| What are the two conditions for total internal reflection? | Angle of incidince greater than critical angle
More optically dense medium to less optically dense medium |
| What is cladding | The less optically dense/ protective medium surrounding the core of an optical fibre |
| What is the core in an optical fibre | The centre part of an optical fibre |
| What is the function of cladding in optical fibres | Stops signals crossing over from one fibre to another
Protects the fibre from scratches which leads to light leakage
Causes total internal reflection because the cladding is a higher index than the core |
| What are the two types of dispersion | Modal
Material/ spectral/ chromatic |
| Describe what modal dispersion is | light travelling in differnt paths/ modes |
| What is material dispersion | The dispersion of light signals due to differnent colours of light having different frequencies and so travelling at differnt speeds |
| Name a way to stop material dispersion | Using monochromatic light (One frequency of light) |
| Name some ways to stop modal dispersion and explain why | Making the core narrower which reduces the possible differnce in modes the light can take |
| Define attenuation | Reduction in intensity as a wave travels through an optical fibre |
| What is attenuation mesaured in? | Decibels |
| What causes attenuation in optical fibres and explain how each can happen | Scattering - variation in the composition of the optical fibre/ when light rays hit the cladding at less than the critical angle
Absorption - electron may absorb a photon of light
Bending losses - Bent too sharply light will hit cladding at less than critical angle |
| What are the applications of optical fibres? | In communications
In medical endoscopes |
| Why is it necessary for the core of an optical fibres to be narrow? | Stops light taking differnt paths which stops light takes different times for the journey which stops modal dispersion |
| What is meant by a coherent fibre bundle? | Fibres are in the same formation at the start of the fibre to the end |
| When light is shone through a slit, explain what the bright and dim patches are | Light patches are where the light waves from the slit are interferring constructivly and the dim patches are where the light waves are interferring destructivly |
| What is monochromatic light | Light of one frequency / colour |
| What is the result of dispersion on the signal being sent | Signal spreads in time/ pulse will blur |
| What is a mode when descussing optical fibres? | A path of light |
| What is the critical angle? | Angle which causes the angle of refelction to be 90 to the normal |
| How can u reduce pulse broadaning | Make the core as narrow as possible to reduce the possible differences in path length of the signal
Use of a monochromatic source so the speed of the pulse is constant
Use of optical fibre repeaters so that the pulse is regenerated before significant pulse broadening has taken place
Use of single-mode fibre to reduce multipath modal dispersion |
| How can u reducde absorption in optical fibres | Use a core which is extremely transparent
Use of optical fibre repeaters so that the pulse is regenerated before significant absorption has taken place |
