4.6 Optical Fibres
Equipment
/ Laser Bench / Protractor, ruler / / Semi-circularlight block
/ Laser Unit / / 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.
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.
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.
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.
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 n2 is the refractive index of the plastic block.
The refractive index n of a material is thespeed of light in a vacuum(air) divided by the speed of light in the material
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.
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 - ANUPage 109/16/2018