Linewidth Sharpening in Optical Frequency Combs via a Gain Switched Semiconductor Laser with External Optical Feedback

In this paper, we propose a novel scheme for an optical frequency comb (OFC) generator which implements a gain switched (GS) semiconductor laser (SL) in the presence of external optical feedback (EOF). Numerical results demonstrate that with appropriate choices of the basic system parameters (i.e., the feedback coupling fraction, the laser bias current and the external cavity length), the linewidths in the OFCs generated by the GS SL with EOF can be significantly reduced below that of the free running continuous wave (CW) SL. Such a significant linewidth reduction can be described by a simple analytic expression presented in this paper which is found to be in good agreement with the numerical results.