2.6 Fibre Optics Flashcards
What does a ray represent?
A ray represents the direction in which a light wave travels and is therefore always at right angles to the wavefronts.
What three things happen to the incident ray?
It partially reflects, partially refracts and partially becomes absorbed at the surface.
What happens with the energy when a ray is reflected, refracted and absorbed?
Since energy must be conserved, the total incident energy will be equal to that reflected refracted and absorbed.
What is the law of reflection?
The angle of the reflection = the angle of incidence
What is Snell’s law?
The ratio of the singe of the angle of incidence (Θ1) to the sine of the angle of refraction (Θ2) is a constant for two different transparent substances.
This constant is the refractive index going from substance 1 to 2.
What happens when e-m waves enter an optically denser substance?
They slow down.
What happens when e-m waves enter an optically less dense substance?
They speed up.
What happens to the direction of the ray when it travels into an optically less dense substance?
It refracts away from the normal, increasing the angle of incidence and this eventually means that the angle of refraction will become 90 degrees and lie along the boundary between the two substances.
Define critical angle.
The angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected.
When is the ray totally internally reflected?
When the angle of incidence becomes greater than the critical angle the ray is totally internally reflected and trapped in the optically denser substance.
What are optical fibres?
Thin threads of glass or plastic designed to carry light waves with the minimum attenuation using total internal reflection.
What happens when a pulse of light is passed into an ideal fibre from a transmitter at one end?
The same pulse will be picked up by the receiver at the other end.
In optical fibres, what is the transparent core surrounded by?
A transparent cladding of slightly lower refractive index than the core.
In optical fibres, what is around the fibre?
A plastic sheath which strengthens the fibre and protects it from damaging scratches which could leak light.
Why is cladding necessary in optical fibres?
To surround the core with a material of lower refractive index and also to make sure that light will not pass directly from one fibre to another if two fibres were to come into contact, this would mean cross over of signals and a very insecure communication channel.
Why are cores made increasingly ever thinner?
To avoid multi path dispersion.
Why would multi path dispersion be significant in a wide core?
Because the rays from a same part of a pulse would travel very different distances when travelling along the axis compared making many reflections off the core/cladding boundary. The effect of this would be to spread the pulse out.
What would the effect of pulse spreading have on the number of pulses per second?
It limits the number of pulses per second that could be transmitted down a fibre and still be recognisable as separate pulses at the other end.
How is the refractive index across the core gradually reduced in a graded index fibre?
From the centre to the edges.
When does light travel faster, low refractive or high refractive index?
Light travels faster in a low refractive index material than in a high index material.
Where does light travel fastest in graded index fibre?
The light rays near the edges of the core travel faster but over a longer distance, meaning that they curve due to refraction and travel through the core in approximately the same time as the straighter rays travelling more slowly near the centre of the core.
Due to interference between the rays, only certain angles of incidence at the fibre/cladding boundary will allow rays to be transmitted. What are these angles called?
Modes- and they satisfy the conditions for constructive interference- other directions will results in cancellation by destructive interference.
When does absorption occur in optical fibres?
When the light encounters impurities or atomic defects in the class.
Some absorption is due to the glass itself whereby infrared and ultraviolet are naturally absorbed because their frequencies match the natural frequencies of the glass atoms.
When does scattering occur in optical fibre?
When the light ‘collides’ with certain molecules in the glass and scatters the beam in all directions, limiting the energy in the forward direction.
What can attenuation be compensated by?
Repeaters- these are positioned at regular intervals and convert the signal to an electrical signal which is fed to a transmitter which sends the optical signal onwards at a higher intensity.
Explain why graded-index fibres can cope with more frequent pulses of information than ordinary fibres.
All rays paths take same length of time
Pulses do not overlap as much
How does the refractive index in the fibre vary with position in the core and in the
cladding of the graded-index fibre?
Cladding- lower than core
Max in centre and reducing towards outside of the fibre
Explain why dispersion occurs in an optical fibre.
different wavelengths/ different speeds/different refractive indices
Explain why graded index fibres can cope with more frequent pulses of information than ordinary fibres.
All Ray paths take same amount of time.
Reduces pulse dispersion.
The cladding increases the critical angle between the two materials. The benefits of this are:
(a) only those rays that are close to the axis of the fibre pass through
(b) the inner fibre is protected from damage
(c) the rays all travel roughly the same distance and so information fed in at one end arrives at the other only very slightly spread out in time
(d) there are fewer reflections and the distance travelled is smaller than the multiple reflection case and so there is less energy loss and the time of transmission is shorter
