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A method for modeling the mode-coupling process in strongly modulated long period gratings (LPGs) is reported. The method is based on calculating the variations of local-mode profiles and propagation constants over the perturbed regions and solving the coupled local-mode equations to obtain a quantitative description of the intermodal energy exchange. The mode-coupling process and the spectral characteristics of a CO2 laser-inscribed LPG in a photonic crystal fiber are numerically modeled and found in good agreement with the experimentally measured results. Compared with the methods based on the conventional coupled-mode and the mode-projection theories, the current method provides a more accurate description of the mode coupling process for LPGs with strong but slow-varying perturbations.