A Measurement of the Saturation Amplitude for Photorefractive Index Gratings by using the Diffraction Efficiency Method

• Main Authors: Yu-Fong Tseng, 曾裕峰
• Other Authors: Shon-Fu Chen
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/3g553s

碩士 === 中原大學 === 應用物理研究所 === 91 === Several photorefractive crystals such as LiNbO3 or BaTiO3 can be used for optical information storage or 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 the interference of two light beams. The amplitude of this index grating grows exponentially as a function of time and stops when it reaches its maximum. The maximum (saturation) amplitude of this index grating, �慨0, determines the information storage capacity. According to the Band Transport model for the photorefractive effect and the Coupled Wave Theory, the amplitude of such an index grating, �慨, is related to the diffraction efficiency of the grating. Several methods reported in the literature quantify �慨0 without using the diffraction efficiency. In one category, the measurements were carried out using a coherent light source, and the accuracy of such measurements is questionable due to the uncertainty of the interaction length. In another category, the measurements were carried out using a non-coherent light source, which requires two apparently identical crystals and again the accuracy is questionable due to the uncertainty of the equivalence of the crystals. There has been no report in the literature for measuring the grating growth speed �� and the saturation amplitude �慨0 precisely using the diffraction efficiency method and using the laser beam for construction of the grating due to the difficulty caused by beam fanning near saturation. Here we report a new method for doing such measurements for a LiNbO3 crystal. First, we measured the beam interaction length using a diffraction measurement under both the Bragg phase-matching condition and the phase-mismatching condition. The measurement and analysis show that a correct interaction length will result in a 4% improvement in accuracy. Second, we used the data of that �慨 as a function of time derived from the phase-matching measurement to compute �慨0 and ��. The results have been verified by measuring the slope during initial build-up of the diffraction efficiency, which is proportional to the ratio of �慨0 and �� within 0.1% of error. The �慨0 and �� are also verified by substituting them back to theoretical formula and the resulted theoretical prediction coincides with the experimental data. The results are also consistent with those derived from the non-diffraction efficiency methods. The results show that �慨0 varies as a function of light modulation as Kukhtarev’s model predicted and extend the range up to unit light modulation. The results also show that �� decreases as the total incident power increases.