We present a theoretical method for calculating small-signal modulation responses and noise spectra of active Fabry-Perot semiconductor waveguides with external light injection. Small-signal responses due to either a modulation of the pump current or due to an optical amplitude or phase modulation of the input field can be calculated. Both responses and noise spectra are given through semianalytical expressions taking into account the longitudinal extent and finite end-facet reflectivities of the active device. Different examples of responses and spectra are presented for semiconductor optical amplifiers and an injection-locked laser. We also demonstrate the applicability of the method to analyze slow and fast light effects in semiconductor waveguides. Finite reflectivities of the facets are found to influence the phase changes of the injected microwave-modulated light.