Optical Fiber

by Rahul Mishra

• Fibre Optic?
  Dielectric waveguide of cylindrical geometry with core and cladding of suitable material. 
  refractive index of core > refractive index of cladding.
• Main Motivation:-
  To meet demand of increase in the telecommunication data transmission
• Physical Principle:- Total internal reflection (critical angle, using Snell’s law.
• Main Advantages:-
• Higher bandwidth (extremely high data transfer rate).
• Less signal degradation.
• Less costly per meter.
• Lighter and thinner then copper wire.
• Lower transmitter launching power.
• Less susceptible to electromagnetic interference.
• Flexible use in mechanical and medical imaging systems.
• Main Applications:- 
• Telecommunications.
• Sensors.
• Fiber Lasers.
• Bio-medical.
• Automotive and many other industries.
• Silica and Plastic as Fibre Optic Materials :- 
• Both core and cladding are of glass.
• – Very pure SiO2 or fused quartz.
• – Germanium or Phosphorus to increase the index of refraction.
• – Boron or Fluorine to decrease the index of refraction.
• – Silica fibers mainly used due to their low intrinsic absorption at wavelengths of operation.
• – Any other remaining impurities cause attenuation and scattering.
•  Fibre Optic?
  Dielectric waveguide of cylindrical geometry with core and cladding of suitable material.
  refractive index of core > refractive index of cladding.
• Main Motivation:-
  To meet demand of increase in the telecommunication data transmission
• Physical Principle:- 
• Total internal reflection (critical angle, using Snell’s law.
• Main Advantages:-
•  Higher bandwidth (extremely high data transfer rate).
• Less signal degradation.
• Less costly per meter.
• Lighter and thinner then copper wire.
• Lower transmitter launching power.
• Less susceptible to electromagnetic interference.
• Flexible use in mechanical and medical imaging systems.
• Main Applications:-
• Telecommunications.
• Sensors.
• Fiber Lasers.
• Bio-medical.
• Automotive and many other industries.
• Silica and Plastic as fibre optic Materials :–
• Silica Fibers:- 
• – Both core and cladding are of glass.
• – Very pure SiO2 or fused quartz.
• – Germanium or Phosphorus to increase the index of refraction.
• – Boron or Fluorine to decrease the index of refraction.
• – Silica fibers mainly used due to their low intrinsic absorption at wavelengths of operation.
• – Any other remaining impurities cause attenuation and scattering.
• Plastic Fibers:- 
• -Plastic core and plastic cladding.
• – Polymethyl Methacrylate (most commonly used).
• – Flexible and Light.
• -Widely used in short distance applications.
• Plastic-clad Fibers:-  
• -Glass as core and plastic as cladding.
• Which is better? (Plastic or Silica):-
•  -Plastic less expensive, flexible, lighter.
• – Plastic is larger in diameter, so easy to connect across joints.
• – Plastic is less efficient then Silica. – Plastic has more attenuation, and less bandwidth making it more suitable for shorter distances.
• Main Characteristics of Optical Transmission Medium:-  
• -The ray entering the acceptance angle will be guided along the core.
• -Acceptance angle is measure of the light.
• -gathering power of the fiber.
• -Higher Numerical Aperature (NA) mean higher coupling from source to fiber, and less losses across joints. 
• NA = (n12 -n22 )½
• Attenuation:- 
• -Limit the optical power reaching the receiver. Power received can be related with the transmitted as: dB = -10 log10 (power out / power input).
• -Lower attenuation mean greater spacing and less cost of the communication system.
• Main Causes of Attenuation:- Scattering:- 
• -Due to interactions of photons with fiber medium.
• Absorption (Intrinsic+Extrinsic):- 
• -By fiber itself (intrinsic) or due to impurities of water and metal, such as iron, nickle and chromium (extrinsic).
•  Bending and Geometrical Imperfections:- 
• -Due to physical stress on fiber.
• -Core-cladding interface irregularities, diameter variations etc.