A semiconductor diode laser works by applying a forward bias across the junction. This allows the electrons and the holes to recombine. They release energy equal to the band gap in the form of electromagnetic radiation, light. The linewidth of this spontaneous emission is on the order of several wavenumbers.
||Schematic of the conduction and valence bands, with a band gap of energy, E.|
||Diagram of recombination of electrons with holes, emitting a photon whose energy is that of the band gap, thus defining the frequency of the light.|
Once a certain current across the diode has passed some critical value, unique to each type of diode, the intensity of the radiation is great enough so that multiple reflections can cause lasing to occur. A temperature change can cause the energy gap to expand through normal thermodynamics thus increasing the energy of the light emitted from the diode cavity.
Because of the fixed cavity length, different wavelengths can exist as standing waves inside the cavity. As a result, different longitudinal modes can be viewed in the output once the current has passed a certain threshold current. As the current is increased, the spectrum should collapse into a single mode.
1.) Demtröder, W. Laser Spectroscopy: Basic Concepts and Instrumentation. Springer-Verlag Berlin Heidelberg. Germany. 1988.
(see also references in intro.)
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