gc truyn lan ca tia cng dc th tiai cng chm. Do c nhng tia thi gian truyn l T min v c nhng tia thi giantruyn l Tmax.. Ta c h s tn sc mode:

c

n

LDm

=

= 1 (2-16)

Gia hai si a mode chit sut nhy bc v chit sut bin i th si chit sutbin i c mo tn hiu t hn. Do chit sut li trong si MM-GI gim dn t trcsi ra pha v, nn cc tia sng c ng i gn ranh gii tip gip v - li s truynvi vn tc nhanh hn cc tia gn trc si cho nn cn bng c thi gian truyn.

- Tn sc vt liu: Nguyn nhn ca loi tn sc ny l do tn sc bn trong. N ls dn xung tn hiu nh sng xy ra trong mt mode. Trong si c s thay i vchit sut do vt liu to ra. T c s khc bit v tc ca cc thnh phn ph(bc sng) khc nhau chy trong mode v vn tc ca cc bc sng ph thuc vo

chit sut theo phng trnh :

)(n

cv = (2-17)

Do vy m c s chnh lch lan truyn ca cc thnh phn bc sng khc nhaudn n tn sc. Ta c h s tn sc vt liu :

2

2 )(1

d

nd

cDM = (2-18)

Vi chit sut ph thuc vo bc sng theo cng thc Sell Miner :


8/2/2019 58722586--an-vin-thong-nhom-5

22/59


=

+=M

i i

iin1

22

2

21)(

Trong : i ,i l cng v tn s cng hng tng ng.

M l tham s ph thuc vo vt liu (v d M thy tinh =3).

- Tn sc ng dn sng : Cng nh tn sc vt liu, tn sc ng dn sng l do stn sc bn trong mode. nh sng truyn trong si khng phi l n sc, n chimmt rng ph no . V hng s lan truyn l hm ca i lng a/ do n

ph thuc vo ph (bc sng ) nh sng v kch thc ca li si. Mt khc theocng thc 2-18, tr nhm chu nh hng ca h s lan truyn . Do vy vn tcnhm ca cc thnh phn ph l khc nhau ( y cha xt n s thay i vn tc dos thay i chit sut). iu ny dn n s chnh lch v thi gian truyn dn v vvy c hin tng tn sc.

T cc phng trnh ca tr nhm ta xc nh c h s tn sc ng dn sng.

])(

[2

21

dV

VbdV

C

nDw

= (2-19)

Vi tn s chun ha c tnh gn ng theo cng thc : 210anKV (2-20)

Trong si a mode tn sc ng dn sng tng i nh so vi tn sc vt liu vvy c th b qua c.

Tn sc tng trong si s bao gm c ba loi tn sc : D = D w + Dm +DM

Ta c hnh 2.15 biu din cc tn sc trong di bc sng 1,1 - 1,7 m ca sin mode. Bc sng ZD l bc sng ti tn sc tng c gi tr 0. R rng tn scng dn sng lun m, cn tn sc vt liu th bt u dng ti bc sng

1.33 m.

Hnh 2.15 Cc h s tn sc trong si n mode[9]

- Tn sc phn cc mode (PMD) :

Tn sc phn cc mode (Polarization Mode Dispertion) l mt c tnh c bn casi quang v cc thnh phn si quang n mode. Trong nng lng tn hiu ti

bc sng cho c chuyn vo hai mode phn cc trc giao c vn tc lan truyn

khc nhau. iu ny dn n dn xung ca tn hiu tng hp u thu v gy chng ph


8/2/2019 58722586--an-vin-thong-nhom-5

23/59

n: H Thng Thng Tin Quangv li bt i vi cc h thng truyn dn tc cao. Khi c ngun pht quang mnh,s dn xung thng lin quan n hiu ng lng chit. Bn knh ca si quang dctheo cc trc x, y khng ng u, chit sut cc mode phn cc theo trc x, y b thayi v sinh ra hiu ng lng chit. Khi xung u vo kch thch c hai thnh phn

phn cc, xung s dn rng u ra ca si v hai thnh phn s phn tn dc do

vn tc nhm khc nhau. Hin tng ny gi l PMD. Lng chit c th tnh tonthng qua s chnh lch gi tr chit sut hiu dng ca hai thnh phn phn cc mode:

= ( ) yxyx Knn = 0 (2-21)

Trong : yx nn , l chit sut hiu dng ca mi mode phn cc

yx , l hng s lan truyn ca mi mode phn cc

i vi mt xung quang th nng lng chia ra thnh hai phn : mt phn mang

bi trng thi phn cc trc nhanh v mt phn mang bi trng thi phn cc trcchm nh hnh 2.16.

Hnh 2.16 Hin tng tn sc do phn cc.

S dn xung c th c xc nh t chnh lch thi gian T gia hai thnhphn phn cc mode trc giao khi xung c truyn. i vi si di L th T cxc nh theo cng thc 2-22:

T = gygx vL

v

L

=L x - y (2-22)

Biu thc 2-22 khng th dng trc tip tnh tham s PMD do tnh ghp ngunhin gia hai mode c sinh ra t s xo trn ngu nhin ca lng chit. Thc tPMD c xc nh bi gi tr cn trung bnh bnh phng RMS ca T. Gi tr trung

bnh ca bin ngu nhin ny l :

( )

+==

h

L

h

LhTT

2exp1

2

2

1 2222

Trong : h l di hiu chnh c gi tr tiu biu nm trong khong 1- 10 m.


8/2/2019 58722586--an-vin-thong-nhom-5

24/59

n: H Thng Thng Tin QuangTa c hm mt xc sut PMD nh sau :

P(T) =3

2

2

2

Texp

2

2

2

T(2-23)

Nh vy kt hp vi phng trnh 2-22 gi tr trung bnh ca tr T lin quan

n tn sc phn cc mode nh sau :

Lh

DT PMD

8=

Trong : DPMD h s tn sc phn cc mode vi cc gi tr tiu biu nm trongkhong 0,1 1ps/ km .

T y ta thy dn xung trong PMD tng i nh so vi cc hiu ng vn tcnhm. Tuy nhin do ph thuc vo chiu di si nn PMD c th nh hng n cc

h thng truyn dn c ly xa.2.3. COUPLER QUANG

Coupler l cc thit b quang th ng n gin, c s dng tch hoc ghptn hiu nh sng u vo hay u ra si. Mt coupler bao gm n cng vo v m cngra. Coupler 1 x n c gi l b tch (splitter), cn coupler n x 1 c gi l b kthp (combiner); c khi coupler kt hp c hai chc nng ghp v tch vi n cng vov m cng ra.

n gin nht l coupler 1x2, 2x1 v 2x2 nh hnh 2.17a, b,c

B chia quang 1x2 nh trn hnh 2.17 a) c t l cng sut u ra c gi l t lchia quang v c th iu khin c. Gi tr ny biu th t l chia quang di

dng dB s cho chng ta suy hao do chia quang. B chia quang hai cng vi t l chiaquang 50:50 l rt ph bin, kt qu l suy hao do chia quang s l 3 dB cho mi cngra.

Cc b coupler c dng tch mt phn cng sut t lung nh sng c thc thit k vi cc gi tr rt gn vi 1, thng l t 0.90 ti 0.95. Khi chngc gi l b r v thng dng cho cc mc ch gim st hoc cc mc ch khc.

Nguyn l hot ng ca coupler c th xt thng qua nguyn l chung cacoupler 2x2.


a, Splitter b, Combiner c, Coupler Hnh 2.17 Coupler 1x2, 2x1 v 2x2.

8/2/2019 58722586--an-vin-thong-nhom-5

25/59

n: H Thng Thng Tin Quang2.4. B LC QUANG

2.4.1. Chc nng ca cc b lc

Vic ghp v lc l mt phn quan trng ca truyn dn quang. Khng c thit bny khng th thc hin bt k s chuyn mch cng nh truyn dn mt vi tn hiu

trong cng mt si quang ti cng mt thi im. B lc quang l phn t th nghot ng da trn cc nguyn l truyn sng khng cn c s tc ng t cc phn t

bn ngoi. Chc nng ca b lc l lc tn hiu khc nhau c truyn trong cng mtsi, trc tin phi tch ring cc bc sng khc nhau khi tn hiu tng. C rt nhiucch thc hin vic tch cc bc sng quang, nhng v nguyn l chng u datrn quan im : cc bc sng s b tr pha so vi bc sng khc khi chng chng qua cc ng dn khc nhau. Ty thuc vo cch nguyn l hot ng catng thit b m ta c hai nhm cc b lc khc nhau nh : B lc c nh v b lciu khin c. B lc quang c nh l cc b lc v nguyn l n loi b tt c cc

bc sng, ch cho php gi li mt bc sng c nh c xc nh trc. B lciu chnh c l cc b lc c th thay i bc sng m chng cho qua ty theoyu cu.

2.4.2. c im, tham s ca b lc

Hai c im quan trng ca b lc cn c nhc n l di ph t do (FSR- FreeSpectral Range) v kh nng phn bit ca b lc hay mn (F - Finesess).

A. mn ca b lc F

mn ca b lc c o bng rng ca hm truyn t. N l t s gia di

ph t do vi rng knh.f

FSRF

=

Trong rng knh (f) c nh ngha l rng 3dB hay rng ph nacng sut ca b lc. f c trng cho hp ca nh hm truyn t.


FSR

1 2 3 N 1 2

ff

.. ..

f1

f2

f3

fn

f

P

Hnh 2.20 FSR v F ca b lc vi N knh khc nhau.

8/2/2019 58722586--an-vin-thong-nhom-5

26/59

n: H Thng Thng Tin QuangS lng knh ca mt b lc quang b gii hn bi di ph t do v mn. Tt

c cc knh phi nm gn trong FSR. Nu gi tr F cao, hm truyn t (nh bngthng) s hp v dn n l c nhiu knh c cha trong di ph t do hn. Khi mn thp, cc knh cn phi c dn cch nhau thm mt khong trnh xuyn m.Do s lng knh trong di ph t do cng gim i.

2.4.3 B lc quang

2.4.3.2 B lc cch t Bragg si

Cch t Bragg si l mnh bin iu ca si quang m trong chit sut ca lisi thay i theo mt chu k dc theo li si quang.

L

Si quang

Hnh 2.23 C ch t Bragg s i

Cch t Bragg hot ng theo nguyn tc : Khi chiu mt chm nh sng a scqua cch t, n cho php phn x duy nht mt bc sng tha mn iu kin phn xBragg c phn x tr li ngun v cho i qua tt c cc bc sng khc. T iukin phn x Bragg ta c :

mmLn =2 (2-34)Trong : n l chit sut li si quang.

Ti cc bc sng khng tha mn iu kin trn th nh sng khng b nh hngv c truyn qua cch t n u thu.

B lc cch t Bragg c suy hao xen thp, c tnh ph c dng b lc bng thng(BPF) vi kh nng t c khong cch gia cc knh l 50 GHz. Hai tham s quantrng nht ca mt b lc cch t Bragg l h s phn x v rng ph. Thng rng ph vo khong 0, 1 nm trong khi h s phn x c th t hn 99 %. u imca chng l n gin v cu to v s dng, ng thi li c h s suy hao xen thp.Cn v nhc im l c ch s chit sut ph thuc vo nhit .

Cch t Bragg c th c s dng nh mt b ghp hay tch khi kt hp vi ccb coupler quang. Nh hnh 2.24 ta c hai cch t Bragg kt hp cng hai couplerquang 3dB.

Khi a chm tia sng a sc c bc sng l 1, 2, vo cng 1, chm sng quacoupler 3dB th nht c chia thnh hai lung n hai cch t. Gi s bc sng 1tha mn iu kin phn x Braggm th nh sng c bc sng 1 s b phn x bicch t v ti cng ra 4 ta tch c bc sng 1.


8/2/2019 58722586--an-vin-thong-nhom-5

27/59

n: H Thng Thng Tin Quang 2.4.4 B Isolator v Circulator

Isolator l thit b cho php truyn dn ch theo mt hng v khng cho truyndn theo hng no khc na. N hot ng da theo nguyn l phn cc ngncch tn hiu. Bng cch s dng cc b ny th cc phn x t cc b khuch i hay

laser c th c cch ly khi tn hiu.Circulator l mt thit b tng t Isolator, nhng n c nhiu cng. Hnh 2.27 m

t mt Circulator vi 4 cng vo v 4 cng ra. Tn hiu t mi cng c hng timt cng ra v b ngn ti cc cng cn li.

ng dng ca b cch ly ny c th l dng trong cc module tch ghp knhquang. Tn hiu ti u ra mi b pht mt bc sng ring, nhng tia sng nyc ghp li v truyn vo si quang. Thit b thc hin chc nng ny gi l bghp knh quang (Multiplexer hay MUX). Ngc li, pha thu c mt thit b tchtn hiu quang thu c thnh cc knh quang c bc sng khc nhau a n mi

b thu quang ring bit.

CHNG 3

CC PHN T TCH CC

3.1. C S VT L CHUNG CA CC PHN T TCH

Khc vi cc phn t th ng, c s vt l chung cho cc phn t tch cc l vtl bn dn. Tuy nhin do tn hiu x l ca cc phn t ny l nh sng nn cc kinthc vt l v nh sng (nh nu chng 1) cng c s dng trong phn t tchcc.

Khi hot ng, cc phn t ny cn phi c ngun kch thch. Cc ngun nylun i km theo nn yu cu ca cc phn t tch cc cng phc tp hn phn t th

ng. V tr t thit b, cc vn v bo dng, an ton v in cng cn c quantm. T a xt n c s vt l chung cho cc phn t tch cc ny.


1 23

2

3 4

1

4

Hnh 2.24 Circulator 4 cng ra v 4 cng vo

8/2/2019 58722586--an-vin-thong-nhom-5

28/59

n: H Thng Thng Tin Quang3.1.1. Cc khi nim vt l bn dn

Vt l bn dn l c s hot ng cho rt nhiu linh kin in t trong c ccphn t tch cc hot ng trong h thng thng tin quang.

3.1.1.1 Lp tip gip p-n

Bn thn cc vt liu pha tp loi p hay n ch nh l nhng cht dn in tt hnso vi bn dn thun. Tuy nhin khi ta s dng kt hp hai loi vt liu ny th s cc nhng c tnh ht sc ng ch . Mt vt liu loi p c ghp vi vt liu loin s cho ta mt lp tip xc c gi l tip gip p-n. Khi tip gip p n c to ra,cc ht mang a s s khuch tn qua n : L trng bn p khuch tn sang bn n, int bn n khuch tn sang bn p. Kt qu l to ra mt in trng tip xc E tx tngang tip gip p n. Chnh in trng ny s ngn cn cc chuyn ng ca ccin tch khi tnh trng cn bng c thit lp. Lc ny, vng tip gip khng c

cc ht mang di ng. Vng ny gi l vng ngho hay vng in tch khng gian.Khi cp mt in p cho tip gip ny, cc dng ngun ni vi vt liu n, cc m

ni vi vt liu p th tip gip ny c gi l phn cc ngc. (Nh hnh 3.2b). Nuphn cc ngc cho tip gip p n, vng ngho s b m rng ra v c hai pha. iuny cng cn tr cc ht mang a s trn qua tip gip. Tuy nhin vn c mt s lngnh ht mang thiu s trn qua tip gip ti iu kin nhit v in p bnh thng.Cn khi phn cc thun cho tip gip (cc m ni vi vt liu n, cn cc dng nivi vt liu p nh hnh 3.2c) th cc in t vng dn pha n v cc l trng vng ha

pha p li c php khuch tn qua tip gip. Lc ny vic kt hp cc ht mangthiu s tng ln. Cc ht mang tng ln s ti hp vi ht mang a s. Qu trnh tikt hp cc ht mang d ra chnh l c ch pht ra nh sng.


Loi n Loi pLoi n Loi p

Loi n Loi p

Vng nghoTip gipp-n

Hnh 3.2 Phn cc cho cc lp tip gip

a) Tip gip p-n

c) Phn cc thunb) Phn cc ngc

8/2/2019 58722586--an-vin-thong-nhom-5

29/59


Cc cht bn dn thng c phn ra thnh vt liu c gii cm trc tip v vt liuc gii cm gin tip ty thuc dng ca di cm (nh hnh 3.3).

Xt qu trnh ti hp ca l trng v in t km theo s pht x photon, ngi tathy qu trnh ti kt hp d xy ra nht v n gin nht khi m l trng v in t c

cng ng lng. Trong trng hp ny ta c vt liu gii cm trc tip. Cn trongtrng hp vt liu c di cm gin tip, cc mc nng lng nh nht vng dn vcc mc nng lng nh nht vng ha li xy ra cc gi tr ng lng khcnhau. Nh vy vic ti kt hp y cn phi c phn t th ba duy tr ng lng

bi v ng lng photon l rt nh.

3.1.2. Cc qu trnh c trng trong vt l bn dn

3.1.2.1. Qu trnh hp th v pht x

Trong vt liu, iu kin bnh thng c xy ra cc qu trnh tng tc gia vt

cht v mi trng xung quanh, v to ra cc hin tng pht x, bc x hay hpth phn tch cc qu trnh pht x v hp th ta xt mt h c hai mc nnglng E1 v E2 vi E2 > E1 nh hnh 3.4 sau. Trong E1 l trng thi c s, cn E2 ltrng thi kch thch.


Nnglngvngcm

trctip

Nnglngvngcm

gintip

Edir

Chuyn dch in t

hf=Edir

Eind

hf=Eind +Eph

Nng lngphoton E

ph

a) b)

Hnh 3.3 S pht photon vi vt liu di cm trc tip (a) v gin tip (b)

Vng dn Vng dn

Vng ha trVng ha tr

8/2/2019 58722586--an-vin-thong-nhom-5

30/59


Khi photon c nng lng hf = E2 E1 i vo vt cht, in t s hp th vchuyn ln mc kch thch E2. y l qu trnh hp th nh sng. Cc in t mckch thch E2, y l trng thi khng bn nn n nhanh chng chuyn v mc c s E1v lc s pht ra mt photon c nng lng l hf = E 2 E1. Ta c qu trnh pht x

t pht. Photon c to ra t pht th c hng ngu nhin v khng c lin h vpha, tc l nh sng khng kt hp. Cn pht x cng bc xy ra khi c mt photonc nng lng ph hp tng tc vi nguyn t trng thi kch thch v pht x racc photon ging ht nhau v nng lng v pha. Ta c cc phng trnh tc ctrng cho cc qu trnh ny nh sau :

Tc pht x t pht : Rspon=A.N2

Tc pht x kch thch : Rstim = B.N2.

Tc hp th : Rabs = C. N1.

Trong : N1, N2 l mt nguyn t ti mc E1 v E2,

l mt ph nng lng chiu x.

iu kin cn bng nhit th mt ph nng lng chiu x phn b theo thngk Boltzman nh sau :

)exp()exp(1

2

Tk

hf

Tk

E

N

N

BB

g== (3-2)

Trong : T l nhit tuyt i ca h nguyn t.

N1, N2 khng ph thuc thi gian trong trng thi cn bng nhit, ngha l tc chuyn di ln xung ca nguyn t phi bng nhau. Do :

A.N2+ B.N2.= C. N1. (3-3)

T cng thc 3-2 v 3-3 ta c mt ph nng lng c tnh nh sau :

1)exp( =

Tk

hf

B

C

BA

B

(3-4)

Nhm 5 : Lp 2_TVT1

E2

E1

hf hf hf1

hf2

hf

a, Hp th b, Pht x t pht c, Pht x kch thch

32

Hnh 3.4 S qu trnh hp th, pht x v pht x kch thch

8/2/2019 58722586--an-vin-thong-nhom-5

31/59

n: H Thng Thng Tin QuangTheo cng thc Plank mt ph nng lng chiu x phi bng mt ph pht

x vt en tuyt i :1exp

8 3

3

=

Tkhf

c

fh

B

Nh vy : Bc

hfA

3

38= v C=B (vi A, B l h s Anhxtanh).

3.1.2.2 Trng thi o mt

nh sng c th pht ra t vt liu bn dn l kt qu ca qu trnh ti hp in tv l trng (e-h). Trong iu kin cn bng nhit, t l pht x kch thch rt nh so vi

pht x t pht, tc l nng e h sinh ra do kch thch rt thp. c pht x kchthch ta phi thc hin tng s lng ln cc in t v l trng trong vng dn v

vng ha tr. Ta xt mt tip gip p n vi hai loi vt liu bn dn loi n v p pha tpcao n mc suy bin. Mc Fermi bn bn dn loi n nm vo bn trong vng dn vmc Fermi trong bn dn p nm vo bn trong vng ha tr. Ti cn bng nhit mcFermi hai bn bn dn loi n v p nm trng nhau, lc ny khng c qu trnh bm htti (hnh 3.5a). Khi phn cc thun ln, cc mc Fermi hai min tch ra, lc nyth cc in t bn bn dn loi n v l trng bn bn dn p c bm in tch khnggian (hnh 3.5b). Khi in th t vo tip gip p-n tng ln qu trnh bm nyt n mc cao th trong min in tch khng gian c rng l d s c mt slng ln cc in t nm trn vng dn v mt s lng ln l trng nm di vngha tr. Trng thi ny gi l o mt .


8/2/2019 58722586--an-vin-thong-nhom-5

32/59


Nh vy iu kin c trng thi o mt l bn dn hai min p v n phipha tp mnh cc mc Fermi nm vo bn trong vng dn v vng ha tr. Thphn cc thun phi ln in t v l trng c th bm vo vng dn v vng

ha tr. Hiu hai mc Fermi hai vng bn dn loi n v p ln hn nng lng vngcm, ngha l : Efc Efv > Eg.

Trn y l cc c s vt l bn dn phn tch c ch hot ng ca cc phn ttch cc trong thng tin quang c cp trong cc phn tip theo.

3.2. NGUN QUANG

Vai tr ca cc b pht quang l bin i tn hiu in thnh tn hiu quang v atn hiu quang ny vo si quang truyn ti pha thu. Linh kin chnh trong b pht

quang l ngun pht quang. Trong h thng thng tin quang cc ngun quang c sdng l it pht quang (LED) v laser bn dn (Laser Diode LD). C s vt l cacc ngun quang bn dn ny nh nu trn. Chng c nhiu u im nh : kchthc nh, hiu sut chuyn i quang in rt cao, c vng bc sng pht quangthch hp vi si quang v c th iu bin trc tip bng dng bm vi tn s khcao.

3.2.1. it pht quang.

LED (Light Emitted Diode) l mt loi ngun pht quang ph hp cho cc h

thn thng tin quang c tc bt khng qu 200Mb/s s dng si dn quang a

Nhm 5 : Lp 2_TVT1

P NE

c

Efv

Ev

Efc

Eg

Ec

Ev

Efv

Eg E

fc

VF

Ec

Ev

Efv

Efc

VF

d

hf

a, Ban u chabm

b, Mc Fermi tchra khi c phn ccthun

c, Bm cao pht xphoton

Hnh 3.5 Gin nng lng ca tip gip p-n vi bn dn suy bin

34

8/2/2019 58722586--an-vin-thong-nhom-5

33/59

n: H Thng Thng Tin Quangmode. Tuy nhin hin nay trong phng th nghim ngi ta c th s dng c tc

bt ti 556 Mb/s do c s ci tin cng ngh cao.

3.2.1.1 Cu trc LED

C hai loi cu trc LED c s dng rng ri l cu trc tip gip thun nht v

cu trc tip gip d th. Trong qu trnh nghin cu v thc nghim, cu trc d thkp mang li hiu qu hn v c ng dng nhiu hn. c im ca cu trc d thkp l c hai lp bn dn khc nhau mi bn ca vng bn dn tch cc, y cngchnh l cu trc khai trin nghin cu LASER . Vi cu trc d th ta c, hai loi l cu trc pht x mt v pht x cnh.

A. Cu trc LED pht x mt

LED pht x mt c mt phng ca vng pht ra nh sng vung gc vi trc casi dn quang (hnh 3.6a). Vng tch cc thng c dng phin trn, ng knh

khong 50m v dy khong 25m. Mu pht ch yu l ng hng vi rngchm pht khong 120o . Mu pht ng hng ny gi l mu Lambertian. Khi quanst t bt k hng no th rng ngun pht cng ngang bng nhau nhng cngsut li gim theo hm cos vi l gc hp gia hng quan st vi php tuyn ca

b mt. Cng sut gim 50% so vi nh khi =60.

B. Cu trc LED pht x cnh

LED pht x cnh c cu trc gm mt vng tip gip tch cc c vai tr l ngunpht nh sng khng kt hp, v hai lp dn u c chit sut thp hn ch s chit

sut ca vng tch cc nhng li cao hn chit sut ca cc vng vt liu bao quanh(hnh 3.6b). Cu trc ny hnh thnh mt knh dn sng hng s pht x v phali si. tng hp c vi li si dn quang c ng knh nh ( c 50- 100m),cc di tip xc i vi LED pht x cnh phi rng t 50m n 70m. di cacc vng tch cc thng l t 100m n 150m. Mu pht x cnh c nh hngtt hn so vi LED pht x mt.

3.2.1.2 Nguyn l hot ng ca LED

Nguyn l lm vic ca LED da vo hiu ng pht sng khi c hin tng ti hp

cc in t v l trng vng tip gip p-n. Do vy, LED s pht sng nu c phncc thun. Khi c phn cc thun cc ht mang a s s khuch tn t qua tipgip p-n : in t khuch tn t pha n sang pha p v ngc li, l trng khuch tn t

pha p sang pha n, chng gp nhau v ti hp pht sinh nh sng. Vi cu trc d thkp, c hai loi ht dn v trng nh sng c giam gi ti trung tm ca lp tchcc (hnh 3.7). S khc nhau v rng vng cm ca cc lp k cn giam gi ccht in tch bn trong lp tch cc. ng thi, s khc nhau v chit sut ca cclp k cn ny giam gi trng quang v cc ht dn ny lm tng bc x vhiu sut cao.


8/2/2019 58722586--an-vin-thong-nhom-5

34/59


mt cht bn dn pht sng th s cn bng nhit phi b ph v. Tc ti hptrong qa trnh ti hp c bc x t l vi nng in t trong phn bn dn p vnng l trng trong bn dn n. y l cc ht dn thiu s trong cht bn dn. tng tc ti hp tc l tng s photon bc x ra th cn phi gia tng nng htdn thiu s trong cc phn bn dn. Nng ht dn thiu s c bm vo cc phn

bn dn t l vi cng dng in ca LED, do cng pht quang ca LEDt l vi cng dng in qua it.


Cht nn

Cht nn

Ta nhit

Kim loi

Lp dn nhsng

Gii tip xc

Min hot tnhSiO

2

nh sng pht ra

Hnh 3.6b Cu trc LED pht x cnh

Si quang

Phin chu nhit

SiO2

SiO2

Cc lptip gip

Ging khchnh trn Vt liu bao ph

in cc

Lp cutrc dth kp

Hnh 3.6a Cu trc LED pht x mt

Cc lp dth kp

8/2/2019 58722586--an-vin-thong-nhom-5

35/59


3.2.1.3. ng dng ca LED

Thng th nh sng pht x ca LED l nh sng khng kt hp v l nh sng tpht. Do cng sut pht x ca LED thp, rng ph rng v hiu ng lng t

thp. N thng ch c p dng cho cc mng c khong cch ngn nh mngLAN. Tuy nhin do cng sut u ra ca n t ph thuc vo nhit v c ch ton gin, n nh cao, LED vn c s dng rng ri trong cc h thng truyntc thp.

3.2.2. Ngun pht laser (Light Amplication By Stimulate Emission ofRadiation )

- C rng ph hp

- Bc song n nh- c s dng vi si n mode

- Cho php s dng vi h truyn dn tc cao v c ly di.

3.2.2.1 Nguyn l hot ng

L kt qu ca 3 qu trnh : hp th proton , bc x t pht v bc x kch thch.

- Hp thu: khi proton c nng lng ln hn E2 E1 c th b hp th bi ccin t ti mc nn v chuyn ti mc kch thch.


Eg

Ti hp in

t v l trnghf

Dng l trng

Dng in t

Nnglngin t

Vng dn sng

Vng tchcc

Ch s

chitsut

Hnh 3.7 Cu trc d th kp hiu sut pht x cao nh chnh lch: a) rngvng cm v b) chnh lch chit sut

a)

b)

8/2/2019 58722586--an-vin-thong-nhom-5

36/59


- Bc x t pht: in t trng thi kch thch c th b chuyn v trng thi nnv pht ra proton c nng lng hv= E2 - E1

- Bc x kch thch: nu proton c nng lng hv tc ng vo h thng trong khiin t ang trng tri kch thch th in t s b ri xung trng thi nn v

pht ra proton c hng cng hng ca proton ti.

3.3. B THU QUANGB thu quang l phn chu trch nhim chuyn i tn hiu quang thu c t mi

trng truyn dn sang tn hiu in v phc hi cc s liu truyn qua h thngthng tin quang ny. Linh kin ch yu thc hin chc nng chuyn i quang introng b thu quang l cc b tch quang cn c gi l detector. Hai b tch quangthng c s dng trong thng tin quang l photodiode loi PIN v APD.


8/2/2019 58722586--an-vin-thong-nhom-5

37/59

n: H Thng Thng Tin Quang3.3.1. Photodiode PIN

y l b tch sng quang thng dng nht c s dng. c im ca ccPhotodiode PIN l c thi gian p ng nhanh v hiu sut lng t cao. N khng ckh nng khuch i dng quang in nhng n li trnh c s khuch i nhiu.

3.3.1.1. Cu trc ca PINCu trc c bn ca Photodiode PIN l bao gm mt lp tip gip p-n v cch

gia hai lp bn dn ny l nt lp bn dn yu loi N t kch hot ni ti hay cn gil lp t dn i. Lp p thng rt mng hp th ht cc photon vp lp bn dn i.Lc ny rng ca vng ngho (W) c tng v chiu di ca lp bn dn i (i cngdy th W cng ln). Thm vo trnh gy tn hao nh sng vo th trn b mtca vng ngho c ph thm mt lp chng phn x. Cu trc ny c m t tronghnh 3.27 sau.

3.3.1.2. Nguyn l hot ng

Nguyn l hot ng ca Photodiode PIN da trn hiu ng quang in. Khi chiumt photon c nng lng ln hn nng lng vng cm vo b mt bn dn caPhotodiode th qu trnh hp th photon xy ra. Khi hp th mt photon, mt in tc kch thch t vng ha tr ln vng dn li mt trong vng ha tr mt ltrng, ta ni photon to ra mt cp in t v l trng (nh hnh 3.28).

Cc cp in t - l trng ny c sinh ra trong vng ngho. Khi c in trngt vo linh kin, s c s chuyn ri cc in tch v hai cc (in t v pha n congl trng ht v pha p nh hnh 3.28) to ra dng in mch ngoi, dng in ny

c gi l dng quang in. Bnh thng mt photon ch c th to ra mt cp int - l trng, vi gi thit hiu sut lng t bng 1, ngha l vi mt lng photon xc

Nhm 5 : Lp 2_TVT1

InGaAsp i n

W

E

di

Au/Au-Sn

InP

InP

pi

n

n+-InP

4m

a) b)

Hnh 3.27 Cu trc PIN vi phn b trng dng hnh xin(a) vcu trc PIN s dng vt liu InGaAs (b).

39

8/2/2019 58722586--an-vin-thong-nhom-5

38/59

n: H Thng Thng Tin Quangnh ch c th to ra mt dng in xc nh. Tuy nhin trong thc t khng c nhvy v nh sng cn b tn hao do nhiu yu t trong c yu t phn x b mt.

L trng intip n

hv

R

pITnhiura

Photon

Vngdn

VnghatrVngnghoL trng

intVngcm

gEh

Hnh3.28S vngnngl-ngcaPIN

Ta c cng thc bc x quang b hp th trong vt liu bn dn tun theo hm msau : P(x)= Pin (1- )(e ) (3-8)

Trong : P(x) l cng sut quang c hp th c ly x

Pin l cng sut quang ti

() l h s hp th ti bc sng .Nh vy kh nng thm nhp ca nh sng vo lp bn dn thay di theo bc

sng. V vy, lp bn dn p khng c qu dy. Min i cng dy th hiu sut lngt cng ln, v xc sut to ra cc cp in t v l trng tng ln theo dy ca minny v do cc photon c nhiu kh nng tip xc vi cc nguyn t hn. Tuy nhin,nu di min i cao th thi gian tri ca cc ht ny di hn, xung nh sng a vocng phi tng ln tng ng vi thi gian tri tng. iu ny khin cho p ngv bng tn iu bin b hn ch. Do , rng ca min i khng c qu ln vnh th tc bt s b gim i. Ta phi chn di min i rng m bo iukin nht nh l hp th ht photon trong vng ngho v khng nh hng thi giantri.

Thng hay chn :

21

8/2/2019 58722586--an-vin-thong-nhom-5

39/59


c = )(24,1

eVEE

hc

gg

=

Tm li PIN hot ng da trn nguyn l hp th nh sng bin i tn hiuquang thu vo thnh dng tn hiu in. Cc thng s bin i ca chc nng ny c

phn tch phn tip theo sau y.

3.3.1.3 c tnh ca PIN

c tnh ca Photodiode thng c c trng bi h s p ng (cn gi l nhy ca ngun thu) v hiu sut lng t .

A. Hiu sut lng t

Hiu sut lng t c nh ngha l xc sut mt photon ri vo b mt linhkin b hp th lm sinh ra mt cp in t v l trng gp phn vo dng in mchngoi. Khi c nhiu photon n b mt bn dn th hiu sut lng t l t s ca

thng lng cc cp in t v l trng sinh ra gp phn to ra dng quang in mch ngoi trn thng lng ca photon ti. Nh vy, hiu sut lng t ca PIN l ts gia s lng ht ti chy trong mch v s photon i vo c b mt PIN trongcng mt n v thi gian.

==e

h

hP

e

I

in

p

vi l p ng ca PIN (3-9)

Theo cng thc 3-8 th hiu sut lng t ph thuc vo bc sng. Khi ta xt nphn nh sng b phn x ti b mt tip xc bn dn th cng sut truyn qua ca nhsng ch cn l : P = P in . We (1-R) vi R l h s phn x ca b mt bn dn. Lc hiu sut lng t ca PIN s c tnh nh sau :

= (1- R) [1-exp(-d)] (3-10)

Thnh phn d ( dy vng t dn) cng thc cho thy rng Photodiode PIN chiu sut lng t cng ln khi kch thc vng i cng ln.

B. nhy ca PINKhi hin tng hp th nh sng xy ra PIN th c mt dng quang in c

sinh mch ngoi. Dng ny t l vi cng sut i vo PIN, v c xc nh theo

cng thc sau : Ip = . Pin

Trong l nhy ca PIN. Theo cng thc 3-9 ta suy ra :

24,1.

.

==

ch

e

h

e[A/W]


8/2/2019 58722586--an-vin-thong-nhom-5

40/59

n: H Thng Thng Tin QuangNh vy nhy PIN t l vi bc sng, vi mt hiu sut lng t l hng s

th nhy PIN tng tuyn tnh theo bc sng. Ta c hnh 3.29 m t s h thuc ca nhy vo bc sng.

Mt khc, hiu sut lng t ca PIN ph thuc vo mt dy W ca vng triv h s hp th ca vt liu bn dn to ra PIN. Do , nhy ca PIN cng phthuc vo h s hp th ca vt liu bn dn hay ph thuc vo vt liu bn dn la

chn lm PIN. 3.3.2. Photodiode quang thc APD

Photodiode APD l loi Photodiode khng ch c kh nng chuyn i quang innh PIN m cn c kh nng hot ng vi c ch khuch i bn trong, tc l dngquang in do APD to ra c kh nng c khuch i ln nhiu ln do mt s c chnhn ht ti.

3.3.2.1. Cu trc ca APD

V c bn, cu trc APD ging nh cu trc ca PIN nhng APD bao gm 4 lp :p+ - i - p - n+ . Bn dn p+, n+ lcc bn dn pha tp mnh. Vng nhn ht ti ca APDc hnh thnh do bn dn p n+

3.3.2.2. Nguyn l hot ng

APD cng da vo hin tng hp th nh cc Photodiode khc to ra cc cpin t v l trng. Bn cnh , APD cn hot ng da trn nguyn l khuch i

dng.

Nhm 5 : Lp 2_TVT1

[m]

Hnh 3.29 S ph thuc ca nhy vo bc sng.

[A/W]

z

Hnh 3.30 Cu trc APD v phn b nng lng.

Vng ngho Vng nhn ht tia)

p+ i p n+R

L

E

b)

p+

n+

ip

Vng va chamin tokhuch i

42

8/2/2019 58722586--an-vin-thong-nhom-5

41/59

n: H Thng Thng Tin QuangBan u, khi cc photon c chiu vo b mt APD, chng c hp th v sn

sinh ra mt cp in t v l trng. t mt in p ngc vo APD nh hnh 3.30a, tathy c hin tng khuch i xy ra khi in p ny t n mt gi tr ln gyhiu ng thc l : Cc ht mang trong vng nhn p- n+ c in trng rt mnh, intrng ny khin cho chng tng nng lng dn dn n khi t c trng thi in

ha, chng c tng tc, va chm vo cc nguyn t trong vng nhn to ra cc cpin t v l trng mi. Cc ht mang in mi ny li tip tc c tng tc, va chmv to ra cc cp in t v l trng mi. V th cc ht mang c tip tc nhn ln vdng quang in pht ra ngoi c khuch i m ta gi l hiu ng thc l. Ta cth xt qu trnh ny thng qua cc biu thc ton hc sau y. Tc sinh ra ht tith cp c c trng bi cc h s in ha e v h [cm-1]. i lng ny cho bit slng ht ti mi c sinh ra hay s lng ht ti dch i trong 1cm chiu di. Lc qu trnh khuch i dng ca APD th hin qua phng trnh tc sau :

=dxdi e e.ie + h.ih v =

dxdie e.ie + h.ih

Trong : ie , ih l dng in t - e v l trng - h (tc l c in t v l trng utham gia vo qa trnh nhn ht ti) v dng tng l : I = i e + ih.

Nu coi dng tng khng i, ta c :dx

di e = (e h )ie + h .I

Xt trng hp kh nng in ha ca in t ln hn ca nng in ha ca ltrng ta c : e>h. Coi nh dng in t chim ch yu, v ch c in t i qua c

vng bin n vng bn dn n, th i h(d) = 0 ie(d)=I. Ta c h s khuch i dng(hay h s nhn M) c nh ngha l t l gia dng c khuch i v dng khicha c khuch i. Nh vy :

)0(

)(

e

e

i

diM = vi d l dy ca vng nhn ht ti.

Kt hp vi phng trnh tc ta c :

AeA

A

kdk

kM

=

])1(exp[

1

vi

e

h

Ak

=

(3-11)

Nh vy, APD thc hin bin i dng tn hiu quang vo thnh dng tn hiuin ra, ng thi khuch i dng ra vi mt h s khuch i l M nh cng thc 3-11.

3.3.2.3. c trng ca APD

Cng nh PIN, APD c cc c trng ca mt Photodiode, tuy nhin v APD ckh nng khuch i so vi PIN nn cc tham s c trng ca n c thm h s nhn

M.


8/2/2019 58722586--an-vin-thong-nhom-5

42/59

n: H Thng Thng Tin Quang nhy ca APD c xc nh theo cng thc sau :

APD = .M =

h

e..M

Trong l nhy ca PIN.

Nh vy nhy ca APD cng ph thuc vo bc sng nh PIN ng thi cngph thuc vo h s khuch i. Thc cht c ch khuch i l mt qu trnh thngk, n ph thuc vo h s in ha ca cc nguyn t khc nhau.

Theo nh cng thc 3-10, th thy rng M rt nhy cm vi cc h s e , v h.Xt i vi cc trng hp khc nhau sau :

+ Khi h = 0 (qu trnh nhn ht ti ch yu ch do in t) th kA = 0.

Lc : M = exp (e.d)

+ Khi e = h tng t ta c : kA = 1.Lc ta c :

M limdkdk

k

eAeA

A

1

1

])1(exp[

1

Cc vt liu khc nhau th h s in ha in t v l trng khc nhau. Khi e.d1 th M , nn APD thng chn e h hoc h e. qu trnh nhn ht ti ch

bi mt loi ht.

Bn cnh M l mt hm ph thuc vo tn s :

M()= M0[1+( e.M0)2

]-1/2

Trong : M0 l gi tr M ti = 0

e l thi gian chuyn tip hiu dng

M cn l hm Vb(V) ph thuc vo nhit . Nh m t trong hnh 3.31 th M tngkhi Vb tng v c tuyn ny ty thuc vo cc nhit khc nhau.


100 200 300 Vb(V)

T1 T2 T31000

100

10

M

Hnh 3.31 S ph thuc ca c tuyn M/Vb

vo nhit

8/2/2019 58722586--an-vin-thong-nhom-5

43/59


3.4 B khuch i

Nh cp trong phn trc, khong cch truyn dn ca bt k h thngthng tin quang si no cng b hn ch bi cc suy hao hay tn sc. Trong cc h

thng thng tin quang ng di cc mt mt quang ny c khc phc bng cctrm lp, trong tn hiu quang suy gim c bin i thnh tn hiu in v ca vo b pht li phc hi tn hiu quang ri tip tc truyn i. Tuy nhin khi sdng cc h thng thng tin quang ghp theo bc sng WDM th cc thit b lp nyli gy ra kh khn, v i hi k thut v vt liu phc tp, tn km hn.

T nm 1980, vn khuch i quang trc tip bng cc linh kin quang cnghin cu v trong nhng nm 1990 cc h thng ng trc thng tin quang sdng cc b khuch i quang trc tip mt cch rng ri. Trong nm 1996, cc b

khuch i quang c s dng trong cc tuyn cp bin xuyn i dng. n nayc nhiu b khuch i quang c nghin cu v ng dng nh : khuch iquang Laser bn dn, cc b khuch i quang pha tp t him, cc b khuch iRaman si, v cc b khuch i Brillouin si. Trong , hai b khuch i c sdng rng ri nht l : khuch i quang bn dn (SOA) v khuch i quang si phatp t him Er (EDFA) v ln lt c xt trong phn ny.

3.4.1. B khuch i quang bn dn.

Cc b khuch i quang bn dn hot ng ch yu da trn nguyn l ca Laser

bn dn, nguyn l khuch i c s dng trc khi xy ra ngng pht x Laser.3.4.1.1. Cu trc b SOA

Cu trc c bn da trn cu trc ca Laser bn dn thng thng, c d rngvng tch cc W, dy d v chiu di L, ch s chit sut l n. Hnh 3.35 m t mt

b khuch i bn dn , tnh phn x b mt u vo v ra c k hiu tng ng lR1 v R2.


d

W L

Pvo

Pra

R1

R2

Hnh 3.35 Cu trc b khuch i bn dn.

8/2/2019 58722586--an-vin-thong-nhom-5

44/59

n: H Thng Thng Tin QuangC hai loi khuch i quang bn dn l khuch i sng chy (Travelling

Wave -TWA) v khuch i quang Fabry- Perot (FPA).

B khuch i quang bn dn TWA l cc Laser bn dn khng c hp cnghng. B ny khuch i c hng v pha trc m khng c phn hi tn hiu. Cn

b khuch i quang FPA s dng cc cnh tinh th l gng phn x trong b cng

hng (vi R 32%), khi dng bm Laser bn dn di ngng pht, n s hotng nh mt b khuch i, tuy nhin cc thnh phn phn x trn ngng vn thamgia vo qa trnh khuch i.

3.4.1.2. Cc thng s ca b khuch i SOA

Cc thng s trong cc linh kin khuch i bn dn bao gm : h s khuch i,tp m v hiu ng nhy phn cc.

a, H s khuch i

H s khuch i trong b khuch i quang bn dn c th hin thng quacng thc : G = Pra/Pvo. Ty thuc vo b khuch i AW hay FP m ta c h skhuch i khc nhau tuy nhin mt h s chung c xt cho cc b SOA ny l hs khuch i bo ha (hay h s tng ch bo ha). Tng ch (khuch i) nh githit s tng dn theo mt ht ti nh sau:

g(N)= ( g/V) (N-N0) (3-12)

Trong : l yu t chn

g l tng ch vi phnV l th tch vng hot tnh

N0 l gi tr ca mt ht ti cn thit mi trng tr nn trong sutvi bc sng.

H s khuch i gim khi yu t chn tnh n s dn n ca cc mode dnsng vt ra khi vng tng ch. Mt ht ti N thay i theo dng bm I. ta c cngthc cho tc thay i N l :

PhNNN

qI

dtdN

m

g

c

)( 0= (3-13)

Trong : P l cng sut quang tn hiu ti

m l in tch tit din ca mode dn sng

c l thi gian sng ca ht ti

Khi chm sng ti l lin tc hoc l xung vi rng ln hn c, trng thi dngca N c th xc nh khi dN/dt = 0 . Thay vo cng thc 3-12 ta tnh c N. Thay

gi tr N trng thi dng vo cng thc 3-11Ta c :


8/2/2019 58722586--an-vin-thong-nhom-5

45/59


s

o

PP

gg

+=1 (3-14)

Trong : g0 = ( g/V)(I c/q-N0)

Ps=h . m/( g c) l cng sut bo ha ca b khuch i.T cng thc ta thy tng ch bo ha ca SOA ging nh h hai mc, lc ta c

cng sut ra bo ha ca b khuch i l :

s

o

S

ra PG

GP

2

)2(ln0

=

Gi tr ca cng sut ra bo ha trong cc b khuch i quang bn dn hin naythng l 5-10 mW.

B. Tp m ca SOATp m ca SOA c nh ngha l t l gia t s tn hiu trn tp m ca tn

hiu vo v tn hiu ra v c k hiu l Fn. H s tp m ny ca SOA ln hn 3dBv ph thuc nhiu vo thng s, c bit l yu t pht x ngu nhin:

0NN

Nnsp

=

S mt mt quang ni nh hp th quang do ht ti t do v tn x s ng gpthm tp m vo hnh nh nhiu thng qua h s tng ch g int. Do ta c tp m

b khuch i c tnh nh sau :

=int0

2g

g

NN

NFn

Thc t gi tr thng thng ca Fn trong b SOA nm trong khong 5-7dB.

C. Hiu ng nhy phn cc

H s khuch i G i vi cc mode TE v TM l khc nhau ( 5-6dB) do G v

g khc nhau vi cc mode phn cc trc giao. Hiu ng trn dn ti h s khuch

i ph thuc vo trng thi phn cc ca chm sng ti. Hiu ng ny khng c licho cc h thng thng tin quang.

C rt nhiu phng n lm gim hiu ng nhy phn cc ca b khuch iSOA v phng n kh thi trong cng ngh ch to l dy v rng ca vng hottnh phi tng thch. Th d chnh lch tng ch gia cc mode TE v TM l 1,3dBkhi dy ca lp hot tnh l 0,26m v dy l 0,4m.

3.4.2.1. Cu trc v nguyn l hot ng ca b EDFA

B khuch i quang si EDFA c cu trc bi mt on quang si pha tpErbium cng cc thnh phn cn thit khc. Cc thnh phn ny gm cc thnh phn


8/2/2019 58722586--an-vin-thong-nhom-5

46/59

n: H Thng Thng Tin Quangthit b ghp th ng WDM, b cch ly quang Isolator, Laser bm Ta c th m tmt b EDFA thc t nh hnh 3.35. y l cu hnh bm xui ca b EDFA.

Thnh phn chnh ca b khuch i quang si EDFA l mt on si quang phatp Erbium, c chiu di t vi mt n vi chc mt. Si ny c xem l si tch ccv chng c kh nng khuch i hoc ti to tn hiu khi c kch thch ph hp. onli si l thy tinh SiO2 Al2O3 pha trn t him Erbium vi nng 100 -2000

ppm. Cc si EDFA thng c li nh hn v khu s NA cao hn so vi si tiuchun. Cc in Erbium tp trung phn trung tm ca li. Ngoi s pha tp Erbium

trong vng li, cu trc on si quang pha tp l hon ton ging vi cu trc ca sin mode. Lp v thy tinh c ch s chit sut thp hn c bao quanh li honthin cu trc dn sng. ng knh ngoi ca lp v ny l 125m.

Nguyn l hot ng ca EDFA da vo c tnh ca nguyn t Erbium- mtnguyn t c tnh nng quang tch cc.

Cc in Erbium c bm ti mt mc nng lng pha trn do s hp th nhsng t mt ngun bm, chng hn nh bc sng 980nm. S chuyn dch ca int t mc nng lng cao ny xung mc nng lng c bn pht ra mt photon,

photon ny c bc x c th l do hin tng bc x t pht hay bc x kch thch .


Tn hiu vo

Hnh 3.36 Cu trc in hnh ca b khuch i quang si EDFA.

Tn hiu ra

EDF

Coupler WDM

LD

Bm nnglng=980nm

Mc kch thchPhn r khng bc x

Mc siu bnTn hiu ckhuch i

Mc c bn

Tn hiu ti

Hnh 3.37 Gin nng lng ca Erbium

8/2/2019 58722586--an-vin-thong-nhom-5

47/59

n: H Thng Thng Tin QuangCc photon tn hiu trong EDFA kch thch s ti nh c trng thi kch thch vkhuch i tn hiu. Thi gian sng ca in t mc nng lng cao vo khong14ms, m bo rng nhiu bc x gy ra do qu trnh bc x t pht thay bng cc

bc x kch thch do cc nguyn t Erbium.

S hp th nh sng bm kch thch cc in Erbium m chng tch tr nng lngs xy ra cho n khi mt cch l tng l c mt photon tn hiu kch thch s chuyni n thnh mt in tn hiu khc. nh sng bm c truyn dc theo si c phaErbium v b hp th khi cc in Erbium c a n trng thi kch thch. Do khitn hiu c truyn vo b EDFA, n kch thch s pht x ca nh sng t cc in trng thi kch thch, v khuch i cng sut tn hiu u ra.

3.4.2.2. c tnh ca b EDFA

B EDFA cng c nhng thng s nh mt b khuch i nh cc thng s v

khuch i, tp m, nhy phn cc. Sau y ln lt xt cc thng s c tnh ny. A. H s khuch i

H s khuch i cng sut ra v nhiu khuch i l cc c tnh quan trng nhtca EDFA trong vic dng n trong h thng thng tin quang. H s khuch i l ts gia cng sut tn hiu ra v cng sut tn hiu vo bc sng 1530nm v1550nm m ti EDFA c kh nng khuch i cao nht. H s ny ca EDFA phthuc nhiu vo thng s ca linh kin nh : nng in Er+3, di khuch i, bnknh li si v bn knh pha tp, cng sut bm xc nh h s khuch i ny

ta xt m hnh 2 mc nng lng ca EDFA (b qua mc trung gian )

Gi mt ht mc nng lng 1, 2, ln lt l N 1, N2. Ta c phng trnh tc c vit nh sau :

1

2121

2 )(T

NNNWNW

t

Nsp =

v N1 = Nt N2 (3-15)

Trong : Nt l mt ca nguyn t

T1 l thi gian sng mc kch thch

Wp, Ws l tc chuyn di ca sng bm v sng tn hiu.

Ta c : Wp = pPp/ap.h p v Ws = sPs/as.h s (3-16)


P P

S S1.Trng thi nn

2. Trng thi kchthch 2

1

Hnh 3.38 Gin nng lng

8/2/2019 58722586--an-vin-thong-nhom-5

48/59

n: H Thng Thng Tin QuangVi p, s l tit din chuyn di ca tn s bm v tn hiu ti tn s bm p v tns tn hiu s

ap, as din tch tit din cu mode bm v mode tn hiu

Pp v Ps l cng sut bm v cng sut tn hiu.

T vic gii cc phng trnh 3-15 v 3-16 ta xc nh c cng sut bo ha:

1T

haP

p

ppsat

p

= v

1T

haP

s

spsat

s

= .

Cng sut bm ph thuc v cng sut tn hiu thay i dc theo di b khuchi bi hp th, pht x cng bc v pht x ngu nhin. Nu coi thnh phn thamgia ca pht x ngu nhin rt nh, c th b qua, th ta c th xc nh c gi trtng ca cng sut tn hiu c khuch i trong EDFA. H s khuch i cn phthuc vo di b khuch i L khi ta c dng bm c nh, khi di L ln hn gitr ti u ca dng bm, on si pha tp tha s khng c bm v trong bkhuch i s xy ra hin tng hp th tn hiu c khuch i trc. Ta c

biu h s khuch i ph thuc vo cng sut bm v chiu di khuch i nhhnh 3.38.

B. Tp m ca b khuch i

Tp m trong b khuch i quang si c nh gi thng qua cc h s tp mFn = 2nsp c phn tch trn. Yu t pht x ngu nhin nsp lun ln hn 1 v vyFn lun ln hn 3dB. V h s pht x ngu nhin l thnh phn ph thuc vo mt ht ti N1 v N2 nn Fn cng nh h s khuch i, n cng ph thuc vo cng sutdng bm, cng sut tn hiu v chiu di khuch i. Thc nghim cho thy : vi F n=3,2dB ca b khuch i EDFA th c di 30m v bm vi cng sut 11mW

bc sng 980nm.

3.5. B chuyn i bc sng

B chuyn i bc sng c s dng nhm chuyn bc sng ca mt tn hiuvo thnh mt bc sng khc. Chng c th c s dng nh mt phn ca chuynmch, b ni cho hay mt b lp 3R. Cc b lp 3R l thit b c kh nng ti sinh

bin tn hiu trong min tn s cng nh min thi gian.


8/2/2019 58722586--an-vin-thong-nhom-5

49/59

n: H Thng Thng Tin QuangB chuyn i bc sng c th l thit b quang in (khng ph bin) hoc c

th l thit b da trn c s cch t quang hoc b trn sng.

3.5.1. B chuyn i bc sng quang in

Trong b chuyn i bc sng quang in, tn hiu quang u tin c chuyn

sang min in thng qua s dng Photodetector (k hiu R). Lung bt in c luli trong mt b m theo nguyn l FIFO. Sau tn hiu in ny c hng vomt Laser iu chnh (k hiu T) c iu khin bc sng mong mun u ra.Hnh 3.39 m t b chuyn i bc sng quang in.

TFIFO

B m haa ch

R

Hnh 3.39 B chuyn i b- c sng quang in

cs

Nhc im ca b ny l nh hng n tnh trong sut ca tn hiu. Ngoi ra ncn yu cu tn hiu phi c dng v tc bt c nh.

3.5.2. B chuyn i bc sng dng cch t quang

Hai kh nng ng dng cch t quang cho b chuyn i bc sng l : Kh nngiu ch li cho (cross-gain converter- XGC) v iu ch pha cho (cross phaseconverter - XPC).

XGC s dng li ca cc b khuch i bn dn SOA theo cng sut u vo.Tn hiu d cng sut thp vi bc sng mong mun c gi ti b SOA. V tnhiu d c li thp hn so vi tn hiu u vo nn n s t c li cao khi tnhiu u vo c trng thi 1 ( li thp). Ngc li tn hiu d s c li thp khitn hiu u vo c trng thi 0 ( li cao). Hnh 3.40 ch ra nguyn l ca XGC.

c

s

cSOA

Lc

Hnh 3.40 Nguyn l ca XGC

XPC s dng pha ca tn hiu d thay i c ban tn hiu vo. Sau chuynsang iu ch mt bng vic s dng giao thoa k. Phng php ny c u imhn XGC v n ch cn cung cp cng sut thp. Thm vo , n khng b nh hngnhiu nu mo xung.


8/2/2019 58722586--an-vin-thong-nhom-5

50/59

n: H Thng Thng Tin Quang3.5.3 B chuyn i bc sng dng b trn sng

tng ca b ny l to mt tn hiu mong mun bng vic s dng cc tn hiud vi cc bc sng m khi i cng vi tn hiu vo c th nh dng c tn hiu u ra vi bc sng mong mun.

u im ca b trn sng l gi c tnh trong sut ca tn hiu. Tuy nhin nli to thm tn hiu u ra, do mun thu c tn hiu mong mun th phi lc.

CHNG 4

THIT K TUYN THNG TIN QUANG


8/2/2019 58722586--an-vin-thong-nhom-5

51/59

n: H Thng Thng Tin Quang4.1 KHI QUTTrnh t thit k tuyn- Chn tuyn- Chn cp quang- Tnh ton chiu di cc i cho php ca thit b

- Kim tra suy haoMng vin thng cng cng thng c chia lm 3 phn:

Mng ng trc (Trunk network) Mng trung k (Junction network) Mng thu bao (Subscriber network)

Si quang hin nay c s dng trong mng ng trc v trung k. Trongtng lai tin ti s dng si quang trong mng thu bao, ngha l ti tn ngi sdng.

Ngoi ra si quang cn c dng trong cc mng khc nh: Mng ring ca cc cng ty ng st, in lc, Mng truyn s liu, mng LAN. Mng truyn hnh.

K thut thng tin quang v ang pht trin vi tc rt nhanh. Cho n nay c nhiu th h h thng quang c s dng. Chng khc nhau v loi si quang,

bc sng hot ng, loi linh kin thu pht quang. Khi thit k mt h thng thngtin quang ngi ta c gng chn cc phn t trong cng mt th h gim chi phcho vic u t v thit b o, phng tin lp t, thit b v cp d tr, hun luynngi s dng Khi cn chn la cc phn t trong cng mt h thng thng tinquang cn cn nhc gia yu cu truyn dn ca h thng v c tnh ca cc phn t.C s ca vic chn la c tm tt nh sau:

Si quang: si n mode (SM) c kh nng truyn dn tt hn si a mode(MM) c v suy hao ln di thng. Ngy nay s chnh lch v gi thnh gia sin mode v si a mode khng ng k. Vic hn ni, o th si n mode cngkhng cn l vn kh khn nn si quang n mode ang c dng rng ri. Chnhng tuyn c ly gn v dung lng thp ngi ta mi ngh n vic dng si a

mode. Bc sng: gi thnh ca cc phn t hot ng bc sng 850nm l thpnht. Nhng suy hao ca si quang bc sng ny qu cao v cc photo it sdng cho bc sng 850nm thng ch to bng silicon khng th thu c bcsng di (1300nm v 1550nm).

Bc sng 1300nm ang c s dng ph bin v tn sc ca si l thpnht. Khi cn c ly tip vn di ngi ta chn bc sng 1550nm do suy hao

bc sng ny thp nht. c bit nu dng si n mode dch tn sc (SMDS) th tn sc bc sng 1550nm cng rt nh.


8/2/2019 58722586--an-vin-thong-nhom-5

52/59

n: H Thng Thng Tin QuangLinh kin pht quang: c hai kh nng chn la l LED v laze, LED hot

ng n nh hn v gi thnh r hn nn c chn dng cho nhng h thng m shn ch v cng sut pht v b rng ph ca n khng nh hng n h thng. Lazec chn dng trong nhng tuyn c c ly di v tc truyn dn cao. Laze cngc nhiu loi vi nhng c tnh khc nhau v d nhin gi thnh cng khc nhau.

Linh kin thu quang: c th chn PIN v APD. PIN hot ng n nh hnnhng b hn ch v nhy v tc lm vic. Ngy nay ngi ta thng chn APDhoc PIN kt hp vi FET.

Cc h thng truyn dn s Vit nam xy dng theo h thng ghp knh caChu u. Mt knh thoi tiu chun c ph gii hn t 0.3 n 3.4kHz c chuynsang dng s c tc 64kbit/s.

4.2. THIT K MT TUYN THNG TIN QUANG

S tuyn thng tin quang n ginMt tuyn truyn dn quang c th hiu n gin l mt ng truyn quang

gia hai im, gm mt b pht, mt cp quang v mt b thu. B pht bin i cctn hiu in thnh nh sng. nh sng c a vo cp quang truyn ti b thu.B thu li bin i tn hiu nh sng ngc li thnh tn hiu in. Lng nh sngc a vo cp quang c hiu l cng sut ghp ni ca b thu. Vi cc cpquang c kch thc khc nhau, lng nh sng a vo cng khc nhau. Cp cng lnth nh sng a vo cng c nhiu. Cp c hai loi chun, n mt (thng dngtrong ng truyn di, hay vin thng) v a mt (dng trong thng tin c ly gndi 4.8 km). B thu nhn tn hiu nh sng t cp quang ti v bin i thnh tn hiu

in tiu chun. S chnh lch lng nh sng a vo cp quang v lng nh sngcn cho b thu c gi l qu cng sut tuyn.Vic tnh ton mt tuyn truyn dn quang c th c tin hnh theo nhiu hngkhc nhau ty theo yu cu t ra, chng hn:

Tnh c ly ti a ca on tip vn khi bit tc bit cn truyn dn vc tnh ca cc phn t trong tuyn. Tnh gii hn c tnh ca cc phn t khi bit tc v c ly cntruyn.


8/2/2019 58722586--an-vin-thong-nhom-5

53/59

n: H Thng Thng Tin QuangHng th nht c thc hin khi cn xc nh v tr v s lng cc trm tip

vn trn cc tuyn di. Hng th hai dng chn loi si quang, thit b thu phtquang khi bit c ly thng tin.

C ly ti a ca on tip vn phi c lu c v qu cng sut ln dithng v ch cn mt trong hai b gii hn th thng tin gia hai trm cng khng thc

hin c. Nu qu cng sut b gii hn th cng sut ca tn hiu quang n u thudi mc ngng ca my thu. Cn nu di thng b gii hn (do tn sc cao) th tnhiu quang n u thu b mo dng, cc xung b ni rng qu mc gii hn nnkhng cn nhn bit c xung 1 hoc xung 0.Thng thng ngi ta tnh c ly ti a gii hn bi qu cng sut ri nghim li xemdi thng c ly c rng cho tn hiu cn truyn khng. Nu di thng ca ontnh c rng hn di thng ca tn hiu cn truyn th c ly gii hn bi qu cngsut c chn. Ngc li phi gim c ly di thng ca tuyn rng cho tn hiucn truyn.

4.2.1. Bc 1: Tnh c ly gii hn bi cng sutCc thng s cn bit:

Cng sut pht ca ngun quang:Cng sut b pht phi c nh ngha l cng sut nh sng a vo mt cp quangcho trc. Khi vng tch cc ca cp quang gim, lng cng sut a vo cng gim.

Ngoi kch thc ca cp quang, cn mt thng s quan trng na ca cp quang lgc nhn nh sng, thng c nh ngha l NA (khu s) ca cp. NA l hmsin ca gc nhn nh sng ca cp quang. NA gim khi vng tch cc gim, do lm

gim gc nhn nh sng.Nu si cp cng nh v gc nhn sng cng nh th lng nh sng i vo cp quang truyn i ti u kia ca tuyn cng nh. Bng 1 cho ta kt qu l thuyt cc t skhi s dng cc cp vi ng knh khc nhau.

Bng 1: Bng h s ghp nh sng t mt LED vo cc cp quang c ng knhkhc nhau (t s/ dBm)

Cng sut ny ph thuc:+ Loi ngun quang: LED, laser.

+ Tc bit hot ng+ ng knh li si quang (lin quan n h s ghp nh sng)

nhy ca my thu quang:Thng tnh mc li BER = 10-9, nhy ca b thu quang biu din theo n vdBm (tng ng vi 1mW) v mW ging nh cng sut ca b pht. Trong hu htcc trng hp, b thu c thit k vi mt nhy nht nh v khng d thay ic.Khi thit k b thu quang, ngi ta tnh n tc v nhy. Thi gian png ca b thu cng nhanh th nhy cng km. nhy ph thuc:

+ Loi linh kin thu quang+ Tc bit


8/2/2019 58722586--an-vin-thong-nhom-5

54/59

n: H Thng Thng Tin Quang+ Bc sng hot ng

Cc yu t suy hao:+ Suy hao trung bnh ca si quang: ph thuc loi si v bcsng hot ng. Thng s ny tnh c bng cch nhn ch s suy haodB.km-1 ca si quang vi chiu di si. Ngi ta thng gi cc suy

hao xy ra trong khong 100m u si quang ni vi LED l suy hao tcthi. Vi cc on si quang ngn hn th suy hao trn mi kilmet lnhn thng s m nh sn xut a ra.+ Suy hao ca cc mi hn (splice) v ca cc khp ni (connector):

ph thuc loi v s lng khp ni c dng. Ti thiu phi c 2 khpni cho hai u si. Suy hao trung bnh ca mi khp ni l 0.5 dB. Ccsuy hao ny c th tnh c d dng bng cch nhn gi tr suy hao oc hoc thng s do nh sn xut cung cp vi s ln xut hin cc

linh kin ny trn tuyn. Vi s lng linh kin nh, s dng ch s suyhao cao nht cho mi linh kin. Vi s lng linh kin ln, s dng chs suy hao trung bnh cho mi linh kin.+ Suy hao ca cp ni i: c th b qua nu chiu di ca cptrong i ngn.

Suy hao d phng:+ D phng cho cp: suy hao ca cp c th tng ln theo thi gians dng do nhit thay i, s tn cng ca m, s xut hin ccmi hn ph khi sa cha. Thng thng nn t suy hao d phng cho

cp khong 0.2 n 0.3 dB/km.+ D phng cho thit b: Cng sut pht ca ngun quang gim dntheo thi gian s dng. nhy ca my thu cng km dn. Cc mchin ca thit b cng c th b lo ha. Do cn t suy hao d phngcho thit b t 3 n 5dB. i khi d phng c tnh trong cngsut pht v nhy my thu.

S phn b suy hao trn tuyn c biu din trn hnh v:


P (dBm)

Cng sut ngun quang phng vo si

Suy hao ca khp ni

Suy hao ca mi hn

Suy hao ca si

Suy hao ca khp ni

Cng sut thu

d phng

Tngsuyha

o(d

B)

C ly ti a (km) nhy camy thu

l (km)

8/2/2019 58722586--an-vin-thong-nhom-5

55/59

n: H Thng Thng Tin QuangHnh: Phn b suy hao trn mt tuyn thng tin quang

Cch tnh:Tnh qu cng sut tng qut:Gi PS: cng sut phng ca ngun quang ti im S (dBm)

PR: nhy ca my thu ti im R (dBm)

Lm: suy hao d phng cho thit b (dB)Lc: suy hao ca cc khp ni (dB)

Qu cng sut ca tuyn l: Pb (dB) = PS PR Lm - LcTnh suy hao ca cp:

Gi f: suy hao trung bnh ca si quang (dB/km) S: suy hao trung bnh ca mi hn trn mi km (dB/km)

C th ly gi tr gn ng:

(km)cpcundichiu(dB)hnmimibinhtrunghaosuy

=s

m: suy hao d phng cho cp (dB/km)Suy hao trung bnh ca cp l: c (dB/km)= f- S - mKhong cch ti a ca on tip vn:

(km)cpcabinhtrunghaoSuy(dB)sutcngQu

(km)vntipcchngKho =

mSf

cmRs

c

b LLPPPkmL ==)(

Bng gi tr suy hao trung bnhCc gi tr suy hao trung bnh trn tuyn thng tin quang

Bc sngv mt

Kchthc si

Suy haotrn si(dB/km)

Suy hao timi hn(dB/mi)

Suy hao tikhp ni

(dB/u ni)Bng thng (MHz x km)

850nm / NM 62.5/125 3dB 0.1dB 1.0dB 160

1300nm /NM

62.5/125 1dB 0.1dB 1.0dB 500

850nm / NM 50/125 3dB 0.1dB 1.0dB 400

1300nm /NM

50/125 1dB 0.1dB 1.0dB 500

1310nm /SM

9/125 0.3dB 0.1dB 1.0dB v cng

1550nm /SM

9/125 0.2dB 0.1dB 1.0dB v cng

4.2.2. Bc 2: C ly gii hn do di thng


8/2/2019 58722586--an-vin-thong-nhom-5

56/59

n: H Thng Thng Tin QuangCc thng s cn bit:

Tc bit cn truyn: 8 34 140- 565 Mbit/s Loi m ng dy c s dng: khi dng m 1B2B thtc bit tng i, khi dng m 5B6B th tc bit tng 6/5 ln.

tn sc mode (nu dng si a mode). Thng thng tn sc mode c cho di dng di thng gii hn bi tc sc mode, n vGHz/km. tn sc sc th: thng ch tnh tn sc cht liu v bqua tn sc dn sng. rng ph ca ngun quang

Cch tnh:

Di thng v tn sc ca tuyn t l nghch vi nhau theo biu thc:tD

B44.0

=

Trong B l di thng, n v GHzDt l tn sc, n v nsDo thay v so snh di thng ca tuyn vi di thng ca tn hiu cn truyn ngita so snh tn sc tng cng ca tuyn vi tn sc ti a cho php ca tn hiu.Tnh tn sc ca tuyn:

chrt DDD2

mod2 +=

trong : Dt: tn sc tng cng

Dmod: tn sc mode, c tnh theo cng thc: )().(44.0

modkmL

kmGHzBD

L

=

BL: di thng gii hn bi tn sc modeL: c ly gii hn bi qu cng sut

Dchr: tn sc sc thDchr= Dmat + Dwg Dwg

8/2/2019 58722586--an-vin-thong-nhom-5

57/59

n: H Thng Thng Tin QuangNu maxDDt : di thng khng b gii hnNu maxDDt > : di thng b gii hnTrng hp sau phi gim c ly ca on tip vn sao cho Dt = Dmax.Si n mode ch c tn sc sc th nn: Dt = Dchr= dmat LDo tn sc ca si n mode rt nh, c bit khi dng bc sng 1300nm, nndi thng ca si n mode rt rng. Trong nhiu trng hp ngi ta khng cn tnhn bc 2 i vi si n mode.

4.2.3 Bi ton c thXt tuyn truyn dn quang ( ng trc H Ni ---- H Nam ). Bit khong

cch tuyn l 80km. Dng si n mode chit sut nhy bc (SI ) truyn tc 140Mbit/s vi m ng dy 5B6B v thng s ca cc phn t nh sau:Cng sut pht ca ngun quang (Laser): PS = - 0.5dB

nhy ca my thu quang: RP = -44dBSuy hao ca mi connector: Lc = 0.5 dB; mi u dng 2 connector.

Suy hao trung bnh ca si: f = 0.4 dB/km ( bc sng 1300nm)Suy hao mi mi hn: S = 0.1 dB, mi cun cp di 2kmTn sc cht liu: dmat = 3ps/nm.km ( bc sng 1300nm) rng ph ca ngun quang: = 3nm.

Suy hao d phng cho thit b:mL = 3dB

Suy hao d phng cho cp: m= 0.25dB/km.CCH TNHC ly ti a ca on tip vn c tnh nh sau:Bc 1: C ly gii hn qu cng sut:- Qu cng sut:Pb = PS - PR - Lm - LcPb = -0.5dB - (-44dB) - 3dB - 4x0.5dBPb = 38.5dB- Suy hao trung bnh ca cp:

c = f- S - m c = 0.4 + 0.1/2 + 0.25 c = 0.7dB/kmC ly gii hn bi qu cng sut:

kmL

km/dB.

dB.PL

c

b

55

70

538

=

=

=

Bc 2:- tn sc tng cng:

chrchrmod

tDDDD =+= 22


8/2/2019 58722586--an-vin-thong-nhom-5

58/59

n: H Thng Thng Tin QuangDt = Dchr = dmat L

= 3ps/nm.km x 3nm x 55km= 0.5ns

- tn sc ti a cho php:

ns.D

s/Gbit.BrD

max

max

51

101404

1

4

1

5

63

=

==

- So snh tn sc:Dt < Dmax: nn di thng khng b gii hn

Nh vy c ly ti a ca on tip vn l: L = 55km.Nh vy m bo cht lng ca tin hiu cng nh bng thng ,tc truyn dn thta phi t mt trm lp trn tuyn.

T tnh ton trn ta chn thit b c th cho tuyn quang.

Ta chn si quang : l si n mode SI, dmat = 3ps/ nm.km suy hao trung bnhca si l 0.4 dB/ Km v Si n mode l si ch cho php truyn dn mt mode trongn nhng kh nng v bng thng ca si kh ln (khong 40GHz). Si quang nmode ph hp i vi h thng ng trc vi gi thnh thp. Si n mode l chghp mt mode nn khng c tn sc mode bng tn ca si tng ln.

Thit b pht quang l: ngun lasez (light Amplication by stimulate Emission of

radiation ) v ngun pht c rng ph hp, bc song pht n nh, s dng vi sin mode, s dng trong h truyn dn tc cao.

Thit b thu quang l: s dng thit b thu quang APD c nhy my thu lPr = - 44 dB.


8/2/2019 58722586--an-vin-thong-nhom-5

59/59


CHNG 5

KT LUN n vin thng trnh by nhng nt c bn nht v mng thng tin quang,

v vic thit k tuyn thng tin quang. Nhng trong qu trnh lm n chng em cn

c phn b ng nn vic hon thnh n cn nhiu thiu st. Chng em rt mong c

c nhng kin nh gi, gp ca thy v cc bn n thm hon thin.

58722586-Đồ-an-viễn-thong-nhom-5

Documents