How is information sent as light along an optical fibre, and why does it outperform copper?
Optical communication: the optical link (LED or laser source, fibre, photodiode receiver), total internal reflection, attenuation and bandwidth, and the advantages over copper.
An Eduqas A-Level Electronics answer on optical communication: the optical link of an LED or laser source, an optical fibre and a photodiode receiver, total internal reflection that guides the light, attenuation and bandwidth, and the advantages of fibre over copper cable.
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What this dot point is asking
Eduqas wants you to describe the optical link (LED or laser source, fibre, photodiode receiver), explain total internal reflection, discuss attenuation and bandwidth, and give the advantages of fibre over copper. Optical communication carries most of the world's long-distance data.
The answer
The optical link
Total internal reflection
Attenuation and bandwidth
Advantages over copper
Examples in context
Optical communication carries the internet's backbone, undersea cables and most long-distance telephony, and increasingly the link to the home. The LED or laser transmitter and photodiode receiver are the optoelectronic devices that bridge electronics and light, and the on-off keying of the light is the digital modulation from the previous topic. Total internal reflection also guides light in endoscopes and sensors, linking this systems topic back to the diode and semiconductor devices studied earlier.
Try this
Q1. Name the device used at the receiving end of an optical fibre link to convert light back to an electrical signal. [1 mark]
- Cue. A photodiode (or phototransistor).
Q2. State the condition for total internal reflection at the core-cladding boundary. [1 mark]
- Cue. The light must hit the boundary at an angle greater than the critical angle (core index higher than cladding).
Q3. State one advantage of optical fibre over copper cable. [1 mark]
- Cue. Much higher bandwidth (or immunity to electromagnetic interference, or lower attenuation).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20216 marksDescribe the three main parts of an optical fibre communication link, and explain how light is kept inside the fibre.Show worked answer →
Three parts (up to 3 marks): a transmitter with a light source (an LED for shorter, cheaper links or a laser diode for long, high-bandwidth links) that converts the electrical signal into modulated light; the optical fibre itself, a thin glass core surrounded by cladding, which carries the light; and a receiver with a photodiode (or phototransistor) that converts the light back into an electrical signal.
Keeping light inside (up to 3 marks): the glass core has a higher refractive index than the cladding around it. Light that strikes the core-cladding boundary at an angle greater than the critical angle undergoes total internal reflection, bouncing back into the core. Repeated total internal reflection guides the light along the fibre with very little loss, even around bends.
Markers reward the source-fibre-receiver chain with LED/laser and photodiode, and total internal reflection at the core-cladding boundary (higher-index core) keeping the light in.
Eduqas 20195 marksState two advantages of optical fibre over copper cable for data transmission, and explain what is meant by attenuation in a fibre.Show worked answer →
Advantages (up to 4 marks, 2 each): optical fibre has a much higher bandwidth (it can carry far more data), and it is immune to electromagnetic interference (light is unaffected by electrical noise and the fibre neither picks up nor radiates interference). Other valid points: lower attenuation so fewer repeaters over long distances, lighter and thinner cables, and greater security (no radiated signal to tap).
Attenuation (up to 1 mark): attenuation is the gradual loss of signal power as the light travels along the fibre (absorbed and scattered), measured in decibels per kilometre; lower attenuation lets the signal travel further before it must be regenerated.
Markers reward two valid advantages (high bandwidth, interference immunity, low attenuation, security, light weight) and attenuation as the loss of optical power with distance.
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Sources & how we know this
- Eduqas GCE AS/A Level Electronics specification (A410QS) — WJEC Eduqas (2017)