| Summary: | 碩士 === 中原大學 === 物理學系 === 87 === Several photorefractive crystals such as LiNbO3 or BaTiO3 can be used for optical information storage as well as key elements for optical signal processing due to their unique property, viz., their refractive index varies according to their incident light distribution. Thus, we can construct phase gratings inside the crystals by using interference of two light beams. According to the Band Transport model for the photorefractive effect, the amplitude of such index grating n is dependent on the electro-optic coefficients of the crystal as well as the polarizations of two incident beams. The index grating amplitude n can be computed from the equation derived from this model. To date, however, there has been no report in the literature for measuring the amplitude n precisely. Here we report a new method for doing such measurements. First, under the Bragg matching conditions, a probe beam was directed incident on the crystal while a phase grating was being built or while a phase grating was decaying away, in order to measure its diffracted light intensity as a function of time. The number of diffraction efficiency peaks from this graph was denoted as n (this n can be a non-integer number). Then we measured the Bragg deviation angle causing the diffraction efficiency to drop to minimum. Using the equation relating n to deriving from Coupled Mode Theory, which also contains the number of efficiency peaks m as its parameter, we can compute the index grating amplitude n. However, there is multiple m which can satisfy the condition that the diffraction efficiency drops to minimum, we, thus, have to use the n obtained from the first experiment in helping us selecting the right m in order to compute the index grating amplitude n precisely. This precise measurement of n , thus, allow us to quantify photorefractive effect.
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