Dispersion in Optical Fibres Revision Notes | A Level AQA Physics

1. Why Signals Degrade in Optical Fibres

Optical fibres carry information as pulses of light — a sharp burst of light represents a binary 1, and no light represents a 0. The challenge is keeping those pulses sharp and strong over long distances. Two separate mechanisms work against you: dispersion smears the pulses out in time, and absorption drains their energy away.

Understanding exactly why each mechanism occurs, and what it does to the signal, is the core of this topic. The AQA specification requires you to distinguish between two distinct types of dispersion — modal and material — and to explain the consequences of both pulse broadening and absorption.

2. Modal Dispersion

Picture a ray of light entering an optical fibre at a shallow angle to the axis. It bounces repeatedly off the cladding boundary via total internal reflection, zigzagging down the fibre. Now picture a second ray entering almost parallel to the central axis — it travels a much more direct, shorter path. Both rays set off at the same instant, but they do not arrive at the far end at the same time.

The word 'mode' here refers to a distinct ray path. A step-index multimode fibre — one with a uniform core refractive index — supports many modes simultaneously, so modal dispersion is most severe in this fibre type. The greater the difference in path lengths between the fastest and slowest modes, the more the pulse spreads.

The practical fix is to use a monomode (single-mode) fibre, which has a very narrow core (typically around 8–10 µm in diameter). The core is so narrow that only one mode — the axial ray — can propagate. With all light travelling the same path, modal dispersion is effectively eliminated.

Modal dispersion is the dominant dispersion mechanism in multimode fibres. Once you switch to a monomode fibre, modal dispersion is no longer significant — but a second, subtler mechanism then becomes the limiting factor.

3. Material Dispersion

Even in a monomode fibre, a real light pulse is never perfectly monochromatic — it contains a small spread of wavelengths. This matters because the refractive index of glass is not the same for every wavelength. Shorter wavelengths (towards the violet end) experience a slightly higher refractive index than longer wavelengths (towards the red end), so they travel more slowly through the glass.

The key phrase to fix in your mind is: the refractive index of glass depends on wavelength. This is the same physical effect responsible for a glass prism splitting white light into a spectrum — the glass refracts different colours by different amounts because it slows them by different amounts.

Material dispersion is minimised by using a light source with a very narrow range of wavelengths — a laser rather than an LED. Lasers emit an extremely narrow spectral bandwidth, so the spread in arrival times between the fastest and slowest wavelength components is tiny.

4. Pulse Broadening and Its Consequences

Both modal and material dispersion produce the same end result: the pulse arriving at the receiver is wider in time than the pulse that was transmitted. This is pulse broadening. Think of it as a sharp spike being smeared into a gentle hump.

The critical consequence occurs when pulses are sent in rapid succession. If the bit rate (the number of pulses per second) is too high, broadened pulses begin to overlap with their neighbours. When two pulses merge, the receiver can no longer distinguish a 1 followed by a 0 from two consecutive 1s — the digital information is corrupted.

This places a hard limit on the maximum bit rate that a fibre can carry over a given distance. The more dispersion a fibre suffers, the lower the maximum bit rate must be kept to avoid overlap. Equivalently, for a fixed bit rate, dispersion limits the maximum transmission distance before a repeater is needed.

5. Absorption and Signal Attenuation

Dispersion is not the only problem. As light travels through the glass core, some of its energy is absorbed by the material — converted into heat by interactions with the glass molecules and any impurities present. The result is that the pulse amplitude (its peak power) decreases with distance. This is absorption, and it causes attenuation of the signal.

Absorption and dispersion are distinct problems with distinct consequences. Absorption reduces the height of the pulse — it gets weaker. Dispersion increases the width of the pulse — it gets broader. Both degrade the signal, but in different ways.

If the pulse amplitude falls below the threshold that the receiver can detect, the signal is lost entirely. This sets a maximum transmission distance for a given fibre and light source. In practice, repeaters are placed at intervals along the fibre to regenerate the signal — they detect the weakened, broadened pulse and retransmit a fresh, sharp, full-amplitude version.

6. Comparing Modal and Material Dispersion

Exam questions frequently ask you to compare the two types of dispersion. The table below sets out the key distinctions clearly — knowing these differences precisely is what separates a top-band answer from a mid-band one.

Notice that both mechanisms share the same consequence — pulse broadening — but their physical origins are entirely different. Modal dispersion is a geometric effect (path length differences); material dispersion is a wave-optics effect (wavelength-dependent refractive index).

7. Exam Technique and Mark-Winning Guidance

Dispersion questions are a reliable source of marks in AQA papers, but they are also a reliable source of dropped marks when students conflate the two types or describe consequences imprecisely. The guidance below addresses the patterns that most commonly cost students marks.

Worked example: Answering a 4-Mark Explain Question on Dispersion

Explain why pulse broadening occurs in a step-index multimode optical fibre and state one consequence for signal transmission. [4 marks]

1. Step-index multimode fibre → modal dispersion is the dominant mechanism [B1] — The question specifies a step-index multimode fibre. This is the fibre type most affected by modal dispersion. Name it explicitly.
2. Different rays (modes) travel different path lengths through the fibre — rays at steeper angles zigzag more and travel a longer total distance [M1] — This is the mechanism mark. You must link the different path lengths to the geometry of ray propagation. Avoid vague phrases like 'light travels differently'.
3. Rays that set off together arrive at the far end at different times, so the pulse is spread out — it is broadened [M1] — This is the second mechanism mark. The key phrase is 'arrive at different times', which directly causes the broadening. Use the word 'broadened' or 'spread out in time'.
4. If the bit rate is too high, adjacent broadened pulses overlap, making it impossible to distinguish individual bits — the signal is corrupted [A1] — This is the application/consequence mark. Overlap of pulses is the key idea. You could alternatively state that the maximum bit rate is reduced, or that a repeater is needed sooner.

Answer: Modal dispersion occurs because different rays travel different path lengths (steeper rays zigzag more). They arrive at different times, broadening the pulse. If pulses overlap, adjacent bits cannot be distinguished, corrupting the signal. [4 marks]

Both dispersion and absorption ultimately limit how far and how fast information can be sent through an optical fibre. Dispersion limits the bit rate by broadening pulses; absorption limits the transmission distance by weakening them. Together, they define why repeaters are needed at regular intervals in long-distance fibre-optic communication systems.