An oscillatory characteristic of diffraction is observed during holographic recording period in an oxidized LiNbO<sub>3</sub>:Cr:Cu
crystal with 514 nm green light as the recording light and 390 nm UV light as the sensitizing light. The optimal
switching time from the recording step to the fixing step for high diffraction of a fixed hologram is studied. It is shown
that switching after the first diffraction maximum leads to higher fixed diffraction efficiency. The theoretical explanation
is presented according to time-space dynamic theory of the nonvolatile holographic recording in doubly-doped LiNbO<sub>3</sub>
Two-dimensional phase unwrapping (PhU) is one of the most important processing steps in optical phase-shifting interferometry. It aims to reconstruct the continuous phase field, but in fact, it becomes very difficult to perform the PhU due to the existence of noise, low modulation, under sampling, discontinuities or other defects. For solving the problem, we present a new PhU method which is based on the local parameter-guided fitting plane. It relies on the basic plane-approximated assumption for phase value of local pixels and is guided by our proposed parameter map- Modulation-Gradient and Pseudo-Correlation (MGPC) parameter map. This new map is the combination of fringe modulation and wrapped phase, thus it's reliable in estimating the quality or goodness of phase data. Meanwhile, the adoption of fitting plane for the 3×3 window of pixels makes the process of PhU very fast. In applications, we offer the simulated and the experimental data to compare our method with the two previously reported path-following unwrapping algorithms. The unwrapped results show our proposed PhU method not only is efficient, but also has higher noise robustness in recovering the true phase field.
Holographic recording experiments of doubly-doped LiNbO<sub>3</sub>:Fe:Ni crystals were conducted by three kinds of different two-color recording schemes. The results show that the saturation diffraction efficiency, the fixing diffraction efficiency, and the recording sensitivity of oxidized LiNbO<sub>3</sub>:Fe:Ni crystal are higher than those of other reported doubly-doped LiNbO<sub>3</sub> crystals. Based on the doped energy-band diagram, the effect of microcosmic optical parameter of the deep trap center on holographic recording properties of doubly-doped LiNbO<sub>3</sub> is analyzed theoretically. LiNbO<sub>3</sub>:Fe:Ni has the potential of being a new highly efficient nonvolatile holographic recording material.
Dopants in doubly doped lithium niobate crystals are crucial for properties of nonvolatile holographic recording. In our experiments, a series of possible doubly doped congruent LiNbO3:Fe:X (X=Mn, Cu, Rh, Ru, Ni) are proposed and investigated for nonvolatile holographic recording. The experimental results demonstrate that the dopants with different distances of energy band to conduction band have different recording efficiency. Further analysis approve that the dopants which have moderate energy-band distance to conduction band can perform recording with both high diffraction efficiency and long lifetime. The compared results show that LiNbO3:Fe:Ni is promising for nonvolatile holographic recording, and its recording conditions are optimized.