Flashcards
Optic Fibres
Cards in this deck (22)
- 1What is a ray?light travelling in a very narrow beam
it represents the direction a light wave travels
always at right angles to the wavefronts - 2What three things happens to a ray upon an air/water boundary1) partially reflects
2) partially refracts
3) partially becomes absorbed at the surface - 3what happens in the energy conservations?total initial energy = reflected + refracted + absorbed energy
- 4What is the law of reflection?the angle of reflection (θ1 on one side of the normal) = the angle (θ1 on the other side of the normal)
- 5What is snells law?the ratio of sine of the angle of incidence (θ1) to the angle of refraction (θ2) is a constant for two different transparent substances
- 6What happens when EM waves enter a denser substance?they slow down
- 7What is the absolute refractive index?n(s) = c / c(s)
- 8What happens when a ray travels into a less optically dense substance?it refracts away from the normal
increasing the angle of incidence
eventually means the angle of refraction will become 90° and lie along the boundary btwn two substances
this is the critical angle (θc) - 9What is total internal reflection?when the angle of incidence becomes greater than the critical angle
it becomes trapped in the optically denser substance
no refracted ray - 10what is the critical angle equation?sinθc = n2 / n1
n1 > n2 - 11What is an optical fibre?they are thin threads of glass/plastic designed to carry light waves with the minimum of attenuation using TIR
- 12How does an optical fibre work?- a pulse of light is passed into a fibre from a transmitter at one end
- the same pulse will be picked up by the receiver at the other end - 13What do they do to try make the ideal situation?a transparent core is surrounded with a transparent cladding of slightly LOWER refractive index than the core
- 14why is there a plastic sheath around the fibre?to strengthen it and protects it from damaging scratches which could leak light
- 15Why is cladding necessary?- to surround the core with a material of lower refractive index
- to make sure light will not pass directly from one fibre to another if they were to come into contact
- this could cause a cross-over of signals and a very insecure communication channel - 16What is multi-path dispersion and how is it avoided?If the diameter was wide then rays from the same part of a pulse would travel in very different distances when travelling along the axis compared with making many reflections off the core/cladding boundary.
- this would spread the pulse out
- limiting the number of pulses per sec that could be transmitted down a fibre + still be recognisable as separate pulses at the end
- reduced by making the core thinner or using a graded-index core - 17Whats a graded-index core?the refractive index across the core is gradually reduced from the centre to the edges
- light travels faster in low refractive index than in high
- light rays near the edges of the core travel faster but over a long distance
- meaning they travel approximately the same distance as the straighter rays travelling slower in the core
- reduces pulse spreading
- can support a bandwidth of over 1 GHz per km - 18what happens w/ interference?due to interference btwn the rays, only certain angles of incidence at the fibre/cladding boundary will allow rays to be transmitted
- the angles allowed are called modes
- modes satisfy the conditions for constructive interference
- other directions will result in cancellation by destructive interferene - 19How does attenuation occur?-it is a result of absorption, scattering and bends in the fibre
- absorption: when light encounters impurities/atomic defects or IR and UV are naturally absorbed bc their frequencies match the natural frequencies of the glass atoms.
- scattering: when light 'collides' with certain molecules in the glass and scatters the beam in all directions, limiting the energy in the forward direction - 20How do you compensate for attenuation?-using repeaters
- these are positioned at regular intervals
- they convert the signals to an electrical signal which is fed to a transmitter which sends the optical signal onwards at a higher intensity - 21Whats the benefits of optical fibre over copper cable?- they are used primarily for systems requiring a higher bandwidth or spanning over longer distances (they are lighter)
- low losses: allowing greater distance between repeaters + greater bandwidth
- exclude EM interference + cross-talk: no signals can induce signals in a nearby fibre
- secure from being tapped into
- high electrical resistance means they are safe near high voltage equipment
- cannot produce sparks when circuits are made/broken - 22What are the benefits of copper cables?-used for short distances + lower bandwidth applications
- low cost of materials, transmitters and repeaters
- easier to join together
- can carry electrical power as well as the signal
- already in place - no need to really replace
Information
- Description
- A Level Physics Flashcards on Optic Fibres, created by Max B on 23/03/2016.
- Created by
- Max B
- Views
- 4
- Rating