Ppt

• Optical fiber


  A glass or plastic fiber that carries light along its length
• Optical fibers


  • Signals travel along them with less loss
  • They are immune to electromagnetic interference
• 3 Regions (3C's) of optical fiber

  • Core - glass or plastic, diameter: 9 um
  • Cladding - glass or plastic, diameter: 125 um
  • Coating - for protection, diameter: 250 um
• Advantages of optical fiber

  • Wider bandwidth and greater information capacity
  • Immunity to crosstalk
  • Immunity to static interference
  • Environmental immunity
  • Safety and convenience
  • Lower transmission loss
  • Security
  • Durability and reliability
  • Economics
• Disadvantages of optical fiber

  • Interfacing costs
  • Strength
  • Remote electrical power
  • Optical fiber cables are more susceptible to losses introduced by bending the cable
  • Specialized tools, equipment, and training
• 3 Variations of optical fiber

  • Plastic core and cladding (PCP)
  • Glass core and cladding (SCS)
  • Glass core and plastic cladding (PCS)
• Advantages of plastic over glass fiber

  • Flexibility and ruggedness
  • Easy to install
  • Less weight
  • More economical
  • Can withstand stress
• Disadvantages of plastic over glass fiber

  • High attenuation
  • Inefficient
  • Limited for short distance applications
• Classifications of optical fibers depend on

  • Mode (path) of propagation
  • Index profile
• Single-mode (monomode) fiber


  Only one path for light to take down the cable, extremely wide bandwidths and low losses, 8um core diameter, placed 3km between repeater
• Multimode fiber


  The light wave rays take many paths between the source and the far end of the fiber, placed 2km between repeaters, diameter: 50 - 200 um, 62.5um most commonly used
• Step index


  The core has a uniform index of refraction providing an abrupt change in refraction index at the core-cladding interface, relatively high dispersion making it useful only at lower rates and shorter distance
• Graded index


  The core has index of refraction that changes continuously from the center to the outside, made of many thin layers with lower index of refraction than the adjacent inner core, light waves are propagated by refraction so they are bent in a sinusoid like curve about the fiber
• Single-mode step-index optical fiber


  • Advantages: Minimum dispersion, can reproduce a pulse of light accurately at the receiver, wider bandwidth and higher information transmission rates

  Disadvantages: Difficult to couple light into and out of, requires a highly directive light source like a laser, expensive and difficult to manufacture
• Multimode step-index fiber


  • Advantages: Relatively inexpensive and simple to manufacture, easier to couple light into and out of

  Disadvantages: Light rays take many different paths down the fiber resulting in large differences in propagation times, lower bandwidths and information rates
• Multimode graded-index fiber

  • Easier to couple light into and out of than single-mode step-index but more difficult than multimode step-index, distortion due to multiple propagation paths is greater than single-mode step-index but less than multimode step-index, considered an intermediate fiber
• Behavior of light

  • Diffraction - occurs when light rays strike a sharp edged obstruction or small opening
  • Interference - occurs when two or more light waves overlap
  • Reflection - return of light waves as it hits an interface
  • Scattering - redirection of light due to inhomogeneities in the material
  • Refraction - bending of light rays as they travel from one medium to another
• Refractive index


  The ratio of light velocity in free space to the velocity of light in a given material
• Refractive index of various mediums

  • Vacuum: 1.0
  • Air: 1.00029
  • Water: 1.33
  • Ethyl Alcohol: 1.36
  • Fused Quartz: 1.46
  • Glass Fiber: 1.5 - 1.9
  • Diamond: 2.0 - 2.42
  • Silicon: 3.4
• Snell's law


  Explains how light reacts when it meets the interface of two transmittive materials with different indices of refraction
• Light travels from a less dense material into a more dense material


  The wave is refracted towards the normal
• Light travels from a more dense material into a less dense material


  The ray bends away from the normal
• Critical angle of incidence


  The minimum angle of incidence at which a light ray may strike the interference of two media and result in an angle of refraction of 90 degrees or when the incident ray is in parallel to the boundary
• Acceptance angle or acceptance cone half angle


  Maximum angle in which external light rays may strike the air/fiber interface and still propagate down the fiber
• Numerical aperture (NA)


  A figure of merit used to measure the light gathering or light collecting ability of the optical fiber
• Requirements for light sources

  • Their light must be as nearly monochromatic (single frequency) as possible
  • Capable of being easily modulated; PCM for better noise immunity
  • High intensity light output so that sufficient energy is transmitted to overcome the losses encountered during transmission down the fiber
  • Devices should be small, compact, and easily couple to the fibers so that excessive coupling losses do not occur
  • Must be inexpensive to manufacture
• Light emitting diode (LED)


  Non-coherent injection light sources which are low-cost, low-heat light sources and are the most promising light sources for optical transmission, can couple about 100uw of power with a coupling efficiency of 2%
• Injection laser diodes (ILD)


  Coherent light sources that can couple a few milliwatts of light power into a fiber, have reduced coupling losses, greater radiant output power, can be used at higher bit rates, and reduced wavelength dispersion
• Advantages of ILDs over LEDs

  • Reduced coupling losses
  • Greater radiant output power
  • Can be used at higher bit rates
  • Reduced wavelength dispersion; monochromatic
• Disadvantages of ILDs

  • Expensive
  • Shorter lifetime
  • Temperature dependence
  • Requires automatic level control circuit to protect the device from power supply transients
• LASER


  Active material to convert energy into laser light, pumping source to provide power or energy, optics to direct the beam through the active material to be amplified, optics to direct the narrow powerful cone of divergence, feedback mechanism to provide continuous operation, output coupler to transmit power out of the laser
• PIN (Positive Intrinsic Negative) diodes


  When photons are absorbed by intrinsic layer's electrons in the valence band, they add sufficient energy to generate carriers in the depletion region and allow current to flow through the device
• APD (Avalanche Photo Diode)


  Light enters diode and is absorbed by the thin, heavily doped n-layer causing a high electric field intensity to be across in p-n junction thus ionization occur and continues like avalanche
• Advantages of APD over PIN

  • APDs give better sensitivity over PIN
  • APDs provide larger amplification
• Disadvantages of APD

  • High bias requirement
  • Temperature dependence
  • Long transit time
• Characteristics of light detectors

  • Responsivity - measure of conversion efficiency of a photodetector; A/W unit
  • Dark Current - leakage current that flows through a photodiode with no light input
  • Transit time - time it takes a light-induced carrier to travel across the depletion region maximum bit rate possible
  • Spectral response - the range of wavelength values that a given photodiode will respond
  • Light sensitivity - minimum optical power a light detector can receive and still gives an electrical output
• Losses in optical fiber cables

  • Scattering losses - due to imperfections in the fiber that are formed during manufacturing process
  • Absorption losses - impurities in the fiber absorb the light and convert it to heat
  • Ultraviolet absorption, Infrared absorption, Ion resonance absorption, Hydrogen effect
  • Dispersion - spreading of pulse out in the time domain, changing its shape so that it may merge into the previous and succeeding pulses
  • Chromatic/wavelength/material dispersion, Waveguide dispersion, Modal dispersion
  • Coupling losses - Lateral misalignment, Gap misalignment or longitudinal displacement, Angular misalignment, Imperfect surface finish, NA mismatch, Unintercepted illumination loss
  • Bending or radiation losses - Microbending, Macrobends