definitions and concepts

• amplitude

  a wave's maximum displacement from its equilibrium position
• antinode

  a position of maximum displacement in a stationary wave
• cladding

  a protective layer on an optical fibre to improve the tensile strenth of the fibre, prevent scratching and to prevent signal transfer between adjacent fibres
• coherence

  waves are coherent if they have the same wavelength and frequency, as well as there being a fixed phase difference between them
• diffraction grating

  a grating with hundreds of slits per millimetre, that results in sharper interference patterns

  • they are used to calculate atomic spacing and to analyse elements
• diffraction

  the spreading of waves as they pass through a gap of a similar magnitude to their wavelength
• electromagnetic waves

  waves that consist of perpendicular electric and magnetic oscillations
• frequency

  the number of waves that pass a point in a unit time period

  • the inverse of the time period
• fringe spacing

  the distance between two adjacent bright fringes or two adjacent dark fringes
• interference

  the name given to the superposition of waves that occurs when two waves meet

  • if the waves are in phase they will constructively interfere
  • if the waves are out of phase, they will destructively interfere
• laser

  a light source that prouduces a collimated and coherent beam
• longitudinal wave

  a wave with oscillations that are parallel to the direction of energy propagation

  • sound waves are an example of a longitudinal wave
• material dispersion

  waves of different wavelengths travel at slightly different speeds through an optical fibre and so reach the end of the fibre at slightly different times, causing pulse broadening

  • the use of monochromatic light fixes this
• modal dispersion

  waves enter an optical fibre at slightly different angles, meaning the distance each beam has to travel is slightly different

  • this leads to the beams reaching the end at different times and so causes pulse broadening
• node

  a position of maximum displacement in a stationary wave
• optical fibre

  a thin glass fibre in which signals are passed through

  • optical fibres are usually surrounded by cladding
• path difference

  a measure of how far ahead a wave is compared to another wave, usually expressed in terms of the wavelength
• phase difference

  the difference in pahse between two points on a wave

  • usually expressed in radians
• phase

  a measure of how far through the wave's cycle a given point on the wave is
• polarisation

  the resitrcition of a wave so that it can only oscillate in a single plane

  • this can only occur for transverse waves
• pulse broadening

  the elongation of a signal passed down an optical fibre, commonly due to modal or material dispersion
• refractive index

  a material property that is equal to the ratio between the speed of light in a vacuum, and the speed of light in a given material
• snell's law

  a law linking a wave's angle of refraction with the use of the refractive indexes of the mediums involved
• speed

  the product of a wave's frequency and wavelength
• stationary wave

  a wave that stores but does not transfer energy
• total internal reflection, TIR

  an effect that occurs in optical fibres, where full reflection occurs at the inside boundary of the fibre, meaning no radiation passes out
• transverse wave

  a wave with oscillations that are perpendicular to the direction of energy propagation

  • electromagnetic waves are examples of transverse waves
• wavelength

  the distance between two identical positions on two adjacent waves

  • commonly measured from peak to peak or trough to trough
• young's double slit experiment

  an experiment that demonstrates the diffraction of light by passing monochromatic light across two narrow slits and observing the resulting pattern of bright and dark fringes