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Researchers are using photorefractivity for dynamic holography & dynamic interferometric detection of ultrasonic motion.

Optical Sensing schematic

We studied the two-wave mixing anisotropic diffraction process in GaAs for demodulation of static and dynamic phase encoded signals. The static results quantitatively agreed with a previous theoretical model for cubic crystals, P. Yeh. This model has been explicitly described for all beam polarizations and crystal rotation angles with respect to the plane of incidence. Dynamic phase modulation, where the signal beam was phase modulated at frequency fs and the reference beam at fr=fs+df, produced a signal at df proportional to the difference between the static beam intensities with and without two-wave mixing under all conditions of polarization and crystal orientation studied. A significant dynamic output signal was produced even when only a shift in polarization but no energy transfer occurred as a result of the anisotropic two-wave mixing process. Therefore, not only the two-wave mixing gain is important for using the photorefractive effect for dynamic phase demodulation, but also the polarization shifts occurring from the mixing process.

Coordinate Diagram

Optical processing of information performed by interfering two waves inside a nonlinear material exhibiting photorefractivity (two-wave mixing) has been extensively studied. Several reviews have been published on the physical effects of two-wave mixing and its many applications [Electro-optic and Photorefractive Materials, P. Günter, editor, (Springer-Verlag, New York, 1987), S. I. Stepanov, International Trends in Optics, (Academic Press, New York, 1991) Ch. 9., P. Yeh, Introduction to Photorefractive Nonlinear Optics, (John Wiley, New York, 1993)]. Phase modulation techniques coupled with photorefractivity have produced a large variety of methodologies for modifying and controlling the space charge field and index of refraction grating established in these materials[M. Vasnetsov, P. Buchhave, and S. Lyuksyutov, “Phase modulation spectroscopy of space-charge wave resonances in Bi12SiO20,” Opt. Comm., 137, 181-191 (1997), J. P. Huignard and A. Marrakchi, “Coherent signal beam amplification in two-wave mixing experiments with photorefractive Bi12SiO20 crystals,“ Opt. Comm. 38, 249-254 (1981)]. An important application of optical interferometry is the detection of phase modulation impressed on an optical probe beam from scattering off of ultrasonic motion at a surface. Photorefractivity has made an impact on this nondestructive evaluation measurement process through its ability to perform optical phase demodulation through homodyne interferometry from rough and diffusely scattering surfaces. The photorefractive process is employed to produce a diffracted reference wavefront with the spatial characteristics of and coaxial with the probe wavefront. Subsequent homodyne interference produces a demodulation of the ultrasonic motion at frequencies greater than the photorefractive cutoff frequency. Another approach is to use the photorefractive process itself to demodulate and image standing waves of a vibrating plate through the two-wave and four-wave mixing processes.

Static signal vs. Polarizer angle

Most photorefractive materials are crystalline exhibiting anisotropic behavior. Two wave-mixing in these materials is inherently complicated by the tensorial character of the optical properties of these materials. Nevertheless, two-wave mixing can occur in these materials and produce interesting effects, such as cross-polarization coupling, that can be used to advantage in situations where polarizing components are used to select particular beams from the output. There is a need to quantitatively describe the anisotropic-diffraction process in crystalline photorefractive materials in order to take full advantage of this effect for demodulation of static and dynamic phase information, as suggested above. A nonlinear model of the static photorefractive two-wave mixing in cubic crystals has been developed by Yeh. Gallium Arsenide (GaAs) is a cubic crystal that can be described by this model. Although GaAs has a smaller coupling constant than other photorefractive materials, such as Bismuth Silicon Oxide, its response time is faster making it particularly suitable for ultrasonic measurements.

Dynamic signal vs. Polarizer angle

Even though GaAs is optically isotropic, the tensor nature of the electro-optic effect in GaAs crystals allows cross-polarization two-wave coupling. This paper compares the results of two-wave mixing in a GaAs crystal with the predictions of the model developed by Yeh for the static case. In addition to the static case, dynamic modulation at and below a given signal frequency was also investigated. Good agreement was found between the two-wave mixing results and the model for the static case and unusual cross-polarization coupling was observed for the dynamic modulation detection.

R. S. Schley, K. L. Telschow and J. Holland, “Static and Dynamic Two-Wave Mixing in GaAs,” Applied Optics: Lasers Photonics and Environmental Optics, 39 (24), 4348-4354 (August 20, 2000).