Lineshape measurement of semiconductor lasers below threshold

A novel heterodyne method for measuring the linewidth ranging from 100 MHz to 100 GHz is developed. The line shape of 1.3- mu m DFB (distributed-feedback) lasers biased below threshold is measured by the use of this method. The center-frequency shift and the linewidth are determined as functions of the bias current, showing linear dependencies on the bias current. From this result, the linewidth enhancement factor is estimated.>