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Summary form only given. The optical impedance mismatch experienced by light propagating across high-low index waveguide-photonic crystal (PC) interface is one of the main issues regarding the application of photonic crystals in integrated optics. This strongly affects the mode coupling efficiency and the polarization insensitivity. We have demonstrated the decrease of impedance in PCs having graded effective index interfaces obtained by modulating the air filling fraction at the input and/or output. Mode propagation across, the resulting asymmetric and/or symmetric graded index interfaces was carefully measured by using a systematic experimental technique. This analysis also allows us to track the propagation of a laser pulse through the modulated structure. The experimental results showed the interplay between efficiency and photonic band gap transmission extinction as a function of the size of the graded index area. They confirm the strong potential for designing PC-waveguides interfaces with reduced loss.