optoelectronics and photonics (18/2) w.-y. choitera.yonsei.ac.kr/class/2018_2_1/lecture/lect 17...
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Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
3-dimentioanl dielectric waveguide?
planar waveguide
- Analysis very complicated
- Contains similar qualitative features
Numerical simulation required
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
n
y
n2 n1
Cladding
Core z
y
r
Fiber axis
The step index optical fiber. The central region, the core, has greater refractiveindex than the outer region, the cladding. The fiber has cylindrical symmetry. Weuse the coordinates r, , z to represent any point in the fiber. Cladding isnormally much thicker than shown.
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Circular Dielectric Waveguide: Optical Fiber
SiO2:Ge
What is special about fiber?
Loss in rectangular metal waveguide
- Extremely low loss: 0.2dB/km
- Can be very long: 100’s of km
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Optical Communication Networks
Total undersea fiber length: 0.4 billion km ( 628 round trips between earth and moon)
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Protective polymerinc coating
Buffer tube: d = 1mm
Cladding: d = 125 - 150 m
Core: d = 8 - 10 mn
r
The cross section of a typical single-mode fiber with a tight buffertube. (d = diameter)
n1
n2
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
for single-mode fiber
(For multi-mode fiber, d: several tens of m)
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
SiCl4 + O2 ->SiO2 + 2Cl2
GeCl4 + O2 ->GeO2 + 2Cl2
(b)
Furnace
Porous sootpreform with hole
Clear solidglass preform
Drying gases
Sintering at 1400-1600 deg C
Vapors: SiCl 4 + GeCl4 + O2
Rotate mandrel
(a)
Deposited sootBurner
Fuel: H 2
Target rod
Deposited Ge doped SiO 2
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Preform feed
Furnace 2000°C
Thicknessmonitoring gauge
Take-up drum
Polymer coater
Ultraviolet light or furnacefor curing
Capstan
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Solving for guided modes for circular dielectric waveguide problem in (r, , z) coordinate is very complicated.
0 2 4 61 3 5V
b
1
0
0.8
0.6
0.4
0.2
LP01
LP11
LP21
LP02
2.405( , ) e lmj zLP lmE E r 12 2 2
0 1 2( )( : fiber core radius)V k a n na
22
202 2
1 2
nkb
n n
n1n2
a
It can be shown that with a little approximation, LP (linearly polarized) mode solutions are obtained
modelmLP
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
m represents the number of peaks along r, 2l represents the number of peaks along
LP01LP11
LP21
lmLP
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Loss in fiber
0.05
0.1
0.51.0
5
10
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Latticeabsorption
Rayleighscattering
Wavelength ()
OH- absorption peaks
1310 nm
1550 nm
Loss(dB/km)
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Dispersion in Fiber
- Multi-Mode Fiber: Modal Dispersion
n1
n2
21
3
nO
n1
21
3
n
n2
OO' O''
n2
(a) Multimode stepindex fiber. Ray pathsare different so thatrays arrive at differenttimes.
(b) Graded index fiber.Ray paths are differentbut so are the velocitiesalong the paths so thatall the rays arrive at thesame time.
23
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Each mode has its own group velocity
If a light pulse is decomposed into many modes, the pulse gets dispersed (broadened) after propagation
Prevention:
( = )gv
Use single mode fiber!
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
- Material (or chromatic) dispersion:Refractive index of any material depends on wavelength (frequency)
Single mode fiber also has small but non-zero dispersion
vg for SMF depends on wavelength(frequency)
- Waveguide dispersionWaveguide solution depends on wavelength
t
Intensity
t
Spread
Intensity
Dispersive SMF
Lect. 17: Optical Fiber
W.-Y. ChoiOptoelectronics and Photonics (18/2)
Homework: (Due on 11/27)