We present the theoretical study and the simulation of the optical field propagation in a guided wave tuneable optical filter using a composite material grating. A numerical analysis of the optical propagation has been performed by using a 3D semivectorial BPM (Beam Propagation Method) based software and the results have been compared with a matrix-transfer approach based on the effective index method. The dependency of the filter response on the index modulation depth (An), the length and the thickness of the grating have been also analyzed. Performance of chirped and apodized gratings were also studied. The optical filter reflection and transmission spectral responses in terms of full width at half maximum (FWHM) have been calculated.