Understanding VCSEL-Based Gain Switching Optical Frequency Combs: Experimental Study of Polarization Dynamics

In this paper, we carry out an experimental study on the polarization properties of a vertical-cavity surface-emitting laser (VCSEL) working under gain switching (GS) regime and the characteristics of the resulting optical frequency comb signal. We have observed that each of the two polarization modes presented in the VCSEL continuous wave emission spectrum generate a separate optical frequency comb (OFC) whose modes are phase correlated thanks to the GS regime. We study how these combs associated with the main and orthogonal polarization modes, respectively, vary depending on the input parameters to the VCSEL (bias current and radio-frequency power and frequency). The correlation between both OFCs in the best operation point as defined by the OFC characteristics is also evaluated. Therefore, this study demonstrates that two orthogonally polarized combs are generated that exhibit a high correlation between each other that combine to produce a wider overall optical comb. Hence, we can predict the feasibility of dual-polarization VCSEL-based OFC generators in the few gigahertz repetition frequency rates, with highly correlated modes and continuously tunable distance between them, in a compact and energy- and cost-efficient system that can find application in ultrafast laser dynamics studies and or in polarization-division multiplexing optical communications.