Notes · ElectronicsUK · A-Levels

Optical communication

Optical communication sends information as pulses of light down a glass fibre, offering enormous bandwidth and very low loss. This chapter develops the optical link and its electrical-to-optical and optical-to-electrical conversion, total internal reflection in the fibre and the critical angle, attenuation and dispersion, and the reasoned evaluation of optical against copper links.

4 sections~15 min reading time3 competenciesLevel Standard 1 · Advanced 3

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Exam profile

AO1 · Describe the optical link, total internal reflection in a fibre, and attenuation and dispersion.AO2 · Calculate the critical angle and fibre attenuation in decibels.AO3 · Evaluate optical communication against copper links and justify a choice.

Operators:describecalculatedeterminecompareanalyseevaluate

basic level

At AS the focus is the optical link and the idea of guiding light by total internal reflection.

higher level

The full A-Level develops the critical angle, attenuation and dispersion, and the quantitative comparison with copper.

Depth

Text

Contents · 4 sections

Optical communication

The optical link#

StandardWJEC/Eduqas A Level Electronics — Unit 4 Digital and Communication (optical link)

The optical communication link

Fig. 1The optical link: electrical-to-optical conversion (LED or laser), the fibre channel, and optical-to-electrical conversion (photodiode).

An optical communication link carries information as light rather than as an electrical signal, so it needs devices to convert between the electrical and optical domains at each end. At the transmitter an electrical signal drives a light source — a light-emitting diode (LED) for shorter, lower-rate links, or a laser diode for long, high-rate links — which converts the electrical signal into light (electrical-to-optical conversion). The light is switched on and off (or modulated) in step with the digital signal.

The light travels down an optical fibre, a very thin, flexible strand of extremely pure glass that guides the light along its length by total internal reflection (developed in the next section). Because the signal is now light confined within glass, it is completely unaffected by the electromagnetic interference that plagues copper cables, and it can carry an enormous range of frequencies. The fibre is the channel of the communication model, and an exceptionally good one.

At the receiver a photodiode converts the arriving light back into an electrical signal (optical-to-electrical conversion): light falling on the photodiode releases charge carriers and produces a current that follows the light intensity, recreating the electrical pulses. This weak signal is then amplified and, for a digital link, regenerated into clean logic levels. The photodiode's speed and sensitivity are important, because they limit how fast and how faint a signal the receiver can handle.

The full link is therefore a clear input-process-output chain: electrical signal in, converted to light, guided along the fibre, converted back to an electrical signal, and recovered. The two conversion stages — the light source and the photodiode — are the price of using light, and they must be fast enough for the data rate and matched to the fibre. This need for conversion at both ends is a recurring point in the evaluation of optical links.

Optical links are almost always digital, sending the light as on/off pulses, so they inherit all the advantages of digital transmission — perfect regeneration, error correction, and immunity to noise — on top of the fibre's huge bandwidth and low loss. This combination is why optical fibre carries the vast majority of the world's long-distance and high-capacity communication, from undersea cables to the backbone of the internet, and why understanding the link's structure is the foundation of the topic.

\text{electrical} \to \text{light (LED/laser)} \to \text{fibre} \to \text{light} \to \text{electrical (photodiode)}

Optical link

Conversion to light, guiding down the fibre, and conversion back.

Worked example

Identifying the conversion devices

In a high-speed long-distance optical link, name the device used to convert the electrical signal to light and the device used to recover the electrical signal, and give a reason for each choice.

Electrical to optical
A laser diode: for a long, high-rate link its narrow, intense, well-defined beam couples efficiently into the fibre and can be modulated at very high speed (an LED would be used for shorter, cheaper links).
Optical to electrical
A photodiode: it responds quickly to the arriving light pulses and converts them back into an electrical current with good sensitivity.
Why light
The fibre offers huge bandwidth, very low loss and immunity to electromagnetic interference, so a laser-and-photodiode link outperforms copper over long distances.

Result: A laser diode converts electrical to optical and a photodiode converts back; light is chosen for the fibre's bandwidth, low loss and interference immunity.

Typical mistakes

Confusing the light source (transmitter) with the photodiode (receiver), or forgetting that conversion is needed at both ends.
Thinking the fibre carries an electrical signal; it carries light, which must be converted at each end.

Active revision

Draw a block diagram of an optical communication link, naming the device that converts electrical to optical and the device that converts optical to electrical, and state one advantage of using light over copper.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

Total internal reflection and the fibre#

AdvancedWJEC/Eduqas A Level Electronics — Unit 4 Digital and Communication (TIR)

Total internal reflection in a fibre core

Fig. 2Light is guided along the core by repeated total internal reflection at the core-cladding boundaries (top and bottom lines).

An optical fibre guides light by total internal reflection. It has a central core of glass surrounded by a cladding of glass with a slightly lower refractive index. When light travelling in the higher-index core strikes the boundary with the lower-index cladding at a large enough angle, it is completely reflected back into the core rather than passing into the cladding. Repeated reflections bounce the light along the fibre, keeping it trapped in the core even as the fibre bends.

Total internal reflection only happens when the light strikes the boundary at an angle to the normal greater than the critical angle. The critical angle

\theta_c

is found from the two refractive indices by

\sin\theta_c = n_2/n_1

, where

n_1

is the (higher) core index and

n_2

the (lower) cladding index. Light striking the boundary at more than

\theta_c

from the normal is totally reflected and guided; light striking at less than

\theta_c

partly escapes into the cladding and is lost. This is a direct application of the physics of refraction (recap).

The small difference in refractive index between core and cladding is what makes the fibre work, and it also limits which rays are guided. Only light entering the fibre within a narrow cone of angles strikes the boundary steeply enough to be totally reflected; light entering at too shallow an angle leaks away. This acceptance cone is why light must be launched into the fibre carefully, and why a laser's narrow, well-directed beam couples in more efficiently than an LED's spread-out light.

The purity of the glass is extraordinary — modern fibre is so clear that light can travel tens of kilometres before it is appreciably weakened. The guiding by total internal reflection is essentially lossless at each reflection, so the tiny attenuation that does occur comes from absorption and scattering in the glass, not from the reflections. This is the physical reason optical fibre has such remarkably low attenuation compared with copper.

Understanding the fibre as a light guide based on total internal reflection, and being able to calculate the critical angle from the core and cladding indices, is the key physics of optical communication. It explains why the fibre confines the light, why the index difference and the acceptance angle matter, and — through the way different ray paths take slightly different times — leads directly to the dispersion that limits the fibre's data rate, developed next.

\sin\theta_c = \dfrac{n_2}{n_1}

Critical angle

n1 is the higher (core) index, n2 the lower (cladding); TIR occurs above this angle.

Worked example

Critical angle of a fibre

An optical fibre has a core of refractive index $n_1 = 1.50$ and a cladding of $n_2 = 1.48$ . Find the critical angle and state which rays are guided.

Apply the formula
$\sin\theta_c = n_2/n_1 = 1.48/1.50 = 0.9867$ .
Critical angle
$\theta_c = \sin^{-1}(0.9867) = 80.6^{\circ}$ .
$\theta_c = \sin^{-1}\!\left(\dfrac{1.48}{1.50}\right) = 80.6^{\circ}$
Which rays
A ray striking the core-cladding boundary at more than $80.6^{\circ}$ to the normal is totally internally reflected and guided; a shallower ray (smaller angle to the normal) partly escapes into the cladding and is lost.

Result: The critical angle is $80.6^{\circ}$ ; only rays hitting the boundary at a greater angle to the normal are guided along the fibre.

Typical mistakes

Inverting the critical-angle formula — $\sin\theta_c = n_2/n_1$ with the smaller index on top.
Forgetting that TIR needs the angle to the normal to exceed the critical angle (a steep ray, not a shallow one).

Active revision

A fibre has a core of refractive index $1.50$ and a cladding of $1.48$ . Calculate the critical angle at the core-cladding boundary and state the condition for a ray to be guided.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

Attenuation and dispersion#

AdvancedWJEC/Eduqas A Level Electronics — Unit 4 Digital and Communication (attenuation, dispersion)

Fibre attenuation over distance

Fig. 3The very low fibre loss (here

0.2\,\text{dB/km}

): the signal falls only

20\,\text{dB}

over

100\,\text{km}

, so repeaters can be widely spaced.

Two effects limit an optical fibre: attenuation, which weakens the signal, and dispersion, which spreads out the pulses. Attenuation in a fibre is remarkably low — modern fibre loses only a few tenths of a decibel per kilometre, against several decibels per kilometre for copper — so a signal can travel tens of kilometres before it must be amplified or regenerated. The loss, measured in decibels per kilometre, comes from absorption and scattering in the glass, and the total loss is simply the loss per kilometre times the length.

The very low attenuation is one of the fibre's greatest advantages: repeaters can be spaced far apart, which cuts the cost and complexity of a long link. A copper cable losing

3\,\text{dB/km}

might need a repeater every kilometre or two; a fibre losing

0.2\,\text{dB/km}

can run for tens of kilometres between regenerators. Calculating the total attenuation over a link, and hence the repeater spacing, is a standard optical-communication exam task using the decibel arithmetic from earlier chapters.

Dispersion is the spreading of a light pulse as it travels, and it limits the data rate rather than the reach. It arises because different components of the light take slightly different times to travel the fibre — different ray paths in a multi-mode fibre (modal dispersion), or different wavelengths travelling at slightly different speeds (chromatic dispersion). As pulses spread, adjacent pulses begin to overlap, and once they merge they can no longer be distinguished, setting a limit on how closely pulses can be sent — the maximum bit rate.

There is therefore a trade-off between the data rate and the distance: the further a pulse travels, the more it spreads, so a fibre that carries a very high data rate over a short distance may carry only a lower rate over a long one. Single-mode fibre, which allows essentially one ray path, greatly reduces modal dispersion and so supports the highest data rates over the longest distances, which is why it is used for long-haul links, at the cost of being harder to make and couple into.

Together, low attenuation and manageable dispersion make the fibre an outstanding channel: it reaches far (low loss) and carries a great deal (high bandwidth, limited by dispersion). Understanding that attenuation limits the reach while dispersion limits the rate — and being able to calculate the total attenuation of a link in decibels — captures the two quantitative limits of optical communication and prepares the ground for the final evaluation against copper.

\text{total loss (dB)} = (\text{dB/km}) \times \text{length}

Fibre attenuation

Loss per kilometre times the length; sets the repeater spacing.

Worked example

Attenuation over a fibre link

A fibre has an attenuation of $0.2\,\text{dB/km}$ and a laser launches $1\,\text{mW}$ into it. Find the total loss over $80\,\text{km}$ and the output power, and state what limits the data rate.

Total loss
$\text{total loss} = 0.2\,\text{dB/km} \times 80\,\text{km} = 16\,\text{dB}$ .
Output power
A $16\,\text{dB}$ loss is a power ratio of $10^{-16/10} = 10^{-1.6} = 0.025$ , so $P_{out} = 1\,\text{mW} \times 0.025 = 25\,\mu\text{W}$ .
$P_{out} = 1\,\text{mW} \times 10^{-1.6} = 25\,\mu\text{W}$
Data-rate limit
Dispersion — the spreading of the pulses as they travel — limits the data rate: once adjacent pulses spread enough to overlap they cannot be distinguished, so a longer link supports a lower maximum bit rate.

Result: The link loses $16\,\text{dB}$ , cutting $1\,\text{mW}$ to about $25\,\mu\text{W}$ ; dispersion, not attenuation, limits the data rate over long distances.

Typical mistakes

Confusing attenuation (limits reach) with dispersion (limits data rate).
Forgetting that the total loss is loss per kilometre times length, in decibels.

Active revision

A fibre has an attenuation of $0.2\,\text{dB/km}$ . Find the total loss over an $80\,\text{km}$ link, and explain what limits how fast data can be sent over a long fibre.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

Evaluating optical communication#

AdvancedWJEC/Eduqas A Level Electronics — Unit 4 Digital and Communication (evaluation)

Optical fibre versus copper

Fig. 4The evaluation: fibre wins on bandwidth, loss, interference immunity and security; copper is cheaper and needs no optical conversion.

Optical fibre has a set of advantages over copper that together explain its dominance of high-capacity communication. It has an enormous bandwidth, so it can carry vast data rates; very low attenuation, so signals travel far between repeaters; and complete immunity to electromagnetic interference, because the signal is light in glass rather than current in a conductor. It is also thin, light and secure — a fibre is hard to tap without detection, and it carries no electrical signal to radiate or pick up interference.

These advantages are decisive for long-distance, high-capacity links. Undersea cables spanning oceans, the trunk routes of the internet, and the connections between telephone exchanges are almost all fibre, because no copper cable could carry the same data rate over the same distance with so few repeaters. The immunity to interference also makes fibre valuable in electrically noisy environments — factories, power stations — where copper would pick up unacceptable interference.

Optical fibre is not without drawbacks, and a balanced evaluation must state them. It is more expensive to install than copper, its connectors and splices must be made precisely, and it is more fragile — it cannot be bent too sharply without losing light or breaking. Crucially, it requires electrical-to-optical and optical-to-electrical conversion at each end, adding cost and complexity that a purely electrical copper link avoids. And dispersion limits the data rate over very long distances.

The choice between fibre and copper therefore depends on the application, and the specification expects a justified evaluation rather than a blanket preference. For a long, high-capacity backbone, fibre's bandwidth and low loss win decisively despite its cost. For a short, low-rate connection between nearby equipment, copper is cheaper and its lower bandwidth is perfectly adequate, and it needs no conversion. Weighing bandwidth, distance, interference, security and cost against the specific need is the reasoning the examiner rewards.

Optical communication brings the whole communications strand of the course to a close by showing the systems approach applied end to end: an electrical signal converted to light, guided by total internal reflection through a low-loss fibre, limited by attenuation and dispersion, and converted back — chosen or rejected for a given link on a clear-eyed weighing of its advantages and drawbacks. This evaluative judgement, built on the quantitative understanding of the earlier sections, is the culminating skill of the topic.

\text{bandwidth} \ \uparrow, \ \text{loss} \ \downarrow, \ \text{EMI} \ \downarrow \quad \text{vs} \quad \text{cost} \ \uparrow, \ \text{conversion needed}

Fibre trade-off

Fibre's bandwidth, low loss and immunity against its cost and conversion complexity.

Worked example

Fibre or copper?

Evaluate optical fibre against copper for (a) a transatlantic cable and (b) a short link between two adjacent circuit boards, and recommend each with reasons.

Transatlantic cable
Fibre: its huge bandwidth and very low $0.2\,\text{dB/km}$ loss allow enormous data rates across an ocean with widely spaced repeaters; copper could not carry the rate over the distance. The higher cost and conversion are easily justified.
Board-to-board link
Copper: the distance is tiny and the data rate modest, so copper's lower bandwidth is ample, and it is cheaper and needs no optical conversion. Fibre would add cost and E-O/O-E converters for no benefit.
Judgement
The requirement decides it: long-haul high-capacity favours fibre; short low-rate favours copper.

Result: Fibre for the transatlantic cable (bandwidth and low loss); copper for the short board link (cheaper, adequate, no conversion needed).

Typical mistakes

Giving only advantages; a balanced evaluation must include the cost, fragility and conversion drawbacks.
Recommending fibre for every situation, ignoring that copper is cheaper and adequate for short, low-rate links.

Active revision

Evaluate the use of optical fibre rather than copper for (a) a transatlantic communications cable and (b) a short link between two circuit boards, giving a justified recommendation for each.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas GCE Electronics specification

Optical communication

4 sections~15 min reading time3 competenciesLevel Standard 1 · Advanced 3

Optical communication

Key points

Identifying the conversion devices

Key points

Critical angle of a fibre

Key points

Attenuation over a fibre link

Key points

Fibre or copper?

Sources

Optical communication

Key points

Identifying the conversion devices

Key points

Critical angle of a fibre

Key points

Attenuation over a fibre link

Key points

Fibre or copper?

Sources