To assess the crystallinity degree for Ge2Sb2Te5 (GST) films the method of reflection and transmission photometry and the approximation of an effective medium of the Brueggemann type were used. It was found that the changes in the energy and duration of the laser pulse lead to the change in the ratio of crystalline and amorphous fractions and change in crystallinity degree. It was found that an increase of crystalline fraction leads to an increase of refractive index and extinction coefficient at telecommunication wavelength of 1550 nm, which also leads to changes of reflectivity and transmissivity of the GST film and can be used to perform modulation and switching of signals.