For advanced photonics networks all kind of optical devices are being developed with the aim of routing high speed optical signals. The multimode interference MMI principle is becoming important because these devices have a simple configuration, compactness and are suitable for integration. MMI couplers based on subwavelength periodical gratings and silicon nanowires are here analyzed based the coupled-mode formulation. Several geometrical configurations were analyzed as a function of the operating wavelength, by varying the width/radius, the period, the segment length (dutycycle) and the nanowires radius. It was observed that for some specific configurations a wavelength independent behavior can be obtained.