Photorefractive crystals have been deeply studied for holographic data storage. A number of approaches have been
studied to improve the storage properties of such materials. In particular, methods to make the photorefractive gratings
nonvolatile, i.e., insensitive to erasure during readout and during storage in the dark, have been developed. Doubly
doped lithium niobate crystals can realize nonvolatile holographic recording by a real time and all optical processing,
which have become a topic of great current interest. Sensitive light with short wavelength, such as UV light, and
recording light are simultaneously applied in the recording process, and only one recording beam is used in the fixing
process. Previous researches of this kind of crystals are always based on 633nm red light or shorter wavelength
recording light. Longer wavelength recording light are more applicable for a practical data storage system. In this paper,
for the first time, near infrared nonvolatile holographic recording is realized in different kinds of doubly doped
LiNbO<sub>3</sub>:Fe crystals. In our experiments, the same sensitive light and recording lights at different wavelengths are
adopted to compare the recording performance. The recording conditions are optimized to improve the near infrared
recording characteristics. In near-infrared two-center holographic recording, the intensity dependence of recording
sensitivity is found to be different with that by recording at 633nm, caused by small bulk photovoltaic coefficient of Fe
traps, long response time and the simultaneous erasure of recorded hologram by sensitizing light.
The recording and readout characteristics in doubly-doped LiNbO<sub>3</sub> crystal with one-color scheme are investigated based
on jointly solving material equations and coupled-wave equations. Asymmetry between grating buildup and readout
process is found when electrons in deep centers can be excited by recording light; the grating is quasi-nonvolatile. The
shorter recording wavelength, and dopant in deep centers with a closer energy level to Fe in LiNbO<sub>3</sub> crystal, can
strengthen such asymmetry. The further investigation shows that two aspects induce the quasi-nonvolatile behavior in
doubly-doped LiNbO<sub>3</sub> crystal, one is the beam coupling between incident and diffracted beams, the other is two grating
form in both centers. This research provides a possible method to prolong the lifetime of grating in doubly-doped
The physical response of photorefractive materials under inhomogeneous illumination is well described by using material equations. Usual solutions to material equations are based on the assumption that the light modulation is small enough to linearize the equations. However, large light modulation, the presence of applied electric field and short time pulses are always required in many applications. A few analytical approaches and numerical solutions are developed for large light modulation. But certain simplifications are applied to the set of material equations and large computational effort is required. In this paper we present a numerical approach based on method of lines for simulating the photorefractive kinetics at high light modulation with an applied electric field. No approximations are made during the simulation and less effort is required during computation. We use different values of light modulation and applied electric field to present the numerical results. Time-space distribution of the carrier density and the space charge field, field amplitude evolution are obtained. Compared to the results under the small light modulation approximation, this method helps to understand the dynamics of photorefractive grating formation at high light modulation. A comparison is also made between the coupling coefficient obtained by this numerical method and that by analytical expressions.
Volume hologram formation in photorefractive materials is a dynamic process which is generally analyzed by using material equations by Kukhtarev et al. and coupled-wave equations. Usual numerical solutions to material equations are based on layered photorefractive materials always using the Runge-Kutta method. We present a new way based on finite element method for photorefractive effect simulation with any light modulation. In this paper material equations are partially decoupled to two equations between the electron density and the electrostatic field (and hence modulate the refractive index via the electro-optic effect). Then these two equations are numerically solved by finite element method for a sinusoidal intensity pattern with an arbitrary modulation depth from the interference of two mutually coherent beams. The numerical solutions allow us to examine the validity of analytical theories for photorefractive effect. We can obtain time-space distribution of photorefractive grating, and the space-charge field buildup. We present results of a number of parameters. These solutions help to understand the dynamics of volume hologram formation in photorefractive materials.