Ya’nan Zhi

Wavelength dependence of light-induced domain nucleation in MgO-doped congruent LiNbO3 crystal

Within the wavelength range from 351 to 799 nm, the different reductions of nucleation field induced by the focused continuous laser irradiation are achieved in the 5 mol % MgO-doped congruent LiNbO3 crystals. The reduction proportion increases exponentially with decreasing irradiation wavelength and decreases exponentially with increasing irradiation wavelength. At one given wavelength, the reduction proportion increases exponentially with increasing irradiation intensity. An assumption is proposed that the reduction of nucleation field is directly related to the defect structure of crystal lattice generated by the complex coaction of incident irradiation field and external electric field. (c) 2007 American Institute of Physics.

Phase mapping of domain kinetics in lithium niobate by digital holographic interferometry

The phase mapping of domain kinetics under the uniform steady-state electric field is achieved and investigated in the LiNbO3 crystals by digital holographic interferometry. We obtained the sequences of reconstructed three-dimensional and two-dimensional wave-field phase distributions during the electric poling in the congruent and near stoichiometric LiNbO3 crystals. The phase mapping of individual domain nucleation and growth in the two crystals are obtained. It is found that both longitudinal and lateral domain growths are not linear during the electric poling. The phase mapping of domain wall motions in the two crystals is also obtained. Both the phase relaxation and the pinning-depinning mechanism are observed during the domain wall motion. The residual phase distribution is observed after the high-speed domain wall motion. The corresponding analyses and discussions are proposed to explain the phenomena.

Ridge-shape phase distribution adjacent to 180° domain wall in congruent LiNbO3 crystal

The digital holographic interferometry is used in the dynamic and static measurements of phase variation induced by domain inversion. For the first time, to the authors’ knowledge, they observe the existence of ridge-shape phase distribution adjacent to 180° domain wall in congruent LiNbO3 crystal. During the domain wall motion, the phase variations are not uniform but have obvious relaxations. In the static measurement, the ridge elevation can vary linearly with the uniform electric field. The reasonable assumptions are proposed to explain these effects.

Electrochromism accompanying ferroelectric domain inversion in congruent RuO2:LiNbO3 crystal.

Electrochromic phenomena accompanying the ferroelectric domain inversion in congruent RuO2-doped z-cut LiNbO3 crystals at room temperature are observed in experiments. During the electric poling process, the electrochromism accompanies the ferroelectric domain inversion simultaneously in the same poled area. The electrochromism is completely reversible when the domain is inverted from the reverse direction. The influences of electric field and annealing conditions on domain inversion and electrochromism are also discussed. We propose the reasonable assumption that charge redistribution within the crystal structure caused by domain inversion is the source for electrochemically oxidation and reduction of Ru ion to produce the electrochromic effect.

Digital holographic interferometry and the angular spectrum method applied to measuring the domain inversion in ferroelectric crystal

The application of digital holographic interferometry on the quantitative measurement of the domain inversion in a RuO2:LiNbO3 crystal wafer is presented. The recorded holograms are reconstructed by the angular spectrum method. From the reconstructed phase distribution we can clearly observe the boundary between the inverted and un-inverted domain regions. Comparisons with the results reconstructed by use of the Fresnel transform method are given. Factors that influence the measurement include the spectrum filter size and the spectrum movement are discussed. The spectrum filter size has an effect on the measurement of the details. Although the spectrum movement affects every single reconstructed image, it has no influence on the final measurement.

Quantitative measurement of domain inversion in RuO2:LiNbO3 crystal by digital holographic interferometry

We quantitatively study the domain inversion in a RuO2:LiNbO3 crystal wafer by the digital holographic interferometry. The crystal wafer is placed into one arm of a Mach-Zehnder-type interferometer to record a series of holograms. Making use of the angular spectrum backward propagation algorithm, we reconstruct the optical wave field in the crystal plane. The extracted phase difference from the reconstructed optical wave field is a well linear function of the applied external voltage. We deduce that the linear electro-optic coefficient of the detected RuO2:LiNbO3 crystal sample is 9.1×10−12m∕V. An unexpected phase contrast at the antiparallel domain wall is observed and the influence of the applied external voltage on it is studied in detail. Also the built-in internal field is quantitatively measured as 0.72kV∕mm.

Reversible electrochromic effect accompanying domain-inversion in LiNbO3:Ru:Fe crystals

A reversible electrochromic effect during the electrical poling process in LiNbO3:Ru:Fe crystals at room temperature is observed. In electrode area, both electrochromism and domain-inversion occur mutually and electrochromism is reversible during back-switch poling, which are experimentally verified, and a microstructure model to explain the mechanism is proposed. In addition, different from the undoped LiNbO3 crystals, the breakdown field (>25.0kV∕mm) is higher than the coercive (21.0kV∕mm) measured in LiNbO3:Ru:Fe, which proves a possible new technique to realize domain-inversion by constant electric field rather than pulsed one.

Optical image processing for synthetic-aperture imaging ladar based on two-dimensional Fourier transform

A two-dimensional (2D) Fourier transform algorithm for the image reconstruction of synthetic-aperture imaging ladar (SAIL) collected data is suggested. This algorithm consists of quadratic phase compensation in azimuth direction and 2D fast Fourier transform. Based on this algorithm and the parallel 2D Fourier transform capability of spherical lens, an optical principle scheme that processes the SAIL data is proposed. The basic principle, design equations, and necessary analysis are presented. To verify this principle scheme, an experimental optical SAIL processor setup is constructed. The imaging results of SAIL data obtained by our SAIL demonstrator are presented. The optical processor is compact, lightweight, and consumes low power. This optical processor can also provide inherent parallel and speed-of-light computing capability, and thus has potential applications in on-board and satellite-borne SAIL systems.

Investigation of effective internal field in congruent lithium niobate crystal by digital holographic interferometry

The phase contrast across the crystal thickness induced by the internal field is measured by the digital holographic interferometry just after the congruent lithium niobate crystal is partially poled. The direction of applied external field is antiparallel to that of internal field, and the measured phase contrast varies linearly with the applied external field. A new internal field is obtained by this method and named effective internal field. The distinct discrepancy between effective and equivalent internal fields is observed. The authors attribute this effect to the new macroscopic representation of elastic dipole components of defect complex in the crystal.

Performance analysis of pupil-matching optical differential receivers in space-to-ground laser communication

In the paper, the principle and structure of a pupil-matching optical differential receiver consisting of double 4f confocal lens groups is introduced to overcome atmosphere turbulences in space-to-ground laser communication. Using the scalar diffraction theory, a systematic analysis of 4f lens groups is formulated mathematically. Based on Seidel aberration, lens aberrations produced by the inherent unideal lens and mutual alignment errors of double 4f lens groups primarily caused by relative axial displacement of the foci and vertical position change of the optical axes are studied mathematically and detailed. Under the effects of varying aberrations on the double 4f lens groups, we evaluate the performance of this receiving system by the model of power penalty for a given 10(-9) bit error ratio. Simulated results of the relationship between power penalty and the different root-mean-square errors are concluded in order to put forward the requirement of machining precision of individual components. That will be helpful in optimizing the design of these groups in the optical receiver.