A semiconductor laser uses a semiconductor as its laser gain medium. Pumping produces carrier populations that permit stimulated emission between suitable electronic states. An optical resonator supplies feedback, and oscillation begins when the gain compensates round-trip losses.
A laser diode supplies pumping by electrical injection; other semiconductor lasers can be optically pumped. The band gap, confinement and resonator determine the possible emission range. A measurement of optical output versus pump strength helps locate the laser threshold, while an emission spectrum tests the modes involved. Increasing output alone does not establish that the device has entered coherent laser oscillation.
For a vertical optical transition between parabolic energy bands with common carrier temperature, net stimulated emission has occupation factor . The monotonic Fermi-Dirac distribution makes this positive exactly when the quasi-Fermi-level splitting exceeds the transition photon energy. An allowed transition must also lie above the band gap. Broadening, band degeneracy and a zero-temperature occupation window require their own qualifications; this condition alone does not establish laser threshold.
A laser diode is an electrically injected semiconductor laser. Forward biasing supplies electrons and electron holes to an active region, where radiative recombination can amplify a compatible optical mode by stimulated emission.
Reflecting facets or other cavity structures provide feedback. Below the laser threshold, emission is dominated by spontaneous processes; above it, stimulated emission into resonator modes becomes important. A light-emitting diode can also use carrier recombination but does not require this feedback-and-gain threshold. Output power, emitted spectrum and terminal current-voltage characteristic together characterize the device more fully than brightness alone.
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