Ru Guo

Observation of composite gap solitons in optically induced nonlinear lattices in LiNbO3:Fe crystal.

We observe experimentally, the first time to our knowledge, two types of composite gap solitons in optically induced one-dimensional nonlinear lattice in LiNbO(3) crystal. We observe the staggered bright composite gap soliton when a single Gauss probe beam is incident at Bragg angle as well as a dipole probe beam is incident at normal incidence. When a single Gauss beam is at normal incidence, the in-phase bright composite gap solitons are observed.

Waveguides and directional coupler induced by white-light photovoltaic dark spatial solitons

We have observed experimentally, for the first time to our knowledge, one-dimensional photovoltaic dark spatial solitons in LiNbO3:Fe crystal by using incoherent white light. We have also fabricated a directional coupler consisting of two waveguides induced by two mutually incoherent white-light photovoltaic dark spatial solitons propagating in parallel in close proximity. It was found that the light field of a probe laser beam launched into one of the two proximate waveguides can be efficiently coupled into the other waveguide because of the presence of evanescent waves. We also studied the dependence of the coupling efficiency on the distance between the two proximate soliton-induced waveguides.

Spatial modulation instability in self-defocusing photorefractive crystal LiNbO3:Fe

We report on the experimental observation of the modulational instability of quasi-plane-wave beams in the self-defocusing photorefractive crystal LiNbO3:Fe. Our analysis shows that this modulational instability is ascribable to the nonlocal response of the refractive index to the optical illumination. Theoretical results indicate that the perturbation period depends on the ratio of the optical beam intensity to that of the dark irradiance and, moreover, the perturbation period rises and reaches a saturated value as the intensity ratio r increases. Experimental and theoretical results have good agreement for the perturbation period.

Interactions of partially incoherent spatial solitons in logarithmic saturable nonlinear media

The interactions of two partially incoherent spatial solitons in logarithmic saturable nonlinear media are investigated numerically based on the coherent density approach. Numerical simulations show that the interaction through cross-phase modulation (XPM) result in attraction, with the solitons going through each other, and these interactions are insensitive to their initial relative phase. Two equal-width solitons with different incoherence may repel each other and exchange energy. If the two equal-width solitons propagate collinearly they can linearly superpose and fuse into a steady composite partially incoherent spatial soliton. The interactions through self-phase modulation (SPM) are very sensitive to their initial relative phase and coherence, and can be changed from attraction to repulsion or vice versa. Some interesting numerical examples are presented to show how these interactions can be controlled by these parameters.

White-light photorefractive phase mask

A volume photorefractive phase mask has been fabricated with incoherent white light from an incandescent source in LiNbO3:Fe self-defocusing photorefractive crystal for the first time, to our knowledge. It can guide and modulate a probe white light or a laser beam and can be used to transmit an incoherent dark image as the guided modes of the waveguides induced by white-light dark spatial solitons. This also proves the existence of photorefractive nonlinearity of white light.

Transmission of digital images consisting of white-light dark solitons

A distortion-free digital image is transmitted through parallel propagation of white-light photovoltaic dark solitons. The waveguide channels induced by white-light dark solitons can guide both the laser beam and the white-light beam well. We determine experimentally the critical separations of the dark solitons in the directions parallel and perpendicular to the crystalline c axis for the given crystal thickness.

Gap solitons in optically induced two-dimensional square photonic lattices in LiNbO3:Fe crystals

We fabricated two-dimensional square photonic lattices with a backbone structure in self-defocusing LiNbO3:Fe crystals through optical induction. We systematically observed four kinds of gap solitons and their power spectra in the nonlinear photonic lattices. Our results show that the lightwave is confined more tightly when it is launched into a low-index region at the input face of the crystal than when it is launched into a backbone ridge waveguide.

Simple optical method for determination of crystal orientation in photorefractive crystals

A simple optical method for determination of crystallographic orientations +/-x, +/-y, and +/-z in photorefractive crystals based on light amplification via backward two-wave mixing (TWM) is presented. We consider a photorefractive Fe-doped lithium niobate crystal (LiNbO(3):Fe) with a standard x-y-z cut as an example to illustrate the principle, manipulation, and experimental results of this method in detail. Also we see that the direction of the light amplification, namely, the direction of the nonreciprocal steady-state energy transfer, strongly depends on the direction of the grating vector with respect to the corresponding crystallographic axes mainly under the diffusion mechanism. Finally, a way of determining the sign of the charge carriers in photorefractive materials is proposed.

Photorefractive phase mask

We propose and demonstrate a new method to fabricate volume phase mask in LiNbO3:Fe crystal by photorefractive effect, for the first time to our knowledge. First, we image an amplitude mask with corresponding intensity distribution at the incident face of a photorefractive LiNbO3:Fe crystal. After an exposure of proper time, due to photorefractive effect, the distribution of refractive index in the crystal is changed. Therefore we get the volume phase mask we want. The influence of the anisotropy of photorefractive nonlinearity on volume phase mask can be overcome by controlling experimental conditions. We can see an intensity-modulation image in the image plane of the volume phase mask. The intensity distribution of the image of the volume phase mask is inverse from the one of input amplitude mask. It can be explained by waveguide theory. Volume phase mask can be used to get modulated amplitude pattern by modulating the phase of incident light. It has the advantage of low energy loss over amplitude mask and real-time. Our method is very simple. It can be used to fabricate many kinds of complex phase masks and phase components used in optical information processing and Integrated Optics.

Propagation of Gaussian beam in self-defocusing photovoltaic photorefractive nonlinear crystal of LiNbO3:Fe

We investigate experimentally the propagation of Gaussian beam in self-defocusing photovoltaic photorefractive nonlinear crystal of iron-doped lithium niobate. In steady-state, the propagating beam in the low-refractive-index optical channel written by the beam itself is affected mostly by linear diffraction and anisotropic divergence of the saturated negative lenslike effect. And the latter affects the incident beam with convergent wavefront much more than that with divergent wavefront. In the case of the incident beam with the convergent wavefront, the angular width of the spatial spectrum of the output beam decreases along with the size of the output beam. We believe that the phase-type spatial filtering of the funnel-type low-refractive-index optical channel written by the propagating beam itself causes the results.