Fibre Optics – SS3 Physics Lesson Note (First Term)

Fibre optics is the use of thin, transparent strands of glass or plastic, called optical fibres, to carry light over long distances.

Behavioural objectives

1. Describe what an optical fibre is and name its two main parts.
2. Explain how total internal reflection keeps light inside an optical fibre.
3. Explain why optical fibres are used in telecommunications.
4. State the role of repeater stations in long-distance transmission.
5. Describe how endoscopes are used in medicine.

What is fibre optics?

An optical fibre is a thin, transparent fibre, usually made of glass or plastic, used for transmitting light.

• Thickness: optical fibres are usually thinner than a human hair.
• Working principle: light is confined inside the fibre by total internal reflection.
• Main uses: telecommunications (carrying signals) and medicine (endoscopes).

What is total internal reflection?

Total internal reflection is the complete reflection of light back into a denser medium when it meets a less dense medium at an angle greater than the critical angle.

• Condition 1: light must travel from a denser medium to a less dense medium, for example from glass to air.
• Condition 2: the angle of incidence must be greater than the critical angle.
• Result: no light passes out, and all of it is reflected back inside.
• Use: it confines light, and fibre optics is one of its most common applications.

What is the structure of an optical fibre?

An optical fibre has two main parts: an inner core and an outer cladding.

What are the parts of an optical fibre?

What are the sizes of an optical fibre?

• Overall diameter: about 125 µm (125 × 10^-6 m).
• Core diameter: about 50 µm (50 × 10^-6 m).

How does light travel along an optical fibre?

Light travels along an optical fibre by repeated total internal reflection at the boundary between the core and the cladding.

1. Light enters the core at one end of the fibre.
2. The core has a higher refractive index than the cladding, so the core is the denser medium.
3. The light meets the core-cladding boundary at an angle greater than the critical angle.
4. Total internal reflection occurs, and the light is reflected back into the core.
5. The light is reflected again and again, and stays trapped in the core.
6. The light travels very quickly down the length of the cable to the other end.

• Key idea: the difference in refractive index between core and cladding makes total internal reflection possible.
• Similar case: it works in the same way as total internal reflection at an air-water surface seen from under the water.

How are optical fibres used in telecommunications?

Optical fibres carry telephone, internet and television signals as pulses of light over long distances with minimal loss of data.

Fibre optic cable. Image credit: iTel Networks

Why are optical fibres better than conventional cables?

• Low data loss: information can be sent over long distances with minimal loss.
• High capacity: a single strand can carry a very large number of transmissions at the same time.
• High speed: the signal is received almost instantaneously because it travels as light.
• Thin and light: a fibre takes up little space compared with a metal cable.

How are signals sent along the fibre?

1. The signal is coded onto a laser.
2. The laser sends it down the fibre as pulses of light.
3. Many signals share one fibre, because individual light pulses are sent at slightly different angles.
4. The pulses reach the other end and are decoded into the original information.

What is the optical fibre compared with a co-axial cable?

• Optical fibre: a glass or plastic strand that carries signals as light pulses.
• Co-axial cable: a conventional metal cable that carries signals as electrical signals.

What is the role of repeater stations?

A repeater station is a station along a cable that amplifies a weakened signal so it can continue to its destination.

1. The signal weakens as it travels over a long distance.
2. It reaches a repeater station.
3. The station amplifies the signal.
4. The amplified signal is relayed towards its destination.
5. It may meet several other repeater stations on the way.

How are optical fibres used in medicine?

Optical fibres are used in medicine in endoscopes, which let a doctor look inside the body.

Fact: Endoscopy means to look inside and refers to looking inside the human body for diagnosing medical conditions.

What is an endoscope?

An endoscope is a medical instrument whose main part is an optical fibre, used to see inside a patient’s body.

1. Light is shone down the optical fibre into the body.
2. The doctor looks inside the body of the patient through the endoscope.
3. To examine the stomach, the endoscope is inserted down the patient’s throat.
4. The doctor uses what is seen to diagnose the medical condition.

What is minimally invasive surgery?

Minimally invasive surgery is surgery in which a person is diagnosed and treated through a small incision (cut).

• Tool: endoscopes make this kind of surgery possible.
• Alternative: open surgery, which needs a large opening in the body.

How does endoscopy compare with open surgery?

Evaluation

1. Name the two main parts of an optical fibre and state the job of each.
2. Explain why light stays trapped inside an optical fibre.
3. Give two reasons why optical fibres are preferred to conventional co-axial cables in telecommunications.
4. Why are repeater stations needed in long-distance transmission?
5. State two advantages of endoscopy over open surgery.

Assignment

Draw and label the structure of a single optical fibre, showing the core and the cladding, and write four short points on how light travels along it.