Measurement of the spatial phase shift in high-gain photorefractive materials

The correct determination of the spatial phase shift ?(p) in photorefractive materials is crucial to the proper characterization of novel materials. It is shown that the grating translation techniques commonly used for the measurement of ?(p) need to be reevaluated for high-gain materials. Strong energy and phase coupling leads to nonuniform slanted gratings, which result in an apparent dependence of the phase shift of the beam ratio and the optical polarization. A revised theory is presented, and analytical solutions are obtained for the special case of ?(p)?pi/2 . Numerical solutions for arbitrary ?(p) are in good agreement with measurements in a photorefractive polymer.     

Transmission behavior in a photorefractive polymer film

When a p-polarized beam propagates through a high-performance photorefractive polymer composite, poly dimethyl-4-(4^′-nitrophenylazo)benzene, its transmission behavior is influenced by three effects: the electroabsorption, the photorefractive coupling with the reflected beam from the rear surface, and the amplified scattering. From the measurements on the incidence angle dependence as well as the applied-electric-field dependence of the three effects, some conclusions are obtained. At a small incidence angle with a low applied electric field, both the amplified scattering and the electroabsorption are small whereas the coupling between the incident beam and the reflected beam plays a principal role. At a large incidence angle or with a high poling electric field, the transmission is influenced mainly by the amplified scattering and the electroabsorption. A poling electric field asymmetric loss to the amplification scattering is also observed. Received: 18 November 1998 / Revised version: 10 February 1999 / Published online: 12 April 1999     

Femtosecond fabrication of waveguide-like micro-structures in a photorefractive polymer

In this letter, we report on, for the first time, the successful femtosecond micro-fabrication of continuous waveguide-like channels in the photorefractive polymer consisting of the nonlinear chromophore 2,5-dimethyl-4-(p-nitrophenylazo)anisole (DMNPAA), the photosensitive compound 2,4,7-trinitro-9-flourenone (TNF), and the plasticiser N-ethylcarbazole (ECZ) all doped in the polymer matrix poly (methyl methacry-late) (PMMA). These channels are caused by the change in refractive index as a result of the localised heating of the polymer and therefore have an important potential for micro-photonic devices in future.     

Manakov solitons in biased photorefractive polymer

The coupled equations for the incoherently coupled soliton pairs in biased photorefractive polymer are provided. It is shown that the coupled soliton equations reduce to Manakov equations when the total intensity of two coupled solitons is much lower than the background illumination. The bright-bright, dark-dark, and grey-grey soliton pair solutions of these Manakov equations are obtained, and the characteristics of these Manakov solitons are also discussed.     

光折变聚合物材料在光电子技术中的应用

光折变聚合物材料与光折变晶体相比，其非线性光学系数大，响应时间快，成本低廉，制备灵活，有可能成为制作光子学集成器件的材料。简要介绍了光折变聚合物材料在图像识别、光信息存储及光学相干层析技术等方面的应用情况，指出光折变聚合物材料中存在光子带隙。最后预测了光折变聚合物材料作为光子晶体在光子学器件中的应用前景。     

Transverse coupling and dynamics of patterns in a photorefractive oscillator

The effect of transverse coupling in a photorefractive oscillator is studied. From the study the condition for stable optical patterns of multimode oscillation is given analytically and verified by numerical simulation. Under the stable condition, the period-doubling route to spatiotemporal chaos is observed.     

Interdigitated coplanar electrodes for enhanced sensitivity in a photorefractive polymer

Organic photorefractive polymer composites can be made to exhibit near 100% diffraction efficiency and fast writing times, though large external slants are needed to project the applied field onto the grating vector. We show here that the use of interdigitated electrodes on a single plane provides similar performance to these standard devices and geometries but without a external slant angle. This new device's structure also greatly improves the diffraction efficiency and sensitivity compared to less slanted standard devices necessary for some real applications, such as holographic displays, optical coherence imaging, and in-plane switching.     

Critical enhancement of photorefractive beam coupling

We show that a hybridization of the optical and material nonlinearities takes place near the threshold of the subharmonic generation in photorefractive crystals. It results in a critical (with a singularity) enhancement of the rate of spatial amplification of light waves and leads to a variety of new optical critical phenomena.     

Optical two-beam coupling for a surface-induced photorefractive effect in undoped liquid crystals

We report beam-coupling measurements of undoped nematic planar cells, for which the surface-induced photorefractive effect (SIPRE) has been confirmed to be due to photoelectric interface activation. We show that the energy transfer strongly depends on experimental geometry. Diffraction efficiency and beam-coupling analyses suggest that the unusual local and (or) nonlocal behavior can be ascribed to an additional in-phase-modulated longitudinal electric field component, unaccountable by standard photorefractivity. We consider a simple charge distribution model of the SIPRE, which produces a space-charge field having two orthogonal modulated components, in phase and pi/2 out of phase. Through the liquid crystal's electric response, the exploited geometries verify the presence of the two components.     

Intensity-dependent coupling strengths in multigrating phase-conjugation using photorefractive crystals

The intensity dependence of the unequal coupling parameters in degenerate four-wave mixing is considered when simultaneous operation of four index gratings is allowed. A simplified form of the theoretical model of Townsend and LaMacchia (J. Appl. Phys. 41 (1970) 5188) is used in the analysis. We employ the shooting and matching technique to obtain a numerical solution of the coupled-wave equations. An adverse effect on the reflectivity of phase conjugation is observed at an intermediate stage of the refractive index modulation.     

Strong beam coupling in mesogenic materials with photorefractive Bragg gratings

Strong beam coupling was observed in mesogenic materials consisting of low- and high-molar-mass liquidcrystal mixtures with high concentration ratios of the latter type of liquid crystal. The beam-coupling effects originate in orientational photorefractive Bragg gratings, and a weak signal beam with an intensity of 1 mW is amplif ied to 8 mW by energy exchange with a strong pump beam after the passage of a 50-mum-thick film under a low operating voltage of 1-4 V/ mum. There is a superior net two-beam coupling gain of more than 600 cm(-1) with the low operating voltage and high resolution.     

Simple method for determining the gain coefficient of a photorefractive polymer film

We propose a simple method for measuring the gain coefficient of two-beam coupling for photorefractive polymer film. To measure the gain coefficient, we attached a phase grating onto the photorefractive polymer film and made an interference pattern between the transmitted and diffracted beams in the photorefractive film by using an illuminating single beam. The gain coefficient values measured at various external fields by our method showed good agreement with those measured by a standard two-beam coupling method. Compared to standard two-beam coupling, this method exhibited fairly improved stability of the output signal to external factors such as human sounds and airstreams.     

外加电场光折变有机聚合物串联回路中独立空间孤子对

对外加电场光折变有机聚合物串联回路中独立空间孤子对进行了理论研究,导出了在光折变有机聚合物串联回路中两光束传播的耦合方程,证明了串联回路中存在暗-暗、明-暗和明-明三种独立空间孤子对.在光束的空间展宽远小于有机聚合物宽度的极限条件下,暗-暗孤子对之间孤子入射光强的改变可相互影响到对方的包络和动态演化.对于明-暗孤子对,暗孤子通过光电流能影响明孤子,但明孤子不能影响暗孤子.明-明孤子对之间没有影响. 关键词： 非线性光学 光折变效应 光折变聚合物 空间孤子对     

Powerful, diffraction-limited semiconductor laser using photorefractive beam coupling

We use semiconductor laser amplifiers and a photorefractive crystal to generate a high-power, diffraction-limited laser beam at 860 nm. Using a single flared amplifier, we obtain 1.09 W in a diffraction-limited beam from 2.2 W of pump power. Using an array of flared amplifiers, we also demonstrate efficient beam coupling, showing that this technique is easily extended to semiconductor amplifier arrays.     

Shaping of photorefractive two-wave coupling by fast phase modulation

Using the distinctive features of the photorefractive nonlinearity, we derive a general self-consistent set of equations to describe two-wave coupling in the presence of fast and arbitrary strong-phase modulation. By considering a number of important particular cases, we show that phase modulation is a powerful and useful tool for shaping the characteristics of two-wave coupling such as the value of the energy exchange, the diffraction efficiency of the recorded grating, and the structure of the grating fringes. Finally, we analyze the role of the phase modulation in the active stabilization of wave coupling by means of an electronically introduced phase feedback.