Zhifang Chai

Investigation for the high recording sensitivity with two center recording in LiNbO3:Fe:Ru crystals

LiNbO3:Fe:Ru crystal is the effective recording media with high recording sensitivity for two-center recording, and the physical mechanism for the high recording sensitivity is investigated theoretically and experimentally. The results show that the energy level of Ru perhaps is closer to that of Fe than that of Mn in LiNbO3 crystal, the electrons in Ru center can be excited more effectively into the conduction band in the same sensitizing conditions, which can induce the improvement of the recording sensitivity. The recording sensitivity 0.044cm/J in LiNbO3:Fe:Ru crystal, which is ten times larger than that obtained in LiNbO3:Fe:Mn reported early. However, the elevation of energy level of deep center will induce that the electron excitation from deep center by the recording light become more effective, and the persistence decreases with the recording sensitivity increase, the grating in Ru center can be erased by red light obviously. In practical application people must take a trade off between the recording sensitivity and persistence.

Two-center recording in LiNbO3:Fe:Ru and LiNbO3:Ce:Ru crystals

In this paper, the two-center recording with 633nm recording and 465nm sensitizing is performed in doubly-doped LiNbO3:Fe:Ru and LiNbO3:Ce:Ru crystals, respectively. The investigation for transmission spectra shows that the better transmission can be found from 400nm to 700nm in LiNbO3:Ce:Ru crystals. Performance measures, such as diffraction efficiency, recording sensitivity, are calculated and compared according to the time evolution curves of diffraction efficiency. The results show that the high recording sensitivity and dynamic range can be obtained in LiNbO3:Fe:Ru crystal. The scattering noise in LiNbO3:Ce:Ru and LiNbO3:Fe:Ru is investigated, the results show that the lower noise can be observed in LiNbO3:Ce:Ru crystal.

Absorption characteristic and two-center holographic recording in LiNbO3:Ce:Ru crystals

The absorption characteristic and nonvolatile holographic storage are investigated for the first time in Ce and Ru codoped LiNbO3 crystals in this paper. The absorption spectra examination shows that there is a good optical transmission in the blue and violet wavelength range in the as-grown and oxidized samples. An absorption peak around 530nm induced by Ru can be found. A photochromic effect is observed through UV illumination experiment. Nonvolatile holographic recording is performed with different sensitizing and recording wavelength strategies. The results show that the highest recording sensitivity (0.050cm/J) and dynamic range (2.7 cm-1) can be obtained with 514nm recording and 405nm sensitizing.

Effect of light intensity on quasi-nonvolatile holography in doubly doped LiNbO3:Fe:Ru crystal

One-color photorefractive holography has been investigated in the oxidized LiNbO3:Fe:Ru crystal with recording wavelength 632.8nm. The crystal shows a high asymmetric behavior in grating buildup and readout erasure rates. This remarkable property shows that the recorded grating is quasi-nonvolatile during readout. In this paper, the effect of the pre-sensitizing process and the readout intensity on the asymmetric behavior is investigated experimentally, the results show that the recording sensitivity can be increased by the pre-sensitizing process, and the grating lifetime can be extended by increasing readout light. At last, the reason for the high asymmetry is explained qualitatively in terms of a multiple center model.

Flat optical lens by use of 90-degree volume holographic grating recording geometry and nonvolatile thermal fixing in LiNbO3:Fe crystal

A new design and fabrication of flat optical lens by use of 90-degree volume holographic grating recording geometry is presented. It is recorded by the interference of a plane wave and a spherical wave in a Fe-doped LiNbO3. The flat optical lens can laterally transfer and focus the plane wave which perpendicularly incidents on the planar crystal surface. The optimal switching from recording to thermal fixing is taken into consideration in order to obtain the nonvolatile hologram with maximum fixing efficiency. The flat optical lens with different recording and reconstructing wavelength has also been discussed. The flat optical lens has the advantages of light weight, laterally transferring and focusing, small duty ratio, and easy for the microstructure integration. The testing results measured verify that the flat optical lens can successfully be used for the free space optical communication.

Investigation for persistence of two-center recording in various doubly doped LiNbO 3 crystals

The effect of dopants in doubly-doped LiNbO3 crystals on persistence of two-center recording is investigated experimentally and theoretically in this paper. Six kinds of doubly-doped LiNbO3 crystals are used in experiments with 633nm recording and 404nm sensitizing, the results show that LiNbO3:Ce:Cu crystal has the largest persistence. The dependence of persistence on bulk photo-voltaic coefficient and photo-excitation coefficient of deep center for recording light (K12 and S12) is discussed based on jointly solving the two-center material equations and the coupled-wave equations. The results show that both the increase of S12 and the decrease of K12 will result in the decrease of the persistence. When a holographic storage system is designed, the appropriate dopant will help to obtain the high persistence and dynamic range simultaneously.

Investigation of two-center recording with variable sensitizing light intensity in doubly-doped LiNbO3 crystal

The effect of variable sensitizing intensity on photorefractive performance in two-center recording is investigated through jointly solving two-center material equations and the coupled-wave equations. Four kinds of schemes for varying sensitizing intensity are designed, the relationship between the sensitizing intensity and recording time is simply assumed to be linear, and at the same time the recording intensity is stable in recording process. The temporal evolution of the space charge field (SCF) is obtained in each scheme. The results show that SCF increase monotonously no more in the recording process. The increase of sensitizing intensity will reduce the peak value of SCF during recording process. However, the decay of sensitizing intensity can enhance the peak value of SCF. The slow decay of sensitizing intensity is also useful to the buildup of high SCF, but the recording time will be long. At last, the effect of the decay mode and the initial value of sensitizing intensity on recording process are discussed. Both of recording sensitivity and the peak value of SCF can be enhanced in decay scheme with the high initial sensitizing intensity. In a real application an appropriate sensitizing light scheme needs to be designed.

Two-wave coupling in LiNbO3:Fe:Ru crystals

Two-wave coupling in LiNbO3:Fe:Ru crystals is investigated experimentally and theoretically in this paper. The dependence of gain coefficient on doped concentration, as well as the light intensity ratio between pump beam and signal is experimentally investigated, the maximal gain coefficient 9.2cm-1 is found while the ratio is 2.2, light gain cannot be observed in the reduced sample because of fan effect. Effect of phase difference between light modulation and the refractive index grating on two-wave coupling is investigated based on jointly solving the two-center material equations and the coupled wave equations, the variation of phase difference is obvious in the crystals with reduced process or high doped concentration.

Holographic scattering noise in LiNbO3:Fe:Ru crystals

Holographic scattering noise in LiNbO3:Fe:Ru crystals is investigated in this paper. The effect of doped concentration and the incident light intensity on the holographic scattering noise is investigated. The experimental results show that the transmitted light intensity is fluctuated in the initial stage of scattering noise evolution, and the sample with highly doped concentration has the lowest scattering noise level among three LiNbO3:Fe:Ru samples. The results are analyzed on the basis of multi-wave coupling amplification mechanism and the saturated space charge field.

Maximized diffraction efficiency for integrated volume grating instruments

High diffraction efficiency of each grating is desired to reduce light intensity loss when gratings are integrated to realize miniaturization for 3-D optical instruments in LiNbO3 crystals. Based on jointly solving the two-center material equations, oscillating behavior of diffraction efficiency during recording and erasing process is investigated theoretically to realize maximized diffraction efficiency for integrated volume grating instruments in weak oxidized doubly-doped LiNbO3:Fe:Mn crystals. Two nonvolatile gratings are integrated and nonvolatile diffraction efficiency for each grating exceeds 60%. These methods depend on the dopant elements and its concentrations, annealing (or oxidation-reduction) in doubly-doped LiNbO3 crystals, and recording and sensitizing intensities and wavelengths. The experimental investigation will be performed to verify the calculated results.