Recording Methods And Demultiplexing Characteristics Of Layered Multiple Bragg Gratings

Dense wavelength division multiplexing(DWDM) is an important technology of choice for expanding capacity in optical fiber communication, and wavelength division multi/demultiplexers is one of the key elements in the fiber-optical networks. The volume Bragg grating is promising greatly to be applied to DWDM system and the demultiplexer based on the VBGs (volume Bragg gratings) shows that it can be used as one of advantageous multi-channel components because of its excellent angle and wavelength selectivities. A kind of multiple VBGs with layered structure and its fabrication method are presented here. According to the method, the layered multiple VBGs (volume Bragg gratings) were recorded in LiNbO₃:Fe crystals, and the infrared demultiplexing characteristics of the formed VBGs were measured and analyzed. The main contents are as follows:(1) The mechanisms of the photorefractive effect and the creation of photorefractive grating are explained. Based on the Kogelnik's coupled wave theory, the Bragg diffraction efficiency, the angle selectivity and wavelength selectivity of VBGs formed in photorefractive crystals are analyzed. According to the Bragg's diffraction formula, the relation of the demultiplexing angle on communication wavelengths to the writing angle of VBG, and the Bragg mismatching value during wavelength demultiplexing with wide spectra by employing VBG are discussed.(2) The spectral transmissivity and absorptivity for three slices of LiNbO₃ crystal were measured, and the optimal one among them was chosen for the writing experiment of VBGs. The holographic time constants were gained by recording and erasing a single grating in a slice of LiNbO₃:Fe crystal. According to the recording and erasing time constants, the exposure time schedule of superimposed gratings was achieved. It was experimentally investigated that the dependence of the diffraction efficiency of VBG on the refractive index modulation and the recording angle. The VBGs were recorded in the crystal by a parallel beam and a spherical beam, and the diffraction experiments were performed subsequently.(3) The recording approach of the layered multiple VBGs in LiNbO₃: Fe crystal was investigated in detail. Eight superimposed VBGs in three layers respectively were recorded alternately in a slice crystal, and the infrared filtering characteristics of 24 formed gratings were measured and analyzed. The experimental results show that the layered VBGs are promising to be employed for a multi-channel demultiplexer in the range of infrared communication.(4) According to the photorefractive properties of LiNbO₃:Fe crystal, the volume holographic grating with the grating vector normal to c-axis of the crystal are obtained hardly.A new method—light erasing method presented in this paper can be applied to fabricate this kind of grating. According to the method, the theoretical analysis as well as experimental investigation was accomplished orderly. The experiment results demonstrate that the light erasing method for fabricating volume holographic grating with grating vectors normal to c-axis in LiNbC>3 crystal is feasible.