A planar optical apparatus for setting the chromatic dispersion in an optical system, which apparatus has: a waveguide input structure with a first free-beam region, a first phase grating, connected to the first free-beam region, for spatially separating the spectral components of a signal, a second free-beam region, connected to the first phase grating, an optical element for spatially dependent modification of the phase of the spatially separated spectral components in the second free-beam region, a second phase grating, connected to the second free-beam region, for combining the spectral components, with their phase modified, and a waveguide output structure with a third free-beam region. In this case, the optical element for spatially dependent modification of the phase of the spatially separated spectral components includes a first phase plate that can be displaced in the second free-beam region.