Fibre Optics
  • Fibre-optics is a method of communication that uses light as a vehicle of information.
    • Light is a great vehicle for communication as it is:
      • Very fast (3 x $10^{8}$ m/s).
      • Has a very short wavelength, allowing it to transmit more information
        • The shorter a wavelength, the more information it can carry.
      • Less affected by interference.
  • The light is sent through optical fibres to deliver information.
  • These are cables made in three or 4 levels:

1. An extremely fine glass fibre core – made from pure silica.
2. It’s enclosed in a glass sleeve/cladding
a. This needs to have a different refractive index to the glass.
3. A tough resin buffer is then applied to protect the cable.
4. Optionally, a jacket may be added to further improve protection and prevent light interference.

  • Optimal properties of the optical fibres include:
    • High flexibility
    • Lightweight
    • Crush resistance
    • Low attenuation or dispersion
    • High bandwidth
    • Unaffected by power surges and electromagnetic interference
  • Compared to copper wires, fibre optics are lighter, safer and carry more information.
    • Lighter means it is easier to install and lay down.
    • Safer as glass does not conduct electricity and thus there is little to no interference
    • It is also harder to tap into, as it can’t be “spiked.”
      • In addition, optic fibres have less attenuation during transmission, meaning very little data is lost, and it’s only at the receiver that data has a chance to be lost.
  • Fibre optic cable transmission consists of 5 steps:
  1. Electrical current is modulated with information
  2. It is then converted into light via a semiconductor laser transmitter.
  3. The fibre optic cable carries this light to its destination.
  4. A Photodiode receiver receives this modulated light
  5. It finally converts it back into current which can bring about information.
  • The optic fibres work on the principle of total internal reflection.
    • As mentioned above, the cladding has a lower refractive index than the glass
    • This causes the glass to reflect off the cladding, zigzagging its way to its destination.

Making the cable

Optical fibres are made through a three step process:

  • Making the perform in a process known as Modified Vapour Chemical Deposition(MVCD)
    • Then drawing the fibres from the collapsed perform
    • MCVD is a method to produce extremely transparent glass:
      • A Silicon tube (ri = 1.46 is heated (1600 to 2000 oC) while a special gas (Oxygen bubbled through Silicon Chloride/Germanium Chloride) flows into the tube.
      • The heat causes the Silicon and Germanium to fuse inside the tube and deposit a layer (ri = 1.47) in the tube.
      • This tube is then heated up to 2500 degrees so that it collapses into the perform material.
    • This provides the core and the cladding.
    • It is then tested.
  • Drawing the fibres also protects it:
    • The collapsed perform is then fed into a drawing tower.
    • This is lowered into a graphite furnace, and then dropped.
    • As it falls, it cools and forms a thread.
    • It then flows through a series of thickness monitors, polymer baths and coating cups. This ensures the thickness isn’t outside acceptable ranges and coats the thread with a protective layer.
    • This method provides the buffer for the Optic Fibre
    • Then it is coated in the jacket material after this method is done.
  • There is an alternate method of drawing known as the double crucible method.
    • This involves feeding the thread into a crucible surrounded by another, larger crucible in which epoxy rods are fed.
    • This does the same thing as the above method, only this empties through a common orifice and does not need to be reformed as many times

Types

  • There are two main types of optic fibre that are used.
  • Step-index is the simple optic fibres where the refractive indices are constant.
  • This can be done in two ways:
    • Single mode: This is when only one beam of light is sent through the cable. This type of transmission usually uses a very thin core (8–12 micrometres).
    • Multimode: This is when more than one beam of light is passed through the core. As a result, it uses a slightly larger core (50-200 micrometres).
      • However, due to the nature of this method (see pic) there is a possibility that some beams will reach the receiver before others.
      • This is known as Intermodal Dispersion
  • To combat the issue of intermodal dispersion, another type known as a Graded index multimode.
    • This is when the refractive index changes from the core outwards.
    • It’s done in a “quadratic” manner in which the refractive index is proportional to the square of the distance from the centre.
    • This causes the light beams to behave in a sinusoidal manner.
    • As a result, the signal received is improved and the beams would arrive in order.