Fabrication method of multiple VBGs with directly layered structure and its experimental demonstration

With the development of the optical technology, wavelength division multiplexing (WDM) is becoming an important technology for the enhancement of bandwidths of fiber-optical networks. Bragg gratings utilize the fact that light is diffracted from a grating only if the Bragg condition is fulfilled. In the paper a new kind of measure called directly layered multiple VBGs (volume Bragg gratings) is presented, which is fabricated by photorefractive volume holographic technology, and the multiple VBGs fabricated by this method with layered structure is more compact, the corresponding system could, more sufficiently, utilize the volume of photorefractive lithium niobate crystal. In our experiment the VBGs can be formed by illumination of interference field of two thin sheet beams expanded and collimated through two cylindrical lenses, and then multiple VBGs can be accomplished at the same location by angle-multiplexing technique. And then the layered structure can be formed by space-multiplexing holographic technique, i.e. after the fabrication of the previous layer, the following layer is able to be created by above multiple illuminations through moving crystal along the sheet beam normal with a certain space. The steady layered multiple VBGs in LiNbO3:Fe crystal can be achieved through thermal fixing technique. Several layered structures are fabricated, that are respectively composed of four superimposed VBGs in LiNbO₃ crystal. The spectrum distribution of each layer gratings transmittance was measured by utilizing Ando AQ6317C optical spectrum analyzer.