Lect 14 Dispersion in Optical Fiber

8/12/2019 Lect 14 Dispersion in Optical Fiber

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High-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics

Lect 14: Dispersion in Optical Fiber

vg (m/s)

(1/s)

TE1

TE2

2.0810 8

2.0710 8

2.0610 8

2.0510 8

0 110 15 210 15 31015

cut-off= 2.31014

c/n2

c/n1

d=10mn1=1.455

n2=1.440

n2

Modal or intermodal)

dispersion:

Waveguide can support

several modes having

different values of v

g

Solution: Use single

mode waveguide

Group velocity depends onFrequency: intramodal dispersion


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Dispersion in single-mode waveguide:

Group velocity depends on frequency.

Limitation on data rate and transmission distance.

t

Intensity

Spectrum

Intensity

f

t

Spread

Intensity

Dispersive SMF


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n

500 700 900 1100 1300 15001700 1900

1.44

1.45

1.46

1.47

1.48

1.49

Wavelength (nm)

Refractive Index for Silica

- Material or chromatic) dispersion

Refractive index of any materialdepends on frequency(wavelength)

vg depends on frequency

(wavelength)

Why does single mode waveguide

have dispersion?


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Waveguide dispersion

Even if refractive indexdoes not change,vg depends onfrequency (wavelength)

dn1

n2

n2Why does single mode waveguide

have dispersion?

2y

dX k=

2

dY =

2

2

2 2 2 2 2

0 1 2( )2

dX Y k n n

+ =

tanY X X=


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Waveguide dispersion

Even if refractive index

does not change,vg depends onfrequency (wavelength)

d=10mn1=1.455

n2=1.440

n2

vg (m/s)

(1/s)

TE1

2.0810 8

2.0710 8

2.0610 8

2.0510 8

0 110 15 210 15 31015

14

c/n2

c/n1

Why does single mode waveguide

have dispersion?


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Dispersion coefficient, D, for silica fiber with a=4.2mDm: material (chromatic) dispersion only

Dw: waveguide dispersion only

Dm+Dw: total dispersion

0

1.2 1.3 1.4 1.5 1.61.1

-30

20

30

10

-20

-10

(m)

Dm

Dm+ Dw

Dw0


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20

-10

-20

-30

10

1.1 1.2 1.3 1.4 1.5 1.6 1.7

0

30

(m)

Dm

Dw

Dch

= Dm

+ Dw

1 2

n

r

Thin layer of cladding

with a depressed index

D

It is possible to control D by changing waveguide structure

Dispersion Flattened Fiber)


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How do we model dispersion

0 0

0

22

0 0 2

2

0 1 0 2 0

20

0

1( ) ( ) 2

1 ( ) ( ) ( )

2

1 1 1 = ( )( ) 2

==> Dispersion exists if is not not linear with

In Silica

g gv v

+ = + + +

+ +

+ +

i i

i i

i i

2

2

2

fiber, 20 ps /km at =1.5 m

(With 0, increases as increases)gv

0)


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Exercises:

Prob. 8, 9, 10, 14

Documents

Lect 14 Dispersion in Optical Fiber