Anomalous linewidth narrowing in microwave beat frequency spectra between adjacent longitudinal modes in semiconductor lasers

Microwave frequency signals resulting from the beats between adjacent longitudinal modes in long-cavity semiconductor lasers have been experimentally observed and numerically modeled by a rate-equation simulation. A periodic collapse of the beat frequency linewidth is observed and attributed to the onset of intermodal correlations in mode competition noise for values of bias current very close to those at which two adjacent longitudinal modes exhibit equal gain.