Abstract: We present experiments and numerical simulations of optical frequency combs generated by gain-switching a single mode laser diode when subject to optical injection. Our analysis focuses on the combs with a frequency separation fR in the GHz range. We present experimental maps in the parameter space spanned by the detuning and the strength of the optical injection that identify the boundaries between regions with different dynamical states. A rich variety of nonlinear behaviors including injection-locked, unlocked and irregular combs are observed and analyzed. The dynamical state corresponding to an injection locked comb, PN, is characterized by an optical phase oscillation with a well defined amplitude in such a way that repeats or changes in 2? with an N/f R period, where N is a natural number. Different regions of locked combs with a tongue shape around detuning values given by multiples of f R appear in the maps. P1 and P2 are the most widespread locked states for large values of the modulation amplitude. As this amplitude decreases a much larger variety of PN states with large values of N appear in small area regions of the map. Our numerical simulations are in very good agreement with experimental results.

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