We investigated optical properties of a polymer dispersed liquid crystal (PDLC) film in-plane switched by a periodic pattern of interdigitated electrodes. Such a system acts as a thin optical phase grating which diffraction efficiency strongly depends on polarization state of the incident light. The intensity and dynamic response of various diffraction orders were analyzed as a function of the applied voltage amplitude. Our results demonstrate that by using a PDLC very effective phase gratings with response times below 10ms can be achieved at the expense of slightly higher driving fields than needed for the similar bulk nematic liquid crystal gratings.