Optogenetic fMRI is an attractive tool for studying neuronal manipulation dynamics at multiple scales, but currently lacks techniques for network (i.e. mesoscale) level investigations due to the small illumination volume of traditional flat-faced optical fibers. Here we show that a recently developed tapered fiber design allowed simultaneous, artefact-free illumination of large cortico-hippocampal areas. We also demonstrated that low-frequency cortico-hippocampal entrainment resulted in a prominent functional desynchronization of the mouse default mode network. These results support the use of this technique for deconstructing network-scale substrates and probing brain-wide functional network dynamics.