PE1-2.3 Optical Sources & Detectors

What this is and why it exists

A fibre carries light. Something must make the light at one end and turn it back into current at the other.

The choices at both ends have the same shape. A cheap simple device, against an expensive one that performs better and needs looking after.

The vocabulary

• LED — a light emitting diode. Emits over a broad range of wavelengths, in all directions.
• Injection laser diode — a laser made from a diode, emitting a narrow range coherently.
• Quantum efficiency — how many photons come out per electron put in.
• Threshold current — the current above which a laser diode starts to lase.
• Rate equations — the pair of equations describing how carriers and photons interact in a laser.
• Relaxation oscillation — the ringing a laser shows immediately after it is switched on.
• Resonant modes — the wavelengths the laser cavity supports.
• PIN photodiode — a detector with an undoped region between the doped ones, and no internal gain.
• APD — an avalanche photodiode, which multiplies each generated carrier internally.
• Excess noise — the extra noise the avalanche process adds beyond simple amplification.

The mental model

The transmitter choice is the design decision of this topic.

An LED emits by ordinary recombination. Its output covers a broad range of wavelengths, it radiates over a wide angle, and it responds relatively slowly. It is cheap, it is robust, and it barely cares about temperature. Its breadth is exactly what causes material dispersion, and its wide emission angle is what makes coupling into a fibre lossy.

A laser diode adds a cavity and enough current to reach threshold. Below threshold it behaves like an LED. Above threshold, stimulated emission takes over and the output becomes narrow, bright and directional. Threshold current is the number every datasheet leads with, and it drifts upward as the device warms. That is why real transmitters carry a control loop watching the output and adjusting the drive.

The rate equations describe the interaction between the carriers being injected and the photons already in the cavity. They explain two behaviours you will actually see. There is a turn-on delay, because carriers must build up to threshold before any light appears. There is also relaxation oscillation, because photons and carriers overshoot each other before settling. That shows as ringing on a fast pulse.

The cavity supports a set of resonant wavelengths, and their spacing and count set the spectral width. Narrow width is what makes long links possible, so this connects straight back to material dispersion. A laser designed to run on a single one of those wavelengths spreads far less than one running on many.

At the receiving end, a photodiode absorbs a photon and produces a carrier pair. A PIN diode does exactly that and no more. Its wide undoped region gives most incoming photons somewhere to be absorbed usefully, and it responds quickly.

An avalanche photodiode adds internal gain. It is biased so hard that each generated carrier accelerates enough to knock more carriers free. Those knock free more, so one photon produces many carriers. That gain lifts the signal above the amplifier's noise, which improves sensitivity.

It is not free. The multiplication is a random process, so its output fluctuates more than plain amplification would, and that is excess noise. It also needs a high bias voltage. Worse, the gain drifts with temperature, so a receiver must control it actively. This is another gain-against-noise bargain of the kind this subject keeps posing.

Response time closes the topic. A detector cannot follow the signal faster than its carriers can cross the depleted region. It is also slowed by its own capacitance working against the load resistance. Raising the load resistance improves sensitivity and lowers bandwidth. The two cannot both be maximised, and the receiver topic makes that a design choice.

What you should now be able to explain or do

Compare an LED with a laser diode on spectral width, speed, coupling and temperature sensitivity. Say what threshold current is and why it needs a control loop. Explain turn-on delay and relaxation oscillation from the rate equations. Say how an avalanche photodiode gains sensitivity and what it costs. Explain what sets detector response time and how it trades against sensitivity.

Check yourself