The non-linear theory of the self-diffraction on the light induced grating of refractive index in electrooptic crystals is developed. The intensities of the diffracted beams, the diffraction efficiency, and the shape of the surfaces of equal index change are calculated analytically for saturation holograms.Holographic storage in nominally pure reduced crystals of LiNbO3 is studied experimentally. It is shown that the developed theory in diffusion approximation satisfactorily describes the experimental data.

Holographic storage in electrooptic crystals. i. steady state

Diffraction characteristics of general dielectric planar (slab) gratings and surface-relief (corrugated) gratings are reviewed. Applications to laser-beam deflection, guidance, modulation, coupling, filtering, wavefront reconstruction, and distributed feedback in the fields of acoustooptics, integrated optics, holography, and spectral analysis are discussed. An exact formulation of the grating diffraction problem without approximations (rigorous coupled-wave theory developed by the authors) is presented. The method of solution is in terms of state variables and this is presented in detail. Then, using a series of fundamental assumptions, this rigorous theory is shown to reduce to the various existing approximate theories in the appropriate limits. The effects of these fundamental assumptions in the approximate theories are quantified and discussed.

Analysis and applications of optical diffraction by gratings

All optical fabrication of Surface Relief Grating (SRG) on azobenzene functionalized polymer films is reviewed in this article. The uniqueness of the single step erasable photofabrication of large amplitude surface relief structures on azo polymer films is mainly due to the photoisomerization and photoanisotropic behavior of the azobenzene group. The design and synthesis of several classes of suitable azobenzene functionalized polymers for the SRG formation is presented. Experimental studies are carried out in a systematic manner to understand the chemical and physical intricacies involved in the SRG formation process. Structural requirements and their limitations for the efficient fabrication of SRGs on azo functionalized polymer films are also discussed. Understanding the fundamental mechanism of SRG formation on the azo functionalized polymer films enables us to make use of these structures in a variety of applications.

Surface relief structures on azo polymer films

PHOTOREFRACTIVE materials are of considerable interest for the development of all-optical devices1. The photoref ractive effect appears in materials that exhibit an electric-field-dependent refractive index and that are photosensitive, such that the spatial distribution of photogenerated charge carriers is modified on irradiation with light. The diffraction pattern formed by the interference of two coherent light beams within such a material generates a non-uniform internal electric field that in turn modulates the refractive index. The resulting refractive-index pattern forms a grating that can diffract light and thereby give rise to two-beam coupling, whereby one of the writing beams gains energy at the expense of the other—a property that can be exploited in photonic devices. Although the best photorefractive materials currently available are inorganic crystals such as LiNbO3, there is considerable interest in the development of photorefractive polymers2–8, owing to their structural flexibility, ease of processing and lower cost. We describe here a polymer composite with excellent photorefractive properties. We have achieved a diffraction efficiency approaching 100% and a net two-beam coupling gain of more than 200 cm–1, making these polymeric materials suitable for immediate application in areas such as dynamic holographic storage and optical information processing1.

A photorefractive polymer with high optical gain and diffraction efficiency near 100

Holography, coherent light amplification and optical phase conjugation with photorefractive materials

The theory of energy transfer between two coupled beams writing holograms in electrooptic crystals is developed. The gain is calculated for different processes causing holographic recording in the crystal.The amplifications of the light beam in the course of recording holograms in nominally pure reduced LiNbO3 crystals is investigated. The amplification of about 10 cm''-1 is obtained for small signals. The gain is found to be independent of the total light intensity and intensity ratio of the writing beams and decreases linearly with the grating spacing.

holographic storage in electrooptic crystals. II. beam coupling—light amplification

A review is given of the present state of theoretical and experimental work on the interaction of two coherent light beams of equal frequency in a nonlinear medium. It is shown that dynamic self-diffraction is substantially different for media with different types of response. Stationary energy transfer between interacting beams is possible in the case of nonlocal nonsymmetric response, whereas this type of transfer is forbidden in the case of local response. Different mechanisms for self-induced changes in the refractive index are examined together with the corresponding processes of energy transfer between the writing beams. The more important applications of dynamic self-diffraction are discussed.

Dynamic self-diffraction of coherent light beams

Transient energy transfer during hologram formation in LiNbO3 in external electric field

Possible applications of dynamic holograms for image amplification and image processing are discussed. The principal physical processes of holographic amplification of coherent light are analysed. Amplification of grey scale images during recording of dynamic holograms in pure reduced and Fe-doped crystals of LiNbO 3 is demonstrated. One-exposure technique of image subtraction or addition is proposed and realised in Fe-doped crystals of LiNbO 3 .

S. G. Odulov

Papers

Holographic storage in electrooptic crystals. i. steady state

holographic storage in electrooptic crystals. II. beam coupling—light amplification

Dynamic self-diffraction of coherent light beams

Transient energy transfer during hologram formation in LiNbO3 in external electric field

Dynamic holography and optical image processing