The disclosure provides a method of forming an erasable optical coupler in a photonic device comprising a conventional optical waveguide formed in a crystalline wafer. The method comprises selectively implanting ions in a localized region of the wafer material adjacent to the conventional waveguide of the photonic device, to cause modification of the crystal lattice structure of, and a change in refractive index in, the ion implanted region of the wafer material to thereby form an ion implanted waveguide optically coupled to the adjacent conventional waveguide to couple light out therefrom, or in thereto. The crystalline wafer material and ion implanted waveguide are such that the crystal lattice structure or composition can be modified to adjust or remove the optical coupling with the conventional waveguide by further modification of the refractive index in the ion implanted region.