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We report optimal phase modulation based on enhanced electro-optic effects in a Mach-Zehnder (MZ) modulator constructed by AlGaAs/GaAs coupled double quantum well (CDQW) waveguides with optical gain.The net change of refractive indexes between two arms of the CDQW MZ modulator is derived by both the electronic polarization method and the normal-surface method.The numerical results show that very large refractive index change over 10-1 can be obtained,making the phase modulation in the CDQW MZ modulator very highly efficient.It is desirable and important that a very small voltage-length product for π phase shift,Vπ × Lo =0.0226 V.mm,is obtained by optimizing bias electric field and CDQW structural parameters,which is about seven times smaller than that in single quantum-well MZ modulators.These properties open an avenue for CDQW nanostructures in device applications such as electro-optical switches and phase modulators.