Photorefractives are able to create real-time holograms through the interference of two or more laser beams within the photorefractive material. The hologram can take the form of a real or virtual image of an object or data, or, in the simplest case, form a diffraction grating. Generally, a diffraction grating is an optical component with a regular pattern, which splits and diffracts light into several beams travelling in different directions. The diffraction grating can be either in phase with the optical interference pattern or out of phase. When in phase, the diffraction grating behaves in the same way as any common diffraction grating, causing equal diffraction of each of the writing beams. However, if the diffraction grating is out of phase with respect to the optical interference pattern, power may be transferred from one or more beams to another. This well established technique is useful for amplifying weak signal beams and for coherently combining two or more beams into a single stronger laser beam.
Diffraction gratings within waveguide structures have previously been used as extremely sensitive optical sensors. The method relies on placing a diffraction grating (a Bragg grating) within a thin optical waveguide at the surface of a transparent substrate. The electric field component of the guided light is able to penetrate a very small distance beyond the confines of the waveguide surface into the surrounding environment. This surface electric field is often called the evanescent field. The propagation of the light within the surface waveguide is highly sensitive to any changes of the surface refractive index, even changes which occur within the evanescent field above the surface. The reflectivity and wavelength selectivity of the surface waveguide diffraction grating are therefore sensitive to any changes of the evanescent field. Materials which come into contact with (or very close to) the surface influence the effective surface refractive index and can be detected by monitoring changes in the optical properties of the embedded diffraction grating. Optical fibers and surface waveguides have both been used successfully for this application and the best devices are capable of detecting fifth decimal place changes in refractive index. Typical applications include biological sensors, refractive index measurement, and product and substance identification.