This paper focuses on numerical analysis of the technological imperfections of the Bragg grating inscription process in the fiber. For such purposes, a Rouard method that allows for simulating a Bragg gratings section-by-section has been employed. The analysis presented in the paper includes considerations on the period instability of the grating together with variance of induced by UV exposure changes of the refractive index. As a result, an impact of these imperfections on the spectral responses has been examined, for both Gauss and Johnson distributions. The general motivation of this work is a proposition for novel process of the phase mask fabrication, what is also described in the following paper.