Lecture 5b

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Lecture 5b Fiber-Optics in Digital Communication
Systems Electronic Interfaces

• 1. Introduction
• Geometric Optics
• Classification of Optical Fibers and Their
  Characteristics
• Multimode Graded Index Fiber
• Single Mode Step Index Fiber
• Dispersion
• Bandwidth of a Single Mode Optical Fiber
• Fiber Optic in Point-to-Point Communication Link
• Lasers
• Distributed Feedback Laser Diode
• Avalanche Photodiode
• Conclusion
• Appendix

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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.
• 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.
  

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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.
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D.    Formats Principle Formats 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.
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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 .

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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Bandwidth of a Single Mode Optical Fiber

• 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

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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.
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Avalanche photodiode
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Point-to-Point Communication Link
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Basics of semiconductor theory
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Laser Beam External Modulation
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L
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2k ?/2, Max
?1500 nm L2cm
K0,1,2.
?1500 nm L2cm
(2k1) ?/2, Min
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Transphasor.
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
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