Short note: Light sources in optical communication

A light source in optical communication converts the electrical signal into optical power for launching into the fibre. Semiconductor sources are used because they are small, efficient, directly modulated by current and emit at fibre wavelengths (850, 1310, 1550 nm).

Requirements

• Emission wavelength in the low-loss windows of the fibre
• Enough output power and efficient coupling (small emitting area)
• Narrow spectral width (less chromatic dispersion)
• High modulation speed, linearity, long life, low cost

Light emitting diode (LED)

• Forward-biased p-n junction (GaAlAs, InGaAsP); light by spontaneous emission.
• Types: surface-emitting (Burrus) and edge-emitting.
• Incoherent, wide spectrum (30–100 nm), wide beam, low power (µW to a few mW), bandwidth up to ~100 MHz.
• Simple, cheap, reliable, less temperature-sensitive. Used with multimode fibre for short links.

Laser diode (LD)

• Light by stimulated emission in an optical cavity (Fabry–Perot, DFB, DBR, VCSEL) once current exceeds a threshold.
• Coherent, narrow spectrum (1–5 nm; DFB < 0.1 nm), narrow beam, high power (tens of mW), modulation to tens of GHz.
• Costlier, needs temperature and power control. Used with single-mode fibre in long-haul, high-speed links.