Fiber-Optic Communications

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Fiber-Optic Communications

• James N. Downing

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Chapter 2

• Principles of Optics

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Chapter 2

• 2.1 Geometrical Optics
• A model by which the nature of light is used to
  explain refraction, reflection, and propagation
  of light
• Refraction
• The bending of light as it passes through a
  medium
• Index of refraction The ratio of the speed of
  light in a vacuum to the speed of light in the
  medium
• Phase velocity The speed of light in a medium
• Optical path length apparent length of an
  optical element

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Chapter 2

• 2.1 Geometrical Optics
• Snells Law
• Mathematical determination of the index of
  refraction at the interface of two media
• Critical angle is the angle at which the
  refracted ray is at 900 to the normal

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Chapter 2

• 2.1 Geometrical Optics
• Reflection
• Bouncing off of rays from a material interface
• Depends on the smoothness of the surface and the
  refractive indices of the media
• Fresnel reflection law
• Determines the fraction of light reflected as a
  function of the incident ray as well as the
  amount of light refracted or transmitted into the
  medium

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Chapter 2

• 2.2 Wave Optics
• Electromagnetic Waves
• Result of the dual properties of electricity and
  magnetism and their relationship
• Derived from Maxwells equations
• Electric waves and magnetic equations are
  perpendicular to each other
• Function of both space and time
• Electromagnetic spectrum consists of all forms of
  electromagnetic energy

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Chapter 2

• 2.2 Wave Optics
• Polarization
• Describes the direction of the electric field
  oscillations
• Induced by preferential reflection, transmission,
  scattering, or passing light through a
  birefringent material
• May be either perpendicular, horizontal, z-axis,
  circular, or elliptical

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Chapter 2

• 2.2 Wave Optics
• Coherence
• Phase difference is the shift between two waves
  along their axis of propagation
• Coherent lightno phase shift
• Incoherent lightphase is continually shifting
• Temporal coherence waves are equal
• Spatial coherencewaves are in phase at a point
  in space

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Chapter 2

• 2.2 Wave Optics
• Interference
• Due to the linear superposition of
  electromagnetic waves such that the amplitude at
  any point is equal to the sum of the individual
  amplitudes at that point
• Constructive interference
• Phase shift is zero
• Destructive interference
• Phase shift is 1800

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Chapter 2

• 2.2 Wave Optics
• Diffraction
• Diffraction describes how light can spread out
  after going through a small aperture.
• Diffraction grating is the separation of the
  diffracted light into different bands of
  different colors.

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Chapter 2

• 2.2 Wave Optics
• Scattering
• Scattering is the spreading apart of light caused
  by interaction with matter.
• Rayleigh scattering, or molecular scattering, is
  caused by small particles of matter (less than or
  equal to 1/10 wavelength) interacting with light.
• Mie scattering is due to interaction with matter
  larger than 1/10 wavelength of light.

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Chapter 2

• 2.3 Quantum Optics
• Bohr Model
• Consists of nucleus and orbitals
• Nucleus contains the protons and neutrons
• The orbital contains the electrons

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Chapter 2

• 2.3 Quantum Optics
• Absorption
• Ground state is the minimum level of energy
  needed to keep an electron associated with its
  orbit.
• Excited state is that in which the electron has
  absorbed some energy.
• Absorption is the process in which light energy
  is converted into electrical energy.
• Beers Law describes the absorption transfer
  function.

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Chapter 2

• 2.3 Quantum Optics
• Emission
• Emission is the process by which electrical
  energy is converted to light.
• Spontaneous emission occurs naturally.
• Stimulated emission occurs when an external
  photon causes a photon to lose energy.
• Linewidth is the length of a wavelength of light
  (defined at the 50 power level).

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Chapter 2

• 2.3 Quantum Optics
• Plancks Law
• This law describes the energy released when an
  electron moves from one energy level to another.

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Chapter 2

• 2.4 Nonlinear Optics
• Four-Wave Mixing
• Four-wave mixing results in a fourth frequency
  when three frequency signals are combined.
• Can be used to generate a fourth frequency, if
  needed.
• Problems arise when the fourth frequency is
  already in use.

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Chapter 2

• 2.4 Nonlinear Optics
• Phase Modulation
• The result of a change in the refractive index
  with a change in light intensity
• Self-phase results in a broadening of the
  linewidth of a particular signal
• Cross-phase occurs when self-phase modulation
  causes phase changes in another signal. which
  results in a linewidth broadening at another
  wavelength.

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Chapter 2

• 2.4 Nonlinear Optics
• Brillouin Scattering
• Occurs at optical powers high enough to generate
  small acoustic waves in the material
• Alters the refractive index, and shifts the
  frequency
• Scattering increases as power increases

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Chapter 2

• 2.4 Nonlinear Optics
• Raman Scattering
• Light is absorbed and some energy is lost or
  gained from molecular vibrations.
• Can be used to transfer energy from one
  wavelength to another resulting in signal
  amplification.
• Cross-talk may be enhanced if more than one
  wavelength is used.

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Chapter 2

• 2.5 Optical Power
• Radiometric and Photometric Quantities
• Photometric quantities describe the visual
  brightness of a light and exist only between
  400nm and 700nm with a peak at 550nm.
• Radiometric quantities are consistent throughout
  the spectrum and are proportional to the square
  of the energy.

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Chapter 2

• 2.5 Optical Power
• Power
• The ratio of energy per unit time (measured in
  watts or dBm)
• Transfer function TdB Pout-dBm Pin-dBm