Three-dimensional nanostructures inside glasses, such as volume nanogratings (VNG), are known to be created by ultra-short laser interactions. These structures have many potential applications in photonics, material marking, nano-machining, etc. However, contrary to surface periodic structures, there are still no clear understanding of the physical origin of these structures. Therefore, a detailed 3D numerical modeling is performed combining both simulation of light propagation effects and the following laser-induced material excitation and modification [1-3]. Some of these processes occur simultaneously (i. e., propagation and ionization), whereas others are much longer (thermal-mechanical effects) and can be taken into account separately. It is demonstrated that light propagation in the presence of nano-defects considerably affects femtosecond laser interactions with transparent materials. In particular, volume nanograting formation induced by femtosecond laser irradiation of fused silica with the presence of randomly distributed nanometric defects, such as holes or nanoparticles, is observed. It is found that local field enhancement leads to formation of nanoplasmas. The characteristics of these nanoplasmas are shown to depend on the concentration of the initial defects and on the irradiation wavelength, in agreement with several experimental observations. The obtained VNG formation can be attributed to two different effects as follows (i) to an interference if the incident laser wave with the scattered one if a sharp interface is present, or (ii) to an intereference of multipole scattered waves in the case of smooth defect concentration profile. References [1] A. Rudenko, J-Ph. Colombier, T. E. Itina, From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser, Phys. Rev. B (2016) . [2] A. Rudenko, J-Ph. Colombier, T. E. Itina, Femtosecond laser irradiation of dielectric materials containing randomly-arranged nanoparticles, SPIE LASE 2016, (2016). [3] A. Rudenko, J-Ph. Colombier, T. E. Itina, Femtosecond laser irradiation of fused silica with a nanometric inhomogeneity, PIERS Proceedings, 1652 - 1656, July 6-9, Prague, (2015).