A method and system for waveguide mode filters are disclosed and may include processing optical signals of a fundamental mode and higher-order modes by filtering the higher-order modes in rib waveguides in a photonic chip. The higher-order modes may be filtered utilizing doped regions and/or patterns in one or more slab sections in the rib waveguides. The patterns may be periodic or aperiodic along the rib waveguides. The higher-order modes may be filtered utilizing varying widths of slab sections, or doped, patterned, and/or salicided ridges on the slab sections in the rib waveguides. The higher-order modes may be attenuated by scattering and/or absorbing the modes. The chip may comprise a CMOS photonic chip.