Chapter 4 FIBER CHARACTERISTICS

Lecturer: Tran Thi Ngoc Dung

Content

Energy Loss in Fiber Fiber Dispersion Data Rate

Fiber Losses

Three fundamental causes1. Material loss. Absorption the energy

by material2. Light Scattering. Light scattered by

the molecules of the material, by structural imperfections, and impurities.

3. Waveguide and bend losses. Losses by imperfections and deformations of the fiber structure.

Loss

Pin Pout

in

outdB

in

out

PPLoss

PPLoss

log10)( =

=

Material Losses

Scattering loss

Waveguide and Bend loss

Change in fiber diameter

Ray leaves fiber

DISPERSION

2/121

22 )( pp ttt =

Cause of Pulse broadening Output Pulse is wider than

Input Pulse

t

tp1

tp2

Input pulse

Output pulse

t

Dispersion

Longer fiber causes more pulse brodeningDispersion for a given length:

)/( kmdispersionLt =

Intermodal Dispersion

1

3 212

3

n1

For Mode zero, travel time:

For critical mode:

Dispersion

===

===

==

cLn

cnnnLn

cLn

cnLnttt

cnLn

cLn

nc

L

t

cLn

ncLt

oc

c

cc

o

1

2

2111

2

21

2

211

1

1

1

)(

sin/sin

/

Intermodal Dispersion (cont.)

2

1

2

1

1

1

)(2

1

)(2

2

NAcnL

t

NAcn

LcLnt

nNA

=

==

=

Example 4-4

A train of light pulses is transmitted through a 400-m fiber with ncore=1.4 and nclad=1.36. Sketch the output pulses for

(1) a pulse rate of 10 x 10^6 pulses per second (10Mb/s)

(2) a pulse rate of 20 x 10^6 pulses per second (20Mb/s)

(3) Find the dispersion per kilometer. Assume that the input pulse is of near zero width.

Ex 4-4 (cont.)

ns

cnnnLnttt oc

9.5410336.1

)36.14.1(4004.1

)(

8

2

211

=

=

==

Dispersion

a) Pulse rate 10.10^6 pulses/sPeriode of pulse: T=100ns

t(ns)50 100 150 200

Relative Amplitude

T=100nsPulses are clearly distingushable

Ex 4-4 (cont.)

50 100 150 200

Relative Amplitude

T=100ns

b) Pulse rate 20.10^6 pulses/s/ Periode of pulse: T=50ns

t(ns)

kmnskmns

kmLt /2.137

4.09.54

)(==

c) Dispersion per kilometer

Pulses are merged and nearly indistingushable

Intramodal Dispersion (Chromatic dispersion) Related to the line width of the light

source, and caused by variations in the refractive index as a function of wavelength n()

Intramodal is often given in terms of picoseconds per kilometer per nanometer of line width.

At 1300nm, intramodal dispersion is minimum

Total Dispersion

2/12int

2int ][ raertot ttt +=

EXAMPLE 4-5/p64

Ex 4-5/p64

A fiber has the following specifications:

DispersionIntermodal 5ns/kmIntramodal100ps/(km x

nm)Linewidth 40nmFiber length 5km1. Find the total intermodal

dispersion2. Find the total intramodal

dispersion3. Find the total dispersion.

.255/5int nskmkmnst er ==

.20405100int nsnmkmnmkmpst ra =

=

nst

ttt raer322025

)(22

2/12int

2int

=+=

+=

CHOICE OF WAVELENGTH

For better performance, the choice of wavelength must be based on minimizing loss and minimizing dispersion.

3 low loss Windows (820-880),(1200-1320), (1550-1610) (nm)

Low intramodal dispersion: 1300nm. Dispersion-shifted fibers: minimum

dispersion is shifted to about 1550nm.

Questions

8. What is the difference between intermodal and intramodal dispersion

10. What are low loss windows? Give the wavelengths for the three low loss windows.

11. Why is it desirable to operate at a wavelength of about 1.3 m?

12. Why is it advantageous to operate at 1.55 m?

13. What is dispersion-shifted fibers?

Problems solving

Do all problems on pp 68,69

PROBLEMS

1. Find the loss, in decibels, for a fiber with input power of 200 W and output power of 10W.

2. Finb the output power for a 5-km fiber with a 4dB/km loss and with Pin=500 W.

3. A fier, 8km in length, has an input power of 300W and an output power of 10 W. What is the loss per kilometer?

4. Two fibers, one 3km long with 4dB/km loss and the other 9km long with 1.2 dB/km loss, are spliced to form a 12km fiber. Ignore losses due to splicing

a) What is the average dB/km loss?b) For an input of 260W, what is the

power out?

5. Given a fiber 5km long, with ncore=1.44 and nclad=1.4, find the intermodal dispersion.

6. Find the intermodal dispersion per kilometer for a fiber with =2% and n1=1.5

7. A fiber has a specified N.A. of 0.22 and ncore=1.4. Find the intermodal dispersion for a 500m length.

8. A system introduces pulse broadening of 50ns. The input pulse is 20ns wide. Find the output pulse width.

9.

Chapter 4 FIBER CHARACTERISTICS ContentFiber LossesLoss Material LossesScattering lossWaveguide and Bend lossDISPERSIONIntermodal DispersionIntermodal Dispersion (cont.)Example 4-4Ex 4-4 (cont.)Ex 4-4 (cont.)Intramodal Dispersion (Chromatic dispersion) Total DispersionEx 4-5/p64CHOICE OF WAVELENGTHQuestionsProblems solvingPROBLEMSSlide Number 21Slide Number 22Slide Number 23