Research Of Diffraction Efficiency In Double Doped Mg:Fe:LN Crystal

Lithium niobate crystal is a nonlinear optical crystals with special structure. The structure and photorefractive properties of lithium niobate crystal can be changed by doping different elements. The photonic lattices made from iron-doped lithium niobate crystals(Fe:LN) has the advantages of quick reaction and high diffraction efficiency, but the photo damage resistance is weak. While magnesium-doped lithium niobate crystals(Mg:LN) has photo damage resistance. In our study we use double doped Mg:Fe:LN crystal not only improve the reaction of LN crystal, but also enhance the photo damage resistance. We usually use light-induced method to make the ideal photonic lattices in the experiments, and then make the corresponding photonic crystal devices. The photonic crystal devices has a good application prospects in the control the spread of light. The key of using light-induced method to making photonic lattices is to find the way of making large-area little-period and higher diffraction efficiency photonic crystal.In this study, laser with different wavelength is respectively used to write one-dimensional photonic lattice in Fe:LN and double doped Mg:Fe:LN crystal, the results are contrastively analyzed and the suitable crystals are selected. The photonic lattice with little-period and higher diffraction efficiency has greater application prospects. We control the periods of written photonic lattice by changing the incident angle. When writing one-dimensional photonic lattice in double doped Mg:Fe:LN crystal, record the diffraction efficiency curve varying with writing time in different angles; record and analyze the influence of the special second-harmonic and the scattering light fanning on the diffraction efficiency. The completed work as follows:1. A 488 nm semiconductor laser and a 532 nm Nd:YAG solid laser are respectively used to write one-dimensional photonic lattice in double doped Mg:Fe:LN(Fe:0.03wt%, Mg:2.0mol%) and Fe:LN(Fe:0.03wt%) crystal with the same doped Fe concentration. The relationship between the maximum diffraction efficiency and time of the photonic lattice in the crystal is compared and researched. Experimental results show that the photonic lattice writing time in double doped Mg:Fe:LN crystal is shorter than that of Fe:LN crystal and the diffraction efficiency is higher than that of Fe:LN crystal.2. In order to improve the diffraction efficiency of light-induced little-period photonic lattices, we control the periods of written photonic lattice by changing the angle between two induced beam when writing one-dimensional photonic lattice in double doped Mg:Fe:LN crystal, record the diffraction efficiency curve varying with writing time in different angles. The results show that the diffraction efficiency of double doped Mg:Fe:LN crystal drops sharply when the angle between two induced beam is more than 24°; The intensity oscillation caused by special second-harmonic increasing the diffraction efficiency of photonic lattices; Greater angle between two induced beam makes shorter time of intensity oscillation.3. By observe the change of diffraction efficiency, generation of special second-harmonic and the formation of scattering light fanning during writing process of one-dimensional lattice. Combined with the theoretical analyses the impact of special second-harmonic and scattering light fanning on of photonic lattice, the writing is divided into three processes: writing-in of one-dimensional photonic crystal lattice; the formation of special second-harmonic; the saturated and split of special second-harmonic. The impact of special second-harmonic on diffraction efficiency is mainly concentrated in the third process, the split of interference fringe make oscillation of the diffraction efficiency. Scattering light fanning have the overall strengthening effect on diffraction efficiency.