Fibre Optics (AQA AS Physics): Revision Note

Fibre optics

• Total internal reflection is used to reflect light along optical fibres

  • Light, that is normally monochromatic, refracts when it enters the optical fibre at one end

  • It undergoes repeated total internal reflection against the sides of the fibre until it reaches the other end

  • Where it is refracted back out

• In this process, the light signals travel long distances without losing information or speed

• These have many important uses, including:

  • communications, such as telephone and internet transmission

  • medical imaging, such as endoscopes

• The three main components of optical fibres are:

  • an optically dense core tube, made of plastic or glass

  • a lower optically dense cladding surrounding the core

  • an outer sheath

• TIR only occurs when n_cladding < n_core

• This type of optical fibre is called a step-index fibre because the refractive index of each component increases moving from the outside to the centre of the fibre

• The role of the cladding is to:

  • protect the thin core from damage and scratching

  • prevent signal degradation through light escaping the core, which can cause information from the signal to be lost

  • keep the signals secure and maintain the original signal quality

  • keep the core separate from other fibres, preventing information crossover

Material & modal dispersion

• Material and modal dispersion both cause pulse broadening 

  • Where the pulses emerging from the fibre are longer than those entering

Material dispersion

• When white light is used instead of monochromatic light inside an optical fibre it is separated into all the colours of the spectrum

  • The white light is therefore dispersed, so the beam gets wider as it travels down the optical fibre

• Each wavelength of light travels at the same speed in a vacuum but at different speeds in a medium

  • Violet light has the shortest wavelength, so it travels the slowest in the fibre 

  • This means its angle of incidence on the fibre boundary is smallest compared to the other colours

  • The angle of incidence is equal to the angle of reflection, so the angle of reflection is also smaller

  • This means it takes longer for the violet colour to travel down the fibre because it undergoes more reflections 

Modal dispersion

• Modal dispersion occurs when the monochromatic light pulses in the optical fibre spread out

  • This is because each part of the wavefront has a different angle of incidence and consequently a different angle of reflection

  • So each part of the wavefront undergoes total internal reflection a different number of times

  • Hence, each part of the wavefront reaches the end of the fibre at a slightly different time

• This effect is more prominent when the core of the fibre is wider

  • So the total internal reflection takes place more times

• To prevent modal dispersion, the core needs to be very narrow

Pulse broadening & absorption

• The absorption of a signal in an optical fibre occurs when the fibre absorbs part of the signal’s energy

  • This reduces the amplitude of the signal, which can lead to a loss in the information transmitted

• Pulse broadening is caused by modal and material dispersion

  • This can result in the merging of pulses, which distorts the information in the final pulse and decreases the amplitude of the signal

Reducing pulse broadening and absorption

• To reduce absorption:

  • Use an extremely transparent core

  • Use optical fibre repeaters so the pulse is regenerated before significant absorption has taken place

• To reduce pulse broadening:

  • Use a core that is as narrow as possible to reduce the possible differences in the path length of the signal

  • Use a monochromatic source so the speed of the pulse is constant

  • Use optical fibre repeaters so the pulse is regenerated before significant pulse broadening has taken place

  • Use a single-mode fibre, where only a single wavelength of light passes through the core, to reduce multipath modal dispersion