Laser Diodes

The fundamental difference between an LED and a laser diode lies not in the basic structure – both are based on a pn junction made from a direct-bandgap semiconductor – but in the emission mechanism. An LED generates light through spontaneous emission: an excited electron recombines with a hole at a random point in time, and the resulting photon has a random phase and direction. A laser, by contrast, relies on stimulated emission: a photon that already exists strikes an excited electron-hole pair and triggers its recombination – the newly generated photon is then identical in phase, direction, and wavelength to the triggering photon. This amplification mechanism (Light Amplification by Stimulated Emission of Radiation) produces coherent light, in contrast to the incoherent light of an LED.

For stimulated emission to dominate over absorption, the active medium must exhibit population inversion: more electrons must occupy the higher-energy conduction band state than the lower valence band state – a condition that never occurs in thermal equilibrium and must therefore be actively forced through high injection current density. In semiconductor lasers, the strongly forward-biased pn junction performs this role: above a certain threshold current, the ratio flips, optical gain sets in, and spontaneous emission increasingly gives way to stimulated emission.

Population inversion alone doesn't yet produce a laser – only an optically amplifying medium. The actual lasing effect additionally requires feedback: an optical resonator, usually realized as two parallel, partially reflective mirror facets on either side of the active region. In simple edge emitters, these mirrors form from the semiconductor's natural crystal cleavage planes (fracture surfaces along specific crystal planes), which already provide a reflectivity of 30–35% due to the refractive-index step to air – enough to circulate light repeatedly through the gain medium.

Only light whose wavelength forms an integer multiple of the resonator length (a standing wave between the mirrors) is amplified constructively – all other wavelengths interfere destructively and are suppressed. This produces a laser's characteristic longitudinal mode structure: instead of a broad, continuous spectrum as with an LED, a comb of sharp, closely spaced emission lines appears, with spacing set by the resonator length. Once the optical gain within the resonator exceeds the sum of all losses (mirror transmission, internal absorption, scattering), laser oscillation begins – the threshold current I_th marks exactly this transition point and is one of the most important parameters of any laser diode.