Coherent optical beams are used to communicate, store, and process information in a growing number of applications. The availability of coherent laser sources is of critical importance for the realization of such optical information systems. An incoherent source randomly populates the available temporal and spatial channels of the optical system, making it difficult to represent information. Coherent laser sources provide a stable carrier on which to encode information, making it possible to realize powerful information-processing techniques such as wavelength division multiplexing in fiber networks, gated holographic imaging, and three-dimensional optical data storage.

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Coherent optical information systems

We propose a novel approach to visualize the light-induced refractive index changes in photorefractive crystals employing digital holography. The holograms formed in a Mach-Zehnder interferometer are recorded by a two-dimensional CCD camera. From these holograms, the phase differences, which contain the information of the index changes in photorefractive crystals, are determined by utilizing digital holographic interferometry. Then the two-dimensional visualizations of index changes in the crystals can be obtained. This method is successfully demonstrated in LiNbO3:Fe, KNSBN:Ce and SBN:Cr crystals.

Visualizations of Light-induced Refractive Index Changes in Photorefractive Crystals Employing Digital Holography

The optical masks prepared by using a LCLV liquid-crystal light valve are first proposed for light-induced photorefractive waveguides in photorefractive materials. Employing this technique, various waveguide structures can be fabricated, e.g., Y- or multiple-branches waveguides, fiber-like waveguides, and Mach–Zehnder-like switches, and even whole optical circuits may be formed. A Y-branches waveguide and a fiber-like waveguide were demonstrated in a LiNbO3:Fe crystal. Several technical problems, such as intensities, resolutions, writing speed, and so on, were also discussed in detail. Using a LCLV with a fast response and a writing beam with a high intensity, the waveguide structures may be changed in real time.

Optical masks prepared by using a liquid-crystal light valve for light-induced photorefractive waveguides

We propose an image cross-connector using an organic photorefractive polymer. With this device, it is possible to perform a reconfigurable connection between multiple input–output image ports by setting the incident angle of individual control beams to minute levels. The use of a polymer can reduce the thickness of the index grating to be induced inside the medium, which can also reduce the distortion of the output image and bring about high precise image connection without an additional optical system. We conduct an experiment with a hole-transport polymer PATPD to verify the operations of an image cross-connector, in which different images from a vertical array of input images would be connected separately to two output ports by two control beams that were injected at different angles.

Image cross-connector using a photorefractive polymer

We demonstrate full-color imaging through a turbid medium by use of photorefractive coherence gating and a technique to separate the recording space of each color from those of the other colors. We found that the recording spaces must be separate when a multicolor image is recorded in a photorefractive crystal to prevent the interference of the holograms with one another. For full-color imaging we used a He-Cd white-light laser, which is compact and useful for full-color holography. Full-color-image retrieval is demonstrated through five mean free paths of a turbid medium.

Full-Color Imaging Through a Turbid Medium by use of Photorefractive Coherence Gating and a Technique to Separate the Recording Spaces

We have demonstrated two-dimensional imaging of photoinduced index change in a photorefractive lithium niobate crystal using Mach-Zehnder interferometry. Refractive index changes induced by both ordinary and extraordinary Ar+ laser beams have been clearly visualized.

Two-Dimensional Visualization of Photoinduced Refractive Index Change in Photorefractive Lithium Niobate Crystal

We propose a new method for multicolor holographic storage system using the dispersive medium which splits the signal beam into several beams of different wavelengths by different angles. Holographic gratings are written in the different area of the crystal by different wavelengths so as not to be erased by the signal beam.

Multicolor image reconstruction using photorefractive holography

We have two-dimensionally visualized a photoinduced planar waveguide produced in a photorefractive LiNbO3 crystal Temporal and spatial evolution of the waveguide was investigated by the interferometric imaging technique The guiding of a probe He–Ne laser in the waveguide was also experimentally observed

Observation of Spatial and Temporal Evolution of Planar Photoinduced Waveguide in LiNbO3 Crystal.

In summary, we have developed a new imaging technique for photo-induced index change in photorefractive crystals Formation of waveguide structures were successfully visualized both spatially and temporally This technique will be an useful tool for monitoring versatile photorefractive phenomena, including photorefractive grating formation

R. Hagari

Papers

Two-Dimensional Visualization of Photoinduced Refractive Index Change in Photorefractive Lithium Niobate Crystal

Multicolor image reconstruction using photorefractive holography

Full-Color Imaging Through a Turbid Medium by use of Photorefractive Coherence Gating and a Technique to Separate the Recording Spaces

Observation of Spatial and Temporal Evolution of Planar Photoinduced Waveguide in LiNbO3 Crystal.

Visualization of photo-induced waveguide formation in photorefractive LiNbO/sub 3/ crystal