Study On The Fabrications Of Layered Bragg Gratings And Their Properties In Wavelength Division Multiplexing

The techniques of dense wavelength-division multiplexing (DWDM) have made a large progress with the appearance of EDFA. But there emerge their own disadvantages of available WDM techniques in optical fiber system with the increasing of the channels. At the present time, volume Bragg gratings (VBGs) used for DWDM attract widely attention due to their excellent angle and wavelength selectivities. On basis of the study of photorefractive volume holographic grating, the multiple VBGs layered by planar waveguide array and the directly layered multiple VBGs are presented. Compared with photorefractive volume holographic grating, not only the layered VBGs have the same characteristics of common VBGs, but also the much more channels can be formed by the layered VBGs. Fabrication of layered Bragg gratings and their properties in DWDM are theoretically and experimentally investigated in the thesis. The main work in the thesis is as following:1. On basis of the Kogelnik's coupled wave theory, the Bragg diffraction efficiency, the angle selectivity and wavelength selectivity, and multiplexing techniques of volume holographic gratings formed in photorefractive crystals are analyzed.2. Under the different experimental conditions, the saturation values of modulation, the recording and erasure time constants are measured by recording single grating in a slice of LiNbO₃:Fe crystal. With exposure time schedules of subsequent gratings given by the preceding experiments, eight multiplexing gratings with the uniform diffraction efficiencies are recorded. The writing method and characteristics of gratings in a single planar waveguide are experimentally investigated, and four multiplexing gratings with the uniformity diffraction efficiencies are recorded in a single planner waveguide. Furthermore, the single grating is recorded in planar waveguide array.3. The Bragg matching and the uniformity of Bragg diffraction efficiencies of multiple volume holographic gratings (MVHGs), which are used for wavelength demultiplexing devices in optical communication based on all wave optical fiber, are theoretically studied and quantitatively analyzed. It is shown that the Bragg mismatching will be caused due to the recording medium dispersion during wavelength demultiplexing with wide spectra by employing MVHGs. Moreover the Bragg diffraction efficiencies of MVHGs with homogeneous refractive index modulation obtained at short writing wavelength will not be uniform at different optical communication wavelengths. And that has been experimentally demonstrated. For this reason, the improved dependence of writing angles on communication wavelengths for fabricating angle-multiplexing transmission MVHGs and the corrected formula for refractive index modulation of individual volume holographic grating at writing wavelength are presented in order to ensure rigorous Braggmatching and the uniformity of diffraction efficiencies.4. It is experimentally investigated that dependence of the angle and wavelength selectivity of photorefractive volume grating on the different crystal thicknesses, recording angles and the refractive index modulation. Experimental results show that the desired angle and wavelength selectivity of the volume gratings can be determined by the choices of the crystal thicknesses, recording angles and the refractive index modulation. Using the photorefractive volume gratings formed in LiNbCO₃ Fe crystal, the demultiplexings in the region of infrared optical communication and visible light have achieved. Furthermore, the four superimposed VBGs in two layers are recorded in a LiNbO3:Fe crystal. The diffraction efficiencies were obtained by respectively measuring the spectrum distribution of each layer gratings transmittance in infrared communication region. The feasibility of layered MVHGs applied to WDM system is experimentally confirmed.