The main control parameters of a single mode semiconductor laser submitted to an injected external signal are the power and the frequency of the injected signal. Following their magnitude, many phenomena can be observed such as phase locking, frequency locking, frequency generation, push-pull effects, hysteresis phenomena and chaos,... We show here that the spectral signature of the slave laser enables a better understanding of the the nonlinear interaction between the two competing sources: the spontaneous emission and the external field for which spectra are equally amplified through the active medium. This amplification is then strongly dependent on their coherency. We describe the role of the injected laser as a filter and an amplifier. It follows that the laser can be used to process information in ways that are not yet completely exploited.