We present a new principle for tuning the diffraction efficiency of an optical grating and its implementation in a micro-optical device. The overlap of two phase gratings is used to vary the effective phase shift and hence the diffraction efficiency. We study the working principle using Fourier Optics to simulate the diffraction pattern in the far field and design and realize a device based on integrated piezo actuation. We find good agreement between simulation and experiment and observe a suppression of the first diffraction order intensity by more than 97% and response times of less than 3 ms.