Abstract: An experimental and theoretical analysis of the effect of optical injection on optical frequency combs generated by gain-switching a single mode laser is performed. Combs with a frequency separation fR in the GHz range are generated by gain-switching a discrete mode laser (DML). The effect of optical injection on the comb has been analyzed for a wide range of the injected power and detuning of the injected master laser from the DML frequency. A rich variety of nonlinear behaviours is found when the injected power is increased for a fixed value of the detuning. At low injected powers two combs due to the gain-switched laser and the injected field are obtained with the same frequency spacing fR. As the injected power is increased, the comb from the gain-switched laser locks to the master laser. In the parameter space spanned by detuning and optical injection strength, we obtain several regions of locked combs with a tongue shape around detuning values given by multiples of the frequency spacing of the comb fR. When injected power is further increased combs with frequency spacing equal to rational fractions of fR are obtained. We also obtain irregular behaviour with low values of the ratio between the intensity of comb lines and noise level. Numerical simulations are in very good agreement with the experimental results.

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