Fiber Optic Transmission

1
Fiber Optic Transmission

• SL/HL Option F.3

2
Reflection/Refraction

• Reflection
• A wave encounters a boundary between two mediums
  and cannot pass through
• The angle of incidence is always equal to the
  angle of reflection
• Refraction
• When a wave passes through boundary into a new
  medium its speed changes
• The wave will change directions based on the
  change in its speed

3
Refraction

• If the wave speeds up it will bend away from the
  normal line
• If the wave slows down it will bend towards the
  normal line
• Snells Law

4
Critical Angle

• When traveling into air from some medium, light
  will always speed up, thus increasing the angle
• If it speeds up enough, the angle of refraction
  will be 90 degrees
• This means that the refracted ray will travel
  along the edge of the boundary

5
Critical Angle

• When light strikes the boundary at the critical
  angle or greater, the wave is totally reflected
  back into the first medium
• Here n1 is the index of refraction of the medium
  the light starts in

6
Total Internal Reflection

• Usually when a wave reaches a boundary between
  mediums it is partially reflected and partially
  refracted
• When the critical angle is exceeded the entire
  wave is reflected back within the medium
• The wave doesnt lose any energy

7
Optical Fiber

• Fiber optic cable is made of thin,
  clear glass or plastic
• Once light enters the cable it is
  totally internally reflected until it reaches
  the far end
• Actual optical fiber is step indexed
• There is another layer between
  the core and the outside
• This is so the fibers can be
  bundled together

8
Dispersion

• Modal
• Not all the waves that enter make it to the other
  end, only certain ones
• The possible paths are called modes
• Material
• Because different frequencies have different
  refractive indices, they have different paths
• These can both cause problems if
  
  the bits of data arrive out of order
• More direct modes are faster
• Laser light and single mode cable reduce
  
  these effects

9
Material Dispersion
10
Attenuation

• Attenuation is the opposite of amplification
• As a signal travels through a cable it will
  slowly lose intensity as energy is lost
• Attenuation is measured in decibels (dB)
• The 10 at the beginning is to convert to decibels
• Generally measured in dBkm-1

11
Wavelength/Attenuation
12
Noise

• One advantage to using fiber optics is that it is
  not particularly susceptible to noise
• Any noise that does occur is generally due to
  random light entering the end of the cable
• The power ratio of noise to signal in fiber
  optics is generally in the range of 10-17 or 10-18

13
Reshapers

• Monomode fibers can eliminate modal dispersion
  and lasers cut down on material dispersion, but
  it is not completely eliminated
• Over a long distance individual pulses can start
  to overlap each other
• Every 40-60km is a reshaper which will detect and
  reshape the signal
• Has its own laser which sends a new signal

14
Amplifiers

• Even with reshaping, signals still attenuate over
  the length of the cable
• Amplifiers along the cable increase the signal
  strength to keep it going