A dynamically tunable filter controls the magnitude of couplings between core and cladding modes of a waveguide by surrounding the waveguide with an overcladding having an adjustable refractive index. The coupled modes are attenuated along the core to produce the desired spectral response. The adjustment to the overcladding index is made in a range that is above the refractive index of the underlying cladding to vary amplitudes of attenuated bands of wavelengths without shifting central wavelengths of the bands.