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• Optical fiber


  A thin, flexible, transparent fiber that acts as a waveguide, or "light pipe", to transmit light between the two ends of the fiber

  
• Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication

  
• Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference
• Fiber-optic applications


  • Long distance communication backbones
  • Inter-exchange junctions
  • Video transmission
  • Broadband services
  • Computer data communication (LAN, WAN etc.)
  • High EMI areas
  • Non-communication applications (sensors etc…)

  
• Advantages of optical fiber communication


  • Wider bandwidth
  • Low transmission loss
  • Dielectric waveguide
  • Signal security
  • Small size and weight
• Fiber optics basics: Principles of optical communication

  1. Information is encoded into Electrical Signals

  2. Electrical Signals are converted into light Signals

  3. Light Travels down the Fiber

  4. A Detector Changes the Light Signals into Electrical Signals

  5. Electrical Signals are decoded into Information

  
• Speed of light


  The velocity of electromagnetic energy in vacuum
• Refraction


  The deflection of light when it passes from one material to another with a different refractive index

  
• Snell's law


  The relationship between the angles of incidence and refraction, when light passes from one medium into another

  
• Critical angle


  The angle of incidence at which the refracted ray emerges parallel to the interface between the two dielectrics

  
• Total internal reflection


  The reflection of light back into the first material when the angle of incidence is greater than the critical angle

  
• Optical fiber structure


  • Core
  • Cladding
  • Coating

  
• Propagation of light through fiber

  1. Light injected into the fiber

  2. Light striking core to cladding interface at greater than the critical angle reflects back into core

  3. Light striking the interface at less than the critical angle passes into the cladding, where it is lost over distance

  
• Propagation of light through fiber is governed by the indices of the core and cladding by Snell's law

  
• Meridional rays and skew rays are the two types of light rays that can propagate through an optical fiber

  
• Light propagation through fiber

  1. Light striking the interface at less than the critical angle passes into the cladding, where it is lost over distance

  2. Cladding is usually inefficient as a light carrier, and light in the cladding becomes attenuated fairly

  3. Propagation of light through fiber is governed by the indices of the core and cladding by Snell's law

  
• Total internal reflection


  Forms the basis of light propagation through an optical fiber

  
• This analysis considers only meridional rays- those that pass through the fiber axis each time, they are reflected
• Skew rays travel down the fiber without passing through the axis, their path is typically helical wrapping around and around the central axis

  
• Skew rays are ignored in most fiber optics analysis
• Optical fibers used in communications

  • Hair-thin fiber consisting of two concentric layers of high-purity silica glass - the core and the cladding, enclosed by a protective sheath
  • Core and cladding have different refractive indices, with the core having a refractive index, n1, which is slightly higher than that of the cladding, n2
  • This difference in refractive indices enables the fiber to guide the light, so the fiber is also referred to as an "optical waveguide"

  
• As a minimum there is also a further layer known as the secondary cladding that does not participate in the propagation but gives the fiber a minimum level of protection, this second layer is referred to as a coating
• Light rays modulated into digital pulses with a laser or a light-emitting diode moves along the core without penetrating the cladding
• The light stays confined to the core because the cladding has a lower refractive index—a measure of its ability to bend light
• Refinements in optical fibers, along with the development of new lasers and diodes, may one day allow commercial fiber-optic networks to carry trillions of bits of data per second

  
• Typical Core and Cladding Diameters

  • 8/125 μm
  • 50/125 μm
  • 62.5/125 μm
  • 100/140 μm
• Fiber sizes are usually expressed by first giving the core size followed by the cladding size
• Attenuation


  Caused by intrinsic factors, primarily scattering and absorption, and by extrinsic factors, including stress from the manufacturing process, the environment, and physical bending
• Windows in Fiber Optic

  • 800nm - 900nm (850nm)
  • 1250nm - 1350nm (1300nm)
  • 1500nm - 1600nm (1550nm)
• Intrinsic attenuation


  Loss due to inherent or within the fiber, may occur as absorption or scattering
• Rayleigh scattering


  Caused by small variations in the density of glass as it cools, affects short wavelengths more than long wavelengths and limits the use of wavelengths below 800 nm

  
• Absorption


  Caused by the intrinsic properties of the material itself, the impurities in the glass, and any atomic defects in the glass, increases dramatically above 1700 nm
• Extrinsic attenuation


  Loss due to external sources, may occur as macro bending or micro bending
• Dispersion


  Spreading of light pulse as it travels down the length of an optical fibre, limits the bandwidth or information carrying capacity of a fibre

  
• Types of dispersion


  • Intermodal Delay or Modal Delay
  • Intramodal Dispersion or Chromatic Dispersion
  • Material Dispersion
  • Waveguide Dispersion
  • Polarization–Mode Dispersion
• Bandwidth is length dependent, longer fibre results in more pulse spreading and leads to lower bandwidth
• Electrical bandwidth (BWel)


  Defined as the frequency at which the ratio drops to 0.707

  
• Optical bandwidth (BWopt)


  Defined as the frequency at which the ratio PLo/PLi drops to 1/2

  
• BWel = 0.707 x BWopt
• Optical fiber


  • Flexible filament of very clear glass capable of carrying information in the form of light, created by forming pre-forms, which are glass rods drawn into fine threads of glass protected by a plastic coating