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In this Letter, an experimental analysis for coherent multiharmonic signal generation in quantum cascade lasers is presented. The underlying principle relies on the direct modulation of the laser with a large sinusoidal wave to drive the laser above and below threshold. This generates a multiharmonic optical signal in the frequency domain as the device is forced to respond in a nonlinear fashion. The impact of the modulation parameters is assessed in the resultant optical spectrum of a continuous-wave distributed feedback quantum cascade laser. Despite not creating pulses due to the ultrafast dynamics of the laser, the results reveal that a comb structure with uniform line spacing and high phase correlation between teeth can be straightforwardly generated.