We　experimentally　demonstrate　how　to　accurately　retrieve　the　refractive　index　profile　of　photonic　structures　by　standard　diffraction　experiments　and　use　of　the　rigorous　coupled-wave　analysis　in　the　multi-wave　coupling　regime,　without　the　need　for　taking　any　auxiliary　data.　In　particular,　we　show　how　the　phases　of　the　Fourier　components　of　a　periodic　structure　can　be　fully　recovered　by　deliberately　choosing　a　probe　wavelength　of　the　diffracting　radiation　much　smaller　than　the　lattice　constant　of　the　structure.　In　the　course　of　our　demonstration,　we　accurately　determine　the　slight　asymmetry　of　the　structure　of　nanocomposite　phase　gratings　by　light　and　neutron　diffraction　measurements.　(C)　2021　Optical　Society　of　America　under　the　terms　of　the　OSA　Open　Access　Publishing　Agreement