Experimental study of optical frequency comb generation in gain-switched semiconductor lasers

A systematic experimental study of the generation of Optical Frequency Combs (OFCs) by gain switching two types of edge emitting semiconductor lasers (Distributed Feedback and Discrete Mode) is presented. High resolution spectral measurements together with linewidth characterization and the analysis of the temporal profiles and the radio-frequency spectra are used to evaluate the evolution of the quality of the OFCs, as a function of the driving conditions. Different types of emission spectra are identified. The physical origin of the OFC properties is discussed. The results indicate that the driving conditions rather than the device properties determine the OFC quality. The optical injection is demonstrated to either improve or deteriorate the quality of the OFCs generated by gain switching, depending on the injection conditions.