在外磁场中的碱金属原子蒸汽在光泵浦的作用下变成非线性介质，线偏振光经过这种介质时会变成椭圆偏振光，且椭偏光的主轴相对于入射偏振光的偏振面会产生一个微小的旋转。分析了碱金属原子能级结构和在光泵浦下产生的二向色性及其对探测偏振激光的偏振面旋转情况，并介绍了法拉第调制技术对这种旋转的测量方法，得到探测光强与旋转角的关系表达式，可以实现10-7rad量级的小角度检测。

The alkali metals atomic vapor which is pumped by the light becomes nonlinear medium in the magnetic field. After passing this medium, a linear polarized light becomes elliptically polarized. The major axis of the polarization ellipse of the transmitted light rotates a tiny angle compared with the incident polarization. The energy level of the alkali metals atomic is analyzed. The dichroic character of the medium under the laser pumping and the rotation of the polarized light of the probe laser are analyzed. The Faraday modulation technique, which can test the rotatory angle, is proposed. The expression of the detective intensity vs. the rotation angle is obtained. The method can test the angle of 10-7rad.