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 is tunable by changing the spacing of the plugs , or by modifying the initial spacing of the plugs during the formation of the microstructured optical waveguide . 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 .