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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]
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
-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)
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
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Siquang
Trm lp
E
O
O
E
Nguntn
hiu.
Phnt
in.
Phnt
in.
O
E
E
O
Biniin-quang
Binisi quang
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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:
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Hnh 2.1: Cc bc sng trong thng tin quang
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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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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n1
1
n2n
122
1
T
P
n2
P
T2
Vng phn x
ton phn
a)
(b)
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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n0
n2
n1
th
t
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
Vi si SI, nu V
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
(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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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:
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[ ]
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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:
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)/(1)()( max2
2
2arNAnrnrNA == = 21max nNA
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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:
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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36
%1
1
21 =
=n
nn
125m
n2
n1
n2
n1
125m
Si SI - MM Si GI - MM
50m
50m
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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9m
125
mn
1
n2
=0,3%
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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:
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z
PzP 100 10)(
=
L
P1=P
0
P2=P(L)
Z
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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:
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2
1lg10)(P
PdB =
)(
)()/(
KmL
dBKmdB
=
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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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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(dB/Km
)
600
1200
1400
1600
8001000
1
0
4
3
2
(nm)
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
44
+
+
=
3/2
22
32
2
21
kRnR
aNNeff
+
=
2
1 )(2
kanN
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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 .
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
46
0
S
E
t
PA
S
E
P
0
Hp th int
Hp th do tp cht
Hp th vtliu
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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)
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
+ 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
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
+
c
Ln
c
Ln
T
opt
2
)2(
)(
2
1
1
opt
=
opt
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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:
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
51
ddD =
2
2
2
21
d
nd
cd
dn
d
nd
d
dn
cd
dDM =
== ps/(nm.km)
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
i: ch s nhm
ca vt liu v siBi: cng daong
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
+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,
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
* 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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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54
DM: Tn sc vt liuDW: Tn sc ng dnsngD: Tn sc tngDR: Tn sc d (nh)DP: Tn sc do mt
DP
DR
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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)
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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1
1
0,5
0 Bfm
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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59
V
Vng nmode
Vng amode
Mode
LP01
LP12
Mode
LP01
LP12
Mode
LP01
, LP11
LP12
, LP21
C1C20
0VC2VC1 =2.40
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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61
250m 125
m
lp vLpph
lpbc
0,9( 2mm)
Li
10(50) m
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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:
Sinh vin: Lng Xun Trng Lp K40VT Email:[email protected]
62
1,2 2mm
siquang
lp ph
ngmchtnhi
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
- 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:
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63
Siquanglp
phlp mmmlpv
0,9mm
Si quang
Lp ph
Lp mmm
Lp v
0,9mm
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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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
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bng 4si
bng 8si
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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
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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.
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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)
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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
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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
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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
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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.
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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:
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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
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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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Trng HKT Cng Nghip Thi Nguyn nThng tin quang
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.
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(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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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
(%)
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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)
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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
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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:
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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:
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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.
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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
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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