PE1-2.3 Optical Sources & Detectors
You can compare LED and injection laser diode structures, materials and quantum efficiency, describe laser threshold and rate equations, and compare PIN and APD photodetectors.
The LED-versus-laser choice is the transmitter design decision: LEDs are cheap, broad-spectrum and slow, lasers are narrow, fast and temperature-sensitive, and the link's reach usually decides it. Threshold current is the laser's defining number and it drifts with temperature, which is why real transmitters carry control loops. On the receiving side, the APD's internal gain buys sensitivity at the cost of excess noise and a high bias voltage — another gain-against-noise bargain of the kind this whole subject keeps posing.
Work through these
LED structures, materials and quantum efficiency
The simpler source, its material system and how efficiently it converts current to light. Its broad spectrum is the limitation the laser below removes.
LED power and modulation
How much light comes out and how fast it can be switched. The modulation bandwidth is what decides whether this source is usable for a given data rate.
Injection laser diodes: modes and threshold conditions
The coherent source, and the condition for it to lase at all. Threshold current is the number every datasheet leads with and the one that varies with temperature.
External quantum efficiency; laser diode rate equations
How efficiently it turns current into output light, and the equations that describe the dynamics. The rate equations explain turn-on delay and relaxation oscillation.
Resonant frequencies
Which wavelengths the cavity supports, which sets the spectral width. Narrow width is what makes long links possible, so this connects straight back to dispersion.
Physical principles of PIN and APD detectors
The two detector types and how they work. The avalanche one has internal gain, which is the whole reason for the comparison that closes this topic.
Detector response time
How quickly the detector can follow the signal, set by transit time and capacitance. It is the receiver's bandwidth limit and it interacts with the load resistance.
Temperature effect on avalanche gain; comparison of photodetectors
Why avalanche gain drifts with temperature, and the summary comparison. The gain has to be controlled actively, which is a real cost in a receiver design.
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