Download 4.6 Optical Fibres

4.6 Optical Fibres
Equipment
Laser
Bench
Protractor,
ruler
Laser Unit
Semi-circular
light block
Lucite rod
Optical fibres are the main links for global communications and formed the
basis of the modern Photonics industry.
Optical fibre is now being used to provide direct links into households to carry
broadband communications. These links can provide many direct and
interactive digital video and data channels.
There are many types of optical fibre. This experiment uses polymer fibre with
a core diameter of 1000microns (1mm). Long distance communication grade
fibre is made of very pure silica and has a core diameter of 5 microns or less,
about the same size as a human hair.
Individual fibre
High index core
Low index coat
Figure 4.26 Optical fibre structure and total internal reflection
Light has to be launched into a fibre. Communication fibre uses lasers but for
local networks light-emitting diodes may be used.
Light travels through the fibre by total internal reflection bouncing off the
interface between the core and cladding. The cladding must have a lower
refractive index compared to the core.
The light signal has to be detected by a sensor and a photodiode or
phototransistor may be used to recover the original signal. These devices are
similar to a solar cell and convert light into an electrical signal.
Acton Instruments - ANU
Page 1
6/27/2017
Activity
Do not stare at the laser beam
Beware of reflections
View the laser from above,
Use card or a screen to trace the path of the laser beam
Demonstrate total internal reflection using the length of smoked Lucite rod
and positioning it so the laser beam passes through the rod as shown in figure
4.27. The smoked Lucite should allow the laser beam to be visualised and
marked out on a sheet of paper underneath the rod.
Lucite
block
Ci
laser
Figure 4.27 Total internal reflection
The refractive index is an important characteristic of a fibre. Consider the
Lucite block as the fibre. Refer to figure 4.28 to measure the angles of
incidence and refraction for a laser beam passing through the material. Mark
out the angles on a piece of paper placed underneath the block and use
Snell's Law of Refraction to calculate the refractive index of the material.
i
r
plastic
block
laser
Figure 4.28 Measuring the refractive index
Snell's Law of Refraction n1 sin i = n2 sin r
where i is the angle of incidence and r is the angle of refraction.
n1 is the refractive index of air (1.00) and n 2 is the refractive index of the
plastic block.
The refractive index n of a material is the speed of light in a vacuum(air)
divided by the speed of light in the material
Acton Instruments - ANU
Page 2
6/27/2017
Activity
As the angle of incidence increases a critical angle is reached at which the
angle of refraction is 90o. Above this critical angle total internal reflection
occurs.
Set up the semicircular plastic block so the laser passes through the centre of
the block.
Rotate the semicircular block until no light passes through and the beam is
reflected out of the front surface. Mark the position of the block and laser path.
c
c
centre
semicircular
block
laser
Figure 4.29 Measuring the critical angle c
Measure the critical angle c.
Why can the effect of refraction be ignored on the front surface?
The critical angle is an important measurement that determines how light is
launched into a fibre.
From the earlier observations explain how optical fibres carry light.
Acton Instruments - ANU
Page 3
6/27/2017