Optical fibres are used to carry signals in the form of pulses of light over distances up to 50km. They do this by Total Internal Reflection. That's why optical fibers can guide light for such long distances - because the walls of the fibre don't absorb any light at all as long as the angle of incidence is greater than the critical angle. Total Internal Reflection causes 100% reflection. In no other situation in nature does this occur, so it is unique and very useful as it is 100% efficient at transfering the light energy.

There are two conditions necessary for Total Internal Reflection to occur:

• The refractive index of the first medium is greater than the refractive index of the second medium
• The angle of incidence must be greater than or equal to the critical angle

Light refracts as expected at low angles of incidence, but at higher angles reflection increases until the internal surface acts as a mirror. This is called TOTAL INTERNAL REFLECTION, because all of the incident light energy is reflected.

Construction (not required at GCSE)

A fibre optic cable is made from a glass or plastic core, that carries the light, surrounded by a glass cladding of lower refractive index, which reflects “escaping” light back into the core. Without the cladding the light would pass between the fibres as they are all made of glass, would not have a lower refractive index boundary for TIR to occur at and therefore would allow light transfer.They are about usually about 120 micrometers in diameter.

Surface scratches lead to light leakage in a single fibre. Scratches in the outer cladding do not matter because it does not carry a light signal. Additional layers of treated paper, PVC or metal may further protect the outside of the fibre.

The core has a higher refractive index than the cladding. Although the cladding does not carry light, it is nevertheless on essential part of the fibre. It is not just a covering. It keeps the value of the critical angle constant throughout the whole length of the fibre

Advantages over Copper Cable

• They can carry signals with much less energy loss than copper cable as copper wires lose signal energy as heat (P=I²R) due to their resistance.
• They are much lighter than copper cables with the same band width, so much less space is required in underground cabling ducts and costs for transportation and handling are therefore less.
• They are immune to electromagnetic interference from radio signals, lightning etc
• They can be routed safely through explosive or flammable atmospheres where are the risk of sparks from electrical cables would be to great for them to be used without a lot of precautions taken.
• The raw materials to make them are plentiful (silicon from sand!) whereas copper supplies are dwindling.

Disadvantages over Copper Cable

• Optical fibres are still more expensive per metre than copper because they are manufactured to a high standard and are made to carry multiple signals.
• They cannot be joined together as easily as copper cable
  

Areas of Application

• Telecommunications
• Sensor Manufacture
• Local Area Networks
• Cable TV
• CCTV
• Optical Fibre Sensors

Telecommunications

A Telecommunications Link is the simplest of fibre optic systems. It consists basically of a transmitter, a fibre link and a receiver. The transmitter will normally be equipped with a laser diode that usually has an output wavelength of 1300nm or 1500nm. The fibre link will be made of single lengths of optical fibres 2km in length, which are fusion spliced (joined) together. The link will be able to carry 1000s of telephone conversations simultaneously by means of TIME DIVISION MULTIPLEXING. This basically means that the data in multiple conversations is split up and sent down the cable. When it reaches the other end of the cable, the individual conversations are put back together again.

Microbending Sensor

A microbending sensor consists of two plates between which passes an optical fibre. The plates have parallel grooves on their facing surfaces and the grooves from the two plates interleave with each other. The fibre passing between the plates is therefore bent alternatively up and down. When a fibre is bent sufficiently the light in the core no longer meets the cladding at an angle equal to or greater than the critical angle. Total Internal Reflection therefore does not occur. This is called microbending loss, and the more a plate is bent, the more loss occurs. This has a military application of submarine detection.

Blood Components Sensor

If we use the correct wavelength we can measure the concentrations of specific components of blood such as total protein, cholesterol, urea and uric acid quickly. When the concentration is high, the output at the detector is less and vice versa. The concentrations of those chemicals are important to doctors in the diagnosis and monitoring of certain disease conditions. Fibre optic sensors can give results very quickly without having to send samples away to an analytical lab.

Endoscopes (or fibroscopes)

Endoscopes are used to look inside people. An endoscope is a narrow bunch of fibres with a lens system at each end. Light is carried down by another bunch of optical fibres. The image is displayed as a full colour-moving image on a television screen. This makes keyhole surgery possible.

(This page has been adapted from a Power Point presentation made by two of my students: Rachel Wylie and Nicola Beesley (2001))