Experimental investigation of the dependence of compact optical frequency comb generated by direct modulation of semiconductor laser on the modulation waveform and mode

Abstract

Optical frequency combs (OFCs) generated by direct injection modulation of semiconductor lasers are attractive for their compactness, simplicity, and low cost. We investigated the dependence of the performance of this type of OFC on the modulation waveform and mode on a distributed feedback laser diode (DFB-LD). OFCs can be achieved with various modulation waveforms (sinusoidal, triangular, sawtooth, and sinc) in continuous wave (CW) mode without the limitation of modulation frequency higher than the relaxation oscillation frequency (ROF). In gain-switching (GS) mode, however, this limitation can be avoided by narrow-pulse modulation with frequency lower than the ROF for high spectral resolution, and have a better performance than that in CW mode. We experimentally demonstrated that OFCs generated by pulse modulation have the best performances over various modulation waveforms mentioned above. Specifically, the OFC parameters improvements of carrier-to-noise ratio (CNR) and spectral width by pulse modulation over common used sinusoidal modulation are 37% and 30% respectively. Further, the OFC performance becomes better with increasing modulation frequency in GS mode. We also evaluated the spectral linewidth of OFC being very closed to that of constant operation, which is the absolute highest limit of spectral resolution of an OFC whether in CW or GS mode. Thus, the requirement for high OFC performance of spectral resolution, CNR, and spectral width could be a simple compromising of modulation waveform, frequency, and working mode on a semiconductor diode laser with our investigation. Optimizing of OFCs performance is achieved by changing the waveform or parameters of the modulating signal during gain-switched laser. This method not only reduces the system cost and complexity, but also improves the flexibility of the system.