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)