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Ultrafast all-optical cross phase modulation (XPM) was enhanced in the InGaAs/AlAsSb coupled quantum wells using new InAlAs coupling barrier. Furthermore, a high XPM efficiency of 0.15 rad/pJ, which is approximately three times as large as that of a previous sample with the AlAs coupling barrier, was obtained by realizing the strong optical confinement in a narrower waveguide. From the analysis of the quantum levels and the measurement of the absorption spectra, the enhanced XPM efficiency was suggested to be contributed by the refractive index dispersion of the interband transition that was modulated by the intersubband transitions (ISBTs). This large XPM efficiency is expected to give a higher performance in Mach-Zehnder interferometer-type ultrafast all-optical switch.