Transcript Option F - Fiber Op..

IB Physics
Option F – 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:
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
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
• Snell’s Law
sin i ni  sin r nr
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
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
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