Laser diode (semiconductor laser)
A laser whose gain comes from electrons and holes recombining in a forward-biased semiconductor junction, pumped directly by electric current. The light source of fiber communication, optical storage, pumping, sensing and most lasers made.
A laser diode is a p-n junction in a direct-bandgap III-V semiconductor, driven with forward current so that electrons and holes are injected into a thin active region where they recombine and emit light. Above a threshold carrier density the active region provides optical gain, and a cavity, in the simplest case the two cleaved facets of the chip, returns enough of the light for oscillation. Because the current pumps the gain directly, with no intermediate pump laser, and the chip is a fraction of a millimeter long, laser diodes outnumber every other kind of laser, and they also serve as the pumps for most of the others, including fiber lasers.
The modern device confines both carriers and light. A double heterojunction, a narrow-gap active layer between wider-gap cladding layers, traps the carriers and, because the narrower-gap material has the higher refractive index, guides the light along the same layer; this structure made room-temperature continuous operation possible in 1970, eight years after the first pulsed diode lasers of 1962. Current devices use one or more quantum wells as the active layer, a ridge or buried waveguide for lateral confinement, and material chosen for the wavelength: GaAs-based for 780 to 1100 nm, InP-based for the 1260 to 1700 nm telecom windows, and GaN-based for violet to green.
The cavity defines the type. A Fabry-Perot laser with cleaved facets oscillates on several longitudinal modes; a DFB laser builds a grating along the active region for a single frequency, and a DBR laser places the grating in passive sections that can be tuned; an external cavity returns light from outside the chip for narrow linewidth and wide tuning; and a VCSEL emits from the surface through mirrors grown above and below the active layer.
The characterization vocabulary follows from the light-current curve. Below threshold the output is weak spontaneous emission; above it, the power rises at the slope efficiency, for differential quantum efficiency . At 1310 nm, where the photon energy is 0.946 eV, a differential efficiency of 50% gives 0.473 W/A. Wall-plug efficiency divides optical output by electrical input and so also counts the voltage drop; the characteristic temperature describes how quickly threshold rises with heat. Spectral figures (side-mode suppression, linewidth) and noise (RIN) complete the set. How each is measured is covered in the laser characterization bench setup, reading a laser diode datasheet, and the T0 and wall-plug efficiency calculators.
References: L. A. Coldren, S. W. Corzine, M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012); G. P. Agrawal, N. K. Dutta, Semiconductor Lasers, 2nd ed. (Van Nostrand Reinhold, 1993).