Recording method and fabrication of layered multiple volume holographic gratings for WDM

For application to optical telecommunication band of layered multiple volume holographic gratings (VHG), the issue of Bragg match and the uniformity of diffraction efficiency are analyzed. Several approaches to fabricate multiple layered VHGs in LiNbO₃:Fe crystal are illustrated. Three layer gratings with 1 mm thickness, 2 mm space and four gratings superimposed in each layer were written in a LiNbO₃:Fe crystal by employing angle and space multiplexing techniques via two sheet beams interference. Measurement and analysis shows that twelve gratings in three layers with homogeneous diffraction efficiency and about 3 nm channel space were obtained.