lect. 17: optical fibertera.yonsei.ac.kr/class/2017_2_1/lecture/lect 17 optical... · 2017. 12....
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Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
3-dimentioanl dielectric waveguide?
planar waveguide
circular w
circular waveguide
Optical fiber- Analysis very complicated
- Contains the same qualitative features
TE, TM modes, cut-off frequency, ...
- Design Exercise 2
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
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)
Optical Fiber: Circular dielectric waveguide made of silica (SiO2)
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
Optoelectronics (17/2) W.-Y. Choi
Optical Communication Networks
Total undersea fiber length: 0.4 billion km ( 628 round trips between earth and moon)
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
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
Optoelectronics (17/2) W.-Y. Choi
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
Optoelectronics (17/2) W.-Y. Choi
Preform feed
Furnace 2000°C
Thicknessmonitoring gauge
Take-up drum
Polymer coater
Ultraviolet light or furnacefor curing
Capstan
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
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 z
LP lmE E r 12 2 20 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
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
E
r
E01
Core
Cladding
The electric field distribution of the fundamental modin the transverse plane to the fiber axis z. The lightintensity is greatest at the center of the fiber. Intensitypatterns in LP01, LP11 and LP21 modes.
(a) The electric fieldof the fundamentalmode
(b) The intensity inthe fundamentalmode LP01
(c) The intensityin LP11
(d) The intensityin LP21
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
For LPlm mode,m represents the number of peaks along r, 2l represents the number of peaks along
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
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
Optoelectronics (17/2) W.-Y. Choi
z
Si and O ions
Light directionk
Ex
Lattice Absorption: EM waves cause vibration of ions inside fiber. Peak absorption occurs at around = 9 m in Silica fiber.
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
Scattered waves
Incident wave Through wave
A dielectric particle smaller than wavelength
Rayleigh scatteringA small portion of EM waves get directed away from small dielectric particles due local fluctuation of fiber refractive index.More scattering with smaller wavelength (inversely proportional to 3).
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
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: Use single mode fiber!
(Group index: ng = c/vg)
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
- Material (or chromatic) dispersion:Refractive index of any material depends on wavelength (frequency)
But 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
Optoelectronics (17/2) W.-Y. Choi
Mathematically,
0 0
22
0 0 2
1( ) ( )2
Dispersion exists because is not not linear with
20 1 0 2 0
1( ) ( ) ( ) 2
22In Silica fiber, ~ 20 ps /km at =1.5 m
0
20
0
1 1 1 = ( )( ) 2g gv v
2With 0, increases as increasesgv
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
Often, dispersion parameter D is used.
1D
t
Intensity
t
Spread (
Intensity
Dispersive medium (L,D)Spectrum (
Intensity
~ D L
1
2
2 c
2
22
2D c
2( )kc c
psD~16 at 1.5km nm m
Approximately,
Lect. 17: Optical Fiber
Optoelectronics (17/2) W.-Y. Choi
Homework