Hi All, could someone please clarify how to observe a phase shift, that is induced by a moving diffraction grating? 

In the layout (see the image), the grating is moved parallel the light source. The defined diffraction orders of the grating are +1 and -1. By recombining the diffracted light, I would expect intensity change due to interference as a function of a gratings position. However, this is not the case, and the intensity remains constant (see the graph).