Mohammad A. Saleh

Self-activated liquid-crystal cells with photovoltaic substrates

We show that photovoltaic fields are capable of efficiently reorienting liquid crystals, leading to new concepts of optically addressable light modulators. Using an arrangement consisting of a liquid-crystal layer between LiNbO3:Fe photovoltaic substrates, we observed spatial filtering due to self-phase modulation in a planar-oriented cell and nonlinear transmission between crossed polarizers in a twist-oriented cell. These processes do not require an external electric field. The substrates are arranged such that light propagates along the +c axis in each substrate, allowing a secondary process of power transfer to occur through contradirectional photorefractive two-beam coupling.

Major improvements of the photorefractive and photovoltaic properties in potassium niobate

Optical and electrical measurements have been made on a new codoped potassium niobate crystal (KNbO3:Fe,Ag) that yields a significant enhancement of the photorefractive and photovoltaic effects when compared with the published results for singly doped potassium niobate crystals. The codoped Ag impurity enters the K site, rather than the typical Nb site, thus changing the local field in the lattice. It is believed that Fe perturbed by the Ag in the K site is responsible for an enhancement of the linear absorption and photocurrent, as well as a probable increase in the effective trap density. An enhanced trap density is likely the cause of the increased photorefractive counterpropagating two-beam coupling efficiency.

Contra-directional two-beam coupling by use of a single input beam in an iron-doped lithium niobate multimode fiber

Experimental results are presented for 1,800 contra-directional two-beam coupling (TBC) measurements in a single crystal fiber of LiNbO8:Fe using a single incident beam and its Fresnel reflection off the back surface of the fiber. To our knowledge, this is the first time that volume gratings have been written in a fiber using this beam coupling geometry. At small f-numbers, the TBC efficiency has been predicted to decrease in bulk LiNbO3:Fe due to the erasure of the weak gratings by the dark conductivity. We present experimental results validating the published theory and show experimentally that confinement of the interfering beams in a fiber geometry overcomes this limitation.

Stimulated photorefractive backscatter leading to six-wave mixing and phase conjugation in iron-doped lithium niobate.

The generation of multiple waves during near-oblique incidence of a 532 nm weakly focused beam on photorefractive iron-doped lithium niobate in a typical reflection geometry configuration is studied. It is shown that these waves are produced through two-wave coupling (self-diffraction) and four-wave mixing (parametric diffraction). One of these waves, the stimulated photorefractive backscatter produced from parametric diffraction, contains the self-phase conjugate. The dynamics of six-wave mixing and its dependence on crystal parameters, angle of incidence, and pump power are analyzed. What we believe to be a novel order analysis of the interaction equations provides further insight into experimental observations in the steady state. The quality of the backscatter is evaluated through image restoration, interference experiments, and visibility measurement. Reduction of two-wave coupling may significantly improve the quality of the self-phase conjugate.

Self-pumped contra-directional two-beam coupling in a photorefractive material using beam propagation simulation

By using split-step beam propagation method, self-pumped contra-directional two-beam coupling is simulated in a photorefractive medium for arbitrary shaped beams. The simulation shows a positive influence of the photovoltaic effect on the two-beam coupling efficiency, in agreement with published experimental observations.

Anisotropic Nonlinear Coupling of Two Counter-Propagating Waves in Photorefractive Fe:KNbO 3

The dependence of self-pumped photorefractive two-beam coupling in Fe:KNbO3with crystal angle has been calculated and measured at 532 nm for the a-c and b-c crystal planes using a combination of oil-immersion and relay imaging.

A Novel Algorithm for Self-Pumped Contra-Directional Gaussian Beam Coupling in Photorefractive Media

Self-pumped contra-directional two-beam coupling is simulated in a photorefractive medium for a Gaussian beam using the split-step beam propagation method. The simulation correctly shows the beam-coupling properties of gratings due to diffusion and photovoltaic field.

Analysis of Stimulated Photorefractive Backscatter during Six-Wave Mixing in Lithium Niobate

Oblique incidence of a weakly focused 532 nm laser beam on LiNbO3:Fe generates stimulated photorefractive backscatter (SPBS) from six-wave mixing in a reflection geometry. SPBS is analyzed through image restoration, interference experiments, and visibility measurements.

Continuous wave generated nanosecond electrical and optical pulses from photorefractive lithium niobate fibers

We have observed nanosecond electrical and optical pulsations from photorefractive lithium-niobate optical fibers using low-power lasers (/spl sim/=532 nm). Fourier spectra of the pulsations have a maximum at /spl sim/900 MHz with peaks separated by /spl sim/30 MHz.

Photovoltaic Induced Instabilities for Contra-Directional Two-Beam Coupling in LiNbO 3 :Fe

In LiNbO3:Fe, quasi-periodic instabilities are observed when recording photorefractive reflection gratings using a contra-directional two-beam coupling geometry. The mechanisms responsible for the noise and a technique used to eliminate these instabilities will be discussed