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Different approaches based on the α-factor to model the phase of the optical fields in bulk semiconductor optical amplifiers are analyzed. Our time-domain numerical model is presented and it is calibrated by means of experimental characterizations of the device gain, of the amplified spontaneous emission spectra, and of the effective α-factor. We then compare the phase modulation at the SOA output for three cases. In the first one, the α-factor is always taken as constant. In the second case, the α-factor is set according to the SOA working point and then kept fixed. In the last case, we take into account the dependence of the α-factor on the carrier density and on the wavelength for every simulated time step. We shall identify when the first two approximations listed above deliver reliable results. In particular cases, the use of a constant value for the α-factor can lead to phase errors up to 50%.