A microstructured optical waveguide is formed to include a periodic sequence of "plugs" of optically active material within the inner cladding air tunnels. The plugs are utilized as a grating structure for generating resonant and periodic structures. The waveguide (in one embodiment, an optical fiber) is tunable by changing the spacing of the plugs (e.g., heating the structure, changing the pressure within the structure, etc.), or by modifying the initial spacing of the plugs during the formation of the microstructured optical waveguide (i.e., by modifying the "dipping frequency" of the waveguide into a reservoir of optically active material). In general, any number of different types of optically active material may be used to form the plugs, where two or more different materials may be used in the same structure, and introduced in an alternating fashion.