Uses of Optical Fibres (WJEC GCSE Physics)

Communication using Optical Fibres

• Optical fibres can be used to transmit:
  • Home (landline) telephone signals
  • Internet signals
  • Cable television signals

• In phone calls from landline phones:
  • Electrical signals are converted to light pulses by an infrared laser that flashes at high speed
  • Repeaters boost the signal every 30 km along the fibre
  • At the receiving end, the digital signal is converted into sound

Landline Telephone Signal Path

Sound from a landline telephone travels through optical fibres and is boosted every 30 km to the landline of the person listening

• Optical fibres can transmit both visible and infrared signals
• These signals travel at a speed of 2 × 10⁸ m/s
  • This is not the same as the speed of light in space

• Optical fibres are installed:
  • In cables attached to telephone (or telegraph) poles in the street
  • Underground from the service box to the telegraph pole or under the sea

Telegraph Poles

Fibre optic cables can be found in the phone cables attached between the telephone poles and the street

• Optical fibres transmit signals over long distances at high speeds
  • The fastest internet available in Wales is called superfast broadband
  • It uses optical fibres to transmit internet signals from the telephone exchange to a roadside cabinet in each area and then to homes or businesses
  • Slower internet speeds are the continued use of old copper cables to carry the internet signal some, or all the way from the telephone exchange

Installation of Optical Fibres for High-Speed Internet

The speed of your internet depends on whether you have all, part or none of your signal transmitted via fibre optic cable from the telephone exchange

• Optical fibres have many advantages over copper cables:
  • They use less energy to transmit the signal
  • They need fewer boosters to increase the signal
  • There is no interference with nearby cables
  • They are difficult to intercept
  • Their weight is lower, so they are easier to install

Fibre Optic and Copper Cables

Copper cables are heavier and more difficult to install than fibre optic cables

Communication Using Satellites

• Many types of communication use satellites or a combination of satellites and cables:
  • Long-distance landline phone calls
  • Mobile phone calls
  • TV broadcasts

• Satellites can orbit the Earth in one of two orbits:
  • Geosynchronous - at an angle between the equator and the north/south poles
  • Geostationary - above the equator

Geosynchronous and Geostationary Orbits

A geostationary orbit is directly above the equator

• Communication involving satellites uses microwaves or radio waves to transmit the signal from Earth to a satellite
• Microwave signals must leave Earth and are reflected back to Earth from the satellite
  • So, signals travel twice the height of the satellite in space
  • They are not reflected from satellite to satellite

• Microwave signals travel huge distances in space
  • This creates a time delay 

Sending & Receiving Signals via Satellite

Signals are transmitted to a satellite and then reflected back to Earth meaning they travel twice the distance of the height of the satellite

• Remember the following equations when answering questions on this topic:

The wave equation:  wave speed = frequency × wavelength

The speed, distance & time equation: 

• Where time can refer to time taken or time delay (s)

• Optical fibres are also used in medicine to see within the human body
• An endoscopy is a medical procedure that uses an endoscope to look inside the body
• An endoscope contains a camera on a long, thin flexible tube containing an optical fibre 
  • It can be inserted down your throat or up where you go to the toilet
  • As a result of these images, doctors can see what might be wrong with a patient
• An endoscopy is invasive, as it involves going inside a person
• It may cause some damage to areas or cells it comes into contact with

An Endoscope

Endoscopes utilise total internal reflection to see inside a patient's body

CT Scans

• CT stands for Computerised Tomography
• CT scans use X-rays to take many images of a body section at different angles
• A 3D image can be built up when all the images are put together
• This 3D image can be rotated and viewed from different angles

A CT Scan

CT scans take 2D images from multiple positions to create a 3D image

An Example Image Created by a CT Scan

This 3D image is viewed on a screen and can be rotated to enable doctors to see clearly inside the body

• Unlike an endoscopy, a CT scan is non-invasive (no instruments are inserted into the body)
• Due to the use of X-rays, it carries an exposure risk as X-rays are ionising
• People should limit their exposure to X-rays because, in high amounts, they can cause damage to cells such as mutations and possibly lead to cancer