Fibre Optics (AQA A Level 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

Optical fibres utilise total internal reflection where the angle of incidence on the side of the fibre is greater than the critical angle

• 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

A light ray is totally internally reflected down an optical fibre against the core-cladding boundary

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

  • It keeps the signals secure and maintains the original signal quality

  • It keeps the core separate from other fibres preventing information crossover

Material & Modal Dispersion

• Material and model 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

White light is dispersed into its spectral components

• 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 

Remember when a light ray is reflected then the angle of incidence = angle of reflection

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

Modal dispersion occurs in a wide optical fibre where it spreads out and broadens

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 increases the amplitude of the signal

Reducing Pulse Broadening & 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 of 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