Fabry-Perot lasers still comprise the hulk of lasers used in optical fiber systems, The spectral envelope, of the Fabry-Perot modes, can be modulated either deliberately or as a consequence of processing stages, This can be beneficial, in the case of modal sculpturing where specific Fabry-Perot modes are suppressed, or a hindrance in the case of poor devices, A time-domain model is used to model 1.3-mu m Fabry-Perot lasers, Simulated power conserving reflective sites are introduced between sections in the model to simulate the effect of reflective sites from etch pits on the output characteristics of real lasers, Spectral modulation of the laser output is reported in agreement with previous experimental results and the strength of reflections required is investigated, We also report the use of the model to investigate the effect of fiber dispersion on the modulated laser output with different spectral mode modulation.