[en] The purpose of this paper is to understand the influence of laser annealing on the index of refraction of Al-doped ferrimagnetic garnet films in the near-infrared domain. It has been shown recently that a similar effect takes place in Ga-doped garnet films. According to theoretical predictions based on the photoelastic origin of the changes of the properties of a film after the local heat treatment, the effect of the variation of the index of refraction in Al-doped films should be several times greater than in the case of Ga-doped films. The experiment was carried out on Y_1.73Bi_1.23Fe_3.69Al_1.31O₁₂ films. Phase diffraction gratings were fabricated in the films by means of laser annealing and thus a diffraction pattern of optical waveguide modes was analyzed with a photodetector. It has been shown that variations of the index of refraction as great as 10^-3 were achieved. Thus it was demonstrated that the investigated effect is much stronger in Al-doped films compared to Ga-doped films. As in Ga-doped films, the dependence of the efficiency of diffraction on the number of an optical waveguide mode has been observed. That confirms the idea that the local value of the film thickness is changed as a consequence of the variation of the lattice parameter by heat treatment