Short note: Optical source and detector

An optical fibre link needs a light source to convert the electrical signal into light at the transmitter and a photodetector to convert light back into an electrical signal at the receiver.

Optical sources

Requirements: emission at fibre low-loss wavelengths (850, 1310, 1550 nm), small emitting area, enough power, fast modulation, narrow spectrum, long life and low cost. Two semiconductor sources are used:

• LED (light emitting diode): forward-biased p-n junction giving spontaneous emission. Incoherent light, wide spectral width (30–60 nm), low power, bandwidth up to ~100 MHz. Cheap and reliable; used with multimode fibre over short distances (LANs).
• Laser diode (LD): stimulated emission in a cavity above a threshold current. Coherent, narrow linewidth (1–3 nm, or < 0.1 nm for DFB lasers), higher power, GHz modulation. Used with single-mode fibre in long-haul high-speed links.

Photodetectors

Requirements: high responsivity at the operating wavelength, fast response, low noise, low bias voltage.

• PIN photodiode: an intrinsic layer between p and n regions widens the depletion zone, so most photons are absorbed there. No internal gain, low noise, fast, cheap.
• Avalanche photodiode (APD): high reverse bias causes impact ionisation, giving internal gain (M ≈ 10–100). More sensitive, used for long links, but needs high voltage and adds excess noise.