IB Physics 11
Option C – Fibre Optics
Mr. Jean
The plan:
• Video clip of the day
• Fibre Optics
– https://ibphysics2016.wikispaces.com/Option+
C+-+Imaging
• Fibre Optic Re-cap
• Fibre Optic Calculations
Learning Objectives:
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
Bending of Light:
• https://phet.colorado.edu/en/simulation/be
nding-light
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
Material Dispersion
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)
Pi
Power in
attenuation  10 log10 (
)  10 log10 ( )
Power out
Po
• The 10 at the beginning is to convert to decibels
• Generally measured in dBkm-1
Wavelength & Attenuation
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
Re-shapers
• Mono-mode 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
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
– https://phet.colorado.edu/en/simulation/lasers
Textbook Questions: P. 638-639
Textbook Questions:
Textbook Solutions:
• https://global.oup.com/education/secondar
y/curricula/ibdiploma/science/phsyicsanswers/?region=i
nternational