From LED to Laser
A semiconductor laser diode is essentially a p-n junction driven hard enough that stimulated emission dominates over spontaneous emission and absorption. Below threshold, the device behaves as an LED, emitting broadband incoherent light. At threshold, the round-trip optical gain exactly compensates mirror and internal losses, and the device transitions abruptly to lasing — producing intense, coherent, monochromatic light from a chip smaller than a grain of sand.
The L-I Curve
The light-current (L-I) characteristic is the fundamental laser diode measurement. Below threshold current I_th, optical power is negligible. Above threshold, output power rises linearly with slope efficiency η_s = η_d × hν/q, where η_d is the differential quantum efficiency. The sharpness of the threshold 'knee' indicates device quality — a soft knee suggests high non-radiative recombination or gain suppression.
Temperature Effects
Temperature is the laser diode's worst enemy. Threshold current rises exponentially with temperature as carriers gain thermal energy and leak out of the active region. The characteristic temperature T₀ quantifies this sensitivity: InGaAsP lasers at 1550 nm have T₀ ≈ 50–70 K (temperature-sensitive), while GaAs lasers at 850 nm achieve T₀ ≈ 120–200 K. Thermoelectric coolers maintain stable operation but add power consumption and cost.
Cavity Design
The Fabry-Perot cavity formed by the cleaved crystal facets determines the laser's spectral and threshold properties. Shorter cavities reduce threshold but increase mirror loss. Reflectivity of the uncoated semiconductor-air interface is about 30% (from Fresnel equations). High-reflectivity coatings on the back facet and anti-reflection coatings on the output facet optimize both threshold and slope efficiency simultaneously.