A method of optimizing a filter response of an arrayed waveguide grating is disclosed. The method includes the step of measuring a respective phase error of a plurality of waveguide cores of the arrayed waveguide grating. Once the phase error is measured, a respective optical path length of the cores is adjusted in accordance with the respective phase error of the cores. Optical path length is adjusted by adjusting a respective refractive index of the cores. The respective optical path length can be controlled to nanometer accuracies by adjusting the respective refractive index of the cores.