1

Lecture 5b Fiber-Optics in Digital Communication Systems

& Electronic Interfaces

1. Introduction2. Geometric Optics3. Classification of Optical Fibers and Their Characteristics4. Multimode Graded Index Fiber5. Single Mode Step Index Fiber6. Dispersion7. Bandwidth of a Single Mode Optical Fiber8. Fiber Optic in Point-to-Point Communication Link9. Lasers10. Distributed Feedback Laser Diode11. Avalanche Photodiode12. Conclusion13. Appendix

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Frequency Wavelength 659 THz Violet 455 nm 612 THz Blue 490 nm 545 THz Green 550 nm 517 THz Yellow 580 nm 484 THz Orange 620 nm 375 THz Red 750 nm 216 THz IR 1300 nm 181 THz IR 1500 nm

1022

1021

1020

1019

1018

1017

1016

1015

1014

1013

1012

1011

1010

109

108

107

106

105

104

103

102

10

1

Visible Light

Cosmic Rays

Gamma Rays

X-Rays

Ultraviolet Light

Infrared Light

Microwave, Radar, UHF, Cellular, PCS

VHF Television, FM

Short wave

Radio

AM Radio

Sound

Subsonic

Wavelength = c

Frequency

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Geometric Optics (Physical Background)

• Dual Nature 

A. Huygens's Principle and Ray Approximation 

• Huygens's principle states that all points on a wavefront can be taken as point sources for the production of secondary wavelets.

We can assume that the wave travels through a medium in a straight line. This is the ray approximation, and it assumes that light behaves like particles traveling in a straight line.

B. Diffraction

• When light passes through an aperture, the ray approximation is valid if the light wavelength is much shorter than the dimensions of the aperture.

4

(a) Straight line propagation

in a wide aperture, d >>

d

(b) Some diffraction at edges

when d =

d

(c) Full diffraction when d <<

5

C. ReflectionWhen light encounters a surface, some light will be absorbed by the surface, some light will be transmitted through the surface, and some light will be reflected by the surface.

(a) Specular reflection (b) Diffuse reflection

θ1 θ1́

θ1 = θ1́

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Light source

n1

Slower transmission

Faster transmission n2 < n1 a

L1

L2

L1́

L2́

Lightdestination

D.    Format’s Principle Format’s principle states that when a light ray travels between two points via surface, its path will be the one that requires the least time. This effect is called refraction.

c

nL

c

nL

c

nL

c

nL 2'

21'

12

21

1

7

Single-mode Step Index Fiber  • long-haul telecommunications; 100 Gbps for 1 km; repeater spacing

of up to 300 km, but these capabilities continue to be improved.

• Axial transmission. For a given core diameter, there is a minimum wavelength c . A single mode fiber will transmit only the single

mode for all wavelengths greater than the cut-off wavelength c .

405.2

22 1

nac

nm1290m29.1405.2

023.02545.1105.12

023.0545.1

510.1545.1;m5.1

2

3

6

1

21

c

n

nnma

Cutoff wavelength c for a fiber with a 3-micrometer core diameter, a core

index of refraction of 1.545, and a cladding index of refraction of 1.510.

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• Single mode fiber has a core diameter 4-9 micrometers, about four times the wavelength of light, allows only one mode (single mode) to exist in the core. No bouncing, destructive or constructive interference occurs.

• Typical bit rate is 100 Gb/s/km. This is 5000 times the bit rate of multi-mode fiber. Theoretical BW limit is 100,000 GHz.

• Single mode is the highest bandwidth optical fiber and is used for long distance communications.

• The single mode fiber bandwidth limitations is due to different light wavelength traveling at slightly different speeds. This phenomena is called chromatic dispersion.

• Using single mode 100 km optical fiber between repeaters bit rate:

Bandwidth of a Single Mode Optical Fiber

sGbkm

kmsGb/0.1

100

//100

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Lasers Light Amplification by Stimulated Emission of Radiation

The laser uses several heavily doped layers of p‑type and n-type materials. When a large forward bias is applied, a large number of free holes and electrons are created in the immediate vicinity of the junction. When a hole and electron pair collide and recombine, they produce a photon of light.

Metallization

n - AlGaAs, Confinement

p - AlGaAs, Active Layer

p - AlGaAs, Confinement

p - GaAs, Contact layer

SiO2 - Insulation Metallization

GaAs Substrate

0.1 - 0.3 m

1 m

1 m

1 m

-

+

10

Current (mA)

Optical Power (mW)

1

2

3

4

5

50 100 150 Ith

Threshold Current

Metallized layer

p-type

Grating

n-type

Active Layer

Cleaved Facet

Output

Metallized layer

typeprefractnndiffractioBragg

ofordermperiodgratingm

n

p

p

.,

,2

0

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Avalanche photodiode - +

+ + + + + +

- - - - - -

e-

e- e-

e- e- e-

e-

e- e-

e- e-

e- e-

e- e-

initial photon collision

electron-electron collisions depletion region (avalanche region)

photodiode ofcurrent p-i-nMch

PeMi

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Point-to-Point Communication Link

Optical Fiber (single-mode or multimode)

Output data

Driver (electronics)

Transmitter

Input data

Optical Source

External Modulator

(a) (b)

Connector

Connector

Photodetector Regenerator

Receiver

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Basics of semiconductor theory

21 EE

hc

(a) Absorption

incoming photon

(b) Emission

E1

E2

emitted photon

E1

E2

Ephoton + E1 = E2 Ephoton = E2 - E1

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Covalent Bond

4 4 4 4

4 4 4 4

4 4 4 4

Silicon Atom

Number of Valence Electrons

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Excess Electron

4 4 4 4

4 4 5 4

4 4 4 4

Silicon Atom

Arsenic Atom

Excess + Charge

4 4 4 4

4 3 4 4

4 4 4 4

Silicon Atom

Gallium Atom

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5 3 5 3

3 5 3 5

5 3 5 3

Arsenic Atom (5)

Gallium Atom (3)

5 3 5 3

3 5 3 3

5 3 3 3

hole

p-type material

(positively charged area)

5 3 5 3

5 5 3 5

5 3 5 3

n-type material

free electron

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Laser Beam External Modulation

c

Ln

v

Lt 1

11

c

Ln

v

Lt 2

22

)(

)(1

21

2121

21

nnL

nnc

L

c

Ln

c

Ln

cfTtt

2k /2, Max

(2k+1) /2, Min

)(2 Vn

=1500 nm; L=2cm

007.021 nn

L

035.021 nn

=1500 nm; L=2cm K=0,1,2….

2

1

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Transphasor.

Crystal block

Partially reflective surfaces

(a) Transphasor is off

Laser beam Output low (near zero)

Reverberating Waves

(b) Transphasor is on

Laser beam Output light

Amplified Waves Weaker beam

indium antimonide

1000 times faster

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By DWDM splitting of a spectrum into hundreds of channels, Decreasing of duration of pulses,

Speed of transfer of the information on a separate line (one single wavelength) now managed to be up to 10 Gb per second,

and 40-3200 Gb on main channel.

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Orbital angular moment of photons

A B C D E F G H I

00

228

180Re solutionAngular

A A

1011001001