采用1.5 mW低功率的He-Ne激光（632.8 nm）正入射向列相液晶TEB30A薄膜, 研究向列相液晶TEB30A薄膜在不同强度的弱磁场（0、0.464 9、0.506 2和0.518 5 T）中的远场衍射特性。实验结果表明, 向列相液晶TEB30A薄膜在4种不同强度的弱磁场（0, 0.464 9, 0.506 2和0.518 5 T）中都能产生远场衍射。衍射图样会随磁场强度的增加而发生明显的变化。衍射环的数目增加, 衍射环宽度变宽, 衍射场向外拓展。当磁场强度为0.506 2 T时, 衍射环上出现了近似相互平行的干涉直条纹。当磁场强度达到0.518 5 T时, 干涉直条纹更加明显的趋于平行。在弱磁场的作用下, 激光束在远场的散度变得明显。基于Kirchhoff-Fraunhofer衍射积分原理的理论模拟结果表明, 当激光通过处在弱磁场中的向列相液晶TEB30A薄膜时, 横向非线性相移会增大。随着弱磁场强度由0 T增加到0.518 5 T, 非线性相移也由4π增加到20π。弱磁场引起样品非线性相移增大, 导致其远场衍射图样发生了变化。远场衍射光强图样的变化也是透射光能量分布的改变, 因此, 可将弱磁场调控向列相液晶这一技术应用于磁控光开关及光限幅等领域。

The far-field diffraction characteristics was studied in the different weak magnetic fields（0, 0.464 9, 0.506 2 and 0.518 5 T）with the low power He-Ne laser of 1.5 mW（632.8 nm）being normal incident on the film of nematic liquid crystals TEB30A.The experiment results showed that the film of nematic liquid crystals TEB30A can produce diffraction in the four weak magnetic fields of 0, 0.464 9, 0.506 2 and 0.518 5.The diffraction pattern could obviously be changed with the increasing of the magnetic strength. The number of diffraction rings increased, the width of diffraction rings became wide, and the diffraction field expanded outward. At the magnetic strength of 0.506 2 T, the interference stripes appeared on the diffracting rings. While the magnetic strength increased to 0.518 5 T, the interference stripes was becoming parallel. Under the effect of the weak magnetic field, the divergence of the laser beam was changed drastically in the far-field. Based on the Kirchhoff - Fraunhofer diffraction integral, the theoretical simulation results showed that, when the laser beam passed through the film of nematic liquid crystals TEB30A in the weak magnetic field, the transverse nonlinear phase would increase. With the weak magnetic strength increasing from 0 T to 0.518 5 T, the nonlinear phase increased from 4π to20π.The weak magnetic field caused the nonlinear phase, resulting in the change of the far-field diffraction pattern. The change of the far-field diffraction pattern is also the change of the distribution of the transmitted light energy. Therefore, the technology of the weak magnetic field modulating the nematic liquid crystals can be applied to the areas of magnetic-optical switch and magnetic-optical limiters.