We investigate the electroluminescence spectra of edge-emitting lasers having self-assembled quantum dots as the active medium. A broad laser emission is observed with a modulation of intensity corresponding to single or bunches (supermodes) of Fabry-Perot modes. The variation of the laser spectra with magnetic field shows that the supermodes originate from laser cavity effects and are not related directly to the electronic properties of the quantum dots. Measurements taken on devices of different cavity height, length, and lateral width indicate that the important parameter controlling the laser multimode emission is the cavity height, effectively the substrate thickness. In particular, the period of the supermodes is inversely proportional to this thickness, indicating that the modulation of the laser emission intensity is due to the leakage of modes into the transparent substrate.
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