do an mon hoc cap soi quang

8/8/2019 Do an Mon Hoc Cap Soi Quang

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Trng HKT Cng Nghip Thi Nguyn nThng tin quang

TRNG HKTCN THI NGUYN

---------------------------------------------------

BN NHN XT N MN H C

THNG TIN QUANG

H v tn sinh vin: ...................................................................... ..MSSV: ........................................................

Ngnh:... Kho: ..2004 2009.

Ging vin hng dn:.Th.S.o Huy Du

Ging vin nhn xt: .................................................................................................................................................

1. Ni dung n:

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2. Nhn xt ca ging vin:

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Ngy thng nm

Ging vin nhn xt

Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]

1


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Mc lc

Trang

Li ni

u...6

Chng 1 c s thng tin quang7

1.1. Lch s pht

trin7

1.2. Cu trc mt h thng thng tin quang ngin9

1.3. u im ca thng tin

quang........11

Chng2 Si

quang....12

2.1. Nhng ng dng ca si

quang.12

2.2. u im ca thng tin si

quang..13

2.3. L thuyt chung v si dn

quang14

2.3.1. Ph ca sng in

t14

2.3.2. Chit sut ca mi tr-

ng17


2


3/106


2.3.3. Hin tng phn x nh sng ton

phn..17

2.4. S truyn dn nh sng trong si

quang182.4.1.Nguyn l truyn dn

chung.18

2.4.2 S lan truyn cc mode trong si

quang...20

2.5. Phn loi si

quang...232.5.1. Si c chit sut nhy bc(SI) v si c chit sut

bin i u (GI)

...24

2.5.1.1.Si quang c chit sut nhy bc (si SI: Step-

Index)24

2.5.1.2.Si quang c chit sut gim dn (si GI:

Graded- Index)..25

2.5.2. Cc dng chit sut khc:

..26

2.5.3. Si a mode v n

mode.27

2.5.3.1.Si a mode (MM: Multi Mode):

.27

2.5.2.2.Si n mode ( SM: SingleMode ):

...28


3


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2.6.Cc thng s ca si

quang29

2.6.1. Suy hao ca si quang:

..292.6.2.Cc nguyn nhn gy suy hao trn si quang:

...31

2.6.2.1. Suy hao do hp th:

..31

2.6.2.2.Suy hao do tn x:

.322.6.2.3.Suy hao do si b un

cong...33

2.6.2.4.Mt s suy hao

khc.34

2.6.2.5. c tuyn suy

hao....35

2.6.3.Tn sc (Dispersion)

..36

2.6.4. Cc nguyn nhn gy ra tn

sc38

2.6.4.1.Tn sc mode (Mode Despersion)

38

2.6.4.2.Tn sc vt

liu.40

2.6.4.3. Tn sc do tc dng ca ng dn sng:

...41


4


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2.6.4.4.. tn sc tng

cng...42

2.6.4.5. Tn sc bc

cao.432.6.4.6.Tn sc mode phn

cc..43

2.6.4.7. tn sc ca mt vi loi si c

bit.45

2.6.5.Di thng ca si

quang..462.6.6. Bc sng

ct.48

2.6.7. ng knh trng mode (MFD:Mode Field Diameter)

...48

2.7.Cu trc si

quang.49

2.7.1. Cu trc ca si

quang49

2.7.1.1.Lp

ph...50

2.7.1.2.Lp

v.50

2.7.2. Yu cu i vi si

quang...52

2.8. Cc phng php ch to si

quang..53


5


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2.8.1.Vt liu ch to

si...53

2.8.2.Cc phng php ch to si

quang.542.8.3.Cc phng php ch to phi

si54

2.8.3.1.Phng php thanh ng c in.54

2.8.3.2.Phng php ni nu i (Double Orucible).55

2.8.3.3.Phng php ng hi ha cht.56

2.8.3.3.1.Phng php ng hi ha cht: (chemical vapourdeposition- CVD)..56

2.8.3.3.2.Phng php ng hi ha cht nh Plasma (Plasma

chemical vapour Deposition- PCVD)58

2.8.3.3.3.Phng php ng hi ha cht bn ngoi ( Outside

Chemical Vapour Deposition- OCVD).59

2.8.3.3.4.Phng php ng hi ha cht theo trc ( Vapour Axial

Deposition- VAD).59

2.8.4.Qu trnh ko si...59

2.8.5. Nguyn tc to ra si quang

mi...60

2.9. Hn ni si

quang.61


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2.9.1. Yu cu k thut

61

2.9.2.Cc phng php hn ni si

quang622.9.2.1. Phng php dng keo

dnh...62

2.9.2.2.Phng php dng h

quang...63

2.9.3.Bo v mi

ni..65Chng 3 Cp

quang...67

3.1. Tng

quan.67

3.1.1. c im, yu cu i vi cp quang

...67

3.1.2. Kh nng ca si v cp

quang...67

3.2. Cu trc cp

quang....68

3.2.1.Cu trc tng qut ca cp

quang69

3.2.1.1.Phn

li...70

3.2.1.2.V cp.

73


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3.3.Phn loi cp quang:

..74

3.3.1. Phn loi theo cu trc:

...743.3.2. Phn loi theo mc ch s

dng75

3.3.3. Phn loi theo iu kin lp

t..75

3.3.3.1.Cp

treo...763.3.3.2.Cp t trong

cng..76

3.3.3.3. Cp chn trc

tip..77

3.3.3.4.Cp t trong

nh78

3.3.3.5. Cp ngp nc v th

bin.78

3.4. m mu trong cp.

79

3.5. o th cp quang v o bo d-

ng...81

3.5.1.Khi

qut..81

3.5.2. Mc ch ca o

th82


8


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3.5.3.o th bo dng.

83

3.6.Cc bin php bo v cp

quang843.6.1. chn su

cp..84

3.6.2. Chng mi v chng

chut.86

3.6.3. Chng nh hng ca

st86Ti Liu Tham Kho.

87

LI NI U

Hin ny cc h thng thng tin quang chim hu ht cc tuyn

truyn dn quan trng trn mng li vin thng quc t, v c coi l

phng thc truyn dn c hiu qu nht trn cc tuyn vt bin v xuyn

lc a. p ng nhu cu truyn ti ln do s bng n thng tin, mng

truyn dn i hi phi c s pht trin mnh v c quy m v trnh cng

ngh nhm to ra cc cu trc mng hin i bao gm c cc h thng thng


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tin quang. Cc h thng thng tin quang trong thi gian ti phi m bo c

tc cao, c ly xa, tin cy cao

Trong ton b h thng thng tin quang phn khng th thiu c

chnh l Cp Si Quang. H thng thng tin quang c nhiu u im hn cch thng khc mt phn chnh l nh mi trng truyn dn l cp si quang.

V vy, em chn ti Cp Si Quang lm n nghin cu gip

em tm hiu su hn .

Do thi gian hn hp v kin thc ca bn thn c hn chnh v vy

n ca em khng th trnh c nhng thiu st. Nn em mong cc thy c

trong b mn v cc bn trong lp nh gi v ng gp nhiu kin ti

su hn v pht trin n mc cao hn na.

Trong qu trnh lm bi, em c nhn c s hng dn chi tit

ca thy o Huy Du v gp ca cc bn trong lp. Em xin gi li cm n

n thy v cc bn.

Em xin chn thnh cm n !

Sinh VinLng Xun Trng

Chng 1

c s thng tin quang

gii thiu tng quan


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H thng thng tin c hiu mt cch n gin l h

thng truyn thng tin t ni ny n ni khc.Khong

cch truyn tin c th l hng trm Km, hng trc ngn Km

hoc xuyn qua cc i dng.Thng tin c th truyn quacc sng in vi cc di tn s khc nhau.

H thng thng tin quang si l h thng thng tin bng

sng nh sng v s dng cc si quang truyn tin. N

pht trin nhanh v cn tim tng kh nng rt ln trong vic

hin i ho cc mng li vin thng trn th gii.

1.1. Lch s pht trinTrong tin trnh lch s pht trin ca nhn loi vic

trao i thng tin gia con ngi vi con ngi tr thnh

mt nhu cu quan trng, mt yu t quyt nh gp phn

thc y s ln mnh tin b ca mi quc gia, cng nh nn

vn minh ca nhn loi .

Cng vi s pht trin ca h thng thng tin hu tuyn

v v tuyn s dng mi trng truyn dn l dy dn kim

loi c in (cp ng ) v khng gian.Th vic s dng nh

sng nh mt phng tin trao i thng tin cng c khai

thc c hiu qu . Cng vi thi gian thng tin quang pht

trin v ngy cng hon thin vi nhng mc lch s nh sau:

-1790 : Clau de Chappe , k s ngi Php, xy dng mth thng in bo gm mt chui cc thp vi cc n bo

hiu trn . Tin tc vt qua chng ng 200km trong vng

15 pht .


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-1870 : John Tyndall nh vt l ngi Anh chng t nh

sng c th dn c theo vi nc un cong vi nguyn l phn

x ton phn . iu vn c p dng trong thng tin quang

hin nay .-1880 : Alexander Graham Bell , ngi M gii thiu h thng

thng tin Photophone. Ting ni c truyn i bng nh sng

trong mi trng khng kh . Nhng cha c p dng trong thc

t v qu nhiu ngun nhiu.

- 1934: Norman R.French, ngi M, nhn bng sng ch h

thng thng tin quang. S dng cc thanh thu tinh truyn dn.

- 1958: Arthur Schawlour v Charles H Tounes, xy dng

v pht trin Laser

- 1960: Theodor H Maiman a laser vo hot ng thnh

cng.

- 1962: Laser bn dn v Photodiode bn dn c tha nhnvn cn li l phi tm mi trng truyn dn quang thch

hp.

- 1966: Charles H Kao v Georce A Hockham, hai k s

phng th nghim Stanrdard Telecommunications ca Anh ,

xut dng si thu tinh dn nh sng. Nhng do cng ngh

ch to si quang thi cn hn ch nn suy hao qu ln (khong 1000dB/Km)


12


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- 1970: Hng Corning Glass Work ch tto thnh cng si

quang loi SI c suy hao nh hn 20 [dB/km] bc sng

1310nm.

- 1972: Loi si GI c ch to vi suy hao 4 [dB/km].

- 1983: Si n mode(SM) c xut xng ti M.

Ngy nay loi si n mode c s dng rng ri vi suy

hao ch cn khong 0,2 [dB/km] bc sng 1550nm.

1.2. Cu trc mt h thng thng tin quang

n gin

Hnh 1.1. Cu trc h thng thng tin quang n gin

Theo s h thng ta c:

- Ngun tn hiu thng tin: l dng thng tin thng thng

l hnh nh , ting ni , fax...hay cn l tn hiu u vo.

- Phn t in: l phn chung ca h thng, x l

ngun tin to ra tn hiu in a vo h thng truyn dn, c

th l tn hiu Alalog hoc Digtal


13

Siquang

Trm lp

E

O

O

E

Nguntn

hiu.

Phnt

in.

Phnt

in.

O

E

E

O

Biniin-quang

Binisi quang


14/106


- B bin i E/O: C nhim v bin i tn hiu t

tn hiu in thnh tn hiu quang vi cc mc tn hiu

in c bin i thnh cng quang, cc tn hiu in

0v 1c bin i ra nh sng tng ng dng khng vc.

Sau tn hiu quang c a vo si quang truyn

i. B bin i in quang thc cht l cc linh kin pht

quang nh LED,Laser diode...

- Si quang: truyn dn nh sng ca ngun bc x (E/O)

c iu bin, n c vai tr nh knh truyn dn.- B bin i O/E: l b thu quang, tip nhn nh sng t

si quang a vo v bin i tr li thnh tn hiu in nh

tn hiu pht i, n c vai tr nh b gii iu ch.

-Trm lp : Khi truyn dn trn tuyn truyn dn, cng

sut b gim i, tn hi trn ng truyn b tiu hao, dng

sng ( rng xung) b gin ra do nhiu nguyn nhn khcnhau. V vy, truyn c i xa cn c trm lp. Hin

nay cha thc hin c khuch i hay ti sinh trc tip tn

hiu quang nn cc trm lp phi thc hin 3 bc sau:

+ Chuyn i t tn hiu quang sang tn hiu in

+ Sa i dng tn hiu b mo hoc ti sinh tn

hiu in.+ Chuyn i tn hiu in c khuch i hoc

ti sinh thnh tn hiu quang tip tc pht i.


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- Ti tin: Trong h thng thng tin in th ti tin l cc

sng in t cao tn, trong h thng quang l nh sng cnh

l sng in t song c tn s rt cao (1014-1015Hz) do vy

ti tin quang rt thun li cho ti cc tn hiu bng rt rng.- Nng lc truyn dn: nng lc truyn dn ca h thng

c nh gi qua hai i lng:

+ rng bng tn c th truyn dn c

+ C ly trm lp hoc di chuyn tip

Xu th ca cc h thng truyn dn quang l truyn dn

di rt rng v c ly trm lp rt ln. Thc t cc h thng

quang hin nay vt qua cc h thng in c 2 yu cu

trn. Cc i lng trn c xc nh bi nhiu yu t lin

quan nh:

+ Tiu hao v tn x truyn dn ca si quang

+ Cng sut bc x v kh nng iu bin linh hot ca

si quang

+ nhy ca my thu quang

+ Tiu hao ph khi x l cc phn t ton tuyn

1.3. u im ca thng tin quangSo vi h thng thng tin in t th h thng thng tin

quang c nhng u im hn hn l nhng u im c

bn nh sau:


15


16/106


1. Suy hao truyn dn thp: dn ti gim c trm lp ,

ko di c c ly truyn dn, cho php truyn dn bng

rng, truyn c tc ln hn cp kim loi khi cng chi ph

xy dng mng nn2. Bng tn truyn dn ln , p ng c thu bao dch v

di rng .

3. Si quang: c ch to t nhng nguyn liu chnh l

thch anh hay nha tng hp nn ngun nguyn liu rt di

do r tin. Si c ng knh nh, trng lng nh, khng c

xuyn m rt d lp t v un cong .4. Dng cp si quang: rt kinh t trong c vic sn xut

cng nh lp t v bo dng. Khng b nh hng ca nhiu

in t, khng dn in, khng gy chp, chy.

5. Tn hiu tuyn trong si quang: khng chu nh hng

ca nhiu t trng bn ngoi (nh sng v tuyn in, truyn

hnh, nh hng ca cp in cao th ...) dn n tnh bomt thng tin cao, khng b nghe trm. N c s dng ti

nhng ni c nhiu in t mnh nh trong cc nh my,

nh my inm khng cn phi che chn in t.

6. Mt cp si quang: c cng kch c vi cp kim loi th c

th cha c mt s lng ln li si quang ln hn s lng kim

loi.7. Si quang: c tnh bo mt cao, v vy vic nh cp

thng tin trn si quang l rt kh khn.


16


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Tuy nhin h thng thng tin quang c mt s hn ch

nh:

+ Kh khn trong vic ghp ni,

+ Khng s dng c trong vng b chiu x

Chnh v c nhng u im trn m cc h thng thng

tin quang c s dng rng ri trn mng li vin thng ca

nhiu quc gia. Chng c xy dng lm cc tuyn ng

trc, trung k, lin tnh. Ti Vit Nam cp quang v ang

lp t vi tuyn truyn dn ng di lin tnh dng cp

ngm.

Tc cc h thng thng tin quang s l mi t ph

v , c ly truyn dn v cu hnh linh hot cho cc dch v

vin thng cp cao trong mng li vin thng.

Chng2

Si quang

2.1. Nhng ng dng ca si quang


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Cng vi s pht trin khng ngng v thng tin vin

thng, h thng truyn dn quang truyn tn hiu trn si

quang v ang pht trin mnh m nhiu nc trn th

gii. Do c nhiu u im hn hn cc hnh thc thng tinkhc v dung lng knh truyn, tnh kinh t,m thng tin

quang gi vai tr chnh trong vic truyn tn hiu cc

tuyn xuyn ng trc avf cc tuyn xuyn lc a, xuyn

i ty dng. Cng ngh nag nay to ra thng tin quang

pht trin v thay i theo xu hng hin i v kinh t nht.

c bit cng ngh si quang n mode c suy haonh lm n gin vic tng chiu di ca ton tuyn

thng tin quang, kt hp vi cng ngh khuch i quang ra

i s lm tng chiu di gp i hoc gp n ln. Cht lng

ca tn hiu thu c trn h thng ny s c ci thin mt

cch ng k.

nc ta thng tin si quang ang ngy cnh chim vtr quan trng, cc tuyn cp quang c hnh thnh, c

bit l tuyn cp quang H Ni H Ch Minh chim mt v

tr quan trng trong thng tin ton quc.

Trong tng lai mng cp quang s c xy dng rng

khp. Tuyn ng trc cp quang s c r nhnh ti cc

tnh, huyn, v xy dng tuyn cp quang ni ht.

* V tr ca si quang trong mng thng tin giai on hin

nay:

- Mng ng trc xuyn quc gia.


18


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- Mng ring ca cc cng ty ng st, in lc.

- ng trung k

- ng cp th bin lin quc gia

- ng truyn s liu, mng LAN

- Mng truyn hnh

2.2. u im ca thng tin si quang

So vi dy kim loi si quang c nhiu u im ng ch

l:

-Suy hao thp: cho php ko di khong cch tip vn do gim c s trm tip vn

-Di thng rt rng: c th thit lp h thng truyn dn

s tc cao

-Trng lng nh, kch thc nh

-Hon ton cch in khng chu nh hng ca sm st

-Khng b can nhiu bi trng in t

-Xuyn m gi cc si dy khng ng k

-Vt liu ch to c rt nhiu trong thin nhin

-Dng h thng thng tin si quang kinh t hn so vi si

kim loi cng dung lng v c ly.

2.3. L thuyt chung v si dn quang

Trong h thng thng tin quang, thng tin c truyn

ti bng nh sng. Trong phn ny, chng ta s nghin cu ti

cc c tnh ca nh sng v rt cn thit hiu c s

lan truyn ca nh sng trong si quang v nguyn l ca dao


19


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ng laser. Ba vn sau s l c s l thuyt cho vic

hnh thnh h thng thng tin quang:

1. Ph ca sng in t

2. Chit sut ca mi trng

3. Hin tng phn x ton phn

2.3.1. Ph ca sng in t

Cc bc x in t ni chung c cng bn cht t nhin

v c th xem nh sng hoc ht (photon). Tnh cht sng

hoc ht ni bt trong tng vng. c trng c bn ca cc

ngun bc x in t l di ph bc x ca n, tc l mt di

tn s ca cc dao ng in t hay cn gi l sng in t

c sinh ra, hoc l di bc sng tng ng. Hai i lng tn s

v bc sng t l vi nhau theo cng thc:

C(m/s)= (m).f(Hz) hoc E(ev) = h.f

Trong :

C l vn tc nh sng trong chn khng

[ C=3.108 m/s ]

H l hng s Planck [ h=6,25.10-34J/s ]

nh sng dng trong thng tin quang trong vng cn hng

ngoi vi bc sng t 800nm n 1600nm. c bit c ba bc

sng thng dng l 850nm, 1300nm v 1550nm.


20


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Hnh 2.1: Cc bc sng trong thng tin quang


21

Tia tngoi

TiaRnghen

TiaGamma

TiaV tr

T ia hngngoi

Tia vtuyn

1,6 1,5 1,4 1,3 1,2 1,1 1 900 800 700 600 500 400m

Tm nh s nh trong thng tin quang

nh s ngnhn thy - c


22/106


Hnh 2.2. Tn s v bc sng dng trong Thng tin

quang

Ta bit nu bc sng cng nh th tn s cng ln m

khi tn s cng ln th suy hao cng ln. Song qua c


22


23/106


tuyn suy hao ca si quang (hnh 2-7) v c bit bc

sng 1550nm th suy hao l di 0,2dB/km. Nh vy l vn

suy hao c gii quyt nn ba bc sng hin nay

ang c dng rng ri m c bit l bc sng 1550nm.Cng ta khai thc thm c cc vng tn s khc m

rng di tn s ng thi khai thc c cc u im ca cp

si quang.

2.3.2. Chit sut ca mi trng

Chit sut ca mi trng c xc nh bi t s ca vn

tc nh sng trong chn khng v vn tc nh sng trong mi

trng y.

V

Cn= trong : n : Chit sut ca mi trng

V : Vn tc nh sng trong mi

trng

M C V nn n 1.

Chit sut ca mi trng ph thuc vo bc sng ca nh

sng truyn cho n.

2.3.3. Hin tng phn x nh sng ton phnCho mt tia sng n sc i t mi trng c chit sut

n1 sang mi trng th hai c chit sut n2 (n1


24/106


Hnh 2.3. Hin tng phn x nh sng ton phn

Tia ti (tia 1) hp vi php tuyn P ca mt phn cch gia

hai mi trng mt gc , khi sang mi trng th hai, tia sng

ny b khc x v hp vi php tuyn P mt gc . Theo

nh lut khc x Snelious ta c:

n1.sin t = n2.sin

Khi tia ti t ti gc thi tia khc x s chy song song vi

mt phn cch gia hai mi trng. Tc l gc khc x t =

900.

Suy ra: Sin t =2

1

n

n

Gc t c gi l gc ti hn, ln ca gc ti hn phthuc vo chnh lch chit sut gia hai mi trng v khi

tia ti vi gc > t th tia phn x ti mt phn cch tr li

mi trng 1.


24

n1

1

n2n

122

1

T

P

n2

P

T2

Vng phn x

ton phn

a)

(b)


25/106


Nh vy, iu kin xy ra hin tng ton phn l:

- Cc tia sng phi i t mi trng chit quang hn (n1)

sang mi trng km chit quang hn (n2) .Hay l Chit sut

n1 > n2

- Gc ti ca tia sng phi ln hn gc ti hn.

2.4.S truyn dn nh sng trong si quang

2.4.1.Nguyn l truyn dn chung

ng dng hin tng phn x ton phn, si quang c ch

to gm mt li (core) bng thu tinh c chit sut n1 v mt

lp bc (cladding) bng thu tinh c chit sut n2 vi n1 > n2

nh sng truyn trong li si quang s phn x nhiu ln (phn

x ton phn) trn mt tip gip gia li v lp v bc.

Ta bit iu kin xy ra hi tng tn phn l:

+ Chit sut n1 > n2

+ Gc ti ln hn gc ti hn.

Do c im cu to ca si quang c iu kin l n1

> n2. Vy ch cn iu kin l gc ti t phi ln hn gc ti

hn th ( t > th). Nn ngi ta a ra khi nim gi l khu

s NA (Numerical Aperture) ngha l kh nng ghp lung

bc x quang vo si.


25


26/106


p dng cng thc : Snelious tnh N:

n0Sin th=n1.Sin . (n0=1 : chit sut ca khng kh)

1.Sin th=n1.Sin =n1Cos 1.

(Sin =Sin(900- t)=Cos t)

===

==

==

2.nnnSinNA

nnnn

1.nSin

nn

1Sin1Cos

121

22th

21

222

1

22

1th

21

22

t2

t

Vi 21

2

2

2

1n.2

nn = gi l lch chit sut tng i

Vy iu kin t c hin tng phn x ton phn

trong li l khi a ngun sng vo li cp phi nm trong

mt hnh nn c gc m = 2.narcsin 1th .

2.4.2 S lan truyn cc mode trong si quang Theo quan im truyn dn sng in t mun bit

c bn cht thc ca cc qu trnh truyn dn nh sng,

cn phi gii phng trnh sng. Mt mode c hiu l mt


26

n0

n2

n1

th

t


27/106


trng thi dao ng in t ng vi nghim ca phng trnh

sng v s lng cc mode c quan h vi cc sng in t

n tho mn cc phng trnh Maxwell v iu kin b t

si quang.Cc mode ca cc sng in t c th chia ra mode vi

tn hao thp. Mode v vi tn hao cao v cc mode r c

c tnh ca c hai loi mode trn. D nhin khi a nh sng

vo si quang th phn ln nng lng tp trung trong rut si,

cn phn nng lng r ra v to ra mode v v mode r b dp

tt ngay. Ngi ta ch n cc mode c truyn dn trongrut si v cc mode lan truyn c nhng c tnh sau :

- Cc mode hon ton c lp vi nhau.

- Mi mode c mt tc lan truyn rng.

- Mi mode ch tn ti cho mt bc sng xc nh ca

ngun sng.

Thc t phi tn ti mt bc sng gii hn g sao cho cc bc

sng ca cc mode u phi tun theo iu kin > g.

S lng cc mode lan truyn trong si quang ph thuc vo

t s dk/ nn dk ln hn nhiu th si cho v s mode

truyn qua, cn khi dk rt nh th ch c mt mode c bn

c truyn qua (si n mode). Ngi ta nh ngha tham s

cu trc V hay cn gi l tn s chun ho:

= .2.

n.d.V 1k


27


28/106


Vi si SI, nu V


29/106


(c). nh sng lan truyn trong li phi tho mn iu kin

phn x ton phn c ngha l nh sng a vo si phi nm

trong mt hnh nn c na gc m th

Si SI c 22211kth nn..2405,2

n.d.Marcsin =

Si GI c 21th )a

r(1..2.narcsin =

M nh ta bit khu s NA = Sin th

Nh vy si SI c == .2.nnnNA 12221

Gi tr ca NA nm trong gii hn t 0,2 0,37.

Cn si GI c 21 )a

r(1.nNA =

m ca si GI lun lun thay i tu gi tr a ra nh

sng vo ti tm li c m l ln nht, cn ra n ch

mt phn cch li th m bng 0.

Mun tng hiu sut nh sng vo si cn c m lnsong l thuyt chng minh l khi tng m th xung

nh sng lan truyn b tn x ln, bng tn truyn dn ca si

b thu hp li.

Theo hnh 2-5 ta khng th m t c trng ca cc

mode v thc t khng phi tt c cc tia sng i vo li

trong phm vi gc m cho php u c lan truyn n

cui si. Do bn thn nh sng c tnh sng, gia cc tia c

hin tng giao thoa.

Hai tia sng s trit tiu nhau nu nh ca mt sng

gp bng ca mt sng khc, hoc hai sng lch pha nhau


29


30/106


mt na bc sng, cn nu hai bc sng c nh gp nh

th s cng tng cng chy n cui ng si m ta gi l cc

mode.

V phng tin truyn sng, c th ni mode c ctrng bi s phn b cng nh sng trn mt ct ngang

ca si v c lan truyn vi tc xc nh.

Xa hn na, xt v phng din truyn dn th mode

s tr thnh ti tin khi iu bin, nh th trn si n mode

c mt ti tin cn trn si a mode th c rt nhiu ti tin,

mi ti tin ng vi mt bc sng nht nh.

2.5. Phn loi si quang

phn bit si quang, ngi ta da vo cc yu t ca si

quang m phn bit thnh cc li si khc nhau.

Danh mc Loi si

Phn loi si theo ch s

chit sut

- Si c ch s chit sut phn

bc

- Si c ch s chit sut

Gradien

Phn loi theo mode

truyn dn

- Si n mode

- Si a mode


30


31/106


Phn loi theo cu trc vt

liu

- Si thu tinh

- Si cht do

- Si thu tinh a thnh phn

Phn loi da theo cc

chc nng c bit

- Si l khng kh

- Si duy tr phn cc

2.5.1. Si c chit sut nhy bc(SI) v si c

chit sut bin i u (GI)

2.5.1.1.Si quang c chit sut nhy bc (si SI:

Step- Index):

y l loi si c cu to n gin nht vi chit sut ca

li v lp v bc khc nhau mt cch r rt nh hnh bc

thang. Cc tia sng t ngun quang phng vo u si vi gc

ti khc nhau s truyn theo cc ng khc nhau


31


32/106


33/106


y n1 khng i m chiu di ng truyn khc

nhau nn thi gian truyn s khc nhau trn cng mt chiu

di si. iu ny dn ti mt hin tng khi a mt xung nh

sng hp vo u si li nhn c mt xung nh sng rnghn cui si. y l hin tng tn sc,do tn sc ln nn

si SI khng th truyn tn hiu s tc cao qua c ly di

c. Nhc im ny c th khc phc c trong loi si c

chit sut gim dn

2.5.1.2.Si quang c chit sut gim dn (si GI:

Graded- Index):Si GI c dng phn b chit sut li hnh parabol, v

chit sut li thay i mt cch lin tc nn tia sng truyn

trong li b un cong dn.

Hnh 2.6 : Si GI ( Grae - Index)

- Mt ct chit sut:


33

[ ]

=


34/106


Trong :a - bn knh li, l h s mt ct xc nh

dng bin i chit sut trong li, = 2 tng ng vi mt

ct parabol.

- Khu s:

- Si truyn nhiu mode hoc v cng c 2 loi tia: tia kinh

tuyn v tia nghing hoc tia xon

- Qu o cc tia lan truyn c dng ng cong hnh sin

do b i hng lin tc khi truyn trong si.

ng truyn ca cc tia sng trong si GI cng khng bng

nhau nhng vn tc truyn cng thay i theo. Cc tia truyn

xa trc c ng truyn di hn nhng li c vn tc truyn ln

hn v ngc li, cc tia truyn gn trc c ng truyn ngn

hn nhng li c vn tc truyn nh hn. Tia truyn dc theo

trc c ng truyn ngn nht v chit sut trc l ln

nht. Nu ch to chnh xc s phn b chit sut theo -

ng parabol th ng i ca cc tia sng c dng hnh sin

v thi gian truyn ca cc tia ny bng nhau. tn sc

ca si GI nh hn nhiu so vi si SI.

2.5.2.Cc dng chit sut khc:

Hai dng chit sut SI v GI c dng ph bin , ngoi ra

cn c mt s dng chit sut khc nhm p ng cc yu

cu c bit:

a.Dng gim chit sut lp bc:


34

)/(1)()( max2

2

2arNAnrnrNA == = 21max nNA


35/106


Trong k thut ch to si quang, mun thu tinh c

chit sut ln phi tim nhiu tp cht vo, iu ny lm tng

suy hao. Dng gim chit

sut lp bc nhm m bo chnh lch chit sut

nhng c chit sut li n1 khng cao.

b. Dng dch tn sc:

tn sc tng cng ca si quang trit tiu bc sng gn

1300nm. Ngi ta c th dch im tn sc trit tiu

n bc sng 1550nm bng cch dng si quang c dng chit

sut nh hnh v:

c) Dng san bng tn sc:

Vi mc ch gim tn sc ca si quang trong mt

khong bc sng. Chng hn p ng cho k thut ghp knh

theo bc sng ngi ta dng si quang c dng chit sut nhhnh v:


35


36/106


Dng chit sut ny qu phc tp nn mi ch c nghin

cu trong phng th nghim ch cha a ra thc t.

2.5.3. Si a mode v n mode

2.5.3.1.Si a mode (MM: Multi Mode):

Si c th truyn c nhiu mode (coi mi mode l mt

tia sng ng vi mt gc lan truyn cho php).

Cc thng s ca si a mode thng dng (50/125 m) l:

- ng knh li: d = 2a = 50 m

- ng knh lp bc: D = 2b = 125 m

- chnh lch chit sut: = 0,01 = 1%

- Chit sut ln nht ca li: n1 =1,46

Si a mode c th c chit sut nhy bc hoc chit sut

gim dn.


36

%1

1

21 =

=n

nn

125m

n2

n1

n2

n1

125m

Si SI - MM Si GI - MM

50m

50m


37/106


2.5.2.2.Si n mode ( SM: SingleMode ):

Cu trc ca si dn quang n mode da trn c s

kch thc ca ng kn li v s khc nhau nh v ch s

chit sut gia li v v si. Kch thc ng knh li si

khong va bc sng.Trong si ch truyn mt mode sng nn

tn sc do nhiu ng truyn bng khng v si n

mode c dng phn b chit sut nhy bc.

c im ca si n mode:

- Mt ct chit sut thng c dng SI

- Kch thc si b, lch chit sut nh

- Si ch truyn mt mode

- Khng c tn sc mode

- hiu c tnh truyn nh sng trong si n mode

phi s dng l thuyt truyn sng

- iu kin n mode V < 2,405


37

9m

125

mn

1

n2

=0,3%


38/106


Cc thng s ca si n mode thng dng l:

ng knh li: d = 2a =9 m 10 m

ng knh lp bc: D = 2b = 125 m

lch chit sut: = 0,003 = 0,3%

Chit sut li: n1 = 1,46

tn sc ca si n mode rt nh, c bit bc

sng = 1300 nm tn sc ca si n mode rt thp ( ~

0). Do di thng ca si n mode rt rng. Song v kch

thc li si n mode qu nh nn i hi kch thc ca cclinh kin quang cng phi tng ng v cc thit b hn ni

si n mode phi c chnh xc rt cao. Cc yu cu ny

ngy nay u c th p ng c do si n mode

ang c s dng rt ph bin.

2.6.Cc thng s ca si quang2.6.1. Suy hao ca si quang:

Cng sut trn si quang gim dn theo hm s m tng

t nh tn hiu in. S thay i cng sut quang trung

bnh truyn trong si tun theo nh lut Beer

Biu thc tng qut ca hm s truyn cng sut c

dng:


38

z

PzP 100 10)(

=

L

P1=P

0

P2=P(L)

Z


39/106


Trong :

P0 : cng sut u si (z = 0)

P(z): cng sut c ly z tnh t u si

: h s suy hao

suy hao c tnh bi:

Trong :

P1 = P0 : cng sut a vo u si

P2 = P(L) : cng sut cui si

H s suy hao trung bnh:

Trong :

A: suy hao ca si

L: chiu di si2.6.2.Cc nguyn nhn gy suy hao trn si quang:


39

2

1lg10)(P

PdB =

)(

)()/(

KmL

dBKmdB

=


40/106


Cng sut truyn trong si b tht thot do s hp th

ca vt liu, s tn x nh sng v s khc x qua ch si b

un cong.

2.6.2.1. Suy hao do hp th:

Gm 2 loi chnh:

+ Hp th ngoi

+ Hp th thun

* Hp th ngoi: Do s c mt ca cc ion tp cht

- S hp th ca cc cht kim loi: Cc tp cht trong

thu tinh l mt trong nhng ngun hp th nh sng. Cc tp

cht thng gp l St (Fe), ng (Cu), Mangan (Mn),

Chromium (Cr), Cobal (Co), Nikel (ni).v.v.. Mc hp th ca

tp cht ph thuc vo nng tp cht v bc sng nh sng

truyn qua n. c si quang c suy hao di 1dB/Km cn

phi c thu tinh tht tinh khit vi nng tp cht khng

qu mt phn t (10-9)

- S hp th ca OH: S c mt ca cc ion OH trong si

quang cng to ra mt suy hao hp th ng k. c bit

hp th tng vt cc bc sng gn 950nm, 1240nm,

1400nm. Nh vy m cng l mt trong nhng nguyn nhn

gy suy hao ca si quang. Trong qu trnh ch to nng

ca cc ion OH trong li si c gi mc di mt phn t (10 -9) gim hp th ca n.


40


41/106


nh hp th chnh (cng hng dao ng) ti 2,7 m

v cc nh hp th iu ho v t hp ca chng vi thu

tinh ti 1.39, 1.24, 0.95 m.

* Hp th thun: do hp th ca thu tinh to nn si.

- Hp th cc tm ( < 0,4 m):

Cc photon kch thch in t trong di ho tr v kch

thch kn mc nng lng cao hn (Cng hng hay chuyn tip

in t).

- Hp th hng ngoi ( > 7 m):

Do tng tc gia cc lin kt dao ng v trng ca tn

hiu quang (Cng hng dao ng).

Do bn cht v nh hnh ca thu tinh cc cng hng

ny dng cc di hp th c ui di m rng vo vng

nhn thy.

Hp th thun trong di 0.8-1.6 m < 0,1 dB/km.2.6.2.2.Suy hao do tn x:

Nguyn nhn:

Sinh ra t:

- S thay i vi m v mt trong vt liu

- S thng ging thnh phn

- Cc khuyt tt hoc cu trc khng ng nht trong

qu trnh sn xut si.


41


42/106


Quan trng: S thng ging mt dn n cc thng

ging ngu nhin ca chit sut n c < bc sng Tn x

Rayleigh

* Tn x do mt phn cch gia li v lp v bc khng

hon ho:

-Khi tia sng truyn n nhng ch khng hon ho

gia li v lp bc tia sng s b tn x. Lc mt tia ti s c

nhiu tia phn x vi cc gc phn x khc nhau, nhng tia c

gc phn x nh hn gc ti hn s khc x ra lp v bc v b

suy hao dn.

* Tn x Raylegh:

-xut hin do nh hng ca nhng ch khng ng nht

cn xt li trong giai on lm ngui si hay nhng ch hn

ni si quang khng chun. Kch thc ca cc ch khng ng

nht cn nh hn bc sng nh sng. Vng hng ngoi nhiu


42

(dB/Km

)

600

1200

1400

1600

8001000

1

0

4

3

2

(nm)


43/106


nn khi bc sng tng th tiu hao ny gim nh rt nhanh, t

l nghch vi s m bc 4 ca bc sng V t l nghch vi

lu tha bc 4 ca bc sng nn gim nhanh v pha bc

sng di.Hnh 2.7: Suy hao do tn x reyleigh

H s suy hao:

Trong hng s C nm trong phm vi 0,7-0,9 (dB/km)- m

R = 0.12 - 0,16 dB/km ti 1,55 m2.6.2.3.Suy hao do si b un cong

Suy hao do un cong si l suy hao ngoi bn cht (khng

c hu). Khi bt k mt si dn quang no b un cong theo

mt ng cong c bn knh xc nh th s c hin tng

pht x tn hiu nh sng ra ngoi v si v gy ra suy hao.

C 2 loi suy hao do un cong si:+ Un cong v m

+ un cong vi m

* Un cong v m: L un cng c bn knh un cong

ln tng ng hoc ln hn ng knh si.

- Khi bn knh R gim dn th suy hao tng theo hm

m.

- si a mode: S lng mode truyn dn trong si b

un cong nh hn si thng.


43


44/106


S lng mode hiu dng:

* Un cong vi m: L si b un cong nh mt cch ngu

nhin

- Do qu trnh sn xut si quang v ch to cp si

quang to lc nn khng u ln b mt.

- gim suy hao vi un cong bc thm lp m chu

nn bng polyme.

- i vi si SM chn tham s V st vi gi tr ct V = 2,0 -

2,4

2.6.2.4.Mt s suy hao khc

- Suy hao do s khng hon ho cu trc si quang- Suy hao do hn ni

- Suy hao trong mi trng hidrogen v chiu x gamma

2.6.2.5. c tuyn suy hao

Tng hp cc loi suy hao trong si v biu din mt t-

ng quan theo bc sng ngi ta nhn c ph ca si. Mi loi

si c c tnh suy hao ring. Mt c tuyn in hnh

ca loi si n mode nh hnh 2.8


44

+

+

=

3/2

22

32

2

21

kRnR

aNNeff

+

=

2

1 )(2

kanN


45/106


Nhn vo hnh 2.8 ta thy c ba vng bc sng suy hao

thp nht, cn gi l ba ca s suy hao.

Hnh 2.8: Ca s suy hao (ph suy hao) ca si quang

* Ca s th nht: bc sng 850nm, suy hao trung bnh

mc t (2-3)dB/Km, c dng cho giai on u.

* Ca s th hai : bc sng 1300nm. Suy hao tng i

thp khong t (0,4 0,5) dB/Km, bc sng ny tn sc

rt thp nn c dng rng ri hin nay.

* Ca s th ba : bc sng 1550nm. Suy hao thp nht

cho n nay khong 0,2 dB/Km, vi si quang bnh thng

tn sc bc sng 1550nm ln so vi bc sng 1300nm. Nhng

vi loi si c dng phn b chit sut c bit c th gim

tn sc bc sng 1550nm. Lc s dng ca s th

ba s c li : Suy hao thp v tn sc nh. Bc sng 1550nm s

c s dng rng ri trong tng lai.


45


46/106


Hnh 2.9: c tuyn suy hao

2.6.3.Tn sc (Dispersion).

Khi truyn dn cc tn hiu Digital quang, xut hin

hin tng gin xung u thu, thm ch trong mt s trng

hp cc xung ln cn ln nhau. Khi khng phn bit

c cc xung vi nhau na, gy nn mo tn hiu khi ti sinh.

Hin tng gin xung ny gi l hin tng tn sc. i vi tn

hiu Analog th nh hng ca tn sc lm bin tn hiu

u thu gim nh v c tn hiu dch pha.

Hnh 2.10. nh hng tn sc ln tn hiu digital(a) v analog(b). S

ch tn hiu pht, A ch tn hiu thu. a: Dn xung, b: xt bin .


46

0

S

E

t

PA

S

E

P

0

Hp th int

Hp th do tp cht

Hp th vtliu


47/106


Hu qu ca tn sc l lm hn ch bin rng bng

truyn dn ca si bi v thu c chnh xc cc xung th

phi ch khi xung th nht kt thc, xung th hai mi n.

Nu hai xung lin tc c pht vi tn s rt ln, uthu b gin rng ln nhau dn ti thu sai.

Ta th xem xt v d hnh trn coi cc xung pht v

thu c dng phn b Gauss gn ng, xung 1 l xung pht,

xung 2 l xung thu. rng xung gi tr bin 0,5 (mc

3dB) l s, e

gin xung l 2e2s =

Xung phn b Gauss c phn b bin l :

)36,0/exp( 22

max

tP

P=

Sau khi truyn qua si quang. Coi gn ng nh mt b

lc thng thp Gauss ti mc suy hao 3 dB, rng bng

truyn dn B c quan h vi nh sau:

26,2

1B

Khi ng thi c nhiu hiu ng tn sc tc ng th tn

sc tng cng l:

1....212 =++= Bv

Ngi ta cng nh ngha mt i lng c trng cho dung

lng truyn dn ca si quang l tc bit c th truyn dn

ln nht C bit/s


47


48/106


Do nh hng ca tn sc, cc xung u vo my thu b

gin rng, nhng hai xung k nhau cn phn bit c khi

gin xung cn nh hn rng xung pht i t c tc

bit l:

C=1/ s=1/ =2,26.B=2.B

Nh vy gin xung, rng bng tn truyn dn B v

tc bit C c quan h nh hng ln nhau. truyn dn 2

bit/s th v l thuyt c rng bng khong 1 HZ nhng

trn thc t cn 1,6HZ cho nn ta c th ni rng tc bit/s

ln nht ca si quang bng rng bng tn truyn dn. T

, si cho php truyn c cc lung bit tc cao hay

l c bng tn rng cn phi gim nh hng ca hin tng tn

sc n mc thp nht thng qua chn loi si hoc chn

cc tham s cu trc ti u ca si.

2.6.4. Cc nguyn nhn gy ra tn sc.

2.6.4.1.Tn sc mode (Mode Despersion)

Nguyn nhn:

- Si truyn nhiu mode

- Mi mode truyn c vn tc nhm khc nhau (mi

mode c hng s lan truyn khc nhau)

Lch thi gian truyn gy ra tn sc mode.

tn sc ca mode ph thuc vo dng phn b chit

sut ca si a mode thng qua s m g trong biu thc hm

chit sut. Tn sc mode ch xy ra si a mode.


48


49/106


V phm vi c hn nn y ch a ra cng thc

tnh ton v tn sc mode : Vi chiu di si quang l L, chit

sut n1, n2 ; Gi s c hai tia i vo si quang, tia th nht i

on ng di hn, tia th hai i on ng ngn hn, tac:

C

n.Lt

n.C

n.Lt

2

12

2

2

21

1

=

=Trong : t1: Thi gian truyn tia th

nht

t2: Thi gian truyn tia th hai.

Thi gian chnh lch gia hai ng truyn t l:

= .1nC

LT vi

2

1

2

2

2

1

n

nn =

tri xung do tn sc mode dmat l:

=

=.

C

n

L

td 1mat

tn sc mode l :

8.1==

C

n

L

tdmat .

+ i vi si MM SI:

lch thi gian truyn gia tia ngn nht (mode bc

thp nht) v tia di nht (mode bc cao nht)


49


50/106


+ i vi si MM GI:

- Cc tia c qung ng ngn lan truyn vi vn tc chm

v ngc li.

- Si GI c mt mt ct chit sut ti u tr thi

gian l nh nht.- lch thi gian truyn gia tia ngn nht (mode bc

thp nht) v tia di nht (mode bc cao nht).

2.6.4.2.Tn sc vt liu

. Nguyn nhn:

- Chit sut thu tinh thay i theo bc sng nn vn tctruyn sng ca nh sng c bc sng khc nhau cng khc

nhau


+

c

Ln

c

Ln

T

opt

2

)2(

)(

2

1

1

opt

=

opt

50


51/106


- nh sng truyn trong si quang khng n sc m c

rng ph xc nh.

- Tc lan truyn ca cc thnh phn ph l khc nhau

(do chit sut l hm ca bc sng)

Cc thnh phn ph c thi gian truyn lch nhau gy ra

tn sc vt liu

V mt vt l, tn sc vt liu cho bit mc ni

rng xung ca mi nm b rng ph ngun quang qua mi km

si quang, n v ca tn sc do cht liu M l

ps/nm.Km.

+ H s tn sc:

+ Ta c:

+ dn xung v ngun c rng ph do tn sc

vt liu l

= DM. .L

+ Chit sut nhm:

+S ph thuc chit sut vo bc sng hay tn s quangc th c tnh bng phng trnh Sellmeier:


51

ddD =

2

2

2

21

d

nd

cd

dn

d

nd

d

dn

cd

dDM =

== ps/(nm.km)


52/106


Hnh 2.11: ch s chit sut n v ch s nhm

ng thay i si thu tinh

bc sng 850nm tn sc vt liu khong (90 120)

ps/nm.Km, bc sng 1300nm tn sc vt liu bng tn

sc ng dn sng nhng ngc du ln tn sc sc th bng

khng. Cn bc sng 1550nm tn sc ny khong 20

ps/nm.Km.

2.6.4.3. Tn sc do tc dng ca ng dn sng:

Nguyn nhn:

- S phn b nng lng nh sng trong si quang ph thuc

vo bc sng.

- nh sng truyn trong si quang khng n sc m c

rng ph xc nh.

- Do hng s lan truyn lan l hm ca a/ nn vn tcnhm ca cc thnh phn ph l khc nhau

Cc thnh phn ph c thi gian truyn lch nhau gy

ra tn sc ng dn sng.


i: ch s nhm

ca vt liu v siBi: cng daong

52


53/106


+H s tn sc ng dn sng:

Hnh 2.12: Tham s

b v cc vi phn ca n

Tn sc do ng dn sng nh v ch ng ch vi si

n mode.

2.6.4.4.. tn sc tng cng

tn sc tng cng c tnh theo cng thc

22

mod1 chrt DCDD += vi Dchr=Dmat+Dwg

Trong : Dchr: tn sc th

Dwg: tn sc ng dn sng.

- Trong si n mode, h s tn sc sc th:D = DM + DW

- Si n mode thng thng: D = 0 ti 1310 nm,


* dV/d =-V/

* dn xung v ngun c

rng ph do tn sc ngdn sng l: = D

w. .L

53

==

2

2

1 )(

dV

VbdV

c

n

dV

dV

d

dDw


54/106


D = 15-18 ps/(nm.km) ti =

1550 nm

Hnh 2.13: Tn sc tng v cc tn sc thnh phn

2.6.4.5. Tn sc bc cao

Nguyn nhn:

Do D 0 trong di bc sng nm trong ph xung quang

quanh ZD.

c trng bi dc tn sc: S = dD/d hoc

Trong :

2.6.4.6.Tn sc mode phn cc

Nguyn nhn:

- Si n mode c hai mode phn cc trc giao


54

DM: Tn sc vt liuDW: Tn sc ng dnsngD: Tn sc tngDR: Tn sc d (nh)DP: Tn sc do mt

DP

DR


55/106


- Do si thc t khng hon ho nn mi mode c ch s

mode khc nhau (birefringence)

* lch ch s mode

- Xung quang truyn trong si mt phn nng lng mang bi

mt trng thi phn cc (trc nhanh), phn khc mang bi

trng thi khc (trc chm) tn sc mode phn cc (PMD).

tr thi gian:

- y cc ch s ph x v y dng phn bit 2 mode

phn cc trc giao vi nhau.

- i vi nhng si quang di ngi ta tin hnh ghp cc

cp mode nhm lm cn bng thi gian truyn v gim

phn cc mode (PMD)


55


56/106


Qu trnh ghp cp mode thay i theo mi trng si

quang, di ghp cp mode bin i theo chiu di si

Gi tr PMD tc thi l mt bin ngu nhin.

- H s PMD: DPMD c n v ps/km1/2

- Nhiu cp ghp mode s tng ng vi mt gi tr PMD

hiu dng

2.6.4.7. tn sc ca mt vi loi si c bit.

Khi thay i tn sc ng dn sng DW dn ti D thay

i => hnh thnh nn cc si quang mi nh:

- Si tn sc phng

- Si dch tn sc

- Si b tn sca.Si dch tn sc.

V suy hao mc sng 1550 nm ch bng phn

na so vi suy hao bc sng 1300 nm nn tuyn cp quang


56


57/106


ng di hay s dng bc sng ny, nhng tn sc bc

sng 1550 nm li ln. gii to tr ngi ny ngi ta lm theo

hai cch:

- Gim rng ph ca ngun quang gim tn sccht liu

Dmar = Dmar. .

- Dch im c tn sc bng 0 n bc sng 1550 nm.

c si dch tn sc th cht liu v tn sc ng dn

sng trit tiu nhau bc sng 1550 nm ( hnh 2-9).

Hnh 2.14: Tn sc th ca cc loi si

b. Si san bng tn scDung lng ca si quang c th c ni rng bng cch ghp

hai hay nhiu bc sng trn cng mt si quang (WDM). Cn

mt si quang c tn sc nh trong khong mt bc sng.


57


58/106


Si nh vy gi l si san bng tn sc v s bin thin tn sc

theo bc sng nh hnh (2-9).

2.6.5.Di thng ca si quang

Si quang c xem nh hm truyn t:

)fm(P

)fm(P)fm(H

1

2=

P1(fm) v P2(fm) l bin cng sut u v cui si

quang vi tn s iu ch fm.

Khi fm = hz tc l c iu ch th hm truyn t biu

din dng chun ho:)o(H

)fm(H

ng biu din ca hm truyn t nh hnh 2.15

Tn s iu ch m ti bin ca hm truyn t

bng c gi l di thng B ca si quang.

2

1

)o(H

)Bfm(H=

=

Nh vy di thng B l tn s iu ch m ti cng sut

quang gim i 50% (hay 3dB).

Hnh 2.15: Hm truyn t ca si quang


58

1

1

0,5

0 Bfm


59/106


Di thng ca si quang t l nghch vi tn sc tng

cng v c tnh

theo cng thc:

).GHZ(D

44,0B

t

=

tnh c di thng chung ca si quang ta s dng thng

s B1 l di thng ng vi mt n v di (thng l 1km).

)km.GHZ(L.BB y1=

Tha s y c gi tr t 0,5 n 1 ph thuc vo chiu di

ca si thng

y = 0,6 0,8

2.6.6. Bc sng ct

Mi mode LP c mt bc sng truyn khc nhau, khi tha s

ct Vc c:

Vcn > V th mode mi c truyn. Trong si n mode

(ch c mt mode l LP01) th bc sng ct c1 l bc sng

ngn nht m si lm vic.

tin ta k hiu c l c1.

S phn chia vng n mode v a mode hnh 2-11.


59

V

Vng nmode

Vng amode

Mode

LP01

LP12

Mode

LP01

LP12

Mode

LP01

, LP11

LP12

, LP21

C1C20

0VC2VC1 =2.40

5


60/106


Hnh 2.16: Vng a mode v n mode

Trn thc t bc sng ct cn ph thuc vo chiu di si

v un cong ca si quang. Si cng di v un cong

cng nh th c cng nh v ngc li.

2.6.7. ng knh trng mode (MFD:Mode Field

Diameter)

Khi s phn tch suy hao ca cc mi ni v iu kin

phng nh sng vo si th s phn b trng l rt quan

trng.

Nng lng nh sng th hin qua nng lng trng bc x F(r)

khng ch tp trung trong li m mt phn truyn ngoi lp

bc. S phn b trng nh hnh 2-12:

Bn knh trng mode P l bn knh ti bin gim i

1/e ln (e = 2,718 nn 1/e 0,37 = 37%). ng knh trng

mode 2P ph thuc vo , nu di th ng knh trng

mode cng tng.

i vi si SI - SM th ng knh trng mode hi ln hn -

ng knh li v tnh theo cng thc gn ng sau

c.Vc.a2.6,2

V

6,2.a2p2

=

2.7.Cu trc si quang

2.7.1. Cu trc ca si quang


60


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Thnh phn chnh ca si quang gm li (core) v lp bc

(cladding). Trong vin thng dng loi si c c hai lp trn

bng thu tinh. Li dn nh sng v lp bc gi nh

sng tp trung trong li nh s phn x ton phn gia li vlp bc.

bo v si quang, trnh nhiu tc dng do iu kin bn

ngoi si quang cn c bc thm mt vi lp na:

- Lp ph hay lp v th nht (primary coating)

- Lp v th hai (Secondary coating)

2.7.1.1.Lp ph.

Lp ph c tc dng bo v si quang:

- Chng li s xm nhp ca hi nc.

- Trnh s try st gy nn nhng vt nt

- Gim nh hng v un cong

Lp ph c bc ngay trong qu trnh ko si. Chit

sut ca lp ph ln hn chit sut ca lp bc loi b cc

tia sng truyn trong lp bc v khi s phn x ton


61

250m 125

m

lp vLpph

lpbc

0,9( 2mm)

Li

10(50) m


62/106


phn khng th xy ra phn cch gia lp bc v lp ph.

Lp ph c th c nhum mu hoc c thm vng nh

du, khi hn ni si hoc ghp nh sng vo si nht thit

phi ty sch lp ph. ng nht, b dy v ngtm ca lp ph c nh hng n cht lng ca si quang.

2.7.1.2.Lp v.

Lp v c tc dng tng cng sc chu ng ca si

quang trc cc tc dng c hc v s thay nhit ,

cho n nay lp v c cc dng chnh sau:

- Dng ng m lng (Loose buffer)

- Dng m kht (tight buffer)

- Dng bng dt (Ribbon)

Mi dng c nhng u nhc dim khc nhau do c s

dng trong tng iu kin khc nhau.

a) Dng ng m lng:

Si quang ( bc lp ph) c t trong mt ng m c

ng knh ln hn ng kch thc si quang.

- ng m lng thng gm hai lp, lp trong c h s ma

st nh si quang di chuyn t do khi cp b ko cng

hoc co li, lp ngoi bo v si quang trc nh hng ca lc

c hc. i vi cp trong nh th bn trong ng m lng

khng cn cht nhi nhng vi cp ngoi tri th phi bm

thm cht nhi c cc tnh cht sau:


62

1,2 2mm

siquang

lp ph

ngmchtnhi


63/106


- C tc dng ngn m

- C tnh nht khng tc dng ho hc vi cc thnh phn

khc ca cp

- D ty sch khi cn hn ni

- Kh chy.

Cu trc ng m lng c nhiu u im nn c

dng trong cc ng truyn dn cn cht lng cao, trong

iu kin mi trng thay i nhiu.

b) Dng m kht:

Mt cch n gin bo v si quang di tc dng ca

nhiu iu kin bn ngoi l bc mt lp v m st lp ph.

Phng php ny lm gim ng knh ca lp v do gim

kch thc v trng lng ca cp, song si quang li chu nh h-

ng trc tip khi cp b ko cng gim nh hng ny ng-

i ta chn thm mt lp m mm gia lp ph v lp v.

Hnh thc ny c gi l cu trc m tng hp. Si quang

c v m kht v m tng hp thng c dng lm cp

t trong nh, lm dy nhy u ni cc trm u

cui...

c) Dng bng dt:


63

Siquanglp

phlp mmmlpv

0,9mm

Si quang

Lp ph

Lp mmm

Lp v

0,9mm


64/106


Cu trc bng dt cung l mt dng v m kht nhng

bc nhiu si quang thay v mt si. S si trong bng c th

ln n 12, b rng ca mi bng tu thuc vo s si trong

bng. Nhc im ca cu trc ny ging nh cu trc mkht, tc l si quang chu nh hng trc tip khi cp b

ko cng.

2.7.2. Yu cu i vi si quang

Cng ging nh trong cp thng tin in, cc si quang c

ch to xong cha c em s dng ngay, m c dng

sn sut cp. Si quang cn p ng c cc yu cu ngt

ngho sau:

1. V c: bn vng, khng b t gy di tc dng ca lc

ko, lc ct ngang v lc un cong. Khng b dn n

qu ln do tc ng ca lc ko thng xuyn. Tc lo

ho chm.

c tnh c hc ca si quang c quyt nh bi vt

liu ch to si v bi lp vt liu bn ngoi. Nu do tc


64

bng 4si

bng 8si


65/106


ng ca lc ko v lc un cong m si b t th c

th do nhiu nguyn nhn, ngi ta chia ra ba loi sau:

- t do lc ko ln, khong > 40 N (Newton), nguyn

nhn c th do cu trc ca thc anh hoc trn b mtsi c i ch b rn nh.

- t do lc ko trung bnh, khong t 15 n 40 N,

thng do cc tp cht nm trong lp v si, gy nn cc

ch khuyt tt.

- t do lc ko yu, di 15 N, thng do cc khuyt tt

trn b mt, do cc bt kh, do c tp cht trong lp bov, do ph lp bo v km.

Khi qun si to thnh cp s xut hin cc lc ko

vi Newton, nu si ri vo trng hp th 3 nh trn th rt

nguy him. V vy, m bo bn ch to cp chc

chn, v trong qu trnh th nghim cp vn bn vng

th si phi chu c lc ko khong 4 5 N trong 1 giy.

Nu si c ph l lp nha bo v Acrylat mt si phi chu

c lc th l 50 N. Khi mi c kh nng ko c cc si di

n 10Km.

2. V c tnh truyn dn nh sng: Vt liu phi c

tnh tinh khit, khng c tp cht. Cu to v v rut

u n, khng c ch khuyt tt trnh lm tn x

nh sng, sinh thm tiu hao ph v mo xung tn hiu.

c tnh truyn dn nh sng ca si c quyt nh bi

kch thc ca rut si, bi chit sut trong rut v v si v bi


65


66/106


ng u ca chit sut, tc l ph thuc vo cng

ngh v k thut ch to si.

2.8. Cc phng php ch to si quang2.8.1.Vt liu ch to si

Cu trc c bn ca si dn quang nhn chung gm c li

v v phn x, ton b to nn si dn quang di v rt mnh,

chng c vai tr truyn thng tin cho c ly xa v tc ln

nn phi c cu to t cc vt liu ph hp vi bn cht truyn

tn hiu ca chng. Nh vy, trong qa trnh chn vt liu si

quang cn phi tho mn yu cu sau:

- Vt liu phi m bo to c cc si dn quang di,

mnh v mm do

- Vt liu phi m bo trong sut ti cc bc sng hot

ng thng dng, to cho si truyn tn hiu tt, t b suy

hao v mo.- Cc vt liu ch to ra li v v si phi c bn cht vt l

tng thch to ra s chnh lch v ch s chit sut

gia li v v kh nh.

Vt liu thch ng v dng nhiu nht l thuy tinh v cht

do trong sut. Cc si lm bng cht do t c dng hn v

suy hao ca n thng l ln hn cc si thu tinh. N ch cdng cho c ly ngn, tc thp, v nhng ni c tc ng

c hc mnh, t quan tm ti cht lng truyn dn, thc cht

th si cht do i khi b lm dng trong cc trng hp s

dng.


66


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2.8.2.Cc phng php ch to si quang

C hai phng php ch to si quang:

Phng php th nht: l vt liu lm rut v lm v c -

a vo l nu chy ri cho ko chy ra ngoi thnh dng si

Phng php th hai: S dng phi c sn cng gm c v

v rut m ko nng thnh si.

Yu cu nhit v tc ko rt n nh m

bo kch thc hnh hc v dng ca ng cong bin thin

theo yu cu.Phi ch trnh bi bn v tp cht khng

lm hng b mt ca si. Khi si ko ra cn ang ng s c

ph lp bo v lun, sau khi ngui c lp bo v hon

chnh v ng u

2.8.3.Cc phng php ch to phi si

2.8.3.1.Phng php thanh ng c in

Phng php ny c s dng vo thi k u th nghim ch to si

quang c tiu hao b. S dng mt thanh thy tinh hoc thanh thch anh, d

kin lm vt liu rut si lun vo mt ng thy tinh c chit sut b hn nh

lm v si. ng knh ngoi ca thanh phi rt kht vi ng knh trong

ca ng, khng cn khe h cho khng kh lt vo. Phng php ny ch

ch to si a mt SI

Phng php ny c nhc im l nu trn b mt phn cch ca v

v rut phi c khuyt tt, khng ng u hoc khng tinh khit th khi kothnh si vn cn li cc khuyt tt , gy nn tn hao tn x ln.

2.8.3.2.Phng php ni nu i (Double Orucible)


67


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Hnh 2.17. Phng php nu ni i

Dng phng php ny ch to ra si ch khng ch phi.

*u im:

- Trnh c cc ch khuyt tt trn lp phn cch- rut si m

phng php thanh ng gp phi

Trn hnh v gii thiu tng qut phng php ny. Thy tinh lm v v

rut si c nu ring thnh cc cht lng ri a vo ni hai lp ring r.

u ra ni nu i ny c van hai lp ko si ra. Nh thm thy tinh

lin tc nn trong qu trnh nu v ko lin tc c th t c si rt di. Si

nng c ko qua b ph cht bo v trc khi c cun thnh cun. Dng

phng php ny c th ch si SI v si GI. L nu ch t cht Platin tinhkhit khng gy tp cht. Tuy nhin ni nu cng vn c th mang tp cht

vo thy tinh nu, lm cn tr vic ch to cc si c suy hao b. S bin

thin ca chit sut si t c nh s khuych tn ca thy tinh nng chy


68


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ti lp phn chia v li v nh s kt hp ca hai loi thy tinh. ch to

si n mt c ng knh rt b th phng php ny cha thc hin c.

2.8.3.3.Phng php ng hi ha cht

Cc phng php ch to si quang hin i u c chung mt nguynl c bn l dung phng php ng hi ch ta phi, sau ko thnh si

tc l ch to phi si t th hi. Yu cu phi t c tc l phi cng phi

c hai lp v v li, kch thc t l v v li cng nh s bin thin ca chit

sut ca phi phi theo ng gi tr nh ca loi si nh ch to. y chnh l

mt thnh cng ca cng ngh mi. Nh th c th ch to c si di

dng th sau ko nh si thnh si cn c. Nh cng ngh mi ny mi c

th ch c si n mt SM c tn hao b v si to thnh t th kh khng

mang ln tp cht. Trong phng php ng hi ha cht xut hin mt s

phng php dn xut khc nhau, nhng u khng chch khi nguyn l c

bn ny.

2.8.3.3.1.Phng php ng hi ha cht: (chemical vapour

deposition- CVD)

S ca phng php ny cho trn hnh


69


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Hnh 2.18. Phng php ng hi ho cht

Khi ch to thay i chit sut ngi ta phi s dng thm cht ph

gia nh GeO2, P2O5, B, F, trong GeO2 v P2O5 lm tng chit sut, cn B, F

th lm gim chit sut cua thy tinh thch anhNguyn liu s dng l mt ng thch anh thun khit. Cc cht lng

Tetra-clorit- silic SiCl4, Tetra-clorit-Giecmani GeCl4, cc cht kh O2, PCl3,

BCl3 (FCl3) ty nh sn xut m c nhng loi phi si vi thnh phn v v

rut nh sau:

a : V l thy tinh SiO2, rut l SiO2 + GeO2

b : V l thy tinh Bor- Silicat SiO2 + B2O3, rut SiO2 + GeO2 + P2O5

ng thch anh c quay trn quanh trc. Quanh ng c ngun t c

th l mt b cng hng sng cc ngn c th t nhit 01500 01600

C. Cc nguyn liu c a vo ng, nhit cao s xy ra phn ng ha


70


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hc. Sau cc phn ng cc vt liu cu thnh v v rut ri vo thnh ng

theo tng lp. Sau nung chy ng ng co li thnh mt phi c.

2.8.3.3.2.Phng php ng hi ha cht nh Plasma

(Plasma chemical vapour Deposition- PCVD)Khi qut v phng php ny nh hnh v:

Hnh 2.19: Phng php PCVD to phi

Cng l phng php ng hi ha cht bn trong. Cc nguyn liu

th hi do mt h thng cung cp vo ng thy tinh t trong l nung

C01150 . Qu trnh phn ng xy ra nh mt vng Plasma sinh ra nh b cng

hng sng cc ngn. B ny c th di chuyn dc theo ng. Bm gi plc trong ng to Plasma v ht kh tha ra. ng bao chit sut rt chnh

xc nh to c hng nghn lp rt mng trn thnh ng sau ng ny

c nung chy C02000 to thnh phi c.


71


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2.8.3.3.3.Phng php ng hi ha cht bn ngoi ( Outside

Chemical Vapour Deposition- OCVD)

Phng php ny c hng Corning Glas (M) pht trin. Hai phng

php trn l phng php ng hi bn trong lng ng thy tinh. Trongphng php ny ng thy tinh c thay bng mt thanh thy tinh. Cho cc

vt liu ng hi xung quanh thanh cng theo phng php trn. Khi ph

cc lp yu cu, cho rt nhanh thanh thy tinh ra cn li mt ng phi

rng. Tip tc nung chy ng C02000 c phi c. Theo phng php

ny c th to ra c phi kh ln ko c si quang di hng chc

kilomet.

2.8.3.3.4.Phng php ng hi ha cht theo trc ( Vapour

Axial Deposition- VAD)

- y l mt phng php rt tt c pht trin Nht Bn v lun c

ci tin.

Cc vt liu to rut v v si c bc hi v ngng t vo u ca mt

thanh thy tinh xoay trn lin tc. Khi di chuyn thanh dc trc s to c

phi. iu chnh hm lng cc cht ph gia c chit sut c chit sut

yu cu. Sau ko phi ny qua l nung to kch thc hnh hc u cho

phi si. u im ca phng php ny l c tc ng hi ln v hiu sut

s dng nguyn liu cao ti 60- 80. Theo phng php ny ngi ta ch

c si c rng bng truyn dn rt ln

2.8.4.Qu trnh ko si

Nguyn l ca qu trnh ko si t phi c m t trn hnh v:


72


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Hnh 2.20. Nguyn l ca qu trnh ko si t phi

Si ko ra t l nung c ng knh ngoi ng yu cu. c kch thc

hnh hc u v ng bao chit sut n nh th nhit ca phi v tc ko phi rt n nh. Lu khng cho bi bn bm vo trn mt si cn nng,

c th sinh ra cc ch gy nh sau ny. Hai lp ph bo v bn trong mm v

bn ngoi cng, l loi quang ha cht. Ngun bc x cc tm gip cho t

c tc ko si rt ln.

2.8.5. Nguyn tc to ra si quang mi

Khi phn tch cc loi si quang ta thy rng si n modec suy hao nh v c bng tn ln. iu khng nh vai

tr v nng lc ca n trong mng vin thng hin ti v tng

lai. Tuy nhin, nng cao kh nng s dng si n mode


73


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thng thng ngi ta tip tc thay i mt s tham s cu

trc ca loi si ny. Nh ti u ho thit k si n mode cng

ngh quang si cho ra i cc loi si mi c suy hao rt

nh, cht lng truyn dn tt hn, tch bng tn c ly ln vtui th cao s p ng c cc h thng thng tin quang

tin tin.

i vi cc h thng thng tin quang c c ly v dung lng ln

th yu t l cc hiu ng phi tuyn, suy hao v tn sc si.

Trong truyn dn n mode, suy hao ca si t gi tr nh

nht vng bc sng 1550nm nhng tn sc c gi tr thpnht ti vng 1300nm. Nu c hai yu t suy hao v tn sc

u t gi tr ti u th s c c tuyn thng tin c ly

truyn dn xa v tc ln.

Tn sc trong si n mode ch yu l tn sc vt liu v

tn sc dn sng. Tn sc vt liu thng khod thay i, tuy

nhin c th thay i tn sc dn sng bng cch sa imt ct ch s chit sut phn bc n gin li si thnh

mt ct ch s chit sut phc tp hn cho ra c gi

tr tn sc mong mun.

2.9. Hn ni si quang

2.9.1.Yu cu k thut Trong k thut thng tin quang, cc si quang ca cc loi

cp quang phi c hn ni vi nhau, hoc cc si quang phi


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c ni vo cc linh kin pht hoc thu quang. Vic ni

khng tt s gy nn tn tht tn hiu quang ti ch hn ni.

Yu cu ca hn ni phi t tn tht quang cng b cng

tt. Nhng v ng knh ca si quang rt b, nht l nhsng ch c lan truyn trong li si quang, do rt kh

phi hp. Trong thc t cc u si quang kh lng khp st

vo nhau nn khng trnh khi b suy hao.

2.9.2.Cc phng php hn ni si quang

Trong phn ny ta khng cp n vic u ni si

quang vi cc linh kin thu, pht quang. Khi ch to cc linh

kin ny th cc nh ch to ch sn thnh cc Mun

vi u si quang th ra ngoi phn si quang ca Mun thu,

pht ny c ni vi si quang bn ngoi nh cc b con-nec-to

tho lp c, ging cc b con-nec-to trong k thut in t

thng thng.

hn ni hai si quang vi nhau ngi ta nghin cu phngphp sau:

- Dng b ni tho ri c v khng tho ri c.

- Hn ni bng si keo dnh v bng h quang in.

Phng php dng cc b u ni thng t chnh

xc khng cao, nn hay s dng cho cc loi si c tc

truyn dn thp, c ly gn nh trong nh, trong my bay hay

tu thu.

Phng php hn ni th hai si dn quang c ni vi

nhau mt cch chc chn c nh, c chnh xc cao.


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Trong hng ny c hai phng php chnh l ni bng keo

dnh v h quang.

2.9.2.1. Phng php dng keo dnh

Cch ny thc hin hon ton bng nhn cng, khng c

s gim st, iu chnh nn chnh xc khng cao v ch

c th p dng ni si a mode c ng knh rut ln,

y khng trnh by c th v iu kin khng cho php.

Hnh 2.18.Hn si bng keo dnh

2.9.2.2.Phng php dng h quang.

c thc hin nh cc my hn. H quang c to ra nh cc

tia la in phng in. Nguyn l ca phng php ny nh hnh 2 -13. cc bc tin hnh nh sau:

1- Dng ho cht tch v ty sch lp v bo v hai

u si cu ni.


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2- Kp hai u si ln b g (hnh a).

3- iu chnh cho hai si li gn nhau, khong cch gia

chng khong 10% ng knh si quang. C th cho tia h

quang lm sch tng u si ring r trc khi cho chng li gnnhau. Qu trnh ny c quan st nh knh hin vi hoc

knh lp, nh b phn gng lp gn theo c th quan st -

c c chiu ngang v chiu ng, sao cho hai u si

quang hon ton ng trc v ng tm vi nhau (hnh b)

4- ng mch tia la in, hai u si nng chy v sau

vi giy dnh cht vo nhau. Qu trnh sy ra t ng, thigian tnh ton l ph hp vi si v kch thc ca si (hnh c).

5- Kim tra mi ni (mi ni tt nh mt si bnh thng ti

ch ni), nu c khuyt tt th phi hn li. Mt ni hn tt

nu nhn thy un nh mt on si bnh thng (hnh

d).

6- Gia c c hc bo v mi ni. Mt phng phphay dng hin nay l dng mt ng bc nha co nng, c

m on vt liu gia cng (hnh e).


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Ngy nay cc nh my hn c thit k hin i,

vic lm ny c thc hin hon ton t ng tng cht

lng.

Hnh 2.19: Phng php hn ni bng h quang

Vic hn ni nh hon ton t ng nn tiu hao ti cc

mi ni rt nh, vi si a mt tiu hao cc mi ni trung bnh

l 0,038 dB, cn vi si n mode th l 0,5 dB. Trong thc

t, suy hao t khong 0,1 dB l chp nhn c.


78

(a)

(b)

(c)

(d)

(e)

1

0

10

10

10

0,0 0,0 0,0 0,00,1

S mihn(n)

Suy

hao(dB)0


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Hnh 2.20: th suy hao thc t mi hn Furukawa SI 475

2.9.3.Bo v mi ni

Khi ni cc si quang c tch ra, khng cn cc lp

bo v ca v cp v vy ngi ta dng cc hp bo v ch

ni. C nhiu loi hp bo v, c loi trong nh, c loi

ngoi tri, c loi chn trc tip, tu theo iu kin mi

trng v mc ch s dng ta chn cho thch hp.

Hnh 2.21. ng mang sng bo v mi ni


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80

Hnh2.22.Mi ni c kp


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Chng 3

Cp quang

3.1. Tng quan

3.1.1. c im, yu cu i vi cp quang

Cng nh cp kim loi, cp quang cng c cc yu cu, c

im cn phi p ng. Trc ht, c lp v bao bn ngoi

bo v si cp quang khi nh hng ca mi trng nh cn

trng, m hoc cc lc c hc tc ng.

Cp cn phi p ng cc yu cu sau:

- Khng b nh hng bi nhiu in t.

- Khng thm nc, lt nc.

- Chng c cc nh hng ca cc tc ng c hc nh va

chm, lc ko, nn, lc un cong

- n nh khi nhit thay i, nht l khi nhit

rt thp c tc ng co ngt si.

- t b bo ho, c thi gian lm vic lu.

- Trng lng nh v kch thc b.

3.1.2. Kh nng ca si v cp quang p ng c nhng yu cu trn th si v cp quang

phi p ng c nhng yu cu rt ngt ngho, va m

bo c bn vng c hc, va m bo c c tnh


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truyn dn. Nu xt trong mt chng mc no th cp

quang th hin tnh u vit so vi cp kim loi nh:

Si quang l vt liu cch in nn hon ton khng

nhy cm vi nhiu in t, do trong cp khng cn

c lp bao che in t nh i vi cp kim loi.

Si quang l rt nh, tn t nguyn liu. Nu xt trong

cng mt khong thi gian truyn dn th 1 gam thu

tinh lm si dn quang thay th c vi kg ng

lm cp kim loi. Nguyn liu ch to cp quang l sn

c trong t nhin, trong khi kim loi mu ch to cpkim loi th ngy cng khan him.

Si quang rt dn v d gy. Th nhng nh lp bo v

trc tip bao quanh si lm co si trnh c m,

tng bn c hc v d un do. Khi ch to thnh

cp th cu trc ca cp cn vt xa nhng c im

ny ca si Xt v phng din truyn sng, nu si b un cong

nh th nng lng ca trng lt t rut ra v gy ra tiu

hao ph. Nu si quang trong cp khng c bo v cn

thn th s chu cc tc ng ca bn ngoi, si b un

cong hoc khi si cp b un cong vi bn knh nh, si

cng b un theo gy suy hao phBi vy, cp quang phi c ch to mt cch bn vng

vi cc tc ng c hc v nhit ca mi trng,


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m bo si va khng b t va khng gim st cc

c tnh truyn dn, trong mi iu kin s dng cp.

3.2. Cu trc cp quang

Cho n nay, cp dn quang c rt nhiu cu trc khc

nhau. Do cng ngh pht trin a ra c nhng mu cu

trc cp quang c c tnh tho mn c nhiu yu cu.

to c cp dn quang, si quang trc ht phi c bc

s b bng cc lp Polime mng ri mi to thnh cp. Cc si

c bc hon thin lm thnh phn chnh ca li cp. Li

cp cn c th thm cc si gia cng bng nilon v cc i

giy bng kim loi cp ngun t xa.

Ngi ta c th t cc si dn cp quang vo li cp vi

nhiu hnh thc. Chnh v vy cp quang c th c dng

hnh trn hoc dt, si dn quang c th c bc cht

hoc bc lng. trnh nh hng c hc lm gy t v

thun li khi hn ni. Thng cc si c t lng t do trong

li cp, hoc t lng c keo mm n vo.

Cc nc Ty u v c thng thit k li cp c cc rnh mi

kh m trong cc rnh l cc si cp quang. Cn cc nc

Chu M v Nht th thng cho si nm lng trong cc ng

ghen nh li cp. Vic cu trc v cp rt phong ph, ph hp

vi iu kin tng mi trng t cp.

3.2.1.Cu trc tng qut ca cp quang


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3.2.1.1.Phn li

a. Phn t gia cng:

- Thnh phn gia cng ca cp l cc phn t to cho cp

c cc lc c hc cn thit to sc cng v co, c bit

l bo m tnh n nh nhit cho cp.

- Yu cu i vi vt liu gia cng:

+ C modul Young cao: tng tnh chu lc, vd: Thp

+ Vt liu phi nh, c mm do ( y l c tnh

quan trng trong qu trnh ko cp trong cng), vd: si

aramid.

Cc c tnh ca mt s vt liu ch to phn t gia cng

:

Vt

liu

Trng

lng

ring

Modul

n hi

(kg/mm2)

ng sut

cong

(kg/mm2)

dn ti

im

cong(%)

ng sut

gy

(kg/mm2)

dn

khi t

(%)


V PE ngoicng

LicpBng nhm

trng

nha hai mtV PE

ngoi cng

Bng thpm km

Cht iny

Li cp

Cpcng

84

Cp chn

- Phn li:. Phn t gia cng. Cc ng m (khi ng m)

. Cc si dy ng cpngun

. Cc phn t n & cht iny- Phn v:

. V PE th nht bc ngoi li(1mm)

. Lp chng m v chng gmnhm. V PE ngoi cng (1,5-2 mm)


85/106


Dy

thp

Si carbon

G-FRPSi t

49

Si t

29

7,86

1,50

2,48

1,44

1,44

20 103

10 2 104

9 103

13 103

6 103

40 150

150 200

300

300

70

0,2 1

1 1,5

3

2

1,2

50 300

150 200

300

300

300

20 25

1,5

2,4

2

4

a. ng m v khi m


85

Sp xp cc phn t gia cng trong cp si quang


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c. Cht n v cht in y

- Trong ng m lng:

. Cht in y phi trung tnh khng to ra hydro,

khng ng bng v khng dn n trong di nhit t -30

+70 0C.

. L cht gien c c tnh l th lng khi b khuyng v th c khi ng yn.

- Trong li cp:


ng m chtmt si

ng m lng mtsi

t trong li rnh ch V

86

Khi cc bng dt


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. Cht in y c ng sut cao, c cc thnh phn

khng nh hng n c tnh ca phn t khc, khng lm

phnh v PE v c h s dn n tng i thp. Lp chn l

sn phm du m c tnh n hi, chu nhit v kt dnhbao bc li cp, va ng vai tr chng thm nc, va gn

kt li v v cp thnh mt khi vng chc nhng khng lm

gim tnh mm mi ca cp.

. Cp trong nh khng phi in y li cp. bo v

cc phn t bn trong li cp s dng bng mng plastic

qun quanh li.. Ngoi ra t thm mt bng giy rng khong 6mm

trn li cp sut dc chiu di cp, trn c ghi khong cch

theo mt. Cng c th ghi chiu di theo mt ngay trn v

ngoi ca cp.

Bn cc phn t bn trong li:

- Khi sn xut cp si quang thng s dng cng ngh bn

cc lp ca li cp. Cc phn t b c sp sp tp trung

trong mt hoc nhiu lp xung quanh phn t trung tm.

- Cc phn t trong mt lp c bn xung quanh phn t

trung tm theo kiu l xo hoc kiu i chiu ZS.

- Mi lin quan gia bc bn S, gc bn v chiu di

phn t bnL:


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3.2.1.2.V cp

V cp si quang c chc nng c bn l bo v cp v c

tnh cht quyt nh n tui th ca cp. V cp c th

c bc m bo v li cp khi b cc tc ng ca

ng sut c hc v mi trng bn ngoi.

Cc vt liu ch to v cp nh:

- V PE:

. Vt liu chu nhit c tng hp t ethylene.

. C mt thp, do, chu lc cng tt, mu en

-V PVC:

. Polyvinyl chloride (PVC) s dng ch yu cho cp trong

nh mu xm

. PVC cng thch hp cho cp ngoi tri lp t nhng

ni t b nhim bn c ho cht n mn.. V PVC c kh nng chng la bng cch tng polymer

halogen

- V halogen t do:


1

2

3

Gc bn

Bc bn S

Chiu diphn

t bn L

Chu vi vng

trn bn 2 R

22

1

+=

S

RSL

R

S

2arctan=

%1001sin

1%1001

21%100

2

=

+=

=

S

R

S

SLL

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. Ch to t cc vt liu cha ethylene vinylacetate

(EVA) v c trn thm 50% trng lng aluminum

trioxydhydrate (c chn khi c yu cu chng la v khng

b halogen ho). Trong cc trng hp ny PE v PVC khngthch hp.

. Di tc ng ca ngn la c nhit cao hn 2500C

nc tch ra khi aluminum trioxyhydrate. Thng qua tch nc

v nc bc hi lm cho ngn la yu dn. Nhit gim

thp hn v nng oxy gim do b sung hi nc nn la

b dp tt. V EVA cng c cc c tnh nh v PE nhng ckh nng chng chy tt hn.

- V bc kim loi:

.Thng l cc bng thp nhn hoc cc si thp sp

thnh v bc

. Cc v ny thng dng cho loi cp chn trc tip

bo v chng cc ng sut c hc, gm nhm, chng s phhu ca cn trng v cc sinh vt.

3.3.Phn loi cp quang:

Ngi ta phn loi cp quang theo nhiu loi khc nhau

thun li cho vic nghin cu.

3.3.1. Phn loi theo cu trc:

Cp c cu trc c in: Cc si hoc cc nhm si

quang c phn b i xng theo hng xoay vng ng

tm, loi cu trc ny rt ph bin.


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Cp c li trc c rnh: Cc si hoc cc nhm si c

t trn cc rnh c sn trn mt li ca cp.

Cp c cu trc bng dt: Nhiu si quang c ghp trn

cng mt bng v trong rut cp c nhiu bng xp chng lnnhau.

Cp c cu trc c bit: Do nhu cu, trong cp c th

c cc dy kim loi cp ngun t xa, cnh bo,

lm cc ng nghip v hoc cp i trong nh ch cn

hai si l .

Hnh 3.1: Cc v d v cu trc cp quang

a.Cp c cu trc c bit c. Cp c cu trc bng dt

b. Cp c li hnh trc c rnh d. Cp si quang dng trong nh

e. Cp c dy ng

3.3.2. Phn loi theo mc ch s dng

- Cp dng trn mng thu bao, ni ht, nng thn.


90

b) c)a)

d) e)


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- Cp trung k gia cc tng i.

- Cp ng di.

3.3.3. Phn loi theo iu kin lp t

- Cp treo ngoi tri.

- Cp t trong ng nc.

- Cp chn trc tip.

- Cp ngp nc v th bin.

- Cp dng trong nh v cp nhy

3.3.3.1.Cp treo


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Thng c cu to m st vo thnh phn gia cng kim loi

hoc phi kim loi c lp, hoc di dng t chu lc.

c dng trong mi trng bng tuyt v gi, c c ly di.

Cp t chu lc i hi phi c sc bn cao v cn phi

dng cu trc bc lng si c khong t do ln hn.

3.3.3.2.Cp t trong cng

- Cp t trong cng phi chu c lc ko v xon, c

trng lng nh d lp t

- C tnh mm do d vt qua cc chng ngi khi ko

cp


Cu trc ca cp treo

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- Chu c m v nc v trong cng cp v b cp thng

hay ng nc.

Chnh v vy m trong cu trc ca cp thng c cht n

jelly v thnh phn chng m bng kim loi. Trong trng hpcp khng c n y th cn bm hi cho cp. Lp bc

thp cng c s dng chng gm nhm ca cn trng.

Cp ko trong cng thng c

do an mon hoc cap soi quang

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